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

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 variousmodel mixtures in HPLC, but moreover, the structure displays a stereo selectivity for difunctionalized cyclohexane molecules. 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. 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”.