Zeolite supported metals

Meriasdeau, P. and Naccache, C. (1997) Dehydrocyclization of alkanes over zeolite-supported metal catalysts monofimctional or bifunctional route. Cat. Rev. Sci Eng., 39, 5-48. 

Zeolite-supported metal catalysts with atoms and clusters existing both inside and outside the zeolite channels are often examples of the situation depicted in Fig. Id. Again, XAFS has been an important tool for characterizing such catalysts (4-10). 

SELECTIVE HYDROGENATION OF UNSATURATED ALDEHYDES OVER ZEOLITE-SUPPORTED METALS... 

Evaluation of Some New Zeolite-Supported Metal Catalysts for Synthesis Gas Conversion... 

This study was initiated in an attempt to produce highly-dispersed, thermally-stable, zeolite-supported metal catalysts and to investigate the effect of acidity and pore size of the zeolite on the products obtained from synthesis gas conversion. As a result of this study, several conclusions can be made. 

Second ingredients including Co, Ca, Cr, Sr, Ag, Ce, Ba, Mg, Mo, Fe, Sn and so forth can be simultaneously employed to improve hydrothermal durability of SCR catalysts such as metal-exchanged zeolites, supported metals and their... 

Recent research on the elementary steps during preparation of zeolite-supported metals has helped to understand the genesis of these particles in considerable detail. For the best studied systems the goal of preparing catalysts by design has been achieved (69-77). 

Some of the most thoroughly characterized supported metal complexes are zeolite-supported metal carbonyls. These have been prepared, for example, by the adsorption of Rh(CO)2(acac) on zeolites (e.g., the faujasite zeolite NaY [26] or dealuminated zeolite Y [27]) followed by CO treatment of the resultant material (Fig. 19.3). The IR spectra (not shown, but found in [26, 27]) of the rhodium dicarbonyl represented in Fig. 19.3 are consistent with a square-planar complex (formally Rh(I)) with the Rh atom bonded to two zeolite oxygen atoms. 

The future of Raman spectroscopy in the research and the development of catalysts appears to be extremely promising. The recent revolution in Raman instrumentation has dramatically increased the ability to detect weak Raman signals and to collect the data in very short times. Thus, it is now possible to perform real-time Raman analysis and to study many catal) c systems that give rise to unusually weak Raman signals. The enormous strides in Raman instrumentation now allow for the characterization of a wide range of catalytic materials bulk mixed oxides, supported metal oxides, zeolites, supported metal systems, metal foils, as well as single crystal surfaces. Few Raman studies have been reported for sulfides, nitrides, or carbides, but these catalytic materials also give rise... 

Zeolites have been used for years as supports for metal catalysts [1-5]. Such catalysts are typically made by impregnation of the zeolite with an aqueous solution of a metal salt, followed by calcination and reduction in hydrogen. Because the metal particles in such catalysts are typically extremely small and nonuniform in size and shape, often being present both inside and outside the zeolite pore structure, their structures are not well understood. This structural complexity provides a fundamental motivation for preparing and investigating structurally simple zeolite-supported metals, those that are so small and uniform as to be nearly molecular in character and located almost entirely within the zeolite pores investigations of well-defined... 

STRUCTURAL CHARACTERIZATION OF ZEOLITE-SUPPORTED METAL CLUSTERS... 

A summary of zeolite-supported metal clusters prepared from metal salts is given in Table 2. Included here are only samples that have been characterized by EXAFS spectroscopy and incorporate extremely small clusters. The additional literature of zeolite-supported metals is reviewed elsewhere [1-5]. 

It must be assumed that the samples listed in Table 2 have distributions of cluster sizes, although the available methods do not provide good evidence of the distributions. It seems likely that zeolite-supported metal clusters made from metal carbonyl clusters (Table 1) incorporate more nearly uniform clusters than samples made by conventional methods from metal salts, but this suggestion is not yet tested. 

In summary, zeolite-supported metal clusters have now been prepared that are so small and apparently nearly uniform in size that they are regarded as nearly molecular. Preparations with metal carbonyl cluster precursors are the best known for making nearly uniform and thus nearly molecular supported clusters, but it is clear that conventional preparation methods based... 

Zeolite-supported metal clusters formed from salt precursors ... 

These samples offer a long-envisioned opportunity to vary metal cluster size and to determine how the catalytic properties depend on it. Results of some of the first experiments with zeolite-supported metal cluster catalysts are summarized below. 

It is likely that metal clusters have been present for years in conventional supported metal catalysts, such as those used for naphtha reforming, but because of the difficulty of distinguishing the small clusters from larger metal crystallites, it has not been possible to identify and define the roles of clusters. Evidence of catalysis by supported metal clusters has arisen only recently in work with catalysts containing the metal almost entirely in the form of clusters. The following section is a summary of catalytic results for zeolite-supported metal clusters that have been characterized by EXAFS spectroscopy. 

Zeolite-supported metal clusters are a new class of catalyst made possible by syntheses involving organometallic chemistry and by precisely controlled treatment of metal complexes in zeolite cages. Elucidation of the preparation chemistry would not have been possible without the guidance of EXAFS spectroscopy. Clusters such as Ir4, Ir, and Pt (where n is about 6) are small enough to be considered quasi molecular rather than metallic. Their catalytic properties are distinct from those of metallic particles, even for structure-insensitive reactions. The zeolite pores seem to confer some properties on the clusters that are not yet well understood. 

The production of hydrogen from methane over zeolite supported metal catalysts can be examined as an alternative to steam reforming because the concomitant aromatization reactions can increase the economic potential of the process. For methane aromatization, Mo/ZSM5 catalysts have been intensively studied since their first report in 1993 (/, 2). In 1997 (3), the promotional effect of ruthenium over Mo/ZSM5 catalysts was reported. Other second metals have also been studied to improve catalyst activity and stability and a review on this topic is available 4). 

Scheme 10. Selective hydrogenation of naphthalene over zeolite-supported metal catalysts.

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