Boosting photocatalytic coupling of amines to imines over BiOBr: Synergistic effects derived from hollow microsphere morphology

Abstract

A detailed study on cooperative effects derived from well-defined hollow microsphere structure of bismuth oxybromide (BiOBr) and corresponding visible-light-driven photocatalytic coupling of primary amines to imines was investigated herein. To clearly demonstrate the benefits of the 3D hollow architecture (B-HMS), BiOBr with microflower (B-MF) and microplate (B-MP) morphologies were also synthesized and compared for their photocatalytic performances. B-HMS shows almost two- and three-times higher imine yield than B-MF and B-MP, respectively, with excellent recycle ability. Photoluminescence and electrochemical studies indicate that the hollow microsphere feature offers strong light absorption, high active surface area, and efficient charge transfer, which were responsible for its excellent activity. High oxygen vacancy content in the hollow microsphere catalyst, evidenced by electron paramagnetic resonance (EPR) spectroscopy, could also be accountable for such enhanced photoactivity. Active radical quenching experiments and Hammett plot suggest that the imine could be produced via 1O2- and O2•–-assisted mechanisms possibly through neutral carbon-centered radical intermediate. This work not only provides a clear understanding on physicochemical, optical, and charge transfer properties of the 3D hierarchical hollow superstructure, which synergistically boost its photocatalytic activity, but also encourages a design and further development of efficient hierarchical photocatalysts for the application in the renewable energy-based organic synthesis of fine chemicals.