Uncovering the Rotation and Translational Mobility of Benzene Confined in UiO-66 (Zr) Metal-Organic Framework by the H-2 NMR-QENS Experimental Toolbox
Résumé
A combination of experimental quasi-elastic neutron scattering (QENS) and deuterium solid-state nuclear magnetic resonance (H-2 NMR) techniques was used to uncover the molecular mobility of benzene confined in UiO-66 (Zr) MOF with a 3D cage-window-cage type porous network topology. We have shown that tetrahedral and octahedral cages of UiO-66 offer notably different states of confinement for benzene. Below 163 K, the guest molecules of benzene are represented by two different dynamic states: in the smaller (tetrahedral) cage the benzene is able to exhibit only anisotropic C-6 rotation and some limited librations. In the octahedral cage, there is enough space for additional C-2 axial rotation and the isotropic random reorientation. Rotational motions have been characterized by the rate constants and corresponding activation energies. The two dynamic states merge as the temperature increases due to translational jump diffusion with the limiting step being the passage through the window between the cages. Both techniques show similar activation barrier for the diffusion of 25-27 kJ mol(-1) and a diffusion coefficient D = 3.3 x 10(-11) m(2) s(-1) at 400 K. The diffusion process is described by a migration via 1 nm long jumps among octa- and tetrahedral cages. The H-2 NMR gives the correlation times for both the elementary step of diffusion (jump exchange between neighboring octa- and tetrahedral cages) and approximate to 58 time slower long-range isotropic migration process.