Functionalized modulators in imine-linked covalent organic frameworks (COFs)

Abstract

Covalent Organic Frameworks (COFs) are porous materials with high surface areas, making them interesting for a large variety of applications including energy storage, chemical sensing, and gas separation. In gas separation and sensing, functionalization beyond the COF linkage can result in selective COF-gas interactions, tailoring the properties for the desired application. However, not all functional groups are compatible with the synthetic conditions needed for COF formation. Modulators, which are typically monovalent building blocks that would terminate the reaction, have been shown to maintain or even improve the COF porosity by slowing down the reaction kinetics. Herein, we report on a series of several para-functionalized (OMe, Me, F, Cl, CF3, NO2) amine modulators to introduce additional functionality into the framework to study the selective CO2/N2 gas separation under flue gas conditions. Thorough characterization of the modulated COFs showed that the modulators are located on the outside of the polymeric sheet and get replaced by benzidine molecules, favoring a regular network formation over a homogeneous modulator distribution. The fractions of eventually incorporated modulator vary per functional group between 2.1% (NO2-modulated COF) and 8.9% (Me-modulated COF) while maintaining high BET surface areas (>1800 m2/g). It was found that all modulated COFs adsorb moderate quantities of CO2 and show comparable CO2/N2 IAST selectivity values under flue gas conditions. A higher number of functional groups in the framework was shown to enhance the IAST selectivity.