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Greenhouse Gases

One-pot process converts CO2 captured from the air into methanol

Scientists use an alkali hydroxide–based system to turn carbon dioxide into a carbon-neutral fuel

by Janet Pelley, special to C&EN
March 11, 2020 | APPEARED IN VOLUME 98, ISSUE 10

09810-scicon11-methanol.jpg
Credit: J. Am. Chem. Soc.
A new one-pot process converts CO2 from air into methanol at moderate temperatures using a solution of potassium hydroxide in ethylene glycol, hydrogen, and a ruthenium catalyst.

Annual carbon dioxide emissions surged to more than 36 billion metric tons last year, causing climate warming and ocean acidification. Capturing some of that CO2 and then converting it into methanol—an alternative transportation fuel and feedstock for chemical synthesis—could be one way to help keep those levels in check. Now, researchers have combined the capture and conversion steps into one continuous process that uses less energy than current methods (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b12711).

Unlike scrubbing concentrated CO2 from smoke stacks, direct air capture (DAC) collects CO2 from ambient air, present at dilute concentrations. DAC uses sorbents such as liquid amines or alkali metal hydroxides that bind CO2 as it contacts the liquid. High temperatures (700–900 °C) drive the CO2 out of the sorbent for recovery, after which the gas can be pressurized and injected underground or hydrogenated to methanol.

In earlier work, Surya Prakash, an organic chemist at the University of Southern California, and his team combined amine-based capture of CO2 and hydrogenation to methanol into a seamless reaction sequence to avoid the energy penalty of the CO2 separation step. “But amines are not well suited for DAC because they are volatile and get oxidized when exposed to the large volumes of air used for DAC,” Prakash says. So the scientists tried integrating the process using alkali hydroxides, which are cheaper than amines and have a much stronger affinity for CO2. “We captured CO2 with potassium hydroxide to form an alkali metal carbonate and reduced it in the presence of a catalyst and H2 to generate potassium formate,” he says, “but then we couldn’t hydrogenate the formate to methanol.”

For the current study, the scientists surmised that alcohols such as ethylene glycol could mediate the hydrogenation pathway from potassium formate to methanol. The researchers bubbled indoor air through a solution of KOH in ethylene glycol in the presence of H2 and a ruthenium catalyst. After 72 hrs at 140 °C, the scientists obtained methanol at 100% yield.

Although most KOH was recoverable, 30% was consumed in an unwanted side reaction. The researchers will aim to avoid that side reaction in future work. Solving these problems and scaling up the process to a commercial level could take 2 to 5 years, Prakash says.

“Industries that already employ hydroxide-based CO2 capture from dilute streams could easily adapt the system to produce methanol, which has many valuable uses,” says David J. Heldebrant, a green chemist at Pacific Northwest National Laboratory. DAC coupled with methanol production could be set up anywhere. If the process is powered with renewable energy, methanol is an ideal way to store excess wind and solar power for later use on calm or cloudy days, he says.

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Comments
Prodyot Sen (March 13, 2020 12:19 PM)
This may be very much helpful in pollution control and power generation field.
Charles Heimerdinger (March 19, 2020 12:52 AM)
It's one thing to be a respected academician and perform tuition-subsidized and/or government-subsidized research at the bench level. It's quite another things to turn this research into a commercial success.

The folks at Liquid Light Company and Skyonic Corporation's Capital Skymine LLC venture attracted private and government money promising that they too would convert CO2 into more valuable products, and guess what? Capital Skymine was liquidated and Liquid Light was also "liquidated." (Get it? Liquidated? Hilarious!) More info can be found at https://www.proofofclaims.com/Skymine/documents/ and https://capricorn.be/avantium-acquires-liquid-light/.

Also see https://www.youtube.com/watch?v=9rc6Yx-fIb0 where I was highly critical of the Skyonic Capital Skymine project five years ago. That the USDOE provided a $28 million taxpayer-backed loan guarantee was just another example of crony capitalism and the ignorance and downright stupidity of the green energy crowd.(Does anyone remember Solyndra?)

Prakash then states, "If the process is powered with renewable energy, methanol is an ideal way to store excess wind and solar power for later use on calm or cloudy days." I beg to disagree. As evidence I cite the following articles:

https://www.leaderherald.com/opinion/letters-to-the-editor/2019/08/so-much-for-the-governors-energy-plan/,
https://www.leaderherald.com/opinion/letters-to-the-editor/2019/09/solar-energy-is-a-losing-proposition-for-the-ratepayers/, and https://www.leaderherald.com/opinion/letters-to-the-editor/2019/09/pumped-storage-will-be-necessary-and-inevitable/ (actually 10 GW, not 5 GW, of pumped storage would be needed, and NY State doesn't have the enough sites to do 10 GW).

So in my opinion Dr. Prakash's goal of commercializing DAC will not succeed.
Dave (March 22, 2020 12:57 PM)
So do we have enough H2 to do this?
Ogaba Philip Obande (March 22, 2020 1:56 PM)

Criticism is good for progrss. I think this investigation should be funded to pilot plant stage if the the present optimistic outlook sustains. No success without trial. The approach is commendable but the unreported fine details might make the difference.
philip (April 3, 2020 3:32 PM)
I think the economics of this current reaction design should be well elaborated even at this scale. Hydrogen required, ruthenium used, access to the CO2, energy released by the methanol, importance of the CO2 consumption to global health temperatures and energy consumed in the process. All these factors should inform the application of this process and reaction.
Vince (April 8, 2020 8:37 AM)
How did they get a 100% yield? I thought that was impossible. Also 100% based on CO2 or 100% based on H2?

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