The Pyrolysis-Gas Chromatography of Simple Organic Molecules Thomas Wolf’ a n d Douglas M. Rosie Department of Chemisrry, Uniuersiry of Rho& Island, Kingston, R. I . 02881 The vapor phase pyrolysis of 20 simple organic molecules was undertaken at eight temperatures i n the range 200°-1000° C. Hydrogen, carbon monoxide, and water were semiquantitatively measured in addition to the products detected by means of flame ionization, which were identified by means of their retention data. The products that were found and their dependence on temperature i s presented i n the form of tables and figures.
THEPRINCIPAL LIMITATION of gas chromatography is that the sample must be brought into the vapor phase. Pyrolysis has been a means of obtaining characteristic gaseous products from polymers and coriplex organic molecules (1-3) that are not themselves volatile. O n the other hand, this method of separation has sparkcd renewed interest in pyrolysis because the method is so w d l suited to dealing with the complex mixtures that often result (4-6). Thus it is apparent that pyrolysis and gas chromatography complement and supplement each other, and a combination of the two is a very powerful analytical tool. However, because information relating temperature and thermal breakdown is scanty and is scattered throughout the chemical literature, much of it from many years ago (7), this analytical tool has not reached its full potential. The present work was undertaken to explore the relationship between temperature and thermal breakdown for typical organic groupings by investigating the pyrolysis of a selection of 20 simple organic compounds. EXPERIMENTAL
Apparatus. The unpacked pyrolysis furnace was inexpensively constructed iny the authors from two concentric stainless-steel tubes, nichrome heating wire, and insulation. The chromatographic analysis system used was novel, and was designed to obtain simultaneous information o n G L C columns of two different polarities and on hydrogen and carbon monoxide-important products at high temperature. The two G L C columns were arranged in parallel, using a plain T joint as splitter and adjusting column length for split ratio. The two gases, in addition to methane, were analyzed, without back-flushing, using a Molecular Sieve column. This coiumn was added in tandem to one of the GLC columns following a nondestructive, thermal conductivity detector. The arrangement is diag-ammed in Figure 1 and the details are shown in Table I. Present address, Colgate-Palmolive Co., Piscataway, N. J. 08854.
(1) G. C. Hewitt and B. T. Whitman, Analyst, 86, 643 (1961). (2) D. F. Nelson, :. L. Yec:,and P. L. Kirk? Microchem. J., 6 , 225 (1962). (3) M. Dimbat and F. T. Eggertsen, Zbid.,9, 5 0 0 (1965). (4) R . J. P. Allen, R . L. Forman, and P. D. Ritchie, J . C k m . Soc., 1955, 2717. ( 5 ) G . M. Badger and J. Novotny, Zbid., 1961, 3400. (6) A. I. M. Keulemans an3 S . G. Perry, Nature, 193, 1073 (1962). (7) C . D. Hurd, “The Pyrolysis of Carbon Compounds,” ACS Monograph No. 50, The Chemical Catalog Co., Inc., New York 1929,807 pp.
In addition to thermal conductivity units B and C used as the nondestructive and permanent gas detectors, a more sensitive flame ionization unit was used for detector A in order t o register the pyrolysis products present in trace quantities. Direct injection into the pyrolysis furnace was accomplished by means of a T joint equipped with a septum (X). Heat-sensitive reference compounds were introduced through the normal injection port of the chromatograph ( Y ) . All pyrolysis samples were reagent grade where possible and were >99% pure or redistilled until they were-xcept ether, from which ethanol was removed by standing over sodium, and acetaldehyde (b.p. 21” C), which was used as received. Pyrolysis Conditions. The main part of the work consisted of the pyrolysis of 17 monofunctional and three difunctional organic materials, each containing no more than six carbon atoms and only hydrogen and oxygen in addition. Pyrolysis was carried out in a n unpacked flow furnace. The three-column system is schematically depicted in Figure 1. Each of the pyrolysis samples was passed through the furnace a t 200” C and a t temperatures of 400” to 1000” C in 100” C intervals. With the exception of a few minor components, all pyrolysis products were identified. Pyrolysis of Methyl Acetate. One of the more simple pyrolysis patterns was shown by methyl acetate. The steps followed for this sample illustrate the operations involved in determining the products from all pyrolyses. Two injections were made a t each temperature. The first gave retentions on two liquid phases, dibutyl phthalate (DBP) and Carbowax 1540 (CW), plus values for hydrogen, methane, and carbon monoxide o n the Molecular Sieve 5A column. The second sample injection was made to determine retention o n a third column, didecyl phthalate (DDP), using both the flame ionization and the thermal conductivity detectors. This was desirable so that the “cross-matching” of detectors was avoided. Cross-matching occurs when the percentage peak areas from a flame ionization chromatogram are compared with those from a thermal conductivity chromatogram, because the responses of the two detectors differ. It was found that use of G L C columns of two polarities was not sufficient for identification purposes, so three were used. This explains the necessity for two sample injections at each temperature. Comparisons of two different columns o n each type of detector were made in a “correlation table.” The purpose of this table was to bring together the peaks on all of the columns that represented the same product. The table contains all of the temperatures studied. The quantities of products formed were found to change in a regular manner with temperature and seldom in a n abrupt, unpredictable way. Therefore the per cent areas a t a single temperature were not required to stand alone, but were related to those obtained at higher and lower temperatures. An average value was then calculated for the retention o n each column and the peak identified by its retention values. The correlation table did not always contain entries for all four columns for each temperature where a pyrolysis product was formed. In some instances, a pair of peaks was not resolved o n one of the columns. I n other instances, the greater sensitivity of the flame ionization detector was able to record peaks that were below the threshold of the thermal conductivity detector. Percentage peak areas were considerably affected by the inorganic components not determined o n the flame detector. The final step to complete the correlation table was to VOL. 39, NO. 7,JUNE 1967
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I--------
I
L_ _ _ _ _ _ _ _ _J ANALYSIS O V E N
I
r----- 1 1
I _ y I
-u
I
He
Figure 1. Mechanical details of pyrolysis apparatus
Gas flow rate Pyrolysis tube Pyrolysis oven Analysis oven and Detectors A, B Detector C Power supplies for B, C
Recorders Analysis columns
Table I. Conditions Used for Pyrolysis and Chromatography" 44 ml/minute of helium, measured at exit of detector C 14 X l/8 inch 0.d. stainless steel, 1/e4-inchwall thickness 12 X l/c-inch 0.d. steel tube, concentric with above Research Specialties Co. Model 1662 gas chromatograph with single flame ionization detector (detector A), equipped in addition with Gow Mac Model 9255 thermal conductivity cell, W2 hot wire filaments (detector B ) Oven temperature 70.0" C Injection port Y 150" C Cow Mac Model 9193 thermal conductivity cell, W hot wire filaments mounted in TR 111 A cell enclosure at 70' C Cow Mac Model 9999C, equipped with step attenuators, using 250-mA current k e d s and Northrup, Model H Mode 1
Mode 2
I . (FI det A ) 2. (TC det B )
10 ft dibutyl phthalate 10 ft didecyl phthalate 8 ft Carbowax 1540 8 ft didecyl phthalate All 20% on 70-80 mesh Gas Chrom P support in '/$-inch 0.d. stainless-steel tubing 3. (TC det C) 4 ft '/d-inch 0.d. stainless steel packed with 30-60 mesh Linde Molecular
Sieve 5A ri
Letters A, B, C, X , Y , and figures I, 2, 3 refer to Figure 1
Nameofproduct Hydrogen Carbon monoxide Water' Methane
Table 11. Complete Pyrolysis Products of Methyl Acetate" Average relative retentionb Pyrolysis temperature in O C DDP(F1) DBP(F1) DDP(TC) CW(TC) 200 400 500 600 700 800 Molecular Sieve Column 0.00 0.00 0.00 1.1 11 45 Molecular Sieve Column 0.00 0.00 0.00 2.0 10 49 ... ... 0.150 3.66 0.00 1.7 0.00 0.00 0.00 0.19 Molecular Sieve Column 0.00 0.00 0.00 0.91 4.2 8.5
900 40 52 46 9.4
lo00 31 52 12
11
Ethane O.Oo0 o.Oo0 O.Oo0 O.Oo0 0.00 0.00 0.02 0.41 2.1 2.9 3.4 3.8 Ethylene) Acetaldehyde 0.089 0.125 0.094 0.270 0.46 0.56 0.67 0.69 0.62 0.10 0.00 0.00 Methanol 0,127 0.214 (0,150) 1.02 1.2 1.8 3.4 7.3 11 1.5 0.00 0.09 88 64 0.98 0.02 0.21 Methyl acetate 0.261 0.360 0,266 0.545 98 96 96 Methyl ethyl ketone 0.626 0.863 0.628 (1.02) 0.00 0.00 0.01 0.14 0.50 0.06 0.00 0.00 ... 1.37 ... 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 Crotonaldehyde 1.36 The following were also detected, but because of the large quantities of Hz and CO, the amounts fall below 0.01 %; benzene (7W1000' C), toluene (900-1000" C ) , isobutyl alcohol (800" C ) , butyl alcohol (700 and 800" C ) , Allene or 2-methyl-propane (700" C ) Expressed as molar per Cent Retention relative to benzene, for each of the four columns given; didecyl phthalate DDP, dibutyl phthalate DBP, Carbowax 1540 CW, and for two detectors flame ionization (FI) and thermal conductivity (TC) * .Measured on CW(TC)
726
a
ANALYTICAL CHEMISTRY
second computer program was written to take the values from the correlation table, in which each peak had been identified, to recalculate them as molar percentages, and to present these as a function of temperature. The results for methyl acetate are shown in Table 11. Mechanisms that could explain the presence of many of the pyrolysis products are indicated in Figure 2,
[C",( h t G H,
I .
+C H
*H3+ Cor, C ' HO ;, +
1"
I
$AH~o+~H, CH,OH \(HC
+1; H,O+ (CHECH)+-
C6 H6
p3
1
GH3CH0
I
1
HO)+H~ I
I 1
RESULTS
CO t H,
CH,CH=CHCHO
Pyrolysis Products. During the course of the investigation, complete product-temperature profiles of the kind shown in Table 11 were obtained for all 20 samples studied. These results are too lengthy to report here except in a partial summary in the tables and figures that follow. Table I11 lists all of the more important products found, arranged by functional group of both sample (columns) and product (rows). The table indicates the level of per cent area found, omitting the Hz, CO, and HzO peaks. Quantities below 0.1 area per cent have been omitted. Stability of Samples. The temperatures at which samples of different functionalities decompose is of great interest for choice of optimum pyrolysis temperature. Figure 3 shows the range over which significant decomposition (10 to 90 %) took place. This was calculated from the averaged values [ lid50 %) il/4(2 % 98 731 for the midpoint and [(98 % - 2 %> 2(90 % - 10 %) 3(75 % - 50 2J1/6 for the 10 to 90% decomposed range. The latter was based on the finding that the range 98 to 2 % decomposed was very similar to twice that from 90 to 10 and three times that from 75 to 25%. Such a result would be expected where the derivative of the per cent decomposed follows a Gaussian curve. The selection of a suitable pyrolysis temperature depends in part on the number of products found at any particular value. In order to compare samples giving many products with those that gave fewer, the number of products was expressed as a percentage of all the products found for that pyrolysis sample. Values for samples of each functional group were found to be quite similar and the averages are shown in Figure 4.
+ HO,
C6H5GHj+ H
Figure 2.
Pyrolysis of methyl acetate
identify each pyrolysis product by means of its retention time on each of the columns on which the peak was found. The retention times were compared t o those for reference compounds injected into the injection port beyond the pyrolysis oven, using unchanged chromatographic conditions. Because the output from the pyrolyzer frequently gave a number of poorly resolved peaks for any one set of conditions for an analysis column under isothermal conditions, and because a single set of conditions was used throughout, the problems of measurement of peak area were very great. Since the recorder trace so often did not return to the base line, neither disc integration nor triangulation of areas was satisfactory. The method of area measurement used was to employ the product cif retention distance and peak height (8, 9). In order to allow for day-to-day variations in flow rate, all retention data was expressed relative t o benzene. The labor of calculating retention values of peaks relative to benzene, peak areas and percentage areas, was considerably reduced by using a suitable computer program. A (8) J. 13. Dhont, Analyst, 89,71 (1964). (9) H. Van Den Do01 m d P. Dec Kratz, J. Chromafog., 11, 463 ( 1963).
IN DEGREES CENTIGRADE 700 aoo 900
TEMPERATURE
300
400
600
500
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A
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,
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ACETIC
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ACETONE METHYL ISOBUTYL KETONE 2 , 5 HEXANEDIONE
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1100
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ACETALDEHYDE BUTYRALDEHYDE
4
, A
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METHYL FORMATE METHYL ACETATE
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1000
METHYL CELLOSOLVE DIETHYL ETHER
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METHANOL ETHANOL ! BUTYL ALCOHOL 3 ISOBUTYL ALCOHOL SEC. BUTYL ALCOHOL TERT. BUTYL ALCOHOL A L L Y L ALCOHOL
4
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+ +
+
i
t
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--y--+-q
Figure 3. Pyrolysis ranges of samples VOL. 39, NO. 7, JUNE 1967
e
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The data indicate that the seven functionalities fall into three distinct classes, with hydrocarbons and ethers giving the least number of decompositions a t lower temperatures, while aldehydes and acetic acid give the most. Alcohols, esters, and ketones form a n intermediate group.
The least stable grouping found was that of a tertiary alcohol, which dehydrated to form the very stable olefin, Primary alcohols, followed by secondary alcohols, were next higher in stability. These dehydrogenated to aldehydes and ketones, respectively, according t o the scheme: CHa
DISCUSSION
RCHPH
Results have been presented that show the effect of temperature on simple organic molecules in a vapor phase flow type of pyrolysis oven. A t temperatures above 800" Cythe products consisted mainly of hydrogen, methane, carbon monoxide, and water, except in the case of benzene, which decomposed only slightly. When a flame ionization detector is used, these large peaks are not recorded. The greatest number of peaks are to be found in 650 "-850" C range.
------f
RCHOH
-+
+ CHI
RCHOH H
(1)
+ RCHO
(2)
The wide occurrence of acetaldehyde was noted. It was always found in the presence of crotonaldehyde, obtained by dimerization followed by dehydration. The wide occurrence of benzene (and toluene) might be explained by condensation of crotonaldehyde with a further molecule of acetaldehyde, followed by elimination of two water molecules and cyclization
Table 111. Products of Pyrolysis by Functional Group
-2 a a 3 -2 5't -2 3-- -e 8 - 5w 3 6 & m 7 5 9 % g E s P a 5 i 0
Sample
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9 8
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-
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ANALYTICAL CHEMISTRY
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Product cp ALDEHYDES Acetaldehyde Isobutyraldehyde Y X Butyraldehyde Y 0 X X Crotonaldehyde 0 Hexaldehyde Acrolein KETONES Acetone Y X X X X 0 0 Methyl ethyl ketone Y 0 Ethyl propyl ketone X Dipropyl ketone ETHERS 0 X X Diethyl ether ACIDS Formic acid ALCOHOLS S Methanol X S Ethanol 0 S sec Butyl alcohol S 0 0 0 Isobutyl alcohol 0 S 0 Butyl alcohol HYDROCARBONS X 0 0 Propane, Propylene X 2-Me-propane, Allene X X X X Butane X Butene-1, 2-Me-propene Butadiene, Neopentane, X 0 X X Butene-2 X X Pentane S X Pentene-2 0 2-Methyl-pentane 0 3-Methyl-pentane 0 X 0 Hexane, Hexene-1 0 Cyclohexane Cyclohexene s Heptane 0 0 0 Methyl-octane Nonane AROMATIC HYDROCARBONS 0 X S Benzene Y X X X 0 X X 0 Toluene X s sample used largest peak for this product found to be above 10 area per cent x largest peak area l-lO% o largest peak area O.l-l.O%
x
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70 60 50 40 30 20
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60 70
1 HYDROCARBCNS
100 200
500 400 500
600 700 800 900
1000
PYROLYSIS T E M P E R A T U R E IN DEGREES CENTIGRADE
Figure 4. Variation of number of peaks with temperature
to the aromatic ring. Alternatively, acetylene could be obtained from acetaldehyde to trimerize and form benzene under pyrolysis condi ;ions. Acetylene would not have been resolved from the methane-ethane-ethylene peak under the conditions used. Another material whose presence would be predicted but which was not detected, was formaldehyde, which is certainly not stable at higher temperatures and which would yield the carbon monoxide and hydrogen found in such large quantities. HC'HO -t HP
+ CO
(3)
light gases but a quantity of acetone, which has a 3 carbon skeleton, and which was a very frequent pyrolysis product. Diethyl ether and methyl ethyl ketone were two further products commonly found that resulted from shorter chain samples. The pyrolysis of aldehydes seems to lead directly to fragmentation of the carbon skeleton. Thus, butyraldehyde yielded ethanol (not resolved from acetone on didecyl and dibutyl phthalate analysis columns). Ketones were almcst as resistant as ether to pyrolysis, and yielded mainly hydrocarbons, indicating complete loss of oxygen: a high carbon monoxide content was found. Diethyl ether breakdown proceeded via acetaldehyde. Hydrocarbons were characterized not only by their high decomposition points, but also by their unusually narrow decomposition range. The superior resistance of the aromatic nucleus t o breakdown was confirmed. It was found that decomposition ranges of the different functional groups were in most cases overlapping, so that decomposition of only one type of functionality in a multifunctional organic molecule by suitable selection of temperature would not appear practical as an analysis method. However, methyl cellosolve (hydroxyl and ether) and allyl alcohol (hydroxyl and double bond) both decomposed entirely in the low-temperature alcohol range. Allyl alcohol containing the thermally stable double bond gave the largest number (36) of pyrolysis products that were found. In conclusion, it should be pointed out that the size of the pyrolysis tube used was sufficient to bring almost the entire sample to the pyrolysis temperature. The less drastic conditions used in microwave discharge (IO)and mercury-sensitized photolysis ( I / ) give far simpler products. I n using pyrolysis as a n analytical tool, it may well prove that a small percentage breakdown in the 700"-900" C temperature range may give the best results. This entails allowing a large proportion of undecomposed sample to pass ( 1 2 , /3) and it minimizes secondary reactions.
RECEIVED for review October 13, 1966. Accepted March 24, 1967. Presented in part at the Pittsburgh Conference o n Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 1966. Financial support was received from a Summer Teaching Fellowship (1964) and a Cooperative Fellowship (1964-65) of the National Science Foundation.
Methyl formate appears to decompose via formic acid, but little methanol. This Fi
+ CH,
(4)
(10) Chem. Eng. News, 42,40 (September 7, 1964). (11) R. S. Juvet, Jr., and L. P. Turner, ANAL. CHEM.,37, 1464
is contrary to the de:omposition of the acetate (Figure 2), which is more stable try 100" C, and which gave much methanol but no acetic acid. The pyrolysis of this acid gave mainly
(1965). (12) W. J. Bailey and H. R. Golden, J . Am. Chem. Soc., 75, 4780 ( 1953). (13) J. H. Purnell and C. P. Quinn, Nuture, 189, 656 (1961).
HCOOCH: --+ HCOOH
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