Modulation of supported Ni catalysts with phosphorus for the hydrogenation of diethyl oxalate to ethyl glycolate
13-Ni/ZrO2, 13-Ni3P/ZrO2, 13-Ni/SiO2, and 13-Ni3P/SiO2 catalysts were used in the hydrogenation of diethyl oxalate (DEO) to ethyl glycolate (Egly) to investigate the modulating effect of phosphorus. XRD, TEM and FT-IR results show that phosphorus act as a divider of Ni metallic particle and shows a segregation effect. The presence of P transformed the linkage of surface metals from the original -Ni-Ni-Ni-Ni- to the P-Ni-P-Ni-P state, and FT-IR shows that the former adsorbed CO in a combination of linear and bridging adsorption mode, whereas the latter predominantly in CO linear adsorption. Bridging adsorption is via dual sites and makes the preliminary hydrogenation products less likely to detach from the catalyst surface thus facilitating deep hydrogenation, while linear adsorption facilitates partial hydrogenation by a rapid detachment of intermediate product like Egly. The various loading Ni3P/SiO2 catalysts show incomplete phosphorylation with increasing nickel loading, giving rise to a state of coexistence of Ni and Ni3P crystalline phases. This may lead to a coexistence of surface -Ni-Ni-Ni-Ni- and P-Ni-P-Ni-P and the corresponding assort of adsorption, e.g. bridging/linear ones, making deep hydrogenation occur partially besides stepwise hydrogenation.
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