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

Abstract

Spin-polarised density functional theory (DFT-B3LYP) energies, harmonic vibrational frequencies, and moments of inertia are used to construct modified Arrhenius rate expressions for elementary steps in chain-propagation and chain-branching pathways for dimethyl ether combustion. Barrierless reactions were treated with variational RRKM theory, and global kinetics were modeled using master equation and perfectly stirred reactor simulations. Our kinetics analysis suggests that the bottleneck along the chain propagation path is the isomerisation of CH 3 OCH 2 OO, contrary to earlier interpretations. Comparing the rate constants for competing decomposition pathways of the key chain-branching intermediate hydroperoxymethyl formate (HPMF), we find that formation of formic acid and the atmospherically relevant Criegee intermediate (CH 2 OO) via a H-bonded adduct may be more favourable than O-O bond scission. Since the latter forms a source of a second OH radical beyond that supplied in chain propagation, which is necessary for explosive combustion, the more favourable pathway to formic acid may inhibit autoignition of the fuel. We predict that the HPMF O-O scission product, OCH 2 OC(¼O)H, most likely directly dissociates to HCO þ HC(¼O)OH. This implies an overabundance of CO at 550-700 K, since HC(¼O)OH is a fairly stable product in this temperature range and facile H abstraction from HCO leads to CO. We find that CO 2 product yields are sensitive to the creation of CH 2 OO and that creation of CH 2 OO is competitive with the O-O scission reaction.

Experimental and kinetic modeling study of the low- and intermediate-temperature oxidation of dimethyl ether

“…They improved the Zhao model to reproduce the measured fuel conversion by adjusting the branching ratio of the second oxygen addition channel and its competitive channel of the important QOOH intermediate to CH 2 O. Moreover, some further advances of chemical kinetics and experimental diagnostics have offered useful insight regarding the key reactions [9,[17][18][19]38,41] that affect the oxidation reactivity of DME in the low-temperature regime.…”

“…With a molecular-beam mass spectrometer, mole fraction profiles of reactants, intermediates and major products were measured. The modeling part started with the Zhao model [7] in which some key reactions and their related rate constants were updated with information from the above-mentioned recent experimental and theoretical investigations [9,[17][18][19]38,41]. Furthermore, theoretical calculations were performed to improve this mechanism.…”

“…Due to its promising properties and potential as an alternative fuel, DME has been investigated extensively in the past decade including experiments such as pyrolysis [5][6][7][8][9][10][11], oxidation [4,5,8,[12][13][14][15][16][17][18][19][20][21][22][23][24] and flames [25][26][27][28][29][30][31][32][33][34][35][36], and theoretical calculations [7][8][9][17][18][19][37][38][39], covering a wide range of temperatures and pressures. Based on these experimental and theoretical studies, detailed kinetic models for DME combustion have been developed [5,7,15,25,40]…”

Analysis of the Kinetics and Yields of OH Radical Production from the CH3OCH2 + O2 Reaction in the Temperature Range 195–650 K: An Experimental and Computational study

“…The latter reaction substantially decreases the steady state concentration of QOOH, thus limiting the rate of the dissociation reaction. Disagreement exists as to the exact chain branching mechanism, 34,59,60,14 but ignition is postulated to occur via the production of O2QOOH followed by subsequent isomerization's and chain branching decompositions, as discussed briefly above. Well-skipping reactions, which do not significantly sample the QOOH well, are important at lower pressures and will reduce the yield of O2QOOH, whereas, at high pressures, excited QOOH species will be stabilized.…”

Global Uncertainty Propagation and Sensitivity Analysis in the CH3OCH2 + O2 System: Combining Experiment and Theory To Constrain Key Rate Coefficients in DME Combustion

“…7 This reaction system is a key step in the combustion of dimethyl ether (DME) [8][9][10][11][12][13] and, with interest in the use of DME as a possible fuel 14 , the CH 3 OCH 2 + O 2 reaction has attracted substantial attention. [15][16][17][18][19] The experiments 6 probed the reaction system by observation of OH, formed by dissociation of CH 2 OCH 2 OOH. Both the time dependence of OH formation and its yield were observed.…”