Visible light-induced partial oxidation of cyclohexane on hydrophobically modified chromium-containing mesoporous silica with molecular oxygen

Photocatalytic oxidation of neat cyclohexane (CHA) with molecular oxygen was carried out using hydrophobically modified Cr-containing mesoporous silica under visible light. Two catalysts (Cr/MOS and Cr/MOS-cal) containing highly dispersed chromate species were synthesized by Cr impregnation onto mesoporous organosilica (MOS) prepared with tetraethyl orthosilicate (TEOS) and 1,2-bis(triethoxysilyl)ethane (BTESE). Cr/MOS was synthesized by Cr impregnation onto MOS followed by calcination, and Cr/MOS-cal was synthesized by calcination of MOS, Cr impregnation, and calcination. Both catalysts promoted partial CHA oxidation with high selectivity (>86%). Cr/MOS-cal synthesized with 10% BTESE showed the highest catalytic activity, due mainly to the higher surface hydrophobicity, which accelerated access of CHA to the chromate species. ESR analysis revealed that the tetrahedrally coordinated chromate species (Td6+) on Cr/MOS-cal were photoexcited and formed highly reduced Td4+∗ species with high catalytic activity. This also contributed to the high catalytic activity of Cr/MOS-cal.