Synthesis of hierarchical ZSM-5 with ultra-small mesopore structure and its catalytic performance during methanol to hydrocarbons

Introduction of mesopore system into zeolite crystals is always deemed to be an effective strategy to improve diffusion and acidic accessibility. In this work, a hierarchically structured ZSM-5 with an ultra-small intracrystalline mesopore structure was prepared by using lysine as a pore-forming agent via a hydrothermal crystallization procedure. The structured and textured properties of the as-synthesized samples were characterized by XRD, SEM, EDS, TEM, NH3-TPD, FT-IR and N2 adsorption-desorption isothermals. The results showed that a purely-phased hierarchical ZSM-5 zeolite with a size of 2-6 nm mesopore can be obtained under the condition of lysine/Al2O3 = 2-8. The addition of lysine in the precursor not only generated an ultra-small mesopore system, but also had a significant effect on the acidity of the synthesized sample: With the increased amount of added lysine, the acid density and acid strength of the as-prepared zeolite catalyst were gradually weakened. The effects of the introduced hierarchical pore structure and the tailored acid sites on the catalytic performance of the catalysts were investigated during methanol dehydration to hydrocarbons reaction. The introduced hierarchical pores and the weakened strong acids significantly improved the stability of the catalyst. Meanwhile, the reduced acid density strongly inhibited hydrogen transfer of light olefins intermediate and then increased the selectivity toward light olefins on the hierarchical zeolite catalysts.Graphical abstractTo overcome phase separation resulted from brittle combining forces between template and zeolite precursor, lysine, which is easily soluble in water and has a stable structure in the temperature range of ZSM-5 synthesis, served as a mesoporous template, and then, a hierarchical ZSM-5 zeolite was prepared in a hydrothermal method.