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

Abstract

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.

Geminal Coordinatively Unsaturated Sites on MOF‐808 for the Selective Uptake of Phenolics from a Real Bio‐Oil Mixture

“…Their structures were characterized by powder X‐ray diffraction (PXRD, Figure S1 in the Supporting Information), thermogravimetric analysis (TGA, Figure S2 in the Supporting Information), and N 2 sorption experiments (Figure S3 in the Supporting Information). UiO‐66 materials with up to four missing linkers per cluster are fairly common . Alternatively, besides the linkers, the nodes themselves may be missing.…”

An insight into the effect of azobenzene functionalities studied in UiO-66 frameworks for low energy CO2 capture and CO2/N2 membrane separation

“…This was also previously observed when synthesizing UiO-66 analogue MOFs by using a non-favorable ligand namely trans-1,4-cyclohexanedicarboxylate. 25 This reduced crystallinity might then also explain the lower thermal decomposition temperature with a higher azobenzene loading in the UiO-66 framework as observed through TGA analysis ( Fig. S8 in the ESI †).…”

High temperature expulsion of thermolabile groups for pore-space expansion in metal–organic frameworks

“…1 H NMR and 13 C NMR spectra were obtained using a Bruker NMR spectrometer operating at 400 MHz for 1 H and 101 MHz for 13 C. 1 H NMR spectra were referenced to the residual protio peaks at 2.50 ppm (d 6 -DMSO) or 7.27 ppm (CDCl 3 ). 13 C NMR spectra were referenced to the solvent peaks at 39.6 ppm (d 6 -DMSO) or 77.7 ppm (CDCl 3 ). For 1 H NMR analysis, MOF samples (~5 mg) were digested by adding 35% DCl in D 2 O (2 μL) and DMSO (500 µL) and waiting until a solution was obtained.…”

“…[10][11][12] Recently, this concept has been extended to deliberate defect introduction by full thermolytic bridging linker removal under oxidising conditions. 13,14 From these examples it is evident that postsynthetic elimination chemistry concurrent delivers new chemical functionality and increases pore size by cleaving groups or bridging ligands from the framework.…”