How MOFs are being, and could be, used to combat climate change

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Metal-Organic Frameworks, or MOFs, are a class of materials which are increasingly being found and used in the world today. They are compounds made up of metal ions and organic molecules (ligands) that form structured frameworks. This gives them the ability to take-up, hold, and release molecules from their pores, similar to sponges. More than 20,000 MOFs have been discovered in the last 20 years, and this number is predicted to grow in the future.

Due to the highly structured framework of pores, MOFs have the largest surface areas per gram of any material. This gives MOFs enhanced abilities in offering more space for chemical reactions and the adsorption of molecules. Not only is the large surface area responsible for the boom in the growth of MOFs, but the metals and organic ligands which make up the framework can be combined in an almost infinite number of ways to create new materials. This creates a lot of

possibilities by changing the functionality of the MOF.

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Use of MOFs in capturing carbon dioxide

Carbon dioxide levels have increased dramatically due to the increased burning o f fossil fuels. One of the most effective ways to combat this issue is to capture and store the carbon dioxide (known as carbon capture, or sequestration). The captured carbon dioxide can then be converted into valuable products, such as chemical fuel (methane) or industrial raw materials (for example, plastic). However, current methods of carbon capture are very expensive, intensive, and they are not able to keep up with the rate at which carbon dioxide is generated.

MOFs are proving to be very effective in capturing carbon dioxide due to the large surface area, their solid state which makes them very easy to handle, the differing pore sizes for selective adsorption of gases from gas mixtures, and the modifiable functional groups for selective adsorption of gases.

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Use of MOFs as a way to store hydrogen

In the transport sector, the use of fossil fuels is also a large contributor to climate change, and hydrogen has been proposed as a way to avoid the use of fossil fuels. Hydrogen as a fuel has many benefits, as when it is burned or consumed in a fuel cell to generate electricity, the only emission is water vapour. Unlike fossil fuels which produce NOx, SOx, and CO2 which pollute the air and contribute to climate change, hydrogen as a fuel is clean.

Hydrogen is going to play a big role in the future as a fuel that can be used to satisfy the world’s energy need, however the challenge with hydrogen as a transport fuel is that it is a very light and low-density gas. If a fuel cell car was to use atmospheric pressure to store 1kg of hydrogen (the amount needed to drive 100km), the fuel tank would have to be 11m3. A solution to this storage issue is to use compressed hydrogen gas, squeezing around 5kg into a smaller reinforced tank. However, this also has problems, as a heavy, awkwardly shaped, expensive fuel container has to be used, and a large amount of energy is required to compress the gas.

Adsorbents are structures which provide a surface for atoms, ions or molecules to bind to, and they have the potential to store hydrogen at a lower pressure. MOFs are examples of hydrogen adsorbents. However, even MOFs have some problems, as hydrogen binds too weakly to the porous material for enough of it to be held. This is because hydrogen does not easily ‘stick’ to adsorbents as, in general, to get molecules to stick to the surface, a polarisation effect is used, however, hydrogen only has two electrons, so it is very difficult to polarise. By adding low-coordinate metal cation sites, the MOF will become ‘stickier’, however there is still a lot of research going on in this area, so it is uncertain whether this will provide an effective solution to the hydrogen storage problem.

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Other uses of MOFs

Not only can MOFs combat the causes of climate change, but they are also increasingly being used to lessen the impacts of climate change. The prevalence of water shortages in hotter areas has risen due to climate change, and MOFs are able to provide a short-term solution to this. Due to the pores in the framework, MOFs can absorb water from the air at night, and during the day they can use solar power to empty the MOF of water.

MOFs are also being used to prevent food waste, as a lot of food is lost between harvest and retail. For example, fruit ripens due to the

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production of ethylene gas, which promotes ripening in other fruits, and 1-MCP (1-methylcyclopropene, a type of MOF) can bind the ethylene and the ethylene action inhibitor, preventing the fruit from ripening.

Carbon dioxide emissions pose the greatest threat to our atmosphere, and MOFs provide a very effective and efficient solution to this problem by being able to selectively adsorb carbon dioxide. Not only do MOFs combat this issue, but they are also increasingly being used to solve other issues related to climate change, such as water shortages, food waste, and the hydrogen fuel storage problem.

By Jess Gilbert

References and further reading: https://www.chemistryworld.com/ features/hydrogen-storage-gets-real/ 3010794.article

https://blog.novomof.com/blog/howmofs-save-the-climate-from-carbondioxide-and-other-greenhouse-gases

https://www.nature.com/articles/ s41467-019-09365-w

https://pubs.rsc.org/en/content/ articlelanding/2009/cs/b802256a#! divAbstract

https://www.sciencedirect.com/ science/article/abs/pii/ S004896971935082X

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