Researchers at Stanford University have shown that porous polymer encapsulation of metal-supported catalysts can drive the selectivity of CO₂ conversion to hydrocarbons. Using this technique, they report orders of magnitude higher turnover frequencies for hydrocarbon formation compared to conventional catalysts. A paper on their work is published in Proceedings of the National Academy of Sciences (PNAS).

CO₂ (black and red) and hydrogen molecules (blue) react with the help of a ruthenium-based catalyst. On the right, the uncoated catalyst produces the simplest hydrocarbon, methane. On the left, the coated catalyst produces longer chain hydrocarbons, such as butane, propane and ethane. (Image credit: Chih-Jung Chen)

The research team encapsulated a supported Ru/TiO₂ catalyst within the polymer layers of an imine-based porous organic polymer that controls its selectivity. Such polymer confinement modifies the CO₂ hydrogenation behavior of the Ru surface, significantly enhancing the C₂₊ production turnover frequency.

We demonstrate that the polymer layers affect the adsorption of reactants and intermediates while being stable under the demanding reaction conditions. Our findings highlight the promising opportunity of using polymer/metal interfaces for the rational engineering of active sites and as a general tool for controlling selective transformations in supported catalyst systems.

—Zhou et al.

We can create gasoline, basically. To capture as much carbon as possible, you want the longest chain hydrocarbons. Chains with eight to 12 carbon atoms would be the ideal.

—Matteo Cargnello, corresponding author

The catalyst produced 1,000 times more butane—the longest hydrocarbon it could produce under its maximum pressure—than the standard catalyst given the same amounts of carbon dioxide, hydrogen, catalyst, pressure, heat and time.

An uncoated catalyst gets covered in too much hydrogen on its surface, limiting the ability of carbon to find other carbons to bond with. The porous polymer controls the carbon-to-hydrogen ratio and allows us to create longer carbon chains from the same reactions. This particular, crucial interaction was demonstrated using synchrotron techniques at SLAC National Laboratory in collaboration with the team of Dr. Simon Bare, who leads Co-Access there.

—Chengshuang Zhou, lead author

Cargnello and his team took seven years to discover and perfect the new catalyst. The longer the hydrocarbon chain is, the more difficult it is to produce. The bonding of carbon to carbon requires heat and great pressure, making the process expensive and energy intensive.

In this regard, the ability of the new catalyst is a breakthrough, said Cargnello. The reactor in his lab would need only greater pressure to produce all the long-chain hydrocarbons for gasoline, and they are in the process of building a higher pressure reactor.

Gasoline is liquid at room temperature and, therefore, much easier to handle than its gaseous short-chain siblings—methane, ethane and propane—which are difficult to store and prone to leaking back into the skies. Cargnello and other researchers working to make liquid fuels from captured carbon imagine a carbon-neutral cycle in which carbon dioxide is collected, turned into fuel, burned again and the resulting carbon dioxide begins the cycle anew.

While long-chain hydrocarbons are an innovative use of captured carbon, they are not perfect, Cargnello acknowledges. He is also working on other catalysts and similar processes that turn carbon dioxide into valuable industrial chemicals, such as olefins used to make plastics, methanol and ethanol, all of which can sequester carbon without returning carbon dioxide to the skies.

If we can make olefins from CO₂ to make plastics, we have sequestered it into a long-term storable solid. That would be a big deal.

—Matteo Cargnello

This work was supported by grants from the Packard Foundation and the Precourt Institute for Energy at Stanford University. Spectroscopy support was provided by the Lawrence Berkeley National Laboratory and by the SLAC National Accelerator Laboratory.

Resources

• Chengshuang Zhou, Arun S. Asundi, Emmett D. Goodman, Jiyun Hong, Baraa Werghi, Adam S. Hoffman, Sindhu S. Nathan, Stacey F. Bent, Simon R. Bare, Matteo Cargnello (2022) “Steering CO₂ hydrogenation toward C–C coupling to hydrocarbons using porous organic polymer/metal interfaces” Proceedings of the National Academy of Sciences 119 (7) doi: 10.1073/pnas.2114768119