Kinetics of Water Vapor Adsorption and Desorption in MIL-101 Metal-Organic Frameworks

The adsorption-desorption isotherms and relaxation curves of water in chromium terephthalate metal-organic frameworks (MOFs), MIL-101, were measured by the gravimetric method at 298 K and 1 atm. The obtained isotherms were compared to those obtained by the volumetric method, which showed good agreement. The measured isotherms exhibited three-step and two-step curves during adsorption and desorption, respectively. The hysteresis between adsorption and desorption isotherms was not pronounced, and the difference in relative humidity between the adsorption and desorption isotherms was 0.05-0.10 at identical adsorbed mass. Regarding the water adsorption relaxation curves, when the relative humidity was varied stepwise from 0.40 to 0.47 and the water supply rate was small, the relaxation curves could be fitted to two straight lines, indicating that, initially, the middle cages filled with water, followed by the large cages at a different adsorption rate. When the relative humidity was changed stepwise from 0.40 to 0.90 and the water supply rate was large, the relaxation curves could be fitted to a single straight line until the equilibrium state, and the relaxation time was about 40 s. The kinetics of water transport inside MIL-101 is strongly dependent on its unique pore structure and hydrophilicity-hydrophobicity spatial distribution.