Porous coordination materials such as metal/covalent organic frameworks (MOFs/COFs) have attracted considerable interest for their potential use in gas adsorption and/or separation. The first example of efficient hydrocarbon separations with a MOF possessing open metal sites was recently reported.1 The extraordinary properties were due to the combination of appropriate pore size and the strong interactions between under-coordinated Fe2+ the metal atoms and the unsaturated hydrocarbons. In this work, we study the interactions of C1-C2 hydrocarbons with the open-metal sites of the M-MIL-127, by means of Density Functional Theory. This framework possesses an oxo-centered trimetalic building unit with the stoichiometry [M3IIIOX]6+, where M = Al, Sc, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Ru, Rh, In and Ir, and X = [F]- counteranion. This trimetallic moiety is very common as isolated species,2 but only the Sc, V, Fe, Ga and In variants have been reported in MOF synthesis.3 A recent computational study, showed how small molecules such as H2, CO and CO2 interact with the open metal sites.4 Here, we study systematically how the metal composition affects the binding of the C1-C2 paraffins and olefins. Binding energies of CH4, C2H2, C2H4 and C2H6 with the open-metal sites are calculated and the interactions are analysed by Natural Bonding Orbitals. A charge transfer from the hydrocarbons to the metal sites is calculated and in some cases a back-donation from the metal to the anti-bonding orbitals of hydrocarbons. A complementary Energy Decomposition Analysis is performed in order to estimate the forward- and back-donation contribution to the bonding energies. Finally, a discussion is made about the orbitals that can back-donate and which metals are promising for separating hydrocarbons. References: (1) Bloch, E. D.; Queen, W. L.; Krishna, R.; Zadrozny, J. M.; Brown, C. M.; Long, J. R. Science 2012, 335, 1606–1610. (2) Cannon, R. D.; White, R. P. In Progress in Inorganic Chemistry, Volume 36; Lippard, S. J., Ed.; John Wiley & Sons, 1988; Vol. 36, pp. 195–298. (3) Wongsakulphasatch, S.; Nouar, F.; Rodriguez, J.; Scott, L.; Le Guillouzer, C.; Devic, T.; Horcajada, P.; Grenèche, J.-M.; Llewellyn, P. L.; Vimont, A.; Clet, G.; Daturi, M.; Serre, C. Chem. Commun. 2015, 51, 10194–10197. (4) Mavrandonakis, A.; Vogiatzis, K. D.; Boese, A. D.; Fink, K.; Heine, T.; Klopper, W. Inorg. Chem. 2015.

Computational investigation of C1-C2 hydrocarbons interacting with the open-metal sites of the MIL-127 framework

251st National Meeting (2016)

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