Suitable location of Lewis acid over ZnOH+/HZSM-5 catalysts effectively enhance dehydrogenation activity and catalyze the aromatization process of C6 olefins in MTA

Abstract

The aromatization of long-chain olefins is a critical step in the methanol to aromatics (MTA) process, which determines the formation of aromatic products. The role of Lewis (L) acid and the key intermediate are needed to be explored in aromatization process. The dehydrogenation is investigated at different L acid location of ZnOH+/HZSM-5 by the density functional theory (DFT) method. It can be concluded that ZnOH+/HZSM-5 exhibits the best activity with a distance of 3.754 Å between Lewis and Brønsted acids. And the dehydrogenation activity of the catalyst becomes increasingly superior with increasing Bader charges (|e|) of Zn on ZnOH+/HZSM-5 at different location of L acid. 2-methyl-1,4-pentadiene most readily generates benzene among the wide range of C6 olefins, suggesting that it is a vital intermediate in MTA. Therefore, C6 olefins prefer to be dehydrogenated to form dienes and then cyclized to form 5MR, followed by ring expansion and dehydrogenation to form benzene, whereas C6 olefins with branched chains are more active as reactants than that with linear chain. These results can provide valuable ideas for the design and prediction of Zn-modified HZSM-5 catalysts to enhance methanol aromatization activity.