Article pubs.acs.org/est
Atmospheric Chemistry of Benzyl Alcohol: Kinetics and Mechanism of Reaction with OH Radicals François Bernard,† Isabelle Magneron,† Grégory Eyglunent,† Véronique Dael̈ e,† Timothy J. Wallington,‡ Michael D. Hurley,‡ and Abdelwahid Mellouki†,* †
Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE), CNRS (UPR 3021), Observatoire des Sciences de l’Univers en région Centre (OSUC), 1C Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France ‡ Systems Analytics and Environmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053, United States S Supporting Information *
ABSTRACT: The atmospheric oxidation of benzyl alcohol has been investigated using smog chambers at ICARE, FORD, and EUPHORE. The rate coefficient for reaction with OH radicals was measured and an upper limit for the reaction with ozone was established; kOH = (2.8 ± 0.4) × 10−11 at 297 ± 3 K (averaged value including results from Harrison and Wells) and kO3 < 2 × 10−19 cm3 molecule−1 s−1 at 299 K. The products of the OH radical initiated oxidation of benzyl alcohol in the presence of NOX were studied. Benzaldehyde, originating from H-abstraction from the −CH2OH group, was identified using in situ FTIR spectroscopy, HPLC-UV/FID, and GC-PID and quantified in a yield of (24 ± 5) %. Ring retaining products originating from OH-addition to the aromatic ring such as ohydroxybenzylalcohol and o-dihydroxybenzene as well as ring-cleavage products such as glyoxal were also identified and quantified with molar yields of (22 ± 2)%, (10 ± 3)%, and (2.7 ± 0.7)%, respectively. Formaldehyde was observed with a molar yield of (27 ± 10)%. The results are discussed with respect to previous studies and the atmospheric oxidation mechanism of benzyl alcohol.
1. INTRODUCTION Aromatic compounds are ubiquitous in the atmosphere; they are emitted by human activities and natural processes. The atmospheric chemistry of toluene, benzene, and xylenes has been intensively studied over the past few years to assess their contribution to urban air pollution. In the presence of NOX (NO2 + NO), the degradation of aromatics leads to the formation of ozone and a number of photooxidants as well as secondary organic aerosol affecting air quality.1−4 Benzyl alcohol (C6H5CH2OH, BzOH) is used in the pharmaceutical, cosmetic, perfume, food flavouring industries, in solvents, and in epoxy resin coatings. It has also biogenic sources and is emitted by fruits such as peachs,5,6 raspberries7 and blackberries8 and in flowers such as petunia.9−11 It has been also identified in indoor air.12 There have been two previous studies of the atmospheric chemistry of benzyl alcohol, both were conducted at the National Institute for Occupational Safety and Health.13,14 In the first investigation, the rate coefficient for reaction with OH radicals, an upper limit for the rate coefficient of reaction with ozone, and identification of benzaldehyde, glyoxal, and 4-oxopentanal as products of OH radical initiated oxidation were reported.14 In the second investigation, the rate coefficient and oxidation products of the reaction of NO3 radicals with benzyl alcohol were reported.13 In the present work, we have conducted an investigation of the © 2013 American Chemical Society
reactions of OH and O3 with benzyl alcohol using the smog chambers at ICARE (Orléans, France), Ford Motor Company (Dearborn, Michigan), and EUPHORE (European photoreactor, Valencia, Spain) which complements the previous studies.
2. EXPERIMENTAL SECTION Three smog chambers were used in the present work. At ICARE, the rate coefficients for the reactions of benzyl alcohol with OH radicals and ozone were measured. At FORD, the rate coefficient for OH reaction with benzyl alcohol and the yield of benzaldehyde were determined. At EUPHORE, the mechanism of the OH-initiated oxidation of benzyl alcohol in the presence of NOX was investigated. ICARE Chamber. Kinetic measurements were performed in the 7300 L ICARE Teflon chamber.15,16 The chamber was surrounded by 24 lamps with a maximum output centered on 365 nm (UV-A T-40 L, Viber Lourmat). Experiments were performed in P ∼ 1013 mbar of air at T = 298 ± 2 K with a relative humidity of ∼5%. Reactants which are liquids under Received: Revised: Accepted: Published: 3182
November 13, 2012 February 27, 2013 February 28, 2013 February 28, 2013 dx.doi.org/10.1021/es304600z | Environ. Sci. Technol. 2013, 47, 3182−3189
Environmental Science & Technology
Article
[BzOH] + k′0, where k (in cm3 molecule−1 s−1) is the rate coefficient of the ozone reaction with benzyl alcohol and k′0 (in s−1) is the pseudo first order decay rate of ozone in the absence of benzyl alcohol. FORD Chamber. The product studies and relative rate measurements were performed in a 140 L Pyrex reactor interfaced to a Mattson Sirus 100 FTIR spectrometer at T = 296 K and P = 933 mbar.17 OH radicals were produced by the photolysis of CH3ONO in the presence of NO in purified air. Ethene was used as a reference compound. The initial reactants concentrations were (in 1014 molecules cm−3): [BzOH] = 1.7− 2.3, [Ethene] = 3.2−5.2, [CH3ONO] = 32−36, and [NO] = 3.2−4.8. Kinetic data for the reaction of OH were derived by monitoring the loss of benzyl alcohol (BzOH) relative to the loss of the references:
ambient conditions were introduced into the chamber by placing a known volume in a bubbler which was then flushed by purified air (gently heated when necessary). Gaseous reactants were injected into the chamber using a calibrated gas cylinder equipped with pressure sensors. Two fans were installed into the chamber to ensure rapid mixing of reactants. Organic reactants were monitored by in situ Fourier transform infrared spectrometer (Nicolet 5700 Magna) coupled to white-type mirror system resulting of 129 m optical path length. Infrared spectra were recorded every 5 min by coadding 130 interferograms with a resolution of 1 cm−1. Temperature and relative humidity data were continuously recorded by a combined sensor. Ozone was produced using a silent discharge in O2 and its concentration was measured by UV absorption (Horiba, APOA-360). NO−NO2−NOX were measured using a chemiluminescence analyzer (Horiba, APNA-360). To prevent contamination from outside air, a flow of purified air was added continuously during all experiments to keep the chamber slightly above atmospheric pressure and to compensate for sampling flows of various instruments connected to the chamber. For OH kinetic studies, hydroxyl radicals were produced by the photolysis of CH3ONO in the presence of NO in purified air: CH3ONO + hυ → CH3O + NO
(1)
CH3O + O2 → HO2 + HCHO
(2)
HO2 + NO → OH + NO2
(3)
OH + reference → products
(5)
⎛ ⎛ [reference]t ⎞⎞ ⎛ [BzOH]t ⎞ k 0 0 ⎟⎟ = BzOH ⎜⎜ln⎜⎜ ⎟⎟⎟⎟ ln⎜⎜ [reference] [BzOH] k ⎝ t ⎠ Ref ⎝ ⎝ t ⎠⎠
(II)
where [BzOH]t0, [BzOH]t, [reference]t0, and [reference]t are the concentrations of benzyl alcohol and reference at times t0 and t, kBzOH, and kref are the rate coefficients for reactions of OH radicals with benzyl alcohol and reference, respectively. EUPHORE. Mechanistic studies of the reaction of benzyl alcohol with OH radicals were performed at EUPHORE in Valencia (Spain) using the large outdoor simulation chamber.18−20 Concentration−time profiles of reactants and oxidation products were monitored using an FTIR spectrometer (Nicolet, Magna 550) interfaced to a multipass reflection optical system with an optical path of 553.5 m. Infrared spectra were monitored every 10 min by coadding 550 interferograms with a resolution of 1 cm−1. To measure the concentrations of polar or phenolic compounds, samples were collected using C18 cartridges treated with NaOH and the stripping technique with a buffer solution of Na2HPO4 at pH 7.0. These samples were then analyzed by HPLC in combination with UV and fluorescence detections (HPLC-UV/FID). In addition, GCPID was also used to analyze the benzyl alcohol and benzaldehyde. Carbonyl compounds were collected on DNPH-coated cartridges and analyzed by HPLC-UV. The samples were passed through an ozone scrubber prior to collection on the sample cartridges to reduce artifact formation. Photolysis frequencies of NO2 and HCHO were continuously measured using a calibrated filter radiometer (Bentham DMT300). NO and NO2 were measured by two specific analyzers equipped with a molybdenum converter (Monitor Laboratories. Inc., ML 9841 A) and a photolytical converter (Eco Physics, CLD 770 AL with PLC 760). Measurement of ozone was made using a specific monitor (Monitor Laboratories ML 9810) based on UV absorption. Temperature
⎛ [BzOH]t ⎞ 0 ⎟⎟ − kL(BzOH) × t ln⎜⎜ ⎝ [BzOH]t ⎠ ⎞ kBzOH ⎛ ⎛ [reference]t0 ⎞ ⎜⎜ln⎜⎜ ⎟⎟ − kL(reference) × t ⎟⎟ kRef ⎝ ⎝ [reference]t ⎠ ⎠
(4)
Control experiments were performed to check for wall or photolytic loss of benzyl alcohol. There was no discernible (99% and >99.9%, respectively.
3. RESULTS AND DISCUSSION 3.1. Kinetic Studies. (i). Reaction of OH Radicals with Benzyl Alcohol. In the experiments conducted at ICARE the organic compounds were monitored over the following wavenumber centered IR bands: benzyl alcohol, 1084 cm−1; propene, 912 cm−1; and di-n-butylether, 1135 cm−1. Experimental durations ranged from 20 to 90 min. The loss of benzyl alcohol through the reaction with OH radicals accounted from 72 to 89% of the total loss. Four runs were performed using din-butyl ether as the reference and two with propene. Using the eq I, plot of the decay rates of the compound versus those of the two references (propene and di-n-butylether) is given in Figure 1. Linear least-squares fits to the data gives the rate coefficient ratios kBzOH/kref, the average values obtained are kBzOH/kref = (0.96 ± 0.12) and (0.93 ± 0.06) respectively with di-n-butyl ether and propene as references. Relative rate coefficient data were placed on an absolute basis using: k(propene + OH) = (2.9 ± 0.7) × 10−11,21 and k(di-nbutylether + OH) = (2.8 ± 0.4) × 10−11 cm3 molecule−1 s−1.22
Figure 2. Loss of benzyl alcohol versus ethene on exposure to OH radicals in 700 Torr of air at 296 K in the Ford chamber.
squares fit which gives a rate coefficient ratio k(OH + BzOH)/ k(OH + C2H4) = (3.65 ± 0.18). Using k(OH + C2H4) = (7.9 ± 0.8) × 10−12,21 gives k(OH + BzOH) = (2.9 ± 0.4) × 10−11 cm3 molecule−1 s−1, which is in excellent agreement with the result obtained in experiments at ICARE. The kinetics of the reaction of benzyl alcohol with OH radicals have been studied previously by Harrison and Wells.14 Using a relative rate method and n-decane and hexanal as reference compounds, these authors have reported a value of k = (2.8 ± 0.7) × 10−11 cm3 molecule−1 s−1 at (297 ± 1) K and at atmospheric pressure (in air). The results from Harrison and Wells14 and from this work (measured at ICARE and Ford) are in excellent agreement. Taking an average of the results from the three laboratories and using standard error propagation techniques gives k = (2.8 ± 0.4) × 10−11 cm3 molecule−1 s−1 at 297 ± 3 K. (ii). Reaction of Ozone with Benzyl Alcohol. Two runs were performed to study the reaction of ozone with benzyl alcohol using the following initial concentrations for benzyl alcohol and ozone (in molecules cm−3), respectively: (0.49 − 3.5) × 1014 and (0.49 − 1.1) × 1013. In both runs, the rate of the loss of ozone in the chamber was unaffected by the addition of benzyl alcohol under our experimental conditions, the loss of ozone in the presence of benzyl alcohol did not differ from that in the absence of compound. Taking the highest concentration of benzyl alcohol used [BzOH]0 = 3.5 × 1014 molecules cm−3 and a loss rate of ozone equivalent to 3 × 10−5 s−1 (which corresponds to the ozone loss rate in the absence of benzyl alcohol), an upper limit of 2 × 10−19 cm3 molecule−1 s−1 for the rate coefficient of the reaction of ozone with benzyl alcohol has been derived at 299 K. The only investigation of this reaction reported previously is that of Harrison and Wells,14 who conducted their work under pseudo first order conditions
Figure 1. Loss of benzyl alcohol (BzOH) versus di-n-butylether and propene on exposure to OH radicals in one atmosphere of air at 298 K in the ICARE chamber. 3184
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([O3]≫[BzOH]) in the presence of cyclohexane as OH scavenger analyzed by SPME-GC-MS. Harrison and Wells reported an upper limit of 6 × 10−19 cm3 molecule−1 s−1, which is consistent with our result. 3.2. OH-Initiated Oxidation Products. (i). Benzaldehyde Yield from OH-Initiated Oxidation Studied at Ford. Following the UV irradiation of benzyl alcohol/CH3ONO/NO/air mixtures the formation of benzaldehyde was observed. Figure 3 shows a plot of the formation of benzaldehyde versus loss of
Table 1. Initial Conditions and Observed Products Following OH-Initiated Oxidation of Benzyl Alcohol (BzOH) for Experiments Performed at EUPHORE (Chamber B) initial conditions
I
II
[BzOH]0a (molecules cm−3) [HONO] (molecules cm−3) [NOX] (molecules cm−3) JNO2 (s−1)
1.6 × 1013 4.3 × 1011 3.7 × 1012 8.4 × 10−3
2.3 × 1013 3.2 × 1011 5.3 × 1012 7.9 × 10−3
irradiation time (hh:mm) Δ[BzOH] (molecules cm−3) temperature (K) pressure (mbar) Products and Yields benzaldehydeb (%) formaldehydec (%) glyoxald (%) o-hydroxybenzyl alcohole (%) o-dihydroxy-benzenee (%)
02:40 9.2 × 1012 304 ± 2 1003 ± 1
03:00 9.9 × 1012 305 ± 2 1000 ± 1
25 ± 4 20 ± 2 3.0 ± 0.2 21.1 ± 0.9 8.3 ± 0.5
25 ± 4 34 ± 3 2.5 ± 0.7 24 ± 1 12 ± 1
Δ[BzOH] is the consumed concentration of benzyl alcohol (BzOH). From FTIR, GC-PID, and HPLC using both DNPH and C18 sampling measurements. cFrom FTIR measurement. dFrom HPLC (DNPH sampling) measurement. eFrom HPLC (Stripping sampling) measurement. a b
Concentration−time profiles of oxidation products were corrected for secondary reaction with OH radicals using IUPAC recommended kOH rate coefficients: formaldehyde, 8.5 × 10−12; formic acid, 4.5 × 10−13; glyoxal, 9.7 × 10−12; acetaldehyde, 1.5 × 10−11; 1,2-dihydroxybenzene, 1.0 × 10−10 and benzaldehyde, 1.26 × 10−11 (cm3 molecule−1 s−1). The rate coefficient for the reaction of OH with 4-nitrophenol was taken as kOH = 6.31 × 10−12 cm3 molecule−1 s−1,24 cited by Kılıç et al.25 The rate coefficient for the reaction of o-hydroxylbenzyl alcohol with OH was estimated using the US Environmental Protection Agency Estimation Programs Interface (EPI) Suite26 based on the structure−reactivity relationship (SAR) methods27 to be k = 4.42 × 10−11 cm3 molecule−1 s−1. Corrections to account for loss via reaction with OH radicals were in the range 0.1−24%. The largest correction was applied to the o-DHB data reflecting its high reactivity with OH radicals. Corrections for loss of products via photolysis were computed and applied. The methods used to estimate photolysis frequencies have been described previously28−30 with some parameters taken from the Master Chemical Mechanism, MCM v3.1.29,30 The solar zenith angle was estimated using the NOAA Solar Calculator.31 The photolysis rates of glyoxal and formaldehyde are expressed relative to that of NO2 as JX = a × (JNO2)2 + b × JNO2 + c. The values of the parameters a, b, and c for each individual experiment can be found in the Supporting Information. Previous investigations of the photolysis of aromatic compounds have shown that this process is of minor importance under the EUPHORE conditions32 and hence corrections were not computed for such compounds. Under our experimental conditions, the correction due to photolysis processes was estimated to be in the range 4−22% for glyoxal and 2−5% for formaldehyde. Ozone was formed during the experiments. The most reactive compounds among the identified products toward ozone are o-dihydroxybenzene and o-hydroxybenzyl alcohol. The rate coefficients kO3 (in cm3 molecule−1 s−1) used were: odihydroxybenzene, 9.2 × 10−18,33 re-evaluated by IUPAC. As
Figure 3. Formation of C6H5CHO versus loss of C6H5CH2OH following the UV irradiation of C6H5CH2OH/CH3ONO/NO/air mixtures in the Ford chamber.
benzyl alcohol. The benzaldehyde data were corrected for secondary loss via reaction with OH using k(OH + benzaldehyde) = 1.26 × 10−11 cm3 molecule−1 s−1,22 using the method described elsewhere.23 Corrections were modest (1−9%) and have been applied to the data in Figure 3. The line through the data is a linear least-squares fit which gives a benzaldehyde yield of (23 ± 2)%. This result is consistent with the previous determination of an approximate yield of 24% by Harrison and Wells.14 (ii). Products Studies at EUPHORE. Known concentration of benzyl alcohol was introduced into the chamber and its temporal behavior monitored by FTIR spectroscopy for one hour before adding HONO (used as OH radical precursor). Identified oxidation products and corresponding formation yields are listed in Table 1. IR spectra obtained during the experiments are shown in Figure 4. Panel A shows the spectrum of a benzyl alcohol/HONO/air mixture before exposure to sunlight, whereas panels B and C show spectra acquired after 30 min and 3 h of exposure to sunlight, respectively (features attributable to benzyl alcohol, SF6, HONO, NO2, HNO3, and water have been subtracted from panels B and C for clarity). Comparison of panels B and C with reference spectra of benzaldehyde (C6H5CHO), formaldehyde (HCHO), ozone (O3), formic acid (HCOOH), and peroxybenzoyl nitrate (PBzN, C6H5C(O)O2NO2) in panels D, E, F, G, and H indicates the formation of these products. Panel I shows the residual IR spectrum after subtraction of reactants and identified products. Carbon monoxide (CO) was also detected as a product using its characteristic IR features. Figure 5 shows the time−concentration profiles of reactants and products in experiment I. 3185
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Figure 4. FTIR spectra recorded before (A) and after exposure of a benzyl alcohol/HONO/air mixture to 30 min (B) and 3 h (C) of sunlight (IR features attributable to benzyl alcohol, SF6, NO2, HONO, HNO3, and H2O have been subtracted from panels B and C). Reference spectra are shown for: benzaldehyde (D), formaldehyde (E), ozone (F), formic acid (G), and peroxybenzoyl nitrate (H). Panel I shows the residual spectrum after subtraction of features attributable to benzyl alcohol, benzaldehyde, formaldehyde, ozone, formic acid, and peroxybenzoyl nitrate from panel (C).
Figure 5. Reactant and product profiles in the OH-initiated reaction of benzyl alcohol in the presence of NOX for the experiment I performed at EUPHORE (Chamber B).
estimated relative to that of NO2 using JNO3 = a × (JNO2)b where the parameters a and b can be found in the Supporting Information. As shown in the Figure 5, the concentration of NO3 radicals increases over the course of the experiment reaching a maximum of approximately 3 × 107 molecules cm−3. The rate coefficients used to correct the product yields for secondary reactions involving NO3 radicals were: 1,2dihydroxybenzene, 9.9 × 10−11,34,35 and o-hydroxybenzyl alcohol, 1.4 × 10−11 cm3 molecule−1 s−1 (assumed same as for o-cresol34). The corrections due to NO3 reactions were 1,2dihydroxybenzene, 8−10% and o-hydroxylbenzyl alcohol, 2− 3%. Benzaldehyde was measured using complementary techniques such as PID, HPLC (C18 sampling), and HPLC (DNPH sampling), and the results are shown in Figure 6. Within the experimental uncertainties, the results from the different analytical methods were indistinguishable. The line through the data in Figure 6 is a linear least-squares fit which gives a yield of benzaldehyde of (25 ± 4) % in the experiments conducted at EUPHORE. This result is in good agreement with that found in the experiments at Ford. Taking an average of the results from the experiments at EUPHORE and Ford gives a
no data exist for o-hydroxybenzyl alcohol, it has been assumed that it reacts at the same rate as o-cresol (kO3 = 2.6 × 10−19 cm3 molecule−1 s−1).34 The loss of products via secondary reaction with ozone reaction was negligible (
