Synergistic effect of Brønsted/Lewis acid in olefin aromatization during MTO over Zn modified H-SAPO-34 zeolite: A periodic DFT study

H-SAPO-34 modified with extra-framework Zn2+ cations improved the selectivity to ethene and ratio of ethene to propene in the early stage of the methanol-to-olefin (MTO) reaction. In order to reveal the promotion mechanism, density functional theory calculations were performed to study the aromatization of 3-methyl-1,3-pentadiene to m-xylene. On three Zn-modified H-SAPO-34, the methylation and dehydrogenation processes, which are the key reactions of olefin aromatization over undoped H-SAPO-34, were studied in detail. It is found that the methylation reaction is more likely to occur at Brønsted acid sites (BAS), and the spatial proximity of BAS and Lewis acid sites (LAS) in zeolite enhances the acidic strength of BAS, thereby improving the methylation activity. The dehydrogenation reaction occurring at the LAS provided by Zn2+ proceeds via the "carbenium" mechanism, with significantly reduced energy barriers and improved dehydrogenation activity. This dehydrogenation mechanism was further testified by the metadynamics (MetaD) simulations. The synergistic effect of Brønsted acid and Lewis acid enables Zn2+ modified H-SAPO-34 to have high olefin aromatization activity, which in turn regulates the selectivity of lower olefins. The present results well interpret the experimental observation that the ethene selectivity is improved at the initial stage of MTO.