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)O-39](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 N-2 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.