Organic-solvent-free oxidation of styrene, phenol and sulfides with H2O2 over eco-friendly niobium and tantalum based heterogeneous catalysts

Abstract

Immobilization of triperoxido derivatives of Nb(V) and Ta(V) on chitosan, an abundant and renewable support material, afforded a pair of stable heterogeneous catalysts, which could be successfully implemented in a variety of organic oxidations under ecologically sustainable organic-solvent-free conditions. The catalysts were extensively characterized by elemental analysis, SEM-EDX, powder XRD, XPS, BET, TG-DTG, FT-IR, Raman and NMR analysis. The supported catalysts showed excellent activity in the non-solvent epoxidation of styrene with H2O2 to provide 96% conversion with nearly 100% selectivity of the desired epoxide and a high TON value of 260 at room temperature. Moreover, phenol was selectively converted into 100% dihydroxybenzene (HQ:CT = 2:1) at ambient temperature, in absence of organic solvent, with a TON value of 80. In addition, both the catalysts were highly active towards the oxidation of a diverse range of sulfides such as aromatic, aliphatic, vinylic, allylic, and alcoholic sulfides to chemoselectively produce the desired sulfoxides in an aqueous medium with a TON value as high as 1900. The heterogeneous nature of the insoluble catalysts was confirmed by a hot filtration test. The catalysts are stable, water tolerant and easily recyclable at least up to five successive cycles of epoxidation and sulfoxidation with undiminished activity and selectivity.