Non-Boltzmann Effects in Chain Branching and Pathway Branching for Diethyl Ether Oxidation

Low-temperature (LT) engine applications
have several potential
benefits, including reduced emissions and increased efficiency. Attaining
these benefits requires accurate kinetic modeling of LT chain branching,
which depends heavily on ketohydroperoxide (KHP) decomposition. For
diethyl ether (DEE), a promising biofuel, current estimates of the
KHP decomposition rate constant are largely based on empirical fits
to data. In this study, we investigate the most important KHP isomer
in DEE LT oxidation by applying variable reaction coordinate transition
state theory to the main pathway for KHP decomposition: OO bond fission
to produce •OH and a keto-alkoxy radical, •OQ′O.
We also use ab initio kinetics methods to investigate the decomposition
of •OQ′O, where we find dominant branching to acetic
acid, with the remaining flux going to CH3C­(O)­OCHO.
Additionally, new time-resolved measurements of DEE and acetic acid
concentrations during LT (450–600 K) DEE oxidation are obtained
in a laser photolysis flow reactor coupled with multiplexed photoionization
mass spectrometry. These new experimental data, along with jet-stirred
reactor data in the literature, are compared with the predictions
of a recent DEE mechanism (Tran
et al. Proc. Comb. Inst. 2019, 37, 511−519) that was modified with the newly calculated ab initio rate constants
for KHP and •OQ′O decomposition. The predictions of
the modified mechanism are quite poor when compared to the experimental
data; this is primarily due to the new KHP ⇄ •OQ′O
+ •OH rate constant, which is 1–2 orders of magnitude
slower than empirical values employed in recent mechanisms. To reconcile
the new KHP rate constant and the experimental data, we explore and
quantify the possible role of non-Boltzmann (nB) reaction sequences.
The nB reactions have a substantial effect on both the overall mechanism
reactivity and the •OQ′O branching to acetic acid. We
also provide guidance on the proper implementation of nB reactions
in kinetic mechanisms.