Influence of Ni-Ag/SiO₂ catalyst preparation method on its performance in hydrogenation of dimethyl oxalate to methyl glycolate

Methyl glycolate (MG), as a high value-added organic chemical intermediate, possesses broad prospects for downstream applications. In this study, nickel-silver bimetallic catalysts, denoted as Ni-Ag/SiO₂-p and Ni-Ag/SiO₂-i, were synthesized via homogenous precipitation-impregnation and impregnation-impregnation methods, respectively, for selective hydrogenation of dimethyl oxalate (DMO) to MG. Various characterization techniques, including X-ray diffraction, infrared spectroscopy, transmission electron microscopy, N₂ physisorption, and X-ray photoelectron spectroscopy, were employed to extensively analyze the catalysts' structures. The results unveiled that the Ni-Ag/SiO₂-p catalyst, prepared through the precipitation of the nickel precursor followed by impregnation of the silver precursor, exhibited a laminated nickel silicate structure and a higher specific surface area compared to the Ni-Ag/SiO₂-i catalyst, synthesized via sequential impregnation of both metal precursors over SiO₂. This enhanced surface area facilitated improved metal-support interaction and the reduction of smaller metal particles, thereby Ni-Ag/SiO₂-p demonstrated superior metal dispersion compared to Ni-Ag/SiO₂-i, providing more active sites for adsorption and activation of reactant molecules. Specifically, Ni species with small particle sizes facilitated the adsorption and activation of DMO molecules , while the introduction of Ag not only promoted the adsorption of DMO molecules but also significantly enhanced the adsorption and activation capacity of H₂. This resulted in H₂ predominating in competitive adsorption with DMO molecules, substantially augmenting the hydrogenation activity of DMO on the catalyst. Remarkably, Ni-Ag/SiO₂-p achieved outstanding results with a low Ag loading of 0.48% under operating conditions of 220 ℃, 2.0 MPa, a liquid hourly space velocity of 0.5 h⁻¹, and a hydrogen-to-ester ratio of 50. Specifically, Ni-Ag/SiO₂-p catalyst demonstrated DMO conversion and MG selectivity of 99.1% and 87.6%, respectively. These findings underscore the substantial impact of catalyst preparation method on the structure and catalytic performance of bimetallic catalysts, offering valuable insights for the design and optimization of catalysts for DMO hydrogenation to MG.