Remarkably improved photocatalytic selective oxidation of toluene to benzaldehyde with O2 over metal-free delaminated g-C3N4 nanosheets: synergistic effect of enhanced textural properties and charge carrier separation

Abstract

The sustainable production of benzaldehyde by the oxidation of toluene with O₂ is a challenging and exciting research area due to the low toluene conversion over the reported catalysts and the importance of benzaldehyde in chemical industries. The photocatalytic toluene oxidation is facilitated by the generation of reactive oxygen species but is limited by charge carrier recombination. Herein a simple metal-free g-C₃N₄ nanosheet photocatalyst is reported for the selective production of benzaldehyde by toluene oxidation with O₂. The efficiency of g-C₃N₄ was enhanced several folds just by increasing the surface area and incorporating carbon vacancies by a simple thermal treatment at different times. The best photocatalyst was obtained by thermal exfoliation at 500 °C for 3 h in a muffle furnace that exhibited 84.4% toluene conversion and ∼100% benzaldehyde selectivity after 8 h using a 250 W high-pressure Hg lamp with good recyclability. The catalyst exhibited excellent activity in sunlight and produced 78.3% toluene conversion and ∼100% benzaldehyde selectivity after 8 h. Toluene derivatives and other aromatic compounds were converted to their corresponding aldehydes or ketones with good yields and selectivity. The structure–activity relationship studied by catalytic investigation, physicochemical characterization, control reactions, and scavenging studies suggest that the exceptionally high surface area with carbon vacancies in exfoliated g-C₃N₄ provides higher numbers of efficiently separated charge carriers with a longer lifetime, and facilitates the generation of various reactive oxygen species and holes responsible for the selective production of benzaldehyde with high toluene conversion and apparent quantum yield. A simple, reproducible, eco-friendly, and sustainable metal-free photocatalytic C–H activation under mild conditions would be fascinating to catalysis researchers and materials scientists to develop simple metal-free catalysts for activating other bonds with high dissociation energy to produce industrially important synthetic intermediates.