Unusual Pore Volume Dependence of Water Sorption in Monolithic Metal-Organic Framework

TL;DR

This study reveals that monolithic MOF-801 has larger pore volume but lower water uptake at moderate humidity compared to powder form, due to capillary condensation effects, informing future MOF-based water harvesting systems.

Contribution

It provides a systematic investigation of water sorption in monolithic versus powder MOF-801, highlighting the impact of pore structure on water adsorption behavior.

Findings

Monolithic MOF-801 has larger pore volume and mesopores than powder MOF-801.

Water uptake in monolithic MOF-801 is lower than powder at 10-90% RH.

Capillary condensation in mesopores explains the water adsorption behavior.

Abstract

Monolithic metal-organic frameworks (MOFs), which have a continuous structure composed of small primary MOF particles and amorphous networks, are demonstrated to possess larger pore volume and thus better larger gas uptake capacity compared to their powder forms. Here, we systematically investigated the water vapor adsorption kinetics in a prototypical MOF, i.e., MOF-801. Our results show that the total pore volume (average pore diameter) of the monolithic MOF-801 is 0.831 cm3/g (5.20 nm) which is much larger than that of powder MOF-801, i.e., 0.488 cm3/g (1.95 nm). Unexpectedly, we find that the water uptake capacity of monolithic MOF-801 is much lower than that of powder MOF-801 when the RH ranges from 10% to 90%. Our molecular dynamics simulations further 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 water molecules can overcome the capillary force when the RH is higher than 90%, and then leads to the increase of the corresponding water uptake capacity of monolithic MOF-801. Our findings reveal the underlying mechanisms for water adsorption kinetics in both powder and monolithic MOFs, which could motivate and benefit the new passive cooling or water harvesting system design based on MOFs.