Quantification of Linker Defects in UiO-Type Metal–Organic Frameworks

Abstract

Metal–organic frameworks (MOFs) are nanoporous materials composed of organic linkers and inorganic nodes. The large variety of linkers and nodes and the multiple ways to combine them make MOFs highly tunable materials, which are thoroughly studied for their use in, e.g., catalysis, gas capture, and separation. The chemistry of MOFs is further enriched by defects, e.g., missing linker defects, which provide active sites for catalysis or anchoring sites for introducing new functionalities. A commonly reported method to quantify linker defects assumes the presence of one type of linker and the complete removal of capping agents, solvents, and other impurities upon activation at high temperature, e.g., 400 °C (M-400). However, attempts to use this method for MOFs containing different types of linkers, also called multivariate MOFs (MTV-MOFs), or capping agents that are not completely removed at 400 °C, give inaccurate results and hamper comparing results from different publications. In this work, we have developed a new procedure to compute missing linker defects in Zr-based MOFs using standard analytical techniques to quantify the capping agents that remain in the MOF upon activation at 200 °C (M-200). This method, which has been tested in UiO-66/67 based MOFs, should be applicable to any MOF that (1) has known decomposition products, (2) has no missing cluster defects, (3) has empty pores or contain species that can be quantified after activation, and (4) has a known node composition at 200 °C.

2-Propanol Dehydration on the Nodes of the Metal–Organic Framework UiO-66: Distinguishing Catalytic Sites for Formation of Propene and Di-isopropyl Ether

“… a Values calculated by methodology reported by Sannes et al using TGA data for quantifying organic content, 1 H NMR spectra for relative quantification of various organic ligands, and EDX spectra for quantifying inorganic ligands. The density of terminal hydroxyl groups was calculated by simultaneously balancing mass and charge of the UiO-66 framework, as determined by above characterizations. b Measured by EDX spectroscopy. c BDC = benzene dicarboxylate, FA = formate, AA = acetate, M = methoxy. …”

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

“…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 .…”

“…However, the current literature lacks investigations of SMG by MAS NMR, a technique that was proven to be very suitable for determining numbers and properties of SMG on zeolites applied in hydrocarbon conversion reactions. , Furthermore, a quantification of SMGs could be an alternative to quantifying the number of surface hydroxyls that are accessible. Note that the current methods dilute the material, which naturally will average quantification over the whole material, also inaccessible sites. , In this contribution, it is thus (i) the formation and reactivity of SMG located on Zr­(OH) groups of UiO-66 Zr-clusters are investigated and (ii) these SMG are then applied as a quantification tool for the amount of accessible defect Zr­(OH) groups present.…”

Palladium nanoparticles confined in uncoordinated amine groups of metal–organic frameworks as efficient hydrogen evolution electrocatalysts

“…Moreover, the excellent oxygen affinity of Zr and the Zr–O bond in the Uio-66 structure also enhance its structural stability. 26–28 Due to these characteristics, this MOF has attracted extensive attention in the field of catalysis. After amine functionalization (Uio-66-NH 2 ), the free amine group (–NH 2 ) can effectively stabilize the metal species through the strong coordination between its lone pair electrons and the empty d orbital of the metal atoms.…”