MOFiosos

MOFiosos

MOFiosos

Berkeley scientists make carbon a structure it cannot refuse

by Zoey Herm

Credit: Leah Anderson

A new class of materials called metal-organic frameworks (or “MOFs”) has injected a surge of energy into the scientific community. These materials are novel because they allow scientists to rationally design nanometer-scale structures and the empty spaces they define. With this control of empty space comes the ability to manipulate the chemistry of any molecules trapped inside, a powerful tool for many important applications. For this reason, chemists around the world are racing to make the newest, most exciting versions, advancing the frontier of this new field at breakneck speeds.

Among scientists in Berkeley and around the world, however, the benefit of MOFs to society is up for debate. While they certainly have furthered our understanding and control of the chemical world, MOFs have yet to be applied in the real world. In particular, while MOFs are regularly touted as a potential solution for global warming, their promise as carbon dioxide (CO2) capturing materials has yet to leave the laboratory. What remains to be seen is how this balance between fundamental and applied contributions will shift in the future.

At UC Berkeley, Professors Berend Smit, Jeff Long, and Omar Yaghi have devoted much of their careers to metal-organic frameworks, and each has his own perspective on the debate between the fundamental and applied benefits of the field. “We are fortunate in MOF chemistry,” says Yaghi. In studying MOFs, he feels he can approach chemistry from a fundamental perspective and let the applications materialize on their own. “We have a responsibility as scientists to be thinking about benefiting society. In this case, we’ve done that.” But Yaghi’s approach has never wavered from a purely fundamental one, despite the fact that he doesn’t forget about his obligation to the greater good. “We sometimes lose sight that the most difficult problems facing society are often solved by those of us who push the frontiers of knowledge, often without having a societal problem in mind.”

These two perspectives—fundamental science aimed at basic understanding and the development of viable applications for the real world—define the extrema of MOF research on campus. In between lies a continuum of many other researchers. But some, including Smit, prefer not to take sides. “The difference between applied and fundamental is overrated,” he says. “The intellectual challenges in applied questions and fundamental questions can be equal.”

As they design, synthesize, and study fascinating new materials on a daily basis, Berkeley’s MOF scientists are breaking down the barriers between these sometimes dichotomous approaches to research. And while the potentially calamitous consequences of global climate change are looming, there is no shortage of inspiration among those trying to bring MOFs to the world.

MOF chemistry 101

Omar Yaghi is widely known for publishing the first report of metal-organic frameworks in 1999. But he has always stated that he wasn’t ever searching for the next big thing, and definitely not trying to create a new field of chemistry. He was simply trying to make materials that were beautiful. In the ensuing years, thousands of MOFs have been made and new bells and whistles are added to them daily. Some can bend, or stretch, or even incorporate movable parts that can be made to spin on command. Many are brightly colored and researchers have taken advantage of this by designing MOFs that will only change color in the presence of toxic molecules like cyanide.

Metal-organic frameworks do their jobs by bringing order to the chaotic realm of small molecules. Ask yourself, what would it take to capture and store a gas like carbon dioxide, to stop it from entering the atmosphere, or hydrogen for use in a zero-emissions vehicle? Now imagine nano-scaled scaffolding (like at a construction site) that can catch or release these small molecules one at a time as they flow through the cages and pores. Metal-organic frameworks are exactly this: perfectly repeating cages filled with empty spaces that are around 10 nanometers across—just the right size to trap a few small molecules. As a result, they represent a new era in materials chemistry in which empty space can be designed for a purpose. “After the first few reports that we made on MOFs where we showed they were indeed porous, and things could be put into the pores and taken out without collapse of the framework, I think it got the attention of a lot of people,” says Yaghi.

MOFs are made of two types of building blocks: linkers and nodes. The long linkers are organic molecules, strings of carbon and hydrogen decorated with oxygen or nitrogen. These have at least two arms that preferentially bind to positively charged metal ions. (Metals occupy a broad swath of the periodic table, from sodium in one corner to livermorium in the other.) The metals form vertices, or nodes, between the organic linkers, making an infinite, repeating structure.

Metal-organic frameworks are made by simply adding linker and metal to a solvent (sometimes water) and heating for a couple of hours or days. During the heating process, the linkers and metals find each other and settle into the final orderly MOF structure. Then a vacuum can be used to pull out any extra solvent molecules that might be trapped inside the vacant spaces. The synthesis requires finding the perfect recipe of linker, metal, solvent, and temperature to get the thermodynamics just right to generate a MOF—otherwise the resulting product can look like a knotted jumble without the pores needed for controlling the small molecules that flow inside.

MOF design for carbon capture

Credit: Leah Anderson

After the discovery of MOFs, research efforts were quickly trained on applications for the new class of materials. One very popular application, so much that all of Berkeley’s MOF scientists are now working on it, is to use MOFs to filter the carbon dioxide from the smokestack of a power plant. Most climate experts calculate that carbon capture and sequestration (CCS), as this filtration is known, will be necessary to reduce CO2 emissions to a sustainable level. If MOFs can successfully capture carbon dioxide, they could be a dream come true in the fight against global warming.

Indeed, metal-organic frameworks could be the perfect answer for CCS applications. Their cages are just the right size to hold carbon dioxide molecules, and they can be modified in almost any way imaginable to take advantage of the unique behavior of CO2. If designed with this functionality, all of the other benign molecules that come out of smokestacks, like water and nitrogen, would pass straight through the MOF scaffolding, while the CO2 remained inside.

Because of these promising possibilities, Professors Yaghi, Long, and Smit are all working to make metal-organic frameworks that selectively bind carbon dioxide. Using their varied expertise—Yaghi and Long focus on experimental methods, while Smit’s group is purely computational—and taking advantage of what they already know about MOFs, their research groups are trying to build new, better MOF materials for CO2 capture.

With the dozens of metals on the periodic table and a virtually limitless number of organic linkers that can be imagined, an essentially infinite number of MOFs are waiting to be created. The shapes, sizes, and chemistry of the spaces inside metal-organic frameworks can be designed for any application where a small molecule needs to be controlled. The astronomical number of combinations begs the question: where does one begin in making new MOFs? Currently, two approaches—rational modification and high-throughput chemistry—span the experimental work done by Yaghi and Long and are bolstered and supplemented by the Smit group’s computational Monte Carlo methods.

Designing for discovery

Like synthetic molecules, MOFs can be tweaked and altered to take a good thing and make it better. The vast majority of MOF research is in these efforts toward rational modification, rather than making brand new MOFs from scratch. For example, imagine that a MOF that is good at CO2 capture also degrades in water. Chemists would search for a way of changing the linker or changing the metal that would render the MOF more stable without sacrificing any CO2 capture performance.

Yaghi works on variations of MOF-5, a flavor of metal-organic framework with zinc oxide nodes and benzene-derived linkers. He has discovered that by adding subtle modifications to the linker, he can drastically alter its ability to capture CO2. All of these modifications involved adding small functionalities like hydroxyl (-OH), chloride (-Cl) or amine (-NH2) groups onto the middle of the linker. Because the linker doesn’t change length or the way it binds to the metal, the skeleton stays the same but different small groups poke out into the channels and are given the chance to interact with the CO2 molecules that float through. One of his most interesting discoveries in this area of study is that by combining many of these modified linkers within the same MOF-5 framework, more CO2 can be captured than with any of the individual linkers in isolation. The underlying principles behind this discovery are not yet completely understood.

Long’s lab is interested in how to modify existing metal-organic frameworks to make them better materials for capturing carbon. One idea takes advantage of a very old method for capturing carbon that can’t be used industrially because it is too inefficient: amine dissolved in water. An amine is a small appendage to a molecule that contains a nitrogen, in addition to a few other specifications. Amines are special because they will react strongly with CO2 but have no interaction with any other component of air.

In practice, these dissolved amines are hugely wasteful because heating and cooling water is energy-intensive. Metal-organic frameworks are a much more promising approach because the gaseous mixture of CO2 and air can flow through the empty space as opposed to bubbling through a mixture of water. To mimic this, the Long lab has installed amines that dangle into the pores of metal-organic frameworks and are perfect for picking up CO2 before it is emitted into the atmosphere.

The reversibility of CO2 capture inside metal-organic frameworks is one of the most promising aspects of the technology. With a little bit of extra heat, the CO2 will float out of the channels as a pure gas that can be sequestered underground, leaving the MOF ready for another cycle.

Casting a wide net

Credit: Leah Anderson

None of the thousands of existing metal-organic frameworks are a perfect carbon capture material yet. For example, there are many MOFs that are great at capturing carbon but bad at releasing it. Industrially, many such criteria must be met for a material to be able to withstand the harsh conditions of a power plant, and near-perfect efficiency of holding CO2 and releasing it at just the right conditions is needed.

Because a viable MOF doesn’t exist, as-yet-unimagined MOFs need to be created and tested. Making new MOFs is a completely different type of chemistry from the rational modification and detailed study of existing MOFs discussed above, and much less developed. Chemists don’t have a good way of designing a structure and then picking linkers and metals that will arrange to create it. Similarly, it is often impossible to predict what the final structure will look like when a new metal-linker combination is reacted for the first time.

Chemists have found a way to make new MOFs even though they are essentially feeling their way through the dark: try everything. This approach is called high-throughput chemistry. The first step involves combinatorial chemistry: reacting dozens of existing metals with dozens or hundreds of linkers, and trying each reaction in many different solvents. To facilitate this approach, both the Yaghi and Long labs use robots, allowing them to make hundreds or thousands of new MOFs in a week.

The Long lab takes this a step further by using a streamlined approach to quickly test dozens of MOFs for their ability to capture CO2. To accomplish this, they couple characterization tools to the high-throughput combinatorial syntheses discussed above. After a large number of new MOFs are made, they are all tested for CO2 capture and the results are entered into a database. Information about promising materials can then be data-mined to distill useful information about what kind of linkers, metals and solvents are effective. This feedback can then guide further synthesis, progressively increasing the likelihood of finding viable new MOFs.

Computational methods

Smit’s lab evaluates the carbon dioxide capture potential of these tailored metal-organic frameworks theoretically, using a computational method called Grand Canonical Monte Carlo simulation. In this method, simulated gas molecules are allowed to flow freely through the structure of a particular MOF and interact with its chemical environment (see “Toolbox”, 53). By running such a simulation, Smit can see exactly where CO2 molecules go and how many can fit within the structure of interest. “It’s like developing a flight simulator for molecules,” he says.

These computational efforts can free the high-throughput approach to MOF design from the confines of synthetic methods. In fact, theoretical studies can calculate structures for hundreds of times more materials than could possibly be done synthetically. The Smit lab recently published a study that used Monte Carlo simulations to compare the CO2 capacity of hundreds of thousands of metal-organic frameworks. Using these vast amounts of data, they were able to identify patterns and important hallmarks of what might make the best carbon capture material.

The role of funding

This crystal is smaller than your fingernail, but it contains at least a billion nanoscale cages! The crystal is made of a MOF called MOF-5, which is composed of phenylene linkers connected by zinc oxide nodes and is a specialty of Yaghi’s lab. Experiments on crystals like this demonstrated that MOFs containing many different functional groups
can be more than the sum of their parts. Yaghi is still not sure why some mixed MOFs outperform any pure MOF that could be made out of just one of the mixed MOF’s many constituent linkers. Credit: Hexiang Deng and Omar Yaghi

The US Department of Energy (DOE) is funding all facets of MOF research at UC Berkeley and Lawrence Berkeley National Laboratory (LBL). In addition to funding a new applied research center at the Molecular Foundry, LBL’s nanoscience facility, the DOE has also funded two multi-lab, multi-year CO2 capture projects. One of them is the Energy Frontier Research Center (EFRC), which funds basic science with the hope that a good carbon capture material will be discovered if scientists are given the freedom to explore the unknown. Thus, while still focused on a particular application, EFRC grants have only limited constraints: researchers are asked to pursue work that is interesting to them, follow up on exciting leads, and test any new materials they make for their ability to capture CO2. This allows them to find new mechanisms of catching CO2 and investigate them thoroughly, without worrying about the commercial viability of their results. Professors Smit, Long, and Yaghi all work together on one EFRC grant that funds most of their work on rationally designed and tailored materials. As Smit explains, the approach encouraged by the EFRC is “very fundamental research, but in addition you make the effort that it’s actually useful in the end. It’s positioning yourself within the field of fundamental research rather than compromising fundamental research.”

In contrast, the Advanced Research Projects Agency-Energy (ARPA-E) is a special funding agency created to take academic technologies and quickly make them industrially viable. Both Long and Smit are part of an ARPA-E funded project that is designed to use robotics and computation to find the perfect MOF for carbon capture. In contrast to a more fundamental approach, materials are tested for CO2 capture before anything is known about their structure, and the funds don’t cover any detailed understanding of what makes one material better than the other. The work for this project entails monitoring and maintaining the robots and organizing the data that they generate.

While both the DOE and EFRC funds ideally will be used to find new MOFs for CO2 capture, the methods of arriving at this endpoint are surprisingly different. Eric Bloch, a chemistry graduate student in the Long group, has contributed to both the EFRC and the ARPA-E projects, and prefers the EFRC. “I came to grad school to do basic science. If it’s ultimately used for an application it’s a bonus.” He sees ARPA-E’s rigid focus on applications as a major drawback since it doesn’t always leave room for curiosity-driven experiments. “There are basic science questions that we have to ignore,” Bloch says.

Outlook

MOFs have yet to be used for CO2 capture and are years away from being a viable technology in that regard. But that doesn’t mean they aren’t close: a large chemical company, BASF, has recently begun making MOFs in large quantities with the intent of selling them to industry.

From a basic science perspective, MOFs already have made an invaluable contribution to science and will continue to do so. Their intrinsic beauty lies in their ability to tailor the walls around empty pores so they can finely control small molecules. And they have pushed the boundaries of what materials can do and can be. “MOFs present a whole new field of chemistry that young people can be dabbling at for the next generation at least,” says Yaghi.

To that end, Yaghi has created a laboratory in Vietnam, which he considers one of his most important projects so far. To him, MOFs are so special because their hierarchical structure allows MOF scientists to have an idea and make a new material that will match the structure that was imagined. He wanted to share that with universities that don’t necessarily have thriving research centers, so that students anywhere could begin to learn how to conduct world-class research.

From this vantage point, Yaghi has a unique perspective when it comes to the most useful applications of MOFs, one he says he’s never mentioned publicly before. When discussing one new material recently published in the journal Science, one that Yaghi had been told could never be made, he said it was “maybe a useless compound in the sense that it’s not going to be put in every power plant to capture carbon dioxide, but it stands for young people as an example of some achievement that was thought impossible but now is possible. That to me should always be part of working in science—to inspire young people. It’s an important application.”

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