Catalytic activity of Cu/ZnO catalysts mediated by MgO promoter in hydrogenation of methyl acetate to ethanol

Abstract

Hydrogenation of methyl acetate is a key step in ethanol synthesis from dimethyl ether carbonylation and Cu-based catalysts are widely studied. We report here that the hydrogenation activity of Cu/ZnO catalysts can be enhanced by the addition of MgO promoter. The evolution of crystal phases during coprecipitation and the physicochemical properties of calcined and reduced catalysts by X-ray diffraction (XRD), thermogravimetric (TG)-mass spectrometry (MS), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), N2O titration, in situ CO-Fourier transform infrared spectroscopy (FTIR) and H2-temperature programmed reduction (H2-TPR) reveal that the promoter effect likely lies in the presence of Mg2+. A proper amount of Mg2+ mediates the precipitation process of Cu and Zn, leading to preferable formation of aurichalcite (CuxZn1−x)5(CO3)2(OH)6 crystal phase and a small amount of basic carbonates such as hydrozincite Zn5(CO3)2(OH)6 and malachite Cu2CO3(OH)2. The presence of aurichalcite strengthens the interaction between Cu and Zn species, and thus enhances the dispersity of Cu0 species and helps generation of Cu+ species on reduced catalysts. Furthermore, the performance of Cu/ZnO catalysts exhibits an optimal dependence on the Mg loading, i.e., 17.5%. However, too much Mg2+ in the precipitation liquid prohibits formation of aurichalcite but enhances formation of basic nitrates, leading to a dramatically reduced hydrogenation activity. These findings may find applications for optimization of other Cu-based catalysts in a wider range of hydrogenation reactions.