Promoting CO₂ hydrogenation to methanol by incorporating adsorbents into catalysts: effects of hydrotalcite

CO₂ hydrogenation, in which CO₂ conversion to methanol plays a key role, is of increasing importance in mitigating the climate crisis. However, high reaction pressures are required to conduct methanol synthesis because of poor catalytic performance of conventional catalysts, leading to high energy consumption. In this study, a novel strategy was applied to promote methanol synthesis by adsorption-enhanced CO₂ hydrogenation. Catalysts made from Cu-ZnO-Al₂O₃ mixed with different hydrotalcite contents (named CZA-HT) were prepared by physically mixing a commercial copper-based catalyst for methanol synthesis with hydrotalcite for high temperature CO₂ adsorption. In these catalysts, only the commercial copper-based component contains the active part for CO₂ hydrogenation and its copper surface area is 48.1 m² g⁻¹ with an optimal reaction temperature of 523 K. Hydrotalcite exhibits no catalytic activity, yet the catalytic performance of the CZA-HT catalysts were clearly facilitated by CO₂ adsorption on HT. The sample containing 40 wt% hydrotalcite and 60 wt% CZA showed the highest methanol selectivity of 73.4% and a methanol yield of 4.4% among all samples. The reaction was conducted at a low reaction pressure of 30 bar (much lower than conventional pressure), so that the methanol yield was not high. However, it is observed that the methanol formation rate based on a unit mass of active CZA always increases as the hydrotalcite content in CZA-HT increases, confirming the promotion effects of CO₂ adsorption on HT on catalytic performance. The mechanism of adsorption enhanced catalytic reaction was also analysed and discussed, in which the well mixed finer particles of CZA and HT perform better than CZA alone with a 73.9% higher methanol yield.