Kinetics and mechanism of the OH-radical and Cl-atom oxidation of propylene carbonate

Abstract

Rate coefficients have been measured at 298 K and atmospheric pressure for the reaction of OH radicals and Cl atoms with propylene carbonate. The measurements were performed in a large volume photoreactor using in situ FTIR spectroscopy for the analysis. The following rate coefficients (in units of cm³ per molecule per s) were obtained: k(OH + PC) = (2.52 ± 0.51) × 10⁻¹² and k(Cl + PC) = (1.77 ± 0.43) × 10⁻¹¹. Product studies performed on the OH-radical and Cl-atom mediated oxidation of propylene carbonate in air support that the major fate of the intermediate cyclo-methyl-pentoxy radicals, formed in the degradation reaction sequence, is unimolecular decomposition. The FTIR product spectra, in combination with the absence of other potential products, suggest that the decomposition probably results to a large extent in the formation of acetyl formyl carbonate, CH₃C(O)OC(O)OCH(O). In product studies performed in N₂, in which ppm levels of O₂ are present, formation of acetic acid was observed in addition to acetyl formyl carbonate. It is postulated that the acid formation occurs via a pathway involving a 1,3-hydrogen shift mechanism with an intermediate alkoxy radical which is able to compete with the unimolecular decomposition pathway of the alkoxy radical at very low O₂ partial pressures.