Seed-directed synthesis of ZSM-5 zeolite via dry gel conversion and its catalytic performance in n-hexane cracking

Seed type controls the physiochemical properties of template-free DGC ZSM-5. • S1 seeding affords mesoporous nanocrystallite aggregates with moderate acidity. • S2 seeding favors Al occupation at straight and intersecting channels. • S2-derived catalyst achieves 94.1% conversion with >90% stability after 36 h. • S1-derived catalyst exhibits ultralow coking rate (0.056 h −1 ) and coke content (∼2.2 wt%). The template-free dry gel conversion (DGC) synthesis of zeolites typically suffers from poor crystallinity and limited structural tunability. In this study, we demonstrated that these limitations can be effectively overcome through a seed-assisted DGC strategy, employing colloidal S1, self-synthesized S2, and commercial ZSM-5 as seeds to direct ZSM-5 crystallization. Comprehensive characterization revealed that seed type critically influences crystallite size, mesoporosity, framework silica-to-alumina ratio (SAR), aluminum siting, and the distribution of Brønsted and Lewis acid sites. Specifically, S1 seeding afforded nanocrystallite aggregates with abundant mesopores, moderate acidity, and the highest SAR (106). S2 seeding resulted in intermediate crystallite dimensions, preferential Al occupation at straight and intersecting channels, and the highest Brønsted/Lewis ratio (6.1). In contrast, the seed-free system yielded dense, large-block crystallites with a lower SAR (65). In n-hexane cracking at 550 °C, the S2-derived catalyst (S2-DG-Z5) achieved the highest initial conversion (94.1%) and light olefin selectivity, while maintaining >90% conversion after 36 h, whereas the seed-free counterpart (DG-Z5) dropped from 90% to <80%. Meanwhile, the S1-derived catalyst (S1-DG-Z5) exhibited the lowest coking rate (0.056 h −1 ) and coke content (∼2.2 wt%), attributable to its moderate acidity, nanocrystallite morphology, and mesoporosity, which synergistically suppress coking and enhance mass transport. Collectively, these findings establish seed type as a pivotal parameter for tuning Al siting and acidity in template-free DG-synthesized ZSM-5, and highlight this seed-assisted route as an effective strategy for producing high-crystallinity, stable zeolite catalysts for hydrocarbon cracking. Download: Download high-res image (288KB) Download: Download full-size image