- Email: [email protected]
Pyrolysis and characterization of the kerogen from the Moroccan Youssoufia rock phosphate
Chemical Geology 186 (2002) 17 – 30 www.elsevier.com/locate/chemgeo
Pyrolysis and characterization of the kerogen from the Moroccan Youssoufia rock phosphate M. Khaddor a, M. Ziyad a,*, J. Joffre b, A. Amble`s b a
Faculte´ des Sciences, De´partement de Chimie, Laboratoire de Physico-Chimie des Mate´riaux et Catalyse, Avenue Ibn Batouta, B.P. 1014, Rabat, Morocco b Laboratoire de Chimie XII, Universite´ de Poitiers, 40 Avenue du Recteur-Pineau, 86022 Poitiers Cedex, France Received 15 March 2000; accepted 19 December 2001
Abstract Kerogen represents the main form of the trapped organic matter (85 wt.%) in the Youssoufia rock phosphate. It is highly aliphatic in nature and contains a substantial amount of oxygen. Its direct pyrolysis leads to a small amount of pyrolysate (0.4 wt.%) and large quantities of gases. Carbon-dioxide evolution proceeded in two waves centered on 300 and 420 jC, attributed to the cleavage of carboxylic and ester functions. Hydrogen production gives rise to a broad peak covering a wide temperature range (400 to 800 jC). The methane evolves in a single peak extending from 300 to 650 jC. It results from the conventional combination of Hb and (CH3)b radicals. The evolution profiles of all the gases are similar to that observed when retorting directly the rock phosphate. Kerogen structure was also investigated by spectroscopic techniques and preparative pyrolysis in presence of tetramethyl ammonium hydroxide (thermochemolysis). The n-alkene/n-alkane doublets indicate the occurrence of crosslinked aliphatic chains, partly originating from preserved, resistant biopolyesters. Ester and probably ether groups are involved in the cross-linking of the matrix. Esterified C14 – C28 fatty acids, cholestanol and 24-ethyl cholesterol are monosubstituents of the kerogen matrix. C29 – C32 a,h hopanes indicate that the kerogen is at the diagenesis evolution stage. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Kerogen; Pyrolysis; Thermochemolysis; Youssoufia phosphate
1. Introduction The natural phosphate of Youssoufia (Morocco) has a pronounced gray color caused by the organic matter trapped in its framework during the burying period that took place around the Montian and the Maastrichtian eras. Propitious physical conditions *
Corresponding author. Tel./fax: +212-37-77-54-40. E-mail address: ziyad@ fsr.ac.ma (M. Ziyad).
have converted most of this organic matter into bitumen and kerogen (Durand and Nicaise, 1980). The total amount of this organic matter in the Youssoufia rock phosphate does not exceed 3 to 4 wt.%. The extractable fractions of it (essentially the lipids) have been previously characterized (Khaddor et al., 1997). The kerogen constitutes the other organic matter fraction that is not extractable by the usual solvents (Tissot et al., 1994; Ensminger et al., 1977).
0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 ( 0 2 ) 0 0 0 0 2 - 5
18
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
The Youssoufia phosphate is mainly composed by carbonated fluorapatite that resulted from deposition of biological systems rich in phosphorus. It also contains several other minor elements, such as Si, V, Mn, Cd, Cr, Fe, that substituted the calcium or the (PO4) groups in the structure (Elliot, 1994). The threedimensional framework of this phosphate (apatite) and the acid-base properties of its surface probably favored the catalytic conversion of the trapped organic compounds. As a matter of fact, the phosphates with similar structures are known, especially those containing transition metals, to catalyze a wide range of organic reactions (Benarafa et al., 2000). Moreover, the apatites framework is crossed by tunnels that have ˚ , which might host small and diameters of around 3 A linear molecules. In the ‘‘wet process’’ that is in use for phosphoricacid manufacturing, the organic matter generates volatile compounds that pollute the environment and foams that complicate the filtration step of the end product. In order to minimize all these problems, the phosphate is calcined around 700 jC before its commercialization. It might, however, be noticed that these organic compounds do not only engender inconveniences. For instance, in the retorting processes, they contribute to lower the energetic expenses. Moreover, if the phosphate is used as it is to directly enrich the soil, the organic matter is beneficial because it provides the required carbon to the biological processes. The exhaustive study of the complex reactions involved in these transformations is a difficult task. The composition and the real structure of the kerogen is indeed, despite all the investigations carried out on the subject, not yet completely established. Its determination requires the use of several complementary heavy techniques of characterization. The main goal of the present work is to increase our knowledge of the kerogen structure and the accumulation processes of the organic compounds in the phosphates. Two pyrolysis techniques were used. Direct pyrolysis in a continuous fixed-bed reactor and pyrolysis in presence of an alkylating agent—or thermochemolysis—which allow the study of gases and light organic compounds formed during the thermal degradation of the kerogen. These techniques can also provide information on the level of thermal maturation of the kerogen as well as its composition.
2. Experimental 2.1. Preparation of the kerogen concentrate The rock phosphate investigated was obtained from the Youssoufia mine (Morocco). It contains 2.0 wt.% of organic carbon. The total organic matter constitutes around 4.0 wt.% (Khaddor et al., 1997). Kerogen concentrate was extracted from the phosphate being as much as possible careful to avoid organic matter degradation. The applied procedure was derived from that used for sediments (Durand and Nicaise, 1980) and that recommended by the International Humic Substances Society (I.H.S.S.) (Amble`s et al., 1989). It is schematized in Fig. 1. The rock phosphate was first crushed then sieved to particles having an average size of 120 Am and finally dried at 80 jC. Free lipids (bitumen) were extracted in a Soxhlet with a methanol/toluene (1:2) mixture. The mineral matrix, which is mainly constituted by carbonates, sulfates, silicates and hydroxides, was carefully treated with portions of a HCl solution at pH 2. The
Fig. 1. Preparation of the kerogen concentrate.
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
dried residue (obtained after centrifugation) was treated with an M/30 HCl solution (pH adjustment), and centrifugation. Bound lipids were then extracted with chloroform in a Soxhlet (Khaddor et al., 1997). Humic acids were removed from the residue with a 0.1 M NaOH solution. Prior to HCl/HF treatment, apatitic calcium was eliminated with 12 M HCl solution to prevent the formation of calcium fluoride insoluble in acidic medium. Silicates were then eliminated by dissolution in HCl/HF (1/1) acid mixture at 70 jC for 24 h. Temperatures superior to 70 jC must be avoided as organic compounds can be oxidized (Amble`s et al., 1989). The residue was thoroughly washed with boric-acid solutions (elimination of neoformed fluorides) and ammonium carbonate, then dried prior to Soxhlet extraction to remove remaining bound lipids, if any. The final residue recovered at the end of all these operations corresponded to the kerogen concentrate. Its X-ray diffraction patterns showed that its structure is amorphous. 2.2. Pyrolysis and characterization techniques Direct pyrolysis of the kerogen concentrate was carried out in a dynamic fixed-bed reactor operated at atmospheric pressure. The sample of the concentrate was maintained in the reactor between two quartz wool plugs and continuously flushed by purified argon at a constant flow rate equal to 40 cm3 min 1 The reactor temperature was monitored by a linear programmer (5 jC min 1) and measured by a thermocouple located within the sample. The evolved gases were analyzed by chromatography (Ziyad et al., 1986, 1993). The pyrolysis used for the structural analysis was a thermochemolysis technique performed using a preparative offline pyrolysis device (Be´har and Pelet, 1985; Largeau et al., 1986). Prior to pyrolysis, the kerogen (20 mg) was moistened overnight with a methanol solution of tetramethylammonium hydroxide (TMAH), which is commonly used as an alkylating agent for thermally assisted hydrolysis and alkylation (THM) (del Rio et al., 1996; Grasset and Amble`s, 1998). The pyrolysate was trapped at low temperature (about 30 jC) in a liquid nitrogen– acetone mixture and then separated by liquid chromatography and thin layer chromatography (Grasset and Amble`s, 1998).
19
2.3. Analysis The products were analyzed by gas chromatography (GC) and gas chromatography coupled to mass spectrometry (GC-MS). The GC separations were carried out on a Packard 438 gas chromatograph using a CPSIL 5CB or CP SIL 88 CB (Chrompack) capillary column (25 m 0.25 mm i.d., 0.12 Am film thickness). The temperature of the column was programmed from 60 to 300 jC (or 260 jC in the case of CP-SIL 88 CB) at 3 jC min 1. The injector and the detector were maintained at 300 jC. GC-MS analyses were performed on a FINNIGAN MAT-INCOS 500 mass spectrometer coupled with a VARIAN 3400 gas chromatograph. The GC conditions were the same as for GC analysis. The mass spectrometer was operated in the electron impact mode (70 eV). Transfer line and ion source were set at 290 and 180 jC, respectively. The mass spectra were collected by scanning the region m/z 50– 600 at a rate of 1 s/decade by a Data general 20 computer. The various products were identified on the basis of their GC retention times and their mass spectra (comparison with standards and literature data). Fourier Transform Infrared (FT-IR) spectra of the kerogen sample were recorded in the range 4000 –400 cm 1 on a NICOLET 510 spectrometer using KBr disks containing around 2 wt.% of kerogen. Prior to spectra recording, the disks were evacuated at 150 jC until the complete elimination of water. Solid state 13C NMR spectra of the kerogen were obtained with a BRUKER spectrometer operating at 4 kHz and equipped with cross-polarization and magic angle spinning (CPMAS). The chemical shifts were calibrated with respect to tetramethylsilane (TMS).
3. Results The extracted kerogen concentrate is made of 83.9 wt.% of pure organic matter, determined by combustion at 800 jC (after pyrite correction). Thus, the original rock phosphate sample contains 34 mg of kerogen per gram of phosphate. 3.1. Elemental analysis The elemental kerogen concentrate analysis is reported in Table 1. The values of H/C (ca. 1.21)
20
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
Table 1 Elemental analysis of the kerogen concentrate (wt.%) C
H
N
O
S
Ash
53.62
5.39
2.44
11.52
10.91
16.12
and O/C (ca. 0.16) ratios indicate that the sample is mostly aliphatic and might be classified in the van Krevelen diagram within the type II kerogens (Tissot and Welte, 1978; Durand and Monin, 1980).
155 ppm (aromatic C bounded to O atoms) (Evans et al., 1971; Amble`s et al., 1994) suggests that the kerogen concentrate probably contains very few phenols or aryl ether groups. The peak appearing between 160 and 200 ppm can be ascribed to CjO bond in ester, carboxylic or amide groups. NMR spectrum also shows clearly that the kerogen structure is mainly made of aliphatic compounds in addition to a significant contribution of aromatic moieties. The carbon aromaticity (fraction of organic carbon in aromatic rings) was found to be fa = 0.29.
3.2. Spectroscopic characterization 3.3. Direct pyrolysis of kerogen The FT-IR spectrum (Fig. 2a) of the kerogen concentrate is characterized by a number of typical absorption bands similar to that found with other oil shale kerogens (Amble`s et al., 1994). The attribution of these bands is not easy because of the complexity of the structures involved and the interference between the individual absorptions. However, the group of bands located between 3000 and 2800 cm 1 and at 1450, 1449, 1377 cm 1 indicate the presence in the sample of methyl and methylene groups. It is difficult here to characterize the aromatic groups. The C – H vibrations of aromatics (and/or of alkenes) ( > 3000 cm 1), if present, are overlapped by the strong O – H vibration at 3000 –3600 cm 1. The bands appearing in the domain 900 – 650 cm 1 can be due to the deformation mode of the aromatic C – H bonds. The absorption band centered on 1706 cm 1 can be assigned to the CjO function of carboxyl groups and/or ketones. The band at 1634 cm 1 may originate from CjC vibration in olefins and possibly in aromatic rings. The broad bands between 1100 and 1300 cm 1 and at 1040 cm 1 can be attributed to C – O bonds present in acids, esters and ethers. The Csp3 – O vibration (in ether, ester, acid groups) normally appears in 1050 – 1150 cm 1 range, while the Csp2 –O band (found in esters, phenols) is observed at higher wavenumbers (1200 –1300 cm 1). The solid state 13C NMR spectrum of the kerogen concentrate is displayed in Fig. 2b. It exhibits, as in coals (Amble`s et al., 1994), two important peaks at 30 ppm and between 100 and 160 ppm, for the n-alkyl chains and for the aromatic carbon atoms, respectively. Different types of aromatic carbons (with protons or alkyl-substituted) can contribute to the second peak. The absence of a well-resolved resonance centered on
The pyrolysis of sediments or kerogens is known to produce a large variety of low molecular weight
Fig. 2. Spectroscopic characterization of the kerogen concentrate from Youssoufia phosphate rocks. (a) FT-IR spectrum (KBr pellets); (b) CP-MAS solid state 13C NMR spectrum.
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
21
Fig. 3. Production of gases (1) and pyrolysate (2) in the pyrolysis of the Youssoufia kerogen.
compounds and complex pyrolysates (Saiz-Jimenez and de Leeuw, 1984, 1986; Larter and Horsfield, 1993; Grasset and Amble`s, 1998). Fig. 3 reports the results obtained with the Youssoufia kerogen concentrate. It shows the quantities of gas and the pyrolysate evolved versus the temperature under a neutral atmosphere (Ar). Between 250 and 600 jC, the weight loss contains 37.2% of gases and 17.9% of the pyrolysate. The amount of gases recovered is superior to that of pyrolysate probably because the kinetics of gases production by the splitting of the organic matter is faster than the diffusion of the heavy compounds out of the reactor. However, since both curves have the same profile (Fig. 3), one might expect the simultaneous production of gases and oil to occur as shown by the following global reaction:
composition and degradation during the phosphate retorting. Moreover, its formation in large amounts may constitute, if not properly evacuated, a danger for the industrial installations. Fig. 4a shows that its evolution from the kerogen starts at 400 jC and continues beyond 850 jC. The maximum of the release rate is achieved around 680 jC. It results essentially from the recombination of hydrogen radicals and represents 7.22% of all the gases produced.
kerogen ! pyrolysate þ gas, where the gases are the noncondensable vapors escaping from the kerogen and the pyrolysate, the condensable hydrocarbons and complex organic compounds formed during the heating period. 3.3.1. Hydrogen evolution Hydrogen plays an essential role in many industrial processes including energy production. The study of its evolution may provide information on the kerogen
Fig. 4. Gases evolution during the direct pyrolysis of the kerogen concentrate: (a) H2, (b) CH4 (c) CO2. The heating rate is 5 jC/min.
22
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
The very low amount of the mineral matter encapsulated in the kerogen concentrate assigns to the pyrolysis curve the symmetrical profile it shows. Hydrogen evolution is not affected by secondary reactions, such as water – gas shift reaction, since no production of CO was detected during the pyrolysis. 3.3.2. Methane evolution Fig. 4b reports the generation rate of methane versus temperature. The evolution profile is similar to that observed during the rock phosphate retorting (Ziyad et al., 1993). Methane appears in the gases between 300 and 650 jC. It probably results from the hydrogenation of CH3 radicals produced by the fragmentation of long chain organic molecules. The maximum rate of CH4 production was reached at 480 jC and was found to be equal to 4.10 5 mol min 1 g 1. 3.3.3. Carbon-dioxide evolution The release rate of carbon dioxide in neutral atmosphere versus temperature is represented in Fig. 4c. The evolution proceeds apparently in two waves of unequal importance that extend from 150 to around 600 jC. The maxima of the peaks are quite distant from each other and are centered on 300 and 450 jC. This suggest that the CO2 originates at least from two different sources that might be for the first peak the splitting of the carboxylic acids in the following manner: R COOH ! R H þ CO2 :
ð1Þ
The carboxylic-acid functions are, as it will be shown latter, abundant in the kerogen concentrate. The second peak that appears above 350 jC can be attributed to the cleavage of ester groups: R COO RV ! Rb þ RVb þ CO2 :
ð2Þ
If the neutral carrier gas (Ar) is replaced by air, the kerogen decomposition changes considerably. It is almost totally oxidized below 600 jC into CO2 (Fig. 5). The heating rate in these conditions has no noticeable influence on the temperature of the maximum, albeit it increases significantly the amplitude of the peak. The quantity of CO2 evolved during the pyrolysis under air is equal to the total amount of the oxidized organic matter. The CO2 recovered probably
Fig. 5. Production rate of CO2 on pyrolysis of Youssoufia kerogen under air at the heating rates 5 and 9 jC min 1.
originates from different competing reactions that might be globally represented by: Cn H2m þ ðn þ m=2ÞO2 ! nCO2 þ mH2 O:
ð3Þ
The kerogen is an assemblage of organic structures that are much more complex than suggested by Eq. (3) and obviously a contribution of the reactions (1) and (2) cannot be excluded from CO2 production. The absence of CO from the evolved gases supports the assumption that reactions, such as Eq. (4), do not occur probably because there is no residual carbon at all. Cresidue þ CO2 ! 2CO:
ð4Þ
3.4. Thermochemolysis of the kerogen Pyrolysates from kerogens generally contain a very complex mixtures of organic compounds. The use of methods, such as pyrolysis (or thermochemolysis) and GC/MS for the analysis of the released products, might provide valuable information for the rebuilding of the original structure of the kerogen and the phosphate genesis. Fig. 6 reports the chromatogram of the apolar fraction from the pyrolysate. It contains series of fatty-acid methyl esters accompanied by octyl esters and C16 fatty acid (palmitic acid), series of n-alkanes and of linear and branched alkenes. Due to the complexity of the fraction, the distribution was determined using specific ion chromatograms (infra-
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
23
Fig. 6. Chromatogram of the apolar fraction obtained after thermochemolysis of Youssoufia kerogen.
vacuum). Tetracyclic and pentacyclic terpenoids, which are considered as biomarkers, were also identified. 3.4.1. Aliphatic hydrocarbons The distribution of n-alkanes is displayed on Fig. 7. The C16 – C33 distribution is clearly bimodal. The short C16 to C23 mode is the most abundant with a maximum at C22. The value of the odd-to-even carbon number ratio is 1.17 and n-alkanes represent 20.5 wt.% of the pyrolysate. The m/z 111 ion fragmentogram (Fig. 8) illustrates the distribution of n-alkenes. They were found in the C17:1 –C33:1 range. The distribution is unimodal (max. C24:1, C25:1) contrarily to that of linear alkanes. Branched C19 – C33 alkenes were also identified (Fig. 8).
3.4.2. Triterpenoid hydrocarbons The pyrolysate contains a large variety of hopanoic structures. Fig. 9 displays the mass fragmentogram of the hopanes (m/z = 191). The distribution extends from C29 to C32 and maximizes at C29. The hopanes detected are 17a (H), 21h (H) hopanes (a, h hopanes). In sedimentary organic matter, 17a (H), 21h (H) hopanes (a, h hopanes) and 17h (H),21a (H) hopanes (h, a hopanes) are known to derive from bacteriohopane tetrol (or related compounds bearing amino groups), which is present in the membranes of bacteria with the natural thermodynamically unstable 17h (H), 21h (H) configuration (h, h hopanes) (Ensminger et al., 1977). During the maturation of the sediment, the formation of the thermodynamically stable a, h and h, a hopanes takes place (Gaskell and Eglinton, 1975). The same
24
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
Fig. 9. Distribution of hopanoic acids (m/z 191 ion fragmentogram).
Fig. 7. Distribution of n-alkanes produced on thermochemolysis of the Youssoufia kerogen.
diagenetic hopanes were detected in the bitumens of the corresponding phosphate rock, but with a wider distribution (C27 to C34) (Khaddor et al., 1997). Two triterpenes C27H 44 (22,29,30 trisnorhop17(21)-ene) and C29H48 (30-norhop-17(21)-ene) were also identified in the pyrolysate, on the basis of their mass fragmentation pattern (Philp, 1985). The double bond is present in the D cycle (D-17,21) and not on the lateral chain (D-22,29). Generally, these two hop17(21)-ene compounds result from the diagenetic
transformation of hop-22(29)-ene (Grimalt et al., 1991). The pyrolysate contains also the two C27 and C29 tetracyclic triterpanes. They are associated with the 5a(H)-cholestane and 24-ethyl-5a(H)-cholestane, respectively. The 24-ethyl-5a(H)-cholestane derives from 24-ethyl cholesterol and has a vegetal origin. An interesting point is that the two steranes are accompanied by the corresponding sterenes. The mass spectrometry indicates that the double bond is present in the A ring. They can probably be identified with mixtures of D-2 and D-3 cholestenes. Steranes were numerous in the soluble fraction (bitumen) of the rock phosphate, but no sterenes were found (Khaddor et al., 1997).
Fig. 8. Distribution of alkenes formed on pyrolysis of the Youssoufia kerogen (m/z 111 ion fragmentogram): (HCx:1: n-alkene; HCx:1br: branched alkene).
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
25
Fig. 10. Distribution of fatty acids (m/z 74 ion fragmentogram).
3.4.3. Methyl esters and higher esters Linear monocarboxylic-acid methyl esters produced on pyrolysis arise from the transesterification of the acids, which were initially esterified in the kerogen matrix. Their distribution in the range C14 – C28 illustrated by the m/z 74 (McLafferty rearrangement) ion fragmentogram (Fig. 10) is dominated by the palmitic (C16) and stearic (C18) acid methyl esters and presents a clear even predominance. Surprisingly, three fatty-acid octyl esters, corresponding to C12, C14 and C16 fatty acids, were found in the pyrolysate. The mass spectrum of the dodecanoic-acid n-octyl ester is given as an example in Fig. 11. These octyl esters are characterized in mass spectrometry by ions at m/z 70, 112, M-129 ( –C8H17Ob) and M-128 atomic mass units (amu). The intensity of the molecular peak is low and the confirmation of the mass value was obtained by chemical ionization (giving [M + H] + ). The mass fragmentation pattern of the fatty-acid n-octyl esters is described in Fig. 12.
McLafferty rearrangement (through the oxygen atom) of the ester leads to the m/z 112 ion radical, which corresponds to ionized n-octene. The m/z 112 ion can eliminate 42 amu (propene) by a McLafferty rearrangement and leads to m/z 70 (n-pentene ion radical). The distribution of the fatty acids combined as n-octyl esters was determined using the characteristic m/z 70 ion fragmentogram (not shown): C12: 8.0; C13: 2.7; C14: 3.5; C15: 1.0; C16: 1.5 in arbitrary units. The origin of the octyl esters in the pyrolysis products is a challenging question. Theoretically, such ester bonds are expected to be broken on pyrolysis.
Fig. 11. Mass spectrum (EI) of dodecanoic acid n-octyl ester.
Fig. 12. Fragmentogram pattern of fatty acid n-octyl esters.
26
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
We observed for a few samples—of soil organic matter or kerogen—that octyl esters are present when using tetramethyl ammonium hydroxide, but absent in the conventional pyrolysis products. Is there a specific (unknown) reaction with esterified fatty acids that were found in the kerogen? It can be noted that the distribution of fatty acids present as octyl esters is different from that given in Fig. 10. The other possibility could be a contamination by n-octyl phthalates (common contaminants). The investigation of such a pollution gave negative results.
4. Discussion The spectroscopic study of the kerogen concentrate extracted from Youssoufia rock phosphate by FT-IR and NMR showed that it has a highly aliphatic character. It is also highly oxygenated. Chemical functions containing oxygen have been identified in its structure and compounds, such as carbon dioxide and esters, were produced by pyrolysis and thermochemolysis. These ester groups were also found in the contemporary Moroccan Timahdit (M and Y layers) and Tarfaya oil shale kerogens (Amble`s et al., 1987; Kribii, 1994; Kribii et al., 1996, 2001). Additional information on the kerogen structure was provided by the evolution sequence of gases upon the temperature increase in the direct pyrolysis. Carbon dioxide was released in the domain of the first pyrolysis. It may originate from the degradation of acids or the decarboxylation of ester groups according to the mechanism given in Fig. 13. As the temperature was increased, the carbonaceous deposits underwent cracking reactions producing methane by hydrogenation of methyl radicals. The dehydrogenation reac-
Fig. 13. Decarboxylation reaction of esters occurring during thermochemolysis.
Fig. 14. Formation of alkenes and alkanes during pyrolysis.
tions started above 400 jC and a maximum of H2 production appeared at 700 jC (Fig. 4a). The kerogen matrix begins to randomly free alkenes and alkanes. Pyrolysis generally affords hydrocarbons as n-alkane/ n-alkene doublets. The breakage of carbon-heteroatom ( –C – X –) bonds with a weak energy must be favored. The terminal unsaturation in n-alk-1-enes arises from the h scission of radicals during pyrolysis (Fig. 14) (Larter and Horsfield, 1993). n-Alkanes and n-alkenes constitute the predominant series of components produced upon kerogen thermochemolysis. Commonly, it is admitted that the origin of n-alkane/n-alkene doublets can be diverse in pyrolysis. They can arise from the scission of C –X bonds (Fig. 13) or from the thermal degradation of esters, which was reported to produce n-alkanes from the acid moiety and n-alk-1-enes from the alcohol moiety (van de Meent et al., 1980). These esters were in the protective layers of higher plants, as suberin,
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
cutin, that were preserved in the kerogen structure (de Leeuw and Largeau, 1993; Macko et al., 1993). They can originate from the aboveground parts as well as from the roots (Nierop, 1998). Usually, n-alkanes and n-alkenes exhibit similar distributions (Larter and Horsfield, 1993; de Leeuw and Largeau, 1993; Macko et al., 1993). However, this was not the case for the Youssoufia kerogen, the distributions were found to be different. The n-alkanes contrarily to that n-alkenes showed a bimodal distribution (Figs. 7 and 8) and presented analogies with the hydrocarbons distributions in the bitumens of the Youssoufia rock phosphate (Khaddor et al., 1997). These differences suggest that a portion of the n-alkanes found in the pyrolysate were present as trapped molecules in the macromolecular network and released after alteration of the matrix. Analogous phenomenon is commonly observed after chemical degradation of kerogens from ancient sediments (Baudet et al., 1991; Halim et al., 1997). In the conventional pyrolysis (without tetramethyl ammonium hydroxide), fatty acids arise from the cleavage of esters (Largeau et al., 1986; Tegelaar et al., 1989a,b) that did not undergo decarboxylation reaction, as illustrated in Fig. 13. Thermochemolysis in presence of an alkylating agent, such as tetramethyl
27
ammonium hydroxide, is recommended to prevent decarboxylation of esters (Challinor, 1989, 1991a,b). It leads only to fatty-acid methyl esters, which mainly result from the transalkylation of ester groups (Martin et al., 1995; Gonzalez-Vila et al., 1996). The strong even over odd predominance of carbon numbers for the monocarboxylic-acid methyl-ester distribution (Fig. 10) indicates that these acids could originate from plant protective polyesters (Tegelaar et al., 1998a,b). The dominant C16 and C18 members (as esters) are ubiquitous, they were found in higher plants, algae, bacteria, animals. The identification of some nonmethylated fatty acids in the TMAH/pyrolysis products could probably indicate that under our experimental conditions, the prevailing transesterification process is accompanied with ‘‘classical’’ pyrolysis cleavage. Other examples were found in the study of ancient sedimentary or soil organic matter by thermochemolysis (Amble`s, unpublished results). Various triterpenoids were found in the pyrolysate products. The identified C29 – C32 hopanes are exclusively ‘‘diagenetic’’ a,h and h,a hopanes, the natural h,h hopanes were not detected. This result is theoretically somewhat surprising for a relatively immature sediment. The same result arose from the study of ether-bound hopanoids from the contemporary Timah-
Fig. 15. Stanols bound to the kerogen matrix with ether or ester groups and yielding sterene/sterane doublets during pyrolysis.
28
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
dit oil shale (Saiz-Jimenez, 1994). The two unsaturated C27 and C2917a (21h)-hopenes were identified. They were also found in the pyrolysate of other kerogens (Seifert, 1978; Largeau et al., 1986; Larter and Horsfield, 1993). They probably originate from the diagenic transformation of hop-22(29)-enes (Ensminger, 1977), which are commonly found, for instance, in soils (Riess-Kaut and Albrecht, 1989). It is impossible to verify if they were bonded to the matrix or present in the kerogen as trapped molecules. They were not identified in the soluble fraction (Khaddor et al., 1997). The 5a(H) cholestane and the 24-ethyl 5a(H) cholestane were formed on thermochemolysis with the corresponding C27 and C29 sterenes. The presence of sterene/sterane doublets indicated the occurrence of 5a(H) cholestanol and 24-ethyl 5a(H) cholestanol chemically bound to the kerogen matrix, probably via an ether or an ester group (Fig. 15).
5. Conclusions The aim of this work was a detailed study of the structure of the kerogen present in the rock phosphate from Youssoufia. In this sediment, the kerogen represents the main form of organic matter (85 wt.%). The study was conducted using spectroscopic methods and two different pyrolysis techniques. CP-MAS 13C NMR and FT-IR spectra indicated that it has a rather aliphatic nature and is rich in oxygen of chemical functions such as esters, ethers, alcohols. Production of carbon dioxide on direct pyrolysis and esters on thermochemolysis evidenced this observation. Direct pyrolysis was used because it provides a quick and reasonable simulation of (i) the processes involved in the evolution (diagenesis, metagenesis, catagenesis) of sediments and (ii) the behavior of organic matter upon heating. In a neutral atmosphere hydrogen evolution covers a wide range of temperatures (400 – 800 jC). Methane gives rise to two peaks suggesting that it originates from at least two different sources. Carbon dioxide appears at temperatures as low as 150 jC in two waves centered at 300 and 450 jC, respectively. The identification of n-alkene/n-alkane doublets formed on thermochemolysis indicates that the kerogen matrix is formed by cross-linked aliphatic chains, partly originating from preserved biopolyesters. Cross-
linking of chains involves ester and probably ether groups. Some aliphatic moieties, which act as monosubstituents of the kerogen matrix, are reflected as aliphatic monocarboxylic-acid methyl esters upon thermochemolysis. Aliphatic compounds, such as alkanes, possibly fatty-acid octyl esters, obviously trapped in the macromolecular structure, are released when the structure of the matrix is altered, as previously observed after chemical degradation of kerogens (Baudet et al., 1991; Halim et al., 1997). The presence of C29 – C32 a,h hopanes and the absence of natural h,h hopanes indicated that the Youssoufia kerogen is at its diagenesis evolution stage. The C27 and C29 sterene/ sterane doublets are indicative of stanols probably bound to kerogen by ether and/or ester groups. They are representative of a mixed animal and higher plant input. It is also interesting to note that, as reported for other pyrolysis techniques, preparative thermochemolysis in the presence of tetramethylammonium hydroxide is a quick and powerful technique for the study of macromolecules. Heating in the presence of an alkylating agent causes additional chemolysis of the resistant aliphatic parts of the matrix, allowing, as an example, transalkylation of polyesters resistant under classical pyrolysis conditions (Gobe´ et al., 2000).
Acknowledgements The authors thank the ‘‘CERPHOS’’ (Morocco) for providing the investigated sample of phosphate. They also gratefully acknowledge CNRS (France) for the financial support provided to this study. [CA]
References Amble`s, A., Dupas, G., Jacquesy, J.C., 1987. Solubilization of Timahdit (Morocco) oil shale kerogen catalysed by 18-Crown-6. Tetrahedron Lett. 28, 6449 – 6452. Amble`s, A., Magnoux, P., Jacquesy, R., Fustec, E., 1989. Effects of addition of bentonite on the hydrocarbon fraction of a podzol soil (A1) horizon. J. Soil Sci. 40, 685 – 694. Amble`s, A., Halim, M., Jacquesy, J.C., Vitorovic, D., Ziyad, M., 1994. Characterization of kerogen from Timahdit shale (Y-layer) based on multistage alkaline permangante degradation. Fuel 73, 17 – 24. Baudet, N., Kribii, A., Halim, M., Jacquesy, J.C., Amble`s, A., 1991. Structural study of kerogens by chemical degradations.
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30 In: Øygard, K. (Ed.), Organic Geochemistry. F. Hurtigtrykk, Oslo, pp. 543 – 546. Be´har, F., Pelet, R., 1985. Pyrolysis gas chromatography applied to organic geochemistry. Structural similarities between kerogens and asphaltenes from related rock extracts and oils. J. Anal. Appl. Pyrolysis 8, 173 – 187. Benarafa, A., Kacimi, M., Coudurier, G., Ziyad, M., 2000. Characterisation of the active sites in butan-2-ol dehydrogenation over calcium – copper and calcium – sodium – copper phosphates. Appl. Catal., A 196, 23 – 35. Challinor, J.M., 1989. A pyrolysis – derivation – gas chromatography technique for the structural elucidation of some synthetic polymers. J. Anal. Appl. Pyrolysis 16, 323 – 333. Challinor, J.M., 1991a. Structure determination of alkyd resins by simultaneous pyrolysis methylation. J. Anal. Appl. Pyrolysis 18, 233 – 333. Challinor, J.M., 1991b. The scope of pyrolysis methylation reactions. J. Anal. Appl. Pyrolysis 20, 15 – 24. de Leeuw, J., Largeau, C., 1993. A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal and petroleum formation. In: Engel, M.E., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum, New York, pp. 23 – 72. del Rio, J.C., Martin, F., Gonzalez-Vila, F.J., 1996. Thermally assisted hydrolysis and alkylation as a novel pyrolytic approach for the structural characterization of natural biopolymers and geomacromolecules. Trends Anal. Chem. 15, 70 – 79, and references cited therein. Durand, B., Monin, J.C., 1980. Elemental analysis of kerogen. In: Durand, B. (Ed.), Kerogen. Technip, Paris, pp. 113 – 142. Durand, B., Nicaise, G., 1980. Procedures for kerogen isolation. In: Durand, B. (Ed.), Kerogen. Technip, Paris, pp. 35 – 53. Elliot, J.C., 1994. Structure and chemistry of the apatites and other calcium phosphates. Studies in Inorganic Chemistry, vol. 18. Elsevier, Amsterdam, pp. 1 – 62. Ensminger, A., 1977. Evolution des compose´s polycycliques se´dimentaires, PhD thesis dissertation, Universite´ Louis Pasteur, Strasbourg. Ensminger, A., Albrecht, P., Ourisson, G., Tissot, B., 1977. Evolution of polycyclic alkanes under the effect of burial (Early Toarcian Shales, Paris Basin). In: Campos, R., Goni, J. (Eds.), Advances in Organic Geochemistry, vol. 1975. Enadimsa, Madrid, pp. 45 – 52. Evans, C.R., Ragers, N.A., Bailey, N.J.L., 1971. Evolution and alteration of petroleum in western Canada. Chem. Geol. 8, 147 – 170. Gaskell, S.J., Eglinton, G., 1975. Rapid hydrogenation of sterols in contemporary lacustrine sediments. Nature 254, 209 – 211. Gobe´, V., Leme´e, L., Amble`s, A., 2000. Structure elucidation of soil macromolecular lipids by preparative pyrolysis and thermochemolysis. Org. Geochem. 31, 409 – 419. Gonzalez-Vila, F.J., del Rio, J.C., Martin, F., Verdejo, T., 1996. Pyrolytic alkylation – gas chromatography – mass spectrometry of model polymers. Further insights into the mechanism and scope of the technique. J. Chromatogr., A 750, 155 – 160. Grasset, L., Amble`s, A., 1998. Structure of humin and humic acids from an acid soil as revealed by PTC-hydrolysis. Org. Geochem. 29, 881 – 891.
29
Grimalt, J.O., Yruela, I., Saiz-Jimenez, C., Toja, J., de Leeuw, J.W., Albaige`s, J., 1991. Sedimentary lipid biogeochemistry of an hypereutrophic alkaline lagoon. Geochim. Cosmochim. Acta 55, 2555 – 2576. Halim, M., Joffre, J., Amble`s, A., 1997. Characterization and classification of kerogen from Tarfaya (South Morocco) based on its oxidation products. Chem. Geol. 141, 225 – 234. Khaddor, M., Ziyad, M., Halim, M., Joffre, J., Amble`s, A., 1997. Characterization of soluble organic matter from Youssoufia rock phosphate. Fuel 76, 1395 – 1400. Kribii, A., 1994. ‘‘Etude structurale des ke´roge`nes par des re´actions chimiques se´lectives’’, PhD thesis dissertation, University of Poitiers. Kribii, A., Baudet, N., Amble`s, A., 1996. Kerogen structural study by a combination of specific reactions. J. Serb. Chem. Soc. 61, 137 – 147. Kribii, A., Leme´e, L., Chaouch, A., Amble`s, A., 2001. Structural study of the Moroccan Timahdit (Y-layer) oil shale kerogen using chemical degradation. Fuel 80 (5), 681 – 691. Largeau, C., Derenne, S., Casadevall, E., Kadouri, A., Sellier, N., 1986. Pyrolysis of immature Torbanite and of resistant biopolymer (PRBA) isolated from extant alga Botriococcus braunii. Mechanism of formation and structure of Torbanite. Org. Geochem. 10, 1023 – 1032. Larter, S.R., Horsfield, B., 1993. Determination of structural components of kerogen by the use of analytical pyrolysis methods. In: Engel, M.E., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum, New York, pp. 271 – 287. Macko, S.A., Engel, M.H., Parke, P.L., 1993. Early diagenesis of organic matter in sediments: assessment of mechanisms and preservation by the use of isotopic molecular approaches. In: Engel, M.E., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum, New York, pp. 211 – 224. Martin, F., del Rio, J.C., Gonzalez-Vila, F.J., Verdejo, T., 1995. Thermally assisted hydrolysis and alkylation of lignins in the presence of tetra-alkylammonium hydroxides. J. Anal. Appl. Pyrolysis 35, 1 – 13. Nierop, K.G.J., 1998. Origin of aliphatic compounds in a forest soil. Org. Geochem. 29, 1009 – 1016. Philp, R.P., 1985. Fossil Fuel Biomarkers. Applications and Spectra. Elsevier, Amsterdam. Riess-Kaut, M., Albrecht, P., 1989. Hopane-derived triterpenoids in soils. Chem. Geol. 76, 143 – 151. Saiz-Jimenez, C., 1994. Analytical pyrolysis of humic substances: pitfalls, limitations and possible solutions. Environ. Sci. Technol. 28, 1773 – 1780. Saiz-Jimenez, C., de Leeuw, J.W., 1984. Pyrolysis – gas chromatography – mass spectrometry of soil polysaccharides, soil fulvic acids and polymaleic acid. Org. Geochem. 6, 287 – 293. Saiz-Jimenez, C., de Leeuw, J.W., 1986. Chemical characterization of soil organic matter fractions by analytical pyrolysis – gas chromatography – mass spectrometry. J. Anal. Appl. Pyrolysis 9, 99 – 119. Seifert, W.K., 1978. Steranes and terpanes in kerogen pyrolysis for correlation of oils and source rocks. Geochim. Cosmochim. Acta 42, 473 – 484. Tegelaar, E.W., de Leeuw, J.W., Holloway, P.J., 1989a. Some mech-
30
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
anisms of flash pyrolysis of naturally occurring higher plant polyesters. J. Anal. Appl. Pyrolysis 15, 289 – 295. Tegelaar, E.W., de Leeuw, J.W., Saiz-Jimenez, C., 1989b. Possible origin of aliphatic moieties in humic substances. Sci. Total Environ. 81/82, 1 – 17. Tissot, B.P., Welte, D.H., 1978. Petroleum Formation and Occurrence. Springer Verlag, Berlin. Tissot, B., Durand, B., Espitalie´, J., Combaz, A., 1994. Influence of nature and diagenesis of organic matter in the formation of petroleum. Bull. Am. Assoc. Pet. Geol. 58, 499 – 506.
van de Meent, D., Brown, S.C., Philp, R.P., Simoneit, B.R.T., 1980. Pyrolysis high resolution gas chromatography and pyrolysis – gas chromatography mass spectrometry of kerogen precursors. Geochim. Cosmochim. Acta 44, 999 – 1014. Ziyad, M., Garnier, J.P., Halim, M., 1986. Non-isothermal kinetics of H2S and H2 generation from Timahdit oil shale. Fuel 65, 715 – 720. Ziyad, M., Khaddor, M., Halim, M., 1993. Non-isothermal retorting of rock phosphate containing organic matter. Fuel 5, 655 – 659.
Pyrolysis and characterization of the kerogen from the Moroccan Youssoufia rock phosphate M. Khaddor a, M. Ziyad a,*, J. Joffre b, A. Amble`s b a
Faculte´ des Sciences, De´partement de Chimie, Laboratoire de Physico-Chimie des Mate´riaux et Catalyse, Avenue Ibn Batouta, B.P. 1014, Rabat, Morocco b Laboratoire de Chimie XII, Universite´ de Poitiers, 40 Avenue du Recteur-Pineau, 86022 Poitiers Cedex, France Received 15 March 2000; accepted 19 December 2001
Abstract Kerogen represents the main form of the trapped organic matter (85 wt.%) in the Youssoufia rock phosphate. It is highly aliphatic in nature and contains a substantial amount of oxygen. Its direct pyrolysis leads to a small amount of pyrolysate (0.4 wt.%) and large quantities of gases. Carbon-dioxide evolution proceeded in two waves centered on 300 and 420 jC, attributed to the cleavage of carboxylic and ester functions. Hydrogen production gives rise to a broad peak covering a wide temperature range (400 to 800 jC). The methane evolves in a single peak extending from 300 to 650 jC. It results from the conventional combination of Hb and (CH3)b radicals. The evolution profiles of all the gases are similar to that observed when retorting directly the rock phosphate. Kerogen structure was also investigated by spectroscopic techniques and preparative pyrolysis in presence of tetramethyl ammonium hydroxide (thermochemolysis). The n-alkene/n-alkane doublets indicate the occurrence of crosslinked aliphatic chains, partly originating from preserved, resistant biopolyesters. Ester and probably ether groups are involved in the cross-linking of the matrix. Esterified C14 – C28 fatty acids, cholestanol and 24-ethyl cholesterol are monosubstituents of the kerogen matrix. C29 – C32 a,h hopanes indicate that the kerogen is at the diagenesis evolution stage. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Kerogen; Pyrolysis; Thermochemolysis; Youssoufia phosphate
1. Introduction The natural phosphate of Youssoufia (Morocco) has a pronounced gray color caused by the organic matter trapped in its framework during the burying period that took place around the Montian and the Maastrichtian eras. Propitious physical conditions *
Corresponding author. Tel./fax: +212-37-77-54-40. E-mail address: ziyad@ fsr.ac.ma (M. Ziyad).
have converted most of this organic matter into bitumen and kerogen (Durand and Nicaise, 1980). The total amount of this organic matter in the Youssoufia rock phosphate does not exceed 3 to 4 wt.%. The extractable fractions of it (essentially the lipids) have been previously characterized (Khaddor et al., 1997). The kerogen constitutes the other organic matter fraction that is not extractable by the usual solvents (Tissot et al., 1994; Ensminger et al., 1977).
0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 ( 0 2 ) 0 0 0 0 2 - 5
18
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
The Youssoufia phosphate is mainly composed by carbonated fluorapatite that resulted from deposition of biological systems rich in phosphorus. It also contains several other minor elements, such as Si, V, Mn, Cd, Cr, Fe, that substituted the calcium or the (PO4) groups in the structure (Elliot, 1994). The threedimensional framework of this phosphate (apatite) and the acid-base properties of its surface probably favored the catalytic conversion of the trapped organic compounds. As a matter of fact, the phosphates with similar structures are known, especially those containing transition metals, to catalyze a wide range of organic reactions (Benarafa et al., 2000). Moreover, the apatites framework is crossed by tunnels that have ˚ , which might host small and diameters of around 3 A linear molecules. In the ‘‘wet process’’ that is in use for phosphoricacid manufacturing, the organic matter generates volatile compounds that pollute the environment and foams that complicate the filtration step of the end product. In order to minimize all these problems, the phosphate is calcined around 700 jC before its commercialization. It might, however, be noticed that these organic compounds do not only engender inconveniences. For instance, in the retorting processes, they contribute to lower the energetic expenses. Moreover, if the phosphate is used as it is to directly enrich the soil, the organic matter is beneficial because it provides the required carbon to the biological processes. The exhaustive study of the complex reactions involved in these transformations is a difficult task. The composition and the real structure of the kerogen is indeed, despite all the investigations carried out on the subject, not yet completely established. Its determination requires the use of several complementary heavy techniques of characterization. The main goal of the present work is to increase our knowledge of the kerogen structure and the accumulation processes of the organic compounds in the phosphates. Two pyrolysis techniques were used. Direct pyrolysis in a continuous fixed-bed reactor and pyrolysis in presence of an alkylating agent—or thermochemolysis—which allow the study of gases and light organic compounds formed during the thermal degradation of the kerogen. These techniques can also provide information on the level of thermal maturation of the kerogen as well as its composition.
2. Experimental 2.1. Preparation of the kerogen concentrate The rock phosphate investigated was obtained from the Youssoufia mine (Morocco). It contains 2.0 wt.% of organic carbon. The total organic matter constitutes around 4.0 wt.% (Khaddor et al., 1997). Kerogen concentrate was extracted from the phosphate being as much as possible careful to avoid organic matter degradation. The applied procedure was derived from that used for sediments (Durand and Nicaise, 1980) and that recommended by the International Humic Substances Society (I.H.S.S.) (Amble`s et al., 1989). It is schematized in Fig. 1. The rock phosphate was first crushed then sieved to particles having an average size of 120 Am and finally dried at 80 jC. Free lipids (bitumen) were extracted in a Soxhlet with a methanol/toluene (1:2) mixture. The mineral matrix, which is mainly constituted by carbonates, sulfates, silicates and hydroxides, was carefully treated with portions of a HCl solution at pH 2. The
Fig. 1. Preparation of the kerogen concentrate.
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
dried residue (obtained after centrifugation) was treated with an M/30 HCl solution (pH adjustment), and centrifugation. Bound lipids were then extracted with chloroform in a Soxhlet (Khaddor et al., 1997). Humic acids were removed from the residue with a 0.1 M NaOH solution. Prior to HCl/HF treatment, apatitic calcium was eliminated with 12 M HCl solution to prevent the formation of calcium fluoride insoluble in acidic medium. Silicates were then eliminated by dissolution in HCl/HF (1/1) acid mixture at 70 jC for 24 h. Temperatures superior to 70 jC must be avoided as organic compounds can be oxidized (Amble`s et al., 1989). The residue was thoroughly washed with boric-acid solutions (elimination of neoformed fluorides) and ammonium carbonate, then dried prior to Soxhlet extraction to remove remaining bound lipids, if any. The final residue recovered at the end of all these operations corresponded to the kerogen concentrate. Its X-ray diffraction patterns showed that its structure is amorphous. 2.2. Pyrolysis and characterization techniques Direct pyrolysis of the kerogen concentrate was carried out in a dynamic fixed-bed reactor operated at atmospheric pressure. The sample of the concentrate was maintained in the reactor between two quartz wool plugs and continuously flushed by purified argon at a constant flow rate equal to 40 cm3 min 1 The reactor temperature was monitored by a linear programmer (5 jC min 1) and measured by a thermocouple located within the sample. The evolved gases were analyzed by chromatography (Ziyad et al., 1986, 1993). The pyrolysis used for the structural analysis was a thermochemolysis technique performed using a preparative offline pyrolysis device (Be´har and Pelet, 1985; Largeau et al., 1986). Prior to pyrolysis, the kerogen (20 mg) was moistened overnight with a methanol solution of tetramethylammonium hydroxide (TMAH), which is commonly used as an alkylating agent for thermally assisted hydrolysis and alkylation (THM) (del Rio et al., 1996; Grasset and Amble`s, 1998). The pyrolysate was trapped at low temperature (about 30 jC) in a liquid nitrogen– acetone mixture and then separated by liquid chromatography and thin layer chromatography (Grasset and Amble`s, 1998).
19
2.3. Analysis The products were analyzed by gas chromatography (GC) and gas chromatography coupled to mass spectrometry (GC-MS). The GC separations were carried out on a Packard 438 gas chromatograph using a CPSIL 5CB or CP SIL 88 CB (Chrompack) capillary column (25 m 0.25 mm i.d., 0.12 Am film thickness). The temperature of the column was programmed from 60 to 300 jC (or 260 jC in the case of CP-SIL 88 CB) at 3 jC min 1. The injector and the detector were maintained at 300 jC. GC-MS analyses were performed on a FINNIGAN MAT-INCOS 500 mass spectrometer coupled with a VARIAN 3400 gas chromatograph. The GC conditions were the same as for GC analysis. The mass spectrometer was operated in the electron impact mode (70 eV). Transfer line and ion source were set at 290 and 180 jC, respectively. The mass spectra were collected by scanning the region m/z 50– 600 at a rate of 1 s/decade by a Data general 20 computer. The various products were identified on the basis of their GC retention times and their mass spectra (comparison with standards and literature data). Fourier Transform Infrared (FT-IR) spectra of the kerogen sample were recorded in the range 4000 –400 cm 1 on a NICOLET 510 spectrometer using KBr disks containing around 2 wt.% of kerogen. Prior to spectra recording, the disks were evacuated at 150 jC until the complete elimination of water. Solid state 13C NMR spectra of the kerogen were obtained with a BRUKER spectrometer operating at 4 kHz and equipped with cross-polarization and magic angle spinning (CPMAS). The chemical shifts were calibrated with respect to tetramethylsilane (TMS).
3. Results The extracted kerogen concentrate is made of 83.9 wt.% of pure organic matter, determined by combustion at 800 jC (after pyrite correction). Thus, the original rock phosphate sample contains 34 mg of kerogen per gram of phosphate. 3.1. Elemental analysis The elemental kerogen concentrate analysis is reported in Table 1. The values of H/C (ca. 1.21)
20
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
Table 1 Elemental analysis of the kerogen concentrate (wt.%) C
H
N
O
S
Ash
53.62
5.39
2.44
11.52
10.91
16.12
and O/C (ca. 0.16) ratios indicate that the sample is mostly aliphatic and might be classified in the van Krevelen diagram within the type II kerogens (Tissot and Welte, 1978; Durand and Monin, 1980).
155 ppm (aromatic C bounded to O atoms) (Evans et al., 1971; Amble`s et al., 1994) suggests that the kerogen concentrate probably contains very few phenols or aryl ether groups. The peak appearing between 160 and 200 ppm can be ascribed to CjO bond in ester, carboxylic or amide groups. NMR spectrum also shows clearly that the kerogen structure is mainly made of aliphatic compounds in addition to a significant contribution of aromatic moieties. The carbon aromaticity (fraction of organic carbon in aromatic rings) was found to be fa = 0.29.
3.2. Spectroscopic characterization 3.3. Direct pyrolysis of kerogen The FT-IR spectrum (Fig. 2a) of the kerogen concentrate is characterized by a number of typical absorption bands similar to that found with other oil shale kerogens (Amble`s et al., 1994). The attribution of these bands is not easy because of the complexity of the structures involved and the interference between the individual absorptions. However, the group of bands located between 3000 and 2800 cm 1 and at 1450, 1449, 1377 cm 1 indicate the presence in the sample of methyl and methylene groups. It is difficult here to characterize the aromatic groups. The C – H vibrations of aromatics (and/or of alkenes) ( > 3000 cm 1), if present, are overlapped by the strong O – H vibration at 3000 –3600 cm 1. The bands appearing in the domain 900 – 650 cm 1 can be due to the deformation mode of the aromatic C – H bonds. The absorption band centered on 1706 cm 1 can be assigned to the CjO function of carboxyl groups and/or ketones. The band at 1634 cm 1 may originate from CjC vibration in olefins and possibly in aromatic rings. The broad bands between 1100 and 1300 cm 1 and at 1040 cm 1 can be attributed to C – O bonds present in acids, esters and ethers. The Csp3 – O vibration (in ether, ester, acid groups) normally appears in 1050 – 1150 cm 1 range, while the Csp2 –O band (found in esters, phenols) is observed at higher wavenumbers (1200 –1300 cm 1). The solid state 13C NMR spectrum of the kerogen concentrate is displayed in Fig. 2b. It exhibits, as in coals (Amble`s et al., 1994), two important peaks at 30 ppm and between 100 and 160 ppm, for the n-alkyl chains and for the aromatic carbon atoms, respectively. Different types of aromatic carbons (with protons or alkyl-substituted) can contribute to the second peak. The absence of a well-resolved resonance centered on
The pyrolysis of sediments or kerogens is known to produce a large variety of low molecular weight
Fig. 2. Spectroscopic characterization of the kerogen concentrate from Youssoufia phosphate rocks. (a) FT-IR spectrum (KBr pellets); (b) CP-MAS solid state 13C NMR spectrum.
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
21
Fig. 3. Production of gases (1) and pyrolysate (2) in the pyrolysis of the Youssoufia kerogen.
compounds and complex pyrolysates (Saiz-Jimenez and de Leeuw, 1984, 1986; Larter and Horsfield, 1993; Grasset and Amble`s, 1998). Fig. 3 reports the results obtained with the Youssoufia kerogen concentrate. It shows the quantities of gas and the pyrolysate evolved versus the temperature under a neutral atmosphere (Ar). Between 250 and 600 jC, the weight loss contains 37.2% of gases and 17.9% of the pyrolysate. The amount of gases recovered is superior to that of pyrolysate probably because the kinetics of gases production by the splitting of the organic matter is faster than the diffusion of the heavy compounds out of the reactor. However, since both curves have the same profile (Fig. 3), one might expect the simultaneous production of gases and oil to occur as shown by the following global reaction:
composition and degradation during the phosphate retorting. Moreover, its formation in large amounts may constitute, if not properly evacuated, a danger for the industrial installations. Fig. 4a shows that its evolution from the kerogen starts at 400 jC and continues beyond 850 jC. The maximum of the release rate is achieved around 680 jC. It results essentially from the recombination of hydrogen radicals and represents 7.22% of all the gases produced.
kerogen ! pyrolysate þ gas, where the gases are the noncondensable vapors escaping from the kerogen and the pyrolysate, the condensable hydrocarbons and complex organic compounds formed during the heating period. 3.3.1. Hydrogen evolution Hydrogen plays an essential role in many industrial processes including energy production. The study of its evolution may provide information on the kerogen
Fig. 4. Gases evolution during the direct pyrolysis of the kerogen concentrate: (a) H2, (b) CH4 (c) CO2. The heating rate is 5 jC/min.
22
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
The very low amount of the mineral matter encapsulated in the kerogen concentrate assigns to the pyrolysis curve the symmetrical profile it shows. Hydrogen evolution is not affected by secondary reactions, such as water – gas shift reaction, since no production of CO was detected during the pyrolysis. 3.3.2. Methane evolution Fig. 4b reports the generation rate of methane versus temperature. The evolution profile is similar to that observed during the rock phosphate retorting (Ziyad et al., 1993). Methane appears in the gases between 300 and 650 jC. It probably results from the hydrogenation of CH3 radicals produced by the fragmentation of long chain organic molecules. The maximum rate of CH4 production was reached at 480 jC and was found to be equal to 4.10 5 mol min 1 g 1. 3.3.3. Carbon-dioxide evolution The release rate of carbon dioxide in neutral atmosphere versus temperature is represented in Fig. 4c. The evolution proceeds apparently in two waves of unequal importance that extend from 150 to around 600 jC. The maxima of the peaks are quite distant from each other and are centered on 300 and 450 jC. This suggest that the CO2 originates at least from two different sources that might be for the first peak the splitting of the carboxylic acids in the following manner: R COOH ! R H þ CO2 :
ð1Þ
The carboxylic-acid functions are, as it will be shown latter, abundant in the kerogen concentrate. The second peak that appears above 350 jC can be attributed to the cleavage of ester groups: R COO RV ! Rb þ RVb þ CO2 :
ð2Þ
If the neutral carrier gas (Ar) is replaced by air, the kerogen decomposition changes considerably. It is almost totally oxidized below 600 jC into CO2 (Fig. 5). The heating rate in these conditions has no noticeable influence on the temperature of the maximum, albeit it increases significantly the amplitude of the peak. The quantity of CO2 evolved during the pyrolysis under air is equal to the total amount of the oxidized organic matter. The CO2 recovered probably
Fig. 5. Production rate of CO2 on pyrolysis of Youssoufia kerogen under air at the heating rates 5 and 9 jC min 1.
originates from different competing reactions that might be globally represented by: Cn H2m þ ðn þ m=2ÞO2 ! nCO2 þ mH2 O:
ð3Þ
The kerogen is an assemblage of organic structures that are much more complex than suggested by Eq. (3) and obviously a contribution of the reactions (1) and (2) cannot be excluded from CO2 production. The absence of CO from the evolved gases supports the assumption that reactions, such as Eq. (4), do not occur probably because there is no residual carbon at all. Cresidue þ CO2 ! 2CO:
ð4Þ
3.4. Thermochemolysis of the kerogen Pyrolysates from kerogens generally contain a very complex mixtures of organic compounds. The use of methods, such as pyrolysis (or thermochemolysis) and GC/MS for the analysis of the released products, might provide valuable information for the rebuilding of the original structure of the kerogen and the phosphate genesis. Fig. 6 reports the chromatogram of the apolar fraction from the pyrolysate. It contains series of fatty-acid methyl esters accompanied by octyl esters and C16 fatty acid (palmitic acid), series of n-alkanes and of linear and branched alkenes. Due to the complexity of the fraction, the distribution was determined using specific ion chromatograms (infra-
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
23
Fig. 6. Chromatogram of the apolar fraction obtained after thermochemolysis of Youssoufia kerogen.
vacuum). Tetracyclic and pentacyclic terpenoids, which are considered as biomarkers, were also identified. 3.4.1. Aliphatic hydrocarbons The distribution of n-alkanes is displayed on Fig. 7. The C16 – C33 distribution is clearly bimodal. The short C16 to C23 mode is the most abundant with a maximum at C22. The value of the odd-to-even carbon number ratio is 1.17 and n-alkanes represent 20.5 wt.% of the pyrolysate. The m/z 111 ion fragmentogram (Fig. 8) illustrates the distribution of n-alkenes. They were found in the C17:1 –C33:1 range. The distribution is unimodal (max. C24:1, C25:1) contrarily to that of linear alkanes. Branched C19 – C33 alkenes were also identified (Fig. 8).
3.4.2. Triterpenoid hydrocarbons The pyrolysate contains a large variety of hopanoic structures. Fig. 9 displays the mass fragmentogram of the hopanes (m/z = 191). The distribution extends from C29 to C32 and maximizes at C29. The hopanes detected are 17a (H), 21h (H) hopanes (a, h hopanes). In sedimentary organic matter, 17a (H), 21h (H) hopanes (a, h hopanes) and 17h (H),21a (H) hopanes (h, a hopanes) are known to derive from bacteriohopane tetrol (or related compounds bearing amino groups), which is present in the membranes of bacteria with the natural thermodynamically unstable 17h (H), 21h (H) configuration (h, h hopanes) (Ensminger et al., 1977). During the maturation of the sediment, the formation of the thermodynamically stable a, h and h, a hopanes takes place (Gaskell and Eglinton, 1975). The same
24
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
Fig. 9. Distribution of hopanoic acids (m/z 191 ion fragmentogram).
Fig. 7. Distribution of n-alkanes produced on thermochemolysis of the Youssoufia kerogen.
diagenetic hopanes were detected in the bitumens of the corresponding phosphate rock, but with a wider distribution (C27 to C34) (Khaddor et al., 1997). Two triterpenes C27H 44 (22,29,30 trisnorhop17(21)-ene) and C29H48 (30-norhop-17(21)-ene) were also identified in the pyrolysate, on the basis of their mass fragmentation pattern (Philp, 1985). The double bond is present in the D cycle (D-17,21) and not on the lateral chain (D-22,29). Generally, these two hop17(21)-ene compounds result from the diagenetic
transformation of hop-22(29)-ene (Grimalt et al., 1991). The pyrolysate contains also the two C27 and C29 tetracyclic triterpanes. They are associated with the 5a(H)-cholestane and 24-ethyl-5a(H)-cholestane, respectively. The 24-ethyl-5a(H)-cholestane derives from 24-ethyl cholesterol and has a vegetal origin. An interesting point is that the two steranes are accompanied by the corresponding sterenes. The mass spectrometry indicates that the double bond is present in the A ring. They can probably be identified with mixtures of D-2 and D-3 cholestenes. Steranes were numerous in the soluble fraction (bitumen) of the rock phosphate, but no sterenes were found (Khaddor et al., 1997).
Fig. 8. Distribution of alkenes formed on pyrolysis of the Youssoufia kerogen (m/z 111 ion fragmentogram): (HCx:1: n-alkene; HCx:1br: branched alkene).
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
25
Fig. 10. Distribution of fatty acids (m/z 74 ion fragmentogram).
3.4.3. Methyl esters and higher esters Linear monocarboxylic-acid methyl esters produced on pyrolysis arise from the transesterification of the acids, which were initially esterified in the kerogen matrix. Their distribution in the range C14 – C28 illustrated by the m/z 74 (McLafferty rearrangement) ion fragmentogram (Fig. 10) is dominated by the palmitic (C16) and stearic (C18) acid methyl esters and presents a clear even predominance. Surprisingly, three fatty-acid octyl esters, corresponding to C12, C14 and C16 fatty acids, were found in the pyrolysate. The mass spectrum of the dodecanoic-acid n-octyl ester is given as an example in Fig. 11. These octyl esters are characterized in mass spectrometry by ions at m/z 70, 112, M-129 ( –C8H17Ob) and M-128 atomic mass units (amu). The intensity of the molecular peak is low and the confirmation of the mass value was obtained by chemical ionization (giving [M + H] + ). The mass fragmentation pattern of the fatty-acid n-octyl esters is described in Fig. 12.
McLafferty rearrangement (through the oxygen atom) of the ester leads to the m/z 112 ion radical, which corresponds to ionized n-octene. The m/z 112 ion can eliminate 42 amu (propene) by a McLafferty rearrangement and leads to m/z 70 (n-pentene ion radical). The distribution of the fatty acids combined as n-octyl esters was determined using the characteristic m/z 70 ion fragmentogram (not shown): C12: 8.0; C13: 2.7; C14: 3.5; C15: 1.0; C16: 1.5 in arbitrary units. The origin of the octyl esters in the pyrolysis products is a challenging question. Theoretically, such ester bonds are expected to be broken on pyrolysis.
Fig. 11. Mass spectrum (EI) of dodecanoic acid n-octyl ester.
Fig. 12. Fragmentogram pattern of fatty acid n-octyl esters.
26
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
We observed for a few samples—of soil organic matter or kerogen—that octyl esters are present when using tetramethyl ammonium hydroxide, but absent in the conventional pyrolysis products. Is there a specific (unknown) reaction with esterified fatty acids that were found in the kerogen? It can be noted that the distribution of fatty acids present as octyl esters is different from that given in Fig. 10. The other possibility could be a contamination by n-octyl phthalates (common contaminants). The investigation of such a pollution gave negative results.
4. Discussion The spectroscopic study of the kerogen concentrate extracted from Youssoufia rock phosphate by FT-IR and NMR showed that it has a highly aliphatic character. It is also highly oxygenated. Chemical functions containing oxygen have been identified in its structure and compounds, such as carbon dioxide and esters, were produced by pyrolysis and thermochemolysis. These ester groups were also found in the contemporary Moroccan Timahdit (M and Y layers) and Tarfaya oil shale kerogens (Amble`s et al., 1987; Kribii, 1994; Kribii et al., 1996, 2001). Additional information on the kerogen structure was provided by the evolution sequence of gases upon the temperature increase in the direct pyrolysis. Carbon dioxide was released in the domain of the first pyrolysis. It may originate from the degradation of acids or the decarboxylation of ester groups according to the mechanism given in Fig. 13. As the temperature was increased, the carbonaceous deposits underwent cracking reactions producing methane by hydrogenation of methyl radicals. The dehydrogenation reac-
Fig. 13. Decarboxylation reaction of esters occurring during thermochemolysis.
Fig. 14. Formation of alkenes and alkanes during pyrolysis.
tions started above 400 jC and a maximum of H2 production appeared at 700 jC (Fig. 4a). The kerogen matrix begins to randomly free alkenes and alkanes. Pyrolysis generally affords hydrocarbons as n-alkane/ n-alkene doublets. The breakage of carbon-heteroatom ( –C – X –) bonds with a weak energy must be favored. The terminal unsaturation in n-alk-1-enes arises from the h scission of radicals during pyrolysis (Fig. 14) (Larter and Horsfield, 1993). n-Alkanes and n-alkenes constitute the predominant series of components produced upon kerogen thermochemolysis. Commonly, it is admitted that the origin of n-alkane/n-alkene doublets can be diverse in pyrolysis. They can arise from the scission of C –X bonds (Fig. 13) or from the thermal degradation of esters, which was reported to produce n-alkanes from the acid moiety and n-alk-1-enes from the alcohol moiety (van de Meent et al., 1980). These esters were in the protective layers of higher plants, as suberin,
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
cutin, that were preserved in the kerogen structure (de Leeuw and Largeau, 1993; Macko et al., 1993). They can originate from the aboveground parts as well as from the roots (Nierop, 1998). Usually, n-alkanes and n-alkenes exhibit similar distributions (Larter and Horsfield, 1993; de Leeuw and Largeau, 1993; Macko et al., 1993). However, this was not the case for the Youssoufia kerogen, the distributions were found to be different. The n-alkanes contrarily to that n-alkenes showed a bimodal distribution (Figs. 7 and 8) and presented analogies with the hydrocarbons distributions in the bitumens of the Youssoufia rock phosphate (Khaddor et al., 1997). These differences suggest that a portion of the n-alkanes found in the pyrolysate were present as trapped molecules in the macromolecular network and released after alteration of the matrix. Analogous phenomenon is commonly observed after chemical degradation of kerogens from ancient sediments (Baudet et al., 1991; Halim et al., 1997). In the conventional pyrolysis (without tetramethyl ammonium hydroxide), fatty acids arise from the cleavage of esters (Largeau et al., 1986; Tegelaar et al., 1989a,b) that did not undergo decarboxylation reaction, as illustrated in Fig. 13. Thermochemolysis in presence of an alkylating agent, such as tetramethyl
27
ammonium hydroxide, is recommended to prevent decarboxylation of esters (Challinor, 1989, 1991a,b). It leads only to fatty-acid methyl esters, which mainly result from the transalkylation of ester groups (Martin et al., 1995; Gonzalez-Vila et al., 1996). The strong even over odd predominance of carbon numbers for the monocarboxylic-acid methyl-ester distribution (Fig. 10) indicates that these acids could originate from plant protective polyesters (Tegelaar et al., 1998a,b). The dominant C16 and C18 members (as esters) are ubiquitous, they were found in higher plants, algae, bacteria, animals. The identification of some nonmethylated fatty acids in the TMAH/pyrolysis products could probably indicate that under our experimental conditions, the prevailing transesterification process is accompanied with ‘‘classical’’ pyrolysis cleavage. Other examples were found in the study of ancient sedimentary or soil organic matter by thermochemolysis (Amble`s, unpublished results). Various triterpenoids were found in the pyrolysate products. The identified C29 – C32 hopanes are exclusively ‘‘diagenetic’’ a,h and h,a hopanes, the natural h,h hopanes were not detected. This result is theoretically somewhat surprising for a relatively immature sediment. The same result arose from the study of ether-bound hopanoids from the contemporary Timah-
Fig. 15. Stanols bound to the kerogen matrix with ether or ester groups and yielding sterene/sterane doublets during pyrolysis.
28
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
dit oil shale (Saiz-Jimenez, 1994). The two unsaturated C27 and C2917a (21h)-hopenes were identified. They were also found in the pyrolysate of other kerogens (Seifert, 1978; Largeau et al., 1986; Larter and Horsfield, 1993). They probably originate from the diagenic transformation of hop-22(29)-enes (Ensminger, 1977), which are commonly found, for instance, in soils (Riess-Kaut and Albrecht, 1989). It is impossible to verify if they were bonded to the matrix or present in the kerogen as trapped molecules. They were not identified in the soluble fraction (Khaddor et al., 1997). The 5a(H) cholestane and the 24-ethyl 5a(H) cholestane were formed on thermochemolysis with the corresponding C27 and C29 sterenes. The presence of sterene/sterane doublets indicated the occurrence of 5a(H) cholestanol and 24-ethyl 5a(H) cholestanol chemically bound to the kerogen matrix, probably via an ether or an ester group (Fig. 15).
5. Conclusions The aim of this work was a detailed study of the structure of the kerogen present in the rock phosphate from Youssoufia. In this sediment, the kerogen represents the main form of organic matter (85 wt.%). The study was conducted using spectroscopic methods and two different pyrolysis techniques. CP-MAS 13C NMR and FT-IR spectra indicated that it has a rather aliphatic nature and is rich in oxygen of chemical functions such as esters, ethers, alcohols. Production of carbon dioxide on direct pyrolysis and esters on thermochemolysis evidenced this observation. Direct pyrolysis was used because it provides a quick and reasonable simulation of (i) the processes involved in the evolution (diagenesis, metagenesis, catagenesis) of sediments and (ii) the behavior of organic matter upon heating. In a neutral atmosphere hydrogen evolution covers a wide range of temperatures (400 – 800 jC). Methane gives rise to two peaks suggesting that it originates from at least two different sources. Carbon dioxide appears at temperatures as low as 150 jC in two waves centered at 300 and 450 jC, respectively. The identification of n-alkene/n-alkane doublets formed on thermochemolysis indicates that the kerogen matrix is formed by cross-linked aliphatic chains, partly originating from preserved biopolyesters. Cross-
linking of chains involves ester and probably ether groups. Some aliphatic moieties, which act as monosubstituents of the kerogen matrix, are reflected as aliphatic monocarboxylic-acid methyl esters upon thermochemolysis. Aliphatic compounds, such as alkanes, possibly fatty-acid octyl esters, obviously trapped in the macromolecular structure, are released when the structure of the matrix is altered, as previously observed after chemical degradation of kerogens (Baudet et al., 1991; Halim et al., 1997). The presence of C29 – C32 a,h hopanes and the absence of natural h,h hopanes indicated that the Youssoufia kerogen is at its diagenesis evolution stage. The C27 and C29 sterene/ sterane doublets are indicative of stanols probably bound to kerogen by ether and/or ester groups. They are representative of a mixed animal and higher plant input. It is also interesting to note that, as reported for other pyrolysis techniques, preparative thermochemolysis in the presence of tetramethylammonium hydroxide is a quick and powerful technique for the study of macromolecules. Heating in the presence of an alkylating agent causes additional chemolysis of the resistant aliphatic parts of the matrix, allowing, as an example, transalkylation of polyesters resistant under classical pyrolysis conditions (Gobe´ et al., 2000).
Acknowledgements The authors thank the ‘‘CERPHOS’’ (Morocco) for providing the investigated sample of phosphate. They also gratefully acknowledge CNRS (France) for the financial support provided to this study. [CA]
References Amble`s, A., Dupas, G., Jacquesy, J.C., 1987. Solubilization of Timahdit (Morocco) oil shale kerogen catalysed by 18-Crown-6. Tetrahedron Lett. 28, 6449 – 6452. Amble`s, A., Magnoux, P., Jacquesy, R., Fustec, E., 1989. Effects of addition of bentonite on the hydrocarbon fraction of a podzol soil (A1) horizon. J. Soil Sci. 40, 685 – 694. Amble`s, A., Halim, M., Jacquesy, J.C., Vitorovic, D., Ziyad, M., 1994. Characterization of kerogen from Timahdit shale (Y-layer) based on multistage alkaline permangante degradation. Fuel 73, 17 – 24. Baudet, N., Kribii, A., Halim, M., Jacquesy, J.C., Amble`s, A., 1991. Structural study of kerogens by chemical degradations.
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30 In: Øygard, K. (Ed.), Organic Geochemistry. F. Hurtigtrykk, Oslo, pp. 543 – 546. Be´har, F., Pelet, R., 1985. Pyrolysis gas chromatography applied to organic geochemistry. Structural similarities between kerogens and asphaltenes from related rock extracts and oils. J. Anal. Appl. Pyrolysis 8, 173 – 187. Benarafa, A., Kacimi, M., Coudurier, G., Ziyad, M., 2000. Characterisation of the active sites in butan-2-ol dehydrogenation over calcium – copper and calcium – sodium – copper phosphates. Appl. Catal., A 196, 23 – 35. Challinor, J.M., 1989. A pyrolysis – derivation – gas chromatography technique for the structural elucidation of some synthetic polymers. J. Anal. Appl. Pyrolysis 16, 323 – 333. Challinor, J.M., 1991a. Structure determination of alkyd resins by simultaneous pyrolysis methylation. J. Anal. Appl. Pyrolysis 18, 233 – 333. Challinor, J.M., 1991b. The scope of pyrolysis methylation reactions. J. Anal. Appl. Pyrolysis 20, 15 – 24. de Leeuw, J., Largeau, C., 1993. A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal and petroleum formation. In: Engel, M.E., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum, New York, pp. 23 – 72. del Rio, J.C., Martin, F., Gonzalez-Vila, F.J., 1996. Thermally assisted hydrolysis and alkylation as a novel pyrolytic approach for the structural characterization of natural biopolymers and geomacromolecules. Trends Anal. Chem. 15, 70 – 79, and references cited therein. Durand, B., Monin, J.C., 1980. Elemental analysis of kerogen. In: Durand, B. (Ed.), Kerogen. Technip, Paris, pp. 113 – 142. Durand, B., Nicaise, G., 1980. Procedures for kerogen isolation. In: Durand, B. (Ed.), Kerogen. Technip, Paris, pp. 35 – 53. Elliot, J.C., 1994. Structure and chemistry of the apatites and other calcium phosphates. Studies in Inorganic Chemistry, vol. 18. Elsevier, Amsterdam, pp. 1 – 62. Ensminger, A., 1977. Evolution des compose´s polycycliques se´dimentaires, PhD thesis dissertation, Universite´ Louis Pasteur, Strasbourg. Ensminger, A., Albrecht, P., Ourisson, G., Tissot, B., 1977. Evolution of polycyclic alkanes under the effect of burial (Early Toarcian Shales, Paris Basin). In: Campos, R., Goni, J. (Eds.), Advances in Organic Geochemistry, vol. 1975. Enadimsa, Madrid, pp. 45 – 52. Evans, C.R., Ragers, N.A., Bailey, N.J.L., 1971. Evolution and alteration of petroleum in western Canada. Chem. Geol. 8, 147 – 170. Gaskell, S.J., Eglinton, G., 1975. Rapid hydrogenation of sterols in contemporary lacustrine sediments. Nature 254, 209 – 211. Gobe´, V., Leme´e, L., Amble`s, A., 2000. Structure elucidation of soil macromolecular lipids by preparative pyrolysis and thermochemolysis. Org. Geochem. 31, 409 – 419. Gonzalez-Vila, F.J., del Rio, J.C., Martin, F., Verdejo, T., 1996. Pyrolytic alkylation – gas chromatography – mass spectrometry of model polymers. Further insights into the mechanism and scope of the technique. J. Chromatogr., A 750, 155 – 160. Grasset, L., Amble`s, A., 1998. Structure of humin and humic acids from an acid soil as revealed by PTC-hydrolysis. Org. Geochem. 29, 881 – 891.
29
Grimalt, J.O., Yruela, I., Saiz-Jimenez, C., Toja, J., de Leeuw, J.W., Albaige`s, J., 1991. Sedimentary lipid biogeochemistry of an hypereutrophic alkaline lagoon. Geochim. Cosmochim. Acta 55, 2555 – 2576. Halim, M., Joffre, J., Amble`s, A., 1997. Characterization and classification of kerogen from Tarfaya (South Morocco) based on its oxidation products. Chem. Geol. 141, 225 – 234. Khaddor, M., Ziyad, M., Halim, M., Joffre, J., Amble`s, A., 1997. Characterization of soluble organic matter from Youssoufia rock phosphate. Fuel 76, 1395 – 1400. Kribii, A., 1994. ‘‘Etude structurale des ke´roge`nes par des re´actions chimiques se´lectives’’, PhD thesis dissertation, University of Poitiers. Kribii, A., Baudet, N., Amble`s, A., 1996. Kerogen structural study by a combination of specific reactions. J. Serb. Chem. Soc. 61, 137 – 147. Kribii, A., Leme´e, L., Chaouch, A., Amble`s, A., 2001. Structural study of the Moroccan Timahdit (Y-layer) oil shale kerogen using chemical degradation. Fuel 80 (5), 681 – 691. Largeau, C., Derenne, S., Casadevall, E., Kadouri, A., Sellier, N., 1986. Pyrolysis of immature Torbanite and of resistant biopolymer (PRBA) isolated from extant alga Botriococcus braunii. Mechanism of formation and structure of Torbanite. Org. Geochem. 10, 1023 – 1032. Larter, S.R., Horsfield, B., 1993. Determination of structural components of kerogen by the use of analytical pyrolysis methods. In: Engel, M.E., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum, New York, pp. 271 – 287. Macko, S.A., Engel, M.H., Parke, P.L., 1993. Early diagenesis of organic matter in sediments: assessment of mechanisms and preservation by the use of isotopic molecular approaches. In: Engel, M.E., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum, New York, pp. 211 – 224. Martin, F., del Rio, J.C., Gonzalez-Vila, F.J., Verdejo, T., 1995. Thermally assisted hydrolysis and alkylation of lignins in the presence of tetra-alkylammonium hydroxides. J. Anal. Appl. Pyrolysis 35, 1 – 13. Nierop, K.G.J., 1998. Origin of aliphatic compounds in a forest soil. Org. Geochem. 29, 1009 – 1016. Philp, R.P., 1985. Fossil Fuel Biomarkers. Applications and Spectra. Elsevier, Amsterdam. Riess-Kaut, M., Albrecht, P., 1989. Hopane-derived triterpenoids in soils. Chem. Geol. 76, 143 – 151. Saiz-Jimenez, C., 1994. Analytical pyrolysis of humic substances: pitfalls, limitations and possible solutions. Environ. Sci. Technol. 28, 1773 – 1780. Saiz-Jimenez, C., de Leeuw, J.W., 1984. Pyrolysis – gas chromatography – mass spectrometry of soil polysaccharides, soil fulvic acids and polymaleic acid. Org. Geochem. 6, 287 – 293. Saiz-Jimenez, C., de Leeuw, J.W., 1986. Chemical characterization of soil organic matter fractions by analytical pyrolysis – gas chromatography – mass spectrometry. J. Anal. Appl. Pyrolysis 9, 99 – 119. Seifert, W.K., 1978. Steranes and terpanes in kerogen pyrolysis for correlation of oils and source rocks. Geochim. Cosmochim. Acta 42, 473 – 484. Tegelaar, E.W., de Leeuw, J.W., Holloway, P.J., 1989a. Some mech-
30
M. Khaddor et al. / Chemical Geology 186 (2002) 17–30
anisms of flash pyrolysis of naturally occurring higher plant polyesters. J. Anal. Appl. Pyrolysis 15, 289 – 295. Tegelaar, E.W., de Leeuw, J.W., Saiz-Jimenez, C., 1989b. Possible origin of aliphatic moieties in humic substances. Sci. Total Environ. 81/82, 1 – 17. Tissot, B.P., Welte, D.H., 1978. Petroleum Formation and Occurrence. Springer Verlag, Berlin. Tissot, B., Durand, B., Espitalie´, J., Combaz, A., 1994. Influence of nature and diagenesis of organic matter in the formation of petroleum. Bull. Am. Assoc. Pet. Geol. 58, 499 – 506.
van de Meent, D., Brown, S.C., Philp, R.P., Simoneit, B.R.T., 1980. Pyrolysis high resolution gas chromatography and pyrolysis – gas chromatography mass spectrometry of kerogen precursors. Geochim. Cosmochim. Acta 44, 999 – 1014. Ziyad, M., Garnier, J.P., Halim, M., 1986. Non-isothermal kinetics of H2S and H2 generation from Timahdit oil shale. Fuel 65, 715 – 720. Ziyad, M., Khaddor, M., Halim, M., 1993. Non-isothermal retorting of rock phosphate containing organic matter. Fuel 5, 655 – 659.