The effect of the amount of ammonia on the Cu~0/Cu~+ ratio of Cu/SiO_2 catalyst for the hydrogenation of dimethyl oxalate to ethylene glycol

The effect of the amount of precipitant ammonia on the Cu⁰/Cu⁺ ratio of Cu/SiO₂ prepared by the deposition-precipitation method is investigated. Species at different preparation stages, resulted from the amount of ammonia used, are identified by the XRD and FTIR techniques. Chrysocolla together with either copper nitrate hydroxide or copper hydroxide coexist in the uncalcined catalysts. Upon calcination, the latter two species are converted to CuO particles while chrysocolla remains. Following reduction, CuO is transformed to metallic Cu and chrysocolla is converted to Cu₂O. The value of Cu⁰/Cu⁺ ratio can be evaluated using the peak areas in their TPR profiles. Hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG) shows that the selectivity of EG depends on the Cu⁰/Cu⁺ ratio. Catalyst prepared with the addition of ammonia solution at n(NH₃)/n(Cu²⁺)=0.9 for precipitation-deposition gains a more suitable Cu⁰/Cu⁺ ratio upon reduction and thus has a higher selectivity for EG.