A flexible pro-porous coordination polymer: non-conventional synthesis and separation properties towards CO2/CH4 mixtures

The novel coordination polymers [Cu(Hoxonic)(H2O)](n) (1) and [Cu(Hoxonic)(bpy)(0.5)](n)*1.5n H2O (2*H2O) (H3oxonic: 4,6-dihydroxy-1,3,5-triazine-2-carboxylic acid; bpy: 4,4'-bipyridine) have been isolated and structurally characterised by ab initio X-ray powder diffraction. The dense phase 1 contains 1D zig-zag chains in which Hoxonic dianions bridge square-pyramidal copper(II) ions, apically coordinated by water molecules. On the contrary 2*H2O, prepared by solution and solventless methods, is based on 2D layers of octahedral copper(II) ions bridoed by Hoxonic ligands, further pillared by bpy spacers. The resulting pro-porous 3D network possesses small hydrated cavities. The reactivity, thermal, magnetic and adsorptive properties of these materials have been investigated. Notably, the adsorption studies on 2 show that this material possesses unusual adsorption behaviour. Indeed, guest uptake is facilitated by increasing the thermal energy of both the guest and the framework. Thus, neither N2 at 77 K nor CO2 at 195 K are incorporated, and CH4 is only minimally adsorbed at 273 K and high pressures (0.5 mmol/g at 2500 kPa). By contrast, CO is readily incorporated at 273 K (up to 2.5 mmol/g at 2500 kPa). The selectivity of 2 towards CO2 over CH4 has been investigated by means of variable-temperature zero coverage adsorption experiments and measurement of breakthrough curves of CO2/CH4 mixtures. The results show the highly selective incorporation of CO2 in 2, which can be rationalised on the basis of the framework flexibility and polar nature of its voids.