Metal-Organic Frameworks from Group 4 Metals and 2,5-Dihydroxyterephthalic Acid: Reinvestigation, New Structure, and Challenges Toward Gas Storage and Separation

Abstract

The reactions of group 4 metals (Ti, Zr, and Hf) and 2,5-dihydroxyterephthalic acid (H(4)dobdc) under solvothermal conditions have been systematically explored, and their major crystalline phases have been investigated by single-crystal diffractions. Ti(IV) forms a layered framework [Ti-2(Hdobdc)(3)] where honeycomb-type sheets are interconnected through strong hydrogen bonding. Various gases are reversibly adsorbed within the straight one-dimensional channels decorated with polar O atoms, and H-2 and CO2 show relatively high isosteric heats of adsorption at 6.6 and 29.4 kJ/mol, respectively. Zr(IV)- and Hf(IV)-based MOFs have also been synthesized using the same ligand and are isostructural with the formula (H3O)(x)[M(dobdc)(bz)(x)] (M = Zr or Hf). They have a unique, nonoxo-trinuclear building block that forms a polyhedral network of 6-connected topology. Unlike the two-dimensional net of Ti, the Zr and Hf metal-organic frameworks are hydrothermally stable as unambiguously shown by variable-temperature X-ray diffraction.