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Engineering of Metal Active Sites in MOFs

Carmen Fernández-Conde, María Romero-Ángel, Ana Rubio-Gaspar and Carlos Martí-Gastaldo

PhD Researchers on Functional Inorganic Materials Team (FuniMat), Instituto de Ciencia Molecular (ICMol) - Universidad de Valencia, C/ Catedràtic José Beltrán, 2, Vivero 1, 46980,, Paterna, Spain

From their appearance, metal-organic frameworks (MOFs), have been an interesting field of research, partly due to the vast possibilities these materials offer [1]. The fact that they can be designed chemically to serve specific applications has already been proved in diverse areas such as gas separation [2], encapsulation [3] carbon capture [4], or catalysis [5-8].

Regarding catalysis, MOFs present many characteristics that make them potential candidates to carry out relevant catalytic processes as well as understanding the mechanisms through which these reactions take place. First of all, MOFs are heterogeneous catalysts, which can have an impact on separation, recyclability, and the possibility to operate in continuous mode, key aspects from an industrial standpoint. Additionally, heterogeneous catalysts often exhibit lower deactivation rates, compared to their homogeneous counterparts, mainly due to the spatial separation of the active sites avoiding its aggregation. However, there are some issues that limit the potential of traditional heterogeneous catalysts. In some cases, diffusion might prevent reagents from reaching the catalytic center, which is often poorly defined and understood. This lack of understanding also impacts the possibilities of modification of the catalyst, resulting in little room for improvement in these catalysts.

MOFs can serve as a platform for overcoming these drawbacks due to their inherent properties. Among the advantages of MOFs, it stands the isolation of the catalytic center and its uniform distribution along the framework, the sizeable porosity that enhances diffusion or the crystalline structure of these materials, which opens the door to the use of advance of characterization techniques. For all of this, research in MOFs as catalysts has become an attractive area. In this chapter, we will focus on metal-based catalysis in MOFs, which have already shown promising results in diverse catalytic processes. We intend to provide the reader with a general perspective of the different strategies used for engineering catalytic active sites in these porous, molecular frameworks. This chapter is divided into three different sections, attending to which component of the framework (metal node, organic linker, or guest) is responsible for the catalytic activity. Finally, different chemical incorporation strategies, characterization techniques, and examples of chemical reactivity of these reactive sites will be provided. This way, the structure of this chapter is shown in Figure 1.1.

Figure 1.1 Schematic view of the three sections of this book chapter focused on engineering of active sites based on metal node, organic linker, or encapsulated guest.

Source: Figure produced by the authors of the chapter.

1.1 Metal Node Engineering

This section will examine the different approaches used to obtain an active catalytic site in the nodes of MOFs, with the aim to be used in metal-based reactivity. Thus, this section of the chapter is divided into three main blocks, as represented in Figure 1.2. The first one comprises MOFs with intrinsically active metal nodes, which can be further divided depending on the composition of the node. This way, this section will go through MOFs containing only one metal and MOFs with more than one (both mixed-metal and heterometallic MOFs).

Figure 1.2 Schematic representation outlining the first section of the chapter: metal-based reactivity based on the engineering of the metallic cluster of MOFs through different strategies: intrinsic active metal sites with one or more than one metal (a), implantation of reactivity through the creation of defects (b), and incorporation of a metallic unit into the node cluster (c).

Source: Figure produced by the authors of the chapter.

The focus of the second part of the chapter deals with the introduction of defects as a way to increase the node performance in catalytic processes. Finally, the third part discusses different approaches that have been used to attach metallic units to the framework nodes. For each sub-section, we describe the different strategies used to tailor activity, the characterization techniques required for controlling and rationalizing activity, and different examples to illustrate their application in heterogeneous catalysis.

1.1.1 Frameworks with Intrinsically Active Metal Nodes

1.1.1.1 Metal-Organic Frameworks with Only One Metal

One of the main reasons why MOFs have attracted that much attention in recent years is their great chemical versatility. In fact, as this chapter explains, there are many ways to introduce specific functionalities to our frameworks, ultimately leading to better-performing materials. Here, we will turn our attention to the different kinds of homometallic clusters that can be found within MOFs, as well as their potential applicability in catalysis. In this part of the chapter, the use of MOFs containing only one metal, the so-called homometallic MOFs, and its application in the catalysis field will be briefly outlined.

In Figure 1.3, some of the most representative clusters are depicted, showing some of the extensive possibilities for constructing the framework. Being metallic nodes as extensive as they are, it is quite challenging to try to explain all their associated reactivity. In fact, MOFs, with certain kind of metals, are being used for redox and photocatalysis. Because of that, we will only consider the reactivity associated with Lewis and Bronsted-based catalysis.

Figure 1.3 Representation of some of the most representative metal nodes that can be found (a) as well as the framework they form upon coordination with a binding organic ligand (b).

Source: Reproduced with permission from Yang et al. [9]/American Chemical Society.

In many cases, the reactivity of the metal cluster mainly comes from uncoordinated metal sites, the so-called open metal site (OMS). The catalytic power of these unsaturated centers is closely related to the Lewis acidity of the metal. This way, highly charged cations at the nodes, associated with high Lewis acidity, are good candidates for carrying out reactions based on Lewis acidity. In order to increase its reactivity, different activation processes can be applied to the pristine material. Furthermore, coordinated molecules can have Brønsted acidic character, as it has been proposed for adsorbed water in MOF-808-SO4 [10].

Enhancing acidity-based reactivity in homometallic MOFs: creating open metal sites. Some post-synthetic treatments can be used to obtain a higher-performing catalyst and there exist different strategies that will be outlined here. The first one consists of a thermal reduction treatment to incorporate new OMS into MOFs as demonstrated by Serre et al. [11]. This strategy corresponds to Figure 1.4a which modifies the MIL-100(Fe) MOF. On the cluster of this framework, two of the terminal molecules coordinated to two Fe centers are water, while the third Fe can bear different molecules, mainly F- or OH-, depending on the synthetic conditions used to obtain it. The water molecules are easily removed, leaving two uncoordinated Fe centers at temperatures higher than 100?°C under vacuum or a gas stream. In their work, researchers demonstrated the possibility of thermally reducing the framework, appearing FeII OMS when heating the framework above 150?°C with a helium stream followed by 12?hours vacuum. Moreover, this work proved the relationship between unsaturated iron sites and the strength of interaction with gases, which has an impact on the performance of the framework for preferential gas sorption.

Figure 1.4 Schematic representation of the different proposed activation pathways by thermal reduction (a) and by the acid treatment (b).

Source: Reproduced with permission from Wei et al. [12]/American Chemical Society.

Another approach was followed by De Vos et al. [13] as a way to tune the catalytic properties of MIL-100(Fe). In this work, they treat the pristine framework with protonic acids, particularly CF3COOH and HClO4. The 1,3,5-benzenetricarboxylic acid, which serves as a ligand, is displaced from the cluster, resulting in the appearance of an OMS and an additional Brønsted acidic site, as represented in Figure 1.4b. In this case, a 2-fold increase in both Lewis and Brønsted acid sites after the acidic treatment of the framework is observed.

In other cases, the goal of the post-synthetic treatment might not be the creation of unsaturated sites within the node but maximizing the acidic character of a metal. This strategy was followed in the case of Lin et al. [14], who developed a synthetic procedure for designing a strongly Lewis acidic MOF. Particularly,...