Engineering monomer structure of carbon nitride for the effective and mild photooxidation reaction

Abstract

Photooxidation plays an important role in organic synthesis. However, deep oxidation and degradation could reduce the product selectivity and the final yield, which is a crucial issue in the photooxidation reactions. Polymeric carbon nitride (C3N4) shows great potentials in the photocatalysis, but most improved methods make C3N4 show strong oxidation ability, which limits their applications in the selective oxidation synthesis. In order to overcome the limits and enhance the photooxidation efficiency, terephthalic acid was used to change the monomer structure of carbon nitride (P-CN). The physical and electrochemical properties of P-CN were studied with TEM, SEM, XRD, XPS, BET, NMR, ESR and theoretical calculations. The conduction band (CB) position of P-CN is reduced while the valance band (VB) is almost unchanged, which benefits for enhancing visible light response, and preventing the deep oxidation or degradation. At the same time, the structure defects (unreacted amino in the copolymerization process of g-C3N4) are reduced. As a result of these enhanced properties, the photocatalytic efficiency of P-CN in the photooxidation reaction of 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (1,4-DHP) is improved 17-fold compared with g-C3N4. More importantly, the pure product could be obtained by simple filtration without any purification, and there is no obvious deep oxidation or degradation.