Synthesis of Highly Porous Coordination Polymers with Open Metal Sites for Enhanced Gas Uptake and Separation

Metal-containing amorphous microporous polymers are an emerging class of functional porous materials in which the surface properties and functions of polymers are dictated by the nature of the metal ions incorporated into the framework. In an effort to introduce coordinatively unsaturated metal sites into the porous polymers, we demonstrate herein an aqueous-phase synthesis of porous coordination polymers (PCPs) incorporating bis( o -diiminobenzosemiquinonato)-Cu(II) or -Ni(II) bridges by simply reacting hexaminotriptycene with CuSO 4 ·5H 2 O [Cu(II)-PCP] or NiCl 2 ·6H 2 O [Ni(II)-PCP] in H 2 O. The resulting polymers showed surface areas of up to 489 m 2 g –1 along with a narrow pore size distribution. The presence of open metal sites significantly improved the gas affinity of these frameworks, leading to an exceptional isosteric heat of adsorption of 10.3 kJ·mol –1 for H 2 at zero coverage. The high affinities of Cu(II)- and Ni(II)-PCPs toward CO 2 prompted us to investigate the removal of CO 2 from natural and landfill gas conditions. We found that the higher affinity of Cu(II)-PCP compared to that of Ni(II)-PCP not only allowed for the tuning of the affinity of CO 2 molecules toward the sorbent, but also led to an exceptional CO 2 /CH 4 selectivity of 35.1 for landfill gas and 20.7 for natural gas at 298 K. These high selectivities were further verified by breakthrough measurements under the simulated natural and landfill gas conditions, in which both Cu(II)- and Ni(II)-PCPs showed complete removal of CO 2 . These results clearly demonstrate the promising attributes of metal-containing porous polymers for gas storage and separation applications.