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

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.