Structural and Acidic Properties of Ion-Exchanged Mazzite

Abstract

A range of modified MAZ and LTL zeolites have been prepared and ion-exchanged with a series of alkali metal cations. It has been shown that K, Rb and Cs can be readily introduced into the MAZ structure with ~2 cations ion-exchanged per unit cell. In contrast, less than one cation of Li or Na per unit cell has been introduced under similar conditions. Ion-exchanged zeolites have been characterised using Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, X-ray fluorescence and N₂ physisorption in order to gain a better understanding of their structural and acidic properties. The FTIR data indicate considerable heterogeneity of the bridging OH groups in mazzite. The concentration of both Brønsted and Lewis acid sites detected in MAZ using pyridine as a probe molecule is lower than expected from its chemical composition, with the relative accessibility of the bridging OH-groups varying from 16% for H-MAZ to 28% for K-exchanged samples. This is in agreement with the N₂ adsorption-desorption data showing a rather low micropore volume for the ion-exchanged materials and with the NH₃-TPD results implying considerable transport limitations. This work demonstrates that the channel structure of mazzite is partially blocked resulting in a decreased micropore volume and limited access to the acid sites.