Directing the Structural Features of N 2 ‐Phobic Nanoporous Covalent Organic Polymers for CO 2 Capture and Separation

A family of azo‐bridged covalent organic polymers (azo‐COPs) was synthesized through a catalyst‐free direct coupling of aromatic nitro and amine compounds under basic conditions. The azo‐COPs formed 3D nanoporous networks and exhibited surface areas up to 729.6 m 2  g −1 , with a CO 2 ‐uptake capacity as high as 2.55 mmol g −1 at 273 K and 1 bar. Azo‐COPs showed remarkable CO 2 /N 2 selectivities (95.6–165.2) at 298 K and 1 bar. Unlike any other porous material, CO 2 /N 2 selectivities of azo‐COPs increase with rising temperature. It was found that azo‐COPs show less than expected affinity towards N 2 gas, thus making the framework “N 2 ‐phobic”, in relative terms. Our theoretical simulations indicate that the origin of this unusual behavior is associated with the larger entropic loss of N 2 gas molecules upon their interaction with azo‐groups. The effect of fused aromatic rings on the CO 2 /N 2 selectivity in azo‐COPs is also demonstrated. Increasing the π‐surface area resulted in an increase in the CO 2 ‐philic nature of the framework, thus allowing us to reach a CO 2 /N 2 selectivity value of 307.7 at 323 K and 1 bar, which is the highest value reported to date. Hence, it is possible to combine the concepts of “CO 2 ‐philicity” and “N 2 ‐phobicity” for efficient CO 2 capture and separation. Isosteric heats of CO 2 adsorption for azo‐COPs range from 24.8–32.1 kJ mol −1 at ambient pressure. Azo‐COPs are stable up to 350 °C in air and boiling water for a week. A promising cis / trans isomerization of azo‐COPs for switchable porosity is also demonstrated, making way for a gated CO 2 uptake.