Dramatic Effect of the Electrostatic Parameters on H2 Sorption in an M-MOF-74 Analogue

Simulations of H-2 sorption were performed in Cu-MOF-74, a recent addition to the M-MOF-74 series. Electronic structure calculations revealed that the Cu2+ ions exhibit an unusually low partial positive charge distribution in Cu-MOF-74, which is a direct consequence of the Jahn-Teller effect. This is in contrast to the charge environment for the metal ions in some of the other M-MOF-74 variants, as determined in previous work [Pham, T.; et al. J. Phys. Chem. C 2015, 119, 1078-1090]. Because of the low magnitude of the partial charges of the Cu2+ ions in Cu-MOF-74, this MOP displays the lowest H-2 uptake and Q(st) values of the M-MOF-74 series, which is consistent with what was observed experimentally for H-2 sorption in this Series of MOFs. Control simulations of H-2 sorption in a nonphysical Cu-MOF-74 variant were performed in which a set of calculated partial charges, appropriate for one of the other M-MOF-74 analogues, were used. These simulations utilize a much higher partial positive charge for the Metal ions and, as a result, a different shape for the simulated H-2 sorption isotherms was obtained compared to that using the normal force field. This shape was not representative of the experimental isotherm for Cu-MOF-74, and thus, confirms the notion that the electrostatic parameters on the metal ions are the key to understanding the H-2 sorption behavior in this MOF. Examining the distribution of the induced dipoles and the Cu2+-H-2 distance via simulated annealing and executing two-dimensional quantum rotation calculations have also verified that the H-2-metal interaction in Cu-MOF-74 is the weakest in the M-MOF-74 series. This study shows the power of using computational modeling to explain certain experimental observables and trends in a series of MOFs.