Mesoporogen‐Free Synthesis of High‐Silica Hierarchically Structured ZSM‐5 Zeolites and their Superior Performance for the Methanol‐to‐Propylene Reaction

A series of hierarchically structured ZSM‐5 zeolites (ZSM‐5 HSZs) with a Si/Al ratio of about 200 have been successfully synthesized in the absence of additional mesoporous agents. XRD, N2 sorption isotherms, SEM, TEM and NH3‐TPD results showed that the obtained HSZs materials possess high crystallinity, interconnected micro/mesoporous structures and similar total acid amounts. Meanwhile, in the methanol‐to‐propylene (MTP) reaction, compared to the microporous counterpart, all synthesized ZSM‐5 HSZs demonstrated superior performance benefitting from their hierarchical porous structures. Interestingly, 27Al MAS NMR and Py‐FTIR results confirmed the inter‐transformation of framework and extra‐framework Al species with the extension of materials crystallization duration and consequently the variation of materials surface acidity. As a result, the optimized catalyst achieved the highest initial propylene selectivity of 54.4 % and the longest catalyst lifetime (t90) of 175 h under the weight hourly space velocity (WHSV) of 1.0 g g–1 h–1. Moreover, a new “in situ alkali etching‐hydrothermal restoring“ mechanism has been proposed to elucidate the structural evolution of here reported high‐silica ZSM‐5 HSZs during the synthesis. A facile and eco‐friendly mesoporogen‐free steam‐assisted crystallization strategy has been developed for the synthesis of hierarchically structured ZSM‐5 zeolites with superior MTP catalytic performance, and a new “in situ alkali etching‐hydrothermal restoring“ mechanism is discussed.