TY - JOUR

T1 - Rotational and translational motion of benzene in ZIF-8 studied by 2H NMR

T2 - Estimation of microscopic self-diffusivity and its comparison with macroscopic measurements

AU - Kolokolov, Daniil I.

AU - Diestel, Lisa

AU - Caro, Juergen

AU - Freude, Dieter

AU - Stepanov, Alexander G.

PY - 2014/6/19

Y1 - 2014/6/19

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=84903184959&partnerID=8YFLogxK

U2 - 10.1021/jp5026834

DO - 10.1021/jp5026834

M3 - Article

AN - SCOPUS:84903184959

VL - 118

SP - 12873

EP - 12879

JO - Journal of Physical Chemistry C

JF - Journal of Physical Chemistry C

SN - 1932-7447

IS - 24

ER -