Confinement effects in the adsorption of simple bases by zeolites

Abstract

An analysis of the heats of adsorption of ammonia and several simple amines on zeolites MOR and MFI, based on recent literature data, shows that these adsorption heats are determined by both chemical (heat of protonation) and physical (confinement) factors. Confinement effects which arise from van der Waals interactions cannot be ignored as they may represent up to 40% of the adsorption heat. Correct heats of protonation which may be related to acid strength are only obtained when heats of confinement are deduced from adsorption heats. Heats of protonation for ammonia do not depend much on zeolite nature for high-silica zeolites (MFI, MOR, FER). The lower value observed for FAU confirms the existence of a media effect which appears, however, to be less important than generally reported or presumed. The heats of protonation of several simple amines adsorbed on zeolites MFI and MOR fall in a rather narrow range (150±10 kJ mol −1), indicating that acid–base interactions for these zeolite/base pairs are comparable. No meaningful correlation is observed between heats of protonation and gas-phase proton affinities or aqueous heats of protonation. The heats of protonation of ammonia and methylamines (NH 3− x R x , x=1–3) show a maximum for dimethylamine, for both MOR and MFI. This observation can be explained by a solvation effect arising from the action of the zeolite framework as a solid solvent. These results thus show that the detailed understanding of the interaction of bases with acid sites in zeolites necessitates a careful accounting of confinement effects arising from van der Waals interactions and the consideration of solvation effects due to the action of the zeolite framework as a solid solvent. Our conclusions are of general relevance for the quantification of molecular complexes in micro- and mesoporous solids and possibly for the description of the action of enzymes.