5166 J. Phys. Chem. A, Vol. 101, No. 28, 1997
Maricq et al.
nonzero offset, with an apparent rate constant comparable to
the true value.
for kro2,∞ and kprod,0, respectively, are in very good agreement
with the present results, with the caveat that the low-pressure
rate constants were determined without consideration of reaction
1b. Their value for kro2,0 of (9.4 ( 4.2) × 10-30 cm6 s-1 is
well below our determination; however, it too suffers from the
data being insensitive to this parameter. It is clearly too small
since a prediction of the formaldehyde yield based on this value
for kro2,0 gives kro2,0/kprod,0 ) 1.6 × 10-18 cm3, a value nearly
one-third that required to fit the formaldehyde yield in Figure
14.
Including the present work, there are three determinations of
the temperature dependence of the CH3OCH2 + O2 reaction;
however, all three are measured under different conditions.
Hoyermann et al.12 examined the reaction at a total pressure of
about 4 Torr, finding a temperature dependence given by 5.6
× 10-13e855/T cm3 s-1. Unfortunately the interpretation of this
result is questionable since they omitted channel 1b. Sehested
et al.11 performed their experiments at 18 bar and over a range
of 296-523 K arriving at a value of (1.07 ( 0.08) ×
10-11e(46(27)/T cm3 s-1 for kr+o2. Our results, near the high-
Two other sets of measurements shown in Figure 15 deserve
comment. First are the values of kr+o2 derived from the rate of
formaldehyde production. That they are smaller than those from
the UV measurements is attributed to vibrational excitation of
the nascent CH2O, which makes a fraction of this product
initially transparent to the IR probe. Its subsequent vibrational
relaxation causes an apparent formaldehyde appearance rate that
is slower than the actual formation rate. Second are rate
constants derived from the relative rate determinations3 of kr+o2
kr+cl2 by scaling them with the current measurement of kr+cl2
/
.
These values underestimate the kinetically derived values of
kr+o2 by about 30%. While the agreement between direct and
relative rate measurements is reasonably good, it does stretch
the combined error bars. The origin of the discrepancy remains
uncertain. Because of the good agreement between the various
direct determinations of kr+o2, this is an unlikely source of the
discrepancy. This leaves the present evaluation of kr+cl2 and
the relative rate measurement as candidates. Were the discrep-
ancies to arise from kr+cl2, its already large value of 6.2 × 10-11
cm3 s-1, as compared to other R + Cl2 reactions, would have
to be elevated to about 1 × 10-10 cm3 s-1. Alternatively, there
may exist some, as of yet unknown, subtlety concerning the
relative rate experiments.
pressure limit, give kr+o2 ) (3.1+-01..08) × 10-12e(326(80)/T cm3 s-1
.
Measurements of kro2,0/kprod,0 at 230, 295, and 350 K yield
lower limits of 4.8 × 10-18, 4.3 × 10-18, and 2.9 × 10-18 cm3,
respectively. At a constant pressure, the branching fraction for
channel 1b increases with increasing temperature. This is
consistent with the CH3OCH2 + O2 reaction proceeding through
a CH3OCH2O2* complex. As the temperature is raised, the extra
available thermal energy enhances dissociation of the complex
into the hydroxyl and formaldehyde products.
Assuming that both channels 1a and 1b proceed through an
intermediate complex that is either stabilized or dissociates
allows, via a Lindemann analysis, the pressure dependence of
the reaction to be parametrized according to
D. Atmospheric and Diesel Fuel Implications. Under
atmospheric conditions the methoxymethyl radical behaves as
a simple organic radical; namely, it adds oxygen to form the
corresponding peroxy radical. The predominant removal path-
way for this radical will likely involve its reaction with NO to
produce CH3OCH2O. The latter molecule either reacts with
O2 or expels a hydrogen atom, in either case producing HO2
and methylformate. The formaldehyde/OH channel is a minor
one at pressures above 100 Torr. As the inset to Figure 14
shows, this remains true over the 230-350 K temperature range.
Consequently the formation of formaldehyde from channel 1b
during the atmospheric degradation of dimethyl ether should
be of only minor importance.
kro2,0[M]
kro2 ≡ k1a
)
)
(14)
(15)
1 + kro2,0[M]/kro2,∞
kprod,0
kprod ≡ k1b
1 + kro2,0[M]/kro2,∞
where kro2,0 is a three-body rate constant that depends on the
nature of the bath gas (here N2), and kro2,∞ is the high-pressure
limit. A fit of kr+o2 ) kro2 + kprod to our pressure dependent
data in Figure 15 leads to values for kro2,0 and kprod,0 of 1.3 ×
10-30 cm6 s-1 and 7.8 × 10-12 cm3 s-1, respectively. However,
f, the branching fraction for channel 1b, is given by
This conclusion about the impact of the formaldehyde channel
is predicated on the assumption that the methoxymethylperoxy
radical is thermally stable. As the temperature is increased
beyond 350 K toward combustion levels, reaction 1a will
become reversible. When this happens, it will dramatically
affect the net hydroxyl radical formation, even at “high”
pressure, and, hence, the combustion of dimethyl ether. Instead
of forming the relatively unreactive peroxy radical, the CH3-
OCH2 + O2 reaction will form the more reactive hydroxyl
radical, which will consume additional dimethyl ether, generate
more heat, and, thereby, engender combustion. While this
provides a plausible scenario for the high cetane number of
dimethyl ether, additional experiments are needed to confirm
the high-temperature behavior of reaction 1.
1
f )
(16)
1 + kro2,0[M]/kprod,0
and fits to eq 16 of the formaldehyde yield pressure dependence
in Figure 14 give kro2,0/kprod,0 ) 4.3 × 10-18 cm3. Clearly, this
ratio is incompatible with the individual parameters deduced
from the pressure dependence of kr+o2 in Figure 15. This
problem is resolved by noting that the three-parameter fits of
kr+o2 to eqs 14 and 15 are insensitive to the choice of kro2,0 and,
consequently, that this parameter is essentially indeterminate
within the accuracy of the measured rate constants. kprod,0 and
kro2,∞, on the other hand, are fit with reasonable accuracy.
Therefore, kro2,0 is deduced from fitting the formaldehyde yield
to eq 16. At 295 K the results are kro2,0 ) (2.6 ( 0.9) × 10-29
cm6 s-1, kro2,∞ ) (11 ( 1) × 10-12 cm3 s-1, and kprod,0 ) (6 (
2) × 10-12 cm3 s-1. The solid line in Figure 15 represents the
predicted pressure dependence of kr+o2 based on these param-
eters.
Conclusion
A detailed kinetic investigation of the chlorine-initiated
oxidation of dimethyl ether has been presented. A combination
of time-resolved UV spectroscopy and transient IR absorption
techniques have been brought to bear on this problem. Record-
ing the entire UV spectrum of the reaction mixture as a function
of time allows us to disentangle contributions arising from the
principal species of interest, CH3OCH2 and CH3OCH2O2, as
Sehested et al.11 also fit the pressure dependence of the kr+o2
rate constant by combining their high-pressure data with the
low-pressure rate constants of Masaki et al.14 Their values of
(11.4 ( 0.4) × 10-12 cm3 s-1 and (6.0 ( 0.5) × 10-12 cm3 s-1