Extreme Low-Temperature Combustion Chemistry: Ozone-Initiated Oxidation of Methyl Hexanoate

The
accelerating chemical effect of ozone addition on the oxidation
chemistry of methyl hexanoate [CH3(CH2)4C­(O)­OCH3] was investigated over a temperature range from 460 to 940 K. Using
an externally heated jet-stirred reactor at p = 700
Torr (residence time τ = 1.3 s, stoichiometry φ = 0.5,
80% argon dilution), we explored the relevant chemical pathways by
employing molecular-beam mass spectrometry with electron and single-photon
ionization to trace the temperature dependencies of key intermediates,
including many hydroperoxides. In the absence of ozone, reactivity
is observed in the so-called low-temperature chemistry (LTC) regime
between 550 and 700 K, which is governed by hydroperoxides formed
from sequential O2 addition and isomerization reactions.
At temperatures above 700 K, we observed the negative temperature
coefficient (NTC) regime, in which the reactivity decreases with increasing
temperatures, until near 800 K, where the reactivity increases again.
Upon addition of ozone (1000 ppm), the overall reactivity of the system
is dramatically changed due to the time scale of ozone decomposition
in comparison to fuel oxidation time scales of the mixtures at different
temperatures. While the LTC regime seems to be only slightly affected
by the addition of ozone with respect to the identity and quantity
of the observed intermediates, we observed an increased reactivity
in the intermediate NTC temperature range. Furthermore, we observed
experimental evidence for an additional oxidation regime in the range
near 500 K, herein referred to as the extreme low-temperature chemistry
(ELTC) regime. Experimental evidence and theoretical rate constant
calculations indicate that this ELTC regime is likely to be initiated
by H abstraction from methyl hexanoate via O atoms, which originate
from thermal O3 decomposition. The theoretical calculations
show that the rate constants for methyl ester initiation via abstraction
by O atoms increase dramatically with the size of the methyl ester,
suggesting that ELTC is likely not important for the smaller methyl
esters. Experimental evidence is provided indicating that, similar
to the LTC regime, the chemistry in the ELTC regime is dominated by
hydroperoxide chemistry. However, mass spectra recorded at various
reactor temperatures and at different photon energies provide experimental
evidence of some differences in chemical species between the ELTC
and the LTC temperature ranges.