Mesoporous metal–organic frameworks (MOFs) are
potential
materials for efficient and reversible water adsorption and desorption
processes. However, pore-filling/pore-emptying processes in mesoporous
MOFs are complicated due to the complex pore geometry, nonuniform
pore surface properties, and polarity of water molecules. In addition,
the temperature effects on these processes are still unknown. Here,
the temperature effect on the adsorption and desorption processes
of water was investigated in a promising mesoporous MOF, namely, MIL-101(Cr).
Experimentally measured water adsorption/desorption isotherms show
a transition from a one-step to a two-step pore-filling/pore-emptying
process with increasing temperature. Theoretical studies revealed
that the corresponding mechanism of this transition is related to
the relative pressures for the transition between two different stable
adsorption states in two different-sized mesopores, the relative pressure
at which the inner surface is completely covered with water, and the
relative pressure at which condensed water overcomes the energy barrier
from the metastable liquid state to the gas state.