Homogeneous catalytic oxidation of styrene and styrene derivatives with hydrogen peroxide in the presence of transition metal-substituted polyoxotungstates

Abstract

The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates with general formula [XW₁₁M(H₂O)O₃₉]^(n−m)−, where X = P, B or Si and M = Mn, Fe or Co, were evaluated as catalysts in the oxidation of styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene, p-chlorostyrene, p-nitrostyrene, and p-methoxystyrene under mild conditions, using aqueous H₂O₂ as an eco-sustainable oxidant. In this study, the influence of the catalysts and of the different styrene substituents on the oxidation reaction profile was evaluated in terms of conversion and selectivity. For all the performed catalytic studies, the main product results from the oxidative cleavage of the vinyl double bond, except in the case of the oxidation of p-methoxystyrene catalysed by BW₁₁Mn, for which p-methoxyphenol is the main product. The catalysts BW₁₁Mn and SiW₁₁Co give rise to 100% conversion for almost all of the substrates, excluding p-methoxystyrene and p-nitrostyrene for both catalysts and α,p-dimethylstyrene only in the case of BW₁₁Mn. The selectivity for CC cleavage products resulting from the oxidative cleavage of the vinyl double bond can be as high as 98%, reaching 98% conversion for p-nitrostyrene when SiW₁₁Co was used as a catalyst. Possible pathways are discussed and the oxidation of a few presumed intermediates was carried out. The systematic study of several substituted styrene derivatives suggests a possible reactivity order for these compounds in the catalytic system considered.