Carbon Dioxide Capture by Using Flexible Coordination Polymers

Abstract

We have assembled two new diamondoid networks, SNU-M21 and SNU-M22, from a flexible organic ligand TCM4- and the Ni(II) macrocyclic complexes incorporating butyl and octyl pendants, respectively, and measured their gas sorption properties together with those of SNU-M20 which has no alkyl pendant. The X-ray structures of SNU-M20, SNU-M21 and SNU-M22 show 8-fold, 5-fold, and 3-fold interpenetrated network, respectively, demonstrating that the degree of network interpenetration decreases as the length of the dangling alkyl pendant increases. All Porous Coordination Polymers (PCPs) hardly adsorb N2, H2, and O2 gases even at 77 K as well as CH4 at 195 K, but they selectively adsorb high amounts of CO2 at 195 K, revealing for the first time that CO2 molecules can open the windows of even the highly interpenetrated networks. SNU-M20 and SNU-M21 show step-wise CO2 adsorption/desorption isotherms at 195 K, with a gate-opening phenomenon in SNU-M20, while SNU-M22 does not show step-wise CO2 adsorption curve since the long octyl chains reduce the flexibility of the networks. Among three PCPs, most highly interpenetrated SNU-M20 with no alkyl pendants shows the highest uptake capacity at 195 K and the highest adsorption selectivity for CO2 over N2, H2 and CH4 gases. On the other hand, SNU-M21 having butyl pendants shows the highest CO2 uptake capacity at 298 K and the highest isosteric heat of the CO2 adsorption. For landfill gas separation using vacuum-swing adsorption process, SNU-M21 exhibits the highest working capacity and regenerability. All three PCPs adsorb high amount of CO2 while hardly adsorbing H2 at 298 K under high pressures (45 atm), exhibiting that they are good candidate materials for pre-combustion CO2 capture.