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.