Kerogen Chemistry 9. Removal of Kerogen Radicals and Their Role in

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Energy & Fuels 2006, 20, 1220-1226

Kerogen Chemistry 9. Removal of Kerogen Radicals and Their Role in Kerogen Anhydride Decomposition§ John W. Larsen,* Ryuichi Ashida, and Paul Painter The Energy Institute, 209 Academic Projects Building, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802

David C. Doetschman Department of Chemistry, Binghamton UniVersity, P.O. Box 6000, Binghamton, New York 13902-6000 ReceiVed NoVember 9, 2005. ReVised Manuscript ReceiVed March 29, 2006

By treatment with CrCl2, the radical populations of Kimmeridge and Bakken kerogens were significantly decreased. When both kerogens are mildly heated, anhydride formation at low temperatures can be detected by infrared spectroscopy only when radicals have been removed by CrCl2 treatment. The “native” kerogen radicals react with thermally formed carboxylic acid anhydrides to form esters and CO. Some of the observable kerogen radicals are persistent and reactive. The Acholla and Orr method for pyrite removal also reduces the radical population of these kerogens.

Introduction Organic free radicals that are observable by electron spin resonance (ESR) spectroscopy are conveniently divided into two groups: persistent and stable. Stable radicals do not participate in chemical reactions because of their great thermodynamic stability, which is usually due to delocalization. If the unpaired electron is delocalized over a large π-system, its thermodynamic stability may be so great that its chemical reaction is prevented. The other class of observable radicals is the persistent ones, radicals whose stability is such that they are capable of reacting, but do not. For example, the structure of a radical may be such that a second molecule cannot approach the unpaired electron closely enough to react. Or, the immediate environment of a radical in a rigid solid may not contain a reactive group. It is important to know what portion of the radicals present in kerogens is persistent and what portion is stable. In this paper, kerogen refers only to Type I and II kerogen, not to Type III. The behavior of coals is different enough to be distinguished by a different name. Some of the work done with coal radicals is necessary background to our work with Type II kerogen radicals. Kerogens and coals contain significant (1 ×1017 to 1 × 1019 spins/g) numbers of radicals that are observable at room temperature by electron spin resonance (ESR). The coal radicals have received attention largely because they significantly complicate obtaining quantitative 13C nuclear magnetic resonance (NMR) spectra, by coupling with 13C nuclear spins. Our concern is with the chemical reactivity of these radicals. Coal radical reactivity was probed by diffusing 4-vinylpyridine vapors into coals and measuring the radical population before and after the 4-vinylpyridine undergoes radical polymerization.1 There were small (529%) reductions in radical populations, with mostly inertinite § Portions of this work were published in Energy Fuels 2005, 19, 22162217. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Flowers, R. A., II; Gebhard, L.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1992, 6, 455-459.

radicals being lost. These results show that a small number of the studied coal radicals are persistent but that most appear stable. A different approach was developed by Muntean et al.2 They showed that potassium in liquid ammonia reduced the radical population of Upper Freeport coal. Alas, it also reduces coal unsaturated structures. But the weaker reducing agent, SmI2 (Sm2+/Sm3+ ) -1.55V) in the good swelling solvent tetrahydrofuran (THF), effectively reduced the radical population of six coals. This was followed by a study that attempted to determine the thermodynamic stability (reduction potential) of coal radicals by using a series of one-electron reducing agents whose oxidation potentials varied from -1.55 to -0.42 V.3 The hope was that regularly increasing portions of the radicals in Wyodak coal would be reduced as the oxidation potentials of the metal reactant became more negative. This hope was not realized. The least-powerful reducing agent tried, CrCl2, was the most effective at removing the coal radicals. CrCl2 was used in the procedure for pyrite removal developed by Acholla and Orr.4 It was suggested3 that accessibility to the radicals by the different reagents might be affecting radical reductions. Reducing agents react with coal radicals. The balance between stable and persistent coal radicals remains unknown. There is no corresponding information on kerogen radicals. When coals are heated, radicals are formed; they play an important role in coal reactions. Petrakis and Grandy5 extensively studied by ESR the thermal formation of radicals in coals heated alone and in different gases and organic liquids. The results are largely as expected. For most coals, the radical concentration remains about constant until 300 °C, when it begins to increase. It continues to increase until about 500 °C and then decreases. The increase is due to bond homolysis and (2) Muntean, J. V.; Stock, L. M.; Botto, R. E. Energy Fuels 1988, 2, 108-110. (3) Larsen, J. W.; Parikh, H.; Doetschman, D. C. Energy Fuels 2001, 15, 1225-1226. (4) Acholla, F. V.; Orr, W. L. Energy Fuels 1993, 7, 406-410. (5) Petrakis, L.; Grandy, D. W. Free Radicals in Coals and Synthetic Fuels; Elsevier: New York, 1983.

10.1021/ef050368n CCC: $33.50 © 2006 American Chemical Society Published on Web 04/26/2006

Kerogen Radicals and Kerogen Anhydride Decomposition

increases with temperature as stronger bonds become cleaved. Above 500 °C, radical reactions in the now at least somewhat fluid coal6 are faster than radical formation and so the radical population decreases. The addition of compounds that react readily with radicals lowers their concentration. Although there are many specifics that are not yet understood, the overall pattern is straightforward and has been successfully modeled.7 There are numerous ESR studies of thermal radical formation in kerogens. Kerogen radicals formed in three different ways have been studied by ESR. The radicals that are normally found in kerogens have been studied, especially regarding their relationship to kerogen maturation. Heating kerogens and then quickly cooling them results in increases in the radical population; this technique has been used often. More rare are the experimentally more difficult ESR studies of kerogens at high temperatures. The number of observable free radicals has been used as a measure of kerogen maturity, and there is a thorough comparison of this ESR-based maturity measure with many other maturity measures.8 The technique usually used to monitor the thermal formation of radicals in kerogens is to heat the kerogen under carefully defined conditions, cool it rapidly, and then record the ESR spectrum.9 The resulting temporal separation of formation and observation requires the assumption that no additional reactions of the radicals occur during and after cooling. We shall use the results obtained using a hightemperature ESR cell to frame our discussion of thermal radical formation in kerogens.10 Bakr et al. observed that the radical population of three Type II kerogens increased continuously with temperature and reached a first maximum between 275 and 350 °C. The population then decreased, reaching a minimum at 350-425 °C before increasing sharply to a second and greater maximum. Between room temperature and the first maximum, radical populations increased by roughly 6-10-fold. During this initial increase in radical population, there was a pronounced loss of H2O and CO2. It was suggested that radical formation was the result of dehydration and CO2 loss. We have doubts about this conclusion. That two changes occur over the same temperature range does not prove that they are related. They may be, but they may also be parallel and unrelated processes. Others who have studied thermal radical formation in kerogens have also claimed that radicals are generated during dehydration and decarboxylation.9 A perusal of any introductory organic textbook11 will reveal that dehydration and decarboxylation are poor candidates for radical formation, with the exception of two reactions. Dehydration is not normally a radical reaction but is heterolytic. Decarboxylation where there is a β-carbonyl group is a lowtemperature reaction proceeding through a cyclic transition state. The several heterolytic decarboxylation mechanisms are all hightemperature processes, as is ketene formation. The C-OH bond (6) Sakurovs, R.; Lynch, L. J.; Barton, W. A. In Coal Science II; Schobert, H. H., Bartle, K. D., Lynch, L. J., Eds.; ACS Symposium Series 461; American Chemical Society: Washington, DC, 1991; Chapter 9. (7) Niksa, S. Combust. Flame 1995, 100, 384. Charpenay, S.; Serio, M. A.; Bassilakis, R.; Landais, P. Energy Fuels 1996, 10, 26-38 and references therein. (8) 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. (9) Marchand, A.; Conard, J. In Kerogen Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980; pp 243-270. (10) Bakr, M.; Akiyama, M.; Sanada, Y. Org. Geochem. 1991, 17, 321329. (11) For example: Jones, M., Jr. Organic Chemistry, 2nd ed.; W. W. Norton: New York, 2000.

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is too strong (104 kcal/mol12) to rupture homolytically at low temperatures. The exception, recognized years ago and then ignored, is one-electron oxidation of a carboxylate anion to the radical followed by rapid loss of CO2 (eq 1).13 This requires the presence of a one-electron oxidant, usually a metal. This reaction will increase the population of organic radicals. slow

fast

RCOO- + M+X 98 M+(X-1) + RCOO• 98 R• + CO2 (1) When heated and quenched, the radical population of kerogens increases.14-17 The peak temperature for thermal oil and gas generation roughly coincides with maxima in radical concentration.14 The activation energy for radical formation has been measured for two Type II kerogens and is 40-45 kcal/mol.15 The kinetics were reported to be first order. There are few bonds this weak in most kerogen structures (see ref 18for example), so simple bond homolysis is unlikely to be the source of the new radicals. Organic radical populations in some kerogens increase continuously on heating above room temperature.10,14 The reactions responsible for this have not been identified. Gaussian ESR line shapes have been observed, indicating that spin exchange is not occurring.15 But the decrease in ESR line width with increasing kerogen maturity has been ascribed to spin exchange.19 This study (ref 19) also reported that the radical population of a kerogen decreased on pyridine extraction. This could be due to the removal of radicals by dissolution in the pyridine. It could also be due to the reaction of persistent radicals freed from their unreactive environment when the glassy kerogen is swollen by the pyridine and becomes rubbery. Some kerogen radicals may be persistent. Mechanical property measurements and NMR relaxation studies indicate that kerogens are primarily glassy at room temperature.20,21 Reactive radicals may be diffusionally trapped in the glassy macromolecular system, unable to find a reactive partner. They are rigidly held in an unreactive environment, consistent with their Gaussian line shape. On the other hand, it seems unlikely that this might persevere for geological time periods. There are only a few published highly detailed kerogen molecular structures, the most detailed being Siskin’s.18 In those kerogen structures, the only molecular structures that would delocalize a radical sufficiently to render it unreactive are a very small population of large polynuclear aromatics. But at 1 × 1018 spins/g, there is one spin per 40 000 carbon atoms. A few statistically insignificant (12) Benson, S. W. Thermochemical Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976; p 309. (13) Cooper, J. E.; Bray, E. E. Geochim. Cosmochim. Acta 1963, 27, 1113-1127. (14) Aizenshtat, Z.; Pinsky, I.; Spiro, B. Org. Geochem. 1986, 9, 321329. (15) Carniti, P.; Beltrame, P. L.; Gervasini, A.; Castelli, A.; Bergamasco, L. J. Anal. Appl. Pyrolysis 1997, 40-41, 553-568. (16) Marchand, A.; Conard, J. Electron Paramagnetic Resonance in Kerogen Studies. In Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1989; Chapter 8. (17) Bakr, M. Y.; Yokono, T.; Sanada, Y.; Akiyama, M. Energy Fuels 1991, 5, 441-444. (18) Siskin, M.; Scouten, C. G.; Rose, K. D.; Aczel, T.; Colgrove, S. G.; Pabst, R. E., Jr. Detailed Structural Characterization of the Organic Material in Rundle Ramsay Crossing and Green River Oil Shales. In Composition, Geochemistry and ConVersion of Oil Shales; Snape, C., Ed; NATO ASI Series, Vol. 455; Kluwer: Boston, 1993; pp 143-158. (19) Bakr, M. Y.; Akiyama, M.; Sanada, Y. Org. Geochem. 1990, 15, 595-599. (20) Zeszotarski, J. C.; Chromik, R. C.; Vinci, R. P.; Messmer, M. C.; Michels, R.; Larsen, J. W. Geochim. Cosmochim. Acta 2004, 68, 41134119. (21) Parks, T. J.; Lynch, L. J.; Webster, D. S.; Barrett, D. 1988 Energy Fuels 1988, 2, 185-190.

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Scheme 1

structures would suffice to produce 1 × 1018 stable spins per gram. The question of the existence of persistent radicals in kerogen will have to be answered experimentally. We have recently published the observation that kerogens form anhydrides in an endothermic process when they are heated at temperatures below 200 °C.22 These anhydrides then decompose by a radical chain reaction initiated by attack of a kerogen radical on the anhydride carbonyl oxygen.23,24 The products are CO and an ester. Thus, kerogens can decarboxylate at low temperatures in a two-step process (Scheme 1) that yields a kerogen ester, water, and CO. Because this process proceeds through anhydride formation, it will not occur in water-wet systems. The formation of anhydrides is an endothermic dehydration reaction that will not occur in the presence of water. This is one difference between the chemistry that occurs in dry and in hydrous pyrolyses. The temperature required for these reactions is experienced by kerogens in petroleum kitchens.25 It is a low-temperature process for kerogen decarboxylation and ester formation that will happen if the kerogens are dry. Because alcohol groups in kerogens are rare and esters are well-known,18 there are reasons to suspect that the chemistry of Scheme 1 occurs in geological systems. This set of reactions requires the presence of reactive radicals at low temperatures. It should be strongly inhibited if those reactive radicals could be removed. In what follows we shall show that a commonly used procedure removes reactive radicals and that this removal blocks the chemistry of Scheme 1, allowing anhydrides to be detected. One of the problems in understanding kerogen chemistry is our ignorance of its physical state. NMR relaxation measurements on a very few kerogens showed them to be predominantly glassy at room temperature and to become rubbery continuously over several hundred degrees when heated.26 A radical chain mechanism requires that the radical be mobile. There are two possibilities. The studied kerogens may have sufficiently mobility and be sufficiently rubbery to permit the reaction. Or, the radicals themselves may be mobile by sequential H• shifts, the occurrence of which has been demonstrated in studies of molecules tethered to silica surfaces to remove translational freedom.27 The evidence that a chain reaction is occurring is strong. Establishing the physical state of a set of kerogens and how this state changes with temperature and pressure would be a major contribution to kerogen chemistry. (22) J. W. Larsen, J. W.; Islas-Flores, C.; Aida, M. T.; Opaprakasit, P.; Painter, P. Energy Fuels 2005, 19, 145-151. (23) Ashida, R.; Painter, P.; Larsen, J. W. Energy Fuels 2005, 19, 19541961. (24) Larsen, J. W.; Ashida, R.; Doetschman, D. C. Energy Fuels 2005, 19, 2216-2217. (25) Hunt, J. Petroleum Geochemistry and Geology, 2nd ed.; W. H. Freeman: New York, 1996. (26) Parks, T. J.; Lynch, L. J.; Webster, D. S.; Barrett, D. Energy Fuels 1988, 2, 185-190. (27) Buchanan, A. C., III; Kidder, M. K.; Britt, P. F. J. Phys. Chem. B 2004, 108, 16772-16779. Buchanan, A. C., III; Kidder, M. K.; Britt, P. F. J. Am. Chem. Soc. 2003, 125, 11806-11807. Kidder, M. K.; Britt, P. F.; Zhang, Z.; Dai, S.; Hagaman, E. W.; Chaffee, A. L.; Buchanan, A. C., III. J. Am. Chem. Soc. 2005, 127, 6353-6360 and references therin.

Figure 1. TGA analysis of Kimmerdige kerogen isolated by dissolving the rock in aqueous HCl and HF. Scan rate ) 20 °C/min. Heating curve to 200 °C is not shown. Table 1. Kerogen Radical Density (spins/g × 10-18) Measured by ESR kerogen

HCl/HF-treated

CrCl2-treated

Kimmeridge Bakken NDGS 105

4.2 ( 1.4 0.48 ( 0.17

0.350 ( 0.099 0.114 ( 0.069

Experimental Section Both kerogens and their isolation have been previously described.22,28 The ESR procedures used have been published.3 Reaction with CrCl2 followed the Acholla and Orr procedure.4 The techniques used to obtain diffuse reflectance Fourier transform infrared (DRIFT) spectra and the techniques used to subtract spectra have been described.21

Results and Discussion Isolation of the kerogen by dissolution of the minerals in aqueous HCl followed by aqueous HF yields a Kimmeridge kerogen with 4.2 × 1018 spins/g and a Bakken kerogen with 0.48 × 1018 spins/g (Table 1). Treatment of these kerogens with CrCl2 using the Acholla and Orr procedure4 reduces the radical population by a factor of 12 for the Kimmeridge kerogen and a factor of 4 for the Bakken kerogen. These reductions in radical concentrations are even larger than those observed with Wyodak coal treated in the same way with the same reagent.3 Immature Bakken kerogen has a low radical concentration, yet even this is reduced. In these kerogens, most of the native free radicals can be removed by treatment with CrCl2 using the Acholla and Orr procedure. Reaction with CrCl2 does not remove all of the radicals. We do not know whether this is due to limited access as with coals3 or to different radical reactivities. The effect of this on the chemistry of kerogens will now be explored. Does removing most of the radicals have any effect on the pyrolysis chemistry? Such effects will be most apparent at low temperatures, because heating the kerogens creates new radicals that possibly can replace the destroyed radicals and initiate the same or similar chemistry. The response of the kerogens before and after radical removal to thermogravimetric analysis (TGA) was determined and is shown in Figures 1-4. The samples were heated at 20 °C/min to a set temperature and then held at that temperature for a total time of 2 h in the TGA. For the (28) Larsen, J. W.; Islas-Flores, C. Fuel Process. Technol. 2006, in press.



Figure 2. TGA analysis of the kerogen from Figure 1 treated with CrCl2 to remove radicals. Scan rate ) 20 °C/min. Heating curve to 200 °C is not shown.

Figure 5. DRIFT spectra of Kimmeridge kerogen isolated using aqueous HCl and HF heated to the indicated temperature at 20 °C/min and held at that temperature for 2 h.

Figure 3. TGA analysis of Bakken NDGS-105 kerogen isolated by dissolving the rock in aqueous HCl and HF. Scan rate ) 20 °C/min.

Figure 4. TGA analysis of the kerogen from Figure 3 treated with CrCl2 to remove radicals. Scan rate ) 20 °C/min.

Kimmeridge kerogen, the differences are small, with less than 2% difference in weight loss at the end of 2 h. The only significant difference in rate of weight loss is for the two samples heated to 225 °C. The differences in the weight-loss curves for the Bakken kerogen are even smaller. It is safe to conclude that

removal of the radicals has at most a small effect on the weight loss from these two kerogens on heating. A more-detailed examination of kerogen radical chemistry is possible by using anhydride formation and decomposition as a probe. In a series of papers, we have shown that as kerogens are heated at low temperature, carboxylic acids form anhydrides; these anhydrides decompose by a radical chain reaction, as shown in Scheme 1. If the reactive radicals are removed, the anhydride decomposition will not occur until new radicals are thermally produced. The anhydrides will persist to higher temperatures and may be observable spectroscopically. We have previously observed them by using diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy.22-24 If sufficient reactive radicals are present in the kerogen, then anhydride decomposition will be faster than in the absence of those radicals. In the extreme case, the anhydrides will react as soon as they are formed and their concentration will never get high enough for them to be observable. Figures5 and 6 show the DRIFT spectra of the kerogens heated to different temperatures after isolation by HCl/HF treatment as a function of temperature. The samples used are from the TGA experiments whose results are shown in Figures 1-4. The heating profiles are available from those figures. Inspection reveals some thermally induced changes, mostly in changes in the relative intensities of bands. Spectral subtraction can be used to make clear the thermally induced changes. To show the different changes occurring as the kerogen is heated to different temperatures, we show a series of subtractions in Figures7 and 8. The spectrum of the kerogen is subtracted from the spectrum of the kerogen after heating at 200 °C to show the changes that have occurred on heating for 2 h at 200 °C. Then the 200 °C spectrum is subtracted from the spectrum of the kerogen heated for 2 h at 225 °C, thereby revealing the changes that have occurred between 200 and 225 °C. This process is continued at 25 °C intervals to 300 °C. The Kimmeridge kerogen shows a strange increase in C-H intensity between 200 and 250 °C that we have previously


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Figure 8. Subtracted spectra from Figure 2.

Figure 6. DRIFT spectra of Bakken NDGS 105 kerogen isolated using aqueous HCl and HF heated to the indicated temperature at 20 °C/min and held at that temperature for 2 h.


discussed and that we plan to examine further.23 There is a lowtemperature loss of hydrocarbons below 200 °C, presumably by evaporation. There is no evidence for the presence of anhydrides. Carboxylic acid anhydrides are easily identified by the presence of a pair of carbonyl bands 60 cm-1 apart in the infrared.29 In acetic anhydride, the two bands occur at 1824 and 1748 cm-1 and other aliphatic 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. The Bakken kerogen shows a steady loss of hydrocarbon and no (29) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; John Wiley & Sons: New York, 1958.

Figure 9. DRIFT spectra of Kimmeridge kerogen (Figure 1) after reaction with CrCl2 and heated to the indicated temperature at 20 °C/ min and held at that temperature for 2 h.

evidence for anhydride formation. With kerogens containing their usual concentrations of radicals, low-temperature anhydride formation is not detected. Figures9 and 10 contain the DRIFT spectra of the HCl/HFisolated kerogen samples after treatment with CrCl2 following the Acholla and Orr procedure.4 These are spectra of kerogens from which most of the radicals have been removed. A comparison of Figures 9 and 10 with Figures 5 and 6 reveals that the CrCl2 treatment resulted in no visible changes in the organic structure (confirming the results reported in ref 4) but did result in the disappearance of a strong band at about 700 cm-1. Several iron-containing minerals have IR peaks in this region.30 The chemistry responsible for the disappearance of the 700 cm-1 band is not known, but a good candidate is the



Figure 12. Subtracted spectra from Figure 6. Figure 10. DRIFT spectra of Bakken NDGS 105 kerogen (Figure 2) after reaction with CrCl2 and heated to the indicated temperature at 20 °C/min and held at that temperature for 2 h.

Figure 13. Scale-expanded Bakken difference spectra obtained by subtracting the spectrum of the sample heated to 200 °C from that heated to 225 °C (top) and by subtracting the spectrum of the original kerogens from that heated to 225 °C (bottom).


removal of an iron-containing mineral by CrCl2 treatment. Again, the subtracted spectra are more revealing. The C-H stretching bands reveal that CrCl2 treatment causes differences in the loss of molecules from the kerogen. Kimmeridge kerogen now shows a large loss between 200 and 225 °C, and the increase in C-H stretch intensity is visible only after heating to 250 °C. The temperature dependences of the Bakken kerogen spectra before and after CrCl2 treatment are different in the O-H stretch region. There is an increase in C-H intensity between 225 and 250 °C after CrCl2 treatment that was not present before. The Kimmeridge kerogen spectra demonstrate anhydride formation between 200 and 225 °C followed by decomposition of those anhydrides between 225 and 250 °C. This anhydride (30) Estep, P. A.; Kovach, J. J.; Karr, C., Jr.; Childers, E. E.; Hiser, A. L. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 1969, 13 (1), 18-34.

formation was not detected with the original kerogen that contained its full complement of radicals. The Bakken kerogen behaves similarly, with anhydride formation visible between 200 and 225 °C followed by its decomposition between 225 and 250 °C. These changes are not entirely clear in the compressed spectra shown in Figures 11 and 12, but are more clearly revealed in the scale-expanded difference spectra shown in Figure 13, which also shows a difference spectrum obtained by subtracting the spectrum of the original kerogens from that of the sample heated to 225 °C. The observed behavior is exactly as expected if the chemistry contained in Scheme 1 is occurring. With all radicals present, anhydrides form and rapidly decompose. Their formation is detected as an endotherm in differential scanning calorimetry. Their concentration never builds up enough for them to be detected by DRIFT. When the reactive radicals have been removed, the decomposition of the anhydrides by a radical chain mechanism (Scheme 1) does not occur because the initiating radicals are no longer present. The anhydrides accumulate and are detected by DRIFT spectroscopy. As the kerogens are heated, radicals are being formed. We do not know the chemistry responsible for their formation, but they are readily detected by ESR spectroscopy. These new radicals eventually initiate


the destruction of the anhydrides by the radical chain process shown in Scheme 1. The results presented here lead to several conclusions. Some of the radicals in kerogens that are routinely observable by ESR are persistent. They are capable of initiating a chain reaction. Likewise, at least some of the reactions formed by warming kerogens to 250 °C are reactive. The Acholla and Orr procedure for removing pyrite can alter kerogen thermal chemistry. Finally,

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the low-temperature thermal decarboxylation of dry kerogen shown in Scheme 1 is consistent with all of the data. Acknowledgment. Grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. EF050368N

Kerogen Chemistry 9. Removal of Kerogen Radicals and Their Role in

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