  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.
