petroleum geology. LIU Dayong & PENG Ping'an. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of. Geochemistry, Chinese Academy of ...
ARTICLES Chinese Science Bulletin 2006 Vol. 51 No. 17 2103—2109
DOI: 10.1007/s11434-006-2070-8
Pyrolysates of raw vitrinites and their residues after CS2-NMP solvent extraction and its significance for petroleum geology LIU Dayong & PENG Ping’an State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Correspondence should be addressed to Peng Ping’an (email: pinganp@ gig.ac.cn) Received November 11, 2005; accepted February 27, 2006
Abstract Binary solvent (CS2-NMP) has extreme high extraction ability to coals, and it can extract most bitumens out of coals and vitrinites. And large amount of messages on side chains and their distribution character in vitrinites should be acquired through flash pyrolysis before and after binary solvent (CS2-NMP) extraction. A few low maturated coals have been selected and vitrinites are handpicked from coals. Then vitrinites have been extracted using different solvents in the order of polarity. Flash pyrolysis-Gas Chromatography/Mass spectrum has been applied to samples. The result shows that CS2-NMP is efficient for the extraction of vitirnites, giving much higher extraction yield than common solvents. Production ratio of liquid hydrocarbons in pyrolysates of vitrinites extracted with CS2-NMP is lower than that of raw vitrinites. And relative ratio of each component in pyrolysates has changed apparently. Production ratio of aliphatic hydrocarbons, especially those long chain aliphatics have decreased much after mixed solvent extraction. It shows that bitumens extracted with CS2-NMP have largely contributed to pyrolysates, especially those aliphatics in pyrolysates. Keywords: vitrinite CS2-NMP solvent flash pyrolysis side chains distribution of alkane/alkenes.
Vitrinite is the main maceral in most huminites[1]. And it accounts for up to 50%―90% in coals of Jurassic Turpan-Hami Basin, with an average value of 70%[2]. And the character of vitrinite dominates the structure and hydrocarbon generation ability of coal[3]. Vitrinite is extremely heterogeneous and composed of a variety www.scichina.com
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of submacerals, mainly desmocollinite and tellocollinite[1]. Tellocollinite is gas-prone while some desmocollinite may generate liquid hydrocarbons besides gas hydrocarbons. Previous studies also showed that desmocollinite may be the dominant source rock in ― some coal-formed oil fields[2,4 6]. So the structure of vitrinite is one of the crucial factors for the exploration of coal-formed hydrocarbons. The structure of aliphatic carbons corresponds to variable hydrocarbon generation character. Oil-prone carbon, gas-prone carbon and aromatic carbon can be deconvoluted from 13C NMR spectra of coal samples. Aromatic carbon in coal and kerogen contributes little to hydrocarbon generation; methylene and methane in aliphatic structure carbons are the main matrix of oil; other aliphatic and carboxyl carbons are assigned as gas-prone carbon[7]. Wealth of information about the structure of side chains of organic matter has been provided through flash pyrolysis. Since pyrolysis is accomplished in extremely short time, secondary reaction is stressed to minimum extent[8]. Basically the pyrolysate may represent those side chains connected directly to macromolecule of organic matter. Kerogen type has been classified according to relative proportion of aliphatic hydrocarbon, aromatic hydrocarbon and phenols in pyrolysates[9]. Based on this, the capacity and the character of hydrocarbon generation of kerogens have been assessed. Also, coal-formed hydrocarbons may be distinguished according to relative proportion of aliphatics with different number of carbons in pyrolysates[10]. And this method has been applied to the study of coal-formed hydrocarbons in several coal-bearing basins in China[11]. Binary solvent is composed of carbon disulfide (CS2) and N-methylpyrrolidone (NMP) at a 1:1 volume ratio. The mixed solvent provides the highest extractability for a variety of coals[12,13]. The mixed solvent neither destroys the chemical structure of organic matters, nor causes the degradation of the macromolecule of organic matter. And CS2-NMP solvent has been found to give high extraction yield to coal by breaking down the linkage of noncovalent bonds, such as H-H bond and π-π bond, and the mixed solvent is called “super solvent”. Noncovalent bonds are more abundant than covalent bonds, and the amount of noncovalent bonds and covalent bonds differ by the order of magnitude. Noncovalent bonds can be broken in lower thermodynamic conditions, and the extraction yield is more than 50% 2103
ARTICLES for some bituminous coals at room temperature using specified solvents. On this basis, researchers stretched out the view that coals have physically associated structures. It means that the macromolecule of coal is composed of kerogen that cannot be dissolved by any solvent and bitumens associated with kerogen tightly with noncovalent bonds[14]. Pyrolysates of vitrinite residues after CS2-NMP solvent extraction are mainly derived from cleavaged side chains of crosslinked molecule of kerogen. So information about distribution of side chains of macromolecule of vitrinite may be acquired through comparison of pyrolysates, especially the distribution of n-alkane/alkenes in pyrolysates, of raw vitrinites and their residues after mixed solvent extraction. And more detailed information may be gained about the contribution of vitrinites with different structures to coal-formed hydrocarbons. 1
Sample and experiment
1.1
Sample
Vitrinite is the main maceral in coal, and purification of vitrinite is conducted with hand picked method. Sample selection in this study is based on the study of microscope study of a great variety of vitrinites in different areas and maturities. Intensive basic study shows that vitrinite samples are of high purity and distinct character[15]. Microscope study shows that vitrinite purity in all samples is up to 95%. Jurassic vitrinites used in this study have low maturity level and relatively high hydrogen content, being mostly composed of desmocollinite and telocollinite, and vitrinite selected in Ordos Upper Carboniferous Basin in this study has higher maturity level compared with other vitrinites (Table 1). 1.2
Experiment
Vitrinites were extracted with dichloromethane, MAB (methanol/acetone/benzene = 2:3:5 v/v/v) and CS2-NMP mixed solvent (carbon disulfide/N-methylpyrrolidone) in sequence to effectively separate differ-
ent parts of vitrinites. Soxlet method was used in extraction of vitrinites with dichloromethane and MAB for 72 h in sequence. Since CS2-NMP mixed solvent has extremely high boiling point, the extraction is carried out ultrasonically. CS2 and NMP used in extraction are analytical purity reagents, and are not further purified. Centrifugal tubes containing about 1g samples and 50 mL CS2-NMP mixed solvent are sealed with lid, and ultrasonically extracted for 30min. After centrifugation at 10000 rpm for 10min the supernatant is separated by decantation. Fresh mixed solvent is added to the residue, which is again extracted ustrasonically for 30min. These procedures are repeated until the color of the supernatant shows no change. The supernatant was filtered and rotatory was evaporated below 50℃ to get rid of CS2. And then the filtrate was acidified with 2N aqueous HCl, and the precipitate extract was separated using a membrane filter. The solid extract was washed with acetone ultrasonically for three times to remove possibly existed NMP, and then dried in vacuo at 60℃ until the weight of the residue had no change. Flash pyrolysis-Gas Chromatography/Mass Spectra (Py-GC/MS) were applied to vitrinites and their CS2-NMP solvent extracted residues. Flash pyrolysis was accomplished on CDS2000 pyrolyzer, which was directly connected with the inlet of Finnigan-GC8000TOP GC/MS. Sample (about 0.5 mg) was placed in a quartz tube, which was seated within the heating coils of the PyrprobeTM. Known amount of Polyα-methylstyrene in dichloromethane was injected into quartz tube as standard for quantitative computation. The coils were rapidly heated (105℃/s) to 600℃, with this temperature maintaining for 10s. A helium sweep was used to carry the volatiles from the pyroprobe directly into the injection port of the GC/MS. The GC temperature program begins at 35℃, maintaining for 5min, then was heated to 300℃ at 3℃/min. Spectra peaks are identified using masslab software. Py-GC/MS experiment was repeated twice or more to ensure the reliability of data.
Table 1 Basic data of samples Sample Zhunji-1
Horizon
Descriptiona)
RO(%)
H/C
Junggar Basin
J2 x
DC(90%), TC(3%), L(1%), I(6%)
0.473
1.01
Area
Zhunji-3
Junggar Basin
J2 x
DC(95%), TC(3%), I(2%)
0.502
0.91
Tujun-1
Turpan-Hami Basin
J2x
TC(99%), I(1%)
0.378
0.95
Eji-2
Ordos Basin
C2 t
DC(80%), TC(15%), L(2%), I(3%)
0.650
1.00
a) DC, Desmocollinite；TC, telocollinite；L, liptinite；I, inertinite.
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ARTICLES Classification and quantification of compounds in pyrolysates were accomplished, and production ratio of C7+ hydrocarbons (Fig. 1), relative proportion of C7+ n-alkane/alkenes, aromatics (C1―C3 alkylbenzenes, alkylnaphthalenes, alkylanthrathene/phenanthrenes) and phenols (Fig. 2) were calculated respectively according to methods of Hartgers [16].
lignite has looser structure. Extraction ratio of CS2-NMP mixed solvent to vitrinite is greatly improved (Table 2). Thus it can be seen that considerable bitumen are bonded to crosslinked macromolecule through non-covalent bonds in the structure of lignite. Due to the low maturation level, CS2-NMP mixed solvent ratio of samples in this study is apparently lower than that of charred coal in Zaozhuang, China[12]. And extraction ratio of each solvent to samples is increasing with maturation level (Table 2). It seems that the proportion of bitumen bonded to crosslinked macromolecules through non-covalent bonds is increasing with maturation level. 2.2 Character of pyrolysates of vitrinites and CS2-NMP mixed solvent extracted residues
Fig. 1. Production ratio of vitrinite pyrolysates before and after CS2-NMP extraction.
2 2.1
Result and discussion Solvent extraction ratio of vitrinites
Previous study shows that the extraction capacity of CS2-NMP solvent relies on the maturation level of coal. CS2-NMP extraction ratio of coal firstly increases with maturation level from the stage of lignite to bituminous coal and then decreases from the stage of bituminous coal to anthracite. Bridged bonds, such as olefinic bond, ether bond and ester bond, are more abundant in the structure of lignite than bituminous coal[17], and solvent extraction ratio of lignite is apparently lower than bituminous coal due to the effect of bridged bonds though
Pyrolysates’ character of vitrinite is dominated by its precursor, sedimentary environment and maturation ― level[18 20]. Desmocollinite may have received relative abundant input of detritus of lipid in deposition stage, and may have rich aliphatic side chains. Pure telocollinite is mainly derived from lignins of high plant, and contains large proportion of methyoxyl phenols and little proportion of aliphatic side chains. Production ratio of pyrolysates and aliphatic hydrocarbons decreased and relative proportion of aromatic hydrocarbons increased with increasing maturation level[15]. Zhunji-1 and Zhunji-3 desmocollinites are in their low maturation level, and proportion of aliphatic hydrocarbons in pyrolysates, such as n-alkane/alkenes, are relatively abundant. Desmocollinite is the main com-
Fig. 2. Production ratios of main components in pyrolysates before and after CS2-NMP extraction ratio. Alka: n-alkanes; Alke: n-alkenes; AB: alkylbenzenes; AP: alkylphenols; AN: alkylnaphthalenes; AN/P: alkylanthracene/phenanthrane.
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ARTICLES Table 2 CS2-NMP solvent extraction ratio of samples Sample Tujun-1 Zhunji-1 Zhunji-3 Dichloromethane extraction ratio 0.675 0.718 0.979 (%) a) MAB solvent extraction ratio (%) 3.945 2.812 4.119 17.41 20.9 CS2-NMP extraction ratio (%) −
Eji-2 1.589
5.397 29.7 a) MAB solvent is composed of methanol: acetone: benzene = 2:3:5 (v/v/v).
ponent in Eji-2 vitrinite in Ordos Basin, and the character of pyrolysates of Eji-2 is similar to that of Zhunji-3 in Junggar Basin. But bicyclic and tricyclic aromatic hydrocarbons are richer in pyrolysate of Eji-2 for its higher maturation level. Pyrolysate of Tujun-1 telocollinite is mostly composed of phenols, and aliphatic hydrocarbons are in low proportion. Production ratio of pyrolysates of CS2-NMP extracted vitrinite decreases apparently, and accounts for 60%―70% of that of vitrinite (Fig.1). It shows that production ratio of pyrolysates of macromolecule is apparently lower than that of bitumen. But the contribution of macromolecule to pyrolysate is nearly equal to that of bitumen since the proportion of macromolecule is much higher than that of bitumen. And the character of bitumen and its proportion in coal vary with maturation level. The content of original bitumen is decreasing with increasing maturation level, and it shows that original bitumen is the main contribution component of low maturated oils. The proportion of secondary bitumen in vitrinite shows positive correlation with maturation level (0.7
