A Two Step Postsynthetic Modification Strategy: Appending Short Chain Polyamines to Zn-NH2-BDC MOF for Enhanced CO2 Adsorption

Functionalizing metal–organic frameworks (MOFs) with amines is a commonly used strategy to enhance their performance in CO2 capture applications. As such, in this work, a two-step strategy to covalently functionalize NH2-containing MOFs with short chain polyamines was developed. In the first step, the parent MOF, Zn4O(NH2-BDC)3, was exposed to bromoacetyl bromide (BrAcBr), which readily reacts with pendant -NH2 groups on the 2-amino-1,4-benzenedicarboxylate (NH2-BDC2–) ligand. 1H NMR of the digested MOF sample revealed that as much as 90% of the MOF ligands could be functionalized in the first step. Next, the MOF samples 60% of the ligands functionalized with acetyl bromide, Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, was exposed to several short chain amines including ethylenediamine (ED), diethylenetriamine (DETA), and tris(2-aminoethyl)amine (TAEA). Subsequent digested 1H NMR analysis indicated that a total of 30%, 28%, and 19% of the MOF ligands were successfully grafted to ED, DETA, and TAEA, respectively. Next, the CO2 adsorption properties of the amine grafted MOFs were studied. The best performing material, TAEA-appended-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, exhibits a zero-coverage isosteric heat of CO2 adsorption of −62.5 kJ/mol, a value that is considerably higher than the one observed for the parent framework, −21 kJ/mol. Although the boosted CO2 affinity only leads to a slight increase in the CO2 adsorption capacity in the low-pressure regime (0.15 bar), which is of interest in postcombustion carbon dioxide capture, the CO2/N2 (15/85) selectivity at 313 K is 143, a value that is ∼35 times higher than the one observed for Zn4O(NH2-BDC)3, 4.1. Such enhancements are attributed to accessible primary amines, which were grafted to the MOF ligand. This hypothesis was further supported via in situ DRIFTS measurements of TAEA-Ac-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8 after exposure to CO2, which revealed the chemisorption of CO2 via the formation of hydrogen bonded carbamates/carbamic acid and CO2δ- species; the latter are adducts formed between CO2 and [amineH]+Br– salts that are produced during the amine grafting step.