Fundamental thermodynamic properties of sorbents for atmospheric water capture

The thermodynamic properties of various sorbents, namely carbon-based sorbents, silica gel and metal-organic frameworks (MOFs) were assessed and compared based on their adsorption isotherms at 25, 30 and 35 degrees C. The isotherms were measured in a custom-made and calibrated environmental chamber using a gravimetric method. Gibbs free energy demonstrated the spontaneity of the adsorption process and the hygroscopicity variation of the sorbents depending on their surface chemistry. The carbon-based sorbent, nanoporous sponges (NPS), and one of the MOFs, Cr-MIL-101, had lower sorbent-adsorbate interactions and thus had integral enthalpies converging rapidly to the heat of vaporization of pure water. As such, these samples would release less heat during an adsorption step with partial filling of the sorbent. Integral entropy showed that, for most of the environmental conditions, adsorbed water molecules had an entropy equivalent to pure water for most of the sorbent materials, except for silica gel, due to its higher energy sites and higher water-sorbent interactions. NPS and Cr-MIL-101 were shown to be entropically advantageous for the recovery/removal of water. Enthalpy and entropy can provide insight to select favorable conditions to perform adsorption-desorption cycles in a practical water capture system.