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

Joeri Denayer1, Vincent Finsy1, Dirk De Vos2, and Gino V. Baron1. (1) Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, B-1050, Belgium, (2) Center for Surface Science and Catalysis, Katholieke Universiteit Leuven, Leuven, Belgium

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.