Atmospheric Chemistry of Two Biodiesel Model Compounds: Methyl Propionate and Ethyl Acetate

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 115; no. 32; pp. 8906 - 8919
• Main Authors: Andersen, Vibeke F, Berhanu, Tesfaye A, Nilsson, Elna J. K, Jørgensen, Solvejg, Nielsen, Ole John, Wallington, Timothy J, Johnson, Matthew S
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18.08.2011
• Subjects: A: Kinetics, Spectroscopy; Acetates - chemistry; Acetic acid; Anhydrides; Atmosphere - chemistry; Atmospheric chemistry; Biofuels; Chlorine; Chlorine - chemistry; Computer Simulation; Esters - chemistry; Ethyl acetate; Hydroxyl Radical - chemistry; Kinetics; Nitric Oxide; Organic Chemicals - chemistry; Oxidation; Oxidation-Reduction; Oxygen; Propionates - chemistry; Radicals; Rate constants; Spectrophotometry, Infrared
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp204819d

The atmospheric chemistry of two C4H8O2 isomers (methyl propionate and ethyl acetate) was investigated. With relative rate techniques in 980 mbar of air at 293 K the following rate constants were determined: k(C2H5C(O)OCH3 + Cl) = (1.57 ± 0.23) × 10–11, k(C2H5C(O)OCH3 + OH) = (9.25 ± 1.27) × 10–13, k(CH3C(O)OC2H5 + Cl) = (1.76 ± 0.22) × 10–11, and k(CH3C(O)OC2H5 + OH) = (1.54 ± 0.22) × 10–12 cm3 molecule–1 s–1. The chlorine atom initiated oxidation of methyl propionate in 930 mbar of N2/O2 diluent (with, and without, NO x ) gave methyl pyruvate, propionic acid, acetaldehyde, formic acid, and formaldehyde as products. In experiments conducted in N2 diluent the formation of CH3CHClC(O)OCH3 and CH3CCl2C(O)OCH3 was observed. From the observed product yields we conclude that the branching ratios for reaction of chlorine atoms with the CH3–, −CH2–, and −OCH3 groups are <49 ± 9%, 42 ± 7%, and >9 ± 2%, respectively. The chlorine atom initiated oxidation of ethyl acetate in N2/O2 diluent gave acetic acid, acetic acid anhydride, acetic formic anhydride, formaldehyde, and, in the presence of NO x , PAN. From the yield of these products we conclude that at least 41 ± 6% of the reaction of chlorine atoms with ethyl acetate occurs at the −CH2– group. The rate constants and branching ratios for reactions of OH radicals with methyl propionate and ethyl acetate were investigated theoretically using transition state theory. The stationary points along the oxidation pathways were optimized at the CCSD(T)/cc-pVTZ//BHandHLYP/aug-cc-pVTZ level of theory. The reaction of OH radicals with ethyl acetate was computed to occur essentially exclusively (∼99%) at the −CH2– group. In contrast, both methyl groups and the −CH2– group contribute appreciably in the reaction of OH with methyl propionate. Decomposition via the α-ester rearrangement (to give C2H5C(O)OH and a HCO radical) and reaction with O2 (to give CH3CH2C(O)OC(O)H) are competing atmospheric fates of the alkoxy radical CH3CH2C(O)OCH2O. Chemical activation of CH3CH2C(O)OCH2O radicals formed in the reaction of the corresponding peroxy radical with NO favors the α-ester rearrangement.