(Oxidation of Hydrocarbons Catalyzed by Hydrogen Bromide)
OXIDATION OF ETHANE AND RELATED COMPOUNDS E. R. BELL, G. E. IRISH, J. H. RALEY, F. F. RUST, AND W. E. VAUGHAN Shell Development Company, Emeryville, Calif. d
.
0
T h e hydrogen bromidecatalyzed oxidation of ethane results in high yields of acetic acid. This is apparently formed via a free radical mechanism involving transitory peroxidic compounds. Ethyl bromide and ethylene can be converted to acetic and bromoacetic acids by the same catalytic process.
0
I/
+
CHSC-
0 2
/I --+ CHsCOz-
(7)
0
/I
CH8C02-
+ HBr +
(8)
An over-all reaction for the reduction of peracetic acid to acetic acid:
T
HE discussions of the hydrogen bromide-catalyzed oxida-
tions of branched- and straight-chain compounds presented in the preceding papers (9, 4, 6) covered essentially the course of reactions following initial attacks on tertiary and secondary carbon atoms. These reactions lead, respectively, to stable peroxides and ketones. The oxidation of ethane, which typifies attack on primary carbon atoms, gives high yields of yet a third kind of final product-namely, acetic acid. Likewise, it will be recalled that propane oxidation gave an appreciable yield of propionic acid. (Methane is so resistant t o hydrogen bromide cataly~iedoxidation that even after 6 hours a t 306" C. a small amount of organic bromides seemed to be the only organic product. ) Acetic acid equivalent to approximately 75% of the consumed oxygen, which constitutes as much as 85 to 90% of that fed, results when 1 to 1 ethane-oxygen mixtures are reacted according to the present process a t temperathes as low as 220" C. This yield is greater than any heretofore reported (3). In common with the other oxidations, considerable quantities of organic bromides are produced, the most important of these being ethyl bromide. However, the latter compound can also be oxidized to acetic acid and, furthermore, it acts as a sensitizer in the oxidation of the parent hydrocarbon.
0
+ 2HBr +CHItlOH + Br2 + HzO
A step analogous to Equation 9 has been postulated previously to explain the formation of terl-butyl alcohol from lert-butyl hydroperoxide [ ($), Equation 51. The exact mechanism is not understood, but it is thought that a t least some of this reaction may occur in the liquid phase in the collection vessels. Peracids are such powerful oxidizing agents that failure to isolate them under the conditions of the present study is not surprising. For example, introduction of gaseous hydrogen bromide into a chloroform solution of perbenzoic acid [preprtred according t o (6)]resulted in immediate liberation of bromine and the evolution of considerable heat. Although Equation 9 shows the formation of 1mole of bromine for each mole of acetic acid, only minor amounts of the free halogen are actually found. The major amount of the bromine apparently is consumed by conversion of ethane to ethyl bromide, which in turn is oxidized to acetic acid. This process may be represented as follows: CHaCHa
+ Br -+- HBr + CH3CH2-
+ CH3CH2Br
(2)
Br
(10)
HBr
I + CH3CHBr
(11)
+ Br2 CH3CHzBr + Br CH8CH2-
MECHANISMS OF ETHANE OXIDATION
It is thought that the acetic acid produced in ethane oxidation originates by chain processes which embrace all features of the
+ Br + (HOn.. . . .?) CHsCHs + Br HBr + CH3CHzCHaCHt- + +CHsCH202CHeCHaOt- + HBr --+Br f [CH&H*O&I] [CH&HzOzH] CHsCHO + HzO 0 2 ----f
(2)
0 2
(3) (4)
(5)
Acetaldehyde is apparently so readily oxidized under the reaction conditions that no more than traces have ever been detected. Its conversion to acetic acid presumably occurs according to the following steps:
0
CH3CH0
+ Br --f CHIE- + HBr
t!
CHn HBr
x' +
CHs HBr
+
HBr
0 2
--+
+Br
+
[
';?"I
(12)
CH3CHBr
(1)
--f
---t
--f
----f
mechanisms previously presented. Although the interpretation is somewhat more complex, it can be readily expressed as a composite of three mechanisms. The f i s t of these concerns the oxidation of ethane to acetaldehyde, the second that of aldehyde to acetic acid, and the third the formation of ethyl bromide and its oxidation to acetic acid. The first of these processes may be represented by the following equations. HBr
(9)
(6)
Counterparts of all of these steps have been previously postulated except for Equation 15 which represents the hydrolysis of acetyl bromide. The foregoing equations, Equations 2 to 15, may be summarized by the net reaction: 2CH8CHa
HBr + 302 + 2CH3COOH + 2H20
(16)
If the reaction were uncomplicated and complete, no hydrogen bromide would be consumed and no alkyl bromides or degrada2609
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2610 ,
100
hydrogen bromide and ethyl bromide remained essentially constant. Inspection of the data, which are summarized in Table 11, will show that approximately as much ethyl bromide was recovered as was introduced in the case of the 26-cc. per minute increment, while in the case of the 64-cc. per minute addition there was a n actual decrease in the amount of the organic halide.
I I
I
U 210
20200
205
215 TemperoturepC.
Vol. 41, No. 11
Method of Analysis. The gases effluent from the reactor were passed through a condenser and the condensate collected in a dry flask. The residual gases were then scrubbed in a wash bottle, passed through a dry-ice cooled trap, and collected in an aspirator bottle for subsequent analysis in an Orsat apparatus. The contents of the dry receiver were analyzed for water (Fischer method), bromine (iodometric), hydrogen bromide (Volhard method), and total acidity. The organic acid was taken as the difference between these last two titrations. Subsequent distillation served to separate acetic acid from bromoacetic. Only minor amounts of organic acid and hydrogen bromide reach the wash bottle. The alkyl bromides were determined by combustion analysis of the liquid in the dry ice trap.
d 220
225
Figure 1. Ethane Oxidation, Isobaric 'I'emperature Profiles Flows, 3:2:1 = CzHe: 0z:HBr; c o n t a c t t i m e , approximatel, 3 min.
TABLE 11. EFFECT OF ADDEDETHYL BROMIDEON
ETHANE OXIDATION
tion products would be formed. Although in practice this ideal state has not been attained, operation under increased pressure seems to approach it. Product Balances. A typical experiment from the hydrogen bromide-catalyzed oxidation of ethane is presented in Table I. The temperature of operation, 220' C., is somewhat higher than the approximate 190" C. for propane oxidation and approximate 160' C. for isobutane oxidation. This balance was made with about 60% oxygen consumption, and i t is noteworthy that there is little loss of reacted oxygen to carbon oxides. Likewise, among the organic compounds in the liquid product, oxygen appears only in acetic and bromoacetic acids. The by-products arise primarily from bromination reactions, the free bromine probably being produced by reduction of peracid. Schiff's reagent gave only a slight positive test for aldehydes and all tests for peroxidic material (after removal of bromine) were negative. Ethyl bromide is the most important by-product of the reaction, but, as mentioned above, this compound can also be oxidized to acetic acid. The balances shown in Table I were made on 2 mixtures which had 26- and 64-cc. per minute added inputs of ethyl bromide, respectively. These increments replaced an equivalent amount of ethane, and the hydrogen bromide concentration was reduced so that the total input of bromine as both
Bath temperature, C. Oxygen consumed % Ethane consumed' % KeQCzHaBr consdmed. cc./min. Organic acid yield (based on consumed organic material), Yo Organic acid yield (based on consumed oxvnen). ?L CO %- COz yield (based on consumed organic material), % CzHiBrz yield (based on consumed organic material) % Bromoacetib acid: organic acid, mole ratio
64 NO 26 Added Cc./Min. Cc. Min. CzHaBr CiH6Br CnkaBr 220 216 212 59 82 52 42 51 40 - 14 -3 4-13
68
74
78
64
60
68
10
1s
17
3
0 06 ~
4 0.06
8 0.06
Operating Variables. The influences of temperature, pressure, and catalyst and reactant concentrations have been studied. The trends with pressure and temperature are shown in Figure 1. The rate of oxygen consumption a t 216 O C. as a function of catalyst concentration is given in Figure 2 for equimolar mixtures of ethane and oxygen at atmospheric pressure. The same variables (oxygen consumption versus hydrogen bromide concentration) are shown in Figure 3 for operation a t 2 atmospheres of pressure and both 215' and 220' C. At the increased pressure, 41y0of the input oxygen can be consumed at 220' C. with only 6.25 mole % hydrogen bromide and 71y0with 9.1% of the halide. I n the oxidation of ethane the reaction of t h e bromine atom with the hydrocarbon has genTABLEI. OXIDATION O F ETHAKG-ETHYL B R O M I D E MIXTURES erally been considered to be the C ~ I - I O240; ~ , os,240, C2Hs, 220; CzHsBr, CzHs, 180; CzHsBr, Flows (cc./min.) rate-determining step of chain HBr, 120 2 6 ; 0 2 , 2 4 0 ; HBr, 64; 02,240; HBr, 100 60 propagation. One might, there220 216 212 Temperature, C. fore, expect the maximum rate of Xoles/lOO Moles oxidation to take place with an .\Ioles/100 Moles Consumed of Moles/100 Moles Consumed of Consumed of ethane-oxygen ratio greater than HBr + c p CaHsBr 0 2 C&Br Products C2H4 0 2 HBr unity. Such seems actually t o Acetic acid be the case. In Figure 4 the Broinoacetic acid Dibromoethane oxygen consumption is shown as Ethyl bromideb a function of the per cent oxyBromine Carbon dioxide gen in the feed. Carbon monoxide Ethylene Water
Hydrogen bromide recovered, % Ethane consumed % Ethyl bromide c o k n e d c , % Oxygen consumed % Input oxygen un$ccounted for, % Input ethane ethyl bromide unaccounted for, Yo Input hydrogen bromide unaccounted for, Yo 5 Purity, 96.2%. b Net production. C Net consumption.
+
63.0 42.0
64.0
59.0 6.3
82.0 6.5
1.6
0.0
6.9
7.3
...
61.0
...
...
39 5 21.0 52.0 hanet,o acetic acid is seen to be much more rapid in the photochemical reaction, even when this process is carried out at a considerably lower temperature than in the dark control experiment. LITERATURE CITED
(1) B a r n e t t , B., Bell, E. R., Dickey, F. H., R u s t , F. F,, and Vaughan, W. E., IND.ENG.CHERI., 4 1 , 2 6 1 2 (1949). (2) Bell, E. R., Dickey, F. H., Raley, J. H., Rust, F. F., a n d Vaughan, W.E., Ibid., p. 2697. (3) Jost, W., “Explosions- u n d V e r b r e n n u n g s v o r g ~ n g ein Gasen,” oo. 416-21. Berlin. Julius Sorineer. 1939. (4) Nawrocki, P. J., RaleZ-, J. II.,R u s t , F. F., and T’aughan, W.E., I S D . E N G . CHEM., 41, 2604 (1949). ( 5 ) Rust. F. F., a n d Vaughan, W. E., I b i d . , p. 2595. (6) Tiffeneau, M., “Organic Synthesis,” Collective Vol. I, 2iid ed., p. 422, New Y o l k , John Wiley & Sons, Inc., 1941. .
I
RECEIVED August 17, 1948. Presented in part a t the meeting of the Gordon Research Conferences of the American Association for the Advancement of Science, Colby Junior College, New London, S . H., June 1948.
(Oxidation of Hydrocarbons Catalyzed by Hydrogen Bromide)
OXIDATIONS OF AROMATIC COMPOUNDS B. BARXETT, E. R. BELL, F. H. DICKEY, F. F. RUST, AND W . E. VAUGHAN Shell Development Company, Emeryuille, Cali$.
The h j drogen bromide-catalj Aed oxidations of aromatic hydrocarbons gire the expected acids or ketones when primary or secondary hydrogen atoms on the side chain are attacked. These reactions are modified, howrerer, by the scission of the postulated peroxide intermediates to yield phenol and aliphatic ketones or acids. When tertiary hydrogen atoms are attacked as, for example, in cumene, the scission reaction is dominant. H j drogen bromide is also a cataljst for the oxidatinn of alkyl benzenes in liquid phase.
T
H E oxidations of aromatic coriipounds appear t o conform to the general principles postulated regarding the mechanisms of oxidation in the presence of hydrogen bromide (3, 4, 11, 12) as based upon the results ohtained from aliphatic substances. Hom ever, the product distributions are more complex, and uniformity of reaction is less evident. Further, a large part of the input hydrocarbon appears to enter into transformations resulting in the formation of inseparable mixtures. By-products from the oxidations of aromatic compounds, as from those of the paraffins, apparently result in some instances by easily interpreted reactions. Thus. halides are probably formed by direct brominations. Less important compounds are produced by competition of one type of hvdrogen bromide-
catalyzed oxidation \\ ith another-for example, propane, by reason of its primary carbon atoms, was converted to propionic acid in an amount equivalent to one tenth of the major product, acetonc. A third and the most important classification of byproduct formation is that resulting from the condensation of primary reaction products-for example, of acetophenone resulting from the oxidation of ethylbenzene or cumene. A fourth group of reactions which leads to by-products is that of degradation or carbon chain rupture. Some of these are responsible for the presence of the small amounts of the carbon oxides and lower hydrocarbons found in some of the experiments. Of particular interest in the following discussions are those reactions which may be attributed to “peroxide degradation.” In the case of ethylbenzene, acetophenone is the expected product; however, comparable amounts of phenols and of acetic acid, probably produced b j splitting, are also formed. It will be recalled that such reactions also occurred in the cases of some of the paraffins. Thus, in the case of isopentan?, acetone and acetic acid are produced (S). Similarly, the oxidation of 2,3dimethylbutane produces acetone and isopropyl alcohol (3). In passing to the oxidations of the individual aromatic compounds, it should be mentioned that experiments have shown that benzene is very resistant to hydrogen bromide-catalyzed oxidation; efforts to oxidize it a t temperatures even as high as
