Adsorption alkene hydrogenation

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

The old and lasting problem of heterogeneous catalysis, the mechanism of alkene hydrogenation, has also been approached from the viewpoint of structure effects on rate. In 1925, Lebedev and co-workers (80) had already noted that the velocity of the hydrogenation of the C=C bond decreases with the number of substituents on both carbon atoms. The same conclusion can be drawn from the narrower series of alkenes studied by Schuster (8J) (series 52 in Table IV). Recently authors have tried to analyze this influence of substituents in a more detailed way, in order to find out whether the change in rate is caused by polar or steric effects and whether the substituents affect mostly the adsorptivity of the unsaturated compounds or the reaetivity of the adsorbed species. Linear relationships have been used for quantitative treatment. 

Garrone et al. (163) have rationalized all these results by proposing the following mechanism for the adsorption of hydrogen and alkenes. Oxygen ions in positions of low coordination on the surface abstract protons from the adsorbed molecules to form OH c ions and a carbanion ... 

The extent of double bond isomerization also varies with the nature of the catalyst. The degree of isomerization over metal catalysts usually decreases in the order Pd > Ni > Rh > Ru > Os =Ir =Pt.5.6 The extensive double bond isomerization observed with palladium and, to some extent, with nickel catalysts can be attributed to the formation of the adsorbed 7t-allyl species with these catalysts. While double bond isomerization may not be important in a routine alkene hydrogenation, it may influence a selective hydrogenation because the isomerized olefin can have different adsorption characteristics from those of the... 

The preparation, atomic and electronic structure, adsorption and reaction properties of metal nanoparticles supported on AljOj/NiAlCllO) were thoroughly explored by Freund and coworkers [11, 43, 52], Most studies were carried out under UHV, including CO dissociation on Rh nanoparticles [39], alkene hydrogenation [53], and CO and methanol oxidation [52],... 

The adsorption constants on the catalytic site for alkene hydrogenation were calculated from the simultaneous reactions of CHE and Q. The transformation between Q and THQl is even faster than that between DHQ and THQ5 which makes it impossible to determine the inhibition adsorption effect of the two molecules separately. Therefore, we assumed that Q and THQl... 

Figure 3.26. A cartoon representing the adsorption of hydrogen and alkene onto a (catalytic) metal snrface. Hydrogen and alkene are almost certainly not (if only becanse they are different) adsorbed in the same way and/or to the same extent. It is presumed that the adsorbed species react and the product is desorbed. Under appropriate conditions, the reaction is known to be reversible for some alkanes and, at least in principle, is reversible for all.

The first step in the reaction is adsorption of Pronto the catalyst surface. Complexation between catalyst and alkene then occurs as a vacant orbital on the metal interacts with the filled alkene tt orbital. In the final steps, hydrogen is inserted into the double bond and the saturated product diffuses away from the catalyst (Figure 7.7). The stereochemistry of hydrogenation is syn because both hydrogens add to the double bond from the same catalyst surface. 

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

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