Carboxyl-, Hydroxyl-, and Nitro-Functionalized Porous Polyaminals for Highly Selective CO₂ Capture

Carboxyl-, hydroxyl-, and nitro-functionalized porous polyaminals PAN-CP, PCN-HP, and PAN-NP are synthesized by using the commercially available 4-carboxylbenzaldehyde, 4-hydroxylbenzaldehyde, and 4-nitrobenzaldehyde as monomers to polymerize with melamine, respectively, by the one-step solution polymerization technique. The effects of functional substituents on CO2 adsorption energy and porosity parameters are studied. Compared with the unsubstituted polymer (PAN-P), PAN-CP and PAN-HP exhibit the increased BET surface areas, total pore volumes, and CO2 adsorption enthalpies, resulting in the simultaneously improved CO2 adsorption capacities and selectivities over other gases. For instance, at 273 K and 1 bar, PAN-HP exhibits 21.6% higher CO2 uptake than PAN-P, while the breakthrough experiment demonstrates that its CO2/N2 selectivity (103.8) is also twice that of PAN-P (51.8). Particularly, the selectivities of CO2/N2 and CO2/CH4 in the nitro-decorated PAN-NP reach 151.3 and 21.8, respectively, which can compete for the highest values with the other porous organic polymers. The facile synthesis method and excellent adsorption/separation properties endow the functionalized PANs with potential applications in CO2 capture.