Selective conversion of methanol into aromatic hydrocarbons over zinc-exchanged ZSM-5 zeolites

Abstract

The conversion of methanol into hydrocarbons over ZSM-5 zeolites has been studied with a special emphasis on improving the selectivity to aromatic hydrocarbons. The introduction of zinc or gallium cations to ZSM-5 appreciably increased the yield of or selectivity for aromatic hydrocarbons. Thus, under the reaction conditions of 700 K, methanol pressure of 40 kPa and W/F= 9.0 g h mol–1, the yields of aromatic hydrocarbons over H-ZSM-5, Ga-ZSM-5, and Zn-ZSM-5 were 40.3, 48.2 and 67.4%, respectively. Over Zn-ZSM-5 the distribution of aromatic products on a molar basis was as follows; benzene (5.8%), toluene (28.0%), xylenes + ethylbenzene (45.6%) and C9+(20.6%). The reaction conditions were optimized to obtain the highest yield of aromatic hydrocarbons over Zn-ZSM-5. The yield of aromatics increased with temperature up to 700 K, but a further temperature increase led to the decomposition of methanol to carbon oxides. The selectivity for aromatics gave a maximum of 70% at a methanol partial pressure of 22 kPa. The yield of aromatic hydrocarbons increased with the degree of cation-exchange of H-ZSM-5 with Zn-cations, indicating the essential role of zinc species in the enhancement of the selectivity to aromatics. It is concluded that the zinc species has a capability of efficiently converting alkene intermediates to aromatic hydrocarbons.