Protonation of an H2O dimer by a zeolitic Bronsted acid site

The potential energy surface for the interaction of a water dimer with the Bronsted acid site in a zeolite represented by a Si4AlO4H13 cluster is examined using the B3LYP density functional method. Local energy minima corresponding to both neutral and ion-pair adsorption structures were located, as well as the transition state for proton transfer to the dimer. The neutral complex is more stable than the ion-pair structure by 2.9 kcal/mol.at the highest level of calculation. In all structures both ends of the adsorbed species form hydrogen bonds (H . . .O) to the zeolitic cluster. The zero point energy corrections cause the energy of the ion-pair structure to rise above that of the transition state, indicating that the ion-pair structure is not a true local energy minimum on the potential energy surface. These results reveal that, like the protonated water monomer complex, the protonated water dimer complex is a transition state for proton exchange between adjacent framework oxygen atoms in our cluster model of the zeolite. However, since the energy differences between the three structures investigated here are so small, the protonated water dimer might possibly be a true equilibrium structure for a particular zeolite framework. The calculated vibrational frequencies for the adsorbed complexes are compared with experimental infrared spectra. This comparison suggests that experimental spectra for zeolite-water systems with loadings of two or more water molecules per acid site are a superposition of features from both neutral and ion-pair-water complexes. This interpretation is consistent with the calculated energies of the two complexes.