Controlled decationization of X zeolite: mesopore generation within zeolite crystallites for bulky molecular adsorption and transformation

Abstract

Controlled decationization (ion exchange with NH₄⁺ and subsequent calcination) of NaX zeolite could produce hierarchically micro-/mesoporous aluminosilicates with systematically variable micro- and mesoporosity. As the Na/Al ratio was decreased from 1 to 0.5, secondary mesoporosity gradually increased up to 0.18 mL g⁻¹ at the cost of initial microporosity while maintaining the total pore volume (>0.41 mL g⁻¹) and BET surface area (>630 m² g⁻¹). On excessive decationization (Na/Al < 0.5), there was no more increase in the mesopore volume, but the microporosity and zeolite crystallinity decreased further. As a result, the fully decationized sample exhibited a solely mesoporous structure but still had a considerable BET surface area (328 m² g⁻¹). TEM and N₂ adsorption–desorption isotherms showed that mesopores were evenly distributed over the entire domain of zeolite crystallites. In contrast, conventional dealumination methods using chemical leaching with HCl and Na₂H₂EDTA showed heterogeneous destruction of zeolite crystallites: the external rim of the crystallites was heavily damaged to form amorphous mesoporous silicates, but the core of the zeolite crystallite remained intact and solely microporous. The present decationization method provides an extremely economic and simple synthesis route to obtain mesoporous and hierarchically micro-/mesoporous aluminosilicates, which are promising as adsorbents and heterogeneous catalysts for large molecular species.