Development of SO42--ZrO2 acid catalysts admixed with a CuO-ZnO-ZrO2 catalyst for CO2 hydrogenation to dimethyl ether

A series of sulfated zirconia (SZ) catalysts were prepared by impregnation of high surface area mesoporous ZrO2 with different H2SO4 concentrations. The resulting SZ catalysts were admixed with a CuO-ZnO-ZrO2 (CZZ) catalyst for direct CO2-to-dimethyl ether (DME) hydrogenation. A sulfated zirconia prepared via sulfation method (SZS), a commercial Zr(SO4)(2) and HZSM-5 catalysts were employed for purpose of comparison. TGA analysis revealed that the interaction between sulfated ions and the ZrO2 surface changed with S contents. At low S content, sulfate ions with tridentate coordination was mainly formed on the ZrO2 surface. Increasing S contents led to an increase of 2-fold coordinated configuration of sulfate ions. The plot of S content against sulfuric concentration indicated that a saturated sulfate monolayer was obtained at 1 M H2SO4 concentration. During the 48 h time-on-stream experiment, the CZZ-Zr(SO4)(2) and CZZ-SZS showed a substantial decrease in DME yield ca. 24.24% and 28.54% compared to their initial activities, respectively, while the CZZ-1SZ catalyst exhibited not only higher activity but also much greater durability, experiencing only a 11.8% reduction under identical reaction conditions. The higher activity of CZZ-1SZ catalyst was attributed to a monolayer coverage of sulfate ions over the high surface area ZrO2, providing most of active sites to be accessible by methanol, while the strong interaction between sulfate ions bonded to the ZrO2 surface could lead to the higher stability of the CZZ-1SZ catalyst.