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

Abstract

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 [Mg₂(L)(H₂O)(DMA)]·DMA (1), [Ca₄(L)₂(DMA)₃] (2), [Ca₄(L)₂(H₂O)₂(DMA)₂]·(3DMA) (3) and [Sr₄(L)₂(DMF)₄]·(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 N₂ over CO₂ molecules, therefore leading to a good CO₂/N₂ separation selectivity. Both Ca-based MOFs were very stable up to 98% relative humidity, while Mg- and Sr-based MOFs were much less stable.