Catalytic and kinetic isotope effect studies of CO2 reduction on Cu-Metalated UiO-66 Metal-Organic framework

Abstract

Building chemicals and energy carriers using CO₂ is a cornerstone of the circular economy. This study presents a comprehensive investigation into the CO₂ hydrogenation to different platform chemicals on Cu clusters supported by a UiO-66 metal–organic framework (MOF). A multitude of characterizations (e.g., PXRD, DR-UV–vis, XPS, and electron microscopy) were conducted to ascertain the nature of dominant Cu species present during the reaction. These analyses, complemented by quantum chemical calculations, revealed that the Cu species predominantly existed as a mixture of tiny Cu clusters and isolated Cu single sites. The catalytic results obtained under varying reaction parameters, supported by kinetic-isotope-effect (KIE_H/D) measurements, indicated that methanol is the predominant product, with the highest space–time yield of 642 g_MeOH·kg_Cu^-1·h⁻¹ at 275 °C and 6.9 % CO₂ conversion. Additionally, other by-products were formed, including CO, ethanol, methyl formate, and dimethyl ether. Kinetic isotope effect (KIE_H/D) measurements indicate the existence of different pathways for the products. The larger KIE_H/D for ethanol (1.4) and CO (1.6–1.1) compared to methanol and methyl formate (0.56–0.63 and 0.40–0.51) suggested that the H-bond formation (hydrogenation) steps are not as detrimental for ethanol/CO as they are for methanol/methyl formate formation.