J. Phys. Chem. 1994, 98, 1161-1170
1161
Isomerization and Decomposition of Propylene Oxide. Studies with a Single-Pulse Shock Tube Assa Lifshitz' and Carmen Tamburu Department of Physical Chemistry, The Hebrew University, Jerusalem 91 904, Israel Received: August 20, 1993@
The thermal reactions of propylene oxide (1,Zepoxypropane) were studied behind reflected shocks in a pressurized driver single-pulse shock tube over the temperaturee range 850-1250 K and overall densities of -3 X mol/cm3. Four isomerization products, acetone, propanal, methyl vinyl ether, and allyl alcohol, and a large number of decomposition products were obtained under shock heating. The major decomposition products in decreasing order of abundance were CO, C2H4, C2H6, and CH4. Studies with free-radical scavengers and with isotopically labeled reactant (C3D60) indicated that the isomerization products and ethylene retain the original skeleton of the reactant whereas all the other products involve free-radical reactions. It is believed that the free-radical reactions, as in the decomposition of ethylene oxide, are initiated by decomposition of thermally excited isomerization products prior to losing their energy by collisions. The rate constants obtained for the isomerization reactions are in very good agreement with the values extrapolated from low temperatures but are by a factor of 7-10 higher than the high-temperature rate constants obtained recently by Qin et al. A reaction scheme composed of 37 species and 68 elementary reactions accounts for the product distribution over the temperature range used in this study. First-order Arrhenius rate parameters for the formation of the various reaction products are given, a reaction scheme is suggested, and results of the computer simulation and the sensitivity analysis are shown. Differences and similarities between the reactions of ethylene oxide and propylene oxide are discussed.
I. Introduction Propylene oxide is an unstable compound in a line of C 3 H 6 0 isomers such as propanone (CH3COCH3), propanal (CH3CH2CHO), and others. When it is subjected to high temperatures, the epoxy ring opens and the molecule isomerizes to the more stable C 3 H 6 0isomers.I4 The two C-0 bonds in propylene oxide are not identical, so that several isomerization channels are expected to follow the rupture of each one of these two bonds. Whereas rupture of the C(2)-0 bond yields acetone as an isomerization product, rupture of the C(3)-0 bond yields propanal. Ethylene oxide (CzHdO), the smallest molecule in the line of epoxides, also isomerizes to a more stable isomer, a~etaldehyde.~.~ With an activation energy of -57 kcal/mol in the ethylene o x i d e acetaldehyde isomerization and an exothermicity of -27 kcal/ mol,7 an acetaldehyde with more than 84 kcal/mol above its ground state is generated in the process, which is just enough for the molecule to split into CH; and HCO' before losing its energy by collision. Therefore, in addition to acetaldehyde, compounds such as methane, ethane, carbon monoxide, and others which result from consecutive reactions of these free radicals are also observed as products in this reactiom6 Similar to the thermochemistry of ethylene oxide decomposition, the thermally excited isomers that are produced in propylene oxide decomposition (except for allyl alcohol) possess enough energy to decompose to unstable fragments which can initiate chain reactions. Figure 1 shows an energy diagram based on an activation energy of 59 f 2 kcal/mol for the formation of the four isomerization products acetone, propanal, methyl vinyl ether, and allyl alcoh01.~ It also shows the unstable intermediates that can be generated by the decomposition of the thermally excited isomers. Whereas the isomerization reactions in propylene oxide have been investigated in the past both at low3 and high4temperatures, there is almost no data on the distribution of the decomposition products and their production mechanisms except for reports of the overall extent of f r a g m e n t a t i ~ n . ~The . ~ purposes of this a Abstract
published in Advance ACS Abstracts, December 15, 1993.
energy level of the thermally excited isomers
I E-59*2 kcal/mol
CH,CH-CH,+Ot
1
C,H,+CHO
I AH-62 kcal/mol
kcal/mol
--m
1
kcal/mol
CHSOCH.
Figure 1. Energy diagram for propylene oxide isomerization and decomposition.
investigationwere to measure the production rates of the individual decomposition products, to examine the effect of free-radical scavengers on their production rates, and to elucidate the kinetic mechanisms. A reaction scheme was constructed, and computer simulations of the isomerizations and decompositions were performed.
11. Experimental Section 1. Apparatus. The thermal reactions of propylene oxide were studied behind reflected shocks in a pressurized driver, 52-mm i.d. single-pulse shock tube. The tube and its mode of operation have been described in an earlier publication* and will be given here very briefly. The 4-m long driven section was divided in the middle by a 52-mm i.d. ball valve. The driver had a variable length up to a
0022-3654/94/2098-1161%04.50/0 0 1994 American Chemical Society
1162
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
maximum of 2.7 m and could be varied in small steps in order to obtain the most rapid cooling conditions. A 36-L dump tank was connected to the driven section at a 45' angle near the diaphragm holder in order to prevent the reflection of transmitted shocks and to reduce the final pressure in the tube. The driven section was separated from the driver section by Mylar polyester film of various thickness, depending upon the desired shock strength. After we pumped down the tube to approximately Torr, the reaction mixture was introduced into the section between the 52-mm i.d. ball valve and the end plate and pure argon introduced into the section between the diaphragm and the valve, including the dump tank. Gas samples were collected from the tube in 150-cm3 glass bulbs through an outlet in the driven section near the end plate and analyzed using a Hewlett-Packard Model 5890A gas chromatograph with a flame ionization detector. Reflected shock temperatures were evaluated in two different ways. In one series of experiments, the temperatures were calculated from the measured incident shock velocities using the three conservation equations and the ideal gas equation of state. The molar enthalpies of propylene oxide for these calculations were taken from Stull et aL7 In the other series, the reflected shock temperatures were calculated from the extent of isomerization of cyclopropanecarbonitrile (cpcn) which was added in small quantities to the reaction mixture to serve as an internal standard. This isomerization is a first-order unimolecular exp(-57.8 reaction9 with a rate constant given by kist = 1014.58 X 103/RT) s-l. Reflected shock temperatures in these series were calculated from the following relation:
where t is the reaction dwell time and X is the extent of isomerization defined as
X = [isomeric products],/( [isomeric products],
+ [cpcn],)
(2) The remaining reflected shock parameters were calculated from the measured incident shock velocities. These were measured with two miniature high-frequency pressure transducers (Vibrometer Model 6QP500) placed 300 mm apart near the end plate of the driven section. The signals generated by the shock wave passing over the transducers were fed through a home-built piezo amplifier to a Nicolet Model 3091 digital oscilloscope. Time intervals between the two signals shown on the oscilloscope were obtained digitally with an accuracy of 2 ps (out of about 450), corresponding to approximately 15 K. A third transducer (P.C.B. Model 113A26) placed in the center of the end plate provided measurements of the reaction dwell times (approximately 1.8 ms) with an accuracy of 5%. Cooling rates were approximately 5 x 105 K/S. 2. Materials and Analyses. Two reaction mixtures containing 0.1% and 1% propylene oxide in argon were prepared manometrically and stored at high pressure in stainless steel cylinders. Both the cylinders and the line were pumped down to better than Torr before the preparation of the mixtures. Propylene oxide was obtained from B.D.H. Chemicals Ltd. and showed only one GC peak. The argon used was Matheson ultra high purity grade, listed as 99.9995%, and the helium was Matheson pure grade listed as 99.999%. All materials were used without further purification. 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 OC was gradually elevated to 190 O C in an analysis which lasted about an hour. A typical chromatogramof 1%propylene oxidein argon shock heated to 1147 K is shown in Figure 2.
-
Lifshitz and Tamburu T=1147K
i l
0
propylene oxide
,
10
I
1
20 30 RETENTION TIME (min)
-
1
I
40
50
Figure 2. Gas chromatogram of a postshock mixture of 1% propylene oxide in argon heated to 1147 K. The numbers by the chromatogram peaks indicate attenuation factors.
Carbon monoxide was analyzed on a 2-m molecular sieve 5A column a t 35 O C . It was reduced at 400 OC to methane prior to detection using a Chrompak methanyzer with a carrier composed of 50% hydrogen and 50% argon. These analyses gave the ratio [CO]/[CHI]. 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 [CO]/ [CH4] in a standard mixture of methane and carbon monoxide was determined periodically in order toverify complete conversion of the latter to methane in the methanyzer. The concentrations of the reaction products Cs(pr)i were calculated from their GC peak areas from the following relations:1° Cs(pr)i = [ A ( ~ r j ) , / s ( ~ r j [C5(propylene )l oxide),/ A(propy1ene oxide),] (3) C,(propylene oxide), = bl%(ProPYlene oxide)(p5/p,)l/100RT, (4) A(propy1ene oxide), = In these relations, Cs(propy1ene oxide)o is the concentration of propylene oxide behind the reflected shock prior to decomposition and A(propy1ene oxide)o is the calculated GC peak area of propylene oxide prior to decomposition (eq 5) where: A(prj), is the peak area of a product, i, in the shocked sample, S(pri) is its sensitivity relative to that of propylene oxide, N(pri) is the number of its carbon atoms, p5/p1 is the compression behind the reflected shock, and T I is room temperature. Identification of the reaction products was based on their GC retention times assisted by a Hewlett-Packard model 5970 mass selective detector. The sensitivities of the various products to the FID were determined reltive to that of propylene oxide from standard mixtures. Theareasunder theGC peaks wereintegrated with a Spectra Physics Model SP4200 computing integrator and transferred after each analysis to a PC for data reduction and graphical presentation. 111. Results
1. Product Distribution. In order to determine the distribution of reaction products, tests were run with mixtures containing 1% and 0.1% propylene oxide in argon, covering the temperature range 970-1250K. Extentsof pyrolysis as low as a few hundredths of 1% were determined. Details of the experimental conditions
Decomposition of Propylene Oxide
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1163
TABLE 1: Experimental Conditions and Postsbock hoduct Distribution in Percent T5 c5x 105 C2H5- (CH3)2- C3H5- C2HY (K) t(ms) mol/cm3
PO
1.54 1.78 1.64 1.50 1.68 1.80 1.92 1.83 1.72 1.98 1.89 2.07 1.89 1.96 1.78 1.89 1.87 1.88 1.97 1.91 1.74 1.97 1.91 1.89 1.79 1.81 1.94 1.95 1.99 1.92 1.97 1.93 1.99 1.97 1.92
1.73 1.81 1.71 1.63 2.45 2.59 2.32 2.35 2.59 2.39 2.34 2.31 2.70 2.58 2.28 2.37 2.41 2.56 2.39 2.41 2.15 2.32 2.45 2.66 2.87 2.23 2.74 2.15 3.04 2.58 2.62 2.50 2.34 2.54 2.42
99.98 99.86 99.77 99.93 99.09 99.75 99.46 99.11 98.92 99.04 99.12 98.55 98.92 98.80 97.63 98.63 97.61 98.58 94.17 87.93 74.69 8 1.96 79.81 38.33 68.56 56.66 10.59 28.43 10.56 7.17 5.63 3.89 3.87 0.49 1.37
908 1.76 915 1.96 949 2.00 962 1.99 972 1.90 1007 1.94 1021 2.06 1104 1.93 1114 1.98 1115 2.01 1117 2.06 1142 1.96 1156 1.98 1165 2.02
3.13 2.44 2.62 2.65 2.79 2.35 2.29 2.21 2.79 2.45 2.82 2.69 2.51 2.81
99.63 98.26 96.53 90.23 92.41 84.11 74.99 30.09 21.89 15.55 20.43 10.25 2.09 0.89
938 941 942 949 955 960 978 984 1006 1009 1012 1036 1037 1054 1093 1144 1161
2.3 1 2.46 2.46 2.45 2.23 2.27 2.19 2.26 2.27 2.30 2.09 2.20 2.08 2.28 2.09 2.42 2.22
97.98 98.16 98.03 96.46 97.56 97.31 94.47 93.51 82.02 85.18 85.41 75.63 74.98 67.08 40.72 10.15
847 857 875 867 905 912 922 925 925 925 927 927 930 930 940 950 952 957 990 998 1010 1015 1022 1034 1034 1045 1062 1062 1125 1130 1135 1187 1210 1217 1308
CHO
CO
OH
OCH3
CH4
C2H4
C2H6
C2H2
C3Hs+ C3Hs
aC3H4
(a) 1% ProDvlene Oxide in Arnon (Ti " . -calculated from shock smed measurements) 0.0l-2' 0.003i' 0.0035 0.0031 O.bO17 0.105 0.0206 0.0040 0.0049 0.0027 0.0046 0.147 0.0301 0.0209 0.0128 0.0101 0.0092 0.026 0.0257 0.0029 0.0050 0.0032 0.0039 0.295 0.112 0.0398 0.0630 0.0155 0.0253 0.0542 0.113 0.0748 0.0137 0.0189 0.0108 0.0152 0.379 0.0598 0.0177 0.0336 0.0201 0.0188 0.619 0.173 0.0459 0.0523 0.0441 0.708 0.151 0.0416 0.073 1 0.0516 0.644 0.123 0.0389 0.0643 0.0428 0.0013 0.0386 0.0801 0.265 0.131 0.0678 0.186 0.0596 0.0013 0.0908 0.986 0.181 0.0504 0.103 0.0589 0.0674 0.0027 0.001 1 0.655 0.170 0.0513 0.0836 0.0586 0.0016 0.516 0.0018 0.799 0.149 0.0644 0.0673 0.0564 0.0437 0.206 1.52 0.1003 0.224 0.107 0.153 0.0058 0.909 0.167 0.0544 0.1 11 0.0598 0.0703 0.0029 1.56 0.089 0.314 0.0961 0.131 0.0035 0.174 0.618 0.295 0.0959 0.200 0.0752 0.134 0.0055 0.218 0.748 0.257 0.517 3.65 0.349 0.0086 7.66 0.444 1.59 0.331 0.728 0.0209 1.15 0.979 4.56 0.649 1.47 0.0524 0.676 3.29 13.13 0.831 2.31 0.534 1.44 0.0457 2.1 1 10.53 0.0028 0.929 2.55 0.659 1.50 0.0490 11.67 2.51 0.0009 5.87 0.811 7.17 3.80 2.48 0.205 0.0054 22.05 2.34 15.47 2.25 6.79 0.939 0.0032 20.47 0.103 2.64 5.30 0.829 21.49 8.04 0.0049 4.09 0.175 2.91 0.255 71.27 1.16 1.41 0.0976 0.689 0.0022 4.04 7.14 6.86 0.755 21.01 3.26 4.19 0.173 12.03 20.09 2.14 0.0077 9.47 9.86 0.263 0.0835 29.05 26.11 11.10 1.02 4.86 0.198 34.90 5.80 23.20 13.51 5.42 0.673 0.0207 2.72 5.81 0.148 34.83 5.91 14.88 5.90 0.555 0.0190 22.07 2.84 5.oo 0.0072 39.67 6.63 0.0289 25.15 8.66 6.33 0.834 2.68 0.235 43.74 7.97 25.50 3.48 4.93 1.96 0.0570 1.68 5.31 0.0824 45.15 8.13 0.0899 26.67 4.24 3.46 5.25 2.52 2.24 0.298 45.85 9.69 27.48 1.15 2.06 1.39 4.24 5.09 0.151 (b) 1% PI*opyleneOxide in Argon (T5calculated from the conversion of an internal standard) 0.286 0.345 0.182 0.017 0.0084 0.0068 0.050 0.086 0.048 0.0015 0.025 0.996 0.152 0.583 0.102 0.359 0.167 0.215 0.152 0.097 0.239 0.138 0.048 2.03 4.46 0.223 0.324 0.618 0.195 0.472 0.279 0.0043 0.108 2.85 0.674 0.339 0.370 0.861 0.217 0.526 0.319 0.113 3.53 1.49 0.632 0.743 2.32 0.577 1.39 0.744 0.014 0.299 7.57 2.05 0.883 1.00 4.93 0.933 2.53 1.37 0.035 0.563 10.37 5.39 1.58 1.31 18.67 3.75 10.73 4.29 0.219 2.03 0.010 20.94 5.05 1.39 1.01 21.50 4.62 14.05 5.25 0.337 2.53 0.024 20.61 5.80 1.68 0.797 25.61 5.13 15.31 5.29 0.381 2.53 0.019 20.71 5.27 1.38 1.05 24.49 4.59 13.62 5.05 0.308 2.37 0.013 20.34 7.48 1.46 0.603 3.47 8.95 27.94 8.93 0.843 4.13 0.033 23.99 3.59 0.465 0.176 39.96 7.84 25.09 6.75 1.32 2.79 0.048 8.59 2.41 0.167 0.190 41.82 8.54 26.53 6.49 1.93 2.43 0.068 4.58 (c) 0.1% Propylene Oxide in Argon
2.08 1.74 1.05 2.00 1.98 2.26 2.01 2.11 1.99 2.19 2.15 2.01 2.25 2.09 2.02 2.32 2.01
CO
1.66 1.68 1.79 2.25 2.13 2.21 4.36 5.24 11.87 9.18 11.22 15.12 14.79 18.30 32.77 42.12 2.09 13.48
pC3H4
oxiran
CHICHO
0.307 0.008 1 0.0024 0.0054 0.0018 0.0056 0.0019 0.0015 0.0037 0.01 15 0.0325 0.0217 0.0044 0.0148
0.0244 0.0137 0.0101 0.0207 0.0168 0.061 1 0.0679 0.0584 0.106 0.109 0.209 0.279
0.0189 0.0409 0.484 0.0557 0.0797 1.33 0.861 0.207 0.383 0.910 0.535 1.28 1.14 0.768 0.805 0.939 0.496
0.019 0.034 0.031 0.023 0.059 0.082 0.110
0.01 1 0.024 0.041 0.048 0.095 0.144 0.351 0.569 0.416 0.381 0.619 0.379 3.07
0.0604 0.106 0.168 0.242 0.0983 0.542 0.0422 0.109 1.93 0.181 0.208 0.193 0.339 0.518 0.444
0.205 0.197 0.595 0.190 0.644 0.869 0.735 0.669 1.24 0.817 0.785
(T5calculated from conversion of an internal standard) 0.277
1.71 3.06 3.99 3.91 7.66 9.23 14.68 7.31
and the distribution of reaction products are given in Table 1. (Data for carbon monoxide and allyl alcohol are available only for one set of experiments.) The percentt of a given product in the total sample, shown in Table 1,corresponds to its mole fraction, irrespective of the number of carbon atoms it contains. Figure 3 shows the product distribution obtained in the postshock mixtures over the temperature range covered. As can readily be seen, in the low-temperature range, the major
0.0834 0.0932 0.0669 0.0991 0.0860 0.0921 0.0878 1.05 0.191 0.115 0.168 0.149 0.167 0.495 0.0933 0.527 0.594 0.565 1.81 1.82 0.578 1.13 0.200 1.13 1.41 1.43 0.412 2.28 2.29 0.421 2.88 2.57 0.542 2.96 2.93 0.682 7.47 7.04 1.64 3.69 14.60 12.34 6.43 28.28 14.99
0.0582 0.0493 0.0933 0.199 0.108 0.117 0.266 0.330 0.0595 0.322 0.239 0.882 1.59 0.817 4.29 23.12
isomerization products, propanal and acetone, have the highest concentrations. At higher temperatures, the decomposition products take over with carbon monoxide, ethylene, methane, and ethane as the major products. 2. Oxygen-Carbon Balance. The balance of oxygen vs carbon among the decomposition products is shown in Figure 4. The concentrations of carbon monoxide, acetaldehyde, and ethylene oxide are plotted against one-third the sum of the concentrations
Lifshitz and Tamburu
1164 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
e v)
2 -
-cn
1 -
0
prop.ox 1 % prop.ox 0.1 %
A
0.9
0.8
a
100
-i
\
propylene oxide
I-
o
1.0
1.1
1.2
107 T (K.')
Figure 5. Arrhenius plot of the first-order rate constant for the overall reaction (isomerizations and decompositions) of propylene oxide.
3 0
10
0 U
a 1 e v)
g cn
0.1
P
: -2 : : -3 : -1
D
.This investigation
\
',, \ \
0
o
prop.ox 1 %
\ '\,
nprop.ox 0.1%
0.01
'\
Flowers (1977)
850
1050
950
1150
TEMPERATURE, K Figure 3. Product distribution in propylene oxide isomerization and decomposition.
-5 08
1 2
1 4
1 6
107 T (K.I) Figure 6. Arrhenius plot of the isomerization rate constant propylene
oxide
-
propanal. 3
1% propylene oxide in argon
h
1 0
2
10 ' 4 0 e ~ p ( - 6 4 . 1 ~ 1 0 1 / R T )
v)
.-
1
0
0
-0
-3 -4
0 1 I/ 0.1
'
'
"""'
'
1
'
"""'
'
,
-5
,,,,',I
10
, , , ,
0.8
prop.ox 1 %
,
',
A,~:~~~~(~g~),,,'~,,"
1 .o
1.2
1.4
1.6
i o 3 1 T (K.') Figure 7. Arrhenius plot of the isomerization rate constant propylene
oxide of all decomposition products (including the oxygen-containing species) each multiplied by the number of its carbon atom. Within the limit of the experimental scatter, there is no deviation from an oxygenxarbon balance over the entire temperature range of the investigation. 3. Overall Reaction. Figure 5 shows the rate constant for the overall reaction (isomerizations and decompositions) of propylene oxide, calculated as a first-order rate constant, ktOtal= -1n([propylene oxide],/ [propylene oxide]o)/r. The rate constant which accounts for both the isomerizations and the decompositions is ktotal= lOI4.O exp(-56.5 X 103/RT)s-I where R is expressed in units of cal/K mol.
o
-6
[Carbon]/ 3 (arbitrary units) Figure 4. Oxygensarbon mass balance among the decomposition
i
',,
10"2sexp(-60.7x1O'/RT)
100
products.
1
.2 :This investgation
-
acetone.
4. Isomerization Products. Figures 6-9 show Arrhenius plots of the first-order rate constants of the isomerization products propanal, acetone, allyl alcohol, and methyl vinyl ether calculated from the relation: k,(isomerization) = [isomerization product] i,l C[isomerization products], + '/,CN(pr,)C(pr,),
ktotal
(6) where N(pri) is the number of carbon atoms in a decomposition
1165
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
Decomposition of Propylene Oxide
2.5
I
I
Qin, et al. (1990)
10'3'exp(-65.7x1 03/RT) 0
-4 -5
,
1,.
,I
-0.5:
~
, , , , Flowers , , , , , , , , (1977) , , , , , , , , , , , , , ,'\, 10'28exp(-571XI O'/RT) /
, ,
-1.5 ~ ~ 0.6
-6
08
1 0
1 2
1 4
16
"
"
'
'
"
"
-
"
'
'
"
"
'
'
'
~
~
'
1 .o
io7T
103/T(Ki)
Figure 8. Arrhenius plot of the isomerization rate constant propylene oxide
'
0.9
1.1
(K.')
Figure 10. Arrhenius plot for the production of methane.
allyl alcohol.
I 2
x.7%.
1
0
.4 -5
E
1,
,
I
: 1 0 ' B 5 e ~ p72x1 ( - 03/RT) s.'
Flowers (1977) , ,
, , , , , ,
,
, , ,
, , , , , ,
,,
, , , ,
,>\,
, , ,
~
.2
10'35'exp(-58.8x103/RT)
-
1 .o
1 2
14
1 0 7 T (K
1 .o
1.2
1,l
1 0 7 T (K.')
I)
Figure 11. Arrhenius plot for the production of ethylene.
methyl vinyl ether.
product and C(pri), is its concentration. These figures show also the Arrhenius lines of two other studies, one in a static reaction vessel at low temperatures3 and one at high temperatures using the single-pulse shock tube t e c h n i q ~ e .It ~ shows also the results of the computer simulation of production of the isomerization products. 5. Decomposition Products. In Figure 10, as an example, the first-order rate constant for production of methane calculated from a relation similar to eq 6:
ken, =
TABLE 2 First-Order Arrhenius Parameters for Product Formation logA E T molecule (sd) (kcal/mol) k(950 K) (K) ~~
total decomposition prop ana 1 acetone allyl alcohol methyl vinyl ether carbon monoxide methane ethane ethylene acetylene propylene propane allene ProPYne acetaldehyde
+
[Chit [isomerization products],
0.9
1 6
Figure 9. Arrhenius plot of the isomerization rate constant propylene oxide
' ~ " ' " " " ' " ' " ' " ' " ' " " " " " ' ' ' ' '
0.6
.6
06
-
-1
ktotal
+ 1/3xN(pri)C(pri)l
(7) is plotted against reciprocal temperature. A similar plot is shown for ethylene in Figure 11. It should be mentioned that these first-order rate constants correspond to the rates at which methane and ethylene are produced (d[CH4]/dt = kce4[propyleneoxide]) and not to the rate a t which propylene oxide decomposes to yield methane. Values of E obtained from the slopes of the lines and their corresponding preexponential factors are summarized in Table 2. They were obtained from the low-temperature-low-conversion range in the figures before curvatures begin to occur. The parameters for the decomposition products do not represent elementary unimolecular reactions but only represent the experimental results. They do not imply that the decomposition reaction of propylene oxide under the conditions of the present experiment is a unimolecular process. Instead, as discussed later,
14.00 14.26 14.00 12.90 13.21 16.57 14.21 13.69 16.50 15.44 13.20 14.94 14.27 12.72
56.5 58.5 59.9 57.1 58.8 73.1 65.0 60.6 72.0 74.9 59.5 78.5 72.8 60.7
1.01 X 6.35 1.66 5.81 X 4.83 X 5.67 X 1.81 X 5.62X 8.65 X 1.62 X 3.26 X 7.62 X 3.33 X 5.71 X
~
lo1
835-1 165 835-1 165 835-1165 10-1 925-1 110 10-' 855-1 110 10-I 950-1 175 10-I 915-1 200 10-I 915-1 170 10-1 875-1 150 915-1 170 10-' 905-1 175 10-4 1020-1215 1035-1215 925-1 170
the decomposition is composed of a large number of elementary reactions involving free radicals.
IV. Discussion 1. Isomerization Reactions. As shown in Figures 6-9, there is good agreement between rate constants deduced in this investigation and values extrapolated from low temperatures except for methyl vinyl ether, which is slightly lower in the present study. The two investigations cover some 8 orders of magnitude variation in the rate constants with almost no change in the Arrhenius parameters. Since the total pressure in the present investigation was around 2 atm, there can not be a significant slide into the falloff region for this 10-atom molecule even a t 1000 K. The rate constants obtained by Qin et a1.4 in a recent
Lifshitz and Tamburu
1166 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 10.~
CH,
H
10'0
A 0
1% 0.1%
10 "
0.0
1.0
0.5
1.5
2.0
-
3
0.03' '
0.8
time, ms Figure 12. Profiles of free radicals obtained by computer simulation.
'
'
"
'
0.9
'
'
"
'
"
1.0
'
"
1.1
'
"
'
1.2
1 0 7 T , K.T
Figure 15. Relative extent of decompositions vs isomerizations as a function of reciprocal temperature.
15
20
25
30
35 40
45
50
55
60 65
m!. Figure 13. Mass spectrum of allyl alcohol in a postshock mixture of propylene oxide and propylene oxide-& in argon. Methyl-Vinyl-Ether
58
15
20
25
64
30 3 5 4 0 4 5 5 0 55 60 65
m?z Figure 14. Mass spectrum of methyl vinyl ether in a postshock mixture of propylene oxide and propylene oxide-& in argon.
single-pulse shock tube investigation are consistently lower, by a factor of about 7-10, compared to the values of the combined low-temperature-high-temperature investigations. The reason for this disagreement is not clear. The mechanisms for the production of both propanal and acetone are rather simple. Both involve C-0 bond rupture, 1,2 hydrogen shift, and addition of a ?r bond to either C(3)-0 (propanal) or C(2)-0 (acetone) to form a C=O double bond: Y
(4)
(4)
In view of the asymmetrical nature of propylene oxide, rupture of the C(2)-0 bond yields a different product than rupture of the C(3)-0 bond. Production of allyl alcohol and methyl vinyl ether, on the other hand, is considerably morecomplicated and requires morecomplex transition structures. For methyl vinyl ether, unimolecular isomerization requires double 1,2 or 1,4 hydrogen-atom migration rather than single 1,2 migration and the C(2)-C(3) bond must remain intact while the migrations take place. This is not required in the production mechanism of propanal or acetone. In allyl alcohol, the situation is still more complicated. In addition to the double 1,2 (or 1,4) hydrogen-atom migration from C(l) to the oxygen, the two C - O bonds in the molecule must be broken and a C( 1)-0 bond must be formed. Such requirements, if they can be fulfilled, must result in the formation of stiff transition structures.
In view of the complexity of the last two processes, we decided to examine more closely the question of the production mechanism of these two isomers, namely, to what extent free-radical reactions are involved in their production. It has been always assumed that the mechanisms are unimolecular, but it has never been unequivocally proven. The relatively high concentrations of methane and ethane in the postshock mixtures (Figure 3) clearly indicate a high concentration of methyl radicals. Profiles of the free radicals in the system, obtained by computer simulation using a reaction scheme that will be discussed later, are shown in Figure 12. Since CH2'CHO C H 2 4 H O ' radicals are also present in the system, methylvinyl ether, for example, can be produced by recombination CHz=CHO' CH2= of these two free radicals: CH3' CH-O-CH3. Toclarify this point, weshock-heated mixturescontaining 1.5% propylene oxide and 1.5% propylene oxide-& in argon. Isomers having either six hydrogen atoms or six deuterium atoms in their skeleton must be obtained by unimolecular isomerizations, while high concentrations of products having formulas such as CsHsDj would indicate free-radical processes.
-
+
+
Decomposition of Propylene Oxide
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1167
TABLE 3: Reaction Scheme for the Isomerization and the Decomposition of Propylene Oxid@ A
reaction 1. 2.
PO --* CH3CH2CHO PO (C2H5CHO#)
3.
PO
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54.
-+
-
+
C2H5 HCO (CzHsCHO#) CH3 + CH2CHO PO CH3COCH3 PO (CH,COCH,#) CH3CO + CH3 PO CHIOCH=CH~ PO CH2=CHCHzOH PO CH3 C2H3O PO C3H50 H PO + H H2 + C3H50 PO H + CH3 EO PO CH3 CH4 C3H50 PO C2H30 EO C3HsO CH3CHzCHO C2H5 HCO CH3CH2CHO CH3 CH2CHO CH3CH2CHO H CzHsCO H2 CH3CH2CHO + H CzHiCHO H2 CH3CH2CHO + CH3 CzHsCO + CH4 CH3CHzCHO CH3 C ~ H I C H O CH4 CHsCHzCHO + C2Hs C2HsCO + CzHs CH3CH2CHO C2H5 CiHdCHO CzH6 CH3COCH3 CH3CO CH3 CH3COCH3 + H CH3COCH2 + H2 CH3COCHs H CH3CHO CH3 CH3COCH3 CH3 CHsCOCH2 CH4 CH3COCHs C2Hs CHjCOCHz C2H6 CHsOCH=CH2 CH, CHzCHO CHsOCH4H2 H CH20CH=CH2 + H2 CH30CH=CH2 + CH, CH20CHxCH2 CH4 CHsOCH=CH2 CzHs C H 2 0 C H 4 H 2 C2H6 CH2=CHCH20H C H y C H C H 2 + OH CH24HCH20H H CH2=CHCHOH + H2 C H 2 4 H C H z O H + CH3 C H 2 4 H C H O H CH4 CH2=CHCH20H CzHs C H 2 4 H C H O H CzHs C2H4CHO C2H4 HCO CH3COCH2 CH2CO CH3 C H 2 O C H 4 H 2 CH20 C2H3 OH C H 2 4 H C H O H -p-C3H4 CH3CO Ar CH3 CO Ar C2H5 C2H4 + H HCO Ar H + CO Ar C2H3 Ar C2H2 H + Ar C2H4 Ar C2H3 + H Ar C2H4 Ar C2H2 H Z+ Ar CH3 CH3 C2Hs CH3 CH3 C2Hs H CH3 C2H4 --* n-C3H7 CH3 C2H2+p-C3H4 + H CH3 C2H2 a-C3H4 + H CH3 + CzH4 CH4 + C2H3 CH3 + C2Hs C3Hs CH3 + C2H3 CH4 C2Hz H C2H3 H2 + C2H2 H C2H4 C2H3 + H2
-
+
+
+
+
+
-+
+
+
+ + +
+
+
+
+ +
+
+
+
+
+
-+
-+
+
-
+ +
+ +
+
-
+
+ + + + + +
+
+
+ + +
+
--
+
+ + + + + + + + +
+ +
-+
+
+
---
-
+
+ + + +
-
-
+
+
+
+ + + + + + + + +
+
n
E
kr(1000 K)
kr(1000 K)
AHro(lOOOK)
source
-23.7 59.9
3 this work
1.84 x 1014 2.45 x 1013
0 0
58.5 58.5
3.06 X 10' 4.08
1.03 X 7.67 X lo8
2.45 x 1013
0
58.8
3.52
6.59
lo8
60.5
this work
1.01 x 1014 4.54 x 1013
0 0
59.9 59.9
8.32 3.74
5.10 X 2.09 x 107
-30.4 51.5
3 this work
1.62 x 7.94 x 8.00 x 8.00 x 5.54 x 2.77 X 1.oo x 2.25 7.25 X 4.78 X
0
58.8 57.1 92.0 92.0 5.2 5.2 10.0 9.1 82.4 84.0
2.32 2.64 6.28 X 6.28 x 1.30 X 6.51 X 6.53 x 2.02 x 7.13 X 2.10 x
3.94 x 9.02 X 9.45 x 4.58 X 5.82 X 2.21 x 5.06 x 3.31 x 4.00 x 1.17 x
-0.9 -9.5 92.1 92.9 -13.3 -9.6 -14.6 -8.8 83.5 84.2
this work 3 est est 16 16 est est 12 12
1013 10'2 1015 1015 102 lo1 10'2 10I6 10l6
0 0 0 3.50 3.50 0 3.65 0 0
10-5
10I2 1O'O 109 109 10-2
X
10-1 10-4 10'0 10l2 lo6 107 105 107 1013 1013
1.00 x 1014
0
9.0
1.08 X 10I2
5.68 x 107
-14.4
est
1.00 x 1014
0
9.0
1.08 X 10l2
6.59 x 109
-5.0
est
5.00 X 10l2
0
8.0
8.93 X 1O'O
8.15 x 107
-15.7
est
5.00 X 10I2
0
8.0
8.93 X 1Olo
9.45 x 109
-6.3
est
1.oo x 1012
0
10.0
6.53 x 109
1.82 X lo8
-10.3
est
1.00 x 10'2
0
10.0
6.53 x 109
2.11 x 10'0
-.9
est
2.48 X 10l6
0
81.4
3.90 X
3.55 x 1012
81.9
16
2.00 x 1014
0
9.0
2.16 X 1OIz
6.45 x 109
-3.0
est
5.54 x 101
3.50
5.2
1.30 X 10"
5.72 x 107
-6.2
est
5.00 X 10l2
0
8.0
8.93 X 1Olo
4.62 x 109
-4.3
est
1.00 x 10'2
0
10.0
6.53 x 109
1.03 X 1O'O
1.o
est
2.60 X 10l6
0
71.0
7.92
8.74 x 109
61.4
est
LOO x 1014
0
9.0
1.08 X 10l2
2.39 X lo8
-8.2
est
5.00 X 10l2
0
8.0
8.93 X 1Olo
3.43 x 108
-9.5
est
1.00 x 10'2
0
10.0
6.53 x 109
7.65 X lo8
-4.1
est
2.00 x 10'6
0
80.0
6.58 X
2.31 x 1013
82.3
est
1.00 x 1014
0
9.0
1.08 X loL2
8.25 x 104
-26.2
est
2.00 x 10'2
0
10.0
1.31 X
1.73 x 104
-27.5
est
1.00 x 1012
0
10.0
6.53 x 109
2.64 x 105
-22.2
est
20.0 26.0 20.0 59.0 12.5 37.2 20.4 32.0 98.2 78.7 0 13.5 7.7 6.0 6.0 21.5 0 0 0 9.0
3.41 x 1.66 X 1.36 X 1.28 X 2.20 x 1.35 x 6.69 X 3.00 X 1.55 X 1.33 2.00 x 3.07 X 6.80 x 2.44 X 2.44 X 6.06 X 1.55 x 3.92 X 9.64 x 8.67 X
1.48 X 1.28 X 8.77 X 5.78 X 4.07 x 1.04 x 1.36 x 2.56 x 1.53 x 6.89 x 2.41 X 1.14 x 2.79 X 2.99 x 1.05 x 6.37 X 1.13 X 3.55 x 5.04 5.26 X
20.2 26.3 19.3 60.0 14.6 37.9 17.3 41.9 108.8 44.5 -91.0 11.1 -23.1 7.9 9.3 1.3 -88.3 -65.6 -64.3 2.6
est est est est 13 15 15 13 16 16 16 14 16 16 16 16 15 16 15 16
8.00 x 8.00 x 3.20 x 1.00 x 1.20 x 4.89 x 5.12 X 3.00 x 4.37 x 2.11 x 2.00 x 2.80 x 3.23 X 5.00 X 5.00 X 3.03 x 4.89 x 3.92 X 9.64 x 8.19 x
1013 1013 1014 1014 1015 109 lo2' 1015 1017 1017 1013 1013 10" 10l2 loL2 1013 1014 10" 1013 1013
0 0 0 0 0 1.19 -2.14
0 0 0 0 0 0
0 0 0 -0.50 0 0 0
lolo
109 lo8 1O'O lo1 10'2 105 1O1O lo8 10-4 1013 1Olo 109 10" 10" lo8 1013 10" 1013 10"
10l2 10l2 10" 10l2 1013 1013 1014 1017 1017 107 1014 lo6 1014 1015 los 10-1
loLo
Lifshitz and Tamburu
1168 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
TABLE 3 (Continued) reaction
-
+ +
A
H CH2CHO CH3CHO H + C2H3O -+ EO H CzHlO CH3CHO C3H6 CH3 C2H3 CHsO CH2O + H n-C3H7 C3H6 H C2H3O CHsCO C3H50 CH3 CH2CO C>H
