Separation of Oxygen and Nitrogen by Porous Cyanometallates

The adsorption-based separation in porous solids takes place through steric, kinetic, or equilibrium effect selectivity. In this contribution the oxygen-nitrogen separation by four porous frameworks representative of cyanometallates was studied by inverse gas chromatography. The following materials were considered: Cd3[Co(CN)6]2 (cubic), Zn3[Co(CN)6]2 (rhombohedral), Zn3K2[Fe(CN)6]2 (rhombohedral) and Co[Fe(CN)5NO] (cubic). Chromatographic separation profiles from gases mixtures using columns of these materials were recorded. For columns prepared from rhombohedral zinc hexacyanocolbaltate(III) excellent separation of O2 and N2 was observed. Such behavior was attributed to a kinetic-based selectivity related to the size and shape for the pore windows of this material. The porous framework of this zinc phase is formed by ellipsoidal cavities (12.5x9x8 angstrom) communicated by elliptical windows of 5 angstrom. For Cd3[Co(CN)6]2 and Co[Fe(CN)5NO] also kinetic-based selectivity was observed while for Zn3K2[Fe(CN)6]2 the K+ ion located close to the cavity windows hinders the porous windows accessibility for nitrogen and oxygen molecules. All the samples to be studied were characterized from X-ray diffraction, infrared spectroscopy, termogravimetric and adsorption data.