The pyrolytic identification of organic molecules

MICROCHEMICAL

JOURNAL

19,229-252 (1974)

The Pyrolytic

identification

Organic

II. A Quantitative

B. C. CAPELIN,’ Deparrmenr

of

Molecules Evaluation

G. INGRAM AND J. KOKOLIS

of Chemistry, Portsmouth Polytechnic, Portsmouth, Received

Burnuby Road,

Hants.. England

November

13, 1973

INTRODUCTION

One of the most promising methods for examination of molecules is the technique of pyrolysis and analysis of the fragments produced. Many polymers and complex molecules, and other organic substances have been examined by this method since Davison et al. (8) in 1954 first applied pyrolysis gas chromatography for the identification of polymer samples. Some attention to such parameters as temperature and mode of pyrolysis, the analysis systems such as gas-liquid and gas-solid chromatography and mass spectrometry, and the amount of sample have been studied. Analytical studies of the pyrolytic behavior of simple molecules have also been made. In many cases complete data of pyrolysis fragments have not been collected, either because of the use of too low a temperature, or the use of inadequate analysis systems. An investigation of the pyrolytic behavior of organic compounds, forming part of a study of the Shiitze-Unterzaucher method for the determination of oxygen, revealed that a high-temperature pyrolysis technique might prove a useful “fingerprint” method (13, 2). A wide range of typical samples were pyrolysed in a nitrogen atmosphere at 95O”C, collecting the volatile products. These were examined by mass spectrometry. The study also included the pyrolysis of polyacrylonitrile at various temperatures between 500-950°C. It was concluded that it was possible to determine the structure, and particularly, the functional identities of a molecule in many cases from its pyrolysis pattern. Ortho, meta, and para disubstituted isomers of benzene could in most cases be distinguished from each other. An account of the investigation has been published by Belcher et al. (3). In this communication we present the results of an investigation (6) I Deceased.

229 Copyright All rkhts

@ 1974 by Academic Press, Inc. of reproduction in any form reserved.

230

CAPELIN,

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KOKOLIS

made in order to set up a system for the study of the pyrolysis of organic compounds, and determine some of the important conditions in this system. Model compounds were chosen for this work. Acetone was selected because of its simple mono-functional nature, and also since its pyrolysis products seemed to be in some confusion in the literature. Methanol, ethanol, and n-propanol were studied because they yield simple pyrolysis products easily identified. A number of different forms of pyrolysis techniques have been devised. Principally these are the reaction chamber pyrolysis (II), the hot filament technique using conventional current heating (4), the dielectric breakdown system (16), and the Currie point filament pyrolyser (20). Materials such as stainless-steel (15), silica (14), copper (9), or gold-plated tubing (7) have been used in the construction of the pyrolyser. Some investigators have employed pyrolysers packed with Chromosorb P (IO), quartz wool (5), or glass beads (15). Catalytic effects of pyrolyser materials could aifect the course of the pyrolysis, and the choice of material is an important consideration. For our experiments we employed flash vaporization into a heated reaction chamber made from silica tubing. The volatile products were analyzed by gas chromatographic techniques. EXPERIMENTAL

METHODS

Apparatus The pyrolyser unit was connected “in line” with the gas chromatographic unit. Samples were injected into the pyrolyser through a septum attached to a side arm close to the carrier gas inlet of the vaporization section of the tube. Samples were injected into the vaporizer section at about 200°C and were evaporated almost instantly into the carrier gas stream and passed as a plug into the hot zone of the pyrolyser. The high gas velocity within the system ensured isolation of the primary products most useful for characterization purposes. The contact time of the samples with the hot zone were generally in the range of 3-6 set, depending on the flow rate of the carrier gases. THE

PYROLYSIS

UNIT

The form of silica pyrolysis tube is shown in Fig. 1. The length of the tube was 25 cm and its internal diameter was 1.0 cm. The narrow connecting tubes had an external diameter of 0.3-0.35 cm and lengths of 3 cm. These were attached to the gas lines by d in. compression couplings. The injection septum was also attached to the side arm by means of an & in. compression coupling, one end of which was closed by the septum.

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FIG. 1. Pyrolysis tube. (A) Septum for sample introduction, (B) carrier gas inlet, (C) connection to gas chromatographic column.

The gas lines to the pyrolysis tube were of 4 in. external diameter copper tubing, while that from the pyrolysis tube to the analyzer unit was of T$in. stainless-steel tubing, cut as short as possible. The vaporization section of the tube was about 10 cm long, and was heated up to 200°C by electrical heating tape in circuit with a Variac controller. The reactor section of about 15 cm in length was mounted in a small electric furnace of a similar length in circuit with a Variac controller to give a temperature range of lOO-1200°C + 5”. The temperature of the hot zone was recorded by means of a thermocouple positioned centrally in the furnace. The exit cooler end of the tube was packed with a short plug of quartz wool to collect tar products and deposited carbon. The carrier gases, and hydrogen, and air for the flame ionization detector were purified by their passage through 5 A molecular sieve material. ANALYZER UNIT

A Pye-Unicam 104 gas chromatographic unit was used for volatile pyrolysate analyses. It was modified to give simultaneous katharometer and flame ionization detection. For this, the outlet from the analysis side of the katharometer was attached directly to the inlet of the flame ionization detector. Each detector was provided with its own recorder. Calibration tests showed that such an arrangement was satisfactory. The katharometer block was installed in the column oven and maintained at the same temperature as the column. The layout of the pyrolyser and analysis units is shown in Fig. 2. Sample volumes of l-10 ~1 were measured by Hamilton micro syringes. Gases for calibration purposes were either measured and injected with Hamilton gas syringes, or with a gas-sampling valve with sample loops of suitable capacity “in-line” with the carrier gas flow and the inlet of the column. Samples for calibration and identification purposes were injected via the pyrolyser maintained at about 200°C or through the inlet port of the column.

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CAPELIN, INGRAM AND KOKOLIS

FIG. 2. Diagram of pyrolysis and chromatographicanalyzer units.

GAS CHROMATOGKAPHIC COLUMN SYSTEMS Column I. Polar molecules. 3 ft X 4 in. o.d. in glass, filled with 100/120 mesh Celite with a loading of 10% PEG 600. Temperature 50°C. Carrier gas, helium at 140 ml/min flow rate. Katharometer and/or flame ionization detection. Column 2. Carbon monoxide and methane. 7 ft X 4 in. o.d. in glass, filled with 80/100 mesh SiO,. Temperature 50°C. Carrier gas, helium at 60 ml/min flow rate. Katharometer and flame ionization detection. Hydrogen. Column as above, but with nitrogen as carrier gas at 60 ml/min flow rate. Column 3. Ethane and ethylene separated in this order. 9 ft X a in. o.d. in glass, filled with 30/60 mesh SiO,. Temperature 50°C. Carrier gas, helium at 250 ml/min flow rate. Katharometer detection. Column 4. Acetylene. 3 ft X 4 in. o.d. in glass, filled with 30/60 mesh SiO*. Temperature 50°C. Carrier gas, nitrogen at 50 ml/min flow rate. Flame ionization detection. Column 5. Nonpolar molecules. 9 ft x 4 in. o.d. in glass, filled with 100/l 20 mesh Celite with 10% loading of squalane. Temperature 50°C. Carrier gas, helium at 50 ml/min flow rate. Katharometer and flame ionization detection. In order to achieve maximum accuracy in the determination of the permanent gases silica gel columns of various lengths were used.

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TABLE I REPRODUCIBILITY

OF

RESULTSOF ACETONE PYROLYSIS

Peak height values Permanent gases

Unpyrolysed acetone 21 21 20 20 20 20 20 21 20

216 205 208 219 216 217 213 212 205

Mean 212.3 SD (%) 2.1

Test Compounds Acetone of ANALAR quality was distilled, dried over anhydrous sodium sulphate, and then redistilled. The sample was stored over 5 A molecular sieve material. Pure quality methanol, ethanol, and n-propanol were triple distilled and stored over 5 8, molecular sieve material. Their purity was checked by gas-liquid chromatography. RESULTS

Pyrolysis of Acetone at Various Temperatures REPRODUCIBILITY OF RESULTS

A series of nine injections of 10 ~1 of acetone were carried out at a pyrolysis temperature of 750°C using column 1 with katharometer detection at an attenuation of 500 and bridge current of 100 mA. The peak heights for the combined permanent gases and residual acetone are given in Table 1. The standard deviation was 2.1% showing that the pyrolysis pattern was reproducible. PYROLYSIS OF ACETONE

An investigation of the effect of temperature on the pyrolysis of acetone was carried out over the range of lOO-1000°C. In this series, a 10 ~1 sample of acetone was injected at selected temperatures with each column system to obtain the different volatile pyrolysis products. Series 1. The peaks found with column 1 and katharometer detec-

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CAPELIN,

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AND KOKOLIS

FIG. 3. Decomposition of acetone on pyrolysis.

tion in order of elution were: (a) peaks caused by unresolved permanent gases, Hz, CO, CH, etc.; (b) a peak representing unpyrolyzed acetone; (c) a peak for benzene formed by pyrolysis. From these experiments Fig. 3 was produced, which shows the variation of the peak due to unpyrolyzed acetone against the temperature of pyrolysis of the constant 10 ~1 sample of acetone. The destruction of acetone appeared to commence at 650°C and to be complete at 850°C. However, the commencement of pyrolysis is best judged by detecting the breakdown products as shown in Fig. 4, in which is produced the curve for the production of the permanent gases. Using maximum katharometer sensitivity the earliest sign of these products was at 48O”C, although it was not until 550°C that sufficient was present for measurement. The fall off of the permanent gases could be due to their breakdown, at least of the methane, or to a completely different mechanism of acetone pyrolysis. Much tar and

FIG. 4. Production of permanent gases by acetone pyrolysis.

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FIG. 5. Production of benzene by acetone pyrolysis.

carbon was deposited on the walls of the pyrolysis tube at temperatures of 850°C and above. Figure 5 shows the production of benzene from acetone, against pyrolysis temperature. The peak identity was confirmed using column 5. Benzene detection with column 1 was just possible at 600°C. The maximum benzene production was at 850°C and at higher temperatures lower yields of the product were obtained presumably due to its decomposition. Series 2. The peaks found with column 2 with helium as the carrier gas were due to carbon monoxide and methane in this order of elution. The curves illustrated in Fig. 6 show the production of the gases from acetone with pyrolysis temperature, with maxima at between 850900°C. The production of both gases increased until this point was reached and then abruptly fell off. This change was accompanied by carbon deposition in the pyrolysis tube indicating that decomposition of some of the primary products was taking place.

FIG. 6. Production of: (A) methane and (B) carbon monoxide by acetone pyrolysis.

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INGRAM

AND KOKOLIS

FIG. 7. Production of hydrogen by acetone pyrolysis.

The fall off of carbon monoxide could not be accounted for. Carbon dioxide or water was not found in detectable amounts. There was no evidence, that at the high temperature, acetone was decomposing in a different manner giving stable oxygenated molecules other than carbon monoxide. However, it has been shown that carbon monoxide may form complexes with active carbon in which oxygen is attached to the carbon as a surface complex (2). Series 3. Hydrogen was detected and its production followed using column 2 with nitrogen as the carrier gas. Carbon monoxide and methane were eluted after hydrogen, but at a low sensitivity. The curve for the production of hydrogen (Fig. 7) shows that the concentration increased rapidly to 83O”C, and then even more rapidly. Series 4. Examination of the pyrolysates using column 3 gave four peaks. Carbon monoxide and methane were eluted first, poorly resolved, followed by ethane and ethylene. Figure 8 records the pro-

FIG. 8. Production of: (A) ethylene and (B) ethane by acetone pyrolysis.

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19) t 8*) r i t ? E H

IA

FIG. 9. Production of acetylene by acetone pyrolysis.

duction of the two products. The maximum concentration of ethane occurred at 75O”C, and ethylene occurred at 800°C. Series 5. Acetylene production was followed using column 4. The results are given in Fig. 9 and show that maximum acetylene production occurred at 850°C. EFFECT OF SAMPLE SIZE AT FIXED PYROLYSIS TEMPERATURES

The effect of sample size on the system (reactor and column) was investigated using from l-10 ~1 of acetone at selected temperatures. The pyrolysates were analyzed using column 1. Figures lo-12 show the variation of responses of the peaks for benzene, residual acetone, and the permanent gases,respectively, with the sample size of acetone employed. Experiments were also carried out using similar amounts of acetone injected on to the column. The results indicated that above a sample size of 5 ~1 nonlinearity occurred due to overloading of the

Fro. 10. Dependence of benzene production from acetone pyrolysis on sample size. Temperature of pyrolysis was 1000°C.

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CAPELIN, INGRAM AND KOKOLIS

FIG. 11. Dependence of acetone pyrolysis on sample size. Pyrolysis at: (A) 700°C; (9) 650°C.

column and not to overloading of the pyrolyser. Smaller samples, e.g., 1 ~1, are preferable in pyrolysis studies employing gas chromatography for quantitative determination of the pyrolysis products. However, when searching for trace quantities such as water, it may be necessary to increase the amount of sample. EFFECT OF PYROLYSIS TEMPERATURE ON COLUMN RETENTION TIMES

It was observed that as the temperature of the pyrolysis tube was increased the retention time of each of the products separated on a column was affected. This effect is illustrated in the curves (Fig. 13) which show the change in retention time with rise in the pyrolysis temperature. The measurements were obtained from the pyrograms of the ethanol pyrolysis experiments (Table 3), in which silica gel

FIG. 12. Dependence of permanent gas evolution from acetone pyrolysis on sample size. Pyrolysis at (A) 700°C; (B) 600°C.

IDENTIFICATION

OF ORGANIC MOLECULES.

24 500

700

900 ‘C

1100

11

239

145mLi ‘C

FIG. 13. Effect of pyrolysis temperature on column retention time. t, for production of carbon monoxide from: (a) ethanol; (b) methane; (c) ethylene; (d) ethane.

columns were used for the analysis of the pyrolysates. Comparison of the curves with their appropriate product concentration values in Table 3 show that the magnitude and direction of the change is dependent on the initial value of the retention time and on the concentration of the product. Pyrolysis

of Some Alcohols

at Various Temperatures

Samples of methanol, ethanol, and n-propanol were pyrolyzed over the temperature range of 300-l 100°C. The volatile products were analyzed by the various chromatographic systems described. Mechanisms for the thermal degradation of these alcohols are to be presented in a future communication. Methanol

Carbon monoxide and hydrogen were the major products, very small amounts of methane and water were also produced together with trace quantities of acetylene and ethylene. The results for the gradual degradation of methanol are given in Table 2 showing the formation of carbon monoxide, hydrogen and, methane over the temperature range examined.

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CAPELIN,

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KOKOLIS

TABLE 2 PRODUCTION OF DEGRADATION PRODUCTS AT VARIOUS TEMPERATURES OF 34.65 % IO-# MOL OF METHANOL T(T) 520 600 630 650 660 680 700 720 730 750 760 770 790 800 825 830 850 870 880 900 920 970 IO50

MeOH

X IO* mol 34.65 34.65 33.95 33.7

HP X 10e6 mol

CO X 10ee mol

CH, X lO-B mol

Tra Tr

30.52 28.4 26.24 22.55

I .94

Tr 1.14

Tr 0.04

3.01 5.45

0.076 0. I

21.13 24.9

0.253 0.27

26.73

0.403

31.19

0.558

4.6

14.95 17.63 14.35 9.43 3.28 Tr X”

25.3 45.1 46.7 46.7 49.1 51.2

n Tr = trace amount. b X = none detected.

Ethanol Carbon monoxide, acetaldehyde, methane, ethylene, and hydrogen were the main products. Small amounts of water, ethane, acetylene, and benzene were also produced. The results for the gradual degradation of ethanol are given in Table 3 together with the amounts of carbon monoxide, methane, acetaldehyde, ethylene, ethane, and hydrogen. A quantitative estimation of the aldehyde was not attempted owing to the difficulty of preparing a satisfactory calibration curve with the very volatile aldehyde. n-Propanol The results for the gradual degradation of n-propanol are given in Table 4. Carbon monoxide, methane, and hydrogen were the main products identified. Ethylene, acetaldehyde, ethane, acetylene, and

IDENTIFICATION

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11

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PRODUCTION OF DEGRADATION PRODUCTS AT VARIOUS TEMPERATURES OF 24.01 X 10e6 MOL OF ETHANOL

T(T)

EtOH x 10-M

460 500 540 550 560 580 600 610 620 650

24.0 I 24.0 23.2

660 700 710 720 730 750 760 770 790 800

18.34 12.18 11.06

810 825 830 850 900 920 940 950 1000 1030 IO50

0.14 Tr X” X

21.98

HI x 10-M

CO x IO-EM

CH, x lo-~&f

CHFH, lo-6M

x

CH,CH, x IO-” M

Tr”

0.18 0.29 0.55 0.92 1.32 2.02 3.82 4.78

CH&HO P.A. cm*

Tr

Tr

Tr

0.02

0.36

0.25

0.1

3.16

3.4

I.14

0.14

3.56 8.47

4.2 10.33

1.25 2.32

0.16 0.44

14.82

15.28

3.67

0.55

16.6

16.55

3.86

0.44

I.8

7.5 10.2 9.3

I I.31 8.4 3.78 3.36

9.0 5.4 5.1

13.82 I.8

I.12 14.99 15.14 16.5 16.4

16.30 15.45

3.93 3.21

0.22 Tr

16.35

14.97

2.0

Tr

15.8

II.55

1.0

Tr

Tr Tr Tr X

IS.81 21.61 23.96 25.14 26.16

a Tr = trace amount. * X = none detected.

water were pr duced in smaller quantities, and carbon dioxide and benzene were %so detected. Small concentrations of water were not easily separated and measured using the PEG 600 column. At least 5 ~1 of the alcohol had to be taken to obtain a measurable quantity of water. This suggested that either the amount of water formed was small, or that water in small concentrations was being retained within the system prior to the column.

242

CAPELIN,

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TABLE 4 PRODUCTION OF DEGRADATION PRODUCTS AT VARIOUS TEMPERATURES OF 18.74 X to-' MOL OF WPROPANOL

T(“C)

450 480 500 530 570 580 600 620 630 650 660 700 725 750 760 800 825 850 900 920 970 1030 1050

rrPrOH X IO+ M

IO-"M

18.74 18.72 18.7 18.74 18.31

Tr”

17.45

0.15 0.22

16.82 II.41

2.29 0.49

H, X

co

x

10-BM

CH, x

CH,CH,

IO-+ M

10-BM

CH&H3

x

IO-" M

Tr

Tr

0.34 0.79

3.08

0.49 0.89 2.47

1.7 2.6 5.22

3.97 6.25

5.93 7.51

6.61 9.19 12.49

0.51 0.88

x

Tr

0.64 1.28

Tr Tr

8.6 10.3

3.43

0.42

8.57

12.68

4.71

0.42

9.7

14.03

4.71

0.42

11.4

16.07

4.07

Tr

11.4

12.9

0.86

X

Tr

Xb

14.04 23.27

blTr = trace amount. b X = none detected.

DISCUSSION

The Contact Time The important variables in any pyrolysis system are: (i) the reactor temperature, (ii) the sample size, and (iii) the flow rate of the carrier gas. One weakness of the apparatus used in this study, which also applies in any other form of pyrolysis system, is that the temperature of pyrolysis affects the carrier gas flow in the reactor. Under a constant column temperature and inlet pressure of carrier gas, the flow rate measured at the column outlet remained constant, even when the temperature of the reactor varied between lOO-1000°C. On entering the pyrolysis tube from the supply line, the carrier gas (helium or nitrogen) expands owing to the high temperature of the reactor. On leaving the tube and passing into the cooler chromato-

IDENTIFICATION

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graphic column the gases must contract in volume. Hence a situation, which is difficult to visualize occurs; the flow rate into the pyrolysis tube is constant, and that through the chromatographic column is constant, but owing to the heat expansion of the gases in the reactor a different flow rate prevails which is temperature dependent. We have calculated the contact time within our system as follows: If V, is the carrier gas flow rate measured at the column exit, V, is the carrier gas flow rate in the reactor, T, is the column oven temperature, T, is the reactor temperature. From Charles law of gas volumes then,

V, = VcT,lT,.

(1)

But T, is constant in any series of pyrolysis (the chromatographic column temperatures are kept constant), V, is constant in any series, therefore,

V, 0~T,.

(2)

The effect of the higher gas flow rate at higher temperatures is to reduce the time passed by the sample to be pyrolyzed in the hot zone of the reactor. The gas velocity V, in the reactor is given by: Y-“=

flow rate in tube cross-sectional area of tube

z-2V lrr2'

(3)

From Eqs. (1) and (3) we see y =$

X $ cmlsec. c

(4)

Substitution of some typical values in Eq. (4) gives r = 0.5 cm (internal radius of the pyrolysis tube),

V, = 50 ml/min, T,=50"C (+273), 50

*'-?'-= 6Od.25

X

T, cmlsec,

323

T= O.O00328T,cmlsec.

(5)

Substitution in Eq. (5) for T, gives with reactor at 500°C + 273”:

Y = 2.54 cmlsec.

(6)

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CAPELIN,

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KOKOLIS

with reactor at 1000°C + 273”: Y = 4.18 cmlsec.

(7) The contact time (t) is the time that the sample is in contact with the hot zone of the reactor and is given by Eq. (8): t

=

t+.

length of heated zone of tube velocity of carrier gas in reactor’

(8)

(9)

For our apparatus 1 = 15 cm, from Eqs. (6) and (7) with (V) we get at 5OO”C, t = 5.9 set and at 1OOO”C,t = 3.6 sec. Our apparatus was initially designed to give contact times of between 3-6 sec., and the dimensions of the pyrolysis tube were calculated assuming a carrier gas flow rate of 50 ml/min in the columns. In some of the experiments higher gas flows than 50 ml/min were necessary in order to obtain good chromatographic conditions in the columns. Hence, lower contact times were produced in these particular experiments. Such a difference might effect the results, and is now considered using benzene pyrolyses as an example. From Fig. 5, depicting the production of benzene from acetone there is a drop in benzene production at 850°C. This could be caused by one of two factors: (i) Above 850°C the gas velocity in the pyrolysis tube is higher, so the contact time of the acetone sample is lower and benzene is formed in smaller quantities, or (ii), above 85O”C, the product benzene is itself decomposed, and there is a drop in its concentration, although more acetone in fact may be pyrolyzed. Since we found in separate pyrolysis of benzene that it begins to decompose at about 800°C giving hydrogen, explanation (ii) is favored. The fact that more, and not less benzene is decomposed at 850°C and above, even though the contact time was becoming smaller due to the higher pyrolysis temperature, substantiates explanation (ii). The occurrence of a pressure change due to the expansion in the pyrolysis tube of gaseous products from the decomposed sample was evident from the pyrograms. Retention times of the products decreased slightly with increased temperatures of the hot zone. Figure 13 shows plots of retention times of the various products obtained from the pyrolysis of ethanol at temperatures between 560-1050°C. Retention times of samples injected directly into a column for identification of pyrolysis products or for calibration purposes also differed slightly from their counterpart in pyrolysates. Retention times on column injection were lower than those obtained from pyrolysis tube injection. Pyrolysate components also gave slightly different values than their

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TABLE 5 RELATIVE RETENTION VALUES OF WATER PEG 600 COLUMN SEPARATION Condition

tR(cm)

0.5 111water through column at 50°C 0.5 4 water through pyrolyser at 760°C Water from pyrolysis of I fi ethanol at 760°C

9.1, 9.2 12.7. 12.8 10.5, 10.6

counterpart injected into columns. Retention values given in Table 5 for the identification of water serve to illustrate this anomaly. Most pyrolysis systems appear to have some weakness and a compromise is always necessary. Nevertheless, the anomalies found do not appear to affect the comparability of one series of our experiments with another. Accuracy and Precision Our study was carried out in the most quantitative manner possible, but the accuracy could not be expected to exceed -+ 10% on absolute figures. The calibration conditions for the chromatograph in the case of gases, could not be exactly the same as for the samples. The gases were released directly into the column head from the gas sampling valve or gas syringe for calibration; the gases were liberated from the reaction zone for the samples. Owing to the lower dead volume during calibration the results obtained from sample pyrolysis tend to be low. Table 6 shows the maximum output of the various pyrolysis products from 10 ~1 of acetone and gives the temperature at which these maxima appear. For hydrogen there was no leveling off and hydrogen continued MAXIMUM Product

HZb co CH, CHiCH CH,CH, CHZCHS W-b co* H,O

TABLE 6 OUTPUT OF PRODUCTS FROM ACETONE”

Maximum

output X IO-* mol

Temp of maximum output

1.5 I 1.14 0.85 0.23 0.12 0.09 0.04 Not detected Not detected

a 1.36 X 10e4 mol of acetone. b H2 production continued to increase with temperature highest calibrated determination.

970” 850 850 850 800 750 850

of pyrolysis,

this was the

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AND

KOKOLIS

7

PYROLYSISPRODUCTSFROM ACETONE" Product

H* co CH, CHiCH CH,CH, CHJIHS GHB

Output

X 1O-4 moi 0.54 1.14 0.85 0.23 0. I 0.07 0.04

a From 1.36 X IO-’ mol at 850°C.

to increase with temperature of pyrolysis in the range investigated. Table 7 shows a list of products and their quantities from the pyrolysis of 10 ~1 of acetone at 850°C. Each of the graphs show katharometer or flame ionization detector response to the products from the pyrolyses. The responses were not quite linear with product quantity. The precision of any single pyrolysis was examined in the experimental work and the standard deviation found to be of the order of 2%. A Mechanistic Interpretation of the Pyrolytic Decomposition of Acetone From the results in Tables 6 and 7 it can be seen that the major gaseous products are hydrogen, carbon monoxide, and methane. At 850°C most of the carbonyl group of the acetone becomes carbon monoxide. Any deficit could be explained by its incorporation as polyketenes in the residue of the pyrolysis tube, since no carbon dioxide or water could be detected in the pyrolysates. The results also indicate that only one of the methyl groups of each molecule gives methane, and the other group gives products such as hydrogen, ethane, ethylene, acetylene, and benzene. The pyrolysis of acetone has received more attention than most organic molecules since it yields as a product, ketene, a useful gaseous acetylation intermediate. The production of ketene from acetone was initially envisaged thus: CH,COCH3 -

CH-C=O

+ CH,.

Many investigators failed to point out that there is much ethylene and carbon monoxide in the product besides the methane. Hinshelwood (12) seems to be one of the few previous workers to find hydrogen in the pyrolysate gas mixture. Hinshelwood in making one of the first

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attempts to study the physical chemistry of acetone pyrolysis, found that the decomposition of acetone was a unimolecular homogeneous reaction. Carbon monoxide, methane, hydrogen, and ethylene were the products obtained. No report of ketene yield was mentioned, presumably since no ketene remained under the pyrolysis conditions used. We also could not detect ketene in the pyrolysates examined. A mechanism of acetone pyrolysis was first suggested by Rice (18) in 1934, who concluded that a free-radical mechanism was involved. Later, Rice and Walters (29) in 1941 put forward a mechanism for the pyrolysis of acetone at 526”, thus, CH,COCH,

-

CH,COCH, + eH, CH,COCH,

-

kH, + CH3C0 -

eH, + CH,-C=O, CH,COeH,

+ CH‘,,

CH,=C=O

+ cH3,

2eH, + CO.

This mechanism was modified slightly by McNesby et al. (17) between 1953-1955 following their examination of the pyrolysis of acetone. These workers detected hydrogen in the pyrolysates at temperatures in excess of 510°C. Wolf and Rosie (21) in recent times made pyrolysis studies of acetone over a wide temperature range. They concluded that pyrolysis of the substance commenced at 500°C and was almost complete at 850°C. The present studies have been made up to higher temperatures than those of previous workers, so that the mechanism of the pyrolysis is somewhat different to that proposed by Rice and Walters (19). Their mechanism is derived from studies made at temperatures of the order of 5OO”C, and does not explain the presence of hydrogen as a major product. The mechanism proposed from the present work involves initiation, propagation, and termination reactions as follows: CH,COCH,

-

CH,COeH, (1)

+ H

(10)

The radical (I) is stabilized by conjugation with the .carbonyl group. This does not occur in the case of the radical CHsCO which is the corresponding radical formed in the Rice-Walters mechanism. The initiation reaction (10) is followed by the propagation reactions: CH,COCH, + H CH,CO&H,

-

CH,CO&H,

+ Hz,

eH, + CH,=C=O,

(11) (12)

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CAPELIN,

CH,COCH,

INGRAM

+ CH, -

AND KOKOLIS

CH3COCH2 + CHI,

(13)

giving the main products, hydrogen, ketene, and methane. The termination reactions are CH, + CH, CH, + ti -

CH&HQ,

(14)

CH,.

(15)

Ketene itself pyrolyses, the initiation reaction being (16), giving carbon monoxide and carbene radicals: CHpC=O

-

EH, + CO.

(16)

Propagation of the reaction gives ethylene and carbon monoxide as the main products as EH, + CHFC=O

-

CH,CH, + CO.

(17)

Other minor products from acetone pyrolysis can be explained also. Acetylene can be formed as CHzCHz CH2CH -

CH$H

+ fi,

CHCH + ii,

(18)

(19)

and benzene may be formed by the following sequence: FH, + CHCH CH,CHCH

+ CH, -

CH,CHCH,

(20)

CH~CHCHCH~,

(21)

CHzCHCHCHz+CHzCHz-CBHB+H2.

c-9)

These reactions explain the formation of products as found in the experimental work. The fact that many of these products are found in maximum concentrations at certain fixed temperatures also indicated that these products also broke down, for example: (i) CH, (ii) CzHs (iii) C,H, (iv) 2C2H1 (v) 3C2H2 Pyrolytic Ethanol,

C + 2Hz, C2H, + HZ, CzHz + Hz, CH4 + 3C, CsHg.

Decomposition and n-Propanol

of Methanol,

METHANOL

From Table 2 it is seen that methanol began to degrade at 630°C with the evolution of hydrogen. This was followed at 700°C with the

IDENTIFICATION

OF ORGANIC

MOLECULES.

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production of trace amounts of carbon monoxide and methane. As the temperature was increased, carbon monoxide production increased, but methane concentrations remained low and only began to increase significantly after 760°C had been reached. At this point there was a significant drop in the concentration of residual methanol with a corresponding rise in the production of carbon monoxide. Methanol destruction was complete at 900°C yet carbon monoxide yields continued to increase until 1050”. At this temperature about 90% of the methanol’s oxygen was converted into carbon monoxide. At the higher temperatures trace amounts of acetylene were detected by the flame ionization detector. In addition, trace amounts of water and ethylene were also detected in the pyrolysates between 700-900°C. The results suggest that most of the alcohol suffers a dehydrogenation process rather than one involving dehydration. Since some methane was formed and water was observed, but not confirmed, and trace amounts of ethylene and acetylene were also produced, indicates that some methanol must degrade through the dehydration route. The two latter hydrocarbons are presumed to be formed from the dehydrogenation of methane with subsequent rearrangement of the free-radicals produced. ETHANOL

The results given in Table 3 show that ethanol began to degrade at 540°C with the production of hydrogen and a trace amount of acetaldehyde. This was followed at 560°C by the production of trace amounts of methane and ethylene. At 610°C carbon monoxide and ethane were produced in measurable amounts. Acetaldehyde continued to increase as the temperature was raised, and reached a maximum at 700°C at which temperature hydrogen production was considerable. Carbon monoxide and methane production is seen to have increased sharply at 750°C whereas, ethylene was only slowly increasing with the rise in temperature. At 800°C both carbon monoxide and methane had reached their maximum, at which time nearly all of the acetaldehyde was gone. At this point about 70% of the possible yield of carbon monoxide had been obtained. Between 850 and 1050°C both carbon monoxide and methane yields decreased somewhat as the temperature was increased. The results suggest that ethanol is degrading through the dehydrogenation route giving acetaldehyde and hydrogen, followed by the decomposition of the aldehyde into methane and carbon monoxide. Hence both yield the same concentration at their maximal. Water was observed at about 7OO”C, but was not determined due to poor resolution on the PEG 600 column. However, ethylene production indicated

250

CAPELIN,

INGRAM

AND

KOKOLIS

that dehydration was occurring. The presence of ethane, acetylene and benzene in the pyrolysates, the two latter arising in trace amounts at about 800°C and increasing with rise in temperature, are presumed to be the products of hydrocarbon decomposition and rearrangement. n-PROPANOL

The degradation of n-propanol (Table 4) commenced at 570°C with the production of trace amounts of hydrogen and methane. Carbon monoxide appeared as a trace quantity at 620°C and slowly increased in concentration until 750°C had been reached. At 800°C the amount of the product increased sharply to coincide with the almost total decomposition of the alcohol. At 850°C when all the n-propanol had gone, carbon monoxide production had reverted to a steady increase with each rise in temperature, and reached its maximum at 920°C. About 60% of the available oxygen was converted into carbon monoxide. The hydrogen production pattern up to 800°C is consistent with decomposition of the alcohol by the dehydrogenation route. Since some acetaldehyde was also detected, but not determined, in the pyrolysates, it seems that the aldehyde is the product of dehydrogenation. Carbon monoxide and the bulk of the methane come from decomposition of the acetaldehyde. The carbon monoxide yield suggests that about 40% of the n-propanol suffers decomposition through the dehydration route. Water production was not followed quantitatively, but a trace of the product was detected at 7OO”C,and found to increase in its concentration with rise in temperature. This rise coincided with the ethylene production pattern. From our results it is not clear how the dehydration route proceeds. Propylene should have been formed as the hydrocarbon product of the dehydration of the alcohol, but was not detected. The amount of carbon dioxide was small and not considered important enough to determine. The examination of this alcohol is continuing. CONCLUSIONS

The investigation has shown this technique capable of examining the pyrolysis pattern of simple organic molecules. It has, however, shown that a long time is required to examine one very simple molecule. The chromatographic analysis might be improved by incorporating simultaneous five column analysis with a splitting system to divide the pyrolysate sample. This would require a similar number of detectors and recorders. The search for more efficient and versatile columns is continuing, and the possibility of the development of a single column containing a mixed packing is being explored.

IDENTIFICATION

OF ORGANIC MOLECULES.

11

251

The investigation has also revealed that high-temperature pyrolysis combined with gas chromatography (or with mass spectrometry) can be employed to define the structure of simple molecules, and in particular, the class of functional group present. This investigation is only a small part of the whole field required for the development of high-temperature pyrolysis as a potential analytical tool for structural identification purposes. Many more simple organic compounds must be thoroughly investigated in order to obtain homologous series of behavior on pyrolysis. The results of our examination of the alcohols by no means represent their true patterns for mechanisms to be constructed. While it is clear that the degradation of a compound proceeds in a reproducible manner under a given set of conditions, more attention must be directed towards the mode of pyrolysis in order to achieve the best possible results. SUMMARY Results are presented of an investigation made to set up a system for the study of pyrolysis of organic compounds, and to determine some of the important conditions in this system. Acetone, methanol, ethanol, and rrpropanol were used as model compounds. Degradation of the compounds was studied quantitatively over a temperature range of 300-1200°C using flash vaporization into a heated reaction chamber. The volatile products were analyzed by gas chromatographic techniques. A mechanism of the thermal decomposition of acetone by its high-temperature pyrolysis is proposed. ACKNOWLEDGMENT We record our thanks to J. Knight, who prepared the diagrams. REFERENCES 1. Belcher, R., Ingram, G., and Majer, J. R., Direct determination of oxygen in organic materials. I. A study of the carbon reduction method. Talanra 16, 881892 (1969). 2. Belcher, R., Ingram, G., and Majer, J.R., The pyrolytic behavior of organic compounds in the determination of oxygen. Mikrochim. Acta 4 18-428 (1968). 3. Belcher, R., Ingram, G., and Majer, J. R., The pyrolytic identification of organic molecules. I. The pyrolytic behavior of organic molecules. Microchem. J. 19, 191-209 (1974). 4. Beroza, M., and Coad, R. A., “The Practice of Gas Chromatography” (L. S. Et& and A. Zlatkis, eds.), p. 473. Wiley (Interscience), New York, 1967. 5. Bumham, H. D., and Legate, C. E., Micro.pyrolytic-gas-chromatographic technique for analysis of organic phosphates and thiophosphates. Anal. Chem. 32, 1042-5 (1960). 6. Capelin, B. C., “An Analytical Study of the Tetracyanoplatinate(I1) Ion and the Pyrolytic Study of Acetone.” Ph.M. thesis, Portsmouth Polytechnic, 1970. 7. Cmmers, C. A. M. G., and Keulemans, A. I. M., Pyrolysis of volatile substances (Kinetics and product studies). J Gas Chromatogr. 5, 58-64 (1967). 8. Davison, W. H. T., Slaney, S., and Wragg, A. L., A method for the identification of PdYmerS by pyrolysis-gas chromatography. Chem. Ind. 1356 (1954). 9. Davison, V. C., and Dutton, H. J., Micro-reactor chromatography. Anal. C/rem. 32, 1302-5 (1966).

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KOKOLIS

IO. Dhont, J. H., Identification of aliphatic alcohols by pyrolysis. Analyst 89, 71-74 (1964). I I. Ettre, K., and Varadi, P. F., Pyrolysis-gas chromatographic technique. Anal. Chem. 35,69-73 (1963). 12. Hinshelwood, C. N., and Hutchison, W. K., A homogeneous unimolecular reaction. The thermal decomposition of acetone in the gaseous state. Proc. Roy. Sac., Ser. A 111, 245-57 (1926). 13. Ingram, G., Direct micro determination of oxygen in organic materials. PhD thesis. Univ. of Birmingham, 1967. 14. Keulemans, A. I. M., and Perry, S. G., “Gas Chromatography” (M. van Swaay, ed.), p. 356. Butterworth, London, 1962. 15. Kosters, B., Smith, G. G., and Wetzel, W. H., Rapid method of qualitative and quantitative analysis of products from pyrolysis. Analyst 86, 480-483 (1961). 16. Lehrle, R. S., and Robb, J. C., Direct examination of the degradation of high polymers by gas chromatography. Nature (London) 183, 167 1 (1959). 17. McNesby, J. R., and Gordon, A. S., Photolysis of acetone. J. Amer. Chem. Sot. 76, 1416-1418 (1954); The pyrolysis and photolysis of acetone. J. Amer. Chem. Sot. 76, 4196 (1954); 77, 4719 (1955). 18. Rice, F. O., Thermal decomposition of acetone. J. Amer. Chem. Sot. 56, 24972498 (1934). 19. Rice, F. O., and Walters, W. D., Thermal reactions promoted by biacetyl. J. Amer. Chem. Sot. 63, 1701-1706 (1941). 20. Szymanski, H., Salinas, C., and Kwitowski, P., Technique for pyrolysing or vaporizing samples for gas chromatography. Nature (London) 188,403-404 (1960). 21. Wolf, T., and Rosie, M., The pyrolysis-gas chromatography of simple molecules. Anal. Chem. 39, 725-729 (1967).