Issue 36, 2024

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

Abstract

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
18 Jul 2024
Accepted
27 Aug 2024
First published
04 Sep 2024
This article is Open Access
Creative Commons BY license

Phys. Chem. Chem. Phys., 2024,26, 23588-23599

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

H. Windeck, F. Berger and J. Sauer, Phys. Chem. Chem. Phys., 2024, 26, 23588 DOI: 10.1039/D4CP02851A

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