Complex Impedance Formalism for Probing Dehydration, Dehydroxylation, and Rehydroxylation Dynamics of UiO-66-Based Metal–Organic Frameworks

Abstract

It is known that the UiO-66 metal–organic framework (MOF) undergoes dehydration of physisorbed water and dehydroxylation of the zirconium clusters from RT to ∼300 °C, while the porous structure remains intact. These two processes are typically studied by thermogravimetric analysis and its derivative method (TGA/DTG), as well as differential scanning calorimetry (DSC). Here, we probe the same phenomena using temperature-dependent complex impedance spectroscopy. Peaks observed in the AC conductivity and in the real and imaginary parts of the dielectric permittivity (σAC, ε′, and ε″) are due to thermally accelerated proton (Grotthuss) hopping competing with the loss of proton carriers during guest removal and dehydroxylation up to 350 °C. Meanwhile, peaks in the loss tangent (tan δ) and phase shift (Θ) identify the temperature at which dielectric-to-heat conversion is maximized. Additionally, peaks in the imaginary part of the impedance (−Z″) and in the real part of the electric modulus (M′) mark the temperature at which the material exhibits its highest insulating character. The MOFs examined include UiO-66 with the 1,4-benzenedicarboxylate linker and UiO-66-Py with the 2,5-pyridinedicarboxylate linker, the latter further modified through trifluoroacetic acid modulation, 1-bromopropane functionalization, and optional intermediate heat treatment. Impedance measurements under cooling reveal composition-dependent rehydroxylation behavior, with the conductivity recovery as early as at 250 °C in some Py-based derivatives vs flat response in pristine UiO-66. Overall, our work provides a comprehensive analysis of several complex electrical quantities and their physical interpretation, offering complementary insights into the dehydration, dehydroxylation, and rehydroxylation dynamics of UiO-66-based MOFs.