Study on the preparation of Cu/ZnO catalyst by sol–gel auto-combustion method and its application for low-temperature methanol synthesis Original Research Article

A series of the as-burnt and the burnt Cu/ZnO catalysts were prepared by a sol–gel auto-combustion method using metal nitrates with the mole ratio of Cu/Zn = 1/1 (noted as M, M = Cu + Zn) and citric acid (noted as CA). When the xerogels were burnt in the argon atmosphere, H2 and CH4 which were came from the decomposition of the citric acid, were the reducing agents and were used in the redox process for synthesizing metallic Cu from Cu2+ in the chelated compound. The XRD patterns revealed that all the as-burnt catalysts with different M/CA molar ratios were converted into pure Cu and ZnO species. TPR analysis of the as-burnt catalyst illustrated that almost no hydrogen was consumed. It proved that Cu2+ in the chelated compound was absolutely reduced to metallic Cu in the as-burnt catalyst. The effects of M/CA molar ratio on the properties of catalysts were studied by TG-DTA, FT-IR, Raman spectrum, XRD, SEM-EDS, BET, and N2O chemisorption techniques. The activity of the as-burnt catalysts without reduction was investigated for low-temperature methanol synthesis from syngas containing CO2 using ethanol as a promoter at 443 K and 5.0 MPa for 12 h. The total carbon conversion increased with increasing the content of citric acid and reached a maximum for the as-burnt Cu/ZnO catalyst C0.8 with M/CA = 1/0.8, and then decreased. The variation trend was in accordance with that of the copper (Cu0) surface area. Comparing with the burnt catalyst C0.8-air after reduction, the methanol selectivity of the as-burnt Cu/ZnO catalyst was much lower owing to lower hydrogenation activity of the ethyl formate. The as-burnt catalyst C0.8 was also used in continuous low-temperature methanol synthesis at 443 K and 5.0 MPa for 40 h. The total carbon conversion was stable after 15 h and no obvious deactivation during 40 h reaction, but the methanol selectivity was still not high.