Experimental and Theoretical Studies of Supercritical Methane Adsorption in the MIL-53(Al) Metal Organic Framework

Abstract

The adsorption equilibrium properties of supercritical methane in the large-pore (lp) structure of the MIL-53(Al) metal organic framework were studied experimentally by gravimetric adsorption and theoretically by grand canonical Monte Carlo (GCMC) simulation. The adsorption experiments span a broad range of pressures (0.01–7 MPa) and temperatures (303–353 K). In our molecular simulation work, MIL-53lp(Al) is assumed to have a perfect, rigid lattice, and both fluid–fluid and solid–fluid interactions are modeled using the TraPPE-UA force field. The adsorption isotherms and isosteric heats of adsorption predicted by GCMC simulation, without any reparametrization of the TraPPE-UA force field parameters, are in good agreement with the experimental measurements. Our molecular simulations predict that the amount of methane adsorbed in the porous framework of MIL-53lp(Al) at 298.15 K and 3.5 MPa is 5.79 mol/kg, yielding a methane storage capacity of 132.6 v/v (volumes of stored gas, measured at standard conditions, per storage volume) for a monolithic block and 107.2 v/v for the theoretical limit of a close-packing of uniform spherical particles. For an isothermal (298.15 K) discharge cycle between 3.5 and 0.136 MPa, the predicted net deliverable capacity is 114.0 (v/v)net for a monolith and 93.1 (v/v)net for a close-packed bed. If, however, the storage system is operated at 253 K, the net storage capacity of a monolithic block of MIL-53(Al) increases to a value that is very close to the DOE target of 150 (v/v)net.