Photocatalytic oxidation of toluene to benzaldehyde over exposed hydroxyl ZnTi-LDH nanosheets with O2/CO2

Abstract

To address the issues of low yield and selectivity of benzaldehyde in the photocatalytic CO₂-toluene reactions, a ZnTi-LDH photocatalyst with exposed hydroxyl groups was developed and a novel co-photocatalytic reaction system involving O₂/CO₂-toluene was established. The structure of ZnTi-LDH catalyst was characterized using XRD, FT-IR, N₂ adsorption-desorption isotherms, XPS and other techniques. The effects of catalyst composition and O₂/CO₂ ratio on the yield and selectivity of benzaldehyde in the O₂/CO₂ co-photocatalytic oxidation of toluene were investigated in a pressurized reactor. The techniques and instruments such as isotope tracing, radical quenching, GC-MS, EPR, and others were employed to elucidate the free radical mechanism underlying the O₂/CO₂ synergistic photocatalytic oxidation of toluene. The results indicate that under solvent-free conditions, with a ZnTi-LDH catalyst composition of 3:1 (ZT-3:1) and an O₂/CO₂ ratio of 2:8, the irradiation by xenon light for 12 h yielded CO and benzaldehyde at rates of 121.37 and 947.89 μmol/(g·h), respectively, with selectivities of 96% and 60%. The total yield was 3.02 times higher than that of the CO₂-toluene reaction alone. Selectivities for CO and benzaldehyde increased by 7% and 11%, respectively. These improvements are attributed primarily to the abundant −OH groups and high specific surface area of ZT-3:1, which promote the activation of CO₂ adsorption on the catalyst, and the synergistic effect of O₂ and CO₂ expands the pathways for free radical reactions and improves the carrier utilization efficiency. This study provides a new approach to enhancing the CO₂ conversion efficiency and co-producing the high-value-added products.