10656 J. Phys. Chem. A, Vol. 102, No. 52, 1998
Lifshitz et al.
In this investigation we present data on the product distribu-
about 50 min. A typical chromatogram of 0.5% 2,5-dimethyl-
furan in argon shock heated to 1238 K is shown in Figure 1.
Carbon monoxide was analyzed on a 2 m molecular sieve 5
Å column at 35 °C. It was reduced at 400 °C to methane prior
to its detection using a Chrompak methanyzer with a carrier
gas composed of 50% hydrogen and 50% argon. These analyses
gave the ratio [CO]/[CH4]. From these ratios and the known
methane concentration obtained in the Porapak N analyses, the
concentration of CO could be calculated for each run. The ratio
tion in shock heated mixtures of 2,5-dimethylfuran. A detailed
mechanism is suggested, a reaction scheme which includes
unimolecular decompositions, dissociative attachments, and free
radical reactions is composed, and computer simulation is
performed.
II. Experimental Section
1
. Apparatus. The thermal reactions of 2,5-dimethylfuran
were studied behind reflected shocks in a pressurized driver,
2 mm i.d. single-pulse shock tube. The tube and its mode of
[CO]/[CH4] in a standard mixture of methane and carbon
monoxide was determined periodically in order to verify a
complete conversion of the latter to methane in the methanyzer.
In the course of analyzing the raw data we encountered some
separation problems which were solved by using the SIM mode
of a Hewlett-Packard model 5970 mass selective detector
connected to a Hewlett-Packard model 5890 gas chromatograph.
The peaks of 1-butyne and 1,2-butadiene were hidden under a
large peak of C4H4. Two peaks of C5H8 were hidden under a
larger peak of cyclopentadiene. The procedure of determining
the magnitude of the hidden peaks is described in detail in the
5
3
operation have been described in an earlier publication and
will be given here only very briefly.
The driven section was 4 m long and the driver had a variable
length up to a maximum of 2.7 m. It could be varied in small
steps in order to tune for the best cooling conditions. A 36 L
dump tank was connected to the driven section at 45° angle
near the diaphragm holder in order to prevent reheating by
reflection of transmitted waves. The driven section was
separated from the driver by a Mylar polyester film of various
thickness depending upon the desired shock strength.
Before each test the tube was pumped down to approximately
1
article on the decomposition of 2-methylfuran.
We have carried out also a separate series of experiments in
order to verify the presence or absence of ketene and/or methyl
ketene in the postshock mixtures. Methylketene can be formed
by a unimolecular decomposition of 2,5-dimethylfuran. Ketene
and methyl ketene tend to react with small quantities of water
absorbed in various location on the way to the GC and produce
acetic and propionic acids which are also absorbed and hard to
analyze. The postshock mixtures in this series of experiments
were collected in bulbs containing small quantities of methyl
alcohol. In this way, methyl acetate is formed from ketene and
methyl propionate from methyl ketene. The latter can be, at
least qualitatively, analyzed. We did not identify any methyl
propionate in the post shock mixtures and only traces of methyl
acetate
-
5
3
× 10 Torr. After performing an experiment, gas samples
were collected from the tube through an outlet in the driven
3
section (near the end plate) in 150 cm glass bulbs and were
then analyzed on a Carlo-Erba Model VEGA-2000 gas chro-
matograph using a flame ionization detector. Reflected shock
temperatures were calculated from the extent of decomposition
of 1,1,1-trifluoroethane which was added in small quantities to
the reaction mixture and served as an internal standard. Its
decomposition to CH2dCF2+HF is a first-order unimolecular
reaction which under the temperature and pressure conditions
4
14.8
of this investigation has a rate constant of kfirst ) 10 exp(-
3
-1
7
4.0 × 10 /RT) s . Reflected shock temperatures were
calculated from the relation:
The concentrations of the reaction products C5(pr)i were
calculated from their GC peak areas from the following
1
AT
5
T ) -(E/R)/ ln - ln(1 - ø)
(I)
relations:
[
{
}]
C (pr) ) A(pr ) /S(pr )(C (2,5-dimethylfuran) /
5
i
i t
i
5
0
where t is the reaction dwell time and ø is the extent of
decomposition defined as
A(2,5-dimethylfuran) ) (II)
0
ø ) [CH dCF ] /([CH dCF ] + [CH CF ] )
C (2,5-dimethylfuran) )
5 0
2
2 t
2
2 t
3
3 t
p %(2,5-dimethylfuran)(F /F )/100RT (III)
1
5
1
1
The additional reflected shock parameters were calculated from
the measured incident shock velocities using the three conserva-
tion equations and the ideal gas equation of state. Dwell times
of approximately 2 ms were measured with an accuracy of (5%.
A(2,5-dimethylfuran) )
0
A(2,5-dimethylfuran) + 1/6ΣN(pr )×A(pr ) /S(pr ) (IV)
t
i
i t
i
5
Cooling rates were approximately 5 × 10 K/s.
2
. Materials and Analysis. Reaction mixtures containing
In these relations C5(2,5-dimethylfuran)0 is the concentration
of 2,5-dimethylfuran behind the reflected shock prior to
decomposition, and A(2,5-dimethylfuran)0 is the calculated GC
peak area of 2,5-dimethylfuran prior to decomposition (eq IV)
where A(pri)t is the peak area of a product i in the shocked
sample. S(pri) is its sensitivity relative to 2,5-dimethylfuran,
and N(pri) is the number of its carbon atoms. F5/F1 is the
compression behind the reflected shock, and T1 is the temper-
ature of the shock tube.
The identification of the reaction products was based on their
GC retention times but was also assisted by a Hewlett-Packard
model 5970 mass selective detector. The sensitivities of the
various products to the FID were determined relative to 2,5-
dimethylfuran from standard mixtures. The areas under the GC
peaks were integrated with a Spectra Physics Model SP4200
0.5% 2,5-dimethylfuran and 0.1% 1,1,1-trifluoroethane in argon
were prepared manometrically and stored in 12 L glass bulbs
at 700 Torr. Both the bulbs and the line were pumped down to
-
5
∼
10 Torr before the preparation of the mixtures. 2,5-
dimethylfuran was obtained from Aldrich Chemical Co. and
showed only one GC peak. The argon used was Matheson
ultrahigh purity grade, listed as 99.9995%, and the helium was
Matheson pure grade listed as 99.999%. All the materials were
used without further purification.
The gas chromatographic analyses of the postshock mixtures
were performed on two columns with flame ionization detectors.
The analyses of all the products except for CO were performed
on a 2 m Porapak N column. Its initial temperature of 35 °C
was gradually elevated to 190 °C in an analysis which lasted