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, N-2 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, Al-27 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 (t(90)) 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.