Metal-containing zeolites, various

A novel method for the preparation of metal containing small pore zeolites is described. The metal is introduced at elevated temperatures by solid state ion exchange. The zeolites obtained by the new method are highly shape selective. As an example, the competitive hydrogenation of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) over various platinum, palladium and rhodium containing 8-membered ring zeolites was studied. 

Various both heterogeneous and homogeneous catalytic systems have been developed for methane oxidation to methanol in gas or liquid phases. Achievements in this field before 2008 are discussed in several previous reviews [10,34,60]. Inspired by the high performance of MMO, Fe- and Cu-containing zeolites were prepared to mimic catalytically active metal-oxide sites and applied for gas-phase oxidation of methane to methanol using N2O [61,62] or O2 [63,64] as oxidizing agents. However, the achieved methanol yields are rather low. 

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. 

Zeolites of type X, containing various metal ions (Pd, Cu, Co, Zn, Ni, Mn, Cr, Fe) were investigated by Gentry et al. [125], Only very small amounts of partial oxidation products are found (acrolein, acetaldehyde, formaldehyde). 

Tn the course of experimentation with formulations of silica, alumina, and A various alkali metal oxides in attempts to prepare new synthetic zeolites, a formulation containing cesium replacing some of the sodium in a typical faujasite preparation yielded a new crystalline zeolitic product which showed a typically cubic powder diagram having a body-centered pattern of... 

Zeolites containing transition metal ions (such as Cr3+, Ag+, and Cu2 + ) are active as oxidation catalysts. Comprehensive reviews dealing with various aspects of the structure (34-37) sorption (35), catalysis (38-42), and other chemical properties of zeolites are available in the literature. 

Since mesoporous materials contain pores from 2 nm upwards, these materials are not restricted to the catalysis of small molecules only, as is the case for zeolites. Therefore, mesoporous materials have great potential in catalytic/separation technology applications in the fine chemical and pharmaceutical industries. The first mesoporous materials were pure silicates and aluminosilicates. More recently, the addition of key metallic or molecular species into or onto the siliceous mesoporous framework, and the synthesis of various other mesoporous transition metal oxide materials, has extended their applications to very diverse areas of technology. Potential uses for mesoporous smart materials in sensors, solar cells, nanoelectrodes, optical devices, batteries, fuel cells and electrochromic devices, amongst other applications, have been suggested in the literature.11 51... 

Minachev et al. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300° and 450°C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84%), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42), which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites. 

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