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

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 À 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 À COOH groups of the linker by titration with LiOH. Comparison of the pK a values with three UiO-66 derivatives confirms distinct pK a value ranges and trends for the different acidic protons. Furthermore, the preparation of ZrÀ CAU-61 membranes and first results on permeation of dyes and ions in aqueous solutions are presented.