A theoretical investigation of the mechanism responsible for the low-temperature autoignition of dimethyl ether

The low-temperature autoignition of dimethyl ether (DME) is the focus of this work. The mechanism for low-temperature autoignition of DME is modeled after the chain reaction mechanism thought to be responsible for alkane autoignition, which includes chain branching. Chain branching involves an exponential escalation in fuel oxidation due to exponential creation of reactive radical species such as ·OH and HOO·. Previous literature work indicates that density functional theory (DFT) using Becke-3-parameter-Lee-Yang-Parr (B3LYP) exchange-correlation gives a reasonable description of the potential energy surface (PES) for ethane oxidation. Thus DFT-B3LYP is used to explore the energetics of the DME autoxidation. In addition to energetics, we calculate rate constants for relevant elementary reactions using standard statistical theories for comparison with literature rate constants. Peroxy radicals and peroxides are important intermediates in alkane autoignition as well as that of DME. One key peroxide involved in DME autoignition, hydroperoxymethyl formate (HPMF, HOOCH2OC(=O)H), is a well-known intermediate in ethylene ozonolysis conducted in the presence of excess formic acid. The lowest energy path for unimolecular reaction of HPMF is to a hydrogen-bonded complex of the ethylene ozonolysis Criegee intermediate (carbonyl oxide, CH2OO) and HC(=O)OH. The Criegee intermediate, well-known in the realm of atmospheric chemistry, has never been considered in the context of DME oxidation, and has the potential to produce ·OH needed to sustain the chain reaction. Both energetics and kinetics show that production of CH2OO is competitive with O-O scission in HPMF. However, while O-O scission directly leads to ·OH, CH2OO most likely decomposes to H2 and CO2. Thus DME chain branching is subdued, which supports low engine noise observations. Master equation simulations show agreement with observed chemical activation phenomena of DME chain propagation, the mechanism for creation of chain-branching precursor intermediates. Further master equation simulations and comprehensive reactor modeling reveal significant sensitivity of relevant reactions to underlying thermochemistry, presenting further challenges for future modeling efforts. Born-Oppenheimer molecular dynamics along selected sections of the chain propagation and branching PES show interesting short-time dynamics with kinetics implications not revealed by statistical rate theories.