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Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2017)

The work presented herein describes the relevance and study of structural heterogeneity in the porous crystalline metal-organic framework (MOF) materials. These frameworks, which consist of metal clusters bridged by organic linking units, frequently contain defects or other components that are not described simply by the unit cell of the framework. Intriguingly, even if the features make up a small part of the overall structure, these can dominate a given material’s properties. This is evidenced in Chapters 2 and 3, which focus on characterizing the structural heterogeneity on a molecular level, specifically of the ‘missing linker’ defects in UiO-66 and the origin of superacidity in sulfated MOF-808 respectively. The former displays Lewis acidic sites and different protonation states while the latter has superacidic Brønsted acid sites following exchange of sulfuric acid into the pristine MOF-808 framework. These sites are dependent on the post-synthetic treatment of the material, and are a direct result of the heterogeneous chemical features that are not necessarily ordered in a long-range manner. Chapter 4 extends the characterization of sulfated MOF-808 by investigating other oxides that may be incorporated into the framework, focusing on their structural characterization.

The concept of structural heterogeneity is not limited to defects and other features that are not ordered over long-range. MOFs can be developed into smart solid-state materials by incorporating multiple types of functional groups with the capacity for different reactivity and chemistry within the same material. This is the focus of Chapter 5, which discusses the synthesis and characterization of MOFs containing phosphonate groups with multiple binding modes and protonation states, carboxylate-based MOFs with disulfide units, and the incorporation of peptide units into the MOF linker to stabilize alkali metal-based clusters.

Finally, the full potential of these materials with complex structural landscapes can only be realized through careful structural characterization and analysis. Chapter 6 highlights the case study of fifteen mixed-linker zeolitic imidazolate frameworks (ZIFs) and how their single crystal structures are characterized from low resolution data. While the information at low resolution is clearly less than what can be uncovered at high resolution, there is still much structural information to be gained. The procedures to obtain the data and model the structures, the limitations and methods used, are described.

    Cover page: Deciphering structural heterogeneity in metal-organic frameworks towards an understanding of structure-property relationships
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2018)
    Particulate methane monooxygenase (pMMO) is an enzyme that oxidizes methane to methanol with high activity and selectivity. Limited success has been achieved in incorporating biologically relevant ligands for the formation of such active site in a synthetic system. Here, we report the design and synthesis of metal-organic framework (MOF) catalysts inspired by pMMO for selective methane oxidation to methanol. By judicious selection of a framework with appropriate topology and chemical functionality, MOF-808 was used to postsynthetically install ligands bearing imidazole units for subsequent metalation with Cu(I) in the presence of dioxygen. The catalysts show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C. Combined spectroscopies and density functional theory calculations suggest bis(μ-oxo) dicopper species as probable active site of the catalysts.
      Cover page: Bioinspired Metal–Organic Framework Catalysts for Selective Methane Oxidation to Methanol
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      Materials development for artificial photosynthesis, in particular, CO2 reduction, has been under extensive efforts, ranging from inorganic semiconductors to molecular complexes. In this report, we demonstrate a metal-organic framework (MOF)-coated nanoparticle photocatalyst with enhanced CO2 reduction activity and stability, which stems from having two different functional units for activity enhancement and catalytic stability combined together as a single construct. Covalently attaching a CO2-to-CO conversion photocatalyst ReI(CO)3(BPYDC)Cl, BPYDC = 2,2'-bipyridine-5,5'-dicarboxylate, to a zirconium MOF, UiO-67 (Ren-MOF), prevents dimerization leading to deactivation. By systematically controlling its density in the framework (n = 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell), the highest photocatalytic activity was found for Re3-MOF. Structural analysis of Ren-MOFs suggests that a fine balance of proximity between photoactive centers is needed for cooperatively enhanced photocatalytic activity, where an optimum number of Re complexes per unit cell should reach the highest activity. Based on the structure-activity correlation of Ren-MOFs, Re3-MOF was coated onto Ag nanocubes (Ag⊂Re3-MOF), which spatially confined photoactive Re centers to the intensified near-surface electric fields at the surface of Ag nanocubes, resulting in a 7-fold enhancement of CO2-to-CO conversion under visible light with long-term stability maintained up to 48 h.
        Cover page: Plasmon-Enhanced Photocatalytic CO2 Conversion within Metal–Organic Frameworks under Visible Light
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2019)
        It remains difficult to understand the surface of solid acid catalysts at the molecular level, despite their importance for industrial catalytic applications. A sulfated zirconium-based metal-organic framework, MOF-808-SO4, was previously shown to be a strong solid Brønsted acid material. In this report, we probe the origin of its acidity through an array of spectroscopic, crystallographic and computational characterization techniques. The strongest Brønsted acid site is shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters. When a water molecule adsorbs to one zirconium atom, it participates in a hydrogen bond with a sulfate moiety that is chelated to a neighbouring zirconium atom; this motif, in turn, results in the presence of a strongly acidic proton. On dehydration, the material loses its acidity. The hydrated sulfated MOF exhibits a good catalytic performance for the dimerization of isobutene (2-methyl-1-propene), and achieves a 100% selectivity for C8 products with a good conversion efficiency.
          Cover page: Identification of the strong Brønsted acid site in a metal–organic framework solid acid catalyst
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