Role of Surface Phenolic-OH Groups in N-Rich Porous Organic Polymers for Enhancing the CO2 Uptake and CO2/N-2 Selectivity: Experimental and Computational Studies

Design and successful synthesis of phenolic-OH and amine-functionalized porous organic polymers as adsorbent for postcombustion CO2 uptake from flue gas mixtures along with high CO2/N-2 selectivity is a very demanding research area in the context of developing a suitable adsorbent to mitigate greenhouse gases. Herein, we report three triazine-based porous organic polymers TrzPOP-1, -2, and -3 through the polycondensation of two triazine rings containing tetraamine and three dialdehydes. These porous organic polymers possess high Brunauer-Emmett-Teller (BET) surface areas of 995, 868, and 772 m2 g-1, respectively. Out of the three materials, TrzPOP-2 and TrzPOP-3 contain additional phenolic-OH groups along with triazine moiety and secondary amine linkages. At 273 K, TrzPOP-1, -2, and -3 displayed CO2 uptake capacities of 6.19, 7.51, and 8.54 mmol g(-1), respectively, up to 1 bar pressure, which are considerably high among all porous polymers reported till date. Despite the lower BET surface area, TrzPOP-2 and TrzPOP-3 containing phenolic-OH groups showed higher CO2 uptakes. To understand the CO2 adsorption mechanism, we have further performed the quantum chemical studies to analyze noncovalent interactions between CO2 molecules and different polar functionalities present in these porous polymers. TrzPOP-1, -2, and -3 have the capability of selective CO2 uptake over that of N-2 at 273 K with the selectivity of 61:1, 117:1, and 142:1 by using the initial slope comparing method, along with 108.4, 140.6, and 167.4 by using ideal adsorbed solution theory (IAST) method, respectively. On the other hand, at 298 K, the calculated CO2/N-2 selectivities in the initial slope comparing method for TrzPOP-1, -2, and -3 are 27:1, 72:1, and 96:1, whereas those using IAST method are 42.1, 75.7, and 94.5, respectively. Cost effective and scalable synthesis of these porous polymeric materials reported herein for selective CO2 capture has a very promising future for environmental clean-up.