Molecular-level insight into water adsorption and projected atmospheric water harvesting performance in a hydrolytically stable MOF

Abstract

Sorption-based atmospheric water harvesting (AWH) promises a solution to the global challenge of water scarcity and, despite the increasing number of promising AWH materials, there remains a need for insight into pore-filling mechanisms and the hydrolytic stability of such desiccants. Here, we report two bnn-topology rod building block (RBB) MOFs, M2F2(tzba)(bpy)2, tzba = 4-(1H-tetrazol-5-yl)benzoate, M = Co or Ni. The MOFs, bnn-1-Co and previously reported bnn-1-Ni, respectively, are built from an RBB comprising three bridging moieties, fluoride, carboxylate and tetrazolate. bnn-1-Ni exhibits promising material-level AWH performance and hydrolytic stability driven by a low uptake threshold (<20% RH), little hysteresis, fast loading kinetics, low regeneration temperature (≤ 60 °C), cycling stability (> 100 cycles) and projected gravimetric water productivity of 0.3044 wt% min−1 (4.38 kg−1 kg−1 d−1) under simulated temperature swing conditions. Loading of water molecules, visualised at the molecular level through single-crystal X-ray diffraction (SCXRD) and density functional theory (DFT) calculations, revealed a binding site with multiple hydrogen bonds for the first water molecule (adsorption energy −75 kJ mol−1) that anchors formation of water layers (average adsorption energy −59 kJ mol−1). bnn-1-Co is less hydrolytically stable, which we attribute to stronger Ni-N/O/F coordination bonds than their Co-N/O/F counterparts. A hydrolytically stable metal-organic framework enables direct visualisation of stepwise water adsorption at the molecular level, providing design insight for efficient, low-energy atmospheric water harvesting materials.