Conventional ethylene production via steam cracking of ethane or naphtha leads to the emission of 1 ton of CO2 for every ton of ethylene produced. Given the carbon intensity of the process and the annual volume of ethylene produced (300 million tons/yr), a lower-energy method for making ethylene from non-fossil feedstock could have large climate implications.
Researchers at Northwestern University (Evanston, Ill.; www.northwestern.edu) have developed an electrochemical route to making ethylene from synthesis gas (syngas) that uses a gas-fed electrolyzer with a solid-state anolyte. This type of electrolyzer, if scaled up, could allow low-energy production of ethylene derived from gasified waste (such as plastic waste) or municipal solid waste, using intermittent renewable energy.
“We wanted to construct an atom-efficient and energy-efficient system that produces chemical building blocks from waste instead of fossil fuels. What’s more, the device works reliably when powered by intermittent electricity,” Sargent said.
The researchers explored syngas (CO and H2) as the starting material because it allows a more thermodynamically favorable starting point than CO2, minimizing the electricity consumption of the process. To allow its use with intermittent energy supply, the Northwestern team also pursued an entirely gas-fed electrolyzer to avoid the liquid anolyte, which corrodes the catalyst material when no power is available.
Eliminating the liquid anolyte presents a problem, however, because the anolyte, such as NaOH, is necessary provide a high concentration of cations (Na+) and high local pH on the electrolyzer’s cathode to activate the conversion from CO to C2+ products. To make the electrolyzer work, the Northwestern team pursued the concept of implanting sodium ions into a solid ionomer material. After trying several strategies unsuccessfully, the researchers, led by Northwestern chemistry professor Ted Sargent and post-doctoral researcher Bosi Peng, arrived at using sodium polyacrylate (PANa), a solid ionomer that can bind the cations needed for the reaction.
“We needed a solid material with exactly the right amount of binding affinity for cations,” Peng explains. “If the binding is too weak, the cations are leached from the system, but if the binding is too strong, the cations cannot help facilitate the CO activation needed for ethylene synthesis.” Ultimately, the team used PANa on a support of carbon black for their electrolyzer, which is described in a recent issue of Nature Energy.
With the success of the electrolyzer concept, the researchers are investigating different catalyst materials and device configurations to further improve the system’s performance, with the goal of achieving energy use comparable to steam cracking.