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Isomerization of cyclopropane to propylene in shock waves
Volume 141, number 3
CHEMICAL PHYSICS LETTERS
6 November 1987
ISOMERIZATION OF CYCLOPROPANE TO PROPYLENE IN SHOCK WAVES Yoshiaki HIDAKA and Takashi OIU Department of Chemistry, Faculty of Science, Ehime University, Bunkyo-rho, Matsuyama 790, Japan
Received 5 June 1987; in final form 10 August 1987
Cyclopropane mixtures diluted with argon were heated to temperatures in the range 1loo-1450 K behind incident and reflected shock waves. The rate of isomerization of cyclopropane to propylene was measured by tracing the time variation of absorption at 3.39 pm. The rate constant expression k=4.60~ 10 14exp( -62.5 kcal/RT) s-’ was determined, in accord with extrapolation from lower-temperature experiments.
1. Introduction
2. Experimental
The isomerization of cyclopropane to propylene at low temperatures (below 1100 K) has been stu$ied by many workers and is extensively used as a test for theories of unimolecular reaction rates. The studies at high temperatures (above 1100 K) have been carried out with a single-pulse shock tube technique [l-4]. The isomerization rate constants k reported at the high temperatures have been lower than Arrhenius lines extrapolated from results reported for low temperatures and also showed a sharp transition at about 1200 K [ 1,3]. When cyclopropane was heated to temperatures above 1200 K, species other than propylene (methane, ethylene, acetylene, etc.) were produced in appreciable amounts during the dwell time [ 11. Because the formation mechanism of these species is complex and still vague, estimation of k using a single-pulse technique above 1200 K may lead to erroneous results. The direct measurement of cyclopropane and propylene concentrations in the earlier stage in the cyclopropane pyrolysis, however, would be more useful for the estimation of k at temperatures above 1200 K, because isomerization may be considerably faster than decomposition [5]. In this paper, we report the rate constant k over the temperature range 1 loo-1450 K estimated with a time-resolved technique.
The shock tube employed in this study has previously been described [ 61. The time variations of cyclopropane and propylene concentrations were traced using the laser absorption apparatus previously described [7]. The transmitted intensities of a 3.39 pm He-Ne laser beam through a 4.1 cm pathlength in the shock tube and through an interference filter (A,,= 3.39 pm, half-width=0.072 ,um) were observed with Fujitsu IV-2OOC4 InSb detector to learn the time variation of C3H6. The gas compositions employed were 5 and 1OWcyclopropane diluted with argon. The argon and cyclopropane, specified as 99.99% and above 99Ohpure, respectively, were obtained from commercial cylinders. The cyclopropane was purified by trap-to-trap distillation before use.
212
3. Results and discussion Thermal isomerization of cyclopropane to propylene was studied over the temperature range 1loo-1450 K and over the pressure range 0.4-5.5 atm. Typical oscillograms of laser absorption at 3.39 km are shown in fig. 1. At temperatures below 1100 K, the absorption intensity after the Schlieren spike remains constant, as seen in fig. 1a. At temperatures between 1100 K and 1350 K, the absorption intensity increases slowly with time as seen in fig, 1b. The
0 009-2614/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 141, number 3
CHEMICAL PHYSICS LETTERS
L
6 November 1987
I
Ilo
I
I
I
En
I l5ul
T/K Fig. 2. Comparison of the extinction coefficients of cyclopropane and propylene. 0: cyclopropane, l: propylene.
Fig. 1. Typical oscillogram of laser absorption at 3.39 pm with a mixture of 5% cyclopropane and 95% Ar. The sweep rate is 50 @div. (a) 1114 K, (b) 1257 K, (c) 1400 K.
rate of the increasing absorption increased with increasing the temperature. Above 1350 K, the absorption intensity increases with time, arrives to maximum value, and then decreases with time as seen in fig. lc. The extinction coefficient of propylene, which was determined with a 5% propylene, 95% Ar mixture, is larger than that of cyclopropane as shown in fig. 2. It is considered, therefore, that the slow increase seen in figs. lb and lc comes nearly exclusively from formation of propylene. At higher temperatures under our experimental conditions, the main prodhcts in addition to propylene were ethylene, methane, propyne, acetylene and allene, etc., which were analyzed with the same method as pre-
viously described [ 71. The concentrations of ethylene, methane, propyne, acetylene and allene produced at the earlier stage (within 5 ps after decay of the Schlieren signal) may be much less than those of cyclopropane and propylene under our experimental conditions: The calculation with an assumed mechanism and rate constant expressions showed that the concentrations of ethylene, methane, propyne, acetylene and allene were below 1% of the initially produced propylene concentration. The extinction coefficients of these species were smaller than that of propylene. The isomerization is also considered to be considerably faster than the decomposition [ 51. It was assumed, therefore, that the absorption intensity change within 5 us after decay of the Schlieren signal came only from cyclopropane and propylene. The first-order rate constant k was calculated from the initial slope of the laser-absorption profiles with the extinction coefficients shown in fig. 2. We obtained the Arrhenius equation k=4.60x
1014 exp( -62.5 kcal/RT) s-l
over the temperature range 1loo-1450 K and over the pressure range O-4-5.4 atm. Both mixtures give the same k versus temperature dependence. The rate constants k obtained from the laser-absorption profiles did not show the transition over the tempera213
Volume 141. number 3
CHEMICAL PHYSICS LETTERS
lU3 '
\y \\
7,o
I
8.0
1200
T/K
I
I
9,O
IO4 K/T Fig. 3. Arrhenius plots for the rate constant of isomerization of cyclopropane to propylene. A: 5% cyclopropane behind incident shock waves; 0: 5% cyclopropane behind reflected shock waves; l: 10% cyclopropane behind reflected shock waves; - - -: reported by Falconer et al. [ 81; - - -: reported by Chambers and Kistiakowsky [ 91; - - - -: reported by Lewis et al. [ 41.
ture range 11OO-1450 K reported in the single-pulse shock tube studies [ 1,3]. The values of k obtained are similar to the extrapolated ones of the Arrhenius lines at low temperatures reported by Falconer et al. [ 81 and Chambers and Kistiakowsky [ 91 as shown in fig. 3. Lewis et al. [ 41 have described that the rate constant obtained by them appears quite consistent with extrapolations of lower temperature data and
214
6 November 1987
that the rate constant does not show a transition over the temperature range 1146-14 16 K, monitoring density gradients in the reacting gases via the laserbeam deflection method. Our results are also in agreement with their results. Lewis et al. [ 4 ] have also reported the rate constant over the temperature range 1041-l 198 K and over the pressure range 3885-5090 Torr using the comparative-rate singlepulse shock-tube method. Our values of k are also consistent with those reported by them as shown in fig. 3. However, the activation energy E=62.5 kcal/ mol and the values of k obtained in this work are somewhat smaller than those of k and E reported at low temperatures [ 8,9]. The isomerization of cyclopropane to propylene under our experimental conditions is inferred to be close to the first-order limit.
References [ 1 ] H. Miyama and T. Takeyama, Bull. Chem. Sot. Japan 38 (1965) 2189. [ 21 J.N. Bradley and M.A. Frend, Trans. Faraday Sot. 67 (197 1) 72. [3] J.A. Barnard, A.T. Cocks and R.K.-Y. Lee, J. Chem. Sot. Faraday Trans. 170 (1974) 1782. [4] D.K. Lewis, SE. Giesler and M.S. Brown, Intern. J. Chem. Kinetics 10 (1978) 277. [S] Y. Hidaka, T. Chimori and M. Suga, Chem. Phys. Letters 119 (1985) 435. [ 61 Y. Hidaka, T. Kataoka and M. Suga, Bull. Chem. Sot. Japan 47 (1974) 2166. [ 71 Y. Hidaka, S. Shiba, H. Takuma and M. Suga, Intern. J. Chem. Kinetics 17 (1985) 441. [ 81 W.E. Falconer, T.F. Hunter and A.F. Trotman-Dickenson, J. Chem. Sot. ( 196 1) 609. [ 91 T.S. Chambers and G.B. Kistiakowsky, J. Am. Chem. Sot. 56 (1934) 399.
CHEMICAL PHYSICS LETTERS
6 November 1987
ISOMERIZATION OF CYCLOPROPANE TO PROPYLENE IN SHOCK WAVES Yoshiaki HIDAKA and Takashi OIU Department of Chemistry, Faculty of Science, Ehime University, Bunkyo-rho, Matsuyama 790, Japan
Received 5 June 1987; in final form 10 August 1987
Cyclopropane mixtures diluted with argon were heated to temperatures in the range 1loo-1450 K behind incident and reflected shock waves. The rate of isomerization of cyclopropane to propylene was measured by tracing the time variation of absorption at 3.39 pm. The rate constant expression k=4.60~ 10 14exp( -62.5 kcal/RT) s-’ was determined, in accord with extrapolation from lower-temperature experiments.
1. Introduction
2. Experimental
The isomerization of cyclopropane to propylene at low temperatures (below 1100 K) has been stu$ied by many workers and is extensively used as a test for theories of unimolecular reaction rates. The studies at high temperatures (above 1100 K) have been carried out with a single-pulse shock tube technique [l-4]. The isomerization rate constants k reported at the high temperatures have been lower than Arrhenius lines extrapolated from results reported for low temperatures and also showed a sharp transition at about 1200 K [ 1,3]. When cyclopropane was heated to temperatures above 1200 K, species other than propylene (methane, ethylene, acetylene, etc.) were produced in appreciable amounts during the dwell time [ 11. Because the formation mechanism of these species is complex and still vague, estimation of k using a single-pulse technique above 1200 K may lead to erroneous results. The direct measurement of cyclopropane and propylene concentrations in the earlier stage in the cyclopropane pyrolysis, however, would be more useful for the estimation of k at temperatures above 1200 K, because isomerization may be considerably faster than decomposition [5]. In this paper, we report the rate constant k over the temperature range 1 loo-1450 K estimated with a time-resolved technique.
The shock tube employed in this study has previously been described [ 61. The time variations of cyclopropane and propylene concentrations were traced using the laser absorption apparatus previously described [7]. The transmitted intensities of a 3.39 pm He-Ne laser beam through a 4.1 cm pathlength in the shock tube and through an interference filter (A,,= 3.39 pm, half-width=0.072 ,um) were observed with Fujitsu IV-2OOC4 InSb detector to learn the time variation of C3H6. The gas compositions employed were 5 and 1OWcyclopropane diluted with argon. The argon and cyclopropane, specified as 99.99% and above 99Ohpure, respectively, were obtained from commercial cylinders. The cyclopropane was purified by trap-to-trap distillation before use.
212
3. Results and discussion Thermal isomerization of cyclopropane to propylene was studied over the temperature range 1loo-1450 K and over the pressure range 0.4-5.5 atm. Typical oscillograms of laser absorption at 3.39 km are shown in fig. 1. At temperatures below 1100 K, the absorption intensity after the Schlieren spike remains constant, as seen in fig. 1a. At temperatures between 1100 K and 1350 K, the absorption intensity increases slowly with time as seen in fig, 1b. The
0 009-2614/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 141, number 3
CHEMICAL PHYSICS LETTERS
L
6 November 1987
I
Ilo
I
I
I
En
I l5ul
T/K Fig. 2. Comparison of the extinction coefficients of cyclopropane and propylene. 0: cyclopropane, l: propylene.
Fig. 1. Typical oscillogram of laser absorption at 3.39 pm with a mixture of 5% cyclopropane and 95% Ar. The sweep rate is 50 @div. (a) 1114 K, (b) 1257 K, (c) 1400 K.
rate of the increasing absorption increased with increasing the temperature. Above 1350 K, the absorption intensity increases with time, arrives to maximum value, and then decreases with time as seen in fig. lc. The extinction coefficient of propylene, which was determined with a 5% propylene, 95% Ar mixture, is larger than that of cyclopropane as shown in fig. 2. It is considered, therefore, that the slow increase seen in figs. lb and lc comes nearly exclusively from formation of propylene. At higher temperatures under our experimental conditions, the main prodhcts in addition to propylene were ethylene, methane, propyne, acetylene and allene, etc., which were analyzed with the same method as pre-
viously described [ 71. The concentrations of ethylene, methane, propyne, acetylene and allene produced at the earlier stage (within 5 ps after decay of the Schlieren signal) may be much less than those of cyclopropane and propylene under our experimental conditions: The calculation with an assumed mechanism and rate constant expressions showed that the concentrations of ethylene, methane, propyne, acetylene and allene were below 1% of the initially produced propylene concentration. The extinction coefficients of these species were smaller than that of propylene. The isomerization is also considered to be considerably faster than the decomposition [ 51. It was assumed, therefore, that the absorption intensity change within 5 us after decay of the Schlieren signal came only from cyclopropane and propylene. The first-order rate constant k was calculated from the initial slope of the laser-absorption profiles with the extinction coefficients shown in fig. 2. We obtained the Arrhenius equation k=4.60x
1014 exp( -62.5 kcal/RT) s-l
over the temperature range 1loo-1450 K and over the pressure range O-4-5.4 atm. Both mixtures give the same k versus temperature dependence. The rate constants k obtained from the laser-absorption profiles did not show the transition over the tempera213
Volume 141. number 3
CHEMICAL PHYSICS LETTERS
lU3 '
\y \\
7,o
I
8.0
1200
T/K
I
I
9,O
IO4 K/T Fig. 3. Arrhenius plots for the rate constant of isomerization of cyclopropane to propylene. A: 5% cyclopropane behind incident shock waves; 0: 5% cyclopropane behind reflected shock waves; l: 10% cyclopropane behind reflected shock waves; - - -: reported by Falconer et al. [ 81; - - -: reported by Chambers and Kistiakowsky [ 91; - - - -: reported by Lewis et al. [ 41.
ture range 11OO-1450 K reported in the single-pulse shock tube studies [ 1,3]. The values of k obtained are similar to the extrapolated ones of the Arrhenius lines at low temperatures reported by Falconer et al. [ 81 and Chambers and Kistiakowsky [ 91 as shown in fig. 3. Lewis et al. [ 41 have described that the rate constant obtained by them appears quite consistent with extrapolations of lower temperature data and
214
6 November 1987
that the rate constant does not show a transition over the temperature range 1146-14 16 K, monitoring density gradients in the reacting gases via the laserbeam deflection method. Our results are also in agreement with their results. Lewis et al. [ 4 ] have also reported the rate constant over the temperature range 1041-l 198 K and over the pressure range 3885-5090 Torr using the comparative-rate singlepulse shock-tube method. Our values of k are also consistent with those reported by them as shown in fig. 3. However, the activation energy E=62.5 kcal/ mol and the values of k obtained in this work are somewhat smaller than those of k and E reported at low temperatures [ 8,9]. The isomerization of cyclopropane to propylene under our experimental conditions is inferred to be close to the first-order limit.
References [ 1 ] H. Miyama and T. Takeyama, Bull. Chem. Sot. Japan 38 (1965) 2189. [ 21 J.N. Bradley and M.A. Frend, Trans. Faraday Sot. 67 (197 1) 72. [3] J.A. Barnard, A.T. Cocks and R.K.-Y. Lee, J. Chem. Sot. Faraday Trans. 170 (1974) 1782. [4] D.K. Lewis, SE. Giesler and M.S. Brown, Intern. J. Chem. Kinetics 10 (1978) 277. [S] Y. Hidaka, T. Chimori and M. Suga, Chem. Phys. Letters 119 (1985) 435. [ 61 Y. Hidaka, T. Kataoka and M. Suga, Bull. Chem. Sot. Japan 47 (1974) 2166. [ 71 Y. Hidaka, S. Shiba, H. Takuma and M. Suga, Intern. J. Chem. Kinetics 17 (1985) 441. [ 81 W.E. Falconer, T.F. Hunter and A.F. Trotman-Dickenson, J. Chem. Sot. ( 196 1) 609. [ 91 T.S. Chambers and G.B. Kistiakowsky, J. Am. Chem. Sot. 56 (1934) 399.