Origin of MnO induced Cu0/Cu+ surface active centers for CO2 containing syngas conversion to DME via tandem catalysis

Abstract

The chemical metamorphosis of a feed comprising carbon oxides (CO + CO₂) in an H₂-lean environment into methanol/DME is a versatile technology to meet sustainable energy demand and partially neutralize the enormous emissions of CO₂ into the atmosphere. However, the process is restricted by product selectivity. Tandem catalysis possessing bifunctional catalytic sites may trigger the activation of CO/CO₂ offering a useful methodology with regulated product selectivity. Herein, a modulating synergy between MnO content and the surface-active copper centers has been established for CO/CO₂ hydrogenation to dimethyl ether (DME) over a series of Cu based bifunctional CZMn_x/γ-Al₂O₃ (x = 0, 5, 10, 20, 30) catalysts derived from a highly substituted malachite precursor phase. The structural changes within the catalytic entity were analyzed as a function of MnO. CZMn₂₀/γ-Al₂O₃ demonstrated an ∼2.7 and an ∼1.8-fold increase in total carbon conversion and DME selectivity, respectively, compared to CZMn₀/γ-Al₂O₃ with optimized reaction parameters. Noticeably, increasing the MnO content beyond 20 mol% in catalyst CZMn₃₀/γ-Al₂O₃ unveiled a detrimental effect on catalytic efficiency owing to its blemished structural properties. The best performing catalyst, CZMn₂₀/γ-Al₂O₃, exhibited an optimum surface Cu : Zn ratio (1.53), the maximum number of (weak + medium) acidic sites (0.368 mmol NH₃ per g_cat), and the highest exposed active metal area (2.72 m² g_Cu⁻¹) and metal dispersion (4.23%) which might be a consequence of improved MnO induced Cu–ZnO synergy. The results also revealed that optimum MnO content precisely tuned the surface Cu⁺/Cu⁰ ratio, probably by stabilizing the active centers for DME formation under the applied reaction conditions.