Energy & Fuels 1991,5,441-444
441
Role of Pyrite during the Thermal Degradation of Kerogen Using in Situ High-Temperature ESR Technique Mohammed Y. Bakr* Department of Geology, Faculty of Science, Alexandria University, Alexandria, Egypt
T. Yokono and Y. Sanada Metals Research Institute, Faculty of Engineering, Hokkaido University, Sapporo, 060 Japan
M. Akiyama Department of Geology, Faculty of Science, Shinshu University, Matsumoto 390, Japan Received October 24, 1990. Revised Manuscript Received December 6, 1990 ~~
The thermal degradation of kerogen during pyrolysis in the presence and absence of pyrite as an additive was investigated and discussed from a free-radical chemistry standpoint using in situ ESR techniques. The added pyrite has pronounced effects on the activity of free radicals of kerogen samples representing the diagenesis stage. In this stage, pyrite enhances and accelerates the process of free-radical formation, where the generation of free radicals in kerogen, in the presence of added pyrite, starta at temperature lower than those observed in its absence. On the contrary, pyrite has negligible effects on the formation of free radicals in kerogen samples representing the catagenesis stage. The overall increase of free-radical activity in the presence of pyrite is consistent with the conversion of pyrite to pyrrhotite. It is suggested that pyrite acts indirectly as a catalyst through the nascent sulfur produced from the pyrite to pyrrhotite conversion process.
Introduction Thermal maturation of kerogen in sedimentary rocks has been studied extensively to better understand the generation of oil and gas.'$ The influence of mineral matrices on kerogen during thermal maturation has also been studied. Studies reported in the literature have shown that the role of catalysis in pyrolysis reactions have been directed toward using minerals, such as kaolinite, montmorillonite, and calcite.*' Analytical techniques applied in these studies did not include in situ ESR spectroscopy. Although marine and brackish water kerogen is always aseociated with diagenetic pyrite, a possible catalytic role of pyrite on the thermal degradation of kerogen has not received attention. The genetic relationship between pyrite and kerogen, in addition to the evidence that pyrite whether indigenous or added enhances coal conversion and improves product quality under hydroliquefaction conditions,"" makes one wonder whether pyrite also plays a role during the course of kerogen maturation. In this work, in situ ESR studies were conducted on kerogen pyrolysates up to 650 O C in the presence and absence of added pyrite. The effect of pyrite on the thermal degra(1) T h o & B. P.; Web, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1984. ( 2 ) Hunt, J. M. Petroleum Geochemistry and Geology;Freeman: San Francisco, 1979. ( 3 ) Evans, €2. J.; Felbeck, Jr., C. T. Org. Geochem. 1983,4,3/4, 145. (4) Fapitalie, J.; Makadi, K. S.; Trichet, J. Org. Geochem. 1984,6365. (6) Tannenbaum, E.; Kaplan, I. R. Geochim. Coemochim. Acta 1987,
49, 2689. (6) Huizinga, B. J.; Tannenbaum, E.; Kaplan, I. R. Org. Geochem.
dation of kerogen from a free-radical chemistry standpoint is discussed.
Experimental Section Materials. Samples studied here include lignin, synthesized kerogen precursor, and natural kerogen. Lignin is a pure reagent supplied from Tokyo Kogyo Co., Ltd. Synthetic kerogen precursor was prepared from a mixture of glucose and casein.I2 The elemental analysis data of lignin and synthetic kerogen precursor are presented in Table Ia. Natural kerogen samples were taken from the MITI-Hamayuchi borehole drilled in Japan. Their characteristic data are given in Table Ib. Hamayuchi kerogens were obtained after demineralization with HCl/HF and treatment with benzenemethanol followed by pyridine extraction. Pyrite is a reagent supplied from CERAC, Inc., and was checked with X-ray diffraction and Mossbauer spectroscopy before use. The particle size of pyrite is -200 mesh. It was mixed well in a mortar with each of the above samples. ESR Measurements. ESR spectra were recorded on a Varian E-109 X-band spectrometer equipped with an Echo Electronics rectangular high-temperature cavity. Experiments were conducted at an operational frequency of 9.35 GHz and a modulation of 100 kHz. A microwave power of 0.2 mW was used throughout. About a 20-mg sample was mounted in a 4 mm 0.d. quartz tube and inserted directly into the high-temperature cavity. Runs were carried out at a heating rate of 10 OC/min in a nitrogen gas flow that was kept at a constant velocity. The spectra were recorded at 25 O C intervals up to a maximum value of 650 "C. Recording of spectra was done at a scan time of 7.5 s, and then data were stored for further investigation in an on-line Hewlett-Packard computer system. Before heating, spin concentrations at room temperature were calculated to two secondary standards: an evacuated and sealed coal sample, and a pitch sample. The two standards were in turn calibrated against diphenylpicrylhydrazyl (DPPH). Corrections were made for Boltzmann factor and weight loss of the sample at elevated temperatures. All experimental procedures were tested for their accuracy and reproducibility. (12) Yamamoto, S. Ph.D.
Thesis, Tokyo Metropolitan University,
1983.
0887-0624/91/2505-0441$02.50/0
0 1991 American Chemical Society
Bakr et al.
442 Energy & Fuels, Vol. 5, No. 3, 1991 LO1
.ignin and 10% FcS2
p
-'p 9
301
? a v
C
.-0
I 20
0-
C
s 0 V
.-C
n
m
10
Temperature ("C) Figure 1. Temperature dependence of radical concentration of
Temperature ( "C Figure 2. Temperature dependence of radical concentration of synthetic kerogen precursor in the presence and absence of FeS?
lignin in the presence and absence of FeS?
FT-IR spectra of kerogen were obtained from KBr pellets (200 mg, 1 w t %). Spectra were recorded on a Nicolet 5DX spectrometer along with a Nicolet series computer and control data disk unit. Software facilities were used for baseline corrections of kerogen spectra which were scaled to a l-mg sample. Since pyrite cannot be fully removed without specific alteration of kerogen structure, no attempts have been made to remove pyrite from the natural kerogen under investigation. Therefore,the role of the indigenous pyrite cannot be distinguished in this study, and discussion will be focused on the added pyrite. To check the catalytic effect of the added pyrite on the formation of free radicals, lignin and synthetic kerogen precursor as pyrite-free materials were tested with and without addition of pyrite.
Results and Discussion Figure 1 shows the temperature dependence of freeradical spin concentrations for lignin with and without addition of 10% pyrite. There are two significant points that should be mentioned (1)the maximum value of spin concentration for pyrite-lignin system is larger than that of the lignin alone; (2) pyrite speeds up the process of radical formation, where the maximum value of spin concentration of the lignin-pyrite system is shifted significantly to lower temperature, from 575 to 550 "C. Figure 2 illustrates the variations in the free-radical formation processes for the synthetic kerogen precursor in the presence and absence of 10% FeS2 as a function of pyrolysis temperature. The main effecta of pyrite on the thermal decomposition of kerogen precursor are manifested by the following: (1)Acceleration of the pyrolysis process which causes shift of spin concentration maxima (Rl and R,) and minimum (R2)to lower temperatures. Lowering in temperatures was measured as 25 "C for both R1and Rz, and 50 "C for RP (2) Drastic enhancement of spin concentration that starts from 225 "C and proceeds throughout the course of pyrolysis. This enhancement becomes intensive at 450 O C (the temperature of R3). Pyrite Effect during Kerogen Maturation. Using the in situ ESR technique, it has been shown that kerogen maturity and hydrocarbon generation are linked to the
100
m m
400 50 600 700
Temperature ("C) Figure 3. Temperature dependence of radical concentration of Hamayuchi kerogen no. 1 in the presence and absence of FeS?
content of free radicals therein.', Five kerogen samples from the MITI-Hamayuchi borehole were mixed with 10 and 30% FeS2 and examined by ESR. These samples ranged in depth from 904 to 4499 m (Table Ib) and cover the maturation stages from diagenesis (samples no. 1and 2) to beginning of catagenesis (samples no. 3-5). The temperature dependence of the free-radical spin concentration of Hamayuchi kerogens in the presence and absence of pyrite as an additive is shown in Figures 3 and 4 for the shallowest (no. 1)and deepest (no. 5) samples, respectively. In sample no. 1 (Figure 3), the free-radicalconcentration markedly increases in the presence of 10% FeSz over that when kerogen was pyrolyzed without the additive. When 30% FeSz was used, an incremental enhancement in reactivity over the 10% FeS2 was observed. The overall (13) Bakr, M. Y. Ph.D. Thesis, Faculty of Engineering, Hokkaido University, Sapporo, Japan, 1989.
Thermal Degradation of Kerogen
Energy & Fuels, Vol. 5, No. 3, 1991 443
fi&
t-
I
speeding up the process of free-radical formation can be observed as the temperature at the position of the maximum value of spin concentration is shifted to lower temperature when pyrite was added. Similar results were obtained for sample no. 2. Figure 4 shows the results of sample no. 5 (similar results were obtained for samples no. 3 and 4). There are no noticeable changes in spin concentrations between the kerogen alone fraction and the kerogen plus 10% FeSz. However, a slight increase in spin concentration was observed in the presence of 30% FeS2. This result suggests that the effect of pyrite in this case is either small (for 30% Fe& system) or negligible (for 10% Fe& system) compared with its effect in samples no. 1 and 2. Spin concentrations measured at room temperature for the samples so far tested are given in Table Ib. Spin concentration of sample no. 5 (deepest, mature, and more aromatic) is larger than that of sample no. 1 (shallowest, immature, and less aromatic). In Figure 5 , FT-IRspectra of samples no. 1 and 5 are illustrated. It is clear from Figure 5 that the peak intensities of the 0-H stretching band near 3400 cm-', the aliphatic hydrogen near 2900 cm-', and the total out-of-plane aromatic hydrogen near 760-850 cm-' (measured from the peak heights) of sample no. 1 surpassed that of sample no. 5. According to the above data, a possible explanation regarding the pyrite activity during kerogen maturity can be drawn. As kerogen matures, it becomes more aromatic and depleted in hydrogen. The more hydrogen a kerogen contains, the higher is the observed activity of pyrite as is evident in sample no. 1. Since the generation of the low molecular weight hydrocarbons proceeds via free-radical chemistry, it is suggested that pyrite may enhance the release of these hydrocarbons from kerogen through its catalytic activity in the diagenesis stage. Effect of Temperature on the Decomposition of Pyrite. Three sets of heating experiments were carried out on kerogen sample no. 1at 325,375, and 450 "C in a N2atmosphere in order to investigate the effect of pyrolysis on the decomposition of indigenous pyrite. Mossbauer spectra in Figure 6 showed that the transformation of indigenous pyrite to pyrrhotite commenced at 450 "C. Five sets of pyrolysis experiments were conducted on reagent pyrite in R N2atmosphere at 250,375,425, and 550 "C. The transformation of pyrite to pyrrhotite starts at 250 "C. This transformation process becomes more sig-
&Kerogen
and 104. FeS2
Kerogen
100
200
300 LOO 500
600
700
Temperature ("C) Figure 4. Temperature dependence of radical concentration of Hamayuchi kerogen no. 5 in the presence and absence of FeS2.
Wave number (cni')
Figure 5.
FT-IRspectra of Hamayuchi kerogens no. 1 and 5.
order of activity in terms of increasing spin concentration is as follows: kerogen-30% FeSz system > kerogen-10% FeS2system > kerogen alone, clearly indicating the strong effect of FeS2 during the pyrolysis of kerogen. Also,
Py r r h o t i t e
Pyrite FcS2 1
1
-6
I
1
-4
-2
1
1
I
0.0 velocity ( m m d
Figure 6. Mhsbauer spectra of Hamayuchi kerogens no. 1 at 450 "C.
I
2
1
1
4
1
1
6
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Bakr et al.
444 Energy & Fuels, Vol. 5, No.3, 1991
Table I. Elemental Analysis (wt %) of L i p h and synthetic Kerogen (a) and Characteristic Data 02 Hamauuchi Kerogens (b) (a) Elemental Analysis sample C H N S O(diff) H/C o/c aah lignin 49.41 4.55 0 3.52 31.67 1.11 0.48 10.85 synthetic kerogen precursor 46.51 6.13 9.81 0.35 23.93 1.57 0.39 13.27 (b) Characteristic Data sample no. depth, m formation name geologic age H/C o/c R,,,w N,x 10*7a 1 904 Wakkanai F. middle Miocene 1.16 0.57 0.38 7.9 1.36 0.51 2 2498 Masuporo F. middle Miocene 9.3 Maauporo F. middle Miocene 1.23 0.25 0.54 12.3 3 3241 middle Miocene 1.15 0.16 31.8 4 4247 Onishibetau F. 1.08 0.11 0.62 35.8 5 4499 Onishibetau F. middle Miocene '
a
Spin concentration per gram of organic matter.
of radical concentration with FeSz may be attributed to the nascent sulfur produced during the process of conversion of pyrite to pyrrhotite. The pyritic sulfur is a strong hydrogen acceptor, where it may abstract hydrogen from kerogen via a free-radical process. Such a process of radical formation may take place as follows:
F e h =z (Pyrite)
Fel+S+S (pyrrhotite)
S + KH F! HS' + K' HS' + K H HZS + K'
-
where K is kerogen. The enhancement of free-radical formation via hydrogen transfer from a hydrogen donor (such as kerogen) to a hydrogen acceptor (as sulfur) is illustrated in Figure 7. Clearly, sulfur is acting as an enhancing agent for the free-radical formation in kerogen at the conditions so far tested.
100
200
300 400 500
600 700
Temperature
("C)
Figure 7. Temperature dependence of radical concentration of Synthetic kerogen precursor in the presence and absence of sulfur.
nificant as the temperature increases to 550 "C. Thus, it is evident that the large enhancement in radical concentration observed in the lignin-pyrite system (Figure l), synthetic kerogen precursor-pyrite system (Figure 21, and natural kerogen-pyrite systems (Figure 3) is consistent with the conversion of pyrite to pyrrhotite. The increase
Conclusions In situ ESR is a practical technique for characterization of pyrite activity during the kerogen maturation process. Based on the data presented in this study, pyrite acta indirectly as a catalytic agent via sulfur. Pyritic sulfur enhances the formation of free radicals in kerogen, representing the diagenesis stage. The catalytic effect of pyrrhotite on kerogen degradation cannot be ignored and is being investigated. Acknowledgment. This work was conducted at the Metals Research Institute, Faculty of Engineering, Hokkaido University, Japan. Registry No. Pyrite, 1309-36-0.
