Van der Waals interactions on acidic centres localized in zeolites nanocavities: A calorimetric and computer modeling study

Adsorption enthalpies of CO, N₂ and Ar on H-BEA and H-MFI zeolites have been measured calorimetrically at 303 K in order to assess the energetic features of Lewis and Brønsted acidic sites. The interaction of the molecular probes with model clusters mimicking Lewis and Brønsted sites has been simulated at ab initio level, and a good agreement between the experimental and the calculated binding energies (BE) has been found. The combined use of the two different approaches has allowed to discriminate among the different interactions (Lewis, Brønsted and dispersive forces) contributing to the measured heat of adsorption. Confinement effects have been investigated on an all-silica MFI zeolite in order to measure the dispersive forces contribution due to the zeolite cavities. For the acidic zeolites both uptake and enthalpy changes correlate well with proton affinity and polarizability of the molecular probes. Whereas CO and N₂ single out contributions from Lewis and Brønsted acidic sites, Ar is only sensitive to confinement effects.