VAPOR PHASE OXIDATION OF HEXANES E. J. KAHLER, A. E. BEARSE,
AND
G. G. STONER'
Battelle Memorial Institute, Columbus, Ohio
T h e purpose of this investigation was to obtain information on reactions occurring in hydrocarbon-air mixtures prior to ignition, with the primary objective of determining the fundamental chemistry of the oxidation process. The ultimate objective of the work was to explain the relationship between hydrocarbon structure and performance in an internal combustion engine. Striking differences were observed in ease of oxidation among the various isomeric hexanes and in the nature of the products formed under different conditions. In the case of n-hexane, oxidation under precool-flame conditions at approximately 280' C. formed relatively stable arganic peroxides with little hydrogen peroxide, while oxidation at higher temperatures formed primarily unstable mixtures of hydrogen peroxide and formaldehyde. The organic peroxides were subjected to various tests in attempts to establish their nature. Oxidation of 2,3dimethylbutane and 2,2-dimethylbutane was slight even at 480' C. The work has thrown additional light on the complex reactions occurring when hydrocarbons are oxidized in the vapor phase and has pointed out the need for additional studies along these lines.
.
describe the isolation, characterization, and identification of stable products from the initial oxidation of hydrocarbons (IS, 18). I n the major portion of the present study, an effort has been made to determine the properties and identity of the initial products which can be isolated from the oxidation of n-hexane. A product was obtained at 276' to 280' C., which is below the temperature a t which a cool flame is present. This material was compared with the different oxidation products formed in the cool-flame oxidation of n-hexane a t 300' to 400' C. Precoolflame oxidation formed relatively stable organic peroxides that contained little hydrogen peroxide; cool-flame oxidation formed primarily unstable mixtures of hydrogen peroxide and formaldehyde. The oxidation product from precool-flame oxidation a t 275' to 280' C. was subjected to vacuum distillation, catalytic hydrogenation, infrared analysis, and liquid-liquid distribution studies. Some of the properties of the oxidation product were determined, but additional work will be required to identify the specific compound or compounds present. Oxidation of the isomeric hexanes, 2,3-dimethylbutane and 2,2dimethylbutane, a t 480" C. was also studied, and found to be slight; however, hydrogen peroxide, formaldehyde, water, and olefins were identified. There was no evidence for the formation of tert-hydroperoxides, APPARATUS
HE purpose of this study of the vapor phase oxidation of
T
hexanes was to provide a better understanding of the reactions occurring in hydrocarbon-air mixtures prior to ignition. Although a large number of reports on the oxidation of hydrocarbons have been published, there is lack of agreement concerning certain phases of the oxidation reactions. Occasionally, experimental evidence is contradictory. The major differences found in the literature are primarily in thk conclusions reached, or the hypotheses proposed. The position of initial attack on the hydrocarbon molecule has been a controversial subject for many years. With straight chain hydrocarbons, the methyl group, the alpha methylene group, and ithe central methylene groups all have been claimed as the point of initial reaction (10, 16, 17). In branched hydrocarbons, the order of decreasing reactivity of hydrogen atoms is stated to be tertiary, secondary, and primary (6,$0); however, it is generally agreed that attack can occur to some extent a t any position. The identity of the initial oxidation product, or products, has also been uncertain. Most investigators have made a kinetic analysis of hydrocarbon oxidation, and supplemented this with a n experimental study ot the end products, or the intermediate products of the oxidation process ( I , b, 7, 8, II,l.C.,61). In such studies it is usually found that several mechanisms of reaction are equally valid on the basis of the thermodynamic and kinetic evidence, Furthermore, although these possible alternate mechanisnp may satisfy the thermodynamic and kinetic requirements, they may not be sufficiently precise to identify theoretisally the oxidation products produced in the first stepof the oxidation process. Therefore] it would be informative to determine experimentally the structure of the first stable product, or products. Unfortunately, there are only a few publications which 1
Present address, General Aniline and Film Corp., Easton, Pa.
The apparatus was designed as a conventional flow system. The Pyrex glass No. 774 reaction tube, 2.5-cm. inside diameter and 90 em. long, was heated electrically by nine individual coils. The desired temperature distribution was obtained by varying the voltage to each coil. Cylinder nitrogen (99.99%) and oxygen (99.5%)were dried by passage over calcium sulfate. Flow rates of 0.28 liter per minute of oxygen and 1.84 liters per minute of nitrogen were used for all of the tests, as measured by calibrated rotameters. Twenty per cent of the nitrogen was mixed with the oxygen prior to being admitted to the reactor, and the remainder was mixed with hydrocarbon vapor. All of the hexanes used in this study were obtained from the Phillips Petroleum Co. The purity of each was specified to be 95 mole % and no further purification was attempted. The hydrocarbon was metered at10.8 to 0.9 ml. per minute by flow through a glass capillary into a vaporizer under pressure from a constant head of liquid fuel. A buret was part of the fuel-metering system and the flow rate was determined at least once during each oxidation test. The fuel was vaporized in a stream of nitrogen. The oxygen-nitrogen and hydrocarbon-nitrogen streams were mixed in a turbulent-flow mixer just before being passed into the reactor, The turbulent-flow mixer caused clockwise and counterclockwise swirling of the two gas streams. Tests showed that the gases were well mixed after passing70 em. into the reactor. The hydrocarbon-oxygen-nitrogen molecular ratios were 1 to 2.04 to 13.4, for 0.8 ml. of fuel per minute, and 1 to 1.82 to 11.9 for 0.9 ml. of fuel per minute. With these ratios, only 21% of the stoichiometric amount of oxygen was supplied for conversion of the fuel to carbon dioxide and water. The average residence time of the gases in the entire reaction tube was calculated to be
2777
I N D U S T R I A L AND ENGINEERING CHEMISTRY
2178
12.9 to 15.2 seconds a t 25" C., 7.1 to 8.3 seconds at 275' C., and 5.7 to 6.7 seconds a t 400" c. During the low temperature oxidations, the total gas stream from the reactor was passed through a system of traps for collection of product and unreacted hydrocarbon. During cool-flame oxidation, the gas sample was nithdrawn through a movable capillary probe that was held in position along the horizontal axis of the reactor by indentations in the glass. This device permitted reproducible sampling of the reactants a t any desired position along the reactor. Vacuum was applied to withdraw samples at a constant rate as measured by a rotameter. OXIDATION PROCEDURE
The approximate temperature distribution desired was established while only nitrogen mas being passed through the reactor. The voltages to the nine heating coils were varied, and the temperature wm measured by means of a movable thermocouple probe. A control thermocouple, 25 cm. from the mixer, extended into the reactor and wm covered with a thin coating of glass. This thermocouple wm used with a photoelectric-type temperature controller, which varied the on-off time of the current to the nine heating coils. I
i
l
l
l
OX I DATION
850
2
2 300 w
I
I
40
!O
I
i 60
I
70
80
9C
DISTANCE FROM GAS M I X E R , CM.
Figure 1.
Temperature Distributions for Oxidation of n-Hexane
After the voltages to the coils and the control temperature were established, hydrocarbon flow was started, followed after several minutes by oxygen. The temperature distribution was determined until equilibrium was reached. Then, if necessary, slight changes in the voltages to the coils were made. During most oxidation tests, liquid condensed on the walls of the exit tube after the gases had passed out of the reactor. This liquid is called "condensate" in the remainder of this paper. The temperature of the tube was 30" to 80" C. a t the region where condensation took place. The condensate was collected in a sump a t room temperature and was then allowed to drip into a receiver cooled by dry ice. After the condensate was removed, the gae stream passed through three dry ice traps. These traps removed unreacted fuel, water, and low boiling oxidation products. During some cool-flame oxidation teets, the remaining gases were analyzed for oxygen, carbon monoxide, carbon dioxide, and illuminants in a Haldane gas analysis apparatus. ANALYTICAL PROCEDURE
The condensate and the material condensed in the dry ice traps mere kept at -70" C. until transferred to small weighing bottles. They were then refrigerated a t 0" C. Condensate from the coolflame oxidation of n-hexane was rather unstable and best results were obtained when the material was analyzed within 1 hour after being warmed t o 0" C. Concentrations of all functional groups were calculated as millimoles per gram. Available active oxygen (AAO) vas determined iodometrically.
Vol. 43, No. 12
A weighed sample of 0.1 gram was added to an iodine flask containing 25 ml. of 40 to 60 chloroform-acetic acid solution. Onehalf milliliter of saturated aqueous potassium iodide solution wt~8 added immediately. The flask was then stored in the dark along with a blank sample for 30 minutes. In some cases, the samples were stored for longer periods of time. After storage, the liberated iodine was titrat'ed with 0.1 N aodium thiosulfate. Starch was necessary for determination of end point only for samples containing less than 1% active oxygen. Hydrogen peroxide (HsOa) was determined colorimetrically by use of titanium sulfate reagent (6). A weighed sample of 0.1 gram was added to 25 to 40 ml. of ethyl alcohol in a 100-ml. voluinet,ric flask. The solution was diluted to 100 ml. with alcohol, and an appropriate aliquot was added to 10 ml. of the titanium reagent. The solution of reagent' and sample n-as diluted to 100 mi. with water, or with alcohol and water to solubilize the sample. A blank solution was used which contained the same amounts of water, alcohol, and reagent. After the prepared solution had stood for 1 hour, the transmission a t 420 mp was measured with a spectrophotometer. If the t,ransmission did not lie between 80 and 20'35, a second solution was prepared using a correspondingly larger or smaller aliquot. The concentration of hydrogen peroxide in the cell solution was determined by comparing the transmission with a standard curve, Both hydrogen peroxide and bis( 1-hydroxyheptyl) peroxide were used in preparing the Etandard curve. Dinitrophenylhydrazone ( D K P ) was one of the tests used for determining the total amount of aldehydes and ketones in the oxidation products. A weighed sample of 0.05 gram was added to 40 ml.of 2 i\; hydrochloric acid contninhg 160 mg. of 2,4-dhitrophenylhydrazine. Xear1y:complet'e precipitation of the hydrazones occurred a t room t'emperature after 24 hours. The solid material was filtered and washed with 2 S hydrochloric acid and water, on a bveighed fritted-glass crucible. The sample was dried to constant %,eightin a desicyator over concentrated sulfuric acid. Calculations were based on formaldehyde hydrazone. Total formaldehyde-formic acid (TFF) was determined by a procedure described by TTalker ( 1 9 ) . -1weighed sample of 0.1 gram was added to a solution of 10 nil. of 3% hydrogen peroxide (10 millimoles) in 50 ml. of 0.1 N sodium hydroxide. This solution and a blank solution were kept a t 60" C. for 30 to 40 minutes and swirled every 5 to 10 minutes. Excess sodium hydroxide wa8 titrated wit,h standard acid aft,er the sample cooled to room temperature. Carboxylic acids (CA) were determined by titration of a weighed sample of 0.1 gram in an aqueous alcohol solution with standard 0.1 N sodium hydroxide. The sample solution was kept a t 0" C. before and during titration of the acids. Liquid-liquid distribution studies were made on condensates by the standard procedure described for other materials by Craig ( 4 , j ) . Mutually satuiated 1-butanol and water were used in the studies. After each equilibration, the two phases were moved countercurrently until the sample ilia8 distributed in eight or nine units. In some studies, each phase of each unit was analyzed for active oxygen and also €or hydrogen peroxide. By this procedure, the distribution coefficient in each unit could be calculated. However, methanol was usually added to each unit t o solubilize the phases, and the total amounts of active oxygen and hydrogen peroxide in each unit m-ere then determined. The fraction of active oxygen and hydrogen peroxide in each unit was calculated, and the results were plotted as a function of unit position. Such a graph indicated the possible t'ypes of peroxides and the relative amounts in the various condewates. EXPERIMENTAL RESULTS
Figure 1 shows the temperature distributions for the two studies made on the oxidation of. n-hexane. Curve A was used for precool-flame oxidation, and could be maintained as long 89
December 1951
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Table 1. Comparison OF Properties of Condensates from Precoolflame Oxidation of n-Hexane with and without Exothermic Reactions Property Color Refractive index, n'%' Water solubility Hexane solubility Available active oxygen, niillimoles/gram Available active oxygen, % HtOi, millimoles/gram HaOa, % Dinitrophenylhydrazonea, millimolea/grain a
No
Several Exothermio Reactions
Exothermic Reaations
Light yellow 1.435 Partial Partial
Colorless 1.440 Partial Partial
3.90 6.24 0.32 1.1
IO 98
9.5
3.2
6.86
0.24 0.8
Calculated as formaldehyde dinitrophenylhydrazone.
the temperature 60 cm. from the mixer did not go higher than 285" to 287" C. When this range was exceeded, the temperature at this position rose 100" to 125" C. Curve B was used for cool-flame oxidation, and could be maintained without any difficulty. By sighting through a movable viewing tube along the axis of the reactor, a stable cool-flame front was first visible with the viewing tube inserted to a position where the temperature of the reactor was 375' C. It is difficult to astimate the depth oi cool-flame necessary for visibility under the particular conditions of observation. However, the position of the cool-flame front very possibly coincided with the position of initial exothermic reaction. KO attempts were made to view cool flames or luminescence during precool-flame oxidation, Precool-Flame Oxidation of n-Hexane. When the temperature distribution shown by curve A of Figure 1 was used, oxidation of n-hexane in the presence of nitrogen yielded condensate with interesting properties. During preliminary oxidation tests, the temperature distribution was difficult to control and several times an exothermic reaction occurred. This happened each time the temperature rose to 285' to 287' C. a t the position 60 em. from the mixer. The condensate obtained irom oxidation tests during which an exothermic reaction occurred several times differed somewhat in properties from that obtained when an exothermic reaction did not occur. Characteristic fogs ( I , 3 ) were observed in the exit tubes whenever the temperature rose suddenly, and the reactor gases acquired the familiar odor of formaldehyde. Table I shows a comparison of condensates from precool-flame oxidations with and without the occurrence of exothermic reactions. The degree of yellow color corresponded to the number of times the temperature rose suddenly during the oxidation. Such exothermic r,eactions also caused the dinitrophenylhydrazone value to increase, which reflects the rapid formation of aldehydes and ketones during the exothermic reaction. Although there was a slight difference in the amount of hydrogen peroxide formed, the exothermic reactions caused a considerable lowering in the amount of active oxygen in the condensate. There was little difference in the refractive index of the t w e condensates, and no difference was noted in their solubility properties. Condensate from the precool-flame oxidation of n-hexane was found to contain 54.5% carbon and 9.10% hydrogen. Calculated values for (C2H40)zare 54.6% carbon and 9.14% hydrogen; however, there was no proof that the condensate was a single compound. As the condensate was stable a t room temperature, a small sample of 1.5 grams was vacuum distilled, and three fractions and the residue were obtained. These were analyzed for active oxygen, hydrogen peroxide, and for their carbon and hydrogen contents. Table I1 includes the results of the distillation and the elemental analyses. The condensate distilled within a relatively narrow temperature range. Also, the carbon and hydrogen contents of the fractions and the residue varied only slightly. These proper-
,
2779
ties indicated that the condensate was primarily one compound
or a mixture of isomers. Single determinations of active oxygen of the fractions and residue gave higher values than those for the original condensate. This could not be explained, and there was insufficient sample for check determinations; however, consistent values were obtained for all fractions and the residue. This supported the evidence from the elemental analyses regarding the uniformity of the condensate. Each fraction and the residue contained less than 0.5% of hydrogen peroxide. The infrared spectrum of fraction I11 showed absorption bands characteristic of hydroxyl, 2 . 9 ~and , carbonyl groups, 6 . 9 ~ . The strong, sharp, hydroxyl maximum at 2 . 9 ~is not of the shape, intensity, and position caused by carboxylic acids. It could be accounted for by water, an alcohol, or tl hydroperoxide. The 5 . 9 ~carbonyl band could be ascribed to aldehydes, ketones, or carboxylic acids. Low acid number and the absence of characteristic carboxylic hydroxyl absorption makes it most likely that the 5 . 9 absorption ~ is caused by an aldehyde or a ketone. Strong absorption a t 8 . 5 ~suggests that the carbonyl present may be ketonic.
Table
II. Results OF Vacuum Distillation of n-Hexane Condensate Boi 1ing
Fraction I
I1
111
Residue Original
Rfnp 43-48 42-44 47 > 47
Pressure Mm. Hg' 0.45 0.25 0 2 0.2
...
% Carbon 52.8 f2.1 53.6 f0.4
9.OQ f 0 . 1 9 9 16 f 0.10
% Hydrogen
54.2 f 0 . 1 5 04.3
9 . 0 4 f 0.08 Q.10
An attempt was made to gain further infbrmation regarding the structure of the condensate by hydrogenation to yield hydroxy compounds which could be identified. A 1.1-gram sample of condensate in diethyl ether was hydrogenated a t room temperature for 48 hours at 60 pounds per square inch gage using platinum oxide catalyst. After this time, negative tests for active oxygen and carbonyl groups were obtained. A residue of 0.9 gram was recovered from the ether solution and distilled at atmospheric pressure. Five fractions were obtained, and the boiling point of each was checked by refluxing.
Table
111.
Fraction
Results of Distillation %,of Distillate
Boiling Rfn8.e.
OF Hydrogenated Condensate
Reflux R%nga,
n%'b
Ca%on
%
Hydrogen
...
10 30-105 75-80 1.383 ... 15 105-150 135-140 1.410 ,,, ... 30 150-205 185-190 1.437 54.3 10.8 IV 30 190-215 205-210 1.441 64.5 10.8 V 16 215-230 212-215 1.451 ... a Eaah fraction of distillate was refluxed in a 3-inch test tube. The reflux range includes the temperatures for initial reflux and vigorous reflux. b nsa : original condensate, 1.440; hydrogenated condensate, 1.435. I
I1 I11
...
Table IT1 includes the boiling point ranges, reflux temperaturea of the distillates, and refractive indexes of the various fractions obtained. The percentage of the total distillate is shown for each fraction. Qualitative iodoform tests for CHO-CO g r o u p ings were positive for all fractions, which indicates the presence of secondary alcohols in fractions I1 through V with terminal 0
0
I / methyl groups. The periodic acid tests for -C -C - groupings were positive for fractions I1 through V; however, the results of the tests suggested that only a small percentage of each fraction reacted with the reagent. Qualitative infrared analysis of fraction I11 showed evidence for strong hydroxyl and weak carbonyl, methyl, and secondary alcohol.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2780
Properties of fraction I suggest low molecular weight materials such as ethyl alcohol, propyl alcohol, and acetal; fraction I1 checks well for secondary hexyl alcohols; and fractions 111and I T T check for both butanediols and hexanediols. The elemental analyses of fractions I11 and IV give the following calculat~ed formulas for four-carbon and six-carbon compounds: C 4 H d h and C~H14.2602.89. These data indicate that hydrogenat,ion of the condensate added two hydrogen atoms per molecule but did not reduce the relative proportion of carbon to oxygen. This was most unexpected. Any hydroperoxide would have been reduced to the corresponding alcohol and water with a loss of one oxygen atom. A dialkyl peroxide would have been reduced to two alcohols of lower molecular jyeight. Only a cyclic peroxide would form a compound containing both oxygen atonis, as Colloi~s: 0--0 1
Ho --f
j( CH,In!
HO-( CH,)n-OH
Liquid-liquid distribution of t,he peroxides in condensate from the precool-flame oxidation of n-hexane was studied according t'o siniilar methods used by Craig (4, 5 ) on other t,ypes of materials. The distribution study indicated that at least two peroxides were in the condensate in addition to the small amount of hydrogen peroxide. One contributed 80% of the active oxygen. This peroxide had a distribution coefficient of 4.1 to 4.2 (organic/ aqueous) for the n-butyl alcohol-water syst'em. Coefficients for tert-butyl hydroperoxide and a sec-heptyl hydroperoxide were found to be 8 and 14, respectively, for this system. Therefore, it, is most unlikely that the major peroxide in the condensate is a siniple hydroperoxide of four to six carbon atoms. The results of the hydrogenation study bear out this assumpt'ion. The minor peroxide in the condensate had a distribution coefficient, of 19 to 24. Such high distribution values are typical of dialkyl peroxides. A siniilar coefficient was found for bis( 1hydroxyheptyl) peroxide. Cool-Flame Oxidation. Cool-flame oxidat'ion of n-hexane led to the formation of coneiderably different types and amounts of oxidation products than precool-flame oxidation. Figure 1 s h o w the temperature distributions for the two types of oxidation of n-hexane. The sudden rise in temperature as shown in curve B mas typical of all cool-flame oxidation tests. The temperature range at which this occurred was invariably 285" to 287" C. for the gas system used in these studies.
Table
IV.
Chemical Analyses of Condensate from Precool-Flame and Cool-Flame Oxidations of n-Hexane Precool-Flame Cool-Flame Available active oxygen hiillimoles/gram 6.86 6.08 I O . 98 9.72 % H90, ~~-~. hIillimoles/gram 0.24 5.90 0.80 17.4 % Carboxylic acids, millimoles/gram 1.0 1.3 Total formaldehyde-formic acid, millimoles/gram ... 15.3 Dinitrophenylhydraaone, millimoles/ gram 3.2 ...
The condensate of the cool-flame oxidation of n-hexane formed at a higher rate than that from precool-flame oxidation. Also, the amount of water and low molecular weight products was greater. The condensate varied in color from colorless to yellow, and with few exceptions was miscible with water; however, an organic layer could be salted out of a n'ater solution. Table IV pyesents results of analyses of condensates from both precool-flame and cool-flame oxidations. Slthough the condensates have the same relative amount of active oxygen, hydrogen peroxide contributes 97% in cool-flame condensate, and only
1
Vol. 43, No. 12
3,5y0in precool-flame condensate. The other major difference between the two is the amount of carbonyl compounds present. Larger amounts are formed by cool-flame oxidation. Chromatographic separation of 2,4-dinitrophenylhydrazones of aldehydes and ketones in condensate from cool-flame oxidation was made according to the procedure of White (ai?). Only small amounts oi unidentified hydrazones were found in addition to the formaldehyde derivative; however, formaldehyde 2,4-dinitrophenylhydrazone was never isolated in similar studies of condensate from precool-flame oxidation. The condensate from cool-flame oxidation of n-hexane was unstable and liberated gas a t room temperature. Hydrogen composed 72 to 87% of the gas. The remainder vias carbon dioxide, carbon monoxide, methane, and olefins. Oxygen was not found. Addition of a clean iron wire or platinum oxide to the condensate caused a rapid exothermic reaction with gas evolution. This reaction w s found to be characteristic of hydroxymethyl peroxides (16).
V.
Results of Gas Analyses for Cool-Flame Oxidation of n-Hexane Distance from Temp., Gas Composition, % ' Mixer, Cm. 0 c. Oa CO COz Illuminants
Table
5 10 15 25 35
300 400 400 400
400
12.9 9.1 7.8
5.0 3.6
0.4 1.4 1.8 2.9
3.2
0.4 0.5 0.6 0.7 0.8
0.4 0.9 1.1 1.4 1.6
Liquid-liquid distribution studies ivere made of condensate from cool-flame oxidation of n-hexane. Distribution curves showed that hydrogen peroxide was piesent in a tree and a combined form. Part of the free hydrogen peroxide, shoun by the study, could have resulted from the hydrolysis of bis(hydroxymethyl) peroxide. The combined form shown by the distribution curve was more soluble in the organic phase, and most likely the condensation product of hydrogen peroxide and a ketone, or an aldehyde other than formaldehyde. Some evidence was obtained that fresh condensate contained a small amount of an organic-soluble peroxide which did not give a positive test for hydrogen peroxide with titanium reagent. Catalytic hydrogenation of condensate from the cool-flame oxidation of n-hexane was not successful in reducing all of the carbonyl compounds. No alcohols were isolated, but acetonyl acetone, 2,5-hexanedione, was isolated and identified from the mixture of hydrogenation products. The presence of the diketone supports the evidence of multiple attack on the n-hexane molecule, as indicated from the properties oi condensate from precool-flame oxidation of n-hexane. In some of the studies of cool-flame oxidation of n-hexane, the residual gas after passage through the dry ice traps was analyzed for oxygen, carbon dioxide, carbon monoxide, and illuminants. The relative concentration of each gas could be determined a t any desired distance from the gas mixer by use of a movable capillary probe. The inert gas was considered to be nitrogen. Table V includes representative values for the concentration of each gas sampled a6 five different positions in the reactor. The temperatures a t these positions are also shown. Carbon monoxide and illuminants are the major gaseous products, with small quantities of carbon dioxide being formed. It is possible that part of the carbon dioxide is formaldehyde which is not completely removed by the dry ice traps. . Oxidation of Branched Hexanes. Oxidations of 2,3-dimethylbutane and 2,2-dimethylbutane were conducted at maximum temperatures of 480' to 500" C. No recognizable exothermic reaction occurred at these temperatures. Small amounts of oxidation products were recovered in dry ice traps, but no condensate was obtained. Yields of 0.44 mole % of hydrogen
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
December 1951
peroxide and 0.06 mole % of formaldehyde were obtained from 2,a-dimet hylbutane Infrared analysis of unreacted fuel from the test with 2,3dimethylbutane showed the presence of approximately 0.5% olefins. The olefinic material was removed from the unreacted fuel by adsorption on -200-mesh silica gel. The original fuel was displaced by washing the silica gel with iso-octane. Desorption of the iso-octane and olefinic material was accomplished with acetone. The acetone was removed from this mixture by repeated washing with water; on distillation, an olefin mixture was obtained. The olefinic material was redistilled to yield two fractions. The first, boiling at 30” to 60” C., was found by infrared analysis to consist principally of 2-methyl-2-butene. Evidence was found for the presence of a small amount of tertiary olefin that was not identified. The second fraction, boiling a t 66’ to 73’ C., contained principally 2,3-dimethyl-a-butene in addition to small amounts of 2methyl-2-butene, and an unidentified tertiary olefin. The identification of 2,3-dimethyl-2-bute?e was based on a single brqnd. This band is the strongest exhibited by 2,3-dimethyl-2-butene and has a well-defined doublet character.
.
DISCUSSION
This study has produced evidence that shows that precoolflame oxidation of n-hexane can lead to considerably different oxidation products from cool-flame oxidation. Precool-flame oxidation produced organic peroxides that may have been the first stable oxidation products. Cool-flame oxidation led primarily to the formation of low molecular weight materials, including hydrogen peroxide. The condensate from precool-flame oxidation was relatively stable and did not evolve gas from samples stored a t room temperature for several months. Small quantities of unidentified carbonyl compounds were present. There was no evidence of formaldehyde in the room temperature condensate, and only small quantities were found in the material trapped at dry ice temperature. The exact structure of the major peroxide from precool-flameoxidation was not determined, but certain aspects of its structure were indicated. On the basis of liquid-liquid distribution studies and of hydrogenation studies, it was concluded that the major peroxide was probably not a primary or secondary hydroperoxide containing six carbons. Because elemental analysis indicated a compound with an empirical formula of CZHIO,and a minimum of two oxygen atoms is necessary per molecule of any peroxide, the molecular formula would be C4Ha02 or CeHl203. Molecular weight determinations did not permit a choice between these formulas, but did eliminate the possibility of a peroxide with more than six carbon atoms. Ubbelohde (18)found evidence from the oxidation of n-pentane for the presence of oxygen-ring peroxides such as: CHz
/ \
C T
C& ‘0’
CH-0 I I
CH-0
1 I
CHt-CHz I
I
GHz
b-CH2
\/I
0 0-0
I
2781’
The exothermic reaction temperature found for n-hexane, under the conditions of the tests, was 285” to 287’ C. The same temperature range was effective whether the concentration of precool-flame oxidation products was allowed to build up or not. This suggests that the exothermic reactions which occurred were not dependent on the concentration of the precool-flame products. It was not established whether cool-flame oxidation of n-hexane took place via precool-flame products, or by an entirely different process. The cool-flame oxidation of n-hexane produced primarily mater, formaldehyde, hydrogen peroxide, and carbon monoxide. Condensate contained considerable active oxygen and was unstable a t room temperature. Hydrogen was evolved and formic acid was formed during the decomposition; this agrees with observations of previous investigators (9, 11, 14). Bis(hydroxymethy1) peroxide has such properties (16). Liquid-liquid distribution studies gave evidence that other alkoxyperoxides were present in the condensate; however, there was no indication of peroxide like that from the precool-flame oxidation of n-hexane. ACKNOWLEDGMENT
This investigation was sponsored by E. I. du Pont de Nemours & Co., . h e . Permission to publish the results is gratefully acknowledged. The authors wish to thank W.T. Reid for valuable suggestions regarding the direction of the study; C. D. Miller for design of the oxidation apparatus; Earl M. Bash for the operation of the oxidation apparatus; Marjorie H. Bennett for many of the analytical determinations; Walter 41. Edwards for assistance in the hydrogenation studies; Glenn W. Kinzer for the separation of olefinic material; and Clara D. Smith for the infrared studies. LITERATURE CITED
Beatty, H. A., and Edgar, G., J.Am. Chem. Soc., 56, 102 (1934). Boord. C. E.. “Third Svmaosium on Combustion and Flame and’ExplosionPhenomena,” p. 416, Baltimore,Rld., Williams and Wilkins Co., 1949. Callendar, H. L., Engineering,123, 147, 182, 210 (1927). Craig, L. C., and Post, O., Ana2. Chem.,21, 500 (1949). Craig. L. C.. and Williamson, B. J.. Biol. Chem.. 168, 687 (1647).
Eiaenberg, G. M., IKD.ENQ.CHEM.,ANAL.ED., 15, 327 (1943). Elbe, G. von, and Lewis, B., J . Am. Chem. Soc., 59, 976 (1937). Friedlander, G., and Grunberg, J., J. Inst. Petroleum, 34, 490 (1948).
Harris, E. J., and Egerton, A., Chem. Revs., 21, 287 (1937). Hinshelwood, C. N,, J . Chem. Soc., 1948,531. Lenker, S.,J. Am. Chem. Soc., 53,2962 (1931). Mardles, E. W. J., J . C h m . SOC.,1928,872. Mardles, E. W. J., and Tett, H. C., Trans. Faraday SOC.,24, 574 (1928).
Pease, R, N., and Munro, W.P., J . Am. Chem. SOC.,56, 2034 (1934).
Pope, J. C., Dykstra, F. J., and Edgar, G., Ibad., 51, 1875, 2203 (1929).
Rieohe, A., “Alkylperoxide und Ozonide,” p. 48, Dresden und Leipzig, Verlag von Theodor Steinkopff, 1931. Ubbelohde, A. R., Proc. Roy. SOC.(London),A152, 354 (1935). Ibid.,p. 372. Walker, J. F., “Formaldehyde,” p. 258, S e w York, Reinhold Publishing Cora.. 1944. Walsh, il. DT, Trans. Faraday Soc., 42, 264 (1946). Ibid., 43,297 (1947). White, J. W., Jr., Anal. Chem.,20, 726 (1948). RECEIVED July 2, 1951.
He did not propose these as initial oxidation products, but as intermediates derived from primary amyl hydroperoxide, or the peroxide radical. This type of peroxide would hydrogenate by the addition of two hydrogen atoms per molecule as was found for the present study. Likewise, there would be no loss of oxygen as water, which also agrees with the elemental analyses. As the hydrogenated condensate from the precool-flame oxidation did not yield any identifiable compounds, it is impossible to assign any definite structure to the peroxides obtained.
The substance of the paper on “Mechanism of Combustion of Hydrocarbons” presented in the Symposium on Combustion Chemistry at the Cleveland meeting by Bernard Lewis and Guenther von Elbe is contained in the book “Combustion, Flames, and Explosions of Gases” by Lewis and von Elbe, published by Academic Press, Inc., New York, N. Y.
