Post-Synthesis Modification I (Molecular Sieves) (v. 1)

Preface to Volume 3

For many purposes, zeolites and related materials are not utilized in the as-synthesized form. Rather, they are only employed after an appropriate post-synthesis modification. Undoubtedly, the classic procedure of zeolite treatment after synthesis is that of ion exchange achieved through treatment of a suspension of the as-synthesized (or natural) zeolite powder (usually in the sodium or potassium form) in an aqueous solution of a salt containing the cations to be introduced. Starting in the 1930s, this type of ion exchange has been extensively studied, not only as a method of preparation, but also with respect to thermodynamics and kinetics. Application on an industrial scale is well developed and, because of its importance, ion exchange in zeolites has been reviewed several times. Thus, the first chapter of Volume 3 of the series “Molecular Sieves – Science and Technology”, which was contributed by R.P. Townsend and R. Harjula, was able to focus on the developments and advances made during the last decade. It emphasizes the need for improvement of theoretical approaches, utilization of the rapidly growing computational power, and the importance of acquiring reliable data as the bases for progress in fundamental studies on conventional ion exchange. The more recent development of solid-state ion exchange and related modification techniques such as reactive ion exchange between solid zeolite powders and solid or gaseous compounds containing the cations we wish to introduce is rather exhaustively dealt with in the subsequent chapter written by H.G. Karge and H.K. Beyer. The concept of solid-state ion exchange is explained and contrasted to the conventional exchange process. Experimental procedures as well as techniques for monitoring the solid-state modification of zeolites are described in great detail and illustrated by a large number of investigated systems. Related methods of post-synthesis modification, possible mechanisms, and first approaches to study the kinetics of solid-state ion exchange are discussed. Post-synthesis modification of zeolites via alteration of the aluminum content of the framework became a most important topic of zeolite chemistry when, in the mid 1960s, the effect of stabilization through dealumination was discovered. In Chapter 3, H.K. Beyer contributes a systematic review on techniques for the dealumination of zeolites by hydrothermal treatment or isomorphous substitution amended by a section on the reverse process, i.e., introduction of aluminum into and removal of silicon from the framework. Methods of post-synthesis modification essentially different from those discussed in the first three chapters are based on the generation of extra-frame-

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work aggregates of metals (as presented in the chapter by P. Gallezot), ionic clusters (as described in the contribution by P.A. Anderson), and oxides and sulfides (treated in the last chapter written by J. Weitkamp et al.). One of the main motivations for studying the generation of such clusters inside the void volume of zeolite structures originates, of course, from possible applications in catalysis. This is most evident in the case of metal cluster/zeolite systems which are successfully employed in heterogeneous catalysis of hydrogenation, hydrocracking, hydroisomerization, etc. However, both ionic clusters and oxidic and sulfidic clusters hosted by the frameworks of zeolites are interestring candidates as catalysts for base-catalyzed, redox, photocatalyzed and perhaps other reactions. In view of cluster formation with zeolites as hosts, questions of size, location, distribution, interaction with the framework, and stabilization of the active aggregates play a decisive role. Thus, in all three contributions on clusters in zeolites, methods of their preparation as well as problems of their characterization and utilization as catalysts and photosensitive materials, as sensors, in optics, and electronics are extensively dealt with. These areas are still challenging for future resarch and promising in view of potential applications. However, not all important phenomena of post-synthesis modification are covered with the present six chapters of Volume 3 of the series ‘Molecular Sieves – Science and Technology’. Topics such as, for instance, ‘Incorporation of Dyes into Molecular Sieves’, ‘Preparation of Ship-in-the-Bottle Systems’, ‘Secondary Synthesis in Zeolites’, ‘Pore Size Engineering’, ‘Modification of Mesoporous Materials’ are equally important and, to a large extent, presently subject to very active research and development. Therefore, such topics will be dealt with in one of the subsequent volumes under the title ‘Post-Synthesis Modification II’. September 2001

Hellmut G. Karge Jens Weitkamp

Ion Exchange in Molecular Sieves by Conventional Techniques Rodney P. Townsend 1, Risto Harjula 2 1 2

Scientific Affairs, Royal Society of Chemistry, Burlington House, Piccadilly, London W1J 0BA, UK; e-mail: [email protected] Laboratory of Radiochemistry, PO Box 55, 00014 University of Helsinki, Finland; e-mail: [email protected]

Dedicated to Professor Gerhard Ertl on the occasion of his 65th birthday

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Introduction

1.1 1.2

The Importance of Ion Exchange Phenomena in Molecular Sieves Origin and Nature of Ion Exchange Behaviour in Molecular Sieves

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The Importance and Utility of Theoretical Approaches

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2.1 2.2 2.3

Preference, Uptake and Selectivity . . . . . . . . . . . . . . . Batch and Column Exchange Operations . . . . . . . . . . . . Thermodynamic Parameters, Non-Ideality and the Prediction of Exchange Compositions . . . . . . . . . . . . . . . . . . . 2.4 Kinetic Processes and the Prediction of Rates of Exchange . . 2.4.1 Hierarchical Model of Zeolite Particle or Pellet . . . . . . . . 2.4.2 Intraparticular Exchange Rate Processes . . . . . . . . . . . . 2.5 Trace Ion Exchange . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Column Models . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.1 3.2 3.2.1 3.2.2 3.2.3

Practical Experiments . . . . . . . Pitfalls . . . . . . . . . . . . . . . . Selectivity Reversal and Ion Sieving Zeolite Hydrolysis Effects . . . . . Colloidal Solids in Suspension . .

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Molecular Sieves, Vol. 3 © Springer-Verlag Berlin Heidelberg 2002

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1 Introduction 1.1 The Importance of Ion Exchange Phenomena in Molecular Sieves

Throughout the 1990s there was a decline in the number of fundamental studies carried out on the ion exchange properties of zeolites and related materials. One has only to examine the content of published conference proceedings on the subject over the last 20 years to observe this trend: the situation has moved from one where whole sessions were devoted to ion exchange studies, to one where the subject is subsumed into sessions covering other areas. Part of this decline is to be expected, as increased attention has been rightly paid to the intriguing possibilities that can arise through the exploitation of newer alternative postsynthesis methodologies, many of which are discussed elsewhere in this volume. Nevertheless, the fact remains that conventional ion exchange techniques continue to be used routinely for post-synthesis modification during the preparation of molecular sieves for major industrial applications. Also, there are now areas where molecular sieves find major application directly as ion exchangers per se. In this respect the situation has changed markedly since the early 1960s, when Helfferich, in his classic book on ion exchange, could justifiably describe zeolites “as ion exchangers they are of little practical importance” [1]. These direct applications are especially detergency [2–7] and also the removal of nuclear waste [8–13] or other environmental pollutants [3]. However, it is generally a combination of properties of a particular zeolite in addition to its ion exchange capability that has tipped the balance in favour of its use, rather than any intrinsic superiority per se, which the zeolite may possess as an ion exchanger. If, therefore, conventional ion exchange remains an important post-synthesis preparative technique, and the materials have in addition major direct applications as ion exchangers, why have the number of fundamental studies decreased? It is certainly not because ion exchange behaviour of molecular sieves is sufficiently well understood and predictable to render further fundamental research studies unnecessary. Two causes are suggested to explain this decline: 1. Many theoretical treatments of the ion exchange reaction within zeolites (both equilibrium and kinetic) are obscure and complicated. This has without doubt rendered inaccessible the real value of the work to those many workers who have a practical need to predict and control ion exchange behaviour during the industrial exploitation of molecular sieves. Although theoretical understanding is important, it is easy to forget that the end purpose of such work should be to provide information and tools that the chemical engineer or other user of the molecular sieve can apply simply and effectively. Obscurities in theoretical treatments mean that users often do not appreciate how basic theory can be used, not just to simplify the number of measurements which need to be made, but also to predict and control behaviour during application. The theory should not be an end in itself!

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2. The second cause is related to the first. Even where the value of theory for the prediction and control of the behaviour of these materials has been recognised, the utility of these approaches has often been greatly reduced because of the experimental methods which have been employed or by the poor experimental data which have been available, or both. Indeed, it is only comparatively recently that a proper recognition has arisen concerning the number of potential pitfalls and difficulties that can militate against the acquisition of meaningful and accurate experimental data. A good example of this is the frequently studied Na/Ca-zeolite A system, which has received much attention because of its importance in detergency applications. Careful and detailed experimental studies over a period spanning some 20 years by different sets of workers [14–20] resulted in calculated values of the standard free energy of exchange (kJ equiv–1*) which ranged from –0.59 [14] to –3.09 [17]. Plots of the corrected selectivity coefficient (defined below; see

E Fig. 1. Plots of the logarithm of the corrected selectivity coefficient ln KG [cf. KA/B in – Eq. (7b)] as determined by different workers for the Na/Ca exchange in zeolite A. ECa is the equivalent fraction of calcium in the zeolite [(Eq. (3b)]. BRW Barrer, Rees and Ward [14]; A Ames [15]; WF Wolf and Furtig [16]; SW Sherry and Walton [17]; BR Barri and Rees [18]; WGC Wiers, Grosse and Cilley [19]; FT Franklin and Townsend [20]. Taken from [8]

* Throughout this paper the term “equiv” denotes 1 mol of unit negative or positive charges.

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Eq. 7b) naturally show a similar diversity but also differ from each other in curve shape and trends (Fig. 1). These marked differences (particularly at the extrema of the plots) were variously ascribed to experimental error [20], to variable quantities of non-exchangeable sodium in the materials employed [20] (the materials differed in their source and in their method of preparation [14–20]) or to variable levels of hydronium exchange depending on the pH and other conditions used [20, 21]. Thus, even for this very important example, not only is some of the published theoretical work difficult to interpret, but also experimental data from different studies are frequently incompatible and incomplete. It is essential therefore that a critical review of advances over the last decade should look at the developments in the context of the field as a whole. This is our intention here. After a discussion of the origin, ubiquity and nature of ion exchange behaviour in molecular sieves, recent advances in the application of thermodynamic and kinetic descriptions of the ion exchange process will be described. This will demonstrate some of the shortcomings of current approaches, together with the relative paucity of reliable literature data that can be applied easily and practically. This whole topic has particular relevance to those industrial applications where zeolites are used directly as ion exchange materials and this will be exemplified throughout the chapter using two main examples. The first of these is the application of A- and P-type zeolites as detergent builders, where the approach is to use a batch exchange approach to remove hardness ions (especially calcium) as fast as is practicable before the indigenous water hardness harms the wash performance of the detergent product. The second concerns the treatment of nuclear waste, where a variety of higher silica zeolites have been employed using a continuous (column) process to remove, and subsequently store, high concentrations of monovalent and divalent radionuclides such as caesium and strontium. For both these major applications, in addition to selectivity, it is noteworthy that the systems are normally multicomponent, that the kinetics of exchange are all important and that the morphology of the exchanger material must be controlled carefully. Post-synthesis modification comes into its own when preparing molecular sieves with desirable and exploitable properties other than those of ion exchange, be they optical, magnetic, catalytic or adsorptive. Here it is not directly the thermodynamic and kinetic ion exchange properties that are of prime importance but rather which experimental, preparative methods are most commonly used. Thus it is important to assess what are the most appropriate experimental methods of preparation, as well as to review the many pitfalls one can fall into which can subsequently give rise to very inaccurate and inadequate experimental data. These experimental problems can include framework hydrolysis, hydronium exchange, dealumination, the presence of key trace impurities, dissolution phenomena, carbonate and bicarbonate interference, colloidal phenomena, metal ion complex formation and cation hydrolysis. Having thus reviewed developments and advances over the last decade, the chapter concludes with some recommendations on directions and topics for this area of research in the future.

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1.2 Origin and Nature of Ion Exchange Behaviour in Molecular Sieves

Ion exchange is a characteristic property manifested by most molecular sieves. In essence, whenever isomorphous replacement of one cation by another of different charge occurs within an initially neutral crystalline framework such as a pure silica molecular sieve, then a net electrical charge remains dispersed over that framework. This is neutralised through the presence, within the microporous channels, of cations of opposite charge (often referred to as counterions). An example of this is seen in the introduction by direct synthesis of small quantities of aluminium into the silicalite framework to give the material ZSM-5. Silicalite, the pure silica analogue of ZSM-5, is then seen to be just the end-member of a set of isomorphous microporous molecular sieves that exhibit ion exchange properties which are a function of the quantity and distribution of aluminium atoms within the structurally similar frameworks. In addition, since one can prepare, through post-synthesis modification of the framework composition, a variety of other isomorphous metallosilicates and metal aluminosilicates, it is obvious that zeolites possessing ion exchange capabilities are a common occurrence. Pure aluminium phosphate molecular sieves are probably more common than are pure silica analogues of zeolites. They resemble pure silica zeolites in that they possess frameworks that are electrically neutral, but there is a significant difference between these two classes of inorganic solids. In topological terms both are 4:2 connected nets of T:O atoms (“T” denoting tetrahedral framework and “O” denoting oxygen). From this it is obvious that it is only required for the T ion to have a charge of +4 for the connectivity of the net to give rise naturally to a neutral framework in concert with the oxide anions. This is fulfilled for pure silicalite. In the case of ALPO molecular sieves the requirement is also fulfilled, but the 4:2 T:O net now comprises two types of strictly alternating T-cations (aluminium and phosphorus, possessing respectively formal positive charges of 3 and 5). Providing the cations alternate strictly throughout the framework, the 4:2 Al,P:O net holds no overall charge; however, in contrast to a pure silica zeolite, where the formal charge at every atomic centre is zero, within a pure AlPO the formal charge is not dispersed homogeneously, but changes from –1 at each aluminium to +1 at each phosphorus. This greater heterogeneity of charge distribution may in part explain the experimental observation that ALPOs frequently exhibit poorer thermal stability than do pure silica zeolites. For a particular ALPO molecular sieve to possess an ion exchange capacity as an intrinsic property, it is necessary to prepare a material where some of the aluminium and/or phosphorus framework atoms have been replaced by other atoms of different charge. This can occur using for example silicon, to form the so-called SAPO materials, or with metals in addition or not to silicon, to form respectively the so-called MeAPSO and MeAPO analogues. However, it is important to note that although silicon could in principle replace either aluminium or phosphorus to give rise to positively or negatively charged SAPO molecular sieves, respectively, in practice only the latter process seems to occur, or another

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process in which two silicons replace one of each of aluminium and phosphorus, which gives rise to no net change in framework charge [22]. In MeAPSOs, divalent or trivalent metal ions replace the aluminiums in the framework. In this way the charge imbalance is minimised as these isomorphous substitutions either make no difference to the overall framework charge (T3+ for Al3+) or only increase it by one negative charge per substitution (e.g. Mg2+ for Al3+), a process analogous to when aluminium replaces silicon in aluminosilicates [22]. Overall therefore, and in common with aluminosilicate zeolites, the norm is for MeAPSOs and MeAPOs to possess cation exchange properties rather than the reverse. In this respect, zeolites and ALPOs resemble many other classes of ion exchangers that are mineralogical in origin, such as the clay minerals. These are layered materials where a cation exchange property can arise primarily from isomorphous replacement of trivalent cations by divalent, or tetravalent cations by trivalent ones, within the layers [23]. However, there is a major exception: these anionic exchangers are the double metal hydroxides, which are also layered structures but which exhibit a net positive charge across the lattice. The “parent” material here is the mixed Mg,Al hydroxide, commonly referred to as hydrotalcite. It would be intriguing to understand better the conditions (if any) under which one might expect to synthesise microporous three-dimensional framework structures which similarly have a net positive charge dispersed over the lattice and hence an anion exchange capacity coupled with a molecular sieve capability. It is important to note that, up to this point, we have been considering the zeolite, ALPO, SAPO, etc., as being described adequately as a 4:2 T:O net. This topological description, which in general terms is, as Smith points out [24], nothing more than a mathematical construct of the human brain, does nevertheless allow us to appreciate both the origin and magnitude of an ion exchange capacity arising from T-atoms being replaced by others of different charge. However, this description is not sufficient to cover the observed differences in ion exchange properties (i.e. selectivity, kinetic rate, level of exchange) that may be seen between various molecular sieves having similar exchange capacities. To understand these differences, one must not only examine more closely the topological properties of the nets but also bring to bear structural considerations. Considering these topological properties in more detail, it is adequate at this point to take as read that all the T-atoms within the microporous net are joined to each other by bridging oxygens. One can therefore concentrate on the Tatoms only and describe molecular sieves in terms of four-connected threedimensional (4-conn.3D) nets of T-atoms [25] that, in turn, can be derived from appropriate 3-conn.2D nets [26]. Considering the latter nets first, these differ from one another in the ways the nodes (T-atoms) link to each other via networks of polygons. Any node can then be described by its “vertex symbol”, viz. by its surrounding polygons with the number of each type of similar polygon surrounding the node being denoted by a superscript [26]. Thus the simplest example of a 3-conn.2D network (the hexagonal net) becomes a 63-net; a more complicated example could be the 4.6.12-net which forms the basis for the gmelinite structure [26]. Note that all the nodes within each of these two separate examples are topologically equivalent. This need not be the case. For exam-

Ion Exchange in Molecular Sieves by Conventional Techniques

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Fig. 2. Structure of mordenite viewed along the main 8-ring and 12-ring channels parallel to the c-axis. Four topologically distinct types of T-atoms are observed within the 3-conn.2D (4.5.8)1 (4.5.12)1 (5212)1 (5.8.12)1 net

ple, consider the case of mordenite (Fig. 2), which is derived from a (4.5.8)1 (4.5.12)1 (5212)1 (5.8.12)1-net containing four topologically distinct types of Tatoms [24]. Similar considerations apply when one considers the 4-conn.3D nets that constitute molecular sieves. Here it is often convenient to describe the structure in terms of polyhedral units or cages, with the polyhedra described topologically in terms of face symbols [25] (not to be confused with vertex symbols defined above). Thus the face symbol for the familiar sodalite unit, which is geometrically a truncated octahedron, is 4668 with all vertices geometrically and topologically equivalent. If these units are then linked together, for example either through their 4-windows or half their 6-windows, one forms respectively the zeolite A and faujasitic structures. Both these structures possess cubic symmetry, with each structure comprising 26-hedral cages connected to each other throughout the microporous zeolite framework, but the vertices of the sodalite units are no longer all topologically equivalent. For zeolite A the sodalite units enclose a cage which is the great rhombicuboctahedron (4126886) [25] whereas for faujasite the cage is the so-called 26-hedron type II, denoted by the face symbol 4641264124 [25]. Why are these matters significant when one considers the ion exchange properties of molecular sieves? The answer is that these topologically non-equivalent T-atoms combined with the overall structural properties of the three-dimensional microporous framework often give rise to several very different types of local environments which repeat themselves regularly throughout the crystalline structure. These different local environments, evidenced by solid state NMR combined with X-ray crystallography [27], are distinct in themselves, differing from each other sterically and electronically, and these differences will be

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manifested not only through their characteristic adsorptive and catalytic behaviour, but also through their ion exchange properties. Formally, therefore, zeolites may be regarded as comprising a set of crystallographically distinct sublattices, each having characteristic selectivities for different exchanging cations, depending on these local environments [28]. The overall ion exchange behaviour of a molecular sieve can therefore be a subtle function of the structural and topological properties combined. An important combination of structural and topological properties concerns the ordering of isomorphously substituted framework atoms [29]: this determines what fraction of the overall framework charge is found on each sublattice. Other significant structural properties can be losses in symmetry through restricted rotation [27], and whether the sites are accessible to exchanging cations (i.e. the sizes of the micropore channels allowing ingress and egress of exchanging cations plus water). A further point is worth emphasising: since site heterogeneity in a particular zeolite is manifested through such a set of crystallographically distinct sublattices, zeolites differ in this respect significantly from some other common classes of ion exchangers, such as the clay minerals or the resins. Whereas in zeolites well-defined sites are repeated regularly through the crystalline matrix, in clay minerals and resins site heterogeneity is often manifested in terms of patches, or regions of the surface where the sorption energies are approximately constant [30]. Thus a statistical thermodynamic model of ion exchange for clay minerals and resins [30] can differ markedly in character from ones developed for zeolites [31, 32]. As a consequence of all these factors combined, both the equilibrium and kinetic aspects of selectivity and uptake of ions within molecular sieves can rarely be understood in a straightforward manner. Phenomena which have received either considerable attention in recent years or deserve further study include the so-called “ion sieve effect”, behaviour of high silica materials, the effects that framework flexibility can have on selectivity and rates of exchange, multicomponent ion exchange, prediction of exchange equilibria, and the possibility of inducing phase transitions within zeolites through ion exchange. Many of these are considered further below. So far we have considered topological and internal structural factors which give the molecular sieve particular ion exchange properties. However, an ion exchange capacity can also be manifested which is not an intrinsic property of the material. The source of this property is unsatisfied valencies occurring at the termination of the crystal edges and faces, or at faults within the crystalline structure. In formal terms, the origin of this is topological, in that this incidental and secondary property arises from disruptions in the net at interfaces, surfaces and faults, but the nature and extent of this incidental property depends essentially on structural and morphological characteristics. For the former, we can take as an example an ion exchange capacity arising either from the presence of silanol groups [33, 34], or from hydroxyl groups attached to aluminium atoms situated at the surface [35]. In clay minerals, as much as a fifth of the total exchange capacity may arise from such sources whereas in the case of zeolites the contribution of such incidental (or secondary) ion exchange properties is usually small compared to the intrinsic, or primary source. The exception here

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can be high silica zeolites [35, 36], whose overall ion exchange properties have received considerable attention over the last decade [37–42]. Interestingly, the external morphology can also be an important factor in determining ion exchange behaviour of molecular sieves. The crystal habit, the average crystallite size, the distribution of crystallite sizes and the properties of aggregates of crystallites can all affect the magnitude of secondary ion exchange characteristics, since these can alter significantly the surface to volume aspect ratio and hence the number of external surface sites available [35]. Also, the kinetic properties may depend on these morphological characteristics, as instanced by recent studies on a highly aluminous form of zeolite P [6, 7].

2 The Importance and Utility of Theoretical Approaches When a zeolite in (say) the sodium-exchanged form is suspended in a solution comprising a mixture of different cations and anions, two properties of the material are brought into sharp focus. The first of these concerns which types of cations are “preferred” over sodium or each other by the zeolite. This property is commonly referred to as the selectivity of a given form of zeolite for another cation, but there are so many definitions of “selectivity” that the term “preference” may be better used for the present. The second key property to which one’s attention is drawn, and which is separate from selectivity (however defined), is the rate at which the mixture of cations achieves its equilibrium distribution between the exchanging phases (viz., the electrolyte solution and the sublattices within the zeolite). 2.1 Preference, Uptake and Selectivity

The preference manifested by a molecular sieve for a particular cation is strongly dependent not only on the character of the material under examination, but also on the conditions of the system as a whole (viz., temperature, perhaps pressure, composition of exchanger and solution phases, pH, nature of solvent, etc.). Given a comprehensive definition of these conditions, the preference of a given form of zeolite for a given cation will then be invariant for that set of conditions because it is essentially an equilibrium property of the system. However, it is important to define clearly what is meant by “preference”. There are numerous selectivity coefficients defined in the literature and, on occasion, “selectivity coefficient” is confused with “separation factor”, a function whose value does depend strongly on the total ion concentration in solution. Similarly,“uptake” or “loading” is often confused with “capacity”. To distinguish these terms, a few basic definitions are required. Considering as an example a binary exchange involving cations A (valency zA) and B (valency zB), the reaction equation is usually written as: – – (1) zA B zB + zB AzA = zA BzB + zB AzA

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where the overbars denote the exchanger phase. The preference displayed by the zeolite for one ion over another is then described by a selectivity coefficient, which is just a mass action quotient. According to the choice of concentration units, a series of these selectivity coefficients may be defined which differ numerically from one another: – –x zB c zA EAzB cBzA c–AzB cBzA A B x E kA/B = 0 ≠ kA/B = 01 (2) – –x zAc zB ≠ k A/B = 01 cAzB c–BzA EBzA cAzB B A where cA , cB are the cation concentrations in solution (mol dm–3) and the corresponding concentrations in the molecular sieve are indicated with an overbar (equiv kg–1 dry exchanger). The definition of kA/B is consistent with IUPAC recommendations [43] but is not very convenient for zeolites because of the signifX and k E are selectivity coefficients in which the zeolite icant water content. kA/B A/B phase cation concentrations are defined in terms of the mole fraction and equivalent fraction E, respectively: –– (3a) X A = c–A / Si c–i . – (3b) E = z c– / S z c– . A

A A

i i i

When zA = zB = zi , then equivalent and mole fractions are numerically identical E . Otherwise, these functions are not numerically identical. In pracand k XA/B = kA/B E tice, kA/B has been used most extensively for studies on zeolites. The selectivity coefficients given in Eq. (2) can be used to derive more fundamental equilibrium properties of the system, such as the standard thermodynamic functions describing the exchange reaction (viz. DGq, DHq, DSq), provided one has information on the nature and extent of all activity corrections for nonideality. However, the key point to note is that having defined the reference states, by contrast with a selectivity coefficient, these standard thermodynamic functions are independent of exchanger composition since they refer by definition to a reaction between components which move from one set of specific, defined standard states to another. The magnitudes and signs of these standard functions therefore give no immediate information whatsoever on the actual preference which a zeolite may display for a particular ion under a given set of experimental conditions. This point, obvious to the thermodynamicist, has often been missed, and effort has been invested uselessly in attempting to relate calculated values of standard thermodynamic functions to mechanistic theories of exchange under real conditions. This has resulted in work being published that is of little practical utility, if not plainly wrong. The issue of misunderstanding and consequently misusing thermodynamic data in this manner is expanded elegantly by McGlashan [44]. The selectivity of a particular molecular sieve for a given ion as a function of exchanger composition is normally measured from an ion exchange isotherm, which is an isonormal [45], isothermal and reversible plot of equilibrium distributions of ions between the solution and zeolite phases. It is emphasised that it is only valid to calculate selectivity coefficients, and derived thermodynamic data, from isotherms which are reversible (that is, the forward and reverse isotherms coincide within experimental uncertainty). The types of isotherms,

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Fig. 3. Examples of ion exchange isotherms exhibiting both unselective and selective behaviour towards the incoming ion A (curves are respectively convex and concave with respect to the ordinate). Clear limits to exchange are also observed which are lower than those expected on the basis of the theoretical exchange capacity of the zeolite. The arrows depict reversible behaviour

Fig. 4. Example of an ion exchange isotherm showing non-reversibility of exchange within a plateau region, characteristic of phase separation and coexistence of two phases over the composition range corresponding to hysteretic behaviour

and the causes for the shapes observed, are discussed elsewhere [45]. However, two isotherm types, which are particularly characteristic of molecular sieves (although not uniquely so), are shown in Figs. 3 and 4. Figure 3 shows isotherms for which only partial exchange for the incoming cation occurs. The isotherm plots enable one to distinguish clearly various basic definitions. Taking, for example, a constant level of exchange or uptake for – an incoming ion (e.g. EA = 0.5, then for this given uptake, the selectivity coefficient can vary from low to high values (cf. the two depicted curves). The abscis– sa of the isotherm ranges from EA = 0 to EA = 1; values of EA are determined by dividing the uptake by the ion exchange capacity, which is the number of exchange sites of unit charge per unit quantity of exchanger (defined as con-

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venient – see comments on this above). However, Fig. 3 shows curves which are – asymptotic to values of EA < 1, demonstrating that the maximum uptake (or loading) under specified experimental conditions for the incoming cation can be less than what would be expected from the value of the ion exchange capacity. The cause of this may be due to inadequate experimental rigour, especially during batch exchange experiments (see Sects. 2.2 and 3.2.1 for further discussion); however, genuine “ion sieve” or “volume steric” effects can also operate as a consequence of the crystalline and microporous nature of molecular sieves. Ion sieving, known for a long time and commonly observed in zeolites, arises when part of the microporous channel network within the molecular sieve is inaccessible to the incoming exchanging cations simply because their ionic diameters exceed the free diameters of the windows through which they must pass [46]. The “volume steric” effect is less common, and arises when the cations have free access to the microporous voids and channels within the crystal but nevertheless the size of the incoming ions is such that the channels are completely filled before 100% exchange for the incoming ion can be achieved [47]. Over the last decade, during a series of studies on high silica zeolites including ZSM-5, ZSM-11 and EU1-1, another possible cause for partial exchange has been identified. Although full exchange of hydronium ion for sodium was observed by Chu and Dwyer for a range of high silica zeolites [37], and ion sieve effects were identified by the same workers to explain partial exchange with some organic-substituted ammonium cations in ZSM-5 [39], Matthews and Rees found more complex behaviour with alkaline earth and rare earth cations in ZSM-5 [38]. Univalent cations exchanged to 100% but this was not the case for multivalent cations. Part of the explanation for the significantly lower maximum loadings found with multivalent cations (especially Ca2+ and La3+) was ascribed to the distribution of the relatively low number of aluminium atoms in the framework, which could make it difficult for multivalent cations to neutralise effectively widely spaced negative charges on the framework [38]. To test this hypothesis, McAleer, Rees and Nowak [40] carried out a series of Monte-Carlo simulations which implied that the charge on divalent cations could only be satisfied adequately by aluminium atoms within the framework which were spaced apart by < 0.12 nm. More recently, similar experimental and theoretical studies were carried out on zeolite EU-1, where analogous behaviour to ZSM-5 was observed, although cut-off values for exchange were much higher in EU-1 [41]. Topological and structural differences between ZSM-5 and EU-1 were proposed as explanations for this different behaviour [41] (see the earlier discussion in Sect. 1.2). Figure 4 shows a type of isotherm shape that is seen with crystalline ion exchangers such as molecular sieves and clay minerals, but is nevertheless relatively uncommon. The shape resembles the type II vapour adsorption isotherm of the Brunauer classification, having a clear “plateau” region and inflexion point. An example is the Na/K exchange in zeolite P [48] that was found to be reversible over the whole range of equivalent fraction of potassium in the crys– tal (EK ). Zeolite P has the gismondine-type structure (GIS [49]). More commonly, isotherms of this type are found to be partially irreversible in the plateau

Ion Exchange in Molecular Sieves by Conventional Techniques

13

region, resulting in a hysteresis loop between the forward and reverse isotherms (Fig. 4). Examples of such hysteretic behaviour include the Na/K and Na/Li exchanges in zeolite K-F [50], which is a framework structure isotype of edingtonite EDI [49], and the Sr/Na exchange in zeolite X [51]. Isotherms of this type (whether fully reversible or not) are characteristic of systems where the process of exchanging one cation for another induces structural distortions and changes in the molecular sieve framework, resulting in the end-members of the exchange – – (EA = 0 and EA = 1, respectively) being different phases. If the framework is flexible and consequently the required structural transformation can occur readily, the plateau region (where the two phases coexist) will be reversible. This is the situation observed for the Na/K exchange in zeolite P [48] which has long been recognised as a material which exists as several structural varieties [52] depending on ion exchange form and level of hydration [53] and which is recognised as having an unusually flexible framework [49, 52]. When a hysteresis loop occurs, this corresponds to a situation where the endmembers of the exchange exhibit limited mutual solid solubility; in other words, over this region of the isotherm two separate phases coexist. Barrer and Klinowski considered the conditions under which phase separation may be expected to occur in a statistical thermodynamic treatment involving an interaction energy for entering ions wAA/kT [31]. When this term is sufficiently negative, so that the cations segregate rather than form a homogeneous phase, they showed that conditions could arise under which a physical mixture of two A- and B-type crystals has a lower free energy than the homogeneous A/B phase [31]. If in addition the nuclei of the A-rich phase grow within the B-rich “parent” phase matrix then two positive free energy terms are involved in the exchange process. These are a strain free energy resulting from the misfit between the new growing phase within the old, and an interfacial free energy. These tend “to delay the appearance of the new phase beyond the true equilibrium points for forward and reverse reactions” [31]. This is the proposed explanation for the hysteretic behaviour seen in systems such as the Na/K and Na/Li exchanges in K-F [50] or the Sr/Na exchange in X [51], and contrasts with P [48, 53]. This has significance for the use of a high aluminium analogue of P in detergency [6, 7]. This material, named “maximum aluminium P” (MAP), has the gismondine framework structure of zeolite P but with a Si/Al ratio of unity [6]. The unusually flexible framework [49, 52] is reported to lead to cooperative calcium binding, as well as to unusual water adsorption/desorption properties that enhance bleach stability [6, 7]. These properties, combined with superior kinetic behaviour, result in a material that reduces water hardness much more effectively than zeolite A (sic, [6, 7, 45]). 2.2 Batch and Column Exchange Operations

Practically all industrial ion exchange applications, except the use of zeolites in detergency, involve column operations (e.g. the removal of radionuclides from nuclear waste effluents). However, basic studies of ion exchange equilibria are usually carried out using the batch method.

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It is instructive at this point to compare these two techniques by considering the conversion of a zeolite from one ionic form (B) to another (A) as shown in Eq. (1) and using the selectivity coefficient kA/B defined in Eq. (2). In batch ion exchange, a given amount m of zeolite in the B-form is contacted with a given volume v of a salt solution of ion A. At equilibrium, the ions are distributed between the solid and solution phase according to: cAzB c–AzB = k . (4) 5 A/B 5 cBzA c–BzA The progress of the reaction is illustrated in Fig. 5 for two univalent cations (zA = zB = 1) assuming a constant selectivity coefficient kA/B = 10 and an ion exchange capacity of 4 mequiv g–1. It is clear that it is difficult to obtain a high degree of conversion by a single batch equilibration. In this example, 430 cm3 of 0.1 equiv dm–3 solution of ion A is required for 99% conversion. This is almost an 11-fold excess even though the exchange equilibrium operates in favour of ions A. In zeolites strong selectivity reversals are often observed and this makes it very difficult to obtain a high conversion to the required ionic form. This problem is discussed in more detail in Sect. 3.2.1. Here, conversion will be discussed in qualitative terms. The solution concentrations of A and B [Eq. (4)] can be written as: – (5) cB = c–A /(V/m) = EAQ/(V/m) and – cA = cA(o) – cB = cA(o) – EAQ/(V/m)

(6)

Loading (meq/g)

Solution concentration (N)

where Q is the ion exchange capacity (equiv kg–1), V/m is the solution volume (dm3) to zeolite mass (kg) ratio in the batch equilibration and cA(o) is the initial

Solution volume (ml)

Fig. 5. Batch exchange: loading of ion A in zeolite (solid curve) and concentration of A in solution (broken curve) as a function of solution volume when contacting 1 g of zeolite in B-form batchwise with 0.1 g equiv–1 solution of A. Selectivity coefficient kA/B and exchange capacity Q have been given values of 10 and 4.0 mequiv g–1, respectively

15 Outlet concentration (N)

Average loading (meq/g)

Ion Exchange in Molecular Sieves by Conventional Techniques

Effluent volume (ml)

Fig. 6. Column exchange: average loading of ion A in zeolite (solid curve) and concentration of A in outlet solution (broken curve) as a function of solution volume passed through the column. Mass of zeolite bed 1 g, inlet solution pure A at 0.1 equiv dm–3 concentration. kA/B and Q as in Fig. 5

concentration of A (equiv dm–3) in the solution. To obtain a high conversion to the A-form in a single equilibration, kA/B and cA must be high and cB must be low. cB can be made low by using a large volume of solution per unit mass of zeolite (maximum value of cB = Q/(V/m)) (Eq. 5). cA can be made large by using a high initial concentration of A and large V/m ratios (Eq. 6). In column exchange, a solution of ion A is passed through a column that contains a given quantity (m) of zeolite. This process is illustrated in Fig. 6 using the same parameters as in Fig. 5 for the batch exchange. In column exchange, the conversion to the A-form proceeds much more easily, as ion B is constantly removed from the system. However, ion A is not homogeneously distributed in the bed, but is first taken up by material near the column inlet and the conversion proceeds in the direction of solution flow. When most of the zeolite has been converted to the A-form, ion A starts to emerge from the column and cA tends to the value of the feed concentration, when the column has become completely exhausted. The important point to note is that by contrast with batch exchange, far less solution is needed for full conversion. In the example of Fig. 6, only 50 cm3 of 0.1 equiv dm–3 solution is required for every gram of zeolite to achieve 99% conversion. This is only a 25% excess. Figures 5 and 6 represent highly idealised cases and serve here only to describe qualitatively the differences between batch and column exchanges. In realistic situations, the selectivity coefficient decreases with increasing loading of A in the zeolite (see Fig. 1). This means that an even higher excess of A must be used under real conditions. In addition, in column exchange, the rate of exchange reaction often tends to decrease at high loadings, which lowers the gradients of the loading and concentration curves (Fig. 5) and increases the solution volume needed for full conversion. Pure synthetic zeolites are fine powders that are usually unsuitable for column operation. Therefore, batch methods are used for the study of ion exchange

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R.P. Townsend · R. Harjula

equilibria. Granular zeolite exchangers that are suitable for column work are manufactured by using suitable binders (e.g. clay, silica, alumina) and care must be taken in extrapolating data obtained from batch experiments to column operation. 2.3 Thermodynamic Parameters, Non-Ideality and the Prediction of Exchange Compositions

To derive thermodynamic parameters of ion exchange, the normal procedure is to correct for solution phase non-ideality first by deriving a corrected selectivity coefficient in which concentrations within the external solution are replaced by activities. The means by which this may be done, for binary or multicomponent systems, is described elsewhere [54, 55]. The corrected selectivity coefficients E are then: corresponding to k XA/B and kA/B –– XAzB aBzA KXA/B = 92 , (7a) –– XBzA aAzB – E zB a zA E = A B . KA/B (7b) – 92 EBzA aAzB E is identical to the function K shown in Fig. 1 and taken from [20]. K A/B G The thermodynamic equilibrium constant Ka is then obtained by integrating the appropriate form of the Gibbs-Duhem equation to give as corresponding expressions for Eqs. (7a) and (7b), respectively, the following: 1

– X dE lnKa – D = Ú lnKA/B A, 0

(8a)

1

– E dE lnKa – D = (zB – zA) + Ú lnKA/B A,

(8b)

0

where D is the water activity term [56, 57]. D is normally ignored on the assumption its magnitude is small; however, it should be noted that for the most commonly employed formulation, corresponding to Eq. (8b) and after Gaines and Thomas [58], D π 0 when the system is behaving ideally if zA π zB but rather equates to (zA – zB) [56, 59]. This must follow since, when the system is behaving ideally, the values of all the activity coefficients are by definition unity for all E = constant [56, 57] since compositions and hence Ka = K XA/B = KA/B – g–AzB f AzB x E Ka = KA/B = K (9) – 5 A/B 5 g–BzA f BzA where fi , gi are the appropriate rational activity coefficients for cations in the exchanger phase in association with their equivalents of anionic charge. Equations (8a) and (8b) provide the starting point for the prediction of ion exchange equilibria in molecular sieves, an activity which has received a significant level of attention over the last decade or so. The basis for prediction comes

Ion Exchange in Molecular Sieves by Conventional Techniques

17

from a principle put forward some time ago [60], viz., that because D is small and changes little with zeolite composition, and providing salt imbibition is negligible (which is true for relatively dilute electrolyte solutions [61]), then for a given zeolite composition, the ratios of activity coefficients fi ,gi will hardly change in value as the total concentration of electrolyte in the external solution is changed [56, 60, 62]. Providing these assumptions hold, then taking as an example a binary exchange process, from Eqs. (8b) and (9), it follows that [62]: 1

– – zB – – zA – E – = (z – z ) + lnK E dE lnKA/B(E Ú B A A – ln (g A(EA)/g B(EA)) A/B A) 0

(10)

– where the subscripted EA in parentheses indicates that the values of the corrected selectivity coefficient and the rational activity coefficients refer to a particu– – lar composition EB , EA and must be invariant since all the terms of the righthand side are constant or hardly change when the total concentration of the external electrolyte solution is changed. The details of the methods which must be employed to predict selectivity trends are described elsewhere [62]; the important point to note is that if the above assumptions hold then for successful predictions it is only required to evaluate the appropriate corrected selectivity coefficient as a function of zeolite phase composition and to have an accurate knowledge of the solution phase activity coefficient g [54, 55, 62]. For binary exchanges, this approach has been used to test a variety of systems over the last decade, including exchanges involving Pb/Na, Pb/NH4 , Cd/Na and Cd/NH4 equilibria in clinoptilolite, ferrierite and mordenite [63–65] using different coanions (chloride, nitrate and perchlorate [62, 66]) as well as the Cd/Na-X and Cd/K-X systems [67], with a high level of predictive success [62, 67]. Recently, a related model has been used with good accuracy for the prediction of K/Na and Ca/Na equilibria over a wide range of total ionic concentrations in solution for natural clinoptilolite [68]. Successful predictions were also achieved for the Ca/Na, Ca/Mg and Mg/Na systems in zeolite A [18, 20, 69]; however, for Mg/Na and Mg/NH4 exchanges in a range of faujasites [70], predictions failed badly in some cases. The failures were attributed at the time to salt imbibition, but further detailed experimental studies involving hydronium exchange in the Ca/NaX, Ca/Na-Y, Cs/Na-MOR and Cs/K-MOR systems [71–75] have shown that the situation is in reality much less straightforward. Failures in predictive methods, particularly at trace levels of exchange, cannot be attributed simply to hydrolysis, hydronium exchange or salt imbibition despite earlier suggestions to this effect [70, 76]. An important factor appears to be the presence of colloid-size zeolite particles [74]. These matters are discussed further in Sect. 3.2.3. To apply the same prediction procedure as that described above for ternary or multicomponent exchanges, it is helpful to derive analogous equations to those shown in Eqs. (8), (9) and (10) for binary exchange. For ternary exchange, this was done by Fletcher and Townsend [77] and this approach was used to predict compositions for Na/Ca/Mg-A [20, 69], Na/K/Cd-X [67] and Na/NH4/ Mg-X,Y ternary equilibria [70]. For the first two of these systems, ternary exchange equilibria were predicted successfully but for the Na/NH4/Mg-X,Y systems, the procedure failed for the higher silica Y materials, as for the corresponding conjugate binary exchanges [70]. In parallel with these studies, the

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model of ternary ion exchange in zeolites [77] was compared with other models published in the literature for clay minerals and resins [78, 79] and a further detailed study [80] came to the conclusion that these other approaches were appropriate under certain specified conditions [80] for the prediction of exchange equilibria in zeolites. A recent criticism of the ternary exchange model [81], on the basis that the equations could have simply been built up from the conjugate binary systems (obviously true), overlooks the main point. If one uses the conjugate binary systems it is necessary to use a model-based approach to predict activity coefficients for the multicomponent exchange equilibrium in the zeolite and the presence of sublattices within the zeolite framework can make this more difficult to do than for clay minerals and resins (Sect. 1.2) [80]. The ternary exchange model of Fletcher and Townsend [77] does not require one to measure at all the activity coefficients, let alone predict them for multicomponent systems from binary data, using some model. All that is required is knowledge of the ternary corrected selectivity coefficients that are obtained by integrating the appropriate Gibbs-Duhem equations over the ternary composition surface [77] in analogy with the binary approach pioneered by Gaines and Thomas [58]. However, acquiring sufficient data for a ternary system is a difficult and time-consuming exercise [20, 67, 70, 82] and simpler approaches can prove quite adequate provided one validates some of the predictions made [83]. Thus, another model, developed originally for clay minerals [84], has been shown after minor revision to work well for ternary anion [85] and cation [86] exchanges in organic resins and has even been extended successfully to a five-component zeolitic system (Sr/Cs/Ca/Mg/Na equilibria in chabazite) [87]. This system is very important in the field of nuclear waste treatment [87]. Accurate prediction is similarly much needed for detergent applications [2, 7, 18, 69]. The level and nature of “hardness” in household water varies extensively from one location to another, as do the conditions under which consumers expect effective laundering to occur (e.g. temperature). Thus accurate selectivity data (i.e. isotherms and selectivity plots as a function of loading), and reliable predictive models that are simple to use, are important, since it would clearly be impossible to measure directly the performance of a given “builder” zeolite for all conceivable situations. Successful predictions have been achieved for the binary Na/Ca-A, Na/Mg-A and Ca/Mg-A systems [2, 18, 69] as well as for the corresponding ternary system [2, 69]. Similar successful predictions were recently achieved also for zeolite MAP [7] once the original iterative procedures of Franklin and Townsend [69] had been modified appropriately. Figures 7 and 8 show examples of such successful predictions in A, for both the binary and ternary cases. Occasionally, isotherms of binary and multicomponent exchanges are described using various empirical adsorption equations. These cannot be used for the prediction of multicomponent equilibria [88]. In fact, a closer inspection of these equations reveals that they have no in-built facility for true prediction (i.e. for the calculation of equilibria over ranges of different total solution concentrations for heterovalent exchanges). Thus these equations are useful in describing the observed isotherm in a mathematical form but the only pre-

Ion Exchange in Molecular Sieves by Conventional Techniques

19

a

b Fig. 7a, b. Predicted isotherms and experimental points for a the Na/Ca-A system and b the Na/Mg-A system. Solid lines are predicted isotherms; experimental points are measured at normalities of 0.025, 0.10 and 0.4 equiv dm–3, shown respectively as solid triangles, circles and squares. Taken from [69]

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R.P. Townsend · R. Harjula

Fig. 8. Ternary experimental and predicted points for the Na/Ca/Mg-A system at a normality of 0.4 equiv dm–3. Measured solution and zeolite phase equilibrium compositions are shown as unfilled stars and filled squares, respectively. The predicted zeolite phase at 0.4 equiv dm–3 is shown as an unfilled circle while the filled circle represents experimental validations at 0.4 equiv dm–3. Taken from [69]

diction these equations can give is the interpolation of the isotherm under one given set of experimental conditions. With such limited utility, these empirical approaches are not recommended for the “prediction” of ion exchange equilibria. 2.4 Kinetic Processes and the Prediction of Rates of Exchange

In direct applications involving zeolites as ion exchangers, it is not normally the case that the system is allowed to reach equilibrium. In batch operations (e.g. in detergency) the time available may be such that the exchange process is interrupted long before equilibrium is reached. Similarly, in column operations (e.g. effluent purification), when the system is operating under steady-state conditions, the balance between throughput of liquid and time of exchange means

Ion Exchange in Molecular Sieves by Conventional Techniques

21

that the system is frequently operating under non-equilibrium conditions. Knowledge of the kinetics of the multicomponent exchange processes (i.e. all of the reaction rates, diffusive mechanisms and hydrodynamic processes which contribute to the overall rates of exchange of all of the different types of ions involved) is therefore of key importance if one is to be able to predict and control behaviour. Unfortunately, this is easier said than done. The kinetics of ion exchange processes in zeolites are extremely complicated even when one focuses on just one mechanistic process [45]; only recently, it was rightly stated that the “picture presented in the literature for diffusion in zeolites is confusing, conflicting and/or inconsistent with theory” [89]. Space permits only a brief overview of the current state of affairs and this is presented here using a hierarchical model [90] for the zeolite particle or pellet. Much more detail is given elsewhere [45]. 2.4.1 Hierarchical Model of Zeolite Particle or Pellet

Whether one is considering an agglomerate of aggregated zeolite crystallites, or a pellet, a hierarchical model [89, 90] allows one to distinguish the different transport and/or rate processes which operate at different length scales. The highest level is concerned with the macroparticle or pellet itself; and the key issue here is whether transport of ions through the fluid film which encompasses the macroparticle is rate-controlling or not. That this process can be ratecontrolling has been recognised for a long time, being favoured by a low concentration of exchanging ions in solution and a small mean particle size; however, it is known that the hydrodynamic regime pertaining can affect its influence markedly, with high levels of agitation (such as are achieved at high impeller speeds in a batch reactor [89]) rendering relatively insignificant any mass transfer resistance through the boundary film. The mechanical integrity of the macroparticle can also be very important. Taking detergent powder particles as an example [which can comprise agglomerates of (primary) zeolite crystalline particles held together by means of adhesive, viscoelastic surfactant bridges], these are designed to break up under shear and/or other hydrodynamic regimes that are imposed as part of the wash cycle. On breaking up and dispersing, some of these dispersed smaller particles may find themselves in regions of low agitation and consequently the rate of removal of hardness ions from the wash liquor can be slower than desired due to the onset of film diffusion control. Generally, however, the aim is to avoid conditions leading to film diffusion control. This means that the focus is shifted towards transport processes that occur at the intermediate level (that is, in the mesopores and macropores within the macroparticle or pellet itself) and those which occur at the smallest dimensional level (viz., in the very micropores of the molecular sieve) [45, 89]. Within the mesopores and macropores between the primary zeolite crystallites transport will be dominated by molecular and ionic intercrystalline diffusion possibly coupled to surface diffusion processes, while, in the zeolite micropores themselves, intracrystalline diffusion occurs, also possibly coupled

22

R.P. Townsend · R. Harjula

with specific exchange rates associated with the different zeolite sublattices [91, 92]. The overall observed kinetics of exchange is of course the result of all the above-described mechanisms working in concert [45, 89]. To cope with the complexities of the system, a simple approach one may adopt is the homogeneous diffusion model, which assumes that the behaviour of each distinct diffusing species within the macroparticle may be described in terms of a single solidphase “effective diffusivity” [89]. More sophisticated approaches include the heterogeneous diffusion models, where the macropore and micropore diffusion processes are described separately and are then assumed in different mathematical treatments either to occur in series or in parallel [45, 89]. In practice, to date, most research activity has focused on the intraparticular diffusion which takes place in the zeolite micropores themselves, on the questionable assumption that these processes are normally the rate-controlling ones. 2.4.2 Intraparticular Exchange Rate Processes

Our understanding of the processes which govern the rates of ion exchange within the micropores of molecular sieves has advanced little over the last decade, yet the imperative to be able to control and manipulate these rates remains as strong as ever. To summarise the current situation it is necessary first to emphasise some basic principles and then to define certain terms and coefficients. To begin, it is important to distinguish the intrinsic dynamic nature of the system from the kinetic processes we actually observe during an ion exchange reaction. An obvious yet important point to remember is that even after exchange equilibrium has been attained, the equilibrium is a dynamic one. Thus transport of all exchangeable cations and of the solvent molecules continues but after equilibrium has been reached there are no net changes in the relative distribution of species between, and hence concentrations in, phases with time. This dynamic character is readily verified by adding to the equilibrated system a trace amount of a radioactive isotope of one of the cation types into (say) the solution phase of the system and then observing the rate at which isotopic exchange between the two phases takes place. The isotopic exchange process may include as a rate-determining step an intracrystalline exchange process [91, 92] but it is also certainly a transport process, which is described in terms of a self-diffusion coefficient D*AA [93]. Self-diffusion coefficients D*AA and D*BB , which can change markedly with temperature [45] or as the equilibrium concentrations of different cations within the zeolite are altered [45, 94], should be sharply distinguished from the exchange diffusion coefficient DAB [95]. DAB describes the kinetics of the A/B exchange process, that is, the observed rates of change of concentrations of ions A and B within each phase as a function of time and as the system moves to equilibrium. Consider therefore a binary A/B exchange between the zeolite and external solution, which is not initially at equilibrium. On mixing the two phases, the A and B cations, which will almost certainly possess different ionic radii and pos-

Ion Exchange in Molecular Sieves by Conventional Techniques

23

sibly charge, will begin to move in their respective directions of negative chemical potential gradient in order to equalise their respective chemical potentials within all phases in the system. However, the mobilities of the two cation types A and B are likely to be different, which means that the more mobile cation type will tend to build its concentration, and hence lower its concentration gradient, faster than the other. If this process were to continue unchecked, charge separation within each phase and between the phases would occur, with a concomitant electrical potential gradient. In practice, of course, the electrical potential gradient that forms as charge separation takes place does not build, but rather acts to slow the faster moving cations and speed the slower ones. Thus it is not adequate to consider only the chemical potential gradients. The net flux JA of (say) the Aexchanging species is actually described by: (11) J = – D [grad c– – (z c– F/RT) grad V] A

AB

A

A A

where F is the Faraday constant and V the electrical potential. An expression for DAB has been derived by Barrer and Rees using an irreversible thermodynamic approach. The form of this is complicated but, if cross-coefficients other than those due to the electrical potential gradient are assumed to be negligible, then [96]: – 2 – – – 2 – – D* AA D* BB [c Az A(∂ ln a B / ∂ ln c B) + c B z B (∂ ln a A/ ∂ ln c A)] DAB = 00000000 (12) 0. – 2 – 2 D* AAc AzA + D* BBc BzB Two points should be noted from Eq. (12). First, the magnitude of DAB depends strongly on the composition of the exchanger not only because it is a direct function of ionic concentrations, but also because it is a function of both D*AA and D* BB, which we have already noted vary with exchanger composition [45]. Secondly, DAB is a function of the non-ideality of the zeolite [data for which can be obtained, as we saw earlier, from the activity coefficients described in Eq. (10)]. One may expect therefore that to describe adequately the kinetic behaviour of even a binary exchange process in a molecular sieve would be a very complicated task. To validate this and other similar models, it is necessary to solve, using appropriate boundary conditions, the differential equations describing overall the transient diffusion process for each ion, of the general form: (13) (∂ c– /∂ t) = div D gradc– i

AB

i

which for spherical symmetry (a good approximation for most primary zeolite particles) becomes [95]: ∂ c–i 1 ∂ 2 ∂ c–i r D (14) = 6 42 5 AB 6 . ∂t r ∂r ∂r





As an example of the above approach, Brooke and Rees [95] studied the Sr/Cachabazite system. Figure 9 shows their computed time-dependent concentration profiles within the zeolite particles both before and after non-ideal behaviour was taken into account. The effect on DAB of taking non-ideality into account was even more dramatic, with a discontinuity appearing in the plot of ∂DAB / ∂ c–A

24

R.P. Townsend · R. Harjula

Fig. 9a, b. Radial concentration distributions at various fractional attainments of equilibrium for the Ca/Sr exchange in chabazite for a the ideal exchanger and b the non-ideal exchanger. The continuous lines represent the Ca/Sr exchange and the broken lines the reverse process. Taken with permission from [95]

(Fig. 10). Even allowing for this non-ideal behaviour, prediction of exchange rates was still poor [95]. Other similar studies, including the measurement of ∂D*ii / ∂ c–i functions, are described elsewhere [45, 94]. It is unfortunate that we are still not able to rationalise adequately the kinetics of ion exchange in zeolites, let alone manipulate rate processes. For example, in realistic detergency applications, the issue can be of prime importance, since the contact time of zeolite suspended in the wash solution is usually shorter than the time required to attain the equilibrium state. Elsewhere in this chapter the strong effects that crystallite size and mesoporosity can have on kinetic rates has been emphasised (Sects. 1.2 and 2.4.1); it is precisely these properties which are identified as being key (in addition to cooperative calcium binding) for the superior performance of zeolite MAP as a builder [6, 7]. For zeolite A, binary and ternary kinetic measurements of the Na/Ca/Mg exchange have been undertaken [21] in addition to equilibrium studies. For the ternary system, the inhibiting effect of magnesium on the uptake of calcium ions was clearly demonstrated (Fig. 11) [21]. 2.5 Trace Ion Exchange

In the preceding sections ion exchange processes involving large changes in the chemical composition of the solution and zeolite phase have been discussed. Under these circumstances, attention has to be paid to the changes in the value of the selectivity coefficient with composition. In the case of exchange of trace ions for the ion present at much higher concentrations (described henceforth as the “bulk ion”), the situation is somewhat different. This brings us to the other important area of zeolite application, viz., the purification of nuclear waste efflu-

25

DAB (¥1014) cm2 sec–1

Ion Exchange in Molecular Sieves by Conventional Techniques

CSr Fig.10. Variation of the exchange diffusion coefficient DAB as a function of equivalent function of strontium (CSr) for the Ca/Sr exchange in chabazite. Taken with permission from [95]

Fig. 11. Examples of ternary ion exchange kinetic measurements within the Na/Ca/Mg-A system. The dashed lines which bound the experimental data represent the simple binary exchange rates (viz, Na/Ca and Na/Mg). When magnesium is added to the system in progressively larger amounts (i.e. O > ■) a progressive slowing of the Na exchange rate is seen. Taken with permission from [21]

26

R.P. Townsend · R. Harjula

ents. This always involves column ion exchange. What is of interest in this application is the capacity (Qv) of the ion exchanger, in terms of the solution volume (V) that can be treated with a given amount (m) of ion exchanger. The maximum (saturation) value of this capacity (Qv,max) is unambiguously given by the distribution coefficient KD of the radionuclide. In general, the distribution coefficient KD of ion A is defined by the equilibrium ratio: K = c– /c . (15) D

A

A

The distribution coefficient is determined by two factors, selectivity and ion exchange capacity. Let us consider here uni-univalent exchange for clarity. Inserting c–B = Q – c–A (where Q = ion exchange capacity) into kA/B in Eq. (2) and combining the resulting expression with Eq. (15) gives for KD KD = Q/((CB/kA/B) + CA) .

(16)

It can be seen that when cB /kA/B  cA (i.e. when the ion A is a radioactive trace ion), then the capacity of the ion exchanger is independent of the concentration of the trace ion A in solution but depends only on Q and CB . Let us consider removal of radioactive Cs ions (e.g. 137Cs) from a waste solution containing sodium salts.As an example, the chemical concentration of 137Cs in solution corresponding to an activity concentration of 1 µCi dm–3 (which is typical in low-active waste) is 8 ¥ 10–11 mol dm–3. Selectivity coefficients kCs/Na are typically in the order of 10–100 in zeolites. Thus, unless the Na concentration in solution is very low ([Na]  10–8 mol dm–3), the KD (and volumetric capacity) of the exchanger is independent of the concentration of caesium in the solution and the familiar relationship is obtained from Eq. (16) (B = Na, A = Cs) for KD , in the logarithmic form log KD = log (kCs/NaQ) – log cNa .

(17)

In other words, the KD of the trace caesium ion is inversely proportional to the concentration of the macro-ion (sodium) in the solution. The selectivity coefficient can be assumed to be constant in this case as the loading of caesium in the exchanger is very low. In general, the logarithmic equation for KD is [75] log KD = (1/zB) log (kA/BQzA) – (zA/zB)log cT

(18)

where CT is the total concentration of exchanging ions in solution (mol dm–3). Thus, plotting log KD against the logarithm of the bulk ion concentration yields a straight line with a slope of –(zA/zB). In experiments this equation is used to determine the selectivity coefficient kA/B , which is obtained from the intercept of the linear plot. A linear plot also confirms the stoichiometry of the exchange reaction over the concentration range of interest. However, quite often the log KD plots are linear in the more concentrated solution of B only. In dilute solutions, leveling-off of the log KD plot is often observed. This can be rationalised when it is kept in mind that Eq. (18) is valid for the free cations with the charges indicated at the equilibrium concentrations of the cations. Many meaningless data have been produced when this point has been forgotten. This issue is discussed further in Sect. 3.2.2.

Ion Exchange in Molecular Sieves by Conventional Techniques

27

Very few data can be found in the literature for the exchange of trace ions in the presence of more than one type of bulk ion. Harjula et al. [75] have studied exchange of 134Cs in mordenite and in mixed salt solutions of sodium and potassium. It was found that at a given constant total concentration CT (cNa + ck) of solution, KD of 134Cs was a linear function of the potassium loading in the zeolite. The equilibria were treated using appropriate pseudobinary selectivity coefficients. From the linear dependence of log KD a more simple treatment can also be obtained, i.e.: ––– ––– (19) log KD = ENa log (kCs/Na Q/CT) + EK log (kCs/K Q/CT) where kCs/Na and kCs/K are the (limiting) binary selectivity coefficients for trace Cs exchange in pure Na- and K-forms of the zeolite. There are too few data to conclude whether the form of Eq. (19) is generally valid for the calculation of trace ion distribution coefficients in the presence of two or more types of bulk ions. If Eq. (19) were always valid the following equation would apply for trace ion A in the presence of M other different ions present in much higher concentrations: M

log KD (A) = Â (1/zi) Ei log (kA/i [Q/CT]zA)

(20)

i=1

where the kA/i are the selectivity coefficients of the trace ion A in the pure i-forms of the zeolite. In order to use Eqs. (19) and (20), additional models for the prediction of bulk ion concentrations need to be used (see Sect. 2.4). 2.6 Column Models

With a few exceptions, industrial applications of zeolites involve column operation in feeds containing more than two counterions. In general, therefore, the prediction of column performance involves the prediction of multicomponent equilibria and kinetics under dynamic flow conditions. Considering the complexity and diversity of these models (see Sects. 2.3 and 2.4), it is obvious that simplifications and approximations need to be made for practical column modelling. For engineering purposes, the most popular approach for column modelling is the “linear driving force – effective plate” concept [97]. Consider again the removal of a radionuclide from a solution as an example of column exchange. By definition (Eq. 15), KD describes the equilibrium distribution of ions between the zeolite and the solution. However, at the same time, it is a measure of the equilibrium distribution of the solution volume and zeolite mass. Thus the total column capacity can be calculated from Eq. (16) or (18) for a simple binary system. The volume that can be treated with the column containing m kg of zeolite is equal to the area above the breakthrough curve (Fig. 12), and can then be calculated from V = mKD . However, in the purification of radioactive effluents, it is necessary to discontinue operation, and change to a fresh column, immediately when the radioactive ion first starts to emerge from the column. The volume at which the breakthrough of the radionuclide com-

28

R.P. Townsend · R. Harjula

Fig. 12. Schematic representation of the effect of plate number N on the breakthrough curve in column exchange

mences is called the breakthrough capacity (QBT) of the column. The breakthrough capacity is usually defined in terms of some chosen level of breakthrough (e.g. at 1%). The efficiency of the column can then be measured by the degree of column utilisation, which is the ratio of breakthrough capacity to the maximum capacity (QBT /KD). According to the plate concept, the number of “transfer units” or “effective plates” (N) is a measure of the column efficiency. Increasing N makes the breakthrough curve steeper and thus improves the degree of column utilisation (Fig. 12). The number of effective plates can be calculated from fundamental data. Thus for film-diffusion controlled exchange: Nf = A(Df)1/2d –3/2 suo–1/2

(21)

where s is the column length, d is the particle diameter, uo is the linear flow rate and Df is the diffusion coefficient. For particle diffusion controlled exchange: Np = BKDDpsd –2uo –1 .

(22)

A and B in Eqs. (21) and (22) are empirical factors. Both mechanisms may contribute simultaneously to the overall exchange kinetics but can be taken into account in an appropriate model [98]. It can be seen from Eqs. (21) and (22) that the degree of column utilisation and the breakthrough capacity increase when the zeolite grain size is decreased and the solution flow rate is decreased (Fig. 12). In the limit, the kinetic performance is determined by the magnitude of the diffusion coefficients. The plate approach has been used in the development and operation of the process for the purification of the highly radioactive solutions that arose in the accident in the Three Mile Island nuclear power station [99]. In this process, a mixed zeolite bed (Linde IE-96 and A51) was used for the removal of 137Cs and 90Sr.

Ion Exchange in Molecular Sieves by Conventional Techniques

29

3 Experimental Approaches It has been emphasised already that accurate and reliable data are essential in the construction of adequate ion exchange models for the industrial applications of zeolite and other ion exchangers. In this section we will discuss ion exchange experimentation and its utility for industrial applications. We also discuss major pitfalls that may lead to unreliable results. Although industrial applications always involve more than two exchanging ions, seeing trends in the overall equilibria under these circumstances may be difficult. Therefore, in this section, only binary ion exchange equilibria are considered in order to keep the major issues in focus. There are basically two major reasons for studying ion exchange equilibria. The first of these is concerned with understanding selectivity and its causes. For these studies, correlations are sought between the properties of exchanging cations (e.g. cation size, charge, acidity, etc.) and the ion exchanger (charge density, pore diameter, acidity, etc.) with the aim of predicting the magnitude of the selectivity. In the case of zeolites, where strong decreases in selectivity are often observed (see, for example, Fig. 1), it is also of great interest to predict how selectivity changes with loading. To date, no useful and general theory has been developed for these predictions, in zeolites or indeed in any other ion exchange materials. Secondly, for application-oriented studies, selectivity data are measured in order to predict the performance of the zeolite under given operational conditions (ion concentration, temperature, contact time, etc.) using appropriate thermodynamic or kinetic approaches and hence to choose and optimise the operating conditions for the application in question. 3.1 Practical Experiments

It should be obvious by now that one of the key tasks in ion exchange experiments is the accurate determination of the selectivity coefficient. In principle, this is straightforward: the zeolite, initially in the B-form, must be equilibrated in solution using an increasing ratio cA /cB and, after equilibrium has been attained, the concentrations of A and B in the zeolite and also in the solution are measured. This can be done either batch-wise, or, in the case of the granular zeolites, column-wise. Both techniques should give in principle an identical result. However, there is one important difference between the two techniques. In column exchange, the equilibrium concentrations of A and B in the solution at equilibrium will be known beforehand since these will be equal to the initial concentrations of A and B in the feed solution. Because of this, it is a relatively simple matter to decide the initial conditions for the experiments in order to determine the selectivity coefficient as a function of the loading. For instance, isonormal (e.g. 0.1 g equiv –1) solution mixtures of A and B may be prepared, containing progressively increasing amounts of A (1%, 5%, 10%, 20%, …, 90%). Each of

30

R.P. Townsend · R. Harjula

these solutions is pumped through the column until the outlet concentrations of A and B are equal to those in the inlet. Equilibrium concentrations of A and B in the zeolite are then determined by direct analysis of the solid, or by analysing a solution containing the dissolved zeolite. In batch ion exchange, the equilibrium concentrations of A and B in the solution will not be known beforehand. These will depend on the experimental conditions (total solution concentration, ion exchange capacity, and the solution volume to zeolite mass ratio V/m). This makes it difficult to decide on the best initial conditions for the experiments. Commonly, the zeolite is converted stepwise from the B-form to the A-form by successive equilibrations in solution mixtures of A and B (low conversion) or in solutions of pure A (high conversion). Only some rough guidelines are available for choosing the initial conditions for the experiment. Thus, after the first measurements have been evaluated, it is often necessary to carry out further experiments in order to fill in gaps in the distribution of the data points across the isotherm. In general, the advantage of batch equilibration is that the experimental apparatus is simple so that a large number of experiments can be carried out in parallel using a minimal amount of solution and zeolite. In column experiments, one column “run” is required for each selectivity measurement and a large number of solution concentration measurements have to be carried out to check that equilibrium has been finally attained. Run times can be very long, especially when the equilibrium is unfavourable at higher loadings or when the uptake of trace ions is being studied. For instance, determination of a KD value of 20,000 cm3 g–1 requires that at least 40,000 cm3 of solution is passed through a 1-g zeolite bed. Such an experiment may take several months. By the batch method, the same information can be obtained by carrying out the experiment in a 20-cm3 plastic bottle in just 1 week. It is clear that the batch method is the preferred option when large quantities of materials are to be assessed in parallel, or when multicomponent equilibria are to be studied. In finely divided zeolites the batch method may be anyway the only alternative. 3.2 Pitfalls

One might infer from the above that the measurement of zeolite ion exchange selectivities is simple. In practice, several factors may interfere which may distort the result. As a consequence, in theoretical work, understanding the selectivity data may become impossible as one tries to rationalise these distortions without knowing their origin. In application-oriented work, a completely wrong picture may be obtained about the performance and utility of a given zeolite due to these problems. In the following these problems are examined.

Ion Exchange in Molecular Sieves by Conventional Techniques

31

3.2.1 Selectivity Reversal and Ion Sieving

Many ion exchanges in zeolites involve incomplete exchange so that some of the ions (usually Na) originally present in the zeolite are not exchangeable for the incoming cations. This may arise from ion sieving, volume steric effects or from very low framework charge densities (see Sect. 2.1). Often divalent cations or large cations are partially excluded. In these cases it is also common to observe very strong selectivity decreases with increasing loading of the incoming cation. A strong selectivity reversal is also common in exchanges that have nearly gone to completion. Due to this selectivity decrease it is very difficult to convert a zeolite from one ionic form to another even though there should be no steric or other hindrance to 100% exchange. In addition, it may be very difficult to detect whether a genuine saturation, or maximum loading, has been achieved, since when the selectivity becomes low, very small changes take place in the ion concentrations even when the exchange is pushed forward by large increases in the amount of incoming cation. Accurate determination of maximum exchange level is very important, since in the determination of the selectivity coefficient one should consider only those ions that are exchangeable [100]. Non-exchangeable cations are obviously not formally involved in the equilibrium, so their presence need not be taken into account directly, although of course their effects may be made manifest indirectly in the values of the activity coefficients of the exchanging ions [100]. The use of the correct value for the maximum exchange limit is especially vital in the determination of the thermodynamic quantities of the exchange reaction (i.e. the thermodynamic equilibrium constant or the ionic activity coefficients in the zeolite), since the determination of these quantities – involves integrating the appropriate selectivity coefficient from EA = O to 1 (Eq. 8) and this scale and the magnitude of the selectivity coefficient strongly depend on the choice of the maximum exchange level fmax . Barrer, Davies and Rees [101] demonstrated the great effect of the choice of the fmax on the – magnitude and variation of the selectivity coefficient with EA . A major aim of many fundamental ion exchange studies in the zeolites has been the rationalisation of the selectivity gradient, since this reflects the non-ideality of the zeolite phase. It is clear that any attempts to do this require a very reliable value for fmax . In addition one should not compare systems from which thermodynamic parameters have been derived using different values of fmax , since the reference states of the systems are different and therefore are not directly comparable. As a general rule, conversion of a zeolite from one ionic form to another in one single batch equilibration is difficult, even when the zeolite is selective for the incoming cation. Considering also the common selectivity reversal exhibited by most zeolites, conversion by a single equilibration becomes a practical impossibility in most cases. Despite this, in many studies in the past, only single equilibrations or at best a few successive equilibrations have been carried out to measure maximal exchange levels in zeolites. It is doubtful whether these results and the selectivity

32

R.P. Townsend · R. Harjula

plots derived from these maximum exchanges are correct. It is also obvious that the inaccuracies in determining the maximum exchange contribute strongly to the frequently observed high levels of scatter in zeolite selectivity data. For instance, zeolites NaX and NaY appear to have very different selectivities for calcium ions at room temperature, when the maximum exchange level of 68%, determined by a single equilibration, is used for CaNaY [102]. However, when the exchange is pushed to a higher level by using 8–12 successive equilibrations, an 85% exchange level is obtained and the pattern of selectivity in X and Y for Ca starts to appear very similar [72], as one would expect intuitively for the two isomorphous zeolites. Another obvious point, easily overlooked, is that in some cases impurities in the salt solutions may cause the exchange to appear not to go to completion. If at high loadings of the incoming ion A the selectivity coefficient leads to a value of (say) 0.01 and the impurity level of B in A within the salt used is (say) 0.1%, it can easily be shown that no matter how many successive equilibrations are carried out, only about 90% conversion to the A-form will be obtained. It is therefore very important to use reagents of very high purity for the experiments. 3.2.2 Zeolite Hydrolysis Effects

Hydrolysis of zeolites gives rise to a range of “impurity” species in both the solution and zeolite phases, which may interfere with the study of binary metal cation exchange. Up to now, most zeolite ion exchange research has been carried out using initially the sodium forms of the zeolites. In this form, zeolites hydrolyse by taking up hydronium ions from the water, viz., ––––– –––+ Na + 2H2O = H3O + + Na+ + OH – .

(23)

The tendency to hydrolyse increases with an increasing aluminium content in the zeolite [103]. An example of this is shown in Fig. 13, which shows the extensive level of hydronium exchange which incidentally occurred during a series of studies on Na/NH4 exchange equilibria in faujasitic zeolites [103]. This can also be seen in the increase of the selectivity coefficients for H3O+/Na+ exchange with the increasing aluminium content of faujasite zeolites [73–75]. Zeolite hydrolysis also leads to several secondary phenomena. First, since the zeolite imparts an alkaline reaction to the water imbibed in the pores, carbon dioxide is picked up from the air. When the zeolite is then immersed in water, carbonate and bicarbonate ions are released into the solution. Secondly, hydroxyl ions released into the solution enhance the dissolution of silica and alumina from the zeolite framework into the solution [73]. As a consequence, the following electroneutrality condition can be found to hold in pure water after it has been contacted with a zeolite such as NaX [73]: [H3O+] + [Na+] = [OH–] + [HCO3–] +2 [CO32–] + [Al(OH)–4] + [SiO(OH)–3] +2 [SiO2(OH)2– 2 ].

(24)

Ion Exchange in Molecular Sieves by Conventional Techniques

33

Fig. 13. Diagrammatic representation of F [F = (mNa + mNH4)/mAl] as a function of equivalent – fraction of sodium (ENa in four faujasitic zeolites with an Si/Al ratio which increased in the – order X < Y2 < Y3 < Y4. The fractional level of hydronium exchange at any composition ENa , – ENH4 is given by (1–F). Taken from [100]

34

R.P. Townsend · R. Harjula

These reactions may have several effects on the binary metal cation exchange process that is primarily under study. These may be summarised as: 1. mass-action effect of hydronium ion exchange on the binary metal cation equilibrium; 2. association of metal cations with bicarbonate, carbonate, silicate and aluminate ions in the solution; and 3. cation hydrolysis (i.e. association of metal cations with the OH– produced by zeolite hydrolysis and/or precipitation of metal hydroxides). Each of these effects will now be examined in more detail, together with a consideration as to how they can be avoided or taken into account. Hydronium Ion Exchange. Considerable quantities of hydronium ions can be exchanged from water into the zeolite, when for example the zeolite is washed after synthesis or even prior to ion exchange experiments. Thus, before the experiments are begun, the zeolite is likely already to be partially exchanged into the hydronium form. Preparing the zeolite in the pure sodium form may be difficult, since hydronium ions can be picked up even from concentrated salt solutions of sodium [73]. When the zeolite is in the process of being converted to another ionic form, further hydronium ion exchange can take place in one direction or the other [103]. For instance, NH4+/Na+ exchange in zeolites X and Y is accompanied by significant hydronium exchange. Initially, almost 20% of the exchange capacity of NaX (Si/Al = 1.26) was taken up by H3O+, and this amount decreased steadily to about 12% upon conversion to the NH4+ form (Fig. 13). In contrast, zeolite NaY (Si/Al = 2.47) contained no H3O+ initially, but conversion to the NH4+ form was accompanied by H3O+ exchange so that in the NH4+ form, about 7% of the exchange capacity was taken up by the hydronium ions [103]. The consequences of this can be profound. In the past, it was common practice for metal ion concentrations in the zeolite phase to be determined from the changes in the corresponding concentrations in the solution phase. It is obvious that this can lead to significant error if significant hydronium exchange also takes place in parallel. However, even if the metal ion concentrations are measured in both phases, the calculated selectivity coefficient will not be that of the pure binary metal exchange if concomitant hydronium exchange occurs. Therefore, kA/B values will not reflect the relative preference of the zeolite framework for the two metal cations. Because a three-component system is actually involved, relative preferences between the metal cations and hydronium ions would be intrinsic in the selectivity coefficient and it is doubtful if this selectivity coefficient could then be used for accurate prediction of the binary ion equilibria. For an accurate description of the binary and overall equilibria one may be forced to use an appropriate ternary model (see Sect. 2.3). In general, one can detect hydronium ion exchange by measuring the balance of the contents of aluminium and exchangeable metal cations in the zeolite [103]. If all the aluminium in the zeolite is present in the framework in tetrahedral coordination, then the degree of hydronium exchange (DH) is given by DH = 1 – Âi zic–i/c–Al  .

(25)

Ion Exchange in Molecular Sieves by Conventional Techniques

35

However, a minor fraction of non-framework aluminium may be present in the zeolite (e.g. in the cation exchange sites). Framework aluminium (tetrahedral) and non-framework aluminium (octahedral) can be distinguished using 27Al NMR although it is very difficult to be quantitative [104]. It is obvious that often the interfering effect of zeolite hydrolysis and hydronium ion exchange cannot be avoided. In general, hydronium ion exchange is favoured in solutions of low salt concentration so one can try to minimise it by carrying out the experiments in moderate salt concentrations. The acid/base nature of high alumina zeolites is in fact very similar to that of weak-acid organic resins. In these materials metal ion uptake depends strongly on the solution pH [105]. This can be seen for zeolites, too. For instance, uptake of caesium and strontium by chabazite or sodium A zeolite depends strongly on solution pH (pH 2–10) at a constant sodium background of 0.1 equiv dm–3 [106]. The effects of hydronium ion exchange and solution pH on metal cation exchange in zeolites have been almost completely overlooked in past studies. As a consequence, there may be considerable systematic error in many published zeolite selectivity data, especially for high aluminium zeolites. Ion Association in the Solution Phase. Ion exchange experiments are usually carried out using anions (chloride, nitrate, perchlorate) that do not interact appreciably with the metal cations under study. However, zeolite hydrolysis produces many anionic species that tend to associate with metal ions (see Eq. 24). For instance, when pure water is contacted with zeolite NaX, between 5 ¥ 10–4 –5 ¥ 10–5 mol dm–3 soluble silica and alumina and between 1 ¥ 10–3–1 ¥ 10–4 mol dm–3 of total carbonates can be found in solution [74], depending on the value of V/m. The pH of the solution contacted with NaX can become markedly alkaline (pH 11.5–9.5) so precipitation of metal hydroxides (or carbonates, aluminosilicates) is also possible. Even if no precipitation of metals takes place, ion association can have a marked effect on the observed selectivities of the zeolites. For example, consider the exchange of a divalent metal ion M 2+ for sodium in a zeolite. If it is assumed that the metal cation associates with a univalent ligand L– to form the complex ion ML+, then this ion association can be characterised by an association constant k1 of the form [ML+] kl = 06 . [M2+][L–]

(26)

Most analytical techniques employed in the measurements of metal ion concentrations in solution yield the total concentration of the metal [M]T , viz., [M]T = [M2+] + [ML+] .

(27)

Assuming that only the free metal cations are exchanged into the zeolite, the observed selectivity coefficient [kM/Na(obs)] that one obtains from the measurement for the exchange is thus: 6 2+][Na+] [M kM/Na (obs) = 00 (28) 6+] [M] [Na T

36

R.P. Townsend · R. Harjula

but since [M]T = [M2+] (1 + k1[L–]), it follows that kM/Na kM/Na (obs) = 06 . 1 + k1[L–]

(29)

It can be seen, therefore, that ion association can apparently decrease the selectivity coefficient of the ion exchange reaction. This apparent decrease in selectivity can take place when k1[L–]>1, that is when k1 is large (strong complexing ligand), and/or when there is a large excess of ligand present in the solution. In the selective exchange of multivalent metal ions the equilibrium concentrations can be very low, well below the concentrations of potential complexing ligands (carbonates, silicates, aluminates, hydroxyl ions), so that appreciable amounts of free ligands can be present in the solution at equilibrium. Carbonate, bicarbonate and silicate ions form moderately strong complexes with most metal ions. For instance, the association constants of alkaline earth cations for carbonate and bicarbonate are in the range of 10–1000 [107, 108]. Similarly, silicate ions complex readily with many metal ions, e.g. with calcium (k1 = 1230) and magnesium (k1 = 1.5 ¥ 104) [108]. There seems to be no data on metal ion association with aluminate ions but it is likely that ion association is moderately strong here also. Cation Hydrolysis. Many metal hydroxides have a low solubility in moderately alkaline solutions arising from the hydrolysis of high alumina zeolites. For instance, most transition metals and magnesium precipitate at pH 9–10 and at this pH range carbonates are likely to precipitate other metals such as calcium, strontium and cadmium [109]. Such precipitation phenomena can seriously distort the measurements of ion exchange selectivities. In addition, even when the metal concentrations are below the limits of hydroxide precipitation, hydrolysed species, such as MOH+, M(OH)2 (aq) and M(OH)–3 , often form the majority of the metal species in solution. For the determination of the ion exchange selectivity coefficient, the concentration of free, non-hydrolysed metal cation should be known. If the concentrations of hydrolysed species are used, a too low value may again be obtained for the selectivity coefficient. 3.2.3 Colloidal Solids in Suspension

Very fine colloidal particles in the nanometre size range may be left suspended in solution when centrifugation or filtration is used for the phase separation operation during the measurement of ion exchange equilibria in zeolites and other inorganic materials [75]. Especially in the study of the ion exchange of radionuclides, which can be present in very low concentrations, the presence of colloidal particles carrying the metal cation under study can bring large errors in the determination of discrete metal cation concentrations in solution. Depending on the analytical technique used, metal ions associated with colloidal particles may be indistinguishable from free metal ions. For instance, in the determination of the distribution coefficients of radionuclides (Eq. 15), large errors may take place [75]. In cases where the metal ion concentration in the col-

Ion Exchange in Molecular Sieves by Conventional Techniques

37

loidally suspended zeolite is much larger than that of the free metal ions, the measurement of KD (cm3 g–1) yields just the reciprocal of the concentration (g cm–3) of suspended zeolite in the solution, instead of the ratio of metal concentrations in the zeolite and solution phases [75]. The presence of colloidal zeolite particles in solution may also apparently decrease the selectivity of the zeolite for a given metal ion. This problem is encountered especially when very low elemental concentrations, corresponding to low degrees of loading, are measured by highly sensitive methods such as by the use of radioactive tracers [75] or by atomic emission or absorption spectrophotometry with plasma or graphite furnace atomisation. Considering again the exchange of a divalent metal cation M2+ for sodium as an example, the observed selectivity coefficient kM/Na (obs) would be given by 3 2+][Na+]2 [M kM/Na(obs) = 0004 4 +]2 2+ ([M ]+[M]c)[Na

(30)

where [M]c is the amount of metal M in the suspended colloids per unit volume of solution. Recent studies indicate that as much as 3 mg dm–3 of suspended zeolite may be present in solution after centrifugation with a low-speed centrifuge (G = 2000) [75]. The results of several studies in the past have shown very low selectivities and “strange” selectivity gradients compared to more recent studies. For instance, in the study of calcium, strontium and barium exchanges in zeolites NaX and NaY, selectivities were low for these ions at low degrees of loading and then increased, finally exhibiting maxima at high loadings [101, 110]. In these experiments initial metal concentrations were very low (and the corresponding equilibrium concentrations even much lower) and cation loadings in the zeolite were increased by increasing the initial metal ion concentrations.Appearance of maxima in the selectivity plots is difficult to rationalise in the absence of a phase change (Sect. 2.1) since one would expect that the most selective cation sites would be occupied first so that the selectivity would steadily decrease with cation loading. This common pattern has been observed in other studies, carried out in isonormal solutions at considerably higher concentrations (0.1–1.2 g equiv–1) for most alkali and alkaline earth cations in NaX and NaY [72, 102, 111]. It is therefore likely that the observed low selectivities at low metal loadings and very low solution concentrations for NaX and NaY are due to ion association or suspended colloidal zeolite in the solution phase, since, at these very low solution concentrations, most of the metal ions in the solution may have been present as other species rather than as free metal cations. Similar decreases of selectivity were observed for the calcium exchange in NaX and NaY in dilute isonormal solutions (NT <0.1–0.001 equiv dm–3), but, in more concentrated solutions, the selectivity was independent of total ion concentration in solution (NT = 0.1–1.2 g equiv dm–3) [72]. Theoretically, after a correction is made to the selectivity coefficients (Eq. 2) for solution phase non-ideality (Eq. 7), the obtained corrected selectivity coefficients are then independent of the total ion concentration in the solution at any given degree of ion loading in the zeolite, provided that no other parallel reac-

38

R.P. Townsend · R. Harjula

tions (salt imbibition, ion association) affecting the metal ion distribution between the zeolite and solution phase take place. This means that one can in principle check whether the selectivity coefficient is unaffected by the above factors by carrying out the determinations of selectivity in isonormal solutions of different total ion concentrations. In general, these interfering phenomena are strongest at low metal concentrations, when the metal ion concentrations become comparable to or lower than the concentration of interfering species (e.g. complexing ligands, suspended particles). In many cases these interferences can be avoided by using sufficiently high total ion concentrations in solution; however, in some cases, interferences may operate even in rather high concentrations (NT = 0.1–0.4 equiv dm–3). Additional interference in the determination of selectivity may be caused in concentrated solutions (NT >1 equiv dm–3) by salt imbibition [61, 112].

4 Concluding Remarks In this chapter an overview has been given of some recent developments in our understanding of ion exchange in molecular sieves, with particular reference to experimental implications and methods. Research activity has declined from what was a quite high level in the 1960s and 1970s; the case for more fundamental studies on this topic seems compelling, especially the kinetic aspects. Specifically, three final remarks are made. The first of these arises out of the complexities one encounters when attempting to compare data from various sources or when predicting exchange behaviour. The need for a simplification and rationalisation of the diverse theoretical approaches and descriptions of exchange behaviour is obvious; it is hoped that current attempts to achieve this [113, 114] will continue and bear fruit. Secondly, the relative neglect of detailed kinetic studies in recent years has been noted. Although our attempts to describe theoretically the rates of ion exchange (let alone predict them) remain simplistic (Sect. 2.4), nevertheless it should be obvious to the reader that most applications involving ion exchange processes in molecular sieves are likely to be kinetically controlled. There remain major theoretical and computational challenges in this area which will entail the utilisation of the current rapidly growing computational power together with increasingly sophisticated models to throw light on exchange rate processes, both at the atomistic and mesoscopic scales. Finally, computational approaches are obviously only as good as the experimental studies which underpin and validate them. Consequently, we hope that the growing awareness of the experimental pitfalls and complexities which can hinder the acquisition of reliable data [115] will encourage further fundamental studies on ion exchange processes, not only in zeolites, but especially in the aluminophosphate families of molecular sieves, where so much unexplored territory remains.

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39

References 1. Helfferich F (1962) Ion exchange. McGraw-Hill, London, p 12 2. Costa E, de Lucas A, Zarca J, Sanz FJ (1987) Lat Am J Chem Eng Appl Chem 17:135 3. Dyer A (1988) An introduction to zeolite molecular sieves, 1st edn. Wiley, Chichester, pp 76, 80, 83 4. Schwuger MJ, Liphard M (1989) Fundamentals of phosphate substitution in detergents by zeolites. In: Karge HG, Weitkamp J (eds) Zeolites as catalysts, sorbents and detergent builders. Elsevier, Amsterdam. Stud Surf Sci Catal 46:673 5. Denkewicz RP Jr, Monino AG, Russ DE, Sherry HA (1995) J AOCS 72:11 6. Adams CJ, Araya A, Carr SW, Chapple AP, Franklin KR, Graham P, Minihan AR, Osinga TJ, Stuart JA (1997) Stud Surf Sci 105:1667 7. Adams CJ, Araya A, Cunningham KJ, Franklin KR, White IF (1997) J Chem Soc Faraday Trans 93:499 8. Sorlie AA, Bowerman BS, Czajkovski C, Dyer RS (1998) Low-level liquid radioactive waste treatment at Murmansk, Russia: facility upgrade and expansion. Proceedings of the Symposium on Waste Management at Tucson, Arizona, paper 56–07 9. Robinson SM, Arnold WD, Byers CH (1990) Design of fixed-bed ion exchange columns for wastewater treatment. Proceedings of the Symposium on Waste Management at Tucson, Arizona, vol 2, p 1635 10. Horsley DMC, Howden M (1990) Trans I Chem E 68(B):140 11. Cauthen BE, Taylor JC (1990) Liquid radwaste process optimisation at Catawba nuclear station. Proceedings of the Symposium on Waste Management at Tucson, Arizona, vol 2, p 305 12. Ekectukwu OE, Loucks LE (1992) Reduction of caesium and cobalt activity in liquid radwaste processing using clinoptilolite zeolite at Duke power company. Proceedings of the Symposium on Waste Management at Tucson, Arizona, vol 2, p 1635 13. James KL, Miller CC (1992) The impact of ion exchange media and filters on LLW processing. Proceedings of the Symposium on Waste Management at Tucson, Arizona, vol 2, p 1575 14. Barrer RM, Rees LVC, Ward DJ (1964) Proc R Soc London Ser A 237:180 15. Ames LL (1964) Am Miner 49:1099 16. Wolf F, Furtig H (1965) Kolloid Z Z Polymer 206:48 17. Sherry HS, Walton HF (1967) J Phys Chem 71:1457 18. Barri SAI, Rees LVC (1980) J Chromatogr 201:21 19. Wiers BH, Grosse RJ, Cilley WA (1982) Environ Sci Technol 16:617 20. Franklin KR, Townsend RP (1985) J Chem Soc Faraday Trans 1 81:1071 21. Drummond D, De Jonge A, Rees LVC (1983) J Phys Chem 87:1967 22. Wilson ST (1991) Synthesis of AlPO4-based molecular sieves. In: Van Bekkum H, Flanigen EM, Jansen JC (eds) Introduction to zeolite science and practice. Elsevier, Amsterdam, Stud Surf Sci Catal 58:137 23. Schoonheydt RA (1991) Clays from two to three dimensions. In: Van Bekkum H, Flanigen EM, Jansen JC (eds) Introduction to zeolite science and practice. Elsevier, Amsterdam, Stud Surf Sci Catal 58:201 24. Smith JV (1989) Towards a comprehensive mathematical theory for the topology and geometry of microporous materials. In: Jacobs PA, van Santen RA (eds) Zeolites: facts, figures, future, part A. Elsevier, Amsterdam, Stud Surf Sci Catal 49:29 25. Smith JV (1988) Chem Rev 88:149 26. Van Koningsveld H (1991) Structural subunits in silicate and phosphate structures. In: Van Bekkum H, Flanigen EM, Jansen JC (eds) Introduction to zeolite science and practice. Elsevier, Amsterdam, Stud Surf Sci Catal 58:35 27. Kokotailo GT, Fyfe CA, Kennedy GJ, Gobbi GC, Strobl H, Pasztor CT, Barlow GK, Bradley S (1986) Zeolite structural investigations by high resolution solid state mas nmr. In Murakami Y, Iijima A, Ward JW, New developments in zeolite science and technology. Kodansha, Tokyo, Elsevier, Amsterdam, p 361

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28. Barrer RM, Klinowski J (1972) J Chem Soc Faraday Trans 1 68:73 29. Engelhardt G (1991) Solid state nmr spectroscopy applied to zeolites. In: Van Bekkum H, Flanigen EM, Jansen JC (eds) Introduction to zeolite science and practice. Elsevier, Amsterdam, Stud Surf Sci Catal 58:285 30. De Kock FP, van Deventer (1995) Chem Eng Comm 135:21 31. Barrer RM, Klinowski J (1977) Phil Trans Roy Soc 285:637 32. Smolders E, van Dun JJ, Mortier WJ (1991) J Phys Chem 95:9908 33. Woolery GL, Alemany LB, Dessau RM, Chester AW (1986) Zeolites 6:14 34. Dessau RM, Schmitt KD, Kerr GT, Woolery GL, Alemany LB (1987) J Catal 104 : 484 35. Handreck GP, Smith TD (1989) J Chem Soc Faraday Trans 1 85:645 36. Chester AW, Chu YF, Dessau RM, Kerr GT, Kresge CT (1985) J Chem Soc Chem Commun 1985:289 37. Chu P, Dwyer FG (1983) Zeolites 3:72 38. Matthews DP, Rees LVC (1986) Chem Age India 37:353 39. Chu P, Dwyer FG (1988) Zeolites 8:423 40. McAleer AM, Rees LVC, Nowak AK (1991) Zeolites 11:329 41. Watling TC, Rees LVC (1994) Zeolites 14:687 42. Watling TC, Rees LVC (1994) Zeolites 14:693 43. Recommendations on ion exchange nomenclature (1972) Pure Appl Chem 29:619 44. McGlashan ML (1979) Chemical thermodynamics, 1st edn.Academic Press Inc., London, p 111 45. Townsend RP (1991) Ion exchange in zeolites. In: Van Bekkum H, Flanigen EM, Jansen JC (eds) Introduction to zeolite science and practice. Elsevier, Amsterdam, Stud Surf Sci. Catal 58:359. See also the revision of this paper by Townsend RP, Coker EN, in print 46. Barrer RM, Townsend RP (1976) J Chem Soc Faraday Trans 1 72:2650 47. Barrer RM, Townsend RP (1978) J Chem Soc Faraday Trans 1 74:745 48. Barrer RM, Munday BM (1971) J Chem Soc A 2909 49. Meier WM, Olson DH (1992) Atlas of zeolite structure types, 3rd edn. ButterworthHeinemann, London 50. Barrer RM, Munday BM (1971) J Chem Soc A 2914 51. Olson DH, Sherry HS (1968) J Phys Chem 72:4095 52. Hansen S, Häkansson U, Landa-Canovas AR, Fälth L (1993) Zeolites 13:276 53. Taylor AM, Roy R (1964) Am Miner 49:656 54. Fletcher P, Townsend RP (1981) J Chem Soc Faraday Trans 2 77:2077 55. Fletcher P, Townsend RP (1983) J Chem Soc Faraday Trans 2 79:419 56. Townsend RP (1986) Pure Appl Chem 58:1359 57. Barrer RM, Townsend RP (1984) J Chem Soc Faraday Trans 2 80:629 58. Gaines GL, Thomas HC (1953) J Chem Phys 21:714 59. Barrer RM, Townsend RP (1985) Zeolites 5:287 60. Barrer RM, Klinowski J (1974) J Chem Soc Faraday Trans 1 70:2080 61. Barrer RM, Walker AJ (1964) Trans Faraday Soc 60:171 62. Townsend RP, Fletcher P, Loizidou M (1984) Studies on the prediction of multicomponent ion-exchange equilibria in natural and synthetic zeolites. In: Olson D, Bisio A (eds) Proceedings of the Sixth International Zeolite Conference. Butterworths, UK, p 110 63. O’Connor JE, Townsend RP (1985) Zeolites 5:158 64. Loizidou M, Townsend RP (1987) Zeolites 7:153 65. Loizidou M, Townsend RP (1987) J Chem Soc Dalton Trans 1911 66. Fletcher P, Townsend RP (1985) J Chem Soc Faraday Trans 1 81:1731 67. Franklin KR, Townsend RP (1988) J Chem Soc Faraday Trans 1 84:687 68. Pabalan RT (1994) Geochim Cosmochim Acta 58:4573 69. Franklin KR, Townsend RP (1985) J Chem Soc Faraday Trans 1 81:3127 70. Franklin KR, Townsend RP (1988) J Chem Soc Faraday Trans 1 84:2755

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71. Harjula R, Lehto J, Pothuis JH, Dyer A, Townsend RP (1991) Hydrolysis and trace Ca2+ exchange in zeolites NaX and NaY. In: Proceedings International Conference Ion Exchange, ICIE ‘91. Kodansha, Tokyo, p 157 72. Harjula R, Dyer A, Pearson SD, Townsend RP (1992) J Chem Soc Faraday Trans 88: 1591 73. Harjula R, Lehto J, Pothuis JH, Dyer A, Townsend RP (1993) J Chem Soc Faraday Trans 89:971 74. Harjula R, Dyer A, Townsend RP (1993) J Chem Soc Faraday Trans 89:977 75. Harjula R, Lehto J, Pothuis JH, Dyer A, Townsend RP (1993) J Chem Soc Faraday Trans 89:1877 76. Franklin KR, Townsend RP, Whelan SJ, Adams CJ (1986) Ternary exchange equilibria involving H3O+, NH +4 and Na+ ions in synthetic zeolites of the faujasite structure. In: Murakami Y, Iijima A, Ward JW (eds) New developments in zeolite science and technology. Kodansha, Tokyo, Elsevier, Amsterdam, p 289 77. Fletcher P, Townsend RP (1981) J Chem Soc Faraday Trans 2 77:965 78. Bajpai RK, Gupta AK, Gopala-Rao M (1973) J Phys Chem 77:1288 79. Brignal WJ, Gupta AK, Streat M (1976) Theory and practice in ion exchange, Soc Chem Ind, London, paper 11 80. Franklin KR, Townsend RP (1988) Zeolites 8:367 81. Zuyi T, Gengliang Y (1995) React Funct Polymers 27:117 82. Fletcher P, Franklin KR, Townsend RP (1984) Phil Trans R Soc London A 312:141 83. Gopala Rao M (1995) Sep Sci Technol 30:1385 84. Elprince AM, Babcock KL (1975) Soil Sci 120:332 85. Smith RP, Woodburn ET (1978) AIChEJ 24:577 86. Shallcross DC, Hermann CC, McCoy BJ (1988) Chem Eng Sci 43:279 87. Perona JJ (1993) AIChEJ 39:1716 88. Robinson SM, Arnold DW, Byers CH (1991) ACS Symp Ser 468:133 89. Robinson SM, Arnold WD, Byers CH (1994) AlChEJ 40:2045 90. Ruthven DM (1994) Principles of adsorption and adsorption processes. Wiley, New York 91. Brown LM, Sherry HS, Krambeck FJ (1971) J Phys Chem 75:3846 92. Brown LM, Sherry HS (1971) J Phys Chem 75:3855 93. Dyer A, Townsend RP (1973) J Inorg Nucl Chem 35:3001 94. Duffy SC, Rees LVC (1974) J Chromatogr 102:149 95. Brooke NM, Rees LVC (1968) Trans Faraday Soc 64:3383 96. Barrer RM, Rees LVC (1964) J Phys Chem Solids 25:1035 97. Hiester NK, Vermuelen T, Klein G (1963) Adsorption and ion exchange. In: Perry JH, Chilton CH (eds) Chemical engineer’s handbook. McGraw-Hill, New York, p 16-1 98. Klein G (1985) AIChE Symp Ser No 242, 81:28 99. Collins ED, Campbell DO, King LJ, Knauer JB, Wallace RM (1985) Evaluation of zeolite mixtures for decontaminating high-activity-level water at the Three Mile Island Unit 2 nuclear power station. Technical Document 337, International Atomic Energy Agency, Vienna, p 43 100. Barrer RM, Klinowski J, Sherry HS (1973) J Chem Soc Faraday Trans 2 69:1669 101. Barrer RM, Davies JA, Rees LVC (1968) J Inorg Nucl Chem 30:3333 102. Sherry HS (1968) J Phys Chem 72:4086 103. Townsend RP, Franklin KR, O’Connor JF (1984) Adsorption Sci Technol 1:269 104. Engelhardt G, Michel D (1987) High resolution solid-state nmr of silicates and zeolites. Wiley, Chichester, p 213 105. Helfferich F (1962) Ion exchange. McGraw-Hill, London, p 16 106. Mimura H, Kanno T (1987) J Nucl Sci Technol 22:284 107. Martell AE, Smith RM (1982) Critical stability constants, vol 5. Plenum Press, New York 108. Hogfeldt E (1982) Stability of metal-ion complexes. IUPAC chemical data series no 21. Pergamon Press, Oxford

42 109. 110. 111. 112. 113.

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Baes CF, Mesmer RE (1976) Hydrolysis of cations. Wiley, New York Barrer RM, Rees LVC, Shamsuzzoha M (1966) J Inorg Nucl Chem 28:629 Sherry HS (1966) J Phys Chem 70:1158 Lowe BM, Pope CG (1989) J Chem Soc Faraday Trans 1 85:945 Harjula R, Lehto J (1997) Harmonisation of ion exchange formulations and nomenclature: what could be done? In: Dyer A, Hudson MJ, Williams PA (eds) Royal Society of Chemistry Spec. Pub. No. 196 114. Soldatov VS (1995) React Funct Polymers 27:95 115. Lehto J, Harjula R (1995) React Funct Polymers 27:121

Solid-State Ion Exchange in Microporous and Mesoporous Materials Hellmut G. Karge 1 and Hermann K. Beyer 2 1 2

Fritz Haber Institute of the Max Planck Society, Faradayweg 4–6, 14195 Berlin, Germany; e-mail: [email protected] Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, Pusztaszeri út 59–67, 1025 Budapest, Hungary; e-mail: [email protected]

Dedicated to Professor Gerhard Ertl on the occasion of his 65th birthday . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

1

Introduction

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Concept of Solid-State Ion Exchange (SSIE) . . . . . . . . . . . . . 49

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Experimental Procedures for SSIE . . . . . . . . . . . . . . . . . . 50

4

Techniques for Monitoring SSIE

4.1 4.2 4.3 4.4 4.5 4.6 4.7

4.10 4.11 4.12

Introductory Remarks . . . . . . . . . . . . . . . . . . . Chemical Analysis (CA) . . . . . . . . . . . . . . . . . . Thermogravimetric Analysis (TGA) . . . . . . . . . . . Temperature-Programmed Evolution of Gases (TPE) . . Combination of TGA and TPE . . . . . . . . . . . . . . . X-ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . Infrared Spectroscopy (IR) and Fourier Transform (FT) Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . Electron Spin Resonance Spectroscopy (ESR) . . . . . . Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS NMR) . . . . . . . . . . . . . . . . . Mössbauer Spectroscopy . . . . . . . . . . . . . . . . . . X-ray Photoelectron Spectroscopy (XPS) . . . . . . . . . X-ray Spectroscopy: EXAFS, XANES . . . . . . . . . . .

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Systems Investigated . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.1.7 5.1.8 5.1.9 5.1.10

SSIE of Alkaline (M+) and Alkaline Earth (M2+) Metal Cations . Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . Application of TPE . . . . . . . . . . . . . . . . . . . . . . . . . SSIE and Lattice Energy . . . . . . . . . . . . . . . . . . . . . . Application of a Combination of TGA and TPE . . . . . . . . . Stoichiometry of SSIE of M+ and M2+ Halides with H-Zeolites . Preservation of Crystallinity upon SSIE . . . . . . . . . . . . . Role of the Nature of the Anions . . . . . . . . . . . . . . . . . SSIE of M+ and M2+ Halides with H-Zeolites Investigated by IR SSIE of M+ and M2+ Halides with Na-Zeolites Investigated by IR SSIE of M+ and M2+ Halides with Na-Zeolites Investigated by MAS NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.8 4.9

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5.4.4

Catalytic Activity of M2+-Zeolites Prepared via SSIE . . . . . . SSIE of Lanthanum (La3+) Cations . . . . . . . . . . . . . . . . Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . SSIE of La3+ Chloride with H-Zeolites Investigated by TPE . . . Stoichiometry of SSIE of La3+Chloride with H-Zeolites . . . . . SSIE of La3+ Chloride with H-Zeolites Investigated by IR . . . . SSIE of La3+ Chloride with Na-Zeolites Investigated by XRD . . SSIE of La3+ Chloride with Na-Zeolites Investigated by IR . . . SSIE of La3+ Chloride with Na-Zeolites Investigated by 23Na MAS NMR . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Activity of La3+-Zeolites Prepared via SSIE . . . . . . SSIE of Other Transition Metal Cations . . . . . . . . . . . . . . Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . SSIE of Copper, Silver and Gold Compounds with Zeolites . . . Introduction of Copper . . . . . . . . . . . . . . . . . . . . . . Introduction of Silver . . . . . . . . . . . . . . . . . . . . . . . Introduction of Gold . . . . . . . . . . . . . . . . . . . . . . . . SSIE of Zinc, Cadmium and Mercury Compounds with Zeolites Introduction of Zinc . . . . . . . . . . . . . . . . . . . . . . . . Introduction of Cadmium . . . . . . . . . . . . . . . . . . . . . Introduction of Mercury . . . . . . . . . . . . . . . . . . . . . . SSIE of Iron, Cobalt, Nickel and Manganese Compounds with Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction of Iron . . . . . . . . . . . . . . . . . . . . . . . . Introduction of Cobalt . . . . . . . . . . . . . . . . . . . . . . . Introduction of Nickel . . . . . . . . . . . . . . . . . . . . . . . Introduction of Manganese . . . . . . . . . . . . . . . . . . . . SSIE of Vanadium, Niobium, Antimony, Chromium, Molybdenum and Tungsten Compounds with Zeolites . . . . . Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . Introduction of Vanadium . . . . . . . . . . . . . . . . . . . . . Introduction of Niobium and Antimony . . . . . . . . . . . . . Introduction of Chromium . . . . . . . . . . . . . . . . . . . . Introduction of Molybdenum . . . . . . . . . . . . . . . . . . . Introduction of Tungsten . . . . . . . . . . . . . . . . . . . . . SSIE of Noble Metal Cations with Zeolites . . . . . . . . . . . . Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . Preparation of Noble-Metal-Containing Large-Pore Zeolite Catalysts by SSIE . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Noble-Metal-Containing Narrow-Pore Zeolites by SSIE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of Bifunctional Zeolite Catalysts by SSIE . . . . . .

6

Modified SSIE and Related Processes . . . . . . . . . . . . . . . . . 162

6.1 6.2

Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . 162 Contact-Induced Ion Exchange . . . . . . . . . . . . . . . . . . . . 163

5.1.11 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.2.6 5.2.7 5.2.8 5.3 5.3.1 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.3 5.3.3.1 5.3.3.2 5.3.3.3 5.3.4 5.3.4.1 5.3.4.2 5.3.4.3 5.3.4.4 5.3.5 5.3.5.1 5.3.5.2 5.3.5.3 5.3.5.4 5.3.5.5 5.3.5.6 5.4 5.4.1 5.4.2 5.4.3

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Solid-State Ion Exchange in Microporous and Mesoporous Materials

6.3 6.3.1

Gas-Phase-Mediated Processes Related to SSIE . . . . . . . . Introduction of Cations into Zeolites Through a Vapor Phase Containing the In-Going Species . . . . . . . . . . . . . . . . 6.3.2 Effect of Additional Molecules in the Vapor Phase on SSIE at Elevated Temperatures . . . . . . . . . . . . . . . . . . . . 6.3.3 Oxidative and Reductive SSIE . . . . . . . . . . . . . . . . . . 6.3.3.1 Oxidative SSIE of Ag0, Cu0 and Pd0 in the Presence of O2 or Cl2 6.3.3.2 Reductive SSIE of Ga2O3 . . . . . . . . . . . . . . . . . . . . . 6.3.3.3 Reductive SSIE of In2O3 . . . . . . . . . . . . . . . . . . . . .

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166 168 168 168 171

7

The Role of Water in and Mechanisms of SSIE . . . . . . . . . . . . 181

7.1 7.2

Role of Water in SSIE . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Possible Mechanisms of SSIE . . . . . . . . . . . . . . . . . . . . . 184

8

Kinetics of SSIE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

9

Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . 191

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

Abbreviations A A AES ALPHA AlPO4-5 At At = • , A• ATS B BET BETA CA CE CLIN DENOX DEXAFS DPPH DRS DTG DH _ ad E EL EDAX EDS

zeolite structure (cf. [180]) absorbance, e.g., of OH groups in IR, A(OH), etc. Auger electron spectroscopy zeolite structure (cf. [180]) microporous aluminophosphate structure (cf. [180]) absorbance of a typical band at time t absorbance after infinite time t microporous aluminophosphate structure (cf. [180]) Brønsted (e.g., acid sites, acidity) Brunauer-Emmet-Teller (method for determination of surfaces) zeolite structure (cf. [180]) chemical analysis conventional ion exchange clinoptilolite, zeolite structure (cf. [180]) process for removal of nitrogen oxides dispersive extended X-ray absorption fine structure 2,2-diphenyl-1-picrylhydrazyl, ESR standard diffuse reflectance spectroscopy (in IR or UV-Vis range) differential thermogravimetry (differential) heat of adsorption most frequent energy of desorption in an energy distribution lattice energy energy dispersive X-ray (spectroscopy) energy dispersive X-ray (spectroscopy)

46 EDX ESEM ESR EXAFS EU-1 FER FTIR GHSV H HFS HT I IR IS L LT M MAPO-36 MAS NMR MCM-41 MFI MM MOR MS nM /nAl nSi /nAl OFF Py QS RSSIE SAPO-n SEM SCR SHFS SSIE STEM T TCD TEA+ TEM TG TGA Tp TPE TPEH TPD

H.G. Karge · H.K. Beyer

energy dispersive X-ray (spectroscopy) electron spin echo modulation (spectroscopy) electron spin resonance (spectroscopy) extended X-ray absorption fine structure zeolite structure (cf. [180]) ferrierite, zeolite structure (cf. [180]) Fourier transform infrared (spectroscopy) gaseous hourly space velocity magnetic induction (related to magnetic field strength), in tesla hyperfine splitting (of ESR signals) high-temperature (peak, etc.) nuclear spin quantum number infrared (spectroscopy) isomer shift Lewis (e.g., acid sites, acidity) low-temperature (peak, etc.) metal microporous Mg aluminophosphate structure, (cf. [180]) magic angle spinning nuclear magnetic resonance (spectroscopy) mesoporous silicate structure (cf. Vol. 1, Chap. 4) zeolite structure (cf. [180]) montmorillonite mordenite, zeolite structure (cf. [180]) mass spectrometry ratio of metal to aluminum atoms ratio of silicon to aluminum atoms in the framework offretite, zeolite structure (cf. [180]) pyridine quadrupole splitting reductive solid-state ion exchange microporous silicoaluminophosphates, n = 5, 34, etc. (cf. [180]) scanning electron microscopy selective catalytic reduction super hyperfine splitting (of ESR signals) solid-state ion exchange scanning transmission electron microscopy tesla, unit of magnetic induction (related to magnetic field strength) thermal conductivity detector tetraethylammonium cation, derived from TEAOH (template) transmission electron microscopy thermogravimetry thermogravimetric analysis peak temperature temperature-programmed evolution (of gases) temperature-programmed evolution of hydrogen temperature-programmed desorption

Solid-State Ion Exchange in Microporous and Mesoporous Materials

TPDA TPO TPR USY UV-Vis X X XAES XAS XANES XPS XRD Y Y ZK-5 ZSM-5 ZSM-35 ZSM-48

47

temperature-programmed desorption of ammonia temperature-programmed oxidation temperature-programmed reduction ultra stable Y-zeolite ultraviolet/visible (spectroscopy) zeolite structure (cf. [180]) conversion of a compound, in percent upon reaction, X(%) X-ray induced Auger electron spectroscopy X-ray absorption spectroscopy X-ray absorption near edge structure X-ray photoelectron spectroscopy X-ray diffraction zeolite structure (cf. [180]) yield of a product upon reaction, in percent, Y(%) zeolite structure (cf. [180]) zeolite structure (cf. [180]) zeolite structure (cf. [180]) zeolite structure (cf. [180])

1 Introduction In zeolites, i.e., crystalline microporous aluminosilicates [1–3], the framework exhibits negative charges as a consequence of the incorporation of trivalent aluminum atoms instead of tetravalent silicon. The same situation is encountered in most cases of related crystalline microporous (cf. [4, 5]) and crystalline-like mesoporous materials, e.g., in isomorphously (by Fe3+, Ga3+, B3+, etc.) substituted zeolite structures and Al-containing M41S materials, respectively (cf., e.g., [6, 7]). These negative charges of the frameworks must be compensated by the positive charges of extra-framework cations or via the interaction of the framework oxygen atoms with protons under formation of acid hydroxyls, i.e., so-called Brønsted centers (cf., e.g., [8]). According to the synthetic procedures (see, e.g., [9, 10]), usually Na+, K+ or template cations are present in as-synthesized micro- or mesoporous materials and play the role of charge-compensating species; proton attack of the oxygen atoms of the framework occurs,e.g.,on removal of the organic template molecules. However, the charge-compensating entities (alkali metal cations, protons, etc.) can be replaced by other cations, and this makes zeolites inorganic cation exchangers (cf., e.g., [11, 12]). The ion-exchange capacity of microporous materials, especially of zeolites such as LTA- and P-type zeolites (see [13,14]),is the basis for their worldwide application as detergent builders [15]. However, ion exchange is also one of the most important processes for post-synthesis modification of microporous materials, for instance, in order to tailor adsorbents and catalysts. Ion exchange is, therefore, carried out not only in research laboratories, but also on an industrial scale. In what follows, we refer to zeolites as the most important representatives of microporous materials, in particular in ion exchange. However, related microporous and mesoporous materials may be treated in an analogous manner.

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Conventionally, ion exchange is carried out by suspending the zeolite powder in aqueous solutions of salts which contain the desired in-going cation, or in mineral acids to introduce protons into acid-resistant zeolites. After stirring the suspension for a period of time (frequently at temperatures higher than ambient), the zeolite powder and the solution must be separated. Generally, the procedure must be repeated several times to achieve a sufficiently high degree of exchange. Ion exchange in aqueous solution is, thus, a time-consuming process and, in any case, it produces large amounts of waste solutions that must be regenerated or discarded in an environmentally friendly manner. Moreover, in several instances, it is very difficult or even totally impossible to introduce particular cations into a given zeolite by means of conventional ion exchange in aqueous solution. Examples are encountered where the cations are available only in complexes which are too bulky to enter the narrow pores of the respective zeolites (vide infra, e.g., Sect. 5.4.3). The situation is significantly different in solid-state ion exchange, where dry powders of zeolites and salts or oxides containing the in-going cation are reacted. In favorable cases, very high degrees of exchange can be achieved in only one step and, also, certain metal cations can be incorporated into narrow-pore zeolites where exchange attempts in aqueous solutions fail (vide infra). Thus, solid-state ion exchange appears to be in many cases an interesting alternative to conventional exchange and has indeed attracted ever-growing attention and experienced increasing application during the past two decades. Scattered in the literature, some early work was reported on solid-state ion exchange as early as the beginning of the 1970s. For instance, Rabo et al. [16, 17] used the reaction between KCl and an acidic form of a faujasite-type catalyst, Ca,H-Y, to remove residual Brønsted acid sites from the microporous solid and, thus, eliminate any catalytic activity for acid-catalyzed reactions. Infrared (IR) spectroscopy was used to confirm the elimination of the acidic OH groups. Another group, viz., Clearfield and co-workers, published a two-page “Recent Research Report” [18] on the solid-state reaction between A-, X-, and Y-type zeolite samples and salts of transition metals such as Ni and Co, monitoring the process by electron spin resonance spectroscopy (ESR). The ion exchange between two physically contacted forms of a zeolite, viz., one sample of zeolite A loaded with Li+ and another sample containing Na+, was studied by Kokotailo et al. [19] and Fyfe et al. [20]; these authors employed X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) to prove the exchange in the solid state. Similarly, it was suspected that, on applying the KBr technique in IR spectroscopy of zeolites, an exchange of K+ of the KBr matrix and the zeolite cations may occur. Systematic work on solid-state ion exchange in zeolites started, however, only in the mid-1980s in the groups of Slinkin in Moscow [21] and Karge in Berlin [22]. Since then, a large body of systems has been investigated, numerous techniques for monitoring solid-state ion exchange in zeolites have been developed, and specific problems related to solid-solid reactions in these systems tackled. Solid-state ion exchange has, thus, become a well-established method for postsynthesis modification of zeolites and related materials. This technique is, therefore, frequently employed in solving special problems of post-synthesis modification of zeolites both in research and industrial processes.

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Solid-State Ion Exchange in Microporous and Mesoporous Materials

Earlier reviews of the field were somewhat limited and focused on the introduction of transition metal cations [23] or alkali metal and rare earth cations into zeolites [24, 25]. The present contribution intends to provide a more extended and almost complete overview of the state of the art and, also, to address more recent developments such as reductive solid-state ion exchange, related processes and questions concerning the role of water in contact-induced ion exchange, comparison of conventional and solid-state exchange, and incorporation of noble metals into narrow-pore zeolites.

2 Concept of Solid-State Ion Exchange (SSIE) Scheme 1 shows the general chemistry of ion exchange in aqueous solution (conventional exchange, CE) and, in contrast, of exchange in the solid state (solid-state ion exchange, SSIE). In the case of SSIE, the main feature is that, usually at elevated temperatures, the pure solids are brought to reaction, whereas in CE the solvent (water) is involved as a third component, and the in-going and out-going cations are solvated. Solvation of the cations in many cases impedes ion exchange; this cannot occur in SSIE. In both SSIE and CE, the exchange process almost always leads to an equilibrium (cf. Schemes 1a and 1b). However, in solid-state ion exchange, a Conventional exchange in aqueous solutions (CE) p M1n1+ (Z–)n1 + q [M2(H2O)w2]n2+ + (n2/m)q [A(H2O)wA]m– + wH2O ¤ x M2n2+ (Z–)n2 + (p – [n1/n2]x) M1n1+ (Z–)n1 + (n1/n2)x [M1(H2O)w1]n1+ +(n2/n1)x

[A(H2O)w]m– + (q – x)

[M2(H2O)w2

]n2+ + (q – x)

n2/m

(1)

[A(H2O)wA]m– + w¢H2O

b Solid-state ion exchange (SSIE) p M1n1+ (Z–)n1 + q M2n2+ (Am–)n2/m ¤ xM2n2+ (Z–)n2 + (p – [n1/n2]x) M1n1+(Z–)n1 m– + (n1/n2)x M1n1+(Am–)n1/m + (q – x) Mn2+ 2 (A )n2/m

(2)

c Solid-state ion exchange under removal of one (volatile) component of the product M1 = H; n1 = 1; M2 = Ca; n2 = 2; A = Cl; m = 1 p H-Z+q CaCl2 Æ x Ca-Z2 + (p-2x) H-Z + 2x HCl≠ + (q-x) CaCl2

(3)

d Solid-state ion exchange with a degree of 100% with respect to H-Z ; p = 2x p H-Z + q CaCl2 Æ p/2 Ca-Z2 + p HCl≠+ (q – p/2) CaCl2

(4)

e Stoichiometric solid-state ion exchange; q = p/2 p H-Z + p/2 CaCl2 Æ p/2 Ca-Z2 + p HCl≠

(5)

Scheme 1. General formulation of ion exchange in zeolites. M1 out-going cation, originally present in the zeolite to be modified; n1 valency of M1; M2 in-going cation to be introduced into the zeolite; n2 valency of M2; A anion of the M2-containing compound (salt, oxide); m valency of A; Z– monovalent fragment of the zeolite framework; p, q, x, w1, w2, w, w¢: stoichiometric coefficients

50

H.G. Karge · H.K. Beyer

the conditions can be chosen such that one component of the exchange product is continuously removed and the equilibrium shifted to the right side, so that an exchange degree of 100% may be achieved. This is illustrated in Scheme 1c for the hydrogen form of a zeolite, H-Z (M1 = H, n1 = 1), where, for sake of simplicity, M2 = Ca, n2 = 2 and A = Cl, m = 1. The solid-state reaction may be conducted in a mixture with the salt being present in a sub-stoichiometric (q

p/2) amount. This gives the possibility of achieving a certain desired degree of exchange.

3 Experimental Procedures for SSIE The procedures for solid-state ion exchange, both in laboratory experiments and on an industrial scale, are relatively simple. The main requirement is to achieve a mixing as intimate as possible of the zeolite powder and the component containing the in-going cation. This can be brought about by careful grinding in a mortar or by milling. However, there are cases where the zeolite lattice is sensitive to mechanical stress (cf. [26, 27]). In such a situation a different way of mixing may be chosen: the zeolite powder and the carefully ground salt or oxide of the in-going cation are suspended together in an inert vaporizable liquid such as n-hexane. After sufficient mixing by stirring or shaking, the liquid is evaporated, which yields a very intimate mixture of the crystallites of the zeolite and the respective salt or oxide (cf. [22]). In a further alternative procedure, the two components, i.e., the salt or oxide and the zeolite, are separately pretreated and ground and subsequently mechanically mixed under application of an ultrasonic treatment [28]. The dried intimate mixture of the zeolite and salt or oxide is then heated at a low rate (5–10 K min–1) to the reaction temperature which is usually between 675 and 875 K. Heating is carried out in a stream of an inert gas (e.g., air, N2 , He) or in high vacuum. In most cases it is sufficient to keep the mixture at the reaction temperature for 2–4 h; in particular cases it might be advisable to extend the reaction time up to 12–24 h. Long reaction times are especially required for systems in which the migration of the in-going and/or out-going species is slow. (cf. Sect. 8). In a number of instances, the first stage of SSIE proceeds easily and starts at relatively low temperatures (followed by a high-temperature process), or it happens that a fraction of the cations is already exchanged during grinding or milling of the mixtures. When the reaction is completed, the mixture is cooled to ambient temperature and analyzed (cf. Sect. 4). If an excess of the compound of the in-going cation has been applied, non-reacted salt can be removed by brief washing with water provided that the respective compound is soluble. Similarly, if the zeolite employed is not in the hydrogen form and an equilibrium has been reached (cf. Scheme 1b), the residual salt can be removed by brief washing and, if required, the procedure may be repeated. However, one has to be aware of the possibility that salts remained entrapped in the zeolite matrix (cf. [16, 17]).

Solid-State Ion Exchange in Microporous and Mesoporous Materials

51

4 Techniques for Monitoring SSIE 4.1 Introductory Remarks

An extremely large armory of techniques has been developed and applied to identify and monitor the process of solid-state ion exchange (SSIE). The respective methods can be distinguished as spectroscopic and non-spectroscopic techniques. Both kinds of experimental tools are usually applicable for both simply confirming the SSIE after completion of the reaction and monitoring the SSIE during the reaction by in situ measurements. Methods described in the following sections will be illustrated by a large number of pertinent examples, provided especially in Sect. 5. 4.2 Chemical Analysis (CA)

In those cases where a stoichiometric conversion has occurred or a removal of an excess of the in-going cations was possible, simple chemical analysis of the exchanged zeolite will be suitable to verify the fact of solid-state ion exchange and will also provide the ratio of nM2/nM1 (M1 = original cation, M2 = in-going, M1 replacing cation) in the product, i.e., the degree of exchange (for methods of analysis, cf. Vol. 5, Chap. 1, this series). 4.3 Thermogravimetric Analysis (TGA)

Since the solid-state ion reaction between hydrogen forms of zeolites and salts (fluorides, chlorides, bromides) or oxides implies the release of volatile products (HF, HCl, HBr, H2O), the exchange can be followed by measuring the weight loss of the mixture. Prior to the reaction, the salt/zeolite or oxide/zeolite mixture must be kept at about 400 K in a sensitive microbalance under dynamic vacuum or an inert gas stream in order to remove physically adsorbed water. Subsequently, the mixture is heated at a rate of 5–10 K min–1 until a constant weight is reached which usually occurs at about 900 K. From the weight loss, the degree of ion exchange can be calculated according to the stoichiometry (cf. Scheme 1). It should be noted, however, that not only the Brønsted acid centers but to some extent also silanol groups may react, so that the gravimetrically determined degree of exchange in some cases may exceed the value corresponding to the amount of ∫Al-(OH)-Si∫ exchange sites. In such a situation at least one additional technique listed in this section must be employed.

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4.4 Temperature-Programmed Evolution of Gases (TPE)

The thermal treatment in this technique is similar to that applied in TGA (vide supra). However, the solid-state reaction may be carried out in a quartz glass oven or in a stainless-steel device. The gases evolved, e.g., HCl or H2O, are determined by mass spectrometry (MS) or with a thermal conductivity detector (TCD). A suitable device including MS is seen in Fig. 1 and is described in more detail elsewhere [29].

A

B Fig. 1. A Scheme of the apparatus for temperature-programmed desorption or temperatureprogrammed evolution of gases: 1, roughing pump; 2, bellows; 3, turbo molecular pump; 4, swing gate valves; 5, ionization gauge and control unit; 6, sample holder and heating device (pan, see B); 7, temperature programmer; 8, ion source and mass filter; 9, secondary electron multiplier (SEM/MS); 10, dosing valves; 11, Baratron gauge, Baratron control unit; 13, pyridine reservoir; 14, cylinder with probe (e.g., ammonia); 15, calibrated volume; 16, cooling finger. B Sample holder and heater (stainless steel): 1, feed through; 2, heating wire; 3, heating block (copper); 4, thermocouple; 5, CF 35 flange (after [29], with permission)

Solid-State Ion Exchange in Microporous and Mesoporous Materials

53

Application of MS to monitor SSIE has the advantage that several simultaneously evolved gases can be separately analyzed, for instance, NH3 , HCl and H2O. In the case of evolved anhydrous acids such as HCl, the vapor can be transferred by a stream of N2 or He into a trap containing an aqueous solution of a base (e.g., NaOH). When the reaction is complete, the degree of exchange can be determined via back-titration of the residual base. Also, the procedure can be designed such that a continuous titration of the HCl evolved and trapped in water is possible. 4.5 Combination of TGA and TPE

A combination of gravimetric analysis (cf. Sect. 4.3) and temperature-programmed evolution of volatile product components (cf. Sect. 4.4) is especially advantageous to monitor SSIE in zeolites. Analysis and determination of the evolved gases by MS, TCD or titration enables us to interpret more specifically the change in weight of the reacting salt (oxide)/zeolite mixture. An instructive example will be discussed in Sect. 5.1.4. Temperature-programmed evolution of gases may also be advantageously combined with XRD, IR, magic-angle spinning (MAS) NMR, and Mössbauer spectroscopy, even though only a few examples have been reported so far. 4.6 X-ray Diffraction (XRD)

X-ray diffraction may be applied to the starting mixture of a salt or oxide and the zeolite and, after completion of the solid-state reaction at a given constant temperature, to the product mixture. A comparison of the respective XRD patterns will then show whether (1) the intensities of reflections typical of the compound containing the in-going cation have decreased or disappeared and (2) the reflections of the zeolitic phase have changed accordingly. In principle, the intensities of the reflections of the zeolitic phase are dependent on the nature of the charge-compensating extra-framework cations. In many cases these intensities are remarkably sensitive to an exchange of the cations. This type of XRD experiment can be carried out at different constant reaction temperatures, which will provide insight into the temperature dependence of the achievable exchange degree. However, if the diffractometer is equipped with a continuously heatable XRD chamber, it is possible to monitor in situ the progress of the solid-state reaction. In those cases where the hydrogen form of a zeolite is reacted, the XRD analysis can be combined with the measurement of the temperatureprogrammed evolution of gases (TPE). Application of XRD in SSIE is illustrated in Sect. 5.2.5 using the introduction of La3+ cations into an Na-Y zeolite as an example.

54

H.G. Karge · H.K. Beyer

4.7 Infrared Spectroscopy (IR) and Fourier Transform (FT) Raman Spectroscopy

IR spectroscopy can be employed to study solid-state ion exchange with and without probe molecules. The application of IR is most straightforward with respect to the exchange of protons of the hydrogen forms of zeolites. Upon reaction with a salt or oxide, the intensity of the band(s) originating from the Brønsted acid hydroxyl groups will decrease. The loss of intensity provides a measure of the degree of exchange. The IR experiments must be carried out with salt (oxide)/zeolite mixtures pressed into self-supporting thin wafers (cf., e.g., [30, 31]) in an appropriate heatable cell (cf., e.g., [31]). If the IR cell is continuously heatable, the progress of exchange can be monitored in situ under high vacuum or in a flow of inert gas passed through the cell; this may be combined with a TPE measurement. Another approach is to employ probe molecules such as pyridine. Pyridine attached to different cations gives rise to IR bands in the region from 1430 to 1460 cm–1 that are indicative of the nature of the cations [32]. This enables IR spectroscopy to be employed not only when the exchange with hydrogen forms of zeolites is investigated, but also when the starting zeolite material contains one kind of cations which should be replaced by another. This is dealt with in Sects. 5.2.6 and 5.3.2 where the exchange of Na+ by La3+ and Cu+, respectively, is discussed. Historically, the IR method was used in the early experiments by Rabo et al. [16, 17] mentioned in Sect. 1. Fourier transform (FT) Raman spectroscopy was applied by Huang et al. [33] to study the solid-state ion exchange between LiCl or CaCl2 and NH4-Y or Na-Y as well as contact-induced ion exchange in the systems Li-A/Na-Y, Li-A/Na-X and Li-A/Ca-A (cf. Sect. 6.2). 4.8 Electron Spin Resonance Spectroscopy (ESR)

The incorporation of transition metal cations both into hydrogen and cationic forms of zeolites (H-Z, M-Z, with M = Na, K, etc.) via reaction with the compounds containing the respective transition metal is, in many cases, easily seen by electron spin resonance (ESR) spectroscopy. In fact, the early studies by Clearfield et al. [18] were conducted using ESR (cf. Sect. 1). Since ESR measurements can also be carried out at elevated temperatures, in situ observations are possible as well.A suitable high-temperature ESR cell was developed by Karge et al. [34]. Using ESR, it can be confirmed that solid-state ion exchange has taken place in a particular system containing transition metal cations and, moreover, the coordination of the incorporated cations can be determined. However, the ESR technique is frequently not capable of providing satisfactory quantitative results. Examples of ESR investigations of SSIE are reported in Sects. 5.3.2, 5.3.4 and 5.3.5.

Solid-State Ion Exchange in Microporous and Mesoporous Materials

55

4.9 Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS NMR)

Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was first applied to problems of solid-state ion exchange when the system LaCl3 ◊7H2O/Na-Y was studied [35]. With the help of 23Na MAS NMR, the fact that exchange had taken place was impressively demonstrated by the appearance of a signal at 0 ppm when the spectrum was referenced to NaCl (cf. Sect. 5.2.7). Moreover, the change in the intensities of the signals indicating Na+ in super- and b-cages was monitored as a function of the reaction temperature and related to the exchange features. However, the behavior of many other cations during SSIE can also be studied by MAS NMR as well (cf. Sect. 5.1.10). Particularly interesting examples are the introduction of Cs+ into H-Y investigated by Weitkamp et al. with the help of 133Cs MAS NMR [36] and the study of the behavior of hydrogen zeolites in SSIE by 1H MAS NMR. Usually, these MAS NMR studies are conducted at room temperature after ex situ preparation of the samples. However, recent developments of the experimental MAS NMR technique have rendered high-temperature investigations possible [37–39]. Thus, this should enable SSIE in zeolites to be monitored by in situ measurements. 4.10 Mössbauer Spectroscopy

There are only a few “Mössbauer nuclei” which are interesting in zeolite chemistry and, thus, candidates for application of Mössbauer spectroscopy in solidstate ion exchange. However, among them is one of the most important elements, viz., iron, which has also attracted much attention in zeolite chemistry as a key component of possible catalyst formulations. Mössbauer spectroscopy proved to be exceptionally successful in discriminating Fe2+ and Fe3+ cations residing on extra-framework sites after introduction of iron via solid-state ion exchange. Moreover, Mössbauer spectroscopy provides information about the various coordinations of Fe2+ and Fe3+ in zeolite lattices (cf. Sect. 5.3.4). 4.11 X-ray Photoelectron Spectroscopy (XPS)

Only a few experiments have been reported where X-ray photoelectron spectroscopy (XPS) has been used to study solid-state reactions between salts and zeolites. XPS enables us to determine changes in the surface composition of a zeolite sample before and after it has been subjected to solid-state ion exchange. This technique is suitable to monitor, for instance, variations in the surface ratios nM/nAl and nM/nSi of the zeolite upon solid-state reaction as a function of temperature, the ratio salt/zeolite and the reaction time. Examples will be provided in, e.g., Sects. 5.3.2.1 and 5.3.4.4.

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4.12 X-ray Spectroscopy: EXAFS and XANES

Analysis of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) of mixtures of salts and zeolites before and after heat-treatment enables us to prove or disprove that a solid-state reaction has occurred. Moreover, EXAFS is a tool to determine the coordination of cations introduced via SSIE into the zeolite matrix. If these cations are reducible to the zero-valent state as, e.g., Pt2+ or Pd2+ cations are, EXAFS also provides information about the size of the metal particles generated by reduction. To date, unfortunately, only a few pertinent studies have been reported (cf., e.g., [40] and Sect. 5.4.2).

5 Systems Investigated Even though the subsequent list may, in fact, not be exhaustive, Table 1 comprises at least most of the systems subjected up to date to solid-state ion exchange. This, however, does not mean that all the systems enumerated in Table 1 were investigated in great detail. With respect to many of them, several important questions still remained unanswered. Certainly, more detailed research on several particular systems will be carried out only if the interest in their solid-state ion exchange is stimulated by the need to solve special problems related to these systems and if there appears to be a chance to successfully solve the problems via SSIE as, for instance, in catalyst preparation (cf. Sects. 5.3 and 5.4). With respect to the types of cations desired to be introduced by SSIE, there is, nevertheless, practically no important group which has not been dealt with so far. Thus, incorporation of cations via solid-state ion exchange has been studied not only for alkaline, alkaline earth and rare earth, but also for transition and noble metal cations. However, there is still not much general insight into why, for instance, the achievable maximum exchange degree strongly depends on the nature of a respective in-going cation or why the nature of the anion of the compound with the in-going ion plays a significant role. In particular, to date, no systematic investigation on the latter problem has been carried out. There is, therefore, still ample room for research in the field of solid-state ion exchange (see also Concluding Remarks, Sect. 9) and only a few of these questions concerning the systems studied to date can be addressed in the context of the subsections that follow.

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Solid-State Ion Exchange in Microporous and Mesoporous Materials

Table 1. Overview of systems investigated for solid-state ion exchange

Compound of the introduced cation Zeolite

References

LiCl, NaCl, KCl, RbCl, CsCl

NR4Cl (NR4: organoammonium) Na-MM (MM: montmorillonite) Li-A, Na-A Li-A/Na-Y Li-A/Na-X Li-A/Ca-X

H-ZSM-5, NH4-ZSM-5, NH4-Y, H-MOR, H-BETA, NH4-BETA, H-EMT, NH4-EMT, Na-Y Na-X NH4-X, NH4-Y, K-Y, K-L NH4-Y H-MOR, MAPO-36 NH4-BETA H-ZSM-5 H-BETA Na-BETA, Cs-BETA, Na-MM (MM: montmorillonite) H-ZSM-5, H-MOR Na-A, Li-A Na-A/Li-Y, Na-A/Li-X, Ca-A/Li-X

[16, 17, 22, 24, 33, 43, 47–49, 58, 60–63, 73] [67–69] [56, 58, 59] [36] [70, 130] [49, 50] [43] [51] [51] [28, 52] [72] [19, 20] [32]

CaCl2 · 2H2O, MgCl2 · 2H2O CaCl2 BeCl2

H-MOR, NH4-MOR NH4-Y Na-Y

[41, 66] [33, 139] [73]

LaCl3 ◊ 7H2O LaCl3 (water-free) LaCl3 ◊ 7H2O CeCl3 , NdCl3, SmCl3 , EuCl3 , YbCl3 EuCl3 CeCl3

NH4-Y, H-ZSM-5, H-BETA NH4-Y, H-MOR, H-L, H-FER Na-Y NH4-Y NH4-Y H-ZSM-5

[78, 83] [287] [79, 88] [83] [85] [84]

Cu0 (+O2)

H-MOR, H-ZSM-5

CuCl2 ◊ 2H2O, CuF2 , Cu3[(OH)CO3]2 , Cu3(PO4)2 CuCl2 CuCl CuCl CuCl CuCl CuO

NH4-A, NH4-X, NH4-Y, H-MOR

RbCl, CsOH NaCl CsCl Cs[PW12O40] NH4Cl

Cu2O Cu2O, Cu2S CuCl, Cu(NO3)2 , Cu(CH3COO)2 CuCl2 CuCrO4 , CuO+CrO3

NH4-Y H-ZSM-5 Na-ZSM-5, Na-ZSM-5/H-ZSM-5 MCM-41 H-ZSM-5

[21, 141, 142, 253, 254] [18, 21, 98, 99, 104, 105] [106, 114–126] [98] [101–103, 112, 113] [107–109] [289, 290] [128, 129, 148] [21, 93, 98, 110, 111, 127] [97, 98, 113] [21, 98] [21, 98, 126] [125] [93, 198]

Ag0 (+O2) AgCl

H, Ag-MOR H-ZSM-5

[141, 253, 254] [130]

AuCl3

Na-Y

[131]

Zn0 ZnCl2 ZnCl2

NH4-Y, NH4-USY, H-MOR NH4-Y H-Y

[136, 140, 247–250] [104, 105, 136, 137] [139]

H-ZSM-5 H-ZSM-5 NH4-Y, H-MOR, H-L H-BETA, H-CLIN Na-Y, Na-MOR H-[Ga]ZSM-5, Fe, H-ZSM-5 H-MOR, H-ZSM-5

58

H.G. Karge · H.K. Beyer

Table 1 (continued)

Compound of the introduced cation Zeolite

References

ZnCl2 ZnCl2 ZnO ZnO

NH4-Y, Na-Y H-ZSM-5 NH4-Y H-ZSM-5

[139, 144–146] [244] [143] [132–135]

Cd0 Cd(NO3)2 , CdCl2 , CdO, CdS, CdSO4

NH4-Y NH4-X, NH4-Y, NH4-MOR

[140, 247–249] [139]

Hg2Cl2

H-ZSM-5

[130]

FeCl2 ◊ 4H2O

NH4-A, NH4-X, NH4-Y

FeCl2 ◊ 4H2O FeCl3 FeCl3 FeO, Fe3O4 Fe2O3 Fe(NO3)3 · 9H2O

H-MFI, H-FER H-ZSM-5 H-ZSM-5, Na-ZSM-5, H-[Ga]ZSM-5 H-ZSM-5 H-ZSM-5, Na-ZSM-5 Na-MM (MM: montmorillonite)

[18, 114, 148–150, 159] [122, 156–160] [118, 155] [129, 148]

CoCl2, CoCl2 · 6H2O

NH4-Y, H-ZSM-5

CoCl2 , Co(NO3)2 CoCl2

NH4-CLIN, H-CLIN H-BETA H-FER H-Y

[106, 114, 121, 139, 162, 164–170, 173–175] [109, 163] [173] [175] [251]

NH4-Y H-ZSM-5 H-ZSM-5 SAPO-42 SAPO-n (n = 5, 8, 11, 34) H-MOR, H-BETA MCM-41 Cu-ZSM-5

[105, 177] [114, 121, 176] [164, 165, 167] [44, 179] [178, 181–185] [169] [186] [187]

MnCl2 , MnSO4 , Mn(NO3)2, Mn3O4 , Mn(CH3COO)2 MnCl2

H-ZSM-5

[147, 188]

NH4-Y

[139]

V2O5

[92, 190–194]

V2O5 V2O5 V2O5 VO(NO3)2 V2O5 VOCl3 V2O5 + CuO

H-MOR, H,Na-MOR, H-ZSM-5, H,Na-ZSM-5 H,Na-X, H-Y, H-ZSM-5, H-MOR Na-Y H-[Ga]ZSM-5 AlPO4-5 NH4-ZSM-5 H-ZSM-5 H-ZSM-5

[208, 209] [195–197] [128, 129] [206] [205] [246] [93, 198]

Nb2O5

NH4-Y, NH4 , Na-Y

[202]

Co2(CO)8 NiCl2 NiCl2 , NiSO4 , Ni(CH3COO)2 , NiO NiCl2 NiCl2 , NiCl2 · 6H2O, NiO NiCl2 NiCl2 Raney nickel

[148] [147] [28]

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Solid-State Ion Exchange in Microporous and Mesoporous Materials

Table 1 (continued)

Compound of the introduced cation Zeolite

References

Sb2O3

Na-Y, La, Na-Y

[210]

CrCl3 CrO3 CuCrO4 CrO3 Cr2O3 CrO2Cl2 CrO3 + CuO CrO3

H-Y H-MOR, H-ZSM-5 H-MOR, H-ZSM-5 H-[Ga]ZSM-5, H-[Fe]ZSM-5 H-MOR, H-ZSM-5 MFI H-ZSM-5 SAPO-11

[100, 199] [92, 190, 199] [92, 93, 190, 199] [128, 129, 148, 151] [92, 190, 213, 214] [246] [198] [215]

MoCl5 (MoOCl4) MoCl3

NH4-Y, NH4-DAY, Co, H-Y H-ZSM-5, H-MOR, H-ZSM-35, H-EU, H-FER, H-ZSM-48, H-L H-ZSM-5, H-USY, H-FER, H-BETA Na-Y, Na-ZSM-5 Na-Y H-USY, H-ZSM-5, H-FER

[200, 201, 216] [23, 44, 179] [92, 190, 201, 216–220] [197, 217] [197] [252]

K-L H-Y NH4-Y, H-ZSM-5

[226] [256] [221–224] [44, 45, 231]

PtCl2 , PtBr2 , PtO2 RhCl3 RhCl3 RhCl3 PdCl2 + CaCl2 , PdCl2 + LaCl3

H-ALPHA, H-RHO, H-ZK-5, H-ZSM-58, H-SAPO-42 H-SAPO-42 H-RHO, H-ZK-5, H-ALPHA, H-ZSM-58, H-SAPO-42 NH4-Y H-ALPHA, H-SAPO-42, H-ZK-5 SAPO-11 DAY H-ZSM-5

Ga0 Ga2O3

H-ZSM-5 H-ZSM-5

MoO3 MoO3 WO3 WO3 (+CCl4) Pt0 Pd0 (+Cl2) PdCl2 , Pd(NO3)2 , PdO, PtCl2 , PtCl4 , PtO2 PdCl2 PdO PtCl2

Ga2O3 Ga2O3 Ga2O3 In0 In2O3 In2O3 In2O3 In2O3 In2O3

[44] [44, 45, 231] [40, 225] [44, 45, 231] [228] [227] [221–224]

[140] [111, 257–262, 265, 266] H-MOR, H-ZSM-5, H-BETA, H-Y [267–269] H-[Ga]ZSM-5 [258] H-SAPO-n (n = 5, 34, 37) [282] (+ template) H-ZSM-5 [140] H-ZSM-5, NH4-Y, H-MOR, H-OFF [111, 260, 261, 267–269] H-ZSM-5, H, Na-Y, NH4-MOR, [271–273, 277, 278] NH4-Y H-BETA (TEA-BETA) [274, 275] H-SAPO-n (n = 5, 34, 37) [282–284] (+ template) MCM-41 [285]

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5.1 SSIE of Alkaline (M+) and Alkaline Earth (M2+) Metal Cations 5.1.1 Introductory Remarks

As already mentioned in Sect. 1 (Introduction), historically, the first experiment on solid-state ion exchange between alkaline metal cations and protons of an acidic zeolite was that described by Rabo et al. [16, 17] for the systems NaCl/Ca, H-Y; NaCl/Ba, H-Y and NaCl/Zn, H-Y, where the authors attempted to remove residual activity from the acidic zeolite. Without any knowledge of this rather obscure report at the beginning of their work, Karge and Beyer started in the mid-1980s systematic investigations on solid-state reactions between alkaline and alkaline earth salts and zeolites [22, 41]. The experimental techniques employed were TPE, IR, CA, XRD, and MAS NMR (cf. Sect. 4). 5.1.2 Application of TPE

The complete series of alkaline chlorides, MCl (M = Li, Na, K, Rb, Cs), was mixed with NH4-ZSM-5 or NH4-Y powder as described in Sect. 3 and subsequently heated. The progress of solid-state ion exchange was monitored via TPE of NH3 , HCl and H2O. A mass spectrometer was used to measure the relative amounts of volatile products evolved (cf. also Fig. 1). Figure 2 displays TPE profiles obtained during solid-state reaction between alkaline chlorides and NH4-ZSM-5. The release of HCl (mass 36) and NH3 (mass 16, NH +2 ) allowed a distinction to be made between a low-temperature (LT) and a high-temperature (HT) regime. The peak temperatures (Tp) of both reaction regimes decreased in the regular sequence Tp (Na)>Tp (K)>Tp (Rb)>Tp (Cs). The amount of cations involved in the HT reaction decreased in a similar sequence, lithium being an exception. This element, however, frequently behaves in an irregular manner in zeolite chemistry (see, e.g., [42]), probably because of the small size of the Li+ cation. Thus, the LT reaction of LiCl predominates and only a small peak around 580 K appears upon solid-state ion exchange of LiCl and NH4-ZSM-5. Very similar results were obtained with the systems MCl/H-ZSM-5 [22] and MCl/NH4-Y ([43]; Fig. 3). 5.1.3 SSIE and Lattice Energy

The ranking of the reactivity of NaCl, KCl, RbCl, and CsCl in the HT regime of SSIE with NH4-ZSM-5, H-ZSM-5 and NH4-Y indicated above can be related to the lattice energy (EL) of those salts. The lattice energies decrease in the same sequence as the peak temperatures. The following explanation may apply: the lower the lattice energy, the lower the energy required to separate salt entities (most likely MCl molecules) from the salt crystallites (cf. Sect. 7.2). This, in turn, reduces the temperature that must be applied to make the solid-state reaction

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61

Fig. 2. Temperature-programmed evolution (TPE) of hydrogen chloride: evolution curves of mixtures containing NH4-ZSM-5 and LiCl (–æ), NaCl (– – –) KCl (–·–·–), RbCl (–··–··–), CsCl (–···–···–); nMCl/nAl = 1.89 (after [22], with permission)

Fig. 3. Temperature-programmed evolution of gases (m/e = 36, HCl, æ; m/e = 18, H2O, ••••; m/e = 17, NH3, ● ● ● ● or – - – -) from pure NH4-Y and MCl/NH4-Y mixtures upon solid-state reaction (after [43], with permission)

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Table 2. Lattice energies and exchange temperatures (high-temperature peak)

Salt

Lattice energy (kJ mol–1)

Exchange temperature (K)

LiCl NaCl KCl RbCl CsCl

834 769 701 680 657

– 895 855 840 815

occur. In contrast, the lower degree of exchange reached through temperatureprogrammed heating in the high-temperature regime when going from NaCl to CsCl is due to the increasing size of the cations (MCl molecules), i.e., their decreasing ability to migrate into the pore system and, in the particular case of Y-type zeolites, to penetrate the six-membered rings to enter the b-cages (vide infra). (With respect to introduction of alkali metal cations via SSIE in static experiments at constant temperatures, where a 100% degree of exchange was achieved, cf. Sect. 5.1.8.) The above-mentioned relationship between EL and Tp is substantiated by the data of Table 2. The relationship between the lattice energy and the reactivity for SSIE of metal compounds in mixtures with zeolites was generally confirmed by the work of Weitkamp and co-workers (cf. [44, 45] and Sect. 5.3) concerning the systems of noble metal chlorides/hydrogen forms of zeolites. It should be noted, however, that such a relationship was not found in SSIE experiments with Mn oxides as compounds of the in-going cation [46]. The studies of SSIE with alkaline salts discussed so far employed A-, X-,Y- and ZSM-5-type zeolites. Interesting investigations were, however, carried out by Barthomeuf and co-workers (cf. [47, 48]) and Mavrodinova (cf. [49]), who used samples of zeolite EMT and BETA. These zeolites were synthesized with a template, e.g., TEA in the case of BETA [49]. Therefore, post-synthesis removal of the template by oxidation resulted in the formation of protons, and a mixed hydrogen-sodium form was obtained. Barthomeuf and co-workers [47, 48] studied solid-state ion exchange of M+ (M+ = Li+, Na+, K+, Rb+, Cs+) into H-BETA and HEMT zeolites under static conditions, i.e., without vacuum or a flow of inert gas. The results were compared with those of conventional exchange in aqueous solutions. It turned out to be difficult to replace all the protons by alkali metal cations via solid-state ion exchange. In contrast, an almost complete replacement was achieved in one step through (conventional) exchange for NH4+ and subsequent SSIE with CsCl, where the solid-state reaction was monitored by TPE and TGA (see Fig. 4 and [47]). The disappearance of the CsCl reflections upon heating the mixture CsCl/H,Na-BETA at 823 K was a further confirmation of the occurrence of solid-state ion exchange. The difference in the results of TPD of 1-propylamine from the BETA sample prior to and after SSIE indicated a 92.7% consumption of the Brønsted acid sites. When NaCl was employed instead of CsCl, in fact only 50% of the protons were replaced by Na+. The reason for this different behavior of the systems

Solid-State Ion Exchange in Microporous and Mesoporous Materials

63

Fig. 4. A Temperature-programmed evolution of 1 NH3 and 2 HCl from a CsCl/NH4-BETA mixture and 3 NH3 from pure NH4-BETA. B DTG curves of CsCl/NH4-BETA mixtures 1 CsCl/NH4-BETA (I) with nSi /nAl = 25.1, 2 CsCl/NH4-BETA (II) with nSi /nAl = 17.06, 3 pure NH4-BETA (I), and 4 pure NH4-BETA (II) (after [49], with permission)

CsCl/H,Na-BETA and NaCl/H,Na-BETA is not yet clear. However, Mavrodinova observed that the rate and degree of SSIE were higher the larger the amount of admixed NaCl (compare the analogous behavior of the system CuCl/H-ZSM-5; Sect. 5.3.2). The XRD pattern showed that the crystal integrity was preserved. Solid-state ion exchange between NH4-BETA and CsCl was also investigated by Xu et al. [50]. These authors monitored SSIE through XRD and TGA/DTA and examined the products with respect to their acid-base properties by IR with pyridine as a probe and by determination of their activity in decomposition of isopropanol. The results showed that it is easy to exchange Cs+ cations into BETA zeolite with a very high degree of exchange. Furthermore, the products of this SSIE process had a higher catalytic activity in base-catalyzed dehydrogenation of isopropanol than conventionally prepared Cs-BETA formulations.

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In the processes described in the preceding paragraphs, solid-state ion exchange was carried out in mixtures of MCl and H-zeolites or NH4-zeolites, where M = Li, Na, K, Rb or Cs. Mavrodinova [51] extended these investigations to solid-state reaction of NH4Cl with M-BETA (M = Na, Cs) or H-BETA. For instance, a low-temperature reaction was shown to occur according to Eq. (6): x NH4Cl + CsyHz – BETA Æ x NH3≠ + Cs(y–x)H(z+x)-BETA + x CsCl (6) This was evidenced by TPE/MS which showed a low-temperature evolution of NH3 but a lack of HCl formation in the same temperature range (Fig. 5). Only at higher temperatures (805 K) was HCl evolved [see Eq. (7)] due to the reaction

Fig. 5. A Temperature-programmed evolution of NH3 from 1 NH4Cl/Cs,H-BETA and 2 NH4Cl/ Na,H-BETA mixtures. B Temperature-programmed evolution of HCl from 1 NH4Cl/Cs,HBETA and 2 NH4Cl/Na,H-BETA mixtures; heating rate: dT/dt = 10 K min–1; isothermal treatment at 823 K for 1 h (after [51], with permission)

Solid-State Ion Exchange in Microporous and Mesoporous Materials

65

between CsCl and Cs(y–x)H(z+x)-BETA generated according to Eq. (6) at low temperature: (7) x CsCl + Cs(y–x)H(z+x)-BETA Æ CsyHz-BETA + xHCl ≠ In the case of NH4Cl/H,Na-BETA, solid-state ion exchange in analogy to Eq. (6) was indicated by an increase in the intensity of the NaCl reflections observed by XRD upon heating the mixture NH4Cl/H,Na-BETA. The resulting composition of the modified zeolite material, i.e., the extent of reactions according to Eqs. (6) and (7), is controlled by both the reaction temperature and the concentrations of the competing cations. Reports on solid-state ion exchange with clays instead of zeolites are still rather rare. In 1990 and 1992, however, two studies were published concerning incorporation of organoammonium (NR+4 ), iron (Fe3+) and aluminum (Al3+) cations into montmorillonite (MM; cf. [28, 52] and also Sect. 5.3.4). Thus, Ogawa et al. [52] observed formation of organoammonium montmorillonite upon reaction of solid organoammonium halides and dehydrated sodium montmorillonite (MM): Na-MM + NR4Cl Æ NR4-MM + NaCl

(8)

5.1.4 Application of a Combination of TGA and TPE

Solid-state ion exchange in the system alkaline chloride/H-ZSM-5 was also monitored via a combination of TPE and thermogravimetric analysis (TGA) [22]. Figure 6 illustrates as an example the results obtained with the system NaCl/H-ZSM-5.

Fig. 6. a TGA and b thermal gas titration curves of a NaCl/H-ZSM-5 mixture (nNaCl/nAl = 1.89) at a heating rate of 2.5 K min–1 (after [22], with permission)

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The thermogravimetric curve consists of three distinct steps. The first step can be attributed to the removal of physically adsorbed water and the concomitant evolution of small amounts of HCl (LT process). The second step is not observed with the pure materials and obviously indicates a solid-state reaction between the two components, i.e., NaCl and H-ZSM-5 (HT process). The last step in the TGA curve probably originates from a volatilization and/or decomposition of NaCl at temperatures above 1100 K. In parallel with TGA, the temperature-programmed evolution of HCl was followed by continuous and automatized titration. The TPE curve also exhibits three steps, a small LT step at about 470 K, a large HT step between ca. 770 and 900 K and a third less distinct one at even higher temperatures. The assignments confirm those given for the TGA curve: the first TPE peak results from the small contribution of the HCl release in the LT regime of the solid-state reaction and proves that most of the first step in the weight loss curve is caused by the desorption of physically adsorbed water; the middle and most pronounced TPE peak gives evidence of the large contribution by the HT regime of SSIE; the last TPE step above 1100 K is indicative of decomposition products (chlorine) of the salt. Analogous to the observations described above, in the system CaCl2 ◊ 2H2O/HZSM-5 the combined TGA/TPE experiment yielded also two steps in the curve of temperature-programmed evolution of HCl [41] (cf. Fig. 7). One (at 125–300°C) was ascribed to the LT regime, the other one (at about 500°C) to the HT regime

Fig. 7. a TGA and b thermal gas titration curves of a CaCl2 · 2H2O/H-MOR mixture (after [41], with permission)

67

Solid-State Ion Exchange in Microporous and Mesoporous Materials

Table 3. Starting materials, mixtures and results of solid-state ion exchange in the system NaCl/H-ZSM-5a. 1, parent zeolite; 2, nSi/nAl ratio of the parent zeolite; 3, Al content of the parent zeolite; 4, NaCl admixed to 1 g (dry) zeolite; 5, HCl evolved on solid-state reaction; 6, Cl– extracted after solid-state reaction; 7, Na+ extracted after solid-state reaction; 8, Na+ irreversibly held in zeolite after extraction; 9, degree of exchange (%) (data in column 8 divided by data in column 3)

1 Zeolite a

2 3 nSi/nAl Al

H-ZSM-5 (I) 155 H-ZSM-5 (II) 23 a

4 NaCl

9 d (employed) (evolved) (extracted) (extracted) (irrev. held) (%)

0.107 0.808 0.691 1.306

5 HCl

6 Cl

7 Na

8 Na

0.549 0.838

0.260 0.478

0.707 0.670

0.101 0.636

94 92

All data in mmol g–1 zeolite fired at 1273 K.

of the solid-state reaction. It was difficult to reach completion of the high-temperature exchange. Only after repeated heating and an isothermal reaction period at 600°C did the reaction cease. Titration of the evolved HCl showed that a total of 2.52 mmol HCl per gram dry zeolite was released (cf. Sect. 5.1.5, Table 4). At variance with the TPE results, the TGA curve of the system CaCl2 ◊ 2H2O/HZSM-5 exhibited only one very steep step of weight loss of the sample, viz., in the LT region. This resulted from the evolution of both water and HCl, as described for NaCl/H-ZSM-5 (vide supra). However, above 200°C, the TGA curve continuously declined and the HT regime of the SSIE could not be clearly discriminated. Obviously, the TGA measurement was markedly disturbed by the presence and release of crystal water. 5.1.5 Stoichiometry of SSIE of M+ and M2+ Halides with H-Zeolites

From the analysis of (1) the starting zeolite material, (2) the gases evolved, (3) the aqueous extracts obtained from the salt/zeolite mixtures after reaction, and (4) the exchanged zeolite, one can determine the stoichiometry of the solid-state ion exchange. This is exemplified by Tables 3 and 4, which provide data for the systems NaCl/H-ZSM-5, CaCl2/H-MOR and CaCl2/NH4-MOR, given in mmol per gram zeolite fired at 1273 K. In the respective experiments, excess amounts of NaCl or CaCl2 were applied, i.e., the employed millimoles of NaCl or milliequivalents of CaCl2 considerably exceeded the Al-content of the starting zeolites (cf. columns 4 and 3 of Tables 3 and 4, respectively). However, the amounts of HCl (NH4Cl) evolved were also markedly higher than the corresponding Al-contents (compare columns 5 and 3) and, consequently, than the amount of Brønsted acid sites per gram. This means that a fraction of the salts has reacted with silanol groups. However, due to hydrolysis of Na bound to silanol groups, i.e., Na+(O–Si∫)– species, and dissolution of the non-converted salt, the excess of NaCl could be extracted. From the results of analyses presented in Table 3 it was derived that, within the limits of error, the amounts of employed NaCl were equal to the sum of (1) evolved HCl

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Table 4. Starting materials, mixtures and results of solid-state ion exchange in the systems CaCl2/H-MOR and CaCl2/NH4-MORa. 1, parent zeolite; 2, nSi/nAl ratio of the parent zeolite; 3, Al content of the parent zeolite; 4, CaCl2 admixed to 1 g (dry) zeolite; 5, HCl evolved on solid-state reaction; 6, Cl– extracted after solid-state reaction; 7, Ca2+ extracted after solidstate reaction; 8, Ca2+ irreversibly held in zeolite after extraction; 9, CaCl2 occluded upon solid-state reaction; 10, degree of exchange (%) (data in column 8 (multiplied by 2 to give mequiv) divided by data in column 3) 1 Zeolite a

2 3 nSi/nAl Al

H-MOR 6.7 NH4-MOR 5.4 a

2.18 2.50

4 CaCl2

5 6 HCl; NH4Cl Cl

(employed) (evolved)

10 d (extracted) (extracted) (irrev. held) (occluded) (%)

1.98 2.48

1.44 1.88

2.52 2.54

7 Ca

8 Ca

9 CaCl2

0.89 0.95

1.09 1.26

– 0.27

100 100

All data in mmol g–1 zeolite fired at 1273 K.

and extracted Cl– in the zeolite matrix (e.g., for H-ZSM-5(I): 0.549 + 0.260 = 0.809 mmol; cf. columns 4, 5, and 6) or (2) extracted NaCl and Na+ irreversibly held in the zeolite matrix (e.g., for H-ZSM-5 (I): 0.707 + 0.101 = 0.808 mmol; cf. columns 4, 7 and 8). Thus, the resulting content of (irreversibly held) Na+ in the exchanged zeolite (0.101 mmol) was close to the amount of Al in the framework (0.107 mmol; cf. columns 8 and 3 of Table 3). In other words, the degree of exchange was almost 100% (cf. column 9). Similar results were obtained with the sample H-ZSM-5 (II), as can be seen from Table 3. The evaluation of the data for mordenites presented in Table 4 was similar to that of the H-ZSM-5 samples; however, with the systems CaCl2/H-MOR and CaCl2/NH4-MOR, the situation was somewhat more complicated. The H-MOR sample contained considerable amounts of extra-framework Al (0.40 mmol per gram dry zeolite), as was proven by IR after adsorption of pyridine and determined by 27Al MAS NMR. From a comparison of columns 3 and 8 it becomes obvious that CaCl2 has reacted with the total Al (framework and non-framework Al), since 1.09 mmol or 2.18 mequiv Ca2+ were irreversibly held. Most likely, the extra-framework Al was connected to OH groups which possibly had been converted according to Eq. (9): 2 AlOOH + CaCl2 Æ Ca(AlO2)2 + 2 HCl≠

(9)

Calcium aluminates are not readily soluble in water and were, therefore, not extracted when the mixture was washed with water after the reaction had been completed. The NH4-MOR sample was essentially free of extra-framework Al. Thus, the Al content given in column 3 of Table 4 exclusively corresponded to tetrahedrally coordinated Al, as confirmed by 27Al MAS NMR. This amount (2.50 mmol Al) was found to equal the amount of Ca2+ introduced via solid-state ion exchange, i.e., 1.26 mmol or 2.52 mequiv of Ca2+ (cf. columns 3 and 8, Table 4). However, careful analysis showed that a certain fraction of the admixed CaCl2 remained occluded after completion of the reaction and subsequent washing with water (cf. column 9). The amount of 0.27 mmol CaCl2 per gram dry zeolite corresponded to about 50% of the side-pockets of the MOR structure where the CaCl2

Solid-State Ion Exchange in Microporous and Mesoporous Materials

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molecules were most probably trapped. No such salt occlusion was observed with H-ZSM-5 samples. Similar observations were made, however, when NaCl was reacted with the NH4-MOR sample. Salt occlusion in zeolites is a wellknown phenomenon [16, 17, 53]. Rabo et al. [53] have shown that the thermal stability of zeolite structures is frequently improved by occlusion of salts. Thus, in many cases, it should be advantageous if ion exchange via solid-state reaction is accompanied by occlusion of molecules of the salt present in the reaction mixtures (cf. also Sect. 5.2.3, incorporation of La3+). 5.1.6 Preservation of Crystallinity upon SSIE

Solid-state ion exchange is usually conducted at higher temperatures and, if metal halides and hydrogen forms of zeolites are reacted, occurs in the presence of hydrogen halides. Therefore, the question may arise as to whether the integrity of the zeolite structure has been preserved when solid-state ion exchange was carried out. There are several ways to check whether the crystallinity of the zeolite material has deteriorated during the solid-state reaction. A most suitable test is provided by X-ray diffraction (XRD). Generally, a number of reflections are selected and the sum of their intensities prior to and after the solid-state reaction compared. However, the absolute intensities frequently change; they may increase or decrease depending on the contributions of charge-compensating, in-going and out-going cations to the structure factors of the reflections. Nevertheless, this method provides reliable results provided these effects are adequately considered (cf. [54]). The XRD tests showed that in the cases described above (Sect. 5.1) no loss of crystallinity had occurred. Another convenient method is that of cation re-exchange. After introduction of the desired metal cations, one can, for instance, in a small representative test sample of the exchanged zeolite, re-exchange the metal cations by NH +4 through repeated treatment in aqueous NH4-salt solution, and subsequently deammoniate the sample, whereby the evolved NH3 may be determined via titration; alternatively, the OH groups generated by deammoniation may be determined via IR or 1H MAS NMR (cf. Sects. 5.1.8 and 5.1.10). If the exchange capacity is the same after solid-state reaction as it was before, one can conclude that the integrity of the structure has remained unaffected. However, the interpretation is difficult if the exchange capacity has decreased, since a fraction of the cations incorporated by SSIE may be irreversibly trapped and, at least under the conditions of treatment with an aqueous solution, hindered to re-exchange. In such a case, the solid zeolite must be chemically analyzed after the re-exchange experiment. 29Si MAS NMR is also an appropriate tool to decide whether or not SSIE has caused a (partial) collapse of the zeolite structure.If such a collapse has happened, it would be accompanied by a loss of framework T-atoms. This would mean that, with T = Si, Al, the framework nSi/nAl ratios [55] of the parent zeolite and the zeolite after SSIE are likely to be different.Therefore,the unchanged nSi/nAl ratio of the Y-type sample after introduction of, e.g., Cs+, confirmed the finding obtained by XRD that no loss of crystallinity had occurred (cf. Sect. 5.1.10).

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If the structure of the zeolite is sensitive to mechanical stress imposed by grinding or milling during the preparation of the salt (oxide)/zeolite mixture, the crystallinity can be preserved by suspending the fine powders of salt (oxide) and zeolite in an inert volatile liquid such as n-hexane (cf. Sect. 3). 5.1.7 Role of the Nature of the Anions

To date, no systematic study of the effect of the anion’s nature in the compound of the in-going cation has been undertaken. Only in the system M+m (Am–)/HZSM-5 (A = anion) has a series of experiments with various anions (A = F, Cl, Br, J, OH, NO3, CO3) been carried out [22]. It turned out that solid-state ion exchange proceeded most easily with the chlorides, whereas with fluorides and bromides the reaction was slower and frequently incomplete. Mixtures of iodides of alkaline metals and H-ZSM-5 turned yellow simply on grinding; obviously, the iodides decomposed in the presence of the hydrogen form of the zeolite. Cesium hydroxide was employed for introduction of Cs+ into Y-type zeolites [36]. Concerning other salts, only a limited number of observations have been reported. For instance, MgF2 mixed with H-MOR reacted only to an exchange degree of about 40%, whereas with MgCl2 an almost 100% exchange was achieved [41]. Reactions of salts with complex anions such as NO –3 and SO42– are more complicated. The anions were more or less decomposed (cf. Sect. 5.3.4). In the case of carbonates no solid-state reaction was observed [22]. Most probably, the carn+ bonates decomposed at the elevated temperatures applied (M n+ 2/nCO3 Æ M 2/nO + CO2 , n = 1, 2), and the oxide was not or only slightly reactive. Solid-state reactions of complex oxo- or chloro-anions (e.g., chromates, vanadates, chloromolybdates) with zeolites will be discussed in subsequent sections (cf., e.g., Sect. 5.3.5). 5.1.8 SSIE of M+ and M2+ Halides with H-Zeolites Investigated by IR

Solid-state ion exchange between alkali halides and hydroxyl groups of the hydrogen form of zeolites was also monitored by infrared spectroscopy.An early example is taken from the study by Rabo et al. [16, 17] mentioned earlier and is reproduced in Fig. 8. A mixture of Ca,H-Y and NaCl was calcined in air. IR spectra of the mixture prior to and after thermal treatment proved the elimination of the hydroxyl groups (disappearance of the OH stretching bands at 3745, 3690 and 3640 cm–1) under evolution of HCl. This resulted in a complete elimination of the catalytic activity of the zeolite sample in the acid-catalyzed isomerization of 1-butene to 2-butene. Similar to the approach of Rabo, Jiang and Tatsumi [56] used SSIE with KCl to eliminate acid centers in K-Y and K-L loaded with Mo3S4 clusters, which had been generated upon activation of the zeolite/Mo3S4 catalysts.After SSIE, the catalysts produced greater amounts of alcohols in the hydrogenation of CO, due to the decreased acidity.

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71

Fig. 8. Solid-state ion exchange between NaCl and Ca,H-Y. OH spectra of Ca,H-Y (A, solid line) and NaCl/Ca,H-Y (B, broken line), both after calcination in air at 825 K for 48 h (after [16], with permission)

In systematic investigations of solid-state ion exchange in the systems MCl/ H-ZSM-5, NH4-ZSM-5 and MCl/NH4-Y (cf. [22, 43]), the decrease in the intensity of the IR bands that are indicative of hydroxyl groups was also used to determine solid-state ion exchange. From Fig. 9 it can be seen that not only the acidic OH groups (Brønsted acid sites, as indicated by the band at 3605 cm–1) have reacted with NaCl but that, to some extent, the weakly acidic silanol groups (band at 3740 cm–1) were also involved as well (cf. also Fig. 8). However, since the silanol groups possess only a low strength of acidity [57], brief washing with deionized water and subsequent drying restored the original intensity at 3740 cm–1. The intensity at 3605 cm–1 re-appeared only after re-exchange with NH4Cl solution, subsequent dehydration and deammoniation in the IR cell. Simultaneously, this was a test for the integrity of the zeolite framework: the reappearance of the bands of the Brønsted acid sites showed that no (partial) collapse of the lattice had occurred during the solid-state reaction. Otherwise, at least part of the tetrahedrally coordinated aluminum would have left the framework and the density of the Brønsted centers would have decreased accordingly. Usually, reaction of MCl with H-ZSM-5, NH4-ZSM-5 or NH4-Y leads to a complete disappearance of the bands of the Brønsted acid sites, thus proving an exchange degree of 100%. An exception was observed with the system CsCl/NH4-Y.However,application of a slight excess of CsCl and a second exchange step also produced a 100% replacement of (acidic) protons by Cs [58] (Fig. 10). In aqueous solution only less than 70% of the original Na+ cations could be exchanged. Thus, the 100% degree of exchange achieved by solid-state reaction is an interesting result in that a high degree of exchange is required in order to increase the basicity of zeolites as catalysts for base-catalyzed reactions. Weitkamp et al. [36], therefore, employed SSIE of cesium salts with Y-type zeolites for the preparation of base zeolite catalysts. As mentioned above, conven-

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Fig. 9. IR spectra of the hydroxyl stretching frequency region after degassing at 723 K for 2 h (final vacuum about 10–3 Pa). a H-ZSM-5; b NaCl/H-ZSM-5 mixture (nNaCl/nAl = 1.89) calcined at 900 K for 1 h; c (b) washed with water; d (b) twice exchanged with 1 N NH4Cl solution (after [22], with permission)

Fig. 10. IR spectra of the hydroxyl stretching frequency region after degassing of a NH4-Y at 725 K for 12 h (final vacuum about 10–5 Pa) and b CsCl/NH4-Y (nCs/nAl = 1.1) at 725 K for 20 h (ex situ SSIE) and subsequently for 2 h in high vacuum (in situ SSIE); degree of exchange in NH4-Y was 98%

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tional exchange of cesium into Y-type zeolites does not yield a very high degree of exchange. This is due to the large diameter of the Cs+ cation (0.37 nm) that does not allow penetration of the six-membered rings (0.22 nm) leading to the small cavities of the structure. Solid-state ion exchange, however, is possible because of the higher temperatures that can be applied during the process and at which the six-membered rings become more flexible (cf. also [53]). The products prepared by Weitkamp et al. via SSIE with KCl and RbCl were characterized by TGA, IR, XPS, 27Al MAS NMR, 29Si MAS NMR, and, particularly for Cs, by 133Cs MAS NMR, as well as by conversion of isopropanol and methanol. A 100% exchange in the case of Rb-Y and an almost 100% exchange in the case of Cs-Y could only be achieved by SSIE. The catalytic behavior of the materials prepared in this way was tentatively explained by an increasing basicity of the oxygen atoms of the framework related to the decreasing electronegativity of the alkaline metal cations. In a study by Xu et al. [50], introduction of Cs+ via solid-state ion exchange was monitored by XRD and TGA/DTA. The Cs-BETA catalyst easily obtained by SSIE was found to have a rather high degree of exchange, viz., 94%, and possess a high activity in base-catalyzed dehydrogenation of isopropanol (vide supra). Systematic studies of the solid-state ion exchange of alkaline halides with NH4-Y were more recently resumed by Jiang et al. [59]. These authors also observed a significantly higher degree of exchange with Rb+ and Cs+ via SSIE than via exchange in aqueous solution. The degree of exchange (by 1-fold solidstate reaction) decreased, however, in the sequence K+ >Rb+ >Cs due to geometric constraints. FT-Raman spectroscopy was employed and proved to be a useful tool in investigating solid-state ion exchange when Huang et al. [33] reacted LiCl or CaCl2 with NH4-Y or Na-Y to produce Li-Y or Ca-Y. The results were verified by XRD. Series of alkaline-metal-containing M+-X and M+-Y zeolites (M+ = Na+, K+, Rb+, Cs+) for detailed IR spectroscopic investigations were prepared by Esemann and Förster [60, 61], Esemann et al. [62] and Geidel [63]. Solid-state reaction between the ammonium forms of the zeolites and MCl proved to be most efficient and provided a high degree of exchange. From the experiments it was concluded that the solid-state reaction occurred to a large extent prior to deammoniation. Since the introduction of cations with a larger diameter caused an expansion of the lattice and a lowering of the bond strength, above 1000 cm–1 a frequency shift of the valence vibrations to lower values was observed (cf. Fig. 11). Also, spectra in the far-infrared region (50–300 cm–1) characteristic of M-X and M-Y zeolites were reported [63]. IR studies were also carried out with mixtures of alkaline earth salts and hydrogen forms of zeolites. As an example, IR spectra of H-MOR prior to and after solid-state reaction with CaCl2 are shown in Fig. 12 (cf. [41]). Comparison of spectra 1a and 2a provided evidence for the complete exchange of the protons of the acidic OH groups as indicated by the disappearance of the band at 3610 cm–1. The decrease in the intensity of the band at 3750 cm–1 shows also that a fraction of the silanol groups have reacted with CaCl2 . Pyridine adsorption confirmed that, after solid-state ion exchange of

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Fig. 11. Spectra in far-infrared of M-Y zeolites with M = Na (a), K (b), Rb (c), and Cs (d) obtained by solid-state ion exchange in the IR cell in a flow of N2 at 600 K (12 h) from mixtures of the respective salts and NH4-Y (after [63], with permission)

CaCl2 and H-MOR, there were no longer any acidic Brønsted sites present, since no band around 1540 cm–1 (which is typical of pyridinium ions and would form in the presence of such acid sites) is seen. Instead, a band at 1446 cm–1 appeared in spectrum 2b, indicating pyridine coordinatively attached to Ca2+ and thus proving the incorporation of these cations into the mordenite structure. A second sample (wafer of a mixture of CaCl2 ◊ 2H2O and H-MOR) was heated in the IR cell at 775 K to obtain a spectrum like 2a in Fig. 12. Subsequently, the wafer was briefly contacted with 1.3 kPa H2O vapor at 400 K. After pumping off the excess water, spectrum 3a was registered. It exhibited an intense band at 3618 cm–1 which originated from acidic OH groups similar to but not identical with those of the parent sample of H-MOR (spectrum 1a). After pyridine adsorption and degassing, spectrum 3b was observed which contained an intense band typical of pyridinium ions (1540 cm–1). It resulted from the reaction of pyridine with acidic Brønsted centers that were generated through the Hirschler-Plank mechanism [64, 65]: Ca2+Z–2 + H2O Æ H+Z– + Ca(OH)+Z–

(10)

where Z– is a monovalent negatively charged framework fragment of the zeolite structure. Additionally, spectrum 3b in Fig. 12 displayed an intense band at 1446 cm–1 due to pyridine interacting with Ca(OH)+, resulting in a coordination complex with pyridine, and a band at 1455 cm–1, typical of pyridine bound to ‘true’ Lewis sites (Al-containing extra-framework species). Very similar results were reported for the system MgCl2/H-MOR [41]. Elimination of (most of) the band arising from OH groups of H-MOR, indication of Mg2+ incorporation via pyridine adsorption (resulting in a band around 1448 cm–1), and rehydroxylation (re-appearance of OH groups and formation of

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Fig. 12. IR spectra of H-MOR and the system CaCl2 ◊ 2H2O/H-MOR with nCa/nAl = 0.5, after thermal treatment at 775 K and 10–5 Pa. Spectra 1a-3a in the OH stretching frequency region, 1b-3b in the region of pyridine ring deformation frequencies. 1a, 1b without and 2a, 2b with admixed CaCl2 ◊ 2H2O; 2b after pyridine adsorption and 3a after rehydroxylation, 3b after rehydroxylation and pyridine adsorption (after [41], with permission)

pyridinium ions) were found. It should be mentioned, however, that solid-state reaction in the system MgCl2/H-ZSM-5 did not proceed as rapidly as in the case of MgCl2/H-MOR. Moreover, in the former system, the replacement of protons by Mg2+ cations was frequently incomplete. Li et al. [66] reported on Mg2+ incorporation into H-ZSM-5 through SSIE as an effective and convenient route for modifying the zeolite. Samples with various Mg2+ contents were prepared. IR spectroscopy as well as XRD and TPD indicated that their structure and crystallinity were not changed in comparison with the parent H-ZSM-5. However, the concentration of Brønsted acid sites decreased with increasing Mg2+ content, whereas the density of Lewis sites was slightly enhanced and the catalytic behavior in alkylation with methanol significantly improved. 5.1.9 SSIE of M+ and M2+ Halides with Na-Zeolites Investigated by IR

Solid-state ion exchange with the sodium form of zeolites can be investigated by IR spectroscopy using probe molecules such as pyridine. In principle, this type of SSIE leads to an equilibrium (cf. Scheme 1b). Solid-state ion exchange between alkaline metal salts and a sodium form of zeolite X (13X) was investigated by Yang and Xu [67–69] using XRD and IR. The

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behavior of the products was compared with that of samples obtained via exchange in aqueous solution: In contrast to the case of MAPO-36 (cf. [70], vide infra), the surface area, pore volume and catalytic activity in isopropanol conversion were found to be independent of the exchange method used for preparation of the samples. The properties of the materials were only determined by the degree of exchange. Samples exchanged with Cs+ exhibited higher basicities and dehydrogenation activities than K+-exchanged catalysts with the same degree of exchange. However, it was more difficult to replace Na+ by the bulky Cs+ than by K+ cations. Figure 13 shows, as an example for the solid-state reaction with a sodium form of a zeolite, the spectrum of a wafer made from a BeCl2/Na-Y mixture, heated in an ultra-high vacuum (p = 10–5 Pa) at 400 K and subsequently contacted with pyridine vapor (spectrum b). An intense band at 1453 cm–1 indicated Be2+ cations on extra-framework sites [71]. No band at 1444 cm–1, typical of pyridine attached to Na+ [32], was observed. However, when the mixture of BeCl2 and Na-Y was heated to 725 K, the band at 1453 cm–1 decreased in intensity and the Py Æ Na+ band at 1444 cm–1 developed. At higher temperatures, the equilibrium (cf. Scheme 1b) is most likely shifted: a fraction of Na+ remigrates from the NaCl crystallites formed (vide infra) (cf. Sect. 5.1.10) to cation sites, thereby replacing Be2+ cations. Similar observations were made in the system LaCl3/Na-Y (see Sects. 5.2.6 and 5.2.7). Akolekar and Bhargava [70] succeeded in modifying MAPO-36 by introduction of Na+ via (1) conventional and (2) solid-state ion exchange and character-

Fig. 13. IR spectra after degassing at 400 K and subsequent pyridine adsorption: a Na-Y; b the mixture BeCl2/Na-Y; c sample of spectrum (b) heated at 725 K (after [73], with permission)

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ized the products by XRD, SEM, FTIR, TGA, BET, 27Al MAS NMR and 31P MAS NMR. Samples obtained by method (1) exhibited the same crystallite morphology and surface area as the parent MAPO-36, whereas these properties were significantly affected by method (2). Moreover, the products of both preparative procedures differed in the 27Al MAS NMR and 31P MAS NMR spectra. Nevertheless, the catalytic activities of the Na+-exchanged materials in o-xylene conversion were similar. Finally, the importance of SSIE with respect to an interaction of zeolite matrices and clays used as binders in catalyst formulations has been stressed several times. Thus, Canizares et al. [72] showed that SSIE between Na-montmorillonite (binder material) and H-ZSM-5 or H-mordenite is the reason for decreased Brønsted acidity in the zeolite matrices as compared to the unbound zeolite. 5.1.10 SSIE of M+ and M2+ Halides with Na-Zeolites Investigated by MAS NMR

Ground mixtures of LiCl, KCl, BeCl2 or CaCl2 with Na-Y were investigated by 23Na MAS NMR [73]. The signals were referenced to crystalline NaCl. Na+ cations residing in the supercages of the Y-structure give rise to a signal at about –9 ppm, whereas a signal at about –13 ppm is assigned to Na+ cations in the small cavities [74]. (This assignment is at some variance to the assignment suggested earlier [73].) Regarding solid-state ion exchange at low temperature (LT regime) or upon mere grinding (“contact-induced ion exchange”, cf. Sect. 6.2) of the mixtures mentioned above, the most striking feature is, however, that the signal at ca. –8 ppm of the parent zeolite disappeared and a sharp signal at 0 ppm developed instead (Fig. 14). The explanation is that the Na+ cations in the supercages were, to a large extent, replaced by Li+, K+, Be2+ or Ca2+ and a fraction of the expelled sodium cations formed with Cl– anions of the admixed salts tiny NaCl crystallites outside the zeolite grains. A broad signal around –13 ppm was left in the spectrum, mainly due to Na+ cations in the small cavities. It cannot be excluded, however, that a small fraction of the Na+ cations are still located in the supercages contributing to the intensity of the –13 ppm signal at its high-field wing. In any event, the intensity of this signal appears enhanced which is most likely caused by a migration during LT ion exchange of a fraction of Na+ cations from the supercages into the small cavities. Thus, these experiments have at least qualitatively proven that solid-state ion exchange has taken place in the above systems. The respective measurements can probably be improved to become suitable for quantitative determinations as well. Weitkamp et al. [36], who prepared a series of alkaline metal zeolites (K-Y, RbY, Cs-Y) via solid-state ion exchange in order to study the effect of the basicity of such zeolites on their catalytic behavior, used 133Cs MAS NMR for the characterization of Cs-Y. Figure 15 shows (a) the spectrum of Cs-Y, (b) the simulated spectrum and (c) the individual components derived from a decomposition of spectrum (b).

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Fig. 14. 23Na MAS NMR spectra of Na-Y and mixtures of halides and Na-Y (see text; after [73], with permission)

Fig. 15. 133Cs MAS NMR spectrum a of Cs-Y prepared by solid-state ion exchange in a CsOH/NH4-Y mixture, b after simulation and c after decomposition of (b) into individual components (after [36], with permission)

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Table 5. Cs+ population of the cation sites in zeolite Cs-Y prepared by SSIE; results of MAS NMR measurements (after [36])

Site

I¢

III

II

Cs(OH)/b-cage

Chemical shift a (ppm) nCs+/u.c.

–78 9

–66 6

–61 17

+57 21

a

133Cs

Referenced vs. 1 M aqueous CsCl solution.

Four signals can be distinguished (referenced to 1 M aqueous CsCl solution), viz., at –78, –66, –61 and +57 ppm. The authors were able to assign, on the basis of earlier work [75], the lines at –78, –66, and –61 ppm to cesium cations on sites I¢ (in the sodalite cages), sites III and sites II (both in the supercages), respectively. The signal at +57 ppm was tentatively attributed to Cs(OH) ◊ H2O, since the solid-state ion exchange was carried out with Cs(OH) · H2O and NH4-Y. The authors were able to derive from the 133Cs MAS NMR results the population of the respective sites by Cs+ and Cs(OH) · H2O. The data are presented in Table 5. Evaluation of the 29Si MAS NMR and 27Al MAS NMR spectra proved that the integrity of the zeolite framework had been preserved. 5.1.11 Catalytic Activity of M2+-Zeolites Prepared via SSIE

The catalytic activity of Ca,H-MOR and Mg,H-MOR obtained via solid-state ion exchange was tested using the disproportionation of ethylbenzene as a test reaction [41, 76]. Expectedly, the modification of H-MOR by incorporation of Ca2+ or Mg2+ (nCa2+/nAl = 0.50 and nMg2+/nAl = 0.65 at 675 and 875 K, respectively; cf. Sects. 5.1.5 and 5.1.8) considerably decreased the activity (by ca. 90 and 77%, respectively). The activity was not completely eliminated, since residual acidic OH groups were, after calcination of the MCl2/H-MOR mixtures, still present (vide supra). For example, as evidenced by in situ IR, 30% of the original Brønsted acid OH groups survived the reaction with MgCl2 . Moreover, interaction with H2O vapor of the ambient air was not excluded in the catalytic experiments following the solid-state ion exchange. However, subsequent rehydroxylation by brief contact with admitted H2O vapor (vide supra) increased the catalytic activity of Ca,H-MOR and Mg,H-MOR obtained via SSIE by about 50%. Thus, in principle, solid-state ion exchange offers an interesting route for preparation of catalytically active zeolites (see also Sects. 5.2.8, 5.4.3 and 5.4.4). 5.2 SSIE of Lanthanum (La3+) Cations 5.2.1 Introductory Remarks

Lanthanum, or virtually all rare earth cations, are important constituents of catalysts employed in cracking of vacuum distillates from petroleum [77]. Thus, it seemed interesting to explore the possibility of introducing La3+ cations into

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zeolites by solid-state ion exchange. Studies were preferentially conducted with hydrogen (ammonium) or sodium forms of Y-type zeolites, since this zeolite is widely employed in cracking catalysis. The solid-state reaction was monitored by chemical analysis (CA), temperature-programmed evolution of gases (TPE), infrared (IR) spectroscopy, X-ray diffraction (XRD), and 23Na MAS NMR [78, 79]. More recently, Jia et al. [80] compared in an extended study the incorporation of lanthanum via conventional and solid-state ion exchange into H-BETA zeolite and investigated the effect of many parameters on this example of SSIE. These authors employed XRD, BET, FTIR TEM, EDS (energy dispersive spectroscopy) and chemical analysis for characterization of the products of ion exchange. The effect of exclusion of water vapor and deliberate hydroxylation was particularly investigated with mixtures of NH4-Y, H-MOR, H-FER with LaCl3 ◊ 7H2O or water-free LaCl3 (cf. also Sect. 7). The catalytic activity of La, H-Y obtained by solid-state reaction was tested in the disproportionation of ethylbenzene and cracking of n-decane. 5.2.2 SSIE of La3+ Chloride with H-Zeolites Investigated by TPE

Figure 16 displays the TPE profiles [78] obtained during temperature-programmed heating of a finely dispersed sample of the parent zeolite, NH4-Y, (for comparison; m/e = 16, deammoniation; m/e = 18, dehydroxylation) and a mixture of LaCl3 · 7H2O with the ammonium (hydrogen) form of zeolite Y (for solidstate reaction; m/e = 16, deammoniation; m/e = 18, dehydration; m/e = 36, evolution of HCl).

Fig. 16. Temperature-programmed evolution of H2O (m/e = 18) from NH4-Y and NH2+ (m/e = 16), HCl (m/e = 36) and H2O (m/e = 18) from a LaCl3/NH4-Y mixture (after [78], with permission)

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Similar to the case of the systems where the exchange occurred with alkaline salts (cf. Sect. 5.1), one can also distinguish between a low-temperature (LT) and a high-temperature (HT) regime (see the curve describing the evolution of HCl, m/e = 36). Comparison of the profiles obtained with NH4-Y (broken line) and LaCl3/NH4-Y (solid lines) shows that, in the latter case, no peak occurs in the region around 950 K [78]. With pure NH4-Y, this well-developed peak originates from the dehydroxylation of NH4-Y or, more precisely, of H-Y, since the NH4-Y sample is deammoniated already at lower temperatures (cf. broken line; NH4-Y, m/e = 16). In the case of the system LaCl3/NH4-Y, such a dehydroxylation cannot occur because the OH groups generated via deammoniation in the LT region are consumed by the solid-state reaction at temperatures below 950 K, i.e., at about 650 to 900 K and are no longer available for dehydroxylation. Both these findings, i.e., the evolution of ammonia and hydrogen chloride as well as the absence of the dehydroxylation peak, proved that NH+4 cations from the NH+4 -zeolite and, at higher temperatures, H+ ions from the deammoniated NH+4 -zeolite, were exchanged for La3+ cations of solid LaCl3 . NH4Cl, which may have formed intermittently, would be thermally decomposed into NH3 and HCl. Combined TPE and TGA experiments (cf. Sect. 5.1.4) could not be carried out because of the simultaneous release of HCl and NH3 . However, titration was possible with the system LaCl3 ◊ 7H2O/H-ZSM-5 (cf. Fig. 17). The curves a (nLa/nAl = 0.33) and b (nLa/nAl = 0.67) reveal that in a low-

Fig. 17. Temperature-programmed evolution of HCl monitored via continuous titration during solid-state ion exchange between LaCl3 · 7H2O and H-ZSM-5 with nLa/nAl ratios of 0.33 (curve a) and 0.67 (curve b) and isothermal steps at 945 K for 2 and 0.5 h, respectively; curve c represents the behavior of similarly treated LaCl3 · 7H2O

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temperature process up to ca. 570 K only a minor fraction of the hydrogen form of H-ZSM-5 was converted and the major part of exchange took place in a hightemperature process between 675 and 950 K, where the sample was kept for 2 h. Curve c describes the behavior of pure LaCl3 ◊ 7H2O. Comparison of curves a, b and c proves that the evolution of HCl is not simply caused by decomposition of the lanthanum salt, but is indeed the result of an interaction of the two solid components of the mixture, i.e., the salt and the hydrogen form of ZSM-5. The exchange was more rapid and led to a higher degree of exchange when an excess of LaCl3 ◊ 7H2O was employed. However, in both cases (a and b), the reaction produced on isothermal treatment an exchange degree of less than 100%. This was at variance with the observations on LaCl3 ◊ 7H2O/NH4-Y (cf. Sect. 5.2.3). Most likely, a 100% exchange is not achieved with H-ZSM-5 because of the difficulty to electrically neutralize the more distant exchange sites in ZSM-5 by a trivalent (naked) cation such as La3+ compared to Y-type zeolites where the exchange sites are closer to each other due to the higher Al content. The difference in the behavior of ZSM-5-type and Y-type zeolites in solid-state ion exchange with La3+ chloride was confirmed by IR investigations (cf. Sect. 5.2.4). 5.2.3 Stoichiometry of SSIE in La3+ Chloride with H-Zeolites

The stoichiometric measurements [79] in the system LaCl3 · 7H2O/NH4,Na-Y via chemical analyses prior to and after solid-state reaction were very instructive (cf. Table 6). The data in Table 6 are the results of chemical analyses obtained for a NH4,Na-Y zeolite with a 89% replacement of Na+ by NH+4 cations. Thus, the first mixture prepared for SSIE of the NH4+ cations contained a small excess of La3+, since its composition corresponded to one La3+ per three Al (nAl/nLa = 0.33), but 11% of the charges generated by the framework Al were compensated by Na+. The experimental results were obtained by titration of the evolved hydrogen chloride and chemical analysis of both the product after extraction with water and the extract solution. The data presented in Table 6 proved an excellent stoichiometry: within the limits of error, the amount of La3+ introduced via solid-state reaction and irreversibly held on washing with water (4.80 mequiv per gram) corresponded exactly to the amount of framework aluminum (4.83 mequiv per gram) or the maximum of bridging OH groups. However, even half of the Na+ cations of the starting material (1.61 mequiv per gram) were removed from the zeolite. Only about 0.7 mequiv Na+ and 0.8 mequiv Cl– per gram remained in the structure. This would correspond to about one NaCl molecule per b-cage. It seems likely that this amount of NaCl is occluded in the structure and would, according to Rabo’s work [16, 17, 53], enhance the thermal stability of the exchanged zeolite (cf. also Sect. 5.1.5). Essentially the same results were obtained when a higher excess of La3+ was applied. Table 7 presents the data for a ratio nLa/nAl = 0.67. The only difference was that, after completion of the reaction, a higher amount of LaCl3 was found in the washing water, viz., the total excess was extracted by the washing water and again only an amount of La3+ corresponding to the Al content of the framework was irreversibly held.

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Table 6. Stoichiometry of solid-state ion exchange in the system LaCl3/NH4-Ya; nLa/nAl = 0.33; heat-treatment at 850 K

La3+

Cl–

Na+

NH+4

Al

Parent zeolite Admixed Evolved as NH4Cl/HCl Extracted with water

– 1.61 – 0.06

– 4.83 3.29 0.72

1.61 – – 0.94

3.29 – 3.29 –

4.83 – – –

Irreversibly held (mmol g–1) (mequiv g–1)

1.60 4.80

0.82 0.82

0.67 0.67

– –

4.83 4.83

a

All data given in mmol per gram, except data of last line (mequiv per gram).

Table 7. Stoichiometry of solid-state ion exchange in the system LaCl3/NH4-Y a; nLa/nAl = 0.67; heat-treatment at 850 K

La3+

Cl–

Na+

NH+4

Al

Parent zeolite Admixed Evolved as NH4Cl/HCl Extracted with water

– 3.22 – 1.57

– 9.65 3.29 5.51

1.61 – – 0.94

3.29 – 3.29 –

4.83 – – –

Irreversibly held (mmol g–1) (mequiv g–1)

1.65 4.95

0.85 0.85

0.67 0.67

– –

4.83 4.83

a

All data given in mmol per gram, except data of last line (mequiv per gram).

With SSIE in the system LaCl3 ◊ 7H2O/H-BETA, however, it appeared that not all of the incorporated lanthanum cations were involved in balancing the charge of the framework [80]. This was mainly inferred from the FTIR data (vide infra) even for those exchange experiments at 773 K, where 100% of the admixed Lasalt was consumed and a ratio of nLa/nAl = 0.33 measured. This finding was largely ascribed to the reaction of LaCl3 with part of the silanol groups and extraframework Al(OH) species. Moreover, it was pointed out that, due to the high atomic ratio nSi/nAl = 15, the distances between the negatively charged exchange sites were generally rather large. Thus, usually one La3+ will not be able to simultaneously balance the negative charges of three exchange sites close to three framework Al atoms (cf. Sect. 5.2.2, SSIE of LaCl3 and H-ZSM-5). However, it was found that, at variance with conventional ion exchange, solid-state reaction between lanthanum chloride and H-BETA led to a higher degree of exchange and a much more homogeneous distribution of the La cations. 5.2.4 SSIE of La3+ Chloride with H-Zeolites Investigated by IR

Further evidence for the solid-state ion exchange with La3+ was provided by IR [78]. An IR transmittent wafer made from a stoichiometric mixture of NH4-Y

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Fig. 18. IR spectra of La,Na-Y obtained by solid-state ion exchange (set S1) and conventional ion exchange in aqueous solution (set S2) (see text; after [78], with permission)

(more precisely, a NH4,Na-Y sample with 75% of the original Na+ cations exchanged by NH4+) and LaCl3 ◊ 7H2O was heated at 673 K. The resulting spectrum is shown as spectrum a in Fig. 18. Essentially none of the OH bands typical of H-Y at 3640 and 3550 cm–1 [8] occurred which showed that all of the available protons of the Brønsted acid sites were replaced by La3+. However, contact of the heat-treated wafer with water vapor (0.65 Pa at 643 K) and subsequent degassing at temperatures increasing from 523 to 773 K provided spectra b (set 1) in Fig. 18. These exhibited two prominent bands in the OH stretching region and were very similar to those of conventionally exchanged samples, the spectra of which are displayed in series c (set 2). Also, the ratios of the intensities of the low-frequency bands and highfrequency bands at 3645 and 3535 cm–1, respectively, were almost identical for sets 1 and 2. The absolute intensities did, in fact, differ. This, however, can be ascribed to the difference in the conditions of dehydration: the spectra of set 1 were run after treatment in high vacuum (10–5 Pa), whereas those of set 2 were run in a flow of dry nitrogen. Later, exchange experiments were carried out starting with NH4-Y materials with a degree of exchange of almost 100% which can be achieved by repeated exchange in aqueous ammonium salt solutions. When these starting materials were subjected to solid-state ion exchange, a 100% replacement of NH4+ (or H+) by La3+ was achieved in one step (cf. Sect. 5.2.8). In contrast to the above findings with the ammonium form of faujasite-type zeolites with a regular low nSi/nAl ratio of 2.5, exchange experiments with HZSM-5 or NH4-ZSM-5 (nSi/nAl >15) and LaCl3 ◊ 7H2O led to incomplete removal of the IR band at 3605 cm–1 which indicates the acidic OH groups. Thus, the IR investigations confirmed the observations described in Sect. 5.2.2.

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Lanthanum introduction into H-BETA by SSIE was further studied by Jia et al. [80] using FTIR as well as XRD, BET, TEM, EDS and AAS. The results were compared with those obtained by conventional exchange in aqueous solution. In contrast to conventional exchange, a 100% incorporation of La3+ was achieved by SSIE. However, on the one hand, the acidic OH groups (band at 3610 cm–1) were not completely removed, not even in the case of a salt excess. On the other hand, it was seen that LaCl3 also reacted with hydrogen bonded and isolated silanol groups (weakening of the bands at 3700–3700 and 3743 cm–1, respectively) and with OH groups on Al totally or partly disconnected from the framework (disappearance of the bands at 3782 and 3660 cm–1). Moreover, the resulting materials were highly homogeneous and did not show any loss of crystallinity. Far infrared spectroscopy was employed by Esemann and Förster [81] as a powerful tool to monitor the incorporation of cations such as La3+ into Y-type zeolites upon solid-state reaction of LaCl3 with H-Y or NH4-Y. This method provided insight into the distribution of the La3+ cations on cation sites of the structure and was also applied on SSIE with alkaline, alkali metal and zinc chlorides (vide infra). Difallah and Ginoux [82] demonstrated that La-Y prepared from NH4-Y via SSIE with LaCl3 exhibited an increased capacity for CO adsorption. Solid rare earth chlorides (CeCl3 , NdCl3 , SmCl3 , EuCl3 , or YbCl3) were reacted with NH4-Y (H-Y) with nSi/nAl ratios of 2.6, 12.5 and 28 [83]. The activity of the exchanged materials for catalytic liquid-phase oxidation of cyclohexane depended on the nSi/nAl ratio, type of cation (CeªYb>Sm>Eu>Nd) and the reaction temperature. A remarkably active Ce-ZSM-5 catalyst for reduction of NOx in diesel exhaust was produced by van Kooten et al. [84] via SSIE. Ce-ZSM-5 did not convert nitric oxide or ammonia into the greenhouse gas nitrous oxide. At 770 K and a GHSV (gaseous hourly space velocity) of 50,000 h–1 a conversion degree of 70% NOx could be reached. The rare earth cation Eu3+ was incorporated into Y-zeolite by solid-state reaction between EuCl3 and NH4-Y [85]. At constant temperature, the degree of exchange depended on the reaction time. A 94% exchange was easily achieved after 4 h. The crystallinity as determined by XRD was little affected. Migration and location of the Eu3+ cations in the structure were discussed on the basis of spectroscopic data. 5.2.5 SSIE of La3+ Chloride with Na-Zeolites Investigated by XRD

Since most zeolites are synthesized in the sodium form, it was interesting to study also the solid-state reaction between LaCl3 and the sodium form of, e.g., Y-type zeolite [35, 74, 79]. It was expected that such a reaction should occur because it was observed that residual Na+ in NH4 ,Na-Y had indeed been replaced by La3+ (cf. Sect. 5.2.3). However, in view of Scheme 1b, it appeared likely that such a reaction would lead to an equilibrium (cf. Sects. 5.1.9 and 5.1.10) rather than to a 100% exchange, because it would not be possible to remove continuously one product component from the reacting mixture, as had been accomplished in the case of M+, M2+, or M3+ halides admixed to H- or NH4-zeo-

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Fig. 19. Schematic representation of XRD patterns of a Na-Y, b LaCl3/Na-Y after SSIE, c La, Na-Y obtained by CE, d (b) heated at 850 K, and e (c) heated at 850 K (after [79], with permission)

lites. Indeed, it was proven by IR, XRD and 23Na MAS NMR that it is possible to replace Na+ in Na-Y by La3+ via solid-state reaction, but that only a fraction of the Na+ cations react until an equilibrium is reached. In Fig. 19 sets of schematized XRD patterns are shown of a (1) the parent Na-Y, (2) a ground mixture of LaCl3 ◊ 7H2O/Na-Y, (3) conventionally exchanged La-Y(74) with an exchange degree of 74%, (4) a ground mixture of LaCl3 ◊ 7H2O/ Na-Y after heat treatment at 850 K, and (5) conventionally exchanged La-Y(74) after heat treatment at 850 K. Figure 19 demonstrates that upon solid-state cation exchange between LaCl3 and Na-Y the reflections of the product NaCl appear (cf. part b of the figure). Moreover, the intensities of other reflections [e.g., those of the reflections (222), (400), (511), and (551) of the faujasite phase] change towards those of La,NaY(74) prepared via conventional exchange in aqueous solution (compare parts a-c of Fig. 19). When, however, the samples were treated at higher temperatures, the intensities of the NaCl reflections markedly decreased (compare parts b and d of Fig. 19) and, also, the reflections of the faujasite phase indicated above changed back toward those shown in Fig. 19, part a. The latter observation was similar to that made with the sample La-Y (74) upon heating to 850 K (compare parts c, e, and a of Fig. 19).

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5.2.6 SSIE of La3+ Chloride with Na-Zeolites Investigated by IR

TRANSMITTANCE

Qualitatively, the formation of OH groups typical of La-Y as a result of SSIE of LaCl3 ◊ 7H2O with the sodium form of Y-type zeolites was substantiated by IR. This is demonstrated by Fig. 20, where the appearance of bands in the OH stretching region at ca. 3650 cm–1 and, after hydration, at 3540 cm–1 (assigned to OH groups associated with La cations) on reaction of LaCl3 ◊ 7H2O and Na-Y is shown (column B) and compared with the features of a conventionally exchanged La-Y(76) (column A) [35]. After pyridine adsorption, the band at 3650 cm–1 was completely eliminated and the intensity of the band around 3540 cm–1 significantly weakened. Instead, the corresponding bands appeared in the deformation region (vide infra). Similar to what was reported for, e.g., alkaline earth/Na-Y systems (cf. Sect. 5.1.9), IR spectroscopy is also suitable to monitor the solid-state ion exchange in LaCl3 ◊ 7H2O/Na-Y mixtures when a probe such as pyridine is employed. On activation of a wafer of a LaCl3 ◊ 7H2O/Na-Y mixture in the IR cell (10–5 Pa, 625 K) and subsequent contact with pyridine (0.6 Pa, 375 K, followed by degassing), a band at 1447–1448 cm–1 developed besides a second one at 1439 cm–1 being indicative of pyridine coordinatively bound to La3+ and Na+ on cation sites, respectively (cf. Fig. 21).

WAVENUMBER [cm–1]

Fig. 20. IR spectra of (A), La,NaY [76] conventionally exchanged and (B) ground mixture of hydrated Na-Y with crystalline LaCl3 after heat treatment at 850 K and washing with water: (a) pretreated at 725 K in high vacuum for 3 h; (b) Sample (a) rehydrated with water (6.5 mbar) at 525 K for 0.5 h and subsequently degassed in 525 K, high vacuum for 1 h; (c) Sample (b) exposed to pyridine (5.7 mbar) at 425 K for 2 h and again evacuated at 475 K for 1 h (after [35], with permission)

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Fig. 21. IR spectra of the pyridine ring deformation frequency region after pyridine adsorption and subsequent degassing at 475 K and 10–5 Pa of La,Na-Y obtained A by conventional and B contact-induced ion exchange (after [35], with permission)

Fig. 22. 23Na MAS NMR spectra of a parent Na-Y zeolite, b mixture of LaCl3/Na-Y, c sample (b) after heat treatment at 850 K

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Similar to the case of BeCl2/Na-Y (cf. Sect. 5.1.9), only at lower reaction temperatures did the Py Æ Na+ band at 1439 cm–1 disappear completely, but at the high temperature of 775 K the equilibrium (Scheme 1b) was shifted toward a partial replacement of Na+ by La3+ only, as can be realized from the partial reappearance of the 1439 cm–1 band. This is in agreement with the XRD and 23Na MAS NMR findings described in Sects. 5.2.5 and 5.2.7, respectively. 5.2.7 SSIE of La3+ Chloride with Na-Zeolites Investigated by 23Na MAS NMR

As described in Sect. 5.1.10, 23Na MAS NMR is a suitable tool for detection and determination of cation migration upon solid-state reaction between M+Cl or M2+Cl2 and sodium forms of zeolites. Similar results obtained for the system LaCl3 ◊ 7H2O /Na-Y are illustrated by Fig. 22 [35, 74]. A sharp signal at about –9 ppm (referenced to NaCl) is indicative of Na+ in the large cavities, and Na+ cations in the small cavities are responsible for a signal at about –13 ppm (cf. [74, 86, 87]). In Na-Y containing sorbed water, both bands overlapped, and the resulting band was found at –8.2 ppm (Fig. 21a).After intense grinding of a mixture of LaCl3 ◊ 7H2O and Na-Y the spectrum b shown in Fig. 22 was observed. The most striking feature was the appearance of a sharp signal at 0 ppm that indicated that Na+ had been replaced by La3+ and tiny crystallites of NaCl had formed. As can be derived from the second signal at –13.1 ppm, all the remaining sodium cations resided in the small cavities. When the ground mixture was subsequently heated to 850 K, the exchange was partially reversed. This was evidenced by a decrease of the NaCl signal at 0 ppm and the reappearance of the signal of Na+ in the supercages (around –9 ppm). The 23Na MAS NMR results, therefore, confirmed the observations made by XRD and IR (cf. Sects. 5.2.5 and 5.2.6). In comparison to conventional exchange, solid-state ion exchange between LaCl3 ◊ 7H2O and the sodium form of Na-Y was also investigated by Hunger et al. [88]. These authors employed 139La MAS NMR,27Al MAS NMR and 29Si MAS NMR and suggested that La3+ migration into the sodalite cages was hindered due to a blockage of the six-membered ring windows by formation of NaCl. Sections 5.1 and 5.2 dealt with solid-state ion exchange with M2+ salts (M = Mg or Ca) and La3+ chloride, respectively. However, in these cases, the salts were initially not water-free but contained physically adsorbed and crystal water. The role of water in SSIE will be discussed in detail in Sect. 7.1. 5.2.8 Catalytic Activity of La3+-Zeolites Prepared via SSIE

As mentioned in the introductory remarks of Sect. 5.2, a particularly interesting question arose as to whether solid-state ion exchange would enable the convenient preparation of catalysts for acid-catalyzed reactions. Various experiments have indeed shown that solid-state ion exchange offers a new route for catalyst production. Some of the pertinent results are described below (cf. also Sects. 5.1.11, 5.3 and 5.4).

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Fig. 23. Disproportionation of ethylbenzene in a flow-reactor at 425 K after catalyst activation at 725 K in a flow of dry helium, subsequent hydration (1.2 · 103 Pa water vapor followed by degassing at 625 K) over La,Na-Y catalysts prepared by A solid-state ion exchange and B conventional exchange; degree of exchange in La,Na-Y about 98% (after [78], with permission)

An almost completely exchanged La-Y sample (0.25 g), viz., La-Y(98) with only 2% of residual Na+, was prepared via solid-state reaction of NH4-Y(98) with LaCl3 ◊ 7H2O at 725 K. This process was carried out in a conventional micro-flow reactor [78], followed by cooling to 455 K, a hydroxylation step (1.2 kPa H2O vapor, 455 K, 2 h), degassing in a helium flow at 625 K and cooling to 425 K (reaction temperature). Upon admission of the feed stream of 1.3 vol.% ethylbenzene in dry helium (30 ml min–1), disproportionation to benzene and diethylbenzenes was observed. The results of the conversion measurements are shown in Fig. 23 and compared with those obtained over a conventionally prepared La-Y(96) catalyst. As can be seen from Fig. 23, the catalytic performance of catalyst A obtained via solid-state ion exchange was even superior to that of the conventionally exchanged material B, in that the conversion was somewhat higher under equal conditions. The selectivity after completion of the induction period was, however, the same for catalysts A and B, i.e., in both cases benzene and diethylbenzenes formed in the ratio 1:1. A very interesting result was obtained when the SSIE in the system LaCl3 7H2O/NH4-Y(98) was conducted under careful exclusion of water vapor and the zeolite produced tested for an acid-catalyzed reaction. Solid-state ion exchange was carried out in situ in a high-vacuum-tight IR flow-reactor cell connected to a gas chromatograph [78] in an extremely anhydrous flow of helium at 455, 575 and 675 K.When the solid-state reaction in the IR transmittant wafer containing the mixture of the salt and the zeolite was completed, a feed stream of ethylbenzene in carefully dried helium was passed through the cell. It is clear from Fig. 24 that, at 455 K, the deammoniation of the NH4-Y component was not yet complete: bands at 3630 and 3549 cm–1, indicative of acidic OH

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Fig. 24. In situ IR spectra of and ethylbenzene conversion over La,Na-Y obtained by heattreatment of a LaCl3/NH4-Y mixture in a flow-reactor IR cell A prior to and B after brief contact with 10 Pa water vapor; a, b and c pretreatment at 455, 575 and 675 K, respectively; d after short contact with 10 Pa water vapor; e after admission of the ethylbenzene/helium feed stream (for details see text; after [78], with permission)

groups, had already developed, but in the NH stretching region a broad peak around 3200 cm–1 with shoulders at 3350 and 3080 cm–1 could still be seen indicating that residual NH+4 was present. Between 455 and 575 K (spectra a and b, respectively), the residual ammonium disappeared, but the OH bands did not develop further. On the contrary, the intensities of the OH bands were diminished because the solid-state reaction between the hydroxyls and LaCl3 has already started to consume the OH groups generated via deammoniation. This solid-state reaction was completed at 675 K (spectrum c). After admission of the ethylbenzene feed (spectrum d) no conversion was measured (curve A).In the IR spectrum only the bands of ethylbenzene appeared (not shown). Only after an extended time on stream (t>1 h) did a very small conversion take place (vide infra). When, however, after completion of the solid-state reaction first a brief (2 min) contact with H2O vapor was allowed (0.1 kPa water vapor injected into the He-stream),followed by degassing, OH bands at 3616 and 3518 cm–1 typical of an acidic La3+-containing Y-type zeolite appeared (cf. Fig. 24, spectrum d). Upon passing the feed stream over this hydroxylated wafer, the ethylbenzene interacted with the acidic OH groups in the large cavities, since the band at 3616 cm–1 disappeared and the bands of ethylbenzene adsorbed onto the catalyst (at 3074, 3032, 2972, 2936 and 2887 cm–1) developed (spectrum e). Moreover, an immediate onset of ethylbenzene conversion was detected by gas chromatography (GC). After a steep increase in the conversion, a steady state was reached at a conversion of 15% (curve B in Fig. 24). Thus, the following conclusions can be drawn: 1. the material produced by solid-state ion exchange in the absence of water is inactive in acid-catalyzed reactions such as ethylbenzene disproportionation; the naked La3+ cations introduced by solid-state ion exchange are not able to generate the necessary intermediates (carbenium ions);

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2. to prepare an active La-Y catalyst, contact with the co-catalyst H2O is a prerequisite for generation of acid Brønsted sites which are able to interact with the hydrocarbon to give the carbenium ions necessary for the catalytic mechanism [76, 89, 90]. The formation of Brønsted sites obviously occurs according to the Hirschler-Plank mechanism (cf. Eq. (10) and [64, 65], vide supra), which assumes that a splitting of H2O molecules in the Coulomb field of multivalent cations such as La3+ takes place: La3+Z–3 + 2 H2O Æ 2 H+Z– + La(OH)+2 Z–

(10)

where Z– denotes a negatively charged monovalent fragment of the zeolite framework. This also explains the slow increase in the conversion from zero to about 0.5% after 2 h on stream in the case without a deliberate hydroxylation step (see curve A in Fig. 24). Here, traces of H2O introduced by the feed stream cause a minor activation of the La-Y wafer, the spectrum of which is shown in Fig. 24, spectrum c. A similar comparison to that discussed above for a catalyst derived from LaCl3 ◊ 7H2O/NH4-Y(98) was made between a conventionally exchanged La, Na-Y(76) catalyst and a catalyst produced by SSIE in LaCl3 ◊ 7H2O /Na-Y, i.e., a system containing the sodium form of Y-zeolite instead of the hydrogen form [79]: After solid-state reaction in a stoichiometric mixture of LaCl3 ◊ 7H2O/Na-Y, the material was briefly washed twice with a few milliliters of water to remove the NaCl crystallites, which had formed (cf. Sects. 5.2.5 and 5.2.7), and non-reacted LaCl3 . Subsequently, the sample (0.25 g) was activated at 625 K in high vacuum (10–4 Pa, 3 h), cooled to reaction temperature (425 K) and contacted with the feed stream (1.3 vol.% ethylbenzene in dry helium, 30 ml min–1). Figure 25 demonstrates that the zeolite prepared from the sodium form of the Y-type zeolite was

Fig. 25. Conversion of ethylbenzene upon disproportionation over La,Na-Y (degree of exchange: 76% of Na+) obtained by A solid-state ion exchange in a LaCl3/Na-Y mixture on heat treatment for 3 h in high vacuum at 625 K and B conventional exchange; reaction temperature: 455 K (after [79], with permission)

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Fig. 26. In situ IR spectra of and decane cracking over highly exchanged La,Na-Y catalysts obtained A, a by solid-state ion exchange and B, b conventional ion exchange; degree of exchange: 98 and 96%, respectively; for details, see text (after [91], with permission)

indeed an active catalyst in an acid-catalyzed reaction such as ethylbenzene disproportionation. In fact, the conventionally produced catalyst, La,Na-Y(76), exhibited a shorter induction period and a higher initial conversion than the catalyst obtained through SSIE. However, both catalysts approached the same steady state conversion and produced benzene and diethylbenzenes in exactly the ratio of 1:1 (not shown). This corresponded to 100% selective disproportionation and excluded concomitant dealkylation that would have resulted in catalyst deactivation [76, 89, 90]. Results similar to those discussed with respect to the disproportionation of ethylbenzene over La-Y catalysts prepared by solid-state ion exchange were obtained when cracking of n-decane was used as a test reaction [91]. Figure 26 displays the in situ IR spectrum of highly exchanged La-Y formed upon solidstate reaction as described before and designated as La-Y(98; SE) and, for the sake of comparison, the IR signals of La-Y produced by conventional exchange with almost the same degree of exchange, designated as La-Y(96; CE). Interestingly, the band at 3520 cm–1 is missing in the La-Y(98; SE) sample. This indicates that the exchange has (preferentially) occurred in the large cavities (cf. Sect. 5.2.7). However, only the acid sites created in the large cavities and seen through the band at 3630 cm–1 play a role in the catalytic process, because only these are accessible for the feed, n-decane. After equal treatment both catalysts, i.e., La-Y(98; SE) and La-Y(96; CE), exhibited the same steady-state activity in n-decane conversion (cf. Fig. 26, curves A and B). The main differences were found in the product distribution: a higher yield of C3 to C5 over La-Y(98; SE) and formation of C1, 1-hexene and a higher yield of hexane over La-Y(96; CE).

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5.3 SSIE of Other Transition Metal Cations 5.3.1 Introductory Remarks

Investigation of the exchange of transition metal cations into zeolites through solid-state reactions is, to a larger extent, the domain of ESR spectroscopy. As already mentioned in the Introduction (Sect.1),Clearfield and co-workers applied this technique in an early study to prove solid-state ion exchange of, e.g., Cu2+ into zeolite Y. Since the respective report [18] is not easily available, some of those early results by Clearfield et al. will be described in more detail (see below). Resuming Clearfield’s pioneering work, Slinkin and his group started in the mid-1980s a systematic investigation of the introduction of copper cations from various copper compounds into zeolites, particularly into ZSM-5 and mordenite-type zeolites [21]. In addition, they carried out the first studies of the reaction of oxides such as CuO, CrO3,V2O5 and Na2CrO4 with zeolites [21, 23, 92, 93]. However, it is not only ESR that can be employed to study transition metal introduction into zeolites via solid-state reaction; as discussed in Sects. 4.2–4.7, chemical analysis, temperature-programmed evolution of product gases, X-ray diffraction and infrared spectroscopy are also generally applicable. In a few particular cases, other methods such as Mössbauer spectroscopy or UV-Vis spectroscopy, etc., have been equally helpful. In fact, the incorporation of transition metal cations into zeolites became very important in view of the generation of redox catalysts, particularly for removal of NOx from exhausts (DENOX processes). Extensive studies were carried out on the modification of zeolites, particularly of H-ZSM-5, by solid-state ion exchange with copper, iron, cobalt, and nickel. Catalysis-related studies of solid-state ion exchange with transition metal cations are reported at the end of Sects. 5.3.2.1, and also in Sects. 5.3.4.1–5.3.4.3. Therefore, the subsequent examples are not categorized with respect to the methods used for monitoring solid-state reactions of compounds containing transition metal cations with zeolites. Rather, a classification of the examples is adopted according to the type of metal cations to be introduced into the zeolite matrix. 5.3.2 SSIE of Copper, Silver and Gold Compounds with Zeolites 5.3.2.1 Introduction of Copper

In their work, Clearfield et al. [18] first converted the sodium forms of zeolites A, X and Y partially (exchange degree of 36%–58%) into the ammonium (hydrogen) forms using the conventional method of exchange in aqueous solutions of ammonium salts. The ammonium forms were deammoniated and reacted with the respective chlorides at 625–725 K in a helium flow or, for the ESR measurements, under vacuum. Figure 27 (curve A) shows the ESR spectrum of

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Fig. 27. ESR spectra of Cu,Na-Y obtained by A solid-state ion exchange in the mixture CuCl2/H,Na-Y and B conventional exchange; C ESR spectrum of the parent Na-Y; degree of exchange for A and B: 15% of the original Na+ cations were replaced by Cu2+(after [18, 24], with permission)

zeolite Y exchanged in the solid state: 15% of the exchangable ions in H,Na-Y were replaced by Cu2+; the sample, however, still contained appreciable amounts of protons due to the preceding deammoniation. For the sake of comparison, curve B presents the ESR spectrum of a Cu,Na-Y sample with the same content of Cu2+ but prepared by conventional exchange in an aqueous solution of copper acetate. Finally, curve C was obtained from the parent Na-Y. Spectra A and B are in good agreement. Both exhibit two sets of lines with four lines belonging to each set. This is indicative of two different environments of the Cu2+ cations. The g-values measured for the first set of lines were g = 2.35 and g^ = 2.06, and for the second one g = 2.30 and g^ = 2.06 (vide infra). These almost coincide with those reported by Krüerke and Jung [94] for conventionally prepared Cu,Na-Y.Very similar spectra were produced for a significantly lower loading of 5%. Solid-state ion exchange with CuCl2 as well as with other chlorides (CoCl2, NiCl2; cf. [18] and Sect. 5.3.4) was also monitored by back-titration of the HCl evolved and trapped in NaOH solution. These experiments showed, inter 2+/n + alia, that the degree of solid-state exchange was increased when the ratio n M H 2+ + was enhanced. Thus, n Cu /n H = 0.4 and 2.5 led to a replacement of 40 and 70% of the protons in H,Na-Y by Cu2+, respectively (vide infra). It was further shown that zeolite A was least resistant against the attack of HCl upon solid-state reaction, whereas Y zeolite was essentially stable and did not measurably lose crystallinity. ESR spectra similar to those reproduced in Fig. 27 were obtained by Slinkin et al. [23] when these authors studied the reactions between copper compounds and H-ZSM-5 or hydrogen mordenite, H-MOR. In reactions with oxides, H2O is formed instead of HCl, as was discussed above for the reactions with chlorides (see below and Scheme 1c–e). The ESR spectrum resulting from a reaction

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Fig. 28. Comparison of ESR spectra of Cu2+-containing ZSM-5 samples. a Sample prepared by solid-state ion exchange in a CuO/H-ZSM-5 mixture in vacuum at 1073 K, b sample obtained by conventional exchange, calcined in air at 1073 K and evacuated at 300 K, c spectrum obtained after contacting sample (a) with air (after [21, 95], with permission)

between CuO and H-ZSM-5 and subsequent evacuation is displayed in Fig. 28 (spectrum b). Obviously, solid-state reactions in systems such as CuO/H-ZSM-5 with oxides of the in-going cation require relatively high reaction temperatures: spectrum b of Fig. 28 was recorded after reaction at 1073 K, and it was reported that increasing the temperature from 793 to 1073 K considerably enhanced the degree of exchange. Spectrum b of Fig. 28 is in complete agreement with that generated by the same treatment of a conventionally exchanged Cu,H-ZSM-5 sample (see spectrum a of Fig. 28). The g-values and the hyperfine splitting (HFS) constants fully coincided. Similar to the system CuCl2/Y-zeolite (vide supra), two sets of gvalues and HFS constants were observed in both spectra a and b: g 1 = 2.29, g 1^ = 2.05, A1 = 15.6 mT, A^1 = 2.3 mT and g 2 = 2.31, g^2 = 2.06, A2 = 15.3 mT, A^2 = 2.25 mT. The computed g-values were found to be in good agreement with experimentally determined ones. Therefore, Kucherov and Slinkin [21] concluded like Clearfield et al. [18] that the Cu2+ cations resided on two differently coordinated sites, with set (1) corresponding to a square planar and set (2) to a pyramidal, fivefold coordinated state (cf. [95, 96]). The first site was visualized as being close to the walls of the straight channels of the ZSM-5 structure, where the Cu2+ cations would be coordinated to three oxygen atoms of the framework and one extra-framework ligand. It was assumed that the second type of Cu2+ cation was surrounded by six oxygen atoms of the framework and at a greater distance from the channel wall. When the authors carried out experiments to contact Cu2+, which had been introduced by solid-state reaction, with adsorbates such as oxygen, they

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Fig. 29. Linear relationship between the maximum ESR intensity of Cu2+ in Cu,H-ZSM-5 (obtained by solid-state ion exchange in a CuO/H-ZSM-5 mixture) and the Al content of the ZSM-5 framework (after [21], with permission)

observed significant modifications of the ESR signals (cf. spectrum c in Fig. 28). The hyperfine splitting (HFS) completely disappeared. This showed that the introduced Cu2+ cations were (i) coordinatively unsaturated and (ii) accessible to the adsorbate molecules. Upon desorption of the adsorbate, the original spectrum was restored, i.e., the adsorption process was entirely reversible. When H-ZSM-5 samples with increasing nSi/nAl ratios were used, a linear relationship between the integrated ESR signal intensity and the Al content of the zeolite matrix was found (Fig. 29). Since an increasing Al content corresponded to an increasing number of Brønsted acid sites (cf. Introduction), this finding proved the important role of those acid centers in the solid-state ion exchange taking place in the system CuO/H-ZSM-5. Interestingly, upon interactions of CuO with hydrogen forms of ZSM-5, unsaturated isolated Cu2+ cations were observed to a much lesser extent when H,Na-ZSM-5 was employed instead of H-ZSM-5. With ZSM-5, which contained significant amounts of Na+ cations, Cu2+ was incorporated in octahedral coordination. No ESR signal at all was obtained after solid-state interaction of CuO with sodium forms of the zeolites, i.e., Na-ZSM-5 or Na-MOR. Slinkin and co-workers reported results similar to those found with CuO for solid-state reactions of CuCl2 , CuF2 , Cu3[(OH)CO3]2 , Cu3(PO4)2 and Cu0 (in the presence of air). However, the reaction proceeded most easily with CuCl2 , whereas it was markedly slower in the other cases and slowest with Cu0 in air. It could be shown by the authors via comparison with ESR measurements of conventionally exchanged Cu-zeolites that Cu2+ was not solely coordinated to oxygen atoms of the framework but also to one negatively charged extra-framework species such as OH–, Cl–, F–, or PO43–. Furthermore, evidence was provided that monovalent Cu+ was incorporated into the zeolite matrix upon reaction of Cu2S with H-ZSM-5. Even when this was carried out in a mildly oxidative atmosphere and CuO formation excluded, the ESR lines typical of isolated Cu2+ on the abovementioned sites were observed.

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Solid-state ion exchange after SSIE at 1073 K in, e.g., CuF2/H-ZSM-5 was checked by XRD: no sign of lattice destruction was detected. Calcination of H-MOR with copper compounds such as CuO, Cu3[(OH)CO3]2 , or with Cu0 (in air) at 823 K yielded ESR spectra (g = 2.325, g^ = 2.055, A = 14.4 mT, A^ = 1.9 mT) very similar to those obtained with H-ZSM-5 [21]. According to Jirka et al. [97], grinding of a mixture of Cu2O and NH4-Y followed by heat-treatment at temperatures up to 770 K caused an exchange of copper cations into the zeolite matrix. The incorporation started around room temperature, but was markedly enhanced on heating between 420 and 620 K. The authors derived from their XPS and XAES results that first the Cu2O particles disaggregated and were oxidized to Cu(OH)2. Subsequently, Cu2+ species migrated into the zeolite channels. The process was facilitated by a pre-exposure of the mixture Cu2O/NH4-Y to water vapor. High-temperature ion exchange between solid, Cu-containing phases and HZSM-5 or NH4-ZSM-5 zeolites was also studied by Karge et al. [98]. CuCl, CuCl2 , Cu2O and CuO were employed and the solid-state reactions monitored by ESR, XRD, TPE and IR. ESR investigations were carried out after oxidation (1.3 kPa O2 , 570 K, 1 h; followed by evacuation). The features of all of the ESR spectra were very similar. The signals were compared with those of conventionally exchanged, equally treated Cu,H-ZSM-5 samples and could be assigned to isolated Cu2+ cations introduced into the zeolite matrix, thus qualitatively confirming that SSIE has occurred. The spectra were essentially in agreement with those obtained by Slinkin et al. [21] and discussed above. Parameters for a square-planar coordination determined were: g = 2.33, g^ = 2.07, A = 12.5–14.0 mT (cf. [95], where data for conventionally exchanged Cu-ZSM-5 samples were reported as g = 2.31, g^ = 2.06, A = 15.3 mT, A^ = 2.25 mT). Two sets of parameters were probably superimposed corresponding to the square-planar and fivefold pyramidal coordinations as reported [21, 95] (vide supra). However, the resolution was not as good as achieved by Slinkin and co-workers: In particular, the splitting at g^ was poorly defined. The intensities of the ESR signals after solidstate reaction with the Cu oxides were, in fact, distinctly lower (by about 50%) compared to those obtained from the heat-treated mixtures of H-ZSM-5 with Cu chlorides. This agrees with the IR results (vide infra). An XRD experiment on solid-state reaction of CuCl2 and CuCl with H-ZSM5 is illustrated by Fig. 30. It shows the XRD patterns of CuCl2/H-ZSM-5 and CuCl/H-ZSM-5 before and after solid-state reaction at 775 K [98]. No sign of damage of the crystal lattice was observed. However, the fact of solid-state ion exchange was clearly indicated by the disappearance of the reflections of the salt components, i.e., of CuCl2 ◊ 2H2O and CuCl. Thus, the XRD experiment confirmed the results of the investigations by ESR (see above), TPE and IR (see below). Similarly, no deterioration of the crystallinity was detected by XRD when, instead of H-ZSM-5, samples of NH4-Y (H-Y) were employed in SSIE with copper compounds (vide infra). A careful and extensive structural analysis of Cu-Y obtained via SSIE (Treact = 698 K, treact = 18 h) between CuCl2 ◊ 2H2O and highly exchanged NH4-Y was conducted by Haniffa and Seff, using pulsed-neutron diffraction [99]. The samples prepared in this manner had the composition Cu24Na5H17Cl15Al55Si137O384 exclu-

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Fig. 30. XRD patterns of CuCl2 · 2H2O/H-ZSM-5 and CuCl/H-ZSM-5 mixtures before and after solid-state reaction at 775 K (after [98], with permission)

sive of water (designated as Cu24-Y and, after interaction with D2O, Cu24-Y◊ D2O). The authors assumed that most or all of the protons stemmed from the decomposition of NH+4 and were retained as H3O+ together with Cl–. Significant salt imbibition was detected (cf. Sects. 5.1.5, 5.2.3 and [53]). From the analysis of the XRD data, the population of sites III¢, I¢ and I by Cu2+ was derived. The Cu2+ cations on site III¢ were found to coordinate to four framework oxygen atoms in a (distorted) square-planar manner and, perhaps, also to one extra-framework species such as H2O, OH– or Cl– to give a distorted square pyramid. This is essentially in agreement with the ESR and UV-Vis results reported by Slinkin et al. [93, 95, 96] and Weckhuysen et al. [100], respectively. Temperature-programmed heating of both CuCl/H-ZSM-5 and CuCl2/HZSM-5 mixtures revealed that above 570 K a substantial amount of HCl was evolved. The solid-state reaction in CuCl/H-ZSM-5 mixtures was strongly controlled by the temperature. It was rather fast in the initial stage but then (T>650 K) proceeded very slowly (cf., e.g., Figs. 31 and 32, Table 8). The TPE profile for the system CuCl2/H-ZSM-5 looked almost identical to that of CuCl/ H-ZSM-5 [98]. Similar features are seen in Figs. 31 and 32: an increasing rate of solid-state reaction between ca. 550 and 650 K followed by slow subsequent conversion. From Fig. 32 it can be seen that the rate (measured through the slope of the ascending part of the curves) and degree of solid-state ion exchange are enhanced by an increase in the content of the Cu-containing compound in the mixture in agreement with the finding by Clearfield et al. [18] (vide supra). + /n When an excess of CuCl was applied (nCu OH = 2.25), 86% of the bridging OH groups were consumed, the protons being replaced by Cu+. Hartmann and Boddenberg [101, 102] used SSIE of CuCl and NH4-Y(70) with an exchange degree of 70% to prepare a Cu-Y sample for a study of CO and Xe adsorption monitored by 13C and 129Xe MAS NMR, respectively. Introduction of

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Table 8. Chemical composition of the original mixtures of H-ZSM-5 with CuCl, CuCl2 , Cu2O, CuO, and their characteristics after heat-treatment in high vacuum (SSIE) Composition of the mixture a,b Salt/oxide Cu m · nCum+/nOH (mmol g–1)

CuCl

CuCl2 Cu2O CuO a b c d e f

0.29 0.92 0.92 0.92 0.46 0.46 0.46

0.32 1.00 1.00 1.00 1.00 1.00 1.00

Heat-treatment c ESR signals d IR results Intensity T (K) t (h) (arb. units) OH con- Absorbance f sumed e (arb. units) (%) PyH+ PyL 770 670 770 770 770 770 770

12 12 0.5 12 12 12 12

– 70 – 120 135 66 50

30 38 47 53 43 17 25

– – – 0.26 – 0.37 0.39

– – – 1.04 – 0.47 0.46

Cu content per gram dry zeolite. Number of Cu cations related to the original number of the zeolite OH groups (0.91 mmol per gram dry zeolite); m, cation valency. Under vacuum at 10–4 Pa. After heat-treatment the mixture was oxidized in 103 mbar oxygen at 570 K for 1 h and evacuated at 420 K for 10 min. Calculated from the intensity of the IR band at 3610 cm–1. Pyridine (6.3 · 102 Pa vapor pressure) adsorption at 470 K for 2 h, desorption at 470 h for 10 min. PyH+ from the intensity of the band at 1545 cm–1 and PyL from that at 1450 cm–1; PyH+ and PyL intensities for the parent H-ZSM-5 were 0.51 and 0.31 (arb. units), respectively.

Cu+ via SSIE proved to be a reliable method. Using SSIE an exchange degree of 70% was achieved (Cu-Y(70; SSIE)). The crystallinity was preserved upon SSIE, as confirmed by XRD, 29Si and 27Al MAS NMR. The properties of a sample prepared by introduction of Cu+ via SSIE, i.e., Cu-Y(70; SSIE), were compared with those of samples obtained by conventional exchange with Cu2+ in aqueous Cu(NO3)2 solution after dehydration, oxidation and subsequent reduction, i.e., Cu-Y(75; CE). Cu-Y(70; SSIE) exhibited by far the highest concentration of Cu+ (30 Cu+/u.c.). About 70% of the Cu+ cations (27 of 29/u.c.) were shown to reside in the supercages, where they quantitatively replaced the Na+ ions. Cu-Y(70; SSIE) also exhibited the highest adsorption capacities for CO (strongly bound to Cu+) and Xe compared with that of a mildly reduced Cu-Y(75; CE) which contained ca. 11Cu+/u.c. More severely reduced Cu-Y(75; CE) contained no Cu+ at all but significant amounts of metallic Cu0. Furthermore, it was shown by 13C and 129Xe MAS NMR that xenon was a more advantageous probe for Cu+ in zeolites than the widely used CO since Xe, in contrast to CO, does not (through adsorption) change the distribution of the cations. Solid-state ion exchange was confirmed by TPE in the systems Cu(I) oxide/ H-ZSM-5 as well [98]. The evolution of H2O was monitored by MS and profiles such as those displayed in Fig. 33 were observed. Interestingly, solid-state reactions with oxides extended to markedly higher temperatures compared to reactions with chlorides. This is in agreement with IR investigations, where up to a temperature of 670 K, which is far above the onset of SSIE with Cu chlorides, no

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Fig. 31. Temperature-programmed (10 K min–1) evolution of HCl, m/e = 36, from a CuCl2/ H-ZSM-5 mixture (2nCu2+/nOH = 1.0) pretreated at 390 K

Fig. 32. Temperature-programmed (10 K min–1) evolution of HCl, m/e = 36, from CuCl/ H-ZSM-5 mixtures as a function of the amount of admixed CuCl with nCu+/nOH = 0.32, 1.00, 1.60, and 2.25 (after [98], with permission)

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Fig. 33. Temperature-programmed (10 K min–1) evolution of H2O, m/e = 18, from Cu2O/ H-ZSM-5 mixtures (nCu+/nOH = 1.0) pretreated at 390 K for 1 h (after [98], with permission)

measurable decrease in the number of OH groups was observed. The profile in Fig. 33 exhibits (i) a maximum at 475 K (removal of strongly held H2O), (ii) a step at 530 K and a shoulder above 550 K (H2O removal overlapping with a low-temperature reaction), and (iii) a feature around 775 K (due to a high-temperature reaction), analogously to what was observed in the systems MCl/hydrogen zeolites or MCl2/hydrogen zeolites (cf. Sects. 5.1.2 and 5.1.4). The introduction of Cun+ into H-ZSM-5 or H-Y was also monitored by the consumption of the acid OH groups indicated by the decreasing intensity of the IR bands at 3610 cm–1 (H-ZSM-5) or 3640 (HF) and 3555 cm–1 (LF) (cf. Fig. 34 and Table 8). Also, to some extent, silanol groups (3745 cm–1) were affected. (For the effect of pyridine adsorption after SSIE see below.) It could be excluded that the changes in the content of bridging hydroxyls was caused by dehydroxylation, since no evolution of H2O was detected by MS during heat-treatment. The quantitative data of Table 8 confirm the above statements derived from TPE. For instance, an increase in the reaction temperature from 670 to 770 K enhanced the degree of exchange (as measured by the consumption of OH groups) from 38 to 53%, respectively (Table 8, rows 2 and 4). Similarly, an extension of the reaction time from 0.5 to 12 h brought about an increase of the degree of exchange from 47 to 53%, respectively (Table 8, rows 3 and 4). Further prolonged heating, however, did not result in further consumption of OH groups. Finally, an elevation of the CuCl content in the mixture from nCu+/nOH = 0.32 to nCu+/nOH = 1.0 raised the degree of exchange from 30 to 53% (cf. Table 8, rows 1 and 4). Analogous effects of the exchange temperature and time on the solid-state reaction between CuCl, CuCl2 , Cu2O and H-Y can be recognized from Table 9. It is worthy to note that after SSIE in the system CuCl2/HY the consumption of OH groups in the large and small cavities was found to be similar, viz., 67%–75% and 69%–75%, respectively.When, however, the heating was stopped at 620 K, both TPE and IR spectroscopy evidenced that only the

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Fig. 34. IR spectra of OH groups and adsorbed pyridine: a parent H-ZSM-5 zeolite heated in vacuum at 770 K for 12 h; b and c CuCl/H-ZSM-5 (nCu +/nOH = 1.0) heated in high vacuum at 670 and 770 K, respectively, for 12 h; d after adsorption of pyridine at 470 K subsequent to (c) (after [98], with permission) Table 9. Chemical composition of the original mixtures of NH4-Y with CuCl, CuCl2 , Cu2O, and their characteristics after heat-treatment in high vacuum (SSIE)

Composition of the mixture a,b

Heat-treatment c

IR results

Salt/oxide Cu m · nCum+/nOH (mmol g–1)

T (K)

OH consumed d (%)

Absorbance e (arb. units)

HF

LF

PyH+

PyL

60 62 75 23

61 64 75 15

– 0.46 – 0.81

– 2.12 – 0.54

CuCl CuCl2 Cu2O a b c d e

0.92 0.92 0.46 0.92

1.00 1.00 1.00 1.00

670 670 670 670

t (h)

2 12 12 12

Cu content per gram dry zeolite. Number of Cu cations related to the number of zeolite bridging OH groups; m, cation valency. Under vacuum at 10–4 Pa. Calculated from the intensity of the IR bands at 3640 cm–1 (HFband) and 3555 cm–1 (LFband). Pyridine (6.3 · 102 Pa vapor pressure) adsorption at 470 K for 2 h and desorption at 470 K for 1 h. PyH+ from the intensity of the band at 1540 cm–1 and PyL from that at 1452 cm–1; PyH+ and PyL (true Lewis sites, [171–172]) intensities for the parent H-Y after similar treatment were 0.81 and 0.45 (arb. units), respectively.

number of OH groups located in the large cavities (supercages) had decreased while those in the small cages were almost unaffected (stage 1). These findings suggest that in the first stage of SSIE the protons located in the supercages, which are more easily accessible, predominantly interact with the salt. Only at higher temperatures are the in-going species (possibly CuCl2 molecules, cf. Sect. 7) able to penetrate the six-membered rings into the b-cages, and/or a rapid migration

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Fig. 35. Temperature-programmed (10 K min–1) evolution of HCl, m/e = 36, from CuCl2/ NH4-Y (nCu2+/nOH= 0.5) pretreated at 390 K for 1 h (after [98], with permission)

of Cu cations from the large supercages into the small b-cages becomes possible (stage 2). This seemed to be reflected in the appearance of two maxima in the TPE profile of the system CuCl2/NH4-Y (cf. Fig. 35). The LT-peak would then correspond to stage 1, whereas the HT-peak would be due to stage 2. From a comparison of Tables 8 and 9 it can also be seen that the exchange of protons of bridging OH groups for Cun+ was, for a given temperature, considerably higher in the case of mixtures of Cu chlorides or oxides with H-Y than with H-ZSM-5. Thus, SSIE seemed to be influenced by the structure type of the zeolite employed. IR investigations of the solid-state reaction of Cu compounds with H-ZSM-5 or H-Y were also conducted using IR spectroscopy and pyridine as a probe (cf. also Sect. 8, where this method was employed for investigations of the kinetics of SSIE). For instance, Figs. 34 and 36 clearly show that introduction of Cun+ is indicated by a band at 1452–1453 cm–1 due to pyridine coordinated to the cation acting as a Lewis acid site (Py Æ L). Correspondingly, the extent of SSIE may be derived from a comparison of the spectra of the parent material and the exchanged zeolite after pyridine adsorption: the intensity decrease of the band at 1540 cm–1, which originated from pyridinium ions (PyH+), is a measure of the degree of exchange. Some quantitative results are included in Tables 8 and 9. From a comparison of the IR data presented in Tables 8 and 9 (consumption of OH groups, formation of Py Æ L, decrease in the intensity of the PyH+ band), it can be concluded that the degree of exchange with H-ZSM-5 decreased, under otherwise identical conditions, in the sequence CuCl>CuCl2 >CuO: for example, for T = 770 K, t = 12 h, m · nCum+/nOH = 1.0, the percentage of OH groups consumed follows the sequence 53 (CuCl), 43 (CuCl2), 25 (CuO). In contrast, with H-Y, the degree of exchange measured via consumption of the OH groups seemed to be slightly higher when the zeolite was reacted with CuCl2 instead of CuCl: 75%

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Fig. 36. IR spectra of OH groups and adsorbed pyridine: a parent NH4-Y zeolite heated in vacuum at 670 K for 12 h; b CuCl2/NH4-Y (nCu2+/nOH = 0.5) heated in high vacuum at 670 K for 12 h; c after pyridine adsorption subsequent to (b) (after [98], with permission)

(CuCl2)>62%–64% (CuCl). Most likely, this was due to the fact that in H-Y there is a considerably lower nSi/nAl ratio than in H-ZSM-5, viz., 2.6 vs. 13.5, which facilitated the incorporation of bivalent cations in H-Y. The PyH+ and Py Æ L results indicated for both zeolite systems a higher degree of exchange in the case of chlorides than oxides. However, after reaction with H-ZSM-5, similar degrees of exchange were observed for both Cu oxides. Another spectroscopic investigation was carried out by Borovkov et al. [103], who investigated by diffuse reflectance IR spectroscopy the vibration modes of CO adsorbed on Cu(I)-Y prepared via SSIE of NH4-Y and CuCl. The various fundamental and 1st overtone bands were compared with those observed with conventionally ion-exchanged Cu-Y, Cu-MOR and Cu-L, which were subsequently auto-reduced at 673 K in vacuum. Esemann and Förster [104] employed far-infrared and X-ray absorption spectroscopy, assisted by computer modeling, to study copper exchange into ZSM-5 by solid-state reaction, the siting of the introduced cations, NO adsorption and decomposition on and redox behavior of the obtained Cu-ZSM-5. Förster and Hatje [105] applied EXAFS techniques to study the incorporation of Cu+ as well as of Zn2+ and Ni2+ (vide infra) into Y-type zeolite. The oxidation state of introduced Cu+ remained unchanged upon SSIE: The determined Cu+oxygen distances were too small as to suggest a coordination to the oxygen

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atoms of the six-membered rings. Therefore, it was tentatively assumed that asymmetric cation positions occur or, more likely, Cu-O-Cu complexes were formed. Introduction of Cu into H-ZSM-5 through solid-state and conventional ion exchange was carried out, and the resulting materials were compared in a study by Auroux et al. [106] using microcalorimetry and XPS. A combination of microcalorimetric determination of the acidity by adsorption of NH3 and XPS measurements of the N1s lines of the adsorbed ammonia allowed these authors to discriminate between the Brønsted (B) and Lewis (L) acidity. The experiments revealed an extreme increase of the density of L-sites as a consequence of Cu incorporation. Besides Y- and MFI-type zeolites, other zeolites such as BETA [107, 108] and clinoptilolite [109] were also loaded with Cu via SSIE. H-BETA was reacted with increasing amounts of CuCl, finally resulting in a complete disappearance of the acid OH groups as evidenced by IR. It was shown that two kinds of Cu(I) sites exist. The equilibria for CO adsorption on these sites were involved in the carbonylation of alcohols which, however, required the simultaneous presence of residual adjacent acid OH groups. Copper could be introduced by SSIE into natural clinoptilolite to an extent which was comparable to that of conventional exchange [109]. The zeolite obtained by solid-state reaction was successfully employed in the catalytic 3-methyl-3-butyn-2-ol decomposition. Cu-ZSM-5 catalysts for the aromatization of light paraffins and conversion of 1-propylamine were prepared via SSIE by Kanazirev and Price [110, 111] (cf. also Sects. 6.3.3.2 and 6.3.3.3). King [112, 113] found that Cu(I)-Y prepared by SSIE was, in contrast to conventionally produced Cu(II)-Y, an active catalyst for oxidative carbonylation of CH3OH to dimethyl carbonate. The presence of NH3 facilitated the migration of copper ions into the zeolite structure. As mentioned in Sect. 5.3.1, Cu-containing zeolites prepared by SSIE have been employed as DENOX catalysts (cf. [114–128]). However, the various studies did not lead to an entirely consistent picture of the catalytic behavior of the resulting materials. Most probably this is due to important but not well-documented differences in preparation and application. For example, Varga et al. [114] compared, in the context of the catalytic removal of NOx from exhausts, the behavior of Cu-ZSM-5 produced by conventional exchange, Cu-ZSM-5(CE), with that of Cu-ZSM-5(SSIE) as prepared via solid-state ion exchange. The latter procedure was carried out with CuCl2/HZSM-5 mixtures which were calcined at 873 K in air for 8 h. The modified zeolite samples were characterized by TGA, XRD, IR and BET measurements, NO adsorption and decomposition. XRD, IR (KBr technique) and BET measurements proved that the crystallinity of the samples was maintained during SSIE. In contrast to Cu-ZSM-5(CE), the samples obtained via SSIE did not exhibit bands due to Brønsted acid sites in the OH stretching region or, after pyridine adsorption, in the pyridine ring deformation region (no pyridinium ions were formed). On adsorption of NO, formation of NO2 was observed with both types of samples, even though it was more pronounced with Cu-ZSM-5(CE). The differences in the behavior (adsorption and reaction of NO) of the samples were

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ascribed to differences in concentration and positions of the introduced copper cations. Similar experiments were carried out with FeCl2, CoCl2, and NiCl2 (vide infra, Sect. 5.3.4). In subsequent work, Varga et al. found that Cu-ZSM-5 produced via SSIE and characterized by ESR [115–116] or IR, BET, derivatographic and acidity measurements [117] exhibited NO adsorption and catalytic behavior different from that of conventionally exchanged Cu-ZSM-5. In particular, the samples obtained by SSIE were reported to be less active in NO decomposition [118]. In contrast, Schay et al. [119] found no difference in catalysis as long as catalysts with the same Cu-loadings were employed.When, however, Cu,H-ZSM5 was prepared with an excess of CuCl (CnCu/nOH = 1.5) and reduced by CO at 773 K, the resulting material was highly active in NO decomposition even at temperatures below 573 K [120]. Similarly, Varga et al. [121] described the preparation of good catalysts for NO decomposition via SSIE of CuCl2 with H-ZSM-5. Selective catalytic reduction of NO by propane in the presence of oxygen over Cu,H-ZSM-5 prepared via SSIE was studied by Halasz and Brenner [122]. Despite its relatively low degree of exchange (63%), the catalyst was found to be rather active and produce more N2 than analogously prepared Ag,H-ZSM-5 and Li,H-ZSM-5 samples. However, the selectivity for N2 was reported to be lower over Cu,H-ZSM-5 since NO2 was also formed. Similarly, Halasz et al. [123] showed that such Cu-ZSM-5 catalysts exhibited a high activity in the selective reduction of NO by ammonia, propane or propene. Another comparative study was conducted by Setzer et al. [124]. These authors investigated Cu-ZSM-5 samples, produced via (i) conventional exchange, (ii) SSIE and (iii) direct crystallization from Cu cations-containing synthesis gels, with respect to their catalytic activity in NO decomposition in diesel exhaust gases. The lower activity of materials produced through method (ii) or (iii) was ascribed to the fact that they contain Cu predominantly in a distorted octahedral coordination, whereas Cu introduced according to (i) exhibits a square-planar or quadratic pyramidal (unsaturated) coordination (see, however, [18, 23, 95, 96]). Poeppel et al. [125] attempted to react siliceous MCM-41 by SSIE with Cu(II) salts. However, investigation with ESEM and comparison with MCM-41 exchanged in aqueous salt solutions proved that Cu2+ ions introduced by SSIE did not show any interaction with the MCM-41 framework. The authors concluded that in the samples modified by SSIE the Cu cations occupy sites different from those in the conventionally prepared Cu-MCM-41 materials. Catalysts for selective catalytic reduction (SCR) with propene were prepared by Liese and Grünert [126] through solid-state reactions of CuCl, Cu(NO3)2 or Cu(CH3COO)2 with Na-ZSM-5 or Na-ZSM-5/H-ZSM-5 mixtures and characterized by XPS and X-ray-induced AES (surface) and IR with pyridine as a probe (bulk). Introduction of copper was most intense for CuCl, but intrazeolite copper was also stabilized through SSIE in the chlorine-free systems. For comparison, Cu-ZSM-5 conventionally exchanged in aqueous solutions was used. It turned out that the reaction rates over both types of catalysts, when normalized to the same degree of exchange, were in the same order of magnitude. Even though incorporation of copper via SSIE predominantly occurred into H-ZSM5, when it was present in a mixture with Na-ZSM-5, the Brønsted acidity proved to be irrelevant in the SCR under study.

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Price et al. [127] prepared Cu-MFI catalysts for NO decomposition by hightemperature treatment of a mixture of CuO and H-MFI in the absence of oxygen. The process included most probably auto-reduction, and the copper cations were introduced as Cu+. The upper limit was close to 1 Cu/1 framework Al. The results of ESR, TPR and thermal desorption of 1-propylamine were consistent with this assumption. The materials produced were very active in the decomposition of NO into N2 and O2 and behaved similarly as so-called overexchanged Cu2+-MFI. Copper cations were also introduced into zeolites by solid-state reaction of a complex oxide such as CuCrO4 [93]. The chromate was mixed with H-ZSM-5 and heated in air to 823–1073 K. ESR spectroscopy yielded two superimposed spectra from Cu2+ and Cr5+ cations (cf. Sect. 5.3.5). These cations were randomly located in the cation positions of the ZSM-5 structure and showed negligible dipole-dipole interaction. In another experiment, a consecutive solid-state reaction was carried out first with CrO3 and then with CuO which resulted first in the appearance of the Cr5+ signals and then, after reaction of CuO, in a decrease of their intensities. Simultaneously, the signals of Cu2+ cations developed. This indicated that a fraction of the initially incorporated Cr5+ cations were replaced by Cu2+ cations, and the latter were more strongly held than Cr5+. Reduction by hydrogen caused both the Cr5+ and Cu2+ signals to disappear. Copper cations were incorporated by solid-state ion exchange into the gallium analog of ZSM-5, i.e., into the gallosilicate H-[Ga]ZSM-5 [128, 129]. The ESR results reported by Kucherov et al. [128, 129], however, were essentially the same as those obtained with the aluminosilicate, i.e., H-[Al]-ZSM-5. For example, after SSIE with a mixture of CuO/H-[Ga]-ZSM-5 at 823 K, the same hyperfine splitting was observed. Also, two types of isolated Cu2+ cations were identified, one in a square-planar and the other in a five-coordinated environment. Interaction with gases, e.g., O2 , showed that the copper ions were accessible. 5.3.2.2 Introduction of Silver

There is, to our best knowledge, only one report on solid-state ion exchange of a silver compound with zeolites. Ag+ was incorporated into ZSM-5 by solid-state reaction of AgCl with H-ZSM-5 [130]. Figure 37 shows the result of a temperature-programmed heat-treatment of an AgCl/H-ZSM-5 mixture. The evolving gases, H2O (M = 18) and HCl (M = 36), were monitored by a mass spectrometer. A very pronounced HCl peak occurred at 890 K, but no dehydroxylation peak was observed. This demonstrated that a solid-state ion exchange, viz., Ag+ for H+, had taken place. It is worth mentioning, however, that this solid-state reaction proceeded with a salt that is insoluble in water. This, in turn, demonstrated that the presence of H2O, which might have been physically adsorbed on the zeolite material, is not a prerequisite for SSIE to occur (cf. Sect. 7.1). Solid-state ion exchange between AgCl and H-ZSM-5 was also proven by IR spectroscopy. Figure 38 demonstrates the significant decrease in the absorbance of the OH band at 3610 cm–1 which occurred when the parent zeolite, H-ZSM-5, was reacted with AgCl at 675 K. The difference in the absorbances corresponded to a degree of exchange of about 70%.

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Fig. 37. Solid-state ion exchange in the system AgCl/H-ZSM-5 monitored by mass spectrometric analysis of the gases evolved from an AgCl/H-ZSM-5 mixture (nAg+/nAl = 1.0) as a function of the temperature; heating rate: 10 K min–1 (after [130], with permission)

Fig. 38. Solid-state ion exchange in the system AgCl/H-ZSM-5 shown by the decrease in the intensity of the OH bands of the parent zeolite (after [130], with permission)

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5.3.2.3 Introduction of Gold

Na-Y zeolites containing gold species were prepared by SSIE by subjecting a physical mixture of Na-Y and AuCl3 to a thermal treatment under vacuum. The product was investigated by XAFS spectroscopy. The XANES spectra provided evidence for the presence of Au0, AuCl and Au2Cl6 (cf. [131] and Vol. 4, Chap. 5, this Series, see [69] therein). 5.3.3 SSIE of Zinc, Cadmium and Mercury Compounds with Zeolites 5.3.3.1 Introduction of Zinc

Introduction of zinc by solid-state ion exchange into ZSM-5 was investigated by Salzer [132], Salzer et al. [133] and Roessner et al. [134] using diffuse reflectance and transmission IR spectroscopy. A mixture of ZnO and H-ZSM-5 was subjected to heat-treatment at 823 K well below the sublimation point of ZnO (2223 K). It was shown that (i) the solid-state ion exchange was completed after 1 h, (ii) ZnO seemed to be the migrating species, and (iii) all types of OH groups present in the zeolite reacted simultaneously rather than consecutively [133]. Zinc-modified ZSM-5 materials were also produced by Hagen and Roessner [135], who thermally treated mechanical mixtures of ZnO and H-ZSM-5 (with and without template) at about 720 K under evolution of H2O, characterized the products containing 2 wt.% Zn by IR using pyridine as a probe, TPD of ammonia to evidence the absence of Brønsted acid sites and X-ray absorption spectroscopy measuring the Zn K edge in transmission of excited synchrotron radiation. The latter technique (XANES analysis) allowed the authors to distinguish between Zn2+ in ZnO and Zn2+ in cationic positions in the zeolite structure.After 5 h reaction time about 0.06 mmol Zn per gram zeolite was located in cationic sites. Zn-Y zeolites were prepared via SSIE by Seidel et al. [136] and Boddenberg and Seidel [137] and subjected to an investigation by quantitative 129Xe NMR spectroscopy. Completely Zn-exchanged Y-zeolite could be obtained by solidstate reaction between ZnCl2 and NH4-Y at 693 K in high vacuum. Zn-Y zeolites obtained via solid-state ion exchange exhibited a more homogeneous cation distribution than samples conventionally prepared from aqueous zinc salt solutions. Analysis of CO and Xe adsorption data in combination with 129Xe NMR results, using a multisite adsorption model [138], yielded the population of sites II and III by Zn2+ in the supercages. It was shown that up to an exchange degree, d, of 50% only sites II were populated, whereas at higher Zn-contents up to d = 100%, sites III were increasingly occupied (cf. Fig. 39). ‘Overexchange’ (d>100%) led to significant salt imbibition (cf. Sects. 5.1.5 and 5.2.3). An excess of ZnCl2 in the mixture with the parent zeolite NH4-Y was occluded in the supercages and, thus, decreased the accessibility of the charge-compensating Zn cations for CO and Xe. A maximum concentration of zinc cations was reached 2+/n = 0.5 [137]. for a ratio of nZn Al

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Fig. 39. Concentrations of Zn2+ cations at supercage positions in samples prepared by solidstate ion exchange in ZnCl2/NH4-Y mixtures by heating the mixtures at a rate of 20 K min–1 to 393 K and maintaining there for 5 h and subsequently heating to 693 K for 24 h in high vacuum (final pressure: p £ 10–3 Pa); a = 2 · nZn2+/nAl · 100, at SIII sites: ●; at SII sites: ▲; at SIII +SII sites: ■ (after [138], with permission)

EXAFS techniques were employed by Förster and Hatje [105] for the determination of the cation coordination in Zn-Y obtained via SSIE; in fact, in a similar way and with similar conclusions as described for Cu-Y (cf. Sect. 5.3.2.1) and Ni-Y (cf. Sect. 5.3.4.3). The authors assumed that upon SSIE Zn2+–O–Zn2+ complexes were likely to form. The distribution of the Zn2+ cations over the various sites of Y-type zeolites modified through solid-state reaction of ZnCl2 with H-Y and NH4-Y was also investigated by far-infrared spectroscopy [81, 104]. In a study by Onyestyák et al. [139], it was also reported that zinc cations were incorporated via SSIE into zeolite Y through reaction of ZnCl2 and NH4-Y. In a recent contribution, Beyer et al. [140] carried out solid-state exchange between metallic zinc (zinc dust) and hydrogen forms of faujasite-type zeolite or mordenite. Ground mixtures of Zn0 (zinc dust) and the respective zeolites were dehydrated at 523 K and then heated to 873 K in vacuum or, preferentially, in a flow of nitrogen. XRD patterns and SEM micrographs taken from the mixtures before and after heat-treatment showed that the reflections of metallic zinc and the zinc particles had disappeared upon heating of the mixture. Moreover, comparison with the XRD patterns of Zn-Y samples obtained by conventional ion exchange revealed changes in the reflections of the zeolite lattice which were obviously due to an incorporation of Zn2+ cations into the zeolite structures. The originally greyish color of the mixture changed to white. Quantitative evidence for a solid-state exchange according to Eq. (11) was provided by measurements of the released hydrogen in temperature-programmed evolution of hydrogen (TPEH): Zn0 + 2 H+Z– Æ Zn2+Z 2– + H2≠

(11)

An analogous reaction had been reported earlier by Jacobs et al. [141] and by Sárkány and Sachtler [142] for the oxidation of small aggregates of Ag0 and Cu0

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in the interior of zeolite structures, respectively. Interestingly, and at some variance with the findings of Salzer [132, 133] and Roessner et al. [134], Beyer et al. [140] reported that they did not observe SSIE between ZnO and acid zeolites when they conducted pertinent control experiments. The hydrogen evolved according to Eq. (11) was measured during heat-treatment of the Zn0/NH4 ,Na-Y mixtures by a heat conductivity detector; alternatively, it may be determined by mass spectrometry. IR spectroscopy with and without pyridine as a probe demonstrated that, on heating the Zn0/NH4 ,Na-Y mixture, the intensities of the OH bands originating from acidic OH groups and those of the corresponding pyridinium ion bands decreased, while simultaneously bands due to pyridine attached to Zn2+ cations (at 1445 cm–1) developed. Similar results were obtained with fine powders of cadmium, indium and gallium. Beyer et al. pointed out the potential of the described reactions for developing a new method to determine qualitatively and quantitatively acidic OH groups in zeolites or related materials. Such a technique would use the amounts of hydrogen released on temperature-programmed heating of the mixtures of fine metal powders and acid solids as a measure of the density of Brønsted acid sites. Moreover, it should provide a tool to distinguish Brønsted acid sites with respect to the acidity strength according to the characteristic temperature peaks seen during TPEH (cf. also [136] and Sect. 6.3). Roessner et al. [134] found that Zn,H-ZSM-5 or Zn-ZSM-5 obtained via SSIE possessed catalytic activities in n-hexane isomerization similar to those exhibited by Zn,H-ZSM-5 catalysts that had been prepared by conventional methods, i.e., either through ion exchange in aqueous solutions of Zn(NO3)2 or by the incipient-wetness technique. The latter method seemed to be in between exchange in aqueous solution and a solid-state reaction (cf. Sect. 6.1). Also, ZnZSM-5 produced via SSIE proved to be almost as equally active in ethane aromatization as conventionally modified Zn-ZSM-5 prepared by exchange in aqueous Zn(NO3)2 solution [135]. Finally, as was shown by Rojasova et al. [143], incorporation of zinc into NH4-Y by solid-state reaction with, e.g., ZnO yielded catalysts active in n-hexane aromatization. Under equal conditions, ZnO alone did not catalyze this reaction. Microwave irradiation during solid-state reactions of ZnCl2 with Y-type zeolites (H-Y or Na-Y) was employed in a series of studies by Yin et al. [144–146] aimed at producing and testing Zn-containing Y zeolite catalysts for special organic reactions. In the case of Na-Y, XRD and IR showed that ZnCl2 was completely dispersed onto the surface of the zeolite. Part of the dispersed salt (up to 25% ZnCl2 loading) underwent SSIE with the Na+ cations of Na-Y, the rest of them coordinated with oxygen atoms of the Y-type framework. The catalysts were used for the Diels-Alder reaction between myrcene and acrolein. At loadings above 25% ZnCl2 and up to 37.5%, when finely dispersed ZnCl2 particles were left after SSIE, a significantly improved performance of the catalysts was observed, suggesting that the activity and selectivity of those particles were even higher than that of Zn-Y generated by SSIE. In the case of the system ZnCl2/ H-Y, the products of SSIE were used to catalyze the a-pinene conversion. The selectivity for carvenol increased with increasing ZnCl2 loading, i.e., with decreasing Brønsted and increasing Lewis acidity.

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Fig. 40. IR spectra of pure NH4-Y and after solid-state reaction of mixtures of cadmium compounds with NH4-Y at 793 K and 10–2 Pa; a prior to and b, c after adsorption of H2S under 6.6 kPa at 298 K for 10 min and subsequent 10 min evacuation at 298 K (after [139], with permission)

5.3.3.2 Introduction of Cadmium

In similar experiments as with zinc, Onyestyák et al. [139] reacted NH4-X, NH4Y and NH4-MOR with various cadmium compounds. The reactions were monitored by IR spectroscopy. Again, very high degrees of exchange were achieved with the chlorides of cadmium. In the case of Cd compounds, the degree of exchange was shown to decrease in the sequence Cd(NO3)2 >CdCl2 >CdO> CdS>CdSO4 (cf. Fig. 40). The Cd-zeolites obtained in this manner were compared with conventionally prepared ones with respect to dissociative adsorption of H2S, hydrosulfurization of olefins and decomposition of ethylthiol (EtSH) or ethylthioether (Et2S). It turned out that Cd-Y samples formed by SSIE were more active in hydrosulfurization, dissociative adsorption of H2S and decomposition of EtSH and Et2S than conventionally prepared Cd-zeolite catalysts. 5.3.3.3 Introduction of Mercury

As an example of solid-state ion exchange with mercury compounds, the result of a reaction between Hg2Cl2 and H-ZSM-5 is reproduced in Fig. 41 [130]. The

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Fig. 41. Solid-state ion exchange in the system Hg2Cl2/H-ZSM-5 shown by the decrease in the intensity of the OH bands of the parent zeolite (after [130], with permission)

mixture was prepared with a ratio of nHg /nAl = 1. From the decrease in the absorbance of the OH band at 3610 cm–1 a degree of exchange of 70% was derived. No attempt was made to enhance the degree of exchange by, e.g., increasing the nHg/nAl ratio, the reaction temperature (Treact) and/or the duration of the solid-state reaction (treact). However, the most interesting result is that, similar to the reaction in the system AgCl/H-ZSM-5 (vide supra), the salt component was insoluble in water, but nevertheless the solid-state reaction took place. This supports the earlier statement that the presence of water does not play a decisive role in SSIE (cf. Sect. 7.1). 5.3.4 SSIE of Iron, Cobalt, Nickel and Manganese Compounds with Zeolites 5.3.4.1 Introduction of Iron

As early as in the work by Clearfield et al. [18] in 1973, solid-state reactions of Fe2+, Co2+, Ni2+ and Mn2+ chlorides with ammonium (hydrogen) forms of zeolites A, X and Y were studied to demonstrate the phenomenon of SSIE. Those authors monitored the reactions through titration of HCl evolved. More recently, interest in zeolites containing these transition metals, especially cobalt and

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iron, has been stimulated by possible applications of the modified zeolites in catalysis (vide infra). Therefore, the possibility of preparation of such catalysts via solid-state ion exchange was explored in more detail. In 1987, solid-state interactions of Fe cations with ZSM-5 were studied by Wichterlová et al. [147]. These authors employed mixtures of Fe2O3 and H-ZSM-5 or Na-ZSM-5, which were heat-treated in a stream of oxygen. The results obtained with the zeolites prepared in this way were compared with those from conventionally exchanged Fe-containing ZSM-5 samples. Measurements of Xe adsorption on the parent zeolites and on the materials obtained via solid-state interaction yielded the same adsorption capacities indicating that no structural degradation or plugging of the zeolite channels had occurred during the procedure of post-synthesis modification. No indication of solid-state ion exchange was observed when Wichterlová et al. [147] used the sodium form of ZSM-5: calcination of a Fe2O3/Na-ZSM-5 mixture did not give rise to an ESR signal indicating isolated Fe3+ ions. In contrast, in the ESR spectra of samples produced via calcination of mixtures of Fe2O3 with the hydrogen form of ZSM-5, a signal appeared at g = 4.27 which was ascribed to Fe3+ ions isolated in the zeolite matrix and in tetrahedral coordination. In fact, the intensity of the ESR signal was significantly lower than in the case of conventionally exchanged Fe-containing ZSM-5 samples. However, those Fe3+ ions were not believed to be part of the framework. Rather, from the ESR results and the observed decrease of the concentration of acid OH groups as evidenced by temperature-programmed desorption of ammonia, the authors concluded that solid-state interaction in the system Fe2O3/H-ZSM-5 led to an incorporation of trivalent Fe3+ ions in extra-framework cation sites, but in tetrahedral coordination. This is at variance with the report of Kucherov and Slinkin [148] (vide infra). The Fe3+ cations in Fe,H-ZSM-5, as described by Wichterlová et al. [147] after solid-state interaction in the system Fe2O3/H-ZSM-5, were easily reduced in vacuum. Part of the tetrahedrally (Td) coordinated extra-framework Fe3+ cations introduced via SSIE retained this ligand field symmetry (Td) even after adsorption of H2O. The catalytic activity in toluene disproportionation and methanol conversion was lower than that of the acidic parent zeolite, H-ZSM-5, but comparable to that of Fe-containing ZSM-5 catalysts that had been conventionally prepared from H-ZSM-5 suspended in aqueous solutions of FeCl3 . Incorporation of trivalent Fe3+ into H-ZSM-5 and Fe2+ into NH4-Y was extensively studied by Kucherov and Slinkin [148] and Lazar et al. [149, 150], respectively. The former authors investigated the possibility of Fe3+ introduction into H-ZSM-5 by calcination of mixtures of FeCl3 , FeO or Fe3O4 and the zeolite. The heat-treatment of the mixtures was carried out in air or vacuum at temperatures up to 1073 K for 4 h. ESR spectroscopy was employed to check whether or not SSIE had occurred. Isolated Fe3+ cations were not detected after calcination of Fe oxides with H-ZSM-5. The authors suggested that this was due to the high lattice energy of these compounds and/or the low mobility of the ions in the oxide lattices even at 1073 K. However, Kucherov and Slinkin [148] provided evidence for a migration of Fe3+ cations into the zeolite matrix when they reacted the chloride, FeCl3, with H-ZSM-5 at 793 K. It was shown that the Fe3+ cations introduced in this fashion were isolated (not clustered) and resided in a strong crystal field

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of low symmetry. Even at 1073 K, XRD did not indicate any amorphization of the zeolite structure upon calcination of the mixture FeCl3/H-ZSM-5. Since the ESR signals were strongly affected by admission of gases and vapors such as O2 , NH3 , H2O, pyridine and p-xylene (vide infra), it was obvious that the Fe3+ cations incorporated via SSIE were accessible to adsorbates. Fe,H-ZSM-5 materials obtained via SSIE with FeCl3 were compared with the ferrisilicate analog of H-ZSM-5, i.e., H-[Fe]-ZSM-5 (nSi/nFe ª 50), where Fe3+ was incorporated into the MFI framework by isomorphous substitution during synthesis and, therefore, tetrahedrally coordinated [148, 151]. In fact, both Fe3+-containing materials, viz., Fe,H-ZSM-5 with Fe residing on extra-framework cation positions and H-[Fe]-ZSM-5 with Fe in tetrahedral positions in the framework, gave rise to similar ESR spectra, even though they showed some differences in the g-values. The main ESR signal of Fe,H-ZSM-5 appeared at g1 = 4.27, which is close to g1 = 4.25 as observed for H-[Fe]-ZSM-5 by Kucherov et al. [148, 151], as well as by Wichterlová et al. [147] for Fe,H-ZSM-5. More severely oxidized Fe,H-ZSM-5 samples exhibited additional ESR lines at g2 = 5.65 and g3 = 6.25; the corresponding g-values for the ferrisilicate analog H-[Fe]-ZSM-5 were g2 = 5.2 and g3 = 7.9. However, the Fe,H-ZSM-5 materials behaved completely differently from H-[Fe]-ZSM-5: (i) Upon admission of the above-mentioned adsorbates, the ESR spectrum of the former zeolite (prepared via SSIE) dramatically changed (vide infra), whereas that of H-[Fe]-ZSM-5 remained practically the same. (ii) Fe3+ on cation sites of Fe,H-ZSM-5 could be replaced by solid-state reaction with CuO, i.e., the spectrum of Cu2+ in ZSM-5 developed (cf. Sect. 5.3.2.1). This did not happen when a mixture CuO/H-[Fe]-ZSM-5 was subjected to heat-treatment. (iii) In contrast to H-[Fe]-ZSM-5, the sample prepared via SSIE, i.e., Fe,H-ZSM-5, exhibited an anomalous temperature effect, i.e., upon cooling to 77 K, the intensity of the main signal at g1 = 4.27 was very much enhanced (cf. Fig. 42), which is typical of Fe3+ on extra-framework cation sites. These criteria (i)–(iii) enable us to distinguish between Fe3+ in extra-framework and Fe3+ in (tetrahedrally coordinated) framework sites. This would even hold if Fe3+ occurred in tetrahedrally coordinated extra-framework positions. However, in the opinion of Kucherov and Slinkin [148], the occurrence of the anomalous temperature effect strongly suggests that Fe3+ introduced via SSIE was in fact not located in tetrahedral but rather in distorted octahedral environment, in contrast to the conclusion by Wichterlová et al. [147] (vide supra). The interaction of Fe3+ in Fe,H-ZSM-5 with NH3 and pyridine led to a complete disappearance of the low-field lines at g2 = 5.65 and g3 = 6.25, and interaction with H2O to their considerable decrease. In any event, the intensity at g1 = 4.27 was markedly enhanced. This was especially pronounced with NH3 and pyridine indicating an increase of the crystal field symmetry upon adsorption of these powerful ligands. Interaction with O2 resulted in a considerable but reversible broadening of the Fe3+ ESR lines caused by dipole-dipole interaction of Fe3+ with O2 . With NH3, the samples of Fe,H-ZSM-5 were reduced at higher temperatures (823 K) as indicated by the disappearance of the signals of Fe3+ and formation of Fe0 clusters. Reoxidation did not fully restore the original spectrum. Interaction with p-xylene yielded an ESR spectrum characteristic of pxylene cation radicals.

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Fig. 42. Changes in the Fe3+ ESR signal upon calcination of the mixture FeCl3/H-ZSM-5 (2.3 wt.% FeCl3) a at 293 K; b after heat treatment in vacuum at 593–793 K for 2 h; c sample (b) measured at 77 K (after [148], with permission)

From the spin density, Kucherov and Slinkin [148] derived an ion concentration of 1 Fe3+ per 30–80 Al. They assumed that it is difficult to compensate by one Fe3+ three rather separated negative charges of the framework and hypothesized that not a naked Fe3+ but one complex cation such as FeCl+2 neutralizes one negative charge. Upon oxidative calcination a gradual substitution of the anionic ligand gives rise to a transformation FeCl+2 Æ FeO+ accompanied by the appearance of signals with g2 = 5.65 and g3 = 6.25. As the signal at g1 = 4.27 was assigned to isolated Fe3+ cations in tetrahedral or orthorhombic coordination stabilized in a crystal field of low symmetry, the development of signals with g > 4.3 (cf. the above g2 and g3 signals) was assumed to be due to further lowering of the symmetry of the environment of the ions. Kucherov and Slinkin also showed that in the solid-state reaction between CuO and Fe,H-ZSM-5 (vide supra) at least 99% of the Fe3+ ions were replaced by Cu2+ cations. The ESR signals with g2 = 5.65 and g3 = 6.25 were entirely eliminated, and only a trace of the signal with g1 = 4.27 was left. This shows that essentially all of the Fe was in extra-framework positions (vide supra) and no or only negligible insertion of Fe3+ into the framework had occurred. Thus, SSIE of FeCl3 and H-ZSM-5 did not result in any isomorphous substitution. The conventional ion exchange in aqueous solutions of easily oxidizable cations such as, e.g., bivalent Fe2+, might require the exclusion of oxygen during the whole exchange procedure. This frequently leads to experimental complications that possibly can be avoided by solid-state ion exchange (cf. Cu+, Sect. 5.3.2.1). Therefore, the incorporation of Fe2+ into zeolites via SSIE was

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investigated in more detail. Similar to earlier work on conventionally exchanged, Fe-containing zeolites (cf., e.g., [152, 153]), in the pertinent studies by Lázár et al. [149, 150] on SSIE of Fe2+ into zeolites, Mössbauer spectroscopy was applied as a powerful diagnostic tool. However, XRD and TPE were also employed. In these studies, an NH4-Y zeolite prepared from Na-Y by almost complete (99.9%) exchange in aqueous NH4Cl solutions was used as a starting material. It was ground with FeCl2 ◊ 4H2O (nFe/nAl, tetr. = 0.5) followed by heat-treatment at temperatures between 420 and 720 K. Most of the sample preparation steps were undertaken without exclusion of ambient atmosphere; subsequent Mössbauer and TPE experiments, however, were carried out in vacuum. Schematic XRD patterns of the parent NH4-Y zeolite and its mixture with FeCl2 ◊ 4H2O as prepared by grinding at ambient temperature and after heattreatment are presented in Fig. 43. As can be seen in this figure, the incorporation of iron (cf. patterns a and b) led to a change of the absolute intensities of the reflections of the lattice, partly caused by changes of the respective structure factors [54] and partly by the higher absorption factor of Fe. However, the intensities re-increased upon heat-treatment, and no broad peak at 2Q = 20–30° was observed which would have been indicative of amorphous silica. Both these findings proved that the zeolite lattice did not measurably deteriorate upon

Fig. 43. Schematic representation of XRD patterns of a the parent NH4-Y zeolite, a = 24.785 Å; b the mixture FeCl2 ◊ 4H2O/NH4-Y ground in air at ambient temperature, a = 24.742 Å; c the material (b) heated in air up to 720 K (heating rate: 10 K min–1), a = 24.542 Å; *FeCl2 ◊ 4 H2O; **NH4Cl

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SSIE. The most striking features in the XRD patterns were, however, the appearance of the (110) reflections of crystalline NH4Cl at Q = 32.68° and weakening of the reflections of FeCl2 ◊ 4H2O in pattern b and their complete disappearance upon heat-treatment at 720 K (cf. pattern c). These features confirmed that solid-state ion exchange had indeed occurred. The system FeCl2/NH4-Y turned out to be a very interesting example of the application of Mössbauer spectroscopy in the field of solid-state ion exchange in zeolites [149, 150]. The spectra (cf. Fig. 44) were decomposed (vide infra; cf., as an example, Fig. 45), and the oxidation states and coordination of incorporated iron were deduced on the basis of assignments reported earlier [154]. Results are presented in Table 10. When the above mixture was investigated as prepared, the Mössbauer spectrum provided evidence that 57% of the iron was oxidized to the trivalent state (cf. Table 10, RI = 57). From the Mössbauer parameters it was concluded that the Fe(III) species were almost perfectly octahedrally coordinated. The remaining non-oxidized iron occurred as three different species, viz., (i) partially dehydrated Fe(II) chloride (21%); (ii) also partially dehydrated, Table 10. Mössbauer parameters of iron species present in FeCl2/NH4-Y mixtures after grinding and subsequent heat-treatment in high vacuum. IS, isomer shift; QS, quadrupole splitting; RI, relative intensity

Species

Fe(III)oct

Fe(III)trig

Fe(II)tetr

Fe(II)trig

Fe(II)oct-1

Fe(II)oct-2

Fe(II)oct-3

FeCl2 ◊ xH2O

Parameter

IS QS RI IS QS RI IS QS RI IS QS RI IS QS RI IS QS RI IS QS RI IS QS RI

As-prepared

0.37 0.63 57 – – – 0.69 0.35 13 – – – 0.83 1.95 9 – – – – – – 1.13 1.83 21

After treatment in high vacuum at 420 K

520 K

620 K

720 K

0.35 0.72 44 – – – 0.72 0.36 11 – – – 0.79 2.00 22 – – – – – – 1.04 1.79 23

0.31 0.75 38 0.25 1.53 11 – – – 1.08 0.78 21 – – – – – – 1.06 2.60 9 1.06 1.90 21

0.30 0.72 5 0.23 1.71 11 – – – 0.88 0.62 21 – – – 0.95 2.20 37 1.23 2.19 26 – – –

0.33 0.58 2 0.22 1.73 2 – – – 0.92 0.68 30 – – – 0.99 2.13 45 1.27 2.16 20 – – –

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Fig. 44. Mössbauer spectra of a FeCl2 · 4H2O/NH4-Y mixture. a ground in air at ambient temperature; material (a) after heat treatment in vacuum at b 420 K, c 520 K, d 620 K, and e 720 K (after [149], with permission)

tetrahedrally coordinated Fe(II) ions (13%) residing in the small and large cavities with probably one H2O molecule in their coordination shell (Fe(II)tetr); and (iii) an octahedrally coordinated Fe(II) species (Fe(II)oct-1 , 9%), where the octahedral environment included H2O and OH ligands. When, however, the FeCl2/NH4-Y mixture was heated in vacuum to increasingly higher temperatures (420, 520, 620, 720 K) and the Mössbauer measurement conducted after cooling to 300 K, the set of spectra shown in Fig. 44 was obtained. For the spectrum obtained after heat-treatment at 720 K, Fig. 45 illustrates the decomposition into individual signals. From the data of the whole set of spectra accumulated in Table 10, one realizes that, after heat-treatment at 720 K, only about 4% of Fe(III) species were left and an almost pure Fe(II)-Y zeolite was obtained exhibiting Fe(II) in trigonal (30%) or octahedral (65%) coordination. The octahedrally coordinated iron was either Fe(II)oct-2 , being due to interaction with oxidic extra-framework Alcontaining species (released from the framework during auto-reduction, vide infra) or Fe(II)oct-3 , being due to coordination with framework oxygen atoms in the hexagonal prisms of the faujasite structure.

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Fig. 45. Mössbauer signals of individual iron species giving the best fit to the spectrum in Fig. 44e of ground FeCl2 · 4H2O/NH4-Y heat-treated at 720 K. (●) Fe(III)oct-1 ; (■) Fe(III)trig ; (ƒ) Fe(II)trig ; (▼) Fe(II)oct-2 ; (ƒ) Fe(II)oct-3 ; QS quadrupole splitting; IS isomer shift (after [149], with permission)

TPE investigations of the system FeCl2 ◊ 4H2O/NH4-Y, where the evolved H2O, NH3 and HCl were monitored by mass spectrometry, revealed some special features: (i) besides the usual H2O peak around 400 K originating from the release of adsorbed water, a second one at 520 K was detected (cf. Fig. 46). This second peak was ascribed to H2O molecules stemming from hydroxyl groups formed intermittently through hydrolysis in which Fe cations and crystal water were involved; (ii) at temperatures above 520 K, the evolution of ammonia declined faster than that of HCl (cf. Fig. 46). The resulting small but significant and reproducible delay in HCl evolution is a peculiarity of the system FeCl2 ◊ 4H2O/NH4-Y in SSIE chemistry and was not observed with, e.g., CaCl2 ◊ 2H2O/NH4-Y. This peculiar behavior can be explained in view of the results obtained by Mössbauer spectroscopy (vide supra): 1. During grinding and/or subsequent heating hydrolysis occurred under formation of hydroxy iron cations according to Eq. (12). Fe2+ + H2O Æ Fe(OH)+ + H+

(12)

2. Mössbauer spectroscopy (vide supra) provided evidence for oxidation of Fe2+ to Fe3+; the latter may also be present as hydroxyl cations, e.g., Fe(OH)2+ or Fe(OH)+2 .

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Fig. 46. Curves of temperature-programmed evolution of HCl (m/e = 36, –––), NH3 (m/e = 17, – – –) and H2O (m/e = 18, – · – · –) evolved A from a ground mixture of FeCl2 ◊ 4 H2O/NH4-Y (nFe2+/nAl = 0.5) and B from crystalline NH4Cl (after [149], with permission)

3. These as well as Fe(OH)+ may react with HCl formed by decomposition of NH4Cl, resulting in the observed evolution of H2O and NH3 at 520 K: Fe(OH) +n–1 + (n–1) HCl Æ FeCl +n–1 + (n–1)H2O

(13)

where n is the valence state of the iron (i.e., n = 2 or 3). 4. Finally, FeCl+n-1 undergoes at somewhat higher temperatures a solid-state reaction with Brønsted acid sites according to: FeCl+n–1 + (n–1)H+ Æ Fen+ + (n–1)HCl

(14)

In summary, a fraction of the HCl which had formed by decomposition of the SSIE product, viz., NH4Cl, is consumed by reaction (13) and only at higher temperatures released according to Eq. (14). This explains the shift of the HCl evolution curve to higher temperatures compared with the evolution curve of NH3 (cf. Fig. 46). It is striking that the decomposition of ammonium chloride observed in TPE experiments around 520 K and the drastic change in the oxidation state of iron at 520–620 K as revealed by Mössbauer spectroscopy (vide supra, Table 10) occurred in almost the same temperature range. The interest in iron exchange into zeolites, particularly into ZSM-5 (MFItype) zeolites, was stimulated by the possible application of the products in DENOX processes. Thus, incorporation of iron by solid-state ion exchange was

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discussed in a series of recent papers in relation to adsorption and catalytic conversion of nitrogen oxides. Fe,H-ZSM-5 samples with different Fe-loadings were prepared by Bell and co-workers via solid-state ion exchange and characterized by IR spectroscopy of NO, TPD of NO and NH3 and TPR with H2 [155]. At low loadings, Fe cations exchanged with protons of the acid Brønsted centers on a one-to-one basis. NO adsorption and TPR experiments suggested that the primary form of incorporated iron was Fe3+, i.e., Fe3+(OH)–2 . At loadings above nFe2+/nAl ≈0.56, small FeOx particles were formed (vide infra, cf. [156, 157]) and at increasingly higher loadings the concentration of Fe2+ cations was found to increase because of autoreduction. Upon NO adsorption at room temperature, three different types of Fe2+ sites were observed. Furthermore, the interaction of NO, NO2 and O2 as well as the reduction of NO by C3H8 on the modified zeolites was investigated. A high activity of Fe-MFI catalysts produced by SSIE in NO decomposition, even though lower than that of conventionally exchanged materials, was reported by Varga et al. [118]. Turek and colleagues [156–160] reported on NOx decomposition and/or reaction under various conditions over Fe-MFI or FeFER (ferrierite) produced via SSIE. The activity for NO oxidation was inhibited by the presence of SO2 or H2O, but could be completely restored by removal of the poisons. Fe-FER was more active but also more sensitive to poisoning than Fe-MFI [158]. Kögel et al. [160] have found that Fe-ZSM-5 prepared through solid-state ion exchange in the presence of air exhibits activities in simultaneous catalytic reduction of NO and N2O comparable to those of Fe-ZSM-5 catalysts obtained under anaerobic conditions. Upon reduction of NO by iso-butane over Fe-MFI catalysts obtained by SSIE, HCN was detected as a substantial product [159]. The catalysts produced by SSIE with FeSO4 · 7H2O to various Fe-contents [156, 157] exhibited a high activity in decomposition of N2O in the absence of reducing agents, but were also active in reduction of NOx with hydrocarbons, especially propane. Above a ratio of nFe2+/nAl ≈ 0.5, however, iron was no longer incorporated as catalytically active Fe2+species, but hematite was formed from the Fe2+ excess (vide supra, [155]), which was inactive in both NO and N2O reduction. Incorporation of iron into zeolites by SSIE was also studied by Varga et al. [114, 118, 121] and Dandl [161] in relation to catalytic decomposition and reduction of NO and N2O. Varga et al. [114] reported the preparation of Fe-, Co- and Ni-containing ZSM-5 zeolites by SSIE for NO adsorption and decomposition (see Sects. 5.3.4.2 and 5.3.4.3). Preparation and characterization were identical to those reported by the same authors for Cu-ZSM-5 (SSIE) in Sect. 5.3.2.1. Also, the results were similar, except for some differences in NO adsorption. The properties of Fe-[Ga]ZSM-5 prepared by SSIE of FeCl3 and the gallium analog of H-ZSM-5, i.e., H-[Ga]ZSM-5, were essentially the same as those of the corresponding Fe-ZSM-5 (i.e., Fe-[Al]ZSM-5) with Al instead of Ga in the framework [128, 129]. The incorporated Fe Cl2+ cations or, after calcination in air, FeO+ cations were replaced by Cu2+ when a mixture of CuO and Fe-[Ga]ZSM-5 was heated. However, by Kucherov et al. [128, 129], the Brønsted acid sites of H[Ga]ZSM-5 provided less stable traps for the introduced cations.

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Crocker et al. [28] reported on the incorporation of Fe3+ and Al3+ into montmorillonite (MM) via solid-state reaction with Fe(NO3)3 · 9H2O and Al(NO3)3 · 9H2O, respectively. Oven-dried Na-MM was co-ground with the metal salt at room temperature for 30 min in air, subsequently dried in vacuum (1 Pa; 24 h) and then analyzed by XRD. Ion exchange was indicated by the appearance of a reflection at 2Q = 29.4°, characteristic of NaNO3. Furthermore, reflections typical of the starting nitrate [Fe(NO3)3 ◊ 9H2O] were no longer observed in the XRD pattern of the ground mixture. From their results, the authors concluded that SSIE had occurred according to Eq. (15), yielding a high degree of exchange: 3 Na-(MM) + Fe(NO3)3 Æ Fe-(MM)3 + 3 NaNO3

(15)

After careful drying, the materials prepared in this way exhibited significant amounts of Brønsted and Lewis acid sites as evidenced by IR spectroscopy and pyridine adsorption. With Al(NO3)3 ◊ 9H2O similar results were obtained. The authors pointed out that the acidity, in particular the high Lewis acidity, may render the Fe-MM (and Al-MM) materials produced via SSIE valuable catalysts for Lewis acid catalyzed Friedel-Crafts acylation and alkylation reactions. Preparation of such catalysts by a solid-state reaction should be preferred over conventional ion exchange since, at least on a large scale, ion exchange with clays in aqueous solutions is hampered by the tendency of aqueous clay suspensions to form intractable gels. 5.3.4.2 Introduction of Cobalt

Sachtler and co-workers [162] studied the redox chemistry of cobalt ions introduced into MFI via SSIE with nCo/nAl = 0.4–1.0, employing IR, ESR and UV-Vis diffuse reflectance spectroscopy. The coordination of Co2+ was found to be tetrahedral; the ions were in their high-spin state, detectable at 60 K. In a recent contribution, Enhbold et al. [109, 163] investigated the incorporation of cobalt into clinoptilolite (CLIN) via solid-state reaction of CoCl2 or Co(NO3)2 with hydrogen and sodium forms of this zeolite obtained through repeated conventional exchange of natural clinoptilolite (from Tzaaga, Mongolia) with 1 M aqueous solutions of NH4Cl and NaCl, respectively. The degree of subsequent solid-state ion exchange was determined by chemical analysis of the starting materials and, after thorough washing, by back-exchange of Co2+ with NH+4 (cf. Sect. 4.1). The degree of exchange was studied as a function of reaction temperature (Treact), reaction time (treact) and the type of cation (Na+, H+) of the parent clinoptilolite. Moreover, the process of SSIE upon heat-treatment of mixtures of the cobalt salts and Na-CLIN or NH4(H)-CLIN was evidenced by formation of NaCl or NaNO3 crystallites and HCl as determined by XRD and chemical analysis, respectively. In fact, introduction of cobalt cations was achieved, but only into those channels of the heulandite-like structure which are formed by 10-membered rings; Co2+ could not enter the channels built by 8-membered oxygen rings. It was shown that SSIE rendered preparation of Co-CLIN possible with cobalt contents similar to that of conventionally exchanged materials. SSIE with clinoptilolite occurred not only with the hydrogen but also with the sodi-

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Fig. 47. Degree of solid-state ion exchange of cobalt into clinoptilolite as a function of the ratio nCo2+/nOH (nCo2+/nNa+). 1 H-form of clinoptilolite; 2 Na-form of clinoptilolite (after [163], with permission)

um form, even though a higher degree of exchange was reached with H-CLIN (cf. Fig. 47). Surprisingly, the amount of Co2+ incorporated into the clinoptilolite structure was not very dependent on the cobalt content in the mixture (cf. Fig. 47). A very fast replacement of the Na+ and H+ cations by Co2+ was observed in the initial stage of SSIE. The subsequent steady state degree of SSIE was temperaturedependent (cf. Fig. 48). Both XRD and IR spectroscopy of the lattice vibrations confirmed that SSIE did not cause a loss of crystallinity. Only slight differences between the IR spectra of Co-CLIN samples prepared via solid-state ion exchange on the one hand and conventional ion exchange on the other were found. No formation of cobalt hydroxide or oxide was detected. At 373 K, SSIE with Co(NO3)2 resulted in an exchange degree twice as high as with CoCl2 in accordance with the lower lattice energy of the former salt (cf. Sects. 5.1.3 and 5.4.3). Adsorption isotherms measured with the parent Na-CLIN and Co-CLIN (prepared through SSIE) as adsorbents suggested that the micropore volume was lower in the case of Co-CLIN due to partial blockage of the 10-membered ring channels. No formation of mesopores was observed. Introduction of cobalt and nickel into zeolites by solid-state ion exchange was investigated by Jentys and colleagues and compared with preparations via conventional exchange in aqueous solution, impregnation and direct synthesis ([164–170], vide infra). For their studies, the authors employed TPD of NH3 (TPDA), IR spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS) and XRD. TPDA revealed that 100% of the strong Brønsted acid sites were eliminated at an nCo/nAl ratio of ca. 1.0 and 0.5 in the case of SSIE of CoCl2 in mixtures with H-ZSM-5 and NH4-Y, respectively. Therefore, the authors concluded

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Fig. 48. Solid-state ion exchange of cobalt into clinoptilolite as a function of reaction time and temperature. A Amount of Co2+ incorporated per gram; B degree of exchange (after [163], with permission)

that in the case of ZSM-5 the negative charge of the framework originating from the Al content was balanced not by bivalent Co2+ cations but by monovalent CoCl+ complexes. This was ascribed to the fact that, due to the high nSi/nAl ratio – tetrain their H-ZSM-5 samples (nSi/nAl = 26), the distance between two AlO4/2 2+ hedra was too large as to be neutralized by one Co cation (cf., e.g., Sects. 5.2.2 and 5.3.5). No attempt was reported [164, 165] to confirm the existence of charge-balancing CoCl+ species by chemical analysis for chlorine of the CoZSM-5 samples prepared via solid-state ion exchange. However, the interpretation given by Jentys et al. was supported by EXAFS results. These indicated in Co-ZSM-5 obtained through SSIE a coordination number of N ª 1 for CoCl and a distance Co–Cl slightly shorter than in CoCl2 where N = 4 was observed. In – tetrahedra contrast, in H-Y (nSi/nAl = 2.5), the spatial separation of AlO4/2 2+ allowed neutralization of two negative charges by only one Co (cf. Sect. 5.2.2, SSIE of La3+ into H-Y vs. H-ZSM-5). This holds true even for lower concentrations of CoCl2 in the mixtures, corresponding to 0.25 £ nCo/nAl £ 0.5; for nCo/ nAl ª 0.25, some Co cations replaced only one proton. This may be the preferred SSIE ratio at very low cobalt salt contents in the mixture. The IR spectra (with and without application of pyridine as a probe molecule) of H-ZSM-5 and Co-ZSM-5 prepared via SSIE confirmed the suggestion that in the case of nCo/nAl ª 1.0 the acid Brønsted OH groups were completely consumed. Two types of newly formed acid sites, most likely Lewis acid sites, were indicated, viz., CoCl+ species (vide supra) and possibly ‘true Lewis sites’, i.e., Al-containing extra-framework species (cf. [171, 172]). Microcalorimetry and XPS measurements of NH3 adsorption were used by Auroux et al. [106] to characterize Co-ZSM-5 obtained through solid-state reaction in a similar way as reported for introduction of copper (vide supra) and

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nickel (vide infra). After ion exchange, a strong increase in Lewis acidity was observed. The results were compared with those observed with Co-ZSM-5 produced through conventional exchange or impregnation. Jentys et al. [168] also compared the properties of Co-ZSM-5 prepared by different methods such as conventional exchange in aqueous solutions of cobalt salts, impregnation and solid-state ion exchange. It is worth mentioning that with samples produced by the impregnation method metal oxide clusters were observed, mainly located on the external surface of the zeolite crystallites [166]. In contrast, SSIE resulted in a highly dispersed distribution of cobalt cations inside the channels. Over reduced bifunctional Co,H-ZSM-5 catalysts prepared via SSIE, high overall activity in hydroconversion of n-heptane and highest selectivity to isomerization to iso-C7 were observed by Lugstein et al. [168]. Main products were, besides 2-methylhexane, iso-butane and propane. An increased metal loading gave rise to pronounced activity in hydrogenolysis upon hydroconversion of n-heptane [167]. Furthermore, it was found that Co-containing zeolites produced via SSIE between H-Y or H-ZSM-5 and cobalt salts were active catalysts in thiophene hydrodesulfurization [166]. Another interesting catalytic application of Co-containing zeolites was reported by Li and Armor [173]. These authors used dealuminated H-ZSM-5, HBETA and H-Y, modified by solid-state reaction with solid cobalt salts or conventional exchange in cobalt salt solution, as catalysts for ammoxidation of ethane to acetonitrile. Even though the catalysts prepared by SSIE had, in general, a lower nCo/nAl ratio, they produced under equal conditions more acetonitrile and also showed higher selectivity for incorporation of NH3 into acetonitrile. Introduction of cobalt into NH4-Y via SSIE has also been reported by Varga et al. [114] and Onyestyák et al. [139]. In the first publication [114], it was reported that Co-ZSM-5 (SSIE) could bind oxygen only loosely, so that oxygen could be easily removed and re-adsorbed in contrast to Cu-ZSM-5. In the opinion of the authors, this behavior renders Co-ZSM-5 produced via SSIE a better catalyst for selective reduction of NO by methane in the presence of oxygen than Cu-ZSM5. In fact, Co-ZSM-5 prepared via SSIE was also employed as catalyst for catalytic NO decomposition and/or reduction by, e.g., propylene (cf. [121, 174]) and other hydrocarbons such as iso-C4H10 in O2-rich or O2-free streams, both with dry and wet feeds. Wang et al. [175] compared the properties of Co-containing ZSM-5 (or ferrierite) catalysts that had been prepared by various methods, viz., conventional ion exchange in aqueous solution of Co-salts, impregnation, solidstate ion exchange and sublimation (cf. Sect. 6.3.1). 5.3.4.3 Introduction of Nickel

Solid-state ion exchange between H-ZSM-5 (nSi/nAl = 13.6, sample I and 22.5, sample II) and nickel compounds such as NiCl2 , NiSO4 , Ni(CH3COO)2 and NiO was studied by Wichterlová et al. [176]. In their investigations, these authors employed IR spectroscopy and TPDA to monitor changes in the concentration of Brønsted acid sites, mass spectrometry and back-titration to determine the gases evolved upon SSIE, e.g., HCl and decomposition products of acetate

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anions.With the system NiCl2/H-ZSM-5 it was observed that almost 100% of the acid OH groups were eliminated through solid-state reaction in a mixture with nNi /nOH = 0.5 at 770 K in a flow of oxygen. In a mixture with nNi /nOH = 0.33, 64 and 66% of the initial OH groups were consumed at 670 and 770 K, respectively. These IR results were confirmed by back-titration of evolved HCl. Wichterlová et al. [176] concluded that SSIE of both H-ZSM-5 samples with NiCl2 resulted in neutralization of two Brønsted acid OH groups by one Ni2+ cation. This finding is different from the interpretation of SSIE of bivalent cobalt with H-ZSM-5 (vide supra), where a ratio of nCo/nAl ª 1.0 was required for complete exchange and the charge balance was assumed to be achieved by replacement of the proton of one Brønsted acid OH group by one CoCl+ species. The reason for this different behavior of the systems CoCl2/H-ZSM-5 and NiCl2/H-ZSM-5 in solidstate reaction has not yet been clarified. Wichterlová et al. [176] demonstrated further that via back-exchange of Ni-ZSM-5 (produced by SSIE) with NH4NO3 solution and subsequent deammoniation the original density of Brønsted acid OH groups was almost completely restored. Similarly, a full regeneration of the original Brønsted acid OH groups was accomplished when the Ni2+ cations introduced via SSIE were subsequently reduced by hydrogen. The fact that the original OH groups were regained in both cases showed that neither SSIE nor subsequent reduction affected the integrity of the zeolite structure or caused a measureable dealumination of the framework. Solid-state reaction of NiSO4 with H-ZSM-5 (nNi /nOH = 0.5) resulted in a consumption of only 50% of the acid OH groups, whereas SSIE did not proceed at all in the systems Ni(CH3COO)2 / H-ZSM-5 or NiO/H-ZSM-5. Nickel acetate decomposed at temperatures above 520 K into CO, CO2 , H2 , CH3COOH, CH3COCH3 and NiO. NiO did not react, most likely because of its rather high lattice energy. Introduction of Ni2+ into Y-type zeolite by SSIE was proven by XAS in experiments similar to that carried out with Cu+ and Zn2+ (vide supra, [105]). In the case of Ni compounds it was observed that solid-state ion exchange started already during compression of the powdered components. Analogously to the cases of Cu+ and Zn2+, the authors tentatively assumed that upon SSIE Ni–O–Ni complexes formed. Partial structures of fully dehydrated Ni-containing Y-zeolite, which was prepared via SSIE, and of its D2O sorption complex, were determined by pulsedneutron diffraction in a study by Haniffa and Seff [177]. In Ni(30)-Y, with the unit cell composition of Ni30Na7Cl12Al55Si137O384 , the Ni2+ cations occupy crystallographically different positions, viz., I, I¢ and II¢ sites with population numbers 4, 18 and 8, respectively. Six of the eight sodalite cages of the unit cell contain (Ni-Cl-Ni-Cl-Ni)4+ clusters. The site population by Ni2+ in the D2O complex is different: 4 at site I, 11 at site I¢, 4 at site II¢ and 11 at site III¢. Solid-state reaction was also reported for the system NiCl2/H-SAPO-34, where Ni(I) was introduced into extra-framework sites, in contrast to as-synthesized samples of NiAPSO-34, where Ni(I) is incorporated into the framework. This difference was evidenced by ESR and ESEM [178]. The study by Wichterlová et al. [176 ] on SSIE of nickel compounds with HZSM-5 was essentially motivated by problems of the preparation of Ni-containing zeolite catalysts for isomerization of C8 aromatics. The authors reported that

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Ni-ZSM-5 was easily obtained with an exchange degree of almost 100% by SSIE, whereas conventional exchange yielded a material with only a maximum of 60% exchange degree. Moreover, Ni-ZSM-5 catalysts, where the protons have been completely exchanged by Ni2+ via SSIE, exhibited after reduction in a hydrogen flow the same catalytic activity as the parent H-ZSM-5 in isomerization and dealkylation of C8 aromatics in a mixture of o-xylene and ethylbenzene. This was ascribed to full restoration of the original acid OH groups upon reduction (vide supra). Furthermore, it was found that the reduced Ni-ZSM-5 samples prepared by SSIE were similarly active in hydrogenation of ethylene as reduced Ni-ZSM-5 materials obtained through conventional exchange of H-ZSM-5 with aqueous solutions of Ni(CH3OOH)2 . Incorporation of nickel into ZSM-5 was also studied by Jentys et al. [164, 165, 2+/n3+ = 1 167], who reacted NiCl2 ◊ 6H2O with H-ZSM-5 and obtained a ratio of nNi Al 2+ in a similar way as reported for the introduction of Co (vide supra). The NiZSM-5 prepared this way was more easily reduced with H2 (at 573 K) than CoZSM-5 (at 773 K) but exhibited a much lower hydrogenolysis activity. Jentys et al. [169] carried out a comparison between the results of nickel introduction via different methods into zeolites similar to that reported for modification by cobalt (vide supra). The samples were prepared by conventional exchange, impregnation and solid-state ion exchange. The comparison was extended to other zeolite structures, viz., H-MOR and H-BETA. A 100% exchange could be achieved only in the case of SSIE. Again, SSIE led to products with highly dispersed metal cations and, after reduction, to small metallic clusters of nickel inside the zeolite structure. The reduced materials showed in the hydroconversion of n-heptane an enhanced selectivity for isomerization. In n-nonane hydroconversion, the differently prepared materials exhibited similar activities and selectivities with Ni,HMFI as the most active and Ni,H-BETA the most selective catalyst. Bock [44] and Bock et al. [179] introduced Ni2+ cations into zeolite-like SAPO-42, a narrow pore silicoaluminophosphate with zeolite A (LTA) structure (cf. [180]), i.e., 8-membered ring pore openings (0.41¥0.41 nm), and the following ratios of the tetrahedrally coordinated framework T-atoms: nSi /(nAl + np) = 1.0, nAl /nP = 2.3 [180]. They reacted powdered mixtures of SAPO-42 and NiCl2 or NiO. Solid-state ion exchange was monitored by TPE of HCl and H2O and characterized by the starting temperature, Tstart , i.e., the temperature of initial gas evolution. Evolved HCl was trapped in an excess of 0.1 M aqueous NaOH solution and determined via back-titration. In the case of NiCl2 , a ratio nCl-,out / nCl-, in = 0.97 of the out-going vs. in-going Cl– was measured. This means that Ni2+ cations were not incorporated as monovalent complex cations, (NiCl)+, in analogy to incorporation of (CoCl)+ into ZSM-5 (vide supra), but just as they were, i.e., as bare Ni2+. This was possible because of the higher Al content compared with ZSM-5. Surprisingly, the starting temperature was lower in the case of SAPO-42/NiO (Tstart = 842 K) than with SAPO-42/NiCl2 (Tstart = 930 K), even though the lattice energies of NiO and NiCl2 are 4010 and 2772 kJ/mol, respectively. This was explained by the fact that the oxide possesses the lower ion pair size (0.30 ¥ 0.38 nm) compared to the chloride (0.38 ¥ 0.84 nm). The higher mobility resulting from the smaller size of the migrating species probably overcompensated the effect of the higher lattice energy.

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Similarly, introduction of nickel into SAPOs was investigated by Kevan and his associates [181–185]. These authors prepared Ni,H-SAPO-n (n = 5, 8, 11, 34) via SSIE between NiCl2 and H-SAPO-n. The materials were characterized by ESR. Ni was incorporated as Ni2+; however, after dehydration (thermal reduction in vacuum at 473–873 K), Ni+ was stabilized in the cation positions of the SAPO structures. Azuma et al. prepared Ni,H-SAPO-5 by SSIE between NiCl2 and HSAPO-5 as well as by conventional exchange using H-SAPO-5 and an aqueous solution of NiCl2 ◊ 6H2O [181]. The Ni-containing materials were investigated in detail by ESR and ESEM (electron spin echo modulation spectroscopy).With the sample prepared via CE no ESR signals were observed. However, in the case of samples produced by SSIE, after thermal reduction, various Ni(I) species could be detected. Their existence and location were dependent on the temperature. In another study, Hartmann et al. compared Ni incorporated into SAPO-5 either by adding Ni(OCOCH3)2 ◊ 4H2O to the synthesis mixtures for the SAPO resulting in NiAPSO-5 or by SSIE of NiCl2 and H-SAPO-5 at 873 K yielding Ni,H-SAPO-5 [182]. The nickel content in the Ni,H-SAPO-5 prepared in this manner, Ni0.01H0.01(Si0.03 Al 0.50 P0.47)O2 , appeared to be the upper limit for SSIE with respect to the silicon content of the sample. Even though the g-values of Ni+ in the synthesized NiAPSO-5 and the Ni,H-SAPO-5 were almost identical, the different incorporations in framework and extra-framework positions, respectively, could be clearly evidenced by 31P nuclear modulations, differences in the ESR spectra after adsorption of ammonia, CH3OH or C2D4 , contrasting coordination properties derived from the ESEM data and the fact that Ni,H-SAPO-5 decomposed water at room temperature to generate O2– (cf. [182]). Similar results were obtained when NiAPSO-11 and Ni,H-APSO-11 were prepared in an analogous manner and investigated by ESR and ESEM [183, 184]. The Ni-containing SAPOs and APSOs were employed as catalysts for ethylene dimerization to n-butenes. The catalytic activity was shown to depend on Ni(I) species incorporated. Compared with NiAPSO-n, the catalysts obtained via SSIE exhibited a lower selectivity with respect to n-butenes [185]. Ni(II) cations were also introduced by SSIE into [Si]MCM-41 and [Al,Si]MCM-41. After thermal or hydrogen reduction, Ni(I) species were stabilized in the products. The results were compared with those obtained with conventionally exchanged MCM-41 samples [186]. Similarly to the zeolite modifications with Fe2+ and Co2+, Ni-zeolites produced via SSIE were also tested as catalysts for NOx decomposition and/or reduction. Varga et al. [114, 121] investigated by IR the adsorption behavior and transformation of NO on the resulting Ni-ZSM-5 and showed that this material is indeed a good catalyst for NO decomposition. In their study on selective catalytic reduction of NO by Raney-nickel-supported Cu-ZSM-5, Ma et al. [187] observed that the NO conversion decreased only at low temperatures but remained constant or even improved at high temperatures. The authors ascribed this effect to a migration of Ni cations into the zeolite matrix via SSIE that led to a stabilization of the lattice.

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5.3.4.4 Introduction of Manganese

A few reports have been published dealing with solid-state ion exchange of manganese cations into hydrogen forms of zeolites. Wichterlová et al. [147] and Beran et al. [188] preferentially employed ESR spectroscopy to monitor introduction of Mn2+ from MnCl2 , MnSO4 , Mn3O4 or Mn(CH3COO)2 into H-ZSM-5. In addition, other techniques such as XPS, IR spectroscopy, TPD of ammonia (TPDA), TPE, back-titration of evolved HCl and test reactions have been used. IR spectroscopy for monitoring the changes of the absorbance in the OH stretching region upon SSIE was also employed in the work by Onyestyák et al. [139] mentioned above on incorporation of various bivalent cations (Ca2+, Cd2+, Zn2+, Co2+ and Mn2+) into faujasite-type zeolites via solid-state reaction in order to prepare adsorbents for H2S (vide supra). Application of XPS to mixtures of Mn(NO3)2 or MnSO4 with H-ZSM-5 after solid-state reaction at 700–920 K in a stream of oxygen or hydrogen revealed that the same concentration of Mn2+ occurred at the external surface of the zeolite crystallites as that determined by chemical analysis for the bulk. Therefore, the authors [147] concluded that the incorporated cations were homogeneously distributed: neither surface enrichment nor depletion had taken place. An instructive set of ESR spectra is displayed in Fig. 49. It is seen that the starting mixture of solid MnSO4 and H-ZSM-5 did not exhibit any hyperfine splitting of the signal at g = 2.0 originating from Mn2+ in crystalline MnSO4

Fig. 49. X band ESR spectra of Mn2+: a a physical mixture of MnSO4 and H-ZSM-5; b sample (a) heat-treated at 770 K, c sample (a) heat-treated at 870 K; and d Mn,H-ZSM-5 prepared by conventional ion exchange and calcined at 770 K (after [147], with permission)

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(spectrum a). Only progressive heating up to 770 and 870 K (spectra b and c, respectively) and subsequent rehydration at ambient temperature caused the hyperfine splitting to appear. This indicated disaggregation of the MnSO4 crystallites and migration of Mn2+ into the zeolite structure. The six-line spectrum with a hyperfine splitting constant of A = 9.8 mT is considered to be characteristic of isolated Mn2+ cations in Oh coordination. This was substantiated by comparison with the ESR spectrum of a conventionally prepared and equally treated M,H-ZSM-5 sample (cf. spectrum d). The low-intensity signal at g = 4.27 was ascribed to Mn2+ in distorted tetrahedral (Td) coordination. This assignment was based on a comparison with ESR spectra of borate glasses [189]. Essentially the same ESR features were observed by Beran et al. [188] when studying the system MnCl2/H-ZSM-5. In contrast, no ESR signals ascribable to isolated Mn2+ cations on exchange sites of ZSM-5 were detected when the sodium form of the zeolite, i.e., Na-ZSM-5, was heated in a mixture with MnSO4 and subsequently hydrated [147]. Solid-state reaction of MnCl2 , MnSO4 , Mn3O4 or Mn(CH3COO)2 with H-ZSM5 was also monitored by IR spectroscopic measurements of the consumption of acidic OH groups, i.e., the decrease in the intensity of the respective IR band at 3610 cm–1 (cf. Fig. 50 and Table 11). In no case, however, was a 100% degree of exchange reached, as measured via the fraction of protons replaced by Mn2+, i.e., as dOH , or through the incorporated fraction of available manganese cations, i.e., as dMn (cf. Table 11). From Fig. 50 and Table 11 it can be seen that an increase in the reaction temperature considerably enhanced the consumption of OH groups, i.e., dOH . In contrast, an increase in the amount of applied MnCl2 from a sub-stoichiometric ratio (0.33 mmol Mn2+ vs. 0.91 mmol OH groups per gram) to a stoichiometric ratio (0.45 mmol Mn2+ vs. 0.91 mmol OH groups per gram) did not bring about a mea-

Fig. 50. Number of bridging OH groups consumed as a function of the reaction time in solidstate reaction in the system MnCl2/H-ZSM-5 (nMn2+/nOH = 0.33) upon heating in vacuum at 570, 670 and 770 K (after [188], with permission)

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Table 11. Chemical composition of the original mixtures of H-ZSM-5 with MnCl2 , MnSO4 , Mn3O4 , and their characteristics after heat-treatment in high vacuum (SSIE)

Composition of the mixture a,b

Heat-treatment c

OH consumed (%)

Salt /oxide Mn (mmol g–1) Mn2+/OH b

T (K)

In vacuumd In N2 flow e

MnCl2

570 670 770 770 770 770

0.21 0.38 0.56 0.57 0.16 0.46

MnSO4 Mn3O4 a b c d e

0.33 0.33 0.33 0.45 0.47 0.60

0.35 0.35 0.35 1.00 0.50 0.66

– 0.41 0.57 – 0.18 –

Mn content per gram of dry zeolite. Number of Mn cations related to the original number of the zeolite bridging OH groups (0.91 mmol per gram of dry zeolite). Under vacuum at 10–4 Pa. Determined from the intensity of the IR band at 3610 cm–1 after heat-treatment in high vacuum (10–4 Pa). Determined from TPDA, high-temperature peak approximately at 700 K.

surable increase in the consumption of OH groups (0.56 to 0.57 mmol OH groups per gram, cf. Table 11). Figure 50 shows clearly that most of the Mn2+ cations were introduced during the initial stage of the reaction (i.e., within the first hour). Subsequently, the reaction proceeded rather slowly (compare also the system CoCl2/H-CLIN, vide supra). This kinetic behavior in solid-state ion exchange was confirmed by TPE measurements of HCl evolution [188]. As in previous cases, the anion bound to the in-going cation also played a significant role in SSIE with manganese compounds. Similar to the observations with solid-state incorporation of alkaline, alkaline earth, copper and iron cations, it was found that SSIE proceeded most easily with manganese chloride and nitrate but to a significantly lesser extent with sulfate and oxide. It is worth mentioning, however, that at variance with the case of nickel (vide supra), an exchange did take place with Mn oxide.As a consequence, reaction with acetate also led to some solidstate ion exchange because Mn oxide resulting from decomposion of Mn acetate was able to react with the Brønsted acid OH groups (cf. Table 11). Consumption of acid OH groups upon introduction of Mn2+ cations led to a decrease in Brønsted and a concomitant increase in Lewis acidity, as indicated by IR using pyridine as a probe. The decrease in Brønsted acidity was indicated by a decrease in the intensity of the pyridinium ion band at 1540 cm–1 observed upon pyridine adsorption on Mn-ZSM-5 samples that had been prepared by SSIE. A corresponding increase in the Lewis acidity effected an enhancement of the absorbance around 1450–1454 cm–1, which is indicative of pyridine coordinated to electron pair acceptors or Lewis sites such as Mn2+. This was compared with similar IR measurements on the parent H-ZSM-5. Figure 51 shows a linear correlation between the density of Brønsted and Lewis acid sites of the parent H-ZSM-5 as well as of Mn,H-ZSM-5 samples measured by this technique.

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Fig. 51. Plot of the maximum absorbance log(T0/T) (T, T0 transmittances at the frequency of the band minimum and the correlated base line) of the PyH+ band against log(T0/T) of the PyL band measured for 1 H-ZSM-5; 2 Mn3O4/H-ZSM-5; 3 MnSO4/H-ZSM-5; 4 MnCl2/H-ZSM-5; and 5 conventionally exchanged Mn,H-ZSM-5; materials heated in vacuum at 770 K for 6, 6, 60, 2, and 6 h, respectively (after [188], with permission)

In fact, the result for SSIE of Mn3O4/H-ZSM-5 deviated from this correlation in that the measured density of Lewis acid sites was lower than was expected according to the measured decrease in the density of Brønsted acid sites. The value of the density of Brønsted acid sites after solid-state reaction of Mn3O4/HZSM-5 was, however, in agreement with IR spectroscopic and TPDA determination of the consumption of OH groups. Therefore, the authors ascribed the deviation to an underestimation of the Py Æ Mn2+ value, and tentatively assumed that this was due to a partial blockage of incorporated Mn2+ by unreacted Mn3O4 species. It is important to note in the context of these investigations that the authors established that SSIE did not generate ‘true’ Lewis sites (cf. [171, 172, 188]) by dealumination of the framework. They showed via re-exchange of the introduced Mn2+ in NH4NO3 solution and subsequent deammoniation that the original density of OH groups could be fully restored. Completeness of reexchange was proven by ESR, which indicated that only traces of Mn2+ were left in the re-exchanged zeolite samples. 5.3.5 SSIE of Vanadium, Niobium, Antimony, Chromium, Molybdenum and Tungsten Compounds with Zeolites 5.3.5.1 Introductory Remarks

Introduction of transition metal cations of high oxidation states via conventional ion exchange is usually rather difficult if not impossible, since the required cations are frequently not available in simple metal salts. If, however,

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such salts are available, the respective cations may exist only in strongly acidic solutions, i.e., the metal cations have to compete with protons for ion exchange and the zeolite lattice is often damaged by acid attack. Therefore, solid-state ion exchange may be a promising route to prepare zeolites containing polyvalent and homogeneously distributed cations with changeable oxidation states. This is particularly true in view of possible applications of such zeolites as valuable redox catalysts. Kucherov and Slinkin [23, 92, 190, 191] were the first to study the incorporation of cations with (possibly) high oxidation states, such as V5+ or V4+, Cr6+ or Cr5+ and Mo6+ or Mo5+, into zeolites via solid-state exchange. Again, they preferentially employed ESR spectroscopy (vide supra). Huang et al. [192–194] as well as Marchal et al. [195–197] extended the investigations on SSIE with vanadium compounds, by also monitoring solid-solid reactions between tungsten and zeolites. Studies on SSIE with chromium cations were conducted by several authors [93, 100, 198, 199] using various experimental techniques. Incorporation of Mo-containing cationic species into zeolites attracted much attention. As various oxidation states of molybdenum may occur in zeolites, the materials resulting from introduction of Mo were expected to exhibit interesting redox properties. Incorporation of Mo-containing species into particular microporous structures could generate valuable shape-selective redox catalysts. Therefore, after the early work by Dai and Lunsford [200], a number of reports on SSIE of zeolites with molybdenum compounds appeared [44, 92, 179, 190, 197, 201]. Finally, systematic research into SSIE using cations with a high oxidation state was extended to solid-state reactions with niobium compounds [202]. Interestingly, in most examples of solid-state reactions between compounds of tetra-, penta- or hexavalent cations and zeolites, a reduction of the cations occurred (vide supra; e.g., V5+ Æ V4+, Cr6+ Æ Cr5+ and Mo6+ Æ Mo5+). Most likely, this is due to an auto-reduction (cf. [203, 204]), i.e., the reduction is accompanied by an oxidation of framework oxygen anions to molecular oxygen, although this was not discussed by the respective authors. 5.3.5.2 Introduction of Vanadium

In order to introduce vanadium into hydrogen forms of zeolites (H,Na-MOR, HMOR with nSi/nAl = 5 and H,Na-ZSM-5, H-ZSM-5 with nSi/nAl = 35), mixtures of V2O5 and the zeolites were subjected to heat-treatment at 1073 K in air [92, 190, 191]. Electron spin resonance spectroscopy (ESR) yielded a spectrum (as shown for the example of H-ZSM-5 in Fig. 52) exhibiting a well-resolved hyperfine (HF) signal of vanadyl cations with g = 1.93, g^ = 2.02,A = 19.8 mT, and A^ = 8.3 mT. These parameters are typical of isolated V4+ cations in an almost square-planar coordination. Identical signals were seen after heating in air, hydrogen or vacuum. When the sample was cooled to 77 K, no change was observed. In particular, vanadyl cations proved to be very stable against oxidative or reductive treatment. However, introduction of oxygen resulted in a considerable line broadening (cf. [190]). This shows that the incorporated vanadium-containing species are accessible to gases.

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Fig. 52. ESR spectra of isolated vanadyl cations (51VIV) introduced into H-ZSM-5 (nSi/nAl ª 35) by solid-state reaction between V2O5 and H-ZSM-5 at 1025 K (after [191], with permission)

In the case of H-MOR with an nSi/nAl ratio of 5, which is considerably lower than in H-ZSM-5 and, thus, gives rise to a much smaller distance between the cation sites, the ESR signals were broadened due to dipole-dipole interaction of the V4+-containing species. In a situation where the isolated paramagnetic ions on extra-framework sites of a zeolite structure are able to electronically interact with adjacent Al3+ ions of the framework, one may expect a super-hyperfine splitting (SHFS) of the signal. This was, indeed, observed with CrO +2 , i.e., Cr(V), in H-ZSM-5 (vide infra and cf. [92, 95]). In view of the analogy of the electronic configuration (3d1), occurrence of a SHF-splitting was also expected for ESR spectra of vanadyl species such as VO(OH)+, i.e., V(IV), in the MFI structure. In fact, Kucherov and Slinkin [191] succeeded in detecting SHF-splitting in the ESR spectra of V,H-ZSM-5 obtained via SSIE in a V2O5/H-ZSM-5 mixture with nV/nAl = 0.3 after calcination at 1023 K in air and subsequent evacuation. A satisfactory resolution was achieved when the spectra were run at 473 K; this produced a signal with a SHF-splitting constant of 0.7 mT (see Fig. 52, spectrum c). Figure 53 shows the proposed possible arrangement of the vanadyl species and its unpaired electron in the dxy orbital adjacent to Al in the framework. As mentioned above, admission of dry oxygen to V,H-ZSM-5 prepared through SSIE led to a line broadening because of dipole-dipole interaction of VO(OH)+ and the paramagnetic O2 molecules. Adsorption of ammonia or pyridine significantly changed the ESR parameters and made the SHF-splitting completely disappear. Kucherov and Slinkin [190] ascribed this to an effect of

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Fig. 53. Schematic representation of the close proximity of a vanadyl cation to framework Al in a zeolite structure giving rise to the super-hyperfine splitting of the ESR signal shown in Fig. 52 (after [191], with permission)

the additionally coordinated strong ligands on the density distribution of the unpaired electron. An attempt was made to co-introduce Cu into the above-characterized VO(OH),H-ZSM-5 via SSIE by calcining a mixture of Cu compounds and the Vcontaining zeolite [93]. This, however, led to a decrease in the VO(OH)+ concentration as was indicated by a decrease in the intensity of the signals related to vanadyl species (cf. Fig. 54) and to the appearance of the ESR signal of isolated Cu+ in ZSM-5 (vide supra, Sect. 5.3.2.1). Obviously, Cu2+ is more strongly held in the zeolite structure than VO(OH)+ and is able to easily replace the vanadyl cation. In a similar way as that used by Kucherov and Slinkin [92, 191], Huang et al. [192–194] prepared vanadium-containing ZSM-5 and mordenite via SSIE between V2O5 and the zeolites mentioned above. The products were characterized by XRD, XPS, and determination of their surface areas. At variance with the interpretation given by Kucherov and Slinkin, Huang et al. [192] and Shan et al. [193] claimed from their ESR results that the vanadyl species existed in two different coordination structures both in ZSM-5 and mordenite, viz., in a compressed hexacoordinated octahedral and a pentacoordinated pyramidal symmetry. Furthermore, the authors studied the acidity of the zeolites after SSIE with V2O5 by FTIR of adsorbed pyridine and titration with butylamine in aprotic solution. In addition, the capacities for adsorption as well as the activities and selectivities for oxidation of toluene to benzaldehyde were investigated. The authors showed that the differences in adsorptive and catalytic behavior were related to the differences in acidity of the materials produced by SSIE [193]. In similar experiments, Ignatovych et al. [205] modified H-ZSM-5 and characterized the oxidation state, coordination and dispersion of vanadium in the products of the solid-state reaction by UV-Vis diffuse reflectance spectroscopy as well as photoluminescence spectroscopy. It was shown that the reaction conditions, i.e., calcination of, e.g., V2O5/NH4-ZSM-5 in (i) a flow of O2 at 973 K or (ii) N2 at 773 K, affected the properties of the products: in the case of (i) isolated VIV and tetrahedral VV were predominant, whereas in case (ii) mainly polyvanadates and clusters with VIV and VV valences were detected. Finally, it is worth mentioning that, in a zeolite in which Al had been isomorphously substituted by Ga, i.e., in H-[Ga]ZSM-5, vanadyl cations could not be

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Fig. 54. ESR spectra of a V2O5/H-ZSM-5 calcined in air at 1073 K for 1 h; b after calcination of sample (a) in a mixture with CuO at 823 K for 1 h; c after calcination of sample (b) at 1023 K for 1 h; and d after reduction of sample (c) with H2 at 400 K for 2 h (after [93], with permission)

stabilized [128, 129]. Thermal treatment of VO(NO3)2 with H-[Ga]ZSM-5 gave rise to only very weak ESR signals of isolated VIV species. Introduction of vanadium into AlPO4-5 was attempted by Whittington and Anderson [206] by grinding a mixture in air at room temperature and subsequent heating at 823 K for 2 h. The products were investigated by XRD and ESR. The results depended on the vanadium loading. For molar ratios of nV/(nV + nAl + nP) = 0.025 the AlPO4-5 structure remained intact, whereas, at higher ratios (0.039 and 0.089), collapse of the lattice and formation of new phases were observed. The ESR spectrum revealed that part of the V was incorporated as isolated vanadyl (= V = O) species and part of it as species with interacting VIV, most probably as some form of oxide. These findings essentially agreed with

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those obtained by Hong et al. [207] for vanadium-loaded AlPO4-5 prepared by impregnation of this aluminophosphate with aqueous solutions of ammonium vanadate and subsequent calcination at 803 K. The possibility of incorporating vanadium into the sodium form of Y-type zeolite, i.e., Na-Y, by solid-state reaction at 690–750 K in air was investigated by Marchal et al. [195, 196] and Thoret et al. [197]. XRD, TEM, 29Si MAS NMR, 129Xe NMR, and ESR techniques were used to study the solid-state reaction between V2O5 and Na-Y (nSi/nAl ª 2.5) and to characterize the products, in particular their crystallinity and phase composition. The authors found that only at low vanadium contents in the V2O5/Na-Y mixtures (i.e., nV/(nSi + nAl) £ 0.05 or nV/nAl £ 0.175) did introduction of V-containing species (V2O5) proceed without modification of the zeolite lattice and/or formation of amorphous phases.After reaction in such a system with nV/nAl = 0.175, ESR showed a signal, the hyperfine splitting of which was hardly visible because of the strong interactions between neighboring tetravalent vanadium ions. A sample with a lower concentration of vanadium was obtained by heating a mixture with a ratio of nV/nAl £ 0.018. This sample produced a spectrum exhibiting a much more resolved HF structure because of the decreased dipole-dipole interactions. The hyperfine splitting was further increased when the observation temperature was lowered from 293 to 77 K (Fig. 55).

Fig. 55. ESR spectrum obtained at 77 K after solid-state reaction in the mixture V2O5 /Na-Y at 690–750 K in air; nV/(nAl + nSi) = 0.005; numbers assigned to maxima and minima indicate the magnetic induction in 10–4 T (after [196], with permission)

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Then, the spectrum looked qualitatively like that reported by Kucherov and Slinkin for the product of a solid-state reaction between V2O5 and H-ZSM-5 (cf. Fig. 52), confirming the axial symmetry of the V4+ ions. However, the characteristic parameters were assumed to be different because of the different environments of the V-containing species introduced either into the fausasite-type (NaY) or MFI (H-ZSM-5) structure. It was claimed that at a somewhat higher loading, corresponding to nV/(nSi + nAl) = 0.2, XRD patterns showed the coexistence of three phases, viz., (i) NaV5VVIVO15, formed by a reaction of weakly acid V2O5 and Na+ of Na-Y, (ii) V2O5 , and (iii) slightly amorphitized Na-Y. The authors suggested that at even higher vanadium loadings, i.e., nV/(nSi + nAl) = 0.6 and T = 870 K, the lattice is completely destroyed and some of the sodium cations of NaY are involved in the formation of vanadium bronzes such as Na5V11VVIVO32 [195, 197]. Analysis of the XRD pattern of the (hydrated) materials after solid-state reaction showed an increase in the cubic unit cell parameter from 2.4639 to 2.4673 ± 0.0015 nm. This effect could originate from an isomorphous substitution of Si4+ by V4+ in the framework, a possibility that cannot be excluded, even though the authors did not obtain further experimental evidence for such a replacement. Narayanan and Sultana [208] as well as Narayanan and Deshpande [209] admixed V2O5 to H,Na-X, H-Y, H-ZSM-5 and H-MOR and subsequently conducted solid-state reaction. Compared to the parent hydrogen zeolites, considerably enhanced activity of the catalysts prepared via SSIE was observed in vapor-phase alkylation of aniline. Introduction of vanadium into microporous materials is also discussed in Sect. 6.3.1. 5.3.5.3 Introduction of Niobium and Antimony

Niobium-containing catalysts have attracted considerable interest because of their activity in reactions such as dehydrogenation of alcohols, photooxidation of propene, oxidative decomposition of methyl-tert-butyl ether and selective catalytic reduction of NO by NH3. Zeolites with niobium as an active component are difficult to prepare since niobium salts are rather sensitive to moisture. Therefore, Ziolek et al. [202] explored the possibility of introducing Nb into hydrogen forms of zeolites via solid-state reactions between Nb2O5 and NH4 , Na-Y, NH4-Y (nSi/nAl = 2.56) and dealuminated NH4-Y (NH4-Y(D), nSi/nAl = 4.25). Mixtures of Nb2O5 and zeolites with ratios of nNb/(nSi + nAl) = 0.03, 0.06 and 0.3 were heated at 975 K in air or in vacuum. For these investigations various techniques were employed, viz., high-temperature in situ XRD, IR, TPE/MS and TPD/MS. TPE/MS and TPD were used to monitor ammonia and water evolved during SSIE and to determine the acidity of the products, respectively. The authors found that SSIE indeed occurred with zeolite samples containing acid protons. Evidence was derived from the decrease in the intensity of the Nb2O5 reflections in the XRD pattern of the heat-treated Nb2O5 /zeolite mixtures and the evolution of water. Also, XRD proved that the crystallinity of the zeolite samples did not deteriorate upon SSIE. TPD of ammonia and IR spectroscopy

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showed a partial elimination of Brønsted acid sites and a concomitant formation of Lewis acid centers. The activity of the niobium-containing samples in Brønsted acid-catalyzed dehydration of isopropanol was distinctly lower than that of the parent zeolites, but introduction of Nb generated redox properties of the zeolite samples that were evidenced by the appearance of acetone among the products of isopropanol conversion. No incorporation of niobium was detected when Nb2O5 was mixed with the sodium form of Y-type zeolite and heat-treated at 975 K. However, in contrast to the findings of Marchal et al. [195, 196] for the case of the system V2O5/Na-Y at high vanadium loadings, no loss of crystallinity was observed with Nb2O5/Na-Y. Thoret et al. also studied the interaction of antimony oxide, Sb2O3, with Na-Y or La,Na-Y [210]. However, similar to their findings with the systems V2O5/Na-Y and MoO3/Na-Y, no real solid-state ion exchange was observed with Sb2O3/NaY. Rather, the oxide is inserted as such. In the case of Sb2O3/La,Na-Y, where the zeolite possesses residual Brønsted acid sites, the oxide can react in a certain range of Sb2O3 content and temperatures to produce Sb2O42+ cations. 5.3.5.4 Introduction of Chromium

There are several reports on solid-state ion exchange of chromium into zeolites [92, 93, 190, 198, 199]. Thus, Kucherov and Slinkin reacted chromium oxides (CrO3 , CuCrO4 , Cr2O3) with hydrogen forms of mordenite (nSi/nAl = 5) and ZSM-5 (nSi/nAl = 35 or nSi/nAl = 140) at 773–1093 K in air or vacuum. The proton-containing samples were H,Na-ZSM-5(40), H-ZSM-5(95), Na,H-MOR(50) and HZSM-5 (95), where the figures in brackets designate the degree of replacement of Na+ by protons, and the gallosilicate and ferrosilicate analogs of H-ZSM-5, i.e., H-[Ga]ZSM-5 and H-[Fe]ZSM-5. Solid-state reaction with Cr(VI) oxide was indicated by the appearance of ESR signals characteristic of isolated Cr(V)-containing ions [92].As an example, the ESR spectrum after SSIE of the system CrO3/H-ZSM-5 (nSi/nAl = 35) is shown in Fig. 56. The signal exhibited a typical hyperfine splitting (HFS). However, in the case of CrO3/H-ZSM-5 (nSi/nAl = 140), a particularly well-resolved splitting occurred with 15 components and a splitting constant of ca. 0.7 mT. This type of splitting was ascribed to the presence of 27Al located in close proximity to the Cr(V) cation (cf. also [211, 212]), and is analogous to the effect of super-hyperfine splitting (SHFS) observed in the system V2O5/H-ZSM-5 (vide supra). The ESR spectrum obtained after SSIE in the system CrO3 /H-ZSM-5 (cf. Fig. 56) was almost identical to that observed with an H-ZSM-5 sample loaded with 1.8 wt.% Cr via impregnation and calcined in air for 3 h at 773 K (cf. [213, 214]). This latter spectrum was characterized by g = 1.96 and 15 SHFS components and a splitting constant of 0.7 mT and ascribed to octahedrally coordinated Cr(V) interacting through its unpaired electron with the nuclear spin (I = 5/2) of the adjacent 27Al in the second coordination sphere of the Cr-containing ion. However, at higher loadings (2.5 wt.%) and calcination in air at 773 K for 1 h, both octahedrally and tetrahedrally coordinated Cr(V) cations were observed with g^1 = 1.98, g 1 = 1.95 and g^2 = 1.91, g 2 = 1.98, respectively. It is quite

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Fig. 56. ESR spectra obtained at 77 K a after solid-state reaction in the mixture CrO3/H-ZSM-5 at 823 K in air followed by evacuation at 293 K; b after calcination of the mixture at 1023 K followed by evacuation at 293 K; and c after admission of air (after [92], with permission)

probable that such coordinations occur with Cr,H-ZSM-5 samples obtained through SSIE as well, in view of the similarity of the ESR spectra of Cr,H-ZSM-5 obtained via solid-state reaction (vide supra) and impregnation. Calcination of CrO3/H-ZSM-5 under more severe conditions (e.g., at 1073 K instead of 823 K) resulted in an increase of the signal intensity (cf. spectrum a and b in Fig. 56). In addition, the intensities were affected by the number of available Brønsted acid sites. From a comparison of the number of spins per gram and the initial number of Brønsted acid sites as given in the report by Kucherov and Slinkin [92], one can deduce that about 3 and 15% of the protons were replaced by CrO+2 in the hydrogen form of ZSM-5 and mordenite samples, respectively. The spins measured per gram, i.e., the absolute number of introduced cations, increased with the number of Brønsted acid centers per gram, and the upper limit of the cation concentration was, indeed, determined by the concentration of these acid sites [92, 190]. However, in no case did the number of isolated Cr(V), i.e., CrO+2 cations located inside the zeolite channels, exeed 20% of the Al cations in the framework corresponding to a maximum of 20% of

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the available Brønsted acid centers. Excess chromium present after completion of SSIE was assumed to be located as compact Cr2O3 aggregates on the external surface of the zeolite crystallites [21, 128, 213, 214]. When, after solid-state reaction yielding the spectrum a or b in Fig. 56, the sample was contacted with air at 293 K, a considerable but reversible line broadening occurred (spectrum c in Fig. 56). Upon admission of water, the ESR signal completely disappeared as in similar cases (vide supra). These effects were taken as proof for the accessibility of the cations incorporated via SSIE. It is worth mentioning that heat-treatment of a mixture of CrO3 and H-MOR did not result in the appearance of an ESR signal with SHFS similar to that of Fig. 56b, whereas such a signal being indicative of SSIE was observed with the system CrO3/H,NaMOR. This was most likely due to the lower density of Brønsted acid sites and, consequently, Cr(V) species in H,Na-MOR. The higher dilution of chromiumcontaining species in H,Na-MOR in turn decreased the signal broadening due to dipole-dipole interaction and, thus, improved the resolution. Similar to their interpretation of the incorporation of V(IV) or Mo(V) cations into zeolites by solid-state reaction, Kucherov and Slinkin suggested that in highsilica zeolites the respective cation sites are too distant to allow a balance of the negative framework charges by bare Cr5+ cations. Rather, they proposed incorporation of Cr(V) in the form of complex cations such as CrO2+ under participation of framework ligands, in analogy to VO(OH)+ (vide supra) or MoCl4+ (vide infra). Co-introduction of Cr(V) and Cu(II) into H-ZSM-5 was possible by SSIE using either CuCrO4 in a one-step solid-state reaction or a consecutive reaction with CrO3 and CuO [93, 198]. In the former case, the ESR signals of Cr(V) (cf. Fig. 56) and Cu(II) (cf. Fig. 28) were superimposed, appeared simultaneously and exhibited well-resolved hyperfine splitting. The Cr- and Cu-containing species were randomly distributed over the same types of sites with negligible dipole-dipole interaction and exhibited the same coordinations as observed with the individual cations after SSIE in CrO3/H-ZSM-5 and CuO/HZSM-5. From the intensities of the signals obtained upon solid-state reaction of CuCrO4 and H-ZSM-5, a ratio of nCu(II)/nCr(V) = 2–3 was estimated. The preference for incorporation of Cu(II) was even more pronounced when Cr(V) and Cu(II) were successively introduced. When Cr,H-ZSM-5, prepared via SSIE of CrO3 and H-ZSM-5 at 1023 K, was subsequently mixed with CuO and calcined in air, the Cr signal dropped considerably and concomitantly the signal of Cu(II) developed. This replacement resulted in a ratio of nCu(II)/nCr(V) = 20–30. When the sequence of the two SSIE steps was reversed, only weak Cr(V) signals were detected. Upon reduction of the samples prepared by cointroduction of Cr(V) and Cu(II), the ESR signals of both cations were rapidly eliminated. There have been no successful attempts to react chromium oxides with sodium forms of ZSM-5 or MOR to incorporate chromium-containing cations. No solid-state ion exchange was indicated by ESR signals of stabilized isolated Cr(V) species similar to those observed upon the solid-state reaction of CrO3 with the hydrogen forms of ZSM-5 or mordenite (vide supra). Kucherov et al. [128, 129] carried out a comparative study of solid-state reaction between CrO3 and the gallosilicate analog of H-ZSM-5, i.e., H-[Ga]ZSM-5,

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Fig. 57. ESR spectra obtained at 293 K of the mixture CrO3/H-[Ga]ZSM-5 a after calcination at 693 K in air for 1 h followed by brief evacuation (5 min) at 293 K; b after calcination at 973 K in air for 2 h followed by brief evacuation (5 min) at 293 K; and c after admission of O2 at 293 K (after [129], with permission)

where via synthesis Al of H-ZSM-5 (i.e., H-[Al]ZSM-5) was isomorphously substituted by Ga. Samples with nSi/nGa = 35 were preferentially used. After calcination of a mixture of CrO3 and H-[Ga]ZSM-5 at 693 K, ESR spectra with two superimposed signals (g^ = 1.99, g = 1.94, A^ = 2.8 mT, A = 2.65 mT) and at 973 K with one signal (g = 1.97, A ª 2.4 mT) were observed (Fig. 57). These spectra indicated formation of isolated Cr(V) cations (CrO+2 ) through SSIE on cation sites of the [Ga]ZSM-5 structure. In analogy to the cases of V(IV)

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and Cr(V) introduced via SSIE into H-[Al]ZSM-5 (vide supra), super-hyperfine splitting with a splitting constant of 2.4–2.8 mT in Cr,H-[Ga]ZSM-5 was ascribed to an interaction of the unpaired electron of Cr(V) with the nuclear spin of closely adjacent Ga3+ (I = 3/2) ions of the framework located in the second coordination sphere of Cr(V). Indeed, the lower nuclear spin number (I = 3/2) of Ga compared to that of Al (I = 5/2) caused, according to the relationship Nrel = (2IGa + 1)/(2IAl + 1), a reduction in the number of components, of the split signal by a factor of 2/3, i.e., from 15 in the case of Cr,H-[Al]ZSM-5 to 10 in the case of Cr,[Ga]ZSM-5. The concomitant increase in the splitting constant from 0.7 to 2.4–2.8 mT is due to the greater radius of Ga3+ (0.062 nm) compared to that of Al3+ (0.05 nm). Admission of O2 to Cr,H-[Ga]ZSM-5 resulted in a considerable but reversible broadening of the ESR signal. Adsorption of NH3 on Cr,H[Ga]ZSM-5 caused an anisotropic spectrum to develop identical to that found after adsorption of NH3 on Cr,H-[Al]ZSM-5 (vide supra). The effects upon adsorption of O2 and NH3 showed that the Cr(V) species incorporated by SSIE into H-[Ga]ZSM-5 were coordinatively unsaturated and accessible to gaseous molecules. The most significant difference between the Cr-containing Al- and Ga-forms is, however, the lower thermal stability of Cr,H-[Ga]ZSM-5 which exhibited a rather rapid decrease in the concentration of isolated Cr(V) on cation positions already at 1073 K. The structure of H-[Fe]ZSM-5 was even less thermally stable than that of H[Ga]ZSM-5 and possessed even weaker acid sites [148, 151]. This zeolite was, therefore, not able to stabilize cations such as Cr5+ or Cu2+ (cf. Sect. 5.3.2.1). The behavior of Cr(III) oxide on heat-treatment in mixtures with hydrogen forms of zeolites was different from that of Cr(VI) oxide [190]. At 353 K, an uncalcined mixture of 1.5 wt.% Cr2O3 produced only a broad ESR signal which was identical to that of bulk Cr2O3 . Also, thermal treatment of Cr2O3/H-ZSM-5 or Cr2O3/H-MOR in vacuum did not give rise to the development of ESR signals of isolated chromium cations. However, when such mixtures were calcined in air at 1093 K, signals of isolated Cr(V) species appeared similar to those shown in Fig. 56. Obviously, Cr2O3 did not react in the solid state, most likely because of the high lattice energy of this compound (Tmelt = 2613 K). Calcination in air, however, produced oxide species of Cr with higher valency that were sufficiently mobile to penetrate into the channels of H-ZSM-5 or H-MOR and react there with the acid Brønsted sites. Weckhuysen and Schoonheydt [100, 199] conducted systematic studies of the preparation of Cr-containing zeolites. These authors employed diffuse reflectance UV spectroscopy (DRS) and ESR spectroscopy. For a comparative investigation, they loaded various zeolites (X, Y, [Ga]Y and MOR-type zeolites) not only by conventional ion exchange or impregnation-incipient wetness techniques but also via solid-state ion exchange with CrCl3 ◊ 6H2O. After calcination of differently prepared samples at 823 K, similar DRS spectra were obtained (cf. Fig. 58). These spectra showed two pronounced bands at 28,000 and 38,000 to 39,000 cm–1, which are typical of chromate-like species ascribed to charge transfer processes (O Æ Cr6+), and were assigned to 1t1 Æ 2e and 6t2 Æ 2e transitions, respectively. Spectrum c of Fig. 58 was obtained from a Cr,H-Y sample prepared

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Fig. 58. Diffuse reflectance spectra (DRS) of chromium-containing zeolites. a Cr-X obtained by conventional ion exchange (CE); b Cr-Y obtained by impregnation; and c Cr-Y obtained by solid-state ion exchange in the mixture CrCl3 ◊ 6 H2O and NH4-Y (H-Y); the samples were calcined at 823 K (after [100], with permission)

via solid-state ion exchange. It showed an additional weak band around 10,000 cm–1 possibly due to d-d transitions of Cr(III). Detailed analysis of the ESR spectra and decomposition of the DRS bands revealed that Cr was present in various oxidation states, viz., Cr(III), Cr(V) and Cr(VI). However, the ESR parameters found for Cr,H-Y obtained via SSIE (g^ = 1.99, g = 1.93, Cr5+ in supercages, site II; g^ = 1.98, g = 1.94, Cr5+ in b-cages, site I¢, cf. [199]) were similar to those reported by Kucherov and Slinkin for Cr,H-ZSM-5 (cf. [92, 190], vide supra) and by Hemidy et al. for Cr-Y [211, 212]. Similar to the interpretation given by Kucherov et al. [129], Weckhuysen and Schoonheydt assumed that Cr(V) species occurred inside the zeolite structure as complex cations such as CrO2+ coordinated to two framework oxygen atoms. Irrespective of the method of preparation, chromate-type species were formed upon calcination, i.e., this holds also for the zeolite exchanged through solid-state reaction. The authors assumed the following two-step process as the most probable pathway of chromate formation [cf. Eqs. (16) and (17)]: CrCl3 + 3 H+Z– Æ Cr3+Z–3 + 3 HCl ≠ Cr3+Z–3

+

3/ 4

O2 +

1/ 2

H2O Æ

(O2Cr=)2+Z–2

(16) +

H+Z–

Scheme 2. Coordination of a complex CrO2+ to two framework oxygen atoms

(17)

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The reaction (Scheme 2) results in the formation of two extra-framework oxygen atoms and anchoring of the chromate-like species to two structural oxygen ions (each bound to one framework Al) and regeneration of one Brønsted acid site, H+Z– [100]. An interesting result of the studies reported by these authors was that Cr,H-Y obtained through SSIE showed the lowest reducibility compared to samples prepared via conventional ion exchange or the impregnation-incipient wetness technique. This behavior was attributed to residual acidity of Cr-containing Y-zeolite produced by solid-state reaction between CrCl3 ◊ 6H2O and NH4-Y. Chromium was also introduced by SSIE into SAPO-11 by Kevan and co-workers [215], and the products were investigated by ESR, ESEM and UV-Vis spectroscopy. The Cr-containing SAPO-11 resulting from SSIE showed Cr(V) in square-pyramidal coordination that could be reduced by H2 to Cr(III). 5.3.5.5 Introduction of Molybdenum

Introduction of molybdenum into hydrogen forms of zeolites by SSIE was investigated by Dai and Lunsford [200], Kucherov and Slinkin [92, 190], Bock [44] and Bock et al. [179]. Mo-containing compounds, such as MoCl5 , (MoOCl4), MoCl3 and MoO3 , were mixed with H-ZSM-5, H-MOR, H-ZSM-35, H-EU-1, H-ZSM-48 and H-L and subsequently subjected to heat-treatment. Solid-state ion exchange with Mo compounds was monitored, and the respective SSIE products were characterized by ESR, XPS, IR, XRD, CA, TPE/MS and titration of evolved HCl. As mentioned above, the earliest experiments of solid-state ion exchange between MoOCl4 and H-Y were those reported by Dai and Lunsford [200]. These studies were occasioned by the interest in Mo-containing solid catalysts because of their possible applications as redox catalysts in, e.g., epoxidation of cyclohexene. MoCl5 (MoOCl4) was reacted with NH4-Y (deammoniated during SSIE to HY), dealuminated, ultrastabilized NH4-Y(D) (deammoniated during SSIE to HY(D)) and Co,H-Y (obtained by impregnation of NH4-Y with Co(NO3)2 and deammoniated during SSIE to Co,H-Y). Since the preparation of the MoCl5/zeolite mixtures was carried out in the presence of ambient air and moisture, the authors assumed that most of the Mo compound was, prior to SSIE, converted to MoOCl4 . From ESR and XRD experiments they concluded that Mo species indeed entered the interior of the zeolite structure upon solid-solid reaction. Differing from the results reported by Kucherov and Slinkin [92, 190] (vide infra), XPS data showed that Mo cations were mainly present as Mo(VI) species, whereas the results of ESR spectroscopy suggested that only about 3.5% of the incorporated molybdenum was in the form of Mo(V). Because the ESR signal was affected by adsorption of O2 and N2O molecules, which cannot enter the small cavities, the authors concluded that a fraction of the Mo-containing species were located in the supercages. This was supported by their finding that twice as many MoOCl4 molecules reacted as the number of OH groups that were available on the external surface of the zeolite crystallites, so that at least part of these entities must have migrated into the interior of the structure. Moreover, from their XRD data, they drew the conclusion that these charge-compensating

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Mo species resided on sites SII. Back-titration of HCl evolved during solid-state ion exchange with MoCl5 (or, in fact, MoOCl4) revealed that four Cl– anions per Mo species had reacted. This would be compatible with either the involvement of four hydroxyl groups (silanol groups and/or Brønsted acid groups) or of four water molecules, since ambient moisture was not excluded, according to Eqs. (18) and (19). MoVIOCl4 + 4 H+Z– Æ [O=MoVI ∫Z–3]+Z– + 4 HCl≠

(18)

MoVIO Cl4 + 4 H2O Æ MoVIO(OH)4 + 4 HCl≠

(19)

where Z– is a negatively charged fragment of the zeolite framework. After heat-treatment at 873 K of mixtures of MoCl5 (MoOCl4) and H-Y or Co,H-Y a new band at 900 cm–1 appeared in the mid-infrared spectra. This band was assigned to a Mo = O bond vibration. It was missing when mixtures of MoCl5 (MoOCl4) and H-Y(D) were heat-treated. Therefore, Dai and Lunsford proposed two model structures for Mo-containing cations in the solid-solid exchanged zeolites, viz., structure I for Mo,H-Y and Mo,Co,H-Y and model II for Co,H-Y(D):

XRD, IR of lattice vibrations and O2 adsorption experiments provided evidence for a partial loss of crystallinity which occurred upon SSIE with MOCl4 , increasing in the sequence Mo,H-Y(D)<Mo,Co,H-Y ª Mo,H-Y. Possible reasons for the degradation in crystal integrity were seen in a partial dehydroxylation of the above hydrogen forms of Y-zeolite followed by rehydration in ambient air as well as in the action of acidity produced in the exchange reaction with MoOCl4 . At variance with the experiments by Dai and Lunsford [200], Kucherov and Slinkin [92, 190] used an experimental procedure which enabled them to exclude ambient air and water vapor. This might explain most of the differences in their results, in particular with respect to the ESR signals and oxidation state of the incorporated Mo cations. After heating at 423 K an evacuated ampoule containing MoCl5/H-ZSM-5, Kucherov and Slinkin observed a color change of the sample to red-violet and the development of the strong ESR signal shown in Fig. 59, indicating uptake of Mo-containing species. The above spectrum is typical of isolated Mo(V) cations. It exhibits a splitting constant of 0.8 mT that is ascribed to the presence of 95Mo and 97Mo (spin quantum number I = 5/2). A maximum of 4 ¥ 1020 spins per gram was reached, which corresponded to about 100% replacement of protons from the acid Brønsted sites of the H-ZSM-5 sample. Increasing the temperature of solid-state reaction to 623 K caused sublimation of excess MoCl5 and a color change to pale yellow. However, the intensity of the ESR signal shown in Fig. 59 did not decrease. This finding suggested that the Mo-containing cations were relatively strongly held in the zeolite structure. At elevated temperatures the cations were stable only in

Solid-State Ion Exchange in Microporous and Mesoporous Materials

149

Fig. 59. ESR spectra (obtained at 293 K) after solid-state reaction in the mixture MoCl5 / H-ZSM-5 a on calcination in vacuum at 423 K and b upon admission of air to sample (a) (after [23, 190], with permission)

vacuum. Oxidation at 723–823 K made the Mo(V) ESR signals irreversibly disappear. A spectrum similar to that shown in Fig. 59 was obtained when H-MOR was used instead of H-ZSM-5. In analogy to the interpretation given for the incorporation of polyvalent cations such as vanadium or chromium cations (vide supra), in the case of SSIE with Mo compounds it was also assumed that not bare Mo5+ but complex cations such as MoCl4+ were the charge-compensating species inside the zeolite structures (cf. [92, 190]). In a systematic study on incorporation by SSIE of molybdenum into hydrogen forms of narrow- or medium-pore zeolites, Bock [44] and Bock et al. [179] investigated the effects of the nSi/nAl ratio, size of the zeolite crystallites, diameter of the pores and dimensionalities of the pore systems of zeolites on their solid-state reaction with MoCl3 . As parameters characterizing the reaction, the temperatures of onset (Tstart) and completion (Tend) of the process were determined by temperature-programmed evolution of the gases (HCl, H2O) released via SSIE. Thus, rather than steady-state experiments,dynamic experiments with a heating rate of 5 K/min in the range 773–1073 K were carried out and, consequently, the kinetics of SSIE likely affected the parameters mentioned above. Bock [44] and Bock et al. [179] obtained the following results: 1. The acid strength of the H-ZSM-5 samples used (characterized by TPD of ammonia) decreased somewhat when the nSi/nAl ratio increased from 15 to 110. In the same sequence Tstart decreased from an average value of 725 K to 697 K. This was surprising, since higher Brønsted acid strength was expected to facilitate SSIE. However, this converse effect was ascribed to the decrease in extra-framework Al-containing species with increasing nSi/nAl ratio as measured by 27Al MAS NMR. Extra-framework species were visualized as barriers for the diffusion of the in-going entities. This would significantly affect the kinetics of SSIE. 2. Tstart and Tend decreased with decreasing crystallite size (cf. Table 12). Again, this effect was attributed to the influence of diffusion. Tstart and Tend were

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affected by both the heating rate and rate of uptake of the in-going Mo-containing species. Thus, at a constant heating rate, one would expect an effect of the diffusion pathways. Since larger crystallites present longer diffusion pathways, higher temperatures were reached before measurable amounts of HCl were evolved indicating the onset of the reaction (Tstart) and, similarly, the evolution of HCl ceased only at higher temperatures indicating the completion of solid-state reaction (Tend). 3. A significant influence of the pore architecture of the zeolites on SSIE existed with MoCl3 . The effect of pore width and/or dimensionality is illustrated by the data compiled in Tables 12 and 13. They were obtained with the following zeolites: ZSM-35 (structure of ferrierite, FER, with intersecting pores or channels built from 8- and 10-membered rings), EU-1 (one-dimensional pore system with channels of 10-membered rings and large side-pockets), ZSM-48 (one-dimensional pore system of channels with 10-membered rings), and L (one-dimensional pore system with channels of somewhat narrowed 12membered rings; cf. [180]). As expected, solid-state ion exchange was facilitated, i.e., Tstart and Tend decreased when the pore size increased (Table 13).An exception seemed to be the case of cylindrical crystallites of the L-type. However, this is most likely due to the increased diffusion pathway along the axes of the cylinders compared to the disk-shaped L-crystallites. A striking result of these investigations were the considerably increased values of Tstart and Tend observed with the one-dimensional channel systems (Table 13) compared to those of the three-dimensional channel system of H-ZSM-5 (cf. Table 12). Obviously, the uptake of the in-going Mo-containing species proceeds more rapidly into pore systems of higher dimensionality. Table 12. Effect of crystallite size on (diameter, d) on the temperature of on-set and completion of solid-state ion exchange between MoCl3 and H-ZSM-5

dcrystallite (µm)

nSi/nAl

Tstart , SSIE (K)

Tend , SSIE (K)

nCl–, out/nCl–, in

10 2 Prevailing 1 (up to 3)

110 110 30

550 499 479

1065 1021 1006

0.65 0.66 0.66

Table 13. Effect of the pore size on the temperature of on-set of solid-state ion exchange between MoCl3 and H-ZSM-5

Zeolite

nSi/nAl

Pore size (nm ¥ nm)

Tstart , SSIE (K)

nCl–, out/nCl–, in

H-ZSM-35 H-EU H-ZSM-48 Zeolite L, disk-shaped Zeolite L, cylindrical

8 15 40 3 3

0.42 ¥ 0.54 0.41 ¥ 0.57 0.53 ¥ 0.56 0.71 ¥ 0.71 0.71 ¥ 0.71

668 631 601 559 653

0.53 0.33 0.65 0.53 0.54

Solid-State Ion Exchange in Microporous and Mesoporous Materials

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Dai and Lunsford [200] as well as Kucherov and Slinkin [92, 190] hypothesized that cations from molybdenum chlorides were not exchanged in solid state as bare Mo6+ or Mo5+ cations but as complex cations, e.g., as [MoOCl2]+ and [MoCl4]+. One of the most important results of the work by Bock [44] and Bock et al. [179] is that this hypothesis was elegantly confirmed for the systems MoCl3/H-zeolites by measuring the numbers of in-going (nCl-, in) and out-going (nCl-, out) Cl– anions. In these experiments the disturbing effect of ambient water vapor (vide supra) was excluded.According to Tables 12 and 13, reaction of MoCl3 (or its dimer) with H-ZSM-5 resulted in an evolution of HCl corresponding to two-thirds of the introduced Cl– anions. This result supports the proposition that not Mo3+ but [Mo2Cl2]4+ or [MoCl]2+cations were incorporated. The chemistry is somewhat more complicated in the cases of H-ZSM-35, H-L and H-EU-1. Here, perhaps only one-half or one-third of MoCl3 (or Mo2Cl6) reacted to form [Mo2Cl3]3+ and [Mo2Cl4]2+, respectively, which reside on cation sites of the zeolite structure. The assumption that, upon solid-state reaction of molybdenum chlorides with hydrogen forms of zeolites, Cl-containing complex cations were generated inside the zeolite pore systems was further supported by EDX analysis after solid-state reaction between MoCl3 and H-ZSM-5. This analysis clearly proved the presence of chlorine in the materials obtained via SSIE between MoCl3 and H-ZSM-5. Many experiments aimed at the introduction of molybdenum via SSIE of a mixture of molybdenum oxide (MoO3) and H-ZSM-5 were unsuccessful, even though MoO3 is volatile and has a relatively low lattice energy (Tm = 1068 K). Kucherov and Slinkin [92, 190] ascribed the failure of this experiment to the fact that MoO3 easily forms large polymeric aggregates that would be unable to enter the zeolite pores. Reduction of precalcined MoO3/H-ZSM-5 mixtures by H2 at 573–673 K produced ESR signals similar to that shown in Fig. 59 but of very low intensity [92]. Yuan et al. [201] and Wang et al. [216] reported that the reaction between MoO3 and H-Y was significantly facilitated by the presence of water vapor. Such a modification of solid-state ion exchange, however, will be dealt with in Sect. 6.3.2. In contrast to the reports by Kucherov and Slinkin [92, 190], Harris et al. [217] claimed to have successfully introduced Mo-containing cationic species via SSIE between MoO3 and H-ZSM-5. At low loadings (£8 wt.%), MoO3 appeared to be highly dispersed on both the internal and external surfaces of the zeolite crystallites, where the oxide species reacted with bridging OH groups (acid Brønsted centers) and terminating silanol groups, respectively. This was deduced from the reduction in the band intensities around 3601 cm–1 (by 70%) and 3740 cm–1. Moreover, in the mid-infrared region bands appeared around 915 and 950 cm–1. The former was seen as indicative of Mo = O stretching vibrations of incorporated cationic Mo-containing species such as:

The band at 950 cm–1 was assigned to T–O–T vibrations. XPS results showed an essentially symmetric doublet originating from Mo(3d5/2) and Mo(3d3/2)

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electrons with binding energies typical of Mo(VI) species. However, ESR spectra revealed that, in addition, a small number of Mo(V) species had formed upon SSIE between MoO3 and H-ZSM-5. In their study on the genesis of methane-activating sites in Mo-exchanged HZSM-5, Borry et al. [218] and Kim et al. [219] prepared the zeolite catalysts by reacting dry solid mixtures of MoO3 and H-ZSM-5 (nSi/nAl = 14.3) powders at 973 K in air. This post-synthesis modification was preferred by the authors over the conventional technique of impregnation with ammonium hexamolybdate because it eliminated the evolution of large amounts of N2 , NH3 , and H2O during subsequent calcination in air. Moreover, it allowed accurate measurements of the nature and rate of MoO3 exchange. Techniques employed to confirm SSIE in the system MoO3/H-ZSM-5 were XRD, CA by AAS, N2 physisorption at 77 K, and TPE of H2O and D2-OH exchange for the determination of remaining acid Brønsted OH groups. Mixtures of MoO3 and H-ZSM-5 with Mo contents below 4 wt.% exhibited spreading of Mo oxide to produce MoOx species on the external surfaces of the zeolite crystallites. These species gradually migrated into the HZSM-5 channels and formed anchored [MoOx]n complexes. More specifically, from the result of TPE of H2O and D2-OH it was derived that each Mo6+ replaced 1.2 (±0.1) protons in H-ZSM-5 as long as nMo/nAl £ 0.37. 27Al MAS NMR, X-ray absorption and Raman spectroscopy (as previously reported [218, 220]) confirmed that, during heat-treatment of the MoO3/H-ZSM-5 mixture, isolated ditetrahedral [Mo2O7]2– dimers formed, which contained framework oxygens associated with two Al sites and were located at exchange centers according to Eq. (20):

(20) Thus, Mo,H-ZSM-5 samples prepared via SSIE proved to be catalyst precursors which, after activation (reduction, carburization), activated CH4 molecules and produced hydrocarbons such as C2H4 , C2H6 , C3H6 , C3H8 , benzene, toluene, and naphthalene. At higher Mo contents, however, (inactive) aluminum molybdates, Al2(MoO4)3 , formed by extracting framework Al that led to a loss of crystallinity. No real solid-state ion exchange was observed by Thoret et al. [197], who used mixtures of molybdenum oxide, MoO3, and sodium forms of zeolites. Similar to what was found by Marchal et al. [196] with the system V2O5/Na-Y, upon reaction of MoO3 with Na-Y at low temperature (790 K) and low loading corresponding to nMo/(nSi + nAl) = 0.075, molybdenum trioxide seemed to be incorporated as such into the porous zeolite structure. This was indicated by an increase in the lattice parameter ao from 2.4649 to 2.4673 nm. For higher load-

Solid-State Ion Exchange in Microporous and Mesoporous Materials

153

ings, i.e., nMo/(nSi + nAl) = 0.125, and higher temperatures (above 750 K), Thoret et al. observed increasing amorphization and finally total destruction of the Na-Y lattice accompanied by the appearance of several new crystalline phases. Harris et al. (vide supra, [217]) also did not observe solid-state ion exchange in the system MoO3/Na-ZSM-5. In particular, no bands at 915 and 950 cm–1 developed upon heat-treatment of the MoO3/Na-ZSM-5 mixture, i.e., no zeolite-anchored MoOx species exhibiting Mo=O stretching vibrations formed. 5.3.5.6 Introduction of Tungsten

As Thoret et al. [197] observed, even tungsten trioxide, WO3 , was unable to migrate into the Na-Y structure. Thus, the initial solid phases in a WO3/Na-Y mixture remained unaffected by heating and coexisted up to 850 K. This was ascribed to the relatively high lattice energy of WO3 (Tm = 1746 K), which was supported by the behavior of oxides having properties similar to those of WO3 . Thus, it was observed that ThO2, UO2, Nb2O5, and Ta2O3 did not enter the pore system of Na-Y whatever the temperature and the composition of the mixture. With respect to Nb2O5/Na-Y, however, this system should be compared with Nb2O5/NH4-Y (vide supra, cf. Sect. 5.3.5.2). It seems possible that in the case of acid Y-zeolite a penetration of WO3 into the structure would occur. In any event, upon heating a mixture of WO3 and Na-Y beyond 850 K and at a loading corresponding to nW/(nSi + nAl) ≥ 0.05, the zeolite lattice was distroyed. 5.4 SSIE of Noble Metal Compounds with Zeolites 5.4.1 Introductory Remarks

A particularly important post-synthesis modification of zeolites is that of ion exchange with noble metal compounds. Zeolites modified this way are, after reduction, frequently employed as catalysts for hydrogenation/dehydrogenation of, e.g., unsaturated hydrocarbons. Introduction of noble metals into zeolite structures can be achieved not only by conventional methods, e.g., ion exchange in aqueous solution or by the incipient-wetness technique (cf. Sect. 6.1), but also by solid-state ion exchange, usually between noble metal halides or oxides and hydrogen forms of zeolites. In fact, SSIE is in several cases the only way to carry out post-synthesis modification of zeolites by incorporation of noble metal cations. Residual Brønsted acidity, which is formed on reduction of the noble metal cations incorporated through SSIE, may be eliminated by subsequent SSIE with alkaline metal compounds (cf. Sect. 5.1). However, for some categories of reactions such as hydroisomerization or hydrocracking of hydrocarbons, both hydrogenation/dehydrogenation and acidic functions are required. The respective bifunctional catalysts may be produced via SSIE as well. Thus, Zhang [221] and Karge et al. [222–224] were the first to prepare noble-metal-containing zeo-

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lite catalysts via solid-state ion exchange in their study aimed at formulating bifunctional catalysts (vide infra). 5.4.2 Preparation of Noble-Metal-Containing Large-Pore Zeolite Catalysts by SSIE

Palladium- and platinum-containing zeolites were prepared by Zhang [221] who reacted successfully PdCl2, Pd(NO3)2 , PdO, PtCl2 , PtCl4 and PtO2 with NH4-Y or H-ZSM-5 in experiments that provided the groundwork for the production of bifunctional catalysts by SSIE (cf. Sect. 5.4.4). Hatje et al. [40, 225] investigated solid-state ion exchange of platinum oxide, chloride and bromide with NH4-Y. The process was monitored by TPR, XRD and, particularly interestingly, by dispersive extended X-ray absorption fine structure (DEXAFS) measurements and time-resolved XAS. It was found that in the presence of oxygen reduction was inhibited but, starting at 555 K, the Pt-Cl shell at 0.175 nm was replaced by a Pt-O shell at somewhat lower distances (0.150 nm). The reflections of PtCl2 disappeared (e.g., at 2Q = 12.85°). Concomitantly, the (111) and (220) reflections increased, mainly due to the occupation of cation sites by Pt2+. In the absence of air, PtO and PtCl2 were reduced at approximately 573 K to Pt0 under formation of highly dispersed Pt clusters. In the case of reacting PtBr2 it was shown that some bromide remained in the zeolite matrix. Thus, from their combined experiments, the authors obtained evidence that the salt or oxide components migrated into the zeolite pores before reduction occurred provided the exchange was conducted in atmospheric air. In contrast, when the reaction was carried out in a flow of helium, the Pt cations were reduced by NH3 to Pt0, forming clusters which could be characterized by EXAFS. Mkombe et al. [226] investigated the dispersion of Pt0 on K-L prepared by various techniques of loading and calcination. They reported that at a level of 1.5 wt.% Pt, different loading techniques, viz., ion exchange in aqueous solution, incipient-wetness impregnation and solid-state ion exchange, resulted in similar Pt dispersions. The dispersion was well correlated to the n-hexane conversion in the aromatization reaction. Solid-state ion exchange of rhodium chloride with the hydrogen form of highly dealuminated Y-zeolite (DAY, nSi/nAl ª 300) was studied by Schlegel et al. [227]. These authors used FTIR to monitor solid-state reaction through the decrease in the intensities of the OH bands at 3631 cm–1 (HF band) and 3568 cm–1 (LF band). In vacuum and oxygen, exchange degrees of only about 25% were obtained as indicated by the weakening of both the HF and LF bands. Because of the low Al content of DAY, introduction of complex Rh cations (vide supra) was proposed according to the following chemistry: (21) [∫Si–(OH)–Al∫] + RhCl Æ [∫Si–(O)–Al∫]– RhCl+ + HCl ≠ 3

[∫Si–(OH)–Al∫] + RhCl3 + 1/2O2 Æ [∫Si–(O)–Al∫]– RhO+ + HCl ≠ + Cl2 ≠

2

(22)

After adsorption of CO, IR bands at 2118 and 2053 cm–1 appeared which were assigned to well-defined Rh(CO)+2 complexes. Simultaneously, the exchange

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Solid-State Ion Exchange in Microporous and Mesoporous Materials

degree increased. The effect of CO on DAY materials modified by SSIE with RhCl3 as described above prompted Schlegel et al. [227] to conduct the solidstate reaction itself in the presence of CO. This will be dealt with in Sect. 6.3.2 on exchange processes related to solid-state ion exchange. Wasowicz and Kevan [228] showed that SAPO-11 has only a low exchange capacity for Rh(III) ions in aqueous solution, whereas via SSIE significantly higher loadings can be achieved, which enabled the authors to detect and study by ESR the stabilized paramagnetic Rh species in SAPO-11. Rh(II) and Rh(I) species were observed which were analogous to those found earlier in X zeolite. 5.4.3 Preparation of Noble-Metal-Containing Narrow-Pore Zeolites by SSIE

With respect to small-pore zeolites, especially those in which the pore mouths are formed by eight-membered oxygen rings (8-MR openings), conventional ion exchange in aqueous solutions of noble metal salts usually fails or provides only low degrees of ion exchange. This is ascribed to geometric constraints: Frequently, the (solvated) cations or complexes such as Pt(NH3)2+ 4 are not able to penetrate the narrow-pore openings. In some cases these difficulties in preparing the desired metal-containing 8-MR zeolites may be circumvented by adding the respective metal compound to the synthesis gel [229, 230].Application of this method is, however, limited because of possible effects of the metal cations on the crystallization process. Solid-state ion exchange offers an alternative route and is frequently the only way for a post-synthesis modification of zeolites by incorporation of noble metals. Systematic studies of the introduction of noble metals (Pt, Pd, Rh) into narrow-pore (8-MR) zeolites have been conducted by Bock [44] and Weitkamp et al. [45, 231]. Zeolites and zeolite-like materials used in the above studies and some of their relevant properties are listed in Table 14. PtCl2 , PdCl2 , PdO and RhCl3 were employed for SSIE. Ground mixtures of the salts and zeolites with a composition such as to achieve a metal loading of 1 wt.% in the case of complete solid-state reaction were heated in a flow of heliTable 14. Properties of narrow-pore zeolites and zeolite-like materials used in solid-state ion exchange with noble metal salts

Zeolite

H-RHO H-ZK5 H-ZSM-58 H-ALPHA H-SAPO-42 a b c

nSi/nAl

3.0 2.2 30; 50; 100 2.5 nAl/nP; nSi/(nAl + nP) 2.3; 1.0

Pore structure

Pore size (nm¥nm)

Ring type a

Dimensionality b

8-MRc 8-MR 8-MR 8-MR

3 3 1 3

0.36 ¥ 0.36 0.39 ¥ 0.39 0.36 ¥ 0.44 0.41 ¥ 0.41

8-MR

3

0.41 ¥ 0.41

Type of oxygen rings forming the channels of the structure. Dimensionality of the channel system. Eight-membered oxygen rings of the framework.

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H.G. Karge · H.K. Beyer

um or nitrogen at 553–623 K. Before conducting a reaction over the exchanged materials, they were dried at 393 K and reduced at 573 K in a current of hydrogen at atmospheric pressure with a typical flow rate of 15 cm3 min–1. The SSIE process and the materials obtained were characterized by measurements of HCl evolved, the temperatures of the onset of the solid-state reaction and, as a test reaction, hydrogenation of a mixture of hex-1-ene and 2,2,4-trimethylpent-1ene. Prior to the hydrogenation experiments, any residual Brønsted acidity of the catalyst was eliminated by poisoning through chemisorption of NH3 . Thus, any effect of a competing acid-catalyzed double-bond isomerization was avoided. Results are presented in Table 15 and in Fig. 60. The effects of (i) the nSi/nAl ratio of the zeolite, (ii) the lattice energies of the metal compounds, (iii) the ion-pair sizes of the metal compounds, and (iv) the pore width of the zeolites were considered. From Table 15 it is evident that SSIE did occur in all of the investigated systems. This was indicated by evolution of HCl except for ALPHA/PtCl2 and H-SAPO-42, where, above 705 and 773 K, respectively decomposition of PtCl2 under evolution of Cl2 was observed. Furthermore, the ratio of nCl–,in/nCl–,out was in almost all cases close to 1, showing that indeed bare Pt2+, Pd2+ or Rh3+ cations were incorporated. For the systems PtCl2/H-ZSM-58 and PdCl2/H-ZSM-58, Tstart increased with increasing nSi/nAl ratio (= 35, 50, 100) in the sequence 598, 614, 636 K and 668, 682, 704 K, respectively. Bock [44] and Weitkamp et al. [45] tentatively ascribed this behavior to a decrease in the driving force for solid-state reaction with increasing nSi/nAl ratio: A lower Al content corresponded to a lower number of extraframe-work cation sites and, therefore, to a lower metTable 15. Introduction of noble metals into narrow pore zeolites via solid-state ion exchange Zeolite

nSi /nAl

Salt

EL of salt (kJ mol–1)

Tstart (K)

nCl–, out /nCl–, in

H-RHO

3.0

PtCl2 PdCl2

2819 2778

639 780

1.01 0.97

H-ZK-5

2.2

PtCl2 PdCl2 RhCl3

2819 2778 5644

677 749 891

1.0 1.0 1.0

H-ALPHA

2.5

PtCl2 PdCl2 RhCl3

2819 2778 5644

–a 738 864

–a 1.01 0.97

PtCl2 PtCl2 PtCl2 PdCl2 PdCl2 PdCl2

2819 2819 2819 2778 2778 2778

589 614 636 668 682 704

0.98 0.98 0.98 1.06 1.06 1.06

PtCl2 PdCl2 RhCl3

2819 2778 5644

–b 776 867

–b 0.6 0.9

H-ZSM-58

H-SAPO-42

a b

30 50 100 30 50 100 nAl/nP; nSi /(nAl + nP) 2.3; 1.0

Decomposition of PtCl2 at ca. 700 K. Decomposition of PtCl2 at ca. 773 K.

Solid-State Ion Exchange in Microporous and Mesoporous Materials

157

Fig. 60. Time-on-stream behavior of palladium-containing microporous catalysts in the competitive hydrogenation of an equimolar mixture of hex-1-ene and 2,4,4,-trimethylpent-1-ene at 343 K and W/Falkenes = 10 g h/mol (after [45], with permission)

al concentration on the surface and a smaller concentration gradient for the ingoing noble metal cations, respectively. In general, one would expect that Tstart increases with increasing lattice energies of the metal compounds, i.e., in the sequence PdCl2
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with hydrogen forms of narrow-pore zeolites and zeolite-like materials such as HSAPO-42 proved to be a valuable route for the preparation of shape-selective hydrogenation catalysts. 5.4.4 Preparation of Bifunctional Zeolite Catalysts by SSIE

Zhang [221] and Karge et al. [222–224] prepared via SSIE bifunctional zeolite catalysts which possessed both a hydrogenation/dehydrogenation and an acid function, i.e., noble metal aggregates and Brønsted acid OH groups, respectively. The following mixtures were used for SSIE: PdCl2/H-ZSM-5; CaCl2, PdCl2/HZSM-5; LaCl3, PdCl2/H-ZSM-5; PdO/H-ZSM-5; Pd(NO3)2/H-ZSM-5; PtCl2/NH4Y and PtCl4/NH4-Y. Solid-state reactions were carried out by heating (10 K/min) the finely dispersed mixtures in high vacuum (10–5 Pa) to the desired reaction temperature of 625 K for 2 h. The reactions were monitored by in situ IR and TPE of evolved HCl. Solid-state ion exchange was successful in all systems indicated above. For several mixtures the data in Table 16 show the decrease in intensity (relative adsorbance, Arel) of the OH stretching band around 3605 cm–1 which is typical of Brønsted acid OH groups in H-ZSM-5. XRD patterns as well as reexchange experiments with aqueous NH4Cl solutions confirmed that no loss of crystallinity occurred upon SSIE (see Table 16). TPE of HCl evolved and subsequent titration of trapped HCl during solidstate reaction between PdCl2 and H-ZSM-5 is illustrated by Fig. 61. The experiment shown in Fig. 61 demonstrates that no complete exchange of Pd2+ for the protons of the Brønsted sites was achieved; rather an exchange degree of about 64% was reached. This was ascribed to the difficulty of establishing a full balance of the negative framework charges by bivalent cations in ZSM-5, if the distances between these charges are relatively long because of a low Al content such as in the sample used (nSi/nAl = 33.7). The Pd,H-ZSM-5 materials obtained via SSIE had a Pd content of 1 wt.% and were subsequently reduced in a flow of H2 (575 K, 3 h, 60 ml H2/min). After reduction, the catalysts were tested for hydrogenation of ethylbenzene and dehydrogenation of ethylcyclohexane. It turned out that, with respect to conversion of ethylbenzene, even in the presence of H2, acid-catalyzed dealkylation and disproportionation of ethylbenzene predominated. Main products were light paraffins, benzene and diethylbenzenes. Only minor fractions of ethylcyclohexane and dimethylcyclohexanes were detected. These results seemed to indicate that the acid and hydrogenation functions were not properly balanced and larger palladium particles had formed at the Table 16. Relative absorbance, Arel , of the OH band at 3610 cm–1 and ion exchange capacity for NH4+ before and after solid-state ion exchange

Sample

Relative absorbance, Arel , (%) NH4+ ion exchange capacity (mmol g–1)

Parent zeolite H-ZSM-5 100 0.37

Mixture of H-ZSM-5 with PdCl2

CaCl2

PdO

Pd(NO3)2

42 0.36

37 0.36

55 40 0.39 0.34

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Fig. 61. Titration of HCl evolved upon temperature-programmed heating of the mixture PdCl2/H-ZSM-5 (after [223], with permission)

external surface of the zeolite crystallites. This was confirmed by TEM images. Earlier work had shown that SSIE of alkaline earth and lanthanum chlorides with hydrogen forms of zeolites was able to reduce the acidity of the zeolite (cf. Sects. 5.1 and 5.3). In fact, solid-state ion exchange of H-ZSM-5 with increasing amounts of, e.g., CaCl2 , reduced the density of Brønsted acid OH groups as indicated by the decrease of the intensity of the OH band at 3605 cm–1, A(OH), and the decrease in the conversion in ethylbenzene disproportionation, X(EB). How– ever, temperature-programmed desorption of NH3 (Tmax , Ed and microcalorimetry of NH3 adsorption (DHad) revealed that the acid strength remained essentially unaffected (cf. Table 17). Thus, to reduce the density of Brønsted acid sites in the bifunctional zeolite catalysts,introduction of both Ca2+ and Pd2+ was carried out,and this was done either simultaneously (method A) or successively (method B). Best results were obtained by method B.As an example, Fig. 62 shows the conversion of ethylbenzene and the yields of ethylcyclohexane, xylenes, dimethylcyclohexanes, alkanes, benzene and diethylbenzenes over a Pd,Ca,H-ZSM-5 catalyst prepared by a two-step solid-state ion exchange. In a first step, CaCl2 was incorporated via SSIE, followed by a second step, viz., SSIE of PdCl2 into Ca,H-ZSM-5 obtained in the first step. Dealkylation (or hydrogenolysis) of ethylbenzene was largely and disproportionation almost completely suppressed. Similarly, dehydrogenation of cyclohexane over Pd,Ca,H-ZSM-5 prepared via successive SSIE proceeded with high selectivity and very low catalyst deactivation (cf. Fig. 63).

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Table 17. Acidic properties of H-ZSM-5 and Ca,H-ZSM-5 samples obtained by solid-state ion exchange.A(OH), maximum absorbance of the band of acidic OH groups at 3610 cm–1; X(EB), conversion of ethylbenzene; Tmax , peak temperature obtained by TPD of NH3 from Brønsted – acid sites (cf. [29]); Ed , most frequent energy of activation for desorption of NH3 from Brønsted acid sites (cf. [29, 222, 223]); DHad , differential heat of adsorption of NH3 (cf. [222, 223])

Sample no.

nCa/nAl

A(OH) (arb. units)

X(EB) (%)

Tmax (K)

– Ed (kJ · mol–1)

DHad (kJ · mol–1

1 2 3 4

0.0 0.2 0.3 0.4

0.30 0.20 0.14 0.11

4.0 2.2 2.0 1.9

585 610 605 600

99 102 101 102

146 146 138 138

Fig. 62. Hydrogenation and hydroisomerization of ethylbenzene over a Pd,Ca,H-ZSM-5 catalyst prepared by successive solid-state ion exchange of (first) CaCl2 and (second) PdCl2 with H-ZSM-5 followed by reduction in H2 (after [223], with permission)

It was assumed that the introduction of Ca2+ preceding the solid-state reaction with PdCl2 not only affected the acidity but also facilitated the generation of a more homogeneous distribution of small palladium particles inside the zeolite matrix on subsequent reduction by H2 . By a test reaction, i.e., competitive hydrogenation of branched 2,4,4-trimethylpent-1-ene vs. slim n-oct-1-ene, it was shown that, indeed, essentially all of the platinum particles were located in the interior of the zeolite crystallites (cf. Sect. 5.4.3 and [45, 221, 232, 233]). Electron micrographs obtained by TEM indicated well-dispersed relatively small metallic palladium aggregates. Figure 64 presents evaluations of such micrographs. It is evident from Fig. 64 that, in contrast to case A of simultaneous SSIE, the two-step solid-state reaction (case B) and subsequent reduction resulted in a shift to smaller particles and narrower particle size distributions with increas-

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Fig. 63. Dehydrogenation and isomerization of ethylcyclohexane over a Pd,Ca,H-ZSM-5 catalyst prepared by successive solid-state ion exchange of (first) CaCl2 and (second) PdCl2 with H-ZSM-5 followed by reduction in H2 (after [223], with permission)

Fig. 64. Size distribution of Pd0 particles from electron microscopy micrographs of Pd,Ca, H-ZSM-5 catalysts obtained by A simultaneous and B successive introduction of Ca2+ and Pd2+ via solid-state ion exchange with CaCl2 and PdCl2 at 675 K (2 h, HV) followed by reduction in H2 (after [223], with permission)

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ing Ca2+ preloading. Concomitantly, the catalytic performance of the materials produced via two-step SSIE improved in that they exhibited higher selectivity and stability. A tentative explanation of this finding was given as follows: Simultaneous SSIE of CaCl2 and PdCl2 with H-ZSM-5 probably generated ‘islands’ of either adjacent Ca2+ or Pd2+ populations, whereas in the case of successive exchange in the first step a relatively homogeneous population of the cation sites by Ca2+ and, correspondingly, a homogeneous distribution of the remaining protons was reached, which reacted in the second step with PdCl2 . Upon reduction of Pd2+ to Pd0, the neighboring Ca2+ cations could act as anchors for the rather mobile Pd0 atoms and, thus, favor the formation of small palladium aggregates. Sachtler and co-workers [234, 235] were the first to advance the idea of polyvalent cations acting as anchors for small aggregates of noble metals in zeolite matrices. Results similar to those reported for SSIE of PdCl2 with H-ZSM-5 were obtained when the other mixtures discussed above were studied. Furthermore, preceding introduction of LaCl3 in a two-step SSIE with palladium compounds and H-ZSM-5 had effects similar to those described for CaCl2 .

6 Modified SSIE and Related Processes 6.1 Introductory Remarks

There are several processes which seem to be more or less closely related to solid-state ion exchange in zeolites. An early reported example is that of the socalled contact-induced ion exchange (cf. Sect. 6.2). In 1986, Kokotailo et al. [19] and Fyfe et al. [20] showed that mere physical contact of crystallites of two samples of the same zeolite structure but loaded with different cations led finally to one sample of crystallites exhibiting a homogeneous distribution of both types of cations. However, later, Karge and Koy [236] demonstrated that the phenomenon of contact-induced ion exchange mentioned above does not occur in the total absence of water. Thus, contact-induced ion exchange is, in fact, mediated by water filling the zeolitic micropores. In essence, the mechanism is the same as that of conventional ion exchange. In the frequently employed incipient-wetness technique of impregnation (cf. [237–241]), two mechanisms are consecutively in operation. In the first stage, i.e., at lower temperatures when the zeolite powder is impregnated with an aqueous solution of a compound of the in-going cation, a fraction of the ions are exchanged as in the conventional process except that, due to omission of the washing step, the out-going cations and the anions of the impregnating salt remain in the product of this procedure. However, when the paste produced via impregnation is subsequently heated to complete the exchange, the mixture of residual salt and zeolite powder is dried and real solidstate ion exchange can then take place. However, if no volatile compounds are formed in this second step, an equilibrium with an exchange degree lower than 100% will be reached (cf. Scheme 1b, c in Sect. 2). In Sect. 7 it will be shown that real solid-state ion exchange does not require any presence of water but will proceed in an absolutely water-free system as well.

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In a sense, even the introduction of cations mediated through the vapor phase of a metal or metal compound is related to solid-state ion exchange (cf. Sect. 6.3). In fact, in the former case, the in-going entities are adsorbed from the vapor phase onto the external surface of the zeolite crystallite. In the case of solid-state ion exchange, i.e., on thermal treatment of a physical mixture of (water-free) salt or oxide of the in-going cation and the (water-free) zeolite powder, small entities (ions or, more likely, molecules; see Sect. 7.2) will be firstly separated from the salt or oxide lattice and then cover the external surface of the zeolite crystallites. Indeed, here also it cannot be excluded that the transport from the solid salt or oxide to the zeolite surface may proceed through the gas phase, even though it is generally assumed that the separation of these small entities is facilitated by the intimate contact of both solids, and their transport may occur via surface diffusion. In any event, once the external surfaces of the zeolite crystallites are covered with the in-going entities, the situation is the same in both cases, i.e., the species must migrate into the (water-free) pores of the zeolite and react there. An interesting modification of solid-state ion exchange as described in Sects. 2 and 5 is to be seen in those cases where the exchange requires in addition to the two solids (zeolite and salt or oxide) the presence of certain vapors or gases such as H2O, O2 , H2 , CO, NH3 , or volatile hydrocarbons (cf. Sect. 6.3). Very important examples are encountered in cases of reductive solid-state ion exchange (RSSIE). 6.2 Contact-Induced Ion Exchange

Employing an interesting experiment, Kokotailo et al. [19] and Fyfe et al. [20] were able to show that even at room temperature cation exchange occurred between, e.g., Li-A and Na-A simply upon intimate contact beween the crystallites of the dry powders of both different cationic forms of zeolite A. This phenomenon was demonstrated by 29Si MAS NMR and XRD measurements. As expected, the mixture of Li-A and Na-A exhibited initially two well-separated 29Si MAS NMR signals, viz., at –85.1 and –88.9 ppm (referenced to TMS). This reflected the different local environments of Si in Li-A and Na-A. Similarly, the XRD patterns showed separated reflections (cf. Fig. 65). After the length of time required for equilibration under ambient conditions, only one sharp 29Si MAS NMR line was observed, and the corresponding splitting of the XRD reflections disappeared. These observations confirmed that after equilibration only one single phase existed where Li+ and Na+ cations were homogeneously distributed over all zeolite A crystallites. Similar findings were reported for the pairs Li-A/NH4-A, Li-X/Na-X, Li-X/NH4X, and Li-A/Na-MOR and by Huang et al. [33] for the systems Li-A/Na-Y, Li-A/Na-X, and Li-A/Ca-X. Koy and Karge [236], however, were able to prove that this type of contactinduced ion exchange requires the presence of residual adsorbed water in the pores of the zeolite crystallites. These authors used completely dried Li-A and Na-A samples and carried out all experimental steps (mixing, filling of the capillaries for XRD measurements, sealing of the capillaries, etc.) in an efficiently working glove box (PH2O <10–7 Pa). The samples then exhibited the expected

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Fig. 65. Contact-induced ion exchange between crystallites of Li-A and Na-A zeolites; 29Si MAS NMR and XRD patterns of Li-A/Na-A mixtures immediately after mixing (initial) and after equilibrating at ambient conditions (equilibrium) (after [20], with permission)

Fig. 66. Check of contact-induced ion exchange between crystallites of Li-A and Na-A: A XRD patterns obtained under exclusion of even traces of water and B after admission of ambient moisture (cf. [236])

splitting of the reflections in their XRD patterns. However, with the samples prepared in this way, even after very extended observation times, no collapse of the two separated sets of reflections was found (Fig. 66A). Only when the capillaries were opened and moisture from the ambient air had access to the samples did the splitting disappear, showing that the presence of water was required to make the Li+ cations of Li-A crystallites migrate into Na-A crystallites and vice versa until an equilibrium was established (Fig. 66B).

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Contact-induced ion exchange was also studied by Fraissard et al. [242]. These authors used the system Rb,Na-X/Na-Y and monitored the exchange by 129Xe NMR. Interestingly, the exchange was not observed upon mixing at 300 K, but only after heat-treating the Rb,Na-X/Na-Y mixture under vacuum to 673 K. 6.3 Gas-Phase-Mediated Processes Related to SSIE 6.3.1 Introduction of Cations into Zeolites Through a Vapor Phase Containing the In-Going Species

In Sect. 6.1 it was indicated that incorporation of cations via an interaction of a vapor phase (containing the in-going species) and the solid phase (the zeolite) is, in a certain sense, related to SSIE in that in both cases the in-going species or their precursors must be first adsorbed onto the external surface of the zeolite crystallites in order to subsequently migrate into the zeolitic pore system. Adsorption may occur in the case of volatile metals, oxides or halides. Examples are the introduction of Zn2+, Ga3+ and Fe3+ into hydrogen forms of zeolites via the reaction of vapors of ZnCl2 , GaCl3 and FeCl3 , respectively, with zeolites [243–245], the incorporation of V-, Ti- and Cr-containing species from VOCl3 , TiCl4 and CrO2Cl2 gas phases, respectively [206, 246], and the reaction of vapors of metallic zinc or cadmium with, e.g., H-Y or H-ZSM-5 [136, 247–249]. For instance, Guisnet et al. [244] prepared Zn-doped ZSM-5 by contacting the degassed zeolite H-ZSM-5 with a flow of ZnCl2 in N2 at 558 K (sublimation temperature of ZnCl2). Incorporation of vanadium into silicalite-1 and H-ZSM-5 was achieved by contacting silicalite and H-ZSM-5 at 593–793 K with a nitrogen stream saturated with VOCl3 vapor at 293 K (pVOCl3 ª 2.1 kPa). IR spectroscopy showed that in silicalite silanol groups were efficiently removed under formation of (∫ Si)3 ∫ VO species, which were relatively resistant to hydrolysis. In addition, in H-ZSM-5 the protons of the acid Brønsted sites associated with the framework aluminum were eliminated. No degradation of the crystallinity was indicated by XRD. According to the ESR spectra of the modified and hydrated MFIs, small amounts of paramagnetic vanadyl species were present. However, most of the vanadium remained as V5+. The materials were active in catalytic gas-phase oxidation of toluene with O2 . The behavior of TiCl4 and CrO2Cl2 was similar, but CrO2Cl2 was less efficient in removal of the silanol groups and Brønsted acid sites [206, 246]. As reported by Boddenberg and co-workers, vapor of Zn0 did not undergo any reaction with Na-Y (at 693 K and pZn ª 0.2 hPa), while contact with H-Y led to a degree of exchange of more than 90% of the initial acidic protons [136]. 129Xe investigation, volumetric determination of the adsorption isotherms of CO and Xe on the resulting Zn-Y samples, and application of the multi-site adsorption model [137] yielded the distribution of the Zn2+ cations on the cation sites in the supercages. Under anhydrous conditions, an unusually high population of the SIII and SII sites was found [247], distinctly higher than in samples prepared through conventional or solid-state ion exchange (cf. [136]). Contact with water

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vapor reduced the population of the supercage sites. In this context, Seidel and Boddenberg discussed the possibility of developing the reaction between zinc vapor and hydrogen forms of zeolites to a titration procedure for zeolitic protons under anhydrous conditions [247]. Similar results to those obtained with zinc vapor were reported for the reaction of cadmium in the system Cd0vapor/H-Y [248].With respect to the interaction of zinc vapor with H-Y and subsequent oxidation of Zn0 to Zn2+ by the protons of the H-Y zeolite, Seidel et al. reported that this technique of zinc incoporation led to a more homogeneous distribution of the Zn2+ cations within the macroscopic sample than conventional ion exchange in aqueous solutions of Zn(NO3)2 [136]. In a more recent contribution, Rittner et al. [248] derived from a 129Xe NMR investigation of Zn-containing Y zeolite, which was prepared via reaction between zinc vapor and H-Y, that in different regions of the sample zinc cations occurred in at least two different formal oxidation states, viz., as Zn2+ cations and Znx2+ clusters with x ≥ 2. The most probable cluster was assumed to be Zn2+ 2 . According to Beyer et al. [250] it is possible to determine the density of the reacting Brønsted acid OH groups by measuring the hydrogen evolved. 6.3.2 Effect of Additional Molecules in the Vapor Phase on SSIE at Elevated Temperatures

In Sect. 5.3.5.5 it was reported that attempts to introduce molybdenum into hydrogen forms of zeolites via SSIE with MoO3 were usually unsuccessful (cf. [92, 190]). The failure of the respective experiments was ascribed to the propensity of MoO3 to form polymeric aggregates which are too bulky to penetrate into zeolitic pores. However, Yuan et al. [201] and Wang et al. [216] reported that an exchange reaction between MoO3 and H,Na-Y did occur in the presence of water vapor. This effect was most likely due to a reaction at higher temperatures (723 K) of water vapor with the molybdenum oxide that prevented MoO3 from polymerization and/or led to a disaggregation of initially formed bulky polymeric species. Thus, it seemed possible that smaller Mo-containing entities formed which were able to migrate into the zeolite pores and react there with the protons of, e.g., H,Na-Y. The fact that incorporation of Mo into the zeolite had occurred was evidenced by XRD, FTIR, ESR and back-exchange of the introduced Mo-containing cations. In the presence of carbon monoxide, CO, metals such as Mo, Re or Ru frequently form carbonyls, which subsequently may be encaged in zeolites and, after decomposition, transformed into small metal aggregates. However, in several cases, the encapsulated carbonyls are transferred into cationic species and anchored as such in the zeolite matrix. For example, Co2(CO)8 was oxidized by the protons of H-Y to Co2+ and held at sites in the supercages, as suggested by IR investigations [251]. Formation of the cations may be due to a reaction as described by Eq. (23). 4Co2(CO)8 + 4H+Z– Æ 2Co2+Z –2 + 12CO + 2H2 + 2[Co(CO)4] + [Co4(CO)12] (23)

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Enhanced solid-state ion exchange in the presence of CO was also observed by Schlegel et al. [227] who attempted to introduce rhodium into dealuminated Y-type zeolite via reaction with RhCl3 (vide supra, Sect. 5.4.2). When SSIE between RhCl3 and dealuminated H-Y was conducted in vacuum or oxygen, the degree of exchange was limited to 25%, even though the reaction temperature was as high as 675–875 K. When, however, SSIE was carried out in the presence of a CO-containing gas phase, the ion exchange occurred at much lower temperatures (375–475 K), and more than 50% of the protons were exchanged for cationic rhodium species. The authors assumed that on the outer surface of the zeolite crystallites during calcination in the presence of residual H2O and/or oxygen rhodium oxychloride or rhodium oxide had formed that was subsequently reduced by CO to yield dicarbonyl species: RhCl3 + H2O Æ RhOCl + 2 HCl ≠

(24)

RhOCl + 3 CO Æ Rh(CO)+2 Cl– + CO2 ≠

(25)

These dicarbonyl species were able to migrate into the zeolitic pores and react there with acidic OH groups, as evidenced by FTIR. Thus, stable Rh(CO)+2 were finally formed on cation positions: Rh(CO)+2 Cl + H+Z– Æ Rh(CO)+2 Z– + HCl ≠

(26)

A new route for introduction of polyvalent cations from the respective oxides into hydrogen forms of several zeolites, viz., H-ZSM-5 (nSi/nAl = 15), H-BETA (nSi /nAl = 8), H-Ferrierite (nSi/nAl = 4), H-USY (nSi/nAl = 5), was found and described by Kucherov et al. [252]. For instance, samples were prepared by grinding precalcined MoO3 with the zeolites mentioned above. Then the mixtures were calcined in an air flow at 773 K for 2 h, cooled to 293 K, heated in an air stream saturated at 293–296 K with tetrachloromethane, CCl4 , (pCCl4 = 10–13 kPa) up to 323–403 K for 2 h and finally purged in pure dry air for 2–3 h. The samples produced in this manner exhibited well-resolved ESR signals typical of isolated Mo5+-containing ionic species, as evidenced by comparison with standard samples prepared via impregnation with aqueous ammonium molybdate solutions. In contrast, no such signals were observed previously when the experiments were conducted without the assistance of a gas phase containing CCl4 (vide supra, Sect. 5.3.5.5). It was proposed that the interaction between MoO3 and CCl4 resulted in the formation of rather mobile reactive species such as molybdenum oxychlorides. In view of the large distances between neighboring framework Al atoms, however, it was assumed that the incorporated isolated Mo species were not bare Mo5+ cations but rather complex entities such as (MoOCl2)+ or MoCl +4 (vide supra). CCl4-assisted incorporation of Mo5+ was much more efficient with H-ZSM-5 and H-BETA than with H-ferrierite and H-USY. In the case of ferrierite, this was explained by the low effective pore diameter (0.4 nm vs. 0.6 nm in ZSM-5 and BETA). For USY (0.8 nm) it was assumed that the low degree of exchange was probably due to a significant aggregation of the Mo5+ species in the large cavities of the structure of the Y-type zeolite.

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6.3.3 Oxidative and Reductive SSIE 6.3.3.1 Oxidative SSIE of Ag0, Cu0 and Pd0 in the Presence of O2 or Cl2

In earlier studies, Beyer and colleagues [253, 254] observed that tiny silver and copper particles (Ag n0 , Cu n0 ) formed on the external surfaces of zeolite crystallites upon reduction by H2 of Ag-Y and Cu-Y, respectively. Concomitantly, acid zeolitic OH groups were restored. Upon calcination in oxygen, Ag0 remigrated into the interior of the zeolite structure and reacted there with the protons of these OH groups under formation of Ag+ on cation sites and water, whereas external copper aggregates were irreversibly oxidized to CuO. In contrast, Kucherov et al. [255] claimed that in the case of reduced Cu,H-ZSM-5 bulky Cu0 aggregates, which had formed upon reduction on the external surface of the zeolite crystallites, could also be reoxidized to cupric species slowly remigrating into the zeolite channels. Similarly, Feeley and Sachtler [256] showed that cation exchange of solid palladium with H-Y was mediated by an oxidative gas phase. These authors assumed that PdCl2 formed from Pd0 and chlorine and that PdCl2 subsequently diffused into the pore system, reacted there and replaced the protons under formation of Pd2+ on cation positions and HCl. 6.3.3.2 Reductive SSIE of Ga2O3

A very interesting and promising modification of solid-state ion exchange is reductive solid-state ion exchange (RSSIE) which proceeds in a reductive atmosphere that is in contact with both solids, usually an oxide of the in-going cation and the hydrogen form of a zeolite. Price et al. [257, 258] and Kanazirev et al. [259–261] were the first to report on RSSIE in the systems Ga2O3 /H-ZSM-5, In2O3/H-ZSM-5 and CuO/H-ZSM-5 in the presence of hydrogen. These exchange processes were assumed to occur according to Eq. (27) or (28): M2O3 + 2 H2 + 2 H+Z– Æ 2 M+Z– + 3H2O

(27)

MO + 1/2 H2 + H+Z– Æ M+Z– + H2O

(28)

where Z– again represents a monovalent negatively charged zeolite fragment. RSSIE in the above systems was investigated by TPR/TG (temperature-programmed reduction monitored by a thermobalance), XRD, TEM/EDAX, XPS and IR. However, in the case of CuO/H-ZSM-5, it turned out that employment of a reducing agent was not necessary for incorporation of Cu cations, since autoreduction occurred. Interest in the preparation of gallium-containing and indium-containing zeolites by reductive solid-state ion exchange was stimulated by the fact that these materials might also be promising catalysts for aromatization of alkanes and redox reactions, respectively.

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Gallium-containing catalysts were obtained by ball-milling b-Ga2O3 with HZSM-5 [257]. Investigations using XRD allowed the exclusion of any phase transformations of the thermodynamically most stable modification, b-Ga2O3 into a-, g-, d- or e-Ga2O3 , and any ion exchange induced by ball-milling. However, it was shown that an intimate mixing of the fine powders of the oxide and zeolite was necessary for subsequent reduction to Ga+Z–: no reduction was observed when Ga2O3 and H-ZSM-5 were placed on the microbalance pan without previous mixing. Also, no reduction was observed in the system Ga2O3/NaZSM-5, which proved that zeolite acidity is likewise required for RSSIE. Under appropriate conditions, Price and co-workers [262] achieved by RSSIE in the system Ga2O3 /H-ZSM-5 a degree of exchange close to 100%, similar to the results obtained for CuO/H-ZSM-5 (vide supra) and In2O3 /H-ZSM-5 (vide infra). TPR experiments further showed that with Ga2O3 the stoichiometry of the process is most likely described by Eqs. (29) and (30): Ga2O3 + 2 H2 Æ Ga2O + 2H2O

(29)

Ga2O + 2 H+Z– Æ 2 Ga+Z– + H2O

(30)

Only a small fraction of Ga2O3 conversion may occur according to Eq. (31) upon calcination under high vacuum: Ga2O3 + 6 H+Z– Æ 2 Ga3+Z–3 + 3 H2O

(31)

Subsequent reduction with H2 would restore two-thirds of the protons replaced according to Eq. (31) (cf. [258]): Ga3+Z3– + H2 Æ Ga+Z– + 2 H+Z–

(32)

Step 1 according to Eq. (29) is consistent with the well-established gallium chemistry in that the suboxide, Ga2O, is a product of reduction of Ga2O3 with H2 . Further reduction of Ga2O to elemental Ga0 could not be completely exluded, but appeared unlikely, since even with a large excess of Ga2O3 it was impossible to exceed an upper limit of reduction. Such an upper limit of reduction, which was significantly lower than what had corresponded to the total amount of admixed Ga2O3 , could not be rationalized if the reduction could proceed from Ga+ to Ga0. Thus, it is obvious that the zeolite was a stoichiometric reactant, and the produced Ga+ was stabilized in the zeolite matrix. Further, it was shown by IR spectroscopy, simply using the KBr pellet technique, that after RSSIE of Ga2O3/H-ZSM-5 the typical IR bands at 648 and 456 cm–1 of Ga2O3 were no longer detectable if Ga2O3 had been admixed in sub-stoichiometric amounts.When Ga2O3 was admixed in excess, its reduction proceeded only to an upper limit corresponding to the Al content of the framework or the respective density of Brønsted acid OH groups [259]. An IR investigation of the OH stretching region confirmed through the decrease of the intensity of the 3610 cm–1 band that the acid OH groups were largely consumed upon RSSIE. This was in contrast to the results of earlier experiments [263] that failed to provide such IR evidence for an incorporation

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of Ga into H-ZSM-5 through the decrease of the OH band intensity. However, in those experiments, an attempt was made to introduce Ga under non-reductive conditions, i.e., by conventional ion exchange or impregnation. TEM/EDAX investigations demonstrated the occurrence of RSSIE in Ga2O3/ H-ZSM-5 in that the Ga2O3 particles of the mixture had essentially disappeared and Ga transfer into the zeolite particles had taken place [257]. Neither Ga-containing phases on the external surfaces of the zeolite crystallites nor Ga enrichment in their surface layers were detected. Finally, XPS of the reduced Ga2O3/H-ZSM-5 mixture before and after Ar+ bombardment showed a remarkable increase in the Ga(3d) signal caused by a large increase in the Ga-containing surface. This was due to the spreading of Ga oxide over the internal zeolite surface and thus clearly indicated the incorporation of Ga into the bulk of the zeolite [259]. In fact, Price et al. [258] also reported on successful introduction of gallium into extra-framework cation sites of the Ga analog of H-ZSM-5, i.e., into H[Ga]ZSM-5. However, in no case was evidence found for incorporation of Ga into the framework via reaction of Ga2O3 with internal silanol-type hydroxyls of ZSM-5, as suggested by Endoh et al. [264]. This should, inter alia, result in an expansion of the unit cell of the zeolite, which was not observed by XRD. Rather, those internal silanol-type groups that were especially abundant in the case of H-ZSM-5 were assumed to also generate Ga+ cations and to be responsible for the observed Ga2O3 reduction beyond the Al content corresponding to bridging acid OH groups. Similar findings were reported for the system In2O3/H-BETA (vide infra). Introduction of Ga into SAPOs is discussed in the Sect. 6.3.3.3 in the context of incorporation of In by RSSIE. Preparation of Ga-containing catalysts through RSSIE of b-Ga2O3 in physical mixtures with hydrogen forms of zeolites was proposed as a particularly interesting technique, because other popular methods such as impregnation with Ga(NO3)3 solutions or conventional ion exchange may not have followed identical phase transformations and reduction pathways, especially when different conditions for calcination and reduction were employed. Thereby, catalysts with quite different properties (that have proved difficult to reproduce) may have been obtained. Ga-containing zeolites, i.e., Ga-ZSM-5 materials prepared from ground mixtures of Ga2O3 and H-ZSM-5, proved to be active and very selective catalysts for the conversion of propane [257, 260, 262, 265] and n-pentane [258] to aromatics under atmospheric pressure and at 750–780 K. In such catalytic experiments it was observed that calcined mixtures of Ga2O3/H-ZSM-5 even without preceding reduction by H2 exhibited, after some time on stream, increasing activity. This suggested that the initial reduction of Ga2O3 was achieved by the hydrocarbons of the feed or H2 generated from the hydrocarbons. However, admission of H2 greatly enhanced the activity. In addition, desorption and decomposition of propylamines on Ga-ZSM-5 were investigated [260, 261, 266]. The features of these processes were completely different from those observed upon interaction of propylamines with H-ZSM-5. Also, Ga-ZSM-5 prepared via interaction of Ga2O3 and H-ZSM-5 or H-MOR in a physical mixture was employed as a catalyst for NO reduction by

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CH4 [267, 268]. According to Kikuchi et al. [267], the activity of such catalysts is lower than that of conventionally exchanged Ga-ZSM-5. 6.3.3.3 Reductive SSIE of In2O3

It has been shown by Kikuchi et al. [267, 268] that ground mixtures of Ga2O3 and H-ZSM-5 as well as of In2O3 and H-zeolites (H-mordenite, H-ZSM-5, H-BETA, H-Y) were, after thermal treatment, active and selective catalysts for the reduction of NO2 (NOx) with hydrocarbons in the presence of oxygen. They assumed incorporated GaO+ to be the catalytically active species. A reduction step leading to Ga+Z– (or In+Z–, vide infra) was not explicitly carried out. However, most likely, the materials were in fact reduced by the hydrocarbon involved in the catalytic reaction. The degree of exchange of InO+ for protons was claimed to be correlated to the strength of the Brønsted acid sites of the zeolites, as characterized by TPD of ammonia, thus determining the observed sequence in catalytic activity for reduction of NO2 by CH4, viz., H-MOR>H-ZSM-5>H-BETA ª HY>SiO2-Al2O3. In contrast to Ga-containing ZSM-5 (vide supra), the activities of In-ZSM-5 prepared via RSSIE on the one hand and by conventional exchange on the other were found to be similar [267–269]. Furthermore, such In-containing zeolites were proven to catalyze the conversion of methanol to hydrocarbons [270]. These observations caused an increased interest in the details of RSSIE in In2O3/H-zeolite systems. Zatorski [270] has suggested that a direct reaction between In2O3 and the hydrogen forms of zeolites discussed above occur that lead to an incorporation of bare In3+ cations into the zeolite according to Eq. (33): In2O3 + 6 H+Z– Æ 2 In3+Z3– + 3 H2O

(33)

However, this was not confirmed by subsequent studies of Kanazirev and associates [260–262] and the work of Beyer’s group [271–275] (vide infra). Kanazirev et al. [258, 259] investigated the incorporation of indium through thermal treatment of mixtures of In2O3 and hydrogen forms of ZSM-5, zeolite Y, mordenite and offretite under reductive conditions, in complete analogy to their studies on the introduction of gallium into zeolites via reductive solid-state ion exchange (RSSIE). Techniques used for these investigations were TEM/EDAX, TPR/TG and IR. Indium was also introduced via RSSIE into large crystallites of H-ZSM-5 when finely powdered In2O3 was loosely mixed with the zeolite crystallites and subjected to reduction in H2 at temperatures as low as 623 K [276]. Similar to what was found with Ga2O3/H-ZSM-5, it was shown by TEM coupled with EDAX that upon heat-treatment of In2O3/H-ZSM-5 under H2 the In2O3 particles disappeared and indium was tranferred into the zeolite. Differential TPR profiles (obtained by subtraction of the DTG curves of the indium-free zeolite) exhibited three peaks, viz., the LTP at low, MTP at medium and HTP at high temperature, corresponding to dehydration, reduction/incorporation of In+ into the zeolite [cf. Eq. (27)] and reduction of (excess) indium oxide to elemental In0, respectively. LTP and HTP always occurred when In2O3 was present in excess and also when samples of pure In2O3 or In2O3-loaded pro-

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Fig. 67. DTG/TPR curves (dehydration, reduction to In+ stabilized in zeolites, reduction to In0) of In2O3 , In2O3/Al2O3 , and mixtures of In2O3 with H- or Na-forms of zeolites (offretite,Y, mordenite). Designation of samples: A sample 1, 25 In/H-OFF, i.e., 0.025 g In2O3 per gram dry zeolite; sample 2, 35 In/H-Y, i.e., 0.035 g In2O3 per gram dry zeolite; sample 3, 25 In/H-MOR, i.e., 0.025 g In2O3 per gram dry zeolite. B sample 1, In2O3 in Ar; sample 2, In2O3 in H2 ; sample 3, 25 In/Na-Y, i.e., 0.025 g In2O3 per gram dry zeolite; sample 4, 25 In/Al2O3 , i.e., 0.025 g In2O3 per gram dry alumina. LTP low-tempreature peak; MTP medium-temperature peak; HTP hightemperature peak (for details see text; after [260], with permission)

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ton-free materials such as Al2O3 or Na-ZSM-5 were employed (cf. Fig. 67). The middle peak (MTP) appeared only if a mixture of In2O3 and the acid hydrogen form of a zeolite was subjected to RSSIE. From the TPR experiments it was concluded that the upper limit of In incorporation was determined by the proton content. The ratio nIn, incorp /nH was always found to be close to 1 despite the fact that various zeolite matrices and In2O3 loadings were used. This ratio was never close to 0.33, which is the value that would be expected if three protons were replaced by one In3+ as suggested by Zatorski for SSIE in a non-reductive atmosphere [270]. Profiles of TPD of ammonia from H-ZSM-5 exhibited two peaks at about 500 and 670 K corresponding to weak and strong Brønsted acid sites [260]. Upon RSSIE of In2O3/H-ZSM-5 at 9 wt.% of In2O3 loading, the high-temperature peak at 670 K disappeared completely. At increasing In2O3 contents, the low-temperature peak at 500 K vanished as well. This decrease in the total concentration of Brønsted acid sites was confirmed by IR spectroscopy through the decrease in the intensity of the 3610 cm–1 band. When pyridine was adsorbed after the reductive solid-state ion exchange, a corresponding decrease in the band at 1540 cm–1 was measured. This band originated from pyridinium ions and was, therefore, related to the density of the acid OH groups. Concomitantly, a band at 1446 cm–1 developed indicating pyridine bonded to Lewis acid sites. In the case of zeolites containing appreciable amounts of (internal) silanol-type hydroxyls, e.g., H-ZSM-5 or H-BETA, these hydroxyl groups were assumed to also react to some extent. The increase in the MTP in the TPR profile mentioned above corresponded well with the decrease in the total density of Brønsted acid sites. The consumption of Brønsted acid sites was explained by the stoichiometry of the solid-state reaction in analogy to the case of Ga incorporation [cf. Eqs. (29) and (30)]: In2O3 + 2 H2 Æ In2O + 2 H2O

(34)

In2O + 2 H+Z– Æ 2 In+Z– + H2O

(35)

As a consequence of the elimination of acid OH groups upon RSSIE in In2O3/ H-ZSM-5 or In2O3/H-Y mixtures, the activity of these materials in acid-catalyzed reactions significantly decreased (vide infra). In an analogous way to Ga2O3 and In2O3, experiments were conducted to prepare Cu-containing catalysts. However, while for preparation of Ga- and Incontaining catalysts the exposure of the oxide/H-zeolite mixture to a reductive environment was an indispensible requirement, during thermal treatment, the system CuO/H-ZSM-5 underwent so-called auto-reduction in that a certain fraction of the framework oxygen anions were oxidized to molecular oxygen under reduction of Cu2+ to Cu+ (vide supra, cf. [203, 204, 261]). Therefore, no admission of a reductant to the system was necessary. In this context, it is worth mentioning that, in contrast to other cations associated with extra-framework oxygen (such as Cu(OH)+, [Cu-O-Cu]2+, FeO+), InO+ residing on cation sites in zeolites did not give rise to auto-reduction, i.e., no molecular oxygen was released.

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The studies on incorporation of indium into zeolites via reductive solid-state ion exchange were considerably amended by the work of Beyer and colleagues [271–275, 277, 278]. To confirm and elucidate RSSIE, these authors investigated mixtures of In2O3 and hydrogen forms of zeolites such as H,Na-Y, H-ZSM-5, HMOR and H-BETA. For their measurements they employed IR spectroscopy with and without pyridine as a probe as well as TPR and TPO, which was monitored either thermogravimetrically by a microbalance or with a thermal conductivity detector determining the hydrogen and oxygen consumed. Furthermore, acidcatalyzed isomerization of m-xylene was used as a test reaction to characterize the density of Brønsted acid OH groups. First, stoichiometric (nIn/nNH4 = 1; sample I) and non-stoichiometric (nNH4/nIn = 2; sample II; nNH4/nIn = 3; sample III) mixtures of In2O3 and H,Na-Y were investigated. In agreement with the findings of Kanazirev and associates [260, 261], the requirement of a reductive atmosphere as well as that of the presence of Brønsted acid OH groups for introduction of In into H,Na-Y in the temperature range up to about 770 K was established by TPR/DTG and IR.At higher temperatures, rapidly progressing thermal dehydroxylation prevented any investigation of the indium incorporation in a non-reductive atmosphere or vacuum. In this context it is worth mentioning that, very recently (cf. [278]), it has been shown that ∫Al-O(H)-Si∫ hydroxyls are more resistant to thermal dehydroxylation in mixtures of In2O3 with H-ZSM5 than in pure H-ZSM-5. Thus, incorporation of indium into this zeolite was found to proceed at 810 K in vacuum. However, strong evidence was provided that the incorporation is preceded by an auto-reduction of In2O3 (vide infra). The DTG peak observed on reduction of pure In2O3 or In2O3/Na-Y with H2 at 730 K was, in the presence of acid OH groups, i.e., in the case of In2O3/H,Na-Y, shifted to a lower temperature of about 630 K. This suggested a modification of the reduction process according to Eqs. (34) and (35) (vide infra). The decrease in the intensities of the OH bands in the IR spectrum of In2O3/H,Na-ZSM-5 upon RSSIE can be recognized from Fig. 68. This figure shows quite clearly that both types of OH groups were affected. They were completely eliminated in the case of the stoichiometric sample I. Correspondingly, with the samples II and III, which possessed a 50 and 66% excess of NH+4 (or H+), only 50 and 33%, respectively, of the total amount of the initial OH groups were consumed during RSSIE. In fact, the OH groups indicated by the HF band at 3640 cm–1 seemed to be somewhat more reactive. After pyridine adsorption, the non-reduced sample I exhibited a strong pyridinium ion band at 1544 cm–1, whereas this band did not appear after RSSIE because of the complete consumption of acid OH groups during reductive solid-state ion exchange. Similar to the observation by Kanazirev et al. in the system In2O3/HZSM-5 ([260], vide supra), a band at 1445–1455 cm–1 was seen instead, which is indicative of pyridine coordinatively bonded to cationic In-containing species. This band was better resolved in other experiments, in particular with H-BETA, and will be discussed below. Concomitantly with the band at 1445–1455 cm–1, a new band was observed at 1441 cm–1. It originated from pyridine attached to Na+ and, thus, demonstrated that residual sodium cations had been expelled from their original sites inside the sodalite cages by the In-containing cationic species which were formed by RSSIE.

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Fig. 68. IR spectra of the OH stretching frequency range of a stoichiometric mixture of In2O3 and NH4-Y (nIn/nNH+4 = 1; sample I) a after calcination at 670 K for 1 h; b sample (a) reduced with hydrogen at 760 K for 1 h; c sample (b) contacted with water vapor (2.7 kPa) at 420 K for 30 min; d sample (a) reoxidized with oxygen at 620 K for 1 h and subsequently degassed at 760 K for 1 h; e sample (d) contacted with water vapor (2.7 kPA) at 470 K for 30 min; and f sample (d) contacted with water vapor (2.7 kPa) at 620 K for 30 min (after [271], with permission)

Reduction was shown by TPR, IR and MS to be also possible to some extent with NH3 and with CO [272]: In2O3 + 2 CO Æ In2O + 2 CO2 ≠

(36)

In2O + 2 H+Z– Æ 2 In+Z– + H2O ≠

(37)

Beyer et al. [271–273] assumed a reduction mechanism for the system In2O3/H,Na-Y similar to that proposed by Kanazirev et al. [cf. Eqs. (34) and (35)], even though no evidence for the intermediate formation of In2O could be obtained; this suboxide is probably unstable and non-existent in the crystalline state [279]. The univalent indium cation, In+, was proven to be absolutely resistant to further reduction by hydrogen at least up to 870 K, whereas In2O3 on nonacidic zeolites (e.g., Na-Y) turned out to be reducible in one step to elemental In0 at temperatures somewhat higher than required for reductive solid-state ion exchange in the system In2O3/NH4,Na-Y. Since In+ cations introduced by RSSIE into zeolites were not further reducible, it was impossible to regenerate the original OH groups according to Eq. (38): M+Z– + 1/2 H2 Æ M0 + H+Z– Cu+, where

(38)

Eq. (38) applied (cf. [280, 281]). in contrast to the case of However, the samples produced via RSSIE (InI-Y from sample I; InI,H-Y from samples II and III) could be reoxidized in a flow of O2/Ar. Reoxidation com-

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menced already at room temperature and was completed at 350 K. Since the original OH groups were not restored, it was suggested that the reoxidation took place according to Eq. (39): 2 In+Z– + O2 Æ 2 (InO)+Z–

(39)

The validity of Eqs. (34), (35) and (39) in describing the reduction and reoxidation process in the system In2O3/H,Na-Y was unequivocally confirmed by measurements of H2 and O2 consumption during the RSSIE and oxidation steps, respectively. During RSSIE of sample I, that started at 370 K and was completed at 670 K, two H2 molecules per In2O3 molecule were consumed. The consumption of oxygen in the reoxidation step corresponded exactly to one O2 molecule per In2O3 molecule or two In+ in agreement with Eqs. (34), (35) and (39). Subsequent reduction of InO+Z– in H2 proceeded at 370–670 K. Thus, in the case of the stoichiometric sample I, a fully reversible redox cycle was confirmed by measurements of oxygen and hydrogen consumption during TPO and TPR, respectively: +O

2 2In+Z– ¨–––––––– Æ 2InO+Z–

(40)

+2H2 ,–2H2O

In the case of samples II and III, where an excess of Brønsted acid OH groups was present, after reoxidation a secondary dehydroxylation was found by IR measurements, which most likely resulted in the formation of [In–O–In]4+ complexes [cf. Eq. (41)]. However, upon reduction of these reoxidized and dehydroxylated samples, IR evidenced a complete restoration of the excess Brønsted acid OH groups [271]. Thus, in this case, the reversible redox cycle is described by the following scheme: +O2

–H2O

2In+Z– + 2H+Z– –––––––Æ 2InO+Z– + 2H+Z– –––––––Æ [In – O – In]4+Z–4

1442443

1442443

+ 2H2 , –H2O | ≠0000000042

(41)

In view of the above redox cycles, In-containing zeolite catalysts prepared by RSSIE are obviously promising candidates for redox reactions, because of their fully reversible changes in the oxidation state of the indium species incorporated and stabilized in the zeolite matrices. RSSIE was also carried out in the system In2O3/NH4(H)-BETA [274]. As in the case of zeolite Y, incorporation of indium was achieved by thermal treatment of mixtures of template-free NH4-BETA and In2O3 in a reductive environment. Reduction proceeded by reaction with H2 or CO and, to a minor extent, by NH3 evolved during deammoniation. RSSIE in the system In2O3/H-BETA was monitored by IR through consumption of OH groups. The acid Brønsted OH groups (band at 3610 cm–1) preferentially reacted. Consumption of acid OH groups was also proven by the decrease in the activity of the heat-treated In2O3/H-BETA samples in the acid-catalyzed isomerization of m-xylene in comparison with equally treated pure zeolite H-BETA. Concomitantly with the pronounced decrease in the bands associated with bridging and silanol-like hydroxyls, upon RSSIE a strong band at 1446 cm–1

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developed after adsorption of pyridine which was shifted to 1453 cm–1 by treatment in O2 at 670 K. The opposite effect occurred when the reoxidized sample was subjected to another reduction. Applying Eqs. (34), (35) and (39), this enabled Beyer et al. [274, 275] to assign the band at 1446 cm–1 to pyridine coordinatively bonded to In+ (Py Æ In+) and the band at ca. 1455 cm–1 to pyridine attached to InO+ (Py Æ InO+). The band at 1446 cm–1 was easily removed by degassing at 380–470 K, i.e., the interactions (Py Æ In+) were similar to (Py Æ Na+). In contrast, the band at 1455 cm–1 disappeared only on evacuation at temperatures higher than 570 K. This suggested that extra-framework InO+ species exhibited higher Lewis acid strength than In+ and behaved more like ‘true’ Lewis sites, i.e., extra-framework AlO+ (cf. [171, 172]). RSSIE in the system In2O3/BETA exhibited some pecularities. When as-synthesized samples of BETA were employed, the zeolite still contained cations (TEA+) derived from the template (tetraethylammonium hydroxide, TEAOH) as used for the synthesis. Thus, RSSIE proceeded without an additional reductant, because the decomposition products of TEA+ (ethene, alkylamines, hydrogen) acted as reducing agents. Thus, In-containing BETA zeolites can be prepared simply by heating a mixture of In2O3 and as-synthesized, template-containing BETA. Furthermore, not only the acidic Brønsted OH groups associated with framework aluminum (band at 3610 cm–1) but also part of the less acidic internal silanol-like hydroxyls occurring in large amounts in template-free H-BETA were involved in the RSSIE process. This was indicated by the decrease in the intensity of the band at 3730–3738 cm–1 that are typical of the latter kind of hydroxyl groups [275]. Simultaneously, the band at 1453 cm–1 assigned to Py Æ InO+ interactions (vide infra) developed to some degree during the RSSIE step, i.e., without admission of an oxidative reactant such as O2 . This was not observed when instead of H-BETA other zeolites that were devoid of internal silanol groups (H-MOR, H-Y) were subjected to RSSIE. Therefore, Beyer et al. [275] assumed that these hydroxyls reacted with the strong reductant In+ similarly to the case of silicalite-1 [278] according to Eq. (42): In+ + 2 [∫SiOH] Æ H2 + InO+ + [∫Si–O–Si∫]

(42)

The incorporation of indium into NH4-MOR, H-ZSM-5 and NH4-Y by RSSIE as well as the redox behavior of the cationic In species introduced in this way were exclusively studied by IR (cf. [277]). Again, the progress of RSSIE was monitored by the change in the intensity of bands typical of pyridine interacting with acid hydroxyl groups and with incorporated indium cations acting as Lewis acid sites. In their study, the authors stressed the outstanding suitability of the n8a ring vibration mode of adsorbed pyridine in the range between 1590 and 1630 cm–1 for the detection of indium cations of different oxidation states and their discrimination from other cations frequently occurring in zeolites, such as Na+ and AlO+. The bands assigned to the n19b ring vibration mode of Py Æ Na+ and Py Æ In+ at 1442 and 1446 cm–1, respectively, and of Py Æ InO+ and Py Æ AlO+ at 1554 and 1556 cm–1, respectively, generally strongly overlap. At variance, the resolution of the spectra in the range of the n8a ring vibration mode is much

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Fig. 69. IR spectra of pyridine retained after adsorption at 470 K and subsequent degassing at 370 K on In2O3/NH4Na-Y which was A thermally pretreated in high vacuum at 720 K for 1 h, B subjected to RSSIE at 720 K in H2 and C oxidized with O2 at 570 K subsequent to RSSIE, and after successive degassing at 420 K (B1), 470 K (B2, C1), 570 K (B3, C2), 670 K (B4, C3) and 770 K (C4) and after pyridine adsorption at 470 K and after immediately subsequent degassing at 670 K (C5) (after [277])

better due to larger differences in wavenumbers: 1591, 1600, 1612 and 1623 cm–1 for pyridine attached to Na+, In+, InO+ and AlO+, respectively. Bands at 1442 and 1591 cm–1 appearing in the spectra of pyridine adsorbed on In2O3/H,Na-Y were low in intensity prior to RSSIE due to the preferential location of the residual Na+ cations in the sodalite cages at sites inaccessible to pyridine. They became intense, however, after RSSIE (Fig. 69). At the same time, when Na+ was indicated, bands typical of In+ incorporated according to Eqs. (34) and (35) developed at 1446 and 1600 cm–1 (Fig. 69). To explain these effects it was suggested that Na+ cations were expelled by a fraction of the in-going In+ cations from sites in the sodalite cages into the large cavities. Upon treatment with O2 at 570 K, the bands characteristic of Py Æ In+ disappeared and intense bands at 1454 and 1612 cm–1 were seen which again proved the formation of InO+ species according to Eq. (39). The broad bands in the range 1440–1480 cm–1, which only appeared at a higher degassing temperature (670 K) in the spectra of pyridine interacting with InO+ (cf. C3 and C5 in Fig. 69), were ascribed to strongly adsorbed compounds formed by oxidation of adsorbed pyridine by InO+. In one report by Beyer et al. [277] the same conclusions were consistingly drawn with respect to the systems In2O3/H-ZSM-5 and In2O3/NH4-MOR from IR spectra obtained after RSSIE, subsequent oxidation and another reduction by H2. However, in the case of In2O3/H-ZSM-5, the bands at 1454 and 1612 cm–1 developed already immediately after the RSSIE process, even though with minor intensities and besides the bands typical of In+. This phenomenon, already

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Fig. 70. IR spectra of pyridine retained after adsorption at 470 K and subsequent degassing at 370 K on In2O3/NH4-MOR which was A thermally pretreated at 720 K in high vacuum for 1 h, B subjected to RSSIE in H2, C oxidized with O2 subsequent to RSSIE, and D reduced with H2 after preceding oxidation (steps B-D were performed at 720 K for 0.5 h) (after [277])

observed upon RSSIE in BETA (vide supra), was also attributed to the reaction of In+ with silanol groups [cf. Eq. (42)] known to be generally present in ZSM-5. An apparently unusual chemical behavior was exhibited by In+ cations introduced into NH4-MOR (Fig. 70) inasmuch as no bands characteristic of Py Æ InO+ were observed after oxidation. Nevertheless, oxidation to the trivalent state must have occurred, since the spectrum after subsequent reduction was identical with that obtained after RSSIE, i.e., In+ cations were restored (Fig. 70). To explain these surprising results it was suggested that InO+ cations may occupy hidden sites in the mordenite structure that are not accessible for pyridine. Alternatively, indium ions may be bound in mordenite to five framework oxygen atoms. In this case, the coordination of trivalent indium cations would be fully saturated due to the additional oxygen atom belonging to InO+. Neinska et al. [282–284] succeeded in introducing gallium and indium into SAPO materials (SAPO-5, SAPO-34 and SAPO-37) via reductive solid-state ion exchange in a flow of H2 . RSSIE was evidenced by TPR/TGA and the acidity properties of the modified SAPOs characterized by TPD of ammonia and TGA/TPD of propylamine [284]. The exchange reactions proceeded according to Eqs. (29), (30) and (34), (35), respectively, where now Z– has to represent a negatively charged fragment of the SAPO framework. It was shown that, in agreement with Eqs. (30) and (35), an upper limit for the incorporation of Ga and In existed, determined by the specific number of exchangeable protons. This number decreased in the sequence H-SAPO-34>H-SAPO-5>H-SAPO-37, as measured by TPR/TG and TPD of ammonia. XRD and TEM/EDAX proved that the

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crystal structures of SAPO-5 and SAPO-34 were not markedly damaged during RSSIE. Ga and In were, indeed, introduced into these silicoaluminophosphates. In contrast, SAPO-37 completely lost its crystallinity during RSSIE and/or upon subsequent rehydration, even though TPR clearly indicated that ion exchange had occurred. In contrast, a procedure similar to the template-induced introduction of indium into BETA zeolite [275] described above seemed to be more promising. Neinska et al. [282–284] carried out such experiments with mixtures of In2O3 and as-synthesized, template-containing SAPO-5, SAPO-34 and SAPO-37. Again, the organic templates (SAPO-5: triethylamine; SAPO-34: tetraethylammonium hydroxide; SAPO-37: tetrapropylammonium hydroxide) or their decomposition products acted as reducing agents. The presence of In2O3 facilitated the decomposition of the templates in a flow of pure inert gas at 873 K. In the case of In2O3/as-synthesized SAPO-5 and In2O3/as-synthesized SAPO-37, the DTG features at ca. 700 K, which corresponded in In2O3-free SAPOs to the decomposition of the most strongly bound template species, were significantly shifted to lower temperatures. Neinska et al. observed a marked decrease in the acidity and activity in m-xylene isomerization of the materials prepared in this way compared with those of H-SAPO-n (n = 5, 34, 37). This clearly indicated that ion exchange of In+ had indeed occurred. The procedure of template-induced RSSIE might be particularly helpful for post-synthesis modification of materials such as SAPO-37, the template-free hydrogen form of which suffers from low stability against hydration. Incorporation of indium into MCM-41 by RSSIE was also studied using XRD, FTIR, TPR and TPD techniques [285]. The process occurred as easily as with zeolites. Some typical differences were associated with the particular acid properties of the mesoporous material and were discussed in terms of the peculiar structure and composition of the MCM-41 framework. In view of the general reaction described by Eqs. (29) and (30) attempts were also made to introduce Fe, Cr, La or Eu cations into zeolites by thermal treatment of mixtures of the hydrogen forms of zeolites with Fe2O3, Cr2O3 , La2O3 or Eu2O3 in a reductive atmosphere [273]. Indeed, in high vacuum at 760–790 K, solidstate reactions were observed, but the respective M+ cations could not be stabilized in the zeolite matrix. Rather, the solid-state reaction was accompanied by a collapse of the zeolite structure. Thus, to the best present knowledge, reductive solid-state ion exchange seems to be restricted to reactions with Ga2O3 and In2O3 . Incorporation of Ga or In via RSSIE into hydrogen forms of zeolites significantly affected their catalytic properties. As already mentioned, Ga-ZSM-5 samples obtained by reductive solid-state ion exchange were active and selective catalysts for aromatization of propane and n-pentane [257, 260, 265]. However, a drop in activity was observed with respect to conversion of n-pentane after loading H-ZSM-5 with In via reductive solid-state ion exchange. Moreover, in sharp contrast to the Ga-ZSM-5 discussed above (cf. [259]), reduced In-ZSM-5 prepared via RSSIE yielded almost no aromatics upon reaction of propane or npentane. Surprisingly, a catalyst prepared by simultaneous introduction of Ga (4 wt.%) and In (3 wt.%) through RSSIE into ZSM-5 proved to be superior in

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aromatization of n-pentane to any Ga-ZSM-5 sample with similar Ga loadings. It seems that the combination of both cations (Ga+ and In+) in one and the same zeolite offers a possibility for regulating the catalytic properties, viz., hydrogen transfer ability (by Ga+) and modification of the acidity (by In+). It was also demonstrated by Kanazirev and Price [261] that Ga- and In-containing ZSM-5 samples prepared via thermal treatment of mixtures of H-ZSM5 with Ga2O3 and In2O3 , respectively, resulted in active catalysts for reactions of amines if heat-treatment was carried out in a reductive atmosphere. When the Ga-, In- or Cu-ZSM-5 materials obtained through reductive or auto-reductive solid-state ion exchange were employed for catalytic amine conversion, the reaction pathways were completely changed compared to those over H-ZSM-5. While over H-ZSM-5 at low coverages unimolecular decomposition of adsorbed alkylamines to alkenes and ammonia occurred, dehydrogenation, transalkylation and condensation reactions were the prevailing processes over Ga-, In- or Cu- containing ZSM-5 catalysts produced by RSSIE. This change in the pathways was ascribed to the elimination of Brønsted acidity and formation of Lewis acidity upon RSSIE [261]. Similarly, the catalytic behavior of hydrogen forms of zeolites in methanol conversion was completely changed upon loading with In2O3 via RSSIE. Instead of acid-catalyzed selective dehydration to olefins, dehydrogenation became predominant resulting in products such as H2 , CO, CO2 and CH4 [260].

7 The Role of Water in and Mechanisms of SSIE 7.1 Role of Water in SSIE

In most experiments on solid-state ion exchange with zeolites as discussed in Sect. 5, the reactants (salts, oxides, zeolites) were indeed ‘dry’, but this does not necessarily mean that water was totally excluded. Usually, these experiments were carried out under ambient conditions, so that water could be adsorbed into the zeolitic pore system. However, in particularly designed experiments, which will be discussed below, it has been shown that the presence of adsorbed water, even though not detrimental to SSIE, was in no case a prerequisite for solid-state ion exchange to occur. Seemingly, an exception was the reaction of MoO3 with hydrogen forms of zeolites; here, however, the reaction with water vapor was required to produce small and mobile Mo-containing species, which were able to easily enter the micropores (cf. Sect. 6.3.2). In fact, there are quite a number of examples that suggest that SSIE proceeds well without any assistance from water. Thus, in Sects. 5.3.2.2 and 5.3.3.3 we saw that salts insoluble in water such as AgCl and HgCl2 reacted (under high vacuum and at elevated temperatures) with H-ZSM-5. Similarly, Kucherov and Slinkin [21, 23, 92] reported on solidstate ion exchange with oxides such as CuO and V2O5 , which are also insoluble in water. Many experiments for SSIE were carried out between hydrogen forms of zeolites and chloride in dynamic high vacuum (p£10–5 Pa). Examples were

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discussed, for instance, in Sects. 5.1, 5.2 and 5.3 with respect to the exchange with MICl (MI = Li, Na, K, Rb, Cs), MIICl2 ◊ xH2O (MII = Mg, Ca, Cu, Fe) and MIII ◊ xH2O (MIII = La, Fe). In these cases one would expect that, at the elevated temperatures of solid-state reaction (T ≥ 670 K), water is removed from the reactants. However, it cannot be completely excluded that traces of residual water persist in the salt/zeolite mixtures or, in the case of salts with crystal water, partial hydrolysis of the chlorides occurs. In this context it is, therefore, worth mentioning that a few investigations have been reported in which special measures were taken to carefully exclude water from the very initial steps of the SSIE experiment and then throughout the whole subsequent procedure: The reactants were separately evacuated and heated at about 670 K in ampoules until water vapor was no longer detected by MS and/or IR. The ampoules were then sealed and transferred into an efficiently working glove box (pH2O £ 10–7 Pa).All the subsequent steps of the sample preparation were then carried out in this glove box, i.e., breaking the ampoules; mixing the salts and zeolite powders; filling the mixtures into capillaries for XRD runs and sealing them; pressing wafers for IR measurements; transferring the wafers into sample holders and these into an ultra-high vacuum-tight IR cell (cf. [286]). One example was the solid-state reaction of a mixture of NaCl and HMOR (cf. [130]). After heating the IR cell, which was connected to an ultra-high

Fig. 71. IR spectra of the OH stretching frequency range of a the parent zeolite, H-L (NH4-L heated at 500 °C in high vacuum), and a mixture of LaCl3 (water-free) and H-L after evacuation (10–5 – 10–6 Pa) at b 300; c 400; and d 525 °C (after [287], with permission)

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Fig. 72. IR spectra of the OH stretching frequency range of a the parent zeolite, H-MOR (NH4MOR heated at 500 °C in high vacuum), and a mixture of LaCl3 (water-free) and H-MOR after evacuation (10–5–10–6 Pa) at b 300; c 400; and d 525 °C (after [287], with permission)

vacuum device, IR spectroscopy revealed that the OH groups of the H-MOR component had completely disappeared. This was ascribed to a 100% exchange of the protons of H-MOR for Na+ from NaCl. Another set of experiments was reported for SSIE with lanthanum chloride under complete exclusion of H2O (cf. [287]). Here, water-free LaCl3 was prepared from LaCl3 ◊ 7H2O by heating the salt in a flow of HCl to avoid any hydrolysis and the formation of lanthanum oxychlorides. Subsequently, this water-free LaCl3 was sealed under high vacuum in an ampoule, transferred to a glove box and further treated as indicated above. Mixtures of water-free LaCl3 with H-L, H-MOR, ultrastabilized (dealuminated) H-Y, i.e., H-S-Y, or ferrierite (FER) were employed. The IR measurements provided clear evidence that the intensities of the bands indicating the reactive OH groups had significantly decreased, i.e., SSIE has occurred in the case of H-L, H-MOR and H-SY under these conditions of absolute exclusion of water vapor (cf. Figs. 71 and 72). The acidic OH groups of H-L (nSi/nAl = 3.18) completely disappeared and were markedly decreased in the case of H-MOR (nSi/nAl = 6.78). The fact that with H-MOR not all of the OH groups were replaced by La3+ is easily understood if one takes into account the high nSi/nAl ratio: similar to H-ZSM-5 (cf. Sect. 5.2.4), the distance between the acid OH groups is larger than in regular faujasite-type H-Y (nSi/nAl = 2.5) or H-L and it is, therefore, more difficult to com-

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Fig. 73. IR spectra of the OH stretching frequency range of a the parent zeolite, stabilized H-Y (H-S-Y), and a mixture of LaCl3 (water-free) and stabilized H-Y (H-S-Y) after evacuation (10–5 – 10–6 Pa) at b 400; c 500; and d 600 °C (after [287], with permission)

pensate the negative charge of three of those sites by one La3+ cation. For the same reason also with ultrastabilized faujasite-type zeolite (H-S-Y, nSi/nAl = 8.9) only a partial solid-state ion exchange was possible. However, this zeolite contained a large amount of silanol groups that also reacted (cf. Fig. 73). In contrast, water-free LaCl3 did not exchange with H-ferrierite at all, i.e., no decrease in the OH bands was observed indicating the failure of SSIE. The latter result will be discussed in Sect. 7.2. By a comparative experiment similar to that reported in Sect. 5.2.5, the incorporation of La3+ cations from water-free LaCl3 into H-L, H-MOR and H-S-Y was qualitatively confirmed by the changes in the intensities of the corresponding framework reflections in the XRD patterns. These were obtained in situ under high vacuum in a heatable XRD chamber [287]. No changes in the XRD reflections of the framework of ferrierite were observed upon calcination of a LaCl3/H-FER mixture which confirms the IR results reported above. 7.2 Possible Mechanisms of SSIE

In solid-state ion exchange, crystallites of salts or oxides are the sources of the in-going cations and must be brought into intimate contact with the zeolite crys-

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tallites into which the cations should be exchanged. In Sect. 6.1 it was already mentioned that the transport of the involved species from the salt or oxide crystallites to the zeolite surface may occur through the vapor phase or via surface diffusion. In most cases the latter possibility appears more likely, since many of the salts and oxides employed in SSIE have, even at the temperatures of solidstate ion exchange of about 530–730 K, a rather low vapor pressure. Another important question arises with respect to the nature of the species separated from the salt or oxide crystallites and subsequently diffusing into the (adsorbate-free) channels and cavities of the zeolite crystallites, viz., as to whether these species are molecules or ions. Usually, the effort required to separate a molecule from the kink of the surface of a salt or oxide crystal (i.e., from a so-called “Halbkristall-Lage”, i.e., the position of the “half-crystal”; cf. [288]) is lower than that necessary to remove a cation and an anion in sequence. This was first computed by Stranski [288] for the case of a sodium chloride crystal in contact with its diluted vapor. A related question is, whether the species migrate from the external surface of the zeolite crystallites to the interior of the structure as molecules or whether cations and ions travel separately? With respect to this question, mainly two mechanistic models seem to be conceivable: (A) Cations and anions of the salt or oxide migrate simultaneously (most likely as molecules) reacting in the interior of the zeolite, in that the original cations are replaced by the in-going ones, combine with anions of the salt or oxide and leave the structure together with them. (B) Cations stemming from the salt or oxide migrate into the pores via hopping from site to site to replace cations of the zeolite which have to move in the opposite direction and combine (possibly outside the zeolite structure) with anions of the salt or oxide. This counter-diffusion of the in-going and out-going cations must proceed in such a way that no excessive electrical gradients occur, i.e., the charge balance must be sustained. Both main models are schematically represented in Fig. 74.

Fig. 74. Schematic representation of two possible models of the mechanism of solid-state ion exchange in microporous materials. Mechanism A, (top): NaCl molecule diffuses. Mechanism B, (bottom): Na+ and H+ counter-diffuse (see text)

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Indeed, modifications of both models may be visualized. Models A and B are difficult to discriminate experimentally. However, there were some indications that at least in several cases model A applied. When, for instance, a salt with a bulky anion such as Cs3 [PW12O40] was reacted with H-ZSM-5, solid-state ion exchange was suppressed to a large extent. This was ascribed to the fact that the large [PW12O40]3– anion prevented the salt molecule from entering the zeolite channels [43]. (In fact, solid-state reaction did occur to a minor extent because of partial decomposition of Cs3 [PW12O40].) Similarly, the failure of ion exchange between LiX and NaX in the absence of adsorbed water (vide supra, Sect. 6.2) could be interpreted as a sign that the cations were unable to separate from the anionic framework of the zeolite (visualized as a bulky polymeric anion) and thus could not migrate. Solid-state ion exchange with a highly covalent compound such as CuCl suggested that most likely intact CuCl molecules migrated into the zeolite structure (cf. Sects. 5.3.2.1 and 8). Finally, we have seen that water-free LaCl3 could be easily introduced into water-free zeolites such as largepore H-L, H-MOR and H-S-Y, but an analogous experiment with H-ferrierite was unsuccessful (Sect. 7.1). In fact, La3+ cations, having a diameter of 0.208 nm (after Goldschmidt) or 0.230 nm (after Pauling), should be small enough to enter the structure through its 10-membered and even through its 8-membered rings with diameters of 0.42¥0.54 nm and 0.35¥0.48 nm [180], respectively. Thus, the failure of SSIE in the system LaCl3/H-FER supports the assumption that the ingoing species are salt molecules rather than separated ions, in that the LaCl3 molecule is obviously too bulky to enter the ferrierite pores [287].

8 Kinetics of SSIE In general, only qualitative observations have been reported with respect to the kinetics of solid-state ion exchange. Thus, it was frequently recognized that SSIE was initially fast and then its rate levelled off. Increasing the amount of the salt or oxide in the mixtures with the zeolite powders usually resulted in an increase in the rate of exchange, possibly due to an enhanced concentration gradient of the in-going cation. Similarly, the exchange was reported to accelerate when the temperature of the solid-state ion exchange was raised. However, systematic investigations of the kinetics of SSIE carried out to date are rather scarce. In principle, kinetics of SSIE could be determined through in situ measurements of, e.g., the time-resolved changes in the intensities of XRD reflections in the pattern of the salt (or oxide)/zeolite mixtures upon heat-treatment (cf. Sects. 5.2.5 and 5.3.2) or of typical IR bands. The IR method may use the signals of lattice vibrations (cf. [60–63]), bands in the OH or NH stretching region (in the case of H- or NH4-forms of zeolites), or IR bands characteristic of interactions between the in-going and/or out-going cations and probe molecules. With respect to the latter method, however, a tacit assumption is made, viz., that the presence of the probe molecules does not affect the kinetics of solid-state ion exchange. IR spectroscopy using pyridine as a probe was employed in the investigation of SSIE of CuCl with Na-Y and Na-MOR (cf. [289, 290]): The respective experi-

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Fig.75. IR spectra of pyridine adsorbed on a CuCl/Na-Y mixture at 533 K as a function of time (for details see text; after [289], with permission)

ments were conducted in a cell, where in the upper part the CuCl/Na-zeolite wafer could be dehydrated at 390–425 K, while the lower compartment with IRtransmittant CaF2 windows was brought to the reaction temperature, Treact (cf. [286, 291]). At zero time, the sample was moved from the upper part of the cell into the lower compartment preheated to the reaction temperature, and simultaneously the probe (pyridine) was admitted. Figure 75 displays a set of selected spectra for the system CuCl/Na-Y run during the solid-state reaction. Initially, only the Na+ ions were indicated by bands at 1592 and 1442 cm–1 originating from Py Æ Na+ complexes. However, when the temperature in the lower compartment was above ca. 450 K, at first shoulders and, after a period of time, bands at 1604 and 1451 cm–1 developed. These were indicative of Cu+ (Py Æ Cu+) populating a fraction of the cation sites in the Y-zeolite structure where they had replaced the Na+ cations. As a consequence, the intensities of the IR bands typical of Py Æ Na+ concomitantly decreased until a steady state was established. At temperatures above 670 K, however, the changes in the band intensities were reversed, i.e., the bands due to Py Æ Cu+ were weakened and those indicative of Py Æ Na+ increased. This was ascribed to a shift of the exchange equilibrium [cf. Eq. (43)], i.e., to a remigration of Na+ to and removal of Cu+ from the cation sites (cf. Scheme 1b; Sects. 5.1.10, 5.2.5 and 5.2.6 with respect to the systems BeCl2/Na-Y and LaCl3/Na-Y): CuCl + Na-Y ¤ NaCl + Cu-Y

(43)

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Fig. 76. Example of a correction of the reaction isotherms (at Treact = 493 K) for the solid-state reaction of CuCl/Na-Y as monitored by the IR spectra of adsorbed pyridine; the correction accounts for the temperature dependence of the pyridine adsorption (for details, see text; after [289], with permission)

Even though the IR bands overlapped, it was possible to determine the proper integrated absorbances after appropriate decomposition of the spectra. This was achieved via fitting the spectra by mixed Gaussian-Lorentzian functions [290]. To obtain curves describing the exchange kinetics, the integrated absorbances had to be plotted as a function of the reaction time. However, one had to be aware of the fact that, for a given exchange temperature, the absorbances, At , of the bands not only depended on the amount of Cu+ introduced and Na+ replaced at a given time, t, but also on the temperature-dependent adsorption equilibrium. Thus, in order to compare the rates of uptake of Cu+ and replacement of Na+ for different temperatures, the influence of the adsorption equilibrium of pyridine on the band intensities had to be taken into account. Since the effect of minor changes of the pyridine pressure during an SSIE experiment (from, e.g., 500 to 400 Pa) turned out to be negligible, the necessary correction could be achieved with the help of an experimentally determined adsorption isobar of pyridine. As adsorbents, CuCl/Na-Y and CuCl/Na-MOR wafers were used which had been previously heat-treated at 533 K until the steady state of exchange was reached. With the isobars obtained in this way, it was possible to relate all the absorbance data measured at lower temperature to the adsorption equilibrium at 533 K; an example of such a correction is illustrated in Fig. 76. The curves with

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Fig. 77. Normalized and corrected integrated absorbances from pyridine adsorption on CuCl/Na-Y during solid-state ion exchange as a function of reaction time (for details, see text; after [289], with permission)

the open symbols are plots of ion exchange kinetics (absorbances vs. reaction time) providing the corrected data that would have been obtained after exchange at 493 K but under the adsorption equilibrium of pyridine at 533 K. From Fig. 76 the correction necessary because of the temperature dependence of the adsorption equilibrium of the probe is obvious. For the correlation temperature of 533 K, the correction would be zero, i.e., the respective curves would coincide. The above correction procedure enabled a comparison of all measurements at Treact £ 533 K. Results for the system CuCl/Na-Y are shown in Fig. 77. The data were normalized to equal sample thickness (5 mg cm–2). The effect of the reaction temperature on the rates of the Cu+ introduction and Na+ replacement alone, i.e., after removal of the temperature effect on the adsorption equilibrium, can be readily recognized. As expected, the rates increase with increasing temperature, whereas the final steady state of SSIE was independent of the temperature. For the temperature of 453 K this was confirmed by extending the reaction time to 25 h. Similar results to those obtained for CuCl/Na-Y were obtained for the system CuCl/Na-MOR [289, 290] and for zeolites containing K+, Rb+ or Cs+ as (out-going) cations [292]. It was tentatively assumed that the exchange kinetics were diffusion-controlled. The reasonable assumption was made that the absorbances of the Py Æ Cu+ bands at a given time, t, and at steady state, i.e., At and At Æ • , are proportional to

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Fig. 78. Description of the kinetics of solid-state ion exchange in the system CuCl/Na-Y through a diffusion model; the symbols represent experimental data derived from the measured integrated absorbances of the probe (pyridine), the broken lines represent results of the fitting to the diffusion model (for details, see text; after [289], with permission)

Fig. 79. Arrhenius plot of the diffusion coefficients evaluated from the description of the kinetics of solid-state ion exchange in the systems CuCl/Na-Y and CuCl/Na-M through a diffusion model (for details, see text; after [289], with permission)

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the amounts of incorporated Cu+, i.e., Mt and Mt = • . Then, an attempt was made to describe the kinetics curves (cf., e.g., Fig. 77) by solutions of Fick’s second law (cf. [290]). Appropriate solutions were provided by Crank [293] for sphere-like (Na-Y) and membrane-like (Na-MOR) adsorbent particles. Indeed, it turned out that such a description is possible (cf. Fig. 78) which, however, does not necessarily mean that SSIE is in fact a process controlled by Fickian diffusion. Figure 79 shows an Arrhenius plot of the thus-determined values of ln (Dt0 /R2) vs. 1/T, where D, t0 , R, T represent the diffusion coefficient, selected time after beginning of the exchange process, particle radius and the exchange temperature, respectively. From the slopes of the straight lines activation energies of EA ~ 70 kJ mol–1 were derived. Even under the assumption that the process is properly described by the diffusion model described above, the magnitude of EA unfortunately did not provide unambiguous support for the above proposal that molecules (CuCl) rather than cations (Cu+) are the diffusing species, since the activation energies for cation diffusion were found to be of about the same magnitude [294, 295].

9 Conluding Remarks As we have seen, a great variety of zeolites and related materials can be modified via solid-state reactions with a similarly broad variety of compounds, i.e., salts or oxides of the desired in-going cations. Solid-state modifications occur most easily when halides (sometimes nitrates) of the cations and hydrogen forms of the materials to be modified are employed. Often a 100% degree of exchange can be achieved in one step. In several cases, however, sodium forms and complex cations may also be used. Also, an extended arsenal of techniques is now available for monitoring and quantitative analysis of solid-state ion exchange. Thus, solid-state reactions of microporous (and mesoporous) materials have become, during the past decade, a well-established method for their post-synthesis modification. Furthermore, related methods such as oxidative or reductive incorporation of cations into microporous solids through solid-state reactions have been developed. Similar modifications of the procedure of cation introduction into zeolites as well as the extension to other systems are likely to come. However, a number of open questions remain to be answered, concerning a deeper understanding of solid-state modifications of zeolites and related materials; pertinent problems are, for instance, the thermodynamics, kinetics and mechanisms.

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261. Kanazirev V, Price GL (1994) In: Weitkamp J, Karge HG, Pfeifer H, Hölderich W (eds) Zeolites and Related Microporous Materials: State of the Art 1994. Proc 10th Int Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17–22, 1994, Elsevier, Amsterdam, p 1935. Stud Surf Sci Catal 84:1935 262. Price GL, Kanazirev V, Dooley KM, Hart VI (1998) J Catal 173:17 263. Kazansky VB, Kustov LM, Khodakov AYu (1989) In: Jacobs PA, van Santen RA (eds) Zeolites: Facts, Figures, Future. Proc 8th Int Zeolite Conf, Amsterdam, The Netherlands, July 10–14, 1989, Elsevier, Amsterdam, p 1173. Stud Surf Sci Catal 49:1173 264. Endoh A, Nishimiya K, Takaishi T (1989) In: Karge HG, Weitkamp J (eds) Zeolite as Catalysts, Sorbents and Detergent Builders – Applications and Innovations (1989) Proc Int Symposium,Würzburg, Germany September 4–8, 1988. Elsevier,Amsterdam, p 779. Stud Surf Sci Catal 46:779 265. Halász J, Konya Z, Fudala A, Kiricsi I (1996) Catal Today 31:293 266. Kanazirev VI, Price GL, Dooley KM (1994) J Catal 148:164 267. Kikuchi E, Ogura M, Terasaki I, Goto Y (1996) J Catal 161:465 268. Ogura M, Aratani N, Kikuchi E (1997) In: Chon H, Ihm S-K, Uh YS (eds) Progress in Zeolite and Microporous Materials. Proc 11th Int Zeolite Conference, Seoul, Korea, August 12–17, 1996. Elsevier, Amsterdam, p 1593. Stud Surf Sci Catalysis 105B: 1593 269. Ogura M, Ohsaki T, Kikuchi E (1998) Microporous Mesoporous Mater 21:533 270. Zatorski LW (1987) Bull Pol Acad Sci 35:325 271. Beyer HK, Mihályi RM, Minchev Ch, Neinska Y, Kanazirev V (1996) Microporous Mesoporous Mater 7:333 272. Mihályi RM, Pál-Borbély G, Beyer HK, Minchev Ch, Neinska Y, Karge HG (1997) React Kinet Catal Lett 60:195 273. Mihályi RM, Beyer HK (1998) In: Rozwadowski M (ed) Proc 3rd Polish-German Zeolite Colloquium, Torun, Poland, April 3–5, 1997, Nicholas Copernicus University Press, Torun, p 309 274. Mihályi RM, Beyer HK, Mavrodinova V, Minchev Ch, Neinska Y (1998) Microporous Mesoporous Mater 24:143 275. Neinska Y, Mihályi RM, Mavrodinova V, Minchev Ch, Beyer HK (1999) Phys Chem Chem Phys (PCCP) 1:5761 276. Kanazirev V, Valtchev V, Tarassov MP (1994) J Chem Soc Chem Commun 1043 277. Mihályi RM, Beyer HK, Neinska Y, Mavrodinova V, Minchev Ch (1999) React Kinet Catal Lett 68:355 278. Mihály RM, Beyer HK, Pál-Borbély G (2001) submitted to J Chem Soc Chem Commun 279. Broch NC, Christensen AN (1966) Acta Chem Scand 20:1996 280. Herman RG, Lunsford JH, Beyer H, Jacobs PA, Uytterhoeven JB (1975) J Phys Chem 79:2388 281. Beyer H, Jacobs PA, Uytterhoeven JB, Vandamme JL (1976) In: Proc 6th Int Congress Catalysis, London, UK, July 12–16, 1976, Chemical Society, London, p 1 (A18) 282. Neinska Y, Minchev Ch, Dimitrova R, Micheva N, Minkov V, Kanazirev V (1994) In: Weitkamp J, Karge HG, Pfeifer H, Hölderich W (eds) Zeolites and Related Microporous Materials: State of the Art 1994. Proc 10th Int Zeolite Conference, GarmischPartenkirchen, Germany, July 17–22, 1994, Elsevier, Amsterdam, p 989. Stud Surf Sci Catal 84:989 283. Neinska Y, Minchev Ch, Kozova L, Kanazirev V (1995) In: Beyer HK, Karge HG, Kiricsi I, Nagy JB (eds) Catalysis by Microporous Materials. Proc ZEOCAT ’95, Szombathely, Hungary, July 9–13, 1995, Elsevier, Amsterdam, p 262. Stud Surf Sci Catalysis 94:262 284. Neinska YG, Mavrodinova VP, Kanazirev VI, Minchev CI (1998) Bulg Chem Commun 30:357 285. Neinska Y, Mavrodinova V, Minchev C, Mihály RM (1999) In: Kiricsi I, Pál-Borbély G, Nagy JB, Karge HG (eds) Porous Materials in Environmentally Friendly Processes. Proc. 1st Int FEZA Conference, Eger, Hungary, September 1–4. Elsevier, Amsterdam, p 37. Stud Surf Sci Catal 125:37

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Dealumination Techniques for Zeolites Hermann K. Beyer Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri út 59–67, 1025 Budapest, Hungary; e-mail: [email protected]

Dedicated to Professor Gerhard Ertl on the occasion of his 65th birthday

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Introduction and Scope . . . . . . . . . . . . . . . . . . . . . . . . . 204

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Extraction of Framework Aluminum by Chemical Agents . . . . . . 205

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Dealumination with Acids . . . . . . . . . . . . . . . . . Dealumination with Complexing Agents . . . . . . . . . Instability of Hydrogen Zeolites Towards Liquid Water . Gaseous Halogen Compounds as Dealuminating Agents

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Hydrothermal Dealumination of Zeolite Frameworks . . . . . . . . 213

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Early Fundamental Investigations Review of Recent Investigations . Faujasite-Type Zeolites . . . . . . ZSM-5 . . . . . . . . . . . . . . . Other Zeolites . . . . . . . . . . . Thermal Dealumination . . . . .

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Dealumination with Silicon Tetrachloride . . . . . . . . . . . Faujasite-Type Zeolites . . . . . . . . . . . . . . . . . . . . . . Other Zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . Isomorphous Substitution with Other Silicon Halides . . . . Dealumination with (NH4)2[SiF6] Solutions . . . . . . . . . . Dealumination of Zeolites in Dry Mixtures with (NH4)2[SiF6]

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Alumination with Gaseous Aluminum Chloride Alumination with Aqueous Fluoroaluminates . Alumination with Aluminate Solutions . . . . . Re-Insertion of Extra-Framework Aluminum .

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1 Introduction and Scope The framework Si/Al atomic ratio of zeolites is an important parameter that exerts a strong influence on properties such as maximum ion-exchange capacity, thermal and hydrothermal stability, hydrophobicity, concentration and strength of acid sites of the Brönsted-type, which may be generated in zeolite structures, and catalytic activity and selectivity. Though in some processes, e.g., in ion-exchange procedures, low Si/Al ratios may be preferred, generally zeolites with low aluminum content (high Si/Al ratio) are more favorable, especially when applied as catalysts. However, the framework Si/Al ratio of zeolites prepared by direct synthesis, such as by direct hydrothermal crystallization in absence of templates, is generally restricted to more or less narrow limits. One of the technically most important members of the zeolite family, faujasite, cannot be directly synthesized with Si/Al ratios substantially higher than 2.5, at least not in economically reasonable crystallization times. It is, therefore, of great importance to find methods that can increase the Si/Al ratio by chemical post-synthesis modification of zeolite frameworks, i.e., by dealumination. In the strict sense of the word, the term “dealumination” refers to the removal of aluminum from zeolite frameworks by chemical reactions generally resulting in lattice deficiencies. However, in its general use, it relates to the more complex process comprising the incorporation of other elements, especially of silicon, into the transient framework vacancies left temporarily by the release of aluminum. Processes which increase the Si/Al ratio of zeolite structures may be subdivided into three categories: 1. Those involving only the removal of framework aluminum by chemical agents or – in case of hydrogen forms of zeolites – by thermal dehydroxylation, thereby resulting in lattice defects. 2. Those including, in addition to the mere extraction of framework aluminum, a second step in which framework vacancies are filled in, e.g., by intrinsic silicon and oxygen atoms migrating in the zeolite lattice under hydrothermal conditions. 3. Those representing true substitution reactions between the aluminum component of the framework and the dealumination agent, being a compound of the element to be incorporated, such as silicon. The present review is aimed at covering first the literature which has appeared since about 1985 on both methodical and mechanistic aspects of dealumination techniques and on their structural and compositional consequences; although fundamental and pioneering contributions published before this date will also be included. Progress made in the last decade in the field of alumination and desilication of zeolites, i.e., of processes closely related in nature to the main subject, will also be reviewed. However, this review does not refer to papers dealing exclusively or predominantly with special properties (e.g., acidity, catalytic activity, hydrophobicity) and applications of dealuminated zeolites and to the isomorphous replacement of framework aluminum by elements other than silicon.

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The older literature on dealumination of zeolites has been extensively reviewed by Scherzer [1], Stach et al. [2] and, with emphasis on mordenite, by Karge and Weitkamp [3]. Recently, Sulikowski [4] dealt with dealumination and alumination of zeolites as part of a more general review.

2 Extraction of Framework Aluminum by Chemical Agents 2.1 Dealumination with Acids

Removal of aluminum from a zeolite framework was first reported in 1964 by Barrer and Makki [5]. They extracted aluminum from clinoptilolite by refluxing with hydrochloric acid. Depending on the acid concentration, up to 100% of the framework aluminum could be removed; however, the thermostability of the products gradually decreased at dealumination degrees higher than 65%. Dealumination with acids was accompanied by ion exchange of lattice cations by protons. The overall process was suggested to proceed according to Eq. (1) under formation of defect sites later generally denoted by the term “hydroxyl nest”.

(1) Dealumination with mineral acids was also successfully applied to erionite [6], mordenite [7, 8], offretite [9] and ZSM-5 [10]. The dealumination of mordenite with mineral acids was monitored by 27Al and 29Si MAS NMR spectroscopy [11] and compared with other dealumination procedures (steaming, reaction with SiCl4). At the beginning of the process, acid leaching generates, in agreement with the stoichiometry of Eq. (1), about four SiOH groups per one Al extracted. Further dealumination was found to lead to a reorganization of the structure, even at 100°C, as shown by the decreasing amount of defects. A hypothesis on the location of framework aluminum in the mordenite structure and the dealumination mechanism was presented. Karge and Dondur [12] used ammonia TPD to study the distribution of acidity in mordenites dealuminated by acid leaching The influence of acid leaching with nitric acid of different concentrations on unit cell parameters, relative crystallinity, adsorption behavior and amount of framework and extra-framework aluminum species of mordenites of different origin was investigated by van Niekerk et al. [13]. It was found that the extent of dealumination and the amount of extra-framework aluminum remaining in the zeolite channels was strongly influenced by the crystallite size and that dealumination is associated with a partial loss in crystallinity. A systematic study of acid leaching of sodium and hydrogen-exchanged mordenite has been reported [14] that deals with the formation and removal of extraframework aluminum species and the creation of acidity upon dealumination.

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The framework of beta zeolites was found to dealuminate upon mild acid leaching procedures [15, 16]. Refluxing in 0.5 N and 1 N hydrochloric acid for 4 h resulted in an increase of the bulk Si/Al ratio from 11.5 to 32 and from 19 to 70 for samples prepared in alkaline and fluoride medium, respectively [15]. The extraction of aluminum from as-synthesized zeolite beta still containing the tetraethylammonium template with nitric acid of different concentrations was studied [16]. Samples of zeolite beta with Si/Al ratios greater than 1000 comprising mesopores and three different types of silanol defect groups were obtained in a single step without significant loss of crystallinity, porous volume or thermal stability. Depending on the acid concentration, Nu-2 zeolite could be progressively dealuminated up to a Si/Al ratio of 90 by treatment with hydrochloric acid at room temperature [17]. The removal of aluminum was found to be associated with a gradual but slight decrease in crystallinity (maximum 20%). ZSM-5 was reported to release framework aluminum completely upon treatment with 1 N hydrochloric acid [18]. In contrast, Kornatowski et al. [19] found that ZSM-5 can be only partly dealuminated by acid treatment. Recently, Kooyman et al. [20] reported that the bulk aluminum content of ZSM-5 zeolites could not be significantly decreased by extraction with 1 N hydrochloric acid even at temperatures as high as 160°C. From this it follows that the framework aluminum content was little affected since the amount of extra-framework aluminum detected by 27Al MAS NMR spectroscopy after acid leaching at 80°C, somewhat dependent on the zeolite preparation mode, was found to be rather small. HBr and H2SO4 proved to be even less effective than HCl. The high stability towards dealumination by acid leaching is attributed to the virtual absence of structural defects in the ZSM-5 samples studied. It is obvious that dealumination of aluminum-rich zeolite frameworks resulting in the formation of high lattice defect concentrations should diminish the stability of the crystal structure. As early as 1958 it was reported [21] that the structure of faujasite-type zeolites collapsed completely upon treatment with strong mineral acids. However, Lee and Rees [22] have shown that the crystal structure of Y zeolite is not significantly affected if the amount of HCl applied in aqueous solutions does not exceed 10 mmol/g Na-Y which results in the release of 56% of the framework aluminum atoms and in the complete exchange of the sodium cations. Thus, at least part of the aluminum in Na-Y zeolite can be extracted without considerable lattice destruction if HCl is applied in amounts that do not yet cause too intense dealumination. For a considerable period of time, contradictory statements have been made in the literature on the thermal stability of zeolites dealuminated by acid leaching. It seems to be self-evident that the thermal dehydroxylation of hydroxyl nests results in the formation of new Si-O-Si bonds and, hence, at least in local rearrangements of the framework atoms in the neighborhood of the vacancies. This reasoning is supported by the partial loss in crystallinity upon acid leaching reported in many of the papers reviewed above. It is to be expected that structures with higher concentrations of such defects are less thermostable than undisturbed lattices. However, there are also several publications (cf. e.g., [8]) in which it is explicitely stated that the extraction of framework aluminum resulted

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in a significant increase in the thermal stability. Most probably, in these studies, the experiments were performed in such a way that the water steam formed during the thermal treatment as a reaction product of the dehydroxylation of hydroxyl nests remained in contact with the zeolite sample for a longer time (e.g., in case of deep-bed calcination). Under these conditions the process of “ultrastabilization” (see Sect. 3.1) may have occurred. This process probably played the decisive role also in the formation of the secondary pore system with channel diameters of 2 and 3.4 nm observed by Wolf and John [8] in mordenite dealuminated by acid leaching. For Na-mordenite it has definitely been shown [23] that the thermal stability significantly decreased after removal of about 80% of the framework aluminum by acid leaching but increased again and even surpassed considerably the stability of the parent material after subsequent steaming. This is illustrated by the dependence of the thus-called “X-ray crystallinity”, i.e., the intensities of selected reflections, on the calcination temperature in Fig. 1. The intensity of the (150) reflection, which proved to be nearly unaffected by acid leaching and steaming, was selected as standard. Though the intensities of XRD reflections do not depend solely on the crystallinity and, hence, do not always give correct information on the lattice destruction, the relative crystallinity values plotted in Fig. 1 may be considered as informative since they are related to the respective starting material and the chemical composition of the samples does not change basically during the calcination process.

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2.2 Dealumination with Complexing Agents

In 1968, Kerr prepared dealuminated Y zeolites by extraction of framework aluminum from Na-Y with H4EDTA solution at ambient temperature [24]. Up to 50% of the aluminum could be removed without any substantial loss in crystallinity and it was claimed that the products showed improved thermal stability. The dealumination mechanism proposed by Kerr [25] comprises: 1. hydrolysis of Si-O-Al bonds, obviously provoked by the acidity of the agent, which results in the extraction of aluminum from the framework, and 2. solubilization and, hence, mobilization of the formed cationic non-framework aluminum species by complexation with EDTA. In line with this dealumination mechanism, no reaction was found to occur between NaY zeolite and the non-acidic Na2H2EDTA. Datka et al. [26] reported that leaching of Na-Y zeolite with a 0.4% solution of H4EDTA at 100°C for 1 h gives defective crystals with a framework Si/Al ratio of about 3.2 in which framework vacancies created by the release of aluminum are not healed by silicon from other parts of the crystals. Ciembroniewicz et al. [27] presented evidence for the creation of a secondary pore system with pore diameters of about 3 nm upon treatment of zeolite Y with H4EDTA, especially if more than 40% of the framework aluminum was removed. It was suggested that these mesopores were associated with a gel phase formed by gradual amorphization of zeolitic material during the process. Thus, this observed phenomenon was not attributed to a secondary mesopore system inside the zeolite crystals, as later found by Lohse et al. [28] in hydrothermally dealuminated Y zeolite. It seems that dealumination with chelating agents is essentially an acid-leaching process where the effectiveness is enhanced by complexing of the aluminum species formed as reaction products. The process proceeds stoichiometrically (see Fig. 2) so that it can be controlled by the amount of H4EDTA calculated for dealumination to the desired level [24, 29]. Other acidic chelating agents, e.g., acetylacetone [30, 31], tartaric acid [32] and oxalic acid [33–35], have been successfully applied for the dealumination of zeolites. According to a patent assigned to the Mobil Oil Corp. [36], up to 40% of the aluminum content of zeolites with Si/Al ratios greater than 1.5 (zeolite Y, zeolite T, erionite, clinoptilolite, phillipsite) can be removed without substantially destroying crystallinity when the parent zeolite is subjected to reflux in an aqueous solution of chromium salts, preferably of CrCl3. The pH of the solution should be less than 3.5. Chromium was found to be incorporated into the zeolite partly by ion exchange but also, e.g., into faujasite up to 6 wt.%, in a non-exchangeable form. This procedure belongs to the family of processes dealt with in this section since the dealuminating effect of chromium salt solutions is assumed to be due to the solubilization of hydrolyzed aluminum species by formation of soluble binuclear aquo-hydroxy complexes comprising both chromium and aluminum as central atoms [37].

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Garwood et al. [36, 38] reported that subsequent to dealumination with EDTA a significant amount of silicon could be digested and removed by refluxing with 1 N NaCl (or other salt) solutions while silicon was less readily removable after dealumination with chromium chloride. This behavior was considered to evidence the incorporation of chromium into framework vacancies left after release of aluminum, resulting in “healing” of the lattice ruptures and, hence, in more stable structures. Liu and Xu [39] reported on a limited increase in the Si/Al ratio of NH4-Y from about 2.5 to 3.4 upon treatment with 0.1 M aqueous solutions of NH4[BF4] at 60°C for 24 h. They suggested that aluminum is released from the framework induced by slow hydrolysis of the boron complex and, subsequently, silicon is incorporated into the lattice vacancies left by dealumination. The silicon involved in the claimed healing process was believed to originate “from dissolution of atoms located on the external surface of the zeolite” and from amorphous silica (if any). However, in this case, the insertion of silicon into the framework

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is doubtful since the authors draw conclusions only from changes in the crystallographic lattice constant and the intensities of 29Si MAS NMR signals obtained without applying 1H cross polarization. These methods alone do not allow definitive conclusions to be drawn concerning the Si/Al ratio after dealumination processes, all the more so as a new IR band found at 3738 cm–1 clearly indicated the formation of silanol groups generally associated with lattice deficiencies. It is highly probable that in the reported dealumination process aluminum is extracted from the framework by acidity created by partial hydrolysis of the boron complex and then removed from the zeolite due to conversion into the soluble complex salt NH4[AlF4]. In conclusion, the complexing agent must be acidic in order to be applicable for the removal of framework aluminum. 2.3 Instability of Hydrogen Zeolites Towards Liquid Water

H-Y zeolite, obtained by thermal deammoniation of the ammonium form, was found to be thermally extremely unstable after resorption of water from the atmosphere [40, 41]. This phenomenon was ascribed to a framework dealumination process similar to that observed upon acid leaching. Later, Beyer et al. [42] observed that full rehydration of H,Na-Y (exchange degree 78%), obtained by thermal deammoniation of the respective ammonium form, resulted in a considerable loss in X-ray crystallinity upon heating at relatively low temperatures (180°C) and virtually affected the crystal structure already at ambient temperature inasmuch as the original ammonium form could not be fully re-obtained by adsorption of ammonia. In contrast, partial re-adsorption of water (77 mg/g) did not show any effect on the thermal stability of the lattice. Furthermore, the crystal structure of hydrogen zeolites stable towards the attack of mineral acids (mordenite, clinoptilolite) proved to be not affected by readsorption of water. Thus, the instability of H,Na-Y was attributed to the removal of framework aluminum or, at least, hydrolytic cleavage of Si-O-Al bonds by the intrinsic acidity of the hydrated H-zeolite which becomes manifest when lattice hydrogen atoms form H3O+ ions in intracrystalline “liquid” water. Confirming these findings, Karge [43] provided infrared spectroscopic evidence for the instability of the lattice of H,Na-Y upon contact with water vapor. Maessen et al. [44] prepared H,Na-Y zeolites with exchange degrees of 70 and 87% from the respective ammonium forms by lowtemperature plasma calcination in a flow of oxygen in order to avoid lattice damage by thermal effects. The products were characterized in the fully rehydrated state. Deammoniation was monitored by IR spectroscopy but not quantitatively evidenced by chemical analysis (e.g., Kjeldahl titration). The products were not subjected to a subsequent heat treatment at low temperatures. Thus, this study is irrelevant to the thermal stability of H-Y zeolite in the presence of intracrystalline water. Nevertheless, the crystallinity of the rehydrated high-exchanged sample was found to deteriorate even at ambient temperature, while the low-exchanged H,Na-Y zeolite proved to be not affected. In any case, rehydrated H-Y zeolite is a highly delicate material the structure of which may be, depending on the exchange degree and the framework Si/Al

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ratio, more or less affected at low or even ambient temperature by the intrinsic acidity. Nevertheless, the influence of intracrystalline water on the structure of hydrogen zeolites is a phenomenon not always considered in experimental practice and interpretation of findings. Results obtained by even highly sophisticated techniques can be only unreservedly accepted, especially in case of sample pretreatments involving a heating step, if the deammoniation was performed in situ in the measuring cells or if the pretreated samples were transferred to the measuring cells under complete exclusion of atmospheric moisture and, of course, if the measurements themselves were performed in absence of water. This problem is well illustrated by the classical debate between Skeels and Kerr [45–47] on the existence of the hydrogen form after thermal decomposition of NH4-Y. The author of the present review is convinced of the soundness of Kerr’s reasoning in favor of the existence of H-Y and its formation by deammoniation of ammonium Y zeolite. Naturally, strict conditions must be observed in order to prevent H-Y coming into contact with water and from thermal dehydroxylation. Thus, Kerr’s decisive argument concerning the destabilization of the framework of H-Y contacted for ion exchange with an aqueous salt solution is surely sound. Even the counter-argument in [48] based on the 27Al MAS NMR spectroscopic detection of some octahedrally coordinated aluminum in deammoniated Y zeolite is not conclusive since the paper referred to does not give any information whether the separately prepared H-Y was transferred to the sample holder (rotor) under complete exclusion of atmospheric moisture and whether dehydroxylation had been completely avoided. 2.4 Gaseous Halogen Compounds as Dealuminating Agents

Dealumination of zeolites with Si/Al ratios >5 by reaction with gaseous chlorine compounds at elevated temperatures was first reported in a patent application filed in 1975 [49]. According to the presented procedure, highly dehydrated zeolites were contacted, at temperatures higher than 400°C, with Cl2 and/or HCl or with a mixture of Cl2 and CO, preferably in a molar ratio of 1:1 as in phosgene. The mechanisms of the reactions between these agents and framework aluminum were not treated in detail; it was only suggested that first defects were created which may be filled up, in a consecutive process, by migrating silicon atoms. Fejes et al. [50–54] extensively studied the extractive dealumination of zeolites (mordenite) with acid halides (phosgene, nitrosyl chloride) at 400–600°C. The reaction of H-mordenite with phosgene, monitored by IR spectroscopic determination of the volatile reaction products HCl and CO2 , was found to proceed in three main steps at temperatures above 100°C (Eq. 2a) and 300°C (Eqs. 2b and 2c): {AlO4/2}– M+ + COCl2 Æ {AlO4/2}– C+ OCl + MCl , {AlO4/2}– C+ OCl

Æ {……} + AlOCl + CO2 ,

AlOCl + COCl2 Æ AlCl3 + CO2

(2a) (2b) (2c)

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where M denotes Na or H and {……} a lattice vacancy left by removal of one Al and two O atoms from the framework.Aluminum chloride is volatile at the reaction temperature and is purged out from the sample if the process is carried out in a stream of phosgene. The dealumination degree can be controlled by the reaction temperature. Also, the sodium form of zeolites can be used as starting material; however, comparable dealumination degrees can be achieved, if at all, only at significantly higher temperatures. NaCl, formed in this case as reaction product, remains in the sample and has to be washed out after the dealumination reaction. The creation of structural vacancies was found to be associated with the appearance of new IR bands of lattice vibrations at 930 and 860 cm–1 for mordenite and faujasite-type zeolites, respectively. The dealumination technique reported was found to result in only minor crystallinity losses (less than 10% [53]) when applied to zeolites with relatively high Si/Al ratios (mordenite, clinoptilolite). Faujasitetype zeolites could be subjected to this procedure without significant loss in crystallinity only after preceding ultrastabilization [52]. Further publications from Fejes’s group dealt with the mechanism of the dealumination with phosgene studied by IR spectrometric and thermoanalytic techniques [54, 55], structural consequences [55], adsorption behavior and catalytic properties of thusmodified mordenite [56]. Gaseous CCl4 and CHCl3 were found to react similarly with the framework aluminum of mordenite [55]. In the case of carbon tetrachloride, the reaction proceeds via phosgene formed as an intermediate, in analogy to the reaction step shown in Eq. (2b), according to Eq. (2d) [55] while, according to [57], CHCl3 gives CO and HCl in a reaction step analogous to Eq. (2c). {AlO4/2}–C+Cl3 Æ {……} + AlOCl + COCl2

(2d)

In the environment of the vacant sites, structural rearrangements were believed to proceed. However, this “framework reconstruction” and the chemical nature of the vacancies denoted in Eq. (2b) by the not very instructive sign {……} are not yet really understood. The removal of one aluminum and two oxygen atoms from the framework according to Eqs. (2a)–(2c) should result in at least local lattice changes (or damages) similar to those occurring upon dehydroxylation of H-zeolites (see Sect. 2.1). Recently, the formation of ∫SiCl (and some =SiCl2) groups in Na- and H-mordenite upon dealumination with phosgene has been suggested since the respective silanol groups were detected in rehydrated products by 29Si MAS NMR spectroscopy [57, 58]. The amount of silanols was found to be close to two per extracted aluminum, thus, the following reaction was believed to proceed: |

|

–Si–

–Si–

|

|

O H+ |

|

|

|

Cl

|

–Si–O–Al––O–Si– + 3COCl2 Æ –Si–Cl |

|

|

Si– + HCl + AlCl3 + 3CO2 (3) 

|

O

O



|

–Si– |

–Si– |

Dealumination Techniques for Zeolites

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The postulation of ∫SiCl species is undoubtedly an original element in the interpretation of the chemical nature of vacancies in zeolites dealuminated with phosgene even if the structural consequences of the severe lattice constraints necessarily associated with the simultaneously formed new Si-O-Si bridge are not considered. However, the crucial point is that the detection of ∫SiOH (and =Si(OH)2) groups in the washed products is not straightforward evidence for the existence of ∫SiCl precursors. The silanol groups could also have been formed by hydrolysis of strongly strained Si-O-Si bonds in defect sites created during the preceding dealumination process. Unfortunately, no attempts were made [57, 58] to detect and quantify by simple titration the HCl that must have been released when the solid product of Eq. (3) was brought into contact with liquid or gaseous water. Even mixed “chlorine-fluorine nests” with four ∫SiCl(F) groups have been suggested by the same authors to be formed upon dealumination of zeolites with CCl2F2 [59, 60]. However, as long as the simple proof of existence of incorporated halogen is not forthcoming, the suggestion of nests (vacancies) comprising two (Eq. 3) or four ∫SiCl(F) groups is rather insubstantial.

3 Hydrothermal Dealumination of Zeolite Frameworks 3.1 Early Fundamental Investigations

In 1967, McDaniel and Maher [61] reported a method to increase the thermal stability of Y zeolite. This so-called “ultrastabilization” procedure immediately became a matter of considerable interest because of the technical importance of Y zeolite as a catalyst. The process consists of two major steps, (1) the practically complete removal of sodium ions by a two-step ammonium ion exchange with intermittent heating and (2) the conversion of this material by heat treatment at 800°C or above to a faujasite-type zeolite resistant to the influence of heat up to about 1000°C. Although McDaniel and Maher noticed that ultrastabilization is associated with a decrease of the ion-exchange capacity and unit-cell size, the question of the framework aluminum content of modified Y zeolite was not explicitely raised in their paper. The credit for recognizing the fundamental features of the stabilization mechanism must go to Kerr [62, 63]. He evidenced that the water formed by thermal dehydroxylation of the hydrogen form plays a decisive role in this process and went on to state that “any technique for keeping this water in the system during the heating process will result in a stable product” [62]. In line with this statement it was shown [63] that thermal treatment of NH4-Y in thin (only a few mm thick;“shallow bed”) layers resulted in thermally less stable products. In contrast, treatment in “deep bed”, i.e., in bed geometries impeding the fast removal of the reaction product water from the bed by diffusion, yielded “ultrastable”zeolites. Kerr suggested a stabilization mechanism comprising (1) hydrolytic cleavage of -O-Al-O- bonds by “self-steaming”, i.e., contact with gaseous water formed

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as reaction product, (2) release of hydroxyaluminum species from the framework and (3) occupation of cation sites by cationic aluminum species [62, 63]. Similarly, ultrastabilized products were obtained also by direct steaming [64, 65], i.e., by contacting the hydrogen zeolite during the thermal treatment with water steam from external sources at partial pressures generally up to 1 bar. Further evidence for removal of aluminum and oxygen from the framework of Y zeolite ultrastabilized according to [61] was found in an X-ray study by Maher et al. [66]. Moreover, it was shown that ultrastabilization is also associated with the incorporation of silica, originating from other portions of the crystal, into the framework vacancies left by dealumination, and that this process is an absolutely necessary step of the stabilization process. Based on IR results, the same interpretation was given by Scherzer and Bass [67]. Gallezot et al. [68] arrived at similar conclusions in an X-ray study of Y zeolite dealuminated with H4EDTA according to Kerr [24] since, even after extraction of 53% of the framework aluminum, all tetrahedral sites and oxygen positions in the framework were found to be completely occupied. Later, as MAS NMR spectrometers became commercially available, impressive evidence for the release of framework aluminum and the refilling of framework vacancies by framework silicon atoms in hydrothermally treated Y zeolite was derived from 27Al and 29Si spectra [48, 69, 70]. Figure 3 shows the 29Si MAS Si(2Al) Si(1Al) Si(3Al)

Si(0Al)

a Si(1Al) Si(2Al)

Si(0Al)

Si(3Al)

b Si(0Al)

Si(1Al) Si(2Al)

cc – 80

– 90

– 100

– 110

– 120

– 80

– 90

– 100

– 110

– 120

ppm from TMS

Fig. 3a–c. Experimental (left column) and computer-simulated (right column) 29Si MAS NMR spectra of NH4 ,Na-Y zeolite a prior to and b after thermal treatment at 400°C and c after steaming at 700°C [48]

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NMR spectra of NH4(0.75)Na(0.25)-Y (a) prior to and (b) after thermal treatment at 400°C and (c) after steaming at 700°C [48]. The experimental spectra are given in the left-hand column and the respective computer-simulated spectra with deconvoluted individual signals (dotted lines) based on gaussian peak profiles in the right one. Upon steaming, spectrum (c) was completely transformed with respect to that of the parent sample. The strong intensity increase of the Si(0Al) signal in spectrum (c) at the expense of the signals typical of framework Si connected via O atoms with 1, 2 or 3 Al indicates the replacement of Al by Si in the framework. The much less pronounced changes in the intensity distribution after heat treatment at 400°C (spectrum b) point to an only slight release of framework aluminum probably not induced by thermal dealumination, which is basically associated with the dehydroxylation of hydrogen zeolites. Since the applied pretreatment temperature of 400°C is high enough for the deammoniation of NH4-Y, but not sufficiently high for the dehydroxylation of the resulting hydrogen form, it seems to be obvious that no precautions were taken to avoid the resorption of atmospheric humidity after the heat treatment and, consequently, the framework was affected by the processes described in Sect. 2.3. Scheme 1 reflects the so-called “T-jump mechanism” (T stands for the tetrahedrally coordinated framework atom) proposed by von Ballmoos [71] for this hydrothermal “healing” process. For nearly a decade not much stress was put on the question whether silicon atoms, known to refill upon self-steaming or steaming the lattice vacancies created by release of framework aluminum, originate from amorphous silica impurities, from the surface of the zeolite crystallites, or from crystalline areas inside the crystals. In 1980, Lohse et al. [28] showed in a study dealing with the adsorption of nitrogen and some hydrocarbons that a secondary pore system (about 0.13 cm3/g) with pore diameters between 3 and 3.8 nm was formed when NH4Y zeolite was subjected to steaming and subsequently extracted with hydrochloric acid. They evidenced that silicon, which reoccupied empty tetrahedral sites, did not (or not exclusively) come from the surface of the crystals, but predominantly from the bulk, probably involving the elimination of entire sodalite units. Essentially based on results of adsorption measurements, further evidence for the formation of mesopore systems upon steaming of hydrogen zeolites was presented for, e.g., Y zeolite [72–74] and mordenite [23, 75]. Using both N2 adsorption and electron microscopy [76], the mesopore structure in steamed Y zeolite was found to be best described by cavities of spherical shape of about 15 nm in diameter connected by narrower openings. A significant proportion of the crystallite surface was covered by an amorphous layer. It is evident that the aluminum released from the framework upon hydrothermal treatment remains in the sample, either as an intracrystalline oxidic or cationic aluminum species or as intercrystalline material, i.e., as a separate crystalline or amorphous aluminum oxide phase [77–79]. Typical signals appear in the 27Al MAS NMR spectra of Y zeolites upon hydrothermal dealumination (Fig. 4). A line at 0 ppm associated with octahedrally coordinated aluminum is indicative of hydrated cationic species [79–82]. Amorphous oxidic aluminum species, which are subject to large second-order quadrupolar

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Scheme 1. T-jump mechanism after von Ballmoos [70]

interactions, are revealed by a broad signal over a wide chemical shift range. A line at about 30 ppm typically found in 27Al spectra of hydrothermally treated zeolites is generally believed to be associated with five-coordinated extra-framework aluminum. More recently, the line at 30 ppm was attributed to tetrahedrally coordinated aluminum species with a sufficiently large quadrupole coupling to cause significant second-order shifts [83]. In any case, no attempts were made to interpret in more detail the chemical nature of such species. In order to prepare high-silica zeolites or even pure silica with zeolite structures, techniques combining hydrothermal treatment and acid leaching were applied. Generally, aluminum was first removed from the framework by steaming or self-steaming resulting in acid-resistant zeolites, and then the extra-framework species were extracted in a second step by leaching with acids. High-

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a 200 200

100 100

0 00

–-100 100

200 200

100 100

0 00

–-100 100

b ppm from Al (H2O)63+

Fig. 4a, b. 27Al MAS NMR spectra of hydrothermally dealuminated Y zeolite after a partial and b complete rehydration [79]

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ly aluminum-deficient mordenites [23, 84, 85], faujasites [86–88], offretites [89–91] and ZSM-5 [10, 92, 93] were prepared in this way. In some instances the materials were also repeatedly subjected to this twostep treatment to enhance the degree of dealumination, e.g., in [89]. In the case of zeolites resistant to acids due to high framework Si/Al ratios, e.g., mordenite and ZSM-5, acid leaching may also precede the hydrothermal treatment [10, 23]. In this case, healing of framework hydroxyl nests left by aluminum release is the lattice-stabilizing process. The hydrothermal stability of steamed Y zeolites was found to increase upon removal of extra-framework aluminum by extraction with acids [94]. Using X-ray photoelectron spectroscopy, Gross et al. [95] studied the surface composition of Y zeolites dealuminated by hydrothermal treatment and by extraction with EDTA according to [24]. In hydrothermally dealuminated samples, it was found that remaining aluminum accumulated at the outer crystal surface of zeolites in the form of oxidic clusters, but not of a dense layer. The existence of cationic aluminum species could not be confirmed. Extraction of Y zeolite with EDTA favored the dealumination of the external crystal shell, probably resulting in an aluminum concentration gradient along the radii of the crystals [95]. It must be emphasized that zeolites can be hydrothermally dealuminated only in their hydrogen form or in cation forms convertible into the hydrogen form upon thermal treatment. Consequently, the dealumination degree is limited by the efficiency of the ammonium or proton ion exchange generally preceding the dealumination procedure. For example, steaming of Na-ZSM-5 at 700°C for 18 h resulted in the transformation into cristobalite, whereas under the same conditions the hydrogen form of that zeolite retained its crystal topology and the framework aluminum content decreased from 1.17 to 0.25 Al per unit cell [93]. 3.2 Review of Recent Investigations 3.2.1 Faujasite-Type Zeolites

A series of papers [96–99] have dealt with the influence of the operating conditions of the hydrothermal dealumination technique (steaming temperature and time, water partial pressure, flow rate of the steam, etc.) on the controlled preparation of zeolitic materials with improved and optimized physical-chemical properties, especially for application as catalysts. Upon steaming at 500°C, dealumination of Y zeolite proceeded in two steps, a fast one up to a dealumination degree of about 50% in the first 30 min and a second slow one resulting in further progressive dealumination [97]. From the results of 27Al MAS NMR spectroscopy it was concluded that during the first step octahedrally coordinated extra-framework aluminum species were formed, while the aluminum expelled during the second step was trapped in the sodalite units in a tetrahedral envi-

Dealumination Techniques for Zeolites

219

ronment. As to the two-step dealumination, Wang et al. [99] experienced a similar behavior of Y zeolite upon steaming at 820°C. For both periods the kinetic order with respect to water was found to be equal to 1. The first hydrothermal step was suggested to correspond to the release of framework aluminum associated with bridged hydroxyls (protons), while the second one (acid leaching) was believed to proceed in zeolite regions where cationic aluminum species compensate negative charges due to residual framework aluminum. Aluminum was found to enrich at the external crystal surface only upon steaming at high water pressure (>50 kPa) and never during self-steaming [99]. Physicochemical characteristics of the dealuminated zeolite samples prepared in this way and their catalytic behavior in the cracking of n-heptane were also reported [100]. Lutz et al. [101] also studied the chemical nature of extra-framework aluminum formed upon steaming and concluded that this process results, concomitantly with the creation of a mesopore system, in the formation of an X-ray amorphous aluminum aluminosilicate on the external crystal surface and of highly condensed Al cations compensating the residual negative charge of the framework. Macedo et al. [102] arrived at similar conclusions in an IR and ammonia TPD study of extra-framework aluminum species in steamed Y zeolites unleached and leached with hydrochloric acid. The acidity in steamed and subsequently acid-leached Y zeolites and mordenites was investigated by calorimetric determination of differential heats of ammonia adsorption [103]. Recently, samples obtained by steaming of Y zeolite at 600°C for different times were characterized with respect to acidity, aluminum distribution and adsorption behavior and tested as catalysts for pentane conversions [104]. Dealuminated HY samples were obtained by varying the steaming time between 16 and 98 h and the temperature between 200 and 500°C [105]. The products, tested as catalysts in 2methylpentane cracking, showed good X-ray crystallinity up to steaming temperatures of 400°C. Li et al. [106] removed aluminum from NH4Y at 95°C with an aqueous solution of oxalic acid and ammonium oxalate prior to heat treatment at 600°C under typical self-steaming conditions. The product with a framework Si/Al ratio of 2.95 was then subjected to acid leaching with 1 M sulfuric acid followed by a second self-steaming treatment. The final zeolite, used as catalyst for alkylation of phenol with long-chain olefins, retained 90% of the initial crystallinity at a final Si/Al ratio of 5.71 and had a catalytically favorable mesopore volume of about 0.3 ml/g. Zhixiang et al. [107] compared typical characteristics of aluminum-deficient Y zeolites prepared by steaming followed by acid leaching and produced by the reverse sequence of the two steps. The sample obtained by the reverse method showed, in contrast to the other one, aluminum enrichment on the external surface of the crystals and lower charge and acid site density on the intracrystalline and mesopore surface. Using X-ray photoelectron spectroscopy, Corma et al. [108] suggested a migration of non-framework aluminum to the surface of the zeolite crystals during deep-bed calcination of NH4Y zeolites. However, the reported high surface Si/Al ratio, based on the Si 2p XPS signal and an Al 2p line at 73.8 eV assigned to extra-framework aluminum species, seems to contradict this suggestion.

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On the other hand, the intensity of an Al 2p line at 75.0 eV attributed to tetrahedrally coordinated aluminum is indicative of a gradient in the framework aluminum content, this value being highest at the surface. In another paper [109] these authors compared amount and distribution as well as the catalytic cracking behavior of extra-framework aluminum species in Y zeolites dealuminated by steaming combined with citric acid leaching and treatment with SiCl4 and (NH4)2[SiF6]. IR bands at 3693 and 3606 cm–1 and the signal at 2.6 ppm in the 1H MAS NMR spectrum were found to be typical of non-framework aluminum species in hydrothermally treated Y zeolites [110]. A re-investigation of the hydroxyl stretching range of IR spectra of hydrothermally treated and acid-leached Y zeolites [111] essentially confirmed the assignments of bands in earlier papers. A comprehensive description and assignment of the 27Al and 29Si MAS NMR signals in spectra of steamed Y zeolites were presented in [112]. Based on results obtained with the X-ray radial diffusion technique, Shannon et al. [113] suggested that, in steamed Y zeolite, non-framework aluminum is present in the form of boehmite-like clusters occluded in supercages or in the mesopores formed during the hydrothermal healing process, but not located on the external crystal surface or, as a separate phase, in the intercrystalline space. To support this hypothesis, these authors relied on similarities in the OHstretching IR spectra of hydrothermally dealuminated Y zeolite and pseudoboehmite along with similar heats of ammonia adsorption. The influence of acid and basic treatments on Y zeolites dealuminated to various degrees was investigated by Aouali et al. [114]. The crystal structure of low dealuminated samples was found to be unstable in acidic media, but did not show drastic modifications in basic solutions. In contrast, Y zeolites highly dealuminated by combined hydrothermal treatment and acid leaching or by SiCl4 (see Sect. 4.1) proved to be highly resistant to acids but easily lost their crystalline structure in basic solutions. In another paper [115], the stability of ultrastabilized Y zeolites towards steaming at 750 and 810°C was the subject of investigation. He et al. [116] and Wan and Shu [117] reported on the influence of calcination and hydrothermal treatment on compositional characteristics and thermal stability of rare earth containing Y zeolites and their performance in catalytic cracking. The alkaline and hydrothermal stability of Y zeolites dealuminated via hydrothermal treatment and by the SiCl4 technique was studied by Lutz et al. [118]. Hydrothermal treatment was found to increase the chemical resistance of Y zeolite to superheated water at 200°C as well as to alkaline solutions due to the formation of a protective layer of extra-lattice oxidic aluminum species on the external surface of the zeolite crystals. The removal of this layer by acid leaching resulted in significantly less stable products. Sulikowski [29] calculated the fractal surface dimension of NH4,Na-Y zeolite steamed at 550°C from adsorption data of short-chain alcohols. This property of matter being a quantitative measure of surface “roughness” was found to be 2.25, which points to a significant deviation from a smooth surface due to the formation of extra-framework aluminum species and mesopores at the external surface.

Dealumination Techniques for Zeolites

221

Fig. 5. Effect of steaming severity (partial pressure of water) on the hexane cracking activity of H-ZSM-5 [119]

3.2.2 ZSM-5

In 1986, Lago et al. [119] reported on the progressive dealumination of the framework of H-ZSM-5 upon hydrothermal treatment at 540°C in a gas stream with increasing partial pressures of water vapor up to 90 kPa. As illustrated in Fig. 5, mild steaming was found to create sites with a catalytic activity for hexane cracking significantly greater than that of the normal acid sites in ZSM-5. The model suggested for the sites of enhanced activity (acidity) involves the transformation of one framework aluminum of a structural unit, comprising two paired tetrahedrally coordinated Al atoms, by partial hydrolysis to the hexacoordinated state. This species acts as an electron-withdrawing center and, hence, increases the strength of the Brönsted acid site associated with the second aluminum atom of the original pair that remains tetrahedrally coordinated. A more detailed model (Scheme 2) of the modification of Brönsted acid sites of the ∫Al-(OH)-Si-O-Al-(OH)-Si∫ type in an acidic zeolite upon “mild” steaming was proposed in [120]. It comprises: 1. the conversion of a framework fragment of a Brönsted acid site from the initial state (I) into configuration (II) by partial hydrolysis, 2. the rearrangement of (II) to a first type (III) and then by subsequent dehydration to a second type (IV) of strong Brönsted acid sites (positive partial charge, d+, on the respective oxygen atom of a ∫Al-(OH)-Si∫ configuration) and, finally, 3. the formation of cationic extra-framework aluminum species (e.g., AlO+) upon more severe treatment (higher temperature and/or H2O partial pres-

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H.K. Beyer

Scheme 2. Proposed model of the modification of Brönsted acid sites of the ∫Al-(OH)-Si-OAl-(OH)-Si∫

sure and/or prolonged reaction time) resulting in a complete separation of Al ions from the framework, after Kühl [121]. H-ZSM-5 zeolites were found to dealuminate upon steaming at 500–600°C, independently of the crystallite size, according to a second-order kinetics with respect to the aluminum concentration of the starting zeolites [122]. Though it was not explicitly stated, data given in the paper allows the conclusion that the obtainable minimum aluminum level amounts, independently of the starting Si/Al ratio, to about 0.3 Al per unit cell. Campbell et al. [123] investigated the effect of hydrothermal treatment at a water vapor pressure of about 2 kPa and at 600–840°C on the structure and adsorption properties of, and on the nature and distribution of aluminum species in, ZSM-5 zeolites with 0.77–7.6 Al per unit cell in the framework of the starting materials.

Dealumination Techniques for Zeolites

223

During steaming of H-ZSM-5 at 550°C two processes were found to influence the concentration of Lewis acid sites in opposite directions, (1) the dealumination resulting in extra-framework aluminum species and (2) the agglomeration of this species associated with a decrease in Lewis acidity [124]. The significance of this observation for the use of steamed zeolites as catalysts was discussed. Depending on the steaming conditions, up to five different types of acidic sites assigned either directly to extra-framework aluminum species or to their interaction with silanols were found in hydrothermally treated H-ZSM-5 by diffuse reflectance infrared spectroscopy [125]. In zeolites of the ZSM-5 type prepared by recrystallization of rare earth containing Y zeolite, hydrothermal dealumination was found to be suppressed, and crystallinity to be better retained by the rare earth constituent [126]. 3.2.3 Other Zeolites

Hydrothermally dealuminated H-EMT zeolites were prepared by steaming in the temperature range of 450–740°C; resulting materials both unleached and leached with 1 N HCl were characterized [127]. In a series of mordenites dealuminated, prior to calcination at 550–650°C, to various degrees by acid extraction, Musa et al. [128] experienced non-monotonic variations of several characteristics (unit cell and crystallite size, Lewis and Brönsted acidity, bulk/surface Al concentration ratio) which were explained by a different dealumination behavior of crystallites differing in size and crystal symmetry. Small crystals having preferentially Cmmm symmetry were assumed to undergo even under mild conditions a fast dealumination distorting the structure and resulting in fragmentation of the crystallites or, under more severe conditions, in partial amorphization. In contrast, crystal structure and size of larger mordenite particles with preferential Cmcm symmetry are believed to be highly resistant to even severe dealumination. Physicochemical and catalytic properties of mordenites dealuminated by both separate and combined thermal treatment and acid leaching and by combined thermal/hydrothermal treatment have been characterized by various techniques [129]. Dry heat treatment at 750°C expelled as much as 70% of the aluminum from the framework resulting for ammonia in nearly complete inaccessibility of strongly acidic sites [130]. Indication of acidity was partly restored upon acid leaching, which showed that sites associated with residual framework aluminum again became accessible to basic molecules by extraction of the extra-framework aluminum species. Roughly 40% of the aluminum remaining after heat treatment at 750°C in the framework was found to be expelled upon steaming at 500°C. In line with the behavior of Y zeolite [99], steam was found to be necessary for extra-framework aluminum species to migrate to, and accumulate at, the crystal surface [131]. Lee and Ha [132] treated Na-mordenite (SiO2/Al2O3 weight ratio = 6.5) with a 6 M HCl solution at 90°C for 3–16 h or a 0.5 M HF solution at room temperature for 192–528 h. HCl was applied in large excess, HF in amounts about equivalent to the aluminum in the sample subjected to acid leaching. After washing with

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water, the acid-leached samples were steamed at 600 and 500°C, respectively. The HCl/steaming procedure resulted in products, the framework Si/Al ratios of which increased with increasing leaching time. In contrast, in the case of HF leaching, a preferential release of aluminum, causing an increase in the framework Si/Al ratio up to about 20, was found to proceed only in the initial period, while later simultaneous removal of Si and Al occurred. Disregarding samples subjected in the first step for longer times to HF leaching, all products prepared by HCl and HF leaching are claimed to retain more than 80% crystallinity and to have also mesopores with diameters around 3.7 nm. Fernandez et al. [133] adapted the combined technique consisting of hydrothermal treatment and successive extraction with acids, already successfully used for the dealumination of Y zeolite [86, 87], for the dealumination of K,TMA-offretite. With an increasing number of treatment cycles (up to 4), the aluminum content could be progressively decreased from 3.41 to 1.68 Al per u.c. The process was found to be associated with a significant increase in the thermal stability of the lattice, but was also accompanied by the formation of defects and holes in the crystal (mesopore system). Carvalho et al. [134] reported on the dealumination of a synthetic K,TMAoffretite by two succesive thermal treatments under self-steaming conditions at 550 °C and higher temperatures up to 850 °C with intermittent removal of potassium cations by ammonium ion exchange. The dealumination, approaching a degree of about 60%, was associated with a strong decrease in crystallinity. Aluminum located in crystallographically different framework sites was found to be removed to different degrees with preference for T2 sites. (As to the loop configurations of T1 and T2 atoms in the structure of offretite, see [135].) As in the case of the solid-state dealumination of the structurally related zeolite L with (NH4)2[SiF6] (see Sect. 4.4 and [198]), only the cell parameter a was significantly reduced by removal of framework aluminum. Offretite behaved similarly to Y [99] and mordenite [129] inasmuch as no migration of extra-framework aluminum species to the outer surface of the zeolite crystals was observed under the applied self-steaming conditions. Contrary to statements in [134], a dealumination preference was found for T1 sites of the offretite lattice in a study dealing with MAS NMR spectroscopic characterization of steamed offretite and erionite [136]. The behavior of zeolite W (synthetic mazzite) was intensely studied by French research groups [137–141]. McQueen et al. [137] found large differences in the size and volume of mesopores depending on the aluminum content of the parent zeolite. Mesopores of 10 nm in diameter with a total mesopore volume of 0.05–0.06 ml/g were found in the dealuminated variety of the aluminum-rich parent material, while dealumination of the aluminum-poor parent zeolite yielded, at about the same final aluminum content corresponding to a Si/Al ratio of about 22, mesopores with diameters of 6–8 nm representing a volume of 0.03–0.04 ml/g. Massiani et al. [138] studied the effects of calcination at 500 °C, subsequent hydrothermal treatment (self-steaming) at 600°C and, finally, acid leaching on the structural parameters, porosity and acidity of zeolite W. Removal of 25 and 50% of the framework aluminum, preferentially from sites located in the four-

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membered rings of the gmelinite cages, was observed upon calcination and selfsteaming, respectively. The consequences of the dealumination were basically similar to those experienced with Y zeolite. Chauvin et al. [141] subjected zeolite W with varying residual sodium contents to hydrothermal treatment at 500°C and subsequent acid leaching at 80°C. Steaming resulted in the formation of mesopores of 10 nm in diameter that were not available to sorbents due to deposits of debris with aluminum in tetrahedral and octahedral configuration. Acid leaching removed all the tetrahedrally and part of the octahedrally coordinated aluminum species. The mesopores were found to be interconnected by narrow necks and not directly linked to the crystallite surface. The influence of sequential steaming and acid leaching on the texture of synthetic mazzite (zeolite W) has also been studied [141]. 3.3 Thermal Dealumination

In the description of the preparation of ultrastabilized Y zeolite reported in [61] it was not explicitly stated that water vapor is involved in the dealumination process. Also, in numerous other papers dealing with “thermal” dealumination and/or “thermal” stabilization of zeolites, the role of water was ignored and the stabilization process was assumed to be governed by a purely thermal effect. As is well known, thermal treatment of ammonium zeolites to about 400– 500°C results in the release of ammonia (deammoniation) and in the formation of the hydrogen form of the respective zeolite. The hydrogen form of a zeolite, especially of zeolite Y, is generally a highly delicate material inasmuch as it undergoes dehydroxylation in a temperature interval slightly exceeding, but sometimes also overlapping with, that of the deammoniation [142]. Moreover, dehydroxylation is accompanied by a release of aluminum from the framework [143] and obviously by a gradual collapse of the crystal lattice (amorphization), as revealed by data reported, e.g., in [123, 130, 144]. In any case, the hydrogen form of Y zeolite obtained by thermal decomposition of the ammonium form was found to be significantly less thermostable than the sodium or other cationic forms [51, 143]. In a series of systematic studies it was clearly shown that heat treatment of hydrogen Y zeolites, cf. e.g., [63, 65], and mordenites, cf. e.g., [23, 145], alone did not stabilize zeolite lattices if the water, formed during the process by dehydroxylation, was efficaciously removed. Thus, in studies claiming dealumination and stabilization of zeolite frameworks by mere thermal treatment (cf. e.g., [146]), obviously water evolved during the process as an intrinsic component of the reaction system was involved in the observed dealumination and/or stabilization by inducing the lattice healing process.

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4 Isomorphous Substitution of Framework Silicon for Aluminum 4.1 Dealumination with Silicon Tetrachloride 4.1.1 Faujasite-Type Zeolites

In 1980, Beyer and Belenykaja [147] reported for the first time on a process in which zeolitic framework aluminum is directly replaced by silicon. The fundamentally new idea was to adapt the reaction between alumina and silicon tetrachloride, Eq. (4), first reported by M. Daubrée [148] nearly 150 years ago in 1854, also for the dealumination of zeolites, i.e., to use gaseous silicon tetrachloride as dealumination agent and, at the same time, as extraneous silicon source. (4) 2Al2O3 + 3SiCl4 Æ 4AlCl3 + 3SiO2 . It was found [147, 149] that, under appropriate experimental conditions, framework silicon is directly isomorphically substituted for aluminum in a strongly exothermic reaction without any substantial lattice damage when the sodium form of faujasite-type zeolites was contacted with silicon tetrachloride vapor at temperatures around 500°C. In a subsequent study [150], the isomorphous incorporation of Si was also evidenced by striking differences in 29Si MAS NMR spectra. Before dealumination, the spectrum of Na-Y (Fig. 6, spectrum a) exhibited the typical signals assigned to silicon atoms with 0, 1, 2 and 3 Al atoms in the second coordination sphere. In contrast, the spectrum of Y zeolite subjected to nearly complete dealumination with silicon tetrachloride (Fig. 6, spectrum b) consisted only of the signal at 107 ppm indicative of Si(4Si, 0Al) ordering. The high crystallinity and structural homogeneity of this material is indicated by the extreme sharpness of this line and also reflected by the high-resolution electron micrograph presented in Fig. 7, which is exactly the same as for the parent Na-Y zeolite. Only part of the AlCl3 , volatile at the reaction temperature, was found to escape from the zeolite bed during the dealumination of Na-Y. This behavior was attributed to the formation of sodium tetrachloroaluminate [147]. Further, a strong exothermic reaction not or scarcely resulting in framework dealumination was already observed at temperatures (about 250°C) substantially lower than those needed for isomorphous substitution and found to result in the formation of surface –SiCl3 groups revealed by a 29Si MAS NMR signal at –45 ppm [151]. Thus, the over-all process {AlO4/2}– Na+ + SiCl4 Æ {SiO4/2} + Na[AlCl4]

(5)

obviously comprises three major steps: 1. the reaction of SiCl4 with sodium cations resulting in surface –SiCl3 groups and NaCl (5a) {AlO4/2}– Na+ + SiCl4 Æ {AlO4/2}– SiCl3+ + NaCl,

227

Dealumination Techniques for Zeolites Si(2Al) Si(1Al)

Si(3Al)

Si(0Al)

aa –-40 40

– -80 80

– 120 -120

d / p.p.m. d / p.p.m.

bb –-40 40

Fig. 6a, b. [150]

29Si

–-80 80 –-120 120 d / p.p.m. d / p.p.m.

MAS NMR spectra of Na-Y a prior to and b after treatment with SiCl4 at 560°C

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b

a

c

Fig. 7. a High-resolution electron micrograph of zeolite Y dealuminated with SiCl4 along the [110] zone axis; b selected area diffraction pattern corresponding to this zone axis; and c [100] projection of the framework [150]

2. the isomorphous replacement of framework aluminum {AlO4/2}– SiCl3+ Æ {SiO4/2} + AlCl3,

(5b)

3. the formation of sodium tetrachloroaluminate first described by Wöhler in 1827 NaCl + AlCl3 ´ Na[AlCl4] (5c) where {AlO4/2}– and {SiO4/2} refer to primary tetrahedral units of zeolite structures containing Si and Al, respectively. The chemistry of this dealumination technique may appear to be simple; however, it is rather difficult and requires exact knowledge of this complex process and great experience in its application to prepare completely and, especially, partially dealuminated Y zeolites without essential damage to the lattice. Depending on the amount of applied zeolite, bed geometry and reaction conditions, the strongly exothermic nature of reaction (5) can mean that considerable overheating may occur in the zeolite bed and result in a partial or total collapse of the crystal structure [147, 149]. At reaction temperatures usually recommended for that dealumination process (about 500°C), Eq. (5c) is not yet significantly shifted to the left side, i.e., most of the aluminum removed from the framework is present in the form of sodium tetrachloroaluminate deposited in the large cavities of the faujasite structure and only a minor part, depending on reaction time and temperature, can escape in the form of AlCl3 via sublimation

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[147]. A signal at 100.7 ppm found in the 27Al MAS NMR spectrum of NaY zeolite treated with SiCl4 was reported to be indicative of Na[AlCl4] [152]. Unfortunately, it has not proved to be practicable to enhance the decomposition of the complex salt and, hence the escape of AlCl3 , simply by temperature increase since the crystal structure of faujasite was found to be strongly affected when contacted with SiCl4 at about 550°C and even to be destroyed under somewhat more severe conditions [149]. Thus, in this dealumination procedure, the increase in the reaction temperature is strictly limited. The amorphization of the product at high reaction temperatures (550°C and above) was also observed by Anderson and Klinowski [152] who reasoned that the rate of aluminum removal could be greater than that of silicon substitution at these temperatures. However, this effect also appeared when the substitution was first carried out, as usual, at about 500°C and then the product was heat-treated at higher temperatures [149]. In contrast, a highly thermostable product was obtained when the complex salt was removed by washing. Thus, it was evident that occluded Na[AlCl4] affects the crystal structure at temperatures of 550°C and above and the extraction of the complex salt by washing with water seems to be the only promising way to remove this reaction product from the dealuminated zeolite. In practice [147], a stream of an inert gas saturated at ambient temperature with silicon tetrachloride was passed at higher temperatures through a bed of the zeolite, preferentially pelleted without any binder. To avoid overheating due to released reaction heat, it was recommended to contact the zeolite with SiCl4 first at a lower temperature (about 200°C), then to heat it in the gas stream containing SiCl4 at a moderate rate up to the reaction temperature of about 500°C and to continue the treatment at this temperature, e.g., for 2 h. Prior to this treatment, the zeolite must be completely dehydrated, e.g., in a stream of dried inert gas in situ in the reactor at about 400°C, to avoid hydrolysis of the reagent. After washing with water, a near-silica analogue of faujasite with Si/Al ratios of 50–60 was obtained. Y zeolite dealuminated with SiCl4 proved to be, after removal of Na[AlCl4] by washing, extremely heat resistant; structure collapse was found to start only at about 1200°C [147, 149]. Furthermore, it proved to be a typically hydrophobic adsorbent. The adsorption isotherms of organic compounds showed the rectangular shape with saturation at very low pressures (see Fig. 8) typical of the adsorption on high-alumina zeolites. In contrast, ammonia and especially water are much less or practically not adsorbed (isotherms (d) and (e) in Fig. 8). [See the striking difference in the adsorption of water on Na-Y (dotted isotherm e) and dealuminated Y (continuously traced isotherm e; Fig. 8).] This distinctly hydrophobic adsorption behavior of highly dealuminated faujasite indicates that the framework must be virtually free of lattice vacancies necessarily associated with hydrophilic internal silanol groups. The isomorphous substitution of framework aluminum in faujasite-type zeolites was evidenced by chemical analysis, XRD (unit cell contraction), mid-infrared spectroscopy (shift of lattice vibrations) and adsorption measurements (hysteresis-free adsorption isotherms) [147] and later by 29Si MAS NMR spectroscopy [149, 150, 153] (see Fig. 5).

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H.K. Beyer

e

0.3 0.3

a

b a

0.2 0.3

0.10 0.10

Adsorbed liquid / cm3 · g–1

3 ·3 g . –1 Adsorbed liquid / cm Adsorbed liquid / cm g–1

c

0.1 0.3

d

0.05 0.10

e

0.1

0.2

0.3

0.4

Relative pressure (p/p0)

Fig. 8. Adsorption isotherms on Na-Y (dotted lines) and Y zeolite (Si/Al = 44) dealuminated with SiCl4 (continuous lines). a n-Hexane; b n-butane; c benzene; d ammonia; and e water [147]

The degree of dealumination depended first of all not only on the final reaction temperature but also on the reaction time [149]. To convey some ideas of the influence of these parameters, maximum dealumination (Si/Al about 50) was obtained at 450°C in 40 min while after 15 min the dealumination degree corresponded, even at a higher reaction temperature (475°C), only to a Si/Al ratio of 19.Although the dealumination degree could be easily controlled by these two parameters, other difficulties associated with the removal of the complex salt arose in case of partial dealumination. The main problem was that the tetr-

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achloroaluminate complex hydrolyzed during the washing process and the thusgenerated acidity might have leached out, depending on the actual aluminum content of the framework, more or less aluminum that had remained in tetrahedral framework positions after the SiCl4 treatment. This undesired consecutive dealumination reaction, which resulted in lattice defects (hydroxyl nests), consumed acidity created by hydrolysis of Na[AlCl4]. Thus, at not too high degrees of dealumination, the pH of the washing water may be higher than 3 and either hydroxyaluminum species, e.g., Al(OH)2+, which occupied lattice cation sites, or even intra-crystalline or extra-crystalline Al(OH)3 might have formed. The extra-framework aluminum content was found to pass through a maximum at intermediate extents of dealumination [149]. The chemical nature of such extraframework aluminum species created in Y zeolite by partial dealumination (up to Si/Al = 20) with gaseous SiCl4 and subsequent washing with water has been intensely studied [154].At least part of this extra-framework aluminum was present as cationic species contributing to the compensation of the skeletal charge and acting as strong electron-acceptor (i.e., Lewis acid) sites. The IR spectra of dealuminated Y zeolites were found to differ from that of the parent zeolite. An additional band at 3620 cm–1 was assigned to amorphous extra-lattice aluminum oxide species, and the intensity increase of the band at 3740 cm–1, typical of non-acidic SiOH hydroxyls, was attributed to lattice defects remaining after dealumination. The dealuminated Y samples contained bridged hydroxyls, the stretching frequencies of which (3630 and 3560 cm–1) were similar to, but nevertheless significantly different from, those present in the HY zeolite (3645 and 3550 cm–1, respectively). Disregarding some slight deviations in hydroxyl stretching band positions, similar observations and assignments were made in [151]. It was stressed that the band found at 3730 cm–1 is associated with the hydroxyl nests formed during the washing process by acid leaching since this band completely disappeared upon steaming. [There is ample proof (see Sect. 3) that steaming anneals lattice vacancies of the “hydroxyl nest” type.] In contrast, the band at 3750 cm–1 developed in intensity as the crystallinity deteriorated. Garralón et al. [155] were inclined to assign the band at 3610 cm–1 in spectra of samples dealuminated with SiCl4 at 350–400°C to amorphous aluminosilicate formed from oxidic extra-framework aluminium species and silica originating from SiCl4 . Using both the Bloch Decay and crossed-polarization techniques in a 29Si MAS NMR spectroscopic investigation of a Y zeolite practically completely dealuminated with SiCl4, Ray et al. [156] detected, besides tetrahedrally coordinated framework silicon as the main constituent, three different types of SiOH groups assigned to defect sites and a small amount of amorphous silica. Steaming of the sample at temperatures up to 700°C resulted in the elimination of the defect sites and the disappearance of the amorphous phase due to the well-known hydrothermal “healing process”. The influence of both reaction temperature and washing conditions on the amount and distribution of extra-framework aluminum species in Y zeolites dealuminated with SiCl4 have been studied [157]. Increase in the reaction temperature and efficiency of washing (water > ethanol > ethanol + buffer) resulted in an increase in the Si/Al ratio both in the bulk and at the surface. Temperature increase and milder washing enhanced the Al-enrichment of the surface with

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respect to the bulk. The higher aluminum content at the surface was due to nonframework aluminum deposits. An attempt was made to separately estimate the degree of dealumination obtained in the first process step upon treatment with SiCl4 and during the subsequent washing procedure for highly dealuminated Y zeolite (Si/Al = 50) [149]. Starting from a Y zeolite containing 54.5 Al per unit cell, 3.5 Al per unit cell were found in the final product after dealumination and, based on thermogravimetric data, it was concluded that four more Al atoms were leached from the unit cell by the acidity created during the washing step. Thus, it may be concluded that the isomorphous substitution stopped, due to product inhibition, at a level of 47 Na[AlCl4] per unit cell corresponding to about 6 per large cavity, provided the amount of reaction product that escaped in the form of AlCl3 could be neglected. For X zeolite (85 Al per unit cell) it was shown by quantitative volumetric measurements [149] that at 210°C the maximum consumption of SiCl4 amounted to about 38 per unit cell.An increase in the reaction temperature to 530°C did not affect the SiCl4 uptake. Thus, the reaction stopped, probably due to product inhibition, at a level of 38 Na[AlCl4] per unit cell (4.7 per large cavities) corresponding to a framework Si/Al ratio of 3.03. The final washed products proved to be X-ray amorphous, obviously due to the relatively high aluminum content which favored framework damage by strong mineral acidity created during the washing process by hydrolysis of the occluded Na[AlCl4]. In contrast, Sulikowski and Klinowski claimed [158] that the lattice destruction observed upon dealumination of Li,Na-X was due to removal of aluminum from six-membered rings containing three aluminum T atoms which are present in larger numbers in the structure of zeolite X than in zeolite Y. However, since the presence of occluded lithium and/or sodium tetrachloroaluminate was not ruled out by the authors, it could be that the lattice collapse of the X zeolite occurred during the washing process by acid leaching rather than during the SiCl4 treatment. In any case, the degree to which both isomorphous substitution and acid leaching contribute to the overall dealumination of Y zeolites obviously depends on experimental details in a way difficult to control. In contrast to hydrothermally dealuminated Y zeolite, extra-framework aluminum could not be detected by XPS at the surface of Y zeolite dealuminated with SiCl4. However, the framework Si/Al ratio was found to be considerably lower (2.4) than that of the bulk (8.0) [108]. Anderson and Klinowski [152] concluded from Lewis acid site levels, found to be low in Y zeolites dealuminated to different degrees with SiCl4 , that little extra-framework aluminum was present in the samples. Shi et al. [159] measured the heat of adsorption of ammonia on a series of Y zeolites with about 35 to 3 framework Al per unit cell prepared by isomorphous substitution with SiCl4 in the reaction temperature range 200–450°C. Extra-framework aluminum was found only in products which were refluxed in hydrochloric acid at pH 2 immediately after the treatment with SiCl4, but not in those washed with water. In another report [160] these authors even questioned the formation of Na[AlCl4] and suggested that all aluminum removed from the framework escapes in the form of AlCl3 since, according to these authors, no non-framework aluminum remained within the porous structure or at the surface even at partial dealumination.

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However, this statement was obviously derived only from 27Al MAS NMR spectroscopic results, and the bulk aluminum content was not considered. Since it is well known [161] that part of the extra-framework aluminum may be “invisible” for 27Al MAS NMR spectroscopy, it may be possible that intracrystalline or intercrystalline oxidic aluminum was overlooked. A recent paper by Stockenhuber and Lercher [162] focused on the characterization of extra-framework species and acid sites in Y zeolites subsequent to dealumination with SiCl4 . Extra-framework aluminum species were found to be present after the treatment with SiCl4, but partially extractable from the product with ammonia solutions. Besides this type of species, a second one, rich in silica and located mainly outside the zeolite channels, was also suggested. The effect of acid leaching subsequent to partial dealumination of NaY zeolite with SiCl4 at 450°C has also been studied [163]. Summing up, depending on the reaction temperature, the dealumination of NaY zeolite with SiCl4 always results in an increase in the framework Si/Al ratio due to direct isomorphous substitution. However, during the following washing step associated with the creation of strong acidity, further framework aluminum is leached out under formation of lattice vacancies and cationic and oxidic extra-framework aluminum species. Under optimal reaction conditions, however, more than 90% of the original framework aluminum is removed by isomorphous substitution. Thus, dealumination with SiCl4 is the favored method if highly siliceous faujasites have to be prepared and it is complementary to the dealumination with (NH4)2[SiF6] in aqueous medium ([180–182], reviewed in Sect. 4.3), that may be more advantageous for the preparation of low-dealuminated zeolites. As shown, dealumination of NaY zeolite by SiCl4 is restricted by product inhibition due to the formation and deposition of Na[AlCl4] in the zeolite cavities. At first glance it may appear to be advantageous to start from the ammonium form, since the respective ammonium complex salt, if it exists at all, completely dissociates at the reaction temperature into its volatile components NH4Cl and AlCl3. Such a modification was already considered in the first paper dealing with this technique [147] and applied, but not studied in detail, by Hey et al. [163]. Recently, the crystallinity of NH4,Na-Y zeolite (ª60% NH4+ exchanged) dealuminated with SiCl4 at 545°C was reported to be only 19% [164]. In our experience (unpubl. results), isomorphous substitution is accompanied by undesired, not yet fully understood, concomitant reactions resulting in partial loss of the lattice integrity if the starting material is an ammonium Y zeolite. However, it has been shown that product inhibition can be avoided if the lithiumexchanged form is subjected to the dealumination procedure [165]. Compared with Na[AlCl4], the corresponding lithium complex dissociates at considerably lower temperatures and the formed AlCl3 escapes almost completely during the reaction at 500°C. As a consequence, products with extremely high crystallinity and structural homogeneity can be obtained, as illustrated by the 29SiMAS NMR spectra of high-silica varieties obtained from Na-Y and Li-Y shown in Fig. 9. Faujasites of extremely low aluminum content (Si/Al >200) and a minimum of framework vacancies and mesopores can be obtained by acid leaching and steaming of Y zeolite previously dealuminated with SiCl4 [147, 149, 166]. Even large-scale production of dealuminated Y zeolite based on this combined proce-

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–-107.5

–-100.9

–-104.6

–-111.1 aa

bb

–-95

–-105 Chemical shift, dTMS (ppm)

–-115

Fig. 9a, b. 29Si MAS NMR spectra of dealuminated faujasites prepared by treatment with SiCl4 from a Na-Y and b Li0.62Na-Y [165]

Dealumination Techniques for Zeolites

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dure has been reported [167]. In a comparative study, Lutz et al. [118] reported on the hydrothermal and alkaline stability of a commercially available high-silica variety of faujasite (Si/Al =150) and hydrothermally dealuminated Y zeolite prepared in this fashion. High-silica Y zeolite (Si/Al about 150) prepared by isomorphous substitution with SiCl4 proved to be poorly resistant to steaming [168]. The loss in lattice integrity was found to start already at temperatures of about 120 °C and approached the 100% value at about 180°C. Hydrothermal resistance was increased by covering the external surface of the crystals with a layer of alumina or alkali aluminosilicates. 4.1.2 Other Zeolites

It is evident that the applicability of this dealumination process is limited to zeolites with pore openings large enough to allow the penetration of the SiCl4 molecule, and that the dealumination may be strongly or completely inhibited by intracrystalline deposition of Na[AlCl4], especially in the case of zeolites with one-dimensional or, with respect to the accessibility for SiCl4 molecules, quasione-dimensional pore systems. Thus, it is not surprising that the dealumination of ZSM-20, structurally closely related to faujasite, has been readily carried out [169]. The framework Si/Al ratio of the lithium form of this zeolite, as obtained from the ammonium form by solid-state ion exchange, could be increased by contact with SiCl4 from 3.6 to >100; the aluminum content of the bulk was reduced to 0.24 mmol Al2O3/g corresponding to a Si/Al ratio of 34. In the first paper dealing with this method [147] it was reported that attempts to use this technique for the dealumination of L zeolite in its as-synthesized K,Na-form and Na-mordenite failed. Later it was claimed [170] that synthetic large-pore Na-mordenite is partially (24%) dealuminated with SiCl4 vapor at 700°C. That is for this type of reaction an extremely high temperature and resulted, at least in the case of faujasites, in complete destruction of the lattice. Though it was stated that the treated material retained high crystallinity, the crucial point was not pointed out in more detail, i.e., it was not evidenced that the relatively low dealumination was not accompanied by a similarly slight loss in crystallinity. Namba et al. [171, 172] subjected H-ZSM-5 to silicon tetrachloride vapor at temperatures between 450 and 650°C and observed only a slight increase in the bulk Si/Al ratio from 19 to 24, while the surface Si/Al ratio determined by XPS increased, depending on the reaction temperature, from 18 to 39. Thus, the external crystal shell was preferentially dealuminated upon contact with SiCl4 , obviously due to diffusion restrictions. In this way the contribution of the surface layer to the catalytic activity of ZSM-5 zeolite could be diminished and the shape-selectivity effect enhanced. In contrast, Thomas et al. [173] reported that aluminum could be removed from the lattice of ZSM-5 by treatment with SiCl4 at 540°C. Na-L zeolite is much easier (i.e., at lower temperatures) to dealuminate than its as-synthesized K,Na-form [174]. This has been attributed to the higher diffusivity of SiCl4 in zeolites containing smaller lattice cations and, hence, that have

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H.K. Beyer

a more open pore structure. Following essentially the procedure given in [147], zeolite beta was dealuminated with SiCl4 at 450°C without any noticeable decrease in crystallinity to a level correponding to Si/Al = 39 [175]. Post-synthetic dealumination of the ammonium form of zeolite MCM-22 by treatment with SiCl4 vapor at 450°C according to [147] led to an increase in the bulk Si/Al ratio from 11 to 20 [176]. The products were found to contain octahedral aluminum detected by 27Al MAS NMR spectroscopy. It was found that five crystallographically non-equivalent T-sites revealed in the structure of MCM22 by 29Si MAS NMR spectroscopy were affected to different degrees upon dealumination with SiCl4 vapor. The framework Si/Al ratio of zeolite W (synthetic mazzite) was increased by treatment with SiCl4 at 500°C from 4.24 to 6.00 without significant loss in crystallinity [177]. The dealumination reaction was accompanied by a slight increase in the hexagonal lattice parameter a while c remained unaffected. This unusual phenomenon, i.e., cell expansion upon isomorphous substitution of silicon for aluminum, as well as 27AL MAS NMR spectroscopic results, pointed to a redistribution of aluminum on at least two crystallographically different framework T-sites. 29Si MAS NMR spectra of offretite, erionite and zeolite W all dealuminated with SiCl4 were presented in [178]. A partial dealumination of ferrierite increasing the bulk Si/Al ratio (determined by EDAX) from 4.6 to 7.0 was found upon treatment with SiCl4 at 550°C [152]. However, the few presented data did not evidence isomorphous substitution and do not give any structural information about the product. Considering the small pore openings of the two-dimensional channel system of ferrierite, it is probable that the observed dealumination is accompanied by a gradual lattice collapse. 4.2 Isomorphous Substitution with Other Silicon Halides

Dealumination of Y zeolite was also performed with silicon chloroform, SiHCl3, at reaction conditions usually used with the SiCl4 technique [147].The reaction is obviously similar to that described by Eq. (5); however, it is not yet known in detail. Special precautions must be taken to avoid the presence of any oxygen in the reactant stream since SiHCl3 is flammable at the reaction temperature. The only advantage offered by this reactant in comparison to silicon tetrachloride may be its potential applicability to the dealumination of zeolites with smaller pore diameters due to its smaller molecular size. In a patent filed to the Union Carbide Corp. [179], the use of silicon tetrafluoride was claimed as dealuminating agent and extraneous silicon source. Dehydrated zeolites were contacted with gaseous SiF4 , preferably highly diluted with nitrogen, at temperatures from ambient to about 200°C and subsequently subjected to an ammonium ion-exchange procedure in order to remove AlF2+ and AlF2+ ions from the treated zeolite. Examples given in the patent specification referred to the dealumination of Y zeolite, mordenite and ALPO4-5. The reported dealumination degrees, reflected by an increase in the Si/Al ratio from 7 to about 10 for mordenite and 2.5 to 3.2 for Y, were rather moderate. Moreover, no

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information was given on the degree of crystallinity of the dealuminated zeolites. The smaller molecular size compared to that of SiCl4 could make SiF4 applicable for the dealumination of zeolites with smaller pore openings. On the other hand, this compound is extremely reactive and, therefore, difficult to handle. That may be the reason why this dealumination method has not been further studied. 4.3 Dealumination by (NH4)2[SiF6] Solutions

Early in the 1980s, Breck and Skeels developed a new method for the dealumination of medium- and large-pore zeolites. It was first described in a patent [180] assigned to the Union Carbide Corp. (application filed in 1981) and then presented at the 6th International Zeolite Conference in 1984 [181]. Their fundamental idea was to treat a zeolite slurried in water with an aqueous solution of an agent which extracts aluminum from the framework, provides ligands for the formation of a thermodynamically strongly favored, soluble aluminum complex and serves as an extraneous source of silicon atoms filling up the framework vacancies formed upon extraction of aluminum. Breck and Skeels realized that only soluble hexafluorosilicate salts, especially the ammonium and lithium salts, meet the requirements of such a process. The overall process of this dealumination process can be described by Eq. (6). (NH4)+x [AlxSi yO(2x + 2y)]x– + (NH4)2 [SiF6] Ø (NH4)+(x–1) [Al(x–1)Si(y+1)O(2x + 2y)](x–1)– + (NH4)3 [AIF6] .

(6)

It is believed that the process proceeds in two steps: (1) the removal of aluminum from the framework and (2) the insertion of Si atoms in the lattice vacancies left by aluminum release. In order to avoid too high concentrations of defect sites leading to unstable products, the reaction rate of the first step should not exceed that of the second one. Thus, the pH of the slurry must be considered as a crucial parameter since it decisively controls the rate of aluminum extraction. Typically, a 1 M solution of (NH4)2[SiF6] is added to an aqueous suspension of a Y zeolite in amounts determined, according to the stoichiometry of reaction Eq. (6), by the desired dealumination degree of the final product, and with a rate of 0.005 moles of (NH4)2[SiF6] per minute and mole of aluminum in the zeolite [181]. This addition rate is crucial in order to maintain the reaction pH at the required value of about 6. Alternatively, the slurry can be buffered, by e.g., ammonium acetate, in order to provide control of the pH. To complete reaction (6), the slurry is refluxed for some hours and the aluminum, removed from the framework and complexed to [AlF6]3–, is washed out from the product with water. The removal of aluminum from, and the isomorphous incorporation of silicon into, the framework of Y zeolite and mordenite was clearly evidenced in the early paper [181] and again in a later publication [182]. It was also shown that this new dealumination method requires ammonium or hydrogen forms of zeo-

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lites and that it is not applicable to sodium forms generally obtained as as-synthesized products in zeolite synthesis. (In this respect the technique resembles the hydrothermal dealumination procedures.) However, questions as to what degree zeolite frameworks can be dealuminated and to what extent dealumination is associated with creation of lattice defects and lattice damages were only stressed in later papers [183–191]. It is also obvious that this technique is only applicable to zeolites whose charge-compensating cations form soluble fluoroaluminate and hexafluorosilicate salts since otherwise such complex salts would be deposited in the pores, thus inhibiting the dealumination reaction or the removal of the reaction product by washing. Garralón et al. [183] found that the final structural and compositional characteristics of NH4,Na-Y zeolite dealuminated by (NH4)2[SiF6] at pH = 6 as described in [181] depended strongly on reaction time, temperature, intermediate treatment (washing and calcination) and the molar ratio of applied (NH4)2[SiF6] to aluminum in the sample subjected to dealumination. Up to the substitution of 30 Al per unit cell, corresponding to a dealumination degree of about 50%, the structure of the final zeolitic product proved to be stable and essentially free of extra-framework aluminum species. However, when (NH4)2[SiF6] was applied in amounts adequate to cause, according to the stoichiometry of Eq. (6), higher dealumination degrees, the crystal structure progressively collapsed. No further dealumination was observed when the (NH4)2[SiF6] treatment was repeated after intermediate washing. On the other hand, calcination of the product at 500°C between two or more subsequent treatments resulted in further dealumination accompanied, however, by a substantial loss of crystallinity. These results are in line with the findings of Zi and Yi [184], who compared the surface acidity and physical properties of Y zeolites dealuminated up to Si/Al ratios of 6.8 by treatment with (NH4)2[SiF6] solution at pH 6 and 70°C with those of a dealuminated sample prepared from Na-Y by H4EDTA extraction (Si/Al = 4.2) and of a commercial ultrastabilized Y zeolite (Si/Al = 2.7). Isomorphous substitution of aluminum for silicon by treatment with (NH4)2[SiF6] solution resulted in products with higher crystallinity and almost free of aluminum debris. In the group of dealuminated zeolites thus prepared, the temperature of lattice collapse was found to increase nearly linearly with the decrease of the number of aluminum atoms in the unit cell. However, IR spectra in the OH-stretching vibration range and chemical analysis gave evidence that, especially at higher dealumination degrees (final Si/Al ratios greater than about 5), up to 30–40% of the vacancies left by aluminum extraction were not refilled by silicon during the secondary synthesis and remained as structural defects. As expected, the surface acidity proved to be preferentially of the Brönsted type and the acid strength increased as compared to the parent Y zeolite. Similar results were reported in a paper of Neuber et al. [185] dealing with the spectroscopic and catalytic characterization of NH4-Y zeolite progressively dealuminated with (NH4)2[SiF6] according to [181]. Again, at higher degrees of dealumination, the process was found to be associated with the creation of defect sites or vacancies which were large enough to admit pyridine even to the soda-

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lite cages, and a substantial loss of crystallinity was observed at dealumination degrees higher than 50%. Further, fluorine species could be detected in the zeolitic product after removal of about 50% of the framework aluminum. With progressive dealumination, higher activation energies were found to be neccessary for the decomposition of the respective ammonium and pyridinium zeolites, i.e., the strength of Brönsted acid sites associated with remaining framework aluminum was increased. In a comparative study of Y zeolites dealuminated by both hydrothermal treatment and (NH4)2[SiF6] solution much lower Lewis and higher Brönsted acid site concentrations were found at comparable aluminum contents in the isomorphously substituted products [102]. More recently, Matharu et al. [186] pointed again to the sensitivity of the dealumination procedure published in [180–182] against reaction parameters such as reaction temperature and time, rate of addition, pH, washing conditions, etc. They also found that the maximum dealumination degree was limited to about 50% and oxy-fluorinated aluminum species, trapped within the zeolite cages, might have been retained which could not be removed by washing. Corma et al. [109] estimated the dealumination limit approachable without essential lattice destruction to be 25 Al per unit cell, corresponding again to a dealumination degree somewhat lower than 50%. The development of strong acidity upon dealumination of Y zeolite according to, and the retention of, not further identified fluorine species was also reported by Lónyi and Lunsford [187]. Using a parent NH4,Na-Y zeolite with 54 Al per unit cell, an abrupt decrease in crystallinity (about 50%) and an increase in the amount of retained fluorine species were observed when the product approached a framework aluminum content of 26 Al per unit cell, i.e., again at a dealumination degree of about 50%. It is worth mentioning that these authors found Y zeolites dealuminated by (NH4)2[SiF6] to be substantially resistant to further dealumination upon steaming at 600°C for 3 h. However, part of the retained fluorine escaped as HF during the hydrothermal treatment. In a series of Y zeolites dealuminated to different degrees (final average framework Si/Al ratios about 3–6) with (NH4)2[SiF6], which retained above 80% of their crystallinity, a much greater Si/Al ratio was found by XPS on the outer surface of the crystallites [188]. The gradient is obviously due both to diffusioncontrolled dealumination and to a selective deposition of silica on the external surface. In another paper [189], this dealumination process was found to be not completely stoichiometric. About 16% of the amount of (NH4)2[SiF6] applied, which was enough to approach a dealumination level of 26.6%, did not react under conditions usually recommended. Non-reacted fluorosilicate, difficult to remove by washing, may have remained in the sample and may have been responsible for the low resistance of the dealuminated products to hydrothermal treatments. Treatment with (NH4)2[SiF6] solutions was found to have no effect on the porosity and total acidity of ZSM-5 zeolites, but it decreased the concentration of aluminum atoms on the external surface of the crystallites and, hence, improved the para-selectivity of ZSM-5 in the catalytic isomerization of m-xylene [190]. Similarly, applying the technique described in [181] to the dealumination

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of NH4-mordenite, only a few extra-framework aluminum species corresponding to a dealumination degree of maximum 12% were formed, but they could not be extracted by washing [191]. It was suggested that aluminum species released from the framework remained blocked in the unidimensional channel system and, therefore, dealumination was limited to the pore mouth. This resulted, due to shape-selective effects, in a large decrease of the amount of adsorbed organic molecules and of the catalytic activity in m-xylene isomerization. The dealumination of faujasite, mazzite and offretite with ammonia hexafluorosilicate and the characterization of the products with various techniques have been reported [192]. The maximum level of dealumination, which could be achieved without loss of X-ray crystallinity, corresponded to 50% for faujasite and 30% for mazzite and offretite. The dealumination capability was found to depend on the texture of the crystals, which may have indicated that the process was diffusion-controlled. NH4-ZSM-5 was dealuminated up to Si/Al ratios of 100 in a multi-step process consisting of (1) steaming at 350–650°C in presence of a not precisely defined admixed “phosphorus compound”, (2) extraction with 0.2 M (NH4)2[SiF6] solution at 80°C, (3) removal of Al and F ions by washing with water and (4) steaming at 800°C [193]. The X-ray crystallinity of the products was fully retained, and, according to expectation, the total acidity decreased with increasing dealumination degree. However, it should be noted that the portion of stronger acid sites is claimed to decrease with the progress in dealumination. Corma et al. [194, 195] removed selectively extra-framework aluminum from ultrastabilized Y zeolites by extraction with 0.4 M aqueous solutions of (NH4)2[SiF6] at 95°C. The complex salt has to be applied in amounts just sufficient for the elimination of these species; application in excess leads to concomitant removal of framework aluminum. In zeolites steamed under severe conditions (at 700–750°C) part of the extra-framework aluminum, probably highly condensed species, proved to be resistant to this treatment. The removal of extra-framework aluminum resulted in characteristic changes in both the catalytic activity of the samples in the isobutane/2-butene alkylation and the deactivation rate. In any case, this process, that proceeds according to e.g., Eqs. (7a) and (7b), must result in silica as a reaction product which should be, if not removed in colloidal form, deposited in the zeolite pores or present as a separate phase. AlOOH + (NH4)2[SiF6] Æ SiO2 + NH4[AlF4] + NH4HF2 ,

(7a)

AlO+ + H2O + (NH4)2[SiF6] Æ SiO2 + NH4[AlF4] + NH4HF2 + H+ . (7b) 4.4 Dealumination of Zeolites in Dry Mixtures with (NH4)2[SiF6]

The literature which has appeared in the field of zeolite dealumination with (NH4)2[SiF6] since the pioneering report by Breck and Skeels [181] has dealt exclusively with the optimization of the reaction conditions and with structural and compositional consequences of this technique as well as with limitations in

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its applicability and not with the modification of fundamental experimental features. Only recently have Beyer et al. [196] reported on an essentially modified procedure in which (NH4)2[SiF6] is applied, ground with the ammonium form of the zeolite in crystalline state. It was found that, upon heating at 140–190°C, the following reaction proceeds in such mixtures from left to right: {AlO4/2}–(NH4)+ + (NH4)2[SiF6] Æ {SiO4/2} + NH4[AlF4] + NH4HF2 + NH3

(8)

where {AlO4/2}– and {SiO4/2} refer to primary tetrahedral units of zeolite structures containing Si and Al, respectively, as T atoms. The escape of the gaseous reaction product, ammonia, is obviously related to the thermodynamic driving force of this reaction. The progress of the dealumination reaction can be easily monitored by titration of the volatile reaction product, ammonia, evolved according to Eq. (8) and depends on reaction temperature and time. Complete conversion of the applied (NH4)2[SiF6] is normally achieved in 0.5–3 h. However, the reaction temperature should not exceed 190°C in order to avoid thermal decomposition of (NH4)2[SiF6] and NH4[AlF4]. It is also recommended that the mixture be heated in a stream of a dry inert gas up to the final reaction temperature at a slow rate (e.g., 5 K/min) in order to remove most of the water adsorbed in the zeolitic component before hydrolytic side reactions can start. Generally, at the reaction temperature, most of the formed NH4HF2 will be stripped off by sublimation; the rest can be extracted together with the reaction product NH4[AlF4] by washing with water. Replacement of Al by Si in tetrahedral framework sites has been evidenced by 27Al and 29Si MAS NMR spectroscopy and XRD (unit cell shrinkage). In the case of Y zeolite, product inhibition was found to occur at a dealumination level corresponding to an incorporation of about 32 silicon per unit cell, i.e., when each large cavity contained 4 [AlF4]–. Supported by XRD observations it was suggested that these anions are located at or near the cation sites SII coordinatively bound to framework oxygen atoms of the six-membered rings connecting sodalite cages and large cavities. Dealumination of L zeolite (with 8.7 Al per unit cell) was limited by product inhibition at a level of 3 [AlF4]– per unit cell corresponding to a dealumination degree of 35%. However, repeated dealumination using again 3 (NH4)2[SiF6] per unit cell resulted in the replacement of a further three Al atoms, i.e., in a dealumination degree of about 70%, and total dealumination was achieved after a third step [197]. It is worthwhile mentioning that in the case of L zeolite the shrinkage of the unit cell was strongly anisotropic; only the cell constant a decreased upon dealumination, while c showed even a small but significant increase. This points to a selective substitution of aluminum in T1 sites, i.e., in the 12-membered rings. Similar dealumination behavior was observed for mordenite with 8 Al per unit cell. Product inhibition appeared at a level of 4 [AlF4]– per unit cell, i.e., at a dealumination degree of 50%, and the unit cell contraction was found to be anisotropic. A practically aluminum-free mordenite was obtained after repeating once the dealumination procedure with 4 (NH4)2[SiF6] per unit cell. ZSM-5 zeolite containing 4 Al per unit cell could be completely dealuminated in one step, obviously due to the low initial aluminum content.

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The dry variant of the dealumination technique based on (NH4)2[SiF6] gives dealuminated zeolites generally poor in lattice defects provided that the reaction temperature is below the decomposition point of ammonium hexafluorosilicate. The dealumination agent is not applied in amounts exceeding the limits due to product inhibition and the formed NH4HF2 is stripped off by sublimation before the product is contacted with washing water. The modified procedure seems to be superior to the original one reported in [180–182] also as far as applicability limitations are concerned, since it can be used, in contrast to the original one (see [190, 191]), without essential restrictions for the dealumination of ZSM-5 and mordenite as well. In ferrierites, solid-state dealumination was found to be inhibited at rather low dealumination degrees [198]. Applying crystalline (NH4)2[SiF6] in an amount equivalent to the ammonium content (1.9 per unit cell) of a completely ionexchanged ferrierite, only 0.7 silicon atoms per unit cell could be substituted for framework Al. However, the zeolite prepared by solid-state synthesis from a crystalline magadiite variety contained a larger amount of extra-framework aluminum (0.52 Al per unit cell) in the form of cationic or oxidic species and lattice defects (vacancies) revealed by IR bands typical of internal silanol groups and of pyridine coordinatively bound to extra-framework aluminum. IR spectroscopic evidence was given that (NH4)2[SiF6] reacted with such aluminum species according to Eq. (7a) and the over-all reaction: 2 AlO+ + (NH4)2[SiF6] Æ SiO2 + 2 AlF3 + 2 NH4+

(9)

The framework vacancies were found to be filled up according to: {(O1/2H)4} + (NH4)2[SiF6] Æ {SiO4/2} + 4 HF + 2 NH4F

(10)

where {SiO4/2} and {(O1/2H)4} refer to primary tetrahedral building units with Si as T atom and vacancies of the hydroxyl nest type, respectively. The limited degrees of dealumination obtained with ferrierite were due to these favored side reactions and to the restriction of migration processes caused, in addition to the formation of NH4[AlF4] in the actual dealumination process (see Eq. 8), by deposition of the products of the reactions (7a) and (9) in the relatively narrow channels of ferrierite.

5 Alumination of Zeolites The use of zeolites as ion exchangers generally requires high ion-exchange capacities and, hence, high framework aluminum contents. Adsorption capacity and selectivity also depends to a certain degree on the aluminum concentration of the framework and may be favored by high framework aluminum levels. Therefore, efforts have been made to find methods for the insertion of aluminum into zeolite frameworks by secondary synthesis.

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5.1 Alumination with Gaseous Aluminum Chloride

Very soon after the dealumination process based on the reaction with silicon tetrachloride (see Sect. 4.1) was first reported [147], attempts were made to reverse this reaction in order to increase the aluminum content of zeolitic frameworks. High-siliceous zeolites of the ZSM-5 and ZSM-11 type were subjected at 500–600°C to a stream of dry nitrogen loaded at higher temperatures (180–375°C) with gaseous aluminum halides [199–203]. 27Al MAS NMR, FTIR and ammonia TPD techniques revealed that the content of both framework and extra-framework aluminum was increased by this treatment, resulting in the generation of both Brönsted and Lewis acidity. This behavior was suggested to be due to the reversibility of reaction (5) [202]. However, Dessau and Kerr [199] and Chang et al. [201] observed that, as far as the aluminum incorporation in tetrahedral framework sites is concerned, internal hydroxyl groups associated with lattice defects were involved in the process. Thus, they suggested the alumination phenomenon to be due to the insertion of Al in lattice vacancies. For thermodynamic reasons it is not possible to consider the reverse of reaction (4) as a pathway of framework alumination (which would formally result in additional {AlO4/2}– tetraeders charge-compensated by Al3+ cations) though that has been done in some publications. Yashima et al. [204] found that aluminum was not incorporated into HZSM-5 heat-treated at 960°C prior to contacting with AlCl3 at 350°C. On the other hand, aluminum was inserted into the framework of this zeolite upon contacting with gaseous AlCl3 at 650°C. Silicon released from the zeolite, obviously in the form of a volatile silicon compound, was recovered by passing the effluent gas through 1 N NaOH and determined by AAS. HZSM-5 pretreated at only 500°C was found to react with AlCl3 already at 350°C. On the basis of these observations, it was suggested [204] that at lower temperatures, e.g. 350°C, alumination proceeded through the reaction of hydroxyl nests with AlCl3 , whereas at higher temperatures (650°C) substitution of framework silicon by aluminum, i.e., the reverse of reaction (5), occurred. However, in a later paper [205], the same authors reported that alumination levelled off within a certain reaction time while the amount of released silicon increased steadily und surpassed that of the incorporated aluminum. Consequently, it was suggested in line with preceding publications that the introduction of aluminum in four-coordinated framework sites proceeded exclusively via insertion in lattice imperfections though no attempts were made to explain the claimed presence of volatile silicon compound(s) in the effluent gas. The formation of extra-framework aluminum species was attributed to the reaction of AlCl3 with silanol hydroxyls on the external surface and/or “the non-intact Si-OSi bonds formed from the SiOH groups on the external surface”. The reaction of AlCl3 with Y zeolites, disclosed in patents assigned to Mobil Oil [206] and to Esso [207], was found to be basically a vapor-phase exchange of the original zeolite cations with aluminum ions.Thus, at least a fraction of the aluminum incorporated in ZSM-5 zeolites probably compensated as lattice cations the negative framework charges created by the incorporation of aluminum into the framework.

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In contrast to findings reported in [201], framework alumination of HZSM-5 was claimed to generate very strong acid sites [205, 208] by a synergetic effect of Lewis acid sites on Brönsted-type sites. However, since this claim was based only on ammonia TPD results that gave no direct evidence for the chemical nature of acid sites, it may be that the observed high-temperature TPD peak was exclusively associated with typical Lewis acid sites. Wu et al. [209] re-inserted aluminum in mordenite previously dealuminated by acid leaching. The optimal temperature for the reaction with gaseous AlCl3 was found to be 600°C. The amount of incorporated aluminum proved to be proportional to the defect site concentration and exceeded the amount of released silicon, depending on the reaction temperature, by a factor of 25–80. Thus, in the case of mordenite also, lattice vacancies are basically involved in the reinsertion of tetrahedrally coordinated aluminum. Using GaCl3 and SbCl3 as reactants, gallium and antimony could be similarly inserted into lattice defiencies created in mordenite by acid leaching [210]. Recently [211], a post-synthesis modification of zeolite beta consisting of separate dealumination and titanation steps has been reported. First hydroxyl nests were formed by removal of up to 90% of the aluminum by leaching with oxalic or nitric acid, than up to 2 wt.% titanium was inserted into the lattice vacancies without formation of TiO2 as a second phase by treatment with gaseous TiCl4 at 500°C. The general conclusion to be drawn is that alumination based on the reaction with gaseous aluminum chloride is restricted to zeolites containing framework vacancies by synthesis and is restricted to the level of such lattice imperfections. 5.2 Alumination with Aqueous Fluoroaluminates

Insertion of aluminum into zeolites by aqueous fluoroaluminates was reported by Chang et al. [201]. The applicability of potential reactants was restricted by the low solubility of the fluoroaluminate salts. The reported alumination procedure for silicalite comprised impregnation with a 0.02 M aqueous solution of (NH4)3[AlF6] and drying the sample, containing about 0.1% AlF3, at 130°C. The pH of the (NH4)3[AlF6] solution had a decisive influence on the incorporation of Al into the framework. In contrast to gaseous AlCl3, alumination with hexafluoroaluminuminates proceeded also in the absence of defect sites. Thus, in this case, direct substitution of framework silicon by aluminum, i.e., the reverse of reaction (6), seemed to occur. 5.3 Alumination with Aluminate Solutions

Shihabi et al. [212] observed that the ion-exchange capacity and acid-catalytic activity of high-silica ZSM-5 significantly increased when the zeolite was extruded with alumina binders. These effects were attributed to the transfer of aluminum as an aquospecies from the binder (g-Al2O3) to the zeolite during extrudation or hydrothermal treatment and their incorporation into framework defects [213].

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Zhang et al. [214] used sodium aluminate solutions as external aluminum source. Y zeolites dealuminated by extraction with EDTA solution and by treatment with SiCl4 were treated with 0.025–1.0 M aluminate solutions adjusted to pH 14 at 60°C for 12 h. MAS NMR and FTIR spectrometry gave evidence that after both dealumination pretreatments aluminum was re-inserted into the framework. The re-alumination mechanism was thought to involve incorporation of aluminum into lattice vacancies in the case of the EDTA-treated samples and direct substitution of framework silicon in high-silica Y zeolite prepared by the SiCl4 procedure. In contrast to this, only alumination of the crystal surface and no significant changes in the bulk composition were found when high-silica Y zeolite, prepared by dealumination with SiCl4, was treated with NaAlO2 solutions under comparable conditions [118, 215, 216]. The products obtained proved to be highly resistant to superheated steam, which was attributed to the formation of a superficial layer of amorphous aluminosilicate upon treatment with sodium aluminate solution. 5.4 Re-Insertion of Extra-Framework Aluminum

In 1980, Breck and Skeels [217] reported that hydroxyaluminum cations, formed during ultrastabilization of Y zeolite through hydrolytic release of framework aluminum, could be at least partly (20%) re-inserted into the framework vacancies by titration with NaOH up to a pH of 10–11.These results were later discounted by Engelhardt and Lohse [218]; applying 29Si MAS NMR spectroscopy, they could not find any re-insertion of aluminum in samples prepared exactly according to the data given in [217]. The experimental conditions applied in [217] and [218] are obviously too mild to achieve a detectable re-alumination, since Liu et al. [219] succeeded in increasing the framework aluminum content of ultrastabilized Y zeolite by about 48% upon treatment of 2 g of sample with 100 ml of a 0.25 M aqueous KOH solution at 80°C for 24 h. This was controlled by 29Si MAS NMR spectroscopy. The reported procedure was claimed to reverse completely the process of ultrastabilization. Bezman [220] and Klinowski et al. [221] confirmed conclusively the aluminum re-insertion reported in [219]. However, it was evidenced [220] that the process is associated with an amorphization of about 20% of the treated zeolite.A study by Hamdan et al. [222] also indicated that aluminum atoms eliminated from the framework of Y zeolite by hydrothermal treatment could be subsequently re-inserted into the framework by treatment with KOH solutions at 80°C. Crystallinity was found to be largely retained in the process. However the Si,Al distribution proved to be significantly different from that in the starting zeolite. The effect of alkalinity on the re-alumination of Y zeolite previously dealuminated by SiCl4 was studied by Zhang et al. [214]. Treatment with NaOH solutions at pH 12 and higher resulted in remarkable re-alumination. However, the crystallinity decreased with increasing pH, reaching about 50% at pH 14. Recently, Liu et al. [223, 224] investigated the effects of 0.025–2.0 M KOH solutions on the structure of Y zeolites previously dealuminated by ultrastabilization and by extraction with EDTA and (NH4)2[SiF6]. As for ultrastabilized

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Y zeolite, the re-insertion mechanism involving the refilling of lattice vacancies was again confirmed. The samples dealuminated by exctraction with both EDTA and (NH4)2[SiF6] were found to be practically free of extra-framework aluminum. Nevertheless, a significant increase in the framework aluminum concentration was observed upon treatment of both samples with KOH solutions. Mainly based on the treatment effects on crystal morphology and concentration of silanol groups, this phenomenon was attributed to the dissolution of the outer silicon-enriched layer in the case of the EDTA-treated sample and to the removal of framework silicon in the zeolite dealuminated with (NH4)2[SiF6]. Partial re-insertion of aluminum into the framework of H-ZSM-5 dealuminated by calcination at 800°C was observed upon treatment with alkaline solutions [225]. Similarly, non-framework aluminum species created by deep hydrothermal dealumination of H-ZSM-5 could be partially re-introduced into the framework upon treatment (2 h, 77°C) with 0.1 M NaOH solution [226]. However, no re-alumination was observed after dealumination under mild hydrothermal conditions when the Si/Al ratio of the dealuminated material was <25. Lietz et al. [227] studied the effects of NaOH treatment on H-ZSM-5 zeolite after both calcination and steaming of the ammonium form at 500°C. The calcined sample with relatively high framework (4.9 Al/u.c.) and low non-framework Al content (0.3 Al/u.c.) released upon treatment with a 0.08 M NaOH solution at reflux predominantly silicon. This desilication resulted in a moderate decrease in the framework Si/Al ratio and in a significant enrichment of non-framework aluminum on the outer surface of the zeolite crystallites. In contrast, alkaline treatment of the steamed sample with low framework (0.8 Al/u.c.) and high non-framework Al content (4.2 Al/u.c.) caused a re-alumination of the framework at the expense of extra-framework aluminum species. In line with these findings, Sulikowski [29] reported on a significant decrease (from 2.24 to 2.03) in the fractal dimension of steamed NH4 ,Na-Y zeolite upon treatment with 0.25 M KOH pointing to a reduction of the surface “roughness” which is due to the formation of aluminate ions and their re-incorporation into the framework. Direct evidence in support of the re-alumination of Y zeolite by (NH4)[AlF4] was obtained from the XRD patterns of NH4 ,Na-Y previously subjected to the first step of the modified dealumination technique reported in [196] (see Eq. 8), i.e., only to the heat treatment of the (NH4)2[SiF6]/zeolite mixture, but not to the subsequent removal of reaction products by washing. According to expectation, after completed dealumination only the reflections typical of the (partly dealuminated) faujasite phase, but not those of the dealuminating agent, were observed.After heating the samples for a longer time at relatively low temperatures (about 150°C) in air, reflections typical of Na2[SiF6] appeared in the XRD pattern [196]. Obviously, sodium ions migrate from sites in the sodalite cages into the large cavities where they react with the occluded reaction products of the previously performed dealumination, i.e., [AlF4]– and NH4HF2, under formation of thermodynamically favored sodium hexafluorosilicate: {SiO4/2} + [AlF4]– + NH4HF2 + 2 Na+ Æ {AlO4/2}– + Na2[SiF6] + NH4+ + H+

(11)

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Storage of zeolite Y, that had previously been subjected to the first step of the dealumination procedure, over an aqueous solution of ammonia resulted even at room temperature within some days in the formation of Na2[SiF6] in amounts detectable by XRD [228]. Thus, disregarding the participation of sodium ions, Eq. (11) can be regarded as the reverse of the process described by Eq. (8). Recently [229], re-insertion of extra-framework aluminum formed in HZSM-5 during preceding calcination at 600°C was claimed to proceed upon treatment with 2 M acidic solutions at 100°C. Hydrochloric acid proved to be the most effective re-alumination agent. The framework aluminum content was found to increase at the expense of extra-framework species from 58% remaining after the thermal dealumination up to 81% of the original amount in the parent material. However, this re-alumination process is obviously restricted to ZSM-5, since the removal of aluminum from other zeolites by acid leaching has been convincingly demonstrated in numerous reports (see Sect. 2.1). Interestingly, Kooyman et al. [20] reported that acid leaching of ZSM-5 with HCl resulted, independently of temperature and time of the treatment, in about the same, though rather insignificant, dealumination degree. Thus, it seems that in the presence of acids some “equilibrium” between framework and extra-framework aluminum is established in ZSM-5.

6 Desilication of Zeolites In principle, desilication of zeolite frameworks must result in the same features, i.e., in the same type of lattice vacancies, as their dealumination. Considering the immense efforts made during the last three decades in the field of dealumination and re-alumination of zeolites it seems astonishing that for a long time no attempts were made to manipulate directly the framework silicon content by secondary syntheses. Only recently have some studies been published aimed at the desilication of zeolites by leaching with alkaline solutions. Removal of framework silicon from Y zeolites (Si/Al = 2.7) upon leaching with alkaline solutions up to pH 12 at 80°C was already discussed in 1988 [230] and connected with the observed increase of the unit cell size (framework Al/Si ratio). Later, Dessau et al. [231] reported that treatment of ZSM-5 with refluxing 0.5 M Na2CO3 solution resulted in partial dissolution of the sample with preferential removal of silica from the outer shell of the crystals and, hence, in aluminum zoning, with aluminum enriched at the exterior crystal surface. Mao et al. [232, 233] extracted X, Y and ZSM-5 zeolites under similar conditions, i.e., twice at 80°C for 4 h with an aqueous 0.8 M Na2CO3 solution (5g/150 ml). According to compositional values given in [233], 41 Si atoms per unit cell could be removed from ZSM-5 with a starting Si/Al ratio of 15.7, while the Si/Al ratio of X and Y zeolite could be reduced from 1.24 to 1.00 (–21 Si/u.c.) and from 2.39 to 1.30 (–62 Si/u.c.), respectively. It was claimed that the original structure, degree of crystallinity, surface area and size of micropores were all essentially preserved. From microporosity measurements the authors concluded that a healing process occurred upon heat treatment resulting in a secondary pore system.

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Removal of framework silicon from Y zeolite dealuminated with (NH4)3[AlF6] and virtually free of extra-framework aluminum species was also suggested by Liu et al. [224], who observed a gradual decrease of the framework Si/Al ratio upon treatment with 0.25 M KOH solution at temperatures between 40 and 100°C and the appearence of soluble silicate in the fluid. Moderate desilication (about 10%) of H-ZSM-5 upon treatment with 0.08 N NaOH at reflux temperature was observed by Lietz et al. [227]. It is highly probable that lattice vacancies created by removal of silicon may be filled up in the same way and under similar conditions as those remaining after release of aluminum. Thus, recrystallization of the desilicated products to particles with well-ordered crystal structure but traversed by nanopores is obviously effected by water steam present as reaction product of the dehydroxylation of “hydroxyl nests”. After the desilication process, the zeolite is in the sodium (or potassium) form and this is known to be highly resistant towards hydrothermal effects. Therefore, it is to be expected that steaming represents the most effective method for the stabilization of desilicated zeolites avoiding the risk of concurrent dealumination.

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214. Zhang Z, Liu X, Xu Y, Xu R (1991) Zeolites 11:232 215. Lutz W, Löffler E, Zibrowius B (1993) Zeolites 13:685 216. Lutz W, Löffler E, Fechtelkord M, Schreier E, Bertram R (1997) In: Chon H, Ihm S-K, Uh YS (eds) Proceedings of the11th International Zeolite Conference, Seoul, Korea, August 12–17, 1996. Elsevier, Amsterdam, p 439. Stud Surf Sci Catal 105A 217. Breck DW, Skeels GW (1980) In: Rees LVC (ed) Proceedings of the 5th International Conference on Zeolites, June 2–6, 1980, Naples, Italy. Heyden, London, p 335 218. Engelhardt G, Lohse U (1984) J Catal 88:513 219. Liu X, Klinowski J, Thomas JM (1986) J Chem Soc Chem Commun 1986:582 220. Bezman RD (1987) J Chem Soc Chem Commun 19870:1562 221. Klinowski J, Hamdan H, Corma A, Fornes V, Hunger M, Freude D (1989) Catal Lett 3:363 222. Hamdan H, Sulikowski B, Klinowski J (1989) J Phys Chem 93:350 223. Liu D-S, Bao S-L, Xu Q-H (1997) In: Proceedings of the International Symposium in China, Nanking, p 3/59 224. Liu D-S, Bao S-L, Xu Q-H (1997) Zeolites 18:162 225. Völter J, Lietz G, Kürschner U, Löffler E, Caro J (1988) Catal Today 3:407 226. Reschetilowski W, Schöllner R, Freude D, Klinowski J (1989) Appl Catal 56:L15 227. Lietz G, Schnabel KH, Peuker Ch, Gross Th, Storek W, Völter J (1994) J Catal 148:562 228. Borbély-Pálné G, Beyer HK, unpublished results 229. Sano T, Uno Y, Wang ZB, Ahn C-H, Soga K (1999) Microporous Mesoporous Mater 31:89 230. Aouali L, Jeanjean J, Dereigne A, Tougne P, Delafosse D (1988) Zeolites 8:517 231. Dessau RM, Valyocsik EW, Goeke NH (1992) In: Higgins JB, von Ballmoos R, Treacy MM (eds) Proceedings of the 9th International Zeolite Conference, Extended Abstracts and Program, Butterworth-Heinemann, Stoneham MA, RP96 232. Mao RLV, Xiao S, Ramsaran A, Yao J (1994) J Mater Chem 4:605 233. Mao RLV, Ramsaran A, Xiao S, Yao J, Semmer V (1994) J Mater Chem 4:533

Preparation of Metal Clusters in Zeolites Pierre Gallezot Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cédex, France; e-mail: [email protected]

Dedicated to Professor Gerhard Ertl on the occasion of his 65th birthday

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Introduction

1.1 1.2 1.3 1.4

Applications of Metal Clusters . . . . . Cluster Characterization . . . . . . . . Scope of the Review . . . . . . . . . . Definitions: Cluster Size and Location

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Bases for Metal Cluster Preparation . . . . . . . . . . . . . . . . . . 262

2.1 2.2 2.3 2.4 2.4.1 2.4.2 2.4.3 2.5 2.6 2.7

Impregnation with Metal Salts and Reduction . . . . . . . . . . . Adsorption and Decomposition of Zerovalent Metal Compounds Preparation from Metal Vapors . . . . . . . . . . . . . . . . . . . Preparation by Ion Exchange and Reduction . . . . . . . . . . . Ion Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generalities on Cation Reducibility . . . . . . . . . . . . . . . . . Mechanism of Reduction and Reducing Agents . . . . . . . . . . Incorporation of Metal Precursors During Synthesis . . . . . . . Preparation of Bimetallic Clusters . . . . . . . . . . . . . . . . . Factors Favoring Redispersion . . . . . . . . . . . . . . . . . . .

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Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.2 3.3 3.4 3.5

Metal Clusters in Faujasite-Type Zeolites Platinum Faujasites . . . . . . . . . . . . Palladium Faujasites . . . . . . . . . . . Rhodium Faujasites . . . . . . . . . . . Ruthenium Faujasites . . . . . . . . . . Iridium and Osmium Faujasites . . . . . Bimetallic Faujasites . . . . . . . . . . . Metal Clusters in L Zeolites . . . . . . . Metal Clusters in ZSM-5 Zeolites . . . . Metal Clusters in Various Zeolites . . . . Carbide, Nitride, and Sulfide Clusters . .

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Concluding Remarks and Prospects . . . . . . . . . . . . . . . . . . 296

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1 Introduction 1.1 Applications of Metal Clusters

Over the last 30 years, increasing attention has been paid to metal clusters hosted in zeolites and related microporous materials. One of the main incentives was the preparation of selective and active catalysts for various types of reactions. Thus, metal-loaded zeolites were first studied in the late 1950s – a notable milestone is the pioneering work of Rabo et al. [1] to obtain efficient bifunctional catalysts for hydrocracking of heavy distillates and paraffin isomerization. Acidic zeolites loaded with platinum-group metals are still used for this purpose, but, despite intensive fundamental investigations and numerous patents in various applications ranging from petroleum refining to fine chemistry, there are still very few industrial processes based on zeolite-supported metal catalysts. A remarkable example is the selective dehydrocyclization of linear hydrocarbons to aromatics on Pt/L zeolites; a process discovered by Bernard [2]. However, because of their small and homogeneous size, metal clusters trapped in micropores of solids are fascinating objects for physical chemistry, organometallic chemistry, and solid-state physics, and future applications could be found in unexpected domains. 1.2 Cluster Characterization

One of the main difficulties in preparing metal clusters is to assess accurately the state of the metal at the different steps of preparation. Thus, after calcination pretreatments, one should be able to locate cations in zeolite framework sites and to distinguish cations bonded to framework anions (charge compensating cations) from cations associated with extra-framework oxygens (oxide clusters). After reduction, parameters such as cluster size, cluster distribution in zeolite grains, composition when bimetallic clusters are involved, and oxidation states have to be determined. Microstructures should be established at high spatial resolution to verify the homogeneity of preparations. These tasks are particularly difficult since physical techniques of characterization, such as TEM, attain their limit of application when cluster sizes become too small. Since samples are never perfectly homogeneous, different techniques can lead to opposite conclusions because they do not probe the same part of the sample. Thus, distinction cannot be made by transmission electron microscopy (TEM) between particles on the external surface and in the bulk unless the zeolite grains are first cut into thin sections to allow detection of internal metal particles. In general, two or more techniques practiced by trained specialists should be applied to obtain a satisfactory characterization of new metal zeolites. A single technique, such as temperature-programmed oxidation and reduction, could complete the characterization of known systems. Oxidation states are also difficult to establish because interpretations of X-ray photoelectron spectroscopy (XPS), X-ray ab-

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sorption edge spectroscopy, and IR spectroscopy of probe molecules are controversial. Besides characterization problems, non-reproducibility of metal cluster preparation can often be attributed to slightly different treatments conditions (e.g., size of zeolite bed, gas flow rate, temperature) that lead to different metal dispersion. Indeed, the final state of metals depends critically on a number of factors that are frequently overlooked. 1.3 Scope of the Review

Several reviews have been published on metal clusters in zeolites: Minachev and Isakov [3], Uytterhoeven [4], Gallezot [5–7], Jacobs [8], Maxwell [9], Delafosse [10], and Sachtler [11–13]. None of them specifically dealt with the preparative aspects. The scope of the present review is limited to the preparation of metal clusters in zeolites and other crystalline microporous materials. Characterization studies of metal clusters are not within the scope of this review but they have been taken into account to apraise the significance of preparation data. Literature data on metal cluster preparation are critically examined taking into account the details given for preparative procedures and the quality of the characterization work. Structural (electronic and geometric) and catalytic properties will be mentioned briefly in a few instances, e.g., when these data assist the characterization of the metal species prepared. Only extra-framework metal atoms will be considered here, which thus excludes metal atoms substituting Si or Al atoms in lattice positions. The preparation of zeolites loaded with cations or oxo-species will not be considered unless they are intermediates in metal cluster preparation. This excludes catalysts based on copper or other transition metal ions, e.g., those used in the reduction of NOx by hydrocarbons. The preparation of molecular metal clusters, (Mm Ll), will not be reviewed in detail; a comprehensive review on this subject has been produced by Ichikawa [14]. There are many reports in the literature on hydrocracking or hydroisomerization catalysts involving zeolites loaded with metals (usually platinum or palladium). Most of these studies have not been considered because the metal phases in these catalysts are of secondary importance with respect to acidic sites and pore geometry which control their properties, and they were therefore usually very poorly characterized. The size and location of metal particles in porous frameworks depend not only on the particular preparation technique employed, but also on the stability of metal clusters once formed. Thus, the following transformations could occur as a result of physicochemical treatments or during catalytic reactions: (i) sintering into larger particles, (ii) re-oxidation into cations or oxo-species, (iii) transformation into molecular clusters. These points have been reviewed briefly elsewhere [7, 13]. Examples of re-oxidation and conversion into molecular clusters will be given in Sect. 3.

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1.4 Definitions: Cluster Size and Location

Metal clusters (Mm) can be roughly described as small pieces of metal, but closer studies show that their atomic and electronic structures deviate from those of bulk metals either because of intrinsic size effects (deviation from normal electronic structure at very small nuclearities) or because of their interaction with framework anions and extra-framework species including cations. Clusters can be naked or bonded to various extra-framework ligands. They may be charged as a result of incomplete reduction or of interactions (inductive and/or field effects) with framework or extra-framework species. Clusters can involve two metals homogeneously alloyed or segregated. The size or nuclearity of metal clusters can vary across a wide range from m = 1 to thousands, and they can be located in various parts of the zeolite crystals, as depicted schematically in Fig. 1: 1. Very small clusters (1<m<4) in small cages (e.g., sodalite cages in faujasites), or side pockets in zeolites with linear pores (e.g., in mordenite). 2. Clusters of low nuclearity (1<m<10–40) whose sizes (<1–1.3 nm) are limited by the space available in cages (e.g., supercages in faujasite) or at the intersections of perpendicular cylindrical pores (e.g., in beta zeolite), or by the diameter of cylindrical pores. Thus, high-resolution TEM pictures showing metal clusters smaller than 1–1.3 nm in Pt/Y [5, 15], Ir/Y [6], Pt/EMT [16], Pt/beta [17], Pt/ZSM-5 [18], Pt/BaKL [19], and Rh/X [20] have been reported. However, in zeolites with cylindrical pores, metal clusters can grow along the axial direction forming small cylinders, as depicted in Fig. 2 corresponding to a TEM view of platinum clusters in mordenite taken with the electron beam perpendicular to the pore axes [7]. 3. Clusters which are not necessarily limited by cage or pore dimension but fill continuously the pore system in a small volume of the zeolite crystal. Thus, in faujasite-type zeolites, metal clusters can fill adjacent supercages forming grape-like particles. These were first evidenced on TEM micrographs taken on a Pd(NH3)42+-exchanged zeolite reduced with H2 at 400 K [21]. Interestingly, comparable grape-like microstructures were reported on Ru/Y zeolite after ammonia synthesis [22]. In Pt(NH3)42+-exchanged beta zeolite reduced

Fig. 1. Scheme of size and location of metal clusters in zeolites (see text for description)

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Fig. 2. TEM micrograph of an ultramicrotome section of zeolite Pt-mordenite (from [7])

at 473 K without prior oxygen treatment, metal atoms fill continuously large domains (50–200 nm) of the pore system [17]. In all these examples, the formation of microstructures characterized by continuous pore filling was favored by the presence of ammonia molecules, which probably enhance the mobility of metal atoms by complexation and transport processes. 4. Metal clusters can grow larger than cages and pores although they still remain encapsulated in the zeolite bulk. This was inferred from TEM images taken on ultramicrotome sections of Y zeolite grains showing that 2–3 nm Pt- or Pdparticles are well encapsulated in the bulk of zeolite crystals [5, 15, 21, 23]. The creation of crystal defects around growing clusters can be expected, but local lattice defects are difficult to detect by TEM. However, according to Exner et al. [24], the growing of nickel, palladium and platinum in Y zeolites into particles larger than cage dimensions could be accompanied by a local recrystallization mechanism that would cure crystal defects and would also account for the very uniform size and spherical form of the occluded particles. 5. Metal clusters can be located in lattice defects. Thus, Fig. 3 gives a TEM view showing iridium clusters precipitated in dislocations or twin boundaries in Y-zeolite crystals [6].

Fig. 3. TEM micrograph of an ultramicrotome section of Ir/NaY (from [6])

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6. Metal atoms or small metal clusters can migrate along the pores until they reach the external surface where they nucleate and usually grow into large particles.

2 Bases for Metal Cluster Preparation With the exception of direct condensation of metal vapors in micropores, the preparation of metal clusters involves at least two steps, viz., the loading of a metal precursor in the microporous material and the decomposition or reduction of the precursor yielding metal clusters. These two steps are highly interdependent and the second is often crucial for governing the final metal dispersion. The different strategies of metal cluster preparation will be examined in this section, and detailed examples of preparations for specific zeolites and metals will be given in Sect. 3. 2.1 Impregnation with Metal Salts and Reduction

Support impregnation with a metal salt is a very general technique widely used for the preparation of supported catalysts which can be applied as well to zeolites. Specifically, the incipient wetness technique (dry-impregnation, pore volume impregnation) consists of mixing a dehydrated – preferably outgassed – support with a volume of solution containing the required amount of metal, equal to the total pore volume of the solid. The impregnated solid is then dried, calcined and reduced under flowing hydrogen. Solutions of chloride, nitrate or carboxylate salts are usually employed. Ideally, the metal compound should fill the pores homogeneously and thus be evenly distributed in the solid. In practice, the diffusion of the impregnating solution is difficult in small micropores, especially if there are residual gas molecules and if the wettability by the solution is poor. Furthermore, impregnated salts are not well anchored on the support surface and thus can redistribute during calcination or reduction treatments. Therefore, this technique is usually not appropriate for obtaining high and homogeneous metal dispersions, the metal frequently ending up concentrated near the external surface of the zeolite grains. Thus, large ruthenium particles were obtained by impregnation of X, L, ZSM-5 zeolites with ruthenium chloride followed by calcination and H2-reduction [25]. However, high metal dispersion was obtained by an impregnation technique at low metal loadings in L zeolites (see Sect. 3.2). Also, impregnation of MCM-41 materials with H2PtCl6 to obtain 0.5 wt.% Pt-loading, followed by calcination and H2-reduction treatment, resulted in well-dispersed platinum [26]. The impregnation technique is useful in different situations: 1. When ion exchange cannot be used because of the lack of cationic exchange sites (e.g., Ni/AlPO4-5, [27]) or because the ion exchange capacity is too low to obtain a required loading.

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2. To load metals available only in anionic form which cannot be ion-exchanged (e.g., molybdenum loaded as molybdate in ZSM-5 [28]). 3. To prepare metal zeolites without Brönsted acidity. Thus, to avoid the generation of protons upon hydrogen reduction of ion-exchanged zeolites, platinum was loaded in L zeolites by impregnation [2, 29–31]. Co-impregnation of a platinum salt with KCl is also efficient for preparing non-acidic Pt/L zeolites since KCl can act as a proton scavenger [32]. Impregnation can be used in combination with ion exchange to improve the reduction of electropositive metals. Thus, impregnation of cobalt-exchanged Y zeolites with alkali hydroxides leads to the formation of cobalt hydroxide species in supercages that are reduced more easily than Co2+ cations in hexagonal prisms or sodalite cages [33–35]. Similarly, by impregnating a Co-exchanged zeolite with sodium acetate, the reduction temperature is lowered by up to 350°C [36]. 2.2 Adsorption and Decomposition of Zerovalent Metal Compounds

Vapor-phase impregnation and chemical vapor deposition are different names for a similar method which is used to load any outgassed microporous materials with vapors of organometallic compounds, where the metal is usually in the zerovalent state. The vapors are produced by sublimation of a solid or by evaporation of a liquid. However, zerovalent compounds can also be dissolved in a solvent and introduced by liquid impregnation. The second step consists of decomposing the adsorbed complex, usually by thermal decomposition under reduced pressure or in a reducing or inert atmosphere. The zerovalent metal atoms, free from all or part of the organic ligands, migrate and nucleate in pores to form clusters of different sizes depending on the mobility of the species formed by thermal decomposition. Organometallic precursors should fulfill a number of requirements: 1. Their molecular dimension should not exceed a critical size to enter and diffuse easily in porous frameworks. 2. Their vapor pressure should be high enough to be easily sublimated at moderate temperatures. 3. The decomposition of metal complexes should not produce intermediate, highly mobile organometallic species which would diffuse to the external surface or produce excessive metal agglomeration. 4. The organic ligands should not decompose into species forming strongly adsorbed deposits blocking the metal or zeolite surfaces. Metal carbonyls have been widely used as metal precursors [37–59] because they are easily available and fulfill most of the requirements mentioned above, but particularly because CO ligands are easily removed from the solid after thermal decomposition. Vapor- or liquid-phase impregnation with platinum acetylacetonate, Pt(acac)2 , which is a comparatively small and easily sublimable complex, has been used by Hong et al. [19] to load platinum in various zeolites. The

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same complex has been used to load metal in KL zeolite [60]. Dossi et al. [61] have loaded KL zeolite via chemical vapor deposition at 70°C of platinum hexafluoroacetylacetonate, Pt(hfa)3, which decomposed at 350°C to yield very small Pt-clusters. Weber and Gates have loaded NaY zeolites with pentane solutions of [Rh(CO)2(acac)] [62] or [Ir(CO)2(acac)] [63]. Subsequent carbonylation at 125°C yielded Rh6(CO)16 or Ir6(CO)16 species from which Rh- or Ir-clusters were obtained by decarbonylation. Ozin et al. [64, 65] have employed easily decomposable metallocene complexes obtained directly from metal vapors (Sect. 2.3). Once loaded, proper thermal treatment conditions should be applied to avoid the desorption of the complex or the formation of transient mobile species. The final state of metal depends also on the environment in the zeolite pores, such as cations acting as anchoring sites for metal atoms or protons which can produce redox reactions leading to the re-oxidation of electropositive metals (see Sect. 2.4.2). This technique presents specific advantages, viz: 1. It can be used to load metals which cannot be introduced by cationic exchange because they are in the form of anions in the pH range where zeolites are stable. Thus, molybdenum and rhenium were loaded in Y zeolites by adsorption and thermal decomposition of Mo(CO)6 [37–42] and Re2(CO)10 [37, 58], respectively. 2. It is useful to load microporous solids with little or no exchange capacity. Thus, various metal clusters have been prepared by adsorption and decomposition of metal carbonyls in AlPO4-5 [27, 42, 43], ZSM-5 [43], and silicalite-1 [44]. Liquid- and vapor-phase impregnations of platinum acetylacetonate were used to load AlPO4-5 and VPI-5 [19]. 3. It is well suited for the preparation of metal clusters which are difficult to prepare by ion exchange and reduction, e.g., when cations are difficult to reduce because of their low redox potential or because they tend to be highly stabilized by zeolite anionic frameworks (e.g., in SI or SI¢ sites of faujasites). There are many reports on the preparation of iron clusters in Y zeolites from Fe(CO)5 [42, 45–51], Fe2(CO)9 and Fe3(CO)12 [52] precursors. Cobalt clusters were prepared by adsorption and thermal decomposition of Co(CO)3NO [42, 53, 54] and Co2(CO)8 [55–57]. Co2(CO)8 was decomposed by microwave discharges allowing better control of the decomposition than is possible with thermal treatments [55]. 4. No reduction step is needed since the metal is loaded in a zerovalent state; thus, it is possible to avoid the formation of Brönsted acidity. Chemical vapor deposition of Pt-acetylacetonate [19, 60] or Pt-hexafluoroacetylacetonate [61] in L-type zeolite was used to obtain platinum clusters in a non-acidic environment. 5. The decomposition of neutral organometallic complexes can be used to prepare egg-shell catalysts where metal particles are sitting on or near the external surface [55].

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2.3 Preparation from Metal Vapors

Direct incorporation of metals in microporous supports by adsorption of metal vapors is possible in the case of alkali metals which have high vapor pressures. In an early piece of work, Rabo et al. [66] reported that faujasites are able to stabilize neutral or ionic sodium clusters by exposing the carefully dehydrated zeolites to sodium vapors under vacuum. The technique was later improved and extended to other zeolites and other alkali metals. Harrison et al. [67] found that Na3+ 4 clusters were located in sodalite cages. Another technique consists of using the decomposition of sodium azide (NaN3) as a source of sodium vapor [68]. Xu and Kevan [69, 70] achieved a detailed characterization of clusters prepared by these methods. Sodium clusters can even be loaded at room temperature merely by stirring the zeolite powder with sodium or potassium in the solid state or in a solution of tetrahydrofuran or hexane [71]. Because they have low vapor pressures, transition metals cannot be loaded by direct adsorption, but their adsorption can be mediated by transient organometallic complexes formed between zerovalent metal atoms and solvent molecules. This is the basis of the solvated metal atom dispersion (SMAD) method developed by Klabunde and Tanaka [72]. Metal vapors condensed in liquid hydrocarbons at low temperatures form weak complexes that are easily decomposed even below room temperature. Microporous supports impregnated with solutions of metal complexes at low temperatures are warmed up to decompose the complex and liberate zerovalent metal atoms which nucleate into clusters. Preparation of Ni- and Co-clusters in HY and HZSM-5 was reported [72]. In the same way, Nazar et al. [64] condensed iron and cobalt vapors in a slurry of dehydrated NaY zeolite in toluene at –120°C, then the mixture was rotated at –78°C. The bis-toluene complex thus formed and adsorbed in the zeolite was decomposed by warming to room temperature yielding clusters small enough to fit into supercages. 2.4 Preparation by Ion Exchange and Reduction

This is by far the most frequently used technique to load metals into zeolites and related materials. It has been applied for the last 40 years, and there are countless reports in the literature on metal zeolites prepared by ion exchange; a detailed description of just a few of these systems will be given in Sect. 3. The preparation of metal clusters involves at least two steps, ion exchange and reduction, but a pretreatment is often needed before reduction to control the final metal dispersion. Indeed, it was shown very early [73–76] that calcination under oxygen before reduction was a key step in the preparation of highly dispersed and homogeneous distribution of metal clusters in zeolite pores. The reasons for this became clear from X-ray studies [75, 76] showing that calcination pretreatments control the position of cations and thus their reducibility and migration capability. There are no standard conditions applicable to all metal zeolites because many factors affect cation location and reducibility and the propensity of metal atoms to migrate and nucleate.

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2.4.1 Ion Exchange

The technique, although very general, suffers from a few limitations, viz.: 1. It cannot be applied to load metals which are only available in anionic form (e.g., Mo, Re), or cations which are only stable at very low pH (e.g., Ga) where most zeolites are unstable. 2. It cannot be used with neutral frameworks such as aluminophosphates or with aluminum-free zeolite frameworks such as silicalite. 3. Only small metal amounts can be introduced in zeolites with low exchange capacities because they contain few framework aluminum atoms (e.g., dealuminated mordenite or faujasite, ZSM-5 and related microporous materials). In these cases, it is possible to obtain higher metal contents by operating in successive steps alternating metal reduction and ion exchange. 4. Complex cations larger than the pore size cannot enter the pore systems. Thus, CaA zeolite could not be exchanged with Pt(NH3)2+ 4 cations so that platinum was loaded during zeolite synthesis [77]. Pt(NH3)2+ 4 cations were more easily exchanged in the pores of H-mordenite than in the slightly smaller pores of Na-mordenite [78]. Typically, ion exchange is performed on the sodium form of zeolites suspended in dilute aqueous solutions of the cations to be loaded. Ion exchange processes are fast enough to ensure within a few hours the exchange levels required in most applications of metal zeolites. The exchange is usually carried out in a single step at room temperature, but higher exchange levels may require two or more exchanges in fresh solutions, and higher temperatures could favor the kinetics of exchange. After exchange, zeolites are filtered and washed with water until washings are free from cations and anions. Attention must be paid to the pH of the exchange solution which can affect the final metal dispersion. Thus, precipitation of metal hydroxides on the external surface of the zeolite, due to the local increase in pH when alkali ions are exchanged, should be avoided. In the case of first-row transition metals, the use of very dilute solutions can be effective in reducing the formation of hydrolyzed species. Precipitation can also be avoided by pH adjustment. For platinum-group metals, Pt(NH3)2+ 4 , 2+, Ir[(NH ) Cl]2+ cations are widely used , Rh[(NH ) Cl] Pd(NH3)42+, Ru(NH3)3+ 6 3 5 3 5 for ion exchange at basic pH. Note that some protonic acidity can be introduced in the zeolite during ion exchange, e.g., as exchanged zeolites are washed with neutral water, which may affect the reducibility of the cations (Sect. 2.4.2) and the catalytic properties of the solid. Ion exchange has also been conducted in a few instances by solid-state reactions (see Chap. 2 of this volume). The approach, developed by Karge and Beyer [79–81], consisted of heating, e.g., up to 675 K in vacuum or in a stream of dry nitrogen, homogeneous mixtures of finely dispersed metal salts (chlorides, nitrates) or metal oxides with zeolites in their hydrogen or ammonium forms. This procedure was used to introduce palladium or platinum in Y and ZSM-5 zeolites that were subsequently reduced by hydrogen to give metal particles. The

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solid-state exchange conditions of palladium in NaX, particularly the influence of grinding the zeolite with palladium chloride, were studied by Stolz et al. [82]. 2.4.2 Generalities on Cation Reducibility

Ion-exchanged zeolites are usually reduced by heating under flowing hydrogen or in the presence of a static hydrogen pressure. To minimize the diffusion and agglomeration of metal atoms, the reduction temperatures should be kept as low as possible depending on the cation reducibilities. Reducing agents stronger than hydrogen can be used to operate at lower temperatures (Sect. 2.4.3). The reducibility of cations depends primarily on their redox potential [83]; thus, cations of the first transition row are less easily reduced than cations of platinum-group metals because of their low redox potential. However, cations interact with framework anions that modify their reducibility since there is an extrastabilization energy that will depend on the precise location and coordination of the cations. Thus, in the absence of extra-framework ligands, cations tend to nest in sites that best fulfill their coordination, e.g., in the case of faujasite, SI sites (hexagonal prism) or SI¢ sites (sodalite cage). An attempt was made to rationalize the reducibility of transition metal cations in terms of electrostatic interaction with the zeolite framework. Thus, a good correlation was reported between the initial rate of reduction of Cu2+ ions in Y zeolites co-exchanged with various alkali and alkali-earth cations and the Sanderson electronegativity of these zeolites [84, 85]. The reducibility of transition ions depends on the Si/Al ratio; it increases in the series Y<X
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2. Hydrolysis of cations can maintain them in the main channels where they are much more easily reduced. This is not only because they are prevented from migrating to remote sites, but also because hydrogen reduction yields water instead of protons, which is thermodynamically more favorable and avoids the formation of protons which may shift the reduction equilibrium (as described above). Thus, nickel hydroxide was precipited in situ by treating a Ni2+-exchanged Y zeolite with Na2CO3, so that nickel was completely reduced at 673 K compared to only 50% reduction in the untreated zeolite [85]. Controlled hydrolysis of Ni2+ and Co2+ cations in zeolite pores facilitated reduction of these cations [96, 97]. Impregnation with sodium hydroxide [33–35] or sodium acetate [36] of Co3+-exchanged Y zeolite induced the formation of hydroxide species in the supercages that were reduced at lower temperatures. The reducibility of Ni2+ ions was found to be higher in the presence of water molecules because of the formation of hydrolyzed species [98]. 3. Extra-framework ligands, such as ammonia molecules, can coordinate the cations and maintain them in unshielded positions. Thus, the increased reducibility of Ni2+ ions in the presence of ammonia [99] can be attributed to a better accessibility of Ni2+ ions since upon adsorption of NH3 they migrate from hidden sites to supercages [100]. Co(NH3)62+ ions exchanged in Y zeolites do not give the same final cobalt dispersion as that obtained by exchanging Co(H2O)62+ ions because, upon thermal activation, the former transform into monoammine species which remain in the supercages even at high temperatures [101], whereas the latter migrate easily into hidden sites upon dehydratation [102] where they are difficult to reduce. The adsorption of CO ligands in Ni-mordenite allowed a reduction of Ni2+ ions at low temperatures [89]. Treatment of FeII, CoIII, NiII, and CuII Y zeolites with aqueous solutions of alkali ferrocyanides led to the formation inside the main zeolite pores of metal ferrocyanides that were reduced by H2 to yield monometallic or bimetallic particles [103]. The same technique was employed by Iton et al. [104] to prepare monometallic and bimetallic clusters in modified ZSM-5 zeolites. 4. The presence of small amounts of platinum or palladium, dissociating hydrogen, favors the reducibility of Ni2+ cations [105]. 2.4.3 Mechanisms of Reduction and Reducing Agents

Auto-reduction processes in the absence of hydrogen can occur during thermal treatments in vacuum or inert atmospheres. In the case of first-row transition cations, reduction stops at low-valent states. Thus, Fe3+ [106] or Cu2+ Y [107] zeolites heated under vacuum were reduced to the Fe2+ and Cu+ forms, respectively. Platinum-group cations can be reduced to metals by heating in the presence of extra-framework species. Thus, Pd2+ ions in Y zeolite were reduced to metal particles by heating at 773 K in the presence of water molecules [108]; since oxygen was formed, the reaction probably proceeded according to the overall reaction: Pd2+ + H2O + 2ZO– Æ Pd0 + 2ZOH + 1/2 O2 .

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Auto-reductions of Pt(NH3)2+ 4 complexes occurring under vacuum were extensively studied. Mashenko et al. [109] detected the presence of a platinum hydride (Pt-H)+ that may have been due to the reduction of Pt2+ by ammonia released by complex decomposition [110], so that the overall reaction could be [111]: 3Pt2+ + 2NH3 Æ 3(Pt-H)+ + 3H+ + N2 . Schulz-Ekloff et al. [112–115] have shown that there is a gradual release of ammonia and Pt2+ ions with the following reactions taking place: Pt(NH3)n2+ Æ Pt0 (NH) + 2H+ + (n–1) NH3 , Pt0 (NH) Æ Pt0 + 1/2 N2 + 1/2 H2 . These auto-reduction processes of platinum tetrammine complexes, whether performed in vacuum or in inert atmosphere, lead to particle sizes within the range 1–4 nm encapsulated in the zeolite grains. Different states of iridium dispersions obtained by auto-reduction of [Ir(NH3)5Cl]2+ in Y zeolites were reported by the same authors [116]. Auto-reduction processes are difficult to control and are not well suited to obtain well-defined and homogeneous states of metal dispersion in zeolites. Metal clusters are generally prepared by hydrogen reduction of ion-exchanged zeolites. The stoichiometry is similar to the reduction of cations in the aqueous phase: Æ M0 + n H+, zeol.n– . Mn+,zeol.n– + n/2 H2 ¨ The kinetics of hydrogen reduction were studied in the case of faujasites exchanged with Ag+ [117], Cu2+ [107], and Ni2+ [91, 98, 118] cations; these data were discussed by Jacobs [85]. Reduction generates protonic acidity, and the equilibrium is more and more displaced to the left with increasing protonic acidity and with decreasing redox potential and nuclearity of the metal clusters. Re-oxidation and re-dispersion of metals in zeolites are discussed in Sect. 2.7. The mechanism given above is not applicable to encaged metal oxides or hydroxides formed by hydrolysis of cations under alkaline conditions. The reduction of these species is described by: Mx Oy + y H2 Æ y H2O + x M . The reduction of ammino cations is a more complex process because intermediate species are formed. The following scheme was proposed for the reduction of platinum tetrammine in Y zeolite [74]: Pt(NH3)42+ + 2H2 Æ Pt(NH3)2 H2 + 2NH3 + 2H+ , Pt(NH3)2 H2 Æ Pt0 + 2NH3 + H2 . This direct reduction gave very poor dispersion because the neutral hydride is very mobile and favors transport processes leading to metal agglomeration in large particles. A calcination step, preferably in pure oxygen, must be conducted to decompose the complex prior to H2-reduction to obtain well-dispersed platinum clusters [74]. This requirement is valid for all other ammino cations of platinum group metals (see Sect. 3 for case studies). The only exception is ru-

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thenium because, upon calcination of zeolites exchanged with Ru(NH3)63+, large particles of ruthenium oxide are formed. Fortunately, thermal treatment of Ru(NH3)3+ 6 -exchanged Y zeolites under inert atmosphere [119, 120] or under vacuum [121] followed by H2-reduction resulted in small ruthenium clusters (see Sect. 3.1.4). The chemistry involved during the calcination treatments prior to hydrogen reduction was studied by Chmelka et al. [122, 123] in the case of Pt/Y zeolite. Based on Raman spectroscopy and product evolution, reactions yielding oxide species were proposed for samples calcined at 673 K: Pt(NH3)42+ + 3/2 O2 Æ PtO + 2NH4+ + N2 + 2H2O , 2NH4+ + 3/2 O2 Æ 2H+ + N2 + 3H2O . At higher calcination temperatures, the oxidic species can react with acidic sites to yield cations: PtO + 2H+ Æ Pt2+ + H2O . Similarly, the calcination of Y zeolite exchanged with Rh[(NH3)5Cl]2+ ions yields (RhO)+ species in the supercages which are in equilibrium with Rh3+ [124]: Æ (RhO)+ + 2H+ . Rh3+ + H2O ¨ The chemistry involved in the calcination and reduction of Pd(NH3)42+ in X and Y zeolites, and of Pt(NH3)42+ in ZSM-5, was re-investigated by Sauvage et al. [125] and by van den Broek et al. [126], respectively; the latter authors reported the transient formation of [Pt(NH3)(H2O)x]2+ species. The importance of treatment conditions (calcination pretreatment and hydrogen reduction) for the control of the final metal dispersion will be illustrated with detailed results of case studies examined in Sect. 3. Because transition cations of the first row are difficult to reduce by molecular hydrogen at moderate temperatures, stronger reducing agents were employed. More complete reduction and smaller nickel clusters were obtained by reduction of NiCaY zeolites with atomic hydrogen produced by microwave discharge, than with molecular hydrogen [93, 127]. Sodium vapors were used to reduce FeY zeolite, but metal sintering could not be avoided [128]. Higher reduction levels of Ni2+ ions in Y zeolites were obtained with ammonia than with hydrogen [99] probably because, in addition to their role as reducing agent, ammonia molecules favor the migration of cations from hidden to more accessible sites. 2.5 Incorporation of Metal Precursors During Synthesis

When metal precursors cannot be prepared by ion exchange (e.g., because of the lack of ion exchange capacity) or by adsorption or impregation (e.g., because the metal precursor is too large to enter zeolites with constricted pores), there is still the possibility of incorporating the precursor during the zeolite synthesis. Thus, in a very early paper on metal zeolites, Weisz et al. [77] reported on the preparation of platinum metal in CaA zeolite. It was realized that this small-pore zeolite cannot be loaded by impregnation with K2PtCl4 or by ion exchange with

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Pt(NH3)42+ ions since the molecular dimensions of these metal complexes exceed the pore apertures. Platinum was incorporated during the synthesis of the zeolite by mixing solutions of K2PtCl4 with solutions of sodium metasilicate and sodium aluminate in the required amounts. More recently, Davis et al. [129, 130] prepared intrazeolitic 2–5 nm ruthenium particles in NaA and CaA zeolites by addition of [Ru(NH3)5Cl]Cl2 to the hydrothermal synthesis mix of zeolite A. This technique of metal loading has never been widely used to prepare metal clusters in zeolites but is frequently employed to substitute lattice aluminum in zeolites or aluminophosphates by metal ions. 2.6 Preparation of Bimetallic Clusters

Bimetallic clusters can be prepared by different routes, viz: 1. Co-impregnation of zeolites with two metal salts followed by hydrogen reduction. This technique does not generally result in well-controlled preparations. Thus, large particles of Pt-Co were obtained by co-impregnation [131]. 2. Co-exchange of zeolites with cations of two different metals followed by hydrogen reduction. This technique is widely used and works well even for metals with different reducibilities because the most easily reduced metal dissociates hydrogen, thus enhancing the reducibility of the other metal cations. Thus, reducibilities of nickel [105, 132], cobalt [133] and copper [134] were found to be higher in the presence of palladium or platinum; however, the proximity of the two metal cations, e.g., in the supercage of faujasite, is necessary to obtain homogeneous bimetallic clusters [133, 134]. Ion exchange of two cations can be performed simultaneously or successively; the total amount of metal cations in solution being fixed onto the solid provided the exchange capacity is high enough. If a limited number of exchangeable sites are available, there is a competitive exchange, and the final respective loadings of the two metals cannot be easily predicted. Another strategy is to ion-exchange one of the metals first, reduce the cations, and proceed to the exchange of the other metal cations; however, atom ordering in the bimetallic particles might not be the same. The co-exchange technique can also be applied to prepare in situ bimetallic molecular clusters. Thus, bimetallic [Rh6–xIrx(CO)16] clusters were obtained by heating a (Rh3+,Ir4+)/NaY zeolite in CO/H2 atmosphere at 473 K [135]. 3. Adsorption and decomposition in a zeolite containing clusters of a first metal of neutral metal complexes, such as metal carbonyls. The metal atoms freed by the decomposition of the complex at moderate temperatures are thus deposited on the surface of the clusters. This technique, used by Tri et al. [136] to prepare Pt-Mo clusters of homogeneous composition from Pt/Y zeolite, was also used to prepare Pt-Re [137] and Rh-Mn [59] clusters.

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2.7 Factors Favoring Redispersion

The redispersion of metal particles can be achieved in three steps: 1. Oxidative complexation of surface metal atoms which are extracted from the particle. 2. Migration of the complexes thus formed in the zeolite pores resulting in a redistribution of the oxidized species in the framework. 3. Reduction of the oxidized species into metal clusters. The first two steps require corrosive oxidizing agents forming mobile metal complexes. Chlorine fulfills these two criteria and can be used effectively to redisperse large particles on the external zeolite surface. Foger et al. [138, 139] have studied the redispersion of platinum by chlorine in various zeolites. However, temperatures in excess of 350°C are required in these processes, and part of the metal may be lost. Oxidative redispersion of metal clusters has been achieved with oxygen treatments. Thus, Herman et al. [140] concluded from ESR and gas uptake measurements that metallic copper in CuY zeolite can be re-oxidized into Cu2+ cations by treatment under oxygen at 673 K provided they are in the form of encaged clusters; larger copper particles were transformed into copper oxide. Beyer et al. [141] showed that silver particles on the external surface of Y zeolite were redispersed into Ag+ cations. For encaged silver clusters, redispersion occurred at temperatures as low as 363 K. Bergeret et al. [21] have shown that Pd0 atoms in sodalite cages or Pd-clusters in supercages can be re-oxidized into cations at 450 and 470 K, respectively. The chemistry of these reactions involves the participation of the zeolite protons (see Sect. 3.1.2). The oxidative redispersion of 2–3 nm palladium particles was achieved with NO acting as oxidant and complexing agent at room temperature [142] (see Sect. 3.1.2), and rhodium particles in Y zeolite were disrupted and redispersed into RhI(CO)2 species by CO adsorption at room temperature [143] (see Sect. 3.1.3).

3 Case Studies 3.1 Metal Clusters in Faujasite-Type Zeolites

Special emphasis will be given to the preparation of platinum and palladium clusters in faujasites since the techniques of metal cluster preparations were first developed for these zeolites and later extended to other zeolites and other platinum-group metals. Case studies concerning the preparation of clusters of iron, nickel and cobalt in faujasites will not be developed since most of the references on these preparations have already been given in Sect. 2.

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3.1.1 Platinum Faujasites

Soon after the discovery, in the late 1950s, of the catalytic properties of zeolites for hydrocarbon conversion, the preparation of bifunctional catalysts for hydrocracking and hydroisomerization was attempted by incorporating metals in porous zeolite frameworks. Patents describing various modes of reduction of ion-exchanged zeolites were published, in particular by Breck and Milton [144]. Rabo and Pickert [1] were the first to report the preparation of Y zeolites loaded with metals in 1960. Sodium Y zeolites exchanged with various bivalent cations were loaded with 0.5 wt.% platinum by ion exchange, but no details were given on the treatments and state of metal. The first basic work on the preparation and characterization of metal clusters in zeolites was achieved by Rabo et al. [145] in 1964. A CaY zeolite (Si/Al = 2.5), prepared by ion exchange of the sodium form, was ion-exchanged with Pt(NH3)2+ 4 cations to obtain a 0.5 wt.% loading, then calcined at 500°C and reduced by hydrogen at 300°C. The metal zeolite was characterized by hydrogen chemisorption, infrared spectroscopy of adsorbed CO and catalytic activity measurements of 2,2-dimethylbutane isomerization in the presence of controlled amounts of thiophene. It was reported that a state close to atomic dispersion had been obtained because the isolated metal atoms appearing upon hydrogen reduction were far apart (statistically: 40 Å), and their mobility at 300°C was sufficiently low. This “atomically dispersed” platinum chemisorbed more hydrogen (H/Pt = 2) and had a higher resistance to sulfur poisoning than larger platinum particles present in zeolites prepared by impregnation or on other supports. These properties of highly divided platinum were the starting point of concepts which three decades later are still vivid and the subject of lively debates. The dispersion of platinum in 0.5 wt.% Pt/CaY was re-investigated by Lewis in 1968 [146] using hydrogen adsorption and X-ray absorption edge spectroscopy; it was concluded that most of the metal was in the form of particles small enough to fit in zeolite supercages. Dalla Betta and Boudart [74] conducted a very complete study on the preparation and characterization by H2-chemisorption and infrared spectroscopy of Pt/Y zeolites ion-exchanged with various bivalent cations. Table 1 gives the H/Pt ratios measured on a 5 wt.% Pt/CaY sample prepared by ion exchange with Pt(NH3)42+ cations and reduced under different conditions. Direct reduction under hydrogen gave a poor Pt dispersion (Table 1) because Pt(NH3)42+ cations decomposed above 90°C forming an intermediate neutral and mobile hydride which favored platinum migration and agglomeration: + Pt(NH3)2+ 4 + 2H2 Æ Pt(NH3)2H2 + 2NH3 + 2H .

In contrast, when the sample was calcined in O2 , Pt(NH3)42+ decomposition released ammonia and Pt2+ cations. These species were subsequently reduced by hydrogen leading to a high dispersion. Indeed, hydrogen adsorption and titration indicated a 100% metal dispersion (Table 1), and further insight on the size of the particles was obtained by infrared measurement of isotopic exchange between the protons of framework hydroxyl groups and deuterium. Assuming

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Table 1. Platinum dispersion in Pt/CaY (from [74])

Treatment(s)

H/Pt (total)

H2 at 350 °C O2 at 350 °C, H2 at 400 °C As above but 5-g batch

0.08 1.19 0.64

that only protons from the hydroxyl groups close to platinum clusters exchange rapidly with deuterium, it was concluded from statistical considerations that the upper nuclearity of the clusters would be ca. six Pt-atoms. Oxygen treatment in deep bed geometry (5 g of sample instead of 0.4 g) favored the mobility of intermediate species and thus led to lower dispersion (Table 1). It was also concluded from catalytic measurements of neopentane isomerization and hydrogenolysis, that the Pt-clusters were electron-deficient as a result of partial electron transfer from the metal to the zeolite, especially in the presence of high electric field multivalent cations. It was proposed that this electron deficiency accounted for the sulfur resistance reported by Rabo et al. [145] because of weaker Pt-S bondings. The importance of calcination temperatures on platinum dispersion in Pt(NH3)42+-exchanged Y zeolites was also demonstrated independently by Kubo et al. [73] who found that 300°C was an optimum temperature for the calcination pretreatment to obtain the highest platinum dispersion. Gallezot et al. [76] determined the positions of platinum cations in Y zeolites by crystal structure X-ray analysis as a function of treatment conditions which helped to understand the effect of calcination temperatures observed in previous works [73,74,145–149].A Pt(NH3)42+-exchanged NaHY zeolite containing 14 wt.% platinum was calcined under flowing oxygen from room temperature to 300°C with a temperature ramp of 0.5°C min–1 to decompose the ammino cations. Failure to use shallow zeolite beds, or a high flux of oxygen to maintain a fluidized bed during this critical step, resulted in heterogeneous metal dispersion. No Pt2+ cations were detected on the SI, SI¢ , and SII¢ cation sites of the Y-zeolite framework, so they were assumed to occupy positions in the supercages. Reduction under flowing hydrogen or under static conditions at 300°C produced metal clusters whose sizes were measured by small-angle X-ray scattering (SAXS) [76] and TEM [15]. SAXS measurements indicated a 0.6–1.3 nm size distribution with a maximum at 1.1 nm, and a mean geometric diameter of 0.7 nm. The TEM study performed on ultramicrotome thin sections showed that the clusters were smaller than 1.2 nm and very homogeneously distributed inside zeolite crystals. Therefore, it was assumed that the Pt-clusters were accommodated in the supercages, and it was shown later [150] that they were most probably in the form of 40-atom, truncated tetrahedra that have the same symmetry and size as the faujasite supercage. This was supported by the similarity between the two radial distribution curves given in Fig. 4, the upper one corresponding to the experimental distribution obtained from wide-angle X-ray scattering measurements and the other to a distribution calculated for the truncated tetrahedra. The Pt-clusters nested in supercages were very stable towards sintering since the H/Pt ratios remained close to 1 up to 800°C [76], and TEM views taken through thin sections

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Fig. 4. Radial distribution of atoms corresponding to platinum clusters in Pt/NaY (from [150]). a Experimental distribution obtained by Fourier transform of X-ray scattering data; b distribution calculated for a 40-atom truncated tetrahedron with the same symmetry and size as the supercage

of a zeolite evacuated at 800°C showed that the clusters were still small enough to be accommodated in the supercages [15]. Sintering of the encaged clusters occurred between 850 and 900°C as the zeolite structure collapsed. Particles of 2.5 nm were measured by SAXS [76] and TEM [15], and the latter technique showed a very homogeneous distribution of the particles encapsulated in an amorphous zeolite matrix. The H/Pt ratio decreased from 1 to 0.15 because most of the particles in the amorphous zeolite were not accessible to hydrogen. After calcination in O2 at 600°C of the Pt(NH3)42+-exchanged NaHY zeolite, crystal structure determination showed that all the Pt2+ cations were located on SI¢ sites in the sodalite cages bonded to three O(3) framework oxygens with 2.20 Å Pt-O(3) distances [76]. After reduction at 300°C, most of the Pt-atoms were still found on SI¢ sites but with Pt-O(3) distances at 2.54 Å, indicating that the platinum atoms were still interacting with the anionic framework as if they were still slightly positively charged. Indeed, it was later shown that the Pt(4f7/2) XPS binding energies were positively shifted by 1.3 eV [151] and that the Pt LIII X-ray absorption edge was shifted by 1.2 eV [152]. Platinum was in a state of atomic dispersion in sodalite cages, but the zeolite chemisorbed very low amounts of hydrogen (H/Pt(total) = 0.25). It was proposed that platinum atoms in sodalite

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Fig. 5. State of platinum in Pt/NaY zeolite (from [76])

cages could not adsorb hydrogen, either because they do not have metallic properties or because hydrogen molecules do not enter the sodalite cages at room temperature. Heating the zeolite above 400°C produced a migration of Pt-atoms out of the cages and the formation of 2–3 nm metal particles growing inside the zeolite as monitored by SAXS and TEM. Accordingly, the H/Pt(total) ratio increased from 0.25 to 0.65 as particles chemisorbing hydrogen were formed at the expense of isolated metal atoms. The different states of platinum dispersion as a function of the treatment conditions employed are summarized in Fig. 5. These results were confirmed by subsequent studies [151–161] at IRC-CNRS (Lyon). It was shown that the optimum calcination temperatures in O2 necessary to maintain platinum in supercages were in the range 300–400°C and that the partial exchange of multivalent cations such as Ce3+ allowed smaller Pt-clusters to be obtained [157]. Foger and Anderson [162] prepared Pt/Y zeolite catalysts exchanged with Na+, Ca2+ and La3+ ions. CaY was prepared by exhaustive exchange in Ca(NO3)2 solutions at 353 K followed by exchange of Pt(NH3)42+ to obtain 3 wt.% platinum loading. The zeolite was calcined in flowing O2 up to 573 K and then reduced in flowing H2 at 623 K. Samples of Pt/NaY and Pt/LaY were prepared from extensive ion exchange of the reduced Pt/CaY zeolite. This strategy was adopted to ensure that metallic platinum particles were of the same size and located in the same position in the zeolite irrespective of the nature of the associated cations. All these samples gave H/Pt ratios of 1.05 in agreement with the mean size of 1 nm measured by TEM. These platinum clusters were found to be electron-deficient by XPS, particularly in samples exchanged with La3+ ions. The state of platinum dispersion in faujasite was re-investigated by Sachtler et al. [90, 163–167] using temperature-programmed desorption (TPD) and tem-

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perature-controlled reduction (TPR) as the main investigation tools and EXAFS measurements. The conclusions reached in these investigations, especially in [90], were in good agreement with the data given in Fig. 5. In addition, these investigations proved that multivalent cations have two effects: they prevent to some extent the migration of Pt2+ cations towards sodalite cages and they act as anchoring sites for Pt0 atoms, these two effects resulting in a better dispersion of platinum. The preparation by auto-reduction of platinum particles larger than zeolite supercages, the characterization of their mechanism of formation, the size and the determination of the morphology and orientation of Pt-particles with respect to the zeolite lattice, were extensively studied by the German group at Bremen [112, 113]. The understanding of the intrazeolitic chemistry taking place during the oxygen pretreatment of Pt/Y zeolites and subsequent H2-reduction was greatly improved by Chmelka et al. [122, 123] who used Raman spectroscopy in conjunction with 129Xe NMR, TEM and hydrogen chemisorption studies. Figure 6 gives the Raman spectra of Pt(NH3)42+-NaY samples calcined at 673, 773 and 873 K, respectively. The strong peaks at 610 and 626 cm–1 observed for the sample treated at 673 K (curve a) were attributed to PtO species in supercages. Based on these data and the results of product evolution measurements, the following reactions were proposed to be involved in the calcination at 673 K: Pt(NH3)42+ + 3/2 O2 Æ PtO + 2NH4+ + N2 + 2H2O , 2NH4+ + 3/2 O2 Æ 2H+ + N2 + 3H2O . At 773 K the decrease in the Raman peaks indicated that there were less PtO species present in supercages, and at 873 K they totally disappeared (Fig. 6, curves b and c, respectively). It was suggested that the PtO species reacted with protons to yield Pt2+ that, at this temperature, migrated towards the sodalite cages: PtO + 2H+ Æ Pt2+ + H2O . Figure 7 gives a summary of the chemistry of platinum cluster preparation under different calcination conditions [123]. These results are in agreement with and complete previous results published by Gallezot et al. [76]; thus, the formation of PtO species in supercages accounts for the fact that platinum could not be located by X-ray structural analysis since platinum did not occupy cation sites. Numerous studies have been devoted to the characterization of platinum clusters in Y zeolites using the chemical shift of 129Xe NMR lines [168–172] or by Xe adsorption isotherms [173]. Details of these studies are beyond the scope of the present review, but it is interesting to note that, following calcination at 573–673 K and reduction at 573–673 K, the nuclearity of metal clusters was found to be between 13 and 55 (in most cases between 20 and 40), and this is in reasonable agreement with the characterization of particle size or coordination numbers obtained by physical methods such as TEM, SAXS, RED, and EXAFS [15, 76, 90, 150].

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Fig. 6. Raman spectra of Pt(NH3)42+-NaY samples calcined at 673, 773 and 873 K (from [123])

An interesting study of platinum dispersion in NaY zeolite as a function of platinum concentration (2, 5, and 10 wt.% Pt) was reported by Ryoo et al. [174]. The platinum clusters were prepared by conventional ion exchange with Pt(NH3)2+ 4 , calcination at 583 K and reduction at 673 K. The Pt-Pt coordination numbers measured by EXAFS were constant in the three samples (5.0, 5.9 and 5.1, respectively), and the cluster nuclearity measured by Xe adsorption isotherms was also constant (ca. 50 atoms). Similar results were also obtained with

Fig. 7. Summary of the chemistry of platinum cluster preparation in Pt/NaY (from [123])

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a 10 wt.% zeolite loaded with platinum in two steps. Although EXAFS data were not in agreement with Xe adsorption measurements, both techniques pointed to the important conclusion that platinum clusters of similar size, i.e., filling the supercages, were formed irrespective of the amounts of metal present. Ihee et al. [16] subjected a Pt(NH3)42+-exchanged EMT zeolite (hexagonal faujasite with two kinds of supercages) to the same treatment conditions (573 K calcination, 573 K reduction) as those applied to Pt/Y zeolites. Characterization by TEM and EXAFS indicated that the size of platinum clusters was comparable to that in Y zeolites. The cluster nuclearity measured by Xe adsorption was 26 ± 4 and 43 ± 7; these clusters were assumed to be in the small supercages and large supercages, respectively. De Mallmann and Barthomeuf [175] demonstrated by IR spectroscopy that carbonyl clusters similar to those of the general formula [Pt3(CO)3(µ2CO3]n2– were synthesized in situ by adsorbing CO at 373 K on a Pt(NH3)42+-exchanged Y zeolite calcined at 573 K. These complexes were formed mainly in the basic zeolites Pt/NaY and Pt/CsNaY. Clusters with n = 3 can indeed be accommodated in zeolite supercages, but higher nuclearity clusters could possibly grow in two adjacent supercages. In situ synthesis of comparable clusters was confirmed by Li et al. [176] on the basis of IR, UV and EXAFS studies and by Chang et al. [177]. 3.1.2 Palladium Faujasites

The preparation of palladium clusters in Y zeolites has been studied extensively at IRC-CNRS (Lyon) [21, 75, 142, 178–180].Y-type zeolites were ion-exchanged with Pd(NH3)42+ cations from ammonia solutions of PdCl2 or aqueous solutions of Pd(NH3)4Cl2 . Calcination in oxygen prior to hydrogen reduction was needed to avoid auto-reduction of Pd(NH3)42+ cations leading to large particles, but the treatment conditions (temperature ramp, O2 flux, zeolite bed thickness) were less stringent than in the case of Pt(NH3)42+-Y zeolite. Structural modifications occurring during calcination and reduction at different temperatures of Pd(NH3)2+ 4 exchanged zeolites were studied by Bergeret et al. [21] by continuous recording of the X-ray pattern, and crystal structure determinations were achieved at specific temperatures to locate palladium ions or reduced atoms in the zeolite framework [21, 75, 178]. Cluster sizes were measured by TEM [21] and SAXS [178]. Calcination at 400 K and reduction at 290 K of Pt(NH3)42+-Y zeolite produced an agglomeration of palladium atoms in the form of grape-like particles filling adjacent supercages [21]. This occurred because the calcination temperature was not sufficient to remove completely ammonia molecules that favored the mobility of reduced species. At higher calcination temperatures, Pd(NH3)2+ 4 cations decomposed, and the Pd2+ ions migrated to sodalite cages at lower temperature than Pt2+ ions. Thus, crystal structure determination after calcination at 490 K showed that 70% of the Pd2+ cations were in sodalite cages and, at 873 K, 10.6 out of 12.5 Pd2+ ions were on SI¢ sites bonded to three framework oxygens, with Pd-OIII distances of 2.01 Å. Hydrogen reduction at room temperature produced isolated atoms which remained in weak interaction with framework oxygens at 2.72 Å. Hydrogen uptake measurements indicated that 80% of the palla-

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dium had been reduced. Interestingly, benzene adsorption at room temperature reduced the Pd2+ cations on SI¢ sites to a lower valence state, as evidenced by the increase of Pd-O3 distances to 2.70 Å [178]. Since benzene molecules do not enter the sodalite cages, the reduction should be due to an electron transfer from benzene to the Pd2+ cations via zeolite framework atoms.A similar charge-transfer process occurring upon adsorption of unsaturated hydrocarbons on Pd and Pt/Y zeolites was proposed by Romannikov et al. [181]. Palladium atoms migrate much more easily out of the sodalite cages than platinum. Thus, at reduction temperatures higher than 550 K, Pd atoms diffused out of the sodalite cages to form palladium particles of 2–3 nm in the bulk of zeolite crystals, as shown in the TEM views through ultramicrotome sections [21]. Due to the ease of migration of Pd2+ ions towards sodalite cages upon calcination and of Pd0 atoms out of the sodalite cages upon reduction, it was difficult to obtain, as in the case of platinum, all the clusters distributed in supercages. The best results were obtained by calcining and reducing at moderate temperatures; thus, after calcination and reduction at 420 K, palladium particles with a mean geometric diameter of ca. 0.6 nm were measured by SAXS indicating the formation of very small clusters in supercages. However, a large fraction of the palladium atoms were still located in sodalite cages [178]. Isolated Pd-atoms in sodalite cages obtained by reducing Pd2+ ions on SI¢ sites were re-oxidized into Pd2+ cations located on SI¢ sites merely by heating the zeolite in oxygen above 450 K [21]. In the same way, Pd-clusters filling adjacent supercages were re-oxidized into Pd2+ cations by calcination in oxygen above 470 K [21]. The mechanism of oxidative redispersion of metal clusters into cations involved the participation of zeolite protons: Pdm(supercages) + 2m H+ + m/2 O2 Æ m Pd2+ (SI¢ site) + m H2O . On the other hand, larger particles were not redispersed by O2-treatment. Thus, 2–3 nm Pd-particles formed by reduction at high temperatures and treated under oxygen above 500 K were oxidized into PdO particles with similar size and location with respect to the parent metal particles [21]. In contrast, a very astonishing oxidative redispersion of 2 nm Pd-particles in Pd/Y zeolite was obtained with NO treatment [142]. The starting material was a Pd/Y zeolite activated in O2 and reduced in H2 at 500°C containing occluded 2 nm particles. Upon adsorption of NO at room temperature, the dark-brown zeolite powder turned to beige pink, and the Bragg peaks corresponding to palladium metal vanished. Simultaneously, the intensities of nOH bands at 3540 and 3640 cm–1 and of the nNO band at 1670 cm–1 decreased with time whilst bands at 1640 cm–1, characteristic of water, and a doublet at 2235–2210 cm–1,probing the formation of N2O,appeared and increased with time. The formation of N2O was also monitored by mass spectrometry. The re-oxidation of palladium into cations was followed by ESR spectroscopy, by the formation of Pd2+ nitrosyl complexes in sodalite cages and supercages (IR bands at 1780 and 1865 cm–1, respectively), and crystal structure determination, after evacuation at 200°C, indicated that the sites were re-occupied by palladium cations. The overall oxidative re-dispersion process could be written: Pdn(encapsulated) + 2n H+ + 2n NO Æ n Pd2+(cation sites) + n N2O + n H2O.

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The Pd2+ species complexed with excess NO molecules to form mobile nitrosyl complexes were redistributed on SI¢ cation sites in zeolite cages. The different states of palladium dispersion in Y zeolites detected by Gallezot et al. [21, 75, 142, 178] as a function of calcination, reduction and the re-oxidation tretaments are summarized in Fig. 8. The elementary steps in the formation of palladium clusters in Y zeolite were re-investigated by Homeyer and Sachtler [182, 183] using TPR, TPD, and temperature-programmed oxidation (TPO) coupled with mass spectrometry. A NaY zeolite was ion-exchanged with [Pd(NH3)4](NO3)2 , then calcined under oxygen. The oxidation of Pd(NH3)42+ ligands was found to be a stepwise process: two ammonia molecules were oxidized by O2 producing N2 and H2O and leaving Pd(NH3)22+ ions in supercages, then, upon further deammination, Pd(NH3)2+ and Pd2+ were formed in sodalite cages [182]. These results indicated that migration of Pd2+ ions towards sodalite cages occurred at lower temperatures than in Pt(NH3)2+ 4 -exchanged zeolites. In agreement with previous work [21, 75], it was found that the final states of metal dispersion (isolated metal atoms which did not chemisorb hydrogen, small clusters in supercages and larger particles) were predetermined by the location and coordination of the Pd cations after calcination. The dispersion of palladium was studied by EXAFS in X zeolites exchanged with [Pd(NH3)4]Cl2 , calcined at 625 K and reduced by H2 at 425 K [184]. The structure described is essentially similar to that found earlier by crystal structure

Fig. 8. State of palladium in Pd/NaY zeolite (from [21])

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analysis [21, 75] for reduced Pd-atoms in sodalite cages. Möller and Bein [185, 186] studied by EXAFS the dispersion of palladium in zeolites co-exchanged with cobalt and palladium. It was shown that, for samples reduced at high temperatures, the dispersion of palladium was higher than in the abscence of cobalt ions. This was not attributed to an anchoring of Pd0 atoms by Co3+ ions because Coedge spectra were not modified; it was rather proposed that Co3+ ions were acting merely by blocking the access of palladium ions to sodalite cages maintaining them in the supercages where they were reduced and formed Pd-clusters. The influence of the basicity of the zeolite and nature of the alkali-metal cations on the decomposition of Pd(NH3)2+ 4 cations and on the reducibility of Pd2+ cations was recently investigated by Sauvage et al. [187] using TPO, TPR and UV-VIS-near IR spectroscopy. These studies showed that the decomposition of the complexes and Pd2+ cation reduction occurred at lower temperatures when the zeolite basicity increased. Zhang et al. [188] carried out an EXAFS study of a Pd/Y zeolite prepared by calcination at 500°C and reduction at 350°C of a Pd(NH3)2+ 4 -exchanged Y zeolite. The presence of clusters with an average coordination number of 4, corresponding to nuclearity of six atoms, was detected. Adsorption of CO resulted in an increase in the coordination number to 6 that was attributed to the formation of 13-atom clusters surrounded by CO ligands. Extensive studies on the formation of Pd13(CO)x clusters in Y zeolite were carried out by Sachtler et al. [13, 189, 190] using IR spectroscopy; the presence of sharp bands of linear and bridging CO were attributed to the formation of Pd13(CO)x clusters.As these clusters were partially decarbonylated, they interacted with protons associated with the zeolite framework. 3.1.3 Rhodium Faujasites

The preparation and chemistry of rhodium clusters in Y zeolite has been extensively studied at IRC-CNRS(Lyon) [143, 191–199] and by others [20, 200–209]. Rhodium-exchange procedures in different zeolites (faujasites, mordenite, erionite, ZK-5, ZSM-34, ZSM-11) were extensively studied by Shannon et al. [197]. Rhodium clusters were prepared from [Rh(NH3)5Cl]2+-exchanged faujasites with a calcination step before reduction, or from aqueous solutions of Rh(H2O)63+ obtained by dissolving RhCl3 or Rh(NO3)2 in water. In the latter case, highly dispersed rhodium was obtained by direct hydrogen reduction at 200°C omitting the preliminary calcination required to decompose [Rh(NH3)5Cl]2+ complexes. Gelin et al. [194] studied rhodium loading by sublimation of Rh6(CO)16 at 353 K onto HY zeolite followed by thermal decarbonylation in H2 or under vacuum at 373 K. Partially stripped rhodium carbonyl species entered the zeolite pores and were re-carbonylated in situ by CO at 373 K to give encaged polynuclear rhodium carbonyls. This loading process was effective only for low metal loadings (0.5 wt.%), otherwise rhodium was mainly loaded on the external surface. Rhodium clusters in Y zeolite were first prepared by Primet [192] starting from a [Rh(NH3)5Cl]2+-exchanged NaY zeolite which was calcined under flow-

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ing oxygen and reduced under hydrogen at 623 K. It was shown by van Brabant et al. [206] that the calcination process produced oxide clusters, and Bergeret et al. [143, 198], using radial electron distribution (RED), characterized oxidic species on a [Rh(NH3)5Cl]2+-exchanged Y zeolite calcined under flowing O2 from 300–620 K at 0.5 K min–1. Tomczak et al. [124] found by TPR and Fourier transform infrared (FTIR) measurements that during O2 calcination, auto-reduction processes occurred yielding rhodium oxide species and rhodyl ions (RhO)+. The latter were in equilibrium with Rh3+ cations: Æ (RhO)+ + 2H+ . Rh3+ + H2O ¨ Because rhodium oxide species, probably located in supercages, were obtained following calcination treatments, subsequent H2-reduction yielded homogeneously distributed, very small rhodium clusters in supercages. This was confirmed by TEM measurements [20, 192] as well as by radial electron distribution (RED) [143] showing no Rh-Rh peaks at distances larger than 7 Å in samples calcined at 620 K and reduced at 470 K (Fig. 9, curve a). Upon adsorption of CO on Rh-clusters at room temperature, IR spectra showed the progressive formation of two doublets near 2100 and 2040 cm–1 which were attributed to gem-dicarbonyl RhI(CO)2 [192]. This did not occur for larger rhodium particles supported on alumina. It was assumed that CO dissociated only on small clusters yielding RhI species that adsorb two CO molecules to form RhI(CO)2. Later, it was shown by RED [143] that CO adsorption at room temperature on Y zeolite containing 1 nm clusters produced an oxidative disruption of the Rh-clusters, as suggested by the disappearance of the Rh-Rh peaks on the radial distribution function (Fig. 9, curve b) and by IR spectroscopy showing the formation of gem-dicarbonyl. Upon heating at 300 K the zeolite containing RhI(CO)2 in the presence of CO and H2O, the monomeric species rearranged into Rh6(CO)16 clusters evidenced by Rh-Rh peaks at 2.77 Å (Fig. 9, curve c). Using the same calcination (up to 620 K in O2) and reduction (573 K in H2) treatments, Martin et al. [20] found that the final metal dispersion was almost similar, regardless of whether [Rh(NH3)6](NO3)3 or [Rh(NH3)5Cl]Cl2 was used as the precursor salt for ion exchange. NaY zeolite-supported rhodium carbonyls with the predominant species being Rh6(CO)16 were prepared by carbonylation at 125 °C of adsorbed [Rh(CO)2(acac)] species [62]. By decarbonylation of the carbonyl clusters at 200 °C in He, the resultant metal cluster had Rh-Rh coordination numbers between 3.5 and 3.9 indicating that the rhodium cluster frame was nearly intact in spite of total decarbonylation. In contrast, some aggregation took place when the decarbonylation was carried out in H2 at temperatures higher than 200 °C. 3.1.4 Ruthenium Faujasites

Lunsford et al. [119, 120] and Verdonck et al. [121] were the first to define the best treatment conditions to obtain small ruthenium clusters in the supercages of Y zeolites. Ion exchange was carried out in aqueous solutions of [Ru(NH3)6]Cl3 .

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Fig. 9. Radial electron distribution of Rh/NaY zeolite (from [143]). a After calcining in O2 at 623 K and reducing in H2 at 473 K; b after contacting with CO at 300 K; and c after contacting with CO/H2O at 300 K (treatments carried out successively)

In contrast to other ammino cations of platinum group metals, the decomposition of Ru(NH3)63+ cations must not be carried out under oxygen, because bulk oxide forms giving large Ru-particles on reduction. The pretreatment was conducted by heating the exchanged zeolite under vacuum from 300–623 K [121] or under a flow of inert gas from 300–673 K [119, 120]. Subsequent reduction by hydrogen at the final calcination temperature gave ruthenium clusters accommodated in supercages. The pretreatment was necessary to avoid auto-reduction processes; thus, heating under hydrogen up to 533 K resulted in 2–4 nm ruthenium particles occluded in the zeolite bulk. Wellenbüscher et al. [22] prepared Ru/NaY catalysts for ammonia synthesis by heating a Ru(NH3)63+-exchanged zeolite in the synthesis gas mixture (N2/H2 = 1/3) from room temperature to 723 K at 1 K min–1. The metal atoms were arranged in the form of grapelike particles filling adjacent supercages, but local damage to the zeolite matrix due to particle growth was not excluded. Cho et al. [210, 211] conducted detailed studies on the preparation and characterization of Ru-clusters in Y zeolite using TEM, Xe adsorption, 129Xe NMR and EXAFS. Ruthenium was loaded by ion exchange of a NaY zeolite in an ammoniacal solution of RuCl3. The exchanged zeolite was heated under vacuum to 673 K with a temperature ramp of 3 K min–1. This treatment led to an autoreduction of the ammine complex yielding Ru-clusters containing ca. 20 atoms. Treatment of this sample at higher temperatures in vacuum or in H2 resulted in a gradual growing of the clusters up to ca. 50 atoms/cluster at 823 K. The cluster nuclearities were estimated from the Xe-adsorption isotherm; however, there were discrepancies with EXAFS results which pointed to lower nuclearities.

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The dispersion of ruthenium in Y zeolite ion-exchanged with Ru(NH3)63+ as a function of treatment conditions was re-investigated by McCarthy et al. [212] who re-confirmed the earlier results of Lunsford et al. [119, 120] and Verdonck et al. [121]. Increasing the H+/Ru ratio from 3 to 10 for Ru/HY resulted in a 75% decrease in hydrogen chemisorptive properties. This was attributed to the larger electron deficiency of the clusters interacting with acidic sites. Another strategy to prepare ruthenium clusters is to adsorb and then decompose Ru3(CO)12 . This was first described by Gallezot et al. [37], who were able to locate the trinuclear carbonyl by X-ray structural analysis. The technique was used in other investigations [45, 46, 199, 213].A comparison between Ru-clusters prepared by ion exchange/reduction and by in situ Ru3(CO)12 decomposition indicated that the latter were smaller (= 1 nm) than the former (1–1.5 nm) [204]. A slightly different procedure consisted of decomposing Ru3(CO)12 into Ru(CO)5 which can diffuse more easily into porous frameworks [214, 215]. Polynuclear Ru-carbonyl species were synthesized by treating with CO ruthenium species partially or totally stripped from their ligands, e.g., Ru3(CO)12 was synthesized in situ from Ru(CO)5 [214] or from bare Ru-clusters [216]. 3.1.5 Iridium and Osmium Faujasites

Dufaux et al. [217] prepared iridium clusters by ion exchange with Ir(NH3)5Cl 2+ cations of a NaY zeolite which was then calcined in flowing O2 from room temperature to 523 K with a temperature ramp of 0.5 K min–1 and reduced under H2 at 383, 773 and 923 K. Dispersion measurements by TEM and chemisorption measurements (H2, O2 and CO) indicated that very small clusters (<1 nm) were obtained upon reduction at 383 K, but, even after reduction at 923 K, the cluster size did not increase appreciably above 1 nm. The smaller clusters gave H/Ir and CO/Ir ratios larger than 1.5 but O/Ir ratios smaller than 1. Pak et al. [218] studied the preparation of iridium clusters by ion exchange and reduction starting either from the Ir(NH3)5Cl2+ cations or from [Ir(NH3)5H2O]3+ present in aqueous solutions of [Ir(NH3)5H2O](ClO4)3 at room temperature or in ammoniacal solutions of IrCl3 at 330 K. Prior to H2-reduction at 573 K, activation in flowing O2 at 573 K was required for the Ir(NH3)5Cl2+-exchanged zeolite, whereas outgassing at 473 K was sufficient for the [Ir(NH3)5H2O]3+ sample. In both cases, TEM, EXAFS and Xe-adsorption measurements pointed to the formation of iridium clusters small enough to fit in zeolite supercages although clusters obtained from the chlorine-free precursor cations gave smaller nuclearities, as deduced from Xe-adsorption [30 atoms/cluster compared to 50 in the case of the sample derived from the Ir(NH3)5Cl2+ precursor]. Iridium clusters were also prepared from polynuclear iridium carbonyl clusters prepared by a so-called ship-in-a-bottle synthesis. Lefebvre et al. [196] were the first to study the in situ synthesis of iridium carbonyls using IR and 13C NMR spectroscopy. Monovalent dicarbonyls were formed by contacting an Ir(NH3)5Cl2+-exchanged zeolite calcined at 300 K with CO. Tetranuclear iridium carbonyls, Ir4(CO)12 , were formed by contacting the zeolite with a CO/H2O atmosphere, and Ir6(CO)16 was synthesized merely by heating the Ir4(CO)12-

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containing zeolite in a H2/CO atmosphere. The synthesis of Ir6(CO)16 , obtained by reacting CO/H2 or CO/H2O mixtures with the IrIIIY zeolite at 440–470 K, was also monitored by the formation of Ir-Ir distances at 2.77 and 3.95 Å on radial function obtained by RED [219]. The decomposition of encaged Ir6(CO)16 clusters at 520 K in a CO/H2 atmosphere yielded mainly 1 nm clusters with a small percentage of larger particles in the 2–3 nm size range. The formation and chemistry of Ir6(CO)16 clusters encaged in Y zeolite was re-investigated in more detail by Kawi et al. [220] using IR and EXAFS spectroscopy. Ir6(CO)16 clusters with edge-bridging or face-bridging ligands were prepared by reductive carbonylation of [Ir(CO)2(acac)]. The structure of the decarbonylated cluster was octahedral Ir6 clusters, since the coordination number measured by EXAFS was very close to the theoretical value of 4. Ir4(CO)12 and Ir6(CO)16 in NaY, obtained by carbonylation of [Ir(CO)2(acac)] at 40 or 175 °C, respectively, were decarbonylated in H2 at 300 °C to yield bare iridium clusters of nuclearities only slightly larger than those of their carbonyl precursors. Somerville et al. [221] employed similar preparation techniques to prepare encapsulated Ir4(CO)12 clusters inside NaY zeolite; they also prepared Ir4(CO)12 clusters on the external surface by impregnation of the zeolite with a slurry of Ir4(CO)12 in cyclohexane. Upon thermal decomposition under hydrogen the carbonyl in NaY yielded 4–6 Ir atom clusters, whereas the carbonyl on NaY sintered into 2 nm iridium particles. The chemistry of osmium carbonyl clusters in basic Y zeolite (NaY treated with NaN3) in CO hydrogenation reactions was studied by Zhou et al. [222]. The possible formation of metal clusters in the course of CO+H2 reactions was mentioned. 3.1.6 Bimetallic Faujasites

Investigations on zeolites loaded with two metals were carried out with different aims, viz: 1. Enhancement of the reducibility of cations of the first transition row, such as nickel, by adding small amounts of platinum-group metals. Thus, Ni2+ [105, 223] and Cu2+ ions [134] were more easily reduced in the presence of palladium. 2. Improvement of the reducibility of a given metal by blocking, with the cations of a second metal, sites of high stabilization energy where cations are difficult to reduce. This was discussed extensively in Sect. 2.4.2. 3. Enhancement of the dispersion of small clusters of platinum-group metals by anchoring them with cations of electropositive metals, thus preventing them from migrating and coalescing, as discussed by Jiang et al. [224]. 4. Preparation of bimetallic clusters of homogeneous composition to obtain specific catalytic properties due to the association of the two metal atoms in the same particles (synergetic effects).

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This section will be restricted to studies of type 4, i.e., the preparation of bimetallic clusters of catalytic interest. One of the first studies on this subject was conducted by Tri et al. [136] to prepare Pt-Mo clusters that were used in hydrogenolysis [225, 226] and CO hydrogenation reactions [227]. Pt-Mo clusters with Pt/(Pt+Mo) ratios ranging from 1 to 0.04 were obtained by adsorption and decomposition of Mo(CO)6 vapors on a Y zeolite containing 1 nm Pt-clusters previously prepared according to treatments described in [76] (see Sect. 3.1.1). The Pt/Y zeolite was dehydroxylated at 900 K under vacuum, the required amounts of Mo(CO)6 vapors were adsorbed at 340 K, and the carbonyl decomposition was achieved by heating at 600 K under reduced H2-pressure. The mean particle diameter measured by TEM on ultramicrotome sections showed that the particle sizes were within 1±0.3 nm on Pt/Y and 1.2±0.3 nm on PtMo/Y samples. Local scanning transmission electron microscopy/energy dispersive X-ray spectroscopy (STEM-EDX) analysis performed at 1.5 nm spatial resolution indicated that the distribution of molybdenum was homogeneous on samples at low Mo-loadings. As the loading increased, the H2 and CO chemisorptive properties were progressively suppressed, suggesting that Mo-atoms were deposited as adatoms on the surface of Pt-clusters. This was later confirmed by a combined study of cluster structure by EXAFS and anomalous wide-angle scattering [228]. The Mo-adatoms were found by XPS to be in a low-valence state and, correlatively, the electron deficiency of the Pt-clusters underneath was suppressed [136]. For an optimum molybdenum concentration [Pt/(Pt+Mo) = 0.48], the turnover frequency for butane hydrogenolysis was 34 times higher than on platinum [225]. The same technique was used to prepare Pt-Re clusters [137]. Platinum clusters were first obtained by treating a Pt(NH3)42+-exchanged Y zeolite in the usual way (temperature ramp up to 300°C in oxygen and reduction in flowing H2). Appropriate amounts of Re2(CO)10 were sublimated in flowing helium at 90°C and adsorbed on the Pt/Y zeolite. The carbonyl was decomposed in flowing H2 , and decomposition was followed by TPR and TPD analysis of the gas evolved. The decomposition of Re2(CO)10 began at 150°C with the evolution of carbon monoxide. Methane and water appeared at higher temperatures, and their evolution was mirrored by a consumption of hydrogen corresponding exactly to the stoichiometry of CO hydrogenation into methane. The PtRe/Y zeolite cooled under H2 was then studied by TPR and TPO experiments and characterized by activity measurements of cyclopentane hydrogenolysis. The whole set of results confirmed that platinum and rhenium were associated in bimetallic particles. Nickel-copper particles were prepared by Maskos and van Hooff [229] by reduction of Y zeolite co-exchanged with Ni2+ and Cu2+ cations and characterized by ferromagnetic resonance. Formation of alloys was detected, but the bimetallic particles were large and located on the external surface. There are different reports on metal clusters associating a Pt-group metal with copper such as Ru-Cu [119, 230], Ir-Cu and Rh-Cu [230], Pt-Cu [230–233], and Pd-Cu [134]. Moretti and Sachtler [232] employed two procedures for the preparation of Pt-Cu clusters. NaY zeolites were either ion-exchanged successively with Pt(NH3)42+ and Cu2+ ions, or Cu2+ ions were exchanged first and the

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zeolite was calcined at 500°C before platinum exchange. The exchanged zeolites were calcined in O2 from room temperature to 300°C at 0.5°C min–1. It was expected that, under these conditions, both Pt2+ and Cu2+ ions occupy supercages in the first case, whereas Cu2+ ions occupy hidden sites in the second case so that the metal clusters should not have the same composition after hydrogen reduction. However, the same TPR curves were obtained indicating that the reduction of Cu2+ ions was favored in both cases by platinum. This could be interpreted by former results of Gallezot et al. [234] showing that Cu2+ ions migrate easily from the SI¢ sites in sodalite cages towards the supercages. Anh et al. [233] prepared Pt-Cu clusters either by simultaneous co-exchange with platinum and copper ammino cations followed by 300°C calcination and reduction, or by reduction of a zeolite obtained by ion exchange of a Pt/NaY zeolite containing Pt-clusters with Cu2+ ions. These samples were characterized by 129Xe NMR, xenon adsorption isotherms, TEM and EXAFS. It was concluded that both samples contained Pt-Cu clusters with copper segregated on the surface of platinum. Zhang et al. [134] studied by TPR, TPD and EXAFS techniques the structure and chemistry of Pd-Cu clusters prepared by co-exchange of a Y zeolite with Pd(NH3)42+ and Cu(NH3)42+ cations followed by calcination at 500°C and reduction at 450°C. The reducibility of copper ions was greatly enhanced by palladium, and the formation of bimetallic clusters was detected. Heating in inert atmosphere at 280°C produced an oxidation of Cu0 into Cu+ that remained on the surface of Pd-clusters and could be re-reduced; however, heating at 500°C produced a complete oxidation to Cu2+ which subsequently migrated into sodalite cages. The oxidative leaching of copper was attributed to the reaction of Cu0 with the zeolite protons yielding hydrogen and copper ions. A detailed study of iron-promoted rhodium clusters was conducted by Schünemann et al. [235] using TPR, FTIR, TEM and Mössbauer spectroscopy. NaY was exchanged with Fe2+ ions in FeSO4 solutions, then rhodium was exchanged from [Rh(NH3)5Cl]Cl2 solutions. The co-exchanged zeolite was calcined from room temperature to 500°C with a ramp of 0.5 °C min–1.After reduction at 500°C most of the iron remained in ionic form, but bimetallic clusters with a low Fe0 content were also formed. Treatment in NaOH of the reduced zeolite followed by calcination and reduction maximized the Fe0 content that attained ca. 50% according to ferromagnetic resonance data. The Rh-Fe clusters were disrupted in a CO atmosphere with formation of rhodium and iron carbonyls. Yang et al. [236] succeeded in preparing platinum-iridium clusters by coexchange of Pt(NH3)42+ and Ir(NH3)5Cl2+ followed by calcination in O2 and reduction in H2 at 573 K. The 129Xe NMR data were sensitive to the number of clusters and to the surface composition. In a subsequent study, Hwang and Woo [237] prepared Pt-Ir clusters by ion-exchanging a PtY zeolite containing Ptclusters with Ir(NH3)5Cl2+ followed by calcination and reduction. From 129Xe NMR and FTIR of CO adsorbed on metal clusters, it was concluded that iridium atoms were located on the surface of platinum clusters. Mériaudeau et al. have prepared platinum-tin [238] and platinum-indium [239] bimetallic clusters in Y zeolite which were characterized using H2 and CO chemisorption, TEM and STEM-EDX analysis, IR and XPS spectroscopies. Pla-

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tinum was ion-exchanged from a solution of Pt(NH3)4(OH)2 at 80°C, and the zeolite was then calcined and reduced in the usual way to obtain 1 nm Pt-clusters; the sample was cooled under H2 and stored under He. Tin was loaded by contacting overnight the Pt/Y zeolite with a hexane solution of tetramethyltin in the required amounts. Under these conditions, the organometallic compound was fixed on the platinum clusters by surface reaction of Sn(CH3)4 with Pt-H. The zeolite was heated in vacuum to 473 K, then in H2 from 473–773 K to remove the solvent and the ligands. Cluster sizes increased indicating the fixation of Sn on Pt-clusters. The formation of Pt-Sn clusters was confirmed by STEM-EDX analysis at high spatial resolution; however, the tin content of the particles was smaller than expected because part of the tin reacted with silanol groups or zeolite protons. The amount of H2 and CO chemisorbed decreased as the tin loading increased, and XPS indicated that tin associated with platinum was in a zerovalent state. The preparation of Pt-In clusters [239] was performed in a more conventional way by co-exchange with aqueous solutions of Pt(NH3)4(OH)2 and In(NO3)3 , and the zeolite was calcined and then reduced in the usual way. The formation of Pt-In clusters was established in the same way as for Pt-Sn clusters. The structure of Pt-Co and Ru-Co clusters prepared by co-exchange in NaY zeolite has been investigated as a function of calcination and reduction treatments [240]. 3.2 Metal Clusters in L Zeolites

Platinum clusters were first prepared in L-type zeolites exchanged with alkali cations by Bernard [2] to obtain new catalysts for paraffin aromatization operating with a monofunctional mechanism on the metal surface. Since it was essential to avoid zeolite acidity, the catalyst was prepared by impregnation rather than by ion exchange which generates protonic acidity upon reduction. Potassium L zeolite was dry-impregnated with Pt(NH3)4Cl2 to obtain 0.6 wt.% Ptloading, then dried at 383 K, and calcined at 753 K in air. The reduction was carried out in a catalytic reactor by heating under flowing hydrogen up to 783 K at 50 K h–1. Amongst various monofunctional catalysts, Pt/KL zeolite was the most selective catalyst for n-hexane dehydrocyclization to benzene (80% selectivity) although non-acidic Pt/NaX also showed good selectivity (66%) [2]. This pioneering work inspired a great deal of scientific and economic interest, which continues to exist 20 years later. Since the present review deals with preparative aspects only, the important patent literature, which is more oriented toward catalytic performance, will not be covered. Besoukhanova et al. [29] prepared 5 wt.% Pt/L zeolites by impregnation of L zeolites exchanged with Li, Na, K, Rb and Cs with Pt(NH3)4Cl2 . A TEM study indicated that the Pt-dispersion was inhomogeneous with large particles on the external surface, 1–2.5 nm particles both inside and outside the zeolite, and cylinders (length: 4–7 nm, width: 1.5 nm) inside the zeolite. It was assumed that IR bands of adsorbed CO near 2000 cm–1 corresponded to Pt-carbonyls formed by the reaction of CO on very small nuclearity Pt-clusters (e.g., to 0.6–0.8 nm) not visible on TEM micrographs. The band near 2060 cm–1 attributed to linear-

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ly adsorbed CO was shifted to lower values as the cation basicity increased in the order Li, K, Rb. This was attributed to an electron transfer from the basic sites that increased the electron density of the platinum clusters. The selectivity to benzene in cyclohexane dehydrocyclization was correlated with the electron properties of the metal. Hughes et al. [30] prepared a 0.8% Pt/Ba2K5 L zeolite by incipient wetness impregation of Ba2K5 L with Pt(NH3)4(NO3)2, drying at 393 K, calcining at 533–573 K and reducing at temperatures ranging from 473 to 773 K. The platinum dispersion measured by CO chemisorption was at a maximum at 590 K (ca. 100%), but the decrease in dispersion at higher temperatures (ca. 70% at 773 K) was attributed merely to a loss in accessibility of CO to the Pt-clusters rather than to a large size increase. However, this interpretation and the final size of the clusters, estimated to be smaller than 1 nm, were based on TEM measurements that were not reported. Larsen and Haller [241] started from a KL zeolite which was ion-exchanged in Pt(NH3)4Cl2 solutions, washed, calcined in O2 from 300–623 K over a period of 5 h, and reduced at the same temperature. At this point, the Pt-loading was 0.8 wt.%, and the H/Pt ratio measured by chemisorption was 1.1. Pt/KHL thus obtained was re-exchanged with BaCl2, CaCl2 or MgCl2 to vary the acidity of the zeolite whilst particle sizes remained unchanged, as suggested by Foger and Anderson [162]. The metal dispersion measured by hydrogen chemisorption in the final catalysts was unchanged. The decrease of the CO chemisorption in the order Ba>Ca>Mg was attributed to a larger electron charge transfer to platinum clusters. The main evidence for charge transfer to platinum was the decrease in the series Ba
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reduced at 300, 450, and 600°C without prior calcination were studied by EXAFS and H2 chemisorption. As the reduction temperature increased, the Pt-Pt coordination number experienced only a slight increase from 4.0 to 4.9 whilst the H/Pt ratio decreased more dramatically from 1.4 to 0.8; it was suggested that, for a slight cluster growth, a larger fraction of the cluster surface in contact with the pore walls was no longer accessible to hydrogen. Using a similar preparative technique, Mojet and Koningsberger confirmed by EXAFS the formation of very small Pt-clusters consisting of five to six atoms [246]; they showed that CO admission at room temperature produced the complete decomposition of platinum particles and the formation of platinum carbonyl clusters. Palladium clusters were prepared in KL zeolite by ion exchange in Pd(NH3)4(NO3)2 solutions, calcination at 573 K, and H2-reduction at 573 K [247]. X-ray absorption spectroscopy measurements detected the formation of ca. 10and 16-atom disc-like particles for 2 and 3 wt.% Pd/KL-zeolite, respectively. Mojet et al. [248] have prepared Pd-clusters in KL zeolites with different potassium content by incipient wetness impregnation with [Pd(NH3)4](NO3)2 . It was concluded from XPS, IR spectroscopy and catalytic activity measurements that the metal clusters became electron-deficient on acidic supports and electron-rich on basic supports. Ir/KL zeolite catalysts were prepared with clusters of about five to six atoms by H2-reduction at 300°C of a KL zeolite loaded with [Ir(NH3)5Cl]Cl2 salt [249]. The IR spectra of adsorbed CO suggested that the environment was slightly basic and that Ir-clusters were electron-rich relative to the bulk metal. Hong et al. [19] compared the preparation of Pt/BaKL zeolites by ion exchange with Pt(NH3)4(NO3)2 (0.2 wt.% Pt-loading) and vapor- or liquid-phase impregnations with platinum acetylacetonate. Pt/BaKL zeolite was obtained by ion-exchanging three times a KL zeolite in 0.3 M Ba(NO3)2 solutions, followed by drying and calcination in air at 600°C. Liquid-phase impregnation was achieved by stirring for 1 h the dehydrated BaKL zeolite in an acetone solution containing the required amount of Pt(acac)2 then by evacuating to dryness. Vapor-phase impregnation was carried out by mixing dehydrated BaKL and Pt(acac)2 in a cell which was evacuated, sealed, and kept at 145°C for 24 h. It was concluded from 13C MAS NMR that the complex did not enter the pores in the case of liquidphase impregnation, whereas vapor-phase impregnation was effective for loading platinum (0.79 wt.% Pt-loading). TEM indicated that the Pt-clusters are intracrystalline, and hydrogen chemisorption measurements (H/Pt = 1.43 and 2.06 for vapor-impregnated and ion-exchanged samples, respectively) confirmed the very low cluster nuclearity. Two series of Pt/KL catalysts were synthesized by incipient wetness and vapor-phase impregnation (VPI) with a Pt(acac)2 complex [60]. The precalcined KL zeolite was physically mixed with the complex that had been sublimed up to 130°C. To decompose the platinum precursor, the sample was heated in air at 350 °C. EXAFS and TEM showed that the VPI sample contains smaller particles than the catalyst prepared by incipient wetness, which are in close contact with the zeolite walls. This results in better diffusion of methylcyclopentane (no collimation effect), better resistance to poisoning and coking, and a lower particle agglomeration rate.

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Sharma et al. [250] carried out combined microcalorimetric and volumetric measurements of H2 adsorption on various samples of Pt/BaKL prepared by impregnation (1 wt.% Pt). Hydrogen chemisorption measured at 403 K gave H/Pt ratios smaller than unity, in contrast with other reports of H/Pt ratios close to 2, especially at very low metal loadings (vide supra). It was shown that on Pt/KL there are weak adsorption sites for hydrogen (at ca. 30 kJ mol–1) not associated with the metal, which may account for hydrogen spillover. Another report from Sharma et al. [251] showed that the Pt-clusters in Pt/BaKL, in contrast to Pt/SiO2, retained their H2 and CO chemisorptive properties after nhexane dehydrocyclization, and it was concluded that clusters in L zeolite are more resistant to poisoning by carbon deposits and keep their high dispersion in the course of hexane conversion. A similar interpretation of the stability of Pt/KL zeolite in aromatization reactions in terms of inhibition of coke deposition on metal clusters inside zeolite pores was also given by Iglesia and Baumgartner [252] on the basis of reaction data. Dossi et al. [61] loaded platinum in KL zeolite by vapor deposition of Pt(hfa)2 (hfa: hexafluoroacetylacetonate). The organometallic precursor was sublimated at 70°C in a flow of argon and adsorbed on the dehydrated KL zeolite. The decomposition of Pt(hfa)2 was achieved at 350°C in a H2-atmosphere. In situ EXAFS measurements suggested the formation of small clusters (Pt-Pt coordination number of 5), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements using CO as the probe molecule indicated the formation of carbonyl clusters of the general formula [Pt3(CO)6] n2– (n = 1 – 4). A series of Pt-Ni bimetallic clusters with varying Ni/Pt ratios were prepared and characterized by Larsen and Haller [253]. KL zeolite was co-impregnated with Ni(NO3)2 and Pt(NH3)4(NO3)2, calcined at 723 K and reduced at 773 K. From EXAFS, X-ray absorption near edge structure (XANES) and chemisorption measurements it was concluded that bimetallic particles were formed with a Pt-rich core and a Ni-rich surface and there was an electron transfer from Ni to Pt atoms in the clusters. 3.3 Metal Clusters in ZSM-5 Zeolites

A detailed study of the preparation of platinum clusters in HZSM-5 zeolites was reported by Engelen et al. [18]. ZSM-5 zeolites synthesized with Si/Al ratios of 50 and 30 in the K+ and NH+4 forms, respectively, were ion-exchanged with the desired amounts of Pt(NH3)4(OH)2 in aqueous solutions. Samples were also prepared by incipient wetness impregnation for comparison. Since the size of the platinum-ammine complex is about the same as that of the aperture of the ZSM-5 pores, care was taken to obtain information on the percentage of the platinum complex effectively introduced by measuring pore volumes before and after complex decomposition and in a Pt-free sample. It was concluded that 65 and 100% intraneous pore loadings were obtained by impregnation and ion exchange, respectively. The decomposition of the ammine complex under He or He/O2 was followed by TGA and TPD, and the final state of platinum dispersion was monitored by TEM. It was shown that large Pt-particles were always formed

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Table 2. Platinum cluster size in ZSM-5 (from [255])

Catalyst, treatment (K)

0.5% Pt, 720 O2 , 790 H2 0.5% Pt, 790 O2 , 790 H2 1% Pt, 790 O2 , 790 H2

N

5.9±1.3 9.3±1.0 10.3±1.0

No. of atoms 13 120 220

Particle diameter (Å) EXAFS

TEM

8 15 18

13 16

on the external surface at high heating rates which was attributed to autoreduction processes. For samples prepared by ion exchange and treated with a low heating rate (1 K min–1), high He/O2 flux (150 ml min–1) and small zeolite batches (0.15 g), the platinum metal (2 wt.%) was kept entirely inside the pore system. In this case, small particles (1–2 nm) were detected on TEM micrographs of remarkably high resolution and contrast, and were attributed to platinum oxide clusters. Subsequent reduction at 573 K in H2 did not change the size or the position of the particles. Unlike external Pt-particles, the Ptparticles in zeolite pores were not deactivated by coke during propane conversion. The catalytic and electronic properties of platinum clusters in Pt/ZSM-5 were studied by Minachev’s group [254], and the formation and structure of platinum particles in ZSM-5 zeolite were studied by Shpiro et al. [255] using EXAFS as the main tool. A NH4 ZSM-5 zeolite was ion-exchanged with Pt(NH3)4Cl2 solutions to obtain 0.5 or 1 wt.% platinum, then heated slowly from room temperature to 720 or 790 K in flowing dry air and reduced at 620 or 790 K. Table 2 gives the EXAFS and TEM data on the state of platinum dispersion. There is a good agreement between the different techniques, and the data illustrate the high dependency of the final dispersion on calcination temperatures and platinum concentration. 3.4 Metal Clusters in Various Zeolites

The formation of platinum clusters in mordenite was studied by Kustov and Sachtler [256] using IR spectroscopy and CO as the probe molecule. It was shown that the presence of water during the reduction treatment modified the acidic sites and the final metal dispersion which was evaluated very indirectly from the spectra of adsorbed CO [formation of gem-Pt(CO)2]. The highest dispersion was obtained in Pt/H-MOR zeolite (0.56 wt.% Pt) prepared by ion exchange of a commercial hydrogen mordenite (H-MOR) sample (Si/Al = 8.8) in Pt(NH3)4(NO3)2 solution which was calcined from 300–770 K at 0.5 K min–1, cooled and saturated with H2O, then reduced in flowing H2 from 300–620 K at 8 K min–1. Zheng et al. [257] studied the auto-reduction of Pt(NH3)2+ 4 ions in K-beta zeolite using TPR, TPO, TEM and FTIR to monitor the elementary processes. It was shown that Pt-particles with sizes ranging from 1–5 nm were formed. The metal

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can be re-dispersed into 2 nm clusters by calcining in O2 at 300°C followed by H2-reduction. Different states of platinum dispersion in beta zeolite were obtained by Gallezot et al. [17]. The starting material was a beta zeolite synthesized with a tetraethylammonium template that was decomposed by heating at 773 K. The zeolite was exchanged thoroughly by Na+ cations in NaCl solution, then the required amounts of platinum were introduced by ion exchange with Pt(NH3)42+. The exchanged zeolite was submitted to two types of treatments: (1) Direct reduction under flowing hydrogen from 298–573 K at 2 K min–1 and (2) calcination from 298–573 K at 1 K min–1 under flowing O2 , cooling under argon down to 373 K, then heating under flowing H2 to 573 K. It was shown by TEM on ultramicrotome sections that treatment (1) yielded metallic platinum filling continuously the three-dimensional network of perpendicular channels, whereas treatment (2) resulted in clusters smaller than the zeolite pores. The latter coalesced during the course of cinnamaldehyde hydrogenation giving clusters of sizes commensurate with the pore diameter. Bimetallic catalysts Pt-Pd/H-beta were prepared by incipient wetness or by ion exchange [258]. The platinum dispersion was improved by the presence of palladium, but the overall dispersion did not exeed 0.59; it was not established whether the two metals were associated or occurred as separate phases. Bécue et al. [259] studied the dispersion of platinum in Pt(NH3)2+-exchanged Na-beta, and Cs-beta, calcined at 300°C and reduced up to 500°C. The mean particle size measured by TEM on the cesium zeolite was 1.7 nm, but the authors did not exclude the presence of undetected smaller particles. Platinum supported on beta zeolites was prepared by ion-exchanging H-beta, Li-beta and Cs-beta zeolites in aqueous solutions of Pt(NH3)4(NO3)2. The platinum particle sizes were in the range 1.2–1.6 nm [260]. Khodakov et al. [261] loaded a dealuminated mazzite (Si/Al=20) by ion exchange in Pt(NH3)4Cl2 solution (0.3 wt.% Pt-loading). Calcination at 723 K and reduction at 823 K yielded platinum clusters fitting in the 7.4 Å zeolite channel (the Pt-Pt coordination number, measured by EXAFS, was 3.7). These clusters grew in size as the zeolite was treated under water vapor at 823 K for various periods of time. Hong et al. [19] studied the loading of AlPO4-5 and VPI-5 aluminophosphates by vapor-phase impregnation with sublimed Pt(acac)2, and liquid-phase impregnation with an acetone solution of Pt(acac)2 , under the same conditions as those described for BaKL zeolite (Sect. 3.2). The second technique failed because the solvated complex did not enter the aluminophosphate pores. On the other hand, 0.80 and 0.68 wt.% Pt in AlPO4-5 and VPI-5, respectively, were successfully loaded by vapor-phase impregnation. It was shown by 13C NMR that the precursor was adsorbed in the pores, and it was verified that the structure of VPI-5 remained stable. A TEM study showed the presence of intrazeolitic Ptclusters, most of them smaller than 1.5 nm, the size of which did not change after n-hexane aromatization. Two methods for the preparation of nickel-loaded AlPO4-5 aluminophosphate were employed by Kraushaar-Czarnetzki and van Hooff [27]. Pore volume impregnations with Ni(NO3)2 or K2[Ni(CN)4]) salts followed by reduction at

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673 K resulted in large nickel particles on the external surface. In contrast, Ni-loading by chemical vapor deposition of nickelocene sublimated at 383 K in helium on AlPO4-5 at 473 K in H2-atmosphere yielded intrazeolitic Ni-clusters, most of them smaller than 2 nm. Weitkamp et al. [262] have loaded small pore molecular sieves, viz. ZSM-58, Rho, ZK-5 and SAPO-42, with platinum, palladium and rhodium using solidstate ion exchange. The solids were heated for 12 h at 500°C in nitrogen and afterwards mixed at room temperature with the required amounts of PtCl2 , PdCl2 , or RhCl3 . The mixture was heated in dry nitrogen to 550 or 625°C whilst monitoring the amount of HCl evolved. Ion exchange occurred quantitatively, and 1 wt.% loading was obtained. The metal zeolites were characterized by competitive hydrogenation of monoolefins of different molecular sizes; the shape selectivity indicated that the metal clusters were located inside the zeolite pores. 3.5 Carbide, Nitride, and Sulfide Clusters

The decomposition under partial pressure of ammonia of Mo(CO)6 loaded in EMT zeolite by chemical vapor deposition and subsequent nitridation under flowing ammonia were used to prepare encaged molybdenum nitride clusters [263]. XPS measurement showed that molybdenum nitride was formed at a nitridation temperature as low as 723 K; TEM measurements indicated that the nitride phase was homogeneously dispersed in the zeolite micropores as clusters smaller than 1 nm. Bimetallic Pt-Mo catalysts were prepared by successive chemical vapor deposition of Mo(CO)6 onto nanometer-size platinum particles dispersed in EMT zeolite [264]. Bimetallic particles of uniform composition in a highly dispersed form were formed. Subsequent decomposition of the Mo precursor at 600 K in hydrogen formed a supported molybdenum carbide phase that was characterized by XPS, TEM and EXAFS. Nitridation at 973 K in flowing NH3 led to a nanometer-sized Pt-core covered by a Mo-nitride layer. The coverage of the Pt-clusters by ca. 0.3 nm of carbide or nitride modified greatly the selectivities in n-heptane isomerization and hydrogenolysis. Molybdenum oxy-carbide species were synthesized in the supercage of NaY zeolite by adsorption of Mo(CO)6 vapor followed by thermal decomposition in vacuum at 573 K [265]. The structure of the encaged species was fully characterized by TPD, EXAFS, XPS, and XRF. The decomposition of methanol at 573 K on the oxy-carbide species yielded H2 , CH4 , and CO2 in an approximate ratio of 2:1:1. Nickel sulfide supported on Y zeolite was prepared by ion exchange with NiCl2 or impregnation with Ni(NO3)2, followed by calcination at 673 K and sulfidation in H2/H2S mixture at 673 K [266]. The main part of the nickel sulfide phase was located outside the zeolite; a higher, more homogeneously distributed amount of nickel sulfide was present in the zeolite pores in samples prepared by ion exchange. The sulfidation of CoNaY was extensively studied by Vissenberg et al. [267–269]. In zeolites prepared by ion exchange an interaction exists between

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protons and the metal sulfide resulting in high dispersion. It was suggested that the high activity in HDS reactions could be attributed to the electron-deficient character of cobalt sulfide. However, upon contact with a base such as NH3, the interaction was broken and migration of cobalt species to the external surface occurred. Ruthenium sulfide particles were prepared by ion exchange of a dealuminated KY zeolite (Si/Al = 6) with Ru(NH3)63+ ions; after washing and drying, sulfidation was carried out in a gas flow containing 15% H2S in He, at 673 K [270]. TEM studies indicated that ruthenium sulfide consisted of 1 nm particles homogeneously distributed in the zeolite grains. From EXAFS data it was concluded that the structure consisted of less than 50-atom clusters of a ruthenium sulfidelike phase with very small domains of ruthenium metal. The high activity of this catalyst in hydrogenation reactions was attributed to surface ruthenium atoms not covered by sulfur. Calcined and sulfided Ni-Mo catalysts supported on ultrastable Y zeolite, NaY, mordenite and ZSM-5 were studied by high-resolution TEM [271]. In USY zeolite, Ni-Mo-S clusters were found in the supercages of the zeolite, whereas, on other zeolites, the sulfide phase was predominantly on the external surface. Bendezú et al. [272] also studied the dispersion and location of Ni-S, W-S and a Ni-W-S phase in US zeolite .

4 Concluding Remarks and Prospects A survey of the literature covering an extensive period of time shows that many experimental results have been rediscovered repeatedly, sometimes with no real improvements. To achieve additional progress in the control of the preparation of intrazeolitic metal clusters, and to avoid the duplication of earlier studies, investigations are needed in the following areas: 1. Influence of the metal concentration and reduction temperature on the metal dispersion and distribution.Very few studies have been devoted to comparing the cluster size and location as a function of the amounts of metal loaded. This parameter is of primary importance for metal zeolites prepared by impregnation with metal salts. Thus, the literature survey of Pt/L zeolites shows that high and homogeneous metal dispersions were obtained at 0.5 wt.% Pt-loading, whereas a very heterogeneous size and distribution of the particles were observed at 5 wt.% loading. This parameter is also critical for metal zeolites prepared by chemical vapor deposition of organometallic compounds. The limits of metal loading attained with these methods – compatible with homogeneous dispersion – have to be determined. On the other hand, most reports show that faujasites can be loaded by ion exchange with 0.5–15 wt.% platinum and still exibit almost the same metal dispersion, i.e., clusters of ca. 1±0.2 nm diameter accommodated in the supercages giving H/Pt ratios equal to or slightly larger than 1. This was confirmed by EXAFS and Xe adsorption measurements of the cluster size in Pt/NaY zeolite as a function of Pt-loading [174]. These findings might be explained in part by the

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rather high reduction temperatures employed in most investigations (typically 300°C), which produce a rapid diffusion of isolated atoms and small clusters, this mobility favoring the growth of the metal cluster until the cage space is almost filled. Reduction of metals with high redox potentials should be conducted at lower temperatures if one wishes to study the effect of platinum concentration. Indeed, reduction at 490 K instead of 673 K resulted in smaller Pt-Pt coordination numbers (7 and 8.8, respectively) [247]; however, this investigation was conducted with a high Pt-loading (10.8 wt.%). To sum up, the effects of reduction temperature and metal concentration on the size of clusters in faujasites need to be clarified by further studies. Investigations directed at attempting to control the cluster size should be extended to other zeolites and, more specifically, to zeolite L. In this latter case, preparation by ion exchange rather than by impregnation should be conducted since a much larger range of concentrations and reduction temperatures can be studied. Protonic acidity can be suppressed, if required, by exchange of alkaline or alkaline-earth cations after cluster preparation. 2. In spite of the difficulty in characterizing high states of metal dispersion in the micropores of molecular sieves, the agreement between many studies conducted by different investigators is generally good. However, there are marked discrepancies between the various characterizations of platinum clusters in the faujasite supercage by hydrogen chemisorption. H/Pt ratios were found to be close to 1 in most investigations,but ratios approaching 2 have also been reported. The different ways of measuring hydrogen uptake should perhaps be closely re-examined to take into account any cause that might contribute to hydrogen consumption, such as reduction by hydrogen spillover of Fe3+ impurities always present in zeolites. Thus, H/Pt ratios significantly higher than 1 were very often observed on zeolites with low metal loading (e.g., 0.5 wt.%) where the molar concentration of impurities may even exceed that of the metal. Still smaller cluster sizes may well account for higher H/Pt stoichiometries, and this could be an indication of extremely small nuclearity. If the absolute hydrogen adsorption stoichiometry for clusters much smaller than 1 nm approaches H/Pt = 2, smaller ratios could be due to the fact that part of the cluster surface in contact with the pore walls is not accessible to hydrogen, as suggested by Vaarkamp et al. [245]. These points need further studies which could equally be carried out with ruthenium or iridium clusters, which have lower mobility (higher melting point) than platinum and can be very easily prepared in the form of clusters with nuclearities smaller than 10. 3. Studies on metal cluster preparation and characterization have been focused mainly on faujasites. There is a need to extend these investigations to other zeolite types, as well as new microporous and mesoporous materials, in order to prepare metal clusters in different geometric and electronic environments. Both medium-pore and mesoporous materials have a potential interest. Thus, conventional zeolites are not well suited for catalytic reactions involving fine chemicals in the liquid phase because of molecular size restriction and/or very low diffusivities. To take advantage of molecular constraints while tailoring the regio-, chemo- and stereoselectivity of metal-catalyzed reactions of

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functionalized molecules, the pore structure should be commensurate with the molecules, i.e., materials with larger pores are required. A few of them are available, thermal stability is not a requirement for low-temperature reactions – so that the preparation of metal clusters or anchoring of organometallic complexes should be carried out on these materials. 4. Platinum zeolites have been the subject of many studies which should be extended to other platinum-group metals, pure or associated with other metals in bimetallic clusters. Rhodium, ruthenium and iridium present several advantages over platinum and palladium to attain ultimate low nuclearities. Calcination of their ammine precursors leads to oxidic species associated with extra-framework oxygens that limit the possibility of migration towards hidden cation sites. Furthermore, these metals have high melting and boiling points (even though small clusters have much lower melting and boiling points than bulk metals), and, therefore, have a comparatively low mobility as a function of temperature. Since they can be reduced by H2 at low temperatures, one can expect to produce much smaller clusters than with platinum. However, their reduction potential being smaller, the clusters could be re-oxidized in the presence of protonic acidity; therefore, the lowest cluster nuclearities should be obtained in a basic environment only. 5. The size and location of metal clusters in working catalysts not only depend on the preparation techniques employed to obtain the fresh catalysts, but also on the stability of metal clusters. In the course of catalytic reactions or during regeneration treatments, sintering of metal clusters frequently happens as a result of complexation and transport processes favored by the presence of molecules such as O2 , Cl2 , CO, and NH3 . Sintering as well as redispersion processes are not well documented and require additional characterization studies, preferably with in situ measurements, to monitor the state of metal clusters as a function of the treatment conditions.

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Jiang HJ, Tzou MS, Sachtler WMH (1998) Appl Catal 39:255 Tri TM, Massardier J, Gallezot P, Imelik B (1984) J Catal 85:244 Tri TM, Massardier J, Gallezot P, Imelik B (1984) J Mol Catal 25:151 Borg F, Gallezot P, Massardier J, Perrichon V (1986) C1 Mol Chem 1:397 Samant MG, Bergeret G, Meitzner G, Gallezot P, Boudart M (1988) J Phys Chem 92:3547 Maskos Z, van Hooff JHC (1980) J Catal 66:73 Tebassi L, Sayari A, Ghorbel A, Dufaux M, Naccache C (1984) J Mol Catal 25:397 Dufaux M, Lokolo M, Meriaudeau P, Naccache C, Ben Taarit Y (1986) In: Proceedings 7th International Zeolite Conference. Elsevier, Amsterdam, p 929 Moretti G, Sachtler WMH (1989) J Catal 15:205 Ahn DH, Lee JS, Nomura M, Sachtler WMH, Moretti G, Woo SI, Ryoo R (1992) J Catal 133:191 Gallezot P, Ben Taarit Y, Imelik B (1972) J Catal 26:161 Schünemann V, Treviño H, Lei GD, Tomczak DC, Sachtler WMH, Fogash K, Dumesic JA (1995) J Catal 153:144 Yang OB, Woo SI, Ryoo R (1992) J Catal 137:357 Hwang IC, Woo SI (1994) In: Weitkamp J, Karge HG, Pfeifer H, Hölderich W (eds) Zeolites and related microporous materials: state of the art 1994. Elsevier, Amsterdam, p 757 Mériaudeau P, Naccache C, Thangaraj A, Bianchi CL, Carli R, Vishvanathan V, Narayanan S (1995) J Catal 154:345 Mériaudeau P, Naccache C, Thangaraj A, Bianchi CL, Carli R, Narayanan S (1995) J Catal 152:313 Guczi L, Bazin D (1999) Appl Catal A: General 188:16 Larsen G, Haller GL (1989) Catal Lett 3:103 Tri TM, Massardier J, Gallezot P, Imelik B (1982) In: Imelik B, Naccache C, Coudurier G, Praliaud H, Meriaudeau P, Gallezot P, Martin GA, Vedrine JC (eds) Metal-support and metal-additive effects in catalysis. Elsevier, Amsterdam, p 141 Dai LX, Sakashita H, Tatsumi T (1994) J Catal 147:311 Vaarkamp M, Grondelle JV, Miller JT, Sajkowski DJ, Modica FS, Lane GS, Gates BC, Köningsberger DC (1990) Catal Lett 6:369 Vaarkamp M, Grondelle JV, van Santen RA, Miller JT, Meyers BL, Modica FS, Lane GS, Köningsberger DC (1993) In: von Ballmoos R, Higgins JB, Treacy MMJ (eds) Proceedings 9th International Zeolite Conference Montréal 1992. Butterworth-Heinemann, Boston, p 433 Mojet BL, Koningsberger DC (1996) Catal Lett 39:191 Menacherry PV, Fernandez-Garcia M, Haller GL (1997) J Catal 166:75 Mojet BL, Kappers MJ, Muijsers JC, Niemantsverdriet JW, Miller JT, Modica FS, Koningsberger (1994) In: Weitkamp J, Karge HG, Pfeifer H, Hölderich W (eds) Zeolites and related microporous materials: state of the art 1994. Elsevier, Amsterdam, p 909 Triantafillou ND, Miller JT, Gates BC (1995) J Catal 155:131 Sharma SB, Miller JT, Dumesic JA (1994) J Catal 148:198 Sharma SB, Ouraipryvan P, Nair HA, Balaraman P, Root TW, Dumesic JA (1994) J Catal 150:234 Iglesia E, Baumgartner JE (1993) In: Guczi L, Solymosi F, Tetenyi P (eds) Proceedings 10th International Congress on Catalysis Budapest 1992. Akademiai Kiado, Budapest, p 157 Larsen G, Haller GL (1993) In: von Ballmoos R, Higgins JB, Treacy MMJ (eds) Proceedings 9th International Zeolite Conference Montréal 1992. Butterworth-Heinemann, Boston, p 441 Bragin OV, Shpiro ES, Preobrazhensky AV, Isaev SA, Vasina TV, Dyusebina BB, Antoshin GV, Minachev KhM (1986) Appl Catal 27:219 Shpiro ES, Joyner RW, Minachev KM, Pudney PDA (1991) J Catal 127:366 Kustov LM, Sachtler WMH (1992) J Mol Catal 71:233 Zheng J, Dong JL, Xu QH (1994) In: Weitkamp J, Karge HG, Pfeifer H, Hölderich W (eds) Zeolites and related microporous materials: state of the art 1994. Elsevier, Amsterdam, p 1641

232. 233. 234. 235. 236. 237. 238. 239. 240. 241. 242. 243. 244. 245.

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258. Blomsa E, Martens JA, Jacobs PA (1997) J Catal 165:241 259. Bécue T, Maldonato-Hodar FJ, Antunez AP, Silva JM, Ribeiro MF, Massiani P, Kermarec M (1999) J Catal 181:244 260. Ramirez S, Viniegra M, Dominguez JM, Schacht P, De Ménorval LC (2000) Catal Lett 66:25 261. Khodakov A, Berthier Y, Oudar J, Barbouth N, Schulz Ph (1994) In: Weitkamp J, Karge HG, Pfeifer H, Hölderich W (eds) Zeolites and related microporous materials: state of the art 1994. Elsevier, Amsterdam, p 1641 262. Weitkamp J, Ernst S, Bock T, Kiss A, Kleinschmit P (1995) In: Beyer HK, Karge HG, Kiricsi I, Nagy JB (eds) Catalysis by microporous materials. Elsevier, Amsterdam, p 278 263. Bécue T, Manoli JM, Potvin C, Djéga-Mariadassou G, Delamar M (1997) J Phys Chem 101:6429 264. Bécue T, Manoli JM, Potvin C, Davis RJ, Djéga-Mariadassou G (1999) J Catal 186:110 265. Shido T, Asakura K, Noguchi Y, Iwasawa Y (2000) Appl Catal A: General 194–195:365 266. WeltersWJJ, Vorbeck G, Zandbergen HW, de Haan JW, de Beers VHJ, van Santen RA (1994) J Catal 150:155 267. Vissenberg MJ, de Pont PW, van Oers EM, de Haan JW, Boellaard E, van der Kraan AM, de Beers VHJ, van Santen RA (1996) Catal Lett 40:25 268. Vissenberg MJ, de Pont PW, Arnouts JWC, van de Ven LJM, de Haan JW, van der Kraan AM, de Beers VHJ, van Veen JAR, van Santen RA (1997) Catal Lett 47:155 269. Vissenberg MJ, de Pont PW, Gruijters W, de Beers VHJ, van der Kraan AM, van Santen RA, van Veen JAR (2000) J Catal 189:209 270. Moraweck B, Bergeret G, Cattenot M, Kougionas V, Geantet C, Portefaix JL, Zotin JL, Breysse M (1997) J Catal 165:45 271. Li D, Xu H, Guthrie D Jr (2000) J Catal 189:281 272. Bendezú S, Cid R, Fierro JLG, Lopez-Agudo A, (2000) Appl Catal A: General 197:47

Ionic Clusters in Zeolites Paul A. Anderson School of Chemical Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK; e-mail: [email protected]

Dedicated to Professor Gerhard Ertl on the occasion of his 65th birthday

1

Introduction

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1.1 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 1.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 2

Alkali Metal Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

2.1 2.2 2.3 2.4 2.5 2.6

Trapped Electrons . . . . . . . . . . . . . . . Chemical Synthesis . . . . . . . . . . . . . . . Structure and Location of M np+ . . . . . . . . Cluster Crystals and the Cationic Continuum Properties and Applications . . . . . . . . . . Beyond the Cluster Model . . . . . . . . . . .

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Silver Clusters

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3.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 3.2 Structure and Location . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 3.3 Properties and Applications . . . . . . . . . . . . . . . . . . . . . . . . 328 4

Other Metals

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4.1 Elemental Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 4.2 Alloy Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 5

Non-metal Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

5.1 Hydrolysis Products and Related Species . . . . . . . . . . . . . . . . . 330 5.2 Occluded Salts and Other Compound Clusters . . . . . . . . . . . . . 331 5.3 Anionic Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 6

Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334

Molecular Sieves, Vol. 3 © Springer-Verlag Berlin Heidelberg 2002

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1 Introduction Many established molecular sieve applications depend either directly or indirectly on the existence of a net anionic charge on the zeolite framework. In order to preserve charge neutrality, the framework charge is normally balanced by the familiar exchangeable cations residing within the zeolite pores. A large and rapidly growing body of literature, however, attests to the possibility of more complex and unusual chemical species fulfilling this charge-balancing role. My task in the following pages is to provide an overview of the preparation and properties of the many ionic cluster species, almost all of them cationic, that can be formed within zeolites. Methods of synthesis range from straightforward ion exchange, dehydration and calcination procedures to direct elemental vapour– zeolite reactions under high vacuum conditions. The resulting clusters, diverse both in chemical composition and in physical and chemical properties, may conveniently be divided into metal and non-metal species. Although it is the burgeoning corpus of published work on the synthesis and characterization of cationic metal clusters in zeolites that primarily shapes this review, research into non-metal clusters, and in particular into clusters of ionic and semiconducting compounds in zeolites, is a major growth area producing many new ionic clusters. A brief outline of this work is also included. A defining feature of zeolite molecular sieves is a regular intracrystalline pore space (see Fig. 1), in which, uniquely, it is possible not only to stabilize unusual cluster species, but also to arrange these with a well-defined geometry relative to each other. Zeolite frameworks thus can act as “molecular scaffolding” for the assembly of ordered cluster arrays or “cluster crystals”, whose properties depend not only on those of the individual clusters, but also, importantly, on the interactions between neighbouring clusters. With cluster sizes limited by the zeolite cages to approximately 1 nm or less, these may be regarded as the ultimate nanoscale materials. It will become clear from the many examples encountered below that zeolite frameworks have enormous potential as hosts for ordered arrays of finely divided fragments of a wide range of solid-state materials, and this concept underpins much current research into the preparation and properties of clusters in zeolites [1]. Attention will therefore be drawn to both established and proposed new applications of ionic clusters in zeolites, in areas such as catalysis, sensors, optics and electronics. 1.1 Stability

The preparation of well-defined entities possessing desirable chemical or physical properties within zeolites is an exciting challenge facing solid-state chemists, and one to which the chemistry of ionic species within the zeolite pore space undoubtedly holds the key. Some years ago Kasai and co-workers [2, 3] considered the ability of zeolites to promote chemical processes such as: (1) Na Æ Na+[zeolite] + e–[zeolite] , NaCl Æ Na+[zeolite] + Cl –[zeolite] .

(2)

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a

b

c

d

Fig. 1. a Sodalite cage (b-cage) structural unit as encountered in the three zeolite structures (b–d) most commonly found to host ionic clusters: b sodalite (SOD), c zeolite A (LTA) and d zeolites X and Y (FAU). Vertices represent silicon or aluminium; oxygen atoms (white in a) are omitted for clarity in b–d

As written these reactions are endothermic, but are found to proceed readily within zeolites. Kasai and co-workers noted that the common feature of several such processes was an increase in the number of charged species, and speculated that the driving force for such reactions was a gain in the electrostatic “Madelung energy” achieved through filling the voids in zeolite crystals with suitably arranged charged species. A natural development of the ideas outlined above is to regard the dehydrated zeolite as a polar solid solvent and reactions (1) and (2) as dissolution processes. When Barrer and Whiteman [4] studied the reaction of mercury metal with a number of different ion-exchanged zeolites, they found that mercury uptake was limited in sodium-, calcium- and lead-exchanged forms and “copious” in silver- and mercury-exchanged forms, where reduction of the exchangeable cations Ag+ and Hg2+ (to Ag0 or Hg22+) by Hg0 would be possible. The implication of these observations is that the ionization process outlined in reaction (1) is necessary for the reaction of elemental metals with zeolites – one of the main synthetic routes to ionic clusters in zeolites – to occur. Reactions (1) and (2), in fact, can usefully be regarded as models for the formation of ionic clusters in zeolites, and detailed calculations of “solvation” energies [1] can help

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provide a more rigorous explanation for the driving force behind these processes. In addition, the reverse reactions represent hypothetical decomposition p+ routes for Na l and (Na+)m(Cl–)n clusters, and so have a bearing on cluster stability. A number of more or less general conclusions may be drawn: 1. the stability of ionic clusters in zeolites depends to a large extent on cation solvation, i.e., the coordination of cationic species to the framework oxygens; 2. the interaction of cations with highly charged frameworks is much stronger than with neutral frameworks; and 3. cation solvation is strongly site-dependent. It follows that through its chemical composition and geometry the zeolite host exerts a considerable influence over the structure and stability of ionic clusters in zeolites and does not act merely as a convenient inert host. In particular, the formation of ionic clusters is much more likely to occur in aluminosilicates with a high aluminium content and a sizable anionic framework charge than in neutral aluminophosphate molecular sieves. In fact, a significant proportion of zeolite ionic clusters are found in just three aluminosilicate frameworks, each of which contains the sodalite cage structural unit (Fig. 1). 1.2 Characterization

The characterization of ionic clusters in zeolites represents a considerable experimental challenge. At least four pieces of knowledge – cluster nuclearity, geometry, location and charge – are needed, a requirement invariably beyond the scope of a single experiment. In addition, the well-known affinity of many zeolites for water provides a severe test of cluster stability, and it is common for cluster-containing zeolites to be air- or moisture-sensitive. This constitutes a significant obstacle to many experimental procedures. The result is that unambiguous information on all four counts is available in only a few cases, and it follows that the picture of ionic clusters in zeolites sketched in this review will necessarily be an incomplete one. It is not unrealistic to hope, however, that after an extensive survey of a well-chosen series of examples, the main features of the subject will have been outlined. With the remarks of the preceding paragraph in mind, the reader is gently reminded that it is advisable to weigh carefully interpretations advanced by authors against the nature of the experimental evidence obtained. Hitherto, the identification of ionic clusters in zeolites has relied heavily on two types of experimental data. The concept of ionic clusters located and stabilized within zeolite cages first arose from early electron spin/paramagnetic resonance (ESR/ EPR) experiments (Sect. 2.1). The great strength of ESR in this context is the ability to provide definitive information on the nuclearity of paramagnetic clusters, but the technique is less effective at providing other information and is inapplicable to diamagnetic species. The characterization of a large proportion of the clusters described in the following pages, therefore, depends on the analysis of single crystal and powder diffraction data. It is important to realize, however, that although crystallography can provide detailed statistical data on

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the location of atoms in zeolites, it is common for non-framework crystallographic sites to be partly rather than fully occupied, and the identification of groups of atoms as clusters is, as a result, rarely unambiguous. A reliable assessment of the valence state of clusters from crystallographic data is particularly difficult. Nevertheless, crystallographic studies have been invaluable in shedding light on rich areas of zeolite cluster chemistry that so far lie beyond the reach of other experimental techniques. Although spectroscopic measurements such as UV/visible spectroscopy and solid state MAS NMR can provide valuable information about clusters in zeolites, specific information relating to cluster nuclearity and charge is in general not available without recourse to theoretical calculations. To date few, if any, ionic clusters have been positively identified in this way.

2 Alkali Metal Clusters 2.1 Trapped Electrons

The story of ionic clusters in zeolites begins in earnest in 1965, when Kasai [5] subjected dehydrated sodium zeolite Y (FAU) to X- and g-rays under vacuum. ESR measurements showed that the resulting pink compounds contained paramagnetic clusters: both the number of lines (2nI + 1 = 13) and the intensity pattern of the observed spectrum were in excellent agreement with that expected from a system of n = 4 equivalent nuclei with nuclear spin I = 3/2 (Fig. 2a). In Na-Y this must mean that the spectrum was due to an electron trapped amongst four equivalent sodium cations, i.e., Na3+ 4 (Fig. 2b). The following year Rabo et al. [6] showed that the same species was formed when dehydrated Na-Y was exposed to sodium vapour at 580°C. The product on this occasion (denoted Na/Na-Y) was bright red indicating a much higher concentration of Na43+ clusters, but, prepared under the same conditions, Na/Na-X (FAU) was found to exhibit a blue colour attributed to Na5+ 6 clusters. Later, Hodgson et al. [7], and Barrer and Cole [8], used UV light and sodium vapour, respectively, to produce Na3+ 4 in the sodalite structure (SOD). Because of their bright colours and paramagnetism, and the initial discovery 3+ 5+ of Na3+ 4 through radiation experiments, Na4 and Na6 were initially described as “paramagnetic centres” or “colour centres” (F-centres). The term “cluster” was first applied some time later by Barrer [9], and gained currency when the reaction of alkali metals with zeolites began to attract renewed attention in the early 1980s. Much of this work was inspired by the idea that size-restricted metal particles, capable of displaying quantum size effects, might be formed within the cages of zeolites [10, 11]. Although, as we shall see (Sect. 2.4), this expectation was not fulfilled, the research that was initiated led to the discovery of a rich variety of cationic cluster species. In 1984, Edwards and co-workers [10, 11] reported the formation of Na43+ in both zeolites Y and A (LTA), and of the potassium analogue K43+ in potassiumexchanged zeolite Y. Using reaction temperatures of 150–350°C, they found that

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a

b Fig. 2a,b. Room temperature ESR spectrum a of Na43+, b in the sodalite cage of zeolite Y (FAU)

Na43+ clusters were formed when Na-Y was exposed to either sodium or potassium vapour, and blue compounds containing K43+ clusters when K-Y was exposed to either vapour. Rb43+ was not observed with Rb-Y under comparable conditions. Thus, the formation of Na43+ and K43+ was found to be independent of the nature of the incoming alkali atom, but dependent on the nature of the host zeolite, implying that the following reaction schemes are appropriate: M0 + 4 Na+[zeolite] Æ M+[zeolite] + Na43+[zeolite]

(3)

+ M0 + 4 K[zeolite] Æ M+[zeolite] + K3+ 4 [zeolite]

(4)

and where M0 is the incoming alkali atom and Na+[zeolite] and K+[zeolite] are exchangeable cations already present in the host material. Reactions (3) and (4) are important as they emphasize that the role of the incoming atoms is confined, at least initially, to providing electrons which can be trapped by groups of zeolite cations. We have already noted (Sect. 1.1) that this ionization process appears to be a prerequisite for the reaction with elemental

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metals to occur. The formation of Mn(n–1)+ clusters may therefore be described by the general reaction: n M+[zeolite] + e– Æ Mn(n–1)+ [zeolite]

(5)

where the “excess” electron may originate from any convenient source. This may include the spontaneous ionization of donor alkali metal atoms, or as first demonstrated by Kasai [5], ionization of the zeolite framework itself through exposure to high-energy ionizing radiation. In the latter case, electromagnetic radiation with wavelength less than ca. 250 nm is required: detectable yields of Na43+, for example, may be achieved through exposure of dehydrated Na-Y in a sealed quartz ampoule to direct sunlight. A third possibility is direct bombardment of the zeolite with electrons whose energy exceeds ca. 6 eV [12]. At relatively low concentrations, the presence of paramagnetic single electron traps such as Na43+ is easily detected through their characteristic “fingerprint” ESR spectra. In the last 10 years, the number of Mn(n–1)+ clusters identified in this way has risen to seven (M = Na, n = 2 – 6; M = K, n = 3, 4), each exhibiting a spectrum consistent with a single electron shared among n equivalent cations. Further work on the Na/Na-X system, for instance, by Kasai and Bishop [13], produced spectra with 16–19 lines, whose intensities did not conform to the expected binomial distribution for a Na65+ cluster. The problem was eventually resolved by Anderson and Edwards [14], who prepared a range of purple compounds, whose spectra could be simulated by assuming that almost equal amounts of 5+ Na4+ 5 and Na6 clusters were present. The formation of higher nuclearity clusters in Na-X seems reasonable as the higher aluminium content of zeolite X results in that zeolite typically containing up to 50% more exchangeable cations than zeolite Y. This logic apparently does not apply in the case of potassium, however, for although K43+ is observed in K-Y, the only cluster to be identified in the brilliant blue compound K/K-X is K32+ [15, 16], which was also found in K/K-A [15]. Despite the identification of a range of sodium and potassium clusters by ESR, to date the technique has uncovered no lithium, rubidium or caesium analogues, nor any mixed alkali species. Possible reasons for this have been suggested by Edwards et al. [10] and by Blake and Stucky [17]. The apparent reluctance of lithium to participate in cluster formation was exploited by Anderson et al. [18], who lowered through lithium ion exchange the number of sodium cations available in zeolites X and Y, and produced pink and purple solids containing Na2+ 3 . Since the residual charge on a lithium ion in dehydrated zeolite Y, for example, has been calculated to be less than one sixth of that on a sodium ion [19], the preference for excess electrons to be trapped at groups of sodium ions is not surprising. Another route to low nuclearity sodium clusters was taken by Kuranova [20], who found that g-irradiation of Na-A at 77 K produced a 7-line ESR spectrum attributable to Na+2 . In a series of similar experiments, Liu and Thomas [21, 22] demonstrated that in zeolite A the cluster formed depends on the temperature, with Na2+ 3 favoured in sodalite, at 196 K. Na+2 was observed in partly lithium-exchanged Na-X, Na2+ 3 2+ in K-X [22], but only Na 3+ in Na-Y was found to in Na-X and Na-Y, and K Na3+ 4 3 4 persist at room temperature. These results illustrate a general observation that clusters formed through exposure to high-energy radiation or electron bom-

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bardment tend to be less stable than those prepared through reaction with metal vapour [6], presumably because of electron–hole recombination. The same applies to clusters generated through the photoionization of arene donor molecules introduced to the pores of zeolites X and Y, where recombination is reported to be even quicker [23]. 2.2 Chemical Synthesis

As described in the previous section, most of the Mn(n–1)+ clusters known can be prepared through exposure of appropriate zeolites to ionizing radiation, but the primary method of alkali metal cluster formation remains direct reaction with the metals themselves. Perhaps the most straightforward procedure, developed by Edwards and co-workers [11, 24], involves the reaction of the dehydrated zeolite with a controlled amount of alkali metal in sealed, evacuated quartz reaction tubes (Fig. 3). A suitable amount of zeolite (typically 1–2 g) is placed in a reaction tube, heated gradually to 450°C, and evacuated overnight to better than 10–5 mbar. With the greaseless tap between reaction tube and vacuum line then closed, the tube is taken into an argon glove box where high purity metal, previously distilled into calibrated capillary tubes, is introduced. The reaction tube is then returned to the vacuum line to be evacuated and sealed with a gas torch. At no stage does the metal come into contact with the atmosphere. When the sealed tube is heated, the alkali metal vapour fills the reaction chamber and spontaneous colouration of the zeolite occurs. Careful annealing results in homogeneous, deeply coloured solids. Caution must be exercised when contemplating reactions involving lithium metal, which can react vigorously with silicate glasses above its melting point (180°C). Although stable indefinitely under vacuum or inert atmosphere, alkali metal clusters in zeolites are nevertheless air- and moisture-sensitive. This inevitably poses problems with regard to characterization. Samples prepared as described above can be returned to the inert atmosphere of a glove box and loaded into airtight cells. Alternatively, the reaction tubes themselves may be designed with sealable side arms appropriate for structural or spectroscopic studies (Fig. 3). A number of groups have found it more convenient to use Pyrex instead of

Fig. 3. Experimental set-up for the preparation of alkali metal clusters in zeolites

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quartz, or design reusable cells for the preparation and measurement of their samples. Examples include those devised by Xu and Kevan [16] and by Srdanov et al. [25]. Heo and Seff [26, 27] developed a procedure for the reaction of dehydrated zeolite single crystals with alkali metal vapour. A feature of this work was the exploitation of combined ion exchange/inclusion reactions, in which the zeolite exchangeable cations are reduced and replaced by those of a more electropositive metal: + – Æ y M¢ 0 + x M +[zeolite] + (x – y) e [zeolite] x M 0 + y M¢[zeolite]

(6)

where x ≥ y. This kind of “zeolite alchemy” is most frequently observed when the reaction system contains an excess of alkali metal. As an alternative to sodium metal, Srdanov et al. [25] used a special sodium dispenser with sodium chromate as a sodium source. Other workers have chosen to use azides as a metal source. Alkali metal azides, which decompose readily to the metal and nitrogen gas, can be introduced into the zeolite from alcoholic solutions, mixed directly with the zeolite powder, or kept entirely separate in another part of the apparatus [16, 28–30]. The relatively high temperatures (350–400 °C) required to decompose the precursor compounds, however, and suggestions of incomplete decomposition [31, 32], can be regarded as a potential disadvantage of using azides as a source of metal. For, although early workers used temperatures of up to 580°C [6, 8], the recognition has gradually grown that in the reproducible production of high-quality samples, long and slow are often best [11, 24]. The limit to the use of low temperatures is defined by the need to have sufficient vapour pressure for the reaction to occur and by the requirement that the rate of diffusion within the zeolite is sufficiently high to produce homogeneous samples in a reasonable time (typically a few days). For sodium it is rarely necessary to exceed 250°C, and reactions with the heavier metals may be successfully carried out at temperatures lower still. One respect in which the use of azides has been regarded as advantageous, is that samples may readily be produced in apparatus suitable for catalytic studies [28, 29]. The desirability of producing samples in a form convenient for further chemistry has also led to the preparation of alkali metal clusters by a number of different chemical means. Yoon and Kochi [33] produced Na3+ 4 through reduction of the sodium ions in Na-X and Na-Y (FAU) with n-butyllithium in hexane, but the preparation of other clusters requires stronger reducing agents such as solvated electron solutions in liquid ammonia [34], or more conveniently in primary amines [18]. The reduction of Na-X in this way results in a purple compo4+ 5+ 3+ und, [18, 33] found by ESR to contain Na3+ 4 , instead of Na5 and Na6 ; Na4 is also detected on irradiation of Na-X [21], highlighting the important point that different methods of cluster preparation can result in different cluster products. For reasons of charge balance, chemical reduction of the zeolite exchangeable cations necessarily introduces additional cationic species into the zeolite structure. When the reducing agent is alkali metal vapour, additional metal cations must be incorporated, and this may account for the stabilization of higher nuclearity clusters. Although Park et al. [35] reported the incorporation of up to 8 sodium or 20 potassium atoms per unit cell into Na-Y by stirring metal and zeolite together in organic solvents such as tetrahydrofuran, these values repre-

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sent only a fraction of the amount of metal that may be introduced through reaction in the vapour phase [24]. In spite of the variety of methods now available for the synthesis of alkali metal clusters in zeolites, the range of zeolites in which clusters have been identified remains small. On the basis of diffuse reflectance spectra, Liu and Thomas + 2+ [36] reported the presence of K2+ 3 in potassium zeolite L (LTL), and Na2 and Na3 in both sodium mordenite (MOR) and the clay laponite, after irradiation with far-UV light. Ikemoto et al. [37] used microoptical spectroscopy to examine the adsorption of potassium in potassium mordenite. With regard to frameworks other than aluminosilicates, Na43+ has been reported in aluminogermanate and gallosilicate sodalites (SOD) [38, 39] and in gallosilicate zeolite Y (FAU) [36]. 2.3 Structure and Location of Mnp+

Despite the ability of ESR spectroscopy to identify unambiguously the presence and nuclearity of Mn(n–1)+ clusters, the technique is not ideally suited to providing information about the structure and location of clusters in zeolites. Early evidence for the tetrahedral arrangement of the sodium ions in Na43+ was furnished by the Raman spectroscopic studies of Sen et al. [40], but the location of this and other alkali metal clusters has been the subject of some debate. On the basis of quenching studies with O2 and probe molecules such as toluene, Thomas and co-workers [22, 23] suggested that in zeolite A (LTA), Na2+ and Na3+ 4 are found in the sodalite cage (b-cage) and Na2+ 3 in the larger a-cage, whereas in zeolites X and Y (FAU), Na43+ resides in the supercage, as originally proposed by Kasai [5]. The observation of fingerprint ESR spectra only in zeolites containing the sodalite cage structural unit, however, provides circumstantial evidence that the clusters are located there. That the ESR spectrum of Na43+ in zeolites A and Y is essentially the same as in sodalite (SOD), has been regarded as strong evidence of a sodalite cage location [10, 11], and a variety of arguments have been presented to suggest that other clusters are also located there [14, 16, 41]. Crystallographic studies of the systems discussed certainly reveal a general tendency for alkali metal cations to cluster within the sodalite cage. Through single crystal XRD studies, Sun and Seff [42–44] located tetrahedral K4p+ clusters in the sodalite cages of zeolites A and X (Fig. 4a). This is interesting because, to date, K43+ has only been found by ESR in zeolite Y. In both cases incomplete occupancy of the sodalite cage sites suggests that some may instead contain triangular K3q+ units identifiable with the K2+ 3 clusters [44] observed by ESR. Triangular K3r+ clusters have also been identified by Anderson et al. [45] in K/K-L (LTL), which does not contain sodalite cages, and an alternative linear K3s+ cluster has been proposed in zeolite X (see Fig. 4a) [44]. Almost exactly 30 years after the initial discovery of Na43+, Armstrong et al. [46] used a combination of Rietveld analysis of powder neutron diffraction data, and ESR and magnetic susceptibility measurements, to locate the tetrahedral clusters in the sodalite cage of Na-Y (see Fig. 2b). Seff and co-workers [47, 48] have also put forward candidates for 2+ Na4+ 5 and Na3 clusters in zeolite X, though in both cases ESR equivalence of the sodium ions could only be achieved through motional averaging. NMR studies

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b

a

c

Fig. 4. a K4p+ (lower) and K3q+ (upper) clusters in zeolite X (FAU); clusters in neighbouring sodalite cages may be linked though the intervening hexagonal prisms to form a “continuum” (see Sect. 2.4); alternatively, the linking atoms (dark) may be regarded as linear K3r+ clusters. b Rb8p+ cluster in zeolite A (LTA) centred on the sodalite cage; clusters of lower nuclearity may be formed if some or all of the outer (dark), and/or one of the inner (light), atoms are absent. c Linear Cs4p+ cluster in zeolite A centred on the sodalite cage; smaller clusters may result if either or both of the terminal atoms are absent

[49] provide some support for the alternative formulation (Na53+) proposed for the pentanuclear cluster, which shares the tetrahedral geometry and sodalite cage location of Na43+, but includes a central sodium atom. Although no rubidium or caesium clusters have been detected by ESR, Seff and co-workers [26, 27, 50–53] have found a range of Csmp+ and Rbnq+ (m = 2 – 6; n = 3 – 8) species in zeolite A, all of which are centred on the sodalite cage. These clusters, the first to be identified crystallographically, are constructed of 2 – 4 atoms within the sodalite cage, each of which may be bonded through a 6-ring to another outside (see Fig. 4b, c). These studies illustrate many of the difficulties in determining cluster nuclearities and valence states from crystallographic data, and in some cases the proposals are acknowledged as tentative. Again diffraction and spectroscopic techniques can sometimes be used in tandem to provide a clearer overall picture. The linear Cs4 unit (Fig. 4c) reported by Armstrong et al. [54] in zeolite A is similar to the Cs43+ cluster previously identified by Heo and Seff [26, 27], but in this case careful measurements of the magnetic suscep-

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tibility were consistent with the presence of diamagnetic (S = 0) Cs42+ species. The same compound was also found to contain Na3p+ groupings [54], illustrating a general tendency for metals to separate rather than form mixed species. 2.4 Cluster Crystals and the Cationic Continuum

For all but the lowest concentrations of sodium metal, the ESR spectrum of Na/Na-Y is dominated not by the fingerprint pattern of Na 3+ 4 , but by a featureless singlet line (Fig. 5). Edwards et al. [10, 11] initially suggested that this line might be attributed to ultrafine, metal particles, confined within the zeolite cages, but, in a much more detailed study, Anderson and Edwards [24] later proposed that the ESR line results from the interaction of unpaired electrons in neighbouring Na43+ clusters. Support for this picture is available from the ab initio molecular dynamics simulations of Ursenbach et al. [55], which estimate the exchange interaction of Na43+ clusters in neighbouring sodalite cages in Na-Y to be comparable to the inverse of the observed 23Na hyperfine coupling constant for Na3+ 4 . A collapse of the hyperfine structure is therefore expected

Fig. 5. Room temperature ESR spectra of Na-Y (FAU) containing 3 (a), 8 (b), 13 (c), and 32 (d) additional sodium atoms per unit cell

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a

319

b

c

d

Fig. 6a–d. Cluster crystals composed of Na43+ clusters located in the sodalite cages of different zeolite structures: a sodalite (SOD), b zeolite Y (FAU) and c zeolite A (LTA). Although the arrangement in c has not yet been synthesized, the potassium analogue is found in d, which 4+) cluster in alternate a-cages consists of a K43+ cluster in each sodalite cage and a larger (K12 (see text)

where each cluster has at least one cluster neighbour. Structural studies by Armstrong et al. [46] clearly point to a network of interacting Na43+ clusters (Fig. 6b), located in the sodalite cages, as the source of the controversial ESR line [56]. Three-dimensional arrays of clusters, whose distances from and orientations relative to each other are fixed by the zeolite host, have been described in various ways; for example, as “cluster crystals” or “supralattices” [57–59]. The formation of a cluster crystal in Na/Na-Y, comprising one Na43+ cluster in each sodalite cage, may be written: 8 Na0 + (Na+)56[Al56Si136O384]Y56– Æ (Na+)32(Na43+)8[Al56Si136O384]Y56–

(7)

where [Al56Si136O384]Y56– represents the zeolite Y framework (FAU). The presence of Na43+ clusters in several zeolite structures [5, 7, 10, 11] offers the possibility of producing a variety of cluster crystals with different geometrical arrangements (Fig. 6). The simplest of these possibilities, the body-centred cubic arrangement (Fig. 6a) in the sodalite framework (SOD), has also been realized in the compound known as “black sodalite” [8, 60]: 6– 6– Æ (Na3+ 2 Na0 + (Na+)6[Al6Si6O24]SOD 4 )2[Al6Si6O24]SOD .

(8)

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A considerably more complex situation is observed in K/K-A (LTA). Edwards and co-workers [56, 61, 62] found crystallographic, ESR and magnetic susceptibility data to be consistent with the formation of a cluster crystal containing 4+ paramagnetic K3+ 4 and diamagnetic K12 clusters (Fig. 6d): 4+ 96– 40 K0 + (K+)96[Al96Si96O384]A96– Æ (K+)56(K3+ 4 )8(K12 )4[Al96Si96O384]A .

(9)

K43+

resides in each sodalite cage in this model, the larger Whilst a tetrahedral 4+ clusters occupy alternate a-cages in a perfectly ordered superlattice. The K12 remaining a-cages contain potassium cations in an arrangement similar to that in dehydrated K-A. Powder neutron diffraction studies by Ikeda et al. [63] on K/K-A revealed a less well-ordered compound, which nevertheless shares a number of important features with the earlier model. These include tetrahedral clusters located within the sodalite cages and an alternation in the contents of 4+ is the first to be reported in zeoneighbouring a-cages. The 12-atom cluster K12 lite A that is not located in the sodalite cage, but recent NMR studies by Moran et al. [64] suggest that there may be other examples. Larger distorted icosohedral p+ q+ and Cs14 have been reported by Sun et al. [65, 66] in the supercages clusters Cs13 of zeolite X (FAU). In this case, because of the short contact distances between clusters in neighbouring cages, the authors viewed the clusters as subunits of a three-dimensional “cationic continuum” of partly reduced caesium cations with a unit cell formula Cs86+ 122 . Sun and Seff [42] have also reported a similar threedimensional continuum in K-X, and a one-dimensional equivalent comprising q+ Kp+ 4 (or K3 ) units located within the sodalite cages linked through intervening potassium atoms (see Fig. 4a). Although to date there are no clear examples of mixed alkali metal clusters in zeolites, the knowledge that the featureless singlet ESR lines often observed from alkali metals in zeolites derive from interacting clusters rather than metal particles [67] provides evidence that such clusters may exist. Blatter et al. [68], for instance, reported ESR signals with a range of g-values characteristic of electrons interacting with both sodium and caesium nuclei. 2.5 Properties and Applications

Alkali metal clusters in zeolites first attracted the attention of researchers through the bright colours observed on exposure of dehydrated zeolites and sodalites to electromagnetic radiation [5, 7]. This photochromism, and the ability to undergo similar changes when exposed to high-energy electrons (cathodochromism), were initially the focus of considerable research effort, directed towards applications in cathode ray tube (CRT) display screens and read–write devices [69]. Work in this area concentrated mainly on halosodalites (SOD) whose cages each contain a halide anion at their centre (Sect. 5.2). In this case Na3+ 4 clusters can be formed only in “defect cages” whose halide anions are absent, but the fact that the surrounding cages are filled renders the colour centres much less sensitive to air and moisture. The reactivity of alkali metal clusters was also apparent from the outset when Rabo et al. [6] noted the reversible reaction of Na43+ with O2 to form adsorbed

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O2– ions in Na-Y. Subsequently, the interaction of various clusters in zeolites X or Y (FAU) with SO2 , N2 ,Ar, Kr, CO, CO2 , NH3 , benzene, n-hexane and several organic halides has been examined [70, 71]. In spite of this reactivity, however, the potential of zeolites containing alkali metal clusters, either as solid base catalysts, or as selective reducing agents, is yet to be fully explored. Martens et al. [29] examined the activity of sodium clusters in zeolites A (LTA), X and Y in the geometrical and double-bond isomerization of cis-2-butene, and the hydrogenation of cis-2-butene, acetylene and benzene. Later Hannus et al. [32] studied the isomerization of 1-butene in a range of sodium-containing zeolite Y samples. Both groups reported activity and product distributions characteristic of strong base catalysts, and Hannus et al. [32] inferred the probable formation of a carbanion intermediate during the isomerization of allyl cyanide. Recently, Simon et al. [72] have shown that Na5+ 6 clusters promote the isomerization of cyclopropane to propene in Na-X. The ordered cluster crystals described in Sect. 2.4 fall into the category of zeolite-based nanostructured materials (Sect. 1). The reaction of alkali metals with zeolites affords a unique opportunity to examine the evolution of optical, electronic and magnetic properties as the excess electron density is varied, continuously and precisely, over a wide range, within tightly confined dimensions, and without significant structural change in the framework [24, 25, 45, 56, 73–75]. These studies are of enormous theoretical and practical importance as they underpin attempts to design tailor-made solids with specific properties [1, 45, 59, 76]. In addition, it is now known that dehydrated Na-X will react reversibly with up to 100 extra sodium atoms per unit cell and no apparent loss in crystallinity. The concentration of excess electrons in this compound falls well within the range 1021 –1022 per cm3 at which Anderson and Edwards [24], using the Mott criterion [77], estimated that a transition to metallic behaviour might occur. The ability to achieve such high concentrations of excess electrons led Edwards et al. [73] to examine the possibility of a matrix-bound insulator–metal transition, and the synthesis of a conducting, metallic zeolite. This is a particularly attractive proposition in the case of zeolites with a one-dimensional channel structure as it raises the possibility of electron delocalization along a single crystallographic direction – in effect the chemical synthesis of a quantum wire array (Fig. 7a) [1, 45, 76, 78]. In 1997 Anderson et al. [45] demonstrated, through microwave cavity loss measurements, that certain zeolites do indeed exhibit an increasing electronic conductivity as more and more excess electrons are introduced into the host structure. The incorporation of potassium into dehydrated potassium zeolite L (LTL) was found to result in an increase in the room temperature conductivity by around a factor of 10,000, to produce the most conducting zeolite-based material known. The strong temperature dependence of the measured conductivities, however, is characteristic of a thermally activated conduction mecha+ nism, thought to involve a K2+ 3 /K3 redox hopping process (Fig. 7b). Indications of metallic behaviour in K/K-A were reported by Terasaki and co-workers [74, 75], who used an itinerant electron model to interpret both the optical properties and observed ferromagnetism [58], but later concluded that the compounds were likely to be ferrimagnetic insulators [37]. The exact details of the

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a

b Fig. 7. a Representation of a quantum wire array based on the one-dimensional channels of zeolite L (LTL). b Randomly oriented K3p+ clusters thought to mediate charge transport along the channels in K/K-L

magnetic behaviour of this compound are still attracting close scrutiny [79], and the observed magnetic behaviour appears to be related to superlattice formation [80]. Similar magnetic properties, extremely rare in compounds not containing a conventional magnetic element, were also found in Rb/Rb-A but not in Na/NaA or K/K-X [81]. In a series of papers modelling the optical properties of Na43+ clusters in sodalite (SOD), Metiu and co-workers [25, 60, 82] suggested that changes in the absorption spectrum at high concentrations of Na43+ might indicate the onset of an insulator–metal transition, but recent work points to the saturated compound (black sodalite) being an antiferromagnetic insulator [83–87]. The observation of antiferromagnetism in sodalite is in keeping with magnetic susceptibility studies on a variety of systems indicating that substantial electron spin pairing is a generic phenomenon in alkali metal zeolites containing medium to high concentrations of excess electrons [88]. Similar behaviour is observed in a number of other chemical systems known to exhibit a composition-dependent insulator–metal transition [73].

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2.6 Beyond the Cluster Model

The question of the nature of alkali metal clusters in zeolites was first examined by Anderson and Edwards [14], who compared the hyperfine splitting of a series of zeolite-based Nan(n–1)+ clusters with a variety of neutral and charged sodium and silver clusters (Fig. 8), and concluded that the single valence electron in Nan(n–1)+ remains rather loosely associated with the cations. The implication of this work was that clusters such as Na43+ are best regarded not as tightly bound molecular clusters trapped within the zeolite host but, in accordance with the original model of Kasai [5], as trapped electrons akin to F-centres in crystalline salts. These observations, along with attempts to model the optical properties of Na43+ in sodalite [17, 82, 83, 89], point to a “hollow” geometry for many smaller (n ≤ 6) alkali metal clusters in zeolites, with a number of cations coordinated to the walls of a zeolite cage and one (or more) electrons occupying the cavity 4+, but, space. This arrangement may also occur in some larger clusters such as K12 in general, these would appear likely to have an increased tendency to contain atoms at their centre [56, 62, 65, 66]. As we have seen (Sect. 1.1), the reaction between elemental metals and zeolites can be viewed as a dissolution process, where the dehydrated zeolite takes the role of a polar solid solvent, with the incoming metal atom as the solute. This process has been found to bear many elements of similarity, both conceptually

Fig. 8. Percentage atomic character, deduced from a comparison of cluster hyperfine splittings with those of the appropriate free atom (sodium or silver), as a function of the number of atoms (n) for different types of clusters: Nan(n–1)+ in Na-X (filled squares), Nan in solid argon (filled circles), Agn+ in zeolite A (open circles) and Agn(n–1)+ in frozen alcohols (open squares)

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and phenomenologically, to the dissolution of alkali metals in non-aqueous liquid solvents [55, 56, 73, 83]: M0 + 4 Na+[zeolite] Æ M+[zeolite] + (Na+)4 e–[zeolite] ,

(10)

M0 + x NH3 Æ M+[solv] + (NH3)x e– .

(11)

Here reactions (10) and (11) highlight the comparison that can be drawn between an electron trapped in a cavity defined by a tetrahedron of sodium cations in the sodalite cage, and an electron trapped within solvent cavities in liquid ammonia. Furthermore, the ability to prepare trapped electrons in zeolites, in stoichiometric or near stoichiometric amounts, has led to the description “inorganic electrides” [88, 90]. Together these ideas have been found to constitute a remarkably robust conceptual framework for rationalizing the properties of a wide range of metal–zeolite compounds [88], whose main strength lies in its ability to do so without recourse to detailed structural information, which is available in only a few cases. Thus, for alkali metal clusters in zeolites at least, the question arises as to what extent the observed species are best described as clusters. Woodall et al. [56] have published a detailed critique of the applicability of the cluster model to alkali metals in zeolites. Although it is clear that the clustering phenomenon observed for alkali metals in zeolites is dictated at least as much by the zeolite framework and the collective coordination chemistry of the zeolite cations as by the strength of bonding between metal atoms, there are still strong pragmatic grounds for considering these compounds within the cluster model. There is no evidence from either experimental or theoretical studies to suggest that the excess electrons, released when metal atoms enter the zeolite and held largely responsible for the observed optical, electronic and magnetic properties of the products, interact to any great extent with the aluminosilicate framework. Confined in close proximity in a zeolite cage, the exchangeable cations, with which the excess electrons are associated, are geometrically predisposed to encourage polaronic trapping, and this has been found to contribute substantially to the stabilization of clusters such as Na43+ [55]. Even if an alternative description may sometimes seem more appropriate, almost all workers have found it convenient to discuss their compounds in terms of alkali metal clusters confined within the various cages. It is perhaps only by viewing these materials as ordered arrays of closely packed, interacting clusters that we can hope to model the essential heterogeneity at the nanoscale level that is a vital determinant of their overall electronic and magnetic properties.

3 Silver Clusters 3.1 Synthesis

A spectacular sequence of colour changes – white (hydrated), to yellow, to orange, to brick red – on dehydration/rehydration of silver-exchanged zeolite A

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a

b

Fig. 9a, b. Ag6 clusters in zeolite A (LTA): a in the sodalite cage coordinated to up to eight Ag+ 8+); b in the a-cage coordinated to up to 14 Rb+ or Cs+ ions (dark). The atomic positions (Ag14 in a have also been interpreted (see text) as linear Ag32+ clusters (dark atoms)

(LTA) was first reported by Rálek et al. [91] in 1962. In 1977 Kim and Seff [92] observed similar colours in single crystals of Ag-A, dehydrated at 400°C under carefully controlled conditions, in which the presence of reduced octahedral Ag60 clusters was detected through XRD studies (see Fig. 9). Subsequently, Jacobs et al. [93] studied the mechanism for the “autoreduction” of Ag+ in completely (Ag-A) and partly (AgNa-A) silver-exchanged Na-A, but concluded that the reduced species was a linear Ag32+ cluster, which they proposed was responsible for the intense colouration of the samples. The reduction process was found to take place in two stages: below ca. 250°C, when residual water may play a role as suggested by Kim and Seff [94]; and at higher temperature under anhydrous conditions. The overall stoichiometry of this second process, considered to be the principal reduction mechanism [93, 95], may be written as follows: 0 1 2Ag+ + O2– [zeolite] Æ 2Ag + /2 O2 + [ ][zeolite]

(12)

where [ ] represents an oxygen vacancy in the zeolite framework (Lewis acid site). In the absence of coordinating water molecules, isolated Ag0 atoms (paramagnetic and therefore in theory detectable through ESR) are not observed, and these are therefore presumed to associate with other silver atoms or cations to form neutral or charged silver clusters [96]. Because of the high selectivity of zeolites for Ag+ ions [97], the introduction of silver through ion exchange is often virtually stoichiometric, and it is comparatively easy to prepare samples either fully exchanged or containing controlled amounts of silver. The autoreduction process thus provides a straightforward route for the preparation of silver clusters in a range of zeolites [92–94, 98–100].A complicating factor is the well-known photosensitivity of silver zeolites (a process also thought to involve cluster formation [101]), which has made many workers store and handle their materials in the dark. The reduction of Ag+ in zeolites can also be achieved in other ways. Beyer et al. [102–104] studied the consumption of hydrogen gas in zeolite Y (FAU), mordenite (MOR), and chabazite (CHA), and concluded that

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charged cluster species were likely to be present. More recently, Seff and coworkers [50, 52] used alkali metals to reduce silver ions in zeolite A. Agm0 and Agnp+ clusters are only detectable by ESR if they are paramagnetic, i.e., if m or (n–p) is odd.Although the Ag2+ 3 clusters proposed by Jacobs et al. [93] fall into this category, Texter et al. [95, 105] demonstrated through magnetic susceptibility measurements that the predominant species present in Ag-A after autoreduction are in fact diamagnetic. This observation was rationalized on the basis that the removal of an oxygen atom from the zeolite framework, as represented in Eq. (12), is necessarily a two-electron process. Nevertheless, paramagnetic Ag6p+ clusters were detected by Hermerschmidt and Haul [106] on exposure of dehydrated Ag-A to hydrogen at temperatures as low as –40°C. The observed 7-line ESR spectrum, characteristic of an unpaired electron interacting with six equivalent silver (I = 1/2) nuclei, is consistent with a cluster charge p = 1, 3 or 5; the actual value must be deduced from other results. As this cluster is apparently formed on further reduction of dehydrated Ag-A, we may conclude that any Ag6 species formed through autoreduction in this case is unlikely to have been neutral. Later, Grobet and Schoonheydt [107] observed both Ag6p+ and Ag8q+ in similarly treated samples of Ag-A and AgNa-A. Both groups reported that the kinetics of cluster formation under these conditions are extremely fast: reaction at elevated temperatures, or even prolonged exposure to H2 at room temperature, resulted in further reduction to silver metal particles. In contrast, the room temperature reduction of silver ions in zeolite rho (RHO) has been found to proceed more slowly, producing paramagnetic Ag4p+ clusters [108]. A second approach to the generation of paramagnetic silver clusters was identified by Narayana and Kevan [109], who observed ESR spectra assigned to species (n = 2 – 4) on exposure of hydrated A, X and Y zeolites to g-rays Ag(n–1)+ n at 77 K or X-rays at 4 K. Paramagnetic clusters with nuclearity less than 6 were also detected by Morton and Preston [110] after g-irradiation of carefully dehydrated Ag-A at 77 K. On annealing to room temperature they found in sequence three paramagnetic clusters, formulated as Ag2+, Ag32+ and Ag+6 , with the ESR equivalence of the three silver atoms in Ag32+ suggesting a cyclic rather than a linear geometry. Michalik and Kevan [111] reported Ag6+ and bent Ag2+ 3 clusters in samples of AgNa-A after dehydration at 450°C followed by hydrogen reduction or exposure to g-rays. On the basis of these observations and diffuse + reflectance spectra, the presence of diamagnetic Ag2+ 6 or Ag3 species in the dehydrated precursors was also inferred.A third ESR signal, tentatively assigned to linear neutral Ag30 , was considered by Morton et al. [112] to be more consistent with Ag54+ clusters. ESR spectra attributed to Ag2+ and linear Ag32+ clusters were observed in AgNa-Y, but only if vacuum dehydration was followed by heating in oxygen at 500°C prior to g-irradiation [113]. The use of g-irradiation and controlled thermal annealing to produce silver clusters in zeolites and related materials was reviewed by Michalik [114]. Unusual stability was attributed to Ag43+ clusters formed in this way in dehydrated zeolite rho, which were found to persist for months at room temperature and could be observed up to 100°C. In the zeolite A analogue, AgH-SAPO-42 (LTA), dehydrated at 300°C in flowing oxygen, clusters such as Ag+2 and Ag32+ were detected in very low yield, but, in the presence of methanol, Ag p+ 4 species were

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observed [115]. Exposure of dehydrated AgH-SAPO-5 (AFI) and AgH-SAPO-11 (AEL) to methanol, ethanol or propanol prior to irradiation resulted in the stabilization of Ag2+ and Ag32+ clusters [116]. Similarly, irradiation of the three smectite clays, hectorite, montmorillonite and saponite, resulted in the production of Ag32+ and Ag43+ clusters only on replacement of the interlayer water with methanol [117]. 3.2 Structure and Location

As mentioned above (Sect. 3.1), the correct model for silver cluster formation in autoreduced Ag-A and AgNa-A (LTA) has been the subject of some controversy, with two conflicting interpretations having been placed on essentially similar crystallographic data [96].After single crystal XRD studies, Kim and Seff [92, 94] proposed that octahedral Ag60 clusters occupied up to two thirds of the sodalite cages. It was found that these neutral species were anchored in the sodalite cages 6+ or by coordination to six or eight silver cations in what might be termed Ag12 8+ Ag14 clusters (Fig. 9a). Kim and Seff, however, considered the distinction between ions and atoms to be clear, and the formulation Ag6(Ag+)n (n = 6, 8) may therefore be more appropriate. A reduced occupancy of the same two silver sites was interpreted by Gellens et al. [118, 119] in terms of linear Ag2+ 3 clusters (Fig. 9a), as proposed by Jacobs et al. [93], but attempts to distinguish between the two models on the basis of crystallographic data alone proved inconclusive [120]. Remarkable ESR spectra recorded by Morton and Preston [121], after dehydration and g-irradiation of a 109Ag isotope-enriched sample of Ag-A, exhibited hyperfine splitting by six silver nuclei, and superhyperfine splitting by up to eight more, thus confirming the cluster geometry proposed by Kim and Seff [92, 94]. A high coordination number for silver atoms in dehydrated Ag-A is also confirmed by recent EXAFS measurements [122]. Nevertheless, it remains possible that clusters of lower nuclearity are formed in the sodalite cages of zeolite A in samples that contain relatively low concentrations of Ag+ or are only lightly reduced [123]. On exposure of partly silver-exchanged zeolite A to rubidium or caesium vapour, Seff and co-workers [50, 52] again located octahedral silver clusters. This time the clusters were found in as many as 80% of the larger a-cages in zeolite A, stabilized by coordination to up to 14 Rb+ or Cs+ ions (Fig. 9b). These compounds are unusual in containing both reduced silver and, in the sodalite cages, either rubidium or caesium clusters [50, 52]. The synthesis of identical octahedral clusters in two very different zeolite cages is a good illustration of the fact that intracluster bonding is much more important in silver species than in alkali metal clusters (Sect. 2.6). The hyperfine splitting data shown in Fig. 8, from a range of paramagnetic silver species, are indicative of comparatively tightly bound molecular clusters, whose unpaired electrons remain closely associated with the Ag+ ions. The reduction of AgNa-A by hydrogen gas rather than alkali metals has been reported to result in the formation of Ag3p+ clusters and a low symmetry Ag63+ species, which might also be regarded as linear Ag30 coordinated to three silver cations, all located in the a-cages [124, 125].

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Dehydration of Ag-Y and Ag-X (FAU) has been shown to increase the number of silver atoms located in the sodalite cages, leading perhaps to the formation of linear Ag32+ clusters that join two neighbouring sodalite cages through the intervening hexagonal prism [99, 126]. The oxidation state of these silver atoms, however, remains unclear, and Butikova et al. [126] have cautioned against the use of short Ag-Ag distances as proof of reduced silver when comparable distances are observed in other univalent silver compounds. 3.3 Properties and Applications

The first potential application of zeolites containing silver clusters was noted by Rálek et al. [91], who proposed the use of red dehydrated Ag-A (LTA) as a water sensor. It is interesting to note that, almost 40 years later, the explanation for the moisture-dependent colour changes observed in Ag-A, and the role of silver clusters therein, remains a topic of current research [127]. The formation of silver clusters may also be involved in the colour changes exhibited by many silver-containing zeolite and sodalite materials in response to diverse physical and chemical stimuli. Ozin et al. [128] have documented barochromic, cathodochromic, hydrochromic, photochromic and thermochromic behaviour in a variety of silver sodalites (SOD), with potential applications in a wide range of sensing devices [129]. The interaction of octahedral silver clusters in Ag-A with a variety of molecules has been examined [112, 130–134]. In most cases the result is a complete break-up of the clusters, but in a few instances lower nuclearity clusters have been observed [112, 134]. Of particular interest is the reaction with ammonia, which was reported to result in the formation at room temperature of the unusual nitrogen hydrides, triazane (N3H5) and cyclotriazane (N3H3) [132]. Perhaps the most promising potential applications of silver zeolites rely on their photosensitivity. Silver-containing sodalites exhibiting laser-induced reversible colour changes, which are thought to result from electron transfer between two different types of tetrahedral silver clusters, may find application in high-density optical data storage [129]. In addition to the photochromic behaviour, the photoconducting, photoluminescence and photochemical properties of silver zeolites are all of considerable interest [128]. Long-lived emissions observed in silver faujasites (FAU) have been associated with a spin-forbidden phosphorescence of Ag2p+ clusters. Changes in this behaviour occasioned by the absorption of various molecules could lead to the development of a new generation of powerful and selective chemical sensors [128]. The formation of silver clusters is thought to play an important part in the self-sensitization phenomenon that accompanies the photochemical splitting of water in a number of silver zeolites [101, 135, 136]. A low frequency (bathochromic) shift in the spectral sensitivity is observed as the photoinduced reduction of silver ions produces new cluster chromophores that absorb at longer wavelengths. Beer et al. [136] have discussed the possible use of this process in a photochemical energy storage system. A similar self-sensitization is observed in the photochemical production of Cl2 from suspensions of silver zeolites in solutions containing Cl–

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ions [137]. Charged silver clusters have also been reported to play a crucial role in the photodimerization of alkanes at room temperature in Ag-A [100, 138], which may prove useful in natural gas conversion.

4 Other Metals 4.1 Elemental Clusters

When Barrer and Whiteman [4] studied the sorption of mercury in mercuryexchanged zeolite X (FAU) they attributed the observed “copious” uptake to the following processes: 2+ Hg0 + Hg2+ [zeolite] Æ Hg2 [zeolite],

(13)

2+ xHg0 + Hg2+ 2 [zeolite] Æ Hg (x+2)[zeolite] .

(14)

Although no further studies of cationic mercury clusters have been reported, evidence for the formation of cadmium and zinc analogues has been presented. Structural and spectroscopic studies of dehydrated Cd-A (LTA) after reaction with cadmium vapour suggest that various clusters such as Cdn2+ (n = 2 – 4) and Cd54+ may be present [139–141]. Boddenberg and co-workers [142–144] have examined the reaction of cadmium and zinc vapour with H-Y (FAU), and con2+ cluded that Cd2+ 2 or Zn2 polycations are produced. Reports of ionic clusters involving other metals in zeolites remain relatively scarce. Single crystal XRD studies by Heo et al. [145, 146] have located indium clusters, formulated as In58+ and In2+ 3 , in zeolite A, and a trapped electron cluster has been observed by ESR following the inclusion of anthracene in copperexchanged zeolite Y [147]. Although many authors have on occasion referred to groupings of monovalent cations as Mnn+ clusters, as in general these “clusters” have no valence electrons, this represents no more than a convenient notation. A notable exception was reported recently by Latturner et al. [148], who suggested that weakly bound Tl 3+ 3 clusters in dehydrated thallium sodalite (SOD) could be partly responsible for the comparatively small lattice parameter observed. 4.2 Alloy Clusters

Until recent work by Zhen et al. [149, 150], there has been little evidence for the formation of mixed metal clusters in zeolites. Through single crystal XRD, they studied the reaction of zinc vapour with Cd-X and Tl-X (FAU) and identified a unique range of possible cadmium–zinc and thallium–zinc clusters. Some evidence has also been presented that the reaction of various alkali metals with zeolites containing the cations of a different metal may result in the formation of mixed metal clusters [68, 71, 151], but, for the most part, these products remain poorly characterized.

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5 Non-metal Clusters 5.1 Hydrolysis Products and Related Species

In the familiar model of zeolite ion exchange chemistry, the charge of the anionic aluminosilicate framework is balanced by hydrated cations located within the zeolite pores, which may be replaced when the zeolite is treated with an aqueous solution of another cation. Experience of ion exchange, however, with all but a few, mainly monovalent, cations (see Chap. 1), leads quickly to an awareness that it is possible for many more complex and unusual chemical species to fulfill the charge-balancing role. It is well established that the heat treatment of zeolites containing divalent and trivalent metal cations can produce clusters of the form [MlOm(OH)n]p+, in whose formation cation hydrolysis plays a crucial role [3]. It is likely that similar species play a part in the introduction to zeolites through solid-state ion exchange of, for example, transition metals in high oxidation states [152]. The broader subject of oxide clusters is considered in detail elsewhere in this volume (Chap. 6), but a few examples of ionic clusters are included here for illustration. The calcination of lanthanum-exchanged zeolite Y (FAU) is known to result in an efficient acid catalyst that is widely used in petroleum refining. The high thermal stability of this catalyst has been attributed to the formation of charged clusters within the sodalite cages, consisting of two or more lanthanum cations connected by bridging hydroxide anions [3, 153]. Although it is likely that the catalytic activity of these materials depends critically on the exact nature of the lanthanum species formed [154], this remains to be firmly established [155]. On the basis of single crystal XRD work, however, Park and Seff [156] reported a number of different species in the sodalite and supercages of dehydrated, partly dehydrated, and hydrated lanthanum-exchanged zeolite X (FAU). In each sodalite cage of La-X vacuum dehydrated at 400°C, a distorted cube comprising interpenetrating tetrahedra of lanthanum and oxygen atoms was located (Fig. 10a). It is thought that these La4O44+ clusters are linked through bridging oxygens to form a neutral tetrahedral network of formula La2O3 . Interestingly, “La4O4” cubes were also present in the partly (vacuum at 22°C) dehydrated compound, but, in this case, the oxygen atoms were thought to represent hydroxide ions. A very similar chemistry has previously been observed for lead in both zeolites A (LTA) and X [157–161], where equivalent “Pb4O4” cubes can be found coordinated to further lead ions resulting in larger clusters such as Pb8O4p+, which are thought to contain both Pb2+ and Pb4+ ions [160, 161]. Although the same arrangement of lead and oxygen sites is found in the sodalite cages of lead-exchanged sodalite (SOD), in this case charge balance demands that the principal species is a dimer of empirical formula Pb2(OH)(H2O)33+, an assignment consistent both with fractional occupancies and spectroscopic measurements [162]. Pseudo-cubic clusters of the formula Ca4(OH)44+ do, however, occupy the sodalite cages of the rare aluminosilicate mineral bicchulite (SOD) [163]. A number of gallium-containing analogues of

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a

b

c

Zn9Onp+

Fig. 10a–c. Proposed La4O44+ (a), (b) and M4Xp+ (c) clusters located in the sodalite cages of zeolite X (FAU), zeolite A (LTA) and sodalite (SOD), respectively. Anion sites (light) are partly occupied in b

this material have also been synthesized using high-pressure hydrothermal conditions [164]. Recent work suggests that a variety of different geometries are possible for oxide clusters in the sodalite cages of zeolites A, X and Y. Fe6On clusters with an octahedral arrangement of iron atoms have been reported [165] in zeolite Y after adsorption of Fe(CO)5 followed by oxygen treatment at temperatures up to 85°C. In contrast, centred cubic Zn9Onp+ clusters (Fig. 10b) may be formed on exposure of dehydrated Zn-A to zinc vapour [166]. These clusters are thought to be oxygen-deficient and may exhibit some direct metal–metal bonding. Lee et al. [167] have reported the formation of linear [(HOPd)2O]4+ units each bridging two neighbouring sodalite cages through a connecting hexagonal prism. Under certain conditions clusters containing bridging water molecules, or even oxonium ions, may also be formed [156, 157, 168, 169]. Examples include tetrahedral hydrogen-bonded clusters of the form (H3O+)2H2O(Mp+)2 reported to occupy the sodalite cages of zeolite X [168, 169]. That clusters of identical form are apparently found for ions as diverse as Na+ and Co2+ suggests that analogous species containing other cations may also occur. Larger linked sodium–water structures have also been reported in partly dehydrated zeolites X and Y [170]. 5.2 Occluded Salts and Other Compound Clusters

The idea of incorporating extra cations and anions into the intracrystalline pore space of zeolites is one for which Nature herself holds the copyright. Taken together these ions may be regarded as common inorganic salts, trapped or “occluded” within the crystal structure, even though in general the “salt” cations are indistinguishable from those that balance the framework charge of the zeolite. Occluded salts are a feature of a number of feldspathoid minerals, including those having the sodalite (SOD) and cancrinite (CAN) structures. Attempts to synthesize analogous materials in the laboratory date back to the work of Barrer and Meier [171] in the 1950s. These authors studied the inclusion of

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silver nitrate in Na-A (LTA) and Na-X (FAU). An informative review and discussion of subsequent early work in this area was given by Rabo [172]. Broadly, two different synthetic approaches may be discerned: to incorporate the occluded material into the framework synthesis, or to introduce material into a previously prepared framework. Both routes can be employed either in an aqueous medium or at higher temperatures in solid-state reactions. The former has the considerable advantage that the presence of salts in a synthesis gel, for example, can promote the formation and stabilization of certain frameworks, but the latter has proved more suited to the incorporation of larger amounts of material into open pore frameworks [171–174]. An enormous range of natural and synthetic compounds based on the sodalite structure are now known, the majority of which conform to the general formula Mm[(TO2)12]Xn and have sodalite cages whose contents may be described as tetrahedral M4Xp+ clusters (Fig. 10c). Examples of species encountered in clusters of this type include: M = Na+, Li+, K+, Ag+, Tl+, Mg2+, Ca2+, Sr2+, Ba2+, Zn2+; and X = Cl–, Br–, I–, OH–, ClO2– , ClO3– , ClO4– , BrO3– , NO2– , NO3– , CN–, SCN–, HCOO–, CH3COO–, B(OH)4–, O2–, S2–, Se2–, Te2–, SO42–, CO32–, C2O42–, MnO4–, CrO42–, MoO42–. A variety of compounds containing a mixture of different M or X ions can be prepared. The TO2 sodalite frameworks may contain atoms such as T = Al, Si, Be, B, P, Ga, Ge, Zn, and materials based on a PN2 framework [175] are also known. A full review of the synthesis and properties of these compounds is beyond the scope of this Chapter. Many have attracted attention because of their optical and electronic properties in particular, and the principle of using the cages of more open frameworks such as LTA and FAU for the assembly of ordered arrays of larger clusters of binary semiconductors such as GaP is now also firmly established [128, 176]. A recent attempt by Zhen and Seff [177] to prepare mercury-exchanged zeolite X through reaction of NH4-X with HgCl2 vapour resulted in the formation of a number of complex clusters of the form (HgCl2)m(NH4+)n located in the supercages. Some of these may be linked through bridging chloride ions to form larger cationic “superclusters”. In contrast nickel-exchanged zeolite Y, prepared by solid state ion exchange methods, was reported to retain Ni3Cl24+ clusters in most of the sodalite cages, illustrating that, even in large pore zeolites, this remains a prime location for the formation of ionic clusters [178]. It is also possible to introduce more complex ionic clusters into larger zeolite cages and channels through direct ion exchange from solution. Interesting examples include the cubane-like clusters [Mo3S4(H2O)9]4+ and [Mo3NiS4Cl(H2O)9]3+, which exhibit activity for the hydrodesulphurization of benzothiophene in zeolites such as Y (FAU), L (LTL), mordenite (MOR) and beta (*BEA) [179]. 5.3 Anionic Clusters

As we have seen in Sect. 5.2, many organic and inorganic anions can be incorporated into the sodalite (SOD) structure, forming tetrahedral anion–cation clusters in the sodalite cages. A particular subset of these compounds comprises those in which the anions themselves may be regarded as cluster species. The

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model for this group of solids is the important industrial pigment ultramarine, whose brilliant blue colour is attributed to the presence of S2– and S3– radical – anions; compounds containing S4–, Se2–, Se2– 2 and Te2 ions, as well as neutral Se2 and Te2 , have also been reported [180]. Attempts to prepare ultramarine analogues by incorporating similar species into zeolites A (LTA), X and Y (FAU) have met with some success [181]. The presence of larger zeolite cages, however, appears to encourage the formation of colourless polysulphides, for example S62– in zeolite A, which exhibits a thermochromic effect on heating to around 520°C, when reversible decomposition to blue S3– is thought to occur [181]: S62–[zeolite] Æ 2 S–3[zeolite] .

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The exposure to laser light of elemental selenium incorporated in neodymiumand lanthanum-exchanged zeolite Y results in the formation of Se2– ions within the zeolite pores [182], and further reduction to Se22– and Se32– can be effected through reaction with rubidium or caesium vapour [183]. In spite of the relative stability of O2– and O3– compared to their heavier chalcogen analogues, there is little evidence for the incorporation of these radical anions into sodalites [184]. The O2– ion, however, detected by ESR, is comparatively common in many aluminosilicate and aluminophosphate framework materials, including the sodalite analogue AlPO4-20 (SOD) [5, 184].

6 Concluding Remarks The subject of ionic clusters in zeolites is growing rapidly. Of the references cited in this review, 60% have been published in the last decade. Among reasons for this surge in interest, two major themes can be discerned. The first of these may be traced to new synthetic advances in the post-synthesis modification of zeolites. Two closely related examples are the development of solid-state and reactive ion exchange processes (see Chap. 2), and renewed interest in salt occlusion as a means of incorporating guest materials into the zeolite pore space [173, 185]. Both separately, and in combination, advances such as these are destined to provide a continuous stream of new zeolite materials – encompassing a wider range of framework structures – in which the cage contents must bear an overall positive charge, unless the framework itself is neutral. The second issue arises from the fact that, even when the contents of the zeolite cages are known, the grouping of atoms into clusters, and the assignment of cluster charge, is seldom straightforward. The question of whether a cluster is neutral or ionic often becomes a matter of interpretation. Many zeolite-based materials have been prepared, containing metal or oxide clusters, in which the clusters have been assumed to be neutral but must share the zeolite cages, and therefore interact, with cations balancing the anionic charge of the framework. Examples include X and Y (FAU) zeolite catalysts containing palladium and platinum clusters said to be “anchored” on Ca2+ or Y3+ ions [186]. This situation mirrors that described in Sects. 3.1 and 3.2, where octahedral silver clusters, presumed to be neutral, are found to be coordinated to rubidium, caesium or silver

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cations. We need only choose to include some or all of these cations to produce an ionic cluster (Sect. 3.1), and, in some cases, this will prove to be a more accurate chemical description. As new and improved techniques of synthesis and characterization become available it seems inevitable that the preparation and identification of new ionic clusters will continue apace. Acknowledgements. My sincere thanks go to the following people: the many collaborators, co-authors and friends, whose names appear in the references below, and in particular Prof. Peter Edwards, for continued support and encouragement; The Royal Society and EPSRC for financial support; Jenny, Mike and Catherine for help with the figures; the editors and publishers, and many others, especially Fiona and Matthew, for their forbearance.

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P.A. Anderson: Ionic Clusters in Zeolites

Haniffa RM, Seff K (1998) J Phys Chem B 102:2688 Tatsumi T, Taniguchi M, Yasuda S, Ishii Y, Murata T, Hidai M (1996) Appl Catal A 139:L5 Reinen D, Lindner G-G (1999) Chem Soc Rev 28:75, and refs cited therein Kowalak S, Stróz˙yk M (1996) J Chem Soc Faraday Trans 92:1639 Goldbach A, Grimsditch M, Iton L, Saboungi M-L (1997) J Phys Chem B 101:330 Armand P, Saboungi M-L, Price DL, Iton I, Cramer C, Grimsditch M (1997) Phys Rev Lett 79:2061 184. Hong SB, Kim SJ, Choi Y-S, Uh YS (1997) Stud Surf Sci Catal 105:779 185. Seidel A, Loos J, Boddenberg B (1999) J Mater Chem 9:2495 186. Kim J-G, Ihm S-K, Lee J Y, Ryoo R (1991) J Phys Chem 95:8546

Preparation of Oxide, Sulfide and Other Chalcogenide Clusters in Molecular Sieves Jens Weitkamp 1, Ute Rymsa 1, Michael Wark 2, Günter Schulz-Ekloff 2 1 2

Institute of Chemical Technology, University of Stuttgart, 70550 Stuttgart, Germany; e-mail: [email protected] Institute of Applied and Physical Chemistry, University of Bremen, 28334 Bremen, Germany

Dedicated to Professor Gerhard Ertl on the occasion of his 65th birthday

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Oxide Clusters in Molecular Sieves . . . . . . . . . . . . . . . . . . 341

2.1 2.1.1 2.1.2 2.1.2.1 2.1.2.2 2.1.2.3 2.1.2.4 2.1.2.5 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.4 2.4.1 2.4.1.1 2.4.1.2 2.4.1.3

Impregnation and Grafting . . . . . . . . . . . . . . . . . Preparation of Basic Catalysts . . . . . . . . . . . . . . . . Preparation of Zeolite-Supported Transition Metal Oxides Molybdenum Oxide Clusters . . . . . . . . . . . . . . . . Titanium Oxide Clusters . . . . . . . . . . . . . . . . . . . Vanadium Oxide Clusters . . . . . . . . . . . . . . . . . . Gallium and Zinc Oxide Clusters . . . . . . . . . . . . . . Miscellaneous Oxide Clusters . . . . . . . . . . . . . . . . Ion Exchange . . . . . . . . . . . . . . . . . . . . . . . . . Application of Oxygen-Containing Salts . . . . . . . . . . Titanium Oxide Clusters . . . . . . . . . . . . . . . . . . . Vanadium Oxide Clusters . . . . . . . . . . . . . . . . . . Hydroxide Treatment of Ion-Exchanged Zeolites . . . . . Nickel Oxide Clusters . . . . . . . . . . . . . . . . . . . . Copper Oxide Clusters . . . . . . . . . . . . . . . . . . . . Zinc and Cadmium Oxide Clusters . . . . . . . . . . . . . Cerium and Aluminum Oxide Clusters . . . . . . . . . . . Ion Exchange with Hydrolyzed Metal Ions . . . . . . . . . Zinc Oxide Clusters . . . . . . . . . . . . . . . . . . . . . Tin Oxide Clusters . . . . . . . . . . . . . . . . . . . . . . Iron Oxide Clusters . . . . . . . . . . . . . . . . . . . . . Solid-State Reactions . . . . . . . . . . . . . . . . . . . . . Molybdenum Oxide Clusters . . . . . . . . . . . . . . . . Vanadium and Tungsten Oxide Clusters . . . . . . . . . . Gallium Oxide Clusters . . . . . . . . . . . . . . . . . . . Miscellaneous Oxide Clusters . . . . . . . . . . . . . . . . Chemical Vapor Deposition and Related Techniques . . . Chemical Vapor Deposition of Chlorides . . . . . . . . . Titanium Oxide Clusters . . . . . . . . . . . . . . . . . . . Vanadium Oxide Clusters . . . . . . . . . . . . . . . . . . Tin Oxide Clusters . . . . . . . . . . . . . . . . . . . . . .

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2.4.1.4 2.4.1.5 2.4.2 2.4.2.1 2.4.2.2 2.4.3

Iron-Molybdenum Oxide Clusters . . . . . . . . . . . . . Molybdenum Oxide Clusters . . . . . . . . . . . . . . . . Chemical Vapor Deposition of Carbonyls . . . . . . . . . Molybdenum and Tungsten Oxide Clusters . . . . . . . . Iron Oxide Clusters . . . . . . . . . . . . . . . . . . . . . Chemical Vapor Deposition of Miscellaneous Compounds

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4.1 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5

Complete Synthesis . . . . . . . . . . . . . . . . . . . . . . . Post-Synthesis Incorporation . . . . . . . . . . . . . . . . . . Cadmium and Lead Sulfide Clusters . . . . . . . . . . . . . . Silver Sulfide Clusters . . . . . . . . . . . . . . . . . . . . . . Molybdenum Sulfide Clusters . . . . . . . . . . . . . . . . . . Cobalt and Nickel Sulfide Clusters . . . . . . . . . . . . . . . Metal Chalcogenide Nanoparticles in Mesoporous Molecular Sieves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 Introduction In zeolites 50 to 80% of the total volume is available for hosting other organic or inorganic phases, and this free volume is highly structured according to the crystallographically defined pore system. Small chalcogenide particles can be incorporated into the pore systems of molecular sieves either during the crystallization process or, in a manner much easier to control, by post-synthesis modification aiming at new functionalities. For example, alkali or alkaline earth metal oxides have been introduced into zeolites to create catalytically active base sites. The entrapment of transition metal oxides aims at the formation of active and selective catalysts for oxidation or ammoxidation reactions, selective catalytic reduction of NOx , or photocatalysis. The formation of isolated, zeolite-anchored oxidic species, which are often less easily reduced than bulk oxides, is desired to favor mild and selective oxidation, i.e., to avoid deep or even total oxidation. Supported metal oxide clusters of nanometer size are also studied with respect to a possible application in redox sensor devices, i.e., for the rapid and selective detection of low concentrations of various gases. The regular pore system of zeolite supports is expected to favor the stabilization of highly dispersed metal oxides of uniform particle size and, thus, uniform properties. The host charge, its hydrophobicity, the

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pore size relative to the guest species, their concentration and three-dimensional arrangement, the guest polarity and its structural properties can all be adjusted to a large extent and in many instances independently from each other. The interest in small, zeolite-entrapped semiconductor particles, e.g., metal sulfides, mainly stems from the occurrence of quantum confinement effects. These effects lead to peculiar optical and electronic properties, which make the materials attractive for applications in optoelectronic devices. In recent years, these new aspects of zeolite chemistry have grown in importance and have become a substantial branch of the increasing research field of nanochemistry. There are various preparation strategies for the entrapment of clusters or nanoparticles of metal chalcogenides, particularly metal oxides and sulfides, in different types of molecular sieves. It is the objective of this chapter to describe these preparation strategies and the pertinent methods for characterizing the resulting host-guest systems.

2 Oxide Clusters in Molecular Sieves 2.1 Impregnation and Grafting

To prepare metal oxide clusters in molecular sieves, impregnation followed by thermal treatment is the method most frequently used. Various metal salts and solvents have been applied. If the impregnation procedure comprises a chemical reaction between functional groups of the molecular sieve and the oxide precursor, this modification technique is frequently denoted as “grafting”. In the following, from the enormous amount of literature dealing with metal oxide modified molecular sieves prepared by impregnation, the focus is on those articles that describe in detail the modification procedure and the characterization of the materials produced. Oxides that are formed as intermediates in the preparation of metal clusters in molecular sieves and are not intentionally prepared and studied will not be included in this review. 2.1.1 Preparation of Basic Catalysts

Impregnation followed by thermal treatment is commonly used to prepare alkali metal oxide or alkaline earth metal oxide clusters in molecular sieves in order to obtain catalysts with basic properties. A first hint at the usefulness of such procedures was published in 1984: Lacroix et al. reported that cesium-exchanged zeolite X is more active in the side-chain alkylation of toluene with methanol when left unwashed after the cation exchange [1]. Hathaway and Davis [2–4] carried out further systematic studies on the preparation of intrazeolitic oxide clusters with basic properties. These authors described two methods for the introduction of alkali metal oxides into the pores of faujasites [2]: When zeolites X and Y, ionexchanged at room temperature with aqueous solutions of alkali metal acetates or

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hydroxides, were left unrinsed after the exchange procedure, the exchange salts remained occluded in the zeolitic pores. Upon calcination at 550°C in helium or air, catalysts were obtained which possessed stronger base sites than faujasites ion-exchanged and washed.Alternatively, the zeolites can be directly impregnated with an alkali salt solution in which the supports are stirred overnight before the water is slowly evaporated. In almost all the examples that follow concerning the preparation of zeolite-occluded alkali metal oxide clusters, one of these two procedures has been applied, only the experimental details differ. At the same time that Hathaway and Davis reported their results in the open literature, patents by Brownscombe et al. were filed describing the same subject [5–7]. Beside the preparation of alkali, alkaline earth, and zinc oxide clusters in zeolites by impregnation in aqueous or organic solutions of the corresponding salts, Brownscombe et al. proposed the use of molten salts for the impregnation. Furthermore, it was suggested that materials impregnated with soluble alkaline earth metal salts should be contacted with precipitating agents, e.g., hydroxides or carbonates, to form the oxide precursor. No investigation on the nature of the guest species formed during calcination of the materials prepared in this fashion has been reported. The authors just assumed that oxidic guest compounds were present. Hathaway and Davis discussed the properties of cesium-modified zeolites in greater detail [2, 3]. Infrared (IR) spectra of unrinsed, uncalcined cesium acetate exchanged zeolite Y confirmed the presence of occluded cesium acetate [2]. In measurements performed by differential thermal analysis (DTA) a sharp exothermic peak was observed at ca. 450°C which was attributed to the oxidation of the acetate ions during the decomposition of the cesium salt. The authors considered cesium metal, carbonate, hydroxide, and oxide as possible guest species generated during the calcination. From the fact that no ESR signals assignable to cesium metal or charged cesium clusters were detected, the presence of metallic cesium was ruled out [3]. IR spectra of impregnated zeolite Y recorded after calcination did not contain bands that could be attributed to carbonates; therefore, the formation of such species was excluded as well. Due to the similarities observed in the kinetics and selectivities of 2-propanol conversion on cesiumimpregnated zeolite Y and on bulk CsxOy , cesium oxide has been invoked as the most likely species formed during calcination [3]. The confinement of the guest compound was confirmed by X-ray photoelectron spectroscopy (XPS) measurements that revealed a surface cesium/silicon atomic ratio very similar to that of the bulk as determined by chemical analysis [4]. Adsorption measurements on unrinsed cesium-exchanged zeolite Y indicated a loss in crystallinity ranging from 4%–11% after calcination at 450°C and 600°C, respectively [2]. In the subsequent decade, several other groups studied the stability of the host zeolites during impregnation with alkali and alkaline earth metal compounds and calcination, as well as the nature and base strength of the guest species formed and host-guest interactions. In this context, it has been reported that X-ray diffractograms of impregnated zeolites do not yield reliable information concerning the integrity of the zeolitic framework due to scattering effects of the bigger alkali cations [2, 8]. However, from the absence of signals due to bulk alkali (alkaline earth) oxides, a high dispersion of the occluded guest oxides

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has been deduced [9, 10]. Evidence for the integrity of the host structure was obtained by 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy or by adsorption measurements [11, 12]. From the results obtained by these characterization techniques, Tsuji et al. concluded that the crystalline structure of zeolite X is not affected by the pH of the impregnation solution or the type of alkali salt used in the impregnation [9]. Preservation of the framework integrity is observed during calcination up to 700°C if the presence of water vapor is excluded. Exposure of ion-exchanged, unrinsed samples to water vapor at high temperature, however, leads to severe damage of the crystalline structure [13]. Laspéras et al. observed a decrease in the micropore volume of zeolite X after impregnation and calcination at 550°C that proceeded linearly with increasing amount of cesium added and was therefore attributed to the incorporation of the larger cations or oxide clusters into the zeolitic cavities [10, 11]. In contrast, the micropore volume of impregnated zeolite Y calcined at 550°C was found to drop significantly above a loading of 9 Cs atoms per unit cell. This was explained by a loss of crystallinity that was also detected by XRD and 29Si MAS NMR measurements [11]. By calcining impregnated zeolite Y at 400°C, this damage to the framework structure can be prevented. Hunger et al. [12] were able to confirm the stability of the zeolitic framework structure under such calcination conditions. However, changes in the local structure of framework AlO4 tetrahedra after the impregnation have been deduced from 27Al MAS NMR spectra of dehydrated samples [14, 15]. Hunger et al., studying modified faujasites in greater detail by MAS NMR spectroscopy, found an increased intensity of the 1H MAS NMR signal attributed to silanol groups after cesium ion exchange of zeolites NaX or NaY. Upon impregnation, this signal was significantly reduced which was explained by an interaction of the guest compound with framework defect sites. No evidence for the presence of Cs-OH groups was obtained, indicating that the guest compound exists in an oxidic form [14, 15]. By 133Cs MAS NMR spectroscopy, Cs cations at sites SI, SIII, SI¢, and SII were identified in ion-exchanged zeolite Y. After impregnation, an additional broad signal was observed with an intensity contribution of approximately 30%, which agreed with the amount of cesium oxide introduced in the impregnation step. The line width and peak position of this signal were significantly different from those of pure cesium hydroxide dehydrated at 400°C and of cesium-impregnated silica gel [12, 14], which has also been reported by Kim et al. [16]. From this it has been concluded that no bulk cesium oxide clusters are formed on the external surface of the zeolite. It has been proposed that the strong broadening of the signal originates from a distribution of chemical shifts for highly dispersed cesium oxide species bound in different ways to the framework. For further characterization of alkali salt impregnated zeolites concerning the nature, location, and base strength of the guest compound, CO2 temperatureprogrammed desorption (TPD) measurements, spectroscopic studies, and different test reactions (such as 1-butene isomerization, dehydrogenation of secondary alcohols, decomposition of methylbutynol, cyclization of acetonylacetone, Knoevenagel condensations) were applied [9–12, 14, 15, 17–26]. In the characterization of ion-exchanged, unrinsed zeolite X samples with TPD of chemisorbed CO2 , Tsuji et al. observed that a greater amount of CO2 is adsorbed

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on impregnated zeolite X than on simply ion-exchanged samples [9, 17, 18]. Furthermore, they found an additional desorption maximum at higher temperatures which they considered to result from the presence of new base sites due to alkali oxides occluded in the zeolite. For cesium-exchanged faujasites additionally impregnated with increasing amounts of cesium acetate, Laspéras et al. concluded from CO2 TPD that, at cesium loadings up to 16 atoms per unit cell, cesium oxide is homogeneously dispersed in the zeolites, whereas, at higher loadings, deposition on the external surface occurs for zeolite X [19, 20]. The CO2 desorption maximum ascribed to cesium oxide internally confined was found at 150°C for impregnated zeolite Y samples and at 250°C for zeolite X [20]. To explain this difference it was proposed that the zeolite host influences the base strength of the cesium oxide species formed inside the cages. The catalytic activity of the modified zeolites in the Knoevenagel condensation of benzaldehyde and ethylcyanoacetate paralleled the trend found by CO2 TPD [21]. The local formation of cesium silicate or cesium aluminate defects in zeolite Y without an overall loss of crystallinity or differences in the host structure concerning occupation of cation sites and cation electric fields were envisaged as explanations for the influence of the zeolite host on the base strength of the guest component. However, by IR spectroscopic characterization of different alkali cation impregnated materials (including CsNa-Y and MCM-41) with deuterochloroform as a probe molecule, Rymsa et al. observed that the base strength of the guest oxide is not influenced by the support material and is only determined by the kind of alkali cation present [22]. Furthermore, as two different bands are detected for the C-D stretching vibration of deuterochloroform adsorbed on base sites of the guest compound, the authors discussed the possibility that different types of oxidic guest species are formed, namely alkali oxides, peroxides or superoxides. Kim et al. have also put forward this proposal based on their CO2 TPD results [16]. The presence of guest species with different base strengths in impregnated faujasites has furthermore been detected by 13C CP MAS NMR spectroscopy of methoxy groups formed on the catalyst base sites during the adsorption of methyl iodide [12, 14, 15]. From their measurements, Hunger et al. concluded that the guest compounds are not as highly dispersed in the pores of zeolite X as in zeolite Y, which has been explained in terms of steric influences originating from the large number of cesium cations located in the supercages [15]. Furthermore, it was proposed that the base strength of the zeolitic framework is enhanced by the presence of the guest compound. To understand the discrepancies between the results of spectroscopic investigations, TPD and catalytic experiments concerning the influence of the zeolite host on the base strength of the guest oxide, one has to take into account that not only the guest oxide is active in adsorbing CO2 or catalyzing the test reactions, but also the zeolitic framework oxygen which possesses a different base strength in zeolites CsNa-X and CsNa-Y. In the test reactions, oxide clusters deposited on the external zeolite surface may influence the activity, because they are more easily reached by the reactants than internally confined cesium oxide. Furthermore, as already stated by Brownscombe et al., impregnation of partially ion-exchanged faujasites most likely results in some counterion exchange between the impregnating alkali metal cations and the

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cations already present in the zeolite [5], so that different alkali oxides may be formed. Kloetstra and van Bekkum showed that the mesoporous material MCM-41 is also suitable as a support for cesium oxide clusters [27, 28]. When MCM-41 is impregnated in an aqueous solution of cesium acetate and subsequently calcined, occluded cesium oxide particles are formed that can be detected by 133Cs MAS NMR spectroscopy [28]. These particles are moisture-sensitive, i.e., they are hydrated by contact with air. Upon repeated calcination, aggregation of the cesium oxide clusters has been found to occur. CO2 TPD revealed that, by impregnation with cesium acetate, base sites of significantly higher strength are created than by mere cesium ion exchange. This can also be deduced from the catalytic activities of ion-exchanged and impregnated samples in the Knoevenagel condensation of various substrates [23]. At cesium loadings around 10 wt.%, desorption of one CO2 molecule per 2.2 cesium atoms was found. Assuming that one CO2 molecule is adsorbed on the oxygen atom of Cs2O, this indicates a homogeneous distribution of cesium. Less CO2 per cesium is desorbed at higher degrees of impregnation, and this was explained by the formation of larger cesium oxide clusters [28]. The stability of the MCM-41 pore structure has been examined by nitrogen adsorption measurements [23, 28]. The results indicate an increase in stability of the MCM-41 materials with decreasing nSi/nAl ratio. After repeated vacuum treatment at 350°C, a significant decrease in BET surface areas was observed for all samples. It has been assumed that occluded cesium oxide particles react with the MCM-41 framework forming cesium silicate species. A poor regenerability was also observed after application of the materials as catalysts in Michael addition reactions. To obtain catalysts of a higher thermal and chemical stability, Kloetstra et al. impregnated MCM-41 samples with cesium acetate and lanthanum nitrate in methanolic solution either in a one- or two-step process [29].With the resulting materials, the formation of homogeneously dispersed binary CsLaO2 was observed after calcination. The preservation of the MCM-41 pore structure was deduced from the shape of the nitrogen adsorption and desorption isotherms. The decrease in the surface area has been attributed to the creation of smaller mesopores by incorporation of the binary guest oxide, indicated as well by the reduction of the mean pore diameter. The BET surface area of the impregnated samples was retained after re-evacuation at high temperature, which confirms that CsLaOx/MCM-41 is more stable than CsxOy /MCM-41 [29]. From 133Cs MAS NMR measurements it was concluded that lanthanum is bonded to the MCM-41 support and that the cesium species have hardly any or no interaction with the silica-type support. The base strength of the MCM-41-supported mixed oxide, as measured by CO2 TPD, was found to be lower than that of CsxOy /MCM-41. Therefore, by incorporating cesium-lanthanum mixed oxide particles into the pores of MCM-41, catalysts are prepared which possess a lower base strength but a higher stability than cesium oxide occluded in MCM-41. Zhu et al. suggested the impregnation of different molecular sieve supports with KF, KOH, and KNO3 to prepare new strongly basic zeolite catalysts which are cheaper than zeolite-supported cesium oxide [30]. From IR spectra of the impregnated materials, the authors concluded that KNO3 introduced into zeolite

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K-L can be decomposed by thermal treatment at 600°C. The formation of a K2O guest species was proposed [31] which was claimed to be strongly basic according to basicity measurements with Hammet indicators, titration of samples immersed in water with HCl, and CO2 TPD. In contrast, KNO3 loaded onto zeolite Na-Y was stable against decomposition at 600°C, and destruction of the zeolite framework was observed at higher calcination temperatures [32, 33]. No true explanation was given for these differences; it was simply proposed that the zeolitic pore structure and the microenvironment around the SiO4 tetrahedra might play a role [31, 32]. Zhu et al. reported that the conversion of KNO3/Na-Y into K2O/Na-Y is feasible by contacting the impregnated zeolite with isopropanol at 400°C, leading to the reduction of KNO3 and the oxidation of isopropanol to acetic acid [31, 33]. A similar treatment was applied to KNO3/MCM-41 [33]. The impregnation of Na-Y with KF or KOH followed by calcination at 400°C produced base sites of similar strengths, although their base strength seems to be lower than that of materials derived from KNO3/K-L [31, 34]. This has been interpreted in terms of the formation of K3AlF6 or K2SiF6 and KOH in a reaction of KF with the supporting zeolite. It has also been proposed that KOH forms species similar to K2SiO3 by reaction with the zeolite framework. These results are contradictory to those published earlier (vide supra) so that independent proof is needed to confirm the higher base strength of calcined KNO3/K-L. Another method for preparing strongly basic zeolite catalysts without use of a cesium compound was reported by Tsuji et al. [17]. They described the formation of magnesium oxide clusters inside zeolite Y by impregnation of dehydrated Na-Y or Mg-Y in a methanolic solution of magnesium dimethoxide under inert atmosphere. X-ray absorption near edge structure (XANES) measurements indicated that magnesium oxide with rock salt structure is present in the zeolites. The different peak intensities observed in the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra of the impregnated zeolites and bulk MgO were interpreted as a sign of the smaller crystallite size of MgO confined in the zeolitic pores. The application of an aqueous solution of Mg(NO3)2 in the impregnation procedure was also studied, but in this case no intrazeolitic MgO was found after calcination [17]. 2.1.2 Preparation of Zeolite-Supported Transition Metal Oxides

Transition metal oxide containing zeolites are used in selective oxidation reactions, ammoxidation, aromatization, photocatalysis and the selective catalytic reduction of NOx . Often, isolated, zeolite-bound oxidic species are identified as the most active sites in these reactions. Therefore, the preparation procedures are usually aimed at creating very small intrazeolitic oxide clusters or even isolated metal oxo-species.

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2.1.2.1 Molybdenum Oxide Clusters

To incorporate molybdenum oxide into the pores of zeolites by impregnation, aqueous solutions of ammonium heptamolybdate are most frequently used, but solutions of Mo(CO)6 in benzene [35, 36] or Mo(p-C3H6) in pentane [37] have been applied as well. After impregnation, the samples are calcined in air. It has been assumed that the heptamolybdate anion mainly adsorbs on the external zeolite surface, because it is too large to enter the pores even of faujasites [38, 39], and that MoO3 and MoO2(OH)2 are formed in the calcination step and diffuse into the zeolitic channels [35, 36, 38]. Studying the impregnated zeolites by X-ray diffraction (XRD), a significant reduction in the signal intensity was observed with increasing amount of incorporated molybdenum, when Na-X and Na-Y were used as supporting materials [40–42]. This was explained by a decrease in zeolite crystallinity, which was also inferred from an intensity loss of structuresensitive TO4 (T=Si, Al) framework vibration bands in the IR spectra of the impregnated zeolites. Furthermore, decreasing BET surface areas and water adsorption capacities were observed [40–42].An additional IR band at 885 cm–1 was attributed to the Mo=O vibration of monomeric MoO42– species attached to the zeolite surface [40, 43]. This bonding to the zeolite framework oxygen has been speculated to be one reason for the framework damage [41]. Kovacheva et al., however, attributed the decreasing peak intensities in the X-ray diffractograms of impregnated Na-Y to a redistribution of Na+ and Mo6+ cations in the zeolite framework, which has also been reported by Khulbe et al. [44] because they did not find any changes in the alumosilicate vibrations [43]. These authors only observed damage to the crystal structure after impregnation of a zeolite Y sample that had previously been ion-exchanged with nickel. This was explained by a strong interaction between molybdenum and the OH-groups of the exchanged zeolite, favored by the higher strength and number of acid sites due to the presence of nickel [43]. In contrast, Leglise et al. reported that the crystallinity of zeolite Na-Y is preserved after accommodation of 15 wt.% Ni and Mo oxides by ion exchange and subsequent impregnation [45]. Thus, it appears that the stability of the host framework essentially depends on the calcination conditions applied by the various groups. Molybdenum oxide particles in zeolites have also been found to be strongly influenced by the calcination step, rather than by the compound type and conditions used in the impregnation [35, 36]. In this context, Fierro et al. reported that, by calcination of impregnated zeolite Na-Y under reduced pressure involving very slow heating (duration of the procedure up to 4 weeks), the crystallinity loss of the zeolitic host can be substantially mitigated [38]. Furthermore, a better dispersion of molybdenum throughout the zeolite crystallites was obtained, even into the sodalite cages. Several authors [35, 36, 39] have reported preservation of the host crystallinity after impregnation of USY with molybdenum compounds. The presence of fully oxidized molybdenum in the zeolites after calcination was proved by XPS [35, 36, 38]. The formation of monomeric Mo6+ ions in tetrahedral coordination with oxygen anions can be detected by UV-Vis diffuse reflectance spectroscopy [40, 41, 46]. In addition, polymeric, octahedral-

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Fig. 1. Molybdenum oxo-species in zeolite USY, after [39]. T denotes Si or Al

ly coordinated Mo6+ oxide species were found when the molybdenum content of the materials was increased. The polymeric species are more easily reduced in hydrogen than tetrahedrally coordinated molybdenum, which confirms a stronger interaction of the latter species with the zeolitic support [41, 43]. A similar trend was found concerning the extractability of molybdenum by ammonia solution [41]. Signals due to bulk MoO3 were neither detected in the X-ray diffractograms nor in the UV-Vis spectra of the impregnated faujasites [40, 41, 43]. Corma et al., by means of IR spectroscopy, observed an interaction of molybdenum species incorporated in USY with all hydroxyls except the external silanol groups [39]. Furthermore, they found similar nSi /nMo ratios at the surface and in the bulk from which they concluded that molybdenum species (as depicted in Fig. 1) are well dispersed in the zeolite pore system. When nickel oxide and molybdenum oxide were present in USY at the same time, a smaller proportion of molybdenum was found to be strongly attached to the zeolite, suggesting an interaction of Ni with Mo [39, 47]. When zeolite ZSM-5 was applied as a support, dispersion of the molybdenum oxide species throughout the pore system was observed for loadings below 10 wt.% after calcination at sufficiently high temperatures [48–52]. This was detected by the interaction of the guest compound with framework hydroxyl groups, leading to a decrease in the OH stretching band intensities [50, 52]. A bonding of guest oxide species to the zeolite was also invoked from Mo-O-T vibration bands found in the IR spectra [50, 51]. Because of the oxide-support interactions, the dispersed molybdenum oxide is less easily reduced than polymeric or bulk oxide species [48]. However, calcination at 500°C for only 2 h did not lead to a homogeneous distribution of molybdenum oxide, even at a molybdenum content as low as 6 wt.%. Instead, accumulation on the outer surface was observed in SEM micrographs [53]. The concentration of molybdenum within the zeolite channels increases with increasing temperature and time of calcination. The presence of moisture during calcination has been reported to promote diffusion of the molybdenum species into the pore system [52]. At high loadings, formation of a bulk MoO3 phase on the external zeolite surface was always observed, and the possibility of pore blocking was mentioned [43, 48–50, 52, 54]. The framework stability of zeolite ZSM-5 is dependent on the calcination conditions [49, 50, 53], e.g., after calcination at 500°C for 2 h, no loss of crystallinity was detected [53], whereas, after 16 h, extra-framework aluminum was found by 27Al MAS NMR spectroscopy [50]. Like MoO3/ZSM-5, zeolite-supported WO3 was prepared by impregnation of H-ZSM-5 with ammonium meta-

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tungstate [55]. The characterization parallels that of molybdenum-impregnated ZSM-5 and tetrahedrally coordinated W 6+ species: polytungstate and WO3 crystallites were found to be present depending on the degree of impregnation. Changes in BET surface areas and micropore volumes were attributed to pore blocking rather than to a loss in crystallinity [55]. When impregnating MCM-41 molecular sieves with aqueous solutions of ammonium heptamolybdate, Cheng et al. observed an increase in the host framework stability during the impregnation and calcination procedure with decreasing nSi/nAl ratio [56]. A similar effect was observed for MCM-41/Al2O3 extrudates, where the presence of the alumina enhanced the stability of the MCM-41 pore structure [57]. By UV-Vis diffuse reflectance spectroscopy it was found that Mo6+ is mainly present as octahedrally coordinated polymolybdena clusters. The appearance of bulk MoO3 was observed by X-ray diffraction for molybdenum contents exceeding 10 wt.% [56]. 2.1.2.2 Titanium Oxide Clusters

Titanium oxide/zeolite composites are of interest as catalysts for photocatalytic or selective oxidation reactions. For applications on a larger scale, the main drawback of pure TiO2 is the small particle size of the powder prepared by the usual industrial synthesis [58]. This has led to a number of attempts to anchor TiO2 on porous supports. One method employed for the impregnation of zeolite catalysts with TiO2 comprises the preparation of a colloidal TiO2 solution by hydrolysis of titanium tetraisopropoxide in acidic aqueous solution. Accompanied by pH adjustment to prevent framework destruction, the zeolite (typically mordenite, L, X, Y, A, or ZSM-5) is added to this sol. After some hours of stirring, either the solvent is evaporated or the solid product is filtered, washed and dried, followed by calcination [58–61]. As under these conditions the particle size of the TiO2 sol is larger than the pores of zeolites and MCM-41 materials, Xu and Langford alternatively proposed the impregnation of MCM-41 at 0°C [61]. Hydrolysis of titanium alkoxides at low temperatures produces titania particles of about 2–4 nm size that can be incorporated into the pores of MCM-41. The earlier studies did not aim at an introduction of TiO2 into the zeolitic pores, and the composites have not been characterized very thoroughly [58–60]. By scanning electron microscopy, a uniform distribution of TiO2 particles over the external zeolite surface was observed that did not lead to a change in the typical morphology of the support [58, 60]. It has been assumed that, at low coverage, the oxide is amorphous, and linkage to the zeolite support was detected by Raman spectroscopy. SEM and XRD measurements confirmed the presence of anatase crystallites at higher titanium content [58]. For MCM-41 impregnated at low temperature, evidence for a partial incorporation of titanium dioxide into the pores has been obtained from XRD and nitrogen adsorption studies. Dispersion of TiO2 in the pore systems of zeolites has been achieved by different impregnation techniques: Hashimoto et al. used titanium tetraethoxide in cyclohexane and impregnated mordenite in an autoclave at 300°C, expecting the alkoxide to be hydrolyzed by water adsorbed on the zeolite [62]. Zhang et al.

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impregnated H-ZSM-5 with titanium isopropoxide in cyclohexane solution [63], whereas Yamashita et al. employed an aqueous solution of titanium chloride or titanium ammonium oxalate for the impregnation of zeolite Y or silicalite-1 [64–67]. All authors reported that, at low loadings, a uniform distribution of the guest oxide in the zeolitic pores is obtained. For TiO2 contents up to ca. 7 wt.%, Hashimoto et al. found no X-ray diffraction peaks attributable to bulk titania [62]. The formation of TiO2 clusters was confirmed by XPS measurements that also showed that a large portion of the titania is incorporated into the mordenite cavities. On the basis of XPS and ESR results, it has been concluded that a strong interaction between TiO2 and the support exists [62]. By photoluminescence, XPS and ESR measurements, Zhang et al. found an increasing interaction between titanium and ZSM-5 when the amount of TiO2 became smaller [63]. Features of bulk TiO2 were observed when the titanium content exceeded 1 wt.%. Yamashita et al. compared titanium-impregnated zeolite Y catalysts with those prepared by ion exchange. UV-Vis DRS, photoluminescence, XANES and EXAFS measurements all indicated a better dispersion of titania in the latter materials. For impregnated zeolites Y and silicalite-1, the presence of aggregated, octahedrally coordinated titanium oxide species has been considered [64–67]. On aluminum-containing MCM-41, both tetrahedrally and octahedrally coordinated titanium oxide species were observed after impregnation with titanium tetraethoxide in ethanolic solution and subsequent calcination [68]. The formation of polymeric Ti-O-Ti linkages is indicated by UV-Vis spectroscopy. The mesoporous host is not affected by the impregnation procedure, as can be seen by XRD, nitrogen adsorption and 27Al MAS NMR measurements [68]. A different approach for the modification of MCM-41 in an attempt to obtain isolated titanium centers needed for epoxidation reactions has been proposed by Maschmeyer et al. [69]. Titanocene dichloride (TiCp2Cl2) dissolved in chloroform was used to graft titanium onto the channel walls of MCM-41. The reaction of this complex with surface silanols, carried out under inert atmosphere, is initiated by addition of triethylamine and proceeds via substitution of the chloride by Si-O groups. The cyclopentadienyl ligands were envisaged to protect the titanium center and prevent dimerization or oligomerization during the grafting process. The modified material was finally calcined in dry oxygen [69]. The structural integrity of the MCM-41 host material was confirmed by X-ray diffraction. EXAFS measurements revealed the change in the type of the titanium neighbors (oxygen for chlorine) during the anchoring and the disappearance of one cyclopentadienyl ligand. After calcination, monomeric, tetrahedrally coordinated titanium oxo-species were detected which were anchored to the support via three Si-O bonds. IR spectra confirmed the formation of Ti-O-Si bonds [70]. Furthermore, a preferential anchoring at geminal and H-bonded hydroxy groups was indicated. Based on these results, the reaction scheme shown in Fig. 2 was proposed. By theoretical calculations it was shown that (∫SiO)3TiOH or (∫SiO)2Ti(OH)2 species are energetically more favorable than (∫SiO)2Ti=O groups [71]. UV-Vis diffuse reflectance and photoluminescence spectroscopy confirmed the presence of two species with different tetrahedral environments [70, 72]. As an alterna-

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Fig. 2. Grafting of Ti oxo-species onto the walls of MCM-41, after [75]

tive, the incorporation of titanium oxo-species into the walls of MCM-41 and the formation of [Ti(-O-Si∫)4] sites were proposed [73]. For samples prepared in air and without dehydration of the support, a small amount of oligomeric Ti4+ sites was also detected [70]. These sites are present in samples prepared under inert atmosphere only at higher Ti content (≥4 wt.%). The characterization of MCM41 first modified by reaction with tetrabutylgermanium and afterwards by anchoring of TiCp2Cl2 confirmed the reported results [74, 75]: The attachment of titanium was concluded from the disappearance of the n (GeO-H) band in the IR spectrum and the appearance of a Ge … Ti shell deduced from the Ge K-edge EXAFS spectrum. 2.1.2.3 Vanadium Oxide Clusters

The impregnation of molecular sieves with vanadium oxide aims at the preparation of selective oxidation catalysts. To suppress total oxidation reactions, isolated active sites are desired [76]. In an attempt to generate such sites, Trifirò, Jiru and co-workers modified Y, ZSM-5 and ZSM-11 zeolites by impregnation with ammonium vanadate or with vanadium phosphorus heteropolyacids in aqueous solution [77–81]. A readily soluble peroxide complex of vanadium, formed by reaction of V2O5 with aqueous H2O2, was also used in order to avoid precipitation of vanadium compounds upon contact with the zeolite crystals and deposition of the guest species on the external surface [76]. The materials obtained were characterized by argon adsorption and IR spectroscopy in the framework and hydroxyl vibration region. From these measurements it was inferred that no damage to the host structures had occurred during the modification. The guest oxide was assumed to be located in the zeolite cavities; only for zeolite HZSM-11 was deposition on the external surface observed [80]. Hong et al. chose AlPO4-5 as support for the incorporation of vanadium oxide by incipientwetness impregnation with aqueous solutions of ammonium vanadate [82]. Characterizing the calcined material by X-ray diffraction, these authors found no damage to the host structure, not even at a vanadium oxide loading as high

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as 14 wt.%. However, signals attributable to V2O5 were detected in this sample that were not observed after incorporating only 2 wt.% vanadium. Interaction of the guest oxide with the support was inferred from a new band found in the lattice vibration region of the IR spectrum [82]. At low vanadium content, the presence of tetrahedrally coordinated V5+ oxide species was demonstrated by UV-Vis diffuse reflectance spectroscopy, whereas, with increasing amounts of zeolite-encapsulated vanadium, absorption bands appeared which were indicative of the formation of small V2O5 crystallites. These observations were confirmed by Catana et al. for USY impregnated in a similar way [83]. Interactions between the supported vanadium oxide and the host zeolite were also found by Kao et al. who observed higher V 2p3/2 binding energies for the supported samples than for bulk V2O5 in the X-ray photoelectron spectra [84]. From oxygen adsorption measurements, these authors concluded that, at vanadium contents around 1 wt.%, the vanadium dispersion is higher on ZSM-5 and silicalite-1 than on AlPO4-5. However, no explanation for these results was given. Adams et al. attempted to form isolated active vanadium sites in zeolite H-Y by carrying out the impregnation with vanadyl chloride in CCl4 under inert atmosphere [85]. Due to the long reaction time of 20 h, a high dispersion of the vanadyl chloride and anchoring at the acid OH groups of the zeolite were expected. After calcination in air at 350°C, the preservation of the crystallinity of the samples was confirmed by XRD [85]. XPS measurements revealed that surface enrichment of vanadium had occurred to some extent. Two different vanadium oxo-species were found by 51V MAS NMR spectroscopy, which were supposed to be tetrahedrally coordinated due to the lineshape of the corresponding NMR signals. It was assumed that these species possess a different number of bonds to the zeolitic support and that linking to the zeolite by a single oxygen bridge predominates. With samples containing up to 6 wt.% vanadium, no NMR or EXAFS evidence was found for the presence of V-O-V bonds [85]. Morey et al. reported on the grafting of vanadium oxide species onto the walls of MCM-48: Under an inert atmosphere, dehydrated MCM-48 was reacted with vanadyl triisopropoxide, (OiPr)3V=O, in hexane, filtered and calcined in anhydrous oxygen [86]. It was stated that, under these conditions, an esterification process between the silanol groups of the mesoporous support and the vanadium compound occurs which leads to V sites anchored to the walls by one Si-O-V bridge. By X-ray diffraction and nitrogen adsorption, only small changes in the host properties were observed. The mean pore diameter was reduced after the impregnation, and this finding is consistent with a location of the guest compound inside the pores [86]. From 51V MAS NMR and UV-Vis diffuse reflectance spectroscopic results it was concluded that, in dehydrated samples, mainly pseudotetrahedral O3/2V=O species are present, probably coordinated by three Si-O-V bonds to the mesoporous walls. In samples containing only a small amount of vanadium, these species change during hydration to an octahedral coordination by adding water ligands. It was assumed that, at higher vanadium content, these hydrated species oligomerize, remaining, however, different from bulk vanadium oxide [86]. Similar results were reported by Grubert et al. for MCM-41 impregnated with vanadyl acetylacetonate in toluene [87]. These authors detected not only the pre-

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sence of mononuclearly distributed vanadium in distorted tetrahedral coordination in calcined, dehydrated samples, but also some formation of chain-like oligomeric vanadium oxide clusters. The amount of these chains increased slightly with repeated impregnation steps, probably forming a larger network [88]. No surface enrichment of vanadium was found by XPS, confirming the location of the guest oxide in the pore system [87, 88]. Oldroyd et al. reported a grafting procedure for obtaining well-dispersed vanadium oxo-species on MCM-41 that was also applied for titanium-modified samples (vide supra) [89]. The dehydrated support was contacted with VCp2Cl2 and triethylamine in chloroform under an inert atmosphere. From EXAFS it was deduced that, during the grafting procedure, vanadium centers are oxidized to the +5 state. The best fit to the spectrum was achieved by assuming one cyclopentadienyl (Cp) ligand still attached to the vanadium center, a vanadyl moiety V=O, and two links to the surface in the form of V-O-Si bonds [89]. After calcination, it was found that the Cp ligand was substituted by an additional V-O-Si link. For samples containing up to 4 wt.% vanadium, no evidence for the formation of V-O-V bonds was obtained. Luan and Kevan [90] and Luan et al. [91] studied the impregnation of MCM-41 materials containing different metals in the framework, i.e., aluminum, titanium and zirconium. After incipient-wetness impregnation with aqueous vanadyl sulfate solution and calcination, no destruction of the regular pore system was observed. Employing electron microprobe analysis, a homogeneous distribution of vanadium was confirmed, and the formation of bulk V2O5 was excluded [90]. From ESR, UV-Vis DRS and Raman spectroscopy, it was inferred that oxidation of VO2+ to V5+ is catalyzed by the titanium centers in V/TiMCM-41 even without calcination (cf. Fig. 3). By studying samples of different titanium and vanadium content, the authors were led to claim a preferred bonding of monomeric guest species to the titanium centers. It was assumed that, when all titanium sites are saturated, further vanadyl species bond to surface siliceous sites near the titanium [90, 91]. The stronger anchoring at titanium centers prevents a change to the vanadium coordination sphere upon hydration of calcined samples, which has been described for siliceous MCM-41 samples (vide supra). This different behavior is illustrated in Fig. 3 [91]. ZrMCM-41 as a support showed similar features to TiMCM-41, although less pronounced [90]. The special properties of V/TiMCM-41 were explained by the strong oxygen affinity of titanium. 2.1.2.4 Gallium and Zinc Oxide Clusters

H-ZSM-5 zeolites modified with gallium oxide and zinc oxide are of interest as catalysts for the aromatization of light alkanes. To prepare gallium-promoted catalysts used, for example, in the M2 Forming or Cyclar processes [92, 93], ionexchange methods as well as impregnation and solid-state reactions (vide infra) have been applied. Interestingly, the promoting effect of gallium was found to be quite independent of the technique used for catalyst preparation [94, 95]. This was explained by the fact that the entrance of trivalent gallium cations into the pores of H-ZSM-5 is hindered by the large radius of the solvated ions and by

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Fig. 3. Bonding of vanadium oxo-species at titanium and silicon centers of TiMCM-41, taken from [91] and slightly modified

constraints arising from the electrostatic imbalance of isolated negative charges of the framework being compensated by polyvalent cations [95–99]. Therefore, ion exchange and impregnation followed by calcination in air both lead to a deposition of Ga2O3 on the external zeolite surface. These catalysts and physical mixtures of Ga2O3 and H-ZSM-5 differ only by the dispersion of gallium oxide and by the average distance between the acid sites of the zeolite and the gallium species [95].

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For ion exchange or impregnation, aqueous solutions of gallium nitrate are usually applied, but the treatment of zeolite ZSM-5 under hydrothermal conditions with sodium gallate solution, formed from Ga2O3 and aqueous NaOH, has been reported as well [100, 101]. Extensive reaction studies have been carried out; however, the number of investigations concerning the stability of the host framework during catalyst preparation or the structure and location of the guest oxide is smaller. Gnep et al. found a slight decrease in the nitrogen adsorption capacity of H-ZSM-5 after impregnation with 6 wt.% gallium and calcination at 530°C. Almost no change was observed in the strength and density of acid sites as determined by thermal desorption of ammonia [102]. However, by IR spectroscopy, Kazansky et al. detected some decrease in the concentration of acidic OH groups after incorporation of gallium oxide [96]. Mériaudeau and Naccache reported similar observations and interpreted their results in terms of a partial ion exchange during impregnation and/or by a solid-solid reaction between SiOH and Ga2O3 during calcination at 500°C [103]. Gallium oxide was shown by transmission electron microscopy to be deposited mainly on the external surface of H-ZSM-5 [97, 99, 103, 104], and this has been confirmed by hexane adsorption measurements [105], XPS [100], and by IR spectroscopy using CO as a probe molecule [98]. A reductive treatment of zeolite H-ZSM-5 modified by gallium oxide led to a better dispersion of gallium within the zeolitic pores as detected by STEM images, IR spectroscopy and XPS [94, 97, 98, 100, 101, 103, 104, 106]. The formation of a mobile Ga2O species was proposed which is believed to migrate into the zeolitic channels where it can undergo a solid-solid reaction with acidic OH groups [94, 97, 98, 103, 104, 107–109]. For a sample containing 1.75 wt.% of gallium, Shpiro et al. estimated the amount of reduced gallium to be about 30% [101]. After impregnation of zeolite H-ZSM-5 with an aqueous solution of zinc nitrate and calcination, a smaller number of Brønsted acid sites was observed by IR spectroscopy using pyridine as a probe molecule [96, 99, 110]. Simultaneously, more Lewis acid sites were found. This was explained by the occurrence of solid-state ion exchange between ZnO and zeolite OH groups during calcination. In IR spectra of adsorbed H2, bands were present which could not be attributed to an adsorption on bulk-like ZnO or on Zn2+ cations [96]. Therefore, the formation of clusters composed of several Zn2+ ions was proposed. In a more detailed study on zinc oxide incorporated in Na-ZSM-5, Xu et al. observed a crystalline phase of ZnO only when the zinc oxide content exceeded 8 wt.% and suggested a high dispersion of the guest species at lower loadings [111]. The adsorption capacity of the modified zeolite for n-hexane and water was reported to decrease continuously up to a ZnO loading of 8 wt.%. The authors concluded that the dispersed zinc oxide is mainly located inside the pore system, while the build-up of a crystalline ZnO phase at higher loadings occurs at the external zeolite surface. The integrity of the zeolite framework structure was confirmed by 27Al MAS NMR spectroscopy. In the framework vibration region a new band was detected which was assigned to Zn-O-T (T=Al or Si) vibrations, indicating a strong interaction of the guest oxide with the support. 23Na MAS NMR spectroscopy revealed an interaction between Na+ and the oxide clusters [111].

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2.1.2.5 Miscellaneous Oxide Clusters

The incorporation of other oxides into the pores of zeolites and related materials has been studied to a much lesser extent. Some reports have been published concerning the preparation of intrazeolitic copper oxide [112–115], cobalt oxide [116–118], nickel oxide and nickel niobate [119, 120], iron oxide [121–124], manganese oxide [118], phosphorus oxide [125, 126], rhenium oxide [127], zirconium oxide [128], tin oxide [129, 130] and rhodium vanadate [120, 131] clusters by impregnation methods. To obtain copper oxide supported on zeolites or MCM-41, impregnation is mostly carried out using cupric nitrate in aqueous solution [113–115]. From ESR and hydrogen-TPR measurements, Lee et al. concluded that, on impregnated and calcined H-mordenite, most of the copper exists as copper oxide and cupric ions inside the pores in a shell near the external surface of the zeolite crystals [113, 114]. Crystalline CuO was found by XRD, regardless of the amount of copper introduced. After high-temperature reduction and re-oxidation, damage of the mordenite framework structure was detected by XRD [113].After evacuation of copper-impregnated MCM-41 at 600°C, Zecchina et al. deduced the presence of highly dispersed Cu(I) species at low copper content from the IR spectra of adsorbed CO [115]. At higher loadings, Cu2O clusters were detected. An interesting approach for the preparation of highly dispersed copper oxide in zeolite Na-X was presented by Berdanova et al. [132]: A tetranuclear complex (µ4-O)L4Cu4Cl6 , where L denotes N,N-diethylnicotinamide, was reacted with dehydrated Na-X in anhydrous CH2Cl2 under strictly inert conditions. Since the complex precursor can be converted into the active component at low temperature, it was hoped that aggregation and enlargement of small particles of the metal oxide during the preparation could be avoided. In aprotic media, only the core of the complex, namely (µ4-O)Cu4Cl6 , was adsorbed on the zeolite. Formation of copper oxide clusters was achieved in a flow of oxygen at 220°C [132]. From pulse oxidation experiments with carbon monoxide it was concluded that the copper oxide was indeed highly dispersed inside the zeolitic pores. Park reported on the formation of a perovskite-type LaCoO3 incorporated into various zeolites by impregnation [116]. The zeolite-encaged mixed oxide was supposed to be well dispersed and mostly amorphous. ESCA results showing shifts of the La 3d5/2 and Co 2p3/2 binding energies relative to the bulk mixed oxide were interpreted in terms of an interaction of the guest oxide with the support [116]. By use of EXAFS, Jentys et al. observed the formation of small cobalt oxide clusters in MCM-41 after incipient-wetness impregnation with cobalt nitrate and calcination [117].As the average oxygen coordination number was found to depend on the pore size of the MCM-41 support, an incorporation of the guest oxide into the pores was claimed. To prepare zeolite-encaged iron oxide, impregnation with aqueous solutions of (NH4)3Fe(C2O4)3 or Fe(NO3)3 and with Fe3(CO)12 in cyclohexane was applied [121,122, 133, 134]. By impregnating Na-Y, Na-ZSM-5 and AlPO4-5 with increasing amounts of (NH4)3Fe(C2O4)3 in aqueous solution, Gao et al. observed a dispersion threshold in the calcined samples by means of XRD phase analysis

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and positron annihilation lifetime measurements [121]. The amount of a-Fe2O3 that could be finely dispersed decreased in the series AlPO4-5 > Na-Y > NaZSM-5, and this was explained by the different pore diameters and charge distributions of the molecular sieves. The zeolite-encaged iron oxide was more difficult to reduce than the bulk oxide, which was interpreted in terms of strong oxide-support interactions [121]. Dadashova et al. reported that a higher dispersion of Fe2O3 on ZSM-5 is achieved when the impregnated zeolite is not calcined but treated with oxygen or argon plasma in a glow discharge [134]. After impregnation of MCM-41 with aqueous iron nitrate solution and calcination, the regular hexagonal lattice structure of the support was maintained, but it had slightly shrunk [122]. No change in the BET surface area was observed at low iron oxide loading. TEM images after photodeposition of Pt on the iron oxide particles revealed that the dispersion of the guest oxide was improved when the impregnated material was suspended in water a second time before calcination. UV-Vis diffuse reflectance measurements showed that Fe2O3 nanoparticles with quite uniform particle sizes were formed [122]. 2.2 Ion Exchange

For the preparation of zeolite-encapsulated oxide clusters via ion exchange, two principally different methods are applied: (1) Cations which already contain oxygen are exchanged into the zeolite. Examples for salts of this type are ammonium titanyl oxalate, (NH4)2TiO(C2O4)2 · H2O, or vanadyl sulfate, VOSO4 . (2) The deposition of hydroxide species within zeolite cavities can be achieved by treating zeolites ion-exchanged in the usual manner with sodium hydroxide solution or moist ammonia gas. Some elements (Sn, Zn, Fe), which tend to be hydrolyzed in aqueous solution, are even reported to form intrazeolitic oxides directly during calcination of ion-exchanged samples. 2.2.1 Application of Oxygen-Containing Salts 2.2.1.1 Titanium Oxide Clusters

Liu et al. proposed the preparation of well-dispersed TiO2 clusters in zeolite Y by ion exchange of NH4-Y with an aqueous solution of ammonium titanyl oxalate monohydrate [135]. This titanium salt is water-soluble and, unlike TiO(NO3)2 or TiOSO4 · H2O, does not form colloidal suspensions with particles too large to enter zeolitic pores [135, 136]. The exact nature of the Ti species in the solution is unknown, but the presence of monomeric TiO+ and dimeric Ti2O32+ forms is anticipated. A pH of 5 was reported for the salt solution, at variance to Klaas et al., who stated that TiO+ ions are stable only in strongly acidic solutions (vide infra, Sect. 2.4.1) [137]. Liu et al. reported that ion exchange in zeolites Y, mordenite and L is possible using ammonium titanyl oxalate solution, because a final pH of 7–8 is reached during the exchange procedure, preventing hydro-

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lysis of the titanium species. By contrast, suspensions of zeolites X and A yield a higher pH and cause the titanium solution to become colloidal [136]. X-ray diffraction studies of the exchanged zeolites confirmed the crystallinity of the host structure and the absence of a bulk TiO2 phase [135, 136]. Furthermore, it was shown by XPS that no enrichment of titanium occurs on the external zeolite surface. IR spectroscopic studies of ion-exchanged zeolite Y indicated the presence of TiO+ and Ti species with Ti-O-Ti linkages at titanium contents higher than 2 wt.%.After calcination, further polymerization of the Ti species was observed. In the UV-Vis diffuse reflectance spectra, an absorption onset between 300 and 360 nm was found for the different ion-exchanged zeolites. This is considerably blue-shifted in comparison to bulk TiO2, confirming the presence of very small guest oxide particles. Calcination and aging of the samples at ambient conditions cause the absorption onsets to red-shift which indicates aggregation of the Ti oxo-species and formation of larger particles [135, 136]. The profiles of the spectra vary slightly when different zeolite supports are applied. This was interpreted as a sign of the different shapes of TiO2 particles in different zeolites, being rather spherical in zeolite Y and rod-like in zeolite L and mordenite [136]. Yamashita et al. did not observe condensed titanium oxide structures in ionexchanged zeolite Y with a TiO2 content of 1 wt.% [64–67, 138]. The absorption onset in the UV-Vis DRS spectrum was reported to be at 270–330 nm, i.e., at higher energies than found by Liu et al. [135, 136]. From XANES and EXAFS measurements, Yamashita et al. invoked the presence of monomeric, tetrahedrally coordinated titanium oxo-species; no Ti-Ti neighbors were found [64–67, 138]. However, in catalysts impregnated with ammonium titanyl oxalate or titanium trichloride, oligomeric titanium oxo-species were detected even at a TiO2 content of 1 wt.%. At higher titanium loadings, the properties of incorporated titanium oxide begin to resemble those of bulk TiO2. 2.2.1.2 Vanadium Oxide Clusters

Wark et al. thoroughly characterized zeolite Na-ZSM-5 samples modified by ion exchange with aqueous vanadyl sulfate solution [139, 140]. They found that mostly vanadyl ions on exchangeable cation positions are present, if the exchange solution is strictly kept at pH 2.8 by addition of sulfuric acid. Some formation of (VO2.5)x clusters was invoked from the V 2p binding energies measured by XPS. However, since no absorption due to clustered, octahedrally coordinated V5+ oxo-species was detected in the UV-Vis diffuse reflectance spectra, it was assumed that the clusters are preferentially deposited near the external zeolite surface [139]. In vanadyl sulfate solutions of pH ª 3, hydrolysis of the cation occurs. In ZSM-5 samples exchanged with such solutions, the formation of clustered (VO2.5)x species inside the zeolite was observed by a characteristic absorption in the UV-Vis spectrum. It appears that the hydrolyzed vanadium oxo-species are small enough to penetrate the zeolitic pores [139–141]. Calcination has significant effects on the kinds of species present in the ZSM-5 channels: After evacuation of ZSM-5 containing predominantly vanadyl cations at 400°C, aggregation of the vanadium species is observed. However, calcination

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of H-ZSM-5 containing clustered (VO2.5)x at 550°C leads to a disruption of the aggregates. Isolated VO2+ ions are identified as active sites for the catalytic reduction of NO by NH3 [139–141]. 2.2.2 Hydroxide Treatment of Ion-Exchanged Zeolites 2.2.2.1 Nickel Oxide Clusters

Sano et al. presented a different approach for the preparation of intrazeolitic transition metal oxide clusters [142–144]: Zeolite Na-Y was conventionally ionexchanged in an aqueous solution of nickel nitrate. The exchanged and dried sample was then treated with aqueous NaOH solution at pH 10.5 [142]. EXAFS measurements of the resulting samples showed that [Ni(H2O)6]2+ ions were present in the exchanged, hydrated zeolite, whereas, after treatment with sodium hydroxide, coagulated nickel ions in small nickel hydroxide oligomers were observed. After calcination at 370°C, separately existing Ni2+ ions were found in the exchanged sample. The Ni-OH-Ni linked species, however, were transformed into small nickel oxide clusters, as shown by the corresponding EXAFS spectrum [142, 143]. No bulk NiO particles were observed in the TEM images, and X-ray diffraction confirmed the absence of a separate NiO phase [145]. An extension of these studies to other transition metals (Co, Cu, Fe, Mn, Cr, Zn) gave similar results [146]. Because of different tendencies of the metal hydroxides to aggregate, adjustment of the hydrolysis conditions is a prerequisite for the formation of well-dispersed oxide clusters. 2.2.2.2 Copper Oxide Clusters

Iwamoto et al. reported that ZSM-5 zeolites with an excess loading of copper ions are readily prepared in a single step through addition of basic compounds such as NH4OH or Mg(OH)2 to the copper nitrate exchange solution [147]. It was suggested that ion exchange occurs between Na+ and Cu2(OH)3+, Cu(OH)+, Cu2(OH)22+, or Cu3(OH)24+ at specific pH values. No destruction of the zeolite framework was observed in the X-ray diffraction pattern after the ion exchange. No information was given concerning the nature and structure of the copper species after calcination [147], but it can be expected that copper oxide clusters are formed in a similar manner to that reported for nickel. 2.2.2.3 Zinc and Cadmium Oxide Clusters

The formation of intrazeolitic zinc or cadmium oxide particles was also achieved by ion exchange of zeolites X, Y or EMT with the corresponding acetate solutions followed by precipitation of zeolite-encaged hydroxides in a sodium hydroxide solution [148–151]. After calcination at 400°C, the formation of CdO particles was confirmed by XPS measurements [151]. The zeolite-encapsulated

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zinc and cadmium oxides give rise to UV-Vis DRS spectra in which the absorption edge is shifted to higher energies in comparison to a sample of CdO or ZnO physically admixed with the zeolites [149, 150]. From this it was concluded that very small oxide particles are present. The particle size was estimated from the UV-Vis spectra and from TEM images to be between 5 and 12 nm; however, this exceeds the dimensions of faujasite supercages. This was rationalized in terms of a formation of mesopores during hydroxide precipitation and calcination. Xray diffraction and nitrogen adsorption measurements revealed that damage of the zeolite host structure had occurred, the extent of which depended on the experimental conditions: Higher concentrations of the sodium hydroxide solution, higher reaction temperatures and a prolonged time of treatment all led to the formation of larger mesopores and of guest oxide particles with larger diameters. Additionally, 29Si MAS NMR spectroscopy revealed that the zeolite nSi/nAl ratio decreases after NaOH treatment, indicating that, in a first step of mesopore formation, hydroxyl ions attack at silicon centers of the zeolite [149]. The leaching of silicon out of the host framework is more pronounced for the silicon-richer Y and EMT zeolites than for zeolite X [150]. As faujasites containing no zinc or cadmium are not damaged under these conditions, it was concluded that the simultaneous precipitation of metal hydroxides favors the formation of mesopores and that a partial destruction of the zeolite framework and the formation of metal oxide particles are interdependent [149, 150]. 2.2.2.4 Cerium and Aluminum Oxide Clusters

Hashimoto et al. reported that Ce3+ and Al3+ ions exchanged into zeolite mordenite from cerium acetate or aluminum nitrate solutions can be hydrolyzed by contacting the filtered and dried sample with moist ammonia gas at room temperature [152, 153]. X-ray diffractograms of the calcined materials confirmed the preservation of the zeolite structure and the absence of signals attributable to the guest oxides. From this it was concluded that the oxides are finely dispersed. Nitrogen adsorption measurements supported this conclusion; neither the generation of mesopores nor pore blocking was observed. The formation of an oxidic guest species was confirmed for the cerium-modified zeolite by XPS measurements that also showed that no enrichment of the outer zeolite surface with cerium had occurred [153]. 2.2.3 Ion Exchange with Hydrolyzed Metal Ions

Di- and trivalent cations exchanged into zeolites, besides occupying cation positions, are known to form zeolite-encapsulated oxidic compounds directly by calcination, because they are partially hydrolyzed in the exchange solution forming polynuclear complexes of the type Mx(OH)y2x–y+ [154–156]. During dehydration, dimeric or aggregated cations bridged by oxygens are formed. By adjusting the experimental conditions during the ion exchange, the formation of larger oxidic clusters can be enhanced.

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2.2.3.1 Zinc Oxide Clusters

The presence of zeolite-encapsulated zinc oxide clusters after calcination of ionexchanged sodalite or zeolites A and Y has been reported by several groups [157–161]. By X-ray diffraction, UV-Vis diffuse reflectance and luminescence spectroscopy, Türk et al. observed the formation of zinc oxide clusters after calcination of zinc chloride exchanged in zeolite A at 600°C [157]. However, no attempt was undertaken by these authors to interpret their results. Wark et al. showed that, due to the basic reaction of zeolite A in water, zinc ions of the exchange solution precipitate as hydroxide at the crystal surface, causing the pH value of the exchange solution to decrease significantly [161, 162]. The presence of precipitates on the zeolite crystals can be detected by scanning electron microscopy [162]. After calcination, the formation of bulk-like ZnO crystals was observed by XRD and UV-Vis DRS measurements. However, the presence of small zinc oxide clusters in zeolite A was also detected when the precipitation of zinc hydroxide during ion exchange was prevented by carefully controlling the zinc content and pH of the exchange solution [149, 159, 160]. In this case, Wark et al. proposed the formation of [Zn4O]6+ cluster ions in the sodalite cages of zeolite A [149]. However, on the basis of EXAFS measurements, Khouchaf et al. excluded this possibility [160]. They found Zn-Zn distances intermediate between those in ZnO and ZnAl2O4 and proposed that the next nearest neighbor environment of part of the Zn2+ located in the sodalite cages is formed by three bonds with oxygen atoms of the framework and one bond with a non-framework oxygen. Kazansky et al. suggested that the stability of [Zn(H2O)n]2+ species present in the supercages after zinc ion exchange in zeolite Na-Y is decreased during vacuum treatment at 300°C because of dehydration [158]. Therefore, the water molecules were assumed to hydrolyze Zn2+ cations resulting in the formation of acidic bridging hydroxyl groups and nanometer-sized clusters of zinc hydroxide which are further decomposed to zinc oxide. The resulting acidic protons were observed in diffuse reflectance IR spectra, while the zinc oxide clusters were detected by their UV-Vis absorption. Vacuum treatment at temperatures above 300°C was reported to result in a further dehydration of the zeolite occurring via reaction of the bridging hydroxyl groups with the ZnO nanoparticles. The process was accompanied by a decrease in the intensity of the IR bands from bridging hydroxyl groups and of the UV absorption bands from ZnO nanoclusters, whereas the increasing amount of Zn2+ cations at SII sites was detected in the IR spectra of adsorbed molecular hydrogen and CO [158]. 2.2.3.2 Tin Oxide Clusters

The formation of tin oxide nanoparticles inside zeolite Y can be achieved by ion exchange with aqueous solutions of SnCl2 and subsequent calcination [130, 148, 149]. Exchange solutions of a suitable pH are required, because exposure of zeolite Y to solutions of pH < 3 leads to severe damage of the crystalline structure. Even at pH > 3, some aluminum is removed from the framework, as shown by a

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signal of extra-lattice aluminum in the 27Al MAS NMR spectrum [149]. The formation of mesopores is detected by nitrogen sorption measurements. From TEM images it can be seen that, at high SnO loadings (>4 wt.%), oxide particles with a broad size range of 2 – 20 nm are formed which are inhomogeneously dispersed [130, 149]. At lower loadings, more homogeneous dispersions with particle sizes of 2 – 10 nm were found. Interactions of the embedded particles with the host were inferred from line broadening effects observed in 119Sn MAS NMR spectra [149]. 2.2.3.3 Iron Oxide Clusters

Iron(III) cations in aqueous solutions are known to undergo hydrolysis forming dimeric [Fe2(OH)2]4+ or [Fe2O2]2+ species at pH >1 [163–165]. When the ion exchange of zeolites Y or ZSM-5 is carried out in such exchange solutions, Fe3+ is not only present on cation positions after calcination, but also small Fe2O3 clusters are formed [163, 165–169]. Joyner and Stockenhuber observed by EXAFS that, in exchanged and calcined zeolite ZSM-5, iron is coordinated to oxygen and has further iron neighbors at short distances [169, 170]. The iron-iron coordination number was determined to be 2–3 and taken as a measure for the iron oxide cluster size. As XPS results revealed no surface enrichment of iron, it was concluded that the clusters are internally confined [169]. The high stability of the clusters against reduction in hydrogen was considered as an indicator for interactions with the zeolitic host. Inamura et al. proposed a detailed preparation procedure to incorporate iron in different environments into zeolite Y [166]. The support was treated in an iron(III) nitrate solution at room temperature, followed by heating to 50°C and slow cooling to room temperature. At different stages of the preparation procedure, samples were withdrawn, filtered and washed. From temperature-programmed reduction and sulfiding experiments, carried out after calcination of the samples in dry air at 380°C, it was concluded that different kinds of iron species are introduced in the different preparation steps [166]: At first, iron is located predominantly on cation positions. In calcined catalysts, withdrawn from the exchange solution during the heating period, small iron oxide clusters, located inside the supercages and featuring a strong interaction with the framework oxygen, substitute the exchanged species. With prolonged treatment, iron oxide without interaction with the zeolite framework oxygen atoms is produced. This model is supported by EXAFS measurements [166], ESR spectroscopy [171] and Mössbauer spectroscopy [164, 172]. However, some loss of zeolite crystallinity by dealumination was observed, becoming more severe with increasing time of the ion-exchange treatment. It was proposed that the dealumination proceeds through attack of the protons which are produced by the hydrolysis of Fe3+ ions and, thus, that the deposition of internally confined, lattice-bound iron oxo-species is coupled with the dealumination process [164, 171, 172].

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2.3 Solid-State Reactions

When ionic or molecular crystalline solids are mixed with a support of high specific surface area and then heated at a suitable temperature below their melting point for several hours, many of them disperse spontaneously onto the surface of the carrier [173]. This can be detected by the disappearance of their X-ray diffraction peaks. The formation of a (sub-)monolayer of the guest compound on the surface of the support has been proposed, and it has been claimed that this process is driven by a gain in entropy [173–177]. As described below, this phenomenon can be utilized for the incorporation of oxides into the pores of zeolites. 2.3.1 Molybdenum Oxide Clusters

Several groups have introduced molybdenum oxide into zeolites by calcining a mechanical mixture of MoO3 and the zeolite, containing sodium or ammonium ions, in air at elevated temperatures [50, 178–181]. Harris et al. reported that, after 16 h of calcination at 500°C, MoO3 is finely dispersed on H-ZSM-5 using oxide loadings of less than 8 wt.%, as inferred from the disappearance of the MoO3 X-ray diffraction peaks [50]. In TEM images, surface agglomeration of molybdenum oxide can only be observed if at least 10 wt.% of the guest oxide is present. Like impregnated ZSM-5, samples modified by a solid-state reaction have suffered framework damage that can be seen in the 27Al MAS NMR spectra. The presence of Mo6+ was confirmed by XPS measurements. From a reduction in the zeolite OH stretching band intensity visible in the IR spectrum, interactions between the guest oxide and the support have been invoked. The appearance of new bands in the lattice vibration region was taken as evidence for the presence of tetrahedral dioxomolybdenum species and the formation of T-OMo bonds [50]. As these new bands were not observed for Na-ZSM-5, Harris et al. proposed an important role of zeolitic (acid) hydroxyl groups in generating the tetrahedral Mo species. However, after calcination of mixtures of MoO3 and Na-ZSM-5 at 450°C for 24 h, Xiao et al. found a dispersion threshold of 8.8 wt.% molybdenum oxide by X-ray diffraction measurements [181]. This is very similar to the value obtained by Harris et al. and, hence, does not necessarily indicate a crucial role of acid sites for the spreading process. Xiao et al. reported that the dispersion capacity of molecular sieve supports is dependent on their pore size: For zeolite Na-A, no spreading of MoO3 was observed, while for MCM-41 and Na-Y, higher dispersion thresholds were found than for Na-ZSM-5 [181]. A homogeneous distribution of the guest oxide within the pores of Na-ZSM-5 was confirmed by electron probe microanalysis. From adsorption measurements with probe molecules of different kinetic diameters, the authors concluded that the guest oxide is located inside the supercages of zeolite Y at low loadings and near the pore windows at higher molybdenum content, thus decreasing the supercage accessibility [181]. Studying the stability of zeolite Na-Y in the solid-state reac-

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Fig. 4. Part of the structure of the Eu4Si4Al8O24(MoO4)2 sodalite, after [182]

tion, Thoret et al. found that, for calcination temperatures between 410 and 480°C, up to 16 wt.% MoO3 can be incorporated into the zeolite without the occurrence of framework damage [179]. Above 480°C and depending on the oxide concentration, progressive framework amorphization and the formation of new phases were observed. A higher amount of molybdenum oxide was inserted into lanthanum-exchanged zeolite Y, which was attributed to the space in the supercages left vacant by migration of La3+ ions into the sodalite cages and the hexagonal prisms during heat treatment [180]. Xenon adsorption measurements on molybdenum-loaded samples indicated a reduction in the pore diameter or even pore blocking, in support of the assumption of molybdenum oxide being incorporated into the pore system. The authors suggested that penetration of the guest oxide into the zeolitic pores is assisted by water released from the zeolite. Though the migrating species were not identified, various possibilities, such as MoO3 , Mo3O9 , or MoO42–, were envisaged [179]. Borgmann et al. reported that a solid-state reaction of Eu3+-exchanged zeolite X with ammonium molybdate at 500°C results in a migration of the molybdate species into the zeolite, which can be followed by decreasing intensities of the corresponding XRD reflexes [182]. Calcination of the modified zeolite at 900–1000°C induces the transformation into a rare earth sodalite containing a tetrahedral MoO4 group connected to four Eu3+ ions. The proposed structure of the resulting material is presented in Fig. 4. Bock prepared Mo-containing zeolites by solid-state ion exchange of the Hforms with MoCl3 [183]. By temperature-programmed oxidation and reduction experiments he was able to show that molybdenum cations thus introduced into zeolite L are oxidized forming MoO3-moieties (as described in Fig. 1) via different intermediate oxidation states. For Mo-containing zeolite ZSM-5, different structures, such as (Mo2O5)2+(Z–)2 (Z– = negatively charged zeolite framework site), were proposed on the basis of the TPO/TPR results.

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2.3.2 Vanadium and Tungsten Oxide Clusters

Thoret et al. studied the solid-state reaction of V2O5 and WO3 with zeolites Na-Y and LaNa-Y [179, 180]. For V2O5, similar results were obtained as in the case of MoO3 , whereas no incorporation of WO3 was achieved. These differences were explained by the significantly lower melting points and higher water solubilities of MoO3 and V2O5 . Results reported by Huang et al. indicate that the incorporation of V2O5 into Na-Y by a solid-state reaction is more difficult than into H-Y [184].As can be seen from the X-ray diffractograms, calcining a mechanical mixture of Na-Y and vanadium pentoxide for 4 h at 450°C does not lead to a fine distribution of the oxide, whereas for H-Y, a significant reduction in the diffraction peaks attributable to bulk V2O5 is observed. Zhang et al. [185] have studied in detail the solid-state reaction of V2O5 with H-ZSM-5. By photoluminescence spectroscopy, these authors showed that, in a sample containing 5 wt.% of V2O5 , highly dispersed V5+ oxide species in a distorted fourfold coordination state are present, probably connected to the framework silicon atoms through a bridging oxygen. The formation of tetrahedrally coordinated vanadium oxide species was also detected in the UV-Vis diffuse reflectance spectrum. From XRD measurements it was concluded that the framework structure of ZSM-5 is not damaged after calcination at 800°C for 5 h. No signals assignable to crystalline V2O5 were observed [185]. These findings were confirmed by IR spectroscopy: In the lattice vibration region, no change in the T-O-T band intensities was detected, and the bands characteristic of bulk V2O5 disappeared. The vanadium K-edge XANES spectrum of the modified H-ZSM-5 zeolite resembled those of reference compounds having an O3V=O unit. In the EXAFS spectrum, a peak due to the presence of neighboring V atoms could scarcely be observed, indicating that aggregated oxide species were not formed. By ESR measurements, Kucherov and Slinkin detected the occurrence of solid-state ion exchange between protons of H-ZSM-5 or H-mordenite and V2O5 during calcination of mechanical mixtures at 520–820°C (cf. also Chap. 2 of this volume) [186]. The authors found vanadium present in the +4 state and suggested the formation of isolated complex cation species like VO(OH)+ located on zeolite cation positions. Kucherov and Slinkin did not, however, address the fact that the vanadium had not been reduced but was present in the +5 state. The partial replacement of strongly acid protons in H-ZSM-5 by vanadyl cations was also observed by Petrásˇ and Wichterlová [187]. As at higher vanadium contents (>1 wt.%) the number of vanadyl species present exceeds the loss of acid protons, these authors suggested that VO2+ is furthermore connected via the surface Si-OH groups and present in thin V2O5 layers formed on the support. Zhang et al. estimated the contribution of VO2+ to the total amount of vanadium species in the solid-state reaction product to be only about 7%. [185]. Similar results were presented for the system V2O5/zeolite Beta: Again, most of the vanadium was found in the form of highly dispersed, tetrahedrally coordinated V5+ oxide species [188].

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2.3.3 Gallium Oxide Clusters

A solid-state reaction between Ga2O3 and H-ZSM-5 was observed when a mechanical mixture of the two components was treated in hydrogen at elevated temperatures [189–194]. By temperature-programmed reduction studies carried out in a microbalance, Price and Kanazirev found that, in an intimate physical mixture of H-ZSM-5 and Ga2O3 , gallium oxide is reduced below 680°C, while the reduction of pure Ga2O3 does not occur in this temperature range [194]. From the finding that hardly any or no reaction at all took place when Ga2O3 and zeolite Na-X or Ga2O3 and silicalite-1 were intimately mixed, it was concluded that the reduction of the gallium oxide phase requires the presence of acid sites. TGA measurements showed that the reduction is limited to one gallium atom per acid site. From X-ray diffractograms of the hydrogen-treated samples, it was deduced that the reflection intensities attributable to Ga2O3 are significantly reduced compared to the starting mixtures. In samples containing up to 5 wt.% gallium and treated with hydrogen at 490°C, no Ga2O3 was detected. The disappearance of a separate gallium oxide phase and dispersion of gallium throughout the zeolite crystallites was also observed in TEM images [193, 194]. The crystallinity of the H-ZSM-5 matrix was reported to remain intact during the solid-state reaction. From XPS measurements it was inferred that the formal oxidation state of gallium is lowered after hydrogen treatment, being between +1 and +3 [193, 195]. IR spectra showed a decrease in the intensity of the band due to acidic OH groups [193, 196]. From these results, Price and Kanazirev concluded that, during the solid-state reaction, volatile Ga2O is formed that migrates into the zeolite crystallites [194]. The presence of Ga2O in the zeolite channels has also been suggested by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies [197]. Price and Kanazirev further suggested that the migrating oxide is trapped by acid sites of the zeolite, undergoing solid-state ion exchange. However, the exact nature of the distributed gallium species has been a matter of debate; alternative proposals include an interaction between acid sites and a dispersed gallium oxo-species [95, 104, 189, 190, 198, 199]. Detailed thermal analysis investigations appear to support the occurrence of solid-state ion exchange [200]. Apart from H-ZSM-5, zeolites USY, H-ferrierite, [B,Al]-Beta and [B,Al]-ZSM5 have been reacted with Ga2O3 in oxygen or hydrogen at temperatures up to 500°C [201–204]. At gallium oxide contents of 5 wt.%, preservation of zeolite crystallinity after hydrogen treatment was observed by XRD and SEM, and no bulk Ga2O3 was detected [204]. From propane transformation studies it was concluded that, with ferrierite, blocking of the pore entrances by GaxOy occurs. For zeolite [B,Al]-ZSM-5, deboronation of the framework and insertion of some gallium into framework positions have been observed by 11B MAS NMR and IR spectroscopy. Si(OH)Ga units were also detected in USY modified by gallium oxide [201–204], but not in [B,Al]-Beta [204]. In the Raman spectra of the modified zeolites, bands were present which were also found after reduction of Ga2O3 with metallic gallium [201, 202, 204]. For zeolite USY, these characteristic bands were independent of the calcination atmosphere (O2 or H2) [202]. From these results the authors concluded that during the solid-state reaction in hydro-

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gen, reduced gallium oxide (Ga2O) is transferred into the channel system of the zeolites. Brønsted acidity of the support was not thought to be a prerequisite to facilitate the dispersion of gallium oxide [204]. Insertion of gallium into the framework of USY was found to be restricted to an amount of 6 Ga per unit cell, independent of the gallium on offer [201, 202]. 2.3.4 Miscellaneous Oxide Clusters

Fewer reports have been published on solid-state reactions for the preparation of other zeolite-encaged metal oxides. Sb2O3, B2O3 and MgO were found to spread spontaneously over H-ZSM-5 and Na-Y after calcination in vacuum or an inert atmosphere at up to 500°C [173, 205]. By X-ray diffraction, a dispersion capacity higher than the amount of oxide needed for a complete ion exchange was observed (12 wt.% of B2O3 on H-ZSM-5 with nSi/nAl =50). From the paraselectivity observed in the methylation of xylene, it was concluded that the pore diameters of the parent zeolites are reduced by the modification [205]. Nickel oxide supported on ZSM-5 (nSi/nAl > 100) was prepared by calcining a mixture of nickel nitrate and the zeolite in air at 650°C [199]. At a nickel oxide content of less than 10 wt.%, no damage of the zeolite structure and no separate NiO phase were observed in the X-ray diffractogram or SEM image. An interaction between the guest oxide and the support was inferred from a shift in the Ni 2p3/2 binding energy compared to bulk nickel oxide [199]. Calcination of a mixture of zeolite ZSM-5 with iron(II) chloride or iron(II) carbonate was reported to yield mainly relatively large, magnetite-like iron oxide clusters on the external zeolite surface [169]. Upon treating a mixture of 5 wt.% ZnO and 95 wt.% Na-ZSM-5 at 500°C, some ion exchange between Na+ and Zn2+ as well as the formation of zinc oxide dispersed inside the zeolite pores were observed by EXAFS measurements [206]. When dehydrated zeolite Na-Y was reacted with Ru3(CO)12 in vacuum at 170°C, metallic ruthenium was formed which was oxidized to RuO2 upon contact with air at room temperature [207]. For a sample with a ruthenium content of 3 wt.%, no peaks attributable to RuO2 were detected in the X-ray diffractogram, nor were RuO2 particles observed in the TEM images. Heating in air caused aggregation of ruthenium oxide that could then be detected as separate particles. Instead of a solid-state reaction initiated by thermal treatment, Zhu et al. proposed the use of microwave irradiation to prepare Al2O3 or MgO dispersed on zeolites Y, L, ZSM-5, Beta, AlPO4-11, and hexagonal mesoporous silica [31, 208–210]. Irradiating a mixture of zeolites K-L or K-Beta and 5 wt.% MgO for 20 min with 2.45 GHz led to the disappearance of the X-ray diffraction pattern attributed to magnesium oxide, indicating a fine distribution of the guest oxide. The host structure was not influenced by the treatment [208]. With Na-ZSM-5 and AlPO4-11 applied as supports, trace amounts of bulk MgO remaining after the treatment were found. This was explained by the smaller pore size of these molecular sieves. Spreading of alumina on zeolite Na-Y can be achieved up to loadings of 29 wt.% [31, 210]. Microwave irradiation was shown to be more efficient for the dispersion of the oxides than thermal treatment. The CO2 TPD

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profiles of irradiated MgO/K-L mixtures are significantly different from those of the pure materials or non-treated mixtures, thus indicating a modification of the zeolite and oxide surface properties and the generation of new strong base sites by the MgO dispersion [208, 209]. 2.4 Chemical Vapor Deposition and Related Techniques

The application of chemical vapor deposition (CVD), denoting the reaction of a zeolite support with a gaseous reactant to form oxides, offers some advantages over the methods described so far: Problems caused by acidity of the ion exchange or impregnation solutions can be ruled out. The gaseous molecules employed for CVD can often be chosen from a reasonably broad variety of candidates. In particular, small enough molecules are often available which can enter the pores of a given zeolite. Furthermore, the anchoring of a guest compound by reaction with zeolite OH groups should lead to a uniform distribution and the formation of small, reactive clusters. The preparation procedures which are described below can be distinguished according to the oxide precursor used in the CVD reaction: Volatile metal chlorides and oxychlorides as well as carbonyl compounds, metals, metal alkoxides, and metal acetates have been applied. 2.4.1 Chemical Vapor Deposition of Chlorides

Volatile metal chlorides are employed above all to incorporate titanium, vanadium, and molybdenum oxides into zeolitic pores; the introduction of chromium [211], tin [149, 212], niobium [213], and zinc [214] has been studied to a much lesser extent. 2.4.1.1 Titanium Oxide Clusters

The chemical vapor deposition of TiCl4 onto various zeolites, i.e., faujasite, L, erionite and mordenite, was described as early as 1976 by Komarov and co-workers [215]. The CVD was carried out in a U-tube by passing a stream of dry air saturated with TiCl4 through the zeolite. The tetrachloride adsorbed by the zeolite was then hydrolyzed in moist air or in ammonia solution. The authors reported that, because of the HCl formed in the CVD procedure, the stability of the zeolite framework is dependent on its acid-resistance. Calcination of the modified zeolites at higher temperatures (860–1000°C) results in an exothermic transformation of supported TiO2 clusters into the crystalline modification of anatase accompanied by a disruption of the zeolite structure [215]. The introduction of TiO2 into zeolites L and mordenite by chemical vapor deposition of TiCl4 and subsequent room-temperature hydrolysis has also been reported by Kim et al. [213, 216]. These authors did not observe damage of the zeolite structure at titanium loadings of 3–7 wt.%. X-ray diffractograms gave no hints at a separate TiO2 phase. The UV-Vis absorption maxima of zeolite-supported tita-

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nium oxide clusters are blue-shifted in comparison to bulk TiO2, which has been explained by the presence of very small oxide particles [213]. Attempts to incorporate titanium oxide into the channels of silicalite-1 by treating the zeolite in a stream of TiCl4-saturated nitrogen at 450°C failed [217]: SEM images of the obtained materials revealed that, after hydrolysis, the titanium oxide particles are located on the external zeolite surface. Schulz-Ekloff, Jaeger and co-workers studied the incorporation of titanium into faujasites by chemical vapor deposition in greater detail [137, 212, 218– 220]. They described their preparation procedure as follows: The dehydrated zeolite powder (Na-Y, Na-X) located in a flow reactor under shallow bed conditions is treated in a stream of dry nitrogen loaded with TiCl4 vapors. The reaction temperature is kept at 100 or 400°C during the treatment that lasts up to 1 h. Dehydration of the zeolite has to be ensured to prevent hydrolysis of the TiCl4 and agglomeration during the CVD treatment. After removing excess TiCl4 in a stream of nitrogen, the chemisorbed TiCl4 is hydrolyzed by a water-saturated nitrogen flow at the reaction temperature. The samples are finally calcined in air at 400°C [137, 219]. The titanium contents reached by this procedure are around 2 wt.% as determined by wet chemical analysis or AAS. As a prolonged reaction time leads to destruction of the zeolite framework structure, the CVD cycles have to be repeated in order to obtain sufficiently high titanium loadings. Even then, a loss of zeolite crystallinity is observed which is attributed to the influence of HCl formed during the hydrolysis step, resulting in a cleavage of Al-O-Si bonds, a local fragmentation of the zeolite framework and the appearance of mesopores [219]. Investigations into the nature of titanium oxide inside the zeolitic pores, carried out by UV-Vis diffuse reflection spectroscopy, revealed the formation of three different species characterized by absorption maxima at 205, 220 and 280 nm [137, 212, 219]. The band positions depend on the oxygen coordination sphere around the titanium: The absorption maximum at 280 nm is attributed to a species bound to the zeolitic framework in a monofunctional manner, as described in Scheme 1a. The reaction of TiCl4 with vicinal OH groups of the zeolite leads to the bifunctional bonding shown in Scheme 1b, indicated by the band at 220 nm. After hydrolysis, the resulting isolated Ti(IV)Ox species are mononuclear, i.e., no Ti-O-Ti bonds are formed. This explains the blue-shift of the UVVis absorption onset in comparison to bulk TiO2 . A formation of bulk anatase can be ruled out from the absence of any absorption at 144 cm–1 in the Raman spectrum, this band being a very sensitive probe of bulk TiO2 .

Scheme1a

Scheme1b

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The relative amount of these two Ti(IV)Ox species formed in the zeolite depends on the reaction temperature during the CVD procedure. At 400°C, the tendency of TiCl4 to bind bifunctionally to vicinal OH groups of zeolite Na-Y is enhanced, presumably facilitated by the higher flexibility of the zeolite framework [219]. XPS analysis of surface nTi /nSi ratios shows an enrichment of Ti on the outer surface of the Na-Y crystals which is more pronounced after low-temperature (100°C) CVD treatment. After loading titanium onto Na-Y at 400°C, a third titanium species was identified by a UV-Vis absorption maximum at 205 nm in a freshly prepared sample [137, 218, 219]. Such an absorption maximum is also detected in the spectrum of TS-1 which possesses Ti(IV) tetrahedrally coordinated in the framework. A similar species may be obtained by multifold coordination of TiCl4 to zeolite NaY that is possible at defect sites, i.e., at hydroxyl nests [221]. These sites originate from a removal of aluminum atoms from framework positions by the HCl formed during the reaction of TiCl4 with the OH groups of the zeolite [137]. The tetrahedral Ti(IV)Ox species in Na-Y, however, is unstable under ambient conditions; after a period of several months, the UV-Vis spectra show a red-shift, indicating an increase in the coordination number of TiOx [137, 218]. Zeolite Na-X is not stable during CVD treatment with TiCl4 at 400°C; an increasing framework collapse is observed with time-on-stream. The silanol groups formed by dealumination enable the attachment of larger amounts of TiCl4 in a monofunctional bonding, probably leading to the formation of Si/Ti mixed oxides. No signal attributable to bulk anatase is detected in the Raman spectrum [137]. Titanium dioxide clusters with Ti-O-Ti bonding can be obtained by multiple loadings of zeolite Na-Y with TiCl4 . During a second and third CVD cycle, the TiCl4 reacts preferentially with OH groups of the already zeolite-bound mononuclear Ti(IV)Ox species under formation of Tix(IV)Oy clusters with x > 1 and sizes smaller than 2 nm which seem to migrate more deeply into the zeolite [212, 219]. In view of a potential application of such materials as sensors, their oxidation and reduction rates at 500°C have been examined with optical detection of Ti(III) d-d electron transitions [212, 219, 220]. While titanium tetrahedrally bound in the zeolite framework is not reducible, the mononuclear Ti(IV)Ox species can be reduced with H2 , and the rate of their reduction increases with increasing amount of bifunctionally bound Ti(IV)Ox .With increasing size of the titanium dioxide clusters, a more and more pronounced induction period is found for the reduction, leading to longer response times. In summary, the zeolite-supported Ti(IV)Ox species show a complete reversibility of the UV-Vis extinctions for multiple redox cycles and response times, which were shortened by a factor of about 10 in comparison to bulk TiO2 [219]. Changes in the ratio of CO/air mixtures could be monitored with only short delay [222]. 2.4.1.2 Vanadium Oxide Clusters

As reported by Whittington and Anderson, treatment of dehydrated zeolites H-ZSM-5 and silicalite-1 with VOCl3 vapor in a nitrogen stream at 520°C followed by room-temperature purging with nitrogen introduces vanadium species

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resistant to room-temperature washing with 1 M HCl [211, 223]. Washing with H2O2 removes most of the vanadium from the zeolites modified in this way, so that they are unsuitable for application as oxidation catalysts when H2O2 is used as an oxidant. The dispersion of the vanadium species in the zeolitic pores has been studied by IR spectroscopy [211]: With increasing amounts of VOCl3 introduced, all of the Brønsted acid sites of H-ZSM-5 and ca. 85% of the silanol groups are removed, indicating that VOCl3 has access to most, if not all, of the zeolite channels. 40% of the original Brønsted sites cannot be regenerated by treatment with water vapor at 320°C, which has been explained by a partial framework dealumination caused by the HCl formed during the CVD treatment or present in the washing step. The loss of aluminum has been confirmed by XPS measurements which showed an increase in the nSi/nAl ratio. Nevertheless, no framework damage could be observed by X-ray diffraction [211].Whittington and Anderson studied the nature of the zeolite-incorporated vanadium species by XP and ESR spectroscopy, and by acidity measurements [211, 223]. They found a vanadium 3p3/2 binding energy of 517.0– 517.2 eV that suggests the presence of V2O5 . This value was later confirmed by Wark et al. [139, 141] who prepared vanadium oxide anchored onto ZSM-5 at 100°C (H-ZSM-5) or 250°C (Na-ZSM-5) in a similar procedure to that described above for TiOx/faujasite. By comparison with vanadyl ion-exchanged zeolites, Wark et al. were able to attribute the measured binding energy to clustered V2O5 on the external zeolite surface. While Whittington and Anderson observed similar nV/nSi ratios on the zeolite surface and in the bulk [211], Wark et al. reported the external zeolite surface to be highly enriched in vanadium [141], which is probably due to the lower CVD reaction temperature in their preparation method. From the results of UVVis diffuse reflectance studies, Wark et al. suggested the coexistence of small intrazeolite (VO2.5)x clusters and V2O5 aggregates on the external surface [141, 220]. Both groups stated that most of the vanadium present in the zeolite is in the oxidation state +5 [139, 211, 223]. Besides, in H-ZSM-5 (and to a lesser extent in silicalite-1), paramagnetic VO2+ species [vanadyl ions or (HO)2VO] can be detected by ESR measurements. According to Whittington and Anderson, a reduction to V4+ occurs by reaction of the V5+ species with HCl generated in the CVD reaction and with Brønsted acid sites [211].Acidity measurements through exhaustive Na+ ion exchange and subsequent titration of the liberated acid showed that the total concentration of acid sites of VOCl3-treated H-ZSM-5 is considerably less than the sum of the residual aluminum and the vanadium introduced. From this, it can be concluded that a substantial proportion of the V5+ introduced does not carry an acidic OH group, but is rather present as (∫SiO)3V=O. When all-silica MCM-41 is subjected to chemical vapor deposition of VOCl3 at 100°C, the reaction time must be restricted: After a treatment for more than 15 min about 20% of amorphous non-porous by-products were found [88]. After a reaction time of 5 min, a vanadium content of 1.5 wt.% was achieved which could be increased to 2.1 wt.% without significant damage to the wellordered MCM-41 structure by triple repetition of the CVD cycle. XP and UV-Vis spectra showed the presence of vanadium atoms mononuclearly dispersed in the pores of MCM-41 and largely tetrahedrally coordinated with oxygen [87]. In

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addition, the formation of chain-like oligomeric vanadium oxide clusters could be identified by a shoulder around 320 nm in the UV-Vis diffuse reflectance spectra. These clusters are more readily reduced by H2 than the mononuclear species [87]. The amount and size of the oligomeric clusters are larger after CVD treatment than in a sample with a lower vanadium content of 0.65 wt.% prepared by impregnation with vanadyl acetylacetonate. Increasing the number of CVD cycles does not lead to the formation of larger oligomers, probably because they are destroyed during the calcination steps due to the influence of HCl formed. It has been suggested that the mononuclear vanadium oxide species is more tightly bound to the MCM-41 framework [88]. Song et al. chose vapor phase adsorption of vanadyl trichloride at room temperature in a vacuum line followed by hydrolysis with water vapor and calcination in air for the preparation of AlPO4-5-supported vanadium oxide [224]. On the basis of ESR studies of samples reduced in H2, they suggest the formation of well-dispersed vanadium species at low loadings forming aggregates when the vanadium content is increased. When the vanadium content exceeds 7 wt.%, a collapse of the AlPO4-5 structure is observed. 2.4.1.3 Tin Oxide Clusters

A CVD procedure as described for TiOx /faujasite has also been applied to introduce tin oxide clusters into zeolites [149, 212]. The loading of faujasites with SnOx particles by reaction of zeolitic OH groups with SnCl4 is, however, restricted to about 2–3 wt.%. Higher loadings as achievable by multiple CVD cycles are accompanied by a significant removal of aluminum from framework sites causing collapse of the zeolite structure. At low loadings, SnO2 nanoparticles with diameters of less than 1 nm are formed which are invisible for transmission electron microscopy. 2.4.1.4 Iron-Molybdenum Oxide Clusters

Yoo and co-workers presented a preparation method for special oxidation catalysts by introduction of iron and molybdenum into molecular sieves via CVD [225–231]. This method was later adopted by Centi et al. [232]. Zeolites of the MFI structure type, such as silicalite-1, H-ZSM-5, and a partially deboronated borosilicate molecular sieve H-AMS-1B-3, denoted as DBH, were applied as supports. The molecular sieve DBH was ion-exchanged with ammonium acetate and calcined prior to being subjected to the chemical vapor deposition reaction. The CVD process involved subsequent loading of the molecular sieves with FeCl3 at 460°C and MoO2Cl2 , MoCl4 , or MoCl5 at 200–300°C in a nitrogen stream [225, 232]. The chloride salts were placed near the support. After consumption of the salts, the stream of nitrogen was reversed to reuse FeCl3 or MoO2Cl2 condensed in cooler zones of the reactor and to achieve a uniform deposition along the axial reactor profile. The catalyst loaded with FeCl3 was washed with deionized water to eliminate chlorine ions and to form iron hydroxide-type species, before the molybdenum chloride was introduced in a second

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Fig. 5. Preparation of ferric molybdate clusters in molecular sieves, after [232]

CVD step. The final catalysts were calcined in nitrogen and air at a maximum temperature of 650°C. The different steps of the preparation procedure along with the species probably formed are summarized in Fig. 5. Zajac et al. prepared catalysts with metal contents around 10 wt.% and nMo/nFe ratios of 1.4–3.3 [231]. By Raman spectroscopic measurements, they showed that DBH samples of higher molybdenum content comprise Fe2(MoO4)3 and MoO3 clusters inside the pores together with surface MoO42– and polymeric surface molybdate. The surface species and part of the MoO3 were sublimed out of the sample by prolonged calcination at 650–690°C. This led to a selective catalyst for the partial oxidation of p-xylene to terephthaldehyde, however, only if DBH was used as support [225, 226, 231]. High-resolution transmission electron images of a fresh CVD sample revealed a fine deposition of iron and molybdenum in rod-like shaped domains with diameters ranging from 1–10 nm along the micropore channels of the support. Upon prolonged calcination, the rod-like structures rearranged into nearly spherical particles with sizes ranging from 2 to 40 nm which were dispersed within the DBH molecular sieve. Some larger particles (>50 nm) were also observed on the external surface of the support. By energy dispersive X-ray emission the particles were shown to consist of Fe2(MoO4)3 and FeMoO4 [226, 231]. Only slight destruction of the molecular sieve support was observed, whereas silicalite-1 and H-ZSM-5 were more severely damaged by the CVD treatment. The ferric molybdate was not formed when the order of CVD treatment was reversed, i.e., when Mo2O2Cl2 was deposited first [230]. The authors presented a model for the high selectivity of the DBH molecular sieve loaded with ferric molybdate in the partial oxidation of

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Fig. 6. Structures of p-xylene and ferric molybdate clusters, after [231]

p-xylene [225, 227, 229, 231]. Because of the good geometrical fit, the p-xylene molecule is expected to align with two Mo centers of the ferric molybdate cluster through interaction with hydrogen atoms of the two methyl groups (cf. Fig. 6). At the same time, the iron atom is proposed to interact with p-electrons of the benzene ring. In this way, the abstraction of two hydrogen atoms, one from each methyl group, can occur in a concerted manner leading to a biradical which is further converted to terephthaldehyde. Centi et al. studied in detail the CVD treatment of silicalite-1 or H-ZSM-5 with FeCl3 and MoO2Cl2 to find out why it does not lead to a catalyst selective in the oxidation of p-xylene [232]. On the basis of UV-Vis-NIR diffuse reflectance measurements they suggested that the reaction of FeCl3 with the Brønsted acid sites of H-ZSM-5 does not lead to a stable anchoring. Instead, the complex is assumed to decompose during the subsequent hydrolysis step causing the formation of small iron oxide and iron hydroxide particles that are supported on the zeolite channel walls but not directly anchored to the zeolite framework. On the other hand, calcination of the ammonium acetate exchanged molecular sieve H-AMS-1B-3 causes removal of a part of the framework boron and the formation of hydroxyl nests into which iron may insert during the chemical vapor deposition. The multiple interaction of iron ions with framework oxygens is expected to stabilize the zeolite-bound species [232]. If a highly defective silicalite-1 sample is used as host zeolite, a selective catalyst similar to the modified H-AMS-1B-3 molecular sieve is obtained, thus indicating the necessity of FeCl3 reacting with defect sites of the support. The differences in the catalytic performance were explained by deformations in the Mo-O-Fe bonds due to the anchoring and by variations in the redox behavior of iron partially incorporated into the zeolite framework. 2.4.1.5 Molybdenum Oxide Clusters

Kucherov and Slinkin reported on the introduction of molybdenum into zeolites H-ZSM-5, H-MOR, and H-Y by an “in situ formed gas phase species” resulting from the reaction of a physical mixture of MoO3 and the zeolite at 200°C with an air flow containing CCl4 [233–235]. The authors suggest the migration of (MoO2)+, (MoOCl2)+, or (MoCl4)+ ions into the zeolitic pore system and the stabilization of these species at cationic positions. The Mo(V) can be quantitatively oxidized by calcination in air at 200–300°C; however, no details about the oxide species formed were given.

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2.4.2 Chemical Vapor Deposition of Carbonyls

A different approach towards the incorporation of metal oxide clusters into zeolitic pores via chemical vapor deposition has been studied extensively by Ozin et al. [236–240]. They developed a method denoted as “intrazeolite metal carbonyl phototopotaxy”. Metal carbonyls are used as precursors to obtain the occluded guest component because of their volatility, fitting molecular dimensions, ease of purification, ready availability, and facile and quantitative conversion to the respective metal oxide materials with minimal contamination by carbon [236, 240]. The metal carbonyl precursors are transformed into the metal oxides by photochemical oxidation. The term “phototopotaxy” is meant to indicate the similarity of this preparation method to epitactical growth of semiconducting oxide layers on planar surfaces commonly used to form low-dimensional quantum nanostructures for applications in electronic and optical devices [238]. 2.4.2.1 Molybdenum and Tungsten Oxide Clusters

Zeolite Y in its H+-form or ion-exchanged with alkali metal cations has been employed as host material. The dehydrated zeolites, pressed into self-supporting wafers, are exposed to molybdenum or tungsten carbonyl vapor under dynamic vacuum. The degree of loading is controlled in situ by IR spectroscopy, taking the carbonyl stretching bands as indicators. To decompose the metal carbonyl, the sample is irradiated at l >240 nm at room temperature in the presence of 53 kPa of oxygen. The photooxidation takes approximately 1 h [236, 240]. Howe et al. showed that Mo(CO)6 adsorbed in Na-Y can alternatively be decomposed at 200–400°C in vacuum [241–243]. The molybdenum formed is oxidized upon exposure to oxygen at room temperature or 400°C. The authors reported that no enrichment of molybdenum oxide on the external surface of the Na-Y host is observed after this kind of treatment. It has been reported that during evacuation of Mo(CO)6 adsorbed in zeolite H-Y at elevated temperatures, oxidation of molybdenum occurs [242–244]. By IR spectroscopy, the formation of covalent Mo-O bonds involving framework oxygen was detected after activation up to 200°C. Ozin et al. studied in detail the location of the guest compounds, the interactions with the zeolitic host and the mechanism of the deposition reaction and the photooxidative decomposition. By elemental analysis they found that a saturation loading of two M(CO)6 precursor molecules (M=Mo,W) per supercage of zeolite Y can be achieved by CVD. XPS indicates the presence of unoxidized M(CO)6 [236]. EXAFS measurements of tungsten or molybdenum hexacarbonyl incorporated into Na-Y showed that the M(CO)6 molecule maintains its structural integrity with only minor perturbation of the ligand bonds compared to the free molecule [238, 240]. The location of the carbonyl molecules inside the pores was proved by XPS measurements showing no metal enrichment on the external surface. When H-Y was applied as support, IR spectra revealed that all supercage protons participate in hydrogen bonding to the carbonyl guests at the saturation level. By X-ray diffraction and 29Si MAS NMR spectroscopy the integrity of the zeolite framework structure was confirmed.After the photooxidation,

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an nM/nO ratio of 1:3 was detected by elemental analysis, suggesting the formation of MO3 clusters [236, 239, 245]. Again, no evidence was found for lattice breakdown or migration of the oxidic clusters to the external surface of the host. A high-resolution transmission electron micrograph lattice image of WO3/ Na-Y showed no segregation of the guest oxide into domains [246]. Raman spectra of WO3/Na-Y revealed the absence of vibrational modes attributable to bulk WO3, even at the heaviest loading of four WO3 per supercage which was achieved by sequential loading/photooxidation steps [236]. Instead, IR, Raman and UV-Vis absorption spectra indicated the appearance of a single kind of intrazeolitic WO3 cluster species over the complete loading range which in H-Y is bound to the supercage protons. EXAFS investigations concerning the structure of the tungsten oxide clusters anchored in zeolite Na-Y revealed two different WO bond lengths with coordination numbers around 2 for Na-Y with a loading of two WO3 molecules per supercage [238]. Furthermore, a relatively short tungsten-tungsten distance was found (3.3 Å). This was explained by the formation of a tungsten(VI) oxide dimer with two terminal and two bridging oxygens, occupying the supercage, as shown in Fig. 7a. The terminal tungsten-dioxo bond lengths of 1.77 Å are intermediate between those having formal bond orders of 2 and 3/2 which provides indirect evidence for the interaction of the terminal tungsten-dioxo groups of the (WO3)2 guest with extra-framework Na+ cations. The same results were found in the EXAFS studies of Na-Y with a higher WO3 content, leading to the conclusion that the sequential addition of WO3 units by CVD/photooxidation steps enhances the (WO3)2 dimer population. This results in an accumulation of supercage-encapsulated dimers-of-dimers, {(WO3)2}2 , rather than in cluster growth to trimers (WO3)3 and/or tetramers (WO3)4 [238]. In contrast to these results, no evidence for dimer formation was found for MoO3/Na-Y [245]. Instead, the EXAFS results were consistent with MoO3 monomers anchored by three oxygen atoms in a zeolite six-ring site, as sketched in Fig. 7b.Anderson and Howe [241] also found evidence for the presence of isolated Mo6+ oxo-species in Na-Y. The interaction of tungsten oxide species with the host zeolite and especially the anchoring site can be studied by 23Na MAS NMR spectroscopy in combination with Na+ far-infrared and proton mid-infrared spectroscopy [247]. For 16 (WO)3/H16Na40-Y it was observed that the OH stretching vibrations of supercage

a

b

Fig. 7a, b. Structural model derived from EXAFS data for a 16(WO3)/Na-Y [238] and b 16(MoO3)/Na-Y [239]

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and sodalite cage protons remain undisturbed, while the Na+ far-infrared and 23Na MAS NMR spectra revealed shifts in the signals of the site II Na+ cations. These shifts were even more pronounced when Na56-Y was employed as host material. If 32 WO3 molecules are incorporated into the unit cell of H16Na40-Y, the intensity of the OH stretching modes is reduced to zero, and a broad band appears which is shifted to lower frequencies indicating the formation of hydrogen bonds to the guest oxide. These results reveal a preferential binding of the (WO3)2 dimers to supercage Na+ cations rather than Brønsted protons in zeolite Y [238]. The protons participate in the anchoring of the guest oxide only at higher loadings. 23Na DOR MAS NMR spectroscopy was used as an even more sensitive tool to prove the interactions between WO3 or MO3 guest molecules and the Na+ site II cations of Na-Y [239, 248]. The chemical shift and the intensity of the site II cation signal are dependent on the kind of guest present (carbonyl or oxide) and on the degree of loading. By combining all methods mentioned (XPS, EXAFS, NMR, IR, Raman, XRD, and chemical analysis), Ozin et al. [239, 245, 246, 249] further investigated the redox behavior of intrazeolitic molybdenum and tungsten oxides and the structures of the incorporated (MO3-x)n clusters. On the basis of their results, they proposed the schemes presented in Fig. 8. Ozin et al. [237, 250–254] and Özkar [255] summarized their results in several reviews, where they also emphasized their understanding of the void spaces in zeolites as macrospheroidal or macrocylindrical multidentate ligands (“zeolates”) structurally comparable to crown ethers. Acid-base interactions of guest molecules with this “zeolate” ligand polarize and activate the adsorbed guest towards a number of reactions involving Brønsted acid sites and metal cations which can lead to anchoring [253]. The channels and cavities of zeolites impose constraints on the spatial arrangement of an occluded guest, as well as a predetermined orientation. A key feature of the topotactic chemistry in the supercage of zeolite Y is seen to be the “essentially” tetrahedral distribution of Na site II cations that enables a kind of lock-and-key docking [250]. 2.4.2.2 Iron Oxide Clusters

Similar to the tungsten or molybdenum carbonyl clusters, Fe(CO)5 entrapped in zeolite Na-Y can be photolyzed by a pulsed laser and further oxidized by introduction of oxygen [256]. XRD measurements and scanning electron micrographs showed no evidence for bulk Fe2O3 particles in the material prepared in this way. In the UV-Vis diffuse reflectance spectrum, the absorption is blueshifted in comparison to bulk Fe2O3 , indicating the formation of small clusters. A more detailed investigation of iron oxide clusters in zeolite Na-Y was reported by Bein et al. [257–259]. They described the preparation procedure as follows: Dehydrated zeolite Na-Y was saturated at 20°C with Fe(CO)5 vapor during an equilibration period of 6 h. Excess carbonyl was removed by degassing. Thereafter, the sample was cooled to liquid nitrogen temperature, and dry oxygen was admitted at a pressure of 15 kPa. In order to avoid overheating during the oxidation reaction and consequently agglomeration of the supported species, the reaction vessel was allowed to warm to room temperature over a period of

Fig. 8. Redox behavior of tungsten and molybdenum oxide clusters in zeolite Na-Y, after [239, 246]. ● Na+ cations

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10 h. The sample was again evacuated, and dry oxygen at a pressure of 100 kPa was admitted. The resulting material contained 9 wt.% iron, as determined by X-ray fluorescence [257]. The sorbed Fe(CO)5 can just as easily be transformed into Fe2O3 by oxidation in the dark at room temperature, using a flow of helium which contains 0.1% of oxygen [258]. TEM images at high resolutions subsequently show no resolved iron-containing clusters on the zeolite support which indicates that the supported guest particles have an average size smaller than 2 nm and are finely dispersed in the intracrystalline void space of zeolite Y. XRD shows no lines other than those attributable to the zeolite. From Mössbauer measurements, the presence of iron(0), iron carbides, Fe(II) and Fe(III) cations at exchangeable positions as well as of unreacted iron pentacarbonyl in the zeolite has been excluded. The values of the hyperfine field and the relaxation behavior of the system, however, suggested that very small particles of either a-Fe2O3 or g-Fe2O3 were present. This finding was confirmed by EXAFS which revealed the main peak at a nearest-neighbor distance of d = 0.18 nm typical for iron(III) oxides [257, 258]. Anderson and Howe confirmed that care has to be taken when zeolite-incorporated iron oxide clusters are to be prepared [241]. They decomposed the adsorbed Fe(CO)5 precursor molecules in vacuum at 400 °C, followed by exposure of the sample to oxygen at room temperature and further oxidation at 400 °C. This treatment leads to iron oxide clusters located mainly on the external surface of zeolite Na-Y. Interestingly, the replacement of Na+ by Co2+ or Ni2+ in zeolite Y inhibits the migration of iron to the external surface of the zeolite, thus indicating an interaction of the iron species formed in the different preparation steps with the zeolitic cations. Okamoto et al. prepared iron oxide clusters within Na-Y, Na-X, H-Y, K-Y, K-L, Na-MOR and Na-ZSM-5 by oxidation of zeolite-confined iron carbonyl at 1.3 kPa of gaseous oxygen in a circulation system made of glass [260]. The oxidation temperature was increased from –15 °C at a rate of 0.3 °C/min to – 3 °C. After consumption of the oxygen, the procedure was repeated until no further oxygen conversion could be observed. Finally, the temperature was raised to 85 °C and kept for 12 h to achieve complete oxidation. The whole incorporation procedure can be repeated in order to obtain higher loadings. The authors showed that iron carbonyl is not able to enter the pores of Na-ZSM-5. For the other zeolites, formation of intrazeolitic Fe2O3 was confirmed by XPS on the basis of the Fe 2p2/3 binding energy. The saturation amount of incorporated iron after several CVD cycles was highest with Na-Y, Na-X, and H-Y (8.4–9.5 wt.%) and decreased with decreasing pore size (K-L: 3.5 wt.%, Na-MOR: 2.5 wt.%). These findings are in good agreement with the results of adsorption measurements reported by Bein et al. [259].As the maximum loading of iron oxide in Na-Y corresponds to only 32% of the total volume of the supercage [260], it was suggested that further adsorption of Fe(CO)5 is limited by pore mouth narrowing caused by the iron oxide deposited.An HREM image of loaded zeolite Na-Y showed that the zeolitic framework was not damaged by accommodation of the oxide and that no agglomerated iron oxide particles were present on the external surface of the host. From the corresponding Mössbauer spectrum it was concluded that the particle size of the supported iron oxide was significantly smaller than 8 nm in diameter. This was supported by EXAFS measurements which showed that

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the Fe-Fe coordination numbers of the iron oxides in Na-Y are much smaller than those of bulk Fe2O3 and do not vary with the Fe content in the supercage, indicating the formation of highly dispersed iron oxide clusters comprising only a few Fe atoms [260]. 2.4.3 Chemical Vapor Deposition of Miscellaneous Compounds

Van der Voort et al. [261–263] prepared vanadium oxide species in the mesoporous material MCM-48 by reacting the support with gaseous vanadyl acetylacetonate [VO(acac)2]. The vapor deposition was carried out in a vacuum reactor (see Fig. 9). VO(acac)2 is sublimed and reacts with the heated substrate at 150°C until a saturation loading is achieved. This takes approximately 16 h, visible by the formation of crystals of the complex on colder parts of the reactor [261]. Subsequently, the sample is purged with dry nitrogen at reaction temperature and calcined in ambient air at 500°C. The uncalcined zeolite-supported vanadium complex and the calcined catalyst were characterized by X-ray diffraction, nitrogen absorption, IR and UV-Vis spectroscopy. The average pore diameter and the total pore volume of the MCM-48 host decreased after loading with the bulky vanadyl acetylacetonate [261–263]. On calcination, both values increased again, and the BET surface area was restored to 80% of its original value. As can be observed in the IR spectrum, all silanol groups of the support form hydrogen bonds with the incorporated VO(acac)2 complex. The saturation loading of 8.7 wt.% vanadium corresponds to 60% of the monolayer capacity. From these results it was concluded that the saturation of the support surface is governed by the availability of surface hydroxyls rather than by geometric and steric constraints. In the calcined material, tetrahedral VOx species are formed which can be identified due to their characteristic

Fig. 9. Vapor deposition reactor described in [261]

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c Fig. 10a–c. Proposed structures of supported vanadium oxide in the pores of MCM-48 [261, 262]. a Monomeric form, b dimeric form, and c oligomeric form

absorption in the UV region. The presence of larger crystallites, square-pyramidal multilayers or non-oxidized V4+ centers was excluded. As the density of silanol groups on the MCM-48 surface is not sufficient to allow the anchoring of monomeric VOx species (bound as displayed in Fig. 10a), the formation of chains of linked tetrahedra is considered to be more likely. This is consistent with the observation that around 80% of the silanol groups are restored after calcination. The authors proposed that vanadium oxide structures (as presented in Fig. 10b and 10c) formed on the host surface [261, 262]. Vapors of zinc metal were employed by Lee et al. for the preparation of zinc oxide encapsulated in zeolites H-Y and Na-A [264]. The zeolitic hosts were exposed to Zn vapor in vacuum at 450°C, the time of deposition being varied from 3 to 12 h. Afterwards, oxidation was carried out in air at 450°C. According to XRD measurements of the loaded zeolites, no framework damage occurred during the deposition and oxidation procedure. For zeolite H-Y, peaks attributable to ZnO appeared when the deposition time exceeded 6 h, whereas, in the case of Na-A, no signs of the presence of zinc oxide were obtained by XRD. With chemical analysis it was confirmed that the zinc content was higher in H-Y than in Na-A after the same time of chemical vapor deposition. From UV-Vis diffuse reflectance spectra, the presence of small zinc oxide clusters could be deduced. The corresponding absorption maximum was again more pronounced in H-Y than in Na-A. From these results, the authors concluded that more zinc had been deposited in the supercage than in the sodalite cage, leading to the presence of larger zinc oxide clusters in zeolite H-Y. Further studies concerning the nature, size, and location of the oxidic clusters were announced [264]. Alyea et al. combined chemical vapor deposition and impregnation in a preparation procedure which they denoted as “metal oxide chemical vapor synthesis” in order to obtain uniform, well-dispersed supported catalysts [265, 266]. In the rotary reaction vessel sketched in Fig. 11, vapors of transition metal oxides

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Fig. 11. Reaction vessel for metal oxide vapor synthesis, after [265]

such as Nb2O5 , WO3 , MoO3 , or V2O5 were generated by evacuation to 10–5 kPa and electric heating of the oxides in a so-called evaporation source (E). The metal oxide vapors were co-condensed at liquid nitrogen temperature with a solvent, usually methanol, which was introduced into the reaction vessel by a heated solvent inlet with shower head (B). When the co-condensate was warmed, a clear product solution was obtained which could be removed into a Schlenk tube [265]. Vacuum-activated H-ZSM-5 was subsequently added to this solution. After 12 h of stirring, the solvent was removed by vacuum evaporation, and the solid obtained was calcined at 500°C. To achieve higher oxide loadings, the procedure was repeated [266]. X-ray diffractograms of WO3-impregnated H-ZSM-5 confirmed the integrity of the zeolite framework and showed no signals of crystalline WO3. Scanning electron micrographs, however, revealed an increase in the particle size of the samples with increasing weight percent of tungsten oxide, indicating the formation of amorphous WO3 on the external surface of the support. From adsorption measurements, the authors concluded that there was no total pore blockage but that WO3 particles were also present to some extent in the channels of the zeolite. However, no quantitative evaluation of the amount of the guest oxide confined internally in the channels or on the outer surface was given.

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3 Chalcogen Chains in Molecular Sieves 3.1 Complete Synthesis

The oldest known host-guest compound, consisting of zeolite material with chalcogens encapsulated in the pores, is the blue mineral “lapislazuli”, known even in the Sumerian civilization about 5500 years ago. The name is a strange mixture of the Latin word for stone (lapis) and the Persian notation for blue (lazur). Lapis lazuli was valued as gold in ancient Egypt and used as a brilliant painting color in the Middle Ages [267]. This ultramarine-type pigment consists · · of the zeolite sodalite containing the color centers S3– and S–2 in its cages. It took until 1806 for the first reliable analysis to be accomplished, suggesting that sulfur species are the color centers, and in 1826 the earliest syntheses of blue ultramarines were reported [268]. In a basic sodalite, each cage with the for3– is centered by a regular [ClNa ]3+ tetrahedron. If, in particular, mula Al3Si3O12 4 ·– S3 substitutes the chloride ions a deficiency is observed on the Cl– and on the Na+ positions as well, since due to the steric constraints caused by the spacious · radical anion only about 35% of the Cl– sites are occupied by S3–. The sulfur radicals have to be generated simultaneously with the formation of the zeolite cages in the synthetic procedure, because the latter are impermeable for the color centers. This explains the extraordinary stability of lapis lazuli containing the normally unstable di- and trisulfur radical anions. For the complete synthesis of lapis lazuli based on the sodalite-type host according to ancient recipes, a mixture of kaolin, quartz, Na2CO3 , coal and sulfur is homogenized in a mill, heated at a rate of about 60°C/h to 780°C and held at that temperature for 6 h [269]. Alternatively, a stoichiometric mixture of CaSO4 , CaCO3 and Al2O3 is ground together thoroughly and heated at 1200°C for 20 h with periodic re-grindings [270]. The products are then heated under H2 at 900°C for 8 h.Weight losses after reduction were found to be 10.64%, which is in good agreement with the value expected (10.47%) for the reaction +H

2 Ca64 [Al96O192] (SO4)16 –––Æ 8Ca8 [Al12O24] S2 + 32H2O

(1)

In the cancrinite structure, which possesses, in comparison to sodalite, more open channels parallel to the hexagonal c-axis, the radicals can be created subsequently by irradiation or heating [271]. For this synthesis a mixture of sodium aluminate, NaOH and Na2S2O3 is dissolved in water, and a solution of water glass (37–40% SiO2) in water is slowly added under stirring. The resulting gel is filled in a platinum-lined autoclave and held for 28 d at 300°C (autogenous pressure about 85 bar). The carbonate counterions normally present in the e-cages of cancrinite are substituted by thiosulfate. The S2O32– ions orient themselves with their C3 axes parallel to the channel direction. On irradiation with X-rays (e.g., CuKa), the white solid turns yellow due to the appearance of an absorption band · around 405 nm, which indicates the presence S2– of radicals.A possible formation mechanism based on an S-S bond breaking in a disordered pair of S2O32– anions

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· in neighboring e-cages leading to S–2 , sulfite anions and sulfite radicals has been proposed [271]. The S-S bonds in the thiosulfate anion are weaker than in the formed disulfur radical [272], providing a driving force for the bond breaking. · Signals assignable to the SO3– radical species could be clearly identified by EPR spectroscopy [271]. Heating the thiosulfate-containing cancrinite at 800°C in air or at 1000°C under flowing Ar produces color changes to green and green-blue · · · caused by the generation of S3– and S–2 . Whereas the S–2 radicals are generated by · the irradiation with X-rays, the formation of triatomic radicals S3– is assumed to appear by a thermally activated diffusive mechanism, where S/S– fragments · migrate within the channels [271]. Quantum chemical calculations on the S2– ·– and S3 radical anions, employing techniques for incorporating electron correlation effects, enabled the assignment of the absorption spectra of the ultramarine-type solids [273]. Another radical identified by EPR and optical spec· troscopy (absorption band around 530 nm) is S4– [269, 274]. Based on EPR studies, as early as 1963, Dudzik and co-workers claimed that the presence of S-containing free radicals in the structure of ultramarines is of importance for their catalytic activity in reactions like the dehydrogenation of isopropanol, tetralin and tetrahydronaphthalene or the hydrodesulfurization of thiophene [275, 276]. Brilliant-red ultramarine-type pigments consist of sodalite with selenium guest species. The synthetic procedure is similar to that of sulfur sodalite except that elemental selenium is used instead of sulfur [277]. The selenium sodalite is as stable as lapis lazuli and does not decompose, even not upon · heating to 800 °C under argon. Following a resonance Raman study Se2– is the only multiatomic selenium species present, exhibiting a typical composition · Na6.5[Al5.9Si6.1O24]Se2.0 [277]. Two different polyhedral species [Se2–Na3]2+ and · [Se2–Na4]3+ in the sodalite cages have been discussed. Furthermore, due to the · steric constraint, the discharge of Se2– in the framework is favored, leaving a · distinctly less voluminous neutral (Se)2 diradical. It is assumed that the detached electron is stabilized by delocalization into the neighboring Na+4 tetrahedra of the sodalite framework. The red color originates from the spectral transparen· cy in the visible region above 600 nm and absorptions of the Se2– radical anion · around 500 nm and intermediate (Se)2 diradicals at ca. 370 nm [278, 279]. Although the color intensity of this material is weaker than that of red Cd(S,Se) pigments, it could partially replace the Cd-containing pigments in the future, · since it is environmentally more benign. The smaller S2– analogs are not subject to steric constraints of this kind and remain unchanged in the sodalite matrix. · In the case of S3– radicals, however, steric strains enforce strongly reduced occupancies of the latter [267]. It is also possible to incorporate ditellurium species into the cages of sodalitetype solids, but, due to the mismatch between the cage size and the dimensions of the color centers, severe steric strain effects appear. Therefore, only tellerium ultramarines have been synthesized in which, in contrast to sulfur and selenium guests, at least half of the cages are still occupied by Cl– anions [280]. Depending on the nT·e–2/nT·e2 ratio in the sodalite cages, the compounds are colored blue to green.

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3.2 Post-Synthesis Incorporation

For zeolites with larger pores, the syntheses and characteristic properties of chalcogen/zeolite composites are distinct from the ultramarine pigments. The sorption of elemental S or Se into small-cage zeolites requires that bonds of the chalcogen bulk structure have to be broken to produce low-molecular species like chains or rings inside the zeolite. For zeolite A, experimental results indicate the formation of isolated clusters, like Se8 rings [281, 282]. Seff reported that sulfur is present in the a-cages of zeolite A as two S8 rings, each in crown configuration [283]. The planes of the two rings are parallel, but 0.496 nm apart, which is considerably more than the van der Waals diameter of sulfur (0.4 nm). They effectively block the six 8-ring windows of the cage. It is impossible for intact S8 rings to enter zeolite A through the 8-ring openings of only 0.42 nm in diameter. Sulfur, therefore, penetrates zeolite A as S2 molecules which then polymerize to S8 within the zeolite. The isosteric heats, therefore, include components from the shifting of vapor equilibria with temperature as well as for processes of sorption and polymerization [284]. In one-dimensional channel systems, like mordenite or AlPO4-5, Se chains exist as isolated, highly ordered helical structures [281, 282, 285, 286]. The threedimensionally linked large pores of the zeolites X and Y impose less rigid constraints on sulfur or selenium rings or chains [287]. Thus, it is possible that more than one Se chain can occupy the same cage, causing deviations from the regular helical conformation [281, 288]. The cations, which counterbalance the negatively charged aluminosilicate framework, are anticipated to be the binding sites for the Se clusters. Furthermore, the cations influence the relative abundance of encaged Se rings or chains [289, 290]. In particular, Raman spectra of Se showed that the fraction of rings formed in cation-exchanged Y-zeolites increases in the series La-Y, Nd-Y, Ca-Y and Sr-Y. A typical synthetic procedure for Se chains in Y-zeolite runs as follows: Selected cations are introduced by twofold ion exchange of Na-Y zeolite in aqueous solution. The product zeolite is dehydrated under vacuum at 550°C. Defined amounts of zeolite and elemental Se are placed in a quartz U-tube, separated by a frit. Both ends of the tube are sealed off under vacuum (p<5 · 10–4 mbar), and the tube is hold at 350 °C for 1 week. The color of the final products changes from yellow to orange with increasing content of Se, which can be determined with a microbalance or by chemical analysis. The incorporation of sulfur into the pores of the zeolite analogs aluminophosphates AlPO4-5 and SAPO-44 occurs already at 250°C [291]. Observed blue-shifts of the optical absorption edges for Se chains encapsulated in zeolites in comparison to bulk Se, as well as the alterations found in the stretching modes in the Raman spectra, are not considered to be a manifestation of a quantum confinement effect, but to reflect mainly the loss of interchain bonding upon isolation of the Se structures in the pores of zeolites. The band structure of amorphous and trigonal Se is determined to a large extent by interactions between the chains [292]. In the case of Nd3+-exchanged Y-zeolites, in which the Nd3+ ions exclusively · occupy the smaller b-cages, Se2– radical anions could be observed beside the

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chain structures after irradiation with a 100-mW krypton ion laser (l = 476.2 nm) [293]. It is suggested that the reducing power of the Nd3+-substituted zeolite lies in the presence of oxide- and/or hydroxide-bridged cation dimers or trimers in the sodalite cages. A plausible scenario involves the homolytic scission of the O-H bond of a bridging hydroxyl in the [Nd2(OH)2]4+ cluster with the resulting hydrogen atom reducing the neutral Se2 molecule and the stabilization · of the unpaired electron on the cluster. The stabilization of the Se2– radical anions may be accomplished by the protons formed: · (2) [Nd2(OH)2]4+ + Se2 Æ [Nd2(OH)(O)]4+ + Se2– + H+ In longer Se chains, the anion produced under irradiation leaves a positively charged hole defect on the terminal Se atom of the chain, stabilized by the very high electrostatic fields in the zeolite pores [294]. Electron transfer from the zeolite to the spin-paired defect results in a neutral chain and a hole in the center of · the zeolite host material, balancing the charge of the Se2– radical anion. In large-cage systems the cluster selectivity begins to break down, as demonstrated by 77Se MAS NMR spectra, which suggest that several different Se allotropes exist in zeolite Y [285]. Selenium in channel systems, e.g., in AlPO4-5 and mordenite, was studied by optical, 77Se MAS NMR and EXAFS spectroscopy. It exists only in a helical chain conformation similar to that in the trigonal allotrope. The lack of spinning sidebands in the NMR spectra also implies that interchain interactions are minimized and that the Se atoms are in a nearly spherical environment, as would be the case for an isolated chain that is free to rotate [281, 295]. In cancrinite, the channels are partially blocked by Na+ or OH– ions. As a consequence of this blockage only Se2 molecules penetrate into the channels where they can generate chains. However, from the observed temperature dependence of Raman scattering spectra, it was inferred that a number of incommensurable phase transitions occur due to a competition between the forces of the Se-Se interactions, leading to the formation of chains, and Sematrix interactions. This results in an inhomogeneous distribution of Se chains within the channels and the stabilization of Se22– ions [296, 297].

4 Sulfide, Selenide and Telluride Clusters in Molecular Sieves 4.1 Complete Synthesis

For aluminate-based sodalite cages, Brenchley and Weller reported the formation of Cd4S6+ units by a two-stage process via the parent sulfate sodalite synthesized at high temperatures [298]. First CdO, CdSO4 and Al2O3 were finely ground in a molar ratio of 3:1:3 and sealed under vacuum in a silica glass tube. The tube was fired at 1000 °C for 2 d and then cooled to yield a white crystalline powder. The sulfate sodalite was then reduced under pure hydrogen at 600°C for 8 h to yield a bright-yellow material in which no S-O vibrational modes could be observed, consistent with a reduction of sulfate to sulfide and the formation of

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Fig. 12. Left Spatial arrangement of Cd4S tetrahedra in Cd8[AlO2]12S2; interactions between the tetrahedra are shown by dotted lines. Right The Al framework of the sodalite is shown to scale in the same orientation as the Cd4S units, after [298]. ● S; ∑ Cd

cadmium sulfide in an aluminate sodalite Cd8[AlO2]12S2 . Figure 12 illustrates the relative orientations of the Cd4S clusters formed in the sodalite cages to each other; the sodalite cages have been omitted for clarity. The central tetrahedron Cd4S group has a Cd-S bond length of 0.2535 nm, the next nearest neighbor cadmium to sulfide distances, which represent the interaction of Cd4S groups between the cages, are 0.5101 nm [298]. In diffuse reflectance UV-Vis spectra, the sharp absorption edge around 500 nm is blue-shifted by approximately 100 nm compared to that of the CdS bulk material. This means that the sodalite Cd8[AlO2]12S2 behaves like colloidal CdS with particle diameters of several hundred nanometers rather than like individual Cd4S units, for which only absorption in the far-UV region should be expected. In contrast, Moran et al. reported for [Zn4Se]6+ clusters in borate sodalite the expected absorption in the far-UV [299]. From an upfield chemical shift of the single 77Se resonance line in 77Se MAS NMR spectra of about 125 ppm relative to that of bulk ZnSe, it was deduced that the electron density at Se2– in the cage center is higher than at the anionic sites in the ZnSe zincblende lattice [299]. In borate or beryllosilicate frameworks possessing the sodalite structure, discrete M4X6+ (X = S, Se) units could be prepared [300]. In zeolites, i.e., in aluminosilicates, the framework does not supply enough negative charge to stabilize ions of the M4X6+ type and, therefore, larger and less highly charged units are formed. 4.2 Post-Synthesis Incorporation

Metal sulfide clusters which are, inter alia, of interest for the study of photoprocesses, can be prepared in molecular sieves either via treatment of ion-exchanged zeolites with H2S [301–303] or by consecutive incorporation of (i) the metal from dimethyl or carbonyl complexes and (ii) the sulfur from H2S [253,

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304, 305]. When the ion-exchange method is applied, chalcogenide clusters are prepared which are located in the sodalite cages of zeolite X or Y, whereas supercage-entrapped clusters are obtained by using the volatile metal complexes. Alternatively, the sulfidation step can be carried out with Na2S in aqueous solution instead of using gaseous H2S. When the metal cations are introduced into the zeolite by conventional ion exchange from aqueous solution, it is well known that the substitution of the cations occurs stoichiometrically. The extent to which this substitution can be achieved depends on various factors. Firstly, the structure of the zeolite and its nSi /nAl ratio define the number of cations and their positions, which have been determined by X-ray analysis and compiled for all common zeolite types [306–309]. Secondly, the nature and charge of the transition metal cation will determine the kinetics and thermodynamics of the ion exchange, i.e., the selectivity of site occupation and the type of exchange isotherm [310, 311]. The degree of ion exchange in combination with the water content strongly influences, for example, the repulsive interactions between cations and, thus, the local coordination and strength of bonding between the metal ions and the zeolite framework. The transfer of charge density from the framework oxygens to the metal ions decreases with increasing electronegativity of the former, i.e., with increasing nSi /nAl ratio [312]. Finally, the exchange process of transition metal cations is affected by interference from different hydrolytic processes [154, 162, 310, 313, 314], i.e., (i) partial hydrolysis of the zeolite, (ii) hydrolysis and solubility of the dissolved transition metal ions, and (iii) reversible hydronium ion exchange resulting in temporal variations in the pH values in the exchange batches. Since metal hydroxide precipitations would bring about undesired large metal sulfide particles on the external surface of the zeolites during the following sulfidation step [162], their formation must be avoided. Different procedures can be applied to suppress hydroxide precipitation, such as (i) stepwise addition of the metal salt solution [315], (ii) use of proton acceptors, e.g., metal acetates to provide buffered solutions, or (iii) application of non-hydrolyzing cation complexes, e.g., amminated cations [162]. Normally, anionic species cannot be introduced into the pores of zeolite-type structures, but, in some cases, the anion is “smuggled” into the cavities together with a cation. This phenomenon is called over-exchange and allows the introduction of more cations than are necessary for charge compensation of the framework. It occurs, for example, during the exchange of lead ions into zeolite A [316] or sodalite [317]; PbOH+ ions are responsible for anion uptake in these cases. Ion exchange via solid-state reaction between the H-forms of zeolites and transition metal oxides or chlorides is an alternative approach, which is, however, frequently restricted to zeolites with low aluminum contents [318, 319]. This technique offers the advantage that metals can be introduced which form large polymeric cations in aqueous solutions, cf. also Chap. 2 of this volume.

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4.2.1 Cadmium and Lead Sulfide Clusters

Cadmium sulfide is one of the most extensively studied chalcogenides as a guest in zeolite hosts. Herron et al. obtained Cd ion-exchanged zeolites by agitating Na-Y in 1 l of distilled water, adjusting the pH to 5 with nitric acid, adding 10 g of Cd(NO3)2 and stirring at room temperature overnight. The Cd-Y zeolite was collected by filtration, washed extensively with distilled water, and then dried and calcined under shallow-bed conditions in flowing oxygen. The temperature of 400°C was reached over 2 h and held for 1 h more giving a dry white powder [301]. The Cd2+ ions are located primarily at the SI¢ sites of the faujasite structure. This site in the sodalite units of the framework is preferred by multivalent cations because their positive charge is partially compensated by hydroxyl anions on SII¢ sites that are located in the sodalite cage as well. The Cd2+ ions are octahedrally coordinated with oxygen, i.e., with three framework oxygen atoms of the six-ring and three extra-framework oxygen atoms from the hydroxyl groups located on SII¢ positions. Since every sodalite cage can host four Cd2+ ions, the overall structure of the sodalite-entrapped species is a distorted cube (CdO)4 [320]. This was confirmed by the presence of Cd neighbors in the second shell of Cd-EXAFS spectra [301]. Herron et al. dried Cd-Y zeolite prior to sulfidation in vacuum and then, after cooling to 100°C, exposed the material to flowing H2S for 30 min. Finally, the still-white zeolite was evacuated, sealed and transferred to a dry nitrogen glove box for storage [301]. During the final evacuation the white zeolite became paleyellow. This color change was found to be reversible, i.e., the addition of H2S turned the color of the zeolite back to white [321]. The reaction Cd2+(Z–)2 + H2S ¨ Æ CdS + 2H+Z–

(3)

where the protons of the H2S reagent become the charge compensators, forcing the CdS clusters to reside in an extremely acidic environment, is completely reversible. This means that CdS is unstable, i.e., it can readily dissociate into Cd2+ ions and H2S. Therefore, the CdS content of the zeolite is dependent on the partial pressure of H2S, the proton activity resulting from Eq. (3) [301] and various subsequent processes, like aggregation, mesopore formation, etc. All samples were found to be water-sensitive (including atmospheric moisture) to a varying extent. Zeolites X and Y with CdS clusters in their pores immediately became deep yellow-orange upon immersion in water or any polar, nonbasic solvent. Zeolite A loaded with CdS clusters, however, slowly developed a yellow-orange color over a longer period of about 1 h when immersed in water [301]. The different coloring rates point to different rates of aggregation or mobility of the CdS species. The concentration dependence of the absorption spectra, measured by Herron et al. for CdS clusters in zeolite Y, has been interpreted by a percolative phenomenon, i.e., an electronic communication between neighboring clusters [301]. The absorption edges in all the CdS/zeolite samples are blue-shifted with respect to bulk CdS (cf. Fig. 13). At low concentrations, CdS exists as isolated molecules or clusters within the zeolite pores leading to an absorption at wave-

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Fig. 13. Diffuse reflectance UV/Vis spectra of bulk CdS and CdS clusters in zeolite Y, after [301]. The spectral intensity at 350 nm increases with increasing loading

lengths below l = 350 nm. Abnormally strong absorption is observed when the CdS-loaded zeolite has an excessively large nS /nCd ratio, probably due to the presence of H2S, since for H2S-loaded zeolites an absorption band in the 280 nm region is known [322], which can interfere with the CdS cluster spectrum. At concentrations of around 4 ± 1 wt.% CdS, the high density of clusters in neighboring zeolite cages start to form a “supercluster” by electronic communication, indicated by an abrupt shift of the absorption edge to l = 400 nm and the steady evolution of an exciton-like feature at 350 nm (Fig. 13). The supercluster concept was advanced on the basis of a series of structural and quantum chemical studies [301–304]. Treatment of the cadmium-exchanged samples with H2S at 100 °C brings about only moderate changes in the EXAFS spectra. It was concluded that (i) the Cd arrangement in the sodalite cages remains unchanged, and (ii) about half of the oxygen atoms in the cubes are replaced by sulfur atoms. The (CdS,O)4 cubes are stabilized in the sodalite cages as shown in Fig. 14, since four double six-ring windows, arranged in a tetrahedral symmetry, match perfectly with the four tetrahedrally arranged Cd atoms of the cube. In the supercage (1.3 nm), however, only one of the Cd atoms of a cube can perfectly coordinate with the framework oxygens, so that only larger clusters should be stable here. Above the critical threshold concentration of 4±1 wt.%, the CdS clusters must populate adjacent sodalite units for statistical reasons. For a three-dimensional percolation process, the percolation threshold is predicted to be around 15 vol.% [323]. If only the free volume of the sodalite cages is taken into account, 4 wt.% CdS corresponds to 14 vol.%, in excellent agreement with the percolation theory. Two cubes point to each other through the Cd atoms at the vertices, the Cd-Cd distance being ª0.6 nm (Fig. 14). Since this is too long a distance for a direct interaction, and since the electron density inside the double six-ring windows was found to be very low by X-ray diffraction, a through-bond interac-

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a

b

Fig. 14. Top Representation of a (CdS,O)4 cube inside the sodalite unit. The dotted lines emphasize the bonds between framework six-ring oxygen atoms (represented as points connected to T atoms by sticks) and Cd atoms of the cluster (solid circles, Cd; open circles, S(O)). Bottom Relative orientation of two (CdS,O)4 clusters in adjacent sodalite units of a the Y structure separated by a double six-ring linkage (solid circles, Cd; open circles, S(O); cubes are viewed edge-on) and b the A structure separated by a double four-ring, after [301]

tion concept was suggested with the six-membered ring windows of the sodalite units as the intervening bridges [301]. Adjacent (CdS,O)4 clusters interact through Cd-O-Al-O-Si-O-Cd bridges, which contain six s-bonds. From the difference of the absorption edges at 290 nm for isolated clusters and at 350 nm for superclusters, a strength for the through-bond interaction of 0.7 eV was estimated, which is very close to other known values for through-bond interaction [324, 325]. It is important to realize that, although the size of the CdS clusters in the sodalite cages is well defined, the size of the supercluster is not. Ion exchange cannot totally fill up the sodalite cages. In addition, a high density of defects (usually extra Cd) and disorder exist. These partial vacancies and imperfections break up the supercluster and introduce inhomogeneity into the supercluster size distribution, which is the primary reason for the observed broad exciton absorption peak [301]. As the semiconductor particle size decreases, the coupling of electronic transition to surface and host phonons becomes important. This gives rise to multi-phonon sidebands and further spectral broadening. In ab initio local density approximation (LDA) calculations, it was found that a cluster orientation with the Cd atoms located on the SI¢ sites and the O or S atoms on the SII¢ sites of zeolite Y is clearly preferred over the reverse geometry, i.e., O or S atoms on the SI¢ sites and Cd on SII¢ sites. Whereas for a (CdO)4 cube the rotation is free, for cubes in which oxygen is partially exchanged by sulfur and

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for (CdS)4 cubes a barrier for rotation was found due to their larger size [326]. The cube arrangement is energetically preferred over four separate diatomic CdS molecules by –0.9 eV per molecule. This indicates that the cubic particles will be formed immediately during the synthesis and that these particles have no tendency to dissociate into smaller units. However, it was also found that the encaged (CdS)4 clusters are strongly deformed [327] and that there is a longrange stress field between the particles, which is slightly repulsive and will act against the formation of a dense packing of (CdO)4 and (CdS)4 cubes within the zeolite pores. Calculations of the energy gap between the HOMO and LUMO in the system (CdS)4-sodalite led to gaps that were about 1.5 times larger than in bulk CdS. It is important to note that Cd-O interactions have to be taken into consideration for the determination of energy gaps in this system [327]. Ozin and co-workers suggested a different method for the formation of faujasite-confined cadmium sulfide clusters [253, 305]. They introduced dimethylcadmium into the cages of faujasites and formed CdS clusters by reaction with H2S. For dimethylcadmium as well as for dimethylzinc adsorbed in zeolite cages, a red-shift in the absorption of about 50 nm was observed compared to their gas-phase analogs. These spectral shifts provide evidence for the interaction of the metal centers in (CH3)2M with oxygen donor sites of the zeolite framework (ZOH, Z: zeolite). It was demonstrated by IR investigations that the methyl groups were able to catch the protons released from the H2S during metal sulfide formation and, thus, protect the zeolite framework against dealumination by the protons [328]. CdSH+ species coordinated to zeolite framework oxygen and occupying supercage sites are formed by reaction with H2S in a first step. Six of these subsequently participate in a cluster self-assembly reaction, constrained and templated by the zeolite ligand to form Cd6S44+ nanoclusters housed in the supercages of the faujasite host [253]. The following two-step chemical vapor deposition mechanism including homolytic bond breakage of O-H and S-H bonds has been suggested [305]: 6ZOH + 6(CH3)2Cd Æ 6ZOCdCH3 + 6CH4 ,

(4)

6ZOCdCH3 + 6H2S Æ 6ZOCdSH + 6CH4 ,

(5)

6ZOCdSH Æ (ZO–)4 (Cd6S4)4+ + 2H2S + 2ZOH .

(6)

The Cd6S44+ nanoclusters are considered to be framework-matched, i.e., coordinated to the zeolite ligand (see Fig. 15). Such a cluster self-assembly is deduced from a monotonic red-shift of the absorption edge during loading, starting around 400 nm and converging to a value around 510 nm. The cluster product in highly loaded samples has an absorption edge identical to that found for partially loaded samples, indicating that the interaction of clusters in adjacent cages (Fig. 15) is only of minor importance for the optical behavior [305]. More recently, the method of metal organic chemical vapor deposition (MOCVD) was also used to form Sn4S64+ cluster ions, possibly possessing adamantanoid geometry, in the supercages of acidic zeolite Y [329]. In this case the NH4+-exchanged zeolite was first dehydrated and deammoniated at 430°C followed by cooling under dynamic vacuum. Subsequently,

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Fig. 15. Left Proposed cubane-like structure of a Cd4S4 cluster anchored with two site II Cd2+ cations within the supercage of zeolite Y. Hatched circles represent cadmium cations, open circles represent sulfide anions. Right Schematic depiction of an ideal diamond superlattice of Cd2(Cd4S4)4+ clusters in the supercage of a zeolite Y host, after [252, 305]

degassed tetramethyltin was introduced incrementally by allowing the vapor above the liquid to expand into a small “titration” volume and then allowing this volume of gas to enter the main body of the closed in situ cell. After a 15-min period allowed for adsorption, the zeolite was heated for 2 h at 150°C. Finally, the sample cell was evacuated again, filled with about 150 mbar of dry H2S, and heated again for 2 h at 150°C. During this last step the color of the zeolite sample turned from white to pale golden-yellow. The authors demonstrated by transmission electron microscopy and different spectroscopic in situ techniques (IR, diffuse reflectance UV-Vis, 119Sn-Mössbauer) that the incremental adsorption of the tin source allows a stepwise, molecule-by-molecule control of the intrazeolite reaction processes [329]. An alternative route for the formation of metal sulfide clusters in the pores of zeolites is the precipitation of the introduced metal cations with Na2S in aqueous solution. An advantage of this method is that the Na+-forms of the zeolites are regenerated. However, in zeolites with small cages, e.g., zeolite A, the large hydrated S2– ions are hindered from diffusing into the zeolite framework, so that counter-diffusion of Cd2+ ions out of the pores and metal sulfide precipitation on the external surface occur. Fox and Pettit tried to stabilize CdS clusters in the pores of zeolites A, X and Y by this method [330]. They immersed Cd2+-exchanged zeolites into 0.1 M Na2S in water. Since clusters with diameters greater than 10–20 nm were found, a growth of these clusters at the external surfaces of the zeolites must be assumed. Nevertheless, such CdS/zeolite composites were found to be suitable for the photocatalyzed evolution of hydrogen in the presence of electron donors, which react with the holes formed during the photoreaction and are, thus, consumed during the reaction (sacrificial donors). By use of S2– and SO32– ions as sacrificial donors and co-catalysts like ZnS or Pt, specific hydrogen production rates of up to 55 cm3/(g · h) have been reported during illumination with a 450-W mercury lamp [330]. The sacrificial donor used might help to generate “electronical contact” between the species located inside the zeolite pores and those on the external surface, but the mechanism is not yet clear. However, it has been demonstrated that an electron transfer between CdS clusters on the external surface and Pt clusters in the pores of zeolite L can be

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obtained by use of methylviologen as mediator [331]. Barnakov and co-workers have also used Na2S solutions to precipitate CdS clusters in the pores of A- and X-zeolites [332]. In this case the absorption data indicate an optical band gap of about 3 eV, very similar to that observed for (CdS)4 clusters in sodalite cages. Since their photoconductivity data and the absorption intensity exhibit the same wavelength dependence between 300 and 600 nm, the authors interpreted the conductivity as a tunneling of electrons between clusters in adjacent sodalite cages. The tunneling is possible when the wavefunctions of the excited electronic states of the clusters strongly overlap. Hydrogen production from zeolites containing CdS clusters in the pores and ZnS co-catalysts on the surface of zeolite Y has also been reported by Telbiz et al. [333]. In that case, however, the CdS clusters were precipitated with H2S and the absorption data showed a distinct blue-shift. A partial destruction of the zeolite framework due to the attack of protons and the formation of mesopores has been reported by Wark et al. after ion exchange of faujasites with Cd2+ or Pb2+ ions from aqueous solutions and subsequent sulfidation with H2S without complete dehydration [334, 335]. In these samples the absorption edges for CdS- and PbS-containing zeolites lie around 530 nm and 620 nm, respectively. Evidence for the growth of sulfide particles within the zeolite lattice to sizes considerably exceeding the dimensions of the supercage was obtained from transmission electron micrographs and adsorption isotherms of cyclopentane. On TEM micrographs, CdS and PbS particles with diameters of 3–5 nm have been observed without indications for an enrichment of particles at the boundaries of the zeolite crystals [336]. Furthermore, the creation of mesopores due to a local destruction of the zeolite framework around the growing particles results in a hysteresis loop in the adsorption/ desorption cycle, as depicted in Fig. 16 for a CdNa-X sample with a degree of ion exchange of a = 20%.

Fig. 16. Adsorption (open symbols) and desorption (filled symbols) isotherms of Cd-X samples (degree of ion exchange: 20%) a before sulfidation, b after sulfidation without complete dehydration and c after additional calcination for 24 h, after [334]

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The absorption edges of diffuse reflectance UV-Vis spectra of X-zeolites containing CdS or PbS nanoparticles, which had been synthesized in the complete absence of water and which were about 1 nm in diameter, lay around 400 nm and 550 nm, respectively [336]. These values are in the same range as the ones shown by Herron and co-workers for samples with interacting (CdS)4 clusters in adjacent sodalite cages [301]. Wark and co-workers described a clear dependence of the diameter of the formed metal sulfide nanoparticles on the loading, i.e., the degree of exchange of metal ions in the faujasites. The color of PbS-containing X-zeolites turns from yellow to ruby, if the degree of ion exchange with Pb2+ ions increases from a=10% to 60%. The ion exchange leads to a slight decrease in the BET surface of the zeolites, indicating a partial destruction of the faujasite structure resulting from the attack of protons formed due to a partial precipitation of Pb(OH)+ species in the aqueous exchange solution [335]. If the theories of the size-quantization effect [337–340] are used for the size determination of nanoparticles in the pores of zeolites, they have to include the strong interaction term between the clusters and the oxygen [341]. This has been demonstrated for PbS nanoparticles in the pores of zeolite X (nSi /nAl = 1.2) [342]. Depending on the Pb content of the zeolites (degrees of ion exchange 10%–60%), PbS particles between 2 and 8 nm were observed in transmission electron micrographs (Fig. 17). For the PbS nanoparticles in the zeolite the absorption edge is blue-shifted in comparison to the bulk materials over the whole temperature range measured (–80 to 250°C). However, the blue-shift is smaller for high temperatures (250°C) than for low temperatures (–80°C). A growth of the nanoparticles could be

Fig. 17. Transmission electron micrograph of zeolite X hosting PbS nanoparticles with diameters of 2–8 nm. 40% of the Na+ ions have been substituted by Pb2+ ions via ion exchange from aqueous solution. Subsequently, the sample was dried and treated with gaseous H2S, after [342]

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excluded as reason for the relative red-shift at higher temperatures, since the shift was fully reversible upon decreasing the temperature. Instead, the temperature dependence could be interpreted as being due to a non-stoichiometric composition of the semiconductor particles. The anionic zeolite framework generates a sulfur deficiency in the particles, i.e., PbSx (x<1) particles are formed. The positive extra-charges shift the Fermi level of the semiconductor (PbS) clusters to energies lower than the highest valence state sub-bands, and thus the particles act like p-type semiconductors [343]. This shift of the Fermi level is called the “Burstein effect” and is valid for all non-stoichiometric (doped) nanoparticles of semiconductors [344]. Especially for PbS with its very low effective electron and hole masses (me,eff ª 0.16 me , mh, eff ª 0.1 me), the Burstein effect occurs at relatively low extra-charge concentrations. The number of extra-defect electrons per particle necessary to explain the observed differences in the band gaps is less then 0.7, a number easily generated under the influence of the anionic framework [342]. With increasing temperature an increasing part of the states in the sub-bands above the Fermi level is filled with electrons. This leads to the observed reversible relative red-shift of the absorption edge with temperature. In the preparation of zeolite-entrapped CdS, considerable attention has been paid to the nCd /nS ratio. A marked non-stoichiometry of zeolite-hosted metal sulfide particles has been found for CdS nanoparticles by X-ray photoelectron spectroscopy [345]. After sulfidation of a zeolite X sample that is partially ionexchanged with Cd2+ ions, the binding energies of Cd 3d5/2 electrons decrease by about 0.3–0.5 eV in dependence on the diameter of the CdS nanoparticles formed. The shift originates from the replacement of ionic interactions between the Cd2+ ions and the zeolitic framework oxygen by more covalent (Cdd+-Sd–) bonds. However, due to the larger effective masses of the electrons and holes in CdS (me, eff ª 0.42 me , mh, eff ª 0.18 me) [339], the absorption of CdS clusters in the pores of zeolites is less affected by the zeolite framework than that of PbS clusters. However, the effect of the zeolite framework on the excited-state relaxation processes, i.e., the luminescence behavior of the CdS clusters, can be very large. In general, the luminescence process in semiconductor clusters is very complex. Broad and Stokes-shifted luminescence appears, caused by the presence of defects [341, 346, 347]. Since trapping of electrons is an extremely fast process (10–13 s), spontaneous fluorescence will not appear, but excitonic fluorescence can arise via detrapping of trapped electrons [348]. High luminescence efficiencies can only be obtained if good surface passivation prevents fast surface trapping [349]. Due to the excitation at 254 nm for CdS clusters in zeolite X and Y, Stokes-shifted luminescence occurs in the yellow-green region (560–580 nm) depending on the CdS concentration and the particle size, i.e., emission was only found above a threshold concentration of CdS of 4 wt.%, and its maximum is, in analogy to the absorption edge, blue-shifted in comparison to bulk CdS (750 nm) [301]. No luminescence was observed near the band edge [321]. The excitation spectra show a pronounced peak at 350–360 nm, which can be attributed to the exciton transition. Thus, an electron-hole recombination is absent, and the observed emission is due to defects. Consistent with the assignment of the emission to defects, the emission intensity increases with decreasing tempe-

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rature and shifts to higher energies. This phenomenon is attributed to shallow defects, which are able to trap charge carriers. For (CdS)4 clusters located in the sodalite cages, three major peaks were found at 440, 580 and 660 nm. The temperature-dependence indicates a multiphonon-induced radiationless process, whereas the emission band at 440 nm quenches rapidly as the temperature increases and disappears above –173 °C. Sulfur-related defects have been suggested to be responsible for this emission [321]. It is absent for the slightly larger clusters in zeolites X and Y, where donor states move into the conduction band. The emissions at 580 and 660 nm result from Cd atoms and sulfur vacancies. Since an effective mediating phonon frequency of about 500 cm–1 was measured, which is much higher than that for bulk CdS (300 cm–1), this is assumed to be responsible for the intense non-radiative relaxation. The value is close to the vibrational frequency of Cd-O and, thus, interactions between the cluster and the zeolite matrix, i.e., host phonons, might be responsible for the radiationless relaxation processes [321]. Thomas and co-workers studied the emission behavior of PbS and CdS clusters in zeolites with cages (A, X and Y) and channels (L, offretite and ZSM-5) [350, 351]. They found that in all zeolites the emission was broad and structureless. The position of the emission maximum increased with increasing excitation wavelength and increasing size of the cages or width of the channels. Since the lifetimes of the excited states at room temperature were measured to be very short (t < 10–9 s), and since the separation of the states in the valence and conduction band should be larger for the small clusters, the broadness of the emission peaks was explained by the appearance of Franck-Condon factors and nonradiative processes. As a consequence, the emission maximum changes with increasing excitation wavelength, in contrast to what is observed in colloidal CdS systems [351]. Thermoluminescence is a very sensitive technique for the detection of defects or traps. Charge carriers trapped at surface states may be excited by heating and can recombine under emission of light. For CdS clusters in zeolite Y, it was found that the thermoluminescence is stronger than that of bulk CdS and increases with decreasing CdS loading [341, 352]. The clusters found at lower loading possess higher surface-to-volume ratios and contain more trapped carriers accessible for thermoluminescence. 4.2.2 Silver Sulfide Clusters

Ag2S particles in the nanometer-size regime are believed to play an important role in the photographic process [353]. Calzaferri and co-workers discovered small silver sulfide clusters in the size regime below 1.5 nm (that are stable under ambient conditions) in zeolite A [354]. The color of the samples depends on the loading density, i.e., they are colorless for low loadings and become yellowgreen for loadings up to two Ag+ ions per a-cage in zeolite A. Bulk Ag2S is a semiconductor with monoclinic structure and a band gap of approximately 1 eV at room temperature [355]. Silver sulfide/zeolite A composites were prepared as follows: Ag+ was introduced into zeolite A by ion exchange using a 0.1 M aqueous AgNO3 solution [356]. After shaking the suspension for 30 min at room tempe-

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rature, the zeolite was centrifuged off, washed and activated at room temperature under high vacuum (5 · 10–7 mbar) for 50 h. Upon progressive dehydration of the Ag+ ions during the activation, the color of the samples changed from colorless to yellow or even red [357]. Seifert et al. explained this in terms of electronic transitions from the lone pairs of the oxygen atoms of the zeolite framework to the empty 5s orbital of the Ag+ ions (ligand-to-metal charge transfer) [358]. The activated sample was then exposed to an excess of H2S for 1 h and subsequently evacuated. The reaction with H2S gave colorless or yellow-green samples. Due to the adsorption of water, a migration of small clusters or silver ions was initiated which led to the formation of larger and energetically more favorable silver sulfide species. This became obvious by the evolution of an absorption band around 320 nm [354]. Upon irradiation at 350 nm, a remarkable shift in the emission with the loading was detected. The observed colors of the emission were blue-green for samples with low silver sulfide content (less then 0.1 Ag ions per pseudo-unit cell), yellow to orange at medium content (0.2–0.5 Ag ions per pseudo-unit cell) and red at higher silver sulfide contents.As in CdS-loaded zeolites, luminescence could only be observed at low temperatures, i.e., below –50 °C. Sugioka and co-workers prepared Ag2S clusters in zeolites Na-A, H-Y, H-mordenite and H-ZSM-5 by ion exchange with aqueous solutions of silver chloride or silver nitrate and sulfidation with H2S at 5.2 kPa and 300°C [359, 360]. In the case of proton-type zeolites, the samples were additionally calcined in air at 500°C for 4 h and tested in the isomerization of 1-butene, cis-2-butene or cyclopropane prior to sulfidation [360]. After sulfidation, remarkably higher activities for the isomerization reactions were found, especially for the A- and the Y-zeolites. The activity enhancement was attributed to the formation of negatively charged [AgSy]xd– (y > 0.5) clusters in the zeolite pores promoting the formation of new Brønsted acid sites, their activity being further enhanced by inductive effects from the silver sulfide clusters [360]. 4.2.3 Molybdenum Sulfide Clusters

The primary interest in the encapsulation of MoS2 clusters in the pores of zeolites lies in the exploitation of these materials for catalytic applications. There is strong evidence that the active sites, e.g., for hydrodesulfurization reactions on bulk MoS2 , are the coordinatively unsaturated molybdenum ions at the planes (100), the number of which should increase with decreasing size of the cluster hosted in zeolite pores [361]. In general, the introduction of metals like molybdenum into zeolite Y risks degradation of the crystallinity of the matrix. Attempts to minimize this risk make use of the adsorption of Mo(CO)6 followed by decomposition and reaction with H2S [362]. Due to its easy hydrolysis, the introduction of molybdenum as well as niobium must be performed by CVD or solid-state procedures [363, 364], such as the adsorption of MoCl5 on the acid form of the zeolite [365]. The NH4-Y zeolite is calcined in dry air up to 450°C to produce H-Y. Dry zeolite H-Y is exposed to MoCl5 at 80°C in vacuum, producing a powder that is brown at first and finally turns green. The brown product ex-

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hibits an EPR spectrum that is characteristic of [MoOCl4]– [366]. The anionic charge of [MoOCl4]– may be compensated by AlO+ produced by dealumination during the preparation. The brown powder can then be activated in an argon stream by heating slowly up to 450°C resulting in a white powder (Mo-Y). The sulfidation of Mo-Y with H2S is also carried out at 450°C, leading to a brown product.A sample prepared with activation and sulfidation below 170°C showed the same color. Bulk sulfur (S8) used as an alternative sulfidation agent at 450°C led to a black powder. The EPR spectra of the Mo-Y samples sulfided with H2S and S8 are quite similar; the signal at higher field can be attributed to oxo-Mo5+ species, the complex spectrum at lower field is dominated by long-chain sulfur radicals [365, 367]. EXAFS shows, in addition to the evidence for Mo-S single bonds, a Mo-Mo backscattering at a distance of 0.279 nm, which is much shorter than that in bulk MoS2 (0.316 nm), indicating the formation of sulfur-bridged molybdenum dimers in the supercages [365]. EPR lines found for sulfided Nb-Y are · · assumed to arise from HS and S3– radicals or monomeric thio-NbIV species [365, 368]. In EXAFS spectra, the dominant backscatterer remains oxygen, demonstrating that sulfidation of Nb is more difficult than that of Mo. Mixed metal clusters of the type Mo3MS4 (M=Ni, Pd) were synthesized in zeolite Y by ion exchange starting from the so-called “incomplete cubane-type” molybdenum(IV) cluster [Mo3S4(H2O)9]4+, which is stable in water and air [369]. Solutions of the chloride salts of the clusters [Mo3NiS4Cl(H2O)9]3+ and [Mo3PdS4Cl(H2O)9]3+ were added dropwise under vigorous stirring at 40 °C to a suspension of the zeolite in water. The composites were used as catalysts in the hydrogenation of CO, and unusually high C2 selectivities were found [369]. 4.2.4 Cobalt and Nickel Sulfide Clusters

Cobalt sulfide is also known to be highly active in hydrodesulfurization, e.g., of thiophene [370]. In ion-exchanged Y zeolites, Co9S8-like clusters are formed in the presence of water, whereas, in dehydrated Na-Y zeolites, smaller ones exist [371]. However, these CoS clusters in zeolites were found to be much less stable in the absence of H2S than, e.g., CoS clusters on alumina [372]. This instability results from the protolysis reaction already discussed for zeolite-based CdS clusters, decomposing CoS into H2S and Co2+ ions at temperatures even lower than 400°C [373]. Essential differences between carbon-, alumina- and zeolitesupported metal sulfide catalysts are the presence of protons and the need for charge-compensating cations in the zeolite [301, 374]. Comparable results have also been obtained for sulfided NiNa-Y; NiS additionally tends to sinter onto the external surface of the zeolites [373]. To prevent protolysis of acidic zeolite-supported metal sulfides, the conditions for the catalytic reactions must be carefully adjusted and catalyst preparation performed by avoiding proton formation, for instance, via the use of carbonyl complexes [375–377]. In some rare cases, e.g., for zeolite-stabilized ruthenium sulfide, it has been reported that an interaction between ruthenium sulfide and the zeolite protons modifies the catalytic properties of the ruthenium sulfide since the protons act as anchors [378]. Such an anchoring by protons has also been envisaged for the stabilization of Pd par-

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ticles in zeolite pores [379]. However, addition of a base leads to a disruption of these interactions, and migration to the zeolite surface followed by sintering occurs on zeolites containing alkali cations. Hydrodesulfurization of ethanethiol and diethyl sulfide was found to occur best with conversions between 50 and 90% and high selectivities towards ethene [380, 381]. However, Brønsted acid sites in the zeolites were assumed as active centers for the elimination of H2S. The activity of the alkali-metal-exchanged zeolites is mainly influenced by their electronegativity, calculated on the basis of Sanderson’s equation [381–383]. 4.2.5 Metal Chalcogenide Nanoparticles in Mesoporous Molecular Sieves

Ihlein et al. studied the preparation of CdTe clusters confined in the pores of Si-MCM-41 [384]. CdTe clusters, the color of which is determined by the particle size, show a relatively strong luminescence, which is increased when a coating with a layer of CdO reduces the number of surface traps [385]. About 0.1 wt.% loading of Si-MCM-41 was achieved by impregnation from a solution of preformed CdTe clusters stabilized in 1-mercaptoglycerol [384]. Under excitation at 254 nm at room temperature, the emission maximum of this composite material was found to be similar to that of the solution around 550 nm, indicating the presence of particles with diameters of approximately 2–2.5 nm. While the luminescence of pure isolated clusters is lost after heating to 180°C due to aggregation, the clusters are strongly stabilized in Si-MCM-41, and the composite retains the green luminescence even after treatment at 400°C. However, it is not clear whether the clusters are really located in the pore system, since clusters supported on fumed silica show a similar behavior. Attempts to incorporate CdTe clusters directly during the synthesis of Si-MCM-41 failed in the sense that, due to the exposure to 90°C for 2 d, CdTe clusters larger than the pore widths of Si-MCM-41 were formed. This was indicated by an emission around 620 nm [384]. A broad photoluminescence spectrum, in which the luminescence arises from electrons in deep trap sites at the particle surface and defects within the confined nanoparticles, was also observed for CdSe particles in Si-MCM-41 [386]. Here, the growth of CdSe nanoparticles was achieved by following a synthetic strategy adopted for the growth of colloidal CdSe nanoparticles [387]. To facilitate the incorporation of nanocrystals, the precursor solutions of Cd(CH3)2 and Se, both dissolved in tributylphosphine, were impregnated inside the pores of Si-MCM-41 under vacuum. After wet impregnation the heterogeneous suspension was injected into a hot (325°C) solvent (trioctylphosphine oxide, TOPO) to initiate the growth of the CdSe nanoparticles. Rapid nucleation and growth of nanoparticles was achieved by maintaining the reaction at 300°C for 15 min. Finally, the CdSe-filled MCM-41 composite was collected as a powder by precipitating it from the reaction mixture with methanol and washing with toluene [386]. An average diameter of the CdSe particles of 4 nm was deduced from an absorption peak at 2.2 eV in diffuse reflectance UV-Vis spectra. EDX analysis and a drastic decrease in the pore volume in the CdSe/MCM-41 composite indicated the incorporation of the nanoparticles into the pores or, at least, at the pore mouths. Although electron diffraction studies proved that the ordered net-

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Fig. 18. Left Nitrogen adsorption isotherms on parent- and thiol-functionalized Si-MCM-41 and right DR-UV-Vis spectra of CdS nanoparticles formed in these materials by impregnation with CdAc2 and reaction with H2S at 130°C. a Parent Si-MCM-41, b Si-MCM-41 functionalized with thiol groups at 20°C and c 110°C

work of the MCM-41 framework was unaffected, the pore volume, analyzed by N2 sorption, decreased to about 30% (cf. Fig. 18). Very defined CdSe clusters with variable diameters of 2, 3 or 3.5 nm and narrow size-distributions were introduced into Si-MCM-41 by impregnation with cadmium acetate solution and subsequent reaction with H2Se at about 130°C [388]. The size of CdS or CdSe clusters formed in the pores of Si-MCM-41 could in this case be adjusted by functionalization of the inner surface of Si-MCM-41 with mercaptopropyltrimethoxysilane prior to the impregnation with CdAc2. The thiol groups act as anchors for the Cd2+ ions, preventing them from migration during the treatment with H2S or H2Se [389]. Thus, the growth of the CdS or CdSe clusters is limited. The decrease of the pore volume, due to prolonged functionalization with thiol groups, and the resulting blue-shifted absorption edge of the CdS particles formed are shown in Fig. 18. The two distinct maxima in the UV-Vis spectrum (Fig. 18c) result from the excitation into two separate exciton levels; their appearance indicates that the size distribution of the CdS nanoparticles in that sample is very narrow, all particles possess diameters of 1.2 ± 0.1 nm. The functionalization of the pore walls of HMS or Si-MCM-41 materials with thiol groups has also been used to achieve a highly effective removal of heavy metal ions, e.g., Ag+ or Hg+, from waste water [390, 391]. Hirai et al. used thiolmodified Si-MCM-41 to anchor CdS nanoparticles that had previously been stabilized in reverse micelles [392]. A bis(2-ethylhexyl)sulfosuccinate/water/isooctane reverse micellar solution containing Cd(NO3)2 was added rapidly to another micellar solution containing Na2S and stirred vigorously at 25°C. The thiolfunctionalized Si-MCM-41 material was added 2 min after the formation of the CdS nanoparticles, and the mixture was stirred for 12 h. The addition of SiMCM-41 suppressed the growth of the CdS particles slightly, probably because the adsorption of water in the mesopores changed the water/tenside ratio in the

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reverse micellar solution. Although the absorption edges in their diffuse reflectance spectra are less steep than that shown in Fig. 18, indicating a much broader size distribution of the CdS nanoparticles, the authors claim that the CdS particles are incorporated in the pore system of Si-MCM-41 and are chemically bonded to the thiol groups via excess Cd2+ ions on the CdS particles. The samples have been tested in the photocatalytic generation of H2 from water. However, the observed reaction rates were so low that a distinct catalytic effect of the CdS nanoparticles seems doubtful [392]. The creation of ordered arrays of identical magnetic clusters was the goal of a recent work dealing with the incorporation of Cd1–xMnxS clusters (x <1) into Si-MCM-41 [393]. Such arrays of clusters may provide a way of studying the link between magnetism on a microscopic atomic level and the macroscopic magnetic state. A suspension of a mixture of (1–x) molar parts Cd(OOCCH3)2 and x molar parts Mn(OOCCH3)2 dissolved in a small quantity of water and Si-MCM-41 grains was stirred for 10 min, centrifuged and dried in an oil pump vacuum. Subsequently, the obtained powder was flushed with H2S gas for 10 min and kept under H2S atmosphere at 70°C in a Schlenk tube for several hours. The successful incorporation of Cd1–xMnxS clusters into the pore volume of the MCM-41 was proven by broad reflections of the wurtzite structure of Cd1–xMnxS in the X-ray diffraction pattern, a slight broadening of Raman peaks, and by quantum confinement effects resulting in a blue-shift of the absorption edge by 80 meV in comparison to bulk reference samples of the same Mn content [393]. Unfortunately, detailed magnetic studies on this sample have not been reported to date.

5 Conclusions In the preparation of zeolite-encaged chalcogenide clusters, two general problems are encountered: Firstly, a homogeneous distribution of the guest species throughout the molecular sieve pore structure is difficult to achieve, when precipitation in contact with the crystals occurs during ion exchange, chemical vapor deposition or impregnation, or when the oxide precursors cannot enter the pores due to size restrictions and have to migrate into the channels during calcination. Secondly, the crystallinity or regular pore structure of the support is often damaged in the modification procedure due to the reaction conditions required, e.g., acidity or basicity of the ion-exchange or impregnation solutions, the formation of HCl during chemical vapor deposition or high temperatures during solid-state reactions. Very often, the dimensions of the guest clusters formed exceed those of the channels and cages of the host. This indicates that, during the incorporation, mesopores may be formed although no overall loss of crystallinity can be detected. In some cases, the formation of intrazeolitic guest compounds and of mesopores is even assumed to be interdependent [149, 171]. Often, the preparation conditions have to be carefully chosen to achieve a uniform distribution of the guest compound without damaging the host structure too much. However, anchoring of guest species at defect sites, e.g., hydroxyl

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nests, appears to favor strong bonding and leads to interesting catalytic properties of the resulting materials. The interaction between the host and the guest oxide as well as the kind of guest species present seem to depend primarily on the loading rather than on the preparation method: At low contents of the guest oxide, anchoring of monomeric oxo-species via more than one bond to the host framework predominates, whereas with increasing amount of oxide introduced, its properties become more bulk-like. For the characterization of the modified materials, EXAFS, IR and – where possible – EPR and UV-Vis spectroscopy seem to furnish the most detailed information concerning the nature, size and location of guest chalcogenide clusters as well as bond formation between the host and the guest. TEM images help to distinguish between internal confinement and external location of the clusters. By 27Al and 29Si MAS NMR spectroscopy, the zeolite crystallinity can be checked. Results obtained by other methods have to be considered more carefully: Nitrogen adsorption measurements cannot distinguish unequivocally between pore blocking from the external surface and pore size reduction due to guest incorporation. The reliability of XPS measurements is limited, because the surface nSi/nAl ratios of a zeolite host may differ from that of the bulk, and large chalcogenide clusters on the external surface may yield nguest /nSi ratios which are too low. Reduced zeolite peak intensities in the X-ray diffractograms of the modified materials do not necessarily indicate destruction of the zeolite framework. For the characterization of electronic and optical properties resulting from quantum confinements and host-guest interactions, diffuse reflectance UV-Vis spectroscopy, XPS, and fluorescence spectroscopy have provided valuable information. A combination of the different methods is needed for a reliable characterization of clusters supported on molecular sieves. The modification of zeolites and ordered mesoporous materials by incorporation of chalcogenides yields materials which are of interest in many fields of modern science, e.g., in catalysis, photochemistry or sensor development. In many instances, the molecular sieve matrix does not only arrange the guest species in a geometrically predetermined manner, which may result in selfassembly effects, but also alters their optoelectronic properties by host-guest and guest-guest interactions leading to superlattice phenomena. The stoichiometry of the nanoparticles is changed by the influence of the charged zeolitic framework, resulting in a modified light absorption. Also, the luminescence behavior is influenced due to phonon coupling with the host and the formation of defects in the particles acting as electron traps. Up till now, these materials have been predominantly tested as catalysts. Before they can be used in optoelectronic applications, problems encountered in the stability of the clusters and in the preparation of large zeolite crystals containing clusters of regular size and distribution in a reproducible manner have still to be overcome. The solution of these problems, along with the requirements for chemical and thermal stability and easy processing, remain fascinating challenges for future research and development in an exciting field of materials science.

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