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 HO/BAS. We consider two approaches to the OAl-O(H)-Si sites: the Brønsted-type approach of HO to the acidic proton and the Lewis-type approach to the aluminium atom of the AlO 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 for loadings of 1, 2, and 3 HO/BAS, respectively. For 1 HO/BAS, the experimental result (70 kJ mol) falls into the range of our predictions, whereas for 2 and 3 HO/BAS, the measured adsorption heats per molecule (74 and 70 kJ mol, respectively) are larger than our predictions. For 2 HO/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 for 1 HO/BAS, 25 kJ mol per HO for 2 HO/BAS, and 18 kJ mol per HO for 3 HO/BAS. Specifically, PBE+D2 overstabilises the ion-pair structure, in many cases the PBE+D2 error is much larger for ionic than for neutral adsorption structures.