Coproduction of methanol and dimethyl ether from biomass-derived syngas on a Cu-ZnO-Al₂O₃/ γ-A₂O₃ hybrid catalyst

The coproduction of methanol (MeOH) and dimethyl ether (DME) from biomass-derived synthesis gas on the coprecipitated Cu - ZnO - Al₂O₃/γ-Al₂O₃ hybrid catalysts has been investigated to overcome the equilibrium conversion of CO on the MeOH only synthetic reaction at the H₂-decificient condition. Two different types of γ-Al₂O₃ were employed as the solid-acid component of hybrid catalyst: one prepared by a precipitation method and the other prepared by a sol-gel method. Several hybrid catalysts were prepared by the coprecipitation method in the slurry of γ-Al₂O₃ with the variation in the weight ratio of MeOH synthesis catalyst (Cu-ZnO-Al₂O₃) to solid-acid catalyst (γ-Al₂O₃) as well as by changing aging time during synthesis. The catalytic activity results revealed that the best yield of MeOH and DME with concomitant decrease in the CO₂ formation was obtained on the hybrid catalyst containing γ-Al₂O₃ prepared by the sol-gel method with an aging time of 6 h. The hybrid catalysts were characterized by using the BET surface area measurements, NH₃ temperature programmed desorption (NH₃-TPD), scanning electron microscopy (SEM), temperature programmed reduction (TPR), and X-ray diffraction (XRD) methods. The observed catalytic properties were related to the surface area of Cu metal, dispersion of Cu - ZnO-Al₂O₃ particles, and reducibility and acidity of hybrid catalysts. The enhanced catalytic activity with low selectivity to CO₂ is attributed to the facile reducibility of well-dispersed copper oxides at low temperatures and also to the benign acid strength of the hybrid catalysts.