Characterisation of Lewis and Brønsted acidic sites in H-MFI and H-BEA zeolites: A thermodynamic and ab initio study

Adsorption enthalpies of N₂, CO, CH₃CN and NH ₃ on H-BEA and H-MFI zeolites have been measured calorimetrically at 303K in order to assess the energetic features of the various interactions occurring within the zeolite nanocavities, namely: (i) specific adsorption on Lewis and Brønsted acidic sites; (ii) H-bonding interactions with hydroxyl nests; (iii) dispersive forces interactions with the walls of the cavities (confinement effects). Confinement effects have been investigated on an all-silica MFI zeolite (silicalite). The interaction of the molecular probes with model clusters mimicking 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 interactions contributing to the measured heat of adsorption (-Δ_adsH). Whereas CO and N₂ single out contributions from Lewis and Brønsted acidic sites, CH₃CN and NH₃ are not preferentially adsorbed on Lewis sites, suggesting that the adsorption on Brønsted sites is competitive with Lewis sites. The zero-coverage heats of adsorption for the different probes on the various systems correlate well with the proton affinity (PA) of the molecular probes.