(577a) High-Performance Separation of Cis/Trans Isomers on MIL-125(Ti)

High-performance
separation of cis/trans isomers on MIL-125(Ti)

Stijn Van der
Perre¹, Ana-Martin Calvo¹, Bart
Bueken², Dirk E. De Vos², Sofia Calero³, Gino V.
Baron¹ and Joeri F.M. Denayer¹

¹Department of Chemical
Engineering, Vrije Universiteit Brussel, Brussels, Belgium

²Centre for Surface
Chemistry and Catalysis, Katholieke Universiteit Leuven, Leuven, Belgium

³Department of Physical,
Chemical, and Natural Systems, Universidad Pablo de Olavide, Sevilla, Spain

In comparison to
conventional stationary phases like (functionalized) silica or alumina in High-Performance
Liquid Chromatography (HPLC), MOFs can lead to chromatographic separations
based on molecular size and shape due to a uniform pore size distribution and
ordered pore structure. Hereby, the versatility of organic linkers and metal
centers used in MOFs introduce different interactions with the analytes than
those obtained with commercial stationary phases in HPLC, which opens
possibilities to separate different kind of mixtures.

In this work,
the MIL-125 (a Ti-based MOF) framework was used as stationary phase in liquid
chromatography to study the separation of different model and stereoisomer
mixtures. MIL-125(Ti) has distorted octahedral (12.5 Å) and tetrahedral (6.1 Å)
cages connected with triangular windows of around 6 Å in size. Monodisperse
MIL-125(Ti) particles were synthesized and slurry packed into a stainless steel
column for HPLC measurements.

MIL-125(Ti)
allowed separation of various model mixtures in HPLC, but moreover, the
structure displays a stereo selectivity for difunctionalized cyclohexane
molecules (Fig. 1). This remarkable selectivity could be attributed to the
shape selective confinement in the cages of the structure, where the
tetrahedral cages act as channels between the octahedral cages, favoring the trans isomer. In a next stage, a Van Deemter curve was measured
and intraparticle diffusion coefficients of some components were calculated
from peak parking experiments in order to characterize and to determine performance
of the crystal packed column, as will be discussed in this work. For a better
understanding of the molecular interactions that occur within the chromatographic
process, simulation techniques have been used. We performed Monte Carlo
simulations to study adsorption properties of the most interesting molecules
inside the framework. For this purpose, models of 4-ethylcyclohexanol and 1,3-dimethylcyclohexane
isomers have been developed to reproduce characteristic properties of the
molecules as well as their interactions with MIL-125(Ti). Adsorption isotherms,
selectivity, distribution of the molecules, and Radial Distribution Functions
were obtained and discussed.

Fig. 1: Chromatographic profiles of cis and trans
mixtures of 4-ethylcylohexanol (left) and 1,3-dimethylcyclohexane (right),
using a mobile phase of ACN at 303 K and 0.2 ml/min. A preference for the trans
isomer was observed.

In conclusion, the
separation potential and applicability of MIL-125(Ti) in chromatography was
demonstrated via an experimental and theoretical way. This could be a start for
the separation of other cis/trans mixtures, giving possibilities for separation
of diastereomers, which is of tremendous importance for the production of
pharmaceuticals.

Acknowledgements The authors are
grateful to IWT Vlaanderen, for financial support for MOFSHAPE “Shaped
Metal-Organic Frameworks for Adsorption, Energy and Catalysis Applications”.