R. P. TAYLOR and F. E. BLACET University of California, 10s Angeles, Calif.
Photochemical Oxidation of Biacetyl Molecular Oxygen
by
Understanding of the complex processes involved in the oxidation of organic compounds in the atmosphere will come from basic investigation, such as this study of chemical reactions under ultraviolet light P H O T O C H E M I C h L oxidation of several simple ketones has been studied (4! 5> 8. 72) by several investigators, but the over-all mechanisms involved are still not clear. The photolysis of pure biacetyl has b-en investigated by Bell and Blacet ( 7 ) who were able to interpret their results by assuming the primary process to be simply the formation of acetyl radicals, although the possibility of the primary production of some methyl radicals was not eliminated. This study was indicated in a n effort to elucidate the fate of acetyl or methyl radicals when they are photochemical1)produced from biacetyl in the presence of molecular oxygen. Apparatus and Materials
The photochemical apparatus was similar to that described by Strachan and Blacet (75),the reaction cell being of fused silica, cylindrical, 20 cm. long and 3 cm. internal diameter. Eastman Kodak \Vhite Label biacetyl dried over calcium sulfate was purified by distillation through a n efficient column under a lo^. pressure of oxygen-free nitrogen. The fraction boiling a t 27.8' C. a t 60 mm. pressure was collected and? without any contact with air, was completely degassed and stored in a blackened reservoir a t -80' C. Oxygen was purified by evaporation of tank oxygen from liquid oxygen temperature several times. Procedure
Biacetyl was admitted to the reaction cell by warming the storage reservoir to a suitable temperature, and the pressure in the cell was measured by a glass diaphragm gage which clicked a t a known pressure difference, reproducible to mm. of mercury. The biacetyl was frozen with liquid nitrogen in a side arm of the cell, and the required pressure of oxygen was measured in, using a mercury manometer outside the cell system. The contents were allowed to attain the temperature of the reaction cell and mixed thoroughly for 15 minutes
before photolysis with a solenoid-operated gas pump. I n determining quantum yields, acetone was used as an internal actinometer. All experiments \\'ere performed a t 3130 -A. Product Analysis
After photolysis, the cell contents Lvere drawn slowly through, first: a glass helix-packed trap a t -195' C. to remove all substances condensible a t that temperature; secondly, a copper oxide-packed tube a t 300' C. to oxidize carbon monoxide (at this temperature methane is not oxidized); and finally. through another helix-packed trap a t -195O C. to remove the carbon dioxide thus formed. The glass helices were necessary to condense efficiently material from the comparatively high pressure oxygen stream. Oxygen passing through the last trap was discarded. I n some experiments a portion of the noncondensibles passing through the first trap was retained in a side tube closed by a stop cock, before reaching the copper oxide tube. This material \vas analyzed for noncondensibles other than carbon monoxide; the latter cannot be analyzed on the mass spectrometer in the presence of oxygen since carbon monoxide is formed by reduction of oxygen near the hot carbonized filament. After removing all noncondensibles, the remaining condensible material transferred to a Le Roy still (70): and the fractions volatile a t the following temperatures were collected a t - 195' C : 4 t -130' C., carbon dioxide and monomeric formaldehyde i i t -90' C., methanol At -65' C., material was collected in three or four parts. Biacetyl \vas the major distillate, but the earlier part contained water and acetic acid. All collected fractions were identified and analyzed with a mass spectrometer. Fractions containing large quantities of biacetyl were analyzed with the sample tube section of the mass spectrometer cooled to -25'. This condensed
most of the biacetyl, reducing its vapor pressure to less than 1 mm. Other substances present in small concentration were not condensed, and the net effect was reduction of the superimposed biacetyl mass spectrum to a workable level. \Vater was analyzed by a quantitative conversion to acetylene. In runs in bvhich this was done the whole of the condensibles remaining after removing the -130' and -90' C. fractions \vas distilled into a tube containing granular calcium carbide. This completely converted small traces of water to acetylene Lvithout reacting with biacetyl. After 15 minutes contact time a t room temperature, all volatile material was distilled back to the Le Roy still; the acrtylene \vas removed a t - 130' C. and run on the mass spectrometer as a n extra fraction. Calibration of the mass sprctrometer was effected by introducing measured quantities of the pure compounds. Volatile liquids were weighed, vaporized by expansion into a large volume, and a knoivn small fraction of this volume taken. \Vater was weighed in this manner and converted to acetylene before running the mass spectrum. -A suitable sample of formaIdeh!.de from a photol>-sis \vas first run on the mass spectrometer under conditions such that only a small amount of it was lost. I t was then recondensed into a sample tube containing ml. of Lvater. The color developed with chromotropic acid ( 3 ) \vas compared tvith that obtained from solutions prepared by dilution of standard formalin solution. Results
The main products detected were carbon monoxide, carbon dioxide, forrnaldehyde, and Lvater. Other products present in lesser quantities were methanol and acetic acid. I n some runs a peak of m/e equal to 78 was found which may be the hydrated dimer of formaldehyde. KO methane, ethane, or formic acid \vas detected. Some polymeric material gradually accumuVOL. 48, NO. 9
SEPTEMBER 1956
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Table I.
Run 28 29 30 31 32 33 34 35
Results of Photochemical Oxidation of Biacetyl b y Molecular Oxygen at 3130 A. (Absorbed intensity, 3-7 X 1 0l2quonta/ml./sec.; biacetyl pressure -1 00 mm.) T e m p .I 02, Quantum Yields O c. Mm. H g CO COS CH20 CHiOH First Series 60 60 60 60 100 100 140 140
..
124 128 125 125 128 124 123 122
0.69 0.64 0.59 0.73 0.81 0.96 1.31 1.55
0.08 0.15 0.12 0.16 0.16 0.29 0.21
0.33 0.31 0.29 0.27 0.41 0.41 0.69 0.61
0.06 0.02
H10
.. .. .. .. ..
*.
0.10 0.04 0.06 0.08 0.06
..
Second Series 40 41 45” 46” 42 43 44“ 47 48
60 60 60 60 140 140 140 140 140
120 120 48 47 115 124 53 49
0.25 0.37 0.25 0.13 0.49 0.55 0.93 0.96 0.24
10
0.81 0.77 0.85 0.70 1.57 1.79 3.86 1.78 1.43
0.38 0.34 0.42 0.30 0.41 0.90 1.46 0.75 0.65
0.04
0.38 0.94
*.
..
*. ..
..
0.07
0.58 0.27
0.04 0.18
.. ..
..
..
0.22
Third Series 53 49 50 51 52 a
60 140 140 140 140
0.51 1.61 100 0.49 101 0.29 111 2.24 Oxygen contained approximately 12%
0.74 1.66 1.70 1.67 1.69
100 102
0.29 0.58 0.94 0.79 0.88
..
0.06 0.03 0.10
* .
*.
..
0.08
..
..
0’601*.
lated on the cell walls, reducing the transmitted light intensity somewhat. This was probably polyformaldehyde. Above 140’ C. a n appreciable thermal reaction was observed. Oxidation seemed dependent on the state of the cell walls, and to obtain reasonably reproducible results, it was necessary to “condition” the walls. Without conditioning, the quantum yields of carbon monoxide, carbon dioxide, and formaldehyde were extremely variable from one experiment to the next; values were obtained both considerably higher and lower than those found subsequently. Three series of experiments were performed and quantum yields are given in Table I. All runs were of 20 minutes duration. I n all cases the absorbed intensity was 5 to 10 X 1014 quanta per second. Acetic acid was found as a product, but quantum yields were small and not sufficiently reliable to be recorded. First Series. Before the first experiment was performed in this series the reaction cell was cleaned with nitric acid, washed, and dried, then conditioned by prolonged photooxidation
which deposited a polymerlike substance on the walls. Analyses were made for carbon monoxide, carbon dioxide, formaldehyde, and methanol only. The quantum yield of carbon dioxide is about twice that of formaldehyde, and four or five times greater than that of carbon monoxide. Results are fairly reproducible between similar experiments. Methanol is a minor product, and its quantum yield does not change noticeably with temperature. Second Series. The cell was conditioned in the same manner as it was for the first series. With two exceptions, the ratio of quantum yields of carbon dioxide and formaldehyde is still about 2 : l a t both temperatures, and the values are comparable to those a t the same temperatures in Series 1. However, the carbon monoxide yields are about three times greater at each temperature compared with those of Series 1. I n this series, with in experimental error, variations of oxygen pressure in the range 10 to 124 mm. had no observable effect on yields. Third Series. I n this series, instead of conditioning by photooxidation, the clean cell was finally washed with a
dilute solution of boric acid and dried to leave a thin film of solid on the walls. Reproducible carbon dioxide quantum yields of 1.68 0.02 are obtained a t 140’ C., and the formaldehyde yields are moderately constant, again being about one half that of the carbon dioxide. However, the carbon monoxide yields are erratic, similar to results obtained without any treatment of the cell walls. Three experiments of the second series were performed with oxygen containing about 127, of 0 1 6 0 0 ’ * with results given in Table 11. Comparing the percentage of 0 ’ 8 in the products with that in the molecular oxygen, it was found that the carbon dioxide contains almost exactly 1 oxygen atom from the molecular oxygen and 1 from the biacetyl. The results for the carbon monoxide are complicated by the fact that it was analyzed as carbon dioxide after oxidation over hot copper oxide in the presence of an excess of the molecular oxygen. T h e relative atom per cent of 0 ’ 8 would be expected to be 50 if the carbon monoxide was derived entirely from the biacetyl carbonyl groups, but any oxygen contributed by the copper oxide would lower this figure. T h e formaldehyde and methanol apparently have oxygen derived solely from the molecular oxygen and the acetic acid has about 1 atom from each source.
+
Discussion The results, although more erratic than one would like, are generally in accord with the following series of postulated reactions. T h e steps given are helpful for discussion purposes, but in most cases they should not be regarded as established. Seither is it claimed that they represent all the postulations which could’be made with reason
+
OH
Table II. Run 44 45 46
Temp., O C. 140
60 60
Results Obtained Using Ols-Enriched Oxygen C02 f r o m C O
Relative A t o m Percentages of Ole C02 CHzO CHsOH
51 35 40
50 43 47 ~~
1 506
INDUSTRIAL AND ENGINEERING CHEMISTRY
77 112 108
CHsCOOH
89
45
101
..
..
..
+
CHaCOCOCH3 hv + 2CHzCO CH3 CH3COCO + CHI CHBCO CO
+
++
+ CHBCOCOCH~ H20 + CHzCOCOCH3 +
CHzCOCOCH3 CHzO
++ CO1 + CH3CO
+ OH CH3CO + OH CHs
OH
0
2
+
+
CH30H CHsCOOH
+ X + wall+
Y
A I R POLLUTION In the photolysis of pure biacetyl, Bell and Blacet (I), for simplicity, assumed only primary process la. However, neither their study nor the presently reported investigation has eliminated process l b . I t could be the source of methyl radicals needed for methanol formation. Oxygen may reduce these primary dissociation processes by deactivation of biacetyl molecules. On the other hand, it may initiate a new primary process, yielding some of the products postulated by reacting directly with metastable, activated biacetyl molecules (Q), which under similar conditions have a mean life of the order of second. This postulate is plausible in view of the fact that in the absence of oxygen the primary dissociation quantum yield of biacetyl a t wave length 3130 is reported ( 7 ) to be 50.10. At temperatures above 100' C. and in the absence of oxygen, acetyl radicals are known to break down rapidly by Reaction 2 (2. 7 ) . However, a t 140' C. the isotope experiment showed that all the carbon dioxide must come from the carbonyl group; so, before decomposition, some acetyl radicals if not all must be oxidized rapidly to carbon dioxide by a reaction such as 3. This observation is substantiated by McDowell and Thomas ( 7 7 ) in a study of acetaldehyde oxidation, although they postulate CH3COS as a product. Possibly 3 and 4 are competing reactions, and in the presence of a comparatively high concentration of oxygen very little carbon monoxide is produced by Reaction 2-i.e., 4 replaces the sequence 2 plus 7. This could explain why the proportion of carbon monoxide with respect to carbon dioxide in the products does not increase a t the higher temperature. An alternate postulate would have all the carbon monoxide come from primary process 16; however, that process alone could not account for the high quantum yields of this compound found in two runs of the third series, Table I. Hydrogen abstraction by CH3O produced in 3 would lead to methanol formation. but the quantum yield of methanol is small and does not increase markedly with temperature, so such a reaction seems unimportant, a t least in the temperature range of these experiments. \Vorking with 1 or 2 mm. pressure of biacetyl, Haagen-Smit and associates ( 6 ) found ozone as a product on irradiating a mixture of biacetyl and air with sunlight. A steady state concentration of ozone of 20 to 30 p.p.m. was attained after ' / 2 hour of exposure. In our experiments it was not feasible to test for ozone except by mass spectrometry. None was found, but this fact does not in any way contradict Haagen.Smit's results, for in the first place
the mass spectrometry is comparatively insensitive to ozone, and secondly, because of the presence of a large amount of biacetyl and the other reaction products, ozone could not be expected to attain a high steady state concentration during irradiation or to survive long enough to give a mass spectrum. Reactions 5 and 9 are offered for consideration as ways in which ozone may be formed. At present, information concerning bond strengths in radicals appears insufficient to determine whether these reactions are energetically possible. Assuming that Reaction 5 can occur, mass action effects involving 3, 5, and 7 could well lead to the steady state concentrations of ozone found b y Haagen-Smit. Reaction 6 probably is a competitor of Reaction 5 and could replace the sequence of Reactions 5, 7, and 8. Possibly it is preferable to Reaction 5 from energetic considerations. The only net difference would be the production of a n HOa radical instead of 0 3 plus O H ; HOt could probably abstract a hydrogen atom in a manner similar to OH as shown in Reaction 11 and so continue the chain. However, other evidence points to the presence of OH radicals in the system, giving slight indirect evidence for ozone formation. In so far as methy radicals are formed under the described experimental conditions, it seems reasonable that they would be removed primarily by Reaction 7. T h e CHsOS intermediate (postulated also in Reaction 4) may decompose directly as in Reaction 8, or it may abstract hydrogen from biacetyl and the resultant hydroperoxide then break down to formaldehyde and water, or it may react with oxygen by Reaction 9 in an alternate path for ozone formation. Direct decomposition would yield the OH radicals in the system which are indicated by the detection of methanol and acetic acid. Reactions comparable to 7, 8, and 11 are given by Christie ( 4 ) and by Finkelstein and Noyes (5) in discussions of acetone and diethyl ketone photooxidations. A chain reaction may be sustained if OH radicals are not removed by Reactions 13, 14, or 15 since hydrogen abstractions probably can occur by Reaction 11. The biacetylyl radical so formed probably would be rapidly oxidized by Reaction 12, the acetyl radical thus formed starting the chain again. There is also the possibility of OH attack on the carbonyl carbon of biacetyl, forming acetic acid and a n acetyl radical as in Reaction 10. This would be analogous to acetone formation, in the photolysis of pure biacetyl, by methyl radical attack a t this point ( 7 ) . Acetic acid formation is preferred by this reaction, rather than 14, since in the presence of much oxygen the acetyl
radical lifetime must be quite short. Any means of removing OH radicals will shorten the oxidation chain. The small amount of methanol formed and possibly some of the acetic acid a t thr lower temperature may arise from Reactions 13 and 14, respectively. However, the quantum yields are not great, so even though the primary process (or processes) may be of low efficiency, the chains cannot be very long. Quantum yields seem to vary with the nature of the cell wall, indicating that chains are being terminated on the walls to some extent. The material deposited on the walls may be a high molecular weight polyformaldehyde, as this is known to be formed rapidly in the presence of traces of water. The variable quantum yields observed with a clean silica cell may be the result of varying amounts of hydroxyl radical adsorption on the walls giving variable chain lengths. I t is possible that this adsorption is considerably reduced when the cell wall is coated with a layer of polyformaldehyde or boric acid. However, there seems to be a qualitative difference in the results obtained with polymer-coated walls compared to boric acid-coated walls. The mechanism demands that the formaldehyde yield should be approximately equal to the sum of the carbon monoxide and carbon dioxide yields. assuming the methanol yield is small. Actually it is less than half, but polymerization would result in the detected yields being low, and it is difficult to estimate what fraction of the formaldehyde polymerizes in an experiment. It does seem unlikely that a definite fraction would polymerize, but the fact that the formaldehyde yield is only about half the carbon dioxide yield in most cases indicates either this or that some other product, so far undetected, is formed by oxidation of the methyl groups in biacetyl. The mechanism suggests the formaldehyde yield should be twice that of water. T h e fact that this was not found also must be attributed to formaldehyde polymerization. The experiments with 0'8-enriched oxygen indicate that none of the carbon of the methyl groups becomes oxidized to carbon monoxide or carbon dioxide. The high percentage of 0 ' 8 in the carbon dioxide resulting from carbon monoxide (Table 11) shows that the original copper oxide contributed very little if any oxygen to the process. Within experimental error, all the oxygen that entered the reaction came from the molecular gas phase. This implies that copper oxide has relatively few active centers which, in the presence of abundant oxygen, function repeatedly, and only in the absence of oxygen gas does it contribute oxygen freely throughout its surface to carbon monoxide oxidation. VOL. 48, NO. 9
SEPTEMBER 1956
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Acknowledgment This investigation was supported by the Office of Ordnance Research. Thanks are extended to R . A. Ogg, Jr.. for the 0’8-enriched oxygen used in this study and to Reed Bell for the suggestion that the cell walls be coated with boric acid.
(2) Blacet. F. E . Blaedel, W.J., J . Am. Chem. SOC.62, 3374 (1 940). ( 3 ) Bricker. C. E.. Johnson, H. R., IND. ENG. CHEM.,AUAL.ED. 17, 400 (1 945). ( 4 1 Christie. M. I.. J . Am. Chem. SOL.76. 1979 (1954). ( 5 ) Finkelstein, A., Noyes. W. A , , Jr., Disc. Faraday SOC.,No. 14, 76, 81; j
_
I n c ?
17JJ.
(6) Haagen-Smit, A. J., Bradley, C. E., Fox: M. M., IND.ENG. CHEM.45,
Literature Cited
2086 (1953). (7)
( 1 ) Bell. W. E., Blacet, F. E., J . Am. Chem. Soc. 76, 5332 (1954); Disc. Faraday SOC.,No. 14, 70, 1953.
Herr, D. S., Noyes, W. A . , Jr.: J .
.4m. Chem. SOC.62, 2052 (1940). ( 8 ) Hoare, D. E., Trans. Faraday SOC. 49, 1292 (1953).
( 9 ) Kaskon, LV. E., Duncan, .4. B. F., J . Chem. Phys. 18, 427 (1950). (10) Le Roy, D. J . , Can. J . Research 28B, 492 (1950). (11) McDowell, C. A,, Thomas, .J. H., J . Chem. Sac. 1949, p. 2217. (12) Marcotte, F. B., Noyes, W. A . J r . , J . Am. Chem. SOC.74, 783 (1952). (13) Sheats, G. F.: Noyes, W. A , , Jr.. Zbid., 77, 1421 (1955). (14) Steacie, E. W. R., “Atomic and Free Radical Reactions,” vol. 2, Reinhold, New York, 1954. ( 1 5 ) Strachan. A . N., Blacet, F. E., J . .4m. Chem. Sac. 77, 5254 (1955).
RECEIVED for review January 14. 1956 .ACCEPTED March 16. 1956
PAUL P. MADER, MERLYN W. HEDDON, MARCEL G. EYE, and WALTER J. HAMMING Los Angeles County Air Pollution Control District, Los Angeles, Calif.
Effects of Present-Day Fuels on Air Pollution Change in olefin composition improves quality of our present powerful fuels but worsens smog problems
R E P O R T S on air pollution in the Los Angeles area have stressed the fact that most of the manifestations of smog such as eye irritation, reduction of visibility, crop damage, and high ozone content of the air may be attributed to the high tonnage of hydrocarbons discharged into the atmosphere. An (3, 7) estimated total of 1600 tons of hydrocarbons are released daily into the air where they are exposed to air oxygen, ultraviolet light, and the effects of ozone and oxides of nitrogen. As a result of a series of photochemical reactions, a number of reaction products are formed which have all the objectionable characteristics with rvhich smog in Los Angeles has become associated. Since hydrocarbons represent the original materials from which the more noxious compounds are formed, a study of individual hydrocarbons was undertaken to determine the relative ability of individual compounds to produce oxidation products upon irradiation with sunlight. After it had been established that certain types of olefins were particularly effective in the formation of oxidation products-a quality found to be closely related to the molecular struclure of the compounds-a series of comparison analyses between thermally and
1508
catalytically cracked gasolines was undertaken. Purpose of this study was to compare the composition of thermally cracked and present-day premium gasolines with respect to those types of hydrocarbons which were found to be highly reactive smog producers. The first part of this paper discusses the irradiation procedures used and the analytical methods employed to measure certain reaction products formed from individual hydrocarbons. In the second part the methods used in the analyses of different types of gasolines are reported and the results obtained are compared and discussed.
Oxidation Capacity of Individual Hydrocarbons Not all hydrocarbons show the same tendency bvith regard to the formation of oxidation products. Saturated hydrocarbons are known to be rather inert. This group forms only very small amounts of oxidation products on exposure to sunlight, even in the presence of oxides of nitrogen or ozone. Unsaturated hydrocarbons, as a group, are far more reactive than the saturated compounds. When exposed to ultraviolet radiation, especially in the presence
INDUSTRIAL AND ENGINEERING CHEMISTRY
of oxides of nitrogen, they readily form oxidation products in abundance. Sincc these products cause the same physiological effects on humans and plants as the constituents of natural smog, it is understandable that a good portion of the blame for the Los Angeles conditions has been attributed to the oxidation products of unsaturated hydrocarbons. The oxidation capacity of a hydrocarbon was considered a convenient means of expressing, in terms of carbonyl compounds, the relative ability of individual compounds to form oxidation products upon exposure to sunlight. Although aldehydes represent only one group of a large number of possible oxidation products, they were selected as a basis for total oxidation products, because of the eye irritating properties of some members of this group and their ability to polymerize to form visibility reducing aerosols (7). When exposed to sunlight in the presence of oxides of nitrogen, the aldehydes are also capable of forming sizable amounts of ozone, thereby adding to the oxidizing characteristics peculiar to the Los Angeles atmosphere (4, 5). The procedure used to measure specific oxidation products formed from individual hydrocarbons consisted of introducing into a 50-liter borosilicate flask 0.25
