FOURIER-TRANSFORM INFRARED STUDY OF THE PROTONATION OF THE ZEOLITIC LATTICE - INFLUENCE OF SILICON - ALUMINUM RATIO AND STRUCTURE

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.