Facile synthesis of MOFs with uncoordinated carboxyl groups for selective CO2 capture via postsynthetic covalent modification

Abstract

A postsynthetic covalent strategy involving dual-acyl chloride has been developed to introduce uncoordinated carboxyl groups into amine containing metal–organic frameworks (MOFs). The carboxyl group functionalized MOFs have been characterized by various techniques, including X-ray diffraction patterning, scanning electron microscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance, thermal gravimetric analysis, and gas adsorption. Results clearly indicated uncoordinated carboxyl groups were successfully grafted to the MIL-101(Cr)–NH₂ framework. In addition, most of the amine groups (>80%) were grafted with carboxyl groups, which indicates this method is very effective. The thermal stability and adsorption selectivity of CO₂/N₂ were substantially enhanced, albeit the BET surface areas and total pore volumes were reduced. These observations could be explained by the effect of elimination of macropores in the framework due to the projecting of new functional groups in pore apertures. Here the successful fabrication of a MOF with uncoordinated carboxyl groups provides the possibility of efficiently modifying other MOFs.