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

Adsorption enthalpies of Ar, N-2, CO, H2O, CH3CN 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 Bronsted acidic sites. The adsorption of the molecular probes with model clusters mimicking surface silanols, Lewis and Bronsted 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 (-Delta(ads)H). Whereas CO and N-2 single out contributions from Lewis and Bronsted acidic sites, Ar is only sensitive to confinement effects. For H2O, CH3CN and NH3 the adsorption on Bronsted 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 -Delta(ads)H vs PA correlation.