Mobility of the 2-Methylimidazolate Linkers in ZIF-8 Probed by H-2 NMR: Saloon Doors for the Guests
Résumé
ZIF-8 is one of the most interesting metal-organic frameworks due to its
high stability and unique capabilities for hydrocarbons separation. Its
porous network is formed by large almost spherical cavities
interconnected by very narrow windows, similar to 3.4 angstrom, which
should be too small even for methane. At the same time the direct
experimental observations show that ZIF-8 cavities are able to host even
such large molecules as benzene. This effect is associated with the
flexibility of the cavity widows, arising from dynamical freedom on the
2-methylimidazole (2-mIM) linkers that form the framework. In this work,
by means of H-2 NMR we show that the 2-mIM linkers of the ZIF-8 are
very mobile and their mobility is sensitive to the presence of benzene
guest. In contrast with other known MOFs based on linearly bonded
carboxylates, in guest-free ZIF-8 the plane of 2-mIM linker exhibits
two-site flips within a sector of 2 phi(f) = 34 degrees with very low
activation barrier (1.5 kJ mol(-1)) and high rates (similar to 10(12)
s(-1)) Above 380 K the linkers begin to demonstrate additional fast
librations with gradually increasing amplitudes gamma(lib) comparable
with the two-site flips (gamma(lib) = +/- 17 degrees above 560 K). This
is direct evidence that the ZIF-8 linkers twist notably, thus increasing
the aperture of the windows sufficiently to fit very large molecules.
Upon benzene loading, the geometry of the observed motions remains
similar but the potential barrier of the linkers flipping rises up to 9
kJ mol(-1).