Hard-and-soft Bifunctional Building Blocks for Porous Frameworks: Water Stability, Catalysis and Heavy Metal Removal

Abstract

Chapter 1 provides a selective overview of open framework materials with regards to the two topical issues of water stability and heavy metal removal. Such background information highlights a virtuous circle of synergizing fundamental research (i.e., building block design as exemplified by sulfur-equipped linkers for framework construction) and technological incentives arising from heavy metal removal practices. On the fundamental side, progresses in linker design (e.g., the thioether-tagged systems) open the possibility for efficient uptake of metal ions (e.g., HgCl2) into the porous framework. On the practical front, the real-world requirements for sorbent reusability stimulate the development of more stable framework materials, e.g., those based on the chemically hard Cr(III) and Zr(IV)-carboxylate interactions. In turn, these hard metal links allow for a versatile execution of the hard-and-soft (e.g., carboxyl-thiol) approach in framework construction, which, besides enriching the functionality, has led to heavy metal sorbent systems with superior binding affinity, selectivity, and uptake capacity/kinetics for practical applications.

Chapter 2 is a fundamental exploration on a novel class of building blocks featuring symmetrically backfolded side arms (Czerwinski dendrimers). These generate complex metal-organic frameworks (MOFs) featuring graphitic ribbons, free-standing carboxylic functions and seemingly intricate topologies. One highlight here is a systematic attempt to correlate these solid-state topologies (connectivity) with the backfolded shape of the molecular building blocks. The discovery is surprisingly simple: as building blocks for extended nets, the backfolded serpins dendrimer behaves as a collection of subsidiary building units of the conventional starburst shape (e.g., the regular tritopic unit); and the resultant networks can be deconstructed into subnets corresponding to the star-shaped subunits of the molecule. Therefore, these seemingly exotic and highly branched molecules are found to not only be compatible with the purpose of network construction, but also present fundamentally new features that open new horizons for molecular design. In practice, the participation in this study had also served to prepare the author for the ensuing studies on open frameworks.

The ligand design in Chapter 3 is intended for addressing the water stability issue of MOF materials, and resulted in a microporous solid featuring the Cu(I)-carboxylate link which is rarely found among MOF structures. Using water as the sole solvent, the bifunctional molecule tetrakis(methylthio)-1,4-benzenedicarboxylic acid (TMBD) was reacted with Cu(CH3CN)4BF4 to form a robust microporous metal-organic framework (MOF, CityU-7) featuring Cu(I) ions being simultaneous bonded to the carboxyl and thioether donors. The MOF solid is stable in air, can be activated simply by heating, without the need for treatment by organic solvents. The sub-nanoscopic pores (ca 0.6 nm) of the host net allows for uptake of CO2 and H2O, but exhibits lesser sorption for N2 at 77K. The microporous net can also be reversibly penetrated by I2 molecules.

Chapter 4 departs from the above strategy of simultaneously engaging the sulfur and carboxyl donors to the metal center (of Cu+). Instead, the design here features distinct “division of labor” between the hard and soft functions, in order to utilize the sulfur functions to improve metal uptake capabilities of the famous Zr(IV)-based metal-organic frameworks (MOFs). The chemically hard thioether side groups can be conveniently designed to take on various sizes and configurations, while refraining from interfering with network formation based on the hard Zr(IV)-carboxylate links. The bulky and hydrophobic sulfur side chains also help to improve the stability of Zr(IV)-based framework solids in the absence of solvent. For better practical impact, here we install sulfur side chains, e.g., tris(methylthiomethyl)methyl, that are conveniently derived from inexpensive pentaerythrityl compounds. One notable advance achieved pertains to a UiO-68-type porous solid (i.e., based on terphenyl dicarlboxyl linkers) featuring long-term stability in the absence of solvents, 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 the adsorption capacity as well as binding strength as measured by the distribution coefficient Kd) over other thioether-equipped MOF materials.

Chapter 5 aims to bring open framework solids a step closer to practical application by means of unique functions and vastly improved stability. In particular, we describe a highly recyclable, 2D aromatic framework that offers a unique and versatile combination of photocatalytic activity and heavy metal uptake capability, as well as other attributes crucial for green and sustainable development technologies. The graphene-like open structure consists of fused tritopic aromatic building blocks (i.e., hexaoxotriphenylene and hexaazatrinaphthylene) that can be assembled from readily available industrial materials without the need for transition metal catalysts. Besides fast and strong binding for Pb(II) ions (e.g., removing aqueous Pb ions below the drinkable limit within minutes), the alkaline N-heterocycle units of the robust and porous host is able to catalyse quantitatively Knoevenagel reactions in water. Furthermore, the fused donor-acceptor aromatic π-systems enable environmentally friendly photoredox catalyses (PRC) utilizing the safe and abundant visible light in a commercial flow reactor. Also discussed is a new metric for benchmarking kinetic performance of sorbents in the context of heavy metal removal from drinking water.

Date of Award	27 Mar 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhengtao XU (Supervisor)