Energy & Fuels 1991,5,885-895
885
Changes to Unbound Biomarkers in Low-Rank Coals during Simulated Coalification' Maowen Li,t Peirong Wang, and R. B. Johns* School of Chemistry, University of Melbourne, Parkville, 3052 Victoria, Australia Received May 17, 1991. Revised Manuscript Received August 27, 1991
A sample of immature Victorian brown coal (R, = 0.31 % ) was subjected to anhydrous and hydrous pyrolysis over a range of temperatures (200-300"C) in a study of structural changes that can be induced in the unbound biomarkers present in the raw coal. Hopanoid hydrocarbons are predominant in the raw coal and occur as the 22R isomers. Although R, rose to 0.99% at 300 O C , isomerization to the 22s epimer was not observed; rather increasing aromatization and degradation to bicyclic aromatics occurred. @,@-Homohopanewas generated on hydrous pyrolysis at 250 "C while 17a(H),21@(H)homohopane showed a decrease over the 200-300 "C range. The interrelationship between these maturity indicators may need review. Ca and Ca benzohopanes were observed to be formed in optimal yield at 250 "C, but not at 300 "C, from the unbound precursor(s). This result substantiates a geochemical pathway during diagenesis for benzohopane formation in addition to the pathway of microbial action already canvassed in the literature. A detailed structural analysis of alkylnaphthalenes, indanes, and tetralins relates them to a wide range of 8,14-seco tetracyclic aromatic hydrocarbons which in part are believed to be their probable precursors. In turn, these seco compounds relate structurally to pentacyclic triterpenoid precursors with skeletons of the oleanane, lupane, ursane, and hopane classes recognized in the unbound fraction in the raw coal. The seco compounds occur with all degrees of aromatization. Beginning with @-amyrinas representative, probable pathways to bicyclic aromatics are proposed on the basis of identified intermediates. The products released by hydrous pyrolysis of this brown coal suggest that the coal itself acts as an acidic catalytic surface. Catalytic effects are more pronounced under anhydrous than under hydrous conditions.
Introduction Previous studies have shown that the unbound (Le., solvent-extractable or thermally desorbable) biomarkers in low-rank coals contain valuable information about biological sources and coal-bearing strata and early diagenesis (see ref 2 for a review). However, the detection of biomarkers specific to a particular sedimentary environment becomes more difficult with increasing coal rank and is nearly impossible in higher rank coals. Several authors,3-6however, have recently demonstrated that in the transition from subbituminous to high-volatile bituminous coals, biological/facies fingerprints may still be recognized. The reconstruction of specific precursor/product relationships of biomarker compounds in a coal is often not an easy task because it is rare to find a series of coals of increasing rank that are derived from the same coal lithotype. Even in cases where such coals can be obtained, contamination by in-migrated hydrocarbons and organic facies variations in the coal seam can obscure the true parent-daughter relationship. Laboratory thermal catalytic experiments offer the possibility of generating geochemical products from a known compound or a mixture without interference due to hydrocarbon migration and organic facies variations. By heating triterpenoid compounds in the presence of clay, Hayatsu et al.6 were the first to provide experimental evidence that pentacyclic triterpenoids are transformed into a variety of hydroaromatic and aromatic hydrocarbons commonly found in coals, oil shales, crude oils, and other sediments. An underlying problem, however, is that on such a clay matrix, reactions may have a closer resemblance to those for dispersed organic matter in shales,and by no means duplicate 'Present address: NRG, Drummond Building, The University, Newcastle-upon-Tyne, NE1 7RU, U.K. * Author to whom correspondence should be addressed.
0887-0624/91/2505-0885$02.50/0
the coalification processes where biomarker compounds are enclosed in an organic matrix. Smith et al.I8 recently observed that the brown coal organic matrix can play a significant role in acting as a hydrogen donor and in promoting chemical reactions such as decarboxylation under hydrous pyrolysis conditions. Other difficulties arise if reactions are simulated under dry conditions. Anhydrous pyrolysis of kerogens often gives n-alkenes that are rare in coal extracts2 or crude oils.' The advantages of hydrous pyrolysis in simulating natural catagenetic processes of organic matter in petroleum source rocks have been discussed by many authors.'-" Because water is ubiquitous (1) A preliminary report appears in the Conference Proceedingsof the 1991 International Conference on Coal Science, Newcastle-upon-Tyne,
U.K.
(2) Chaffee, A. L.; Hoover, D. S.; Johns,R. B.; Schweighnrdt,F. K. In Biological Markers in the Sedimentary Record: Johns, R. B., Ed.; Elsevier: Amsterdam, 1986; pp 311-346. (3) Villar, H. J.; Piittmann, W.; Wolf, M. Org. Geochem. 1988, 13, 1011-1021. (4) Radke, M.; Willsch, H.; Teichmijller, M. Org. Geochem. 1990,15, 539-564. (5) Chang, H. C. K.;Nishioka, M.; Bartle, K.D.; Wise, S. A.; Bayona, J. M.; Markides, K.E.; Lee, M. L. Fuel 1988,67,45-57. (6) Hayatsu, R.; Botto, R. E.; Scott, R. G.; McBeth, R. L.; Winans, R. E. Org. Geochem. 1987,11, 245-250. (7) Lewan, M. D.; Winters, J. C.; McDonald, J. H. Science 1979,203, 897-899. (8) Lewan, M. D.; Bjoroy, M.; Dolcater, D. L. Geochim. Cosmochim. Acta 1986,50, 1977-1988. (9) Winters, J. C.; Williams, J. A.; Lewan, M. D. In Advances in Organic Geochemistry 1981; Bjoray, M., Ed.; Wiley: New York, 1983; pp 524-533. ~~. (10) Hoering, T. C. Org. Geochem. 1984,5, 267-278. (11) Lewan, M. D. Philos. Tians. R. SOC.London 1985, A315,123-134. (12) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985,8, 275-292. (13) Comet, P. A.; McEvoy, J.; Giger, W.; Douglas, A. G. Org. Geochem. 1986,9, 171-182. (14) Rullkotter, J.; Marzi, R. Org. Geochem. 1988, 13, 639-646. ~
0 1991 American Chemical Society
886 Energy & Fuels, Vol. 5, No. 6,1991 Table I. Analytical Data of a Victorian Brown Coal (Rosedale Field) from Core R327 and Its Hydrous Pyrolysates at 200,250, and 300 "C" sample raw coal 200 "C 250 O C 300 "C 5.2 3.7 TSE (70in dry sample) 11.4 9.5 20.5 13.7 8.7 8.1 thermal desorption yield ( % in dry sample) neutrals ( % in dry sample) 0.7 0.6 1.1 1.3 vitrinite reflectance (Ro, % ) 0.31 0.48 0.69 0.99 gross composn of neutrals, % 14 16 14 16 aliphatic 52 59 73 73 aromatic 34 25 13 11 polar aliphatic hydrocarbons, re1 % n-a1kanes 62 61 83 92 b b 2.3 1.5 acyclic isoprenoid8 sesqui- and diterpenoids 4 3 1 0.7 triterpenoids hopanes 13 19 11 6 hopenes 11 7 2 b nonhopanoid triterpenes 9 11 1 b steranes 0.5 0.2 b a Thermal desorption yields given are the average of five determinations. TSE = total solvent extract. *Below the level of detection.
in peats up through medium-volatilebituminous coals even to the anthracite ranks, simulated coalification experiments under hydrous conditions may be more realistic. Recent hydrous pyrolysis work in our laboratory at the University of Melbourne has been directed toward a better understanding of the fate of the unbound terpenoid biomarkers present in Victorian brown coals (Australia) during thermal maturation (geochemical coalification). As part of this larger project, the objectives of the present paper were (1) to document and compare the effects of hydrous and anhydrous pyrolysis on the composition of the unbound terpenoid biomarkers in the coal; (2) to search for a number of reaction-intermediate products that can be used to trace biological precursor/geochemicd product relationships; and (3) on the evidence collected, to assess hydrous pyrolysis as a technique for simulating natural geochemical coalifications. Experimental Section Sample Description. The Victorian brown coal sample was a medium-light lithotype, taken from a bore core (R327) in the Rosedale field at the depth of 171.6 m. The detailed chemical analyses of this sample have been reported.lg Other analyses are shown in Table I. The moisture content of this coal was 59.5%. On a dry, mineral-free basis the coal contained 68.4%; H, 4.9%; N, 0.6%; S, 0.3% and 0,25.7%. The inorganic content of the dry coal was 0.8% with Ca, 0.07%;Mg, 0.11%, Na, 0.08%; C1,0.12%; S, 0.28%; and Fe, 0.09% being the major components. Experimental Procedures. Twenty grams of the raw coal (undried, with a moisture content of around 55%, w/w) and 10 mL of deionized, chloroform-prewashed and distilled water was transferred to a 70-mL stainless steel reaction vessel to ensure the presence of water at all stages. The reaction vessel was closed after purging with nitrogen gas. The vessels were heated rapidly to 200, 235, 250, 265, or 300 "C and maintained at the selected temperature (A5 "C) for 72 h. For comparison (1)to clarify the role of water in the thermal transformation reactions, three aliquots (each of 10 g) of (15) Jones, D. M.; Douglas, A. G.; Connan, J. Org. Geochem. 1988,13, 981-993.
(16)Peters, K. E.; Moldowan, J. M.; Sundararaman, P. Org. Geochem.
1990.15. 249-265. (17) Marzi, R.; Rullkotter, J.; Perriman, W. S. Org. Geochem. 1990, 16, 91-102. (18)Smith, J. W.; Batts, B. D.; Gilbert, T. D. Org. Geochem. 1989,14,
365-373. (19)Li, M.; Johns, R. B. J . Anal. Appl. Pyrol. 1991, 20, 161-170.
Li et al. freeze-dried coal sample were heated anhydrously for 72 h at 200, 235, and 265 "C, respectively; and (2) to distinguish between the contributions of pyrolysates from the original solvent extractable (unbound) material and from that derived from the coal matrix, aliquots of solvent-extracted coal (9.0 g) were heated separately with 10 mL of water for 72 h a t 250 "C. After cooling, the vessels were opened and their contents removed and washed with chloroform/methanol(2:1,w/w). The low molecular weight gases were not investigated. The washings were dried under a gentle nitrogen flow and then combined with the related solid residues, the combinants were then freeze-dried and ground (
