Capture and Convert

PSC Helps University of Pittsburgh (Pitt) Team Design Material to Capture and Turn CO2 into Useful Products

A new material may be able to capture carbon dioxide and turn it into a commercially useful substance, according to a team at the University of Pittsburgh. Using PSC’s Bridges system, they simulated two “metal oxide framework” materials that simulated removal of carbon dioxide from exhaust gas. Better, the material also converted it into formic acid, which can be used to make products like methanol fuel. If the material works as well in the lab and factory as it does in the computer, it could fundamentally alter the economics of limiting human CO2 release and avoiding climate change.

Why It’s Important

In 2017, human release of carbon dioxide (CO2) gas into the atmosphere increased by 1.6 percent. That may not seem like much. But it ended a two-year period in which global emissions were flat. Given projections of another increase of 2.7 percent in 2018, that means a record 37.1 billion tons of extra CO2 going into the air each year. Tons of a gas. That makes the goal of holding the average global temperature increase by 2030 to 1.5° Celsius (2.7° Fahrenheit) vastly more difficult—and expensive. Technologies for capturing carbon as it emerges from power plant and factory exhausts are attractive. But they add another headache. What do you do with the captured CO2? How do you store it so that it can’t escape? Will disposing of billions of tons of carbon only add another cost?

“The basic idea here is that we are looking to improve the overall energetics of CO2 capture and conversion to some useful material, as opposed to putting it in the ground and just storing it some place. But capture and conversion are typically different processes.”—Karl Johnson, University of Pittsburgh

Karl Johnson and his team at the University of Pittsburgh wondered if it couldn’t be possible to make carbon capture a “win win.” They thought they might be able to design a material that could both capture CO2 and turn it into something valuable. Such a converter could generate a saleable product that might improve the economics of its installation and operation. It could also allow us to continue using fossil fuels as carbon-neutral fuel technologies mature and become less expensive. To pursue this goal, the Pitt team turned to PSC’s Bridges.

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The simulated candidate metal oxide framework (MOF) converts carbon dioxide molecules (gray and red balls) to formic acid by reducing them with hydrogen (white balls). Image reproduced by permission of Karl Johnson and The Royal Society of Chemistry from Catal. Sci. Technol., 2018, 8, 4609-4617, DOI: 10.1039/C8CY01018H.

How PSC and XSEDE Helped

Postdoctoral fellows Jingyun Ye and Lin Li of Johnson’s team began work designing a metal oxide framework (MOF) to achieve their objective. MOFs are particularly attractive for this kind of work. Scientists can alter the materials’ meshwork molecular structure fairly easily. By adding specific chemical groups, they can make the MOF perform desired functions. As a first stage in this work, the scientists created candidate materials virtually, in the computer. In these simulations, they watched how the new materials interacted with simulated CO2 molecules. Bridges was particularly useful for this work because its 800 “regular shared memory” nodes made it easy for them to explore many candidate reaction pathways at the same time.

“We had to do a bunch of calculations—what are the reaction pathways, what are the barriers, what are the possible products and their electronic structures. That requires a pretty huge computational effort, with a lot of computations in the pipeline over a relatively short time.”—Karl Johnson, University of Pittsburgh

The Bridges simulations produced some eye-opening insights into how CO2 behaves in the presence of their candidate materials, called MIL-140B-NBF2 and MIL-140C-NBF2. Normally, chemists convert the gas into formic acid by first activating a hydrogen molecule. Then they make this hydrogen react with the CO2. But the Pitt scientists found that, by putting the right chemical groups into their MOFs, they could also squeeze the CO2 molecules. This primed the CO2 for the reaction as well. The extra step made the reaction more efficient and easier to achieve at a lower temperature. This is important because higher temperatures mean more energy must be added to make it work. A lower temperature, then, reduces the cost.

“The thing about MOFs is they’re highly tailorable. You can design new MOFs with the properties you want; it’s a matter of picking the right building blocks that will determine size and functionality.”—Karl Johnson, University of Pittsburgh

The temperature needed for the reaction in their simulated MOFs is modest. It might be achievable by heating the exhaust with solar energy. It might even be possible with captured waste heat from the process that’s generating the CO2 in the first place. The team reported their results in the journal Catalysis Science and Technology in September 2018. They’re now working with colleagues at Pitt to convert their simulated chemicals into practical materials.