Alkyne and Thiol-Equipped Metal-Organic Frameworks for Robust Catalysis and Energy Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date1 Sep 2020


The first chapter provided a brief introduction of the origin, development and applications of metal-organic frameworks (MOFs), especially the secondary-donor-functionalized (e.g., thiol/thioether groups) MOFs reported in the past few years. Because of the high crystallinity, large porosity and flexible chemical functionality, MOFs and their derivatives have attracted much attention for catalysis and energy application. With the continuous investigation of the synthetic principles and structural studies, the performance and applications of these novel solid-state materials keep developing rapidly. Our group, who has been working across solid state chemistry and organic chemistry in the past few years, will take on the challenge of driving the functionalization and application of MOF systems. Our research progress combined the design of organic linkers and the construction of functional networks. Particularly, we have improved the synthetic protocol for a group of thiol-equipped ligands to assembly MOFs and explore their multiply functionality.

The second chapter presented an alkyne-equipped Zr-MOF featuring the topology of NU-901 (denoted as ZrL1). Finely dispersed Co(0) and CoO species were efficiently loaded into this stable metal-organic framework, to impart catalytic activities to the porous solid. The metalation of the MOF host was facilitated by the dense arrays of accessible alkyne units that boosted alkyne-Co2(CO)8 interaction. The tetrakis(4-carboxylphenylethynyl)pyrene linker, with eight symmetrically backfolded alkyne sidearms, displayed strong fluorescence and a dendritic Sierpinski shape. The resultant Zr(IV)-MOF features scu net (with rhombus channels) and breathing properties: e.g., the contracted (porous) phase reverted to the as-made phase upon contact with DMF (dimethylformamide). The inserted Co2(CO)8 guests quickly reacted with air to form dispersed CoO species (non-diffracting) atomically, and subsequent thermal treatment at 600 ºC of the CoO-loaded solid generated an electrocatalyst for oxygen evolution reaction (OER).

The third chapter presented a thiol-equipped Zr-MOF featuring the topology of UiO-67 (denoted as Zr-DMBPD). Thiol groups (-SH) offer versatile reactivity for functionalizing metal-organic frameworks, and yet thiol-equipped MOF solids remain underexplored due to synthetic challenges. On the basis of the recent breakthrough using benzyl mercaptan as the sulfur source and AlCl3 for uncovering the thiol function, herein we reported the synthesis of thiol-equipped linker 3, 3'- dimercaptobiphenyl- 4, 4'- dicarboxylic acid and its reaction with Zr(IV) ions to form a UiO-67-type MOF solid with distinct functionalities. The thiol-equipped UiO-67 scaffold showed substantial structural stability towards oxidation, e.g., it could be treated with 30% H2O2 to afford oxidation of the thiol to the strongly acidic sulfonic function while still maintaining the ordered porous MOF structure. The thiol groups could also take up palladium(II) ions from solutions effectively to allow for comparative studies on catalytic activities, which is of great value to help elucidate how the spatial configuration of the thiol groups could be engineered to impact the performance of heterogeneous catalysis in the solid state. Comparative studies on the stability in the solvent-free (activated) state also help to highlight the steric factor in stabilizing UiO-67-type frameworks.

In the fourth chapter, we employ facile aromatic nucleophilic substitution between the mercapto (-SH) and arylfluoro (Ar-F) groups to achieve an extensive and robust crosslinking of a coordination host by porphyrin guests that also serve the purpose of versatile post-synthetic functionalization. For this, a tritopic linker with three trident-like thiol-flanked carboxyl units are reacted with ZrOCl2·8H2O to afford a two-dimensional (3,6-connected) net. The wide aperture of the porous framework solid, together with its stability in both air and boiling water, facilitates the entry of even bulky metalloporphyrin guests and the subsequent property studies. On the porphyrin side, four pentafluorophenyl (C6F5-) groups offers multiple fluoro groups to facilitate their replacement by the thiol groups from the host net. The inserted metalloporphyrin bridges impart to the MOF host stable and cyclable activities for photocatalytic hydrogen production. We will also disclose an improvement in synthetic methodology, in which BBr3 is used to simultaneously dealkylate the ester and benzyl thioether to more efficiently access thiol-equipped carboxylic acid building block.