Water enhanced photo-oxidation of alcohols on colloidal quantum dots

Abstract

Organic transformations via photocatalysis in water medium, especially for alcohol oxidation, are highly attractive but remain challenging. Herein, we report an efficient and green alcohol oxidation strategy in water medium using metal-cocatalyst-free colloidal CdS QDs as photocatalysts. The production rate of benzaldehyde in water medium reaches up to 24.8 mmol g⁻¹ h⁻¹, which is one order of magnitude higher than that in acetonitrile solvent. Interestingly, we found that the conversion of benzyl alcohol is markedly increased when we add water to pure acetonitrile solvent, implying that water has a positive effect on the oxidation of benzyl alcohol. Detailed experiments reveal that hydroxyl radicals, the dominant reactive oxygen species (ROS), are derived from superoxide radicals which are responsible for accelerating the production of benzaldehyde via a radical pathway. Additionally, the adsorption strength between benzyl alcohol and CdS QDs is further enhanced after the pre-adsorption of water on the CdS QD surface, contributing to more efficient charge transfer. The high selectivity is favored by the superior adsorption of the reactant alcohol and the rapid desorption of the desired product. This photocatalytic reaction is eco-friendly with 100% atom utilization.