JOHNT. NEU
320
Vol. 60
INPltAltED SPECTROGRAPHIC STUDIES OF PREFLAME REACTIONS OF wBUTANE BYJOHN T. NEU Contribution from the California Research Corporation, Richmond, Cal. Received August 16, 1366
This work reports the use of infrared spectroscopy to determine directly the behavior of the products and reactants aa a reaction is occurring without the necessity of sampling. The method was applied to the oxidation of n-butane under conditions which caused a cool tlame to occur aa well aa under slightly milder conditions which did not result in the formation of a cool flame. The products observed initially in both caaes were formic acid, acetic, acid, carbon monoxide, carbon dioxide and methyl alcohol. With the occurrence of a cool flame, ethylene, propene, but.enes and acetylene formed, and formic and acetic acid were partial1 consumed. I n the case in which no cool flame occurred, all products found initially increased throughout the reaction andrin the latter stages of the reaction, methane formed. This work shows that infrared spectra can be used advantageously to study moderate temperature hydrocarbon oxidations and will be of considerable value as a complement to other methods of study.
Thomas and Crandall' employed a recording ultraviolet spectrophotometer to carry out kinetic studies of the reactions occurring during the preflame combustion of hydrocarbons. I n this work, the reaction was conducted directly in the light path of the spectrophotometer. Barusch, et a1.,2v3 utilized similar techniques t o identify products and intermediates of the preflame combustion reactions. This paper describes an analogous investigation of the formation of reaction products and intermediates, as determined by the infrared spectra of mixtures of n-butane and oxygen during preflame reactions. The application of infrared spectroscopy t o the analysis of certain fractions of butane oxidation products has been reported p r e v i ~ u s l y . ~However, the procedures involved condensing and fractionating the products prior to subjecting them t o analysis and suffer the disadvantage of allowing possible reactive and unstable combustion products t o undergo further reaction prior to examination. Infrared spectroscopy has been used for the direct determination of intermediates in other types of reactions.b I n general, the infrared spectral region is more useful than the ultraviolet because all pertinent molecular species, except the homonuclear diatomic molecules (02, Hf, NJ, absorb in the infrared region. Experimental A Perkin-Elmer Model 21 infrared spectrometer was used for these studies. A detailed description of the apparatus was published elsewhere.6 The fuel in all experiments consisted of n-butane and oxygen in a 1 :1 mole ratio. Phillips Research Grade n-butane (99% pure) and Linde commercial oxygon were used without purification. Cancellation techniquca, applicable to d u d beam spectrometers.6 were used to identify weak absorptions overlapped by xpectra of. known components and to measure quantitatively the .gases known to be present in the sample cell. The quantitative analyses of products, with the exception of acetic acid and acetone, were made by this cancellation technique. Acetic acid and acetone were measured by conventional spectroscopic methods.
The first set of experiments was conducted at a temperature of 270" and an initial pressure of 500 mm. These conditions were t800mild to produce a cool flame. The reaction proceeded over an interval of about 20 minutes, and this relatively slow rate permitted four scans of the entire rock Bait region of the spectrum while the reaction was progressing. A scan from 5000 to 650 cm.-l and return to 5000 cm.-l required about five minutes. I n the second set of experiments, the same pressure and a slightly higher temperature (280")were employed. Under these conditions the reaction resulted in the formation of a cool flame after an induction period of about two minutes. The scanning rat,e of the spectrometer was not high enough to allow scanning of the entire rock salt region during the two-minute induction period. To provide a representative picture of the kinetics of the reaction, it was necessary to survey this region several times during the reaction. Consequently, a number of combustion reactions were carried out under identical conditions, and the rock salt region was scanned by segments RO that, during a given combustion reaction, several scans of a particular segment could be made. Each segment of the rock salt region was covered, and the scans were then pieced together to give a complete representation of the spectra during oxidation. Table I shows the estimated detection limits of pertinent compounds in the presence of n-butane and also the estimated limits in the presence of n-butane combustion products. The increased difficulty of detection exhibited by the latter is caused by the overlapping of absorptions from the various products. The detection of water vapor is especially difficult because the water vapor spectrum is weak and because the sensitivity of the spectrograph in the water vapor region is limited due to the absorption caused by water in the atmosphere.
TABLE I ESTIMATED LIMITSOP DETECTIONOF COMPOUNDS AT 270' (EXCLUDING THE USE OF C=O A N D C-H FREQUENCIES FOR IDENTIFICATION)
+
(1) J. R. Thomas and H. W. Crandall, I n d . Eng. Chem., 43, 2761 (1951). (2) M. R. Barusch, H. W. Crandall, J. Q.Payne and J. R. Thomas, ibid., 43, 2764 (1951). (3) M. R. Barusch, J. T. Neu, J. Q. Payne and J. R. Thomas, ibid.. 48, 2766 (1951). (4) W. G. Appleby. W. H. Avery. W. K. Meerbott and A. F. Sarter, J . Am. Chem. Soc., 1 6 , 1809 (1953). (5) G. L. Simard, J. Steger, T. Mariuer. D. J. Salley and V. 2. Williams. J . Chem. Phys., 16, 836 (1948). (6) J. T. Neu, J . O p t . SOC.Am., 43, 520 (1953).
Compound
Methane Ethane Formic acid Acetic acid Formaldehyde Acetaldehyde Acetone Methyl ethyl ketone Methyl alcohol Ethylene Ethyl alcohol
Pressure, mm. In presence of In presence of 250 mm. of Combustion n-butane products
5
10
60
100
5 8 1 5 25 50 25 75 5 5" 5 8" 4 8 2 2 5 30 8 8 co 2 2 c02 100 150 HnO Using room temperature spectrum for identification.
Mar., 1956
SPECTRA STUDIES OF PREFLAME REACTIONS OF WBUTANE
321
Fig. 2.-Oxidation in absence of a cool flame, successive spectra (2” = 270°, initial P = 500 mm., final P = 525 rnm.; fuel: 1:l mixture n-butane-02, cell length = 5.8 cm.): 1, time 0 to 3.6 min.; 2, time 5 to 9.6 min.; 3, time 10.9 to 15.7 mn.; 4, time 17 to 21.8 min.; 5, background. Room temperature spectra of certain compounds may vary sharply from their elevated temperature spectra; and, consequently, elevated temperature reference spectra were required in some cases for identification of products. As an example, the contrast in the spectrum of acetic acid a t 25” and at 300”,due to dimerization phenomena, is shown in Fig. 1.
Results The general features characteristic of hydrocarbon oxidations carried out at moderate (250-350’) temperatures as discussed by Jost’ were observed in these experiments. I n all cases, there was an induction period during which little or no change was evident, followed by an autocatalytic reaction. I n cases in which a cool flame did not occur, the rates of pressure rise reached maxima and then decreased to zero, giving the usual “S” shaped time-pressure curves. When cool flames occurred, the induction periods of the over-all oxidation reactions were shorter and were followed by accelerating pressure rises which apparently became discontinuous at the instant of cool flame occurrence. The heterogeneous nature of these reactions was indicated by the fact that a “conditioning” of the cell walls by several reactions was necessary before reproducible induction periods could be obtained. The successive spectra of oxidation without a cool flame are shown in Fig. 2. For a period of (7) W. Jmt, "Explosion and Combustion Processes in Gasea,” MoGrew-Hill Book Co., New York, N. Y.,1946. p. 367.
several minutes after introduction of the gases into the heated cell, the only spectrum apparent was that of n-butane, as shown in Trace 1 of Fig. 2. It was noted that this spectrum differed from the room temperature spectrum of n-butane in that the absorption bands were somewhat broader, and there was a continuous absorption above the background. Trace 2 shows that formic acid (1100, 1780 cm.-I), acetic acid (1186, 1780 cm.-l), and carbon dioxide (2300 cm.-l) were forming. The carbonyl absorption (1780 cm.-’) due to the acids was so intense that it prevented the identification of carbonyl absorption caused by compounds expected in small concentrations, e.g., formaldehyde and acetaldehyde. Trace 3 shows a general increase in absorption, and bands due to methyl alcohol (1034 cm.-l) and carbon monoxide (about 2123 cm.-l) appear. Trace 4 shows an increase of the products present, except formic acid which remains about the same. Methyl alcohol and carbon moiloxide increased sharply, and a small amount of methane (1306 cm. -’) appeared. Trace 3 shows a weak band centered at about 850 cm.-’ which is not evident on trace 4. While this absorption is weak, it does indicate the transitory existence of an intermediate. The hydroperoxide group (-OOH) gerierally absorbs in this region, and an alkylhydroperoxide or peracid may account for this absorption; however, positive identification was not made. A
JOHNT. NEU
322
Vol. 60
Fig. 3.-Oxidation in absence of a cool flame, high resolution scans (2" = 270", P = 525 mm., cell length = 5.8 cm.): 1, high resolution scan, vacuum in ref. cell: 2, hiah resolution scan, followinrr...-eases in ref. cell For& acid 11 Methane 13 Carbon dioxide 65 Methyl alcohol 138 38 n-Butane 180 Nitrogen Carbon monoxide 68 I
,
-
0 FREQUENCY IN WAVE NUMOLRSo ~. ~
Fig. 4.-Oxidation
in absence of cool flame, product spectr'um at room temperature (T = 25', cell length = 5.8 cm.).
subsequent scan, not represented on Fig. 2, established the fact that the reaction was essentially complete a t the time scan 4 was taken. A slow, high resolution scan of the final products and also the spectrum recorded with certain products present in the reference cell are shown in Fig. 3. The high resolution scan shows definite but weak lines due to the presence of water vapor. I n a separate but similar experiment after the reaction was complete, the cell was allowed to cool to room temperature. The spectrum of the product gases, as shown in Fig. 4, indicates the presence of acetone (1260 cm. -l). To check for any possible effects of the rock salt windows on the oxidation reactions, a combustion experiment was conducted in a Pyrex cell with Pyrex window under conditions otherwise comparable t o the oxidation in the spectrometer cell. The spectrum a t 25' of the gases removed from this cell was essentially the same as the room temperature spectrum noted above. A summary of the combustion products from nbutane is shown in Table 11. Formaldehyde was not detected by infrared absorption, and the value listed was taken from the data of similar experiments conducted in the ultraviolet spectrophotometer.3 The water vapor spectrum was not stroiig enough t u allow the determination of the ainount of
water vapor, and the value of 140 mm. was assumed so as t o give an approximate material balance. The quantitative values in Table I1 are subject to some uncertainties but are useful to indicate the general distribution of products. Almost certainly, small amounts of other products, such as the p-dicarbonyl compounds, ethyl alcohol, acetaldehyde, etc., were present but not detected. TABLE I1 PRODUCTS OF OXIDATION IN ABSENCEOF A COOLFLAME Compound
n-Butane Formic acid Acetic acid Methyl alcohol Methane Carbon monoxide Carbon dioxide Acetone Water Formaldehyde Total
P,mm.
Yo of reactant elements found in products Oxygen Hydrogen
at 270°
Carbon
180
72.0
11
1.1
20 38 13 68 65 8 140 (20)
4.0 3.8 1.3 7.1 6.5 2.4
4.4 8.0 7.6 0.0 13.6 26.0 1.6 28.0
-
(2.0) -(4.0)-
563
100.2
93.2
72.0 0.9 3.2 6.1 2.1 0.0 0.0 2.4 11.2 (1.6) 99.5
The products obtained prior to the occurrence of a cool flame are, so far as can be determined from the infrared absorption spectrum (Fig. 5 ), ideiitical
Mar., 1956
SPECTRA STUDIES O F PREFLAME REACTIONS OF %-BUTANE
323
FREQUENCY IN WAVE NUMBERS. Fig. 5.-Spectra before and after a cool flame (5” = 280°, initial P = 500 mm., final P = 580 mrn.): 1, gases immediately after induction; 2, gases with induction period one-half expired; 3, gases directly before cool flame; 4, gases after cool flame occurred; 5, background.
to the products obtained when no cool flame occurs. The course of the changing spectrum, just prior to the cool flame, showed a general acceleration in infrared absorption, but there was no indication of a rapid build-up of a critical material, such as a peroxide. A comparison of the spectra recorded just before and just after the cool flame (Fig. 5 ) shows sharp differences. Some compounds appeared which were not observed prior to the occurrence of the cool flame. A relatively large amount of ethylene formed, together with propene, butenes, acetylene and methane. Coincident with the cool flame, there was a sharp drop in the concentrations of n-butane, formic and acetic acids, a large amount of carbon monoxide was formed, and the carbon dioxide concentration increased sharply. Following the occurrence of a cool flame, the spectrum showed essentially no change, as shown by subsequent scans. Discussion The large number of compounds formed and the order of appearance of the products suggest that no single mechanism is operative; rather, it is likely that several paths of degradation occur and that, as the population of the various molecular species changes, the predominant reactions change. The study of the reactions of certain simple oxy formed in peroxide decomposition and the researches on the mechanism of the oxidation of aldehydeslo establish modes by which these radicals react. Applying this knowledge to the data obtained in these experiments permits the formulation of a reasonable explanation of the reaction mechanism for the oxidation of n-butane in the absence of a cool flame. The occurrence of a number of the products formed in the absence of a cool flame is explained by assuming that methyl radicals are formed and react in the presence of a diminishing supply of oxygen. Raley and his co-workerss showed that the products formed from methyl radicals in the presence of oxygen are methyl alcohol, carbon mon(8) J. H. Raley, L. M. Porter, F. F. Rust and W. E. Vaughan, J . Am. Chsm. Soc., 78, 15 (1951). (9) F. F. Rust, F. H. Seubold and W. E. Vaughan, ibid.. 72, 338
(1950). (10) J. Grinner. “Abstract of Papers Fifth Symposium (International on Combustion)” University of Pittsburgh, 1954, p. 41.
oxide, formaldehyde, formic acid, carbon dioxide, water and hydrogen, while the products formed in the absence of oxygen are methane and ethane. Although Raley’s conditions differed somewhat from the conditions in the present work, there is a striking parallel between the products found. Apparently, in the presence of a high concentration of oxygen, the production of formic .acid and carbon dioxide from methyl radicals predominates; but, as the oxygen concentration diminishes, oxygenated products containing less oxygen per molecule begin to form. Finally, when the oxygen concentration is low, methane forms. The mechanisms for the formation of these compounds are discussed by Raley.* A possible mechanism for the formation of methyl radicals, as well as acetic acid and acetone, is of interest. It is considered that all of the products stem predominantly from the secondary butoxy 0. radical, CH3CHC2H6. This radical is assumed to form through the intermediate of an alkyl peroxy radical, as CHsCE-IzCHzCHa
+
0 2
+
+
CHaCHCH2CHa HOz. (1) 0-0. CHsCHCHaCHa 0-0.
+
CHaAHCH2CHa RH
+
0 2
+CHa
A
HCHzCHa (2)
+
+ R.
(3)
+ .OH
(4)
CH3:-OHCH2CH3 H 0--OH
c:
CHa HCHzCHa
8’
CHo HCHzCHs
The formation of the alkyl peroxy radical (equations l and 2) is generally accepted as an initial step of the hydrocarbon oxidation chain reaction. The conversion of the peroxy radical to the butoxy radical was demonstrated in the case of the t-butyl group by Seubold and his collaborators,” though the mechanism for the conversion was not definitely established. The formation of the butoxy radical through an alkyl hydroperoxide (equations 3 and 4) (11) (1951).
E’. H. Seubold. F. 1’. Rust and W. E. Vaughan, ibid., 78, 18
JOHNT.NEU
324
is probable in oxidation reactions. On the basis of the products determined, the butoxy radical is thought to decompose according to the equations 0.
c:
CHs HCzH) +CHaC
Vol. 60
ucts might be expected t o increase at the expense of ethyl alcohol formation. The formation of acetone can be postulated as occurring by the reaction of methyl and acetyl radicals 0
0.
CH&O
CW!XIC~H& +CHS.
+ C Z H &
