Structure and thermodynamics of water adsorption in NU- 1500-Cr

The mechanism of water adsorption in NU-1500-Cr, a high-performance atmospheric water harvest- ing metal-organic framework (MOF), is investigated using a combination of molecular dynamics simulations and infrared spectroscopy. Calculations of thermodynamic and dynamical properties of water as a function of relative humidity allow for following the adsorption process from the initial hydration stage to complete filling of the MOF pores. Initial hydration begins at the water molecules that saturate the open Cr3+ sites of the framework, which is then followed by the formation of water chains that extend along the channels connecting the hexagonal pores of the framework. Water present in these channels gradually coalesces and fills the hexagonal pores sequentially after the channels are completely hydrated. The development of hydrogen-bond networks inside the MOF pores as a function of relative humidity is characterized at the molecular level using experimental and computational infrared spectroscopy. A detailed analysis of the OH-stretch vibrational band indicates that the low- frequency tail, which is common in the experimental infrared spectra of water in MOFs, stems from strongly polarized hydrogen-bonded water molecules, suggesting the presence of some structural dis- order in the experimental samples. Strategies for designing efficient water harvesting MOFs are also proposed based on the mechanism of water adsorption in NU-1500-Cr.