Researchers identify new pathways in low-temp oxidation of hydrocarbons; important to fuel combustion, atmospheric chemistry and biochemistry
|The diagram illustrates the newly-described reaction that transforms molecules of ketohydroperoxide into acids and carbonyl molecules, after going through intermediate stages. Credit: ACS, Jalan et al. Click to enlarge.|
Researchers at MIT, with colleagues at the University of Minnesota, have provided evidence and theoretical rate coefficients for new pathways in the low-temperature oxidation of hydrocarbons. Their paper is published in the Journal of the American Chemical Society.
The newly explained reaction—the basic outlines of which had been first hypothesized by Korcek and co-workers more than 30 years ago but the workings of which had never been understood in detail—is an important part of atmospheric reactions that lead to the formation of climate-affecting aerosols; biochemical reactions that may be important for human physiology; and combustion reactions in engines. The new study provides theoretical confirmation of Korcek’s hypothesis that ketohydroperoxide molecules (KHPs) are precursors to carboxylic acid formation.
The new pathways lead to the conversion of KHPs [γ-ketohydroperoxides] into carboxylic acids and carbonyl compounds that are well-known products in liquid-phase oxidation and have also recently been observed in low-temperature combustion and atmospheric oxidation.
The Korcek reaction sequence involves the isomerization of the KHP species to the more stable five-membered cyclic peroxide and can occur via unimolecular and acid-catalyzed bimolecular pathways. The second step in the sequence involves concerted fragmentation of the O−O and C−C bonds in CP accompanied by 1,2 H-shift to directly yield carboxylic acids and carbonyl products.
...Based on our calculations, unimolecular cyclization via reaction C was found to be the rate-limiting step in the overall sequence. For gas-phase systems, in the absence of catalyzed cyclization, bond dissociation (D) is expected to be the main unimolecular route for KHP consumption at temperatures above 400 K [126.85 °C]. As oxidation products build up, acid-catalyzed and other lower-energy bimolecular channels are expected to dominate and lead to rapid equilibrium between KHP and CP. Even trace amounts of acids in the reacting mixture can rapidly catalyze the cyclization process making fragmentation of CP the rate-limiting process and leading to the autocatalytic formation of even more acid and carbonyl products. Incorporation of acid-catalyzed cyclization and solvent effects leads to excellent agreement between theoretical predictions and the limited experimental data reported by Korcek and co-workers.
In addition to liquid-phase oxidation (corresponding to the high-pressure limit with solvent effects), the new pathways are also relevant in gas-phase ozonolysis chemistry which operates at substantially lower pressures. Preliminary master equation calculations suggest that the chemically activated decomposition of CP formed by reaction between the Criegee intermediate and vinyl alcohol can lead to formation of organic acids (underestimated by existing kinetic models) under atmospheric conditions. The new pathways qualitatively explain acid formation in recent low-temperature alkane combustion experiments in jet-stirred reactors and are also expected to be relevant in lipid peroxidation chemistry.—Jalan et al.
The new analysis is explained in a paper by MIT graduate student Amrit Jalan, chemical engineering professor William Green, and six other researchers.
I think this may be the best paper I have read this year. It uses a multitude of theoretical methods … to explore multiple aspects of a novel discovery that has important ramifications in atmospheric chemistry, combustion kinetics and biology. As a result of this clear exposition and the high level of theory that was applied, I believe this work will be widely accepted immediately. I certainly am already convinced by their conclusions.—Stephen Klippenstein, a senior scientist at the Argonne National Laboratory in Illinois who was not involved in this research
When he first described the reaction in the scientific literature 30 years ago, Stefan Korcek of the Ford Motor Company proposed a hypothesis for how the reaction might take place. The new work shows that Korcek had the right concept, although some details differ from his predictions.
The original discovery was the result of analyzing how engine oils break down through oxidation—part of an attempt to produce oils that would last longer. That’s important, Green points out, since waste oil is among the largest hazardous waste streams in the United States.
In analyzing the problem, Korcek realized that “there were fundamental things about the way even simple hydrocarbons react with oxygen that we didn’t understand,” Green said. By examining the products of the reaction, which included carboxylic acids and ketones, Korcek outlined an unusually complex multipart reaction. But for the next three decades, nobody found a way to verify whether the reaction or the steps he outlined could work.
In collaboration with the Minnesota researchers—including Donald Truhlar, a co-author of the new paper and a leading expert in such calculations—Jalan and Green were able to demonstrate exactly why the reaction works as it does. But they also found that part of the process must differ slightly from Korcek’s original hypothesis.
Green says that understanding how this “very important reaction” works could be significant in several fields. The researchers’ initial impetus was, in part, a colleague’s exploration of biofuel combustion. The new understanding of the degradation that can take place as different fuels oxidize—sometimes producing toxic or corrosive byproducts—could help narrow the choice of fuel types to pursue, he says.
The process is also related to oxidations that take place in the body, contributing to the tissue damage and aging that antioxidant vitamins seek to combat, Green says.
Green points out that because this is an entirely new type of reaction, it opens the door to research on other variations.
Once you discover a new type of reaction, there must be many similar ones. It’s very odd to have so many reactions at once in such a small molecule. Now that we know that can happen, we’re searching for other cases.—William Green
Anthony Dean, dean of the College of Applied Science and Engineering at the Colorado School of Mines, who also was not involved in this study, says, “A particularly nice aspect of this work is to then consider how this finding might be applicable to other systems. In a broader context, this combined effort by two very prominent research groups illustrates the power and potential for electronic structure calculations [in] quantitatively important problems in chemical kinetics.”
The research was supported in part by the US Department of Energy, and used computing facilities at the Pacific Northwest National Laboratory and the Minnesota Supercomputing Institute.
Amrit Jalan, Ionut M. Alecu, Rubén Meana-Pañeda, Jorge Aguilera-Iparraguirre, Ke R. Yang, Shamel S. Merchant, Donald G. Truhlar, and William H. Green (2013) New Pathways for Formation of Acids and Carbonyl Products in Low-Temperature Oxidation: The Korcek Decomposition of γ-Ketohydroperoxides. Journal of the American Chemical Society 135 (30), 11100-11114 doi: 10.1021/ja4034439