January 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
(1) Egloff, G., Morrell, J. c-8 Thoma*$c. L.1 and H. s . ~ J. Am. Chem. SOC., 61, 3571 (1939). (2) Good, G. M., VOge, H. H., and Greensfelder, €3. s., I N D . E N G . CHEM.,39, 1032 (1947). (3) Greensfelder, B. S., Voge, H. H., and Good, G. M., Ibid., 41, 2573 (1949). (4) Grosse, A. V., and Wackher, R. C., IND. ENG.CHEM.,ANAL. E D . , 11, 614 (1939). (5) Mateen, F. A., Chem. Eng. News, 28, 4552 (1950). B1ochl
147
(6) Rice. F. 0.. and Kossiakoff. J . Am. Chem. Soc.. 65. 590 (1943). (7j Rice; F. O., and Teller, E., J . Chem. Phys., 6, 489 (1938): (8) Richardson, R. W., Johnson, F. B., and Robbins, L. V., IND. ENG.CHEM.,41, 1729 (1949). (9) Thomas, c . L., bid,, 41, 2564 (1949). (10) Voge, H. H., Good, G. M., and Greensfelder, B. S., Ibid., 38, 1033 (1946).
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~,h~~,",n,~~~~,2566
RECEIVED for review May 14, 1952.
ACCEPTED August 21, 1952.
Isomerization of Saturated Hydrocarbons in Presence of Hydrogenation= Cracking Catalysts NORMAL HEXANE F. G . CIAPETTAl AND J. B. HUNTER The Atlantic Refining Co.,Philadelphia, Pa.
P
REVIOUS investigators have shown that in the presence of hydrogenation catalysts such as nickel, cobalt, or platinum, saturated hydrocarbons undergo ( a ) an exchange reaction with deuterium (9); (b) hydrocracking t o lower molecular weight hydrocarbons in the presence of hydrogen (22, 28, 3'7, 46); (c) cracking to methane, hydrogen, and carbon (41); ( d ) dehydrogenation to alkenes and aromatics which includes dehydrogenation of Cp to C6 alkanes to alkenes ( I S , 32, 33), dehydrogenation of Ce cycloalkanes to aromatics (48), and dehydrocyclization of alkanes with six or more carbon atoms to aromatics (26); and ( e ) isomerization of alkylcyclopentanes t o cyclohexanes and normal alkanes to isoalkanes in the presence of hydrogen (7,
21, 93).
The catalytic cracking of pure saturated hydrocarbons in the presence of catalysts such as silica-alumina or silica-zirconiaalumina a t atmospheric pressure and temperatures of the order of 500" to 550" C. has been previously reported (20). Good and coworkers ( 1 4 , on the basis of their extensive analyses of the products obtained by passing the hexane isomers over a silica-zirconia-alumina catalyst a t 550" C., concluded that isomerization under the conditions investigated occurs only to a small extent. It has been found, on combining a hydrogenation agent with a cracking agent, that the resulting complex catalyst, in the presence of hydrogen, is very active and highly selective for the isomerization of alkane and cycloalkane hydrocarbons. The present paper describes the experimental data obtained in the investigation of these catalysts, and includes a study of the isomerization of n-hexane in the presence of nickel-silica-alumina catalysts, and the extension of this work to catalysts containing other hydrogenation components in combination with silicaalumina. Also included in this paper is the investigation of the effect of reaction variables and catalyst composition of nickelsilica-alumina catalysts on the activity and selectivity of these catalysts for the isomerization reaction. EXPERIMENTAL
Apparatus. A flow diagram of the laboratory high pressure dynamic unit used for this investigation is shown in Figure 1. Electrolytic hydrogen, freed of oxygen by passing through a 1
Present address, Sooony-Vaouum Laboratories, Paulsboro, N. J.
Deoxo gas purifier (Baker & Co., Inc.) and freed of water by a dryer containing 4-8 mesh silica gel, flows a t the desired rate through one of two calibrated high pressure capillaries, which is automatically controlled by a Hammel-Dah1 valve, to the mixing block near the top of the reactor. There i t is joined by the n-hexane whose rate is controlled by a small high pressure displacement pump. The liquid feed is charged to the pump from one of two calibrated glass burets of 50-ml. capacity. The gas and liquid feed enters the top of the reactor (O/18 inch i.d.), passes through a preheating section and then through the catalyst (50 ml.). The reactor is heated by an automatically controlled aluminumbronze block furnace (19). The products are cooled by a water condenser and collected in a 300 ml. high pressure receiver. The gaseous product passes through a back pressure, automatically controlled Hammel-Dah1 valve to a glass trap cooled in dry ice-alcohol mixture and through a wet gas test meter. When desired, gas samples are collected in a gas bottle by displacement of brine solution. The liquid product is collected in a low pressure receiver. The catalyst and furnace temperatures were measured by iron-constantan thermocouples. The thermocouple in the catalyst bed could be moved to obtain temperatures along the full length of the catalyst bed. Procedure. After activating the catalyst as described later, the furnace temperature -was adjusted t o the initial reaction temperature and the unit brought t o reaction pressure (usually 24.8 atm.) with hydrogen. After pressure testing the unit, the desired hydrogen flow (usually 4 moles of hydrogen per mole of hydrocarbon) was established and the feed pump started. n-Hexane was passed over the catalyst with hydrogen for a 1-hour pretest period (during which period the feed rates and temperatures were carefully adjusted t o the desired values). After removal of the p r e t e s t eriod liquid product, a 1/2-hour test run was begun. At the enzof the test run, the liquid product was collected from the high pressure receiver, and the reaction temperature was increased approximately 25" C. The reaction conditions were adjusted a t this higher temperature during a 1hour pretest period, after which a second '/%-hour test run was made. This same procedure was used to make test runs a t higher temperatures. Since these catalysts show little or no deactivation under the conditions used, it is possible to make as many test runs as desirable. Usually a series of four test runs (representing a total of 6 hours on stream) a t successively higher temperatures was made on each catalyst. At the end of each test run, the dry-ice trap was disconnected from the unit and immediately connected to a gas collecting bottle and the trap liquid permitted t o weather to room temperature. The stabilized liquid in the trap was then added to the primary liquid product. A t regular periods, samples of the primary gas passing through the gas meter were obtained for
148
INDUSTRIAL AND ENGINEERING CHEMISTRY
analysis. The total liquid weight and the gas volume were recorded at the end of each test period. Method of Analysis. The liquid product was first fractionated in a high efficiency, low temperature Podbielniak column to remove and determine the concentration of C1to CShydrocarbons. T h e hexane residue was analyzed by a Consolidated Engineering Co. mass spectrometer. Samples of the primary gas and also trap gas (if sufficient) were analyzed for hydrogen and hydrocarbons by the mass spectrometer. In this manner, a complete analysis of all the products was obtained from which could be calculated the yields of all the hydrocarbons produced. The accuracy of mass spectrographic analyses of these hydrocarbons has been discussed by Brown et al. ( 2 ) . For the hexanes the average deviation in the absolute concentrations is = t l . O % . Where the concentrations of the individual isomers is greater than E%,this deviation may be as high as 1 1 . 5 % . n-Hexane. Technical n-hexane supplied by the Phillips Petroleum Corp. was used as received in the present investigation. Mass spectrographic analyses of all the batches used showed that the n-hexane concentration varied from 96 to 98 mole 70,the remainder being methylcgclopentane. The analysis of the charge was used to calculate the conversion of n-hexane and the yields of other hydrocarbons on a pure n-hexane feed basis. CATALYSTS
The following code identifies the various catalyst preparations:
W = tungsten oxide SA = silica-alumina E M = nickel molybdate N = nickel NW = nickel tungstate Co = cobalt NCr = nickel chromate Fe = iron N P = nickel phosphate Pt = platinum N B = nickel tetraborate Cu = copper Mo = molybdenum oxide Arabic numbers signify concentration (wt. %) of metal in finished catalyst. For the nickel salts, number shows concentration of nickel. Roman numbers signify catalyst numbers. Capital letters signify successive batches of the same catalyst. Nickel Catalysts. Standard catalysts, SA-5N (VII) and S A d N (VII-D), used for a major part of this work and referred to as the standard nickel catalyst, were prepared by the following procedure: 198 grams of Ni(?rTO3)~.6H20was dissolved in 1700 ml. of water, and 760 grams of ground fresh silica-alumina (commercial cracking catalyst produced by Davidson Chemical Corp., 12.8% dlz08,surface area = 420 square meters/gram) was added with vigorous stirring. To the slurry was added 1 liter of 1.84 AT ammonium carbonate solution; the mixture was stirred for 15 minutes, filtered, and dried a t 110" C. for 16 hours. The amount of ammonium carbonate used in this preparation corresponds t o approximately 40 to 42% in excess of the theoretical quantity needed to form the nickel carbonate. Catalyst S d d N (VII-E) was prepared in an analogous manner to the above catalysts except that only a 5% excess of ammonium carbonate over the theoretical quantity was employed. Catalysts SA-2.51\' (XXI), SA-5K (XXII), S A d N (XVI), SA-7.SN (XYIII), SA-1ON (XXIV), SA-5N (XXV), SA4-20X (XXVI) were prepared by mixing a knovn volume of a nickel carbonate slurry with a slurry of silica-alumina. The nickel carbonate \Tas prepared by adding 6.7 liters of 1 29 N ammonium carbonate solution to 6.0 liters of 1.36 N nickel iiitrate solution with vigorous stirring. The mixture was stirred for 0.5 hour, filtered, and washed five times with water (each wash was carried out with 8 liters of water), and the mashed cake was reslurried in 8 liters of water. The slurry was analyzed for nickel content. Each catalyst was prepared by adding the proper amount of nickel carbonate to give the desired final nickel concentration to a slurry of a known weight of silica-alumina contained in 2.0 liters of water. After stirring for 0.5 hour, the mixture was filtered and dried a t 110" C. for 16 hours.
Vol. 45, No. 1
Catalysts SA-5N (XI), (XII), (XIV), (XV), and (XXVII) were prepared by mixing the required amounts of freshly precipitated and washed gels of nickel carbonate, silica, and alumina. The nickel carbonate gel was prepared as described. The silica gel was made by adding 2 iV hydrochloric acid to a 1.0 N sodium silicate solution to give a final pH of 5.5 to 6.0. The gel was washed free of sodium, reslurried in water, and analyzed for silica content. The alumina gel was prepared by adding 10% ammonia to a 0.9 hr aluminum nitrate solution until the mixture was basic to litmus. The gel was washed six times, filtered, and reslurried in water and then analyzed €or alumina content. Catalyst SA-5N (X) was prepared from 750 ml. of 1.36 X nickel nitrate solution and 750 ml. of 1.96 iV sodium carbonate solution, which were heated separately to 90" C. and then added together rapidly. The hot mixture mas filtered immediately and the precipitate mashed five times with water (each wash was with 2 liters of water). The filtered cake was slurried in 2 liters of water and analyzed for nickel content. The required amount was added to a slurry of silica-alumina in water, and the resulting mixture was filtered and dried a t 110" C. for 16 hours. Catalyst SA-5K (T'III) was prepared by dissolving 74.3 grams of Ni(N0&.6Hz0 in 250 ml. of water; the resulting solution was absorbed on 300 grams of silica-alumina, and the product was dried a t 110" C. for 16 hours. Cobalt Catalyst. Catalyst SA-5Co (111) was made from 98.7 grams of C O ( N O ~ ) ~ . ~dissolved H ~ O in $50 ml. of lyater, t o which 380 grams of silica-alumina was added and thoroughly slurried in the solution. T o the stirred mixture there was added 480 ml. of 1.84 N ammonium carbonate solution. The mixture was stirred for 15 minutes, filtered. and dried a t 110' C. for 16 hours. Iron Catalyst. Catalyst SA-5Fe (11) was prepared by dissolving 144.5 grams of Fe(NO3)3.6H2Oin 850 ml. of water and adding 380 grams of silica-alumina to the solution. To the resulting slurry there was added 758 ml. of 1.84 N ammonium carbonate solution; the mixture was stirred for 15 minutes, filtered, and dried a t 110" C. for 16 hours. Platinum Catalyst. Catalyst SA-0.5Pt (11) was prepared by dissolving 2.66 grams of chloroplatinic acid (H2PtCls.6H20) in water and diluting the solution to 150 ml. This solution was absorbed on 200 grams of silica-alumina, and the product was dried a t 110' C. for 16 hours. Copper Catalyst. Catalyst SA-5Cu ( I )contained 46.4 grams of copper nitrate, C~(;?rT03)~.6H~0, dissolved in water and the volume of the solution made up to 150 ml. The resulting solution was absorbed on 200 grams of silica-alumina. and the product was dried at 110' C. for 16 hours. Metal Oxide Catalysts. Molybdenum oxide catalyst SA-10 Mo (I) was made by dissolving 111 grams of ( N H ~ ) B R I O ~ O ~ ~ . ~ H ~ O in 860 ml. of mater to which 69 ml. of concentrated ammonia was added. The resulting solution was used to impregnate 540 grams of silica-alumina. The product was dried at 110' C. for 16 hours. Molybdenum oxide catalyst A-lObIo (I) was made by using a solution of ammonium molybdate (prepared as given above) to impregnate 540 grams of commercial granular activated alumina (#F-1, Aluminum Co. of America). The product was dried a t 110' C. for 16 hours and used in the granular form. Tungsten oxide catalyst SA-5W (I) was started with 44.8 grams of NazWO4.2H20dissolved in 405 ml. of water. T o the stirred solution there was added rapidly 39.6 ml. of 10% hyclrochloric acid. The etirrer was turned off and the resulting solution allowed to gel (usually 2 to 3 minutes). The gel was slurried in 500 ml. of water, filtered, and washed until free of sodium ions The cake was then slurried in 1 liter of water and 468 grams of silica-alumina was added. The mixture was filtered and dried a t 110" C. for 16 hours. Nickel Salt Catalysts. Catalyst SA-1ONRI ( I ) was made from a solution of 24 grams of ( N H ~ ) B M o ~ O ~ dissolved ~ . ~ H ~ Oin
I N D U S T R I A L A N D E N G I N E E R I N G CHEM ISTRY
January 1953
149
Catalyst SA-1ONB (I) was prepared from a solution of 149 grams of Ni(N03)2.6H20 dissolved in 750 ml. of water and heated to 90" C. This solution was added to 750 ml. of a hot solution (90' C.) of sodium tetraborate (1.36 N ) . The resulting slurry was stirred for 30 minutes with the temperature at 90" C. The precipitate was filtered and washed five times with water (1500 ml. of water for each wash). The filter cake was slurried in 1500 ml. of water, and 270 grams of silica-alumina was added. After stirring for 15 minutes the slurry was filtered and dried a t 110' C. for 16 hours. Activation of Catalysts. Prior to use, the catalysts were pelleted X inch) ; 50 ml. of the pellets was placed in BLOCK the reactor and activated in a stream of FURNACE hydrogen (H2 rate = 6 liters per hour) a t atmospheric pressure and 538" C. for CONDENSER 16 hours, with the exception of those HIGH PRESSURE catalysts used for the study of the effect SEPARATOR of activation temperature on activity. LOW T E M P E R LOW PRESSURE Calculations. The molar conversion TURE TRAP of n-hexane was obtained b y subtracting RECEIVER the number of moles of n-hexane in the products from the number of moles of Figure 1. High Pressure Dynamic Laboratory U n i t n-hexane charged over the catalyst during the test period. The yields of the 200 ml. of water to which 15 ml. of concentrated ammonia was various products are expressed as moles per 100 moles of n-hexane added and the solution heated to 80" C.; 44 grams of Ni(NO&.charged. Since the amount of carbonaceous material on the 6Hz0 was then dissolved in 200 ml. of water and this solution catalyst is normally extremely low, conversions of n-hexane and heated to 80" C. The nickel nitrate solution was then added to yields of products have been calculated on a no-loss basis. I n the hot ammonium molybdate solution after which 270 grams of the majority of runs the over-all weight per cent recovery are silica-alumina was added. The mixture was stirred for 1 hour usually of the order of 95% or higher, with the temperature a t 80' C., after which it was filtered and RESULTS WITH NICKELSILICA-ALUMINA CATALYST the filter cake washed five times with water (each wash was with 500 ml. of water). The washed solid was filtered and dried a t Nickel and Hydrogen. Table I contains the results obtained 110" C. for 16 hours. on passing n-hexane over a commercial synthetic silica-alumina Catalyst SA-1ONW (I) was started with a solution of 149 catalyst a t 24.8 atmospheres in the presence of 4 moles of hydrograms Ni(N08),.6H20 dissolved in 720 ml. of water, which was added slowly to 750 ml. of 00 1.36 N sodium tungstate solution with stirring / i 440, washed for 15 minutes. five timesThe (each precipitate wash waswas with filtered 1500 and ml.
I
T
h
=dl
$-
of water). The washed nickel tungstate was slurried in 1500 ml. of water and 270 grams of silica-alumina added. The mixture was stirred for 10 minutes, filtered, and dried a t 110" C. for 16 hours. Catalyst SA-IONiCr (I) was prepared from a solution of 149 grams of Ni(N0&.6Hz0 dis-
/,,/- /-\-
d e 2 400-
-
$0-
P
r 40-
F s z
'f
L
:3m300
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- -5:
/-/
eo-
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point and held a t this temperature for 3.5 hours. The mixture was filtered, washed five times with water (1500 ml. of water for each wash), filtered, and dried at 100' C for 16 hours. Catalyst SA-1ONP (I) was started with a solution of 149 grams of Ni(N0&.6HzO dissolved in 750 ml. of water which was added to 750 ml. of 0.91 N disodium acid phosphate solution. The mixture was stirred for 15 minutes, filtered, and the filter cake washed five times with water (1500 ml. of water for each wash). The washed cake was slurried in 1500 ml. of water and 270 grams of silica-alumina was added. After stirring for 10 minutes, the slurry was filtered and dried at 110" C. for 16 hours.
20
40
60
80
100
20
40
60
80
lod
gen to 1 of n-hexane. The reaction temperature was varied from 315" to 485" C. In agreement with the work of Good ( I S ) , less than 4 mole yo of the charge was converted to isomeric hexanes. The main products formed a t all conversions of nhexane were lower molecular weight hydrocarbons. Thus, at higher pressures and in the presence of hydrogen the products from n-hexane are similar to those obtained at atmospheric pressure. However, under similar conditions, if we use a catalyst consisting of 5% nickel deposited on the same silica-alumina as a
INDUSTRIAL AND ENGINEERING CHEMISTRY
150
-
I""
- - _ _ _ -m_ _
a 6 0 - .--_ .__.
6
8
60-
r+--9
0
c
g
40-
0
6 p
a
eo--
0
t 2.0
40
TABLE I.
6.0
60
0.5
ISOMERIZATION O F
Catalyst Pressure L.S.V. HdHC
Temperature ' C. Recovery, wd. % of charge
15
I O
n-HEXANE
= SiOz-AlzOa = 24.8 atm. (hydrogen) = 1.0 vol./vol./hr. = 4
Moles/100 moles charge (no-loss basis) Cl-CS CB isomers n-Hexane Mole % of charge Convn. n-hexane Ce isomer yield Wt. % C on Catalyst
315 94.7
374 97.7
431 95.8
485 98.5
3.6 1.4 96.2
4.7 2.1 95.3
9.8 1.8 92.4
35.4 3.4 76.2
3.8 1.4
4.7 2.1
7.6 1.8
...
...
24.8 3.4 2.1
...
carrier, we obtain the results shown in Table 1 ~ (runs 1 416419). With this catalyst the conversion of n-hexane occurs a t much lower temperatures, but of greater interest, the results show that under these conditions the catalyst is very active and highly selective for the isomerization of normal hexane to isohexanes The catalyst showed no loss in activity and a t the end of these runs contained only o.O5y0by weight of carbon. If instead of hydrogen we use nitrogen (Table 11) under the same conditions, this catalyst shows no ability to convert nhexane to the other hexane isomers. Over the temperature range of 311' to 422' C. less than 1.5 mole yo of the n-hexane reacted. These results clearly demonstrated that in the absence of hydrogen a nickel-silica-alumina catalyst sho>\-slittle ability t o isomerize n-hexane.
a function of the reaction temperature in Figure 2. The data show that under the conditions employed the conversion of n-hexane commences at 300" to 310" C. and increases to approximately 90% a t 420" C. The reproducibility of the various batches of catalysts is evident from Figure 2. As revealed in Table 111, the major products formed in the presence of this catalyst, up to 75 to 80% conversions of n-hexane are primarily the hexane isomers, 2- and 3-methylpentanes and 2 , s and 2,2-dimethylbutanes. The total hexane isomer yields are plotted in Figure 3 as a func20 tion of the n-hexane conversion. The 45" line represents 100 mole % conversion of the reacted n-hexane to isohexanes. At n-hexane conversions higher than SO%, hydrocracking reactions become prominent. As shown in Table 111, demethylation to form methane and iso- and n-pentanes occurs as well as center cracking of the n-hexane to form two molecules of propane. As the hydrocracking becomes more severe, demethylation of the pentanes also occurs to form iso- and n-butane, propane, and ethane. Hydrocarbons higher in molecular weight than n-hexane were not found in any of the products. Hydrogen to Hydrocarbon Ratio. T o determine the effect of the molar ratio of hydrogen to hydrocarbon on the conversion of n-hexane and the isomer yield, a series of runs were made in which the ratio of hydrogen to hydrocarbon was varied from 0.5:1 to 8:l. I n order to maintain the total gas (hexane plus hydrogen) space velocity constant, the liquid space velocity was varied from 0.6 ml./ml./hour for a H2/HC ratio of 8:l t o 2.2 for a ratio of 0.5:l. The conversion of n-hexane a t constant reaction temperature is plotted as a function of the hydrogen to hydrocarbon ratio in Figure 4. The results show that a t constant reaction temperature. the conversion of n-hexane is independent of the hydrogen and the hydrocarbon partial pressures in the feed'
\
I: A
%-HEXAXEIX ABSENCEO F HYDROGEN
ISOMERIZATION
Catalyst Pressure L.S.V. Nz/HC Temperature, C. Recovery, wt. 70of charge Moles/100 moles of charge (no-loss basis) C1-Ca CBisomers n-Hexane Mole % of charge Convn. n-hexane Cs isomer yield
= = = =
01 0
I
10
422 94.3
0.3 0.1 99.7
2.3 0.1 98.8
1.1 0.1 99.6
3.9 0.1 98.6
0.3 0.1
1.2 0.1
0.4 0.1
1.4 0.1
Reaction Temperature. The effect of temperature on the conversion of n-hexane and the yields of the various products, using three different preparations of the standard nickel (5 wt. %)-silica-alumina catalyst, is shown in Table 111. The molar conversion of n-hexane based on the charge is plotted as
I
I
20
30
40
50
Figure 6.
Effect of Reaction Pressure on Conversion of n-Hexane Reaction temp., ' C.: 0 = 320, 0 = 350, A = 380, = 410 ZIw'HC
396 98.7
c* -A-
REACTION PRESSURE (ATM)
OF
SA-5 N (VII) 24.8 atm. (nitrogen) 1.0 vol./vol./hr. 4 311 367 96.4 96.3
\
2 ' w
TABLE11.
Vol. 45, No. 1
=
4
The experimental data reveal that there is little or no effect of the molar ratio of hydrogen to hydrocarbon on the isomer yield a t the same conversion of n-hexane. Thus, provided hydrocracking is lo'ly, and since there is no consumption of hydrogen during the isomerization reaction, these catalysts will show high selectivity for the isomerization of n-hexane even at low molar ratios of hydrogen to hydrocarbons. Liquid Space Velocity. The results plotted in Figure 5 show the effect of varying the rate of n-hexane from 0.2 to 2.0 volumes per volume of catalyst per hour on the conversion of n-hexane, maintaining the other variables constant. I n these runs, the total gas space velocity (hydrogen plus n-hexane) was varied from 172 to 1716 volumes of gas a t standard temperature and
January 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
---
151
OF REACTION TEMPERATURE TABLE 111. EFFECT
Pressure
L.S.V.
H2/HC Run No. Temperature, ' C. Total recovery, wt. 3 ' % of charge Catalyst
416 328 92.6
418 417 355 385 90.4 89.4 SA-5N (VII)
Product distribution moles/100 moles of charge (no-loLs basis) Methane Ethane Propane Isobutane n-Butane Iaooentrtne ilijentane 2 2-Dimethylbutane 2:3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane
0.5 1.1 15.0 10.8 71.8
3.6 5.0 29.6 19.6 41.2
4.3 0.9 0.6 3.7 1.9 5.1 5.6 34.1 23.0 23.2
Mole % of charge Convn. n-hexane CS isomer yield
28.2 27.4
68.8 57.8
76.8 67.8
Selectivity factor Wt. % C on catalyst
... 1 . .
1.2
... ...
... .
I
.
0.97
...
24.8 atm. 1.0 vol./vol./hr. 4
...
...
1.6
...
...
... 0.1
0.98
...
.
... ...
0.88 ...
pressure per volume of catalyst per hour. As the liquid space velocity of the feed increases, the conversion of n-hexane decreases a t constant reaction temperature. The rate of decrease of conversion with space velocity is less a t the higher reaction temperatures due t o the hydrocracking reaction which becomes the major reaction above conversions of n-hexane of 75 to 80 mole %. Since this reaction is highly exothermic, control of the catalyst temperature becomes more difficult. As the catalyst temperature increases, the hydrocracking reaction is accelerated so that smaller changes in conversion are observed with increase in liquid space velocity than a t lower reaction temperatures where isomerization to CS isomers is the major reaction. There is little effect of liquid space velocity on the selectivity of the isomerization reaction up to conversions of 70 t o 80%. The data reveal that within the contact times employed these catalysts show a preferential activity for the isomerization reaction and are specific for the formation of isomers. The apparent energy of activation of the isomerization reaction, calculated a t a constant conversion of 50 mole %, is of the order of 52 t o 56 kcal. per mole. Total Pressure. The effect of total reaction pressure on the isomeriaation of n-hexane in the presence of a standard nickel catalyst was investigated a t pressures from 7.1 to 48.5 atmospheres. I n order to maintain all the other variables constant, the liquid space velocity was varied from 0.25 ml. per volume of catalyst per hour a t 7,1 atm. pressure to 2.0 ml./ml./hour a t 48.5 atm. pressure. I n this manner, using a constant hydrogen to hydrocarbon ratio of 4:1, it was possible t o maintain the total gas contact time (hydrogen plus n-hexane) constant. Correlation of the conversion of n-hexane as a function of the reaction pressure, a t the same reaction temperatures, is shown in Figure 6. The results show that a t constant gas contact time, the conversion of n-hexane decreases as the total reaction pressure increases. Since the reaction is apparently of zero order in respect to the partial pressure of hydrogen and n-hexane, increasing the partial pressure of the reactants by increasing the total pressure should cause a decrease in conversion provided the total gas contact time is maintained constant. Thus, in changing the pressure from 24.8 to 48.5 atm. the partial pressure of n-hexane has been increased by a factor of two. Hence, in order to obtain the same conversion of n-hexane, it would be necessary t o double the contact time. This has been found to be the case in other experiments in which the liquid space velocity and hydrogen to hydrocarbon were maintained the same a t 24.8 and 48.5 atm. pressure.
419 412 93.7
489 316 98.9
44.5 4.0 19.1 4.0 5.9 16.3 11.5 2.1 3.2 18.7 12.9 12.7
0.5 0.3
0.2 0.6 0.5 8.3 5.2 84.6
87.3 36.9
15.4 14.6
0.42 0.05
0.8
... ... ...
0.95 ...
490 491 343 370 96.9 95.5 , SA-5N (VII-D)
0.5 1.1 2.0
557 314 97.1
558 342 96.2 -SA-BN (VI1-E)-
0.5
0.2 1.3 1.5 25.8 17.3 52.2
1.2 0.4 0.3 1.3 2.0 5.3 4.5 33.9 24.3 27.4
36.6 2.9 10.0 2.7 3.7 12.5 9.1 4.7 4.5 23.0 16.9 16.1
... ... 7.1
6.0 86.5
1.8 0.5 3.3 21.2 19.3 53.4
47.8 45.9
72.6 68.0
83.9 49.1
13.5 13.1
46.6 44.3
... 0.4 ...
0.96
...
3.2
492 400 96.1
'
...
0.94
...
0.59 0.05
1.6
...
...
... ... ...
...
...
0.4
.0.. 9. 7
0.4 0.4
...
0.95
...
The selectivity of the isomerization reaction is not affected by the total reaction pressure within the pressure range investigated. , At atmospheric pressure, hydrocracking is the primary reaction observed on passing n-hexane with hydrogen over this catalyst. At 330" C. and a liquid space velocity of 0.5 ml./ml./ hour 20% of the n-hexane was converted to other products, The yield of isohexanes was only 4.4y0 based on the charge; the remainder of the converted n-hexane appears as lower molecular weight hydrocarbons (predominantly methane). Although reaction pressures higher than 48.5 atm. were not investigated, the data plotted in Figure 6 indicate that this catalyst would still be active a t higher pressures. Catalyst Preparations. Methods other than the standard procedure were investigated for the preparation of the nickel (5 wt. %)-silica-alumina catalyst. The results obtained, using catalysts SA-5N (XVI), SA-5N (VIII), and SA-5N (X) for the isomerization of n-hexane, were in excellent agreement with those shown in Table I11 for the standard nickel-silica-alumina catalysts. Catalyst Activation Temperature. The initial choice of 538" C. as the temperature for activating these catalysts prior to use was a fortunate one, since it has been found that the activity and isomerization ability of these catalysts depend on the temperature a t which they are activated. Several samples of the same batch of catalyst SA-5N (VII) were activated a t temperatures of 371", 427', 496", 593', and 649" C. for 16 hours using the same hydrogen flow rate. The activity of these catalysts was then investigated for the isomerization of n-hexane. The conversion of n-hexane a t the same reaction temperature is plotted as a function of the activation temperature in Figure 7. At reaction temperatures of 300' t o 390" C. the conversion of n-hexane increases as the activation temperature is increased, passes through a maximum between activation temperatures of 500" and 550' C., and then decreases for catalysts activated a t higher temperatures. Plotting the yields of isomers obtained a t the same reaction temperature as a function of the activation temperature of the catalyst gives the interesting results shown in Figure 8. Since extensive hydrocracking occurs a t conversions higher than 75 to SO%, Figure 8 shows only the isomer yields for conversions of n-hexane below this value. For all reaction temperatures, the isomer yield is low with catalysts activated a t low temperatures (371' t o 427" C.), increases to a maximum for activation temperatures of 500" t o 550' C., and then decreases for higher activation temperatures. The experimental data show that the catalysts
INDUSTRIAL AND ENGINEERING CHEMISTRY
152
Figure 7. Effect of Catalyst Activation Temperature on Conversion of n-Hexane
Figure 8. Effect of Catalyst Activation Temperature on Hexane Isom e r Yield
350
400
450
500
550
600
650
71
Activotim Tempemture "C.
0 = 330, 0 = 360, A = 390, = 4.20 Pressure = 24.8 atm.; L.S.V. = 1.0 vol./vol./hr.;
HdHC
=
4
activated at the lower reaction temperatures possess a higher hydrocracking activity than the catalysts activated a t the higher temperatures. Determination of the extent of reduction of the catalyst at 427" C. showed that under the conditions used (16 hours and 6 liters of hydrogen per hour) only 40 to 45 % reduction of the nickel oxide occurred compared to 90 to 95% for the catalyst activated at 538' C. These results indicate that the extent of reduction of nickel oxide is important for the isomerization ability of these catalysts. The effect of activation temperature on the B.E.T. surface area of the catalyst is shown in Table IV. The unreduced catalyst possessed a surface area, as measured by nitrogen adsorption, of 336 square meters/gram which decreased to 280 square meters/ gram after activation with hydrogen at 538" C. Thus, these catalysts retain much of the porous structure of the silica-alumina cracking catalyst.
TABLE IV. EFFECT OF ACTIVATION TEMPERATURE ON SURFACE AREA Catalyst = S A - 5 S ( V I I ) T e m p e r a t u r e a , C. Unred.b 427 B.E.T. a r e a o , sq. m./g. 336 324 Catalyst heated in Hz for 16 hours. Evacuated a t 316' C. for 0.5 hour. Surface area of fresh silica-alumina, 420 sq m./g.
538 279
593 282
Nickel Concentration. A series of catalysts --as prepared containing 2.5, 5, 7.5, 10, 15, and 20% nickel by weight, by adding the required amount of nickel carbonate slurry t o a water slurry of silica-alumina. The catalysts were all treated in the same manner prior to use and tested under similar conditions. With catalysts of high nickel content, it was necessary to use lower reaction temperatures due to the increased hydrocracking tendencies shown by these catalysts. Figure 9 shows the conversion of n-hexane as a function of nickel concentration in the catalyst a t constant reaction temperatures. Plotting the hexane isomer yields as a function of the nickel content of the catalyst, a t constant reaction temperatures, gives
Vol. 45, No. 1
the series of curves shown in Figure 10. Only isomer yields for n-hexane conversions below 75 to 80% are shown. As espected, the isomer yields for the catalyst containing 2.5% nickel are low a t all temperatures, but as the nickel content is increased the Isomer yield passes through a maximum at approximately 4 to 670 nickel and then decreases a t higher nickel contents. This masimum becomes more evident a t the higher reaction temperature. Figure 11: in which the methane yields are plotted as a function of the nickel content of the catalyst at constaiit reaction temperatures, shows very clearly that as the nickel content increases above 5%, a greater amount of hydrocracking to methane occurs which accounts for the l o ~ e isorncr r yields obtaincd. These results suggest that a complex type of surface compound is formed between the nickel and silica-alumina, and that the isomerizat,ion activity is a inaximuin a t about 5% nickel. Lower amounts of nickel are insufficient to give an active conversion catalyst. while higher amounts of nickel produce products n-hich are more characteristic of nickel alone. Alumina Content. I n order to determine the effect of alumina content of the catalyst on the isomerization activity and selectivity, a series of catalysts in which the alumina cont,rntswere 2, 13, 60, 80, and 95% was prepared. The results obtained with these catalysts are shown in Table V. The conversion of nhexane for catalysts containing 13 and 80% alumina are approximately the same, but the catalysts containing 2 and 60% alumina are less active at the same reaction temperature. Increasing the alumina content to 95% gives a poor conversion catalyst. Both the 13 and 60% alumina catalyst show a high selcctivity for isomerization, w]iile the 2 and 80% alumina catalj higher hydrocracking activity resulting in loir-er yields of hcsane isomers a t the same conversion of n-hexane. RESULTS WITH OTHER HYDROGENATION COMPONENTS
Metals. Other metals of Group YIII of the periodic systenm such as cobalt, platinum, palladium, et,c., have been kiioir-11 for a long time to possess high activity for hydrogeriation react,ions. Catalysts containing 5% by weight of cobalt, 0.5% by weight platinum, and 5% by weight of iron were prepared arid tested using n-hexane as the feed. Both cobalt and platinum in conibination with silica-alumina show high activity for the convcrsion of n-hexane to other Cg isomers and high se1ectivit)- similar to that found for the nickel catalyst. This is shown in Figure 12 where the n-hexane conversion is plot.ted as a function of the reaction temperature. For comparison purposes the previous results obtained with the standard nickel catalyst are also shown. A catalyst containing 0.5% by weight of platinum appears to be almost equivalent to the catalyst. containing 5% by Wright of nickel. The catalyst containing 5% by weight of iron dors not show much activity for tJhe conversion of n-hexane; at 438' C. the conversion of n-hexane was only 16 mole %. For both platinum- and cobaltrcontaining selectivity for the isomerization of 11-hexane to the other C, isomers is comparable to that found for nickel at t,he same conversion of n-hexane. This is shown in Figwe 13. The 4 5 O line, as previously mentioned, represents 100 mole yo conversion of the reacted n-hexane to isohexanes. However, in the iron catalyst the isomer yield is very low a t all conversions. Although pure copper is a poor hydrogenu tion catalyst, a run was made using a silica-alumina catalyst containing 5 9 : by weight of copper. As revealed in Figurcas 12 a n d 13, this catalyst showed little activity for the reaction. Metal Oxides. Metal oxides such as molyhdonuiii oxide, tungsten oxide, and vanadium pentoxide are active hydrogenation catalysts a t hydrogen pressures of the order of 100 to 300 atm. and high temperatures. Two of these oxides, molybclcnuin and tungsten oxides, in combination with silica-alumina ivere tested for the isomerization of n-hexane. The mol~-lxlenumoside
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1953
153
TABLE V. EFFECT OF ALUMINACONCENTRATION Pressure = 24.8 atm. L.S.V. = 1.0 vol./vol./hr. Hl/HC = 4 Run No. Temperature, C. Total reoovery, wt. charge Catalyst Wt. % Ah01
of
Product distribution, moles/100 moles of charge (no-loss basis) Methane Ethane Propane Isobutane n-Butane
432 330
433 361
95.6 96.1 -SA-BN (XI)2-
5.4
452 392
454 332
93.6 95.8 93.9 -SA-5N (XI1)___ 13-
99.3
450 329
0.4 92.5
20.4 0.9 1.0 0.1 1.9 0.7 8.0 0.6 1.0 3.4 3.3 78.5
9.8 5.8 84.3
Mole % of charge Coavn. n-hexane Ca isomer yield
7.5 2.5
21.5 8.3
15.7 15:6
Selectivity factor Wt. % C on catalyst
0.33
...
.., ... ...
Isnnnnt,nne r--------
0.2 4.4
;:Pentane 2,2-Dimethylbutane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane
... ...
2.1
1.6
3.8
... ...
...
2.0 28.1 25.3 40.1
37.6 2.3 4.7 2.5 5.9 8.9 6.3 2.0 3.1 28.8 22.1 16.5
3.7 1.4 94.9
59.9 55.4
83.5 56.0
5.1 5.1
.. .. .. .. .. .. ... ...
0.39 0.03
...
451 359
...
1.0 1.0
...
1.3 2.0
...
0.99
. . . . 0.. 9.3
.. .. .. ... ... ... ... ... .
.
I
455 357
457 414
456 386
95.8 96.0 SA-5N (X1V)60
96.4
441 329
442 359
95.0 93.5 98.0 -SA-5N (XXVI1)80-
94.0
99.3
433 387
444 442
99.1
96.9
-SA-5N (XV)95
... ... 0.4 ... ... ..*
0.5
0.5
1.1
3.8
... ...
0.4
...
...
...
...
13.0 6.4 79.1
0.2 1.6 2.3 23.9 14.1 56.0
5.5 0.2 0.1 0.2 0.6 4.1 6.2 29.2 20.4 36.3
0.4
.,.
1.0 1.5 0.4
0.7 1.7 96.8
1.5 1.3 93.0
4:i 2.9 54.0
1.7 1.1 8.0 88.4
0.9 5.9 88.0
0.4 0.8 2.3 95.6
4.9 2.9 90.9
20.9 20.0
44.0 41.9
63.7 59.9
3.2 2.4
7.0 2.8
46.0 7.6
11.6 10.8
12.0 8.0
4.4 3.5
9.1 7.8
0.6
...
...
...
. . . .0.96 . . . 0.95 ..
catalyst contained 10% by weight of molybdenum as the metal, while the tungsten oxide contained 5% by weight of the metal. The results are plotted in Figures 14 and 15. To show the importance of the silica-alumina carrier for the isomerization activity of these catalysts a molybdenum oxide catalyst was prepared on an activated alumina base by the same procedure. Catalysts of the latter type have found considerable use in the "hydroforming" of virgin naphthas (24). The final catalyst contained the same amount of molybdenum (10% by weight) as the silica-alumina catalyst. The marked difference in activity of these two catalysts is shown in Figures 14 and 15. For the alumina base catalyst a conversion of 19.5% was obtained at a reaction temperature of 485" C. With silica-alumina as the carrier and at the same temperature, a conversion of approximately 89% was obtained. However, the alumina base catalyst possesses some isomerization activity since the product obtained at 485" C. showed an isomer yield of 14% for a conversion of n-hexane of 19.5%. The observed isomerization ability of molybdenum oxide on aIumina is in agreement with the results reported by Greensfelder and coworkers (15). Nickel Salts. Catalysts containing various nickel salts in combination with silica-alumina were also investigated for the isomerization of n-hexane. These included nickel phosphate, nickel tetraborate, nickel tungstate, nickel chromate, and nickel
572 348
569 316
0.5 0.6 1.0
1.00
0.67 0.07
571 287
...
... ...
...
... 3.9 .. .. ..
.., 0.5
... ...
0.94 0.07
0.75
. . . . 0.. 4.0
45.1 1.4 1.4 1.2 5.5 2.4 26.4
...
0.5
...
1.0
...
... ... ... ...
0.17 0.04
1.6 0.9 3.9 1.0 1.0
... .1. ... ..2
0.6 1.0
0.93
. . . .0 ..6 7. . 0. . 8. 0
0.5 0.6 1.4
...
0.4
...
0.1
... ...
0.86 0.06
molybdate. All these catalysts contained 10% by weight of nickel as the metal. The experimental data are shown in Figures 16 and 17. As revealed in Figure 17, these catalysts show, in general, fairly high selectivity for the isomerization of n-hexane. The high selectivity of the nickel phosphate-silica-alumina catalyst is particularly noteworthy. DISCUSSION
Starting with the early work of Sabatier, the catalytic properties of reduced nickel catalysts have been under intensive investigation. The activity and selectivity of nickel as a catalyst for the hydrogenation of unsaturated organic compounds a t low temperatures (25" to 200" C.) and pressures is well known, and this catalyst has been employed in several commercial processes involving the addition of hydrogen to unsaturated carbon-carbon bonds. At temperatures above 250" to 300" C., nickel catalysts cause extensive degradation of hydrocarbons, with the formation of methane and hydrogen and the deposition of carbonaceous materials on the catalyst surface. Haensel and Ipatieff (22) found that the demethylation reaction could be controlled in the presence of hydrogen b y carefully controlling the reaction temperature and partial pressure of hydrogen. Several investigators have shown that the cracking activity of nickel can be 2001
I
I
e .-*-
I
I
5
I
10
I
I
15
20
Nickel Concentration (Wt. Yo)
.Figure 9.
I
I
10
5
eo
I5
Nickel Concentratim (Wt. %)
Figure 10. Effect of Nickel Concen- Figure 11. Effect of Nickel Contration on Hexane Isomer Yield centration on Methane Yield Temp., O C.: 0 = 290, tf 320 0 350, A = 380 = 410
Effect of Nickel Concentration on Conversion of n-Hexane
-
-
Pressure = 24.8 atm.; L.S.V. = i.0 vol./vol./hr.;
Hs/Hk
=
4
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
154 - 0 6 4
Figure 12.
n-Hexin Presence of Other Metal Silica Alum i n a Catalysts ane Conversion
I' Figure 13. Hexane I s o m e r Yield in Presence of Other Metal Silica Alum i n a Catalysts
N-Hexane Converslm (Mol %fig) = Fe st. A = Pt. 0 = Co; tf = Cu. Pressure = 24.8 atm.; E.SrV. = l.O'vol./vd./hr.; H d H k = 4
Vol. 45, No. 1
The formation of a "nickel clay" a t the surface provides an explanation for the unexpected high surface area of the unreduced catalyst and also its increased activity. X-ray diffraction patterns of the unreduced 5 and 10% nickelsilica-alumina cataIgsts used in this investigation, in agreement with deLange and Visser, did not show any of the characteristic lines of hydrous nickel oxide or the carbonate. Very diffuse patterns were obtained indicating that the nickel is probably in combination with the hydrous aluminum silicate. The formation of a hydrous nickel-aluminum silicate would be one possible explanation for the unexpected activity these catalysts show for the isomerization of saturated hydrocarbons. Further work on the chemical nature of the catalyst, which is discussed in the fourth paper of this series, lends further support to the incorporation of the nickel into the hydrous aluminum silicate lattice. Role of Hydrogen. I n the absence of hydrogen these catalysts show no activity for the conversion of n-hexane. It has also been found, as shown in Figure 4,that over the range of hydrogen to hydrocarbon molar ratios investigated (0.5:1 to 8 : l), the conversion of n-hexane and the selectivity of the isomerization reaction is independent of the hydrogen partial pressure. Thus, provided free hydrogen is present, these catalysts are active for the reaction and maintain their activity.
0 = StandardNi catal
modified by depositing the nickel on a suitable carrier. Thus, Zelinskii and Komarewsky (49) found that by coprecipitating nickel and aluminum hydroxides, a nickel catalyst was obtained which shoved good selectivity for the dehydrogenation of cyclohexane a t 300' C. As shown by this investigation, if nickel is deposited on a silicaalumina cracking catalyst, so that the final catalyst contains 4 to 6% nickel by weight, the resulting catalyst after reduction at 500' t o 550' C. possesses the ability to catalyze the isomerization of n-hexane in the presence of hydrogen. The fact that it does this with a high degreeof selectivity a t temperatures above 300' C. is rather surprising. The investigation of the effect of alumina content shows that the isomerization activity of these catalysts cannot be attributed to a combination of nickel with alumina or silica alone. Both the high alumina and high silica containing catalysts caused excessive hydrocracking of n-hexane. Thus, on the basis of the present work and the extensive investigations of the acidic properties of silica-alumina catalysts (H) it,appears more reasonable to conclude that the isomerization activity of these catalysts is due to compound formation between the nickel and hydrous aluminum silicate. Further experimental support for this conclusion was obtained from the investigation of the effect of nickel content on the activity and selectivity of the catalyst for the isomerization reaction. Since the synthetic silica-alumina catalyst contained 12.8% by weight of alumina and 87.2% silica, the molar ratio of nickel t o alumina to silica (for the optimum concentration of nickel-i.e., 4 to 6% b y weight) would vary from approximately 1:2:20 to 1:1:13, Thus on the assumption that all the alumina is in combination with silica in the form of a hydrous aluminum silicate, there is approximately 1 atom of nickel for every 2 or 4 atoms of aluminum. DeLange and Visser (6) concluded fromx-ray analysis, temperature of reduction, and surface area measurements that the typical '[nickel on Kieselguhr" catalyst cannot consist of a layer of nickel on the surface of the siliceous carrier. The x-ray diffraction pattern of the unreduced catalyst indicated that a synthetic nickel hydrosilicate was produced in which the atoms close to the surface formed a typical mineral lattice consisting of a monomolecular layer of hydrous nickel oxide sandwiched between plates consisting of silicon and oxygen (or hydroxyl groups).
*
soo/
460
I/"
I
/i Figure 14. " n - H e x a n e Conversion i n Presence of Metal Oxide Catalysts
-
-
80
,
I
0
i
V
-
2
.9 i 4 0 1
/// 1 '
o :
Figure 15. Hexane Isomer Yields i n Presence of Metal Oxide Catalysts
1
PO 40 60 80 N-Hexane Conversion (Mol % Chg)
100
= Standard Ni catalyst; = 41o0z-SiO~A1~0~; 0 = MoOz-Alr03; 0 = WOa-SiOpAlzOa Pressure = 24.8 a t m . ; L.S.V. = 1.0 vol./vol./hr.; Hz/HC
=
4
The previous work on the interaction of hydrocarbons a t the surface of metals hasindicated that in the exchange and deniethylation reactions, the initial step involves the disruption of a carbon-hydrogen bond. The resulting hydrocarbon radical and hydrogen atom are attached to the metal surface atoms.
H
H
I
I n the absence of hydrogen the alkyl radical is firmly attached to the metal surface. At high temperatures the alkyl radical will
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1953 4401
1
I
Figure 16. n-Hexane Conversion in Presence of Ni SaltSilica-Alumina Catalysts
Figure 17. Hexane I s o m e r Y i e l d s 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 t o 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.
387 2-Methylhexane uentanea n-
-412Calcd. (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 t o 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
