Hydrophobic Cr–Si mixed oxides as a catalyst for visible light-induced partial oxidation of cyclohexane

Abstract

Photocatalytic oxidation of cyclohexane (CHA) with molecular oxygen has been performed with hydrophobic Cr–Si mixed oxides as a catalyst under visible light irradiation (λ > 400 nm). The Cr–Si catalysts with highly dispersed chromate species were prepared by hydrolysis of tetraethyl orthosilicate (TEOS), organoalkoxysilane (1,2-bis(triethoxysilyl)ethane: BTESE), and Cr(NO₃)₃ followed by calcination. All of the Cr–Si catalysts promote partial oxidation of CHA to cyclohexanol (CHA-ol) and cyclohexanone (CHA-one) with high selectivity (>99%). Among the Cr–Si catalysts prepared with different TEOS and BTESE compositions, the catalyst prepared with 10% BTESE demonstrates the highest catalytic activity. The CHA vapor adsorption and electron spin resonance analysis reveal that the enhanced catalytic activity is due to the increased hydrophobicity of the catalyst surface, which enhances the access of CHA to the excited state chromate species.