Vapour phase dehydration of crude methanol to dimethyl ether over Na-modified H-ZSM-5 catalysts

A series of Na-modified H-ZSM-5 catalysts, with Na content varying from 0 to 80 mol%, were prepared by an impregnation method, characterized by TPD of ammonia and FT-IR pyridine methods, and tested for the dehydration of crude methanol (i.e. pure/anhydrous methanol containing 20 mol% of H₂O) in a fixed-bed micro-reactor under normal atmospheric pressure. Though the unmodified H-ZSM-5 catalyst was more active and stable in the presence of water than the commercial γ -Al₂O₃, the Na modified H-ZSM-5 catalysts showed the optimum activity (C_MeOII > 80%) and 100% selectivity for DME in a wide range of temperatures: 230-340 °C. The catalysts were tested for the time-on-stream (TOS) and remained least sensitive towards water for a total reaction period of 65 h. The superior performance of Na-modified H-ZSM-5 catalyst is attributed mainly to the elimination of strong surface acid-sites by partial substitution of Na in H-ZSM-5, resulting in the prevention of coke and/or hydrocarbon formation.