Bimetallic Cu-Ag/SiO2 catalysts with tunable product selectivity and enhanced low-temperature stability in the dimethyl oxalate hydrogenation

Abstract

We developed two types of silver promoted Cu-Ag/SiO2 catalysts by sol-gel coupled with two different impregnation methods, which exhibited distinct catalytic activity and tunable selectivity in the dimethyl oxalate (DMO) hydrogenation. Impregnation method A was introduction of Ag promoter after calcination of Cu/SiO2 while impregnation method B was before its calcination. As a result, the Cu-Agx/SiO2 (A) catalysts possessed inferior catalytic activity but higher methyl glycolate (MG) selectivity, while the Cu-Agx/SiO2 (B) catalysts exhibited superior low-temperature catalytic activity and higher ethylene glycol (EG) selectivity under the same conditions (P = 2.0 MPa, T = 463 K, H2/DMO molar ratio = 50). Experimental results revealed that doping with silver species into Cu/SiO2 catalysts by using impregnation method A would aggravate the agglomeration of Cu particles and decrease Cu dispersion, thereby reducing the catalytic activity. In contrast, the introduction of silver promoter by using impregnation method B was conducive to inhibiting the growth of Cu particles, promoting Cu dispersion, and enhancing the interaction between Cu and Ag species. Moreover, the distribution of Cu species was regulated and thus maintain suitable Cu+/(Cu+ + Cu0) ratio after Ag addition, thereby improving the catalytic activity over the Cu-Ag0.04/SiO2 (B) catalyst. In addition, the monometallic Cu/SiO2 catalyst deactivated rapidly within 100 h, while the average EG selectivity of the optimal Cu-Ag0.04/SiO2 (B) catalyst was no less than 97% even after 150 h stability test at a low temperature of 463 K. Systematic study found that the Ag promoter in Cu-Ag0.04/SiO2 (B) catalyst played an essential role in stabilizing the catalyst structure and reducing the sintering and agglomeration of copper particles, thus greatly improving the stability of the Cu/SiO2 catalyst.