Kerogen Chemistry 4. Thermal Decarboxylation of Kerogens - Energy

1954

Energy & Fuels 2005, 19, 1954-1961

Kerogen Chemistry 4. Thermal Decarboxylation of Kerogens Ryuichi Ashida, Paul Painter, and John W. Larsen* The Energy Institute, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802 Received April 17, 2005. Revised Manuscript Received July 12, 2005

Infrared spectra confirm that, when heated to 200 °C, Bakken kerogens form anhydrides from carboxylic acids. The anhydrides in turn thermally decompose at temperatures below 250 °C. The thermal production of CO, the absence of CO2, and the low temperature of the anhydride decomposition are all consistent with a radical chain mechanism initiated by a kerogen radical. Anhydride decomposition occurs by addition of a radical to the anhydride carbonyl to form an ester and to liberate an acyl radical that rapidly loses CO to form an alkyl radical to continue the chain. This pathway is one of several by which kerogens thermally decarboxylate.

Introduction It is apparent from the way that Type I and II kerogens follow the van Krevelen diagram that kerogen maturation begins with deoxygenation.1 Loss of oxygen occurs under very mild conditions. There now exist analytical techniques that make it possible to follow closely the changes in oxygen functionality that occur during kerogen maturation and when kerogens are heated. This raises the possibility of identifying the reaction pathways that are responsible for the initial oxygen loss during kerogen maturation. These are especially intriguing because CO and sometimes CO2 are produced under very mild conditions. This paper describes one deoxygenation reaction pathway, one responsible for low-temperature CO formation. This paper deals with the decarboxylation of dry kerogens. Because acid anhydrides are important to this chemistry and they would not exist in an aqueous environment because they are rapidly hydrolyzed, the work that follows is only tangentially relevant to hydrous pyrolysis. Furthermore, we are concerned with reactions that occur at temperatures below 300 °C. Such reactions may be relevant to kerogen maturation and petroleum formation. During kerogen maturation and also when heated, kerogens lose carboxylic acid groups.1 We address here the reaction pathways responsible for their loss. Carboxylic acids and their derivatives must be considered because any or all of them may be involved in the decarboxylation pathways. Because this paper is directed to an audience that includes geochemists and fuel scientists who may not be intimate with organic reaction mechanisms, the possible reaction mechanisms will be outlined in the Introduction. Because of significant mechanistic differences and the large difference in the stability of aromatic and aliphatic * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1984.

radicals (the bond dissociation energies for C-H bonds in ethane and benzene are, respectively, 98 and 110 kcal/mol),2 the decarboxylations of aromatic and aliphatic carboxylic acids are considered separately. Aromatic carboxylic acids decarboxylate most rapidly by an acid-catalyzed reaction in which the aromatic ring is first protonated at the ipso position.3 Loss of CO2 then restores aromaticity. Radical decomposition of aromatic carboxylic acids was not observed at 400 °C.3 Radical decomposition is expected to be faster for aliphatic carboxylic acids than for aromatics because the aliphatic radical produced is more stable. Yet even with aliphatics, radical decarboxylation is a high-temperature process. If the carboxylic acid or more probably the carboxylate anion is oxidized to a radical, its departure to produce CO2 and an aliphatic radical, as in the familiar Kolbe reaction, is exothermic and fast.4 As suggested first by Cooper and Bray,5 this is the only reasonable pathway for direct low-temperature CO2 loss from kerogens, and we plan to deal with this in future papers. β-Ketocarboxylic acids decarboxylate rapidly at low temperatures by a cyclic mechanism.6 The probability of this functional group occurring in kerogens is low. Unless aided by a one-electron oxidation, the aliphatic carboxylic acids present in kerogens will not lose CO2 at temperatures under 300 °C unless they are first converted to a more reactive derivative. Simple aliphatic carboxylate anions will also be stable at temperatures below 300 °C. Aliphatic carboxylic acid esters pyrolyze by a cyclic six-membered ring mechanism to give an olefin and a (2) Benson, S. W. Thermochemical Kinetics, 2nd ed.; John Wiley & Sons: New York, 1976. (3) Eskay, T. P.; Britt, P. E.; Buchanan, A. C., III. Energy Fuels 1997, 11, 1278-1287. (4) Kochi, J. K. In Oxidation-Reduction Reactions of Free Radicals and Metal Complexes in Free Radicals; Kochi, J. K., Ed.; John Wiley & Sons: New York, 1973; Vol. 1, pp 651-660. (5) Cooper, J. E.; Bray, E. E. Geochim. Cosmochim. Acta 1963, 27, 1113-1127. (6) Jones, M., Jr. Organic Chemistry, 2nd ed.; W. W. Norton & Co.: New York, 2000; pp 983-987.

10.1021/ef0501086 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/17/2005

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Energy & Fuels, Vol. 19, No. 5, 2005 1955

cannot claim thermal anhydride formation in this case or others (for example, ref 12) where there are shoulders in the IR carbonyl that might indicate anhydride formation because the published spectra in the figures are too small to interpret reliably. There is enough evidence to conclude that anhydrides are formed from many kerogens on heating to low temperatures.8-10 We believe that the primary route for low-temperature CO formation from kerogens and an important route for thermal decarboxylation of kerogens starts with anhydride formation. Decomposition of anhydrides by homolytic bond cleavage (eq 1) is a slow reaction. The OdC-O single bond strength in aromatic anhydrides has been estimated3 to be 84 kcal/mol, and the value for aliphatic systems will be close to this. It is much too strong to cleave at measurable rates below 300 °C. Note that thermal cleavage of this bond leads to the formation of both CO and CO2. Anhydrides also react by a cyclic mechanism to form a ketene and a carboxylic acid (eq 2) in another high-temperature (∼500 °C) process.13 The highly reactive ketene will react with any available nucleophile. The anhydride itself is subject to attack by nucleophiles, but this will not result in decarboxylation. As shown by Eskay et al., electrophilic attack by a radical on the carbonyl oxygen will result in the formation of an ester, CO, and an alkyl radical (eq 3).3 This radical chain reaction has the potential to partially decarboxylate kerogens at low temperatures provided there is a source of radicals. One caution, the use of steel reactors is to be avoided because the decomposition of carboxylic acids in water is strongly catalyzed by stainless steel.14,15

Figure 1. TGA measured weight loss for NDGS 105 and NDGS 1858 kerogen.

carboxylic acid.7 This reaction is not possible with compounds not having a hydrogen β to the ester. Carbon dioxide is not formed. This is a high-temperature reaction often carried out at temperatures around 500 °C. It is faster than homolytic bond cleavage to form CO or CO2 by a radical pathway. The rapid formation of acid anhydrides from kerogen carboxylic acids by heating at low temperatures has been reported.8 We have found only two other published observations of anhydride formation in kerogens. Lowtemperature thermal formation of anhydrides in Zap North Dakota lignite and in Nagoorin kerogen was observed using hot stage FTIR microscopy.9 Anhydrides were also reported in a paper that used IR emission from kerogens during oxidation to follow the oxidation.10 In an earlier study of IR emission from heated kerogens and bitumens, there is no specific mention of anhydride formation, although some of the spectra show weak peaks that disappear on heating to around 350 °C.11 We (7) Ansell, M. F.; Gigg, R. H. Monobasic Carboxylic Acids. In Rodd’s Chemistry of Carbon Compounds, 2nd ed.; Coffey, S., Ed.; Elsevier: New York, 1965; p 145. (8) Larsen, J. W.; Islas-Flores, C.; Aida, M. T.; Opaprakasit, P.; Painter P. Energy Fuels 2005, 19, 145-151. (9) Taulbee, D. N.; Sparks, J.; Robl, T. Fuel 1994, 73, 1551-1556. (10) Rose, H. R.; Smith D. R.; Vassallo, A. M. Energy Fuels 1998, 12, 682-688. (11) Rose, H. R.; Smith D. R.; Vassallo, A. M. Energy Fuels 1993, 7, 319-325.

Except for eq 3, none of the reactions listed is expected to play a significant role in decarboxylation at temperatures below 300 °C, and eq 3 requires a source of radicals. Thermal radical formation at temperatures as low as 300 °C requires weak bonds. Kerogens are known to contain radicals in amounts between 1017 and 1019 spins/g, and radical populations increase with temperature even at temperatures well below 300 °C.16-23 (12) Huang, W.-L.; Otten, G. A. Org. Geochem. 1998, 29, 1119-1137. (13) March, J. Advanced Organic Chemistry, 4th ed.; John Wiley & Sons: New York, 1992; p 1011. (14) Kharaka, Y. K.; Carothers, W. W.; Rosenbauer, R. J. Geochim. Cosmochim. Acta 1983, 47, 397-402. (15) Palmer, D. A.; Drummond, S. E. Geochim. Cosmochim. Acta 1986, 50, 813-823. (16) Marchand, A.; Conard, J. In Electron paramagnetic resonance in kerogen studies in Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; Chapter 8. (17) Bakr, M. Y.; Yokono, T.; Sanada, Y.; Akiyama, M. Energy Fuels 1991, 5, 441-444. (18) Silbernagel, B. G.; Gebhard, L. A.; Siskin, M.; Brons, G. Energy Fuels 1997, 1, 501-506. (19) Requejo, A. G.; Gray, N. R.; Freund, H.; Thomann, H.; Melchior, M. T.; Gebhard, L. A.; Bernardo, M.; Pictroski, C. F.; Hsu, C. S. Energy Fuels 1992, 6, 203-214. (20) Bakr, M.; Akiyama, M.; Sanada, Y. Org. Geochem. 1991, 17, 321-328. (21) Carniti, P.; Beltrame, P. L.; Gervasini, A.; Castelli, A.; Bergamasco, L. J. Anal. Appl. Pyrolysis 1997, 40-41, 553-568.

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Ashida et al.

Figure 2. DRIFT spectra of NDGS 105 kerogen heated to the indicated temperature together with IR band assignments.

Next, the thermally induced emission of CO2 and CO from kerogens and anhydride-containing polymers will be summarized. The polymers are model systems of known structure. As we shall presently document, many polymers form anhydrides before they decarboxylate. Both the temperature at which these gases are formed and the ratio of CO2 to CO have mechanistic significance (see eqs 1-3). Data on CO2 evolution from seven different maleic anhydride copolymers are available. The copolymers of maleic anhydride and allyl acetate and vinyl acetate both begin to lose CO2 at about 200 °C.24,25 Copolymers with isopropylene and R-methylstyrene begin to form CO2 at about 250 °C.26,27 The acynaphthalene-maleic anhydride copolymer begins to liberate CO2 slowly at (22) Ishiwatari, R.; Ishiwatari, M.; Kaplan, I. R.; Rohrback, B. G. Nature 1976, 264, 347-349. (23) Ishiwatari, R.; Rohrback, B. G.; Kaplan, I. R. AAPG Bull. 1978, 62, 687-692. (24) McNeill, I. C.; Polishchuk, A. Yu.; Zaikov, G. E. Polym. Degrad. Stab. 1992, 37, 223-232. (25) McNeill, I. C.; Ahmed, S.; Gorman, J. G. Polym. Degrad. Stab. 1999, 64, 21-26.

250 °C and more rapidly at 300 °C.28 There is another report that a styrene-maleic anhydride copolymer is stable to 330 °C.29 A highly aromatic coal model polymer with a low concentration of carboxyl groups formed anhydrides on heating to 250-300 °C.30 The anhydrides react at temperatures higher than 375 °C, temperatures that cause radical formation. Some cyclic anhydride groups in polymers are capable of decomposing at 250 °C or lower temperatures. The mechanisms of these decompositions have not been studied. It is apparent that there are low-temperature routes to CO2 loss from anhydrides. There are some data on the thermal decomposition of kerogens and shales to give CO2 and CO. The (26) McNeill, I. C.; Ahmed, S.; Rendall, S. Polym. Degrad. Stab. 1998, 62, 85-95. (27) Aida, H.; Urushizaki, M.; Maegawa, H.; Okazaki, S. Kobunshi Ronbunshu 1988, 45, 333-338; CAN 109:38378. (28) McNeill, I. C.; Musarrat, M. H.; Davydov, E. Ya.; Zaikov, G. E. Polym. Degrad. Stab. 1997, 58, 61-67. (29) Moore, E. R. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 315321. (30) Britt, P. F.; Mungall, W. S.; Buchanan, A. C., III. Energy Fuels 1998, 12, 660-661.



Figure 3. NDGS 105 spectra resulting from subtracting the unheated kerogen spectrum from the heated kerogen spectra.

evolution of CO2 and CO from 15 oil shales heated at 10 °C/min was studied by triple quadrupole mass spectrometry.31 It is immediately clear that there are large differences in the temperatures at which CO2 evolution commences. For the high carbonate shales, there is only occasionally a little CO2 formed below 350 °C. Its evolution commences at about 400 °C. The siderite shales are similar. The low carbonate shales show steady slow CO2 evolution beginning around 200 °C, except for a Montana marine shale (PHOS) whose CO2 evolution peaks at temperatures lower than 400 °C. The mineral matrix appears to play a significant role in CO2 evolution. None of these shales lost CO at temperatures lower than 300 °C, and only one-half of them showed significant CO loss at temperatures below 500 °C. It is apparent that there exist different reaction pathways for thermal kerogen deoxygenation. Another study used gas chromatography to measure the gases produced when shales were heated.32 Three shales showed CO2 evolution starting at about 200 °C when heated at 2 °C/min.32 Green river shale forms CO2 and small amounts of CO on heating at 200 or 300 °C.33 These data confirm the existence of multiple pathways for thermal decarboxylation. Both CO and CO2 are formed at very different temperatures in different shales and therefore are formed in different chemical reactions. (31) Reynolds, J. G.; Crawford, R. W.; Burnham, A. K. Energy Fuels 1991, 5, 507-523. (32) Sato, S.; Enomoto, M. Fuel. Process. Techn. 1997, 53, 41-47. (33) Tannenbaum, E.; Kaplan, I. R. Geochim. Cosmochim. Acta 1985, 49, 2589-2604.

We are studying the differences in kerogen and shale chemical structure that are responsible for these reactivity differences. Experimental Section Sample Preparation. Two Bakken shales, NDGS105 (immature) and NDGS1858 (mature), were used in this work. The shales were demineralized using aqueous HCl and then aqueous HF followed by pyrite removal using aqueous CrCl2 to obtain kerogens.34,35 A Perkin-Elmer TGA7 thermogravimetric analyzer was used for heating the kerogens under a nitrogen atmosphere. About 15 mg of the kerogen was heated to selected temperatures between 200 and 300 °C at a heating rate of 20 °C/min, and then it was kept for 2 h at the final temperatures. Fourier Transform Infrared Spectroscopy. Infrared spectra were recorded on a Digilab model FTS-45 spectrometer at a resolution of 2 cm-1 by using the diffuse reflectance (DRIFT) technique. Here, 10 mg of kerogen or heat-treated kerogen was mixed with 200 mg of dry KBr powder and ground with a WIG-L-BUG for 2 min. Pure ground KBr was used to obtain a reference spectrum. All spectra were recorded using 200 scans. To obtain difference spectra, the intensity of each spectrum was first normalized to the concentration of sample in KBr and then multiplied by the yield of the sample during the heat treatment, which was determined by TGA, followed by subtraction. (34) Saxby, J. D. Chem. Geol. 1974, 6, 173-184. (35) Acholla, F. V.; Orr, W. L. Energy Fuels 1993, 7, 406-410.

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Figure 4. NDGS 105 shale spectra resulting from subtracting the unheated shale spectrum from the heated shale spectra. Scheme 1

Results A pair of Bakken kerogens, one immature (NDGS 105) and one of intermediate maturity (NDGS 1858), were studied. See ref 8 for characterization data and geological background information. Figure 1 shows the weight loss measured by thermogravimetric analysis (TGA) from Bakken NDGS 105 kerogen as well as the temperature profile. TGA analysis of another sample of kerogen isolated from the same shale showed similar, but not identical, results. The samples from the TGA runs were used to obtain the DRIFT spectra. It is impossible to distinguish between evaporative weight loss and weight loss due to chemical reaction form these curves. A sharp increase in weight loss from kerogen occurs between 275 and 300 °C. The DRIFT spectra of NDGS 105 kerogen heated as indicated are shown in Figure 2. Figure 3 shows the results of subtracting the unaltered kerogen spectrum from each of the heated samples’ spectrum. Figure 4 shows the changes that occur at 25 °C intervals. Similar data are shown for NDGS 1858 in Figures 5-7. To get

good quality subtracted spectra, the individual spectra were normalized to kerogen sample weight. To test this, illite was added as an internal standard and the spectra were normalized to completely remove illite peaks form the subtracted spectra. The two different normalization procedures gave identical results. In this paper, we are dealing with the decarboxylation mechanism and will concentrate on changes in the carbonyl region, reserving discussion of other changes for future papers. Discussion Before anhydride formation and reaction is discussed, two other features of the IR spectra require brief mention. The first is the increase in the intensity of the C-H stretching band (∼2930 cm-1) that occurs between 225 and 250 °C that can most easily be seen in Figures 4 and 7. One expects evaporative losses, not gains. This phenomenon has been observed previously and ascribed to movement of alkanes from the interior of the kerogen particles to the surface where they become visible to the IR instrument.9,36 This is a reasonable explanation. The weight loss curves in Figure 1 show that most of the weight loss occurs quickly, within the first 20 min as the temperature is increasing. The weight loss on heating the immature kerogen to 300 °C is much greater than that on heating to 275 °C. The more mature kerogen has a smaller weight loss at 300 °C, as expected. The IR spectra of both kerogens reveal the occurrence of numerous chemical changes at 300 °C that (36) Rouxhet, P. G.; Robin, P. L. Fuel 1978, 57, 533-540.



Figure 5. DRIFT spectra of NDGS 1858 kerogen heated to the indicated temperature together with IR band assignments.

do not occur at lower temperatures. Significant thermal decomposition of these kerogens begins at roughly 300 °C. The focus of this paper is the formation and disappearance of carboxylic acid anhydrides that occurs below 250 °C. We hope to report in detail in a future paper on the reactions occurring at 300 °C. The IR spectra in Figures 3 and 4 show that the immature Bakken kerogen (NDGS 105) has formed anhydrides at temperatures lower than 225 °C. These anhydrides disappear on heating to 250 °C. Carboxylic acid anhydrides are easily identified by the presence of a pair of carbonyl bands 60 cm-1 apart in the infrared.38 In acetic anhydride, the two bands occur at 1824 and 1748 cm-1, and other aliphatics have bands close to this. Conjugation lowers these bands by 20-40 cm-1. If the anhydride is part of a five-membered ring, the values are shifted with succinic anhydride absorbing at 1865 and 1782 cm-1. With the more mature kerogen, anhy(37) Daly, A. R.; Peters, K. E. AAPG Bull. 1982, 66, 2672-2681. (38) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; John Wiley & Sons: New York, 1958; pp 127-129.

dride formation is complete at 200 °C and they have disappeared on heating to 225 °C. The spectra leave no doubt that carboxylic acid anhydrides are formed as a result of mild heating and do not survive past 250-300 °C. The disappearance of the anhydrides at this low temperature limits the possible reactions. The most reasonable is radical addition to the anhydride carbonyl followed by β scission to give an ester and an acyl radical. The acyl radical will immediately decompose to give CO and an alkyl radical to continue the chain reaction. The formation of CO when Bakken kerogens are heated has been thoroughly studied.37 Daly and Peters (DP) studied the thermal evolution of CO from a Bakken kerogen maturation series and from thermally altered Bakken kerogen.37 They followed a Rock-Eval temperature profile in which the CO evolution was measured by infrared spectroscopy as the kerogen was heated at 300 °C for 3 min and then heated at 25 °C/min to 550 °C. We are concerned with CO evolution during the 3 min at 300 °C. For the maturation series, it decreased as expected with increasing

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Figure 6. NDGS 1858 spectra resulting from subtracting the unheated kerogen spectrum from the heated kerogen spectra.

maturity. Yet some CO was produced by all of the shales, even those having H/C 0.5. A sharp decrease in CO evolution occurred between H/C 1.0 and H/C 0.8. Heating the Bakken shale to 300 °C followed by the Rock-Eval analysis resulted in the almost total disappearance of the initial CO loss. Similar behavior was observed for Green River shale and a sub-bituminous coal. Daly and Peters37 observed CO2 loss from Bakken shale during the Rock-Eval S3 peak at 550 °C. A TGAMS study of Bakken NDGS-105 kerogen confirmed the absence of CO2 below 300 °C. The next question to be addressed is the mechanism by which the anhydrides decompose. There are two strong and independent evidences that the mechanism is the radical chain decomposition shown in Scheme 1. One is the low temperature at which the reaction occurs. Homolysis of the OdC-O single bond (∼84 kcal/mol) will not occur below 300 °C. There is no evidence for nucleophilic attack on the anhydrides. The data are entirely consistent with the radical chain mechanism. Further evidence is provided by presence of CO and the absence of CO2 from the decomposition products. Only decomposition initiated by radical attack produces only CO as the sole gaseous product. Thus, the low-temperature thermal decarboxylation of Bakken kerogen appears to occur by a radical chain reaction initiated by radical attack on anhydrides formed from carboxylic acids at low temperature. This reaction should produce one-half as many esters as there were carboxylic acids reacted. Aliphatic esters

have a strong carbonyl band at 1735-1750 cm-1. There is also a strong C-O stretching vibration at 1150-1200 cm-1.39 There are too many undifferentiated peaks in both the carbonyl and the C-O region of the spectra to allow the behavior of esters to be determined reliably. The DRIFT evidence for their formation in the subtracted spectra is weak. The spectra in Figures 2 and 5 are consistent with the presence of aliphatic ketones, but the subtracted spectra provide no information on their formation and decomposition other than that there is a decrease in carbonyl intensity with increasing temperature. The IR evidence suggests that esters are formed at low temperatures but are not thermally stable. The low-temperature anhydride decomposition chain reaction requires a radical initiator. Kerogens contain radicals, and their concentration increases with dephth.40 Values on the order of 1018 spins/g are common, although populations as large as 1019 spins/g have been reported.41 The radical population increases with temperature with increases beginning at about 100 °C and reaching a maximum somewhat at a temperature that varies strongly with the identity of the kerogen, but is almost always above 300 °C.20,40,41 Heating at a constant temperature of 350 or 375 °C results in the continuous (39) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; John Wiley & Sons: New York, 1958; pp 178-183. (40) Bakr, M. Y.; Akiyama, M.; Sanada, Y. Org. Geochem. 1990, 15, 595-599. (41) Aizenshtat, Z.; Pinsky, I.; Spiro, B. Org. Geochem. 1986, 9, 321329.



Figure 7. NDGS 1858 shale spectra resulting from subtracting the unheated shale spectrum from the heated shale spectra.

increase in kerogen radical population over several hours.18 The radicals necessary to initiate the decomposition chain reaction are present in the kerogen, and their concentration increases with temperature. The reaction pathway that best fits the data is shown in Scheme 1. It begins with anhydride formation. The anhydride then decomposes to give CO and esters by a radical chain reaction initiated by a kerogen radical. This reaction pathway will not occur during hydrous pyrolysis when the kerogen is submerged in water that has access to at least the polar groups in the kerogen.42 In the presence of water, the anhydride will not form. The alternative decarboxylation pathways are all expected to be slower than the pathway that proceeds through the anhydride. Thus, there is an identifiable difference between thermal kerogen reactions in hydrous and anhydrous pyrolysis. There is a low-temperature pathway that converts carboxylic acids to esters and CO that occurs in anhydrous pyrolysis but does not (42) Larsen, J. W.; Aida, M. T. Energy Fuels 2004, 18, 1603-1604.

occur during hydrous pyrolysis. In source rocks, there exists a pathway for low-temperature kerogen decarboxylation of dry kerogens that does not exist for wet kerogens. This is certainly not the only decarboxylation pathway that occurs in kerogens. When heated, some kerogens begin to liberate CO2 at temperatures as low as 250 °C and others form CO2 only at much higher temperatures without forming CO.31 Green River kerogen liberates CO2 and small amounts of CO at 200 °C.33 There are several different patterns of thermal CO evolution from kerogens.31,37 These different CO and CO2 emission pathways require that there are at least two other decarboxylation pathways available to kerogens. Acknowledgment. Grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. EF0501086

Kerogen Chemistry 4. Thermal Decarboxylation of Kerogens - Energy

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