Enols Found Lurking in Flames

COMBUSTION RESEARCH

After more than 150 years of playing with fire, chemists might have assumed they knew most everything there was to know about combustion processes. Not so: An international team of scientists has now discovered that enols--a class of compounds previously undetected in flames--are produced in significant amounts (Science, published online May 12, dx.doi.org/ 10.1126/science.1112532).

This observation likely will require substantial revision of standard models of hydrocarbon oxidation, according to Craig A. Taatjes of Sandia National Laboratories, one of the lead investigators.

Carbonyl compounds, such as aldehydes, are known intermediates in hydrocarbon combustion pathways that lead to CO₂ and water. Enols are the less stable isomers of carbonyls that have a hydroxyl group attached to a doubly bonded carbon atom. The simplest one is ethenol.

The researchers found the enols using burners operated under low pressure at research facilities in the U.S. and China. Oxidation products and intermediates from the combustion of various fuels were collected from different locations in the burner flame and collimated into a molecular beam. A tunable synchrotron-generated vacuum UV beam was used to ionize the molecular beam, and the resulting ions were analyzed by mass spectrometry.

Concentration profiles of molecules generated in the flames show formation of ethenol, propenols, and butenols. According to the researchers, formation of these enols can't be explained purely by keto-enol tautomerization--the simple isomerization, for example, of acetaldehyde into ethenol. They believe that separate formation mechanisms must be at work. But explaining how the enols form and what happens to them during combustion is going to require considerably more experimental and theoretical study, Taatjes adds.

The discovery could impact many areas of science, the researchers note, including pollution formation in fuel combustion, partial oxidation in fuel cells, oxidations carried out in supercritical water, and the chemistry of planetary atmospheres.