Cutting-edge research on MOFs opens for future solutions

It's important that we develop different kinds of MOFs that can meet different needs, says Xiaodong Zou, Professor at Kemikum, Stockholm University.

“Some need to withstand very harsh environments, others need to tolerate high temperatures or operate in water. Some should last a lifetime, while others need to be flexible and break down after fulfilling their function—for example, controlled release of anticancer drugs to cancer cells inside the body,” she continues.

Visualizing MOFs

It was Xiaodong Zou who brought the research on metal-organic frameworks, MOFs, to Stockholm University twenty years ago, and much has happened since. There are currently six research groups working with MOFs at Stockholm University and in the labs at Kemikum, activity is constant. A MOF is a combination of an organic and a metallic component that can be created in test tubes, combining metal ions and organic molecules into a nanoporous material. With the help of an electron microscope — capable of imaging the crystals at a molecular scale — researchers can understand their structures and assess the potential of a newly created MOF.

“Many MOF crystals are so small that it’s difficult to determine their structures with conventional X-rays diffraction. That’s why I realized early on that electron microscopy has a large potential and focused on developing new methods to study the MOF structures,” says Xiaodong Zou.

Purifying wastewater

One of the aims of MOF research is to identify functionality in the structures — that is, useful things that different types of MOFs can do. In 2023, research at Stockholm University resulted in a MOF capable of purifying wastewater from pharmaceutical residues. Toxic molecules in the water can be soaked up and captured by the MOF like a sponge, enabling its removal.

“We found out that the combination of an organic molecule from pomegranate and the metal ion zirconium(IV) formed a new highly water-stable MOF. Since the MOF is electronically charged, it has an extra high capacity for attracting oppositely charged environmental pollutants,” says Ken Inge, research leader of one of the five groups.

Unexpected inspiration

He constantly searches for materials that could be suitable for creating MOFs. On the shelves in his office are bags of organic materials — substances he has found online or in health-food stores.

“It’s important to look for solutions that don’t already exist and to keep creativity alive. If we follow others, we will not make any large steps in the direction we want to go in our research. But if we look for inspiration outside of our immediate surroundings, the research can move in its own unique direction,” he continues.

Together with research colleague Tom Willhammar, who also leads one of the five research groups, he maintains an ongoing dialogue about possible applications for new MOFs. Everyone in the group is free to test their ideas.

“Sometimes that leads to new discoveries. If we see that there’s something new here, we let that idea grow. I think it’s very important for creativity that you’re free to explore things and test your ideas,” says Tom Willhammar.

Replaces harmful gases

However, not all MOFs are equally porous. Hanna Boström, assistant professorin inorganic chemistry, works with linker molecules that create short distances between the metals in the framework and thus smaller cavities.

“Shorter distances between the metals make them interact more with each other. This leads to the spin crossover effect,” says Hanna Boström.

Materials created from such MOFs could be used as sensors, for example to detect changes in pressure or temperature, or to detect chemicals in water or air. Another possible application is in cooling and heating systems.

“They could potentially be used in ordinary refrigerators or in cooling and heat pump systems to replace environmentally harmful gases that damage the ozone layer,” says Hanna Boström.

Safety is crucial

For refrigerants, combining safety and performance is the major challenge for both solids and gases. The material must not catch fire or leak.

“If you use a solid material made of MOFs rather than gas, for example a powder or pellets, it is less likely to leak to the atmosphere and thus more environmentally friendly,” says Hanna Boström.

MOFs for catalysis

In the corridor next door is Belén Martín-Matute. She was hired at Stockholm University twenty years ago, an international top recruitment brought in to study MOFs as catalysts in organic synthesis — that is, to speed up chemical processes. Catalysis can also initiate chemical processes that cannot be triggered naturally and can direct chemical processes to produce exactly what is needed, for example to obtain a specific pharmaceutical.

“That’s the amazing thing about MOFs — we can fine-tune them to create precisely the MOF that is needed for a particular purpose. And we have the chemical expertise required to do that. This makes the possibilities for MOFs endless,” says Belén Martín-Matute.

Chemical challenges

To strengthen the research in the area of MOFs as catalysts even more, Miguel Rivero Crespo, professor of organic chemistry, was recruited two years ago. He works at the intersection of organic chemistry, sustainable chemistry, and materials science. Together with his research team, he searches for new materials that, through catalysis, can solve difficult chemical challenges — for instance by finding MOFs with multiple functions within a single material.

“Such MOF-based catalysts could collaborate even more efficiently than ordinary catalysts to tackle truly difficult challenges,” says Miguel Rivero Crespo.

Transforming waste

A current challenge is the shortage of materials needed for products essential to humanity, such as pharmaceuticals or agricultural pesticides. One solution could be to convert materials that are easy to obtain anywhere into something that can serve as a replacement. The team recently published their first scientific paper, on oxidation of several organic molecules using ordinary oxygen from air instead of toxic chemicals. Oxidation is a chemical process used, among other things, to produce metals and chemical compounds. With the right MOF, the entire process can become both fast and environmentally friendly. The team is also working on converting biomass from nature into compounds that are difficult to obtain in industry.

“Our goal is to transform materials and substances that are readily available everywhere — or even considered waste — into something useful. Chemistry often relies on highly reactive substances like noble metals, which are both expensive and scarce. But through catalysis with MOFs containing abundant elements we can convert ordinary oxygen, plastic waste or natural biomass into valuable materials,” says Miguel Rivero Crespo.

Future challenges

And while research continues in the labs at Kemikum, the team is now searching for new paths forward, always on the lookout for methods that open the door to further breakthroughs.

“We will continue designing and discovering new MOFs catalysts that can address new challenges in organic synthesis. In the near future we will make use of high-throughput experimentation guided by machine learning in the new facilities of CircuLab at Stockholm University”, says Belén Martín-Matute.

“We want, among other things, to develop new methods that, using electron diffraction, can analyze materials faster. Identifying useful MOFs is like finding a needle in a haystack and often takes a long time — but with our methods, we can catch them,” says Xiaodong Zou.

Se a short video on MOF with Tom Willhammar and Ken Inge