M . H . G O R IN , C. S. K U H N , JR., AND C. B . M IL E S M a g n o l ia P e t r o l e u m C o m p a n y , D a lla s , T e x a s
Supporting evidence is presented for a simple m echanism which explains the products o f the primary alkylation reaction o f isobutane with olefins, catalyzed with acids and alum inum halides. This m echanism excludes cer
tain products which arc not experimentally detected in important quantities. Primary alkylation products are defined as those alkanes which arc formed preferentially and in high yields at low temperatures and short residence times. Formation o f these products and the secondary isomeric products is ascribed to the simple addition of methyl and isopropyl fragments (from isobutane) to the various permissible forms o f an olefin-catalyst complex.
T
HE mechanism of the alkylation of isoparaffins with olefins has received much attention in recent years. Other workers (1, 4, 10) have frequently adapted Whitmore’s concept of the“carbonium ion” to this reaction with considerable success. In criticism of these theories, it may be contended that they are too broad in allowing the formation of almost any alkane isomer by means of methyl group or hydride shifts within the intermediate carbonium ion. In order to explain the observations made in our laboratory under various conditions during an extensive study of alkylation reactions catalyzed with both hydrogen fluoride and aluminum bromide, it has been convenient to differentiate be
tween the possible mechanisms by which “primary” and “ second
ary” isomeric products are formed and to introduce the concept of “ direct” alkylation as that process which leads to the prefer
ential formation of a single primary alkane isomer. This theo
retical situation has been approached closely by proper adjust
ment of the experimental conditions employed in conducting the reactions. A mechanism for isobutane alkylation is presented where the relative amounts of the principal alkane isomers are qualitatively accounted for on the basis of an approach to thermo
dynamic equilibrium of the permissible product isomers via (o) simple_one-carbon hydride shifts in the catalyst-olefin complex and (6) addition to this complex of methyl and isopropyl frag
ments from the polarized isobutane molecule or from the primary product. The theory is substantiated by pertinent experimental data.
i
PROCEDURE
The experiments were for the most part conducted in a small (2.8-liter capacity) steel batch reactor provided with a mercury- sealed motor-driven agitator and with a jacket through which a fluid heat transfer medium was passed to control reaction temperature. Accessory equipment included pressure burets for measuring the desired quantities of hydrocarbons into the reactor and special facilities for charging hydrogen fluoride or aluminum bromide.
With the exception of the experiments in which olefins were treated with hydrogen fluoride prior to alkylation, the standard procedure consisted of adding the olefin at a controlled rate to a well agitated mixture of isobutane and catalyst. Although paraffin-olefin ratios and catalyst concentrations were studied independently, these variables were held substantially constant in most of the studies reported here. In the case of investiga
tions with hydrogen fluoride as the catalyst, the usual paraffin- olefin mole ratio was about 4.0 and the acid content of the reac
tion mixture was about 20 liquid volume % .
At the end of an experiment the hydrocarbons were washed free of catalyst with water (after separation from the liquid catalyst phase in the case of hydrogen fluoride), and a sample of the total hydrocarbons was analyzed by fractionation on a Podbielniak Hydrobot column (Ileligrid packing) to determine the content of individual light paraffin hydrocarbons through the pentanes. A second sample was stabilized by removal of the bulk of the isobutane and lighter hydrocarbons on a low tem
perature column, and was then fractionated on a special precision column (4 feet long and 0.5 inch in diameter) containing 0.5-inch diameter conical packing constructed of 40 X 60 mesh stainless steel gauze. The column efficiency was approximately eighty theoretical plates. Components were identified by boiling points and by refractive index (nr?) values for selected cuts.
Densities were occasionally employed. It is felt that each of the components recorded account for at least 9 0-95% of the fraction boiling in the range of the component. The possibility of the presence of small quantities of unrecorded components is, there
fore, not excluded.
D IR E C T A LK YLA TIO N
At an early stage in' the development of processes for the manufacture of aviation alkylates by the acid-catalyzed reaction of isobutano with butenes, a close similarity in composition of the products obtained with the several butene isomers was observed.
This similarity was ascribed, among other things, to isomerization of the olefins or rapid equilibration of intermediate carbonium ions prior to alkylation. Such concepts implicitly discounted the idea of producing a single alkylate isomer in high yield and conse
quently led to either an oversimplified or an overly complex pic
ture of the mechanism. More recently, experimental work car
ried out in this laboratory has shown that the nature of the prod
ucts obtained from the alkylation reaction wi£h both hydro
fluoric acid and aluminum bromide catalysts is strongly influenced by the operating variables, particularly contact time, tempera
ture, and catalyst concentration1. Thus, by operating at short contact times and low temperatures with relatively low ratios of hydrogen fluoride to hydrocarbon, marked differences were ob
tained in the composition of the products from the individual butenes (1-butcne, 2-butene, and isobutene).
As Table I shows, at —10° C. and a contact time of 5 minutes a single octane isomer, 2,3-dimethylhexane, constituted about 60% of the octane fraction of the product from 1-butene, whereas this same isomer constituted only about 3 % of the product from 2-butene and was not detected at all in the product from iso
butene. Correspondingly, under the same conditions 2,2,4-tri- methylpentane constituted about 75% of the octane fraction isobutane with propylene, the concentration of 2,3-dimcthylpentane in the heptane fraction could be maintained substantially constant at about 85 to 95% b y progressively decreasing the catalyst concentration from about 0.9 to 0.3 weight % as the temperature was increased from 17° to 49° C. With the hydrogen fluoride catalyst, a critical concentration of about 20 volume % hydrogen fluoride in the acid-hydrocarbon emulsion was observed at 10° C.
with 1-butene; below this concentration the reaction shifted rapidly to one of polymerization and hydrofluorination, and above it the products began to approach the composition of 2-butene alkylate.
795
Ta b l e I . Ef f e c t o f Re a c t io n Tim e a n d Te m p e r a t u r e a n d Ap p r o a c h t o Th e r m a l
14.0 34.6 32.5 38.9 46.3 37.0 74.8 39.7 35.2 38.5 29.6
5.2 17.7 43.6 4 .5 14.5 33.9 8 .4 31.5 37.4 52.0 56 .8
21.3 23.8 12.0 39.1 19.9 11.9 16.8 17.8 18.2 3 .6 4 .5
14.2 8 .6 8 .3 5 .0 5.3 2.5 3 .4
59.5 23.8 11.9 3.3 10.7 8.9 6 .0 3.9 3 .4 5 .7
63.1 55.3 39.2 80.4 61.7 35.7 64.5 41.2 39.8
34.0 34.6 34.1 14.7 21.0 30.5 26.3 31.7 28.6
these conditions 2,3,4-trimethylpentane was found to constitute about 3 9 % of the octane fraction from 2-butene, but only 21 and 17% of the octane fractions from 1-butene and isobutene, respec
tively. Furthermore, by increasing the temperature or the residence time, a sharp drop in the concentration of 2,3-dimethyl- hexane, 2,2,4-trimethylpentane, and 2,3,4-trimethylpentane in the products from 1-butene, isobutene, and 2-butene, respectively, was observed. Accordingly, these three octane isomers are de
noted the primary products of the three olefins, respectively, and the reactions which lead to their formation are defined as the direct alkylation reactions.
Similar evidence has been accumulated in the study of the alkylation of isobutane with other olefins which indicates that 2,3-dimethylalkanes are the primary products from all normal olefins in which the double bond is in the terminal position2.
Thus, 2,3-dimethylbutane, -pcntane, and -heptane have been established as the primary products from alkylation with ethyl
ene, propylene, and 1-pentenc, respectively. In the case of propylene, for example, 2,3-dimethylpentane may be made to constitute as much as 95% of the heptane fraction by proper choice of reaction conditions with either hydrofluoric acid or aluminum bromide as catalysts.
Table II summarizes results which are illustrative of the direct alkylation of isobutane with ethylene, propylene, and 1-butene in the presence of aluminum bromide. Particularly noteworthy is the need for a progressive decrease in the temperature (at con
stant catalyst concentration) with increasing molecular weight of the olefin in order to obtain high concentrations of the primary product (2,3-dimethylalkane) in the C6-C8 fraction3. This re
quirement is largely dictated by the steadily decreasing stability of the primary product as the molecular weight and, conse
quently, the reactivity of the olefin are progressively increased.
SECONDARY PRODUCTS
In line with the definition of primary products, then, secondary products are defined as those isomeric products whose concentra
tions increase as the contact time or temperature is increased above the values required for formation of maximum concentra
tions of primary products. Thus, by this definition (Table I) 2,2,4-trimethylpentane is a secondary product from 1-butene and
* 2,3-Dimethylbutane is undoubtedly the true primary product from the direct alkylation of isobutane with ethylene in the presence of aluminum bromide. The appearance of small amounts of ^-methylpentane and con
siderable quantities of octanes from the hydrogen fluoride-catalyzed reaction of isobutane with ethylene is ascribed to a polymerization-hydrogen exchange mechanism rather than to direct alkylation. The production of octanes by this latter process was considerably increased by the addition of promoters to the hydrogen fluoride catalyst.
* The high temperature propylene experiment (Table II) resulted in con
siderably more reforming of the primary product than occurred during the ethylene experiment even though the time-temperature combination was more severe in the latter case.
2-buteue, 2,4-dimcthylhexano is a sec
ondary product from all three butenes, 2,3,4-trimethylpentane is a secondary product from 1-butene and isobutene, and 2,3-dimethylhexane is a secondary product from 2-butene and isobutene. The fifth principal octane isomer found in the isobutane- butene nlkylation products, 2,3,3- trimethylpentane, was obtained in appreciable amounts only from 2-butene; since its concentration was found to increase with decreasing temperature, it could be considered as a primary product from this olefin along with 2,3,4-trimethyl
pentane.
Only minor amounts of other octanes are found in the products from the hydrogen-fluoride-catalyzed isobutane-butene reactions even at relatively long contact times and high temperatures.
With more active isomerization catalysts such as aluminum bromide, however, considerable amounts of other octane isomers are formed. Such reactions, as well as reactions involving the formation of paraffins of lower and higher molecular weight than can be accounted for on the basis of a net addition of one mole of starting isoparaffin to one mole of olefin, can bo more properly considered as proceeding by mechanisms which, though perhaps basically related to the primary alkylation phenomena, are readily distinguishable from them. Thus, the formation of pro
pane and octanes in roughly equimoleeular amounts from iso- butane and propylene is an important competing reaction in the formation of 2,3-dimethylpentane by direct alkylation; the formation of isopentane in the reactions of isobutane with ethyl
ene, propylene, and the butenes is known to be accompanied by a net drop in the production of the principal isomeric alkylation products and, in some instances, by a net increase in isobutane consumption as well. Products derived via such phenomena are really secondary or by-product in the ordinary sense; but they are not defined here as secondary products in order that they may, for present purposes, be readily differentiated from the true secondary products derived by equilibration of the isomeric pri
mary products of direct alkylation.
While the importance of the various competing and side reac
tions in obtaining.a completely integrated picture of both the acid-catalyzed and the aluminum-halide-catalyzed processes is fully appreciated, discussion of these phenomena is beyond the
Isobutane/olefin mole ratio 3 .3 10 10 10
Initial AlBrj concn., wt. % 0 .9 0 .9 0 .9 0 .9
Residence time, min. 225 4 34 30
Alkylate compn., Cs-Cs basis, wt. %
° Contains small amounts of 2-inethylpentane.
b No trimethylpentanes were detected in the product from 1-butene by precision fractionation and by measurement of physical properties of selected cuts.
August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 797 T H E R M O D Y N A M IC E Q U I L I B R I U M O F P R I M A R Y A N D
S E C O N D A R Y P R O D U C T S
Table I shows that at long residence times and relatively high temperatures the octane fractions of the products from the alkylation of isobutane with each of the three butenes approach one another in composition. It also shows that, wliile minor amounts of other octanes may be present, the five octanes listed as primary or secondary products from direct alkylation constitute around 90% , at least, of the total octane fraction in each case.
The relative amounts of these five principal octanes at long residence times or high temperatures apparently are determined largely by thermodynamic considerations. Thus, Table I indicates that the ratios of the concentrations of these five isomers are approximately as required by the calculated free energies.
It is probable that the experimental ratios are somewhat nearer the true,equilibrium ratios than the theoretical values reported (8); by alkylating with each of the individual butene isomers, the equilibrium concentrations were in effect approached from regions of both high and low concentrations of the five permissible oc
tanes. Particularly noteworthy is the absence of large amounts of. certain thermodynamically stable octane isomers such as 2,5- and 2,2-dimethylhexancs and 2-methylheptnne. Together these should constitute about 50% of the total octane fraction if com
plete thermodynamic equilibrium were established in the system.
As previously suggested by Caeser and Francis (3), ultimate com
positions of direct alkylates may then bo determined by purely thermodynamic considerations, with the restriction imposed by the absence of routes whereby certain alkylate isomers can be formed via processes characterized by high reaction rates.
In this same connection, it was observed that the apparent equilibrium ratio of the two principal heptane isomers from the isobutane-propylene reaction— i.e., 2,3- and 2,4-dimethylpen- tane— approached a value of about 1.0 at 25° C. with both the hydrogen fluoride catalyst and the aluminum bromide catalyst.
Although this value is roughly one third of that of the reported (9) thermodynamic ratio, it is nevertheless within the maximum limits of uncertainty of the theoretical value.
IIY D R O F L U O R IN A T IO N O F I S O M E R I C O L E F IN S
As further information to support the concept of the direct alkylation reaction, hydrofluorination of 1-butene, followed by alkylation of the sec-butyl fluoride under mild conditions of resi
dence time, temperature, and hydrogen fluoride/hydrocarbon ratio, gives products which are essentially identical with those obtained from 2-butene; however they are very different from those obtained by direct alkylation of 1-butene under the same conditions (Table III). This information furnishes strong evi
dence that the mechanism of alkylation of 1-butene to give the primary product, 2,3-dimethylhexane, does not involve fluoride formation prior to alkylation and therefore supports the concept of the direct alkylation reaction. It leaves open the question of whether the mechanism of the alkylation of 2-butene involves the formation of sec-butyl fluoride prior to alkylation.
Under certain conditions the hydrofluorination reaction can be made to occur almost exclusively. For example, by the addition of only 7 % of water to the hydrogen fluoride catalyst, the reaction of 1-butene at 10° C. shifts completely from one of isobutane alkylation to one of hydrofluorination. A similar result is ob
tained with propylene in the presence of excess isobutane simply by lowering the temperature below about 0 ° C. In this same connection it has been observed that, by progressively increasing either the water content of the hydrofluoric acid catalyst (up to 7 weight % ) or the ratio of essentially anhydrous hydrofluoric acid to hydrocarbon, the composition of the products from the 1-butene-isobutane reaction can be made to approach the com
position of the products from the 2-butene-isobutanc reaction.
All of these facts tend to indicate that alkylation mechanisms other than the direct process presented here are operative under conditions which depart appreciably from those previously
de-Ta b l e III. Ef f e c t of Hy d r o g e n Fl u o r id e Pr e t k k a t m k n t o f 1 -Bu t e n e o n Co m p o s it io n o f Al k y l a t e
(Pretreatment: temperature, 20° C.; HF/olefin final mole ratio, approx. 0.9;
HF added to olefin over approx. 10-minute period. Alkylation: tempera
ture, approx. 10° C.; average residence timç, 5 minutes; other condition?
scribed as being necessary for primary product formation. The critical nature of some of the variables in this regard is particu
larly striking.
P R O P O S E D M E C H A N IS M
The following mechanism is proposed largely as a means for cataloging the more important facts concerned with the direct alkylation reaction and the isomerization of the primary products.
It attempts also to give to the alkylation catalyst a definite role in the mechanism, an aspect which has been largely ignored in the earlier literature on the subject. There are two major steps in the mechanism: (1) The catalyst and the olefin form an addition complex via coordination with two electrons from the double bond, and (2) the addition product reacts with isobutane to pro
duce the final product and release the catalyst. In step 2 the isobutane molecule is assumed to behave as if it becomes polarized into a negative methyl fragment and a positive isopropyl frag
ment as it approaches the olefin-catalyst complex; the reaction is completed by the addition of the negative methyl group to the carbon atom deficient in electrons with the positive isopropyl group displacing the catalyst. The formation of the olefin- catalyst complex is illustrated for the case of isobutene, 2-butene, and I-butcne in Figure 1. The catalyst, which may be alkylation mechanisms proposed, by these investigators. In the opinion of the writers, however, such a rupture of the isobutane molecule is scarcely less likely, a priori, than the similar alpha- beta shift of a methyl group embodied in the mechanism of Schmerling or required, for that matter, to account for the isomerization of alkanes. There seems to be little to choose be
tween the two proposals on the basis of information now available.
It is assumed also that hydride shifts (5) occur within the olefin- catalyst complex. In the case of the isobutene complex (Figure 1) the central carbon atom is first rendered deficient in electrons through capture by the catalyst of one of the electron pairs of the double bond. The positive carbon can then capture a hydride from either of the adjacent carbon atoms to give two new forms of the olefin-catalyst complex which react with isobutane. Thus with isobutene, the original form of the olefin-catalyst complex gives 2,2,4-triincthylpentane (primary product). By shifting a hydride from the same carbon atom that holds the catalyst, an olefin-catalyst complex results which gives
2.3.4-trimethyl-pentane; finally, by shifting a hydride from one of the methyl groups adjacent to the electron-deficient carbon atom, the re
sulting olefin-catalyst complex gives 2,4-dimethylhexane. It is not necessary to assume formation of transient octane molecules or carbonium ions whose isomerizations (by methyl group shifts) account for the actual products of isomerization. By this mechanism, on the other hand, the relative yields of the isomeric products of alkylation under primary conditions are determined by the relative stabilities of the various possible forms of the olefin-catalyst complex.
The following rules governing the mechanism of direct alkyla
tion of isobutane with olefins in the C2-C5 range are consistent with the data obtained in our study of the hydrogen fluoride and aluminum bromide alkylation systems:
1. In the formation of the catalyst-olefin complex, the catalyst adds to that member of the double bond required by Markownikoff’s rule for the positive addend. The more electro
negative carbon atom of the olefin thus becomes a donor of an electron'pair which is accepted by the positive pole of the catalyst.
In the case of the monomer, aluminum bromide, the aluminum atom possesses an incomplete electron shell and is, therefore, the probable acceptor. With hydrogen fluoride catalyst a weak
In the case of the monomer, aluminum bromide, the aluminum atom possesses an incomplete electron shell and is, therefore, the probable acceptor. With hydrogen fluoride catalyst a weak