Theoretical study on the influence of ZSM-5 zeolite with different structures for methanol to aromatics

The influence of hierarchical structure and SiO2 modification of ZSM-5 on the mechanism of methanol to aromatics was studied. The formation of benzene and toluene through reaction pathways of carbenium ions with high carbon number was discussed over ZSM-5 and modified ZSM-5, respectively. The results demonstrated that the energy barriers for the formation of aromatics over hierarchical ZSM-5 were higher than that over ZSM-5, indicating that aromatics were easier to form in micropores thermodynamically. Moreover, higher activation energy was observed over mesoporous ZSM-5 with SiO2 modification indicating greater difficulty in the reaction. A decrease in the acid strength by desilication then SiO2 modification was benefit to suppressing the production of aromatics. In addition, the reactions related to hydride transfer needed overcome higher barriers and thus were the rate-determining steps for aromatization compared with those in deprotonation and cyclization steps. Meanwhile, the length of C-O bond for alkoxy groups increased obviously from ZSM-5 to modified ZSM-5 zeolite in such step, which was ascribed to the reduction of acidic site strength of modified ZSM-5.