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Thermal decomposition of isobutane in shock waves. Rate constant of initiation reaction
Volume 144, number 5,6
CHEMICAL PHYSICS LETTERS
11 March 1988
THERMAL DECOMPOSITION OF ISOBUTANE IN SHOCK WAVES. RATE CONSTANT OF INITIATION REACTION Yoshiaki HIDAKA, Minoru FUJIWARA, Takashi OKI and Hiroyuki KAWANO Department of Chemistry, Faculty ofscience, Ehime University,Bunkyo-eho, Matsuyama 790, Japan Received 30 November 1987; in final form 8 January 1988
Isobutane mixtures diluted with argon were heated to temperatures in the range 1000-l 560 K at pressures of 1.3-2.9 atm behind reflected shock waves. IR laser absorption measurements at 3.39 Km were carried out in order to learn the consumption rate of isobutane. The rate constant expression k, = 4.5 x lOI exp( - 8 1 kcal/RT) s- ’with error limits of + 50% for the initiation reaction was estimated from the initial slope of the laser absorption profiles.
1. Introductioii The thermal decomposition of isobutane has been studied by several workers [ l-61 and rate constants k, for the initiation reaction i-C4H10+i-C3H, +CH,
(1)
were reported. Rate constants kl at low temperatures (below 1200 K have been reported by Brooks [ 11, Konar et al. [ 21, Golden et al. [ 31 and Pratt and Rogers [ 41. Though the activation energies reported are in fairly good agreement, the rate constant values show a considerable scatter. The rate constants k, at high temperatures have been reported by Bradley [ 51 and Koike and Morinaga [ 61. Bradley studied the pyrolysis in a single-pulse shock tube over the temperature range 1200-l 500 K at total pressures of 190-690 Torr and obtained k, = 8.7 x lo9 exp( - 48.4 kcal/RT) s-‘. Koike and Morinaga traced an intermediate species CH3 produced in isobutane pyrolysis by UV absorption over the temperature range 1300-1800 K and about half atmospheric pressure andreported k,=6.3X 1012exp( -60.8 kcal/RT) s-l. There is a considerable disagreement between the two results obtained at high temperatures. The values of k, obtained at high temperatures are also smaller than extrapolations of the expressions of Golden et al. and Konar et al. even though the kl values at high temperatures are considered to be in the fall-off region under their conditions. Data at both high tempera570
tures and higher pressures would be required to clarify this point. The direct measurement of the reactant concentration using IR laser absorption should be useful for the estimation of kl in the thermal decomposition of isobutane at high temperatures. In this paper, we report the rate constant k, over the temperature range 11OO- 1560 K estimated with the IR laser absorption technique.
2. Experimental The shock tube employed in this study was described previously [7]. Time variations of the isobutane concentration were traced using the IR laser absorption apparatus described previously [ 8,9]. The transmitted intensities of a 3.39 pm He-Ne laser beam after a 4.1 cm pathlength in the shock tube and through an interference filter (&,,,=3.39 km, halfwidth=0.072 km) were observed with a Fujitsu IV2OOC4InSb detector to learn the time variation of the concentration of isobutane. The gas compositiqns employed were 1 and 2.5% isobutane diluted with argon. The argon and isobutane, specified as 99.99% and above 99.7% pure, respectively, were obtained commercially. The isobutane was purified by trap-to-trap distillation before use.
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume
144,number5,6
11March1988
CHEMICAL PHYSICSLETTERS
3. Results and discussion Thermal decomposition of isobutane ( i-C+H,o) was studied over the temperature range 1000-l 560 K and over the pressure range l-3-2.9 atm. A typical oscillogram of IR laser absorption at 3.39 pm is shown in fig. 1. At temperatures below 1100 K, the absorption intensity after the Schlieren-spike remained constant. At temperatures above 1100 K, the absorption intensity after the Schlieren-spike decreases with time as seen in fig. 1. The rate of decreasing absorption increased with increasing temperature. Assuming first-order behavior, the observed rate constant expression kob = 6.4 x 10L6 x exp( - 8 1 kcal/RT) s- ’ was calculated from the initial slope of the laser absorption profiles after the Schlieren spike. Both mixtures give the same k versus temperature dependence, as shown in fig. 2. The products under conditions similar to this experiment were examined behind reflected shock waves using the same gaschromatographic method as mentioned previously [ 81. It was found that the main products in addition to propylene were methane, ethylene, isobutene and ethane. A small amount of these species is inferred to be produced at an earlier stage. The IR absorption observed may include absorption by the products. These contaminations may result in an apparent lowering of the values of the estimated rate constant. The influence of these products upon the rate constant was examined using
Fig.1. Typicaloscillogram of IRlaserabsorptionat 3.39pmwith the mixture2.5%isobutane,97.5%Ar. Sweeprate is 50 @div. T,= 1504 K, p,=2.7 atm.
1500
1300
T/K
105 rl I VJ \
Y
104
lOi
7.0
8.0 lo6 K/T
Fig. 2. Arrhenius plots of the observed rate constant kobsobtained from the initial slope of the IR laserabsorptioncurvesand k, estimated.0,2.5% &butane;l,l .o% isobutane; - . -. - , k,.
the extinction coefficients of these species and a mechanism assumed for the isobutane pyrolysis. The influence upon the inferred rate constants was found to increase with increasing temperature. The error limits coming from the contamination were derived to be within +30%. It is considered that decomposition of isobutane at an earlier stage (within 1 ps after decay of the Schlieren signal) mainly comes from the initiation reaction (1) . Under our experimental conditions, however, part of isobutane would also be decomposed by the following reactions: i-GH,, +CH3 -t&H9 +CH4 ,
(2)
i-C,H,o +H+C4H9 +Hz ,
(3)
where C4H9 includes both i-C4H9 and t-C,H,. The values of k,, therefore, would be somewhat smaller than those of kobS.i-CsH7 would decompose to C2H4, CH, and C3H6, H by reactions i-C3H7-‘C2H4+CH3,
(4) 571
Volume 144, number 5,6
CHEMICAL PHYSICS LETTERS
and i-&H, +C3H6 i-H ,
(5)
respectively [ 2,4-61. C4H9would also decompose to C3H6, CH3 and C4H8, H by reactions C4H9 +C3H6 +CH3
(6)
and CdH9 -‘C4Hs tH
(7)
respectively [ 2,4-61. By using reactions (1 )-( 7) and the rate constant expressions reported for reactions (2)-(7) [2,4-61, the amount of CCdH1Odecomposition within 1 Ps was calculated. We obtained the Arrhenius equation k, = 4.5 x 10I6exp( - 8 1 kcal/RT) s-l with error limits of * 50% as the rate constant of reaction ( 1). The values of k, obtained are larger than those of Bradley [ 51 and Koike and Morinaga [ 61, but they are in good agreement with the extrapolated ones of the Arrhenius line at low temperatures reported by Konar et al. [ 21 and Golden et al. [ 31, as shown in fig. 3. Konar et al. [ 21 investigated T/K 1600
800
1000
11 March 1988
the formation of propane in static vessels in the ranges of 770 to 855 IS and 20 to 150 Torr and estimated the k, expression at infinite pressure. Golden et al. [ 31 studied the decomposition under VLPP conditions and estimated the high-pressure limit rate constant used in fig. 3 according to RRKM theory. The activation energy obtained is also close to those reported at low temperatures [ 2-41 and to the AH value derived using thermal data at 800 K [ 21. The values of k, obtained under our experimental conditions are inferred to be close to the fust-order limit. As mentioned above, a part of isobutane is decomposed by reactions (2) and (3). So, it is very interesting to see how the chain process intrudes. The amount within 1 ps was examined with the mixture 2.5% iCOHIO, 97.5% Ar by simulation. When both reaction (2) with k2= 1.0~ 1014exp( -6.79 kcal/RT) cm3 mol-’ s-’ [5] and reaction (3) with k3= 3.9x1014exp( -26.1 kcal/RT) cm3 mol-’ s-l [5] are omitted, the decomposition of isobutane at 1400 K and 2.4 atm decreases to 62.5% from 100%. Accordingly, the amount of chain decomposition by reactions (2) and (3) within 1 ps is 37.5Oh. The estimated values of k, depend on the values chosen for k2-k,. Simulations using various reported values of kz-k, indicated that the error limit of k, was within - 50 through + 10% because the reported values of the rate constants k,-k, are also scattered. This work was supported by a Grant-in-Aid for Science Research No. 62540338 from the Ministry of Education, Science and Culture.
References
\w \
(f)
-5
(d) \ I
I
6,O
I 1080
I
III 14,o
lo4 K/T Fig. 3. Comparison of our k, values with those reported for isobutane dissociation. (a) Bradley [ 5 1, (b) Koike and Morinaga [6],(c) Brooks [1], (d) Konaretal. [2],(e) Goldenetal. [3], ( f) Pratt and Rogers [ 41, and (g) this work.
572
[ I] C.T. Brooks, Trans. Faraday Sot. 62 (1966) 935. [2] R.S. Konar, R.M. Marshall and J.H. Pumell, Intern. J. Chem. Kinetics 5 (1973) 1005. [ 31 D.M. Golden, Z.B. Alfassi and P.C. Beedle, Intern. J. Chem. Kinetics 6 (1974) 359. [4] G.L. Pratt and D. Rogers, J. Chem. Sot. Faraday Trans. I 76 (1980) 1694. [ 51 J.N. Bradley, Prec. Roy. Sot. A337 (1974) 199. [ 61 T. Koike and K. Morinaga, Bull. Chem. Sot. Japan 55 (1982) 690. [ 71 Y. Hidaka, T. Kataoka and M. Suga, Bull. Chem. Sot. Japan 47 (1974) 2166. [8] Y. Hidaka, S. Shiba, H. Takuma and M. Suga, Intern. J. Chem.Kinetics 17 (1985) 441. [9] Y.HidakaandT.Oki,Chem.Phys.Lettersl41 (1987) 212.
CHEMICAL PHYSICS LETTERS
11 March 1988
THERMAL DECOMPOSITION OF ISOBUTANE IN SHOCK WAVES. RATE CONSTANT OF INITIATION REACTION Yoshiaki HIDAKA, Minoru FUJIWARA, Takashi OKI and Hiroyuki KAWANO Department of Chemistry, Faculty ofscience, Ehime University,Bunkyo-eho, Matsuyama 790, Japan Received 30 November 1987; in final form 8 January 1988
Isobutane mixtures diluted with argon were heated to temperatures in the range 1000-l 560 K at pressures of 1.3-2.9 atm behind reflected shock waves. IR laser absorption measurements at 3.39 Km were carried out in order to learn the consumption rate of isobutane. The rate constant expression k, = 4.5 x lOI exp( - 8 1 kcal/RT) s- ’with error limits of + 50% for the initiation reaction was estimated from the initial slope of the laser absorption profiles.
1. Introductioii The thermal decomposition of isobutane has been studied by several workers [ l-61 and rate constants k, for the initiation reaction i-C4H10+i-C3H, +CH,
(1)
were reported. Rate constants kl at low temperatures (below 1200 K have been reported by Brooks [ 11, Konar et al. [ 21, Golden et al. [ 31 and Pratt and Rogers [ 41. Though the activation energies reported are in fairly good agreement, the rate constant values show a considerable scatter. The rate constants k, at high temperatures have been reported by Bradley [ 51 and Koike and Morinaga [ 61. Bradley studied the pyrolysis in a single-pulse shock tube over the temperature range 1200-l 500 K at total pressures of 190-690 Torr and obtained k, = 8.7 x lo9 exp( - 48.4 kcal/RT) s-‘. Koike and Morinaga traced an intermediate species CH3 produced in isobutane pyrolysis by UV absorption over the temperature range 1300-1800 K and about half atmospheric pressure andreported k,=6.3X 1012exp( -60.8 kcal/RT) s-l. There is a considerable disagreement between the two results obtained at high temperatures. The values of k, obtained at high temperatures are also smaller than extrapolations of the expressions of Golden et al. and Konar et al. even though the kl values at high temperatures are considered to be in the fall-off region under their conditions. Data at both high tempera570
tures and higher pressures would be required to clarify this point. The direct measurement of the reactant concentration using IR laser absorption should be useful for the estimation of kl in the thermal decomposition of isobutane at high temperatures. In this paper, we report the rate constant k, over the temperature range 11OO- 1560 K estimated with the IR laser absorption technique.
2. Experimental The shock tube employed in this study was described previously [7]. Time variations of the isobutane concentration were traced using the IR laser absorption apparatus described previously [ 8,9]. The transmitted intensities of a 3.39 pm He-Ne laser beam after a 4.1 cm pathlength in the shock tube and through an interference filter (&,,,=3.39 km, halfwidth=0.072 km) were observed with a Fujitsu IV2OOC4InSb detector to learn the time variation of the concentration of isobutane. The gas compositiqns employed were 1 and 2.5% isobutane diluted with argon. The argon and isobutane, specified as 99.99% and above 99.7% pure, respectively, were obtained commercially. The isobutane was purified by trap-to-trap distillation before use.
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume
144,number5,6
11March1988
CHEMICAL PHYSICSLETTERS
3. Results and discussion Thermal decomposition of isobutane ( i-C+H,o) was studied over the temperature range 1000-l 560 K and over the pressure range l-3-2.9 atm. A typical oscillogram of IR laser absorption at 3.39 pm is shown in fig. 1. At temperatures below 1100 K, the absorption intensity after the Schlieren-spike remained constant. At temperatures above 1100 K, the absorption intensity after the Schlieren-spike decreases with time as seen in fig. 1. The rate of decreasing absorption increased with increasing temperature. Assuming first-order behavior, the observed rate constant expression kob = 6.4 x 10L6 x exp( - 8 1 kcal/RT) s- ’ was calculated from the initial slope of the laser absorption profiles after the Schlieren spike. Both mixtures give the same k versus temperature dependence, as shown in fig. 2. The products under conditions similar to this experiment were examined behind reflected shock waves using the same gaschromatographic method as mentioned previously [ 81. It was found that the main products in addition to propylene were methane, ethylene, isobutene and ethane. A small amount of these species is inferred to be produced at an earlier stage. The IR absorption observed may include absorption by the products. These contaminations may result in an apparent lowering of the values of the estimated rate constant. The influence of these products upon the rate constant was examined using
Fig.1. Typicaloscillogram of IRlaserabsorptionat 3.39pmwith the mixture2.5%isobutane,97.5%Ar. Sweeprate is 50 @div. T,= 1504 K, p,=2.7 atm.
1500
1300
T/K
105 rl I VJ \
Y
104
lOi
7.0
8.0 lo6 K/T
Fig. 2. Arrhenius plots of the observed rate constant kobsobtained from the initial slope of the IR laserabsorptioncurvesand k, estimated.0,2.5% &butane;l,l .o% isobutane; - . -. - , k,.
the extinction coefficients of these species and a mechanism assumed for the isobutane pyrolysis. The influence upon the inferred rate constants was found to increase with increasing temperature. The error limits coming from the contamination were derived to be within +30%. It is considered that decomposition of isobutane at an earlier stage (within 1 ps after decay of the Schlieren signal) mainly comes from the initiation reaction (1) . Under our experimental conditions, however, part of isobutane would also be decomposed by the following reactions: i-GH,, +CH3 -t&H9 +CH4 ,
(2)
i-C,H,o +H+C4H9 +Hz ,
(3)
where C4H9 includes both i-C4H9 and t-C,H,. The values of k,, therefore, would be somewhat smaller than those of kobS.i-CsH7 would decompose to C2H4, CH, and C3H6, H by reactions i-C3H7-‘C2H4+CH3,
(4) 571
Volume 144, number 5,6
CHEMICAL PHYSICS LETTERS
and i-&H, +C3H6 i-H ,
(5)
respectively [ 2,4-61. C4H9would also decompose to C3H6, CH3 and C4H8, H by reactions C4H9 +C3H6 +CH3
(6)
and CdH9 -‘C4Hs tH
(7)
respectively [ 2,4-61. By using reactions (1 )-( 7) and the rate constant expressions reported for reactions (2)-(7) [2,4-61, the amount of CCdH1Odecomposition within 1 Ps was calculated. We obtained the Arrhenius equation k, = 4.5 x 10I6exp( - 8 1 kcal/RT) s-l with error limits of * 50% as the rate constant of reaction ( 1). The values of k, obtained are larger than those of Bradley [ 51 and Koike and Morinaga [ 61, but they are in good agreement with the extrapolated ones of the Arrhenius line at low temperatures reported by Konar et al. [ 21 and Golden et al. [ 31, as shown in fig. 3. Konar et al. [ 21 investigated T/K 1600
800
1000
11 March 1988
the formation of propane in static vessels in the ranges of 770 to 855 IS and 20 to 150 Torr and estimated the k, expression at infinite pressure. Golden et al. [ 31 studied the decomposition under VLPP conditions and estimated the high-pressure limit rate constant used in fig. 3 according to RRKM theory. The activation energy obtained is also close to those reported at low temperatures [ 2-41 and to the AH value derived using thermal data at 800 K [ 21. The values of k, obtained under our experimental conditions are inferred to be close to the fust-order limit. As mentioned above, a part of isobutane is decomposed by reactions (2) and (3). So, it is very interesting to see how the chain process intrudes. The amount within 1 ps was examined with the mixture 2.5% iCOHIO, 97.5% Ar by simulation. When both reaction (2) with k2= 1.0~ 1014exp( -6.79 kcal/RT) cm3 mol-’ s-’ [5] and reaction (3) with k3= 3.9x1014exp( -26.1 kcal/RT) cm3 mol-’ s-l [5] are omitted, the decomposition of isobutane at 1400 K and 2.4 atm decreases to 62.5% from 100%. Accordingly, the amount of chain decomposition by reactions (2) and (3) within 1 ps is 37.5Oh. The estimated values of k, depend on the values chosen for k2-k,. Simulations using various reported values of kz-k, indicated that the error limit of k, was within - 50 through + 10% because the reported values of the rate constants k,-k, are also scattered. This work was supported by a Grant-in-Aid for Science Research No. 62540338 from the Ministry of Education, Science and Culture.
References
\w \
(f)
-5
(d) \ I
I
6,O
I 1080
I
III 14,o
lo4 K/T Fig. 3. Comparison of our k, values with those reported for isobutane dissociation. (a) Bradley [ 5 1, (b) Koike and Morinaga [6],(c) Brooks [1], (d) Konaretal. [2],(e) Goldenetal. [3], ( f) Pratt and Rogers [ 41, and (g) this work.
572
[ I] C.T. Brooks, Trans. Faraday Sot. 62 (1966) 935. [2] R.S. Konar, R.M. Marshall and J.H. Pumell, Intern. J. Chem. Kinetics 5 (1973) 1005. [ 31 D.M. Golden, Z.B. Alfassi and P.C. Beedle, Intern. J. Chem. Kinetics 6 (1974) 359. [4] G.L. Pratt and D. Rogers, J. Chem. Sot. Faraday Trans. I 76 (1980) 1694. [ 51 J.N. Bradley, Prec. Roy. Sot. A337 (1974) 199. [ 61 T. Koike and K. Morinaga, Bull. Chem. Sot. Japan 55 (1982) 690. [ 71 Y. Hidaka, T. Kataoka and M. Suga, Bull. Chem. Sot. Japan 47 (1974) 2166. [8] Y. Hidaka, S. Shiba, H. Takuma and M. Suga, Intern. J. Chem.Kinetics 17 (1985) 441. [9] Y.HidakaandT.Oki,Chem.Phys.Lettersl41 (1987) 212.