The use of small and weakly basic probe molecules for the investigation of Brønsted acid sites in zeolites by NMR spectroscopy

Abstract

Publisher Summary This chapter highlights the use of small and weakly basic probe molecules for the investigation of Bronsted acid sites in zeolites by NMR spectroscopy. The catalytic behavior of H-zeolites, with respect to Bronsted-acid catalyzed reactions is determined by the strength of acidity, the concentration, and the accessibility of the bridging hydroxyl groups which are known to act as Bronsted acid sites. Small and weakly basic probe molecules are used for the characterization of Bronsted acid sites in zeolites by variable-temperature 1H and 13C NMR spectroscopy. It could be shown that the induced 1H NMR chemical shift Δδ of free surface hydroxyl groups caused by the adsorption of probe molecules as, e.g., CO and C2C14 reflects the strength of acidity of the hydroxyls. The deprotonation energy ΔEDp of surface hydroxyl groups can be determined from Δδ, if the influence of rapid thermal motions and/or exchange processes is suppressed. Furthermore, the use of the H NMR chemical shift δH..M of surface hydroxyl groups influenced by adsorbed probe molecules M as a sensitive qualitative measure for the strength of acidity is suggested. The geometry of the adsorption complexes formed by CO molecules hydrogen bonded to Bronsted acid sites could be determined by NMR spectroscopy.