ALKYLATION OF ISOBUTANE WITH 1-BUTENE USING SULFURIC ACID A S CATALYST A T HIGH RATES OF AGITATION J A M ES F
.
M0S BY
A
N D LY LE F. A LBR IGHT
,
Purdue University, Lafayette, Ind.
lsobutane was alkylated with 1-butene in the presence of sulfuric acid in a small ( 2 4 ml.) stirred autoclave a t various rates of agitation, 10" to 40" C., isobutane-1 -butene ratios of 2.5 to 1 and 5 to 1, average residence time in the reactor of 15 to 60 seconds, and volumetric acid-hydrocarbon ratios of 0.5: 1 to 2 : l . The sulfuric acid used as feed varied from 88 to 99% acid containing 1 to 2% water and the remainder organic materials formed in the alkylation reaction. The hydrocarbon product was analyzed using a gas chromatographic procedure for all major compounds through the CS range. Several C ~ and O higher peaks were noted but not identified. Numerous runs were made to determine quantitatively the effect of operating variables on the product quality, yields, and drop in acid strength. In all runs, the 1-butene reacted completely by polymerization, esterification, or alkylation reactions. Reasons for certain phenomena are postulated.
of isobutane (2-methylpropane) with an olefin, as practiced commercially to produce high-octane gasoline. involves two liquid phases, a n organic phase, and an acid phase (sulfuric acid, hydrogen fluoride, or aluminum chloride). Although olefins are soluble in all of the commonly used catalysts, isobutane is much less so. The isobutane solubility in the catalyst may be a n important factor in determining the type and rate of reaction. Agitation in the reactor is an important variable in promoting contact between the two liquid phases, but the quantitative effrcts of neither agitation nor residence time in the reactor have been reported in the literature. The nonhomogeneous nature of alkylation has made the determination of the reaction mechanism most difficult, and numerous uncertainties concerning the mechanism still exist. Investigators have studied the effect of varying the olefin (2, 7, 8 ) , the isoparaffin ( 2 ) ,and the catalyst (7) ; the use of radioactive-tagged olefins ( 9 ); exchange reactions with alkyl halides ( 7 ) ; disproportionation tests (2, 75) ; and alkylations involving butylene dimers and trimers ( 6 ) . Various reaction schemes have been proposed (3, 4, 73), but the generally accepted mechanism, proposed in 1953 by Schmerling ( 7 4 , involves carbonium ions. A chain reaction is supposedly initiated by the protonation of the olefin followed by hydrogen exchange with the isobutane to form the tert-butyl cation and a paraffin corresponding to the feed olefin. This tert-butyl ion then adds to the double bond of the olefin, forming a carbonium ion of higher molecular weight which may rearrange via hydride and methide ion shifts. The rearranged ion can then undergo hydrogen exchange with more isobutane to produce the product molecule and regenerate the tert-butyl cation which re-enters the reaction. Proposed side reactions of the carbonium ions include destructive alkylation (fission of the heavier product ions to form a n olefin and a lighter carbonium ion), self-alkylation (dehydrogenation of some of the tert-butyl cations to form isobutylene and subsequent reaction of the isobutylene), polymerization of the olefin. and formation of alkyl esters of the catalyst acid. Den0 et al. (5) have recently reported that the tert-butyl cation is very unstable in strong sulfuric acid. The Schmerling mechanism assumes that all or a t least most dimethylhexanes produccd during isobutane-butene alkylations
A
1
LKYLATION
Present address, American Oil Co., Whiting, Ind.
result from the direct alkylation of 1-butene (72). Zimmermann, Kelly, and Dean (76), on the other hand, think that most dimethylhexanes are produced after the isomerization of the trimethylpentyl carbonium ions which result from the direct alkylation of 2-butene. The Schmerling mechanism does not explain the observed variation in product composition with changes in acid strength. Hofmann and Schriesheim investigated the product composition as a function of time, using both carbon-14-tagged butenes ( 9 ) and nonradioactive butenes ( E ) . Since the sulfuric acid strength declined with time (although they did not report the acid strengths), the information obtained in this manner was a rough measure of the effect of acid strength on product composition. T h e product composition varied significantly with time, and initially the Schmerling mechanism did not predict the correct type of product. In the present study, isobutane was alkylated with 1-butene a t high agitation rates and short contact times. The hydrocarbon product was carefully analyzed to identify all major constituents up to the C9 materials. The effect of operating variables on the various hydrocarbons produced is reported here Experimental
Equipment and Materials. The equipment consisted of feed systems for isobutane, 1-butene, and sulfuric acid, a reactor, and a sample-collecting system. .4 flowsheet of the total system is shoivn in Figure 1. The isobutane and 1-butene feed systems were alike and each consisted of a feed tank, a Matheson Series 600 rotameter with stainless steel fittings, and a stainless steel Hoke precision metering needle valve. Nitrogen was used to pressurize the reactants in the system. The isobutane and 1-butene were Matheson technical grade gases. The isobutane was of 96.07, purity and contained 3.57, of n-butane plus traces of ethane and propane. T h e 1-butene was 98.07, pure and contained 1.5% of isobutane plus traces of ethane and 2-butene. The isobutane and 1-butene streams were combined before they entered the reactor. The sulfuric acid feed system was identical to that used for the hydrocarbons, except that the rotameter used was an F and P precision-bore Flowrator equipped with Teflon packing. Reagent grade sulfuric acid of about 97.5% strength, 307, fuming sulfuric acid, and two used alkylation acids of about 94.5 and 85.07, strength were used to prepare the catalysts. These acids were blended in various proportions to obtain the desired acid for each run. The reactor, shown in Figure 2, was machined from 11/2inch stainless steel rod and consisted of the reaction chamber VOL. 5
NO. 2
JUNE 1 9 6 6
183
Valves a r e numbered.
111 Condenser A U
Magnetic Stirrer
Figure 1 .
U
Dry Ice Trap
Teflon Gasket
Reactor closure
Flowsheet of alkylation system
itself, the reactor closure, and a packing nut used to tighten the packing around the agitator shaft of about 0.5-inch diameter. The reaction chamber, with an inside diameter of 1 inch and a height of 2 inches. had a capacity of 24.0 f 0.5 ml. with the agitator in place. The acid and hydrocarbon feed lines entered the bottom of the reactor and the product stream left the reactor a t the top. A pressure tap for a 400-p.s.i.g. pressure gage was also located a t the top of the reactor. A thermocouple was positioned in a thermowell about halfway up the side wall of the reactor and extended about 3,'16 inch into the fluid. The agitator was a four-bladed impeller of about 0.75-inch diameter, driven by a '/r-hp. electric motor equipped with a variable-speed pulley. T h e agitator speed could be varied from 500 to 3100 r.p.m. T h e motor and reactor were rigidly mounted on a frame so that the reactor and packing gland were totally immersed in a water bath during an alkylation run. This well agitated bath served the dual purpose of controlling the reactor temperature and cooling the agitator shaft packing gland. .4Grove back-pressure regulator of 15- to 150-p.s.i.g. range \vas connected to the outlet line of the reactor and was used to control the reactor pressure to about 90 p.s.i.g At this pressure, the hydrocarbons were maintained in a liquid state. The sample-collection system, connected to the regulator with a short Teflon sleeve, consisted of a 500-ml. three-necked flask containing 230 ml. of saturated sodium sulfate solution, and immersed in an ice bath to prevent excessive heating of the sample as the acid catalyst was quenched. The flask was equipped with a magnetic stirrer and a water-cooled condenser, the top of which was connected to a 100-ml. collection tube immersed in a dry ice bath in a Dewar flask. A hydrocarbon product sample was collected by diverting the entire reactor effluent stream into the three-necked samplecollecting flask containing a cold (approximately 15' C.) saturated solution of sodium sulfate, which helped "salt out" all hydrocarbons from the water layer. The more volatile hydrocarbons in the effluent stream passed over and condensed in the dry ice trap. A sample of the exit acid was obtained during a run by allowing the acid and organic phases to separate without using the quenching technique. Analytical Procedures. The material in the samplecollecting flask was transferred to a separatory funnel to separate the hydrocarbon and water layers. The hydrocarbon layer was then washed with 10% sodium carbonate solution. The hydrocarbon layer from the sample-collecting flask and the volatile hydrocarbons in the dry ice trap were both analyzed by gas chromatography using a 1/4-inch 0.d. X 12-foot column of 307, di-2-ethylhexyl sebacate on 60- to 80-mesh Columnpak a t 120' C. A total of about 30 peaks was obtained, but only the first 18 peaks (up to 2,2,5-trimethylhexane) could be identified. The remaining peaks were undoubtedly nonanes or higher and were present in only small quantities. The iodine number of the nonvolatile layer was determined using a Hanus solution (iodine monobromide in glacial acetic acid). The unsaturation is thought to have occurred primarily in the heavy ends of the alkylate product. 184
Bronze Bearing
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
Reaction v e s s e l
Figure 2.
Alkylation reactor
The exit acid was diluted with water and titrated with a standard caustic solution (approximately 2,V sodium hydroxide) to determine the weight per cent of sulfuric acid in the exit stream. The volume of the water layer in the samplecollecting flask was measured and a sample titrated with the standard sodium hydroxide solution. These values were used to calculate the amount of acid in the reactor effluent and, thus, the acid material balance. Results
The effects of operating variables on yield, product composition, and acid consumption were investigated in 79 runs under the following conditions: Retention time. 15, 30, or 60 seconds Agitator speed. 600, 2000, or 3000 r.p.m. Reactor temperature. lo', 25', or 40' C. Isoparaffin-olefin ratio. 2.5:l or 5 : l Acid strength. 85 to 99% H2S04 The yield is defined as the grams of product formed per gram of olefin consumed. Product was considered to be the total hydrocarbon exit stream less any unreacted isobutane, n-butane, or butenes. Unless reported otherwise, the system operated with the acid phase continuous and at a volumetric acidhydrocarbon ratio of 1 to 1. Effect of Residence Time. Thirteen series of runs were made to determine the effect of residence time on the product, 0.7, 97.4, drop of acid strength, and yield for runs with 99 94.5, and 91.9% feed acid. The residence time was calculated by dividing the reactor volume by the volumetric rate of the entering reactants. Figure 3 shows product composition as a function of time for four series of runs with 99% feed acid and a t 3000 r.p.m. for both 10' and 40' C. and with isoparaffin-lbutene ratios of 2.5 to 1 and 5 to 1. Figures 4 and 5 indicate the effect of time on octane products and other reaction products for a series of runs with the 9970 acid, 3000 r.p.m., and a ratio of 5 to 1. The acid concentration decreased as the acid passed through the reactor by about 1 to 3y0 and the hydrocarbon product had a n unsaturation ranging from about 0 to 100 iodine value. 1-Butene was not detected in significant quantity in the
*
Effect of Increasing Residence Time from 15 to 60 Seconds 6 7 8 9 10
Table 1. 1 Comparison 99.7 Feed acid, % 10 T , C. 600 R.p.m. 2.5 1 / 0 ratio 1 HCIacid ratio Total light ends 30-25 22-26 Total octanes Total heavy 48-50 ends Yield 11.2-1.3
2 99.7 10 600 5
3 4 5 98.5 99 99 10 10 10 3000 3000 2000 5 2.5 5 1 1 1 38-32 30-27 37-30 26-40 30-39 32-48
1
37-33 24-24
~~
Yield
5 1 42-35 29-37
1
1
34-36 23-27
40-40 29-32
97.4 10 3000 5 1
33-29 28-50
11
94 10 600 5
12 13 94.5 91.9 10 10 3000 3000 5 5
94 10 2000 5 1
1
35-27 26-27
1
35-31 31-29
35-29 36-50
1
35-28 35-46
~
Table II.
Total octanes Total heavy ends
99 99 40 40 3000 3000 5 2.5
40-44 37-28 39-33 30-22 29-28 42-38 32-38 40-21 40-47 34-30 30-21 30-26 1.3-1.6 1.7-2.3 1.8-2.2 1.5-2.5 1.8-2.0 1.5-2.0 1.7-1.9 1.4-2.6 1.3-1.8 2.0-2.3 2.4-2.6 2.2-2.5
~
Comparison Feed acid, 7~ T , O c. Residence time, sec. 1 / 0 ratio HC/acid ratio Total light ends
99 25 3000
10 15 5
2 99 10 30 5
1
1
1 99
37-37 24-33 40-30 1 3-2 2
35-34 25-40 40-26 1 2-2.2
Effect of Increasing Agitation from 600 to 3000 R.P.M.
3 99 10 60 2.5 1 24-27 26-40 50-33 1.4-2.2
4 99 10 60 5 1
32-30 23-48 44-22 1 5-2.5
product in any run of this investigation; apparently essentially 1007, reacted in all cases. Table I indicates how the composition of the product changed as the residence time in the reactor increased from 15 to 60 seconds for all 13 series of runs; the first number in the rows for composition or yield is the value for the run a t 15 seconds in the series and the second is for the value of the 60-second run. An intermediate value was generally obtained for the 30-second run. I n general, the following trends were notrd with increased residence time : Increased fraction of octanes particularly trimethylpentanes in the product. Decreased fraction of heavy ends in the product. Small decrease of light ends in product. Decreased degree of unsaturation in the product. Increased yield of product. Decreased drop of acid strength. T h e only exceptions were: All series of runs a t 600 r.p.m. showed a n increased amount of heavy ends and little or no increase of octanes with time. Runs a t 40' C. had a small increased fraction of light ends in the product. T h e changes of the octane and heavy end contents were less pronounced because of time than in runs a t lower temperatures.
Effect of Agitation. Agitation had a n important effect on the alkylation reaction, as was indicated by ten series of runs with 99 i 0.7 or 94.57, feed acid. Figure 6 indicates the results for four series of runs a t 60 seconds' residence time and 99% feed acid made a t 10' and 40' C. and a t an isobutane1-butene ratio of 2.5 to 1 or 5 to 1 ; the exit acid strengths were lower than 99% by about 1 to 3%. Figures 7 and 8 demonstrate the effect of agitator speed on the production of octanes and other major components for runs with 99% acid, 3000 r.p.m., 5 to 1 ratio, and 10' C. Table I1 shows the major trends in product composition as the agitator speed increased from 600 to 3000 r.p.m. for all ten series of runs. T h e first number in the rows for the composition or yield is the value of the 600-r.p.m. run and the second number is the value a t the 3000-r.p.m. run. I n all cases, increased agitation caused increased octane production (especially trimethylpentanes) and
5 99 40 30 5 1 41-45 24-30 35-26 1.3-1 8
6 99 40 60 2 5 1
35-36 21-27 44-37 1.5-2 0
7 99 40 5
8 94.5 10 15 5
1
1
60
40-41 21-32 39-27 1.5-2 0
9 94.5 10 60 5 1 27-29 27-48 47-22 1.8-2.6
35-35 25-35 40-30 1.3-2.3
10 94 10 60
5 1 32-35 19-28 49-37 1.4-2.3
yields, and decreased heavy ends, unsaturation in the product, and drop in acid consumption. I n general, agitation had little effect on the total amount of light ends in the product; however, increased agitation as a rule decreased the amount of n-butane formed and increased the production of isopentane and 2,3-dimethylbutane. Effect of Acid Strength. Several runs were made using various acid strengths at 10' C., an agitator speed of 3000 r.p.m., an isobutane-1-butene ratio of 5 to 1, and retention times of 60 and 15 seconds. First runs were made with fresh 98 and 99% acid-Le., 2 and 1% water, respectively; the results of these runs were very similar. Feed acids of less than 987, strength were prepared by blending a used alkylation acid with fresh 98% sulfuric acid. The acid strengths reported in Figure 9 are for the exit acid assumed the same as that of the acid in the reactor. The percentage of total octanes in the product reached a maximum with about 94Y0 acid in the reactor, a t which point the total nonanes and heavier reached a minimum. T h e percentage of heptanes and lighter increased only slightly as the exit acid strength increased from 90 to 98%. Figures 10 and 11 indicate in detail the effect of exit acid strength on the product composition. While most of the octanes increased with increasing acid strength, the 2,3,4trimethylpentane plus 2,3-dimethylhexane content decreased sharply a t the higher acid strengths of about 98%. Detailed
Table 111.
Effect of Increasing Temperature from 10' to 40' C.
Comparison Feed acid, 70 R.p.m. Residence time, sec. 1 / 0 ratio HC/acid ratio Total light ends Total octanes Total heavy ends Yield
1
2 94 2000 60 5
3
4
99
99
3000 15 5
3000 60 5
1
1
94 600 60 5 1 27-32 27-19 47-49 1.8-1.4
31-35 37-41 39-28 33-29 30-37 29-30 2.3-2 3 2.2-1.7
VOL. 5
NO. 2
1
30-41 48-32 23-28 2.5-1.9
JUNE 1 9 6 6
185
:
-~
H e p t a n e s and lighter
40
18
-
16
-
4-
014-
a
U
212
a
.-
-
c10-
-
50
I
I
:IA, 0
3 40
"
r\
20
IO
I
I
I
I
I
I
I
I
I
Nonanes and
2,2.3-T M C 5
30
Figure 4. tion
3
2.5
I
0
50
60
Effect of reaction time on product composi-
10.0' C., 3000 r.p.m., 99.0 =k 0.7% acid, 5 to 1 1/0 ratio
heavier
"I
20 I
40
Retention Time (sec.)
n
Q20 I 50
-
$6
a
Octanes
0
-
'Ea al
,
f
1
I
I
'
I
I
1 --
-
I
41b
20
30
I
40
50
60
Retention T i m e (sec.) Figure 5. Effect of retention time on product composition IO
20
30
40
50
60
10.0' C., 3000 r.p.m., 99.0 z!c 0.7% acid, 5 to 1 1/0 ratio
Retention Time (sec.) Figure 3. product
Effect of retention time on yield a n d 3000 r.p.m. A 10' c., 5 to 1 I/O ratio
A 0
10' C., 2.5 to 1 1/0 ratio 40' C., 5 to 1 1/0 ratio 40' C., 2.5 to 1 1/0 ratio
analyses by the American Oil Co. indicate that the 2,3-dimethylhexane content of several products increased only slightly as the acid strength increased, and therefore the observed change is almost entirely caused by 2,3,4-trimethylpentane. The amount of high-boiling residue decreased sharply as the exit acid strength increased from 90% to about 93%, and then was nearly constant as the acid strength continued to rise. The 2,2,5-trimethylhexane content increased with increasing acid strength, causing the total C9 f material to go through a minimum. The yields for this series of runs varied from 2.4 to 2.7 and attained a maximum a t about 94% exit acid strength. The iodine number increased about tenfold, from near 5 to over 55, as the exit strength dropped from 987, to 90%. 186
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Effect of 'Temperature. Most of the runs were made at 10' and 40' C., but two runs were made at 25' C. The yield dropped sharply as the temperature rose to 25' C. and then continued to drop more slowly as the temperature continued to rise. Figure 12 indicates that as the temperature rose for runs with 99'37,, feed acid, the total octanes decreased sharply, and the total heptanes and lighter increased. The amount of nonanes and heavier reached a maximum at 25' to 30' C., and remained essentially constant a t higher temperatures. The effect of increasing temperature from 10" to 40" C. for two sets of runs using 94.57, feed acid is shown in Table 111. The trends are the same as for the 99% feed acid, except that the yields were identical for one comparison. Effect of Isobutane-1-Butene Ratio. Changing the ratio from 2.5 to 1 to 5 to 1 had a significant effect on the reaction, as 12 comparisons indicated. In all cases, increased ratios caused increased amounts of octanes, especially trimethylpentanes, decreased amounts of heavy ends, higher yields, decreased iodine values of product, and less drop in acid strength. The amount of light ends produced was relatively independent of the external ratio, however. The apparent increase of the total light ends in Figures 3 and 6 is caused
1-
$ e a .-t
6
Heptanes and lighter
40
=J
-
1
W
20 -
-
30
1 0
500
1000
1500
2500
2000
WOO
Agitator S p e e d (rpm.)
Figure 7. Effect of agitator speed on product composition
___---
a
Nonones
and
1 0 . O o C., 60 seconds, 99.0 21 0.7% acid, 5 to 1 1/0 ratio
heavier
-
2.0
2 W
.-
2.
1.0
590
I
1000
1
1500
Agitator Figure 6.
1
I
2000 2500 Speed .(rpm.)
I
3000
Effect of agitator speed on yield and product 60 seconds A 10' C., A 40" C., 0 40' C., 0 10' C.,
0200
Id00
15b0
2;oo
A00
3c
Agitator Speed (rpm.) 5 to 1 1/0 ratio 2.5 to 1 1/0 ratio 5 to 1 1/0 ratio 2.5 to 1 1/0 ratio
almost entirely by the additional n-butane in the isobutane feed. In one run, only 1-butene and sulfuric acid were fed to the reactor. Four immiscible liquid layers were formed but were not analyzed. Presumably the top and bottom liquid layers were hydrocarbons and acid phases, respectively. The two middle layers may have been mono- and di-esters of sulfuric acid. Acid-Hydrocarbon Ratio. All but two runs were operated a t volumetric acid-total hydrocarbon feed ratios of 1 to 1. I n general, the system operated acid-continuous. At lower acid concentrations and higher temperatures, the system tended to operate hydrocarbon-continuous, as indicated by the change in appearance of the product mixture and by the more rapid separation of the acid and hydrocarbon phases in the separa-
Figure 8. Effect of agitator speed on product composition 10.0' C., 60 seconds, 99.0 i: 0.7% acid, 5 to 1 1/0 ratio
tion flask. I n general, the system could be changed back to an acid-continuous system by momentarily increasing the acid feed rate. When the system operated hydrocarbon-continuous, the hydrocarbon product was very low in octanes and other products desired for a high quality gasoline. A run was made a t acid-hydrocarbon ratios of both 0.5 to 1 and 2 to 1. As long as the system was maintained acidcontinuous, the products were rather similar. A small increase of nonanes and heavy components occurred a t lower ratios. Discussion of Results
Reactions of 1-butene such as polymerization or ester formation were obviously important in many runs of this investigation, as is indicated by the low yields, even though most, if not VOL. 5
NO. 2
JUNE 1966
187
I
I
I
I
I
I
I
I
I
I
I
L I
2o
t 911
le'
92
93
I
I
94
95
96
A c i d Strength (wt.%) Figure 9. Effect of acid strength on product
t
I
97
98
10.0' C., 60 seconds, 3000 r.p.m., 5 to 1 1/0 ratio
2 A
1 L I -
I
A
I
2,2,3-T M C 5
A -
rrr
I
I
I
&
10.0' C., 60 seconds, 3000 r.p.m., 5 to 1 1/0 ratio
all, of the 1-butene reacted. Such reactions produce heavy ends which were often unsaturated, light ends by fragmentation of heavier components (8, 9 ) , or organic materials that built u p in the acid phase. Whenever the yields increased, relatively more trimethylpentanes were produced than dimethylhexanes. T h e theory of Zimmerman, Kelly, and Dean (76) that dimethylhexanes are formed by the isomerization of trimethylpentyl carbonium ions is not supported by the present results. In such a case, the dimethylhexane content would be expected to follow the trimethylpentane content. An alternative mechanism for production of dimethylhexanes has recently been postulated by Hofmann and Schriesheim (8, 9). These compounds are produced from an intermediate obtained by reaction of a branched allylic ion (formed from either isobutylene or tert-butyl cations) and 2-butene or isobutylene (2-methylpropene). This mechanism explains some facts very well. Dimethylhexyl carbonium ions (and subsequently dimethylhexane) are produced by the reaction of tert-butyl cations with 188
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
1-butene, whereas trimethylpentyl ions are produced with 2-butene. Isomerization of the double bond of 1-butene will of course be rapid in the acid, but it seems surprising that conditions producing higher yields and presumably higher amounts of dissolved isobutane in the acid phase also produce more olefin isomerization. O n the contrary, 1-butene would be more likely to react before isomerization when the concentration of tert-butyl ions was high. Results of the present investigation hence seem to indicate that the two types of branched octanes are produced by two different mechanisms. Experimental runs of this investigation were all a t average residence times in the reactor considerably less than those in most commercial units. Hence the products obtained may be more representative of the initial products of the reaction. Based on this line of reasoning, 1-butene seems to have reacted preferentially because of higher solubilities in the acid to produce dimers, trimers, etc., or esters. With increased time in the reactor, these initial products perhaps reacted with iso-
c
IOC
-i 41 2
0
I
I
I
92
91
Figure 1 1.
93 Acid
t
I
I
I
94
95
96
97
98
I
Strength (wt.o/ol
Effect of acid strength on product composition
10.0' C., 60 seconds, 3000 r.p.m., 5 to 1 1/0 ratio
-c
35-
I
10
15
Figure 12.
I
Effect
I
I
20 25 30 Temperature ("C.1
35
40
of temperature on product
10.0' C., 6 0 seconds, 3000 r.p.m., 5 to 1 1/0 ratio, 99% feed acid
butane to form the numerous saturated products. I n this respect, alkylations with butene dimers or trimers are reported to produce products essentially indistinguishable from those obtained by the alkylation of butenes (6). The higher drop in acid strength for runs a t short residence times could be caused by dilution of the acid with polymeric materials or by the formation of esters. Runs a t longer residence times would be expected to have higher dissolved ratios of isobutane to 1-butene in the acid phase. Longer residence times were obtained by decreasing the flow rates, and hence more acid was recirculated past the hydrocarbon feed line relative to the amount of 1-butene entering. Since there was always a n excess of isobutane in the reactor, the acid would always contain dissolved isobutane. The ratio of dissolved isobutane to entering olefin would hence increase with residence time. By similar reasoning, the dissolved ratio would increase with increased agitation. Obviously, the dissolved ratio would also increase with increased ratios of isobutane to 1-butene in the feed.
Agitation and residence time each had in many cases a large effect on the octane production but a lesser effect on both the heavy ends and especially the light ends. The ratio of isobutane to 1-butene in the feed significantly affected production of octanes but to a n ever greater extent heavy ends. Such observations emphasize the complexity of the over-all alkylation involving not only complicated chemistry but, in addition, many transfer steps of the materials in the two-phase fluids. T h e C 9 material in the alkylation product almost certainly did not result from the fission of a Clz ion, since only a small amount of propane was formed. T h e most generally accepted opinion is that this material is formed by the alkylation of byproduct isopentane (2-methylbutane) with feed butenes (70, 7 7 ) . The results of this work support this hypothesis. I n general, the production of 2,2,5-trimethylhexane (the only individual Cg measured) followed the amount of isopentane formed. An exception to this was noted a t lower agitator speeds (600 r.p.m.) where the 2,2,5-trimethylhexane content increased while the isopentane content decreased. Perhaps a t the lower agitator speeds, where the polymerization rate was high, some of the C9materials were formed by fragmentation of CI6or C20ions. Based on the available information, the following three reactions seem important for 1-butene: 1. Polymerization and/or ester formation to produce CS to Czo low polymers or ions, principally in the dimer, trimer, and tetramer range. Although such reactions have been suggested in the past, they are apparently more important than previously considered. The carbonium ions produced may undergo several different reactions. Many of the ions will fragmentate and produce C1 through C I compounds, ~ some of which will be olefins that will eventually react with tert-butyl cations. Isopentyl ions produced may react with butenes to form nonanes. Some heavy carbonium ions will eject a proton to '?rm olefins and some will be saturated by hydride transfer to form heavy paraffins. Some dimethylhexanes are probably formed in these reactions.
2. Formation of dimethylhexanes by alkylating isobutane :
C
C
e-A+ + c=c-e-c
c:
+
I
+
c-c-c-e-c-c I C
VOL. 5
NO. 2
JUNE 1 9 6 6
189
The product ion can isomerize by rnethide ion shifts to produce any of the dimethylhexyl ions, which then form dimethylhexanes by hydride transfer from isobutane. 3. Isomerization to 2-butene, followed by alkylation of the isobutane :
c c I / + + c-c-c-c-c
C
I c-C' I
+- c-c=c-c
C
C
The product ion will also isomerize to produce any of the trimethylpentyl ions, which then form trimethylpentane by hydride transfer from isobutane.
The concentration and average life of the carbonium ions in an alkylation system are ordinarily determined by both the acid strength and the solubility of the isobutane in the acid catalyst. Den0 et al. (5) have recently reported on the stability of tert-butyl cation. The solubility of isobutane in the acid phase is important because it determines the rate a t which product carbonium ions are saturated by hydride transfer from isobutane. The effect of acid strength on the composition can be explained by considering the following mechanism for producing the various trimethylpentanes. The product from the reaction of a tert-butyl cation and 2-butene is a 2,2,3-trimethylpentyl carbonium ion. If a hydride or methide ion cannot shift from any position other than alpha to the electron-deficient carbon atom, and if tertiary ions are much more stable than secondary ions, then the 2,2,3-trimethylpentyl ion would rearrange by the following series of hydride and methide shifts: I
I
C
c c C-A-C-C-C
I
+
Jr (B)
1
(A)
+
I
C
(E)
I*\c
c
(C) C-A-C-d-C
I
C
C
+ C-A-C-C-C
f
*
'
c
I
f
(D)
I
l
(F) C-C-C-C-C
l
+
A hydride transfer to any of these ions would produce a trimethylpentane. At higher acid strengths, there is a relatively greater tendency to produce trimethylpentanes from ions A, B, C, or D than from ions E and F. At lower acid strengths, the tendency is to produce relatively larger quantities from ions E and F. Apparently a t the higher acid strengths, the rate of saturation of the carbonium ions by hydride transfer from isobutane is more rapid. Afiother point which supports the theory that the rate of hydride transfer is more rapid a t higher acid strengths is the very high iodine numbers that are obtained a t low acid strengths. The lower rate of saturation of the carbonium ions by hydride transfer permits more of the heavier ions to stabilize by ejecting a proton to form an olefin.
190
An important reaction in alkylations using sulfuric acid and 1-butene is the polymerization or formation of esters with 1-butene to produce low polymers or C8-C20 carbonium ions, followed by reaction with isobutane to form the final product. Trimethylpentanes are apparently sometimes produced a t a later stage of the reaction after dimethylhexanes and Cs-C7 paraffins. The rate of transfer of hydride ions from isobutane, and thus the product composition, is determined by the acid strength. Agitation and residence time in the reactor have a significant effect on the alkylation reaction. Ac knowledgrnent
The authors thank the American Oil Co. for supplying the used alkylation acids used in this study, and the Esso Research and Development Co. for analytical assistance. literature Cited
11 0101 ,~,-,,.
i-
c c c C-C-C-C-C
Conclusions
(1) Bartlett, P. D., Condon, F. E., Schneider, A., J . Am. Chem. Soc. 66, 1531 (1944). ( 2 ) Birch, S. F., Dunstan, A. E., Trans. Faraday Soc. 35, 1013
c c C-C-C-C-C
Hofmann and Schriesheim ( 9 ) postulate that, for the case of relatively low agitation which they studied, a polymerization sequence predominates until the buildup of an organic activator in the acid phase makes the isobutane available for reaction. They suggested that the activation may be caused by either the increased solubility of isobutane in the used acid, or the buildup of a hydride transfer intermediate that makes the dissolved isobutane more available for reaction. Since the rate of hydride transfer from isobutane was apparently higher in stronger acids, the activation noted by Hofmann and Schriesheim a t lower acid strengths was probably a solubility effect.
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
(3) Birch, S. F., Dunstan, A. E., Fiedler, F. A., Pim, F. B., Tait, T., Znd. Eng. Chem. 31,1079 (1939). (4) Ciapetta, F. G., Zbid., 37, 1210 ( 1 9 4 5 ) . (5) Deno, N. C., Boyd, D. B., Hodge, J. D., Pittman, C. U., Turner, J. O., J . Am. Chem. Soc. 86,1745 (1964). (6) Field, H. W., Gould, D. W., Petrol. Refiner 25, No. 11, 575 (1964). (7) Gorin, M. H., Kuhn, C. S., Miles, C . B., Znd. Eng. Chem. 38, 795 (1946). (8) Hofmann, J. E., Schriesheim, A., J . Am. Chem. Soc. 84, 953 (1962). (9) Ibid., p. 957. (IO) Ipatieff, V. N., Grosse, A. V., Zbid., 57, 1616 (1935). (11) Ipatieff, V. N., Schmerling, L., Advan. Catalysis 1, Chap. 2, 27-64 (1948). (12) Iverson, J. O., Schmerling, L., Advan. Petrol. Chem. Refining 1, Chap. 7, 337-85 (1958). (13) Schmerling, L., Znd. Eng. Chem. 45,1447 (1953). (14) Schmerling, L., J . Am. Chem. Soc. 67, 1778 (1945). (15) Schneidcr, A., Kennedy, R. M., Ibid., 73, 5013, 5017, 5024 (1951). (16) Zimmerman, C. A., KelIy, J. T., Dean, J. C., IND.END. 1,124 (1962). CHEM.PROD.RES.DEVELOP.
RECEIVED for review October 5, 1965 ACCEPTEDMarch 25. 1966 Division of Petroleum Chemistry, 150th Meeting, ACS, Atlantic City, N. J., September 1965. \Vork supported by the National Science Foundation through its Cooperative Fellowship Program.
