Conclusions
The catalytic, metal synergism, and metal-antioxidant interaction effects on the oxidation of styrene-butadiene rubber were adequately demonstrated by the high pressure DTA method of evaluating rubber samples for oxidation characteristics. The electron transfer mechanism of catalytic activity of metals in the oxidation of hydrocarbons as stated by Chalk and Smith (1957) was corroborated. The three antioxidants studied interacted with the metals to some extent as deactivators but, for the most part, retained their traditional role of free radical traps and peroxide scavengers. The phenolic antioxidant had some effect on the catalytic activity of the metals, indicating that the antioxidants interact with metals through mechanisms other than metal deactivation. Some mild metal synergism effects were observed, but none was in the extreme, in either the positive or negative direction. literature Cited
Barnhart, R. S., Newby, T. H., “Introduction to Rubber Technology,” M. Morton, ed., p. 130, Reinhold, New York, 1959. Chalk, A. J., Smith, J. F., Trans. Faraday Soc. 53, 1214, 1235 (1957). Duff, E. D., J . Chem. Soc. 1968a, 836. Duff, E. D., J . Inorg. Nucl. Chem. 30, 861, 1257 (1968b). Jones, M., “Elementary Coordination Chemistry,” p. 226, Prentice-Hall, Englewood Cliffs, N.J., 1964.
Jones, P., Tobe, M. L., Wynne-Jones, W. F. K., Trans. Faraday Soc. 55, 91 (1959). Kruse, W.,-Atalia, R. H., Chem. Comm. 1968, 921. Kuz’minskii, A. S., Zaitseva, V. D., Lezhnev, N. N.,Dokl. Akad. Nauk SSSR 125, 1057 (1959). Lange, N. A., ed., “Handbook of Chemistry,” p. 1213, McGraw-Hill, New York, 1961. Lee, L. H., Stacy, C. L., Engel, R. G., J . Appl. Polymer Sei. 10, 1699, 1717 (1966). Leto, J. R., Leto, M. F., J . Am. Chem. Soc. 83, 2944 (1961). Malatesta, L., Angoletta, M., J . Chem. Soc. 1957, 1186. Marks, D. R., Phillips, D. J., Radfern, J. P., J . Chem. Soc. A1967, 1464. May, W. R., Bsharah, L., IND. ENG.CHEM.PROD. RES. DEVELOP.8, 185 (1969). May, W. R., Bsharah, L., Merrifield, D. B., IND.ENG. CHEM.PROD. RES. DEVELOP. 7, 57 (1968). May, W. R., Matthew, W. R., Bsharah, L., IND. ENG. CHEM.PROD. RES. DEVELOP.6, 185 (1967). Raevski’l, A. A., Kovrizhko, L. F., Shishkina, U. V., Malyugina, A. L., Smyslova, A. F., Khim. Vysokomol. Soedin. 4, 160 (1966); C A 68, 79303m. Vinal, R. S.,Reynolds, L. T., Inorg. Chem. 3, 1062 (1964). Vol’pin, M. E., Kolomnikov, I. S., Dokl. Akad. Nauk SSSR 170, 1321 (1966). RECEIVED for review May 28, 1969 ACCEPTED October 29, 1969 Division of Rubber Chemistry, ACS, Los Angeles, Calif., April 29 to May 2, 1969.
ISOBUTANE-OLEFIN ALKYLATION WITH INHIBITED ALUMINUM CHLORIDE CATALYSTS A .
K .
R O E B U C K ’
A N D
B .
1.
E V E R I N G ’
Research and Development Department, American Oil Co., Whiting Ind. 46394
Addition of certain aromatic hydrocarbons or certain metal chlorides to a solution of aluminum chloride in aluminum chloride-dialkyl ether complex gives a n alkylation catalyst that is much more active and selective than present commercial catalysts. Parasitic side reactions such as isomerization, disproportionation, hydrogen transfer, and cracking are dramatically reduced. The effect of the inhibitor seems to be connected w i t h the reduced acidity of the inhibited catalyst.
THE alkylation
of isoparaffins with monoolefins is catalyzed both by strong Bransted acids such as sulfuric acid and hydrogen fluoride and by strong Lewis acids such as aluminum chloride. In the commercial alkylation of isobutane (2-methylpropane) with C3 and C4 olefins to produce C iand Cs isoparaffins of >90 Research octane number, sulfuric acid and hydrogen fluoride have long been preferred; aluminum chloride catalyzes side reactions I Present address, Calumet Campus, Purdue University, Hammorid, Ind. ’Present address, 337 Homeland South Way, Baltimore, Md.
76
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
such as isomerization, cracking, and disproportionation that produce either hydrocarbons of poorer octane number or excessive amounts of sludge (Iverson and Schmerling, 1958). The octane number of alkylate is primarily a direct function of the trimethylpentane content (Stiles, 1956). If alkylate consisted only of an equilibrium mixture of trimethylpentanes, its Research octane number would be 102.5 (American Society for Testing and Materials, 1958, 1964; Posen et al., 1945). Since the Research octane number of today’s commercial alkylate is 92 to 96, there is still considerable incentive to improve selectivity.
Consideration of the accepted carbonium-ion chain mechanism (Schmerling, 1945, 1955) for catalytic alkylation suggests that one of the important factors determining selectivity is the solubility of isobutane in the acid phase, where the reaction undoubtedly takes place. If 2-butene is taken as the olefin, the steps that lead to 2,2,3trimethylpentane are as follows:
CHICH = CHCH,
+ H-
(from acid) + cH cH ,‘c HCH
-
CH~CH~CHCH + ?( C H J ~ C H CHrCHrCH2CHj + (CH)?C+ ( C H I ) ~ C+- CH?CH=CHCH? + C HiCHi
I
CH~-C--
I
CH--+CH-CH~(CJ
CH? CEt
+ (CH3)jCH * (CH,),C- + 2,2,3-trimethylpentane
Steps 1 and 2 provide the initial supply of tert-butyl carbonium ion, the chain transfer agent. Step 3 produces C8 carbonium ion. Step 4 produces 2,2,3-trimethylpentane and simultaneously regenerates tert-butyl carbonium ion. 2-Butene competes with isobutane for C8 carbonium ion, as shown in Step 4a, and forms the undesirable “heavy ends” and “light ends”: ‘28-
+ CHiCH=CHCH?* +
C,?
-
+ CH ICH = C H C H I
C5, C6, Ci (light ends)
C16(heavy ends)
(44
The higher the concentration of isobutane in the acid phase, the faster should be Step 4 and the greater the selectivity. Since the solubility of isobutane is only 0.07 weight % in 98.7% sulfuric acid a t 13.3”C. (Cupit et al., 1961) and 3.1 weight 7c in hydrogen fluoride (Butler et al., 1946), it appeared worth while to look for an acid phase that would dissolve more isobutane, yet not favor the side reactions. The desired solvency was provided by the equimolar complexes of aluminum chloride with dialkyl ethers. These dissolve excess aluminum chloride to form a highly active but poorly selective alkylation catalyst (Francis, 1945, 1950). We found that isobutane is very soluble in them. The complexes of individual ethers melt a t about room temperature (Frankforter and Daniels, 1915). T o reach the low temperatures most desirable for alkylation, we used mixtures of the complexes of dimethyl, diethyl, and methyl ethyl ethers, which form low-melting eutectics--like those formed by the aluminum bromide-ether complexes (Walker, 1961). Finally, the addition of certain aromatic hydrocarbons or certain metal chlorides inhibited the side reactions without impairing alkylation activity. The selectivity of the combination of aluminum chloride, inhibitor, and aluminum chloride-mixed ether 1 to 1 complex was then tested in the alkylation of isobutane with various light olefins. Isobutylene and %butene gave high yields of alkylates of 98 and 103 Research octane, respec-
tively. Propylene and 1-butene gave substantially higher yields of alkylate, with slightly better octane number, than present commercial processes. Ethylene did not react in the presence of inhibitor, but gave high yields of 103 Research octane alkylate in its absence. Experimental
The alkylation reactions, both batch and continuous, were carried out in a 1-liter stirred pressure reactor shown schematically as part of Figure 1. I n batch alkylations, isobutane (300 grams) and catalyst (20 ml.) were charged to the reactor and brought to proper temperature. Olefin (120 ml.) was charged over a 1-hour period. The product was discharged from the reactor, fractionated to remove isobutane, and distilled in a Claisen flask. Octane number and chloride content were determined by standard methods, and in most cases the complete hydrocarbon composition was determined by gas chromatography. The batch reactor was converted to a continuous unit by the addition of a Lapp pump to pass 750 ml. of isobutane per hour continuously through the stirred reactor. Because the aluminum chloride-ether complexes are slightly soluble in isobutane, the isobutane charged to the alkylation unit must be presaturated with complex; the quantity carried into the reactor must just balance that carried out. This was accomplished by bubbling the isobutane through aluminum chloride-ether complex maintained a t an appropriate temperature in a saturator. The continuous unit is shown in Figure 1. Aluminum chloride-ether complexes are best prepared by adding the ether to a stirred suspension of aluminum chloride in isopentane which refluxes to dissipate the heat of reaction. The complex i s separated and distilled a t about 1 mm. pressure and 90” to 95°C. Complexes of aluminum chloride with dimethyl, diethyl, and methyl ethyl ethers are stable below 110°C. An equimolar mixture of the complexes of dimethyl and diethyl ethers melts a t 2°C. and an equimolar mixture of all three a t about -20°C. Another important physical property is the high solubility of isobutane in aluminum chloride-ether complex. This was measured to be 8.5 weight 9; in the mixed complexes of dimethyl and diethyl ethers a t 24” C. The active catalyst is prepared by dissolving aluminum chloride in mixed ether complex; about 6% will dissolve at room temperature, and about 12% if anhydrous HC1 is added. The most convenient method is to mix the complex, aluminum chloride, and metal chloride inhibitor (when used) under an HC1 pressure of 25 p.s.i. in a glass
ISOBUTANE PURIFIER
I I1
DRY ICE
SATUGTOR I U RUSKA PUMP OLEFIN FEED
Figure 1. Schematic of continuous alkylation Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1 , March 1970
77
pressure bottle; a portion is transferred to the reactor for use. Isobutane, olefins, alkyl chlorides, ethers, and metal chlorides were used as received; however, isobutane for continuous alkylation was purified by passage through Drierite, Ascarite, and Linde 5A Molecular Sieves. Results and Discussion Successful use of aluminum chloride-ether catalyst necessitates controlling its high activity. One obvious method of moderating excessive catalyst activity is to decrease the amount of aluminum chloride dissolved in the complex. Figure 2, A , shows, however, that as the amount of aluminum chloride is decreased the octane number increases, but the yield declines sharply. The preferred way to suppress side reactions without decreasing yield is to use aromatic hydrocarbons or metal chlorides as inhibitors. Aromatic Inhibitors. Aluminum chloride-ether catalysts are particularly susceptible to inhibition by aromatic hydrocarbons. Figure 2, B , shows the effect of 0.25 weight % hexaethylbenzene. Both the octane number, which is a measure of the trimethylpentane content, and the yield of alkylate are high. Benzene, toluene, xylenes, and some trimethylbenzenes, as well as some dicyclics such as indane, naphthalene, diphenyl, and dibenzyl, are also good inhibitors. Table I shows the effect of rn-xylene on the composition of the alkylate produced with aluminum chloride-ether catalyst and 2-butene. Without an inhibitor substantial amounts of isopentane and other side-reaction products are formed by isomerization, cracking, and disproportionation. The rn-xylene so modifies the catalyst that the initial trimethylpentane structure is preserved.
I
1
I
I
I
I
r0 1
l c
- zoo*
G
W
- 180d -65 v, z 4 0
- 160s - 4
0 5 0 - I40 2' - 2 Y
W
rK
m 2
92
,
w>
6'
I
I
1
I
102,
-J 120 -_
6?
-
-0
s
1
1
- 220
-105
c
v)
a
t
-2002-80
r"
h
b -180
=
I o -6
O S -160 6 ? - 4 2
w v, w
S
0 P
92-
0
2
1
I
I
1
4
6
8
IO
I
1 2 1 4
WT 70 AlC13 ADDED TO D I S T I L L E D COMPLEX
Figure 2. Effect of aluminum chloride concentration on 2-butene alkylation A. No inhibitor
78
6. 0.25 wt.
Yo
HEB
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
Table 1. Effect of rn-Xylene on Alkylate Composition
(Alkylation temperature, 21" C.)
None Isopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2,4-Dimethylpentane 2,3-Dimethylpentane 2,2,4-Trimethylpentane 2,4-Dimethylhexane 2,S-Dimethylhexane 2,3,4-Trimethylpentane 2,3,3-Trimethylpentane 2,2,5-Trimethylhexane Higher boiling
]
Total trimethylpentanes Yield, weight % on olefin Research octane number
Inhibitor m-Xylene
15.8
1.7
10.5
3.4
4.4 4.4 15.8
3.5
... 46.6
7.O 14.0 14.1 14.0
37.9
...
100.0
6.9 100.0
29.8 197 87.8
84.5 184 98.5
The fate of the inhibitor was determined in the case of toluene. Toluene was not present in the alkylate but was converted to a high-boiling material that appeared in the alkylate bottoms, which was also a good inhibitor. Infrared analysis of a crystalline component of this material indicated that two indane rings had been formed by cyclization and two tert-butyl groups had been added to give a completely substituted aromatic. I n later work hexaethylbenzene was the preferred inhibitor because it did not consume olefin and could be recovered unchanged from the alkylate bottoms. Although the aromatic inhibitors are effective in controlling side reactions, the fmmation of an acid-soluble oil (red oil) with the dissolved aluminum chloride gradually deactivates the catalysts. This deactivation probably occurs through the formation of the heptaalkylaryl carbonium ion, as found in the alkylation of aromatics (Doering et al., 1958). Metal Chloride Inhibitors. Some metal chlorides are even more effective inhibitors than the aromatic hydrocarbons, and also reduce the undesirable formation of red oil which deactivates the catalyst. As shown in Table 11, the metal chlorides vary considerably in their effectiveness for 2-butene alkylation. Sodium chloride in equal molar quantities with aluminum chloride destroys the activity of the catalyst; only less than equal molar quantities showed activity and some inhibiting action. Lithium, lead, cuprous, and silver chlorides are very good inhibitors. The alkaline earth chlorides and zirconium chloride are a little less effective. Although some of the metal chlorides are not completely soluble in the catalyst, this does not appear to harm their effectiveness. Ferric, zinc, cupric, platinum, antimonous, stannic, and arsenous chlorides are inactive. The composition of the 2-butene alkylate produced with metal chloride inhibitors is shown in Table 111. The selectivity is even greater than that obtained with the aromatics; with cuprous chloride the trimethylpentane content of the alkylate is 9 7 r r , indicating that side reactions have been virtually eliminated. Effect of Inhibitors on Various Olefins. Ethylene is the most difficult olefin to alkylate and requires the strongest catalyst. Aluminum chloride-ether catalyst containing the
Table II. Metal Chloride Inhibitors
Metal Chloride
Moles MCl ~Moles AlC1,S
Yield, Wt. % ' on Olefin
Research Octane Number
Red Oil, W t . % on Catalyst
At 10" C. None NaCl XaCl LiCl KC1 BeC12 CaCL BaC1, SrC1, MgC1, PbCL CuCl ZrCl,
...
210 174 None 187 203 194 188 201 192 179 193 193 195
0.67 1.0 1.0 0.50 1.o 0.50 0.50 0.50 1.0 1.0 1.0 1.0
93.4 99.3
8.8 2.1
100.0 95.4 95.5 98.8 94.6 99.3 99.5 99.8 101.0 100.8
1.9 10.0 5.0 2.9 6.2 4.1 3.5 1.2 1.7 2.5
103.1 103.0 101.9
0.4 0.2 0.9
...
At -7" C. AgCl CuCl PbCly
196 195 191
1.0 1.0 1.o
Table 111. Effect of AgCl and CuCl on Alkylate Composition
(Alkylation temperature -7" C.)
None Butanes Isopentane Pentenes and butenes 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 2,2-Dimethylpentane 2,4-Dimethylpentane 2,3-Dimethylpentane 2,2,4-Trimethylpentane 2,4-Dimethylhexane 2,5-Dimethylhexane 2,2,3-Trimethylpentane 2,3,4-Trimethylpentane 2,3,3-Trirnethylpentane 3,4-Dimethylhexane
}
}
Total trimethylpentanes Yield, wt. 5 on olefin Research octane number
Inhibitor AgCl
cue1
1.1 5.4
...
0.1 0.8 0.3
0.2 0.6 0.5
5.1
0.8
0.6
0.3 0.1 2.0 0.4 42.3
0.2 0.7
0.3 0.6
0.3 35.1
40.4
3.0
...
...
7.2 11.4 20.7 0.4 -
2.4 31.2 27.5 0.6
3.0 26.3 27.3
100.0 99.4
100.0
100.0 99.8
81.6 186 98.8
96.2 190 102.8
97.0 192 103.0
...
...
... -
maximum amount of dissolved aluminum chloride (12 weight 5;) gave very $,electivealkylation without an inhibitor. The addition of even small amounts of any inhibitor virtually destroyed its activity for ethylene alkylation. Propylene, isobutylene (2-methylpropene) and 1-butene, as well as 2-butene, alkylated readily and gave more selective products in the presence of an inhibitor. The first three, particularly isobutylene, gave more heavy ends than 2-butene. Analysis of the alkylate produced with various olefins at optimum conditions is shown in Table IV. Ethylene gives 2,3-dimethylbutane in high yields. Propylene with cuprous chloride inhibitor gives 96.1% dimethylpentanes, mostly 2,3-dimethylpentane. The same inhibitor with 1-butene gives high yields of dimethylhexanes, indicating they are primary products. Since they are undesirably low in octane number, virtually all 1-butene must be
removed from the feed to obtain the maximum octane number. Isobutylene produces 80% trimethylpentanes which are predominantly 2,2,4-trimethylpentane. The somewhat lower trimethylpentane content is caused by the tendency of isobutylene to polymerize and form highermolecular-weight ions which crack to products other than trimethylpentanes (Schneider, 1952). 2-Butene gives the most selective product, and as expected, it gives less 2,2,4trimethylpentane than isobutylene. Alkylation of Isopentane. Isopentane and ethylene give predominantly 2,4-dimethylpentane (Table V) . The primary product should be 2,3-dimethylpentane, as obtained with isobutane and propylene; but it was partially converted to 2,4-dimethylpentane because the ethylene alkylation was not inhibited. Isopentane and propylene give a mixture of 2,3- and 2,4-dimethylhexanes (as predicted by the Schmerling theory), along with some 2,4-dimethylhexane probably formed by isomerization. Reaction Conditions. Alkylation temperature, catalysthydrocarbon ratio, and isobutane-olefin ratio were investigated to establish optimum conditions. The effect of temperature on ethylene and 2-butene alkylation is shown in Figure 3. I n each case the selectivity, as measured by Research octane number, increases with decreasing temperature, because fewer side reactions occur a t the lower temperatures and more of the firstformed 2,3-dimethylbutane or trimethylpentanes survive. However, the decline in yield limits the desirability of going to lower temperatures. For the most selective alkylation, the lowest temperature consistent with good yield is 10°C. for ethylene and -7°C. for 2-butene. Raising the isobutane-olefin ratio is a well recognized method of improving alkylation selectivity. Figure 4 shows that over a limited range with 2-butene, the metal chloride-inhibited catalyst behaves similarly to conventional alkylation catalysts. The greatest departure from conventional alkylation conditions is the hydrocarbon-catalyst ratio. The aluminum chloride-ether catalyst is so active that ratios as high as 120 to 1 may be used. As shown in Figure 5, the selectivity increases as the ratio increases until a limit is reached where the alkylation yield suffers. This is very marked for ethylene. Because inhibitors cannot be used in ethylene alkylation, the hydrocarbon-catalyst ratio is the most useful variable, along with temperature, in maximizing the octane number of the alkylate. A similar but less striking effect with 2-butene is also shown in Figure 5. Continuous Alkylation. The products of continuous alkylation with 2-butene using cuprous chloride inhibitor were investigated by analyzing the alkylate a t various intervals during a run carried out to 120 gallons of alkylate per pound of aluminum chloride. The uniformity of the product indicated that the catalyst activity remained remarkably constant. As shown in Figure 6, the trimethylpentane content remained steady at about 95%. The side reactions, producing products other than trimethylpentanes, increased from 2 to 4% at 20 gallons per pound of aluminum chloride, leveled out until the catalyst began to lose activity a t 100 gallons of alkylate per pound of aluminum chloride, and then increased sharply. During the continuous runs HCl must be added continuously to activate the catalyst. Because it is difficult to Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1 , Morch 1970
79
Table IV. Composition of Alkylates from Various Olefins
Ethylene
Olefin Isobutane n-Butane Isopentane n-Pentane tert-Butyl chloride 2,3-Dimethylbutane 2,4-Dimethylpentane 2,3-Dimethylpentane 2,2,3-Trimethylpentane 2,2,4-Trimethylpentane 2,3,3-Trimethylpentane 2,3,4-Trimethylpentane 2,3-Dimethylhexane 2,4-Dimetbylhexane 2,5-Dimethylhexane 3,4-Dimethylhexane Higher boiling
...
2.2
... ...
... ...
... 100.0 10 None 281" 102.5
Theoretical yield 300%;. * Theoretical yield 238%
e
2-Butene
0.2
0.4
...
...
0.7
0.2
4.0
0.6
...
... ...
0.7
2.6 3.2 1.1 1.9 60.7 11.1 6.8 2.8
...
... ... ...
1.7
1.7
...
...
1.o
Isobutylene
...
0.1 0.4 4.1 92.0
... 0.2 ...
Temperature, C. Inhibitor Yield, wt. 5% Research octane number a
0.6 0.3 0.8
2.2 0.6 1.2 0.1 0.7 91.8
}
Composition 1 -Butene
Propylene
... ... ... 0.6 0.9
...
3.0 40.4 27.3 26.0
... ... ... ... ... -
2.8 78.2
100.0
100.0 102.8
100.0
100.0 99.5
-7 CUCl 21gb 91.7
10 CUCl 153' 70.3
10 CUCl 184' 98.2
-7 CUCl 192' 103.0
...
... ...
... ...
13.2 1.2 4.6
...
5.4
__
Theoretical yield 204%
Table V. lsopentane Alkylation
Composition Ethylene Propylene
Olefin Isopentane 2,3-Dimethylbutane 2- and 3-methylpentanes 2,4-Dimethylpentane 2,3-Dimethylpentane 2-Methylhexane 3-Methylhexane 2,2,3-Trimethylpentane 2,5-Dimethylhexane 2,4-Dimethylhexane 2,3-Dimethylhexane 3,4-Dimethylhexane 2,2,5-Trimethylhexane
Inhibitor Yield, wt. L7,
Theoretical yield 350%
5.3 2.4 4.3 52.6 26.9 3.2 2.4 0.2 0.4 0.5
...
4.6 3.5 2.6 0.2 1.1
... ...
0.2 1.3 11.7 54.4 20.0
1051
I
I
I
ETHYLENE I00
2z
3
z
1 0 INHIBITED
0 NOT
95
INHIBITED
1I
105F I
W
_
_
Z
a
g+
100
I
0.2 1.4 -
... -
5
100.0 99.8
100.0 99.6
W
None 350"
CUCl 230*
' Theoretical yield 275% .
I
1
0
Lz
95
VI
Lz -10
0
T E M PER AT U R E ,
10
20
30
O C
Figure 3. Effect of temperature on alkylation
control the addition of gaseous HC1 in small quantities, tert-butyl chloride was added to the feed as an activator. This low boiling alkyl chloride is readily removed from the alkylate by distillation. Figure 6 shows that the chloride in the product dropped rapidly from 2% and leveled out a t 1%. Only 1 to 2% red oil accumulated in the catalyst. Since cleanup of the feed with molecular sieves increased catalyst life, a substantial portion of the red oil may have come from sulfur and oxygen compounds and diolefins in the feed. Similar results were obtained in the continuous alkylation of propylene using cuprous chloride as inhibitor and tert-butyl chloride or hydrogen chloride as activator. A 200 weight 5% yield based on olefin (theoretical 238%) of 91 Research octane number alkylate was obtained; it contained 88% 2,3-dimethylpentane. The composition 80
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1 , March 1970
remained essentially constant out to 87 gallons of alkylate per pound of aluminum chloride. I n the alkylation of propylene using conventional catalysts, propane is produced and more than theoretical quantities of isobutane are consumed. This is due to a hydrogen transfer reaction between isobutane and propylene to form propane and isobutylene which alkylates. When metal chloride inhibitors are used with aluminum chloride-ether catalyst, most of this hydrogen-transfer reaction is stopped. This makes the catalyst especially suitable for propylene alkylation because it is strong enough to catalyze the alkylation of propylene without the formation of isobutylene. Mechanism. At question is the mechanism by which the inhibitors so drastically alter the composition of the alkylate obtained from the aluminum chloride-ether sys-
+ Z W
0
I
90
,
TOTAL
TRIMETHYLPENTANES
I
I
ATOTAL CRACKED
L1:
0
w
PRODUCTS t - B U T Y L CHLORIDE
a
c I
2 w
3
3
5
4
6
ISOBUTANE- OLEFIN RATIO
Figure 4. Effect (of isobutane-2-butene ratio
20
60
40
120
I00
80
G A L L O N S A L K Y L A T E P E R POUND OF A L U M I N U M CHLORIDE
Figure 6. Continuous alkylation, product variation ‘--I
LT W
m
H
AIC13- COMPLEX t AICIg
90
3
z
I
I
0 AIC13- COMPLEX
NO INHIBITOR 600 -
85
W
z 4
+ 0
500
0
-
BUTENE-2
I
0
4 W Ln W
IL
I
-
a
-
94
BUTENE- 2 NO INHIBITOR
90 I
,
I
I
23
4Cl
60
80
I
I00
I
I
120
140
i
H Y D R O C A R B O N TO CATALYST R A T I O
Figure 5. Effect of hydrocarbon-catalyst ratio 2
1
3
tem. No interaction occurs between pure aluminum chloride and pure HCI (Brown ~t al., 1950), but in the presence of a basic substance- Le., a proton acceptor-a complex is formed in which the aluminum chloride and hydrogen chloride are associated stoichiometrically.
PJC1, + HC1 + proton acceptor= [proton acceptor]+AlCl,H I t is postulated that the resulting ionized complex acts as the effective catalyst (Evering et al., 1953). T h e aluminum chloride-ether complex with its pair of unshared electrons is a good proton accelptor. Association occurs as follows:
C1 C H , C1 CHI C1:Ai:O: -c HC1+ AICli 2 Cl:Ai:O:H] AlCK c i CFL ci C H I
[
+
Thus, the availability of the electron pair in the ether determines the extent of interaction of HC1 and aluminum chloride. The equilibrium HCl vapor pressures over aluminum chloride-ether complex with and without dissolved aluminum chloride (shown in Figure 7) support these ideas.
4
5
6
7
I
MILLIMOLES HCI ADDED
Figure 7. Hydrochloric acid absorption Equilibrium vapor pressure of hydrogen chloride-aluminum chloride-ether complex
With the pure complex each addition of gaseous HC1 produces a proportionate increase in pressure. The slope of this line is the solubility of HC1 in the complex. However, when 10.5 weight % dissolved aluminum chloride is present, the curve is displaced downward, showing definite association between HCl and aluminum chloride. No association between HC1 and aluminum chloride occurs when the aluminum chloride-ether complex is absent. The mechanism of inhibition is determined by the same electron pair. I n the case of aromatic inhibitors, the aromatic is more basic than the aluminum chloride-ether complex and tends to tie up the proton more strongly, forming in turn a carbonium ion of lower acid strength as follows:
c1 c1
H:C1 + AlCl
aromatic
aromatic H ’ [AlCl.,]
The net effect is to buffer the acidity a t a lower level and thereby slow down the alkylation reaction. Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1, March 1970
81
The inhibiting action of the metal chlorides which overcame the fundamental difficulty with the aromatic inhibitors may be pictured in an analogous manner as follows:
Metal chloride
c1 + H:C1: + AlCl c1
-+
[metal chloride HI’ [A1C14]This metal chloride is competing with the ether complex to modify the acidity of the catalyst. One can readily see how variations in the basicity of the metal cation may have different inhibiting effects. Both aromatic and metal chloride inhibitors appear to act as bases toward the aluminum chloride-ether catalyst. Some understanding of the acid strength of these catalysts was obtained by studying the reaction of benzene with the isomeric butyl chlorides both with and without inhibitors. Aromatic hydrocarbons are more difficult to alkylate with n-butyl chloride than with sec-butyl chloride, while alkylation with tert-butyl chloride is relatively easy. The uninhibited aluminum chloride-ether catalyst is sufficiently strong to alkylate benzene with all the butyl chlorides; but when the metal chloride-inhibited catalyst is used, only the tert-butyl reacts, indicating that the acid strength has been reduced. This accounts for the selective production of the highly branched trimethylpentanes in the inhibited catalyst system; ions other than the desired tert-butyl are not produced, and the side reactions such as isomerization and cracking do not occur. Conclusions
Aluminum chloride-ether catalysts are exceptionally active for alkylation of isobutane with low-molecularweight olefins, but have only limited value unless this high activity is controlled. Aromatic hydrocarbons and metal chlorides modify this activity so that the first products of alkylation are not altered by subsequent isomerization, disproportionation, hydrogen transfer, and cracking. The inhibitor appears to act through control of acidity. Metal chloride inhibitors are more selective than aromat-
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ic inhibitors and reduce the formation of high-molecularweight compounds (red oil) to well below that obtained with an uninhibited catalyst. Red oil appears to originate in impurities in the olefins and isobutane. To obtain maximum benefits from the inhibited catalyst, the hydrocarbon catalyst ratio must be high (50 to 1) and temperature must be low (-7°C.). The rate of alkylation is about 30 times that of sulfuric acid. Literature Cited
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RECEIVED for review March 14, 1969 ACCEPTED November 3, 1969
