Vanadyl heterogeneous oxidation

Vanadyl (V02 +) is an ideal cation for the study of heterogeneous oxidations for several reasons (1) the electron-transfer behavior of VO2 + is in many aspects similar to that of Fe2+ (Rosseinsky, 1972, Wehrli et al., in press), (2) the experimental conditions can be chosen so that vanadyl is completely adsorbed at pH >4 (Fig, 2), (3) the adsorbed V(IV) species have been characterized by ENDOR spectroscopy as inner-sphere surface complexes (=MO)xVO (Motschi and Rudin, 1984). Adsorption experiments are compatible with x = 2. The oxygenation rates of V02+ adsorbed to anatase and <5-Al203 follow the empirical rate law... 

Figure 7. Oxygenation kinetics for 1 atm 02. (a) Oxidation of Fe(II). Open circles represent data by Singer and Stumm (1970), dots are data from Millero et al. (1987). The solid line was calculated with Eq. 26. Small dotted lines represent heterogeneous rate constants for Fe(II) adsorbed to Fe(OH)j (upper line) and a-FeOOH (lower) data from Tamura et al. (1976). (b) Oxygenation of vanadyl in solution and as surface complexes data from Wehrli and Stumm (1988).

Vanadium compounds have been widely explored as catalysts for H O -based hydroxylation of benzene and other arenes [11, 77, 78]. A vanadyl complex grafted on to periodic mesopotous organosilicas, [VO(acac)j]/PMO, was used as recyclable heterogeneous catalyst for benzene hydroxylation, producing phenol with nearly 100% selectivity at 27% benzene conversion but with a low oxidant utilization efficiency [79]. 

Centi, G. and Busca, G. (1989). Surface dynamics of adsorbed species on heterogeneous catalysts Evidence from the oxidation of C4 and C5 alkanes on vanadyl pyrophosphate, J. Am. Chem. Soc., Ill, pp. 46-54. 

The chiral vanadyl salen complex was anchored on mesoporous materials by a covalent grafting method. These heterogenized complex catalysts were evaluated as asymmetric catalysts for the asymmetric oxidation of sulfides to sulfoxides [92]. 

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