Atmospheric Water Sorption Kinetics in Powder and Monolithic Metal–Organic Frameworks

Abstract

Metal–organic frameworks (MOFs) have been widely applied for adsorption applications owing to their high surface area and porosity. In this paper, the atmospheric water adsorption kinetics in a prototypical MOF with two forms, that is, powder and monolithic MOF-801, are systematically investigated. It is shown that the total pore volume (average pore diameter) of the monolithic MOF-801 is 0.831 cm³ g⁻¹ (5.20 nm) which is much larger than that of powder MOF-801, that is, 0.488 cm³ g⁻¹ (1.95 nm). Monolithic MOF-801 absorbs more water than powder MOF-801 at a relative humidity (RH) above 90%. However, between the RH ranges from 10% to 90%, its water uptake is significantly lower than that of the powder form. Molecular dynamics simulations demonstrate that the unexpected water uptake capacity of monolithic MOF-801 at RH of 10%∼90% is caused by the water film formed by the capillary condensation in these mesopores of monolithic MOF-801. The capillary force of the formed film can be overcome by water vapor pressure when RH is over 90%. These findings reveal the underlying mechanisms for water adsorption kinetics in both powder and monolithic MOFs, which can motivate and benefit the new passive cooling or water harvesting system design based on MOFs.