Adsorption of CO2 on a micro-/mesoporous polyimine modified with tris(2-aminoethyl)amine

Abstract

Amine-modified sorbents are relevant to the capturing of dilute carbon dioxide from gas mixtures, and micro-/mesoporous polymers are promising substrates due to their rich chemistry. Here, we prepared an aldehyde-rich polyimine with micro- and mesopores by a Schiff-base condensation of 1,3,5-tris(4-aminophenyl)benzene and 1,3,5-benzenetricarboxaldehyde using an excess of aldehyde. The micropores were crucial to the physisorption of CO₂, while the mesopores provided space for the post-modification with tris(2-aminoethyl)amine (tren) that induced the chemisorption of CO₂. The amine modified polymer showed a high uptake of CO₂ at low pressures (1.13 mmol g⁻¹ at 0.05 bar and 273 K) and a high estimated CO₂-over-N₂ selectivity (1.04 × 10³ at 273 K for 5 v%/95 v% CO₂/N₂ mixture). CO₂ both physisorbed and chemisorbed on the amine-modified polyimine, which we confirmed by studying the CO₂-amine chemistry using in situ FTIR spectroscopy and solid state ¹³C NMR spectroscopy. Carbamic acid formed during the chemisorption of CO₂, as the CO₂ reacted with the amine groups. Due to the formation of carbamic acid, the isosteric heat of adsorption was high, with values up to 80 kJ mol⁻¹ at a low coverage of CO₂. It appears that amine-modified porous polymers could be relevant to the removal of CO₂ from gas streams with low concentrations.