Pore structure modulation of Cu-ZnO-ZrO₂ catalysts for methanol production from CO₂ hydrogenation

Copper-based catalysts have attracted much attention for the hydrogenation of carbon dioxide (CO₂) to synthesize methanol, however, problems including low methanol selectivity, easy sintering of the active components of the catalysts, and poor stability are commonly encountered. In this study, Cu-ZnO-ZrO₂ (CZZ) catalysts with macroporous and nonporous morphology were prepared by the colloidal crystal template method and the conventional co-precipitation method, respectively, and their CO₂ hydrotreating to methanol performance was investigated. In the colloidal crystal template method, polymethyl methacrylate (PMMA) was chosen as the template structure, and the diameter size of the macropores was regulated by controlling the PMMA particle size, so that samples with different pore sizes were prepared. The results show that compared with the bulk samples prepared by the co-precipitation method, the samples prepared by the template method have a permeable macroporous structure, and due to the special three-dimensional ordered structure of the macroporous holes, the ZnO can be uniformly dispersed around the pore wall formed by Cu, which effectively prevents the growth of ZnO particles. Moreover, by changing the pore size of the macropores, the regulation of ZnO particle size can be realized, and smaller ZnO particle size shows more excellent catalytic performance. Among them, excellent catalytic performance and application potential were demonstrated on a (CZZ-55) sample with a pore size of 55 nm, a ZnO particle size of 14.5 nm, a CO₂ conversion of 14.83%, a methanol selectivity of 78.8%, and a methanol yield up to 345.8 g/(kg·h) which is 1.52 times higher than the performance of the nonporous catalyst. The results of in situ diffuse reflectance infrared Fourier transform spectroscopy showed that the methanol synthesis from CO₂ hydrogenation over the CZZ catalyst followed the formate pathway, and the ZnO-ZrO₂ interface was the active site for CO₂ adsorption and activation. Moreover, the three-dimensional ordered macroporous structure provides an ideal “pedestal” for the creation of abundant interfaces and active sites, which contributes to the formation of more dispersed ZnO-ZrO₂ active sites, thus significantly increasing the CO₂ conversion rate. At the ZnO-ZrO₂ interface, CO₂ can be stably adsorbed and activated to form carbonates and bicarbonates and to adsorb formates generated by subsequent hydrogenation reactions. It is worth noting that the pore size of the catalyst has a significant effect on the conversion of the reaction intermediates. Specifically, smaller pore sizes were more favorable for the formation of the key intermediates formate and methoxide, which provided the necessary conditions for effective conversion to methanol. In addition, samples with three-dimensionally ordered macroporous structures play a unique role in the CO₂ hydrogenation to methanol process compared with the non-porous bulk samples. The three-dimensional ordered macroporous structure of the pores provides a “high-speed channel” for the rapid diffusion of the reaction products (water vapor and methanol), which effectively inhibits the toxicity of the by-products of CO₂ hydrogenation, water vapor, to the active components of Cu and ZnO, and improves the stability of the catalyst to a large extent. Under the actual reaction conditions of 4 MPa and 220 ℃, no obvious deactivation phenomenon was observed within 600 h, and the catalyst has a promising application. This work emphasizes the importance of catalyst morphology for the design of catalysts for methanol synthesis from CO₂ hydrogenation and provides new ideas for the controlled synthesis of efficient methanol synthesis catalysts.