deficient catalyst had a higher activity for hydrocracking n-decane alone us. hydrocracking Decalin alone. However, in a mixture of n-decane and Decalin, there was virtually no hydrocracking of decane, which suggests that Decalin almost completely denied the n-decane access to any of the active sites. Literature Cited
Adams, C. E., Kimberlin, C. N., Shoemaker, D. P., Proceedings of Third International Congress on Catalysis, 1964, p. 1310, Wiley, New York, 1965. Benesi, H. A. (to Shell Oil Co.), U.S.Patent 3,190,939 (June 22, 1965). Breck, D. W., J.Chern. Educ. 41, 678 (1964). Crank, J., “The Mathematics of Diffusion,” Chapt. VI, p. 86, Clarendon Press, Oxford, England, 1956. Eberly, P. E., Jr., J . Phys. Chem. 6 7 , 2404 (1963).
Eberly, P. E., Jr., Ind. Eng. Chem. Fundamentals 7, in press (1968). Frilette, V. L., Ruben, M. K., J . Catalysis 4, 310 (1965). Gerhard, E. H., Chem. Tech., Berlin 13, 718 (1961). Keough, A. H., Division of Petroleum Chemistry, 144th Meeting, ACS, Los Angeles, March 31-April 5, 1963, Preprint Vol. 8, No. 1, 6.5 (1963). Satterfield, C. X., Sherwood, T. K., “The Role of Diffusion in Catalysis,” p. 65, Addison-Wesley, Reading, Mass., 1963. Voorhies, A., Jr., Bryant, P. A., A.1.Ch.E. Meeting, Houston, Tex., February 1967. Weisz, P. B., Hicks, J . S., Chem. Eng. Sci. 17, 265 (1962). Weisz. P. B., Prater, C. D., Aduan. Catalysis 6 , 143 (1954).
RECEIVED for review January 8, 1968 ACCEPTED April 26, 1968 Division of Petroleum Chemistry, 154th Meeting. ACS, Chicago, Ill.. September 1967.
DEHYDROGENATION OF MONOCYCLIC NAPHTHENES OVER A PLATINUM ON ALUMINA CATALYST WITHOUT ADDED HYDROGEN A .
W .
R I T C H I E
A N D
A .
C .
N I X O N
Shell Deuelopment Co., Emeryuille, Calif. 94608 The dehydrogenation of cyclohexane and a number of its methyl and ethyl homologs has been investigated over a platinum on alumina catalyst at 10 atm. pressure in the temperature region 840’ to 1200’F. at liquid hourly space velocities to 150. Generally, selectivity for the corresponding aromatic and catalyst stability were good. The introduction of alkyl groups into the cyclohexane ring generally increased reactivity, but the magnitude of the effect was dependent on the position of the groups in the ring and in space, as w e l l as on the temperature in which the reaction took place. Thus, at 842’F., the introduction of either a methyl or an ethyl group generally increased the rate of dehydrogenation but the rate relative to methyl cyclohexane varied from 0.42 (for frans1,4-DMCH) to 2.84 (for cis-1,P-DMCH). Because the value of the energy of activation varied from 15.2 kcal. (for CH) to 7.2 kcal. (for 1,3-DMCH) the relative reactivities were a function of temperature. Much greater variations in the values of E were observed for individual geometric isomers-e.g., 6.3 for cis- and 20.4 for trans-1,4-DMCH-but this may have been due to concurrent isomerization.
THEAir Force is considering the use of vaporizing and endothermic hydrocarbon fuels for cooling high-speed aircraft in the speed range above Mach 3. Previous papers arising out of work on this project (Ritchie et al., 1965; Ritchie and Nixon, 1966) showed experimentally that a large heat sink could be provided by the datalytic dehydrogenation of methylcyclohexane (MCH) using a platinum on alumina catalyst. With this naphthene a total heat sink of about 2000 B.t.u. per pound was obtained, about 950 B.t.u. per pound of which came from the catalytic reaction. The catalyst was active, and it was possible to operate the reactor without added hydrogen a t a liquid hourly space velocity of 100, a t 10-atm. pressure and 1100”F., and obtain 95+% MCH conversion with 99%
selectivity for toluene and with good catalyst stability. These studies have since been extended to other monocyclic naphthenes, and the results are presented in this paper, in which the emphasis is on the relative reaction rates and reaction behavior rather than on the heat sinks obtained from these reactions. Thermal reaction studies on these naphthenes will be presented in another paper. Experimental Cyclohexane (CHI, ethylcyclohexane (ECH), the three dimethylcyclohexane (DMCH) isomers, a mixture of the three diethylcyclohexane isomers (DECH), and the three trimethylcyclohexane (TMCH) isomers were tested a t 10-atm. pressure in the temperature region of 840” to 1112°F. The various naphthenes were VOL. 7
NO. 3 SEPTEMBER 1968
209
compared on the basis of first-order rate constants, although the reaction kinetics may not necessarily be first-order. Apparatus. The apparatus was a tubular flow reactor equipped with conventional devices for measuring feed flow rates and for collecting liquid and gas products. The reactor was a stainless steel tube (No. 347, %-inch IPS) 32 inches long, %-inch i d . , heated by an electric furnace. The catalyst was contained in the annular space between the thermowell and the reactor wall. In order to reduce the heat transfer effects in the reactor as much as possible, the annular distance between the thermowell and the reactor wall was made % a inch or about one pellet diameter. This gave a catalyst bed length of 4 % to 4 % inches for a 7-mi. volume of catalyst. The catalyst was 1% platinum on alumina and was prepared in the laboratory by impregnating alumina granules (Harshaw 0104, 10 to 20 mesh). Prior to carrying out the experiments, the catalyst was reduced in situ with hydrogen for 30 minutes a t 572°F. (300°C.) and then for one hour at the reaction temperature. (Argon can be substituted for hydrogen during catalyst reduction.) The complete apparatus was described in detail in a previous paper (Ritchie et ul., 1965). Product analyses were carried out by mass spectrometry and by GLC, from which conversions and selectivities were calculated. Assuming first-order kinetics, rate constants were calculated based on the rate of disappearance of the starting material according to the following equation:
kspC =
LHSV ~
3600
X
~ ~ 2 2 , 4 1 2T 1 x - x 2.3 log 273 1-f MW x P ~
(1)
where LHSV = liquid hourly space velocity-Le., volumes of feed per volume of catalyst per hour MW = molecular weight P = reactor pressure, atmospheres T = reaction temperature, K. (reactor wall temperature) liquid density P = f = fraction reacted O
Relative reactivities were computed taking the rate constant of MCH as unity. Apparent activation energies were computed from the first-order rate constants. Feed Materials. Cyclohexane: Phillips pure grade, passed over silica gel prior to use. Ethylcyclohexane: Matheson, Coleman and Bell, practical grade, passed over silica gel prior to use. Analyzed 97.7% ECH, 2.3% MCH. Dimethylcyclohexane isomers were prepared by hydrogenation of the corresponding xylenes. The various isomers had the following composition (weight per cent):
1,4DMCH 1,3-DMCH 1,2-DMCH 40.8 32.1 30.6 57.8 67.6 68.3 1.4' 0.3" l.lb 0.706 0.475 0.456 2.93 1.98 0.470 * truns-1,2-DMCH.
trans cis Other components trans/&, feed trans/cis, equil. (1200" F.) Xylene truns-1,4-DMCH. Diethylcyclohexane: a Shell Chemical Co. preparation containing a mixture of the three isomers with both cis and trans species, having the following composition (weight per cent) :
trans cis Total
I,4-DECH
1,3-DECH
1,2-DECH
16.2 13.4 29.6
16.6 38.2 54.8
6.0 9.6 15.6
Trimethylcyclohexane: prepared by hydrogenation of the corresponding trimethylbenzenes (composition given below). 210
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Results
All of the naphthenes tested dehydrogenated readily to the corresponding aromatics over the platinum on alumina catalyst without added hydrogen. At 842°F. the reaction was clean and selectivities for aromatics of 98+% were obtained. Similar high selectivities were obtained at 1022°F. except for ECH. With ECH considerable side chain splitting to form toluene was observed. Reaction rates varied between naphthenes, between the various naphthene isomers, and between the cis and trans species. Conversions, first-order rate constants, the relative reactivities at 842" and 1022"F., and apparent activation energiesare tabulated in Table I. Complete data for the various naphthenes are shown in Tables I1 to VI which have been deposited with the AD1 Auxiliary Publications Service of the American Society for Information Science. Cyclohexane. Under the test conditions cyclohexane (CH) reacted to give benzene with 98+% selectivity. Cracked product make was low, being 1.2% a t 1112°F. and lower at lower temperatures (Table 11). Catalyst stability appeared good and only a 3% decline in conversion was observed over a 90-minute reaction period (run 169). An apparent activation energy of 15.2 kcal. per mole was observed for the temperature region of 842" to 1112°F. The reactivity of this naphthene was slightly less than that of MCH at 842"F., but slightly more a t 1022°F. as Eactfor CH > Eacl MCH. Ethylcyclohexane. Ethylcyclohexane was the least stable of the naphthenes, under our test conditions. At 842°F. and at moderate to high conversions, high selectivities for ethylbenzene (EB) were obtained. At higher temperatures, however (1022" and 1112" F.), selectivities for E B were only 40 to 60%. Under these latter conditions considerable side-chain splitting and naphthene cracking were observed. The complete data, including product analyses, are tabulated in Table 111. No styrene formation was observed a t 932°F. (run 58). Attempts to produce this compound by operating the reactor at higher temperatures and lower space velocitiesLe., increased contact time-were not successful, and only increased side-chain and naphthene cracking. For example, increasing the temperature from 932" to 1022" F. and reducing the space velocity from 30 to 20 reduced the selectivity for E B from 89.7 to 60.5%, and increased toluene production from 4.3 to 21.1%, benzene from 0.6 to 5.3%, and light gas from 4.8 to 11.6%. Only 0.7% styrene was found at the higher temperature (runs 58 and 40). Increasing the temperature still further to 1112" F. resulted in rapid catalyst deactivation and only a slight increase in selectivity for styrene (run 36). From the distribution of products in the run a t this higher temperature, it appears that a concurrent thermal reaction occurred along with the dehydrogenation reaction. Presumably, this concurrent thermal reaction led to catalyst deactivation. However, the possibility that styrene was formed on the catalyst, followed by polymerization and subsequent decomposition, cannot be discounted. (Attempts to produce styrene by the dehydrogenation of ethylbenzene under these reaction conditions were not successful.) First-order rate constants were calculated from the data at 842" F. only, where dehydrogenation was essentially the only reaction. At higher temperatures it was not possible to obtain rate constants for the dehydrogenation reaction. The relative reactivity of ECH a t 842" F. was greater
Table I. Relative Reactivities of Various Naphthenes for Dehydrogenation Pressure LHSV Reaction period
Conversion Wt Naphthene
84P F
10 atm Catalyst Catalyst volume 100 30 min. Catalyst size
k , Sec.
cc
'
2022O F.
842O F.
1022" F.
1'1 Pt on A1203 7 ml. 10-20 mesh
Relatice Reactictties k kMCH 1022" F 842" F
Log Freq Factor
Eact ' kcal. mole
842" F
1022" F
3.57 4.67
1.OO 1.25
11.7 15.2
4.03 4.68
...
1.00 0.90 1.43
0.95 1.58 0.26
2.13 2.71 1.48
1.53 2.55 0.42
1.47 1.87 1.02
9.5 6.3 20.4
...
...
2.31 6.22
2.30 6.30
74.4 73.2 77.2
0.84 0.79 0.95
1.61 1.56 1.75
1.35 1.27 1.53
1.11 1.08 1.27
72 6.6 6.8
...
...
2.04 2.21
2.08 2.20
59.0 79.6 29.6
81.2 88.8 69.8
0.99 1.76 0.56
2.06 2.73 1.49
1.60 2.84 0.90
1.42 1.88 1.03
9.2 5.5 12.2
...
...
2.01 3.96
2.01 3.65
DECH (total)
62.2
81.4
0.85
1.68
1.37
1.16
8.2
...
...
1,4-DECH cis trans
61.5 73.9 51.3
81.4 85.8 77.8
0.85 1.19 0.64
1.68 1.95 1.51
1.37 1.92 1.03
1.16 1.34 1.04
8.2 5.8 10.2
...
...
1.95 3.09
1.94 3.09
1,3-DECH cis trans
63.7 61.7 68.5
81.1 80.2 83.6
0.90 0.85 1.02
1.67 1.62 1.81
1.45 1.37 1.64
1.15 1.12 1.25
7.4 7.7 6.7
...
...
2.40 2.16
2.41 2.17
1,2-DECH cis trans
58.4 70.8 38.3
79.5 83.3 73.3
0.78 1.08 0.43
1.58 1.79 1.32
1.26 1.76 0.69
1.09 1.23 1.09
8.5 5.9 13.4
...
...
1.94 3.93
1.93 3.94
1,3,5-TMCH 1,2,4-TMCH 1,2,3-TMCH
58.2 56.2 60.3
73.8
0.80 0.77 0.89
...
..,
1.24 1.24 1.44
... ...
...
7.0 8.5
2.22 2.73
2.22 2.71
MCH CH ECH
40.8 55.4" 53.3
66.6 89.9O
1.45 1.81
...
0.62 0.56 0.89
1,4-DMCH cis trans
60.5 78.4 22.7
84.6 90.5 72.3
1,3-DMCH cis trans
55.0 53.1 59.5
1,2-DMCH cis trans
LHSV
=
... 77.5
...
.
I
...
...
.
50.
than that of MCH but lower than DMCH and about equal to that of DECH. Dimethylcyclohexane. The DMCH isomers were the most reactive of all naphthenes tested (842" to 1022"F., liquid hourly space velocity of 100). Further, for each isomer the reactivities of the cis and trans species were different. Hence, the total over-all reactivity of each isomer depended upon the cis and trans concentrations. For the composition of the DMCH isomers tested (at 842" and 1022"F.), the over-all reactivities of the 1,4 and 1,2 isomers were about equal; the cis species being more reactive than the trans. The 1,3 isomer, however, was less reactive than the other two and with this isomer the cis species was less reactive than the trans. The complete data are shown in Table IV. Of the various DMCH isomers the cis-1,2 and &1,4 isomers were the most reactive, being 2.84 and 2.55 times more reactive than MCH at 842"F., while the truns-1,4 species was the least reactive (Table I ) . Apparent activation energies varied from 5.5 to 20.4 kcal. per mole (Table I ) . Figure 1 shows Arrhenius plots for the data. The trans-cis equilibria for the DMCH isomers are shown in Figure 2 (Hawthorne, 1965). Comparison of the trans-cis ratios of the feed and product materials (Table IV) suggests that for the 1,4 and 1,3 isomers (and probably for the 1,2), the rate of dehydrogenation is
greater than the rate geometric isomerization. Thus, for the 1,4 isomer a t high conversions, the concentration of the cis species was greater than equilibrium in the feed and less than equilibrium in the product. Similarly, for the 1,3 isomer the concentration of the trans species was less than equilibrium in the feed and greater than equilibrium in the product. These observations are valid over the temperature region of 842" to 1022°F. At other temperatures the rate of isomerization may predominate, depending upon the relative activation energies for the dehydrogenation and isomerization reactions. With all three isomers the reaction was highly selective for the corresponding xylenes. (In our GLC analysis system the m- and p - xylenes emerged almost as one peak. Consequently, these isomers are reported as n-p-xylene.) Further, good naphthene stability was observed, and high conversions with no naphthene cracking were obtained a t 842" and 932°F. (runs 88 and 89, Table 11). Diethylcyclohexane. This feedstock was a mixture of the three DECH isomers and their cis and trans species. As the reactivity of each species was different, the over-all DECH reaction rate was dependent upon the concentrations of the various components. For the DECH mixture tested the over-all reactivity was greater than MCH and only slightly lower than DMCH. Further, the reactivities of the cis and trans species appeared to follow the same VOL. 7 NO. 3 S E P T E M B E R 1 9 6 8
211
I Pressure:
IO atm
LHSV
100
:
IJ-DMCH
~
4
I
I
I
I
1,4-DMCH
o.2
t-
0
cis DMCH
V
trans
DMCH
0 O v e r a l l DMCH
Figure 1 . Dehydrogenation of dimethylcyclohexane isomers
pattern as was observed with DMCH. Thus, the cis species was more reactive than the trans for the 1,4 and 1,2 isomers, while the trans species was more reactive than the cis for the 1,3 isomer (Table V). The most reactive species were the &1,4 and the cis-1,2 isomers. Based on first-order rate constants, the former was 1.92 and the latter was 1.76 times more reactive than MCH a t 842°F. (Table I ) . The computed values of the conversions, first-order rate constants, and activation energies for the DECH isomers are shown in Table V. Apparent activation energies ranged from 5.8 to 13.4 kcal. per mole. Complete product analyses are shown in Table V. Reaction products were mainly diethylbenzenes (DEB, 9 6 + 5 selectivity) with smaller amounts of material not identified. A small amount of material was formed whose GLC emergence time was greater than DEB, and was presumed to be ethylvinylbenzene and divinylbenzene. Trimethylcyclohexane. Three trimethylcyclohexane isomers (TMCH) were tested a t 842" to 1022" F. These feedstocks were obtained by the hydrogenation of the corresponding trimethylbenzenes (TMB), but were not pure components, since the hydrogenations were incomplete. The feed compositions are tabulated below (weight per cent) : 1,3,.5TMCH
1,2,4-TMCH
1,2,3-TMCH
l , c i ~ - 3cis-5, TMCH = 67.5 l,cis-3, trans-5T M C H = 21.8 C9cyclene = 4.5
l,trans-Z,trans-4 = 0.9
l,cis-2,trans-3= 76.4
l,trans-2,cis-4= 7.5
1, trans-2, cis-3 = 5.8 l,cis-2,cis-3 = trace C9 cyclene = 3.8 1,2,3-TMB= 13.6 Cracked = 0.3
1,3,5-TMB= 6.2
l,cis-2,cis-4 = 89.8 l,cis-2,trans-4 Cycyclene = 1.8
Temperature coefficient
4
0 0
1,ZDMCH 1,4DMCH
1,3 D M C H
3
KP 2
1
c
1 5 00
1
1
1000
1500
T e m p e r a t u r e , 'F
Figure 2. Dimethylcyclohexane trans-cis equilibria Calculated from d o t a of API tables, Project 44 (Hawthorn, 1965)
212
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Under the test conditions these naphthenes were considerably less stable than the other naphthenes and catalyst deactivation was observed during all of the runs as shown by the increase in catalyst bed temperatures during the 30-minute reaction periods (Table VI). At 842" and 932" F. the deactivation was slight (5" t o 15" increase in bed temperature), but was considerable a t 1022" F., where increases in bed temperature of 90" to 140" F. were noted. The magnitude of the temperature increase was taken as a measure of catalyst deactivation. Based on this criterion, the 1,3,5 isomer was the most stable and the 1,2,4 isomer was the least stable (Table V I ) . First-order rate constants for each isomer were computed at 842" and 932" F., based on the total isomer conversions and the average wall temperature during the run. Based on the first-order rate constants, the relative reactivities of the TMCH isomers a t 842°F. were greater than MCH but less than DMCH. Further, the order of reactivity was 1,2,3 > 1,3,5 -1,2,4. Apparent activation energies were 7.0 and 8.5 kcal. per mole for the 1,3,5 and 1,2,4 isomers, respectively. N o rate constants were calculated from the data a t 1022" F. because of extensive catalyst deactivation a t this temperature (runs 140-3 and 140-1, Table V I , in which the conversions a t 1022" F. were lower than those a t 932" F.). There appeared to be some cis-trans isomerization within a given isomer species. Thus, with the 1,2,4 isomer, the concentration of the l,truns-2,cis-4 species increased during the run (runs 139, 140-1, Table VI). Similarly, with the
1,2,3 isomer the l,truns-2,cis-3 concentration increased during the run (run 147, Table VI). Inspection of the data of Table VI shows that for any one isomer the various cis-trans species react a t different rates. The absolute reactivities of the various cis-trans species for dehydrogenation, however, cannot be determined from the conversions due to the concurrent cis-trans isomerization reaction. N o attempt was made to identify the deactivating reactions. I n separate experiments it was shown that there was essentially no thermal reaction a t 1022" F. and LHSV 20. Thus, in the catalytic system with both catalyst and hydrogen present, it appears that the deactivation was due to a hydrocracking-type reaction. High selectivities for aromatics were observed, which suggests that the deactivation was probably due to strongly adsorbed feed or product being degraded to coke. I t is possible that use of a less acidic catalyst would have reduced deactivation, but this was not investigated. Gas products were mainly hydrogen, but contained about 0.1 to 0.2 weight % of methane. Discussions and Conclusions
The relative reactivities and apparent activation energies for the various monocyclic naphthenes and their cis and trans species are summarized in the bar graphs of Figures 3 and 4. I n general, successive addition of an alkyl group to the naphthene ring increased the rate of dehydrogenation and reduced the apparent activation energy. I n this respect ethyl groups were no more effective than methyl groups. There was considerable variation in reactivities between the cis and trans species of the 1,2- and 1,4-DMCH and DECH isomers, however, and
-
Q e a c l i v i l y of M C H = 1 p r e r r u r e : IO a t m LHSV: C H : 50 O t h e r s : 100 C a t a l y s t : 1% P t on A 1 2 0 J Ca'alyrt Volume: 7 ml
the cis species were more reactive than the trans. Further, the activation energies appeared greater for naphthenes with lowest reactivities. As an example, the trans-1,4and truns-1,2-DMCH and DECH were less reactive than the cis species, while their Ea,, were greater than those of the corresponding cis species. The 20 kcal. Eactfor the truns-1,4-DMCH species was obtained from poor data, and appears to be much too high. For example, in the Arrhenius plot for this isomer (Figure l), the points for the trans species did not fall on a straight line and the 20-kcal. value was based on the best straight line through all three points. Calculating the activation energy from the data a t 932" and 1022°F. only, however, gave a value of 11 kcal. per mole, which seems more reasonable. Frequency factors, A , for the dehydrogenation of the various naphthenes varied from about 10' to l o 6 (Table I). A plot of log,, A as a function of E,,, gave a straight line (Figure 5 ) . This straight-line relationship between log A and E is a good example of what has become known as the "compensation effect" in the catalysis literature (Cremer, 1955). The observed results suggest that the dehydrogenation of naphthenes goes via a dissociative adsorption process in which formation of the carbon-metal bond on the catalyst surface is concurrent with hydrogen removal from the ring. For CH and MCH presumably the slow step is the initial adsorption of the naphthene. For cyclohexane, adsorption would involve breaking a secondary carbonhydrogen bond. For a methyl-substituted naphthene, adsorption would involve breaking a tertiary carbonhydrogen bond. As bond strengths of tertiary hydrogens are lower than those of secondary hydrogens, activation
p r e s s u r e : IO a t m Black Temperature Range: 842- 1022°F C a t a l y s t : 1 % P' on A I 2 0 3 C a t a l y r t Volume: 7 ml LHSV: C H : 50
Block Tempera'ure kMc
1022'F = 1.45 i e c - '
Others:
100
0
E
1. Block Temperature 842'F k M C H = 0.62 i e c - '
L
1 Y
11I
I
DMCH
