Structure of adsorption complexes of water in zeolites of different types studied by infrared spectroscopy and inelastic neutron scattering

Abstract

We combined the results of diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurements (4000–1500 cm⁻¹) and inelastic neutron scattering (INS) (5–100 meV) to study the structure of adsorption complexes of water in zeolites of types FAU and MFI exchanged with alkali metal cations. In the case of faujasites, at relatively high water loadings we observe correlation between the position of the broad band at about 3400–3500 cm⁻¹ arising from OH-stretching of hydrogen bonded water molecules and the basicity of framework oxygen atoms of the zeolite. This result indicates that at these water loadings a significant number of water molecules forms hydrogen bonds to framework oxygen. Temperature-programmed DRIFT studies show that depending on the type of exchanged cation (Li-LSX, NaX) the position of the band at about 3400–3500 cm⁻¹ changes suddenly in the temperature region 400–450 K and then remains stable upon further increase of the temperature. The observed behavior is caused by the formation of smaller, more strongly bonded water-cation clusters from the large web-like structures as a significant number of water molecules is removed. Bands due to hydrogen bond stretchings (O–H⋯O) and up to three librations of water molecules are observed in the INS spectra in the regions 15–44 meV and 44–89 meV respectively. The observed shifts of the band positions for different water loadings are in agreement with the results of DRIFT studies.