Active centers chemical deactivation

The global polymerization rate changes with time. A period of increasing rate is usually followed by a decline and eventually by a stationary state. The rate decay is often attributed to the change in concentration of active sites in the context of the above noted expression. The initial increase may be due to the progressive activation of new active centers. The deactivation, particularly evident in supported catalysts (211,212), is attributed to variations in both number and chemical nature of the centers (213). 

Organosilanes, especially dimethyldichlorosilane (M2), are important chemicals used in the silicone industries. The direct reaction of silicon with an organic halide to produce the corresponding organosilanes as a gas-solid-solid catalytic reaction was first disclosed by Rochow [1]. In the reaction, a copper-containing precursor first reacts with silicon particles to form the catalytically active component, which is a copper-silicon alloy, the exact state of which is still under discussion. As the reaction proceeds. Si in the alloy is consumed, which is followed by the release of copper. This copper diffuses into the Si lattice to form new reaction centers until deactivation occurs. The main reaction of the direct process is ... 

Rate decay is mainly ascribed to a chemical deactivation of active centers. Nevertheless, in the case of ethylene, it appears that diffusive phenomena play also a certain role in the drop of the polymerization rate88 94. Moreover, diffusivity of monomer in the reaction medium may restrict polymerization rate, as can be concluded from the dependence of catalytic activity on catalyst concentration 95... 

Such experimental results have been rationalized by assuming a chemical deactivation of some of the active centers and the presence of at least two types of species on the catalytic surface These two are isospecific polymerization centers which are unstable with time, and only slightly specific polymerization centers which, in turn, are stable with time. The latter appear to be preferentially and reversibly poisoned by the outside donor. 

Nevertheless, it is possible that such analytical fittings of the catalyst decay curve are too oversimplified to take into account the complexity of the phenomena which take place during polymerization. On the other hand, the kinetic studies are only able to measure the average constants of the reaction and not those for each individual species. Thus, although the mechanism of deactivation of the active centers, or part thereof, has clearly been shown to be of a chemical nature, it can only be explained in hypothetical terms. In agreement with the 2nd order decay law they had proposed, Keii and Doi98 99) speculated on a bimolecular disproportionation of the active species with a consequent reduction of Ti3+ to Ti2 1 due to the action of the cocatalyst. 

The density of the free active centers per unit of the sorbent surface area also depends on the chemical structure of the sorbent, and additionally on the number of molecules other than those of the solute or mobile phase occupying sorbent active centers. These are mostly water molecules, which block (deactivate) active centers on a sorbent surface, and the degree of deactivation usually depends on the storage conditions of the precoated chromatographic plates. The density of the free active centers can also be measured and expressed numerically. 

The nanoparticle assemblies can also be used to enhance the chemical reactivity of biomolecules. Au nanoparticles assembled on polyurethane nucrospheres are used as permeable high-surface supports for the immobilization of enzymes such as pepsin to provide easy access of the substrate molecules to the enzyme active centers in the multilayer enzyme assembly. Proteins immobilized in this way exhibit biocataly tic activity higher than that of the free enzyme in solution and significantly enhanced temperature and pH stability [108]. In another approach, the layer-by-layer deposition of enzymes and magnetic particles is applied to prepare a bioreactor, which allows the biocatalytic layer to be stripped out with an external magnet when it is deactivated, so that the surface could be reloaded with a new active biocatalyst layer [109]. 

A recent elegant example of the tailoring the chemical properties of encapsulated metal complexes is the work of Balkus etal. who prepared and studied perfluorinated phthalocyanine complexes of Fe, Co, Cu and Ru (Scheme 25)[230] in NaX. Perfluorinating the complexes enhances the stability and catalytic activity of the catalysts in the oxyfiinctionalisation of light alkanes. The rapid deactivation of the catalysts based on Fe, Co and Cu Fj Pc complexes was overcome by using Ru as the metal center. Similar catalysts, i.e.,Co-phthalocyanine (Co-Pc) encapsulated in zeolite Y, are active catalysts for cyclohexene and 1-hexene epoxidation (Scheme 27)[231]. Comparison of the activity of free and encapsulated Co-Pc has shown that the interaction with the zeolite stabilizes the complex. Co-Pc is still active after 24 hrs reaction whereas the free complex in solution is virtually inactive after 15 minutes. 

Enzyme deactivation may also result from chemical modifications which alter the protein three-dimensional structure or which destroy the catalytic center. The destruction of the active site may result from an alteration of the reactive moieties or from a change in the local conformation. 

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