Confinement effects in the adsorption of simple bases by zeolites

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 d er 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 a cid strength are only obtained when heats of confinement are deduced from a dsorption heats. Heats of protonation for ammonia do not depend much on zeo lite nature for high-silica zeolites (MFI, MOR, FER). The lower value obser ved for FAU confirms the existence of a media effect which appears, however , to be less important than generally reported or presumed. The heats of pr otonation 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 int eractions for these zeolite/base pairs are comparable. No meaningful correl ation is observed between heats of protonation and gas-phase proton affinit ies or aqueous heats of protonation. The heats of protonation of ammonia an d methylamines (NH3-xRx, x=1-3) show a maximum for dimethylamine, for both MOR and MFI, This observation can be explained by a solvation effect arisin g from the action of the zeolite framework as a solid solvent. These result s thus show that the detailed understanding of the interaction of bases wit h acid sites in zeolites necessitates a careful accounting of confinement e ffects arising from van der Waals interactions and the consideration of sol vation effects due to the action of the zeolite framework as a solid solven t. Our conclusions are of general relevance for the quantification of molec ular complexes in micro- and mesoporous solids and possibly for the descrip tion of the action of enzymes. (C) 2000 Elsevier Science B.V. All rights re served.