(489c) Separation of CO2 On Amino-MIL-53: Effect of Operating Conditions

The affinity of adsorbents for CO2 can be improved by grafting of amines on surfaces of porous materials [1]. An example of such a material is amino-MIL-53, a Metal Organic Framework with one-dimensional pores, containing 2-amino terephtalic acid as organic linker [2]. This adsorbent has free standing amino groups pointing inwards the pores. Recently, we have demonstrated that amino-MIL-53 has an extremely large selectivity for CO2 over methane [3]. Adsorption isotherms of CO2 show 2 steps, caused by the flexibility of the framework. At low CO2 pressure, the pores of the material contract as a result of the strong interaction with CO2. Only at pressures above 10 bar (at 30°C), the pores reopen. Methane is nearly not adsorbed at 30°C at pressures below 5 bar. It was demonstrated that this material show extra-ordinary separation performance for CO2 / CH4 mixtures. In the present work, the adsorption properties of amino-MIL-53 were studied in closer detail. Binary adsorption isotherms of CO2 / CH4 were measured at 0.1, 1, 2, 5, 10, 15, 20 and 25 bar, at temperatures between 30°C and 110°C. Even at high pressure, the material shows high selectivity for CO2, almost excluding CH4 from adsorbing (Figure 1, left). The steepness of the breakthrough profiles is extremely large, pointing at very high selectivity and very good mass transfer. A peculiar property of amino-MIL-53 is that the breakthrough profile of CH4 shows two steps in presence of CO2 (Figure 1, right, bottom), even enhancing product purity. The occurrence of this step is attributed to breathing of the framework. While the CO2 front is propagating through the column, it causes contraction of the framework above a critical amount adsorbed. Due to the reduction in pore volume, the amount adsorbed suddenly decreases and pre-adsorbed molecules are evacuated from the pores. As a result, the CH4 exit flow increases. The occurrence of this effect depends on pressure, temperature and composition of the gas mixture. For example, at 30 °C and 10 bar, a normal breakthrough profile showing the usual roll-up is observed at a CO2 fraction of 0.5, while a second step is observed at a CO2 fraction of 0.3 (Figure 1, right). Typically, the second step in the breakthrough curve disappears with increasing temperature and pressure. The stability of the open and closed forms is related to temperature and pressure [4]; this phenomenon will analyzed in this paper. Besides, this paper discusses separation of CO2 from other gases such as N2, H2, CO and the effect of water. It is demonstrated that also for synthesis gas mixtures, extremely large selectivities are obtained. Acknowledgements Senter Novem is gratefully acknowledged for financial support through the project EOSLT-04008 (Lange termijn EOS-onderzoeksprogramma). References: [1] (a) Zelenak, V.; Badaninicova, M.; Halamova, D.; Cejka, J.; Zukal, A.; Murafa, N.; Goerigk, G. Chem. Eng. J. 2008, 144, 336. (b) Xu, X.; Song, C.; Andresen, J.M.; Miller, B.G.; Scaroni, A.W. Micropor. Mesopor. Mater. 2003, 16, 1463. (c) Huang, H.Y.; Yang, R.T. Ind. Eng. Chem. Res. 2003, 42, 2427. (d) Hiyoshi, N.;i Yogo, K.; Yashima, T. Micropor. Mesopor. Mater. 2005, 84, 357. (e) An, J.; Geib, S.J.; Rosi N.L., JACS 2010, 132, 38,. (f) Chew, T.L.; Ahmad A.L., Bhatia S., Adv. Coll. & Interface Sci. 2010, 153, 43. (g) Belmabkhout, Y.; Serna-Guerrero, R.; Sayari, A., IE&C 2010, 49, 359. (h) Yaghi, O.M.; O'Keeffe, M.; Ockwig, N.W.; Chae, H.K.; Eddaoudi, M.; Kim, J. Nature, 2003, 423, 705. (i) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O''Keeffe, M.; Yaghi, O.M. Science, 2002, 295, 469. (j) Rowsell, J.L.C.; Yaghi, O.M. J. Am. Chem. Soc. 2006, 128, 1304. (k) Arstad, B.; Fjellvag, H.; Kongshaug, K.O.; Swang, O.; Blom, R. Adsorption, 2008, 14, 755 (i) Devic et al, JACS, 2010, 132, 1127. [2] Gascon J.,Aktay, U., Hernandez-Alonso, M.D., van Klink, G.P.M., Kapteijn, F. J. Catal., 2009, 261, 75. [3] Couck, S., Denayer, J.F.M., Baron, G.V., Rémy, T., Gascon, J., Kapteijn, F., JACS, 131, 6326?6327, 2009 [4] Coudert, F.X.; Mellot-Draznieks, C.; Fuchs, A.H.; Boutin, A., JACS, 2009, 131, 11329.