Alkane oxidation acidic surface

Reactions catalyzed by silica and silica-alumina surfaces (with or without added acid) closely parallel the carbonium ion reactions in homogeneous acid. The principal advantage of such acidic surfaces is that they allow alkylations and dealkylations to be conducted at temperatures in the 200 00°C region where dealkylation becomes thermodynamically favorable relative to alkylation. They are thus the catalysts of choice for cracking of petroleum distillates to branched olefins and branched alkanes. The acidic surfaces also possess the advantage of little oxidizing action. 

With both reactions occurring at the same rate, a surface composition of CjHjOj would correspond to these strongly adsorbed species. However, other compositions with noninteger i, p, and q are likely, depending on the reactivity of each site. The oxidation of these intermediates is considered difficult, resulting in electrocatalyst deactivation 195). Formation of [CHO] and [COOH] has also been postulated for the reverse reaction, that is, the electroreduction of COj 202, and for formic acid electrooxidation 201, 203,204. One should remember that species [CHO] was also proposed as a surface intermediate of alkane oxidations (type I), reacting only with difficulty (Section IV,E,1). 

Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield ftom isobutyric acid was successfully enhanced by the stabilization of the vanadium-substituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (V0)2P207 was reversibly oxidized to the Xi (8) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12-molybdates in solution showed 93% conversion on H2O2 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. 

Most of the catalysts employed in the chemical technologies are heterogeneous. The chemical reaction takes place on surfaces, and the reactants are introduced as gases or liquids. Homogeneous catalysts, which are frequently metalloorganic molecules or clusters of molecules, also find wide and important applications in the chemical technologies [24]. Some of the important homogeneously catalyzed processes are listed in Table 7.44. Carbonylation, which involves the addition of CO and H2 to a C olefin to produce a + 1 acid, aldehyde, or alcohol, uses rhodium and cobalt complexes. Cobalt, copper, and palladium ions are used for the oxidation of ethylene to acetaldehyde and to acetic acid. Cobalt(II) acetate is used mostly for alkane oxidation to acids, especially butane. The air oxidation of cyclohexane to cyclohexanone and cyclohexanol is also carried out mostly with cobalt salts. Further oxidation to adipic acid uses copper(II) and vanadium(V) salts as catalysts. The... 

Centi, G., Colinelli, G., and Busca, G. Modification of the surface pathways in alkane oxidation by selective doping of Broensted acid sites of vanadyl pyrophosphate.,/ Phys. Chem. 1990, 94, 6813-6819. 

Spectra of the greasy wool are more complicated than in the previous study. In negative ion mode, different fatty acids, fatty alcohols and alkanes are detected, whereas the positive ion mode shows mainly the presence of cholesterol and the cholesterol oxidation product (Figure 15.6). These ions are attributed to the presence of wool wax on the surface of raw wool. 

---