Translational and Rotational Motion of C8 Aromatics Adsorbed in Isotropic Porous Media (MOF-5): NMR Studies and MD Simulations

V. J. Witherspoon, L. M. Yu, S. Jawahery, E. Braun, S. M. Moosavi, S. K. Schnell, B. Smit, and J. A. Reimer, Translational and Rotational Motion of C8 Aromatics Adsorbed in Isotropic Porous Media (MOF-5): NMR Studies and MD Simulations J Phys Chem C  121(28) 15456 (2017) http://dx.doi.org/10.1021/acs.jpcc.7b03181

Abstract: We combined nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation to study xylene behavior in MOF-5, probing the effects of adsorbate geometry in a weakly interacting model isotropic MOF system. We employed NMR diffusometry and relaxometry techniques at low field to quantify the self-diffusion coefficients (Ds) and the T1 of xylenes in MOF-5 as a function of temperature at the saturated loading for each xylene. These experiments reveal the translational motion activation energies to be 15.3, 19.7, and21.2 kJmol-1and the rotational activation energies to be 47.26,12.88, and 11.55 kJmol-1for the (p-,m-,o-) xylene isomers respectively. Paraxylene exhibits faster translational motion yet shows four times the activation energy barrier for rotational motion vis-a-vis the other isomers. MD simulations performed on these model systems corroborate the findings for paraxylene and suggest that paraxylene has the lower free energy barrier for hopping away from its binding sites, yet has the slowest rotational motion in the plane of the xylene molecule.

© Berend Smit 2019