If bifunctional catalysis occurs at neutral or close to neutral pH values , i.e., the interaction between uncharged imidazole groups , three mechanisms can be proposed to describe the interactions between a polymer catalyst and a substrate. [c.297]
Catalytic processes are widespread in nature and are effectively used in various industries , science and technology. Thus, in the chemical industry , tens of millions of tons of ammonia are produced from heterogeneous catalytic processes from air nitrogenand hydrogen, nitric acid by oxidation of ammonia, sulfur trioxide by oxidation with 50 g of air, etc. In the petrochemical industry, more than half of the oil produced by catalytic processes of cracking , reforming, etc. is processed into more valuable products – high-quality motor fuel , various monomers for the production of polymeric fibers and plastics. To multi-tonnage catalytic processes include the processes of hydrogen production by conversion of carbon dioxide and methane, the synthesis of alcohols , formaldehyde andmany others . It can be argued that a catalyst can be created for any reaction . The theory of catalysis should disclose the regularities of an elementary catalytic act, the dependence of catalytic activity on the structure and properties of the catalyst and the reacting molecules, and thus create the necessary prerequisites for predicting the structure and properties of the catalyst for a particular reaction , and indicate the ways of its production. The description of the rate of the catalytic process can be approached using the main provisions of formal kinetics and the transition state method. At the same time, it is expedient to first isolate the general laws of catalysis , inherent in all types of catalytic processes, and then consider some specific features of individual groups of catalytic processes. [c.617]
One of the problems of molecular bionics – is the creation of polymeric catalysts of various reactions , which operate on the principle of enzyme and approaching the enzymes on the activity and selectivity of action . It is well known that enzymes are incommensurably more productive than the best catalysts of non-biological origin used by the chemical industry . It is also clear that proteins are very complex molecular structures, the precise reproduction of which by non-biological methods is a very difficult task. Overcome the huge gap between synthetic and biopolymers in the foreseeable future until recently it seemed almost impossible. At the same time, there is no need to prove how attractive is the prospect of designing artificial non-protein enzymes that are tuned to the catalysis of practically important reactions . This would make it possible, with colossal efficiency, to obtain industrially important products in small reaction volumes and without significant energy costs. [c.284] The decisive role indevelopment of production of HDPE , as before, remains behind the catalysts. In recent years, the search for catalytic systems that are fundamentally different from the known ones. For such systems, i are, in particular, immobilized on polymeric supports (heterogenized catalyst systems ) . Of significant interest -component catalysts operating at po- Vyshen temperatures (up to 200 ° C) and bifunk- tional catalysts . Studies in the field of highly active catalytic olefin polymerization systems adjointhe general problem of catalysis – the use of catalytic systems, is close to [1x to biocatalysts-enzymes.
The above examples relate to homogeneous systems in which both the starting materials and polymer catalysts dissolve uniformly . Such systems are very convenient from the point of view of the study of reaction mechanisms , the main regularities of catalysis by polymer additives , etc. However, in this case, it is always difficult to separate the reaction products and catalysts. Forproduction goals more promising are insoluble polymer catalysts. [c.90]
An example of such substances are ion exchange resins . Columns filled with sulfonated polystyrene have been used for the hydrolysis of esters since 1960. At the same time, the course of the reaction is almost independent of the flow rate of the reacting liquid , but is related to the size of the resin beads . The catalytic effect is determined mainly by the rate of diffusion inside the polymer beads, and not by the polymer proper nature. For comparison, we give the ratio of the catalytic efficiency of a cation exchange resin and a low molecular weight catalyst ( sulfuric acid ) for several reactions . This value is 0.5 for methyl acetate, 0.3 for ethyl acetate and 0.05 for ethyl-n-caproate. A lot of data are known about the advantages (in comparison with catalysis with low molecular weight agents) of catalysis by polymer additives in heterogeneous systems of esterification, alcoholysis, condensation of acetals , inversion of sugars, etc. [c.90]