Environ. Sci. Technol. 1997, 31, 1496-1504
OH Radical-Initiated Oxidation of 2-Butoxyethanol under Laboratory Conditions Related to the Troposphere: Product Studies and Proposed Mechanism KONRAD STEMMLER, WOLFGANG MENGON, DAVID J. KINNISON, AND J. ALISTAIR KERR* EAWAG, Swiss Federal Institute for Environmental Science and Technology, ETH Zu ¨ rich, CH-8600 Du ¨ bendorf, Switzerland
This type of study provides information on the reaction mechanism of the conversion of a substrate molecule, in this case a glycol ether, into its oxidation products under polluted tropospheric conditions. Such detailed pathways for the breakdown of the substrate molecule lead to the generation of photooxidants, mainly ozone, and are essential input data for computer modeling studies used to derive ozone-creating potentials of volatile organic compounds released into the atmosphere. The products formed by the hydroxyl radical-initiated oxidation of 2-butoxyethanol (C4H9OCH2CH2OH) have been investigated by irradiating synthetic air mixtures containing the substrate, methyl nitrite, and nitric oxide at ppm levels in a Teflon bag reactor at room temperature. The decay of reactant and the formation of products were monitored by gas chromatography and by mass spectrometry. The molar yields of the major products (mol of product formed/mol of 2-butoxyethanol consumed) were as follows: butyl formate (HC(O)OCH2CH2CH2CH3), 0.35 ( 0.11; ethylene glycol monoformate (HC(O)OCH2CH2OH), 0.39 ( 0.18; butoxyacetaldehyde (CH3CH2CH2CH2OCH2C(O)H), 0.12 ( 0.09; 3-hydroxybutyl formate (HC(O)OCH2CH2CHOHCH3), ∼0.20; and propionaldehyde, ∼0.2-0.3. The yields of minor products were as follows: 2-propyl-1,3dioxolane (CH3CH2CH2CHOCH2CH2O), 0.025 ( 0.005; ethylene glycol monobutyrate (CH3CH2CH2C(O)OCH2CH2OH), ∼0.02-0.03; 2-hydroxybutyl formate (HC(O)OCH2CHOHCH2CH3), ∼0.05; acetaldehyde, 99%), butyl glycolate (Fluka, ∼95%), hydroxyacetaldehyde dimer (Fluka, 98%), n-hexanol (Fluka, >99%), propionaldehyde (Merck, >98%), butyraldehyde (Fluka, >99%), acetaldehyde (Fluka, 99.5%), butylmethyl ether (Fluka, >99.5%) and butyl formate (Fluka, ∼95%). Apparatus. The product studies were carried out in an experimental system that has been described previously (911) and is summarized here. Hydroxyl radicals were produced from the photolysis of methyl nitrite in air containing NO:
CH3ONO + hν f CH3O + NO
(1)
CH3O + O2 f CH2O + HO2
(2)
HO2 + NO f OH + NO2
(3)
The experiments were performed at room temperature (297 ( 3 K) and at atmospheric pressure (725 ( 10 Torr) in a 200 dm3 Teflon bag surrounded by 16 black lamps (Philips L20/ 05), which provide UV radiation in the region of 350-450 nm. Reactant mixtures were prepared by sweeping measured amounts of 2-butoxyethanol and methyl nitrite vapors from a calibrated volume into the Teflon bag with a stream of synthetic air. The bag was filled with up to 200 dm3 with synthetic air, and nitric oxide was added to minimize the formation of O3. Once the gas mixture was prepared, the bag was agitated and left to stand for about 0.5-1 h to mix the reactants. The bag mixture was irradiated for up to 1.5 h, during which time samples were periodically removed and analyzed. Product Analyses. The decay of the starting material 2-butoxyethanol; the major products butyl formate, ethylene glycol monoformate, and 3-hydroxybutyl formate; and the
minor products ethylene glycol monobutyrate and 2-hydroxybutyl formate were measured by gas chromatography (Carlo Erba HRGC 5300) with a flame ionization detector (FID). Vapor samples were injected into the gas chromatograph via a 3 cm3 stainless steel loop and gas sampling valve onto a 30 m DB-Wax fused silica column (J&W Scientific) with 0.32 mm internal diameter and 0.25 µm film thickness operated with temperature programming from 313 to 453 K. The product butoxyacetaldehyde was withdrawn from the bag using a 3 cm3 gas sampling valve and was injected on a DB-1701 capillary column (J&W Scientific, 30 m, 0.25 mm i.d., 1 µm film) with temperature programming from 313 to 433 K. Detection was performed with a mass spectrometer detector (Fisons MD800, electron impact ionization) operated in the single ion mode, recording ions with m/z ) 57.2 ( 0.2, 73.2 ( 0.2, and 87.2 ( 0.2. Calibrations were carried on the ion mass trace m/z ) 87.2. The minor products propyl nitrate, 2-propyl-1,3-dioxolane, and butyraldehyde were measured by preconcentration on Tenax TA adsorption tubes. An air sample of 212 cm3 was withdrawn from the bag at a flow rate of 53 cm3 min-1 through adsorption tubes (100 mg, Tenax TA, Chrompack). The contents of the tubes were thermally desorbed (Carlo Erba TDAS 5000) at 473 K for 4 min in a stream of helium and reconcentrated by means of a cold trap at 123 K. The trap was rapidly heated to 523 K to inject the sample onto the DB-1701 capillary column. The column was kept at 313 K for 1 min and then heated to 393 K at a heating rate of 5 K min-1 and finally to 473 K at 10 K min-1. The column was connected to a mass spectrometer (Fisons MD800, electron impact ionization) operated in the total ion mode. Several samples were drawn through two Tenax TA adsorption tubes in series, which were both analyzed in the same way to check for breakthrough of the compounds in the adsorption tube. The carbonyl products propionaldehyde and acetaldehyde were quantitatively determined by derivatization with 2,4dinitrophenylhydrazine (2,4-DNPH) followed by HPLC separation with a 250 × 4 mm i.d. Zorbax ODS 5 µm column (Sa¨ulentechnik Knauer) using a three-component gradient solvent program and UV detection at 360 nm (12). Samples (2 dm3) of the gas mixture were withdrawn from the bag at a flow rate of 0.5 dm3 min-1 through a impinger containing 4 cm3 of acidified 2,4-DNPH solution (12). To identify the observed carbonyl compounds, 2,4-DNPH derivatives of several carbonyl products were prepared (12), and liquid samples were injected into the HPLC. The sample efficiency for propionaldehyde was derived by calibrating the detector with liquid standards and comparing with the calibrations obtained from known gas mixtures. In general, the above products were calibrated by preparing at least four bag mixtures containing standards in air at mixing ratios comparable to those in the experiments and by analyzing with the same procedures as in the experiments to obtain response factors from a linear least squares fit of the data. Such calibrations were not possible for ethylene glycol monobutyrate, 2-hydroxybutyl formate, and 3-hydroxybutyl formate. The response factor of ethylene glycol monobutyrate was measured relative to that of 2-butoxyethanol by liquid injection of known mixtures of these compounds. The response factors of 2-hydroxybutyl formate and 3-hydroxybutyl formate relative to 2-butoxyethanol were assumed to be equal to those of butane-1,2-diol and butane-1,3-diol relative to 2-butoxyethanol. This approximation is based on the equivalent carbon number, as calculated by Scanlon and Willis (13), which predicts no contribution of a formate group to the total FID response of such a compound. Clearly the derived results for these three compounds not calibrated by gaseous standards involve considerably larger experimental errors than for the other products. For qualitative GC-MS analysis, several experiments were carried out with high initial mixing ratios of 2-butoxyethanol,
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methyl nitrite, and NO. Gas samples of up to 10 dm3 of the mixture were withdrawn through an impinger cooled to 178 K in an acetone-liquid nitrogen sludge. The trapped material was dissolved in 1 cm3 of diethyl ether and injected onto the DB-Wax fused silica column connected to a high-resolution mass spectrometer detector (VG AutoSpec-Q), involving either positive chemical or electron impact ionization systems. Rate Coefficients of OH Reactions with Products. Rate coefficients for the gas phase reaction of the OH radical with butoxyacetaldehyde and 2-propyl-1,3-dioxolane were determined by the relative rate method as described previously (2) in which the decays of the test compounds (TST) and the reference compound n-hexanol (REF) were measured in the presence of OH radicals:
OH + TST f products
(4)
OH + REF f products
(5)
Assuming that there are no loss processes for the test and reference compounds other than by reaction with OH radicals, the rate expressions for these processes can be integrated and combined to give
ln ([TST]0/[TST]t) ) (kTST/kREF) × ln ([REF]0/[REF]t) (6)
TABLE 1. Room Temperature Rate Coefficients for Reaction of OH Radicals with Oxygenated Compounds Relevant to This Study
compound
rate coefficient (10-12 cm3 molecule-1 s-1)
2-butoxyethanola butyl formate 2-propyl-1,3-dioxolanea
19.4 3.1 10.8
butoxyacetaldehydea
20.6
ethylene glycol monoformate ethylene glycol monobutyrate n-propyl nitrate 3-hydroxybutyl formate 2-hydroxybutyl formate propionaldehyde acetaldehyde butyraldehyde
methodb (ref)
4.9
RR, n-hexanol (2) FP-RF (15) RR, n-hexanol (this work) RR, n-hexanol (this work) estimation, see text
8.8
estimation, see text
0.73 10.5 12.2 19.6 15.8 23.5
data evaluation (26) estimation, see text estimation, see text data evaluation (31) data evaluation (31) data evaluation (31)
a The ratio k(OH + oxygenate)/(k(OH + hexanol) was used in correcting for secondary OH reactions. b FP-RF, flash photolysisresonance fluorescence; RR, relative rate method, reference compound.
where the subscripts 0 and t indicate concentration at the beginning of the experiment and at time t, respectively. A plot of ln ([TST]0/[TST]t) versus ln ([REF]0/[REF]t) thus yields the rate coefficient ratio kTST/kREF. Synthetic air mixtures (200 dm3) containing methyl nitrite, NO, the test compound, and the reference compound were prepared in the Teflon bag reactor. The bag mixture was irradiated for up to 2 h, during which time samples of the mixture were periodically removed via the 3 cm3 sampling valve and the test and the reference compound were analyzed by gas chromatography (Carlo Erba HRGC 5300) with a FID detector equipped with the DB-Wax fused silica column operated, as described above, with temperature programming from 313 to 393 K.
Results The observed product mixing ratios were corrected for secondary removal by the OH radical according to the method of Atkinson et al. (14) using the following expressions:
F[prod] ) f∆[BE]
(7)
where
(
)(
)
(8)
FIGURE 1. Sample plots of ln ([TST]0/[TST]t) for the reaction of OH with following test compounds (TST): (9) butoxyacetaldehyde, (b) 2-propyl-1,3-dioxolane, versus ln ([hexanol]0/[hexanol]t).
where kBE and kprod are the room temperature rate coefficients of the OH radical reactions with 2-butoxyethanol and with the given product and ∆[BE] ) [BE]0 - [BE]t, where [BE] refers to the concentration of 2-butoxyethanol and the subscripts denote the time at which it was measured. The fractions of 2-butoxyethanol that form the given product, f, were derived by linear least squares fits of the slopes of the data plotted according to eq 7. The rate coefficients of the OH radical reactions used in these calculations are listed in Table 1. The rate coefficients of the OH radical reactions with butoxyacetaldehyde and 2-propyl-1,3-dioxolane were determined each in one separate relative rate measurement involving n-hexanol as the reference compound. The initial mixing ratios in the bag reactor were as follows: 2-propyl1,3-dioxolane, 2.5 ppm; butoxyacetaldehyde, 1.6 ppm; hexanol, 1.6 and 1.9 ppm; NO, 12 and 20 ppm; and methyl nitrite, 14 and 18 ppm. The ln-ln plots of the present data according to eq 6 are shown in Figure 1. The rate coefficient ratios derived from the slope of a linear least squares fit of the data
are as follows: k(OH + 2-propyl-1,3-dioxolane)/k(OH + hexanol) ) 0.86 ( 0.08, k(OH + butoxyacetaldehyde)/k(OH + hexanol) ) 1.65 ( 0.15. The error limits correspond to 95% confidence intervals. These rate coefficient ratios were converted to absolute rate coefficients on the basis of the evaluated data (21) for k(OH + hexanol) ) 12.5 × 10-12 cm3 molecule-1 s-1 at 298 K and are shown in Table 1. The estimated uncertainty in both absolute rate coefficients is about (40% mainly due to uncertainties in the rate coefficient of the reference compound with the OH radical. However, the absolute rate coefficients were not needed in this particular case, since the rate coefficient of 2-butoxyethanol was measured by the competitive method against n-hexanol in the same system using the same analytical conditions (2). The rate coefficients of OH radical reactions with ethylene glycol monoformate and ethylene glycol monobutyrate were estimated by modifying the literature data for ethyl formate (15) and ethyl butyrate (15), respectively, by replacing the CH3 group by a CH2OH group according to the structure
F)
1498
kBE - kprod 1 - [BE]t/[BE]0 kBE ([BE]t/[BE]0)kprod/kBE - [BE]t/[BE]0
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FIGURE 2. Typical GC-FID chromatogram from the present study (experiment 4) of an irradiated bag mixture containing initially 1.8 ppm of 2-butoxyethanol. The labeled peaks correspond to the following compounds: 1, butyl formate; 2, 2-propyl-1,3-dioxolane; 3, butoxyacetaldehyde; 4, 2-butoxyethanol; 5, 3-oxobutyl formate; 6, unknown; 7, ethylene glycol monoformate; 8, 2-hydroxybutyl formate; 9, unknown nitrate; 10, 3-hydroxybutyl formate; 11, unknown nitrate; 12, ethylene glycol monobutyrate. Peaks 9 and 11 show high abundances of m/z ) 46 in MS analyses and are thought therefore to be organic nitrates. activity relationship (SAR) of Kwok and Atkinson (16). For 3-hydroxybutyl formate and 2-hydroxybutyl formate, the literature result for butyl formate (15) was similarly modified. A typical gas chromatogram of an irradiated mixture is shown in Figure 2 where all the observed products are labeled. These products were identified by retention time and in an experiment with a higher initial mixing ratio of 2-butoxyethanol (∼20 ppm) by connecting the column to a MS detector. Not all the identified products could be quantified in any given experiments. Thus, three experiments (nos. 1-3) were carried out to measure principally the formation of ethylene glycol monoformate and a further three (nos. 4-6) to determine butyl formate. In these experiments the initial mixing ratios of 2-butoxyethanol, methyl nitrite, and NO were 1.7-2.6, 3-6, and 2.5-6 ppm, respectively. In a third set of experiments (nos. 7-9), the product butoxyacetaldehyde was measured, starting with initial mixing ratios of 2-butoxyethanol, methyl nitrite, and NO of 1.6-1.7, 2.5-6, and 3-6 ppm. Because the sample of 2-butoxyethanol used contained about 1% butoxyacetaldehyde, a significant amount of butoxyacetaldehyde was present in the mixtures prior to the irradiation, but a distinct buildup of butoxyacetaldehyde was observed during the course of the irradiation. At high conversion ratios of 2-butoxyethanol, the mixing ratio of butoxyacetaldehyde decreases again due to its fast reaction with the OH radical. Figure 3a shows typical yields of ethylene glycol monoformate, butyl formate, and butoxyacetaldehyde, corrected for secondary OH reactions, against the mixing ratios of 2-butoxyethanol removed by OH reaction during the course of the given experiment. Figure 3b shows similar typical yields of 2- and 3-hydroxybutyl formate and ethylene glycol monobutyrate measured, as described above, for experiment no. 5. The minor products n-propyl nitrate, 2-propyl-1,3-dioxolane, and butyraldehyde were measured in four separate runs (nos.10-14) by irradiation of bag mixtures with initial mixing ratios of 2-butoxyethanol, methyl nitrite, and NO of 0.6-1.9, 10-12.5, and 7.9-13.2 ppm. Additionally 2-butoxyethanol, butyl formate, ethylene glycol monoformate, 3-hydroxybutyl formate, butoxyacetaldehyde, propionalde-
hyde, ethylene glycol monobutyrate, and 2-hydroxybutyl formate were identified in these chromatograms as primary products. Significant artifacts were observed in the analyses of several of these compounds by the adsorption tube method although it has been used successfully in several previous studies (10, 11). Thus butoxyacetaldehyde appears to be unstable under the adsorption-desorption conditions in the presence of methyl nitrite and gave rise to a significant amount of butyl methyl ether by an unknown mechanism. In addition, ethylene glycol monoformate and possibly also other high boiling products could not be quantitatively desorbed and lead to subsequent background signals in the next analysis. A further problem arose in the analysis of 2-butoxyethanol by the adsorption tube method in the presence of methyl nitrite leading to reduced and scattered signals in relation to the direct vapor injection of 2-butoxyethanol. It was not possible, however, to quantify the above minor products in any other way. Although the presence of these compounds was also observed in the GC-FID or the HPLC analyses described above, the signals were too small without preconcentration on Tenax TA. Clearly, systematic errors were to be expected in these results, but these could not be estimated. For a typical experiment, no. 10, plots of the yields of n-propyl nitrate, 2-propyl-1,3-dioxolane, and butyraldehyde, corrected for secondary OH removal, versus the amount 2-butoxyethanol reacted are shown in Figure 3c. Five of the experiments above included measurements of the carbonyl products propionaldehyde and acetaldehyde via derivatization with 2,4-DNPH reagent. The initial mixing ratios of these experiments for 2-butoxyethanol, methyl nitrite, and NO were 0.6-2.6, 2.9-8.6, and 2.5-10 ppm. The sample efficiency for propionaldehyde was found to be 95 ( 3%. A typical plot for the corrected yields of acetaldehyde and propionaldehyde versus the mixing ratio of 2-butoxyethanol removed is shown in Figure 3d corresponding to experiment no. 5. The results of all the experiments are summarized in Table 2. As seen in Figure 3, several of the observed products do not build up linearly with the amount of 2-butoxyethanol consumed. This fact indicates that additional sources for
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uncertainties in the correction factors F. For these compounds, estimates of their primary formation yields were derived at low conversion ratios of 2-butoxyethanol. Secondary formation of ethylene glycol monobutyrate is not expected from OH-initiated oxidation of any of the observed products but may arise from the reaction of ozone with 2-propyl-1,3-dioxolane. A preliminary experiment in which ozone (∼6 ppm) was reacted with 2-propyl-1,3dioxolane (∼8 ppm) in the bag reactor gave rise to ethylene glycol monobutyrate as the sole product. This finding is in accordance with the experiments of Deslongchamps (17), which showed that the reaction of ozone with 2-alkyl-1,3dioxolanes proceeds readily in organic solvents and leads to the formation of the respective ethylene glycol monoesters. Kuramshin et al. (18) reported a similar reaction, proceeding with oxygen and hydroperoxides as oxidants. The importance of this reaction in the present work is not clear due to the unknown mixing ratios of ozone during the course of a normal experiment and the unknown rate coefficient of this reaction. Another interference comes from the reaction of butoxyacetaldehyde with the OH radical, which gives rise to butyl formate. Although this secondary production of butyl formate contributes significantly to the total observed yield of butyl formate, this causes no pronounced curvature in the plot (Figure 3a) of butyl formate produced versus 2-butoxyethanol removed due to the high primary yields of this compound. The secondary production of butyl formate is therefore included in the total observed fraction of butyl formate (35% of 2-butoxyethanol removed). Based on the amount of butoxyacetaldehyde reacted in a typical experiment and an estimated yield of ∼50% butyl formate from the reactions following the OH attack on butoxyacetaldehyde, a contribution of ∼6% in terms of percent of 2-butoxyethanol reacted was estimated for the secondary formation of butyl formate. The yield of butyl formate from the reaction of butoxyacetaldehyde with the OH radical under the experimental conditions was approximated to be equal to the yield of ethyl formate from 2-ethoxyethanol reported previously (3). The major product 3-hydroxybutyl formate is expected to undergo secondary reactions to form mainly 3-oxobutyl formate, which is relatively stable and should not interfere with other products. Relatively large amounts of a compound, clearly produced in secondary reactions, were observed (see Figure 2), the compound and is believed to be 3-oxobutyl formate by reason of its high-resolution mass spectrum. The following characteristic ions were observed in this mass spectrum: m/z ) 43 (100, C2H3O1), 47 (8, C1H3O2), 55 (38, C3H3O1), 70 (23, C4H6O1), 73 (6, C3H5O2), 88 (5, C4H8O2), 101 (3, C4H5O3), and 116 (1.5, C5H8O3); the relative intensities and the most probable composition of this ions (high-resolution MS analysis) are shown in parenthesis. In addition, 2-oxobutyl formate was observed in low concentrations and identified by injections of liquid standards and ascribed to secondary reactions of 2-hydroxybutyl formate.
Discussion
FIGURE 3. Plots of the mixing ratios of products, corrected for secondary reactions with OH radicals, against the mixing ratios of 2-butoxyethanol reacted with the OH radical. The maximum correction factor F used for each compound is given in parentheses. these compounds (i.e., formation by reactions of primary products with the OH radical) play a significant role. For acetaldehyde, secondary production clearly dominates the buildup. For propionaldehyde, ethylene glycol monobutyrate, and butyraldehyde, the secondary formation is less important and the observed curvature may be partly due to
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Reaction Schemes 1-3 show a proposed mechanism for the breakdown of 2-butoxyethanol following the initial H-atom abstraction by the OH radical from each of the five CH2 groups, including all the possible major channels. The observed products, upon which the proposed reactions are based, are highlighted in boxes. No products were detected that suggest the occurrence of H-atom abstraction from the -OH or the -CH3 group in 2-butoxyethanol. In all schemes, the alkyl peroxy radicals produced by the reaction of the initial alkyl radicals 1-5 with O2 have been omitted for clarity. The formation of organic nitrates has also been neglected, although small amounts of several unidentified nitrates were observed (see Figure 2). Scheme 1 shows the reactions occurring at the CH2 groups adjacent to O-atoms in the substrate molecule; these CH2
TABLE 2. Correcteda Yields of Major Products from Photooxidation of 2-Butoxyethanol and Comparison with Corresponding Results of photooxidation of 2-Ethoxyethanol products from 2-ethoxyethanol + OH (3)
products from 2-butoxyethanol + OH yieldb,c butyl formate 2-propyl-1,3-dioxolane butoxyacetaldehyde ethylene glycol monoformate ethylene glycol monobutyrate propyl nitrate 3-hydroxybutyl formate 2-hydroxybutyl formate propionaldehyde acetaldehyde butyraldehyde
35 ( 11 2.5 ( 0.5 12 ( 9 39 ( 18 ∼2-3 3.8 ( 1.8 ∼20 ∼5 ∼20-30
