Nitrate-Mediated Alcohol Oxidation on Cadmium Sulfide Photocatalysts

Developing photocatalysts capable of organic oxidations enables the generation of value-added products from biomass feedstocks through visible light irradiation. Through a series of nonaqueous photocatalytic experiments, we have uncovered that CdS nanowires catalyze benzyl alcohol (BnOH) oxidation and 5-hydroxymethylfufural (HMF) oxidation. The rate can be improved by introducing nitrate salts that act as a redox mediator in solution. Specifically, nitrate salts of lithium, magnesium, calcium, and manganese promote the selective photooxidation of BnOH to benzaldehyde on CdS in 70-100% yields at rates up to 13.6 mM h(-1), compared to 8% yield at 3 mM h(-1) in the absence of a nitrate mediator. Kinetic analysis reveals that, in the absence of nitrate salts, the reaction is first-order with respect to BnOH, while in the presence of nitrate, the reaction is half-order in BnOH. This rate law disparity, along with radical trapping and kinetic isotope experiments, suggests that nitrate-mediated alcohol oxidations proceed through a mechanism involving the catalytic generation of a nitrate radical, NO3 center dot. The generation of this radical also enables the selective photooxidation of HMF to 2,5-diformylfuran at a rate of 2.6 mM h(-1) using CdS nanowires.