Preparation of hierarchical mordenite zeolites by sequential steaming-acid leaching-alkaline treatment

• Sequential steaming-acid leaching-alkaline treatment on Al-rich mordenite.
• Intra-crystalline mesopores are introduced by this sequential method.
• Elevated steaming temperature enhances the final mesoporosity.
• An extraction-realumination is observed for Al sites with Si(2Al) configuration.

A three-step method consisting of steaming-acid leaching-alkaline treatment is used to introduce mesoporosity in a commercially available Al-rich mordenite zeolite (Si/Al = 4.8). It has been found that the three-step sequential treatment results in the intra-crystalline mesoporosity with a pore size distribution centered at around 20 nm. Single step or binary combination among steaming, acid leaching (treated in 3.0 M HNO3 solution at 80 °C for 1 h) and alkaline treatment (treated in 0.1 M NaOH aqueous solution at 85 °C for 2 h) hardly induces the mesoporosity with a controlled mesopore size distribution. Among the involved three steps, the impact of steaming temperature on the mesoporosity development is studied. The elevation of steaming temperature in the range of 300–600 °C improves the susceptibility to the successive alkaline treatment and thus enhances the mesoporosity formation. Moreover, transitions of 29Si NMR spectra demonstrate an extraction-realumination of framework Al species with Si(2Al) configuration during the synthesis of hierarchical mordenite zeolite.

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