Tuning product selectivity and visible-light-driven activity in oxidative coupling of amines to imines: A case study of BiOI<inf>x</inf>Cl<inf>1−x</inf> photocatalyst

Abstract

BiOCl has shown a promising photocatalytic activity in non-selective oxidation reactions, however its application in selective photocatalytic organic transformations is often limited by the strong oxidizing ability of photogenerated holes along with inefficient visible-light absorption. Herein, we showed that the poor visible-light-harvesting ability and low product selectivity of BiOCl in the selective oxidation of primary amines to corresponding imines can be alleviated by band energy level modification using a solid solution strategy. We combined an efficient visible light absorption performance of BiOI with a strong oxidizing ability of BiOCl to achieve BiOIxCl1−x solid solution catalysts with substantial improvements in imine yield. Among the BiOIxCl1−x catalysts, BiOI0.2Cl0.8 delivers the highest benzylamine conversion of ~84% with a selectivity of ~96% towards the imine, while pure BiOCl shows much lower conversion (~65%) and product selectivity (~81%). Such excellent performance could be attributed to electronic structure modifications induced by iodine atom incorporation into BiOCl structure as supported by UV–vis DRS, Mott-Schottky, and VB-XPS studies. Based on photoelectrochemical studies and material characterizations, band energy diagram of the BiOI0.2Cl0.8 is proposed and compared with that of pristine BiOCl and BiOI. Radical scavenging study, EPR spin trapping result, and Hammett plot suggest that the imine formation mechanism may occur via both 1O2- and O2•–-mediated pathways. This work highlights a rational catalyst design for which the benefits from each individual components are used to maximize photocatalytic performance toward the selective synthesis of value-added organic compounds.