Effects of silicon to aluminum ratio and crystal size of zeolite on catalytic properties of Cu-ZnO-Al₂O₃/HZSM-5 for the direct synthesis of dimethyl ether from syngas

A series of HZSM-5 zeolite samples with different n(SiO₂)/n(Al₂O₃) ratios and crystal sizes were hydrothermally synthesized and physically mixed with a methanol synthesis component (CuO-ZnO-Al₂O₃) to form hybrid catalysts. The direct synthesis of dimethyl ether (DME) from syngas was carried out over the prepared hybrid catalysts in a pressurized fixed-bed under continuous flow conditions. The results indicated that the selectivity for CO₂ decreased with the increase of n(SiO₂)/n(Al₂O₃) ratios in HZSM-5 zeolite due to the decrease in the amount and strength of the acid sites, so the selectivity for DME was enhanced greatly. On the other hand, although the activity was not enhanced on the smaller sized HZSM-5 compared to the larger sized one, the selectivity for CO₂ and hydrocarbons was decreased evidently.