Competitive Adsorption of NH₃ and H₂O in Metal-Organic Framework Materials: MOF-74

Elucidating the interaction between coadsorbed H₂O and NH₃in metal-organic frameworks (MOFs) is of paramount importance to uncover mechanistic details of their competitive coadsorption behavior as well as to guide the design of new materials for enhanced NH₃adsorption in humid environments. Nevertheless, molecular competition between NH₃and H₂O within the confined nanopores of MOFs was rarely explored and is poorly understood due to challenges in characterization. Here, we combine in situ infrared spectroscopy with ab initio calculations to unveil the competition of NH₃and H₂O for occupying active adsorption sites in the representative MOF-74 material by analyzing the kinetics and energetics of the molecular exchange process. We find that at a high NH₃/H₂O ratio, the incoming NH₃is capable of displacing metal-bound H₂O and moving it to secondary adsorption sites due to the stronger binding of NH₃compared with H₂O. Interestingly, the reverse process of H₂O displacing metal-bound NH₃is also possible upon increasing water concentration. Our calculations show that H₂O exchanging the preabsorbed NH₃at the metal site is driven not only by a reduced kinetic barrier but also by a favorable energetical state resulting from the formation of water clusters at metal sites and intermolecular H-bonding between the metal-coordinated H₂O and displaced NH₃. Our finding emphasizes that the description of molecular occupation in MOFs at equilibrium cannot simply be established by comparing molecules' binding energies at their strongest binding sites derived by single-component measurements; rather, intermolecular interactions can greatly affect molecular distribution at equilibrium. Furthermore, we show that vibrational modes of adsorbed NH₃are markedly perturbed upon contact with water molecules, accompanied by a large frequency shift (>30 cm^-1) and considerable intensity decrease, which arises from the freezing of NH₃vibrations by coadsorbed H₂O. The mechanistic insight obtained through our study sheds light on molecular coadsorption processes in MOFs and helps to assess NH₃removal efficiency of MOFs containing open-metal sites under realistic conditions, particularly in the presence of humidity.