Chemically accurate predictions for water adsorption on Brønsted sites of zeolite H-MFI

We investigate the adsorption of water molecules in the zeolite H-MFI at isolated Brønsted acid sites (BAS) for loadings of 1, 2, and 3 H₂O/BAS. We consider two approaches to the O₃Al-O(H)-Si sites: the Brønsted-type approach of H₂O to the acidic proton and the Lewis-type approach to the aluminium atom of the AlO₄ tetrahedron. From the twelve crystallographically inequivalent framework sites for Al, a representative set of six active site positions is chosen. For them, we calculate CCSD(T)-quality adsorption energies at MP2-quality adsorption structures for different approaches, 48 in total. The Brønsted-type approach is favoured for most cases but the Lewis-type approach has similar stability for some framework positions. We predict heats of adsorption per molecule ranging from 60 to 76, 56 to 65, and 56 to 64 kJ mol^-1 for loadings of 1, 2, and 3 H₂O/BAS, respectively. For 1 H₂O/BAS, the experimental result (70 kJ mol^-1) falls into the range of our predictions, whereas for 2 and 3 H₂O/BAS, the measured adsorption heats per molecule (74 and 70 kJ mol^-1, respectively) are larger than our predictions. For 2 H₂O/BAS, the ion-pair structure generated by proton transfer to the water dimer competes with the neutral adsorption complex. The DFT adsorption energies (PBE+D2) deviate significantly from the CCSD(T)-quality reference energies, by up to 25 kJ mol^-1 for 1 H₂O/BAS, 25 kJ mol^-1 per H₂O for 2 H₂O/BAS, and 18 kJ mol^-1 per H₂O for 3 H₂O/BAS. Specifically, PBE+D2 overstabilises the ion-pair structure, i.e. in many cases the PBE+D2 error is much larger for ionic than for neutral adsorption structures.