Visible-Light-Induced Partial Oxidation of Cyclohexane by Cr/Ti/Si Ternary Mixed Oxides with Molecular Oxygen

Cr/Ti/Si ternary mixed oxides (CrTiSi) were prepared by a hydrolysis of tetraethyl orthosilicate (TEOS), Cr(NO 3 ) 3, and titanium tetraisopropoxide (TTIP) followed by calcination. These oxides were used as catalysts for partial oxidation of cyclohexane with molecular oxygen under irradiation of visible light (λ > 400 nm). The CrTiSi catalysts produce cyclohexanol and cyclohexanone with high selectivity (>91%) and show much higher activity than the CrSi binary oxides. Among them, the catalyst with equimolar amounts of Cr and Ti shows the highest activity. Visible-light irradiation of the CrSi catalyst promotes a reduction of tetrahedrally coordinated Cr oxide species ( T d 6+ ) and produces the excited state ( T d 4+ *), which behaves as an active site for oxidation. In contrast, the CrTiSi catalyst contains tetrahedrally coordinated Cr and Ti oxide species that are connected through the Cr–O–Ti bond. The photoformed T d 4+ * species is strongly stabilized due to delocalization of excited electrons on the Cr–O–Ti species. This suppresses rapid deactivation of T d 4+ * and results in enhanced photocatalytic activity.