DEHYDROGENATION OF HYDROCARBONS OVER A CHROMIA-ALUMINA CATALYST IN T H E ABSENCE OF ADDED HYDROGEN A. W . R I T C H I E , R. D. H A W T H O R N , A N D A . C . N I X O N
Shell Development Go., Emeryville, Calif. The catalytic dehydrogenation of pure paraffin and naphthene feeds has been investigated over a chromiaalumina catalyst in the absence of hydrogen to determine if these endothermic reactions would provide sufficient heat sink to be useful in augmenting the cooling capacity of hydrocarbon fuels for aircraft in the Mach 3-6f regime. The feeds consisted of six paraffins from Ca to C16 and a naphthene. At high conversions cracking generally increased with increasing molecular weight and with increasing length of alkyl side chain. The maximum endothermic heat observed was 460 B.t.u. per pound with propane. A brief study of the mechanism of coke formation with propane suggested that coking proceeds via unstable secondary propylene reaction products. s AIRCRAFT speeds increase, the problem of coping with
A aerodynamic heating becomes more and more critical.
T h e stagnation temperature rises from 600" F. a t Mach 3 to 1000" F. a t Mach 4 ; 1600' F. a t Mach 5 to 2500" F. at Mach 6 ; and so forth ( 3 ) . The only heat sink on board the plane is the fuel and this is already being utilized in a minor way with present aircraft to cool the engine oil. More elaborate schemes for utilizing the heat sink in higher speed aircraft have been suggested ; these take advantage of latent, sensible, and endothermic reaction heat (4, 5). Heretofore, only thermal (uncatalyzed) cracking reactions have been seriously examined for this purpose (70). These have the inherent disadvantages of requiring high temperatures to achieve sufficiently high reaction rates and of suffering decreasing heat sink as the conversion increases above 60% ( 7 7). Accordingly, in the present study we have been considering catalytic reactions as a possible way of circumventing these limitations, although it is realized that some other disadvantages are introduced. I n this paper we report on a study exploring the reactivity of several paraffins and a naphthene over a chromia-alumina catalyst at high conversions in the absence of hydrogen. The catalyst used is typical of existing state-of-the-art catalysts which are now commercially available. Various endothermic reactions can be written which should be susceptible of catalysis. These all involve the breaking of carbon-hydrogen or carbon-carbon bonds. The former type are generally more desirable because of the greater energy of the C-H bond. The enthalpies involved in some typical reactions are shown below.
Feed Propane
Propane n-Octane n-Octane Methylcyclohexane Decalin
Calculated Equil. Heat of Conversion Reaction at 1000" K., at 10 Products Atm. B.t.u./Lb. C311.5, HZ 40 510 (1275 at 100%) 740 CzH4, C H I 95 99 970 Ethylbenzene, H P 99 1440 Styrene, H P Toluene, H z 99 940 (100) (1000) Naphthalene, H z
Of course, all the feeds listed can undergo many other reactions. These are mainly endothermic cracking reactions but hydrogen transfer, which is exothermic, can also occur, so
that the endothermicity of the over-all reaction is reduced. All of the hydrocarbons listed would have a sensible and latent heat capacity of about 1000 B.t.u. per pound from 70" F., so that if extensive dehydrogenation can be achieved, total heat sink will be increased in the most favorable case by about 150%. The total enthalpy required for cooling a high-speed aircraft will be a function of many factors, including aircraft speed, altitude, mission requirements, design, and engine configuration. At present it is considered that most of the heat sink will be required to cool the engines. The leading edges, electronic equipment, and living space which also require cooling would impose only a minor load. I t is generally thought that any speed greater than about Mach 4.5 would require some endothermic reaction augmentation for adequate cooling. Although the dehydrogenation of paraffins and naphthenes over chromia-alumina catalysts has been studied by many workers, in many cases hydrogen is included with the feed and the reaction is studied only at low conversions (7, 2, 79, 74). I n the present instance, the objective is to achieve as high a conversion as possible in order to provide maximum heat sink capability. Vapor-phase catalytic dehydrogenation of propane and methylcyclohexane and dehydrocyclization of six normal and branched hydrocarbons were studied at 1- to 10-atm. pressure, and 1022" to 1283" F. at LHSV's of 5 to 50 in the absence of hydrogen. A commercial potassium-promoted chromiaalumina catalyst (Catalysts and Chemicals, Inc., Catalyst C-94) (2% K20, 8% Cr203 on A1203; 10- to 20-mesh size) was the standard catalyst used in these studies. Tests were for 90 minutes' duration. Product material was analyzed by mass spectrometry (gas products) and by gas-liquid chromatography, from which conversions and selectivities to desired products were calculated. Coke and polymer formed during reaction were neglected and were not included in the conversion computations. Heat sinks produced were not measured directly but were calculated from thermodynamic heats of reaction based on the experimental conversions and product distributions and from the sensible and latent heats for the various hydrocarbons. The bulk of the study was carried out on propane and methylcyclohexane Experimental
T h e system used for carrying out the catalytic studies was a hot tube reactor with conventional devices for measuring feed VOL. 4
NO.
2
JUNE 1 9 6 5
129
i .
1600
6 Total Pressure
IZOZ°F.
\
Y
6
1400
r
-
3 r
:
g\
7 . 8 Atm.
_.I--I.:-,
012OZ'F:
1200
41.8 Aim.
700
980
1160
800
900
1340 1000
,
Pressure: 1 Atm. Selectivity f o r Propylene: 9%
1 5 2 0 "F.
1
1100 OK,
Temperature
I
%
1
I
I
1:" 0
Figure 1 . Equilibrium conversions of propane to propylene and hydrogen with pure propane feed
flow rate and for collecting and measuring reaction products. The reactor was a furnace-heated stainless steel tube ('/2-inch IPS) 33 inches long. (A few experiments were done with a Vycor tube of about the same dimensions.) The catalyst bed (ca. 5 inches long) was located in the lower portion of the tube; the top portion served as a preheater. The nonhydrocarbon gases (used for catalyst regeneration) were metered through conventional rotameters and entered the reactor a t the top. Liquid hydrocarbon feed was forced from a liquid reservoir through a rotameter by means of argon pressure and then into a heated line. T h e hydrocarbon vapor entered the reactor through a separate tube that terminated just above the catalyst bed. The hot exit gases from the reactor passed through a condenser and then a gas-liquid separator. The liquid was collected in a Jerguson gage; the gas products were passed through a Grove pressure regulator and a wet-test meter and then were vented. Gas samples were taken up-
Table 1.
8279-
Press., Atm. LHSV
92b 93b 59" 94; 101 61 105c 62b
1 1
5
Contact Time, Sec.
1 1 1
0.79 0.40 0.40 0.75 0.38 0.19 0.35 0.18
1022 1022 1022 1112 1112 1112 1202 1202
131"
1
20
0.19
1112
94b 1440 145b 148b 15lC 153~ 155~ 1580
1 4 6 8 10 10 10 10
5 20 30 40 50 20 20 50
0.75 0.75 0.75 0.75 0.75 1.87 1.77 0.71
1112 1112 1112 1112 1112 1112 1202 1202
1 1
At 30-minute reaction time. reaction time. a
130
2% K O , 8% Crz03, on 13.3 j=0.2g. 20 ml. Pure propane
Propane Selectivity for Catalyst Bed Block Temp., OF. Conversion, % Propylene, yo Temp., 90 min. "F. 30 min. 90 min. 30 min. 90 min. 30 min. STAINLESS STEELREACTOR
10 10 5 10 20 10 20
1
The dehydrogenation of propane was equilibrium limited over the range of our study (Figure 1). Thus, at 1- and 10atm. pressure equilibrium conversions of 50% could be obtained only a t temperatures greater than 1150' and 1340' F., respectively. At these temperatures coking of the catalyst became appreciable, thereby reducing catalyst life and lowering selectivity for the desired product propylene. For example, at 1-atm. pressure, LHSV 5 (liquid hourly space velocity = volume of liquid per volume of catalyst per hour), and 1112' F.
Dehydrogenations of Propane over Chromia-Alumina Catalyst
Catalyst. Catalyst weight. Catalyst volume. Feed.
Run No.
Dehydrogenation of Propane
28.5 25.6 94.5 95.0 94.0 97.6 22.1 19.5 22.5 21.0 94.5 94.1 ~. 93.5 97.2 37.7 33.5 30.6 23.6 94.7 92.0 19.0 17.0 98.7 96.5 75.Bd 41.5 29.0d 89.0 ... 27.0" 25.5.' 89.5' 80.0f VYCOR REACTOR 977 997 20.6 16.5 97.7 97.2 ELEVATED PRESSURE, STAINLESS STEELREACTOR 1044 1048 37.7 33.5 93.5 97.2 979 990 19.9 17.0 90.9 93.1 1009 1019 9.3 7.8 91.9 93.1 981 989 10.1 9.7 95.6 94.5 939 ... 8.2 ... 93.4 ... 1000 998 15.7 14.8 76.2 81.8 1092 1112 24.6 16.0 73.0 68.6 1033 90.5 ... 12.9 ... ... 968 946 928 1044 1002 977 1085 1035
Regenerated catalyst.
972 948 937 1048 1034 988 1125
Fresh catalyst.
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
Pressure, 2Op.s.i.g.
Propane Converted to Coke,
%
Heat Sink, B.t.u./Lb.a Reac- Total Total equil. tion exp.
0.2 0.1 0.7 1.o 0.9 0.1 1.7 2.1
339 262 268 444 365 236 466
1339 1262 1268 1504 1425 1296 1683
1365 1365 1365 1640 1640 1640 1873
0.0
252
1312
1640
1 .o 0.5 0.4 0.3 0.6 1 .o 0.4 0.0
444 228 108 122 95 151 226
1504 1288 1168 1182 1155 1211 1343
1640 1375 1337 1299 1274 1274 1441
Pressure, 25p.s.i.g.
f
...
...
...
...
...
...
Pressure, 65p.s.i.g., 60-minute
I60
k . 5 3
140
$ i?
+
5
120
m
.w
E 1oc
1112°F.
Figure 3. pressures
Dehydrogenation
of
----
propane
at
various
-
Experimental conversions and corresponding heat sinks -Equilibrium values (1 1 12' F.) Angular points an right-hand side indicate corresponding values a t 1 2 0 2 ' F.
Table 11.
Product Distribution in Dehydrogenations of Propane
Temp. LHSV.
1112OF. 20
Run No. Pressure, atm. Product components, % Hz CH4 C2H4 CZHE C3HE C3H8 Table 111.
8279-61
82 79-153
1
10
16.2 0.4
16.2 2.3
...
0.1
1.8 9.9 69.7
0.1
15.8 67.6
Effect of Oxygen on Thermal Cracking of Propane 1 atm. Pressure.
Reaction time. 20 min. LHSV. 20
Run No. 8219-
40 160
128 162
Propane Reaction 70 0 2 Temp., OF. in Feed Convn., % METALREACTOR 1206 0 0.4 1265 0 3.5 -~.. 2.6 0.95 1202 4.6 0.95 1283 VYCORREACTOR 1202 0 2.6 1283 0 9.5 1202 0.95 12.6 1283 0.95 34.0 Product Distribution
Run No.
Temp., O F . % 0 2 in feed Products formed, % Hz CH4 CZH4 CZHE C3HE C3Hs
8219-162
1202 0.95 4.1 4.9 4.9
...
4.4 81.9
82 19- 128
1283 0.95 7.6 8.9 8.8 0.5 7.7 66.5
1202 0
1.7 1.5
... ...
2.0 94.8
1283 0 4.4 4.6 2.3 0.3 4.5 83.7
an initial propane conversion of 37.7% (86% of equilibrium) was obtained that declined by about 11% during the 90minute process period. At 1202' F. (LHSV = lo), however, the conversion declined by about 30% during the 90-minute run (initial conversion = 41.5%; 72% of equilibrium, T a ble I). Further, at the lower temperature selectivity for propylene was 93.5 to 97%, while a t 1202' F. the selectivity declined from 89% to 76% during the run. At this higher temperature the reactor plugged because of coking, which increased the pressure drop in the catalyst bed. Similar results were obtained at increased pressures, although propane conversions were lower because of equilibrium limitations on the reaction. Thus, at 10-atm. pressure and 1112' F. an activity decline of only 5% during the 90-minute run at a propylene selectivity of 94.5y0 was observed, while at 12020LF. the activity decline during the run was 35y0 at a propylene selectivity of only 73.5y0 (Table I). The data for conversions, propylene selectivities, and coke formed at various temperatures, pressures, and space velocities together with the corresponding heat sinks are tabulated in Table I and are further summarized in Figure 2, which shows conversion and heat sinks generated as a function of space velocities at several temperatures, and in Figure 3, which shows the effect of pressure on propane conversion and heat sinks generated. (Heats of reactions and sensible and latent heats were calculated using data of American Petroleum Institute Research Project 44.) Reaction products were mainly hydrogen and propylene. At comparable conversions increased pressure gave more cracked products and more coke. Typical product analyses are shown in Table 11. The coke formed during reaction was not measured directly but was estimated from the amount of hydrogen formed in excess of the propylene. Carbon-hydrogen analyses showed the coke to have the empirical formula C,Ho.49,. The propane converted to coke was then taken as 0.31 m, where m = % Hz formed - % C3H6 formed. This is a reasonable assumption for the experiments at 1-atm. pressure, where little or no methane or ethane was formed (Table 11). However, the values of propane converted to coke shown in Table I are minimum values for the experiments at 10 atm. The used catalyst could be completely regenerated in situ by passing a 570 oxygen in argon gas mixture over the catalyst bed at 1022' F. Thermal Reaction
The extent of the noncatalytic thermal decomposition of propane at 1-atm. pressure, 850' to 1290' F., and space velocities of 1 to 40 was determined with quartz chips substituted for the catalyst. The space velocities were calculated on the basis of the 20-ml. volume of quartz chips. At an LHSV of 1 essentially no propane conversion was observed at 1074' F. and lower, but at 1282' F. about %yoconversion was observed. At an LHSV of 20, propane conversions of about 0.5% and less were observed at 1157' F. and 3.5% at 1274' F. The complete data are shown in Figure 4. The propane conversions observed with the thermal reaction at LHSV of 20 are far too low to be of practical interest. However, the reaction rate can be accelerated considerably by free radical "initiators." For example, addition of 1% oxygen to the propane feed accelerated the rate of cracking by a factor of 3 to 6 a t an LHSV of 20 (contact time = 0.2 second, 1 atm., 1202' to 1283' F., Table1 11). This effect of added oxygen has been well studied by other researchers (6,12,73,75), who determined that the oxygen is consumed during reaction. VOL. 4
NO. 2
JUNE 1 9 6 5
131
P r e s s u r e : I Atm. Catalyst: Quartz Chips, 10-20 Mesh Catalyst Volume: 2 0 mi.
1600
___---
-
,0---
-/:
C
Atrn.
P . 1400
i
0 LHSV = 1 0 LHSV = 5 0
LHSV = 10
i
LHSV = 20
/
.wi4
/
c
-B
i
/
/
/
/
_+-_ / - - -
. e -
/
00 7 10 Atm.
0
1200-
c
+
/
100
/
/
/
/ /
-
80
c 60
d
c 3 40
E,z
20
Reaction Time: 30 Minutes
11 800
A
I 900
n
n 1000
"
0 0
550
600
IO22
1112
h50 1202
700 1292
Reaction Temperature
Figure 5. Dehydrogenation of methylcyclohexane over chrornia-alumina catalyst
Other possible initiators are organic peroxides and azo-type compounds. The decomposition products from the thermal reaction were mainly propylene, ethylene, and methane (Table 111). With addition of oxygen to the feed an increase in the relative amount of ethylene in the product was observed (Table 111). Mechanism of Coke Formation
Coke formation on the catalyst is one of the main factors that limit the usefulness of the propane dehydrogenation reaction. For example, considerably greater conversions would be obtained if the reaction could be carried out at higher temperatures with good selectivity. With the present catalyst, this is not possible because of rapid coke deposition at the higher temperatures. T o elucidate the mechanism of coke formation, a brief study was carried out to determine possible coke precursors. I n the metal reactor coke was deposited not only on the catalyst bed but also on the walls of the reactor and on the metal spacers just below the catalyst bed. Independent experiments, however, had shown that both propane and propylene were thermally stable in the metal reactor as well as in the Vycor reactor (1 atm., 1112' F., LHSV = 20). This suggested that coke was being formed from thermally unstable propylene reaction products. The view that propylene reacts further over the catalyst to form coke was also supported by the observation that the catalyst regeneration periods were five to 10 times longer when the catalyst was in contact with propylene than when in contact with propane (Table IV). The most logical secondary reaction product from propylene appeared to be allene or methylacetylene. Indeed, when mixtures of 5 or 10% allene in argon were passed through the metal reactor at 1112' F. with no catalyst present, conversion of allene to coke and cracked products was almost 100% (Table V). Under the same conditions in the Vycor reactor 132
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
allene formed a yellow-green liquid (probably a polymer) that collected in the cool part of the tube (Table V). A similar polymer deposit was observed in lesser amounts when propane or propylene was passed over the chromia catalyst in the Vycor reactor (Table IV). In the Vycor reactor coke was observed only on the catalyst particles. Thus, it appears that one main source of coking in the propane dehydrogenation reaction is from secondary propylene reaction products, presumably allene or methylacetylene. These products form polymers that are unstable on both the catalyst and the walls of the stainless steel reactor. This suggests that less coking might be obtained by using a catalyst that contains very large pores. [Wheeler (76, p. 317) has shown that for consecutive reactions of the type A B C the selectivity for the intermediate B will be greater with catalysts with larger pores.] Further, it is possible that coking may occur primarily on sites of high surface energy where propane is held very tightly to the surface until it reacts to form allene. Thus, reduced coke formation might be achieved by moderating the catalyst surface with promotors to eliminate sites of high energy, or by adding small amounts of foreign material in the feed that would preferentially adsorb on sites of highest energy. This is being investigated. -+
-+
Methylcyclohexane
Thermal Reaction. Methylcyclohexane (MCH) reacted thermally ~hthe metal tube reactor to give toluene, cracked products, and intermediate dehydrogenation products such as methylcyclohexenes and methylcyclohexadienes. At 10-atm. pressure and 1283' F., extensive reaction occurred, with over 50% of the M C H fed being converted (Table V). At this temperature and pressure the reaction products were principally cracked material with lesser amounts of dehydrogenation products. Lower conversions were observed a t lower temperatures and lower pressures (Table VI).
Table IV.
Reaction of Propane, Propylene, and Allene over Chromia-Alumina Catalyst
Temperature. 1112' F. Pressure. 1 atm.
Run No.
20
Catalyst vol.
20 ml.
Reaction Time, Min.
Catalyst Temp., OF.
Feed Convn., %
131
Propane
5960
VYCOR REACTOR 90 977
132 135
Propylene 5% Allene in argon
5960 1500
90 30
1108 1114
0.6 100
136
10% Allene in argon
1500
30
1175
100
90
METALREACTOR 982
20.6
Regeneration Time, Min.
Reaction Products
97.7 % Propylene" 2 . 3y0 Cracked products Cracked productsa 6870 to coke or polymern 32W to cracked products 66y0to coke or polymer 34y0 to cracked productsa
65 600 127
...
96-98y0 Propylene 105 4 2 % Cracked products 90 1092 Coke plus cracked prod510 5960 108 Propylene ucts Considerablepolymer material collected in cool part of tube with allenefeed. Polymer in lesser amounts also observed withpropylene and propane feeds. 61
a
GHSV
Feed
8219-
LHSV.
5960
Propane
Dehydrogenation. The dehydrogenation of methylcyclohexane (MCH) is not equilibrium limited in the region of our study and conversions of 99+% are theoretically possible. However, the chromia catalyst was only moderately active, hence high M C H conversions (70 to 80%) were possible only a t the higher temperatures (1202' to 1283' F.). Under these conditions, selectivities for toluene plus benzene and catalyst stabilities were only fair under 1-atm. pressure operation. At 10-atm. pressure improved catalyst stabilities were observed but selectivities were considerably less. As an example, at 1283' F. 81% conversion and 73% selectivity with a decline in activity of 18.5% during the 90-minute run were observed a t 1-atm. pressure; at 10-atm. pressure and 70% conversion the activity decline was 13.501, but the selectivity for toluene plus
19.0 17 .o 1
benzene was only 48y0 (Table VII). Higher selectivities and improved catalyst stabilities were observed at lower reaction temperatures (1022' to 1112' F.) a t both 1- and 10-atm. pressures. Similar results were obtained in one experiment using a Vycor reactor. From the data for the thermal reaction it appears that the loss in selectivities at the higher temperature and pressure was due to the thermal reaction occurring simultaneously with the dehydrogenation reactions. The experimental data for runs at various temperatures, pressures, and space velocities together with the initial heat sinks based on conversions and selectivities at 30-minute reaction times are presented in Table VII. Product analyses are given in Table V I I I . The data are further summarized in Figure 5, which shows M C H conversion and total heat sinks
Run Data Pressure: 1 Atm. Temperature: 1 1 12% LHSV: 5 Reaction Time: 30 Minutes Internal Standard
Air
Figure 6.
Gas-liquid chromatography product analysis of run 8277-40-1 Dehydrogenation of methylcyclohexane, little cracking evident
VOL. 4
NO. 2
JUNE 1965
133
Table V.
8219-
Reaction Time, Min.
Block Temp. OF.
113
30
1022
Run No.
)
1112 116
20
1022
Thermal Reaction of Allene
Pressure. 1 atm. GHSV". 1500 Feed. Allene in argon Catalyst Temp., % Allene Allene OF. in Feed Conm., 7 0 STAINLESS STEELREACTOR 1035 5 89.7 10 95.6 1036 100.0 5 1125 VYCOR REACTOR 1024 5 14.0
1112 1202
...
23.0 5 26.0 ... 10 44.0 GHSV = gas hourly space velocity = volume of gas (at STP)per volume of catalyst per hour.
...
Table VI.
Run No. 827724 1112 Temp., OF. 1 Pressure, atm. LHSV" 5 MCH conversion. % 0.4 Product com onen& % 99.6 Methylcycrohexane Toluene 0.1 Benzene ... 1-Methylcyclohexene 0.1 1.9 3-Methylcyclohexene 4-Methylcyclohexene 0.1 ... Cyclohexadienes ... Heavier than toluene 0.1 Cracked a LHSV calculated basis 20-ml. volume.
10
Coke and cracked material Coke and cracked material Coke and cracked material Polymer plus coke Polymer plus coke Polymer plus coke Polymer plus coke Polymer plus coke Polymer plus coke Polymer plus coke Polymer plus coke Polymer plus coke
Product Distribution in Thermal Reaction of Methylcyclohexane
Reaction time. 20 min. Catalyst. Quartz chips Catalyst volume. 20 ml. 25 27 1293 1202 1 1 5 5 14.0 3.2 96.8 0.2 0.1 0.6
... 1.8 ... ...
0.4
Run Data P r e s s u r e : 10 Aim. T e m p e r a t u r e : 1283°F. LHSV: 2 0 R e a c t i o n T i m e : 9 0 Minutes
86.0 2.5 1.5 1.5 1.9 2.5
...
57 1112 10 20 11.2
58- 7 1202 10 20 26.3
58-2 1283 10 20 51.1
88.8 0.2 0.1 0.7 1.5 0.6
73.7 0.6 0.9 1.9 4.1 1.8 0.6 0.9 16.4
48.9 2.2 3.5 2.3 4.6 2.3 1.3 1.3 35.3
...
...
...
8.2
4.0
Internal Standard
Figure
7. Gas-liquid chromatography product analysis of run 8277-53- 1 Dehydrogenation of methylcyclohexane, considerable cracking
134
Reaction Products
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Table VII.
Run No.
Press., Atm.
8277-
LHSV
Block Temp., "F.
29 31 32 65
1 1 1 1
5 5 5 5
1022 1112 1202 1283
40
1
5
1112
10
20 20
1022 1112 i 202 1283
43 45 48 52 a
Dehydrogenation of-Methylcyclohexane over Chromia-Alumina Catalyst
10
io
20
20 10 30-minute reaction time.
2% KzO, 8% CrzOa, on AMh Catalyst. 13.3 S t 0.2 g. Catalyst wt. Catalyst volume. 20 ml. Feed. Pure methylcyclohexane Selectivity for Catalyst Bed Methylcyclohexane Conversion, % Toluene Benzene, yo Heat Sink, B.t.u./Lb." - Temp., "F. 30 90 30 90 30 90 Total Total min. min. min. min. min. min. Reaction exp. equil. STAINLESS STEELREACTOR 938 41.0 39.3 99.0 99.94381 1231 1790 930 1013 61.2 56.9 95.7 92.8 550 1470 1860 1000 1116 70.3 53.5 91.1 91.1 600 1590 1930 1096 1206 81.3 66.4 73.5 61.6 ... ... ... 1182 VYCORREACTOR 998 55.6 44.6 99.9+ 99.94552 1442 1860 986 STAINLESS STEELREACTOR 182 1032 1790 75.6 88.1 21.2 16.8 943 923 310 1230 1860 93.0 87.4 948 34.2 959 35.4 340 1330 1930 1058 50.1 46.2 72.4 67.8 1042 48.1 20.9 315 1365 2005 69.9 60.6 1168 1141
+
Table VIII. Product Distribution in Dehydrogenation of Methylcyclohexane over Chromia-Alumina Run No. 827729 31 40a 32 65 43 45 48 52 Temp., "F. 1022 1112 1112 12Q2 1293 1022 1112 1202 1283 Pressure, atm. 1 1 1 1 1 10 10 10 10 LHSV 5 5 5 5 20 20 20 20 20 Product components, Methylcyclohexane 59.0 38.8 44.4 29.7 18.7 78.8 64.6 49.9 30.1 60.5 54.2 16.7 32.6 34.2 27.5 57.6 56 .O Toluene 40.5 3.5 5.5 0.1 0.4 0.9 0.7 2.3 6.1 Benzene 0.1' 1. o 1.2 1.3 0.5 0.7 1.5 1.1 0.8 0.6 1-Methylcyclohexene 3-Methylcyclohexene ... ... ... ... 0.2 ... ... ... 1.1 1.3 0.9 1.2 1.8 1.5 1 .o 1.4 0.9 1.1 4-Methylcyclohexene 1.2 19.8 0.4 0.8 0.3 0.4 6.0 27.2 0.1 Cracked After 90-minute reaction time. a Vycor reactor. ~
~~~~
52Ab
1283
~~
Table IX.
n-Octane Iso-octane Mixed dimethylhexane 2,2,5-Trimethylhexane n-Dodecane n-Hexadecane
39.4 8.0 4.8 2.8 3.1 2.3 39.2
Dehydrogenation of Paraffins over Chromia-Alumina Catalyst Pressure. 10 atm.
Reaction period.
Hydrocarbon n -Heptane
10
20
30 min.
Catalyst Temp.,
Block Temp., aF. 1112 1202
LHSV 20 20
31 .O 49.4
973 1065
Olejin 34.8 11.1
1112 1112
20 8.7
61.2 92.0
957 1065
9.5 1.5
Convn.,
%
OF.
as a function of temperature for 1- and 10-atm. pressure. The solid lines show MCH conversions; the broken lines are the corresponding heat sinks. T h e unusual shape of the conversion curves in which the conversion values at the higher temperatures are greater than would be expected from extrapolation is attributed to the occurrence of the concurrent thermal reaction. Reaction products from the dehydrogenation of MCH were mainly toluene with small amounts of benzene and methylcyclohexanes. Of the cyclohexene isomers, the 4-methyl appeared most abundant (Table VIII). Further, the 3methyl isomer appeared to be the product of the thermal reaction (compare runs 65, 52, and 52A in Table VI11 with 58-1, 58-2, and 27 in Table V I ) . More cyclohexenes were produced at the higher reaction pressure (cf. runs 40 and 45, Table VIII). This is illustrated by the GLC curves shown in Figures 6 and 7. [GLC analyses were carried out with a
Selectivity for, yo Diolejns Aromatics 4.5 18.1 3.2 10.7
4.1
,..
Cracked 38.7 68.4
5.9
...
81 .O 98.5
Heat Sink, B.t.u./Lb. Reaction Total 553 1391 413 1330
2
120
685 945
20-foot column (10% Carbowax on Chromosorb W at a column temperature of 70' C.). Carrier gas was helium at a flow rate of 60 cc. per minute.] T h e used catalysts were readily regenerable with 5% oxygen in argon at 1022' F. Dehydrogenation of Higher Paraffinic Hydrocarbons
Appreciable endothermic heats of reaction are possible when cyclization accompanies the dehydrogenation reaction. Even greater heats are obtained when the side chains on the cyclized products are also dehydrogenated. As noted above, the dehydrocyclization of n-octane to ethylbenzene absorbs 940 B.t.u. per pound, while if the product is further dehydrogenated to styrene a 50% greater heat sink (1440 B.t.u. per pound) is obtained. We have made a very brief study of seven different paraffins at elevated space velocities (8.7 to 22) and in the VOL. 4
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absence of hydrogen (most of the literature data are at low space velocities and with hydrogen). Both normal and branched paraffins in the C7 to C I 6region were tested. The results are given in Table IX. I n general, none of these hydrocarbons gave over 25y0 selectivity to aromatics (30 to 90% conversion). Cracking appeared to be the main reaction, enhanced cracking being observed with increasing carbon number. The effect of branching on paraffin reactivity was not clearly evident from the results of our experiments. I n the case of n- and iso-octane (at 1112’ F.), the extent of reaction and the product distributions were practically the same. With n-heptane the principal aromatic product was toluene, while the mixed dimethylhexanes and the trimethylhexane gave principally m- and p-xylenes. The dimethylhexanes gave the greatest yield of aromatics. Dodecane gave mainly hexylbenzenes. In the region of our study the reactivities of the various hydrocarbons appeared to increase with increasing carbon number. Of the higher paraffins surveyed over the chromia-alumina catalyst, none appeared to give sufficiently encouraging product distributions to warrant further study with this catalyst. The cracking reaction appears to be a concurrent reaction ; hence greater selectivities to the desired products may be possible with a more active catalyst that can be used at a lower temperature, assuming that the heat of activation is greater for cracking than for dehydrogenation. Conclusions
Of the feeds examined, only propane and methylcyclohexane gave substantial endothermic reaction. For the various paraffins the extent of reactivity increases with increasing molecular weight. No general effect of branching on reactivity could be discerned. A catalyst more selective for dehydrocyclization will be needed to achieve worthwhile heat sinks with the higher paraffins.
For propane the mechanism of coke production appears to involve propylene dehydrogenation products, perhaps allene and methylacetylene. Acknowledgment
Helpful discussions with Jack Fultz, Air Force Aero-Propu$ion Laboratory, are gratefully acknowledged, and also with our colleagues R. B. Olney, H. T. Henderson, and I. S. Bjorklund. Permission to publish was extended by the U. S. Air Force and by Shell Development Co.
literature Cited
(1) Ciapetta, F. G., Dobres, R. M., Baker, R. W., “Catalysis,” P. H. Emmett, ed., Vol. VI, Reinhold, New York, 1958. (2) Derbensev, Yu. I., Balandin, A. A., Isagulyants, G. V., Kinetics Catalysis ( U S S R ) 2, 667 (1961). (3) Dugger, G., A R S (Am. Rocket SOC.)J . 29, 819 (1959). (4) Hibbard, R. R., Division of Petroleum Chemistry, ACS, Preprints 5 , No. 4, C-5 (September 1960). (5) Hibbard, R. R., NASA Lewis Research Center, private communication. (6) Martin, R., Niclause, M., Dzierzynski, M., Compt. Rend. 254, 1786 (1962). (7) Pines, H., Chen, C. T., J . Am. Chem. SOC.82, 3562 (1960). (8) Pines, H., Chen, C. T., J . org. Chem. 26, 1057 (1961). (9) Shabtai, J., Chem. Ind. (London) 1962, p. 1282. (10) Smith, J. O., et al., U. S. Air Force, WADD TR 60-841, Part I1 (December 1961). (11) Zbzd.,‘Part I11 (October 1962). (12) Steacie, E. R. W., “Atomic and Free P.adical Reactions,” 2nd ed., Vol. I, p. 157, Reinhold, New York, 1954. (13) Stepukhovich, A. D., Tatarientsev, V. V., Dokl. Akad. Nauk SSSR 99,1049 (1954); C.A. 49,13630h. (14) Timofeeva, E. A., Shuikin, N. I., Dobrynina, T. P., Bull. Acad. Sei. U S S R Diu. Chem. Sci. 1961, p. 797. 15) Voevodsky, V. V., Trans. Faraday SOC.55, 65 (1959). 16) Wheeler, A., Aduan. Catalysis 3,249-327 (1951).
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RECEIVED for review October 28, 1964 ACCEPTED February 23, 1965 Division of Petroleum Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964. Work done under Air Force Contract AF 33(657)-11096.
VAPQRiPHASE DEHYDROCYCLIZATION OF SOME AROMATIC HYDROCARBONS DANIEL A. SCOLA’ Boston Laboratories,Monsanto Research Corp., Everett 49, Mass. Treatment of diphenylmethane; bibenzyl; trans-stilbene; lf4-diphenylbutadiene; and 1,I ’-binaphthyl over 0.6% platinum-silica gel catalyst at 550” C. in an atmosphere of hydrogen yielded fused ring systems indicating that the major reaction occurring was abstraction of nuclear hydrogen followed by intramolecular bond formation (dehydrocyclization). The dehydrocyclization reaction has potential application for the preparation of fused ring systems, such as substituted naphthalenes and substituted fluoroanthenes, from properly substituted aromatic hydrocarbons.
program designed to screen catalysts to promote hydroof the carbon-carbon bond between aromatic nuclei (73), it was observed that 0.6% platinum-silica gel catalyst promoted the dehydrocyclization of o-terphenyl to triphenylene (72). I t was of interest to determine whether this catalyst would also promote dehydrocyclization of other aroN A
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1 Present address, Research Laboratories, Corp., East Hartford, Conn.
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matic systems that have ortho-hydrogens’ capable of being abstracted and that would lead to the formation of fused ring systems. Although similar work has been reported by other investigators, only a few aromatic compounds have been studied (2, 8, 70, 77, 76). This article presents the results of the vapor-phase treatment of diphenylmethane; bibenzyl; trans-stilbene; 1,l-diphenylbutadiene; and 1,l ’-binaphthyl over 0.6% platinum-on-silica gel at 550” C. in a hydrogen atmosphere.
