An Active Fluorinated Cu/ZnO/MgO Carbon Dioxide-to-Methanol Hydrogenation Catalyst with Long-Term Stability

The oxidative fluorination of a ternary CZMg (Cu/ZnO/MgO) methanol catalyst resulted in a 5%-10% catalyst improvement within the first 3 to 4 days on a CO2/3 H2 stream reaching a stable and improved performance over 14 days on stream with respect to methanol productivity (at 40 bar, 250 °C, GHSV 19,800 NL kgcat-1 h-1). By contrast the powerful commercial (but more expensive) CZZ (Cu/ZnO/ZrO2) and the industrially used CZA (Cu/ZnO/Al2O3) system optimized for CO/CO2/H2 streams lost 30% (CZA) / 12% (CZZ) of their initial methanol productivity and were surpassed in productivity by a fluorinated CZMg system within a few hours (CZZ) or after a few days on stream (CZA). This (fluorinated) CZMg catalyst system was characterized using methods including XPS, XAS, in situ pXRD, in situ EPR, and HRTEM. Hence, oxidative fluorination of the pristine CZMg system reduced the apparent activation energy for CO2 hydrogenation EA,app from 52 to 43 kJ mol−1 (CZMg versus CZMg_F1250), removed the volcano shape of the methanol production under integral conversion in a stoichiometric (1 + x)H2 / (COx)-variation stream (x = 1…2) and led to stable performance even with a CO2-rich or pure CO2-stream with stoichiometric amounts of H2 present (at 40 bar, 250 °C, GHSV 19,800 NL kgcat-1 h-1). This long-term stability is most likely attributed to the formation of mixed oxo fluorides MgO1-xF2x during oxidative fluorination. Magnesium and fluoride are presumably incorporated into the ZnO1-x overgrowths of the Cu nanoparticles, stabilize them against sintering and apparently prevent the catalyst from deactivation by water, thus acting as a structural support.