Abstract
An increase in demand for energy efficient processes for the separation of a mixture of hydrocarbons drives the need for understanding metal–organic frameworks (MOFs) that can provide better noncryogenic alternatives for the fractionation of hydrocarbon mixtures. Here we study the structure and properties of a metal–organic framework, Fe-MOF-74, and its effectiveness to separate C1–C3 hydrocarbon mixtures. The binding of these hydrocarbons to the open metal sites of Fe-MOF-74 has been investigated using a meta-generalized gradient approximation density functional, M06-L, which has previously been validated for systems containing transition metals. For interpretive purposes, charge model 5 (CM5) is used to determine the partial atomic charges on the metal cations and the oxygen atoms of the ligands surrounding these metal centers. Our computations show preferential binding to the metal center of Fe-MOF-74 of unsaturated hydrocarbons over saturated ones in agreement with experimental results, and the calculated binding energies are in semiquantitative agreement with experiment. The results are analyzed in terms of various factors contributing to the binding, including structural distortion, electrostatics, damped dispersion, charge transfer, back bonding, and ligand field effects on the d orbitals. The CM5 charges are not sensitive to small differences in structure.
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