Fourier-transform infrared study of the protonation of the zeolitic lattice. Influence of silicon : aluminium ratio and structure

Abstract

An in situ Fourier-transform infrared study is presented for the spectral region from 250 to 1400 cm^–1. Using supported samples the changes for the infrared-active lattice modes after protonation are studied for zeolites A, X, L, ZK-5, mordenite and for zeolites Y with Si : Al ratios from 2.4 to 40.

Protonation of the lattice results in a shift to higher wavenumbers for the asymmetric and symmetric T—O stretching modes (where T = Al or Si). These shifts are attributed to coupling between T—O lattice modes and in-plane bending modes of acidic hydroxyls. Inter-tetrahedral and intra-tetrahedral T—O stretching modes show a different coupling. Coupling between T—O lattice modes and out-of-plane hydroxyl bending modes appears to be much weaker. Protonation further results in changes of the infrared band intensities for the symmetric T—O stretching modes and for several modes in the region below 650 cm^–1.

The changes observed for the asymmetric T—O stretching modes are a function of the proton concentration and do not depend significantly on the lattice structure. Changes after protonation for the symmetric T—O stretching modes and especially for the modes in the region below 650 cm^–1 are most sensitive to variation in zeolite structure.