Mono-substituted silicotungstates as active catalysts for sustainable oxidations: homo- and heterogeneous performance

Abstract

A series of tetrabutylammonium (TBA) salts of the transition metal mono-substituted silicotungstates [SiW₁₁M(H₂O)O₃₉]ⁿ⁻, M = Co^II, Fe^III, Mn^III, (SiW₁₁M) were explored as homogeneous catalysts for the oxidation of geraniol and styrene with H₂O₂. The most active homogeneous catalysts (SiW₁₁Co and SiW₁₁Fe) were immobilized onto an amine-functionalized SBA-15 (aptesSBA-15) and the resulting composites were characterized using several techniques (FT-IR, FT-Raman, UV-Vis/DRS, elemental analysis, powder XRD, SEM and N₂ adsorption–desorption isotherms). The catalytic performance of the new composites SiW₁₁Co@aptesSBA-15 and SiW₁₁Fe@aptesSBA-15 was investigated under similar experimental conditions to those used for homogeneous counterparts. 2,3-Epoxygeraniol and benzaldehyde were the main products obtained from geraniol and styrene oxidation, respectively, for all the catalysts. SiW₁₁Co and SiW₁₁Co@aptesSBA-15 showed to be the most active catalysts for the oxidation of geraniol and styrene. The recyclability of the composite SiW₁₁Co@aptesSBA-15 was investigated for three reaction cycles. The stability of the composites was confirmed using several techniques after catalytic cycles.