Understanding of the dissolution-crystallization fabrication strategy towards macro/microporous ZSM-5 single crystals

In this study, detailed investigations about a novel dissolution-crystallization synthetic route for the fabrication of hierarchically macro/microporous ZSM-5 single crystals were conducted. During the fabrication, pre-prepared mesoporous aluminosilicate spheres (MASS) acted not only as raw materials but also as quasi scaffold templates simultaneously to construct additional macropores. Factors like the amount of alkali sodium hydroxide (NaOH), crystallization temperature and Si/Al ratios of MASS could affect the successful fabrication of such hierarchical zeolites via varying either the dissolution behaviors of precursors or the crystallization of zeolite crystals. Generally, only with suitable amounts of alkali NaOH that matched the Si/Al ratios of precursors and at appropriate crystallization temperature could the balance between the dissolution and crystallization of MASS precursors be achieved and thus the intracrystalline macropores be constructed. Owing to the introduction of rich intracrystalline macropores, the diffusional properties of the hierarchical ZSM-5 crystals were significantly enhanced, which has also been well proved through the toluene adsorption experiment with an intelligent gravimetric analyzer. Thus, with a similar Zn-modification, all the three hierarchical samples with different Si/Al ratios exhibited a much longer lifetime and higher aromatics selectivity in the MTA reactions than the conventional microporous sample.