794
Energy & Fuels 1988,2, 794-801
Natural and Artificial Maturations of a Coal Series: Infrared Spectrometry Study Marc Monthioux* Laboratoire Marcel Mathieu, U A 1205 CNRS- Universitd 2, Avenue du Prdsident P. Angot, 64000 Pau, France
Patrick Landais Centre de Recherche sur la Gdologie de l’llranium, BP 23, 54501 Vandoeuvre-16s-Nancy Cedex, France Received April 4, 1988. Revised Manuscript Received June 6, 1988
A lignite from the Mahakam delta (Indonesia) was heated for 24 h from 250 to 550 OC in a confined system, under pressures of 500 and 1000 bar with the presence or absence of water. Experiments were also performed in an open-pyrolysis system. Natural samples from the same provenance with varying maturities were selected as a series of natural references. In addition to previous studies on the solid organic residues and on the chloroform extracts, an infrared spectrometry examination was performed. Confined-system pyrolysis appears to be able to satisfactorily simulate the natural in situ chemical evolution of the solid organic macromolecule, while open-system pyrolysis, which seems to enhance aromatization reactions, does not. It was found that natural coals are able to retain free but trapped hydrocarbons while solid residues from artificial maturation are not.
Introduction Despite the numerous studies already performed on this important research topic, the laboratory simulation experiments of the natural organic matter evolution rarely gave satisfactory results. However, we recently showed that applying strong confinement conditions during the pyrolysis experimentswas a determinative factor for widely improving the simulation results. This statement is deduced from the systematic comparison between the natural data from a homogeneous series of reference coals and the experimental data from various pyrolysis conditions. Both the solid residue~l-~ and the chloroform extracts4+ were examined. These results have shown that the distinction between “closed” and “confined” systems is important. Confinement implies that the dead volume is minimal in the reaction system. This allows the pressure to be mainly ensured by the effluents released and a close contact to be ensured between the solid, liquid, and gaseous reacting phases. This paper describes the chemical evolution of the solid residues from natural (in situ) and artificial maturations, as followed by infrared spectrometry. The goal is to confirm the good agreement between the evolutions of the natural series of reference coals and a series of artif‘icially derived coals from confiied-systemexperiments, which was observed by preliminary work using elemental analysis and Rock-Eval analysis1and vitrinite reflectance.2 The expected originality of the work is, in addition to the use of a confined-pyrolysis system, to closely compare the results of the artificial maturation from an immature sample with those of a natural series from the same basin. (1) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985,8, 275-292. (2) Pradier, B.; Monthioux, M.; Bertrand, P.; Landais, P. C. R. Acad. Sci., Ser. 2 1986, 303, 171-176. (3) Landais, P.; Dereppe, J. M.; Monthioux, M. C. R . Acad. Sei., Ser. 2 1988,306,1093-1097. (4) Monthioux, M.; Landais, P.; Durand, B. Org. Geochem. 1986,10, 299-311. (5) Monthioux, M.; Landais, P., submitted for publication in Chem. Geol. (6) Monthioux, M., submitted for publication in Chem. Geol.
0887-0624/88/2502-0794$01.50/0
Experimental Section The following is a brief description of the experimental and analytical methods. A more detailed discussion is given in previous papers.’Jr8 Selection of Samples. All the samples are non-acid-treated borehole cuttings of raw coals or lignites from the Mahakam delta (Kalimantan,Indonesia). On the basis of numerous geochemical studies,“* the type I11 organic matter from this basin is assumed to derive from the same type of vegetation (equatorial forest) since the Mio-Pliocene. The most highly evolved coals in the sample set reach a maturity level corresponding to about 1% Ro (vitrinite reflectance). The starting material of the pyrolysis experiments is a lignite taken at the top of the series (sample no. 32362). ExperimentalProcedure. Heat treatments were performed on powdered samples that were sealed in thin-walled (0.4 mm) gold tubes and then placed in a steel autoclave. For some experiments, 50 WLof distilled and deoxygenated water was added. The gold tube is then placed under a hydrostatic pressure of 500 or 1000 bar (3000 bar for 550 “C experiments). Inside the gold tube, the dead volume is minimal and the confinement is high. This method is called “confined-system pyrolysis”. In order to (7) Monthioux, M.; Huc, A. Y.; Durand, B. Presented at the 188th National Meeting of the American Chemical Society, Philadelphia, PA, 1984. (8) Monthioux, M. These ,Doctorat d‘Etat, UniversitB d’OrlBans, France, 1986. (9) Muller, J. Ancient Pacific Floras: The Pollen Story; Cranwell, L. M., Ed.; Hawaii University Press: Honolulu, HI, 1964; pp 33-42. (10) Muller, J. The Quaternury Era in Malaysia; Ashton, P., Ashton, M., Eds.; Geological Department Series 13; Hull University: England 1972; pp 6-16. (11) Anderson, J. A. R.; Muller, J. Rev. Paleontol. Palynol. 1975,19, 291-351. (12) Morley, R. J. Proc. Annu. Conu.-Indones. Pet. Assoc. 1977,6th, 255-276. (13) Combaz, A.; de Matharel, M. AAPG Bull. 1978,62, 1684-1695. (14) Caratini, C. L.; Tissot, C. Notes et Mgmoires; Compagnie Francaise des PBtroles: Paris, 1979; Vol. 15, pp 152-154. (15) Durand, B.; Oudin, J. L. Proc. World Pet. Congr. 1980, IOth(2), 3-11. (16) Boudou, J. P. These Doctorat d’Etat, Universite d’orleans, France, 1981. (17) Schoell, M.; Teschner, M.; Wehner, H.; Durand, B.; Oudin, J. L. Aduances in Organic Geochemistry 1981; Bjoroy, M., et al., Eds.; Wiley: Chicheater, England, 1983; pp 156-163. (18) Hoffmann, C. F.; Mackenzie, A. S.; Lewis, C. A.; Maxwell, J. R.; Oudin, J. L.; Durand, B.; Vandenbroucke, M. Chem. Geol. 1984,42,1-23.
0 1988 American Chemical Society
Natural and Artificial Maturations of Coals
Energy & Fuels, Vol. 2, No. 6, 1988 795
COC = 81,8
given in arbitrary units and calculated as K = S X 100/(C + H + 0 + iV) where S is the integrated area of the band and C, H, 0, and N are the percentages of organic elements in the sample (C,H, 0, and N are given by elemental analysis). Uncertainties on the absorption coefficient values were not estimated. The corrected organic carbon content COC = % C X 100 (% C + % H + % 0 + % N) of the solid residues, which increases continuously with increasing maturation, was chosen as the common maturity index for both artificial and natural evolution series. The accuracy of COC values is estimated to be *0.6%. The validity of such a maturity parameter for the Mahakam delta was verified by Boudou,lBwho found the COC value for the diagenesis/catagenesislimit to be about 80%.
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compare the results to those using low-confinement pyrolysis methods, other heat-treatmentswere performed in an open glass tube where the powdered sample is continuously swept by an inert gas. This method is called “open-systempyrolysis” and is commonly used.for simulation purposes. Standard conditions of heat treatment were 25-30 OC/min temperature programming and then 24 h isothermal at the final temperature. For open-system pyrolysis, nonisothermal heating was also used. Temperatures ranged from 250 to 550 O C . Sample sizes were about 150 mg. Natural reference coals or solid residues from artificial maturation were chloroform extracted according to the standard conditions described by Monin et al.’@ All the extracted samples, except those from nonisothermal open-system pyrolysis, were submitted to hydrochloric acid attack (HC11N, 1h, 50 OC, with magnetic stirrer). Often, samples were examined by infrared spectrometry before and after this acid attack. Samples were powdered again together with KBr (1mg sample for 300 mg of KBr), in order to make a KBr pellet with a 200 kg/cm2 load pressure under vacuum for 5 min. Pellets were previously dried in a desiccator for 1 week before being submitted to infrared spectroscopy analysis; 1week is assumed to be a sufficient time for eliminating the adsorbed water.m The infrared spectrometer (FTIR) was a DIGILAB 15 FTE instrument. The infrared bands examined were attributed according to Smith,22and VogeI.= Figure 1illustrates the infrared bands we studied and the way they were integrated. Essentially bands numbered 1,2, 3,4, and 9,for which the attributions are unquestionable,are discussed. The manner of integration (for example for the VOH band) does not correspond always to what was chosen by other authors. However, we considered that the purpose was not to perform an absolute titration of the chemical functions but only a semiquantitative analysis by comparing,for an equivalent manner of integration, solid residues from artificial maturation with natural coals. On the other hand, infrared bands in coals are generally composite, which makes absolute quantitative studies illusive if complex mathematical processes (deconvolutions) are not employed. Absorption coefficients K are (19) Monin, J. C.;Pelet, R.; FBvrier, A. Rev. Znst. Fr. Pet. 1978, 33, 233-240. (20) Robin, P.L.; Rouxhet, P. G. Fuel 1976,55, 177-183. (21) Robin, P. L. T h h e Doctorat d‘Etat, UniversiU de Louvain. Belgium, 1975. (22) Smith, A. L. Applied Infrared Spectroscopy; Wiley: Chichester, England 1979. (23) Vogel, A. Text Book of Practical Organic Chemistry; Longman: London, 1979; pp 1271-1282.
Natural in Situ Maturation. Some infrared spectra for Mahakam natural samples are given in Figure 8 in order to be easily compared with the results from artificial evolution in a confined system. All of the figures are for HC1-treated lignites and coals. Band 1 (YOH). This wide asymmetrical band is mainly due to the stretching vibration of the 0-H bond in alcohols (-3500-3200 cm-l) then carboxylic acids (-3100-2500 cm-l). It also contains a slight contribution of phenol functions. OH groups from molecular water may also absorb in this region (-3400 cm-’), if water is not eliminated by desiccator drying.20 Figure 2a shows how the reference natural envelope is determined from 17 Mahakam samples of increasing maturity. Amounts of hydroxyl groups decrease with increasing maturity, as a consequence of the increasing defunctionalization of kerogen. Band 4 (vco). This band characterizes the stretching vibration of C=O bonds in esters, ketones, aldehydes, carboxylicacids, quinones, etc. This band is not as strongly represented in Mahakam coals as in other more “classical” type I11 organic mattersz4 Nonetheless, despite ita low absorption coefficient value, the integration of the band gives accurate results. Thus, Figure 2b shows the strong decrease in carbonyl groups with increasing maturity, which occurs earlier (i.e. for lower COC values) than for hydroxyl groups (Figure 2a). Band 2 (Aromatic Y C ~ ) . Even though its intensity is low, this absorption band is only imputable to the stretching vibration of the C-H bonds in aromatics25i2eand allows the evolution of these groups to be rather well followed. Only CH2 and CH groups in alkenes can also absorb in this region, which is without consequences for the quantities of alkenes in kerogens are low. Figure 2c shows an increase in the amounts of aromatic CH groups that corresponds essentially to a relative enrichment and maybe also to a saturation by protons of aromatic sites, where various functional groups that have been eliminated were attached (hydroxyls, carboxyls, aliphatic chains). Despite its low intensity, this band is so specific that it can help to check the purity of the deformation band 9 (6cH). Indeed, this latter band seems more interesting as it is far more intense and better resolved, but it is also more likely to be disturbed by absorptions from nonaromatic origins (particularly minerals). Band 9 (Aromatic SCH). In fact, this band is made of three smaller bands (at 865, 815, 750 cm-’1, as visible in Figure 1and some spectra of Figure 8. These three bands (24) Durand, B.;Nicaise, G.; Roucach6, J.; Vandenbroucke, M.; Haggemann, H.W. Advances in Organic Geochemistry 1975; Campos, R., Goni, J., Eds.; Enadisma: Madrid, 1977; pp 601-631. (25) Kuehn,D. W.;Snyder, R. W.; Davis, A.; Painter, P. C. Fuel 1982, 61, 682-694. (26) Riesser, B.; Starsinic, M.; Squires, E.; Davis, A.; Painter, P. C. Fuel 1984,63, 1253-1261.
Monthioux and Landais
796 Energy & Fuels, Vol. 2, No. 6, 1988
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Energy & Fuels, Vol. 2, No. 6, 1988 797
Natural and Artificial Maturations of Coals
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of CHSand CH2 groups (for long alkyl chains) are superimposed, for which the extent of the contribution remains undetermined. So, resolving well this band appears to be rather difficult and complex, and we chose to integrate it as a single entity. Moreover, the correlation (Figure 3) between U C H , (band 2) and 6CH, (band 9) is not so bad. This shows on one hand the participation of alkyl groups to be minor and on the other hand the residual mineral absorptions to be not very disturbing. Therefore, the evolution of band 9 with increasing maturity (Figure 2d) is similar to the evolution of band 2 (Figure 2c). Band 3 (Aliphatic YCH). This double band is only imputable to the aliphatic CH. The peak at =2920 cm-' correspondsto the asymmetric stretching vibration of C-H bonds in aliphatic CH2, while the peak at =2860 cm-l corresponds to the symmetric stretching vibration of C-H bonds in aliphatic CH2 and CH,. The shoulder at about 2960 cm-* on the left side of the doublet is due to the asymmetric stretching vibration of C-H bonds in CH3 groups. In fact, a deconvolution studyn determined that, as for aromatic 6CH, these peaks are resulting from several subsidiary peaks, as a function of the various environments in which these groups are placed. Figure 2e shows a strong enrichment in aliphatic groups from a COC value of -72%. Already, it appears surprising to not observe any decrease of this band for the most evolved coals of the reference series, while these samples are in the middle of catagenesis, i.e. the maximum oil generation. One explanation could be that the relative enrichment due to the loss of nona(27) Wang,
S.H.;Griffithe, P.R.Fuel
liphatic groups is able to compensate for the aliphatic CH consumption. Further comments and another explanation will be given in the discussion section. Finally, note the good linear correlation (Figure 4) between band 3 and band 6 (the latter characterizes the deformation vibration of aliphatic C-H, noted as aC%), showing that there are no disturbing minerals remaining after the hydrochloric acid attack (indeed, carbonates for instance may strongly interfer with band 6, at 1400 cm-', and band 9, at 880 cm-'). Anyway, no natural samples presented a contribution of calcite, before HC1 treatment. On the contrary, silicates (clays for instance) resist the hydrochloric acid attack and are sometimes still present in samples. They give band 7 at 1110 and 1030 cm-ls and band 10 at 470 and 540 cm-'. These bands vary in a parallel manner and do not disturb the organic bands studied, as shown by the comparison, in Figure 8, of sample no. 37246, which is a quasi-pure organic matter (no bands 7 and lo), and sample no. 32323, which contains a few percent of silicated minerals (bands 7 and 10 are intense). Artificial Maturation in Open-System Pyrolysis. Infrared spectra of solid residues from open pyrolysis are given in Figure 5, either with no isothermal conditions and no HC1 treatment (on the left), or with a 24 h isothermal stage and HC1 treatment (on the right). The variations of the integrated area of the different absorption bands are given in Figure 6a-d. The artificial decrease in C=O functions (Figure 6b) rather well follows the natural behavior, while the artificial decrease in 0-H groups (Figure 6a) is too fast. The integration of band 9 (Figure 6c) could indicate that the variation of aromatic C-H amounts, as seen by the variation of the 6 c L absorption band, rather well follows the natural behavior. However, band 2 ( v c ~ ) is not detected on any spectrum of solid residues from open-system pyrolysis, except for the highest heat-treatment temperature for both series (respectively 400 OC for the isothermal conditions series and 550 "C for the nonisothermal condition series). Therefore, the correlation ~ C H , f VCH for open-system experiments cannot be reported inpigure 3. This shows that band 9 essentially comes from another origin for these artificially derived samples (maybe due to residual minerals), which then are much more depleted in aromatic CH groups than could be deduced from the mere examination of Figure 6c. The lack of aliphatic CH groups (band 3) also appears quickly. Figure 6d shows that, from a COC value of about 70%, the loss of aliphatic functions starts and then increases, by following an artificial trend that is strongly different from the natural one. The bad correlation bC&/vCH.hph (Figure 4) for open-system pyrolysis under nonisothermal conditions (black circles, non HCL treated) clearly shows that band 6 (6cH&h) is enhanced by inorganic absorptions, for heat-treatment temperatures higher than 400 OC. The abnormally high integrated values for bands 6 (6~1z"lf) and 9 (acH,) suggests that these disturbances may be ue to calcite CaCO3 This calcite is probably formed from the calcium carboxylates initially contained in the starting sample, no. 32362 (see below). HCl-treated artificial samples (residues from isothermal open-system pyrolysis, black and white circles) give the best ~ c H . , ,f~V~C H , , , ~correlation. A widespread statement is that, at equivalent COC values, the solid residues from 24-h isothermal experiments defunctionalize more quickly than those from nonisothermal experiments. This has been already observed by elemental analysis or Rock-Eva1 analysis.' Even though comparisons are not easy, these results are compatible with those obtained with similar experiments by others
1986,64, 229-236.
798 Energy & Fuels, Vol. 2,No. 6, 1988
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Artificial Maturation in Confined-SystemPyrolysis. Before HC1 treatment, all of the spectra of the solid residues from confined-system pyrolysis showed a contribution from calcite (Figure 7). Probably, this is due to the calcium initially contained in the starting sample, no. 32362 (and certainly in the other coals also but to various extents),in the cation form Ca2+associated to carboxylate groups COO-, which appears as CaC03 under the effect of heat treatment. These carboxylate groups would be in sufficientlylow quantities for their characteristic absorption bands (at 1575-1589 and 1395-1400 cm-l) not to be detected on infrared spectra. The absorption coefficients of calcite are so high when compared to those of organic matter that a few percent of calcite content is enough to make it visible in infrared spectra. Figure 8 shows the infrared spectra of solid residues after HCl treatment was performed in order to remove calcite and to protonate the possibly remaining carboxylate groups. Infrared spectra from artificial maturation and from natural maturation evolve following a rather similar behavior. Anyway, the (28)Robin, P.L.;Rouxhet, P. G. Geochim.Cosmochim.Acta 1978,42, 1341-1349. (29)Robin, P.L.;Rouxhet, P. G.; Durand, B. Advances in Organic Geochemistry 1975; Camp,R.; Goni, J., Eds.; Enadimsa: Madrid, 1977; pp 693-716. (30)Rouxhet, P.G.;Robin, P. L. Fuel 1978,57,533-540.
agreement is better than with the infrared spectra from artificial maturation in open-system pyrolysis (Figure 5). However, two main discrepancies can be observed: The former is the behavior of the aliphatic C-H band, which decreases faster with artificial maturation than with natural maturation. The latter is the importance of the background, which absorbs much more in the artificial series and more as the wavenumber is higher. The phenomenon is commonly encountered both in artificial and natural series. It is expected to be due to a diffusion effect (so-called Christiansen effect31) that would increase in proportion as the wavelength value of the incident radiation approaches the particle The powdering conditions being standardized for both artificial and natural samples, these differences in the intensity of the base-line drift may reveal structural differences, which cannot be determined more precisely because this phenomenon still remains unstudied in literature. Figure 9a shows that the decrease in OH content varies similarly in natural coals and in solid residues from confined-system pyrolysis with increasing maturation. How(31)van der Maas, J. H. Basic Infrared Spectroscopy; Heyden: London, 1969. (32)Rouxhet, P. G.;Robin, P. L.; Nicaise, G. Kerogen; Durand, B., Ed.; Technip: Paris, 1980;pp 163-190.
Natural and Artificial Maturations of Coals
Energy & Fuels, Vol. 2, No. 6, 1988 799
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Discussion
As most of the samples of the natural series, the starting sample (no. 32362) of the pyrolysis experiments is not a pure organic matter (ash content of 5.9%). The IR examination of the natural samples before HC1 treatment has shown that mineral impurities were not calcite but rather silicates (clays). Clays have been shown to enhance catalytic and/or retention effects during pyrolysis experiments.*% In fact, heat treatments have been previously (33) Brooks, B. T. Am. Assoc. Pet. Geol. Bull. 1948, 32, 2269-2286. (34) Spiro, B. Chem. Geol. 1980,31,27-35. (35) EspitaliB, 3.; Madec, M.; Tissot, B. AAPG Bull. 1980,64,59-66. (36) Homfield, D.B.; Douglas, A. G . Geochim. Cosmochim. Acta 1980, 44,1119-1131. (37) HeGnyi, M. Acta Mineral.-Petrogr. 1983,26,73-85. (38) Evans, R. 3.; Felbeck, Jr., G. T. Org. Geochem. 1983,4,145-152.
800 Energy & Fuels, Vol. 2, No. 6,1988
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Figure 9. Variations of infrared absorption bands VOH (a), vco (b), ~CH, (c), and VCHin confined-system pyrolysis. performed on the one hand on a more mineral-rich sample from the Mahakam delta (ash content of 17%) and on the other hand on sample 32362 mixed with illite (CoalJillite = 10/90,and 2/98).8 These experiments have shown that 5.9% of clay content in a coal is not sufficient to disturbing the chemical reactions of organic matter during pyrolysis experiments, at least with regard to the accuracy of the analytical methods used. Obviously, the artificial maturation obtained with confmed-system experiments is in much better agreement with natural data than artificial maturation obtained with open-system pyrolysis. The mere fact that 24 h of isothermal open-pyrolysisgives worse results than nonisothermal open pyrolysis suggests that keeping the effluents in the reactional system is essential in maturation processes. Results above show that artificial residues from open-system pyrolysis are depleted in aliphatic C-H bonds, which indicates that alkyl compounds are removed too early from the kerogen. In addition, these solid residues are also strongly depleted in aromatic C-H bonds and slightly in O-H bonds. This may reveal that aromatic C = C bonds, and bridges such as C-C bonds or C-0-C bonds are enhanced in these solid residues when compared to natural coals. Unfortunately, such bonds are not clearly
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(d) in solid residues from artificial maturation
detectable on infrared spectra, for C-C or C-0-c bridges generally absorb within band 8, which is a complex and unresolved infrared band. In addition, their absorption coefficients decrease more as the bridged groups are heavy aromatic molecules. Furthermore, the absorption coefficients of the stretching vibration of aromatic C=C bonds (which absorb around 1600 and 1580 cm-l) are rather low and lower as the number of condensed aromatic-rings increases and as the polarity of the substituting groups decreases. Thus, open-system pyrolysis would enhance aromatization reactions. This was already observed with the chloroform extracts: and relevant mechanisms were proposed elsewhere.39 Moreover, the development of C-C and C-O-C bridges, for which the activation energies are high, should be expressed by a greater thermal stability of the solid residues from open pyrolysis when compared to natural samples of equivalent COC values. This may be estimated from the T ,, values from the Rock-Eval analysis. Indeed, T ,, is commonly used as a maturity indexu but more generally reveals the thermostability of (39) Monthioux, M. Fuel 1988,67, 843-847. (40)EspitaliB, J.; Madec, M.; Tiesot, B.; Mennig, J. J.; Leplat, P. hoc.-Annu. Offshore Technol. Conf. 1977,9th, 439-444.
Energy & Fuels, Vol. 2, No. 6, 1988 801
Natural and Artificial Maturations of Coals ,r
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INFRARED SPECTRA OF RESIDUES FROM CONFINED PYROLYSIS (GOLD TUBES ISOTHERMAL STAGE 24hl
WAVENUMBER (cm-')
Figure 10. Comparison of the infrared spectra for evolutions of solid residues from confined-system pyrolysis under various conditions (24-h isothermal stage).
organic samples.41 Actually, 2"' values are found to increase more quickly for artificially derived residues from open-system pyrolysis than for the coals from the reference series.8 On the contrary, the agreement between the evolutions of T ,, in artificial (conFined) and natural systems is satisfactory.8 The single strong discrepancy between the results from artificial maturation in confined-system pyrolysis and from the natural maturation lies in the evolution of the uC-H aliphatic infrared absorption band. However, we have noticed in the Resultshow surprising the fact was that the higher concentration of aliphatic groups in natural coals occurred for COC = 84% (Figure 2e). This COC value corresponds to the middle of catagenesis, which is the main zone of oil generation, i.e. the zone of intense loss of aliphatic groups. The best explanation, we believe, is that the slow defunctionalization induced by the natural ma(41) Landais,p.; Monthioux, M.; Meunier, J. D. Org. Geochem. 1984, 249-260.
turation (which uses low temperatures and long times) gradually transforms the organic network into a condensed phase.39 Thus, natural coals would become able to retain nonnegligible amounts of free hydrocarbons. Actually, if most of the hydrocarbons released by the natural thermal evolution are retained inside the organic solid macromolecule as a phase unaccessible by solvents, the C-H bonds of these free but trapped hydrocarbons will be analyzed as C-H bonds from the solid macromolecule. Therefore, the aliphatic C-H infrared band (band 3) will decrease only tardily, when the organic network will be dislocated under the effect of the widespread thermal breakdown of C-C bonds, C-0 bonds etc., allowing the free hydrocarbons to escape or to be accessible to solvents. This point of view is supported by other analytical data.42 In reality, the maximum of the aliphatic UC-H band in the organic matrix should occur just before the phase of hydrocarbon release, i.e. for COC = 80% (end of diagenesis). As a matter of fact, this is the COC value observed for the beginning of the decrease in the aliphatic Y G H band obtained with confined-system pyrolysis (Figure 9d). In addition to other features proving the solid residues from artificial maturation are not able to trap free hydrocarbons,3e this indicates that all the aliphatic C-H groups detected by infrared spectrometry in residues from confined experiments truly appartain to the solid organic macromolecule. Thus, the COC/uCHCph relationship obtained from confined-system pyrolysis (Figure 9d) is assumed to be much more representative of what the natural behavior really is. Contrarily, the part of the natural COC/ucH relationship (Figure 2e) located between COC = 80% a$ COC = 84% is assumed to be widely due to the free but trapped hydrocarbons.
Conclusion This infrared spectrometry study demonstrates that confined-systempyrolysis is able to satisfactorily simulate the natural evolution of solid organic matrix from a chemical standpoint. The single disagreement is rather structural and lies in the ability of natural evolution to transform the organic network into a condensed phase while artificial maturation does not. Consequences are that natural coals retain free but unextractable hydrocarbons (with standard conditions), which the infrared absorptions superimpose on those of the chemical groups of the solid macromolecule. In confined-system pyrolysis, adding or doubling the external pressure value did not produce significant effects. In contrast, using a pyrolysis method where the confinement conditions are low (as open-system pyrolysis) badly simulates the natural behavior and seems to enhance aromatization reactions. Acknowledgment. We wish to thank the Institut FranGais du PBtrole, which provided the natural samples and the analytical techniques. (42) Monthioux, M.; Landais, P. Fuel 1987, 66, 1703-1708.
