Towards the photocatalytic production of cinnamaldehyde with phosphorous-tailored graphitic-like carbon nitride

Photocatalysis has emerged as an environmentally friendly technology to develop selective reactions such as the oxidation of alcohol to aldehydes. Graphitic carbon nitride (CN) is a metal-free polymeric structure easily photoactivated under radiation. This work focuses on the enhancement of the photocatalytic activity of bare CN by doping it with phosphorous (P-CN) under a one-pot synthesis basis. Different doping levels of P (2-12 wt.) have been explored and the samples were fully characterized by XRD, FTIR, N2 physisorption, elemental analysis, XPS, DRS-UV-visible, and photoluminescence. A better activity and enhanced selectivity were observed in the P-CN samples if compared to the undoped CN in the oxidation of cinnamyl alcohol to cinnamaldehyde in aqueous solution. The presence of P was demonstrated to contribute to a better delocalization of photo-generated charges. Moreover, the reactivity and selectivity of the CN and P-CN samples were analyzed in water-acetonitrile mixtures, appreciating a better selectivity in the presence of acetonitrile to the detriment of the conversion of the cinnamyl alcohol. The photocatalytic oxidation mechanism over P-CN in aqueous media has been tentatively proposed based on the influence of the reactive oxidative species generated in the process by chemical scavenger tests. They suggested the contribution of superoxide radicals with more selectivity than the photo-generated holes, the second reactive species of importance in the overall oxidation scheme. The contribution of hydroxyl radicals was discharged since its presence was negligible in the probe tests based on the formation of 2-hydroxy-terephthalic acid.