A fundamental discovery into how the Fischer-Tropsch process works could improve control of both its reaction rate and yield.
Researchers at Washington State University (WSU), Pullman, Washington, have discovered the catalytic reaction at the heart of the more than 100-year-old process to convert syngas into hydrocarbons doesn’t have one steady state but rather undergoes self-sustained oscillations between high and low activity states. They also discovered why this happens.
“Usually, rate oscillations with large variations in temperature are unwanted in the chemical industry because of safety concerns," said Norbert Kruse, Voiland distinguished professor in WSU’s Gene and Linda Voiland School of Chemical Engineering and bioengineering and corresponding author in a recent Science article about the work.
Kruse has been working on oscillatory reactions for more than 30 years and says the discovery of the oscillatory behavior with the Fischer-Tropsch reaction was very surprising because the reaction is mechanistically extremely complicated.
For the study, the researchers demonstrated the Fischer-Tropsch reaction in a lab using a frequently used cobalt catalyst, conditioned by adding cerium oxide, and then modeled how it worked.
Experimental and theoretical oscillations were in agreement over an extended range of reactant pressure ratios, and phase portraits for hydrocarbon production supported a thermokinetic origin of the rate-and-selectivity oscillations.
What’s happening is that as the temperature of the reaction goes up due to its heat production, the reactant gases lose contact with the catalyst surface and their reaction slows down, which reduces the temperature. Once the temperature is sufficiently low, the concentration of the reactant gases on the catalyst surface increases and the reaction picks up speed again. Consequently, the temperature increases to close the cycle.
According to Kruse, his researchers’ current work focuses on showing that oscillatory behaviors can also be obtained for cobalt-based catalysts using supports/promoters other than cerium oxide. “The goal is to reach a consensus on the mechanistic scenarios governing the Fischer-Tropsch reaction. “This knowledge is indispensable when it comes to designing new process variants of the reaction,” he said.
While using oscillatory states to boost selectivities of the Fischer-Tropsch reaction in process applications is still in its infancy, Kruse notes that from the viewpoint of already developed non-linear physics theory, this should be possible.
“The Fischer-Tropsch reaction, with its large variety of process variants, would be a marvelous example for passing the test. Other reactions similarly complicated as Fischer-Tropsch might follow and leverage non-linear theory in catalysis science and its applications,” he concluded.