(708f) Unusual Patterns in the Adsorption and Diffusion of C1-C6 Alcohols and Other Organic Compounds in the ZIF-68 Metal-Organic Framework

Zeolitic
Imidazolate Frameworks (ZIFs) are crystalline microporous materials, consisting
of transition metals (M = Co, Cu, Zn, etc.) linked by imidazolate (Im) or
functionalized Im ligands. This subfamily of the Metal-Organic Frameworks
(MOFs) displays good chemical and structural stability, which make them
attractive as separation media in liquid phase separations. ZIF-68 is an
interesting member of this family, with a complex pore structure consisting of two
one-dimensional channels formed by three different types of cages. The small
channel is composed of alternating small and medium cages, while the large
channel comprises the large cages [1].

In the present
work, the adsorption behavior of ZIF-68 is studied via vapor phase kinetic and
equilibrium experiments with C1-C6 alcohols and other organic compounds
(acetone, acetonitrile, C6-isomers, tri-isopropylbenzene) and kinetic batch
experiments of alcohol mixtures in water. S-shape isotherms are obtained for
polar adsorbates (fig.1), which is related to (1) the presence of two
chemically different channels in the ZIF-68 framework and (2) the rather apolar
nature of the material, which results in unfavorable adsorption of polar
compounds at low concentration. Depending on the component, the adsorption
capacity varies between 0.25 and 0.30 g/g. A comparison to the Argon isotherm
indicated that both types of channels of ZIF-68 are densely filled with the
different organic compounds.

Uptake of the
different alcohols occurs slowly on the large crystals (around 70 µm) used in
this study. Moreover, two different diffusion regimes can be observed, in which
molecular transport in the two-pore systems occurs at different rates. Besides,
it was observed that bulky molecules, with a kinetic diameter significantly
larger than the pore windows, are adsorbed in large amounts, which gave reasons
to think that also this ZIF material, just as ZIF-7 [2] and ZIF-8 [3-5], has a
certain degree of structural flexibility to enlarge the free aperture of the
channels, enhancing the diffusion process. Liquid phase experiments indicate
the potential of ZIF-68 in the selective uptake and separation of alcohol/water
mixtures.

Fig.1:
Adsorption isotherms of C1-C6 alcohols, acetone and acetonitrile at 50 °C.

References

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Acknowledgements

The authors are
grateful to the Agency for Innovation by Science and Technology in Flanders
(IWT) for support in the SBO MOFshape project.