Selective oxidation of anthracene using inorganic–organic hybrid materials based on molybdovanadophosphoric acids

Abstract

Inorganic–organic hybrid materials were synthesized by immobilization of molybdovanadophosphoric acids onto mesoporous silicas, such as MCM-41, MCM-48, and SBA-15, through an organic linker. 12-Molybdovanadophosphoric acids of the general formula H 3+ x PMo 12− x V x O 40 ( x = 0 – 3 )⋅ nH 2O, such as H 4[PMo 11VO 40]⋅32.5H 2O, H 5[PMo 10V 2O 40]⋅32.5H 2O, and H 6[PMo 9V 3O 40]⋅34H 2O (represented as V1PA, V2PA, and V3PA, respectively) were prepared and immobilized onto mesoporous silica. All the catalyst materials were characterized by elemental analysis, FT-IR, N 2 sorption measurements, SAXS, UV–vis, XPS, MAS-NMR, and TEM for their structural integrity and physicochemical properties. It was found that the structure of the polyoxometalates was retained on immobilization over mesoporous materials. The catalytic activities of these inorganic–organic hybrid materials were tested in the liquid-phase oxidation of anthracene (AN) with 70% aqueous tert-butylhydroperoxide (TBHP) oxidant in benzene. Among the catalysts, V2PA immobilized onto amine-functionalized SBA-15 was highly active in the oxidation of AN (turnover frequency [TOF] = 21 mole AN converted per mole of catalyst per h) with TBHP oxidant in benzene at 80 °C and gave 100% selectivity to 9,10-anthraquinone. Catalyst leaching studies indicated the absence of leaching into reaction medium and the catalyst truly functioned as a heterogeneous catalyst in the oxidation reaction.