Insights of precursor phase transition of (Cu-Zn-Al)/γ-Al2O3 hybrid catalyst for one step dimethyl ether synthesis from syngas

Abstract

The concerns on environmental pollution and climate change, along with ever-growing energy demands, have driven interest in dimethyl ether (DME) as a clean fuel alternative and a versatile chemical intermediate. Due to the advantages of single-step synthesis of DME, we investigated the influence of precursor phase transition of methanol synthesis catalyst from hydrotalcite to zincian malachite on the hybrid catalyst’s activity for direct syngas to DME process by varying its Cu/Al content. A series of Cu-Zn hybrid catalysts were prepared via a kneading extrusion process by physical mixing of methanol synthesis catalyst (CuO:ZnO:Al2O3) prepared via facile forward precipitation route with CuO/Al2O3 ratios 1.5–6 and the dehydration component boehmite (γ-AlO(OH)). The physicochemical aspects of prepared hybrid catalysts were studied in detail by BET, XRD, N2O chemisorption, H2-TPR, NH3-TPD, SEM, and TEM. The activity studies revealed that the catalyst with a lower CuO/Al2O3 (1.5) ratio derived from the phase pure hydrotalcite precursor showed enhanced catalytic activity with a high CO conversion 60.5% and DME selectivity 71.5%. Whereas the catalyst with a high CuO/Al2O3 (6) ratio showed relatively lower DME selectivity, although it was derived from the phase-rich zincian malachite, which is a preferred active phase for methanol synthesis. The higher yield of DME with hydrotalcite-derived hybrid catalyst was most likely attributed to the well dispersed nanostructured active Cu sites and the higher number of weak and medium strength acidic sites. Moreover, the phase transition of dehydration component boehmite to γ-Al2O3 helps in methanol dehydration with its characteristic Lewis acidity.