Methanol aromatization over Zn-modified HZSM-5 catalysts derived from ZIF-8

Abstract

Methanol to aromatics (MTA) is an effective way to produce aromatics by non-petroleum route. The acidity and structure of zeolite catalysts are the key to aromatics yield and stability of catalysts in MTA reaction. Here, Zn-modified HZSM-5 catalysts were prepared by physical mixing method (PM), in-situ hydrothermal synthesis method (SH) and steam-assisted crystallization method (SAC) using ZIF-8 as sacrificial template and Zn source. XRD, SEM, TEM, N2 adsorption-desorption, ICP-OES, NH3-TPD, Py-IR, XPS, H2-TPR, TG characterization methods were used to investigate the physical and chemical properties of the catalysts. The influence of preparation methods of catalysts, the addition amount of ZIF-8, different Zn sources and reaction conditions on the products distribution in MTA reaction were investigated. The results showed that the crystallinity of ZnZ8(SAC)/Z5 catalyst was improved and the hierarchical pore structure was obtained successfully after introduced ZIF-8 by steam-assisted crystallization method. Among them, ZnZ8 mainly exists as ZnOH+ species, which improved the utilization rate of Zn atoms and more L acid sites and moderate acid concentration were formed. The addition amount of ZIF-8 can affect the crystalline size, pore structure and the concentration of ZnOH+ species of ZnZ8(SAC)/Z5. An appropriate ratio of B/L acid concentration was formed when adding an appropriate amount of ZIF-8. Compared with the traditional Z5 catalyst modified with Zn(NO3)2 as Zn source (1% ZnZN(SAC)/Z5), there are smaller particle size, larger specific surface area and mesopore volume as well as suitable acid distribution in 1% ZnZ8(SAC)/Z5 catalyst. The preparation method of catalyst, the addition amount of ZIF-8, different Zn sources and reaction conditions have a significant effect on the selectivity of BTX.