… upgrading ethanol into an advanced biofuel, such as butanol, offers a more efficient alternative fuel from renewable biomass sources. Consequently, butanol is considered a “drop-in” replacement for gasoline. Despite these advantages, the incorporation of butanol into the energy economy remains underexplored. Common synthetic routes to produce butanol from ethanol, including bacterial fermentation (also known as the “A.B.E.” process, which produces a mixture acetone, butanol, and ethanol and hydroformylation/hydrogenation of propylene, and the Guerbet reaction suffer from poor selectivity, separation issues, and low conversion and/or yield. Based on our recent reports on the reversible dehydrogenation–hydrogenation processes involving alcohols, we became interested in converting ethanol into n-butanol via the Guerbet process.

—Chakraborty et al.

Converting ethanol to butanol involves creating a larger chemical molecule with more carbon and hydrogen atoms. Although both molecules have a single oxygen atom, the higher carbon-to-oxygen ratio in butanol gives it a higher energy content, while the larger size make it less volatile.

The Guerbet process comprises three main steps:

• Dehydrogenation of a primary alcohol;

• A base-catalyzed aldol coupling reaction; and

• Hydrogenation of an α,β-unsaturated aldehyde, allowing for the coupling of two primary alcohols into a longer-chained alcohol through “borrowed hydrogen” chemistry with no net loss of the hydrogen gas.

Most typical Guerbet processes feature precious metal catalysts for the de/hydrogenation steps and an inorganic base to aid in the aldol coupling step. While most of these processes have been performed with some longer-chain primary alcohols, the same reaction utilizing ethanol as a substrate remains a great challenge for several reasons, the researchers said.

These include the uphill thermodynamic challenges associated with the dehydrogenation of ethanol and the formation of side-products due to the uncontrolled aldol reaction involving acetaldehyde, which can react with both itself and the butanol product to create unwanted molecules.

To modify the process, the researchers used iridium as the initial catalyst and nickel or copper hydroxide, instead of potassium hydroxide (KOH), in the second step. While the best current conditions for the Guerbet reaction convert ethanol to butanol with about 80% selectivity, the new system produced butanol with more than 99% selectivity. No undesirable side products were produced.

These sterically crowded nickel and copper hydroxides catalyze the key aldol coupling reaction of acetaldehyde to exclusively yield the C₄ coupling product, crotonaldehyde. Iridium-mediated dehydrogenation of ethanol to acetaldehyde has led to the development of an ethanol-to-butanol process operated at a lower temperature.

—Chakraborty et al.

There’s still more work to do. We’d like to have a catalyst that’s less expensive than iridium. Also, we want to make the conversion process last longer, which means figuring out what currently makes it stop. Once we solve the remaining problems, we may be able to start looking for ways to apply the conversion process in the making of renewable fuels.

—Prof. William Jones, University of Rochester

Jones says the process currently terminates after one day because one or more of the substances—the iridium, nickel, and copper—has broken down.

The research by Jones was carried out under the NSF (National Science Foundation) support of the Center for Enabling New Technologies through Catalysis, an NSF Center for Chemical Innovation program.

Resources

• Sumit Chakraborty, Paige E. Piszel, Cassandra E. Hayes, R. Tom Baker, and William D. Jones (2015) “Highly Selective Formation of n-Butanol from Ethanol through the Guerbet Process: A Tandem Catalytic Approach” Journal of the American Chemical Society 137 (45), 14264-14267 doi: 10.1021/jacs.5b10257