Leighanne C. Gallington

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.

Structural Transitions of the Metal-Oxide Nodes within Metal–Organic Frameworks: On the Local Structures of NU-1000 and UiO-66

In situ pair distribution function (PDF) analyses and density functional theory (DFT) computations are used to probe local structural transitions of M6O8 nodes found in two metal organic frameworks (MOFs), NU-1000 and UiO-66, for M = Zr, Hf. Such transitions are found to occur without change to the global framework symmetry at temperatures within a range relevant to many potential MOF applications. For the particular M6(O)8 nodes studied here, the observed distortions can be mapped to polymorphic forms known for bulk ZrO2. In the MOF framework, however, node distortions are found to occur at substantially lower temperature than analogous distortions in bulk ZrO2 owing to the nanoscale nature of the former.

Regioselective Atomic Layer Deposition in Metal-Organic Frameworks Directed by Dispersion Interactions.

The application of atomic layer deposition (ALD) to metal-organic frameworks (MOFs) offers a promising new approach to synthesize designer functional materials with atomic precision. While ALD on flat substrates is well established, the complexity of the pore architecture and surface chemistry in MOFs present new challenges. Through in situ synchrotron X-ray powder diffraction, we visualize how the deposited atoms are localized and redistribute within the MOF during ALD. We demonstrate that the ALD is regioselective, with preferential deposition of oxy-Zn(II) species within the small pores of NU-1000. Complementary density functional calculations indicate that this startling regioselectivity is driven by dispersion interactions associated with the preferential adsorption sites for the organometallic precursors prior to reaction.

Thermal Stabilization of Metal-Organic Framework-Derived Single-Site Catalytic Clusters through Nanocasting.

Metal-organic frameworks (MOFs) provide convenient systems for organizing high concentrations of single catalytic sites derived from metallic or oxo-metallic nodes. However, high-temperature processes cause agglomeration of these nodes, so that the single-site character and catalytic activity are lost. In this work, we present a simple nanocasting approach to provide a thermally stable secondary scaffold for MOF-based catalytic single sites, preventing their aggregation even after exposure to air at 600 °C. We describe the nanocasting of NU-1000, a MOF with 3 nm channels and Lewis-acidic oxozirconium clusters, with silica. By condensing tetramethylorthosilicate within the NU-1000 pores via a vapor-phase HCl treatment, a silica layer is created on the inner walls of NU-1000. This silica layer provides anchoring sites for the oxozirconium clusters in NU-1000 after the organic linkers are removed at high temperatures. Differential pair distribution functions obtained from synchrotron X-ray scattering confirmed that isolated oxozirconium clusters are maintained in the heated nanocast materials. Pyridine adsorption experiments and a glucose isomerization reaction demonstrate that the clusters remain accessible to reagents and maintain their acidic character and catalytic activity even after the nanocast materials have been heated to 500-600 °C in air. Density functional theory calculations show a correlation between the Lewis acidity of the oxozirconium clusters and their catalytic activity. The ability to produce MOF-derived materials that retain their catalytic properties after exposure to high temperatures makes nanocasting a useful technique for obtaining single-site catalysts suitable for high-temperature reactions.

Bridging Zirconia Nodes within a Metal–Organic Framework via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires

Metal-organic frameworks (MOFs), with their well-ordered pore networks and tunable surface chemistries, offer a versatile platform for preparing well-defined nanostructures wherein functionality such as catalysis can be incorporated. Notably, atomic layer deposition (ALD) in MOFs has recently emerged as a versatile approach to functionalize MOF surfaces with a wide variety of catalytic metal-oxo species. Understanding the structure of newly deposited species and how they are tethered within the MOF is critical to understanding how these components couple to govern the active material properties. By combining local and long-range structure probes, including X-ray absorption spectroscopy, pair distribution function analysis, and difference envelope density analysis, with electron microscopy imaging and computational modeling, we resolve the precise atomic structure of metal-oxo species deposited in the MOF NU-1000 through ALD. These analyses demonstrate that deposition of NiOxHy clusters occurs selectively within the smallest pores of NU-1000, between the zirconia nodes, serving to connect these nodes along the c-direction to yield heterobimetallic metal-oxo nanowires. This bridging motif perturbs the NU-1000 framework structure, drawing the zirconia nodes closer together, and also underlies the sintering resistance of these clusters during the hydrogenation of light olefins.

Sinter-Resistant Platinum Catalyst Supported by Metal-Organic Framework

Single atoms and few-atom clusters of platinum are uniformly installed on the zirconia nodes of a metal-organic framework (MOF) NU-1000 via targeted vapor-phase synthesis. The catalytic Pt clusters, site-isolated by organic linkers, are shown to exhibit high catalytic activity for ethylene hydrogenation while exhibiting resistance to sintering up to 200u2009°C. Inu2005situ IR spectroscopy reveals the presence of both single atoms and few-atom clusters that depend upon synthesis conditions. Operando X-ray absorption spectroscopy and X-ray pair distribution analyses reveal unique changes in chemical bonding environment and cluster size stability while on stream. Density functional theory calculations elucidate a favorable reaction pathway for ethylene hydrogenation with the novel catalyst. These results provide evidence that atomic layer deposition (ALD) in MOFs is a versatile approach to the rational synthesis of size-selected clusters, including noble metals, on a high surface area support.

Catalytically Active Silicon Oxide Nanoclusters Stabilized in a Metal–Organic Framework

Post-synthetic modification of the zirconium-based metal-organic framework (MOF) NU-1000 by atomic layer deposition (ALD), using tetramethoxysilane (Si(OMe)4 ) as a precursor, led to the incorporation and stabilization of silicon oxide clusters composed of only a few silicon atoms in the frameworks pores. The resulting SiOx functionalized material (Si-NU-1000) was found to be catalytically active despite the inactivity of related bulk silicon dioxide (SiO2 ), thus demonstrating the positive effects of having nanosized clusters of SiOx . Moreover, Si-NU-1000 showed activity greater than that found for aluminum oxide based catalysts-oxides known for their high acidity-such as an aluminum oxide functionalized MOF (Al-NU-1000) and bulk γ-Al2 O3 . X-ray photoelectron spectroscopy and infrared spectroscopy measurements unmasked the electron donating nature of Si-NU-1000, explaining the unusual electronic properties of the nanosized SiOx clusters and supporting their high catalytic activity.

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.