Active sites hydrogenolysis

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

Next, we investigated the experimental parameters for hydrogenolysis of Cbz-protected amino acids. It is important to carefully select the experimental parameters so that the reactions are not limited by diffusion of hydrogen to the catalytically active sites. The diffusion of hydrogen can be affected by temperature, agitation speed, as well as the number of catalytically active sites... 

The kinetic studies of the hydrogenolysis of DPM indicate that both the DPM and hydrogen are adsorbed on the same kind of active sites on the catalyst. Also, the rate-determining step of the hydrogenolysis is a surface reaction between adsorbed DPM and dissociatively adsorbed hydrogen. When the rate equation for DPM is applied to asym DAMs, their reactivities can be satisfactorily explained, and it is suggested that the product selectivity is proportional to the ratio of the adsorption equilibrium constants of the two aryl groups. 

Finally, the active sites for the hydrogenolysis of asym DAM are Mo(IV) species that originated from the reduction of the octahedral Mo(VI) species. The adsorption of the aryl group occurs on the coordinatively unsaturated molybdenum sites, which have acidic properties this fact, in turn, leads to the reaction mechanism of the interaction between the active species and the substrates. 

The concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

The hydrogenolysis and isomerization of methyloxirane were studied over various Pt catalysts in order to determine the number and nature of the active sites. The steps were found to be the probable active sites and the transformation is structure-sensitive. The regioselectivity is not affected by variation in the catalyst structure, so it is determined by the nature of the metal. 

The main minor product is ethane. (The distribution of the minor products on 0.48% Pt/Si02 catalyst CH4 = 2.6%, CO = 10.6%, C = 32.4%, C = 54.4%.) The minor products are produced by the decarbonylation of methyloxirane, but only the hydrocarbons desorb, the CO remaining adsorbed on the surface. During the decarbonylation process, C-D and C-C bond ruptures occur. It is well known that kink sites are the active sites of C-C hydrogenolysis, so it is understandable that decarbonylation will poison the kink sites. 

We have been able to identify another active site by studying the ratio of the dehydrogenation rate to hydrogenolysis rate of cyclohexane to benzene and /i-hexane, respectively (36a). While the benzene /j-hexane ratio is 3 1 on a stepped surface (with roughly 17% of the surface atoms in step positions), the ratio decreases rapidly with increasing kink density (Fig. 21b). Using a set of catalyst surfaces that were cut to maintain the same terrace width (step density equal to 2.5 x 1014/cm2), but with variable kink density in the steps, we have found that the hydrogenolysis rate increases linearly with kink... 

For any given catalytic reaction the active surface area is normally only a small fraction of the area of the active component (active phase). The term active sites is often applied to the sites effective for a particular heterogeneous catalytic reaction. The terms active site and active centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. There is evidence that the centres required for some catalytic reactions are composed of a collection of several metal atoms (ensemble). This appears to be the case for such reactions as, for example, hydrogenolysis, hydrogenation of CO, and certain deuterium-exchange processes with hydrocarbons. 

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