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Energy & Fuels 2001, 15, 8-13
Identification of Organic Sulfur Compounds in Supercritical Extracts from Polish Lignite Graz˘ yna Gryglewicz* and Piotr Rutkowski Institute of Chemistry and Technology of Petroleum and Coal, Wrocław University of Technology, ul. Gdan´ ska 7/9, 50-344 Wrocław, Poland Received April 25, 2000. Revised Manuscript Received August 1, 2000
Supercritical fluid extraction (SFE) of a Bełchato´w lignite (Ro ) 0.26%) was carried out with four different solvents, i.e., a toluene, 2-propanol, toluene/2-propanol, and toluene/THF mixtures, at 360 °C and 10 MPa in an apparatus with continuous flow of solvent. The type of solvent has an influence not only on the extract yield but also on the sulfur extractability. For the 2-propanol/ toluene mixture the highest process yield was obtained, 43.1%. The toluene/THF mixture was shown to be the most effective solvent in terms of the extraction of organosulfur compounds (OSC). Gas chromatography-mass spectrometry (GC-MS) was used to study the distribution of OSC in the extract obtained on the SFE process. The results demonstrate that the preparation of a sample prior to GC-MS analysis has a great influence on the OSC distribution of the extract. For the raw extract with solvent, aliphatic and aromatic thiols, aliphatic sulfides, polysulfides, and the C1-C4 alkylthiophenes were detected, whereas only diphenyl sulfide, benzothiophene, and dibenzothiophene were identified after solvent removal by evaporating and final drying of the sample in a vacuum oven. Methyl thiol and methyl mono-, di-, and trisulfides seem to be originally present in the lignite. The presence of thiol and mono-, di-, and trisulfides substituted with an isopropyl group in the extract can be explained as due to the reaction of elemental sulfur with the 2-propanol used for the extraction
Introduction A significantly higher extract yield than that produced by Soxhlet extraction can be obtained by extracting coal with organic solvents in supercritical states (SFE).1 This is highly relevant for the study of organic sulfur-containing compounds (OSC) which are present in low concentrations in coals. The coal extract contains also a large number of different compounds in much higher concentrations, which tend to interfere in the identification and quantification of OSC. The OSC in coal extracts or pyrolysis products have been investigated by means of different analytical methods. High-resolution mass spectrometry (HRMS) is widely applied to whole extracts.2-4 In most cases, usually after fractionation to concentrate OSC, gas chromatography equipped with different detection systemss flame ionization detection (FID), flame photometric detection (FPD), atomic emission detection (AED), and mass spectrometry (MS)sis used to analyze extracts.5-8 Although only the volatile part of the extract * Corresponding author. E-mail:
[email protected]. WROC.PL (1) Vahrman, M. Fuel 1970, 49, 5-16. (2) Palmer, S. R.; Kruge, M. A.; Hippo, E. J.; Crelling, J. C. Fuel 1994, 73, 1167-1172. (3) White, C. M.; Collins, L. W.; Veloski, G. A.; Irdi, G. A.; Rothenberger, K. S. Energy Fuels 1994, 8, 155-171. (4) Sinninghe Damste, J. S.; White, C. M.; Green, J. B.; de Leeuw, J. W. Energy Fuels 1999, 13, 728-738. (5) Bartle, K. D.; Martin, T. G.; Williams, D. F. Fuel 1975, 54, 226236. (6) Andersson, J. T.; Schmid, B. J. Chromatogr., A 1995, 693, 325338.
may be analyzed using the chromatographic technique, the latter is very useful for identifying OSC. To facilitate gas chromatography and mass spectrometry identification, methods of isolating the fraction of coal liquids enriched with a given class of OSC have been developed. Identification of polycyclic aromatic sulfur heterocycles (PASH) is usually greatly facilitated if PASH are isolated from polycyclic aromatic hydrocarbons (PAH) by means of ligand exchange chromatography with PdCl2.6,9,10 Due to their low stability and thermal sensitivity, the separation of aliphatic sulfur compounds from the hydrocarbons and others components in coal-derived products and the subsequent identification of them by gas chromatography is much more difficult.11,12 To our knowledge, little information concerning the exact nature of the aliphatic sulfur compounds in coal is known. The aliphatic OSC are thought to have a sizable share in low maturated coals.13 There are much more reports on the molecular structures of aromatic sulfur compounds, particularly thiophene derivatives.3,7-9,14,15 However, knowledge about organi(7) White, C. M.; Douglas, L. J.; Perry, M. B.; Schmidt, C. E. Energy Fuels 1987, 1, 222-226. (8) White, C. M.; Lee, M. L. Geochim. Cosmochim. Acta 1980, 44, 1825-1832. (9) Nishioka, M.; Lee, M. L.; Castle, R. N. Fuel 1986, 65, 390-395. (10) Nishioka, M. Energy Fuels 1988, 2, 214-219. (11) Calkins, W. H. Fuel 1994, 73, 475-484. (12) Davidson, R. Organic sulphur in coal, IEA Coal Research, 1993. (13) Maes, I. I.; Gryglewicz, G.; Machnikowska, H.; Yperman, J.; Franco, D. V.; Mullens, J.; van Poucke, L. C. Fuel 1997, 76, 391-396. (14) White, C. M.; Douglas, L. J.; Anderson, R. R.; Schmidt, C. E.; Gray, R. J. ACS Symp. Ser. 1990, 429, 261-286.
10.1021/ef000085p CCC: $20.00 © 2001 American Chemical Society Published on Web 11/11/2000
Organic Sulfur Compounds in Polish Lignite Extracts
Energy & Fuels, Vol. 15, No. 1, 2001 9
Table 1. Analysis of Bełchato´ w Lignite proximate analysis (wt %) moisture ash (db) volatile matter (daf) ultimate analysis (wt %) C (daf) H (daf) N (daf) S (db) Odiff (daf)a sulfur forms (wt %, db) organic sulfatic pyritic elemental a
29.4 23.9 54.5 61.5 5.3 1.3 8.5 28.2 3.74 0.59 4.08 0.09
Odiff(daf) ) 100 - (Cdaf + Hdaf + Ndaf + Sodb).
cally bound sulfur in coal on the molecular level is still relatively skimpy. Mild conditions of the SFE process involve slight changes in the structure of the extractable material.5,16 Thus the SFE is quite often used as a coal structure investigation procedure, in our case to gain knowledge of the structure of organosulfur compounds. It has been reported that methyl polysulfides are present in the supercritical methanol extract of coals.17,18 However, the methyl polysulfides can be also formed via the reaction of methanol with elemental sulfur as has been found by White et al.19 Therefore, in our work attention has been focused on the consequences resulting from the presence of elemental sulfur in the initial coal. Every coal sample, being not well protected against air, contains a small amount of elemental sulfur due to the atmospheric oxidation of pyrite.20 The main purpose of this investigation was to identify organic sulfur-containing compounds in extracts obtained from the supercritical fluid extraction of a Polish lignite with different solvents. Another goal was to determine the effectiveness of the solvent used in sulfur extraction. Also, the impact of the elemental sulfur present in the lignite on the distribution of OSC in the supercritically extracted material was to be assessed. Experimental Section Materials. Lignite was collected from an unexplored deposit of the Bełchato´w mine (Poland). The lignite is of Tertiary age. The sample was selected as being rich in organic sulfur according to geological documentation. Proximate, ultimate, and sulfur form analyses of the Bełchato´w lignite are presented in Table 1. Elemental Analysis. A Perkin-Elmer 2400 CHN elemental analyzer was used for CHN determinations. The total sulfur was determined by combusting the sample and absorbing the sulfur oxides in a hydrogen peroxide solution. The oxygen concentration was determined from the difference. Pyritic, sulfatic, and organic sulfur contents were determined using ASTM methods. Elemental sulfur was extracted from the (15) Sinninghe Damste, J. S.; de Leeuw, J. W. Fuel Process. Technol. 1992, 30, 109-178. (16) Kershaw, J. R. J. Supercrit. Fluids 1989, 2, 35-45. (17) Lee, S.; Fullerton, K. L. Fuel Sci. Technol. Int. 1992, 10, 11371159. (18) Muchmore, C. B.; Chen, J. W.; Kent, A. C.; Liszka, M. Proceedings of the International Conference on Coal Science; New Energy and Industrial Development Organization: Tokyo, 1989; pp 193-196. (19) White, C. M.; Rohar, P. C.; Shaw, L. J.; Collins, L. W. Energy Fuels 1996, 10, 1187-1188.
Figure 1. Apparatus for coal extraction with supercritical solvent. 1, solvent reservoir; 2, solvent pump; 3, reactor heater; 4, extraction vessel; 5, valves; 6, cooler; 7, product collector lignite with cyclohexane in the Soxhlet apparatus for 6 h and analyzed by GC-MS. Apparatus and Extraction Procedure. A series of SFE runs were carried out with four different solvents, i.e., toluene, 2-propanol, toluene/2-propanol mixture (50:50; v/v), and toluene/ tetrahydrofuran mixture (90:10 v/v). All solvents used were pure p.a. For the toluene/2-propanol mixture, the pseudocritical temperature and pressure are 275 °C and 4.8 MPa, for the toluene/THF mixture, 312 °C and 4.3 MPa. These parameters were calculated according to Kay’s method.21 Extraction was carried out in an apparatus with flowing solvent at 360 °C and under a pressure of 10 MPa (Figure 1). The constant solvent flow rate was 0.6 dm3 h-1. The reactor was charged with 10 g of lignite with a grain size range of 0.315-0.5 mm. The extracted material was continuously removed from the extraction cell, cooled, and collected in a collector which was filled with argon. The extraction process was continued until the condensate was clear. The extraction yield was determined after solvent removal in a rotary evaporator followed by drying in a vacuum oven at 60 °C till a nearly constant weight, and was calculated on a dry and ash-free basis. Extraction was carried out in duplicate. The extracts were subsequently analyzed by GCMS and characterized by elemental composition. The extract samples were stored in an inert gas atmosphere. Reaction of Elemental Sulfur with Solvent. To check if polysulfides detected in the Bełchato´w extracts could have been formed during the extraction process, 0.2 g of elemental (20) Stock, L. M.; Wolny, R.; Bal, B. Energy Fuels 1989, 3, 651661. (21) Kay, W. B. Ind. Eng. Chem. 1936, 28, 1936.
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Energy & Fuels, Vol. 15, No. 1, 2001
Gryglewicz and Rutkowski
Table 2. Extraction Yield and Ultimate Analysis of the Extracts Obtained from Bełchato´ w Lignite on SFE with Different Solventsa Bse-T Bse-iP Bse-TiP Bse-TTHF ultimate analysis (wt %) C H N S Odiff extraction yield (wt % daf)
73.0 6.8 0.4 3.3 16.5 14.5
72.3 7.2 0.5 2.6 17.4 19.3
75.1 6.8 0.5 2.6 15.0 43.1
72.9 6.4 0.6 3.5 16.6 35.0
a Bse represents the supercritical extract obtained from Bełchato´w lignite with the following: T, toluene; iP, 2-propanol;TiP, toluene/2-propanol mixture; TTHF, toluene/tetrahydrofuran mixture
sulfur was placed in a SFE reactor and extracted with 2-propanol for 1 h in the same conditions as in the case of the lignite. The collected condensate was analyzed by GC-MS in conditions identical to those used to generate the chromatogram of the supercritical coal extract. Elemental sulfur was also extracted with toluene under supercritical conditions. Additionally, 2 mg of elemental sulfur and 0.02 mL of 2-propanol were placed in small glass tubes (60 mm long, 3 mm i.d.) and sealed after pumping the air out. The duplicate tubes were heated in an oven set at 200, 250, and 275 °C for 5 h. Then the tubes were cooled in a refrigerator. Finally, the reaction mixture was dissolved in 2-propanol and analyzed by GC-MS. Gas Chromatography-Mass Spectrometry Analysis. Organic sulfur compounds in the extracts were identified using an HP 6890 gas chromatograph equipped with an HP 5973 mass selective detector. An HP-5 capillary column (30 m × 0.25 mm i.d., 0.25 µm film thickness, cross-linked 5% PH ME siloxane) was used with He as the carrier gas. The column temperature was programmed from 40 to 250 °C at 10 °C/min after an initial 2 min isothermal period and kept at the final temperature for 10 min. The inlet was set at 250 °C. Sample injection was made in the split mode (1:20). The mass spectrometer was set at an ionizing voltage of 70 eV with mass range m/z 15-400 and cycle time 1.8 s. The identification of OSC was accomplished by comparing GC retention times and mass spectra of the resolved components using computerized library search routines. For each extract two GC-MS analyses were performed. The first analysis was made for the condensate just after the extraction process. The second one was made for the extract after solvent removal according to the procedure described above. For the GC-MS analysis the extract was prepared as a 0.1 vol % solution in the solvent used for SFE.
Results and Discussion Ultimate Analysis of Extracts. The results of the ultimate analysis of the extracts obtained from the Bełchato´w lignite by SFE with different solvents are presented in Table 2. A comparison of the elemental composition of the starting lignite (Table 1) and that of its supercritical extracts shows a marked increase of carbon and hydrogen contents followed by a decrease in the oxygen content in the extracts. The much lower oxygen content in the extract indicates that during SFE the oxygen-containing compounds in the lignite seem to be sparingly soluble and some of them are decomposed to form gaseous products, e.g., carboxylic groups evolve CO and CO2 during heat treatment. The highest sulfur content in the extract is observed when the toluene/THF mixture was used for the extraction process. It was reported earlier12,22 that THF is a very effective solvent for the extraction of OSC.
Extractability of Lignite under Supercritical Conditions. The extraction yield for different solvents is shown in Table 2. The results indicate that the highest yield of lignite extraction can be obtained when a mixture of solvents is used. For the toluene and 2-propanol extractions the process yield was 14.5% and 19.3%, respectively. It is considered that lignite may contain ester linkages.23 Thus the higher conversion for 2-propanol can be explained by ester bond scission by alcohol and the solubilization of the resulting fragments. The use of a mixture of toluene and 2-propanol gives an extraction yield as high as 43.1%. This is consistent with the data of Kershaw and Bagnell24 who reported that a mixture of hydrocarbon and polar solvents such as aliphatic alcohols is beneficial for the supercritical fluid extraction of brown coals. The enhanced ability of the mixture of solvents to dissolve the coal matrix is probably related to the synergistic effect which the solvents may have on coal during the SFE process. Taking into account the extraction yield level and the sulfur content of the extract it can be concluded that the 10% vol THF/toluene mixture is the most effective solvent for the extraction of sulfur containing compounds under supercritical conditions. It should be noted that in our work the SFE experiments were performed under flowing conditions which limit retrograde reactions leading to the reincorporation of the extracted sulfur compounds into the coal matrix.25 Influence of Solvent Removal on Organosulfur Compound Distribution in Extract. This part of the present work resulted from the failure of our attempt to identify aliphatic sulfur compounds which should be present in the obtained extracts in view of the overall knowledge about the chemical structure of the lignites themselves. In many works7,14,15 it was reported that the difficulty in identifying this group of OSC arises from the fact that they are thermally unstable. In general, no evidence has been provided for the presence of aliphatic thiols and disulfides in coal when an investigation consists of the analysis of solvent extracts by chromatographic techniques. On the other hand, a statement that aliphatic OSC are not being extracted with a solvent during extraction is rather less reliable. Moreover, according to the model of the macromolecular constitution of coal26,27 low-molecular-weight aliphatic OSC should be preferentially extracted from the micropore structure, whereas macromolecular network is untouched. Damste et al.28 identified dimethyl tetra-, tri-, and disulfide in a Spanish brown coal by flash pyrolysis-gas chromatography-mass spectrometry, but these results were weakened by White et al.19 The commonly used post-extraction procedure includes solvent removal from the extract in an evaporator and drying the extract in a vacuum oven at 50-70 °C for 12-24 h. We have found that the above procedure (22) Riley, J. T.; Zhu Mingshe; Coffey, D. S.; Sadler, S. G.; Stidam, J. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36 (2), 820829. (23) van Krevelen, D. W. Coal, Typology-Physics-Chemistry-Constitution; Elsevier: Amsterdam, 1993. (24) Kershaw, J. R.; Bagnell, L. J. Fuel 1987, 66, 1739-1741. (25) Meffe, S.; Perkson, A.; Trass, O. Fuel 1996, 75, 25-30. (26) Larsen, J. W.; Kovac, J. Organic Chemistry of Coal; Larsen, J. W., Ed.; ACS Symp. Ser. No. 71, Washington, DC, 1978. (27) Marzec A. Fuel Process. Technol. 1986, 14, 39-46. (28) Sinninghe Damste, J. S.; de las Heras, F. X. C.; de Leeuw, J. W. J. Chromatogr. 1992, 607, 361-376.
Organic Sulfur Compounds in Polish Lignite Extracts
Figure 2. Total ion chromatogram of supercritical 2-propanol extract obtained from Bełchato´w lignite; (a) without solvent removal; (b) after solvent removal. Italic numerals (i.e., 6‚) refer to organic sulfur compounds identified in extracts and are listed in Table 3. Non-sulfur compounds: 1) phenol; 2 ) methyl phenol; 3 ) hydroxy anisole; 4 ) ethyl phenol; 5 ) methoxy methyl phenol; 6 ) isopropyl phenol; 7 ) dihydroxy benzene; 8 ) ethyl methoxy phenol; 9 ) methyl dihydroxy benzene; 10 ) hydroxymethylbenzene acetic acid; 11 ) ethyl methoxy phenol; 12 ) diphenyl ethane; 13 ) butyl hydroxy anisole; 14 ) diphenyl ethylene.
leads to a loss of a significant part of the volatile aliphatic and aromatic compounds, including OSC. Figure 2 shows a total ion chromatogram (TIC) for the condensate (a), i.e., extract dissolved in the whole amount of the solvent used for SFE extraction, and for the extract (b), i.e., after solvent removal according to the classic procedure. It can be clearly seen that a dramatic change in the extract composition was caused by the solvent removal procedure. The GC-MS data indicate that alkyl sulfides, diphenyl ethane, diphenyl ethylene, and most phenols were lost. Sinninghe Damste et al.4 in their recent publication concluded that compounds with a boiling point
