Environ. Sci. Technol. 1991, 25, 410-415
Atmospheric Chemistry of Diethyl Ether and Ethyl tert -Butyl Ether Timothy J. Wallington" and Steven M. Japar Research Staff, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053
The mechanisms for the C1-initiated and OH-initiated atmospheric oxidation of diethyl ether and ethyl tert-butyl ether (ETBE) have been determined. For diethyl ether the products are ethyl formate and formaldehyde and its atmospheric oxidation can be represented by CzH50CzHS + OH + 2 N 0 CzH50C(0)H+ HCHO + 2NO2 + HOz The mechanism for the atmospheric oxidation of ETBE is more complex, with 80% of the reaction being accounted for in terms of tert-butyl formate and formaldehyde. The remaining 20% we ascribe to 2-ethoxy-2-methylpropanal. The atmospheric oxidation of ETBE can be represented by ETBE + OH + 1.8NO 0.8HCOOC(CH3)3 + 0.2CzH,0C(CH3),CH0 + HOZ + 0.8HCHO + l.8NOz The subsequent atmospheric chemistry of 2-ethoxy-2methylpropanal we estimate to be represented by CZHSOC(CHJzCH0 + OH + 3 N 0 COZ HZCO + C,H5OC(O)CH, + HOZ + 3N02 These results ae discussed in terms of the reactivity of these compounds in urban atmospheres. -+
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Introduction To prevent or minimize knock in spark-ignited engines, several gasoline additives have been used to boost the octane number of automotive fuel. These included mainly organolead compounds such as (CHJ,Pb and (C2H5)4Pb, until the wide-spread use of catalytic convertors forced their replacement by alternatives such as aromatic compounds. Most recently, oxygenated organic compounds such as the ethers methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) are gaining prominence as high-octane fuel additives. The switch to such oxygenated compounds as motor fuel additives has the added benefit of reducing CO emissions ( I ) . The use of substantial quantities of these volatile organics in gasoline makes their emission to the atmosphere an important consideration. Following such release, the main atmospheric fate of ethers is expected to be reaction with OH radicals, since photolysis ( 2 ) , reaction with O3 (3),and reaction with NO3 radicals ( 4 ) are negligibly slow. To assess the atmospheric impact of release of these oxygenated fuel additives, accurate kinetic and mechanistic data concerning their reactivity toward OH radicals are needed. The kinetic data base for the reaction of OH radicals with ethers has improved substantially in the past five years (5-8) to a point where there are now methods that can be used to predict the reactivity of most ethers toward OH radicals to &20% (9-11). However, in contrast to the much improved kinetic data base, there is scant information available as to the products of these reactions. Thus, the only detailed studies reported are those of Cox and Goldstone (7)on MTBE and our recent study dealing with dimethyl ether and MTBE (12). As a consequence of the lack of product information there are significant uncertainties associated with the prediction of the environmental impact of use of these fuel additives. In particular, it is difficult to assess the ozone-forming potential of these compounds in urban atmospheres. As part of our ongoing 410
Environ. Sci. Technol., Vol. 25, No. 3, 1991
program aimed at addressing the environmental impact of automotive and manufacturing emissions, we report herein the results of the first product study of the oxidation of diethyl ether and ethyl tert-butyl ether under simulated atmospheric conditions. While the oxidation of oxygenated hydrocarbons in the atmosphere is initiated by the OH radical, in this work we make considerable use of C1 atom initiated oxidation. Our reasons for employing C1 atoms as initiators are as follows: (i) C1 atoms are more easily generated in our reactor than OH radicals; (ii) product analysis is simpler in C1-initiated systems since there are no complications due to organic reactants and products associated with the usual modes of OH radical formation (e.g., methyl nitrite or ethyl nitrite photolysis); (iii) C1 atoms are, in general, less selective in their attack of organics, hence reducing problems frequently encountered with OH radicals where the OH radicals preferentially attack the products rather than the reactants leading to consumption of products; (iv) the modes of attack by C1 and OH are generally the same for saturated organics. Experimental Section The apparatus and experimental techniques used have been described in detail previously (12, 13) and are only briefly discussed here. All experiments were performed in a 140-L Pyrex reactor surrounded by 22 fluorescent blacklamps (GE F15T8-BL), which were used to photochemically initiate our experiments. The loss of reactants and the formation of products were monitored by Fourier transform infrared spectroscopy. The path length for the analyzing infrared beam was 26.6 m. The spectrometer was operated at a resolution of 0.25 cm-'. Infrared spectra were derived from 16 coadded interferograms. Reference spectra were obtained by expanding known volumes of the reference material into the long path length cell. Three sets of experiments were performed to delineate the oxidation products of diethyl ether and ETBE. First, the reactant (either diethyl ether or ETBE) was mixed with CH30N0 and NO and the mixture irradiated to give OH radicals: C H 3 0 N 0 + hv CH30 + NO (1) CH30 + O2 HCHO + HOz (2) HO? + NO OH + NO2 (3) reactant(RH) + OH R + HzO (4) R + O z + M-ROz + M (5) ROz + NO products (6) This approach most closely simulates the oxidation of DEE and ETBE in the atmosphere. However, this method is limited in that reaction 2 produces HCHO and hence obscures any formation of this important species from the reactant. The second set of experiments used the irradiation of mixtures of the ether with C1, and NO to specifically search for HCHO and other trace species: C12 + hu 2C1 (7) reactant(RH) + C1- R + HC1 (8) R+O,+M+RO,+M (5) ROZ+ NO products (6)
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A [diethyl ether] (rntorr) Figure 2. Plot of the observed yield of ethyl formate versus the loss of diethyl ether following irradiation of DEE/CH,ONO/NO mixtures in air. The solid line is a linear least-squares fit.
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Flgure 1. Infrared spectra taken before (A) and after (B) a 1.5-min irradiation of a mixture of diethyl ether with CH,ONO and NO. HCHO product features have been removed from spectrum B for clarity. Figure C is a reference spectrum of ethyl formate.
0,CHpCH2
OEt
02,.= L.*'
OCHCH20Et
Finally, to gain further mechanistic insight into the mechanism of oxidation of DEE and ETBE, the ethers were individually mixed with C1, and irradiated in the absence of NO. In each of the three sets of experiments at least two gas mixtures were prepared, irradiated, and analyzed to check the experimental reproducibility. In all cases indistinguishable results were obtained from successive experiments. Diethyl ether and ETBE had stated purities of >99.9% (confirmed by gas chromatography/mass spectroscopy) and were used after repeated pump/thaw cycles. CH30N0 was generated by the dropwise addition of 50% sulfuric acid to methanol saturated by sodium nitrite, as described previously (11). Formaldehyde, ethyl formate, tert-butyl formate, tert-butyl acetate, methyl nitrite, methyl nitrate, ethyl acetate, and acetaldehyde spectra were obtained from our standard reference library. Products were quantified by fitting reference spectra of the pure compounds to the observed product spectra by using integrated absorption features. We estimate uncertainties associated with quantitative analyses using these reference spectra to be
