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TCAL C H E M I S Registered in U.5.Palsnt Ofice @ Copyright, 1971, by the American Chemical Society
VOLUME 75, NUMBER 1 JANUARY 7,1971
Shock Tube Experiments on the Pyrolysis of Deuterium-Substituted Ethylenes by Gordon B. Skinner,* Ronald C. Sweet, and Steven K. Davis Department of Chemistry, Wright Bate University, Dayton, Ohio
(Received February 16,1970)
Publication costs assisted by the Petroleum Research Fund
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Dilute mixtures of CZH4, cZH4 CzD4, trans-1,2-CzHzDz,and CzHz DZin argon were pyrolyzed in a singlepulse shock tube. Empirically, in ethylene pyrolysis the order with respect to argon mas 0 f 0.2, and that with respect to ethylene 1.2 f 0.2. Pyrolysis data are summarized in terms of first-order (in ethylene) rate constants as follows: 8.9 x lO8e-463400’RT sec-1 for 0.1% CzH4, 1.05 X 1 0 Q e - 4 8 ~ ssec-1 u u ~ ~for ~ 0.05% CzH4 0.057l, CzD4and 0.1% C2HzDz,and 1.8 x 109e-4Qoo’Er sec-’ for 0.25% CzH4 0.25% CzD4 and 0.5% C~HZDZ, st 3 atm total reaction pressure, and 1100-1500°K. The isotopic distribution of ethylene after reaction indicated that isotope exchange is nearly first order in ethylene, the rate constant for the step producing CzHsD and CzHD3from C2H4and CzD4 being 3.4 x 107e-Sz3°u0/RTsec-I. Rates of subsequent steps are governed by the statistics of the reactions. Isotope distributions of acetylene and hydrogen (the chief decomposition products of ethylene) and of ethylene produced by hydrogenation of acetylene indicate that both ethylene decomposition and acetylene hydrogenation occur by freeradical rather than molecular mechanisms. Isotope exchange reactions were considerably faster than either ethylene decomposition or acetylene hydrogenatioii.
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Introduction During the past 10 years several experimental studiesl-&of ethylene pyrolysis have been made using the shock tube technique. The temperature range covered has been 1150-2250°K. I n all cases the ethylene was diluted with an inert gas, the partial pressure of ethylene ranging from 8.02 to 1 atm during the reaction. There is general agireement that under these conditions ethylene decomposes primarily to acetylene and hydrogen, and that the rate of the reaction shows approximately first-order dependence on the ethylene concentration. Gay, Kern, Kistiakowsky, and Niki4 included some experiments with deuterium-substituted ethylene, while Bauer6 has reported on the reaction between ethylene and deuterium. However, all t h e s e experimental results have been insuEcient to establish what the mechanism of ethylene pyrolysis is. Benson and Haugen’ made a kinetic analysis of the reaction, concluding that a free-radical chain should predominate in the lower part of the temperature range, while elimination of molecular hydro-
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gen should predominate a t higher temperatures. They made an effort to correlate all available data, both kinetic and thermodynamic, in their analysis, but did not predict the temperature dependence of the pyrolysis reaction very well. It seemed highly desirable to carry out further pyrolysis experiments using deuterium-substituted ethylenes. The product distributions from these experiments can provide direct evidence as to the type of reaction occurring (free radical or molecular) and also provide addi-
(1) G. B. Skinner and E. M. Sokoloski, J . Phys. Chem., 64, 1028 (1960). ( 2 ) T. Asaba, K. Yoneda, and T. Hikita, Kogyo Kagaku Zassh?:,65, 1811 (1962). (3) G. I. Kozlov and V. G. Knorre, Kinet. Katal., 4, 189 (1963). (4) 1. D. Gay, R. D. Kern, G. B . Kistiakowsky, and H. Niki, J . Chem. Phys., 45, 2371 (1966). (5) J. B . Homer and G. B. Kistiakowskg, ibid., 47, 5290 (1967). (6) S. H. Bauer, Symp. (Int.) Combust. Proc., 11, 105 (1967). (7) S. W. Benson and G. R. Haugen, J . Phys. Chem., 71, 1735 (1967).
2 tional data of a kind not previously available for testing proposed reaction mechanisms. Study of the hydrogenation of acetylene is another approach to the understanding of this reaction system. Skinner and Sokoloski' carried out some hydrogenation experiments, and Benson and Haugen' included them in their kinetic analysis. Kuratani and Bauer8studied the reaction of CzHzwith Dz in terms of the exchange reaction, but reported only a cursory look, with essentially negative results, into the formation of ethylenes. Accordingly, it seemed useful to study the isotope distribution of ethylenes produced by this reaction.
Experh e ntal See tion A single-pulse shock tube of the general nature of that described by Lifshitz, Bauer, and Reslerg was used. The reaction section was 1.8 m long and 3.9 cm in diameter, with an expansion tank of 110-1. capacity attached by a short length of 3.9-em pipe a t a point 15 cm downstream from the diaphragm. The driver section was also 3.9 cm in diameter and of variable length, but a typical length was about l.S m. For measuring the incident shock speed, SLR!I Model 603 pressure transducers spaced 75 cm apart! near the downstream end of the reaction section were used. A third transducer could be mounted midway between these two to permit measurements of shock attenuation. Transducer outputs were amplified by identical circuits and used to start and stop a microsecond timer and also to start an oscilloscope. This recorded the pressure, by means of mother SLM transducer mounted 5 cm from the downstream end of the reaction section. Directly opposGte the transducer was a valve through which samples could be drawn for analysis. The temperature (Tb) and pressure (Pb) behind the reflected shock wave 8,s it began to move away from the end plate of the Lube were calculated from the shock speed, corrected for the attenuation observed in calibration experiments. The pressure record showed that the pressure immediately behind the reflected shock wave was within 2% of that calculated from the speed of the incident wave (allowing for attenuation). For the first millisecond after passage of the reflected wave the pressure rose BlOWly to about 10% above its initial value, and subsequently fell slowly back to near its initial value The pressure drop is probably due to some loss of gas into the expansion tank. Typical dwell times were 5 msec, followed by a rapid expansion at a typical initial rate of 3 X 105'K per second. The cooling part of the process would contribute approximately 2% of the totai reaction for an activation energy of 50 kcal, and 12% for 20 kcal, in our temperature range. To allow for these pressure changes, it was assumed that TS and Po were actually realized just behind the reflected shock wave, and that subsequent pressure The Journal of Ph.ysiccal Chemistru, Vol. 76, -Vo. 1, 1971
G. B. SKINNER, R. C. SWEET,AND S. I
