Acidity and accessibility studies on mesoporous ITQ-4 zeolite

Abstract

Desilication of ITQ-4 (IFR code, 1-dimensional 12-MR channel system of 0.62 nm × 0.72 nm) in aqueous NaOH solutions was practiced in order to produce the zeolite in the hierarchical form, i.e. combining the native micropores with a secondary network of intracrystalline mesopores. The parent material (Si/Al = 32) was synthesized using benzylquinuclidinium as the structure-directing agent in the presence of fluoride anions. The samples were characterized by ICP-OES, XRD, TEM, N 2 adsorption, and FTIR spectroscopy. In a representative treatment, the mesopore surface area of the ITQ-4 experienced a sevenfold increase (from 29 to 206 m 2 g −1) upon optimized NaOH post-treatment, whereas the microporosity was reduced by 70% (from 0.20 to 0.06 cm 3 g −1). Chemical analysis and infrared spectroscopy of adsorbed pyridine showed that after alkaline treatment the amount of Brønsted acid sites remains practically constant whereas a significant increase in Lewis acid sites was obtained. Infrared studies with adsorbed 2,4,6-collidine concluded that the accessibility of acid sites in the hierarchical zeolite increased substantially due to an increase in the available active sites at the pore mouths of the hierarchical zeolite.