Effects of Lithium Content on the Structural Properties and Catalytic Activities of CuLi/AC Catalysts in the Oxidative Carbonylation of Methanol to Dimethyl Carbonate

The CuLi/AC(active carbon) catalysts with different lithium contents were prepared by microwave heating and tested in the oxidative carbonylation of methanol to dimethyl carbonate(DMC). The bulk and surface properties of the catalysts were characterized by means of X-ray diffraction(XRD), H-2-temperature program reduction(H-2-TPR), X-ray photoelectron spectrum(XPS), scanning electron microscope (SEM), and CO-temperature programe desorption (CO-TPD) techniques. The results show that the addition of moderate content of lithium into the Cu/AC catalyst enhances the auto-reduction of copper species to Cu-0, leading to more well-dispersed Cu nanoparticles with more uniform grain size on the surface of active carbon. The catalytic performance is related to the concentration of Cu-0 in the surface of CuLi/AC catalysts, indicating that Cu-0 species is the main active centers for the synthesis of DMC. The addition of lithium enhances CO weak adsorption centers, which favors the insertion reaction between CO molecules and Cu-OCH, intermediates, and thus effectively improve the catalytic activity of CuLi/AC catalysts. The STY(space time yield) of DMC firstly increases and then decreases with the content of lithium, and reaches the maximum of 540.6 mg.g(-1).h(-1) when the mass fraction of lithium is up to 0.15% in the catalysts, and the corresponding MeOH conversion and DMC selectivity are 4.5% and 81.4%, respectively.