Modeling–Experiment–Theory Analysis of Reactions Initiated from Cl + Methyl Formate

Methyl formate (MF; CH3OCHO) is the smallest
representative
of esters, which are common components of biodiesel. The present study
characterizes the thermal dissociation kinetics of the radicals formed
by H atom abstraction from MFCH3OCO and CH2OCHOthrough a combination of modeling, experiment,
and theory. For the experimental effort, excimer laser photolysis
of Cl2 was used as a source of Cl atoms to initiate reactions
with MF in the gas phase. Time-resolved species profiles of MF, Cl2, HCl, CO2, CH3, CH3Cl, CH2O, and CH2ClOCHO were measured and quantified using
photoionization mass spectrometry at temperatures of 400–750
K and 10 Torr. The experimental data were simulated using a kinetic
model, which was informed by ab initio-based theoretical kinetics
calculations and included chlorine chemistry and secondary reactions
of radical decomposition products. We calculated the rate coefficients
for the H-abstraction reactions Cl + MF → HCl + CH3OCO (R1a) and Cl + MF → HCl + CH2OCHO (R1b): k1a,theory = 6.71 × 10–15·T1.14·exp(−606/T) cm3/molecule·s; k1b,theory = 4.67 × 10–18·T2.21·exp(−245/T) cm3/molecule·s over T = 200–2000
K. Electronic structure calculations indicate that the barriers to
CH3OCO and CH2OCHO dissociation are 13.7 and
31.6 kcal/mol and lead to CH3 + CO2 (R3) and
CH2O + HCO (R5), respectively. The master equation-based
theoretical rate coefficients are k3,theory (P = ∞) = 2.94 × 109·T1.21·exp(−6209/T) s–1 and k5,theory (P = ∞) = 8.45 × 108·T1.39·exp(−15132/T) s–1 over T = 300–1500
K. The calculated branching fractions into R1a and R1b and the rate
coefficient for R5 were validated by modeling of the experimental
species time profiles and found to be in excellent agreement with
theory. Additionally, we found that the bimolecular reactions CH2OCHO + Cl, CH2OCHO + Cl2, and CH3 + Cl2 were critical to accurately model the experimental
data and constrain the kinetics of MF-radicals. Inclusion of the kinetic
parameters determined in this study showed a significant impact on
combustion simulations of larger methyl esters, which are considered
as biodiesel surrogates.