Effect of Enhanced Accessibility of Acid Sites in Micromesoporous Mordenite Zeolites on Hydroisomerization of n-Hexane

Abstract

This paper describes a study of the nature and the accessibility of the acid sites in micromesoporous mordenite zeolites obtained by desilication and dealumination and analysis of their activity and selectivity in the hydroisomerization of n-hexane. Alkaline–acid, acid–alkaline–acid, and fluorination–alkaline–acid postsynthesis treatments were employed for the preparation of micromesoporous mordenites. The FTIR spectra of adsorbed d3-acetonitrille, 27Al MAS NMR, HR-TEM, and N2 adsorption were used for quantitative analysis of the Bronsted and Lewis sites, the coordination of Al atoms, and the textural properties. The alkaline treatment causes desilication, preferably occurring along the crystal defects and resulting in the formation of a secondary mesoporous structure characterized by 5–20 nm cavities and the formation of extraframework (AlEx) species and terminal Si–OH groups. The AlEx species formed by hydrolysis of perturbed or dislodged framework Al easily restrict part of the pseudomonodimensional ch...