Al_2O_3 Effect on the Catalytic Activity of Cu-ZnO-Al_2O_3-SiO_2 Catalysts for Dimethyl Ether Synthesis from CO_2 Hydrogenation

摘要

The effect of Al_2O_3 on the Cu-ZnO-Al_2O_3-SiO_2 catalysts prepared by a pseudo sol-gel method has been investigated and these catalysts were characterized by XRD, H_2-TPR, XPS, NH_3-TPD and CO_2-TPD techniques. As revealed by XRD and H_2-TPR, the added alumina produces high dispersion of CuO and makes the reduction of CuO difficult. XPS analysis detects a remarkably high Al~(3+) enrichment at the surface of calcined samples, along with a decrease of E_b of Cu 2p_(3/2), which confirms the Cu-Al interaction. Another important role of Al_2O_3 would be to incorporate into the SiO_2 structure to form the acid-base sites for ether formation. The reaction results shows that the addition of Al_2O_3 exhibits a promoting effect on the CO_2 conversion only when its content is below 1.4%, and an optimal DME selectivity is obtained when 4.0%Al_2O_3 is added, indicating a better 'synergistic effect' is present between the methanol forming component and the acidic component in bifunctional catalysts. Possible relationship between the catalytic activity and the Cu-Al interaction as well as the surface acidity is discussed.