Acidic Properties of Known and New COOH-Functionalized M(IV) Metal-Organic Frameworks

Abstract

Abstract Herein, we report two new COOH‐functionalized metal‐organic frameworks (MOFs) of composition [M 6 O 4 (OH) 6 (PMA) 2 (H 2 PMA)]×H 2 O, M=Zr, Hf), denoted CAU‐61, synthesized by using pyromellitic acid (H 4 PMA), a tetracarboxylic acid, as the linker and acetic acid as the solvent. The structure was determined from powder X‐ray diffraction data and one‐dimensional inorganic building units are connected through tetracarboxylate as well as dicarboxylate linker molecules, resulting in highly stable microporous framework structures with limiting and maximum pore diameter of ∼3.6 and ∼5.0 Å, respectively, lined with −COOH groups. Thermal stabilities of up to 400 °C in air, chemical stability in water at pH 1 to 12 and water uptake of 17 mol/mol prompted us to study the proton exchange of the μ 2 ‐OH, μ 3 ‐OH of the IBU and −COOH groups of the linker by titration with LiOH. Comparison of the p K a values with three UiO‐66 derivatives confirms distinct p K a value ranges and trends for the different acidic protons. Furthermore, the preparation of Zr−CAU‐61 membranes and first results on permeation of dyes and ions in aqueous solutions are presented.