Water Adsorption and Insertion in MOF‐5

The high surface areas and tunable properties of metal−organic frameworks (MOFs) make them attractive materials for applications in catalysis and the capture, storage, and separation of gases. Nevertheless, the limited stability of some MOFs under humid conditions remains a point of concern. Understanding the atomic-scale mechanisms associated with MOF hydrolysis will aid in the design of new compounds that are stable against water and other reactive species. Toward revealing these mechanisms, the present study employs van der Waals-augmented density functional theory, transition-state finding techniques, and thermodynamic integration to predict the thermodynamics and kinetics of water adsorption/insertion into the prototype compound, MOF-5. Adsorption and insertion energetics were evaluated as a function of water coverage, while accounting for the full periodicity of the MOF-5 crystal structure, that is, without resorting to cluster approximations or structural simplifications. The calculations suggest that the thermodynamics of MOF hydrolysis are coverage-dependent: water insertion into the framework becomes exothermic only after a sufficient number of H2O molecules are coadsorbed in close proximity on a Zn−O cluster. Above this coverage threshold, the adsorbed water clusters facilitate facile water insertion via breaking of Zn−O bonds: the calculated free-energy barrier for insertion is very low, 0.17 eV at 0 K and 0.04 eV at 300 K. Our calculations provide a highly realistic description of the mechanisms underlying the hydrolysis of MOFs under humid working conditions. ■ INTRODUCTION Metal−organic frameworks (MOFs) are crystalline, microporous compounds with potential applications involving adsorption (i.e., gas storage, capture, and separation) and catalysis. MOFs hold the record for their specific surface area, and their structure and composition can be varied extensively, resulting in many thousands of known and hypothetical MOFs with a wide range of properties. Despite this promise, several MOFs are known to exhibit limited stability to humidity, which would restrict their use in applications such as CO2 capture from flue gas or in contexts where exposure to air is likely. For example, Chanut et al. examined CO2 adsorption in the presence of humidity across a wide range of MOFs using thermogravimetry. Most MOFs exhibited at least a 10% reduction in CO2 uptake in the presence of water. Similarly, Zuluaga and co-workers reported on the stability of MOF-74 in humid environments and on how this stability impacts CO2 capture. It was shown that CO2 uptake is reduced because of the presence of hydroxyl groups formed from the dissociation of water. The hydroxyl attaches to the coordinatively unsaturated metal sites, thereby blocking CO2 adsorption on these favorable sites. Perhaps the best-known example of water stability issues in MOFs occurs in the prototype compound MOF-5, which has attracted considerable attention as a gas storage material because of its high storage densities. The degradation of MOF-5 following exposure to humid air has been reported in several studies. For example, Long and co-workers reported on the hydrolysis of MOF-5 by measuring the X-ray diffraction spectrum and the hydrogen uptake isotherm of a sample before and after exposure to air. Schröck and coworkers identified the water loading threshold for the degradation of MOF-5; irreversible decomposition was observed after an uptake of 8 wt % water. Cychosz and Matzger analyzed the structure of several MOFs following exposure to dimethylformamide (2 mL) solutions containing 50 to 2000 μL of water; they concluded that the MOF stability was related to the composition and geometry of the metal cluster of MOFs. Experiments conducted by the present authors observed a sudden increase in water uptake (type V isotherm) in MOF5coinciding with a rapid, irreversible structure change upon exposure to air with a relative humidity of 50% or higher. Below this threshold, water uptake was limited and little structure change was observed for exposure times lasting up to several hours. The faster rates of decomposition observed Received: August 3, 2017 Accepted: August 11, 2017 Published: August 24, 2017 Article http://pubs.acs.org/journal/acsodf © 2017 American Chemical Society 4921 DOI: 10.1021/acsomega.7b01129 ACS Omega 2017, 2, 4921−4928 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.