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

[Display omitted] ► For the first time, a novel sol–gel auto-combustion method was proposed to prepare the metal and metallic oxide catalyst (Cu/ZnO) without additional reduction. ► Cu particle size was strongly influenced by the initial M/CA molar ratio. ► More carbon residues were left with the increased citric acid amount. ► The methanol selectivity increased with the decreased Cu particle size. 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, H 2 and CH 4 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 Cu 2+ 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 Cu 2+ 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 N 2O chemisorption techniques. The activity of the as-burnt catalysts without reduction was investigated for low-temperature methanol synthesis from syngas containing CO 2 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 C 0.8 with M/CA = 1/0.8, and then decreased. The variation trend was in accordance with that of the copper (Cu 0) surface area. Comparing with the burnt catalyst C 0.8-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 C 0.8 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.