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

Abstract

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 ketoalkoxy 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 CH 3 C(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.

Bimolecular Peroxy Radical (RO2) Reactions and Their Relevance in Radical Initiated Oxidation of Hydrocarbons

“…Therefore, since RO 2 + OH reactions are a dominant OH consumption pathway at the early stages of autoignition, they should be considered in detailed kinetics models, particularly in simulations of O 3 -enriched and plasma-enhanced combustion where early pulses of OH accelerate oxidation. These reactions may also be relevant in Cl-initiated oxidations of hydrocarbons under conditions where substantial RO 2 can persist at temperatures below which traditional NTC chemistry begins to dominate. − Figure (b) highlights the sensitivity of C 2 H 6 autoignition delay to key reaction steps consuming C 2 H 5 O 2 at varying temperatures from 600 to 1000 K. It shows that the competition between reactions –, , and – have the highest relevance in the NTC regime, while their significance diminishes as temperatures increase above 700 K.…”

Spiers Memorial Lecture: Theory of unimolecular reactions

“…144 For example, in a collaborative study of diethyl ether oxidation with Goldsmith we explored the coupled sequence of prompt dissociations of first the O 2 QOOH, then the KHP, and finally the ketoalkoxy radicals. 145 This sequence had a strong effect on the chain branching through an effective increase in the KHP dissociation rate, as well as in the final product branching from the ketoalkoxy decomposition. Notably, the importance of the prompt processes increased significantly as more reactions were included in the prompt analysis sequence.…”

Virtual Special Issue of Recent Advances in Fundamentals of Biomass and Biofuel Combustion

“…Belmekki et al 18 used a shock tube to investigate reaction kinetics of hydroxyl radicals with dimethyl ether, diethyl ether, di-n-propyl ether, and di-n-butyl ether. Mulvihill et al 19 proposed a high-level theoretical study of non-Boltzmann effects on the formation of ketohydroperoxides during the lowtemperature oxidation of diethyl ether. Fan et al 20 investigated the pyrolysis of di-n-butyl ether with experiments in a flow reactor with photoionization molecular beam mass spectrometry and detailed kinetic modeling.…”