Structural and Acidic Characteristics of Multiple Zr Defect Sites in UiO-66 Metal–Organic Frameworks

Abstract

Although defects are prevalent in metal−organic frameworks (MOFs) and usually play a crucial role in modulating their performance in various applications, detailed structural characterizations of various defects remain a challenging task mainly due to their disordered, heterogeneous, and local nature. In this work, by using solid-state nuclear magnetic resonance spectroscopy (SSNMR) techniques in conjunction with density functional theory (DFT) calculations, it is clearly elucidated that the trimethylphosphine (TMP)-assisted 31 P NMR strategy is capable of greatly facilitating the qualitative and quantitative description of the detailed structural and acidic characteristics as well as the evolution process of various Zr defects with subtle distinctions in UiO-66 upon moderate thermal treatment, hence surpassing most conventional analytical techniques. These results offer a fundamental understanding of the defect chemistry in MOFs.

Quantifiable Surface Methoxy Groups on Zr(OH) Groups of UiO-66 Metal–Organic Framework: Generation from Methanol-13C and Reactivity

“…30 Similar conclusions were drawn from the adsorption of the probe molecule trimethylphosphine (TMP). 19 Thus, our material is highly comparable to the UiO-66 investigated in the literature, stable, and largely defect-free.…”

“…Peaks below 50 ppm are usually explained by remains from synthesis. 19,48,49 After the adsorption of methanol-13 C and after deconvolution of the respective peak area, a 1 H MAS NMR peak appears at δ 1H = 3.1 ppm. This chemical shift is assigned to methyl protons of the methanol- 13 C, which agrees well with former findings of methyl protons of adsorbed methanol in the δ 1H range between 3.3 and 2.8 ppm.…”

“…The chemical shift of peaks shifts thereby downfield with the decreasing oxygen coordination at zirconium. 19 No signs of TMP protonation on Zr(OH) groups were found. Thus, the use of TMP is suitable if the decomposition of the UiO-66 structure shall be monitored as this will lead to exposure of reactive zirconium acting as LAS.…”

“…Thus, stability is a major issue when MOFs are investigated. One of the rather stable MOFs is the UiO-66 material, constructed by stitching Zr 6 O 4 (OH) 4 clusters with terephthalic acid (BDC) linkers as SBUs. , Note that the structure-immanent bridged μ 3 -(OH) groups start to dehydroxylate at elevated temperatures above 523 K. ,− In contrast, missing linkers (here BDC) can cause the presence of defect Zr­(OH) groups. − For a maximized stability of UiO-66, a low number of defects was identified as a crucial prerequisite. − , The stability against chemicals and elevated temperatures enables even the postmodification of SBUs by anhydride compounds . It also leads to the application of UiO-66 in catalysis, for example, for Lewis-catalyzed reactions .…”

Surface Hydrophobicity and Guest Permeability in Polydimethylsiloxane-Coated MIL-53 as Studied by Solid-State Nuclear Magnetic Resonance Spectroscopy

“…Solid-state NMR is frequently utilized to analyze the chemical composition of various MOF composites. − The relative molar ratio between MIL-53 and PDMS in a series of PDMS-coated MIL-53 is directly determined from the 1 H MAS NMR spectra, which is summarized in Table S1. The molecular formula for representing the chemical composition of MIL-53@PDMS-10, MIL-53@PDMS-20, MIL-53@PDMS-30, MIL-53@PDMS-40, and MIL-53@PDMS-60 can be described as [Al­(OH)­(BDC)]­[Si­(CH 3 ) 2 O] 0.3 , [Al­(OH)­(BDC)]­[Si­(CH 3 ) 2 O] 1.0 , [Al­(OH)­(BDC)]­[Si­(CH 3 ) 2 O] 1.5 , [Al­(OH)­(BDC)]­[Si­(CH 3 ) 2 O] 2.7 , and [Al­(OH)­(BDC)]­[Si­(CH 3 ) 2 O] 3.0 , respectively (see Table S1).…”