Temperature Effect on Water Adsorption and Desorption Processes in the Mesoporous Metal–Organic Framework MIL-101(Cr)

Abstract

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.

Elucidating Improvements to MIL‐101(Cr)’s Porosity and Particle Size Distributions based on Innovations and Fine‐Tuning in Synthesis Procedures

“…[ 33,50,84,169,171 ] Specifically, the coordination stems from Coulombic interactions between the unsaturated Cr 3+ sites of MIL‐101(Cr) and the oxygen atom of water molecules. [ 50,172 ] Molecular simulation results support these adsorbent–adsorbate interactions, including the weak coordination of water to saturated Cr 3+ sites. [ 30,50 ] 1D water chains form within MIL‐101(Cr)’s pores as more water molecules interact with the unsaturated Cr‐bonded water.…”

“…intervals, two adsorption steps may be visible as water vapor fills the 29 Å and then the 34 Å mesoporous cages of MIL-101(Cr). [33,50,62,172,[174][175][176] This difference is because the 29 Å cages have a smaller inner surface with greater curvature than the 34 Å cages. [50] Furthermore, MIL-101(Cr)'s heat of adsorption decreases rapidly from about 80 kJ mol −1 at low RH to slightly above the water's heat of evaporation (40.69 kJ mol −1 ) with a minute increase in RH, demonstrating that water-water interactions govern adsorption after the Cr 3+ sites are saturated.…”

“…[ 33,171 ] Interestingly, water molecules do not interact with the carboxylate oxygen in MIL‐101(Cr) with a low atom charge. [ 50,172 ]…”

“…[33,171] Interestingly, water molecules do not interact with the carboxylate oxygen in MIL-101(Cr) with a low atom charge. [50,172] As RH increases to 35-50%, MIL-101(Cr) has a steep rise in water vapor uptake. [33,84,171] This enhanced water uptake occurs because water undergoes capillary condensation in the pores of MIL-101(Cr).…”

“…[ 30,50 ] 1D water chains form within MIL‐101(Cr)’s pores as more water molecules interact with the unsaturated Cr‐bonded water. [ 50,172 ] Within this low RH range, MIL‐101(Cr)’s water sorption capacity varies linearly with RH. [ 33,171 ] Interestingly, water molecules do not interact with the carboxylate oxygen in MIL‐101(Cr) with a low atom charge.…”