Effect of Hydrophobic Surface Modification and Metal Doping on Cu/ZnO Catalysts for Ethanol-Assisted CO2 Hydrogenation to Methanol

Abstract

Carbon dioxide (CO2) hydrogenation to methanol is a route for converting greenhouse gases into value-added chemicals. However, conventional CO2 hydrogenation requires harsh conditions because of kinetic and thermodynamic limitations. Ethanol-assisted CO2 hydrogenation offers a milder route, but side reactions remain challenging. Water formed during the reaction can reduce catalyst performance. Ethyl acetate from ethanol can lower methanol selectivity and complicate separation because it forms an azeotrope with methanol. Therefore, this study investigated co-precipitated Cu/ZnO catalysts modified with 0-6 mM stearic acid, together with 5Mg-Cu/ZnO and 5Ce-Cu/ZnO prepared with and without 4 mM coating. Catalytic testing was performed in a batch reactor using ethanol as the catalytic solvent. Stearic acid increased surface hydrophobicity and helped suppress ZnO coarsening after reaction. However, excessive coating created a trade-off by reducing surface area and pore volume. Cu/ZnO-4mM gave the highest methanol yield of 35.0% and selectivity of 87.0%. It also showed the lowest CO and ethyl acetate selectivity. This suggests that methanol-selective performance requires a balance of hydrophobicity, reducibility, surface area, and acid-base properties. Mg and Ce doping affected catalytic performance differently. In-situ DRIFTS showed that Cu/ZnO-4mM retained ethanol adsorption. But ethoxy species were readily removed during purging, and acetate accumulation was limited. Therefore, hydrophobic modification can limit the ethanol dehydrogenation pathway and improve methanol-selective performance over Cu/ZnO catalysts.