(339d) Can Compressing MOFs Lead to Increased Separation Properties ? Gas Adsorption on Mechanically Constrained Soft Porous Crystals

Gas separation processes involving adsorption may have advantages over other separation methods (simplicity, environmental, cost) although one drawback still remains which is the need for increased selectivity. A simple way to improve the selectivity of a separation process may be to increase the pore confinement by applying an external pressure to compress the porous solid to induce molecular sieving. Indeed, some MOFs can be considered as soft porous crystals whose structure transits when they are exposed to stimuli such as adsorbed molecules¹, pressure² or temperature. This structural flexibility, leading to a change in the pore size, strongly influences the selectivity of the adsorbent for some gas mixtures. For example, the MIL-53 solid in its narrow pore form exhibits a very good CO₂/CH₄ selectivity, which is lost when the structure switches to the large pore form with the increase of gas pressure³. By maintaining the narrow pore form via mechanical compression, it may be possible to control its selectivity over a wider range of pressure. Releasing the mechanical pressure will lead to the large pore form which could be easier to regenerate.

For this purpose, we have developed a novel methodology to tune the adsorption behavior of mechanically responsive materials by coupling the effects of internal gas pressure and external mechanical pressure. In order to pilot the structural flexibility of the adsorbent during the gas adsorption, we built an experimental device to apply a mechanical pressure (up to 25 tons) on the porous material via a uniaxial press system which equally allows gas adsorption up to 15 bars.

Promising applications of this concept will be explored in this communication including mechanically induced desorption, thus avoiding thermal treatment, and pressure induced gate opening.

¹ S. Bourrelly, P.L. Llewellyn, C. Serre, F. Millange, T.Loiseau, G. Ferey, J. Amer. Chem. Soc., 2005, 127(39) 13519-13521.

² I. Beurroies, M. Boulhout, P.L. Llewellyn, B. Kuchta, G. Ferey, C. Serre, R. Denoyel, Angew. Chem. Int. Ed., 2010, 49(41), 2010, 7526-7529.

³ L. Hamon, P.L. Llewellyn, T. Devic, A. Ghoufi, G. Clet, V. Guillerm, G.D. Pirngruber, G. Maurin, C. Serre, G. Driver, W. Van Beek, E. Jolimaitre, A. Vimont. M. Daturi, G. Ferey, J. Amer. Chem. Soc., 2009, 131(47) 17490-17499.