Thermal Reactions of 4=Vinylcyclohexene, Cyclohexene, and 4-Methylcyclohexene. Kinetics and Mechanism Tomoya Sakai,"' Toshiaki Nakatani, Natsuki Takahashi, and Taiseki Kunugi Department
01Synthetic
Chemistry, Faculty of Engineering, I'niuersity of Tokyo, Hongo, Tokyo, J a p a n
Thermal reactions of 4-vinylcyclohexene (VCH), cyclohexene (HCH), and 4-methylcyclohexene (MCH) were vinylcycloconducted at 530 to 665°C. The main primary products in molal quantities were: butadiene hexadiene from VCH; ethylene = butadiene cyclohexadiene from HCH; propylene = butadiene > methane = cyclohexadiene > ethylene cyclopentene > methylcyclohexadiene from MCH. The reverse Diels-Alder reaction and dehydrogenation occurred simultaneously, and their first-order rate constants were obtained for the respective feed stocks. It was suggested from the kinetic parameters for the formation of each product from MCH that the reverse Diels-Alder reaction proceeds via biradical intermediate at the high temperatures adopted. Our proposal on the reaction mechanism includes the rapid isomerization of the biradical intermediate before the second cleavage of the C-C bond. Kinetic analyses for HCH and VCH pyrolyses also supported the above scheme.
2
A kinetic study on thermal reactions of 4-vinylcyclohesene
(VCH), cyclohexene (HCH), and 4-methylcyclohesene ( X C H ) was carried out to obtain more accurate information on the rates of dehydrogenation and decomposition of these compounds to aromatics and olefins, respectively. + i n idea was confirmed by the authors (Sakai, et al., 1970) that VCH, HCH, and M C H are the important intermediate compounds to aromatics in pyrolysis of paraffinic hydrocarbons. Olefins, mainly ethylene, propylene, butenes, and butadiene, formed a t the initial stage of paraffin pyrolysis, are converted to cyclic compounds by the following reactions.
C+ll
scission
pi-* (3)
C+< -a
(Sakai, et al., 1971), and these cycloolefins, will be helpful in getting more complete understanding of the complex reaction scheme. Much effort has been exerted on the pyrolysis of HCH, but little has been done on VCH and MCH. Smith and Gordon (1961) pyrolyzed H C H a t 425 to 535"C, obtaining the conclusion that three parallel reactions took part in this reaction, two unimolecular reactions and one free radical reaction. Overall reaction can be formulated in first-order kinetics with the rate constant h.0 = 7.7 X 10'5 esp(-67,600/RT) sec-l. The rate constants for two unimolecular reactions are 121 = 1.4 X lo1' esp(-72,700/RT) and kl = 1.9 X 10lG exp(-71,20O/RT) sec-l. They proposed the formation of the c-c
-0
(+r-0^
>>
>>
(4)
(5)
Generally, these reactions proceed very fast at the conditions of pyrolysis. .inlong these, reactions I, 2, and 3 are 5-100 times faster than reactions 4 and 5. This is the reason why VCH, HCH, and M C H were adopted in this study. I n the case of gas-phase reactions, it is expected that discrete kinetic studies of several intermediate compounds of hydrocarbon pyrolysis, e.g., thermal reactions of ethylene (Kunugi, et al., 1969), propylene (Kunugi, et al., 1970a,b), benzene 1 Present address, Department of Chemical Reaction Engineering, Faculty of Pharmaceutical Sciences, Nagoya City University, llizuho-ku, Xagoya, Japan.
1
c
+
,cyclizatio; f r e e radical mechanism (31
0
+
H2
c1, cz, c3, c4, c5
biradical intermediate a t the first step of the reaction. This biradical then undergoes several scission and/or cyclization reactions. The bases of their arguments are (a) the coincidence in s,ctivation energies of reactions 1 and 2, (b) the fact that the hesadienyl biradical is rather stable and easily converted to cyclohexadiene, and (e) also the fact that the biradical intermediate can naturally be converted t o CI-C~ fragments through free radical mechanism. Cchiyama, et al. (1964), conducted the same reaction a t 541 to 629'C. Ethylene and butadiene \yere obtained a t a yield of mcre than 96%. First-order rate constant for the Ind. Eng. Chern. Fundarn., Vol. 1 1 , No. 4, 1972
529
Table I. The Thermal Reaction of VCH (Typical Data)
Temperature, OC Residence time, sec
530 0.58
530 0.90
550 0.18
Butadiene Vinylcyclohexadiene
4.66 0.19
6.91
3.93 0.20
550 1.02
570 0.32
570 0.87
585 0.21
585 0.90
14.75 0.35
65,40 2.89
Product yield, moles per 100 moles of feed
0.45
18.44 1.21
14.68 0.60
29.88 1.97
clarify the propriety of the two mechanisms mentioned above.
A similar situation exists in the liquid-phase Diels-hlder
L
I
a
0
I
I
l
10 20 30 40 Reoclion depth, mol of producl / 100 moles of feed
Figure 1. Distribution of product from VCH. 0 0 , 530°C; 0 550OC; $+, 570°C; 9 #, 585OC
+,
reaction about the biradical and circular activated complexes (Charton, 1966; Grieger and Eckert, 1970; Kwart and King, 1968; Seltzer, 1965; Stewart, 1971). It is, however, adequate for the present study to restrict mechanistic discussions to high-temperature gas-phase reactions. Detailed analysis of primary products, as well as kinetics for their formation by use of three different feedstocks, especially those for MCH, rendered it possible to postulate that the biradical mechanism fits better for the cleavage reaction, as was stated by Smith and Gordon (1961). However, dehydrogenation products such as cyclohexadienes were, in contrast to those of Smith and Gordon, not related with this biradical intermediate, but were formed through another simultaneous reaction. Experimental Section
-3
'
115
120
125
1000 / 1 ('K 1
Figure 2. Arrhenius plots (VCH)
reverse Diels-Alder reaction was reported to be k = 1.5 x 1015exp(-66,000/RT) sec-l. They stated that their rate equation also fitted the data obtained by Smith and Gordon. A shortage in the molal yield of butadiene, as compared to ethylene, is not explained clearly in this paper. However, the dimerization of butadiene to form VCH is not the cause of this shortage. Uchiyama, et al., used a circular activated complex rather than a linear biradical one, based on the comparison of activated entropies calculated for the two postulated complexes with that from their experiment. The calculation of activated entropies of the two postulated complexes was made by use of the bond frequency data by Rowley and Steiner (1951). Kuchler (1939) and Kraus, et al. (1956), found the firstorder rate constants for H C H pyrolysis to be k = 9.0 X lo1* exp(-57,500/RT) sec-l and k = 1.22 X 10l2 exp(-55,100/ R T ) sec-l, respectively. Tsang (1965) studied the reverse Diels-Aklder reaction of HCH, MCH, and VCH in a single pulse shock tube by use of the rate of dehydrogenation of hydrogen chloride from isopropyl chloride as the reference reaction. First-order rate constants were reported as kHCH = 101b.02 esp(-66,700/RT) sec-I, ~ M C H= 1015.'3exp(- 66,600/ R T ) sec-l, and ~ V C H= 10~5~~0esp(-62,000/RT) sec-1. The present paper, dealing with the thermal reactions of VCH, HCH, and MCH, intends to make a clear distinction between the primary and secondary reaction products, obtain correct kinetic parameters for the primary reactions, and 530 Ind.
Eng. Chem. Fundarn., Vol. 1 1 , No. 4, 1972
A conventional flow type apparatus was used in these experiments. The reactor was an annular quartz cylinder, 330 mm long, outer cylinder 14-mm i.d., and inner cylinder 8-mm 0.d. It was placed in an electrically heated copper block, 162 mm long. Feedstocks, VCH, HCH, and MCH were rectified by use of the 60-stage rectification column to the purities of 99.6, 99.5, and 99.8 mole yo,respectively. I n HCH, 0.5 mole yo of cyclohexane and, in MCH, 0.2 mole yo of methylcyclohesane were detected by a gas chromatographic analysis rrhich \vas equipped with a capillary column of more than 10,000 transfer units. These quantities were vaporized a t constant temperatures, carried on a nitrogen stream, and flowed t o the reactor. Commercially available nitrogen was used after the removal of trace oxygen over reduced copper gauze a t 350 t o 400°C. Concentrations of feed materials in nitrogen flow ranged from 0.7 to 3.3 mole %. Reaction temperatures (530 t o 665°C) and residence times (0.18 to 4.55 sec) were determined by the method of Hougen and Watson (1943). Outlet gas was analyzed by the use of gas chromatographs. KOcondensate was found in the outlet gas stream. Kinetic Results
Thermal reaction of VCH was conducted a t temperatures of 530 to 585OC and residence times of 0.18 to 1.53 sec. Concentration 6f VCH in nitrogen flow was kept a t a constant value around 0.66 mole yo.Typical data are summarized in Table I. Figure 1 shows the distribution of two main products, butadiene and vinylcyclohexadiene, in mole yo against the reaction depth, which was defined as total moles of C-containing products over 100 moles of feed. These two products were obtained from the initial stage of the reaction. There was about ten times as much butadiene as vinylcyclohesadiene. Trace amounts of cyclohesene, cyclohesadiene, benzene, cyclopentene, cyclopentadiene, and (seemingly) ethylcyclohexene, vinylcyclohexane, and ethylcyclohesadiene n-ere noticed in the gas chromatogram. At higher temperatures, small amourits of ethylene and propylene were also detected.
Table II. The Thermal Reaction of HCH (Typical Data)
Temperature, "C Residence time, sec
585 2.71
585 3.85
625 0.65
625 0.81
665 0.33
0.51
685 1.00
22.15 0.24 21.20 Trace 1.03 0.29 0.25
39.39 0.81 37.49 0.29 3.19 1.35 0.78
665
Product yield, moles per 100 moles of feed
4.93 ... 4.71 Trace 0.32 Trace 0.10
Ethylene Propylene Butadiene C5 compounds Cyclohexadiene Benzene Methylcyclopentene
7.48 ...
6.62 Trace 0.42 Trace 0.14
6.92 ... 6.68 Trace 0.32 Trace 0.08
9.47 ...
9.17 Trace 0.45 0.09 0.11
14.34 0.18 13.66 Trace 0.66 0.15 0.16
I
0
log k~ = 15.05 - 60J600/4.575T =
10.60
1,
15
585°C;
Butadiene is produced by the reverse Diels-*\lder reaction, while vinylcyclohexadiene is produced by dehydrogenation. Both reactions follow the first-order reaction kinetics. Participat'ion of backward reaction (Le., butadiene formed bimolecularly to VCH) is not anticipated in this case because of a very low concentration of butadiene. A\rrhenius plots for the two parallel react'ions are shown in Figure 2, from which the kinetic parameters of the two first-order rate const'ants in sec-' are described as below.
log k~
1O I
1000 / I ( ' K 1 Figure 4. Arrhenius plots (HCH)
Reactioi d e p t h , mol 01 prcduct / 100 ~ n o l e s01 feed
Figure 3. Distribution of product from HCH. 0 0 , U +,625°C; +,665°C
,
1
I05
- 43,400/4.575T
Difference in activation energies shows that the ratio of the rate of dehydrogenation to that of the reverse DielsAlder reaction becomes smaller a t higher temperatures. This obtained fact, coupled with the very short residence time adopted, seems to support the success of the high severity cracking technique, which demonstrates no marked coke formation inside the tubular heater a t higher temperatures. Thermal reaction of H C H was conducted a t 585 to 665°C and 0.28 to 4.48 sec. Concentration of H C H in nitrogen flow was kept constant a t about 3.34 mole 7,.Typical experimental results are summarized in Table 11. Primary products of the reaction were mainly ethylene and butadiene, accompanied by small amount's of cyclohexadiene, hydrogen, and methylcyclopentene. Benzene and propylene were the secondary reaction product's. Figure 3 shows the distribution of main products against the reaction depth. A very small amount of cyclohexane was detected in the product, but no methylcyclopentane was noticed, contrary to the report by Smith and Gordon. Moreover, propylene, as the secondary product of the reaction, increased along with t'he increasing
difference between the amounts of ethylene and butadiene production. It is well known that propylene, methane, and ethylene are produced easily in the thermal reaction of butadiene (Sakai, et al., 1970). First-order kinetics also fit in this case for the two primary reactions, Le., the formation of ethylene and cyclohehadiene plus benzene. Xrrhenius plots for the reverse Diels-Alder reaction and dehydrogenation are shown in Figure 4. Kinetic data from Uchiyama, et al., are plotted in the same figure. The formulation of the two first-order rate constants in sec-' are decided as below. log k~
=
15.01 - 65,700/4.575T
log k~
=
11.50 - 54,900j4.575T
Similar differences are obtained in A factors and activation energies, as in the case of VCH. The two parallel reactions are substantially different in their reaction schemes. Thermal reaction of AICH was conducted a t 5i5 to 650OC and 0.20 to 4.55 sec. Concentration of MCH in nitrogen flow was maintained a t about 1.05 mole %. The reaction products were somewhat complicated, as shown in Table I11 for typical data and in Figure 5 for the distribution of main primary products. Other than propylene, butadiene, and methylcyclohexadiene, which were expected to be the main primary products, considerable amounts of methane, ethylene, cychlohesadiene, and cyclopentene were also produced from the initial stage of the reaction. Toluene, cyclohexene, benzene, and cyclopentadiene were found to be the secondary products. Inspection of Figure 5 shows that the nearly-close couples exist in their production curves, Le., propylene and butadiene, methane and cyclohexadiene, ethylene and cyclopentene. Shortage in the production of butadiene to propylene, cyclohesadiene to methane, or cyclopentene to ethylene is partly due to the formation of secondary products, and partly, maybe, to the decomposition of feed itself to methane, Ind. Eng. Chem. Fundam., Vol. 1 1 , No.
4, 1972 531
Table 111. The Thermal Reaction of MCH (Typical Data)
Temperature, "C Residence time, sec
575 2.17
575 4.55
600 1.13
600 1.97
625 0.56
625 0.90
650 0.20
650 0.39
2.67 1.48 6.66 6.14 0.94 0.18 Trace 2.08 Trace 0.73 Trace
4.79 2.56 12.07 12.01 1.94 0.39 1.40 3.85 0.31 1.05 0.57
Product yield, moles per 100 molar of feed
1.45 0.79 3.36 3.09 0.39 Trace 0.17 1.13 Trace 0.26 0.06
M~ethane Ethylene Propylene Butadiene Cyclopentene Cyclopentadiene Cy clohexene Cy clohexadiene Benzene Methylcyclohexadiene Toluene
3.98 1.50 6.60 5.86 1.36 0.21 0.64 2.57 0.21 0.46 0.18
2.57 1.32 5.95 5.67 0.77 0.24 0.17 1.60 0.15 0.53 0.21
4.16 2.38 9.50 8.92 1.57 0.27 0.71 2.84 0.16 1.24
.oo
1
2.95 1.53 6.58 6.33 1.10 0.19 0.24 2.47 Trace 0.72 0.29
5.07 2.81 12.05 10.01 1.65 0.25 0.92 4.05 0.23 0.91 0.54
~
~~
~
~
Table IV. Kinetic Parameters for the Formation of Primary Products Feed stock
Product
MCH
Propylene Methane Ethylene Ethylene Butadiene Methylcyclohexadiene plus toluene Cyclohexadiene plus benzene Vinylcyclohexadiene
HCH VCH MCH Reaction depth, mol of product / 100 m o b of led
Figure 5. Distribution of product from MCH. 0 0, 575°C;
0 $,65O0C
-0-+ , 6 O O 0 C ; ~ + , 6 2 5 " C ;
I
110 115 1000 / 1 ('KI
* i
Figure 6. Arrhenius plots (MCH)
ethylene, or propylene, as suggested by Smith and Gordon. I n this case, therefore, the Arrhenius plots are taken for the formation of propylene, methane, ethylene, and methylcyclohexadiene plus toluene, as shown in Figure 6. From the figure, the first-order rate constants in sec-1 can be described. log k D
=
14.94 - 64,900/4.5752'
log kc, = 13.61
- 61,100/4.575T
- 62,400/4.575T = 10.49 - 49,550/4.575T
HCH VCH
Activation energy, kcal/mole
log Alrec-')
64.9 61.1 62.4 65.7 60.6 49.6
14.94 13.61 13.70 15.01 15.05 10.49
54.9
11 50
43.4
10.60
parameters for the formation of methane and ethylene deviate from those of the two reactions above, they are rather near to the parameters of the reverse Diels-Alder reaction. I n other words, the formation of propylene plus butadiene, methane plus cyclohexadiene, and ethylene plus cyclopentene is probably coming through the same reaction route, which is substantially different from the route for the formation of methylc yclohexadiene. Discussions on Mechanism
The results of kinetic analyses are summarized in Table IV. Both activation energies and A factors for the formations of products listed in the upper part of the table are substantially different in their values from those in the lower part. Our proposal on the mechanism for the formation of primary products in the case of M C H pyrolysis is summarized in Figure 7. Two parallel reactions proceed, Le., the formation of biradical and the dehydrogenation. As for the reverse Diels-Alder reaction, the first step is the formation of the briadical
log kc, = 13.70 log k H
The formation of propylene and methylcyclohexadiene plus toluene has similar values in both A factors and activation energies to those of the reverse Diels-Alder reactions and dehydrogenations, respectively, in the cases of VCH and HCH pyrolyses. On the other hand, although the kinetic 532 Ind. Eng. Chem. Fundam., Vol. 1 1 , No. A, 1972
and the step is rate determining. This biradical isomerizes easily to three possible isomers through the one-step hydrogen shift. I n succession, the second C-C bond cleavage of three biradicals proceeds through p-scission rule. These three reaction routes proceed competitively to give the primary products.
t
0 6/-
O+ll
Figure 7. Mechanism for the thermal reaction of MCH
The amounts of each product are subjected to the energetical and kinetical characteristics of three biradicals mentioned above, namely, their stabilities and reactivities of @ scission. From the general rule, the stabilities of three biradicals are in the order
Figure 8. Mechanisms for the thermal reactions of HCH and VCH
spond to the stabilities of three biradicals, which are generally considered to be in the order and are expected to be in line with the concentrations of these biradicals. The reactivities of p scission are generally in the order
which \vi11 be related to the rate constants of three different scissions. The combination of these two factors, concentration and rate constant, fixes the amounts of primary products qualitatively in accordance with the experimental results. In any case, it is difficult for the circular activated complex theory to explain the formations of methane, cyclohexadiene, ethylene, and especially cyclopentene a t the very initial stage of the reaction. Figure 8 shows similar mechanisms for H C H and VCH pyrolyses. Stabilities of two biradicals from H C H are in the order
The experimental results suggest that the rate of p scission of the unstable biradical is considerably larger than that of the cyclopentene ring formation from the stable biradical. Anyway, the important evidence to support the biradical theory is that a small amount of methylcyclopentene is found as one of the primary products in H C H pyrolysis. ;1biradical from VCH has two allyl-type reasonances. It is difficult to expect any other stable biradical through hydrogen shift, so that, both in the sense of stability and reactivity of @ scissioii, the rate of butadiene formation is overwhelming. There exist no significant differences in A factors for the formations of propylene from M C H , ethylene from HCH, and butadiene from VCH, as seen in Table IV. Small differences in act’ivation energies, Le., 64.9, 65.7, and 60.6 kcal/mole, corre-
Tsang (1965), in his study of the thermal reaction of M C H , obtained small amounts of methane and ethylene. H e attributed the formation of methane and ethylene t o 3-methylcyclohexene, which was contained in fed 4-methylcyclohexene (MCH) as much as 12 mole yo.Therefore, in conclusion, he preferred the loose cyclic complex as the intermediate of the reaction. If methane and ethylene were formed from 3-methylcyclohexene, the activation energies for their formation should be larger than that of the propylene formation from 4-methylcyclohexene. The present kinetic data for the formation of methane and ethylene show a much smaller value in activation energies than that of propylene formation. literature Cited
Charton, M., J. Org. Chem. 31,3745 (1966). Grieaer, R. A., Eckert, C. A., J. Amer. Chem. SOC.92, 7149 (1370). Hougen, 0. A., Watson, K. M., “Chemical Process Principles,” p 884, Wiley, New York, N.Y., 1943. Kraus, M., Varruska, AI., Bogart, V., Chem. Listy 50, 553 (1956). Kuchler, L., Trans. Faraday SOC.35, 874 (1939). Kunugi, T., Sakai, T., Soma, K., Sasaki, Y., IND.ENG.CHEM., FUND.4M. 8, 374 (1969). Kunugi, T., Sakai, T., Soma, K., Sasaki, Y., IND.ENG.CHEM., FUNDAM. 9. 314 (1970a’i. Kunugi, T., Soma,‘K., Sakai, T., IND.EKG.CHEM.,FWDAM. a. --219 (iwnh) \--.-Kwart, H., King,’ K., Chem. Rev. 68, 415 (1968). Rowley, D., Steiner, H., Discuss. Faraday SOC.10, 198 (1951). Sakai. T.. Soma. K.. Sasaki. Y.. Tominaaa. H.. Kunum. T., Adian. Chem. ker. No. 97, 68 (1970). Sakai, T., Wada, S., Kunugi, T., Ind. Eng. Chem., Process Des. Develop. 10, 305 (1971). Seltzer, S., J. Amer. Chem. SOC.87, 1534 (196*5). Smith, S.R., Gordon, A. S., J. Phys. Chem. 6 5 , 1124 (1961). Stewart, Jr., C. A., J . Amer. Chem. SOC.93,4815 (1971). Uchivama, M.. Tomioka. T., Amano, A., J . Phys. Chem. 68, 1878 (1964).’ Tsang, W., J . Chem. Phys. 42, 1805 (1965). - I
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RECEIVED for review October 4, 1971 ACCEPTEDJuly 28, 1972 Ind. Eng. Chem. Fundam., Vol. 1 1 , No.
4, 1972
533
