Nanoporous covalent organic polymers incorporating Tröger's base functionalities for enhanced CO2 capture

Abstract

The CO₂ uptake capacity and CO₂/N₂ selectivity of Tröger's base-bridged nanoporous covalent organic polymers (TB-COPs) were investigated. The TB-COPs were synthesized by reacting the terminal amines of tetrahedral monomers – namely, tetraanilyladamantane and tetraanilylmethane – with dimethoxymethane in a one-pot reaction under relatively mild conditions. Interestingly, these two tetrahedral monomers formed nanoporous polymers with substantially different surface areas. While the polymer resulting from the Trögerization of the tetraanilyladamantane monomer (TB-COP-1) exhibited a high surface area of 1340 m² g⁻¹, that from the tetraanilylmethane monomer (TB-COP-2) was found to be only 0.094 m² g⁻¹. This unusual phenomenon can be explained by the proximity of the amino moieties to each other within the monomeric unit. A shorter distance between the amino groups enables intramolecular as well as intermolecular cyclization, thus resulting in a much lower porosity. TB-COP-1 exhibited significant CO₂ uptake capacities of up to 5.19 and 3.16 mmol g⁻¹ at 273 and 298 K under ambient pressure, and CO₂/N₂ selectivities of 79.2 and 68.9 at 273 and 298 K at 1 bar for a gas mixture of CO₂ : N₂ at a ratio of 0.15 : 0.85. It is noteworthy that TB-COP-1 showed remarkable selectivity retention when increasing the temperature from 273 to 298 K.