Cu(II) supported on crosslinked chitosan-cellulose beads as efficient and recyclable catalysts for oxidative self-coupling of amines to imines

Abstract

To address environmental and sustainability goals, catalytic processes must be efficient, economical, and practical. Chitosan-supported metal catalysts face challenges in mechanical strength and chemical stability, which can be improved by crosslinking and blending with cellulose. This study developed Cu(II) supported crosslinked chitosan-cellulose beads as a cost-effective and sustainable catalyst for green synthesis of imines, focusing on mechanical robustness to extend catalyst lifetime. The catalyst beads were prepared by mixing medium molecular weight chitosan and microcrystalline cellulose in HCl solution (2:1 w/w), crosslinking with 0.17 wt.% glyoxal, and depositing copper using 30 mM Cu(OAc)₂ solution. The interactions between Cu(II) ions and the crosslinked chitosan-cellulose matrix were investigated. The Cu(II) species formed a stable square planar geometry, coordinating with oxygen and nitrogen donor atoms in the crosslinked matrix. The resulting structure combined the strength of cellulose with the chemical stability provided by glyoxal crosslinking, outperforming pure chitosan in mechanical strength and stability. The Cu/chitosan-cellulose beads catalyzed the oxidative self-coupling of amines to imines (14 examples), achieving yields of 45–97 % and retaining activity over 13 cycles with simple separation and recycling. Therefore, this Cu-based catalyst, with its robust structure and bead form, is a promising option for sustainable and efficient synthesis of imines.