Selective synthesis of Cu–Cu2O/C and CuO–Cu2O/C catalysts for Pd-free C–C, C–N coupling and oxidation reactions

Abstract

In this study, the preparation of economical Cu-based catalysts for Pd-free coupling and oxidation reactions was demonstrated. For the preparation of the Cu-based catalyst, first, a Cu-BTC MOF was prepared under ambient conditions. The Cu-BTC MOF was then carbonized in N₂ to obtain Cu–Cu₂O-supported carbon materials. Moreover, the Cu–Cu₂O-supported carbon material was subjected to heating in O₂ to obtain the CuO–Cu₂O-supported carbon material. Formation of Cu-BTC-MOF, Cu–Cu₂O, and CuO–Cu₂O was confirmed via powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The amount and types of carbon present in the catalyst were confirmed using thermogravimetric analysis and Raman spectroscopy, respectively. The textural properties of the materials were determined using N₂-sorption measurement. The morphology, porosity, and elemental composition were confirmed via the scanning and transmission electron microscopy images. The Cu–Cu₂O-supported carbon material successfully catalyzed the Sonogashira cross-coupling (C–C cross-coupling reaction), Ullmann amination (C–N coupling reaction), and A³ coupling (decarboxylative C–C and C–N coupling) reactions; moreover, the CuO–Cu₂O-supported carbon material successfully oxidized diphenylmethane in O₂ at 1 atm. The catalysts were found to be stable and recyclable. Heterogeneity of the reaction was confirmed via the CS₂ poisoning and hot-filtration tests. The presence of the optimum amount and the suitable oxidation state of Cu in the stable nanoporous carbon matrix having graphitic conducting nature were responsible for the excellent activity and recyclability. The results demonstrate that the applications of these Cu-based catalysts can be extended to several catalytic reactions that are known to be catalyzed using Pd and other costly metals.