Insight into Selected Reactions in Low-Temperature Dimethyl Ether Combustion from Born−Oppenheimer Molecular Dynamics

Abstract

Dimethyl ether is under consideration as an alternative diesel fuel. Its combustion chemistry is as yet ill-characterized. Here we use Born-Oppenheimer molecular dynamics (BOMD) based on DFT-B3LYP forces to investigate the short-time dynamics of selected features of the low-temperature dimethyl ether (DME) oxidation potential energy surface. Along the chain propagation pathway, we run BOMD simulations from the transition state involving the decomposition of (*)CH(2)OCH(2)OOH to two CH(2)=O and an (*)OH radical. We predict that formaldehyde C-O stretch overtones are excited, consistent with laser photolysis experiments. We also predict that O-H overtones are excited for the (*)OH formed from (*)CH(2)OCH(2)OOH dissociation. We also investigate short-time dynamics involved in chain branching. First, we examine the isomerization transition state of (*)OOCH(2)OCH(2)OOH --> HOOCH(2)OCHOOH. The latter species is predicted to be a short-lived metastable radical that decomposes within 500 fs to hydroperoxymethyl formate (HPMF; HOOCH(2)OC(=O)H) and the first (*)OH of chain branching. The dissociation of HOOCH(2)OCHOOH exhibits non-RRKM behavior in its lifetime profile, which may be due to conformational constraints or slow intramolecular vibrational energy transfer (IVR) from the nascent H-O bond to the opposite end of the radical, where O-O scission occurs to form HPMF and (*)OH. In a few trajectories, we see HOOCH(2)OCHOOH recross back to (*)OOCH(2)OCH(2)OOH because the isomerization is endothermic, with only an 8 kcal/mol barrier to recrossing. Therefore, some inhibition of chain-branching may be due to recrossing. Second, trajectories run from the transition state leading to the direct decomposition of HPMF (an important source of the second (*)OH radical in chain branching) to HCO, (*)OH, and HC(=O)OH show that these products can recombine to form many other possible products. These products include CH(2)OO + HC(=O)OH, H(2)O + CO + HC(=O)OH, HC(=O)OH + HC(=O)OH, and HC(=O)C(=O)H + H(2)O, which (save CH(2)OO + HC(=O)OH) are all more thermodynamically stable than the original HCO + (*)OH + HC(=O)OH products. Moreover, the multitude of extra products suggest that standard statistical rate theories cannot completely describe the reaction kinetics of significantly oxygenated compounds such as HPMF. These secondary products consume the second (*)OH required for explosive combustion, suggesting an inhibition of DME fuel combustion is likely.

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

“…The other peak at m/z = 46.005 u appears above 11.33 eV, as indicated by the recorded PIE curve (Figure 5b) and can thus be assigned to formic acid [HC(O)OH]. 81 The Criegee intermediate (CH 2 OO), which is postulated to be part of the low-temperature combustion of DME, 10,14,17 has the same exact mass as formic acid but was not detected in this experiment (IE = 9.98 eV 82 ). A possible explanation is that the Criegee intermediate is very reactive and thus could be present only in very low concentrations that might be below the detection limit of the current experiment (ppm range).…”

“…The low-temperature oxidation of dimethyl ether (DME) receives continuous attention in the combustion community and many experimental, theoretical, and modeling studies have been reported. − The motivation for these efforts has been varied: because innovative engine designs like homogeneous charge compression ignition (HCCI) engines rely on the control of low-temperature ignition timing, , it becomes increasingly necessary to explore the low-temperature combustion (LTC) of transportation fuels. DME has been proposed as an alternative and/or additive to conventional diesel fuel, and it is of particular interest because it has a high cetane number (≥55), low soot emission characteristics, and interesting physical–chemical properties (such as a low boiling point, easy liquefaction under pressure, and photochemical inertness). − Also, it can be massively produced both from biomass and from fossil fuel re-forming.…”

First-principles-derived kinetics of the reactions involved in low-temperature dimethyl ether oxidation

“…within the first 500 fs of its formation, based on DFT-B3LYP molecular dynamics simulations that are discussed elsewhere [66]. A rough estimate rate constant for reaction (14) using the simple Gorin model as implemented in KHIMERA is 4.64 Â 10 16 s À1 at 600 K (i.e.…”

Tunneling effect in 1,5 H-migration of a prototypical OOQOOH

“…10,11 This appears to be true of -hydroperoxyalkyl radicals, generally, 12 including some oxygen-substituted variants. 13,14 The barrier height to reaction (1) has been computed in previous work, 10,11,15 and Goldsmith et al used Born-Oppenheimer Molecular Dynamics to study the energy distribution of the products of this reaction. 15 Previous studies of the kinetics of H-migration reactions in •OOQOOH (e.g., Refs.…”