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Aqueous Mediated Heterogeneous Catalysis
Asit K. Chakraborti, Bubun Banerjee, Asit K. Chakraborti, Bubun Banerjee
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eBook - ePub
Aqueous Mediated Heterogeneous Catalysis
Asit K. Chakraborti, Bubun Banerjee, Asit K. Chakraborti, Bubun Banerjee
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Heterogeneous catalysts are an important tool for greener catalytic processes due to the ease of their removal from the reaction mixture and feasibility of reuse. When these catalysts can operate in the ideal green solvent, water, they improve the sustainability of the process. This book explores aqueous mediated heterogeneous catalysts and their use in synthesis. Topics covered include nanomaterials, quantum dots, metal organic frameworks, and their use as catalysts.
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1 Recent developments about metal-organic frameworks as heterogeneous catalysts in aqueous media
Arruje Hameed
Department of Biochemistry, Government College University Faisalabad, Pakistan
Tanveer Hussain
Department of Applied Chemistry, Government College University Faisalabad, Pakistan
Muhammad Fayyaz Farid
Department of Applied Chemistry, Government College University Faisalabad, Pakistan
Tahir Farooq â¶
1.1 Introduction
At the interface of coordination chemistry and material sciences, metal-organic frameworks (MOFs), a novel class of crystalline materials, have proved their value and worth as promising catalysts with unique structure and functionality. Their extraordinary surface area, ultrahigh porosity, and other topological attributes have made them attractive for applications in sensing, drug delivery, catalysis, and so on. They show a porous and periodic framework with self-assembled organicâinorganic hybrid units. The unique properties of MOFs are controlled by the structural chemistry of organic legends and metal ions. The MOFs can be designed and functionally tuned by adopting different approaches. The functional sites in MOFs could be developed by pre- and post-functionalized methodologies using organic legends, inorganic metals, and various guest functionalities. The MOFs could be functionalized using enzymes, metallic nanoparticles (NPs), and organometallic compounds [1].
In the perspective of catalysis, the MOFs are promising candidates due to adjustable framework and well-defined geometry, high porosity, and structural diversity of like metallic nodes and organic linkers [2]. The high structural tenability and uniform porous environment present them as advantageous catalytic materials compared to the traditionally used mesoporous silica, zeolite, and clays.
1.1.1 Synthesis and structural diversity of MOFs
The synthetic strategies and pre- and post-synthetic functionalization have put MOFs in three main categories. The structural nature of inorganic moieties and organic linkers represents the preparation of first-generation MOFs. Some chemical modifications allow the conversion of first-generation MOFs into second-generation MOFs with novel characters and properties. The applications of bioactive molecules, drugs, organic cations, and biomolecules allow the development of third-generation MOFs.
According to another classification, the MOFs could be rigid or flexible depending upon the structural nature of their framework. The MOFs could be crystalline or amorphous through coordination bonds; the metals are connected with organic linkers and the strength of the bond controls the crystalline nature, geometry, and symmetrical shape of the resulting MOF. The MOFs could be prepared by a range of synthetic methods including sonochemical, mechanochemical, solvothermal, and electrochemical methods (Figure 1.1). Various post-synthetic functionalizations are applied to modify the physiochemical properties of MOFs for their selective applications [3].
Figure 1.1: Synthesis and applications of MOFs.
In MOFs, the inorganic units (the metal ions) are also called secondary building units (SBUs) and the organic linkers could be heterocyclic compounds, carboxylates or anions like sulfonates or phosphonates (Figure 1.2). The chemistry of the functional groups, geometry, and coordination number of metal ions determine the final framework topology of MOFs. The nature of SBUs and unique characteristics of organic linkers confers highly critical attributes like specific recognition, chirality, and catalytic activity in the MOFs [4].
Figure 1.2: A few known organic linkers for MOF preparation.
The MOFs represent promising feature of a heterogeneous catalyst. They could display linker-mediated as well as cluster-mediated catalytic activity. Therefore, the MOFs are considered the most promising heterogeneous catalyst for synthetic reactions and degradation studies [5, 6]. As described earlier, the MOFs could be tuned with desirable surface functionality and a porous environment. Thus, MOFs are attractive materials for effective decontamination of emerging contaminants (ECs) and environmental remediations. Over the last few years, they have become the promising choice for organic pollutant management due to their wide applications in adsorption and catalysis. They develop hydrogen bonding, hydrophobic interactions, ÏâÏ stacking, and electrostatic interactions for the adsorption of different organic pollutants.
Over the last few decades, the photodegradation of ECs has received immense importance because it eliminates pollutants in a simple and practical way. The MOFs are also attractive for photocatalytic applications because they generate a charge-separated state with a wide adsorption spectrum due to the presence of organic linkers. In recent decades, heterogeneous photocatalytic degradation has received considerable attention because of the economical and facile applications of the catalyst with high reusability. In general, photocatalytic degradation proceeds through adsorption, surface reactions, and desorption of the products. The MOFs have a highly porous structure, tunable surface functionalities, and wide adsorption spectrum and hence are capable of exhibiting high photocatalytic activity [7, 8].
1.1.2 The stability of MOFs in water
Despite great options for diverse applications of MOFs, there are some serious challenges in their scope of applications due to water stability issues. In an aqueous environment, they show structural destruction because water molecules could replace metal-coordinated linkers. Such stability issues limit their applications as heterogeneous catalysts in aqueous media. Great attempts have been made to prepare water-stable MOFs using suitable constituents. The water-mediated decomposition could be avoided using hydrophobic functionalities. The destruction of the MOF framework has been avoided using bulky alkyl groups.
As a part of post-synthetic strategies, the modification of functional groups has also been exploited to enhance the water stability of MOFs. Similarly, the reactivity of metal clusters could be controlled by post-synthetic methods. As a new strategy, MOFs are coated with protective layers to induce water and moisture stability. Very recently, Ding et al. [9] introduced one-step surface polymerization as a new post-synthetic modification strategy to increase the stability of MOFs in aqueous media. Advantageously, the post-synthetic functionalization of MOFs for water stability does not undermine thei...