Sodium thiosulfate modified graphitic carbon nitride for enhancing the photocatalytic production of aldehydes

Abstract

The oxidation of alcohols to aldehydes is one of the most relevant reactions in organic chemistry. The currently implemented methods based on expensive noble metallic catalysts, toxic solvents, and high temperature and pressure conditions have released the seek for softer and cheaper alternatives such as photocatalysis. In this sense, graphitic carbon nitride has been modified with sodium thiosulfate as a source of S and Na⁺ incorporation in the structure, aimed at enhancing the photocatalytic performance on the oxidation of alcohols to aldehydes, i.e. cinnamaldehyde, benzaldehyde, and vanillin in aqueous solution. Three g-C₃N₄ samples synthesized from different precursors, i.e. melamine, thiourea, and urea, were treated with sodium thiosulfate. Urea led to the g-C₃N₄ with the highest mesoporosity (surface area, 69 m² g⁻¹) and photocatalytic activity. The modification with 5 % (wt.) of Na₂S₂O₃ enhanced the pseudo-first order rate constant of cinnamyl alcohol oxidation from 0.265 h⁻¹ (bare sample) to 0.792 h⁻¹ (Na₂S₂O₃-modified). The characterization of the material suggests a better charge separation of the photogenerated charges after S and Na⁺ incorporation in the structure, minimizing the recombination rate of photogenerated charges. The optimum photocatalyst, tested in aqueous solution, was most selective in the production of benzaldehyde (selectivity, >100 %) > cinnamaldehyde (>23 %) > vanillin (∼5 %). The selectivity was considerably boosted under acetonitrile as the solvent medium, raising in the case of cinnamaldehyde the 23 % recorded in water to 51 % in pure acetonitrile. The degradation mechanism suggests a strong influence of the photogenerated holes and the superoxide radical, the latter being more selective in the oxidation of the alcohol.