Water Adsorption at Pairs of Proximate Brønsted Acid Sites in Zeolites

We model water adsorption
at pairs of proximate Brønsted acid
sites (BASs) in zeolites H-MFI, H-FAU, and H-CHA. We use machine-learning
potentials to explore the potential energy surface, combined with
quantum mechanical methods for chemically accurate energies of selected
structures. We identify BAS pairs that adsorb water cooperatively,
forming an H-bonded chain that connects the two BASs and provides
additional stabilization. The formation of such a water bridge requires
at least two molecules, making the adsorption of the second water
molecule stronger than the first, e.g., by 20 and 44 kJ mol–1 for an Al9–Al10 and an Al4–Al6 pair, respectively,
in H-MFI, and by 11 kJ mol–1 for H-FAU. The magnitude
of this extra-stabilization depends on the relative alignment of the
BASs. Both Al pairs separated by just one SiO4 tetrahedron
(next-nearest neighbor sites) and pairs across a 10-membered ring
are included. The increase of the heat of adsorption with the water
loading per BAS contrasts with the decrease obtained for isolated
BASs and aligns with observations in some experiments.