Molecular Mobility of Simple Aromatics and the Framework Flexibility in Microporous Zeolitic Imidazolate Framework ZIF-8 Probed by Solid State 2H NMR
Molecular mobility of simple aromatics and the framework flexibility in microporous zeolitic imidazolate framework ZIF-8 probed by solid state 2H NMR
D.I. Kolokolov,a,b,* A.E. Khudozhitkov,a,b A.G. Stepanov,a,b H. Jobic,c D. Freude,d J. Haased
aBoreskov Institute of Catalysis,SB RAS, Novosibirsk,630090 ,Russia
bNovosibirsk State University, Novosibirsk, 630090, Russia
cInstitut de Recherches sur la Catalyse et l’Environnement de Lyon, CNRS, Université de Lyon , 69626, Villeurbanne, France .
d Universität Leipzig, Leipzig, 04103, Germany
*Corresponding author: firstname.lastname@example.org
ZIF-8 is a highly stable microporous zeolitic imidazolate framework (2-MiM) envisioned as a base porous material for the new generation of molecular sieves and sorbents. It shows a diffusion driven separation selectivity for short alkanes/alkenes  and simple aromatics [1,2]. For the practical use it is essential to know how the guest molecules mobility and transport depend on the molecular size. The ZIF-8 framework is characterized by an LTA type topology with spherical cages connected by narrow windows of 3.4 Å and was assumed to behave as a typical zeolite, sieving off all guests with kinetic diameter larger than 3.4 Å. However it was experimentally shown, that ZIF-8 framework is able to host much larger molecules like benzene or even xylenes . This finding raised the question of the effective flexibility of the ZIF-8 framework.
In this work we report how structural dynamics of the framework and the mechanistic details behind the molecular mobility of adsorbed species in the ZIF-8 porous network can be experimentally probed by solid state 2H NMR.
By means of 2H NMR spectra line shape analysis we have demonstrated, that the 2-MiM linkers of ZIF-8 are in fact involved in a very fast but restricted angular liberations whose amplitude grows with the temperature. This fact indicates that the aperture of the ZIF-8 cage windows is effectively much larger than crystallographic data suggest due to the possible twist of the linkers (see Figure 1). Furthermore, the application of the 2H NMR spin relaxation allows us to characterize the molecular mobility of adsorbed in ZIF-8 simple aromatics: benzene, toluene, o-/p-/m-xylene and isobutane. These experimental data suggest
Figure 1. ZIF-8 linkers dynamics probed by 2H NMR
that the guest molecules have an additional adsorption sites located near the cage window in ZIF-8. The observed exchange between the windows site and the central part of the cage is a slow process that directly depends on the size and geometry of the particular guest. In addition to the intracage dynamics of the all studied guest molecules it was possible to characterize the exchange of benzene between the ZIF-8 cages.
This work was supported in part by Russian Foundation for Basic Research (grant no. 14-03-91333) and by the Deutsche Forschungsgemeinschaft (grant HA 1893/16).
1) Pimentel, B.R., Parulkar, A., Zhou, E. Brunelli, N.A., Lively, R.P., ChemSusChem, 2014, 7, 3202 – 3240.
2) Zhang, K.; Lively, R. P.; Zhang, C.; Chance, R. R.; Koros, W. J.; Sholl, D. S.; Nair, S. J. Phys. Chem. Lett. 2013, 4, 3618-3622.