Heterogeneous catalytic systems distributed

Wesslau was the first to demonstrate that, along with certain heterogeneous catalytic systems for polyethylene, there is a certain MWD dependence on the type and number of ligands altogether distributed between catalyst and cocatalyst. With TiCl -aluminium alkyls it was concluded > that, by varying the type of aluminum alkyl, polymers are obtained with a very similar MWD curve, but with the maximum shifted according to the nature of the alkyl group. Instead, different results were found by Russian researchers 59-ieo)... 

X. Some Aspects of the Dynamic Behavior of the Distributed Heterogeneous Catalytic Systems... 

X. SOME ASPECTS OF THE DYNAMIC BEHAVIOR OF THE DISTRIBUTED HETEROGENEOUS CATALYTIC SYSTEMS... 

Comparison of this new type of catalyst with known cataljdic heterogenized systems, for example, Ru-BFNAP-catalyst deposited on zeolite-6eto showed that these catalysts are very effective, despite the metal complex not being covalently bonded to the carrier but distributed in the liquid film on the surface and thus representing a "hybrid genuine" homogeneous-heterogeneous catalytic system (Davis ). 

For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

In real systems, especially in heterogeneous catalytic and biological sys terns, the reactants are often arranged irregularly in space. Therefore, an arising instability may cause simultaneous diffusion of substances from one point to another inside the system to make the reactant concentration oscillations arranged in a certain manner in space during the occurrence of nonlinear chemical transformations. As a result, a new dissipative structure arises with a spatially nonuniform distribution of certain reac tants. This is a consequence of the interaction between the process of diffusion, which tends to create uniformity of the system composition, and local processes of the concentration variations in the course of nonlinear... 

A way to overcome this difficulty is catalyst heterogenization. With this technique the catalyst, whose activity has been identified in a homogeneous phase, is distributed onto a relatively inert, highly dispersed, solid support, thus originating the catalytic system ... 

Vizen and Kissin (94) suggested that the distribution of active centres of a heterogeneous catalyst determines the distribution of compositions in the copolymer they derived formulas for calculating both composition and weight of copolymer fractions in relation to the stereospecificity of the active centres. The theoretical relationships were compared with experimental data, obtained in part by IR measurements, on composition distribution in a copolymer prepared with the catalytic system VC13—A1(C2H5)3. 

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