In the presence of nickel and other metal catalysts, CO2 and ethylene gas form an acrylate precursor configured in a five-membered ring. The challenge has been to crack that ring open, allowing a carbon-carbon double bond to form, creating acrylate. Lewis acids do the trick.
Credit: Berkskoetter lab/Brown University
The process for making acrylate, an important commodity chemical used to make materials from polyester fabrics to diapers, could soon be less expensive and more sustainable if a development discovered by researchers at Yale and Brown proves to be an enduring "enabling technology."
Chemical companies produce billions of tons of acrylate each year, usually by heating propylene, a compound derived from crude oil. “Everything that goes into making it is from relatively expensive, nonrenewable carbon sources,” says Wesley Bernskoetter, assistant professor of chemistry at Brown University in Providence, R.I., who led the research.
Since the 1980s, researchers have been investigating how to make acrylate by combining carbon dioxide (CO2) with ethylene gas in the presence of nickel and other metal catalysts. Both of the raw materials ― CO2 and ethylene ― are lower cost than propylene: CO2 is essentially free, and ethylene can be made from plant biomass.
A persistent obstacle to the approach, however, has been that instead of forming the acrylate molecule, CO2 and ethylene tend to form a precursor molecule with a five-membered ring made of oxygen, nickel and three carbon atoms. In order to finish the conversion to acrylate, that ring must be cracked open to allow the formation of a carbon-carbon double bond, a process called elimination. That step had proved elusive.
Research by Bernskoetter and his colleagues, recently published in the journal Organometallics, shows that a class of chemicals called Lewis acids can break open that five-membered ring, allowing the molecule to eliminate and form acrylate. Basically electron acceptors, Lewis acids steal away electrons that make up the bond between nickel and oxygen in the ring. That weakens the bond and opens the ring.
“We thought that if we could find a way to cut the ring chemically, then we would be able to eliminate very quickly and form acrylate,” Bernskoetter says. “And that turns out to be true.”
He calls the finding an “enabling technology” that could eventually be incorporated in a full catalytic process for making acrylate on a mass scale. “We can now basically do all the steps required,” he says.
Going forward, the team must tweak the strength of the Lewis acid used. To prove the concept, they used the strongest acid that was easily available, one derived from boron. But that acid is too strong to use in a repeatable catalytic process because it bonds too strongly to the acrylate product to allow additional reactions with the nickel catalyst.
There is a spectrum of Lewis acid strengths, so Bernskoetter is confident that one will work. “We think it’s possible,” he says. “Organic chemists do this kind of reaction with Lewis acids all the time.”
The ongoing research is part of a collaboration between Brown and Yale supported by the National Science Foundation’s Centers for Chemical Innovation program. The work is aimed at activating CO2 for use in making all kinds of commodity chemicals. Other authors on the paper were Dong Jin and Paul Willard of Brown and Nilay Hazari and Timothy Schmeier of Yale.