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

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• 作者：Vibeke F. Andersen ; Tesfaye A. Berhanu ; Elna J. K. Nilsson ; Solvejg J酶rgensen ; Ole John Nielsen ; Timothy J. Wallington ; Matthew S. Johnson
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：August 18, 2011
• 年：2011
• 卷：115
• 期：32
• 页码：8906-8919
• 全文大小：1205K
• 年卷期：v.115,no.32(August 18, 2011)
• ISSN：1520-5215

文摘

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 卤 0.23) 脳 10^鈥?1, k(C₂H₅C(O)OCH₃ + OH) = (9.25 卤 1.27) 脳 10^鈥?3, k(CH₃C(O)OC₂H₅ + Cl) = (1.76 卤 0.22) 脳 10^鈥?1, and k(CH₃C(O)OC₂H₅ + OH) = (1.54 卤 0.22) 脳 10^鈥?2 cm³ molecule^鈥? s^鈥?. 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鈥? 鈭扖H₂鈥? and 鈭扥CH₃ groups are <49 卤 9%, 42 卤 7%, and >9 卤 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 卤 6% of the reaction of chlorine atoms with ethyl acetate occurs at the 鈭扖H₂鈥?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 鈭扖H₂鈥?group. In contrast, both methyl groups and the 鈭扖H₂鈥?group contribute appreciably in the reaction of OH with methyl propionate. Decomposition via the 伪-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 伪-ester rearrangement.