Effect of Surface Hydroxyl Content of Support on the Activity of Cu/ZSM-5 Catalyst for Low-Temperature Hydrogenation of Dimethyl Oxalate to Ethylene Glycol

Cu/SiO2 catalyst prepared by the ammonia-evaporation (AE) method is the potential preferred catalyst for hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG). Although significant advancements have been obtained in the confirmation and influence factors of active species in the hydrogenation process, the relationship between the catalytic activity and the sensitive factors in the preparation or pretreatment process of the catalyst is still uncertain. In this paper, Cu/ZSM-5 catalysts for DMO hydrogenation to EG were prepared by the AE method using ZSM-5 molecular sieve with high silicon-alumina ratio (1500) as a support. The ZSM-5 support was pretreated by drying at 393 K for different hours and it was found that the surface hydroxyl group of the support had significant influence on the structure and catalytic hydrogenation performance of the prepared Cu/ZSM-5 catalyst. The distribution of surface hydroxyl groups could be significantly changed by pre-drying the carrier, which further resulted in the change of the copper dispersion and surface properties of subsequent copper-supported catalysts. With the decrease of hydroxyl content on the surface of the ZSM-5 support, the prepared reduced Cu/ZSM-5 catalyst possessed smaller Cu0 particles size, higher copper dispersion, higher surface area of Cu0 and Cu+ species, but weakened surface acidity of the catalyst, which resulted in the great improvement of the catalytic activity. The DMO conversion and EG selectivity could reach 100% and 93% even under the low reaction temperature at 448 K over the Cu/ZSM-5-24 catalyst (based on the ZSM-5 support pretreated by drying for 24 h). In addition, the catalytic activity did not show obvious change after 300 h of reaction, probably due to the low temperature reaction and suitable surface properties of the catalyst.