Turning carbon pollution into ethanol
Not only do we need to cut carbon emissions to curb climate change, but many scientists say we also need to capture excess CO2 from the air to prevent continued climate impacts. Currently, however, carbon removal tends to be costly, and polluters may lack incentives to clean up their emissions.
One solution is to create useful products from captured CO2, thus increasing the value of drawing it out of the air. Stanford engineers are proposing a way to do just that.
In two recent papers, published in the Journal of the American Chemical Society and Angewandte Chemie, a team of Stanford researchers demonstrated that it’s possible not only to convert carbon dioxide to ethanol but also to do so with very few byproducts. Previous efforts have not been able to design a reaction converting the gas with such high selectivity.
The researchers chose to focus on ethanol – the same stuff that’s in alcoholic beverages – because the chemical is useful for many industries such as fuels, polymers, and pharmaceuticals. But right now ethanol is mostly made from crops such as corn harvested from vast acres in regions like the Midwest – the process is both land and resource intensive.
The researchers propose a win-win: taking captured carbon dioxide and making ethanol, which would further reduce emissions by replacing more energy-intensive ethanol sources. “If we are able to turn CO2 into valuable chemicals, we close the cycle,” said the lead author of the two papers, Chengshuang Zhou, recent PhD graduate in chemical engineering. “We have the opportunity of not producing more carbon, but instead circulating the carbon that is already out there.”
Creating the catalyst
To test this idea, the researchers needed to combine carbon dioxide with hydrogen. But you can’t just put the two gases in a cylinder and shake them to make ethanol, because the reaction isn’t favorable. So the team needed to find a catalyst that would provide an extra kick, moving the reaction in the desired direction.
To do so, they first considered the molecular structure of ethanol. “How do we cut several bonds to reconstruct CO2 so that we can retroactively come up with ways to synthesize ethanol from CO2?” Zhou asked. “This reaction would necessarily involve not only hydrogenation, essentially adding hydrogen to CO2, but also coupling two carbon atoms into one single molecule.”
He identified that an ideal catalyst must produce at least two fragments, one deeply hydrogenated like methane (-CH3) and one partially hydrogenated like methanol (-CH2OH), which can then come together to form ethanol. He scoured the scientific literature for elements that can both synthesize methanol and encourage hydrogenation, arriving at ruthenium and indium. “Ruthenium has been well documented to be a great catalyst for the CO2 hydrogenation reaction to produce methane,” he said, while “indium oxide-based catalysts have been documented to catalyze CO2 hydrogenation to methanol.” The perfect match.
Using nanoscale design and synthesis, the researchers created a ruthenium-indium oxide (Ru/In2O3) catalyst and put it to the test. In a stainless steel tube, the team flowed carbon dioxide and hydrogen across a small scoop of the powdered catalyst, adding pressure and heat to speed the reaction, and analyzed the gases that came out the other end.
In their initial run, they produced mostly methanol, a promising first step reported in the Journal of the American Chemical Society that provided clues to the catalyst’s function. Now that they had one of two components, they could tweak the catalyst’s structure to encourage more stability, and thus produce ethanol.
After the changes, they ran the experiment again and found that the input gases converted to 70% ethanol and only carbon monoxide as a byproduct, a finding reported in Angewandte Chemie. “That’s the biggest discovery,” said Matteo Cargnello, senior author of the papers and an associate professor of chemical engineering. “There was a way to direct this reaction to make mostly ethanol rather than other products.”
This high selectivity of conversion is important because separating out ethanol from a mixture would be more energy intensive, which would somewhat defeat the purpose of the research. “If you can selectively make a simple, single product, that will absolutely reduce emissions,” said Cargnello.
Scaling up to smokestacks
While the team performed the experiment in a small tube, they believe the process could scale to settings like industrial chimneys. Before this work, “there hasn’t been a concise guiding principle” for converting carbon dioxide to ethanol, said Zhou. By demonstrating the potential to produce a high rate of ethanol, he believes the work sets the stage for future efforts to use metals that are more abundant. Ruthenium and indium are scarce metals, and using them for mass production of a catalyst would be costly.
That’s Cargnello’s next step. He will work with elements in the first row of transition metals on the periodic table, which are far more common and therefore cheaper to scale. He plans to develop a catalyst that can be widely used in conjunction with various sources of carbon dioxide – including captured from the air or from a smokestack – to turn the gas into ethanol, which can then be bottled and sold.
“Our hypothesis is that we can make it work with cheaper metals and then be able to scale it up,” Cargnello said. “Because we certainly want our discovery to not just be an academic discovery, we would like it to be translated into a larger scale process.”
Cargnello is also a member of Stanford Bio-X.
Additional Stanford co-authors of the papers include graduate students Gennaro Liccardo, Jinwon Oh, and Sindhu Nathan; and postdoctoral researchers Michael Stone, Eric J. McShane, and Baraa Werghi.
Other co-authors include researchers at the California Institute of Technology, SLAC National Accelerator Laboratory, and Lawrence Berkeley National Lab.
This work was funded by the Novo Nordisk Foundation through the CO2 Research Center, and benefited from initial work funded by the Packard Foundation, the Sloan Fellowship, and the Precourt Institute for Energy at Stanford.
