Evidence of Hydronium Formation in Water–Chabazite Zeolite Using Inelastic Neutron Scattering Experiments and ab Initio Molecular Dynamics Simulations

A combined
study of inelastic neutron scattering and ab initio molecular dynamics simulations has been performed
in order to study the water–acid site interaction in zeolite
chabazite with a ratio Si/Al = 16 that corresponds to 2 protons/uc
with two different water coverages with the number of water molecules
being lower and higher than that of proton sites. These results have
provided a clear picture of the water–acid site interaction,
and it has been demonstrated that there are two regimens of water
adsorption, which depend on the water loading. (i) At low water coverage
(water/acid site ∼0.5), the main interactions between water
and the zeolitic acid sites are established through hydrogen bond
and there is no proton transference to water. (ii) At relatively high
water loading (water/acid site ∼3), the clustering of water
molecules and hydronioum cations formed by the complete transference
of the zeolitic proton to the water molecules has been observed. The
formation of water–hydronium clusters interacting with oxygen
atoms of the zeolite framework provides the stabilization energy needed
for the protonation of water molecules confined in the cavities of
chabazite. These results are the experimental evidence obtained from
INS of proton transfer from the zeolitic acid site and the hydronium
formation and are in agreement with a previous computational study
(Phys. Chem.
Chem. Phys. 2009, 11, 1702−1712) and very recent
solid state NMR spectroscopy studies (J. Am. Chem. Soc. 2019, 141, 3444−3455). The inspection of the low energy bands (translational
and optic modes) and librational bands of the experimental results
allows obtaining information about the H-bond network of the hydronium–water
cluster.