Highly stable hierarchical ZSM-5 zeolite with intra- and inter-crystalline porous structures

• Hierarchical ZSM-5 nanocrystals with meso- and macropores are synthesized.
• Each ZSM-5 nanocrystal contains intra-crystalline mesopores generated by PEG.
• The aggregation of ZSM-5 nanocrystals produces inter-crystalline macropores.
• Hierarchical zeolites with high crystallinity are achieved in only 12 h at 160 °C.
• Hierarchical ZSM-5 shows higher catalytic activities than conventional ZSM-5.

Highly stable hierarchical ZSM-5 zeolites with intra-crystalline mesopores and inter-crystalline macropores have been synthesized via a steam-assisted conversion in the presence of polyethylene glycol. XRD, SEM, TEM, N2 sorption and mercury intrusion measurement revealed that the irregular intra-crystal mesopores (2–10 nm) in each zeolite nanocrystal were generated by the removal of encapsulated PEG, and the inter-crystalline macropores (50–90 nm) were formed by the aggregation of the well crystallized ZSM-5 nanocrystals with a small size (50–200 nm). The ZSM-5 zeolite nanocrystals were achieved by a dense-gel with high TPA+/SiO2, highly concentrated TPA+ can induce a burst in nucleation rate during the first step of steam-assisted conversion, which makes the dry gel easy to convert into zeolite nanocrystals, PEG arrested in the gel can be easily encapsulated by zeolite crystals during the steam-assisted conversion. Finally, these nanocrystals containing intra-crystalline mesopores aggregate into the bulky materials with an inter-crystalline macroporous network. In addition, the mesopore volume and external surface areas of Hier-ZSM-5 can be easily adjusted by changing the amount of PEG. Catalytic tests showed that Hier-ZSM-5 had a similar activity to Nano-ZSM-5, which must be collected by high-speed centrifugation; while had a higher activity than Con-ZSM-5, especially for the reactions involving large molecules. The high catalytic activity and easy separation from liquid mixture of Hier-ZSM-5 would facilitate its practical applications in chemical industry.

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