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

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• 作者：Tony Pham ; Katherine A. Forrest ; Juergen Eckert ; Brian Space
• 刊名：Crystal Growth & Design
• 出版年：2016
• 出版时间：February 3, 2016
• 年：2016
• 卷：16
• 期：2
• 页码：867-874
• 全文大小：427K
• ISSN：1528-7505

文摘

Simulations of H₂ 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.; 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 MOF displays the lowest H₂ uptake and Q_st values of the M-MOF-74 series, which is consistent with what was observed experimentally for H₂ sorption in this series of MOFs. Control simulations of H₂ 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 H₂ 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 H₂ sorption behavior in this MOF. Examining the distribution of the induced dipoles and the Cu²⁺–H₂ distance via simulated annealing and executing two-dimensional quantum rotation calculations have also verified that the H₂–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.