Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl and NO3

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Article 3

Gas-Phase Oxidation of Allyl Acetate by O, OH, Cl and NO: Reaction Kinetics and Mechanism 3

Shuyan Wang, Lin Du, Jianqiang Zhu, Narcisse Tchinda Tsona, Shijie Liu, Yifeng Wang, Maofa Ge, and Wenxing Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10599 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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The Journal of Physical Chemistry

Gas-phase Oxidation of Allyl Acetate by O3, OH, Cl and NO3: Reaction Kinetics and Mechanism Shuyan Wang,† Lin Du,*,† Jianqiang Zhu,† Narcisse T. Tsona,† Shijie Liu,† Yifeng Wang,‡ Maofa Ge,§ and Wenxing Wang†

† ‡

Environment Research Institute, Shandong University, Jinan 250100, China Key Lab of Colloid and Interface Science of the Education Ministry, Department of

Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China §

Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory

for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

Correspondence to: Lin Du ([email protected])

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ABSTRACT Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O3, OH, Cl and NO3) have been investigated in a Teflon reactor at 298±3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm3 molecule-1 s-1) are (1.8±0.3)×10-18, (3.1±0.7)×10-11, (2.5±0.5)×10-10, and (1.1±0.4)×10-14, for reactions with O3, OH, Cl and NO3, respectively. While results for reactions with O3, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO3 is reported for the first time in this study. Based on determined rate constants, the tropospheric lifetimes of AA are: τO3 = 9 days, τOH = 5 h, τCl = 5 days, τNO3 = 2 days. Based on the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants, and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, both at daytime and nighttime.

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1. INTRODUCTION

Volatile organic compounds (VOCs) are ubiquitous in the environment and are directly emitted into the atmosphere from a variety of natural and anthropogenic sources.1 Natural sources include biogenic emissions, while anthropogenic sources include emissions from vehicular exhaust, fuel evaporation, industrial processes, household products and solvent usage.2-4 As a subset of VOCs, oxygenated volatile organic compounds (OVOCs) are formed as oxidation products of all hydrocarbons present in the atmosphere and play a significant role in atmospheric chemical processes.2 OVOCs may undergo complex chemical reactions with the main atmospheric oxidants, O3, Cl atoms, OH, and NO3 radicals, leading to the formation of low volatile products, which contribute to the formation of secondary organic aerosols (SOA).5 SOA are a major component of atmospheric fine particles (PM2.5), which pose serious health risks and influence Earth’s climate.6, 7 In addition, OVOCs are key precursors of O3. Both high concentration of PM2.5 and O3 in the atmosphere can be used to characterize the photochemical smog, which is one of the top environmental concerns.1 Moreover, OVOCs are generally more reactive than parent alkanes and play an active role in the sequence of chemical reactions.2 Therefore, the atmospheric chemistry of OVOCs is of relevance in processes related to air quality and climate change.8

Among OVOCs, acetate esters (CH3C(O)OR, R can be an alkenyl group) are used on a large scale in industry, and many of them identified as high-productive volume (HPV) chemicals.9 Acetate esters are potential replacements for traditional solvents and diesel fuels or fuel components, and are emitted into the atmosphere during their production, processing, storage and disposal.10 A number of kinetic data have been published concerning the gas-phase reactions of short chain esters such as methyl acetate, ethyl acetate, and isopropenyl acetate with OH, Cl, and O3.11-13 The rate constants of reactions of the isopropyl acetate, n-propyl acetate, isopropenyl acetate, n-propenyl acetate, n-butyl acetate, and ethyl butyrate with OH were determined by relative rate experimental method in purified air under atmospheric conditions, at 750 Torr and 295±2 K.13 3



Tropospheric lifetimes of these esters ranged from 5 hours for isopropenyl acetate to 5 days for n-propyl acetate. Atmospheric oxidations of these esters by OH may also generate O3. The rate coefficients have been measured for the reaction of Cl with a series of alkyl esters at 298±2 K and atmospheric pressure in a large volume photoreactor using the relative kinetic technique, which suggested that the values of the substituent factor F(RC(O)O-) used in structure activity relationship (SAR) may depend on the nature of the alkyl group R on the acyl side of the ester functionality.14 Rate coefficients for the reactions of O3 with n-butyl methacrylate, ethyl crotonate and vinyl propionate have been determined at 298±1 K and atmospheric pressure.15 The following room temperature rate coefficients (in units of cm3 molecule-1 s-1) were obtained: kCH2=C(CH3)C(O)OC4H9+O3 = (1.0±0.3)×10-17, kCH3CH=CHC(O)OCH2CH3+O3 = (8.0±1.8)×10-18 and kCH3CH2C(O)OCH=CH2+O3 = (5.3±1.3)×10-18. Furthermore, the reactivity trend of esters toward O3 was comparable to that of reactions with other tropospheric oxidations such as OH and Cl. It was found from the same unsaturated esters that reactions with O3 were more sensitive to the presence of substituent groups than reactions with Cl and OH. For reactions of O3 with n-butyl methacrylate and ethyl crotonate, it could be observed that the replacement of a -CH3 group of the double bond by a -H atom produces a decrease of the rate coefficient in connection with low susceptibility of π electrons of the double bond towards O3 electrophilic attack. Kinetics of the gas-phase reactions of O3 with unsaturated esters (methyl methacrylate, vinyl acetate and methyl crotonate) have been investigated by comparing their rates of decay with those of three corresponding reference compounds at atmospheric pressure and ambient temperature (285-295 K).12 Atmospheric half-lives with respect to removal by O3, calculated from the measured rate constants, suggested that removals by O3 and OH were of comparable magnitude.12 Allyl acetate (CH3C(O)OCH2CH=CH2, AA) has been detected as one of the products in the combustion of esterified rape oil.10 As a monomer, co-monomer or intermediate, AA can be used in the synthesis of several chemicals, such as in the preparation of copolymers with acrylonitrile, ethylene, methyl cyanoacrylate as well as in the production of fire resistant rigid polyurethane foams.16-18 The healthy problems of 4


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irritation for human eyes, nose, skin and respiratory system could also be caused by the release of AA. Once emitted to the atmosphere, AA would be subject to degradation by photolysis and reaction with main atmospheric oxidants. It is therefore important to assess the chemical outcome of AA through photooxidation, by exploring the kinetics and mechanism of its atmospheric degradation by these oxidants.

Previous studies on AA reactions with some atmospheric oxidants have been reported. The rate constant of the reaction of AA with OH was measured by laser photolysis-laser induced fluorescence technique over the temperature range 243-372 K.10 The kinetic data of this reaction were used to derive an Arrhenius expression (in units of cm3 molecule-1 s1

), k = (2.33±0.27)×10-12 exp[(732±34)/T], and the observed rate constant was

(2.71±0.30)×10-11 cm3 molecule-1 s-1 at 298 K.10 Using the relative rate method with propene and n-octane as reference compounds, the rate constants for AA + OH reaction were determined to be (2.89±0.45)×10-11 and (2.46±0.24)×10-11 cm3 molecule-1 s-1, respectively.19 The main oxidation products were 2-oxoethyl acetate and formaldehyde for the reaction of AA with the OH radical.19 By investigating the reaction mechanism, it is seen that reactions proceed mainly by the addition of the oxidant to the C=C bond, whereas, the initiation caused by H-atom abstraction is negligible.20, 21 Additionally, the yields of main products, 2-oxoethyl acetate and formaldehyde, decreased in the absence of NOx, which indicated that under such experimental conditions, 1,2-hydroxyalkoxy radicals formed after addition of OH to the double bond.18 The AA reaction with O3 was investigated both theoretically and experimentally and rate constants of (2.40±0.70)×10-18 and 2.43×10-18 were obtained, respectively.10,

22

The experimental reaction was

performed with the absolute rate method, and methyl vinyl ketone was used as the reference compound.

While only one study has been reported on the kinetics and mechanism of the atmospheric oxidation of AA by Cl,11 to our knowledge, the reaction of AA with NO3 has not yet been investigated. Although the NO3-initiated degradation is not the prime loss process for AA in the atmosphere, kinetic data and mechanism for the NO3 reaction with 5



AA is needed to have a better understanding about the role of AA in the atmosphere where its reaction with NO3 could potentially lead to night-time source of OH. On the other hands, significant levels of NO3 have been detected over a wide range of atmospheric conditions, indicating a potential role for NO3 in the oxidation chemistry over large regions of the atmosphere. Moreover, NO3 reactions with VOCs are important, or in some cases, even dominant in the atmosphere, therefore impacting the budgets of these species and their degradation products. These reactions together with the ozonolysis of VOCs are also responsible for the nighttime products and cycling of OH and peroxy (HO2 + RO2) radicals. Reactions of NO3 with biogenic hydrocarbons are also known to be responsible for the production of organic nitrates and SOA.23

The present work determines the kinetics for reactions of AA with four atmospheric oxidants (OH, NO3, Cl and O3) using relative rate and absolute rate methods. The reaction with NO3 is investigated for the first time. Atmospheric lifetimes of AA based on its reactions with the four oxidants were determined, and the reactions products were characterized using thermal desorption-gas chromatography-mass spectrometry (TD-GCMS) and Fourier transform infrared spectroscopy (FTIR) for the purpose of proposing degradation mechanisms for AA in the atmosphere.

2. EXPERIMENTAL SECTION

2.1. Kinetics Study All experiments were performed in a 100 L Teflon reactor in a chamber at room temperature (298±3 K) and atmospheric pressure. This reactor was made of 0.05 mm thick FEP Teflon film. An inlet and an outlet made of Teflon were used for the introduction of reactants and sampling. Three UV lamps (F40T6BL, center wavelength 254 nm) mounted in parallel were used for radical precursors photolysis. The light intensity and hence the radical production rate could be regulated by adjusting the number of lamps and the time of irradiation. The reactor was flushed with purified air from a zero air supply (111-D3N, Thermo Scientific) three times before reactants were 6


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added into the reactor. The purified air flowed into the reactor through a mass flow controller (D08-8C/ZM, Beijing Sevenstar Electron Corporation) which could record the volume of purified air in the reactor. The background O3 concentration in zero air was below 1.5 ppb, and the NOx concentration was below 1 ppb. Homogeneous mixing of the reactants in the reactor was needed before starting the experiment. The concentrations of reactants in the reactor were measured during the experiments by gas chromatography with flame ionization detector (GC-FID) (7890B, Agilent Technologies). In all experiments, a capillary column (30 m × 0.32 mm id × 1.8 µm film thickness, DB 624, Agilent Technologies) was used to separate the compounds. The chromatographic conditions used for the analysis were as follows: injector 250°C, detector 300°C, column temperature 110°C.

The relative rate technique was used to determine rate constants for reactions of OH, Cl and NO3 with AA. A number of factors were considered in the choice of reference compounds: (i) the rate constants should be convinced, preferably the data from the IUPAC recommendation, (ii) the rate constants of reference compounds are different from one another, (iii) the wall loss and photolysis of the reference compounds should be very low to minimize errors during the kinetics experiments. Cyclohexane, isoprene and propene were used as reference compounds for the reaction with OH, propene and cyclohexane were used for the Cl kinetic experiments, whereas, propene or acetaldehyde were used for the NO3 kinetic experiments. Ratios of initial concentration of AA and reference compounds were set as 1:1, 1:2, 2:1, 2:3 and 3:2 in order to eliminate the influence of secondary reactions in the reaction process.24 The initial reactant concentrations are listed in Table 1. The techniques and the reference compounds used in the reactions of AA are listed in Table 2.

In order to verify the influence of precursor to the Cl atoms, the photolysis of thionyl chloride (SOCl2) and trichloroacetyl chloride (CCl3COCl) were used to generate Cl atoms. OH radicals were produced by the photolysis of hydrogen peroxide (H2O2). NO3 radicals were generated by the thermal decomposition of N2O5 in the dark:25, 26 7



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SOCl2 + hν(λ = 254nm) → SOCl ⋅ +Cl ⋅

(1)

CCl3COCl + hν(λ = 254nm) → CCl3CO ⋅ +Cl ⋅

(2)

H 2 O2 + hν(λ = 254nm) → 2OH ⋅

(3)

N 2 O5 + M → NO3 ⋅ +NO 2 + M

(4)

In the experiments performed with GC-FID, the reactant mixtures were photolyzed for 625 s for SOCl2, 4-10 min for CCl3COCl and 5-15 min for H2O2 and a gas chromatogram of the reactor contents was recorded. This photolysis-sampling procedure was repeated until 74-92% depletion of AA and 65-92% depletion of the reference compounds were achieved. Typically, 8-17 photolysis-sampling steps were carried out during each experiment. During GC-FID experiments successive additions of N2O5 were performed. Sampling was carried out approximately 6 min after the addition of N2O5 and at subsequent intervals of 13 min. This procedure led to 33-55% depletion of AA and 26-48% depletion of the reference compound. Overall, 7-9 additions were typically made to the reactor during an experiment.

N2O5 was prepared via addition of 80-120 g phosphorus pentoxide powder to 30 mL fuming nitric acid. Phosphorus pentoxide powder, whose number of moles was kept seven times higher than that of nitric acid, was slowly added into fuming nitric acid which was kept in a three-neck flask at 233 K and mixed thoroughly with a stirrer. The flask was then heated and kept at 313 K for 40 min. After that the volatile compounds including N2O5 and NO2 in the three-neck flask were collected by a cold trap. The mixture in the cold trap was yellow powder. At last, the cold trap was continuously extracted out by a pump at 263 K until the mixture became white powder. This indicated that, NO2 had been extracted out, and the white powder was pure N2O5, exclusively. N2O5 was kept in cold trap, held in liquid nitrogen in case of breakdown. The produced Cl atoms, OH and NO3 radicals initiate the decay of AA and the reference compounds through the following reactions: kAA X + AA → products

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(5)

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kRef X + R → products

(6)

where X represents the oxidant (Cl, OH or NO3), kAA and kRef are the rate constants of reactions (5) and (6), respectively.

The decay of AA and reference compounds is governed by the following rate equations: d[AA] = − k AA [AA][X] dt

(7)

d[R] = − kRef [R][X] dt

(8)

Integration and rearrangement of equation (7) and equation (8) lead to the expression: ln

[AA]0 k AA [R]0 = ln [AA]t kRef [R]t

(9)

where [AA]0 and [AA]t are the concentrations of AA at time t = 0 and time = t. [R]0 and [R]t are the concentrations of the reference compound at time t = 0 and time = t. The plot of ln([AA]0/[AA]t) versus ln([R]0/[R]t) yields a straight line passing through the origin and whose slope gives the ratio of rate constants, kAA/kRef. Rate constants of reactions of the reference compound with Cl, OH and NO3 are known and hence, rate constant kAA can be readily calculated.

The relative rate technique relies on the assumption that both AA and the reference compound are removed solely by reactions with the oxidants (Cl, OH and NO3).27 To verify this assumption, mixtures of AA and reference compounds with H2O2 or SOCl2 or CCl3COCl were prepared and allowed to stand in the dark. The reaction between AA and reference compounds with the precursor was found to be negligible over the period of the experiments. Possible photolysis loss of AA and the reference compounds was tested by irradiating mixtures of AA and reference compounds with the UV lamps in the absence of the radical precursors. Reactions with precursors (H2O2 or SOCl2 or CCl3COCl) and photolysis were found to be negligible for AA and the reference compounds.

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The rate constant for the reaction of AA with O3 was determined using the absolute rate method. The decay of O3 was measured in the presence of excess concentrations of AA. The concentrations of O3 were governed by the following processes: O3＋wall → loss of O3

(10)

O3＋organic → products

(11)

With the initial concentration of [AA]0 being in large excess over the initial O3 concentration ([AA]0/[O3]0 ＞ 50), the AA concentration essentially remains constant throughout the reaction,28 and reaction (11) can be treated as a pseudo-first-order reaction. The following equation could be obtained: −d ln[O3 ] = k1 + k2 [AA]0 dt

(12)

where k1 and k2 are the rate constants of wall loss of O3 (equation 10) and O3 reaction with AA (equation 11).

The total error, σAA, for the rate constants quoted in relative rate measurement is combination of the 2σ statistical error ( 2σ kAA

k Ref

) from the regression analysis σ (slope)

and the error in measuring the rate constant for the reaction of AA with the reference compound ( σ kRef ), according to equation (13):

σ AA = kAA [

2σ kAA

kRef

kAA kRef

]2 + [

σk

Ref

kRef

]2

(13)

Since relative measurements do not depend on the absolute concentrations of the reactants, calibration errors can be ignored.29 The concentrations of AA and reference compounds were measured using the same device, the GC-FID. The calibration curves for reactants concentration versus their signals were linear, and the precision of the measured signal contributes to the precision of the measured rate constants.30 The quoted errors in the rate constant ratios (kAA/kRef) are twice the standard deviation ( 2σ kAA

k Ref

) in

the linear least-squares fit of the measured losses. In addition to the precision due to this

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ratio, the estimated uncertainty in the rate constant for reactions of OH, Cl and NO3 with the reference compounds were included.

O3 was produced using oxygen from an oxygen cylinder by high voltage discharge, which was saved at a 50 L Teflon bag temporarily. For each reaction investigated, one run was conducted in the presence of cyclohexane to scavenge OH radicals, which could be produced during ozonolysis. The accurate concentration of O3 was measured by ozone analyzer (Model 49i, Thermo Electron Corporation) with a flow rate of 0.7 L/min. To insure that O3 concentration was not affected by the background air prior to experiments, we measured its concentration in zero air in the reactor (below 1.5 ppb). There is no obvious change after injection of 13.5×1014 molecules cm-3 AA (the highest concentration used in the kinetics experiments) into the reactor, suggesting no or negligible effect of AA on O3 concentration. This further indicates that the current technique used to determine the rate constant for the reaction of O3 with AA is reliable. During the experiments, O3 concentrations were recorded every 1 min, until the final O3 concentration in the reactor decreased to be lower than 30% of the initial concentration. The measurement lasted for 40-50 min. The initial AA concentrations in the reactor were in the range of (2.7-13.5)×1014 molecules cm-3, while the initial O3 concentrations were in the range of (2.8-6.7)×1012 molecules cm-3.

2.2. Products Study Product experiments were carried out at room temperature and atmospheric pressure in a 100 L Teflon reactor with three 254 nm UV lamps except NO3-initiated and O3-initiated oxidation experiment. Products were detected using the TD (ATDS-3400A, Huashenpuxin) coupled with GC-MS (7890-5977B, Agilent Technologies) and a Bruker Vertex 70 FTIR. In the product experiments the initial concentration of AA in the reactor was 44 ppm (1 ppm = 2.46×1013 molecules cm-3). The quantitative analysis was performed using SOCl2 as the Cl source. The total photolysis time was 5-10 min for SOCl2 and 2-3 h for H2O2, with about 80-90% conversion of AA. The NO3-initiated and O3-initiated oxidation experiment lasted for 2-3 h to achieve 80-90% conversion of AA 11



without the UV lamps. The qualitative determinations of products for the AA reactions with OH, Cl and NO3 were carried out using TD-GC-MS. The samples were collected into a Tenax TA (Markes International, 3.5-inch length and 0.25-inch O.D.) by a sampler and the sampling lasted for 30 min. After that, the adsorption tube was put into the TD instrument to desorb the sample into the capillary column by applying the following temperature program: from 30°C (3 min) to 100°C (0 min) at a rate of 5°C min-1, then to 230°C (10 min) at a rate of 10°C min-1. The qualitative determination of products for AA reactions with O3 and Cl was carried out using FTIR spectrometer equipped with a KBr beam splitter and a DLaTGS detector. The IR spectral range was 370-7500 cm-1 with a resolution of 1 cm-1.

2.3. Chemicals The chemicals used in the experiments were obtained commercially except N2O5. The chemicals were used as received and their purities were as followed: AA (Aladdin, 99%), cyclohexane (Aladdin, 99.9%), isoprene (Alfa Aesar, 99%), acetaldehyde (Aladdin, 99.5%), SOCl2 (Damas-beta, 99+%), CCl3COCl (Damas-beta, 99%). The following gases were supplied by Jinan De Yang Special Gas Co., Ltd.: propene (1%, N2 as bath gas), N2 (> 99.999%), O2 (> 99.999%). Acetaldehyde and H2O2 were supplied by Beijing Fine Chemicals Co., Ltd. with the purity of 98% and 30% respectively. Commercially available phosphorus pentoxide (P2O5, ≥98.0%, Tianjin Damao) and fuming nitric acid (HNO3, ≥98.0%, Sinopharm) were used to synthesize N2O5.

3. RESULTS AND DISCUSSION

3.1. Kinetics Studies 3.1.1. O3-initiated Reaction The gas phase reaction of O3 with esters produces OH, which can further react with the reactants and induce errors to the rate constants of the reaction.10 In this study, the concentration of AA was almost constant during the reaction since a large excess of the AA was used. As a result, the effect of AA loss by reaction with OH can be negligible. 12


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Nevertheless, to be more accurate, a certain amount of cyclohexane was added as OH scavenger. In order to reduce the interference from the wall effect, the reactor operated for at least 24 h in the presence of O3 prior to each set of experiments. Attenuation experiment of O3 was performed to study the wall effect, which lasted 9 h, and the O3 decay rate constant of 3.9×10-6 s-1 was obtained. This value is consistent with previously reported value (6.09×10-6 s-1), and is about 2 orders of magnitude lower than the pseudofirst-order rate constant of AA reaction.31 Thus, the background O3 decay accounts for only a small part of all the loss of O3 in the reactor during experiments. Obviously, the O3 loss caused by background decay in this study was negligible.

The rate constants were determined under pseudo-first-order conditions. Decay of the O3 concentration was obtained as a function of time at 298±3 K. The logarithms of the ratios of the O3 concentration in the presence of AA were plotted for different reaction times. For reaction (11), straight lines were obtained for all pseudo-first-order plots for five AA concentrations. An example of the typical loss of O3 versus reaction time at different concentrations of AA at 298±3 K is shown in Figure S1 in the Supplementary Information. The slope of such plots yields the pseudo-first-order rate constant, dln[O3]/dt. The value of -dln[O3]/dt versus [AA]0 data were subjected to linear leastsquares analyses to obtain the rate constant of AA reaction with O3, kAA+O3. The plot of dln[O3]/dt versus the initial concentration of AA was linear with intercept close to zero (see Figure 1). This indicates that if secondary reactions take place in the experiment, their effect is negligible. Rate constants for the reaction of AA with O3 from this work and from other references are listed in Table 2 for comparison. The errors are two standard deviations from the least-squares analysis of the kinetic data plot. In some previous studies, errors for rate constants determination included 2σ from the leastsquares analysis of the kinetic data plot and an estimated systematic error of 5-10%.32, 33 Considering these, the error for the rate constant of O3 reaction with AA includes 2σ from the least-squares analysis and 10% systematic error.

13



We determined a rate constant of (1.8±0.3)×10-18 cm3 molecule-1 s-1 for the reaction of O3 with AA, in agreement with other reports listed in Table 2. Using the relative rate method, a value of (2.40±0.70)×10-18 cm3 molecule-1 s-1 was derived at 298±4 K,10 while quantum chemical calculations were used to determine a rate constant of 2.43×10-18 cm3 molecule-1 s-1 at 298 K.22 The slight discrepancy between our results and available literature values might be due to the use of different experimental techniques10 and the error associated with theoretical calculations.10, 22 Rate coefficients for O3 reactions with other unsaturated esters have also been reported: (3.0±0.4)×10-18, (0.95±0.06)×10-18, and (1.28±0.06)×10-18 cm3 molecule-1 s-1 for O3 reactions with CH3C(O)OCH=CH2, CH2=CHC(O)OCH3, and CH2=CHC(O)OCH2CH3, respectively.34, 35 These values are of similar magnitude as the (1.8±0.3)×10-18 cm3 molecule-1 s-1 value reported in this study for the O3 reaction with CH3C(O)OCH2CH=CH2, indicating a similar mode of reactivity in O3 reactions with unsaturated esters. Moreover, unsaturated acetates are slightly less reactive towards O3 than corresponding alkenes (kCH3CH2CH=CH2+O3 = 9.64×10-18 cm3 molecule-1 s-1 and kCH3CH2CH2CH=CH2+O3 = (9.6±1.6) ×10-18 cm3 molecule-1 s-1),36 and the electron withdrawing effect of the C(O)OR substituent in unsaturated alkenes and reactions of corresponding alkenes being electrophilic additions of O3 at the C=C bond can generally explain the difference of reactivity.37

3.1.2. OH Radical-initiated Reaction Reported rate constants of the reaction of OH with reference compounds are as follows (in units of cm3 molecule-1 s-1): (6.97±1.39)×10-12 for cyclohexane, (1.00±0.06)×10-10 for isoprene, 2.63×10-11 for propene.36, 38, 39 Several samplings, performed at short and steady time intervals (~1 min), were executed to accurately determine the initial amount of AA and reference compounds. The Teflon reactor was irradiated using UV lamps. From the amounts of AA and reference compounds measured during experiments, ln([AA]0/[AA]t) versus ln([R]0/[R]t) was plotted as shown in Figure 2.

The kAA/kRef ratio was obtained from least-squares regression analysis of the plots of kinetic data for all the experiments performed on OH reaction with AA (see Table 2). 14


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Since the rate constants for AA reaction with OH obtained using three reference compounds are in good agreement with each other, the final reaction rate constant was quoted as the average of all rate constants determined using the different reference compounds. This resulted in a rate constant value of (3.1±0.7)×10-11 cm3 molecule-1 s-1 for the AA reaction with OH. This value is in very good agreement with available data reported in the literature.10,

19

Using the pulsed laser photolysis-laser fluorescence

technique over the temperature range 243-372 K, the kinetic data of OH reaction with AA were used to derive an Arrhenius expression for the rate constant: (2.33±0.27)×10-12 exp[(734±34)/T] cm3 molecule-1 s-1. At 298 K the rate constant obtained was (2.71±0.30)×10-11 cm3 molecule-1 s-1.10 The rate constant of AA reaction with OH, determined in purified air 750 Torr and 295±2 K using a relative rate method and different reference compounds (n-heptane, n-octane, and n-nonane with respective rate constants for reactions with OH of 7.12×10-12, 8.42×10-12 and 9.70×10-12 cm3 molecule-1 s-1), was found to be (2.46±0.24)×10-11 cm3 molecule-1 s-1.13 Slight discrepancies between our results and literature results are likely due to differences in experimental conditions.

The reaction of R1C(O)OR2 with OH seems to occur at OR2 entity and R1CO. For compounds with same R1 as AA, the reactivity increases if the length of R2 chain increases, as observed when comparing the rate constant for the reaction of vinyl acetate with OH ( (2.3±0.3)×10-11 cm3 molecule-1 s-1) with that of AA reaction with OH.34 Also, the rate constant for reaction with OH is higher than when R2 is an alkenyl than when it is the corresponding alkyl, as indicated by the rate constant of OH reaction with n-propyl acetate ((1.97±0.24)×10-12 cm3 molecule-1 s-1), compared to the rate constant for AA + OH reaction.13 For compounds with same R2 as AA, the reactivity increases slightly if the length of R1 chain increases: the rate constant of allyl butyrate is (2.89±0.31)×10-11 cm3 molecule-1 s-1.40 For an isomeric group, the branched chain is more reactive than linear chain, as illustrated by rate constant value for the OH reaction with isopropenyl acetate, (6.25±1.24)×10-11 cm3 molecule-1 s-1, more than twice higher than that of the AA + OH reaction.13 Furthermore, it was observed that rate constants of OH reactions with AA and the corresponding alkene are very similar (kCH3CH2CH2CH=CH2＋OH = 2.91×10-11 15



cm3 molecule-1 s-1).41 This indicates that the OH radical-initiated oxidation of AA proceeds mainly by OH addition to the C=C bond, forming hydroxyalkyl radicals which can react with O2 to form corresponding hydroxyalkyl peroxy radicals. These observations are consistent with previous studies.42 There are two distinct pathways for the methylacrolein reaction with OH: abstraction of the hydrogen atom, or addition of the OH on the C=C double bond.43 The reactivity of the unsaturated esters is significantly higher than those of the corresponding saturated esters. This can be explained by the mechanistic difference of OH attack.13 At room temperature, H-atom abstraction is the minor channel with 5-10 % reaction probability.42

3.1.3. Cl Atom-initiated Reaction The kinetic data obtained from experiments, plotted according to equation (9) for the reaction of Cl with AA, were measured relative to two reference compounds (see Figure 3). Good linear relationships with practically zero intercept were obtained in all experiments, suggesting that secondary reactions could be neglected.

In order to calculate the rate constants for the reaction of Cl with AA, the following values for rate constants of Cl reaction with the reference compounds at 298 K were used: (3.50±0.25)×10-10 cm3 molecule-1 s-1 for cyclohexane, and (2.3±0.3)×10-10 cm3 molecule-1 s-1 for propene.44,

45

Results obtained from linear least-square analysis of the data are

shown in Table 2. The values for rate constants obtained using two different Cl precursors and two reference compounds were in very good agreement within the experimental uncertainties: kCl＋AA = (2.45±0.40)×10-10 cm3 molecule-1 s-1 (using SOCl2 as Cl precursor), kCl ＋ AA = (2.60±0.40)×10-10 cm3 molecule-1 s-1 (using CCl3COCl as Cl precursor). The final rate constant for AA reaction with Cl was taken as the average of the individual determinations with error limits, and is determined to be (2.5±0.5)×10-10 cm3 molecule-1 s-1. This value differs from the rate constant in a previous study by a factor of ~2, which found a rate constant of (1.30±0.45)×10-10 cm3 molecule-1 s-1, determined at 298±3 K and a total pressure of 760 Torr using isobutene (kisobutene＋Cl = (3.40±0.28)×1016


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10 1

cm3 molecule-1 s-1) and 1,3-butadiene (k1,3-butadiene＋Cl = (4.2±0.4)×10-11 cm3 molecule-1 s-

) as reference compounds in a quartz-glass reaction chamber.11 The discrepancy might

originate from the choice of the reference compounds.

On the other hand, the rate constant for the reaction of vinyl acetate with Cl, (2.68±0.91)×10-10 cm3 molecule-1 s-1, is slightly higher than that of AA + Cl reaction. This observation can be attributed to the activation of the double bond toward Cl addition by the conjugation effect of the lone pair electrons on the O atom of the vinyl acetate with the olefin π electron system.11 However, in the AA structure, the -CH2- entity separates the O atom from the olefin π system, which makes the conjugation effect impossible.11 The rate constant for the AA + Cl reaction is 3 times higher than that of the Cl reaction with n-propyl acetate (kCH3C(O)OCH2CH2CH3+Cl = 7.76×10-11 cm3 molecule-1 s-1), which suggests that the double bond is highly reactive toward Cl addition and that addition is seemingly the major reaction pathway.46 The reactivity of the -OCH2- group in OCH2CH=CH2 and the rate of allylic H-atom abstraction from AA have been studied in the Cl-initiated oxidation of AA, vinyl acetate and butyl acrylate. Results show that Cl addition to the C=C bond in AA dominates the reaction mechanism with a contribution between 70 and 85%.11

3.1.4. NO3 Radical-initiated Reaction The relative rate method was used to determine the rate constant for the AA reaction with NO3. AA, the reference compound and N2O5 were introduced into the Teflon reactor without UV lamps. The rate constants for the reaction of NO3 with reference compounds were as follows: (6.1±1.3)×10-15 cm3 molecule-1 s-1 for propene, and (2.62±0.29)×10-15 cm3 molecule-1 s-1 for acetaldehyde.25, 47 Typical plots of the kinetic data were obtained from investigations on the reaction of NO3 with AA at 298±3 K and atmospheric pressure (see Figure 4). The data give straight lines with zero intercepts (indicating that secondary reactions are not important), having slopes of kAA/kRef. The weighted average kAA values and the rate constant ratio kAA/kRef obtained by the GC-FID technique are given in Table 2. The rate constants for the AA reaction with NO3 obtained using propene and 17



Page 18 of 39

acetaldehyde as reference compounds are in good agreement with one another. Hence, the final rate constant for this reaction, quoted as the average of the values obtained using the two reference compounds, was determined to be kAA ＋ NO3 = (1.1±0.4)×10-14 cm3 molecule-1 s-1.

To our knowledge, the rate constant of the AA + NO3 reaction has not been determined previously and no comparison with previous work is possible. However, comparison of NO3 reactions with other unsaturated esters can be made. Using both absolute and relative rate methods at atmospheric pressure and 293±3 K, the rate constant for the NO3 reaction with vinyl acetate was determined to be (7.1±1.9)×10-15 and (7.5±1.7)×10-15 cm3 molecule-1 s-1, respectively, leading to an average value of (7.3±1.8)×10-15 cm3 molecule-1 s-1.34 This is lower than the rate constant of AA reaction with NO3 reported in the present study. Another study also reported rate constants, using the absolute rate method, of NO3 reaction with some aliphatic esters: (5±2)×10-17, (0.7±0.2)×10-17, and (1.3±0.3)×10-17 cm3 molecule-1 s-1 for propyl acetate, methyl acetate, and ethyl acetate, respectively.48 It follows from these data that the rate constant of NO3 reaction with AA is higher than the rate constants of NO3 reactions with other aliphatic esters mentioned above, including vinyl acetate. Furthermore, the rate constant of the vinyl acetate reaction with NO3 being about two orders of magnitude higher than rate constants of NO3 reaction with the three other aliphatic esters is a strong indication that the process of NO3 attacking the C=C bond is the main reaction pathway. This conclusion can also be observed when comparing

rate

constants

of

NO3

reactions

with

AA

and

propyl

acetate

(CH3C(O)OCH2CH2CH3), and rate constants of NO3 reactions with vinyl acetate (CH3C(O)OCH=CH2) and ethyl acetate (CH3C(O)OCH2CH3).48 Reactions of NO3 with unsaturated esters proceed predominantly by addition to the carbon double bond. The reactivity of NO3 toward esters could be made by applying the group reactivity factors for alkanes, which show that the reactivity of acetate esters decreases with decreasing number of carbon atoms,49 so that, in a 24 h period, the nighttime reaction with NO3 can become an important factor in the budget. A common way of assessing the importance of 18


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NO3 versus OH, O3 and Cl reactions is to use averaged radical concentrations, daytime and nighttime periods, and calculate the AA lifetime for the AA + radical reactions.

As mentioned above, the rate constants of AA reaction with O3, Cl, OH and NO3 are summarized in Table 2. The rate constant of AA reaction with Cl is 8 times higher than the reaction with OH, 4 orders of magnitude higher than that for the reaction with NO3, and 8 orders of magnitude higher than that for the reaction with O3. It is necessary to mention that the rate constant for the reaction with Cl is close to the gas kinetic limited value from gas collision theory, which means that the reaction probability in this case is high regardless of the collision site and that the compound chemical structure plays a limited role in determining the reaction rate constant.50

3.2. Atmospheric Implications Once emitted into the atmosphere, a volatile organic compound can be removed by different atmospheric processes including photolysis, homogeneous and heterogeneous reactions and wet and dry deposition. The residence times of AA in the atmosphere with respect to reactions with O3, OH, NO3 and Cl can be estimated using the rate constants obtained in this work. The tropospheric lifetime of AA is defined as the reciprocal of the sum of loss rates of each removal process, and it is estimated by applying the following equation:

τX =

1 kX [X]

(14)

where X is the typical atmospheric concentration of O3, OH, NO3 or Cl, kx is the rate constant of the reaction of AA with the oxidant X.

Typical atmospheric concentrations of the four oxidants are as follows: a 24 h average O3 concentration of [O3] = 7×1011 molecules cm-3,51 a 12 h daylight average OH concentration of [OH] = 2×106 molecules cm-3,52 a 12 h night-time average NO3 concentration of [NO3] = 5×108 molecules cm-3,53 and an average of Cl atom concentration in the marine boundary layer, coastal areas, and some urban regions of [Cl] 19



= 1×104 atoms cm-3.54-56 As Table 3 shows, the reaction with OH is the main tropospheric loss process of AA during the daytime. Based on this process, the lifetime of AA is 5 h, while in areas with high O3 concentrations (as high as 5×1012 molecules cm-3), the reaction with O3 could equally become important. In coastal areas and in the marine boundary layer, with peak concentrations of Cl atoms as high as 1×105 atoms cm-3, the Cl degradation of AA could compete with the OH degradation process.57 The lifetime of AA based on the reaction with NO3 is two days. This reaction is an important degradation process for AA at nighttime. The calculated short lifetimes of AA indicate that it may be degraded close to the emission source areas, causing a local impact potentially leading to its photo-oxidation in the atmosphere, which is responsible for the formation of photochemical smog.

3.3. Mechanism Study To facilitate the discussion of the results, reaction schemes are shown in Schemes 1, S1, S2, and S3 for AA reactions with OH, O3, Cl and NO3, respectively. The products of the reactions of AA with O3 and Cl were detected using FTIR, whereas, the products of the reactions of AA with Cl, OH and NO3 were detected using TD-GC-MS. The identified products are marked in the oxidation schemes. Besides, compared to the loss that occurred on photo-induced degradation, the dark reactions, photolysis of AA in the absence of oxidant precursors, and wall losses were negligible.58

2-Oxoethyl acetate (CH3CO(O)CH2CHO) and formaldehyde have been detected as main products in the four reactions investigated. Their IR spectra were readily available from previous study for comparison.18 Scheme S1 shows a proposed mechanism for the ozonolysis of AA, based on the general knowledge on alkene ozonolysis reactions.59 The reaction of AA with O3 will initially lead to the formation of the primary ozonide (POZ), which can further decompose through two pathways, each forming a carbonyl compound and a Criegee biradical. It has been shown that the Criegee biradical, [·CH2OO·]* can form in the ozonolysis of VOCs of the type R-CH=CH2.34 The Criegee biradical formed from the first pathway, [·CH2OO·]*, is believed to undergo a variety of reactions which 20


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can lead to the formation of formic acid, formic anhydride (HC(O)OC(O)H) and hydroperoxymethyl formate (HOOCH2OCHO) under dry conditions.60, 61 However, there was no evidence of the formation of these species from our spectra. The Criegee biradical formed from the second pathway, [CH3C(O)OCH2C(·)H2O2·]*, could rearrange to 2oxoethyl acetate, although no plausible mechanism can be provided.

Due to the electron deficiency of OH relative to the C=C bond of AA, the reaction with OH proceeds essentially through addition of OH to the C=C bond, accounting for over 95% of the reaction while the H-atom abstraction is expected to account for about 5%.36, 62 The addition of OH is mainly to the terminal carbon atom in the double bond,18, 36 forming 1,2-hydroxyalkyl (1) and 2,1-hydroxyalkyl (2) radicals.18 Analogous to alkyl radicals, these hydroxyalkyl radicals are expected to react rapidly with O2 to yield the 1,2hydroxyalkyl peroxy (3) and 2,1-hydroxyalkyl peroxy (4) radicals, respectively.63 Radicals (3) and (4) can then react with themselves to form 1,2-hydroxyalkoxy (5) and 2,1-hydroxyalkoxy (6) radicals, respectively. These hydroxyalkoxy radicals can in principle react via three routes: reaction with O2, decomposition, and isomerization.63 Obviously, isomerization can only occur in the alkyl chain side with more than two carbon atoms and, hence, does not appear to be of importance for products (5) and (6).64 The two hydroxyalkoxy radicals (5 and 6) may react further through four pathways: i) decomposition to acetoxyacetic acid (CH3C(O)OCH2COOH) and formaldehyde (route A1); ii) decomposition to form CH3C(O)OCH2· and glycolaldehyde (HC(O)CH2OH), after which CH3C(O)OCH2· would undergo further reaction to form acetoxy formaldehyde (CH3C(O)OCHO), acetic acid and CO (route B2); iii) decomposition to formaldehyde and 2-oxoethyl acetate (CH3C(O)OCH2CHO) (route B1); iv) reaction with O2 to form CH3C(O)OCH2CH2OHCHO and CH3C(O)OCH2COCH2OH (routes A2 and B3). CH3C(O)OCH2COOH and CH3C(O)OCHO have not been detected in previous studies. It was also shown that both reaction with O2 and decomposition occur, with the reaction with O2 occurring with 22±5% probability at 760 torr total pressure of air and 298 K.65, Peroxy radicals were shown to undergo self-peroxy reactions in the absence of NOx, which resulted in the formation of 1,2- and 2,1-hydroxyalkoxy to a large extent. 21



However, the molecular channels possibly contribute to the formation of multifunctional products including hydroxy, carbonyl and ester functionalities.66 On the other hand, in the presence of NOx, the peroxy radicals react with NO to mainly form 1,2-hydroxyalkoxy radicals.19, 34

2-Oxoethyl acetate, acetoxyacetic acid, acetoxy formaldehyde and acetic acid were detected by TD-GC-MS (see Scheme 1 and Figure S2). CH3C(O)OCH2CH(·)OH (7) reacts with O2 to form acetoxyacetic acid. The acetoxy formaldehyde can be formed by CH3COOCH2· (8) reaction with O2, and CH3COOCH2· (8) forms the acetic acid by rearrangement. The NIST library does not include 2-oxoethyl acetate. However, on the basis that weakest bonds in a molecule break first, especially for long lived excited intermediates where there is time for the energy to be randomized, combined with the exploration of the mass spectrum fragments, the compounds can be conjectured.67 Aldehyde, ketone and ester which contain the carbonyl group could easily form by breaking the C-O and the =CH-CH2- bonds as can be observed by analyzing the mass spectrum. Hence, the main mass peaks at m/z 43 and 73 were tentatively assigned to CH3CO+ and CH3COOCH2+, the characteristic fragment ions, which possibly result from the 2-oxoethyl acetate fragmentation.

The mechanism of OH-induced oxidation of AA has been studied previously using FTIR spectrometer,19 and 2-Oxoethyl acetate and formaldehyde were primary oxidation products whereas acetic acid and acetoxy formaldehyde were secondary products. The level of NOx affects the yields of 2-oxoethyl acetate and formaldehyde, which are smaller in experiments performed at low NOx. The different mechanisms observed with and without NOx can be explained by the differences in exothermicities of the reaction producing the alkoxy radicals.18

For the reaction of Cl with AA, the products CH3C(O)OCH2CHClCH2Cl (1), HCl (4), CH3C(O)OCH2CHO (5), HC(O)CH2Cl (6) and CH3C(O)OCH2CH2OC(O)CH3 (7) (see Scheme S2 and Figure S2) were detected by TD-GC-MS and the products CO and HCl 22


Page 22 of 39

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were

detected

by

FTIR.

CH3C(O)OCH2CHClCH2Cl,

HCl,

and

CH3C(O)OCH2CH2OC(O)CH3 have not been detected in earlier studies.68 The tropospheric chemical degradation of AA by Cl was also studied using FTIR in previous studies.68 The results showed that the molar yields of 2-oxoethyl acetate (CH3C(O)OCH2CHO), acetoxy formaldehyde (CH3C(O)OCHO), acetic acid and formaldehyde were

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl and NO3

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