Thermodynamic and kinetic analysis using AB initio calculations on dimethyl-ether radical+O 2 reaction system

Abstract

Reaction pathways and kinetics are analyzed on CH3OC·H2+O2 reaction system using ab initio calculations to determine tehrmodynamic properties of reactants, intermediate radicals, and transitionstate (TS) compounds. Enthalpies of formation (ΔHf298o) are determined using the CBS-q//MP2(full)/6-31G(d,p) method with isodesmic reactions. Entropies (S298o) 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 calculated energy-dependent rate constants, k(E), and the master equations is used to account for collisional stabilization. The dimethyl-ether radical CH3OC·H2 (ΔHf298o=0.1 kcal/mol) adds to O2 to form a peroxy radical CH3OCH2OO·(ΔHf298o=−33.9 kcal/mol). The peroxy radical can undergo dissociation back to reactants or isomerize via hydrogen shift (Ea,rxn=17.7 kcal/mol) to form a hydroperoxy alkyl radical C·H2OCH2OOH, (ΔHf298o=−26.5 kcal/mol). This alkyl radical can undergo β-scission reaction to formaldehyde (CH2O)+hydroperoxy methyl radical (C·H2OOH), (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. O H + C H 3 O C H 3 → C H 3 O C ⋅ H 2 + H 2 O ( 1 ) + ) C H 3 O C ⋅ H 2 + O 2 → 2 C H 2 O + O H ¯ ( 2 ) C H 3 O C H 3 + O 2 → 2 C H 2 O + H 2 O Comparison of calculated falloff with experiment indicates that the CBS-q calculated Ea, 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) exp(−Ea/RT}), A in cm3/(mol s), EA in kcal/mol): k1, (2.33×1063)(T/K)−16.89e−11.89/RT for CH3OC·H2+O2⇒CH3OCH2OO·; k3, (T/K)−5.46e−8.59/RT for CH3OC·H2+O2⇒CH2O+CH2O+OH at 1 atm..gif>