Combined Nuclear Magnetic Resonance and Molecular Dynamics Study of Methane Adsorption in M2(dobdc) Metal–Organic Frameworks

Author(s): Witherspoon, Velencia J; Mercado, Rocio; Braun, Efrem; Mace, Amber; Bachman, Jonathan; Long, Jeffrey R; Blumich, Bernhard; Smit, Berend; Reimer, Jeffrey A | Abstract: © 2019 American Chemical Society. We examine the diffusion of methane in the metal-organic frameworks M 2 (dobdc) (M = Mg, Ni, Zn; dobdc 4- = 2,5-dioxido-1,4-benzenedicarboxylate) as a function of methane loading through a combination of nuclear magnetic resonance and molecular dynamics simulations. At low gas densities, our results suggest that favorable CH 4 -CH 4 interactions lower the free energy barrier for methane hopping between coordinatively unsaturated metal sites and thus enhance the translational motion of methane down the c-axis. At higher gas densities, CH 4 -CH 4 interactions become more significant, CH 4 -CH 4 collisions become more frequent, and the gas self-diffusion begins to decrease. Finally, we observe that the self-diffusion coefficient of methane is inversely related to the binding energy at the coordinatively unsaturated metal sites, such that diffusion is most rapid in the Zn 2 (dobdc) framework.