Energy & Fuels 1993, 7, 988-993
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Carboxylic Acids Obtained by Alkaline Hydrolysis of Monterey Kerogen Assem 0. Barakat Department of Chemistry, Faculty of Science, Alexandria University, P.O.Box 426, Alexandria 21312, Egypt Received September 29,1992. Revised Manuscript Received December 11, 1992
In order to obtain more information on the structural entities bonded to the macromolecular structure of Monterey kerogen, the solvent-extractable acidic products obtained by mild alkaline hydrolysis were investigated by gas chromatography and combined gas chromatography-mass spectrometry analysis. On the basis of the results obtained, the previously proposed structural feature of Monterey kerogen is expanded to include saturated normal monocarboxylic acids (CS-C~S), ~ ) ,and anteiso saturated normal a,o-dicarboxylic acids (CrC29), isoprenoid acids (c14-c17,C I S - C ~ is0 acids (Cll-ClB), unsaturated acids (C16:1,Clad, aromatic acids, hopanoid acids, and 3-carboxycholeatane, as well as a complex mixture of alkylated thiophene carboxylic acids (C&&). The significance of the lipid composition of the kerogen is discussed in terms of lipid diagenesis and biomarker information.
Introduction Kerogen is the most abundant form of organic carbon on earth and is generally considered to be a major source of catagenetically formed petroleum.' Intensive studies of the molecular structure of kerogen are valuable in the assessment of the source of the precursor organic material, the depositional environment and the thermal histories of organic matter in sediments and of petroleum. Molecular characterization of kerogen, however, has been hampered by the insolubility and presumed macromolecular nature of this heteropolymeric material. A wide variety of methods have been applied293to the study of kerogen. Direct structural investigation by bulk analyses (Rock-Eva1pyrolysis, elemental analysis, IR, NMR etc.) often reveal only gross inf~rmation.~ Specific chemical degradation technique^,^^ on the other hand, allow a better insight into kerogen structure as they can reveal information on the molecular level and provide details on the nature of linkage by which the released molecules were attached. In this respect, chemical degradation reactions selective to oxygen-containing functionalities are important to assess the extent of lipid incorporation into young kerogen via biologically-derivedpolar functionalities such as hydroxyl and carboxyl groups. Although Monterey kerogens have been investigated previously, much less is (1)Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 19W, Chapter 4. (2)Rullketter, J.;Michaelis; W. In Aduances in Organic Geochemistry, 1989; Durand, B., Behar, F., Eds.; Pergamon: Oxford, England, 1990;pp 829-852. (3)Durand, B. Kerogen, Insoluble Organic Matter fromSedimentary Rocks; Editions Technip: Paris, 1980. (4)Vandenbroucke, M. In Kerogen, Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980;pp 420-425. (5)Chappe, B.; Michaelis, W.; Albrecht, P. In Aduances in Organic Geochemistry 1979; Douglas A. G., Maxwell, J. R., Eds.; Pergamon: Oxford, England, 1980, pp 265-274. (6)Vitorovic, D. In Kerogen, Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980;pp 301338. (7) Mycke, B.; Michaelis, W. Org. Geochem. 1983,5,111-119. (8) Barakat, A. 0.;Yen, T. F. Fuel 1987,66,587-593. (9)Boucher,R. J.; Standen, G.;Patience, R. L.;Eglinton, G. In Aduances in Organic Geochemistry, 1989; Durand, B., Behar, F., Eds.; Pergamon: Oxford, England, 1990; pp 951-958.
known about their constituents than, for example, those of the widely studied Green River Formation? The geochemistry of Monterey oils and their source rocks in the coastal areas of California is of considerable interest not only because this system has unique characteristics but also because of large reserves discovered in recent years in the offshore. From previous organic geochemical studies,lOJ1it has been concluded that the organic matter in Monterey sediments in the Santa Maria Basin is immature and derived from the remains of marine (algal) organisms deposited in a highly reducing environment. Monterey kerogens are exceptionally rich in organic sulfur (8-14%), and would be termed "type II-S" kerogens in a strict sense (atomic SordCog> 0.04).12The relatively high amount of sulfur in kerogen from marine sediments has been explained by reaction of the products of sulfatereducing bacteria (such as hydrogen sulfide or polysulfides) with organic matter involved in the formation of kerogen.' There is some evidence to suggest that sulfur-rich kerogens begin to generate oil under very mild geothermal conditions, which is probably related to the preferential cleavage of weak sulfur 1inkage.l2J3 In an earlier paper,14 a controlled stepwise oxidative degradation (NazCr207tglacial CH3COOH) was used to characterize the chemical structure and origin of macromolecular material in Monterey oil shale kerogen. The acidic oxidation products (64% of the original organic matter) were examined by capillary gas chromatography (GC) and computerized capillary gas chromatographymass spectrometry (GCIMS) of their methyl esters. On the basis of the evidence obtained from the qualitative and semiquantitative variation of the products with duration of oxidation, it was suggested that the nucleus of Monterey kerogen is composed mainly of a long-chain cross-linked polymethylene structure, to which n-alkyl and (10)Didyk,B.M.;Simoneit,B.R.T.;Braesel,S. C.;Eglinton,G.Nature 1978,272, 216-222. (11)Milner, C. W.D.; Rogers, M.A,; Evans, C. R. J. Geochem. Ezplor. 1977,7,101-153. (12)Om,W.L. Organic Geochem. 1986,10,4%&516. (13)Baskin, D. K.;Peters, K. E. AAPG Bull. 1992,76,l-13. (14)Barakat, A. 0.; Yen, T. F. Geochim. Cosmochim. Acta 1988,52, 359-363.
0887-0624/93/2507-0988$04.00/00 1993 American Chemical Society
Alkaline Hydrolysis of Monterey Kerogen
isoprenoid chains and minor amounts of non-isoprenoids branched hydrocarbons are attached. The results also indicated the presence of subordinal structures of phenyl and tolyl groups, as well as bicyclic and heterocyclic moieties. For reasons as yet not understood, the stepwise oxidation method did not give any information about the nature of sulfur moieties in this sulfur-rich kerogen. Furthermore, the presence of hopanoids, steroids, and aromatic structures (other than mono- or disubstituted benzene rings) was not established. It is evident that degradation studies of kerogen should involve the use of a variety of reagents of different selectivities and comparison of the nature and yield of the products if a meaningful structural interpretation is to be achieved. In this communication, I present detailed results of the products obtained by alkaline hydrolysis of Monterey kerogen. It was anticipated that this mild and specific type of degradation would help illustrate the distribution and nature of structural entities bonded by ester linkage to the macromolecular structure of kerogen.
Experimental Section The Miocene Monterey Formation sample used in this study was supplied from the Union Science and Technology Division, Brea, CA. This sample, rich in carbon and sulfur, was taken at 1350 m depth in the Santa Maria Basin (Leroy 51-18 well). Materials. All solvents were either distilled-in-glass grade solvents or high-purity solvents. All chemicalswere reagent grade. The water was distilled in an all-glassapparatus after deionization. Procedure. The kerogen concentrate was prepared by Soxhlet extraction of the pulverized shale sample (140 mesh) with an azeotropic CeHe/CHsOH mixture for 72 h, followedby treatment with 20 % HC1to remove the carbonate minerals. The carbonatefree shale was then digested with a 1:l mixture (v/v) of concentrated HF (48%) and HC1 (20%) for 72 h at room temperature, followed by filtration and washing with distilled water. The residue was again treated with 20% HCl(2 h, 20 OC) and the acid-leached material was washed successively and hot distilled water until the filtrate was free of C1-, dried, and again exhaustively extracted with the same solvent. Finally, the kerogen was ultrasonically extracted with CHzC12/CHaOH (1:l) and dried in a vacuum for 1 h. The elemental analysis was as follows: 58.3% C, 6.20% H, 13.5% 0, 2.67% N, 12.7% S, and 6.16% ash. A 2.6855-g portion of the kerogen concentrate was stirred at reflux temperature under Nz with 250 mL of 0.2M KOH in 97% aqueous CHsOH for 4 days. At the end of this period, the mixture was cooled, filtered, and the precipitate was thoroughly rinsed with distilled water and extracted ultrasonically with CHzCl2 (3 X 50 mL). Bound lipids were extracted from the alkali hydrolysate with CH2Cl2 after acidification with HCl to pH = 1,and the combined CHzClz extracts were washed with distilled water and dried over anhydrous NaZSOI. Carboxylic acids were then separated by medium-pressure liquid chromatography (MPLC) on a column of Si02 impregnated with KOH.15 They were washed down from the column with CHzCldHCOOH (991) (formic acid was used to enhance desorption of carboxylic acids from the strongly basic silica column). In order to remove formic acid from the solvent mixture, distilled water was added followed by extraction and phase separation. The aqueous layer was further extracted with CHzClz and the combined total extracts were washed with water, dried over NazS04, and concentrated on a rotaryevaporator, and the residue was transferred to a preweighed 1-mL vial and dried under Nz. (15) Radke, M.;Willsch, H.; Welte, D.H.Anal. Chem. 1980,52,406411.
Energy & Fuels, Vol. 7, No. 6, 1993 989 Table I. Yields of Products from Saponification of Monterey Kerogen w t of acidic wt of kerogen % of GC concentrate wt of kerogen products amenable used in residue after recovered on componenta saponifn, g saponifn. g SiO-KOH (1) in (1) 2.68545 2.46134 47.9 mg (1.8%
