The adsorption behavior of xylene isomers in MIL-47 from a theoretical perspective

Recently, the spectrum of nanoporous materials like zeolites and zeotype structures was further expanded through the occurrence of a new class of hybrid porous porous solids [1, 2]. Those materials are nowadays also known as metal organic frameworks or MOFs and consist of inorganic and organic moieties. Certain MOFs exhibit a very interesting adsorption and even catalytic behavior. Our study focuses on one of them: MIL-47. MIL-47 was first synthesized by Barthelet [3], later on Alaerts [4] studied the ability of MIL-47 to separate several xylene isomers. This separation behavior of MIL-47 could be better understood with adsorption studies [4, 5]. The adsorbates were located inside the MIL-47 host by Rietveld refinements of the XRD patterns of MIL-47 samples saturated with each of the C8-isomers. The packing of each C8-pair was then discussed on the basis of the interactions between those two aromatic compounds and MIL-47 [4]. In this case, theoretical modeling could predict the relative energy difference between several possible geometries. The stability of a unit cell of MIL-47 fully loaded with four adsorbates was investigated with periodic calculations. Starting form crystallographic information (supp. info of [4]), two possible geometries were cut out for different C8-isomers: para-xylene, meta-xylene, ortho-xylene and ethylbenzene. The optimized structures are displayed in the figure below. The relative energy differences are shown relative to meta-xylene (II), the most unstable structure. [1] Ferey, G., Chemical Society Reviews 37 (2008) 191. [2] Tranchemontagne, D.J.L. , M. O'Keeffe, Z. Ni and Yaghi O.M., Angewandte Chemie-International Edition 47 (2008) 5136. [3] Barthelet, K., Marrot, J., Riou, D. and Ferey G. Angewandte Chemie-International Edition 41 (2002) 281. [4] Alaerts, L., Kirschhock, C.E.A., Maes, M., van der Veen, M.A., Finsy, V., Depla, A., Martens, J.A., Baron, G.V., Jacobs, P.A., Denayer, J.E.M. and De Vos, D.E. Angewandte Chemie-International Edition 46 (2007) 4293. [5] Finsy,V., Verelst, H., Alaerts, L., De Vos, D., Jacobs, P.A., Baron, G.V. and Denayer J.F.M. Journal of the American Chemical Society 130 (2008) 7110