Coupling Molecular and Nanoparticle Catalysts on Single Metal–Organic Framework Microcrystals for the Tandem Reaction of H₂O₂ Generation and Selective Alkene Oxidation

A molecular catalyst, (sal)­Mo^VI, and a heterogeneous catalyst, either Pd or Au nanoparticles (NPs), were integrated into one UiO-66 MOF microcrystal. The resulting dually functionalized catalysts, <b>Pd@UiO-66-(sal)­Mo</b> and <b>Au/UiO-66-(sal)­Mo</b>, have been utilized for a one-pot tandem reaction of H₂O₂ generation and selective liquid-phase alkene oxidation. The NPs serve as catalysts for the production of H₂O₂ from H₂ and O₂ gases, while the (sal)Mo moieties function as the oxidation catalyst. When the metal NPs are fully encapsulated within the MOF microcrystals, the alkene hydrogenation side reaction is largely suppressed, with a 6-fold decrease in the hydrogenation/oxidation product ratio for 5-bromo-1-cyclooctene favoring the epoxide as the major product. For <b>Au/UiO-66-(sal)­Mo</b>, where the two catalysts are in close proximity on the MOF microcrystal, the enhancement in oxidation productivity is increased by 10 times in comparison to the [<b>Au/UiO-66-NH</b>_<b>2</b> + <b>UiO-66-sal­(Mo</b>)] physical mixture of the two singly functionalized MOFs

Optimized Metal–Organic-Framework Nanospheres for Drug Delivery: Evaluation of Small-Molecule Encapsulation

We have developed a general synthetic route to encapsulate small molecules in monodisperse zeolitic imid-azolate framework-8 (ZIF-8) nanospheres for drug delivery. Electron microscopy, powder X-ray diffraction, and elemental analysis show that the small-molecule-encapsulated ZIF-8 nanospheres are uniform 70 nm particles with single-crystalline structure. Several small molecules, including fluorescein and the anticancer drug camptothecin, were encapsulated inside of the ZIF-8 framework. Evaluation of fluorescein-encapsulated ZIF-8 nanospheres in the MCF-7 breast cancer cell line demonstrated cell internalization and minimal cytotoxicity. The 70 nm particle size facilitates cellular uptake, and the pH-responsive dissociation of the ZIF-8 framework likely results in endosomal release of the small-molecule cargo, thereby rendering the ZIF-8 scaffold an ideal drug delivery vehicle. To confirm this, we demonstrate that camptothecin encapsulated ZIF-8 particles show enhanced cell death, indicative of internalization and intracellular release of the drug. To demonstrate the versatility of this ZIF-8 system, iron oxide nanoparticles were also encapsulated into the ZIF-8 nanospheres, thereby endowing magnetic features to these nanospheres

Yolk–Shell Nanocrystal@ZIF‑8 Nanostructures for Gas-Phase Heterogeneous Catalysis with Selectivity Control

A general synthetic strategy for yolk–shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk–shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu₂O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu₂O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk–shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis

Selective Deposition of Ru Nanoparticles on TiSi₂ Nanonet and Its Utilization for Li₂O₂ Formation and Decomposition

The Li–O₂ battery promises high capacity to meet the need for electrochemical energy storage applications. Successful development of the technology hinges on the availability of stable cathodes. The reactivity exhibited by a carbon support compromises the cyclability of Li–O₂ operation. A noncarbon cathode support has therefore become a necessity. Using a TiSi₂ nanonet as a high surface area, conductive support, we obtained a new noncarbon cathode material that corrects the deficiency. To enable oxygen reduction and evolution, Ru nanoparticles were deposited by atomic layer deposition onto TiSi₂ nanonets. A surprising site-selective growth whereupon Ru nanoparticles only deposit onto the b planes of TiSi₂ was observed. DFT calculations show that the selectivity is a result of different interface energetics. The resulting heteronanostructure proves to be a highly effective cathode material. It enables Li–O₂ test cells that can be recharged more than 100 cycles with average round-trip efficiencies >70%

Surfactant-Directed Atomic to Mesoscale Alignment: Metal Nanocrystals Encased Individually in Single-Crystalline Porous Nanostructures

Composite nanomaterials are attractive for a diverse range of applications in catalysis, plasmonics, sensing, imaging, and biology. In such composite nanomaterials, it is desired, yet still challenging to create a controlled alignment between components with lattices in disparate scales. To address this challenge, we report a new concept of colloidal synthesis, in which self-assembled molecular layers control the alignment between materials during the synthesis. To illustrate this concept, self-assembled cetyltrimethylammonium bromide (CTAB) molecules are used to control interfaces in a core–shell nanocomposite with a well-defined metal nanocrystal core and a metal–organic-framework (MOF) shell, which differ in structural dimensions by orders of magnitude. We show that single metal nanocrystals are captured individually in single-crystalline MOFs, and an alignment between the {100} planes of the metal and {110} planes of the MOFs is observed. By utilizing the same concept, a layer of mesostructured silica is formed over MOF crystals. These multilayered core–shell structures demonstrate a controlled alignment across a wide range of materials, from the metal nanocrystals, extending to nanoporous MOFs and mesostructured silica

Molecular Encapsulation beyond the Aperture Size Limit through Dissociative Linker Exchange in Metal–Organic Framework Crystals

Under linker exchange conditions, large guests with molecular diameters 3–4 times the framework aperture size have been encapsulated into preformed nanocrystals of the metal–organic framework ZIF-8. Guest encapsulation is facilitated by the formation of short-lived “open” states of the pores upon linker dissociation. Kinetic studies suggested that linker exchange reactions in ZIF-8 proceed via a competition between dissociative and associative exchange mechanisms, and guest encapsulation was enhanced under conditions where the dissociative pathway predominates

Imparting Functionality to Biocatalysts via Embedding Enzymes into Nanoporous Materials by a <i>de Novo</i> Approach: Size-Selective Sheltering of Catalase in Metal–Organic Framework Microcrystals

We develop a new concept to impart new functions to biocatalysts by combining enzymes and metal–organic frameworks (MOFs). The proof-of-concept design is demonstrated by embedding catalase molecules into uniformly sized ZIF-90 crystals via a <i>de novo</i> approach. We have carried out electron microscopy, X-ray diffraction, nitrogen sorption, electrophoresis, thermogravimetric analysis, and confocal microscopy to confirm that the ∼10 nm catalase molecules are embedded in 2 μm single-crystalline ZIF-90 crystals with ∼5 wt % loading. Because catalase is immobilized and sheltered by the ZIF-90 crystals, the composites show activity in hydrogen peroxide degradation even in the presence of protease proteinase K

Driving CO₂ to a Quasi-Condensed Phase at the Interface between a Nanoparticle Surface and a Metal–Organic Framework at 1 bar and 298 K

We demonstrate a molecular-level observation of driving CO₂ molecules into a quasi-condensed phase on the solid surface of metal nanoparticles (NP) under ambient conditions of 1 bar and 298 K. This is achieved via a CO₂ accumulation in the interface between a metal–organic framework (MOF) and a metal NP surface formed by coating NPs with a MOF. Using real-time surface-enhanced Raman scattering spectroscopy, a >18-fold enhancement of surface coverage of CO₂ is observed at the interface. The high surface concentration leads CO₂ molecules to be in close proximity with the probe molecules on the metal surface (4-methylbenzenethiol), and transforms CO₂ molecules into a bent conformation without the formation of chemical bonds. Such linear-to-bent transition of CO₂ is unprecedented at ambient conditions in the absence of chemical bond formation, and is commonly observed only in pressurized systems (>10⁵ bar). The molecular-level observation of a quasi-condensed phase induced by MOF coating could impact the future design of hybrid materials in diverse applications, including catalytic CO₂ conversion and ambient solid–gas operation

Highly Enantioselective Catalysis by Enzyme Encapsulated in Metal Azolate Frameworks with Micelle-Controlled Pore Sizes

Encapsulating enzymes within metal–organic frameworks has enhanced their structural stability and interface tunability for catalysis. However, the small apertures of the frameworks restrict their effectiveness to small organic molecules. Herein, we present a green strategy directed by visible linker micelles for the aqueous synthesis of MAF-6 that enables enzymes for the catalytic asymmetric synthesis of chiral molecules. Due to the large pore aperture (7.6 Å), double the aperture size of benchmark ZIF-8 (3.4 Å), MAF-6 allows encapsulated enzyme BCL to access larger substrates and do so faster. Through the optimization of surfactants’ effect during synthesis, BCL@MAF-6-SDS (SDS = sodium dodecyl sulfate) displayed a catalytic efficiency (Kcat/Km) that was 420 times greater than that of BCL@ZIF-8. This biocomposite efficiently catalyzed the synthesis of drug precursor molecules with 94–99% enantioselectivity and nearly quantitative yields. These findings represent a deeper understanding of de novo synthetic encapsulation of enzyme in MOFs, thereby unfolding the great potential of enzyme@MAF catalysts for asymmetric synthesis of organics and pharmaceuticals