MOF: Metal Organic Frameworks

1. Introduction

Metal organic frameworks (MOFs) are crystalline, porous materials that consist of metal building blocks connected with organic linkers. They are often termed as superzeolites or the next generation of porous materials. Their three-dimensional structures are self-assembled by coordination of suitable metal ions/clusters with organic ligands. The connectivity is driven by coordinative chemistry between the metal centers and a functional group of the linker. An attractive property of this type of materials is their tunability. The whole organic toolbox can be used to pre- or post-functionalize the linkers to best suit the desired application. Another interesting feature of these materials is the presence of single metal sites. These can prove valuable in catalysis and sorption applications. In general MOFs have applications in gas storage and separation, catalysis, sensors and electronics next to some more exotic properties such as magnetism and luminescence.

2. Metal Organic Frameworks for (photo)catalysis and adsorption

Researchers: drs. Hannes Depauw, drs. Kevin Hendrickx, dr. Karen Leus

 

The use of metal-organic frameworks in photocatalysis

 

The ever increasing demand for more green and sustainable reaction pathways and energy conversion incites researchers to develop a whole range of new and improved photocatalysts. One of the new interesting materials for these applications are metal-organic frameworks, a class of hybrid crystalline materials, built from both inorganic bricks, and organic linker units. Their tuneability makes it possible to incorporate several functionalities in the framework, or to encapsulate different species that are active in several conversions (like for example nanoparticles). Unfortunately, most MOF materials absorb only in the UV region of the spectrum and are thus not active under solar irradiation. In this research project we will investigate different possibilities to tune MOFs (more specifically UiO-66 frameworks) by modification of the building units in order to make them solar active and increase their activity in general. The well-defined structures make it possible to study on a detailed level the different processes and gain basic insights in these materials and photocatalysis in general. In order to reach this goal, a combined experimental and theoretical approach is used, whereby an exchange of data in both directions will allow us to get a complete picture.

 

In situ synthesis of nanoparticles in metal-organic frameworks towards their use in various applications ranging from catalysis to adsorption

For utilizing MOFs in heterogeneous catalysis, active sites can be introduced employing several strategies. In the most studied and widely explored strategy, the framework metal ions (saturated or unsaturated) act as the catalytically active sites. In the second approach, the metallated or metal-free organic ligands can be used as active sites. The third approach involves the incorporation of guest moieties inside the void cavities of MOFs, which act as the active sites. Apart from the encapsulation of homogeneous complexes and polyoxometalates, an increasing interest has grown recently toward the incorporation of nanoparticles (NPs) within the pores of MOFs. To stabilize the NP many different supports have been examined as host materials. The most commonly employed support materials are metal oxides, e.g., CeO2, TiO2 and Al2O3 but also carbon and silica based materials have been extensively investigated. In this project different types of nanoparticles, more specifically Au NP, Fe NP, Pt NP are synthesized in situ in different metal organic frameworks (see Figure 1) towards their use in different applications ranging from catalysis (oxidation, hydrogenation..) to adsorption of different metallic species (like for example As). 

The incorporation of nanoparticles inside MOF materials can be clearly seen in the movies below.

 

 

Synthesis of doped V/Al metal-organic frameworks and their catalytic and breathing properties

Recently, we have demonstrated that MIL-47, with VIV=O nodes and benzene dicarboxylate (BDC) linkers, is an efficient catalyst in the liquid phase oxidation of cyclohexene.[1] However, this catalyst exhibits limited framework stability under ambient conditions, more in particular, water exposure should be avoided. This problem may be circumvented by doping the highly stable MIL-53-Al (AlIII(OH)BDC), practically isostructural with MIL-47) with catalytically active VIV ions (see Figure 2). In this project we will synthesize different V/Al based metal-organic frameworks having the COMOC-2(V) and MIL-53(Al) topology (see Figure 2). Both MOFs have shown a certain breathing behavior in which lattice movements were observed driven by changes in temperature, gasses and under pressure. The aim of this research is to investigate this phenomenon referred to as “breathing”. Doping of the frameworks opens the opportunity to play around with the framework flexibility which will result in an in-depth investigation of this behavior. One may also expect that such doping procedure could lead to an increased catalytic efficiency and selectivity. 

Figure 2. Schematic representation of doping the aluminium based metal-organic frameworks with vanadium.

 

3. Chiral Metal Organic Frameworks for Gas Storage, Separation and Catalysis

Researcher: dr. Asamanjoy Bhunia

Design and synthesis of chiral metal-organic frameworks (CMOFs) have attracted huge attention because of their potential applications in gas storage, separation and heterogeneous catalysis depend on their large surface areas, well-ordered porous structures, and available for functionalizations. Chiral MOFs are ideally suited for heterogeneous asymmetric catalytic conversions because they impose size- and shape-selective restrictions through readily fine-tuned chiral pores. So far, there are several approaches have been used for chiral MOFs construction: (1) from achiral linker via self-resolution during crystal growth, (2) from achiral metal salt and organic linker under chiral influence, (3) from chiral linker as a building block Recently, we are focusing on a direct approach to isolate the CMOFs from chiral linker. In this project, you will synthesize the chiral ligand as well as new chiral MOFs and then investigate the heterogeneous catalytic properties based on metallosalen environments for epoxidation or sulfoxidation reactions. 

The solid-state structures will be determined by single-crystal X-ray diffraction. At the same time, you will characterize the new CMOFs through IR, NMR, UV-VIS, TGA, CHN analysis and so on.

 

4. References

[1] Leus, K. et al., Chem. Commun., 2010, 46, 28, 5085.