Direct Synthesis of Dimethyl Ether from Syngas Over Hybrid Catalyst with Hierarchical ZSM-5 as the Methanol Dehydration Catalyst.

Commercial ZSM-5 zeolites were submitted to alkaline treatment in order to prepare hierarchical ZSM-5 zeolites with hierarchical porosity and moderate acidity. Hybrid bifunctional catalyst comprising a CuO-ZnO-Al₂O₃ (CZA) and zeolite was prepared by physical mixing method, used for the direct synthesis of dimethyl ether (DME) from syngas. The correlation between the textural and acid properties of the zeolite and the catalytic performance of hybrid catalyst has been assessed from the combined results of N₂ physisorption, FTIR of adsorbed pyridine, elemental analysis ICP, online-GC and other characterization techniques. The results indicate that the hybrid catalyst with hierarchical ZSM-5 zeolite exhibits higher activity and stability than that with parent ZSM-5 zeolite due to the synergy effect of moderated acidity and enhanced mass transport capability of the hierarchical ZSM-5 sample. Under 260 °C, 2.0 MPa and 3600 h, the conversion of CO and the DME selectivity to DME over hybrid catalyst CZA-ZSM-5(25)-A achieved 65.2% and 67.8%, respectively.