Organosulfur Compounds in Sulfur-Rich RaÅ¡a Coal - American

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Energy & Fuels 1999, 13, 728-738

Organosulfur Compounds in Sulfur-Rich Rasˇ a Coal Jaap S. Sinninghe Damste´,*,†,‡ Curt M. White,§ John B. Green,| and Jan W. de Leeuw†,‡ Department of Geochemistry, Institute of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands, Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands, Federal Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, and BDM Petroleum Technologies, P.O. Box 2543, Bartlesville, Oklahoma 74005 Received November 2, 1998. Revised Manuscript Received February 11, 1999

The organosulfur compounds in the extract and pyrolysate of the unusually organic sulfurrich (11.4 wt %) Upper Palaeocene Rasˇa coal have been identified by gas chromatography-highresolution mass spectrometry. The major organosulfur compounds (OSC) present in the extract are alkylated benzo[b]- and dibenzothiophenes and in the pyrolysates alkylated thiophenes and benzo[b]thiophenes. In addition, a large suite of sulfur-containing polyaromatics were identified, which sometimes contain more than one sulfur atom per molecule. The degree of alkylation of many homologous series was found to maximize at either three, four, or five alkyl carbons. The dominance of polyaromatic sulfur compounds is consistent with the relatively mature stage of the coal (Ro ≈ 0.7%), and their abundance at this rank indicates that the initial peat must have been extremely organic sulfur-rich (atomic Sorg/C ratio ≈ 0.15). This together with the very low abundance of lignin-derived components in the coal pyrolysate indicates that Rasˇa coal should not be considered a typical coal. Nevertheless, our results represent a starting point that can be used as a guide for analysis of other coals.

Introduction The characterization of organically bound sulfur in fossil fuels has been and still is a major topic of interest, both from the point of view of environmental concerns with respect to the application of sulfur-rich fossil fuels as well as for its geochemical significance.1-5 The concentration of organically bound sulfur in fossil fuels is often relatively low, which makes it difficult to determine the structures of organosulfur compounds (OSC) in low-molecular-weight fractions or pyrolysates of high-molecular-weight fractions due to the high concentrations of non-sulfur compounds. One way to overcome this problem is to study “end-members”, i.e., samples which contain much higher concentrations of organically bound sulfur than average. An example of this approach is the study of the Rozel Point oil, a crude oil which contains ca. 7.5 wt % organic sulfur.6 The very * To whom correspondence should be addressed. [email protected]. † Utrecht University. ‡ Netherlands Institute for Sea Research. § Federal Energy Technology Center. | BDM Petroleum Technologies. (1) Stock, L. M.; Wolny, R.; Bal, B. Energy Fuels 1989, 3, 651-661. (2) Sinninghe Damste´, J. S.; de Leeuw, J. W. Org. Geochem. 1990, 16, 1077-1101. (3) Orr, W. L.; Sinninghe Damste´, J. S. ACS Symp. Ser. 1990, 429, 2-29. (4) Sinninghe Damste´, J. S.; de Leeuw, J. W. Fuel Process. Technol. 1992, 30, 109-178. (5) Vairavamurty, M. A.; Orr, W. L.; Manowitz, B. ACS Symp. Ser. 1995, 612, 1-14. (6) Sinninghe Damste´, J. S.; de Leeuw, J. W.; Kock-van Dalen, A. C.; de Zeeuw, M. A.; de Lange, F.; Rijpstra, W. I. C.; Schenck, P. A. Geochim. Cosmochim Acta 1987, 51, 2369-2391.

high amount of sulfur in this oil made it possible to identify ca. 1000 novel OSC, unidentified previously. Subsequently, these compounds were also identified in less sulfur-rich samples.2 Since the knowledge about organically bound sulfur on a molecular level in coal is relatively scant,1,4 the success of this end-member approach prompted us to study the OSC of a very organic sulfur-rich coal. Rasˇa coal, which contains 11.4 wt % organic sulfur,7 was considered to be an excellent choice in this respect. The Upper Palaeocene Rasˇa coal is from Istria (Slovenia). It is a high-volatile bituminous coal with a mean vitrinite reflectance (R0) of 0.68% (ref 7). The dominating maceral is vitrinite with some exinite or liptinite and very little inertinite.7 Elemental composition data are listed in Table 1 and reveal the unusually high amount of sulfur, dominated by organic sulfur.7 The aromaticity, as determined by solid-state NMR, of 65% (ref 7) is in agreement with the data of Kreulen8 who reported that 59% of the carbon of this coal is aromatic. Several workers have determined the organic sulfur speciation in Rasˇa coal. Kavcic9 reported that ca. 75% of the sulfur was thiophenic and 25% reacted with methyliodide and was thus considered to be non-thiophenic. Ignasiak et al.10 determined that ca. one-third of the organic sulfur in Rasˇa coal is present in sulfide linkages using two (7) White, C. M.; Douglas, L. J.; Anderson, R. R.; Schmidt, C. E.; Gray, R. J. ACS Symp. Ser. 1990, 429, 261-286. (8) Kreulen, D. J. W. Fuel 1952, 31, 462-467. (9) Kavcic, R. Bull. Sci. Yugosl. 1954, 48, 5809. (10) Ignasiak, B. S.; Fryer, J. F.; Jadernik, P. Fuel 1978, 578-584.

10.1021/ef980236c CCC: $18.00 © 1999 American Chemical Society Published on Web 03/31/1999

Organosulfur Compounds in Sulfur-Rich Rasˇ a Coal

Energy & Fuels, Vol. 13, No. 3, 1999 729

Table 1. Elemental Composition (daf) of Rasˇ a Coal and Its Solvent Extracta

whole coal coal extract a

C (wt %)

H (wt %)

O (wt %)

N (wt %)

Stot (wt %)

Sorg (wt %)

Ssulf (wt %)

Spyr (wt %)

H/C

80.23 78.42

5.21 5.80

1.54 3.58

1.23 1.66

11.79 10.98

11.44 10.98

0.02 0.00

0.33 0.00

0.78 0.89

atomic ratios O/C Sorg/C 0.014 0.034

0.053 0.053

N/C 0.013 0.018

Data from ref 7.

independent techniques. More recent studies using X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS)11-13 established that the Rasˇa coal contains 70 mol % thiophenic sulfur and 30 mol % sulfide sulfur, in agreement with a high-pressure temperature-programmed reduction study.14 Oxidation of this coal followed by subsequent determination of organic sulfur forms by XANES and XPS led to the speculation that about one-half of the so-called sulfide sulfur is actually thiol or disulfide sulfur.15 These latter findings are at variance with an earlier study,10 reporting chemical tests which ruled out the presence of thiols in Rasˇa coal. Brown et al.13 postulated that the 30% sulfide sulfur is mainly composed of aryl sulfides. This has been the subject of discussion16,17 since it was shown that upon pyrolysis Rasˇa coal evolves a significant fraction of its sulfur as hydrogen sulfide at relatively low temperatures,18-19 which is difficult to rationalize as originating from aryl sulfides. Few investigations have been performed to characterize organically bound sulfur in Rasˇa coal on a molecular level. White et al.7 have reported molecular formulas of OSC in a solvent extract of the Rasˇa coal using lowvoltage, high-resolution mass spectrometry (LVHRMS). Many families of OSC were detected, some of which contained more than one sulfur atom per molecule, but no exact structures could be determined. The degree of alkylation of these families of OSC was found to maximize at three, four, or five alkyl carbons. In the current study, the OSC of the Rasˇa coal were determined in both the extract and in the pyrolysate of the residue after extraction. This latter aspect is important since the residue contains ca. 75% of the total organic sulfur of the whole coal. Experimental Section Extraction and Fractionation. The ground coal (142.9 g) was mixed with an equal amount of Celite and extracted for 5 days in a Soxhlet apparatus using the pyridine/toluene azeotrope (22:78), which has a boiling point of 110 °C. The extract was filtered through a 10 µm Teflon filter. The filtrate was subjected to rotary evaporation to remove bulk solvent, (11) Keleman, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939 (12) Gorbaty, M. L.; George, G. N.; Keleman, S. R. Fuel 1990, 69, 945. (13) Brown, J. R.; Kasrai, M.; Bancroft, G. M.; Tan, K. H.; Chen , J-.M. Fuel 1992, 71, 649-653. (14) Mitchell, S. C.; Snape, C. E.; Garcia, R.; Ismail, K.; Bartle, K. D. Fuel 1994, 73, 1159-1166. (15) Gorbaty, M. L.; George, G. N.; Keleman, S. R. Fuel 1990, 69, 1065-1067. (16) Calkins, W. H.; Gorbaty, M. L.; Keleman, S. R. Fuel 1993, 72, 900. (17) Brown, J. R.; Kasrai, M.; Bancroft, G. M.; Tan, K. H.; Chen, J-.M. Fuel 1993, 72, 900-901 (18) Keleman, S. R.; Gorbaty, M. L.; George, G. N.; Kwiatek, P. J.; Sansone, M. Fuel 1991, 70, 396. (19) Calkins, W. H.; Torres-Ordonez, R. J.; Jung, B.; Gorbaty, M. L.; George, G. N.; Keleman, S. R. Energy Fuels 1992, 6, 411-413.

followed by further evaporation of solvent using a gentle stream of nitrogen over the warmed (50 °C) extract. Subsequently, the extract was placed in a vacuum oven at 50 °C until the extract reached nearly constant weight (yield 26.5 wt %). The residue of the extraction was also dried in a vacuum oven. The extract was dissolved in a mixture of THF/benzene/ ethanol (4.5:4.5:1), and an insoluble fraction precipitated (31 wt %). A neutral fraction was isolated (11 wt %) using nonaqueous ion-exchange liquid chromatography.20 The initial THF/benzene/ethanol-soluble portion was passed through a pair of anion and cation columns (70 × 1.5 cm). Material eluting through these columns was recovered via rotary evaporation under nitrogen and subsequently redissolved in a mixture (3:1) of cyclohexane/benzene. This cyclohexane/ benzene solution was subsequently passed through a fresh pair of anion and cation columns of the same dimensions, which had been preequilibrated with this same solvent mixture. Material eluting from this second pair of columns constituted the neutral fraction (11 wt % of the extract). Strong acid, strong base, weak acid, and weak base fractions were subsequently recovered under nitrogen from the respective ion-exchange resins as described elsewhere;20 however, these materials were not analyzed further in this work. Both nonaqueous ionexchange separations were carried out at 40 °C to improve sample solubility. The whole neutral fraction was fractionated into saturated hydrocarbon (1 wt % of neutrals) and neutral aromatic (82 wt % of neutrals) fractions using a combination of chargetransfer and adsorption HPLC techniques. Approximately 1 g of whole neutrals was dissolved into a 7:3 mixture of dichloromethane/hexane, filtered (3 wt % insoluble residue recovered), and charged onto a column (30 × 2.5 cm) packed with 2,4-dinitroanilinopropyl silica (DNAP).21-22 Saturates plus 1-ring aromatics were collected by eluting the column with 5 vol % dichloromethane in hexane in the forward direction until the detectors (UV and RI) indicated the onset for elution of g2-ring aromatics. The balance of the g2-ring aromatics were recovered via backflushing with 7:3 dichloromethane/hexane. The evaporated saturate/1-ring aromatic fraction was subsequently redissolved in hexane and reseparated on silica (30 × 2.5 cm dimensions, hexane eluent) to obtain the final saturated hydrocarbon fraction. The recovered l-ring aromatics from the silica separation were combined with the g2-ring aromatic fraction from the DNAP column to obtain the final neutral-aromatic fraction. Approximately 14 wt % of the initial neutrals was lost in the course of the work, presumably due to incomplete elution from the DNAP column. Both HPLC separations were carried out at 50 °C, largely in an attempt to improve sample solubility. Solvent removal was carried out under a nitrogen atmosphere to reduce degradation. Curie Point Pyrolysis Gas Chromatography. The whole coal and coal fractions were thermally degraded using a Curie point pyrolyzer and ferromagnetic wires with a Curie temperature of 610 °C. In addition, thermal extracts were generated using ferromagnetic wires with a Curie temperature of 358 (20) Green, J. B.; Hoff, R. J.; Woodward, P. W.; Stevens, L. L. Fuel 1984, 63, 1290-1301. (21) Grizzle, P. L.; Thomson, J. S. Anal. Chem. 1982, 54, 10711078. (22) Thomson, J. S.; Reynolds, J. W. Anal. Chem. 1984, 56, 24342441.

730 Energy & Fuels, Vol. 13, No. 3, 1999 °C. At this temperature, no significant cleavage of carboncarbon bonds occurs and only low-molecular-weight compounds are released from the sample investigated. The whole coal, extracted coal, and total extract were applied to the ferromagnetic wire by pressing the samples on the wire.23 The aromatic fraction was analyzed by applying a few droplets of a solution of the fraction in dichloromethane to the wire using a syringe and evaporating the solvent. The pyrolyzer was mounted on the injection port of a Varian 3700 gas chromatograph. Online separation of the flash pyrolysate was accomplished by using a fused silica capillary column (25 m × 0.32 mm i.d.) coated with CP Sil-5 CB (film thickness 0.40 µm). The oven of the gas chromatograph was temperature programmed from 0 °C, using a cryogenic unit, to 300 °C at 3 °C min-1. The oven was first held at 0 °C for 5 min and finally at 300 °C for 15 min. Helium was used as the carrier gas. Pyrolysis products were detected by simultaneous flame ionization detection (FID) and sulfur-selective flame photometric detection (FPD) using a stream splitter at the end of the capillary column. Curie Point Pyrolysis Gas Chromatography-Mass Spectrometry. The Curie point pyrolyser (FOM-3LX) was directly connected to a gas chromatograph (Hewlett-Packard 5840) in tandem with a magnetic sector mass spectrometer (VG-70S) by direct insertion of the capillary column into the ion source. Gas chromatographic separation was performed as described above. The mass spectrometer was set at an ionizing voltage of 70 eV and operated at a cycle time of 1.8 s over the mass range m/z 40-800 at a resolution of 1000. Data acquisition was started 1 min after pyrolysis. Pyrolysis gas chromatography-high-resolution mass spectrometry (Py-GC-HRMS) was performed on a Hewlett-Packard 5890 gas chromatograph, also equipped with a Curie point FOM-3LX pyrolyser, interfaced to a VG Autospec Ultima Q mass spectrometer operated at 70 eV with a mass range m/z 120-400 and a cycle time of 2.2 s (resolution 6000). Gas chromatographic separation was performed as described above. Helium was used as the carrier gas.

Results

Sinninghe Damste´ et al.

Figure 1. Partial (0-90 min) FID (a) and FPD (b) chromatograms of the thermal extract of the whole Rasˇa coal generated by flash evaporation (358 °C, 10 s). The letters and numerals refer to Tables 2 and 3, respectively. Filled circles indicate n-alkanes. Italic numerals indicate the total number of carbon atoms. The FID chromatogram is normalized on 2-methylnaphthalene (compound R) and the FPD chromatogram on 2,3dimethylbenzo[b]thiophene.

Whole Coal Thermal Extract. Figure 1 shows the FID and FPD chromatograms of the thermal extract of the untreated Rasˇa coal. These were obtained by flash evaporation of the whole coal at 358 °C for 10 s. Compounds were identified by mass spectrometry. Letters and numbers in Figure 1 refer to identified compounds listed in Tables 2 and 3, respectively. The FID trace (Figure 1a) is dominated by aromatic hydrocarbons (i.e., alkylated benzenes and naphthalenes) and, to a lesser extent, aliphatic hydrocarbons (i.e. n-alkanes, branched and isoprenoid hydrocarbons). Sulfur compounds are also present and are dominated by alkylbenzo[b]thiophenes and, to a lesser extent, alkyldibenzothiophenes. Their distribution is exemplified by the FPD trace (Figure 1b). It should be noted, however, that the FPD has a quadratic response, which leads to an optical overestimation of the abundance of the dominant compounds (in this case the C2 alkylated benzo[b]thiophenes). At 358 °C, low-molecular-weight components rapidly volatilize from the coal while no cleavage of C-C bonds occurs,24 except for low-energy rearrangement processes such as, for example, the generation of prist-1-ene.24

Weaker bonds, such as S-S and C-S bonds, can be (partially) broken at these conditions25 and partly explain the presence of relatively high amounts of hydrogen sulfide (i.e., the gas peak in the FPD trace), which is formed from decomposition of organic (poly)sulfide moieties and, perhaps, pyrite. The presence of organic polysulfide moieties in the coal matrix is deemed unlikely, however, since they are preferentially lost during the earliest stages of thermal maturation (i.e., at R0 ) 0.3-0.4).26 The Rasˇa coal is significantly more mature. The thermal extract of the solvent-extracted coal (see below), apart from hydrogen sulfide, did not contain any OSC, indicating, indeed, that all C2+ components present in the thermal extract are indeed solvent extractable and do not originate from C-C or C-S bond cleavage in the macromolecular matrix of the coal. GC-MS was used to study the distribution of compound classes in the thermal extract in more detail. The mass chromatogram of m/z 57 (Figure 2) reveals that, apart from straight-chain acyclic hydrocarbons, the thermal extract contains quite abundant C8-C20 regular isoprenoid hydrocarbons. It is noteworthy that the isoprenoid C20 hydrocarbon (phytane) is more abundant

(23) Venema, A.; Veurink, J. J. Anal. Appl. Pyrol. 1985, 7, 207213. (24) Crisp, P. T.; Ellis, J.; de Leeuw, J. W.; Schenck, P. A. Anal. Chem. 1986, 58, 258-261.

(25) Sinninghe Damste´, J. S.; Eglinton, T. I.; Rijpstra, W. I. C.; de Leeuw, J. W. ACS Symp. Ser. 1990, 429, 486-528. (26) Koopmans, M. P.; de Leeuw, J. W.; Lewan, M. D.; Sinninghe Damste´, J. S. Org. Geochem. 1996, 25, 391-426.



Table 2. Non-sulfur Compounds Identified in Thermal Extracts and Flash Pyrolysatesa A B C D E F G H I J K L M a

benzene methylcyclohexane toluene 2-methylheptane 2,6-dimethylheptane ethylbenzene m- and p-xylene o-xylene 2,6-dimethyloctane 1,2,4-trimethylbenzene phenol 2,6-dimethylnonane o-methylphenol

N O P Q R S T U V W X Y

m- and p-methylphenol C2-alkylphenols naphthalene 2,6-dimethylundecane 2-methylnaphthalene 1-methylnaphthalene C2-alkylnaphthalenes cadalene (1-isopropyl-4,7-dimethylnaphthalene) norpristane (2,6,10-trimethylpentadecane) pristane (2,6,10,14-tetramethylpentadecane) prist-1-ene phytane (2,6,10,14-tetramethylhexadecane)

Letters refer to compounds in Figures 1, 6, 7, and 9. Table 3. Sulfur Compounds Identified in Thermal Extracts and Flash Pyrolysates 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

hydrogen sulfide thiophene 2-methylthiophene 3-methylthiophene 2-ethylthiophene 2,5-dimethylthiophene 3-ethylthiophene 2,4-dimethylthiophene 2,3-dimethylthiophene 3,4-dimethylthiophene 2-propylthiophene 2-ethyl-5-methylthiophene 2-ethyl-4-methylthiophene ethylmethylthiophene 2,3,5-trimethylthiophene 2,3,4-trimethylthiophene 3-isopropyl-2-methylthiophene 2-methyl-5-propylthiophene 2,5-diethylthiophene unknown sulfur compound 2-butylthiophene 2-ethyl-3,5-dimethylthiophene ethyldimethylthiophene 5-ethyl-2,3-dimethylthiophene ethyldimethylthiophene ethyldimethylthiophene

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Figure 2. Mass chromatogram of m/z 57 of the thermal extract of the whole Rasˇa coal generated by flash evaporation (358 °C, 10 s). n-Alkanes are indicated by filled circles, and their total number of carbon atoms are denoted by italic numerals. Regular isoprenoid alkanes are indicated by i-, followed by a numeral, indicating the total number of carbon atoms.

than its C19 counterpart (pristane) since, in general, coal extracts are characterized by high pristane/phytane ratios of 5-10 (ref 27).

2,3,4,5-tetramethylthiophene C5-thiophene 2-ethyl-5-propylthiophene C5-thiophene 2-butyl-5-methylthiophene 2-pentylthiophene dimethyl-5-and 2,3-propylthiophene 2-ethyl-3,4,5-trimethylthiophene benzo[b]thiophene C1-benzo[b]thiophenes 7-methylbenzo[b]thiophene 2-methylbenzo[b]thiophene 5- and 8-methylbenzo[b]thiophene 4- and 3-methylbenzo[b]thiophene C2-benzo[b]thiophenes 2- and 4-ethylbenzo[b]thiophene 2,6-dimethylbenzo[b]thiophene 2,4-dimethylbenzo[b]thiophene 2,3-dimethylbenzo[b]thiophene C3-benzo[b]thiophenes C4-benzo[b]thiophenes dibenzothiophene C1-dibenzothiophenes 4-methyldibenzothiophene 2- and 3-methyldibenzothiophene 1-methyldibenzothiophene C2-dibenzothiophenes

The distribution pattern of the most abundant compound class of OSC (Figure 1b), the alkylbenzo[b]thiophenes (I; see Appendix), is exemplified by a summed, accurate mass chromatogram of m/z 134 + 147 + 148 + 161 + 162 + 175 + 176 + 189 + 190 (Figure 3a). Since these ions are the major ions in the mass spectra of benzo[b]thiophene and its C1-C4 alkylated derivatives, this summed mass chromatogram provides a quantitative representation of the abundance of these compounds in the thermal extract of the Rasˇa coal. An accurate mass chromatogram was used to eliminate contributions from other compounds with mass spectra containing the same nominal masses but accurate masses outside the mass window used (see caption of Figure 3). The distribution pattern of the alkylbenzo[b]thiophenes is very complex, as expected from the large number of theoretically possible isomers for C0C4 alkylated benzo[b]thiophenes (i.e., 224 isomers). The C2- and, to a lesser extent, C3-alkylbenzo[b]thiophenes dominate in the thermal extract. Specific isomers were identified by coelution with authentic standards when available. The major C2-isomer is tentatively identified as 2,3-dimethylbenzo[b]thiophene. Py-GC-HRMS provided circumstantial evidence for their identification as alkylbenzo[b]thiophenes (Table 4). These results (27) Powell, T. G.; McKirdy, D. M. Nature 1973, 243, 37-39.

732 Energy & Fuels, Vol. 13, No. 3, 1999

Figure 3. Partial, summed, accurate (mass window 0.02 dalton) mass chromatogram of m/z 134.02 + 147.03 + 148.04 + 161.04 + 162.05 + 175.06 + 176.07 + 189.07 + 190.08 revealing the distribution of benzo[b]thiophene and its C1-C4 alkylated derivatives in (a) the thermal extract of the whole Rasˇa coal and (b) the flash pyrolysate (610 °C) of the solvent extracted Rasˇa coal. Numbers refer to compounds listed in Table 3.

confirm the findings of White et al.,7 who suggested the presence of C1-C6 alkylated benzo[b]thiophenes on the basis of low-voltage, high-resolution mass spectrometry (LVHRMS) analysis of the solvent extract of the Rasˇa coal. The presence of alkylated dibenzothiophenes (IIa) in the solvent extract of the Rasˇa coal7 was also confirmed by our results. Figure 4 shows accurate mass chromatograms of m/z 184, 198, 212, 226, and 240, revealing the distribution patterns of dibenzothiophene and its C1C4 alkylated derivatives and, possibly, C0-C4 alkylated naphthothiophenes (IIb). These latter components probably represent the smaller peaks in the mass chromatogram shown in Figure 4b. The three major peaks represent the four possible C1 methyldibenzothiophenes (of which two coelute). The other remaining smaller, not labeled peaks thus must represent other components with the same molecular formulas. Again, the clusters of isomers become increasingly more complex with increasing number of alkyl carbons (Figure 4). Py-GCHRMS confirmed their elemental composition (Table 4). Mass chromatography of m/z 234 + n14 (n ) 0-3) revealed the presence of benzonaphthothiophenes (IIIa), thienophenanthrenes (IIIb), thienoanthracenes (IIIc), and C1-C3 alkylated derivatives. Their elemental compositions were confirmed by GC-HRMS (Table 4). White et al.7 also suggested the presence of these components in the Rasˇa coal extract. The mass chromatogram of the parent components (m/z 234) already revealed the presence of at least four peaks, which can be explained since 18 isomers are theoretically possible. The clusters of higher homologues are accordingly complex. Less complicated are the mass chromatograms


(m/z 208 + n14, n ) 1-3) of another group of monosulfur components with four rings, the phenanthro[4,5-bcd]thiophenes (IV) also proposed to be present in the Rasˇa coal extract.7 Their elemental compositions were confirmed by GC-HRMS (Table 4). The mass chromatogram of the parent component shows only one peak. Various series of OSC containing two sulfur atoms per molecule were also tentatively identified. Small amounts of thienothiophenes (VIa and VIb) and bithiophenes (VIIa and VIIb) and their alkyl homologues were detected, and their elemental compositions were confirmed by GC-HRMS. A series of thienobenzo[b]thiophenes (e.g., VIIIa) was also detected by mass chromatography and GC-HRMS (Table 4). Figure 5 shows accurate mass chromatograms of m/z 240, 254, and 268. These chromatograms probably reveal the distribution of thienodibenzothiophenes (e.g., IXa) or thienonaphthothiophenes (e.g., IXb) with elemental composition C14H8S2 + nCH2 as confirmed by Py-GCHRMS (Table 4). White et al.7 identified a homologous series with the same elemental composition in the extract of the Rasˇa coal by LVHRMS analysis and suggested alkylthiophenanthro[4,5-bcd]thiophenes as possible stuctures. Since the mass spectra of the components revealed in Figure 5 showed significant doublecharged molecular ions (which is not expected in mass spectra of the compound type proposed by White et al.7), thienobenzothiophenes or thienonaphthothiophenes seem more logical since these components belong to the family of polycyclic aromatic hydrocarbons (PAH) which are known to produce substantial double-charged ions upon electron impact ionization.28 The presence of at least four components in the trace of the parent of the class of components (Figure 5a) is to be expected since nine theoretically possible isomers exist. Whole Coal Pyrolysate. Figure 6 shows the FID and FPD chromatograms of the flash pyrolysate of the untreated coal using a ferromagnetic wire with a Curie temperature of 610 °C. The pyrolysate obtained contains, in addition to compounds which are present as such and simply evaporate, components which result from thermal breakdown of the macromolecular fraction of the coal. The pyrolysate contains the same components as the thermal extract (i.e., aliphatic and aromatic hydrocarbons, benzo[b]- and dibenzothiophenes) and in addition a number of other compounds (e.g., n-alk-1enes, prist-1-ene, alkylphenols, alkylthiophenes), which results in an even more complicated FID chromatogram (e.g., cf. Figures 1a and 6a). These latter compounds must be pyrolysis products, but the presence of compounds in the thermal extract does not exclude the possibility that a significant amount of a compound in the pyrolysate is due to thermal degradation of the macromolecular matrix. This hypothesis is confirmed by changes in the composition of the products which are both present in the thermal extract as well as in the pyrolysate (e.g., cf. compounds B and C in Figures 1a and 6a). Sulfur compounds are somewhat more dominant in the pyrolysate compared to the thermal extract. More importantly, the distribution of the sulfur compounds has changed considerably (cf. Figures 1b and 6b). In addition to the alkylated benzo[b]thiophenes and (28) Lee, M. L.; Novotny, M. V.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic: London, 1981; 440 pp.



Table 4. HRMS of Organosulfur Compounds in Rasˇ a Coal Thermal Extract no. of alkyl carbons

mol formula

0 1 2 3 4

C8H6S C9H8S C10H10S C11H12S C12H14S

0 1 2 3 4


0 1 2 3

measured mol weighta

calcd mol weight

Benzo[b]thiophenes (I) 134.0185 134.0190 148.0349 148.0347 162.0495 162.0503 176.0652 176.0660 190.0804 190.0816 Dibenzothiophenes (IIa) and Naphthothiophenes (e.g., IIb) 184.0349 184.0347 198.0486 198.0470 212.0649 212.0660 226.0808 226.0816 240.0964 240.0973

∆M (mDa)

min no. of isomersb

0.5 0.2 0.8 0.8 1.2

1 4 10 20 30

0.2 -1.6 1.1 0.8 0.9

2 7 25 40 60

Benzonaphthothiophenes (e.g., IIIa), Thienophenanthrenes (e.g., IIIb), and Thienoanthracenes (e.g., IIIc) C16H10S 234.0504 234.0503 -0.1 C17H12S 248.0655 248.0660 0.5 C18H14S 262.0810 262.0816 0.6 C19H16S 276.0964 276.0973 0.9

4 8 25 40

0 1 2 3

Phenanthro[4,5-bcd]thiophenes (IVa) and Thienoacenaphthenes (e.g., IVb) C14H8S 208.0337 208.0347 1.0 C15H10S 222.0496 222.0503 0.7 236.0644 236.0660 1.6 C16H12S C17H14S 250.0795 250.0783 2.1

0 1 2 3 4


Phenylbenzothiophenes (e.g., V) 210.0504 210.0503 224.0655 224.0660 238.0812 238.0816 252.0970 252.0972 266.1121 266.1129

0.1 0.5 0.4 0.2 0.8

1 5 20 40 80

0 1 2

C6H4S2 C7H6S2 C8H8S2

Thienothiophenes (VIa and VIb) 139.9733 139.9754 153.9894 153.9911 168.0035 168.0067

2.1 1.7 3.2

2 3 5

0 1

C8H6S2 C9H8S2

Bithiophenes (VIIa and VIIb) 165.9908 165.9911 180.0058 180.0067

0.3 0.9

1 8

0 1 2 3 4 5 6

C10H6S2 C11H8S2 C12H10S2 C13H12S2 C14H14S2 C15H16S2 C16H18S2

Thienobenzo[b]thiophenes(e.g., VIII) 189.9905 189.9911 204.0046 204.0067 218.0199 218.0224 232.0396 232.0380 246.0528 246.0537 260.0683 260.0693 274.0845 274.0850

0.6 2.1 2.5 -1.6 0.9 1.0 0.5

5 15 20 20 30 50 80

0 1 2

Thienodibenzothiophenes (e.g., IXa) or Thienonaphthothiophenes (e.g., IXb) C14H8S2 240.0069 240.0067 -0.2 C15H10S2 254.0201 254.0224 2.3 C16H12S2 268.0404 268.0380 -2.4

4 12 35

1 8 16 40

a Weighted average over all chromatographic peaks with a mass window of 200 ppm. b As determined by the number of peaks in specific mass chromatograms (e.g., Figures 3-5).

dibenzothiophenes identified in the thermal extract, alkylthiophenes are quite important components in the flash pyrolysate. Furthermore, the distribution of the alkylbenzo[b]thiophenes is changed. 2-Methylbenzo[b]thiophene is the most dominant benzothiophene in the pyrolysate, while an unspecified C2-alkylated benzo[b]thiophene is the most abundant benzo[b]thiophene in the thermal extract (Figure 3). Extracted Coal Pyrolysate. The FID and FPD chromatograms of the flash pyrolysate of extracted coal using a ferromagnetic wire with a Curie temperature of 610 °C are shown in Figure 7. The pyrolysate contains the same suite of compounds present in the pyrolysate of the whole coal (cf. Figures 6a and 7a). The two major peaks labeled with an asterisk in the first part of the FID chromatogram are due to residues of solvents used to extract the coal. The residues were

incompletely removed by drying the extracted coal. The FID chromatogram (not shown) of the thermal extract (358 °C) of the extracted coal only revealed the presence of these two peaks, indicating that the extraction with pyridine/toluene has been extensive, as expected from the high extract yield (26.5 wt %). This experiment confirms that the compounds present in the thermal extract of the nonextracted, whole coal (Figure 1a) are only formed by thermal evaporation and not by C-C bond cleavage. The FPD chromatogram (Figure 7b) reveals the distribution of the sulfur compounds in the pyrolysate, which is dominated by the same compounds as the flash pyrolysate of the whole coal, although the alkylthiophenes are more abundant than in the pyrolysate of the whole coal (cf. Figures 6b and 7b). This can be explained by the fact that the pyrolysate of the whole coal also contains a contribution


Figure 4. Partial, accurate (mass window 0.02 dalton) mass chromatograms of m/z (a) 184.05, (b) 198.05, (c) 212.06, (d) 226.07, and (e) 240.10 revealing the distribution of dibenzothiophene and naphthothiophenes and their C1-C4 alkylated derivatives in the thermal extract of the whole Rasˇa coal generated by flash evaporation. Numbers refer to compounds listed in Table 3.


Figure 6. Partial (0-90 min) FID (a) and FPD (b) chromatograms of the flash pyrolysate (610 °C, 10 s) of the whole Rasˇa coal. For peak labeling, see the caption of Figure 1. n-Alk-1enes are indicated with filled squares. The FID chromatogram is normalized on toluene (compound C) and the FPD chromatogram on 2-methylbenzo[b]thiophene (compound 37).

Figure 5. Partial, accurate (mass window 0.02 dalton) mass chromatograms of m/z (a) 240.00, (b) 254.02, and (c) 268.04 revealing the distribution of thienyldibenzothiophenes (e.g., IXa) and thienylnaphthothiophenes (e.g., IXb) and their C1C2 alkylated derivatives in the thermal extract of the whole Rasˇa coal generated by flash evaporation.

The distribution of the C0-C5 alkylthiophenes is shown by a summed accurate mass chromatogram of m/z 84 + 97 + 98 + 111 + 112 + 125 + 126 + 139 + 140 + 153 + 154 (Figure 8). Since these m/z values are the dominant peaks in the mass spectra of the alkylthiophenes, this summed mass chromatogram provides a quantitative representation of the alkylthiophene distribution. The alkylthiophenes were identified by comparison with data reported previously.29 An accurate summed mass chromatogram of m/z 134 + 147 + 148 + 161 + 162 + 175 + 176 + 189 + 190 (Figure 3b) shows the distribution of the C0-C4 alkylated benzo[b]thiophenes exclusively generated by thermal breakdown of the macromolecular network. Comparison with the distribution of these compounds in the thermal extract (cf. Figures 3a and 3b) confirms the observation from the flash pyrolysis experiment with the whole coal that 2-methylbenzo[b]thiophene is mainly generated by pyrolysis. Furthermore, significant changes in the abundance of specific isomers in the C2 and C3 cluster can be noted. Interpretation of these differences is, however, hampered by the incomplete identification of isomers. GC-HRMS of the flash pyrolysate of the solvent extracted coal confirmed the presence of the same OSC as in the solvent extract (cf. Table 4). Solvent Extract. To compare the results of the solvent-extract of the coal with those obtained for the

from free alkylbenzo[b]thiophenes but not from alkylthiophenes (Figure 1b).

(29) Sinninghe Damste´, J. S.; Kock-van Dalen, A. C.; de Leeuw, J. W.; Schenck, P. A. J. Chromatogr. 1988, 435, 435-452.


Figure 7. Partial (0-90 min) FID (a) and FPD (b) chromatograms of the flash pyrolysate (610 °C, 10 s) of the solvent extracted Rasˇa coal. For peak labeling, see the caption of Figure 1. n-Alk-1-enes are indicated with filled squares. The FID chromatogram is normalized on the peak consisting of mand p-xylene and 2,5-dimethylthiophene (compounds G and 6) and the FPD chromatogram on 2,5-dimethylthiophene (compound 6). Peaks denoted by asterisks in the FID chromatogram indicate residues of solvents used for extraction of the coal.

Figure 8. Partial, summed, accurate (mass window 0.02 dalton) mass chromatogram of m/z 84.00 + 97.01 + 98.02 + 111.03 + 112.03 + 125.04 + 126.05 + 139.05 + 140.06 + 153.07 + 154.08 revealing the distribution of thiophene and its C1-C5 alkylated derivatives in the flash pyrolysate (610 °C, 10 s) of the solvent extracted Rasˇa coal. Numbers refer to compounds listed in Table 3.

whole coal and extracted coal, the coal extract was analyzed in a similar manner. Figure 9 shows the FID and FPD chromatograms of the thermal extract of the coal extract generated using a ferromagnetic wire with


Figure 9. Partial (0-90 min) FID (a) and FPD (b) chromatograms of the thermal extract of the solvent extract of the Rasˇa coal generated by flash evaporation (358 °C, 10 s). For peak labeling, see the caption of Figure 1. The FID chromatogram is normalized on the most abundant C2-alkylnaphthalene eluting in the cluster of peaks denoted by T + 40 and the FPD chromatogram on the most abundant C2-benzo[b]thiophene (2,3-dimethylbenzo[b]thiophene). Peaks denoted by asterisks in the FID chromatogram indicate residues of solvents used for extraction of the coal.

a Curie temperature of 358 °C. The FID trace is dominated by alkylated naphthalenes, alkylated benzo[b]thiophenes, alkylated dibenzothiophenes, and alkylated phenanthrenes. In comparison with the thermal extract of the whole coal (Figure 1a), the loss of lowmolecular-weight components is apparent. This is probably due to the prolonged time (300 h) in a vacuum at elevated temperature (50 °C) to remove the solvents from the extract, which is assumed to have led to extensive losses of low-molecular-weight components. The FPD chromatogram (Figure 9b) shows the distribution of sulfur compounds. In comparison with the thermal extract, an enrichment of the alkylated dibenzothiophenes relative to the alkylated benzothiophenes can be observed, which is also most probably due to evaporative losses during sample workup. The composition of the flash pyrolysate (610 °C) of the solvent extract of the Rasˇa coal is very similar to that of the thermal extract of the solvent extract, indicating that the solvent extract does not contain significant amounts of high-molecular-weight material from which pyrolysis products can be generated. For example, the FPD chromatogram (not shown) contains only traces of alkylthiophenes (cf. Figure 6b) and is still dominated by a 2,3-dimethylbenzo[b]thiophene instead of 2-methylbenzo[b]thiophene.



Figure 11. Gas chromatogram of the saturated hydrocarbon fraction of the solvent extract of the Rasˇa coal. Filled circles denote n-alkanes with italic numerals indicating their total number of carbon atoms. Key: a ) pristane, b ) phytane, c ) 17R-trinorhopane, d ) 17R,21β(H)-30-norhophane, e ) 17R,21β(H)-hopane.

Figure 10. Partial (0-90 min) FID (a) and FPD (b) chromatogram of the thermal extract of the aromatic fraction of the solvent extract of the Rasˇa coal generated by flash evaporation (358 °C, 10 s). For peak labeling, see the caption of Figure 1. The FID and FPD chromatograms are normalized on dibenzothiophene (compound 47).

Saturated Hydrocarbon and Neutral Aromatic Fractions. The thermal extract of the neutral aromatic fraction of the solvent extract of the Rasˇa coal is shown in Figure 10. It reveals a further loss of lower-molecularweight compounds since alkylated dibenzothiophenes dominate over alkylated benzo[b]thiophenes, whereas this is the opposite in the thermal extract of the whole coal. Both compound classes are expected to be present in the neutral aromatic fraction. Gas chromatography and gas chromatography-mass spectrometry of the saturated hydrocarbon fraction confirmed this further loss of volatile compounds: the FID chromatogram (Figure 11) contains no compounds

Organosulfur Compounds in Sulfur-Rich RaÅ¡a Coal - American

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