2440 J. Phys. Chem. A, Vol. 101, No. 13, 1997
Arif et al.
with additional work. The observation of propene and propa-
diene is conceivable with the subsequent decomposition of
isobutene. The formation of acetone and other oxygenated
species indicates that other reactions, perhaps initiated by loss
of CH3 from CH3OC(CH3)2CH2, are significant. These routes
may result in formation of large ring species incorporating
oxygen in their structures. Products of this type have recently
been identified using multidimensional gas chromatography-
mass spectrometry of the reactor effluent from the oxidation of
reformulated gasoline in our laboratories.31
Conclusions
New atmospheric pressure, absolute rate measurements are
reported for the reaction of hydroxyl radicals with dimethyl ether
(k1) and methyl tert-butyl ether (k2). Arrhenius plots of the data
indicated significant curvature. The following modified Ar-
rhenius expressions (in units of cm3 molecule-1 s-1) are thus
recommended for use by both atmospheric chemistry and
combustion modelers:
k1(295-650 K) ) 1.05 × 10-17T2.0 exp[328/T]
k2(293-750 K) ) 1.11 × 10-17T2.04 exp[266/T]
Figure 6. Total ion chromatogram of the stoichiometric oxidation of
CH3OC(CH3)3. Reactor temperature ) 918 K, residence time ) 0.85
s, initial gas-phase CH3OC(CH3)3 concentration ) 500 ppm. Legend:
1, carbon dioxide; 2, ethane; 3, propene; 4, 1,2- propadiene; 5, water;
6, methanol; 7, isobutene; 8, acetone; 9, 1,1-dimethylcyclopropane; 10,
C4 oxygenate; 11, methyl tert-butyl ether.
Atmospheric pressure product analysis studies suggest that the
dominant fate of the CH3OCH2 radical produced in k1 is â C-O
bond cleavage yielding formaldehyde and methyl radicals as
the major products. For CH3OC(CH3)3, product analyses
suggest that OH attack on the tert-butyl group is the dominant
initial reaction channel at elevated temperatures. Furthermore,
it appears that the primary mode of decomposition of CH3OC-
(CH3)2CH2 is â C-O bond cleavage yielding isobutene and
methoxy radicals as the major products.
temperature smog chamber studies indicated that H abstraction
from the methoxy site was dominant.12,13 The major reaction
products under these simulated atmospheric conditions were tert-
butyl formate, formaldehyde, and methyl acetate. At higher
temperatures where the differences in bond strength become
less important, one must also consider the entropy favored H
abstraction from the tert-butyl site, thus resulting in the
formation of both CH2OC(CH3)3 and CH3OC(CH3)2CH2 radi-
cals. The fate of these oxygenated radicals is not well
understood. H abstraction via reaction with molecular oxygen
results in the formation of two diradicals, CH2OC(CH2)(CH3)2
and CH3OC(CH2)2CH3. The only conceivable stabilization
routes for these diradicals are formation of substituted three-
or four-member ring compounds. An alternative fate for CH2-
OC(CH3)3 and CH3OC(CH3)2CH2 radicals is â C-O or C-H
bond cleavage. For CH2OC(CH3)3, this results in formation of
formaldehyde and tert-butyl radicals. For CH3OC(CH3)2CH2,
the corresponding products are isobutene and methoxy radicals
(C-O cleavage) or methylpropenyl ether and methyl radicals
(C-H cleavage).
A total ion chromatogram from the stoichiometric oxidation
of CH3OC(CH3)3 (T ) 918 K) is illustrated in Figure 6. The
major reaction products were isobutene and methanol. Minor
reaction products included ethane, propene, 1,2-propadiene,
acetone, 1,1-dimethylcyclopropane, and a C4 oxygenate. The
major atmospheric products, tert-butyl formate, formaldehyde,
and methyl acetate, were not detected in these high-temperature
experiments. The distribution of products suggests that OH
attack on the tert-butyl group is the dominant initial reaction
channel. Furthermore, it appears that the primary mode of
decomposition of CH3OC(CH3)2CH2 is â C-O bond cleavage
yielding isobutene and methoxy radicals. The lack of detection
of three- or four-member ring compounds suggests that molec-
ular oxygen may not play a dominant role in reactions of the
initially formed radicals, although this must be substantiated
Acknowledgment. The support of this work by the National
Renewable Energy Laboratory (XAU-3-12228-02) is gratefully
acknowledged. The constructive comments of the reviewers
are appreciated.
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