Zeolite-based bifunctional catalysts for the single step synthesis of dimethyl ether from CO-rich synthesis gas

Zeolite-based bifunctional catalysts for the production of dimethyl ether (DME) from CO-rich synthesis gas (H2:CO = 1) were prepared via various preparation methods including co-precipitation–impregnation, impregnation, co-precipitation sedimentation and oxalate co-precipitation. The catalysts comprise a Cu/ZnO/Al2O3 component for methanol formation and zeolite H-MFI 400 as acidic component for methanol dehydration to DME. An admixed catalyst system was used as reference and all catalysts were characterized by SEM-EDX, XRF, N2-physisorption, XRD, H2-TPR, NH3-TPD, N2O-pulse chemisorption and TEM. The catalysts were tested in a continuously operating laboratory plant and the experiments revealed that the synthesis strategy strongly influences catalyst properties and hence catalyst activity. Especially a bifunctional catalyst prepared via oxalate co-precipitation proved to be highly active and could compete with conventional admixed catalyst systems. High CO-conversion and a high DME-selectivity could be reached. Catalyst characterization revealed that high Cu surface areas, small Cu particle sizes and a high number of moderate acidic sites are crucial to achieve maximum catalytic performance.