Computational investigation of small toxic molecule adsorption on water-stable metal-organic frameworks

Efficient capture of toxic gases like NH₃, H₂S, NO₂, and SO₂ is crucial for environmental and human health. Metal-organic frameworks (MOFs) are promising sorbents, but undetermined links between their structure, electronic properties, and adsorption performance limit their rational design. Moreover, their application in humid conditions warrants the design of water-stable MOFs for the removal of target toxic molecules. In this work, we investigate the adsorption mechanisms of selected toxic gases on water-stable MOFs, namely UiO-66, UiO-66-NH₂, UiO-67, MIL-53 (Al, Cr, Fe), and MFM-300 (Al, Cr, Fe, V), using density-functional theory (DFT) with Hubbard U correction. Inclusion of the Hubbard U correction significantly improves the accuracy of the prediction of properties such as bandgaps and binding energies, bringing computational results in closer agreement with experimental data. Adsorption is primarily governed by weak physisorption and hydrogen bonding between the adsorbates and hydroxyl groups of water-stable MOFs, except for NO₂, NH₃ and SO₂ on UiO-66-NH₂, where NH₂ hydrogens are the most reactive binding sites. Functionalization of UiO-66 with amino linkers slightly increases its affinity toward NH₃, NO₂ and SO₂, indicating the role of functional groups in modulating interaction strength and selectivity. Quantum theory of atoms-in-molecules (QTAIM) and non-covalent interaction (NCI) analyses confirm non-covalent hydrogen bonding interactions between water-stable MOFs and target molecules, without direct bonding to metal centers. However, water may compete effectively for adsorption sites, potentially limiting the selectivity of the water-stable MOFs investigated here in humid conditions. This study highlights the critical role of the Hubbard U correction in improving both electronic and adsorption property predictions for transition-metal MOFs. It also underscores the need for targeted linker modifications to enhance weak binding of toxic molecules on water-stable MOFs while preserving their structural stability.