Water vapor adsorption in microporous and mesoporous frameworks for atmospheric water harvesting: Isotherms, kinetics, and thermodynamics insights

This study investigates the fundamental water vapor adsorption mechanisms in microporous MOF-801 and mesoporous Al-Fumarate for atmospheric water harvesting applications. Using the ASTM E104 methodology, we analyzed the adsorption behavior tested at different temperatures across a 10 to 80 % relative humidity range. Comprehensive characterization (XRD, FTIR, TGA, BET, SEM) was complemented by rigorous statistical evaluation of seven isotherm models using six metrics. The Frenkel-Halsey-Hill (FHH) model provided the best fit (R-2 > 0.82) for both MOFs, revealing distinct pore-filling mechanisms through surface fractal dimensions (D, = 1.40, D-2 = 2.83 for Al-Fumarate; D, = 1.32, D-2 = 2.81 for MOF-801). Thermodynamic analysis demonstrated stronger water interactions in Al-Fumarate's mesoporous framework (-43.2 [kJ/mol]) compared to MOF-801's microporous structure, which facilitates monolayer physisorption with milder enthalpy changes (-28.1 [kJ/mol]). Kinetic studies using the linear driving force model (LDF) revealed humidity-dependent transitions from monolayer to multilayer adsorption. MOF-801 exhibited superior performance at low humidity, while AlFumarate showed faster kinetics and higher capacity at moderate-to-high humidity levels, providing insights for humidity-specific MOF optimization.