Selective photocatalytic conversion of guaiacol using g-C3N4 metal free nanosheets photocatalyst to add-value products

Abstract

Valorization of lignin into high valuable chemical is a critical challenge. Its availability is a key factor for the development of viable lignocellulosic processes to replace fossil derived compounds. In this work, new insights on the high photocatalytic conversion of guaiacol (82%) as a lignin model compound was achieved, also, high selectivity to p-benzoquinone (59%), catechol (27%), and pyrogallol (6%) was obtained using metal-free pyrolyzed g-C3N4 under visible light irradiation. To highlight the new insights, experimental parameters were modified to control the reaction mechanism to increase selectivity and photo-conversion. g-C3N4 photocatalyst was synthesized through urea calcination at 550 °C and the photocatalytic performance was assessed in terms of pyrolysis time, where higher time resulted in better photocatalytic activity. This effect was attributed to smaller structures and therefore better quantum confinement of the charges. The oxidation was promoted by OH radicals, which were detected through EPR operando mode and the addition of radical scavengers. A reaction pathway was proposed, in which the ·OH attacks guaiacol through a methoxy group. The photocatalytic reaction can be tuned using external oxidant agents such as O2 and/or H2O2 to promote certain radical formation, enhancing conversion rates and promoting selectivity for a specific product, where yield shifting from p-benzoquinone to pyrogallol was experimentally observed.