(662c) Cage Effects in the Adsorption of Hydrocarbons On the Metal-Organic Framework Uio-66
AIChE Annual Meeting
2013
2013 AIChE Annual Meeting
Separations Division
Characterization of Adsorbent Materials I
Thursday, November 7, 2013 - 1:10pm to 1:30pm
Cage effects in the adsorption of hydrocarbons on
the Metal-Organic Framework UiO-66
T.
Duerinck1,2, 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
(~11Å) and pyramidal pores (~8Å). 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 Cn and Cn+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