Solid state NMR — a powerful tool for the investigation of surface hydroxyl groups in zeolites and their interactions with adsorbed probe molecules

Abstract

Recent developments in the NMR spectroscopic investigation of surface hydroxyl groups in zeolites are reviewed in the present paper. The composition and porous structure of zeolites are briefly described. The structure and basic properties of surface hydroxyl groups (especially of bridging hydroxyl groups) in zeolites are explained. The influence of the magic angle spinning (MAS) technique on the 1H NMR spectrum of activated zeolites is described in detail. Different NMR spectroscopic methods which allow the measurement of the HAl distance r HAl between the protons and the 27Al nuclei in bridging hydroxyl groups are compared. The limitations of spectral resolution of the 1H MAS NMR spectra are discussed quantitatively on the basis of extended quantum chemical calculations. It is shown that the 1H NMR chemical shift δ H allows the measurement of the deprotonation energy of free surface hydroxyl groups in zeolites. The influence of electrostatic interactions upon δ H is discussed and a correlation between the 1H NMR chemical shift and the wavenumber of the stretching vibration of surface hydroxyl groups is stated. Further, 1H MAS NMR and IR spectroscopy are compared especially with respect to concentration measurements. Applications of low-temperature 1H MAS NMR spectroscopy to the investigation of the interaction between surface hydroxyl groups and adsorbed probe molecules are demonstrated. Finally, the present paper reviews recent NMR spectroscopic studies of probe molecules under the influence of bridging hydroxyl groups.