Salt Confined in MIL-101(Cr)-Tailoring the Composite Sorbents for Efficient Atmospheric Water Harvesting

Abstract

The adsorption method for atmospheric water harvesting (AWH) is considered as a promising heat-driven technology for potable water supply in arid regions. This research is focused on novel composite sorbents based on hygroscopic salts loaded in the pores of MIL-101(Cr) developed for AWH. The composites based on LiCl, LiBr, CaCl2, and Ca(NO3)2 were synthesized and comprehensively studied by SEM, XRD, N2 adsorption, and thermogravimetric methods. We evidence that the CaCl2/MIL-101(Cr) composite demonstrates a high net water uptake of 0.52–0.59 g_(H2O)/g_(composite) per cycle under conditions of Saudi Arabia and the Sahara desert as the reference regions with extra-dry climate, which exceeds the appropriate values for other adsorbents. It is shown that water adsorption on the composite cannot be presented as a combination of the adsorption on the components, thus indicating a synergistic effect. A detailed characterization of water coordination, mobility, and hydrogen bonding within the confined CaCl2 hydrates and salt solution using solid-state 2H NMR spectroscopy has been performed. It is established that pore confinement promotes a prolonged transition to a dynamically melted state of the hydrated salt and a notable decrease of the melting temperature, which facilitates the molecular transport of water and causes the alteration of sorption properties of CaCl2 inside MIL-101 pores. Finally, the performance of AWH employing CaCl2/MIL-101(Cr) was evaluated in terms of the fractions of water extracted and collected, and the specific energy consumption, demonstrating its high potential for AWH.