Turning CO2 into vinegar with a clever catalyst

Researchers have made a catalyst that can turn carbon dioxide into acetic acid, a tremendously useful industrial chemical and food additive.

The Australian, US, and Japanese researchers, who published their research in Nature Communications, say their method points to a scalable way to turn carbon dioxide emissions into useful materials.

There's worldwide demand for about 6.5 million tonnes of acetic acid each year, to make a range of products including pharmaceuticals, vinyls, textiles, and cosmetics.

It's also the main component of vinegar, and is often used as a food preservative.

In the food industry, acetic acid is mostly made by fermenting, but in other industries it's made from fossil fuels, releasing greenhouse gas emissions in the process. The production typically also needs expensive precious metals like cobalt, iridium and rhodium to work.

Now this team has figured out how to make acetic acid from carbon dioxide and hydrogen, using (cheaper) iron as a catalyst.

The iron catalyst also stays solid for the entire reaction, meaning the process doesn't need extra equipment and energy to purify the acetic acid once it's made.

"From theory we knew iron should be a good candidate for catalysing this reaction, but the challenge is to keep it stable under acidic water conditions," says senior author Associate Professor Akshat Tanksale, a chemical engineer at Monash University.

Making acetic acid produces – unsurprisingly – acid dissolved in water.

"As is commonly known, iron rusts – oxidises – in water, whereas we wanted it to remain at least partially in the metallic form," says Tanksale.

The researchers’ solution was to use a metal-organic framework (MOF): a substance made from metallic atoms (in this case, iron), linked with carbon-based bridges, forming a sort of sponge with molecule-sized holes in it.

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They then heated the MOF, allowing some of the iron atoms to fuse together and form particles a few nanometres in size, embedded in a porous layer of carbon.

The resulting catalyst could make acetic acid (CH3COOH) out of CO2 and hydrogen (H2) very efficiently.

Tanksale says that it took his team more than a year, with some trial and error, to land on this catalyst.

"We started working on this project at the start of COVID-19 pandemic in 2020, so my research staff and students weren't allowed in the lab every day and they had to work alone in shift rotations," he says.

"It took us another 18 months to provide definitive evidence for how this catalyst works at the molecular level, while having to deal with a number of lockdown periods in Melbourne."

The catalyst is cheaper than those currently used, and the researchers are working on commercialising it.

The bottleneck, says Tanksale, is not the catalyst itself but the feedstocks: CO2 and hydrogen.

"While they are readily available today, their cost is significantly higher if it is derived from green sources," he says.

"To reap the true benefits of our technology, i.e. to achieve negative carbon emissions, carbon dioxide must be captured from air, and hydrogen must be made from water using renewable energy (green hydrogen).

"These enabling technologies are yet to reach their full commercial potential."

Originally published by Cosmos as Taking carbon dioxide from the atmosphere and turning it into vinegar

Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.

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