3 Similar Compositions of Alkanes from Coal, Petroleum, Natural Gas, and Fischer-Tropsch Product R. A. FRIEDEL and A. G. SHARKEY, JR.
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Bureau of Mines, U.S. Department of the Interior, Pittsburgh Coal Research Center, Pittsburgh, Pa.
Possible interrelationships of natural substances are important. Similarities of the low molecular weight alkane isomers from crude oil and Fischer -Tropsch synthesis product have been reported. A similar composition for high temperature coal car bonization has been found. The C to C 4
7
alkane
isomers from these sources can be calculated quan titatively with equations developed for Fischer -Tropsch products. A reversal of the concentrations of the monomethyl isomers from C (2 Me > 3 Me) 6
to C (3 Me > 2 Me) occurs in all three products; 7
comparisons at higher carbon numbers indicate some dissimilarities. Naphthene isomers for crude oil and high temperature coal carbonization also have similar compositions. Aliphatic hydrocarbons from low temperature coal processes are consid erably different. The C
13
isotopic composition of
pure compounds from the various sources are being compared in order to provide information on their origin.
^
similarity between the l o w molecular weight alkane isomers i n crude o i l and Fischer-Tropsch catalytic synthesis products has been reported previ ously (8), along with a discussion of the possible significance for the origin of petroleum. It was shown that the composition of the C 4 - C 7 alkane isomers i n crude oils (13) can be calculated quantitatively w i t h equations previously reported for calculating the alkane isomers i n Fischer-Tropsch products (1, 6, 21). T h e equation for calculating isomer distribution is θ = kFn a "\ where 0* is the number of moles containing η carbon atoms, k is a constant, a n d F« is a function of /, a constant representing the ratio of chain branching, b, over η
n
32 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
3.
FRIEDEL AND SHARKEY
Alkanes from Coal, Petroleum, and Gas
33
chain lengthening, a. If b = af, the lengthening and branching of chains is illustrated as follows: a a a" C C -> C C C -> C C C C -> C C C C C 2
and Ce alkanes were very similar to those obtained from thermal cracking of petroleum. These are data on only a few compounds and no dimethyl isomers were found. It is perhaps not surprising that a slight similarity exists since petroleum compounds i n high molecular weight ranges are primarily straight chained, apparently branched only on one end. In fact some researchers have found that the waxes from tobacco have similar structures (14). Also, the 2-methyl and 3-methyl isomers predominate alternately. Thermal cracking of higher components of 'crude oils can produce l o w molecular weight alkanes having isomeric compositions similar to the alkanes found i n crudes. Table III.
Alkane and Naphthene Compositions: Crude Oil and Coal Derivatives, Mole Percent
Alkanes n-Heptane 2-Methylhexane 3-Methylhexane 2,3-Dimethylpentane 2,4-Dimethylpentane 3-Ethylpentane 2,2-Dimethylpentane 3,3-Dimethylpentane 2,2,3-Trimethylbutane
Predicted, F.-T. equation f = 0.1
Crude No. 1 (13)
Coal, high temp. carb. (10)
Coal hydrogénation (7,12)
54.6 16.4 22.9 3.4 1.1 1.2 0.2 0.2 0.03
56.5 15.5 19.9 5.1 1.4 1.1 0.2 0.2 0.1
75.6 14.6 15.2 9.1
60
41.4 58.6
90.5 9.5
28.6 71.4
100.0
100.0
100.0
58.5 1.8 16.0 7.9
64.4 2.0 14.5 13.6 5.5
35.6
— — 0.5 —
40
3.0
J
Naphthenes Cyclohexane Methylcyclopentane
Methylcyclohexane 1-1 -Dimethylcyclopentane l-tram-2-Dimethyîcyclopentane l-irafw-3-Dimethylcyclopentane l-cw-2-Dimethylcyclopentane l-cw-3-Dimethylcyclopentane Ethylcyclopentane
7.9 7.9
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
47.8 16.6
37
COAL GEOCHEMISTRY
38
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Table IV.
Isomeric Analysis of a Natural Gas Condensate* Natural Gas
Crude No. I
n-Pentane i-Pentane 2,2-Dimethylpropane
65.5 35.5
65.8 33.8 0.4
n-Hexane 2- M ethy lpentane 3-Methylpentane 2,3-Dimethylbutane 2,2-Dimethylbutane
37.5 45.5 15.8 1.2
56.7 23.3 16.8 2.9 0.3
n-Heptane 2-Methylhexane 2,3-Dimethylpentane 3-Methylhexane 2,4-Dimethylpentane 2,2-Dimethylpentane 2,2,3-Trimethylbutane 3-Ethylpentane 3,3-Dimethylpentane
12.6
56.5
17.8 12.6
20.6 19.9
23.6
1.7
19.3 14.1
1.1 0.2 present present present present
CH Isomers Cylohexane Benzene Toluene
—
—
t
present present present present
* Institute of Gas Technology
Carbon-13 Isotope
Analyses
It is generally accepted that the lower alkanes i n petroleum and i n coal products are obtained from cleavage of bonds in higher components. O n the other hand, the lower alkanes i n crude oil may have originated from synthesis or from chemical reactions of nonhydrocarbons. In coal it is possible that the carbonaceous constituents may have small alkanes adsorbed on them and that these are desorbed at elevated temperatures. In this case scission of bonds w o u l d not be involved. It seems important then to attempt to find out whether or not low molecular weight alkanes are really derived from high molecular weight molecules by decomposition processes. One possible way of approachi n g this problem is to study the content of carbon-13 isotopes in nature. This figure is taken to be 1 . 1 % , and from isotopic work on petroleum it is known that i n nature this value varies i n small but measurable amounts (18). A l l such measurements have been for the purpose of making total carbon-13 isotopic determinations i n a compound w i t h one exception. T h e work of Stevenson (19, 20) and co-workers on small molecules has made it possible to determine the isotopic content of various carbon atoms in these molecules. Calculations involved for other than small molecules appear to be prohibitively complex. T h e problem has been approached empirically; a standard has been used w h i c h is known to be a molecule not derived from any bond scission, namely, n-hexane from Fischer-Tropsch synthesis. Fischer-Tropsch products provide the only source of molecules that could not have been formed by decomposition. A n y other hexane obtained from nature or from synthesis with smaller molecules may or may not have a history involving bond cleavage.
Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
3.
Alkanes from Coal, Petroleum, and 6 0 s
FRIEDEL AND SHARKEY
Preliminary measurements have been made on Phillip's n-hexane, k n o w n to be derived from natural gas, a n d comparisons have been made w i t h the Fischer-Tropsch standard, n-hexane. Both products were purified b y D r . B . D . Blaustein of the Pittsburgh C o a l Research Center b y means of gas chromatog raphy. It is known from the work of Stevenson that breakage of a carbon-12carbon-12 bond is more probable than breakage of a carbon-12-carbon-13 bond. This means that if the n-hexane from natural gas were formed b y a bond
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