Cage Effects in the Adsorption of Hydrocarbons On the Metal-Organic Framework Uio-66

Cage effects in the adsorption of hydrocarbons on the Metal-Organic Framework UiO-66

T. Duerinck^1,2, J.F.M. Denayer¹

¹ Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, 1050 Brussel, Belgium.
² Georgia Institute of Technology, School of Chemical & Biomolecular Engineering, Atlanta, GA 30332 USA.

The metal-organic framework UiO-66 (Zr- terephthalate) has a complex structure of octahedral (~11Ĺ) and pyramidal pores (~8Ĺ). UiO-66 shows excellent chemical and thermal stability.¹ Analogue materials using modified BDC-linkers are fairly straightforward synthesizable (e.g. Me, NO₂, NH₂, halides...).² Several reports in the adsorption properties of this framework for permanent gasses or water exist but only a few on hydrocarbons exist. The separation potential for hexane and xylene isomers was studied for several UiO-66 variants.^3-5

In this work, we tried to get a deeper insight in the adsorption mechanisms at pore level by studying a wide range of molecules: n-, single and double branched alkanes, cycloalkanes and –alkenes, aromatics and hetero atom functionalized derivatives. A pulse gas chromatographic study, using columns packed with pellets of UiO-66, UiO-66-Me, UiO-66-NO₂ and UiO-66-Me₂ was performed in the range of 200-300°C. Using the van ‘t Hoff equation, adsorption properties (enthalpy and entropy values, pre-exponential factors, Henry constants) were determined.

Remarkably, the adsorption enthalpy of n-alkanes does not linearly increase with carbon number. Non-linearity effects are observed around C₅-C₇ chain lengths. Significant entropy effects define the overall favourable Gibbs free energy. The UiO-66 framework shows significant adsorption preference for cyclic and branched alkanes with maxima around cyclo-octane for cycloalkanes and 3,3-dimethylpentane compared to the linear counterparts. Remarkably the adsorption enthalpy and entropy factors show a drop in absolute value at C₆-C₇ on UiO-66-Me and C₇-C₈ UiO-NO₂.

Further analysis by investigating the relative confinement factors⁶ between C_n and C_n+1 isomers shows changes that can be related to change in relative closeness to the pore wall. The functionalized UiO-66 materials have a relatively higher confinement, indicating that the presence of an additional group leads to a reduction in effective pore size. This change in effective pore size was further investigated by comparing cage sizes and average molecular dimensions of the pore size. In addition, Monte Carlo simulations were performed to investigate the molecular position of the adsorbates in the structure. It was shown that most of the selectivity effects arise from the interplay between the molecular dimension of the adsorbate and the adsorbent's cage size. Adsorbates preferentially adsorb in the smallest, tetrahedral cage whenever possible. Larger alkanes (typically C₈ or bigger) do not optimally fit in the tetrahedral cage and preferentially sit in the octahedral cage. Not only the largest dimension but also overall shape of the adsorbate plays an important role. More spherical molecules interact stronger with the cage walls than linear molecules due to the relatively small cage dimension of smallest cage. In some cases, entropy effects play a major role in the separation of stereoisomers. This is especially true for cyclic adsorbates (e.g. dimethylcyclohexanes).

Acknowledgements
 T. Duerinck and J.F.M. are grateful to FWO Flanders for financial support.

References

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