(248a) Cu-Btc: a Shape Selective Mof Material for the Adsorptive Separation of N- and Iso-Alkanes

Metal organic frameworks (MOFs), a relatively new class of ordered microporous material, are formed by a network of transition metal ions linked by bridging organic ligands. In this way a crystalline, 3D open framework with large pores and large pore volume can be formed. These materials have recently gained a lot of interest due to their high storage capacities for hydrogen and methane. Beside the storage of hydrogen and methane, these metal organic frameworks bear a number of characteristics which make them attractive for other adsorption applications. They are relative simple to synthesise, they have exceptionally high specific surface areas and pore volumes. Altering the organic linkers allows to vary the size of the pores without affecting the underlying topology and to introduce functionalities quite easily. Despite these interesting characteristics, there are only very few papers discussing the experimental adsorption properties and the potential use of metal organic frameworks in adsorptive separation processes (Wang et al., 2002). In our study, we determined adsorption properties of the copper (II) benzene-1,3,5-tricarboxylate (CuBTC) metal organic framework using the puls chromatografic technique. Experiments were performed at temperatures between 298 K and 493 K. Adsorption properties of C1-C8 n- and iso-alkanes, aromatics (benzene, toluene and xylene), CO2, CO, H2, SO2 and NO were determined. Using the van ?t Hoff equation, adsorption enthalpies and adsorption entropies at low loading were calculated. Considering the pore size (1.1-1.2 nm) we found unexpected high adsorption enthalpies for linear and branched alkenes. Adsorption enthalpies were much higher as compared to zeolites with similar pore size, which results from the different chemical composition of the MOF materials containing many organic functions. Unexpected preference of adsorption for linear over branched alkanes was observed proving the shape selective properties of the MOF materials.

Wang Q.M., Shen D., Bulow M., Lau M.L., Deng S., Fitch F.R., Lemcoff N.O. and Semanscin J. (2002) Metallo-organic molecular sieve for gas separation and purification. Microporous and mesoporous materials 55, 217-230