Zinc-Rich Copper Catalysts Promoted by Gold for Methanol Synthesis

In this study, we gathered further understanding of the function of the components in the Cu-ZnO-Al2O3 catalyst for methanol synthesis from mixed syngas feeds (CO/CO2/H2) to rationally develop systems displaying superior performance. In order to unravel the role of ZnO in the hydrogenation of the preferred methanol source, CO2, and in the (reverse) water–gas shift ((R)WGS) reaction, we tested coprecipitated materials with variable surface zinc content under industrially relevant conditions (5.0 MPa, 503–543 K). We found that a surface enrichment in zinc leads to higher activity and selectivity due to (i) the enhancement of the unique synergistic Cu-ZnO interactions boosting CO2 hydrogenation, (ii) the inhibition of the RWGS reaction which produces the undesired CO, and (iii) the electronic stabilization of the Cu sites against reoxidation by CO2 or H2O. Thus, a catalyst with a surface Zn/(Cu + Zn) ratio of 0.8 displayed superior catalytic properties than a commercial benchmark sample, which featured only half of the ratio. An even more performing catalyst was obtained utilizing oxalates instead of hydroxycarbonates as precursors. The better thermal degradation of the former minimizes the content of residual carbon on the surface of the activated catalyst improving the amount of Cu-ZnO contacts. The retention of the metallic state of copper was greatly favored by the deposition of an electron-withdrawing metal such as gold. The Cu-based activity in mixed syngas and CO2 hydrogenation of the zinc-rich gold-promoted catalyst was ca. 2 and 4 times higher, respectively, than that of the commercial system.