Size-controllable synthesis of ZSM-5 molecular sieves and their catalytic performance in the conversion of methanol to hydrocarbons

ZSM-5 molecular sieves with a uniform particle size from 0.1 to 14 μm were hydrothermally synthesized by using tetrapropylammonium hydroxide (TPAOH) as the template and polyethylene glycol (PEG) as a co-template. The ZSM-5 samples were characterized by XRD, SEM, BET, Py-IR, NH₃-TPD and ICP and their catalytic performance in the conversion of methanol to hydrocarbons were investigated. The results indicated that the crystal size of ZSM-5 molecular sieves can be regulated by adjusting the quantity of PEG, silica source and water content in the precursor gel as well as controlling the aging and crystallizing conditions. The ZSM-5 molecular sieves with smaller crystal sizes exhibit higher conglomeration degree, Si/Al ratio and surface area than those with large crystal sizes, but lower crystallinity, BrØnsted acid density and total acid quantity. When used as the catalysts in methanol conversion to hydrocarbons, the crystal size of ZSM-5 molecular sieves has a significant effect on their catalytic stability and the products distribution. The catalyst with smaller crystal size shows much higher stability and selectivity to the light alkanes of C₁～C₄; an increase in the crystal size can enhance the formation of C₅⁺ and aromatic products but lead to a significant decline in its stability.