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The nature of straight-chain aliphatic structures in green river kerogen
Geochimica et Cosmochumca
Acta,1917. Vol 41,pp 1411to 1417PergamonPress PrIntedin GreatBrltaln
The nature of straight-chain aliphatic structures in Green River kerogen D. K. YOUNG and T. F. YEN* Departments of Chemical Engineering and Medicine (Biochemistry), and Program of Environmental Engineering, University of Southern California, Los Angeles, CA 90007, U.S.A. (Received 3 November 1975; accepted in revised form 6 June 1977) Abstract-Previous studies of the Green River kerogen only provide apparently contradictory conclusions about the size of the straight-chain aliphatic structures as well as the manner in which these structures form part of the kerogen matrix. The present investigation is an attempt to resolve this contradiction. A mild stepwise oxidation procedure was followed so that extensive degradation of kerogen-derived intermediates could be prevented. Products isolated from each oxidation step were analyzed by conventional GLC techniques, GC-MS, and proton-NMR measurements in order to ascertain the significance of the straight-chain aliphatic structures present in the Green River kerogen. The following results were obtained: (a) Green River kerogen contains a substantial portion (ca 24 carbons out of every 10) of straight-chain aliphatic structures which are longer than C4, (b) the kerogen matrix forms a three-dimensional network of non-straight-chain clusters interconnected by long polymethylene cross-links, (c) the ‘core’, in comparison with the ‘periphery’ of the kerogen matrix, contains a greater proportion of straight-chain and branched aliphatic structures which are attached to the kerogen matrix at one terminus, (d) some of the straight-chain structures may exist as physically entrapped components in the kerogen matrix.
INTRODUCTION
portions of the kerogen matrix, (e.g. linkage, terminal attachment, or IN THE past, several investigators have employed difthe kerogen structure). To achieve ferent methods to elucidate the structure of kerogen mild stepwise oxidation procedure isolated from the Green River oil shale. The basic (ROBINSONet al., 1963; DJURICI et al., 1971) was utilapproach of these methods invariably involves the ized which prevented extensive degradation of interdegradation of the kerogen matrix into smaller strucmediate products released from the kerogen matrix. tural components which can be analyzed using conventional laboratory instruments. The complexity of The products of each oxidation step were analyzed. Conventional gas chromatographic techniques were the kerogen structure renders the choice of suitable supplemented with proton-NMR and elemental methods for studying the kerogen difficult. Depending analysis for many of the larger structural units deon the extent of kerogen degradation and the techrived from kerogen degradation in order to estimate nique of analysis employed, apparently inconsistent results have been reported. For example, ROBINSON the proportion of straight-chain structures in each product fraction. The pattern of kerogen degradation, (1976), based on the analyses of products from solvent as revealed by the composition of the fractions from extraction (ROBINSONand CUMMINS, 1960), proposed successive oxidation steps, was studied in order to that Green River kerogen consists mainly of aliphatic arrive at a reasonable reconstruction of the kerogen ring structures held together by short (less than C,) matrix. methylene interconnections with very few long aliphatic chains attached. On the other hand, DOUGLAS et EXPERIMENTAL al. (1968) and DJURICIC et al. (1971) suggested the The Green River shale sample (from the Anvil Point presence of significant amounts of long-chain aliphatic structures in the kerogen matrix. In contrast to oil shale mine) was supplied by the Laramie Energy Research Center, Wyoming. The raw shale was pulverized Robinson, DJURICIC et al. (1971, 1972) envisaged the to pass a lOO-mesh screen and pretreated with 10% hydrokerogen ‘nucleus’ as consisting of long (greater than chloric acid to remove the carbonate mineral. The carbonate-free shale was Soxhlet-extracted exhaustively with C,) polymethylene bridges. The present investigation provides quantitative ex- solvent (benzene/methanol at azeotropic ratio) before and after undergoing the HF/HCl treatment outlined by BURLperimental evidence which could help determine the INGAME and SIMONEIT (1969). The kerogen concentrate thus nature of the straight-chain structures in Green River obtained was free of solvent soluble material and contained kerogen, especially with regards to: (a) the relative the following: 69.0% carbon; 8.9% hydrogen and 8.9% ash. Approximately 2.1 g of this kerogen concentrate was proportion of straight-chain to non-straight-chain then subjected to the stepwise alkaline permanganate oxistructures, (b) the size distribution of the straightdation procedure depicted in Fig. 1. In each step, 40ml chain structures and (c) the manner in which these of KMn04-KOH solution (2% KMn04 in 1% KOH) was *-To whom communications
should be addressed.
structures form through cross entrapment in these ends, a
added and the qixture was allowed to react at a constant temperature of 75°C while steady mechanical stirring was 1411
G.C.A. 41/10-A
1412
D. K. You~c and T F. YEN Combmed gas chromatography-mass spectrometry (GC-MS) analyses were carried out m the Finnigan Laboratory at Sunnyville, California. A Finnigan model 9500 GC coupled to a Fmmgan quadrupole model 1015 D mass-spectrometer and a Fmnigan model 150 data reduction system was used; the chromatographlc column was stmtlar to the SE-30 column menttoned earlier. Separated components were identtfied by co-injection of standards, comparison of mass spectra with standard spectra. and by the interpretation of unknown spectra. Proton-NMR spectroscopy was performed on a Vartan A-60 NMR spectrometer. The fractions were dissolved in pyridine-d, to gave a concentration range of 6-109; (w/w). Areas under the NMR absorption bands were integrated with a planimeter (average of 10 measurements with a 95”; reproducibility). RESULTS
applied. At the termmation of each oxidation step (marked by the appearance of brown MnOz precipitate and the disappearance of the purple coloratton of the solutton), the aqueous phase was separated from the solid layer by centrifugatton and collected in a separate container for later analysis. From previous experience we found that removal of the accumulated MnOz precipitate facilitated subsequent oxidation of the remammg kerogen. The MnO, precipitate was removed by allowing the reactton mixture to react with an oxalat+H,SO., solution (all the oxalic acid would become COZ) and the kerogen residue rinsed thoroughly prior to the next oxidation step. The average time for each oxidation step was less than one hour. At the end of the ninth oxrdatton step, only a trace amount of inorganic material remained and the reaction mixture did not show signs of further oxidation even when subjected to more than 20 hours of oxidatton: therefore the trace residue was discarded. The aqueous fractions collected from each of the nine oxidation steps were analyzed separately according to the following procedure. Each aqueous fraction was acidified with HC1 to a pH _ 1, the precipitated orgamc acids were collected by centrifugation. rmsed three times and lyophihzed. These prectpitated acid fractions were denoted as F-fractions (Fig. 1I. After the removal of the precipitated acid fractions. the aqueous layer was also lyophilized and the solid residue thoroughly extracted with a 1:l mtxture, by volume, of methanol/ethyl ether to remove the soluble organic acids. The soluble acid fractions were denoted as the E-fractions (Fig. 1). Both the precipitated and the soluble acid fractions were esterified with BCla in methanol; hexane extracts of each esterified fraction were analyzed by gas--liquid chromatography (GLC). GLC analyses were carried out on a Hewlett Packard research model 5750 chromatograph equipped wrth a hydrogen flame ionization detector. The columns used were: (1)274 x 0.32cm stainless steel column packed with 3”,/,SE-30 on 100/120 mesh Gas-Chrom Q. helium carrier gas flow 35-40 ml/min., temperature program at 8C/min., from 100 to 300°C. (2) 152 x 0.32 cm stainless steel column packed w&h 5?, XE-60 on SO/l00 mesh Chromosorb G. AW. DMCS, helium carrier gas 35-40 mI/min.. temperature program at 8’C/min. from 100 to 25O’C.
The quantity of organic product recovered from each oxidation step is shown in Fig. 2. The total weight was 1.54 g (about 74?, of the original kerogen concentrate) of which 0.97 g was the precipitated acids (F-fractions) and 0.57 g the soluble acids (~-fractions). The largest fractions were F-7, F-8, and F-9, which accounted for nearly 400, of the total product weight. The F-fractions were solid powders and the E-fractions were semi-solids. Also, the color of these fractrons changed gradually from the dark brown of the earlier oxidation steps to a pale yellow at the later steps. Results of elemental analyses of representative F-fractions are listed in Table 1. GLC analysis of the fractions revealed primarily the same homologous series of acids as reported by BURLINGAMEand SIMONEIT (1969) and DJURICIC et al. (1971). However, the major difference between the l
- F fraction
tPmlplrat*d Acid*)
Ftg. 2. The weight of product recovered from each step of oxtdation. The F-fraction represents the material that preciprtated out upon acidification (pH _ 1) while the E-fraction is the material that remained soluble. The initial wetght of kerogen concentrate was about two grams.
1413
Straight-chain aliphatic structures Table 1. Elemental analysis of selected fractions recovered from stepwise oxidation of Green River kerogen
F-Z
66.7
8.19
2.29
1.85
21.0
1.47
F-4
67.8
8.77
2.09
2.19
19.1
1.55
F-7
69.5
9.32
2.11
1.15
18.0
1.61
F-9
69.9
9.57
1.78
1.20
17.6
1.64
* Wt% of oxygen by difference, ash content is less than 0.2% for all samples. yields of this work and those of BURLINGAME et al. (1969) is the distribution of monocarboxylic acids and dicarboxylic acids. As shown in Fig. 3, the dicarboxylic acids were produced very early and continued to be produced in approximately the same quantities throughout the entire sequence of oxidation steps. On the other hand, negligible quantities of monocarboxylit acids (normal and isoprenoid) were detected in the earlier F-fractions but gradually became dominant in
F5
the later F-fractions (Fig. 3). The E-fractions (soluble acids) were primarily discarboxylic acids. From the GC-MS analysis, a minor series of n-alkanes (C25-C35r maximum at Cz6) was detected. The possibility of impure hexane solvent as the source of this series was excluded, since high sensitivity GLC analysis of lOO-fold concentrated hexane solvent failed to reveal this series. The amount of this series increased gradually from the early oxidation steps,
D
Dl4 D12 I
‘I
F9
Fig. 3. The gas chromatogram (on XE-60 column packing) of material extracted by hexane from the BCl,-methanol esterification mixture of (a) oxidation step F-5, (b) oxidation step F-9. The cross-hatched peaks represent the straight-chain monocarboxylic acids; the letter D denotes the straight-chain dicarboxylic acids; n denotes the normal alkanes; and b denotes the branched (isoprenoid) acids. The number of carbons in the structure are indicated by the Arabic numbers in all cases.
1414
D.
K YOUNGand T. F. YEN
F4
I
I
I
I
I
I
12
11
10
9
8
7
I
I
5 6 6 PPM fraction F-4 dissolved
I
I
I
I
I
4
3
2
1
0
Fig. 4. Proton-NMR spectrum of total in pyridine-d,. The residual pyrldine protons are denoted by 1, fi, and )‘. The absorption band centered at 6 - 1.25, range 6 _ 1.05-1.45 has been assigned to straight-chain methylene protons, HR. The 6 - 0.5-3.5 band has been assigned to the total protons excluding the ionizable ones. The band centered at 6 _ 9 IS due to the presence of water in the pyrldine solvent. apparently reached a maximum near the intermediate steps (e.g. F-5, Fig. 3) and then decreased with further oxidation steps. The proton-NMR spectra of the F-fractions were very similar; in Fig. 4 a represtative spectrum (fraction F-4) is shown. The resonance peak at 6 w 9 ppm was due to the presence of water in the pyridine solvent (RETCOFSKYand FREIDEL, 1970); addition of D20 shifted this peak upfield to 6 c 5.8. Almost all the protons were included in the resonance region of 6 _ 0.5-3.5 ppm. The aromatic proton resonance region (6 w 6.5-8.5 ppm) did not show any discernible peak. This indicated that there were very few aromatic protons in the kerogen derived products. Detailed carbon type distributions could be estimated from the proton-NMR spectrum (SPEIGHT, 1970a; YEN and ERDMAN,1962). In the present study, however, the carbons were divided into only two major types: (a) the straight-chain methylene carbon, &, and (b) the non-straight-chain carbon CN, which included the saturated ring carbons and the unsaturated ring carbons. The fraction of straight-chain methylene carbon, CR, relative to total carbon, C, was estimated by the equation:
6 - 1.25 ppm) to the total area of the peak ranging from S z 0.5 to 3.5 ppm (Fig. 4). (HJH) is the correction for ionizable protons, primarily the carboxylic acid protons. This quantity was estimated from the oxygen weight percent, Oo;, and the hydrogen weight percent, HPd, by the expression (Hi/H) = l/32 (O:;/HS,). The (CR/C) estimations, in percentages, for selected F-fractions are presented in Table 2.
DISCUSSION
The complex structure of kerogen makes it extremely difficult to understand the mechanism of its degradation by oxidation. The degradative process probably involves the addition of oxygen to functional linkages and carbon-carbon branch points. Therefore, the oxygen content should be expected to increase as oxidation progresses (SIMONEIT and BURLINGAME,1973). However, in the present investigation, the oxygen content decreased slightly with oxidation, from 21% in Fraction F-2 to 17.6% in the last fraction, F-9 (Table 1). This may indicate that components with unknown larger hydrocarbon structures CR/C = 1/2(H,/H),,, (1 - Hi/H) (H/C) were degraded from the kerogen during the later oxiwhere H/C is the atomic ratio calculated from ele- dation steps. This is supported by GLC analysis (Fig. mental analyses (Table 1). (HR/H)NMRis the ratio of 3) where the long-chain monocarboxylic, acids (range the area of straight-chain methylene proton, HR, C10-C35, maximum C&were predominant in later resonance peak (6 z 1.05-1.45 ppm, centered at fractions (e.g. F-9).
1415
Straight-chain aliphatic structures Table 2. Results of calculations based on proton-NMR (see text for details)
and elemental analysis
Fraction nunhr
F-2
0.0802
0.356
24.1
F-4
0.0682
0.407
29.4
F-7
0.0602
0.439
33.2
F-9
0.0575
0.486
37.5
* Ionizable protons. ** Straight-chain methylene protons. *** Straight-chain methylene carbons. Without further degradation, many of these unknown larger hydrocarbon structures derived from kerogen oxidation were too complex for conventional GLC or GC-MS analysis. The situation is perhaps analogous to the difficulties involved in the studies of the humic and sub-humic acids (VAN KREVELEN, 1961) derived from coal oxidation (DAVIES and LAWSON, 1962). Based on the results of GLC analysis alone (Fig. 3). it would seem that the oxidation products were mainly composed of straight-chain aliphatic components. On the other hand, the H/C atomic ratios (Table 1) of these products suggested the presence of non-straight-chain structures, such as fused aliphatic or naphthenic and aromatic hydrocarbon structures. In order to estimate the fraction of straight-chain carbon structures in the products derived from kerogen oxidation, we employed proton-NMR spectroscopy. The NMR method has previously been used to characterize the structural types in bitumen (SPEIGHT, 197Oa), asphaltene (SPEIGHT, 1970b; YEN and ERDMAN, 1962) and extracts derived from coal (RETCOFSKY and FRIJZDEL, 1970). Although this method does not provide the kind of detailed quantitative information on individual components in a sample mixture as the GLC method does, it is, however, useful in estimating the relative proportion of structural types present in the whole sample. The NMR method is especially useful in characterizing large hydrocarbon structures derived from degradation of geo-organic materials (e.g. coal and kerogen) which are too complex for GLC or GC-MS analysis. In the present application of NMR spectroscopy we did not detect any aromatic protons in the oxidation products (Fig. 4). This could be the result of: (a) the existence of condensed aromatic or excessively alkyl-substituted aromatic structures since the resonances around 6 = 1.s3.5 ppm may have significant contributions from protons tl and fi to aromatic carbons (Fig. 4); and (b) a low content of aromatic protons in the kerogen. e.g. below the detection limit of the NMR technique. Working with model compound mixtures, we were able to detect an aromatic proton content of approximately 1% (based on unpublished studies by the authors). The aromaticity If,)
of the kerogen from Green River oil shale that has been determined recently seems to support (b). The _& value of kerogen determined by solid state 13C NMR is about 0.15 (BATUSKAand MACIEL, 1976). The X-ray diffraction value is also 0.15 (KWAN and YEN, 1976). The strongest NMR peak was centered at 6 = 1.25 ppm; it was primarily due to the straightchain methylene protons, HR (SPEIGHT, 1970a; YEN and ERDMAN, 1962). The percentage of straight-chain methylene carbons, C,/C, in each F-fraction was estimated from the area of this peak (Table 2). The (C,/C) percentage increased from 20% in the earlier F-fractions to almost 407; in the last fraction, F-9. This increase in straight-chain aliphatic carbon structures paralleled the increase in the (H/C) atomic ratios (Table 1) and in the quantities of long-chain carboxylit acids produced from the kerogen as oxidation progressed (Fig. 3). These results suggested that the straight-chain aliphatic structures present in the kerogen were mostly longer than Cq. The pattern of carboxylic acid distribution in each oxidation step revealed certain features of the kerogen structure: (a) The kerogen matrix may be thought of as a three-dimensional network of non-straight-chain clusters interconnected by long polymethylene bridges (Fig. 5). Proton-NMR measurements revealed substantial amounts (cu. 60-80%) of non-straight-chain carbon structures in each of the oxidation fractions. Also, GLC analysis showed that dicarboxylic acids constituted a significant portion of the long chain fatty acids produced in each of the oxidation steps (Fig. 3). The dicarboxylic acids were postulated to be derived from oxidation of the polymethylene bridges at their sites of connection to the kerogen matrix (DJURICIC et al., 1971). (b) Near the ‘core’ of the kerogen matrix were attached, via one terminus only, long-chain aliphatic structures and branched aliphatic structures which upon oxidation, gave rise to the normal monocarboxylic acids and isoprenoid acids respectively. This is because more monocarboxylic acids (both straight-chain and branched) were produced toward the end of the stepwise oxidation sequence (e.g. F-9 in Fig. 3).
D. K. YOUNGand T. F. YEN
1416
rJcM
ENTRAPPEDSPECIES
-
UNBRANCHED ALIPHATICSTRUCTURE
-
BRANCHEDALIPHATIC STRUCTURE
_---
POLYYETHYLENEBRIDGES ‘CYCLIC’ SKELETAL CARBON STRUCTURE (MAINLY ALIPHATIC RINGS)
Fig. 5. Hypothetical
structure
of Green
River kerogen.
(c) The presence of the n-alkane series among the oxidation products suggested that there were entrapped components in the kerogen matrix (Fig. 5). This series was produced from the kerogen in a significant quantity only during the intermediate oxidation steps (e.g. F-5). However, the quaqtity was small when compared with the carboxylic acid products (Fig. 3). The carboxylic acids were especially dominant in the late oxidation steps (e.g. F-7, F-8 and F-9). If all the fractions were combined and analyzed as a whole, the presence of this n-alkane series would have been relatively negligible (DJURICIC et ul. 1971). SCHMIDT-COLLERUSand PRIEN (1976) reported the isolation of similar entrapped materials from kerogen that had previously been extracted for soluble materials. They speculated that those entrapped materials were residual protokerogen or post-kerogen components which gradually diffused to the kerogen surface over the course of two years. The normal carboxylic acids in the oxidation products were probably not derived from the oxidation of the entrapped n-alkanes since intermediate oxidation products (e.g. alcohols and ketones) were not detected. On the other hand, some of the carboxylit acids may have been physically entrapped components. Based on the results of previous investigations (BURLINGAMEand SIMONEIT,1969; and DJURICIC et al., 1971), the existence of entrapped materials were somewhat unexpected. More studies on the composition of these entrapped materials are needed to establish their importance in the diagenesis of kerogen. SCHNITZER and NEYROUND (1975) extracted an n-alkane series (major range C18-C36) from naturally formed humic acids and fulvic acids in soil. The most effective methods of extraction found were methyla-
tion (or esterlfication) and hydrolysis at 17U’C combined with ultrasonic dispersion. They suggested a blological origin for the n-alkanes, since the distribution and the odd/even carbon ratio of 1.0 resembled that of microbial hydrocarbons (JONES, 1969). The material entrapped in the Green River kerogen was probably very tightly confined so that during oxidative degradation this material remained physically entrapped m ‘humic-like’ Intermediate oxidation products. Mild solvent extraction alone could not release it from the entrapping macromolecular structure without prior treatments (e.g. esterification). This physical entrapment may explain how the long n-alkane molecules could ‘dissolve’ in the alkaline aqueous oxidation medium. At present the origin of the entrapped alkanes in this relatively young kerogen (Eocene) is not clear. Biological contamination as well as diagenesis of kerogen are possible causes. The straight-chain aliphatic acids only account for part of the organic products As shown by the proton--NMR measurements, the majority of the organic carbons belong to non-straight-chain structures (ca. 6&800,,). Preliminary GC-MS and X-ray diffraction studies indicate that these non-straight-chain structures are mostly fused-rmg cycloalkanes. Since these structures were not detected in significant quantities in GLC, It is suggested that they are produced by kerogen oxidation as macromolecular components (i.e. similar to humic acids from coal oxidation). Further understanding of the kerogen structure would entall the study of these macromolecular components (WEN et d.. 1977). .~cknowlrdgemmtsThls Grant No. GI-35683. BR-48-12.
work PRF
was supported by NSF 6272-AC2 and A.G.A.
REFERENCES BATUSKA V. J.
and
MAC~EL G. E. (1976) 13C NMR
of solid coal samules. Chemistry Workshop, Institute.
studies of the 1976 Coal Stanford Research
In Preermts pp. 22@228
BURLINGAME A. L and SIMONEI?‘ B. R.
(I 969) High resolutlon mass spectrometry of Green River formation kerogen oxldatibn. Nature-222, 252-256.
DAVIES C. and LAWSON G. J. (1962) Chemical
constitution
of coal XII--optimum conditions for the preparation of subhumic acids from humic acids by oxidation with alkaline potassmm permanganate, Fuel 41, 131. DJIJRICIC M., MURPHY R. C.. VITOROVIC D. and BIEMANN K. (1971) Organic acids obtained by alkaline permanganate oxidation of kerogen from the Green River (Colorado) shale, Geochim. Cosmoclzrm. Acta 35, 1201-1207. DJURICIC M., VITOROVIC D., ANDERSEN B. D.. HERTZ H. S.. MURPHY R. C., F%ETI G and BIEMANN K. (1972)
Acids obtained by oxldatlon of kerogens of ancient sediments of different geographic origin. In Advances zn Organzc Geochemistry 1971. Proceedmgs of 5th International Meetmg on Organic Geochemistry. (H. R. V Gaertner and H. Wehner. editors), pp. 305-321. Pergamon Press. DOUGLAS A. G., EGLINTON G. and HENDERSON W. (1968) Thermal
alteration
of the orgamc
matter m sediments.
In Organic Geochemzstry. Proceedings of 7th International Meeting on Organic Geochemistry (G. D Hob-
Straight-chain aliphatlc structures son and G. C. Speers, editors), pp. 369-388, Pergamon Press. JONESJ. G. (1969) Studies on lipids of soil microorganisms with particular reference to hydrocarbons. J. Gen. Microbiol. 59, 145-152. KWAN J. T. and YEN T. F. (1976) Aromaticity determination of coal, oil shale, and their derivatives by X-ray diffraction. A.C.S. Div. Fuel Chem., Preprint. 21(7), 61-16. RETCOF~KY H. L. and FRIEDELR. A. (1970) Spectra of coals and coal extracts: proton magnetic resonance spectra of pyridine and carbon disulfide extracts. In Spectrometry ofFuels,(R. A. Friedel, editor), pp. 7&89. Plenum Press. ROBINSONW. E. and CUMMINSJ. J. (1960) Composition of low temperature thermal extracts from Colorado oil shale. J. Chem. Eng. Data, 5, 74-80. ROBINSON W. E., LAWLORD. L., CUMMINSJ. J. and FESTER J. I. (1963) Oxidation of Colorado Oil Shale. Bureau of Mines Report of Investigations, 6166, l-33. ROBIN~CJN W. E. (1976) Origin and characteristics of Green River oil shale. In Oil Shale (T. F. Yen and G. V. Chilingarian, editors), pp. 61-79. Elsevier. SCHMIDT-C• LLERUS J. J. and PRIENC. H. (1976) Structural investigations on Green River oil shale kerogen. In
1417
Science and Techndogy
of Oil Shale, (T. F. Yen, editor), pp. 183-192. SCHNITZERM. and NEYROUNDJ. A. (1975) Alkanes and fatty acids in humic substances. Fuel 54, 17-19. SIMONEITB. R. and BURLINGAME A. L. (1973) Carboxylic acid derived from Tasmanian tasmanite by extractions and kerogen oxidations. Geochim. Cosmochim. Acta 31, 591610. SPEIGHTJ.
G. (1970a) A structural investigation of the constitutents of athabasca bitumen by proton magnetic resonance spectroscopy. Fuel 49, 7690. SPEIGHTJ. G. (1970b) Structural analysis of athabasca asphaltenes by proton magnetic resonance spectroscopy. Fuel 49, 102-112. VAN KREVELEND. W. (1961) Coal, pp. 225231. Elsevier. WEN C. S., CHILINGARIANG. V. and YEN T. F. (1977) Properties and structure of bitumens. In Bitumens, Asphalts and Tar Sands (G. V. Chilingarian and T. F. Yen, editors), chapter 7. Elsevier. YEN T. F. and ERDMANJ. G. (1962) Invesbgation of the structure of petroleum asphaltenes and related substances by proton magnetic resonance. A.C.S., Div. Petrol. Chem., Preprints, 7, 99-111.
Acta,1917. Vol 41,pp 1411to 1417PergamonPress PrIntedin GreatBrltaln
The nature of straight-chain aliphatic structures in Green River kerogen D. K. YOUNG and T. F. YEN* Departments of Chemical Engineering and Medicine (Biochemistry), and Program of Environmental Engineering, University of Southern California, Los Angeles, CA 90007, U.S.A. (Received 3 November 1975; accepted in revised form 6 June 1977) Abstract-Previous studies of the Green River kerogen only provide apparently contradictory conclusions about the size of the straight-chain aliphatic structures as well as the manner in which these structures form part of the kerogen matrix. The present investigation is an attempt to resolve this contradiction. A mild stepwise oxidation procedure was followed so that extensive degradation of kerogen-derived intermediates could be prevented. Products isolated from each oxidation step were analyzed by conventional GLC techniques, GC-MS, and proton-NMR measurements in order to ascertain the significance of the straight-chain aliphatic structures present in the Green River kerogen. The following results were obtained: (a) Green River kerogen contains a substantial portion (ca 24 carbons out of every 10) of straight-chain aliphatic structures which are longer than C4, (b) the kerogen matrix forms a three-dimensional network of non-straight-chain clusters interconnected by long polymethylene cross-links, (c) the ‘core’, in comparison with the ‘periphery’ of the kerogen matrix, contains a greater proportion of straight-chain and branched aliphatic structures which are attached to the kerogen matrix at one terminus, (d) some of the straight-chain structures may exist as physically entrapped components in the kerogen matrix.
INTRODUCTION
portions of the kerogen matrix, (e.g. linkage, terminal attachment, or IN THE past, several investigators have employed difthe kerogen structure). To achieve ferent methods to elucidate the structure of kerogen mild stepwise oxidation procedure isolated from the Green River oil shale. The basic (ROBINSONet al., 1963; DJURICI et al., 1971) was utilapproach of these methods invariably involves the ized which prevented extensive degradation of interdegradation of the kerogen matrix into smaller strucmediate products released from the kerogen matrix. tural components which can be analyzed using conventional laboratory instruments. The complexity of The products of each oxidation step were analyzed. Conventional gas chromatographic techniques were the kerogen structure renders the choice of suitable supplemented with proton-NMR and elemental methods for studying the kerogen difficult. Depending analysis for many of the larger structural units deon the extent of kerogen degradation and the techrived from kerogen degradation in order to estimate nique of analysis employed, apparently inconsistent results have been reported. For example, ROBINSON the proportion of straight-chain structures in each product fraction. The pattern of kerogen degradation, (1976), based on the analyses of products from solvent as revealed by the composition of the fractions from extraction (ROBINSONand CUMMINS, 1960), proposed successive oxidation steps, was studied in order to that Green River kerogen consists mainly of aliphatic arrive at a reasonable reconstruction of the kerogen ring structures held together by short (less than C,) matrix. methylene interconnections with very few long aliphatic chains attached. On the other hand, DOUGLAS et EXPERIMENTAL al. (1968) and DJURICIC et al. (1971) suggested the The Green River shale sample (from the Anvil Point presence of significant amounts of long-chain aliphatic structures in the kerogen matrix. In contrast to oil shale mine) was supplied by the Laramie Energy Research Center, Wyoming. The raw shale was pulverized Robinson, DJURICIC et al. (1971, 1972) envisaged the to pass a lOO-mesh screen and pretreated with 10% hydrokerogen ‘nucleus’ as consisting of long (greater than chloric acid to remove the carbonate mineral. The carbonate-free shale was Soxhlet-extracted exhaustively with C,) polymethylene bridges. The present investigation provides quantitative ex- solvent (benzene/methanol at azeotropic ratio) before and after undergoing the HF/HCl treatment outlined by BURLperimental evidence which could help determine the INGAME and SIMONEIT (1969). The kerogen concentrate thus nature of the straight-chain structures in Green River obtained was free of solvent soluble material and contained kerogen, especially with regards to: (a) the relative the following: 69.0% carbon; 8.9% hydrogen and 8.9% ash. Approximately 2.1 g of this kerogen concentrate was proportion of straight-chain to non-straight-chain then subjected to the stepwise alkaline permanganate oxistructures, (b) the size distribution of the straightdation procedure depicted in Fig. 1. In each step, 40ml chain structures and (c) the manner in which these of KMn04-KOH solution (2% KMn04 in 1% KOH) was *-To whom communications
should be addressed.
structures form through cross entrapment in these ends, a
added and the qixture was allowed to react at a constant temperature of 75°C while steady mechanical stirring was 1411
G.C.A. 41/10-A
1412
D. K. You~c and T F. YEN Combmed gas chromatography-mass spectrometry (GC-MS) analyses were carried out m the Finnigan Laboratory at Sunnyville, California. A Finnigan model 9500 GC coupled to a Fmmgan quadrupole model 1015 D mass-spectrometer and a Fmnigan model 150 data reduction system was used; the chromatographlc column was stmtlar to the SE-30 column menttoned earlier. Separated components were identtfied by co-injection of standards, comparison of mass spectra with standard spectra. and by the interpretation of unknown spectra. Proton-NMR spectroscopy was performed on a Vartan A-60 NMR spectrometer. The fractions were dissolved in pyridine-d, to gave a concentration range of 6-109; (w/w). Areas under the NMR absorption bands were integrated with a planimeter (average of 10 measurements with a 95”; reproducibility). RESULTS
applied. At the termmation of each oxidation step (marked by the appearance of brown MnOz precipitate and the disappearance of the purple coloratton of the solutton), the aqueous phase was separated from the solid layer by centrifugatton and collected in a separate container for later analysis. From previous experience we found that removal of the accumulated MnOz precipitate facilitated subsequent oxidation of the remammg kerogen. The MnO, precipitate was removed by allowing the reactton mixture to react with an oxalat+H,SO., solution (all the oxalic acid would become COZ) and the kerogen residue rinsed thoroughly prior to the next oxidation step. The average time for each oxidation step was less than one hour. At the end of the ninth oxrdatton step, only a trace amount of inorganic material remained and the reaction mixture did not show signs of further oxidation even when subjected to more than 20 hours of oxidatton: therefore the trace residue was discarded. The aqueous fractions collected from each of the nine oxidation steps were analyzed separately according to the following procedure. Each aqueous fraction was acidified with HC1 to a pH _ 1, the precipitated orgamc acids were collected by centrifugation. rmsed three times and lyophihzed. These prectpitated acid fractions were denoted as F-fractions (Fig. 1I. After the removal of the precipitated acid fractions. the aqueous layer was also lyophilized and the solid residue thoroughly extracted with a 1:l mtxture, by volume, of methanol/ethyl ether to remove the soluble organic acids. The soluble acid fractions were denoted as the E-fractions (Fig. 1). Both the precipitated and the soluble acid fractions were esterified with BCla in methanol; hexane extracts of each esterified fraction were analyzed by gas--liquid chromatography (GLC). GLC analyses were carried out on a Hewlett Packard research model 5750 chromatograph equipped wrth a hydrogen flame ionization detector. The columns used were: (1)274 x 0.32cm stainless steel column packed with 3”,/,SE-30 on 100/120 mesh Gas-Chrom Q. helium carrier gas flow 35-40 ml/min., temperature program at 8C/min., from 100 to 300°C. (2) 152 x 0.32 cm stainless steel column packed w&h 5?, XE-60 on SO/l00 mesh Chromosorb G. AW. DMCS, helium carrier gas 35-40 mI/min.. temperature program at 8’C/min. from 100 to 25O’C.
The quantity of organic product recovered from each oxidation step is shown in Fig. 2. The total weight was 1.54 g (about 74?, of the original kerogen concentrate) of which 0.97 g was the precipitated acids (F-fractions) and 0.57 g the soluble acids (~-fractions). The largest fractions were F-7, F-8, and F-9, which accounted for nearly 400, of the total product weight. The F-fractions were solid powders and the E-fractions were semi-solids. Also, the color of these fractrons changed gradually from the dark brown of the earlier oxidation steps to a pale yellow at the later steps. Results of elemental analyses of representative F-fractions are listed in Table 1. GLC analysis of the fractions revealed primarily the same homologous series of acids as reported by BURLINGAMEand SIMONEIT (1969) and DJURICIC et al. (1971). However, the major difference between the l
- F fraction
tPmlplrat*d Acid*)
Ftg. 2. The weight of product recovered from each step of oxtdation. The F-fraction represents the material that preciprtated out upon acidification (pH _ 1) while the E-fraction is the material that remained soluble. The initial wetght of kerogen concentrate was about two grams.
1413
Straight-chain aliphatic structures Table 1. Elemental analysis of selected fractions recovered from stepwise oxidation of Green River kerogen
F-Z
66.7
8.19
2.29
1.85
21.0
1.47
F-4
67.8
8.77
2.09
2.19
19.1
1.55
F-7
69.5
9.32
2.11
1.15
18.0
1.61
F-9
69.9
9.57
1.78
1.20
17.6
1.64
* Wt% of oxygen by difference, ash content is less than 0.2% for all samples. yields of this work and those of BURLINGAME et al. (1969) is the distribution of monocarboxylic acids and dicarboxylic acids. As shown in Fig. 3, the dicarboxylic acids were produced very early and continued to be produced in approximately the same quantities throughout the entire sequence of oxidation steps. On the other hand, negligible quantities of monocarboxylit acids (normal and isoprenoid) were detected in the earlier F-fractions but gradually became dominant in
F5
the later F-fractions (Fig. 3). The E-fractions (soluble acids) were primarily discarboxylic acids. From the GC-MS analysis, a minor series of n-alkanes (C25-C35r maximum at Cz6) was detected. The possibility of impure hexane solvent as the source of this series was excluded, since high sensitivity GLC analysis of lOO-fold concentrated hexane solvent failed to reveal this series. The amount of this series increased gradually from the early oxidation steps,
D
Dl4 D12 I
‘I
F9
Fig. 3. The gas chromatogram (on XE-60 column packing) of material extracted by hexane from the BCl,-methanol esterification mixture of (a) oxidation step F-5, (b) oxidation step F-9. The cross-hatched peaks represent the straight-chain monocarboxylic acids; the letter D denotes the straight-chain dicarboxylic acids; n denotes the normal alkanes; and b denotes the branched (isoprenoid) acids. The number of carbons in the structure are indicated by the Arabic numbers in all cases.
1414
D.
K YOUNGand T. F. YEN
F4
I
I
I
I
I
I
12
11
10
9
8
7
I
I
5 6 6 PPM fraction F-4 dissolved
I
I
I
I
I
4
3
2
1
0
Fig. 4. Proton-NMR spectrum of total in pyridine-d,. The residual pyrldine protons are denoted by 1, fi, and )‘. The absorption band centered at 6 - 1.25, range 6 _ 1.05-1.45 has been assigned to straight-chain methylene protons, HR. The 6 - 0.5-3.5 band has been assigned to the total protons excluding the ionizable ones. The band centered at 6 _ 9 IS due to the presence of water in the pyrldine solvent. apparently reached a maximum near the intermediate steps (e.g. F-5, Fig. 3) and then decreased with further oxidation steps. The proton-NMR spectra of the F-fractions were very similar; in Fig. 4 a represtative spectrum (fraction F-4) is shown. The resonance peak at 6 w 9 ppm was due to the presence of water in the pyridine solvent (RETCOFSKYand FREIDEL, 1970); addition of D20 shifted this peak upfield to 6 c 5.8. Almost all the protons were included in the resonance region of 6 _ 0.5-3.5 ppm. The aromatic proton resonance region (6 w 6.5-8.5 ppm) did not show any discernible peak. This indicated that there were very few aromatic protons in the kerogen derived products. Detailed carbon type distributions could be estimated from the proton-NMR spectrum (SPEIGHT, 1970a; YEN and ERDMAN,1962). In the present study, however, the carbons were divided into only two major types: (a) the straight-chain methylene carbon, &, and (b) the non-straight-chain carbon CN, which included the saturated ring carbons and the unsaturated ring carbons. The fraction of straight-chain methylene carbon, CR, relative to total carbon, C, was estimated by the equation:
6 - 1.25 ppm) to the total area of the peak ranging from S z 0.5 to 3.5 ppm (Fig. 4). (HJH) is the correction for ionizable protons, primarily the carboxylic acid protons. This quantity was estimated from the oxygen weight percent, Oo;, and the hydrogen weight percent, HPd, by the expression (Hi/H) = l/32 (O:;/HS,). The (CR/C) estimations, in percentages, for selected F-fractions are presented in Table 2.
DISCUSSION
The complex structure of kerogen makes it extremely difficult to understand the mechanism of its degradation by oxidation. The degradative process probably involves the addition of oxygen to functional linkages and carbon-carbon branch points. Therefore, the oxygen content should be expected to increase as oxidation progresses (SIMONEIT and BURLINGAME,1973). However, in the present investigation, the oxygen content decreased slightly with oxidation, from 21% in Fraction F-2 to 17.6% in the last fraction, F-9 (Table 1). This may indicate that components with unknown larger hydrocarbon structures CR/C = 1/2(H,/H),,, (1 - Hi/H) (H/C) were degraded from the kerogen during the later oxiwhere H/C is the atomic ratio calculated from ele- dation steps. This is supported by GLC analysis (Fig. mental analyses (Table 1). (HR/H)NMRis the ratio of 3) where the long-chain monocarboxylic, acids (range the area of straight-chain methylene proton, HR, C10-C35, maximum C&were predominant in later resonance peak (6 z 1.05-1.45 ppm, centered at fractions (e.g. F-9).
1415
Straight-chain aliphatic structures Table 2. Results of calculations based on proton-NMR (see text for details)
and elemental analysis
Fraction nunhr
F-2
0.0802
0.356
24.1
F-4
0.0682
0.407
29.4
F-7
0.0602
0.439
33.2
F-9
0.0575
0.486
37.5
* Ionizable protons. ** Straight-chain methylene protons. *** Straight-chain methylene carbons. Without further degradation, many of these unknown larger hydrocarbon structures derived from kerogen oxidation were too complex for conventional GLC or GC-MS analysis. The situation is perhaps analogous to the difficulties involved in the studies of the humic and sub-humic acids (VAN KREVELEN, 1961) derived from coal oxidation (DAVIES and LAWSON, 1962). Based on the results of GLC analysis alone (Fig. 3). it would seem that the oxidation products were mainly composed of straight-chain aliphatic components. On the other hand, the H/C atomic ratios (Table 1) of these products suggested the presence of non-straight-chain structures, such as fused aliphatic or naphthenic and aromatic hydrocarbon structures. In order to estimate the fraction of straight-chain carbon structures in the products derived from kerogen oxidation, we employed proton-NMR spectroscopy. The NMR method has previously been used to characterize the structural types in bitumen (SPEIGHT, 197Oa), asphaltene (SPEIGHT, 1970b; YEN and ERDMAN, 1962) and extracts derived from coal (RETCOFSKY and FRIJZDEL, 1970). Although this method does not provide the kind of detailed quantitative information on individual components in a sample mixture as the GLC method does, it is, however, useful in estimating the relative proportion of structural types present in the whole sample. The NMR method is especially useful in characterizing large hydrocarbon structures derived from degradation of geo-organic materials (e.g. coal and kerogen) which are too complex for GLC or GC-MS analysis. In the present application of NMR spectroscopy we did not detect any aromatic protons in the oxidation products (Fig. 4). This could be the result of: (a) the existence of condensed aromatic or excessively alkyl-substituted aromatic structures since the resonances around 6 = 1.s3.5 ppm may have significant contributions from protons tl and fi to aromatic carbons (Fig. 4); and (b) a low content of aromatic protons in the kerogen. e.g. below the detection limit of the NMR technique. Working with model compound mixtures, we were able to detect an aromatic proton content of approximately 1% (based on unpublished studies by the authors). The aromaticity If,)
of the kerogen from Green River oil shale that has been determined recently seems to support (b). The _& value of kerogen determined by solid state 13C NMR is about 0.15 (BATUSKAand MACIEL, 1976). The X-ray diffraction value is also 0.15 (KWAN and YEN, 1976). The strongest NMR peak was centered at 6 = 1.25 ppm; it was primarily due to the straightchain methylene protons, HR (SPEIGHT, 1970a; YEN and ERDMAN, 1962). The percentage of straight-chain methylene carbons, C,/C, in each F-fraction was estimated from the area of this peak (Table 2). The (C,/C) percentage increased from 20% in the earlier F-fractions to almost 407; in the last fraction, F-9. This increase in straight-chain aliphatic carbon structures paralleled the increase in the (H/C) atomic ratios (Table 1) and in the quantities of long-chain carboxylit acids produced from the kerogen as oxidation progressed (Fig. 3). These results suggested that the straight-chain aliphatic structures present in the kerogen were mostly longer than Cq. The pattern of carboxylic acid distribution in each oxidation step revealed certain features of the kerogen structure: (a) The kerogen matrix may be thought of as a three-dimensional network of non-straight-chain clusters interconnected by long polymethylene bridges (Fig. 5). Proton-NMR measurements revealed substantial amounts (cu. 60-80%) of non-straight-chain carbon structures in each of the oxidation fractions. Also, GLC analysis showed that dicarboxylic acids constituted a significant portion of the long chain fatty acids produced in each of the oxidation steps (Fig. 3). The dicarboxylic acids were postulated to be derived from oxidation of the polymethylene bridges at their sites of connection to the kerogen matrix (DJURICIC et al., 1971). (b) Near the ‘core’ of the kerogen matrix were attached, via one terminus only, long-chain aliphatic structures and branched aliphatic structures which upon oxidation, gave rise to the normal monocarboxylic acids and isoprenoid acids respectively. This is because more monocarboxylic acids (both straight-chain and branched) were produced toward the end of the stepwise oxidation sequence (e.g. F-9 in Fig. 3).
D. K. YOUNGand T. F. YEN
1416
rJcM
ENTRAPPEDSPECIES
-
UNBRANCHED ALIPHATICSTRUCTURE
-
BRANCHEDALIPHATIC STRUCTURE
_---
POLYYETHYLENEBRIDGES ‘CYCLIC’ SKELETAL CARBON STRUCTURE (MAINLY ALIPHATIC RINGS)
Fig. 5. Hypothetical
structure
of Green
River kerogen.
(c) The presence of the n-alkane series among the oxidation products suggested that there were entrapped components in the kerogen matrix (Fig. 5). This series was produced from the kerogen in a significant quantity only during the intermediate oxidation steps (e.g. F-5). However, the quaqtity was small when compared with the carboxylic acid products (Fig. 3). The carboxylic acids were especially dominant in the late oxidation steps (e.g. F-7, F-8 and F-9). If all the fractions were combined and analyzed as a whole, the presence of this n-alkane series would have been relatively negligible (DJURICIC et ul. 1971). SCHMIDT-COLLERUSand PRIEN (1976) reported the isolation of similar entrapped materials from kerogen that had previously been extracted for soluble materials. They speculated that those entrapped materials were residual protokerogen or post-kerogen components which gradually diffused to the kerogen surface over the course of two years. The normal carboxylic acids in the oxidation products were probably not derived from the oxidation of the entrapped n-alkanes since intermediate oxidation products (e.g. alcohols and ketones) were not detected. On the other hand, some of the carboxylit acids may have been physically entrapped components. Based on the results of previous investigations (BURLINGAMEand SIMONEIT,1969; and DJURICIC et al., 1971), the existence of entrapped materials were somewhat unexpected. More studies on the composition of these entrapped materials are needed to establish their importance in the diagenesis of kerogen. SCHNITZER and NEYROUND (1975) extracted an n-alkane series (major range C18-C36) from naturally formed humic acids and fulvic acids in soil. The most effective methods of extraction found were methyla-
tion (or esterlfication) and hydrolysis at 17U’C combined with ultrasonic dispersion. They suggested a blological origin for the n-alkanes, since the distribution and the odd/even carbon ratio of 1.0 resembled that of microbial hydrocarbons (JONES, 1969). The material entrapped in the Green River kerogen was probably very tightly confined so that during oxidative degradation this material remained physically entrapped m ‘humic-like’ Intermediate oxidation products. Mild solvent extraction alone could not release it from the entrapping macromolecular structure without prior treatments (e.g. esterification). This physical entrapment may explain how the long n-alkane molecules could ‘dissolve’ in the alkaline aqueous oxidation medium. At present the origin of the entrapped alkanes in this relatively young kerogen (Eocene) is not clear. Biological contamination as well as diagenesis of kerogen are possible causes. The straight-chain aliphatic acids only account for part of the organic products As shown by the proton--NMR measurements, the majority of the organic carbons belong to non-straight-chain structures (ca. 6&800,,). Preliminary GC-MS and X-ray diffraction studies indicate that these non-straight-chain structures are mostly fused-rmg cycloalkanes. Since these structures were not detected in significant quantities in GLC, It is suggested that they are produced by kerogen oxidation as macromolecular components (i.e. similar to humic acids from coal oxidation). Further understanding of the kerogen structure would entall the study of these macromolecular components (WEN et d.. 1977). .~cknowlrdgemmtsThls Grant No. GI-35683. BR-48-12.
work PRF
was supported by NSF 6272-AC2 and A.G.A.
REFERENCES BATUSKA V. J.
and
MAC~EL G. E. (1976) 13C NMR
of solid coal samules. Chemistry Workshop, Institute.
studies of the 1976 Coal Stanford Research
In Preermts pp. 22@228
BURLINGAME A. L and SIMONEI?‘ B. R.
(I 969) High resolutlon mass spectrometry of Green River formation kerogen oxldatibn. Nature-222, 252-256.
DAVIES C. and LAWSON G. J. (1962) Chemical
constitution
of coal XII--optimum conditions for the preparation of subhumic acids from humic acids by oxidation with alkaline potassmm permanganate, Fuel 41, 131. DJIJRICIC M., MURPHY R. C.. VITOROVIC D. and BIEMANN K. (1971) Organic acids obtained by alkaline permanganate oxidation of kerogen from the Green River (Colorado) shale, Geochim. Cosmoclzrm. Acta 35, 1201-1207. DJURICIC M., VITOROVIC D., ANDERSEN B. D.. HERTZ H. S.. MURPHY R. C., F%ETI G and BIEMANN K. (1972)
Acids obtained by oxldatlon of kerogens of ancient sediments of different geographic origin. In Advances zn Organzc Geochemistry 1971. Proceedmgs of 5th International Meetmg on Organic Geochemistry. (H. R. V Gaertner and H. Wehner. editors), pp. 305-321. Pergamon Press. DOUGLAS A. G., EGLINTON G. and HENDERSON W. (1968) Thermal
alteration
of the orgamc
matter m sediments.
In Organic Geochemzstry. Proceedings of 7th International Meeting on Organic Geochemistry (G. D Hob-
Straight-chain aliphatlc structures son and G. C. Speers, editors), pp. 369-388, Pergamon Press. JONESJ. G. (1969) Studies on lipids of soil microorganisms with particular reference to hydrocarbons. J. Gen. Microbiol. 59, 145-152. KWAN J. T. and YEN T. F. (1976) Aromaticity determination of coal, oil shale, and their derivatives by X-ray diffraction. A.C.S. Div. Fuel Chem., Preprint. 21(7), 61-16. RETCOF~KY H. L. and FRIEDELR. A. (1970) Spectra of coals and coal extracts: proton magnetic resonance spectra of pyridine and carbon disulfide extracts. In Spectrometry ofFuels,(R. A. Friedel, editor), pp. 7&89. Plenum Press. ROBINSONW. E. and CUMMINSJ. J. (1960) Composition of low temperature thermal extracts from Colorado oil shale. J. Chem. Eng. Data, 5, 74-80. ROBINSON W. E., LAWLORD. L., CUMMINSJ. J. and FESTER J. I. (1963) Oxidation of Colorado Oil Shale. Bureau of Mines Report of Investigations, 6166, l-33. ROBIN~CJN W. E. (1976) Origin and characteristics of Green River oil shale. In Oil Shale (T. F. Yen and G. V. Chilingarian, editors), pp. 61-79. Elsevier. SCHMIDT-C• LLERUS J. J. and PRIENC. H. (1976) Structural investigations on Green River oil shale kerogen. In
1417
Science and Techndogy
of Oil Shale, (T. F. Yen, editor), pp. 183-192. SCHNITZERM. and NEYROUNDJ. A. (1975) Alkanes and fatty acids in humic substances. Fuel 54, 17-19. SIMONEITB. R. and BURLINGAME A. L. (1973) Carboxylic acid derived from Tasmanian tasmanite by extractions and kerogen oxidations. Geochim. Cosmochim. Acta 31, 591610. SPEIGHTJ.
G. (1970a) A structural investigation of the constitutents of athabasca bitumen by proton magnetic resonance spectroscopy. Fuel 49, 7690. SPEIGHTJ. G. (1970b) Structural analysis of athabasca asphaltenes by proton magnetic resonance spectroscopy. Fuel 49, 102-112. VAN KREVELEND. W. (1961) Coal, pp. 225231. Elsevier. WEN C. S., CHILINGARIANG. V. and YEN T. F. (1977) Properties and structure of bitumens. In Bitumens, Asphalts and Tar Sands (G. V. Chilingarian and T. F. Yen, editors), chapter 7. Elsevier. YEN T. F. and ERDMANJ. G. (1962) Invesbgation of the structure of petroleum asphaltenes and related substances by proton magnetic resonance. A.C.S., Div. Petrol. Chem., Preprints, 7, 99-111.