GASOLINE PROCESSING literature Cited (1)
(2) (3) (4)
(5) (6)
Bailey, G. C., and Reid, J. A. (Phillips Petroleum C o . ) , U. S. Patents 2,581,228 (Jan. 1, 1952) and 2,605,940 (Aug. 12, 1952). Burk, R. E., Baldwin, B. G., and Whitacre, C. H., IND.ENG. CHEM.,29, 32630 (1937). Cheney, H. A., McAllister, S. H., Fountain, E. E., Anderson, J., and Peterson, W. H., Ibid., 42, 2580-6 (1950). Egloff, G., and Weinert, P. C., World Petroleum Congr., Proc. 3rd Congr., Hague, 1951, Sect. IV, p. 201. Evans, A. G., and Polanyi, M., J . Chem. SOC., 1947, p. 252. Hogan, J. P. (Phillips Petroleum Co.), U. S. Patent 2,642,467 (June 16, 1953).
(7) Ipatieff, V. N., and Pines, H., IND. ENG. CHEM.,27, 1364-9 (1935). (8) Langlois, G. E., Ibid., 45, 1470-6 (1953). (9) Langlois, G. E., arid Walkey, J. E., World Petroleum Congr., Proc. 3rd Congr., Hague, 1951, Sect. IV, p, 191. (10) Schmerling, Louis, and Ipatieff, V. N., "Advances in Catalysis," Vol. 11, p. 65, ilcademic Press, New York, 1950. (11) Steffens, J. H., Zimmerman, XI. U., and Laituri, iM. J., Chem. Eng. Progr., 45, 269 (1949). (12) Storch, H. H., J . Am. Chem. SOC.,57, 2598-601 (1935). (13) Thomas, C. L., IND.ENG.CHEM.,37, 543-5 (1945). (14) Whitmore, F. C., Ibid., 26, 94 (1934). RECEIVED for review September 10, 1954.
$CCEPTED January 11, 1955.
Dehydroalkylation of Aromatics with lsoparaffins JOE T. KELLY
AND
ROBERT J. LEE,
Pan American Refining Corp., Texas Cify, rex.
The alkylation of aromatic hydrocarbons by isobutane in the presence of olefins has been studied in considerable detail. By this reaction, tert-butyl aromatics can be prepared in good yield and high purity. The olefins are found to act primarily as hydrogen acceptors under acidcatalyzed conditions at 0" to 30" C. when excess isobutane is employed and an alkylatable aromatic i s present in the system. The olefins are thereby converted to paraffins rather than alkylating the isobutane or the aromatic. In the case of Cq and higher normal olefins, hydrogen transfer and saturation of the olefin i s accompanied by isomerization of the carbon skeletoni.e., hydroisomerization-so that a branched chain paraffin is produced. A carbonium ion mechanism i s presented which i s in accord with the experimental facts and the products produced.
T
HIS paper describes the alkylation of aromatic hydrocarbons
%
by isoparaffins in t h e presence of an olefinic hydrogen acceptor. Primary emphasis has been on the alkylation of benzene and toluene with isobutane to produce tert-butyl aromatics. Excess isobutane is required as alkylating agent under acid catalyzed conditions a t 0' to 30" C. Olefins must be present to accept hydrogen in order for t h e reaction to proceed. T h e reaction is therefore termed dehydroalkylation. The olefins are converted to paraffins by t h e hydrogen transfer reaction involved in dehydroalkylation. I n t h e case of C4 and higher n-olefins, hydrogen transfer and saturation of t h e olefin is accompanied b y simultaneous isomerization of the carbon skeleton-Le., hydroisomerization-so t h a t a branched chain paraffin is produced. Studies of numerous reaction combinations indicate that isobutane is converted to tert-butyl ions which alkylate the aromatic. By this process, p-di-tert-butylbenzene and isoparaffins can be produced in near quantitative yields from a reaction mixture consisting of isobutane, benzene, and olefins. Similarly, tert-butyltoluene is obtained from toluene. Yields of tert-butylated aroinatics and hydroisomerized products vary considerably, depending on the structure of the olefin hydrogen acceptor and the catalyst. Data are presented on a variety of olefins using sylfuric acid, boron fluoride monohydrate (BF3. HzO), and hydrogen fluoride catalysts. On the basis of these results, a carbonium ion mechanism has been developed which is in accord with t h e experimental facts and t h e products produced. The dehydroalkylation reaction may be illustrated by equation presented herewith. T h e over-all result may be pictured as t h e reaction of isobutane with an aromatic (in this case toluene), with a net transfer of t h e tert-hydrogen from isobutane and a hydrogen from t h e aromatic to t h e olefin. tert-Butyltoluene and a paraffin are t h e major products. T h e reaction is thus seen to involve dehydroalkylation in contrast to t h e conventional alkylation of an aromatic b y an olefin. However, an olefin is necessary as hydrogen acceptor;
*
April 1955
in the abeence of olefin, no reaction of isobutane and toluene could be induced with these acid catalysts. Literature references to the direct alkylation of aromatics with olefins are voluminous. However, the alkylation of aromatics with isoparaffins by a hydrogen transfer reaction was first reported by Condon and Matuszak in 1948 ( 4 ) , and subsequently in a Condon patent ( 3 ) . Condon and Matuszak described experiments in which the relative rates of alkylation of benzene and isobutane were studied, by reacting mixtures of benzene in excess isobutane with propylene. Their major products were direct olefin alkylation products, namely isopropylbenzenes. However, some dehydroalkylation was evidenced by moderate yields of mono-tert-butylbenzene and terl-butyl-isopropylbensene. Since no isobutylene was charged to this reaction, the krt-butyl aromatics must have been formed from isobutane through a hydrogen transfer reaction. I n the present work, dehydroalkylation becomes the major reaction, and the competing reactions of direct olefin alkylation become of minor importance when the reaction is carried out with excess isobutane and with certain combinations of catalysts and olefin types. Further, even when a n-olefin is used as hydrogen acceptor, it is converted to isoparaffins almost to the exclusion of n-paraffins. However, the yields are generally lower when using a n-olefin as hydrogen acceptor, and with some catalysts the yields are much loner than obtained with branched chain olefins. CH CH3-C-?
FH3
+
t
CH3 ISOBUTANE
TOLUENE
G3
4-
IOOC
CHJ-CH-CH=CH-CH~-
OLEFIN
ACID CATALYST
CH3 CH3-hH-CHz-CHz-CH3
CHJ-C-CH~ CH3
2 - M E T H Y LPENTANE
t - B U T Y LTOLUENE
INDUSTRIAL AND ENGINEERING CHEMISTRY
151
ENGINEERING. DESIGN. A N D PROCESS DEVELOPMENT Equipment and Experimental Procedure Equipment. The reaction was studied under flow conditions, using equipment shown in Figure 1. The major parts of this unit were a 10-gallon weigh tank for charge, a 4600-ml.-capacity steel reactor, a settler, and a 10-gallon weigh tank as product receiver. The reactor was of the Stratco type, and was equipped with mechanical stirrer, internal baffles, and cooling jacket.
Anhydrous hydrogen fluoride was the commercial grade supplied by Harshaw Chemical Co. The aluminum chloride-toluene complex was prepared by heating (75" C.) anhydrous aluminum chloride with excess toluene while bubbling a slow stream of anhydrous hydrogen chloride through the mixture. The catalyst complex was a viscous reddish-brown oil.
TAc M '
Dehydroalkylation of Benzene
Both olefin type and catalyst are important in causing the dehydroalkylation reaction to predominate over the competing reactions of direct alkylation of aromatic and of isobutane by the olefin. In the preliminary phase of the work, a variety of olefins were tested as hydrogen acIO GAL. CHECK ceptors for the dehydroalkylation of benzene with CHARGE VALVE PRODUCT 10 GAL. isobutane. The major product was di-Lert-butylRECEIVER benzene; only trace amounts of mono-tertFLOWRATOR butylbenzene were found even when the yield of CONTROL the di-tert-butylbenzene was low. 'The results VALVE from twelve of these runs are presented in Table I. I The first conclusion which can be drawn from this MOTOR work is that the dehydroalkylation reaction proceeds in good yield with a variety of olefins as REFRIGERANT SCALES hydrogen acceptors. Secondly, substantially CIRCULATION better yields of di-tert-butylbenzene were obtained when using a branched chain olefin. For example, Figure 1. Bench s c a l e alkylation unit in runs 4, 7, 9, and 12 using branched olefins, the yield of di-tert-butylbenzene ranged from 70 The isobutane, olefin, and aromatic charge mixture was preto 91 mole % based on benzene charged. Some branched olefins are more effective as hydrogen acceptors than others. blended in the charge tank. The catalyst (2600 ml.) was charged As shown in the preceding reaction equation, the olefin is cont o the reactor and the reactor cooled t o the desired operating verted to a paraffin of the same number of carbon atoms under temperature, in the 0" t o 30" C. range, by circulation of coolant idealized conditions. The extent t o which this occurs in actuaI through the jacket. The reaction system was pressured to an practice is shown in the sixth line of Table I. I n many of t h e operating pressure of about 60 pounds per square inch gage, runs, there was a good correlation between the yield of di-tertwhich pressure was sufficient t o maintain the reactants in the butylbenzene and the amount of paraffin produced from t h e liquid phase throughout the system. The hydrocarbon charge olefin by hydrogen transfer. Generally, a higher yield of paraffin was pressured in liquid state from the charge vessel, through a flowrator, a manual control valve, and into the reactor. Resiproduct was obtained when using branched chain olefins than dence time in the reactor was 25 t o 30 minutes. The total from the n-olefins. A notable exception was in run 5 using 2period of operation was generally 6 to 8 hours for each run. The pentene where a much higher yield of paraffin (isopentane) was observed. However, in this run an excess of olefin was emreactor effluent passed to the settler, where the catalyst was ployed-Le., 5 moles/mole of benzene instead of the 2/1 ratio separated and returned by gravity t o the reactor, while the prodused in most of the other runs. The excess olefin was undoubtuct passed to the product receiver. Reaction pressure was edly involved in isobutane alkylation reactions. As pointed out maintained by manually bleeding gaseous product from the reby Schmerling ( 1 1 ) and others (9),this alkylation reaction leads ceiver through a gas meter and sampler. to the formation of an isoparaffin of the same number of carbon The product was fractionated on Podbielniak low temperature atoms as the starting olefin. Other secondary reactions (disproand high temperature distillation columns. Analysis of terrportionation and cracking) further contribute t o isopentane proalkylaromatics was by infrared spectroscopy. The analysis of CS duction. These reactions would account for the formation of a product when using hexenes as hydrogen acceptors was by mass considerable portion of the isopentane found in this run, even spectrometer. though the yield of dehydroalkylation product (i.e., di-tertMaterials. The isobutane was a concentrate from a commerbutylbenzene) was relatively low. I n run 2, wing 2-pentene a t cial isobutane-butylene alkylation unit. Typical analysis was as a 2/1 ratio, only small amounts of isopentane and di-tert-butylfollows: 80.7 volume % isobutane, 16.5% n-butane, 2.5% propane, benzene were obtained. I n contrast, when 2-methyl-1-butene or 0.3% C5 and other components. Technical grade isobutane 2-methyl-2-butene were employed as hydrogen acceptors (runs 3 (95+y0 purity) was employed in a few of the runs for compariand 4),considerably higher yields of di-tert-butylbenzene and isoson purposes. Technical grade olefins of 95+% purity and aropentane were observed. In judging the effectiveness of an olefin matics of 98 to 99% purity were used. as hydrogen acceptor in dehydroalkylation, the authors prefer The boron fluoride-monohydrate catalyst was prepared by t o use the yield of di-tert-butylbenzene or tert-butyltoluene (Table saturating water with boron fluoride (Harshaw) a t room tempera111) as a basis of comparison. One reason for this is the diffiture. The resultant complex was a heavy fuming liquid, conculty of obtaining quantitative separation of small amounts of taining 72 to 76y0boron fluoride. "The concentrated sulfuric acid Cg or C6 paraffins from the large excess of isobutane. (98 to 99yo) was commercial white acid supplied by Consolidated The composition of the paraffin product produced from the Chemical Co. (Houston). The 90% alkylation acid ( H ~ S O I ) olefin by hydrogen transfer was also determined. This is shown was spent sulfuric acid catalyst from a commercial isobutanein the bottom section of Table I. In all cases, this product was butylene alkylation unit. This acid had a titratable acidity of found to be predominantly a branched chain paraffin. Although 90 t o 91%, contained 5% water, 1.0 to 1.5% nonvolatile neutral the yield of the paraffin was low in some runs, as when starting oil, and 1.5 t o 2.0% carbon. T h e 90% aqueous sulfuric acid was with a n-olefin, the paraffin produced was always predominantly prepared by dilution of ACS grade concentrated sulfuric acid
f
OD
758
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 4
-
"
~
GASOLINE PROCESSING
___
a branched chain structure. More precise data on this point were obtained in connection with t h e work with toluene which follows.
h
Effect of Olefin Strucfure on Dehydroalkylation of Benzene
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Some interesting results from the dehydroalkylation of benzene with isobutane, using various Ce olefins as hydrogen acceptors, are shown in summary form in Table 11. These data show the importance of thc structure of the olefin. T h e low yield of hydrogen transfer products obtained with the n-olefin (1-hexene) as hydrogen acceptor should be noted as compared to the near quantitative yields obtained with branched chain olefins. I n all cases, the dehydroalkylation product from benzene was predominantly p-di-tert-butylbenzene, with only traces of the meta isomer being present. Only small amounts (less than 2 mole %) of mono-tert-butylbenzene were formed. I n the runs described in Tables I and PI, substantially all the benzene reacted, and all the olefin present in the charge was converted to other products. The olefin not converted to paraffin by hydrogen transfer took part in the competing reactions of polymerization, direct alkylation of aromatic or isobutane, or formation of acid oils. It is believed that in those runs where low yields of the paraffin hydrogen transfer products were obtained that the predominant competing reaction was the direct alkylation of the aromatic by the olefin. For example, in the run with 1-hexene (Table 11), larger amounts of high boiling aromatic residues(Le., above di-tert-buty1benzene)-were observed. This material is believed to be a mixture of dihexylbenzenes of unknown structure although it was not characterized. Such an explanation appears reasonable in view of the fact that virtually all the benzene disappeared, even in runs where low yields of di-tert-butylbenzene were produced. The difference in yields obtained with 4methyl-I-pentene and 4-methyl-2-pentene, as shown in Table 11, is believed to be significant. The distance of the olefinic double bond from the tert-carbon atom is thought to be the important factor here. The formation of the tert-carbonium ion from the I-olefin requires more hydride ion shifts than in the case of the 2-olefin.
'2A Dehydroalkylation of Toluene 0) 0 v
v
April 1955
More detailed study of the dehydroalkylation reaction was carried out using toluene as the aromatic and various hexene isomers as hydrogen acceptors. Six different catalysts were investigated. Hexenes were chosen as hydrogen acceptor for the major part of the investigation for several reaaons.
INDUSTRIAL AND ENGINEERING CHEMISTRY
759
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT 1. Recovery and analysis of the c6 product could be done accurately and easily. 2. Secondary degradation or cracking of any paraffin alkylate product would yield only minor arnounts of c6 paraffins, nhereas considerable Cq and Cb products niight be produced by reactions of this type. This is an important consideration in studying the mechanism of the reactions. 3. There are more possibilities of chain branching with the c6 hydrocarbons than with the C4 or Cg members; consequently, use of the C6 olefins enables a more complete study of the amounts of isomerization which may occur in the course of the hydrogen transfer reactions. 4. Any alkylated aromatic product, produced by direct alkylation of toluene with the c6 olefin, could be easily segregated b y distillation from the tert-butyltoluene product produced from isobutane.
The results of these dudies are presented in Table 111. Since toluene was used as the aromatic in this work the alkylate product was tert-butyltoluene. This material was segregated from the reaction product by fractionation. A 185' to 200' C. fraction was collected and subsequently analyzed by infrared for content of p- and m-tert-butyltoluene; no ortho isomer was found. This fraction generally contained 95% or more of tert-butyltoluene. The total tert-butyltoluene yield for each of the runs is reported in Table 111, and directly below these figures the isomer distribution (% para- and % meta-) is shown. I n the lower section of Table 111, the yields of Ce paraffins produced by hydrogen transfer are shown. The detailed analytical breakdown of this paraffin product is listed below the yield values. The work with toluene again showed that a branched olefin is more effective than a n-olefin as a hydrogen acceptor as judged by the higher yields of tert-butylated aromatic. I n runs 23, 24, and 25 (Table 111) using 2-pentenc and 2-hexene as hydrogen acceptors, the yield of tert-butyltoluene was only 7 to 15 mole %. On the other hand, in runs 26 and 27 using 4-methyl-2-pentene, the tert-butyltoluene yield was 72 to 74%. The amount of c6 paraffin product indicates a similar trend, but these data are not as clear cut. The anomalies in this regard are believed to be due in part to the difficulties of separating and recovering all of the I n the run with 2c 6 product from the large excess of isobutane. pentene, a large amount (15OO1,)of iso-pentane was produced, although the yield of tert-butyltoluene was low (7%). The high yield of pentane must be due to side reactions as mentioned previously. The data in Table I11 clearly show that isomerization of the carbon skeleton occurs in the hydrogen transfer process. For example, the Ce product fron 2-hexene (runs 24 and 25) was mainly 2-methylpentane and 3-methylpentane in 52/36 ratio, with only a small amount of n-hexane being formed. I n run 26 with 4methyl-2-pentene, the olefin appears to be converted more directly to the corresponding paraffin, since the paraffin product contained 80% Zmethylpentane and 15.6% 3-methylpentane. Review of the data in Tables I and I11 discloses that the paraffin product always possesses a predominantly branched structure.
Comparison of Catalysts for Dehydroalkylation The six catalysts investigated in the work reported in Tables
I and I11 were BFvHzO UP
Boron fluoride monohydrate appears to be the best catalyst of this group. It produced high yields of tert-butyl aromatics with n-olefins as hydrogen acceptors. The remainder of the catalysts produced low yields when using n-olefins. Boron fluoride monohydrate is as effective with a n-olefin as hydrofluoric or the sulfuric acid catalysts are with branched olefins. However, the latter give good yields of tert-butyl aromatics when branched olefins are employed. The effectiveness of the boron fluoride mono-
760
Table II.
Effect of Olefin Type on Dehydroalkylation of Benzene Alkylation Acid (HzSO4) Catalyst 4-Methyl4-.Metliyl2-pentene 1-pentene 1-Hexene
YO%
Olefin (hydrogen acceptor) Charge composition (molar) isobutane/olefin/benzene Yield of C6 paraffin (mole olefin charged)
Yo
40/2/1
50/2/1
30/2/1
S8Q
67"
lBo
91
70
42
on
Yield of p-di-tert-butylbenzene (mole yo on benzene charged)
Predominantly (>95%) methylpentanes, by infrared or mass spectrometer analysis; only traces of olefins were present. a
hydrate catalyst is likely due to its greater ability to induce branching of the carbon skeleton of the starting olefin to a more reactive branched structure. This structure is apparently essential for the hydrogen transfer reaction. On the other hand, i t is generally accepted that the chain branching or isomerization ability of hydrofluoric and sulfuric acid catalysts is very limited. This is reflected by the low dehydroalkylation yields when using n-olefins with these catalysts. Concentrated sulfuric acid produced appreciably lower yields than 90% H2S04-10% H2O or than 90% alkylation acid. This may be seen by comparing runs 26 through 29 in Table I11 and runs 7, 8, and 9 in Table I. The 90% H2SOa-10% H2O catalyst appears to be about equivalent to the 90% alkylation acid, based on comparison runs with a branched olefin-Le., runs 26, 27, and 28 in Table 111. Yields of 72 to 79% of tert-butyltoluene were obtained in these runs, and this product contained 95+% of the para isomer. Anhydrous hydrogen fluoride seems to be equivalent to the 90% sulfuric acids on the basis of limited comparative data-i.e., compare run 31 with 24 and 25. Low yields (15%) of tert-butyltoluene were obtained in these runs using 2-hexene as hydrogen acceptor. However, both the C6 product and the tert-butgltoluene produced with hydrofluoric acid showed more isomerization than was evident with the sulfuric acid catalysts. The nluminum chloride catalyst also produced a high degree of isomerization in the products, but the product yields were disappointingly low.
Orientation of the tert-Butyl Group The effect of the catalyst type on the orientation of the tert-butyl group in the alkylated aromatic product is summarized in Table IV. With sulfuric acid the tert-butyltoluene product is predominantly-Le., 96 to 98%-the para isomer a t reaction temperatures betuTeen 0" and 30" C. With boron fluoride monohydrate the amount of meta isomer increases substantially with increase in temperature from 0" to 30" C.; a t 0' the alkylated toluene is 89% para, whereas a t 30" C. the product contains roughly 50% of both the meta- and para-isomers. With t h e hydrofluoric acid catalyst, a 50:50 ratio of meta-para isomers was obtained a t 10" C. 3,5-Di-tert-butyltoluene can also be produced in moderateljgood yield by the dehydroalkylation reaction. This of course requires the use of additional olefin as hydrogen, acceptor. This is illustrated in run 32, where a 2/1 ratio of a-hexene/toluene was employed, together with excess isobutane and boron fluoride monohydrate catalyst a t 25' C. A 40 mole % yield of 3,5-ditert-butyltoluent. and 53 mole yo of mono-tert-butyltoluene were obtained. It is believed that a catalyst having isomerization activity, so as to produce an appreciable amount of meta isomer, is necessary if di-tert-butyltoluene is desired. Sulfuric acid would not be expected to be effective because it produces predominantly p-tert-butyltoluene, which is very difficult to alkylate further with lert-butyl groups.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47,NO.4,
GASOLINE PROCESSING 0
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Effect of Isobutane/Olefin/ Aromatic Ratio I n the work reported in the previous sections, a large excess of isobutane was employed so as to minimize the possibility of direct alkylation of the aromatic b y the olefin, or olefin polymerization reactions. For example, in Table I11 isobutane/ olefin/toluene ratios of 15/1/1 to 50/1/1 were used. T o determine if lower ratios would be satisfactory, a reaction was run at a 4.7/1/1 molar ratio of isobCtane/ a-hexene/toluene, using boron fluoride monohydrate as catalyst. The conditions and product yields are shown in Table V. For comparison, a similar run at 40/0.9/1 ratio is included in this table. Table V indicates that a nioderattl reduction in yield from 85 mole % to 58% of tert-butyltoluene occurred by operating at the lower ratio. There was a lesser reduction in the amount of hydrogen transfer product (c6 paraffins) produced from the olefin. The c 6 paraffin product was predominantly 2- and 3-methylpentanes in both runs, indicating a similar hgdroisomerization mechanism. The 2hexene starting olefin was virtually cornpletely converted as shown by the absence of olefins in the C6 product fraction. The amount of c6 paraffin produced has invariably been lower, on a mole yo basis, than the amount of tert-butylaromatic. Theoretically, one mole of olefin should be converted to one mole of paraffin for each mole of tert-butyltoluene produced. h-o experimental data are available to account for the observed difference. However, i t is probable that disproportionation reactions, involving the c6 paraffins or carbonium ions, lead to the formation of C d , CL, C7, and C8 paraffins. Owing to the complexity of these and other competing reactions, it did not appear worth while to try to obtain a quantitative analytical mass balance. However, the formation of C4, C5,C7,and Ca paraffins was observed in minor amounts in all runs, substantiating the hypothesis that some disproportionation of the c 6 products did occur. This type of secondary reaction is! of course, commonplace in acid-catalyzed isobutane-olefin alkylations, and olefin polymerization reactions. -
Dehydroalkylation with Other lsoparaffins I n view of the successful results obtained on dehydroalkylation of aromatics with isobutane, i t was of interest t o determine if this reaction would go with other isoparaffins containing a lert-hydrogen. Consequently, runs were made with both isopentane and with 2,3dimethylbutane. The run with isopentane was carried out under flow con-
April 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
76 1
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Table IV.
Effect of Catalyst and Temperature on Orientation of ferf-Butyltoluene Product from Dehydroalkylation Catalyst SOYo Spent HaSOa BFa. Hz0 HF
Maximum reaction temperature, C. Hydrogen acceptor Comvosition of tert-butvltoluene product, % ' Para Meta
Table
v.
10
30 2-Hexene
1-Hexene
10 2-Hexene
30 2-Hexene
10 2-Hexene
98 2
96 4
89
84 16
52 48
50 50
4-Methyl-2nentene
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32 92
yo
42.5
15.5 -
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25
89
72
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58.0
85.0
48.0
51.0
1.4 3.2 64.8 28.0 0.6 1.0 1.0
65:7 31.4 1.6
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STEP I
ditions a t 10' C. The run with 2,3-dimethylbutane was a batch experiment a t 25" C. in which the olefin and 2,3-dimethylbutane were slowly added t o a stirred flask containing toluene, catalyst, and excess 2,3-dimethvlbutane, Data from these runs are shown in Table VI along with results from a run with isobutane for comparison. Isobutane appears to be more reactive than isopentane for this reaction, while 2,3-dimethylbutane did not appear to react a t all under the conditions employed. It is possible that the latter reaction could be induced under other reaction conditions.
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Dehydroalkylation of Toluene with Various Is0 pa ra ff ins
Isoparaffin Hydrogen acceptor
With boron fluoride monohydrate, which is a more active
chain /reaction ~ branching l ~ catalyst f i than ~ sulfuric / ~ or hydrofluoric ~ ~ ~ acid, ~ the~ goeF nearly as well with n-olefins as with iso-olefins. These facts suggest a mechanism, based on the carbonium ion theory, which will explain the formation of the major products. This mechanism is centered around the intermediate formation of a tert-carbonium ion from the olefin charge, and the selective acceptance of hydrogen by this tertiary ion. This mechanism is illustrated in Figure 2 using 2-hexene as the hydrogen acceptor and toluene as the aromatic. I n the first step, a sec-hexyl ion is formed from the olefin by addition of a proton supplied by the catalyst. I n step 2, the secondary ion is isomerized to the tertiary ion. Published equilibrium data for olefins show the equilibrium to favor strongly the branched structure a t these low temperatures (8). These hexyl ions may react with aromatics, olefins, or isobutane. However, when an excess of isobutane is present, analysis of the reaction products indicates that the hexyl ions react predominantly
Molar Ratio of Reactants 4.7/1/1 40/0.9/1 2-Hexene 2-Hexene BFa. H20 BFa.Ha0
Total tert-butyltoluene product (mole % on toluene reacting) Hexane product (mole Yo on olefin charge) Composition of Cs product (vol. % by mass spectrometer) 2,2-Dimethylbutane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane Cyclopentane 2-Hexene
Catalyst
11
4.
Effect of ~
Isobutane/olefin/toluene Olefin Catalyst Temperature, OC. Reaction time, minutes Toluene converted, mole Product yields, mole % p-tert-Butyltoluene m-teTt-Butyltoluene
Table VI.
0
Isobutane 4-Methyl-2pentene 90% Spent Has04 51/0.9/1
Isopentane 4-Methyl-2pentene 90% Spent HaSOa 78/1/1
Isoparaffin/olefin/toluene Conversion of olefin to. corn=....spending paraffin GO 68 Yield of tert-alkyltoluene (mole % on toluene reacting) 73 51 a No hydrogen transfer products were found.
2,3Dimethylbutane Dimethvlheptene BFa. Hi0
.
STEP 3
- HYDRIDE
Y 3
CH3:C:CH3
t
CY3
CH3CH2CH2:C:CH3
@ t
- H E f Y L ION
ION TRANSFER
CH3 CH3CH2CH2:C:CH3
Q-____-
/
c
*@
6/0.9/1 nila
2 - MaPENTANE
nila
Mechanism of the Reaction
STEP 4
-
t-BUTYL ION
ALKYLATION
The experimental data have brought out these significant points
1. When n-olefins were charged to the reaction, they were hydroisomerized to singly branched paraffins. 2. When a singly branched olefin, such as 4-methyl-Z-pentene, was charged, no further chain branching occurred; however, some migration of t h e methyl group was observed. 3. With sulfuric acid or hydrofluoric acid catalysts, which are relatively inactive for chain branching, iso-olefins gave much higher yields of hydrogen transfer products than did n-olefins. 762
t - B U T Y L ION
Figure 2.
TOLUENE
t-BUTOLUENE
PROTON
Hydroisomerism and dehydroalkylation mechanism
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No, 4
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GASOLINE PROCESSING by accepting a hydride ion from isobutane. Both the secondary and the terthexyl ions would have an opportunity to react with isobutane in this manner. The secondary hexyl ion would yield n-hexane by addition of a hydride ion, whereas the tert-hexyl ion would yield a methylpentane. The experimental results show that the Ce product is almost exclusively methylpentanes, irrespective of the catalyst used. Therefore, the tert-hexyl ion must be formed by isomerization, and must react much more readily with isobutane than does the secondary ion. The preferential reaction of the tert-hexyl ion is shown as step 3 in the reaction mechanism. This preferential acceptance of hydride ions by tertiary ions under acid-catalyzed conditions was also discussed in a paper by Greensfelder, Voge, and Good ( 5 ) and more recently by Maury (IO)and Burwell (2). . The fact t h a t virtually no n-hexane is produced indicates that the sec-hexyl does not participate in a hydride ion exchange reaction with isobutane, or t h a t such reaction is very slow. The overall rate of the dehydroalkylation should therefore be dependent on the rate of the isomerization reaction (step 2), when a n-olefin is used as hydrogen acceptor. Catalysts which possess chain branching ability should, therefore, be more effective than those which do not have this ability or which are only weakly active in this respect. Boron fluoride (BF3), boron fluoride monohydrate (BF,.H20), and hydrofluoric acid-boron fluoride combinations (BFa.HF) are known to be isomerization catalysts ( 1 , 6, 7). Moreover, boron fluoride monohydrate was found in the present work to be a good catalyst for the combined hydroisomerization-dehydroalkylationreactions. Sulfuric and hydrofluoric acids, on the other hand, were very poor catalysts when n-olefins were employed, presumably because of their inability to catalyze, effectively, the isomerization step of the reaction. Further evidence of preferential acceptance of hydrogen by tert-carbonium ions is afforded by the higher yields obtained with iso-olefins as hydrogen acceptors than with n-olefins when using sulfuric acid catalyst. I n the cases where an iso-olefin was charged, a tert-carbonium ion could be formed from the initial ion by one or more simple hydride ion shifts.
.... .... C
c:c,:c;:c:c: H
OLEFIN
Ht
.. .. ..
9
ti
.,C
,. ..
C :g:c,:c : c : H H .I
t - C A R B O N IUM I ON
No further chain branching would be necessary t o form the tertiary ion, and the experimental data indicate that no further isomerization occurred. The fact t h a t better yields were obtained with 4-methyl-2-pentene than with 4methyl-1-pentene (Table 11) was mentioned earlier. This suggests that an appreciable time is required for the series of hydride ion shifts leading to the formation of the tert-carbonium ion structure. Up to this point the formation of the isoparaffin and tertbutyl ions has been shown. Finally, in step 4, the tert-butyl ions react with the aromatic to form the tert-butyl aromatic and a proton.
April 1955
The reaction discussed in the previous sections can be applied to produce a variety of tert-butylated aromatics. tert-Butyltoluene, tert-butylxylenes, p-di-tert-butylbenzene, and terl-butylated naphthalenes are examples of compounds t h a t can be produced. The method should also be useful for producing tertbutyl derivatives of a variety of organic compounds such m tert-butylphenol, and tert-butyl benzoic acid. By employing a branched chain olefin as hydrogen acceptor, yields of tert-butyl aromatics in excess of 75 mole % are produced with spent alkylation acid (HzS04) as catalyst. When using a mild isomerization catalyst, such as boron fluoride monohydrate, high yields of tert-alkyl aromatics are formed even with a nolefin as hydrogen acceptor. I n the hydrogen transfer process, the n-olefin is hydroisomerized to an isoparaffin. This combination dehydroalkylation-hydroisomerization reaction may be developed into a practical process for producing tert-butyl aromatics and isoparaffins. The advantage of this method is t h a t low cost isobutane concentrates can be used as tert-butylating agent to yield alkylated products which are almost exclusively the tert-butyl derivatives. The hydrogen acceptor may be a low grade olefin concentrate, a mixture of olefins, or a cracked fraction containing olefins and the desired aromatic. I n contrast, the direct alkylation method, using the olefin requires high purity isobutylene or diisobutylene which is considerably more expensive than isobutane. A carbonium ion mechanism is proposed for these reactions. The mechanism is centered around the formation of tert-butyl ions by selective transfer of hydride ions from isobutane to higher molecular weight tert-carbonium ions derived from the olefins.
Acknowledgment The authors wish to acknowledge with thanks the cooperation and work of C. J. Robinson, B. M. Drinkard, 0. P. Funderburk, and N. R . Taylor in the fractionation and analysis of the reaction products from this study. The authors also express their gratitude to the management of Pan American Refining Corp. for encouraging and approving the publication of this work.
literature Cited
CARE. ION
SEC.
-
(:HI . .~S H I F T
C
c:c:&:c,:$.:
Conclusion
-4xe, W. N., U. S.Patent 2,370,118(1945). (2) Burwell, R. L., Jr., Petroleum Division, 125th Meeting ACS, Kansas City, Ma., March 29 to April 1, 1954. (3) Condon, F. E., U. S.Patent 2,653,980 (1953). (4) Condon, F. E., and Matusaak, A I . P., J . Am. Chem. SOC..70, 2539 (1948). (5) Greensfelder, B. S., Voge, H. H., and Good, A. M , , IND.ENG. CHEM..41. 2573 (1949). (6) Ipatieff, V. k., and'Pines, H., U. S. Patent 2,283,142(1942). (7) Ibid., 2,325,122(1943). (8) Kilpatrick, J . E., Prosen, E. J., Pitzer. K. S..and Rossini. F. D.. J . Research L\7atl. Bur. Standards, 36, 559 (1946). (9) Jfarschner, R. F., and Carmody, D., Petroleum Division, 122nd Meeting ACS, Atlantic City, N. J., April 1946. (10) Maury, L. G., Doctoral Dissertation, Korthwestern University, 1953. (11) Schmerling, L., J. Am. Chern. SOC.,68,279 (1946). (1)
RECEIVED for review Septembex 20, 19.54.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDFebruary 17, 1955.
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