Tackling the Defect Conundrum in UiO-66: A Mixed-Linker Approach to Engineering Missing Linker Defects

Abstract

© 2017 American Chemical Society. Over the past decade, the zirconium-terephthalate UiO-66 has evolved into one of the most intensely studied metal-organic frameworks (MOFs) to date. Among the most fascinating and pervasive features of this material are defects, and their influence on a multitude of its properties. However, the simultaneous occurrence of two defect types, missing linkers and missing nodes, limits the extent to which certain material properties can be accurately matched to the framework's defect structure. In this contribution, we present a strategy to unequivocally create missing linker defects in UiO-66, by first synthesizing terephthalate frameworks doped with a thermolabile linker, trans-1,4-cyclohexane-dicarboxylate (cdc), followed by postsynthetic thermal decomposition of the latter. Characterization of the mixed-linker materials before and after cdc removal by powder X-ray diffraction, thermogravimetric analysis, N2 physisorption, and NMR spectroscopy confirmed a homogeneous distribution of cdc, and thus also of the formed defects, throughout the materials. The UiO-66 structure is shown to tolerate up to 4.3 missing linker defects per [Zr6O4(OH)4]12+ node, with higher defect densities compromising the framework's structural integrity and porosity. Importantly, no increase in specific surface area was seen after additional missing linker defects were formed, providing compelling evidence that high porosity often observed in modulated UiO-66 samples should rather be attributed to missing cluster defects.