Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory

Abstract

a b s t r a c tThe thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH 3 OCH 3 . An evaluation of the available experimental results for H + CH 3 OCH 3 can be expressed by a three parameter Arrhenius expression as, k ¼ 6:54  10 À24 T 4:13 expðÀ896=TÞ cm 3 molecule À1 s À1 ð273 À 1465 KÞ The potential energy surface is explored with high level ab initio electronic structure theory. The dynamics of roaming versus radical formation is studied with a reduced dimensional trajectory approach. The requisite potential energy surface is obtained from an interpolative moving least squares fit to wide-ranging ab initio data for the long-range interactions between methyl and methoxy. The predicted roaming and radical micro-canonical fluxes are incorporated in a master equation treatment of the temperature and pressure dependence of the dissociation process. The tight (i.e., non-roaming) transition states leading to a variety of additional molecular fragments are also included in the master equation analysis, but are predicted to have a negligible contribution to product formation. The final theoretical results reliably reproduce the measured dissociation rate to radical products reported here and are well reproduced over the 500-2000 K temperature range and the 0.01-300 bar pressure range by the following modified Arrhenius parameters for the Troe falloff format: k 1;1 ðTÞ ¼ 2:33  10 19 T À0:661 expðÀ42345=TÞ s À1 k 1;0 ðTÞ ¼ 2:86  10 35 T À11:4 expðÀ46953=TÞ cm 3 molecule À1 s À1 F cent ðTÞ ¼ expðÀT=880ÞThe experimentally observed branching ratio of 0.19 ± 0.07 provides a direct measure of the contribution from the roaming radical mechanism. The theoretical analysis predicts a much smaller roaming contribution of 0.02.

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]…”

Experimental and kinetic modeling study of the shock tube ignition of a large oxygenated fuel: Tri-propylene glycol mono-methyl ether

“…6) were taken by analogy to dimethyl ether [19][20][21]. For abstraction rate constants at sites that were adjacent to O-atoms, the rate constants at sites 5, 9, and 13 ( Fig.…”

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

“…18,22,25 Given its great application potential and the simplicity in molecular structure, the hightemperature combustion chemistry of DME has also been extensively studied. [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] Because of the importance of low-temperature chemistry in determining fuel effects in autoignition, the present paper focuses on the key reactions and intermediates of the low-temperature regime, and the results of the high-temperature chemistry studies will not be reviewed here.…”