Light-assisted coupling of phenols with CO2 to 2-hydroxybenzaldehydes catalyzed by a g-C3N4/NH2-MIL-101(Fe) composite

Abstract

The present work described a novel photocatalytic approach for the synthesis of 2-hydroxybenzaldehydes from the coupling of phenols and CO₂ in the presence of a base using a graphitic carbon nitride/NH₂-MIL-101(Fe) composite under mild conditions. The loading of NH₂-MIL-101(Fe) (FEMO) on g-C₃N₄ in the composite was found to be an important parameter, and among the various compositions, FEM-10 prepared by using 10 wt% Fe-MOF showed maximum efficiency. Similarly, the presence of a base was found to be vital and in its absence the reaction did not proceed. Among the various organic and inorganic bases studied, only potassium tert-butoxide remained effective for the desired transformation. Furthermore, the phenol derivatives substituted with electron-donating groups showed higher efficiency than their electron-deficient analogues. Controlled experiments and computational studies based on activation energy calculations suggested the formation of CO from the iron-catalyzed photoreduction of CO₂ that reacted with hydrogen to give formaldehyde as a reactive intermediate for the further reaction with phenol to give 2-hydroxybenzaldehyde in the presence of a base.