Polyfunctional catalyst selectivity

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. 

Much effort has been made in the last few years and continues to be devoted to the development of catalysts containing multiple types of active sites. Polyfunctional catalysts may be used to perform several steps in a reaction sequence, working in a cooperative manner to alter the characteristics of a single reaction, e.g. rate and selectivity. 

With only acid sites, the intermediate actually plays no role, but the metal sites permit the alternate, and then apparently dominant, reaction. Many further aspects of polyfunctional catalyst conversion and selectivity behavior were also discussed by Weisz [16], but our main goal is to develop kinetic rate expressions. 

Preparation of new aiuminophosphate oxynitride (AlPON) and nickel modified AlPON are presented. Such basic and polyfunctional catalysts are tested in Knoevenagel condensation and one step synthesis of methylisobutylketone (MIBK) from acetone. It is evidenced that the nitrogen content of the AlPON, which controls the rate of benzaldehyde-ethylcyanoacetate condensation depends on the activation procedure. 90% selectivity in the MIBK synthesis is obtained on Ni formate impregnating AlPON activated at 400 °C for 4 hours. TPD of CO2 and NH3, DRIFT analysis and quantum mechanical calculation evidence that the 0/N substitution creates strong basic sites. 

The knowledge currently available allows us to make useful predictions of which metals (pure or alloyed) and in which form (small or large particle size) have the best chance to be good catalysts for a new reaction with simple (monofunctional) molecules which have not yet been studied. However, much less can be predicted at the moment with regard to the polyfunctional molecules, in which (for example) a C=C bond stands in the neighbourhood of a C=0, C=N or other bond. The only general theory of selectivity in these reactions is that of Ballandin [86], but this theory does not seem to be satisfactory from the modern point of view. However, useful information is available for some individual reactions. For example, with regard to the a,(1-unsaturated aldehydes, of which acrolein is the simplest example. Let us describe this in more detail. 

An important aspect of polymerization is the ability to obtain different products from the same monomer by proper choice of catalyst and reaction conditions. These products may differ in stereochemical configuration or in the nature of the repeating unit. For example, it has been possible to polymerize selectively one or all of the functional groups of polyfunctional monomers. A subject of particular fascination has been... 

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. ... 

Air-oxidized Ni-graphite (obtained by air exposure of dispersed Ni on graphite) is a less active catalyst not affected by aging. A bond selectivity in the hydrogenation of polyfunctional compounds is observed, with partial reduction of j3-ionone, 22, to dihy-droionone, 23. The same reduction is accomplished by the use of triethylsilane in the presence of catalytic amounts of RhCl(PPh3)3 followed by hydrolysis ... 

One of the biggest challenges in the development of the fine chemistry, is the selective reduction of polyfunctional organic molecules. Heterogeneous catalysis is playing an important role in this kind of reactions (2). The use of solid catalysts presents the advantage of handling and separation properties. 

ABSTRACT. Molecular sieve catalysts offer a broad range of possibilities for carrying out reactions with high selectivity in the synthesis of organic intermediates and fine chemicals, i. e. compounds possessing functional groups. The chemical composition of the molecular sieves can be different to a certain extend, in addition to their large number of known and unknown structures. Furthermore, acidic, basic, polyfunctional and redox properties can be combined with the structural features of these materials such as shape selectivity. The potential afforded by such molecular sieve catalysts is illustrated by examples taken from both academic publications and the patent-literature. An extremely important aspect is the fact that molecular sieves contribute to the development of processes which are environmentally more friendly. 

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 ... 

One component of a two-part urethane adhesive typically contains an NCO-terminated prepolymer and possibly fillers. The second component consists of polyfunctional, NCO-reactive materials, such as polyols and/or polyamines, and may also contain fillers and catalysts. The work life and rate of cure can be adjusted by the selection of catalysts, the catalyst level, and the starting materials used to prepare the resins. Table III lists the cure times for three adhesives which have identical bond strengths when cured, but different work lives. Two-part urethane systems usually cure at room temperature, but the cure can be accelerated by the use of heat. Although... 

October 1998 A mefliod is described for the pr aration of cyclic or polymer products by selective olefin metathesis of bi- or polyfunctional substrates in the presence of homogeneous or heterogeneous catalysts in compressed CO2. A claim includes control of the product distribution by tuning the density of the reaction medium. 

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