Methane Storage: Molecular Mechanisms Underlying Room-Temperature Adsorption in Zn4O(BDC)3 (MOF-5)

In this paper we study how the methane adsorption properties of the ionic MOF-5 are derived from the local structure of its coordinated metal-cluster. Density functional theory is used to study the adsorption process and identify the key interactions which drive it at ambient temperatures. A detailed adsorption model which represents the adsorption process is derived and used to extract thermodynamic properties from previously reported adsorption isotherms. We find that after adsorption of a single molecule to the face of the metal cluster, a nanostructured surface is formed which enables adsorption of additional CH4 molecules at reduced entropic penalty thanks to on-surface hopping motions and retention of significant translational freedom. Binding of the CH4 molecules to the MOF is dominated by electrostatic interactions with negatively charged carboxylate groups, while CH4-CH4 dispersion interactions are important only at high pressures. Last, the MOF-specific adsorption model is compared against the single-site Langmuir model. (Graph Presented)