The active site for dehydrogenation and cyclization on Zn2+/HZSM-5 catalyst aiming at long-chain C6 mono-olefins aromatization

[Display omitted] •The Zn2+/HZSM-5 catalyst with Zn2+ at T5-T3 site exhibits excellent dehydrogenation performance.•Brønsted acid site shows higher activity for cyclization than Lewis acid site.•The Lewis acid enhances the strength of Brønsted acid.•The stronger Brønsted acid strength, the cyclization easier occur.•The favorable route for the benzene formation on Zn2+/HZSM-5 with the highest TOF base on the microkinetic analysis. The Zn modified HZSM-5 (Zn2+/HZSM-5) has emerged as effective catalysts for the methanol to aromatics (MTA) reaction, owing to its superior activity and selectivity in producing the desired target products. The intricacies of the internal environment within Zn2+/HZSM-5, have led to uncertainties regarding active centers responsible for dehydrogenation and cyclization reactions. In this study, we investigate the active centers involved in the dehydrogenation and cyclization reactions during the aromatization of long-chain C6 mono-olefins over the Zn2+/HZSM-5 catalyst using density functional theory (DFT) method. Our findings reveal that for dehydrogenation, the Lewis acid site (Zn2+) serves as the active center. Specifically, Zn2+ located at the T5-T3 site exhibits optimal dehydrogenation activity due to its lower energy barrier, more Mulliken charge transfer, and a smaller energy barrier gap between the high occupied molecular orbit and the low occupied molecular orbit. In addition, for cyclization, the active center is identified as the Brønsted acid site. Remarkably, the Zn2+/H-Al-ZSM-5 (Zn2+ at T5-T3 site) catalyst exhibits superior cyclization activity with the strongest Brønsted acid strength. The dehydrogenation and 1,6 ring closure process of n-hexene for benzene formation on Zn2+/HZSM-5 possesses the highest TOF based on the microkinetic analysis. This study offers valuable theoretical insights that can guide the exploration and development of catalysts for the aromatization of long-chain C6 olefins.