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Dynamic acidity in defective UiO-66.

ABSTRACT: The metal-organic framework (MOF) material UiO-66 has emerged as one of the most promising MOF materials due to its thermal and chemical stability and its potential for catalytic applications. Typically, as-synthesised UiO-66 has a relatively high concentration of missing linker defects. The presence of these defects has been correlated with catalytic activity but characterisation of defect structure has proved elusive. We refine a recent experimental determination of defect structure using static and dynamic first principles approaches, which reveals a dynamic and labile acid centre that could be tailored for functional applications in catalysis.

SUBMITTER: Ling S

PROVIDER: S-EPMC6016445 | biostudies-literature | 2016 Jul

REPOSITORIES: biostudies-literature

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Dynamic acidity in defective UiO-66.

Ling Sanliang S Slater Ben B

Chemical science 20160322 7

The metal-organic framework (MOF) material UiO-66 has emerged as one of the most promising MOF materials due to its thermal and chemical stability and its potential for catalytic applications. Typically, as-synthesised UiO-66 has a relatively high concentration of missing linker defects. The presence of these defects has been correlated with catalytic activity but characterisation of defect structure has proved elusive. We refine a recent experimental determination of defect structure using stat ...[more]

PMID: 30155119

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Dynamic Interplay between Defective UiO-66 and Protic Solvents in Activated Processes.

Project description:UiO-66, composed by Zr-oxide inorganic bricks [Zr6 (μ3 -O)4 (μ3 -OH)4 ] and organic terephthalate linkers, is one of the most studied metal-organic frameworks (MOFs) due to its exceptional thermal, chemical, and mechanical stability. Thanks to its high connectivity, the material can withstand structural deformations during activation processes such as linker exchange, dehydration, and defect formation. These processes do alter the zirconium coordination number in a dynamic way, creating open metal sites for catalysis and thus are able to tune the catalytic properties. In this work, it is shown, by means of first-principle molecular-dynamics simulations at operating conditions, how protic solvents may facilitate such changes in the metal coordination. Solvent can induce structural rearrangements in the material that can lead to undercoordinated but also overcoordinated metal sites. This is demonstrated by simulating activation processes along well-chosen collective variables. Such enhanced MD simulations are able to track the intrinsic dynamics of the framework at realistic conditions.

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Engineering a Highly Defective Stable UiO-66 with Tunable Lewis- Bronsted Acidity: The Role of the Hemilabile Linker.

Project description:The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of ?-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity.

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The Remarkable Amphoteric Nature of Defective UiO-66 in Catalytic Reactions.

Project description:One of the major requirements in solid acid and base catalyzed reactions is that the reactants, intermediates or activated complexes cooperate with several functions of catalyst support. In this work the remarkable bifunctional behavior of the defective UiO-66(Zr) metal organic framework is shown for acid-base pair catalysis. The active site relies on the presence of coordinatively unsaturated zirconium sites, which may be tuned by removing framework linkers and by removal of water from the inorganic bricks using a dehydration treatment. To elucidate the amphoteric nature of defective UiO-66, the Oppenauer oxidation of primary alcohols has been theoretically investigated using density functional theory (DFT) and the periodic approach. The presence of acid and basic centers within molecular distances is shown to be crucial for determining the catalytic activity of the material. Hydrated and dehydrated bricks have a distinct influence on the acidity and basicity of the active sites. In any case both functions need to cooperate in a concerted way to enable the chemical transformation. Experimental results on UiO-66 materials of different defectivity support the theoretical observations made in this work.

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UiO‐66 and hcp UiO‐66 Catalysts Synthesized from Ionic Liquids as Linker Precursors

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PANI@UiO-66 and PANI@UiO-66-NH2 Polymer-MOF Hybrid Composites as Tunable Semiconducting Materials.

Project description:This investigation explores optimum synthetic conditions for novel polymer-metal organic framework hybrid composites composed of Zr-terephthalate-based MOF UiO-66 and conductive polyaniline (PANI) nanofibers in an effort to optimize conductivity while minimizing MOF structural deformation. Successful syntheses of self-assembled PANI nanofibers in PANI@UiO-66 and PANI@UiO-66-NH2 composites were confirmed using scanning electron microscopy, infrared spectroscopy, and powder X-ray diffraction. The polymer-MOF composites show different bonding synergies to the PANI nanofibers depending on the organic linker used. Electronic properties of the post-synthetically modified PANI@UiO-66 and PANI@UiO-66-NH2 were investigated using UV-vis diffuse reflectance spectroscopy. Sheet resistivity of the self-assembled polymer-MOF composites was determined under an inert atmosphere at room temperature using four-point probe measurements to confirm tunable semiconductivity ranging from 40 to 2 mS/sq. Furthermore, the effects of aniline oxidation on the crystallinity and coordination of UiO-66 and UiO-66-NH2 were determined through analysis of these results.

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Evidence for a chemical clock in oscillatory formation of UiO-66.

Project description:Chemical clocks are often used as exciting classroom experiments, where an induction time is followed by rapidly changing colours that expose oscillating concentration patterns. This type of reaction belongs to a class of nonlinear chemical kinetics also linked to chaos, wave propagation and Turing patterns. Despite its vastness in occurrence and applicability, the clock reaction is only well understood for liquid-state processes. Here we report a chemical clock reaction, in which a solidifying entity, metal-organic framework UiO-66, displays oscillations in crystal dimension and number, as shown by X-ray scattering. In rationalizing this result, we introduce a computational approach, the metal-organic molecular orbital methodology, to pinpoint interaction between the tectonic building blocks that construct the metal-organic framework material. In this way, we show that hydrochloric acid plays the role of autocatalyst, bridging separate processes of condensation and crystallization.

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Missing Linkers: An Alternative Pathway to UiO-66 Electronic Structure Engineering.

Project description:UiO-66 is a promising metal-organic framework for photocatalytic applications. However, the ligand-to-metal charge transfer of an excited electron is inefficient in the pristine material. Herein, we assess the influence of missing linker defects on the electronic structure of UiO-66 and discuss their ability to improve ligand-to-metal charge transfer. Using a new defect classification system, which is transparent and easily extendable, we identify the most promising photocatalysts by considering both relative stability and electronic structure. We find that the properties of UiO-66 defect structures largely depend on the coordination of the constituent nodes and that the nodes with the strongest local distortions alter the electronic structure most. Defects hence provide an alternative pathway to tune UiO-66 for photocatalytic purposes, besides linker modification and node metal substitution. In addition, the decomposition of MOF properties into node- and linker-based behavior is more generally valid, so we propose orthogonal electronic structure tuning as a paradigm in MOF design.

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Free Energy of Ligand Removal in the Metal-Organic Framework UiO-66.

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Adsorption and controlled release of three kinds of flavors on UiO-66.

Project description:Delivery systems for controlled release of fragrances are significantly essential in the flavor and fragrance industry due to a limited life span (premature evaporation and degradation) of fragrance compounds. Recently, several adsorption materials such as porous materials have been developed in delivery systems for targeted fragrance release. In this work, UiO-66, a member of metal-organic framework (MOF) family with high porosity and greater adsorbability, was selected as a prospective alternative to traditional porous adsorbents for controlled release of fragrances. Isophorone, eugenol, and ?-ionone with strong aroma are widely used as perfume flavors, soap flavor, cosmetic flavors, and even as a food-flavoring agents, and were chosen as representative fragrances for adsorption and controlled release studies. The adsorption and release behavior of fragrances on UiO-66 was evaluated by high-performance liquid chromatography (HPLC). The UiO-66 with high surface area (1,076 m2/g) achieved effective storage and controlled release for isophorone, eugenol, and ?-ionone. The adsorption rates of isophorone, eugenol, and ?-ionone can reach 99.4%, 99.9%, and 60.2%, respectively. Additionally, the release of these fragrances from UiO-66 can sustain over 20 days. UiO-66 exhibited higher release rate over eugenol with desorption rates of 95.2% than that of ?-ionone (52.6%) and isophorone (49.6%), respectively, suggesting a good adsorption-release selectivity of UiO-66 to different fragrances. This study further confirms the usability of UiO-66 in fragrance release and extends the application of MOF porosity in aroma release.

| S-EPMC7174235 | biostudies-literature

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