Herein, a post-synthetic modification strategy is used to covalently graft polyamines, including ethylenediamine (ED), diethylenetriamine (DETA), tris(2-aminoethyl)amine (TAEA), and polyethyleneimine (PEI) to the amino-ligand inside of a Cr-MOF, NH2-Cr-BDC, for post-combustion carbon capture applications. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and ion chromatography (IC) reveal that approximate to 45% of the MOF ligands are grafted with polyamines. Next, assessment of CO2 uptake, CO2/N-2 selectivity, isosteric heats of CO2 adsorption, separation performance during humid CO2/N-2 (15/85) breakthrough experiments, and cyclability, reveals an enhanced performance for the polyamine-containing composites and the following performance trend: NH2-Cr-BDC