Heterogenization of a macrocyclic cobalt complex for photocatalytic CO2 reduction

Abstract

Heterogenization could potentially improve the stability and recyclability of molecular catalysts. In this study, a Co(III) cyclam complex, where cyclam is 1,4,8,11-tetraazacyclotetradecane, was grafted on a mesoporous silica surface via two different covalent linkages. The resulting heterogenized catalysts were characterized with a variety of techniques and tested in photocatalytic CO2 reduction in the presence of p-terphenyl as a molecular photosensitizer. Linking strategies were shown to be important for the preparation of highly active surface Co(III) sites. In particular, derivatizing one of the amines on the cyclam ligand resulted in a detrimental effect on the activity of the molecular Co(III) catalyst. Improved activity was achieved by reacting the Co(III) catalyst with surface silanol groups, forming Si–O–Co linkages. A catalyst loading effect was observed, where the best catalytic activity was achieved when the surface Co(III) sites likely formed a monolayer in silica mesopores. Our results also indicate that the cyclam ligand was essential for the observed activity using the heterogenized Co(III) catalysts.