Aug., 1956
DIFFUSION FLAMES OF SIMPLE ALCOHOLS
two strong components in the crystal. The 1051 cm.-' band has a definite shoulder a t 1066 cm.-l, while the 780 cm.-l band is too weak to be certain of the number of components. This would leave the weak band at 2305 cm. -' as perhaps a combination involving a lattice mode. I n summary, it is clear that our data do not establish the structure of biphosphine. A detailed examination of the fine structure obtained under high resolution is needed to settle the problem. Other information, such as dipole moment and
1059
Raman spectrum, would be most helpful but this is likely to be difficult to obtain because of the instability of the compound. Biphosphine decomposes in light and various attempts to prepare solutions showed that biphosphine decomposes completely in benzene and in carbon tetrachloride. Acknowledgments,-The author wishes to acknowledge the assistance of Dr. Walter 0. Freitag in recording the infrared spectra and the advice and aid of Dr. Robert E. Hughes in the X-ray investigation.
STUDIES OF DIFFUSION FLAMES. 11. DIFFUSION FLAMES OF SOME SIMPLE ALCOHOLS BY S. RUVENSMITHAND ALVINS. GORDON Contribulionfroin Chemistry Division, Research Department U.S. Naval Ordnance Test Station, China Lake, California Received February 67, 1.966
Using a quartz probe technique, samples of recombustion products have been removed from the diffusion flames of methyl, ethyl, normal and isopropyl alcohols antanalyzed with a mass spectrometer. Analyses of these products for all the flames are self consistent, and indicate that the mechanism of burning involves a pyrolysis of the alcohol followed by an oxidation of the pyrolysis products. The mechanisms of these pyrolyses are discussed.
Introduction Previous spectroscopic work1 has shown that the pyrolysis of the hydrocarbon must play a role in hydrocarbon diffusion flames. Other workers probed a methane diffusion flame2 with mass spectrometric analysis of the samples. They showed in detail that the hydrocarbon diffusion flame mechanism was a pyrolysis of the hydrocarbon followed by oxidation of hydrogen and very small carbonaceous particles. The present work is an extension of the work t o simple alcohol diffusion flames. It will be shown that the burning mechanism is a pyrolysis of the alcohol, followed by oxidation of the pyrolysis products, as in the hydrocarbon flames. I n the alcohol flames, aldehydes and ketones as well as hydrocarbons are formed by pyrolysis, and these oxygenates break down to give CO, so that CO, H2 and carbon particles are the ultimate species oxidized. Fletcher3 studied the pyrolysis of methanol in a static system a t pressures less than atmospheric and temperatures between 626 and 730". He proposed that the pyrolysis occurs in two stages; the first stage gives formaldehyde and hydrogen, and the formaldehyde is pyrolyzed in the second stage to GO and Hf. Soineno4 studied the thermal decomposition and slow combustion of monohydric alcohols. He worked in quartz vessels a t subatmospheric pressure at temperatures up to 650". Identification of products was made spectrographically. Ketones, aldehydes, ketenes and unsaturated hydrocarbons were found in the products. Formaldehyde was found only in the products from the dom oxidation. (1) H. G. Wolfhard a n d W. J. Parker, J . Chem. Soc., 2058 (1950). (2) S.R. Siiiith and A . S. Gordon, THISJOURNAL, 60, 759 (1956). (3) J. C . N. Fletcher, P i o c . Roy. Soc. ( L o n d o n ) , 8147, 119 (1934). (4) Fujiko Sotueno, Bull. I n s . P h y 8 . Chem. Research (1942).
(Tokyo),2 1 , 277
The pyrolysis reactions in the diffusion flame give information on the homogeneous mechanism of the reactions, but none on the kinetics of the processes. Experimental The apparatus and procedures are similar to those described previously.2 An alcohol burner with a Pyrex glass wool wick was set up on a micromanipulator in a Plexiglas hood to eliminate the effects of air currents on the flame. A quartz probe with a ground diaphragm opening was sealed into position above the burner, and the samples probed from the flame were expanded into previously evacuated flasks. Samples have been probed a t the central axis near the wick and in the neighborhood of the tip of the inner cone of the flame in all cases. The probed samples were analyzed with a Consolidated Analytical mass spectrometer. Analyses were carried out in triplicate after proper equilibration and conditioning of the sample system. The procedure gives reproducible remlts for the water and alcohol components. Identification of the components was aided by thermal fractionation of the samples from a series of temperatures from liquid nitrogen to room temperature and the use of high mass resolution. The summation of the partial pressures of the components of the sample always checked within 1% of the total pressure reading of the sample introduced into the mass spectrometer. The partial pressures were used to calculate the reported mole percentages. The temperature profiles of the inner pyrolysis zone have been determined with a hairpin shaped 1 mil quartz coated Pt-Pt 10% Rh thermocouple. This type of thermocouple eliminates errors due to catalysis on the surface of the platinum. An accurate correction for conductivity errors is not possible; however, with low temperature gradient8 near the central axis of the flame any error in temperature measurement in this zone is probably small. The temperatures measured in the flames varied from about 200" at the wick to about 1400" at the tip and the edge of the flame. The temperature profile of these flames corresponds closely to that of the methane flame.
Discussion The following alcohol diffusion flames were studied : methanol, ethanol, n-propyl and isopropyl alcohol. We believe that the pyrolysis is free radical induced, probably by the small percentage of 0% which dif'fuses in near the base of the
S. RUVENSMITHAND ALVINS. GORDON
1060
Vol. 60
burner. I n part, the O2 may react with H2and the resulting H and OH radicals can also induce pyrolysis of the alcohol. The heat for the pyrolysis is supplied by hot CO, C02, N2 and H 2 0 molecules which diffuse from the edge of the flame where H2, CO and carbon particles burn. Methanol Diffusion Flame.-The analysis of samples probed from various regions of a methanol flame are reported in Table I. Samples no. 1 and 2 were probed from the zone near the wick of the flame, while the successive samples were probed along the central axis of the flame toward the tip.
methyl radical combinations. Ethylene, which is more stable than ethane, is present in quite small concentrations, so that the ethane concentrations are below the limit of mass spectrometer identification. It should be noted that methane is present in much smaller concentration than formaldehyde, even though methane is much more stable to pyrolysis than is formaldehyde. This shows that formaldehyde production is the most favorable path for methanol pyrolysis. The importance of the formaldehyde production reaction is also shown by the high steady-state concentration of its pyTABLE I rolysis products CO and Hz compared to the conSAMPLES PROBED FROM METHANOL FLAME centration of these materials in the methane difSample no. 1 2 3 4 5 fusion flame.2 Carbon dioxide 7.56 6 . 9 8 8.17 9.04 11.21 Ethanol Diffusion Flame.-The samples probed Argon 0.55 0.51 0.55 0.61 0.63 from an ethanol diffusion flame are shown in Table Methyl alcohol 16.29 14.76 6 . 0 3 3.04 0.04 11. Both samples were taken on the flame axis. Oxygen 0 . 0 3 0.04 0.06 Sample I was probed from just above the wick and Formaldehyde 0.51 0.52 0.27 0.27 0.02 sample I1 from just inside the luminous cone. Ethylene 0.05 0.02 0.03 0.03 0 . 0 3 The products formed indicate that ethanol decomAcetylene 0.04 0.03 0.02 0.03 poses via four primary processes. Water Methane Carbon monoxide Nitrogen Hydrogen
14.08 19.Gl 25 09 21.18 26.59 0 . 2 8 0 . 2 4 0.21 0.06 0.01 8.58 7.93 7.51 9.19 4.61 46.09 43.41 47.23 51.33 54.71 5.97 5.94 4.83 5.23 2.07
----100 .oo 100.00 100.00 100.00 100.00 350 375 980 1150 1300
T,"C.
The primary attack on methanol is via R R
+ CHiOH + CHIOH
+RH +RH
+ CHzOH + CH30
(1) (la)
where R is a free radical including H atoms and 0 2 . Reaction (1) has an energy of activation of about 10 kcal. and reaction (la) has an energy of activation of about 25 kcal. I n addition, there is an a priori factor of about 3 in favor of reaction (1). These reactions are followed by
+
CHzOH + CHzO H C&O + CHzO H
+
(2) (24
The heat of reaction of (2) may be estimated; the strength of the bond between H and oxygen is not over 115 kcal., and the heat of formation of the doubly bonded C and 0 from singly bonded C and 0 in an alcohol is about 85 kcal. The heat of reaction is thus about 30 kcal. From similar reasoning reaction (2a) has a heat of reaction of about 15 kcal. Some of the formaldehyde may also result from a radical attack on the CHzOH CHzOH
+R
+ CHzO
+ RH
(3)
The presence of methane in the reaction products shows that CH3 radicals are present in this flame. l'hus a small percentage of the methanol decomposes via H
+ CHaOH
+
CH3
+ HzO
TABLE I1 SAMPLES PROBED FROM ETHANOL FLAMES Sample no.
1
Ethyl alcohol Benzene 1,&Butadiene Vinylacetylene Diacetylene Acetaldehyde Propylene Carbon dioxide Argon Oxygen Propyne Formaldehyde Ethane Ethylene Acetylene Water Methane Carbon monoxide Nitrogen Hydrogen
T, "C. CHaCHzOH CHiCHzOH CHICHzOH CHaCHzOH
+R
2
9.47 0.02 0.02 0.04 0.01 0.55 0.04 7.84 0.59 0.05 0.04 0.03 0.20 1.70 1.16 14.49 1.20 7.41 51.24 3.72
2.05 0.02 0.02 0.04 0.02 0.51 0.04 8.49 0.63 0.04 0.04 0.03 0.27 1.54 1.23 16.17 1.52 7.55 55.13 3.76
100.00 350
100.00
+ + + +
+
+
+
,
1050
CHdCHOH RH CHzCHzOH RH + CH3CHz0 RH H -,CHaCHz HzO
+R +R
(5) (6)
(7) (8)
when R may be a free radical including H and 0 2 . These reactions are followed by reactions of the free radicals formed in the primary processes.
(4)
The strength of the H3C-OH bond is too great to permit a split into CH3and OH a t the temperatures inside the flame cone where CH8OH is present in reasonably high concentrations. Ethylene and acetylene probably result from the pyrolysis of ethane which in turn may result from
B
+H
CHICHOH 4 C&=!o
(9)
H
+
CHsCH20 + CHad=O H CHiCHzO + CH3 CHzO CHZCHzOH + CzHl OH
+
+
(9a) (9b) (10)
DIFFUSION FLAMES OF SIMPLE ALCOHOLS
Aug., 1956
lOGl
+ +
cause McNesby, et al.,' found no evidence of propyne when propylene was pyrolyzed in the presence of methyl radicals a t ca. 500". The CH2CH=CH2 Reactions (5) and (6) are low energy of activation radicals formed diallyl instead of propyne or allene. reactions compared with reactions (7) and (8). n-Propyl and Isopropyl Alcohol Flames.-The Reactions (9) and (10) should be the preferred analysis of the samples probed from the normal stabilization reaction for each of the primary free and isopropyl alcohol flames are reported in Tables radicals since a sizable portion of the necessary 111 and IV. n-Propanol may decompose along energy to split the C-H or C-OH bonds is simul- four main primary paths taneously available from the formation of the C=O H or C=C-bond. The formation of the ethyl free radical is via an R CHsCHzCHzOH + R H + CH3- --CHzOH (17) unfavorable path. The radical stabilizes itself by R + CHaCHzCHzOH + RH + CH2-CHZ-CH20H (18) reaction (111 or (12). At room temperature reCH3CHzCHzOH RH CHsCHnCHOH (19) combination of ethyl radicals and disproportiona- R (20) tion of ethyl radicals are the predominating reac- R CH3CHzCHzOH + RH + CHSCHzCHzO tions but as the temperature is increased abstrac- followed by tion of hydrogen becomes increasingly important.6 At still higher temperatures there is evidence that the ethyl radical can pyrolyze6 to ethylene and H. CHI- --CHzOH + CHICH=CHz OH (21) Since ethylene is considerably more stable than CHzCHzCHzOH + CzH4 + CHSOH (22) acetaldehyde, the fact that ethylene/acetaldehyde CH3CHzCHOH + CH3 + CHZ=CHOH (23) ratios are about 3 must mean that both reactions CH3CHzCHZO -+ CHiCHz + CHzO (24) (5) and (6) are important pyrolysis paths at flame temperatures. The ethoxy radical will stabilize The CHzOH decomposes to CH20 H and the itself as acetaldehyde in part (9a), but mostly as vinyl alcohol appears as acetaldehyde, presumably formaldehyde (9b). The steady-state concentra- by rearrangement 011 the wall of the sample flask. tion of formaldehyde is quite low and less than 10% TABLE I11 of the acetaldehyde. Thus reaction (7) does not form an important reaction path. Acetaldehyde in SAMPLES PROBED FRON IIL-PROPANOL FLAMES Sample no. 1 2 the diffusion flame will rapidly pyrolyze to CHI CO via its well known free radical chain mechanism. Benzene 0.04 0.10 The possible reactions which could lead to ethane n-Propyl alcohol 13.50 0.69 formation are Toluene 0.01 0.01 CH3CHz
CHjCHz + CzH H (11) + CzH6 CHaCH OH (12)
+ CH~CHZOH
c:
+ + +
+
+
1
+
+
+
CH,
+ CH3
CzHs
+R
+
+
CZHB
CZH6
+R
(13) (14)
Both reactions (13) and (14) are important paths for ethane production. With increasing temperature, abstraction processes are favored over recombination. However, the CH3 concentration is so high in these flames that reaction (13) is an important process. Ethane pyrolysis gives rise successively to ethylene, acetylene, and the reactions of acetylene. The reaction products of acetylene with itself and with ethylene form the vinylacetylene, diacetylene, 1,3-butadiene and benzene found in the product gas. A 4-carbon compound such as vinylacetylene is a reasonable intermediate for benzene formation. The methane in the ethanol flame is the result of the extremely fast pyrolysis of acetaldehyde formed in reaction (9). Methyl radicals from this pyrolysis stabilize themselves by abstracting hydrogen and by recombination. A small percentage of propylene and propyne are formed in the ethanol pyrolysis. These species are probably via radical-radical reactions.
+
CH3 HC=CHz C=CH CHI
+
CHaCH=CHz + CH~CECH +
(15) (16)
We feel that the formation of propyne by propylene pyrolysis is not an important reaction be( 5 ) M. H. J. Wijnen a n d E. W. R. Steacie, Can. J. Chem., 29, 1092 (1951). ( G ) J. R. MoNesby and A. S. Gordon, J. A m . Chem. Soc.. 77, 4719 (1955).
0.01 0.02 0.06 0.06 0.06 0.02 0.41 7.31
2,3-Cyclohexadiene Cyclopentadiene Butene-1 1,3-Butadiene Vinylacetylene Diacetylene Acetaldehyde Carbon dioxide Propylene Propyne Ethyl alcohol Argon Oxygen Ethane Ethylene Acetylene Formaldehyde Water Methane Carbon monoxide Nitrogen Hydrogen
0.01
...
0.57 0.04 0.32 2.10 1.36 0.lG 11.93 I .88 6.24 49.65 3.59
0.05 0.07 0.09 0.03 0.35 8.49 0.44 0.12 0.09 0.64 0.06 0.22 2.25 1.85 0.15 12.49 2.31 7.78 58.02 3.75
100.00 320
100.00 925
0.60 0 06
...
T , "C.
Isopropyl alcohol decomposes along three main primary paths R
+ CH~CH-CHI HA
+
RH
+ CHzCHCHI
dH
(25)
(7) J. R. MoNesby, T. W, Davis and A. 9, G o r d w , ibid., 76, 823
(1954).
S. RUVENSMITH A N D ALVINS. GORDON
1002 R
+ CHs-CH-CHa
+
RH
+ CHaC-CH3
AH H
bH H
c:
R
(26)
-,RH
CH3 --CHI
c:
+ CH3
-CHs
(27)
A
OH I H I
.-f
I
OH CH3C-CH3 OH I H I CH&CHa
CH3-b=CH2
+
CH&-CH,
01
+
CH3C-CHd
I1
I
0
c: A
CHI -CHa
+ OH
(28)
+H
(29)
+H
(30)
0
H
H +
CH3
+ CHnA=O
(31)
It is probable that reactions (20) and (27) have the slowest rates, and that reactions (19) and (26) have the fastest rates for n-propyl and isopropyl alcohol, respectively. TABLE IV SAMPLES PROBED FROM ISOPROPYL ALCOHOL FLAMES Sample no.
Isopropyl alcohol Benzene Toluene Cyclohexadiene Cyclopentadiene Acetone Ethyl alcohol Isobutylene 1,3-butadiene Vinylacetylene Diacetylene Acetaldehyde Carbon dioxide Propylene Propyne Argon Oxygen Ethane Ethylene Acetylene Water Methane Carbon monoxide Nitrogen Hvdrogen
T,"C.
1
We note that propionaldehyde is not present in detectable amounts in the products from n-propyl alcohol. This indicates that the CH3CH2C-OH and the CH3CH2C0 radicals decompose exclusively via reactions (23) and (24). In both the n-propyl and the isopropyl alcohol flames, methane is formed via hydrogen abstraction by CH3 and ethane is formed via CHs CH3. I n the n-propyl alcohol flame ethane is formed in addition by C2H6abstracting H. In the isopropyl alcohol flame the formation of ethylene is probably exclusively via the dehydrogenation of ethane. Acetylene is formed from the dehydrogenation of ethylene. The usual hydrocarbons which result from reactions involving acetylene, including 1,3-butadiene, vinylacetylene, diacetylene and benzene are also present. Ethylene is roughly twice as large in the npropyl alcohol flame as in the isopropyl alcohol flame, even though the ethane concentration is quite close in the two flames. The higher concentration of ethylene in the n-propyl alcohol flame is probably caused by reactions (22) and (24). Reaction (22) generates ethylene while reaction (24) generates ethyl radicals which stabilize themselves both by H abstractlion to form ethane and by loss of H to form ethylene. There are no analogous sources of ethylene in the isopropyl alcohol flame. A small concentration of formaldehyde results from reactions (22) and (24) in the n-propyl alcohol flame. As mould be predicted, no formaldehyde is found in the isopropyl alcohol flame. A small concentration of ethyl alcohol has been found in both the n-propyl and isopropyl alcohol flames. Carbon Formation.-Observation of the flames studied indicated an increasing amount of carbon production as one proceeds from C1 to C3 alcohol. The methanol flame produces no carbon, the ethanol flame a small amount of yellow tipping, and the propanol flames produce relatively large amounts of carbon. Analysis shows no benzene and less than 0.004% acetylene in the methanol flame, but the amounts of these precombustion products in the ethanol and n- and isopropyl alcohol flames are very similar-benzene O.l%, and acetylene (1-2%). The temperature profiles of these flames are quite similar so that this lack of correlation indicates that the tendency toward carbon formation does not depend only on the steady state concentration of these constituents in the flame. Acknowledgment.-We wish to acknowledge the cooperation of Mr. Maynard H. Hunt, who helped in carrying out the temperature measurements. We wish also to acknowledge the assistance of Mr. Andreas V. Jensen for the operation of the mass spectrometer, and Mrs. Helen R. Young, for assistance in the reduction of the mass spectral data.
+
followed by CHsCH-CHZ
Vol. 00
2
17.63 0.05 0.01 0.01 0 02 0.34 0.06 0.08 0.09 0.07 0.03 0.41 7.04 1.09 0.18 0.47 0.04 0.30 1.08 1.05 15.75 1.78 4.78 45.37 2 31
4.06 0.09 0.01 0.03 0.58 0.09 0.10 0.12 0.09 0.04 0.42 8.02 1.34 0.18 0.56 0.03 0.33 1.47 1.33 18.31 2.63 5.88 51.36 2.95
100.00 300
100,00 825
?
.
