Selective CO2 adsorption in water-stable alkaline-earth based metal-organic frameworks

Four novel metal-organic frameworks (MOFs) built from alkaline-earth metal ions and the flexible tetrahedral carboxylate ligand tetrakis[4-(carboxyphenyl)oxamethyl]methane acid (H4L) were synthesized using solvothermal methods. A variety of three-dimensional frameworks were obtained when employing different alkaline earth ions with the formula [Mg-2(L)(H2O)(DMA)]center dot DMA (1), [Ca-4(L)(2)(DMA)(3)] (2), [Ca-4(L)(2)(H2O)(2)(DMA)(2)]center dot(3DMA) (3) and [Sr-4(L)(2)(DMF)(4)]center dot(2DMF) (4) reflecting the variation in the ionic radius of alkaline-earth ions as well as the key role of the synthetic conditions used. By removing the guest molecules, a framework shrinking was observed driven by the structural flexibility of the H4L ligand. This resulted in large diffusional resistances towards N-2 over CO2 molecules, therefore leading to a good CO2/N-2 separation selectivity. Both Ca-based MOFs were very stable up to 98% relative humidity, while Mgand Sr-based MOFs were much less stable.