Mechanism Study On Oxidative Degradation Of Vinyl Ethers In The Atmosphere

Vinyl ethers (CH=CHOR, VEs), an essential type of volatile organic compounds (VOCs), are emitted into environment from anthropogenic sources. In general, VEs have low boiling point and high vapor pressures at room temperature. These compounds are widely used in various industries (adhesive, coating, plasticizer and motor oil additives, etc.). With the increment of their usage, the quantities of VEs in the atmosphere are increased. Since the existence of C=C double bond, VEs are easily oxidized by the tropospheric oxidants (OH, NO3, O3 and Cl). These oxydative degradation reactions are of vital importance to the formation of ozone and secondary organic aerosols (SOAs). Moreover, the secondary organic pollutants generated during reaction processes have detrimental effects on human. Thus, the investigations on the oxidation of VEs are essential in discussion of atmospheric chemistry, climate change and environmental assessment.Experimental investigations are mostly focus on the detection of products and rate coefficients. Therefore, complex degradation mechanisms, high-activity intermediates and some tiny products are not detected. Theoretical methods were employed for the calculation of structures of VEs, the degradation processes, transformation mechanisms and kinetic studies. In this paper, author has done the following works:1. Mechanistic and kinetic study on the ozonolysis of ethyl vinyl ether, propyl vinyl ether and butyl vinyl etherDensity functional theory (DFT) and ab initio methods were employed to elucidate the mechanisms for 03-initiated oxidation of ethyl vinyl ether (EVE), propyl vinyl ether (PVE) and butyl vinyl ether (n-BVE, i-BVE and t-BVE). For each VE, the reactions proceed via O3 cycloaddition resulting in the formation of primary ozonides (POZs) and then two self-decomposition pathways of POZs are followed, result in the formation of Criegee radicals. Major products were identified to be formaldehyde and formates. The strong polarity of these products makes them important in the formation of secondary organic aerosol. In different temperatures and pressures, the total and individual rate constants were calculated using the modified multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) and transition state theory (TST). At 298 K and 760 Torr, the calculated total rate constants are KEVE+O3=1.80×10-16 cm3 molecule-1 s-1, KPVE+O3=2.45×10-16 cm3 molecule-1 s-1,kn-BVE+O3=2.50×10-16 cm3 molecule-1 s-1, ki-BVE+O3=3.41×10-16 cm3 molecule-1 s-1 and kt-BVE+O3=4.17×10-16 cm3 molecule-1 s-1, which are in perfect agreement with experimental results. The order of the rate coefficients are EVE< PVE<n-BVE<i-BVE< t-BVE, which could be explained by the combined electron-donating and steric effect of alkoxy groups. With respect to ozonolysis reactions, the atmospheric lifetimes of EVE, PVE, n-BVE, i-BVE and t-BVE are 2.20 h,1.62 h,1.59 h,1.16 h and 0.95 h。The short lifetimes indicate that the reactions of VEs and O3 are easily to occur and lead to the harmful secondary compound pollutants.2. Mechanistic and kinetic study on the hydroxylation of ethyl vinyl ether and butyl vinyl etherComputational methods were employed to investigate the mechanisms for the hydroxylation of ethyl vinyl ether (EVE), n-butyl vinyl ether (n-BVE), iso-butyl vinyl ether (i-BVE) and tert-butyl vinyl ether (t-BVE). The geometry optimizations and frequency calculations were carried out at the MPWB1K/6-31+G(d,p) level, and the accurate energetic parameters were obtained by the MPWB1K/6-311++g(3df,2p) method. According to the calculated energy barriers and reaction heats of elementary reactions, both OH-addition pathways and H-abstraction channels of a-alkyl are favorable reactions. The major products are ethyl formate, n-butyl formate, iso-butyl formate, tert-butyl formate and HCHO. The rate constants of primary reactions at different temperatures and pressures were computed by employing the MESMER program. At 298 K and 760 Torr, the total rate constants of EVE+OH, n-BVE+OH, i-BVE+OH and t-BVE+OH are 4.53×10-11 cm3 molecule-1 s-1,12.3×10-11 cm3 molecule-1 s-1,9.32×10-11 cm3 molecule-1 s-1 and 5.75×10-11 cm3 molecule-1 s-1, respectively. The order of rate constants are EVE<n-BVE<i-BVE< t-BVE. In addition, the atmospheric lifetimes of VEs with respect to OH radical are τ(EVE)= 3.07 h,τ(n-BVE)=1.13 h,τ(i-BVE)=1.49 h and τ (t-BVE)=2.41 h. Thus, the OH-initiated degradation reactions of VEs are competitive with ozonolysis reactions.3. Mechanistic and kinetic study on the reactions of methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether and butyl vinyl etherThe Cl-initiated oxidation reactions of methyl vinyl ether (MVE) were analyzed by using the high-level composite method CBS-QB3, while the reactions of ethyl vinyl ether, propyl vinyl ether and butyl vinyl ether with Cl atoms were investigated by MPWB1K/6-31++G(3df,2p)//MPWB1K/6-31+G(d,p). Cl-addition channels are favorable pathways during the reaction processes. In accordance with the further investigation of the dominant routes, formyl chloride and formaldehyde are major product, other products are chloracetate, formate and chloroacetaldehyde. Over the temperature range of 200-400 K and the pressure range of 100-760 Torr, the rate constants of primary reactions were calculated by employing the MESMER program. H-abstraction channels are negligible according to the value of rate constants. At 298 K and 760 Torr, the total rate coefficients for the reactions of VEs with Cl atoms are kMVE+CI= 1.25×10-10 cm3 molecule-1 s-1,kEVE+CI=2.49×10-10 cm3 molecule-1 s-1, kPVE+CI= 4.40×10-10 cm3 molecule-1 s-1,kn-BVE+CI=4.62×10-10 cm3 molecule-1 s-1,ki-BVE+CI= 2.43×10-10 cm3 molecule-1 s-1 and kt-BVE+CI= 2.45×10-10 cm3 molecule-1 s-1. The order of rate constants are MVE<i-BVE< t-BVE< EVE< PVE< n-BVE. Finally, the atmospheric lifetimes of MVE, EVE, PVE, n-BVE, i-BVE and t-BVE with respect to Cl were estimated to be 22.3 h,11.16 h,6.31 h,6.01 h,11.43 h and 11.34 h. Although the value of lifetimes are longer than ozonolysis and hydroxylation reactions, the degradation processes of VEs initiated by Cl atoms are also important during atmospheric chemistry.