INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1953 4401
1
I
Figure 16. n-Hexane Conversion in Presence of Ni SaltSilica-Alumina Catalysts
Figure 17. Hexane Isomer Yields in Presence of Ni SaltSilica-Alumina Catalysts
x
'
. A
-
20
40
60
80
100
N-Hexane Conversion (Mol %Chg)
Standard Ni catalyst; W = NidP04)z; 0 = NiBzO7; = NiW04; 0 = NiCrO; -0- = NiMor Pressure = 24.8 atm.; L.S.V. = 1.0 vol./vol./hr.; Hn/HC
-
equilibrium were not in complete agreement with the values calculated by Rossini and coworkers (40)from entropies and heats of combustion. The calculated and experimental values were in agreement for n-hexane and 2,3-dimethylbutane, but in the methylpentanes and 2,2-dimethylbutane large discrepancies were observed. This disagreement in the equilibrium concentrations of the hexanes was also reported by Kock and Richter (SI). I n the present investigation the observed molar ratio of isohexanes to n-hexane (Table 111) increases with increasing reaction temperature to a maximum value of 3.03 to 3.05 which remains constant over the temperature range 385" to 413" C. The selectivity of the isomerization reaction (ratio of Ce isomer yield to n-hexane converted) changes from 0.45 to 0.91 over this same temperature range. Thus, it appears that equilibrium between the hexane isomers has been established a t these temperatures. Table VI shows the average molar concentrations of the hexanes obtained a t 387" and 412" C. The calculated values from Rossini data a t these temperatures are also shown. Also included in Table VI are the concentrations of the hexanes obtained a t 385' C. using 2-methylpentane as the feed in the presence of the standard nickel catalyst. The results show that both n-hexane and 2methylpentane give the same concentrations of hexanes in the product
.
4
undergo degradation and hydrogen disproportionation with the resulting formation of lower molecular weight hydrocarbons and carbonaceous material, the latter causing loss in activity of the metal. However, in the presence of deuterium or hydrogen, exchange reactions (9) occur a t low temperature, while a t higher temperatures demethylation is the predominant reaction (21),and there is little if any carbonaceous material left on the surface of the catalyst. Thus, the role of hydrogen appears to be primarily to keep the catalyst surface clean of hydrocarbon residues and thus maintain its activity. The possibility that hydrogen is also necessary for the isomerization reaction itself cannot be ignored, but other experiments would be necessary to prove this. Hexane Equilibrium. Experimental data on the equilibrium concentrations of the hexanes over a temperature range of 25" to 200" C. have been reported by a number of investigators (11, 31, 43). Evering and D'Ouville (12) found that the experimentally determined concentrations of the isomeric hexanes a t
155
TABLE VI. EQUILIBRIUM CONCENTRATIONS FOR ISOMERIC HEXANES
Temperature, Feed
a
C.
-412-
387 2-Methylhexane uentanea n-
Calcd.
(40)
nhexane
Calcd. (40)
Temperature = 385' C.
The present data at higher temperatures show the same discrepancies between the calculated and the experimental values of the hexane concentrations as was observed by Evering and D'Ouville. For n-hexane and 2,3-dimethylbutane the calculated and experimental values are in fair agreement, but there are large differences in the values for the methylpentanes and 2,2dimethylbutane.
(Isomerization of Saturated Hydrocarbons)
NORMAL PENTANE, ISOHEXANES, HEPTANES, AND OCTANES F. G. CIAPETTAl
I
N T H E first paper of this series (p. 147), it was revealed
that complex catalysts, consisting of a hydrogenation cataIyst in combination with a silica-alumina cracking catalyst, are active and highly selective for the isomerization of normal hexane in the presence of hydrogen. The activity of the standard nickel-silica-alumina catalyst was further investigated for the isomerization of other alkane hydrocarbons. These include n-pentane, 2-methylpentane, 2,3and 2,2-dimethylbutanes, n-heptane, 2,3- and 2,4-dimethylpentanes, 2,2,3-trimethylbutane, n-octane, and 2,2,4-trimethylpentane. Under conditions similar t o those employed for the isomerization of n-hexane, this catalyst was found to be quite 1
Present address, Socony-Vacuum Laboratories, Paulsboro, N. J
AND
J. B. HUNTER
active and highly selective for the isomerization of all these hydrocarbons with the exception of 2,2,4-trimethylpentane. From the data obtained, a comparison has been made of the effect of carbon content and structure on the ease of isomerization of these paraffin hydrocarbons. EXPERIMENTAL
The apparatus, experimental procedure, and method of analysis are described in the preceding paper. The standard nickelsilica-alumina catalyst SA-5N (VII), containing 5% by weight of reduced nickel was used in this investigation. n-Pentane, 2-methylpentane, 2,3- and 2,2-dimethylbutanes, and 2,3- and 2,4-dimethylpentanes were obtained from the
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
156
Phillips Petroleum Corp. They were either the pure grade (99yo purity) or technical grade (>95% purity). +Heptane and 2,2,4trimethylpentane were >98% purity. n-Octane (99.5% purity) was obtained from the Humphrey-Wilkinson Corp., and 2,2,3-trimethylbutane (997u purity) from Eastman Kodak Corp Each of these hydrocarbons was analyzed by the mass spectrometer in order to calculate conversions and isomer yields on the basis of the pure compound. I n the present investigation the reaction pressure, hydrogen t o hydrocarbon molar ratio, and liquid space velocity were maintained constant a t 24.8 atmospheres, 4:1, and 1.0 volume of charge per volume of catalyst per hour, respectively. EXPERIMENTAL RESULTS
n-Pentane. The isomerization of n-pentane was investigated over the temperature range 340' to 410" C. The experimental data are given in Table I. Because of the volatility of the feed and the products, the total recoveries in several runs were not as high as normally obtained with the heavier hydrocarbons.
TABLE I.
ISOMERIZATION O F '%PENTANE
Catalyst = standard nickel-silica-alumina Pressure = 24.8 atm. __L.9.V. = 1.0 vol/vol./hr. np/nc= 4 271 272 273 Run No. 371 382 393 Temperature, O C. Total recovery, wt. % of charge 85.5 84.8 99.7 105.0
274 407
&!?
Product distribution, moles/ 100 moles of charge (noloss basis) Methane Ethane Propane Isobutane n-Butane Isopentane n-Pentane Mole 7Qof charge Convn. n-pentane Yield isopentane Selectivity factor
0.4
...
2.3 0.6 0.1 5.4 92.3
7.7
s.4 0.70
93.4
1.5 2.5 1.9 31.3 63.1
5.4 0.5 2.3 1.4 4.1 41.8 51.1
15.7 1.9 5.4 3.0 9.3 43.4 39.4
24.7 3.4 7.9 1.9 13.2 41.1 35.3
36.9 ' 31.3 0.85
48.9 41.8 0.86
60.6 43.4 0.72
64.6 41.1 0.64
3.1
...
The analysis of the products showed that the standard nickel catalyst is quite active for the isomerization of n-pentane to isopentane. The selectivity factor (molar ratio of isomer yield to n-pentane conversion) is not as high as found previously for n-hexane, but up to conversions of approximately 50% of npentane this factor is 0.85 to 0.86. At higher conversions the isomer yield decreases and hydrocracking to lower molecular weight hydrocarbons becomes more prominent. The absence
TABLE
Run No.
Temperature, ' C. Total recovery, wt. yo of charge Hexane Product distribution moles/lOO moles of charge (no-lom basis) Methane Ethane Propane Isobutane n-Butane Isopentane %-Pentane 2 2-Dimethylbutane 2:3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane Mole % of charge Convn. of hexane Cd isomer yield Selectivity factor Wt. % C on catalyst
11.
of neopentane is in agreement with the results reported by other investigators (34, 39) who studied the isomerization in the presence of aluminum chloride. Hoffever, the absence of higher molecular weight hydrocarbons indicates that in the presence of the standard nickel catalyst, destructive alkylation reactions do not occur as is commonly found with aluminum halides. Hexane Isomers. The isomerization of 2-methylpentane and 2,2- and 2,3-dimethylbutanes was investigated over the standard nickel catalyst. The experimental data are shoir-n in Table 11. The high selectivity of the standard nickel catalyst for isomerization, &st observed with n-hexane, is also found in the isomerisation of the isohexanes. Hydrocracking to lower molecular weight hydrocarbons is low in 2-methylpentane up to a conversion of 64.0 mole 7uand practically negligible for the dimethylbutanes even up to conversions of 68.0%. Hydrocarbons higher in molecular weight than the hexanes were not found in the products. Heptane Isomers. The data given in Table I11 reveal that the standard nickel-silica-alumina catalyst is very active and highly selective for the isomerization of n-heptane, 2,3- and 2,4dimethylpentanes, and, 2,2,3-triniethylbutane. The selectivity factors for the isomerization reaction a t high conversions (60 to 75%) show that this catalyst is vastly superior to the usual Friedel-Craft catalysts such as aluminum chloride and bromide (3, 87, 50) and to hydrogenation catalysts such as molpbdenum oxide or sulfide (6)and tungsten sulfide (37). At low conversions of n-heptane the only isomers formed are the 2- and 3-methylhexanes. At higher conversions, 2,4-dimethyl pentane appears in the product. The absence of the other heptane isomers, even at, a conversion of 81.1% is rather surprising. More recent data on similar catalysts showed that several of the other dimethylpentanes are also produced at the higher conversions. The isomerization of 2,3-diniethylpentane produces all the heptane isomers with the exception of n-heptane and 3-ethylpentane, The presence of 2,2-dimethylpentane in the products suggested that 2,2,3-trimethylbutane might also be present. Because of the inability of the mass spectrometer to differentiate between these two compounds when their concentrations are low, the product from run 254 was also analyzed by infrared. Of the reported 7.3% of 2,2-dimethylpentane, approximately 1% ' was 2,2,3-trimethylbutane. 4 s indicated in Table 111, hydrocracking of the heptanes a t conversions below 80 to 90% occurs preferentially a t the center of t,he molecule t o form propane and butanes. The high ratio of isobutane to n-butane observed in the product's using n-heptane as the charge (run 325) indicates that isomerizat'ion precedes the cracking reactions.
ISOMERIZATION OF
HEXAXE ISOMERS
Catalyst = standard nickel-si!ica-alumina Pressure = 2 4 . 8 atm. L.S.V. = 1 . 0 vol./vol./hr. Hn/HC = 4 661 663 778 779 659 660 776 777 328 343 371 357 385 288 314 302 329 287 95.1 96.8 100.2 97.8 94.4 95.3 98.0 98.1 97 4 _ , -2-Methylpentane 2,3-Dimethylbutane0.5
...
0.6
... ..
0.4 0.4 0.4 0.4
...
1.3
0.4 0.4 0.4 0.4
..
...
...
0.1 ,..
...
,
0.4 0.4
0.7 87.8 10.9
...
1.5 3.8 73.9 15.6 4.1
1.5 4.8 57.8 25.8 9.0
6.1 7.6 39.7 25.7 20.0
12.2 11.6 0.95
22.0 20.9 0.95
39.0 37.9 0.97
58.1 57.1 0.95
...
...
...
...
Vol. 45, No. I
2.1 0.4 0.8 1.2
...
,..
... 0.4 ...
.
.
I
...
0.4
,..
...
...
...
...
1.3 1.6 5.9 6.9 34.1 24.7 24.1
0.4
3.2 96.1
64.0 59.6 0.93 0.03
0.1
...
...
0.4
...
...
0.8
...
1.1
... ...
...
...
...
0.5 98.6 1.0
...
1.8
... ...
10.1 64.2 11.2 7.8 5.7
11.7 32.0 23.2 14.7 15.9
3.9 3.2 0.82
13.6 13.3 0.98
35.8 34.8 0.97
68.0 65.5 0.96
...
...
...
...
0.4
, , .
6.7 86.4 2.8 1.6 2.2
...
656 657 6.58 316 344 373 95.0 96.1 96.8 -2,2-Dimethylbutane655 287 92.1
0.05
I . .
...
... ... ... ... 1.3 ...
0.5 93.7 4.9
...
..
0.4
... ...
...
...
85.2 7.5 3.2
...
... 1.4 1.0 0.72
..
.. 0.4 ...
3.7
6.3 4.9 0.78
...
14.8 14.4 0.98
...
0.4
0.5 0.8 63 .5 10 3 10.1 6.2 8.3 36: 34.9 0 06 0.21
January 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
157
OF HEPTANE ISOMERS TABLE111. ISOMERIZATION
-
Run No. Temperature O C. Total recove&, wt. '%of charge Heptane Product distribution, moles/100 moles of charge (no-loss basis) Methane Ethane Propane Iscbutane n-Butane Isopentane n-Pentane Hexan e8 2 ~,3-Triinetliylbutane 2.2-Dimethylpcntane 2 3-Diiiiethylpentane P
322 288 97.5
... ...
1.1
... ...
Catalyst = standard nickel-silica-alumina 24.8atm. Pressure L.S.V. = 1.0 vol./vol./hr. Hl/HC = 4 323 324 325 252 253 254 313 314 315 316 315 332 353 270 291 316 256 286 313 343 97.2 98.7 97.5 95.5 98.8 98.7 92.5 96.7 97.5 97.6 n-Heptane2,3-Dimethylpeptane -2,4-Dlmethylpentane-
...
1.6 0.5
4.4
2.5
9.3 7.6 1.4 0.4 0.3 3.8
...
0.2 0.1 0.1 0.6
...
...
... ...
...
0.7'
... ... . . I
... ... ...
1.3
...
... ] . . I
...
0.7 1.6
...
0.3 .
.
I
0.1 1.3 87.0 6.0 1.3
4.8 6.6 3.4 2.4-dimethyl pentane ... ... ... 3.3-Dimet hylpentane ,.. ... ... 3-Ethylpentane 1 7 . 8 23.3 0.3 1.4 6.4 2-Methylhexane 2.6 39.1 32.4 9.1 4.0 3-Methylhexane 40.7 18.9 81.2 94.1 %-Heptane Mole % of charge 13.0 59.3 81.1 5.9 18.8 Convn. of heptane 11.5 56.8 67.2 18.9 C, isomer yield 5.4 0.92 1.00 0.96 0.83 0.89 Selectivity factor 0.12 Wt. % C on catalyst Approximately 1% of 2,2,3-trimethylbutane by infrared analysis.
...
I
1..
.
.
...
...
0.7 3.4 1.8
...
... ... ...
... ...
.... .. ... ... ... -.... ..
0.6
1.2
1.4
4.1 3.8 0.9
... ...
...
... ...
...
... ...
...
...
0.8
5.5 70.0 9.5 2.9
0.1 7.3" 45.6 14.2 5.6
1.3 11.3 85.5 0.3
7.5 19.7 59.0 2.7
3.6 5.7
8.9 13.3
0.9 0.7
3.9 6.5
13.0 22.7 24.9 8.2 0.8 11.3 12.0 1.9
30.0 27.2
54.4 49.3
14.5 14.5
...
..
. . . . 0.91 ..
.........
...
0.7 5.3 4.4 0.2
...
...
0.91 0.15
... ..
1.00
0.1
41.0 40.4
75.1 69.9
. . . .0 ..9 8. . 0.93 ..
20.0 0.7 17.7 15.8 2.2 1.0 1.8 14.3 5.5 9.2 6.9 4.5
408 409 410 411 304 320 338 354 94.2 99.S 95.1 93.6 -2,2,3-Trlmethylbutane-
0.6 0.7
... ... 0.2 ... ... ...
88.8 4.7 3.1 2.9
14.5 15.9 6.6
... ... ... ... ...
93.1 57.2
11.2 10.7
1.0
0.61 0.11
0.96
0.6 1.3
...
0.6 1.7 0.2 1.6 0.3 0.1
1.3 5.9 5.7 0.2 0.2
0.1 81.9 8.5 4.7 3.9
0.3 76.6 6.4 3.8 4.4
0.2 62.2 5.9 4.4 4.3
...
...
1.9 5.5
5.3 11.1
18.1 17.1
23.4 22.0
37.8 310
...
0.3
...
0.1 0.1
...
...
...
..
...
...
. . . .0.95 . . . 0. . 9.4
...
...
...
0.82 0.08
most completely converted into two molecules of isobutane Octanes. Only two of the octanes, n-octane and 2,2,4-triin the presence of the standard nickel catalyst. This behavior methylpentane, were studied in the presence of the standard of 2,2,4-trimethylpentane is peculiar in view of the results obnickel catalyst. The results are given in Table IV. tained with 2,2,%trimethylbutane. The high coke content of Two series of runs were made using n-octane as the feed In the used catalyst (0.57 wt. %) suggests that the isomerization the &st series (runs 605 t o 608) an upset occurred in the hydrogen activity of the catalyst may have been poisoned early in the run flow prior to the second test run (606) which caused a decrease series. As pointed out in the first paper, insufficient hydrogen, in the activity of the catalyst. At the end of this series of runs or rapid hydrocracking resulting in a momentary deficiency of the carbon content of the catalyst was 0.32% by weight. The hydrogen, can poison these catalysts for the isomerization reacsecond series of runs (runs 609-612) was started at a higher tion. temperature (316" C.), and the results showed that the catalyst had a much higher activity than the first batch of catalyst at DISCUSSION this same temperature. At the end of the second series of runs The literature on the isomerization of saturated hydrocarbons the carbon content was 0.09% by weight. is too extensive to review in this paper (7, 38) Probably the The selectivity of the standard catalyst for the isomerization of n-octane appears to be similar to that for the heptanes and most active catalysts available for the selective isomerization of alkanes are the aluminum halides, and aluminum chloride and hexanes. Even a t 97.9% conversion the selectivity factor is bromide. With these catalysts the isomerization of the lower 0.71. At a conversion of 63.0'%, the mass spectrometer analysis of the liquid product revealed that the only isomers present were molecular weight alkanes, butanes, pentanes, and hexanes has 2-, 3-, and Pmethylheptanes and possibly 2,Bdimethylhexane. been extensively studied, and several commercial processes for the isomerization of these hydrocarbons have been developed A t higher conversions 2,4-dimethylhexane is formed. Since the (4, &). Recently, Haensel and liquid product was not prefractionDonaldson (g1) have reported data ated before analysis by the mass 440 on the isomerization of n-heptane spectrometer, there exists the possiin the presence of a platinum conbility that small amounts of other taining catalyst. octane isomers may be present. par400It is well known that starting with ticularly a t the higher conversions. pentanes, degradation reactions to As in the heptanes, hydrocrackform lower molecular weight hydroing of n-octane occurs primarily at carbons and destructive alkylation the center of the chain. The prereactions to form higher molecular dominance of isobutane and isoweight hydrocarbons occur simulpentane shows that isomerization taneously with these catalysts. Supof n-octane precedes the hydropression of these side reactions by cracking reaction. The relative the addition of hydrogen or aromatic molar quantities of butanes and propane indicate that approxiand cycloalkane hydrocarbons has 0 been extensively studied (38). For mately three molecules of octane are cleaved into two butane mole240 I 4 I the isomerization of butanes and 0 20 40 60 80 loo pentanes these additives are quiteefcules for every one reacting to N-Paraffin Conversion (Mol %Chg.) fective. However, in the heptanes give a molecule of propane and one Figure 1. Effect of Reaction Temperature and higher molecular weight alkanes of pentane. on Conversion of n-Paraffins these cracking suppressors have been The data in Table I V reveal -0- n-Pentane 0 = n-Heptane that 2,2,4trimethylpentane is al0 = n-Hexane 0 n-Octane of little value. Extensive degrada-
-
5
INDUSTRIAL AND ENGINEERING CHEMISTRY
158
Vol. 45, No. 1
TABLE IV. ISOMERIZATION OF OCTANE ISOMERS
Run KO. Temperature C. Total recoveiy, wt. % of charge Octane Product distribution, moles/100 moles of charge (no-loss basis) Methane Ethane Propane IsoJbutane Isobutane n-Butane Isopentane n-Pentane Hexanes Heptanes 2 5-Dimethylhexane 2:4-Dimethylhexane 2,3-Dimethylhexane 4-Methylheptane 4-MethyJheptane 3--Methylheptane 3--Me 2-Me 2-Methylheptane n-Octane n-Oct 2,2,4-Trimethylpentane Mole % of charge Convn. of octane Ca isomer yield Selectivity factor Wt. % C on catalyst Predominantly 4-methylheptane.
Catalyst = standard nickel-silica-alumina Pressure = 24.8 atm. L.S.V. = 1.0 vol./vol./hr. Hz/HC = 4 606 607 608 609 610 287 316 348 316 348 970 98.8 95.3 96.6 98.1 97.0 n-Octane
605 260 97.8
... 0.8
...
1.4
0.0 0.6
...
...
... ...
I
...
.. .. ..
. I ,
,..
..,
)l.05 . . 3.5 3.9 91.1
... ,.. ...
99.2
-
0.8
..
.
14.7 63.2
...
8.9 8.4 0.95
36.8 34.5 0.94
...
...
5.7=
14.1
...
. t .
,
...
... ...
... .
0.8 2.2 1.2 0.6 0.8 0.2
...
... ... ... ... .
...
..,
0.8
...
1.4
...
10.1 21.6 13.63 1 0 . 6j 2.8
...
... 1.6
9.0 10.0' 18.0 -- 3 16.1 14.6
...
85.4 54.7 0.64 0.32
... ...
0.7
0.7
10.6 17.9 12.4 10.1 2.5 0.5 0.2
222.6 2.6 33.8 23.0 17.4 5.4 0.3 0.3
...
1.0 1.0 0.2 0.8 0.2 0.7 0.3
... ...
...
10.5 1 7 . ~ 5 ~1 6 , 1 a 32.1 21.9 11.2 21.2 37.0 2.1
...
...
63.0 60.8 0.97
97.9 69.7 0.71
...
611 364 9 55 .66
...
I
.
635 636 637 638 336 259 288 320 96.9 93.,3 95.1 884 -2,2,4-Trimethylpentane--
2.8
I
...
11.9 8.8a 11.9 14.9 0.0
... ..
100.0 47.5 0.48
32.8 49.2 36.0 26.3 10.9 0.8 0.3
,..
0.8 12.2 0.2
...
0.6
1.2
1.2
92.9
69.0
59.1
7.1 0.6
31.0 1.2
40.9 1.8
...
...
.
...
...
...
1.8 0.4
...
...
...
I . .
...
0.8 76.3 0.2
... ... ... ... ...
.
...
...
1
..,
0.8 57.7 0.6
... ...
I
... ... ... ...
3.4 5,0n 0.4 5.0 5.1 0.6 I 98.2
99.4 17.4 0.17 0.09
...
..I
0.8 3.0 0.2
...
...
...
.
...
,..
.
. t .
612 380 9 65. 2
...
...
0.57
of the hexanes is shown in Figure 2. At conversions below 3 . 3 mole %, 2-methylpentane isomerizes at a faster rate than nhexane, showing that the presence of a tertiary hydrogen atom increases the reactivity of the molecule. However, 2,a-dimethylbutane isomerizes a t a slightly lower rate than n-hexane although it contains two tertiary hydrogen atoms. Neohexane is the most stable of the hexane isomers studied showing that the presence of the gem methyl group (or quaternary carbon) deactivates the molecule for isomerization (14). Thus, the order of activity found for the isomerization of hexanes ie Zmethylpentane > n-hexane > 2,3-dimethylbutane > 2,2-dimethylbutane. This order of activity for the isomerization of the hexanes is different from that observed by Evering and Waugh ( l a ) in the presence of an aluminum chloride-hydrogen chloride catalyst at much lower temperatures. '2401
I
20
I
40
I
60
I
80
1
100
4001
Hexane Conversion (Mol. Yo Chg.) Figure 2. Effect of Reaction Temperature on Conversion of Hexane Isomers
-($- = 2,2-Dimethylbutane Q
-
2,S-Dimethylbutane
I
1
I
0 = 2-Methylpentane
---
= n-Hexane
tion of heptanes and octanes resulting in poor isomerization selectivity has prevented any systematic study of the effect of molecular size and structure on the relative rates of isomerization of paraffin hydrocarbons. The data obtained on the isomerization of n-pentane, n-hexane, n-heptane, and n-octane tvith the standard nickelsilica-alumina catalyst (Figure 1) show that as the carbon content increases, the temperature necessary to obtain the same conversion decreases significantly. Comparing these n-alkanes at 50 mole % conversion, where the isomerization selectivity is over 95% for n-hexane, n-heptane, and n-octane and 86% for n-pentane, the reaction temperatures necessary to obtain this conversion are 384" C. for n-pentane, 346' C. for n-hexane, 323' C. for n-heptane, and 302" C. for n-octane. Thus, a t a constant liquid space velocity, n-octane gives the same conversion as n-pentane a t a temperature which is approximately 80" C. lower than that used for the pentane. The increase in reactivity with increase in carbon content is similar t o that observed for the catalytic cracking rates of the n-alkanes in the presence of cracking catalysts (16). The effect of carbon structure on the rate of isomerization
L.-=
I I
20
I
1
40
60
80
IOd
C7 Conversion (Mol 'YoChg.) Figure 3. Effect of Reaction Temperature on Conversion of Heptane Isomers -0- = 2,2,3-Trimethylbutane
0
E
2,3-Dimethylpentane
0 = 2,4-Dimethylpentane
---
= n-Heptane
The relative rates of isomerization of the four heptanes, ag shown in Figure 3, decrease in the order 2,4dimethylpentane > 2,3-dimethylpentane > n-heptane > 2,2,%trimethylpentane. As in the hexanes, the introduction of a tertiary hydrogen atom increases the activity of the molecule. However, the presence of a quaternary carbon atom in the molecule decreases the activity of 2,2,3-trimethylbutane for the isomerization reaction.
January 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY CH~-CHZ-CH~-CHZ-CH~--CH~
As shown in Figures 2 and 3, the addition of a methylene group
Jl"
to 2,3-dimethylbutane increases the reactivity of the molecule. For 50 mole % conversion of 2,3-dimethylpentane a reaction temperature of approximately 310" C. is necessary compared to 350" C. for the mme conversion of 2,3-dimethylbutane. However, the substitution of a methyl group for a secondary hydrogen atom in 2,2-dimethylbutane does not result in as large a change in the reactivity of the molecule. The data show that 2,2,Strimethylbutane requires a reaction temperature of approximately 20" C. lower than 2,2-dimethylbutane for the same conversion. The data presented in the previous paper on n-hexane and that shown in Tables I11 and IV reveal that the initial products formed in the isomerization of n-hexane, n-heptane, and n-octane are the methyl isomers. n-Hexane isomerizes to 2- and 3-methylpentanes; n-heptane to 2- and 3-methylhexanes; and n-octane t o 2-, 3-, and 4methylheptanes. These results substantiate the conclusions of Evering and Waugh (12) that the isomerization reaction occurs in a stepwise manner. For n-hexane the isomerization to the isomers appears to occur as shown herewith. Since the isomerization of Smethylpentane was not investigated, the direct formation of 2,2-dimethylbutane from this isomer is not indicated. The data contained in Tables I11 and IV show that hydro-
159
I
CHs-CH-CH2-CH2-CHa
LT
11 r3
+ CHa-CHz-
CHI CHI CHI-
(!A
H-
H-CH3
H--CHz-CHs
CH3 72
I
CH3-C-CHz-CH3
I
CH3 cracking of the heptanes and octanes at the higher conversions results primarily in the formation of propane, butanes, and pentanes. The absence of substantial amounts of methane and ethane reveals that cracking of these hydrocarbons occurs primarily at the center of the molecules. Thus the nickel-silica-alumina catalyst does not show the typical demethanation reaction of nickel-containing catalysts (22). This behavior of the nickelsilica-alumina catalyst is similar t o that observed by several investigators (8, 18) in the catalytic cracking of alkane hydrocarbons in the presence of silica-alumina-zirconia catalysts. The pronounced tendency of these catalysts to form butanes and pentanes in the cracking of higher molecular weight hydrocarbons is well known.
(Isomeriza t ion of Saturated Hydrocarbons)
CYCLOALKANES F. G. CIAPETTA'
T
H E activity of the standard nickel-silica-alumina catalyst was further investigated for the isomerization of cycloalkane hydrocarbons. These include methylcyclopentane, cyclohexane, methylcyclohexane, and ethylcyclohexane. Under conditions similar to those employed for the alkane hydrocarbons, this catalyst was found to be quite active and highly selective for the isomerization of these cycloalkanes. The formation of 1,ldimethylcyclopentane from methylcyclohexane and 1,l-dimethylcyclohexane and 1,1,2-trimethylcyclopentane from ethylcyclohexane in appreciable yields shows that the cycloalkanes containing a quaternary carbon atom are normal products of the isomerization of the alkylcyclohexanes ($1). EXPERIMENTAL
The apparatus, experimental procedure, reaction conditions, and method of analysis were similar to those described in the first paper of this series (p. 147). The standard nickel-silicaalumina catalyst was used in this investigation. Methylcyclopentane (95% purity) and cyclohexane (99% purity) were obtained from the PhillipsPetroleum Corp. Methylcyclohexane and ethylcyclohexane were of 99% purity and were obtained from Eastman Kodak Corp. Each of these hydrocarbons was analyzed by the mass spectrometer in order to calculate conversion and isomer yields on the basis of the pure compound. EXPERIMENTAL RESULTS
Methylcyclopentane. The isomerization of methylcyclopentane in the presence of the standard nickel-silica-alumina catalyst was investigated over the temperature range of 280" to 370" C. The experimental data are given in Table I. The molar conversion of methylcyclopentane as a function of the reaction temperature is plotted in Figure 1. 1 Present
addreas, Socony-Vaouum Laboratories, Paulsboro, N. J.
Analysis of the reaction products revealed that isomerization to cyclohexane was the predominant reaction a t all conversions, but a t the higher reaction temperatures, hydrocracking to hexanes and lower molecular weight hydrocarbons and isomerizationdehydrogenation to benzene also occur. The selectivity factor (molar ratio of isomer yield to methylcyclopentane conversion) is high initially, having a value of 0.97, which drops off to 0.65 a t the highest conversion. Since side reactions are not too extensive, the apparent low reactivity of methylcyclopentane is understandable from the standpoint of the equilibrium concentrations of methylcyclopentane and cyclohexane a t these conditions. I n Table I1 are the observed and calculated ($9) values for methylcyclopentane and cyclohexane. Comparison of these values show that the observed concentrations of cyclohexane and methylcyclopentane obtained above 316" C. are in good agreement with the calculated values. Cyclohexane. The data shown in Table I and plotted in Figure 1 reveal that cyclohexane is selectively isomerized to methylcyclopentane in the presence of the standard nickelsilica-alumina catalyst. Because of a partial hydrogen failure prior to run 626, the conversion decreased a t the highest temperature studied. The selectivity factor a t the highest conversion obtained-i.e., 76.9%-is 0.95 showing that side reactions are not too extensive under these conditions. At this conversion only 2.4 mole % of the cyclohexane is converted to hexanes and lower molecular weight alkane hydrocarbons, and 1.9 mole yo is dehydrogenated to benzene. Hydrocarbons higher in molecular weight than the feed were not found. As shown in Table 11,the observed mole per cent concentrations of cyclohexane and methylcyclopentane were rapidly approaching the calculated equilibrium values as the reaction temperature was increased. However, due to the decrease in the catalyst activity prior to run 626, the equilibrium concentrations were
