|Enol Intermediates Unexpectedly Found in Flames|
For those studying flame chemistry and the properties of combustion intermediates by means of molecular beam mass spectrometry, the addition of tunable vacuum ultraviolet (VUV) from a synchrotron to photoionize the beam for mass spectrometry makes for a powerful technique capable of differentiating between isomers with the same molecular weight and composition. With the help of a unique experimental apparatus, an international team of American, Chinese, and German researchers has exploited this selectivity to identify chemical compounds known as enols as apparently ubiquitous intermediates in flames burning a variety of fuels. This surprising observation will require combustion modelers to revise their models to account for the presence of these compounds.
After more than 150 years of study, combustion seems to be well understood in terms of average energy output, high-concentration intermediates, and major products. However, for improving combustion efficiency and controlling pollution, it is necessary to understand flame chemistry at the parts-per-million level while simultaneously facing the turbulent fluid dynamics of a "real" flame. As a result, many important rate constants have never been measured directly, nor have all the species included in mathematical flame models been directly observed. To this end, a joint collaboration between Sandia National Laboratories, Cornell University, the University of Massachusetts (Amherst), and Berkeley Lab has developed a low-pressure-flame photoionization mass spectrometer that allows experimenters to isolate the chemistry.
In the apparatus, premixed reagent gases enter the flame chamber through the porous flat face of a burner that translates horizontally relative to a fixed quartz sampling cone and nickel skimmer, which allows the temperature profiles and concentration profiles to be mapped to very high precision. A well-collimated molecular beam from the skimmer enters a differentially pumped (10-6 Torr) chamber, where it is photoionized by a crossed tunable VUV beam. Photoions are mass-analyzed using a time-of-flight (TOF) mass spectrometer (MS).
Among enols, the simplest is ethenol (vinyl alcohol). It is thermodynamically unstable relative to its isomer acetaldehyde and has only recently been observed as an intermediate in an ethene flame [T.A. Cool et al., J. Chem. Phys. 119, 8356 (2003)]. The team launched a systematic search for enols among 24 different flames of 14 prototypical single fuels found in modern fuel blends; they also studied commercial gasoline. Experiments were conducted with similar flame chambers operating at a branch of ALS Chemical Dynamics Beamline 9.0.2 and at the National Synchrotron Radiation Laboratory (Hefei, China).
Photoionization efficiency curves taken for m/z = 44 ions sampled from four representative flames showed that ethenol is present in all. The 0.9-eV difference between the photoionization thresholds for ethenol and its isomer acetaldehyde made them easily distinguishable. In addition to ethenol, larger enols also occur in flames of both simple fuels and commercial gasoline. Photoionization measurements of m/z = 58 ions show the presence of propenols, while butenols occur in measurements for m/z = 72 ions sampled from a gasoline flame.
In the case of ethenol, not only are the concentrations far too high to be explained as the isomerization of acetaldehyde, but the data suggest that ethenol kinetics in flames are distinct. Markedly differing distributions of ethenol and acetaldehyde with distance from the flame burner suggest either separate formation mechanisms or differential removal of ethenol as it diffuses toward the burner. And the increasing fraction of ethenol relative to acetaldehyde with distance from the burner for two flames suggests that the chemical fates of the two are not at all the same.
While the practical impact of these findings on combustion remains speculative for the moment, understanding the fundamental chemistry of enols, important not only in combustion but also in other forms of hydrocarbon oxidation important in such widely varied settings as fuel cells, planetary atmospheres, and interstellar space, clearly requires much more theoretical and experimental study.
Research conducted by C.A. Taatjes (Sandia National Laboratories and JILA); N. Hansen, A. McIlroy, J.A. Miller, J.P. Senosiain, and S.J. Klippenstein (Sandia National Laboratories); F. Qi (Sandia National Laboratorires and National Synchrotron Radiation Laboratory, China); L. Sheng and Y. Zhang (National Synchrotron Radiation Laboratory, China); T.A. Cool and J. Wang (Cornell University); P.R. Westmoreland and M.E. Law (University of Massachusetts, Amherst); T. Kasper and K. Kohse-Höinghaus (Universität Bielefeld, Germany).
Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES) and National Nuclear Security Administration; the U.S. Army Research Office; the Chinese Academy of Sciences; the National Natural Science Foundation of China; and the Deutsche Forschungsgemeinschaft. Operation of the ALS is supported by BES.
Publication about this research: C.A. Taatjes, N. Hansen, A. McIlroy, J.A. Miller, J.P. Senosiain, S.J. Klippenstein, F. Qi, L. Sheng, Y. Zhang, T.A. Cool, J. Wang, P.R. Westmoreland, M.E. Law, T. Kasper, and K. Kohse-Höinghaus, "Enols are common intermediates in hydrocarbon oxidation," Science 308, 1887 (2005).