Energy & Fuels 1989,3,613-616
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The Nature of Porphyrins in Kerogen. Evidence of Entrapped Etioporphyrin Species Assem 0. Barakat Department of Chemistry, Alexandria University, Alexandria, Egypt
Teh Fu Yen* Environmental & Civil Engineering, University of Southern California, Los Angeles, California 90089-0231 Received September 1 , 1988. Revised Manuscript Received May 26, 1989
Computerized capillary gas chromatography-mass spectrometry (GC-MS) was employed to investigate maleimides (lH-pyrrole-2,Bdiones) produced from a controlled stepwise oxidation (Na2Cr20,/glacial CH3COOH) of bitumen-free kerogen samples isolated from Green River and Monterey oil shales. The results indicate major concentrations of 3-ethyl-Cmethyl-lH-pyrrole-2,5-dione and minor amounts of 3,4-dimethyl-lH-pyrrole-2,5-dione in the oxidation products of both kerogens. On the basis of the evidence obtained from the analysis of all fractions, it is suggested that etioporphyrin species are most likely entrapped in the molecular sieve type network of kerogen.
Introduction Petroporphyrins represent one of the major groups of geochemical fossils and are of great interest to geologists and geochemists as they provide information on the original organic They have been widely used for interpretation of the environment of deposition and diagenetic history of oil sha1esa4+ Moreover, information about the petroporphyrins and their constituent metals is needed in order to provide more efficient processes for converting oil shale to energy sources with minimum environmental contamination and refinery problems.' It is now generally accepteds that porphyrin precursors, which are known to be resistant to microbiological and chemical alteration occurring during early diagenesis, are locked in the kerogen matrix (thus insuring a better preservation from outside contamination) and are later released from kerogen with increasing depth and temperature. However, the manner of incorporation of these porphyrins into the kerogen structure still needs to be investigated. It is not certain whether porphyrin species are bound to the kerogen, trapped in a molecular sieve type network, or strongly adsorbed. Fossil porphyrins have been analyzed by a variety of analytical techniques; the most important ones are thinlayer chromatography (TLC), high-performance liquid chromatography (HPLC) fractionation followed by mass spectra, capillary gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance (NMR) spectroscopy.+12 Numerous investigationsl2-l' have been directed toward the characterization of porphyrins in oil shales; however, most of the data available so far have been obtained from constituents of shale oil produced upon retorting and from the organic solvent extractable from the oil shale. Because of the inherent insolubility of kerogen, which forms the greater part of the organic material in sediments, direct analysis of porphyrins is difficult. Little is known about the constitutional aspects and distribution of porphyrins in the kerogen matrix. Oxidative degradation is a classical method in kerogen structural s t u d i e ~ 'which ~ aims to degrade the kerogen matrix into smaller identifiable fragments that can be *Author to whom correspondence should be addressed.
analyzed by conventional analytical techniques. Oxidative degradation has also been used to degrade porphyrin mixtures isolated from petroleum samples into alkylmaleimides ( l ~ - p y r r o l e - 2 , 5 - d i o n e ~ )These . ' ~ ~ ~low-molecular-weight products are easily separable by gas chromatogaphy (GC) and provide valuable information on the alkylation pattern of the original porphyrins.
(1)Triebs, A. Angew. Cheni. 1936,49,682-686. (2)Baker, E. W.; Yen, T. F.; Dickie, J. P.; Rhodes, R. E.; Clarke, L. F.J. Am. Chem. SOC.1967,89,3631-3639. (3)Corwin, W. H. R o c . World Pet. Congr. 1960,6th,119-129. (4)Barwise, A. J. G.; Park, P. J. D. In Advances in Organic Geochemistry, 1981; Bjory, M., Ed.: Wiley: Chichester, U. K., 1983,pp 668-674.
(5) Mackenzie, A. S.;Quirke, J. M. E.; Maxwell, J. R. In Advances in Organic Geochemistry, 1979;Douglas, A. C., Mazwell, J. R., Ede.; Pergamon Press: Oxford, England, 1980; pp 239-248. (6)Didyk, B.; Alturki Y. I. A.; Pillinger, C. T.; Eglinton, G. Nature 1975,256,563-565. (7) Silver, H. F.; Wang, N. H.; Jenaen, H. B.; Pouleon, R. E. In Science and Technology of Oil Shale; Yen, T. F., Ed.; Ann Arbor Science: Ann Arbor, MI, 1976;pp 163-175. (8)Tieeot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: West Berlin, 1984;pp 93-130. (9)auirke, J. M. E.; Eglinton, G.; Maxwell, J. R. J. Am. Chem. SOC. 1979,101,7693-7697. (10)Marriott, P. J.;Gill, J. P.;Evershed, R. P.; Hein, C. S.; Eglinton, G. J. Chromatogr. 1984,301,107-128. (11) Quirke, J. M. E.; Maxwell, J. R.; Eginton, G.; Sanders, J. K. M. Tetrahedron Lett. 1980,21,2987-2990. (12)Hajibrahim, S. K.;Quirke, J. M. E.; Eglinton, G. Chem. Geol. 1981,32,173-188. (13)Baker, E. W.; Palmer, 5. E. In Porphyrina; Dolphin, D.,Ed.; Academic Press: New York, 1978;Vol. I, pp 486-552. (14)Hcdgaon, G. W.; Baker, B. L.; Peake, E. In Fundamental Aspects ofPetrokum Geochemistry;Nagy, B., Colombo, V.,Ede.;Elsevier: Amsterdam, 1967;pp 1977-2260. (15)Vitorovic, D. In Kerogen, Insoluble Organic Matter from Sedimentary Rocks, Durand, B., Ed.; Editions Technics: Park, 1980; pp 301-338. (16)Hodgson, G. W.; Stroeher, M.; Caeagrande, D.J. In Advances in Organic Geochemistry, 1971; von Gaertner, H. R., Wehner, H., E&, Pergamon: Oxford, England, 1972;pp 151-161. (17)Barwise, A. J. G.; Whitehead, E. V. In Advances in Organic Geochemistry, 1979, Maxwell, J. R., Douglas,A. G., E&.; Pergamon Press: Oxford, England, 1980,pp 181-192. (18)Quirke, J. M. E.; Shaw, G. J.; Soper, P. D.; Mazwell, J. R. Tetrahedron 1980,36,3261-3267.
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614 Energy & Fuels, Vol. 3, No. 5, 1989 wa
Barakat and Yen
GREEN RIVER KEROGEN
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Figure 1. Identical sections of reconstructed ion chromatograms illustrating the variation in the distribution of the maleimides (R and Q)isolated from stepwise oxidation of Green River and Monterey kerogens. M9 = nonanoic acid methyl ester; B2 = 3or 4-methylbenzoic acid methyl ester; D6 = hexanedioic acid dimethyl ester. Figure 3. Mass spectrum of peak (Q)in Figure 1.
In this work, an oxidative degradation approach was used to study the nature of porphyrins in kerogen. Computerized capillary gas chromatography-mass spectrometry (GC-MS) was employed to investigate maleimides produced from mild stepwise oxidation (Na2Cr2O7/glacial CH,COOH) of bitumen-free kerogen samples isolated from Green River and Monterey oil shales. Detailed discussions of the identification of the various oxidation products and the proposed structures of Green River and Monterey kerogens have been given e1se~here.l~ Experimental Section The Green River shale sample (kerogen type I, from the Anvil Point oil shale mine) was supplied by the Laramie Energy Research Center, Laramie, WY, and the Monterey shale (type 11, 1350 m from the Santa Maria basin, Leroy 51-18 well) was obtained from the Union Science and Technology Division, Brea, CA. The kerogen concentrates were prepared according to a reported pro~edure.'~To insure that the kerogen samples were free from any solvent-soluble material, exhaustive Soxhlet extractions (CeHe/CHaOH a t azeotropic ratio, 72 h each) were carried out before and after the mineral removal step. The procedure for NapCrzO,/glacial CHSCOOH stepwise oxidation and a detailed account of the analytical and computerized GC-MS analyses employed are given e1se~here.l~ Mass spectral data acquisition (19) Barakat, A. 0.;Yen,T.F. Fuel 1987,66,587-593. Barakat, A. 0.; Yen, T.F. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1987, 32(1), 43-47.
was performed by an INCOS 2300 computer system.The INCOS system was also used in fully automated peak finding, spectral matching, and integration.
Results Five steps were necessary for complete degradation of Green River kerogen, while Monterey kerogen was almost completely oxidized in only four steps. The total weights of oxidation products represent about 71 and 64% of the ash-free Green River and Monterey kerogens, respectively. Green River shale has 15.4 wt % total organic carbon (TOC) and contains 21.3 wt % total organic matter with 5.8 wt % solvent extractable bitumen, while the Monterey shale sample had 20.7 wt % TOC and contained 33.2 wt % organic matter and 15.8 wt % bitumen. The ash content and elemental analyses of the resulting kerogens were as follows: Green River, 72.3% C, 9.1% H, 6.6% 0, 2.4% N, 4.8% S, 4.3% ash; Monterey, 62.2% C, 6.4% H, 9.2% 0, 2.3% N, 13.5% S, and 5.6% ash. Identical sections from the reconstructed ion chromatograms obtained from the oxidation fractions (I-IV) of the two kerogen samples studied are shown in Figure 1. GC-MS revealed major concentrations of 3-ethyl-4methyl-lH-pyrrole-2,5-dione (compound R, Figure 1). In (compound Q, addition, 3,4-dimethyl-lH-pyrrole-2,5-dione Figure 1)was identified but in far less abundance. Mass spectra of these two compounds are presented in Figures 2 and 3, respectively.
Energy & Fuels, Vol. 3, No. 5, 1989 615
The Nature of Porphyrins in Kerogen
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(a) RESULTS SCAN No. 608(COMPOUND 31331 SPECTRA N I L i m a i m c SEARCHED FOR 2 8 4 mTCHED AT LERST 5 OF
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Figure 4. Results of the computer comparison of a library of 31 331 authentic mass spectra with the masa spectra of (a) compound R and (b) compound S. r
(S)
Oridation Step
Figure 6. Percentage abundance of 3-ethyl-4-methyl-1Hand 3,4-dimethyl-lH-pyrrole-2,5-dione (a), pynole-2,5-dione(0) relative to the total oxidation products, obtained in the individual
oxidation steps of Green River and Monterey kerogens.
Mey(Et methyl-lH-pyrr01e-2,5-dione.'~J~ It is characterized by an
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Figure 5. Mass spectrum of peak S in Figure 1.
Examination of the fragmentation pattern presented in Figure 2 shows a molecular ion at m / z 139, and characteristic ions at m / z 124,121, 111,110,106,96,81, 67, and 53, which correspond to the molecular ion C 7 H a 0 2 and fragments C6H6N02, C7H7N0,C6H9N0,C5H4N02,C&4NO, C6H80, C&, C5H7, and C4H5,respectively. The M-15, M-18, M-28, and M-29 fragment ions indicate a loss of CHS, HzO, CO, and CHO from the parent structure. Furthermore, the results obtained from a computerized library search (Figure 4a) clearly indicate that the compound eluting from the gas chromatograph at spectrum 607 is 3-ethyl-4-methyl-lH-pyrrole-2,5-dione (compound R). I t should be mentioned that 2-ethyl-3-methyl-2-butenedioic acid dimethyl ester (compound S; see fragmentation pattern, Figure 5), which is probably obtained by further oxidation of compound R, was tentatively identified; however, the results for its computer spectral matching (Figure 4b) show very poor purity data that are attributed to a problem of coelution. On the other hand, a computerized library search for Compound Q was not possible because its spectra was not available in the spectrum collection (31331 spectra); nevertheless, the mass spectral features shown in Figure 3 were found to match published spectra of 3,4-di-
intense molecular ion at m l e 125, and base peak at m l e 54, which corresponds to the fragment C4H6.
Discussion Fossil porphyrins were shown2rz0to consist mainly of nickel and vanadyl complexes of deoxophylloerythroetioporphyrins (DPEP) and etioporphyrins (Etio). In addition, three minor types were observed: Rhodo-Etio, RhodoDPEP, and Di-DPEP. 3-Ethyl-Cmethyl-lH-pyrrole-2,5-dione (R) and 3,4-dimethyl-lH-pyrrole-2,5-dione(Q)are classical oxidation products of etioporphyrins. DPEP types would, however, produce, in addition to compounds R and Q, a 3methyl-lH-pyrrole-2,5-dione originating from the pyrrole unit with the isocyclic ring.21 The mass spectra of the latter compound is characterized by a base peak at m l z 111 and an intense ion at m l z 68.18 The absence of such a spectrum in our GC-MS results indicates that etioporphyrins are probably the major porphyrin types present in both Green River and Monterey kerogen samples. Furthermore, the absence of maleimides bearing a -COOH or a -(CH2)COOH group (e.g. hematinic acid), which would be produced from oxidation of porphyrin molecules bonded to the kerogen matrix at the fl-position, strongly suggests that the etioporphyrin species are not likely to be chemically bonded to the kerogen matrix. The approximate relative percentage abundance of each of the compounds Q and R was normalized from its peak height compared to the total of all components in the chromatograms. The estimated proportions in the individual oxidation steps of both kerogen sample are presented in Figure 6. Inspection of the latter figure shows no evidence for a smooth Gaussian distribution of the product throughout the steps. Instead, it is clear that, for the two kerogen types, Q and R are released mainly in the second oxidation step. It is therefore evident that the porphyrin molecules are not homogeneously distributed (20) Yen, T. F.; Boucher, L. J; Dickie, J. P.; Tynan, E. C.; Vaughen, G. B. J. Znst. Pet. 1969, 55(542), 87-99. (21) Ellsworth, R. K. J . Chromatgr. 1970,50, 127-131.
Energy & Fuels 1989,3,616-620
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throughout the kerogen matrix. On the basis of the evidence thus obtained, it seems most likely that the etioporphyrin species are entrapped in the molecular sieve type network of kerogen. "Looseningnof the kerogen matrix, however, liberates the entrapped molecules, which eventually results in the absence of a smooth Gaussian distribution of the porphyrin oxidation products, as depicted in Figure 6. This is analogous to the generation of entrapped n-alkanes by mild stepwise oxidation of Green River kerogen.22 The origin of these entrapped compounds is not clear. Diagenesis of kerogen is probably a very important factor. Finally, the present results substantiate previous as(22) Young, D. 1411-1417.
K.;Yen, T. F. Ceochim. Cosmochim. Acta
1977,41,
sumptions8 that geochemical fossil molecules may be trapped in the kerogen network or alternatively attached to kerogen by chemical bonds and are released later by thermal or thermocatalytic cracking.
Acknowledgment. We are grateful to E. Ruth of the Department of Earth and Space Sciences at the University of California, Los Angeles, CA, for his invaluable assistance with the GC-MS facilities and Dr. J. Curiale of the Union Sciences and Technology Divisions for providing the Monterey shale sample. Acknowledgement is also made to the Binational Fulbright Commission for the research grant received by A.O.B. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for funding. Registry No. R,20189-42-8; Q,17825-86-4.
Some Aspects of Autoignition Limits and Delays of Timahdit Oil Shale A. Alaoui Sosse, J. P. Vantelon," C. Breillat, and F. Gaboriaud Laboratoire de Chimie Physique de la Combustion, U A 872 au CNRS, Domaine du Deffend, Mignaloux-Beauvoir, University of Poitiers, 86800 Saint Julien I'Ars, France Received March 27, 1989. Revised Manuscript Received June 13, 1989
Oil shale deposits ,of Morocco represent an important energetic potential. Different engineered projects are expected; among them, direct use as a fuel is a promising technique. One important problem associated with using oil shales in combustors such as utility boilers is the conditions of particle ignition. The ignition and combustion of solid fuels are complex phenomena involving homogeneous and' heterogeneous reactions and heat and mass transfer. In this paper are presented the main parameters that describe ignition limits and delays of small size samples of two oil shale varieties which originate from the Timahdit deposit. Work is under way to describe and interpret the experimentally observed results. In particular, it has been possible to consider the influence of temperature and pressure, to specify the nitrogen effect on the ignition process, and to correlate ignition delays vs oxidant pressure.
Introduction Oil shales are comparatively poor fuels. However, recent energy problems prompted countries having important deposita to utilize them. Morocco, in particular, has very substantial potential resources (19 million tons, or about 15% of the known reserves in the world) and intends to use them as alternative fuel. There are several ways to exploit the energy potential of crude oil shales. Thus, in the past few years, real advances have been made with retorting processes, providing a set of usable liquid fuels. But it is also possible to burn directly solid oil shales. With this aim, fluidized-bed combustion and burning under conditions characteristic of pulverized combustion appear as promising techniques.' Thus, it is very important to develop fundamental knowledge associated with the combustion of this type of material, particularly ignition studies of small size samples. Indeed, to use this fuel in the form of particles injected into a boiler, correlation of furnace residence time and ignition delays are needed.
* To whom correspondence should be addressed.
Contrary to coals, which yield glowing combustion of the solid carbonaceous material that remains after the rapid consumption of the volatiles, oil shales burn with a sustained flame of diffusional character. The present work is the study of the spontaneous ignition limits and delays for two varieties of Moroccan oil shales. The ignition of a solid exposed rapidly to a radiative and convective heat flux results in a set of physicochemical processes. The combustible gases, arising from the surface, mix with the surrounding oxidizer, leading to exothermic reactions. Then, ignition may occur spontaneously if critical conditions are reached. A large number of studies have been performed on flammability limits and delays. Exhaustive reviews of the basic aspects of the problem can be found in ref 2 and 3. Previous works (Breillat et al."') concern ignition of dif(1) Payne, R.; Michelfelder,S.;Rem, U.; Leikert, K. Reu. Gen. Therm. 1978,NO.196, 331-339. (2) Jellinek, H.H.G. Aspects of Degradation and Stabilization of Polymers; Elsevier Scientific Publishing Company: .~ Amsterdam, 1978; pp 501-514. (3) Drysdale, D.D.An Introduction to Fire Dynamics, John Wiley and Sons La.:1985; pp 186-225.
0887-0624/89/2503-0616$01.50/00 1989 American Chemical Society
