Density-functional theory study on hydrogenation of dimethyl oxalate to methyl glycolate over copper catalyst: Effect of copper valence state

The development of high-performance copper (Cu)-based catalysts is critical to achieve the industrial hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG). To understand the effect of Cu valence state on MG formation, a theoretical study was performed over Cu+-perfect, Cu+-defective, Cu0, and Cu0/Cu+ catalysts by density-functional theory calculations. Calculations showed that the rate-limiting step (RLS) on the Cu+-perfect catalyst was H2 dissociation, whereas CH3OOCCHOH hydrogenation becomes the RLS for the Cu+-defective catalyst. For Cu0 and Cu0/Cu+, DMO dissociation was the RLS. Compared with other three catalysts, Cu0/Cu+ bicomponent catalyst needed to overcome the lowest barrier (168.6 kJ/mol), which is suggested to be an optimal catalyst for selective DMO hydrogenation. The remarkable catalyst efficiency was ascribed to the synergistic effect between the Cu0 and Cu+ sites. The calculation results indicated that Cu0 was beneficial to the dissociation of H2 and was primarily responsible for the hydrogenation (CH3OOCCO-MG), whereas Cu+ was beneficial to DMO dissociation. Besides, Cu+ stabilized intermediates. In summary, we have found that the adsorption energy of CH3OOCCO+H can be considered as the catalyst performance descriptor through the calculation results. Because Cu0/Cu+ agreed with the index of moderate adsorption energy, Cu0/Cu+ presented the best catalytic performance.