Roles of Structural Promoters for Direct CO2 Hydrogenation to Dimethyl Ether over Ordered Mesoporous Bifunctional Cu/M-Al2O3 (M = Ga or Zn)

The different behaviors of structural promoters such as gallium or zinc oxides on the acidic gamma-Al2O3 surfaces were investigated for a direct CO2 hydrogenation to DME by using highly ordered mesoporous bifunctional Cu/Al2O3 (Cu/m-MAl, M = Ga or Zn) to verify their gradual changes of product distribution as well as catalytic stability with time on stream. The oxidation states of Cu nanoparticles with their extent of aggregations and amounts of acidic sites on the mesoporous gamma-Al2O3 were significantly affected by the phase transformations of the structural promoters, which simultaneously changed the DME production rate and stability on the Cu/m-MAl. The partial formations of the surface spinet-type CuAl2O4 phases, still acting as active sites for methanol dehydration to DME, enhanced the catalytic stability as well as DME productivity, which were found to be steadily increased on the most stable Cu/m-Al and Cu/m-ZnAl with time on stream. However, the thermally stable CuAl2O4 phases on the less stable Cu/m-GaAl were steadily decreased due to the preferential phase transformation of gallium oxide promoter from stable tetrahedral Ga-O sites to octahedral Ga-O sites, which were strongly interacted with acidic Al2O3 surfaces by suppressing the formation of CuAl2O4 phases with a fast reconstruction of Cu species by water formed by methanol dehydration reaction. Among those structural promoters on the Cu/m-MAl, the ZnO promoter on the Cu/m-ZnAl revealed an enhanced catalytic stability by preferentially forming the thermally stable CuAl2O4 phases, which further stabilized the Cu nanoparticles with their less aggregations during CO2 hydrogenation compare to the unmodified mesoporous Cu/m-Al and nonordered mesoporous Cu/Al2O3.