Mechanistic And Kinetic Study On Degradation Of Two Typical Alkenyl Acetates In The Troposphere

Alkenyl acetates, one kind of OVOCs, are unsaturated ester. The major applications of alkenyl acetates include the following:manufacture of polyalkenyl acetate and alkenyl acetate copolymers, which in turn are used in latex paints, plastics, adhesives, film paper coatings, and synthetic fibers. It is also available as a synthetic supplement in foods, beverages and cosmetics. The broadly uses of alkenyl acetates caused the considerable emissions into the troposphere. Once being released in troposphere they can be degraded by oxidations such as OH and NO3 radicals, O3 molecules and Cl atoms. These oxidation processes result in secondary pollution (e.g. anhydrides, aldehydes, ketones and carboxylic acid). To shed light on the oxidation processes quantitively, theoretical calculation is performed on the reaction of two kinds of alkenyl acetate, vinyl acetate (VAC) and allyl acetate (AAC), with different oxidations. Several specific conclusions can be drawn from this study.1. Mechanism and Kinetics of OH-initiated Atmosphere Oxidation of VACThe reaction mechanisms of OH-initiated reaction of VAC are investigated at the MPWB1K/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) level. The calculated data show that the addition-elimination mechanism dominates the reaction of VAC with OH. The main products of the OH-initiated atmospheric oxidation of VAC are formic acid, formaldehyde, acetic formic anhydride, 1-hydroxy-2-oxoethyl acetate, acetic acidand acetic 2-hydroxyacetic anhydride in the present of O2/NO. Using MESMER program, the rate constants of the primary reactions over the temperature of 250-400 K and the pressure range of 7.6-76000 Torr are gained. The total rate constant is ktot= 4.01 × 10"11 cm3 molecule-1 s-1 at 298 K and 760 Torr. The total rate constants show negative temperature dependence in the temperature range of 250-400 K.2. Mechanism and Kinetics of NO3-initiated Atmosphere Oxidation of VACThe NO3-initiated atmosphere oxidation of VAC is performed at the M06-2X/6-311+G(3df,2p)//M06-2X/6-31G(d,p) level of theory. Two types of channel are identified for primary reactions, as follows:NO3-addition and H-abstraction. According to the calculated results, the NO3-additions dominate the reaction of NO3 with VAC, whereas H-abstractions are negligible. Further reaction of the two adducts are discussed in the presence and absence of NOX. The major reaction products are formic acetic anhydride, formaldehyde, 1,2-bis(nitrooxy)ethyl acetate,2-(nitrooxy) acetic anhydride and 1-(nitrooxy)-2-oxoethyl acetate in the presence of O2/NOx. The rate constants are estimated by using the MESMER program. The total rate constant of NO3-initiated oxidation of VAC is 7.53 × 10-15 cm3 molecule-1 s-1 at 298 K and 760 Torr, which is in good agreement with the experimental data. The atmospheric lifetime of VAC is about 12.2 days, as determined by NO3 radicals.3. Mechanism and Kinetics of OH-initiated Atmosphere Oxidation of AACThe detailed mechanisms for hydroxylation of AAC are performed at the level of M06-2X/6-311+G(3df,2p)//M06-2X/6-31+G(d,p). Two reaction patterns (OH-addition and H-abstraction) and the subsequent reactions of the primarily produced intermediates (IM1,IM2 and IM4) have been proposed. The OH-addition reactions are more favorable than the H-abstraction reactions, but H-abstraction from CH2 can be competitive. The major degradation products are identified as 3-hydroxy-2-oxopropyl acetate, acetoxyacetaldehyde, formaldehyde, formic acid, 2-hydroxy-3-oxopropyl acetate and acetic acrylic anhydride. The rate constants are obtained over the temperature of 200-500 K and the pressure range of 0.76-760000 Torr. The total rate constant is ktot= 3.16 × 10-11 cm3 molecule-1 s-1 at 298 K and 760 Torr. With respect to the typical concentration of OH radical (2.0 × 106 molecule cm-3), the atmospheric lifetime of AAC is estimated to be 4.40 hours.4. Mechanism and Kinetics of Cl-initiated Atmosphere Oxidation of AACThe reactions of AAC with Cl are calculated at the same level of AAC with OH. The calculated results show that the two OH-additional channels dominant the reaction of AAC with Cl. Further investigations of two Cl-adducts are performed in the presence of O2 and NO, and 3-chloro-2-oxopropyl acetate, chloroacetaldehyde, formaldehyde, acetoxyacetaldehyde and 2-chloro-3-oxopropyl acetate are identied as main products. The rate constants are calculated by employing the MESMER program. The total rate constant (1.39×10-10 cm3 molecule-1 s-1) is well consistent with experimental data at 298 K and 760 Torr The atmospheric lifetime of AAC with respect to Cl atoms is 6.8 hours at high-chlorine region. Comparing with the lifetime of AAC with OH, the degradation of AAC initiated by Cl atoms can be competitive in troposphere.