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Title: Diol isomer revealed as a source of methyl ketene from propionic acid unimolecular decomposition

Journal Article · · International Journal of Chemical Kinetics
DOI:https://doi.org/10.1002/kin.21532· OSTI ID:1976352
 [1];  [1];  [2]; ORCiD logo [1]
  1. Univ. of Colorado, Boulder, CO (United States)
  2. Univ. of Colorado, Boulder, CO (United States); National Inst. of Standards and Technology (NIST), Boulder, CO (United States)

Carboxylic acids are important combustion intermediates, especially regarding the combustion of oxygenates such as ethyl esters. The purpose of this study is to discern the unimolecular decomposition pathways of one such carboxylic acid, propionic acid, using microreactor flow experiments with line-tunable photoionization mass spectrometry and infrared spectroscopy, along with high-level electronic structure calculations (CCSD(T)/cc-pV∞Z//M06-2X/cc-pVTZ level of theory) and master equation theory. Here, microreactor experiments were performed at 300–1500 K, pressures decreasing from roughly 300 Torr to vacuum pressure, and around 100 μs residence times. Primary products are methyl ketene, ketene, ethylene, and methyl radical. Theory suggests the two lowest energy bond fissions are active at these conditions and responsible for the production of ketene, methyl radical, and some ethylene. Importantly, theory revealed the significance of the isomerization of propionic acid to propene-1,1-diol, and the subsequent dehydrogenation reaction of the diol as an alternative explanation for the unimolecular formation of methyl ketene. This is predicted to be the dominant thermal decomposition pathway at lower temperatures (up to ~850 K). Line-tunable VUV photons allowed for isomer resolution of the products and showed no evidence of the diol of propionic acid surviving until detection, suggesting propene-1,1-diol decomposes either to methyl ketene and water rapidly, fragments upon ionization to a distonic ion inseparable from methyl ketene through our detection methods, or never stabilizes as propionic acid well-skips directly to methyl ketene. Infrared spectroscopy showed no evidence of the decarboxylation of propionic acid at these conditions. Updated rate constants and branching ratios for propionic acid decomposition are calculated and provided for future modeling studies.

Research Organization:
Univ. of Colorado, Boulder, CO (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
Grant/Contract Number:
SC0018627; ACI-1532235; ACI-1532236
OSTI ID:
1976352
Journal Information:
International Journal of Chemical Kinetics, Vol. 53, Issue 12; ISSN 0538-8066
Publisher:
WileyCopyright Statement
Country of Publication:
United States
Language:
English

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