Adding a dash of the fullerene C60—that iconic ball of carbon atoms—to a copper catalyst boosts yields of ethylene glycol under mild conditions, potentially opening a route to manufacture this commodity chemical from sustainable feedstocks such as biomass (Science 2022, DOI: 10.1126/science.abm9257).
“This is a breakthrough in the synthesis of ethylene glycol,” says Eric Doris, who studies carbon nanomaterials and catalysis at CEA Paris-Saclay, a part of the French Alternative Energies and Atomic Energy Commission (CEA) and was not involved in the work.
Ethylene glycol is used as an antifreeze and to make polymers. Most ethylene glycol is made from ethylene derived from crude oil or natural gas, and global production is about 42 million metric tons per year.
But a decade ago, companies in China commercialized an alternative route starting from syngas, a mixture of carbon monoxide and hydrogen produced by gasifying coal. Their process uses a palladium catalyst to turn CO into dimethyl oxalate, which then reacts with hydrogen over a copper-silica catalyst to make ethylene glycol. But this reaction requires expensive infrastructure to supply hydrogen at high pressure. The conditions can also deactivate the catalyst and produce a lot of unwanted by-products.
A team led by Youzhu Yuan and Su-Yuan Xie at Xiamen University has now found that loading the copper-silica catalyst with 10% C60 by weight dramatically improves its performance, boosting yields 10-fold to about 98% at ambient pressure. The reaction forms only 2 by-products, far fewer than the 20 or so generated under conventional conditions, and the catalyst can be reused with no loss of activity.
In principle, the syngas that feeds the process could come from the gasification of biomass, rather than coal, making it a more sustainable route to ethylene glycol, Yuan says.
The researchers used a battery of experimental techniques and theoretical modeling to understand C60’s role. They found that C60 molecules offer a reservoir of electrons to maintain the right balance of copper oxidation states.
Two different forms of copper are active during the reaction. Elemental Cu(0) is involved in dissociating H2 into hydrogen atoms, while Cu(I) helps add these atoms to dimethyl oxalate. Cu(I) is unstable to oxidation and reduction, so C60 protects it by acting as an electron buffer— it can accept electrons from Cu(0) or donate electrons to Cu(II) to achieve the ideal mix of catalytic copper states.
C60 is widely used as an electron acceptor in organic solar cells but rarely used as an electron buffer in conventional catalysis, Doris says. The Xiamen researchers found that C60 also enhanced the copper-catalyzed hydrogenation of other molecules, and Doris thinks it could have a similar benefit for other metal catalysts.
Using C60 in an industrial process would once have been considered prohibitively expensive, but several Chinese firms now produce it on a large scale, which has lowered its cost considerably, Doris says. Yuan’s team is now working with a company to scale up the reaction.
This article was updated on April 25, 2022, to correct the description of C60's role in solar cells. The fullerene acts as an electron acceptor, not as an electron buffer in that context.