Origin of long-chain alkylcyclohexanes and alkylbenzenes in a coal-bed wax

0016-7037/93/$6.00

Geochmica et Cosmmhimica Ada Vol. 57,, pp. 837-849 Copyright 0 1993 Pergamon Press Ltd. Pnnted in U.S.A.

+ NJ

Origin of long-chain alkylcyclohexanes and alkylbenzenes in a coal-bed wax JI-ZHOU DONG, WILLIAMP. VORKINK, and MILTON L. LEE* Department of Chemistry, Brigham Young Unive~ty, Provo, UT 84602, USA (Received October 16, 1991; acceptedin revisedform August2 1, 1992) Abstract-A coal-bed wax was fractionated and analyzed using capillary column GC and combined GC/ MS. It was found that the major components in the wax were n-alkanes (55.6%), cyclic /branched alkanes (2&O%), and several homologo~ series of alkylbenzenes (5.7%). All alkylbenzene isomers (except 6-n-alkyl-m-xylene) were positively identified by comparison with the retention times and mass spectra of newly synthesized authentic standards. 5-n-Alkyl-m-xylene, 2-n-alkyl-p-xylene, 4-n-alkyl-m-xylene, 4-n-alkyl-o-xylene, 2-n-alkyl-m-xylene, and 3-n-alkyl-o-xylene were identified for the first time from geological sources. All of these long-chain alkyl compounds (e.g., n-alkylcyclohexanes, n-alkylbenzenes, n-alkyl-o-toluenes, n-alkyl-p-toluenes, and 5-n-alkyl-m-xylenes) have similar total carbon number distributions and maxima with a slight even over odd carbon number preference between C&&. Moreover, the carbon number dis~butions of these compounds resembled those of the n-alkanes found in the same wax with slight odd over even carbon preference between C,,-C,, . This indicates that the ~kylcyclohexan~ and alkylbenzenes may have the same fatty acid precursors as the n-alkanes. The alkylcyclohexanes and alkylbenzenes could have been formed by direct cyclization and aromatization, while the n-alkanes could have been formed by decarboxylation of the straight chain fatty acids. This explanation is further supported by the identification of homologous series of tetramethyl-n-alkylbenzenes and pentamethyl-n-alkylbenzenes with relatively high abundances at C,s, C&, and Cl*, and a fatty acid distribution with maxima at Cl6 and Cis. Based on these findings, mechanisms for the conversion of fatty acids or alcohols to alkylcyclohexanes and alkylbenzenes are proposed. INTRODUCTION

tentative identifications of five alkylbenzene structural isomers ranging from C&-C& in the aromatic fraction of a hydropyrolysis coal liquid. Proton and 13CNMR was also used to confirm the presence of long-chain alkane moieties in the coal. Using similar techniques, GALLEGOS( 198 1) found homologous series of long-chain alkylbenzenes in the pyrolysis products of three coals. The same alkylbenzenes have been found in relatively high concentrations in crude oils from both West Texas Midland and Michigan Basin (WILLIAMS et al., 1988). Figure I summarizes the three general class structures of these compounds and their possible precursorproduct relationships. 1,1,3-Trimethyl-2-alkylcyclohexanes (Fig. 1, Ia) and trimethylbenzenes (Fig. I, Ib) have been found in crude oils and sediments ( SCHAEFLEet al., 197 7; OSTROUKHOVet al., 1983; HALL and DOUGLAS, 1983; POWELLet al., 1984; SUMMONSand POWELL, 1987). Due to their similar structures, they are thought to be derived from carotenoids. BYERSand ERDMAN ( 1983) have shown that low-temperature de8radation of &carotenoids results in the production of low-molecular-weight alkylbenzenes and alkylnaphthalenes. Carotenoids are pigments which are ubiquitous components ofthe biosphere, occurring in continental plants and in algae and bacteria ( LIAAEN-JENSEN,1978; LIAAEN-JENSEN et al., 1982; SUMMONSand POWELL,1987). These compounds have been found in ancient and recent sediments from a variety of different environments ( SCHWENDINGERand ERDMAN, 1963; ANDERSand ROBINSON,1971; JIANG and FOWLER, 1986), both lacustrine and marine. Most of them do not survive early diagenic defunctionalization in a recognizable form. @Carotane, however, is a distinct exception and has been detected in geological samples.

SINCEn-alkylcyclohexanes were discovered in crude oils and sediments in the late 1950s (MAIR et al., 1958; HQOD et al., 1959) and early 1960s (LEVY et al., 196 1; NAGY and GAGNON, 1961; JOHNS et al., 1966; HOEVENet al., 1966), there have been many reports concerning their separation and identification in geological samples such as coal. Their origin, however, is not well established ( RUBINSTEINand STRAUSZ, 1979; BASET et al., 1980; GALLEGOS, 1981; INGRAM et al., 1983; FOWLER et al., 1984, 1986; CONNAN et al., 1986; HOFFMANNet al., 1987; WILLIAMSet al., 1988). The analysis of n-alkylbenzenes in samples of geological origin, such as fossil fuels, has also received considerable attention. This is because these straight-chain substituted aromatic compounds do not seem to have logical precursors in living organisms, unlike the series of ~kylcyclohexanes and alkylbenzenes with isoprenoid side chains which are apparently derived from carotenoids (HALL and DOUGLAS,1983; JIANG and FOWLER, 1986; SUMMONSand POWELL, 1987). Isolation and detailed identification of such isomers is essential to obtain information about the origins and transformation mechanisms of fossil fuels such as coal, shale oil, and petroleum ( DURAND, 1980; T&SOT and WELT, 1984). It is difficult to achieve positive identification of the various isomers of the alkylbenzenes, even when using high-resolution chromatographic and mass spectrometric methods, because of the large number of interfering compounds, their similar retention behavior and ~agmentation patterns, and the absence of standard compounds. BASETet al. ( 1980) reported * Author to whom correspondence may be addressed. 837

J.-Z. Dong, W. P. Vorkink, and M. L. Lee

838

Biological Markers

I

(4

R

R=%-CT, ( in isoprcnoid sidechain)

R=C13-C;1 ( in isoprenoid

II

(4

R

Precursors

side chain)

o-(

cH310 CooTi

R= n-C,,-n-C,,

ll-Cyclohcxyl undccanoic acid

R-C-R’

o-

O%h2coOH

1%Cyclohuyl txideanoic acid

-jj-j

R


R=n-Cs-n-C35

@ R‘=H @I R’= CHj @) R’= (CH&

(1) AIkyhuion andacylafica ofbenzu~cs (or cyclohcxancs) withslcoholsor fatty adds.

(2)~Clivltionof fatty acids, n-alcohols, and

R R

R=n-C6-n-r&

n-kctoacs, et!2

6) R’=H

FIG. 1. Examples of types of alkylcyclobexanes and alkylbenzenes and their presumed precursors. For further explanation, see text.

The second class of alkyl~yclohexanes and alkylbenzenes listed in Fig. 1 are those with narrow ranges (e.g., CL&&) of alkyl chain lengths. JOHNS et al. ( 1966) first reported the presence of n-C,,, to n-C,, alkylcyclohexanes (Fig. 1, IIa) in Nonesuch Seep oil. The identification of four groups of alkylbenzenes in a low-volatile bituminous coa1 has recently been reported (Dong et al., unpubl. data) (Fig. 1, IIb), two of the groups having four isomeric series (Fig. 1, IIa, i, iv) and two of them having five isomeric series (Fig. 1, IIa, ii, iii). Bacterially occurring 11-cyclohexyl undecanoic acid and 13-cyclohexyl tridecanoic acid have been suggested to be their precursors (Dong et al., unpubl. data). Diagenetic oxidation and reduction of the carboxyl functionai groups in the alkyd chains were proposed as intermediate steps in the formation of the compounds from their biological precursors. This has also been demonstrated in a thermal experiment in which cyclohexanepentanoic acid was heated with THF-extracted coal and water at a temperature as low as 80°C for six days (Dong et al., unpubl. data). This type of reaction would be plausible because both 1l-cyclohexyl undecanoic acid and 13-cyclohexyl tridecanoic acid have been identified in several

biological samples, such as butter fat, rumen bacteria, organic matter in hot springs, and acidophilic thermophilic bacteria ( SCHOGT and BEGEMANN,1965; HANSEN, i 967; DE ROSA et al., 197 1, 1972; OSHIMAand ARIGA, 1975; SUZUKI et al., 1981; KANNENBERG et al., 1984). The third class of alkylcyclohexanes and alkylbenzenes listed in Fig. 1 is primarily encountered in geological samples, and there have been several publi~tions that propose possible identifications ( RUBINSTEINand STRAUSZ, 1979; BASET et al., 1980; GALLEGOS, 1981; OSTROUKHOVet al., 1983; CONNAN et al., 1986; FOWLER et al., 1986; REED et al., 1986; HOFFMANNet al., 1987; KISSIN, 1990). However, only recently has their positive identification become possible; and as reported in this paper, eleven isomers were confirmed. Even when using standard compounds for comparisons, it was still difficult to detect each homolog of every series because of the large number of interfering compounds and their similar retention behavior and mass spectra. There are widely differing opinions concerning the origin of the Group III compounds (Fig. 1). GALLEGOS( 1981) postulated gamma carotene and chlorohactene to be the sources of alkylbenzenes

Long-chain alkylcyclohexanes and alkylbenzenes found in coal pyrolysis products. He proposed that in early diagenesis, bacteria clip off the methyl groups on the isoprenoid chain, but the work by CANTWELL ( 1978) does not support Gallegos’ proposition. CONNAN et al. ( 1986) and WILLIAMS et al. ( 1988) reported concentrations of specific n-alkylbenzenes, mostly n-pentadecylbenzene, in different sources of rocks and crude oils. These findings encouraged thermal treatment experiments by WILLIAMS et al. ( 1988), who found that benzene, toluene, and cyclohexane can be alkylated by long-chain alcohols in the presence of kerogen. Direct cyclization from fatty acids was also proposed by several authors (RUBINSTEIN and STRAUSZ, 1979; FOWLER et al., 1986; HOFFMANN et al., 1987). RUBINSTEIN and STRAUSZ ( 1979) heated octadecanoic acid and 9octadecenoic acid with a clay catalyst and found homologous series of both n-alkylcyclohexanes and methyl-n-alkylcyclohexanes among the products. These findings were later confirmed by HOFFMANN et al. ( 1987 ) . More recently, SINNINGHE DAMST~?et al. ( 199 1) identified several homologous series of 1,2di-n-alkylbenzenes (C&&) in an immature Amposta crude oil and found that monoalkylbenzenes and 2-alkyltoluenes dominated the alkylbenzene distribution. Cyclization and aromatization reactions of linear, functionalized precursors in the subsurface were suggested as the mechanism of alkylbenzene formation. Meta and para alkyltoluenes encountered in more mature crude oils were explained to be due to isomerization reactions of initially formed 2-alkyltoluenes during catagenesis (SINNINGHE DAMSTI? et al., 1991). SPIRO (1984) suggested that alkylcyclohexanes may be derived from cracking of kerogen at high temperature under the influence of certain minerals. In this paper, the isolation, separation, and identification of alkylcyclohexanes and alkylbenzenes from a complex coalbed wax are described. A series of standards was synthesized and verified for use in identification of these groups of compounds. Further, their possible diagenic relationships with n-alkanes and fatty acids were investigated, and a possible mechanism of alkylcyclohexane and alkylbenzene formation was proposed. EXPERIMENTAL Source of Coal-Bed Wax The coal-bed wax sample was obtained from Resource Enterprises, Salt Lake City, Utah. The sample was collected at the gas and water separator from the Hamilton No. 3 well. The well location is at 1184 ft FSL, 1112 ft FWL section 30, Township 32 north, Range 10 west, Cedar Hill Field, San Juan County, New Mexico (San Juan Basin). The measured mean maximum vitrinite reflectance of the coal at 2880.8-2881.6 ft was 0.85%. It was reported by KBUGE (1989) that the wax had dark green blebs embedded in a lighter green matrix when first received, but no significant differences were found from the analysis of the saturate and aromatic fractions of each colored material. Chemical Class Fractionation The coal-bed wax was fractionated into four chemical classes on a neutral alumina (Aldrich, Milwaukee, Wisconsin) column using the methods described by LATERet al. ( 198 1). Briefly, 500 mg of wax were first adsorbed onto neutral alumina by dissolving the wax in a few milliliters of methylene chloride, then adding the dissolved wax to 3 g of alumina, followed by elimination of the solvent under a stream of Nz. The alumina containing the sample was then introduced into an 1l-mm i.d. column on top of 6 g of previously packed neutral alumina. The fractions were eluted using the following sol-

839

(6)

d 100

1p

Xl

120

140

180

so Time (mln)

40

30

160

200

220

240

Temp (Tc)

FIG. 2. Capillary column gas chromatogram of the six synthesized dodecylxylene isomers. Conditions are as follows: temperature pronrammed from lOO-240°C at 3°C min-’ after an initial 2-min isoihermal period; split ratio of 100: 1; for other conditions, see text. vents: 20 mL of hexane (aliphatic), 50 mL of benzene (aromatic), 70 mL of chloroform (containing 0.75% ethanol as preservative; Polar 1 ), and 50 mL of THF/ethanol (9:1 v/v) solution (Polar 2). The Polar- 1 and Polar-2 fractions were combined and methylated by diazomethane from N-methyl-N-nitroso-N’-nitroguanidine before analysis (FALESet al., 1973). The alkylcyclohexanes were separated and concentrated from the aliphatic fraction using the method of O’CONNORet al. ( 1962 ) . Molecular sieve type 5 A was dried at 250°C under vacuum for 12 h. Approximately 200 mg of the ahphatic fraction were dissolved in 150 mL of isooctane, and 40 g of the molecular sieves were added. The contents of the flask were then vigorously refluxed for 6 h. The normal alkanes penetrated into the molecular sieves, while the other more bulky compounds were occluded. The alkylbenzenes were isolated and concentrated by further alumina adsorption chromatography of the aromatic fraction. The original aromatic fraction was added to a second alumina column, prepared as described above, and the solutes were eluted with four 6mL aliquots of hexane/benzene (8:2 v/v). Capillary column GC/ MS analysis of these four subfractions showed that the alkylbenzenes eluted in subfractions 1 and 2. These two fractions were combined for the detailed analysis of the alkylbenzenes. Syntheses of Standards The six isomers of the dodecyl xylenes were synthesized from reactions of I-bromododecane with each of the bromoxylene isomers using sodium at 20°C according to the method reported by BACKMANN ( 1965; see Eqn. 1). After reaction, vacuum distillation was used to obtain pure 5-dodecyl-m-xylene, 2-dodecyl-p-xylene, 4dodecyl-m-xylene, 4-dodecyl-u-xylene, 2dodecyl-m-xylene, and 3dodecyl-m-xylene (see structures in Fig. 2 ) Mass spectra were obtained for all of the synthetic compounds using GC/MS. 1-Bromododecane, 5-bromo-m-xylene, 2-bromop-xylene, 4-bromo-m-xylene, 4-bromoc-xylene, 2-bromo-m-xylene, and 3-bromo-u-xylene were obtained from Aldrich and used without further purification. GC and GC / MS Analysis The analysis of the fractions of the wax for n-alkanes, fatty acids (methyl esters), n-alkylcyclohexanes, n-alkylbenzenes, n-alkyltolu-

+

Na.Ether

CH&X-Iz)i+r

20 oc

-@

\ I

x

(CRZ),lCR, W&h

03 (1)

840

J.-Z. Dong, W. P. Vorkink, and M. L. Lee

enes, and n-alkylxylenes was performed using an HP 5890 gas chromatograph interfaced to an HP mass selective detector. The gas chromatographic separation was accomplished using an SE-54 fused silica capillary column ( 17.5-m X 0.2-mm i.d. X 0.2-pm film thickness). The oven temperature was programmed from 40-300°C at 3°C min-‘. The initial temperature (40°C) was held for 2 min before programming to the final temperature of 300°C which was held for 20 min before ending the chromatographic run. Helium was used as carrier gas, and the MS was operated at an ionization voltage of 70 eV. Quantitation of homologous series of n-alkylbenzenes, n-alkyltoluenes, and n-alkylxylenes was accomplished by comparing peak areas of the resolved compounds with a standard injection of eicosane. Identifications were made by comparing retention times of sample components with those of standards and by matching their mass spectra. Single ion chromatograms were obtained based on the following ions: m/z 7 1, n-alkanes; m/z 74, fatty acids (as methyl esters); m/z 83, n-alkylcyclohexanes; m/z 97, methyl-n-alkylcyclohexanes; m/z 91 and 92, n-alkylbenzenes; m/z 105 and 106, n-alkyltoluenes; m/z 119 and 120, n-alkylxylenes; m/z 133, trimethyl-n-alkylbenzenes; and m/z 147, tetramethyl-n-alkylbenzenes. In this way, nearly all of the peaks were identified; and the relative amounts of the components in each homologous series were routinely determined by

integration. RESULTS AND DISCUSSION The quantitative results of the chemical class fractionation are given in Table 1. The yields are expressed in wt% of the original wax sample. It is clear that the wax was comprised primarily of n-alkanes (55.6%). The branched and cyclic aliphatic hydrocarbons were the second largest group (26.0%) and included isoprenoids (Ci+& and Cl*); pristane; phytane; and homologous series of 2-methylalkanes, 3-methylalkanes, n-alkylcyclohexanes, and methyl-n-alkylcyclohexanes, etc. Several important homologous series including the nalkylbenzenes, n-alkyltoluenes, and n-alkylxylenes were also found in the aromatic fraction. The polar fraction was comprised primarily of fatty acids. Synthetic Mixture of Dodecylxylenes

Figure 2 shows a chromatogram of the six dodecylxylenes synthesized to provide positive identification of the isomers present in the wax sample. Except for peaks 4 (2-dodecyl-mxylene) and 5 (4-dodecyl-o-xylene), which are only partially separated, all other compounds were well resolved. Figure 3 shows the mass spectra of these synthetic dodecylxylenes.

Very similar mass spectra are observed; all of them have a molecular ion at m/z 274 and, except for tidodecyl-m-xylene (Fig. 3a), all were characterized by a base peak ion at m/z 119. In their individual mass spectra, differences in relative abundances are seen for mf z I 19 and 120. The reason for the differences in the relative abundances of m/z 119 and 120 has been explained to be a result of the inhibitory effect of various methyl substitutions to the ortho and para positions in the site-specific y-hydrogen rearrangements (KINGSTON et al., 1988). In particular, this hydrogen rearrangement from the y-position (3-position) on the alkyl group to the ring does not occur appreciably if both ortho positions and the para position are methyl-substituted. The highest degree of hydrogen transfer is seen in those compounds where all ortho and para positions to the n-alkyl group are unsubstituted (Fig. 3a). If only one ortho position is substituted, there is less hydrogen transfer (Fig. 3b,f ) but significantly more than in those isomers where two of the three positions are methyl substituted (Fig. 3c and e). The various isomers can thus be distinguished from their m/z 120/ 119 intensity ratios (2dodecyl-p-xylene, 79.5; 4-dodecyl-m-xylene, 15.9; 4-dodecylo-xylene, 42.0; 2-dodecyl-m-xylene, 20.5; and 3-dodecyl-oxylene, 89.8). However, it should be pointed out that this alone is not sufficient for positive identification of the isomers of the alkylbenzenes because the mass spectra normally encountered when analyzing complex samples are often contaminated by coeluting compounds. Matching chromatographic retention data are required to provide the necessary complementary information for positive identification. Occurrence and Structural Characterization of Alkylcyclohexanes

and Alkylbenzenes

Analysis of the aliphatic hydrocarbon fractions derived from the coal wax indicated the presence of several homologous series of alkylcyclohexanes. Figure 4 shows selectedion chromatograms of m/z 83 (Fig. 4a), m/z 97 (Fig. 4b), and m/z 111 (Fig. 4c), showing the distributions of n-alkylcyclohexanes, methyl-n-alkylcyclohexanes, and dimethyln-alkylcyclohexanes, respectively. The identifications of the n-alkylcyclohexanes were based both on GC retention times and mass spectra of authentic standards. The three isomer& methyl-n-alkylcyclohexanes were identified by comparison

Table 1 Weight Percentages of Chemical Class Fractions of the Coal-Bed Wax’

Aliphatic hydrocarbons n-Akanes Othe&

81.6% 55.6% 26.0%

Aromatic hydrocarbons Naphthalene derivatives Alkylbenzenes

8.6%

Polar-l

1.7%

Polar-2

7.1%

Total column recovery

2.9% 5.7%

99.0%

‘Based on the total mass of the wax. bIncludes isoprenoids (C,,, C,,, C,s, C,,, C,& pristane, phytane, 2-methyklkaoea, 3methylalkanes, n-alkylcyclohexanes, methyl-n-alkylcyclohexanes, erc.

Long-chain alkylcyclohexanes and alkylhenzenes

loo1 (4

loo1W

\ 0 Cfh

v.%),,CH,

‘1

274

HJ

214

loo- (d)

119

so-

uwIlC~3

2,4

105 91 50

lo

100

A

1. -* L 150

200

841

thesized in this laboratory (see the following text). Dimethyln-alkylcyclohexanes were tentatively identified from their mass spectra, and their side chains were assigned from their parent ions. Both the n-alkylcyclohexanes and methyl-n-alkylcyclohexanes have been recognized as significant components of the hydrocarbon fractions of various sedimentary rocks and petroleum (RUBINSTEIN and STRAUSZ, 1979; FOWLER et al., 1986; REED et al., 1986; HOFFMANNet al., 1987; KISSIN, 1990). It can be seen (Fig. 4a) that the side chains of the n-alkylcyclohexanes range from Cr-C3, (C+& total carbon number) with an even over odd preference from CZ2-CZ4 ( C28-C30 total carbon number). The methyl-n-alkylcyclohexane (Fig. 4b) and dimethyl-n-alkylcyclohexane (Fig. 4c) series are, however, very complex and show several isomer series distributions. From detailed GC/MS analysis, it is known that the distribution of the methyl-n-alkylcyclohexanes is similar to that of the n-alkylcyclohexanes with a maximum at Cz,-C& (C27-C29 total carbon number). Although it is difficult to identify each compound in the homologous series of dimethyl-n-alkylcyclohexanes, there are two significant peaks in the single-ion chromatogram of m/z 111 (Fig. 4~). Based on mass spectral data (shown in Fig. 5)) they have been tentatively identified as dimethyl-n-Cs-cyclohexanes (peak 1) and dimethyl-n-ClO-cyclohexanes (peak 2). If we consider their total carbon numbers, they correspond to Cl6 and Cls, respectively. As discussed in the following text, it is interesting to note that such prominant compounds reflect their possible origin from biological markers. Figure 6a shows the total-ion-current chromatogram of the aromatic fraction isolated from the coal-bed wax. It is

250

W

I&&_.‘& I %5----J

50

91 105

50

100

100

VI

CHl

: ’ CH’ 0

50

(CH,),,CH,

174

JLJ-JT 50

100

200

250

m/z

FIG. 3. Mass spectra of the synthesized (a) S-dodecyl-m-xylene, (b) 2-dodecylp-xylene, (c) 4-dodecyl-m-xylene, (d) 4-dodecyl-oxylene, (e) 2-dodecyl-m-xylene, and ( f) 3-dodecyl-o-xylene standard compounds. Min

with published mass spectra (FOWLER et al., 1986; HOFFMANN et al., 1987); and their elution order was determined from comparison of their boiling point order, which is similar to the corresponding alkylbenzenes which have been syn-

FIG.

4. Selected-ion-chromatograms of the branched/cyclic hydrocarbon fraction at m/z values of (a) 83, (b) 97, and (c) 111. Numbers in each chromatogram denote the number of carbons in the alkyl chain.

J.-Z. Dong, W. P. Vorkink, and M. L. Lee

842

(4

50

40

60

80

100 120 140 160 180 200 220 240

(b)

160

150

2ilO

ZSO

Ill/Z FIG.5. Mass spectra of (a) Peak 1 and (b) peak 2 in Fig. 4(c).

apparent that, except for the presence of naphthalene (peak 1) and its methyl derivatives (peaks 2 and 3), dimethylnaphthalenes (peaks bracketed by 4)) trimethylnaphthalenes (peaks bracketed by 5 ), and tetramethylnaphthalenes (peaks bracketed by 6), there are also other compounds in a broad and unresolved hump which may represent several other homologous series. In order to further concentrate these compounds, the aromatic fraction was separated into subfractions by liquid chromatography (see the Experimental section). Figure 6b shows the total-ion-current chromatogram of the aromatic alkylbenzene subfraction. It appears that each group contains several peaks with each group of isomers, giving similar mass spectra. They represent n-alkylbenzenes, n-alkyltoluenes, and n-alkylxylenes, etc. In order to clarify this,

Fig. 7a shows an expanded section of the total-ion-current chromatogram with retention times between 36 and 50 min; Fig. 7b-d shows the selected-ion chromatograms of m/z 274, 288, and 302, respectively. Each group of alkylbenzene isomers now clearly indicates the presence of at least eleven isomers. Except for the 6-n-alkyl-m-xylenes, each series was positively identified by comparing its mass spectra and retention times with the newly synthesized standards, and these identifications are found in Table 2. Although the 6-n-alkylm-xylenes were not identified by comparison to actual standards, their identifications are considered to be reliable because six of the seven possible standards were available; and they have similar mass spectra. From previous mass spectral studies, it is known that nalkylbenzenes can be characterized by a base peak at m/z 92 (McLafferty rearrangement) and a second highest peak of m/z 9 1. However, in the case of the three isomers of the n-alkyltoluenes, the para isomer was characterized by m/z 105 base peak; and both the ortho and meta isomers were characterized by one of m / z 106. In the case of the six xylene isomers, five isomers were characterized by a base peak of m/z 119 and one isomer (5n-alkyl-m-xylene) by m/z 120. By monitoring variations in intensity of these characteristic ions, the mass spectrometer can be selectively used to determine the isomer series. This is illustrated by a comparison of Figs. 8 and 9. Similar intensities for the selected-ion chromatograms at m/z 9 1 and 92 (Figs. 8a and 9a) were observed, and their regular elution patterns in the mass chromatograms indicate that the alkyl chains are linear. Triplet peaks in the single-ion chromatograms of m/z 105 and 106 (Figs. 8b and

(a)

36

~38

40

4i

44

46

48

(b)

10

10

20

30

40

50

60

70

80

90

20

30

40

50

60

70

80

90

Min

FIG. 6. (a) Total-ion-current chromatogram of the aromatic fraction of the coal-bed wax; (b) total-ion-current chromatogram of a concentrated homologous series in the aromatic fraction. Peak identifications: ( 1) naphthalene, (2 and 3) methylnaphthalenes, (4) dimethylnaphthalenes, (5) trimethylnaphthalenes, and (6) tetramethylnaphthalenes.

36

38

40

42

44

46

48

Min FIG. 7. Partial mass chromatograms showing the complexities of the isomer groups of the alkylbenzenes. (a) total-ion-current chromatogram and mass chromatograms at m/z values of(b) 274, (c) 288, and (d) 302. Numbered peaks refer to Table 2.

Long-chain alkylcyclohexanes and alkylbenzenes

843

Table 2 Alkylbenzenes Identified in tbe Coal-Bed Wax (see Fig. 7)

Compound Peak No. (Fig. 7C)

(Fig. 78)

Vig. 7D)

1

5-Dcdecyl-m-xylene

5-Tridecyl-m-xylene

5-Tetradecyl-m-xylene

2

6-Dodecyi-m-xylene”

6-Tridecyl-m-xylend

6-Tetradecyl-m-xylem?

3

2-Dodecyl-p-xylene

2-Tridecyl-p-xylene

2-Tetradecyl-p-xylene

4

o-Tridecyltoluene

o-Tetradecyltoluene

o-Pentadecyltoluene

5

4-Dodecyl-m-xylene

4-Tridecyl-m-xylene

4-Tetradecyl-m-xylene

6

m-Tridecyltoluene

m-Tetradecyltoluene

m-Pentadecyltoluene

7

Tehadecylbenzene

Pentadecylbenzene

Hexadecylbenzene

8

p-Tridecyltoluene

p-Tetmdecyltoluene

p-Pentadecyltoluene

9

4-Dodecyl-o-xylene

4-Tridecyl-o-xylene

4-Tetradecyl-o-xylene

10

2-Dodecyl-m-xylem?

2-Tridecyl-m-xylene

2-Tetndecyl-m-xylene

11

3-Dodecyl-o-xylem?

3-Tridecyl-o-xylene

3-Tetradecyl-o-xylene

%-Dodecyl-m-xylene, 6-hidecyi-m-xylene, and interpretation of their mass spectra.

6-tetradecyl-m-xylene

were

identified only by

9b) are apparent, which correspond n -aIkyl-m-toluenes,

J,

36

38

36

38

40

42

44

42

44

46

48

(b)

40

and

to the n-alkyl-p-toluenes, n -aIkyl-o-toluenes, respectively.

More than seven xylene isomers were identified in the singleion chromatograms of m/z 119 and 120 (Figs. 8c and SC). This technique was used in this way to monitor the distributions of n -alkylbenzenes, n -alkyltoluenes, and n -alkylxylenes in the full scan mode; the results are shown in Figs. 10 and 11. The carbon chain distributions and peak maxima were determined from measuring the relative areas of the peaks in a selected ion chromatogram for each series; these data are presented in Table 3, along with their concentrations in the wax. The n-alkylbenzenes had alkyl chain lengths in the range of C&a2 with a maximum at CZ2; the three isomeric alkyltoluene series had chain lengths in the range of C&s1 with maxima at around Car ; and the six isomeric alkylxylene series had chain lengths in the range of C7-C3a with maxima in the range of C1s-Ca2. Biological Precursors of the n-Alkylcyclohexanes n-Alkylbenzenes

J.

36

38

40

42

44

46

48

Min FIG. 8. Expanded selected-ion-chromatograms at m/z values of (a) 91, (b) 105, and (c) 119, showing the distribution of several homologous series of alkylbenzenes. Numbered peaks refer to Fig. 7 and Table 2.

and

As was previously discussed, the coal-bed wax is comprised primarily of n-alkanes. Therefore, the distributions of the alkylcyclohexanes and alkylbenzenes were compared to the n-alkane distribution. Figure 12 shows plots of these comparisons. The relative abundances were integrated from the following homolog series, n-alkylcyclohexanes from m/z 83 (Fig. 12a), n-alkylbenzenes from m/z 92 (Fig. 12b), n-alkylo-toluenes from m/z 106 (Fig. 12c), n-aIkyl-p-toluenes from m/z 105 (Fig. 12d), and 5-n-alkyl-m-xylenes from mjz 120 (Fig. 12e). It can be seen that the normal alkanes showed an odd over even carbon number preference in the range of CZ5CXI, which is similar to observations in plants in which waxy

J.-Z. Dong, W. P. Vorkink, and M. L. Lee

844

(4

38

36

42

40

46

48

I

lb)

38

36

40

42

44

46

48

(a) 92, (b) 106, and (c) 120, showing the distribut’ion of several homologous series of alkylbenzenes. Numbered peaks refer to Fig. 7 and Table 2.

40

20

30

40

60

70

80

50

60

70

80

50

60

70

80

SO

(4

CC)

20

30

40

Min

FIG. 10. Selected-ion chromatograms at m/z values of(a) 91, (b) 105,and(c) 119.Numbers in each chromatogram denote the number of carbons in the alkyl chain.

40

50

60

70

80

20

30

40

50

60

70

80

20

30

40

SO

60

70

80

Min

FIG. 9. Expanded selected ion chromatograms at m/z values of

30

30

(W

Min

20

20

FIG. 11. Selected-ion chromatograms at m/z values of (a) 92, (b) 106,and(c) 120.Numbers in each chromatogram denote the number of carbons in the alkyl chain.

layers are formed to coat and protect the stems, leaves, flowers, and fruit. This result is consistent with the obvious circumstantial conclusion that a coal-bed wax would have a terrestrial origin. Comparing these results with the n-alkylcyclohexanes (Fig. 12a), it was found that a remarkable similarity in the distributions became apparent; the n-alkylcyclohexanes had chain length maxima at Czz and Cz4 (total carbon numbers of C&aand C3,,), one less carbon number than the n-alkanes maxima. This similarity in compound distribution is significant from a geological viewpoint and suggests that they have similar biogenic precursors. These are most likely terrestrial fatty acids which occur in the wax of vascular lipids with a dominance of even carbon numbers at Cz6, Cza, and C3,, ( free or as esters). These fatty acids have been identified in low-rank coals ( CHAFFEE et al., 198 1; SNAPE et al., 198 1; DONG et al., 1987, 1988; NIWA et al., 1988) and were suggested as an important source for the formation of long-chain alkylaromatics in coals ( DONG and OUCHI, 1989a,b; KOMORI et al., 1990). The n-alkylcyclohexanes could have been formed by direct cyclization, while the n-alkanes could have been formed by decarboxylation of fatty acids (see the following discussion). The n-alkylbenzenes are similar to the n-alkylcyclohexanes (Fig. 12b), showing maxima at C22 (total carbon number of Cza), and they could have been derived from the aromatization of the n-alkylcyclohexanes. Similarly, it was found that the other three homologous series of n-alkyl-o-toluenes and n-alkyl-p-toluenes (Fig. 12c and d) and S-n-alkyl-m-

Long-chain alkylcyclohexanesand alkylbenzenes

845

Table 3 Carbon ChainDistributions andConcentrations of n-Alkylbenxenes, n-Akyltoluenes, andn-Alkylxylenesin the Coal-bedWax

Compound series

Carbon-&sin distxibution”

Maximum”

concenhatmn (mg g-‘P 6.4

n-Alkylbenzenes

C&z

CU

n-All@-o-toluenes and 2-n-alhyt-p-xylenesc

G-C,,

c,9-c21

n-Alkyi-m-toluenes

G-G L

%C22

3.5

n-Alkyl-p-toluenes

Cl-C,,

CZI

6.9

4-n-Alkyl-o-xylenes, Z-n-Alkyl-m-xylem%, and5-n-Alkyd-m-xylenes’

c7-Go

Cw%

5.3

6-n-A&y&m-xylenes*

c,-c,

%&,4

1.1

3-n-Alkyl-0-xylenes*

C7-%

%c,

1.0

4-n-Alkyl-m-xylenes*

C?*cm

Cl9

0.6

10.5

pDetermined frommeasuringtherelativeareasof thepeaksin the selected-ion c~~tog~. bApproximate eoncentsation in mg g’ of theoriginalwax. %terminedtogetherbecauseof poorresohnion. *Concentration wss estimatedby comparing withthe distribution of the S-n-alkyl-m-xyknes.

xylenes (Fig. 12e) had maxima at around Cza (total carbon num~r), which again could be derived by aromatization

from corresponding alkylcyclohexanes. A similar correlation was found by FOWLER et al. ( 1986) in Ordovician organic matter. They found that both n-alkylcyclohexanes and one homologous series of methyl-n-alkylcyclohexanes were distributed similarly as the n-aikanes found in the same samples, having a pronounced odd carbon number preference at total carbon numbers of Cl7 and Cr9 _ They suggested that the monocyclic alkanes in these Ordovician samples might be primarily derived from the cyclization of the straight-chain algal fatty acids by mechanisms that involve decarboxylation. Moreover, they claimed that some methyl-n-alkykyclohexanes could also be derived from fatty acids by a less-preferred mechanism that does not involve d~ar~xy~ation or from other precursors. Results from the present study favor the involvement of a direct cyclization mechanism of straightchain fatty acids or perhaps alcohols in the formation of these compounds in this coal-bed wax. The structural differences observed between the coal-bed wax and the Ordovician samples suggest that their organic precursors were different. The Ordovician samples are presumed to have organic precursors dominated by the extinct marine algae G. prisca ( FOWLER et al., 1986; REED et al., 1986; HOFFMANN et al., 1987), not terrestrial plants. Several other observations support the direct cyclization mechanism. The distribution of fatty acids found in the wax (Fig. 13) shows that Cr6 and C,s are clearly the most abundant fatty acids (96%). The two most abundant alkylcydohexanes found in this wax were those with total carbon numbers of Cl6 and C18 (Figs. 4c and 5a and b). Palmitic ( Cr6) and stearic ( Cr8) acids have also been shown to exist in high concentrations in coals ( CHAFFEEet al., 198 I). Finally, the single ion chromato~ms of the t~methyl and tetramethyl-~-a~kyl~nzenes show four peaks that predom-

inate in these series. The most prominent peak (No. 4 in Fig. 14b) is also a Ci6 compound (Fig. 15d). The three other peaks (Fig. 14) are Crs peaks that may have been derived from predominantly Cl6 or C18 compounds by mechanisms described below. As mentioned above, the Clb and Cl8 fatty acids make up the overwhelming majority of the fatty acid fraction of the wax. RUBINSTEINand STRAUSZ( 1979) carried out incubation experiments with stearic and oleic acids and clay catalysts. After detailed analysis and characterization, they found that, in addition to the presence of a homologous series of n-Cr~ to n-Cj5 alkanes with predominantly n-C,, products of decarboxylation of the acids ( JURG and EISMA, 1964), other homologous series of n-alkylcyclohexanes and methyi-n-alkylcyclohexanes (total carbon numbers of C1+& with predominantly Cls products) were found. Therefore, they speculated that the n-alkylcyclohexanes and methyl-n-alkylcyclohexanes were formed directly from an intermediate in which the carboxyl group was chemically bound to the aluminosilicate crystal structure, and cyclization was concomitant with the release of the whole carbon chain system. Later, the same experiments were repeated by HOFFMANN et al. ( 1987 ), and the same results were obtained. Considering their greater reactivity compared to fatty acids and their widespread high abundance in plants, animals, and microorganisms, n-alcohols (probably n-ketones as well) could also produce these kinds of com~unds by a similar mechanism. Based on the above findings and discussion, a proposed route for conversion of fatty acids and alcohols to alkylcyclohexanes and alkylbenzenes is given in Fig. 16. Acidic clays such as montmorillonite, bentonite, and kaolin, etc., are the principal minerals in fossil fuels, can significantly effect the rates at which some organic compounds rearrange or isomer& and have been used to simulate natural reactions

J.-Z. Dong, W. P. Vorkink, and M. L. Lee

846

12

10

8

6

4

2

0 10

n-Alkyl-Chain

0

10

n-Alkyl-Chain

40

30

20

n-Alkyl-Chain

Carbon Number

20

40

30

Carbon Number

0

10

0

n-Alkyl-Chain

10

n-Alkyl-Chain

20

Carbon Number

20

30

Carbon Number

3-o

Carbon Number

FIG. 12. Comparisons of the distributions of n-alkanes with (a) n-alkylcyclohexanes, (b) n-alkylbenzenes, (c) nalkyl-o-toluenes, (d) n-alkyl-p-toluenes, and (e) 5-n-alkyl-m-xylenes. The n-alkylcyclohexanes were measured by integration of the peak areas in the mass chromatograms of m/z 83; n-alkylbenzenes from m/z 92; n-alkyl-o-toluenes from m/z 106; n-alkyl-p-toluenes from m/z 105, 5-n-alkyl-m-xylenes from m/z 120, and n-alkanes from m/z 71. q represents the n-alkanes.

40

Long-chain alkylcyclohexanes and alkylbenzenes

847

61.2

35.3 I

60

40

80

100 120 140

160

200

180

lb)

204 I.

Distribution of Fatty Acids FIG.

60

40

80

L 140 160 II

loo

120

II

180

..I r m

220

13. Distribution of fatty acids isolated from the wax.

I@

100

( JURG and EISMA, 1964; RUBINSTEINand STRAUSZ, 1979; DONG and OUCHI, 1989a,b; Dong et al., unpubl. data). The reduction of the fatty acids or alcohols (I) to cu-carbonium ions (II) are well known (MARCH, 1985) and may also be involved in the formation of some compounds that are found in fossil fuels during early diagenesis. Especially in the presence of water, clay minerals can promote this type of carbonium ion mechanism ( GREENSFELDERet al., 1949; GALWEY, 1969a,b; SHIMOYAMAand JOHNS, 1971). The a-carbonium ion (II) then cyclizes to form n-alkylcyclohexane (III) and further aromatizes to n-alkylbenzene (IV). Moreover, at the same time, the ol-carbonium ion (II ) may undergo H + migration to form a ,&carbonium ion (V) and/or may undergo a carbonium ion isomerization to form either secondary (VIII) or tertiary carbonium ions (IX). These types of migration and isomerization are common in synthetic organic chemistry ( MARCH, 1985 ) , and diagenesis of fatty acids and alcohols in the presence of minerals and water under geological conditions may also follow this path. From a thermodynamic viewpoint, the tertiary carbonium ion (IX) has greater stability than the secondary carbonium in the laboratory

(a)

.

2

20

30

40

50

60

70

80

Min FIG. 14. Selected ion chromatograms at m/z values of (a) 133 and (b) 147. The mass spectra of the numbered peaks are given in Fig. 15.

100

147 ((=,)I

-3

KWS

CH,

(d

(d)

147

‘I

0

‘(C&k

5

d

I 218 Ir 40

60

80

100

120

140

160

180

UH)

,

220

m/z FIG.

15. Mass spectra of peaks (a) 1, (b) 2, (c) 3, and (d) 4 in

Fig. 14.

ion (VII). Thermodynamic equilibration of these carbonium ions (II, V, VIII, and IX) under specific conditions will determine the production of different alkylcyclohexanes and alkylbenzenes (as discussed in the following text); therefore, their yields in different samples will provide information on the environmental conditions (e.g., pH). Both the &carbonium ion (V) and the tertiary carbonium ion (IX) could cyclize to produce cu-methylcyclohexane and a-dimethylcyclohexane, respectively. However, these structures are not stable and could undergo rearrangements of the methyl groups to produce three isomeric methyl-n-alkylcyclohexanes (VI) and seven isomeric dimethyl-n-alkylcyclohexanes (X), which further aromatize to produce three isomeric n-alkyltoluenes (VII) and seven isomeric n-alkylxylenes (XI), respectively. The identifications of the three homologous series of methyl-n-alkylcyclohexanes (Fig. 4b), n-alkyltoluenes (Figs. 8b, 9b, lob, and 1 lb), seven homologous series of dimethyl-n-alkylcyclohexanes (Fig. 4c), and n-alkylxylenes (Figs. 8c, 9c, 1Oc, and 11c) could be the results of such rearrangements. Other n-alkylcyclohexanes and nalkylbenzenes with trimethyl and tetramethyl substituents identified in the present study (see Figs. 14 and 15 ) could also be produced according to the above scheme. Our data, therefore, also support previous hypotheses ( RUBINSTEINand STRAUSZ, 1979; HOFFMANNet al., 1987).

J.-Z. Dong, W. P. Vorkink, and M. L. Lee

848

RCH,CH,CH,CH,CH,CH,Cft,COOH Or RCB,CB,CH,CH*CB,CH3CH,CH,10H I

H+migl%liOll RCH,CH,CHICH,CH,CH,~WCH,

-

-

r@l carboniumion + iSOlDZli?2tiOIl RCH,CH,CH~CA,CIT,CHICH,CH,*~

V Cyclization and rean=angment

+

%

RCH,CH,CH,CH,CH, % CH, +

II Cyclization, -H? +

G

t

CH&!H,R

III

1x

Cyclization and rearrangrnent of a-methyl, -JX+ R

WW,

X Aromatization

t

CH'

IV

VII

KH3z XI

FIG. 16.Proposed mechanisms for conversion of fatty acids and alcohols to ~kylcyciohexanes and alkylbenzenes in geoiogical envimnmen~.

CONCLUSIONS A wax obtained from a coal-seam methane well was shown to be a rich source of substituted alkylcyclohexanes and aikylbenzenes (5.7% of the wax). Eleven alkylbenzene homologous series were identified using authenticated standards. The eleven alkylbenzene homologous series identified were characterized by total carbon number distribution, total carbon number maximum, and concentration in the wax. A unified, clay catalyzed mechanism of formation of these alkylcyclohexanes and alkylbenzenes has been proposed invotving direct cyclization of predominantly Ct6 and Cl8 straight-chain precursors (fatty acids or alcohols), ubiquitous in terrestrial plant materials that served as organic precursors of coals. How the wax was generated from the coal is an interesting question for further study. work was supported by the Gas Research Institute (GRI), Contract No. 5084-260-1129, Any opinions, findings,

Acknowledgments-This

conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of GRI. The authors wish to thank Drs. M. G. Fowler, R. E. Summons, and M. A. Kruge for their reviews and comments. Editorial handling: S. A. Macko

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BYERSJ. D. and ER~MANJ. G. ( 1983) Low temperature degradation of carotenoids as a model for early diagenesis in recent sediments. In Advances in Organic Geochemistry 1981 (ed. M. BJOROYet al.), pp. 725-732. Wiley-Heyden. CANTWELLS. G., LAN E. P., WATT D. S., and FALL R. R. ( 1978) Biodegradation of acyclic isoprenoids by pseudomonas species. J. Bact. 135,324-333. CHAFFEEA. L., PERRY J. J., and JONES R. B. ( 1981) Carboxylic acids and coal structure. In Coal Structure (ed. M. L. GORBATY and K. OUCHI); Adv. Chem. 192, pp. 113-l 3 1. CONNAN J., BOUGOULLEC J., DESSORTD., and ALBRECHTP. ( 1986) The microbial input in carbonate-anhydrite facies of a Sabkha Pale~nvironment for Guatemale: A molecular approach. In Advances in Organic Geochemistry 198s (ed. D. LEYTHAEUNSER and J. RULLK~TTER);Org. Geochem. 10,29-50. DE ROSA M., GAMBACORTA A.,MINALE L., and BU’LOCKJ. D. ( 197 I) Cyclohexane fatty acids from a thermophilic bacterium. Chem. Cammun., S334. DE ROSA M., GAM~ACORTAA., MINALE L., and BU’LOCKJ. D. (1972) The formation of w-cycle-hexyl fatty acids from shikimate in an acidophilic thermophilic bacillus. Biochem. .I. 128,75 1-754. DONG J.-Z. and OUCHIK. ( 1989a1Geochemical oriein of lone chain alkyl~omatics in coal: i. Model reaction of fatty acid or alcohol with phenol. Fuel@, 710-716. DONG J.-Z. and OUCH1 K. ( 1989b) Geochemical origin of long chain alkylaromatics in coal. 2. Model reaction of lignin with alcohol. Fuel68, 1354-1357. DONG J.-Z., KATOH T., XTOH H., and OUCHI K. ( 1987) Origin of alkanes in coal extracts and liquefaction products. Fuel 66, 13361346. DUNG J.-Z., KATOH T., ITOH H., and OUCHI K. (1988) Structure in Wandoan coal from analyses of mild hydrogenation products: 1. First stage products. Fuel 6‘7,284-293. DURANDB. ( 1980) Kerogen, pp. 4 15-472. Ed. Technip. FALESH. M., JAOUNIT. M., and BABASHAK J.F.(1973)Simple device for preparing ethereal diazomethane without resorting to codistillation. Anal. Chem. 45, 2302-2303. FO~L.ERM. G. and DOUGLAS A. G. ( 1984) Di~bution and structure of hydrocarbons in four organic-rich Ordovician rocks. In Advances in Organic Geochemistry 1983 (ed. P. A. SCHENCK,J.W. DE LEEUW,and G. W. M. LIJMBACH);Org. Geochem. 6, 105-l 14.

Long-chain alkylcyclohexanes and alkylbenzenes FOWLER M. G., ABOLINSP., and DOUGLASA. G. ( 1986) Monocyclic

alkanes in Ordovician organic matter. In Advances in Organic Geochemistry 1985 (ed. D. LEYTHAEUSER and J. RULLK~TTER); Org. Geochem. 10, 815-823. GALLEGOSE. J. ( 1981) Alkylbenzenes derived from carotenes in coal by GC/MS. J. Chromatogr. Sci. 19, 177-182. GALWEYA. K. ( 1969a) Hydrogenous reactions in petroleum genesis and maturation. Nature 232, 1257-1260. GALWEY A. K. ( 1969b) Reactions of alcohols adsorbed on montmorillonite and the role of minerals in petroleum genesis. J. Chem. Sot., 577-578. GREENSFELDER B. S., VOGEH. H., and GOLD G. M. ( 1949) Catalytic and thermal cracking of pure hydrocarbons. Mechanisms of reactions. Ind. Eng. Chem. 41, 2573-2584. HALL P. B. and LMUCLAS A. G. ( 1983) The distribution of cyclic alkanes in two Iacustrine deposits. In Advances in Organic Geochemistrv 1981 (ed. M. BJOROYet al.). DD.576-587. J. Wilev. HANSENR: P. ( 1967) I 1-cycIohexyl undkcanoic acid: Its occurrence in bovine rumen bacteria. Chem. Ind., 1640- I64 1. HOEVENW. Y. P., HAUG P., BURLINGAMEA. L., and CALVINM. ( 1966) Hydrocarbons from an Australian oil 2000 m.y. old. Nature 211, 1361-1365. HOFFMANNC. F., FOSTERC. B., POWELLT. G., and SUMMONS R. E. ( 1987) Hydrocarbon biomarkers from Ordovician sediments and the fossil alga Gloeocapsomorpha prisca Zalessky 19 17. Geochim. Cosmochim. Acta. 51,2681-2697. HOOD A., CLERC R. J., and O’NEAL M. J. (1959) The molecular structure of heavy petroleum compounds. Inst. Petrol. 45, 168173.

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