Metal Cluster in Zeolites

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

The application of zeolite-entrapped rhodium carbonyl clusters [prepared by exchanging Rh(NH3)6Cl3 into NaY zeolite followed by reduction in (CO H2) mixtures] as catalysts for the liquid-phase hydroformylation of alkenes has been discussed. More recently, infrared spectra of Rh6(CO)i6, supported on NaY zeolite by sublimation and treated with carbon monoxide at 100 C, have been found to be virtually identical to those obtained in the hydroformylation experiments.  

Qusters apparently devoid of ligands and encaged in zeolite structures may exhibit electrophilic properties. Platinum clusters containing fewer than six atoms, when encaged in a Y zeolite exhibited enhanced activity for ethene 

The activity of platinum clusters in zeolites for the hydrogenolysis of neopentane, ethane, and butane exceeds that of conventional platinum catalysts, and in two cases the results were interpreted in terms of an intrinsic change of electronic properties with particle size. In butane hydrogenolysis the sticking probability of alkane on the electrophilic platinum cluster was high compared with that on conventional platinum crystallites.  

Reduced sensitivities to sulphur poisoning and enhanced sensitivity to nitrogen poisoning have been claimed for metal encaged in zeoHte the position is confused.  

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There does seem to be some quite good evidence for the existence of small electron-deficient metal clusters in zeolites (101-105), which may be related to their increased resistance to sulfur poisoning. Further studies are, however, necessary in order to provide a more detailed understanding in this area. 

Metal clusters in zeolites can be regarded as intermediate forms between the naked clusters and clusters which are stabilized chemically by a ligand shell. Fe and Co clusters with diameters of 4-11 A have been prepared from bis(toluol)metal complexes and NaY zeolite93 ). 

The interaction of metal clusters in zeolites with protons has been studied by isotope exchange with D2 280-283). The presence of metals or metal impurities is essential for this exchange to occur at moderate temperatures absolutely no exchange between D2 and zeolite protons in metal-free HY is detected at room temperature. In the presence of Pt or Pd, however, exchange is fast and includes all protons in the zeolite, which are detectable by their O—H vibration bands in FTIR. 

A. Ship-in-Bottle Synthesis of Metal Clusters in Zeolites and Clay Minerals Structures and Reactivity of Metal Clusters in Zeolites... 

These and other aspects of catalysis by metal clusters in zeolites have been reviewed recently by Gallezot, and by Uytterhoeven. ... 

The size of metal clusters and the number of atoms in the cluster can vary in a large range. The number of nuclei m can vary from 1 to thousands. In a zeolite, metal clusters can be located in different cages and defects. And the metal clusters in zeolites can be roughly classified as the following types (Figure 9.1) ... 

Figure 9.1 The size and location of metal clusters in zeolites. Reproduced from [3], Copyright (2002) Springer-Verlag...

Generally two approaches can be used to prepare metal clusters in zeolites. One involves direct evaporation and deposition of metals into the pores of zeolites, whereas the other is to load the metal-containing precursors into the zeolite pores followed by formation of metal clusters in the zeolite pores through decomposition or reduction.[3] According to the preparation techniques used, these two approaches can be further divided into the following sub-approaches ... 

P. Gallezot, Preparation of Metal Clusters in Zeolites. In Molecular Sieves, Vol. 3., Springer-Verlag, Berlin, Heidelberg, 2002, 257-305. 

Transition metal clusters in zeolite cages form another important class of supported species. Zeolites are very suitable supports/hosts for small metal particles because the dimensions of their cavities affect the formation of encaged moieties of nanometer and sub-nanometer scale, thus stabilizing clusters of desired sizes and shapes. Experimental investigations in this direction resulted in preparation procedures that yield nearly uniform small encapsulated metal moieties in zeolites [18-20]. 

Quantum chemical calculations on metal clusters in zeolite A [12] and semi-empirical ligand field interpretations of spectroscopic data of transition metal ions [6] have proven to be successful in structural characterizations of molecular sieves and their guest species. The present tendency in catalysis towards a more fundamental approach justifies the expectation that ESR, combined with other spectroscopic techniques, will become important. However, this requires an accurate and unambiguous parameterization of the ESR spectra. The parameter set thus obtained forms a firm basis for a theoretical investigation of the coordination environment of the paramagnetic entity. 

The development of environmentally friendly solid catalysts for the production of fine chemicals has known a recent growing interest and recent review articles have been devoted to catalysis by solid bases. Several solid bases have been proposed such as alkali ion-exchanged zeolites [1], alkaline oxides supported on microporous [2] and mesoporous solids [3], sodium metal clusters in zeolites... 

Metal clusters in zeolites an intriguing class of catalysts Zeolite-supported transition metal catalysts Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides Silver clusters and chemistry in zeolites Structure and reactivity of surface species obtained by interaction of organometallic compounds with oxidic surfaces infra-red studies... 

Metal clusters in zeolites. Zeolite NaY-supported Ir4(CO),2 was prepared by direct deposition onto the outside surface of the zeolite crystals and alternatively by reductive carbonylation of IrfCOjjfacac) sorbed in its pores. Measurements of infrared and EXAFS spectra during the formation of the entrapped metal carbonyl clusters gave evidence of reaction intermediates, suggested to be These... 

In ship-in-a-bottle syntheses, metal ions, metal complexes, or small metal clusters in zeolite cages can be converted in reactions with CO, or with CO + H2 or CO + H2O, to give (larger) metal carbonyl clusters that fit within the cages but are too large to pass through the pore windows. [3-5, 30] Similar reactions for metal cluster synthesis takes place in solution [31, 32] and on surfaces [33-37]. They are called reductive carbonylation reactions and are discussed below. 

Metal-catalyzed reduction of methylviologen metal clusters internal or external location of metal cluster in zeolite Characteristic blue color of the reduced viologen radicals makes it ea to locate dusters. 

Metal clusters in zeolite cages are small and structurally well defined relative to the metal crystallites (often called clusters) present in typical metal oxide supported metal catalysts used in industry. Thus, researchers have investigated zeolite supported metals in attempts to better understand the structures of supported metals, the interactions of metals with supports, and the dependence of catalytic properties on cluster size and the nature of the interactions with the support. [105, 107, 108]... 

X-ray diffraction crystallography can be used to determine not only the framework structures but also the locations of the isolated metal ions in the zeolites. The method is applicable when the metals are periodically isolated within the crystalline zeolite structure and not agglomerate outside of it. Although the method is, in principle, appropriate for the characterization of all types of metal clusters in zeolites, its use has been largely restricted to ionic clusters, [138, 139] which are beyond the scope of the present review. 

Sorption and desorption of gases, usually H2, have been widely used to characterize metal clusters in zeolites. [165] The methods are standard for supported metal catalysts and provide reliable measures of surface area and metal dispersion. However, when the clusters are as small as those in zeolite cages, there are important limitations to the method. For example, the stoichiometries of the reactions of H2 with metal clusters are not well known and vary from metal to metal. The method will be mentioned again in Section 4.3.3.5 for Pt dusters in the zeolite L, where the results are in good agreement with the EXAFS results. 

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