Metal-Organic Framework Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane at Low Temperature

Zr-based metal-organic frameworks (MOFs) have been shown to be excellent catalyst supports in heterogeneous catalysis due to their exceptional stability. Additionally, their crystalline nature affords the opportunity for molecular level characterization of both the support and the catalytically active site, facilitating mechanistic investigations of the catalytic process. We describe herein the installation of Co(II) ions to the Zr₆ nodes of the mesoporous MOF, NU-1000, via two distinct routes, namely, solvothermal deposition in a MOF (SIM) and atomic layer deposition in a MOF (AIM), denoted as Co-SIM+NU-1000 and Co-AIM+NU-1000, respectively. The location of the deposited Co species in the two materials is determined via difference envelope density (DED) analysis. Upon activation in a flow of O₂ at 230 °C, both materials catalyze the oxidative dehydrogenation (ODH) of propane to propene under mild conditions. Catalytic activity as well as propene selectivity of these two catalysts, however, is different under the same experimental conditions due to differences in the Co species generated in these two materials upon activation as observed by in situ X-ray absorption spectroscopy. A potential reaction mechanism for the propane ODH process catalyzed by Co-SIM+NU-1000 is proposed, yielding a low activation energy barrier which is in accord with the observed catalytic activity at low temperature.

Addressing the characterisation challenge to understand catalysis in MOFs: the case of nanoscale Cu supported in NU-1000

We explore the dynamic structure and reactivity of Cu species supported on NU-1000. By combining pair distribution function (PDF) analysis and difference envelope density (DED) analysis of in situ synchrotron-based X-ray scattering data, we simultaneously probe the local structure of supported Cu-species, their distribution within NU-1000 and distortions of the NU-1000 lattice under conditions relevant to catalysis and catalyst activation. These analyses show that atomic layer deposition (ALD) of Cu in NU-1000 (Cu-AIM) leads to the formation of Cu-oxo clusters within the small pores that connect the triangular and hexagonal channels. Exposure of Cu-AIM to a reducing atmosphere at 200 °C produces metallic Cu⁰ of two distinct particle sizes: ∼4 nm nanoparticles and small sub-nanometer clusters. The size of these nanoparticles appears to be constrained by NU-1000 pore dimensions, with evidence of the sub-nanometer clusters being bound within the triangular channels flanked by pyrene rings. This supported Cu⁰–NU-1000 system is catalytically active for gas-phase ethylene hydrogenation. Exposure of the catalyst to oxidative atmosphere re-oxidises the Cu species to a Cu₂O cuprite phase. The dynamic restructuring of the system in different chemical environments underscores the importance of probing these systems in situ.

Tuning ethylene gas adsorption via metal node modulation: Cu-MOF-74 for a high ethylene deliverable capacity

Co and Cu-MOF-74s are promising candidates for efficient ethylene abatement or storage and delivery, respectively.

Assembly of dicobalt and cobalt–aluminum oxide clusters on metal–organic framework and nanocast silica supports

NU-1000, a mesoporous metal–organic framework (MOF) featuring hexazirconium oxide nodes and 3 nm wide channels, was infiltrated with a reactive dicobalt complex to install dicobalt active sites onto the MOF nodes. The anchoring of the dicobalt complex onto NU-1000 occurred with a nearly ideal stoichiometry of one bimetallic complex per node and with the cobalt evenly distributed throughout the MOF particle. To access thermally robust multimetallic sites on an all-inorganic support, the modified NU-1000 materials containing either the dicobalt complex, or an analogous cobalt–aluminum species, were nanocast with silica. The resulting materials feature Co₂or Co–Al bimetallated hexazirconium oxide clusters within a silica matrix. The cobalt-containing materials are competent catalysts for the selective oxidation of benzyl alcohol to benzaldehyde. Catalytic activity depends on the number of cobalt ions per node, but does not vary significantly between the NU-1000 and silica supports. Hence, the multimetallic oxide clusters remain site-isolated and substrate-accessible within the nanocast materials.

Metal–organic frameworks for the removal of toxic industrial chemicals and chemical warfare agents

Toxic gases can be captured or degraded by metal–organic frameworks.

From 2-methylimidazole to 1,2,3-triazole: a topological transformation of ZIF-8 and ZIF-67 by post-synthetic modification

We report the first examples of the simultaneous non-assisted replacement of ligands and the conversion of topology in MOFs by applying the PSM approach.

Tuning the properties of metal–organic framework nodes as supports of single-site iridium catalysts: node modification by atomic layer deposition of aluminium

The metal–organic framework NU-1000, with Zr₆-oxo, hydroxo, and aqua nodes, was modified by incorporation of hydroxylated Al(iii) ions by ALD-like chemistry with [Al(CH₃)₂(iso-propoxide)]₂ followed by steam (ALD = atomic layer deposition). Al ions were installed to the extent of approximately 7 per node. Single-site iridium diethylene complexes were anchored to the nodes of the modified and unmodified MOFs by reaction with Ir(C₂H₄)₂(acac) (acac = acetylacetonate) and converted to Ir(CO)₂ complexes by treatment with CO. Infrared spectra of these supported complexes show that incorporation of Al weakened the electron donor tendency of the MOF. Correspondingly, the catalytic activity of the initial supported iridium complexes for ethylene hydrogenation increased, as did the selectivity for ethylene dimerization. The results of density functional theory calculations with a simplified model of the nodes incorporating Al(iii) ions are in qualitative agreement with some catalyst performance data.

Enzyme encapsulation in metal–organic frameworks for applications in catalysis

Various methods for encapsulating enzymes in metal–organic frameworks are discussed and the catalytic activity of biocomposites prepared using these methods is highlighted.

Ni(ii) complex on a bispyridine-based porous organic polymer as a heterogeneous catalyst for ethylene oligomerization

A Ni(ii) catalyst incorporated into a new porous organic polymer, Ni(ii)-POP-1, is prepared via a click reaction followed by metalation with NiCl₂. It shows good catalytic activity for ethylene dimerization.

Engendering Long-Term Air and Light Stability of a TiO2-Supported Porphyrinic Dye via Atomic Layer Deposition

Organic and porphyrin-based chromophores are prevalent in liquid-junction photovoltaic and photocatalytic solar-cell chemistry; however, their long-term air and light instability may limit their practicality in real world technologies. Here, we describe the protection of a zinc porphyrin dye, adsorbed on nanoparticulate TiO₂, from air and light degradation by a protective coating of alumina grown with a previously developed post-treatment atomic layer deposition (ALD) technique. The protective Al₂O₃ ALD layer is deposited using dimethylaluminum isopropoxide as an Al source; in contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum isopropoxide does not degrade the zinc porphyrin dye, as confirmed by UV-vis measurements. The growth of this protective ALD layer around the dye can be monitored by an in-reactor quartz crystal microbalance (QCM). Furthermore, greater than 80% of porphyrin light absorption is retained over ∼1 month of exposure to air and light when the protective coating is present, whereas almost complete loss of porphyrin absorption is observed in less than 2 days in the absence of the ALD protective layer. Applying the Al₂O₃ post-treatment technique to the TiO₂-adsorbed dye allows the dye to remain in electronic contact with both the semiconductor surface and a surrounding electrolyte solution, the combination of which makes this technique promising for numerous other electrochemical photovoltaic and photocatalytic applications, especially those involving the dye-sensitized evolution of oxygen.

Liquid Water- and Heat-Resistant Hybrid Perovskite Photovoltaics via an Inverted ALD Oxide Electron Extraction Layer Design

Despite rapid advances in conversion efficiency (>22%), the environmental stability of perovskite solar cells remains a substantial barrier to commercialization. Here, we show a significant improvement in the stability of inverted perovskite solar cells against liquid water and high operating temperature (100 °C) by integrating an ultrathin amorphous oxide electron extraction layer via atomic layer deposition (ALD). These unencapsulated inverted devices exhibit a stable operation over at least 10 h when subjected to high thermal stress (100 °C) in ambient environments, as well as upon direct contact with a droplet of water without further encapsulation.

Atomic Layer Deposition of Ultrathin Nickel Sulfide Films and Preliminary Assessment of Their Performance as Hydrogen Evolution Catalysts

Transition metal sulfides show great promise for applications ranging from catalysis to electrocatalysis to photovoltaics due to their high stability and conductivity. Nickel sulfide, particularly known for its ability to electrochemically reduce protons to hydrogen gas nearly as efficiently as expensive noble metals, can be challenging to produce with certain surface site compositions or morphologies, e.g., conformal thin films. To this end, we employed atomic layer deposition (ALD), a preeminent method to fabricate uniform and conformal films, to construct thin films of nickel sulfide (NiS_x) using bis(N,N'-di-tert-butylacetamidinato)nickel(II) (Ni(amd)₂) vapor and hydrogen sulfide gas. Effects of experimental conditions such as pulse and purge times and temperature on the growth of NiS_x were investigated. These revealed a wide temperature range, 125-225 °C, over which self-limiting NiS_x growth can be observed. In situ quartz crystal microbalance (QCM) studies revealed conventional linear growth behavior for NiS_x films, with a growth rate of 9.3 ng/cm² per cycle being obtained. The ALD-synthesized films were characterized using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess the electrocatalyitic activity of NiS_x for evolution of molecular hydrogen, films were grown on conductive-glass supports. Overpotentials at a current density of 10 mA/cm² were recorded in both acidic and pH 7 phosphate buffer aqueous reaction media and found to be 440 and 576 mV, respectively, with very low NiS_x loading. These results hint at the promise of ALD-grown NiS_x materials as water-compatible electrocatalysts.

Toward Metal-Organic Framework-Based Solar Cells: Enhancing Directional Exciton Transport by Collapsing Three-Dimensional Film Structures

Owing to their ability to act as light-harvesting scaffolds, porphyrin-containing metal-organic frameworks (MOFs) are in the forefront of research on the application of highly ordered molecular materials to problems in solar-energy conversion. In this work, solvent-assisted linker exchange (SALE) is performed on a pillared paddlewheel porphyrin containing MOF thin film to collapse a 3D framework to a 2D framework. The change in dimensionality of the framework is confirmed by a decrease in the film thickness, the magnitude of which is in agreement with crystallographic parameters for related bulk materials. Furthermore, NMR spectroscopy performed on the digested sample suggests a similar change in geometry is achieved in bulk MOF samples. The decreased distance between the porphyrin chromophores in the 2D MOF film compared to the 3D film results in enhanced energy transfer through the film. The extent of energy transport was probed by assembling MOF thin film where the outermost layers are palladium porphyrin (P2) units, which act as energy traps and fluorescence quenchers. Steady-state emission spectroscopy together with time-resolved emission spectroscopy indicates that excitons can travel through about 9-11 layers (porphyrin layers) in 2D films, whereas in 3D films energy transfer occurs through no more than about 6-8 layers. The results are difficult to understand if only changes in MOF interlayer spacing are considered but become much more understandable if dipole-dipole coupling distances are considered.

Nanosizing a Metal-Organic Framework Enzyme Carrier for Accelerating Nerve Agent Hydrolysis

We report the synthesis and characterization of a water-stable zirconium metal-organic framework (MOF), NU-1003, featuring the largest mesoporous aperture known for a zirconium MOF. This material has been used to immobilize the nerve agent hydrolyzing enzyme, organophosphorus acid anhydrolase (OPAA). The catalytic efficiency of immobilized OPAA in nanosized NU-1003 is significantly increased compared to that of OPAA immobilized in microsized NU-1003 and even exceeds that of the free OPAA enzyme. This paper highlights a method for rapid and highly efficient hydrolysis of nerve agents using nanosized enzyme carriers.