Metal-organic frameworks (MOFs) are new micro-porous materials with exceptionally high porosity, uniform pore size, well-defined molecular adsorption sites and large surface area. MOFs have two main components: the organic linkers considered as organic secondary building unit, act as struts that bridge metal centers known as inorganic primary building units and act as joints in the resulting MOF architecture. The two main components are connected to each other by coordination bonds, together with other intermolecular interactions, to form a network with defined topology.

Up to 90% of a MOF consists of empty space that could be filled with hydrogen, carbon dioxide, medications for slow-release in the human body or a range of other materials. MOFs have such an enormous internal surface area – up to 7,000 square meters per gram – that a single ounce, unraveled and spread out, could cover the surface of 280 football fields.[1]

Since their discovery, MOFs have attracted extensive and continually increasing interest from both academia and industry owing to their unprecedented porosity, structural and functional diversity. Proposed applications of MOFs include gas separation, gas storage, catalysis, and carbon capture, as well as in emerging medical technologies.[2]

Uniform structures, adjustable porosity and a wide variety of chemical functionalities offer solutions to various industries and to many applications. The porosity of MOFs was discovered through a series of gas adsorption experiments carried out by Professor Susumu Kitagawa in 1997.[3]

Our planet is in crisis. The impact of human activity has now reached a scale at which it interferes profoundly with Earth’s atmosphere, ice sheets, ocean, forests, land and biodiversity.[4]

Greenhouse gas emissions have risen at alarming rates and in April 2018, levels of carbon dioxide in the atmosphere reached an average of 410 parts per million (ppm) across the entire month – the highest level in at least 800,000 years.[5]

MOFs may be small, but we believe their impact on these issues could be huge. Their unique properties mean they show immense promise for tackling a range of environmental issues. Here are a couple of important examples:

Given the alarming levels of carbon dioxide now present in our atmosphere, developing efficient carbon capture and storage techniques is vital.But carbon dioxide is not the only potentially problematic gas being released into our atmosphere. Globally, nearly 150 million tons of ammonia (NH₃) are produced every year to be used in manufacturing fertilizers, pharmaceuticals, commercial cleaning products, refrigerants, and more. Meanwhile, sulphur dioxide and nitrogen oxides in flue gas are well-known for their damaging effects on the environment, causing the formation of haze and acid rain, as well as contributing to climate change.Their high porosity, variable pore size, and high concentrations of active adsorption sites make MOFs a promising class of materials for use in capturing many of these gases during industrial activity and production processes – making it far less likely they will pollute our atmosphere.

MOFs also have a role to play in promoting the use of cleaner fuels. For example, oxyfuel combustion requires the delivery of oxygen rather than air to a combustion chamber, so that the gaseous product of the reaction is near-pure carbon dioxide, rather than a mix of gases. The advantage of this is that no separation of gases is required for carbon capture. However, widespread implementation of oxyfuel combustion technology requires industrial-scale quantities of high purity O₂. Processes for producing this are currently both costly and energy intensive, but metal-organic frameworks have the potential to change that. Other promising renewable fuels – hydrogen (H₂) and methane (CH₄) – stand poised to offer cleaner, greener alternatives to gasoline for powering motor vehicles. However, it has so far been impossible to store these gases at a high enough capacity to enable their widespread adoption. MOFs are the most promising materials for achieving the hydrogen and methane storage capacities needed to make them viable alternatives to current fuels.

The numerous advantages of MOFs, particularly their high surface area and modular composition, place them at a multidisciplinary crossroads. For good reason, MOFs are one of the most active research fields today, with aspects of their fundamental and applied properties permeating into disciplines as varied as electronics, medicine, chemical engineering, and optics. They have the potential to make significant contributions to everything from fighting climate change to beating cancer. It feels highly appropriate, given their composition, to say ‘watch this space’!

[1] https://www.acs.org/content/acs/en/pressroom/presspacs/2012/acs-presspac-february-22-2012/mofs-special-review-issue.html [2] https://pubs.rsc.org/en/content/articlelanding/2018/ta/c8ta02919a#!divAbstract [3] https://www.emdgroup.com/en/research/grants-and-awards/emanuel-merck-lectureship.html [4] https://www.nature.com/articles/415023a [5] https://scripps.ucsd.edu/programs/keelingcurve/2018/05/02/carbon-dioxide-in-the-atmosphere-hits-record-high-monthly-average/