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Atmospheric Chemistry of i-Butanol V. F. Andersen,† T. J. Wallington,*,‡ and O. J. Nielsen† Copenhagen Center for Atmospheric Research, Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen, Denmark, and Systems Analytics and EnVironmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122, Dearborn, Michigan 48121-2053, United States ReceiVed: August 22, 2010; ReVised Manuscript ReceiVed: October 21, 2010
Smog chamber/FTIR techniques were used to determine rate constants of k(Cl + i-butanol) ) (2.06 ( 0.40) × 10-10, k(Cl + i-butyraldehyde) ) (1.37 ( 0.08) × 10-10, and k(OH + i-butanol) ) (1.14 ( 0.17) × 10-11 cm3 molecule-1 s-1 in 700 Torr of N2/O2 diluent at 296 ( 2K. The UV irradiation of i-butanol/Cl2/N2 mixtures gave i-butyraldehyde in a molar yield of 53 ( 3%. The chlorine atom initiated oxidation of i-butanol in the absence of NO gave i-butyraldehyde in a molar yield of 48 ( 3%. The chlorine atom initiated oxidation of i-butanol in the presence of NO gave (molar yields): i-butyraldehyde (46 ( 3%), acetone (35 ( 3%), and formaldehyde (49 ( 3%). The OH radical initiated oxidation of i-butanol in the presence of NO gave acetone in a yield of 61 ( 4%. The reaction of chlorine atoms with i-butanol proceeds 51 ( 5% via attack on the R-position to give an R-hydroxy alkyl radical that reacts with O2 to give i-butyraldehyde. The atmospheric fate of (CH3)2C(O)CH2OH alkoxy radicals is decomposition to acetone and CH2OH radicals. The atmospheric fate of OCH2(CH3)CHCH2OH alkoxy radicals is decomposition to formaldehyde and CH3CHCH2OH radicals. The results are consistent with, and serve to validate, the mechanism that has been assumed in the estimation of the photochemical ozone creation potential of i-butanol. 1. Introduction Recognition of the importance of energy security and climate change has led to growing interest in the use of biofuels in transportation. Biofuels are typically used in blends with diesel or gasoline. The Renewable Fuel Standard established by the Energy Independence and Security Act in 2007 in the U.S. mandates the use of 36 billion gallons (136 billion liters) of renewable fuel by 2022. This corresponds to replacement of approximately 17% of the projected gasoline use for light-duty vehicles in 2022.1 In Europe, the Renewable Energy Directive calls for use of 10% renewable energy in the transportation sector by 2020.2 To reduce competition with food crops and to increase the yields per hectare, research is focusing on the development of second-generation biofuels. Second-generation alcohol biofuels can be produced from biomass via either biotic routes3 (e.g., pretreatment of cellulose and hemicellulose to release sugars that can be fermented to give ethanol, butanol, or higher alcohols) or abiotic routes4 (e.g., gasification followed by thermochemical synthesis giving a mixture [typically C1-C4] of alcohols). There is interest in the use of i-butanol as a second generation biofuel. The chemical and physical properties of i-butanol are similar to those of gasoline. Unlike smaller alcohols such as ethanol, modest amounts (5-20%) of i-butanol can be blended into gasoline without substantially changing the energy density, water-induced phase separation performance, or volatility of the fuel blend.5-7 The use of alcohol biofuels will result in their release into the atmosphere. In the atmosphere, alcohols undergo photochemical oxidation initiated by the OH radical. Prior to the large scale use of alcohols, an assessment of their atmospheric * To whom correspondence should be addressed. E-mail: twalling@ ford.com. † University of Copenhagen. ‡ Ford Motor Company.
chemistry and environmental impact is needed. There is a substantial kinetic and mechanistic database for smaller alcohols such as methanol, ethanol, and propanol and the atmospheric chemistry of these compounds is well understood.8 The kinetic and mechanistic database for larger alcohols is sparse and the details of their atmospheric oxidation are unclear. There have been two studies of the kinetics of the reaction of OH radicals with i-butanol9,10 and one study of the kinetics of the reaction of chlorine atoms with i-butanol.9 The mechanism of the atmospheric degradation of i-butanol has yet to be studied. To improve our understanding of the atmospheric chemistry of alcohols, we conducted a study of the kinetics and the oxidation mechanism of i-butanol initiated by chlorine atoms and OH radicals.
2. Experimental Section Experiments were performed in a 140 L Pyrex reactor interfaced to a Mattson Sirius 100 FTIR spectrometer.11 The reactor was surrounded by 22 fluorescent blacklamps (GE F40T12BLB), which were used to photochemically initiate the experiments. Chlorine atoms were produced by photolysis of molecular chlorine.
Cl2 + hν f Cl + Cl
(1)
OH radicals were produced by photolysis of CH3ONO in air.
CH3ONO + hν f CH3O + NO
10.1021/jp107950d 2010 American Chemical Society Published on Web 11/04/2010
(2)
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CH3O + O2 f HO2 + HCHO
(3)
HO2 + NO f OH + NO2
(4)
Relative rate techniques were used to measure the rate constant of interest relative to a reference reaction whose rate constant has been established previously. The relative rate method is a well established technique for measuring the reactivity of Cl atoms and OH radicals with organic compounds. Kinetic data were derived by monitoring the loss of i-butanol relative to one or more reference compounds. The method assumes that the reaction under investigation is the only significant loss process for the reactant and reference. The following relation is then valid:
(
ln
[i-butanol]t0 [i-butanol]t
)
)
(
[reference]t0 kreactant ln kreference [reference]t
)
(I)
where [i-butanol]to, [i-butanol]t, [reference]t0 and [reference]t are the concentrations of i-butanol and the reference compound at times t0 and t, and ki-butanol and kreference are the rate constants for reactions of Cl atoms, or OH radicals, with i-butanol and the reference compound. Plots of ln([i-butanol]t0/[i-butanol]t) versus ln([reference]t0/[reference]t) should be linear, pass through the origin, and have a slope of ki-butanol/kreference. Unless stated otherwise, quoted uncertainties represent the precision of the measurements and include two standard deviations from the regression analyses and uncertainties in the IR analysis (typically (1-2% of the initial reactant concentrations). CH3ONO was synthesized by the dropwise addition of concentrated sulphuric acid to a saturated solution of NaNO2 in methanol. i-Butanol and i-butyraldehyde were obtained from Sigma-Aldrich at purities of >99.5% and >99%, respectively. Experiments were conducted in 700 Torr total pressure of high purity O2/N2 diluent at 296 ( 2 K. Concentrations of reactants and products were monitored by FTIR spectroscopy. IR spectra were derived from 32 coadded interferograms with a spectral resolution of 0.25 cm-1 and an analytical path length of 26 m. To check for unwanted loss of reactants and reference compounds via heterogeneous reactions, reaction mixtures were left to stand in the chamber for 60 min. With the exception of (CH3)2CHCH(Cl)OH, there was no observable (60%) consumption of i-butanol reflects the loss of ibutyraldehyde via reaction with chlorine atoms. The data in Figure 3 contain information on the initial yield of ibutyraldehyde (which we equate to k5a/k5) and the rate constant ratio k6/k5. Assuming that the formation and loss of i-butyraldehyde are determined by reactions 5, 12, 15, and 6, then it can be shown25 that [(CH3)2CHCHO]t [(CH3)2CHCH2OH]t)0
)
R k6 1k5
(1 - x)[(1 - x)k6/k5-1 - 1]
(II) Where R is the yield of (CH3)2CHCHO following reaction of chlorine atoms with (CH3)2CHCH2OH (k5a/k5), k6 is the rate constant for reaction 6, and x is the fractional loss of (CH3)2CHCH2OH defined as:
x)1-
[(CH3)2CHCH2OH]t [(CH3)2CHCH2OH]t0
(III)
The best fit was obtained with R ) 0.53 ( 0.03 and k6/k5 ) 0.64 ( 0.06. Quoted errors correspond to two standard deviations from the fit. The rate constant ratio k6/k5 ) 0.64 ( 0.06 is consistent with the results reported in section 3.1; k6/k7 × k7/k5 ) k6/k5 ) 0.69 ( 0.09 and k6/k8 × k8/k5 ) k6/k5 ) 0.66 ( 0.05. From the value of R we conclude that 53 ( 3% of the reaction of chlorine atoms with i-butanol occurs via H-atom abstraction from the R-carbon (reaction 5a).
Figure 4. IR spectra obtained before (A) and after (B) 30 s of irradiation of 31 mTorr of i-butanol and 129 mTorr of Cl2 in 700 Torr of air. Panel (C) is the product spectrum obtained by subtracting 65% of panel (A) from panel (B). Panels (D) and (E) are reference spectra for i-butyraldehyde and acetone.
3.4. Products of Cl Atom Initiated Oxidation of i-Butanol in 700 Torr of N2/O2. The mechanism of Cl atom initiated oxidation of i-butanol was investigated by irradiating mixtures of 30-31 mTorr of i-butanol, 117-1296 mTorr of Cl2, and 15-150 Torr of oxygen in 700 Torr total pressure of N2 diluent. Figure 4 shows spectra acquired before (A) and after (B), a 30 s irradiation of a mixture of 31 mTorr of i-butanol and 129 mTorr of Cl2 in 700 Torr of air. The consumption of i-butanol in this experiment was 35%. Panel (C) shows the product spectrum derived by subtracting the IR features of i-butanol from the spectrum in panel (B). Comparison with the reference spectrum in panel (D) shows the formation of i-butyraldehyde. As with other R-hydroxyalkyl radicals, reaction with O2 giving i-butyraldehyde will be the sole fate of the radical generated in reaction 5a.
(CH3)2CHCHOH + O2 f (CH3)2CHCHO + HO2
(16) The circles in Figure 5 show the formation of i-butyraldehyde versus loss of i-butanol observed following the UV irradiation
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Andersen et al. 20-60% there was a substantial increase in the observed yield of acetone and HCHO, and we conclude that these compounds are formed as secondary products. For i-butanol consumptions of >60% the yield of HCHO reaches a maximum and then declines; in contrast, the yield of acetone continues to increase with i-butanol consumption. Acetone is relatively unreactive towards chlorine atoms (k(Cl + acetone) ) 2.2 × 10-12 8), whereas HCHO is reactive k(Cl + HCHO) ) 7.2 × 10-11.8 The acetone yield for i-butanol consumptions >90% tends towards a value that is similar to the initial i-butyraldehyde yield (see previous sections). The simplest explanation for the acetone formation shown in Figure 6 is that it is formed via chlorine-initiated oxidation of i-butyraldehyde:
Figure 5. Formation of i-butyraldehyde vs the loss of i-butanol following UV irradiation of i-butanol/Cl2 mixtures with 15 Torr of O2 (open symbols) or 150 Torr of O2 (closed symbols) in 700 Torr total pressure of nitrogen diluent in the presence (triangles) and absence (circles) of NOx.
(CH3)2CHCHO + Cl f (CH3)2CHCO + HCl
(6a) (CH3)2CHCO + O2 f (CH3)2CHC(O)O2
(17)
(CH3)2CHC(O)O2 + RO2 f (CH3)2CHC(O)O + RO + O2
(18) (CH3)2CHC(O)O f (CH3)2CH + CO2
(19)
(CH3)2CH + O2 f (CH3)2CHO2
(20)
(CH3)2CHO2 + RO2 f (CH3)2CHO + RO + O2
(21) (CH3)2CHO + O2 f CH3C(O)CH3 + HO2 Figure 6. Formation of acetone (circles) and formaldehyde (triangles) vs the loss of i-butanol following UV irradiation of i-butanol/Cl2 mixtures with 15 Torr of O2 (open symbols) or 150 Torr of O2 (closed symbols) in 700 Torr total pressure of nitrogen diluent in the absence of NOx. The dashed line shows the expected yield of acetone based on the mechanism discussed in Section 3.4.
of i-butanol/Cl2/O2/N2 mixtures. The filled circles are for experiments with 150 Torr O2, open circles are for experiments with 15 Torr O2. There was no discernible impact of [O2] on the i-butyraldehyde yield. A fit of eq II to the data gives k6/k5 ) 0.70 ( 0.05 and R ) 0.48 ( 0.03. These results are consistent with those presented in Section 3.3 obtained in 700 Torr N2. The rate constant ratio k6/k5 ) 0.70 ( 0.05 is consistent with the results presented in Section 3.1. As seen from comparing panels C and E in Figure 4, a small amount (0.84 mTorr) of acetone was observed. The sharp feature at 1746 cm-1 evident to the left of the i-butyraldehyde Q-branch in Figure 4C is attributable to the formation of HCHO (1.05 mTorr). Figure 6 shows the formation of acetone and formaldehyde versus the loss of i-butanol. Filled symbols were obtained using 150 Torr O2, and open symbols were obtained using 15 Torr O2. There was no discernible effect of [O2] on the yield of acetone, but the yield of HCHO was lower in the experiment using higher [O2]. For small consumptions (
