Structural Transitions in the MIL-53(Al) Metal-Organic Framework upon Cryogenic Hydrogen Adsorption

The energetics and phase behavior of the MIL-53(Al) metal-organic framework upon low-temperature (15-260 K), subatmospheric H₂ adsorption are studied experimentally using a volumetric technique and theoretically by grand canonical Monte Carlo simulation. The adsorption equilibrium data are recorded for a fixed amount of H₂ in the system at stable increasing temperature steps starting from 15 K while recording the equilibrium pressure attained at each step. The adsorption isotherms are generated by repeating the experiments for different fixed amounts of adsorbate in the system and connecting the equilibrium points obtained at the same temperature. The solid-fluid interactions are modeled using the TraPPE-UA force field and the fluid-fluid interactions using a parametrization consistent with the same force field; quantum effects on H₂ adsorption are taken into account via a quartic approximation of the Feynman-Hibbs variational approach. The use of a consistent force field with proven transferability of its parameters provides an accurate description of the experimental adsorption equilibria and isosteric heats of adsorption. Because of the weak solid-fluid interaction, the Henry constant for H₂ adsorption in the large-pore (LP) form of MIL-53(Al) surpasses that for H₂ adsorption in the narrow-pore (NP) form at a temperature lower than that at which the dehydrated structure of the material collapses. However, the saturation capacity of the LP form is always higher than that of the NP phase. The phase behavior of MIL-53(Al) upon temperature-induced H₂ desorption is interpreted in terms of the osmotic thermodynamic theory. For the conditions spanned in the experiments MIL-53(Al) exhibits at most a single structural transition and its phase behavior depends not only on pressure and temperature but also on the thermal history of the bare material.