Oxidation of Hvdrocarbons Catalvzed bv Hydrogen Bromide J
J
SUMMARY FREDERICK F. RUST AND WILLIAM E. VAUGHAN Shell Development Company, Emeryville, Calif. *
T h e gas phase, homogeneous oxidation of the lower hydrocarbons is greatly modified by the presence of hydrogen bromide. In the presence of this catalyst ethane is converted principally to acetic acid, straight-chain paraffins mainly to ketones, and branched-chain compounds chiefly to stable peroxides. The character of the product is determined principally by the most reactive carbon-hydrogen linkage in the molecule. In these reactions the reactivities of these linkages increase markedly in the order-primary, secondary, and tertiary. The introduction of oxygen under these conditions is accompanied by a low degree of carbon-carbon bond scission, the operating temperatures are low, and the conversions and yields high.
guishes these oxidations from others of the past, inasmuch as such a product in major amount has not been reported heretofore. For example, the oxidation of isobutane, the simplest member of the series, a t temperatures as low as 160" C. produces tert-butyl hydroperoxide in yields as high as 75% based on the consumed oxygen from a 10 to 10 to 1 mixture of isobutane, oxygen, and hydrogen bromide under oonditions where 87% of the oxygen is reacted. tert-Butyl alcohol and di-tcrtbutyl peroxide ( 6 , IO) are also formed. Breakdown of the carbon skeleton during the oxidation is minor. Further, the hydrogen bromide catalyst is regenerated semiquantitatively ; losses are attributable to oxidation to bromine or to formation of organic bromides, the possibilities for the production of which will be apparent from a consideration of the following mechanism developed to explain the principal reaction:
T
HE availability of vast quantities of natural gas and petro-
leum hydrocarbons attaches more than ordinary importance ,to their economic utilization as sources of industrial chemicals. An obvious and especially attractive method of using the lower paraffins is their direct oxidation with gaseous oxygen. Extensive prior research has given evidence of the mechanism of oxidation and it is thought by many to involve the generation of free radicals which react with oxygen to form peroxy bodies. These are, apparently, generally unstable under the extreme reaction conditions necessitated by the inherent stability of the paraffin toward oxygen and thus decompose, perhaps violently, with carbon skeleton breakdown. The present study has revealed that addition of a third component, hydrogen bromide, lowers the reaction temperature, stabilizes the reactive peroxy intermediates, and, thus, so modifies the oxidation chains that the undesired degradation reactions are minimized. These oxidations not only are characterized by discreteness of reaction, but also in some cases lead to products not previously derived in quantity from reactions with oxygen. High yields of relatively simple mixtures are obtained a t temperatures far below those usually encountered in hydrocarbon oxidations. The joint participation of hydrogen bromide and oxygen in photochemical chain reactions involving olefins was demonstrated earlier in these laboratories (9). This paper summarizes the laboratory results obtained with several types of hydrocarbons and the mechanisms deveIoped to explain the various kinds of principal products derived therefrom. The experimental data will be given in greater detail in succeeding papers ( 1 - 3 , 7 ) . The hydrogen bromide-catalyzed oxidation of paraffins containing tertiary carbon-hydrogen linkages (9)appears to proceed by a relatively simple chain reaction, which is fundamental to the oxidations of the other types of compounds. (In accordance with general acceptance, tertiary carbon-hydrogen bonds or tertiary hydrogen atom involve a carbon-hydrogen linkage wherein the other three valences of the carbon are satisfied by three other carbon atoms. The same interpretation is applicable to secondary and primary.) I n general i t can be said that branched chain compounds are converted in high yield to stable organic hydroperoxides; this is a characteristic which distin-
HBr
4k R-
-H
+ O2
A- +
I R
Br
+ (HOz.. .. . .?)
+ B r e H B r + R-
I
R
I
-
(2)
R
0 2
+R-(3-0-I
(3)
I R
R R-&&OP
1
(1)
R
R R-
----f
+ HBr
----f
R-
z I
-02H
+ Br
(4)
R
It is seen that the over-all reaction is simply:
Equation 1, a chain-initiating step which probably occurs largely a t the wall, generates a bromine atom. The bromine atom, in reacting with a molecule of a tertiary hydrocarbon or derivative, attacks virtually exclusively the tertiary hydrogen atom, forming hydrogen bromide and the tertiary-alkyl radical (Equation 2). The latter may, with due consideration for the probable reversibility of the previous reaction, undergo an association reaction with oxygen (Equation 3) and the peroxy radical thus produced is stabilized as the hydroperoxide molecule by an exchange reaction with hydrogen bromide (Equation 4). By this process a bromine atom is regenerated and reactions 2, 3, and 4 may be repeated. Of course, the chain can be interrupted at any point by destruction of the carrier radicals, as by inelastic collision with the walls or by the association of two radicals. It is apparently reaction 4 which distinguishes these oxidations from others which are less clean-cut, for without such a hydrogen donor, the peroxy radicaI apparently cannot become a s b b l e molecule and instead breaks down with varying degrees of carbon-carbon bond scission. Further and important is the specificity of the
2595
INDUSTRIAL AND ENGINEERING CHEMISTRY
2596
attack of the bromine atom on the hydrocarbon. I n these studies no other material has been found which approaches hydrogen bromide in its ability t o control the reaction. If the compound undergoing oxidation contains only secondary and primary carbon-hydrogen linkages, the principal product is a ketone (7). Propane, for example, can be converted to acetone in a yield of %yobased on the consumed propane in a 2 to 2 to 1 mixture of propane, oxygen, and hydrogen bromide. Approximately 75y0 of both of the first two components are converted while 83% of the catalyst is recovered. The operating temperature is somewhat higher than that required for isobutane (190" as compared to 160" C.) and the catalyst requirements are somewhat greater. An interesting by-product is propionic acid which accounts for about 8% of the consumed propane; the bearing of this on the oxidation of ethane will be mentioned later. The mechanism of ketone formation strictly parallels that previously given for the branched chain compounds:
R
+ Br
I
AI
H-
R
H-b-
1
+ Br C;= R-d-
R-A-H
I
H
+ HBr
I
(11)
H H
H
+ 02
1 I
----f
R-d-02-
(12)
1
H
H
H
R
H-C-HI
R
H
H
R-C-
Vol. 41, No. 11
-
+ HBr
There is good evidence for the dehydration shown in Equation 14, as it is known that aldehydes are among the major products of decomposition of normal alkyl hydroperoxides ( 4 , 8). However, the expected aldehyde is, for all practical purposes, only a transitory intermediate which is presumably converted to the end product, organic acid, thus:
R
R
+ O2 -+-H-C-08-I I
R
H
R
+ Br+R-&=O
R--di=O
(15)
02-
I
+ 02 --+
R-C-0
[
+ HBr I
R-C=O
R
H-l-O-B]
--+
i=O
+ H20
(9) The peracid in the atmosphere of hydrogen bromide or in the collection vessels is reduced:
The bromine atom attacks principally a t the secondary carbonhydrogen bonds and the isoradical thus formed reacts with oxygen. In turn the peroxy radical is stabilized by hydrogen bromide but in this instance the hydroperoxide molecule spontaneously undergoes elimination of water, giving the major product. The chain steps 6 to 9 may be summarized: RaCHz
+
HBr
0 2
+R1C=O
+ HzO
(10)
The hydrogen bromide-catalyzed oxidative attack on primary carbon-hydrogen bonds leads to the formation of organic acids (3). (Attempts to oxidize methane usefully by the present process have been unsuccessful to date.) Just as propane required somewhat higher temperatures than isobutane for comparable amounts of reaction (190" C. instead of 160' C. for the cited example), so the oxidation of ethane is a still slower reaction and the temperatures of operation are somewhat greater than those for propane (220' C. in a comparable case). However, even this temperature is far below that usually required for ethane oxidation, and, whereas in the present instance there are realized yields of acetic acid equivalent to 75% of the consumed oxygen (which is 85 to 90% of the input), the uncatalyzed reactions at 800" C. and higher lead to complex mixtures (6). Operation at elevated pressures appears to be particularly beneficial; under such conditions there results an increased amount of product, retardation of side reactions, and a lowering in catalyst requirements. Surface plays an important role in these chain processes and its effects are lessened by an increase in the total pressure in the system. The reactions have been interpreted by the following mechanism:
0
+ 2 HBr +R-
IC1-OH + Br, + H 2 0
(18)
Step 18, as written, is an over-all reaction, not a step in a chain, and its mechanism is not known. Actually free bromine in considerable quantity is found in the product, in contrast to the propane oxidation where the final product can be formed without reduction of the peroxy compound. However, the amount of free halogen is far less than that required by the stoichiometry of Equation 18 and the major portion o f it apparently reacts to form alkyl bromide which, in turn, is also converted t o acid. Thus:
H
H
+ Br2
R - L
I
----f
H
+ Br
R-C-
Br
R-C-
I
I H
Br
+
I
0 2
--f
R-6--0I
H
Br
H
I I
H
H
I
+ Br
(19)
+ HBr
(20)
Br
bI -Br
R--~-02--
I
H
H R-
I
R-C-Br
+ HBr e
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
November 1949
0
I/ + H,O +R-C-OH
*
+ HBr
(24)
All of these steps, excepting reaction 24 which simply represents the hydrolysis of an acyl bromide, have been proposed previously in the discussion. Verification of the formation of acyl bromide was obtained from a liquid phase oxidation of benzyl bromide wherein benzoyl bromide was isolated as an intermediate ( I ) . All of the equations, 11 through 24, dealing with the oxidations of primary carbon-hydrogen bonds may be summarized by the following equation: H ~~
I
2R-C-H
I
0
HBr
+ 302 +2R-
e
+ 2H20
-OH
2-phenyl-2-hydroperoxypropane is, by analogy with isobutane, to be expected from the oxidation of cumene (2-phenylpropane); actually phenol and acetone are obtained, probably by degradation of the predicted product. Similar cleavages have been revealed by the investigations on other hydrocarbons. I n support of these mechanisms of hydrogen bromide-catalyzed oxidations, a t least one example of every postulated nonfree radical intermediate or end product has been isolated or identified a t some point during these studies. These compounds are: hydroperoxide, ROOH; carbonylic, R,R,C=O; acid, RCOOH; alcohol, ROH; acid bromide, RBrC=O; and bromide, RBr. ACKNOWLEDGMENT
The authors wish to thank A. E. Lacomble, chairman of the board of Shell Development Company, for his encouragement and criticism of their efforts. To many of their colleagues in the company they are indebted for the benefit of helpful discussions.
(25)
H and it is seen that under ideal conditions there would be no loss of hydrogen bromide which then would function strictly as a catalyst. I n actual operation this condition has been closely approximated. The formation of by-product alkyl bromides, particularly wmonobromides, is not harmful because, as shown, these compounds themselves are oxidized to acid. The fifth paper in this series covers the oxidations of some aromatic compounds; i t also briefly treats some liquid phase oxidations. Because of greater structural complexities, the oxidations discussed therein are less clean-cut than those of the simpler aliphatic hydrocarbons. However, the processes apparently conform to the theories developed for the simpler cases. Thus, toluene, by reason of its primary carbon-hydrogen bonds, is converted in good yield to benzoic acid, while ethyl benzene with its methylenic group gives the anticipated acetophenone. Further, the studies on the aromatic compounds reveal a general reaction, peroxide degradation. As an example of this, the hydroperoxide,
2597
LITERATURE CITED
Barnett, B., Bell, E. R., Dickey, F. H., Rust, F. F., and Vsughan W. E.. IND. ENQ.CHEM..41.2612 (1949). Bell, E. R., Dickey, F. H.,‘ Raley, J. ‘H., Rust, F. F., and Vaughan, W. E., Ibid., p. 2597. Bell, E. R., Irish, G. E., Raley, J. H., Rust, F. F., and Vaughan, W. E., Ibid., p. 2609.
Harris, E. J., Proc. Roy. SOC.(London), 173A, 126 (1936). Jost, W., “Explosions- und Verbrennungsvorganpe - in Gasen,” pp. 416-21, Berlin, Julius Springer, 1939. Milas, N. A., and Surgenor, D. M., J . Am. Chem. Soc., 68, 205 (1946).
Nawrocki, P. J., Raley, J. H., Rust, F. F., and Vaughan, W. E., IND. ENG.CHEM.,41,2604 (1949).
Reiche, A., “Alkylperoxyde und Ozonide,” Leipzig, Steinkopff, 1931.
Rust, F. F., and Vaughan, W. E., J . Org. Chem., 7, 491 (1942). Vaughan, W. E., and Rult, F. F., U. S. Patent 2,403,771 (July 9, 1946). RECEIVED August 17, 1948. Presented in part a t the meeting of the Gordon Research Conferences of the American Association for the Advancement of Soienoe, Colby Junior College, New London, N. H., June 1948.
(Oxidation of Hydrocarbons Catalyzed by Hydrogen Bromide)
OXIDATION OF BRANCHED-CHAIN COMPOUNDS E. R. BELL, F. H. DICKEY, J. H. RALEY, F. F. RUST, AND W. E. VAUGHAN Shell Development Company, Emeryville, Calif. a
T h e hydrogen bromide-catalyzed oxidation of isobutane and related branched-chain compounds gives high yields of organic peroxides as primary products. The mechanism of the reaction appears to be a free radical chain, in which the catalyst functions both in initiation and in propagation.
H
YDROGEN bromide-catalyzed oxidation of the lower hydrocarbons (1%)is unique in its high degree of selectivity and its low order of carbon-carbon bond rupture. The oxidation of lower branched-chain hydrocarbons (19)is of particular interest because i t represents the first isolation in major amount of organic peroxides formed by the reaction of paraffin hydrocarbons with oxygen. The mechanism of this peroxide synthesis, as well as of some of the side reactions, is suggestive of steps occurring during noncatalyzed hydrocarbon oxidation.
EXPERIMENTAL
APPARATUS.The hydrocarbon oxidations summarized in the first paper have for the most part been carried out in a Pyrex glass coil reactor of 25-mm. inside diameter and 2940-m1. capacity, immersed in a thermostated oil bath regulated to =t0.5’ C. The reactants, if gases, were metered through ordinary differential flowmeters. I n other cases liquid reactants flowing from a constant-pressured reservoir passed through a rotameter and a glass regulating stopcock into a vaporizing unit. To minimize explosion hazards, oxygen was introduced through one arm and the other gases through the other arm of a W-shaped preheater. Mixing occurred in the middle ascending tube and the mixture was then introduced directly into the top of the coil reactor. The effluent material was collected in various ways, depending upon the nature of the products and other materials to be handled. The only modification in the reactor system for operation at pressures other than atmospheric was the installation on the exit line of a commercial regulator loaded for from 5- t o 50pound gage pressure. I n the series of experiments where reduced
