Improvement of Crystalline Structural Order, Mercury Removal and Framework Stability in Sulfur-functionalized Zr(IV)-based Metal-organic Frameworks

提高硫功能化鋯(IV)金屬有機框架材料的晶態結構有序性, 金屬去除能力和框架穩定性

Student thesis: Doctoral Thesis

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Award date9 Aug 2018

Abstract

The opening chapter provides a brief introduction for metal-organic frameworks (MOFs) and sulfur–functionalized MOF developed in the past decades. Recently, MOFs become a dominant subject in the field of porous materials because of their porosity and flexible chemical functionality. As the synthetic principles and structural studies continue to be extensively investigated, ever stronger emphasis is being shifted to the properties and applications of these systems as novel solid state materials. Ours is a group working across solid state chemistry and organic chemistry, and we are thus well-poised to take on the challenge of pushing forward the functionalization and application of the MOF systems. Our research advances in the past few years encompass both organic synthesis of new ligands and the construction of functional networks. In particular, we have significantly improved the synthetic protocol for a group of large aromatic thioethers and thiols as MOF building blocks.

Chapter 2 builds on the hard-soft approach developed earlier in the group for functionalizing metal-organic frameworks (MOFs). Such an approach is embodied in the carboxyl-thioether combination, and here we aim to further demonstrate its efficacy and generality in connection with the prototypic UiO (the type of zirconium-based metal-organic framework made in Universitetet i Oslo) system. Specifically, the thioether side chain (CH3SCH2)3CHCH2O- with free binding sites was embedded into a porous, fcc (face-centered cubic) network [Zr6O4(OH)4(L)6] topologically equivalent to UiO prototype. Compared with the methylthio (CH3S-) unit used previously, the side chain used in this research provides more tunable binding strength to allow the bound Hg species to be extricated in a porous MOF matrix. We report here the dramatic triggering of structural order in a Zr(IV)-based metal−organic framework (MOF) through docking of HgCl2 guests. The crystal structure revealed elaborate HgCl2-thioether aggregates nested within the host octahedra to form a hierarchical, multifunctional net. The chelating thioether groups also promote Hg(II) removal from water, while the trapped Hg(II) can be easily extricated by 2-mercaptoethanol to reactivate the MOF sorbent.

Chapter 3 moves on to further address the practical issues of framework stability and metal sorption capability, by means of installing free-standing, highly designable thioether functions (-SCH3) in a series of zirconium-based robust MOFs with various sizes and configurations. These frameworks, assembled by reacting ZrOCl2/ZrCl4 with sulfur-functionalized linkers, provide an especially versatile platform for such systematic investigations. One notable advance achieved pertains to a UiO-68-type porous solid (i.e., based on terphenyl dicarboxyl linkers) featuring long-term stability in direct exposure to air, while maintaining significant mercury uptake capability from water and organic solutions. Also discovered is a cubic NU-1100-type net that offers more efficient mercury removal capability (with regards to the adsorption capacity as well as binding strength as measured from the distribution coefficient Kd) than other thioether-equipped MOF materials.

Chapter 4 carries on from such a strategy and addresses the fundamentally important issues of chemical and thermal stability and linker-based chemical transformation in the crystalline porous network. We employed a two-step strategy for accessing crystalline porous covalent networks with highly conjugated π-electron systems. For this, we first assembled a crystalline metal-organic framework (MOF) precursor based on Zr(IV) ions and a linear dicarboxyl linker molecule featuring backfolded, highly unsaturated alkyne backbones; massive thermocyclization of the organic linkers was then triggered to install highly conjugated, fused-aromatic bridges throughout the MOF scaffold while preserving the crystalline order. The formation of cyclized carbon linkers not only greatly strengthen the stability of Zr-MOF in concentrated sodium fluoride and phosphorous acid, strong base, but more importantly, enhance electroactivity and charge transport throughout the polycyclic aromatic grid.

Chapter 5 investigates the unique luminescent properties of the Zr-MOF with flexible thioether functions for the preparation of rare earth (RE)-free material capable of white light-emission. Here we combine a luminescent Zr(IV)-based metal-organic framework with interesting liquid-solid sensing property with blue-light-emitting carbon dots (CDs) for the achievement of white LED materials through one-step assembly. The white LED lamp constructed by coating this type of RE-free CDs/MOF nanocomposite capable of white light-emission exhibit excellent white color-emitting properties, featuring a CIE (created by International Commission on Illumination in 1931) chromaticity coordinate at (0.28, 0.31), high color rendering index (CRI) of 82, desirable fluorescence quantum yields (QYs) of up to 16%, and no obvious luminous degradation after exposure to air for three months.