Designer Porous Frameworks with Sulfur-Based Functions for Precious Metal Sensing/Recovering and Advanced Catalytic Applications

Abstract

The first chapter gives a brief introduction for metal-organic frameworks (MOFs). MOFs have entered a stage where it is ever more crucial to fine-tune the porosities and functionalities within MOFs through judicious choices of bridging ligands and metal centers. We present in particular interest in bi-functional organic molecules based on thiol or thioether-carboxylate as the building blocks (e.g. 2,5-dithioalloxyterephthalic acid, 2,5-dimercapto-1,4-benzenedicarboxylic acid), which can offer richer functional diversity to provide materials with advanced properties (like sensing, separation and catalysis) in the prospective solid. The key thing here is using chemically hard carboxylate group bonding with hard metal ions (e.g., Zr4+, Al3+) to build up the host framework, while leaving the free standing thiol/thioether groups to encapsulate soft guest species (e.g., Hg2+, Pd2+, Ag+) to form metal-thiolate functions into the resultant frameworks.
The second chapter carries on with the long-standing studies in our group, focusing on the interaction between metal guests and MOFs that are functionalized by sulfur groups (e.g. thioethers and thiols). Here a highly specific, distinct color change in the crystals of a metal−organic framework with pendant allyl thioether units in response to Pd species was discovered. The color change (from light yellow to orange/brick red) can be triggered by Pd species at concentrations of a few parts per million and points to the potential use of these crystals in colorimetric detection and quantification of Pd(II) ions. The swift color change is likely due to the combined effects of the multiple functions built into the porous framework: the carboxyl groups bonding with Zn(II) ions to assemble the host network and the thioether and alkene functions for effective uptake of the Pd(II) analytes (e.g., via the alkene−Pd interaction). The resultant loading of Pd (and other noble metal) species into the porous solid also offers rich potential for catalytic applications, and the alkene side chains are amenable to wide-ranging chemical transformations (e.g., bromination and polymerization), enabling further functionalization of the porous networks.
Chapter 3 moves on to a more robust, water-stable system of Zr(IV)-based MOFs, in order to approach real-world applications. A robust metal–organic framework (MOF) for convenient recovery of Pd(II) from acidic nitric solutions which emulate high-level liquid wastes (HLLW) generated from the reprocessing of spent nuclear fuel is reported in this chapter. The highly stable framework solid (ASUiO-66) was constructed from Zr(IV) ions and the multifunctional linker 2,6-bis (allylsulfanyl) terephthalic acid (H2L1), featuring the well-known UiO-66 topology. Herein the robust Zr(IV)-carboxylate bonds impart structural strength to the host net, while the alkene and thioether units provide efficient and selective binding to the Pd(II) ions. For example, over 95% of the Pd(II) ions can be adsorbed from a simulated HLLW (1.0 M HNO3, containing about 20 different types of metal elements), with Ag(I) being the only other metal ion taken up significantly by the ASUiO-66 sorbent. Moreover, the adsorbed Pd(II) species can be effectively stripped by a dilute solution of thiourea (0.01 M); and the regenerated framework solid can be used for additional cycles of Pd extraction, with the sorption capacity of Pd(II) being slightly changed (38–41 mg g-1). The isotherm adsorption data fit well with the Langmuir model with a saturation capacity of 45.4 mg g-1, being equivalent to each octahedral cage in the UiO-66 net containing roughly one Pd(II) ion. In a broader perspective, the alkene and thioether combination could be anchored onto other sorbent systems (e.g., porous polymers and resins) to impart versatile adsorption properties for the retrieval of noble metal ions.
Chapter 4 continues with such a stable and versatile thiol-functionalized metal-organic framework. In this chapter, we report a widely useful, stepwise strategy for anchoring strongly Lewis acidic metal centers within porous frameworks—all by means of thiol-functionalized Zr(IV)-based metal-organic framework (MOF). First, the strong-binding thiol (-SH) groups take up the metal ion guest (e.g., Hg2+); the resultant metal-thiolate units (tethered to host net via the S atom) were then oxidized by H2O2 to form the metal sulfonate functions. Owing to the very ionic nature of the sulfonate groups, the metal ions imbedded in the pores offer strong Lewis acidity to enable potentially powerful reactivity within the solid state pores. For example, the Hg(II) ions imbedded within the well-known UiO-66 host net effectively catalyze the industrially important acetylene hydration in plain water at room temperature. Apart from the ease of product separation and minimizing Hg pollution, the MOF catalyst can be reactivated and reused for multiple cycles of operation.
Chapter 5 summarizes the above fundamental studies of structures and properties, and suggests exemplary directions for future studies.

Date of Award	16 Nov 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhengtao XU (Supervisor)