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 ( D₁ = 1.40, D₂ = 2.83 for Al-Fumarate; D₁ = 1.32, D₂ = 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 Al-Fumarate showed faster kinetics and higher capacity at moderate-to-high humidity levels, providing insights for humidity-specific MOF optimization. • MOF-801 and Al-fumarate were synthesized and tested for AWH at varying T/RH. • The Frenkel-Halsey-Hill (FHH) model best fits MOFs via surface fractal pore-filling. • Al-Fumarate shows stronger H₂O interaction (−43.2 kJ/mol) vs. MOF-801 (−28.1 kJ/mol). • Kinetics show humidity-based adsorption: MOF-801 at low RH, Al-Fumarate at high RH. • Six metrics validate seven isotherm models, revealing optimal AWH mechanisms.