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M. UCHIYAMA, T. TOMIOKA, AND A. AMANO
Varian A-60 spectrometer. Samples were allowed to equilibrate for 10 min. in the probe after which time the temperature remained a t 35.9 f 0.3' as measured by a thermocouple in the sample tube. The spectra of samples of concentration less than 0.05 M were obtained by repeated timed integration at various con-
stant field strengths. The internal reference, 0,2-0.5% tetramethylsilane, was added to the solvent prior to itsuse.
Acknowledgment. We thank Dr. James Resh for the computer programs used in the calculations.
Thermal Decomposition of Cyclohexene
by Masao Uchiyama, Tadao Tomioka, and Akira Amano The Department of Applied Chemistry, Tohoku University, Sakura-.koji,Sendai, J a p a n
(Received February 7, 1964)
Thermal deconiposition of cyclohexene was studied in a flow system a t a pressure of 25 min., temperatures ranging from 814 to 902"K., and contact times ranging from 0.57 to 5.4 sec. The main reaction is a simple uniinolecular splitting to form ethylene and butadiene with selectivity of no less than 96%. From the temperature dependence of the first-order rate constants, values of the A-factor and activation energy are assigned to be 1.5 X 1OI5 sec. - l and 66 kcal./niole, respectively. These values are consistent with those of the reverse reaction and also with the assumption of a six-membered cyclic activated complex. It is further suggested by calculation that the secondary dimerization of butadiene is not a serious cause of a small stoichiometric deficiency of butadiene as compared with the amount of ethylene.
Introduction The predominant mode of thermal decomposition of cyclohexene a t moderately high temperatures (400850') has been known as a simple unimolecular splitting to form ethylene and butadiene.'V2 I n spite of the apparent simplicity of the reaction, there has been disagreement on the values of kinetic parameters. According to Kiichler3 (485-565') and to Kraus, Vavrugka, and Bazant4 (750-850'), the first-order rate constant could be expressed by the following equations: log k (sec.-l) = 12.776 - 57,500/4.575T and log k = 12.133 - 55,100/4.575T, respectively. Smith and Gordon6 (425-535'), on the other hand, have recently determined the rate constant to be expressed by log k = 17.138 - 72,700/4.575T. There is thus a 10b-fold variation in A-factor and a 17.6 kcal./ mole difference in activation energy. The disagreement such as mentioned above is obviously beyond any The Journal of Physical Chemistry
conceivable experimental uncertainties. As has been pointed out by Benson,6in the light of modern therinodynamic data, none of the kinetic expressions quoted above is compatible with kinetic parameters assigned for the reverse reaction by Rowley and Steiner.' In the present study, the rate of the reaction is measured a t intermediate temperatures which have not (1) N. D. Zelinski!, B. M. Mikhallov. and Yu. A. Arbuzov, J . Gem. Chem. U S S R , 4, 856 (1934). (2) F. 0. Rice, P. M. Ruoff, and E. L. Rodowskns, J . Am. Chem. SOC.,60, 955 (1938). (3) L. KPtchler, Trans. Faraday SOC.,35, 874 (1939). (4) M. Kraus, M .Vavrugka, and 1 ' . Bazant, Collection Czech. Chem. Commun., 22, 484 (1957). (5) S. R. Smith and A. S. Gordon, J . Phys. Chem., 65, 1124 (1961). (6) S. W. Benson, "The Foundations of Chemical Kinetics," McGraw-Hill Book Go., Inc., Ken, York, N. Y., 1960, pp. 259. (7) D. Rowley and H. Steiner, Discussions Faraday Soc., 10, 198 (1951).
THERMAL DECOMPOSITIOTS OF CYCLOHEXESE
been covered in the previous works. The A-factor and activation energy a,re evaluated, and the thermodynamic consistency of the values is then examined with reference to thiose for the reverse reaction and also to the structure of the activated complex. The limitation of the experimental evaluation of the kinetic parameters is then discussed in connection with some possible complicatioiis expected in the reaction, i.e , the secondary polymerization of butadiene.
Experimental A conventional flow system operated under atmospheric pressure was employed. Details of the apparatus and procedures have been described in our previous communication.8 Definitions of effectivle reaction temperature and the corresponding reactor volume have also been given in the same communication. Reactant cyclohexene, stored in a saturator bottle immersed in a,n ice bath, was introduced into a reaction vessel a t it5 vapor pressure of approximately 25 mm. with a stream of helium. The flow rate of the latter was precisely controlled for residence times of the reactant vapor in the reaction vessel ranging from 0.57 to 5.4 sec. The reaction vessel of approximately 8-ml. free volume was made of a quartz cylinder. Cyclohexene was prepared by the vapor phase dehydration of cyclohexanol a t 300' using activated alumina as a catalyst, and was used after being fractionated and dried over magnesium perchlorate. Cylinder helium of 99.9% purity was used without further purificg'it'ion. Analyses were carried out by gas--liquid chromatography. A dirnethylxulfolane column 3 ni. in length was used at, 40' for the determination of ethylene, butadiene, and unreacted cyclohexene. Used for the identification of by-]product gases and liquids were a 15-m. acetonylacetone column and a 5-ni. polyethylene glycol column, respectively. Hydrogen and methane were not analyzed in the present study.
Results and Discussion Product Distributaons. The main products were ethylene and butadiene in accordance with the equation, G H I O C2H4-t C4H6,which accounts for more than 96Y0 of the cyclohexene pyrolyzed. The amount of butadiene formed was, however, consistently less than that of ethylene by a'few per cent. Possible causes for the small deficiency of butadiene will be discussed later. Other products include ethane (0.5%), propane (0.5%), propylene (1,5%), butenes (trace), benzene (0.3oj,), 1,3-cyclohexadiene (3%), 1,4-cyclohexadiene (trace), and cyclohexane (trace). The distributions rnentioned above, taken from a typical
-
1879
run whose conversion was limited to about 7Y0,are in agreement with a general conclusion drawn by the previous investigators. Kinetics. Summarized in Table I are rate data used for calculation of kinetic parameters. The firstorder rate constant can be evaluated from the contact time ( t ) and the conversion of cyclohexene to ethylene ( 2 ) based on the following integral equation: kt = -In (1 - z). As listed in the fourth colunin of Table I, sufficiently constant values are obtained a t each temperature to assure the proposed first-order rate law. Table I : Thermal Decomposition of Cyclohexene Temperature, OK.
Contact time, sec.
Ethylene, mole 7'
k X 108,
814 829 829 858 858 859 879 879 879 879 879 879 879 879 899 899 899 900 900 902
5.42 2.95 519 2.55 1.81 5.11 1.02 1.59 2.15 3.75 1.89 1.21 0.93 0.66 1.74 1.34 3.34 0.77 4.90 0.57
2.00 2.20 3.71 7.31 4.70 13,42 7.59 11.22 13.06 23.8 10.81 8.00 5.67 4.05 29.0 21.5 37.1 10.62 61.7 8.38
3.63 7.34 7.33 29.9 26.3 28.2 77.2 74.6 65.0 72.4 60.4 69.1 60.7 62.1 197.0 180.1 138,7 145,7 195,8 152.7
880. -1
Arrhenius plots of the first-order rate constants, illustrated in Fig. 1, correspond to the values of A-factor and activation energy of l O I 5 . l 6 see.-' and 66.2 kcal./ mole, respectively. Results of the rate measurement reported by the previous investigators are also illustrated in Fig. 1 for comparison. It can be clearly seen that the plots representing the results obtained by Smith and Gordon5 lie on the line extrapolated froin those of the present study. In excellent agreement with the values mentioned above, the A-factor and activation energy from these two independent studies, covering approximately a 200' temperature interval, are calculated by the method of least squares to be l O l 5 . I 4 sec. -l and 65.9 kcal. /mole, respectively. The values reported by Smith and Gordon were, however, (8) A. Amano and ?VI. Uchiyama, J . Phys. Chem., 67, 1242 (1963).
Volume 6 8 , .Vumber 7
July, 1964
1880
M. UCHIYAMA, T. TOMIOKA, AND A . AMANO
T,*C
800
700
600
.
500
I
I
I A
a
b
Figure 2. Two forms of activated complex: (a) cyclic complex, ( b ) open-chain complex.
I / T x IO? O K " Figure 1. Arrhenius plots for the rate of decomposition of cyclohexene. Filled circles, present study; empty circles, Smith, et al.; circles with dot,, Kraus, et al.; filled triangle, Rice, et al.. Lines A, B, and C correspond to t'he values of kinetic parameters reported by Kraus, et al., Kiichler, and Smith, et al., respectively.
set.-' and 72.7 kcal./mole as indicated by the solid line C in Fig. 1. We are unable to trace the reason for
the contradictory assignment of the kinetic parameters. Activated Complex. The A-factor obtained above corresponds to 8.0.3 ~ a l . / m o l e - ~ Kin. entropy of activation at 600°K. based on the following relation: A = (tdT/h)e"S*/R, where the transmission coefficient The entropy of activation ( K ) is assumed to be unity. also relates with t.he structure of the activated complex, and can be estimated according to the forniulations in statistical thermodynamics if niechanical properties of the given complex are known with reasonable accuracy. The structure of the complex in this case may be represented by the two isomeric fornis as illustrated in Fig. 2. The values of total entropy at 600°K. are computed as 108.8 cal.,/mole-°K. and 123.3 cal.1 Inole-OK. for cyclic and open-chain complexes, respectively. The same frequency assignment as used by Rowley and Steiner7 is adopted in the calculations, except that the symmetry number of the cyclic coniplex The Journal of Physical Chemistry
is taken as 2 and t,hat the electronic degeneracy is considered for the open-chain complex. Similar calculations give 100.5 cal./mole-OK. for the entropy of cyclohexene at 600°K., in excellent agreement with the . ~ puts reported value of 100.04 ~ a l . / m o l e - ~ KThis the entropy of activation of 8.3 cal./mole-"K. for t'he case of the cyclic complex, a value sufficiently close to our experimental finding. Therefore, of the two forms t'he cyclic complex (Fig. 2a) must be preferred. Equilibrium System: Cyclohexene 5 Ethylene iButadiene. Rowley and Steiner? obtained the values of A-factor and activation energy for the reverse reaction to be l O 7 . I 9 l./mole-sec. and 27.5 kcal./mole, respectively. These figures have recently been supported by Skinner and Sliepcevich.lo From the four kinetic parameters discussed so far, the values of A S o and A H o for the reversible system can be calculated as being 44.7 cal./mole-OK. and 40.1 kcal./mole a t 800"K., respectively. They are in excellent' agreement with A S O R o o = 45.80 cal./inole-OK. and AHOsoo = 40.75 kcal./mole deduced from the API compilation of thermodynamic data.ll Thermodynamic consistency is therefore observed between the independent, studies of the rates of the forward and backward reactions. Dimerization of Butadiene. It has already been pointed out that, the amount of butadiene produced is always less than that of ethylene. This may be attributed to an extremely high reactivity of butadiene at the temperatures studied. The thermal reaction of butadiene has been reported by Kistiakowsky and Ransom1* a t temperatures beheen 173 and 387" and by Rowley and Steiner' at temperatures between 445 and 644O, and was shown to produce 4-vinylcyclohexene. Thus, the reaction of cyclohexene can be (9) M. B. Epstein, K. S. Pitzer, and F. D. Rossini, J . Res. Natl. Bur. Std., 42, 379 (1949). (10) J. L. Skinner and C. M. Sliepcevich, Ind. Eng. Chem. Fundamentals, 2 , 168 (1963). (11) F. D. Rossini, et al., "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds," Carnegie Press, Pittsburgh, Pa., 1953. (12) G. B. Kistiakowsky and W. FV. Ransom, J . Chem. Phys., 7 , 725 (1939).
THERMAL DECOMPOSITION OF CYCLOHEXENE
described more precisely by the following consecutive scheme. I.
cyclohexene
ethylene 2
+ butadiene
3
2butadiene
4-vinylcyclohexene 4
The rate of the change in the concentrations of cyclohexene and butadiene can be expressed by the following differential equations
- d(cyclohexene)/dt
== kl(cyclohexenle) k2(butadiene)[ (cyclohexene)o - (cyclohexene)1, d(butadiene)/dt = kl(cyclohexene) kz(butadiene)[ (cyclohexene)o - (cyclohexene)] k3(butadiene)2 k4[(cyclohexene)o(cyclohexene) - (butadiene) 1
+
where the subscript zero denotes the initial concentration. In the above equations, the values of kl and kz have already been discussed, and those of k3 and k4 were evaluated as follows
1881
log k3 (l./mole-sec.) log k4 (set.-')
=
=
8.14 - 26,800/4.5757'
15.70 - 61,800/4.5755" l 3
Unfortunately the above simultaneous differential equations cannot be solved explicitly, and the integration has been performed numerically following the method proposed by Runge. According to the calculations, only about 3% of the butadiene formed would dimerize to form 4-vinylcyclohexene a t temperature 477O, contact time 1000 sec., and initial concentration of cyclohexene 1.2 X 10-3 niole/l. Under the same reaction conditions, however, Smith and Gordon5 observcd that the deficiency of butadiene amounted to 28% of the latter. The trial calculations mentioned would clearly indicate that the deficiency has to be explained by some reactions other than the secondary dimerization of butadiene. Acknowledgmenl. The authors wish to thank Professor Hiroshi Tokuhisa of the Tohoku University for helpful suggestions and also Mr. Takeo Iwania for assistance in the experimental work. (13) N. E. Duncan and C . J. Jana, J . Chem. Phys., 20, 1644 (1952).
Volume 68,Number 7
J u l y , 1964
