Zeolites metal species encapsulated

Metal macrocycles encapsulated in zeolites seem to be a solution to overcome the above mentioned problem because they combine successfully the advantages of homogeneous catalysts, especially their selectivity and controllability, with the ease of the separation of heterogeneous catalysts. In these catalysts the large, electroneutral metal macrocycle species is held in the zeolite cavities topologically rather than chemically. 

Metal Species and Oxide Clusters Encapsulated in Zeolites Basicity and General Aspects... 

As platinum is especially important in catalysis, strong experimental efforts have been made to characterize small Pt particles entrapped in zeolite cages, using the whole arsenal of spectroscopic, structural and chemical methods (e.g. Refs. 200, 241, 242 and references therein). However, the interpretation of experimental data often remained ambiguous because the measured values commonly reflect more than one effect as a rule, when the electronic state of an encapsulated metal species changes due to interactions with the zeolite host, so do its size and shape. Model calculations enable to separate these effects. 

Conventional preparation techniques result in non-uniform supported metal species, which are difficult to characterize structurally. Therefore, Gates developed a strategy [266,267] to prepare nearly uniform metal clusters, either on metal oxide support or encapsulated in zeolite cavities. Starting from supported metal carbonyl clusters as precursors, the synthesis proceeded with decarbonyla-tion aiming at a minimal perturbation of the metal frame. However, fragmentation or aggregation of metal clusters may occur. 

Entrapment or intercalation of metal species in pores and cavities of solid supports has frequently been used for the immobilization of catalysts in inorganic materials such as zeolites, clays, charcoals, silicas, aluminas, and other solids. Though this review article focuses on the immobilization of palladium complexes on polymer supports via covalent and/or coordination bonds, recent novel approaches to polymer-supported palladium species (including palladium nanoparticles) via nonbonding immobilization, such as encapsulation and incarceration, are intriguing because of their high potential for utility. In this section, several representatives are introduced. 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

There has been considerable debate about whether intrazeolitic species are elec-trochemically accessible and recent results on zeolite-encapsulated metal complexes are of interest in this regard. Before we discuss this topic, the role of ion exchange in electrochemical response is discussed. Electroactive transition metal ions can readily be ion exchanged into zeolites. Baker and co-workers reported that a Co +-zeolite Y-ZME resulted in reduction of cobalt ions if Li" was used as the supporting cation in the electrolyte solution, whereas the voltammetric response was absent in the presence of the larger tetrabutylammonium ion (TBA) [166]. Figure 32 compares the voltammetric data for Li+ and TBA. There are two ways to interpret these data ... 

From the seminal work of Lunsford et al. in the early 1980s (DeWilde et al., 1980 Quayle and Lunsford, 1982), ship-in-a-bottle synthesis of metal complexes in the zeolite supercages, encapsulation of catalytically, optically, and/or electrochemically active species within micro- and mesoporous aluminosilicates, has received considerable attention (Alvaro et al., 2003). Site isolation of individual guest molecules, combined with shape and size restrictions imposed by the supercage steric limitations. 

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]. 

Zeolite catalysts incorporated or encapsulated with transition metal cations such as Mo, or Ti into the frameworks or cavities of various microporous and mesoporous molecular sieves were synthesized by a hydrothermal synthesis method. A combination of various spectroscopic techniques and analyses of the photocatalytic reaction products has revealed that these transition metal cations constitute highly dispersed tetrahedrally coordinated oxide species which enable the zeolite catalysts to act as efficient and effective photocatalysts for the various reactions such as the decomposition of NO into N2 and O2 and the reduction of CO2 with H2O into CH3OH and CH4. Investigations on the photochemical reactivities of these oxide species with reactant molecules such as NOx, hydrocarbonds, CO2 and H2O showed that the charge transfer excited triplet state of the oxides, i.e., (Mo - O ), - O ), and (Ti - O ), plays a significant role in the photocatalytic reactions. Thus, the present results have clearly demonstrated the unique and high photocatalytic reactivities of various microporous and mesoporous zeolitic materials incorporated with Mo, V, or Ti oxide species as well as the close relationship between the local structures of these transition metal oxide species and their photocatalytic reactivities. 

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