A core-shell structured S-1@ZSM-5 catalyst with thin shell and weak surface acidity for stable methanol aromatization to para-xylene
A core-shell structured S-1@ZSM-5 was prepared by a seed-induced method, and its physicochemical properties and catalytic performance in methanol aromatization were studied based on the growth control of ZSM-5 shell. Results revealed that the ZSM-5 shell can be well coated on the surface of the silicalite-1 (S-1) seed under a low temperature crystallization, which induced the formation of worm-like morphology with a controllable shell thickness of 30 nm. Importantly, the weak acid sites dominated by Lewis acid sites can be constructed based on the control of growth degree. For methanol aromatization, the weak acid sites not only ensured the aromatization reaction, but also inhibited the isomerization and alkylation of para-xylene. The selectivity of aromatics and para-xylene in xylene reached 25.96% and 47.53%, respectively. The thin shell promoted the rapid removal of the coke precursor, which gave the catalyst a relatively long lifetime. Interestingly, on such a shell with weak acid sites, the limited sites can be easily covered by coke deposition along with the reaction progress, and thus the para-xylene selectivity can be gradually increased to 73.81%. Additionally, protective desilication was implemented for core-shell catalysts, and the aromatic selectivity was decreased while the catalytic lifetime and para-xylene selectivity remained unchanged. This reflected the inherent catalytic stability for methanol aromatization due to the thin shell and weak acidity. This work provides a new preparation strategy for ZSM-5 with high para-xylene selectivity and catalytic lifetime based on the direct synthesis of suitable acid sites and the diffusion channels.
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