Rotational and Translational Motion of Benzene in ZIF-8 Studied by 2H NMR: Estimation of Microscopic Self-Diffusivity and Its Comparison with Macroscopic Measurements

In relation to unique properties of metal–organic framework (MOF) ZIF-8 to adsorb and separate hydrocarbons with kinetic diameters notably larger than the entrance windows of the porous system of this microporous material, the molecular dynamics of benzene adsorbed on ZIF-8 has been characterized and quantified with 2H nuclear magnetic resonance. We have established that within the ZIF-8 cage the benzene molecule undergoes fast rotations, hovering in the symmetric potential of the spherical cage and relatively slow isotropic reorientations by collisions with the walls. Benzene performs also translational jump diffusion between neighboring cages characterized by an activation barrier ED = 38 kJ mol–1 and a pre-exponential factor τD0 = 4 × 10–10 s. This microscopic measurement of benzene mobility allows us to estimate the self-diffusion coefficient for benzene in ZIF-8 (D0self ≈ 4 × 10–16 m2 s–1 at T = 323 K). Macroscopic measurements of diffusivities derived from membrane permeation studies (3.5 × 10–15 m2 s–1 at T = 298 K for fractional occupancy Θ ≈ 0.99) and sorption uptake (Di,MS ≈ 10–20 m2 s–1 at 323 K) are several orders of magnitude larger or smaller than the microscopic self-diffusion coefficient D0self, which was derived from relaxation time analysis. This experimental finding is attributed to the limits of macroscopic measurements.