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

Elucidating the interaction between coadsorbed H2O and NH3 in 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 NH3 adsorption in humid environments. Nevertheless, molecular competition between NH3 and 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 NH3 and 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 NH3 is capable of displacing metal-bound H2O and moving it to secondary adsorption sites due to the stronger binding of NH3 compared with H2O. Interestingly, the reverse process of H2O displacing metal-bound NH3 is also possible upon increasing water concentration. Our calculations show that H2O exchanging the preabsorbed NH3 at 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 NH3 are 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 NH3 vibrations by coadsorbed H2O. The mechanistic insight obtained through our study sheds light on molecular coadsorption processes in MOFs and helps to assess NH3 removal efficiency of MOFs containing open-metal sites under realistic conditions, particularly in the presence of humidity.