D I M E T H Y L H E X A N E FORMATION IN BUTENE ALKYLATION C O R T A . Z I M M E R M A N , J O E T. K E L L Y , A N D J. C. D E A N ' Cilies Service Research and Development Co., Lake Charley, La.
The primary products of alkylation of isobutane with mixed butenes are high octane trimethylpentanes, but lesser amounts of low quality dimethylhexanes are also produced. The objective of this study was to establish the source of these low octane components when sulfuric acid was used as catalyst. This was approached experimentally b y alkylating the individual butenes with isobutane and comparing the composition of the C g fraction using improved chromatographic techniques. It was concluded that the dimethylhexanes are formed primarily b y isomerization of trimethylpentyl carbonium ion intermediates and to a lesser extent b y direct alkylation of 1 -butene with isobutane.
HE purpose of this work was to establish the source of Tdimethylhexanes formed in sulfuric acid-catalyzed isobutane (2-methylpropane) alkylation of mixed butenes. This was approached by the alkylation of isobutane with the individual butene isomers, followed by careful analysis of the CSproduct using the new gas chromatographic techniques. One of the most important high octane gasoline components is produced by the sulfuric acid-catalyzed alkylation of isobutane with mixed butenes. The main constituents of this alkylate product are trimethylpentanes, which have research octane ratings in the 100 to 110 range; ho\vever, substantial quantities of low octane dimethylhexanes are also formed. T h e variations in research octane ratings of these individual hydrocarbons are shown in Table I along with the composition of a typical sulfuric acid alkylate (6). \Vork published by Iverson and Schmerling (7) and Linn and Ipatieff ( 8 ) indicated that the dimethylhexanes were the result of the direct reaction of isobutane with 1-butene, which was either present in the feed or formed by rapid isomerization of 2-butene. While this explanation appeared plausible, it was felt that a study of the effect of olefin type on alkylate composition might be useful in more firmly establishing the route by which the dimethylhexanes were formed, This work was also of interest because, at the time it was done, no published d a t a were available showing detailed composition of alkylate produced from isobutane and the individual butene isomers when sulfuric acid was used as catalyst. Such data were available, however, for hydrogen fluoride-catalyzed alkylation ( 5 , 8'). For these reasons: sulfuric acid-catalyzed alkylation of isobutane with 1-butene. 2-butene, and isobutene (2-methylpropene) was carried out and alkylate composition determined using improved gas chromatographic procedures. Based on these data, a modified mechanism for formation of dimethylhexanes is suggested.
Procedures Experimental. Isobutane (95OjO), 1-butene (99%), 2butene (99%), and isobutene (99%) from the Phillips Petroleum Co. were used without further purification. T h e sulfuric Present address, Petro-Tex Chemical Corp., Houston, Tex. 124
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PRODUCT RESEARCH A N D DEVELOPMENT
acid (98%) used was as received from the Cities Service Refining Corp. acid plant. The alkylation reactions were carried out in a 1-gallon, stirred autoclave equipped with the usual accessories. The sulfuric acid and isobutane were charged to the evacuated reactor and cooled to 50' F. The olefin was charged a t a constant rate, over a 20-minute period, into the vigorously stirred acid-isobutane emulsion. The quantity of olefin charged gave an external isobutane-olefin molar ratio of 3 to 1. The acid-total hydrocarbon ratio was 2.38 to 1 by weight. \\'hen olefin addition was complete, the reaction mixture was allowed to separate and the acid removed. The hydrocarbon remaining in the reactor was water-washed, dried, and then distilled using a 30-plate Oldershaw distillation column at a 20 to 1 reflux ratio. Analytical. The alkylate analyses were made with a Barber-Colman Model 20 chromatograph utilizing 200 feet of squalane-coated 0.01-inch capillary column and a hydrogen flame ionization detector. T h e normalization technique was used and no detector sensitivity correction factors were necessary ( 4 ) . Elution time and peak broadening prevented the determination of the C g + fraction on this instrument. The C g - fraction was therefore determined on a Perkin-Elmer 154B instrument equipped with a reverse-flow valve and using a 20foot by 1/4-inch phenylbenzylamine column. Iso-octane (2,2,4-trimethylpentane)was common to all the samples and was used in the following equation to determine the C g + area equivalent to enter into the capillary normalization calculations.
where
AEg+ = area of C,, fraction to be entered into capillary normalization analyses
F
= response correlation factor for the two instruments
A': Ais*
used, determined on known blends on capillary column = area of iso-octane determined on packed column = area of C,, determined on packed column
A:,+
= area of iso-octane determined
Although the method necessitates a run on the PerkinElmer instrument to account for the C 9 +material, it eliminates the constant sample size requirements of other methods and requires a minimum of calibration. A synthetic alkylate blend was prepared and analyzed on different days by different operators to illustrate the quantitative aspects of the method. The results of these analyses and the operating conditions used are given in Table 11.
Table I.
Table II.
Typical Sulfuric Acid Alkylate
Component Pentane and lighter 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2,2-Dimethylpentane 2,4-Dimethylpentane 2,2,3-TrimethyIbu t ane 2,3-Dimethylpentane 2-Methylhexane', 3-Methylhesane!
Val. To (6)
Research Octane 'VO. of Indiuidual Component ( 9 )
8.9 4.7 1.1 0.4 0.2 3.4 0.2 2.3 0.3
103.7 73.4 74.5 92.8 83.1 112.2 91.1 42.4 52.0
24.3 1.2 13.0 12.3
100.0 109.5 102.7 106.1
2,2-Dimethylhexane 2,5-Dimethylhexane' 2,4-Dimethylhexane,r 2,3-Dimethylhexane 3,4-Dimethylhexane
0.2 6.6 3.0 0.4
72.5 55.5 65.2 71.3 76.3
2,2,5-Trimethylhexane 2,3,5-Trimethylhexane
4.5 0.9
2,2,4-Trimethylpentane 2,2,3-Trimethylpentane 2,3,4-'r'rimethylpentane
2,3,3-Trimethylpentane
Isoparaffins ( Cg+ )
12.1
Table 111.
Component
iCi 2,3-DMC 2-MC5 3-MCj 2,4-DMCj 2,2,3-TMC4 2iMCs 2,3-DMCj 3-MCn 2,2,4-TMC, 2,5-DMCs 2,4-DMC6 2,2,3-TMCs 2,3,4-TMCa 2.3.3-TMCS 2 3-D;CIC,j 3.4-DMCe 2;2,5-TMCs C,+ except 2,2,5-TMCs
;
%d$ 4.62 4.84 1.67 1.39 3.68 0.13 0.05 1.78 1.23 36 22 232 279 251 11 04 0 52 1.47 1.44 4.27 17 87
Weight 7 0 Found Run 7 Run2 Run 3 4.6 4.7 5.0 5.1 5.2 4.9 1.9 2.2 2.0 1.3 1.4 1.4 3.7 3.6 3.5 0.13 0.13 0 12 0.05 0.04 0.04 1.6 1.7 1 .7 1.2 1.2 1.2 36 3 36 1 36 9 2 2 2 3 2 2 2 7 2 6 2 7 2 4 2 5 2 3 10 0 10 6 10 6 0 44 0 54 0 36 1.3 1 5 1.2 1.3 1 2 1.2 3.8 4 0 3 9 18 8 18 5 18 4
Operating Conditions Column, squalane Carrier flow rate, 2 to 3 ml./min. Detector, Hz flame ionization Column length, 200 feet HPflow, 14 ml./min. Column diameter, 0.01 inch Air flow, 300 ml./min. Column temp., 30" C. Flash heater temp., 250' C. Splitter ratio, l / 2 0 0 Sample size, 0.5 ml. (approx.)
Results of Alkylating Isobutane with Individual Butenes Temperature,' F. Time, minutes Acid-hydrocarbon wt. ratio charge
50 20 2 .38-1
2-Butene
1-Butene
Octane rating (F-1 clear) 1 / 0 mole ratio Olefin conversion, wt. %, Yield based on olefin, wt. % Components, wt. % c 6 2,3-D51C1 2-MC5 3-MCj c 7 2,2,3-T1ICi 2,3-DMCj 2.4-DMCa 2-MCs 3-MCs C8 2,2,4-TMCj 2,2,3-TMCj 2;3,3-TMC:s 2,3,4-TMC:j 2,3-DMCs 2,4-DMCs 2,5-DMCs 3,4-D C 6 c,+ 2,2.5-T;CiCs Others
Component Analyses of a Synthetic Alkylate Blend
Isobutene
Run 7 94.1 3.0 98.2 186,3
Run 2 95.0 3.0 97.6 197.8
Run 1 95.3 3.0 99.8 187.1
Run 2 95.9 3.0 99.4 194.3
Run 1 94.5 3.0 100.0 161.2
Run 2 94.8 3.0 100.0 165.8
4.6 1.6 0.6 0.4 1.5 3.2 0.2 0.1 29.7 2.8 15 1 11 .o 3.3 3.5 3.4 0.1 4.5 14.4
5.2 1.6 0.5 0.4 1.8 3.2 0.3 0.3 27.8 1.8 15.3 12.7 3.3 3.4 3.4 0.4 4.0 14.7
4.1 1.5 0.5 0.2 1.6 3.3 0.4 0.2
5.0 1.4 0.5 0.4 1.5 3.3 0.1 0.3
5.1 2.0 0.7 0.2 I .9 3.9 0.2 0.1 23.9 1.5 9.7 6.1 1.8 2.3 3.3
6.0 1.6 0.8 0.4 2.1 4.2 0.2 0.1 25.5
Discussion of Results T h e results of duplicate runs for alkylation of isobutane with the individual butenes are shown in Table 111. I n agreement with previously published data, there is little difference in octane ratings of the alkylates produced with the three olefins ( 7 ) . Their compositions, with the exception of the high Cg+ material produced with isobutene, are also very similar. as was recently shown by Cupit, Gwyn, and Jernigan ( 3 ) . This high yield of heavy product was expected because of the relatively high rate of polymerization of isobutene as compared to the normal olefins and this polymerization reduced the overall yield of alkylate. T h e other more significant difference is
2.5 2.8 3.1 0.8 4.7 12.1
1.8 2.5 3.2
...
3.9 10.7
1.9
9.3 6.5 1.4 2.7 3.7
...
...
5.5 31.8
6.0 27.6
-
the slightly higher yields of dimethylhexanes formed with 1 butene. For easier comparison the normalized Cg fractions of the alkylates are shown in Table IV. (A fraction of alkylate from commercial production using mixed butenes as feed is also given to show the similarity between laboratory and commercial alkylates.) Here the similarity of products with different butenes is even more evident. Based on stability of carbonium ion intermediates the isobutane alkylation of 1butene would be expected to give dimethylhexanes while from isobutene and 2-butene, trimethylpentanes should result. Schmerling ( 1 2 ) and Linn and Ipatieff ( 8 ) explained this similarity of product and the divergence from expected product VOL. 1
NO. 2
JUNE 1 9 6 2
125
Table IV.
Composition of CSAlkylate Fractions 72IsoMixed Butenea Butenea butenea Butenesb
2,2,4-Trimethylpentane 2,2,3-Trimethylpentane 2,3:3-Trimethylpentane 2,3,4-Trimethylpentane Total trimethylpentanes 2,3-Dimethylhexane 2,4-Dimethylhexane 2,5-Dimethylhexane 3,4-Dimethylhexane Total dimethylhexanes a
Average of iwo runs.
41.8 3.4 22.2 17.3 84.7 4.8 5.0 5.0 0.5 15.3
45.2 3.2 23.6 16.4 88.4 3.0 3.6 4.4 0.6 11 , 6
49.6 3.4 19.1 12.7 84.8 3.2 5.0 7.0 ...
15.2
43.4 2.1 22.1 20.3 87.9 3.6 3.8 4.6 0.1 12.1
dlkylate from commercial unit.
distribution with different olefins, as due to the formation of a n ester intermediate and the preference for 2-butene in the very fast equilibrium reaction:
c-c-c=c
HX
c-c-c-cx
minor C-C=C-C major
+ HX
The tert-butyl ion from isobutane reacts with 1-butene, 2-butene, or the ester as shown; thus similar products are formed regardless of whether the starting olefin is 1-butene or 2-butene. Slightly higher dimethylhexane yields with 1butene have been explained on the basis of direct reaction of the tert-butyl ion with this olefin before equilibrium is reached (72). While this can explain the reason for the similarity of the products from the two normal olefins, it does not account for their similarity to the CSproduct obtained with isobutene when sulfuric acid is used as the catalyst. Here it is difficult to envision the isobutene entering into an equilibrium of the sort shown above at a very significant rate, and even if it did so, the equilibrium would strongly favor the branched olefin under these low temperature conditions. The formation of dimethylhexanes from isobutene can more plausibly be explained as occurring through the isomerization of the trimethylpentyl ion intermediates. C
I I
c-c+ C
C
+ C=c-c
__f
I I
c-c-c-c-c C
C
I +
A
dimethylhexanes The thermodynamic equilibrium for the octanes highly favors the dimethylhexanes ( 70) and trimethylpentanes will undergo isomerization under alkylation conditions. This is shown by the data in Table V, which were obtained by contacting iso-octane with concentrated sulfuric acid under the conditions indicated. The actual conversion to dimethylhexanes is low here, but the conversion of 2,2,4-trimethylpentane to the other trimethylpentane isomers is also low. This indicates that isomerization by methyl shift as well as change in degree of chain branching is more favored under alkylation conditions than under conditions used in this test. This is probably due to the presence of intermediate trimethylpentyl carbonium ions as well as the continuous source of olefin to act as a carbonium ion chain initiator. Work by Roebuck and Evering ( 7 7 ) as well as by Burwell et al. (2) has demonstrated the ability of sulfuric acid to catalyze the rearrangement 126
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
Table
V. Sulfuric
Acid-Catalyzed Degradation of Isooctane= (2,2,4-Trimethylpentane) Normal Cg fraction
Temperature, O F. Hydrocarbon-catalyst wt. ratio Reaction time, hr. 2,2,4-Trimethylpentane 2,2,3-Trimethylpentane 2,3,3-Trimethylpentane 2,3,4-Trimethylpentane 2,3-Dimethylhexane 2,4-Dimethylhexane 2,5-Dimethylhexane 0.1 DO/. yo 3-heptene added as initiator.
50 0.4 1 94.0 0.4 1.9 1.3 0.2 0.8 1.4
of paraffins to both more highly and less highly branched isomers, thus adding credence to isomerization as the source of dimethylhexanes. If this isomerization mechanism is correct for isobutene, then we would expect the formation of dimethylhexanes in alkylation with normal olefins to follow a similar path. These data support the generally accepted theory that with sulfuric acid catalyst, under normal alkylation conditions, a very rapid isomerization of 1-butene to 2-butene occurs, and that isobutane reacts predominantly with 2-butene or with the ester intermediate. Dimethylhexanes are believed to be formed to only a very small degree by direct alkylation of I-butene, but are probably largely formed by isomerization of the intermediate trimethylpentyl ions.
Conclusions Under conditions normally practiced in sulfuric acidcatalyzed alkylation of isobutane with butenes : The extent of formation of dimethylhexanes is essentially independent of olefin type; the primary product of alkylation is a trimethylpentyl carbonium ion, regardless of the butene charged; and the dimethylhexanes are formed to only a small extent by direct reaction of 1-butene with tert-butyl ion, but are presumably formed by isomerization of trimethylpentyl ion intermediates.
Literature Cited (1) Birch, S. F., Dunstan, A. E., Fidler, F. A., Pim, F. B., Tait, T., IND.EKG.CHEX31, 884-91 (1939). (2) Burwell, R. L., Jr., Scott, R. B., Maury, L. G., Hussey, A. S., J . Am, Chem. SOG.76, 5822-7 (1954). (3) Cupit, C. R., Gwyn, J. E., Jernigan, E. C., PetrolChem. Engr. 33, 43-55 (December 1961). (4) Durrett, L. R., Simmons, M. C., Dvoretzky, I., “Quantitative Aspects of Capillary Gas Chromatography of Hydrocarbons,” Division of Petroleum Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961. (5) Gorin, M. H., Kuhn, C. S., Jr., Miles, C. B., IND.ESG.CHEM. 38, 795-9 (1946). (6) Hengstebeck, R. J., “Petroleum Processing Principles and Applications,” p. 225, McGraw-Hill, New York, 1959. (72, Iverson, J. O., Schmerling, L., “Alkylation of Paraffins,” Advances in Petroleum Chemistry and Refining,” Vol. I, pp. 337-85, Interscience, New York, 1958. (8) Linn, C. B., Ipatieff, V. N., “.4lkylation of Isobutane by Pure Olefins in the Presence of Hydrogen Fluoride,” Preprints of ACS Division of Petroleum Chemistrv PaDers. March 1949. (9) Lovell, W. G., “Knocking Characteiistics‘of Hydrocarbons,” Technical Papers of Ethyl Corp., 1953. (10) Prosen, E. J., Pitzer, K. S., Rossinl, F. D., J . Research Natl. Bur. StandQrdS 34, 255 (1945). (11) Roebuck, A. K., Evering. B. L., J . A m . Chem. SOL.75, 1631-5 (105’3\ \----/.
(12) Schmerling, L., Ibid., 68, 275-81 (1946). RECEIVED for review September 26, 1961 ACCEPTEDMarch 1, 1962
