Introduction

Metal-organic frameworks (MOFs) are a class of crystalline porous materials that has been extensively researched in last two decades. Structurally close to zeolites, MOFs were first synthesized in 1999 by O.M. Yaghi et al.^[1]MOFs can be described as coordination polymers of positively charged metal ions and organic ligands, often referred to as linkers. The free space between the metal ions and organic ligands shows up as channels in the final structure and gives the materials their high surface area (500-2000m²/g). By varying the metal atoms and organic linkers, researchers can tailor the size of the pores for optimal properties towards the application and a huge number of different structures have been developed since. MOF-5 and HKUST-1 are some of the earliest and by now most studied structures. Their structures will be discussed more further in the page. Nowadays several hundred different MOF structures have been synthesized and studied.

Applications of MOFs rely on the cage-like structure of the compounds, such as gas storage and separation, catalysis or sensor applications.^[2]

Structure

When describing the structure of MOFs, it's easier to break them into different building blocks, often referred to as secondary building units (SBUs). These building blocks are either inorganic or organic structures. One of the most studied MOFs,  MOF-5 is represented in Figure 1.

Figure 1. Structure of MOF-5 in Vesta. Carbon atoms in brown, oxygen in red, hydrogen in white and zinc represented by the  light blue coordination polyhedra. (Figure: Peter Sinkkonen)

 MOF-5 consists of Zn₄O(CO₂)₆ clusters linked together with six 1,4-benzenedicarboxylate (BDC) bridges (space group Fm3m). The void in the center of the unit cell makes MOFs highly porous materials.  The inorganic SBUs affect the resulting topology the most. As we can see from Figure 1 the zinc clusters have an octahedral shape, and they are linked together with the organic BDC units. The size of organic linkers corresponds to the void in the center of the unit cell. Using larger molecules as linkers results in a larger empty space in the middle of the structure and thus it is possible to control the size of the pores in the final structure.  MOFs can be built from few different types of building blocks and thus their parts are relatively simple units. 

Inorganic SBUs control the topology of the final structure. Using different inorganic SBU can result in the structure becoming more complex. Another prominent MOF structure is presented in Figure 2 using different organic linkers and a copper based cluster.^[3]

Figure 2. Structure of HKUST-1 in Vesta. Carbon atoms in brown, oxygen in red, hydrogen in white and copper  represented by the dark blue coordination polyhedra. (Figure: Peter Sinkkonen)

Commonly used organic SBUs are carboxylate compounds such as 1,4-benzenedicarboxylate (BDC) or 1,3,5-tricarboxylate (BTC)^[4]. The carboxylate part of the molecules bonds to the metal atoms in the inorganic part of the framework. Organic ligands are usually classified by the amount of anchoring sites in them, so BDC would be called a bitopic SBU with two sites and BTC tritopic with three sites as seen from the structures in Figure 3. 

Figure 3. Common organic SBUs from left to right: 1,4-benzenedicarboxylic acid and 1,3,5-benzenetricarboxylic acid. (Figure: Peter Sinkkonen)

Both of the before mentioned organic SBUs are part of the two MOF structures presented in Figure 1 including the bitopic BDC linkers and Figure 2 the tritopic BTC.

Further modifications to the structure can be accomplished through post-synthetic modifications through reactions with the organic linkers. A wide array of different organic reactions is available to modify the relatively simple organic linker molecules. One such modification is presented in Figure 4.^[5]

Figure 4. Postsynthetic modification of IRMOF-3  (Figure: Peter Sinkkonen)

A known MOF named IRMOF-3 was prepared by reacting Zn(NO₃)₂ with an organic linker molecule R₃-BDC, which is a previously mentioned BDC structure with and added 2-amino group.  As the 2-amino group is not part of the coordination to the Zn₄O node of the final framework it is possible to modify it further after the synthesis by simple organic chemistry reactions.  In the study IRMOF-3 crystals were treated with acetic anhydride at room temperature with DCM as the solvent for hours to days resulting in the final modified product of IRMOF-3-AM1. Modifying the final MOF structures after the synthesis allows us further tailor the organic part of the structure more easily.

Synthesis

When talking about synthesis of MOFs it is referred that they are synthesized through reticular chemistry, which constitutes synthesizing net-like structures^[6]. It is based on how the building blocks come together to form the netlike structures of the MOFs. While synthesis of MOFs is similar to organic copolymers, in that the building blocks are carefully chosen for specific properties in the final material, the final properties in a MOFs arise from the way the building units are connected together in the network. The nets don't always form uniformly and controlling the formation of the pores is an important part of the synthesis. Because of the large space in middle of the structure, it is sometimes possible for two MOFs to grow and intertwine together^[7]. This can be limited by designing MOF topology that doesn't allow interpenetration to occur.

MOFs are synthesized through solvothermal synthesis where both solvent and precursors are heated to a higher temperature than the boiling point of the solvent in a closed system  or nonsolvothermal synthesis^[8] . Thus temperature is one the major parameters of MOF synthesis as heating removes the solvent molecules in the center of the structure and results in the porous structure of the materials. Some of the more prominent materials, such as MOF-5 and HKUST-1 have even been synthesized in room temperature.

It is also possible to synthesize MOFs through ALD/MLD, since the precursors to the MOF are relatively simple and thus work well with thin film deposition techniques, but the research is still new.^[9]

Applications

Most of the applications of metal-organic frameworks stem from their porous structure and high surface area. Easiest to see applications are from gas storage, such as hydrogen storage for instance. MOFs have a much higher surface area when compared to other alternatives for gas absorption such as zeolites or activated carbon.

Hydrogen storage involves pressurized and cooled tanks, chemisorption or physisorption.  Pressurizing and cooling tanks has safety and economic implications and this is where MOFs can potentially be utilized to help. Hydrogen storage in MOFs is based on physisorption. It has been established that physisorpion increases when surface area of the material increases. In the study it was found that MOF-5 can adsorb hydrogen up to 4,5 wt% at 77K and 1,0 wt% at room temperature. Even higher surface areas of 4500m²/g were found in study with the compound MOF-177. ^[10]There is also some interest in using MOF structures to store other alternative fuel sources such as natural gas (methane). To further tailor MOFs for hydrogen storage their pore size can be adjusted for better gas absorption. One such method would be to use longer organic linkers for a larger pore size, which naturally has more adsorption sites for the gas.

References