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

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-BDC^2-) 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 CO₂^δ- species; the latter are adducts formed between CO₂ and [amineH]⁺Br^- salts that are produced during the amine grafting step.