Zeolites polyfunctional catalysts

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

Feolite catalysts modified by transition metals are interesting and difficult subjects to study. In one of the first studies of zeolites as catalysts, Rabo and co-workers (1) used a zeolite catalyst containing 0.5% platinum for isomerization of n-paraffins. In this reaction the metal-zeolite system acted as a typical representative of the bifunctional catalysts. Studies of zeolites modified by transition metals ( 2, 3, Jf) showed that their polyfunctional properties are determined by the structural and chemical properties of the zeolite and by the state of the metal in it. In this paper we discuss new data on the metal state after reduction as well as the catalytic functions of zeolite catalysts containing nickel and platinum. 

Nickel catalysts on a support have been and continue to be a subject of multilateral studies, the results of which have helped the development of the theory of the polyfunctional catalyst. The catalytic properties of Ni molecular sieves in the presence of H2 2, 7, 8) have not been studied completely. Study of zeolite structural changes and the condition of the metal after thermal and hydrogen treatment have been comparatively poor. The same is true for the influence of zeolite type on the catalytic activity of the metal. This study aims to give additional information on the subject. As type A molecular sieve modified with Ni proves to be a suitable subject for clarifying some of the structural and catalytic peculiarities of zeolites, it is given main consideration. 

Investigation of methods to control zeolite properties and composition, to modify these properties to obtain the highest specificity in their application as catalysts for various processes. Development of new multi-component polyfunctional catalytic systems. 

A similar acid catalyzed reaction (equation 41), called hydrodimerization, was found for benzene and alkylbenzenes, which are converted over polyfunctional zeolite catalysts to phenylcyclohexane derivatives89. 

Polyfunctional and composite catalysts offer many new shape selective uses for zeolites and other microporous materials. In metal-loaded zeolites, monoatomic dispersion is usually preferred. Pt- and Pd-zeolites are usually prepared by ion-exchange or impregnation. 

The effect of the acidity of polyfunctional zeolite catalysts on their activity in benzene alkylation by propene has been examined.Crystalline zeolites exist with a variety of characteristic pore and channel sizes. In the case of ZSM-5 zeolite, the interconnected channels formed by 10-membered rings of oxygen atoms allows certain benzene derivatives to fit rather closely, diffuse into the pores, and, after undergoing a reaction, diffuse out. This is the origin of the high para selectivity in reactions such as alkylation of toluene by methanol catalyzed by zeolites like ZSM-5 and related modifications. ... 

In this review article, the significance and the potential 6f the zeolite catalysts in this second field of use will be discussed in more detail and in connection to their chemical i.e. acidic, basic, polyfunctional and redox properties as well as their structural features ie. reactant-, product- and restricted transition state shape selectivity. Industrially, zeolite catalysts offer the following possibilities ... 

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