Thomas J. Schmidt is head of the PSI Center for Energy and Environmental Sciences and co-director of the Swiss Center of Excellence on Net-zero Emissions (SCENE). © Paul Scherrer Institute PSI/Mahir Dzambegovic

Mr. Schmidt, what exactly is a MOF?

Thomas J. Schmidt: MOF stands for metal-organic framework. To put it simply, a MOF is a tiny but highly effective molecular sponge – with properties very similar to a kitchen sponge which absorbs liquids into pores within its structure and releases them again. A MOF does the same thing, but with individual molecules.

What does such a molecular sponge look like?

At first glance, it’s quite unspectacular. MOFs are usually crystalline powders, like table salt. But their name reveals what’s inside: They are structures made up of a metallic component and an organic component. Metal ions form nodes, and complex organic molecules, so-called linkers, connect these nodes to each other. The result is a three-dimensional, hybrid crystal framework.

Are they similar to crystals found in nature?

There’s one crucial difference: MOFs are artificial structures, often designed on a computer and grown in the laboratory. Here at PSI, too. The key is that by carefully selecting the nodes and linkers, we can produce crystals with very specific properties. That’s because, depending on which metal ions are combined with which organic molecules, different properties result, such as pores of different sizes and shapes.

Is it that we no longer have to search for suitable materials in nature, but can simply assemble them ourselves from the appropriate building blocks?

That’s the basic idea, though in practice it’s not quite that simple. For example, we have to find the right metal ions and linkers so that the framework will be stable and won’t decompose at high temperatures or in humid conditions. And this is very important: For the MOF to later do what we want it to do, we have to deliberately select and produce the linker. This preliminary work, which we carry out at the PSI Center for Energy and Environmental Sciences, is often more complex and time-consuming than the actual synthesis of the MOF, for which established methods can now be used. 

What purposes does your MOF research aim to serve?

Marco Ranocchiari, head of the new Laboratory for Energy Technologies and Innovation in our Center since the beginning of the year, brought MOF research to PSI more than ten years ago. Since then, research on these materials has been conducted in many different laboratories. Among other things, we are interested particularly in MOFs that can be used to capture greenhouse gases such as CO₂, or that could help in the production of alternative fuels.

Capturing CO₂ – to dispose of it underground like hazardous waste?

This is actually being considered. MOFs could be ideal for CO₂ capture and storage due to their highly porous structure: While each individual pore, depending on the linker molecule used, is only a few millionths of a millimetre in size, a single gram contains so many pores that their combined surface, spread out, could cover several football fields. And CO₂ and methane are relatively small molecules. A teaspoon of MOF material could easily store several litres of those gases.

But we want to go a step farther: We want to make it possible to reuse the CO₂. We are developing MOFs that can not only absorb CO₂ but also can easily release it again. This work is taking place within the Swiss Center of Excellence on Net-zero Emissions, SCENE.

What is SCENE?

SCENE is one of six joint initiatives of the ETH Council focusing on energy, climate, and ecological sustainability. Here more than 100 researchers from PSI, Empa, WSL and Eawag as well as ETH Zurich and EPFL Lausanne are working to avoid or eliminate greenhouse gas emissions, or to utilise them further. PSI leads SCENE, and I serve as one of its co-directors. Our research on CO₂ capture with MOFs in the Center for Energy and Environmental Sciences is funded by SCENE.

You said one goal of SCENE is the reuse of CO₂. Why?

We shouldn’t forget that CO₂ is a carbon source for the chemical industry. In the development of pharmaceuticals, plastics and basic chemicals, carbon is needed as a raw material. And at the moment it comes largely from natural gas. If instead CO₂ could be extracted from the air and reused, natural gas would no longer be needed. That would truly be a major step towards a climate-friendly industry and net-zero emissions, which Switzerland aims to achieve by 2050.

SCENE (Swiss Center of Excellence on Net-Zero Emissions) is a Joint Initiative, co-financed by the ETH Board. The four research institutes of the ETH Domain, PSI, Empa, WSL & Eawag, as well as ETH Zurich & EPFL are involved. SCENE covers a wide range of research areas related to the goal of net-zero emissions and provides a network for cross-institutional collaboration. © Empa

You also mentioned alternative fuels.

It's clear that if we’re serious about climate goals, we can’t keep on powering our cars, ships, and airplanes with fossil fuels in the future. Hydrogen will play a crucial role: not only as a fuel, but also in storing renewable energy, as well as in producing ammonia and other important chemicals. To obtain hydrogen, water is split into its components through electrolysis. This involves applying an electrical voltage between two electrodes in a water bath: oxygen forms at the positively charged anode, and hydrogen at the negatively charged cathode.

Where do MOFs come into play?

MOFs can act as catalysts, simplifying the formation of these gases. So the right MOFs could help make water electrolysis more cost-effective. Conventional catalyst materials often use expensive precious metals, and they can’t be custom-made. A research group led by Emiliana Fabbri and myself is investigating which MOFs are best suited for this purpose and developing corresponding prototypes. But catalysts aren’t just needed for water electrolysis; they’re also essential for using hydrogen and carbon to produce synthetic, and therefore sustainable, jet fuel. Marco Ranocchiari’s group has done pioneering work in this area and is currently building a demonstration plant on the PSI campus in collaboration with the industrial partner Metafuels AG from Adliswil.

Cables and gas pipes are an integral part of this test rig, used for the electrolysis of water and CO₂. It is operated at PSI as part of SCENE. Catalysts from the class of materials called MOFs are amongst the materials that are investigated here. © Paul Scherrer Institute PSI/Mahir Dzambegovic

In concrete terms, what does the research on MOF catalysts entail?

We investigate MOFs on several different levels. For example, Vitaly Sushkevitch and Jeroen van Bokhoven use the Swiss Light Source SLS at PSI to map out and better understand the individual steps of MOF synthesis. In the field of water electrolysis we want to know, for instance, how catalysts change during electrolysis. SLS plays a crucial role in this: its X-rays penetrate the electrolysis cells and show us what’s happening inside on the molecular level. And this happens in real time, while the chemical reactions are taking place at the catalyst. You can see how SLS offers globally unique opportunities for addressing a wide range of research questions, especially in the fields of X-ray absorption spectroscopy and X-ray imaging tomography. This is crucial in helping us identify the strengths and weaknesses of different MOF variants and to develop increasingly better catalysts. Last but not least, once we have a new MOF catalyst, it must be brought into the correct macroscopic form before it can be used in technical applications. This essential step is carried out by Andrea Testino, who among other things makes it possible to use PSI catalysts in the demonstration plant I mentioned earlier.

Has the SLS upgrade improved your research capabilities?

With the new SLS, we can take measurements much more rapidly thanks to the more brilliant X-ray beam. We now can capture complete X-ray spectra in less than a second! And if we also want to measure the ultrashort-lived molecular intermediates during a chemical reaction, we use the X-ray free-electron laser SwissFEL at PSI.

Could you single out a specific research result from your Center that you’re especially proud of?

Last summer Julia Linke, one of our doctoral candidates in the Electrocatalysis and Interfaces research group, completed her PhD with a very exciting result: She was able to show that a functional catalyst for electrolysis can be produced from an intermediate stage of MOF synthesis that omits the final crystallisation step and operates at lower temperatures. This means we are now able to significantly shorten and simplify the manufacturing process. This promises additional cost savings for potential commercial applications.

Are you optimistic that MOFs will develop into the miracle materials some say they’ll prove to be?

You have to keep in mind that CO₂ storage and catalysis are just two of countless potential applications. MOFs have the ability to separate, store, and release any combination of gases. Or consider potential applications in medicine: MOFs could be used to transport active ingredients and deliver them precisely within the body. We already know of 90,000 different types of MOFs, and computer models predict hundreds of thousands more. Even if not all of them prove to be stable, every combination of a metal ion and an organic compound brings its own specific properties. We’re far from knowing everything that can be done with MOFs.

Interview: Jan Hattenbach

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