Catalysts, “active centers

Reactions that are catalyzed by solids occur on the surfaces of the solids at points of high chemical activity. Therefore, the activity of a catalytic surface is proportional to the number of active centers per unit area. In many cases, the concentration of active centers is relatively low. This is evident by the small quantities of poisons present (material that retards the rate of a catalytic reaction) that are sufficient to destroy the activity of a catalyst. Active centers depend on the interatomic spacing of the solid structure, chemical constitution, and lattice structure. 

If the catalyst active centers are nonuniform, a time variation of the average value of Kp may be caused by the change of the proportion between the centers with various reactivity during polymerization. However, in the case of chromium oxide catalysts the experimental data show that the... 

GP 3] ]R 3h] [R 4a] Safe operation in the explosive regime was demonstrated [103]. Catalytic runs with 1-butene concentrations up to 10 times higher than the explosion limit were performed (5-15% 1-butene in air 0.1 MPa 400 °C). A slight catalyst deactivation, possibly due to catalyst active center blockage by adsorption, was observed under these conditions and not found for lower 1-butene concentrations. Regeneration of the catalyst is possible by oxidation. 

This is namely a thermodynamic conjugation of two processes that allows the conjugated stepwise reaction 22 to proceed in a forbidden direction when K y.2 and V22 are of opposite signs, and thus, the stepwise reaction 22 leads, formally, to a decrease in the entropy. Typical examples of the conjugating and conjugated reactions are, respectively, a reaction catalyzed by some catalyst and the closed chain of chemical transformations of the catalyst active center. The simplest combination of such reactions is the catalyzed stepwise reaction... 

As mentioned earlier, the acceleration type behavior is explained by an increase in surface due to breakup of catalyst particles subjected to mechanical pressure of growing polymer chains anchored to the catalyst active centers. The smaller the particle size, the greater the mechanical energy required for further size reduction, and so the particle size—and hence the specific surface area—would reach some asymptotic value. The stationary polymerization rate would correspond to this catalyst particle size. 

The concentration of the functioning active centers for example turns out to be much lower than the initial catalyst concentration. When initiation takes place by Lewis acids and oxonium salts, the concentration of active centers is determined by the catalyst - active centers equilibrium, which is established during a period of time much shorter than the overall time of polymerization. 

The electrochemical activity of the catalyst is normally measured in the electrolyte solution saturated with inert gas such as pure N2 or Ar and the electrode is at static state (not rotated). The purpose is to obtain information about the redox activity of the catalyst itself or some information about the catalyst surface behavior, from which the catalyst electrochemical active surface area (for Pt-based catalysts) or the concentration of catalyst active center (for non-noble metal catalysts) an be obtained. To give some basic sense about these measurements, we will give two examples as follows. 

Catalytic activity values for the catalysts differing in the natiu of organic plasticizer are compared in Table 3. Evidently, the increase of plasticizer amount does not affect significantly the NO conversion degree. A more considerable influence on is provided by changing one plasticizer to another. The samples with PEO and cfilC are comparable in activity, while on the samples with PAA and natural resin, decreases to 58-76%. The sample with PVA has a mediiun value. The influence oi plasticizer on X q is not yet explicable. Perhsqps, the final products of destruction of these plasticizers affect the catalyst active centers. It is also possible that CMC and PEO decompose to form the minimum amount of carbon in the catalyst and natiunl resin causes carbonization of catalyst surface upon thermal treatment. 

The vast majority of papers reporting stereoselective epoxide polymerization focus on isospecific propylene oxide polymerization. For clarity, this chapter is organized by the type of metal of the catalyst active center. The three most commonly used metals for discrete stereoselective epoxide polymerization catalysts are aluminum, zinc, and cobalt, and research using these metals forms the foundation of this chapter. 

Figure 5.14 Carbon monoxide incorporation into polyethelene M is the catalyst active center.

Information about alkoxy silane donors of various structure is mostly to be found in the patent literature. l l Though some effects of the most common external alkoxy silane donors on polymerization of propylene have recently been published,scientific studies on the relationship between structure and polymerization behavior are few. Soga et al. l and liskolal l have recently reported some interesting results for propylene polymerization in heptane medium with various external alkoxy silane donors. Considerable dependence was found between the polymerization behavior and the structure of the alkoxy silane. In our previous publication ) we examined the effect of the structure of external alkoxy silane donors on catalyst activity and the isotacticity of the formed polymer. Effects on catalyst active centers were also discussed. Our results were broadly similar to those of Soga and liskola. ... 

The polymer chain is a "fingerprint of the catalyst active centers, go that study of microstructure of the polymer chain handily provides information about the structure of active centers in the catalyst and the effect of different additives on them. The microstructure of polypropylene is most easily studied in the methyl carbon region of NMR spectra.With modem high field NMR instruments even heptad configurations of PP chain can be determined rather easily, Microstructures of polypropylene up to hexads can be examined in the methylene region of the spectra,... 

An unoccupied coordination site and titanium alkyl bond in the titanium complex are fundamental requirements for its catalytic activity in olefin oligomerization. The titanium-alkyl bond is formed in the reaction of a titanium compound with alkyl or alkylchloroaluminum compounds when, for example, halide atoms or alkoxy groups of titanium compounds are replaced by alkyl groups of aluminum derivatives. There are many proposals concerning the structure of the Ziegler-Natta catalyst active centers. These are presented in structural formulas (16)-(21) [291. 

---