Dr. Hong-Cai “Joe” Zhou has spent decades conducting fundamental research in coordination chemistry. His work on has helped shape the development of metal-organic frameworks (MOFs) that can be used in large-scale commercial production. Despite the misconception that MOFs are too expensive for commercial use, Dr. Zhou believes new pore engineering tools in development now mean it is only a matter of time before we see more MOF breakthroughs move from the lab to commercial use.
Material Insights spoke with Zhou, who leads the Zhou Research Lab at Texas A&M University, about his pioneering work in pore engineering, the role of MOFs, and the growing field of interdisciplinary “framework chemistry.”
Insights: Tell us a little bit about your work has evolved in the last few years.
Zhou: In the last 5 years, we have focused on creating a toolkit for modifying zirconium MOFs, which includes “linker labilization,” linker installation, linker reinstallation, and retrosynthetic design. Generally speaking, when you have a relatively stable MOF, you can modify that with a lot of different methods. When you use organic chemistry to modify a linker, it’s straightforward; you go directly from organic chemistry to MOF chemistry. What we were trying to do was to take advantage of the metal-linker coordination bonds, especially the kinetic “lability” of the bonds. Linker labilization is a method to increase the MOF porosity and pore size.
Most of the coordination bonds are kinetically unstable and constantly subject to change. For example, water exchange rates of coordination cations can reach as fast as ten to the twentieth power times per second. We now realize that in MOF chemistry, we can take full advantage of that. We can not only exchange the bonds but also selectively remove, for example, some bonds to create a cavity or defect or insert a linker inside the defect. We have even seen work that expands the whole crystal or shrinks the crystal based on ligand substitution.
In the past, that would be unimaginable because, normally you have a network of strong bonds in the solid state, so you need a lot of energy and have to go to extremely high temperature and pressure, to go from one structure to another, such as in the case of converting graphite to diamond. But in a MOF, due to the porosity, you can achieve it easily through pore engineering. So, we now have a toolkit to modify the porosity, functionality, and other features of the pores almost at will. It’s a very powerful set of tools, which enables a new way of arranging atoms in 3-D space. The pores generated rationally can control what’s happening inside the pores physically, chemically, and even biologically.
Insights: How did you first become interested in MOFs?
Zhou: My background goes back to my Ph.D. work with Al Cotton. My main focus at the time was to study metal-metal multiple bonds, mostly chromium-chromium quadruple bonds. And then I did my post-doctoral work with Dick Holm at Harvard and worked on synthetic model compounds for the active sites of iron-sulfur proteins.
When I started my independent work, I realized it was important to branch out into work different from my mentors’. So, I started searching in the literature, and at that time, in the late 1990s, there were initial reports from researchers on MOFs as a porous material. When I looked at the research, although I had never worked on porous materials before, I saw papers published by the pioneers and realized this is something that’s going to be big. I also realized that my training from graduate school and my postdoctoral training in synthetic techniques and x-ray crystallography were actually quite useful for this new research field. That’s how I got started, and I began hiring graduate students and postdoctoral researchers in 2002.
Recently, we have seen the potential for biomedical applications of these technologies. This is just the beginning of a very exciting era in MOF applications.
Insights: What drives your work now?
Zhou: The whole field of MOFs has shifted toward real-world applications, as well as the discovery of new chemistry. We still have a long way to go but we have made a lot of progress. There’s been a change of mentality and focus and is part of my lab’s current drive.
Insights: What are you most excited about in the MOF field right now?
Zhou: Because I’m a chemist, I’ve always wanted to change the rate and shift the direction of chemical reactions, which means working on improving the selectivity and activity of catalysts. Currently, I am most interested in gas and energy storage and separation. For selective adsorption and separation, my lab has participated in the Center for Gas Separation, led by Jeff Long and Omar Yaghi and funded by the DOE Energy Frontier Research Center program, which has played a leading role in MOF separation research over the last 10 years. It is very satisfying to see MOF applications in separation develop so quickly. My research group has written some of the first reviews to stimulate work in the engineering community. Recently, we have seen the potential for biomedical applications of these technologies. This is just the beginning of a very exciting era in MOF applications.
Insights: When you think about applications for catalysis, what do you have in mind?
Zhou: I think for catalysis, especially with large industrial processes, most of the time they are heterogenous gas phase procedures, and that’s how you use a flow system and then scale it up to a mass production level. Big companies such as DowDuPont, BASF, and Exxon-Mobil play a major role in our daily lives, but their processes have not yet been affected by MOFs. I am excited and interested to see those types of catalysis reactions in real-world applications and how they can be improved down the road.
Insights: What do you think are the biggest challenges facing commercialization of MOFs?
Zhou: I think the biggest hurdle is time. Given time, we will get there. A lot of times, people say we have a lot of MOF researchers, so why haven’t we seen more discoveries? A true effort toward applications with MOFs has only been happening for 20 years. For a lot of this technology, especially in big industries, you need a lot more time than that. Sometimes we have to be patient and keep doing what we’re doing, and we will get there.
In terms of science or chemistry, there are no real hurdles preventing us from getting there. And progress has been accelerating in the past several years. You can tell from new results coming out of MOF labs worldwide: Newer generations of researchers, even ones not in chemistry–from materials science/engineering, chemical engineering, biological engineering–they are all working on MOFs. With the collaborative effort and across several disciplines, we are going to see a lot of surprising breakthroughs in the near future.
Insights: What is the biggest misconception people in industry have about MOFs?
Zhou: There are a couple: One is that MOFs are unstable. A lot of MOFs are really stable. Some others are unstable, but not all. The stability, obviously, is a requirement for a lot of applications, but not for every application. Some may need the MOF to be kinetically labile, so you can change it easily. That’s one big misconception.
Another misconception is that MOFs are expensive. If you work on a milligram-scale on a bench, it’s going to be expensive. But once you scale for an application, the prices come down and it becomes affordable. A lot of petrochemical products, for example, when first discovered were prohibitively expensive, and that has dramatically changed as they become more broadly adopted in the marketplace.
Insights: What do you think MOFs may look like in society 20-30 years from now?
Zhou: It’s obviously difficult to see ahead, but I believe that when you compare MOFs to a lot of other materials, MOFs have better prospects for real-world applications. It will even help us to understand chemistry itself a little better. The biggest advantage for MOFs is that you can design this small environment such that you can control a reaction or a physical process happening inside. With recent developments in electron microscopy, together with the rapid improvements of X-ray diffraction methods, we can expect more breakthroughs in MOF chemistry and pore engineering in the near future. Ultimately, we want to develop a mature technology.