Low-Temperature CO₂ Hydrogenation to Liquid Products via a Heterogeneous Cascade Catalytic System

Research described in this paper targeted a cascade system for the hydrogenation of CO 2 to methanol via formic acid and/or formate intermediates, a reaction sequence that has been accomplished previously using homogeneous catalysts. On the basis of results for the hydrogenation of CO 2, formic acid, and ethyl formate over a series of Cu- and Mo 2 C-based catalysts, we selected a Cu chromite catalyst for CO 2 hydrogenation to the formate and a Cu/Mo 2 C catalyst to convert the formate to methanol. These catalysts worked cooperatively in the presence of ethanol, yielding a methanol turnover frequency of 4.7 × 10 –4 s –1 at 135 °C, 10 bar of CO 2, and 30 bar of H 2 in 1,4-dioxane. The performance for this Cu chromite:Cu/Mo 2 C cascade system surpassed the additive production of the individual catalysts by ∼60%. The results also allowed an investigation of the reaction pathways. The hydrogenation of CO 2 to formic acid appeared to be the rate-limiting step for most of the catalysts. This is not surprising given the thermodynamics for this reaction. Finally, the hydrogenation of CO 2 to dimethyl ether was also demonstrated using a system consisting of the Cu/Mo 2 C catalyst to produce methanol from CO 2 and HZSM-5 to produce dimethyl ether from methanol. The systems described in this paper are, to our knowledge, the first demonstrating cascade CO 2 hydrogenation via heterogeneous catalysts.