Dimethyl Ether From Methanol

A number of HZSM-5 catalysts modified with 5wt% Mg, Na, Zr, and Al as well as others modified with 5-60wt% Zn prepared by wet impregnation. These materials were characterized by the NH3-TPD, XRF, and XRD analyses and tested in a slurry reactor to determine their activities in dehydration of methanol solution in kerosene. Reactions were carried out at 230 degrees C and 19 bar for 4h of residence time in the reactor. Results showed that in the first series, the catalyst modified with Zr and in the second series, the one modified with 10wt% Zn led to the highest methanol conversion. It was deduced that elimination of strong acid sites and partial replacement of active cations in the HZSM-5 zeolite lattice enhanced the performance and improved resistance against formation of hydrocarbons and other by-products such as olefins all of which acted as coke precursors. In order to determine the activation energy, the rate equation of Bercic and Levec was utilized and assumptions consistent with reaction conditions considered. Hence, rate constants determined empirically through fitting experimental data to the aforementioned rate equation. Ultimately, the related activation energies were calculated. In comparison with common dehydration catalysts such as alumina, the activation energies of the modified catalyst decreased. Nonetheless, the superior performance of the modified HZSM-5 attributed mainly to the elimination of strong surface acid sites by partial substitution of cations resulted in prevention of coke and undesired hydrocarbon formation.