Molecular Insight into the Swelling of a MOF: A Force-Field Investigation of Methanol Uptake in MIL-88B(Fe)-Cl

Volume changes are observed in the metal-organic frameworks (MOFs) of the MIL-88 family when they are exposed to certain solvents. We investigate here, at the atomic level, the swelling behavior of MIL-88B absorbing strongly interacting guest molecules, methanol, for which the largest changes are found. The MOF is positively charged and possesses open metal sites at the trimetallic inorganic building units (M3O), with which the counterions and guests coordinate. We develop an extended MOF-FF-type interaction model and perform the first molecular dynamics (MD) simulations to describe the structural changes of the flexible MIL-88B(Fe)-Cl upon insertion of methanol. The newly developed interaction model according to the MOF-FF scheme consists of (I) the intra-MOF interactions, (II) a fully MOF-FF-compatible model for the methanol and the solvated Cl- ion, which was recently published, and (III) specific new terms developed for the interactions between a trimetallic building unit (Fe3O) connected with six benzoate rings and these species. We report the free energy versus volume profiles as a function of loading and temperature, which are matched with the evolution of the unit cell volume versus the methanol loading profile. We discuss radial pair distribution functions (rdf) and some three-dimensional distributions of the counterions around Fe3O. We find that the pore opening is accompanied by characteristic structural changes in the arrangements of the counterions near the central Fe-3 units and also of the solvent coordinating these counterions: this illustrates the role of the solvated counterions in the swelling process.