The Dissociation of Diacetyl: A Shock Tube and Theoretical Study

summary

Xueliang Yang,† Ahren W. Jasper,‡ John H. Kiefer,§ and Robert S. Tranter*,†
Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 South Cass AVenue,
Argonne, Illinois 60439, Combustion Research Facility, Sandia National Laboratory,
P.O. Box 969, LiVermore, Califo
ia 94551, and Department of Chemical Engineering, UniVersity of Illinois at
Chicago, 810 South Clinton Street, Chicago, Illinois 60607
ReceiVed: April 22, 2009; ReVised Manuscript ReceiVed: May 22, 2009
The dissociation of diacetyl dilute in krypton has been studied in a shock tube using laser schlieren densitometry
at 1200-1800 K and reaction pressures of 55 ( 2, 120 ( 3, and 225 ( 5 Torr. The experimentally determined
rate coefficients show falloff and an ab initio/Master Equation/VRC-TST analysis was used to determine
pressure-dependent rate coefficient expressions that are in good agreement with the experimental data. From
the theoretical calculations k∞(T) ) 5.029 × 1019 (T/298 K)-3.40 exp(-37665/T) s-1 for 300 < T < 2000 K.
The laser schlieren profiles were simulated using a model for methyl recombination with appropriate additions
for diacetyl. From the simulations rate coefficients were determined for CH3 + CH3 ) C2H6 and CH3 +
C4H6O2 ) CH3CO + CH2CO + CH4 (k(T) ) 2.818T4.00 exp(-5737/T) cm3 mol-1 s-1). Excellent agreement
is found between the simulations and experimental profiles, and Troe type parameters have been calculated
for the dissociation of diacetyl and the recombination of methyl radicals.
Introduction
The dissociation of diacetyl, 2,3-butadione, is initiated by
C-C fission (1) to form two acetyl radicals which rapidly
dissociate (2) to a methyl radical and CO.
Thus, from each diacetyl two methyl radicals are obtained
and (1) should be a clean, efficient pyrolytic source of methyl
radicals at shock tube temperatures and it may be superior to
some other sources we have used including ethane, acetaldehyde,
and acetone. Acetone is the next easiest to dissociate of these
other precursors and has a C-C bond strength that is about 10
kcal/mol higher than that of the central C-C bond in diacetyl.
Consequently, diacetyl can be used for methyl generation at
somewhat lower temperatures than the other precursors.
Recently, we have also investigated the dissociation of CH3I
as a source of methyl radicals1 and developed a mechanism for
methyl radical reactions that simulates both methyl recombination
and ethane pyrolysis reactions very well over the temperature
range 1500-2200 K. It is likely that the experimental
range that (1) is observable over will overlap the CH3I
experiments providing another test for the methyl radical
submechanism.
The earliest reports on the thermal decomposition of diacetyl
are by Rice and Walters2 (420-470 K, 38-458 Torr) and
Walters3 (383-436 K, 147-287 Torr) who studied the reaction
in bulb experiments. Product analyses were performed, and a
reaction mechanism proposed along with rate coefficients for
(1). Subsequent thermal experiments were carried out in a stirred
flow reactor (677-776 K, 0.6-45 Torr) by Hole and Mulcahy4
and in a flow tube by Scherzer and Plarre5 (822-905 K,
0.6-430 Torr). Knoll et al.6 investigated (1) in static cells
(648-690 K, 43-183 Torr). The rate coefficients for reaction
1 obtained by Knoll et al., Hole and Mulcahy, and Scherzer
and Plarre are in good mutual agreement.
On the basis of product analyses from the above investigations,
an investigation of acetone formation in diacetyl pyrolysis
by Guenther et al.,7 and photochemical studies by Blacet and
Bell,8,9 a reaction mechanism for the low temperature pyrolysis
of diacetyl has been elucidated that satisfactorily explains the
main products ketene, methane, acetone, ethane, and CO. The
previous studies indicate that (1) is the sole dissociation path
for diacetyl which is followed by the rapid dissociation of acetyl
radicals via (2) promoting a chain reaction mechanism propagated
by methyl radicals.