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Exploring Crystals and their Environments: Susumu Kitagawa on the Evolution of MOFs

Susumu Kitagawa is a researcher renowned for his seminal 1997 paper describing a porous metal-organic framework (MOF) for adsorbing small molecules. Still a pioneer in synthesizing and developing novel applications for MOFs, Kitagawa’s journey began with simpler structures in the late 1980s.

The evolution of his work parallels many of the monumental changes that have occurred in the field of “coordination chemistry” over the past 30 years — starting with early work by Kitagawa and others to explore not only the materials themselves but also the spaces in between. This body of work has uncovered materials with exciting surfaces and environments such as MOFs that enable novel applications in a variety of industries.

Material Insights spoke with Kitagawa, who is now at the Institute for Integrated Cell-Material Sciences (iCeMS) of Kyoto University, about his early work, the current trends he sees in adoption of MOF technology, and his dream application of MOFs (hint: it involves creating something out of the air).

Insights: You have said the turning point for your work was in 1989 at Kindai University in Osaka. What happened then?

Kitagawa: I was an assistant professor at Kindai University, studying extended structures in coordination chemistry. I was interested in materials for electrical conductivity and was looking at the copper ion, which has a monocation with a spherical electron configuration and can readily form single crystals with 3-D extended structures when bound to organic molecules.

We had synthesized some linear chain compounds and wanted to look at molecular-level structure in the crystals, but Kindai did not have the necessary computer for analysis. So I took several students to Kyoto University, where I had gotten my Ph.D., to use its X-ray crystallography analysis software and high speed computer. The high-speed computer was the only one in the university’s computing center.

The computer for the X-ray crystallography was in high demand, so we had to wait hours for the result. In the meantime, my students started to look at some preliminary results to see if they could determine the structure. And it was beautiful honeycomb structure and had holes with organic guest molecules! And this was really the start of looking beyond the frameworks themselves to the empty spaces. This result was published in 1992.

Insights: And this then led to your paper in 1997 on MOFs. How?

Kitagawa: Over the next several years, many research groups would synthesize and show new coordination networks but had yet to show it was possible to make solid pore structures with organic matter. The early coordination networks were fragile —removing the pores would cause the structures to collapse — or were highly dense with no porosity. I struggled to find a robust framework that would be porous. But in 1996, I finally had an experiment that showed this was possible with a MOF — and this was epoch-making. The soft porosity of the MOF was different than the zeolite, which was stiff.  Our MOF system was very clear, very beautiful but had a soft nature, showing crystals changing from one state to the other state in response to chemical and physical stimuli.  I thought “this is a very new compound, a big discovery.”

Since then, I have created new concepts in MOFs and porous coordination polymers, for example: soft porous crystals (local and global dynamics) as mentioned above, polymer synthesis, capture and storage of dangerous gases (acetylene, 2015), and MOF melting, among others. Recently, we’ve been able to observe crystal surfaces using atomic-force microscopy. It’s amazing because these surfaces are very active.

Insights: After this rich history of synthesizing and developing MOFs as a new material, what do you see as the next big challenge?

Kitagawa:These are completely new materials. So even in the case of industries that are interested in using them, they have to set up new devices and equipment to accommodate them. For example, in Japan, many companies have special vessels for storing and transporting gases, and they don’t want to change them. That’s a big issue, but it is gradually changing, as more companies come in to help provide not just the materials but also the know-how and processes to drive the scaling and application of MOFs. Over time, we will change this mindset.

Insights: What do you think is the most compelling case for industries to make this investment in MOFs?

Kitagawa:There are certain efficiencies that MOFs have that make them superior to their competitors. For example, in separation, using porous structures like MOFs could greatly increase capacities for separation in comparison to activated carbons.  Ultimately showing these advantages in more products over time will help change the mindset.

Insights: What is the general trajectory you have seen in the field of MOF research and development?

Kitagawa:We have seen interest in MOFs surging at different times. First it was storage, then separation, and then catalysis in terms of applications of this new chemistry. Right now, use of MOFs in storage is maturing. The trends are always changing.  In the history of humans and the activities in the sciences and industrialization, it’s too early to make good predictions. One of the biggest changes is the switch to companies bringing in expertise on how to synthesize, characterize, and integrate these materials. This is important step. In Japan, many of the people who run chemical companies have social science backgrounds and so, understandably, do not have the background to evaluate MOFs and new materials. So having other companies that can bring the materials along with the expertise and processes is an important step.

Insights: What is your dream application of MOFs?

Kitagawa:I have a big dream.  My dream is to make amino acids from the air.  Air contains CO2 and nitrogen, of course, and oxygen, and hydrogen to make water. So, the elements are carbon, oxygen, nitrogen, hydrogen, and from these elements, we can get amino acids.  We just need to separate these substances and condense them because when there are diluted in the air, no reaction will occur. Condensing them in small pores will allow them to combine. And then if we use an active catalyst or something like that, we can convert these materials into useful ones.

In Japan, we say we are a country with natural resources. But right now, we don’t have natural resources like petroleum and natural gas, so creating new resources out of the air is my dream.

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