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Amine Dynamics in Diamine-Appended Mg2(dobpdc) Metal-Organic Frameworks.

  • Author(s): Xu, Jun
  • Liu, Yifei Michelle
  • Lipton, Andrew S
  • Ye, Jinxing
  • Hoatson, Gina L
  • Milner, Phillip J
  • McDonald, Thomas M
  • Siegelman, Rebecca L
  • Forse, Alexander C
  • Smit, Berend
  • Long, Jeffrey R
  • Reimer, Jeffrey A
  • et al.
Abstract

Variable-temperature 15N solid-state NMR spectroscopy is used to uncover the dynamics of three diamines appended to the metal-organic framework Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), an important family of CO2 capture materials. The results imply both bound and free amine nitrogen environments exist when diamines are coordinated to the framework open Mg2+ sites. There are rapid exchanges between two nitrogen environments for all three diamines, the rates and energetics of which are quantified by 15N solid-state NMR data and corroborated by density functional theory calculations and molecular dynamics simulations. The activation energy for the exchange provides a measure of the metal-amine bond strength. The unexpected negative correlation between the metal-amine bond strength and CO2 adsorption step pressure reveals that metal-amine bond strength is not the only important factor in determining the CO2 adsorption properties of diamine-appended Mg2(dobpdc) metal-organic frameworks.

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