Interface tuning of Cu+/Cu0 by zirconia for dimethyl oxalate hydrogenation to ethylene glycol over Cu/SiO2 catalyst

Abstract

An efficient ZrO2–doped Cu/SiO2 catalyst was fabricated through hydrolysis precipitation method (HP) and used to produce ethylene glycol (EG) through dimethyl oxalate (DMO) hydrogenation. The states for zirconia on copper catalyst and roles in DMO hydrogenation were investigated through various characterization tools, including N2 physical adsorption, XRD, H2–TPR, Methyl glycolate–TPD–MS, XPS, XAES as well. Compared with common ammonia evaporation and co–precipitation methods used in catalyst preparation, this HP method is found to effectively suppress the agglomeration and further size growth of copper nanoparticles by enhancing the interactions between copper and zirconia species. More importantly, uniform distribution of ZrO2 dopant is achieved due to the pseudo-homogeneous reactions in the mixing step of catalyst preparation. A proper amount of zirconium dopant helps achieve the desirable proportion of Cu+/(Cu++Cu0) for surface copper species, especially promotes the production of Cu+ species originated from Cu–ZrO2 species at the interface of copper and zirconia particles. In comparison with Cu+ species formed from copper phyllosilicates reduction, the Cu+ sites derived from Cu–ZrO2 species show higher adsorption ability of MG, an important intermediate species in ethylene glycol production. These adsorbed MG molecules further react with atomic hydrogen shifted from adjacent metallic copper surface, leading to a higher catalytic behavior. For the EG production via DMO hydrogenation, the turnover frequency (TOF) normalized by Cu0 species on CuZr/SiO2 catalyst is 1.8 times than that of traditional Cu/SiO2 counterpart. Due to the enhanced synergy effect between Cu+ and Cu0 active sites, a lower activation energy of ester hydrogenation on this ZrO2–doped Cu/SiO2 catalyst is believed to be responsible for the significant improvement.