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

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.