Synthesis of ZSM-5 and its application in butylene catalytic cracking

The template-free ZSM-5 was prepared by hydrothermal synthesis and then modified by tetramethoxysilane (TMOS). The structure, morphology, and acidity of all samples were studied by various techniques, such as XRD, SEM, 29Si MAS NMR, 27Al MAS NMR, NH3-TPD, BET and UV-vis DRS. Comparing with the zeolites synthesized by traditional method, the template-free ZSM-5 exhibited the analogical acidity, morphology and structure, but obvious difference in acid distribution. Due to the absence of the structure directing agent, the template-free ZSM-5 possessed more acid sites situated at pore channels of catalyst and less acid sites stayed at the intersection of straight and sinusoidal channels. Consequently, the shape selectivity enhanced significantly. After TMOS modification, the non-shape acid sites located at the external surface were covered by a single SiO2 layer. The template-free ZSM-5 achieved the highest total yield of ethylene and propylene, longest working life-span and lowest level of coke deposition among the studied catalysts, ascribing to the suppression of the side reactions.