Zeolites and Ordered Mesoporous Materials: Progress and Prospects: The 1st FEZA School on Zeolites, Prague, Czech Republic, August 20-21, 2005

Studies in Surface Science and Catalysis 157 ZEOLITES AND ORDERED MESOPOROUS MATERIALS: PROGRESS AND PROSPECTS

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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Series Editor: G. Centi

Vol.157

ZEOLITES AND ORDERED MESOPOROUS MATERIALS: PROGRESS AND PROSPECTS The 1st FEZA School on Zeolites, Prague, Czech Republic, August 20-21, 2005

Edited by J. Cejka J. Heyrovsky Institute of Physical Chemistry Academy of Sciences of the Czech Republic Prague, Czech Republic H. van Bekkum Laboratory of Organic Chemistry and Catalysis Delft University of Technology Delft, The Netherlands

2005

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Preface At the beginning of the 21st century zeolites are the most frequently used industrial catalysts, their applications ranging from oil refining, petrochemistry and the synthesis of special chemicals up to environmental catalysis. The rapid progress of basic research and the development of new processes using zeolites led us to the proposal to organize the 1st FEZA School on Zeolites and to prepare this book. Some two thirds of this book reflects the programme of the 1st FEZA School on Zeolites held in Prague on August 20 and 21, 2005. FEZA stands for Federation of European Zeolite Associations, which was established in 1996 and currently includes 15 national zeolite associations. It is the first time that a FEZA Conference was preceded by a Summer School and, in view of the large number of young researchers (mostly PhD students) attending the School, the idea was well received. Factors of importance were a low financial threshold and an attractive programme. When the prospect of a book was raised, all School lecturers reacted positively and agreed to have their contributions ready five months in advance of the School, enabling the organizers to provide the School participants with the book. When designing the School programme, the focus has been on relatively new and exciting areas of zeolite and molecular sieve chemistry. Thus, four chapters deal with the synthesis of the ever-expanding spectrum of zeolites, zeotypes and ordered mesoporous materials. Delaminated materials, germaniumcontaining zeolites as well as micro/mesoporous composites are shifting the borders here. The synthesis of mesoporous materials is covered by two of the inventors and there is also an account of the frontier topic of high-throughput techniques. Two chapters deal with the analytical side. Quite new is the use of electron tomography, providing inside views of zeolite crystals and mesoporous molecular sieves. The techniques of UV-VIS spectroscopy, photoluminescence and X-ray photoelectron spectroscopy are also not so often applied to zeolites. One chapter reviews the application of quantum-chemical methods to zeolite science to show the necessity of combining experimental and theoretical approaches. In the field of applications, one chapter overviews the progress in zeolitic membranes which slowly, but steadily, are coming into practical use. Three chapters deal with the use of zeolites and mesoporous materials as catalysts in organic conversions. These range from the fascinating ship-in-bottle systems via cascade reactions to bulk applications in oil refining and petrochemistry. A two-day school programme has its time limits. In order to arrive at a publication that can serve as a micro-handbook for students, we invited several world experts to contribute chapters. This has strengthened and enriched the book with fine chapters on trends in the molecular sieves field, zeolite structures, ion-exchange properties of zeolites, advanced applications (with many unique technologies and opportunities) and last but not least a chapter on natural zeolites. Finally, we would like to thank all contributors for timely review of their topics and to acknowledge the support of Elsevier in publishing this book, the financial support of sponsors for enabling the low registration fee and a bursary programme, and particularly the enormous effort of Dr. Nadezda Zilkova (J. Heyrovsky Institute of Physical Chemistry, Prague) in editing and formatting the whole book. Prague/Delft - April 2005

Jifi Cejka and Herman van Bekkum

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SUPPORT AND SPONSORING (asofApril!5,2005)

The Organizers of the 1st FEZA School on Zeolites wish to thank various Institutions and Companies for their support to the School. Their contributions allowed a reduced registration fee for students and a bursary program.

INSTITUTIONS J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague International Visegrad Fund and Ceramic Membrane Centre 'The Pore", Delft University of Technology, Delft

COMPANIES Eurosupport Micromeritics Research Institute of Inorganic Chemistry, Czech Republic Sigma-Aldrich, Czech Republic Total Tricat Zeolites

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ORGANIZING COMMITTEE Chairman J. Cejka H. van Bekkum

J. Heyrovsky Institute of Physical Chemistry, Prague University of Delft

Secretary N. Zilkova

J. Heyrovsky Institute of Physical Chemistry, Prague

Treasurer J. Ondrackova

J. Heyrovsky Institute of Physical Chemistry, Prague

Members G. Kosova I. Nekoksova J. Klisakova Z. Pavlackova J. Pawlesa J. Nedvedicka Z. Mlynska

J. Heyrovsky Institute of Physical Chemistry, Prague J. Heyrovsky Institute of Physical Chemistry, Prague J. Heyrovsky Institute of Physical Chemistry, Prague J. Heyrovsky Institute of Physical Chemistry, Prague J. Heyrovsky Institute of Physical Chemistry, Prague Orgit, Ltd. Orgit, Ltd.

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List of contributors Thomas Bein Department of Chemistry and Biochemistry, University of Munich, Butenandtstr. 5-13 (E), 81377 Munich, Germany Herman van Bekkum Ceramic Membrane Centre "The Pore", Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Christian Baerlocher Laboratorium für Kristallographie, CH-8093, ETH Zurich, Switzerland JiríCejka J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3,182 23 Prague 8, Czech Republic Carmine Colella Dipartimento d'Ingegneria dei Materiali e della Produzione, Università Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy Avelino Corma Instituto de Tecnología Química, Av. de los Naranjos s/n, 46022 Valencia, Spain Colin S.Cundy Centre forMicroporous Materials, School of Chemistry, University of Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom Krijn P. de Jong Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands Francesco di Renzo Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRSENSCM-UM1, Institut C. Gerhardt, FR1878, ENSCM, 8 rue Ecole Normale, 34296 Montpellier, France Alan Dyer Institute of Materials Science, University of Salford, Cockcroft Building, Salford M5 4WT, United Kingdom. Francois Fajula Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRSENSCM-UM1, Institut C. Gerhardt, FR 1878, ENSCM, 8 rue Ecole Normale, 34296 Montpellier, France Pierre A. Jacobs Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, B-3001 Leuven (Heverlee), Belgium

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Andries H. Jannsen Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands Anne Julbe Institut Européen des Membranes de Montpellier (UMR 5635 CNRS-ENSCM-UMII) UMII (CC 047), Place Eugène Bataillon, 34095 Montpellier cedex 5, France Kamil Klier Lehigh University, Bethlehem, PA 18015, USA Abraham J. Koster Department of Molecular Cell Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Herman W. Kouwenhoven Ceramic Membrane Centre "The Pore", Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Lynne B. McCusker Laboratorium für Kristallographie, CH-8093, ETH Zurich, Switzerland Agustin Martinez Instituto de Tecnología Química, Av. de los Naranjos s/n, 46022 Valencia, Spain Svetlana Mintova Department of Chemistry and Biochemistry, University of Munich, Butenandtstr. 5-13 (E), 81377 Munich, Germany PetrNachtigall Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, Flemingovo nam. 2, 166 10, Prague 6, Czech Republic WieslawJ.Roth ExxonMobil Research and Engineering Co., 1545 Route 22 East, Annandale, NJ 08801, USA Ferdi Schüth Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1,45470 Mülheim, Germany James C. Vartuli ExxonMobil Research and Engineering Co., 1545 Route 22 East, Annandale, NJ 08801, USA Ulrike Ziesse Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

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Contents Preface List of Contributors Chapter 1 Introduction to molecular sieves: trends of evolution of the zeolite community F. Fajula and F. di Renzo

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Chapter 2 Natural zeolites C. Collela

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Chapter 3 Zeolite structures L B. McCusker and Ch. Baerlocher

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Chapter 4 Synthesis of zeolites and zeotypes C.S. Cundy

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Chapter 5 Synthesis of mesoporous molecular sieves W.J. Roth andJ.C. Vartuli

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Chapter 6 Recent trends in the synthesis of molecular sieves J. Cejka

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Chapter 7 Zeolite membranes - A short overview A. Julbe

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Chapter 8 High-throughput experiments for synthesis and applications of zeolites F. Schüth

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Chapter 9 Ion-exchange properties of zeolites A. Dyer

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Chapter 10 Spectroscopic studies of zeolites and mesoporous materials K. Klier

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Chapter 11 Electron tomography of molecular sieves K.P. de Jong, A.J. Koster, A.H. Janssen and U. Ziese

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Chapter 12 Applications of quantum chemical methods in zeolite science P. Nachtigall

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Chapter 13 Advanced applications of zeolites T. Bein and S. Mintova

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Chapter 14 An evaluation of the potential of the Ship-in-Bottle approach for catalyst immobilization in microporous supports P.A. Jacobs

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Chapter 15 Zeolites in organic cascade reactions H. van Bekkum andH.W. Kouwenhoven

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Chapter 16 Zeolites in refining and petrochemistry A. Corma and A. Martínez

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Subject index

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Introduction to molecular sieves: trends of evolution of the zeolite community Francesco Di Renzo and Francois Fajula Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRSENSCM-UM1, Institut C. Gerhardt, FR 1878, ENSCM, 8 rue Ecole Normale, 34296 Montpellier, France, [email protected] 1. THE DEVELOPMENT OF A CLASS OF HIGH-TECHNOLOGY MATERIALS 2. FIELDS OF APPLICATION AND VOLUMES OF PRODUCTION 2.1. Synthetic zeolites 2.2. Natural zeolites 2.3. Mesoporous molecular sieves REFERENCES

1. THE DEVELOPMENT OF A CLASS OF HIGH-TECHNOLOGY MATERIALS At which extent the traditional definition of zeolites is still valid? Are zeolite scientists still dealing with "crystalline aluminosilicates containing pores and cavities of molecular dimensions" [1], or did they create new materials original enough to render this timehonoured definition obsolete? Indeed, the zeolite community has pushed afar the borders of his field of interest, as any healthy body of scientists has to do. Zeolite researchers presently deal with nanopores instead of micropores, self-assembly instead of synthesis, and they prepare periodical structures from any corner of the periodical table, well beyond the limits of the class of ordered silicates. The evolution of the subject (and of the vocabulary used to describe it) has been astounding and somewhat refreshing but the core of the activity of the zeolite scientist is still the same as it was when Barrer described the first documented synthetic zeolite in 1948 [2]: to apply up-to-date characterisation techniques to the design, synthesis and application of periodical self-assembled objects. The history of the synthesis of zeolites has been the object of recent review articles [1, 4,18] and some landmarks are cited in Table 1. The story of the development of zeolites from a mineralogical oddity to a commodity subtly present in the everyday life of each of us truly began with the discovery of zeolite A and synthetic faujasites by Breck and Milton in 1949 [3, 4]. A cheap and reliable synthesis method was available for new materials, original by their structure or their composition. The stage was set for the development of industrial processes based on the outstanding properties of the zeolites: microporosity, cation exchange and reactivity. An appropriate way to peering inside the properties of zeolites is to follow their development from discovery to process. The first relevant property of zeolites is microporosity. Zeolites, like any other tectosilicate, are formed by a continuous network of oxygen-sharing SiO4 or AIO4" tetrahedra. Alkali or alkaline earth cations are trapped inside

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the network and compensate the negative charge of trivalent aluminium atoms sharing four oxygen atoms with their neighbours. Zeolites are formed by hydrothermal synthesis at temperature levels much lower than feldspar and, for this reason, the cations of the zeolites are hydrated. After the crystallization, the hydration water of the cations can be evaporated by heating. The aluminosilicate framework is rigid enough to avoid collapsing and the micropore volume previously occupied by water is made available to adsorb other molecules. The first commercial application of zeolites was the adsorption of water molecules to dry gas flows [4]. It was early realized that the pore size of zeolites can be tailored, and the selective adsorption of molecules of different size and polarity is nowadays at the basis of many industrial separation processes [19]. The definition of the zeolites as "molecular sieves" correctly describes the screening of molecules of different size by their microporosity [20]. The molecular-size microporosity of the zeolites is also important for catalytic applications. The shape selectivity effect in catalysis corresponds to the selection between possible reaction paths on the basis of the match between shape and size of the reacting molecules and the micropores [21]. Shape selectivity was early recognised [22] and was at the basis of the earliest applications of zeolites in catalysis, specifically as isomerisation catalysts [23]. Table 1 Landmarks in the history of zeolites and related materials 1948 syntheses of mordenite and chabazite 1949 syntheses of zeolites A and X 1954 application of zeolites for gas drying 1959 application of zeolites for paraffin separation 1959 application of clinoptilolite for cation exchange 1961 organic cations as templates 1962 zeolites as FCC catalysts 1967 syntheses of zeolite beta and ZSM-5 1974 zeolites as detergent builder 1978 synthesis of silicalite 1982 aluminophosphate molecular sieves 1983 incorporation of titanium in the TS-1 1992 micelle-templated mesoporous silicates 1994 micelle-templated mesoporous oxides 1999 mesoporous carbons by silica replica 1999 microporous metalloorganic frameworks

R.M. Barrer [2] Union Carbide [3, 4] Union Carbide [4] Union Carbide [4] Ames [5] Barrer and Danny [6] Mobil Oil [7] Mobil Oil [8, 9] Henkel [4] Union Carbide [10] Union Carbide [ 11 ] Eniricerche [12, 13] Mobil [14] G.D. Stucky [15] R. Ryoo [16] Yaghi and O'Keeffe [17]

Natural zeolites have been known, in some ways, since more than two thousand years and have been often used as light building materials since the Roman times [24]. The first industrial applications of synthetic zeolites suggested that natural zeolites could also present a significant added value and prompted the search and discovery of exploitable zeolite deposits [25]. If available natural zeolites have smaller pore opening than synthetic faujasite, they present easily exchangeable charge-compensating cations, and their first application was indeed based on cation exchange: the removal of radionuclides from the wastewater of the nuclear industry [5]. This was the first of many environment-friendly applications of zeolites based on cation exchange, like ammonia removal from wastewater and replacement of phosphates as water softeners in laundry washing. [26] Another environment-friendly application was the use of acidic zeolites as heterogeneous catalysts in replacement of homogeneous acid catalysts, an important source of

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industrial pollution. The properties of acidic zeolites, in which charge compensation of the framework anion is provided by an acidic proton, are related to the way in which their silicate network relaxes when the proton is involved in the catalytic reaction [27]. Due to the elasticity of their structure, the zeolites present a stronger Bransted acidity than amorphous silica-aluminas and became the standard catalyst in FCC (fluid catalytic cracking), at the core of petrochemistry and refining [28, 29]. It was early realised that the acidity of zeolites is affected by their composition. Zeolites with a low aluminium content, hence a low density of lattice anions, present isolated acidic sites, which strength is not diminished by mutual interaction [30]. In this way, catalytic applications usually demand zeolites with a lower aluminium content than applications in water softening or in the separation of air gases, for which a high density of cations is demanded. The aluminium content of zeolites can be controlled by post-synthesis treatment, like in the dealumination treatments which lead to the ultra-stable Y (USY) used in the FCC catalysts [31]. The aluminium content can also be controlled by modifying the conditions of synthesis, albeit each zeolite structure presents a preferential field of composition. It was early shown that the Si/Al ratio is affected by the pH of the synthesis system, the lowest Si/Al ratio being obtained in the most alkaline systems, in which silica is largely depolymerised and is incorporated as isolated tetrahedra [32]. An important method to form zeolites with a high Si/Al ratio was introduced by the research of Barrer on the use of organic cations as templates for the porosity of zeolites [6]. The idea at the basis of this pioneering work was that tetraalkylammonium cations are larger than alkali or alkaline earth cations and can be surrounded by a larger number of SiCU tetrahedra per AIO4" anion. The application of this principle led to the synthesis of new zeolite structures with low aluminium content, usually defined as high-silica zeolites, like ZSM-5 [9] and zeolite beta [8]. Further developments allowed to form all-silica zeolites, the first one being silicalite-1, a siliceous analog of ZSM-5 [10]. The synthesis of these microporous silica polymorphs, happily unaffected by the toxicological hazards of other crystalline silicas [33], allowed to develop applications of zeolites as hydrophobic adsorbents [34]. The introduction of organic templates brought about a multiplication of the number of different zeolite structures [35-37]. The number of stable configurations of SiCU tetrahedra around template molecules is extremely large [38] and each configuration of tetrahedra corresponds to different shape and size of the micropore system. The nomenclature of zeolites rapidly became a headache for the scientific community, especially due to the attribution of different names to zeolites differing by composition or synthesis method albeit presenting the same configuration of tetrahedra. A welcome clarification came through the activity of the Structure Commission of the International Zeolite Association. Since many years the Atlas of Zeolite Structures of the IZA provides an extensive listing of zeolite framework types, characterised by a three-letter code. This classification does not take into account the composition of the framework, which has to be provided as a side information. The last edition of the Atlas [39], published in 2001, reported 133 framework types and, at the end of 2004, 26 more have been synthesized, characterised and accepted by the Structure Commission of the IZA, which report them on its website [40]. The natural zeolite community has followed a different path towards a systematisation of its nomenclature. An historically-determined nomenclature in which the crystal structure, the nature of the charge-compensating cation and some thermal properties were taken into account in a non-systematic manner [41] has been replaced by a nomenclature in accordance with the rules of the International Mineralogical Association (IMA) [42], in which both the structure and composition of the mineral are considered. This has led to a substantial increase

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of the number of natural zeolites, as zeolites with the same structure but different main compensating cation have been considered as individual mineral species. A general acceptation of the IMA nomenclature by the natural zeolite community seems to require some induction time [43]. To come back to the effects of the introduction of organic templates in the zeolite synthesis, the formation of silica-rich zeolites was not the only change in composition allowed by the use of organic cations. The incorporation in the silica framework of elements other than aluminium, virtually impossible with small alkali cations, was made easy by the use of large organic cations [44, 45]. This research field was opened by the synthesis of borium containing zeolites (boralites) [46, 47] and titanium silicalites [12] by the ENI scientists and has brought to the development of the processes of catalytic oxidation based on TS-1 catalysts [13]. The incorporation of transition metals in the zeolite framework is still a thriving research field, as it has been shown by the recent development of the old-established fluoride synthesis route [48, 49] towards the incorporation of germanium in silicate zeolites and the discovery of new zeolite types [50]. The research of new compositions led to an important development when the Union Carbide researchers synthesized the aluminophosphate molecular sieves known as AIPO4 [11]. The framework of these zeotypes is formed by alternating AIO4" and PC>4+ tetrahedra and often corresponds to structural types already known for silicic zeolites. However, several original structural types have been synthesized for the first time as AIPO4 and the structures presenting the largest pore size can only be synthesized as gallophosphate or aluminophosphate [51, 52]. Most AIPO4 are prepared in the presence of an organic cation, albeit the first syntheses of these materials have been realized in the absence of organics [53]. The presence of organic cations has proven to be needed to modify the composition of the AIPO4 by substituting a fraction of the PO4+ tetrahedra with SiO4 tetrahedra [54]. The SAPO (silicoaluminophosphate) materials formed in this way present an anionic framework and the corresponding acidic properties, exploited in industrial catalysis. The number of tetrahedra which form the channel or window of a microporous material provides a first indication of the accessibility of the pores. Zeolites are usually defined as small pore, medium pore or large pore zeolites if their pores are delimited by, respectively, 8, 10 or 12 tetrahedra. Typical examples of these classes are zeolite A (structural type LTA, small pores of 4.1 A diameter), ZSM-5 (MFI, medium pores of 5.3 A), and the zeolites X or Y (FAU, large pores of 7.4 A). Zeolites with pores delimited by 14 or more tetrahedra are defined extra large pore zeolites. The structures with pores delimited by the largest number of tetrahedra present 20 tetrahedra around their pores [51, 52]. However, the structures with 20 member rings present interrupted frameworks, with protruding oxygens which significantly decrease the practical pore opening [55]. The most accessible zeotypes are the 18 member ring gallosilicate ECR-34 (structural type ETR, pores of 10.1 A diameter) [56] and VPI-5 (VFI, pores of 12.7 A) [57]. Zinc phosphates or nickel phosphates with pores delimited by 24 member rings have been obtained [58-60]. However, their structure presents both tetrahedra and octahedra and the pore size is somewhat decreased by corner sharing of the framework polyhedra. The development of the oil production towards the processing of heavier crudes has generated a demand for zeolites with still larger pores, able to catalyse the conversion of very large hydrocarbon molecules. Attempting to cope with this demand, the researchers of Mobil led the way towards a new class of molecular sieves [14]. If the available molecular templates, be them hydrated inorganic cations or organic cations, were too small for the desired pore size, the logical answer was to organise silica tetrahedra around larger objects, in

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fact molecular aggregates. The aggregation properties of surfactants are well known, and the addition of silicates to micellar systems has allowed to form mixed organic-inorganic mesophases corresponding to liquid crystal structures, with silica walls separating surfactant micelles. The removal of the micelles allows to form mesopores with an extremely narrow pore size distribution. A huge body of knowledge about the properties of micellar systems had been made available by the community of colloid science, providing methods to finely tune the size of the micelles and the size of the corresponding mesopores in the final material. The first micelle-templated molecular sieves were formed by using cationic alkylammonium surfactants (cetyltrimethylammonium) and received the collective name of M41s, a definition lumping together several materials: for instance, MCM-41 with parallel mesopores organised in a hexagonal array or MCM-48 with a tridimensional pore system and a cubic symmetry [14]. In a few years, a variety of surfactants were used as templates and mineral phases other than silica or aluminosilicates were used to form the walls between micelles [15,61,62]. The variety of pore geometries and wall compositions made feasible by the collaboration between the colloid scientists and the material scientists is astounding [63]. The contact with the community of surfactants and colloids has not only brought new ideas but also a new language and the discovery of new meanings for old practices. The members of the zeolite community suddenly realized that they had practised self-assembly all their life, when they thought they were just making hydrothermal synthesis, exactly as the would-be gentleman of Moliere was amazed to discover that he has been speaking prose all his life, when he thought he was just talking [64], The blossoming of new materials has been so rapid that the nomenclature of ordered mesoporous materials is in the wild state in which zeolite nomenclature was thirty years ago. A set of rules for writing a standardized crystal chemical formula for both microporous and mesoporous materials has been established by the Physical Chemistry Division of the International Union for Pure and Applied Chemistry, through its Commission on Colloid and Surface Chemistry including Catalysis [65]. The impact of this nomenclature on the activity of the scientists dealing with mesoporous materials has still to be verified. Indeed, crystallography provides a less complete information in the case of the mesoporous materials than in the case of zeolites. At the nanometer scale, the mesoporous materials can present an ordered pore system with a defined space group. However, at the Angstrom scale, the walls between the mesopores are amorphous and diffraction methods are unable to define the position of each atom, as in the case of microporous zeolites. The organisation of the walls has important consequences on the properties of the materials. The inner surface of the pores corresponds to an interrupted framework and mesoporous aluminosilicates present a much smaller acid strength than aluminosilicate zeolites [66]. This situation logically prompted studies on the incorporation of zeolite-like entities inside the walls of ordered mesoporous materials [67-70]. The field of application of ordered mesoporous materials as catalysts has been largely studied [71, 72]. Their main assets are the accessibility to large molecules, the ease with which their surface can be modified by grafting large active groups [73], and the variety of possible compositions of their walls. Materials with silica-based walls are still the easiest to synthesize [62] but the preparation of materials with transition oxide-based walls has been considerably improved by the use of evaporation-induced self assembly, a method in which a surfactant-inorganic mixed mesophase is formed by controlled drying of a liquid film [74-76]. The variety of chemical compositions in which ordered nanocomposites can be prepared has been extended virtually beyond any limit by the nanocasting method, in which a new solid phase is formed inside the porosity of an ordered mesoporous silica [16, 77-80].

6

The possible compositions for the walls of the ordered mesoporous materials go beyond the field of inorganic chemistry, and materials with hybrid organosilica walls have been prepared [81-84]. Some mesoporous benzene-silica hybrids are stable at a temperature higher than 500 °C [84]. Mesoporous materials prepared from polysilazanes and nonionic surfactants can be activated to form silicon carbonitride ceramics, which retain an ordered mesoporosity up to 1500 °C [85]. A parallel expansion of the chemistry of ordered porous materials has been achieved by the contribution of another scientific community, the coordination chemistry scientists, who were at the basis of the development of the MOFs, the metallo-organic frameworks. The preparation of MOFs with permanent porosity has been a long standing challenge. The assembly of rigid molecular building blocks consisting of metal-organic fragments has generated a huge number of 2D and 3D structures, which failed to produce porous materials due to interpenetration of the networks or poor stability leading to collapse upon removal of the guest solvent. Recently, Yaghi and coworkers have reported the successful synthesis of novel rigid and highly porous non-interpenetrating materials based on the coordination of metal ions by carboxylate groups and their assembly into 3D frameworks by organic linkers [17]. This new field of research is rapidly developing, largely due to the accessibility of the metal cations, which provide interesting adsorption sites for the storage of hydrogen and methane [86-92]. The appropriate choice of the organic linkers allows to prepare materials with a surprising thermal stability [93]. 2. FIELDS OF APPLICATION AND VOLUMES OF PRODUCTION 2.1. Synthetic zeolites The world market for synthetic zeolite in 2001 represented some 1.6 million tons (Mt) with 1.3 Mt in the field of detergents, 120 000 tons in catalysis and 85 000 tons in desiccation/adsorption. Zeolites (A-type, LTA) in detergents are basically water softeners, which prevent carbonate precipitation from wash water by extracting hard calcium and magnesium cations via sodium ion-exchange [26]. Zeolite X is also widely used, its large pore volume allowing to include liquid soaps in washing powders and avoid caking. The large expansion of the zeolite detergent market in the past decade has been driven by the growing public sensitivity to environmental issues and the resulting ban on the use of phosphate builders. To date, nearly 90% of the production of zeolite A for detergent uses is consumed in Europe, USA and Japan and growth potential is expected mainly in the Asia-Pacific region [94]. Most of the current large scale commercial processes using zeolite catalysts are in the petroleum refining and the petrochemical industry (Table 2). The unique properties of zeolite catalysts, and their technological and economical impacts on this sector of activity have been reviewed several times [28, 29, 95-97]. Zeolite catalyst consumption is dominated by far by zeolite Y (FAU) for FCC application which accounts alone for nearly 95-97% of the total zeolite catalyst share in terms of tonnage. The remaining share is distributed among a small dozen of different structural types from which over 70 catalysts are derived and used in key processes, including emerging innovative technologies for the synthesis of fine and commodity chemicals [95-99].

7

Table 2 Zeolite structural types used in commercial and emerging catalytic processes Structural type Catalytic process (zeolite or zeotype) FAU (Y) Catalytic cracking, Hydrocracking, Aromatic alkylation, NOx reduction, Acetylation MOR (Mordenite) Light alkanes hydroisomerisation, Hydrocracking, Dewaxing, NOx reduction, Aromatic alkylation and transalkylation, Olefm oligomerisation MFI (ZSM-5, TS-1, Dewaxing, Methanol to gasoline, Methanol to olefins and products, Silicalite) FCC additive, Hydrocracking, Olefin cracking and oligomerisation, Benzene alkylation, Xylene isomerization, Toluene disproportionation, Aromatisation, NOx reduction, Oxidations, Hydration, Amination, Beckmann rearrangement, Cyclodimerisation, BEA (Beta) Benzene alkylation, Aliphatic alkylation, Acetylation, Baeyer-Villiger reaction, FCC additive, Etherification LTL (KL) Alkane aromatisation MWW (MCM-22) Benzene alkylation CHA (SAPO-34) Methanol to olefins AEL (SAPO-11) Long chain alkane hydroisomerisation, Beckmann rearrangement FER (Ferrierite) Olefin skeletal isomerisation ERI (Erionite) Selectoforming RHO (Rho) Amination TON (Theta-1, Long chain alkane hydroisomerisation ZSM-22) Though the global consumption of zeolite catalysts represents less than 10% of the volume consumed as detergents, it constitutes approximately 55% of the global synthetic zeolite market on a value basis [94]. Zeolite adsorbents (mostly A-type and X-type, LTA and FAU, respectively, and to a lesser extent Mordenite, MOR and Y, FAU) are used in a number of processes related to bulk gas separations, dilute impurity removal, drying and chromatography (Table 3). Table 3 Main commercial gas separation and purification processes using synthetic zeolites [100]. Bulk gas separation O2, N2 industrial production (A, X, Mordenite) Medical oxygen production (A, X) H2 production from off-gases (A,X) CO2/CH4 separation (X) Alcohol dehydration (A,X) n-/iso-paraffm separation (A) Xylenes separation (X, Y) Gas analysis r-., .. V . / A V » J J - * \ Chromatography (A,X, Mordenite)

Impurity removal, drying Industrial and natural gas drying (A,X) Insulated glass windows (A) Drying acid gases (Mordenite) Desulfurization of gases (A, Y) CO2 removal (A,X) Solvent recovery (A,X,Mordenite) Silanes removal (A, Mordenite) Removal of rare gases (A) Trapping Hg vapors, SO2, NOx, HC1 (Mordenite) Air Fpollution control (A, X, Mordenite) » , . .„;,' ' Removal of trace NH3 (A)

8

New developments for niche applications of zeolite adsorbents are expected in various domains where they can provide an alternative to carbon filters to meet ultra-low VOCs emission requirements in confined environments (such as houses, vehicle habitacles, painting cabins, workshops ...) [34]. 2.2. Natural zeolites The worldwide production of natural zeolites (mainly clinoptilolite and chabazite) in 1999 was estimated to be between 3 and 4 million tons, on the basis of production reports by some countries and production estimates reported in business journals. China and Cuba definitely dominate the production, with 70% and 15%, respectively, of the total volume Table 4). By far, the largest use of natural zeolites in terms of tonnage is found in the construction industry, for the preparation of dimension stones, pozzolanic cements and concrete and lightweight aggregates. Though exact figures for production and uses are difficult to ascertain, zeolite building materials would account for nearly 90% of the total natural zeolite share. Other applications (ca 350 kt per year) include, in decreasing order by tonnage, pet litter, animal feeding, horticulture, wastewater treatment, odor control, desiccant, fungicide and pesticide carrier, water filtration, gas adsorbent, catalysis, aquaculture, medicinal and pharmaceutical applications, heat storage and solar refrigeration, and many miscellaneous other uses [24]. Table 4 Production estimates of natural zeolites for individual countries in 1999 (Thousands of tons) [101] China 2 500 Greece 4.7 Cuba 500-600 Canada 4 Japan 140-160 4 Italy USA 43 Australia 3.9 Hungary 10-20 Bulgaria 2 Slovakia 12 South Africa 1-2 6 Georgia Other 5 New Zealand 5 TOTAL ca 3 500 The demand for natural zeolites in the past decade has diversely grown, with a high 10% increase for agricultural uses [102], whereas markets for pet litter and water treatment applications leveled in Europe and the US. A main characteristic of the natural zeolite market is the very small volume of imports/exports. Prices of natural zeolites range from 25 to 200 EUR per metric ton, depending on purity and granulometry, making shipping costs prohibitive. The global consumption of natural zeolites is estimated to reach 4.6 Mt in 2005 and 5.5 Mt in 2010 [94]. 2.3. Mesoporous materials The time from discovery to process for synthetic zeolites has been about ten years. This lag has already elapsed since the discovery of the ordered mesoporous materials and times seem ripe for their industrial development. The main obstacle towards viable applications is the presence on the market of much cheaper amorphous alternatives, mainly based on silica gels. Micelle-templated materials has to compete for new applications, to obtain results that can be achieved only thanks to their narrow pore size distribution.

9 Many catalytic applications have been proposed [71, 72]. Mobil announced in 2004 that a mesoporous catalyst was implemented on an industrial plant, but details on the nature of the application are still lacking at the moment in which this notes are written. Hydrotreating or mild hydrocracking processes seem to be the most likely candidates. Other potential applications of ordered mesoporous materials are in chromatographic applications and in the separation of molecules of biological interest, for which their uniform large pores can allow the development of new ways to products of high added value [103105]. The field of sensors and solid-state materials are also promising [106-109]. REFERENCES [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [II] [12] [13] [14]

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

Natural zeolites Carmine Colella Dipartimento d'Ingegneria dei Materiali e della Produzione, Universita Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy

1. INTRODUCTION 2. FROM THE PAST TO PRESENT TIMES 2.1. The early times 2.2. Development of physical-chemical studies 2.3. The discovery of sedimentary zeolite deposits 2.4. The international natural zeolite organisations 3. DEFINITION AND NOMENCLATURE 4. FORMATION 4.1. Zeolite genesis 4.2. Zeolite occurrences 4.2.1. Diagenesis in hydrologically open systems 4.2.2. Diagenesis in hydrologically closed systems 4.2.3. Diagenesis in marine sediments 4.2.4. Burial diagenesis 4.2.5. Genesis from pyroclastic flow 4.3. Zeolites in volcaniclastic deposits 5. PROPERTIES 5.1. Mineralogy 5.2. Chemical and physical-chemical properties 5.2.1. Cation exchange 5.2.2. Reactions with alkalies 5.2.3. External surface reactivity 5.2.4. Physical adsorption 5.3. Other properties 5.3.1. Thermal properties 5.3.2. Physical-mechanical properties 6. APPLICATIONS 6.1. Applications based on ion exchange 6.2. Agricultural uses 6.3. Applications based on adsorption 6.4. Uses as building materials 6.5. Miscellany 7. CONCLUSION AND OUTLOOK BIBLIOGRAPHY REFERENCES

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1. INTRODUCTION As anybody knows, research in the field of zeolite science and technology made its first steps with natural zeolites and was mostly focussed on natural zeolites until the beginning of the 1950s. With the introduction of zeolite synthesis and synthetic materials, interest moved towards these last products, which offered a series of advantages, including (a) wider versatility, due to the large variability of their structures, (b) more open frameworks, suitable for adsorption and catalysis, two of the pillars of the modern chemical technology, and (c) quality, i.e., constancy in constitution and chemistry. From then on, research on natural zeolites, apart from geological and mineralogical studies, was, therefore, mainly devoted to ion exchange, with the purpose to find applications in the removal of toxic and noxious cations from wastewaters of industrial, agricultural or nuclear origin. Successful efforts were also made in agricultural and building applications. The most recent frontier in the application of natural zeolites is in the field of life sciences. One of the drawbacks of natural zeolite research for application purposes is the belief that minerals are in any case a precious resource, compared to the synthetic counterparts, having as major advantage a large availability and, therefore, a lower cost. That is why much research, often with a low degree of innovation, has been and is being performed in unprofitable fields mainly in less favourite or emerging countries, where, for a strange coincidence, a large part of economically valuable zeolite formations are concentrated. Actually, considering natural zeolites as resources of economical value, may be correct, if we look at the raw material, but it may be absolutely untrue if we refer to the cost/benefit ratio, especially in applications for which specific structures and (relative) constancy in composition are needed. Effective usage of natural zeolites should be determined therefore not by the low cost of the material, but rather by the fulfilment of a unique or superior ability, in comparison with the synthetic species.

Fig. 1. Increment of the number of natural zeolites discovered, starting from their appearance on the mineralogy scenery.

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On these bases, in the following, after some introductory notes on history, constitution, occurrences and formation of natural zeolites, a selection of real or realistic applications will be presented. In other words, instead of listing lots of hypothetical applications of no or doubtful economic value, emphasis will be given to uses in which (a) specific advantages of technical character are evident and/or (b) massive utilizations due to their inexpensiveness are expected. Given the vastness and the variety of the matter dealt with, in order to provide useful reference tools for untreated applications, a comprehensive bibliography of useful books and conference proceedings is included at the end of this chapter. 2. FROM THE PAST TO PRESENT TIMES 2.1. The early times The history begins about 250 years ago in Sweden. A famous mineralogist, Axel F. Cronstedt, was the first man who handled a sample of zeolite mineral, studying its apparent properties and discovering its strange behaviour upon heating [1]. Somebody says that the unknown mineral was stilbite, but there is no certain proof of its identity. On the contrary, the first zeolite, a clear evidence of which is found in the literature is chabazite, described by Bosch D'Antic in 1792 [2]. Several other zeolites were discovered in the following years, but rather slowly if one considers that one hundred years later, around 1850, only about twenty zeolite types were reported in mineralogy books, including analcime, brewsterite, chabazite, edingtonite, epistilbite, faujasite, gismondine, gmelinite, harmotome, heulandite, laumontite, levyne, mesolite, natrolite, phillipsite, scolecite, stilbite, and thomsonite. Starting from the middle of the 19th century until about 1975, there was a moderate increment in the number of zeolites discovered (about one new type every 6-7 years) and a clear acceleration in the last twenty five-thirty years (one mineral, in average, every year) (see Fig. 1). 2.2. Development of physical-chemical studies Hydrothermal zeolites were considered for long time mineralogical curiosities and their beautiful, euhedral, sometimes coloured, crystals had certainly prominent positions in private and public mineral collections. But, in the same time, their "strange" properties attracted the attention of scientists, who started investigations on their physical-chemical studies very early [3]. As regards adsorption properties, Damour, starting from 1840, observed in a series of studies that about all the zeolites known at that time could be reversibly dehydrated with no apparent structural damage [4]. Physical adsorption of gases, besides water vapour, on dehydrated zeolites (especially chabazite) was discovered by Friedel at the end of that century [5] and further investigated by Grandjean [6] and Seeliger [7], who incremented the number of gases and enlarged the experimental conditions. Early observations on adsorption selectivity by dehydrated chabazite (uptake of methanol, ethanol and formic acid; exclusion of acetone, ether and benzene) were made by Weigel and Steinhof in 1924 [8], but this important phenomenon was mainly studied by McBain, who first introduced the concept of "molecular sieving" [9]. Natural zeolites (essentially chabazite and mordenite) were further studied as adsorbents until the middle of the 1950s by Barrer and co-workers [10], before leaving space to the supervening synthetic zeolites. The ion exchange phenomenon was discovered around 1850, studying the behaviour of soils in contact with electrolytic solutions [11,12]. A few years later, Eichhorn [13] observed that zeolites, namely chabazite and natrolite, behaved as ion exchangers and

16 demonstrated the reversibility of the relevant reactions. Specific research was then suspended for long time and concentrated on products, called "permutites", a class of partially crystalline synthetic aluminosilicate exchangers, considered more promising than zeolites as water softeners [14]. Nevertheless some research on ion exchange selectivity of a variety of zeolites for peculiar cations, e.g., ammonium, was performed in the early decades of the 20th century (references in [15]). After a period of obscurity, in coincidence with the introduction of ion exchange resins [16], more prompt in exchanging cations, zeolites found a renewed valorisation, starting from the end of the 1950s, in various sectors of environmental relevance, e.g., treatment of wastewaters and soil rebuilding and remediation (see Sec. 6). 2.3. The discovery of sedimentary zeolite deposits For about two hundred years, zeolites were considered minerals of no practical interest, because of their exiguity in known occurrences in vugs and cavities of basalts (rocks with silica content ranging between 45% and 52%) and other trap-rock formations. However, the discovery of their unique and attractive properties useful for many applications of environmental and industrial relevance, promoted search campaigns to find more significant natural zeolite formations. Evidence for the existence of microcrystalline zeolite occurrences of volcanic origin had sporadically been reported in the literature (see, e.g., [17]), but nobody would have expected that so huge zeolite deposits would have been found in a few decades all over the world (see Sec. 4). Contrary to zeolite samples previously known, said of hydrothermal origin, which were characterized by beautiful, very pure, euhedral crystals of centimetre size (Fig. 2a), the new zeolite minerals, occurring in volcaniclastic rocks and said of sedimentary origin, formed usually at lower temperatures and through slower processes. They were crypto-crystalline (crystal size from 0.1 to a few micrometers, usually <10 um) (Fig. 2b), often co-crystallised with other zeolite species and constantly accompanied by conspicuous quantities of impurities, usually amounting to several tens percent. The early discovered sedimentary zeolite deposits, during the 1950s, were the Green tuff formation of Yokotemachi, Akira Prefecture, Japan, [18], the low-grade metamorphic rocks of Southland, New Zealand [19], and the Neapolitan yellow tuff, around Naples, Italy [20]. Many zeolite occurrences were made known a few years later in the western USA, as a result of a capillary five-year exploration campaign promoted by Union Carbide and other chemical companies [21]. After these, hundreds of zeolite formations were and are being discovered in many countries around the world. 2.4. The international natural zeolite organisations An important role in the diffusion of natural zeolites culture around the world and the promotion of the relevant scientific and technological research has been played by an international organisation, ICNZ (International Committee on Natural Zeolites), founded in 1976 with the purpose to serve as a focal point of worldwide interest in natural zeolites and to be instrumental in the organisation of conferences, field trips, symposia, and special sessions at national and international meetings on the subject of natural zeolites. Creator and propulsor of the Committee for over twenty years was Frederick A. Mumpton (1932-2004), who represented the soul of natural zeolites during his entire academic and scientific life. He organised and/or served as co-editor of the Proceedings of several international conferences and meetings, in particular those of the series: Occurrence, Properties and Utilization of Natural Zeolites (see Bibliography). In 2004 the Committee ceased and changed its name in INZA (Int. Association on Natural Zeolites), saving purposes, functions and activities.

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Fig. 2. Phillipsite crystals, (a) macrocrystals of hydrothermal origin (sample from Collezione Vesuviana, Arcangelo Scacchi hall, Real Mineralogical Museum, Naples, Italy); (b) microcrystals of sedimentary origin covering a pumice wall in Neapolitan yellow tuff (sample from Rione Sanita, Naples, Italy) (SEM, Oxford-Cambridge S440) Recently, in 1991, a second international organisation has been established: the Natural Zeolites Commission of the International Zeolite Association (IZA-NZC). The purpose of this Commission, like the other IZA Commissions, is that of taking care of a specific sector of the zeolite world, preparing compilations, atlases, handbooks, maintaining data banks, establishing standard procedures. Peculiar objectives of the Natural Zeolites Commission are: (a) preparation of a worldwide catalogue of sedimentary zeolites with information on their occurrence, properties, use, and including also a series of standard analytical procedures for application purposes; (b) preparation of a reference book providing a summary of all our knowledge about natural zeolites, including genesis, mineralogy, crystal chemistry, physical properties and historical notes. 3. DEFINITION AND NOMENCLATURE The concept of natural zeolite and the connected classification underwent numerous changes in the last seventy years. The first definition, taking care not only of the physical-chemical behaviour, but also of the peculiar structure, is due to Hey in the early 1930s [22]. Further proposals on this subject were made by Smith [23] and Liebau [24], but the discovery of new minerals having only partly the structural features, the chemistry and the properties of the "traditional" zeolites, made necessary the redefinition of the term "zeolite" and the proposition of novel nomenclature rules. A Committee was constituted for this in 1993 by the International Mineralogical Association. After a capillary work lasted about five years a Report was produced on the Recommended nomenclature for zeolite minerals [25]. According to this Report "A zeolite mineral is a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. The channels are large enough to allow the passage of guest species. In the hydrated phases, dehydration occurs at temperatures mostly below about 400°C and is largely reversible. The framework may be interrupted by (OH, F) groups: these occupy a tetrahedron apex that is not shared with adjacent tetrahedra". It is to be observed

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that (a) a natural zeolite is no longer necessarily an alumino-silicate, since the tetrahedral (T) position in the framework may be occupied by other cations, than Si or Al, such as Be, P, Zn or others, (b) a natural zeolite is no longer necessarily a hydrated compound able to give rise to ion exchange, because also anhydrous and/or not exchanging minerals can be regarded as zeolites and (c) a natural zeolite is no longer necessarily characterized by a continuous tetrahedral framework, as T cations may sporadically be bound to monovalent species, such as OH or F. The new definition and the connected rules [25] allowed to recognise and to list, as for 1997, 57 zeolite types (Table 1). A few other zeolites have been discovered in the meanwhile, such as nabesite, a berillo-silicate (Na2BeSi40io-4H20) [26]. Table 1 List of natural zeolites* Name

Year of discovery

Amicite Ammonioleucite Analcime Barrerite Bellbergite1 Bikitaite Boggsite Brewsterite5 Chabazite5 Chiavennite2 Clinoptilolite3§ Cowlesite Dachiardite5 Edingtonite Epistilbite Erionite§ Faujasite45 Ferrierite8 Garronite Gaultite Gismondine Gmelinite5 Gobbinsite Gonnardite Goosecreekite Gottardiite Harmotome Heulandite3§ Hsianghualite Kalborsite Laumontite Leucite Levyne5

1979 1986 1797 1975 1993 1957 1989 1822 1792 1983 1923 1975 1906 1825 1826 1898 1842 1918 1962 1994 1817 1825 1982 1896 1980 1996 1801 1822 1958 1980 1805 1791 1825

Idealised formula K4Na4[Al8Si8O32]-10H2O NH4[AlSi206] Na[AlSi2O6]-H2O Na2[Al2Si7Oi8]-6H2O Me2Sr2Ca2M>'4[Al18Sii8O72]-30H2O Li[AlSi2O6]-H2O Ca8Na3[Al19Si77O,92]-70H2O (Sr,Ba)2[Al4Sii2O32]-10H2O (Ca0.5,Na,K)4[Al4Si4O24]- 12H2O CaMn[Be 2 Si 5 Oi 3 (OH) 2]-2H2O (Me/,Me//0.5)6[Al6Si3o072]-~20H20 Ca[Al2Si3O10]-5.3H2O (Ca0.5,Na,K)4.5[Al4.5Si2o-i9048]-~13H20 Ba[Al2Si3O10]-4H2O (Ca,Na 2 )[Al 2 Si 4 O| 2 ]-4H 2 O K2(Na,Cao.5)8[Al,oSi26072]-~30H20 (Na,Ca0.5,Mgo.5,K)x[AlxSii2-x024]-16H20 (K,Na,Mgo.5,Cao.5MAl6Si3o072]-18H2O NaCa 2 .5[Al 6 Siio0 32 ]-14H 2 0 Na 4 [Zn 2 Si 7 O, 8 ]-5H 2 O Ca[Al2Si2O8]-4.5H2O (Na2)Ca,K2)4[Al8Si ,6O48] -22H2O Na5[Al5SinO32]-12H2O (Na,Ca)6.8[(Al,Si)2o04o]-12H20 Ca[Al2Si6Oi6]-5H2O Na3Mg3Ca5[Al19Si117O272]-93H2O (Bao.5,Cao.5,K,Na)5[Al5Si,i032]-12H20 (Me//0.5,Me/)9[Al9Si27O72]-~24H2O Li2Ca3[Be3Si3O12]-F2 K 6 [Al 4 Si 6 0 2 o]B(OH) 4 Cl Ca4[Al8Sii6O48]-18H2O K[AlSi2O6] (Ca0.5,Na,K)6[Al6Si12O36]-~17H2O

3-letter code GIS ANA ANA STI EAB BIK BOG BRE CHA -CHI HEU DAC EDI EPI ERI FAU FER GIS VSV GIS GME GIS NAT GOO NES PHI HEU ANA EDI? LAU ANA LEV

19 Table 1 (continued) Name

Ye;ir of discove;ry

Lovdarite Maricopaite 2 Mazzite Merlinoite Mesolite Montesommaite Mordenite Mutinaite Natrolite Offretite Pahasapaite 5 Partheite 2 Paulingite 5 Perlialite Phillipsite 5 Pollucite 7 Roggianite 2 Scolecite Stellerite Stilbite§ Terranovaite Thomsonite Tschernichite Tschortnerite 8 Wairakite Weinebeneite Willhendersonite 9 Yugawaralite

1973 1988 1974 1977 1813 1990 1864 1997 1803 1890 1987 1979 1960 1984 1825 1846 1969 1813 1909 1801 1997 1820 1991 1997 1955 1992 1984 1952

Idealised formula K4Nai2[Be8Si28O72]-18H2O Pb7Ca2[Ali2Si36(0,OH)IOo]-~32(H20,OH) Mg2.5K2Cai.5[Al10Si26O72]-30H2O K5Ca2[Al9Si23O64]-22H2O Nai6Cai6[Al48Si72O240]-64H2O K9[Al9Si23O64]-10H2O (Na2,Ca,K2)4[Al8Si4o096]-28H20 Na3Ca4[AliiSi85Oi92]-60H2O Na 2 [Al 2 Si 3 O i0 ]-2H 2 O CaKMg[Al5Sii3O36]-16H2O (Me//5.5,Me/)Li8[Be24P24O96]-38H2O Ca2[Al4Si4Oi5(OH)2]-4H2O (K,Cao.5,Na,Bao.5) io[Al i oSi32084] -27-44H2O K9Na(Ca,Sr)[Al 12Si24O72] • 15H2O (K,Na,Ca0.5,Bao.5)x[AlxSi,6.x032]-12H20 (Cs,Na)[AlSi2O6]-nH2O Ca2[Be(OH)2Al2Si4Oi3]-<2.5H2O Ca[Al2Si3Oi0]-3H2O Ca[Al2Si7Oi8]-7H2O (Cao.5,Na,K)9 [Al9Si27O72]-28H2O Na,Ca[Al 3 Sii 7 0 4 o]->7H 2 0 Ca2Na[Al5Si502o]-6H20 Ca[Al2Si6Oi6]-~8H2O Ca4Me3Cu3(OH)8[Ali2Sii2O48]-nH2O Ca[Al2Si4O,2]-2H2O Ca[Be3(PO4)2(OH)2]-4H2O KxCa(i.5-o.5x)[Al3Si3Oi2]-5H20 Ca[Al 2 Si 6 0 16 ]-4H 2 0

3 -letter code LOV -MOR MAZ MER NAT MON MOR MFI NAT OFF RHO -PAR PAU LTL PHI ANA -RON NAT STI STI TER THO BEA ANA WEI CHA YUG

T h e reported zeolite minerals are 61 instead of 57 as referred to in the text. This is due to the fact that, for historical and opportunity reasons, different names have been saved for the isostructural couples clinoptilolite-heulandite and harmotome-phillipsite and for the isostructural triplet analcimepollucite-wairakite. As regards the 3-letter framework-type codes, see [27]. § Zeolite types for which the existence of series has been recognised. A series is constituted by two or more isostructural individual species each characterized by the prevailing presence of one specific extra-framework cation. 1 Me = (K,Ba,Sr); Me' = (Ca,Na).2 A hyphen before the structural code indicates an interrupted framework. 3 Me' = (Na,K); Me" = (Ca,Sr,Ba,Mg). 4 x = 3.2-4.4.5Me" = Ca; Me' = Li3.6KL2Nao.2.6x ~ 4-7. 7 Cs + n = 1. 8 Me = (K2,Ca,Sr,Ba);n £ 2 0 . 9 0 ^ ^ .

4. FORMATION 4.1. Zeolite genesis Zeolite formation in nature follows pathways which are familiar in laboratory synthesis. Zeolite nucleation, crystallisation and crystal growth take place as a result of slow

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to fast cooling of warm to hot magmas, which are basic, oversaturated in silicate and aluminate species and contain alkaline and/or alkali-earth cations. Magmas may be of volcanic origin or may originate from the interaction of warm to hot fluids with loose volcaniclastic materials, usually fine ashes, deposited in more or less far times as a product of volcanic eruptions. In general, the series of reactions which lead to the formation of zeolite crystals can be schematised as follows: hot fluid + volcanic ash — dissolutlon > oversaturated basic magma — coo/ '" g > zeolite crystals {solution + gel)

The basic character of the magma, deriving from the interaction of the original glassy material with water (hydrolysis) [28], favours the tetrahedral coordination of aluminium, which contributes with silicon to the formation of zeolite framework. The chemistry of the ash and therefore the chemical composition of the resulting solution, together with temperature, are the main parameters playing a structure-directing role. An intermediate stage of glass-to-zeolite conversion is the formation of a gel [28-30], which appears in the early stages of the glass-water interaction, but remains along the whole zeolite crystallisation pathway [29,30]. Its role, however, is not directly connected to nucleation and growth, as there is evidence that zeolite nuclei form from the oversaturated solution at the glass shards / solution interface [31]. Two main parameters differentiate natural zeolitisation from laboratory synthesis: temperature, which ranges from room conditions to several hundred degrees Celsius, and time, which may be very short [32], but also very long, in the case of low kinetics of glass-to-zeolite conversion, especially at low temperatures. 4.2. Zeolite occurrences Despite the apparent simplicity of the process of natural zeolitization, geologists and mineralogists were able to distinguish various zeolite occurrences which correspond to different formation mechanisms in relation to different physical, chemical and geological situations. This matter has been recently reviewed by Hay and Sheppard [33], but an easier and linear approach to this subject may be found in an older paper by Gottardi [34]. As already mentioned, natural zeolites may be classified as hydrothermal or sedimentary, in reference to processes which are not ever very clearly distinguishable from each other. Basically and very schematically, the main differences involve two parameters: distance between the sites of glass dissolution and the sites of crystal deposition, which are usually far in the hydrothermal process and close in sedimentary process (also said diagenesis), and temperature, which is usually higher in the former than in the latter [34]. As regards in particular diagenesis, various distinct situations have been recognised by the geologists [33,34]. The main diagenetic zeolite occurrences are briefly described in the following. 4.2.1. Diagenesis in hydrologically open systems Meteoric and surface waters, percolating through the thick layers of thin volcanic material, become always more basic and saline, as they penetrate in depth. This results in a vertical zonation with formation of clays near the surface and then, going downwards, zeolites, from open types (clinoptilolite, chabazite, phillipsite) to narrow-pore (analcime), and finally alkalifeldspars. Generally, only one or two zeolite species are formed in open systems. A typical example of this type of occurrence is the John Day Formation, Oregon, characterised by fresh glass and clinoptilolite zones, plus sporadic K-feldspar [35].

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4.2.2. Diagenesis in hydrologically closed systems This system is represented by a closed basin, made of impermeable rocks and filled in the past by a saline alkaline lake. Water in this case could not permeate downwards but only evaporate, so the deposit develops horizontally, instead of vertically as in the previous occurrence. Here pH and salinity in the fluids tend to increase, giving rise to brines, e.g., basic, alkali-rich solutions. Concentric zones of authigenic minerals are so formed, from an outer and upper ring of little altered glass and clay minerals, to zeolites, analcime and a finally alkali-feldspars. A good example for this type of occurrence is Lake Tecopa, California, where the zeolitic ring is constituted by phillipsite, clinoptilolite and erionite, followed by the central feldspar zone [36]. 4.2.3. Diagenesis in marine sediments There are many sub-occurrences belonging to this type of zeolite deposition [34]. One of the most important (essentially from a scientific point of view) is that occurring in ocean floors Here, the contact between sea water and deep sea sediments results in very long times, due to the low temperature, in zeolite formation. Phillipsite usually prevails in surface layers [37], whereas clinoptilolite is preferred in the deeper layers [38]. 4.2.4. Burial diagenesis The burial of surface levels through sedimentation processes and the consequent increase of the geothermal gradient are cause of zeolite formation. Generally, the most open (and hydrated) zeolites, e.g., clinoptilolite and heulandite, prevail in the most recent layers, whereas more compact zeolites (e.g., analcime and laumontite) are found in the ancient and deeper ones. This type of occurrence is typical of several Circum-Pacific countries, such as Japan, New Zealand and the west coast of the United States [39]. 4.2.5. Genesis from pyroclastic flow This zeolitisation process is not ascribable stricto sensu to a real diagenesis, resembling someway hydrothermal genesis. The original products, made of pyroclastic material mixed with abundant fluids, derived from freatomagmatic activities and were emplaced at temperatures of a few hundred degrees Celsius. Zeolites, mostly phillipsite and chabazite with minor analcime, presumably formed at high temperatures, very shortly after the emplacement, through the action of the fluids entrapped inside the mass. This type of occurrence is typical of some European deposits, e.g., those of the Neapolitan area, south Italy [40]. 4.3. Zeolites in volcaniclastic deposits Contrary to strictly hydrothermal types, characterised by large crystals, spread in rocks where their presence is absolutely sporadic (see, e.g., Fig. 2a), diagenetic zeolites are concentrated in enormous formations, generally called tuff deposits, from which they can be readily quarried and, after suitable beneficiation processes, utilised. Only a small part, about one third, of the natural zeolite types reported in Table 1 have been found in sedimentary occurrences, as those described above [33,41]. In some instances such occurrences are rare, as in the case of faujasite, whose only known sedimentary formation is in the Aritayn region, Jordan [42]; in other instances they are very frequent [3]. Clinoptilolite-rich tuffs, for instance, are widely spread in numerous locations all over the world and reserves are being continuously discovered. At present exploited deposits (or in view of possible exploitation) are present in Europe (Bulgaria, Greece, Hungary, Italy, Romania, Slovakia, Slovenia, Turkey, Yugoslavia) [41], in Russia and several states of the

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former Soviet Union (Georgia, Ukraine, Azerbaijan), in Asia (China, Iran, Japan, Korea), in Africa (South Africa), in Australia and New Zealand and in many countries of America, such as Argentina, Cuba, Mexico and the United States [43]. Apart from clinoptilolite, other frequent, technologically relevant, sedimentary zeolite occurrences are those of mordenite, phillipsite and chabazite. Mordenite is present in several deposits in eastern Europe (Bulgaria, Hungary, Slovakia, Yugoslavia), in Japan, Russia, the United States and New Zealand [43,44]. Monomineralic deposits of phillipsite and chabazite are infrequent [21,43,44]. On the contrary these two zeolites are rather common in joint occurrences, especially in Italy and in a few other locations, in central Europe (Germany) and in Canary Islands (Spain) [40]. Apart from the described types, it is difficult to establish with certainty which sedimentary zeolites, among those discovered [33,41], may have a potential economic interest, because exploitation and utilisations have in some instances a merely local value. Table 2 (first column) lists some zeolite types which, due to relative abundance and/or technical interest, are being exploited or may have chances for future exploitations. 5. PROPERTIES 5.1. Mineralogy Zeolite tuffs are rocks which contain, besides zeolites, various other crystalline or amorphous phases. One parameter that is especially important in determining the nature of both the formed zeolites and the secondary phases is the silicon content of the original rock. More siliceous rocks gave rise, in fact, to siliceous zeolites, such as clinoptilolite, ferrierite and mordenite, whereas intermediate-silica materials led to the formation of the less siliceous analcime, chabazite and phillipsite. The overall mineralogical composition may vary greatly, even in the single deposit, especially in the contents of the various phases, less as regards their nature. Such an inconstancy is ascribable to the chemical and mineralogical heterogeneity of the parent volcanic rocks and also to the different mechanisms of zeolite genesis. Table 2 Principal zeolites in volcaniclastic rocks* Zeolite species Tsi

Analcime Chabazite Clinoptilolite Erionite Faujasite Ferrierite Laumontite Mordenite Phillipsite 1

0.60-0.74 0.58-0.80 0.80-0.85 0.72-0.79 0.68-0.74 0.83-0.85 0.66-0.71 0.80-0.85 0.52-0.77

Chemistry1 Cations CEC (meqg"1) Na Ca, Na, K Na, K, Ca K, Na, Ca Ca, Na, Mg Ca Mg, K, Na Na, Ca K, Na, Ca

3.57-5.26 2.49-4.66 2.04-2.60 2.68-3.42 2.98-3.42 2.09-2.30 3.82-4.31 2.05-2.43 2.92-5.64

Structural features2 Window s>ize (A) Void vol. 1.6 x 4.2 3.8 3.1 x 7.5 3.6 x 5.1 7.4 4.2 x 5.4 4.0 x 5.3 6.5 x 7.0 3.8

0.18 0.48 0.34 0.36 0.53 0.24 0.35 0.26 0.30

Chemical data mostly taken from [33]. Tsi = fraction of tetrahedral positions occupied by Si. Extraframework cations are those more frequently found. Cation exchange capacity (CEC) was calculated from idealised formulae [33,44], having Na as the only extra-framework cation.2 Window size is that of the largest channel [27]; void volume is expressed as cm3 of liquid water per cm3 of crystal [45].

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The mineral assemblages of the most common zeolite occurrences is as follows. Clinoptilolite- and mordenite-containing tuffs are usually zeolite-rich with contents higher than 50%, up to 80% and over [44]. According to the high silicon content of the parent rock, usual accompanying phases are smectite, quartz, cristobalite and unreacted glass. Sometimes a limited amount of mordenite in clinoptilolite-rich tuffs or clinoptilolite in mordenite-rich tuffs may also be present. Calcite, muscovite and feldspar are other sporadic accompanying phases. Erionite deposits are very rarely monomineralic [46]. It is usually associated to other zeolites, such as clinoptilolite, chabazite and phillipsite and, besides, some clay minerals and feldspars. Its content in the rock is usually low, 20% or less, but the use of these materials is discouraged, because their presence in the environment, due to the thread-like morphology of the erionite crystals, has been associated with the incidence of mesothelioma [47,48]. As regards zeolite formed from less siliceous original magmas, typical examples are those of phillipsite and chabazite, whose parent rocks were usually of trachytic nature. In these cases, the most frequent mineral assemblages include, apart from the two zeolites (often co-present in the same deposit), minor amounts of analcime, biotite, pyroxene and some unreacted glassy material; clay minerals may be sometimes present, too [49]. Zeolite contents are usually high (50-70% or more). Most Italian deposits, spread in the central-southern part of the peninsula, are chabazite-rich with subordinate phillipsite contents, with the important exception of the tuff surrounding the town of Naples (the so-called Neapolitan yellow tuff) in which the situation is opposite. Inconstancy of composition of this type of tuff is a rule, also within the same deposit [50]. 5.2. Chemical and physical-chemical properties Table 2 (second to fourth columns) summarizes some chemical features of the main sedimentary zeolites. It is evident that the mentioned zeolites are medium- to high-silica types. Phillipsite and chabazite belong to the first category, especially if we consider that the values of T$i in the most common joint occurrences [see, e.g., 40] are in the ranges of 0.710.73 and 0.69-0.72, respectively. Instead, clinoptilolite, ferrierite and mordenite are to be considered siliceous zeolites. This chemical character results naturally in different cation exchange capabilities, as it can be seen in the fourth column of Table 2 (CEC values). It is worth to observe that these values are merely indicative, as they refer to pure zeolite species, not to zeolite-rich rocks. On the basis of the theoretical CEC values (Table 2) and zeolite contents in tuff occurrences (see Sub-sec. 5.1), more realistic CEC values for the most common types of zeolitic tuffs may be worked out, as follows: • clinoptilolite-rich tuffs: CEC values ranging between 1.2 and 2.0 meq g"'; • mordenite-rich tuffs: CEC values >1.0 meq g"1 with sporadic much higher values, up to 1.82.0 meq g"1; • phillipsite- and chabazite-rich tuffs: CEC values in the range 1.8-2.5 meq g"1. 5.2.1. Cation exchange The chemistry of natural zeolites may have important effects on their ion exchange properties, mainly in terms of selectivity. It is well known that selectivity is a function of various parameters, depending on (1) framework topology, (2) ion size and shape, (3) charge density on the anionic framework, (4) ion valence and (5) electrolyte concentration in the aqueous phase [51]. Within the same zeolite type, the variation of the framework composition (in practice, Si/Al ratio) and therefore of the framework charge density, affects the cation selectivity [52], as it has experimentally been proven for phillipsite [53]. It is improper, stricto sensu, to compare with each other, in terms of selectivity behaviour, different zeolites having

24

different framework compositions. It is, however, undeniable, according to the EisenmanSherry theory [54], that zeolites with medium to weak anionic field (medium to high silicon content), as most natural zeolites are, prefer, in uni-univalent exchange, cations with low charge density, e.g., NH/, Cs+, whereas in uni-divalent exchange, monovalent cations are preferred, unless divalent cations are characterised by low energy of hydration. Fig. 3, reporting some cation exchange isotherms of clinoptilolite (Fig. 3a) and phillipsite (Fig. 3b), show, although from a purely qualitative point of view, how these two zeolites satisfactorily conform to the predicted behaviour. Curves above the diagonal demonstrate selectivity for NH4+, Cs+, and Pb2+; minor selectivity or unselectivity for Sr2+, Cd2+ and Zn2+ (curves partially or abundantly beneath the diagonal) are mostly explainable by the values of the hydration energy of the three cations (-1443, 1806 and 2044 kJ mol"1, respectively), compared to that of Pb2+ (-1480 kJ mol"'). 5.2.2. Reactions with alkalies Zeolites are generally unstable in alkaline environments, as they tend to transform, similarly to glassy systems, into more stable phases, usually into other framework silicates. This behaviour has suggested in some occasions to utilise zeolitic tuffs as raw material for zeolite synthesis [see, e.g., 60]. The interaction of zeolite-rich materials with Ca(OH)2 is of special interest, because zeolites, like other reactive aluminosilicate systems, e.g., crushed bricks, give rise to calcium silicates and aluminates, which are able to harden upon hydration in both aerial and aqueous environments. This behaviour, already known in ancient times, is typical of a volcanic, mostly glassy material, called pozzolana, which is the genetic precursor of the mentioned Neapolitan yellow tuff, widely spread in the surroundings of Naples, Italy [61]. That is why every material able to behave as pozzolana is called "pozzolanic material" and the property to react with lime is called "pozzolanic activity".

Fig. 3. Profiles of some ion exchange isotherms of Na-exchanged clinoptilolite from Pentalofos (Greece) (a) [55] and Na- exchanged phillipsite from various sites near Naples (Italy) (b) [53,56-59], at 25°C. £(z) and E(S} = equivalent cation fraction in the solid and in the liquid phases, respectively.

25

Fig. 4. Reactivity test of various tuff materials with Ca(OH)2 at 25°C [63]. ERT = sample from Agua Prieta (Sonora, Mexico) (erionite 60%, clinoptilolite 10%); CHT = sample from Marano (Naples, Italy) (phillipsite 31%, chabazite 27%, smectite 4%); CLT = sample from Eskisehir (Anatolia, Turkey) (clinoptilolite 91%); PHT = sample from Tufino (Naples, Italy) (phillipsite 46%, chabazite 5%, analcime 9%, smectite 10%). Residual percentages: inert phases.

Pozzolana has been used since ancient times to obtain, in mixture with Ca(OH)2, hydraulic mortars, sometimes referred to as Roman cement, characterised by a memorable durability, which allows some civil works of about two thousands years ago be still preserved nowadays (e.g., the Pantheon in Rome) [62]. The ability to act as a pozzolanic material is typical of any zeolitic tuff, but the activity is function of different chemical parameters, especially nature, chemistry and content of tuff constituents. Fig. 4 summarizes the results of the reactivity test of four tuff samples with Ca(OH)2, i.e., the ability of these materials to fix lime more or less readily [63]. It is evident that (a) all the tuffs denote a remarkable reactivity for Ca(OH)2; (b) the fastest kinetics is presented by the erionite-rich tuff, which is able to fix in 15 hours the same amount of lime fixed by the other three tuffs in 3 days. The reactivity of the four tuffs is comparable to or ever higher than that of the same pozzolana. Although this statement can not be generalised, it is interesting to note that a typical pozzolana, subjected to the same test, was able to fix the same maximum amount of Ca(OH)2, reported in Fig. 4, in times of the order of 90 days [64]. 5.2.3. External surface reactivity It is well known that zeolites, apart from being microporous solids, exhibit large, negatively charged, external surfaces. Surface charges are balanced by weakly bonded inorganic cations, which results in possible interactions either with large organic cations, unable to enter the internal channels, or with polar molecules. Charge-balancing surface cations in natural zeolites (typically alkaline and alkali-earth cations) can be replaced quantitatively, and essentially irreversibly, by long-chain organic cations, e.g., hexadecyltrimefhylammonium (HDTMA+), ethylhexadecyldimethylammonium, and cetylpyridinium, having also surfactant properties [65]. This involves only the "external"

26

cation exchange capability, leaving unchanged and still available the "internal" cation exchange capability, i.e., most of the original CEC. Surfactant modifications of zeolites, inspired by analogous studies on clay minerals [66], strongly alter the chemistry of the external surface, which becomes enriched in organic carbon, acquires a positive charge and displays anion exchange properties. Accordingly, these materials, even though they save their original ability to exchange cations, are effective to remove inorganic anions from water and, in addition, they exhibit also the capacity to sorb organic molecules. In a series of experiments, a clinoptilolite-rich material from Winston (New Mexico, USA), containing 74% zeolite and 5% smectite, was able to take up by cation exchange an amount of HDTMA as high as 200 meqkg"1. This modified material, while retaining its original ability to exchange Pb2+, displayed a notable affinity for CrC>42~ and also for organic soil pollutants as benzene and perchloroethylene [67]. Humic substances, which are very complex, molecular flexible polyelectrolytes, are common constituents of soil, where they play the fundamental role to store nutrients and to assure their transfer to rhizosphere. These substances, called also humic acids (HA), but normally present in soil in anionic form [68], are stabilised through the formation of organomineral aggregates with colloidal mineral phases, usually clay minerals, C. The aggregation, which can be schematised as [(C-M-HA)x]y, is realised by a cationic bridge, constituted by a polyvalent cation, M [68]. Zeolitic tuffs demonstrated to have an analogous behaviour as clay minerals. Fig. 5 reports some adsorption isotherms of HA on untreated and cation-enriched phillipsite-rich tuff [69]. It is evident that, considering the original untreated tuff as reference, adsorption is more effective, when zeolitic tuff is in Ca2+-enriched form, whereas preexchange with Na+ depresses the formation of the organo-mineral aggregate.

Fig. 5. Profiles of some sorption isotherms of humic acid (HA) on a phillipsite-rich tuff from Marano (Naples, Italy) (see composition in the caption of Fig. 4) at 30°C [69]. q = amount of sorbed HA; C = equilibrium HA concentration in the contact solution; Ca = Ca-enriched tuff; Na = Na-enriched tuff; UNT = untreated tuff.

27

Other large organic molecules may favourably interact with natural zeolite or clay surfaces. Of interest is the ability of these materials, e.g., clinoptilolite- or montmorilloniterich rocks, to adsorb on their hydrophilic, negatively charged surfaces complex substances, such as aflatoxins, which are toxic secondary metabolites of several agricultural products, containing polar functional groups [70,71]. Adsorption, which has been proven either in-vitro or in-vivo, is effective and amounts to some hundred ug per g of adsorber. 5.2.4. Physical adsorption Comprehensive review papers on the sorption properties of natural zeolites can be found in literature [72,73]. Referring in particular to the main sedimentary zeolites, the last two columns in Table 2 show some structural features of interest for sorption applications. Chabazite, clinoptilolite, faujasite and mordenite, which couple reasonably large to large window sizes with wide inner volumes (except mordenite), appear the most suitable materials for adsorption processes. Fig. 6 (a, b), showing some adsorption isotherms of gas and vapours of environmental interest, on chabazite-, clinoptilolite- and faujasite-rich tuffs [74], confirms the expectations. In fact, the best results are presented by the material containing the large-pore faujasite, which displays a normalised adsorption capacity roughly twice that of chabazite-rich tuff capacity and even better, compared to the clinoptilolite-rich tuff. Adsorption rates appear rather high for all the three samples. As regards sulfur dioxide, the performances of the faujasite sample are still the most favourable, whereas those of the other two zeolites are comparable with each other. The results of ammonia adsorption on faujasite are of interest, too. Adsorption capacity is, however, only about half of that displayed for sulfur dioxide and also kinetics appears slower.

Fig. 6. (a) Adsorption isotherms of water vapour on a -90% faujasite-rich tuff (enriched sample) from Aritayn (north-east Jordan) (squares); a chabazite-rich tuff (47% chabazite, 16% phillipsite) from Vulsini (central Italy) (triangles); and a 44% clinoptilolite-rich tuff from Palestra (north-east Greece) (circles), at 25°C [74]. Non-adsorbent phases are not reported, (b) Adsorption isotherms of sulfur dioxide on the same materials (same symbols) at the same temperature. Filled symbols: ammonia adsorption on Aritayn faujasite.

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5.3. Other properties 5.3.1. Thermal properties The thermal behaviour of zeolites has thoroughly been investigated. When heated, a zeolite powder undergoes a series of physical and chemical changes, which include water loss, decomposition and gas evolution, phase transition, structure breakdown, re-crystallisation, melting, and others [75]. The thermal characterisation of natural zeolites has been carried out by various techniques and the relevant data may be found in several publications [44,76-78]. One of the more interesting thermal properties of natural zeolites, similarly to other hydrated materials, such as perlite, is that they expand on heating, due to rapid water evolution at temperatures at which matter is plastic. This enables, starting from zeolite grains, to obtain lightweight materials, having unit weight up to only one third of the parent tuff stone and, in addition, displaying good sound-proofing and heat insulation. Tuff expansion is obtained by quick heating at temperatures of 1250°C or above, depending on zeolite nature, chemical composition and rock constitution, followed by a rapid quenching to room temperature. Expanded materials have been prepared from clinoptilolite- or mordenite-rich rocks [3], but interesting results have been collected more recently starting from phillipsite- or chabazite-rich tuffs, having a zeolite grade >50% and a SiO2/fluxing (Fe2O3 + Na2O + K2O + MgO + CaO) ratio >3 [79]. The treatment was made at temperatures ranging between 1380 and 1400°C for 2-3 minutes (soaking time) with subsequent quenching. Obtained products were practically not permeable to water, presented a remarkable expansion (>100%) and were characterised by low bulk densities, in the range of 0.9-1.1 g cm"3. Zeolite tuff stones display interesting thermal-insulating properties, due to the combination of the physical-chemical features of their zeolitic constituents with the properties connected to their nature of porous stones. This makes the thermal conductivity of tuff stones particularly low, in the range 0.32-038 W m'1 K"', compared to that of other natural or artificial stone materials, whose values are at least 2 up to 8-10 times bigger [80]. Table 3 Main physical-mechanical properties' of some zeolitized tuffs and other stone materials [80] Tuff deposits Italian3 Rhenish4 Transylvanian5 Former USSR6 Clay brick Limestone Sandstone

Zeolites2 Phi and/or cha Cha + phi or cha Cli Cli or cli + mor -

^(kN m"3) 10-16 13-17 16-23 6-12 15-24 23-26 18-26

P(%) 37-61 31-43 26-31 19-44 8-42 5-15 4-20

cr(MPa) 1-13 5-20 2-4

24-79 10-45 49-147 39-127

y = unit weight; P = percent porosity; a = uniaxial compressive strength. Cha = chabazite; cli = clinoptilolite; mor = mordenite; phi = phillipsite. 3 Samples from various tuff deposits in centralsouthern Italy [80]. 4 This material is locally called trass (Eifel region, Germany). 5 Extending from northern to central Romania. 6 Deposits covering the entire territory of the former USSR from west (Ukraine) to far east (Kamchatka, Russia).

29 As far as heat capacity is concerned, zeolitic tuff has a thermal storage capacity that is greater than that of other non-adsorbent building materials as a result of water adsorption phenomena and the associated heat of adsorption [80]. This explains the ability of zeolitic tuffs to act as heat and humidity regulators. The interior of buildings made with zeolitic tuff stones is, in fact, cool during the day and warm at night, as zeolite removes heat from the environment by desorbing water vapour during the warmest hours of the day and returns this heat during the coolest hours of the night by re-adsorbing water. 5.3.2. Physical-mechanical properties The finely crystalline zeolitic matrix of tuffs cements the other non zeolitic particles and is responsible for the overall mechanical properties of the material. The compactness and the consequent physical-mechanical properties are somewhat variable as they are function of various parameters, such as mineral composition, tuff genesis, and original conditions of the deposit, especially as regards looseness of the parent material, grain size distribution, and others. Table 3 summarizes some data on unit weight, y, porosity, P, and uniaxial compressive strength, a, of a few tuff samples and other stone materials. Compared with other building materials, zeolite tuffs display higher porosities and weaker mechanical properties, which accounts for their inferior durability. It is noteworthy that, unlike other building materials, zeolitized tuffs show no strict relationship between P and a, possibly due to the heterogeneity of the materials. 6. APPLICATIONS There is a long tradition of applied research on natural zeolites in several sectors of industrial, agricultural or environmental significance. Updated reviews on these subject have frequently appeared in Proceedings of Conferences on natural zeolites (see Bibliography). Some additional useful papers, reporting the state of art on natural zeolites utilisations, may also be found in the current literature [see, e.g., 3,81-83]. In analysing such literature, care should be used to avoid to mistake real applications for potential or even hypothetical applications. It is obvious that every zeolite, natural or synthetic, possesses a series of basic properties, i.e., it is potentially a catalyst, an adsorber and a cation exchanger, but this does not mean that it can be really used as a catalyst, an adsorber or a cation exchanger. Application, in fact, implies not only possessing the specific property, but having also a series of other requirements, such as specificity, efficiency, reliability, high cos^enefit ratio. Several uses, which have been or are continuously being proposed for natural zeolites fail in one or more of the previous requirements. Often it is believed that natural zeolites are inexpensive, compared to synthetic zeolites. This is certainly true if the specific application requires little or no tuff processing, but sometimes processing is necessary. Ackley et al. [84], for instance, have demonstrated that the global expenses to process a natural zeolite for a hypothetical use in the purification and separation of gases, would raise its original cost up to 60% over the cost of a commercial zeolite 13X, which is , in addition, definitely more effective. On the basis of the preceding considerations, reported applications will be limited to those, realised or potential, in which natural zeolites display either specificity and/or real inexpensiveness. 6.1. Applications based on ion exchange As mentioned in the Sub-sec. 5.2, natural zeolites display exclusive ion exchange properties for some cationic pollutants of water. This is due to both excellent selectivities (see

30

Fig. 3) [54,85] and reasonable exchange rates, K, ranging between 10" -10" 1 meq" min" [86]. For these reasons, on the basis of laboratory and pilot plant experiments performed by Ames and co-workers [e.g., 87], uses in wastewater purification were proposed since the beginning of the 1960s. Relevant applications have been reviewed in the recent literature [3,82,83,88,89]. Only the main (and most successful) uses will be presented here. Clinoptilolite-rich tuffs have been and are being largely used for removing ammonium from municipal sewages. Large-scale facilities have been constructed, especially in the United States, e.g., in Tahoe Truckee Sewage District, California, and in Rosemont, Minnesota, [90], but smaller plants are reported also in other countries, such as Japan and Hungary [3]. Bed regeneration is performed by NaCl and CaC^ solutions. The exhausted regenerant is recovered and recycled, after ammonia air-stripping by alkalinisation. Stripped ammonia is absorbed by a sulphuric acid solution to give a concentrated ammonium sulphate solution used as fertiliser. An improved procedure to recover either ammonium or phosphate from municipal sewages has been proposed, but the procedure did not go beyond the pilot-plant stage. According to Liberti et al. [91], the above nutrients are separately removed from the sewage by ion exchange with a clinoptilolite-rich tuff and an anionic resin, respectively, and recovered from the mixed, exhausted regenerant solutions as precipitated MgNHUPO^FhO, usable as slow release fertiliser. Processes based on ion exchange are successfully utilized in several countries, e.g., United States, Hungary, Russia and Ukraine, also to remove or reduce ammonia in drinking water [82]. Similarly, natural zeolites of different nature are used in aquaculture to remove from re-circulating hatchery waters ammonium produced by the decomposition of excrement and/or unused food [82]. Further, a Ca-exchanged clinoptiloliterich tuff has been proposed to remove ammonium from NASA's advanced life-support wastewater system [92]. Application of natural zeolites in the treatment of nuclear wastewaters is also widely reported in the technical literature [3,82,88]. For reasons of selectivity (Fig. 3), the most frequently treated radio-nuclides have been 137Cs and 90Sr [87]. Natural zeolites are attractive especially because of their cheapness and much more elevated resistance to nuclear degradation, compared with organic resins. Decontamination by ion exchange allows to transfer radio-nuclides from low-level waste streams of nuclear reactors into zeolitic tuff, which is then stored directly or after incorporation into concrete for long-term burial. Natural zeolites utilised for nuclear waste treatments are chiefly chabazite- and clinoptilolite-rich tuffs. Nuclear sites where applications have been and/or are being realised are spread in the United States, e.g., Richland, Washington, [87], Idaho Falls, Idaho, [87], Oak Ridge, Tennessee [93], West Valley, New York, [94], but a few examples are also reported elsewhere, e.g., Sellafield, U.K. [95]. Natural zeolites have also helped in decontaminating waters and soils after nuclear fallout. Mixtures of natural chabazite and synthetic zeolite A were used in 1979 to remove Cs and Sr, respectively, from the contaminated waters after the Three Mile Island accident [96]. Clinoptilolite was used to inhibit Cs uptake in contaminated Bikini Atoll soils [97]. The same zeolite was used to counteract the fallout from the 1986 Chernobyl disaster [82,98]. The recourse to ion exchange to remove heavy metal cations and other toxic or noxious cations from industrial wastewaters is potentially interesting, given the good to reasonable selectivities that natural zeolites display for most of the above cations [86,99]. Actually, no substantial applications are reported. The failure in passing from basic science to technology is mainly due to unsuitability of many wastewaters to be treated by ion exchange for reasons such as (1) too elevated concentrations of polluting cations, (2) low pHs, and (3) excessive interference of the cationic matrix, all reasons of them resulting in low process

31

performances. A possible overcoming of the above drawbacks, al least in some cases, is to renounce to the continuous treatment in column, adopting a discontinuous process (addition of zeolite to liquid wastes). This further reduces the process effectiveness, but gives the advantage to better modulate the solid-to-liquid ratio to the specific process requirements. Obviously, this practice requires that the (abundant) spent zeolitic sludge is processed to avoid pollutant leaching and dissemination in the environment. Research demonstrated that this is possible through a solidification-stabilisation in a consolidated cement matrix (a safely practice used also to confine radioactive material), taking advantage of the excellent pozzolanic activity displayed by zeolitic tuffs (see Sub-sec. 5.2.2.) [100]. In the range of environmental applications of natural zeolites based on ion exchange, also of interest is the possible use of surfactant-modified zeolites (see Sub-sec. 5.2.3.) for the removal of a series of water contaminants. A recent multipurpose investigation demonstrated good performances of these materials for chromate removal from water in a pilot-plant. In addition, laboratory tests demonstrated excellent selectivities for organics present in oilfield wastewaters and even for bacteria and viruses present in sewage effluents [101]. 6.2. Agricultural uses Soils of volcanic origin, sometimes containing significant amounts of natural zeolites [102], have long since demonstrated to be extremely productive, giving strong indications for a specific role of zeolitic minerals in improving their fertility. Experiments on soil amendment by zeolite are, however, rather recent. They, in fact, began in the 1960s in Japan, but gained a rapid success all over the world in the following years. Natural zeolites proved to be beneficial for the growth of a large variety of plants, including spinach, radish, sugar beet, tomato, maize, rice, potato and cucumber [99]. Although zeolites may contribute to fertilisation with their K content (and sometimes with NH4 content, introduced by ion exchange), their working mechanism in soil is largely undisclosed. It is, however, undeniable that zeolites, with their properties of water retention and cation exchange, enhance the plant conditions, particularly in the neighbourhood of rizosphere. On one hand, they behave, in fact, as a sort of reservoir of available water, on the other they increase the global cation exchange capacity of the soil, ameliorating and modulating the dynamics of the nutrient circulation. In addition, their buffering action involves a substantial pH stability in the case of increase of acidity due to alkali depletion and/or leaching. According to recent investigations [69], a further positive action of natural zeolites might be their ability to stabilise the organic components of soil through the formation of organo-mineral aggregates. This property opens the way to massive uses of natural zeolites also for re-building of depleted and/or desertified soils and recovery of soils polluted by potential toxic elements [103,104]. Natural zeolites have recently been used also for the preparation of soilless substrates [105,106]. This practice allows the cultivation of plants in artificial growing media, composed by NH4- and K-exchanged zeolites, e.g., clinoptilolite, and natural or synthetic apatite, which supply a balanced diet for many production cycles without fertiliser addition. 6.3. Applications based on adsorption Uses of sedimentary zeolites based on adsorption are limited for the reasons reported at the beginning of Sec. 6. They regard essentially the more "open" phases among those listed in Table 2, i.e., chabazite, clinoptilolite and mordenite (faujasite would be ideal for adsorption, but its presence in sedimentary deposits is limited to the only Arytain Jordanian occurrence).

32

Most applications concern water vapour adsorption (see Fig. 6a). There are, in fact, many examples of use of the above mentioned materials as drying media of air [107], sour natural gas and other artificial gas streams [82]. Effective, although more limited as regards quantities, is the use as desiccant, in place of silica gel, in many different applications, such as Freon drying in refrigerating circuits, air drying in the frames of the double window-panes and in packaging of optic instruments. Much research has been carried out in recent years on the possibility to use natural zeolites in water desorption-adsorption cycles, in order to capture thermal energy from waste or discontinuous heat sources and to store it until a possible use for heating and/or drying purposes [see, e.g. 108], but no full-scale devices seem to have been realised. Water adsorption-desorption cycles with chabazite- and clinoptilolite-rich materials proved to be useful also in air-conditioning and refrigeration. This gave rise to the production of refrigerators and other heating/cooling devices by a small company in the United States [109]. Prospects of use for selective adsorption of pollutant gases, such as SO2 (see Fig. 6b), from dehydrated combustion gas effluents are sound, considering the good selectivity of natural zeolites for polar molecules compared to non-polar molecules, such as CO2 [74]. Selectivity and sorption capability for NH3 are also elevated [74,110,111], so that applications are predictable for deodorisation of gaseous mixtures. It is noteworthy that raw tuffaceous materials are already sold for this purpose as house's consumer products in the United States, Japan, Hungary, Cuba and Germany [82]. Massive uses are also reported for the treatment of animal wastes in stables and poultry-houses and as deodorising additions to bentonite-made pet litters [82]. Gas separation processes are limited to the production of high-grade O2 from air in generators operating by the pressure-swing adsorption method. Full-scale plants based on mordenite-rich tuffs are operating in Japan since the end of the 1960s [112]; pilot plants are reported working in other countries, e.g., Bulgaria [107]. Other interesting separation processes, still at level of laboratory scale, are feasible, e.g., the water-ethanol vapour separation by phillipsite-rich volcanic tuff, based on the moderate selectivity of this zeolite for ethanol, compared to water [113]. 6.4. Uses as building materials Tuff stones, as many other stones of different nature, have a traditional local usage as building materials, which dates back to prehistory. In historical times (e.g., in Greek and Roman times), until today, the profitable utilisation of tuffs as dimension stones, in side of clay bricks or other artificial stones, was supported by their reasonable mechanical properties, acceptable durability, but especially excellent ability of temperature and humidity control in houses (see Sub-sees. 5.3.1. and 5.3.2.). This is still the most relevant natural zeolite utilisation, although limited to a restricted number of countries (essentially Bulgaria, Cuba, Germany, Greece, Italy, Hungary, Japan, Mexico, Romania and Turkey). The estimated present tuff consumption as stone material amounts to roughly 3 x 106 tons per year [80]. Zeolitic tuff is also utilised in the cement industry as pozzolanic addition (see Sub-sec. 5.2.2.) to portland cement. This application recalls the use of pozzolana, since the beginning of the 1900s, to obtain blended cements, able to fix the lime formed by the hydration of the calcium silicate components of the portland clinker. The utilisation of zeolitic tuff, as substitute of pozzolana, to obtain pozzolanic cements is based on both economic and technical considerations. On one hand, manufacturing blended cements allows a 40% fuel savings, without reducing the quality of the produced binder (it is to bear in mind that the mixture lime-pozzolana is itself a cement), on the other, it involves some advantages, e.g., the

33

improvement of resistance to lime leaching by flowing waters and to expansion due to sulphate attack [114]. An additional advantage is the slower and decreased heat evolution during hydration, which is beneficial in the setting-up of massive structures [115]. The use of natural zeolite materials in the cement industry is well established in many countries, such as Bulgaria, China, Cuba, Germany, Jordan, Russia, Slovenia, Spain, the United States and Yugoslavia [80]. Research is quite active in Greece [e.g., 116] and in Italy [e.g., 63], but in the latter country a possible application is delayed by the still abundant availability of natural pozzolana and by the consolidated use of other artificial pozzolanic materials, such as fly ash, condensed silica fume and heat-treated clay. The application of natural zeolites in the field of cement manufacture is destined to grow in the next years. In fact, the "next revolution in materials of construction" is to go back to the past. The current concrete construction practice is reportedly unsustainable, because the present production of about 1.7 x 109 tpy portland cement has a not negligible environmental impact from standpoint of energy consumption and global warming. So, the expected trend in the concrete industry in the next years is to recur to ancient practices, such as that to re-use the Roman concretes. This would increase the resort to raw, recycled and waste materials, enabling cost reduction and manufacture of environment-friendly products [117,118]. On the basis of the property to expand on heating (see Sub-sec. 5.3.1.), zeolitic tuffs appear very promising as raw materials for manufacturing lightweight aggregates for concrete. A recent investigation demonstrated that mixtures of such expanded materials, prepared from Neapolitan yellow tuff [79], cement, sand and water, gave rise to concretes, whose unit weight and uniaxial compressive strength after 28 days curing, were quite comparable with those obtained using common aggregates, available on the market [119]. In this same sector, also of interest is the possibility to obtain cellular materials by the use of foaming agents (e.g., treating powdered clinoptilolite-rich rocks at temperatures ranging between 1100 and 1400°C in the presence of silicon carbide [120]. These materials, which can be obtained as granules or pre-shaped blocks, display, besides relevant lightness, very good thermal-insulating properties (thermal conductivities are in the range of 0.06-0.21 W m"1 K"'), much better than that of the parent tuffaceous material (see the relevant data in Sub-section 5.3.1.). 6.5. Miscellany Sedimentary zeolites, e.g., clinoptilolite and mordenite, are widely used as dietary supplements for a variety of animals. This subject has been reviewed recently [121], but abundant previous literature is included in most Proceedings of Natural Zeolite Conferences (see Bibliography). The practice to use natural zeolites for animal nutrition began in Japan some forty years ago in imitation of the successful use of montmorillonite to slow down the passage of feed in the digestive system of the chicken with the result to improve nutrients assimilation. Tests were initially made with swine and poultry, but they were then extended to other animals. The results were in general good to excellent, as faster growth and minor incidence of diseases and death were achieved. The action mechanisms are not completely understood, either because of the complexity of the matter (likely the good results depend on various co-causes involving ion exchange and adsorption properties of the zeolites) or the completely different physiology of the various animals treated. Some actions played by zeolites which are generally believed valid are the following: • better nutrients assimilation and feed efficiency (analogously to the above mentioned action played by montmorillonite);

34

• reduction of ammonium amount in circulation, which is taken up by zeolite and gradually released with the result of a better conversion of feed-stuff nitrogen to animal protein; • stimulation of antibody production with consequent diseases inhibition; • reduction of ammonia levels in blood, • withdrawal of toxic substances (essentially cations) from the vital circle; • withdrawal of aflatoxins ingested with contaminated feed (see Sub-sec. 5.2.3.). Natural zeolites are used also for drug preparation. This practice is widespread in various countries, such as Bulgaria and Cuba. The first application regarded clinoptilolite-rich materials which were tested as antacids on the basis of their alkaline nature. At present blends of natural zeolites associated with other gastric adjuvants, such as sodium carbonate [122] or aspirin [123], are sold as Pharmaceuticals having neutralising properties. The administration of natural zeolites proved to have also anti-diarrheic effects, similarly to the recognised action played by some clay minerals, which are for that reason already used for drug formulations. This and other claimed abilities, possibly due to the capacity of zeolites to adsorb endogenous and exogenous substances responsible for this and other diseases [see, e.g., 124,125], opened the way to the preparation of specific drugs, which are on the market, for instance in Cuba. Another massive application of clinoptilolite-rich tuff, reported in Japan and in Hungary, is that of paper filler in place of kaolin. The zeolitic material, which is needed to be white, is claimed to give a paper bulkier, more opaque, easier to cut and less susceptible to ink blotting than that filled with clay [126]. 7. CONCLUSION AND OUTLOOK Natural zeolites are the ancestors of the present synthetic materials, but it should be a mistake to consider them only for historical reasons. Research on and applications of natural zeolites are still alive and vital fields which deserve attention and offer interesting prospects. What should be borne in mind, however, is that natural zeolites can not be considered as substitutes of the synthetic materials; on the contrary, they should be used for some unique peculiarities, when the inconstancy of composition does not represent an obstacle and when their inexpensiveness accords with the obtained benefit. So, fields of environmental or industrial interest, such as water pollution control, manufacture of cement and lightweight aggregates, agriculture, including soil amendment, decontamination or re-building, animal husbandry and aquaculture, should be regarded as preferential fields both for research and application. Am emerging sector, in which more research is needed, is that of life sciences and medicine. Recent results have, in fact, demonstrated that the application of natural zeolites as dietary supplements for animals, which was originally proposed on an empirical basis, has solid fundaments. Research is, therefore, destined to deepen and widen, with the objective to provide specific therapeutic remedies also for man. BIBLIOGRAPHY Proceedings of Conferences and Workshops • Natural Zeolites: Occurrence, Properties, Use L.B. Sand and F.A. Mumpton (Eds.), Pergamon Press, Elmsford, NY, 1978, 546 pp. • Zeo-Agriculture: The Use of Natural Zeolites in Agriculture and Aquaculture W.G. Pond and F.A. Mumpton (Eds.), International Committee on Natural Zeolites, Brockport, NY, 1984,305 pp. • Occurrence, Properties, and Utilization of Natural Zeolites D. Kallo and H.S. Sherry (Eds.), Akademiai Kiado, Budapest, Hungary, 1988, 858 pp.

35 • Zeolites '91: Memoirs of the 3rd International Conference on the Occurrence, Properties and Utilization of Natural Zeolites, G. Rodriguez Fuentes and J.A. Gonzales Morales (Eds.), International Conference Center, Havana, Cuba, 1991, Vols. I-II, 310 + 322 pp. • Natural Zeolites '93: Occurrence, Properties, Use D.W. Ming and F.A. Mumpton (Eds.), International Committee on Natural Zeolites, Brockport, NY, 1995,622 pp.. • Natural Zeolites Sofia '95 G. Kirov, L. Filizova, and O. Petrov (Eds.), Pensoft Publishers, Sofia, Bulgaria, 1997, 300 pp. • Natural Microporous Materials in the Environmental Technology P. Misaelides, F. Macasek, T. Pinnavaia, C. Colella (Eds.), NATO Sciences Series E 362 (Applied Sciences), Kluwer A.P., Dordrecht, The Netherlands, 1999, 516 pp. • Natural Zeolites for the Third Millennium C. Colella and F.A. Mumpton (Eds.), De Frede - Editore, Napoli, Italy, 2000, 484 pp. • Zeolite '02: Proceedings of the 6th International Conference on the Occurrence, Properties and Utilization of Natural Zeolites Microporous and Mesoporous Materials (Special Issue), P. Misaelides, D. Zamboulis and M. Stocker (Eds.), 61 (2003) 1-294. Clays & Clay Minerals, D.L. Bish (Ass. Ed.), 51 (2003) 589-643. Volumes • Mineralogy and Geology of Natural Zeolites MSA Short Course Notes, Vol. 4, F.A. Mumpton (Ed.), Mineralogical Society of America, Washington, DC, 1977, 233 pp. • Natural Zeolites Clays & Clay Minerals, 29 (5) (1981) 321-416 (Special Issue). • G. Gottardi and E. Galli Natural Zeolites Springer-Verlag, Berlin-Heidel berg, Germany, 1985, 409 pp. • Zeolite Deposits Mineralium Deposita, 31 (6) (1996) 451-599 (Special Issue). • Natural Zeolites: Occurrence, Properties, Applications Reviews in Mineralogy and Geochemistry, Vol. 45, D.L. Bish and D.W. Ming (Eds.), Mineralogical Society of America, Washington, DC, 2001, 654 pp.

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A. Hedstrom, J. Environ. Eng. 127(8) (2001) 673. F.A. Mumpton, in: L.B. Sand and F.A. Mumpton (Eds.), Natural Zeolites: Occurrence, Properties, Use, Pergamon Press, Elmsford, NY, 1978, p. 3. L. Liberti, A. Lopez, V. Amicarelli and G. Boghetich, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 351. C. Galindo, Jr., D.W. Ming, M.J. Carr, A. Morgan and K.D. Pickering, in: C. Colella and F.A. Mumpton (Eds.), Natural Zeolites for the Third Millennium, De Frede - Editore, Napoli, Italy, 2000, p. 363. S.M. Robinson, T.E. Kent and W.D. Arnold, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 579. D.C. Grant, M.C. Skirba and A.K. Saha, Environ. Progr. 6(2) (1987) 104. Anonimous, British Nuclear Technology Paper No. 9, Risley, Warrington, U.K., 1987. E.D. Collins, D.O. Campbell, L.J. King and J.B. Knauer, AIChE Symp. Ser. 213 (1982) 9. W.L. Robinson and G.R. Stone, Bikini Atoll Rehabilitation Committee Summary Report No. 6, BARC, Berkeley, CA, 1988, Appendix A, A1-A48 pp. N.F. Chelishev, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 525. C. Colella, in I. Kiricsi, G. Pal-Borbely, J.B. Nagy and H.G. Karge (Eds.), Porous Materials in Environmentally Friendly Processes, Studies in Surface Science and Catalysis No. 125, Elsevier, Amsterdam, The Netherlands, 1999, p. 641. D. Caputo, R. Cioffi, B. de Gennaro, M. Pansini and C. Colella, in: Proc. 4th Int. Congress on Energy, Environment and Technological Innovation, Universita di Roma "La Sapienza", Rome, Italy, 1999, Vol. I, p. 73. R.S. Bowman, Microporous Mesoporous Mater. 61 (2003) 43. J.L. Boettinger and R.C. Graham, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 23. A. Buondonno, E. Coppola, M. Bucci, G. Battaglia, A. Colella, A. Langella, C. Colella, in: R. Aiello, G. Giordano and F. Testa, (Eds.), Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium, Studies in Surface Science and Catalysis No. 142B, Elsevier, Amsterdam, The Netherlands, 2002, p. 1751. E. Coppola, G. Battaglia, M. Bucci, D. Ceglie, A. Colella, A. Langella, A. Buondonno, C. Colella, in: R. Aiello, G. Giordano and F. Testa, (Eds.), Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium, Studies in Surface Science and Catalysis No. 142B, Elsevier, Amsterdam, The Netherlands, 2002, p. 1759. E.R. Allen and D.W. Ming, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 477. D.W. Ming and E.R. Allen, in: C. Colella and F.A. Mumpton (Eds.), Natural Zeolites for the Third Millennium, De Frede - Editore, Napoli, Italy, 2000, p. 417. I.M. Galabova, in: R. Aiello (Ed.), Proc. 3° Convegno Naz. Scienza e Tecnologia delle Zeoliti, Associazione Italiana Zeoliti, Naples, Italy, 1995, p. 253. R. Aiello, A. Nastro, G. Giordano and C. Colella, in: D. Kallo and H.S. Sherry, Occurrence, Properties and Utilizations of Natural Zeolites, Akademiai Kiado, Budapest, Hungary, 1988, p. 763.

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[109] [110] [Ill] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126]

D. Tchernev, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 613. D.T. Hayhurst, in: L.B. Sand and F.A. Mumpton (Eds.), Natural Zeolites: Occurrence, Properties, Use, Pergamon Press, Elmsford, NY, 1978, p. 503. D. Kallo, J. Papp and J. Valyon, Zeolites 2 (1982) 13. H. Minato and T. Tamura, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 509. C. Colella, M. Pansini, F. Alfani, M. Cantarella and A. Gallifuoco, Microporous Mesoporous Mater. 3(1994)219. G. Malquori, in: Proc. 4th Int. Symp. On Chemistry of Cement, National Bureau of Standards, Monograph No. 43, Vol. II, U.S. Department of Commerce, Washington, D.C., 1962, p. 983. M.I. Sanchez de Rojas, M.P. Luxan, M. Frias and N. Garcia, Cem. Concr. Res. 23 (1993)46. Th. Perraki, G. Kakali and F. Kontoleon, Microporous Mesoporous Mater. 61 (2003) 205. P.K. Mehta, Concrete International, ACI, 23(10) (2001) 61. P.K. Mehta, Concrete International, ACI, 24(7) (2002) 23. R. de Gennaro, P. Cappelletti, G. Cerri, M. de' Gennaro, M. Dondi and A. Langella, Appl. Clay Sci. 28 (2005) 309. B.A. Fursenko, L.K. Kazantseva and I.A. Belitsky, in: C. Colella and F.A. Mumpton (Eds.), Natural Zeolites for the Third Millennium, De Frede - Editore, Napoli, Italy, 2000, p. 337. W.G. Pond, in: D.W. Ming and F.A. Mumpton, (Eds.), Natural Zeolites '93. Occurrence, Properties, Use, Int. Committee on Natural Zeolites, Brockport, NY, 1995, p. 449. A. Rivera, G. Rodriguez-Fuentes and L.A. Montero, Microporous Mesoporous Mater. 24 (1998)51. A. Lam, L.R. Sierra, G. Rojas, A. Rivera, G. Rodriguez-Fuentes and L.A. Montero, Microporous Mesoporous Mater. 23 (1998) 247. G. Rodriguez-Fuentes, M.A. Barrios, A. Iraizoz, I. Perdomo and B. Cedre, Zeolites 19 (1997) 441. B. Conception-Rosabal, J. Balmaceda-Era and G. Rodriguez-Fuentes, Microporous Mesoporous Mater. 38 (2000) 161. Y. Kobayashi, Japan Kokai No. 45,041,044 (1970).

Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Zeolite structures Lynne B. McCusker and Christian Baerlocher Laboratory of Crystallography, ETH, CH-8093 Zurich, Switzerland

1. INTRODUCTION 2. ZEOLITE FRAMEWORK TYPES 2.1. Characteristics of zeolite framework types 2.2. Selected zeolite framework types 2.2.1. SOD (Type material: Sodalite, |Na8 C12| [Al6 Si6 O241 - SOD) 2.2.2. LTA (Type material: Linde Type A, |Na,2 (H2O)27|8 [Al,2 Si,2 O48]8 - LTA) 2.2.3. FAU (Type material: Faujasite, |(Ca,Mg,Na2)29 (H2O)240| [Al58 Sii34 O384] - FAU) 2.2.4. EMT (Type material: EMC-2, [Na21 (Ci2H24O6)4| [Al21 Si75 Oi92] - EMT) 2.2.5 CHA (Type material: Chabasite, |Ca6 (H2O)40| [Al,2 Si24 O72] - CHA) 2.2.6. GIS (Type material: Gismondine, |Ca4 (H2O)16| [Al8 Si8 O32] - GIS) 2.2.7. MFI (Type material: ZSM-5, |Nax (H2O)16| [Alx Si96-x O,92| - MFI, x < 27) 2.2.8. MEL (Type material: ZSM-11, |Nax (H2O),6| [Alx Si96.x O,92] - MEL, x < 16) 2.2.9. MOR (Type material: Mordenite, [Nag (H2O)24| [Al8 Si40 O96] - MOR) 2.2.10. MWW (Type material: MCM-22, |H2.4 Na3.,| [Al04 B5.i Si66.5 O,44] - MWW) 2.2.11. *BEA (Type material: Zeolite Beta, |Na7| [Al7 Si57 O,28] - *BEA) 2.2.12. AFI (Type material: AlPO4-5, |(C,2H28N)4 (OH)(H2O)X| [Al12 Pi2 O48] - AFI) 2.2.13. VFI (Type material: VPI-5, |(H2O)42| [Al18 P18 O72] - VFI) 2.2.14. -CLO (Type material: Cloverite, |(C7H,4N)24|8 [F24 Ga96 P96 O372 (OH)24]8 - -CLO) 2.2.15. ETR (Type material: ECR-34, |H,.2 K63 Na4.4| [Gan.6 Al 03 Si36., O%] - ETR) 2.2.16. UTL (Type material: IM-12, [Ge,3.8 Si62.2 O152] - UTL) 2.3. Searching the Zeolite Structure Database on the internet 3. ZEOLITE STRUCTURES 3.1. Framework composition 3.2. Extra-framework species 3.3. Stacking faults 4. POWDER DIFFRACTION 4.1. Information in a powder diffraction pattern 4.2 Common applications 5. CONCLUSIONS REFERENCES

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1.

INTRODUCTION

The fascinating and wonderfully exploitable properties of zeolitic materials, such as their ionexchange properties, their sorption capacity, their shape selectivity, their catalytic activity or their role as hosts in advanced materials, are essentially determined by their structures. For example, sorption characteristics depend upon the size of the pore openings and the void volume; ion-exchange selectivity upon the number and nature of the cation sites and their accessibility; catalytic behavior upon the pore openings, the dimensionality of the channel system, the cation sites, and the space available for reaction intermediates; and host applications on the size and spacing of the cages. Consequently, structural analysis is a fundamental aspect of zeolite science. Information on the framework type alone can elucidate many of the observed properties of a zeolite. The framework type, which just describes the connectivity (topology) of the framework tetrahedral atoms in the highest possible symmetry without reference to chemical composition, defines the size and shape of the pore openings, the dimensionality of the channel system, the volume and arrangement of the cages, and the types of cation sites available. Nonetheless, the chemical composition of the framework, the nature of the species within the channels, and the type of post-synthesis modification also play a very important role in determining the specific properties of a particular zeolitic material. For example, an aluminosilicate framework has a negative charge whereas an aluminophosphate is neutral, a large cation can block or reduce the effective size of a pore opening, a small cation might distort a pore opening, or a sorbed species can influence the catalytic, optical, magnetic or electronic properties of a zeolite. Precise structural details, such as the nature of the distortion of a framework from ideal symmetry or the exact location of non-framework species, are often needed to fully understand the properties of a specific zeolite. Unfortunately, most synthetic zeolitic materials are polycrystalline. That is, single crystals of a size suitable for the application of traditional crystallographic methods of structure analysis (i.e. ~ 50-100 p.m on an edge) are rare. However, zeolites are crystalline with well-defined periodicity even if the crystallites are small, so structural elucidation using powder diffraction data is possible, though not quite so straightforward. Usually a number of analytical techniques are combined to probe the structure of a zeolite. These include sorption experiments (pore size and accessibility), solid state NMR (short range order and connectivity), electron microscopy (symmetry, faulting), and powder diffraction. The following sections of this chapter will cover (1) descriptions of selected zeolite framework types, (2) a discussion of some aspects of real zeolite structures, and (3) a summary of the information that can be extracted from a powder diffraction pattern. 2.

ZEOLITE FRAMEWORK TYPES

Because zeolite scientists recognized very early on that zeolite framework structures are fundamental to the understanding of zeolite chemistry [1], classification of zeolitic materials by framework type, first proposed by Meier and Olson in 1970 [2], has gained wide acceptance in the zeolite community. A framework type, as opposed to a framework structure, simply describes the connectivity of the tetrahedrally coordinated atoms (T-atoms) of the framework in the highest possible symmetry. The framework composition, the observed symmetry, and the actual unit cell dimensions are not considered. In this way, many different materials can be classified under one designation. For example, amicite, garronite, gismondine, gobbinsite, Na-Pl, Na-P2 and SAPO-43 all have the gismondine framework type

43

(GIS). A three letter code (e.g. GIS) is assigned to confirmed framework types by the Structure Commission of the International Zeolite Association according to rules set up by an IUPAC Commission on Zeolite Nomenclature [3,4]. The codes are normally derived from the name of the zeolite or "type material", e.g. FAU from the mineral faujasite, LTA from Linde Type A, and MFI from ZSM-5 (Zeolite Socony Mobil - five). Information pertinent to these framework types is published in the Atlas of Zeolite Framework Types [5] and on the internet at http://www.iza-structure.org/databases/. As new codes are approved, they are announced on the IZA Structure Commission's WWW pages (http://www.iza-structure.org/) and included in the internet version of the Atlas. As of January 2005, 161 zeolite framework types had been confirmed by the Structure Commission. In this chapter, all references to materials whose framework types are known will be accompanied by the appropriate three letter code in boldface type. 2.1. Characteristics of zeolite framework types The feature that is common to zeolite or zeolite-like materials is that they all have a 3dimensional, 4-connected framework structure constructed from corner-sharing TO4 tetrahedra (basic building unit or BBU), where T is any tetrahedrally coordinated cation. This framework structure is relatively open and characterized by the presence of channels and cavities. A description of a zeolite structure almost always begins with a description of the framework type in terms of the size of the pore openings and the dimensionality of the channel system. Pore openings are characterized by the size of the ring that defines the pore, designated an »-ring, where n is the number of T-atoms in the ring. An 8-ring is considered to be a small pore opening, a 10-ring a medium one, and a 12-ring a large one, with free diameters or effective pore widths (calculated using an oxygen radius of 1.35 A) of approximately 4.1, 5.5 and 7.4 A, respectively. Of course, rings can be distorted considerably so these numbers should only be used as a rough guide. A number of structural features (cages, channels, chains, sheets) are common to several different zeolite framework types, so designations such as a-cavity and (3-cage, pentasil unit, crankshaft and double crankshaft chain, and 4.82 sheet or net have crept into common usage. To help the reader, some of these subunits are shown in Figs. 1, 2 and 3. In these drawings, oxygen atoms have been omitted for clarity. Since polyhedral building units are sometimes described in terms of the «-rings defining their faces, these designations are also given in Fig. 1. For example, a truncated octahedron (sodalite cage), whose surface is defined by six 4-rings and eight 6-rings, would be designated a [4668] cage. The three double chains in Fig. 2 also occur as single chains in many zeolites, but these are so common that they are seldom discussed as a characteristic feature of a structure. The Narsarsukite chain is found more often in AlPCVstructures than in silicates, whereas the pentasil chain of edge-sharing [58] cages is characteristic of a family of high silica zeolites (MFI, MEL). The channel walls of zeolites with 1-dimensional pores are often composed entirely of 6-rings. The two possible orientations of the 6-rings in such "6-ring wraps" are shown for the 12-ring channels in AFI and CAN in Fig. 3. A nomenclature similar to that used for cages has also been developed to describe 2dimensional, 3-connected sheets or nets. In this case, the sizes of the three n-rings associated with each node are used for the designation. In the net shown in Fig. 3, for example, each node is associated with one 4-ring and two 8-rings and is therefore called a 4.8 net. To complete the 3-dimensional description, the orientation of the fourth connection can also be

44

Fig. 1. Some subunits and cages/cavities that recur in several framework types.

given as U or D (i.e. pointing Up or Down from the sheet). The example given in Fig. 3 describes the 4.82 sheet found in the GIS framework type, where the connections from half of each 8-ring point up and the other half point down. The 8-rings containing the letters correspond to the central 8-ring (front layer) in the GIS framework type shown in the next section in Fig. 9. Another example of a framework type that can be described in terms of a 4.82 sheet is that of ABW, which has an UUDUDDUD orientation of tetrahedra around the 8nngs. Some frameworks consist only of cages with a maximum ring size of six and have no channels (e.g. the pure-silica clathrasils), but the majority have at least 8-ring channels. These channels can intersect to form 2- and 3-dimensional channel systems, and this can be a critical feature for catalytic or sorption applications. For example, a 1-dimensional channel is much more easily blocked by the formation of coke deposits than is a higher dimensional one where "detours" are possible. The stacking sequence of layers, cages or rings in zeolite frameworks is often described using the "ABC-system". This crystal chemistry terminology, which is usually used to describe the stacking of layers of closest packed spheres (atoms) in metals or oxides, has been adapted to describe stackings in certain types of zeolite structures. For example, seventeen of the 161 zeolite framework types can be described in terms of stackings of hexagonal arrays of 6-rings (Fig. 4), and are known as the ABC-6 family of zeolite frameworks (see SOD and CHA in the following section). The longest stacking sequence reported for the ABC-6 family is that of Giuseppettite (GIU) with sixteen layers (ABABABACBABABABC) [6]. Other stackings described using the ABC terminology involve sheets of sodalite cages (see FAU and EMT in the following section). This concept of stacking sequences is not only an elegant

45

double double double zig-zag sawtooth crankshaft Fig. 2. Some chains that recur in several framework types.

Narsarsukite chain

pentasil chain

way of describing a family of frameworks, but also appears to reflect the way nature builds real materials with such frameworks (see section 3.3). Zeolite frameworks can be classified according to various schemes (e.g. by pore opening, by structural subunit, by channel system, by framework density, by loop configurations, by vertex symbols, and/or by coordination sequences). Most of these features are defined in the introductory pages of the Atlas of Zeolite Framework Types and then given

channel wall in AFI

channel wall in CAN

4.8 2 sheet or net

Fig. 3. Two types of channel walls composed of 6-rings (left), and the GIS 4.82 sheet (right)

46

Fig. 4. ABC-stacking of hexagonal arrays of 6-rings viewed (a) in projection, and (b) from the side. for each framework type. It is perhaps worth noting that the set of coordination sequences and vertex symbols for each of the T-atoms in a given framework type is unique, so this is a good way of determining whether or not the framework of a new zeolite is novel. 2.2. Selected zeolite framework types Although there are 161 confirmed zeolite framework types, only a few of them describe zeolites or zeolite-like materials that are actually used in industrial applications. Sixteen have been selected for a more detailed description here. Some have been chosen because of their industrial relevance, and some because they illustrate specific structural features. They are presented approximately in the order of the historical development of zeolite synthesis from aluminosilicates to high-silica zeolites to aluminophosphates to gallophosphates to gallosilicates to germanosilicates. No ranking is implied. For each framework type, the name and IUPAC crystal chemical formula [4] of the type material is given. In the drawings of the frameworks (Figs. 5-18), the nodes represent T-atoms and the lines oxygen bridges. For clarity, most rings with fewer than eight T-atoms have been made opaque. The selected aluminosilicates are sodalite (SOD), zeolite A (LTA), faujasite (FAU), EMC-2 (EMT), chabasite (CHA), gismondine (GIS). With the exception of EMT, all of these framework types have also been synthesized as aluminophosphates. The high silica zeolites, with a Si/Al ratio of at least 5, are ZSM-5 (MFI), ZSM-11 (MEL), mordenite (MOR), MCM-22 (MWW) and zeolite beta (*BEA). The common feature of these framework types is the presence of 5-rings. To complete the spectrum, two aluminophosphates, AlPO4-5 (AFI) and VPI-5 (VFI), a gallophosphate, cloverite (-CLO), a gallosilicate, ECR-34 (ETR), and a germanosilicate, IM-12 (UTL) will be discussed. 2.2.1.

SOD (Type material: Sodalite, |Na8 C12| |A16 Si6 O24] - SODJ

In the strictest sense of the word, sodalite is not a zeolite, because it has only 6-ring windows and thus has very limited sorption capacity. However its framework density of 17.2 T-atoms per 1000A3 is well within the zeolite range. It is an important material for creating simple periodic arrays of clusters, and is one of the most seriously investigated hosts for advanced materials [7]. The blue pigment ultramarine is a sodium aluminosilicate with a SOD-type framework and sulfide ions replacing the chloride ions inside the cages. Sodalite has much in common with some of the zeolites used in industrial applications. The SOD framework type (Fig. 5a) is best described as a body-centered cubic arrangement of (3 or sodalite cages (see Fig. 1) joined through shared 4- and 6-rings. It is also a member of the ABC-6 family of

47

Fig. 5. (a) the SOD framework type, and (b) the LTA framework type. zeolites [8], and can be viewed as an ABCABC stacking of hexagonal arrays of single 6-rings in the [111] direction (the body diagonal of the cubic unit cell). 2.2.2 LTA (Type material: Linde Type A, |Nai2 (H2O)27|8 [Al]2 Si12 O48]8 - LTA) The LTA framework type (Fig. 5b) is related to SOD, but in this case, the sodalite cages, in a primitive cubic arrangement, are joined via oxygen bridges to form double 4-rings rather than sharing a single 4-ring. This creates an a-cavity (see Fig. 1) instead of a P-cage in the center of the unit cell, and a 3-dimensional, 8-ring channel system. Alternatively, the framework can be described as a primitive cubic arrangement of a-cavities joined through common 8rings (producing a sodalite cage in the center). This is one of the more open zeolite framework types with a framework density of only 12.9 T-atoms per 1000A3. Zeolite A is used as a desiccant both in the laboratory and between the panes of glass in double-glazed windows, and as an ion-exchanger (water softener) in laundry detergents. 2.2.3. FAU (Type material: Faujasite, |(Ca,Mg,Na2)29 (H2O)240| [Al58 Si134 O384] - FAUJ There are also sodalite cages in the FAU framework type (Fig. 6). In this case, they are arranged in the same way as the carbon atoms in diamond, and are joined to one another via double 6-rings. This creates the so-called supercage with four, tetrahedrally-oriented, 12-ring pore openings, and a 3-dimensional channel system along <110>. The framework density, at 12.7 T-atoms per 1000 A3, is even lower than that of LTA. There is a center of inversion in each of the double 6-rings, so the puckered layers of sodalite cages are related to one another by inversion. The framework type can also be described as an ABCABC stacking of such layers. The combination of large void volume (ca 50%), 12-ring pore openings and 3dimensional channel system makes the thermally stable silicate materials with the FAU framework type ideal for many catalytic applications. 2.2.4. EMT (Type material: EMC-2, |Na2i (Ci2H24O6)4| [Al2, Si75 O]92] - EMTj In the same way that lonsdaleite is a hexagonal analog of diamond (or wurtzite one of zinc blende), the EMT framework type (Fig. 7) is the simplest hexagonal analog of FAU. In EMT, the puckered sodalite cage layers are stacked in an ABAB sequence and the layers are related to one another by a mirror plane. This arrangement of sodalite cages creates a medium cavity with three 12-ring pore openings and a larger cavity with five. As in FAU, the

48

Fig. 6. The FAU framework type and its supercage. The three different layers of sodalite cages are indicated with the letters A, B and C. Layer A is highlighted in gray.

resulting channel system is 3-dimensional with 12-ring pores, but the nature of the channel system and of the larger cavities in the EMT framework type is significantly different. As might be expected, this framework type is also well-suited for catalytic applications.

Fig. 7. The EMT framework type showing the medium and larger cavities separately. The two different layers of sodalite cages are indicated with the letters A and B. Layer A is highlighted in gray.

49

Fig. 8. The CHA framework type (AABBCC 6-ring stacking indicated) and its cavity.

2.2.5. CHA (Type material: Chabasite, |Ca6 (H2O)40| [AI12 Si24 O72] - CHAj The CHA framework type (Fig. 8) is another member of the ABC-6 family of zeolite frameworks. While SOD can be described in terms of an ABC stacking of hexagonal arrays of single 6-rings, CHA has an ABC stacking of double 6-ring arrays (or an AABBCC stacking of single 6-ring arrays). This stacking produces an elongated cavity with six 8-ring pores and a 3-dimensional channel system. Unlike the previous examples, the channels in CHA are not straight. The silicoaluminophosphate with this framework type is used in the conversion of methanol to olefins and in the aldol condensation of aldehydes. 2.2.6. GIS (Type material: Gismondine, |Ca4 (H2O)i6| [Al8 Si8 O32] - GISj The GIS framework type (Fig. 9) can be described as a stacking of 2-dimensional arrays of double crankshaft chains (Fig. 2). There are 8-ring channels running parallel to x and y, displaced with respect to one another along z. They intersect to form a 3-dimensional channel system. The double crankshaft chains are very flexible, and so is the GIS framework. Materials with this framework type have symmetries varying from monoclinic (e.g. gismondine) to orthorhombic (e.g. gobbinsite) to tetragonal (e.g. garronite) and the lattice parameters can differ by as much as 6%. The framework type can also be described in terms of 4.8 nets stacked along the x or y direction (see section 2.1). The "maximum aluminum P" zeolite (or MAP for short), which is used as an ion-exchanger in laundry detergents, has this framework tvoe.

Fig. 9. The GIS framework type with a double crankshaft layer highlighted.

50

Fig. 10. The MFI framework type with pentasil chains running parallel to z. One corrugated sheet perpendicular to z has been highlighted in gray. Adjacent sheets are related to one another by inversion centers (in the 6- and 10-rings).

2.2.7. MFI (Type material: ZSM-5, |Na* (H2O)i6| [Alx Si96-x Oi92] - MFI, x < 21) The framework type of the high silica zeolite ZSM-5 (Fig. 10) can be described in terms of [54] units, but it is easier to use pentasil units (Fig. 1). These [58] units are linked to form pentasil chains (Fig. 2), and mirror images of these chains are connected via oxygen bridges to form corrugated sheets with 10-ring holes (e.g. the gray sheet perpendicular to x in Fig. 10). Each sheet is linked by oxygen bridges to the next to form the 3-dimensional structure. Adjacent sheets are related to one another by an inversion center. This produces straight 10ring channels parallel to the corrugations (along y), and sinusoidal 10-ring channels perpendicular to the sheets (along x). The latter channels link the straight channels to one another to form a 3-dimensional 10-ring channel system. Because the pore openings are 10rings rather than 12-rings, the shape selectivity for sorption and catalysis is distinctly different from that of FAU- or EMT-type zeolites, and this fact is exploited in catalysis applications. ZSM-5 has found many applications in refinery and petrochemical processes. 2.2.8. MEL (Type material: ZSM-11, |Nax (H2O),6| [Alx Si96-x O192] - MEL, x < \6) In the MEL framework type (Fig. 11), the corrugated sheets of pentasil chains that are found in MFI are also present (one is highlighted in gray in Fig. 11). However, in MEL, adjacent sheets are related to one another by a mirror plane rather than by a center of inversion. This produces straight 10-ring channels along both x and y. Because these channels are displaced from one another in z, a 3-dimensional channel system is formed. As might be expected, intergrowths of the MEL and MFI framework types can and do occur (see section 3.3). 2.2.9. MOR (Type material: Mordenite, |Na8 (H2O)24| [Al8 Si40 O96] - MOR) In the MOR framework type (Fig. 12), units of four 5-rings [54] (Fig. 1) are joined to one another via common edges to form chains. Mirror images of these chains are connected via oxygen bridges to form corrugated sheets (lying horizontally in Fig. 12). These sheets, displaced by half a translation in c, are then connected to one another to form oval 12- and 8rings along the corrugations. The lining of the 12-ring channels contains 8-rings, but the 8ring openings of adjacent 12-ring channels are displaced with respect to one another, so only

51

Fig. 11. The MEL framework type with pentasil chains running parallel to z. One corrugated sheet perpendicular to x has been highlighted in gray. Adjacent sheets are related to one another by mirror planes (running through the 6- and 10-rings).

very limited access from one channel to the next is possible, Consequently, the channel system is effectively one dimensional. 2.2.10. MWW (Type material: MCM-22, |H2.4 Na3.,| |A1O.4 B5., Si66.5 OM - MWWj The high-silica zeolite MCM-22 has a rather unusual framework structure (Fig. 13). It can be viewed as a stacking of double layers joined by single oxygen bridges. The single layers consist of [435663] cages sharing 4-ring faces, and are joined to a second layer via double 6rings. The two layers of the double layer are mirror images of one another. The framework has two non-intersecting, 2-dimensional, 10-ring, channel systems. One of these lies within the double layer, and the second between the double layers. The latter also has two side

Fig. 12. The MOR framework type (left) and the chain composed of edge-sharing [54] units (right). The chains in the first layer (related by mirror planes) are highlighted in gray.

52

Fig. 13. The MWW framework type showing the double layer, the small [435663] cage and the side pockets at the intersections of the channels running between the double layers. pockets (12-ring access) at each channel intersection that form large cages (see Fig. 13, right). While the [435663] cage with a T-atom inside the cage may appear a little unusual, the geometry is quite reasonable. 2.2.11. *BEA (Type material: Zeolite Beta, |Na7| [Al7 Si57 Oi28] - *BEA;

Zeolite beta is disordered in the c-direction. That is, well-defined layers are stacked in a more or less random fashion. Since no ordered material has yet been produced, the three letter code is preceded by an asterisk to indicate that the framework type (Fig. 14) described in the Atlas is an idealized end member of a series. [54] units are joined to one another via 4-rings to form layers with saddle-shaped 12-rings. Adjacent layers are related to one another by a rotation of 90°. The disorder arises because this rotation can be in either a clockwise or a counterclockwise sense. If the counterclockwise or clockwise rotation were maintained throughout the crystal, the structure would be ordered and chiral. Interestingly enough, whatever the stacking sequence, a 3-dimensional 12-ring channel system results, so for catalytic applications, the stacking sequence is not important (unless, of course, the chirality of the channel system were to be exploited in some way). In 2000, Conradsson et al. synthesized a germanate, FOS-5 [9], with a strict alternation of clockwise and counterclockwise rotations of the *BEA layers (beta polymorph C, |(C3H9N)48 (H2O)36| [Ge256 O5i2] - BEC), and this ordered (non-chiral) framework has been assigned the code BEC. A silicogermanate material, ITQ-17 [10], and a pure silicate overgrowth on ITQ-14 [11] with this framework type have also been reported. It is interesting to note that while the pure germanate contained single crystals, the silicogermanate was polycrystalline, and the pure silicate was only nanometers in size. Germanium is known to stabilize double 4-rings, and these are prevalent in BEC. Consequently, the more germanium in the material, the larger the crystals.

53

Fig. 14. The idealized *BEA framework type with all layers related to one another via a counterclockwise rotation (connections between layers shown as dotted lines). The well-defined layer, and its building unit are shown separately.

2.2.12. AFI (Type material: AlPO4-5, |(Ci2H28N)4 (OH)(H2O)X| [Al12 P12 O48] - AFI) As for all AlPO4-based molecular sieves, the framework of AIPO4-5 (Fig. 15a) contains only even numbered rings, since Al and P alternate throughout the framework. In the AFI framework type, 6-rings are connected to three neighboring 6-rings via oxygen bridges to form 4-rings between the 6-rings and a hexagonal array of 12-rings. The tetrahedra are oriented in a strictly alternating fashion, so that every other one points up to the next layer while the others point down to the previous one. Mirror images of these layers are stacked on top of one another to form a 1-dimensional 12-ring channel system. Unlike the aluminosilicate molecular sieves, which tend to favor double crankshaft chains for connecting 4-rings in adjacent layers (e.g. tetrahedra oriented in an UUDD fashion), the aluminophosphates seem to prefer the Narsarsukite chain (Fig. 2), in which diagonally related corners of the 4-rings form the bonds to the next layer (e.g. UDUD connections). The 12-ring channel in AFI is lined with 6-rings (Fig. 3). 2.2.13. VFI (Type material: VP1-5, |(H2O)42| [Al18 P18 O72] - VFIj The framework of the aluminophosphate VPI-5 (Fig. 15b) is closely related to that of AIPO45. Instead of being linked via 4-rings, the 6-rings in the VFI framework type are linked via two 4-rings sharing a common edge (fused 4-rings). This produces an 18-ring in place of the 12-ring found in AFI. The tetrahedra are oriented in the same manner, and layers are stacked similarly. The 18-ring channel, with an effective width of ca 12 A, is also lined with 6-rings. One feature of the VFI framework type worth noting is the unusual conformation of the fused 4-rings. The geometry is highly strained if the T-atoms are assumed to be tetrahedral. The Al atom on the edge shared by the two 4-rings relieves this unfavorable situation by coordinating to two water molecules in addition to the four framework oxygens, and assumes an octahedral geometry [12]. Upon dehydration, these water molecules are lost, and VPI-5 transforms very easily into the related molecular sieve AIPO4-8 (AET) with 14-rings and fewer fused 4-rings

54

Fig. 15. (a) The AFI framework type, and (b) the VFI framework type. [13,14]. Under carefully controlled conditions, VPI-5 can be dehydrated and retain its framework type (albeit with considerable reduction in symmetry [15]). 2.2.14. -CLO (Type material: Cloverite, |(C7Hi4N)24|8 [F24 Ga 96 P96 O372 (OH) 24 | 8 - -CLOJ Of the gallophosphate molecular sieves synthesized, probably the most exciting material from a structural point of view is cloverite. The -CLO framework type (Fig. 16) consists of a primitive cubic array of a-cavities joined to one another via two [ 4 6 8 ] or rpa units to produce an enormous cavity with a body diagonal of ca 30 A in the center of the cube. However, not all of the T-atoms in the framework are 4-connected. One eighth of the Ga and one eighth of the P form only three bonds to framework oxygens. The fourth bond is to a terminal OH-group. That is, the framework is interrupted. The fact that not all T-atoms are 4-connected is indicated by a dash "-" in front of the three letter code. The terminal OHgroups protrude into the pore openings, and produce an unusual pore shape reminiscent of a

Fig. 16. The -CLO framework type (left) and its large central cavity (right).

55

4-leafed cloverleaf (hence the name cloverite). The ring is composed of 20 T-atoms and 24 oxygens. There are two non-intersecting, 3-dimensional channel systems: one 20-ring (with cloverleaf-shaped pores) and one 8-ring (passing through the a-cavities). The cavity in the center is by far the largest yet observed, and the framework density (11.1 T-atoms / 1000A3) the lowest. A further interesting aspect of the structure is that it can be constructed entirely from double 4-rings. In the structure of the as synthesized material, there is a fluoride ion in each double 4-ring, and this may be suggestive of a synthesis mechanism, since several other gallophosphate materials synthesized in the presence of HF have also been found to contain this unit (e.g. gallophosphate-LTA). 2.2.15. ETR (Type material: ECR-34, |H,.2 K6.3 Na4.4| [Gan.6 Alo.3 Si36.i O96] - ETRj Until quite recently, the largest pore opening in a silicate material seemed to be limited to a 14-ring. By including gallium in a silicate synthesis mixture, Strohmaier and Vaughan were able to break through this apparent ceiling, and produce the gallosilicate ECR-34 with 18-ring channels (Fig. 17) [16]. These 18-ring channels are connected to one another via the 8-rings in [466286] cavities to form a three-dimensional channel system. 2.2.16. UTL (Type material: IM-12, [Ge^.g Si62.2 O,52] - UTL) Over the years, considerable effort has been put into synthesizing silicates with extra-large pores (i.e. larger than 12-rings), and a number have been made. However, the large channels were either one-dimensional or only connected via 8-rings until the germanosilicates IM-12 [17] and ITQ-15 [18] were synthesized in 2004. Both of these have the UTL framework type with 14- and 12-ring channels intersecting to form a two-dimensional channel system (Fig. 18). As was noted earlier, germanium has been observed to stabilize the formation of double 4-rings [19], and indeed in these structures, Ge is located only in the double 4-rings that connect the pure silica layers, which contain primarily 5-rings. The layers themselves consist of chains of [4]58] units, and these chains are linked to one another via one or two additional tetrahedra. 2.3. Searching the Zeolite Structure Database on the internet As mentioned at the beginning of this chapter, essential structural information for all zeolite framework types to which the Structure Commission has assigned a three-letter code is

Fig. 17. A projection of the ETR framework type (left) and its [466286] cavity and 18-ring channel (right).

56

Fig. 18. The UTL framework type. Projection (a) down the z axis, and (b) down the >> axis, (c) The [4158] cage found in the layers, (d) The intersection of the 14-and 12-ring channels.

published on the internet under http://www.iza-structure.org/databases/. If the three-letter letter code of the zeolite of interest is not known, the material name can also be used to find the data. For each framework type, the database contains information such as crystal data (unit cell, space group and coordinates of T-atoms for an idealized SiC>2 composition), framework density, rings present, dimensionality of the channel system, secondary building units (SBU's), coordination sequences and vertex symbols. All of these data are also searchable (under Advanced Search). For example, all framework types with «-rings with n > 14 and a framework density of less than 16-T-atoms/1000A3 can be extracted very easily (Fig. 19). The resulting list shows that there are now several low-framework-density structures with multidimensional channel systems and rings larger than 12. A search without the limitation on the framework density yields 10 framework types. The pore dimensions given in the channel description are calculated from the crystal structure of the type material. The Atlas database also contains drawings of the pore openings (windows), the framework and framework projections, and provides a window in which the framework structures can be manipulated (rotated, zoomed, choice of display styles, range of atoms) and interatomic distances measured. Several other databases are also available on this website, including schemes for building models of the frameworks, a catalog of disordered zeolite FTC

Type Material

FD

FDsi

SBU

Rings present

Dim.

Channel Description

-CLO

Cloverite

11.1

11.1

4-4

20 8 6 4

3

<100> 20 4.0 x 13.2*** | <100> 8 3.8 x 3.8 ***

ETR

ECR-34

14.7

15.4

18 8 6 4

3

1 [001] 8 2.5 x 6.0** o [001] 18 10.1*

OSO

OSB-1

13.4

13.3

14 8 3

3

[001] 14 5.4 x 7.3* o 1 [001] 8 2.8 x 3.3**

14 12 6 5 4

2

[001] 14 7.1 x 9.5* <-> [010] 12 5.5 x 8.5*

18 6 4

1

[001] 18 12.7 x 12.7*

UTL

IM-12

15.2

15.6

VFI

VPI-5

14.2

14.5

3

6

Fig. 19. Results of the framework type search described in the text.

57

structures, and data for simulating powder diffraction patterns for all framework types. This is a powerful resource for zeolite scientists and is frequently accessed. 3.

ZEOLITE STRUCTURES

The framework types discussed in the last section describe only the connectivities of the frameworks. While these characterize the basic framework structure in terms of approximate pore opening, cage arrangement and channel system, and facilitate comparison of related materials, they do not describe real materials. That is, the influence of framework composition, extra-framework cations, organic species, sorbed molecules, or structural defects, is not considered. These aspects are addressed in the following sections. 3.1. Framework composition Many of the interesting properties of zeolites are based on the fact that the framework is anionic and the balancing cations exchangeable. A pure silica (SiCb) framework is neutral, but if some of the tetravalent Si are replaced by trivalent Al to produce an aluminosilicate, the framework becomes negative and counterions such as Na are needed to balance its charge. The neutral aluminophosphate or gallophosphate frameworks can be made anionic in a similar manner by inserting other elements into some of the T-sites. Even a small amount of a transition metal ion in the framework can make the material useful for catalysis applications. Many elements have now been incorporated into zeolite framework structures. What was originally the realm of aluminosilicates has expanded to include a significant portion of the periodic table. In some cases, only a few percent of the element is incorporated, while in others it is a major constituent. The framework composition also affects the stability of a material. For example, a high silica zeolite usually has a higher thermal stability than does the corresponding aluminosilicate, an aluminosilicate tends to be more stable than an aluminophosphate, and a gallophosphate is generally more sensitive to moisture than is an aluminophosphate. As has been indicated in the discussion of framework types, the chemical composition of a framework is sometimes reflected indirectly in certain features of the framework type. For example, double crankshaft chains are prevalent in aluminosilicates, 5-rings in high silica zeolites, Narsarsukite chains in aluminophosphates, 3-rings in zinco- and beryllosilicates, and double 4-rings in germanosilicates. Materials with strictly alternating T-atoms, such as Al and Si in aluminosilicates with a Si/Al ratio of 1, Al and P in aluminophosphates or Ga and P in gallophosphates, also require that only even numbered rings be present. If there are two or more types of T-atoms and these are ordered (i.e. not randomly distributed over all T-sites), the ideal symmetry of the framework type is likely to be reduced. For example, Al and Si alternate in the framework structure of zeolite A (LTA). To illustrate the effect of this ordering on the symmetry, the LTA framework type with all nodes identical and with alternating nodes marked are shown in Figs. 20a and b, respectively. The lattice constant (repeat distance) a and one of the mirror planes for the former is shown in Fig. 20a. In Fig. 20b, the symmetry reduction dictated by the ordering of Si and Al is readily apparent. Two obvious effects of the alternation are that (1) the mirror planes between sodalite cages are gone, and (2) the unit cell has to be doubled along each of the axes. Similar effects are observed in other materials in which the T-atoms are ordered.

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Fig. 20. The LTA framework type (a) with all nodes identical, and (b) with alternating nodes marked. In (a) the repeat distance a and one of the mirror planes are indicated. In (b) the mirror plane shown in (a) is lost and the repeat distance is doubled in all directions (a'=2a). For simplicity, the necessary doubling of the unit cell in (b) is shown in only one direction.

3.2. Extra-framework species The channels and cages of a zeolite framework are usually filled with extra-framework species such as exchangeable cations, which balance the negative charge of the framework, removable water molecules, and/or organic species. These may come from the synthesis mixture or they may be the result of a post-synthesis treatment. Whatever their origin, it is often of interest to know where they are located. Modern crystallographic techniques generally allow such information to be extracted from diffraction data, but there are some limitations that should be appreciated. The primary problem is the fact that extra-framework species do not generally follow the high symmetry of the framework, so they are what is called "disordered". For example, the Na+ ion in an 8ring of zeolite A is located off-center where it can approach three framework oxygens (Fig. 21), but because there is a 4-fold axis running through the center of the 8-ring, there are four equivalent positions for the Na+ ion. However, there is only room for one Na+ ion per 8-ring. This Na+ ion may hop between the four equivalent positions (dynamic disorder) or it may be stationary but occupy different positions in different 8-rings (static disorder). Conventional X-ray analysis cannot distinguish between these two possibilities, but whichever is the case,

Fig. 21. The 8-ring in zeolite A (LTA) showing a Na+ ion position (gray) with its bonding to 3 framework oxygens, and the 3 unoccupied symmetry equivalent Na+ ion positions (dotted circles).

59 an electron density map generated from the diffraction data will show 1/4 of a Na ion (e.g. 10/4 electrons) at each equivalent position rather than one ion (10 electrons) at a single position. This means that the peaks in the electron density map are weak, and that chemical sense (e.g. known chemical composition, feasible coordination numbers, sensible interatomic distances and angles, no fractional atoms possible, etc.) must be used to interpret them. In the case above, the interpretation is relatively simple, but for more complex molecules, the interpretation of the electron density map becomes more difficult and ambiguous. Nonetheless, very useful information regarding the location of extra-framework species can be gleaned from a diffraction experiment. Examples include the location of the 18-crown-6 molecule required for the synthesis of pure EMC-2 (EMT) [20], the location of sorbed m- and p-xylene in Ba-exchanged zeolite X (FAU) at different loadings [21], and the location of naphthalene sorbed into ZSM-5 (MFI) [22]. 3.3. Stacking faults Closely related zeolite framework structures often form under very similar conditions, and this can lead to the formation of stacking faults or intergrowth structures. For example, both ZSM-5 (MFI) and ZSM-11 (MEL) contain pentasil sheets. The only difference between the two is the linkage between adjacent sheets (they are related by a center of inversion in MFI and by a mirror plane in MEL, see sections 2.2.7 and 2.2.8), and it is not uncommon for an occasional stacking fault to occur [23]. If substantial domains of two framework types are formed and these domains share a common face, the material is referred to as an intergrowth. One of the first zeolite intergrowths to be examined was that of the natural zeolites offretite (OFF) and erionite (ERI), which are members of the ABC-6 family of structures (see section 2.1) with AABAAB and AABAAC 6-ring stacking sequences, respectively [24]. In this case, the stacking is critical, because a single stacking fault in offretite (i.e. a C instead of a B) blocks the 12-ring channel. As might be imagined, the ABC-6 family of zeolites is quite prone to stacking "mistakes". If a stacking fault occurs regularly, a new framework type with a new repeat period is formed. The two structures are then called the end members of an intergrowth series. The Catalog of Disordered Zeolite Structures on the internet describes a number of such families. High resolution electron microscopy is the technique of choice for the investigation of such structural defects. The high-resolution images of a faulted material will show the local stacking sequences and domain sizes quite clearly. As was mentioned in sections 2.2.3 and 2.2.4, both zeolite structures can be built by stacking layers of sodalite cages in an ABCABC (for FAU) or ABAB (for EMT) sequence [25,2]. In this case, the stacking faults do not block the channels, but the local environments are slightly different, so some of the properties of the intergrowth materials can differ from those of the pure end members. Many such systems have been studied using electron microscopy techniques. Examples include studies of faulting in the zeolites beta (*BEA) [27], ferrierite (FER) [28], and NU-86 [29]. For further examples and experimental details, the reader is referred to the review by Terasaki et al. [30]. 4.

POWDER DIFFRACTION

Since zeolite structural information is very often derived from laboratory X-ray powder diffraction data, it is perhaps appropriate to outline a few aspects of the technique. Additional

60

Fig. 22. The relevant features of a powder diffraction pattern and their origin. information can be found in the book Modern Powder Diffraction edited by Bish and Post [31] and in the papers by Langford and Louer [32] and by Baerlocher and MeCusker [33]. 4.1. Information in a powder diffraction pattern A powder diffraction pattern has several features that can be of interest to a zeolite scientist: the peak positions, their relative intensities, their widths, and the background (Fig. 22). Each of these features can be interpreted relatively easily to yield useful information. The peak positions in a powder pattern (usually measured in °26) are determined only by the geometry of the unit cell. Each peak represents at least one reflection (and often several that happen to have similar 29 values). The 28 value is related to the
61

directions. For example, if the short dimension is assumed to be parallel to the c-axis, then the 001 reflections will be broader than the hkO reflections. In principle, this information can be used to establish how the channels of a zeolite of known structure are oriented in a membrane, because it is very thin in one direction. Finally, the background in a powder pattern indicates whether or not an amorphous material is present in the sample. A high background relative to the peaks is usually caused either by a large amount of amorphous material (e.g. unreacted gel) or by X-ray fluorescence. The latter is observed, for example, if an Fe-containing sample is irradiated with CuKa radiation. It can be avoided by changing the wavelength (e.g. to CoKa radiation). 4.2. Common applications Powder diffraction techniques are used on a routine basis by many zeolite scientists. Probably the most common application is the use of a powder diffraction pattern as a "fingerprint" in the identification of synthesis products. Ideally, a laboratory should have a set of "standard" zeolite patterns measured on the in house instrument for direct comparison. For laboratories without such a set of patterns or for those whose set is incomplete, the Synthesis Commission of the IZA has published experimental patterns for some zeolites in the book entitled Verified Syntheses of Zeolitic Materials [35] (also available on the internet at http://www.iza-synthesis.org/), and the Structure Commission has published a book entitled Collection of Simulated XRD Powder Patterns for Zeolites [36], which contains at least one representative powder diffraction pattern for each known framework type. An up-to-date internet version of the latter that includes data for newly approved framework types, is maintained at http://www.iza-structure.org/databases/. Even a cursory examination of the patterns in the Collection shows that different materials with the same framework type can have markedly different diffraction patterns, so direct comparison is not always straightforward. The identification of an unknown phase can sometimes be facilitated if the lattice parameters are determined, and these are compared with those of known zeolites. Of course, lattice parameters can also be used to study the effects of post-synthesis treatment (e.g. ion exchange, calcination, dealumination, sorption, etc.), to estimate Si/Al ratios in well-calibrated systems such as faujasite, to monitor a phase transition as a function of temperature, or to begin the structural characterization of a new material. Indexing a pattern can also serve to establish whether or not a phase is pure. If all lines can be indexed on a single unit cell, there is probably only one crystalline phase present. However, if there are unindexed lines, either the indexing is incorrect or there is a crystalline impurity present. A simple method for the evaluation of whether or not a post-synthesis treatment has induced structural change is to look at the effect on the powder diffraction pattern. Changes in the relative intensities of the peaks indicate that a structural modification has occurred, changes in the positions of the lines indicate that the unit cell has deformed in some way, and broader (or narrower) lines indicate that the crystallinity has deteriorated (or improved). In general, non-framework species have a pronounced effect on the low-angle region of the pattern. For example, a calcined material will tend to have higher relative intensities in this region than the corresponding as-synthesized or loaded sample. The high-angle region is usually less sensitive to the presence or absence of electron density in the channels and cages and more sensitive to distortions of the framework. More detail can be extracted from the powder pattern if a full Rietveld (whole-profile) structure refinement is performed [37,38], but even the simple qualitative evaluation of the pattern described above can be extremely

62

useful. Similarly, the presence of an amorphous phase can be established very easily simply by looking at the diffraction pattern. The determination of the structure of a zeolite with a new framework type remains a challenge to the powder method. Nonetheless, significant advances have been made in this area, and an increasing proportion of the new framework types are solved this way. While the techniques are rather sophisticated and beyond the scope of this chapter, it is perhaps important to know that methods of structure determination from powder diffraction data do exist and that all is not lost if single crystals of a new material cannot be synthesized. It may still be possible to solve the structure from the powder data [39,40,41]. Faulted materials (section 3.3) can also be recognized from their unusual powder diffraction patterns. The patterns are quite complicated, often with broadened lines for certain classes of reflections, and, until quite recently, could not be simulated easily. The calculation requires a knowledge of the structures of the end members, an estimate of the planar fault probability and an accurate description of the faulting involved. Given this information though, the program DIFFaX [42] can be used to simulate a powder pattern, which can then be compared with the observed one. In this way, the degree of faulting can be estimated. Such simulations for a number of zeolite families can be found in the Catalog of Disordered Zeolite Structures. It should be noted that the powder pattern of an intergrowth of two phases will be quite different from that of a physical mixture of those two phases. The latter is simply a summation of the powder patterns of the constituent phases in proportion to the amount present, while the former involves a much more complicated calculation. 5.

CONCLUSIONS

The key to understanding the properties of zeolites and zeolite-like materials lies in their structures. In this chapter, we have tried to convey the beauty and diversity of zeolite framework structures, to introduce some of the jargon used to describe them, and to outline some of the techniques used to characterize them. The main feature of a zeolite structure is its framework type, which describes the arrangement of the cages, the dimensionality of the channel system and the approximate size of the pore openings. A few framework types, selected for their industrial relevance and/or to illustrate some of the more common structural nomenclature, have been presented. However, there are many more, and for more information about a specific framework type, the reader is referred to the relevant references in the Atlas and the Collection. To fully understand the properties of a real zeolitic material though, not only the framework type, but also the composition and true geometry of the framework, the location and nature of the extraframework species, and the number and type of defects must be investigated. Since most zeolites are only available as polycrystalline phases, powder diffraction is an essential structural characterization technique. A powder diffraction pattern can be used to identify a material, to determine unit cell parameters, to estimate the quality of a sample, to monitor phase transitions, to evaluate whether or not a post-synthesis treatment has induced structural changes, to establish whether or not impurities (amorphous or crystalline) are present, or to recognize the presence of a significant level of faulting.

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T. Conradsson, M.S. Dadachov and X.D. Zou, Microporous Mesoporous Mater. 41 (2000) 183.

[10] A. Corma, M.T. Navarro, F. Rey, J. Rius and S. Valencia, Angew. Chem., Int. Ed. 40 (2001) 2277. [II] Z. Liu, T. Ohsuna, O. Terasaki, M.A. Camblor, M.-J. Diaz-Cabanas and K. Hiraga, J. Am. Chem. Soc. 123(2001)5370. [12] L.B. McCusker, Ch. Baerlocher, E. Jahn and M. Billow, Zeolites 11 (1991) 308. [13] R.M. Dessau, J.L. Schlenker and J.B. Higgins, Zeolites 10 (1990) 522. [14] J.W. Richardson, Jr. and E.T.C. Vogt, Zeolites 12 (1992) 13. [15] J. de Onate Martinez, L.B. McCusker and Ch. Baerlocher, Microporous Mesoporous Mater. 34 (2000) 99. [16] K.G. Strohmaier and D.E.W. Vaughan, J. Am. Chem. Soc. 125 (2003) 16035. [17] B. Harbuzaru, J.-L. Paillaud, J. Patarin and N. Bats, Science 304 (2004) 990. [18] A. Corma, M.J. Diaz-Cabanas, F. Rey, S. Nicolopoulus and K. Boulahya, Chem. Commun. (2004) 1356. [19] T. Blasco, A. Corma, M.J. Diaz Cabanas, F. Rey, J.A. Vidal Moya and CM. Zicovich Wilson, J. Phys. Chem. B 106 (2002) 2634. [20] Ch. Baerlocher, L.B. McCusker and R. Chiappetta, Microporous Mater. 2 (1994) 269. [21] C. Mellot, D. Espinat, B. Rebours, Ch. Baerlocher and P. Fischer, Catal. Lett. 27 (1994) 159. [22] H. van Koningsveld and J.C. Jansen, Microporous Mater. 6 (1996) 159-167. [23] G. Perego, M. Cesari and G. Allegra, J. Appl. Crystallogr. 20 (1987) 356. [24] J.M. Bennett and J.A. Gard, Nature (London) 214 (1967) 1005. [25] M.W. Anderson, K..S. Pachis, F. Prebin, S.W. Carr, O. Terasaki, T. Ohsuna and V. Alfredsson, J. Chem. Soc, Chem. Commun. (1991) 1660. [26] M.M.J. Treacy, D.E.W. Vaughan, K.G. Strohmaier and J.M. Newsam, Proc. R. Soc. London A 452(1996)813. [27] J.M. Newsam, M.M.J. Treacy, W.T. Koetsier and C.B. de Gruyter, Proc. R. Soc. London, Ser. A 420 (1988) 375. [28] R. Gramlich-Meier, W.M. Meier and B.K. Smith, Z. Kristallogr. 169 (1984) 201. [29] M.D. Shannon in "Proc. 9th Int. Zeolite Conf, Montreal, 1992," R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (eds.), Butterworth-Heinemann, Boston, MA, 1993, pp. 389-398.

64 [30] O. Terasaki, T. Ohsuna, V. Alfredsson, J.-O. Bovin, D. Watanabe and K. Tsuno, Ultramicroscopy 39 (1991) 238. [31] D.L. Bish and J.E. Post (eds.), Modern Powder Diffraction, Reviews in Mineralogy 30, 1989. [32] J.I. Langford and D. Louer, Rep. Prog. Phys. 59 (1996) 131. [33] Ch. Baerlocher and L.B. McCusker, Stud. Surf. Sci. Catal. 85 (1994) 391. [34] J. Bergmann, A. Le Bail, R. Shirley and V. Zlokazov, Z. Kristallogr. 219 (2004) 783. [35] H. Robson and K.P. Lillerud, Verified Syntheses of Zeolitic Materials, 2nd edn., Elsevier, London, 2001. [36] M.M.J. Treacy and J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, 4th edn., Elsevier, London, 2001. [37] R.A. Young (ed.), The Rietveld Method, Oxford University Press, Oxford, UK, 1993. [38]

L.B. McCusker, R.B. Von Dreele, D.E. Cox, D. Louer and P. Scardi, J. Appl. Cystallogr. 32 (1999)36.

[39] W.I.F. David, K. Shankland, Ch. Baerlocher and LB. McCusker (eds.), Structure Determination from Powder Diffraction Data, Oxford University Press, Oxford, UK, 2002. [40] L.B. McCusker in Abstracts 14th Int. Zeolite Conf., E.W.J. van Steen, L.H. Callanan, M. Claeys and C.T. O'Connor (eds.), Transformation Technologies, Cape Town, South Africa, 2004, pp. 42-52. [41] A. Burton, Z. Kristallogr. 219 (2004) 866. [42] M.M.J. Treacy, J.M. Newsam and M.W. Deem, Proc. R. Soc. London, Ser. A 433 (1991) 499.

Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

65

Synthesis of zeolites and zeotypes C. S. Cundy Centre for Microporous Materials, School of Chemistry, University of Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom

1. SCOPE OF THIS ACCOUNT 2. HISTORY 3. SOURCES OF INFORMATION 3.1. Background 3.2. Experimental aspects 4. MAKING ZEOLITES AND ZEOTYPES IN THE LABORATORY 4.1. Outline of typical observations 4.2. Following the course of a synthesis 4.3. Microwave heating 5. MODELLING THE PROCESSES OF ZEOLITE SYNTHESIS 5.1. Mathematical models of synthesis reactions 5.2. Molecular Modelling 5.3. Use of a simple reaction model to clarify an experimental concept: how fast is my reaction? 6. STUDIES OF ZEOLITE AND ZEOTYPE CRYSTALLISATION 6.1. The induction period 6.2. Nucleation 6.2.1. Basic concepts 6.2.2. Data and measurement 6.2.3. The nature of nucleation in zeolite systems 6.3. Crystal growth 6.3.1. Experimental observations 6.3.2. Growth models 7. EXPLAINING THE EXPERIMENTAL OBSERVATIONS 7.1. Equilibria and kinetics 7.2. An overall model for the synthesis process 7.3. A stepwise reaction model 7.4. The mechanism in chemical terms 7.5. Alternative views of the synthesis mechanism 7.5.1. Solution-mediated or solid phase? 7.5.2. The nature of growth species 7.5.3. Other porous materials 8. CONCLUSION APPENDIX: SOME EXAMPLES OF ZEOLITE AND ZEOTYPE SYNTHESES 1. Synthesis of MFI materials (silicalite-1, ZSM-5) 2. Microwave synthesis of A1PO4-5 by a fluoride route REFERENCES

66 1. SCOPE OF THIS ACCOUNT This short review of zeolite and zeotype synthesis is written for those who are relatively new to the field. It aims to present an overall introduction to some fundamental aspects of the subject and to indicate where further information can be found. An account of experimental practice is followed by a summary of mathematical modelling procedures. Observations from crystallisation studies then introduce basic principles of the synthesis process. 2. HISTORY Aluminosilicate zeolites and their analogues are almost invariably prepared by the process of hydrothermal synthesis, i.e. by the action of heat upon an aqueous reaction mixture, usually under pressure. Early work was based on attempts to synthesise materials previously observed as natural minerals. However, the resulting claims are difficult to substantiate since there were no reliable methods for fully identifying and characterising the products. The first studies having a sound scientific basis were those of Richard Barrer and Robert Milton, commencing in the late 1940s. Barrer began his work by investigating the conversion of known mineral phases under the action of strong salt solutions at fairly high temperatures (ca. 170-270 °C) and quickly discovered the first synthetic zeolite unknown as a natural mineral (species P and Q [1], later found to have the KFI structure [2]). Robert Milton pioneered the use of more reactive starting materials (freshly precipitated aluminosilicate gels), enabling reactions to be carried out under milder conditions and leading to the discovery of zeolites A and X [3]. Early work on zeolite synthesis utilised only inorganic reaction components but in 1961 the range of reactants was expanded to include quaternary ammonium cations [4,5]. This in turn led to the discovery of silica-rich phases (high-silica zeolites) such as zeolite beta [6] (1967) and ZSM-5 [7] (1972). Many more synthetic zeolites were subsequently discovered [2], as well as new families of zeolite-like or zeolite-related materials [8] known as zeotypes. Examples of the latter include the microporous alumino- and gallo-phosphates (AIPO4S and GaPO4s) [9,10] and the titanosilicates (such as ETS-10) [11]. Zeotype materials display great compositional diversity and frequently have frameworks unknown for zeolites. This increased structural flexibility originates from the intrinsic heteroelement properties and in particular the expanded range of atomic radii, bond lengths and bond angles, leading also to coordination numbers greater than four. Even further removed from the realm of microporous aluminosilicates are the major new class of zeolite-related phases discovered in the early 1990s. Mesoporous materials, synthesised with the aid of surfactant molecules and typified by the M41S and SB A series, have periodic structures with far larger pore sizes (up to ca. 200 A) but are not conventionally crystalline [12-14]. Investigative work aimed at gaining an understanding of the synthesis process has its origins in the 1960s. These studies continue today, aided by discoveries of new materials, advances in synthetic procedures, innovations in theoretical modelling methods and, especially, by the development of new techniques for the investigation of reaction mechanisms and the characterisation of products.

67 3. SOURCES OF INFORMATION 3.1. Background Essential facts and figures on the synthesis of crystalline microporous materials can be found in the textbooks by Breck [15], Barrer [16] and Szostak [8], and also within previous volumes on zeolite science and technology published in collaboration with the International Zeolite Association (IZA) [17,18]. These sources contain (besides much other useful background material) chapters on the synthesis of zeolites, metallophosphates and other porous framework structures. Further background information is available in a number of reviews and surveys and a representative selection of the more recent of these is given in Table 1. 3.2. Experimental aspects Descriptions of the syntheses of zeolites and zeotypes are very widely disseminated in the journal and patent literature. Searches carried out on, for example, zeolite A or ZSM-5 would reveal hundreds of examples. In general, it is often instructive to look both at some of the original synthesis procedures (often as patent examples) and also at some of the most recently published methods. For all materials, the extent of information increases with the length of time since first disclosure. This means that the amount of data on (for example) recently-discovered zeotype phases may be quite limited. A very useful source for experimental methods and reliable synthetic procedures for zeolites and the more common zeotypes can be found in the handbook issued by the Synthesis Commission of the International Zeolite Association (IZA) [51]. Some practical aspects of zeolite synthesis are described in the following section, whilst some illustrative examples of laboratory preparations are given in the Appendix. Scale-up and industrial production are outside the scope of this work. 4. MAKING ZEOLITES AND ZEOTYPES IN THE LABORATORY 4.1. Outline of typical observations A typical hydrothermal zeolite or zeotype synthesis can be described as follows: 1. Sources of the main compositional components (e.g. silica, alumina, heteroatom source) are selected, using criteria such as availability, chemical reactivity and cost. Such reactants are usually in an oxide-like form and amorphous. 2. The chosen reactants are then mixed together with a cation source, usually in a basic (high pH) water-based medium. 3. The aqueous reaction mixture is heated, often (> 100 °C) in a sealed autoclave. 4. For some time after raising to synthesis temperature, the reactants remain amorphous. 5. After the above "induction period", crystalline zeolite or zeotype product can be detected. 6. Gradually, essentially all amorphous material is replaced by an approximately equal mass of product crystals, which are recovered by filtration, washing and drying.

68

Table 1 Selected reviews and surveys relating to zeolite and zeotype synthesis Author(s)

Year

Title (abbreviated)

Ref.

Occelli, Robson (Eds)

1989

Zeolite synthesis (symposium proceedings)

[19]

Gilson

1992

Organic and inorganic agents in molecular sieve synthesis

[20]

Davis, Lobo

1992

Zeolite and molecular sieve synthesis

[21 ]

Occelli, Robson (Eds)

1992

Synthesis of microporous materials (symposium)

[22]

Feijenetal.

1994

Zeolites and their mechanism of synthesis

[23]

Kessler et al.

1994

Fluoride route in the synthesis of microporous materials

[24]

Davis

1995

Strategies for zeolite synthesis by design

[25]

Morris, Weigel

1997

Synthesis of molecular sieves from non-aqueous solvents

[26]

Occelli, Kessler (Eds)

1997

Synthesis of porous materials (symposium proceedings)

[27]

Thompson

1998

Recent advances in the understanding of zeolite synthesis

[28]

Francis, O'Hare

1998

Kinetics & mechanisms of zeotype crystallisation

[29]

Brock etal.

1998

Porous manganese oxide materials

[30]

Oliver et al.

1998

A new model for aluminophosphate formation

[31 ]

Cheetham et al.

1999

Open-framework inorganic materials

[32]

Millinietal.

1999

Synthesis & characterisation of Boron molecular sieves

[33]

Cambloretal.

1999

Synthesis of high-silica molecular sieves in F media

[34]

Matsukata et al.

1999

Conversion of dry gel to microporous crystals in gas phase

[35]

Guth, Kessler

1999

Synthesis of Al,Si-zeolites & related silica-based materials

[36]

Ma et al.

2000

Zeolite-like porous materials

[37]

Fricke et al.

2000

Gallium zeolites: syntheses, properties, catalysis

[38]

Serrano, van Grieken

2001

Heterogeneous events in the crystallisation of zeolites

[39]

Balkus

2001

Synthesis of large pore zeolites and molecular sieves

[40]

Taulelle

2001

Crystallogenesis of microporous metallophosphates

[41]

Eddaoudi et al.

2001

Modular chemistry: SBUs in porous carboxylate design

[42]

Nishi, Thompson

2002

Synthesis of classical zeolites

[43]

Patarinetal.

2002

Synthesis of AlPOs and other crystalline materials

[44]

Anderson, Rocha

2002

Synthesis of titanosilicates and related materials

[45]

Serre, Taulelle, Ferey

2003

Rational design of porous titanophosphates

[46]

Cundy, Cox

2003

Hydrothermal synthesis of zeolites: history, development

[47]

Corma

2004

Towards a rationalisation of zeolitic materials synthesis

[48]

Corma, Davis

2004

Issues in the synthesis of crystalline molecular sieves

[49]

Cundy, Cox

2005

Hydrothermal synthesis of zeolites: reaction mechanism

[50]

69

This is illustrated schematically in Fig. 1 for an aluminosilicate system. The elements (Si, Al) which will make up the microporous framework are imported as precursors containing Si-0 and Al-0 bonds. During the hydrothermal reaction in the presence of a "mineralising" agent, most commonly an alkali metal hydroxide (but occasionally fluoride), the crystalline zeolite product (e.g. zeolite A or ZSM-5) containing Si-O-Al linkages is formed. No radically new bond types are created in this reaction so that the resulting enthalpy change (AH) is small. Whilst the reactants actually employed may include the simple oxides or hydroxides mentioned above (e.g. precipitated silica or alumina trihydrate), it is also very common for the reagents to represent some degree of precombination of components, as for example in sodium silicate solution or solid sodium aluminate. These materials may present advantages in cost or ease of processing and may also offer optimum routes to particular materials. 4.2. Following the course of a synthesis Although it is possible to learn something about the pathway followed in a synthesis reaction from examination of the start (reactants) and the end (products) of the process, the only really satisfactory method is to make observations throughout the course of the experiment. Ideally, this should be by some in-situ, non-invasive method, such as microscopy or spectroscopy. However, there have been only a few such investigations (see e.g. section 6.3.]), largely because of the difficulty of making the necessary measurements on caustic mixtures under conditions of elevated temperature and pressure. More commonly, analyses are carried out on samples isolated at intervals during the course of the synthesis. The best procedure is for material to be periodically extracted from the reaction vessel via a sampling device during a single reaction run. Stirring is usually essential for the provision of homogeneous and representative samples.

Fig. 1. Schematic representation of hydrothermal zeolite synthesis.

70

An alternative procedure is for the original batch of reaction mixture to be divided into a series of aliquots. These are then heated in separate vessels which are removed from the reaction oven at known time intervals. This procedure has the advantage of convenience but suffers from the disadvantage that the reaction conditions may not be exactly the same in every case. Differences may be caused by, for example, trace contaminants in the reaction vessels or small variations in temperature from one vessel to another. (Running each sample in duplicate provides a check on this.) The products from each sample are analysed by some selective technique which recognises a characteristic product property, such as XRD (phase, crystallinity), IR (characteristic band frequencies) or sorption (micropore volume). In each case, a plot of the defining property against time will give an S-shaped growth curve which reflects the rate of product formation. In some cases, other measurements (e.g. crystal size) can also be carried out (see section 6.3.1). A measurement of fundamental importance is the rate of size increase of a crystal with time (the linear growth rate). 4.3. Microwave heating In many synthesis reactions, heating is carried out by direct thermal methods, most usually by placing the reaction vessel in a conventional oven. However, a very useful alternative procedure is microwave dielectric heating [52,53]. In its simplest form, this entails the use of a suitable, microwave-transparent glass or plastic (e.g. Teflon) reaction vessel, which is placed in a laboratory microwave oven. The benefits to zeolite synthesis have been discussed in a review [54]. In conventional (thermal) heating, reactions may be strongly influenced by the rate of heat transfer through the walls of the containing vessel and the resulting thermal gradients across the reaction volume. These limitations are modified when a different form of energy input is applied. The most apparent advantage of microwave heating is the significant shortening of reaction time which is found in virtually every case. The factors underlying the reduced timescale have been analysed [54,55] in terms of the contributions from (i) thermal lag, (ii) the induction period and (iii) crystal growth. The major accelerations derive from (i) and (ii) by virtue of the rapid heating rate to the working temperature and also from nonequilibrium (e.g. differential heating) effects. The rapid heating rate also means that the effects of seeding and of reactant history (e.g. ageing) can also be important [55,56]. In some cases, the special conditions of microwave synthesis can lead to product selectivity, if some component of the system is selectively sensitive to microwave energy input or to heating rate. Such a situation might arise in a heterogeneous reaction mixture containing components of different dielectric properties, or through differences in the rival rates of nucleation and growth of competing phases. Phase selectivity in zeolite Y synthesis has been demonstrated by the suppression of undesired co-products in microwave-mediated preparations, even under conditions (150 °C) where zeolite P would normally dominate in a thermal synthesis [54,57]. 5. MODELLING THE PROCESSES OF ZEOLITE SYNTHESIS Not all our knowledge of microporous materials synthesis has come from laboratory experimentation. Valuable understanding has also been achieved from the development and use of a variety of mathematical modelling procedures. These have increased greatly in

71

importance with the huge growth in computing capability in recent years. Computational methods have been applied both to the thermodynamics and the kinetics of the synthesis process and also to the molecular basis of the synthesis reactions. Important insight can be gained where laboratory and modelling studies are used together in a synergistic way. 5.1. Mathematical models of synthesis reactions Reaction models as used by chemists and engineers are of two basic types: (i) those using a kinetic approach and (ii) those founded on a thermodynamic approach. Those in the first category [58] range from simple empirical correlations to complex computer programs. Of the more complex treatments, the most important are those based on particle numbers, such as the population balance model [59,60] developed extensively by Thompson and coworkers. By the use of groups of equations which describe the evolution with time of the mass and the numbers of particles, hypothetical reactions can readily be explored to assess the effect of changing reaction variables and introducing other components such as seed crystals. For example, predictions of crystal size and size distribution can be developed to reflect changes in nucleation and growth behaviour brought about by gel ageing [61]. A significant thermodynamics-based treatment of zeolite synthesis is the equilibrium model of Lowe [62]. This was initially developed to provide insight into the pH changes which occur in the course of high-silica zeolite syntheses [63]. The model considers the zeolite synthesis process as a series of pseudo-equilibria: amorphous solid <-> solution species «-> crystalline zeolite progress of reaction

>

At the start, amorphous solid is in equilibrium with solution species. This initial equilibrium is then maintained while product crystals grow from the supersaturated solution. Finally, when all the amorphous precursor has been consumed, the crystalline zeolite equilibrates with its mother liquor. This simple analysis enables the solution chemistry, and in particular the effects of solubility and pH, to be understood at a fundamental level [62]. 5.2. Molecular modelling Molecular modelling methods [64-66] have provided insight into (i) the determination of the location and energies of the templating agents occluded within zeolite structures during synthesis and (ii) the detailed investigation of small framework fragments. The key advantage modelling methods offer over experimental techniques is that they permit investigation of the energetics of template-framework interactions, usually through the use of molecular mechanics calculations. This basic methodology has been employed in several different ways to investigate the relationship between template molecules and the zeolite product. For example, Zones et al. have made effective use of modelling methods to aid their search for bulky molecules that are too large, or have the wrong geometries, to synthesise well-known materials which might otherwise crystallise from their reaction mixtures. This has resulted in the discovery of several new structures [67]. The properties of small silica fragments which may be critical to the synthesis process are also difficult to study via experimental methods. Cluster calculations, usually based on

72

quantum mechanical approaches, have proved highly valuable as a tool to probe their detailed structure, geometry and reactions [68,69]. Pereira et al. [70,71] have developed these procedures to examine the interaction between solvated zeolite fragments and template molecules, demonstrating that the organic molecule plays a key role in stabilizing clusters of framework material and preserving their structure under the influence of water. Recently, Wu and Deem [72] have developed a Monte Carlo procedure to model a silicate solution on the atomic scale in the absence of a template molecule. This study has yielded an insight into some of the fundamental processes associated with nucleation such as an estimation of the nucleation barrier and the critical cluster size. 5.3. Use of a simple reaction model to clarify an experimental concept: how fast is my reaction? The statement is frequently made that one zeolite synthesis is "faster" than another but the measurement criteria may be extremely loosely defined. Often, there is no distinction between the induction time and the growth period (section 6), so that it is impossible to tell whether a reaction is "slow" because it takes a long time to nucleate or because the crystals grow slowly in the given circumstances. A comparison of some hypothetical synthesis reactions is shown in Table 2. The data are derived from a computer simulation of zeolite growth based on a very simple kinetic crystal growth model [50,73], i.e. radial growth rate = dr / d^

product mass (m) = 4n7ipr3/ 3

where m is the mass of product crystals (density p, equivalent spherical radius r) per unit volume and n is their number per unit volume. The data are not specific to any particular system but are chosen to be reasonably realistic and could, for example, be related to MFItype syntheses operating at around 100 °C. Following an initial single nucleation event, the monodisperse crystal population is allowed to grow at the stated rate up to a nominal 100% Table 2 A simple model for zeolite growth: demonstration of comparative rate criteria Starting parameters

Run 1

Run 2

Run 3

Crystal radial growth rate (jim h"1) Nucleation rate (# cm'3 h"1)a Induction period (h)

0.01 10" 0

0.02 109 5

0.01 1012 15

Simulation results Time to reach 100% yield, tf (h) Yield at 0.25 tf (%) Yield at 0.50 tf (%) Yield at 0.75 tf (%) Crystallisation rate (% h"1) b Crystal size (um) a applied during first hour of reaction onl;

20 1.6 12.5 42 9.6 4

15 17 0 0 1.8 0 25 0 19.5 100 4 0.4 gradient at 50% conversion

73

conversion of starting materials. (Note that there are no terms to account for a more realistic reaction termination in which growth slows gradually as reactants become exhausted.) The three simulations all reach completion within 15-20 h but have completely different growth profiles. Depending on the criterion chosen (e.g. yield at t/2, total reaction time, crystallisation rate), each one of the three could be claimed as the fastest synthesis! Such ambiguities can be avoided by specifying (a) some form of quantitative growth curve (section 4.2) and (b) a reliable product crystal size distribution (e.g. from a calibrated SEM). From these basic data, an estimate can be made of (i) how long the reaction took to start, (ii) when the first crystals appeared, (iii) the time at which crystallisation stopped and (iv) how to interpret the growth period in the light of the likely nucleation profile. 6. STUDIES OF ZEOLITE AND ZEOTYPE CRYSTALLISATION: A STEP-BY-STEP DESCRIPTION OF THE SYNTHESIS PROCESS The synthesis of crystalline microporous solids is a complex reaction-crystallisation process, usually involving a liquid phase and both amorphous and crystalline solid phases. Described below (section 6) are the main elements of this transformation: the induction period, nucleation processes and crystal growth. Having established this background, the mechanism underlying these changes is then examined in more detail (section 7). 6.1. The induction period The induction period (r) is the time (t) from the start of the reaction to the point at which crystalline product is first observed. It therefore depends on the moment chosen for setting t = 0 (often taken as the time at which the reactants reach the working temperature) and upon the method of analysis used to detect the product (most usually X-ray diffraction). In this time, the zeolitic product is born. Firstly, the reactants interact with one another and begin to equilibrate. During this period, there is an increase in structural ordering within the amorphous phase. There is then a step change leading to the establishment of the zeolite lattice itself, i.e. nucleation. At this point, a statistical selection of the reconstructed areas reach a critical nuclear size and degree of order such that a periodic structure is able to propagate. Crystal growth begins and the newlyformed nanocrystal grows to a detectable size. 6.2. Nucleation 6.2.1. Basic concepts We may imagine a sub-nuclear unit of radius r. As this grows, it gains in cohesive energy, a quantity proportional to its volume (AGV a r"), but also has to expend energy in creating a surface to achieve separation from its surroundings (AGS a r2). As r increases, a point is reached (at the critical nuclear size (rc) [74]) where AGV equals and then exceeds AGS, leading to an overall gain in free energy. This enables a stable unit to be formed: a viable nucleus, capable of growth. For zeolite systems, a critical size of 1-8 unit cells has been estimated by Thompson and Dyer [59]. However, the chemical and physical processes leading up to critical nucleus formation are governed by the structure being formed and by the experimental conditions. If these conditions change, then the necessary critical size will also vary. A notional

74

nucleation rate (J) can be defined as the rate of production of units having radius r > rc. A simple measure of J is often taken by assuming J a 1/r [47,75]. However, since the induction period is a compound term containing components of equilibration time and early growth as well as the time required for nucleation itself, this procedure is subject to error [76,77]. The nomenclature for the most usually recognised types of nucleation can be summarised as follows [74]: primary nucleation may be homogeneous (from solution) or heterogeneous (induced by foreign particles); secondary nucleation (induced by crystals) may be considered as a special case of heterogeneous nucleation in which the nucleating particles are crystals of the same phase. Good general accounts of nucleation in zeolite systems (along with further references) will be found in reviews by Barrer [16] and Thompson [28,60]. 6.2.2. Data and measurement Obtaining direct experimental data on nucleation processes is very difficult due to the extremely small fraction of the total mass involved and the problems of distinguishing this from surrounding reactants of very similar nature and composition. Consequently, much of the information available is more that of early growth behaviour rather than of nucleation itself. Recent cryo-TEM [78] and HRTEM studies [79-81] represent the most direct evidence of nucleation phenomena currently available. However, much useful data can be obtained from "circumstantial evidence", i.e. measurements of the appearance, growth and size distribution of the crystals resulting from the earlier, hidden patterns of nucleation. The procedure of Zhdanov and Samulevich enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal size distribution [16,82]. Originally implemented in analyses of zeolite Na-A [83] and Na-X [82] crystallisation, the method has subsequently been applied to other zeolite systems, including silicalite [84,85]. If it is supposed that all the crystals in a batch have the same (known) growth rate behaviour, the total growth time of each crystal can be calculated. Assuming also that the nucleation point for each crystal can be obtained by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined from their final sizes. Detailed information on nucleation behaviour can also be obtained from work on the ageing of reaction mixtures. The crystallisation behaviour of a synthesis composition is often dependent upon the time-temperature history of the sample in advance of the principal heating step. The first studies to link these time-dependent effects with control of crystal size were those of Freund [86] and of Zhdanov and coworkers [82]. These investigations showed that roomtemperature ageing of aluminosilicate Na-A and Na-X gels resulted in both an acceleration of the crystallisation and a decrease in the crystal size of the product, thus linking the ageing process directly with the number of nuclei formed. More recently, related studies by Cizmek et al. [87], Cundy et al. [88] and Li et al. [89] have provided detailed insights into nucleation processes in the silicalite-1 system. In addition, modelling work has allowed the simulation of gel ageing by population balance methods [61]. Assuming that the ageing step results in the formation of viable nuclei that effectively lie dormant until the temperature is raised, it has been shown that the number of nuclei generated during ageing strongly influences subsequent crystallisation behaviour, whilst their size is relatively unimportant.

75 6.2.3. The nature ofnucleation in zeolite systems It seems reasonable to suppose that the construction of the nucleus of a zeolite crystal may involve a more complex assembly process than is necessary for simpler substances whose unit cells are smaller and contain far fewer atoms, although there should be no difference in basic principles. The nature of the nucleation process in zeolite systems is also complicated by the nature of zeolite synthesis sols. These contain observable solid phases (amorphous gel, crystals) and components (cations, anions) in true solution. However, there is frequently also a colloidal component, invisible to the naked eye, which itself may be amorphous or crystalline. In zeolite synthesis, there is evidence for the participation of both primary and secondary (crystal-catalysed) nucleation, although the former is believed to be the most prevalent [28,60]. Even so, there is often disagreement on whether the predominant mechanism is homogeneous or heterogeneous. However, it can be seen from the foregoing paragraph that in zeolite systems an apparently homogeneous nucleation may actually involve the participation of colloidal amorphous material. Supporting evidence for this came initially from microscopic observations on dilute reaction mixtures [84,90] but the process has recently been directly demonstrated for the "clear solution" syntheses of zeolites A [80], Y [81] and Si-MFI [79]. Using highresolution transmission electron microscopy (HRTEM) in conjunction with in-situ dynamic light scattering (DLS), X-ray diffraction and other techniques, Mintova et al. have imaged the development of crystalline structure within amorphous gel particles of nanometre dimensions. Single zeolite A crystals were observed [80] to nucleate in amorphous gel particles of 40-80 nm in size within 3 days at room temperature. The embedded zeolite A nanocrystals grew at room temperature, consuming the gel particles and forming a colloidal suspension of 40-80 nm crystals. A broadly similar picture was found for the zeolite Y [81] and Si-MFI [79] phases. It has therefore been concluded that (i) the most common process of zeolite nucleation relies on a primary nucleation mechanism and (ii) the most probable primary nucleation mode is heterogeneous and centred upon the amorphous phase of the reaction mixture (which for most "clear solution" syntheses is colloidal in nature). 6.3. Crystal growth 6.3.1. Experimental observations In a typical zeolite synthesis, the first definite evidence for a successful reaction is the appearance of crystals of the product. As noted above (section 6.1), this signal for the end of the induction period is dependent upon the method of detection - most commonly a combination of visual inspection or microscopy with X-ray diffraction. Thereafter, crystal growth can be monitored by the same techniques and the resulting S-shaped growth curve of bulk crystallinity against time is by far the most commonly reported measurement of zeolite crystallisation kinetics (Fig. 2). More closely related to the growth mechanism itself is the crystal linear growth rate and to determine this some form of size measurement is made as a function of time. Suitable data can be obtained from samples taken during the course of small-scale experiments, through the use of particle counters relying on (for example) light scattering, conductivity across an orifice, or diffraction effects. More directly, the sizes of individual crystals can be measured by optical or

76

electron microscopy. These methods were used in the first determinations of linear growth rates (for zeolites A and X) by Zhdanov and coworkers [82,83] and were later applied to ZSM-5 synthesis by Nastro and Sand [91]. In these cases, measurements were carried out on syntheses sampled or terminated at a series of different reaction times. The first in-situ growth rate determinations were made by Lowe et al. [92,93], who used optical microscopy to study the growth behaviour of ZSM-5 and other high-silica zeolites by direct observation of reaction mixtures contained in glass capillaries. This work was extended by Sano, Iwasaki and coworkers using a specially-constructed cell in which a sample of ZSM-5 reaction mixture maintained at temperatures up to 170 °C could be directly observed using an optical reflection microscope [94]. The plots of crystal size against time typically show a constant linear growth rate for most of the reaction with a final tailing-off due to nutrient depletion (Fig. 2). From this and also from the magnitude of the activation energies (ca. 45-90 kJ mol"'), it is very probable that the rate controlling process is the surface integration step of crystal growth itself [16,28,93]. The activation energies are typical of those found for making and breaking T - 0 bonds [95,96] and much higher than would be expected for a process controlled by diffusion from the bulk liquid phase to the crystal surface (see Ref. [16], p. 152). Zeolite crystal linear growth rates show a strong temperature dependence. However, even for a given structure type, the rates are also influenced by other synthesis variables, particularly reaction composition. In a few cases, systematic studies have been reported, although most of these relate to the ZSM-5 system. Some typical zeolite and zeotype crystal growth rates are given in Table 3 (although data on the latter are very scarce). These growth rates are 2-A orders of magnitude smaller than comparable values for simple ionic salts (such as alums or alkali halides) or simple molecular compounds (e.g. sucrose) [74]. This large difference reflects the

Fig. 2. Zeolite or zeotype synthesis: crystal linear growth plot (solid line) and corresponding bulk growth curve (broken line). The crystallinity curve has here been calculated (by cubing the linear growth values).

77 Table 3 Some published values for zeolite and zeotype crystal linear growth rates System investigated

Zeolite or zeotype

Temperature range (°C)

Other parameters investigated

Na-A Na-A

50-100 60-80

Alkalinity, seeding Alkalinity, SiO2 source, Na/K Ageing Si/Al variation SiO2 source Na, TPA, OH, SiO2 pH, TPA / Na (F system) — Composition & conditions Seeding, stirring (growth rate estimated) (growth rate estimated) Si/Al, Al source

Na-X 70-100 NaX,Y 88 Na-analcime 130-160 Na,TPA-ZSM-5 170 Na,TPA-ZSM-5 170 Na,TPA-ZSM-5 150-200 Na,TPA-ZSM-5 90-175 Dodecasil 1H 160-200 AIPO4-5 170-200* CoAPO-5 170-200* SAPO-5 210 * microwave heating

Range of measured linear growth rates 0.5 Al/At (urn h"1)

Ref.

0.011-0.13 0.02-0.06

[83] [97]

0.02-0.11 0.0005 - 0.2 1.2-5.5 0.5-1.1 0.08-1.0 0.4-3.2 0.01-1.17 0.47 - 0.98 6 0 - 120 60-120 30-50

[82] [98] [99] [100-102] [103] [104] [84,93] [105] [54] [54] [106]

nature of the zeolite synthesis reaction, in which a largely covalent, polymeric structure is being assembled piece by piece as T-O-T bonds are formed one after another. This is clearly a more complex and intricate process than the packing of relatively simple units in ionic or molecular crystals where bonding is electrostatic or van der Waals and relatively non-directional. However, the high values for the AlPO4-5-type materials (although enhanced by the high measurement temperatures) would appear to reflect their more polar nature (see section 7.5.3). 6.3.2. Growth models The slow linear growth rates of zeolite crystals at fairly high supersaturations [107] and the rarity of the observation of growth spirals [108] suggests that the predominant growth mode is that of the adsorption layer type [74], controlled by surface nucleation rather than by the screw dislocations necessary for the Burton-Cabrera-Frank spiral growth mechanism [109]. A simple picture of this is given in Fig. 3(a). Following Gibbs-Volmer theory [74], a growth unit is adsorbed on the growing crystal face and migrates to its optimum location (i), a high-energy kink site at which the number of points of attachment are maximised. Following completion of a given layer (ii), no further growth can occur in the absence of a dislocation. The next stage is made possible by the creation of monolayer island nuclei (2-dimensional nucleation) (iii) from which the growth process can continue on a layer-by-layer basis. Direct evidence for the layer growth model is emerging from AFM studies (Fig. 3(b)), the growth terraces sketched in Fig. 3(a) being clearly identifiable. Using a representation of the Gibbs-Volmer type, Agger et al. have modelled the growth of zeolite A in terms of the features seen in AFM images [110]. Growth probabilities are

78

Fig. 3. Layer-by-layer crystal growth: (a) adsorption layer model, showing (i) addition of growth unit with migration to high-energy kink site, (ii) completion of a layer, (iii) further growth through surface nucleation; (b) AFM image of growth terraces on a zeolite A surface (image area 2.5 x 2.5 um2).

assigned to the potential growth sites (edge, kink, etc.) and the relative values for these probabilities are adjusted until the computed unit-by-unit growth pattern matches that observed experimentally. It is found that, to match the observed features, growth at kink sites has to be 500 times faster than growth on the clean surface and 15 times faster than growth at an edge site. It is also apparent that the overall advance of a growth face must be controlled by the surface nucleation rate, the lateral spreading rate being much faster. 7. EXPLAINING THE EXPERIMENTAL OBSERVATIONS: EQUILIBRIA, KINETICS AND MECHANISM 7.1. Equilibria and kinetics As noted earlier (section 4.1), the bond type (e.g. Si-O, Al-O, P-O) of the crystalline product in a synthesis reaction is very similar to that present in the precursor oxides, so that no great enthalpy change (AH) would be anticipated. In fact, the overall free energy change (AG) for such a reaction is also usually quite small, with little difference between the pathways to a number of potential products. The outcome is therefore most frequently dominated not by the prevailing equilibria (thermodynamics) but by the relative rates of various competing reactions (kinetics) [47,49,111-113]. Kinetic control is a pervading influence throughout zeolite synthesis, where the desired product is frequently metastable. Much of the know-how in this industrially important area centres around choice of the exact conditions for product optimisation, so that the

79 required material can be prepared reproducibly and to the same specification (see Ref. [16], p. 175). Quantitative information relating to microporous materials is rare but recent papers by Petrovic et al. and Piccione et al. present data on the thermochemistry of high-silica zeolites [114,115] and the thermodynamics of their synthesis [113]. The observed range of enthalpies of transition is quite narrow at only 6.8-14.4 kJ/mol above that of quartz. Correspondingly, Gibbs free energy changes for the crystallisation of microporous silica phases from amorphous silica are estimated for R4N+-templated zeolites ZSM-5 and beta to be in the range -4.9 to -8.5 kJ/mol SiC>2. No single thermodynamic factor was found to dominate the overall Gibbs free energies and the small energetic differences were taken to suggest that kinetic factors were of major importance in molecular sieve preparation. It is possible to reach an analogous conclusion from calculations based on the Lowe equilibrium model for zeolite synthesis [47,62,92]. Molecular modelling calculations (section 5.2) also point to the similarity in template binding energy between different products of a common organic guest molecule. 7.2. An overall model for the synthesis process We have seen (Fig. 1) how, in the conversion of oxidic starting materials to a microporous crystalline framework, it is necessary both to break chemical bonds and to make new ones, e.g. (Si-O-Si), (A1-0-A1) [reactants] -» (Si-O-Al) [products]. To achieve this, it is necessary to dismantle the original polymeric structures (silica, alumina), confer upon the fragments some reactivity and mobility, and re-combine the resulting elements into a new crystalline framework. The active constituent which achieves this is sometimes known as a "mineraliser" and, since its mode of action is at least partly cyclic, its behaviour resembles that of a catalyst. This is shown diagramatically in the scheme below.

The reactants (designated "A") are attacked by the mineraliser (XT) to give mobile anionic species (AX~) which carry mass to the growing product crystals. These are the monomeric and oligomeric silicate and aluminosilicate anions known to be present in zeolite synthesis solutions [8,16,19], The mineraliser is detached in the crystallisation process, whence it returns to combine again with further reactants. In this scheme, the mineraliser is most commonly hydroxide ion, but it may be (for example) fluoride ion, or even an uncharged complexing agent. Again, the process may not be a simple cycle. In many cases, a portion of the X~ ion is retained within the product together with a charge-balancing cation. In some cases, the mineraliser and associated species may have restricted mobility. However, the general principle is a useful one for the many zeolitetype syntheses which conform to the pattern of nucleophile-mediated condensations. It is not applicable to those metal-organic open framework materials (section 7.5.3) which resemble coordination complexes and whose preparation is essentially an acid-base reaction.

80 7.3. A stepwise reaction model The model given above provides a description of the overall synthesis process. To reach the next level of detail, we may consider the reaction as comprising three stages based on the concepts of the Lowe equilibrium model [62,92] (section 5.7). For any given system, changes in the solid and solution species can be monitored by chemical and physical methods and are reflected in quite simple properties such as reaction pH [63]. Although there will be considerable variation between one system and another, a reasonable generalised mechanism can be proposed (Fig. 4) [50]. 1. The reactants equilibrate with each other and with the reaction solution During the first part of the induction period (section 6.1), the different components of the reaction interact with one another, moving towards a pseudo-equilibrium under the prevailing conditions of temperature and pressure. In many cases, a visible gel phase is present. In other cases, the same type of material is present as colloidal particles, invisible to the naked eye. A fragment or domain of this amorphous component is shown in Fig. 4(a) as a random network of linked tetrahedra [36], This amorphous material equilibrates with the cations and anions of the solution phase in such a way that local areas of increased order are created in which the structure begins to resemble that of the eventual product (Fig. 4(b)).

Fig. 4. Zeolite and zeotype synthesis: order is established ((a)-(d)) within a fragment of the solid phase through interaction with solution species in a cation-mediated self-assembly process (inset (e)).

81

2. Product crystals nucleate and grow from nutrient solution species provided by the reactants, which themselves dissolve further to maintain the equilibrium At some point in the equilibration process (1), areas of sufficient order and regularity are established for them to achieve a new type of growth that is completely regular and periodic. This is the process of crystal nucleation (section 6.2.1) in which the first viable fragment of the crystal lattice itself is established (Fig. 4(c)). This crystal embryo is able to grow in a uniform and systematic way. The same solution species which were involved in the initial equilibration now contribute to the growth, adding mass to the growing crystal. Amorphous areas dissolve to maintain the solution concentration (Fig. 4(d)). 3. When all the reactants have dissolved, the product crystals stop growing and equilibrate with the reaction solution Unless the processes of synthesis are interrupted (e.g. by removing the reaction vessel from the oven), the reaction will continue until a steady state is reached. Normally, this will occur when the reactants are exhausted so that there is no further nutrient available to continue the growth of the crystals. The product crystals are now in equilibrium with the residual species in their mother liquor. 7.4. The mechanism in chemical terms In the foregoing sections (7.2, 7.3), an outline mechanism for the assembly of zeolitelike materials is given. The underlying chemistry which enables these changes to occur comprises two principal elements: (a) a reversible mechanism for the cleaving and re-forming of framework chemical bonds and (b) the cation-assisted construction of ordered regions. The first of these processes has been highlighted by Flanigen and Breck [47,116] and by Chang and Bell [117], namely the breaking and remaking of Si,Al-O-Si,Al (T-O-T) bonds catalysed by hydroxyl ion, the related condensation reaction also playing a part: T - O H + " O - T ^ = ^ T - O - T + OFT T-OH + HO-T ^

T - O - T + H2O

However, the anions do not work in isolation and a crucial role in the progressive ordering process is played by the cations present in the synthesis system. These will act to attract around themselves energetically favourable coordination spheres of oxy-species and in so doing will generate certain preferred geometries. In this way, cation-dependent elements similar to those present in the eventual zeolite product will gradually be assembled. These reactions are responsible for the incremental increase in order leading up to the nucleation step and also subsequently for the related construction of the zeolite or zeotype crystal lattice itself. The notion of cation-assisted ordering derives from the ideas of Breck [118] and forms the basis for the structuring of silicate and aluminosilicate units through electrostatic and (for organo-cations) van der Waals forces as proposed by Flanigen [119,120], Chang and Bell [117] and Burkett and Davis [121-123]. This is illustrated diagrammatically by the central insert (e) in Fig. 4. Here, the hydrated cations are pictured as coordination centres (templates) for the growth-unit additions and ring-closures necessary for the construction of the framework. It is envisaged that this process of structure generation occurs at growth points situated at the solution-solid interface,

82 thus constituting an in-situ self-assembly process for the growing crystal. The nature of the growth units or building blocks is discussed further below (section 7.5). 7.5. Alternative views of the synthesis mechanism The general account presented above (sections 7.2-7.4) is intended to provide a flexible guide to the understanding of synthesis mechanism. Whilst it is certainly misleading to prescribe a single set of unchanging rules for every situation, it nevertheless seems likely that the models given earlier provide a reasonable basis for the interpretation of many experimental observations. However, there are several topics on which reference to the published literature will reveal considerable differences of opinion. 7.5.1. Solution-mediated or solid phase? Throughout the history of the synthesis of zeolite-like materials, there has been much debate about the location of the structure-forming activity [28,36,39,50]. At one extreme, there is the possibility of solution-mediated crystallisation. In this view, the reactants dissolve (to a greater or lesser extent) in the reaction mixture to afford active species, from which the product is formed as it crystallises from the solution. In the opposite view, the solution is seen as more or less inert with the product being formed within the gel phase by a process described as a "solid state transformation". Whereas the solution-mediated route is well known in the science of crystallisation, the alternative is a somewhat shadowy phenomenon for which no chemicallyspecific mechanism has ever been published. There are many transformations which appear to take place in the absence of a welldefined liquid phase but in nearly all cases a liquid-like phase is indeed present, either within the gel or as an adsorbed component. This provides the required mobility for the transport of active species and the construction of the crystal lattice, although active species may not be freely mobile within a bulk solution phase. With this caveat in mind, a mechanism based upon the solution-transport concept still seems the most useful general model. 7.5.2. The nature of growth species Concerning growth species, there are two main areas for controversy: (i) whether the growth units are solution species or particulates, and (ii) whether the formation of a particular structure requires the presence of specific building units. Several authors have postulated mechanisms for zeolite nucleation and growth which involve the aggregation of particles [78,124—127]. Much of this speculation arises from recent work on "clear solution" syntheses in the TPA-silicalite system, using advanced scattering and diffraction techniques. The basic argument is that primary sub-colloidal particles with diameter 3-4 nm aggregate and "density" in a series of steps to give clusters with a size of 6-7 nm and which are amorphous to X-rays. After this, the densified clusters aggregate to particles of around 50 nm in size which display X-ray crystallinity. No detailed explanations have been provided as to how a "growth by aggregation" model operates in chemical terms. For the reaction sol itself, a degree of agglomeration of sub-colloidal or colloidal amorphous particles seems very probable. However, a zeolite or zeotype growth mechanism based upon the aggregation of amorphous with crystalline particles [126], or the agglomeration of non-viable nuclei with other growing nuclei [128], is more problematic. Some form of

83 depolymerisation or in-situ reconstruction step involving mobile growth units is also necessary for the generation of an ordered periodic structure. Direct experimental evidence relating to the significance of aggregation processes comes largely from HRTEM studies. For example, a weakness of an aggregation growth mechanism involving additive entities of ca. 5 nm in size has been highlighted by Davis [129]. This is its inability to account for the existence of ordered crystalline mtergrowths of structurally different phases - a fact which is readily explained by a normal layer-by-layer mechanism of crystal growth (section 6.3.2). There exists a structural similarity between some species which are found in zeolite and zeotype synthesis solutions and the building units which appear in zeolite and zeotype frameworks. Several groups of workers have accordingly adopted the idea that a zeolite containing (for example) double-5-rings is constructed by the extraction and polymerisation of these units from the distribution of such species available in solution, the equilibrium then rapidly shifting to make up the deficit [124,130,131]. Although attractive, this theory, for zeolites at least, cannot be substantiated [132,133]. Predictions made on its basis (for example, on the density of defects in the resulting structures) are incorrect and there is no correspondence between the occurrence of particular solution anions and the nature or formation rate of a particular zeolite. However, such a theory can never be completely rejected since there is always the possibility of an active component present at a low (and possibly undetectable) level. The most generally applicable crystal growth model for zeolite systems is that based on mass gain from simple species (predominantly monomers) in solution [20,28,84,92,93,128,133135]. Following nucleation on or near the surface of an amorphous particle, the crystalline region is extended by the acquisition of growth units from solution. These are replenished by the adjustment of solution equilibria and the dissolution of amorphous (or less ordered) material. The identity of the growth species will depend on the reaction chemistry (e.g. OFT- or F~-based) but the same general principles should apply to every system. This is the type of growth assumed for the general mechanism given in Fig. 4. However, investigation of the nature of the species which carry mass to the growing crystal is a challenging experimental problem, which may in due course be resolved by (for example) a combination of AFM and Raman microscopies. 7.5.3. Other porous materials The majority of investigations on synthesis mechanism have been carried out on the materials which have been known for the longest time, i.e. the zeolites and zeolitic polymorphs. However, some studies are now available on the newer zeotypes such as the metallophosphates (AlPO4s, SAPOs, MeAPOs) [31,41,46,54,106,136-139] and the metal-organic [42,140,141] and other [142] open framework materials. The most significant difference in thinking on the two types of system concerns the importance of specific precursor units. Some workers in the zeotype field have adopted the idea of pre-nucleation building units (PNBUs) [41,143,144], suggesting that the products have been formed by condensation of such units pre-existing in the solution. Complex models of zeotype formation by linkage of 1- to 3-dimensional precursor units have been proposed [31,42,46]. Based largely on NMR evidence of solution species and the commonality of similar structural units in the products, a recent review of metallophosphate formation discusses a synthesis mechanism in which significant elements of the structure exist in solution as identifiable building blocks before being "clipped" together to form the precipitated product [41].

84

Some zeotype systems certainly behave very differently from their zeolite counterparts. One of the most striking demonstrations of this is found in the framework zinc phosphates, which can be crystallised very rapidly under very mild conditions, suggesting a more nearly ionic assembly route. For example, the sodalite analogue Na6(ZnPO4)6.8H2O can be made from a mixture of ZnO, H3PO4, NaOH and water in 24 h at room temperature [145], whilst Na,TMA-ZnPO4-faujasite can be synthesised in 30 min at 4 °C [146]. Even further along the polarity scale are the inorganic-organic hybrid materials [140] such as the complex metal carboxylates. In a recent report [147], a single reaction mixture produced five different cobalt succinates, the structure obtained depending only upon the reaction temperature. As the temperature was increased from 60 to 250 °C, the products became less hydrated and more dense, the structures changing from chains to sheets to frameworks across the series. However, whilst chemical differences between the different classes of microporous materials clearly do exist, it is not yet clear to what extent these affect the mechanism of synthesis. Possibly the products do derive directly from PNBUs but there seems no reason at present to reject the alternative possibility that much of this chemistry goes on at the liquidsolid interfacial growth points rather than (as is implied) independently of the growing crystal. It seems unlikely that any completely new concepts will be necessary to explain the formation patterns of zeotypes. However, just as the range of behaviour observed for zeolites requires a flexible (but coherent) mechanistic scheme, this spectrum will need to be further extended in order to allow for the differences in composition, structure, polarity and solution chemistry found in zeotype synthesis. 8. CONCLUSION Successful syntheses of a wide variety of zeolite and zeotype materials have been developed over the last 50 years. Much is known about the way in which these structures are formed, particularly from recent detailed studies using advanced techniques of microscopy (HRTEM and AFM). However, a number of issues surrounding the mechanism of synthesis remain incompletely resolved. This is partly due to experimental difficulty but is also a reflection of the fact that not all systems are the same. Nevertheless, in the foregoing account an attempt has been made to review our existing knowledge in terms of basic similarities between one reaction regime and another. In this way, it is hoped to establish some overall principles which, with appropriate modification, may be found to be generally applicable, or at least to provide a framework for further analysis. ACKNOWLEDGEMENTS The author would like to acknowledge the contribution of his colleagues at the University of Manchester Centre for Microporous Materials in their advice and constructive comments. Particular thanks are due to Jonathan Agger and Itzel Meza who also provided Figure 3, and to Paul Cox (University of Portsmouth) for his sustained help and enthusiasm.

85 APPENDIX: SOME EXAMPLES OF ZEOLITE AND ZEOTYPE SYNTHESES Given below are two examples of flexible zeolite and zeotype syntheses which have been chosen for their merit in demonstrating a variety of techniques and underlying principles. Minimal description is provided (although some references are given to further information), the experiments providing scope for individual exploration and discovery. Important note It is assumed in the following examples of synthesis procedures that those carrying out the experiments will be adequately supervised by experienced scientists and that they will follow established procedures of good practice and safe working [51]. No specific warnings of individual hazards (e.g. caustic substances, pressure in reaction vessels) are here given. 1. Synthesis of MFI materials (silicalite-l, ZSM-5) The target reaction composition is jcNa2O : 60SiO2 : yA\2Oi : 6.0TPABr : 6000H2O : 240EtOH. The primary procedure (x = 3) is as follows. Tetraethyl orthosilicate (225 g) is added to a solution of sodium hydroxide (4.5 g), tetrapropylammonium bromide (TPABr) (28.5 g) and deionised water (1840 g) in a polypropylene container. The vessel is then sealed and the twophase mixture stirred at room temperature until the organic layer has reacted completely to give a clear, homogeneous sol (ca. 48 h). Silicalite. If the above sol is heated at 90-95 °C (preferably with stirring), silicalite is obtained in about 7-14 days as rounded, lozenge-shaped crystals (mostly twinned) of broad particle size distribution (0.5-10 um). Raising the reaction temperature to 130 °C shortens the reaction time (2-3 days) and changes the morphology, the product now being largely untwinned lozenges (3-20 um, aspect ratio 6:3:1). Ageing the reaction mixture before carrying out the

Fig. 5. Silicalite synthesised from hydrolysed tetraethyl silicate sols at («- left) 90 °C, (right -») 130 °C.

86 heating step encourages the generation of "proto-nuclei" [50,88]. For example, over a period of weeks (at room temperature) the crystal size distribution of the final product gradually narrows, eventually (ca. 1 year) reducing to a uniform value of around 2 um. The necessary heating time shortens correspondingly. The nucleation process can also be accelerated by raising the base level (preferably after completion of the initial hydrolysis step). At x = 8, a product of nearmonodisperse 2-4 um crystals may be obtained from unaged reaction mixtures as near-spherical twins (90-95 °C, 4-7 days) or lozenges (130 °C, <2 days), and both reaction time and crystal size can again be further reduced by ageing (Fig. 5). At these high base levels, the reaction mixtures will eventually yield crystalline silicalite at room temperature on storage (>2 years). ZSM-5. The above procedure can readily be adapted for the synthesis of the isostructural ZSM-5 by the inclusion of an aluminium source. The reaction time and product morphology are source-dependent. The most efficient incorporation of aluminium is obtained from soluble sources such as sodium aluminate, perhaps made in-situ [148]. As with other heteroatoms [149], there is a correlation between increasing temperature and increasing ease of aluminium incorporation into the framework of the product, the maximum readily attainable at around 90 °C being Si/Al = ca. 90. Very rapid crystallisation can be achieved by the synergistic use of nanocrystal seeding and microwave heating [55]. 2. Microwave synthesis of AlPO4-5 by a fluoride route [54,150,151] The target reaction composition is 1.0Al2O3 : 1.3P2O5 : 1.6TEA : 1.3HF : 320H2O. Phosphoric acid (85%, mass) (9.2 g) is diluted with water to 50% concentration and triethylamine (TEA) (5.0 g) is then added dropwise. At 0 °C, aluminium triisopropylate (12.6 g) is slowly added to the resulting mixture with intense stirring. After stirring the slurry at room temperature for 2 h, HF (40% (mass) aqueous) (2.0 g) is added and the mixture stirred for a further 2 h. The resulting homogeneous white gel is diluted with the remaining water (170 g), placed in a fluoropolymer autoclave and heated in a temperature-controlled microwave oven at 180 °C. With a CEM MDS-2100 laboratory microwave oven, a highly crystalline product was obtained in 15-30 min as hexagonal rods 6x2 - 45x12 um. In a comparative synthesis using a conventional oven, the minimum reaction time for a well-crystallised product of similar particle size was 12-24 h. In a variant to the microwave procedure, the reaction was stopped on reaching 175 °C (2 min) and the supernatant decanted from the settled solids. Further microwave heating (175 °C, 20 min) of the decanted solution produced a very clean product of uniform size distribution (24x7 um). Similar procedures can be used for CoAPO-5 synthesis [54,150,152] although with this more heterogeneous reaction mixture the microwave/decantation procedure is less effective in narrowing the size distribution. REFERENCES [1] [2] [3] [4] [5]

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Synthesis of mesoporous molecular sieves Wieslaw J. Roth and James C. Vartuli ExxonMobil Research and Engineering Co., 1545 Route 22 East, Annandale, NJ 08801, USA. e-mail: [email protected] 1. INTRODUCTION 2. DISCOVERY AND FUNDAMENTAL PROPERTIES OF THE M41S FAMILY OF MESOPOROUS MOLECULAR 3. SURFACTANT AND LIQUID CRYSTAL TEMPLATING 4. EXPLOITING SURFACTANT SYSTEM PHENOMENA IN THE SYNTHESIS OF MESOPOROUS MATERIALS 4.1. Variation of surfactant type 4.2. Different structures 4.3. Pore size variation 4.4. Framework composition 5. DIFFERENCES BETWEEN MESOPOROUS MATERIALS AND ZEOLITES 6. SYNTHESIS OF MESOPOROUS MATERIALS 6.1. Synthesis of MCM-41 6.2. Synthesis of cubic MCM-48 6.3. Synthesis of SB A-15 6.4. Synthesis of mesoporous alumina 7. SYNTHESIS OF MICRO- AND MESO-POROUS HYBRID MATERIALS 8. ORGANO-INORGANIC MESOPOROUS COMPOSITES 9. CONCLUDING REMARKS REFERENCES 1. INTRODUCTION The title materials are synthesized in the presence of surfactants and were recognized first in the silica-cationic surfactant systems. The area expanded rapidly becoming an independent, though closely connected to zeolites, interdisciplinary field with potential of practical applications in catalysis and sorption. The discussion outlines the major advances in the synthesis of these materials from discovery to the present, stressing the general trends and interplay between surfactant and oxide chemistries. Representative materials are discussed in detail from the standpoint of convenient synthesis of a high quality useful product. 2. DISCOVERY AND FUNDAMENTAL PROPERTIES OF THE M41S FAMILY OF MESOPOROUS MOLECULAR SIEVES Mesoporous molecular sieves have uniform pores like zeolites but with a diameter greater than about 20 A. They were unknown until beginning of 1990's [1,2]. The area rapidly

92

ZSM-5 ~5 Å Pores

VPI-5 ~12 Å Pores

MCM-41 ~40 Å Pores

Fig. 1. TEM images of MCM-41 and representative microporous materials, ZSM-5 and VPI-5.

expanded into an extensive interdisciplinary field following a series of reports from Mobil R&D Corp, which included a communication at 9th IZC in Montreal, two pioneering publications [3,4], two principal composition of matter patents [5,6] and numerous other patent disclosures [6-9]. This initial work demonstrated that cationic surfactants, such as hexadecyltrimethylammonium cation (CTMA+), can act like equivalents of the organic structure directing agents of zeolites and can generate aluminosilicate and other oxide materials with uniform channels in the mesopore region (>20 A) [7]. As a particularly notable development the periodic structure and enormous pore openings of some of the newly discovered mesoporous molecular sieves was visualized directly by electron microscopy. A

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representative TEM image, shown in Fig. 1, revealed a remarkable hexagonal honeycomb with pore openings approximately 40 A in diameter. The new material with proven hexagonal arrangement of apparently unidimensional channels was designated MCM-41. The corresponding X-ray powder diffraction pattern (XRD) could be indexed and assigned a hexagonal unit cell (without c direction), see Fig. 2. Two additional discrete structures were identified at higher surfactant/silica ratios. These structures were identified based on unique and indexable XRD patterns: MCM-48, a cubic structure with Ia3d symmetry and 3dimensional pore system, and MCM-50, a lamellar species. Not all of the apparently mesoporous products generated in the presence of surfactants could be assigned a discrete structure. Many materials exhibited only one or two XRD peaks, which could not be unambiguously indexed and used for structure assignment. As a diverse class these new mesoporous molecular sieves were referred to as the M41S family and could be identified by the following fundamental and general properties: •

Thermally stable periodic structure manifesting itself after calcination by at least one low angle line in X-ray diffraction pattern above 18 A d-spacing (Fig. 2)

•

High adsorption capacity with unprecedented uniform mesopores resulting in the presence of capillary condensation in the adsorption/desorption isotherms for small sorbate molecules (Fig. 3).

The detailed accounts of the discovery of the M41S family and subsequent developments have been published recently [7,8]. The discovery, embodied by the unprecedented honeycomb pattern observed in TEM images, was unexpected but resulted from a deliberate and systematic effort to prepare large pore zeolitic or related materials. One of the efforts focused on exploiting the recent discovery that zeolite MCM-22 formed via a layered precursor [10-12]. A conversion of the MCM-22 precursor into a pillared derivative could afford a large pore structure with zeolitic active sites and/or internal microporosity. However, none of the special treatments known at that time enabled sufficient extent of exfoliation (layer separation) in MCM-22-P, which was the essential first step. The introduction of custom made cationic surfactants in the hydroxide form overcame the problems with low efficiency of layer separation encountered initially with MCM-22-P. This new reagent enabled essentially complete swelling of the MCM-22 precursor and afforded the pursued 'pillared zeolite' MCM-36 upon additional treatment [12]. The use of the available surfactant hydroxide was expanded further: as a swelling agent in the presence of active silica sources, such as tetramethylammonium silicate, and directly in the hydrothermal reaction of aluminosilicate gels as the templating reagent. This work produced the first examples of the M41S materials and the accompanying electron microscopy characterization revealed the underlying regular structure. This demonstration, that the combination of surfactants and silica can produce mesoporous materials resembling zeolites in terms of uniform pores and well-defined structure stimulated a systematic exploration of the new area. The behavior of surfactant system was exploited to vary pore size of the generated materials and to produce novel discrete structures. An indepth characterization showed some remarkable and unprecedented physical properties, such as adsorption of gases with capillary condensation in the mesopore region [12]. A comprehensive catalytic testing showed potential for practical applications in hydrocarbon conversion and other processes [7,9].

94

Fig. 2. M41S family of ordered mesoporous materials - X-ray powder diffraction patterns, representative TEM images and proposed structures.

95

Fig. 3. Representative nitrogen adsorption/desorption isotherm for an ordered mesoporous material with approximately 40 A pore.

The initial broad effort established basic foundations and served for further expansion while the principal tenets have remained valid to date. When reported in the open literature, the new large pore materials immediately attracted attention of the scientific community, especially in catalysis and adsorption fields. Subsequent effort by many groups led to unforeseen expansion not only in terms of the overall effort and abundance of materials and phenomena but also new directions pursued. There is probably close to 10,000 publications on the subject [1]. This presentation will focus primarily on fundamental concepts and ideas related to synthesis. For detailed and comprehensive treatments one is referred to the review publications that have been appearing regularly [13-20]. For up-to-date overview of state of the art several recent reviews are especially recommended [17-20]. As the new field expanded and novel concepts were recognized other names were being used to designate the entire class or its subgroups. The examples include ordered or periodic mesoporous materials, mesostructures, mesophases, etc. and they have been used interchangeably. In here we adopted the abbreviated form 'mesoporous materials' as referring to materials with uniform mesopores. In the early 1990's Kuroda et al. reported preparation of porous materials resulting from interaction of layered silica kanemite with alkyltrimethylammonium salt solutions [21]. Later on [22] they proposed the mechanism labeled FSM (Folding Sheet Mechanism; see Fig. 4) to rationalize the nature of the products [23,24].

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Fig. 4. Proposed mechanism of the formation of a mesoporous material by bending and condensation of surfactant treated kanemite (Folding Sheet Mechanism) ([22]; reproduced by permission of The Royal Society of Chemistry], 3. SURFACTANT AND LIQUID CRYSTAL TEMPLATING

M41S materials were obtained as the result of incorporation of surfactants as structure directing agent in the microporous-type preparations. The identified well defined structures resembled some of the lyotropic liquid crystal phases observed in solutions of the corresponding surfactant compounds [25-28], thus prompting a name for the entire phenomenon as liquid crystal templating (LCT). In the refined version the concept proved all-embracing by proposing two possible pathways, as shown in Fig. 5. One pathway was referred to as liquid crystal initiated. It postulated diffusion of silicate ions into the aqueous region of preformed liquid crystals resulting in assembly of an ordered silica framework. The second pathway was silicate anion initiated and entailed self-assembly upon mutual attraction between silicate and surfactant ions. The latter proved operative in almost all cases. It was further elaborated and in a detailed form is referred to as the cooperative self-assembly [29]. Later on the first pathway representing true LCT was also proved viable [30]. It must be stressed that there is a big difference between the liquid crystal and the templated solid mesoporous phases. The former are equilibrium systems, while the latter are apparently transient. As far as we know the solid mesophases are not produced through equilibration, which makes their formation and existence quite astounding and a bit of a surprise (V. Luzzatti, personal communication). Another new concept formulated in conjunction with the formation of the M41S structures was supramolecular templating, i.e. one involving assemblies of molecules (surfactant aggregates, micelles, liquid crystals) [31]. In contrast, microporous materials are templated by isolated molecules acting as structure directing agents. 4. EXPLOITING SURFACTANT SYSTEM PHENOMENA IN THE SYNTHESIS OF MESOPOROUS MATERIALS The expansion and diversification of the ordered mesoporous materials can be attributed to bringing together two advanced disciplines, i.e. microporous solids and amphiphilic compounds. The latter turned out particularly rich in phenomena that could be exploited for innovation and synthesis of novel and/or designer products. The following developments can be considered as exploiting the principles of surfactant science in the preparation of porous solids.

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Fig. 5. Schematic representation of the liquid crystal templating mechanism of the ordered mesoporous materials - original with added modifications

Fig. 6. Phase diagram for the cetyltrimethylammonium bromide (C]6H33N(CH3)3-Br; CTAB) in water (based on reference [28])

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4.1. Variation of surfactant type The initial use of cationic surfactants producing negatively charged frameworks has been extended to amphiphiles with other charges and even ionic pairs. These various possibilities have been represented by the notation system specifying charge for the surfactant (S), inorganic component (I) and auxiliary small counterions, which may be mediating in some systems between the S and I (X - halide, etc, M - alkali metal) [32]. Nonionic surfactants (amines, poly-ethers) also produced mesoporous structures and were usually used with alkoxides as the inorganic component. Novel mesophase structures (see below) were identified when silica (tetraethyl orthosilicate, TEOS) was templated by triblock copolymers in acidic media, apparently through small ion mediation (S°H+)(XT °) [33]. The major supramolecular templating routes are summarized in Table 1, which is based on Ref. [17,18] 4.2. Different structures The first mesoporous structure identified, i.e. the hexagonal unidimensional MCM-41, resembled one of the liquid crystal phases observed in the surfactant-water systems. The corresponding phase diagram indicated existence of other phases at higher surfactant concentrations, namely a cubic and a lamellar one (Fig. 6) [25-28]. This trend was reproduced in the aqueous silica-surfactant system upon changing surfactant/silica ratio with nominally constant surfactant concentration [34]. As the amount of silica added decreased and the ratio surfactant/silica increased, the following sequence of changes occurred: the initially dominant hexagonal MCM-41 gave way to the product with cubic Ia3d symmetry and 3-dimensional channels, MCM-48, through lamellar MCM-50 and finally a soluble molecular silicate was obtained. This experiment can be interpreted as providing the evidence against the liquid crystal initiated pathway, at least in this case. The observation of different structures formed at essentially constant surfactant concentration is inconsistent with templating by a pre-existing liquid crystal phase. The three M41S family silica phases obtained with cationic surfactants in a basic medium represent the primary set of principal mesoporous structures. The observed 'phase' transitions are attributed to changing surface curvature resulting from decreasing packing factor, g, which is related to the surfactant molecule dimensions and defined as [35]: g=V/(ao-l) V - volume of the surfactant chain including contribution from co-solvent or other molecules between the chains ao - effective head group area at the surface / - kinetic length of the surfactant side chain The values of g at which a phase transforms to another are: 1 for a lamellar phase, between 1/2 and 2/3 for the cubic Ia3d phase and 1/2 for the hexagonal phase [35]. Not surprisingly other cationic surfactants and the alternative surfactant systems listed in Table 1 were also able to generate the three basic structures. The cubic MCM-48 type was most difficult to synthesize and consequently quite rare. Other types of surfactants and/or alternative media (acidic or neutral) allowed preparation and recognition of additional structural kinds of mesoporous materials (which could also be correlated with certain g values). More than 20 types of silica mesoporous materials are listed in a recent compilation as unique in terms of structure and/or method of assembly [18]. Several new cubic structures

99 Table 1 Classification of the major supramolecular templating routes according to surfactant charge Surfactant charge Cationic

Surfactant type Alkyl-NR3+

Interaction type +

Alkyl-NR3+

S+XT+

Alkyl-NR3+, R=Et

S+XT+ +

Di-(N-quaternary) Anionic

Neutral

sr

2+

M41S-MCM-41,-48,-50 FSM-16 SBA-3 SBA-1 MCM-48, SBA-6, SBA-8

sr +

Examples/structures

+

Alkyl-OPO3H2" Alkyl-OSO3" R-CO2H

si , S"M r

Amine

N°I°

Poly(ethylene)oxide Alkyl-(EO)n, EO ethylene oxide Amine

gUjU

Alkyl-(EO)n, EO = ethylene oxide Triblock copolymers

(S U H + )(XT + )

SBA-11,-12,-14

(S U H + )(X1 + )

SBA-15,-16 FDU-1 Al(sec-OBu)3 - organized mesoporous alumina

ST+

SV ST

gUjU

Mainly lamellar or disordered; Al, Pb, Fe, Mn, Ni, Co, etc Al (sec-OBu)3 - organized mesoporous alumina HMS and MSU silicas - wormlike structures, similar to M41S and can be modified similarly With TMOS - true LCT Ligand assisted; amine bonding to Nb and Ta alkoxides

with different symmetries were obtained using Si-ethoxide (TEOS, tetraethyl orthosilicate) as the silica source [1,18,33]: SBA-1, Pm3n, with supercage cavities; templated by a cation surfactant with a larger head group SBA-6, Pm3n, like above templated by a divalent template SBA-16, Im3m, cages, templated by a triblock copolymer SBA-2, intergrowth of hexagonal and Fm3n cubic; divalent cationic template SBA-15, a hexagonal (MCM-41-like) silicate obtained with (EO)n(PO)70(EO)n copolymers; attracted attention because of thick silica walls (up to 60-70 A), the opportunity for pore tailoring up to 300 A and microporous connection between mesoporous channels [33]. Only a handful of the identified types of mesoporous materials have been extensively explored and used. A noteworthy occurrence has been that some of the preparations afforded well developed faceted particles, a la ordered crystals. The examples are MCM-48, SBA-1 andSBA-16[36].

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4.3. Pore size variation The length of surfactant side chain determines dimensions of the hydrophobic domain and consequently unit cell of a liquid crystal. Pores of different diameter can be obtained by changing surfactant size. This initially allowed MCM-41 pore adjustment from about 20 A up to 50-60 A. Further expansion was possible based on another phenomenon, i.e. micelle swelling by solubilized organic compounds. MCM-41 with pores up to 120 A was obtained with mesitylene as an auxiliary organic (expander) in a hexadecyltrimethylammonium templated preparation [4]. Triblock copolymers were found to afford hexagonal structure, SBA-15, with a pore diameter up to 300 A [33]. Pore dimensions can also be tailored by adjusting the synthesis temperature and post-synthesis treatment with amines [18]. 4.4. Framework composition Composition of the inorganic framework can be varied just like the surfactant use has been expanded beyond the original cationic species. The number of explored element combinations has been enormous and often driven by specific catalytic needs and prior experience with amorphous or crystalline compositions [37]. One of the leading approaches was doping of silica formulation with appropriate activating element, such as aluminum (to impart acidity) and titanium or vanadium for redox potential. Based on analogy of AlPO's and SAPO's to zeolites these compositions were also synthesized in the mesoporous form as were other non-silica inorganic oxides and their combinations [38]. Pure metal mesoporous product was obtained via the pre-formed liquid crystal route [39]. Many of the non-silica compositions showed problems with the stability and quality of the structure. Efforts to address these issues have been on going and quite successful in some cases such as all-alumina compositions (see below). Silica-based materials remain dominant as the most versatile and best quality molecular sieves (structure and stability) available by a facile synthesis. These attributes, especially the convenient synthesis made mesoporous silicate attractive for post-synthesis functionalization with other elements as well as organic moieties with active groups/centers. Recently the compositional diversity has been extended further to include both silica and organic moieties within the framework. The new class is referred to as periodic mesoporous organosilicas (PMOs). The synthesis involves surfactantassisted assembly by hydrolysis of organo-silicon compounds. Additional discussion of the PMOs is presented below. Recently carbon based mesoporous materials have been synthesized via a technique using the inorganic mesoporous framework as a template [40]. The approach is designated nanocasting [17]. In fact, these materials were the first to illustrate the existence of the mesoporosity of SBA-15 materials [41]. 5. DIFFERENCES BETWEEN MESOPOROUS MATERIALS AND ZEOLITES The surfactant templated mesoporous materials were treated as an extension of microporous solids and have been adopted by the scientific community as part of the zeolite endeavor. On the other hand there are fundamental differences between the two classes and this has had a significant impact and consequences in the areas of synthesis, characterization, and practical applications, especially catalysis. The ordered atomic structure of typical microporous materials makes them easy to identify and characterize by XRD. The structural information obtained by XRD combined with a few additional characterization data may effectively define the properties and expected behavior

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of a particular preparation. Furthermore, the ordered structure of zeolites imparts inherent higher stability and catalytic activity in comparison to analogous amorphous compositions. In the case of the ordered mesoporous materials, XRD informs primarily about the presence of ordered pores. In many cases the pattern cannot be indexed (too few and/or broad peaks) to assess symmetry and channel dimensionality. Additional characterization is essential to determine pore size and distribution and to learn other basic properties. Electron microscopy proved quite informative when appropriate images could be obtained. This has been the leading technique for discovery and identification of new products [3,4,36]. It is also very useful when a high quality uniform product is available. In general, the use of electron microscopy as a bulk analytical/characterization tool has been limited and, in view of labor intensive sample preparation, rather impractical. The basic description of a mesoporous sample requires two types of determinations: X-ray diffraction and gas adsorption/desorption isotherm. The latter are usually represented as the amount of gas adsorbed by the sample as the function of relative pressure. This information characterizes pore size distribution. Nitrogen adsorption/desorption isotherm at 77 K is most often used and relatively convenient to carry out. The adsorption of noble gases is used if accurate in-depth pore characterization is attempted, especially quantitative. The calculation of pore size distribution from the isotherms is carried out using appropriate formulas such as Kelvin and Horwath-Kawazoe equations (e.g. as in Ref. [5] and [6]), which involve assumptions and approximations. A more detailed and rigorous treatments have been developed, as for example KJS (Kruk-Jaroniec-Sayari), which is relatively simple and accurate [42]. In practice, the diameter of mesopores can be quickly estimated directly from the position of the capillary condensation or, if not vertical, the p/p0 of the inflection point. The conversion table of p/po values to pore diameters can be found in Ref. [43] and is partially reproduced here in Table 2. The advances in synthesis and characterization of the ordered mesoporous materials also translated into new findings associated with adsorption fundamentals and pore description. Novel features such as hystereses with unusual shapes have been observed and required interpretation. Significant theoretical progress has been made regarding understanding and quantitative treatment of the gas adsorption data in the mesopore region. A comprehensive review of the subjects has been published recently [43]. Table 2 Estimating pore size from nitrogen isotherm - selected value of the nitrogen capillary condensation relative pressure and the corresponding pore diameters [43] P/Po 0.1 0.2 0.3 0.4

Pore diameter (A) 24 29.4 34.9 41.3

P/Po 0.5 0.6 0.7 0.8

Pore diameter (A) 49.6 61.3 79.7 114.i

6. SYNTHESIS OF MESOPOROUS MATERIALS The above discussion concerned some general trends and principles governing the formation of mesoporous materials. On a practical level the formation/synthesis of products with uniform mesopores by self-assembly, especially the silica-based ones, appears quite facile

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and deceptively simple. Solids with low angle XRD peak indicative of ordered pore structure and narrow pore size distribution have been found to form spontaneously if enough surfactant is available and conditions not particularly harsh. Even a room temperature precipitation can afford products that meet the criteria for mesoporous materials [44] and which might be suitable (e.g. stable enough) for some applications. However, the nature of a mesoporous product, particularly in terms of quality and stability, is strongly influenced by the synthesis conditions. Solids with specific properties (e.g. structure or pore size), high quality XRD, good stability, etc, typically required more constrained synthesis parameters. Non-silica based compositions presented a separate problem altogether because of frequently occurring structural collapse upon surfactant removal. In short, on the one hand the synthesis has appeared quite straightforward but, on the other hand, obtaining a good/desired product by a convenient route required a considerable developmental effort. Among the many varieties of mesoporous materials several stand out as most extensively studied. The initial 3 structures [3,4] remain dominant although MCM-50 has attracted least attention and found little practical value. It has an unstable structure collapsing upon calcination but still exhibits considerable microporosity, which is intriguing and possibly deserving closer attention. However, if a post treatment by adding a reactive silica source, e.g. TEOS, to the as-synthesized MCM-50 is done prior to air calculations, then a uniform mesopore structure (after calcination to remove the surfactant) with retention of the lamellarlike XRD is observed. MCM-41 and MCM-48 type structures and SBA-15 are considered here as representative of the entire class and to reflect the expected range of characteristics and behavior. Another practical aspect of the synthesis of mesoporous materials concerns balancing product quality with simplicity and economics of the procedure [7]. The approaches discussed below were selected primarily because they afford high quality products (structure, stability) from common reagents and by a simple hydrothermal synthesis. 6.1. Synthesis of MCM-41 MCM-41 has unidimensional, presumed to be straight channels. It can be identified by XRD pattern with at least 3 peaks corresponding to hkO reflections of a hexagonal unit cell. It has been possible to vary the dimensions and compositions of the hexagonal framework over a broad range. Many facile synthesis procedures have been identified and developed to produce MCM-41, sometimes to address a particular application need. The synthesis approach described here can afford a high quality representative product under wide range of temperature and time conditions [45,46]. It evolved from the fundamental studies of the interaction of silicate solutions with surfactants and supplied a convenient laboratory synthesis route. In addition, it is based on certain stoichiometric relationships between reagents that justify its characterization as a rational method. Finally, with essentially unchanged approach upon simple lowering of the silica to surfactant ratio a convenient method of MCM-48 preparation is obtained [47]. In this procedure the synthesis mixture is reduced to three basic components: silica (solid), surfactant (halide) and hydroxide as mineralizer (tetramethylammonium hydroxide, TMAOH), all supplied as the convenient reagents. The key parameter is the essentially stoichiometric 1:1 ratio of surfactant and hydroxide, which allows treatment of the product (MCM-41) as formally a surfactant silicate resulting from interaction between silica and surfactant hydroxide. The latter reagent is not a conveniently available or cost effective one to use. Consequently the most desirable surfactant source is its halide salt. The presented

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method of MCM-41 synthesis demonstrated flexibility and the following important benefits [45]: 1. A high quality product judged by XRD and nitrogen sorption is obtained with the silica/surfactant ratio around 5-6:1. 2. Virtually all surfactant used ends up incorporated in the solid MCM-41 product, reinforcing the rational (stoichiometric) character of the synthesis. 3. Increasing severity of the synthesis conditions (higher temperature, longer time) results in a more robust framework and pore diameter expansion. The latter represents a simple method of tailoring pore size by synthesis time and temperature adjustment. 4. Upon reduction of the silica/surfactant ratio to -3.5/1 a smooth transition to the cubic MCM-48 as the favored phase occurred. This was one of the first demonstrations of 'pure' MCM-48 synthesis by a simple method with common reagents and analogous to the MCM-41 preparation. Synthesis of MCM-41: the synthesis mixture is prepared by mixing 140 g of water, 30 g of silica, 30 g of 25 % tetramethylammonium hydroxide and 80 g of 29 % hexadecyltrimethylammonium chloride solution. The hydrothermal reaction affording MCM-41 can be carried out under variety of conditions, e.g. 16 hrs at 95°C [45]. An alternative robust laboratory method recommended [F. Schuth, personal communication] is a modification of the published procedure [48]: The synthesis mixture comprises: 120 g water, 10 ml 25% ammonia solution, 2.4 g CTAB (hexadecyltrimethylammonium bromide), 0.6 g 1-butanol and 10 ml TEOS. Ammonia solution is mixed with water, then CTAB and BuOH are added. The mixture is stirred at 30°C under magnetic stirring until the solution is clear. 10 ml TEOS is added, and stirring is continued for 1 more hour. Then the precipitate is isolated by filtration. The solid is washed with 200 ml water. Drying 90°C for eight hours, calcination 5h, 550°C, heating rate to final temperature ldeg./min. 6.2. Synthesis of cubic MCM-48 In contrast to MCM-41, which seemed ubiquitous, the cubic MCM-48 was obtained initially under special circumstances relying on controlled hydrolysis of the silica source, TEOS [34]. In addition, the initial procedures employed less desirable raw materials, namely TEOS (high cost, handling hazard) and surfactant hydroxide (had to be custom prepared), making the method unsuitable for a routine application and scale-up. The improvements of the MCM-48 synthesis methodology were gradual until around the year 2000 when several reports of facile MCM-48 synthesis using common reagents were published [49-52]. The method was very similar to MCM-41 synthesis but with adjusted silica/surfactant ratio. Alternative effective method of synthesizing MCM-48 included the use of Gemini surfactants (not readily available) [51]. A method allowing tailoring pore size of MCM-48 relied on the inclusion of additional neutral organic component as the co-template [52]. Synthesis of MCM-48: 31 g of silica, 57 g of 25 % tetramethylammonium hydroxide and 165 g water are digested at 100°C. 172 g of 29 % hexadecyltrimethylammonium chloride solution is added and the mixture reacted for 12 hrs. at 150°C yielding MCM-48. The desired product is transient and upon prolonged heating slowly transforms into the lamellar MCM50. The latter is initially difficult to observe since its most prominent XRD peak overlaps with the MCM-48's (220) reflection. The lamellar impurity in MCM-48 can be detected and

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quantitatively appraised using nitrogen isotherm plot based on the presence of a characteristic horizontal-triangular hysteresis loop above p/po > 0.45 [53]. 6.3. Synthesis of SBA-15: SBA-15 is prepared under acidic conditions using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers, (EO)n(PO)7o(EO)n, as templates, e.g. Pluronic PI23 (n = 20), Pluronic L121 (n = 5) or Pluronic F127 (n = 106). In a typical synthesis [54,55] based on the original procedures [33], 4.0 g of (EO)2o(PO)7o(EO)2o copolymer was dissolved in 150 g of 1.6 M HCL To this solution was added 8.50 g of TEOS, and the resulting mixture was stirred until TEOS was dissolved. The mixture was heated in an oven for 24 h at 308 K and then 24 h more at 373 K under static condition. Silica products were filtered, dried (without washing), and calcined at 823 K. 6.4. Synthesis of mesoporous alumina One of the directions pursued with ordered mesoporous materials has been changing the framework composition. Many different elements could be incorporated in the walls. Among the numerous variations the all-alumina composition has been of special interest. This is related to the wide spread use of alumina as support in catalysis. Successful syntheses with a neutral and anionic template have been achieved [56], see Table 1. The details will be provided in one of the following presentations [56b]. 7. SYNTHESIS OF MICRO- AND MESO-POROUS HYBRID MATERIALS Extensive catalytic testing of mesoporous materials revealed promising performance with practical implications in some process [7,9]. However the observed improvements were mostly incremental compared to state of the art catalysts. This fact in combination with the initial lack of commercially synthesized product hindered practical implementation. In most high value petroleum processes the catalytic activity of mesoporous materials was lower in comparison to zeolite-based catalysts resulting in inadequate conversion levels. The second frequently invoked deficiency of the mesoporous materials was low stability, especially hydrothermal. The apparent lack of stability appears overemphasized and represents a serious problem only for some applications, albeit rather important such as FCC. In general, a particular type of mesoporous material can be made in a quite stable form. Unlike zeolites, the stability of mesoporous materials is very sensitive to synthesis conditions due to amorphous nature of the walls. As shown above, while stability can be improved by adjusting severity of the synthesis (temperature and time) this has little effect upon improving the activity. It has been considered from the very beginning that zeolite-like walls may render mesoporous materials more active and stable. The challenge of mesoporous structures with ordered walls remains outstanding but a number of innovative strategies have been advanced in order to combine zeolitic and mesoporous structures. The following general approaches have been pursued [19]: 1. Simultaneous crystallization of a zeolite and mesoporous solids by using a mixture of structure directing agents for both components [57] 2. Assembly of zeolite seeds, protozeolite clusters, etc, into mesoporous frameworks zeolite synthesis mixture is crystallized for a certain period of time to generate zeolite fragments and then treated with surfactant to assemble the mesoporous structure [58] 3. Simultaneous crystallization of zeolite and mesoporous materials precursor phases [59]

105 4. Recrystallization of mesoporous solid walls into zeolitic domains [60] 5. Coating of mesopore material walls with zeolite fragments/seeds by treatment with a solution containing the latter [61]. Zeolites selected for these attempts were usually ZSM-5, faujasite and zeolite beta. Some promising leads were reported but a real validation through more rigorous systematic testing or commercial implementation remains to be demonstrated. The determination of the real nature of zeolite/mesoporous material composites represents a serious challenge. To date it does not appear to have been adequately resolved. It remains difficult to distinguish physical mixtures (separated phases) versus both structures combined in one phase. Short of a fully refined structural model based on XRD characterization all other characterizations appear not sensitive enough to discriminate between the two possibilities unequivocally. In this situation a functional differentiation, e.g. based on catalytic testing might be the ultimate criterion. This leaves a performance test as a real validation and clear indication of success will be a result exceeding those achieved by each component separately. 8. ORGANO-INORGANIC MESOPOROUS COMPOSITES Because of the unique pore size in the mesopore range of the M41S and related molecular sieves and how these materials are formed, a novel class of materials has been generated. This class of materials has been broadly labeled as organo-inorganic mesoporous composites. The large uniform pore structure allows for functionalization of molecular complexes inside the pores using of the silanol groups present within the pore walls of the material as anchoring sites. Alternatively, because the materials are formed under mild synthesis conditions and via a polymerization mechanism, reactive components such as organosilanes can be used to incorporate, in situ, organo-species within the wall structure, as shown schematically in Fig. 7. In both cases, these functionalized products provide opportunities for designing unusual catalytic/sorptive materials for various applications and present a class of materials significantly different than other uniform porous supports and from the unfunctionalized starting materials. For the post-functionalized materials, the silanols present within the channels provide a suitable anchoring site for chemical attachment. Although the wall composition contains random interatomic bond angles and atomic location, there exists an uncharacteristically large and uniform density of silanol groups (other than the silanols that exist due to incomplete condensation) within the channels due to surfactant packing requirements. NMR studies showed that up to 40% of the silicon atoms located on the surface of the walls could be associated with hydroxyls [62]. An obvious example of an organo-inorganic mesoporous composite would be the assynthesized form of these molecular sieves in which the surfactant template remains attached to the walls of the channels. Although the pores are filled with the surfactant, these assynthesized materials still exhibit significant amount of hydrocarbon sorption capacity. The pore diameter can be changed by changing the surfactant chain length. The hydrocarbon sorption capacity of the resultant calcined versions increased with increasing pore size. However, for the as-synthesized materials, the benzene uptake appeared to be constant, -25 weight %, regardless of the increasing pore size [63]. It appears that the accessible void volume in these as-synthesized materials is constant regardless of the increasing surfactant chain length used in the synthesis or the pore size.

106

Fig. 7. Two primary modes of organic (functional) group distribution in mesoporous frameworks .

The initial example of a functionalized mesoporous molecular sieve was prepared using trimethylsilylchloride as a reagent to react with the silanol groups [4]. The initially present silanol groups were replaced with trimethylsilyl groups which affected on both pore size and sorption characteristics. The pore size, obtained by argon physisorption, did decrease by approximately 10 A consistent with the addition of two trimethylsilyl groups per unit diameter. The pore volume of the functionalized material decreased by -40%, whereas the water sorption capacity was reduced by 67%. It was concluded that this unusual decrease in the water adsorption was a result of both pore size reduction and that the addition of the trimethylsilyl groups affected the hydrophilicity of the walls of the mesoporous material. An approach termed 'functionalization with concomitant surfactant extraction' can be implemented by using an appropriate organic solvent (e.g. alcohol) with dissolved functionalizing reagent. It has a number of advantages such as being a one step procedure, allowing surfactant recovery, having the higher number of available silanols than in a template-free (calcined) version [64,65]. There are many examples of researchers using functionalized mesoporous molecular sieves as separation media. Workers at Batelle Memorial Institute have provided some of the earliest work, adding such groups as mercaptopropylsilane (via reaction with the trimethoxide), ethylenediamine, diethylenetriamine, cyanopropyl, and others [66]. The impetus for their work was the separation of mercury by selective adsorption via complexation of the mercury with the attached sulfur group. These functionalized materials adsorbed significantly higher amounts of mercury per weight of adsorbing material than other available amorphous based adsorbents. The same concept, use of complexation for selective adsorbance, was used for adsorption of copper via an amine functionalized material [67]. Post-functionalized materials have also been used in catalytic reactions. The concept is to "heterogenize" homogeneous catalysts by tethering metal complexes within the mesopore. An example of this is the work of Sir J. M. Thomas and R. Raja [68]. These researchers tethered a metal complex near the surface of the wall. The mesopore allowed free access of the reactant molecules with the catalytic site while the close proximity of the wall (due to the length of the tethered complex) limited and directed the reactants resulting in asymmetric synthesis of products. Yamamoto and coworkers combined both the concept of tethering of a reactive metal site, titanium, with the changing of the hydrophilicity of the wall, addition of a

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silane, to create more reactive oxidation catalyst. The researchers concluded that the change of hydrophilicity of the pore wall excluded water that would deactivate the oxidation catalyst [69]. The alternative method of producing these organo-inorganic mesoporous composites is by in situ reaction using organosilanes. Organosilica versions of M41S materials were first introduced, almost simultaneously, by groups led by Ozin [70], Stein [71], and Inagaki at Toyota [72]. Also called PMOs, for Periodic Mesoporous Organosilicas, these materials are prepared by hydrolysis of substituted tetra-alkyl orthosilicates. So hydrolysis of bis(triethoxysilyl)-ethane or -benzene, for example, along with TEOS with a surfactant, provides a mesoporous material with an organic link between silica, i.e. the organic groups replace oxygens. A wide number of such materials have been prepared. The methods would seem to be ideal for incorporation of stable functional groups for catalysis. For example, sulfonation of benzene links could give catalytically active sites. Inagaki and coworkers used a benzene molecule as the linker in the silica-based composition. They produced a uniform benzene-silica mesopore material which by HREM imagery suggested a very ordered wall composition [73]. Other researchers used in situ functionalization to enhance the heavy metal adsorption, Cu, Zn and Ni, of functionalized materials using imprinting techniques. These researchers functionalized the walls of the mesoporous materials using a reactive aminosilane metal complex as a reagent. After removing the metals that were used in the synthesis, these functionalized materials demonstrated enhanced metal capacity and rate of uptake compared to equivalent materials prepared except without the metal complex. The authors reasoned the use of the metal complexes as functional groups optimized the density of the functionalized groups within the mesopore channel resulting in more effective uptake of the metals in subsequent sorption testing [74]. The apparent ease of producing these organo-inorganic composites has provided a class of materials that have demonstrated promise in both separations and catalytic applications. The compositions and applications appear to be only limited by the imagination of the researcher. 9. CONCLUDING REMARKS The discussed area expanded considerably with many interesting and exciting developments from the standpoint of fundamental science and potential uses. The synthesis progressed in many directions affording both novel materials and methods for a large scale manufacture. The M41S based catalyst is used in a commercial application [7] and in one commercial process MCM-41 offers advantage over the state of the art catalyst [75]. The inherent flexibility and potential for modifications are the main drivers for continuing interest, innovation and search for improvements. REFERENCES [1 ] [2]

[3]

O. Terasaki (ed.), Mesoporous Crystals and Related Nano-structured Materials, Elsevier, Amsterdam, 2004. (a) F. Schuth, in A. Galarneau, F. DiRenzo, F. Fajula and J. Vedrine (eds.), Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. No. 135, Elsevier, Amsterdam, 2001, p. 1. (b) F. Schuth in Ref. [1] pp. 1-13. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature (London), 359 (1992)710.

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Recent trends in the synthesis of molecular sieves Jiff Cejka J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic 1. INTRODUCTION 2. BEYOND 12-MEMBERED RINGS 2.1. Synthesis of VPI-5 2.2. Synthesis of UTD-1 2.3. Synthesis of SSZ-53 3. GeO2 - INORGANIC STRUCTURE-DIRECTING AGENT 3.1. Synthesis of ITQ-15 3.2. Synthesis of Polymorph C of zeolite Beta (ITQ-17) 4. DELAMINATED AND PILLARED ZEOLITES 4.1. Synthesis of MCM-22 4.2. Synthesis of ITQ-2 5. MESOPOROUS NON-SILICEOUS MOLECULAR SIEVES 5.1. Synthesis of organized mesoporous alumina - "neutral" pathway 5.2. Synthesis of organized mesoporous alumina - "anionic" pathway 6. MESOPOROUS ZEOLITE SINGLE CRYSTALS 6.1. Synthesis of mesoporous ZSM-11 with carbon Black pearls 6.2. Synthesis of mesoporous ZSM-5 with carbon nanotubes 7. MICRO-MESOPOROUS COMPOSITE MATERIALS 7.1. Synthesis of micro/mesoporous composites via re-crystallization 7.2. Synthesis of micro/mesoporous composites using seed crystals (Beta/MCM-48) 8. APPLICATION IN CATALYSIS 9. SUMMARY AND OUTLOOK ACKNOWLEDGEMENT REFERENCES 1. INTRODUCTION Zeolite-based molecular sieves represent one of the most important groups of inorganic materials with an extremely high impact on industrial applications as adsorbents, ionexchangers and catalysts [1,2,3]. In particular, the number of applications of zeolites as highly active, selective and stable catalysts in large-scale technologies steadily increases. It includes oil refinery, petrochemistry, production of fine chemicals and environmental catalysis [4]. While zeolites and later on zeotypes have been well known for more than 20 years, a remarkable boom in the preparation of new types of molecular sieves started at the beginning of 1990s [5,6]. Much synthetic effort has been devoted in recent fifteen years in particular to the enlargement of the size of zeolite channels in order to allow larger molecules to reach the

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active sites. This approach could have a particular importance in fine chemical and pharmaceutical industry and in petroleum refining. In addition, the increase in the accessibility of acid sites located inside the channel systems of zeolite-based molecular sieves would result in an increase in the number of reactions catalyzed by these catalysts. Fig. 1 shows schematically the required size of pores needed for upgrading different petrochemical raw materials. Recently, several new zeolite structures have been reported consisting of 14membered rings (UTD-1 [7], SSZ-53 and SSZ-59 [8]), bi-directional intersecting 14- and 12membered rings (ITQ-15 [9]) and even 18-membered rings (ECR-34 [10]). Further attempts to synthesize zeolites with at least 20-membered ring channels unfortunately failed. The synthetic effort in many laboratories around the world is focused also on utilization of GeC>2 in mixtures resulting in the synthesis of a number of new structural types of zeolites, e.g. polymorph C of zeolite Beta [11]. However, the main goal is to synthesize molecular sieves with pore sizes of at least several nanometers preserving the advantages of zeolites, in particular their strong acidity and high thermal stability. New synthetic approaches have been also developed to prepare mesoporous molecular sieves with amorphous channel walls and various chemical compositions, pore sizes and surface areas [12,13,14], delaminated zeolites [15,16], mesoporous molecular sieves with crystalline zeolite phases embedded in the channel walls [17,18,19] and mesoporous zeolite single crystals [20,21]. Understandably, these materials allow faster diffusion of reactants and products and transformation of bulkier molecules compared with classical zeolites or zeotypes.

Fig. 1. The relationship between the type of hydrocarbon substrate and the pore size of molecular sieve required for its transformation

This contribution briefly highlights current trends in the synthesis of new types of zeolite-based molecular sieves showing recent development in this very topical area. We have tried to cover the most important examples of the development of these molecular sieves, in which a high potential for future applications of molecular sieves is expected. At the end of

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each chapter typical synthesis recipes are given. Some interesting catalytic examples of application of these molecular sieves are also included. 2. BEYOND THE 12-MEMBERED RINGS The application of zeolites in catalysis is limited due to molecular size restrictions imposed by currently known zeolites, which significantly limits the possibility of transforming larger organic compounds with larger molecules. To-date only several extra-large pore zeolites and zeotypes (having more than 12 tetrahedrally coordinated T atoms in the ring) have been synthesized. Their catalytic properties are very rarely described due to their complex synthesis procedures and usually rather complicated and costly structure-directing agents. This is the reason why no big potential of these zeolites and zeotypes in practical applications can be foreseen at the moment. A list of zeolites and zeotypes possessing larger than 12membered rings is provided in Table 1 together with the codes (if available) of these structures used by the Structure Commission of the International Zeolite Association, type of T-atoms forming the main channels and pore sizes. All these microporous materials possess structures with one-dimensional channel systems. It is clearly seen that we still have not succeeded to escape from the "1-nm prison" of zeolite and zeotype channel systems. While fully symmetric channels having more than 18 rings should possess pore sizes much larger than 1 nm, the actual shapes are elliptic or highly puckered, which leads to a significant decrease in the effective pore sizes of the microporous molecular sieves. Fig. 2 shows the perspective views of the main channels of gallophosphate cloverite, aluminophosphate VPI-5, and two aluminosilicates CIT-5 and SSZ-59.

Fig. 2. Schematic pictures of main channels of cloverite (A), VPI-5 (B), CIT-5 (C), and SSZ-59 (D)

The first silica-based 14-membered ring zeolite, UTD-1, was synthesized by Balkus et al. [7,22] using bis(pentamethylcyclopentadienyl)cobalt(III) hydroxide as a structure-directing

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agent. This zeolite is a product of an intergrowth of two or more closely related polymorphs and is characterized by one-dimensional elliptic pores of 0.74 x 0.95 nm dimensions. UTD-1 was synthesized as alumino- and later on as titanosilicate. Based on refinement of X-ray data, the [(Cp*)2Co]+ ion is oriented with its five-fold axis parallel to the channel direction. The second 14-membered ring zeolite, CIT-5 [23], was synthesized using N(16)methylsparteinium. The presence of Li cations seemed to be decisive for its synthesis while without Li other zeolites were formed. The synthesis was carried out at 175 °C for 12 days. Recently two novel zeolites with one-dimensional 14-membered rings were synthesized by researchers of ChevronTexaco (SSZ-53 and SSZ-59 [8,24]). For their syntheses, organic cations derived from (phenylcyclopentylmethyl)ammonium, azocanium or piperidinium were used. Their pore sizes lie between those of CIT-5 and UTD-1 (Table 1). The successful synthesis of silica-based microporous material possessing the largest pore size so far was reported by Strohmaier and Vaughan [10]. Using tetraethylammonium cations and an alkali-metal containing reaction gel they synthesized gallosilicate ECR-34 with hexagonal symmetry and one-dimensional 18-membered ring pores with a diameter of 1.0 nm. The synthesis of ECR-34 was performed from low-silica seeded gallia gel at 100 °C for 17 days. This is the first silicon containing microporous sieve with pore openings larger than 14-membered rings. The successful synthesis shows the potential of preparing thermally stable silicate molecular sieves with extra-large pores. Table 1 List of extra-large pore zeolites and zeotypes Name T-membered ring Code A1PO-8 AET 14 (Al, P) 20 (Ga, P) Cloverite CLO VFI 18 (Al, P) VPI-5 ND-1 24 (Zn, P) NTHU-1 24 (Ga, P) FDU-4 24 (Ge) OSO OSB-1 14 (Be, Si) ETR ECR-34 18 (Ga, Si) DON UTD-1 14 (Al, Si) CFI CIT-5 14 (Al, Si) SFH SSZ-53 14 (Al, Si) SFN SSZ-59 14 (Al, Si)

Pore size (nm) 0.79x0.87

1.32x0.40 1.21 1.05 1.04 0.97 0.73x0.54 1.05 0.74 x 0.95 0.72 x 0.75 0.64 x 0.87 0.62x0.85

References [25] [26] [27] [28] [29] [30] [31] [32] [7]

[33] [34] [34]

2.1. Synthesis of VPI-5 15.00 g of pseudoboehmite was dispersed in 64.60 g of water. After that 22.20 g of phosphoric acid was added under vigorous stirring until the mixture became homogeneous and then it was aged for 2 h. Finally, 10.11 g of dipropylamine was added and the viscous gel formed was stirred for another 2 h. The synthesis was performed in Teflon-lined autoclave at 142 °C for 4 h without agitation. The reaction product was diluted with water, filtered off, washed and dried in air at temperatures below 50 °C [35]. 2.2. Synthesis of UTD-1 A silica slurry, prepared from 27.8 g of Cab-O-Sil M5 and 68.4 g of water, was stirred until it became homogeneous. 6.60 g of this slurry was added to 49.15 g of an 8.95 wt. % aqueous solution of bis(pentamethylcyclopentadienyl)cobalt(III) hydroxide and the mixture

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was stirred for 90 min. A solution of 0.67 g of aqueous 38 wt. % hydrofluoric acid in ca. 70 g of water was slowly added to the reaction mixture. After 20 min of vigorous stirring, the homogeneous gel was concentrated by removing excess of water under reduced pressure. The remaining 24.2 g of the uniform gel was transferred to a Teflon-lined autoclave and heated at 170 °C for 25 days. Finally, the zeolite was recovered by filtration and washed extensively with water [36]. 2.3. Synthesis of SSZ-53 As to borosilicate SSZ-53, a solution of 6.33 g of 0.475M {[l-(4fluorophenyl)cyclopentyl] methyl}-trimethylammonium hydroxide, 1.2 g of aqueous 1 M NaOH and 4.45 g of water was stirred in a 23-mL Teflon-lined autoclave. Then 0.06 g of sodium borate decahydrate was added and stirred until complete dissolution. Finally, 0.9 g of Cab-O-Sil M5 was added to the solution and the mixture was thoroughly stirred. The resulting gel was placed into a Parr reactor and heated at 160 °C for 18 days while rotating at 34 rpm. The zeolite was recovered by filtration and extensively washed with water. [34]. Recently, two new extra-large pore zeolites were synthesized by Corma [9] and Patarin [37] groups. Their channel system consists of two perpendicular channels having 14membered and 12-membered rings, which is the first example of zeolitic microporous molecular sieve exhibiting intersecting extra-large and large pores. In both cases the synthesis was performed in the presence of germanium dioxide as discussed in more detail in the following chapter. 3. GeO2 - INORGANIC STRUCTURE-DIRECTING AGENT Although several very interesting new structural types of zeolites have been recently synthesized using novel types of organic templates, it was found that addition of GeO2 to the synthesis mixture can completely change the synthesis pathways leading to new structural types of zeolites. In the absence of GeC>2 already known zeolites are formed [38,39]. Corma and his coworkers have found that the addition of GeC>2 into the reaction mixture changes the preferential formation of some secondary building units, which results mainly in the formation of double-four-rings (D4R) containing Ge. Therefore, particularly those zeolites having in their structures significant concentrations of D4R are easily formed, without a special preference of the formation of narrow, medium, large or extra-large pore zeolites.

A

B

Fig. 3. Schematic pictures of zeolite structures ITQ-22 (A) and ITQ-17 (B) [39,11]

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Addition of GeC>2 enabled the first synthesis of pure polymorph C of zeolite Beta (ITQ-17 [11], Fig. 3), which was predicted by Newsam et al. [40] but also of a new family of related materials (like ITQ-16), in which the ratio of polymorphs A, B, and C can be varied [41] (in zeolite Beta polymorphs A and B are in the ratio 60 : 40). The structure-directing role of the GeC>2 in the synthesis of polymorph C has been clearly demonstrated. Fig. 4 depicts a SEM image of the crystals of polymorph C. A number of different templates (from both the structural and sterical points of view) was tested under the same hydrothermal conditions (temperature range 135-175 °C, 15-120 h, Si/Ge 0.5-30) and in all cases pure polymorph C was synthesized despite the fluoride or hydroxide medium employed. However, in the absence of germanium zeolite Beta, with a typical intergrowth of polymorphs A and B, ZSM12 or ITQ-4 were produced [11].

Fig. 4. Scanning electron micrograph of ITQ-17 (Polymorph C of zeolite Beta)

Another interesting new zeolite with a potential application in oil refining, ITQ-21 [42], was synthesized using N(16)-methylsparteinium cation as a bulky and rigid organic structure-directing agent, in combination with fluoride anions and germanium oxide. ITQ-21 is a zeolite with a three-dimensional channel network containing large cavities at the intersections of the channels. Each cavity is accessible through six circular windows of 0.74 nm size. Recently, ITQ-21 was also synthesized in a fluoride-free medium [43].

A

B

Fig. 5. Perspective views of the 14- (A) and 12-membered (B) rings of zeolite ITQ-15 (IM-12 [9,37])

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For the first time, a zeolite having three interconnected channel systems of different pore entrances (ITQ-22) was synthesized possessing 8-, 10 and 12-membered ring channels [39]. l,5-bis-(methylpyrrolidinium)-pentane was used as a structure-directing agent. It was found that germanium oxide controls and probably also stabilizes the formation of D4R units of this zeolite structure. Two extra-large pore zeolites ITQ-15 [9] and IM-12 [37] both possessing intercrossed 14- and 12-membered rings were independently synthesized using l,3,3-trimethyl-6-azoniumtricyclo-[3.2.1.46'6]dodecane or (6i?,10>S)-6,10-dimethyl-5-azoniaspiro[4.5]decane cations, respectively, as templates. The schematic pictures of their 14- and 12-membered rings are given in Fig. 5 and some of structure-directing agents mentioned in this chapter are in Fig. 6. Detailed structural analysis showed that these two zeolite structures are identical. The presence of GeCh in the synthesis mixture can sometimes lead to the synthesis of a very special zeolite. ITQ-13 [44] synthesized in the presence of doubly charged hexamethonium cations exhibits a three-dimensional channel system with intersecting 10- and 9-membered rings. This is for the first time that a zeolite possesses both 10- and 9-membered rings. Table 2 List of some new zeolite structures synthesized in the presence of GeC>2 Name T-membered ring Pore size (nm) References Code ITQ-13 ITH 10x10 x 9 0.48 x 0.53, 048x0.51, 0.40x0.48 [45] ITQ-15 UTL 14 x 12 0.95 x 0.71, 0.85x0.55 [9] IM-12 14x12 UTL 0.95 x 0.71, 0.85x0.55 [37] ITQ-17 12x12 x l 2 0.63 x 0.75, 0,60 x 0.69 BEC [11] 12 ITQ-21 0.74 (cavity- 1.18) [42] IWW ITQ-22 12 x 10 x 8 0.66, 0.59 x 0.49, 0. 45 >c 0.33 [39] ITQ-24 IWR 12x12 x l O 0.77 x 0.56, 0.72 x 0.62, 0.57 x 0.48 [46]

Fig. 6. Structure-directing agents used for the synthesis of SSZ-53 (A), SSZ-59 (B), polymorph C (C), and ITQ-21 (D)

On the basis of MAS NMR data it can be summarized that in Ge-containing zeolites Ge atoms preferentially occupy crystallographic sites in D4R units. When the synthesis was

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carried out in fluoride medium, the Ge content increased with decreasing concentration of incorporated fluoride anions, which are preferentially located in D4R units. The effect of substitution of silicon for germanium on the synthesis of these microporous materials was investigated by high-temperature oxide melt solution calorimetry [47,48] and density functional theory [49]. It was found that the enthalpies of formation of Ge-containing zeolites are by ca. 15-20 kJ/mol less energetically favored compared to quartz. The range is higher than that for pure silica zeolites (7-14 kJ/mol), microporous aluminophosphates (5-9 kJ/mol) and pure silica mesoporous materials (14-15 kJ/mol). This clearly indicates that the incorporation of Ge into zeolite frameworks decreases their stability. The destabilization generally increases with increasing Ge content and is more pronounced for structures with lower framework density [48]. On the other hand, the computational results have shown that Ge substitution increases, from the thermodynamic point of view, the probability of synthesizing frameworks containing D4R units with respect to all other types of small cages [49]. As to the related materials, recently three-dimensional GeC^-zeotypes having interconnected network of 14-, 12- and 12-membered rings [50] or a three-dimensional 12membered ring system [51] were synthesized using 4-(aminomethyl)piperidine or 1,4-diazabicyclo[2.2.2]octane, respectively. The second GeO2-zeotype possesses an isotypic framework structure with polymorph C of zeolite Beta. 3.1. Synthesis of ITQ-15 GeO2 (0.60 g) was dissolved in 52.95 g of a 0.6M solution of l,3,3-trimethyl-6azonium-tricyclo[3.2.1.46'6]dodecane hydroxide. Then, 12.60 g of tetraethyl orthosilicate and 0.27 g of aluminum isopropoxide were added and left under stirring until the alcohol and the amount of water necessary to reach the final composition were evaporated. The final gel was homogenized and put into an autoclave for 18 days at 175 °C. After cooling down to room temperature, the solid phase was recovered by filtration and washed. Calcination was carried out in a stream of air at 540 °C for 3 h [9]. 3.2. Synthesis of Polymorph C of zeolite Beta (ITQ-17) 2.93 g GeO2 was mixed with 12.1 g H2O and 2.225g HF (40%) and stirred for 10 min. Then 43.99 g of 0.3 M solution of 1-benzyl-l,4-diazoniabicyclo[2.2.2]octane was added and stirred for another 10 min. Finally, 11.22 g of Ludox AM-30 was added dropwise and homogenized for 20 min. The synthesis was carried out in Teflon-lined autoclaves at 150 °C for 4-6 days. 4. DELAMINATED AND PILLARED ZEOLITES Most of the zeolite syntheses carried out under hydrothermal conditions directly results in the formation of three-dimensional crystalline frameworks. However, several zeolites, like MCM-22 or ferrierite, can be synthesized in the form of layered precursors, which can be transformed by further thermal treatment into the three-dimensional crystal structure. These layered solids arouse an interest due to their ability to intercalate guest molecules between two neighboring zeolite layers. Using a proper treatment, layered zeolite materials can be delaminated while the structure of layers is preserved, which makes accessible all active sites located on the external surface of such catalyst. By adding proper inorganic guest molecules functioning as pillars, the control of the interlayer distance can be achieved. Such materials

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exhibit strong acidity and thermal stability, which is characteristic for zeolite materials. In addition, the accessibility of acid sites is much higher even for larger organic molecules. At present, MCM-22 and ferrierite represent the only two described examples of zeolites, which can be synthesized in the layered form and further transformed to 3-D crystal structures. Delamination of ferrierite led to the formation of ITQ-6 and ITQ-20 [52], while ITQ-2 can be prepared from the lamellar precursor of MCM-22 [15]. Not only aluminum but also titanium was incorporated into the siliceous ITQ-6 and evaluated in acid and oxidation reactions [52,53]. Particularly, the MCM-22 family represents a very interesting group of interrelated materials, including layered, delaminated, pillared, as well as three-dimensional structures. The schematic picture providing basic structural relationships among known representatives of this family is depicted in Fig. 7. Typical MCM-22 with three-dimensional crystal structure can be synthesized via its lamellar precursor, MCM-22(P), followed by thermal treatment at about 580 °C. However, a direct synthesis method was also described leading to a zeolite designated MCM-49, which is isostructural with MCM-22 but produced in one synthesis step [54]. Although MCM-22 has been already exploited for large-scale benzene alkylation, much more interest has been devoted to its lamellar precursor, which can serve for further applications as a delaminated or pillared material. Fig. 8 shows the X-ray diffraction patterns of MCM-22(P), MCM-22, MCM-56 and MCM-49. The formation of three-dimensional crystalline framework of MCM-22 and MCM-49 can be clearly distinguished from lamellar phases MCM-22(P) and MCM-56. MCM-56 was recognized as an identifiable intermediate in the crystallization of MCM-49 within a few hour time window [55].

Fig. 7. Structural relationships among the individual types of the MCM-22 family (modified from ref. [55])

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The lamellar precursor can be swollen and subsequently transformed into pillared (MCM-36 [56]) or delaminated (exfoliated, e.g. ITQ-2 [15]) material. The delamination can be performed via addition of a proper surfactant (usually aqueous solution of hexadecyltrimethylammonium salt) under ultrasonic treatment. Strict control of pH and temperature seems to be critical to ensure proper delamination and to avoid the formation of undesired phases [57,58]. X-ray diffraction used to monitor the swelling process clearly showed the diminishing and removal of (001) and (002) diffraction lines due to the partial loss of well-ordered lamellar solid while the FTIR spectra of these materials remained practically unchanged. The resulting delaminated material (ITQ-2) exhibits significantly higher mesopores surface areas compared with MCM-22 (up to 700 m2/g) with preservation of randomly oriented layers, ca. 2.5 nm thick. The introduction of pillars into the interlamellar space of MCM-22(P) results in the formation of pillared material MCM-36 [56]. Roth et al. used tetraethyl orthosilicate or tetramethylammonium silicate solution to form silica pillars in MCM-36. They described that the swelling caused the disappearance of the characteristic diffraction line (002) corresponding to the c-parameter of about 2.7 nm. On the other hand, a new intensive line appeared at low angles, which represents new c-parameter. In general, pillaring of MCM22(P) to MCM-36 caused a shift of (hkl) reflections while those of (hkO) remained unchanged. As a result, the pillaring generated pores with maxima between 3.0 and 3.5 nm, which significantly enhances the catalytic possibilities of this type of zeolite-based materials.

Fig. 8. X-ray powder diffraction patterns of MCM-22(P) - A, MCM-22 - B, MCM-56 - C, and MCM-49 - D

4.1. Synthesis of MCM-22 0.92 g of sodium aluminate and 0.60 g of sodium hydroxide were dissolved in 124.20 g of water to form a clear solution. Then 7.61 g of hexamethyleneimine was added and

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thoroughly mixed to produce a slightly yellow clear solution. Finally, 9.23 g of silica (Aerosil 200) was added and mixed until white gel was formed. Crystallization was carried out in a Teflon-lined stainless steel autoclave at 150 °C for 7 days with agitation. After cooling down to room temperature, the solid phase was recovered by filtration and washed. Calcination was performed at 580 °C for 6 h [59]. 4.2. Synthesis of ITQ-2 MCM-22(P) was synthesized in the following way: 0.23 g of sodium silicate and 0.81 g of sodium hydroxide were dissolved in 103.45 g of distilled water, after which 6.35 g of hexamethyleneimine and 7.86 g of silica Aerosil 200 were added successively. The mixture was stirred for 30 min at room temperature followed by a 11-day reaction in a stirred Teflonlined autoclave at 135 °C. Swelling of the solid product was carried out by refluxing mixture of 27 g of the slurry with 105 g of a 29 wt. % aqueous hexadecyltrimethylammonium bromide solution and 33 g of a 40 wt. % aqueous solution of tetrapropylammonium hydroxide at 80 °C for 16 h. Calcination was performed at 540 °C [15]. 5. MESOPOROUS NON-SILICEOUS MOLECULAR SIEVES The first successful synthesis of siliceous mesoporous molecular sieves opened a new era in the investigation of this type of inorganic materials and also initiated developments in the preparation of new types of non-silica mesoporous molecular sieves. The mesoporous molecular sieves are characterized by well-defined pore sizes of narrow distribution in the range between 2.0 and 30 nm and surface areas larger than 1000 m2/g. These materials have been synthesized using self-assembled molecular aggregates or supramolecular assemblies as structure-directing agents [12,60,61,62,63,64]. In contrast to a huge number of papers dealing with synthesis, characterization and catalytic utilization of silica-based mesoporous molecular sieves, synthesis of organized mesoporous alumina was announced several years later and still not many reports can be found in the open literature [64,65,66,67,68]. This is despite the fact that alumina is a very important material being used as a support of various catalytically active phases with application in large scale industrial processes [69,70]. Preparation of titanium dioxide represents a special field of synthesis of templated porous solids. Various synthesis routes proposed in the literature are not described in this review because titanium dioxide is used in photocatalysis, photoelectrochemistry and sensorics [71,72,73]. Synthesis of organized mesoporous aluminas is based on the same approaches as those successfully used for the synthesis of mesoporous molecular sieves ("anionic", "cationic", and "neutral") using aluminum alkoxide as the source of aluminum and in the absence of any silicon source. In contrast to the synthesis of siliceous MCM-41, the "cationic" route to organized mesoporous aluminas using hexadecyltrimethylammonium cations is the least applied and understood. Cabrera et al. [68] described the possibility to tailor the pore dimensions from 3.3 to 6.0 nm by modifying the ratio of surfactant, water and triethanolamine; however, this synthesis route seems to be less reproducible compared with "anionic" and "neutral" routes. Vaudry et al. [66] described the employment of long-chain carboxylic acids, the "anionic" route, for the synthesis of organized mesoporous aluminas. These syntheses are carried out in alcohols, formamide, chloroform or diethyl ether. While aluminas prepared by Vaudry et al. exhibited surface areas between 500 and 700 m2/g and pore sizes around 2.0 nm, we have reported [74] the effect of the hydrocarbon chain length on the pore size diameter of organized mesoporous aluminas prepared using long-chain carboxylic acids of different chain

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lengths. It was observed that with increasing length of the chain the pore size diameter increased as well. Poly(ethylene) oxide surfactants serve as structure-directing agents for the synthesis of organized mesoporous aluminas using the "neutral" pathway [17,114,75]. Pinnavaia et al. have shown that alkylamines or poly(ethylene oxide)-poly(propylene oxide) block copolymers can be used for the synthesis of organized mesoporous aluminas [65,75]. Mesoporous aluminas with pore diameters up to 10 nm were prepared depending on the type of the surfactant used and stability of the alumina at high temperatures was achieved by addition of small amounts of La3+ or Ce3+ cations. One of the main advantages of this synthesis route is a relatively weak bonding between surfactant and alumina species not only during the synthesis itself but also in the final product. This allows extraction of surfactant used without the necessity of using a new surfactant for the next synthesis. Such weak interaction between the surfactant micelles and surrounding alumina walls was demonstrated by EPR spectroscopy using 5-doxyl stearic acid as a probe molecule [76]. The final stabilization of the structure of mesoporous alumina proceeds, thus, during thermal posttreatment at temperatures of ca. 120 °C, which allows to change the textural parameters of organized mesoporous alumina by washing with various solvents. To meet the requirements for high-temperature applications of organized mesoporous alumina, the dependence of their textural properties on the temperature of calcination was investigated. It has been shown that organized mesoporous aluminas exhibiting amorphous walls after calcination at 500-600 °C are transformed into a crystalline phase in the temperature range from 800 to 1000 °C, which is accompanied by a decrease in the surface area and void volume. Formation of a-alumina was reported by Liu et al. [77], while we have described the formation of 5-alumina particles [78]. The changes of the textural properties of organized mesoporous aluminas with increasing temperature of the thermal treatment are depicted in Figs 9, 10 and 11. Organized mesoporous aluminas exhibit a typical worm-like structure without the formation of clear hexagonal or cubic structure well known for silicate mesoporous sieves (Fig. 9A). With temperature of calcination increasing above 800 °C, diffraction lines of crystalline alumina phase are found evidencing a phase transformation proceeding at this temperature (Fig. 9B). The increase in the temperature of calcination leads also to significant change of textural properties of organized mesoporous aluminas like a decrease in surface area and void volume (Fig. 10) or increase in the pore size and broadening of pore size distribution (Fig. 11).

Fig. 9. Transmission electron micrographs of organized mesoporous alumina synthesized using the "anionic" pathway and calcined at 420 (A) and 800 °C (B) [78]

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Mesoporous molecular sieves having chemical compositions different from silicates and alumina including aluminum phosphates, various transition-metal oxides, chalcogenide and nitridic framework, metals, and carbons were recently nicely reviewed by Schiith [79]. 5.1. Synthesis of organized mesoporous alumina - "neutral" pathway A solution containing deionized water (42 mmol) in anhydrous sec-butyl alcohol (10 mL) was added very slowly to a stirred homogeneous solution containing Pluronic 64L a triblock copolymer of composition (PEO)o(PPO)3o(PEO)i3 (2.1 mmol) and aluminum secbutoxide (25 mL). The resulting gel was stirred for 3 h and then diluted with sec-butyl alcohol to react for additional 16 h. The solid product was washed with ethanol and dried at room temperature for 16 h and at 100 °C for 6 h. The calcination was carried out at 500 °C for 4 h. To recover the structure-directing agent (Pluronic 64L) from the as-synthesized solid, extraction with ethanol can be successfully used [65].

Fig. 10. Nitrogen adsorption isotherms of organized mesoporous alumina synthesized using "anionic" pathway and calcined at 420 (A), 600 (B), 800 (C) and 1000 °C (D)

Fig. 11. Pore size distribution of organized mesoporous alumina synthesized using "anionic" pathway and calcined at 420 (A), 600 (B), 800 (C) and 1000 °C (D)

5.2. Synthesis of organized mesoporous alumina - "anionic" pathway 5.1 g of stearic acid was dissolved in 100 ml of 1-propanol. After addition of 3.1 ml of water, the solution was stirred for 30 min. Finally, 13.7 g of aluminum sec-butoxide was added and the reaction mixture was stirred for 20 min. The prepared gel was aged at 100 °C for 50 h under static conditions. After cooling, filtration and washing with ethanol, the solid product was dried overnight at 50 °C. Calcination was carried out in a stream of nitrogen at 410 °C and then in air at 420 °C [74]. 6. MESOPOROUS ZEOLITE SINGLE CRYSTALS Zeolites as crystalline microporous materials are widely used in chemical industry in catalysis, adsorption and separation. Utilization of zeolites should be attributed to the

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presence of micropores in their structures, inducing various types of shape selectivity [2] based on exact shapes and sizes of their micropores related to the kinetic diameters of organic reactants, intermediates or products. However, the presence of micropores can also limit the catalytic performance of zeolite catalysts, especially when the diffusion is the ratedetermining step of the overall chemical reaction. For that reason different synthetic strategies were investigated to overcome diffusion limitations. One of these approaches is based on the formation of mesopores in zeolite single crystals. Usually mesopores in zeolites were formed by dealumination [80] or desilication [81] and these methods including industrial applications of mesoporous zeolites were recently nicely reviewed by the group of de Jong [21]. A new direct and controlled synthesis of mesoporous zeolite crystals is based on the utilization of small carbon particles (carbon black) having sizes in the range 5-50 nm, which are added to the synthesis mixture for the preparation of the zeolite. Using excess of a zeolite gel it is possible for the zeolite to grow around these carbon particles forming the inert matrix within the whole pore system [20]. This means that after the synthesis the zeolite contains a large number of carbon particles inside the individual crystals. Such particles are removed by combustion leading to the formation of well-defined zeolite single crystals with mesopores the size of which is close to the size of the carbon particles used. A similar approach to the synthesis of mesoporous single crystals includes utilization of multiwall carbon nanotubes as mesopore-forming agents. Schmidt et al. [82] synthesized mesoporous ZSM-5 (silicalite-1) using multiwall carbon nanotubes. As the carbon nanotubes with different diameters can be now synthesized in a controlled way, this method could result in zeolite single crystals having mesopores of variable diameter. In contrast to the use of carbon black beeds, mesopores formed using carbon nanotubes are not spherical and it can be expected that the active sites will be more easily accessible. The external surface of zeolites synthesized with carbon particles differs significantly from that one prepared without them as many carbon particles were localized just at the surface of these crystals. (Fig. 12).

Fig. 12. Scanning electron micrographs of ZSM-5 synthesized without (A) and with Carbon Black Pearls (B)

The first mesoporous zeolite single crystal (ZSM-5) was synthesized by Jacobsen et al. [20] and since that time a particular attention has been devoted to characterization of this material [ 83 , 84 ]. Combination of nitrogen adsorption isotherms, scanning electron

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microscopy, and three-dimensional transmission electron microscopy used for investigation of ZSM-5 zeolites synthesized from different sources of silicon provided clear evidence that both carbon nanotubes and carbon black were capable of templating the formation of mesopores in ZSM-5 zeolite. The crystallization of ZSM-5 using dissolved silica (Aerosil 200) resulted in much larger ZSM-5 crystals compared with those prepared from tetraethyl orthosilicate. With increasing zeolite crystal size, more carbon particles were surrounded by zeolites while the tortuosity of the mesoporous system was much higher for carbon black pearls compared with carbon nanotubes. The size of zeolite crystals was found to depend also on the heating rate of the crystallization mixture. The higher was the rate the larger were resulting crystals probably due to the shortening of the nucleation time [84]. Nitrogen adsorption isotherms clearly show the increase in the mesoporous volume for ZSM-5 samples synthesized with carbon particles (Figs 13 and 14). Tao et al. [85] synthesized zeolite Y with a significant portion of mesopores by adding the resorcinol-formaldehyde polymer to the zeolite synthesis mixture. Nitrogen adsorption isotherms showed the presence of mesopores with a pore diameter close to 10 nm and a pore volume of 1.37 cnvVg. The presence of mesopores was also confirmed by field-emission scanning electron microscopy.

Fig. 13. Sorption isotherms of nitrogen of ZSM-5 synthesized without (A) and with Carbon Black Pearls (B)

Fig. 14. Pore size distribution of ZSM-5 synthesized with Carbon Black Pearls

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6.1. Synthesis of mesoporous ZSM-11 with Carbon Black pearls The Carbon Black Pearls labeled BP 2000 was supplied by Cabot Corp. 10 g of carbon black pearls (BP-2000) was dried in an oven at 110 °C overnight. In a 200-mL flask, 17.2 g of 40 % tetrabutylammonium hydroxide, 2.5 g of H2O and 15.1 g of ethanol was stirred to obtain a homogeneous solution. The dried carbon black was impregnated with this solution to incipient wetness. After evaporation of ethanol at room temperature for 12 h, carbon particles were impregnated with 19.3 g of tetraethyl orthosilicate. This mixture was left at ambient conditions to hydrolyze overnight. The hydrothermal synthesis was performed in a Teflonlined autoclave at 180 °C for 72 h. Then the autoclave was cooled, a solid product was filtered and washed with distilled water. Calcination was carried out in a muffle oven at 550 °C for 18 h [86]. 6.2. Synthesis of siliceous mesoporous ZSM-5 with carbon nanotubes Carbon nanotubes were purified by reflux with concentrated hydrochloric acid for 24 h. After that 1.0 g of carbon nanotubes was successfully impregnated with 1.0 g of tetrapropylammonium hydroxide (40 wt. %) and 1.0 g of tetraethyl orthosilicate. Samples were digested for 3 h and then heated to an autoclave at 175 °C for 24 h. After the hydrothermal synthesis, the solid phase was filtered, washed with water and ethanol, and dried at 110 °C. The carbon nanotubes were completely removed by calcination in air at 600 °C for 20 h [82]. 7. MICRO-MESOPOROUS COMPOSITE MATERIALS The acidity and thermal stability of aluminosilicate mesoporous molecular sieves of the MCM-41 and MCM-48 types are relatively low compared to zeolites. These mesoporous materials also suffer from low concentration of Broensted acid sites on the surface of their walls. Low acidity and insufficient thermal stability limit the potential of these catalysts in their application in petroleum refining and fine chemicals manufacture. For these reasons new approaches are sought to increase thermal stability of mesoporous molecular sieves and to induce a strong acidity particularly of the Broensted type. Several approaches were recently described in the literature. Modification of siliceous support with heteropolyacids results in the preparation of highly active acid catalysts [87]; however, lack of stability in polar systems as well as leaching are the main drawbacks of this type of catalysts [88]. In a similar way, attachment of sulfonic groups via organic spacer provided interesting and active acid catalysts but their use is rather restricted due to the presence of thermally unstable organic spacers limiting not only the catalytic application at higher temperatures but also the regeneration [89,90]. Two probably the most promising ways of synthesizing hierarchic micro/mesoporous composite systems are based on partial re-crystallization of originally amorphous mesoporous molecular sieves or on the employment of zeolite seeds as a source of building blocks for their assembly to form the mesoporous material. These hierarchic systems should benefit from the acid strength and concentration of acid sites related to the presence of zeolite nanoparticles and faster diffusion combined with higher accessibility of active sites for bulky molecules. The first approach (recrystallization) was applied to form the ZSM-5/MCM-41 system by impregnation of or ion-exchange in MCM-41 with a solution of tetrapropylammonium hydroxide followed by classical hydrothermal treatment [91]. In this case some portion of originally mesoporous walls should recrystallize into zeolite particles. The other approach stems from the possibility to use zeolite seeds as building units, which are

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further self-assembled into mesoporous molecular sieves. Therefore, a higher stability and acidity of this composite materials should be obtained using zeolite seeds compared with composite materials prepared by a partial recrystallization [18,19,92,93]. Verhoef et al. provided experimental evidence on the recrystallization of a part of originally amorphous walls of MCM-41 into crystalline particles of ZSM-5 after impregnation of or ion-exchange in MCM-41 with a solution of tetrapropylammonium hydroxide [88]. X-ray diffraction and transmission electron microscopy provided evidence of the formation of particles of ZSM-5 with a diameter of 3 nm. These particles are not mobile to agglomerate to larger particles and it was suggested that they grew from the MCM-41 pore walls. Unfortunately, the formation of ZSM-5 particles was accompanied by a partial collapse of the mesoporous structure. To retain the mesoporous structure either addition of template molecules is proposed or MCM-41 with thicker walls should be used. This approach was confirmed by other experiments showing its applicability in the systems like mesoporous material/Pluronic P123/tetrapropylammonium hydroxide leading to ZSM-5/mesoporous sieve or mesoporous material/Pluronic P123/tetraethylammonium hydroxide forming Beta zeolite particles in mesoporous material [94,95]. Various nanocrystal zeolites were used to assemble in mesoporous material (ZSM-5, Beta, zeolite Y, etc.) [18,19,96, 97 ]. The first steam-stable hexagonal mesoporous aluminosilicates was successfully synthesized from zeolite Y type seeds by Liu et al. [98]. Seeds of zeolite Y were prepared by reacting sodium hydroxide, sodium aluminate, and sodium silicate under vigorous stirring at 100 °C overnight. 27A1 MAS NMR indicated the exclusive presence of tetrahedrally coordinated aluminum, although no diffraction lines of these seeds were observed. The hexagonal mesostructure was achieved by addition of hexadecyltrimethylammonium bromide as a structure-directing agent. The same method was employed to prepare a micro/mesoporous composite of ZSM-5 and Beta types [99,100,101,102]. All these materials show characteristic features of microporous as well as mesoporous materials. As the further step in the synthesis of this hierarchic materials, micro/macroporous composites were synthesized although it is not clear at the moment whether such materials can be used in catalysis [103,104]. 7.1. Synthesis of micro/mesoporous composites via recrystallization MCM-41, synthesized using hexadecyltrimethylammonium hydroxide, was ionexchanged with tetrapropylammonium hydroxide-MCM-41 by shaking a suspension of 2.00 g of MCM-41 in 1.22 g of 25.0 wt. % of tetrapropylammonium hydroxide solution in 1 L at room temperature overnight. The solid was recovered by centrifugation, washed in water and subjected to a hydrothermal treatment at 170 °C in a 15-mL Teflon autoclave. The recrystallized MCM-41 was recovered by centrifugation, washed with deionized water and dried under vacuum at 60 °C. The template was removed by calcination in a stream of air at 500 °C for 6 h [91]. 7.2. Synthesis of micro/mesoporous composites using seed crystals (Beta/MCM-48) Zeolite Beta seeds were prepared from 0.816 g of aluminum isopropoxide mixed with 40.0 g of silica sol and 53.02 g of tetraethylammonium hydroxide in a polypropylene bottle. The resulting mixture was stirred at room temperature for 1 h. The clear precursor solution was hydrothermally treated in an oven at 100 °C and the crystallization was stopped when a milky solution containing colloidal particles of zeolite Beta was obtained (at least after 24 h). Simultaneously, solution for the synthesis of MCM-48 was prepared from 0.868 g

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of hexadecyltrimethylammonium bromide dissolved in a solution containing 50 mL of ethanol, 12 mL of aqueous ammonia and 120 mL of water. Then 1.776 g of tetraethyl orthosilicate was added. Immediately after cooling the colloidal Beta solution to room temperature, 3.50 g of the Beta seed solution was mixed with 6.40 g of the freshly prepared precursor solution of MCM-48. The crystallization was carried out at 100 °C for 40 h [19]. 8. APPLICATION IN CATALYSIS Most of the advanced zeolite-based materials described in the previous chapters have been synthesized very recently. For that reason not much effort has been devoted to investigation of their catalytic properties. However, at least some examples of interesting reactions or promising results are given in this chapter. The extra-large pore zeolite UTD-1 with incorporated titanium was studied in oxidation of alkanes and olefins with hydrogen peroxide or organic peroxides. Particular attention was paid to the oxidation of cyclohexane to adipic acid [105] or oxidation of 2,6-di-tertbutylphenol [106]. Acid form of UTD-1 exhibited the isomerization-to-disproportionation ratio and the distribution of trimethylbenzene isomers in the m-xylene isomerization test reaction similar to those obtained from other large-pore zeolites [7]. m-Xylene isomerization was proposed as a test reaction to distinguish among medium, large and extra-large pore zeolites including UTD-1 and CIT-5 [107]. UTD-1 was found superior to CIT-5 in cracking of triisopropylbenzene; however, both 14-membered ring zeolites exhibited lower catalytic activities in cracking of complex feedstock compared with traditional three-dimensional zeolites Beta or Y [108]. Aluminum-containing ITQ-21 was compared with commercial USY and Beta zeolites in cracking of light vacuum gasoil [42]. The results showed that the catalytic activity of ITQ21 was higher than that of USY and much higher than that of zeolite Beta. On the other hand, ITQ-21 produced more propylene. The gasoline analysis showed that much less olefins were formed compared with USY and Beta while the octane number was the highest with ITQ-21. In acid-catalyzed acylation of 2-methoxynaphthalene to 2-acetyl-6-methoxynaphthalene (2AMN) polymorph C of zeolite Beta was found more selective than zeolite Beta due to a slighly smaller size of its channels. Molecular dynamic calculation confirmed the higher selectivity showing an increase in the ratio of diffusion coefficients for 2-AMN / 1-AMN of about one order of magnitude between polymorph C and zeolite Beta [109]. Delaminated zeolites like ITQ-2, benefiting from greater accessibility of acid sites and shorter diffusion pathways, exhibited a higher catalytic activity in cracking of 1,3diisopropylbenzene or vacuum oil compared with MCM-22 [15]. However, it seems that a high potential of delaminated catalysts can be found in synthesis of special chemicals. Corma and Fornes showed the synthesis of 2-methyl-2-naphthyl-4-methyl-l,3-dioxolane, which has an orange blossom fragrance [110,111]. The most interesting examples employing organized mesoporous alumina as a support for catalytically active species include the utilization of molybdenum oxide for hydrodesulfurization of thiophene [112] and rhenium oxide for metathesis of linear a-olefms [113]. In the former case at least 30 wt. % of M0O3 can spread over the alumina support, which results in almost twice higher conversion compared with the commercial catalysts. In the latter case, rhenium oxide supported on organized mesoporous alumina exhibits high conversions in metathesis of various a-olefms even at room or at only slightly elevated temperature. The high activity in both reactions has been explained by the absence of

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micropores in organized mesoporous aluminas [114] and the spreading of significantly higher concentrations of active phases compared with commercial aluminas. Mesoporous zeolite single crystals of ZSM-5 were investigated in gas-phase benzene alkylation with ethylene and compared with standard ZSM-5 [115]. Both mesoporous and standard ZSM-5 exhibited the same size and shape of crystals as determined by SEM (2 jim) while the concentration of acid sites based on FTIR and 27A1 NMR measurements was much higher in standard ZSM-5 with no mesopores. Nevertheless, the catalytic activity of mesoporous ZSM-5 was significantly higher compared with standard ZSM-5 due to improved mass transport in mesoporous crystals. Higher selectivities for ethylbenzene observed in mesoporous crystals can be explained by shortening of the diffusion path of products in mesoporous ZSM-5 decreasing, thus, the rate of the second alkylation step [115]. In a similar way, higher activities of mesoporous ZSM-11 in K-hexadecane isomerization and cracking, and of mesoporous Ti-ZSM-11 in epoxidation of 1-octene and styrene were reported compared with standard ZSM-11 and Ti-ZSM-11, respectively [86]. An increase in the activity of mesoporous MCM-41 after the incorporation of seeds of zeolite Beta resulted in a higher conversion of toluene in toluene alkylation with propylene to cymenes compared with Al-MCM-41 [116]. The resulting order of conversions was as follows: nanocrystalline Beta [117] > industrial zeolites Beta > micro/mesoporous composites (Beta/MCM-41) > MCM-41 (Fig. 15). Beta-TUD-1 catalysts with different loading of nanocrystals zeolite Beta were tested in n-hexane cracking [93]. A higher activity of these composite catalysts was observed compared with zeolite Beta and a physical mixture of zeolite Beta and TUD-1. The higher activity of composite material was ascribed to the isolated particles homogeneously dispersed in the mesoporous matrix making easier transport of reactants and preserving acidity of zeolite Beta. A similar composite catalyst (Beta/hexagonal mesostructured silicate) exhibited a high long-term activity and stability in the alkylation of isobutane with 2-butene, which were even higher than over zeolite Beta and much higher compared with MCM-41 or ZSM-5 [100,118].

Fig. 15. T-O-S values of the toluene conversion in alkylation of toluene with propylene over different Beta related catalysts (toluene/propylene molar ratio of 2.0, WHSV = 20h'', reaction temperature 250 °C) over nanosized Beta Bl - 0, industrial Beta iB2 - o, micro/mesoporous composite BM3 - V, and mesoporous Al-MCM-41 - x.

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9. SUMMARY AND OUTLOOK The continuous effort of numerous academic and industrial laboratories around the world has resulted in recent years in successful synthesis of a number of new porous materials including new structural types of zeolites and zeotypes, siliceous and non-siliceous mesoporous molecular sieves, mesoporous zeolite single crystals, and micro/mesoporous or micro/macroporous composite materials of different chemical compositions. As a consequence of the success of basic research in this area, zeolites have found new industrial applications. In the case of zeolites and zeotypes, several new extra-large pore microporous sieves have been synthesized like UTD-1, CIT-5, SSZ-53, and ECR-34. Their synthesis procedures are rather complex and catalytic data are usually lacking. Some effort has been exerted to enhance the accessibility of catalytically active sites by delamination and pillaring of zeolite precursors related mainly to the MCM-22 type while preserving the structure and properties of zeolitic layers. Some promising catalytic examples appeared in this area showing the potential of delaminated materials in transformation of organic compounds with larger molecules. The role of germanium dioxide as inorganic structure-directing agent has been investigated, which resulted in the successful synthesis of several new types of zeolites with so far not observed structural features (e.g. polymorph C of zeolite Beta, zeolite UTL with intercrossed 14- and 12-membered rings, ITQ-22 possessing 8-, 10-, and 12-membered rings). New types of mesoporous molecular sieves (their first synthesis opened a new subfield of molecular sieve chemistry) have been prepared over the last ten years by new synthetic approaches, different from those known for zeolites. The variety of the synthetic procedures described and the differences in the textural properties due to different synthetic procedures, as well as to the high temperature treatment, give evidence that mesoporous molecular sieves of different chemical compositions are very interesting materials not only in materials science. They could be important also for the application as heterogeneous catalysts, support for immobilization of homogeneous catalysts, adsorbents or materials for synthesis of new types of inclusion compounds. The first successful preparation of micro/mesoporous or micro/macroporous molecular sieves as well as mesoporous zeolite single crystals started an intensive search of optimization procedures for their synthesis, to increase their thermal stability and to tailor their acid, base and redox properties for possible applications in heterogeneous catalysis. There is no doubt that mastering of synthesis of these hierarchic materials is an important challenge in the area of porous materials. It can be expected that many new molecular sieve materials will be synthesized in a near future. These materials as catalysts or adsorbents are promising in different respects. Synthesis of novel molecular sieves is more complex compared with the classical synthesis of zeolites. It usually consists of several steps, e.g. preparing zeolite seeds followed by their assembly into mesoporous molecular sieve. This is of course time-consuming and costly and, thus, their catalytic or adsorption properties have to be superior to those of existing materials. In this respect, we are only in the beginning. In a similar way the acid properties of these molecular sieves have to be tailored as regards the acid strength and the concentration and type of acid sites. This means preparation of zeolite-based molecular sieves with pre-defined concentrations of strong Broensted acid sites and decreasing as much as possible the presence of Lewis acid sites. Lewis sites were shown in many reactions to catalyze undesired reactions causing a decrease in the selectivity or accelerate deactivation of the catalyst. For application of these materials in chemical industry their advantages have to be explored and their

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syntheses more facile even in large-scale facilities. With increasing pore sizes and decreasing dimensions of crystals, the advantages of traditional zeolite shape-selectivity will probably be smaller; on the other hand, these catalysts might be beneficial due to the possibility of transformation of large organic substrates. Finally, after fifty years of intensive investigation of synthesis, properties and applications of zeolites and related molecular sieves resulting in a number of industrial applications, it is clear that this research area is still very topical and many interesting and useful materials and processes will certainly be developed in future. ACKNOWLEDGEMENT The long-term support of our projects devoted to the synthesis, characterization and applications of zeolite-based molecular sieves by the Grant Agency of the Academy of Sciences of the Czech Republic (A4040411, B4040402) and the Grant Agency of the Czech Republic (104/02/0571, 203/03/0804, 104/05/0192, 203/05/0197) is gratefully acknowledged.

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Zeolite membranes - A short overview A. Julbe Institut Europeen des Membranes de Montpellier (UMR 5635 CNRS-ENSCM-UMII) UMII (CC 047), Place Eugene Bataillon, 34095 Montpellier cedex 5, France

1. INTRODUCTION 2. MEMBRANE SYNTHESIS METHODS 2.1. Polymer-zeolite composite membranes 2.2. Supported zeolite membranes 2.2.1. In-situ direct crystallization 2.2.2. Dry or wet gel conversion method 2.2.3. Seeding and secondary growth 2.3. Template removal and formation defects 3. MODIFICATION OF ZEOLITE MEMBRANES 4. CHARACTERIZATION METHODS 4.1. Static characterization methods 4.2. Dynamic characterization methods 5. TRANSPORT MECHANISMS AND MODELING 6. APPLICATIONS OF ZEOLITE MEMBRANES 6.1. Pervaporation 6.2. Gas and vapour separations 6.3. Membrane reactors 6.4. Sensors 7. INDUSTRIAL DEVELOPMENTS AND LIMITATIONS 8. CONCLUSIONS REFERENCES

1. INTRODUCTION Most of the classical batch separation processes involving zeolite pellets require the sequential regeneration of the adsorbing material. Because a zeolite membrane can potentially sieve out molecules in a continuous way, the development of zeolite membranes has attracted the attention of many research and industrial groups, since the beginning of the 80's. Zeolite membranes are the only membranes able to perform Angstrom-sharp separations and which are stable both thermally and in organic solvents. However, although about 152 zeolitic framework types have been indexed by the IZA, only about 15 structure types were prepared as membranes. A number of examples is reported in Table 1.

136 Table 1 Main zeolite and zeolite-related membrane structures studied in the literature Recent Oxygen Channel Max. Remarks Zeolite type Example of referen number at system "Pore" applications window ces size (A) (*) 1-2 SOD 6 2.8 GS, PV "Zeo-like" structure 3D CO2/CH4 separation 3-4 CHA 8 3.8 GS SAPO-34, SSZ-13 "Zeo-like" structure 3D 4.2 PV, MR, 5-9 LTA 8 Commercial (Mitsui) Na-A (GS?) 2D 4.4 CO2/CH4 separation 10-11 DDR GS 8 Decadodecasil 3R Steam resistant, Studied by NGK 12-18 MFI 10 3D 5.6 GS, PV, VP, MR Silicalite-1 Hydrophobic ZSM-5 Hydrophilic FER 10 2D 5.4 PV, GS 19 ZSM-35 3D MEL 10 5.4 PV 20 ZSM-11 12 FAU 3D 7.4 PV, GS, MR (X : Si/Al= 1-1.5) 21-23 Na-X, Na-Y (Y : Si/Al > 2) 12 ID 24-27 MOR 7.0 PV, GS Acid resistant & hydrophilic BEA (Beta) 12 3D 7.6 PV, GS, MR 28-29 * PV : Pervaporation; VP: Vapor Permeatiorl, GS: Gas separation; MR : Membrane Reactor The MFI membranes have been the most largely investigated, due to their large attractive industrial properties: suitable pore sizes, with high thermal and chemical stability, easy synthesis and possible modification of its chemical composition. These membranes are typically suitable for the separation of commercially important species, such as p-xylene from its isomers. The insertion of Ti in the silicalite-1 structure has also generated a number of studies on TS-1 [30] or ETS-10 membranes [31,32]. Other elements were also inserted in MFI such as Fe [33], B [34], Ge [35], V [30,36]. Pt was inserted in FAU for catalytic applications [21]. A number of other zeolite membrane systems have been reported such as ANA [37], GIS (P) [38], T [39], TON (ZSM-22) [40], KFI, or LTL (L), but also phase mixtures or composite membranes. A number of zeolite-related materials such as MCM [41], A1PO [42], or pillared clays [43] have been investigated as well. The annual number of Elsevier publications on "zeolite membranes" increased from 4 in 1991 to over 50 in 2004. A large number of book chapters and review articles have been published during the last five years, entirely or partially dedicated to zeolite membranes and films [44-55]. An international conference is also specifically dedicated to zeolite films and membranes. Initiated in Gifu (Japan) in 1998, its second edition was held in 2001 in Delft (The Netherlands) and its 3 rd edition (3 rd Int. Zeolite Membrane Meeting) was held in Breckenridge (CO-USA) in 2004. A website is also dedicated to Zeolite and Molecular Sieve Membranes [56]. The objective of this review article is to provide students and young researchers in the field, with a general but relatively short overview of the current state of the art in zeolite membranes. It will cover zeolite membrane preparation, characterization and main applications, while referring to the recent literature. For more detailed reviews, the lecturer will refer to a

137 listing of specialized articles, specialist reviews, and book chapters which have been published over the last five years [44-55]. 2. MEMBRANE SYNTHESIS METHODS There are roughly three types of zeolite membranes: polymer-zeolite composite membranes, self-supported zeolite membranes and supported zeolite membranes. Although the first type has still a number of niche applications, the latter are by far the most largely studied and will be the most largely detailed here. Free standing compact zeolite film with limited thickness (<1 ^m) have been also grown on temporary supports like Teflon® or cellulose [57,58] but this concept can not be easily up-scaled because of the poor permeability and high fragility of the derived self-supported zeolite membranes. 2.1. Polymer-zeolite composite membranes One of the first zeolite based membranes were composite membranes, obtained by dispersion of zeolite crystals in dense polymeric films in order to make "zeolite filled polymeric membranes" [59,60,61]. These membranes have been developed at the end of the 80's for both gas separation and pervaporation. The clogging of zeolite pores by the matrix and the quality of the interface between the zeolite crystals and the polymer matrix (non-selective diffusion pathways) were key points. Although this concept and related studies are relatively old, a number of research teams are still working in this field [62,63], because of the constant improvements in membrane material science. The recent discovery of zeolite colloidal seeds and the evolution of both zeolite surface grafting and ceramic/polymer interfaces knowledge are typical recent advances which are attractive for the development of improved zeolite-filled polymeric membranes. The pervaporation team of the US Environmental Protection Agency is currently developing zeolite filled polymeric membranes with improved ethanol/water selectivities [64]. Their studies include the development of silicalite-l-Poly(DiMethylSiloxane) membranes with optimised amount of zeolite in the membranes and optimised zeolite particle sizes. Polymer-zeolite composite membranes are also studied as interphase contactors in liquid phase oxidation processes [65] or for improving the properties of Nafion® in fuel cells applications [66]. 2.2. Supported zeolite membranes Various types of supports have been used for the synthesis of supported zeolite membranes, whose shape, chemical composition, porous structure (pore size, porosity), micro-, macro-structure and pre-treatment can influence considerably the membrane characteristics. Typical examples of porous support suppliers mentioned in the zeolite membrane literature are reported in Table 2. C1AI2O3 is by far the most largely studied, although other ceramic materials were also reported such as YA1 2 O 3 [67], TiO2 [68], SiC [69], or mullite [70]. Metals (essentially stainless steel (SS)) are also largely considered because of their lower price and easier sealing [3,32,71,72]. Laser perforated stainless steel [73] and etched silicon [74] have been also recently considered for the development of micromembranes A number of groups also considered carbon [75] or glass [76] supports but their poor resistance to either oxygen or basic media is restrictive. Indeed when selecting the support, its thermal and chemical resistances to the synthesis media, to the thermal treatment or to the application conditions, as well as the difference in thermal expansion coefficients between the support and the zeolite layer are important parameters which have to be considered. Depending on whether the substrate surface is hydrophilic or hydrophobic, the amount of surface OH and its stability in high pH conditions,

138 different results are obtained. The surface roughness is also an important parameter which influences the quality of the layer (flat supports can even be polished). In order to promote a good adhesion of the membrane layer, the supports are subjected to various cleaning treatments (e.g. washing with water, acetone, acids, bases, sometimes assisted with ultra-sonic treatment) to remove loose particles, adsorbed salts, or organic compounds. Subsequent to cleaning the supports can be stored in water to prevent adsorption of organics from the atmosphere [51]. Table 2 Typical example of inorganic porous support suppliers mentioned in the zeolite membrane literature Support Supplier Material Support shape Typical pore sizes (nm) Pall-Exekia (USA) Tube 200, 800 aAl 2 O 3 Tube 5 yAl2O3 Inocermic GmbH (Germany) Tube 60, 100, 1900, 3000 aAl 2 O 3 Disk 60, 1800 aAl 2 O 3 Tube 5,60 yAi 2 o 3 Poco Graphite (USA) Tube 700 aAl 2 O 3 Tube 200 Sulzer ChemTech LTD aAl 2 O 3 (Switzerland) Tube 5 yAl2O3 GKN Sinter Metals Filters GmbH Disk 100, 200 aAl 2 O 3 (Germany) Disk 270 ss NGK Insulators Co. (Japan) Tube 100, 200 aAl 2 O 3 Mott Metallurgical Company (USA) SS Tube 200, 500 NOK Corp. (Japan) Tube 120-150, 150-170 aA! 2 O 3 Nikkato Corp. (Japan) Tube 1300 aAl 2 O 3 Golden Technologies (USA) Tube 200 aAl 2 O 3 US filters (USA) Tube 100 aAl 2 O 3 Trumem international (USA) Plate SS 2000-500 Dong-su Co. (Korea) Tube SS 900 Most of the studies have been performed on flat (discs) or tubular supports. It is now a current challenge to increase the surface to volume ratio in modules (from a few m2/m3 to more than 500 m2/m3) by using capillaries [77], hollow fibres [78] or honeycomb supports [69,79,80]. The influence of the support layer characteristics on the flux limitations is also an important concern for industrial applications [81]. There are three main routes to synthesize continuous supported zeolite membranes: the "insitu direct crystallization", the "dry or wet gel conversion method" and the "seeding and secondary growth". 2.2.1. In-situ direct crystallization The first investigations related to the development of supported zeolite membranes concerned the in-situ direct crystallization of zeolites on top of a support, by direct contact of the support with the precursor solution. The method was initially patented in 1987 by H. Suzuki [82]. The membrane is synthesized under autogenous pressure, by both the nucleation and growth of zeolite crystals on the surface of the support grains. A relatively thin zeolite layer crystallizes on the surface and/or in the pores of the porous support. One of the most important problem with this method is the long induction period and the homogeneous nucleation which often alter the membrane quality. The sol compositions which reveal successful for making supported

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membranes are usually different from the classical ones leading to powders. The influence of the type of precursor, sol composition and support type are discussed in [51] ; examples of successful sol compositions for making MFI, A type and Y membranes are also given. Clear sols are often used in order to limit homogeneous nucleation. In the case of MFI membranes, the molar ratios OH7SiO2, TPA+/SiO2, Na+/SiO2, Al/Si, and H2O/SiO2 strongly influence the membrane characteristics. The support type and pore size can influence the growth of the zeolite material on top of the support or within its pores [36,83]. The separating layer of the final membrane can then be located either outside of the support pores (non infiltrated membrane) or inside the pores (infiltrated composite membranes) as shown in Fig. 1. If the starting sol becomes below supersaturation before the membrane is fully developed, growth must be repeated with a fresh sol. Two or three hydrothermal syntheses are often required to get "good quality" membranes. In many cases, large quantity of non connected zeolite crystals can be observed on the surface of infiltrated composite membranes, after several synthesis steps. The methods for evaluating the membrane quality will be detailed in the "Characterization" section. In the case of MFI membranes, a very simple method can be used at this step: the membrane is washed, dried at about 150-200 °C and tested by single gas permeation (He or N2). As far as the TPA+ ions are blocking the zeolite channels, no gas permeance should be detected if there are no defects. 2.2.2. Dry or wet gel conversion method The second type of method includes the so-called dry gel [84] and wet gel [85] conversions. Since its first description by Xu et al. in 1990 [86] for the synthesis of MFI membrane, the method principle has been extended to ANA, MOR and FER [87,88]. It involves the deposition of an amorphous aluminosilicate gel on the support and its subsequent crystallization under vapours of triethylamine, ethylenediamine and water. When the templating agent is not included in the gel, the crystallization is said to occur by "Vapour Phase Transport" (VPT) [86,89]. The method is called "steam-assisted crystallization" when the dry gel already contains the templating agent [90]. This type of synthesis method allows to avoid homogeneous nucleation, it has also the advantages of improved controllability, minimal waste generation, and reduced chemical consumption, that are desirable for large-scale production of zeolite membranes. However, the large volume shrinkage during the transformation from gel to zeolite often leads to defects in the films. 2.2.3. Seeding and secondary growth The third method, called "seeding and secondary growth" involves the growth of a layer of seed crystals previously nucleated and deposited on the support. This method has been first investigated for growing LTA supported membranes, whose direct synthesis from a sol in contact with a support was highly difficult because of the absence of an SDA (Surface Directing Agent) [91]. The seeding and secondary growth method is now considered as one of the most attractive and flexible approaches for orienting the formation of consolidated thin membranes including A, L, ZSM-5, SOD, FAU and highly oriented silicalite-1 films [92-96]. Bypassing the nucleation and going directly to crystal growth eliminate a serious source of irreproducibility due to adsorbed species or support dissolution, it expands the range of synthesis composition and allows the use dilute sols. The derived membranes are generally highly permeable and selective.

140

Fig. 1. Schematic representations of a thin top layer membrane (Al) and a composite infiltrated

membrane (Bl) on/in asymmetric supports. The corresponding FE-SEM observations of MFI membranes on/in an aAl2O3 support (Pall-Exekia, with 200 nm top-layer pore size) are shown in A2 (seeding/secondary growth) and B2 (in-situ direct crystallisation). The synthesis of seeds (suspension of colloidal crystals) is generally performed at low temperature according to methods reported 10 years ago by various groups for silicalite-1 [97,98]. The seed size is usually in the range 0.05-2 |^m. Depending on the selected temperature, the induction period and synthesis can be very long and can reach a few months. Two steps synthesis allows to control the nueleation and growing steps and consequently the seed size, homogeneity and yield [99,100]. Discrete colloidal monodispersed zeolite seeds are synthesized from clear homogeneous solutions and purified by repeated centrifugation and redispersion. The solid content and the pH of the seed suspensions are adjusted. In most cases the suspensions are filtered and the experiments are carried out in a laminar flow bench to reduce the amount of dust. The size, homogeneity, concentration and dispersion of the seeds are important parameters which directly impact both the penetration of seeds within the support and the quality of the final membrane. Commercial zeolite crystals can also be used and grinded to obtain small seeds [92]. Coating the support with the seeds is a critical task. Different strategies are proposed in the literature. The supports can be seeded by simple contact (deep coating for a few minutes) with a suspension of zeolite crystals at an appropriate pH, and subsequent washing to keep only a surface monolayer [51]. Fig. 2 shows a thick layer of silicalite-1 seeds on CCAI2O3 support after 3 h contact with the seed suspension. The seeding can be performed under vacuum [102] or by electrophoretic deposition in aqueous or non-aqueous medium [103]. The latter method has been applied to the rapid synthesis of A-type zeolite membranes. Two strategies can be used for an electrostatic attachment of the seeds to the support: either a fine-tuned surface charge by pH control and measurement of the support zeta-potential or the adsorption of positively charged polymers [104], and immersion in a suspension whose pH is such that the seed particles are negatively

141

charged. The quality of the seed suspension is important to avoid the deposition of aggregates on the support. The seeded support is then heated at about 150-200 °C to bind the particles on the surface by condensation of hydroxyl groups. A higher temperature (typically 400 °C) is needed when a cationic polymer has been used.

Fig. 2. CCAI2O3 support (200 nm pore size) covered with a thick and close pack layer of silicalite-1 seeds (3 h contact time) and dried for 6 hours at 155 °C [101].

For operating the secondary growth, the seeded support is hydrothermally treated in a diluted synthesis mixture (with or without template) to grow the seed crystals, resulting in a compact zeolite film. A simple steaming of the seed layer can also partially consolidate the zeolite membrane by local dissolution and recrystallization processes in the seed layer. The films are finally dried or calcined if an SDA was used. The MFI system is orthorhombic (or even monoclinic after calcination). Its multidimensional channel network is an excellent model system for fundamental studies related to the influence of preferred crystal orientation on membrane performance. A schematic representation of the channel directions and specific crystal orientations is shown in Fig. 3. Obviously it can be predicted that c orientation is less favourable to high fluxes than b orientation. The habit of MFI crystals is determined by the hydrothermal conditions. Large differences in preferred orientation can be obtained by using different sol concentration, hydrothermal conditions, seeding density, seed sizes and type of SDA [105]. The growth of the crystals in the film is competitive: crystals oriented with the fastest growing direction perpendicular to the substrate surface are able to grow both perpendicular to the substrate and in the lateral direction. These crystals dominate in thick films and the number of crystal faces per unit area decrease with increasing film thickness. A number of growth models have been proposed, including those explaining oblique preferred orientations ; most of them are reviewed in [106]. Without any specific precaution, the MFI membranes are polycrystalline, intergrown and with different preferred crystal orientations. Several strategies have been used to prepare oriented zeolite seed layers such as slow dip-coating [107], seed deposition after cationic polymer adsorption [104], adsorption of positively charged additives on the seeds to compensate the negative surface charge of the crystal faces and change their orientation on the support [108], and finally covalent bonding on functionalised surfaces [109]. The implementation of crystal shape engineering by use of SDA polycations (dimerTetraPropylAmmonium or trimer-TPA instead of regular TPA) as shape/habit modifiers has been clearly demonstrated in [110]. The b-oriented 1 um thick silicalite-1 membranes revealed excellent performance for 0,/5-xylene separation (high p- and low o-xylene fluxes). The morphology and separation performance of both c- and b-oriented MFI membranes are shown in Fig. 4 [110].

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Fig. 3. Schematic representation of MFI crystal orientation along c or b axis on a porous support.

In order to avoid the infiltration of seeds in the support and to develop ultra-thin membranes (typically 500 nm thick) with a high permeability, a masking techniques has been recently developed in Lulea University [111]. A solution of poly methyl methacrylate (PMMA) in acetone was applied and dried on the support top surface. The interior of the support was subsequently filled with wax and the protective PMMA layer was dissolved in acetone. The masked support was then seeded with a monolayer of silicalite-1 crystals before being submitted to the classical hydrothermal and calcination steps.

Fig. 4. ZSM-5 membrane performance in xylene isomer separation, p-xylene, o-xylene permeance and mixture separation factor (SF) are plotted versus temperature of permeation for typical c-oriented (A) and b-oriented (B) films [110],

In order to accelerate the synthesis of both zeolite seeds and membranes, microwave-assisted hydrothermal synthesis (MW-HT) revealed an attractive method. Colloidal zeolite seeds can be successfully prepared by MW-HT synthesis [100,112,113]. Starting from a seeded support,

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the MW-assisted secondary growth yields good quality MFI membranes within a few hours, while limiting the dissolution of the DA^C^ support. Typical examples of MFI membranes prepared by this method are shown in Fig. 5. The membranes were obtained very rapidly, i.e. after a synthesis time as short as 30-60 min, at 120-180 °C. Depending on the synthesis conditions, the membrane thickness varies typically from 200 nm to a few microns, with glassy like or columnar morphologies respectively. The performance of these promising membranes which are not yet optimized is still under study [101].

Fig. 5. FESEM observations of the MFI membranes prepared at different temperatures, by MWassisted secondary growth, from a layer of MW derived silicalite-1 seeds [101].

A number of other strategies have been investigated in order to improve the quality (selectivity and fluxes) or the synthesis efficiency of zeolite membranes by pre-treatment of the support. Most of these treatments are listed in [49], and include laser ablation, utilisation of diffusion barriers to limit sol infiltration, oxidation of stainless-steel supports, deposition of iron oxide to favour heterogeneous nucleation on the support and the use of acid attacks to activate the surface. A series of original synthesis strategies has been also reported recently such as flow-through reactors for the homogeneous synthesis of zeolite membranes [77], centrifugal force field [114] or electrophoresis [115] for the preparation of A-type membranes, and pulse laser deposition (PLD) for the secondary growth of oriented MCM-22 membranes [116]. 2.3. Template removal and formation defects When the membrane synthesis requires a template, it has to be removed from the zeolite pores. Chemical leaching or ion exchange with ammonium ions is only possible for big pore sizes (e.g. MOR, Beta, MCM). In most cases, and namely for the MFI structure, the template is removed by thermal treatment in strictly controlled conditions (atmosphere, heating rate, temperature and duration). We have to note that an efficient and gentle low temperature ozone treatment was recently proposed for organic template removal from MFI zeolite membranes [117]. The removal of the template molecules by calcination is often responsible for the formation of cracks which are detrimental to the membrane selectivity. The formation of defects during template removal has been largely studied in MFI-type membranes and also in individual large crystals [118]. A very low heating rate (typically 0.1-0.5 °C/min) up to 400 °C is generally recommended in order to limit the formation of defects in supported polycrystalline MFI membranes. The orientation of the MFI crystals seems also to influence the membrane integrity during the thermal treatment [119]. The formation of cracks is attributed to a compression stress within the film during the cooling step, when the zeolite film is firmly attached to the support [120]. Indeed, most of the silico-aluminate zeolites have a reversed thermal expension coefficient compared to the classical behaviour of ceramic or metallic supports: zeolites contract

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when heated and expand when cooled. The quality/integrity of the supported zeolite membrane is generally improved by selecting a suitable support and temperature program. An other method to limit the influence of differential expansion coefficients is to grow the zeolite crystals within the pores of the support. This "pore plugging method" [36,83] leads to infiltrated membranes (figure IB) which can withstand heating cooling cycles up to 500 °C without any modification of their properties. It is not difficult with this method to avoid support dissolution and to orient the zeolite crystals. Obviously the growth of zeolite crystals within the support pores limits the membrane permeability, but a precise control of the synthesis conditions, support thickness and pore sizes allows to limit the apparent membrane thickness to a few microns and to get substantial permeability. The contradictory findings which have been reported concerning the influence of infiltration on the membrane selectivity [51 ] are attributed to the difficult comparison of thicknesses, crystallinity and composition of the membranes which are usually modified by a partial support dissolution. An other approach to solve the problem of cracks is to repair them after they have formed. This can be done namely by a selective coking procedure [121]. However in most cases, the increased membrane selectivity is coupled with a substantial decrease of the membrane flux. 3. MODIFICATION OF ZEOLITE MEMBRANES Among the numerous strategies which have been adopted to produce zeolite membranes with either a reduced number of defects or with specific properties, a series of modification techniques have been tested. Most of them are listed in [49]. One can namely mention isomorphous substitutions of e.g. Al, Fe, B, or Ge in the structure, ion exchange, silylation, atomic layer deposition (ALD), chemical vapor deposition (CVD), coking, catalytic cracking, and reactions with adsorption sites in the zeolite structure. Silylation is very attractive for increasing the selectivity of zeolite membrane to H2 [122]. MFI and SAPO-34 membranes have been recently modified by silylation and atomic layer deposition (ALD) to create H2 selective membranes [123]. The multistep synthesis is classically used to repair defects or to increase the zeolite membrane thickness. This technique also allows to superimpose layers with different zeolite structures. We have also to notice that for certain zeolite structures such as FAU or LTA, when the synthesis duration is too long, other phases can appear by transformation of the initial zeolite structure. FAU or MFI membranes can also be modified in order to allow an optical switching of gas transport [124]. The switch (guest) is azobenzene and acts as a nanovalve which is inserted in FAU or MFI membranes (hosts). The flux of single components through the trans-state azobenzene modified membrane is increased by a factor 3 to 6 compared to the cis-state membrane. 4. CHARACTERIZATION METHODS The zeolite membranes quality and the development of methods able to rapidly identify the defects are key-factors for large scale applications. A large number of static and dynamic methods can be used to evaluate the quality of membranes [125]. Static methods allow to study the physico-chemical characteristics of the membrane material. Dynamic methods allow the detection of defects affecting the transport properties and consequently the membrane performance. For zeolite membranes the quality of the adhesion between the zeolite layer and the support, the orientation of zeolite crystals, the layer thickness, and the number and quality of

145 crystal boundaries are key parameters. Defects can occur from an incomplete growth of crystals, a non uniform seeding, a difference between the thermal expansion coefficients of the support and the zeolite, or from the development of stresses during the calcination step. An amorphous or a crystalline phase can also form within the support pores during the synthesis and modifies its transport properties. Depending on the selected application (high or low temperature, for gas, vapour or liquid separations) the influence of defect size, number, location and connectivity are very different. 4.1. Static characterization methods Scanning Electron Microscopy (SEM), coupled with energy dispersive X-ray analysis is the most classical method used to study zeolite membranes. It gives access to the homogeneity, morphology, adhesion, infiltration and thickness of the zeolite layer. Crystal sizes can be also evaluated (the length of crystal boundaries influences the permeation properties). HR-TEM and coupled analysis are useful to study the zeolite structure and composition, also within the pores of the support. Atomic Force Microscopy can also be used to study the growth of zeolite films. X-ray diffraction, including grazing incidence XRD, is needed to study the crystalline phases and the orientation of zeolite crystals grown on supports. Gas adsorption (N2, Kr) can be used to estimate the relative quantity of zeolite deposited on the support (BET- or Langmuir equation). When a dense substrate is used, ellipsometry gives the film thickness and void volume fraction. Absorption spectroscopies such as FTIR, are adapted to study the membrane material short range structure. The novel extension of Fluorescent Confocal Optical Microscopy (FCOM) has also to be underlined as a qualitative, non-destructive, three dimensional imaging tool, to view polycrystalline features of MFI membranes. The coupling of this technique with simultaneous reflectance and SEM provides a critical link between the grain structure of MFI membranes and the more qualitative FCOM images. The selective adsorption of large dye molecules (diameter-tuned) within the grain boundaries throughout the membrane thickness, allows to evidence "quantitatively" non-zeolite pores [126]. This very promising technique has yet to be fully explored. 4.2. Dynamic characterization methods Gas permeation measurements with single components or mixtures need to be performed for a real evaluation of zeolite membranes quality, although a large number of contradictory results can be found in literature depending on the conditions used for these measurements. First of all the membrane must be carefully outgassed at high temperature and/or under vacuum before the measurements (in order to eliminate CO2, H2O, organic solvent vapours...). The type of cell and measurement conditions (Wicke-Kallenbach, sweep gas, side of gas feed...) can also influence the results [127] as well as the support characteristics. The single gas permeance of He, H2, N2, O2, Ar, CO2, CH4, C3H8, SF6, butane, hexane, or xylene isomers is classically studied in order to evaluate the quality of zeolite membranes and their molecular sieving or shape selective transport properties. Transient gas phase permeation also allows to measure the effective membrane thickness non-destructively [128]. A series of more sophisticated permeation techniques have been recently proposed in order to better characterise the quality of the membrane and to evaluate qualitatively the contribution of non-zeolite pores to the transport. A number of methods derived from permporometry [125] are under study [111,129]. The method used in [111] consists in measuring the permeance of an inert gas such as He through a MFI membrane, while increasing the partial pressure iplpo) of a condensable gas which can enter into the zeolite pores, such as w-hexane (0/dneHc = 0.43 nm). Zeolite pores are then blocked first by «-hexane adsorption, and when the proportion of n-

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hexane increases, larger and larger pores are blocked by condensation and become unavailable for He transport (Fig. 6). Around plp0 = 1, essentially macroporous defects participate to He transport. The Kelvin equation can be used to estimate roughly the size of defects, keeping in mind that the Kelvin approximation breaks down for micropores. Typically «-hexane activities of 0.025 and 0.85 correspond to pore sizes of respectively 1.1 nm and 24 nm. Then the He permeance atp/p0 « 0.025 can be used as an estimate of the total permeance via defects larger than micropores [111].

Fig. 6. Principle of permporometry for the dynamic characterisation of zeolite membranes. When the partial pressure of n-hexane (p/po) increases, it blocks first zeolite pores and then larger and larger defects sizes, thus blocking progressively the permeation of He.

Other groups also work on similar principles for the characterisation of zeolite membranes by the so-called Dynamic Desorption Porosimetry or by dynamic desorption of adsorbing species [130]. The single gas permeance ratios N2/SF6 or H2/SF6 are often considered as representative of the zeolite membrane quality. However these values can be quite low even for high quality membranes [131]. Indeed, these ratios are affected by the defect distribution but can also depend on other factors such as the applied feed pressure, the film thickness and the support type. For example the N2/SF6 permeance ratio increases with pressure due to non-linear adsorption of SF6 and also increases with the film thickness due to the mass transfer resistance of the support. In the same way, the ideal or mixture selectivity 2-butane/w-butane is often considered as a good indication of the membrane quality, but varies largely (from 10 up to 200) even for good quality membranes. It is then difficult to use only one of these criteria to compare zeolite membranes.

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An other interesting characteristic of zeolite membranes is their single gas permeation behaviour at high temperature. Once again, contradictory results were reported in the literature for apparently good quality MFI membranes. Most of the gas permeation studies at high temperature fit with the curve ABCDi reported in Fig. 7, i.e. with the permeance increasing with temperature above about 300 °C [36, 132].

Fig. 7. Schematic evolution of the permeance of single gas through MFI membranes, as a function of temperature. ABC: Activated configurational diffusion, AB: the mobility of adsorbed species increases and occupancy decreases, B: the increase in mobility cannot compensate the decrease in occupancy. C: adsorption is negligible. CD, and CD2 :activated transport through zeolite pores and/or through grain boundaries (?).

This behaviour was described by an activated transport yielding a "Knudsen like" selectivity at high temperature but does not fit with the classical behaviour of fluids at high temperature in zeolite channels. Indeed a continuous decrease of the permeation with temperature should rather be expected for a zeolite membrane made with only one single crystal. A possible hypothesis for explaining an increase of permeance with temperature, while maintaining a good selectivity at low temperature, could be related to the reversible and continuous contraction of zeolite crystals when temperature increases. Such a contraction creates spaces between the crystals, which become available to the gas phase. This behaviour seems to occur for both top-layers [132] and infiltrated membranes [36] and seems to be enhanced by the crystallinity of the zeolite material, i.e. when the synthesis and thermal treatment are performed at higher temperature. The number and quality of crystal boundaries seem to orient either an ABCDi or an ABCD2 behaviour but does not seem to influence directly the separation performance of the membranes at low temperature. As an example, the separation IVw-butane is compared in Fig. 8 for two infiltrated membranes prepared from similar sol compositions but with slightly different synthesis conditions (synthesis temperature and thermal treatment). In both cases, the diffusion of H2 is blocked at low temperature by the strong adsorption M-C4H10. When the temperature increases the quantity of adsorbed n-butane decreases and the selectivity is reverse, in favour of H2. Such a behaviour is classical and was reported earlier by Geus et al. for a CH^w-butane mixture [134]. When there is no more adsorption, the membrane selectivity for the mixture come close to the value for single gases. However the permeances continuously increase for the membrane in Fig. 8a, although they

148 decrease for the one in Fig. 8b. Specific studies have still to be performed in order to clarify the origin(s) and impact(s) of such opposite behaviours at high temperature.

Fig. 8. Separation performance for a mixture H2/«-butane as a function of temperature for two MFI/aAl2O3 infiltrated membranes prepared by the « pore plugging method »: a) membrane prepared at 185 °C for 3 days and fired at 600 °C in air [36], b) membrane prepared at 170 °C for 3 days and fired at 500 °C with 5 % O2 by the group of J.A. Dalmon [133].

5. TRANSPORT MECHANISMS AND MODELING Most of the separation applications of zeolite membranes are based on preferential adsorption, differencial diffusion rates or molecular sieving effects. Key parameters for transport in zeolites by surface transport and micropore diffusion include temperature, pressure, molecular weight, kinetic molecular diameter, pore diameter, molecular collision free path, heat of gas adsorption, and thermal activation energies for both surface and microporous diffusion. Adsorption is a nonactivated, exothermic phenomena which is driven by fugacity. It depends on the adsorption enthalpy (-AHajs) and on the molecular size of the adsorbate. It is a competitive phenomena. Diffusion is an activated process which depends on (-AHa{is), on the molecular size of the adsorbate and on the coverage. Diffusion is generally described by a Maxwell-Stefan approach and can be measured by Pulse-Field Gradient NMR, Quasi-Elastic Neutron Scattering, chromatography uptake, frequency response, or transient permeation measurements. The Maxwell-Stefan model is recognized as the most adapted for explaining the experimental results and the subtle interplay between adsorption and diffusion, as well as the coupling between species diffusion [55]. Prediction of macroscopic transport properties for diffusion of molecular species through microporous crystalline membranes is the key to the development of both classical applications, such as separations and innovative ones such as chemical sensors. However transport in zeolite is a too vast subject which will not be treated here. In addition to specialized articles, readers should also refer to review articles and book chapters such as [50-52,55]. The reference [54] reviews separations by pervaporation through zeolite membranes. Some aspects of these separations are similar to gas-phase separations, except that feed-side coverages are close to saturation during pervaporation, making competitive adsorption and molecule-molecule interaction more important during multicomponent diffusion.

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6. APPLICATIONS OF ZEOLITE MEMBRANES The unique properties of zeolites coupled with the continuous separation properties of membranes make zeolite membranes and films attractive for a wide variety of industrial applications. When zeolites are grown as films, they offer the potential for continuous separation of industrially important gas, vapour or liquid mixtures, and can be used for applications in membrane reactors, as sensitive chemical sensors, as electrodes (for selective ion transport), for solar energy conversion, as batteries, as optical and data storage materials, or in electronic and thermoelectric applications. The inherent properties of zeolites which are attractive for membrane applications are: regular pore sizes with molecular dimensions (enabling shape or size selective catalysis or separation), a high thermal stability, acid or basic properties, hydrophilic or organophilic properties, possible ion exchange, dealuminationrealumination, isomorphous substitution, and insertion of catalytically active guests (transition metal ions, complexes or chelates, basic alkali metal or metal oxide clusters and even enzymes in ordered mesoporous materials). Zeolite membranes can potentially sieve out molecules in a continuous way. By combining their adsorption and molecular sieving properties, they are useful for the separation of mixtures containing non-adsorbing molecules, or different organic compounds, or mixtures of permanent gases/vapors or water/organics. Hydrophilic Na-A zeolite membranes have been commercialised at the end of the 90's for alcohol dehydration and solvent dewatering. Hydrophobic membranes are attractive to remove organic compounds from water, to separate organic mixtures and to remove water from acidic solutions on the lab scale. The hydrophilic or catalytic properties of zeolites are also exploited in membranes reactors for either conversion or selectivity enhancement. A large number of examples are reported in the literature [44-55] and only few of them will be reported here. 6.1. Pervaporation Pervaporation is a process in which a liquid stream -containing two or more componentscontacts one side of a non-porous or ultramicroporous membrane, while vacuum or sweep gas is applied to the other side. The components in the liquid stream sorb into the membrane, permeate through it and evaporate into the vapour phase. The fundamentals and applications of pervaporation through zeolite membranes are reviewed in [54]. Dehydration of organics is the first, and still the only, commercial application of zeolite membranes [8]. Indeed water is a small molecule {0kinetic(^2O)= 2.9 A, 0fane,,c(C2H5OH)= 4.5 A) which adsorbs very strongly on hydrophilic membranes such as LTA (window size= 4.2 A). This explains why non-zeolite pores have very little effect on the selectivity of hydrophilic membranes. Indeed these membranes behave as poorly selective gas separators (with a Knudsen selectivity only) because of a relatively high concentration of non-zeolite pores (small mesopores). The removal of organics from water with zeolite membranes is generally much less efficient as far as non-zeolite pores are highly detrimental to the currently available hydrophobic membranes (e.g. silicalite-1) which still contain silanol groups and adsorb water. The behaviour of hydrophilic and hydrophobic membranes in a pervaporation experiment for water/ethanol is sketched in Fig. 9. Organic/organic separations are generally more efficient for molecules with different dipoles and sizes.

150 A- HYDROPHILIC ZEOLITE MEMBRANE

B- HYDROPHOBIC ZEOLITE MEMBRANE

Fig. 9. Schematic representation of the contribution of both zeolitic and non-zeolitic pores in: A) hydrophilic (LTA) and, B) hydrophobic (silicalite-1) membranes during pervaporation experiments for a water/ethanol mixture.

The pervaporation-based separation processes are typically attractive for sustainable biofuel production, pollution prevention and wastewater treatment. Membrane technologies can be easily integrated into existing plants in order to separate, fractionate and concentrate contaminants or process components. Membrane technologies require minimal temperature changes and chemical additions, they operates in either continuous or batch modes, they use significantly less energy than traditional separation processes, they do not alter the chemical structure of the processed materials and they can be easily integrated into existing processes due to their modular nature and compact size. The membranes typically used for pervaporation experiments are listed in Table 1. Among the new promising membranes currently considered for more aggressive media and for more selective and long term pervaporation applications, we have to mention MOR, Ttype, ETS-10 and SOD membranes. The development of very thin, strongly hydrophobic membranes with a low number of non-zeolite pores (e.g. by masking or by changing the silanol groups) is the current challenge. 6.2. Gas and vapour separations A number of zeolite membranes have been studied for their attractive gas separation performance. A huge number of results can be found in the literature, in relation with the separation of small molecules like H2, CO2, O2, N2, CO and CH4. Separation can occur by adsorption and/or mobility. In the case of MFI, there can be no strong mobility difference or molecular sieving. As far as the adsorption differences are not important, MFI membranes do not provide high separation selectivity for these small gases [51]. In principe zeolite A with its smaller pore opening would be ideal for separation by molecular sieving but the density of defects is too high in this type of membrane. The modification of MFI membranes by catalytic cracking of silane is an attractive way to increase the membrane selectivity for H2 separation [122,123]. The transport of H2 is also studied in zeolite micromembranes [135]. On the other hand, SAPO-34 [3] and DDR [10-11] membranes revealed particularly attractive for CO2/CH4 separations. Certainly the most promising application of MFI membranes is in hydrocarbon separations. Adsorption increases with the carbon number for linear compounds and steric effect allows to separate n-butane from i-butane. Consequently at low temperature the permeance decreases when the carbon number increases and at high temperature, differences in diffusion factors

151 dominate [132]. A number of attractive separations have been considered such as the extraction of C2-C5 hydrocarbons from natural gas or the extraction of H2 from hydrocarbons in refinery streams from hydrotreating units [51]. The separation of liquid hydrocarbons such as Cs-Cs paraffins (or olefins) and aromatics such as benzene, toluene and xylene is also an attractive application of zeolite membranes. These separations are performed at relatively high temperature and/or low partial pressure (typically 10 % hydrocarbon in He carrier) so that both the feed and permeate sides are in the gaseous state. Highly selective separations were obtained between straight chains and branched alkanes with MFI membranes [16]. The linear hydrocarbons fit well in the zeolite channels, although the branched hydrocarbons have lower adsorption and much lower mobility. The model mixture is «-hexane/2,2-dimethylbutane, but more complex ternary mixtures were also studied [51]. Finally the separation of the three xylene isomers is of high industrial importance because the p-xylene is used to make terephtalic acid and the o-xylene is used for the production of phtalic anhydride. High selectivities (as high as 300) can be obtained for this separation with MFI membranes [51]. Depending on the considered gas, membrane thickness, crystal orientation and experimental conditions, the gas permeance through zeolite membranes typically lies in the range 10"5-10"9 mol.rnV.Pa" 1 . 6.3. Membrane reactors The concept of combining membranes and reactors is being explored in various configurations, which can be classified in three groups, related to the role of the membrane in the process [136]. The membrane can act as : a) an extractor : the removal of product(s) increases the reaction conversion by shifting the reaction equilibrium, b) a distributor : the controlled addition of reactant(s) limits side reactions, c) an active contactor: the controlled diffusion of reactants to the catalyst can lead to an engineered catalytic reaction zone. Typical applications of zeolite membranes in reactors include i) conversion enhancement either by equilibrium displacement (product removal) or by removal of catalyst poisons/ inhibitors and ii) selectivity enhancement either by control of residence time or by control of reactant traffic. A large number of examples are reported and discussed in [49,50,52]. Several of them are reported in Table 3. The use of a zeolite membrane as a distributor for a reactant has been attempted for the partial oxidation of alkanes such as propane to propene [137], or nbutane to maleic anhydride [138]. Limited performances were obtained because the backdiffusion of the alkane is hardly controllable with this type of microporous membrane [139]. Table 3 Examples of applications of zeolite membranes in membrane reactors. Preferential References Type of Reaction Type of zeolite permeating species membrane CONVERSION ENHANCEMEN[T- Product removal Dehydrogenation of /-butane MFI [140] H2 of ethylbenzene MFI, Fe-MFI [141] H2 Syngas production Na-A, Silicalite-1 [6,142] H2 Metathesis of C3H6 Silicalite-1 trans-2-butene [143] Esterification H2O [144,145] Na-A, T, H-ZSM-5 Ammonia synthesis [1471 ZSM-5 NH3

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Table 3 (continued) Examples of applications of zeolite membranes in membrane reactors References Preferential Type of Reaction Type of zeolite membrane permeating species Fermentation Silicalite-1 ethanol ri49i SELECTIVITY ENHANCEMENT- Control of residence time Pt-Y [21,150] H2 + CO Oxidation of CO [151] z'-butene oligomerization Beta (-butene Conversion of methanol to olefins ZSM-5 methanol [152] [153] Ti-silicalite pollutants Photocatalytic oxidation of trichloroethylene SELECTIVITY ENHANCEMENT- Control of reactant traffic [154,155] Competitive hydrogenation of 1 - Silicalite on catalysi: reactant heptene and 3,3-dimethyl-l-butene pellets (heptene) Isomerization of paraffins Silicalite-1 unconverted [156] reactants Isomerization of xylene is also a very attractive application for MFI zeolite membranes (Fig. 10). The commercialisation of a new zeolite-based membrane is being planned by NGK Insulators for />-xylene production from other xylene isomers. p-xylene molecules which are smaller than those of m- or o-xylene, are sieved from the mixture using the membrane, thus cutting significantly the production costs [157].

Fig. 10. Schematic representation of the p-xylene selective extraction through a MFI membrane from an isomerization reactor.

The concept of zeolite membrane encapsulated catalyst (ZMEC) is an original way to use zeolite membranes in catalytic reactors [158,159]. The concept of coating catalyst particles with a selective zeolite layer can reveal useful for either increasing reactant selectivity (e.g. selective hydrogenation of linear molecules) or product selectivity (e.g. amination of methanol). It also protects the catalyst from poisoning. Polymer-zeolite composite membranes are also studied in rectors, either as interphase contactors in liquid phase oxidation processes [65] or for improving the properties of Nafion in fuel cells applications [66]. Finally reaction and separation microdevices based on zeolite membranes are now also seriously considered [160]. Lab-on-a-chip (scaled-down analog of a chemical processing plant) is becoming a familiar concept to express the miniaturization of chemical and biological analyses, with a drastic reduction of reactant consumption [161]. According to the large number

153 of studies emerging in this field in the recent literature [49], microseparators and microreactors with zeolite membranes will certainly develop very rapidly during the coming years. 6.4. Sensors Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantileverbased sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitated capacitors for sensing w-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 7. INDUSTRIAL DEVELOPMENTS AND LIMITATIONS Industrial applications of zeolite membranes can be considered only for separations where they offer some unique advantage in terms of flux, selectivity, or thermal and chemical stability. The very high fluxes obtained with LTA membranes (typically 100 times higher than those obtained with a polyacrylonitrile membrane at the same H^O/alcohol selectivity) explain the rapid expansion of this type of application at the end of the 90s' [8]. According to a recent conference given by Prof. Kita [162], the classical synthesis method currently used by Mitsui allows to produce about 250 zeolite membranes per day. Both the LTA and T types (Na & K) membranes are now commercial and more than 80 pervaporation and vapor permeation plants are operating in Japan for the dehydration of organic liquids [163]. A typical pervaporation system, similar to the one described in [8], is shown in Fig. 11. One of the most recent applications concerns the production of fuel ethanol from cellulosic biomass by a vapour permeation/ pervaporation combined process. The required heat is only 1 200 kcal per liter of product, i.e. half of that of the classical process. Mitsui has recently installed a bio-ethanol pilot plant based on tubular LTA membranes in Brazil (3 000 liters/day) and a plant with 30 000 liters/day has been erected in India. The operating temperature is 130 °C, the feed is 93 % ethanol, the permeate is water and the membrane selectivity is 10 000. We have also to mention the very recent European alliance of The Smart Chemical Company Ltd (Great-Britain) and Inocermic GmbH (Germany) for the production of multichannel NaA zeolite membrane modules with 5.8 m . The other considered industrial applications concern MFI membranes for molecular sieving (cut-off at 5.5 A). In fact, only MFI and a few other high-Si types are really shapeselective because of their reduced number of defects. NGK Insulators announced the industrial development of both MFI membranes for xylene separation [157] and DDR-type membranes for CO2/CH4 separation [11]. One of the main problems limiting the industrial development of zeolite membranes is their low fluxes. Typically a few m3.m"2.h"'.bar"' are achieved by most of the zeolite-based membranes whose thickness has to be sufficient (currently in the range 0.5-5 urn), in order to limit the influence of non-zeolite pores. An other problem is related to the chemical stability of membranes in both acidic (Al is leached) and basic media (Si is leached). The recent developments of both MOR [31] and ETS-10 [31,32] membranes will contribute to solve this latter problem.

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A number of technico-economic evaluations clearly show the need for the development on industrial scale of large surface area and high quality MFI membranes [164]. In parallel, research and development of new families of efficient membranes are also required. In spite of their poor hydrothermal stability, silicate and aluminosilicate mesoporous membrane materials like MCM-41 or MCM-48, are attracting much attention in industry because they give access to large calibrated and ordered pore sizes (2-10 nm). These large pores are highly favourable for surface chemistry (grafting of organic, inorganic and metallo-organic molecules, incorporation of heteroelements or transition metals in the framework structure, or by surface modification). This type of zeo-type materials are considered as potentially attractive for a large number of applications [41], including catalysis [165].

Fig. 11. Pervaporation system from Mitsui & Co, a) pilot containing 8 modules with 10 m2 of zeolite membrane in each of them, b) Modular unit containing 10 m2 of zeolite membrane. (From http://www.mitsui.com/Dallas/zeolite. This web site is now closed).

The cost of the support element and the lengthy steps involved in zeolite film synthesis makes zeolite membranes very expensive compared to the well established polymeric membranes. MFI membranes are relatively expensive and their price (about 5.000 €/m2) is typically due by 50% to the support and by 50 % to chemicals. Consequently the development of template-free synthesis on cheaper supports (multi-channels, ceramic foils/plates, capillaries, hollow fibers) is seriously considered, in parallel with the development of continuous, automatic, reliable, and reproducible production methods. For high temperature applications of zeolite membranes the problem of module sealing has also to be considered. Gas-tight sealing of membranes at low temperature is classically performed with polymeric o-rings including Viton®, Kalrez®, although Teflon®, graphite, or

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water-cooled polymer rings have to be used at high temperature. The full ceramic module currently developed by CEPAration-TNO is a smart attractive concept for high temperature applications, although studies of module reliability under extreme temperature cycling and vibrations will be required. 8. CONCLUSION Zeolite membranes have clearly demonstrated a high efficiency to carry out separations that are not attainable by conventional methods. Even if our understanding of the formation of zeolite structures on porous supports is still poor, the first zeolite membranes (NaA) are already commercialized at large scale for pervaporation applications. A new generation of affordable membranes with improved quality, higher flux and long term stability will be soon available, thank to the increasing interest of industry in this field, for applications in liquid, vapor and gas media. Nevertheless, the control and fine tuning of the properties of the zeolitecontaining membrane configurations remain a challenge. Indeed, despite the use of improved methodology, it is still often difficult to obtain all types of membrane structures with consistent and predictable properties at large scale. Great improvements have been obtained during the last ten years concerning the quality and performance (permeability, selectivity, stability, reproducibility) of zeolite membranes. A number of synthesis process variables have been clearly identified which control the final characteristics of the membranes (crystal size, shape, orientation, membrane thickness and infiltration). This effort, involving a systematic study of process variables, must be maintained to further increase membrane reproducibility and performance. In parallel, the progress of zeolite membrane technology for industrial gas separation applications still needs the development of a generally applicable, « dynamic » and certified quality control method. The possibility to repair non-zeolite pores in already formed membranes could also compensate the current lack of reproducibility of large scale zeolite membranes. The development of low cost, reproducible and simple synthesis methods adapted to large scale synthesis on low cost supports will be a necessary breakthrough for larger industrial development. Finally zeolite membranes with tunable permeation properties will probably be the next important challenge for zeolite membrane researchers in the coming years.

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High-throughput experiments for synthesis and applications of zeolites F. Schiith Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Miilheim, Germany 1. INTRODUCTION 2. SYNTHESIS 3. CHARACTERIZATION 3.1. X-ray diffraction 3.2 X-ray fluorescence 3.3 IR analysis 3.4 Gas adsorption 3.5 Miscellaneous techniques 4. CATALYTIC TESTING 4.1. Reactors 4.2. High-throughput studies of the performance of zeolite catalysts 5. LIBRARY DESIGN AND DATA ANALYSIS 6. PERSPECTIVES REFERENCES 1. INTRODUCTION High-throughput experimentation has entered materials science and - with a slight delay catalysis in the middle of the nineties, when the concepts originally developed in the pharmaceutical industry for drug discovery were adapted and applied in these fields [1], although some of the initial ideas date back to the 1970s with the so-called compositional spread approach [2]. High-throughput experimentation is a concerted approach, comprising several different elements which need to be integrated in the high-throughput workflow. These are (i) the automated, often parallel synthesis of solids, (ii) the parallel or fast sequential testing of the relevant properties of the solids, (iii) data management to integrate all the data and to extract knowledge, and (iv) often a robotics environment. In addition, to some extent high-throughput characterization technology is necessary, if a correlation of physicochemical parameters of the synthesized solids with the performance is desired. All scientists active in the field of zeolites know that often wide ranges of parameters and synthesis conditions have to be screened in order to optimize the synthesis of a zeolite or to discover novel materials. The same holds for the evaluation of performance data of zeolites in various applications. Zeolite science and technology is therefore a field where there is a high need to accelerate experimentation and to achieve a higher throughput. Thus, relatively early on in the development of high-throughput experimentation, efforts were made to use the

162 concepts in the zeolite field. The state-of-the-art in this field, possible directions of future work, and the limitation as far as they can be perceived at this stage will be discussed in the following. 2. SYNTHESIS Zeolite synthesis is one of the fields in solid state chemistry, where predictability of the outcome of the synthesis is rather low. It is in most cases not clear, why specific structures do form with certain template molecules and under certain synthesis conditions. Some success has been reached in a more rational zeolite design [3], either by computational design of specific templates or by judicious addition of heteroatoms to induce the formation of certain ring sizes in the structures. However, empirical studies to optimize the formation of certain structures, or systematic exploration of the parameter space often still remains the method of choice in zeolite synthesis. Due to the number of experiments necessary in such studies which take typically times of several hours to several days, there is a strong incentive to work on acceleration of synthesis experiments in zeolite science. On the other hand, there are also major obstacles to overcome in such an endeavour. Zeolite syntheses are often carried out at elevated pressure, reaction media are typically corrosive, precursor solutions and the synthesis gel itself may be highly viscous and difficult to handle, the synthesis is very sensitive to preparation conditions, such as sequence of reagent addition, stirring (difficult for small volumes) may be necessary for some formulations, aging conditions may differ from batch to batch, if automated sequential preparation is chosen, the work-up involving many steps is complex, and the resulting materials are often not phase-pure and difficult to characterize. Nevertheless, in spite of these problems some of the earliest examples for the synthesis of catalytically relevant solids in parallelized and - some points -automated equipment were reported for zeolites [4]. The first report on parallelized zeolite synthesis was published by the Sintef group [5]. They have used a 100 well autoclave which in the simplest case was realized as a Teflon block, but also a metal frame with Teflon linings is possible (Fig. 1). Since only 0.5 ml of synthesis solution was present in each well, the pressures could be handled without any metal reinforcement in this case. For dispensing of the precursor solutions a commercial pipetting robot was chosen. The autoclave block was sufficiently small to be fitted onto the working stage of typical robots. In order to parallelize even more, the blocks could be stacked on top of each other with stainless steel plates and Teflon seals in between different blocks. For product isolation, top and bottom covers are removed from the block and the autoclave content was directly flushed onto filter paper, washed with water and then removed for further Fig. 1. Typical multiautoclave block with Teflon inserts as used by the Sintef team, showing the stacking (reprinted with permission from ref. [51).

analysis. This is probably the biggest disadvantage of system used in this publication, since the isolation and subsequent XRD of the samples is very time consuming and takes away some of the advantages which the system provides. Nevertheless, this

the

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equipment was used for a systematic investigation of the synthesis field Li2O - CS2O (TMA)2O - Na2O - A12O3 - SiO2 (TMA = tetramethylammonium). Analcime was formed from all cesium containing gels, while five different zeolitic materials, which all had been obtained before in isolated experiments resulted in the Li2O - Na2O part of the synthesis space diagram. The problem of the time consuming analysis was addressed in the work of Maier and coworkers [6]. They crystallized minute amounts of zeolite samples (for the example of TS-1 syntheses) in wells formed by a silicon wafer as bottom plate on which a Teflon block with drilled holes was placed. Each bore formed the wall of a separate reactor chamber. The system was sealed by a top plate made of stainless steel. The amounts of reaction mixture are only several microliters. Reaction solution is removed by placing porous rods into each well, thus removing the supernatant (and possibly also some solid) by capillary forces. The innovative aspect of this publication was the XRD analysis which was performed directly on the wafer without manual intervention. For this purpose a Bruker AXS GADDS X-ray diffraction system was used which allows focussing the X-ray beam by Gobel mirrors onto a small spot with a size as low as 50 urn diameter. Very satisfactory signal to noise ratio could be obtained by this kind of analysis, and identification of the TS-1 phase which had been synthesized with various different templates was possible without any problems. For screening experiments such equipment is certainly very valuable, since it allows synthesis and analysis of materials in an automated fashion in relatively short periods of time. A drawback of the system is the fact that agitation of the reaction mixture is quite difficult, a problem which is not as severe as in conventional autoclaves, since the wells are very small and thus diffusion can provide some homogenization. The use of the silicon wafer as bottom plate could also become a problem for more basic reaction media. In the study on TS-1 formation, no corrosion of the substrate had been observed. However, this should be a solvable obstacle, either by replacing silicon by another material or by coating the silicon wafer. Methods combining the two approaches to some extent were used by Bein and coworkers [7]. They designed a parallel autoclave block with wells drilled into a Teflon block with a size allowing solution volumes of some 100 ul. For product isolation, a filtration module could be integrated into the system after the reaction. In a later embodiment of the multiclave system, a centrifuge setup had been developed which allows separation and washing of the solid. The final product in both cases is collected Fig. 2. X R D patterns for samples on filter paper and is either analyzed manually or isolated from different wells of a sequentially in an automated STOE transmission multiclave. Samples were prepared with XRD setup [8, 9]. identical gel composition expected to Later on, a study was published by Lai et al. [10], in give AIPO4-5, which formed in all which the approach of Klein et al. [6] was essentially cases, demonstrating the reproducibility of the procedure (reprinted with inverted. Also in this study a library of zeolite permission from ref. 11). samples was crystallized onto one substrate, an

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alumina wafer. However, the setup was integrated in a conventional Parr reactor. The wells of the multiclave were drilled into a Teflon block fitting into the autoclave, each well having a volume of about 40 ul. The top was sealed with the alumina wafer on which an MFI seed layer had been deposited before. The autoclave was then inverted and the crystallization started by heating the autoclave. The product was found to be crystallized onto the alumina wafer. The system was used for the exploration of the synthesis space for MFI type films synthesized in absence of an organic structure directing agent. In the initial phase at the end of the 1990s, parallelized zeolite synthesis was carried out more in form of feasibility studies. The target in the published papers was normally not the discovery of novel zeolites, but to show that the systems did work. Thus, typically known zeolite systems were explored in some more detail as had been done previously in single publications, but no novel discoveries were made. One of the broadest screening studies in the initial phase with the goal of discovering novel materials was performed by Akporiaye et al. [11], primarily directed at the synthesis of aluminophosphates. In this study several novel layered materials were discovered, which are probably formed by the intercalation of the different organic template molecules between aluminophosphate layers. One interesting aspect in this study was the check of the reproducibility of the synthesis of a porous inorganic material in such a multiclave setup. As an example, the authors show a series of diffraction patterns obtained for a set of replicated syntheses in various positions in the autoclave block. AIPO4-5 with rather similar diffraction patterns was formed in all wells, demonstrating the reproducibility of the method (Fig. 2). This may be different for a phase with a more narrow range of synthesis conditions under which it forms, but the experiments nevertheless prove that one can obtain fairly reliable data in such a system. Very interesting novel zinc phosphate structures were discovered in the group of Yu and Xu [12] using a combinatorial approach. The setup resembles the one initially described by Akporiaye [5] in that the well are formed in a Teflon block, with each well having a volume of about 800 ul. Product analysis was done with an automated powder XRD setup after the samples had been transferred to a sample holder. Solids were formed from zinc solution, phosphoric acid and N,N'-dimethylpiperazine as precursors. The most interesting material formed was a zinc phosphate with 16-membered ring channels in one direction, but two other structures with smaller pore openings were also described in this publication. Recently, studies directed at the discovery of phosphonate organic/inorganic hybrid materials were published [9], in which several novel zinc phosphonates were described, in addition to a novel structure which could be realized with various divalent cations and the zwitterion [(HO3PCH2)2(H)N(CH2)4N(H)(CH2PO3H)2]2' in which each phosphonate group and each N-atom is protonated. The MO6 and PO3C polyhedra form eight-membered rings within layers of the structure. The layers are linked by the organic groups which effectively block the eight-membered rings. This explorative study was performed in two steps which is rather typical for a high-throughput program, starting with an exploratory library in order to investigate which cations gave (possibly) microporous solids. The search was focussed in a subsequent step to optimize the synthesis and to create crystals with a size sufficient for structure solution. As successful as these studies were in showing the general possibility of high-throughput synthesis of microporous materials in minute amounts, one should not underestimate the problems associated with such an approach. Especially if the volume of the wells decreases to the range of 10 ul or below, there are additional effects which need to be considered. These problems are probably one reason why most high-throughput discovery studies were rather directed at the formation of metal phosphates or phosphonates instead of at classical zeolites.

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Precursor solutions for the phosphates are much easier to handle, since the pH is not extreme, all components are liquid, and typically no two phase systems which are difficult to mix are formed, as could happen with alkoxysilane precursors. The additional problems encountered when doing zeolite synthesis on miniature scale have been discussed by Newsam et al. [4], and include: inhomogeneities of precursor mixtures, dispensing of ul of fluids of various different viscosities and vapour pressures. mixing of initial solutions or gels, mixing of multiclave well content under reaction conditions, evaporative fluid losses, oxidative degradation, such as of organic additives, contamination from debris, dirt or chemical attack on multiclave components. Several of these problems can also occur during zeolite synthesis on a larger scale. However, due to the much larger volumes involved, their effect is typically negligible, which is not the case for syntheses carried out on the ul scale. There are thus groups who rely on more conventional autoclaves also for parallelized studies, but the transition to conventional zeolite synthesis are not sharply defined. At hte-company an autoclave system is used which allows larger sample volumes (Fig. 3). 16 inserts are used in one autoclave housing. These inserts can be filled by a dispensing robot and simultaneously placed in a heating unit. Centrifugation in a module, which is adapted to the autoclave set, is used for product isolation. From the centrifuge module the product is transferred to an XRD sample holder where it can automatically be analyzed.

Fig. 3. Autoclave setup for zeolite synthesis. Synthesis module in the disassembled stage and the assembled state (left). Centrifugation module (right). For centrifugation a filter paper between two Teflon elements is placed on top of the 16 Teflon inserts and the system is placed upside down in the centrifuge to remove the liquid (courtesy of hte Aktiengesellschaft, Heidelberg). Even more strongly related with conventional zeolite synthesis are systems consisting of several individual autoclaves. Also in conventional zeolite synthesis, researchers typically

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place several autoclaves in one oven. Thus, the multiclave system described by Tagliabue et al. [13] in which up to 39 15 ml autoclaves are mounted on one oscillating rack to provide agitation is rather closely related to conventional zeolite synthesis, especially since also filling and work-up has to be done as in conventional zeolite synthesis. Nevertheless, if syntheses are carried out on this scale, results are easier to transfer to technical dimensions. 3. CHARACTERIZATION In the preceding chapter it had already been discussed that it is less the synthesis itself which may be the bottleneck in high-throughput zeolite science but rather the analysis of the solids formed in a high-throughput program. There are several standard characterization techniques which are typically employed to characterize zeolitic materials. These include powder XRD for phase identification, X-ray fluorescence analysis (XRF) or atomic absorption spectrometry to analyze elemental composition, sorption analysis to study the pore system, IR-spectroscopy, typically using adsorbed probe molecules to characterize the acid sites, NMR spectroscopy and many others. For some of these techniques parallelized solutions have been developed and described in the literature, other properties are more difficult to assess in a parallelized or even a fast sequential fashion. However, when considering the development of parallel methods for sample characterization, one has to ask oneself what the purpose of the characterization might be. Often solids are characterized by physico-chemical methods in order to draw conclusions with respect to potential catalytic activity or other desired performance. Only such samples having the proper physico-chemical parameters would then be selected for in-depth catalytic studies. This is done because a catalytic experiment used to involve much more efforts than physicochemical characterization of solids. However, with the availability of high-throughput reactors for catalytic testing this is no longer necessarily the best approach. If catalytic testing takes less time than the analysis, the order may be reversed. After synthesis, firstly all samples prepared could be catalytically evaluated, and only those showing promising or unusual catalytic performance would then be characterized in detail, for instance to establish propertyactivity correlations. If such an approach is chosen, demands for high-throughput characterization are substantially less severe than if every sample is analyzed in detail. However, the exact nature of a specific project will determine, which sequence is the more appropriate one, and so in general there is some need for the development of suitable highthroughput analysis techniques for solid samples and in particular for zeolites. In the following the approaches which have been published so far will be discussed in more detail. 3.1. X-ray diffraction XRD has already been briefly addressed in the preceding section in connection with zeolite synthesis, since this is the standard technique to determine whether a microporous or at least possibly microporous material has formed. Since the existence of channels allowing access of molecules into the structure requires a certain size of the unit cell, a zeolitic material will typically have low angle reflections below around 10° (2©) for Cu-Ka radiation. A very simple way of quickly analyzing a library for the possible presence of zeolitic material is to scan only a narrow angle range between 3° and 10° (2©) and only record the full range for such materials having at least one reflection in this range. This can speed up the analysis by a factor of five to ten, but still requires manual intervention, either for each sample, or when a sample changer is used, at least during isolation of the synthesis product or filling of the sample changer. For larger library sizes, this is thus not a practical solution.

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Thus, nowadays mostly diffractometers equipped with microdiffraction possibilities and xy-sample stages are used for automated collection of diffraction patterns. The key element in these setups is a Gobel-mirror [14], a parabolical multilayer device which allows the generation of a precisely collimated beam which can be focussed to spot sizes below 100 um. This makes it possible to analyze many different spots on a single substrate with reasonable signal to noise ratio as shown in the initial work by Klein et al. [6]. With a spatially resolving detector, recording of a diffraction pattern is possible within a few minutes. However, also less sophisticated methods are useful, such as covering all but one spot with lead foil which allows the use of an unfocussed beam [10]. However, even if samples can be analyzed in an automatic fashion by XRD, automated phase analysis by computer based methods is still in its infancy. Typically, XRD patterns have to be inspected manually, and especially if phase mixtures are formed, as it often happens in zeolite chemistry, the analysis of the patterns can become the bottleneck. Work is in progress in different laboratories to solve this problem, but no suitable solution has been published yet. 3.2. X-ray fluorescence XRF is one of the standard techniques used for the analysis of the elemental composition of zeolites. However, normally sample preparation is rather complicated, involving the preparation of the zeolite in a ceramic flux or in other matrices in order to have a standard sample geometry and size which is a requirement for precise analysis. This kind of sample preparation is not possible if the analysis is to be performed in high-throughput mode. An alternative method to conventional XRF makes use of focussing the X-ray radiation as in the case of diffraction, however, not the diffracted X-rays are collected, but the element specific fluorescence. Since the matrices and the sample geometries are rather ill-defined in this measurement mode, analysis is more of a semiquantitative character than quantitative. Nevertheless, for screening purposes this is a quite valuable method. So far this technique has not been used for the analysis of zeolites, but rather for libraries of sol-gel derived materials [15], or beads obtained from a split-and-pool protocol [16,17]. It makes use of different micro-XRF analyzers which focus the X-ray beam by a multi capillary system to a spot of around 50 urn diameter, as for instance in the Eagle II uprobe from Roentgenanalytik [15,16], or the ThermoNORAN Omicron system [17]. One can expect that analysis of zeolites will be possible with about the same level of precision as reached for the sol-gel derived libraries. However, this is not sufficient to determine Si/Alratios reliably, especially since it is typically not known whether the sample is fully crystalline when analyzed by XRD in high-throughput mode. Nevertheless, some initial information on elemental composition can be achieved by micro-XRF. 3.3. IR-analysis IR spectroscopy is one of the most useful techniques for the characterization of zeolitic materials. It provides information on the nature of OH-groups in the materials and on the framework vibrations. Even more useful is the IR analysis of adsorbed species. Basic probe molecules, such as pyridine, ammonia, or benzene, allow the analysis of acidic sites. CO adsorbed at low temperature also helps to analyze acidic sites, but can also be used for the analysis of noble metal particles in zeolites. In principle, several samples could be analyzed simultaneously, using an xy-table with which the zeolitic materials could be positioned in the focus of the IR-beam. However, one problem is encountered using such procedures: Typically, background correction is necessary,

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if adsorbed species are to be analyzed in order to detect small changes in the spectra. Normally, first a spectrum of the activated zeolite is recorded, then the probe molecule is admitted, another spectrum is recorded, and finally the spectrum of the activated zeolite is subtracted. If several samples shall be analyzed simultaneously, this procedure is not as straightforward, since for the background subtraction to work, one has to find exactly the same position for the recording of the background spectrum and the spectrum with the adsorbed probe molecule. We have used this method both in a conventional FTIR setup with a measurement cell which could be evacuated as well as in an FTIR microspectrometer. In both cases we have not been able to record satisfactory spectra in a consistent way. Such a system is extremely sensitive to the conditions of the measurement, and in practice, it is probably too difficult to realize such an approach for routine operation. In addition, superior alternative technology became available over the last years, i.e. focal plane array detector (FPA) systems. These systems were originally developed for imaging in different frequency ranges, mostly for military and astronomical purposes. Such detectors consist of an array of small IR detectors, typically arranged in a rectangular pattern, and a system often used consists of 64x64, i.e. 4096 pixels altogether. This array is part of an imaging setup; thus, radiation of each part of the area to be imaged falls on a defined part of the detector array. Readout of the detector elements can be coupled to the modulation of an FTIR spectrometer, so that simultaneously 4096 spectra are obtained, which allow rather high spatial resolution. This means that experiments which are typically carried out for one sample using IR analysis can now be expanded to encompass a number of samples which have to be imaged on the FPA detector. Such FPA detector setups were first used by the group of Lauterbach for the parallel characterization of solid samples and the product gas stream from catalytic reactors [18,19]. These authors also changed the mode of operation from the previously used step-scan mode to the rapid scan mode which made it possible to even record transient processes [20,21]. The group of Lauterbach was also the first to apply FPA IR spectroscopy to a problem from zeolite science, even if it was only in form of a feasibility study. They investigated the adsorption of CO on Cu-ZSM-5 and on Pt/SiC>2 in order to prove that it would be possible to detect the absorption bands of adsorbed species [19]. Since experiments were carried out at room temperature, bands for CO on the Cu-ZSM-5 would be expected to have very low intensity, and indeed, no spectra for CO on this solid were shown. The band of CO on the noble metal, on the other hand, could clearly be detected without problems, and a signal-tonoise ratio not much different from that obtained for a conventional experiment. We have developed a sorption cell for use in connection with a FPA IR setup for the analysis of eight samples in parallel [22,23]. Fig. 4 shows a photograph of the cell. It is integrated into an imaging mirror system which uses the beam coming out of the external port of an FTIR spectrometer. In order to facilitate sample handling, the samples are placed on a horizontal CaF2 window, IR light passes through the cell from top to bottom. The whole cell can be evacuated and heated, so that sample activation is possible. Several problems are encountered in carrying out such parallelized experiments, (i) The samples have been placed in small apertures in a bigger metal disk which serves as spacer between the two CaF2 windows. Otherwise, the sample will move during evacuation of the cell so that cross contamination between different samples could occur, or sample could even be sucked into the pump, (ii) Temperature homogeneity of the cell can be a problem. Since heating has to be provided from the circumference of the cell, radial gradients may exist, (iii) Most severe is the fact that scattering losses are more severe than in conventional experimentation. All scientists experienced in the analysis of zeolites are aware of often

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strongly reduced intensity and sloping background due to scattering losses. These losses are an even bigger problem in a parallelized cell, since the necessity to integrate the cell in the imaging system makes the beam path much longer so that even slightly divergent rays caused by scattering are not captured by the mirrors and do not reach the FPA detector. This is most severe for samples with particle sizes in the range of the wavelength of the radiation, and for some samples with unfavourable shapes, such as mordenite with elongated shapes, we did not succeed in recording useful spectra. The problem could be reduced to some extent by pressing the material into small wafers as in conventional IR analysis of zeolitic materials, typically with a thickness of about 10 mg/cm2. Producing wafers with defined thickness also helped in semi-quantitative analysis, and compared to conventional experiments the wafer fragments used can be much smaller which facilitates handling. So even taking into account the problems with parallel IR analysis of zeolites, for many samples the system proved to be useful and analysis times can be strongly reduced by such systems.

Fig. 4. Schematic drawing of the vacuum cell for parallelized IR analysis with adsorbed gases (top) and photograph of the cell (bottom). The light comes from the front, hits the top mirror, passes the cell and is reflected towards the detector by the bottom mirror which is turned away from the viewer. Upper left insert in the photo shows the distance element with the eight apertures in which the samples are placed (from ref. [22]).

One of the most tedious experiments in zeolite analysis is the assessment of acidity using pyridine as probe molecule. Activation, pumping and equilibration times are very time consuming, and thus such experiments typically take up to a day per sample, especially, if

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thermal desorption experiments are included. With the setup shown above, we have analyzed the acidity of different zeolite samples, such as H-ZSM-5 or H-Y. The spectra were compared to those recorded with a conventional spectrometer and overall were quite similar (Fig. 5). The small deviations are probably caused by small differences in the temperature measurement in the parallel and the conventional cell. If one uses published extinction coefficients, the concentration of acidic sites determined with the parallel and the conventional setup agreed within a few percent.

Fig. 5. IR spectra of pyridine adsorbed on H-ZSM-5 recorded with a conventional setup (top trace) and in the parallelized cell (bottom trace). Samples had been heated to 250°C for 15 minutes before recording the spectra.

The system also proved to be useful for the analysis of CO adsorbed on noble metal particles in different zeolites [23]. Series of samples could be analyzed in relatively short periods of time, and it could be estimated, that the parallel setup could be used for samples with a platinum content as low as 0.1 % at an acceptable signal-to-noise ratio. One can expect that with the rapid scan capabilities implemented by Lauterbach and coworkers even kinetics of fast adsorption and desorption processes could be monitored in a parallel fashion. 3.4. Gas adsorption It would be extremely attractive to also parallelize gas adsorption measurements. There are commercial multiport systems available, but the savings in equipment is rather moderate for these systems. One may think about using FPA IR setups for parallelized gas adsorption, for instance by adsorbing hydrocarbons at their boiling point and analyzing the amount adsorbed via the integral absorption of selected IR bands. We have attempted to use this approach, but extinction coefficient of conventional hydrocarbons are so strongly coverage and pore size dependent, that not even semi-quantitative analysis was possible. Similar problems are expected for other vibrational spectroscopies, since the interatomic potentials are modified by the adsorption process. Thus, a truly parallelized adsorption measurement is still not in sight. 3.5. Miscellaneous techniques: temporal analysis of products (TAP) and temperature programmed desorption (TPD) For two other methods often used for the characterization of zeolitic materials, parallelized implementations have been described in the literature, i.e. TAP studies and TPD experiments. Also these experiments, as is the case for IR measurements with probe molecules, need a long time for sample loading, pumping, pre-treatment and conditioning steps, while the actual experiment is relatively short. These experiments can therefore

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tremendously be sped up, if the time consuming steps are done in parallel for several samples, while the TAP or TPD experiment itself is analyzed for each sample separately. TPD of basic probe molecules is a method which is often used for the analysis of zeolitic acidity [24]. In a parallelized implementation [25] of this technique, the samples to be studied were placed in a parallel channel reactor-body built analogous to the one described by Hoffmann et al. [26]. By means of a multiport valve, the effluent from each channel could be switched to a mass spectrometer which was used for the analysis of the desorbed gas. In a typical experiment, after conditioning of the samples and adsorbing ammonia, a temperature ramp was started and in a sequential manner the effluent of each catalyst was fed into the mass spectrometer. Flushing and analysis times per channel were 8 s, so that all 10 samples could be analyzed in somewhat more than one minute. This proved to be sufficient to obtain well resolved TPD curves at a heating rate of 10 K/min. The recorded data corresponded well to curves measured with conventional setups. TAP reactor systems are used to obtain transient kinetic data. As in TPD and IRanalyzed adsorption of probe molecules, loading of the system and pre-treatment of the samples is the most time consuming step. In order to solve this problem, van Veen et al. [27] integrated a multisample holder into a TAP setup which is capable of holding 12 samples. The holder is connected to a pulse valve manifold, which allows addressing sequentially each of the samples with a gas pulse. As a case study, 10 different zeolite samples were loaded into the TAP system, and by pulsing 50/50 mixtures of neon and ethane, heats of adsorption of ethane on the different zeolites could be determined. The expected trends in the heats of adsorption (decreasing enthalpy with increasing Si/Al ratio) were found using the TAP system. However, the absolute figures were substantially lower than those measured with a conventional setup, which may be related to the modelling of the signal, in which diffusion and adsorption were lumped together in the study. 4. CATALYTIC TESTING One of the most important steps in high-throughput development of catalysts is the catalytic test itself. For catalytic testing, typically stage I (or "discovery") and stage II ("optimization") systems are discriminated [28]. In a stage I project, typically little is known about the catalytic system, screening is very broad and massively parallelized, the depth of information is rather low, the reaction conditions are often far away from technical conditions, and often the format of the reactor is determined by the format of the analytical assay. This allows a very high sample throughput, but in most cases no exact analysis of the performance of a catalyst. In a stage II project, the reactor and the conditions approach conventional testing and technical processes, more detailed analysis is possible, typically selectivity information is obtained with a rather high level of precision, but on the expense of throughput. The degree of parallelization in stage I screening can reach several hundred samples, while stage II screening typically employs 16 - 64 channel reactors operated in fixed bed arrangement. Catalytic evaluation of zeolitic materials will in most cases require stage II testing, at least with the technology available today. Most zeolite catalyzed reactions are characterized by a complex product spectrum and the danger of deactivation by coking of the catalyst, which is an extremely important factor for practical application. Complex product spectra and changing activity with time are typically difficult to handle in stage I setups. The general discussion in the following will therefore focus on stage II techniques, stage I approaches are well covered in recent review articles [29,30]. Then some examples will be given, which also include stage I systems. However, they are presently mostly of academic interest. To the best

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of the knowledge of the author, by far the majority of the high-throughput projects in industry - not only those on zeolites - are carried out in stage II mode. 4.1. Reactors Most of the reactors used in stage II systems are variants of the ones described by Hoffmann et al. [26,31,32]. Some types of parallel reactors are shown in Fig. 6. They are characterized by a number of common features: common gas inlet for all channels; catalyst placed in cartridges which can be easily exchanged; outlet via capillaries which provide the major flow restriction in order to minimize the influence of differences in the packing of the catalyst beds; outlet capillaries mostly connected to a multiport valve for switching one channel to analytical systems, the others are vented; flexibility in the analytical system (GC, mass spectrometry, FTIR, etc.); in most cases sequential addressing of the channels; typically all channels integrated into a metal block which is heated.

Fig. 6. Different stage II parallel reactors. Left: 49 channel reactor for normal pressure and temperatures up to 450°C (from ref. 31). Middle: 49 channel reactor for pressures up to 50 bar and temperatures up to 260°C (from ref. 32). Right: Modular slice reactor which can be expanded in multiples of four (courtesy of hte Aktiengesellschaft).

Different implementations of such reactors may differ in detail. For instance, in some cases the flow to the analytical instrument is additionally controlled by a mass flow controller, so that at least during the analysis all catalysts are evaluated at exactly the same space velocity. In other setups, which will be discussed below, a truly parallelized analysis is integrated into the reactor, which allows the elimination of the multiport valve. The general features of such reactors, however, can be summarized as those of a multitubular fixed bed. Critical issues in operating such parallel reactors have been discussed by Moulijn et al. [33], who also had published one of the first prototypes of a parallel reactor, the so-called six-flow reactor [34]. As mentioned above, zeolite catalyzed reactions are complicated by the fact that deactivation easily occurs. This can only be fully solved by truly parallelized analysis, such as provided by the FPA IR systems. Alternatively, if sequential analysis is used, sequential start of the reaction in the different channels is an alternative, so that all catalysts are compared at the same time on stream. However, this requires rather complex valve schemes which are difficult to implement. As a simpler solution, the catalysts performance is analyzed several times in different cycles, so that for each catalyst an activity pattern over time is obtained.

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One has to keep in mind in these cases, though, that the times-on-stream at which performance is sampled, differ for each catalyst. One other problem, which is not unique for zeolites, but occurs in many zeolite catalyzed reactions, is the fact that gas volumes change during reaction. This makes it more difficult to ensure equal space velocity over all zeolites. If the flow restrictors, for instance capillaries, are placed in front of the catalyst bed, the influence of changing volumes can be minimized. If the restrictors are downstream of the catalyst, additional flow controlling elements have to be incorporated, such as a mass flow controller in the analysis line (to a first approximation not sensitive to changed volume flow). A last problem is the fact that in zeolite catalyzed reactions often high boiling products are formed, either as the desired product, but more often via unwanted side reactions. Such products can be deposited in the multiport valves and lead to clogging of individual ports and build-up of pressure. Most commercial multiport valves capable of handling high pressures cannot be heated to more than 80°C which is not sufficient for many problems. We are operating home built valves which can be used up to 100 bar pressure and a temperature of 200°C. Such systems, however, are complex and require a high level of maintenance. 4.2. High-throughput studies of the performance of zeolite catalysts A number of papers have already been published where high-throughput methods were used for the evaluation of the catalytic performance of zeolitic materials. However, so far these investigations can be considered more as feasibility studies than as targeted at real catalyst discovery. Questions of significance of the recorded data and reproducibility of the results were in the focus of the attention while for real discoveries the programs typically did not have sufficiently broad scope. A particularly thorough study of the influence of the mode of operation of parallel reactors, such as bed dilution, for the example of N2O decomposition over ZSM-5 based catalysts was published by Moulijn et al. [33].

Fig. 7. Main reactions in the aldol condensation of acetone. The different products could reliably be detected in the products stream of an 80 channel reactor filled with different zeolite catalysts by quantitative mass spectrometry (reproduced with kind permission from [35]).

Wang et al. [35] studied various different zeolite catalysts in the aldol condensation of acetone. This reaction is an interesting benchmark, since several different products can be

174

expected for different zeolite catalysts (Fig. 7). As a special feature of the reactor, which was of the general type as discussed above, a low melting alloy was used as heat transfer medium. This ensured a very homogeneous temperature distribution over the 80 channels of the reactor. Equal contact time for catalysts in all channels was ensured by measuring firstly the flow through each channel (deviations between channels around 10 %) and then adjusting the amount of catalyst accordingly. Changes in the gas volume due to reaction were not considered, however, acetone diluted in argon was used, and the system was operated at rather low conversions of around 10 %. Volume changes can therefore be neglected when catalysts are evaluated. Products were analyzed by quantitative mass spectrometry, which needed 8 seconds per channel (3 s measurement time and 5 s flushing times). Duplicates were tested in this system, and the selectivities were rather well reproduced with the exception of compounds formed at low conversion with a low selectivity. Such compounds are present in the gas at concentration well below 1 % and quantitative mass spectrometry becomes difficult in such cases. Trends in selectivities for different zeolites with acidic or basic sites agreed with previous reports. Further conclusions, however, were not drawn from this study. Jacobs et al. showed in a feasibility study that it is possible to obtain meaningful kinetic data using a high-throughput reactor [36]. Decane hydroisomerization was studied over Pt-loaded ultrastabilized Y type zeolites. Complete analysis of the complex product mixture could be achieved in 3.2 minutes using a multicapillary column, and the kinetic data obtained agreed well with those of a kinetic model derived for a conventional reactor. The group of Corma used a high-throughput reactor more as a tool to develop a deeper understanding of a catalytic system, i.e. the dealkylation/transalkalation of alkyl-aromatics over zeolite catalysts by screening a number of zeolites with different pore topology [37]. This is the state which should eventually be reached when high-throughput systems are standard laboratory tools, as they are becoming in a number of groups dealing with heterogeneous catalysis. It is possible to use them as discovery tools, screening many different samples and reaction conditions, but they can also strongly accelerate the understanding of catalytic processes by providing a broader database from which correlations between catalyst properties and performance can be established. The authors used a 16 channel reactor at a pressure of 25 bar and a temperature of 400°C and studied zeolite catalysts with different ring sizes and pore connectivity. A clear correlation between the dealkylation selectivity for ethylbenzene and the zeolite pore volume was observed. Dealkylation selectivity increased with decreasing pore volume. In addition, toluene, which is formed by a bimolecular reaction as a primary product, is also favoured by specific pore systems, with that of zeolite beta and ITQ-23 being the most favourable ones. This study showed, that the combination of high-throughput reactors with properly designed experiments can accelerate the understanding of the catalytic action, in this case of zeolites. The papers discussed so far used the general setup described under section 4.1, i.e. multiport valves for flow selection. However, as stated above, for reactions were deactivation may be a problem, truly parallelized analysis techniques are superior. Some techniques had been described in the earlier publications on high-throughput catalysis research, such as IR thermography [38,39] or Resonance Enhanced Multi Photon Ionization (REMPI) [40]. However, while the first technique does not provide selectivity information and is therefore not suitable for a stage II screening, the second one is rather specific and complex to use, and therefore does not seem to have been used in a real discovery program. Also other parallel methods, such as photoacoustic detection [41] and selective reagent detection [42] have not been used in connection with zeolite catalysts, since also these techniques are rather specific and not generally applicable.

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Somewhat related to both the REMPI method and the photoacoustic detection is laser induced fluorescence imaging (LIFI) analysis, since it also uses selective optical excitation of molecules in the gas phase leaving a catalyst bed. This technique has been developed as a parallel method to be used for the analysis of catalyst libraries and was also applied for zeolites by Su and Yeung [43]. Here a laser beam tuned to an absorption band of the molecule to be detected is passed through the gas streams leaving the reactor, and in those plumes where the molecule is present fluorescence light is emitted, which can be detected using an imaging system. The authors studied the acylation of benzene by phthalic anhydride over various different zeolites. Only zeolite beta was found to be active in this reaction, and fast deactivation processes on time scales of minutes were observed at high temperatures around 300°C. Picking up such fast deactivation was only possible due to the parallel mode of the analysis. However, the data recorded in this mode are not highly precise, and LIFI is more an analytical method for stage I screening. We have recently used FPA IR imaging to analyze the products from pentane hydroisomerization over different zeolite catalysts. The problem in the use of IR imaging to detect products of a catalytic reaction in a parallel flow-through cuvette, as described by Lauterbach and coworkers [18,19,20,21] and us [44] is the fact that degrees of parallelization going substantially beyond 16 creates a major plumbing problem in connecting the outlets of the parallel reactor channels to the parallel gas cuvette. In order to study pentane hydroisomerization catalysts in a higher integrated system we have developed a reflection setup [45]. Since in a reflection setup the reactor can directly be attached to the cuvette, much higher degrees of parallelization are possible, in our case we realized a 7x7 channel array, i.e. 49 channels. The gases leave the channels of the reactor through a pinhole in a gold coated metal piece which serves as the reflection mirror in the gas cuvette (Fig. 8).

Fig. 8. (a) Schematic drawing of the reflection cell for the FPA IR system. On the left is the reactor. Gas leaves the reactor through gold coated nozzles and directly enters the 49 gas cuvettes. The gold coated nozzles serve also as mirror for reflection of the IR beam. At the outlet of the individual cuvette tubes the gases mix, but the pathlength of the IR beam through the mixed gas is only about l/50th of the pathlength in the cuvette, so that the error is negligible, (b) FPA readout. The 7x7 pattern plots the integrated intensity. Upper spectrum is for one pixel, lower spectrum is the averaged spectrum for a group of pixels belonging to one channel. The ratio of the bands at around 1400 cm"1 gives information on the ratio of n-pentane and i-pentane. Reproduced with permission from ref. [45].

In an evaluation of the setup, pentane hydroisomerization at temperatures up to 300°C over different zeolite based catalysts Pt-MOR was carried out. The catalysts had been pretreated at different temperatures, ranging from 300°C to 800°C. As expected, pre-calcination at intermediate temperature between 500 and 600°C produced the most active catalysts. At lower temperatures autoreduction of platinum and generation of the H-form of the zeolite are

176

not complete, at too high temperatures, the platinum particles sinter and the zeolite framework is destroyed. Conversions could be determined with a relative error of 20%. This is not satisfactory for stage II screening, but for stage I screening it is more than sufficient. In addition, for other reactions much lower errors are expected, since the strongly overlapping bands of iso- and n-pentane made the product analysis very difficult. Finally, a non-catalytic high-throughput study of zeolitic materials shall be mentioned. Atienzar et al. [46] have developed a parallelized photoluminescence setup, which allows spatially and time resolved recording of photoluminescence spectra. This system was used to study various zeolites in which luminescent poly (p-phenylenevinylene) was encapsulated as well as zeolites with 2,4,6-triphenylpyrylium which can be used as photocatalysts. The highthroughput photoluminescence setup was used to quickly optimize the synthesis of the two ship-in-a-bottle systems. 5. LIBRARY DESIGN AND DATA ANALYSIS The more advanced a high-throughput program in catalysis becomes the more important are intelligent design of the libraries and efficient methods for "knowledge extraction" which are typically computer based. These fields are at present still in their infancy, and applications in zeolite science are scarce. Nevertheless, as these problems are highly relevant in highthroughput experimentation, they should at least briefly be addressed. When a library of catalysts for a specific problem shall be created, the question arises, how the members of the library should be selected. Established procedures are those based on factorial design rules or other methods to effectively screen a large parameter space. The only previous information such an approach needs is the range of parameters to be explored, i.e. knowledge about synthesis conditions which are likely to result in the formation of zeolites is necessary in synthetic work, or ideas what kinds of catalysts would perform well in a given transformation have to be supplied. If the boundaries of the search space are given, then well known design-of-experiment (DoE) tools can be used for optimal mapping of the search space. ouCii a idCtondi ucSigii Siuuy ui z-cuiiicS udS, IUI in&ttiiit;c, uccn pciiuiincu uy lagjuituuc ci di.

recently, using the multiclave setup already mentioned above [13]. More chemical information enters the planning of zeolite libraries, if the method suggested by Chen and Deem is used [47]. These authors calculate a figure of merit which serves as the basis for the library design, based on the contributions of structure directing agent and zeolite composition. This study, however, so far only provided a concept, and no experimental implementation has been reported, yet. If several rounds of optimization, either in synthesis or in catalysis, are envisaged, then other, possibly more efficient methods are available. Genetic algorithms, which try to mimic natural evolution, are such a method. The concept shall briefly be described for a catalytic reaction using a catalyst which contains up to eight different elements (Fig. 9). First a set of catalysts is prepared, for instance by a random approach, and evaluated in the target reaction. Catalysts are encoded like a gene, as indicated in Fig. 9. The best performing catalysts are then selected for further optimization. Based on these selected samples, a new library is generated by two operations, mutation, i.e. adding or removing single elements, or cross-over, i.e. exchanging larger parts of the "genes". Then the new library is evaluated, the best samples selected, modified by mutation and cross-over, and so on, until no further significant improvement in performance is reached. Such schemes have been implemented for the development of different catalysts (for instance in [48,49]). As far as the author is aware, no genetic algorithm scheme has been used for the optimization of zeolitic materials, but the

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work of Serra et al. [49] was directed at least at the optimization of related systems, such as modified acidic zirconia, titania and alumina, used for hydroisomerization of n-pentane.

Fig. 9. Principle of a genetic algorithm in catalyst development. An initial library is created. Elements present in a catalyst are encoded by the light grey color. In a first evaluation, the most active catalysts are selected, indicated by the arrows, as the basis for the next generation. By using "mutation", exchange of one element, or "cross-over", exchange of parts of the code for two catalysts, a new generation is created, which is subjected to the same process.

Genetic algorithms can be useful methods to efficiently approach (at least a local) optimum in a given system, because it feeds-in information from previous experiments in a way which well balances preservation of information and introduction of new elements, if the parameters of the algorithm, such as mutation rate, are properly chosen. Another software based method, which helps to discover connections between parameters describing the catalysts and their performance, are artificial neural networks (ANNs). These techniques seem to have first been introduced by Hattori in heterogeneous catalysis [50], but were not widely used subsequently. However, in recent years, with the advent of high-throughput experimentation, ANNs are receiving increasing attention as a means to predict the performance of catalysts. In an ANN, parameters describing the solids, such as composition, are used as the input layer. The input layer is connected by so-called "hidden layers" with the output layer, which gives the performance of the catalysts. Nodes in the layers are connected by links, which can be considered as transfer functions, influencing data flow from input to output layer. Like in a biological neural network, pathways between input and output layers are amplified, if they are often used, and become weaker, if they are not used. During the training of the network, both input and output layers are known, and by entering for each sample parameters and catalytic performance, the connections in the hidden layers are established. These connections then are able to predict with some degree of certainty also the performance

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of catalysts, which have not been used in the training step. In zeolite science, such a system has already been used in the 1990s, where the performance of Cu-ZSM-5 catalysts in NO decomposition has been predicted [51]. However, the data set used in this study was rather small, and much bigger data sets and thus more sophisticated ANNs can be generated using the now available techniques of high-throughput experimentation. A more fundamental method to guide the search for novel material relies on so-called descriptors. Descriptors are combinations of properties of solids which are correlated with a target performance. For predicting structural properties of zeolites, descriptors have been developed by Rajagopalan et al. [52]. The descriptor includes properties such as lattice constants, space group, point group, crystal system, presence of certain symmetry elements, ratios of lattice constants and others, and this set of descriptors allowed the prediction of ring sizes of zeolites or M-O bond length. The descriptors described by Rajagopalan et al. were created for a rather specific problem, the prediction of structural features of zeolites. Recently, more comprehensive approaches to the design of libraries and knowledge extraction have been suggested. We proposed a method, by which a so-called descriptor vector is generated for solid catalysts based on the performance of the catalysts in a target reaction [53]. This method has been implemented and has been shown to work for the example of propene oxidation [54,55]. Using the method, the performance of solids in the oxidation of propene to different products could be predicted with a certainty by far exceeding statistical expectation values, although extremely different materials were investigated. A full description of the approach is beyond the scope of this contribution, and thus the reader is referred to the original literature. A somewhat different, but also very comprehensive method was suggested by Caruthers et al. [56,57]. They describe a "knowledge extraction engine" which models the reaction network based on a set of reaction rules. These reaction rules are formulated by a human expert and then implemented in a simplified, but still very complex kinetic model. Using software based methods, this initial set of rules is refined to model the actual behaviour of the reaction system. The part of the knowledge extraction engine describing the kinetics is already well advanced in this system. However, for a full understanding of the system, also the features of the catalyst have to be correlated with the kinetic constants. In order to achieve this kind of modelling, suitable descriptors have to be found to capture the relevant properties of the catalysts, which brings the problem back to the one formulated in the previous paragraph. The case study provided by Caruthers et al. was directed to a problem from zeolite science, i.e. the aromatization of propane over H-ZSM-5. For this particular catalytic problem, the kinetic model was in fact developed to a point, where predictions for different reaction conditions agreed quite well with the experimental data. 6. PERSPECTIVES The different components of an integrated high-throughput approach for zeolite science are in different stages of development. While synthesis, part of the characterization, and catalytic testing are already well developed and in several laboratories used on a routine basis, software based tools for the design of libraries and the extraction of knowledge from the vast amounts of data are still in their infancy. Depending on the type of the problem to be studied, it can be useful to employ conventional methods or to resort to high-throughput experimentation. However, it seems clear that high-throughput experimentation is a powerful methodology in zeolite research, and over the next years, probably most laboratories active in zeolite research will implement such methods to some extent, either in a fully integrated mode, which for cost

179 reasons will mostly be restricted to industrial laboratories, or with selected elements only, which provide the crucial functionality for a given project. While still many of the systems are home built and specifically developed to solve a problem at hand, more and more systems, such as those for synthesis or for catalytic testing, are becoming commercially available, which will significantly speed up the penetration of research laboratories with these technologies. REFERENCES [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [II] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

Ion-exchange properties of zeolites A. Dyer Institute of Materials Science, University of Salford, Cockcroft Building, Salford M5 4WT,United Kingdom.

1. INTRODUCTION 2. ZEOLITE ION EXCHANGE THEORY 2.1. Cation exchange equilibria 2.1.1. Introduction to the theory 2.1.2. The binary ion exchange process 2.1.3. Analysis of isotherms to provide thermodynamic data 2.1.4. Eisenman theory 2.1.5. Zeolites as model cation exchangers 2.1.6. Zeolite cation exchange as a diffusion controlled process 2.1.7. Column models 3. CATION EXCHANGE PROPERTIES OF INDIVIDUAL ZEOLITES 3.1 Introduction 3.1.1. Analcime ANA 3.1.2. ChabaziteCHA 3.1.3. Clinoptilolite HEU 3.1.4. Edingtonite EDI 3.1.5. ErioniteERI 3.1.6. FaujasiteFAU 3.1.7. Ferrierite FER 3.1.8. Gismondine GIS 3.1.9. KFI Zeolite ZK5 (previously described as P and Q) 3.1.10. Laumontite LAU 3.1.11. LTA Zeolite A 3.1.12. Merlinoite MER 3.1.13. MFI ZSM-5 (and other " high silica" zeolites) 3.1.14. Mordenite MOR 3.1.15. Phillipsite PHI 3.1.16. StilbiteSTI 3.1.17. Summary 4. USE OF ZEOLITES AS CATION EXCHANGERS IN COMMERCE AND THE ENVIRONMENT 4.1. Detergency 4.2. Water treatment 4.3. Nuclear waste treatment and accident legacy solutions 4.4. Other comments REFERENCES

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182

1. INTRODUCTION The phenomenon of ion exchange is thought to be the oldest physical process known to man, dating back to descriptions in the Bible and the writings of Aristotle. It was also the first property of zeolites to be studied scientifically when, in 1858, Eichorn [1] showed that the natural zeolites chabazite and natrolite were capable of reversible cation exchange. Gans, at the turn of the 19* century, synthesised aluminosilicate materials capable of water softening. He called them " Permutits" and for many years they were employed as domestic and industrial water softeners, as well as in the treatment of nuclear waste. This, together with the first identification of "base exchange" in soils (Way [2] and Thompson [3]), led to the misunderstandings perpetuated in elementary texts that zeolites are responsible for the ion-exchange in soils, and that they are widely used as water softeners. In truth the clay minerals contribute mainly to the ability of soils to take up cations, such as ammonium, and Permutits were amorphous materials with low cation exchange capacities and limited chemical stability. Zeolites are, of course, crystalline by definition and the cation exchange properties of traditional aluminosilicate zeolites arise from the isomorphous positioning of aluminium in tetrahedral coordination within their Si/Al frameworks. This imposes a net negative charge on the framework (Si4+-» Al3+) counterbalanced by cations held within the cavities and channels. This is said to create facile cation exchange " lubricated" by the water molecules also present in the voids within the framework. This certainly is so for zeolites with open frameworks but in some of the more narrow-pored frameworks, such as natrolite, cation replacement is slow and difficult. R.M. Barrer commenced detailed studies of zeolites in the mid 1930s, and was responsible for the establishment of the basic equilibrium and kinetic thermodynamic interpretation of zeolite cation exchange properties [5]. Part of the driving force for these early studies came from the discovery that the introduction of different cations into zeolites was of critical importance to the modification of their catalytic, molecular sieve and drying properties. An additional interest came from the realisation that zeolites were ideal models for the development of cation exchange theory because, unlike organic resin exchangers, their framework structures did not appreciably swell on contact with water. The bulk of these investigations were directed to synthetic species, in accord with their commercial significance. Interest on natural zeolites has, in the main, focussed on their availability as economic, and abundant, materials for water treatment and environmental clean-up. Overall, study has shown that zeolites exhibit unique abilities to selectively take up specific cations from mixtures with other cations- even when the competing cations are present at much higher solution concentrations. Zeolites also have high cation exchange capacities (CEC). This chapter will first provide an insight into cation exchange theory as it relates to zeolites, with details of individual properties of the common synthetic and natural species, including details of those having industrial significance 2. ZEOLITE ION EXCHANGE THEORY 2.1. Cation exchange equilibria 2.1.1. Introduction to the theory The following theoretical approach assumes;

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(a) the system is in a true state of equilibrium (b) the exchange process is fully reversible. If the system is not reversible then there will be two different reactions (one representing the forward exchange, and another the reverse exchange) that need to be examined separately. They will have different standard Gibbs energies. It is worth noting here that the following theory is a rigorous thermodynamic approach and is, by its nature, exact and so can be applied to all cation exchangers [6,7]. 2.1.2. The binary ion exchange process The exchange between a cation A 2A , initially in solution (the ingoing ion), and a cation B ZB , initially in the zeolite (the out-going or counter ion) can be written as: ZB^2-*

+ZABZB

<->

ZB~AZ*

+Z

A

5

Z

*

(1)

ZA,B are the valencies of the ions , and characters with a bar as a superscript refer to a cation inside the zeolite lattice. If the zeolite is contacted with a series of isonormal solutions, constituted from differing proportions of the in-going and counter cations, an ion exchange isotherm can be constructed at a standard condition of temperature and pressure, to yield the distribution of cations between zeolite and solution phases when equilibrium has been reached (checked by prior kinetic tests). It is also imperative that the ratio of the volume of solution (mL) in contact with a known mass of zeolite (g) be at least 20, and larger if experimentally feasible. The correct experimental methodology has been fully described by Dyer et al. [8], and reinforced by Townsend [9], who emphasises the importance of carrying out complete analyses of both solid and liquid phases. This enables an isotherm to be plotted recording the equivalent fraction ( A s ) of the in-going cation in solution against its equivalent fraction in the zeolite (A z ). These quantities can be defined as,

As = ZAmA/(ZAmA + ZBmB) and Az

= ZAMA/( ZAMA + ZBMB)

(2)

mAjniB and MA,MB are the cation concentrations (mol dm"3) in solution and solid phases respectively. Idealised isotherm shapes are shown in Fig.l. They give a pictorial indication of the relative preferences of the cations for the solid and solution phases. When the zeolite phase has equal affinity for both cations the isotherm is a straight line — the dotted line in Fig.l. Isotherm (2) is the shape adopted when A remains in solution i.e, does not readily displace B from the zeolite phase, at the conditions of temperature and concentration of the experiment. Isotherm (3) occurs when A replaces B from the zeolite. The sigmoidal shape of (1) arises when a change in selectivity occurs, in the concentration range being studied, and is indicative of more than one exchange sites being available in the zeolite for the competing cations.

184

The selectivity of a zeolite for ion A can be expressed quantitatively as a separation factor (a) viz:

a=AzmB/BzmA

(3)

where, by definition,

Bz=l-Az

(4)

Fig. 1. Idealised cation exchange isotherms In Fig.l a can be estimated for isotherm (1) from the ratio of area (a) to area (b). Often isotherms representing incomplete exchange are observed. An example of this is sketched in Fig.2. Here the exchange process has reached an equilibrium without the in-going ions being able to replace all the counter ions from the zeolite phase, under the temperature and concentration conditions employed. Under these circumstances it is necessary to " normalise" the isotherm to gain thermodynamic information. This does not compromise the solution activity correction but it clearly does influence the equilibrium constant and zeolite activity coefficients, and the definition of the standard state. In effect the normalisation procedure regards those counter ions not involved in the exchange as part of the exchanger framework [6]. A fuller consideration of the normalisation, and its theoretical justification, can be found in Barrer et al. [10]. As emphasised earlier, to apply thermodynamic analyses to isotherm data it is imperative that the process be demonstrably reversible. In some zeolites the path of the reverse isotherm deviates markedly from that of the forward exchange (Fig.3). This is described as "hysteresis" and can arise from small errors in analysis, but often it is a consequence of changes in occupancy of heteroenergetic cation sites created by drying the zeolite samples prior to their use in generating the reverse curve. Other causes are the

185

incidence of unsuspected ternary exchange, the presence of phase separation and the coexistence of two phases over the composition range studied, or even the precipitation of metal oxide/hydroxides on the zeolite external surface. Further discussion of these points will be considered later.

Fig. 2. Incomplete exchange of Na<-»l/2 Zn in zeolite EUO, forward isotherm • reverse +.

Fig. 3. Isotherms showing hysteresis

Not all isotherms in the literature have been constructed in the formal ways described above. Often they arise from contacting the zeolite with solutions containing only the in-going cation, only one ion is analysed in one phase and various units of concentration are used. These simple approaches can be useful to compare practical selectivities, but when thermodynamic data is required from interpretation of the isotherms, the more rigorous methodology must be followed, with confirmation of full reversibility in order to apply the laws of mass action. When measurements are attempted to construct the reverse leg of the

186

isotherm it is appropriate not to dry zeolites as heating can change the number of cation sites partaking in the ion exchange process. Another simple approach comes from the realisation that each point on the isotherm represents the distribution of ions between the phases. At each point a distribution coefficient (K D ) can be defined for ion A : KD= CJCS

(5)

C z is the concentration of ion A per unit mass of anhydrous zeolite and Cs the concentration of A per unit volume of external solution Distribution coefficients are widely used as convenient checks of practical selectivities under fixed experimental conditions, provided equilibrium has been reached. Examples of this approach are often concerned with comparisons of the ability of zeolites to scavenge toxic metals from solution. 2.1.3. Analysis of isotherms to provide thermodynamic data For a fully reversible isotherm the mass action quotient (Km) can be used to define the process, as with any reversible chemical reaction, namely: Km= Az" mzBA/ BZA mz"

(6)

This leads to the determination of the thermodynamic constant (Ka); Ka=KmT{ fz"lfz')

(7)

where, F=yz//yz/

(8)

yA and yB are the single cation activity coefficients of A Z/> and B ZB , respectively, in solution, and/,, B the activity coefficients of the same cations in the zeolite phase. Ka can be obtained by graphical integration of a plot of In Km T against Az ( or by an analytical integration of the polynomial that provides the computed best fit to the experimental data). The quantity Km T can be described as: Kc=KmT

(9)

Kc is known as the Kielland coefficient and is related to Ka by the simplified Gaines and Thomas [11] equation:

187

\nKa=

(ZB-ZA)+

J lnKcdAz

(10)

0

Values for y A B cannot be determined but Y is accessible from the mean stoichiometric activity coefficients in mixed salt solutions via equation (8) thus:

r-rz,'ir'"-

ttr'£'}*A'-'z''nr'Sl

*•*''*-z'h1'2*

(n)

In equation (11) Zx is the charge on the common anion. an T+BV d Y(±AX can be calculated from y±AX and y±BX (Glueckauf [12]). fA B are available from the Gibbs-Duhem equation. Having obtained Ka, a value of AGe arises from: AGe = -(RT In

tfJZ^Z,

(12)

AGe is the standard free energy per equivalent of charge; R and T have their usual meanings. The standard states of the zeolite exchanger relate to respective homoionic forms of the zeolite immersed in an infinitely dilute solution of the corresponding cation. This implies that the water activity in the solid phase, in each standard state, is equal to the water activity in the ideal solution, and the standard states are defined as the hypothetical ideal molar (mol dm"3) solutions or the pure salts according to Henry's Law definition of an ideal solution. The approach outlined so far is based upon a simplified version of the Gaines and Thomas treatment. In the complete version of equation (10) the LHS should be h\Ka -A, where A is a water activity term. For most selectivity studies the simplified equation suffices, as A has been shown to be negligible. Values of free energy can be used to generate a selectivity series. This is done by taking a zeolite in a pure homoionic form (say Na) and using it to construct isotherms by contacting it with isonormal solutions of in-going ions (say Li,K,Rb,Cs). The AG6 values obtained provide an assessment of the affinity that the Na zeolite has for the other alkali metals. If experimentation is extended over a range of temperature then enthalpy and entropy changes can be obtained to help in the further interpretation of the cation exchange mechanisms in zeolite frameworks. 2.1.4. Eisenman theory Brief mention should be made at this point of the approach made by Eisenman [13] to explain observed selectivities in ion exchange systems. He noted that;

where ^^exchange

= crian

ge in free energy for a binary cation exchange reaction,

188

G* + G + = Gibb's free energies of the cations A1', Bz" in the zeolite phase ACB'

G*

A/

G + = Gibb 's free energies of the cations Az*, Bz" in the solution phase

The first term in equation (12) represents the difference between cation free energies in the zeolite, and the second that in the solution phase so is essentially the change in free energy of hydration. Translated into zeolite terms this means that when the charge on the framework is high (low Si/Al ratio) the first term dominates and the zeolite shows selectivity for small highly hydrated cations. An example of this is the use of LTA as a detergent builder (see later). For zeolites with high Si/Al ratios in their frameworks the second term is influential and they will prefer larger cations which are less hydrated, such as in the use of clinoptilolite to scavenge radioactive Cs from aqueous nuclear waste. Detailed studies by Barrer and co-workers [14,15] have strengthened this approach by using dielectric theory to elucidate binary exchanges in FAU frameworks with different Si/Al ratios. Fletcher and Townsend [16] employed a dielectric approach to rationalise selectivities observed for aqueous and amminated silver1 ion uptake into X,Y and mordenite. 2.1.5. Zeolites as model cation exchangers Zeolites usually retain their structural integrity and do not appreciably swell in solution, or experience dimension changes related to the size of the cations within the zeolite phase. These properties are in contrast to those of organic resin exchangers, and have attracted interest in modelling cation processes using zeolites as "ideal" substrates. Details of this are beyond the scope of this review and readers who wish to pursue this further are referred to the excellent summaries available [5,6,9,17]. They include cases where progress has been made in the theoretical approach to multi-component (e.g., ternary) systems by using well-characterised zeolites [18]. Some authors have taken a different approach by studying the kinetics, rather than the equilibrium aspects, of cation exchanges and a short summary of the background to these follows. 2.1.6. Zeolite cation exchange as a diffusion controlled process [7] The rate controlling step in a zeolite cation exchange can be mediated by, either, the diffusion of the participating ions through the internal channels and cavities of the zeolite internal structure (particle diffusion) or, their hindered movement through the film of ordered water which exists on the surface of the zeolite particle (film diffusion). Most studies endeavour to work under conditions governed by particle -controlled mechanisms as this enables good use to be made of the unique selectivity exhibited by zeolites as well as maximising their capacity. Film control can be significant (i) when concentrations in the contacting solutions are small (less than 10~5M is a "rule of thumb" estimate), or (ii) in the pores of a pelletised zeolite created by geological genesis or the industrial forming of an agglomerate of small crystallites. It also is of importance in the theoretical assessment of column performance. When considering the movement of cations within the zeolite a multiplicity of diffusion processes must be considered; (i) diffusional barriers experienced by the individual cations encountered by the steric restrictions of the zeolite framework,

189

(ii)

the influence the cations can have on one another as they progress through the same framework, (iii) barriers arising from the self- exchange of the individual cations, (iv) flux of water as it moves within the framework, both as a self diffusion process and when cooperative movements between cations and water molecules influence diffusion. Perhaps not surprisingly only small progress has been made in the provision of an adequate foundation on which to base an understanding of the kinetics of zeolite cation exchange. The pioneering work of Brooke and Rees [19] on the Sr/Ca chabazite system, which highlighted the problems involved, remains incomplete due to the requirement to take into account the variation of cation diffusion coefficient with concentration in the zeolite and to consider the effects of water transport during the exchange process. Both Sherry [20] and Barrer [5] have reviewed this, and earlier work, which included estimations of self-diffusion parameters and cation diffusivities. More recent work has extended this theme, with assessments being made of the mathematical equations available to compute diffusion values. Barrer et al. [21] used an equation developed by Ash [22] to study cation diffusivity in RHO. This takes the form:

h= fl(-e,/ejdt=ro2/15JD

(14)

Q,n are the extents of exchange at times t = t, t = a, ro the radius of the zeolite particles, and D the diffusivity. A plot of Qt I Qm vs t for an ion exchange is constructed, and then Io is defined as the area between the plot and the horizontal asymptote provided that, t is large, the zeolite crystals are of uniform size and can be approximated as spheres of radius ro . This proves to be a simple and useful method. White and Dyer [23] examined its utility for the estimation of exchange diffusivities in clinoptilolite and concluded it to be convenient and reliable in comparison to the equation of Carman and Haul [24] used in other zeolite studies [25], or to that of Boyd et al. [25], More recently Inglezakis and Grigoropoulou [26] have compared equations that can be used to estimate ion exchange diffusion coefficients in clinoptilolite. The large scale use of natural zeolites as selective cation exchangers continues to grow and the need to provide cation self - and exchange -diffusion coefficients increases in importance especially to feed into computer models mimicking the performance of zeolite columns. Some progress is evident in this area employing dynamic modelling [28]. Tsitsishvili et al., in their book on natural zeolites [29] tabulate values of cation diffusion coefficients. 2.1.7. Column models A simple review [6] of the definition of a distribution coefficient (equation 5) enables the calculation of the column capacity for a cation from equilibrium batch tests, provided that the concentration of the ion being scavenged by the zeolite is low in the solution being treated. This is particularly useful when removal of radioisotopes from aqueous nuclear waste is the intended use [30].

190 A typical representation of column performance plots removal of the species from solution against the volume of effluent passed through the column. When measurable quantities of the cation to be removed exceed a predetermined level in the solution exiting the column, "breakthrough" is said to have occurred. Harjula and Townsend [6] point out that the application of plate theory can be used to quantify column efficiency via the estimation of the number of effective plates (Nf,p). For a film diffusion controlled exchange, N f =A(D f ) l / 2 d- 3 / 2 su 0 1 / 2

(15)

s is the column length, d the particle diameter, u0 the linear flow rate and Df the film diffusion coefficient. When particle diffusion is controlling, Np = B(D p ) 1/2 sd- 2 u 0 " 1

(16)

A, B are empirical factors , Dp the particle diffusion coefficient and Nf>p the numbers of effective plates under film and particle control. In any one case both rate controlling processes may contribute to the overall exchange kinetics. Equations (15) and (16) show that the degree of column utilisation and the breakthrough capacity can be expected to increase when zeolite grain size and the flow rate are decreased, although, clearly, column design parameters must be taken into account. In the limit the column kinetics are determined by the diffusion coefficients hence the need for data outlined above. 3. CATION EXCHANGE PROPERTIES OF INDIVIDUAL ZEOLITES 3.1. Introduction Some general points need to be made at this stage. Work by Drummond et al. [31] showed that water washing of LTA zeolite resulted in exchange of the Na cations initially present by hydronium ions from the water via the hydrolysis: Na*z +2H2O

= H30+z + Na+ +OH~

(17)

Hydrolysis increases as the aluminium content of the zeolite in contact with the water increases, so the cation exchange becomes ternary rather than binary. This phenomenon can be easily confirmed by monitoring the pH of water in contact with a zeolite. The initial high value (pH ~10) drops as reaction (17) proceeds. Examples of this are seen in Harjula et al.[32] Other secondary reactions now arise; (i) hydroxyl ions enhance the dissolution of Si and Al from the framework and this has major consequences viz; extra framework aluminium species carry a charge that will, again, introduce ternary exchange processes, additionally the leaching of Si/Al forms an "inert" layer on the zeolite surface, and hydrolytic damage produces colloidal material in solution, so tested centrifugation and filtration protocols must be employed, (ii) carbonate and bicarbonate ions are formed at external and internal zeolite surfaces by the uptake of carbon dioxide from the air facilitated by the alkaline conditions,

191

(iii)

alkaline conditions increase the possibility an apparent over exchange, caused by metal oxide/hydroxides/carbonates precipitation on the external zeolite surface, and, perhaps, a cation valency change.

The effects listed above are most evident at low salt concentrations, so cation exchange is best studied at moderate salt concentrations when thermodynamic data is to be measured. However salt concentrations must be kept below the conditions at which neutral salt imbibition (ingress of salt molecules into the internal zeolite structure via a Donnan mechanism) can occur. This has been suggested [33] to be below 1M. A further point to be made is that, in studies where cation concentrations are very low such as the uptake of radioisotopes when they will be in picograms/L, even anions such as chloride, sulphate and organic anions can form ion-association complexes that alter the cation exchange processes [6,34]. Another difficulty is that of obtaining sufficiently accurate analytical data at low cation concentrations in both solution and solid phases with which to construct reliable isotherms. The majority of these factors have been recognised comparatively recently so the earlier cation exchange work must be reviewed in the light of the above. Detailed discussions of the difficulties that can be experienced are in a series of papers by Harjula and co-workers [32,36-38]. Turning now to literature describing exchange in natural zeolites, additional care is needed to assess the value of early work (Note some work has used synthetic analogues of natural species). Initial work on natural zeolites often was deficient in essential information. Authors neglected to describe the geological origin of the zeolites, their cation and framework compositions, and frequently infer that they are dealing with pure materials, ignoring the fact that many impurity minerals will be present. Clearly impurities such as forms of silica, calcite and feldspars will alter analytical data, and clay minerals, or other zeolites, affect observed cation exchange properties. In addition natural zeolites formed in a sedimentary environmental can have higher Si/Al framework compositions than those arising from other geological geneses, making exact comparisons, even for the same zeolite, problematic. These comments are particularly true of the early work on clinoptilolite. This occurs widely in the earths crust and is commercially mined in at least 14 countries. It is recovered as volcanic tuffs that have very wide variations in clinoptilolite content and cation composition. Reviews are available for consultation, as listed in the references to this chapter [29,39-42]. Pabalan and Bertetti [40] provide an excellent listing of the known thermodynamic parameters collected from studies on chabazite, clinoptilolite, erionite, ferrierite, heulandite, laumontite, mordenite and phillipsite. Interested readers are referred to their review, as space does not allow the inclusion of this information here. Individual zeolites will now be discussed. A later section will be devoted to their commercial usage. 3.1.1. Analcime ANA Barrer and Hinds [43] recorded the first observation of hysteresis created by cation exchange in a zeolite. The sodium form of analcime has one water molecule per cation in its framework, whereas the potassium form is anhydrous in its homoionic form. This K form is another phase, leucite, and the hysteresis is a consequence of the co-existence of the two phases within the solid. Leucite is now classified as a zeolite.

192

The analcime framework is unique in that it contains 3 unconnected channels [44] such that cations and water molecules cannot move from one channel to another. Dyer and Yusof [25], using a synthetic analcime, have shown that independent cation and water molecule movements exist within each of these channels, and have measured the diffusional energy barriers that define these separate processes. Another significant result from the early work of Barrer [45] was the recognition that Cs+ (3.30A diameter) could not enter the ANA framework whilst Rb+(2.96A) and the smaller alkali metal cations could. This introduced the concept that zeolite internal architectures were capable of selectively removing cations from water on the basis of cation size- the property described as "ion-sieving". Barrer [45] also measured a selectivity series: Ag»Na>Li>K which illustrates that the size is not the only factor defining zeolite cation preferences, as clearly Li is the smallest cation in the series, but behaves as a larger moiety because it retains part of its hydration shell during the exchange process. More recently [46] another series has been obtained on analcime sythesized from a natural volcanic glass: Pb> Cu> Zn> Ni The preference of analcime for the large Pb cation over the Na cation initially present in comparison to the smaller transitional element cations was thought to be due to the large polarisability of the lead cation. 3.1.2. Chabazite CHA Several authors have determined selectivity series for chabazite: Tl>K>Ag> Rb>Na=Ba >Sr>Ca>Li (Barrer, Davies, Rees [47]:Breck [48]) Tl>K>Rb>Ag>Na (Barrer and Sammon [49]) Cs>K>Na>Li (Barrer and Klinowski [50]:Ames [51]) Cs>Rb>K>Na>Li (Sherry [52]) Ba>Sr>Ca>Mg (Ames [51]) NH4>K>Pb>Na (Torracca [53]) Ames [54] has also studied a commercially modified chabazite, Linde AW-500, (AW -acid washed) and confirmed its preference for Cs over K and Na. Subtle variations in cation preferences are due the reasons mentioned earlier. Chabazite has been commercially mined in the USA and as a mixture with phillipsite in Italy. 3.1.3. Clinoptilolite HEU This zeolite has been commercially mined in USA, Australia, Russia, Slovenia, Cuba, Indonesia , South Africa, Greece, Bulgaria, New Zealand, Turkey, China, Mexico, Hungary, Jordan and Outer Mongolia. Deposits in other countries are under consideration. This widespread occurrence has encouraged the determination of many selectivity series: Cs>K>Rb>Na>Li (Sherry [52]) Rb>K>Na>Ba>Sr>Ca>Li (Filizova [55]) Cs>K»Na (Ames [54]) Cs>NH4>Na (Howery and Thomas [56]) Ba»Pb»Cd»Zn»Cu (Semmens and Seyfarth [57]) Pb»Cd>Cu»Zn (Fujimori and Moriya [58]) K>NH4>Na»Ca>Mg (Sherman and Ross [59]) Cs>K>Sr=Ba>Ca»Na>Li (Vaughan [60])

193 Cs>Rb>K>NH4>Na>Li Ba =Sr>Ca>Mg Pb>Ag>Na Pb>Ag>Cd>Zn>Cu>Na (At low loadings) Zn>Ca=Cu>Fe">H>Mg>Al Pb>Zn>Mn>Cd

(Ames [61]) (Ames [61]) (Chelishchev et al. [62]) (Chelishchev et al. [62]) (Bremner and Schultze [63]) (Cerjan-Stefanovic and Curkovic [64])

Tsitsishivili et al. [29] list more examples. Uniquely Barrer et al. [65] record a series for the uptake of organic cations into clinoptilolite: C2H5NH3+ >NH4+> nC3H7NH3+> nC4H9NH3+ Larger cations, such as (CH3)4N+ and CH3CH(NH3)CH2CH3+, were completely excluded by ion sieving. It is convenient to record series determined for the other natural species with the same framework- namely heulandite itself: K>Rb>Na>Li>Sr>Ba>Ca (Filizova [55]) Sr>Ca (Hawkins [66]) Cs>Rb>Pb=K>NH4>Ca>Na>Li (Al'shulterand Shkurenko [67,68]) Mn>Na>Zn>Cu>Ni (Al'shulter et al. [69]) A recent study [70] using a Siberian clinoptilolite serves to illustrate an example of column use for the removal of NH4, Pb, Cs and Sr from effluent streams. In conclusion the attention of the reader is drawn to the elegant and detailed examination of the thermodynamics of ion exchange in clinoptilolite from Death Valley Junction, California, carried out by Pabalan [71], and by Pabalan and Bertetti [72]. 3.1.4. Edingtonite EDI A synthetic analogue of this mineral (Linde F) showed the following selectivity series [73]: low loadings Cs>K>Na>Li high loadings K>Li>Na>Cs Ba»Sr>Ca>Na Sherman and Ross [74] found; NH 4 »Ca>K>Na>Mg 3.1.5. ErioniteERI The existence of potentially workable deposits, at Jersey and Pine Valleys, Nevada and Rome, Oregon in the USA prompted early work by Sherry [52] and Ames [75,76]. Chelishchev and Volodin studied an erionite from Georgia (formerly USSR). The selectivity series determined were: Rb>Cs>K>Ba>Sr>Ca>Na>Li (Sherry [52]) (at low loading) Cs>K>Na>Li (Ames [75]) Ba>Sr>Ca>Mg (Ames [75]) Cs>K>Na (Ames [76]) Ca>Rb>K>Na>Li (Chelishchev and Volodin [77]) Ames [76] also used acid washed erionite (Linde AW-300) to construct the series: Cs>K>Na. Finally Sherry [20] examined Linde T, an early synthetic product subsequently shown to be a mixed erionite/offretite phase. This exhibited the following preferences: Cs>Rb>Ag>K>NH4> Ba»Na>Ca>Li

194

3.1.6. Faujasite FAU No work has been recorded on the natural zeolite apart from some preliminary assessments of the faujasite-rich tuff found in the Northern Badia region of Jordan [78]. This material has a total zeolite composition of 47% of which 30% is faujasite. It can be benificated to 92 % zeolite content (faujasite and phillipsite) with a cation exchange capacity of 3.24 meq/g. The synthetic analogues, zeolites X and Y, have received considerable attention because of their value as industrial catalysts. This has seen virtually every cationic species from the Periodic Table introduced into FAU frameworks. The following is a listing of selectivity series observed: (a) Linde X below 40% exchange Ag»Tl>Cs>Rb>K>Na>Li ( Sherry [20]:Breck [49]) at 50% exchange Ag»Tl>Na>K>Rb>Cs>Li ( Sherry [20]:Breck [49]) Cu>Ca>Mg (Wolf et al. [79]) Cu>Zn>Co>Ni (Maes and Cremers [80]) Zn>Na (Maes and Cremers [80]) Cd>Cu>Zn>Co>Ni (Gal et al. [81 ]) K>Na>Rb>NH4>Cs>Li>(CH3)2NH2+(Tolmachev and Federov [82]) K>Na>Cs (Ames [76]) H>NH4>K>Na>Li (Thomas [83]) Ba>Sr>Ca>Mg (Thomas [83]) Sr>Na, Ce>Na, Na>Cs (Ames [84]) at low loadings Ba>Ca>Sr>Mg (Sherman [42]) Cs>Sr>Na (Ames [84]) Ag»K= Na>NH4>Ag(NH3)2>Li (Sherman [42]) (b) Linde Y up to 68% exchange Tl>Ag>Cs>Rb>NH4>K>Na>Li (Breck [48]) partial exchange Tl>Cs>Rb>Na>Sr>Ca (Barrer et al. [47]) complete exchange Ag>K>Na>Li (Barrer et al. [47]) Ba>Ca=Sr>Mg (Katan and Bravo [85]) at low loading Cs>Rb>K>Na>Li (Sherry [20]) at 50% exchange Cs>Rb>K>Na>Li (Sherry [20]) Cu>Zn>Co>Ni (Maes and Cremers [80]) Cd>Cu>Zn>Co>Ni (Gal et al. [81,86]) Cs>Na,Sr>Na, Ce>Na (Ames [87]) Where selectivity variations with loadings have been noted this, in general, relates to the number of sodium cations available in the hetro-energetic sites within the X,Y frameworks. Fletcher and Townsend [88] presented a detailed analysis of the competitive exchange between Na and NH4 cations in X and Y samples with SiCVAlOs in the range 2.52-4.25, showing how ion sieving and framework charge control exchange equilibria. They also demonstrated [89] that the X framework is much more selective for the hydrated silver cation than its Y counterpart (and mordenite) but the lower charged Y framework preferred the amminated silver cation more than X. Y was inferior in its preference for this complex cation to that shown by mordenite.

195

3.1.7. Ferrierite FER The earliest record of ion exchange in ferrierite used a synthetic material that showed selective uptake of Sr over Ca (Hawkins [66]). Later Dyer and Ahmad [90] examined a purified ferrierite from the Lovelock, Nevada deposit in a homoionic K form and found the following affinity sequence: Tl=Cs>Rb>K>NH4>Ag>Na>Ba>Sr>Ca>Mg Loizidou and Townsend [91 ] studied ferrierite from the same source with a view to its utility to scavenge NH4, Pb, and Cd from binary mixtures. They found that interpretation was hindered as the participation of K inherently present in the initial sample rendered the Pb systems ternary rather than binary. They were able to define thermodynamic parameters in the Cd/Na and Cd/NH4 systems that remained binary. They showed that Cd was not preferred over Na and NH4. The presence of Cl" altered Cd uptake due to Cd/Cl complex formation whereas presence of the nitrate anion improved uptake. 3.1.8. Gismondine GIS Barter and Munday [92] produced the following preferences for a synthetic zeolite designated as "zeolite P" which has subsequently found to have the GIS framework: Cs>Rb>K>Na>Li Ba>Sr>Na More recently the commercial zeolite "MAP", also with the GIS framework, has attracted interest as a detergent builder. Allen et al. [93] have examined the way that the incursion of different cations into this zeolite causes structural changes. They present reasoned arguments, supported by PXRD, MASMR and thermal analysis, to explain how they affect the observed selectivity series, based upon isotherm data, viz: Ba>Sr>Ca»Na>K>Rb>Cs At ambient temperatures exchange of Na present in the as-synthesised MAP for Li and Mg was limited to 12 and 45 %, respectively, even after contact for one month with solutions of total normality TN =1. 3.1.9. KFI Zeolite ZK5 (previously described as P and Q) Barrer and Munday [92] showed this synthetic species to have the following affinity series: Cs>Rb>K>Na>Li Ba>Sr>Ca Linde B is a similar phase for which Sherman and Ross [42] quote the following: K>NH4>Na=Ca>Mg 3.1.10. LaumontiteLAU Laumontite from Bernisdale, Isle of Syke, Scotland, was carefully purified and converted to the homoionic Ca form. This proved to have the following selectivity series [94]: Sr>K>Na=Cs=NH4 3.1.11. LTA Zeolite A The discovery that this zeolite had molecular sieve properties controlled by the introduction of different cations prompted cation exchange studies. Selectivity series measured are: Ag>Tl>Na>K>NH4>Rb>Li>Cs (Breck [[48]) Zn>Sr>Ba>Ca>Co>Ni>Cd>Hg>Mg (Breck [48])

196

Na>K>Rb>Li>Cs (Sherry [52]) Ag>Zn>K>Ca>Na>Cd>Mg (Thomas [83]) Ag>Ca>K>Na>NH4>Li (Sherman [42]) Ca>Na>Li>Mg (Sherman [42]) Sr>Ca>Mg (Sherman [42]) Sr>Cs>K>Zn>Li (Sherman [42]) Cd>Pb>Zn>Na (Sherman [42]) Cd>Cu>Zn»Co>Ni (Gal et al. [81,86] Maes and Cremers[80]) Sr>Na>Cs (Ames [87]) Like FAU, the LTA structure can be prepared with a lower Al content. This is known as ZK4 and Ames [87] showed that it preferred both Cs and Sr to the Na cation present in the asprepared material. 3.1.12. Merlinoite MER A synthetic phase, zeolite Linde W, with the MER structure exhibited cation preferences as: K>NH4»Na=Ca>Mg (Sherman [42]) 3.1.13. MFI ZSM-5 (and other " high silica " zeolites) Chu and Dwyer [95] were the first workers to take an interest in the cation exchange properties of the high silica zeolites being developed as catalysts. Their assessment of the affinity of ZSM-5 was that it was unaffected by S1O2/AIO3 ratios in the range 40-206 and the following selectivity series was generated from a values: Cs>H3O>NH4>K>Ag>Na>Li Matthews and Rees [96] found a similar series for a sample with Si/Al=39, constructed from free energy measurements, viz: Cs>Rb=NH4=H3O>K>Na>Li When exchanges involving divalent cations were examined, these were limited by the extent that the divalent cation was able to " bridge" adjacent Al lattice sites in the MFI framework [97]. Exchange in EUO (EU-1) also showed similar pattern [98]. Dyer and Emms [99] found the following selectivity series for high silica zeolites, initially in their sodium form, as reflected by free energy measurements: *BEA (beta) Cs>Rb>K>Na>Li; Ni>Ba>Sr>Ca>Mg>Zn EUO Cs>Rb>K>Na>Li: Ba>Sr>Ca>Zn>Ni>Mg TON (NU-1) K>Cs>Rb>Na>Li: Sr>Ba>Zn>Ni>Ca>Mg 3.1.14. Mordenite MOR Ames [100] studied natural mordenite from Nova Scotia, Canada and concluded that showed selectivities for monovalent ions as Cs>K>Na>Li and divalent ions Ba>Sr>Ca>Mg. Other series observed on natural samples were: Cs>Ag>K>H>Na>Li (Wolfe/al. [79]) Ba>Sr>Ca>Mg (Wolfe/al. [79]) Cs>K=Rb>Na>Li (Sherry [20]) Townsend and Loizidou [101], working with samples from Lovelock, Nevada, found that these sample were unable to exchange more than ~ 50% of the ammonium CEC expected from its Al content. This was in contrast to synthetic mordenites whose capacities approached 100% [102]. They concluded that this arose from stacking faults in the natural crystals. These

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workers also examined the competitive uptakes of Na/NHVPb in the same natural zeolite [103]. Linde marketed an acid washed natural mordenite (AW-300) and Ames [76] showed, at low loadings, Cs>K>Na. He found the same sequence for the synthetic mordenite marketed as " Zeolon" by the Norton Company, and referred to as "small port mordenite". Barrer and Klinowski [102] noted the thermodynamic affinity sequence for this material as Cs>K>NH4>Na>Ba>Li. Barrer and Townsend [103] also used it to investigate the ability of its ammonium form to exchange transition metal cations. They showed that the sequence of Mn>Cu>Co=Zn>Ni, was little affected by pH (4-7) and the presence of various anions (acetate, formate, chloride). Other work on synthetic mordenites have been carried out by Golden and Jenkins [104], Kutnetsova et al. [105], and Susuki et al. [106] who provide the following selectivity series: Cs>NH4=K~H> Ba>Sr=Ca>Rb>Mg 3.1.15. Phillipsite PHI Natural phillipsite has been found to show cation affinities as follows: Cs>Rb>K>Na>Li (Barrer and Munday [ 107]) Ba>Na>Sr>Ca (Barrer and Munday [ 107]) Cs>K>Na (Ames [76]) K>NH4>Na>Ca (Sherman and Ross [59]) 3.1.16. StilbiteSTI Ames [108] showed this zeolite to have the preferences: Cs>K>Na The New South Wales Government, Australia, have described preliminary selectivity measurements for Na/NFLt and K/NH4 on stellerite (a zeolite with the STI framework that contains only calcium as its exchangeable cation) from Tambar Springs,NS W [ 109]. 3.1.17. Summary Consideration of the selectivity series listed above, provides the following generalised comments, (1) the propensity for zeolites with silica rich frameworks to prefer large monovalent cations is confirmed, as is the zeolite preference for smaller multivalent cations when the Si/Al~l . (2) cations with large heats of hydration (eg., Li, Mg) tend not to be readily exchanged at the temperatures close to ambient normally used to determine isotherm, or similar, data. (3) transition metal cations are often not preferred- but this must be qualified by their known precipitation on zeolite external surfaces [110] The next section will give a brief account of the commercial and environmental uses of zeolites as cation exchangers. Further information can be found in recent reviews by Sherry [111] and by Townsend and Coker [112].

4. USE OF ZEOLITES AS CATION EXCHANGERS IN COMMERCE AND THE ENVIRONMENT 4.1. Detergency The largest scale production of synthetic zeolites is that of LTA for use as "builders" in domestic and commercial detergents to remove the calcium and magnesium "hardness" that

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hinders washing efficiency [113]. Traditionally this function has been performed mainly by phosphates, now discouraged by environmental pressures. Latterly zeolite MAP has joined a market that currently requires a zeolite production in excess of 1 million tons per annum. This has generated a need to understand the ion exchange process over the wide range of Ca and Mg concentrations encountered in natural waters, together with the complex nature of the domestic detergent compositions, and by the variation in home washing conditions used by consumers throughout the world (particularly in washing temperatures used). Studies designed to provide a deeper understanding of the relevant cation exchange processes, with an ability to achieve accurate predictive models, have been carried out on MAP [114], LTA [115,116] and FAU [117,118]. They involve both binary and ternary cation systems. These zeolites are preferred because of their high capacity and affinity for small hydrated cations. In practice magnesium still presents a minor problem due to the modern trend towards washing cycles at lower temperatures that are not high enough to disturb its " tight" hydration shell. Some detergents have been marketed as "environmentally friendly" by making use of natural zeolites. Smolker and Schwuger [119] have made an assessment of the detergent capability of both natural and synthetic zeolites. Synthetic zeolites cation selectivities otherwise have been little exploited because of small, as-synthesised, crystallite sizes. Although commercial robust composites of synthetic zeolites are available their use in columns has been slow to progress. 4.2. Water treatment Natural zeolites have the advantages of being a cheap resource (especially when local deposits can be used) and most mined materials are available as robust crystal aggregates that can be processed to sizes appropriate to column use. This has resulted in a large literature describing actual and potential use of the natural species for water and wastewater treatment, as reviewed by Kallo [41]. The ability of zeolites to selectively scavenge ammonium cations has been employed to good advantage throughout the world to purify waters. Clinoptilolite has been the zeolite of choice and functions best in a sodium rich form. A small-scale example is in the increasing use of clinoptilolite as a replacement for sand in the filters of domestic and commercial swimming pools. A Ca-rich clinoptilolite tuff from Sweetwater County, Wyoming has successfully been used in columns to remove ammonium from the wastewater of the National Space Administration (NASA) regenerative life-support system test bed [120]. This forms part of the Lunar-Mars Life Support Test Project, and may be used on the International Space Station. Water treatment using natural zeolites is capable of producing potable water and drinking water plants exist, for example, in Budapest, Hungary, and Tblisi, Georgia. In Tenerife, Canary Islands, use has been made of a percolator reactor containing a local phillipsite-rich tuff, which has been shown to favourably remove bacteria [121]. On a larger scale clinoptilolite has been used in plants to treat municipal wastewaters. Sherman [42] has reviewed some of the earlier work in the USA. Kallo [41] gives details of plants, using local clinoptilolites, at Lake Tahoe, California (27,000 m3/day), and Alexandria, Virginia, (245,000 m3/day). In some cases plants involve methodology to remove other contaminants. Prominent among these are the RIM-NUT (at West Ban and Manfredonia in Italy) [122] and the ZeoFlocc processes. The latter was developed at a plant near Budapest, Hungary and is now in use in Austria, Australia, and Germany treating an estimated 400,000 m3/day of waste water [41].

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Further information on similar applications of clinoptilolite and mordenite- rich tuffs in Eastern Europe can be found in Tsitishvili et al. [24] The selectivity series listed previously do not reflect the wide use that has been found for the readily available natural zeolites as cheap materials for the removal of toxic metal cations, such as Fe, Mn, Cu,Co, Zn, Pb,Cd, Ni, and Hg from industrial wastes to protect the environment. Kallo [41], Tsitshishvili et al. [24] and Pansini [123] list many instances of advantageous treatments of effluents from the electroplating, tannery, mining and photographic industries. Sherman [42] has included some instances of comparison to the effectiveness of synthetic zeolites in similar areas. This usage illustrates the difficulties of using data acquired under laboratory conditions to predict efficacy under column use, and at trace concentrations. There are many examples of the use of zeolites (again especially clinoptilolite) for reducing the potential environmental damage arising from animal wastes associated with cattle, swine and poultry production, by removing ammonium, and related solution species. Other examples of the use of zeolites in effluent treatment can be found in conference proceedings [64,90,120,122]. 4.3. Nuclear waste treatment and accident legacy solutions This area of use is clearly predicted from the consistency that the series suggest affinities for Cs and Sr. It has been shown to be of critical value in the treatment of radioactive aqueous wastes arising from the nuclear industry. This has a literature extending back to 1947 and has been extensively reviewed by Dyer [124]. Some 20 natural zeolites have been studied for their ability to scavenge Sr, Cs and other radioisotopes from nuclear effluents. The other radioisotopes taken up include those of uranium, thorium, radium , americium and cobalt. British Nuclear Fuels (Sellafield, Cumbria, UK) have used their Site Ion Exchange Effluent Plant (SIXEP) to treat wastewater from the ponds used to store spent fuels rods, and other effluents, for some 20 years [125]. Clinoptilolite from Mud Hills, near Barstow, California, successfully treats 4700 m3/day, in SIXEP, to remove Cs and Sr radioisotopes prior to direct discharge to the Irish Sea. In this process it has been calculated that, in a typical nuclear effluent, clinoptilolite has the ability to selectively take up 1 mole of Sr and 20 moles of Cs in the presence of 7.5xlO5 moles of Na, 6.5xlO3 moles of Mg and 5xlO3 moles of Ca. The Italian Commission for Nuclear and Alternative Energy (ENEA) has developed a process [126] to treat high nitrate salt solutions emanating from CANDU and MTR reactors and also the US Elk River nuclear fuels reprocessing campaigns. This uses pelletised chabazite (Union Carbide/Dow IE-95, IE-96) from the Bowie, Arizona deposit to take up Cs radioisotopes. Robinson et al. [125] also used similar materials to treat various low level wastewaters arising from the Oak Ridge National Laboratory, Tennessee. Chabazite (as AW-500) was used to treat pond water at the Trawsfynydd Nuclear Power Station, Bala, N.Wales.UK (now closed). Natural zeolites have played important roles as in clean-up from nuclear accidents. After the Three Mile Island incident, the SDS (Submerged Demineraliser System) made use of a 60/40 mixture by volume of IE-96 and LTA zeolite (A-51) from the then Union Carbide Corporation to immobilise 340,000 Ci of fission products from >1.5 million gallons of water [128]. Phillipsite tuff, from Pine Valley Nevada, clinoptilolite, A-51, and IE-96 have all been used at pilot plant scale to deal with high salt, high activity, aqueous wastes at West Valley, New York- site of the PUREX plant used for reprocessing nuclear fuels from 1966 to 1972.

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Chelitshchev [129] reports the use of some 50,000 tons of clinoptilolite from the Sokamitsa (Ukraine), Tedsami (Georgian Republic) and Holinmskoe and Shivirtui (Russia) deposits to treat contaminated water in the Chernobyl reactor, and its environment. This was apart from the many thousands of tons from the Beli Plast, Bulgaria, deposit dropped onto the burning reactor to ameliorate the release of radiocaesium to the atmosphere. The cation exchange of natural zeolites is being studied in relationship to their potential use as barriers to nuclear waste repositories, as well as their presence in the tuffs at Yucca Mountain, which is the proposed high-level radioactive waste storage facility in Nevada [130]. It is appropriate to mention again the detailed studies that have been carried out in relation to the potential use of clinoptilolite and chabazite in the US nuclear industry. In this context Pabalan and Bertetti [72] have produced a thermodynamic model based on a Margules formulation of activity coefficients, for the zeolite components, and Ptizer equations, for solution cation activity coefficients. This was successful in modelling fully the Na / 1/2 Sr, K/ l/2Sr and K7 l/2Ca binary systems for clinoptilolite exchange. Perona [131] modelled a five cation equilibria in chabazite (Sr/Cs/Ca/Mg/Na). 4.4. Other comments The use of solid state ion exchange is now a well-established route to achieve the high cation exchange levels important to some zeolite catalytic uses as reviewed by Karge and Beyer [132]. Numerous references claim anion exchange properties for zeolites. These have been concerned, in the main, with the use of clinoptilolite as a potential remover of fluorine, arsenic species and phosphate from contaminated waters. It has been suggested that salt imbibition into Ca rich zeolites can cause the internal precipitation of insoluble Ca fluoride or phosphate hence the observed anion removal. Another alternative might arise from the hydrolysis of the water molecules on zeolite surface leading to the creation of OH" moieties capable of exchange. As previously noted the high zeolite surface pH commonly leads to surface oxide/hydroxide deposition with certain metal cations so it may be that this can provide an explanation, albeit that exchange capacities would be very small. Some authors have introduced organic entities into the zeolite internal structure, or on the zeolite external surface to create usable zeolite "anion exchangers" [134]. Finally the cation exchange properties of a beryllophosphate, with a zeolite type framework have been reported [135]. REFERENCES [1] [2] [3] [4] [5] [6]

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[78] H.N. Khoury, K.M. Ibrahim, A.M. Ghrir and T.N. Ed-Deen, Zeolites and Zeolitic Tuff in Jordan, University of Jordan, Amman, 2003, p.79. [79] F. Wolf, H. Fuertig and H. Knoll, Chem. Tech. (Leipzig), 23 (1971) 273. [80] A. Maes and A. Cremers, J. Chem. Soc. Faraday Trans.I, 71 (1975 265. [81] IJ. Gal, O. Jankovic, S. Malcic, P. Radanova and M. Todorovic, Trans. Faraday Soc, 67 (1971) 999.I.J. Gal and P. Radanova/Trans. Faraday Soc, 71 (1975) 1671. [82] A.M. Tolmachev and V.A. Federov, Russian J. Phys. Chem., 39 (1965) 1204. [83] T.L. Thomas, U.S. Patent, 3,3033,641 (1962). [84] L.L. Ames, Am. Mineral., 49 (1964) 1099. [85] L. Katan and O. Bravo, Acta Cient. Venez., Suppl., 24 (1973) 136. [86] S. Lomic and IJ. Gal, Croat. Chem. Acta, 44 (1972) 403. [87] L.L. Ames,Can. Mineral, 8 (1966) 325. [88] P. Fletcher and R.P. Townsend, J. Chem. Soc. Faraday Trans.I, 78 (1982) 1741. [89] P. Fletcher and R.P. Townsend, J. Chem. Soc. Faraday Trans.I, 77 (1981) 497. [90] Z.B. Ahmad and A. Dyer, in D. Kallo and H.S. Sherry (eds.), Occurrences, Properties, and Utilization of Natural Zeolites, Akademiai Kiada, Budapest, 1988, p.431. [91] M. Loizidou and R.P. Townsend, Zeolites, 7 (1987) 153. [92] R.M. Barrer and B.M. Munday, J. Chem. Soc, 1971, 2909. [93] S. Allen, S. Carr, A. Chappie, A. Dyer and B.R. Heywood, Phys. Chem. Chem. Phys, 4 (2002) 2409. [94] A. Dyer, A.S.A. Gawad, M. Mikhail, H. Enamy and M. Afshang, J. Radioanal. Nucl. Chem. Letters, 154(1991)265. [95] P. Chu and F.G. Dwyer, in G.D. Stuckey and F.G. Dwyer (eds.), Adv. Chem. Ser, 218 (1983) 59. [96] D.P. Matthews and L.V.C. Rees, in T.S.R. Prasada Rao (ed.), Advances in Catalysis Science and Technology, Wiley, East Delhi, 1985, p.493. [97] A.M. McAleer, L.V.C. Rees and A.K. Nowak, Zeolites, 11 (1991) 329. [98] T.C. Watling and L.V.C. Rees, Zeolites, 149 (1994) 687. [99] A. Dyer and T.I. Emms, unpublished work. [100] L.L. Ames, Am. Mineral, 46 (1961) 1120. [101] M. Loizidou and R.P. Townsend, Zeolites, 4 (1984) 191. [102] R.M. Barrer and J. Klinowski, J. Chem. Soc, Faraday Trans.I, 70 (1974) 2362. [103] R.M.Barrer and R.P. Townsend, J. Chem. Soc, Faraday Trans.I, 72 (1976) 2650. [104] T.C. Golden and R.G. Jenkins, J. Chem. Eng. Data, 26 (1981) 366. [105] E.M. Kutznetsova, A.V. Sinev and A.J. Krasovskii,Vest. Mosk. Univ. Ser. 2 Khim, 39 (1998) 159. [106] N. Suzuki, K. Saitoh and S. Hamada, Radiochem. Radioanal .Lett, 32 (1978) 121. [107] R.M Barrer and B.M. Munday, J. Chem. Soc, 1971, 2904. [108] L.L. Ames, Can. Mineral, 8 (1966) 582. [109] D.W. Emerson and H.K. Welsh, Proc Seminar Nat. Zeolites in New South Wales, Geol. Survey Rep. GS 1987/146, New South Wales Gov. Australia, 1987, p 47. [110] W. Lutz, H. Fichter-Schmittler, M. Bulow, E. Schierhorn, N. Van Phat, E. Sonntag, I. Kosche, S. Amin and A. Dyer, J. Chem. Soc. Faraday Trans, 86 (1990) 61. [111] M.J. Schwuger and M. Liphard, in H.G. Karge and J. Weitkamp (eds.), Stud. Surf. Sci. Catal, 46 (1989) 673.

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[112] H.S. Sherry in S.M. Auerbach, K.A. Carrado and P.K. Dutta (eds.), Handbook of Zeolite Science, Marcel Dekker, New York, 2003, p. 1007. [113] R.P. Townsend and E.N. Coker, Stud. Surf. Sci. Catal, 137(2001) 467. [114] C.J. Adams, A. Araya, KJ. Cunningham, K.R. Franklin and IF. White, J. Chem. Soc. Faraday Trans., 93(1997)499. [115] K.R. Franklin and R.P. Townsend, J. Chem. Soc. Faraday Trans.,1, 81 (1985) 1071. [116] K.R. Franklin and R.P. Townsend, J. Chem. Soc. Faraday Trans.,1, 81 (1985) 3127. [117] K.R. Franklin and R.P. Townsend, J. Chem. Soc. Faraday Trans.,1, 84 (1988) 687. [118] K.R. Franklin and R.P. Townsend, J. Chem. Soc. Faraday Trans.,1, 84 (1988) 2755. [119] H.G. Smolka and M.J. Schwuger, Tenside Deterg., 4 (1977) 222,169 (1979) 233. [120] C. Galindo, Jr., D.W. Ming, M.J. Carr ,A. Morgan and K.D. Pickering in C. Collela and F.A. Mumpton (eds.), Natural Zeolites for the Third Millenium, De Frede Editore, Napoli, 2000, p.363. [121] J.E. Garcia, M.M. Gonzalez and J.S. Notario, Appl. Clay Sci., 7 (1992) 323. [122] L. Liberti, A. Lopez, V. Amicarelli and B. Grancarlo, in D.W Ming and F.A. Mumpton (eds.), Natural Zeolites'93, Int. Comm. Natural Zeolites, Brockport, New York, 1995, p.351. [123] M. Pansini, Mineralium Deposita, 31 (1996) 563. [124] A .Dyer in J.D. Cotter-Howells, L.S. Campell, E. Valsami-Jones and M Batchelder (eds.), Environmental Mineralogy:Anthropogenic Influences,Contaminated Land and Waste Management, The Mineralogical Society, London, Ser.9, 2000, p.319. [125] M. Howden, Inst. Chem. Eng. Symp. Ser., 77 (1983) 253. [126] A. Marrocchelli and L. Pietrelli, Solvent Extr. Ion Exchange, 7 (1989) 159. [127] S.M. Robinson, T.E. Kent and W.D. Arnold, in D.W. Ming and F.A. Mumpton (eds.), Natural Zeolites'93, Int. Comm. Natural Zeolites, Brockport, New York, 1995, p.579. [128] K.J. Hofstetter and C. Hintz, ACS Symp. Ser., 293 (1986) 228. [129] N.F. Chelishchev, in D.W. Ming and F.A. Mumpton (eds.), Natural Zeolites'93, Int. Comm. Natural Zeolites, Brockport, New York, 1995, p.525. [130] D.T. Vanniman and D.L. Bish, in D.W. Ming and F.A. Mumpton (eds.), Natural Zeolites'93, Int. Comm. Natural Zeolites, Brockport, New York, 1995, p.533. [131] J.J. Perona, AIChEJ , 39 (1993) 1716. [132] H.G. Karge and H.K. Beyer, in H. Karge and J. Weitkamp (eds.), Post-Synthesis Modification 1, Molecular Sieves, 3, Science and Technology, Springer, Berlin Heidelberg, 2002, p.43. [133] R.S. Bowman, E.J. Sullivan and Z. Li, in C. Collela and F.A .Mumpton (eds.), Natural Zeolites for the Third Millenium, De Frede Editore, Napoli, 2000, p.287. [134] E.N. Coker and L.V.C. Rees, J. Chem. Soc. Faraday Trans., 88 (1992) 263,273.

Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

205

Spectroscopic studies of zeolites and mesoporous materials K. Klier Lehigh University, Bethlehem, PA 18015 1. INTRODUCTION 2. OPTICAL METHODS 2.1. Reflectance standards and background subtraction 2.2. Site uniformity vs. distribution 2.3. Hydroxyls and water 2.4. Correlation motion 2.5. Intrazeolite complexes 2.5.1. Complexes with no known analogues in metal-organic chemistry 2.5.2. Photoluminescence 2.5.3. Intrazeolite complexes analogous with metal-organic chemistry 3. PHOTOELECTRON SPECTROSCOPY 4. SUMMARY ACKNOWLEDGEMENT REFERENCES

1. INTRODUCTION Zeolites and mesoporous materials are high-surface area solids of considerable industrial importance and at the same time may be used as objects of fundamental scientific investigations. They can exist in both "perfect" and, more often, imperfect forms with cations distributed among different sites and structural defects that include local framework deformations and external surfaces. Synthesis, structure, and study of electronic properties are important aspects of the materials science of these dispersed systems and provide a basis for applications involving equilibria with sorbates and dynamics in separation and catalytic processes. Against the large background of zeolite science, we will focus on optical and photoelectron spectroscopic methods which aim at the analysis of electronic structure that can be validated with reference materials. A quantitative determination of stability of the surface molecular complexes in various states of bonding to the zeolite or mesoporous "wall" framework can be obtained by a combination of spectroscopy, sorption, and theory. 2. OPTICAL METHODS A quantitative spectroscopic analysis of dispersed materials requires a precise understanding of the physical basis of the optical phenomena in optically inhomogeneous media. From the physical viewpoint, a polycrystalline zeolite powder is a turbid, optically inhomogeneous system. A comprehensive theory of radiative transfer, which governs the distribution of a

206

radiation field in a turbid medium that absorbs, emits, and scatters radiation, has been formulated and elaborated by the 1983 Nobel Prize laureate, astrophysicist Subrahmanyan Chandrasekhar [1]. His fundamental equation of radiative transfer for stratified media (1) is applicable to scattering-absorbing-emitting media of all possible spatial distributions of absorbers, scatterers, and emitters.

tf^

=/ (

}

1J ( . W , ) / (

,)rf ,

(1)

Here Iv is the irradiance of frequency between ^and v + dv, r = \KVP&Z is the optical path length along inward normal z , p is the local density of the medium and KV is the effective absorption coefficient which includes radiative losses by absorption and scattering from the inclination to the z-axis, /u = cos(9). The angular distribution of the scattered radiation is specified by the so called phase function p^\/Li,iu') which is proportional to the rate at which light is being scattered from the direction // to n'. Solutions of the integro-differential equation (1) have been found by Chandrasekhar for a variety of phase functions and boundary conditions. For each physical problem specified by the boundary conditions the angular distribution of the light intensity becomes known at any point of the medium and outside. If the turbid medium consists of oriented anisotropic particles, various spectacular color and glitter effects can be achieved by angleviewing of a paint layer, a coating, a printed page, a hologram, or an optoelectronic display. The phase function for isotropic scattering is an angle-independent constant and is called the albedo ("whiteness") GT0 which assumes values between 0 and 1. We shall proceed assuming that the zeolite powder is an isotropic medium with constant average density p. A number of experimental designs are possible based on measurements of I/z,fi) outside the turbid medium, either in the "forward" (9 < n/2) or "back-reflected" (8 > n/2) directions. The former case is that of diffuse transmission spectroscopy (DTS) and the latter case, of diffuse reflectance spectroscopy (DRS). The most common forms of DRS are those in which I/z,/u) is integrated over a range of angles 9, using the "preying mantis" arrangement for collecting a part of the reflected light or an integration sphere for collecting all of the diffuse reflected light at angles ranging from 8 = n/2 to n. The forward and i

backward fluxes Fv(z)* = \/uIv{z,/j)d^i 0

o

and Fv(z)~ = \lulv(z,ju)djj I

define the reflectance R from the front (illuminated) surface of a plane parallel specimen as R = FJOf /Fy(0)+ and the transmittance T at a thickness Z as T = F/Z)+ /F/0)+ , where the illumination strikes the front surface at z = 0 and light escapes the back surface at z = Z. Equation (1) has been solved by Chandrasekhar for the case of semi-infinite plane parallel medium and expressions for R and T have been obtained by Klier [2] in the form

R=

T=

\ + Rs (bcothY-a) a + bcothY-R b

b cosh Y + a sinh Y

(2)

(3)

207

where Y = % Kvp Z and £, is the positive real root of the characteristic equation m

°

ln[(l + £)/(l-£)]

The coefficients a andb in equations (2) and (3) are a = -{\+(j>2)l{2(p) andb = ^ a 2 - l ) , where g + ln(l-g) £-ln(l + | ) and Rg is the reflectance of the background, Rg = F/Z)~ /F^Zf. Equations (2) and (3) are formally identical with the earlier Kubelka's hyperbolic solutions of differential equations for forward and backward fluxes [3], although the two theories start from different assumptions and employ different definitions of constants characterizing the scattering and absorption properties of the medium. The constants a, b and Y are related to what has become known as the Schuster-Kubelka-Munk (SKM) absorption K and scattering S coefficients as K/S = a - 1 and SbZ = Y. In Chandrasekhar's theory, the true absorption coefficient av = Kvp (1 - mo) and true scattering coefficient av = Kvp mo- There are simple relations between the Chandrasekhar and the SKM coefficients av = r/K

and


(4)

and the "correction factors" r/ and % were tabulated as a function of the albedo mo. The ratio (z/rf) is fairly constant and equal, to within 1.5%, to 8/3 for values of K/S between 0 and 0.3. The ratio K/S is most easily experimentally determined from the reflectance Roofrom a semi-infinite specimen (Z -> oo) as

£J±^L. S

(5)

2RK

This is the widely used SKM equation, often introduced in commercial software. Herein we provide a web link to one of the commercial DRS spectrometer with accompanying software: http://www.varianinc.com/cgi-bin/nav7products/spectr/uv/ However, if absorbance in the specimen yields reflectance values Roo < 0.5 with the concomitant K/S > 0.3, corrections to the SKM equation by the factors T] and % become significant. The author has derived the relation for the ratio of the true absorption and scattering coefficients (av /av) as

^JI£\*=-*ZE£-2+_L av

{Z)S

m0

(
wltM<0

(6)

The "corrected SKM" relation (6) has so far not been imbedded in any commercial software, although its application is essential for strongly absorbing media. The correction factor (TJ/Z) has the limits 3/8 for small absorptions (co0 -> 1, or K/S -> 0, or | - > 0) and [l-ln(2)] for large absorptions (coo ~^ 0, or K/S -> oo, or S, -> 1 ). A graph of this correction factor is shown in Figure 1. For practical purposes, a convenient fitting function for correcting the

208

SKM function of equation (5) to obtain the true ratio {av/av) of equation (6) is a triexponential form (77/^) = 0.30685 +0.03645 exp(-K/S/\0.08966) + 0.0298 exp(A75/2.4111) + 0.0019 exp(-A/S/0.68328). An overall error associated with this fitting function is chiA2 = 6e-13 compared with the exact values of (rj/%) derived from expressions (6). Figure 1 shows the highly non-linear relationship between the reflectance and the K/S ratio, marked on the upper abscissae axis, and the correction factor {rj/%) that reduces the SKM ratio by 3/8 = 0.375 at low absorbances and sharply decreases at high absorbances, or small reflectances. This correction becomes important in a quantitative analysis using spectral signals from dark samples or intense absorptions in the NIR and UV regions. 2.1. Reflectance standards and background subtraction Variations of instrumental background are in most cases removed by using proper "white" standards. Detailed description and selection of early reflectance standards is given in Kortiim [4] and some practical implementations by this presenter [5]. The more recent and in many ways superior white standards are the US NIST Halon, basically Teflon disks that are inert to most atmospheric contaminants and are hydrophobic, an advantageous property over a wide spectral range including the near-infrared (NIR) region in which DRS is particularly useful for analysis of water, hydroxyls, and organic compounds with C-H, N-H and other hydrogen containing bonds. While both double-beam and single-beam arrangements are commercially available, the current stability of sources and detectors makes

Fig. 1. The factor (r//x)tor calculation of the true absorption/scattering (av/av) reflectance via the SKM function K/S according to equation (6).

from the

209 practical single-beam experiments wherein the reference baseline is recorded and stored once for many sample recordings performed subsequently. Due to the non-linear character of

50000

Fig 2. (a) Top: DRS spectrum of MnCl2.4H2O in the K/S and O.V/KV coordinates vs. transition energy in wavenumbers v, (b) Bottom: Region of the d -> d* transitions in MnCl2.4H2O before and after background removal. The apparent instrumental "band" around 12000 cm"1 has been removed by step (iii), and the relative intensity of the CT band in the UV was reduced by the correction (v). DRS, the method of "subtraction" of the baseline requires special attention, especially because instructions in various commercial manuals might lead to erroneous results. The procedure adopted by the author involves (i) digital acquisition and storage of the reflectance spectrum of the "white" standard, Ro(v) over the range of wavenumbers v (cm" ) of interest, where v = I/A., A, is the wavelength in cm and the conversion factor for the photon energy in

210

eV is 1 eV = 8065.5 cm"1; (ii) acquisition and storage of the reflectance spectrum of the sample, Rsampie(v); (iii) calculation of relative reflectance R(v) = RsamPie(v)/Ro(v); (iv) calculation of the ratio of the absorption to scattering coefficients for the plane parallel medium, in case of semi-infinite thickness of the SKM function K/S = [1- R<x(v)]2/[2Ro<(v)]; (v) correction of K/S by multiplying with the factor (77/^) to obtain the true ratio of the absorption and scattering coefficients (av/av). The process and results are exemplified by the analysis of diffuse reflectance spectrum of the Mn(II) compound MnCl2.4H2O with the vibrational NIR overtones and combination bands of its crystal water in the range 4500 - 8000 cm"', the weak spinforbidden d -> d* transitions in the nearly octahedrally coordinated Mn(II) ion between 8000 and 30000 cm"', and the Cl" -> Mn(II) charge transfer band in the near UV region. Figure 2 shows (a) the overall spectrum following steps (i) - (v) above, and (b) the SKM spectrum in the region of the d->d* transitions before and after the background removal. It is noted that the d -> d* transitions between 15000 and 43000 cm"' are sensitive to the local structure, in this case near-octahedral coordination by the Cl anions and water, as evidenced by an excellent agreement with the Orgel term diagram [6] for octahedral complexes with Aoct = 8200 cm"1 to within 1% of the transition energies from the ground state 6 A,g (^ 6 S) to 4 T lg (18890 cm"1), 4T2j, (22640 cm"1), 4A,g and 4Eg (24580 cm"') (<^4G), 4T2g (27650 cm"1), 4Eg (29110 cm"1) ( d* term diagrams applied to observed optical spectra of transition-ion complexes with olefins, dioxygen, mono-oxygen, and a number of other intrazeolite sorption assemblies. Predictive diagrams have been calculated for the specific locations of the Mn(II) ions in zeolites, which have been analyzed for magnetic properties [8] and structure [9]. A particularly salient feature of the d5 -> d5* transitions is the independence of some of the terms on the ligand field strength, such as that of the 6Aig -> 4Aigband at 24580 cm'in Figure 2(b). Such transitions are displayed as sharp peaks and provide an excellent internal reference for the interpretation of the rest of the absorption spectrum, wherein other transitions are sensitive to the local structure. A general question of the relation between the LFT and all-electron density functional theory (DFT) has also been addressed and comparison with experimental energies made where available. Since DFT is basically a ground state method, interest is focused on variations of the ground state energies with the position of the metal ions in the periodic table, known in the LFT as ligand-field stabilization energy (LFSE) pattern. A successful relationship has been established for compact oxides and to lesser extent for zeolites, primarily for lack of experimental data [10]. The DFT-LFSE pattern has also been established theoretically for transition metal atoms sorbed on oxygen six-ring windows on silicas [11], together with predictions of magnetic moments as a function of the metal cluster size [12]. The strategy deemed useful for the interpretation of optical spectra involves DFT optimization of the ground state geometry which fixes LFT parameters for the low-symmetry complexes, followed by the construction of the LFT term diagrams with that particular set of parameters and subsequent identification of the optical transitions. A convergence between DFT, LFT, and experiment is considered an important goal, as one theoretical approach reinforces the other.

211

2.2. Site uniformity vs. distribution This is an issue that challenges all characterization of the zeolite and mesoporous materials by structural, chemical, as well as spectroscopic analysis. In spectroscopy, evidence for uniformity is usually taken to be the independence of the spectra on concentration, linear dependence of spectral intensities on concentration of the absorbing species, and single-exponential decay of photoluminescence lifetimes of emitting species. These conditions require non-interacting centers. Examples involve linear integrated K/S vs. Co(II) concentration in Co(II) LTA for electronic transitions [13] and part-wise linear K/S vs. H2O concentration in zeolites [14]. hi the case of a distribution, two or more kinds of species can be resolved with increasing difficulty and interpretative level of sophistication. Successful examples entail the analysis of the NIR spectra of water in Na LTA and silicas, where the H20-Na and -OH...H2O adducts were distinguished from clusters by band center shifts [15], oscillator strengths, and rotational time correlation motion [16]. hi each of these cases, gravimetric or volumetric analysis of the sorbate provides additional helpful information. 2.3. Hydroxyls and water A specific application involving the analysis and distinction between various states of intrazeolitic water and hydroxyls is the DRS in the NIR region of overtone and combination bands H2O (v + 5) near 5000 cm"1, H2O (2v) near 6800 cm"1, -OH (2v) near 7300 cm"1, -OH (2v + 8) near 8100 cm"1. These bands are due to transitions (000) -> (011) and (000) -> (101) for water, where (vi V2 V3) are the vibrational quantum numbers of the symmetric stretching (vi), bending (v2) and antisymmetric stretching (V3) modes, and due to transitions 0 -> 2 for the stretching modes of the hydroxyls, the band at 8100 cm"1 being combined with the bending of the OH group with respect to the substrate. Higher overtones are also observed with a sufficient instrumental sensitivity, for water (2v + 5) near 8320 cm"1 and (3v + 5) near 10025 cm"1 (see Fig. 3). The state of bonding, agglomeration, interaction, and dynamical motion of these species is reflected in the band center positions, line widths and shapes [16]. The water bands involving the bending mode v2 are absent if all the OH vibrations are due to hydroxyls only. In general, the NIR region has proven useful for study of water adsorbed on various solids, crystal water, liquid water with solutes, extraterrestrial water and zeolites [17]. Here we give selective examples that involve quantitative aspects of the state of adsorbed water. The NIR DRS of water and hydroxyls gave rise to the following results, to list a few from a large amount of data: (i) proof of a complete dehydration and dehydroxylation of certain zeolites, including those containing transition-metal ions, in contrast to incomplete dehydroxylation of silicas; (ii) evidence for 1:1, 2:1 adducts with water oxygen-down on the surface hydroxyls; (iii) stoichometric equivalences with metal ions, starting with the framework-ion-monoaquo complexes; (iv) onset of water clustering on hydrophobic and hydrophilic surfaces. 2.4. Correlation motion Dynamical properties of sorbed molecules can be studied by converting absorption spectral bands to the time-correlation function. The method involves a Fourier transform of an absorption band

I{co) = YJP, \{l\z.mY\u)\2x8[{Eu-E,)lh-(o\ l,u

(7)

212

where the final rotational state of energy Eu, mv the vibrational transition dipole, e the exciting field, and co the angular frequency. Equation (7) describes the band lineshape including roto-vibrational interactions in hindered or frustrated rotations and corresponding changes in phase transformations [16]. If the roto-vibrational coupling is negligible, then and only then is a> the frequency displacement from the vibrational band center at o)o-

Fig. 3. The DRS-NIR spectrum of combination bands of water in MnCl2.4H2O.

The construction of the time-correlation function involves representing the 8-function in (7) by its Fourier integral, expressing the vibrational transition dipole by the Heisenberg moving dipole operator, ensemble averaging, and averaging over all directions of e in an isotropic sample, normalizing the intensity as I{co) = I(co)l \l(ca)dco

(8)

band

and introducing a unit vector « along the direction of the transition dipole moment, « = mv /<( mv )2 > 1/2 , to obtain the fundamental relation < K(0). «(t)> =

[l(G))eiMda>

(9)

band

The Fourier transform of the normalized band intensity thus describes the time development of the projection of the transition dipole onto its value at an arbitrarily chosen zero time. It is also noted that normalization removes the scattering coefficient ov if the experimentally acquired ratio (av /ov) of DRS is used. The expression

213

C(t) =< K(0). «(t)>

(10)

is the transition dipole time correlation function. Since the transition dipole rotates with the molecule, the correlation function describes the progress of randomization of the rotational motion over ensemble average that appears as an envelope on the vibrational oscillations of the particular band that is being analyzed. The relation of C(t) and the continuous wave (CW) spectrum is analogous to that in NMR. The time scale is very different in IR, however, in general less than picoseconds, while the NMR radiofrequencies cover a much longer time scale of nano- to microseconds. The IR dipole evolution at short times is revealing in that it detects roto-vibrational interactions in condensed systems, including surfaces. The angularly unperturbed correlation function for rotation in an average spherically symmetric viscous field evolves at short times as [17] C(t) =1 - (kT/IR) t2

(11)

where /« is the moment of inertia about an axis perpendicular to the vibrational transition dipole, whereas the time evolution involving roto-vibrational coupling described as [16] C(t) «1 - {kT/IR) t2 - Vi t2Z,p,(Aco/- Acov)2

(12)

contains an additional term, quadratic in time and negative, showing that the angular perturbation may produce faster than the inertial decay of the correlation function. Here the quadratic multipliers (Aco/- Acov)2 include the average rotational perturbation of the excited vibrational state Aco/ and the band center shift Acov due to such perturbations. For infinitely narrow band, just the opposite holds: /(«) in Equation (9) is a 5-function 5(co-coo) and C(t) given by Equation (10) oscillates for any length of time as exp(icoo t) with the vibrational frequency coo, without rotational perturbation. The inertial time evolution has been reported for critical water, faster than inertial motion for bulk liquid water and surface water on fluorine micas, and significantly slower motion of a nearly stationary water bound to the Na ions in LTA zeolite and SBA-15. The comparison is shown in Figure 4. It should be noted that different IR bands have their transition moments pointing in different directions: the symmetric mode Vi of water has the transition moment along the molecular axis, while the antisymmetric V3 moment is perpendicular to the molecular axis, and the moment of the bending mode V2, although also symmetric, is comparatively small. This results in the mapping C(t) using the (v+8) mode mainly of the roto-vibrational motion about the molecular axis, while the C(t) of the 2v mode describes time evolution that involves both the rotation about the molecular axis and flipping of the molecular plane about the H-H direction. Although the rotational envelope of the correlation function of water can be quite complex, primarily due to hindered or accelerated rotational motion resulting from repulsive or attractive interactions with neighbors, the average correlation time can still be defined in a number of ways, as in Texter et al. [19]. Long roto-vibrational correlation times are associated with narrow lines and short times with broad lines. Line asymmetry indicates either inhomogeneous broadening or presence of beats in the time evolution as the rotating water molecule makes and breaks hydrogen bonds with neighbors. There is a relation between the two physical pictures in the case of uniformly adsorbed layers. Examples in zeolites include the observation of symmetric narrow H2O (v+8) bands at low occupancies in

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Fig. 4. Time dipole correlation functions C(t) of water in critical state (left top), in bulk liquid water at 30°C (left center), in a monolayer on fluorophlogopite mica (left bottom), in LTA bonded to the first 4 Na+ ions (right top), in SBA-15 heated to 300°C for 2 hrs (right center), and in fully hydrated SBA-15 (right bottom). The normalized total correlation functions, obtained according to Eq. (9) involve vibrations of the transition dipole of the (v+8) band displayed as rapid oscillations. Rotational correlations including angular perturbations appear as envelopes of the vibrational correlation functions. The inertial rotational motion about the least rotational axis of the water molecule is indicated as a quadratic decay C(t) =l - (kT/I) t2 at times 0 - 0.05 psec in each C(t) vs. t graph. The graphs on the left are reproduced from ref. 18. LTA which evolve into broad asymmetric bands, tailing to low frequencies at occupancies exceeding the hydration of the floating Na+ ions. In the FAU zeolite, weakly framework-bonded water monomers were clearly distinguished from those bonded to cations after a proof of dehydroxylation was obtained. Furthermore, an abrupt increase of oscillator

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strength concomitant with the onset of clustering and increase of dielectric constant was also observed. When the material contains hydrogen bond acceptors such as fluorine micas [18], water adsorbs hydrogen-down, which is reflected in large shifts of the CW spectra and dramatic differences in C(t), as shown in Fig. 4. 2.5. Intrazeolite complexes Identification of adsorption centers in zeolites and specificity of interactions of those centers with molecules is facilitated by DRS, but for quantitative analysis the spectroscopies should be combined with gravimetric or sorption methods. In many cases intrazeolite complexes are formed with ligands no other than the zeolite framework that do not have analogues in solution metal-organic chemistry. A number of examples were given in the literature, including in publications from the host institution, selected here with focus on the Co(II) and Cu(I)/Cu(II) chemistry [20-35], and a summary of the presenter's early studies [36]. For a good identification and characterization of the complex, uniformity of the sorption centers is desired, but in some cases multiple centers could be resolved. We will not present an exhaustive review, but will rather group the complexes into classes with some of their properties identified. 2.5.1. Complexes with no known analogues in metal-organic chemistry These complexes involve adducts of the metal ions planted in exchange sites with ligands no other than the zeolite framework and the guest molecule. Probably the weakest transition-ion complex reported in zeolites is that of Co(II) ions with N2O, whose formation is accompanied with a profound change of optical spectrum but no specific 1:1 complex has been identified by adsorption which gradually accumulated N2O in the CoA cavity [37]. However, weak site-specific complexes have been observed. These comprise olefin and diene 71-complexes with transition metal ions exchanged into the zeolites. The d->d* transitions in the transition metal respond to the changes of the ligand field generated by the olefin or diene to a detail that reflects the ligand strength in the order ethyne =ethene > propene > cis-2-butene > trans2-butene [38]. Semiempirical ligand-field theoretical (LFT) codes have been produced by coworkers and the author for any structure and ligand strength, following the earliest calculation by Polak and Cerny [39]. While useful for octahedral, tetrahedral, and planar trigonal structures, these codes have too many parameters and become less practical for all possible low-symmetry complexes. However, the parameters and in fact the complexation energies can now be obtained from structures optimized by all-electron methods. In zeolites, the optimization is still a formidable task because of the size of the unit cell. Cluster approximations may be used (with caution recognizing errors due to cluster termination) when the interactions are local and the cluster is "sufficiently large". Here we give examples relating to a number of experimentally observed zeolite-transition metal-molecule complexes calculated using a trigonal alumino-silicate Me(II) or Me(I) cluster terminated by fluorine, with structural parameters summarized in Figure 5. Experimental enthalpy of formation of the specific Co(II)-ethene complex is -68 kJ/mol [38], and this is compared with the calculated DFT sorption energy of-118 kJ/mol for the cluster of Figure 5. The theoretical value is higher, which seems to be a systematic trend for the olefin complexes. However, the interpretation of spectroscopic data that this is a 71-complex, based on digonal perturbation of tetrahedral geometry, and theory agree with each other. Similar, slightly more strongly bonded CoA-ethyne 71-complex, has been identified both by crystallography, spectroscopy, and analyzed on the present model. The calculated DFT bonding energy of -121 kJ/mol is nearly equal to that of the CoA-ethene

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complex, consistent with nearly equal experimental spectroscopic splitting of the d -> d* transitions in these two complexes [38]. Furthermore, the strength of the 7t-ligand field decreases with steric hindrances in the order ethyne = ethene > propene > cis-2-butene > trans-2-butene, as reflected by decreasing digonal splitting of the excited d-states of the complexes. An example is provided by comparing the sorption energies of ethene with the more weakly bonded cis-2-butene, -76 kJ/mol, consistent with the smaller spectral splitting of the digonal bands in the Co(II) LTA of the latter.

Fig. 5. Molecular complexes with Me ions in oxygen six-ring windows (Si3Al3O6Fi2). Optimization is carried out for Me with 3 proximal O atoms and the molecule, leaving the "skeletal" S13AI3O3F12 framework frozen. In structures (a)-(c) the color distinguishes the AIO2F2 and S1O2F2 tetrahedra. In structures (d)-(f) spin isosurfaces (0.002 a.u.) are shown for the Cr(II)-O2 (spin triplet and quintet of comparable energies) and Cu(I)-C>2 complexes. Selected interatomic distances are shown in nm. The C=C bond in adsorbed olefins is slightly stretched from free ethene, 0.133 nm, and cis-2-butene, 0.134 nm, the C=O bond in the adsorbed Cu(I) complex from free CO, 0.113 nm, and dioxygen from free O2, 0.121 nm. Calculations are at the DFT/B3LYP/6-31G** level. (a) CoA-ethene 71-complex (S=3/2), calculated AEads = -118 kJ/mol; (b) Co(II)-cis-2-butene n-complex (S=3/2), AEads = -76 kJ/mol; (c) Cu(I)-CO complex (S=0) optimized to upright, carbon-down geometry, AEads=-144kJ/mol; (d) Cr(II)-O2 it-complex (S=l), AEads = - 53 kJ/mol, (e) Cr(II)-O2 re-complex (S=2), AEads = -40 kJ/mol, (f) Cu(I)-O2 7t-complex (S=l), AEads = -106 kJ/mol

An issue of the electronic state of the Co(II) ions in the trigonal sites is that of degeneracy of the ground state. Early LFT indicated crossing of a non-degenerate A| with doubly degenerate state E at ligand field strengths thought reasonable for the zeolite, and it was argued that the high orbital magnetic moment favored the doubly degenerate state E [36]. This picture was challenged by Schoonheydt et al. [40] based on DFT cluster

217 calculations that favored the non-degenerate A| ground state and interpreted optical transitions based on this result. To address this issue in a broader framework, we have now calculated the energetic and structural patterns for the entire series of the 3d metal ions in the trigonal sites of the [MeSi3Al3O6F12]"n (n=l,2,3) clusters at the DFT/B3LYP/6-31G** level. Pertinent results are summarized in Table 1. In this table, we define a composite index characterizing the Jahn-Teller distortions as IJT = A(distance)*A(angle)* A(vib), where A(distance) is the maximum difference in Me-O distances in nm, A(angle) the maximum difference in O-Me-0 angles in degrees, and A(vib), the average splitting of the vibrational E-modes in cm"1. Clearly Ti(II), Cr(II), Fe(II), Co(II) and Cu(II) suffer Jahn-Teller distortion and vibronic splitting of the vibrational E-modes, as is seen from the values of IJT that are 3-4 orders of magnitude larger than the rest. Of those ions, Co(II) shows the smallest IJT but nevertheless still orders of magnitude larger than the undistorted ions Ca(II), V(II), Mn(II), Ni(II), Zn(II) and Cu(I). The calculated energy stabilization of the Co(II) asymmetric cluster compared to that restricted to C3 symmetry is 14.2 kJ/mol. Thus the present calculations support the original picture wherein the ground state of Co(II) stems from the degenerate electronic E-state, at variance with ref. [40]. The calculated geometry is consistent with crystallographic results [41,42], although averaging of the ionic positions is assured in the latter case. The magnitude of the presently calculated Jahn-Teller distortion of Cu(II), 8e-4 nm, is to be compared with (4-8)e-3 nm estimated for Cu(II) in the SI' site in FAU from spectroscopic data [43]. It is also noted that we have completed calculations, to be published elsewhere, of the entire vibrational spectra, entropies, enthalpies and Gibbs free energies of formation of the [MeSiaAbOeF^J-n (n=l,2,3) clusters, which establish the relationships between the numerical DFT calculations and the lowsymmetry ligand-field theory - relationships that are suitable for assessment of properties and patterns in any open-site geometry that may occur in zeolites. Table 1 Structural and energetic values for 3d metal cations (Me) in trigonal zeolite sites O-Me-0 O-Me-0 O-Me-0 A(angle), A(vib), A (distance), Cation n m x 1 e4 1-0-2 2-0-3 3 - 0 - 1 degrees cm"1 7.9 Ca(II) 110.76 110.52 110.71 0.24 0.5 Sc(II) 0.8 117.44 117.42 117.35 0.09 0.8 Ti(II) 36.7 117.22 118.78 117.23 1.56 17.8 V(II),S=3/2 115.73 115.85 115.88 0.15 0.8 2.1 Cr(II) 5.9 118.41 117.72 121.19 3.47 13.7 Mn(II) 3.6 119.32 0.19 1.0 119.16 119.35 Fe(II) 51.9 120.31 5.3 117.78 119.77 2.53 Co(II) 119.66 11.3 7.1 119.80 118.31 1.50 Ni(Il) 2.5 118.12 118.06 118.05 0.07 0.8 8.2 Cu(II) 120.91 119.94 118.02 2.89 14.3 Zn(II) 2.9 119.72 119.75 119.86 0.14 1.3 3.3 Cu(I) 119.10 119.22 119.05 0.17 0.8 Ti(III) 1.6 118.72 118.76 118.81 0.09 0.5 V(II),S=l/2 1.6 116.47 0.02 116.46 116.48 1.3 Co(II), C 3 0.0 119.89 119.89 0.00 0.0 119.89

IJT

x Ie4 0.95 0.06 1019.08 0.25 280.48 0.68 695.93 120.35 0.14 338.88 0.53 0.45 0.07 0.04 0.00

Another interesting case is that of the reversible dioxygen complex with the Cr(II) ions in LTA [28]. Sorption-desorption of O2 is Cr(II)-specific and is accompanied with reversible change of spectra between the pale blue Cr(II)A, grey charge-transfer complex

218

Cr(II)A-O2, and back to the pale blue Cr(II)A upon oxygen desorption. The theory based on the cluster model predicts sorption energy of -52.6 kJ/mol-O2 for the spin triplet, -39.6 kJ/mol-O2 for the spin quintet, and highly endothermic heptet, +212.5 kJ/mol and singlet, +148.7 kJ/mol. These sorption energies are compared with that determined experimentally, 62 kJ/mol, to favor structure (d) in which two xc-electrons of the adsorbing O2 molecule cancel spin with 2 out of 4 unpaired electrons of Cr(II). Thus for this weak complex the theory accounts well for the observed reversibility, and moreover identifies the Cr(II)-O2 complex as "peroxo", 71-bonded, 4 spin-paired complex with 2 remaining unpaired electrons located on the Cr(II) center. The intrazeolitic Cr(II)-O2 complex is metastable, however, and converts to an irreversible mono-oxygen complex of Cr(IV) at elevated temperatures [45]. The dioxygen complex formed with the Cu(I) centers is tilted, however, into a "superoxo" geometry, and the net spin is distributed between the O2 "anion radical" and the electron hole in the Cu center (Figure 5(f)). 2.5.2. Photoluminescence Excited states are probed by photoluminescence, in addition to optical absorption. Excitation, emission and absorption spectra, together with time-resolved luminescence, provide a wealth of information when there is an emitting center in the zeolite. In the inorganic variety, Cu(I) ions proved to be centers characterized by quantum efficiency near 100% [46]. The emission wavelengths were found site-sensitive (for 3 types of sites in Cu(I) FAU, at 540 nm (site I'), 470 nm (site II'), and 560 nm (site II)), as demonstrated by pulling the Cu(I) ions in FAU from "buried" to exposed positions by adsorption and desorption of CO. The luminescence lifetime of 120 [xs with single-exponential decay provided evidence for uniformity and identity of the Cu(I) centers in the zeolite either exchanged with Cu(I) from liquid ammonia solution of Cul or reduced from Cu(II) by hydrogen. Upon gradual oxidation of the Cu(I) centers by molecular oxygen, the newly formed Cu(II)-O2~ species became acceptor centers for the resonance energy transfer, dramatically shortened the luminescence decay time which became multiexponential due to the distribution of the sensitizer-activator distances, and could be converted to structural distances of 8 and 11 A between the sensitizer Cu(I)* and activator Cu(II)-C>2~ at near-completion of the adsorption process. Resonance energy transfer was also observed and interpreted in FAU loaded with different cations, the Cu(I) sensitizers and Co(II), Ni(II), and Mn(II) activators at different concentrations, and an excellent correlation between observed and predicted relative quantum efficiencies of the Cu(I) emission was reported [47]. The energy and structural relationships for the Cu(I) centers are summarized in Figure 6. The long luminescence lifetime of the bare Cu(I) center indicates that the downtransition is between the excited Cu(I)* spin triplet and the ground Cu(I) singlet. Furthermore, the 120 us lifetime allows a full structural relaxation in the excited state before the emission takes place. Notwithstanding the well known challenges of the theory of the excited states, especially the DFT theory that essentially is that of the ground states, attempt has been made to use DFT with spin constraints to calculate the energies of the Cu(I)* triplet in unrelaxed and relaxed state. The electronic structure in two optimized geometries is important to consider, that of the ground state Cu(I) singlet, and of the excited Cu(I)* triplet. The up-transition is a two-step process, a rapid Franck-Condon excitation of Cu(I) into UV singlets followed by radiationless decay into the Cu(I)* triplet. We divide the latter further into the calculation of the Cu(I)* triplet energy with no structural change followed by relaxation to equilibrium geometry of the Cu(I)* triplet. The orbital structures of the ground and excited states are important for the understanding of this excited state: analysis of the

219

Fig. 6. Diagram for photoluminescence of Cu(I) centers in oxygen six-rings of an aluminosilicate zeolite framework. Energy in eV on the vertical scale is shown as a function of distances in the [x,y] plane as well as along the z-axis. The ground state is Cu(I) [Ar]3d10 spin singlet and the photoemitting excited state is a strongly distorted Cu(I) [Ar]3d94sp triplet. Calculations (spinrestricted DFT/B3LYP/6-31G**) are compared with experiment in text.

HOMO in the Cu(I) ground state yields 95% occupancy of the 3d-orbitals, primarily the in-plane {dxy, dx2-y2}, 5% occupancy of the 4p orbitals, and none of 4s, while the HOMO of the unrelaxed Cu(I)* excited triplet has 13% occupancy of the d-orbitals (primarily dZ2) , 81% of 4s, and 6% of 4p, which undergoes a rehybridization to a 11% mix of dX2-y2 and dZ2 orbitals, 78% of 4s, and 11% of 4p in the relaxed Cu(I)* triplet. The calculated energy change between the unrelaxed and relaxed Cu(I)*, -0.48 eV, is associated with two mechanisms: first, the movement of the in-plane Cu(I) ions along the 3-fold axis due to the

220

repulsion between the voluminous 4s electrons in Cu(I)* and the lone pairs of the proximal oxygens, and a large off-axial Jahn-Teller distortion in a configuration which is due to the degeneracy of the excited E-state of Cu(I)* originating from opening up the 3d shell upon photoexcitation. The pertinent electronic structures are graphically represented by spin and orbital isosurfaces in Fig. 7. Because of the open structure of the cluster model employed, we consider the closest analogy to be between this geometry and that of the SII site in FAU characterized by Cu(I) emission wavelength of 560 nm, or 2.21 eV energy [46]. The cluster model gives 2.88 eV for the unrelaxed Cu(I)* triplet -> Cu(I) singlet transition and 1.88 eV for the relaxed Cu(I)* triplet -> Cu(I) singlet emission. While there appears to be a qualitative agreement between the current implementation of theory and experiment, the theory underpredicts the emission energy by 18%. Obviously more precise treatment of the excited states, as well as a larger cluster or periodic lattice model, will be required for a further refinement of numbers, although the original and the current physical description of these luminescence centers appears to be correct. Thus, despite some discrepancies in detail, the spectroscopy and theory converge toward one another, which provides a background for analysis of more complex zeolites with multiple sites.

Fig. 7. Isosurfaces (0.002 a.u.) centered at Cu. The unbalanced electron spin (S=l/2) in the ground state of the Cu(II)A [Ar]3d9 (left) resides in the singly occupied in-plane dxy-orbital, which becomes doubly occupied in the HOMO of the closed-shell Cu(I)A [Ar]3d10 (center). Upon excitation, Cu(I)A* [Ar]3d94s (right) (S=l) forces the Cu(I) ion out of the plane along the 3-fold axis due to repulsion between the voluminous 4s orbital and the lone pairs of the nearest oxygens, and moves off that axis by the strong Jahn-Teller distortion due to opening up the 3d-shell as in the upper surface depicted in Fig. 6. All the Cu3d and 4s interactions with the cluster environment are antibonding, as demonstrated by the orbital nodes along the Cu-0 bonds, with a slight nephelauxetic spill-over of the spin and orbital density to the proximal oxygens.

2.5.3. Intrazeolite complexes analogous with metal-organic chemistry A vast number of such complexes have been reported in the literature, and a review of these is outside the scope of the present paper. In addition to X-ray structures, NMR, etc., the

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identification by DRS may involve a comparison with known compounds and detect subtle changes in electronic structure due to intrazeolite fields, anionic framework, and dynamic structural distortions. Some examples involve intrazeolite octahedral and tetrahedral complexes [36] in which the d^d* transitions are slightly shifted from those in solutions due to compression or expansion of the coordination sphere of the ligands in the zeolite. The power of DRS rests in a very quick determination of the local structure, oxidation state of the cation, and loss/gain of ligands resulting from changes in gaseous environment, including the option to follow those changes during a reaction. Near future studies are anticipated in mesoporous materials, in which reactions of larger molecules can be catalyzed. In most general terms, however, there are different schools of thought as to whether known metalorganic complexes anchored in zeolites and mesoporous silicas would have advantage to solution chemistry which has led to many recent important discoveries, including asymmetric syntheses and single-site polymerization catalysis.

3. PHOTOELECTRON SPECTROSCOPY X-Ray and UV photoelectron spectroscopy (XPS and UPS) yields valuable information about the chemical and electronic structure of zeolites and mesoporous materials, similarly to that for compact solids and other surfaces. Chemical information resides in the binding energy core-level shifts (CLS), in valence-band (VB) spectra, and in Auger lines. Analytical measurements aim at a quantitative assay of concentrations of elements in their various valence states using calibrations based on the knowledge of instrumental factors, ionization cross-sections and photoelectron escape depths. Detailed discussion of all the factors that enter into XPS analysis is available in the literature, of which we single out Briggs and Seah [48]. It is within the escape depth of 2 - 20 nanometers that the material is analyzed, and this imposes demands on careful inclusion of all the factors relating the composition and distribution of the elements to the XPS signals observed. In the case of flat specimens, a great advantage is gained by angle-resolved XPS and UPS, and in the case of regular crystal structure of regions within the spatial resolution of the instrument, by photoelectron diffraction, as exemplified in some of the author's recent work [49]. However, XPS analysis of amorphous and polycrystalline materials requires angle averaging, such as by the Dreiling's factor g which enters formulas for quantitation of the XPS peaks [50], in addition to the corrections for different kinetic energies of the escaping photoelectrons. In the simplest case the elements are distributed uniformly throughout the material, including the surface region. There are several criteria for interpretation of XPS emissions from materials of uniform composition, one of which involves comparing two or more XPS lines of the same element with different BEs, therefore with different kinetic energies and escape depths. The analysis is also facilitated by internal standardization of one element against another. In recent applications involving zeolites and the SBA materials, Hunsicker et al. [51] was able to distinguish between framework-substituted and cation-exchanged Zn in FAU. In synthesized Zn-FAU the XPS and chemical analyses agreed to within relative 15%. Acidity in zeolites was also probed by XPS through the Nls CLS of pyridine [52]. Johansson et al. [53] outlined the methodology in greater detail and noted for the first time the relationships between the CLS of S2p emissions of sulfonic or sulfate acid groups and Nls of sorbed nitrogen bases upon acid-base interactions of different strengths that moved the S2p CLS to lower binding energies (BE) concurrently with those of Nls in the base to higher BEs, thus identifying the donors and acceptors of hydrogen bonds. This approach was later used in the analysis of SBA-15 for the oxidation state of sulfur, stoichiometry of the pendant propyl-

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sulfonic groups, and their adducts with the nitrogen bases [54, 55], and currently in the analysis of ion-exchanged SBA-15 [56]. 4. SUMMARY Optical methods for electronic and roto-vibrational transitions and photoelectron spectroscopy for chemical core-level shifts and valence band analysis can yield quantitative information regarding the electronic structure of intrazeolite "active centers" and their adducts with molecules. Theory and its advancement play a crucial role in the interpretation of the spectra, but practical analysis without extensive theoretical involvement is also feasible. Advances are seen in both the precise applications of spectroscopic methodologies and the synthesis of a variety of novel structures, as well as in examination of systematic patterns of properties that are governed by the composition and structure of the important zeolite and mesoporous materials.

ACKNOWLEDGEMENT The author wishes to express gratitude to the organizers for the invitation to present selected topics on spectroscopy of zeolites and mesoporous materials at the 2005 FEZA school.

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Electron tomography of molecular sieves Krijn P. de Jong", Abraham J. Kosterb, Andries H. Janssen" and Ulrike Ziese" a

Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

b

Department of Molecular Cell Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

1. INTRODUCTION 2. FROM 2D TO 3D-TEM 3. CASE STUDIES WITH MOLECULAR SIEVES 3.1. Mesopores in zeolite crystals 3.2. Pore architecture of mesoporous materials 3.3. Metal particles in ordered mesoporous materials 4. NEW DEVELOPMENTS 4.1. Quantitative image analysis 4.2. Element specific electron tomography 4.3. Study of complex (industrial) catalysts 5. CONCLUSIONS REFERENCES 1. INTRODUCTION Research on zeolites and mesoporous materials depends critically on the availability of characterization techniques that provide information on their electronic and structural properties. Many techniques (e.g. XRD, NMR, XAFS, UV-VIS, IR, Raman) provide information about bulk properties whereas surface sensitive techniques (e.g. XPS, SIMS, LEIS) will provide information from the surface of the particles of porous materials. For modern research spatially resolved information is indispensable, in particular with the advent of complex hierarchical materials that combine micropores and mesopores. For the latter sake, electron microscopy is of growing importance for the study of molecular sieves as is also apparent from the number of papers published on this topic over the last ten years (Fig. 1). Please note that the almost four-fold increase in papers over the last ten years about electron microscopy on molecular sieves far outnumbers the relative increase of the total number of papers on molecular sieves (increase by factor 1.4).

Present address: Shell Research & Technology Centre Amsterdam, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands

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Fig. 1. Number of publications per year for the period 1993-2003 that involve electron microscopy studies on molecular sieves (Sci Finder, August 2004).

In 1933 Ruska built he first electron microscopy with a magnification that surpassed that of the light microscope. For this achievement Ruska received the Nobel Prize for Physics in 1986. Since then an enormous development of electron microscopy has taken place that has led to a wealth of innovations. The modes of operation that are now used in modern electron microscopy — amongst others — are summarized in Fig. 2. Scanning Electron Microscopy (SEM) is particularly powerful to study the topology of solids, whereas Transmission Electron Microscopy (TEM) is needed to study the interior of solid specimens. Both types of microscopes can be used to carry out elemental analysis using Energy Dispersive analysis of X-rays (EDX). For TEM operation in Bright Field (BF) one makes use of the transmitted electrons following both coherent and incoherent scattering. Dark Field images are obtained by detection of either coherently scattered electrons (DF) or Rutherford scattered electrons

Fig. 2. Modes of operation of electron microscopy. See text for explanation.

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using a high-angle annular dark field (HAADF) detector. Finally, inelastically scattered transmitted electrons can be used to carry out element specific Electron Energy Loss Spectroscopy (EELS) with sub-nanometer resolution. Excellent reviews on electron microscopy for the study of porous materials have appeared over the years. The reviews by Datye [1], Thomas and Midgley [2], Anderson et al. [3] and De Jong and Koster [4] and Ziese et al. [5] provide an introduction to the field. The different modes of electron microscopy summarized in Fig. 2 have inherent prospects and limitations as will be discussed in this paper. First we will discuss these limitations and then introduce the use of 3D-TEM, in particular electron tomography, for the study of molecular sieves (section 2). The use of electron tomography in materials science is quite recent and exciting information on zeolites and mesoporous materials has already been obtained. These case studies will be dealt with in section 3. New and future developments are summarized in section 4. 2. FROM 2D TO 3D-TEM In Fig. 3 images of hydrotalcite platelets with a lateral size of-200 nm obtained by SEM and TEM are shown. SEM provides information on the spatial organization of the material and its surface topology while TEM gives details about the interior of the specimen. By comparison of both images it appears that the TEM image does not provide spatial information along the beam axis (z-axis). In fact in TEM the spatial resolution in the third dimension is not better than the thickness of the sample, which may be several 100 nanometers. This fundamental limitation of TEM is further illustrated in Fig. 4. Since the electron beam transverses through the sample a two-dimensional (2D) projection image is obtained from a three-dimensional (3D) object. For many years efforts have been made to collect more detailed information about the 3r dimension in TEM. For thin samples (< 50 nm) through-focus series [6] and electron holography [7,8] have been applied successfully. In this paper we mainly restrict the discussion to electron tomography (ET) that can be applied to thicker samples (50-500 nm). For an overview of the several modes of 3D-TEM that elaborates on the microscopy aspects we refer to an earlier review [4]. Here we will summarize the principle and practical aspects of ET for the study of molecular sieves.

Fig. 3. Electron microscopy images of hydrotalcite platelets of about 200 nm lateral size, a - SEM; b TEM.

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Fig. 4. Projection of three-dimensional objects Fig. 5. Schematic presentation of the principle of by a transmitted electron beam leading to two- tomography. Collection of a range of TEM projections at different tilt angles and consecutive dimensional patterns. Fourier transformation fills up the 3D Fourier space. Inverse Fourier Transformation yields the 3D object in real space.

In tomography a series of 2D-projections of a sample is collected. The Fourier transformations of the projections to fill up the 3D Fourier space and a back-transformation result in a reconstruction of the 3D object (Fig. 5). Examples involve X-ray and Positron Emission tomography used extensively in medical science and X-ray micro-tomography in materials science. Tomography enables a non-destructive investigation of the interior of truly unique structures. The tomography techniques mentioned above provide resolution in the millimeter to micrometer range. Electron tomography allows one to obtain reconstructions in 3D with nanometer resolution. The early developments of the basic aspects of ET date back to 1968 with three independent papers [9-11] published in the same year. The progress for imaging larger structures in 3D had to await automated data acquisition and image processing as has been reported by Koster and co-workers [12,13]. In the year 2000 our groups have published the application of ET using Bright Field TEM in materials science [14,15]. One year later, Midgley et al. [16] extended the technique to High-Angle Annular Dark Field (HAADF) detection. Hereafter we provide a short summary of the technical aspects of application of electron tomography for materials science. In Electron Tomography used in materials science one typically collects a series of images of a single specimen by tilting the sample from +70° to -70° with 1 ° increments using a computer-controlled microscope. Automated track and focus shift corrections followed by data collection using a CCD camera allow restriction of the electron dose and thus sample damage and of the measurement time (20-60 min. typically). Improvements of the automation procedures by Ziese et al [17] have allowed the very short data collection times mentioned. Following data acquisition accurate alignment of the so-called data stack is mandatory. The use of fudicial markers on the grid - often small gold particles - facilitates this alignment process followed by 3D-reconstruction and visualization [5,12,13]. Visualization may involve surface or volume rendering often in colors assigned to bands of gray values or slices through the 3D reconstruction.

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The resolution (R) of the reconstructed volume in the third dimension according to the Crowther criterion [18] reads T

R = TT—

N with T being the thickness of the specimen and N the number of images collected. For a specimen of 7=100 nm and iV=150 one obtains a value of R=2 nm. In practice resolutions along the z-axis have been obtained better than predicted by the Crowther criterion [5,19]. In performing TEM studies one should always be concerned with statistics, that is whether or not the specimen studied is representative for the sample as a whole. A zeolite crystal of ~ 500 nm has a weight of about 10"13 gram; in other words 1 gram of zeolite contains 1013 crystals. For this reason, conclusions from TEM in general or ET in particular should be carefully compared to results from macroscopic techniques such as XRD, XPS and physisorption that provide properties averaged over large samples. 3. CASE STUDIES WITH MOLECULAR SIEVES First-of-its kind results [14,15] with electron tomography for the study of zeolites have been obtained with Ag/Na-Y. The location of the Ag particles of about 10 nm could be unequivocally established with respect to the surface and the interior of the crystals. The moderate resolution of 5 nm of the reconstructed images in this study related to the rather large zeolite crystals (500 nm) involved. Hereafter, we first discuss in more detail the use of electron tomography to study mesopores in zeolite crystals and, secondly, case studies involving metal particles and pore architecture of ordered mesoporous materials. 3.1. Mesopores in zeolite crystals The beneficial role of mesopores in zeolite crystals has been known for a long time [20,21] and has been reviewed recently [22]. Mesopores alleviate mass transfer limitations for reactants and products in catalysis by effectively reducing the path length for diffusion. In this respect mesoporous zeolite crystals are competitors of small zeolite crystals that can be produced in many ways. More recently combinations of mesoporous materials with zeolites have been studied to arrive at supported nanocrystals [23,24]. Up till now for industrial applications mesoporous zeolites are the preferred option based on considerations of (hydro)thermal stability and production costs. The most important routes to obtain mesoporous zeolites involve post-synthesis treatments such as steaming and acid or base leaching [22]. More recently secondary templating involving carbon has been explored [25,26]. The leaching treatments lead to both chemical changes of the system (e.g. Si/Al ratio of the lattice) and the generation of a mesopore network. The nature of the mesopores, i.e. shape and tortuosity of the pores, are important factors for the effectiveness to facilitate mass transfer. Up till now the details of the mesopores network could not be studied in detail. Electron tomography has revealed many of the intricate details published in a series of papers [27-31 ] that are summarized below.

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Table 1 Physical characterization of zeolite Y samples. Zeolite NaY USY XVUSY HMVUSY

Si/Al (at/at) 2.6 2.6 39.3 5.0

Crystallinity %Y 100 87 72 71

vV

• micro

(ml g 1 ) 0.34 0.26 0.28 0.15

* meso

(ml g 1 ) 0.05 0.11 0.25 0.47

c ^ meso

(mV) 8 63 120 146

The physical properties of the zeolite Y samples that have been studied are summarized in Table 1. The as-synthesized NaY does not contain intra-crystalline mesopores, whereas the steamed USY as well as the steamed and leached samples (XVUSY and HMVUSY) do show a significant mesopore volume as deduced from nitrogen physisorption. A conventional TEM image together with a slice through the 3D reconstruction of a crystal of NaY (Fig. 6) confirms the absence of mesopores. Please note that the resolution of the images is such that micropores and the lattice planes cannot be discerned. The images of a crystal of USY (Fig. 7) diplay great detail about mesopores and density variations in the lattice. First, the conventional TEM image of USY (Fig. 7a) displays contrast variations different from the much more homogeneous NaY crystal. From the contrast variations as well as high-resolution images [32] in the past one has been able to infer the presence of mesopores. Electron tomography results as displayed in Figs. 7b and 7c much more clearly visualize the mesopores as well as the density variations. The dark rim that surrounds the USY crystal studied consists of alumina-rich deposits as has been confirmed by XPS measurements [27], The dark spots in the crystals are thought to consist of extra-framework alumina that fills up potential mesopores in the crystal. The light grey zones are thought to be microporous, crystalline regions. The lightest zones in Figs. 7b/c are empty mesopores that lead to the mesopore volume observed with nitrogen physisorption. For the sake of illustration in Fig. 7d a USY crystal damaged by the electron beam is shown. Clearly, beam damage leads to blurred images that differ vastly from reconstructions of intact crystals (Figs. 7b/c).

Fig. 6. Conventional TEM image of a NaY crystal (a) and a slice through the 3D reconstruction of this crystal (b).

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Fig. 7. Conventional TEM image of a USY crystal (a), two slices through the 3D reconstruction of this crystal (b,c) and a slice through the 3D reconstruction of a radiation-damaged USY crystal (d).

Fig. 8. Conventional TEM image of an XVUSY crystal (a) and a slice through the 3D reconstruction of this crystal (b).

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After steaming and acid leaching extra-framework alumina has been removed from both the exterior of the crystals and the mesopores as can be deduced from the reconstructed image of XVUSY in Fig. 8b as well as from the Si/Al ratio (Table 1). The light areas in this slice of the 3D reconstruction are interpreted as mesopores whereas the grey zones relate to the crystal lattice. The size of the mesopores (4-40 nm) is in excellent agreement with results from physisorption [27,29]. Please note that these detailed conclusions cannot be obtained reliably from a conventional TEM image (Fig. 8a). In Fig. 9 three orthogonal slices through the reconstruction of the XVUSY crystal are displayed. The x-z projection shows a cylindrical mesopore that connects the interior of the crystal with the 'outside world'. For one and the same mesopore marked with a white arrow in all three projections it is clear that no connection to the external surface via the mesopore network exists. In other words, this mesopore is a cavity in the crystal and will hardly contribute to reduction of mass transfer resistance. From independent measurements based on physisorption and mercury intrusion [29] it has been found that 30% of the mesopore volume in this material is present in cavities that are connected to the external surface only via micropores. More recently elegant proof from thermoporometry experiments for the existence of these cavities has been published [31].

fig. y. inree orthogonal slices through the 3D-1LM reconstruction ot an A V U M crystal showing a cylindrical mesopore as well as cavities. One of the cavities is marked with a white arrow.

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Fig. 10. 3D-TEM slices through the reconstructions of two HMVUSY crystals.

To allow for better accessibility of zeolite Y crystals extensive acid and/or base leaching has been applied. The physical properties of the thus obtained HMVUSY are shown in Table 1 while electron tomography results are shown in Fig. 10. Clearly cylindrical mesopores predominate at the expense of the number of cavities in these crystals. A disadvantage of these extensive steaming and leaching treatments is a loss of crystallinity and the filling up of micropores with amorphous material [29]. Furthermore, independent control of the desired Si/Al ratio of the zeolite is difficult. For this reason methods have been devised to generate mesopores during synthesis of zeolites. Workers from Haldor Tepsoe were the first to propose the use of a second (carbon based) template during zeolite synthesis [25,26]. Using several EM modes we have studied a silicalite sample [30] the results of which are summarized in Fig. 11. Tremendous mesoporosity is apparent albeit with appreciable tortuosity due to the use of carbon black. For this reason carbon nanotubes [26] and carbon nanofibers [30] with high aspect ratios have been used as templates too. Mesopores with more straight, cylindrical shapes were thus obtained. 3.2. Pore architecture of mesoporous materials In the early 1990-ies researchers in Japan and the USA have opened the field of ordered mesoporous materials (OMM). Important examples of these materials comprise MCM-41 [33] and SBA-15 [34]. The advent of OMM has stimulated an enormous research effort of groups previously involved in zeolite research. Applications in catalysis, separation and chemical sensing, to name a few, have been envisaged at an early stage. The often silicabased OMM have been used more recently to study confinement effects in crystallization and to obtain nanoparticles and nanorods of metals, metal oxides and metal sulfides. Going one step further, negative replicas from silica OMM based on carbon have been introduced by Ryoo et al. [35] and stimulated imagination on other possibilities. In this respect nanocasting has been emphasized by Schiith in a recent review [36]. From all these research efforts and developments interesting applications are emerging.

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Fig. 11. Electron microscopy of mesoporous silicalite after burning off the carbon matrix: a, b Secondary Eletron images (SEM), c TEM image, d 3D-TEM slice. Detailed knowledge of pore size, topology and architecture is indispensable in exploiting the benefits of ordered mesoporous materials. Electron microscopy has been used extensively to obtain this information and in many cases TEM is the technique of choice. An example of a conventional TEM image of MCM-41 is shown in Fig. 12. The hexagonal ordering over an extensive domain size of the mesopores is apparent with pores of ~3 nm and silica walls of ~1 nm. Realizing that the resolution in the third dimension is not better than the thickness of the sample we do not obtain information of the pore architecture in detail. Furthermore, a precise measurement of pore diameter and pore wall topology cannot be deduced from this type of micrographs. Rather than electron tomography, electron crystallography (EC) is used to obtain the details of pore size, shape and topology. Electron crystallography of zeolites [37] and ordered mesoporous materials [3, 38-41] has developed to an impressive level over the last decade. The great advantage of EC in general is that much smaller crystals can be used than with Xray diffraction. In particular the Terasaki group has made great strides in resolving the structures of materials such as MCM-48, SBA-1, SBA-6 and CMK-4 [38-41]. In Fig. 13 we have reproduced the outcome of a study on MCM-48. The pore structure of MCM-48 is elucidated in detail as is apparent from the electron density maps. Electron crystallography can be used to study well-ordered crystals as it relies on periodicity within the object. To collect information for unique objects that lack periodicity electron tomography may be used. For the study of ordered mesoporous materials electron

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Fig. 12. TEM image of MCM-41, pore size ~3 nm.

Fig. 13. Pore topology of MCM-48 obtained from electron crystallography [38],

Fig. 14. TEM images of an agglomerate of SBA-15 particles at different tilt angles suggesting straight or slightly bended pores (a) and showing strongly curved pores (b).

tomography has been used to study pore architecture at mesoscopic length scales (SBA-15). Furthermore, disordered mesoporous materials can be studied (TUD-1). Both studies are summarized below. The Stucky group [34] has synthesized SBA-15 by using block-copolymers as templates. Similar to MCM-41, the mesopores (5-10 nm) of this silica-based material display hexagonal ordering. From a limited number of TEM images it is tempting to conclude that the pores are straight channels over large length scales. In Fig. 14a for example a side-on view of the channels suggests a modest curvature of the pores [42]. In many cases with SBA-15 particles, however, an agglomerate of smaller particles is in fact studied. When acquiring a tilt-series of images over appreciable angles, one often obtains a few images that display pore curvature [43]. So-called 'noodle-shaped' pores are apparent in Fig. 14b. Depending on the synthesis conditions of the SBA-15 material the extent of pore bending varies largely [42-44]. For applications that involve mass transfer 3D mesoporous materials may have benefits over ID materials such as MCM-41 and SBA-15. Jansen et al. have recently published the synthesis of TUD-1, a material that holds promise because of attractive catalytic properties and low synthesis costs [45,46]. As the mesopores of TUD-1 do not display long-

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range order, electron tomography is the technique of choice to acquire knowledge of its pore system. A surface-rendered visualization of TUD-1 with an average pore diameter of about 10 nm is shown in Fig. 15. The Pt-particles in the pore network have been used as markers for data alignment. The tremendous porosity of this material is apparent with clear 3D pore architecture. It has been shown that this pore architecture is beneficial for e.g. liquid-phase epoxidation catalysis [45,46]. 3.3. Metal particles in ordered mesoporous materials Ordered mesoporous materials are attractive hosts for nanoparticles and nanorods of metals, metal oxides and metal sulfides. Most often the mesoporous materials consist of amorphous SiC>2 whereas the particles or rods involve heavy elements. In this context we briefly comment on the use of bright field (BF) versus dark field (DF) techniques in electron tomography. As mentioned in section 2, researchers at Cambridge have developed HAADFtomography. This technique is particularly suitable to study small particles of heavy elements in a matrix of light elements as it relies on the so-called Z-contrast (signal increases approximately linear with Z2, Z being the atomic number of the element). The advantages involve (i) less contribution from diffraction contrast that allows more reliable reconstructions and (ii) better contrast between metal and matrix. In theory the ultimate resolution of HAADF-tomography should be better than that of BF-tomography but in practice 'the jury is still out'. Hereafter we will discuss some results based on the HAADF technique together with results that have been obtained in our departments using BF tomography. Using STEM/HAADF tomography excellent results have been obtained for PdRu nanoparticles dispersed in MCM-41 [16]. Deposition of the particles had been done using Pd6Ru6 carbonylate clusters [47]. Due to thermal treatment and/or effects of the electron beam both small clusters (1 nm) and larger particles are noted in the 3D reconstruction. A representative volume rendering of the reconstruction is shown in Fig. 16. By assigning colors to ranges of gray bands the metal particles could be highlighted in red. From projections among a number of orientations [48] it is evident that some particles are located inside the mcsoporcs whereas many reside on the outer surface of MCM-41 particles. These results show the unique capability of electron tomography to characterize nanostructured catalysts in great detail in 3 dimensions. Janssen et al. [49] have done similar studies on SBA-15 loaded with either ZrC"2 or with Au particles. In their paper it is shown that not every pore in SBA-15 is equally loaded with particles: some pores are empty and others contain a row of particles. In this work ET

Fig. 17. Adsorption-desorption isotherms for N2 at 77 K of Au/SBA-15 and ZrCVSBA-15.

237

has been complemented with nitrogen physisorption data (Fig. 17) that show a two-step desorption branch. As has been discussed before by Van der Voort [42] this desorption characteristic proves pore blocking of the mesopores with nitrogen escaping via the micropores that are abundantly available in the silica walls of the material. A more detailed ET study by Ziese et al. [5] for Au/SBA-15 has lead to renderings that are shown in Fig. 18. From the orientation in Fig. 18a it is clear that gold particles fit just into the mesopores in line with results from physisorption. The orientation in Fig. 18b shows the hexagonal arrangement of the mesopores while all together they provide evidence for the Au particles to be inside the mesopores and not at the outer surface.

Fig. 15. Volume rendering of a Pt/TUD-1 particle (height 216 nm) showing the 3D mesopore network in orange with Pt particles in red.

Fig. 16. Projection of the reconstructed volume of PdRu/MCM-41 showing the hexagonal order of the mesopores and the metal particles [16].

Fig. 18. Rendering of an Au/SBA-15 particle (256x256x166 nm3) at two different orientations, i.e. side on view of the pores (volume rendering, a) and parallel to the pores (surface rendering, b).

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Fig. 19. Surface rendering of a 50 nm sized cube inside an Au/SBA-15 particle viewed at different angles showing Au particles (a) and small pores connecting the mesopores (b).

Fig. 20. Surface rendered view of the reconstruction of an XVUSY zeolite crystal; edge size ~ 400nm (a) and pore size distribution calculated from the reconstruction of USY and XVUSY crystals (b).

In Fig. 19 we show again Au/SBA-15 but now with more detail with respect to the pore wall topology. At some orientations it seems that smaller pores connect the micropores (e.g. Fig. 19a). We should keep in mind, however, that the grey-level thresholds used for the definition of the surfaces (gold - yellow, pore wall - orange) affect the detailed topology of the pores and only certain ranges lead to the visualization of micropores connecting the mesopores. Here we are at the limits of what these data sets can bring and we are cautious to make firm statements about direct proof for the micropores in the silica walls of the mesopores of SBA-15. We expect that improvements in the techniques will make it possible to image these types of complex micro/mesopore systems in the near future.

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4. NEW DEVELOPMENTS 4.1. Quantitative image analysis With the information rich 3D-reconstructions of porous materials it becomes very attractive to extract quantitative information such as: • • •

The average distance between particles inside a pore The size of particles outside and inside pores Pore size distributions, specific surface areas

These quantitative data can underpin the results of macroscopic techniques as a first step. Moreover, it provides the microscopic basis for models to describe accessibility of porous materials. A first example to extract quantitative data from ET reconstructions has been obtained by workers in Liege [50]. For a crystal of USY and a crystal of XVUSY that have been discussed both in section 3.1 they have derived size distributions of the cavities inside the crystals (Fig. 20). The size range fits very well with those obtained from nitrogen physisorption. Please note that we could not obtain the size range of the cavities from the physisorption directly; in other words the ET results have been key to allow for interpretation of the physisorption data. 4.2. Element specific electron tomography Using commercially available electron energy loss spectroscopy (e.g. GIF by Gatan) one can obtain element-specific images in TEM (EFTEM) or STEM (ESI) mode. The use of STEM-EELS has been explored for some time and quantitative information on element distribution can now been obtained [51]. Thomas and Midgley have published an excellent overview of this technique as applied to materials science in general and solid catalysts in particular [52]. The first applications of electron tomography using EFTEM detection have been published by Weyland and Midgley [53] and Mobus et al. [54]. A trend in this direction will be of great importance in tomography of molecular sieves. However, limitations relating to beam damage have to be taken into account. 4.3. Study of complex.(industrial) catalysts Following the developments of electron tomography in catalysis using mainly model or experimental catalysts (vide supra) we expect applications for complex (industrial) catalysts that often consist of many components, e.g. zeolite, silica-alumina, binder and hydrogenation function in hydrocracking catalysts. Recently we have undertaken a structural study of an industrial hydrotreating catalyst that consists of MOS2/Y-AI2O3 using bright-field electron tomography [55].

5. CONCLUSIONS •

Electron tomography (ET) can now be applied on a routine basis using an automated electron microscope in combination with image processing and visualization software. Three-dimensional information with nanometer resolution has been obtained for structural studies of molecular sieves.

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•

• •

•

•

With zeolites application of electron tomography has provided unique information on the nature of mesopores that have been obtained by secondary treatments or the use of carbon templates. For ordered mesoporous materials it has been found using electron tomography that channels can be strongly bend (noodle shaped pores). Metal particles that fit the pores of SB A-15 precisely have been studied using bright field ET while very small particles in the pores of MCM-41 have been imaged using HAADFET. Novel developments involve (i) quantification of structural information from reconstructed images, (ii) element specific ET employing EFTEM and (iii) the study of more complex (industrial) catalysts at high resolution. Structural information obtained from ET should be complemented with other characterization techniques in view of the intrinsically poor statistics of electron microscopy.

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Applications of quantum chemical methods in zeolite science P. Nachtigall Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, Flemingovo nam. 2, 166 10, Prague 6, Czech Republic 1. SCOPE 2. INTRODUCTION 3. METHODOLOGY 3.1. Methods 3.1.1. Traditional ab initio method 3.1.2. DFT methods 3.1.3. Basis sets 3.2. Models 3.2.1. Periodic models 3.2.2. Cluster models 3.2.3. Hybrid models 4. MODEL/METHOD RELIABILITY 4.1. Interaction energies 4.2. Structure determination 4.3.Chemical reactions 5. CONCLUSION REFERENCES

1. SCOPE The reliability of method for electronic structure calculations and models used in zeolite modeling is briefly reviewed. Only traditional ab initio methods and methods based on the density functional theory are discussed. Periodic, cluster, and combined models are described and their suitability for investigation of various properties is discussed. This contribution is written for non-experts in computational chemistry. The author hopes that it will help them to gain a basic orientation in the field. 2. INTRODUCTION Enormous developments in experimental techniques led to increased resolution of numerous experiments. As a result, detail information about various properties is now experimentally available. However, interpretation of experimental data is often very difficult or at least not straightforward. One of the major problems is a lack of knowledge about the framework

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aluminum distribution and about the structural details of extraframework species. It is a big challenge for computational chemistry to supply the missing information, in particular, to find a correlation between structural details and experimentally observed properties. A large development in the computational hardware and software makes it now possible to use significantly more realistic models and more reliable methods for modeling of zeolite properties. One of the greatest challenges is to maximize the overlap between theory and experiment in the zeolite science. Combined experimental and computational studies aiming to interpret the experimental data at the atomic scale level are now appearing. Despite the large progress achieved in recent years there are still many interesting questions open. Computational techniques in general are used in various areas of zeolite research. In fact, just such a basic question as what is the structure of a zeolite relies on the zeolite modeling. Currently, the methods of computational chemistry are used in investigation of almost any property of zeolites, including, e. g., zeolite structures, zeolite characterizations (modeling, e. g., the UV-vis, IR, NMR, or ESR spectra), and catalytic activities. Number of different methods and models were used for the zeolite modeling. Due to a large number of variables in a model definition (e. g., a number and arrangement of atoms representing the zeolite, boundary conditions, geometry constraints, electronic structure description, basis set used) not only there is often a big disagreement between theory and experiment but often even the results of various theoretical approaches are dramatically different. Understanding the assumptions in computational models is crucial for judgment about the reliability of obtained results. It is a goal of this text to help the reader in this respect. Models used in the zeolite science can be divided into two categories: (i) models that do not explicitly consider any electron in the system (molecular mechanics, interactions described with interatomic potential functions) and (ii) models that explicitly consider part of the electrons in the system (either at semiempirical level or at ab initio level). This text should serve as an introductory overview of quantum chemical approaches (excluding semiempirical methods) and models available for zeolite modeling. It is impossible to review the quantum chemical calculations in zeolite science on pages available here. Only qualitative description of methods will be given, avoiding mathematical equations. More details can be found, e. g., inRefs. [1-5]. 3. METHODOLOGY Due to the large size of the zeolite crystals the rigorous quantum chemical methods cannot be used for the description of the entire zeolite crystal. Therefore, simplified models are used for the zeolite description. Model is defined as a set of simplifying approximations adopted for the description of a specific system. In general, model definition includes: (i) specification of the set of atomic nuclei representing the system, (ii) set of constraints applied (e. g., boundary conditions, constraints used in geometry optimization, etc.), (iii) number of electrons explicitly treated, and (iv) interaction potentials between particles in the system. It is sometimes advantageous to limit the model definition to items (i) and (ii) and refer to items (iii) and (iv) as "method". The notation model/method defined by items (i)-(ii)/(iii)-(iv), respectively, is adopted here. First we describe quantum chemical methods applicable on zeolites followed by the discussion of various models used for the zeolite representation.

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3.1. Methods In order to solve the stationary Schrodinger equation for a molecule a number of approximations must be adopted. With the increasing number of atoms (electrons) in the system more approximative methods must be adopted. In other words with the increasing size of the system the reliability of methods applicable on such system decreases. When a particular zeolite topology is properly respected by a model it is obvious that such model consists of hundreds of atoms. (The size of the models suitable for the zeolite representation will be discussed in the next section.) Only few ab initio methods can be used for the description of such large systems. The reliability of applicable methods is somewhere between "poor" and "good" depending on the method used and studied property. In this section we want to give a brief overview of DFT and traditional ab initio methods and to discuss their reliability. Term "traditional ab initio" methods is used for the Hartree-Fock (HF) method and for post-HF methods accounting for the electron correlation. In early applications of quantum chemistry in zeolite science the traditional ab initio methods were dominantly used. It has been predicted in Ref. [2] that DFT will become a major computational method in the zeolite modeling. Indeed, due to a very promising development of exchange-correlation functionals most of the quantum chemical calculations on zeolites rely currently on DFT methods. There is a certain overlap between approximations adopted in HF and DFT methods (summarized below). HF and DFT methods differ in the way they handle the electron-electron interaction. Approximations specific for traditional ab initio and DFT methods are briefly described in sections 3.1.1. and 3.1.2., respectively. Details can be found in many textbooks, e. g., in Ref. [5] and Ref. [6] for DFT and ab initio methods, respectively. The major approximations used in DFT methods and in traditional ab initio methods are also summarized in Fig. 1. Following approximations are common to both DFT and traditional ab initio methods: (i) Non-relativistic Hamiltonian is adopted. (ii) Born-Oppenheimer approximation separating electronic and nuclear degrees of freedom is used. (iii) One-electron wavefunction (molecular orbital or band in cluster or periodic calculations, respectively) is expressed as a linear combination of functions of the basis set (MO LCAO approximation) and variational principle is used. The same basis set can be used for the wavefunction expansion in both, traditional ab initio and DFT calculations. In cluster calculations the basis set is typically defined as a set of atom-centered gaussians while in periodic calculations also a planewave basis set can be used for DFT. (iv) The number of electrons explicitly considered can be reduced by replacing the core electrons with a pseudopotential. 3.1.1. Traditional ab initio method The electron-electron interaction is in the HF method treated within the model of independent electrons. Within this approximation each electron moves in the average potential of other electrons. As a consequence, there is a non-zero probability that two electrons are located at the same point in the space. Error resulting from this approximation is known as a correlation energy. The advantage of the model of independent electrons is that it allows to search for a wavefunction in the form of the product of one-electron functions (orbitals). Instead of a simple product function the Slater determinant is used in order to maintain anti-symmetry of the wavefunction. The solution of an n-electron Schrodinger equation can then be found in

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terms of n Fock equations that can be solved in iterative manner. Molecular orbitals (also

Fig. 1. Traditional ab initio and DFT methods.

denoted as Hartree-Fock orbitals) are just one-electron functions found by solution of Fock equations. The Hartree-Fock method neglects the electron correlation. The reliability (and applicability) of the HF method itself is rather small. However, the electron correlation can be recovered by post-HF methods based on the wavefunction expansion in terms of Slater determinants constructed from the one-electron Hartree-Fock orbitals.

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The Coupled-clusters (CC) method[7] based on the cluster expansion of the wavefunction has been established as a highly reliable method for calculations of ground state properties of small molecules with the spectroscopic accuracy. When this method is used together with a flexible basis set it recovers the dominant part of the electron correlation. Typically, CC variant explicitly considering single and double excitations (CCSD) is used. In order to save computer time the contributions from triple excitations are often calculated at the perturbation theory level (notation CCSD(T) is used in this case). CCSD(T) method can be routinely used only for systems with about 10 atoms at present. Therefore, it cannot be used directly in zeolite modeling, however, results obtained at CCSD(T) level for small model systems can serve as an important benchmark when discussing the reliability of more approximate methods. Only the computationally cheapest post-HF method can be currently applied on zeolites. Computationally the fastest and most popular post-HF method is perturbation theory considering up to the second order terms (MP2 method, using a IVMler-Plesset formulation for the correlation energy).[8] This method is not variational and typically it overestimates the effect of the electron correlation. When the resolution of identity (RI) approximation^] is used the RI-MP2 method can be used for calculations on systems consisting of more than hundred atoms. 3.1.2. DFTmethods Hohenberg and Kohn showed that the ground-state energy and other properties of the system were uniquely determined by the electron density. [10] The practical realization is based on the use of one-electron orbitals (Kohn-Sham orbitals) and the variational principle.[ll] This formalism leads to a set of Kohn-Sham equations for one-electron functions that are solved iteratively. Kohn-Sham equations are formally very similar to Fock equations, both include terms for the kinetic energy of electrons, electron-nuclei interaction, and classical Coulomb interaction between electron densities. In Fock equations the non-classical exchange term is evaluated exactly within the model of independent electrons approximation. In Kohn-Sham equations there is an exchange-correlation functional (Fxc) instead. This functional includes the electron exchange interaction, electron correlation, and part of the electron kinetic energy. Hohenberg and Kohn theorems state that the exact exchange correlation functional exists, however, its form is not known. There is a number of exchange correlation functionals described in the literature. In most cases the exchange correlation functional is expressed as a sum of exchange (Fx) and correlation (Fc) parts. Notation for the majority of functionals is derived from the initials of functional authors in some cases supplemented by a year of publication, the exchange functional being described first. Exchange-correlation functionals are divided into three groups (only those combinations of exchange and correlation functionals that have been repeatedly used in the zeolite science are shown as examples, for more details see Ref. [5]): (i)

Local density approximation (notation LDA or SVWN is used)[ll-13]. Exchangecorrelation functional depends only on the electron density (Fxc =Fxc[p]). (ii) Generalized gradient approximations (e. g., BLYP,[14,15] BP86,[15,16] PW91,[17] PBE[18]). Exchange-correlation functional depends also on the gradient of electron density (Fxc =¥xc[p,Vp]). (iii) Hybrid exchange-correlation functionals that partially mix in the exact exchange (e. g., B3PW91 or B3LYP)[19]. The exchange-correlation functional depends in addition to p and Vp also on HF exchange calculated exactly. Therefore, four-center integrals must be

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calculated (N4 integrals, N is the number of basis functions). Calculations with hybrid functionals are currently feasible with atom-centered basis sets only.[20] DFT calculations are computationally less demanding than traditional ab initio calculations. Formally DFT calculations using LDA or GGA functionals roughly scales as N3 (N is the number of basis functions) when Coulomb repulsion between electrons is evaluated via overall electron density integration. When Coulomb repulsion is explicitly evaluated from four-center integrals or when a hybrid functional is used the calculations formally depend on N4 just like in the HF method. MP2 and CCSD(T) methods are significantly more computer time consuming, scaling formally as N5 and N7, respectively. DFT methods appear to be even more computationally favorable over the traditional post-HF methods considering the fact that DFT does not require the use of such large flexible basis sets as is usually required for the post-HF calculations. Number of techniques can be used in order to improve the computational efficiency. The actual scaling is far better than formal scaling reported above, however, the relative efficiency of individual methods remains unchanged. Despite all the advantages of the DFT method one should be aware of well known failure of this method. Exchange-correlation functionals currently available are not capable to account for dispersion interaction, e. g., interaction between zeolite channel wall and hydrocarbons cannot be properly described at the DFT level (see section 4.3). 3.1.3. Basis sets Any method described above can be used together with atom-centred basis sets. Slater type orbitals (STO) or numerical basis functions defined over a spherical polar grid centred on atoms can be used. STO and numerical basis sets are used, e. g., in program suites ADF[21] and Dmol3,[22] respectively. However, the most common approach currently is to use Gaussian type orbitals (GTO) since a majority of program suites suitable for the zeolite modeling relies on the use of the GTO basis sets. The reliability of calculations increases with the increasing size of the basis set used. STO and numerical basis sets converge faster than GTO basis sets, however, due to the computational advantages of the GTO basis set it is often easier to carry out the calculations with a larger GTO basis set than with a medium size STO or numerical basis set. The basis set used in DFT calculations should be at least a valencedouble-f quality augmented with polarization functions (VDZP notation). Post-HF methods require the use of an even larger basis set, e. g., a correlation consistent valence-quadruble-fplus-polarization functions (cc-pVQZ) basis set. The use of the atom-centred basis set is inherently connected with the basis set superposition error (BSSE). Calculated interaction energies should be always corrected for BSSE, e. g., using a counterpoise correction method.[23] In addition to the atom-centred basis sets the DFT calculations on periodic systems can be carried out with the planewave basis set (see section 3.2.1. for details). Since the basis set definition is related to the size and shape of the periodic unit cell and not to particular atoms the BSSE does not occur in planewave calculations. 3.2. Models Three types of models were repeatedly used in the theoretical description of zeolites: (i) periodic models, (ii) cluster models, and (iii) combined (named also hybrid or embedded) models. Cluster models have dominated in the zeolite science in the eighties and nineties. [2] However, with ever improving computational hardware and with a progress in the software development (various embedding schemes applicable for zeolites, e. g., Refs. [24,25] and

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periodic models, e. g., Refs. [26,27]) all three models are successfully used in the zeolite science at present. Each of these models has certain advantages and disadvantages (summarized in Fig. 2) that will be discussed below together with the description of basic characteristics of these models. 3.2.1. Periodic models A periodic model appears to be very easy to accept since the periodicity of the zeolite structure is well known from X-ray crystallography. The translational symmetry of the crystal is exploited in a natural way. Within the periodic approach the zeolite is represented by a unit cell that periodically repeats in all three crystallographic directions. All atoms in the system are treated at the equal level and the long-range electrostatic interactions are implicitly included in this model. Model specification for a particular system is rather straightforward: (i) definition of the periodic unit cell, (ii) unit cell composition (Si/Al ratio, framework Al atom placements, charge compensating cations, adsorbed species), and (iii) initial geometry of the system. In addition, the method used for the electronic structure calculations needs to be specified (see above). Periodic quantum chemical calculations can be carried out either with the atom-centered basis set or planewave basis set. First calculations of the electronic structure of periodic models of zeolites were carried out with atom-centered basis sets (e.g., Ref. [28-30]). First applications of the periodic model of zeolites employing a planewave basis set appeared just a year later [31-33]. Majority of applications use the planewave basis sets at present. Two implementations of the periodic model employing the atom-centered basis sets are currently available. The numerical basis is used in Dmol3 program [22] while the Gaussian type atomic orbitals are employed in CRYSTAL program [26]. The wavefunction of the system is expressed as a linear combination of functions in the basis set. The basis set used in the periodic model may consist of the same set of atomic orbitals as it is used in the calculations on a gas phase molecule or cluster model. Therefore, effects of the boundary conditions and long-range interactions can be investigated in a straightforward way. Number of basis functions used in the wavefunction expansion depends only on the number of atoms in the unit cell and not on the unit cell volume. The method is thus suitable for calculations on systems with a large unit cell provided that the number of atoms in the unit cell does not exceed the current threshold on the number of atoms (basis functions) that can be managed with available computational resources. Another consequence of the relatively small number of atom-centered basis functions in the wavefunction expansion (compared to plane-wave implementation) is that the exchange integrals can be explicitly calculated. Therefore, hybrid density functionals as B3LYP or even post-HF methods as MP2 can be used for the electronic part of the Hamiltonian. In spite of all the advantages of periodic calculations with the atomcentered basis set routine applications in the zeolite science are rare due to the large computational expenses of this model. A foreseen progress in software and hardware development will probably result in larger number of applications of this model for zeolites. More details about periodic models with atom-centered basis sets can be found, e. g., in Refs. [22,34]. Planewaves exp(ik-r) are a natural choice for the description of systems with periodicity. On one hand the simple mathematical form of planewaves makes calculations of electronic integrals in Schrodinger equation very fast, on the other hand, a very large number of planewaves must be used in the linear expansion of one-electron functions. Not all the planewaves satisfying Bloch's theorem are used in practical calculations, instead only the planewaves with a kinetic energy smaller than a specified energy cutoff are used. Planewaves

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Fig. 2. Models commonly used for zeolite representation. Figures show an example of an extraframework cation site on the channel intersection in FER. Advantages and disadvantages of individual models are summarized.

with a very high kinetic energy must be used for the description of core electrons. In order to use the planewave basis set in periodic calculations on "real" systems it is necessary to make additional approximations to save a computer time. Since most of the properties depend dominantly on valence electrons the pseudopotential approximation can be used. Only valence electrons are treated explicitly while the pseudopotential is used for the representation of nuclei and core electrons. Within the pseudopotential approximation the nodal structure in the radial part of the wavefunction in the core region is lost. As a consequence, the properties depending on the core region (e. g., hyperfine structure) cannot be calculated within this approximation. Another possibility is to use the projector augmented waves (PAW) potentials[35] that reconstruct the exact valence wavefunction with all nodes in the core region. Excellent review of planewave pseudopotential methods can be found in Ref. [36]. Examples of application of periodic models employing the planewave basis set in the zeolite science can be found, e. g., in chapter 5 of Ref. [4].

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In addition to the number of electrons in the model the computational requirements depend also on the volume of the model unit cell (the number of planewaves satisfying Bloch's theorem increases with the increasing volume of the unit cell for a given energy cutoff). Therefore, calculations using the planewave basis set are feasible for zeolites with small and medium size unit cells. Calculations on zeolites with a large unit cell (e. g., MFI) are computationally very demanding. Clearly, investigation of zeolites with a small unit cell is computationally advantageous. However, with the exception of purely silicious structures and zeolites with Si/Al=l, the periodicity of the zeolite is not absolute since the distribution of framework Al is expected to be more random. The same applies for the position of charge-compensating cations. Special care must be taken when investigating adsorption or chemical reactivity. Calculations using a unit cell where at least one dimension is small can lead to potential problem. It is always very valuable to perform some test calculations with the unit cell doubled along the shortest cell dimension. Calculations with planewave basis set are currently limited on the use of LDA or GGA density functionals (see above). 3.2.2. Cluster models Cluster models consist of several TO4 units arranged in a way to mimic a part of the zeolite structure. In addition, charge compensating cations and adsorbed species can be specified. The simplest cluster model of a zeolite is 1-T model (Fig. 3a). Small cluster models such as 3T and 5-Tj (Fig. 3b, d, respectively) were often used in zeolite modeling. None of these models represents a particular zeolite framework. A large variety of medium size cluster models was also used, e. g., 6-Tr or 12-TD6R clusters, shown in Fig. 3e and 3f, respectively. In order to model a particular zeolite framework large cluster models must be used, e. g., a 28-T model consisting of two ten-member rings and additional atoms was used to model a channel of theta-1 zeolite.[37] A simple notation for clusters giving only a number of framework Tatoms in the cluster and possibly additional structural information given in the subscript (e. g., d, 1, and r for tetrahedral, linear, and ring structures, respectively, or D6R abbreviation for double-six-member ring) is used. It should be pointed out that other notations for clusters were also used, e. g., notation based on a number of shells around central atoms [38], or chemical nomenclature terms [39]. Cluster models consisting of just few T-atoms are crude models for a description of the zeolite crystal. There are at least three potential sources of errors inherent to cluster models: (i) wavefunction perturbation due to the cluster boundary, (ii) structural constraints, and (iii) neglect of the long range interactions. (i)

When the cluster is cut out from the zeolite crystal structure it introduces the artificial "dangling bonds" on the cluster boundaries. The most common approach used in the zeolite modeling is to saturate dangling bonds with hydrogen atoms. Hydrogen atoms are placed along the direction to the neighboring framework atom (not present in the cluster model). Either silicon or oxygen atoms can be replaced by hydrogen (OH and H termination, respectively). Examples of OH- and H-terminated 3-T clusters are depicted in Fig. 3b and 3c, respectively. Since the electronegativity of H and Si atoms is relatively similar (compared to the electronegativity of O and H atoms) the perturbation on the wavefunction due to the cluster boundary is smaller for the OH-terminated cluster than for the H-terminated cluster of the same size. The boundary effect on the wavefunction decreases with the increasing cluster size. The critical cluster size depends

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on the investigated property. Pseudo-atoms (instead of hydrogen atoms) can be also used for the cluster termination.

Fig. 3. Small cluster models used in zeolite modeling. 1-T (a), 3-T (b,c), 5-Td (d), 6-Tr (e), and 12-TD6R (f) cluster models. All clusters are OH terminated except 3-T cluster (c) that is H terminated. Oxygen atoms are depicted white, Si/Al and cluster terminating H atoms are grey.

(ii) Most of the properties investigated with the zeolite models requires the geometry optimization (within a specified set of constraints) of the model. Difficulties connected with the geometry optimization of the cluster model represent probably the largest drawback of the cluster model. In general, three approaches can be adopted for cluster models: (a) geometry is fixed at the experimental geometry, (b) geometry is fully relaxed, and (c) only some degrees of freedom are optimized (a set of geometry constraints is defined). The first two approaches can be used only in some special situations, while their use is very problematic in the majority of applications (see Section 4.2. for examples). (a) Experimental geometries are not available in most cases since positions of Si and AI atoms are not distinguishable from diffraction analysis. (b) Full geometry optimization can lead to structures that are far from those observed in zeolites, e. g., cluster terminating OH groups can form artificial intramolecular hydrogen bonds or geometry changes of six-member (and larger) rings due to the interaction with extraframework cation are significantly overestimated. (c) The most common set of constraints used in the cluster model optimization is to fix positions of cluster terminating atoms (either H terminating atoms or OH terminal groups). It is apparent that this approach is far more realistic than the full geometry optimization, however, the constraint geometry cluster model is not suitable for the description of processes accompanied by a large geometry relaxation. (iii) Long-range interactions are neglected when the cluster model is used. The importance of the long-range interactions depends on the properties investigated. Despite all the problems mentioned above cluster models can be advantageously used for special tasks in zeolite modeling. With decreasing cluster model size the post HF methods of

253 increasing accuracy (including the CCSD(T) method) can be employed. Small cluster models can be, therefore, used for the method reliability benchmarking. Calculations with cluster models of various sizes can bring information about the importance of the details of the zeolite topology to the structure and properties of active sites. 3.2.3. Hybrid models Hybrid models were developed in order to overcome main drawbacks of cluster models. Within a hybrid model (denoted also embedded or combined model) the system (S) is partitioned into two parts (Fig. 2): inner part (I) containing the atoms and molecules of a particular interest (e. g., active site and adsorbed molecule) and outer part (O) containing the remaining atoms in the system (S = I + O). The inner part of the system is treated at the higher level of theory while the outer part of the system is described at the lower level of theory. There are numerous possibilities how to account for the cross interactions between atoms from different parts of the system. In addition, the I/O boundary region selection and handling can be done in a number of ways. Contrary to the periodic and cluster models where the model description is equally valid for zeolites as well as for other fields of applications, the combined models are somewhat application dependent. Our description of hybrid models is limited to embedding schemes designed for or used in the zeolite science. One example of each, mechanical embedding, electrostatic embedding, and embedding accounting for both electrostatic and "mechanical" interactions across the boundaries is given. Embedding schemes can be also classified as either subtraction or additive schemes defined in Equations (1) and (2), respectively. •^

~~ nlow-level ^ nhigh-level

pS _ pi ^

~ E'high-level

+

po ^low-level

^low-level +

(1)

p'-° ^coupling

(2)

The subtraction scheme can be easily understood in the following way: the whole system is described at the low level of theory (Efow_level) and the description is upgraded for part of the system (inner part) to the high level (E'Ugh_level - E'lm_level). The additive scheme is simply sum of low and high level descriptions of outer and inner parts, respectively, augmented by a cross-interaction term (E^°ling ). Embedding based on the subtraction scheme is exploited in the QM-pot approach of Sauer and coworkers[24,40]. (ONIOM model of Morokuma et al. that is available in GaussianO3 package[41] is also based on the subtraction scheme [42]). In the QM-pot scheme the dangling bonds on the inner part boundary are saturated with the hydrogen atoms (link atoms) that are placed along the direction of the broken bond towards the adjacent atom from the outer part (similarly as cluster model termination). Inner part atoms and link atoms form a cluster the electronic structure of that is described at the higher level of theory (typically DFT). The interactions between atoms in the outer part and cross-interactions between atoms from inner and outer parts are treated at the interatomic potential function (IPF) level. Within the subtraction scheme it is assumed that the effect of the link atoms is approximately the same at the high and low levels of theory (subtraction cancellation). Therefore, it is convenient to use IPF fitted to the ab initio data obtained at the same level of theory as it is used for the description of the inner part [40,43]. The cross-interactions are treated at the IPF level, therefore, the wavefunction of the cluster is not effected by the Coulomb field of the environment. The periodic boundary conditions can be applied within the subtraction scheme

254

and the geometry optimization can be performed without any artificial constraints. Therefore, this approach is sometimes denoted as "mechanical" embedding. Another type of the embedding scheme focuses only on the long-range electrostatic effect of the environment on the wavefunction of the cluster. This can be considered as a special case of the additive scheme (E s = E'hjgh_level + E'c~°pUng). The electrostatic potential of the environment defined by a set of point charges (localized typically at the crystallographic positions of surrounding zeolite atoms) is included in the Hamiltonian of the inner part. Difficulties of this "electrostatic" embedding consist in the necessity to use terminating atoms on the inner part boundaries. The electrostatic potential in the vicinity of the inner part must be modified in order to avoid artificially large electrostatic interactions between (artificial) terminating atoms and nearby point charges. Electrostatic potential of the zeolite framework obtained with the SCREEP method was used in number of applications in zeolites [44,45]. In general, electrostatic embedding correctly accounts for the effect of the Madelung potential on the wavefunction of the inner part, however, this scheme does not allow a full geometry optimization of the zeolite (only the atoms in the inner part of the system can be relaxed). The elastic polarizable environment (covEPE) cluster embedding approach that allows for both mechanical and electrostatic interactions across the I/O boundary was developed recently [25]. This approach attempts to overcome the deficiencies of methods using the artificial H link atoms on the inner part boundary. The inner part is terminated with specially constructed monovalent pseudoatoms (border atoms) that simultaneously belong to both inner and outer parts. The boundary between inner and outer parts cuts through the framework oxygen atoms. Border oxygen atoms are replaced by monovalent seven-electron pseudoatoms in the inner part description. In the outer part description the border atoms are considered as lattice centers with modified charge. Within covEPE approach the energy minimization can be carried out without any constraints while the electron density in the inner part reflects the Coulomb field of the environment. 4. MODEL/METHOD RELIABILITY Methods described in section 3.1. provide in general a qualitatively correct description of the system in the electronic ground state. However, a quantitative agreement between theory and experiment is often being sought. It is therefore important to understand the reliability of methods and models for quantitative calculations of various properties. As we mentioned above electron correlation is neglected at the Hartree-Fock level, therefore, some of the postHF methods must be used in order to recover the electron correlation error and to describe the system reliably. A great advantage of traditional ab initio methods is that the description of the electron correlation can be improved in a systematic way, extending the wavefunction in terms of increasing number of Slater determinants and using a more flexible basis set. It has been shown that the coupled clusters method gives excellent results (quantitative description) for ground state properties of small molecules when it is used with sufficiently flexible basis set. In most cases the CCSD(T) results can serve as a benchmark calculation for more approximative methods. We will focus the discussion on the reliability of DFT methods since these are most often used in zeolite modeling. It is much more difficult to assess the reliability of various exchange-correlation functionals since these cannot be systematically improved and the form of the exact functional is not known. Many benchmark calculations on small molecules were performed and results were compared with the reliable post-HF calculations and experimental

255

data. An excellent compilation can be found in Ref. [5], here we will focus only on calculations carried out on zeolites. 4.1. Interaction energies The quality of various methods for calculations of interaction energies was tested many times. The performance is usually judged based on the mean absolute deviation (MAD) of atomization energies calculated for a so called G2 set of molecules [46]. It is well known that LDA strongly overestimates bonding (MAD = 370 kJ/mol). GGA functionals perform significantly better giving about an order of magnitude smaller MAD (e. g., about 30 kJ/mol for BLYP functional). Hybrid functionals perform even better, e. g., MAD = 13 kJ/mol was found for B3LYP functional [46]. More discussion can be found in Ref. [5]. It should be pointed out that only few molecules in G2 sets contain Si or Al atoms or metal cations. It is therefore important to investigate the reliability of the DFT method for the description of zeolites. Reliability of various methods for the description of molecules interacting with cationic sites in zeolites can be judged based on the data summarized in Table 1. The interaction energies of CO with the cationic site in the zeolite were calculated at various levels of theory. Results obtained for Na+ and Cu+ cations are reported. In order to compare DFT data with the results of reliable (but computationally demanding) CCSD(T) method, the simplest model of a zeolite (1-T cluster model, Fig. 3a) was adopted. This model is a rather crude model of the intersection site in zeolites where the metal cation is coordinated to the two oxygen atoms of a single AIO4 tetrahedron. Due to the simplistic character of the model it is not meaningful to compare calculated energies with the experimental data. The suitable method that can be used with more realistic models of zeolites is selected based on the agreement with CCSD(T) results [47]. CO interaction with Na+ cation is almost entirely electrostatic and can be reliably described with almost any method listed in Table 1. The MP2 method and DFT employing PBE, BLYP, or B3LYP functionals give interaction energies in excellent agreement with the CCSD(T) value (within 2 kJ/mol). The tendency of LDA to overestimate the interaction is apparent even for this relatively simple system (interaction energy is 8 kJ/mol, about 30%, overestimated). Only the BSSE corrected results are reported for Na+ cation since the BSSE is almost constant (2-4 kJ/mol) for all methods and basis sets used (except planewave calculations). In addition to the electrostatic component the interaction of CO with Cu is stabilized by the covalent contribution (both, the donation and the back-donation effects take place). As it is apparent from the wide spread of calculated energies summarized in Table 1 the description of the CO interaction with the Cu+ cation is much more complicated. Calculated interaction energies (upon the inclusion of the BSSE correction) range from 9 kJ/mol at the HF level to 262 kJ/mol at the LDA level. The LDA and HF methods completely fail to describe Cu+.. .CO interaction. GGA functionals provide significantly better interaction energies (in the range from 186 to 211 kJ/mol) but still up to 30% overestimated. In the same range is also the interaction energy calculated at the MP2 level. The hybrid exchange-correlation functionals (B3LYP and B3PW91) give interaction energies in excellent agreement with the CCSD(T) value (within 3%). Note that interaction energies must be corrected for BSSE. Except for the HF method, the BSSE correction is in the range from 23 to 35 kJ/mol. It is apparent from Table 1 that the reliability of a particular method can differ even for seemingly similar systems. It is therefore always very valuable to benchmark the quality of the method used in zeolite modeling. It is not unusual that for a given property of a particular

256 system a rather large range of theoretical values can be found in the literature. This can be demonstrated by the example of NO interaction with the Cu+ cation in zeolites. Calculated Table 1 Interaction energies (in kJ/mol) of CO adsorbed on extraframework Na+ and Cu+ ions in zeolites. The simple 1-T model used. CO/Na7A1(OH)4 CO/Cu A1(OH)4 Method Basis set Geometryd E(BSSE) E E(BSSE) CCSD(T) CCSD MP2 HF HF

cc-pVQZa cc-pVQZa cc-pVQZa cc-pVQZa VTZPb

CCSD(T) CCSD(T) CCSD(T) CCSD(T) BLYP

-26 -26 -28 -22 -21

-192 -158 -242 -26 -22

-159 -128 -209 -21 -9

LDA VTZPb BLYP -34 -288 -262 PW91 VTZPb BLYP -30 -241 -211 PBE VTZPb BLYP -29 -240 -210 PBE 400/600° PBE -26 -229 -229 B3LYP VTZPb BLYP -26 -180 -155 BLYP cc-pVQZa BLYP -25 -196 -191 BLYP VTZPb BLYP -25 -216 -186 B3PW91 VTZPb BLYP -23 -186 -163 BP86 VTZPb BLYP -23 -232 -205 a Correlation consistent valence-quadruple-f basis set with polarization functions[48], effective-core relativistic pseudopotential and valence (8s7p6d2flg)/[6s5p3d2flg] basis set for copper [49]. b Valence-trip le-f basis set with polarization functions [50]. c Calculations performed in supercell 19.1x14.3x7.6 A and energy cutoff 400 and 600 eV for Na+ and Cu+, respectively. d Level of theory used in geometry optimization. interaction energies range from 21 to 213 kJ/mol while experimentally a heat of adsorption 100 kJ/mol was found [51]. Very weak interaction was found by HF method combined with the constraint cluster model [52]. On the contrary, interaction energies obtained with GGA functionals are severely overestimated. Adsorption energy in very good agreement with experiment was found by a combined model employing the B3LYP functional for the inner part description [53]. 4.2. Structure determination The LDA and GGA functionals predict the equilibrium geometries of organic molecules with the same accuracy (up to 0.02 A and up to 2° deviations in bond lengths and angles, respectively) [5]. In general, LDA underestimates and GGA overestimates bond lengths. Hybrid exchange-correlation functionals (B3LYP in particular) perform significantly better, reducing error of LDA and GGA for bond lengths to about 0.01 A. Hybrid functionals provide better geometries than MP2 for many systems. Bond lengths optimized at the GGA level for compounds containing third-row elements deviate slightly more from experimental structures (about 0.03 A)[54]. Going from LDA to GGA results in elongation of Si-0 and Al-

257

O bonds while the OH bond is shortened [39]. It can be concluded that the DFT methods can provide structures with a good accuracy. The direct comparison between the experimental and computational structures of zeolites is complicated. X-ray data provide averaged T-0 bond lengths and averages T-O-T angles not distinguishing between Si and Al T-atoms. Therefore, except for pure silica materials, the experimental Si-O bond lengths are influenced by the presence of aluminum. It is assumed that for high-silica zeolites this influence is small. The zeolite structures obtained with periodic DFT models and GGA functionals are in a good agreement with the available experimental data. Periodic DFT models were used either with planewave or with atomcentered basis sets for the structure optimization of various zeolite matrices (e. g., Refs. [32,55-57]). It is difficult to compare directly EXAFS data with computed structures in the majority of cases, e. g., the bond lengths between framework oxygen atoms and extraframework metal cation obtained from EXAFS are averaged over all existing sites of the metal cation in the zeolite (different sites may have rather different coordination and bond lengths). The structure and coordination of extraframework cations in high-silica zeolites were studied theoretically for many systems. Only few examples are given below for each of the models discussed above. The periodic DFT model was used in investigation of the localization and coordination of Zn2+ ions in chabazite [58]. It was found that two Bransted sites located on the four member ring are more stable than those located on the six member ring. On the contrary, the Zn2+ sites on the six member ring are more stable than those located on the four member ring. The Zn2+ site stability is strongly influenced by the framework deformation. It was shown that the substitution of Zn2+ for two H+ ions is accompanied by a significant framework distortion. In addition, interaction of Zn2+ with a strongly bound water ligand led to the changes in the Zn2+ coordination with the framework and to the framework relaxation [58]. The large framework deformation was also found for Cu2+ ions in ZSM-5 framework investigated by the hybrid QM-pot model [59]. The strong preference of the Cu2+ ion for nearly square planar coordination drived the framework deformation. It was shown that the geometry of Cu2+ environment (and extent of the framework distortion) strongly depends on the localization of the framework aluminum pair. There were many theoretical investigations of the structure and coordination of extra-framework metal cations (including the divalent cations) using cluster models. When the zeolite interaction with the cation leads to a large framework distortion the results obtained with a cluster model should be interpreted with care. Larger distortion can be expected for cations of decreasing size and increasing charge. A full geometry optimization using the cluster models has been shown to be problematic [1,39]. Terminal hydroxyl groups form intramolecular hydrogen bonds in many cases and the resulting structures are often far from those found by a periodic model. It has been suggested that in some cases this artificial intramolecular hydrogen bonding can be reduced when symmetry constraints are used [60]. Potential problems of cluster models in the geometry optimization are demonstrated in Fig. 4 where the structure and coordination of Cu2+ cations in MFI obtained with various models are compared. The Cu2+ coordinated on top of the six member ring on the channel wall (M6 site according to notation of Ref. [59]) with framework Al atoms in T7 and T12 positions was investigated. Geometry was optimized with (i) hybrid QM-pot model using the 6-T cluster for the inner part and the core-shell model potential for the interactions of atoms in the outer part (Fig. 4c), (ii) constrained cluster model with frozen positions of OH terminal atoms (only Cu, Al, Si, and six O atoms are relaxed during the optimization, Fig. 4b), and (ii) unconstrained cluster model (Fig. 4d). All geometry optimizations started from the geometry optimized at the QM-pot level for the 6-T ring without Cu2+ cation. Calculations were carried

258

out with B3LYP functional and valence triple-^* basis set with polarization functions (see Ref. [59] for details). As it is apparent from Fig. 4 there are large differences in the Cu2+ coordination obtained with various models. The QM-pot model predicts the Cu + coordination to four oxygen atoms of AIO4 tetrahedra (Fig. 4c). In order to achieve such coordination the geometry of the 6-T ring is significantly changed. Two AIO4 tetrahedra move towards each other upon the interaction with Cu2+. Cu-0 bonds are not equivalent: two short bonds (-1.95 A) and two longer bonds (2.11 and 2.14 A) were found. The longer bonds are found for framework oxygen atoms that are 6.06 A apart in the original geometry (Fig. 4a). The distance between these two oxygen atoms decreases by almost 2 A upon the interaction with the Cu + ion. The unconstrained cluster model calculations lead to the four almost equivalent Cu-0 bonds (Fig. 4d). There are several intramolecular hydrogen bonds found in this model (six atom O-Al-O-Si-O-H ring with 1.9 A hydrogen bond). The main problem of unconstrained cluster model is that the same geometry would be found for any six member ring containing two AIO4 tetrahedra separated by two SiC>4 tetrahedra regardless the zeolite topology. If Al atoms are placed in two Tn positions (Fig. 4a) the unconstrained cluster model leads to exactly the same geometry as found for the Al pair at T7 and T12 (Fig. 4d). However, using periodic or hybrid models it was shown that coordination of cations depends on the position of framework AIO4 tetrahedra [58,59]. On the contrary, the constrained cluster model does not allow two framework AIO4 tetrahedra to approach (Fig. 4b) and, therefore, the Cu2+ ion cannot be coordinated to four

Fig. 4. Structure and coordination of the Cu2+ cation located on top of the sixmember ring on the zeolite channel wall. Geometries optimized with various models.

259 oxygen atoms of AIO4 tetrahedra. Instead, one of the four oxygen atoms in coordination with Cu2+ is between two framework Si atoms. The structure obtained with constrained search is rather asymmetric and it is very different from structures obtained with other models. The use of different models for the geometry optimization can lead to rather different structures (Fig. 4). Calculated electron densities and spin densities on Cu2+ are different for structures obtained with the combined model or with the constrained or unconstrained cluster model optimizations (Fig. 4c, 4b, and 4d, respectively). Therefore, the calculated properties (e. g., electronic spectra, ESR, IR of probe molecule, or adsorption energies) will depend on the choice of the model and method. 4.3. Chemical reactions Conclusions about the reliability of methods for calculations of interaction energies cannot be extrapolated to the calculations of activation barriers of reactions. It should be noted that it is rather difficult to assess the reliability of methods for the activation energy calculations. In general, GGA functionals underestimate barriers by tens of kJ/mol[5]. The problem is partially due to the self-interaction of electrons that is not properly treated by the approximate exchange-correlation functionals. The problem is partially reduced when the hybrid functionals are used [61]. Reliable calculations of activation barriers for reactions taking place in the zeolite are very challenging but also difficult. In many cases there are several possible reaction paths between reactants and products. If the barrier calculated at the DFT level for one of them agrees with experimental observation there still remains uncertainty about the barrier underestimation due to the DFT deficiency. Barriers of proton jumps between adjacent Bransted sites were calculated for FAU, MFI, and CHA zeolites [62]. Combined QM-pot model employing B3LYP functional was used and the coupled clusters correction was introduced. Barriers calculated at the B3LYP and CCSD(T) levels for the small 1-T model were compared. It was shown that the B3LYP barriers are 5-15 kJ/mol underestimated. Localization of stationary points along the reaction path for reactions taking place inside the zeolite pores is one of the greatest challenges in zeolite modeling. The reactions of hydrocarbons are particularly difficult to model since the hydrocarbon...zeolite interaction can be dominated by the dispersion interaction that is not properly accounted at the DFT level. Only one example is presented here. Clark et al. investigated the role of benzeniumtype carbenium ion in the bimolecular m-xylene disproportionation reaction in zeolite faujasite.[63] The benzenium-type carbenium ion 1 was identified in zeolite catalyst for the

first time. In order to describe the dispersion interaction Clark et al. used combined QM-pot model with relatively small inner part definition (3-T cluster). Thus, the hydrocarbon interaction with the Bransted acid site was described at the DFT level while the hydrocarbon interaction with the rest of the zeolite was described with the Lennard-Jones type force field. To verify the reliability of the QM-pot model they also used the periodic DFT model where all the atoms in the system were treated at the same (DFT) level of theory. Interaction energies of m-xylene with faujasite calculated at the QM-pot level (-61 and -51 kJ/mol for 0(4) and O( 1) positions of Bransted hydrogen, respectively) were in good agreement with the

260

estimate from experimental data (between -67 and -73 kJ/mol). On the contrary, the interaction energy calculated at the periodic DFT level (-28 kJ/mol for 0(4) hydrogen) is severely underestimated since the dispersion interactions are neglected at this level. Both models used (combined QM-pot and periodic DFT) gave comparable reaction energies for the alkoxide 2 to carebenium ion transformation (53 and 63 kJ/mol, respectively). Apparently, the van der Waals interactions between the hydrocarbon and the zeolite are approximately the same for species 2 and 3. The carbenium ion 3 and the benzenium ion 1 were identified as stationary points at both the combined QM-pot and the periodic DFT levels. Experimental evidence of a protonation of an aromatic ring by a zeolite was reported recently (hexamethylbenzenium ion in H-beta).[64]

6. CONCLUSIONS Presented text does not give a final word about what model is the best suited for applications in the zeolite science. We have attempted to show that the choice of the model depends primarily on the property under investigation. It is important to stress that other factors must be also taken into consideration, e. g., software and hardware limitations. It is always a difficult decision to find a suitable compromise among the size of the model and reliability of the method (under the constraint of available computational resources). It is equally difficult to judge the reliability of any computational study in the zeolite science. We hope that the present manuscript helps the reader to acquire some initial orientation in the computational zeolite science. ACKNOWLEDGEMENT This work was supported by Grants of the Ministry of Education of the Czech Republic No. LC512 REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Advanced applications of zeolites T. Bein and S. Mintova Department of Chemistry and Biochemistry, University of Munich, Butenandtstr. 5-13 (E), 81377 Munich, Germany 1. INTRODUCTION 2. ADVANCED APPLICATIONS OF ZEOLITE CRYSTALS AND POWDERS 2.1. Storage materials 2.2. Encapsulation of organic dye molecules 2.3. Zeolite hosts for nanoclusters 2.4. Biological and medical applications 3. ZEOLITE FILMS, LAYERS AND COMPLEX MORPHOLOGIES 3.1. Synthesis of zeolite films and membranes 3.2. Applications of zeolite films 4. CONCLUSIONS REFERENCES

1. INTRODUCTION Over the past decade crystalline microporous materials, in particular zeolites, have continued to find new applications in their traditional areas of use such as catalysis, separation and ion exchange due to their unique physicochemical properties and improved structuring in appropriate morphologies.[1,2,3,4,5] The current impact of molecular sieve materials is significant, with applications ranging from petroleum refining for fuels and petrochemical processes for various chemicals to air separation and nuclear waste management. The well-defined porous structure of zeolitic materials makes these materials true shapeselective molecular sieves. The presence of charge-compensating cations such as alkaline and alkaline earth cations, protons, etc. within the inorganic frameworks adds ion exchange and catalytic properties. Moreover, the hydrophobic nature of pure silica zeolites or the hydrophilic nature of aluminosilicates makes these solids useful as specific adsorbents of organic molecules or water in the gas or liquid phase. In addition to the more traditional applications, new applications of these versatile materials are now being explored by an increasing number of research groups. These applications often utilize the unique spatial structuring of the zeolite channel system for novel concepts such as the stabilization of nanoscale forms of matter, size-selective chemical sensing, or separation of reaction spaces in electron transfer processes, to name just a few. [6,7] Furthermore, a number of novel applications depends not only on the control of pore structure and intrazeolite chemistry, but also on the ability to control the external morphology, for example in thin membranes for separations or in the design of sensor

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devices. These systems are often excellent examples of our increasing control of the structure of materials at the nanoscale, using "bottom up" chemical strategies. It is the aim of this review to present these new developments in zeolite science and to point to successful or emerging applications in this field. We have structured the review according to the morphology of the zeolite involved, i.e., first we will deal with isolated zeolite crystals or powders, followed by a discussion of zeolite films, membranes and other more complex morphologies. 2. ADVANCED APPLICATIONS OF ZEOLITE CRYSTALS AND POWDERS 2.1. Storage materials Hydrogen storage Hydrogen storage on porous materials has attracted considerable attention because of the great importance of hydrogen as a potential substitute for fossil fuel. Various methods for storing hydrogen including gaseous, liquid and solid-state storage based on molecular sieves have been considered. The particular interest in using zeolite-type materials as gas storage media is due to the fact that by changing the size and charge of exchangeable cations, the ionmolecule interactions can be adjusted, and the diameter of the channels can be controlled, thus enabling the effective trapping of differently sized gas molecules. The large internal surface area of zeolite Y leads to a relatively high hydrogen uptake. In hydrogen storage applications, the structures of the host materials play a major role in achieving reasonable hydrogen adsorption capacities. [8] Synthetic zeolites such as A, Y, X, mordenite, chabazite, sodalite, gismondine and thomsonite, etc. were used as powder carriers for hydrogen storage (Fig. 1).

Fig. 1. Framework structures of (a) sodalite, (b) zeolite A, (c) zeolites X and Y, and (d) zeolite RHO. The corners represent Si or Al and the lines represent oxygen bridges. One working principle is based on forcing the guest molecules into the cavities of the molecular sieves at elevated temperatures and pressures, and upon cooling to room temperature, hydrogen is trapped inside the cavities and could be released again by raising the temperature. [9] It was found that sodalite could store 9.2 cnrVg (0.082 wt. %) of hydrogen if loaded at about 300°C and a pressure of 100 bar. However, at very low temperatures, the storage capacity is found to be 1.2 wt. % (at -196°C).[10] Additionally, physical adsorption at low temperature is significant due to the large specific surface area of the zeolites. The specific surface area measurements of the zeolites under consideration can be plotted against

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hydrogen uptake values (Fig. 2). As expected, the zeolites X and Y offer a maximum hydrogen storage capacity; these can be considered to be low-cost media for stationary hydrogen storage applications. [11,12]

Fig. 2. Correlation between hydrogen adsorption at -196 CC and the BET surface area of different zeolites. Ref. [12]

A critical re-examination of the sorption properties of carbon nanotubes, filaments and fibers with respect to H2 storage did not show more than about 1 wt.% uptake of the gas, and metal-exchanged zeolites have been suggested as alternative candidates for that application. The reversible storage of molecular hydrogen is critical for large-scale applications of hydrogen as fuel, in particular for mobile applications. In zeolites where Si atoms can be substituted by Al atoms, metal cations act as charge-balancing ions of the negatively charged framework by coordinating to the lattice oxygen atoms such that their electronic properties can experience changes that ultimately could affect their interactions with H2 molecules.[13,14] These changes vary depending on the coordination environment of the metals, thus resulting in very different abilities of cations in different zeolite coordination sites for binding hydrogen.[15] The molecular sieves can also be used as storage medium for ammonia.[16] The ammonia adsorption capacity of zeolites has been increased substantially by ion exchange with transition metal ions due to the increase in the number of ammonia adsorption sites. The ammonia adsorption capacity of zeolite Y (FAU structure) with Co and Cu was higher than that of Na-Y; on the other hand the adsorption on K-Y, Rb-Y and Cs-Y was lower than on Na-Y due to the decrease of electrostatic attraction between ammonia and the zeolite surface. Methane storage Alternative fuels should be substantially non-petroleum in order to provide energy security and environmental benefits, and to substitute for conventional fuels such as gasoline and diesel. Natural gas consisting mainly of methane fits this definition, and the natural gas is mainly stored as compressed natural gas in pressure vessels or as adsorbed gas that can be stored in a porous solid at a low pressure. An important advantage of zeolites over activated

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carbon is their high crystallinity, a large accessible surface area with a high free volume, a low framework density and relatively strong interactions between the framework and the methane molecules. [17] Storage of heat Zeolites are attractive for the possible use in heating and cooling systems which utilize solar energy as a heat source (Fig. 3). When heated, zeolites release water vapor and as long as they stay dry, they can store large amounts of "latent" heat. When zeolites absorb moisture, they release the stored heat, and this regenerative cycle can be repeated numerous times.

Fig. 3. Heating and cooling systems based on highly hydrophilic zeolites using solar energy as a heat source.

Thermochemical storage based on sorption/desorption of water in porous materials appears to be suitable for seasonal storage in so-called low energy houses. In this application one aims for materials with a high sorption capacity towards water. The relationships between the composition and the structure of zeolites upon modification and the sorption/desorption properties of water such as the sorption capacity, the heat of sorption, and the desorption temperature are the most important parameters for solar applications. The standard procedures used for the modification of the zeolites for this application include ion exchange of Na with Li, Ca, Mg, Zn, Co, Al, Fe or additional impregnation of the porous hosts with hygroscopic salts such as MgCb and CaC^. The replacement of two monovalent sodium ions by one bivalent magnesium ion for charge compensation can enlarge the pore volume of the zeolite, and consequently, provide more space for the adsorption of the water molecules in the pores, which increases the potential to store more energy. In this case the water is more strongly bonded to the bivalent ions with their higher charge density. However, high heats of adsorption also imply higher temperatures for desorption of the water from the zeolites. [18] An arrangement involving zeolite coatings synthesized on metal gauzes was applied in order to remove limitations originating from the existence of temperature and condensation gradients within the solar collector. [19] 2.2. Encapsulation of organic dye molecules The nanosized void spaces in zeolites have been exploited to serve as hosts for interesting organic guest molecules. A large variety of organic dye molecules have been incorporated in the defined cages and channels of uniform size of the zeolites. Control of the morphology and the size of zeolite hosts allow for the optimisation of the dye-zeolite composites for a specific application. [20]

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Large crystals, mainly with the one-dimensional AFI type channel structure (AIPO4-5), have been used for studying the optical properties of the dye-zeolite composites by means of optical microscopy,[21,22,23,24] while small crystals are of interest for high-efficiency photonic antennae. Furthermore, nanosized dye-loaded crystals have been used for the generation of dye-sensitized solar cells, light emitting nanocomposites, as well as fluorescent probes in cell biology and in analytical chemistry (Fig. 4).[25] These materials exhibit great stability against chemical attack, photobleaching and thermal decomposition. For example, zeolite LTL has been synthesized in a large range of sizes (30 nm up to 3000 nm) by changing the composition of the starting precursor solutions and reaction conditions. [26] The morphology, pore volume and the size distribution of zeolite nanocrystals strongly affect the behaviour of optical antenna host-guest systems for light harvesting and energy transport.

Fig. 4. a) Schematic representation of zeolite with donor molecule modified with acceptor stopcocks and b) magnification showing details of zeolite channel and the shape of the stopcock molecule. Ref. 25b

Another interesting example among the many uses of porous solids as a host material is the zeolite-dye microlaser, where the incorporation of organic dyes as guest species in microporous crystalline AIPO4-5 gave the first lasing molecular sieve with the size, shape and arrangement of the porous crystals affecting the performance of the system. [27] Organic dye guest molecules such as l-ethyl-4-[4-(p-dimethylaminophenyl)-l,3-butadienyl]-pyridinium perchlorate were inserted into the 0.73-nm-wide channel pores of a zeolite AIPO4-5 host. The dye molecules were not only aligned along the host channel axis but also they have been oriented (Fig. 5).

Fig. 5. Dye moleculel-ethyl-4-[4-(p-dimethylaminophenyl)-l,3-butadienyl]-pyridinium perchlorate inserted in the one-dimensional channel of A1PO-5 molecular sieve. Ref. 27b

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The substantial increase in the stability of organic dyes in zeolites opens the possibility to prepare new pigments with a large variety of colours via combination of two or more dyes. For a recent review on the incorporation of dyes in zeolites see ref. [28]. Zeolites appear to be a promising starting material for the synthesis of ultramarine analogues. [29] It has been found that the structures of SOD, LTA and FAU containing high aluminium levels were the most suitable for the synthesis. The main advantage of zeolites and polysulphides applied for the preparation of ultramarine consisted in a substantial reduction of volatile sulphur compounds upon calcination. The colour of the samples can be modified markedly by using different polysulphides and by altering the conditions for hydrothermal and post-synthesis treatments. The latter affect the oxidation state of the sulphur species. Different stages of organization have been accomplished ranging from the spatial arrangements of dyes in the zeolite channels to the specific adsorption of molecules at the channel entrances as well as coupling of the dye-zeolite crystals to external devices. In most of the cases zeolites with LTA, FAU, MFI, AFI and LTL type structures have been used for these supramolecular assemblies. The dye 2-(2'-hydroxyphenyl)benzothiazole (HBT) was successfully incorporated into the voids of nanosized FAU-type zeolite through a hydrothermal treatment of colloidal precursor solutions. The photoconversion from the keto to the enol form of the HBT molecule incorporated in the HBT-FAU sample upon UV excitation at 385 nm is shown in Fig. 6.[30] This work demonstrates, with HBT as an example, the great potential presented by nanoscale zeolites stabilized in solutions for developing host/guest systems with medium-size organic molecules where optical in-situ investigations of ultrafast photochemical processes are possible. The combination of nanosized porous hosts containing organic compounds such as dyes or photochromic molecules and ultrafast spectroscopy opens new perspectives for the development of nanoscale devices.

Fig. 6. Changes of the absorption in sample HBT-FAU upon excitation at 385 nm. [30]

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2.3. Zeolite hosts for nanoclusters Metal clusters The importance of small metal clusters in catalysis and in many advanced applications is due to the significant physical changes that occur when reducing the size of a material down to a few nanometers; these systems often display unique nanochemical and nanophysical properties and allow the creation of nanoscopic magnets, spatially ordered nanostructures, nano-electronic devices, quantum electronics, etc. The stringent requirements for novel optical and electronic materials with precise nanoparticle size, geometry, and dimensionality are stimulating research efforts aimed at the formation of metal nanoclusters in confined matrices such as polymers, micro- and mesoporous materials. Metal nanoclusters or nanowires can be prepared in confined spaces by using the internal structures of micro- and mesoporous hosts. The size and the shape of the pore opening could determine the properties of the metal nanostructures. In Fig. 7 we give three examples where Ag nanoclusters are formed in partially exchanged zeolite A, a silver network is formed in highly silver-exchanged crystals, while nanowires are formed in a onedimensional mesoporous matrix. In recent years, molecular sieves have found new uses as hosts for the preparation of small metal- and semiconductor clusters that can be grown in confined zeolite spaces and are envisioned for uses in photo-catalysis, non-linear optics, sensors, flat panel displays, etc..[31] The framework-type structures of zeolites can be described with the presence of the n- rings, e.g., 4-, 6-, 8-, 12-rings or double 5-, 6-, 8- rings, or secondary building units, e.g. the sodalite cavity, super cages, and others. These confined spaces can be used for the preparation of optical and electronic materials with desired properties.

Fig. 7. Schematic representation of ordered Ag nanostructures formed as (a) nanoclusters in LTA zeolite, (b) spatial network in LTA, and (c) wires in a mesoporous matrix.

Optical spectroscopy is being used for the characterization of the quantum electronic state of nanoscale materials, particularly alkali metal clusters incorporated in the space of zeolite crystals. For example, potassium clusters were incorporated into cage-type LTA and channel-type MOR zeolites that demonstrated even ferromagnetism at low temperatures.[32] The LTA framework is constructed of p-cages interconnected with double 4-rings among which the a-cages are formed. The latter are connected via 8-membered rings having a size of 4.1 A. The effective inner diameter of the cages depends on the size of the cations, and it is about 11 A and 7 A for the a-cages and P-cages, respectively. In the case of MOR-type zeolite, there are two types of elliptical channels with size of 6.5 x 7.0 A and 2.6 x 5.7 A called main and sub-channels, respectively. When the alkali metals are adsorbed in the dehydrated zeolites, different types of cationic clusters are generated in the free spaces. Ferromagnetism has been observed in the zeolites with a medium loading density of K

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although no magnetic elements are present - this effect was attributed to the inter-cluster interactions. The dynamics and the local environment of silver cations in different types of zeolites during hydration/dehydration/rehydration cycles can be followed with optical methods. The size of metallic clusters in zeolites is controlled by the position of the preferential adsorption of silver cations in the zeolite framework. A colour change from white to orange is due to a decreased number of water ligands coordinated to the silver cations.[33] Silver clusters have been prepared in various zeolite hosts, mainly in LTA, LTL, MFI, FAU, and MOR via impregnation, ion exchange, diffusion at elevated temperatures, and spontaneous monolayer dispersion. The quantum confinement effect of the zeolite channels endows these host-guest nanocomposites with interesting optical properties. Metal clusters in general have been prepared in the cavities and channels of zeolites A, X, Y, chabazite, mordenite, etc., by vacuum dehydration, by reduction with reducing agents, or by X-ray or gamma-irradiation. The most-studied Ag- and Cu- clusters were prepared from various types of molecular precursors, while Au clusters have been less studied because the gold ions are unstable and must be stabilized by specific ligands.[34] Gold-loaded zeolites have been used as catalysts for removing harmful materials such a NO, CO, etc. Photoconductors The nanocomposite materials of FAU-AgI and MFI-AgI with high guest-contents in the form of quantum wires or three-dimensional nanocrystals exhibit optical properties different from quantum dots. In addition, silver-modified zeolites and silver halide encapsulated zeolites offer potential applications as fast-ion conductors, information storage materials, or photoconductors. The properties of (Ag2S)n clusters formed in zeolite A hosts depend on the amount of guest, and up to a loading of 4 Ag+ per a-cage, the properties of the material are mainly determined by the presence of isolated clusters and by short-range interactions.[35] The photoluminescence spectra of (Ag2S)n zeolite composites contain two bands, one around 480 nm (blue-green) and the other at 600 nm (orange-red); the former was observed in samples with low concentration while the latter dominates the spectra at high silver sulphide content. The luminescence bands were assigned to (Ag2S)n monomers (n = 1, blue-green) and Ag4S2 (orange-red), where energy transfer between the Ag2S-donor and Ag4S2-acceptor was proposed.[36,37] Another interesting property of the luminescence of (Ag2S)n zeolite composites relates to its distinct temperature dependence, where the orange-red emission shows stronger quenching with increasing temperature. These samples can be employed in thermometry applications by observing the luminescence colour and intensity and the luminescence lifetime. A great advantage of such device is that a contact with an electric device is not required. The performance of semiconductor clusters in sensing and thermometry applications does not rely on the high level of organization that is required for optoelectronic devices. [3 8] Semiconductors CdS, PbS, CdSe, and ZnS clusters are the semiconductor compounds studied most extensively in zeolites.[39,40] In addition, ternary systems such as ZnCdS have attracted the interest of many researchers. [41] An ordered array of cadmium clusters assembled in zeolite A was prepared in an attempt to produce heavy metal clusters with different distributions and geometries. [42] (Fig. 8)

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Fig. 8. Cadmium cluster (Cd98+) in the sodalite cage of zeolite A. Ref. [43]

The samples were quite stable under the electron beam and their images are consistent with a cubic array of high nuclearity clusters occupying most of the sodalite cages, separated by nearly empty a-cages. However, Cd clusters located not only in the sodalite cages but also in the 8-rings of zeolites result in different properties.[43] Sub-nanometric Co-clusters have been produced in MFI and BEA hosts demonstrating superparamagnetic-paramagnetic phase transitions. Regarding the magnetic properties, a similar behaviour is observed with the two zeolites, with a large magnetic moment for the samples with high Co content. The most remarkable property of these materials is a change in the magnetic behaviour without structural modification under variation of the temperature. At 10 K, the materials behave as typical superparamagnetic nanoparticles, while at 300 K they behave as pure paramagnetic materials. This property may open the possibility for using these materials as temperature-dependent magnetic switches.[44,45,46] Supercapacitors Nanostructured electrode materials have attracted great interest as a basis for supercapacitor charge storage devices, as they show better discharging rates and higher capacities than traditional materials, where the distance within the material over which electrolyte must transport ions is dramatically smaller than with conventional electrodes composed of chemically similar bulk materials. In addition, larger surface areas in these materials can lead to higher currents during charge and discharge compared with conventional electrodes. From a materials point of view, there are three main categories of electrochemical capacitors: carbon/carbon, metal oxides and electronically conductive polymers.[47] Microporous frameworks might offer alternative electrode materials with promising characteristics and performance, for example ultra-stable Co-containing zeolite Y.[48] The zeolite Y was used as a high-surface area template on which a redox-active Co(OH)2 nanostructure was synthesized through ion exchange, chemical precipitaton, and self-directed growth processes. During the ion-exchange reaction, Co2+ ions undergo exchange with H+ ions to balance the charge of the negatively charged AIO4" tetrahedral units embedded in the structure; the zeolite host causes a concentration gradient of Co + ions decreasing from the

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surface of the USY particles towards the surrounding solution, and forming whisker-type Co(OH)2. The loosely packed nanometer-scale whiskers create electrochemical accessibility for electrolyte OH" ions and a fast diffusion rate through the bulk Co(OH)2, which is of fundamental importance for materials intended for supercapacitor applications. 2.4. Biological and medical applications The diverse and highly controlled properties of zeolites make them interesting candidates for a variety of biological and medical applications. Zeolites have been applied to regenerate artificial dialysis solutions.[49] Another promising application is diagnostic magnetic resonance imaging (MRI), which relies on administering contrast agents. The contrast agents contain high-spin metals that bind water molecules and thereby yield proton spin relaxation times that are orders of magnitude faster than those obtained with free water. Gadolinium ions, Gd3+ perform particularly well as contrast agents, but cannot be administered directly owing to their inherent toxicity. Zeolites with enclosed transition and rare earth metal ions are being envisioned as contrast agents for the gastrointestinal tract.[50] The zeolite immobilizes the Gd + and thus mitigates its toxicity (zeolites themselves are not toxic when introduced into the gastrointestinal tract), while still allowing dynamic coordination of water. Many proteins that are removed from biological structural matrices become very unstable. Protein stabilization by binding to the external surface of zeolites is therefore being investigated. The proteins tend to bind well at or around their isoelectric point to zeolite particles with high silica content suggesting that the hydrophobicity and the three-dimensional structures of zeolites have the strongest impact on adsorption. In order to utilize zeolites as potential chromatographic carriers, it is important that the proteins adsorbed on zeolites must be recovered easily and the recovered proteins must have the same activities as before. For example, lysozyme activity has been preserved when using zeolite beta as a carrier. Moreover, the zeolite carriers are resistant to extreme pH, high and low temperatures, denaturant, and detergents. [51] The molecular sieves ITQ-1 and ITQ-6 with low aluminum content have been used as supports for enzyme immobilization; the zeolites exhibit a very ordered external surface and good thermal stability. Different enzymes including Penicillin G acylase were stabilized via electrostatic and covalent interactions on zeolite carriers. The possibility of using molecular sieves for processes involving enzymes as catalysts or sensors has been demonstrated.[52] Zeolites have also been used as absorbers for perfluorocarbons (PFC), which are applied in the treatment of respiratory diseases, to control the PFC content in breathing gases.[53] By preventing water condensation in the zeolite and PFC condensation, the weight gain of the zeolite absorber over time allows precise measurements with volume errors less than 1 % for PFC volumes grater than 1 ml. Molecular sieves are used in various applications in nuclear medicine. For example, small beads of zeolites were soaked in a solution of radioactive ions. These zeolite beads are employed as point source markers for the identification of anatomical landmarks and for gamma camera uniformity. Due to their small size and relatively high uptake they provide excellent devices for measuring spatial resolution, detector uniformity and energy resolution. [54] Zeolites are also utilized as binding agents for toxic compounds and antioxidant for selenium, vitamins, and provitamins, and are also used as mineral additive in various dietary strategies. [5 5] Zeolites can be very effective as a vehicle for transferring foreign compounds into a living cell by optical manipulation.[56] Zeolite particles can hold foreign chemicals in their fine pores. One example is based on the transport of a zeolite particle doped with a

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fluorescent dye into a protoplast cell, and when the particle is delivered the emission from the dye complexed with intracellular calcium ion is observed. On the other hand, optically driven transport has great potential since the necessary laser manipulation is remotely controllable. In the examples given above, the bioactive molecules were adsorbed on the outer surface of the zeolites. Important other applications of zeolites in this area include CO2 removal in the breathing atmosphere in space, and recycling of wastewater. Recently the NASA conducted research on the removal of gas components under ultra clean room conditions in satellite systems, where zeolites can reduce the outgassing rate inside of instruments and absorb molecules without producing additional particles by themselves. 3. ZEOLITE FILMS, LAYERS AND COMPLEX MORPHOLOGIES 3.1. Synthesis of zeolite films and membranes A number of novel applications of zeolites depend on the ability to create thin, adhesive films on various substrates. While zeolite films or layers are commonly prepared on dense substrates such as silicon wafers, zeolite membranes are made on porous supports in order to permit permeation through the zeolite layer. Numerous synthetic studies have addressed the goal of obtaining adhesive layers of zeolites on various substrates such as noble and nonnoble metals, glass, ceramics, silicon, and even biological substrates such as cellulose fibers. For a more detailed discussion of zeolite membranes the reader is referred to the article by Julbe in this book. Pertinent reviews to this subject are given in the following references.[57,58] 3.1.1. Synthesis of zeolite films by in situ crystallization In the synthesis by in situ crystallization, the zeolite crystals grow directly from a synthetic solution on the surface of an appropriate substrate such as silicon. For example, continuous multilayered ZSM-5 films were grown on cordierite modules. [59] Similar films were generated on as-prepared and acid-treated honeycomb substrates. [60] The latter treatments led to silica-rich surface layers; their composition affected the Si/Al ratios of the zeolites crystallized on the cordierite. Thin, defect-free MFItype films were also made on porous alpha-alumina and yttria-doped zirconia substrates using tetrapropylammonium hydroxide (TPAOH) as a structure-directing agent. [61] Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9). [62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 3.1.2. Synthesis of zeolite films by seeded growth In a second approach for thin film synthesis, a layer of pre-synthesized seed crystals is deposited on the substrate, followed by some means of binding it to the surface and subsequent hydrothermal synthesis of the desired zeolite film on the seed layer. This and the previous growth technique are advantageous if a very strong bond to the substrate is desired. Furthermore, for a select group of zeolites it has been shown that substantial crystal

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intergrowth can occur during film synthesis such that the resulting film becomes a dense zeolite phase that can even be gas-tight. As tightness is a key factor for zeolite membranes, the above growth techniques have also become the most important methods for membrane preparation.

Fig. 9. SEM images of the fracture surface of SiOC microcellular foam coated with silicalite-1 crystals synthesized at 150 °C (a, b) and 125 °C (c, d).[62]

Thin layers of oriented (with their (hOO) faces parallel to the substrate) zeolite A crystals were prepared by adsorption on oppositely charged substrates by electrostatic attraction. [64] A silylation reaction was used to modify the external surface of the negatively charged zeolites such that they exhibited a positive zeta potential. An alternative slow dip-coating process on substrates bearing the same charge as the zeolites resulted in highly ordered, closepacked hexagonal colloidal crystals. Such films could also be used as seed layers for secondary growth, first by local epitaxy on the seed layer and later by incorporating particles from solution. A detailed study of the growth process and the structural evolution of silicalite-1 (MFI) films was undertaken with the aid of grazing incidence synchrotron X-ray diffraction.[65] The diffraction data of the adsorbed and grown zeolite films at different incident and exit angles reflect the distribution of the crystal orientation along the film thickness. The films were prepared via assisted adsorption of nanoscale MFI seed crystals, followed by calcination and subsequent hydrothermal synthesis on the seed layers. The adsorbed (multi-) layer of seed crystals consists of randomly oriented crystals. With progressing hydrothermal growth, the film surface becomes smoother and a preferred crystal orientation with the b-axis close to vertical to the substrate develops. 3.1.3. Synthesis of zeolite films by vapour phase transport A third approach, sometimes called vapour transport synthesis, usually relies on the deposition of a gel-layer on the substrate followed by treatment with a (humid) vapour of some agents missing in the synthesis mixture, for example a volatile amine, resulting in the crystallization of the desired zeolite film. One approach deals with the formation of zeolite ZSM-5 or silicalite-1 from dry precursor material (dried alumino/silicate gels) via a gas phase transport process in the

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presence of dry ammonium fluoride and tetrapropylammonium bromide. [66,67] The authors suggest that silicon tetrafluoride might act as a mobile species for mass transfer between the amorphous precursor and the zeolite crystals formed. The vapour-phase transport method was also successfully used for the preparation of membranes and bulk material of the following zeolites: MCM-22 membrane (used for the alkylation of toluene with MeOH to p-xylene),[68] FER-membrane (for the separation of benzene/p-xylene),[69] EMT crystals using a crown ether as structure-directing agent,[70] and zeolite beta (BEA).[71] In the latter two examples, a conversion of agglomerated nanoparticles to highly crystalline zeolite was observed. A reversed approach using a volatile Si-source (tetraethoxysilane, TEOS) was applied to prepare MFI membranes by wetting a porous alumina substrate with TPAOH-solution and exposing it to vapors of TEOS at elevated temperatures. [72] In this case the TEOS must hydrolyze and gel in situ in the thin liquid film on the porous substrate. 3.1.4. Deposition of zeolite films by laser ablation It has been shown that pulsed UV-lasers can be used to ablate certain fragments from a zeolite target; these fragments can be deposited on a substrate and subjected to a subsequent hydrothermal treatment resulting in crystalline zeolite layers. The initially deposited layers appear to be X-ray amorphous, although they can still be used to nucleate crystals of the original zeolite target material. [73,74,75] In some cases the subsequent hydrothermal growth of the target zeolite results in preferential crystal alignment in the films. For example, oriented UTD-1 membranes were obtained via laser ablation from UTD-1 zeolite and deposition on silicon or porous stainless steel, followed by hydrothermal treatment (Fig. 10).[76] The resulting films showed densely packed plank-like crystals of UTD-1 with the large one-dimensional pore system oriented away from the substrate.

Fig. 10. SEM images of a UTD-1 film obtained by laser ablation followed by hydrothermal growth for 72 h. [76]

3.1.5. Deposition of zeolite films from pre-synthesized crystals and growth on molecular layers If the demands for tightness of the films are less stringent or irrelevant, other methods can be used for the formation of zeolite films. Spin-coating of zeolite suspensions (with nanoscale zeolite crystals) as well as molecular assembly of pre-synthesized zeolite crystals are viable methods for the deposition of rather smooth films. Furthermore, electrophoresis was successfully used to attach zeolite crystals to electrically conducting substrates.

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The molecular assembly of zeolite layers has been developed in an elegant series of studies by the group of Yoon. In one of their earlier papers, fullerene (C6o) was used as a covalent linker for the attachment of cubic zeolite A (LTA) crystals and ZSM-5 crystals on glass. This was achieved by tethering (3-aminopropyl)triethoxysilane on the glass surface, and attaching a layer of fullerene molecules by addition of the terminal amino groups to fullerene double bonds.[77] The zeolite crystals were separately treated with (3aminopropyl)triethoxysilane on their external surface and were allowed to react with the fullerene-covered surface, thus forming a rather close-packed zeolite monolayer. Other binding modes for zeolite monolayers include diisocyanates (they readily react with surface hydroxyl groups on glass or on the zeolite crystals by forming urethane linkages),[78] 3-halopropylsilyl groups,[79] the reaction between tethered epoxy-groups and primary amines,[80] and hydrogen bonding. As an example of the latter, the adenine-thymine binding pair was used to bind thymine-tethered ZSM-5 or zeolite A crystals to adeninetethered glass substrates (Fig. 11).[81]

Fig. 11. Scheme for the attachment of zeolite crystals by thymine/adenine-mediated hydrogen-bonding (left). The thymine and the adenine were tethered to the respective surfaces through 11trimethoxysilyl-«-undecyl-linkers. [81] SEM image of a monolayer of cubic thymine-zeolite-A crystals assembled on adenine-glass at 50 °C (right).

Well-defined zeolite layers can also be prepared via synthesis in the presence of molecular or polymeric templates. Some of the first examples in this regard were the syntheses of porous aluminophosphate and zincophosphate crystals on molecular layers of alkylphosphonic acids.[82,83] In extending this general templating approach to polymers, a recent study shows that uniformly aligned polyurethane films turn out to be excellent templates for the synthesis of well-aligned two-dimensional arrays of silicalite-1 crystals. [84] It is possible to extend the assembly techniques for zeolite layers to three-dimensional bodies. For example, the layer-by-layer deposition of a variety of zeolites such as LTA and BEA on negatively charged polystyrene beads was achieved by first modifying the bead to adsorb the zeolite particles, followed by the electrostatic layer-by-layer assembly of alternating zeolite and polyelectrolyte layers of opposite charges (Fig. 12). [85]

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Fig. 12. Presentation of the layer-by-layer technique used for the construction of multilayer zeolite coatings on spheres. [85]

Electrophoresis An electric field can be used to move charged particles onto a surface, thus resulting in a thin film. This approach has been used for the deposition of zeolite films on conducting surfaces. For example, films of zeolites NaA, NaY, and H-ZSM-5 were electrophoretically deposited from solvents such as acetonitrile and acetone on bodies of various shapes - in this study an important ingredient was the addition of colloidal silica as a binder that was used to strengthen the films upon heating to 400 °C.[86] Hydrothermal growth can be used to further enhance the stability of the resulting zeolite films. [87] 3.2. Applications of zeolite films We develop the different types of applications based on their underlying physical principles. As mentioned above, for a discussion of zeolite membranes (see, e.g., [88,89,90]) the reader should consult the chapter by Julbe in this book. 3.2.1. Electrochemistry with zeolite-coated electrodes Based on their regular pore structure, it should be possible to impart the molecular sieving capabilities of zeolites on the surface of electrodes in electrochemical reactions. Several research groups have addressed this issue by (i) developing ways to modify electrodes with thin zeolite films and (ii) by studying the resulting changes in electrode behaviour in electrochemical reactions. The subject has been reviewed.[91] Mechanically stable faujasite-type films were grown hydrothermally on glassy carbon electrodes at low temperature.[92] The authors succeeded in demonstrating charge- and size-

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selectivities of such films (given a sufficient thickness) for redox reactions of model systems such as Ru(NH3)63+, Fe(CN)63", Ru(bpy)32+, dopamine, and ascorbic acid. Zeolite films were also explored for their use in potentiometric reactions.[93] The authors determined the potentiometric response of zeolite-modified electrodes towards cations in aqueous phase. Three different preparation routes were used for the formation of the zeolite films; zeolites Y (FAU), A (LTA) and mordenite (MOR) were pressed into discs, sodalite (SOD) was grown in a free-standing membrane, and for the pressed discs of LTA a secondary growth phase was added in order to heal defects in the pressed discs. The authors could demonstrate size-selective behaviour in these systems, i.e., cations with diameters exceeding those of the zeolite window openings resulted in no detectable potential response. The various tunable properties of zeolites have inspired a great variety of concepts in electrochemistry with zeolite-modified electrodes. For example, silver ions inside the zeolite pore system are not electrochemically active in amperometric detection. However, indirect analyte detection can occur when the analyte causes the removal of silver ions into the solution where they are electrochemically detected.[94] This indirect approach was extended to different copper-exchanged zeolites and demonstrated for the detection of several nonelectroactive ions including alkali metal, ammonium and calcium.[95] A zeolite-modified electrode (ZME) with high selectivity towards Pb over Cd in cyclic voltammetry was prepared via electrophoretic deposition of zeolite Y, coated with Nafion.[96] Electrodes modified with zeolites and mesoporous periodic aluminosilicate (MCM-41) were used for the investigation of encapsulated polyacetylene and polypropyne obtained by in situ polymerization of the respective monomers on Ni(II) exchanged hosts. [97] The authors observed high charge uptake during (irreversible) oxidation of the polymers. The reduction of methylviologen dications to the cation-radicals at ZME (zeolite Y) interfaces shows a concentration overpotential due to the ion exchange equilibrium with sodium at the zeolite-solution interface.[98,99] In the process, methylviologen dications diffuse from the zeolite channels into solution. The authors suggest that ion exchange accounts for the electron transport mechanism. A striking example of molecular sieving in a stable, continuous b-oriented silicalite-1 film (having pores of about 0.55 nm) on an electrode was recently demonstrated with redox probe molecules of different sizes (Fig. 13). [100] Specifically, the smaller complex Ru(NH3)63+ with a diameter of ca. 0.5 nm was shown to travel through the film, while the larger complex Co(phen)32+ with a diameter of ca. 1.3 nm was completely excluded from the zeolite film and thus from redox processes. 3.2.2. Zeolite films as dielectric layers for semiconductor applications In the relentless quest for ever faster computer circuitry, the dielectric constant of the insulating layers between conductors on the chip is becoming a major issue. This constant should be as small as possible while the mechanical properties of the dielectric material must withstand the subsequent processing steps and ensure the integrity of the computer microprocessor. Nanoscale zeolite crystals, particularly pure silica zeolites, have been proposed as candidates for thin films with low dielectric constant (low k). As an example, suspensions of nanoscale crystals of the pure zeolite silicalite-1 (MFI-type) were used for spin-on deposition of thin dielectric layers.[101] The as-deposited films were subsequently calcined at 450 °C in order to remove organic molecules and to consolidate the films. The authors report low dielectric constants (although the adsorption of humidity must be controlled) and satisfactory mechanical properties of their films.

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Films from the same research group were subsequently characterized with regards to their porosity, showing both zeolite microporosity and textural mesoporosity.[102] The above concept can be extended towards films with different binders, including organic polymers. Thus, two-component films comprised of nanoscale silicalite-1 and acrylic latex were deposited on silicon wafers via spin-coating. [103] In this case, a purified suspension of colloidal zeolites with sizes of 30 or 60 nm were first deposited followed by calcination. In a second step, a layer of acrylic latex was deposited, resulting in layers with dielectric constants between 2.0 and 2.5.

Fig. 13. Structure and concept of size-selective zeolite-coated electrode, (a) Electron micrograph of boriented silicalite-1 film (top view), (b) side view, (c) schematic illustration of molecular sieving of electro-active species having different sizes. [100]

The dielectric constant of a film of nanozeolites can be reduced further if the volume fraction of the zeolite is reduced. This was demonstrated by adding a porogen, i.e., gammacyclodextrin, into an MFI-nanoparticle suspension used for spin-coating the film. [104] The resulting films still had an elastic modulus of about 14 GPa and a dielectric constant of 1.8 after calcination. The most stable, adhesive and hydrophobic silicalite-1 films can be obtained by in situ crystallization on the silicon substrate, followed by calcination to remove the organic template.[105] Their elastic modulus reaches 30-40 GPa, but the dielectric constant was measured to be 2.7-3.3. These findings suggest an inverse relationship between the mechanical strength of the films and the lowest achievable dielectric constant. 3.2.3. Optical applications of zeolite films So far, only few studies have been reported on the use of porous zeolite films for optical applications. While significant efforts have been extended towards the encapsulation of various dye molecules into zeolite crystals or powders (see section 2.2), integration of such systems into thin films has not been pursued by many groups yet. As one of the few examples reported so far, we discuss the inclusion of oriented hemicyanine dyes into thin zeolite films aimed at Second Harmonic Generation (SHG).[106] The zeolite film plays the important role

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of organizing the orientation of the optically active molecules. Thus, silicalite-1 films with a thickness of 400 nm were prepared on glass plates, and hemicyanine dyes with different alkyl chain lengths (between 3 and 24 carbon atoms) were incorporated into the film via diffusion from a methanolic solution. It was concluded from the optical data that the dyes form only a small average tilt angle of about 8 degrees with respect to the channels of the zeolite host. The high degree of uniform alignment in these films (albeit at low dye densities) is being viewed as a promising feature for the future design of host-guest films for nonlinear optical applications. Photocatalytic reactions in heterogeneous media rely on efficient energy transfer to the reacting molecules- this calls for effectively reducing the scattering at interfaces. One possible strategy for achieving this relies on the use of nanoscale zeolites in thin films. For example, the photocatalytic isomerization of cis-2-butene to the trans-form at 2 °C over a transparent TS-1 film consisting of nanoparticles was demonstrated. [107] The TS-1 nanoparticles were also used for the preparation of fibers and porous monoliths, thus providing a variety of formats for possible optical applications.[108,109] Reviews covering this diverse area are available.fi 10,111] The former review covers the photochemistry of nanostructured materials (including zeolites) for energy applications, the latter the preparation and properties of chromophores in porous silica and minerals such as zeolites. 3.2.4. Chemical sensors based on zeolite films Chemical sensors are small devices for the detection and quantification of gaseous or solvated species. This is an active research area based on the need to obtain increasing amounts of data in chemical and food process streams as well as environmental monitoring. Most sensors consist of an appropriate transduction principle such as the quartz-crystalmicrobalance (QCM) and a chemically sensitive layer that imparts the desired chemical response behaviour. Most often a chemically selective response is desirable. Zeolite molecular sieves offer size- and shape-selective adsorption behaviour that can be combined with appropriate transduction concepts in order to construct chemically selective sensor devices. Acoustic devices Acoustic wave devices such as the quartz-crystal-microbalance or surface-acoustic wave devices have been combined with thin zeolite layers in order to impart the molecular sieving capabilities onto the sensor device. The formation of acoustically coupled zeolite layers was achieved according to different methods including dip-coating from a zeolite-binder suspension, spin-coating from such a suspension, growth of a crystal layer, or attachment of nanoscale zeolite crystals via electrostatic or covalent binding methods. Over the years it has been demonstrated that different types of chemical selectivity in the sorption behaviour of zeolites could be transferred to the acoustic wave device; this includes size- and shape selectivity as well as hydrophilic-hydrophobic interactions. Typically, differences in uptake between molecules entering the zeolite pores and those adsorbing only on the external surface can amount to a factor of 100.[l 12] Thin layers of colloidal silicalite-1 were electrostatically pre-assembled on the gold electrode of a QCM, followed by hydrothermal growth of a dense film of the zeolite.[113] This sensor system was highly sensitive for the detection of hydrocarbon vapors. A recent example shows the high sensitivity of LTA-type zeolite films on QCMs towards low levels of humidity.[114] The films were obtained using secondary growth on a precursor

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seed layer consisting of zeolite seeds with a mean size of 40 nm that were adsorbed on silanemodified sensor surfaces. The seed layers were subjected to hydrothermal treatment, thus resulting in LTA-films with thicknesses of ca. 60-1000 nm. The thicker films showed, not surprisingly, slower equilibration rates than the thinner films. However, reversible water vapour sorption isotherms could be measured at room temperature in these thin films. Zeolite Beta (BEA) films of 250 nm thickness were synthesized on the gold electrodes of QCM-devices according to a similar approach and their sorption behaviour towards different vapors was compared with that of zeolite LTA.[115] Due to the larger pore size of the BEAtype films, the latter could adsorb larger organic molecules such as pentane, hexane and cyclohexane, while the LTA-type films showed a selective response to water vapour only. Further examples of acoustic sensors modified with zeolites include a QCM sensor with silver-exchanged ZSM-5 that responds selectively to acetone (in diabetic's breath) in the ppm-range,[l 16] principal component analysis of multiple QCM-sensor responses (with LTA, MFI, SOD) for the detection of NO/SO2 mixtures,[117] MFI-zeolite-coated microcantilevers with ppm-sensitivity for Freon detection [118,119] and other zeolite-coated cantilevers for humidity sensing. [120] Electrochemical and electronic devices In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coated sensor device, thus resulting in an increase of the sensor selectivity towards

Fig. 14. Zeolite-modified Pd-doped S11O2 sensor design (left; DVM: digital voltmeter). The plots on the right display the sensor resistance as a function of the type and concentration of the organic vapour (methane or ethanol). The measurements were done at 350 °C with 0% relative humidity in the feed. The data for the unmodified (reference) sensor and the sensors modified with layers of silicalite-1 and zeolite A are shown. [121] Another type of zeolite-filter was employed for the detection of hydrocarbons; in this case a Pt-doped zeolite filter changes the selectivity of a titania conductivity sensor towards

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propane in the presence of CO, presumably due to the resulting water of combustion. [122] Related concepts were used for the detection of NOX.[123,124] Humidity sensors are often based on conductivity changes of hydrophilic substances, as in the case of LiCl. This salt can be included into zeolites such as Y, and at suitable loading levels a humidity-sensitive composite is obtained that is stable towards high humidity and high temperatures and less susceptible to external contamination than bulk LiCl.[125] The nature and mobility of ions and solvent in zeolite cages will affect the ac-impedance of the material. This effect can be utilized for zeolite-based sensor concepts where a zeolite film is coated on interdigitated electrodes. For example, it was shown that the impedance of a film of proton-conducting H-ZSM-5 is influenced by the presence of ammonia (Figs. 15, 16).[126,127] The ammonia is protonated in the zeolite, thus producing much larger ammonium ions with different mobilities in the zeolite that can be detected by impedance spectroscopy. The detection of ammonia is of interest for automotive applications where the selective catalytic reduction of NOX by ammonia is envisioned.

Fig. 15. Schematic presentation of the sensor; (a) measuring electrode design coated with zeolite film (top) and heater electrodes (bottom), (b) cross section. The resistance of the Pt heater is also used as temperature sensor. [1271

Fig. 16. Resistance changes of a zeolite-coated (H-ZSM-5) sensor exposed to different ammonia concentrations. The resistance was derived from an RC equivalent circuit at 1 MHz measurement frequency. [127]

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Optical sensor concepts Optical responses of molecules or materials can offer several advantages in sensor design: The response is often very fast, allowing for rapid response if needed, the response can be transferred through space, across optical windows or fibers thus protecting the analytical system or the measurement device, and, depending on the transitions involved, the analyte may have highly structured spectroscopic features that may help to identify or quantify it. A few recent examples will serve to illustrate these points. The redox behaviour of highly dispersed mononuclear and clustered Ti-, Sn- and V-oxide species in the pores of zeolites and mesoporous hosts was investigated as a means for oxygen detection.[128,129] The authors found that the reversible optical changes of these materials could be correlated to the number of oxygen vacancies in the particles, and that these changes can be used for the design of an oxygen sensor. Another approach is to encapsulate photoactive molecules in the cages of zeolites. The complex tris(bipyridyl)ruthenium(II), Ru(bpy)3]2+, was entrapped in highly siliceous zeolite Y obtained by a dealumination reaction with silicon tetrachloride.fi 30] This was achieved by adsorption of neutral Ru(bpy)Cl3 as a starting material. The oxygen sensing mechanism of Ru(bpy)3]2+ is based on emission quenching by dissolved oxygen. It was found that the dealumination leads to a hydrophobic environment that favors oxygen diffusion from the water into the zeolite. A thin film format where the zeolites are included in a siloxane layer was also investigated. Aromatic molecules such as naphthalene will emit phosphorescence when adsorbed in zeolites exchanged with heavy ions such as thallium. This was exploited in a convenient, zeolite TlY-coated optical fiber format in order to detect naphthalene.[131] Solvatochromic dyes offer the possibility to detect changes in the polarity/dielectric constant of a solvent, resulting in a significant spectral change. If such a dye is included in the cages of a zeolite, the solvent loading in the nanoscale cages in combination with the molecular sieving behaviour of the zeolite will control the spectral signature of the dye, thus acting as a sensor (Fig. 17).[132,133] This concept was realized with a number of solvatochromic dyes such as nile red in dealuminated zeolites that showed fast and reversible changes in absorption and fluorescence upon exposure to a variety of different molecules.

Fig. 17. Encapsulation of the dye nile red in siliceous zeolite Y in a multi-step synthesis. [132] If a redox reaction causes a reversible optical change, this reaction could be used to detect the concentration of the oxidant. This approach was incorporated into an elegant sensor

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design for oxygen-detection at high temperatures. [134] The Cu(I) ions in the zeolite Cu-ZSM5 show a strong fluorescence at 550 nm, while reversible oxidation with oxygen produces Cu(II) ions that do not emit in this region. This zeolite was embedded into a thin sol-gel derived silica film on the tip of an optical fiber used for both excitation and detection of the emission. This sensor was shown to detect oxygen at the level of a few hundred to a few thousand ppm at 425 °C with a fast response time of a few seconds and good reversibility. Other concepts - microcalorimetry Finally, we mention a novel transduction concept based on the heat evolved from a reaction such as combustion. Microcalorimetric devices can now be made using lithographic techniques. One of the two sensitive areas of such a device (were evolved heat can be measured) was coated with a thin film of CoAlPO4-5, the other was kept open as a reference.[135] The additional benefit of a zeolite with catalytic activity for such a device is the molecular sieving effect that can be combined in the response of the sensor (a molecule too big to enter the catalytically active interior of the zeolite should only show a weak response). The change in temperature was measured with a meandering Pt-wire resistor. This device was examined in the detection of CO and cyclohexane, and sensitivity and selectivity in the low ppm-range was observed. 4. CONCLUSIONS The well-defined nanoscale pore systems featured by zeolites are the basis for several established types of applications such as heterogeneous catalysis, ion-exchange and gas separation by selective adsorption. In the more recent past, however, researchers have also explored opportunities in additional areas. This review has explored recent developments in advanced applications of crystalline molecular sieves. It is apparent that the zeolite pore systems with their extremely well-defined crystalline cages and channels, their ion-exchange capability, and their tunable properties with respect to acid/base behaviour, hydrophilicity/hydrophobicity can offer a platform for many novel functional concepts. In addition, the growing ability to control the size and morphology of either zeolite single crystals or zeolite intergrowths has opened up entirely new applications such as zeolite membrane separations, chemical sensors or spatial confinement. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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An evaluation of the potential of the Ship-in-Bottle approach for catalyst immobilization in microporous supports Pierre A. Jacobs Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, B-3001 Leuven (Heverlee); [email protected] 1. SCOPE 2. METALLO-PHTHALOCYANINES IN FAUJASITE-TYPE ZEOLITES 2.1. Encapsulation of MePc 2.2. MePcY zeolites as oxidation catalysts 2.3. MePc faujasites in various reactions 3. NON-HEME TYPE ZEOLITE ENTRAPPED COMPLEXES 3.1. Bipyridyl complexes in zeolites 3.2. Poly-aza macrocyle complexes 3.3. Metal Schiff base complexes 3.4. Dimeric Cu and Co-Mn mixed acetate complexes in Y zeolite 4. ZEOLITE ENCAPSULATED CHIRAL COMPLEXES 5. GENERAL CONCLUSIONS ACKNOWLEDGMENTS REFERENCES 1. SCOPE Encapsulation is a unique catalyst immobilization technique as it does not require interaction between the host (support) and the guest (complex) and in principle allows to mimic the (catalytic) properties of homogeneous complexes [1]. All other methods, such as covalent linking of the ligand with the substrate, might induce changes in the ultimately formed complex. The only prerequisite for successful encapsulation requires that the size of the complex should exceed that of the pores / cages of the support, precluding the leakage of the complex into solution in working conditions of the heterogenized catalyst. As a consequence, faujasitic-type zeolites have been the preferred hosts for encapsulation of complexes. i The encapsulated catalyst can be obtained by

r

assembling the catalytic complex within the pores or by building the support around the isolated complex 1 (vide infra). Both methods today are known in • - ' —~^^^B' literature as the flexible ligand and zeolite synthesis method, respectively. In the flexible ligand method the ligand or its precursors should be able to diffuse freely through the zeolite pores exchanged with the preferred transition metal ion.

290

while for the zeolite synthesis method the transition metal complexes should be stable and dissolved under the conditions of zeolite synthesis, i.e. at high pH (pH > 12) and elevated temperature (> 100°C). Unfortunately, this does not preclude leaching of the transition metal from the occluded complex in cases where its coordination with the substrate or reaction product is stronger than with the ligand. Much work with encapsulated complexes mainly in zeolites appeared in the past decade. Surprisingly, the last few years, a revival of the activity in this area has occurred, mainly by groups that entered the field rather recently. As this area is often reviewed as part of larger reviews on catalyst immobilization, this particular area of 'Complex Immobilization in Microporous Hosts' usually is treated in less detail [2]. It is the aim of the present work, given the clear revival of activity in this area, to critically overview the major achievements. Emphasis will be on such complexes which have catalytic potential. The very early work, which appeared in the late seventies-early eighties, dominated by the Lunsford group, will only occasionally be referred to. It is hoped that this approach will help new workers to define new objectives and approaches. 2. METALLO-PHTHALOCYANINES IN FAUJASITE-TYPE ZEOLITES 2.1. Encapsulation of MePc Phthalocyanine (Pc) complexes of transition metals have received much attention in the scientific literature of the last two decades, not only as mild catalysts for selective oxidation reactions but also as functional models for enzymes. Unfortunately, their use is hampered by their reduced solubility in solvents and their tendency to form adducts even when used in solution. Provided such complexes can be immobilized individually on a catalyst carrier, it is expected that an enhanced dispersion of the complex will be achieved. The use of heterogenized Pc complexes will also no longer be restricted by the nature of the reaction medium or solvent. The issue has been reviewed as early as 1986 [3]. Among the many possible candidates for catalyst support, some zeolite topologies constitute a particular group of carriers. It seems that Romanovski et al. [4] were the very first to report an in situ synthesis of transition metal Pc (MePc) complexes in the supercages of faujasite type zeolites. The synthesis was later successfully repeated by Schulz-Ekloff et al. [5] and Herron [6]. It is assumed that formation of MePc out of four 1,2-dicyanobenzene (DCB) molecules in the supercages of Me-exchanged faujasite (FAU)-type zeolites is accompanied by a two-electron oxidation of (residual) water molecules: Me+++ H 2 O+ 4DCB

»~ MePc + 2H + +

1/2O2

The in-situ synthesis of MePc in zeolite Y supercages can be schematically represented as follows:

291

The key issue of such synthesis is that DCB molecules as chelating agents may enter freely the 12-membered ring windows of the supercages, where Me ions reside. Once formed, the MePc complexes with at least a diameter of 1.2 nm are stuck irreversibly within theses cages. To this irreversible encapsulation, the term ship-in-bottle approach was coined by Herron [6]. The individual complexes are now isolated in the supercages and at a sufficient degree of dilution, say one complex per every 10 supercages, they are accessible by substrate molecules that have access to the zeolite intracrystalline space. An important aspect of encapsulation is to prove that such complexes really reside within the zeolitic cages as they seem to interact strongly with the external surface of zeolite crystals also [7]. Similarly, it has been shown that the pores of VPI-5 are completely filled with MePc [8]. Heating of hydrated VPI-5, an extra large pore AIPO4 molecular sieve, in presence of ferrocene and 1,2dicyanobenzene, there was advanced ample evidence that encapsulated FePc was formed. More in particular the material was now stabilized thermally as the usual transformation into the less open AIPO4-8 structure, which occurs upon removal of the stabilizing triple helix of adsorbed water, fails to be observed, pointing to the extra-stabilization offered by pore occluded stacks of FePc. The use of 'H-27A1 CPDOR NMR (cross-polarization - double rotation) shows a remarkable enhancement of the NMR signal caused by the tight fit between arrays of stacked metallophthalocyanines and the VPI-5 pore walls. Such phenomena fail to be observed after impregnation of pre-synthesized FePc on VPI-5. The schematic representation of this system is given below. Rather recently, attention was paid again to this issue, using a multitude of techniques yielding complementary information on FAU occluded MePc. Chemical analysis of a CuPcY sample prepared with the in situ ligand synthesis method {vide supra) using DCB and CuY, shows that next to 0.8 Pc ligands per unit cell, 0.5 unmetallated ligands are occluded [9]. The characteristic n - n* transitions (Q bands) in the visible region of Pc are red-shifted from the soluble complex to the encapsulated one. The band at 545 nm e.g. shifts to 581 nm upon encapsulation. This has been interpreted as the result of puckering of the planar geometry of Pc upon encapsulation [7].The 2nd derivative ESR spectrum of CuPcY shows 9 hyperfme features, due to 4 equivalent N, indicating that the CuPc complexes are isolated as intermolecular interactions are negligible. The same ESR spectrum allows also to resolve the hyperfme features attributed to Cu. The spin Hamiltonian parameters of CuPcY (gparaiiei : 2.246; gperpendicuiar : 2.056; ^paraiiei(Cu) : 189.5 G; ^perpendicuiar(Cu) : 14.2 G) point to the existence of a tetragonally elongated square pyramidal geometry for copper and puckering of the planar Pc in the zeolite supercages [9].

292

From mere geometric considerations and mechanical modelling, Herron [10] anticipated that encapsulated Pc should have a saddle-type deformed structure. Unfortunately, the parallel IR shifts and band splittings for such puckered structure with decreased symmetry, have not been reported yet. The zeolite Y occluded MePc complexes are ideally accommodated in the supercages as appears from their unusual thermal stability in these hosts. Only above 400°C, Pc ligand is seen to distil out of the zeolite and to loose its structural integrity [11]. An attempt to encage nitro-substituted Pc in NaY, starting from adsorbed ferrocene and 4-nitro, 1,2-dicyanobenzene failed, as the formed complexes were exclusively found at the external crystal surface as they easily were removed upon Soxhlet extraction with dimethylformamide [12]. The synthesis of Me-porphyrins (MePOR) in situ in zeolites has been reported out of a mixture of pyrrole and an aldehyde (acetaldehyde) (see scheme) [13]. As this synthesis is a much less clean process than the corresponding one with DCB, the abundant debris filling the zeolite pores is difficult to remove. In a study of carrier effects on adsorbed tetraphenylporphyrin (Ph4POR) complexes, the association with NaX has been reported [14]. Unfortunately no preparational details were given, though it seems that impregnation of preformed complexes is used. Therefore, adsorption of Fe(III)Ph4POR at the external surface of the crystals may be expected. Given the acceptable methane monoxygenase activity with H2O2 of up to 100 TON per hour at 180°C with such systems, careful design of encapsulated complexes seems worthwhile. The synthesis of faujasite type zeolites around MePc and MePOR complexes has been reported as well [15, 16]. This method is suited in particular when Pc complexes with more bulky substituents as tetra nitro-Pc are used. Such substitution on the Pc plane will result in a significantly enhanced activity and an even more secure encapsulation. It will allow reduction of the degree of pore filling and pore blocking, thus facilitating the mass transport in the zeolite cages of a working catalyst. Consequently, only relatively small amounts of MePc have to be added to a traditional zeolite synthesis mixture. It is not clear whether in such cases the MePc complexes are functioning as templates and therefore reside exclusively in the supercages rather than in lattice imperfections or crystal holes. A disadvantage of the former method consists in the synthesis, as occluded or demetallated Pc is always formed. For FePcY an improved method was reported, using adsorbed ferrocene rather than ion exchanged Fe. This method exclusively generates FePc [8]. The encapsulated MePc in Y zeolite can be considered as a formal mimic of cytochrome P450, the rigid zeolite environment becoming a substitute for the flexible aminoacid environment of the natural enzyme [17]. After embedding in a polydimethylsiloxane (PDMS) membrane, the MePcY is able to function in a membrane reactor, and mimic the performance of the natural cytochrome P450 in a cell membrane. 2.2. MePcY zeolites as oxidation catalysts To mimic the Cytochrome P450 properties with MePc embedded in zeolites, the oxygenation of alkanes and cycloalkanes has been studied frequently. Occasionally, iodosobenzene (PhOI) has been used as oxidant [6] Alternatively, t-butyl hydroperoxide

293

(tBuOOH) allowed the conversion of secondary C atoms into the corresponding alkanol /alkanone mixtures [19, 20]. Oxidation with hydrogen peroxide or dioxygen failed. The reaction of highvalent Me such as Fe(V)O and Mn(V)O species [21] with an sp3 C-H bond occurs via H° abstraction, followed by a fast recombination of °OH and C° radicals to form an alcohol: Mn(V)=O+ H - R - » -

Mn(IV) - OH +

R°

»-

Mn(lll)+ ROH

This "oxygen — rebound" mechanism assures that the radicals formed remain with the complex, and are not allowed to start a radical chain reaction in solution. Kinetic isotope effects (KIE) in the order of magnitude of 10-12, clearly indicate that the C-H bond breaking event is rate determining as in the natural enzyme [17]. Oxygenation via free radical chains in such systems, frequently shows KIE of around 2-4. Although the mobility of MePc complexes should be strongly reduced in the intracrystalline faujasite supercages, truly site-isolated metal centers can be obtained this way. Moreover, Fe and Mn (salts or exchanged zeolites) which are active for one-electron redox processes as in Fenton free radical H2O2 decomposition (see reaction), LFe(ll)+ H2O2 —*-

LFe(lll)+ OH° + OH~

upon ligation with heme-type macrocycles such as phthalocyanines (and porhyrins) are able to activate mono-oxygen donors such as H2O2, tBuOOH, NaOCl, or PhOI in a two-electron redox reaction [23]. Me(lll)Pc + XO

»-

X +

Me(V)PcO

On the other hand, in some cases hydroperoxides have been detected as oxygenation products, stemming from a radical trapped by dissolved dioxygen. This is a particularly evident phenomenon with subambiant oxygenations as shown for cw-pinane [21]:

FePcY

tBuOOH

The strong coordination with ligands in apical positions, explains the sensitivity of the reaction to solvent effects. Another advantage of the zeolite encaged MePc resides in their stability through site isolation. In solution, MePc are known to easily form aggregates, catalyzing their selfoxygenation. For the same systems haloperoxidase activity has been reported with H2O2 / O2 as oxidant and HC1 / HBr as halogen source [22]. In this way oxychlorination / oxybromination of benzene, toluene, phenol, aniline, anisole and resorcinol could be achieved. The CoPc based materials are the established catalysts for the autoxidation of thiols to disulfides in basic aqueous medium [23]. First thiol is deprotonated in the reaction medium. Then the deprotonated thiol coordinates to the Co-macrocycle. When the thiol and dioxygen are coordinated in trans to the MePc, Co mediates the transfer of an electron from RS" to coordinated O2:

294

RS

+ 0 2 —*• RS° + 0 2

Two thiol radicals then recombine to form a disulfide: 2 RS°

»-

RSSR

The room temperature autoxidation of mercaptoethanol and dodecanethiol with zeolite encapsulated CoPc has been investigated in detail [24. In basic aqueous medium enhanced hydrophobicity obtained with hosts with increasing Si / Al framework ratios is the key activity determining parameter, thus giving CARBON supports substantial advantage. For the autoxiation of long chain thiols in organic medium CoPc in dealuminated Y is the preferred catalyst. As the weak ionization of thiol into thiolate in such conditions is difficult and the oxygen solubility at higher reaction temperatures is low, the combination of a hydrophobic support consisting of a dealuminated faujasite or a VPI-5 AIPO4 with a substituted Pc such as tetra-nitro-Pc, perfluoro- or perchloro-Pc, should yield an optimally designed catalyst. Oxidation of mercaptanes is possible as well [25], while applications in the sweetening of the petroleum fractions in the MEROX process [26] seem possible. For several reactions an enhanced specific oxygenation activity per MePc has been reported [3]. In the oxidation of cumene, cysteine, and CO, the zeolite specific activity increases by a factor of 25, 3, and 4, respectively, compared to homogeneous conditions. This has been attributed to zeolite physisorption properties, resulting in an enhanced substrate concentration in the immediate neighborhood of the active site. In this respect the polarity of the zeolite cage is relevant. It has been reported that in the oxygenation of cyclohexane with tBuOOH and FePcY, strong adsorption of the polar reaction products (cyclohexanol and cyclohexanone) causes catalyst deactivation which can only be restored by excessive solvent extraction [27]. As this phenomenon is absent with apolar carbon black as support [28], it must be possible to obtain a suitable faujasitic support for design of a stable catalyst, after extensive dealumination of the zeolite. There is an amazing report in literature claiming that CuClnPcY in presence of dioxygen [29] is able to oxidize primary C atoms in a selective way, excluding the occurrence of a free radical pathway, normally expected for a Cu-catalyzed autoxidation [30]. This important observation (see table) urgently needs experimental confirmation.

The oxidation of styrene around 60°C with tBuOOH [9] yields the epoxide, phenylacetaldehyde and benzaldehyde as main products amongst some side-products (see table). The turnover frequency (per Cu or CuPc) of the oxygenation is superior for the

295 encapsulated CuPc complex, as already suggested for other similar systems. Benzaldehyde is possibly obtained via secondary (radical-type) oxidation, while phenylacetaldehyde seems to be an isomerization product of the primarily obtained aldehyde. Compared to the homogeneous phase, consecutive (acid-catalyzed) isomerization is favored, probably by the presence of residual protons generated during the Cu exchange and subsequent dehydration. It cannot be concluded that apart from the enhanced turnover frequency, attributable to zeolitic sorption, the oxidation mechanism is significantly different from that in homogeneous conditions. The activity order for MePcY [31]: V=O < Co < Cu in parallel with a decreased epoxide selectivity, simply confirms the increasing ability of transition metal hydrolysis during the first stages of catalyst preparation and the enhanced presence of residual protons. The hydroxylation reaction of phenol with hydrogen peroxide and zeolite encapsulated MePc has received considerable attention. With the perchlorinated phthalocyanine (Cl^Pc) and tetra-nitro ((NCh^Pc) substituted ligands, catalysts with superior activity have been obtained [32]. Such catalysts have been prepared via the zeolite synthesis method around the individual complexes. With the former more bulky complex only the slimmer hydroquinone (HQ) has been obtained, while with the encapsulated perchloroPc equal ratios of catechol (CAT) and the para-isomer have been obtained (see table). The unsubstituted Pc in zeolite Y both with Co and Cu as metallating ion, show an excess of the ortho-isomer (CAT) [32], corresponding to the approximate thermodynamic ratio. This points to the critical importance of the available space close to the encapsulated Pc as selectivity determining parameter: when there is more space, the catalyst yields more catechol.

TON

EH2O2W

CAT/(HQ+ PBQ)

Cu (NO 2 ) 4 PcY

0: 1

CuCI14PcY

1 :1

CuPcY

235

79

2: 1

CoPcY

196

65

1-8=1

FePc-VPI-5 [8, 18] is an active catalyst in the alkane oxygenation with tBuOOH at room temperature. For cyclododecane and cyclohexane as substrate, TON of 125 and 313 have been obtained, respectively, assuming that all FePc are accessible. When the real situation is taken into account that FePc are present in the monodimensional pores as stacks of Pc with only the pore mouths being accessible, real TON should be in the range of a few hundred thousand, corresponding to a TOF of a few hundred turnovers per minute, a value that largely surpasses all literature values on related systems. These piles of buried FePc

296

should very much enhance the properties of the complex in the pore mouth in direct contact with the substrate. How this unsurpassed catalytic effect should be rationalized mechanistically and to what large sized (bio)molecules this may be extrapolated, remains unsolved. 2.3. MePc faujasites in various reactions Next to oxygenation, zeolite entrapped MePc complexes show also activity in dehydrogenation and hydrogenolysis reactions, and NO reduction [3]. More recently, the carboxylation of epoxides into organic carbonates has been reported [33] (see scheme).

The table again shows that upon encapsulation of CuPc in zeolite Y, also for the carboxylation of propylene oxide (PO) enhanced reaction yields are generated rather than new chemistry, in perfect agreement with the conclusions drawn from the oxygenation chemistry. The reaction was run for 2700 TON and the catalyst found to be regenerable. 3. NON-HEME TYPE ZEOLITE ENTRAPPED COMPLEXES Next to the rich oxygenation chemistry of Mn in Mn-Pc and Mn-POR complexes, there exists catalytic chemistry of Mn with non-heme-type ligands, mostly bioinspired. In Photosystem II, a non-heme multinuclear Mn redox center allows to oxidize water, while in catalase the active center is a dinuclear Mn species [34]. Biomimetic models for these biological redox centers use ligands such as 2,2'-bipyridine (BPY), triaza- and tetraazacycloalkanes and Schiff bases such as Me(Salen) and Me(saloph) (structure see below) [23]. Usually, the complexes activate heterolytically peroxides, with Mn valency changes such as: Mn(ll)

9- Mn(IV)

Mn(lll)

*- Mn(V)

3.1. Bipyridyl complexes in zeolites In solution, complexes of manganese ions with BPY easily form dimeric complexes, the link of the nuclei occurring either via a /u,-oxo or a /n-hydroxy ligand [23]. Such complexes are very active in the decomposition of H2O2. With the ligand method, i.e. via addition of bipyridyl ligand to Mn(II) exchanged Y zeolite, a monomeric form of the complex is stabilized in the supercages (Mn(BPY)2) [35].

297

There is ample spectroscopic evidence that this occluded complex has exclusively cissymmetry (see model), allowing Mn coordination with the organic ligands as well as with the oxygens of the zeolite cage walls via Me solvatation. Entrapment shows a clear adaptation of the complex to the zeolite geometry and transforms the peroxide decomposition catalyst into an epoxidation catalyst. Residual zeolite acidity is found to open the epoxide and form a series of consecutive oxidation products, resulting ultimately after 800 turnovers in the filling of the pores with adipic acid (reaction sequence below) [36].

It seems that practical implementation of this type of selective catalysts will require a medium in which (very) polar products can be removed from the zeolite phase. Unfortunately, no attention has been paid in literature to such issues. On the contrary, some attention has been devoted to host modification after exchange of NaY with other alkali metal cations [37]. The cyclohexene epoxidation activity increases with decreasing size of the charge compensating cation pointing to the influence of steric effects or of electrostatic effects on the activity. In competitive experiments using cyclohexene and 1-octene as feed, the reactivity of the smaller substrate is suppressed, indicating that competitive sorption is involved as well [37]. [VO(BPY)2]2+ complexes can be formed via ligand addition to vanadyl-exchanged Y zeolite [38]. There is strong spectroscopic proof with FT-Raman, FT-IR, DRS, EPR, that individual complexes up to 0.5 mmol/g are occluded in the intracrystalline zeolite space and are homogeneously distributed across the crystals (XPS; TGA). The activity of the system, up to 0.6 TON/minute, comparable to that of dioxygenase enzymes, is much higher than either the VO-zeolite or the homogeneous complex, which is catalytically inert in the same conditions. Thus it seems that the zeolite renders the complex catalytically active upon entrapment. The primary product formed with cyclohexane and H2O2 is cyclohexyl hydroperoxide, yielding equal amounts of ol / on. The results parallel those of vanadatepyrazinecarboxylic acid complexes and allowed to advance a possible mechanism, involving an open V-peroxo species with O-O stretching band at 873 cm"1, that abstracts H from an alkane substrate (see scheme below).

298

The same encapsulated complex is a good epoxidation catalyst, showing no evidence for allylic oxidation. It follows that heterolytic rather than homolytic dissociation of the V-peroxo species constitutes the dominating mechanism. Whereas with hydrogen peroxide in acetone the catalyst yields high diol selectivity, giving cis/trans isomers at equilibrium, with ffiuOOH in tBuOOtBu mainly epoxide is obtained. Thus the role of residual acidity in such systems seems crucial. Another way of reported host modification consisted in the post-treatment of zeolite Y with various acids, or by synthesis of zeolite Y in presence of large templates of the cetyltributylammonium type [39]. While in the oxygenation of cyclohexane with H2O2 an ol /on mixture is obtained in all cases, the acid-treated Y as well as the zeolite synthesized in presence of bulky alkylammonium template molecules and subsequently loaded with Mn(BPY)2 via the ligand method, show enhanced activity compared to the encapsulated complex prepared via addition of the ligand to MnNaY. All complex containing catalysts show in their turn superior activity compared to that of MnY, devoid of any ligand. Although the porosity of such hosts has been insufficiently characterized, it follows that the generation of secondary mesopores in zeolite Y crystals results in enhanced activity and consequently in better diffusivity. The emission characteristics of Mn-diimine complexes occluded in various faujasite hosts has been studied in detail [40]. The key factor influencing the emission properties of Mn(BPY)2 is found to consist of matrix induced complex distortion, determined by cation stabilization of the complex stabilizing anion. As host for Mn(BPY)2 are compared the cubic faujasites X, Y and the hexagonal EMT faujasite. The high-spin complexes show emission spectra with decreasing intensity of the metal to ligand charge transfer absorptions (MLCT). With 490 nm excitation the emission is blue shifted for NaX (570 nm) with increased quantum yield, while for NaEMT the emission is red shifted to 591 nm with decreased quantum yield. It seems that the zeolite cages impose steric constraints on the ligands in the sequence: NaX > NaY > NaEMT. This sequence follows the framework Si/Al ratio and thus the cation population of the cages. Undoubtedly, the zeolite functions as supramolecular cryptating agent that enhances coordination sphere stability of the occluded complex against photo-dissociation and is a valuable alternative to the use of organic cage-type ligands. Unfortunately, all this does not allow to decide that in EMT for accommodation the complexes prefer the hypercages (see model) over the supercages. When over-stoechiometric amounts of ligands are used, it has been known since long that tris bipyridyl complexes [41, 42] are formed which are not active in 'dark' catalysis but show interesting properties as photocatalyst [43]. Photoinduced electron transfer from encaged [Ru(BPY)3]NaX to methylviologen is 2 orders more

299

efficient than in other crystallites of the same size [42]. A similar electron transfer upon photo-excitation has been illustrated for ITQ-2 zeolite [44]. Upon photo-excitation, encapsulated cationic viologen2+, accommodated in the 10-membered ring channels of this zeolite receives electrons from (polypyridyl(ruthenium) complexes located in the open cups of this zeolite topology. This phenomenon of "through-framework-electron transfer" has been illustrated for ferrocene and [Fe(BPY)3]NaY as well [45]. The same ferrous tris encapsulated complex can be oxidized to its ferric analogue with the help of chlorine. For the photocatalytic degradation of 2,4-xylidine a system consisting of ferrous [Fe(BPY)3]NaY codoped with TiC>2 clusters has been designed, and prepared via consecutive encapsulation with the ligand method and hydrolysis of adsorbed TiCU [46]. The chemistry of phenanthroline (PHEN) complexes of e.g. manganese, is comparable to that of BPY. The relative stiffness of the former ligand, however, yields complexes that accept much less steric constraints from the environment. A spectroscopic study of [Mn(PHEN)2] + in NaY shows significantly broadened MLCT bands (at 524 and 480 nm) compared to [Mn(BPY)2]2+, resulting from the different degree of distortion of the coordination sphere [47]. Whereas with BPY rotation around the C2 - C2 axis is allowed, with the rigid phen, no deviation from the planar ligand structure is possible. As a result, the phen ligand is less capable of adaptation to different coordination requirements, while in a given constrained environment distortions of the coordination sphere will be more pronounced for PHEN than for BPY. This has immediate obvious catalytic consequences. The catalase activity (H2O2 decomposition rate) is much higher for encapsulated Mn(PHEN)2. Moreover, in cyclohexene reaction with the same oxidant, the presence of allylic oxidation products (2-cyclohexene-1-ol and -1-one) is significant. With styrene, benzaldehyde is an important product. With PhIO [Mn(PHEN)2]2+ in NaY is a better epoxidation catalyst than [Mn(BPY)2]2+ NaY. 3.2. Poly-aza macrocyle complexes Several polydentate N-ligands were shown to make macrocyclic poly aza complexes in the supercages of FAU-type zeolites [7, 48]. In order to fully understand the coordination chemistry of Ni2+ and Co2+, ESR, UVVIS-NIR, Raman, and magnetic techniques were applied. The formation of [Co(II)cyclam]Y is clearly established, the cis complexes outnumbering by far the transligated ones. This behavior is quite general and is observed for several tetradendate ligands forming pseudo-octahedral complexes. It thus seems that the zeolite surface has only low tendency to bind as a monodentate ligand to planar metal surfaces. It forms a new microporous redox-solid acting as a reversible, highaffmity, and high capacity (> 90 jiimol/g) dioxygen-sorbing material [23] and ranks among the best compared to [Co(II)(bipyridine-terpyridine)] and [Co(II)(CN)5]3"NaY [49, 50]. On [Co(II)(tetren)]2+-NaY oxygen sorption is irreversible, due to /i-peroxo formation. The latter probably is related to the presence of a ligand with extremely high electron density. In contrast to Co, with [Ni(II)cyclam]Y the trans-form

300

dominates, keeping the axial positions available for ligand exchange. For intercalated complexes between clay layers, the latter is blocked [51]. The prevention of H2O2 decomposition is an issue for Mn(Me3triazacyclononane) complexes. This complex [Mn2+(Me3tacn)] when accommodated in a supercage of zeolite Y is stabilized as a dimer [52]. Through this way the catalase, i.e. H2O2 decomposition activity is suppressed, epoxidation is only possible in acetone. Indeed, at low temperature acetone perhemiketal is formed, which serves as a reservoir of active oxygen keeping the H2O2 concentration in solution very low, thus showing low catalase activity:

It should be mentioned here that when the same complex is immobilized on support by covalent tethering, isolated monomeric complexes are exclusively obtained, thus yielding a heterogeneous catalyst that works in any solvent! Compared to the Mn complexes, the catalytic activity of Cu-aza macrocycles has not been explored much. Among the different Cu triand tetra-aza complexes investigated [53], Cu(tacn)Y, Cu(Me4Cyclen)Y, and Cu(cyclam)Y show high TOF of around 50-70 h"1 at 55°C in the oxidation of ethylbenzene with tBuOOH. With CuY mainly benzaldehyde is formed, pointing to the Me(R3tacn) importance of radical pathways. In homogeneous conditions side-chain alkylations as well as ring hydroxylation occur. Upon encapsulation, the ring hydroxylations are significantly suppressed, with acetophenone as major product. This has been ascribed to the differences in active Cu sites in the homogeneous compared to the zeolite medium. From Cu-protein work, it is known that side-on peroxo Cu(II)2 species as in tyronase enzyme is present, while in Dopamine (l-monooxygenase hydroperoxo species intervene. ESR and UV-VIS spectra from a Cu(cyclam)Y catalyst after tBuOOH addition show evidence for diamagnetic side-on peroxo and bis /i-oxo species. Taking into account the easy dimerization of Mn(Me3tacn) in Y zeolite [52], this is a very logical rationalization for Cu(cyclam) too.

301

The oxidation of dimethyl sulfide to the corresponding sulfoxide on different zeolites has been reported recently, using zeolite entrapped Cu-ethylenediamine ([Cu(en)2]2+) complexes. Spectroscopic comparison between the neat and the NaY, KL, and NaBETA entrapped complexes, shows that the square planar complex undergoes distortion in the zeolite crystal [54-56]. Changes in redox properties of the complexes in the zeolites are due to decrease of the HOMO / LUMO levels of the metal complexes upon encapsulation under influence of the electric field existing inside the zeolite [56]. The high activity in ZSM-5, however, points to the existence of extrapore complexes, probably strongly adsorbed at the external surface. 3.3. Metal Schiff base complexes Schiff bases have been synthesized in an amazing structural variety. This flexibility has been used also to design zeolite based oxidation catalysts. Though initially limited TON were obtained with PhIO [57], good oxidation catalysts are available now, in particular when the phenol groups in salen are replaced by the more oxygen resistant pyridyl groups, resulting in e.g. a pyren ligand [47]. With Mn Schiff bases in zeolites, peroxide decomposition is very fast. With PhIO good Me(salen) selectivity is obtained, whereas tBuOOH is the more active alternative. Anyway, the product distribution with olefins shows that the mechanism is not purely of the oxo transfer type, but has some radical character, which is evident from the presence of significant amounts of (cyclo-2-hexenyl) t-butyl peroxide from cyclohexene:

An axially coordinated base in case of Co(smdpt) enhanced the oxygen sorption capacity [58], while this is much less the case in catalysis with Mn(smdpt), possibly due to the constraints of the zeolite supercage. This partial radical character does not preclude these systems from being good alkane oxygenation catalysts. In Me(smdpt) a comparative experiment involving Mn(salen)Y, Mn(pyren)Y, Mn(pyrpn)Y, all catalysts Me(pyrpn)

302

show comparable activity and selectivity, though for long time-on-stream the relative sensitivity of the salen ligand to self-oxidation is apparent. This work confirms the behavior of the zeolite Y encapsulated poly-aza macrocycle complexes, as in all these zeolite-catalyzed oxidation reactions less selectivity for allylic oxidation is observed [59]. It can be clearly shown for CoY, that the formation of square-planar complexes with 4-coordinated Co-like salen is very difficult in faujasites, while complexes with 5-coordinated Co-like smdtp are easily obtained. It seems as though the additional tertiary N in smdpt provides the necessary donor strength to lift the Co ions from their zeolite coordinated positions, while the softer salen donor atoms are less suited for doing this. The difficulties in bringing an axial base ligand into the right position when the square planar complex is encapsulated in zeolite Y is circumvented by using this pentadentate smdpt [60]. The use of EMT zeolite for the same encapsulation work yields spectroscopically more relaxed spectra of Co(smdpt) with a better similarity to the corresponding homogeneous complexes. This a posteriori indicates that such complexes are preferentially located in the hypercages rather than in the supercages of such structures. Cu(II), Ni(II), Zn(II) [61], as well as Cr(III), Fe(III), Bi(III) [62, 63] pentadentate saldien complexes encapsulated in zeolite Y have been reported as well. The encapsulated complexes are more active in the phenol hydroxylation with H2O2 than the homogeneous ones with 2:1 catechohhydroquinone selectivity being at thermodynamic equilibrium. The encapsulation of dimethylated (Me2salen) complexes of Co, Ni, Cu in zeolite Y has been tested in the oxidation of ethylbenzene with hydrogen peroxide [64]. The tetrahedrally distorted Cu(II) square planar complexes show superior activity and selectivity for acetophenone, the oxo-product, while the octahedral symmetry of Co(II) and Ni(II) complexes may have rendered them only weakly active. Generalization of these zeolite effects is obtained with the N,N'-bis(2-pyridinecarboxamide) Mn and Fe complexes encapsulated in zeolite Y, for which the chemical link between the pyridinecarboxamide parts is varied. Suitable ligands are: bpenH2, bppnH2, bpchH2, bpbH2 [65, 66]. The Mn(bpen), Mn(bppn), Mn(bpch), Mn(bpb) complexes turn out to be acceptable oxidation catalysts with H2O2 and tBuOOH. Invariably, not only decreases the catalase activity of each of the zeolite encapsulated complexes significantly upon entrapment, but also with cyclohexene the epoxide selectivity depends strongly on the nature of the ligand. The changes in the nature of the bridging group, determine the selectivity for allylic oxidation, the Mn(bpch) and Mn(bppn) encapsulated complexes being the superior epoxidation catalysts.

303

ESR spectroscopy shows that all catalysts, in contrast to what happens with Mn(Me3tacn), fail to dimerize upon encapsulation as they do not show the typical Mn'"-Mn!V signal. ESR simultaneously shows complex distortion, caused in the Mn coordination sphere by the adsorbed ligands. The LMCT bands are intense especially for the Mn(bpb) complex, but less pronounced than with the corresponding Fe complexes, pointing to a less pronounced Mn-N(pyridine) interaction. IR inspection of the amide region shows that multiple amide bands resulting from coupling between C-N and N-H bending modes around 1400-1300 cm"' are shifted to higher wavenumbers. As this shift gives an indication on the degree of Mn-4N coordination, it follows that imperfect Mn-4N coordination and therefore differences in the degree of ligand protonation increase in the following sequence:

Mn(bppn) = Mn(bpch) < Mn(bpen) « Mn(bpb). Thus N4 square planar coordination occurs with the propyl- and cyclohexyl-linked ligands, while with ethyl- and benzyl-linked ligands, N2 ligand coordination and double protonation is more likely. It is evident that the imperfect coordination in the latter two cases yields lower epoxide and higher yields of products from allylic oxidation. When this class of bis(2-pyridinecarboxamide) ligands is complexed in the intracrystalline space of zeolite NaY, potential catalysts for alkane oxidation are obtained. A combination of XPS, TGA and SEM points to a homogeneous distribution of Fe across the zeolite crystals upon encapsulation. The MLCT bands in UV-VIS spectra confirm the absence of multinuclear Mn aggregation. FT-IR shows variable amide deprotonation of the ligand upon complexation, depending on the nature of the link between the two 2pyridinecarboxamide units. The increased degree of ligand deprotonation and the parallel increased degree of 4N square planar coordination occurs in line with an increased participation of an oxo mechanism with a highvalent Fe=O intervening, at the expense of a radical pathway. Moessbauer spectroscopy allows to discriminate among ligated Fe(II), free Fe(II) and Fe located in the hexagonal prisms (HP) of the faujasite structure (see table below) [66]. The superior quality of the bpch ligand as coordinating agent as evident from the yield of oxo-product in catalysis is confirmed physico-chemically. The determination of the amount of high-spin Fe(II) and the molar susceptibility with SQUID is also in line with the amount of complexed iron.

Fe(bpch) Fe(bppn) Fe(bpen) Fe(bpb)

Ligated Fe(II) %

Fe HP %

Free Fe(II)

89 66 70 71

6 9 20 11

12 25 10 18

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The oxo-oxidation products from cyclohexane with tBuOOH are cyclohexyl peroxide and its decomposition products, cyclohexanol and cyclohexanone. The relative ratio of the hydroperoxide decomposition products depends on its decomposition mechanism (see scheme). After a homolytic O-O bond cleavage in the peroxide, the formed alkoxy radical can undergo disproportionation, yielding equal amounts of ol/one. A high ketone yield results from the peroxide dehydration with a Lewis acid, such as Fe(OH) formed by H2O2 decomposition on free Fe. The use of adamantane to probe the reaction mechanism yields consistent information. This molecule with 12 H atoms on secondary and 4 H atoms on tertiary C atoms, upon radical oxidation with tBuOOH yields product ratios for attack at Csec/C,er, varying between 0.05 and0.15. For FeY a ratio of 0.05 has been obtained, confirming the radical nature of the oxidation reaction. For the samples containing ligated iron, substantially higher ratios adamantane are obtained. Zeolite Y encaged Co and Ru(salophen) were very active catalysts in the oxidation of a-pinene (see scheme). For Ru at 100°C and 3 MPa of air, TOF of over 18,000 have been observed, which were a factor 2 higher than the homogeneous complex [67]. The presence of a highvalent Ruv=O was proposed for the pinene epoxide product, while the presence of the allylic oxidation product (D-verbenone) points to the presence of a radical pathway. The same encapsulated Co(salophen) complex has also been used in the electron chain designed Me(salophen) for the oxidation of alcohols [68]. The acetoxylation of 1,3dienes with dioxygen, uses the Pd(II)/Pd(0) redox couple, To reoxidize Pd(0) a redox chain is used, shuttling electrons from Pd(0) to paraquinone and further to a Comacrocycle complex which activates a | p h a . p i n e n e D-verbenone pin ene epoxide dioxygen. As a particularly stable Co-macrocycle, the Co(salophen) complex encaged in zeolite Y has been proposed (see cycle) [69, 70].

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When Mn(salen) is substituted with ethoxy-groups in position 3 of the phenol moiety, selective hydroxylation catalysts for 1-naphthol or phenol are obtained with the NaY encapsulated version [71]. However, when in Mn(saldp) (see scheme) methoxy-groups are placed in the same position, no acceptable (cyclohexene) epoxidation catalysts are obtained [72]. The abundant formation of allylic oxidation products points to the existence of highly distorted Mn complexes or of free Mn(II). Only the Mn(3methoxy-salen) entrapped in zeolite X shows medium epoxidation selectivity [72]. It is clear that subtle changes in host-guest size / configuration can cause drastic catalytic selectivity changes. 3.4. Dimeric Cu and Co-Mn mixed acetate complexes in Y zeolite Dimeric copper acetate complexes encapsulated in zeolite Y (and MCM-22) exhibited increased activity in the phenol hydroxylation with dioxygen, compared to the homogeneous system [73]. Again a dimeric complex is stabilized in the zeolite cages, as can be derived from the reduced coupling constant between the 2 Cu ions as is derived from ESR [7]. Mixed /x-oxo Co-Mn acetate complexes were synthesized in the supercages of zeolite Y [74]. This is one of the few examples showing a reduced activity upon encapsulation. Nevertheless, the catalyst shows excellent selectivity in the oxidation of p-xylene into therephthalic acid, as no 4-carboxybenzaldehyde is formed, on e of the nuisances in the traditional process. Such an important observation makes experimental verification is desirable. 4. ZEOLITE ENCAPSULATED CHIRAL COMPLEXES The catalytic chemistry of chiral complexes entrapped in zeolite cages has been the subject of recent reviews. [75-78]. Much attention has been devoted to the encapsulation of the Jacobsen

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/ Katsuki chiral salen complex for the epoxidation of prochiral alkenes. The state of the art can be summarized as follows. Although the initial work showed some difficulties to synthesize intact forms of this complex (see scheme), now most of the authors succeed in making encapsulated versions of the complex that reach enantioselectivity comparable to that of the homogeneous phase. This raises the expectation that it should be easily feasible to entrap chiral Me(salen) in super-/ hypercages of faujasitic zeolites. Careful consideration however, of Jacobsen's system [76] consisting of chirally substituted Me(salen), in which an enantioselective oxygen transfer occurs from NaCIO (hypochlorite) or PhIO as mono oxygen atom donor to a prochiral epoxide, shows that it is necessarily a two-phase system. Indeed, the catalyst resides in the organic phase containing the prochiral substrate and the enantiomerically enriched products, thus protecting them from overoxidation with aqueous hypochlorite. The use of an oxidant soluble in the organic phase, like tBuOOH, is a misconception that never can yield high epoxide chemoselectivities. Immobilization on zeolite supports will yield a hydrophilic catalyst, positioned necessarily in the wrong phase. The Cu(ll)(oxazoline)2 encapsulation of Cu(II) bis(oxazoline) (see scheme) in the supercages of zeolite Y has been reported for enantioselective Diels-Alder reactions [79, 80]. The oxidation of organic sulfides into sulfoxides or sulfones with high chemoselectivity but rather low enantioselectivity has been reported using Mn(II) or Cu(II) ligated with tetradentate ligands with C2-symmetry (see scheme) [81-82]. 5. GENERAL CONCLUSIONS Metallo-phthalocyanines can be embedded in zeolite pores via an in situ synthesis technique comparable to but less tedious than the artisan ship-in-bottle technique. The supercages of the faujasite topology are particularly suitable as host of the Pc complex. Around transition metal ions exchanged in the zeolite four 1,2-dicyanobenzene molecules tetramerize, forming the occluded MePc complex. The latest information indicates that the planar ring is puckered due to the steric limitation imposed by the supercage walls. At relatively low loading corresponding to less than 1 Pc complex per 10 supercages, a catalytic system is generated with sufficiently fast diffusivity. Alternatively, the MePc complexes, in particular substituted ones like tetra-nitro and perchloro, can be used as template for zeolite synthesis. Such MePc-Faujasites show isolated MePc complexes as active sites that are no longer susceptible to auto-oxidation as in solution, where Pc complexes have tendency to form aggregates. The zeolite encapsulated MePc are not only formal but also mechanistic analogues of the Cytochrome P450 enzyme in oxygenation reactions of alkanes with monooxygen donor oxidants. Their use as heterogeneous catalysts circumvents all problems encountered in homogeneous catalysis, related to their limited solubility. As far as nature of oxidation reaction catalyzed and reaction selectivity is concerned, the behavior in both phases is identical. In some cases, selectivity changes observed with the encapsulated complexes can

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be attributed to zeolitic shape-selective effects, due to steric constraints around the active site. The zeolite based catalyst invariably shows significantly enhanced activity compared to the homogeneous phase, which is attributed to concentration effects in the zeolite pores. The selective retention of the more polar products is the only origin of catalyst deactivation. The tendency to obtain incomplete degrees of transition metal chelation starting from transition-metal exchanged zeolites, thus yielding ions that can start a radical cycle, may be circumvented, at least for iron, starting from ferrocene adsorbed NaY. The entrapment of the Pc complexes in the zeolitic environment renders them thermally more stable. The only weakness of such systems consists in the limited access of large molecules via the 12-membered ring orifices of the faujasite structure. The unprecedented properties of VPI-5 as support for MePc, showing a limited number of MePc complexes at the pore mouths, though unclear at the mechanistic level, might remediate this reduced accessibility, though the phenomenon requires more work to clarify its mechanism and estimate its potential. The entrapment of non-heme ligands in the intracrystalline phase of mainly transition metal-exchanged faujasite-type zeolites to form the corresponding encapsulated complexes invariably shows a significant enhancement or generation of catalytic activity. In particular, this is true for bipyridyl and aza macrocyclic ligands and manganese. The powerful hydrogen peroxide decomposition activity, identical to that of the catalase enzyme in the constrained zeolitic environment is suppressed and converted into epoxidation power. For transition metals like iron, alkane oxygenation catalysts yielding hydroperoxides and their decomposition products are obtained. Copper holds an intermediate position, while cobalt works with dioxygen and sulfides. In every case, the selectivity is significantly enhanced, as secondary reactions of radical nature yielding allylic oxidation products, are suppressed. As a general fashion, it is seen that optimal symmetry of the occluded complexes is distorted as a result of adaptation of the guest symmetry to the pore architecture of the host. The degree of deformation and in parallel the increase of the selectivity for allylic oxidation, depends on the ligand size and structure, its substitution (degree), as well as the Si/Al ratio of the faujasite framework and the nature of the charge compensating cations. The presence of residual acidity can favor the occurrence of secondary reactions, such as opening of primarily formed epoxides. Though there is agreement on the general observations, that have been experimentally confirmed by many groups now, there still remain a few challenging observations. Most of the catalysts in the reaction conditions described show very acceptable stability, though in most cases no hard data are available and one has to rely on author's statements that the "catalyst is stable in time or can be regenerated easily during at least five cycles". The encapsulation of chiral complexes in zeolitic hosts has been a less successful story. A number of complexes have been shown to be susceptible to encapsulation with acceptable chemoselectivity, though the enantioselectivity has never been very high and in no case higher than that obtained with the corresponding complex in homogeneous conditions. Most of the work has been devoted to attempts of immobilization of the Jacobsen Me(salen) catalyst. After all, it seems that a misconception of the operational principle of this catalyst might have been at the basis of this work. Indeed, the enantioselective epoxidation of prochiral substrates with hypochlorite is a two-phase reaction in which the catalyst resides in the organic phase and this way is protected against overoxidation. A zeolite entrapped complex invariably yields a hydrophilic catalyst residing in the hypochlorite-rich phase.

308 ACKNOWLEDGMENTS The experimental results and insight into this topic could never have been achieved without the help of a large number of postdocs and Ph.D students and without sponsoring in the frame of an IAP framework by the Ministery of Science Policy of the Belgian Federal Government. In particular, the author is grateful to Rudi Parton, Dirk De Vos, Fred Thibault-Starzyk, Ivo Vankelecom and Peter-Paul Knops-Gerrits. REFERENCES [I] [2] [3]

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Zeolites in organic cascade reactions H. van Bekkum and H.W. Kouwenhoven Ceramic Membrane Centre 'The Pore", Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands 1. INTRODUCTION 2. AROMATIZATION REACTIONS 2.1. Carbohydrates-to-hydrocarbons 2.2. Alkanes to aromatics 2.3. Cyclocondensation of carbonyl compounds with ammonia to pyridines 2.4. Bi- and tricyclic aromatic N-systems 3. COUPLED CARBOHYDRATE CONVERSIONS 3.1. Cascade of hydrolysis and hydrogenation 3.2. Combined isomerization / hydrogenation in the preparation of D-mannitol 3.3. One-pot synthesis of dilaurylisosorbide 4. AROMATIC SUBSTITUTION REACTIONS 4.1. Alkylation / Isomerization in the reaction of toluene with cyclohexene 4.2. Synthesis of cyclohexylbenzene by trapping intermediate cyclohexene in benzene hydrogenation 4.3. Alkylation - Cyclization 4.4. Selective para-halogenation of biphenyl 4.5 Chlorination / isomerization / separation 5. TWO-STEP REACTIONS WITH EPOXIDATIONS AS THE FIRST STEP 5.1. Dihydroxylation of unsaturated alcohols and halides over TS-1 5.2. Oxidation of furan and methylfurans to dicarbonyl systems 5.3 Oxidation of furfuryl alcohols to 6-hydroxypyranones 5.4. Epoxidation and ring closure 5.5. Epoxidation - Isomerization 6. TWO- AND THREE-STEP CYCLIZATION REACTIONS TO NONAROMATIC RINGS 6.1. One-pot transformation of citronellal to menthol 6.2. Synthesis of coumarin derivatives 6.3. Rearrangement of allyl benzyl ethers 6.4. Synthesis of 2,2,6,6-tetramethyl-4-oxopiperidine (triacetonamine) 6.5. Preparation of 1-substituted tetrahydroisoquinolines via the Pictet-Spengler reaction using a natural zeolite catalyst 6.6. One-pot synthesis of pyrano- and furoquinolines over zeolite HY 6.7. Synthesis of tetra-arylporphyrins 7. CONCLUSIONS REFERENCES

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1. INTRODUCTION Following the successful application of zeolites in the major oil refining processes, zeolites entered the field of bulk chemicals synthesis, e.g. ethylbenzene, cumene and recently caprolactam. Their applications in the areas of fine chemicals are also growing. An important factor is that zeolites, compared to conventional Broensted and Lewis acid catalysts, are lowwaste catalysts and are regarded as 'green'. After the early review on zeolite-catalyzed organic reactions by Venuto and Landis [1] the field has been reviewed several times [2-11]. An obstacle for fast further introduction of zeolites in synthetic organic chemistry is the fact that the average organic chemist is not sufficiently acquainted with zeolites, their handling and tuning, and their potential other than their use as drying agent. Illustrative is that the major laboratory chemicals catalogue does not mention zeolites at all. Just under the name 'molecular sieves' a selection of zeolites is listed [12]. A forthcoming booklet [13] might be of help to the organic chemist. Many target organic molecules are too bulky to enter or leave zeolites. For this category the ordered mesoporous materials, like MCM-41, MCM-48 and SBA-15, became available. The latter material can even accommodate enzymes. Quite recently the use of these ordered mesoporous materials in catalysis has been reviewed [14]. The review covers no less than 447 references. Another recent review [15] deals with alkylation, hydrogenation and oxidation by mesoporous materials. Zeolites are crystalline but versatile materials. They may be modified in many ways; they can be tuned over a wide range of acidity and basicity, and of hydrophylicity and hydrophobicity, many cations can be introduced by ion exchange and isomorphous substitution is possible also allowing build-in of isolated redox centers (e.g. Ti) in the lattice. Moreover metal crystallites and metal complexes can be entrapped within the microporous environment. There is for instance much progress in enantioselective synthesis on chiral catalysts immobilized in microporous or mesoporous materials [16]. This chapter will focus on the use of zeolites in cascade reactions, i.e. combined catalytic reactions without intermediate recovery steps [17]. Here nature serves as the shining example; numerous multistep cascade syntheses are executed in the cells of living organisms without separation of intermediates. By contrast, in fine chemicals syntheses generally a stepby-step approach is applied in which intermediate products are isolated and purified for each next conversion step. Nowadays interest in mankind-designed cascade reactions is rapidly growing. A recent review of Bruggink et al.[18] lists a total of 62 cascade reactions, of which 21 examples involve bio-bio catalysis by mixtures of enzymes, 26 examples involve bio-chemo catalysis, combined action of an enzyme and a chemo-catalyst, and 15 examples pertain to the action of two or more chemo-catalysts. In most bio-bio examples two enzymes are applied but four examples already exist where eight enzymes operate in concert. Cascade record holder is a 12-step synthesis of hydrogenobyrinic acid, the corrin moiety of vitamin B12, starting from 5-aminolevulinic acid [19]. An example in which enzymes are switched on and off by pH variation is the four-step one-pot synthesis of ketoses from glycerol [20]. It may be noted that a modern laundry detergent formulation contains up to six different enzymes. A fine example in the bio-chemo cascade category is the work of Schoevaart and Kieboom, who combined an enzyme, a homogeneous chemo-catalyst and a heterogeneous chemo-catalyst [21]. In the 3-step reaction sequence B-methyl galactoside is selectively oxidized (oxygen, galactose oxidase, 25 °C), dehydrated (L-proline, 70 °C) and hydrogenated

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(Pd/C, 25 °C) to give methyl 4-deoxy-6-aldehydo-B-D-glucoside. Water is the medium and for all reactions pH is 7. Furthermore the enzyme catalase is added to decompose the hydrogen peroxide formed in the first step. An interesting example from the chemo-chemo cascade category is the combined use of an acid and a base catalyst entrapped in two different materials [22]. The acid catalyst is e.g. Nafion (perfluorinated sulfonic acid resin) entrapped in a sol-gel silica and the base catalyst is an Ormosil (organically modified silica sol-gel) carrying e.g. H2N(CH2)2NH(CH2)3 groups. These two catalysts do not desactivate each other and were successfully applied in a pinacol-pinacolone rearrangement (acid catalyzed) followed by a Knoevenagel condensation with malononitrile (base catalyzed) towards l,l-di-cyano-2,3,3-trimethyl-l-butene. As mentioned above zeolites can be tuned in many ways, the same holds for ordered mesoporous materials. Some well-known types of bifunctional zeolitic catalysts are: zeolites in the H-form containing noble metal crystallites titanium containing zeolites or mesoporous materials equipped with mild Bransted or Lewis acidity The following conversion areas provide examples with zeolite participation, in which two or more steps are involved without separation of intermediates. 2. AROMATIZATION REACTIONS 2.1. Carbohydrates-to-hydrocarbons There is a growing interest in processes that convert biomass to (green) transportation fuels or to boosters thereof. Some options are given in the Scheme. On a large and rapidly further increasing scale hydrolysis of the polysaccharide starch and the disaccharide sucrose to the monosaccharides is carried out, followed by fermentation to ethanol. Two ethanol molecules and two CO2 are formed from one C6-monosaccharide (glucose). The largest ethanol producers are Brazil [23, 24] and the USA [25]. Zeolites play a role (adsorption (KA) or membrane techniques) in the dewatering of ethanol (required for mixing with gasoline). Forthcoming are processes in which cellulosics, as present in agricultural waste streams (wheat straw, corn stalks, bagasse), are also hydrolyzed (enzymatically) and converted to ethanol [26] by a yeast. When the ethanol is not blended but used as such as fuel, hydrous ethanol (e.g. 95%) can be applied. In a cascade-type continuous set-up [27] a partial evaporation of the fermentation liquid was carried out and the ethanol-water mixture was passed over a H-ZSM5 zeolite (350 °C, 1 atm). As in the MTG (methanol to gasoline) [28, 29] process a mixture of alkanes (mainly C3-C5) and aromatics (mainly toluene, xylenes and C9-compounds) was obtained. In this approach the ethanol-water separation is avoided: the hydrocarbons and water are nonmiscible and separate by gravity. Though its octane number is good there is no future in MTG-gasoline because of the trend to lower the aromatics content. Mechanistically the ethanol-to-gasoline process is easier to understand than the MTG process because the still somewhat mysterious first C-C bond formation step is absent. When passing the ethanol-water mixture over H-ZSM5 at lower temperature (200 °C) or over zeolite H-Y, only dehydration occurs and ethene is obtained [30]. The track to gasoline is then oligomerisation to Ce-Cg alkene, hydrogenation/isomerization e.g. over the TIP-catalyst Pt-H-mordenite (TIP is Total Isomerization process).

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Recently Dumesic disclosed [31] another route from glucose to n-hexane (together with some pentane and butane) consisting of hydrogenation to sorbitol followed by stepwise hydrogenolysis over a Pt-catalyst in acid medium (pH 2). The hydrogen required is obtained by Dumesic et al. [32] by aqueous-phase reforming of sorbitol or glycerol over a relatively inexpensive Raney Ni-Sn catalyst. Roughly 1.6 mol of glucose are required per mol hexane. The Scheme also shows routes via hydroxymethylfurfural (HMF), a compound which can be selectively made from fructose using a dealuminated H-mordenite (Si/Al 11) [33]. HMF might well develop to a new key chemical its chemistry (and that of furfural) has recently been reviewed by Moreau et al. [34]. When preparing HMF it is advantageous to apply a cascade reaction by using a fructose precursor. Thus the hydrolysis of the fructan inulin (glucose (fructose)n) or of the glucose-fructose combination sucrose is coupled with the dehydration to HMF. In the case of sucrose as starting compound, HMF and the remaining glucose can be easily separated. HMF may be hydrogenated over a Pd-catalyst [35] to 2,5-dimethylfuran, a compound with the extremely high blending research octane number (BRON) of 215 [36]. Another interesting HMF-derived compound is levulinic acid formed together with formic acid by solid acid catalysis. A one-pot cascade route from e.g. inulin seems feasible. Manzer et al. [37] give examples in which an esterification step with an alkene is coupled as well. The

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levulinic esters exhibit good octane numbers. The by-product formic acid might also be esterified or used as hydrogen source. 2.2. Alkanes to aromatics Mono- as well as bifunctional zeolite catalysts are applied in the multistep conversions of: (a) hexane and heptane to benzene and toluene, respectively; (b) lower alkanes (propane, butane) to aromatics; (c) methane to aromatics. The field has been reviewed by O'Connor [38]. Ad (a), as has been found by Bernard [39], platinum on the nonacidic zeolite KL is superior to conventional reforming catalysts in the dehydrocyclization of hexane to benzene (460 °C, molar ratio H2 : hexane 6). It is proposed that zeolite KL is unique in its ability to prevent agglomeration of the small Pt particles required for the reaction. Chevron workers partially exchanged KL towards a Pt-BaKL catalyst [40]. The aromatization selectivity over Pt-BaKL was high and nearly constant (between 90 and 82 %) for n-alkanes having six to nine carbons. A process was developed by Chevron under the name AROMAX. Equilibria to methylcyclopentane and the methylpentanes play a role in the reaction network together with a route to benzene via cyclohexane. Initially shape selectivity exerted by the zeolite was assumed, but equally small Pt particles on magnesia gave similarly good results [41]. Ad (b), by incorporating gallium or zinc into zeolite H-ZSM5, bifunctional catalysts are obtained for the aromatization of light alkanes (LPG). See ref. [42] for an overview. Ga can be introduced by impregnation with aqueous Ga(NO3)3 or by reaction of acidic zeolite OH-groups with trimethylgallium [43] (Aldrich). Zinc is incorporated by ion exchange with a Zn (NC>3)2 solution. First step in the aromatization will be dehydrogenation, e.g. propane to propene, though the mechanism on Ga or Zn is assumed to be different [43]. The olefin joins an olefin pool formed by oligomerization and depolymerization, much alike the MTG process. Then cyclization and further H-transfer reactions follow. With propane as feed a typical product spectrum over Ga-H-ZSM5 is 64% aromatics (of which 89% BTX (benzene, toluene, xylenes, ethylbenzene)), 6% hydrogen and 30% fuel gas [44]. Reaction temperatures are in the range 450-500 °C. BP and UOP jointly developed the CYCLAR process based on Ga-HZSM5 as the catalyst. Ad (c), in 1993 Wang et al. [45] first reported on the dehydroaromatization of methane over Mo-modified H-ZSM5 catalysts (Si/Al 50 and 25). Operating at 700 °C and 200 kPa, benzene was the only product at a methane conversion of 7.2%. The catalysts were made by impregnating NH4-ZSM5 with ammonium molybdate, then drying at 110 °C and calcining at 500 °C for 4 hours. The Mo-loading was 2 wt%. Since then much research was devoted to this process [46]. Mechanistically there is a consensus that at the Mo-site methane dimerisation takes place to ethane, followed by dehydrogenation. The ethene is subsequently - as in the MTG process - aromatized in the H-ZSM5 via an olefin pool. An induction period of up to 1 hour is observed, possibly a Mo-carbide is formed first. 2.3. Cyclocondensation of carbonyl compounds with ammonia to pyridines Aldol condensations of aldehydes and aldehydes or aldehydes and ketones, lead in the presence of NH3 over acid catalysts to pyridine and alkylated pyridines [47]. In some cases high selectivities are obtained. A fine example is the synthesis of 3- alkylpyridines [48] by reaction of acrolein, an alkanal and ammonia over MFI zeolites.

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When passing a mixture of ammonia, acrolein and butanal (molar ratio 3:1:1) over a B-MFIzeolite at 400 °C one obtains P-ethylpyridine with 72 % selectivity. For R = n-C4H9 and n-CeHn the selectivities are 78 % and 90 %, respectively. In all cases conversion is complete and catalyst lifetime > 48 h. The increasing selectivity with increasing chain length indicates shape selectivity; the zeolite might induce the long chain alkanals to adopt - on average favourable positions with respect to the other reactants. In industry pyridine and (3-picoline (3-methylpyridine) are generally coproduced by condensing formaldehyde, acetaldehyde and ammonia. Also small amounts of a- and ypicoline and of higher alkylated pyridines are formed.

The conventional amorphous silica-alumina catalysts have been substituted here by zeolites, especially of the H-ZSM-5 type [49]. Higher yields and higher pyridine/p-picoline ratios are obtained with zeolite catalysis. The micropores will reduce the formation of higher alkylated pyridines. The zeolites can be further improved by incorporating metal oxides (e.g. Pb, Tl, Co) or noble metals or by applying both types of promoters. As an example, a Pb-MFI catalyst, operated at 450 °C in a fixed bed reactor and fed with CH2 O/CH3CHO/NH3 in a 1.0 : 2.0 : 4.0 molar ratio gave 79 % total pyridines with a pyridine/p-picoline ratio of 7.5. Also zeolites MCM-22 and Beta [50] perform well in combined pyridine/p-picoline synthesis. Pyridines and picolines are manufactured on a fairly large scale (together over 50.000 t/a) whereas 3,5- and 2,6-dimethylpyridines (lutidines) are made on a 10 - 100 t/a scale. The combination acetone/formaldehyde/NH3 leads to 2,6-lutidine. Zeolite ZSM-5 and beta [51] have been studied as catalysts here. Instead of formaldehyde also methanol can be used. When feeding a mixture of acetone, l3C-labelled methanol, ammonia and water (molar ratio 2 :1: 4:13.7) to H-ZSM-5 (Si/Al = 96) at 450 °C, 2,6-lutidine is formed (13 % selectivity) which is exclusively labelled at the 4-position [51].

3,5-Lutidine can be prepared from propionaldehyde, formaldehyde and ammonia over a zeolitic catalyst, H-ZSM-5 or H-Beta, but in practice this chemical is isolated from the higher methylpyridines fraction in the pyridine/p-picoline manufacture. Cyclocondensation of 2-butanone or 2-butanol with formaldehyde and ammonia is claimed to give high yields of 2,3,5- and 2,3,6,-collidines (trimethylpyridines) and 2,3,5,6-

317 tetramethylpyridine [52]. A ZSM-5 catalyst is applied at 350 - 420 °C. Compared to the conventional organic syntheses this is a direct route using cheap starting chemicals. The tetramethyl derivative fully reflects the three reactants. One-pot synthesis of 2-phenylpyridine Upon feeding a mixture of acetophenone allyl alcohol and ammonia at 360 °C to a fixed bed of MCM-41 material (Si/Al = 15) or of zeolites HY, H-Beta or HZSM-5 2-phenylpyridine is formed in good yield [57]. Major side product is 3-methylpyridine (P-picoline). Best results are obtained with the MCM-41 catalyst: 67.1 % selectivity at 99.7 % conversion. Of the zeolites, HY (Si/Al = 2.4) gave the best performance: 58.3 % selectivity at 97.8 % conversion.

A 3-step mechanism is proposed by the authors, obviously P-picoline is formed from allyl alcohol and ammonia. Oxidative pyridine synthesis The reaction of ethanol with ammonia on zeolite catalysts leads to ethylamine. If, however, the reaction is carried out in the presence of oxygen, then pyridine is formed [53]. MFI type catalysts H-ZSM-5 and B-MFI are particularly suitable for this purpose. Thus, a mixture of ethanol, NH3, H2O and O2 (molar ratio 3:1:6:9) reacts on B-MFI at 330 °C and WHSV 0.17 h"1 to yield pyridine with 48 % selectivity at 24 % conversion. At 360 °C the conversion is 81% but there is increased ethylene formation at the expense of pyridine. Further by-products include diethyl ether, acetaldehyde, ethylamine, picolines, acetonitrile and CO2. When applying H-mordenite, HY or silica-alumina under similar conditions pyridine yields are very low and ethylene is the main product. The one-dimensional zeolite H-Nu-10 (TON) turned out to be another pyridine-forming catalyst 54]. A mechanism starting with partial oxidation of ethanol to acetaldehyde followed by aldolization, reaction with ammonia, cyclization and aromatization can be envisaged. An intriguing question is why pyridine is the main product and not methylpyridines (picolines). It has been suggested in this connection that zeolite radical sites induced Ci-species formation. Improved yields of pyridine and picolines are obtained when instead of ammonia methylamine is applied as the N-reactant in the reaction with ethanol over H-TON in the presence of oxygen. Here, 4-methylpyridine is a major side-product.

2.4. Bi- and tricyclic aromatic N-systems Quinoline synthesis By reacting an arylamine with lower aldehydes quinoline derivatives can be obtained. Various Bransted acids, including large pore zeolites can be applied as catalysts [55].

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The quinoline structure reflects the reactants. One-pot synthesis of octahydroacridine over CeH-ZSM-5 catalyst Octahydroacridine and its derivatives are of great interest as they play an important role in the preparation of alkaloids, dyes, drugs and other biologically active compounds. Conventional synthesis methods so far reported in literature are multistep and homogeneously catalyzed and require tedious work-up procedures. Several zeolites (HY, H-Beta, H-ZSM-5) and the mesoporous material MCM-41 were found to catalyze the one-pot synthesis of octahydroacridine from the low cost compounds cyclohexanone, formaldehyde and ammonia [56].

Presumably 1-aminocyclohexene and 2-hydroxymethylcyclohexanone are intermediates. Generally dicyclohexylamine and 9-methyloctahydroacridine show up as by-products. Catalyst of choice is CeH-ZSM-5 with Si/Al 30 and a Ce-content of 3 wt %. Operating at 350 °C, this zeolite gives 92 % selectivity to octahydroacridine at 100 % conversion of cyclohexanone. The two by-products mentioned above are absent. Conditions: a mixture of cyclohexanone, formaldehyde and ammonia (molar ratio 2 : 1 : 3) is fed from the top to a fixed bed reactor containing the catalyst, with WHSV= at temperatures of 250 - 350 °C and at atmospheric pressure. Product analysis by GC-MS and NMR. The new zeolite-catalyzed synthesis of octahydroacridine represents an eco-friendly and simple method.

3. COUPLED CARBOHYDRATE CONVERSIONS 3.1. Cascade of hydrolysis and hydrogenation An interesting example of a single-stage zeolite-catalyzed method for a two-step reaction is the direct conversion of polysaccharides of the glucan type, especially starch, towards D-glucitol (sorbitol). Below this is formulated for the amylose component of starch.

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The usual industrial process requires purification of the intermediate glucose because the enzymatic hydrolysis does not reach completion. A recently commercialized process combines hydrolysis and hydrogenation by using Ru-loaded H-USY (3 wt % Ru) as a dualfunction catalyst [58]. The outer zeolite surface provides the Brensted acidity required for the hydrolysis of the polymeric substrate. Surface roughness and crystal size are expected to be important factors. Pressure accelerates hydrolysis as was recently found in the hydrolysis of inulin over H-Beta [59]. The Ru hydrogenation component of the catalyst can exert its action at the inner as well as at the outer surface of the zeolite as the Y pore system is accessible to glucose. Typical reaction conditions are: 180 °C, batch autoclave, 5.5 MPa H2, starch concentration 30 wt %, Ru/starch wt/wt 0.002. With this formulation a reaction time of 1 h suffices to obtain essentially quantitative conversion. The selectivity to sorbitol is > 95 %. Just minor amounts of mannitol and pentitols are formed. The catalyst can be re-used. Similar excellent results are obtained by combining a 5 % Ru-on-carbon catalyst with an acidic zeolite catalyst (H-USY, H-mordenite or H-ZSM-5). Ru-H-USY preparation: zeolite NaY is exchanged with aq. NH4CI (100 molar excess) at room temperature, washed, calcined (12 h, 450 °C) and the procedure repeated twice to obtain an essentially Na-free H-USY. Ru is incorporated by ion exchange with 0.05 M aq. Ru(NH3)gCl3. The material is reduced by heating in H2 at 2 °C / min to 400 °C to obtain 3 % Ru in H-USY with a Ru dispersion of 0.73 (by CO adsorption). 3.2. Combined isomerization / hydrogenation in the preparation of D-mannitol Simultaneous action of a bio- and a chemo-catalyst is an attractive possibility for certain catalytic processes in solution. For example, a starting compound could be enzymatically equilibrated with a product that could be irreversibly transformed by the chemo-catalyst into the final product. Problems associated with such double catalytic systems are the limited range of conditions for enzyme activity, the preferential conversion of the intermediate by the chemo-catalyst, and the possible mutual poisoning of the two catalysts. Two groups have studied a glucose-to-mannitol process in which D-glucose is isomerized under hydrogenation conditions, aiming at selective hydrogenation of the fructose to a mannitol / sorbitol mixture. It may be noted that the price of mannitol is about 5 times that of sorbitol.

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For the isomerization the enzyme glucose isomerase is the catalyst of the choice. The enzyme can be purchased in several immobilized forms. As hydrogenation catalyst, Ruddlesden and Stewart applied RuNaY zeolite [60]. In this way the Ru is protected against enzyme fragments leaching from the glucose isomerase catalyst. Conditions: 60 °C, 70 bar H2, pH 8.5.

As Cu appeared to give a higher stereoselectivity than Ru, Makkee et al. applied a copper-on-silica catalyst together with glucose isomerase immobilised on silica. Up to 66% yields of mannitol were obtained. Conditions: glucose (60.0 g), the 2 catalysts, small amouts of MgHPO4, Na2B4O7 and EDTA, water (200 ml), 70 °C, 50 bar H2, pH 7.1-7.6. The Mg(II) prevents Mg-leaching from the enzyme, the borate enhances the stereoselectivity and the EDTA catches any Cu-ions thus protecting the enzyme. Also CuNaY can be applied as catalyst. Note that the system is complex because glucose and fructose exist as equilibria of 6and 5- ring hemi-acetal structures (mutarotation). Each structure has its adsorption strength to and hydrogenation rate at the metal catalyst. By contrast the enzyme accepts and produces only on mutarotation structure of glucose and fructose.

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3.3. One-pot synthesis of dilaurylisosorbide In acid conditions, sorbitol is dehydrated to 1,4-sorbitan and subsequently to the bicyclic isosorbide.

Fatty esters of sorbitan are commercially available as "Span" surfactants and have many applications. A common industrial route for the production of the Span esters is based on a two-step procedure, with acid-catalyzed sorbitol cyclization to sorbitan, followed by hightemperature alkali-catalyzed transesterification. Alternatively, the cyclization and esterification can be conducted exclusively with acid catalysts, resulting in the isosorbide diester, which can be applied as plasticizer. Conventional catalysts include p-TsOH and a sulfonic acid resin. It was found that a sulfonic acid functionalised ordered mesoporous material of the MCM-41 type is a good catalyst for the one-pot reaction of sorbitol and lauric acid [62,63]. Selectivity to dilaurylisosorbide was 95 % at 33 % lauric acid conversion. A H-Beta zeolite (Si/Al = 12.5) gave no conversion, presumably because it is monopolized by sorbitol.

Conditions: Sorbitol (3.64 g) and lauric acid (24.0 g) (molar ratio 1 : 6) were heated in the presence of the mesoporous sulfonic acid catalyst (0.36 g) for 24 h at 110 °C. Analysis by size-exclusion HPLC and NMR

4. AROMATIC SUBSTITUTION REACTIONS 4.1. Alkylation / Isomerization in the reaction of toluene with cyclohexene 3-Methylbiphenyl is an expensive intermediate in the pharmaceutical industry. The conventional route (with Suzuki coupling as final step) brings along an inorganic waste stream. A relatively inexpensive new route would include cyclohexylation of toluene, separation of isomers, dehydrogenation of the 3-isomer and recycling of the 2- and 4-isomer (see scheme) by isomerization.

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Several zeolites in the H-form, two activated clays, a silica-alumina, a sulfonic acid resin and a silica-occluded heteropoly acid were tested in the reaction of cyciohexene and toluene (excess) at 110 °C [64]. The ortho / meta / para ratio of the mixtures strongly depends on the structure of the catalysts involved. With zeolite H-USY and Filtrol-24 as active catalysts the meta I para ratio is found to be about 2 : 1, in agreement with the thermodynamic equilibrium, and the ortho-isomer is essentially absent.By contrast H-Beta and H-mordenite gave a meta I para ratio of 1:4.5. As H-USY appeared to be a good isomerization catalyst for the cyclohexyltoluenes, the mechanism may involve ortho I /wra-alkylation followed by isomerization. Researchers of UOP (Des Plaines, USA) found a separation method for meta I para cyclohexyltoluene (undisclosed technique). Altogether the results open a new low-waste route to 3-methylbiphenyl. It may be noted that a large scale p-xylene process also involves an isomer separation (Parex process [65] using an X-type zeolite) followed by isomerization and recycle. 4.2. Synthesis of cyclohexylbenzene by trapping intermediate cyciohexene in benzene hydrogenation When using a bifunctional catalyst combining a hydrogenation function (Pt,Pd,Ni) and a Bransted acid support (silica-alumina, zeolite) good selectivities to cyclohexylbenzene are obtained upon hydrogenating benzene [66]. Apparently the intermediate cyciohexene alkylates benzene (present in excess). Cyclohexylbenzene is of interest because it can be

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converted - via the hydroperoxide - into phenol and cyclohexanone. A major side product is cyclohexane other side-products are cyclohexylcyclohexane and trimeric Cis products.

Nickel supported on Y zeolite (made by ion exchange of NaY with nickel acetate) was a particularly good catalyst giving selectivity to cyclohexylbenzene of 79 % at 20 % conversion. Conditions: stirred autoclave containing 0.45 mole benzene and 7 g of catalyst, hydrogen pressure 6 bar, temperature 200 °C. 4.3. Alkylation - Cyclization Aromatic alkylation followed by alkylation of the side chain and cyclization can lead to interesting new compounds. In the following example zeolite Beta appears to be a unique catalyst. Conditions:

Autoclave charged with naphthalene (10 mmol), isopropyl alcohol 20 mmol, 0.5 g zeolite H-Beta (Si/Al = 12.5), cyclohexane (100 ml), undecane (10 mmol) as internal standard, 200 °C, 2 MPa N2. After 1 h the selectivities to 2-isopropylnaphthalene and to the cyclic compound I are 50 and 46 %, respectively, at 19 % naphthalene conversion. Mechanism presumably involves

which intermediate cation reacts with propene (ex iPrOH) to

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which cation undergoes fast cyclization

Small amounts of

and

and their regioisomers were also present in the reaction product. On the more spacious zeolite HY the isopropylation of naphthalene is faster but the main procucts are mono-, di- and tri-isopropylnaphthalene. 4.4. Selective para-halogenation of biphenyl 4,4'-dichlorobiphenyl is an interesting starting compound for high performance polymers. Conventional Lewis acids applied in the chlorination of biphenyl mainly lead to the 2,2'-and 2,4'-dichloro derivatives. Onedimensional zeolites of the L-type turned out to be highly selective in giving the linear 4,4'-dichlorobiphenyl[68]. This is a fine example of shape selectivity.

Of various counter cations tested K and Li gave the best performance. As zeolite KL is commercially available this is the zeolite of choice. Some seven solvents were tested, best results were obtained with methylene chloride. Operating in methylene chloride at 40 °C with 10 % excess chlorine over KL as catalyst 96.4 % selectivity to the 4,4'-isomer was obtained. By-products were the 2,4'-isomer (2.4 %) and the monochlorinated 2-chlorobiphenyl (1.0 %). Conversion was 100 %. Conditions: Zeolite KL (15 g, freshly heated in air at 400 °C) is added to a solution of biphenyl (77.1 g , 0.5 mol) in methylene chloride (160 ml). Under stirring at 40 °C chlorine (78.0 g, 1.1 mol) is introduced in the course of 6 h. The 4,4'dichlorobiphenyl starts to crystallize in this period. Stirring under nitrogen is continued for 15 min. Also a solvent-free procedure was applied, at 140 °C (m.p. biphenyl is 70 °C). Selectivity was distinctly lower, however, than when applying dichloromethane solvent. In a similar way p-terphenyl (1,4-diphenylbenzene) could be converted over KL to 4,4dichloro-p-terphenyl with high selectivity (97 %). Its isomers, o- and m-terphenyl cannot enter zeolite KL. The same type of zeolite was applied in selective bromination of biphenyl.

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Applying a solventless process (100 °C) and zeolite NaKL as the catalyst the desired 4,4'-dibromo compound was obtained in a selectivity of 75 % at 100 % conversion (17 % of 4-mono-Br), [69]. 4.5. Chlorination / isomerization / separation Several patents deal with isomerization (and separation) of chlorinated aromatics such as the dichlorobenzenes and the chloro- and dichlorotoluenes, over zeolites. Often integrated processes can be designed combining zeolite-catalyzed isomerization with separation over zeolites or A1PO molecular sieves and including recycle of unwanted isomers. The isomerization of the dichlorotoluenes may serve as an example. Upon direct chlorination of toluene 2,4- and 2,5-dichlorotoluene are the main products. The 2,6-, 2,3- and the 3,4-isomer are present in low amounts and the 3,5-isomer is absent in such mixtures. Isomerization covering all dichlorotoluenes is achieved over the zeolites Beta, omega and mordenite in the H-form at temperatures 300-350 °C in hydrogen. By doping the zeolite with Re, Ag or Ni a stable catalyst is obtained [70]. Thus AgH-mordenite at 350 °C remained completely stable over 150 h as measured by the amount of 2,6- dichlorotoluene formed (8.6 %).

Specific isomers can be separated from dichlorotoluene mixtures by adsorptive separation using a simulated moving bed, wherein a faujasite type zeolite is used as the adsorbent. An industrially made isomer is 2,6-dichlorotoluene which is the starting material for the herbicide 2,6-dichlorobenzonitrile (Casoron). When using an MFI-type zeolite, the attainable equilibrium is limited to 2,4-,2,5- and 3,4-dichlorotoluene. The slightly larger 2,3-, 2.6- and 3,5-isomers are assumed to take part in the equilibrium at the crossings in the zeolite but are too bulky to diffuse through the channels and leave the zeolite crystal. This represents a fine case of product selectivity. 5. TWO-STEP REACTIONS WITH EPOXIDATIONS AS THE FIRST STEP The discovery of TS-1 (Ti-MFI) by Enichem workers opened a wide spectrum of catalytic oxidations using hydrogen peroxide or t-butyl hydroperoxide as the oxidant. Recently a

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review on microporous and mesoporous titanium silicate molecular sieves has been compiled by Ratnasamy et al. [71]. The review covers synthesis, reactions, mechanisms and spectroscopy and is most informative (343 references). Besides being a redox center Ti can also act as Lewis acid site. This enables us to combine epoxidation with a consecutive reaction in which the epoxy-oxygen is cordinated to Ti, thus activating the system. Also mild Bransted acidity may be generated, e.g. by Ticoordinated water. Some examples follow. 5.1. Dihydroxylation of unsaturated alcohols and halides over TS-1 Titanium silicate molecular sieve, TS-1, is an efficient and selective catalyst for the dihydroxylation of olefins under triphase reaction conditions in the presence of aqueous hydrogen peroxide. Unsaturated alcohols (> C4 ) give the corresponding triol in a high yield with high H2O2 efficiency [72]. With allyl alcohol (miscible with water) the epoxide (96 %) is obtained and the consecutive reaction does not occur. Unsaturated halides give the corresponding diols even at a faster reaction rate. (In the presence of a cosolvent (acetone) to homogenize the liquid layers) epoxide is obtained as the major final product). Acidity generated in situ over TS-1 in the presence of water (used as the dispersion medium) is responsible for the bifunctional (oxidation and acidic) behavior of the titanium silicate. Protonation of the epoxide activates the system for attack by water.

Conversion 95 %, Triol selectivity 95.5 %, H2O2-selectivity 95.5 %. Reaction conditions: 0.02 mole substrate, 0.02 mole H2O2 (30 wt % aqueous), catalyst (TS-1, Si/Ti = 27) 20 wt % with respect to the substrate, H2O 10 ml, temperature 333 K. Unsaturated halides like allyl chloride and crotyl chloride gave also high yields of diols with the TS-1 / aqueous H2O2 system. Thus crotyl alcohol gave the diol with 96 % selectivity at 99.5 % conversion in 4 h. Ti-containing MCM-41 material has been tested in the reaction of cyclohexene and hydrogen peroxide [73]. Because methanol is applied as the solvent also ether products are observed resulting from reaction of the epoxide with methanol, moreover cyclohexanone is found resulting from epoxide isomerization (cf. section 5.5). 5.2. Oxidation of furan and methylfurans to dicarbonyl systems Upon oxidation of furan, 2-methylfuran and 2,5-dimethylfuran with hydrogen peroxide over TS-1 (Ti-MFI) in acetonitrile at 25 °C essentially complete conversion was attained in 3 h. The main products are dicarbonyl alkenes as a cis/trans mixture with the cisisomer dominating [74]. Thus 2,5-dimethylfuran gives cis- and trans- 3-hexene-2,5-dione with a selectivity of 85 % at 94 % conversion (cis/trans ratio 68/32). Mechanistically, it is proposed that the oxidation proceeds via epoxidation of one double bond of the furan ring. An unstable epoxide is formed, which rearranges to a cis dicarbonyl

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system. The epoxide intermediate itself was not observed. Isomerisation yields the corresponding trans-isomer. Furan and 2-methylfuran yield 2-butene-l,4-dial and 4-oxo-2-pentenal, respectively, with cis/trans ratios 95/5 and 97/3. Due to the presence of water the aldehydes enter equilibria towards hydrates and were not isolated in a pure state. Conditions: Furan or the (di)methylfuran (10 mmol) with H2O2 (15 mmol as 35 wt % aqueous solution) and 0.1 g of TS-1 in acetonitrile (10 ml) are stirred for 3 h at 20 °C. Analysis was carried out by GC. 5.3. Oxidation of furfuryl alcohols to 6-hydroxypyranones In an analogous way furfuryl alcohol and 1 -(2-furyl) ethanol are converted to 6hydroxy-2H-pyran-3 (6H)-one and its 2-methyl derivative, respectively, upon treatment with hydrogen peroxide in acetonitrile in the presence of TS-1. Here dicarbonyl formation is followed by cyclization towards a stable hemiacetal [74].

Conditions: substrate (10 mmol), TS-1 (0.1 g) H2O2 (35 wt % in water, 12.5 mmol), in CH3CN (10 ml) were stirred at 40 °C. For furfuryl alcohol after 3.5 h the conversion was 99 % and the selectivity to 6-hydroxypyranone 93 %. The methyl derivative required 6.5 h for a conversion of 93 % and a selectivity of 80 %. Upon calcination of the used TS-1 at 550 °C the catalytic activity is fully restored. 5.4. Epoxidation and ring closure Interesting cyclic ether formations by 2-step Ti-catalyzed epoxidation/ring closure reactions have been reported by Corma et al. and by Tatsumi et al. Thus linalool was converted by treatment with t-butyl hydroperoxide over Ti-Beta and Ti-MCM-41 via the mono 6,7-epoxide to a mixture of five- and sixmembered cyclic ethers [75]. In the more limited space of TS-1 (Ti-MFI) essentially complete regioselectivity to the fivemembered ring ethers was achieved in the 2-step reaction of 4-pentene-l-ol and 4-hexenl-ol with hydrogen peroxide [76].These reactions represent fine examples of a zeolite acting as bifunctional regioselective catalyst. Conditions: 4-pentene-l-ol, room temperature, TS-1, acetone, 2-butanol or water as the solvent, 30 % aq. H2O2. 4-Hexen-l- ol, acetone 60 °C.

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5.5. Epoxidation - Isomerization Epoxides can be isomerized to carbonyl compounds. Industrial examples include the isomerization of styrene oxide to phenylacetaldehyde and that of a-pinene oxide to campholenic aldehyde. As Ti-containing zeolites as TS-1 [77] and the Ti-Beta [78] are good catalysts for these isomerizations it is tempting to combine the epoxidation and the isomerization step. Several substituted styrenes have been subjected [79[ to such a two-step one-pot procedure.

X = H, CH3, CH2OH Methanol, acetone and t-butyl alcohol were applied as solvents. Selectivities to aldehyde were up to 90 % at styrene conversion 95 %. Conditions: a mixture of substrate (20 ml), H2O2 (75 ml, 36 wt %) and TS-1 (2.5 g) in 40 ml of solvent was stirred at 80 °C. One-pot synthesis of campholenic aldehyde from a-pinene has been achieved using tbutyl hydroperoxide and a Ti-containing mesoporous material of the HMS-type. [80]. Selectivity to campholenic aldehyde was 82.4 % under the following. Conditions: a-pinene (5 mmol), t-butyl hydroperoxide (5 mmol, dried over MgSO4), Ti-HMS (0.1 g) in acetonitrile (30 ml) stirred 24 h at 75 °C.

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6. TWO- AND THREE-STEP CYCLIZATION REACTIONS TO NONAROMATIC RINGS 6.1. One-pot transformation of citronellal to menthol H-Beta zeolite, loaded with 3 wt % Ir.catalyzes the one-pot full conversion of citronellal to menthol by consecutive acid catalysed cyclization and Ir-catalyzed hydrogenation, with 95 % selectivity for the menthol isomers of which 75 % is the desired (-)-menthol [81]. Conditions: The catalytic tests were performed at 80 °C in a stirred stainless steel autoclave using H2 pressures in the range 0.5-1.5 Mpa, 50 mg catalyst, 1 ml citronellal and 7 ml solvent (cyclohexane or 2 propanol).

Cyclohexane was the preferred solvent. In 2-propanol the menthol yield was just 47 % due to more pronounced formation of citronellol. Alcohol solvents favour the hydrogenation of the carbonyl bond. The Ir was introduced by impregnation of H-Beta with a solution of Ir (acac)3 in toluene followed by calcination (300 °C) and reduction (H2, 450 °C). The 3 wt % Ir appears to be an optimum. When other metals are used instead of Ir in similar conditions, the results were less successful. 3 % Ru/H-Beta forms citronellol in a 70 % yield, with 3 % Pd/Hbeta, 3,7-dimethyloctanal is the dominant product (69 %). Considerable menthol yields are obtained with 3 % Pt/H-Beta (56 % yield) and 3 % Rh/H-Beta (55 % yield) but 3 % Ir/H-Beta is the catalyst of choice. Earlier reported bifunctional catalysts (Ru-ZnBr2-silica [82] and Cu-silica [83]) require much more catalyst. The 3 % Ir/H-Beta catalyst can be recycled by simple cyclohexane washing and retains its full activity. 6.2. Synthesis of coumarin derivatives By reaction of phenols with bifunctional molecules, which are able to alkylate and to esterify, several coumarin derivatives have been prepared using H-Beta as the catalyst. Thus resorcinol and propynoic acid react at 150 °C in p-chlorotoluene as the solvent towards the perfumery ingredient umbelliferone (7-hydroxycoumarin) in good yield, 60 %, [84]. Amberlyst-15 gave 40 % yield. H-Beta catalyzes the esterification as well as the ring closure alkylation step.

With acrylic acid and methacrylic acid dihydrocoumarins are obtained in yields of 70 and 81 % respectively, H-Beta is again the catalyst, toluene is used as a solvent at reflux, so

330

conditions are milder than in the case of propynoic acid. Supposedly the system first enters esterification equilibrium. Another route to coumarins is the Pechmann condensation of phenols with pketoesters. The reaction proceeds via transesterification and intramolecular hydroxyalkylation, followed by dehydration, and is conventionally conducted using sulfuric acid as catalyst. Resorcinol and ethyl acetoacetate reacted solvent-free over H-beta at 150 °C to give 7-hydroxy-4-methylcoumarin (78 % isolated yield) [85]. This is another perfumery ingredient which is also an intermediate for the insecticide Hymecromone.

Similar results have been obtained with Amberlyst-15 as the catalyst. Recently it was shown that Nafion - on - silica composite catalysts gave even beter results (up to 96 % selectivity) in the Pechmann reaction [86]. 6.3. Rearrangement of allyl benzyl ethers In some cases, the steric constraints of the zeolite pores can enable or impede secondary reactions. A good example is found in the rearrangement of benzyl allyl ethers over H-Beta zeolites. In the normal course of this reaction, catalyzed by H-Beta or boron trifluoride etherate, the benzyl allyl ether is transformed to a 4- arylbutanal, probably via a five- membered ring intermediate; e.g. methallyl 2,5-dimethoxybenzyl ether:

331

However, with the slimmer 3,4-dimethoxybenzyl ether or the 3,4-dioxymethylene compound consecutive cyclization and dehydration occurs, to give a dihydronaphthalene [87].

The reaction is conducted at room temperature 6.4. Synthesis of 2,2,6,6-tetramethyl-4-oxopiperidine (triacetonamine) The conventional way of preparing the TEMPO-precursor 2,2,6,6-tetramethyl-4-oxopiperidine (triacetonamine) involves reacting acetone with ammonia in the presence of a substantial amount calcium chloride for nine days, thereby obtaining a yield of about 20 %. A recent patent application describes the use of zeolite CaY as the catalyst, [88] which has the advantages of a low-salt method and a re-usable catalyst.

A 3:1 molar ratio of acetone and ammonia is applied together with 3 wt % CaY. Yield of acetonamine is 22 % with 99 % selectivity. TEMPO is obtained from acetonamine by reducing the CO group to CH2 and by oxidizing the NH function to an N-O» group. TEMPO is a fine homogeneous catalyst for oxidation of primary alcohols to aldehydes or carboxylic acids. It can be turned to a heterogeneous catalyst by attaching TEMPO to MCM-41 material [89].

6.5. Preparation of 1-substituted tetrahydroisoquinolines via the Pictet-Spengler reaction using a natural zeolite catalyst One of the most powerful methods for the construction of tetrahydroisoquinoline systems is the Pictet-Spengler cyclisation. The reaction consists of the condensation of a b-phenylethylamine derivative with a carbonyl compound, generating an imine (Schiff's base), which undergoes cyclisation via an intramolecular electrophilic aromatic substitution yielding the isoquinoline derivative. The Pictet-Spengler reaction is traditionally carried out in a protic solvent with acid catalysts, usually acetic acid or trifluoroacetic acid. A one-step synthesis was achieved using the natural zeolite E4 (clinoptilolite type) modified towards a more acidic form E4a. Reactants studied include b-phenylethylamine and its 3,4dimethoxy-derivative and substituted acetophenones as well as a series of benzaldehydes:[90]

332

In the case of acetophenone reactants 1-aryl-l-methyl-tetrahydroisoquinolines are obtained. Conditions: a mixture of 5 mmol of the b-phenylethylamine derivative, 5 mmol of the aldehyde/ketone and 0.8 g E4a in ethanol was heated at 80 °C for 20/40 h. The solid was filtered off, the filtrate was evaporated and the residue characterised. Yields, according to NMR, are from 75-100 %. The catalyst could be easily recycled without significant loss of activity. In view of the relatively narrow 8- and 10- ring channels of clinoptilolite (code name HEU) it is assumed that the reaction takes place at the outer surface of the zeolite crystals. In view of the relatively narrow 8- and 10- ring channels of clinoptilolite (code name HEU) it is assumed that the reaction takes place at the outer surface of the zeolite crystals. 6.6. One-pot synthesis of pyrano- and furoquinolines over zeolite HY Pyranoquinoline derivatives are known to possess important biological properties such as antiallergic, antiinflammatory and estrogenic activities. Two solid acids, Fe3+-K-10 montmorillonite clay and HY-zeolite have been found [91] to catalyze efficiently the singlestep synthesis of pyrano- or furoquinolines in high yields by coupling of three components: anilines, benzaldehydes and 3,4-dihydro-2H-pyran or 2,3-dihydrofuran. Isolated yields are 71-92 %. Conditions are mild; room temperature in CH3CN for the clay and reflux in CH2CI2 for HY.

Aromatic substituents are H,CH3,C1 and OCH 3 . Solvent is MeCN or CH2C12.

333

Possibly the sequence of reaction steps first involves reaction between the aniline and the benzaldehyde to form an imine, which adds- as carbenium ion- to the dihydrofuran or- pyran to form an oxocarbenium ion that alkylates the aromatic ring. In view of the size of the product molecules, the reaction is assumed to occur at the outer surface of the zeolite. In the product the aromatic ring is positioned trans or cis with respect to the oxygen-containing ring. The trans: cis ratio varies from 74 : 26 to 92 : 8 6.7. Synthesis of tetra-arylporphyrins A recent patent [92] claims the synthesis of tetra arayl porphyrins by subjecting an equimolar mixture of pyrrole and (a substituted) benzaldehyde to microwave radiation in the presence of MCM-41 material No solvent is applied. Averaged yield of tetraphenylporphyrin was 23.5 %. Here 4 molecules of each reactant form a 16-membered ring in MCM-41. For the synthesis of metallo-porphyrins in zeolite Y, see the chapter of P. A. Jacobs in this book. 7. CONCLUSIONS It may be concluded that zeolite and ordered mesoporous materials offer many opportunities to operate in two- and multi-step organic cascade conversions. Some fine examples exist already. The modification possibilities and tenability of the microporous and mesoporous systems are impressive and allow design of new bi- and multifunctional catalysts. Moreover bi- and multizeolite systems can be envisaged in which catalysis and separation (membranes) operate in concert. Such new catalysts and methods will certainly contribute to eco-friendlty low-waste organic conversions.

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Studies in Surface Science and Catalysis 157 J. Cejka and H. van Bekkum (Editors) © 2005 Elsevier B.V. All rights reserved.

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Zeolites in refining and petrochemistry Avelino Corma and Agustin Martinez Institute de Tecnologia Quimica, UPV-CSIC. Avenida de los Naranjos s/n, 46022 Valencia, Spain 1. INTRODUCTION AND SCOPE 2. ZEOLITES IN REFINING 2.1. Production of aromatics 2.1.1. Non-oxidative conversion of methane to aromatics 2.1.2. Aromatization of short (C2-C4) alkanes 2.1.3. Catalytic reforming of naphtha 2.2. Skeletal isomerization of linear butenes 2.3. Isobutane/butene alkylation 2.4. Isomerization of light straight run (LSR) naphtha 2.5. Catalytic dewaxing and isodewaxing 2.6. Fluid catalytic cracking (FCC) 2.7. Catalytic hydrocracking 3. ZEOLITES IN PETROCHEMISTRY 3.1. Production of para-xylene 3.2. Alkylation of aromatics 4. CONCLUSIONS AND FINAL REMARKS REFERENCES

1. INTRODUCTION AND SCOPE Since their introduction in the formulation of catalytic cracking catalysts about four decades ago, zeolites, and more generally molecular sieves, have been playing an increasingly role in the development of cleaner and more efficient refinery processes for the production of transportation fuels and petrochemicals. The successful application of zeolites as catalysts arises from the possibility of controlling the pore diameter and topology, as well as the nature and concentration of active sites and adsorption properties, allowing in many cases a priori design of catalysts with enhanced activity and/or selectivity for particular applications. The number of zeolites used has been continuously expanding. More than eight types are used today in oil refining processes, and the number will be larger if other catalytic applications in the field of chemicals and fine chemicals are considered. There is a limitation for applying new potentially interesting structures, which derives from the manufacturing cost. Indeed, for application in many multi-tone processes, the cost of the template and/or too long synthesis time can be limiting factors for commercial application.

338 Here, we will present the important catalytic processes in the field of oil refining and petrochemistry that use zeolites as catalysts. Catalyst activity and product distribution will be discussed on the bases of the most important zeolite variables: pore dimensions and topology, acid site density and strength, and crystallite size. 2. ZEOLITES IN REFINING 2.1. Production of aromatics 2.1.1. Non-oxidative conversion of methane to aromatics Direct catalytic conversion of methane, the principal component of natural gas, to more valued chemicals and fuels still remains as one of the most important challenges in the field of heterogeneous catalysis [1]. In this respect, Wang and co-workers reported for the first time in 1993 the dehydro-aromatization of methane (MDA) in the absence of oxygen at 700°C and atmospheric pressure using a Mo/HZSM-5 catalyst [2]. The MDA reaction is thermodynamically limited, with an equilibrium conversion of methane of about 12 % and 24 % at 700°C and 800°C, respectively, and 1 atm. Besides the production of aromatics, the interest of the MDA reaction also resides in the co-production of COx-free hydrogen which may be useful for fuel cell applications. Since the initial work of Wang and co-workers, many efforts have been directed to the optimization of the Mo/HZSM-5 catalyst and reaction conditions, to understand the interactions between the transition metal and the zeolite host, and to determine the nature of the active sites and possible reaction mechanisms, as well as the role of the carbonaceous deposits formed during the reaction [3,4]. It was shown [5] that the channel structure and acidity of the HZSM-5 zeolite and the state and location of Mo species play a vital role in the performance of Mo/HZSM-5 catalysts for the MDA reaction. The groups of Solymosi [6] and Lunsford [7] observed that the formation of hydrocarbons was preceded by an induction period, during which methane was mainly converted into coke and gaseous products (CO, H2O, H2 and CO) with very little hydrocarbon formation. During this period, a partial reduction of Mo6+ by methane occurred with formation of Mo2C/MoOxCy species highly dispersed in the zeolite channels. Following the initial activation period, a benzene selectivity of about 70 % at a methane conversion of 8-10 % could be sustained for more than 16 h over a 2 wt% Mo/HZSM-5 catalyst at 700 °C [7]. It was then proposed that the methane aromatization takes place through a bifunctional mechanism, where the initial activation of methane occurs on the molybdenum carbide/oxycarbide species leading to the formation of ethylene as the primary product, which then oligomerizes and cyclizes on the acid sites of the HZSM-5 zeolite to form the aromatics [7,8]. Later on Meriaudeau et al. [9] observed the formation of acetylene as a primary product during the MDA reaction on Mo/HZSM-5, and suggested that the aromatization of methane occurs via acetylene as intermediate rather than via ethylene. Deactivation of the Mo/HZM-5 catalyst by carbonaceous deposits is a serious drawback for the possible commercialization of the MDA process, and therefore, great efforts have been devoted to improving the stability of the catalyst during the reaction by reducing the formation of coke. This has been achieved through the addition of small amounts of CO, CO2, O2, or NO into the reactant stream [10,11,12,13,14] and by reducing the density of Bronsted acid sites of the HZSM-5 zeolite by steam dealumination followed by acid washing before incorporation of Mo

339 species [15,16,17]. Inactivation of the external acid sites by silanization of the external zeolite surface [18,19,20] or by selective dealumination of the external Bronsted acid sites by treatment with oxalic acid [21 ] resulted in an improved activity, benzene selectivity and stability of the Mo/HZSM-5 catalyst. Improvements in the MDA performance of Mo/HZSM-5 catalysts have also been achieved by the addition of promoters, such as Ru [22] and Co or Fe [23]. The partial exchange of H+ in HZSM-5 with Cu2+ ions markedly increased the methane conversion and benzene yield (from 15 to 70 %) by suppressing the dealumination of the zeolite framework upon incorporation of Mo and decreasing the formation of coke on the catalyst [24]. The porous structure of the zeolitic support plays an important role in the reaction [25]. A higher benzene selectivity and stability as compared with Mo/HZSM-5 has been reported for a Mo/MCM-22 catalyst [26]. The good performance of Mo/MCM-22 catalyst was related to the unique pore structure of MCM-22 zeolite. Thus, the enhanced benzene selectivity was ascribed to the smaller size of the sinusoidal 10-MR channel system in MCM-22, while the presence of the large supercages contributed to its higher coke tolerance by acting as a trap for coke molecules. 2.1.2. Aromatization of short (C2-C4) alkanes Zeolites are being used in the formulation of commercial catalysts for the conversion of short C2C4 alkanes (LPG) into aromatics, mainly benzene, toluene and xylenes (BTX), as in the Cyclar process developed by BP/UOP which employs a bifunctional Ga/HZSM-5 catalyst [27]. An important aspect of the Cyclar process is the co-production of H2, a high-value refinery byproduct. Both the density of acid sites in the HZSM-5 zeolite, given by the Si/Al ratio, and the metal content has an on the aromatization performance. Typically, optimum performance is found for catalysts containing 1-5 wt % Ga and Si/Al ratios in the range of 15-30. Yields of about 65 % and 5 % to aromatics and hydrogen, respectively, are typically obtained in the Cyclar process. In order to achieve high yields of aromatics, it is essential that both Ga and H + sites are in close proximity to each other. The effect of the distance between Ga and H + sites in the aromatization of propane can be observed in Table 1 by comparing the results obtained on a single catalyst bed containing the Ga/HZSM-5 sample and on a two bed system comprising a first bed of Ga/y-AkOs and a second bed of zeolite. Table 1 Influence of the proximity between Ga and H+ sites on propane aromatization

Conversion, % Selectivity, % C,-C 3

HZSM-5 29

52 8 BTX a 39 C9+ 1 a BTX= benzene, toluene, and xylenes C4-C6

Two beds Ga/y-Al2O3 + HZSM-5 23

Ga/HZSM-5 20

51 15 33 1

25 3 65 7

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It is seen in Table 1 that although aromatization can also proceed on mono-functional HZSM-5 catalysts [28], much higher yields of cracking products are produced in detriment of aromatics as compared to the bi-functional Ga/HZSM-5 catalyst. Experimentally, it has been observed that the rate of propane dehydrogenation is faster when H + and Ga sites are present in Ga/HZSM-5 catalysts, suggesting that both sites are probably involved in the initial activation step [29,30]. It has also been proposed that propane activation occurs through a synergetic effect at the interface between the Ga2C>3 and the zeolite [31]. Gallium can be introduced by impregnation, ion exchange, or directly incorporated into the zeolite framework during the synthesis [32,33]. By either of these methods, a similar catalyst performance is obtained after calcination, which suggests that the active Ga sites are extraframework rather than framework Ga species. However, the exact nature of the active Ga species still remains unclear, and Ga oxides, probably in the form of Ga2C>3 and/or GaO(OH) [34,35], highly dispersed GaHx hydride species generated upon reduction of Ga3+ by H2 pretreatment or by the propane reactant [36], or even Ga+ in exchange positions [37,38,39] have been proposed. There is, nevertheless, a general consensus on the inactivity of framework Ga species for the alkane aromatization reaction [37,40]. The alkane aromatization performance of the bi-functional Ga/HZSM-5 catalysts depends to a great extent on the balance between the H + and Ga sites, which is a function of: the Ga content, Si/Al ratio of the zeolite, the method of catalyst preparation, and the pre-treatment conditions [41,42,43,44,45,46,47,48,49,50]. It has been shown that the performance of Ga/HZSM-5 can be improved by subjecting the catalyst to a series of red-ox cycles or by stream treatment under controlled conditions [51]. Catalysts with a high concentration of H + sites were shown to display a higher initial activity and aromatics selectivity, though they deactivate faster with time on stream [52]. The reaction temperature is expected to significantly influence the thermodynamics of the different reactions occurring during propane aromatization (dehydrogenation, cracking, hydrogenolysis, aromatization, etc.), and therefore, it has a strong effect on product selectivity [53,54]. Higher reaction temperatures decreased the relative aromatization activity of a H-GaAl/ZSM-5 sample (prepared by hydrothermal synthesis) as compared to the cracking and dehydrogenation activities, and increased the concentration of benzene in the BTX fraction, in detriment of xylenes [55,54]. Besides Ga other metals such as Zn [56,57,58], Pt [56,59,60], Mo [61,62], Cr [61], and In [63] have been used in combination with HZSM-5 zeolite for the aromatization of light alkanes. Generally, higher yields to aromatics starting from propane are reported for Ga and Zn, while Pt also catalyzes other undesired side reactions, such as hydrogenolysis, hydrogenation, and dealkylation leading to methane and ethane, and promote the formation of coke resulting in a faster catalyst deactivation [64]. Ga is usually preferred over Zn due to the high volatility of ZnO, which may be partially lost from the catalyst at the high temperatures applied in the aromatization reaction [65]. Pt/HZSM-5 catalysts are, however, more appropriate for the aromatization of ethane, since Pt provides a stronger dehydrogenation function required for the activation of this refractory alkane [35]. Bimetallic Pt-Ga [64,66], Pt-Ge [67], and Pt-Re [68] HZSM-5 catalysts have also been studied for propane aromatization. Inui et al. [66] reported a selectivity to aromatics of 60.7 % at 92.3 % propane conversion at 600 °C and atmospheric pressure over a Pt-GaMFI sample prepared by ion exchange of the gallosilicate with of

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Cl2. However, the aromatics distribution in the BTX fraction was shifted towards benzene (48.8 %) and toluene (35.7 %) as compared with Ga/HZSM-5. 2.1.3. Catalytic reforming of naphtha The catalytic reforming process is, together with catalytic cracking, one of the most important processes in modern refinery schemes. It is used to convert low octane n-alkanes and cycloalkanes with 5 to 10 carbon atoms contained in the petroleum naphtha into high-octane isoalkanes and aromatics gasoline components and hydrogen. Typically, reformer reactors operate at temperatures of 425-525 °C and hydrogen pressures of 0.5-3.0 MPa. Naphtha reforming catalysts are mostly based on metals (Pt, Pt-Re, Pt-Ir, Pt-Sn, PtRe-Sn) supported on chlorinated-A^C^ or on a KL zeolite. Non-acidic KL zeolite in combination with Pt has been applied in a new reforming process. The non-acidic zeolite support inhibits undesired isomerization and hydrocracking reactions leading to enhanced aromatization selectivities [69]. Besides the absence of acidity, the presence of highly dispersed Pt clusters inside the zeolite channels and the shape-selective effects imposed by the monodirectional channel structure (0.71 nm diameter) of the zeolite may also contribute to the excellent aromatization performance of Pt/KL catalysts. One of the main drawbacks of Pt/KL catalysts is their high sensitivity to sulfur poisoning, resulting in a rapid catalyst deactivation unless sulfur trapping beds are introduced. The method of Pt incorporation has a strong influence on the distribution of Pt clusters size and location, and therefore, on the catalyst activity and stability. In this respect, it was shown [70] that Pt/KL samples prepared by incipient wetness impregnation (IWI) contained, after reduction, small Pt particles located inside the zeolite channels. Meanwhile, those prepared by ion exchange (IE) contained larger particles, some of which were located on the external zeolite surface. The catalysts obtained by the IWI method were more selective for dehydrocyclization and less selective for hydrogenolysis, and they deactivated at a lower rate than those prepared by IE. More recently, Pt/KL samples prepared by vapor phase impregnation (VPI) showed a lower tendency to agglomerate under both clean and sulfur-poisoned conditions leading to more active and stable catalysts than those prepared by IE or IWI [71]. Introduction of Pt into the zeolite channels by chemical vapor deposition of platinum hexafluoroacetyl acetonate in a flow of Ar at 70 °C, followed by thermal removal of the organic ligands under H2 at 350 °C also produced very small Pt clusters with a nucleophilic character [72]. The Pt/KL thus prepared showed high activity and selectivity in the conversion of methylcyclopentane to benzene at 500 °C, with a low deactivation rate owing to a reduced coke formation and low sintering rate. Modification of Pt/KL with heavy rare earth elements, and particularly with Tm, has been shown to improve the aromatization activity and the Stolerance of the zeolite catalyst [73,74]. Besides Pt/KL, other zeolite-based reforming catalysts have been studied and patented. Pt supported on dealuminated ZSM-12 and Beta zeolites have been studied for reforming a naphthene-rich feedstock [75]. The one-directional large-pore ZSM-12 catalyst displayed a higher stability with time-on-stream owing to its pore structure, which does not favor coke formation. Good results have been reported by researchers at Chevron for a sulfided Pt/Cs-Beta catalyst [76]. Reforming catalysts prepared from large-pore borosilicates (e.g., B-Beta, B-SSZ-33, B-SSZ-24, B-SSZ-31) in which the B is partially replaced by a Group IIIA metal or a first raw transition metal (e.g., Co, Zn, Al, Ga, Fe, Ni, Sn, or Ti) have also been patented by Chevron [77].

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2.2. Skeletal isomerization of linear butenes Skeletal isomerization of the much more abundant n-butenes has been recognized as an attractive alternative for increasing the availability of the more demanded isobutene [78], which is widely used in the petrochemical industry in the fabrication of various monomers, polymers, and chemicals, and MTBE (methyl-/-butyl ether) employed as an oxygenate additive for gasoline. The n-butenes skeletal isomerization is an acid-catalyzed reaction that proceeds via carbenium ion intermediates formed upon protonation of the double bond [79]. The reaction is thermodynamically favored at low temperatures. Although solid acids such as phosphoric acid and halogenated aluminas were initially applied [80], the interest has been directed towards the utilization of more environmentally friendly zeolites. Medium pore zeolites have shown better isomerization performance than large pore zeolites because the competing oligomerization reaction is hindered in the narrower channels of the former [81]. Moreover, medium pore (10-MR) monodirectional zeolites and zeotypes, such as ZSM-22 (and the isostructural TON), ZSM-23, and AEL perform better than ZSM-5 possessing intersecting channels. The effect of the zeolite pore topology on the isobutene selectivity can be observed in Fig. 1 for a n-butene conversion of 41-47 % [82].

Fig. 1. Influence of zeolite pore topology on the isobutene selectivity obtained at 350°C and 1 atm (5% of 1-butene in N2) and at a conversion of 41-47% (reproduced from Ref. [82])

In general, the n-butenes skeletal isomerization is carried out at moderate temperatures (350-450 °C) and with diluted butene feeds in order to inhibit the undesired oligomerization reactions. However, a high isobutene yield (ca. 40-45 %) and excellent stability with time on stream has been reported for ferriedte zeolite at relatively low temperatures (350 °C) and without any feed dilution [83,84]. This behavior has been attributed to the particular topology of the ferrierite zeolite having intersecting 10-MR and 8-MR, which according to the authors promoted the selective formation of isobutene through a bimolecular dimerization-cracking mechanism involving the formation of trimethylpentene intermediates at the channel intersections, and their consecutive cracking into two butene molecules (including isobutene)

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as the highly branched octenes can not diffuse out of the zeolite through the 10- and 8-MR pores [84]. The question about the mechanism of formation of isobutene in ferrierite, and in general in zeolites, has raised a very interesting open debate in the literature between different research groups, as reviewed by Houzvicka and Ponec [85]. In fact, the bimolecular dimerization-cracking mechanism was initially proposed to overcome the formation of the energetically unfavorable primary carbenium ion that would be required in the monomolecular isomerization mechanism occurring via a protonated cyclopropane intermediate. Nevertheless, there are several experimental observations that are in favor of the monomolecular pathway as the prevailing mechanism for the skeletal isomerization of nbutenes. By using 13C-labeled n-butene as reactant, Meriaudeau et al. [86] concluded that isobutene is mainly formed through a monomolecular process on a selective ferrierite sample (aged catalyst) while the bimolecular mechanism is operative on a fresh non-selective ferrierite. Similar conclusions were obtained by Cejka et al. [87] by reacting l3C-labeled 1butene over CoAlPO-11 and ferrierite catalysts. These authors concluded that high selectivity to isobutene is only possible through a monomolecular mechanism, although the dimerization-cracking pathway can also contribute to the non-selective formation of isobutene together with by-products. A number of kinetic data obtained by different authors also provide indirect evidence that the prevailing mechanim of isobutene formation is monomolecular, and that the bimolecular path mainly leads to the formation of undesired byproducts (mostly C3 and C5 hydrocarbons) and thus has to be suppressed as much as possible. For instance, it has been shown that the isobutene selectivity increases when increasing the reaction temperature and when decreasing the n-butene partial pressure in the feed stream, while the selectivity to C3+C5 by-products (which are necessarily formed by a bimolecular process) follows the opposite trend [88,89]. The effect of n-butene partial pressure and temperature on isobutene selectivity can be clearly seen in Table 2 for a MCM-22 (Si/Al= 15) zeolite [88]. Table 2 Influence of 1-butene partial pressure and reaction temperature on isobutene selectivity at ca. 50% conversion on an MCM-22 zeolite (Si/Al= 15). Reaction conditions: WHSV= 28 h'1, 30 min TOS Temperature (°C) 350 350 350 400 500

1-Butene partial pressure (atm) 0.1 0.5 1.0 0.1 0.1

Isobutene selectivity (mol%) 41.0 31.7 26.6 49.7 59.4

In addition, it was found that 10-MR zeolites and zeotypes having non-intersecting channels (e.g. ZSM-22, ZSM-23, SAPO-11 and MeAPO-11) do not allow the formation of bulky trimethylpentenes inside the channels despite they display high selectivity to isobutene (Fig. 1) suggesting some kind of shape selectivity. Increasing the Si/Al ratio of the zeolite, that is, decreasing the density of Bronsted acid sites, has a positive effect on isobutene selectivity [90,91,92]. This effect is illustrated in Fig. 2 for a ZSM-22 zeolite [91]. Again, the

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fact that a low density of acid sites favors the selective formation of isobutene in detriment of C3+C5 hydrocarbons suggests that both isobutene and by-products are formed by different mechanisms, i.e., monomolecular in the case of isobutene and bimolecular in the case of byproducts.

Fig. 2. Influence of Si/AI ratio on isobutene selectivity at ca. 50% n-butenes conversion over AlZSM-22 catalysts.

The mechanism of skeletal isomerization of n-butenes has also been investigated by means of theoretical calculations. By using ab initio methods, Boronat et al. [93,94] found that the primary cation is the transition state for the branching isomerization of n-butyl into tbutyl cation, whereas the protonated cyclopropane ring is the transition state for the scrambling reaction. Later on, these authors performed theoretical studies based on density functional theory where the acid site in the zeolite was simulated by a cluster consisting of two silicon and one aluminum tetrahedral [95]. They found that the unimolecular isomerization of n-butene to isobutene occurs through a transition state having a cyclic structure in which the transferring methyl group is halfway between its position in the linear and in the branched alkoxide species, and in which the positive charge is stabilized by the two oxygen atoms of the zeolite. The activation energy obtained for this process was ca. 30 kcal/mol, indicating that the unimolecular isomerization can take place in zeolites without involving the formation of a free primary carbenium ion [95], The particular behavior of ferrierite during the skeletal isomerization of n-butenes (increase of isobutene yield with time-on-stream coinciding with the formation of relatively large amounts of coke) led Guisnet and co-workers to propose an additional isomerization mechanism (pseudo-monomolecular) involving as active sites carbenium ions resulting from the adsorption on the H+ sites of carbonaceous deposits (coke) located near the pore mouths [96,97]. Later, these authors attributed the high isobutene selectivity characteristic of aged ferrierite to an autocatalytic pseudo-monomolecular pathway in where n-butene isomerization

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occurs on t-butyl cations (the active species) originated from isobutene molecules adsorbed on the protonic sites [98]. The pseudo-monomolecular mechanism has been questioned by different groups, and it has been further discussed in the reviews by Houzvicka and Ponec [85] and by van Donk et al. [99]. Very recently, a new proposal has been advanced by the group of Guisnet in order to explain the effect of aging of ferrierite and the influence of Si/Al ratio and n-butene partial pressure on isobutene selectivity [100]. According to them, the observed increase in isobutene yield/selectivity and the intrinsic activity (turnover frequency) of the protonic sites with TOS could be explained by a simple model that considers the 10MR channels of ferrierite as non-interconnected nano-reactors in where the reactant molecules have to diffuse without any possibility of desorption before their exit. Thus, channels (i.e. nano-reactors) containing a large amount of protonic sites will lead to a thermodynamic equilibrium mixture of propene-pentenes-butenes and to deactivating carbonaceous deposits through consecutive bimolecular processes, while in nano-reactors containing a few protonic sites (as in high Si/Al ratio sample or in aged catalyst) isobutene is selectively formed by the autocatalytic mechanism. Besides the effect of acid site density (that is, of Si/Al ratio) discussed before, the acid strength of the protonic sites also has a pronounced influence on isobutene selectivity. In general, better isobutene selectivities have been reported when decreasing the strength of the Bronsted acid sites by exchanging part of the protons by alkaline earth cations [101,102], or by replacing Al in the zeolite framework by other trivalent cations, such as B [103,104], Ga [91,105], and Fe [91,105,106]. This was explained by considering that the strongest Bronsted acid sites associated with framework Al are more active for catalyzing undesired oligomerization and cracking reactions mainly leading to by-products and coke. Then it is not surprising that medium pore silicoaluminophosphates such as SAPO-11 and SAPO-31 [78,107,108,109] and substituted aluminophosphates having the structure of AIPO4-H and AIPO4-3I (MeAPOs, Me=Mn, Fe, Co) [108,110] were found active, highly selective, and stable catalysts for the skeletal isomerization of n-butenes. 2.3. Isobutane/butene alkylation Alkylate gasoline produced by the reaction of isobutane with light olefins, mainly nbutenes, is an ideal component of reformulated gasolines as it is mainly formed by C$ isoparaffins having high octane number, acceptable volatility, and is almost free of sulfur and aromatics. The process is commercially carried out in the presence of strong liquid acids, HF and H2SO4, which impose serious limitations for further increasing the alkylation capacity owing to environmental and safety concerns associated to the use and handling of these liquid acids. Therefore, there is a strong incentive for replacing the harmful liquid acids by more environmentally benign solid acids [111]. In this respect, zeolites have been extensively investigated as potential solid alkylation catalysts, and special mention deserves the pioneering works by Kirsch et al. [112,113], Chu and Chester [114], and Weitkamp [115,116] who used large pore faujasites (X, Y) either in the protonic form or exchanged with rare earth cations. Weitkamp combined a differential sampling system downstream of the reactor with highresolution gas chromatography to obtain detailed information of the evolution of conversion and product yields with time on stream (TOS) [115,116]. By using this methodology, he observed that a very high conversion (100 %) with high selectivity to the desired trimethylpentanes (TMPs) could be achieved over a CeY zeolite during the initial stages of the reaction (first 30 min on stream), but then the activity rapidly declined with TOS while the product composition changed from mainly TMPs to mainly octenes, indicating a change in the reaction pathway from alkylation to oligomerization. This methodology was later applied by the group of Corma, who

346 observed similar trends over a wide variety of solid acids, including sulfated metal oxides [117,118,119], heteropolyacids and their salts [120,121], and zeolites [122,123,124]. Key parameters affecting the activity, selectivity and stability of zeolites during isobutane/butene alkylation are the pore topology and the zeolite composition (both framework and extraframework) which will determine the number and strength distribution of the protonic sites. In general, large pore zeolites (Y, EMT, Beta, ZSM-12) are preferred over medium pore ones (ZSM-5, MCM-22) since the formation of bulky TMPs is strongly impeded in the narrower pores of the latter zeolites [124]. It has been shown that Beta and ZSM-12 are more coke resistant than USY because their favorable pore structures limited the accumulation of carbonaceous compounds that lead to pore plugging during isobutane alkylation [125]. In the case of USY zeolites, Corma et al. [122] found a maximum initial conversion of 2-butene for samples with unit cell size (ao) between 2.435 and 2.450 nm, while the initial trimethylpentaneto-dimethylhexane ratio (TMP/DMH), which can be taken as the ratio between alkylation and oligomerization, continuously increased with ao, that is, with increasing number of framework Al atoms (Fig. 3). This was associated with a fast rate of hydride transfer (HT) on the zeolites presenting a high density of Bronsted acid sites. A fast rate of hydride transfer is essential during isobutane alkylation to rapidly desorb the TMP cations and to avoid consecutive alkylations of TMP + with olefin feed molecules that would lead to the formation of large hydrocarbons retained on the acid sites, and finally, to catalyst deactivation. In fact, a correlation has been recently observed between the hydride transfer activity of solid acids, including zeolites and sulfated zirconia, and the amount of TMPs produced per acid site [126]. Feller et al. [127] concluded by studying a series of X and Y zeolites in their protonic form that a high HT activity required for long catalyst life is favored over zeolites with a high ratio of Bronsted to Lewis acid sites and a high concentration of strong Bronsted acid sites.

Fig. 3. Influence of USY zeolite unit cell size on the initial (TOS= 1 min) 2-butene conversion and TMP/DMH ratio during isobutane/2-butene alkylation at 50°C, 2.5 MPa total pressure, i-C4/2-C4" molar ratio of 15, and WHSV (referred to the olefin) of 1-4 h"1. The method of zeolite synthesis, the Si/Al ratio in the synthesis gel, and the crystallite size of Beta zeolite are factors to be considered for optimum alkylation performance [123,128,129[. For both USY and Beta zeolites, the amount and type of extraframework Al species (EFAL) remaining in the zeolite after calcination and/or post-synthesis treatments also affected the catalyst performance [130,131]. For both zeolites, elimination of highly dispersed cationic-type EFAL formed under mild (hydro)thermal treatments decreased the alkylation

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activity and increased the rate of deactivation with TOS, while removal of polymeric-type EFAL had a beneficial effect on both zeolite activity and stability. Special mention merits the work carried out on the use of hexagonal faujasite (EMT) as alkylation catalyst [132,133,134]. EMT was found more selective toward the formation of TMPs and more stable with TOS than the corresponding cubic faujasite (Y). These differences were attributed to the slightly larger size of the second type of supercage and a to the stronger Bronsted acid sites in the hexagonal EMT zeolite [132]. As it has been said before, a fast deactivation with TOS is the main drawback for possible commercialization of zeolites as solid alkylation catalyst. In this respect, lower coke formation and deactivation rates have been claimed by performing the alkylation reaction under supercritical conditions in the presence of a co-solvent or diluent (e.g., CO2) in molar excess with respect to the hydrocarbon reactants [135]. Deactivated zeolites can be easily regenerated by burning off the coke deposits at temperatures above 500 °C in the presence of air. However, it would be desirable to regenerate the catalyst at much lower temperatures that are more compatible with the alkylation conditions (the alkylation reaction over zeolites is typically performed at 80-120 °C). In this sense, it has been shown that the initial alkylation activity of the zeolite can be completely restored by a combined treatment with ozone at 125 °C followed by air treatment at 250 °C [136,137]. The incorporation of small amounts (0.4 wt%) of a noble metal such as Pt to the zeolite allows to perform a hydrogenative regeneration of the coked catalyst under mild conditions [138]. Thus, under suitable regeneration conditions, e.g. at 300 °C and 15 bar hydrogen pressure, the alkylation activity of a LaY catalyst could be fully restored. Treatment of the completely deactivated zeolite under supercritical isobutane (one of the reactants) conditions has also been used to remove coke and to recover most of the initial alkylation activity [139,140,141]. 2.4. Isomerization of light straight run (LSR) naphtha The skeletal isomerization of C5-C6 n-paraffins contained in LSR naphtha has become a key process in modern refineries for increasing the octane number of gasoline. Commercial LSR isomerization catalysts are bifunctional and typically contain Pt as the hydrogenatingdehydrogenation function and halogenated aluminas or zeolites as acidic carriers. The former catalysts are more acidic and thus are able to operate at lower temperatures (ca. 150 °C) which are more favorable for the formation of high-octane isoparaffins. Mordenite is the preferred zeolite for LSR naphtha isomerization. Bifunctional Pt/mordenite catalysts typically operate at temperatures around 250 °C, which limit the maximum isomerization conversion that can be achieved per pass. However, Pt/mordenite catalysts are significantly more resistant toward deactivation by sulfur and water poisons present in the LSR naphtha feed than Pt/chlorinatedAI2O3. Consequently, expensive feed pretreatment can be often omitted in Pt/mordenite based LSR isomerization processes resulting in lower capital investments. Moreover, Pt/mordenite catalysts are easier to regenerate than the chlorinated catalysts and do not require the continuous addition of small amounts of chlorine to maintain high catalyst activity. These properties often compensate for the lower activity of Pt/mordenite catalysts as compared to Pt/Cl-Al2O3 and are the main reasons for their successful commercialization [142].

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An important parameter controlling the activity and selectivity of Pt/mordenite catalysts for LSR naphtha isomerization is the zeolite framework composition, that is, the framework Si/Al ratio. Thus, a maximum of activity for n-pentane isomerization was found for a Si/Al ratio of about 10 [143]. This would correspond to a situation where all the framework Al atoms are isolated and therefore to the maximum Bronsted acid strength. The decrease of activity at higher Si/Al ratios is thus due to a decrease in the number of strong Bronsted acid sites available for the reaction. Increasing the Si/Al ratio of the mordenite zeolite by dealumination also has a beneficial effect on catalyst deactivation by reducing the coking rate on the zeolite [144]. Therefore, in order to achieve optimum activity, selectivity and stability for light naphtha isomerization the mordenite zeolite has to be dealuminated to the desired Si/Al ratio. If the framework Si/Al ratio was the only parameter controlling the catalyst performance, one would expect to obtain the same activity irrespective of the method used to dealuminate the mordenite. However, this was not the case, and it was found experimentally (Fig. 4) that the maximum of isomerization activity is attained at different Si/Al ratios depending on the dealumination procedure used [145,146,147,148[. This clearly indicates that there are other parameters, besides the framework Si/Al ratio, that determine the isomerization activity. As observed in Fig. 4, dealumination by acid treatment followed by steaming at mild conditions produces the most active catalysts and the maximum of activity is attained at a higher Si/Al ratio as compared to the other dealumination methods [149,150].

Fig. 4. Influence of the method of dealumination of mordenite on the n-C5 isomerization activity as a function of the Si/Al ratio.

It is well known that dealumination by acid treatment produces samples which are almost free of extraframework Al species (EFAL), while dealumination by steaming leaves all the Al extracted from the framework in extraframework positions. In the case of the acid + steaming procedure, a small and controlled amount of highly dispersed EFAL is formed which produces a synergetic effect on the Bronsted acid sites associated to framework Al enhancing their acid strength [149]. In this way, it should be possible to produce highly active

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isomerization catalysts at high Si/Al ratios, and thus with high stability, by optimizing the amount and nature of the EFAL species left on the zeolite during dealumination. These findings constituted the base for the development of an improved generation of Pt/mordenite naphtha isomerization catalyst (HYSOPAR) that is being commercially used in the CKS (Cepsa-Kellog-Siid Chemie) ISOM process [151]. Besides mordenite, other large pore zeolites such as offretite [152], mazzite [153], and Beta [154,155,156,157] have also been used for light naphtha isomerization. The isomerization activity of Pt/Beta catalysts was seen to depend on the synthesis Si/Al ratio, and better results were obtained for a sample with Si/Al ratio of 6.7 and loaded with 0.3 wt% Pt [155]. The use of Pt/Beta would be advantageous for feeds with a high proportion of n-C6, since these catalysts generally lead to higher yields of higher octane dimethyl butane isomers as compared with Pt/mordenite [156,157,159]. With the new legislation limiting the amount of aromatics, and particularly of benzene, in the gasoline pool it could be of interest to process the benzene and benzene precursors (C6+ n-paraffms) from the reformers together with the LSR naphtha in the isomerization units. In this case, isomerization of C7+ n-paraffms should be accomplished with high selectivity toward the high-octane multibranched isomers, while benzene should be hydrogenated to cyclohexane and then selectively isomerized to its higher octane methylcyclopentane isomer. However, this ideal situation would be difficult to achieve with the current Pt/mordenite LSR isomerization catalysts, whose isomerization activity becomes strongly inhibited by the presence of even low amounts of benzene in the naphtha feed [158]. Moreover, the addition of C7+ n-paraffms to the C5/C6 naphtha causes a significant increase of the gas yield by extensive cracking of the multibranched isomers occurring on the strong acid sites of mordenite at the typical LSR isomerization temperatures [159]. In this case, it is clear that zeolites with milder acidity and larger pores would be more adequate to limit recracking and to facilitate the diffusion of the multibranched isomers. Thus, Pt supported on Beta, USY, and ZSM-12 zeolites produced higher isomer yields during n-C7 isomerization than Pt loaded on mordenite or ZSM-5 [160]. Relatively high selectivities to multibranched isomers can be achieved during the isomerization of C7-C8 n-paraffms with Pt/Beta catalysts [161], even in the presence of relatively large amounts (25 wt%) of benzene [162]. Moreover, the isomerization selectivity of Pt/Beta (either in the absence and presence of benzene) could be improved by decreasing the crystallite size of the zeolite owing to a faster diffusion of the branched isomers preventing their recracking into lighter compounds [161,163]. Moreover, the nanocrystalline zeolite also produced a higher yield to multibranched iso-C7 products (about 22 % at ca. 90 % n-C7 conversion). 2.5. Catalytic dewaxing and isodewaxing Catalytic dewaxing over shape selective medium pore zeolite catalysts is commercially used to improve the cold flow properties (pour point, freezing point, cloud point) of distillate fuels through the selective cracking of linear and slightly branched (waxy) paraffins. This is the basis of the Mobil Distillate Dewaxing (MDDW) process using a ZSM-5 zeolite which was first publicly announced in 1977 and commercialized in 1978 [164]. In 1988 Akzo-Fina developed an improved catalytic dewaxing process, the Cold Flow Improvement (CFI) process, by combining a hydrotreating catalyst placed upstream of the ZSM-5 dewaxing catalyst to produce a low-sulfur (typically below 0.05 wt%) diesel fuel with good cold flow properties. The CFI process was brought together with MDDW process in 1992 under the Mobil Akzo Kellog (MAK) hydroprocessing alliance.

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Premium quality jet fuels requiring high flash point (> 170 °C) and low freezing point can also be produced from a variety of petroleum feedstocks by combining catalytic dewaxing with product hydrogenation over a small pore Ni-erionite catalyst at high (> 69 atm) hydrogen pressures [165] or over a metal-containing ZSM-5 catalyst [166]. Catalytic dewaxing can also be used to produce high viscosity index (VI) lube oils, as in the Mobil's Lube Dewaxing (MLDW) process. This process uses a two-reactor system, where a ZSM-5 based catalyst is placed in the first reactor, and the product stream is then sent without intermediate separation to the second reactor containing a hydrofinishing catalyst to meet the desired final product specifications [167]. The dewaxing performance of ZSM-5 zeolite was shown to depend on the Al content [168] and of crystallite size [168,169]. Moreover, isomorphous substitution of Al by other trivalent cations (e.g. B, Fe, Ga) leading to weaker acid sites decreased the activity though it significantly increased the dewaxing selectivity by reducing the extent of secondary cracking [170]. Besides ZSM-5, other zeolites such as ferrierite and mordenite loaded with a noble metal (Pt, Pd) have also been found to be effective lube dewaxing catalysts [171,172]. In this case, high hydrogen pressures are applied to saturate the cracked by-products and to ensure a stable catalyst activity by reducing coke formation, particularly in the case of mordenite which is more susceptible to coking owing to its unidirectional pore system. Bifunctional Pt/zeolite based catalysts have been developed to produce high quality distillates and lube oils by selective isomerization of the n-paraffins. This is the base of several dewaxing processes developed by Mobil, including the Mobil Isomerization Dewaxing (MIDW) process, Mobil Wax Isomerization (MWI) process, and Mobil Selective Dewaxing (MSDW) process. Catalytic dewaxing via n-paraffin isomerization has inherent advantages over dewaxing via cracking such as a higher product yield. This is even more important in the case of lube oils, where any cracking to lighter products represents a significant economic penalty. This can be seen in Table 3 by comparing the product distribution produced by lube dewaxing via cracking over a ZSM-5 based dewaxing catalyst (MLDW-1) and over a bifunctional metal-zeolite dewaxing catalyst (MSDW-2) via isomerization [173]. Moreover, the incorporation of a noble metal into the shape selective dewaxing catalyst increases catalyst activity and results in extended catalyst cycle length. Table 3 Product distribution produced by lube dewaxing via cracking (MLDW) and isomerization (MSDW)[173] Pour point (°C) Yield (wt%) Viscosity index Kinematic viscosity, @ 100°C, cSt By-product selectivity (wt%) 166-343°C, distillate 52-165°C, naphtha C4-C5

c,-c3

MSDW-1 -12 90.5 105 10.2

MLDW-2 -12 82.9 92 10.7

17.5 37.9 33.0 11.6

9.9 28.2 43.1 18.8

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Generally, ZSM-5 catalysts display a low isomerization activity, which is ascribed to a slow diffusion of the branched alkane product through the zeolite pores as compared to the linear alkane favoring secondary cracking into lighter compounds. For that reason, other zeolites with larger pores such as Beta and MCM-22 have also been studied by Mobil for dewaxing via isomerization [174]. In particular, higher product yields and lower pour points have been claimed over a low acidic Pt/Beta catalyst for gas oil dewaxing as compared with ZSM-5 based catalysts [174]. Chevron has also developed its Isodewaxing process using a bifunctional Pt/SAPO-11 catalyst [175]. This process is used to produce high yields of high VI (viscosity index) and low pour point lube oils from waxy feedstocks. Usually, the feed need to be hydrotreated prior to the isodewaxing step to lower nitrogen and sulfur contents and thus to ensure high product yields and long cycle lengths. The catalytic performance of Pt/SAPO-11 for nhexadecane isomerization, taken as a representative molecule for the long-chain n-paraffins present in real feeds, is compared with that of an amorphous Pt/SiC>2-Al2O3 catalyst in Table 4 [176]. Table 4 Isomerization of n-hexadecane at 1000 psig, 3.1 WHSV, and 30 Hb/hydrocarbon molar ratio at a 96% conversion over Pt/SAPO-11 and Pt/SiO2-Al2O3 catalysts [176] Temperature, °C Isomerization Selectivity, wt% C|6 Product Composition, wt% 7- + 8-MCl5 6M-C15

5M-C,5 4M-Ci5 3M-C 15 2M-C 15

n-Cie Other C l6 Pour Point, °C

Pt/SAPO-11 340 85

Pt/SiO2-Al2O3 360 64

22.9 7.1 7.5 8.1 7.7 4.7 29.8 12.2 -51

6.1 3.2 3.1 3.2 3.3 6.0 37.8 34.6 -28

Clearly, a higher selectivity towards monobranched isomers is achieved over the Pt/SAPO-11 catalyst. Moreover, this catalyst favors branching at the center of the hydrocarbon chain (7- + 8-MC15) resulting in a lower pour point. The outstanding catalytic performance of Pt/SAPO-11 has been related to its mild acidity and strong hydrogenation function, in combination with a constrained one-dimensional pore system which minimizes secondary cracking of the branched products. Other one-dimensional medium pore zeolites such as ZSM-22 and ZSM-23 have also shown high performance as isodewaxing catalysts as claimed in several patents from Chevron [177,178]. 2.6. Fluid catalytic cracking (FCC) Fluid catalytic cracking (FCC) is probably the most important conversion unit in modern refineries and the largest user of zeolite catalysts [173]. Essentially, catalytic cracking involves the rupture of C-C bonds in heavy hydrocarbon feeds such as vacuum gas oils and

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residues to produce more valuable lower molecular weight hydrocarbons, including diesel, gasoline, and light olefins for petrochemistry. The greatest revolution in catalytic cracking occurred in 1962 when faujasite zeolites replaced amorphous silica-alumina as the main component of FCC catalysts. Since then, zeolite Y continues to be the primary component of current FCC catalysts, and main efforts have concentrated on modifying the Y zeolite for improving activity, gasoline selectivity and octane number, coke selectivity, and stability. In this respect, one important parameter controlling the cracking behavior of the Y zeolite is the framework composition (given by the unit cell size, ao), which determines, in principle, the amount and strength distribution of the Bronsted acid sites associated to framework Al. Thus, a maximum in activity for gas oil cracking was found for a framework Si/Al ratio of 5-8 (ao= 2,436 - 2,440 nm), which corresponds to the situation where all framework Al atoms in the faujasite structure are isolated. This clearly indicates that gas oil cracking requires the presence of strong Bronsted acid sites. The product selectivity during gas oil cracking also depends on the framework composition. Dealuminated Y zeolites with low framework Al content (ao ^ 2.430 nm) are more selective to diesel in detriment of gasoline, give a higher product olefinicity (and thus a higher research octane of the gasoline), and are more stable under the severe conditions (temperatures above 700 °C in the presence of steam) used in the regenerator of FCC units. Due to this, it was considered that the unit cell size (ao) was the unique parameter determining the activity and selectivity of Y zeolite during catalytic cracking [179]. However, it was found that the cracking behavior of dealuminated Y zeolite with a given ao value strongly depended on the method of dealumination used, indicating that there are other factors affecting the cracking performance of Y zeolite [180,181]. These factors are the nature and amount of extraframework Al (EFAL) species and the mesoporosity generated during the dealumination treatments [180,181]. Therefore, great efforts have been devoted to the optimization of the cracking performance of Y zeolite through the manipulation of both EFAL and mesoporosity. Mesoporosity plays an important role in catalytic cracking since the bulky molecules present in commercial gas oil feedstocks can hardly access the acid sites located in the micropores and the first cracking events will take place, predominantly, on the external zeolite surface. Consequently, the activity and selectivity of the zeolite will be mainly determined by the surface composition and the accessibility to the acid sites, which greatly depends on the method and severity of the dealumination treatment. Another way of increasing the accessibility of the zeolite pores by the gas oil molecules is by decreasing the mean diameter of the zeolite crystallites. Thus, it was shown that both the activity and gasoline selectivity of afresh catalyst increased when decreasing the crystallite size of a Y zeolite synthesized with a Si/Al ratio of 2.5 [182]. Unfortunately, the benefits of reducing crystallite size were lost after steaming of the zeolite at high temperatures (above 700 °C) to simulate the equilibrated catalyst owing to the lower stability of the small crystallites. This problem could be overcome by synthesizing a NaY zeolite with a higher Si/Al ratio (3.1 instead of 2.5) and small crystallites in a narrow distribution of sizes [183]. Besides Y zeolite, the addition of ZSM-5 zeolite to the FCC unit has been recognized as the simplest solution for improving gasoline octane and increasing the yield of light olefins (propylene and butenes) at the expense of gasoline. Significant advances have been made in improving the hydrothermal stability of the ZSM-5 zeolite under the regenerator conditions at higher concentrations in the additive [184]. The potential use of MCM-22 as a cracking additive for FCC has also been studied [185], MCM-22 showed higher steam stability, lower gas yield and lower gasoline loss than ZSM-5, thought it displayed a lower activity, thus requiring higher levels of the additive to observe the expected benefits [185].

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2.7. Catalytic hydrocracking Hydrocracking is widely used in petroleum refining to convert heavy oil fractions (which may include residues) into lower molecular weight hydrocarbons through C-C bond breaking. Catalysts used for hydrocracking are bifunctional, as they comprise a hydrogenation-dehydrogenation function provided by metals, and an acidic function given by the support. The metal function is typically provided by noble metals (e.g. Pt, Pd) or by a combination of Group VIA (Mo, W) and Group VIIIA (Ni, Co) metals in their sulfided state. The acidic support may consist of amorphous silica-alumina, a zeolite, or a combination of zeolite and amorphous oxides. As compared to amorphous silica-alumina, zeolite based hydrocracking catalysts show a higher activity (owing to a higher acid strength), higher thermal/hydrothermal stability, better selectivity to the naphtha fraction, better resistance to poisoning by sulfur and nitrogen impurities, and lower coking rate resulting in longer catalyst life [186,187]. The most widely used zeolites in commercial hydrocracking catalysts are of the Ytype. The hydrocracking activity and selectivity of Y zeolite is mainly controlled by the zeolite composition. Catalysts based on Y zeolites with high density of framework Al (i.e. high density of Bronsted acid sites) display high activity and high selectivity to gasolinerange (C5-C11) hydrocarbons, but also produce high yields of gases (C4.). By contrast, USY zeolites with low framework Al content produced by steam dealumination are less active but more selective to middle distillates. Besides the framework composition, other factors such as the presence of extraframework Al (EFAL) species generated during the steam dealumination step, and the mesoporosity, which facilitates the accessibility of the large reactant molecules to the acid sites, are also important in zeolite-based hydrocracking catalysts. Thus, removal of EFAL by acid leaching was seen to increase the activity and selectivity to middle distillates as compared to the parent USY zeolite [188]. On the other hand, the presence of mesopores in USY zeolites improves the selectivity to middle distillates by reducing secondary cracking reactions that would lead to naphtha and gases. This is also an important parameter in designing active and stable zeolite-based catalysts for hydrocracking heavy feeds and residues [189,190]. The accessibility of the bulky feed molecules can also be increased by using Y zeolites synthesized with small crystallites [191,192]. Mordenite, L, Omega, and Beta large pore zeolites have also been studied for hydrocracking. Mordenite, having a one-directional system of channels and strong acid sites, produces a deep cracking and is therefore suitable for hydrocracking naphtha into LPG, but not for hydrocracking heavier fractions [186]. Zeolite Beta, alone [193,194] or in combination with Y zeolite [195,196], has also been used as support for preparing active hydrocracking catalysts. An improvement in activity and selectivity to middle distillates has been observed when decreasing the average crystallite size of the Beta zeolite [197]. Recently, it has been shown that catalysts prepared by supporting NiMo sulfides on the new delaminated 1TQ-2 zeolite display a higher activity than those based on amorphous silica-alumina (ASA) and USY zeolite for mild hydrocracking of vacuum gas oil [198]. Moreover, NiMo/ITQ-2 catalysts presented a selectivity to middle distillates intermediate between that of NiMo/ASA and NiMo/USY. Finally, it would be worth to mention that ordered mesoporous aluminosilicates like MCM-41 have also been studied as the acidic component of hydrocracking catalysts. Thus, NiMo/MCM-41 catalysts showed higher activity than NiMo/ASA and NiMo/USY for the one-stage hydrocracking of a vacuum gas oil feed containing 2.5 wt% sulfur and 2900 ppm nitrogen, despite the Bronsted acidity of MCM-41 was much lower than that of USY and similar to that of ASA [199]. In addition, catalysts based on MCM-41 provided better

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hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities than those based on ASA and USY, as well as a selectivity to middle distillates comparable to the amorphous catalyst (Table 5) [199]. These results were explained by the combination of a very high surface area (> 900 m2/g) and the presence of uniform pores in the mesopore range (ca. 3.0 nm), which facilitated the diffusion of large feed molecules, together with a mild acidity and high stability. Table 5 One-stage hydrocracking of vacuum gas oil over NiMo catalysts supported on MCM-41, USY and amorphous silica-alumina (ASA) Catalyst NiMo/MCM-41 NiMo/ASA NiMo/USY

Conversion at 400 °C 48 35 27

Product distribution at 50 % conversion (wt%) Middle distillates Gases Naphtha 16.2 25.8 58.0 18.9 23.1 57.9 19.7 52.0 27.3

3. ZEOLITES IN PETROCHEMISTRY Although still the most important applications of zeolite catalysts are in the field of refining, as described in the previous section, the use zeolites in petrochemistry is expected to grow in the near future and several petrochemical processes based on zeolite catalysts have been already developed and commercialized. Moreover, zeolites can offer new opportunities for the development of new petrochemical processes replacing harmful and corrosive mineral acids which are still used in several processes and thus would lead to more efficient, selective, and cleaner processes [200,201,202]. In this section, we will describe the application of zeolites in some relevant petrochemical processes, with especial emphasis in aromatics transformation processes, including well established technologies as well as potential applications in new processes. 3.1. Production of para-xylene Para-xylene (PX) is the most valuable xylene isomer which is used in the production of terephthalic acid (TPA) and dimethyl terephthalate (DMT), which are intermediates in the manufacture of polyethylene terephthalate (PET) used in polyester fibers, molded plastics, and films. The main source of xylenes is the C8 alkylaromatic mixture (also known as mixed xylenes stream) obtained from reformate and pyrolysis gasoline, a by-product of the steam cracker units. Besides xylenes, the mixture also contains ethylbenzene (EB) in amounts that vary depending on the origin of the cut, ranging from about 20 % for reformate up to 50 % or higher for pyrolysis gasoline. Para-xylene is separated from the Cs alkylaromatics mixture by crystallization or selective adsorption on molecular sieves. Then, the remaining PX-depleted mixture can be further upgraded to increase the yield of the desired PX via catalytic isomerization. In the xylenes isomerization units, EB must be converted in order to avoid its build up in the recycle stream. There are two types of xylene isomerization processes depending on the way EB is converted: EB-isomerization and EB-dealkylation processes. In the former, EB is isomerized into an equilibrated mixture of xylenes and is thermodymanically limited to ca. 30% EB conversion per pass. Most of the today's EBisomerization processes are based on bifunctional Pt-supported on alumina-mordenite catalysts [203]. The activity and isomerization selectivity of mordenite-based catalysts has

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been shown depend on the zeolite composition (i.e. acidity) and porosity. Thus, higher selectivities to xylenes have been reported for mordenites with a reduced density of Bronsted acid sites and having no mesoporosity [204,205,206]. Partial exchange of the H+ sites of mordenite by Ca2+ [205] or Na+ [207] also resulted in an enhanced isomerization selectivity owing to a suppression of the secondary reactions, mainly disproportionation and transalkylation. On the other hand, the medium-pore ZSM-5 zeolite is the preferred zeolite component in the catalysts used in EB-dealkylation processes, as in those developed and commercialized by Mobil. In these processes, EB is converted into high-purity benzene and ethylene (which can be hydrogenated to ethane). The reaction is not thermodynamically limited allowing higher EB per pass conversions (70 % or higher), though the final PX yields are lower than in EB-isomerization processes. The ZSM-5 zeolite allows high isomerization selectivities with PX selectivities close or slightly above the equilibrium values due to the combined effect of configurational diffusion and restricted transition state selectivity which favors isomerization against the competing xylene disproportionation reaction resulting in lower xylene losses [204]. The paralortho (P/O) ratio in the xylenes stream obtained over ZSM-5 catalysts can be increased by using zeolites with larger crystallites [208] and by eliminating the acid sites at the external surface through selective surface dealumination, chemical deposition of bulky organosilicon compounds, and coke-selectivation treatments [209,210,211,212]. The addition of a metal (e.g. Pt, Ni) to the ZSM-5 zeolite increases the rate of EB dealkylation at the expenses of disproportionation (occurring through a deethylation-ethylation mechanism) due to the rapid hydrogenation of ethylene [209,213]. Para-xylene (p-X) can also be produced by disproportionation of toluene [214]. The commercial processes utilize ZSM-5 based catalysts, as in the MTD (Mobil Toluene Disproportionation) and MSTD (Mobil Selective Toluene Disproportionation) processes developed by Mobil [215]. The disproportionation of toluene is an-acid catalysed reaction occurring through a bimolecular pathway and is therefore favored by a high density of Bronsted acid sites [201]. In the normal MTD process a thermodynamic mixture of xylenes (containing ca. 24 % p-X) is produced, while in the MSTD process a high selectivity to the PX isomer (> 80 %) is attained owing to a proprietary coke-selectivation procedure which increases the rate of diffusion of p-X from the zeolite pores as compared to the o-X and m-X isomers. Selectivation can also be accomplished by deposition of silica on the zeolite surface [216]. MCM-22 zeolite was also seen to be attractive for selective p-X formation via toluene disproportionation [217,218]. Very recently, an improved selectivity to p-X could be achieved by replacing the protons in MCM-22 and ZSM-5 with InO+ cations through reductive solidstate ion exchange with I112O3 [219]. An advantage of the toluene disproportionation processes is the low concentration of EB in the C$ alkylaromatics product stream, which facilitates the separation of p-X from the reaction products. Xylenes can also be produced by toluene/C9+ aromatics transalkylation. In this case, large-pore zeolites, and particularly mordenite, are preferred over medium-pore zeolites [201]. The addition of a metal function to the catalyst can improve catalyst life and selectivity by hydrogenation of the light olefins formed by dealkylation thus avoiding futher alkylations of the desired xylenes. Alkylation of toluene with methanol can also be a source of xylenes. As in the toluene disproportionation processes, the use of unmodified ZSM-5 zeolite leads to an equilibrium mixture of xylenes, while p-X can be selectively formed by proper modification of the zeolite, for instance by incorporating Mg or P [220] or by deactivation of the external surface. Besides ZSM-5, zeolites such as mordenite, Y, and SAPO-11 among others have also been used for the alkylation of toluene with methanol [221,222]. An improved selectivity to p-X has been recently reported for a MCM-22 modified with La(NO3)3 [223].

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3.2. Alkylation of aromatics Alkylation of aromatics is the main commercial route for the production of important petrochemical intermediates, such as ethylbenzene, isopropylbenzene (cumene), paraethyltoluene, linear alkylbenzenes, and alkylnaphthalenes [200,202,224]. Alkylation of aromatics is an acid-catalyzed reaction which traditionally employs Friedel-Crafts type (AICI3, BF3) and Bronsted acid (HF, H2SO4, H3PO4) catalysts. Substitution of such corrosive and environmentally unfriendly acids by zeolites would represent a great benefit for the processes and may also improve the selectivity toward the desired alkylated product by taking advantage of their discrete pore size and adjustable Bronsted acidity. An excellent review on the production of mono- and dialkylbenzenes over zeolites, with especial emphasis on the effect of the type of acid sites, zeolite structure, and reaction conditions, has been recently published by Cejka and Wichterlova [225]. Ethylbenzene (EB) is predominantly used as a precursor for the production of styrene monomer. In the first step of the process, benzene is alkylated with ethylene to produce EB, which is then dehydrogenated to styrene. Several technologies using zeolites are now available for the production of EB. The first industrial application of zeolite in the alkylation of benzene with ethylene was realized by Mobil-Badger in 1980 by a ZSM-5 based catalyst [226]. The reaction occurs in the vapor phase at 400-450 °C, 2-3 MPa, and high molar ratio of benzene to ethylene (5-20) to ensure almost complete ethylene conversion and high EB selectivity. The main by-product of the reaction is di-ethylbenzene (DEB), which can be converted to EB by transalkylation with benzene in a separate step. Alternatives to the vapor phase EB process are the liquid phase alkylation of benzene with ethylene using large pore zeolites, such as Beta and MCM-22 [227,228]. The advantages of the liquid phase processes are a reduced formation of poly-alkylated benzenes, resulting in a higher EB selectivity and longer catalyst life-time. Ethylbenzene can also be formed by side-chain alkylation of toluene with methanol over basic catalysts, such as alkali-exchanged X and Y zeolites [229,230]. The cross-coupling of toluene and methane to produce EB/styrene at high temperatures (> 700 °C) in the presence of oxygen has also been attempted by using basic zeolites, such as CsX and BaO/NaX [231, 232,233]. Isopropylbenzene (IPB or cumene), produced by the alkylation of benzene with propylene, is predominantly used in the co-production of phenol and acetone, and to a minor extent as a source for a-methylstyrene via dehydrogenation. Although first industrial processes for cumene synthesis used supported phosphoric acid (SPA) or AICI3/HCI catalysts, zeolites have been commercially applied since 1994 [234]. Main advantages of using zeolites are the non-corrosive nature of the catalyst and the possibility of regeneration, with the corresponding savings in catalyst costs. In general, the cumene selectivity of zeolite based catalysts is lower than that of SPA, as the former give more poly-alkylated products, mainly para-diisopropylbenzene (DISPB). However, this by-product can be converted to cumene in a separate transalkylation reactor in the presence of zeolite catalysts. Main concerns in the alkylation of benzene with propylene are the formation of n-propylbenzene, which is difficult to separate from IPB by distillation, and the formation of oligomers from propylene, which are responsible for the rapid catalyst decay. In the alkylation of benzene with propylene, large pore zeolites are more suitable than medium pore zeolites (e.g. ZSM-5), as the latter have a higher tendency to form oligomers and polyalkylated products resulting in lower selectivities and higher deactivation rates. Thus, a process based on modified mordenite catalyst was developed and commercialised by Dow Chemical [235,236]. In this process, two highly dealuminated mordenites are used for the alkylation and transalkylation steps. The alkylation

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takes place in liquid phase at low temperatures (130 °C), which are more favourable for avoiding the formation of M-propylbenzene. Catalysts based on zeolite Beta have also been developed by Chevron [237] and EniChem [238]. Interesting results have also been reported by using a ZSM-12 zeolite with Si/Al ratio of 60 [239]. This zeolite displayed a total yield of cumene+DIPB of 99.3 % and showed a higher stability than catalysts based on low Si/Al ratio LaHY and mordenite zeolites. Recently, Mobil has developed a process for the alkylation of benzene with propylene using MCM-22 [228], a medium pore zeolite comprising two independent 10-MR pore systems, one consisting of sinusoidal pores and the other containing large supercages (1.76 nm long x 0.71 nm diameter) interconnected by slightly elliptical pores. This zeolite displayed a better selectivity than other large pore zeolites at the high propylene conversions typically used in commercial operations. The unusual high selectivity of MCM-22 (as compared to other 10-MR zeolites) has been explained by considering that the alkylation reaction occurs preferentially on the hemi-cages resulting from the truncation of the supercages at the external surface. The contribution of the external surface of MCM-22 to the reaction was confirmed by computational studies [240] and by selectively poisoning the external acid sites by bulky basic molecules, such as collidine [241]. Even better results have been achieved with a new zeolitic ITQ-2 material, formed by delamination of the lamellar precursor of MCM-22 and containing a larger amount of hemi-cages on its very large (> 900 m2/g) and accessible surface [242]. Another relevant alkylaromatic product is isopropyltoluene (cymene), an important intermediate in the production of cresols and also used as an industrial solvent. Commercially, cymene is produced by alkylation of toluene with propylene using, as in the case of cumene, solid phosphoric acid (SPA) as catalyst [243]. Again, for environmental and economical reasons, there is a strong incentive to develop a process using more benign and regenerable solid catalysts, and in this sense, the use of zeolites has been explored. By taking advantage of the shape-selectivity of zeolites the production of para-cymene, the precursor of para-cresol, can be enhanced as compared to the process using SPA as catalyst. Further improvement in the selectivity to para-cymene can be achieved by surface modification of the zeolite, as in the case of ZSM-5 modified by chemical vapor deposition of silicate esters. The alkylation of toluene with isopropanol over zeolite catalysts possessing different acidity (Al- and Fesilicates) and structure (Y, mordenite, ZSM-12, Beta, ZSM-5) has also been investigated [244,245]. The best compromise between activity and selectivity leading to highest yields to the desired para-cymene product was obtained for the medium pore H-(Fe)ZSM-5 metallosilicate possessing a low number of Bronsted acid sites and low acid strength [244, 245]. The alkylation of polyaromatics, such as naphthalene and methylnaphthalenes, to produce the desired isomer through shape-selective effects over zeolite catalysts has also been investigated. An interesting contribution from zeolites in this area is the synthesis of 2,6dimethylnaphthalene (2,6-DMN), which is an important precursor for the production of polyethylene naphthalate (PEN), a polyester that can be an alternative to the PET. The alkylation of pure 2-methylnaphthalene (2-MN) with methanol using MCM-22 zeolite has been claimed in a patent from Mobil [246]. This zeolite yielded a product containing ca. 10 % 2,6-DMN, with a total amount of dimethylnaphthalenes of ca. 34% and a 2,6/2,7 ratio of 2.3. However, the 2,6/2,7 ratio significantly decreased when a mixture of DMN isomers, as it is the case for real feeds, was used instead of pure 2-MN [247]. Interesting results have been reported for the alkylation of naphthalene with methanol over a ZSM-12 zeolite in the presence of trimethylbenzene (TMB) [248]. In the best case, a 2,6/2,7 ratio of 2.6 and a 2,6DMN yield of up to 20 % could be achieved. A high selectivity to the 2,6-isomer (up to 82 %)

358

has been also obtained during the isopropylation of naphthalene over mordenite and ZSM-12 catalyst [249]. This probably occurs by a faster diffusion of the 2,6-diisopropylnaphthalene (2,6-DIPN) isomer in the zeolite channels due to a smaller critical diameter and a more linear structure of this isomer as compared with 2,7-DIPN, as supported by molecular modelling studies [250]. The selectivity to 2,6-DIPN over mordenite was substantially increased upon dealumination [251]. This was ascribed, at least partially, to a lower reactivity of 2,6-DIPN on dealuminated mordenites, which avoided its consecutive transformation via isomerization, transalkylation, and further alkylation [252]. In a recent study, it has been found that a USY catalyst is more active and stable for the isopropylation of naphthalene with isopropyl alcohol than mordenite, though the 2,6-/2,7-DIPN ratio was lower in the former zeolite, suggesting the absence of shape selectivity effects in the larger pores of USY [253]. Finally, it would be worth to mention that zeolites have been also investigated as potential solid acid catalysts to replace the harmful HF catalyst commercially used for the production of linear alkylbenzenes (LAB), which are important intermediates for alkylbenzenesulfonates used on a large scale as anionic detergents. LAB having 10-14 atoms in the alkyl chain is produced by alkylation of benzene with C10-C14 a-olefms obtained by dehydrogenation of the corresponding linear paraffins [254]. LAB in where the phenyl group is attached at the end of the alkyl chain (for instance 2-phenyl decane in the case of alkylation of benzene with 1-dodecene) are preferred because of their better emulsifying properties. As observed in Table 6, zeolites display a higher selectivity to 2-phenyldecane, the least bulky of the linear phenyl dodecane isomers, than HF due to steric restrictions in the zeolite pores [255, 256]. In general, higher selectivities to the 2-phenyldodecane isomer were observed for zeolites with smaller pores and one-dimensional structure. Table 6 Comparison of different zeolites with HF for the alkylation of benzene with 1-dodecene [255, 256] Catalyst HF REY

Linde L Beta ZSM-5 Offretite Mordenite (dealuminated) ZSM-12

2-Ph 17 25 40 57 57 79 85 92

Distribution of pheny ldodecane isomers (%) 3-Ph 5-Ph 4-Ph 16 18 24 20 19 18 18 16 15 18 10 7 25 8 5 14 1 5 15 0 0

6-Ph 25 18 11 8 5 1 0

0

0

8

0

However, the alkylation activity of zeolites with relatively small pores such as ZSM-5 and ZSM-12 was seen to be generally low [257]. In the case of mordenite, the activity was increased upon dealumination, probably because of the formation of mesopores which facilitated the diffusion of reactants and products through the pores. In the case of Y zeolite, a good correlation between the rate of benzene alkylation with 1-dodecene and the concentration of framework Al was observed, though a high density of framework Al was detrimental for catalyst stability. The higher stability of dealuminated samples was attributed to the presence of mesopores created during the dealumination process, and therefore, an optimum performance should be obtained by balancing both acidity and mesoporosity. A fast

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deactivation resulting in low 1-dodecene conversions was also observed for zeolite Beta [258]. To date, the only commercialised technology using a solid acid catalyst is the Detal process offered by UOP [254], although the type of solid acid used was not disclosed. 4. CONCLUSIONS AND FINAL REMARKS As we have seen, the number of zeolite catalysts used is still expanding. There is room for stable ultra-large pore bi- or three-dimensional zeolites for catalytic cracking and hydrocracking. The benefit is expected not only from the point of view of increasing bottoms conversion, but especially for increasing the yield to diesel. In the same field, the synthesis of bi- or three-dimensional zeolite structures with 10 and 10 x 9 member ring pores should further increase propylene and ethylene. "Layered" zeolites offer new possibilities for catalytic conversion processes, as well as for petrochemicals production. Finally, new possibilities for zeolites will be opened in separation processes and for combined reaction/separation. In our opinion, there is no reason to believe that all major things that could be done with zeolites have already been done. This belief can only be a limit for our imagination. ACKNOWLEGEMENT The authors are grateful to the Comision Interministerial de Ciencia y Tecnologia (CITY) of Spain for financial support through the projects MAT2003-07945-C02-01 and CTQ200402510/PPQ.

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367

SUBJECT INDEX A Abinitio Acoustic devices Adsorption physical AFI Agriculture Alkane Alkylation aromatics Alkylation isobutene/butane Alkylation long-chain Alkylpyridines AlPO4 AlPO4-5 Alumina mesoporous Amylose ANA Analcime Application catalytic Application ion-exchange Aromatization

Background subtraction BEC .. 1 1 Benzene alkylation Beta/MCM-48 _. ... , Bipyridyl complexes ™ n u • • Born-Oppenheimer approximation Building materials .. . . . Butene linear isomerization

245 280 27 53, 86,267 31 339 356 345 358 315 4 53, 86,267 99,121 319 191 191 128 29 339

208 116, 117 7

Cluster models Collidine Correlation motions Coumarin Crystal growth models Crystal growth Cytochrome P450 D Definition Density functional theory Dewaxing Dielectric layers Di-imine complexes Di-laurylisosorbide Dimethylnaphthalene Dopamine ß-monooxygenase Dye molecules

ECR-34 ETR EDI

4,55 4,55 193

Edingtonite Electron microscopy

193 226

129 127 __, 296 ^AA 244 32

Electron tomography Electronic devices Embedding

225, 239 281 251

Encapsulation

266,290

Enomte

193

342

C

Carbon pearls Catalytic processes Cation exchange equilibria Cation exchange isotherm CHA Chabazite Chemical properties Chemical sensors Chiral complexes Citronellal Clinoptilolite CLO Cloverite

17 245 349 278 298 320 357 300 266

E

Extra-framework species

Carbon nanotubes

250 316 211 329 77 75 292

58

F 126 124 7 182 184 49,192 49, 192 23 280 305 329 192 54 54

FAU

47, 194

Faujasite FCC catalyst FCC FER Ferrierite Fluid catalytic cracking FSM-16 Furoquinoline

47,194 3 351 195 195 351 99 332

G Gas separation Germanium oxide

7 115

368 GIS Gismondine

49,195 49, 195

H Hamiltonian non-relativistic HEU High-throughput adsorption High-throughput catalysis High-throughput characterization High-throughput IR High-throughput reactors High-throughput synthesis High-throughput XRD High-throughput XRF HMS Hybrid models Hydrocracking catalytic

245 192 170 171, 173 166 167 172 161 166 167 99 250,253 353

1

ICNZ IM 12

Immobilization Induction period Interaction energy Intrazeolite complexes Ion-exchange theory Ion-exchange Isodewaxing ITQ 15 ITQ-17 IT

Q-2

IZA

16

55,116 289 73 255 215 182 181 349 116,118

116,117 119 3

„ „„

-. „ ,

L LAU Laumontite Library design Linde type A Liquid crystal templating LTA Lutidine

195 195 176 47, 195 96 47, 195 316

M M41S Mannitol MCM-22

94 319 51,119,343

MCM-22 precursor MCM-36 MCM-41 MCM-48 MCM-49 MCM-50 MCM-56 MEL Membrane application Membrane characterization Membrane composite Membrane industry Membrane polymer-zeolite Membrane reactors Membrane supported zeolite Membrane synthesis Membrane template removal Membrane transport in Membrane zeolitic Menthol MER Merlinoite Mesoporous sieves discovery Mesoporous sieves Mesoporous spectroscopy Mesoporous synthesis Mesoporous ZSM-11 Mesoporous ZSM-5 Mesoporous Methane storage Methane MFI

Micro/meso composites Microcalorimetry Microwave heating Mixed acetate complexes Modeling of synthesis Molecular sieve Molybdenum MOR Mordenite MSU

MWW

119 119 92,94 94 119 94 119 50 149 144 137 153 137 151 137 138 138 148 135 329 196 196 91 91,233 205 96,101 126 126 1,4 265 338 3,50,85,92,196,338 104,126 284 70 305 70 1 338 50, 196 50, 196 " 51,119,343

N Nanoclusters Naphtha isomerization Naphtha reforming Naphthalene acylation Nomenclature Non-heme complexes Nucleation

269 347 341 128 17 296 73

369

o Octahydroacridine Octene epoxidation Olefine metathesis One-electron wavefunction Optical methods Organic cascade reactions Organo/inorganic composites

S 318 129 128 244 205 311 105

p p-Cymene Periodic models Petrochemistry Phenylpyridine PHI

Phillipsite Photoconductors Photoluminescence Phthalocyanine Poly-azamacrocycle complexes PolymorphC Porphyrins Powder diffraction Properties thermal Purification

357 249 337,354 317 197

197 270 218 290 299 116,117 292 59 28 7

p-Xylene

354

Pyranoquinoline

332

u • i chemical methodology , . , , , Quantum chemical methods . 24 Quinoline

.,„„ 244 243 3 317

vQuantum

R

Reaction alkylation Reaction alkylation-cyclization Reaction aromatic substitution Reaction aromatization Reaction cyclocondensation Reaction expoxidation Reaction halogenation Reaction hydrogenati on Reaction hydrolysis Reaction isomerization Reaction Pictet-Spengler Recrystallization Refining Reflectance

321 323 321 313 315 325 324 318 318 321 331 127 337 208

SBA-1 SBA-15 SBA-16 SBA-3 Schiff base complexes Schuster-Kubelka-Munk Sedimentary deposits Semiconductors Ship-in-the-bottle S ite uniformity Sodalite Sorbitol Stacking faults

99 99 99 99 301 207 16 270 289 211 46 319 59

STI

197

Stilbite Storage materials Storage of heat Structural Commission Supercapacitors Supramolecular templating Surfactant templating Synthesis - recent trends Synthesis alumina Synthesis course Synthesis equilibria Synthesis history Synthesis ITQ-15 Synthesis ITQ-17 Synthesis ITQ-2 Synthesis kinetics Synthesis MCM-22 „ , . s i s , .. Synthesis eMCM-41 . ,.„,..„ Synthesis MCM-48 S Synthesis mechanisms Synthesis of AIPO4-5 Synthesis of ZSM-5 Synthesis reviews Synthesis SBA-15 Synthesis SSZ-53 Synthesis UTD-1 Synthesis VPI-5

197 264 266 3 271 99 96 111 123 69 78 66 118 118 121 78 120 ,„„ 102 1__ 103 78 86 85 68 104 115 114 114

T Tectosilicate TEMPO Tetra-arylporphyrin Tetrahydroisoquinoline Theory Eisenman Thiophene Toluene alkylation

1 331 333 331 187 128 129

370

Transmission electron microscopy TS-1 TUD-1

227 4 129,237

U

Ultra-stable Y UTL

UV photoelectron spectroscopy

3 55,116 221

v

yp, yp, 5

. , 92 3, 92

X X-ray photoelectron spectroscopy Xylene isomerization

221 128

Z Zeolite advanced applications Zeolite bioapplications Zeolite coated electrodes Zeolite delaminated Zeolite detergency Zeolite films Zeolite framework types

263 272 277 118 197 273 42

Zeolite genesis 19 Zeolite history 2 Zeolite host 269 Zeolite ion-exchange 181 Zeolite layers 273 Zeolite medical applications 272 Zeolite membranes 135 Zeolite mesoporous 123,229 Zeolite mineralogy 22 Zeolite natural 1,8,13,18 Zeolite nuclear waste treatment 199 Zeolite occurrences 20 Zeolite optical applications 279 Zeolite pillared 118 Zeolite production 6 Zeolite spectroscopy 205 Zeolite storage materials 264 Zeolite structure database 55 Zeolite structures 41 Zeolite synthesis 65 Zeolite water treatment 198 Zeolite Y supercage 290 Zeolite ZK5 195 Zeolite 1 Zeolites extra-large pore 113 Zeotype synthesis 65 ZSM-11 50 ZSM-5 3,50,85,92,196,338

371 STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.

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Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1 4 - 1 7 , 1 9 7 5 edited by B. Delmon, P.A. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, w i t h Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts II.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-laNeuve, September 4 - 7 , 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32 n d International Meeting of the Societe de Chimie Physique, Villeurbanne, September 2 4 - 2 8 , 1979 edited by J. Bourdon Catalysis by Zeolites.Proceedings of an International Symposium, Ecully {Lyon), September 9 - 1 1, 1980 edited by B. Imelik, C. Naccache.Y. BenTaarit, J.C.Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, A n t w e r p , October 1 3 - 1 5 , 1 9 8 0 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7 th International Congress on Catalysis, Tokyo, June 3 0 - J u l y 4 , 1980. Parts A and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.l.Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyne, September 2 9 - O c t o b e r 3,1980 edited by M. Laznicka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 2 1 - 2 3 , 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 1 4 - 1 6 , 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A.Martin and J.C.Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 2 2 - 2 4 , 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jiru and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1 - 4 , 1982 edited by C.R. Brundle and H.Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets

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Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6 - 9 , 1982 edited by G. Poncelet, P. Grange and PA. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 1 2 - 1 6 , 1983 edited by G.M. Pajonk.S.J.Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9 - 1 3 , 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kaiansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9' h Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1 9 8 4 edited by S. Kaliaguine and A.Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 2 5 - 2 7 , 1984 edited by B. Imelik, C. Naccache, G. Coudurier.Y. Ben Taarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 2 8 - 2 9 , 1984 edited by M. Che and G.C.Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J.Koukal Zeolites:Synthesis, Structure.Technology and Application. Proceedings of an International Symposium, Portoroz-Portorose, September 3 - 8 , 1984 edited by B.Driaj.S. Hocevar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4 - 6 , 1985 edited by T.Keii and K.Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 1 5 - 1 9 , 1985 edited by DA. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerveny New Developments in Zeolite Science and Technology. Proceedings of the 7 th International Zeolite Conference, Tokyo, August 1 7 - 2 2 , 1986 edited by Y.Murakami.A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8 - 1 1 , 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-laNeuve, September 1 - 4 , 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P.Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A.Martens Catalyst Deactivation 1987. Proceedings of the 4 th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine

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Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 2 7 - 3 0 , 1987 edited by D.M. Bibby, C.D.Chang, R.F.Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J.Mortier, E.F.Vansant and G. Schuli-Ekloff Catalysis l987.Proceedings of the 10 th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of t h e IUPAC Symposium (COPS I), Bad Soden a, Ts., April 26-29,1987 edited by K.K.Unger, J. Rouquerol, K.S.W.Sing and H. Krai Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7 - 1 1 , 1987 edited by J.Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule.D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Pail Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30 th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H.Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wiirzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C.Morterra.A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8 * International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC,

November 27-December 2, 1988 V o l u m e 51 Volume 52

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edited by ML. Occelli and R.G.Anthony New Solid Acids and Bases.Their Catalytic Properties by K.Tanabe, M. Misono.Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by ]. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining I989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5 - 8 , 1989 edited by D.L.Trimm.S.Akashah, M.Absi-Halabi and A. Bishara Future Opportunities in Catalytic and Separation Technology edited b y M. Misono,Y.Moro-oka and S. Kimura

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Mew Developments in Selective Oxidation, Proceedings o f an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F.Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 2 3 - 2 5 , 1989 edited by T.Keii and K.Soga Spectroscopic Analysis of Heterogeneous Catalysts. Part A; Methods of Surface Analysis edited by J.L.G. Fierro Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.LG. Fierro Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of t h e 2 n d International Symposium, Poitiers, October 2 - 6 , 1990 edited by M. Guisnet, ] . Barrault, C. Bouchoule.D, Duprez, G. Perot, R.Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui.S. Namba and T.Tatsumi Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S.Kolboe Characterization of Porous Solids II. Proceedings of t h e IUPAC S y m p o s i u m (COPS II), Alicante, May 6 - 9 , 1990 edited by F, Rodriguez-Reinoso, ] , Rouquerol, K.S.W. Sing and K.K.Unger Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3 - 6 , 1990 edited by G. Poncelet, PA. Jacobs, P. Grange and B. Delmon New Trends in CO Activation edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings o f ZEOCAT 9 0 , Leipzig, August 20-23, 1990 edited by G . Ohlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of t h e Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonfured, September 10-14, 1990 edited by LI. Simandi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 2 2 - 2 7 , 1990 edited by R.K. Grasselli and AAVSIeight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Evanston, IL, June 2 4 - 2 6 , 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8 - 1 3 , 1991 edited by P.A. Jacobs, N.I. Jaeger, L.Kubelkova and B.Wichterlova Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova

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Catalysis and Automotive Pollution Control II. Proceedings of t h e 2 n d International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8 - 1 0 , 1991 edited b y P, Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12 th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 2 5 - 2 8 , 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission.Theory and Current Applications edited b y S.D. Kevan Mew Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P.Tetenyi Fluid Catalytic Cracking: Science and Technology edited by j.S.Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited b y T. Inui, K. FujimotoJ.Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings of t h e 3rd International S y m p o s i u m , Poitiers, April 5 - 8 , 1 9 9 3

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e d i t e d b y M. Guisnet, J. Barbier, j . Barrault, C. Bouchoule,D. Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited b y j.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, July 4 - 9 , 1993 edited by H.E. Curry-Hyde and R.F.Howe New Developments in Selective Oxidation II. Proceedings of t h e Second W o r l d Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 2 0 - 2 4 , 1993 edited b y V. Cortes Corberan and S.Vic Bellon Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 2 2 - 2 5 , 1993 edited byT. Hattori and T.Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited b y J.Weitkamp, H.G. Karge,H. Pfeifer and W. Hdlderich Advanced Zeolite Science and Applications edited b y J.C. Jansen, M. Stocker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slinko and N.I. jaeger Characterization of Porous Solids III, Proceedings of t h e IUPAC S y m p o s i u m (COPS III), Marseille, France, May 9 - 1 2 , 1993 edited by J.Rouquerol, F. Rodriguez-Rdnoso, K.S.W. Sing and K.K.Unger

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Catalyst Deactivation 1994. Proceedings of the 6 th International Symposium, Ostend, Belgium, October 3-5, 1994 edited b y B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings o f t h e International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K.Soga and M.Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2 - 4 , 1993 edited b y H. Hattori.M. Misono and Y.Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited b y G. Poncelet, j.Martens.B. Delmon, P.A. Jacobs and P. Grange Science and Technology in Catalysis 1994. Proceedings of t h e Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H.Arai and M. I warn o to Characterization and Chemical Modification of the Silica Surface by E.F.Vansant, P.Van Der Voort and KX.Yrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9 - 1 3 , 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B.Nagy Catalysis by Metals and Alloys by V. Ponec and G.C.Bond Catalysis and Automotive Pollution Control III. Proceedings of the Third International Symposium (CAPoC3S, Brussels, Belgium, April 2 0 - 2 2 , 1994 edited by A. Frennet and J.-M. Bastin Zeolites:A Refined Tool for Designing Catalytic Sites. Proceedings of the International Symposium, Quebec, Canada, October 15-20, 1995 edited b y L. Bonneviot and S. Kaliaguine Zeolite Science I994: Recent Progress and Discussions. Supplementary Materials t o t h e 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited b y H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A.Dabrowski and V.A.Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 199S. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26, 1995 edited by M.Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus I I * International Congress on Catalysis - 4 0 * Anniversary. Proceedings of the 11 t h ICC, Baltimore, MD, USA, June 30-July 5, 1996 edited by J.W. Hightower.W.N. Delgass, E. Iglesiaand A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon.S.I.Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited b y P.V. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W . Rudziriski.W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings of the 11 t h International Zeolite Conference, Seoul, Korea, August 12-17, 1996 edited by H. Chon,S.-K. Ihm and Y.S.Uh

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Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 1 s ' International Symposium / 6 th European Workshop, Oostende, Belgium, February 17-19, 1997 edited b y G.F. Froment.B. Delmon and P. Grange Natural Gas Conversion IV Proceedings of the 4 th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23, 1995 edited by M. de Pontes, R.L. Espinoia, C.P. Hicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings of the 4 th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12, 1996 edited by H.U. Blaser, A. Baiker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings of the International Symposium, Antwerp, Belgium, September 15-17, 1997 e d i t e d b y G.F. Froment and KX.Waugh Third World Congress on Oxidation Catalysis. Proceedings of the Third World Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited b y R.K. Grasselli.S.T.Oyama, A.M. Gaffney and J.E. Lyons Catalyst Deactivation I997. Proceedings of the 7"1 International Symposium, Cancun, Mexico, October 5-8, 1997 e d i t e d b y C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings of the 4 th International Conference on Spillover, Dalian, China, September 15-18, 1997 e d i t e d by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings of the 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4, 1997 e d i t e d b y T.S.R. Prasada Rao and G.Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings of the 4 th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997 e d i t e d b y T. Inui, M.Anpo,K. lzui,S.Yanagida and T.Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis and Automotive Pollution Control IV. Proceedings of the 4 th International Symposium !CAPoC4), Brussels, Belgium, April 9-11, 1997 edited b y N. Kruse, A. (rennet and J.-M. Bastin Mesoporous Molecular Sieves 1998 Proceedings of the 1 s ' International Symposium, Baltimore, MD, U.S.A., July 10-12, 1998 e d i t e d b y L.Bonneviot, F. Beland, C.Danumah, S. Giasson and S. Kaliaguine Preparation of Catalysts VII Proceedings of the 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-Ia-Neuve, Belgium, September 1-4, 1998 e d i t e d by B. Delmon, P.A. Jacobs, R. Maggi, J.A-Martens, P. Grange and G. Poncelet Natural Gas Conversion V Proceedings of the 5th International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25, 1998 edited by A. Parmaliana, D. Sanfilippo, F. Frusteri, A.Vaccari and F,Arena Adsorption and its Applications in Industry and Environmental Protection. Vol I: Applications in Industry edited by A. Dijbrowski

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Adsorption and its Applications in Industry and Environmental Protection. Vol II: Applications in Environmental Protection e d i t e d b y A. Dsjbrowski Science and Technology in Catalysis 1998 Proceedings of the Third Tokyo Conference in Advanced Catalytic Science and Technology, Tokyo, July 19-24, 1998 edited by H. Hattori and K. Otsuka Reaction Kinetics and the Development of Catalytic Processes Proceedings of the International Symposium, Brugge, Belgium, April 19-21, 1999 edited by G.F. Froment and K.C.Waugh Catalysis:An Integrated Approach Second, Revised and Enlarged Edition edited by R.A. van Santen, P.W.N.M. van Leeuwen, J.A. Moulijn and B.A.Averill Experiments in Catalytic Reaction Engineering by J.M. Berty Porous Materials in Environmentally Friendly Processes Proceedings of the 1 s t International FEZA Conference, Eger, Hungary, September 1-4, 1999 edited b y I. Kiricsi, C. Pal-Borbely, J.B.Nagy and H.G. Karge Catalyst Deactivation 1999 Proceedings of the 8th International Symposium, Brugge, Belgium, October 10-13, 1999 e d i t e d b y B. Delmon and G.F. Froment Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2nd International Symposium/7th European Workshop, Antwerpen, Belgium, November 14-17, 1999 edited by B. Delmon, G.F. Froment and P. Grange Characterisation of Porous Solids V Proceedings of the 5 th International Symposium on the Characterisation of Porous Solids (COPS-V), Heidelberg, Germany, May 30- June 2, 1999 e d i t e d by K.K.Unger,G.Kreysa and J.P. Baselt Nanoporous Materials II Proceedings of the 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari.M. Jaroniec and T.J. Pinnavaia 12 "' International Congress on Catalysis Proceedings of the 12 th ICC, Granada, Spain, July 9-14, 2000 edited byA. Corma, F.V. Melo.S. Mendioroz and J.L.G. Fierro Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Natta and Ring-Opening Metathesis Polymerization By V. Dragutan and R. Streck Proceedings of the International Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8,2000 25 ' h Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan e d i t e d b y Y. Iwasawa, N.Oyama and H.Kunieda Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings of the 3rd International Symposium, Oostende, Belgium, April 2225, 2001 e d i t e d b y G.F. Froment and K.C.Waugh Fluid Catalytic Cracking V Materials and Technological Innovations edited by M.L Occelli and P. O'Connor

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Zeolites and Mesoporous Materials at the Dawn of the 21 "Century. Proceedings of the 13lh International Zeolite Conference, Montpellier, France, 8-13 July 2001 edited by A. Galameau, F. di Renso, F. Fajula ans J. Vedrine Natural Gas Conversion VI Proceedings of the 6lh Natural Gas Conversion Symposium, June 17-22, 2001, Alaska, USA. edited by J.J. Spivey, E. Iglesia and T.H. Fleisch Introduction to Zeolite Science and Practice. 2nd completely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, P.A. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces edited by A. Guerrero-Ruiz and I. Rodriquez-Ramos Catalyst Deactivation 2001 Proceedings of the 9lh International Symposium, Lexington, KY, USA, October 2001 edited by J.J. Spivey, G.W. Roberts and B.H. Davis Oxide-based Systems at the Crossroads of Chemistry. Second International Workshop, October 8-11, 2000, Como, Italy. Edited by A. Gamba, C. Colella and S. Coluccia Nanoporous Materials III Proceedings of the 3* International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by A. Sayari and M. Jaroniec Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium Proceedings of the 2"a International FEZA (Federation of the European Zeolite Associations) Conference, Taormina, Italy, September 1-5, 2002 edited by R. Aiello, G. Giordano and F.Testa Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the 8th International Symposium, Louvain-la-Neuve, Leuven, Belgium, September 9-12, 2002 edited by E. Gaigneaux, D.E. De Vos, P. Grange, P.A. Jacobs, J.A. Martens, P. Ruiz and G. Poncelet Characterization of Porous Solids VI Proceedings of the 6lh International Symposium on the Characterization of Porous Solids (COPS-Vi), Alicante, Spain, May 8-11, 2002 edited by F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. linger Science and Technology in Catalysis 2002 Proceedings of the Fourth Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, July 14-19, 2002 edited by M. Anpo, M. Onaka and H. Yamashita Nanotechnology in Mesostructured Materials Proceedings of the 3rd International Mesostructured Materials Symposium, Jeju, Korea, July 8-11, 2002 edited by Sang-Eon Park, Ryong Ryoo, Wha-Seung Ahn, Chul Wee Lee and Jong-San Chang Natural Gas Conversion VII Proceedings of the 7th Natural Gas Conversion Symposium, Dalian, China, June 6-10, 2004 edited by X. Bao and Y. Xu Mesoporous Crystals and Related Nano-Structured Materials Proceedings of the Meeting on Mesoporous Crystals and Related Nano-Structured Materials, Stockholm, Sweden, 1-5 June, 2004 edited by O. Terasaki Fluid Catalytic Cracking VI: Preparation and Characterization of Catalysts Proceedings of the 6th International Symposium on Advances in Fluid Cracking Catalysts (FCCs), New York, September 7 - 1 1 , 2003 edited by M. Occelli

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Coal and Coal-Related Compounds Structures, Reactivity and Catalytic Reactions edited by T. Kabe, A. Ishihara, E.W. Qian, I.P. Sutrisna and Y. Kabe Petroleum Biotechnology Developments and Perspectives edited by R. Vazquez-Duhalt and R. Quintero-Ramlrez Fischer-Tropsch Technology edited by A. P. Steynberg and M.E. Dry

Volume 153

Carbon Dioxide Utilization for Global Sustainability Proceedings of the 7th International Conference on Carbon Dioxide Utilization (ICCDU VII), October 12-16, 2003 Seoul, Korea edited by S.-E. Park, J.-S. Chang and K.-W. Lee

Volume 154

Recent Advances in the Science and Technology of Zeolites and Related Materials Proceedings of the 14th International Zeolite Conference, Cape Town, South Africa, 25-30th April 2004 edited by E. van Steen, L.H. Callanan and M. Claeys

Volume 155

Oxide Based Materials New Sources, Novel Phases, New Applications edited by A. Gamba, C. Colella and S. Coluccia

Volume 156

Nanoporous Materials IV edited by A. Sayari and M. Jaroniec