Entrapping molecules in zeolites nanocavities: A thermodynamic and ab-initio study

Adsorption enthalpies of Ar, N₂, CO, H₂O, CH ₃CN and NH₃ on H-BEA and H-MFI zeolites and on Silicalite, have been measured calorimetrically at 303K in order to assess the energetic features of dispersive forces interactions (confinement effects), H-bonding interactions with surface silanols and specific interactions with Lewis and Brønsted acidic sites. The adsorption of the molecular probes with model clusters mimicking surface silanols, Lewis and Brønsted sites has been simulated at ab-initio level. The combined use of the two different approaches allowed to discriminate among the different processes contributing to the measured (-Δ_adsH). Whereas CO and N₂ single out contributions from Lewis and Brønsted acidic sites, Ar is only sensitive to confinement effects. For H₂O, CH₃CN and NH₃ the adsorption on Brønsted sites is competitive with the adsorption on Lewis sites. The energy of interaction of H₂O with all considered zeolites is surprisingly higher than expected on the basis of -Δ _adsH vs PA correlation.