Controlled structural collapse of high-Al zeolite via decationization and its applications in adsorption

Abstract

In zeolite chemistry, it was known that the decationization ($NH4^+$ ion-exchange followed by calcination) of highly aluminous zeolites (Si/Al<1.3) results in a loss of zeolite crystallinity. Although such structural transformation has been known for several decades, the pore structures of the amorphous phases have not been investigated. The reason might be attributed to the lack of scientific interests in the ill-defined amorphous materials and to the hasty conclusion that the atomically ‘collapsed’ phase would be nonporous. In this thesis, two phenomena, generation of mesopores and reduction of micropore size, was observed via decationization of two different zeolites (FAU and LTA structures). In case of NaX zeolite (FAU structure, Si/Al = 1.2), the controlled decationization of NaX zeolite can produce hierarchically micro-/mesoporous zeolites and solely mesoporous aluminosilicates having an amorphous framework. The hierarchically porous NaX zeolite has significantly higher adsorption capacities and catalytic activities for bulky molecular species compared with conventional NaX zeolite due to the presence of secondary mesoporosity that allows the facile diffusion of bulky molecules. The controlled collapse or atomic disordering of NaA zeolite (LTA structure, Si/Al = 1.0) with decationization can systematically narrow the effective pore size below 0.4 nm and tune the molecular sieving effect. As the zeolite is gradually disordered, the adsorption amounts for all gas molecules decrease

however, larger molecules show a much faster decrease than that of the smaller ones. Consequently, the adsorption selectivities could be remarkably enhanced for various gas pairs. Moreover, the decationized NaA zeolites were applied for mixed matrix membrane (MMM) to improve their $CO_2$/$CH_4$ permselectivity by enhancing the diffusion selectivity. This simple and controllable post-modification method would be industrially suitable for generation of mesopores and control of molecular sieving effect, which are promising as adsorbents and heterogeneous catalysts.