Friday, 23 February 2018 15:49

A lifelong search for new catalysts

Richard Schrock, trailblazer in organometallic chemistry, delivers annual Killian Lecture.
MIT Killian Lecture 2018 01 PRESS Web
MIT professor of chemistry Richard Schrock delivers the annual Killian Lecture. Photo, Jake Belcher

On a summer evening in 1973, Richard Schrock came home from his job at DuPont’s Central Research Department and told his wife, Nancy, “I think I’ve done something important.”

As part of his research exploring synthesis of new polymers, Schrock had created a novel type of molecule with a double bond between a metal and a carbon atom. This type of compound, known as an alkylidene, had never been seen before.

Schrock, this year’s recipient of the James R. Killian Jr. Faculty Achievement Award, described at yesterday’s Killian Lecture how that discovery set him on a path that would ultimately lead to the development of catalysts that can control the formation of many kinds of organic compounds. This work, which has been applied in the chemical industry to efficiently produce pharmaceuticals, plastics, fuels, and other substances, also earned Schrock the Nobel Prize in Chemistry in 2005.

Established in 1971 to honor MIT’s 10th president, the Killian Award recognizes extraordinary professional achievements by an MIT faculty member. The award citation noted that Schrock, the Frederick G. Keyes Professor of Chemistry at MIT, has also made valuable contributions to MIT’s educational mission: He has mentored more than 185 graduate students and postdocs, and continues to serve as a lecturer in MIT’s freshman chemistry course.

“He began this challenging teaching assignment in the early 1990s and returned to it after winning the Nobel Prize, inspiring undergraduates to unravel the beauty of molecular structures and chemical reactions,” said Susan Silbey, chair of the MIT faculty, in presenting the award before yesterday’s lecture.

“Magical little machines”

Schrock’s interest in chemistry was kindled at age 8, when his older brother, Ted, gave him a chemistry set. After earning his bachelor’s degree in chemistry from the University of California at Riverside, he did his graduate studies at Harvard University, where he worked in the lab of inorganic chemist John Osborn.
“John got me interested in catalysis,” Schrock recalled. “Catalysts are these magical little machines that can make something over and over again.”

After earning his PhD, Schrock spent a year at Cambridge University, where he met a scientist on sabbatical from DuPont, and that connection eventually led to a job offer for Schrock. When he got there, “They told me, why don’t you go make a new polymer?” he said. “That means you’ve got to make a new catalyst.”

DuPont had done some previous work using the metals chromium, vanadium, and titanium as catalysts, but Schrock turned his attention to tantalum. That summer in 1973, he realized that he had created a compound containing a double bond between tantalum and carbon.
Schrock joined the MIT faculty in 1975, and through further research there he uncovered the role that these metal-carbon bonds play in a type of reaction known as olefin metathesis. This reaction had been first seen in the 1950s but was not well understood.

Yves Chauvin, who shared the 2005 Nobel Prize with Schrock, first proposed the mechanism for this kind of reaction in 1971. Olefin metathesis involves breaking and making double bonds between carbon atoms, with help from a catalyst that contains a metal-carbon double bond, which forms a ring that contains the metal and three carbon atoms.

In 1986, Schrock developed the first catalyst that could perform this type of reaction: an atom of the metal molybdenum attached to organic structures known as ligands. He won the Nobel Prize for this work in 2005 but said yesterday, “At the time of the Nobel Prize, the most important problems in this area weren’t solved.”
One major issue to be resolved was how to control the configuration of the olefin products, which can occur in one of two configurations. Since 2005, Schrock has developed new catalysts that can control these configurations, making it easier for chemists to design possible new drugs and other useful compounds.
There is always room for improvement in designing new catalysts and reactions, noted Schrock, who described himself as “a molecular engineer.”

“Once you have a reaction that you know works, you want to build on it, tinker with it, and make it go in the direction you want it to go,” he said.

Beyond Haber-Bosch

Schrock also described his other major research focus, which centers on nitrogen. Nitrogen, found in proteins, DNA, and RNA, is essential for all life on Earth, but for most organisms to use it, it has to be converted to ammonia. Many microbes can perform this conversion, and in the early 1900s, scientists began seeking their own methods.

Two German scientists, Fritz Haber and Carl Bosch, developed a chemical process that is now used to produce 300 million tons of ammonia every year, for use in fertilizer and other compounds. It is estimated that 1.4 percent of all energy used by humans goes into producing ammonia through this process, in part because the reaction must be performed at very high temperatures and pressures.

The process also releases carbon dioxide as a byproduct, so Schrock and others have been seeking alternative ways to convert nitrogen to ammonia.

In a landmark 2003 Science paper, Schrock reported that using a molybdenum catalyst he could produce ammonia in a multistep reaction that can take place at room temperature and atmospheric pressure. The process is not yet efficient enough for industrial use, but Schrock hopes that with further refinement, it could be useful within the next 20 years.

“I don’t think it’s going to replace Haber-Bosch, but I think it can reduce the amount of carbon dioxide we release into the atmosphere,” he said.

– Anne Trafton | MIT News Office
February 16, 2018

Tuesday, 28 November 2017 16:50

Turning emissions into fuel

MIT-developed method converts carbon dioxide into useful compounds.
MIT Fuel From CO2 Wu Web
XiaoYu Wu pictured with the reactor his team used for the research. MIT researchers have developed a new system that could potentially be used for converting power plant emissions of carbon dioxide into useful fuels. The method may not only cut greenhouse emissions; it could also produce another potential revenue stream to help defray its costs. Image, Tony Pulsone

MIT researchers have developed a new system that could potentially be used for converting power plant emissions of carbon dioxide into useful fuels for cars, trucks, and planes, as well as into chemical feedstocks for a wide variety of products.

The new membrane-based system was developed by MIT postdoc Xiao-Yu Wu and Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering, and is described in a paper in the journal ChemSusChem. The membrane, made of a compound of lanthanum, calcium, and iron oxide, allows oxygen from a stream of carbon dioxide to migrate through to the other side, leaving carbon monoxide behind. Other compounds, known as mixed ionic electronic conductors, are also under consideration in their lab for use in multiple applications including oxygen and hydrogen production.

Carbon monoxide produced during this process can be used as a fuel by itself or combined with hydrogen and/or water to make many other liquid hydrocarbon fuels as well as chemicals including methanol (used as an automotive fuel), syngas, and so on. Ghoniem’s lab is exploring some of these options. This process could become part of the suite of technologies known as carbon capture, utilization, and storage, or CCUS, which if applied to electicity production could reduce the impact of fossil fuel use on global warming.

The membrane, with a structure known as perovskite, is “100 percent selective for oxygen,” allowing only those atoms to pass, Wu explains. The separation is driven by temperatures of up to 990 degrees Celsius, and the key to making the process work is to keep the oxygen that separates from carbon dioxide flowing through the membrane until it reaches the other side. This could be done by creating a vacuum on side of the membrane opposite the carbon dioxide stream, but that would require a lot of energy to maintain.

In place of a vacuum, the researchers use a stream of fuel such as hydrogen or methane. These materials are so readily oxidized that they will actually draw the oxygen atoms through the membrane without requiring a pressure difference. The membrane also prevents the oxygen from migrating back and recombining with the carbon monoxide, to form carbon dioxide all over again. Ultimately, and depending on the application, a combination of some vaccum and some fuel can be used to reduce the energy required to drive the process and produce a useful product.

The energy input needed to keep the process going, Wu says, is heat, which could be provided by solar energy or by waste heat, some of which could come from the power plant itself and some from other sources. Essentially, the process makes it possible to store that heat in chemical form, for use whenever it’s needed. Chemical energy storage has very high energy density — the amount of energy stored for a given weight of material — as compared to many other storage forms.

At this point, Wu says, he and Ghoniem have demonstrated that the process works. Ongoing research is examining how to increase the oxygen flow rates across the membrane, perhaps by changing the material used to build the membrane, changing the geometry of the surfaces, or adding catalyst materials on the surfaces. The researchers are also working on integrating the membrane into working reactors and coupling the reactor with the fuel production system. They are examining how this method could be scaled up and how it compares to other approaches to capturing and converting carbon dioxide emissions, in terms of both costs and effects on overall power plant operations.

In a natural gas power plant that Ghoniem’s group and others have worked on previously, Wu says the incoming natural gas could be split into two streams, one that would be burned to generate electricity while producing a pure stream of carbon dioxide, while the other stream would go to the fuel side of the new membrane system, providing the oxygen-reacting fuel source. That stream would produce a second output from the plant, a mixture of hydrogen and carbon monoxide known as syngas, which is a widely used industrial fuel and feedstock. The syngas can also be added to the existing natural gas distribution network.

The method may thus not only cut greenhouse emissions; it could also produce another potential revenue stream to help defray its costs.

The process can work with any level of carbon dioxide concentration, Wu says — they have tested it all the way from 2 percent to 99 percent — but the higher the concentration, the more efficient the process is. So, it is well-suited to the concentrated output stream from conventional fossil-fuel-burning power plants or those designed for carbon capture such as oxy-combustion plants.

“It is important to use carbon dioxide to produce carbon monoxide for the conversion of sustainable thermal energies to chemical energy,” says Xuefeng Zhu, a professor of chemical physics at the Chinese Academy of Sciences, in Dalian, China, who was not involved in this work. “Using an oxygen-permeable membrane can significantly reduce the reaction temperature, from 1,500 C to less than 1,000 C, indicating a great energy saving compared to the traditional carbon dioxide decomposition process,” he says. “I think their work is important to the field of sustainable energy and membrane processes.”

The research was funded by Shell Oil and the King Abdullah University of Science and Technology.

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David L. Chandler | MIT News Office
November 28, 2017