Analysis of Vapor-Phase Pyrolysis Products of the Four Trimethylpentane Isomers J. Q. Walker McDonnell Douglas Research Laboratories, McDonnell Douglas Corp., St. Louis, Mo. 63166 J. B. Maynard Research Laboratory, Shell Oil Co., Wood River, IN. 62095 High-resolution capillary gas-liquid chromatography has been employed to determine the individual C1-Cs vapor phase pyrolysis products of the four isomers of trimethylpentane (TMP)-Le., 2,2,3-TMP, 2,3,3-TMP, 2,3,4-TMP, and 2,2,4-TMP. The pyrolyses were conducted in helium at 680 “C in a gold-tube reactor. The amount of parent compound degraded was less than 10 wt %, minimizing side reactions. Comparison of empirical data with the products predicted by classical theory was good. A linear relationship between the per cent weight parent molecule degraded in the pyrolysis chamber and the Research Octane Number has been established. In addition, pyrolysis products of 2,2,4-TMP i n an environment of 20 vol % O2and 80 vol % N P have been found to be identical with those formed from oxidative degradation of 2,2,4-TMP in a nonfiring, motored engine, suggesting that such pyrolyses might be used to study the blending octanequality of various hydrocarbons.
VAPOR-PHASE PYROLYSIS gas chromatography is a technique that has proved useful in the identification of individual hydrocarbons when only small amounts of sample are available. Previous gold-tube, vapor-phase pyrolyses of 83 C,-Clo hydrocarbons have shown that different pyrolysis patterns were obtained for alkane, alkene, and alkyne hydrocarbons examined except the cis and trans olefin isomers ( I ) , and in general, the technique has been useful for identification of types of compounds (2-4). Keulemans and Perry (5) and Cramers (6) investigated the rates and mechanisms of the gas-phase thermal decomposition of hydrocarbons to determine the experimental parameters necessary to use vapor-phase pyrolysis gas chromatography as a hydrocarbon identification method. Wolf and Rosie (7) have also studied the effect of temperature on the vapor-phase pyrolysis of 20 simple organic molecules. The work described in this paper is a study of the individual pyrolysis products of four Cs hydrocarbon isomers. Identification of the individual reaction products was facilitated by use of a high-resolution gas chromatographic system similar to that described previously (8). Application of this chro( I ) D. L. Fanter, J. Q. Walker, and C. J. Wolf, ANAL.CHEM., 40
2168 (1968). (2) J. Q. Walker and C. J. Wolf, ibid.,p 711. (3) D. L. Fanter and C. J. Wolf, “Vapor Phase Pyrolysis of Amines, Furans and Nitriles,” 158th National Meeting, American Chemical Society, New York, N.Y., September 1969. (4) W. D. Dencker and C. J. Wolf, J. Chromarogr. Sci., 8 , 534 (1970). ( 5 ) A. 1. M . Keiilemans and S. G. Perry, “Gas Chromatography, 1962,” M. VanSwaay, Ed., Butterworths, Washington, D.C., 1962, p 356. (6) C. A. M. G. Cramers, “Some Problems Encountered in High Resolution Gas Chromatography Pyrolysis,” Thesis, Technological University, Eindhoven, The Netherlands, 1967. (7) T. Wolf and D. M. Rosie, ANAL.CHEM., 39,725 (1967). (8) W. N. Sanders and J. B. Maynard, ibid.,40, 527 (1968). 1548
0
matographic system has allowed us to identify the individual pyrolysis products from each of the four isomers. The hydrocarbons chosen for this study were the four isomers of trimethylpentane (TMP)-Le., 2,2,3-TMP, 2,3,3TMP, 2,3,4-TMP, and 2,2,4-TMP. These particular comof a typical premium pounds account for as much as 10 wt gasoline. There is a 20-fold greater amount of these four TMP isomers in typical premium gasolines than in regular gasolines (8). With the increased blending of low-lead and nonleaded gasolines, these TMP isomers may comprise a much greater percentage of future fuels, especially if a ceiling is placed on the aromatic content of gasolines. Thus, a study of the thermal decomposition products of these TMP isomers may be a useful technique for determining their contribution to the octane quality of low-lead and nonleaded gasolines. The pyrolyses of two of the isomers reported in this study (2,3,4-TMP and 2,2,4-TMP) were reported previously by Doue and Guiochon (9). However, their studies were carried out at 443 OC and 8 atmospheres compared to our work at 680 OC and essentially atmospheric pressure. As a result, significant differences in the product distributions for these two isomers are observed. EXPERIMENTAL
Apparatus. The gas chromatographic-pyrolysis system used was similar to that used by Fanter et al. ( I ) , except for the use of high-resolution open-tubular columns and a precolumn inlet sample splitter. A block diagram of this system is shown in Figure 1. The system consists of a flowcontrolled, carrier-gas supply, gas sample valve, injector, vaporizer, variable temperature pyrolyzer, heated column splitter, temperature-programmed gas chromatograph with flame ionization detector, recorder, and digital integrator. Pure liquid TMP samples (99.5 wt) were introduced through a silicone septum with a syringe (Hamilton Model CR-700-20). The sample size was 0.6 111. A gas sample valve was used to introduce calibration gas blends into the system. The vaporizer was constructed of 92-cm by 0.074-cm i.d. stainless steel tubing. The pyrolyzer consisted of 5.1-cm by 0.102-cm i.d. gold tube wound in a 1.27-cm diameter helix. The pyrolyzer temperature was controlled to 5 “C. The carrier gas flow-rate through the pyrolyzer was 60 cm3/min. Since the optimum flow-rate through our capillary column system was about 2 cm3/min, the pyrolyzer effluent was split 30:l prior to the column. The sample residence time in the pyrolyzer was 1.6 sec. The pyrolyzer transfer lines and the column inlet splitter (Hoke No. 1345 G2Y) were heated to 225 “C. The analytical column consisted of a multidiameter arrangement on the order of that described previously by Walker (IO). A 15.5-m by 0.051-em i.d. squalane SCOT column 9
*
(9) F. Doue and G. Guiochon, J. Phys. Chem., 73, 2804 (1969). (IO) J. Q.Walker, ANAL.C H E M . ,226(1968). ~~,
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
,
Vaporizer
h-[
, ,
ZOO'C
Pyrolyzer
ZOO'
- IOOOOC
,
I Regulator
flowmeter
Sulitter to M.S. or vent
1 I i
it!
Carrier e
Integrator
I
Recorder
1 I
I I
_
-
I ,J
F. I.O_
w
Gas Chromatograph ~
Figure 1. Pyrolysis-gas chromatographic system. The dashed line indicates the heated carrier gas transfer lines (Perkin-Elmer Corp.) was used in series with a 62.0-m by 0.025-cm i.d. column which was coated with 8 wt % squalane in pentane using the dynamic method described by Sanders and Maynard (8). This two-diameter column system allows enhanced resolution of normally difficult separations for capillary columns such as methane, ethylene from ethane, and propylene from propane. The 62.0-111 squalane column was wound on a mandrel with each layer of tubing separated by about 2 mm to allow adequate circulation of oven air around all parts of the column, and thus, produce more uniform heating. A gas chromatograph (Hewlett-Packard Model 5750) with a flame ionization detector (FID) was used in all experiments. The FID was maintained at 250 "C. The column oven temperature was held isothermally at 25 "C for 20 min and then heated to 90 "C at 2 "C/min. Using the multidiameter column arrangement under these conditions, essentially all of the CI-Cs hydrocarbons were separated at 25 "C, eliminating the need for column operation at subambient temperatures or packed columns to separate these materials. Since all pyrolyses used in quantitative structure calculations were done in helium, the possible effects of small amounts of 0 2 and Nzeluting with CHI and possibly affecting the performance of the FID were minimal. Calibration. A highly-olefinic gasoline sample was used for column calibration of the C5-Ca olefin products. A qualitative synthetic mixture of C1-CS paraffins and olefins (consisting of methane, ethene, and Phillips Petroleum Co. Mixture No. 4&4% ethane, 17% propene, 16% propane, 19 % 2-methylpropane, 14% 2methylpropene, 9 1-butene, 8 % n-butane, 8 % trans-2-butene, 3 % cis-2-butene, 1 2methylbutane, 1 1-pentene, and 1 n-pentane) was used for light hydrocarbon calibration. The sensitivity of the flame detector for each product was calculated using the method described by Dietz (11). The compounds studied were obtained from Chemical Samples Co., Columbus, Ohio. The purity of each sample was determined by decreasing the pyrolyzer temperature to 200 "C and injecting (1 1) W. H. Dietz, J. Gas Chromatogr., 5,68 (1967).
an aliquot of the material into the system. The purity of all compounds was equal to or greater than the manufacturer's stated value of 99.5 wt %. The area and elution time of each product peak was measured and the time recorded with a digital integrator (Infotronics CRS-100). The area of each peak was converted into moles and the moles were normalized to 100 excluding the parent material. With the pyrolysis system at 680 "C, about 2 to 7 wt of the parent compounds were thermally fragmented. This low decomposition percentage minimized secondary reactions. Helium was used as the carrier gas for pyrolysis of each TMP isomer. In addition, a mixture of 20 vol oxygen and 80 vol nitrogen was used as the carrier gas for pyrolysis of 2,2,4-TMP.
x
RESULTS AND DISCUSSION Pyrolysis Patterns. The olefinic pyrolysis products obtained by the vapor-phase pyrolysis of the four TMP isomers at 680 "C are shown in Table I. Inspection of the data in Table I shows considerable differences in the product distributions for the four isomers. In instances where both ethene and ethane as well as propene and propane were observed as pyrolysis products, the amounts of ethane and propane were so small they were added to the ethene and propene peaks for the purpose of calculations. In the case of 2,2,3-TMP, the major pyrolysis products are methane and 2-methylpropene (27.1 and 32.2 mol %, respectively). The 2,3,3-TMP isomer shows propene as the major component (21.1 mol %), while the 2,3,4-TMP isomer has propene (30.2 mol %) and 2-methyl-2-butene (22.5 mol %) as the major products. The 2,2,4-TMP isomer has only two major pyrolysis products, 2-methylpropene (59.1 mol %) and methane (19.0 mol %). Although butene-1 and 1,3-butadiene are not separated from 2-methylpropene under our chromatographic conditions, the production of these materials by the thermal degradation mechanisms considered is remote, and thus, in most cases, this peak is considered to be only 2-methylpropene.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
1549
Table I. Thermal Fragmentation Patternso from the Vapor-Phase Pyrolysis of Different Trimethylpentanes in Helium Peak Component 2,2,ETMP No. 2,3,3-TMP 2,3,4-TMP 2,2,4-TMP BP, "C Methane -161.5 1 14.7 27.1 16.2 19.0 Ethene -103.7 2 2.9 13.6 ... ... ethane - 88.6 Propene 3 - 47.7 9.7 21.2 30.2 8.5 propane - 42.1 - 11.7 2-Methylpropane 4 ... ... 1.o 1.8 - 6.9 2-Methylpropene 5 32'. 2 9.5 59.1 - 6.3 1-butene 1,Ebutadiene - 4.4 trans-2-Butene 6 0.9 7.6 3.6 ... 7 3.7 cis-2-Butene 7.3 ... 1.8 20.1 3-Methyl-1-butene 8 2.5 ... 2-Methyl- 1-butene 9 31.2 13.0 ... ... 2-Methyl-2-butene 38.6 ... 10 22.5 ... 13.7 11 3,3-Dimethyl-1-butene 41.2 2.3 *.. ... 12 73.2 2J-Dimethy1-2-butene ... 10.9 ... 13 72.5 4,4-Dimethyl-l-pentene ... 0.7 4,CDimethyl-truns-2-pen14 76.7 ... ... ... 4.2 tene 15 77.9 2,3,3-Trimethyl-l-butene 8.1 ... ... 16 80.4 4,4-Dimethyl-cis-2-pntene ... ... 1.3 ... 2,4-Dimethyl-2-pentene 17 83.3 ... Trace 4.8 3,4-Dimet hyl-cis-Zpentene 18 87.9 Trace Trace ... 1.3 90.5 19 3,4-Dimethyl-trans-2-pen... 6.0 tene 20 97.4 2,3-Dimethyl-2-pentene 3.4 4.1 1.8 0.2 21 101.4 ... ... ... 2,4,4Trimethyl- 1-pentene 22 104.9 2,4,4-Trimethyl-2-pentene ... ... 0.4 23 116.3 2,3,CTrimethyLZpentene ... ... 11.8 Total 100.0 100.0 100.0 100.0 Normalized mol %.
+ +
+
0
+
Table 11. Repeatability of Thermal Fragmentation Patterns from 680 "C Pyrolysis pentane in Helium (Normalized mol %) RunNo. 1 2 3 4 5 6 7 8 Component Methane 19.2 19.0 19.1 18.8 18.9 19.0 18.9 19.2 Propene & propane 8.6 8.5 8.6 8.4 8.5 8.4 8.5 8.5 2-Methylpropane 1.8 1.7 1.7 1.8 1.8 1.8 1.7 1.7 2-Methylpropene 1Butene 1,fButadiene 58.9 59.3 58.5 59.7 59.3 59.1 59.4 58.7 0.5 0.8 0.7 0.7 0.8 0.6 0.8 4J-Dimethyl-l-pentene 0.7 4,4-Dimethyl-trans-2-pentene 4.3 4.2 4.6 4.1 4.0 4.0 4.2 4.3 4,4-Dimethyl-cis-2-pentene 1.3 1.4 1.3 1.3 1.3 1.3 1.3 1.4 4.7 2,4-Dimethyl-2-pentene 4.8 4.8 4.6 4.9 5.0 4.6 4.9 0.2 0.2 2,4,4-Trimethyl-Zpentene 0.2 0.2 0.1 0.2 0.2 0.2 0.5 0.4 2,4,4-Trimethyl-1-pentene 0.4 0.5 0.4 0.4 0.3 0.4 Total mol 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Degradeda 6.5 6.6 6.7 6.5 6.5 6.6 6.7 6.6 % Degraded = Total area - area of 2,2,4-TMP x 100. Total area
+
+
of 2,2,4Trimethyl9
10
Av.
18.9 8.4 1.9
19.1 8.6 1.8
19.01 8.50 1.77
0.12 0.03
59.1 0.8 4.2 1.2 4.9 0.2 0.4 100.0 6.4
58.9 0.6 4.1
59.09 0.70 4.20 1.33 4.81 0.19
0.11
1.5
4.9 0.2 0.3 100.0 6.5
U
...
0.02 0.01
0.40
...
6.56
...
0.01
@
The difference in product distributions for the four TMP isomers is shown graphically in Figure 2. Here the data are presented in bargraph form showing the relative mole per cent as a function of carbon number. The numbered bars in Figure 2 refer to the individual hydrocarbon pyrolysis products and are identified by number in Table I. Repeatability of Pyrolysis Data. The thermal fragmentation patterns observed from 10 separate pyrolysis determinations of 2,2,4-TMP in helium carrier gas a t 680 "C are shown in Table 11. These data were obtained over a period of 10 days. The repeatability of the normalized molar areas is excellent. The average value of the 10 determinations and the standard deviation (a) are shown in the two columns at the right. The last line of Table I1 shows the relative per cent 1550
wt of the 2,2,4-TMP that decomposed for each determination. This relatively low per cent (average 6.56 wt %) decomposition of the parent molecule decreases further reactions between the initially formed pyrolysis products, and therefore reduces the formation of secondary products in the pyrolysis chamber. Theoretical Thermal Fragmentation Produc'ts. The major pyrolysis products as well as those of lower concentration can be predicted by the thermal pyrolysis mechanisms described by Kossiakoff and Rice (12). Their approach t o thermal decomposition assumes a chain-reaction, free-radical (12) A. Kossiakoff and F. 0.Rice, J. Amer. Chem. Soc., 65, 590 (1943).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
2, 2, 3-Trimethylpentane (TMP)
\
Peak number
number
40
t
I
2,3,3-TMP
10 8 Carbon
r 40 30
20 10
2,3,4-TMP
3
1
23
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112131
60 50
Peak number
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-
number
\
50
r
7
8
I
1
1
19 20 18 I
5
1
6
I
7
1
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I
Carbon number
\
40 30
Peak number
20 10
14 4
13
I
16
17 I
21 22
0
number
Figure 2. Experimental thermal fragmentation patterns from the vapor phase pyrolysis of different trimethylpentanes type of degradation. The larger free radicals which are formed decompose to alkenes and smaller free radicals. This theory assumes that radicals formed by hydrogen abstraction usually undergo C-C bond rupture beta to the radical site. Another possible reaction path would be abstraction of H atoms from neutral molecules to form saturated hydrocarbons. These and other assumptions of the Kossiakoff-Rice mechanism have been discussed in terms of vapor-phase pyrolysis GC by Fanter el al. (I). Further, the removal of a secondary hydrogen requires 2 kcal less than a primary hydrogen, and the abstraction of a tertiary hydrogen requires 4 kcal less than a primary (12). At our pyrolysis temperature of 680 OC, the relative rates of hydrogen abstraction from primary, secondary, or tertiary sites should be 1 :3 :8, respectively, compared with 1 :3.2 :10 determined previously for pyrolysis at 600 "C (1). Based on these assumptions, the probable mole per cent formation of each type of radical is calculated by multiplying the relative rate factor and the number of C-H bonds of that type present in the parent molecule. If there are two or more paths by which a given radical may degrade to an olefin and radical, equal amounts of these moieties
should be formed unless the resulting radical is a methyl free radical. In this case, reactions resulting in ethyl and larger radicals are favored over the reaction resulting in the methyl radical by a ratio of about 3 :1 according to the experimental data of Kossiakoff and Rice (12). Since few saturated hydrocarbons other than methane were observed as pyrolysis prodducts (only 2-methylpropane and small amounts of ethane and propane), the mechanism of hydrogen abstraction from neutral molecules t o form C2-Cs saturates is evidently of less significance than loss of a hydrogen or larger radical to form olefins. Free radical reaction mechanisms for the thermal degradation of the four T M P isomers are shown in Tables 111-VI. In each table, the theoretical and experimental values for each product are listed for comparison. Table I11 shows the mechanisms using the theoretical approach of Kossiakoff and Rice (12) to obtain the products formed from the 2,2,3-TMP isomer. The theoretical and experimental product distributions for the 2,3,3-TMP, 2,3,4-TMP, and 2,2,4-TMP isomers are shown in Tables IV-VI, respectively. This study indicates that the amount of the thermal frag-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
1551
Table III. Tbennal Jkgradatioa Route of 2,2,3-Trimethylpeotane
c c I I I c-c-c-c-c + c-c + c-d-c-c I1
I
c c
C
I C
I C
I
Mol Z Theoretical
Compound
primary C
Found
Isobutylene Propylene Methane
10,8 10.8 10.8
2,ZDimethyl-1-pentene Methane
2.7 2.7
...
3,ZDimethyl-1-butene
2.3
2.3
1-Butene
2.2
b
Isobutylene
2.2
b
+ C=C
Ethylene
4.5
2.9
c
2-Methyl-2-butene
4.5
...
Methane
4.5
...
cis and trans-2-Butene
7.2
14.9
+ .H
Isobutylene
7.2
+ *CHs
4,4-Dimethyl-2-pentene
1.8
...
Methane
1.8
. . .a
6.0
8.1
I
b
9.7
...
(I
I
I
C
+
I c-c. + c--c--c-c I c
c
1
L+C-c-C
c c I I I I c-c-c--c-c*+ c-c-c. I I
+ .H
c c
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c c I I c--c--c
