Competitive coadsorption of CO₂ with H₂O, NH₃, SO₂, NO, NO₂, N₂, O₂, and CH₄ in M-MOF-74 (M = Mg, Co, Ni): The role of hydrogen bonding

The importance of coadsorption for applications of porous materials in gas separation has motivated fundamental studies, which have initially focused on the comparison of the binding energies of different gas molecules in the pores (i.e., energetics) and their overall transport. By examining the competitive coadsorption of several small molecules in M-MOF-74 (M = Mg, Co, Ni) with in situ infrared spectroscopy and ab initio simulations, we find that the binding energy at the most favorable (metal) site is not a sufficient indicator for prediction of molecular adsorption and stability in MOFs. Instead, the occupation of the open metal sites is governed by kinetics, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Specifically, the displacement of CO₂ adsorbed at the metal center by other molecules such as H₂O, NH₃, SO₂, NO, NO₂, N₂, O₂, and CH₄ is mainly observed for H₂O and NH₃, even though SO₂, NO, and NO₂ have higher binding energies (∼70-90 kJ/mol) to metal sites than that of CO₂ (38 to 48 kJ/mol) and slightly higher than that of water (∼60-80 kJ/mol). DFT simulations evaluate the barriers for H₂O → CO₂ and SO₂ → CO₂ exchange to be ∼13 and 20 kJ/mol, respectively, explaining the slow exchange of CO₂ by SO₂, compared to water. Furthermore, the calculations reveal that the kinetic barrier for this exchange is determined by the specifics of the interaction of the second guest molecule (e.g., H₂O or SO₂) with the MOF ligands. Hydrogen bonding of H₂O molecules with the nearby oxygen of the organic linker is found to facilitate the positioning of the H₂O oxygen atom toward the metal center, thus reducing the exchange barrier. In contrast, SO₂ molecules interact with the distant benzene site, away from the metal center, hindering the exchange process. Similar considerations apply to the other molecules, accounting for much easier CO₂ exchange for NH₃ than for NO, NO₂, CH₄, O₂, and N₂ molecules. In this work, critical parameters such as kinetic barrier and exchange pathway are first unveiled and provide insight into the mechanism of competitive coadsorption, underscoring the need of combined studies, using spectroscopic methods and ab initio simulations to uncover the atomistic interactions of small molecules in MOFs that directly influence coadsorption.