Saturated hydrocarbons chemisorption

In most cases the slow step of the reaction is not simply the activation or chemisorption of hydrogen, but involves other chemisorbed species. Thus, the exchange of deuterium with methane and with other saturated hydrocarbons is much slower than with hydrogen and probably proceeds through dissociative adsorption of the hydrocarbon. 

Several important studies have been conducted by various research groups regarding the chemisorption of various saturated hydrocarbons on the Ir(l 1 0) and Pt(l 11) surfaces, providing much insight into the dynamics of alkane activation on transitional metal surfaces. 

Saturated hydrocarbons other than methane do react with metal surfaces at 20°C. Clean metal surfaces such as ruthenium absorb saturated hydrocarbons and effect dehydrogenation of the hydrocarbon, e.g., cyclohexane is converted to benzene. In these cases, perhaps the initial chemisorption process comprises the formation of several C-H-metal three-center two-electron bonds. 

We seek in our cluster chemistry further examples of facile C-H bond activation and bond scission. Other established examples of such hydrocarbon chemistry had been reviewed earlier (23). In the following section the chemisorption of saturated hydrocarbons is considered. 

Extensive studies of hydrocarbon chemisorption have been made by Eischens and Pliskin (1). In a series of studies on olefins and paraffins chemisorbed on silica-supported nickel they were able to show that both associative and dissociative adsorption could occur, depending on catalyst pretreatment. Associative chemisorption of olefins is observed when hydrogen is left on the nickel surface dissociative absorption, when the hydrogen has been pumped off at an elevated temperature before chemisorption. The associative mechanism is deduced from the fact that the only absorption bands found when ethylene is added to a hydrogen-covered surface are in the C-H stretching region characteristic of saturated hydrocarbons, and that a C-H deformation band at 1447 cm-1 characteristic of two hydrogens on a carbon is also observed. 

The chemisorption of unsaturated and saturated hydrocarbons on various simple semiconducting oxides, namely vanadium pentoxide and cuprous oxide, and on spinels (CuCr204 and MgCra04) was investigated by Margolis (94). 

B. M. W. Trapnell Liverpool University) Ni, Co, and Fe are relatively inactive in most saturated hydrocarbon exchange reactions. This may be because of the activation energy of chemisorption being high on ferromagnetic metals. At 0° evaporated films of these metals chemisorb no CH4 and very little C2H6, whereas on all other metals I have studied (W, Mo, Ta, Cr, Rh, Pd, Ti) coverages up to 30 % may be achieved. 

Here a surface Lewis acid (denoted by j) abstracts a hydride ion from the methylene group adjacent to the double bond. This mechanism is in accord with the essential Lewis acid nature of the silica-alumina surface and is consistent with the previously demonstrated ability of this surface to abstract hydride ions from tertiary hydrocarbons. Since an alkenyl carbonium ion is stabilized by resonance to a greater extent than is a saturated carbonium ion, it may well be the most stable species which could form in the chemisorption of an aliphatic olefin or its precursor. It seems reasonable, therefore, to presume that such species may be involved in heterogeneous acid catalysis to a greater extent than has been generally recognized. This chemisorption process does not, of course, exclude the more conventional acid addition to the double bond which may occur under suitable circumstances but rather, it introduces an alternate path which may well exert a considerable influence on the overall course of catalytic reactions. Thus, for example, since a substituted ally lie carbonium ion may be converted to a conjugated diene by loss of a proton, it may be an important intermediate in the formation... 

The mechanism of the dehydrogenation of cyclohexane, which may be taken as typical, is therefore pictured as follows. Chemisorption of both saturated and unsaturated hydrocarbons takes place with dissociation of hydrogen. With cyclohexane, this probably occurs with formation of a benzene nucleus, bound to the surface by six bonds similar to those postulated by Twigg and Rideal (45) for ethylene chemisorption,... 

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