(311a) Breathing Effects In the Adsorption and Separation of Xylene Isomers on the Metal Organic Framework MIL-53

Xylene isomer separation is one of the most important bulk scale adsorptive separation processes. Currently, faujasite based adsorbents are used in this process. Recently, we discovered that MIL-47 is a very effective adsorbent for the liquid phase separation of para-xylene from the other xylene isomers [1]. MIL-47 is a porous terephthalate built from infinite chains of V4O6 octahedra, held together by the dicarboxylate groups of the terephthalate linkers [2]. In this way a 3-dimensional microporous framework with 1-dimensional diamond shaped channels with free internal diameter of about 0.85 nm is formed.

In the present work, adsorption and separation of xylene isomers on a similar structure, MIL-53, was investigated. Low coverage adsorption properties were determined using gas phase pulse chromatography, vapor phase adsorption isotherms were obtained by gravimetry and the separation of C8 alkylaromatics (o-xylene, p-xylene, m-xylene and ethyl benzene) was evaluated by performing breakthrough experiments.

Zero coverage adsorption enthalpies were found to be comparable to those obtained on MIL-47. However, adsorption isotherms showed completely different behavior. All C8 alkylaromatics have 2-step isotherms and show hysteresis. This is attributed to breathing of the MIL-53 framework, as reported before by Llewellyn et al [3]. This breathing effect strongly influences the separation of the C8 alkylaromatics in breakthrough conditions.

[1] 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. F. M.; De Vos, D. E. Angewandte Chemie-International Edition 2007, 46, (23), 4293-4297.

[2] Barthelet, K.; Marrot, J.; Riou, D.; Férey, G. Angewandte Chemie - International Edition 2002, 41, (2), 281-284.

[3] Llewellyn PL, Bourrelly S, Serre C, Filinchuk Y, Ferey G, How hydration drastically improves adsorption selectivity for CO2 over CH4 in the flexible chromium terephthalate MIL-53, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, 45(46), 7751-7754, 2006.