162
INDUSTRIAL AND ENGINEERING CHEMISTRY
hexane. Schuit and his coworkers (45)found that 1,l-dimethylcyclohexane isomerizes in the presence of aluminum chloride at 80" C. to give only the dimethylcyclohexanes. However, it cannot be concluded that these isomers are the only ones formed at higher reaction temperatures since the equilibrium is favorable for the formation of alkylcyclopentanes. Furthermore, in order to explain the formation of isopropylcyclopentane, it is necessary to conclude that 1,l-dimethylcyclohexane can isomerize further to form a five-membered ring isomer since the direct
Vol. 45, No. 1
formation of isopropylcyclopentane from any of the other isomers is highly improbable. The production of aromatic hydrocarbons a t the higher reaction temperatures shows t h a t t h e hydrogenation agent, nickel, retains its ability to catalyze the dehydrogenation of cyclohexanes Hence, in the presence of this catalyst the direct production of aromatic hydrocarbons in high yields from alkylcyclopentanes by simultaneous isomerization and dehydrogenation may be possible at higher temperaturesthan those used in the present investigation,
(Isomerization of Saturated Hydrocarbons)
NATURE OF THE CATALYST AND MECHANISM OF THE REACTION F. G. CIAPETTA'
I
K THE previous papers of this series it has been established t h a t on combining a hydrogenation catalyst with a silica-
alumina cracking catalyst, a complex catalyst is obtained which is very active and highly selective for the isomerization of saturated hydrocarbons. Early in this investigation it was observed t h a t the introduction of sodium ions, b y an exchange reaction, into the catalyst during ita preparation poisoned the catalyst for the isomerization reaction. The results of these experiments and the significance of the poisoning effect of sodium in relation to the chemical nature of the isomerization catalyst and the mechanism of the reahtion are described in this paper. Also included are the results obtained on passing 1-pentene and 1-hexene over the standard nickel-silica-alumina catalyst in the presence of excess hydrogen. The rapid hydro-isomerization of these alkenes a t lower reaction temperatures than previously found for the corresponding saturated hydrocarbons, suggests t h a t the initial and rate-controlling step in the isomerization of saturated hydrocarbons is the dissociation of a carbonhydrogen bond a t the metal surface. Consideration of all the factors involved suggests t h a t the mechanism of the isomerization reaction involves the formation of carbonium ions on the surface of the catalyst. EXPERIMENTAL
The apparatus, experimental procedure, preparation of the standard nickelsilica-alumina catalyst, reaction conditions, and method of analysis were similar to those described in the first paper of this series (p. 147). The alkenes, 1-hexene, and 1pentene (95% purity) were obtained from Humphrey-Wilkinson, Inc. Each of these hydrocarbons was analyzed by the mass spectrometer in order to calculate conversions and isomer yielda on the basis of the pure compound. Catalyst SA-5N (I) was prepared by rapidly mixing 750 ml. of a hot (90' C.) solution of sodium carbonate (78.5 grams of Na&Os) with 750 ml. of a hot (90' C.) solution of nickel nitrate [148.9 grams of Ni(N03)2.6Hi0]. To the resulting mixture was added 570 grams of fresh, ground synthetic silica-alumina catalyst. The slurry was stirred for 10 minutes and filtered; the cake was washed five times (each water wash was 1500 mL)> and then dried a t 110' C. for 16 hours. The amount of sodium carbonate used represents a 45% excess over the theoretical re uirement. This catalyst contained 1.24% sodium by weight. %atalyst SA-5N (V) was prepared in a manner similar to that used for the standard nickel-silica-alumina catalyst. 1
Present address, Socony-Vacuum Laboratories, Paulsboro, N. J.
EXPERIMENTAL RESULTS
Effect of Sodium. The experimental data for the reaction of n-hexane in the presence of catalyst SA-5n' (I) are shown in Table I. At reaction conditions similar to those used with the standard nickel-silica-alumina catalyst, the products obtained revealed that the catalyst prepared in the presence of sodium ions showed little activity for the isomerization of n-hexane. The main products formed were hydrocarbons of lower molecular weight due to hydrocracking reactions. The large amounts of methane produced revealed that this catalyst was active for selective demethylation of n-hexane. The predominance of n-pentane and n-butane in the products showed that little isomerization of n-hexane occurred prior to demethylation. I n order to determine if sodium ions would also poison an active isomerization catalyst, catalyst SA-5N (V) was soaked in a 10% solution of sodium carbonate, washed free of absorbed sodium iofis, and dried a t 110' C. Chemical analysis of the catalyst showed i t contained 2.02% by weight of sodium. Table I gives the results obtained with n-hexane for this catalyst prior to treating with sodium carbonate solution and after treating. The sodium-poisoned catalyst was active primarily for hydrocracking reactions and the products formed were similar to those obtained using catalyst SA-5N (I). Hydro-Isomerization of Alkenes. The hydrogenation of 1-pentene and 1-hexene in the presence of the standard nickelsilica-alumina catalyst was investigated to determine if skeleton isomerization occurs simultaneously with hydrogenation. These alkenes were contacted with the catalyst under conditions similar to those used for the isomerization of saturated hydrocarbons. Preliminary experiments showed that rapid hydrogenation occurred a t low temperatures, and because of the exothermic nature of the reaction, the temperature of the upper portion of the catalyst bed increased from 50' to 85" C. above the furnace block temperature. The results obtained with these alkenes are shown in Table 11. Included in the d a t a are the highest and lowest temperatures observed in the catalyst bed. T h e reaction products were completely saturated, The conversions of the alkenes are based on the concentrations of the corresponding saturated hydrocarbon in the products. The conversions of 1-pentene and 1-hexene as a function of the highest observed catalyst temperature are shown in Figures 1 and 2. Also shown for comparison purposes are the results previously obtained for n-pentane and n-hexane over the same catalyst. The data show that the conversion of alkene hydro-
January 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE I. EFFECT OF SODIUM IN Pressure
L.S.V.
HV/HC ., 156 157 366 371 98.0 94.2 -SA- -5N (I) ~
Run No. Temperature, C. Total recovery, wt. 70of charge Catalyst Product distribution, moles/100 moles of charge (no-loss hasis) Methane Ethane Propane Isobutane n-Butane Isowntane n-Pentane 2,2-Dimethylbutane 2.3-Dimethvlbutane
17.7 1.1 1.0 1.0 0.4 1.6 8.4 0.9 0.2
ISOMERIZATION OF ?&-HEXANE
24.8 atm. = 1.0 vol./vol./hr. = 4 158 226 382 338 97.4 92.9 =
~
23.1 0.6 1.0 0.4 1.0 1.3 12.1 0.8 0.2
45.6 1.7 2.7 1.6 4.2 2.0 25.0 1.2
...
0.6
...
116.0 6.9 10.1 1.0 11.4 4.3 23.6 1.0 1.0
85.5
ii.'6
61'.'3
36.6
14.5 1.1 0.08
18.4 1.0 0.05
38.7 1.8 0.05
63.4 2.0 0.03 0.05
...
...
3-Methj.lientane n-Hexane Mole % of charge Convn. n-hexane Ca isomer yield Selectivity factor Wt. % C o n catalyst
.
155 354 94.0
...
...
163
...
carbons is obtained a t substantially lower reaction temperatures than found for the corresponding saturated hydrocarbons. Analysis of the products obtained from I-pentene and I-hexene showed that isomerization also occurred along with hydrogenation. The selectivity factors (molar ratio of isomer yield to conversion) for the alkenes are lower than found in the alkanes, but this is not due to increased hydrocracking reactions. The sroducts from both 1 - p e n t e n e and 1-hexene contained small a m o u n t s (1 t o 10oJo)of saturated h y d r o c a r b o ns heavier i n molecular weight t h a n t h e feed. These hydrocar/. bons are probably e60' i o io io CIO ioL! formed b y polyHydrocarbon Conversion @,lol. %Chg.) merization reacFigure 1. Hydro-Isomerization of tions at the sur1-Pentene-Effect of Reaction face of the silicaTemDerature on Conversion alumina (48) 0 = 1-Pentene 0 = n-Pentane followed by subs e q u e n t hydrogenation, This is also indicated by the higher carbon contents found in the used catalyst. I n the investigation of the isomerization of alkanes and cyoloalkanes, hydrocarbons heavier than the charge were not found. DISCUSSION
I n recent years there has been general recognition that the hydrocarbon cracking activity of silica-alumina catalyst is due to its acidic properties (17,20, 36, 45, 47) The introduction of alkali metal ions such as sodium or potassium b y ion exchange techniques results in a marked decrease in catalytic cracking activity and in measurable acidity (36). The preparation of the standard nickel catalyst involves the addition of ground silica-alumina to a water solution of nickel nitrate. A partial exchange of the hydrogen ions with nickel ions occurs as indicated by a lowering of the p H on addition of the silica-alumina. However, the addition of excess ammonium carbonate to precipitate nickel carbonate should cause almost complete replacement of nickel ions with ammonium ions. Under the conditions employed to activate these catalysts (16 hours a t 538' C.), decomposition of the ammonium ions occurs so that
... ...
227 228 229 374 352 366 94.8 99.4 95.5 SA-5N (V) Na f-
189 370 93.0
+
9.7
48.3 2.0 2.9
1.1
5.1
6.2 1.7 23.8
12.0 1.6 20.4
0.5 1.1 90.5
0.9 1.1 62.3
1.8 0.8 54.8
1.1 0.8 58.0
3.7 24.9 15.1 53.5
2.7 0.6 3.3 0.3 0.1 0.8 2.2 2.8 6.2 32.5 20.4 33.0
37.7 2.0 0.05
45.2 2.6 0.06
42.0 1.9 0.05 0.73
46.5 45.5 0.98
67.0 61.9 0.93
... 0.6 ... 1.1
... ...
... ...
9.5 1.6 0.17
...
240
...
,..
0.6
(L
71,O 2.9 3.9
... ...
...
...
58.6 2.3 3.3 0.1 8.4 1.8 22.7
191 190 393 421 96.0 99.6 SA-5N (V)
. ..
1.6
.. .. .. .... .. 1.8
,..
0 0
zoo
I
I
I
. ..
...
192 439 98.3
10.7 1.1 10.7 1.2 1.5 5.5 5.4 3.1 6.0 27.9 20.6 23.4
23.6 2.3 22.7 2.5 2.4 7.4 9.0 2.9 4.9 23.2 17.5 17.4
76.6 57.6 0.75
82.6 48.5 0.59 0.09
...
ions on the isomerization act i v i t y of t h e s t a n d a r d nickel
INDUSTRIAL AND ENGINEERING CHEMISTRY
164
Vol. 45, No. 1
TABLE 11. HYDRO-ISOMERIZATIOS OF ALKENES
651
Alkene Product distribution, moles/100 moles of charge (no-loss basis) Methane Ethane Propane Isobutane n-Butane Isopentane n-Pentane
Isohexanea
n-Hexane Higher mol. wt. HC. Mole % of charge Convn. alkene Iso-Cn yield Selectivity factor Wt. ’% on catalyst
Catalyst = standard nickel-silica-alumina Pressure = 24.8 atm. L.S.V. = 1.0 vol./vol./hr. Hz/HC = 4 652 653 654 591 592
283 212 97.4
301 32 1 239 268 97.6 98.4 ____ 1-Pentene
... , . .
0.3 0.5 0.5 23.1 67.8 . .
.
I
I
.
6.2 32.2 23.1 0.72
...
...
...
0.5 0.7 1.0
30.1 67.9
...
196 121 94.4
211 126 95.8
232 149 98.5
...
...
... ...
...
... ...
... ...
... ...
1.0 1.2 1.7 35.4 51.0
-
0.7 1.5 2.0 40.2 44.9
...
9.0
9.0
42.1 30.1 0.72
49.0 35.4 0.72
55.1 40.2 0.73 4.2
...
...
(9, $6, 35). The formation of aromatics from cycloalkanes in the presence of the standard nickel-silica-alumina catalyst, a t the higher reaction temperatures, clearly indicates that dehydrw genation to form unsaturated hydrocarbons can follow the initial dissociation of the carbon-hydrogen bond (21,g.S). The dependence of the isomerization activity on the acidity of the complex catalyst suggests that the rearrangement of the absorbed molecule involves the intermediate formation of a carbonium ion (1). The hydrogenation-dehydrogenation activity of these catalysts further suggests that an alkene or potential alkene is formed initially which combines with the hydrogen ion of the complex catalyst to form the carbonium ion. The observed effect of reaction pressure at constant contact time on the isomerization of n-hexane (see first paper) and the lower reaction temperatures required to isomerize n-octane compared with those found for n-pentane support this mechanism. The lower reaction temperatures found for the hydro-isomerization of alkenes compared with those required for the corresponding alkanes also lends support to the suggestion that a n alkene or potential alkene is necessary before isomerization can occur. Further substantiation of a carbonium ion mechanism is provided by the lower molecular weight hydrocarbons formed by hydrocracking during the isomerization of heptanes and octanes. Instead of methane, the major constituents of these products are propane, butanes, and pentanes, which shows that cracking occurs preferentially a t the center of the molecule instead of at the end of the chain. This is typical of the hydrocarbon cracking activity observed for silica-alumina catalysts (20). This extensive investigation of hydrogenation-isomerization catalysts has shown that they are in reality polyfunctional catalysts. Their activity for isomerization, hydrogenation and dehydrogenation, and hydrocracking shows that these catalysts are quite versatile for hydrocarbon reactions in the presence of hydrogen. ACKNOWLEDGMENT
The authors wish to express their appreciation to R.A. Brown who was in charge of the mass spectrometer analysis, to L. Berger, P. Holloway, L. Thaler, and C. Shipley who carried out part of the experimental work, and to The Atlantic Refining Co. for permission to publish the results of this investigation. LlTERATURE CITED
(1) Block, H. S., Pines, H., and Schmerling, L., J . Am. Chem. Soc., 68, 153 (1946). (2) Brown, R. A., Taylor, R. C., Melpolder, F. W., and Young, W.S., Anal. Chem., 20, 5 (1948).
,..
. . I
... ...
8.2
593
346 296 95.1
0.4
...
.. .. ..
0.4
...
...
99.2 0.3
t . .
0.8 0.0
... ...
0.4
...
0.2 2.2 97.0 0.4 3.0 2.2 0.73
...
0.6
0.4
0.1 0.5 6.4 90.9 1.5 9.1 6.4 0.70 3.1
594 286 204 97.6 1-Hexene-
... ... 0.8
596
597
59s
309 229 97.6
337 258 98.2
357 288 97.8
--
...
...
.. .. .
1.1 1.1 17.0 77.6 2.7
0.8 1.2 0.6 1.6 0.5 29.0 64.1 3.3
2.5 2.8 0.4 2.5 1.3 41.1 48.5 3.4
22.4 17.0 0.76
35.9 29.0 0.81
51.5 41.1 0.80
0.4
...
...
...
...
__-_ ...
0.3 3.1 5.6 1.0 4.3 2.4 45.4 36.8 5.1
63.2 45.5
0 72 1.32
(3) Calingaert, G., and Beatty, H. A., J . Am. Chem. Soc., 58, 51 (1936). (4) Cheney,’H. A., and Raymond, C. L., Trans. A m . Inst. Chem. Eng., 42, 595 (1946). (5) Cowley, C. M., and Hall, C. C., J . Soc. Chem. I d . (London). 63, 33 (1944). (6) delange, J. J., and Visser, G. H., Ingenieur (Utrech), 5 8 , No. 26, 24 (1946).
(7) Egloff, Cr., Hulla, G., and Komarewsky, V. I., “Isomerization of Pure Hydi ocarbons,” New York, Reinhold Publishing Co., 1942. ( 8 ) Egloff, G., Moriell, J. C., Thomas, C. L., and Block, H. S., J. Am. Chem. Soc., 61, 3571 (1939). (9) Eley, D. D., Advances in Catalysis,” Vol. I, p. 181, New York, Academic Press Inc., 1948. (10) Epstein. M. B., Barrow, G. h l . , Pitzer, K. S., and Rossini, F. D , J . Research Natl. BUT.Standards, 43,249 (1949). (11) Evering, B. L., and D’Ouville, E. L., J. Am. Chem. Soc., 71, 440 (1949).
(12) Evering, B. L., and Waugh, R. C., IND.ENC). CHEM.,43, 1820 (1951). (13) Frey, F. E., and Huppke, W.F., Ibid., 25, 54 (1933). (14) Good, G. M., Voge, H. H., and Greensfelder, B. S., Ibid., 39, 1032 (1947).
(15) Greensfelder, B. S., Archibald, R. C., and Fuller, D. L., Chem. Eng. Progress, 43, 561 (1947). (16) Greensfelder, B. S., and Voge, H. H., IND.ENQ.CHEM.,37, 514 (1945). (17) Greensfelder, B. S., Voge, H. H., and Good, G. M., Ibid., 41, 2573 (1949). (18) Grenall, A.,Ibid., 41, 1485 (1949). (19) Grosse, A. V., Morrell, J. C., and Mattox, W. S., Ibid., 32, 528 (1940). (20) Haensil, V.; “Advances in Catalysis,” Vol. 111, p. 179, New York, Academic Press, Inc., 1951. (21) Haensel, V., and Donaldson, G. R., IKD,ENG. CHEX, 43, 1881 (1951). (22) Haensel, V.,and Ipatieff, V. N., Ibid., 39,853 (1947). (23) Heinemann, H., Mills, G. A,, Hattman, J. B., and Kirsch,
(24) (25) (26) (27)
F. W., presented before the Division of Petroleum Chemistry at the 12156 Meeting, AMERICAN CHEMICAL SOCIETY, hlilwaukee, Wis. Hill, L. R., Vincent, G. A , , and Everett, E. F., Trans. Am. Inst. Chem. Engrs., 42, 611 (1946). Hoop, H., Verheus, J., and Zuiderweg, F. J., Trans. Faraday Soc.. 35, 993 (1939). Horiuchi, J., and Polanyi, kl , Ibid., 30, 1164 (1934). Ipatieff, V. N., and Grosse, A. V., IND.ENG CHEM.,28, 461
(1936). (28) Kemball, C., and Taylor, H. S., J . Am. Chem. Soc., 70, 345 (1948). (29) Kilpatrick, J. E., Werner, H. G., Beckett, C. W., Pitzer, K. S., and Rossini, F. D., J . Research Natl. Bur. Standards, 39, 523 (1947). (30) Koch, H., and Gilbert, W., Brennstof Chem., 30,413 (1949). (31) Koch, H., and Richter, H., B E T . , 77, 127 (1944). (32) Lazier, W.A., and Vaughn, J. V., J . Am. Chem. Soc., 54, 3080 (1932).
January 1953
P
INDUSTRIAL AND ENGINEERING CHEMISTRY
(33) Matignon, C., Kline, A., and Florentin, D., Comp. rend., 194, 1040 (1932). (34) Moldawski, B. L.,J . Gen. Chem. (U.S.S.R.), 10, 1183 (1940). (35) Morikawa, K.,Trenner, N. R., and Taylor, H. S.. J. Am. Chena. Sos., 59, 1103 (1937). (36) Oblad, A. G., Milliken, T. H., and Mills, G. A., “Advances in Catalysis,” Vol. 111, p. 199, New York, Academio h a , Inc., 1951. (37) Pier, M., 2. Elektrochem., 53, 291 (1949). (38) Pines, H.,“Advances in Catalysis,” Vol. I, p. 201, New York, Academic Press, Inc., 1948. (39) Pines, H.,Kvetinskas, B., Kassel, L. S., and Ipatieff, V. N., J. Am. Chem. Soc., 67, 631 (1945). (40) Rossini, F. D.,Prosen, E. J., and Pitzer, I(. S., J. Research Natl. Bur. Standards, 27, 529 (1941). (41) Sabatier, P., and Senderens, J. B., Ann. chim. et phys., 4, 435 (1905).
165
(42) Schmerling, L., and Ipatieff, V. N., “Advances in Catalysis,” Vol. 11, p. 21, New York, Academic Press, Inc., 1950. (43) Schuit, G. C. A., Hoog, H., and Verheus, Rec. trap. chim., 59, 793 (1940). (44)Swearingen, J. E., Geckler, R. D., and Nuperwander, C. W., Trans. Am. Inst. Chem. Engrs., 42, 573 (1946). (45)Tamele, M.W., Disc. Faraday SOC.,8,271 (1950). (46) Taylor, H.S.,J . Am. Chem. SOC.,61, 503 (1939). (47) Thomas, C.L.,IND. ENG.CHEM.,41, 2564 (1949). (48) Trapnell, B. M. W., “Advances in Catalysis,” Vol. 111, p. 4, New York, Academic Press, Inc., 1951. (49) Zelinskii, N. D.,and Komarewsky, V. I., Ber., 57, 667 (1924). RBCEIVED for review March 22, 1952. ACCEPTEDSeptember 1 1 , 1952. June 20, Presented at A.A.A.S. Gordon Research Conference, C d b y , N. H., 1951.
Coumarone-Indene Resins MOLECULAR WEIGHT DISTRIBUTION A. C. ZETTLEMOYER AND E. T. PIESKI’ Lehigh Unioersity, Bethlehem, P a ,
S
Y N T H E T I C polymers have been found to be heterogeneous with regard to molecular weight. I n technical applications, the distribution of molecular size in the polymer may be as important as the average itself. Many methods have been proposed for the study of molecular weight distribution (S), b u t a t best all methods are too time consuming for frequent use. Fractionation by precipitation from dilute solution is by far the most generally used method for size distribution studies because it has the advantage of supplying samples for examination of properties. I n fact, this method is very often used solely for the purpose of obtaining fractions with a narrower distribution of molecular weights than the original polymer. This study was designed t~ secure the information necessary to establish approximately the distribution curves for the resins under investigation and at the same time to produce a series of samples with a narrow range of molecular size and with quantities large enough for a study of their melt viscosities. Few data on the distribution of molecular weighb in the commercial low molecular weight coumarone-indene resins have been published ( I ) . The approximate distributions of molecular weights for several of these resins were determined in this investigation.
sulfuric acid wash, the material was polymerized by using aluminum chloride as a catalyst between 0” and 30” C. Washing with mater to remove the salts was followed by vacuum and steam distillation to remove the unpolymerizable materials and heavy oils (low molecular weight polymers). Since t h e starting material contained a much greater proportion of indene than coumarone, the resin consists mostly of polyindene (22). The raw material for resin I1 was a fraction of solvent naphtha boiling between 160’ and 200’ C. Such a fraction contains no styrene (boiling point 146’ (3.). This material was polymerized using aluminum chloride as a catalyst between 20” and 50’ C. Washing was followed by vacuum and steam distillation. This resin consists largely of polymers of indene and methyl indene.
EXPERIMENTAL
MATERIALB.The three polymers (two coumarone-indene and one indene polymer) used in this study were commercial resins, T h e raw material for resin I was a naphtha, 94% of which distilled between 160’ and 180’ C. After the undesirable polymerizable material had been removed by means of a dilute 1 Present address, Experimental Station,
E. I. du Pont de Nemoms & Co., Inc., Wilmington. Del.
0
100
600 700 900 MOLECULAR WEIGHT
200
1100
Figure 1. Experimental Fractionation for Resin I 1. Integral weight curve
2. 3. 4. 5.
Differentialwei htcurve Step differentia? weight curve Differential number curve Distribution curve
1300
1500
