The ability to generate materials with specific properties is changing what’s possible in fields like energy storage and biomedicine. Metal-organic frameworks (MOFs) are an example of materials with seemingly limitless possibilities for developing specific structures and properties. Thanks to the research growth into these materials over the last 20 years, they’re now poised to drive breakthroughs in fields ranging from gas separation and storage to catalysis and biomedical sensors.

MOFs are comprised of metal ions or metal clusters and organic linkers attached to each other by coordination bonds, which results in a porous crystalline material. Since their discovery in 2002 by Dr. Omar M. Yaghi and his team, nearly 90,000 MOFs have been synthesized and hundreds of thousands more predicted. We analyzed the CAS Content Collection^TM, the largest human-curated repository of scientific information, to see where MOFs were making significant impacts, and we found that the number of MOF-related publications has skyrocketed in just a few years (see Figure 1).  

While the majority of publications have been in journals, the growth of patent publications suggests that wider commercialization of this technology is imminent. This could mean key breakthroughs are on the horizon for decarbonization efforts, clean water access, sensors, drug delivery systems, and more.

What are metal-organic frameworks?

MOFs are primarily composed of transition metals and organic linkers containing functional groups such as carboxylate, pyridyl, sulfonate, and cyanide (see Figure 2). The combinations of the metal ions in the metal clusters and organic linkers can be different, and this is what drives the variety of possible MOFs — they can have variable pore size, pore shape, and chemical and physical properties.

The surface areas for MOFs are the highest reported for any material, reaching up to 7000 m²/g and predicted to reach up to 14600 m²/g. They also have high porosity and tunable chemical functionalities. This diversity makes MOFs useful in many applications, but this modular nature also makes it challenging to predict their structure-property relationship and choose one for a specific application. Advanced technologies may be the key to addressing this challenge.

How AI drives metal-organic frameworks research

AI methodologies including machine learning can screen large numbers of existing MOFs and even predict the properties of new MOFs, thereby reducing the time and effort to successfully identify the right MOFs for a given application. For example, a machine learning method used to screen MOFs for hydrogen gas uptake was found to be more than three orders of magnitude faster than conventional molecular simulations.  

Similarly, MOF synthesis typically involves a trial-and-error approach, which drains time and resources. However, in another study, machine learning trained on previous experiment data to predict the outcome of planned synthesis experiments had an 89% success rate.

By evaluating the nearly unlimited possibilities in MOF structure and properties quicker than human researchers, AI-driven technology can facilitate bringing these materials from theory to practical usage. In the CAS Content Collection, we can see the growth of AI-related publications in the MOF space in the last 10 years. When we look at the top 15 CAS-indexed concepts in these AI-related documents, we’re starting to find subjects like adsorption and separation increasing in interest along with the technological aspects (see Figure 3).

Potential decarbonization applications for metal-organic frameworks

The diversity of properties in MOFs makes them suitable candidates for gas storage and separation, catalysis, energy storage, and other applications that support decarbonization efforts. Figure 4 shows the many fields in which MOF research is taking place according to publications in the CAS Content Collection. We also found notable patent percentages across decarbonization-related applications like energy and gas storage.

Why are MOFs such promising candidates for these applications? Their tunable properties are the key factors:

• Gas separation and storage: High surface area and porosity combined with tunable chemical properties give MOFs the ability to adsorb gases and volatile organic compounds (VOCs). This is attracting enormous interest in gas separation and storage, particularly for CO₂. MOFs could play an important role in carbon capture technologies, which are vital for lowering industrial emissions and controlling global warming.  
  
  ExxonMobil has filed patents for MOFs relating to carbon capture and naphthalene separation. King Abdullah University of Science and Technology has also filed patents for carbon capture and gas separation, and startups like Nuada are exploring how MOFs can assist cement manufacturers in capturing CO₂.

• Energy storage: MOFs are being studied as cathode and anode materials for lithium-ion batteries. They’re also being researched for supercapacitor applications due to their large surface area. These energy storage devices are often used in hybrid vehicles and trains.

• Catalysis: MOFs’ surface area and the choice of active sites or chemical functionalities, resulting from their modular composition, also make them suitable for catalysis applications. These are important for energy storage and conversion technologies like fuel cells and water electrolysis. One study found that nitrogen-doped carbon nanotubes derived from MOFs exhibited improved electrocatalytic activity and stability compared to other electrocatalysts.

We can see the patent landscape relating to these and other technologies in Figure 5, particularly the International Patent Classification (IPC) codes Performing Operations (B) and Electricity (H):

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Additional fields for MOF applications

That same structure and property variability makes MOFs promising in many other fields besides decarbonization efforts. For example, MOFs’ adsorption capabilities are useful for dehumidification in air conditioning and generating water from air, known as atmospheric water harvesting. This could be crucial for developing clean water supplies in regions that lack clean water sources. We found in our patent analysis that the Regents of the University of California have been exploring MOFs for water and air purification as well as water harvesting.

Biomedical applications of MOFs are also growing. The unique structure of MOFs allows them to exhibit luminescence by various pathways such as linker emission, ligand-to-metal charge transfer, and metal-to-ligand charge transfer. This has led to their use in luminescence-based sensors, especially as biosensors, due to their low toxicity and biodegradable nature.

These luminescent properties and their ability to host other luminescent nanoparticles is driving MOF application in biomedical imaging. Researchers are also exploring MOFs for drug delivery systems, a promising field for the application of nanotechnologies to more effectively deliver medications in the body.

With all of their potential applications, MOFs are driving important breakthroughs in some of our most challenging scientific fields. Recent improvements in AI and machine learning are making the screening and evaluation of these materials faster and easier, which means more advances and commercial uses may be close.