Thermodynamic and kinetic analysis using ab initio calculations on dimethyl-ether radical + O2 reaction system

Document Type

Conference Proceeding

Publication Date

1-1-1998

Abstract

Reaction pathways and kinetics are analyzed on CH3OC · H2 + O2 reaction system using ab initio calculations to determine thermodynamic properties of reactants, intermediate radicals, and transition-state (TS) compounds. Enthalpies of formation (ΔH f 298o) are determined using the CBS-q//MP2(full)/6-31G(d, p) method with isodesmic reactions. Entropies (Sf 298o) and heat capacities (Cp(T) 300 ≤ T/K ≤ 1500) are determined using geometric parameters and vibrational frequencies obtained at the MP2(full)/6-31G(d,p) level of theory. Quantum Rice-Ramsperger-Kassel (QRRK) analysis is used to calculate energy-dependent rate constants, k(E), and the master equation is used to account for collisional stabilization. The dimethyl-ether radical CH3OC·H2 (ΔH f 298o = 0.1 kcal/mol) adds to O2 to form a peroxy radical CH3OCH2OO · (ΔH f 298o = - 33.9 kcal/mol). The peroxy radical can undergo dissociation back to reactants or isomerize via hydrogen shift (E a,rxn = 17.7 kcal/mol) to form a hydroperoxy alkyl radical C·H2OCH2OOH, (ΔHf 298o = - 26.5 kcal/mol). This alkyl radical can undergo b-scission reaction to formaldehyde (CH2O) + hydroperoxy methyl radical (C · H 2OOH), (Ea,rxn = 24.7 kcal/mol). The hydroperoxy methyl radical rapidly decomposes to a second CH2O plus OH. The reaction barriers for CH3OCH2 + O2 to 2 CH2O + OH are lower than the energy needed for reaction back to CH3OC · H2 + O2, and provide a low-energy chain propagation path for dimethyl-ether oxidation. (Equation presented) Comparison of calculated falloff with experiment indicates that the CBS-q calculated E a,rxn for the TS of C · H2OCH2OOH → C · H2OOH + CH2O needs to be lowered in order to match the data of Sehested et al. Rate constants of important reactions are (k = A(T/K)nexp(- Ea/RT), A in cm3/(mol s), EA in kcal/mol): k1, (2.33 × 1063)(T/K) -16.89e-11.89/RT for CH3OC · H 2 + O2 → CH3OCH2OO ·; k3, (6.42 × 1029)(T/K)-5.46e -8.59/RT for CH3OC · H2 + O2 → CH2O + CH2O + OH at 1 atm.

Identifier

0032277333 (Scopus)

Publication Title

Symposium International on Combustion

External Full Text Location

https://doi.org/10.1016/S0082-0784(98)80406-2

ISSN

00820784

First Page

201

Last Page

209

Issue

1

Volume

27

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