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

Herein, we report two new COOH-functionalized metal-organic frameworks (MOFs) of composition [M6O4(OH)(6)(PMA)(2)(H(2)PMA)]xH(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 & SIM;3.6 and & SIM;5.0 & ANGS;, respectively, lined with -COOH groups. Thermal stabilities of up to 400 & DEG;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 & mu;(2)-OH, & mu;(3)-OH of the IBU and -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 Zr-CAU-61 membranes and first results on permeation of dyes and ions in aqueous solutions are presented.