Metal Insertion in a Methylamine-Functionalized Zirconium Metal-Organic Framework for Enhanced Carbon Dioxide Capture

The reaction of ZrCl₄ with 2',3',5',6'-tetramethylamino-p-terphenyl-4,4″-dicarboxylic acid (H₂tpdc-4CH₂NH₂·3HCl) in the presence of NaF affords Zr₆O₄(OH)_2.1F_1.9(tpdc-4CH₂NH₂·3HCl)₆ (1), which is a new member of the Zr₆O₄(OH)₄(dicarboxylate linker)₁₂ or UiO-68 family, and exhibits high porosity with BET and Langmuir surface areas of 1910 m²/g and 2220 m²/g, respectively. Remarkably, fluoride ion incorporation in the zirconium clusters results in increased thermal stability, marking the first example of enhancement in the stability of a UiO framework by this defect-restoration approach. Although material 1 features four alkylamine groups on each organic linker, the framework does not exhibit the high CO₂ uptake that would be expected for reaction between CO₂ and the amine groups to form carbamic acid or ammonium carbamate species. The absence of strong CO₂ adsorption can likely be attributed to protonation at some of the amine sites and the presence of counterions. Indeed, exposure of material 1 to acetonitrile solutions of the organic bases 1,8-bis(dimethylamino)naphthalene (DMAN) or trimethylamine, affords a partially deprotonated material, which exhibits enhanced CO₂ uptake. Exposure of basic amine sites also facilitates the postsynthetic chelation of copper(I) ([Cu(MeCN)₄]·CF₃SO₃) to yield material 2 with an enhanced CO₂ uptake of 4 wt % at 0.15 bar, which is double that of the parent framework 1.