Synthesis of Cu–Mg/ZnO catalysts and catalysis in dimethyl oxalate hydrogenation to ethylene glycol: enhanced catalytic behavior in the presence of a Mg2+ dopant

Abstract

Mg²⁺ doped nanoscale Cu–Mg/ZnO catalysts prepared by the co-precipitation method have been systematically characterized focusing on the amount of Mg²⁺ ions incorporated. The amount of Mg²⁺ dopant was demonstrated to have profound influence on the evolution of textural and structural properties, the functionality of active phases and the catalytic behavior of the as-synthesized ternary catalysts (Cu, ZnO and Mg²⁺). The Cu–1Mg/ZnO catalyst with 1 wt% MgO loading was found to be helpful for enhanced Cu dispersion and an increased amount of active surface Cu⁰ sites, which promoted catalytic activity in dimethyl oxalate (DMO) hydrogenation to ethylene glycol (EG) effectively. Further increasing the Mg²⁺ concentration results in the aggregation of surface metal Cu nano-particles (NPs), and thus causes the reduction in the number of surface active Cu⁰ sites and the activity of the Cu/ZnO based catalyst. However, the high density of the surface Cu⁺ sites and O²⁻ centers generated in the Cu–4Mg/ZnO catalyst with 4.0 wt% MgO loading facilitates superb hydrogenation activity. Under the optimized reaction conditions, the Cu–4Mg/ZnO catalyst shows 100% DMO conversion and an EG yield of 95% for longer than 300 h. During the DMO hydrogenation process, Cu⁰ sites are assumed to afford atomic hydrogen by dissociative adsorption and spillover. The reaction rate greatly depends on the dissociative adsorption of DMO molecules by the surface Cu⁺ and oxygen vacancies, originating from tight contact between the Cu NP ZnO matrix and Mg²⁺ dopant. Additionally, the strengthened metal-support interaction (MSI) originating from the enhanced chemical interaction between the Mg²⁺ modified ZnO substrate and the Cu NPs leads to excellent stability.