When you hear the word "epoxide," what do you think? If anything, likely "glue." But epoxides are quite common in our everyday lives. You might be sitting on a foam seat cushion made from epoxides. There is a good chance the synthetic textiles in your clothing involve epoxides in their production. The same is true of the paint on your car and the printed circuit boards in your electronic devices.
"It's surprising how we think of epoxides as this really narrow category, but they're actually in so many things around us," says Karthish Manthiram , Bren Professor of Chemical Engineering and Chemistry at Caltech. As it turns out, he says, if you add up the carbon footprint related to the manufacture of all those epoxides worldwide, the total is equivalent to the carbon footprint of all the cars driving in Southern California. "So, it's huge. I mean, Southern California has a gazillion cars," says Manthiram, who is also a William H. Hurt Scholar and executive officer for chemical engineering at Caltech.
Now, Manthiram's research group has figured out how to drive a series of chemical reactions that produce epoxides, such as propylene oxide, a three-member ring involving two carbon atoms and one oxygen atom, using a much greener process and an Earth-abundant, or widely available, catalyst. Manthiram and his co-authors from Caltech and UCLA describe the new system in a paper that appears in the journal Nature Catalysis. The lead author of the paper is Kalipada Koner, postdoctoral scholar research associate in chemical engineering at Caltech.
"A lot of what motivated us is that if you look at the ways that people have made epoxides, you're really picking your poison," Manthiram says. "There's a problem with virtually every method that's practiced today."
Historical Approaches
The most straightforward approach to epoxidation, the creation of epoxides such as propylene oxide, is to take an oxygen atom from the oxygen gas in the air to bind with double-bonded carbon in propylene at high temperature. Unfortunately, this often over-oxidizes the material, making the process chemically inefficient. As a result, it is not practiced commercially today.
For decades, the industry standard for epoxidation was to treat propylene with chlorine gas in water. During this process, a chlorohydrin-an organic compound that contains both a chlorine and a hydroxyl (-OH) group-is created. If the chlorohydrin is then treated with a base such as calcium hydroxide, the chemical chain closes, forming the desired epoxide ring. The problem here is that the introduction of calcium hydroxide also leads to the production of the salt calcium chloride. For a long time, people thought this was OK and simply discharged the salt into rivers and oceans. "But folks have realized that these salts are really harmful to aquatic life and also to humans," Manthiram explains. The process also creates organohalides, carbons connected to chlorines that are extremely toxic. As a result, permits for the chlorohydrin process are disappearing around the world.
There has been a shift toward peroxide-based processes where propylene is treated with hydrogen peroxide). In these processes, reactive oxygen is taken from the hydrogen peroxide to create the epoxide ring, leaving water as a side product. The process is clean in terms of environmental damage, but peroxides in contact with organic compounds are potentially explosive. The necessary safety precautions have made the capital costs for these epoxidation techniques prohibitively expensive.
In 2024, Manthiram's group reported a palladium-platinum oxide catalyst that could drive an electrochemical approach to epoxidation, transferring oxygen from water to produce an epoxide and hydrogen gas. This has been a promising approach, with groups around the world working to further develop that catalyst. But palladium and platinum are rare and extremely expensive.
Tackling Cost and Sustainability
The new system the Caltech and UCLA scientists have come up with relies on inexpensive lanthanum cobaltite, an Earth-abundant transition metal-based catalyst, to enable the transfer of oxygen atoms from water in an electrified process. The system also uses a phosphate-based electrolyte rather than the usual halogenated electrolytes, which are toxic and explosive.
The scientists developed a platform that allowed them to methodically test many combinations of possible catalyst materials. The researchers ultimately settled on a catalyst structure for epoxidation called a perovskite oxide- with the form ABO3, where B is a metal actively involved in the catalysis, and A is a so-called spectator atom that allows the scientists to tune the chemical environment.
"As chemists, we want the sustainability to be there. We're dreaming big about new catalysts and how they can enable things, but we also wear the hat of chemical engineers. The engineer's mindset tells us that we have to think about the economics of this process and how it all pans out," Manthiram says. "So, we want to make sure that making it more sustainable actually allows us to achieve a cheaper process."
An Eye Toward Commercialization
Manthiram says the team still has ideas about how to improve the rate at which the new process produces epoxides. He notes that the support his team has received from the Gordon and Betty Moore Foundation has been crucial in helping them to more thoughtfully develop new catalysts, as well as prototypes with an eye toward commercialization. "It's sustainability with techno-economics," Manthiram says. "Those are the two things we have to stay laser-focused on to ultimately get something like this out of the lab."
The paper is titled "Direct Electrochemical Propylene Epoxidation over Amorphized Perovskite Oxide in Non-halogenated Aqueous Electrolyte." Additional Caltech authors are former postdoctoral scholars Jason S. Adams and Justin C. Bui; research scientist and lab manager Evan V. Miu, graduate students Spencer P. Delgado-Kukuczka and Chenyu Jiang; and Paul H. Oyala, manager of the Electron Paramagnetic Resonance Facility at Caltech. Co-authors from UCLA are Jung Tae Kim, Hayoung Park, and Yuzhang Li. In addition to the Moore Foundation, the work also received support from the Catalysis Science Program of the US Department of Energy's Office of Basic Energy Sciences.
