Lyophilization-Induced Structural Changes in Solvent-Swollen and

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Lyophilization-Induced Structural Changes in Solvent-Swollen and Supercritical Carbon Dioxide Treated Low-Rank Turkish Coals and Characterization of Their Extracts Serkan Bas¸ ,† Billur Sakintuna,† Burak Birkan,† Yusuf Menceloglu,† Alpay Taralp,† Zeki Aktas¸ ,‡ and Yuda Yu¨ru¨m*,† Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, Istanbul 34956, and Department of Chemical Engineering, Ankara University, Bes¸ evler, 06100 Ankara, Turkey Received November 18, 2004. Revised Manuscript Received February 17, 2005

In the present work lyophilization was employed to recover the readily volatile solvent fraction in previously impregnated, solvent-swollen low-rank Turkish coals as well as to potentially remove inherently volatile components of the coal matrix. Experiments were performed subsequently to assess if there existed a correlation between the conformational stability of swollen coal in certain solvents and the magnitude of lyophilization-induced structural changes. Lyophilization-induced alterations in the macroscopic and macromolecular structure of the coal and the effect of lyophilization on the structure of supercritical carbon dioxide extracts of the coal are reported in the present work. Freeze-dried samples were treated with supercritical carbon dioxide in a supercritical system at 50 bar and 80 °C. Then the soluble components accessible within raw samples and supercritically treated samples were digestively extracted in tetrahydrofuran, for 24 h at 20 °C. Extracts obtained were analyzed using a GC-MS system. Structural changes of the coal particles upon lyophilization were observed by SEM. Lyophilization seemed to increase the BET surface area of the coal samples. Lyophilization did not change the pore size distribution of the coal samples, but it mechanically reduced the particle size of the coal particles. In both types of coal, the amount of material that had extracted into THF after supercritical carbon dioxide treatment was greater in the case of samples that had been previously lyophilized.

Introduction Extensive knowledge based on coal structure is fundamental to comprehend the physical properties of coal and the chemistry of conversion processes. Inter- and intramolecular interactions have long been recognized as principle determinants of the overall physical and chemical properties of coal. One key experimental strategy to investigate inter- and intramolecular association forces has been based on solvent swelling and extraction. By way of this method, insight has been gained into the macromolecular network structure of coal. Effort1-6 has been invested to elucidate the correlation between the extent of swelling and parameters related to the macromolecular network. In particular, some noteworthy investigations have focused on better understanding the influence of parameters such as average molecular weight between cross-link points, * To whom correspondence should be addressed. Phone: 90-216483 9512. Fax: 90-216-483 9550. E-mail: [email protected]. † Sabanci University. ‡ Ankara University. (1) Sanada, Y.; Honda, H. Fuel 1966, 45, 295. (2) Kirov, N. Y.; O’Shea, J. M.; Sergeant, G. D. Fuel 1968, 47, 415. (3) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structures; Mayers, R. A., Ed.; Academic Press: New York, 1982. (4) Nelson, J. R. Fuel 1983, 62, 112. (5) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (6) Lucht, L. M.; Peppas, N. A. Fuel 1987, 66, 803.

using the theories of polymer physical chemistry.7 This readily observable fact, namely, solvent-induced swelling of coal, has long been related to the macromolecular structure of coal. In particular, the Flory-Rehner theory and variants thereof, which have been used to approximate the molecular weight between cross-link points,5,8-12 have been frequently utilized to associate the macromolecular network parameters with the degree of swelling in good solvents. The Flory-Rehner7 theory assumes that the deformation of the elementary chains of the network is closely related, even in proceeding down to the molecular level. Thus, it would follow to reason that any observed macroscopic deformation of a given sample should correlate linearly with a change in the statistical distribution of chain lengths of the coal macromolecule. In other words, the coal macromolecule would be anticipated to expand consistently on the segmental scale whenever the macroscopic swelling is related to molecular characteristics such as the cross-link density. (7) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (8) Sanada, Y.; Honda, H. Fuel 1966, 45, 295. (9) Kirov, N. Y.; O’Shea, J. M.; Sergeant, G. D. Fuel 1968, 47, 415. (10) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structures; Mayers, R. A., Ed.; Academic Press: New York, 1982. (11) Nelson, J. R. Fuel 1983, 62, 112. (12) Lucht, L. M.; Peppas, N. A. Fuel 1987, 66, 803.

10.1021/ef049706v CCC: $30.25 © 2005 American Chemical Society Published on Web 04/07/2005

Structural Changes in Low-Rank Turkish Coals

With due consideration of the types of potentially significant inter- and intramolecular interactions and their contributions to swelling behavior, some key points should be emphasized. In particular, (1) covalent crosslink bonds (>50 kcal/mol in strength) act as point interactions, (2) reversible cross-links can be dislocated at elevated temperatures or by suitable exchange reactions and solvent swelling, and (3) hydrogen bonds (∼5 kcal/mol) act to brace together molecules in a continuously associated form. Multiple bonds with such cooperative interactions are established in the macromolecular structure and are estimated to have bond strengths on the order of 20 kcal/mol or more. Indeed, many studies indicate that noncovalent bonds in coal influence strongly the physical properties of coal.13,14 A mixed solvent, such as N-methyl-2-pyrrolidinone/carbon disulfide (CS2/NMP), has been noted to give high extraction yields of more than 50 wt % (daf) for several bituminous coals at room temperature, even though no noteworthy chemical reaction was shown to happen during the extraction.15,16 This finding supplies a provision in support of the noncovalent network model of coal as was proposed originally by Nishioka,17 namely, the associated or physical network model. A cross-linked (covalent bonds), three-dimensional macromolecular model has been extensively recognized to elucidate the polymeric nature of coal.18 It was, however, accepted that intraand intermolecular interactions, known as noncovalent bonds, play an important role in coal structure.17,19-25 Noncovalent bonds in coal contain ionic forces, chargetransfer interactions, and interactions due to δ-electrons in polycyclic aromatic compounds.17 The large quantity of these interactions is highly rank-dependent. These interactions are considered to be more stable than hydrogen bonds and dispersion forces, and to merely partially be solvated even with one of the best recognized solvents, pyridine.20 It has been proposed that major sites are cross-linked by these noncovalent bonds and act as if they are covalently cross-linked.17,21-24 The real structure of coal may be a mixture of assemblies covalently cross-linked and structures connected by noncovalent interactions. The level to which coal molecules may have these two types of structures is unidentified. However, several appearances of evidence for noncovalent networks were attained from the surveys of solvent swelling of coal between 1992 and 1993.17,21-24 These studies emphasized the (1) irreversibility of swelling,21 (2) reliance of swelling on coal concentration,22 and (3) larger swelling of coal residue than coal extract.23 (13) Iino, M.; Takanohashi, T. Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; p 203. (14) Iino, M. Fuel Process. Technol. 2000, 62, 89. (15) Iino, M.; Takanohashi, T.; Osuga, H.; Toda, K. Fuel 1988, 67, 1639. (16) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588. (17) Nishioka, N. Fuel 1992, 71, 941. (18) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1961. (19) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (20) Nishioka, M. Energy Fuels 1991, 5, 487. (21) Nishioka, M. Fuel 1993, 72, 997. (22) Nishioka, M. Fuel 1993, 72, 1001. (23) Nishioka, M. Fuel 1993, 72, 1719. (24) Nishioka, M. Fuel 1993, 72, 1725. (25) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788.

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The perfusion of solvent into coal has been a subject of much debate. Solvent molecules incrementally intercalate into the compliant, noncovalent network and infiltrate into the coal structure. As the solvent molecules make their way into coal, the coal swells.26 The use of an appropriate solvent as swelling agent aids both in the penetration of solvent within pores and in the exit of solubilized coal compounds migrating outward.27 In a number of reports, it was noted that the solubility parameters of coal can supply systematic knowledge related to the physical interactions of any solid fuel.28 There are two factors which considerably complicate the measurement of coal swelling by solvents: the porous nature of coals and the macromolecular network.29 This network contains a noteworthy amount of extractable organic material,30 which is related to cross-linked and non-cross-linked structures of coal. To separate the solvent in the solvent-swollen lignite, and to investigate the effect of swelling on the macroscopic structure of coal and the effect of supercritical carbon dioxide extraction, solvent-swollen lignite was dried by lyophilization. Lyophilization, commonly referred to as freeze-drying, is the process of removing water from a product by desorptive sublimation under reduced pressure. In the present work, lyophilization was employed to separate the organic solvent which was used to swell the coal. The solvents were separated by vacuum sublimation during lyophilization, leaving behind a solvent-free coal. To extend our understanding of the fundamentals governing the nature of the swelling, we designed experiments to investigate further the influence of the conformational stability of swollen coal in suitable solvents on the magnitude of lyophilizationinduced structural changes. The magnitude of lyophilization-induced structural changes was investigated typically by employing scanning electron microscopy and 13C NMR and FTIR spectroscopic techniques. Lyophilization-induced alterations in the macroscopic and macromolecular structure of the coal and the effect of lyophilization on the structure of the supercritical carbon dioxide extracts of the coal are reported in the present work. Experimental Section Turkish Elbistan and Beypazari lignite samples were used in the present work. Elemental and proximate analyses of these are given in Table 1. The lignite samples were ground to -100 mesh size before use. The swelling behavior of the lignite samples was measured by Liotta’s method.31 In this method, a volumetric swelling method using a glass tube is commonly used. Volumetric swelling ratios, Q, are calculated by the ratio of heights or volumes before and after swelling. Approximately 100 mg of a sample was placed in a 6 mm o.d. tube and centrifuged for 10 min at 5000 revolutions/min. The height of the sample (h1) was measured. Excess ethylenediamine or dimethyl sulfoxide (∼1 mL) was added into the tube, the contents of the tube were mixed, the tube was centrifuged after 24 h, and the height of the sample in the tube (h2) was measured. Coal swelling kinetics was followed until equilibrium was established. (26) Giri, C. C.; Sharma, D. K. Fuel 2000, 79, 577. (27) Branner, D. Fuel 1982, 62, 1347. (28) Jones, J. C.; Hewitt, R. G.; Innes, R. A. Fuel 1997, 76, 575. (29) Hombach, H. P. Fuel 1980, 59, 465. (30) Weinberg, V. L.; Yen, T. F. Fuel 1980, 59, 287. (31) Liotta, R.; Brons, G.; Isaacs, J. Fuel 1983, 62, 781.

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Table 1. Proximate and Elemental Analyses of Beypazari and Elbistan Lignites Beypazari lignite

Elbistan lignite

Proximate Analysis moisture, as received (%) 22.3 fixed carbon (% db) 25.4 volatile matter (% db) 31.8 mineral matter (% db) 42.8

34.6 27.7 46.7 25.6

Elemental Analysis carbon (%, dmmf) 62.7 hydrogen (%, dmmf) 4.7 nitrogen (%, dmmf) 0.8 sulfur, total (%, dmmf) 4.0 oxygen, by difference (%, dmmf) 27.8

65.2 5.4 2.1 5.4 21.9

The swollen Elbistan and Beypazari lignite samples were dried in a vacuum oven until a constant weight was obtained. The dried lignite samples were frozen directly in liquid N2 and then were freeze-dried at room temperature and a pressure of 0.120 mbar for 60 h in a Christ brand ALPHA 1-2 LD lyophilizer. The freeze-dried lignite samples were weighed after the experiments. Each experiment was repeated at least two times. The freeze-dried and raw samples were examined with a Gemini brand scanning electron microscope. The change in the structure after freeze-drying was determined by comparison with the structures of the raw samples. All samples were coated with gold. Freeze-dried samples were packed in a 50 mL TharTech Design high-pressure reaction view cell equipped with a stirring bar. An Isco model 260 D automatic syringe pump was used to pressurize the view cell with carbon dioxide (99.99% pure) to approximately 3 MPa, and the view cell was heated to a reaction temperature of 80 ( 1 °C. Then the remaining carbon dioxide was slowly added to the system until the desired temperature and pressure (5 MPa) were reached. After settling to the final extraction conditions, the reaction was allowed to proceed with stirring for 2 h. At the end of the reaction CO2 was slowly vented from the view cell into a solution of dichloromethane. The raw samples and supercritically treated samples were then extracted in tetrahydrofuran (THF) digestively, for 24 h at 20 °C. THF extracts obtained were analyzed using a Shimadzu brand QP5050A gas chromatography-mass spectrometry (GC-MS) system. Pure helium was used as the carrier gas in the GC-MS system. The flow rate of the carrier gas was 3 mL/min. A capillary DB-5 ms column (length 30 m, diameter 0.25 mm, and thickness 0.25 µm) was used in the analyses. Both the temperatures of the injection port and column oven were constant at 135 °C. To measure the changes in the surface area and the pore size distribution of the coal samples, adsorption isotherms were measured with a Quantachrome NOVA 2200 series surface area and porosimetry system. The determination is based on the measurements of the adsorption isotherms of nitrogen at 77 K. The surface areas of the samples were determined by using the BET equation in the relative pressure range between 0.05 and 0.3, at five adsorption points. Before the measurements were started, moisture and gases such as nitrogen and oxygen which were adsorbed on the solid surface or held in the open pores were removed under reduced pressure at 100 °C for 5 h. The pore volume and the pore size distribution of the treated coal samples were calculated using the Barret, Joyner, and Halenda (BJH) method.

Results and Discussion In this work, swelling of coal in EDA and DMSO has been investigated by the whole coal swelling strategy. Changes of the volumetric swelling ratios of Beypazari

Figure 1. Change of volumetric swelling ratios of (A) Beypazari and (B) Elbistan lignites in ethylenediamine and dimethyl sulfoxide with time.

and Elbistan lignites in EDA and DMSO are presented in parts A and B, respectively, of Figure 1. It was found that the rates of solvent uptake and kinetics of swelling are strongly influenced by factors such as the nature of the coal, the size of the coal particles,32,33 the nature of the solvent used,22,28,34-36 the size and shape of the solvent molecules,37,38 the accessibility of solvents to coal macromolecules, solvent sorption and diffusion processes in coals,39,40 the temperature of heat treatment,35,41,42 the moisture content of the coal,42 and other features related to its pretreatment.42,43 Coals do not dissolve; rather they swell when they come into contact with a good solvent. The best solvents contain atoms with an available unshared electron pair, such as nitrogen or oxygen.44 Coals swell more in hydrogenbonding solvents45 such as EDA and DMSO. Swelling ratios measured in the present study for Beypazari lignite reached equilibrium constant values of 1.27 after about 42 h in EDA and 1.42 in DMSO after about 45 h. Elbistan lignite, a much younger lignite (PleistocenePliocene, 2-5 million years B.P.),46 reached equilibrium swelling ratios of 1.46 in EDA after 41 h and 1.23 in DMSO after 45 h. These values were relatively lower (32) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379. (33) Krzesin´ska, M. Erdo¨ l, Erdgas, Kohle 1998, 114, 388. (34) Larsen, J. W.; Shawver, S. Energy Fuels 1990, 4, 74. (35) Hall, P. J.; Thomas, K. M.; Marsh, H. Fuel 1992, 72, 1271. (36) Otake, Y.; Suuberg, E. M. Fuel 1989, 68, 1609. (37) Ndaji, F. E.; Thomas, K. M. Fuel 1995, 74, 842-845. (38) Aida, T.; Fuku, K.; Fujii, M.; Yoshikara, M.; Maeshima, T.; Squires, T. G. Energy Fuels 1991, 5, 79. (39) Green, T. K.; Selby, T. D. Energy Fuels 1994, 8, 213. (40) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 11, 1155. (41) Ndaji, F. E.; Thomas, K. M. Fuel 1993, 72, 1525-1530, 1531. (42) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. C. Energy Fuels 1993, 7, 384. (43) Yun, Y.; Suuberg, E. M. Fuel 1993, 72, 1245. (44) Dryden, I. G. C. Fuel 1951, 30, 39. (45) Quinga, E. M. Y.; Larsen, J. W. In New Trends in Coal Science; Yu¨ru¨m, Y., Ed.; NATO ASI Series C, Vol. 244; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988; p 85. (46) Karayigit, A. I.; Akdag, Y. Turk. J. Earth Sci. 1996, 7, 1.



Figure 2. Micrographs of raw Beypazari lignite samples.

than those measured for higher rank coals in the same solvents.47 In light of the outcome of some preliminary solvent-swelling experiments, the use of DMSO was adopted prior to lyophilization of the Beypazari lignite while the use of EDA was adopted prior to lyophilization of the Elbistan lignite, as the degree of swelling of the coal samples utilized in the present work had been comparatively superior in the above-mentioned solvents. Nishioka et al.19 and Larsen et al.48 stated that coal swelling provides the macromolecule with the opportunity to undergo conformational rearrangements and to adopt a lower free energy, more highly associated structure. The swelling of coal refers to an increase in volume due to absorption of solvent. When such a coal sample is dried, a high distortion is observed.49 These changes can be interpreted as a consequence of the reorientation of the macromolecular chains, the driving force coming from the free energy of mixing of the (47) Kirov, N. Y.; O’Shea, J.; Sergeant, G. D. Fuel 1967, 47, 831. (48) Larsen, J. W.; Flowers, R. A., II; Hall, P. J.; Carlson, G. Energy Fuels 1997, 11, 998. (49) (a) Brenner, D. Fuel 1983, 62, 1347. (b) Brenner, D. Fuel 1984, 63, 1324.

solvent and the coal structure. The coals seem to expand greatly in size and crack. The effect of the lyophilization technique under atmospheric conditions on the physical and chemical structure of the lignite samples was investigated in the present work. The mass of the dry coal samples stayed almost unaltered after lyophilization. The structural changes concomitant with this technique were observed by scanning electron microscopy. Images of the raw and lyophilized coal samples are presented in Figures 2-5. In Figures 2 and 3, the micrographs of raw Beypazari and Elbistan lignite samples are shown, respectively. The particles of the raw Beypazari lignite samples seemed to be irregular in shape and contained some layered structures, and the particles were free of cracks. The much younger Elbistan lignite did not show particles with layered structures, but it once again appeared sponge-looking and crackless. Micrographs of the lyophilized Beypazari and Elbistan lignite particles are presented in Figures 4 and 5, respectively. All of the particles in these figures contained extensive cracks. Cracking of the particles was very severe, and it seemed

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Figure 3. Micrographs of raw Elbistan lignite samples. Table 2. Surface Areas and Pore Diameters of the Coal Samples sample

BET surface area (m2/g)

pore diameter (nm)

raw Beypazari lignite lyophilized Beypazari lignite raw Elbistan lignite lyophilized Elbistan lignite

5.8 5.3 2.9 4.7

2.20 2.20 2.19 2.20

that the particle size of the samples had decreased by an order of magnitude after lyophilization. Changes in the BET surface area of the raw and lyophilized coal samples are presented in Table 2. While lyophilization seemed to increase the BET surface area of the Elbistan lignite from 2.9 to 4.7 m2/g, it did not cause any change in the BET surface area of the Beypazari lignite, which was indicated by the almost constant value of about 5.0 m2/g. Pore diameters of the two lignite samples, both raw and treated, stayed at a constant value of 2.2 nm. The pore size distributions of both raw and lyophilized lignite samples are presented in Figure 6. The pore size distribution seemed to be unaltered after lyophilization in all of the samples. In the case of the lyophilized Elbistan lignite sample, the differential pore volume

appeared to be expanded slightly relative to that of the raw lignite sample for the low-diameter pores of approximately 25 Å pore diameter. The immediate conclusion that could be reached at this point was that lyophilization did not change the pore size distribution of the coal samples but it mechanically reduced the particle size of the coals, and the effect was particularly pronounced in the case of Elbistan lignite, which was relatively much younger in age compared to the Beypazari lignite. The lyophilization experiments conducted in this work were instructive in this instance and showed clearly that there was a strong relation between mechanical stability of the coals in swelling solvents and the magnitude of lyophilization-induced structural alterations. A significantly higher extract yield than that produced by Soxhlet extraction can be obtained by extracting coal with organic solvents in supercritical states.50 Carbon dioxide is preferred as the supercritical solvent for workable applications since it is inexpensive, nontoxic, inflammable, and environmentally tolerable and has a low critical temperature and a moderate critical (50) Vahrman, M. Fuel 1970, 49, 5.



Figure 4. Micrographs of DMSO-swollen and lyophilized Beypazari lignite samples. Table 3. Supercritical Carbon Dioxide and Tetrahydrofuran (THF) Extraction Yields of the Coal Samples sample

material loss after supercritical CO2 treatment (%)

yield of THF extraction (%)

total yield (%)

raw Beypazari lignite lyophilized Beypazari lignite raw Elbistan lignite lyophilized Elbistan lignite

0.4 7.1 2.9 3.5

0.9 7.5 4.7 4.3

1.3 14.6 7.6 7.8

pressure. The attention surrounding supercritical carbon dioxide treatment may be attributed to the exceptional advantages offered by the process, such as high mass-transfer rates, favorable selectivities, and low operating temperatures. Some volatile material was carried with supercritical carbon dioxide treatment. The percentage of the material loss after this treatment is given in Table 3. It seemed that material carried with the supercritical carbon dioxide increased for the lyophilized samples. In the case of raw Beypazari lignite while the material lost with supercritical carbon dioxide was abou 0.4%, the material loss increased in the case of the lyophilized sample of the same lignite. The values for the material loss after supercritical carbon dioxide treatment for raw and lyophilized Elbistan lignite samples were 2.9% and 3.5%, respectively. In the

present work, the amount of material extracted with THF from coals after supercritical carbon dioxide treatment was greater in the case of lyophilized samples in both of the coals. THF extraction yields increased from almost 1% to 8% in Beypazari lignite and stayed nearly constant at about 4% in Elbistan lignite samples (Table 3). Supercritical extraction often forms a basis to carry out coal structure investigations. In the current investigation, this method was used to gain insight into the structure of matrix-resident incipients. The mild conditions of the supercritical extraction process give rise to only slight changes in the structure of the extractable material.51,52 GC-MS was used to study the distribution (51) Bartle, K. D.; Martin, T. G.; Williams, D. F. Fuel 1975, 54, 226.

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Figure 5. Micrographs of EDA-swollen and lyophilized Elbistan lignite samples.

of organic compounds in the extract obtained from the supercritical process. The compounds identified in the total ion chromatograms (TICs) of the extracts (Figures 7 and 8) and their relative percentages are presented in Tables 4 and 5. The first result that could be observed from the data presented in these tables was that most of the compounds were oxygenated compounds in the extracts of the raw and treated Beypazari and Elbistan lignite samples. Among the oxygenated compounds, alcohols, phenols, aldehydes, carboxylic acids, esters, and ether were present. Amines, amides, sulfanilic acids, and pyrazinyl compounds were among the nitrogenous compounds detected. Thiophenes and sulfides described were the majority of the sulfur compounds. n-Hexene, 2-methylnonane, n-tridecane, heptacosane, and hexadecane were the aliphatic hydrocarbons detected in the extracts. The fluorescence spectra of the solubilized coal in the paper by Kashimura et al.53 give an indication of the presence of aromatic ring systems with three to six condensed rings. In contrast, the (52) Kershaw, J. R. J. Supercrit. Fluids 1989, 2, 35. (53) Kashimura, N.; Hayashi, J.; Li, C. Z.; Sathe, C.; Chiba, T. Fuel 2004, 83, 97.

aromatic compounds determined in the present study were usually monoaromatic compounds, though phenanthrene and fluorene compounds were also observed in the extracts of the Elbistan lignite. It seemed that there was little evidence for highly condensed aromatic rings in the extracts from raw and treated coal samples. The extracts also contained cyclic aliphatic ring compounds. The presence of 1-aminoadamantane in the THF extract of the lyophilized supercritical carbon dioxide treated Elbistan lignite sample is quite interesting. Swelling and lyophilization of the coal samples facilitated the extraction of a greater number of oxygenated, nitrogenous, and sulfur-containing compounds from the coal structure. Phenol, neopentyl phenyl ether, octadecanoic acid, 1-(2-pyrazinyl)-4-methyl-2-pentanol, 1methylallyl phenyl ether, and 2,4-di-tert-butylanisole were some of the compounds present in the extracts of the treated Beypazari lignite but absent in the extracts of the raw Beypazari lignite. The extracts of the treated Elbistan lignite contained 7-norbornanol, 1,4,6-trimethyl-2-azafluorene, R-(dimethylamino)-4-ethyl-o-cresol, trimethylene bisethyl sulfide, dihydrocarveole, hexade-



Table 4. Chemicals Identified and Their Relative Percentages in the TICs of THF Extracts of Raw or Lyophilized and Then Supercritical Carbon Dioxide Treated Beypazari Lignite Samples peak number in the TIC of the THF extracta raw Beypazari lignite (A)

lyophilized supercritical CO2-treated Beypazari lignite (B)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 2 3 4 5 6

a

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

relative percentage of compound A B

empirical formula

chemical name

C7H14O2 C5H13N C6H12 C7H10S3 C15H24O C9H18O2 C5H8N2O2 C10H14O C6H7NO3S C10H16N2O C10H22 C8H11NO3 C11H14O3 C8H18O2 C11H14O2 C11H14O2 C11H15NO2S C9H10O3 C16H26O C17H26O3 C6H6O C9H16O C11H16O C19H38O2 C10H16N2O C10H12O C15H24O

DL-trans-1,3-dimethoxycyclopentane methyldiethylamine 1-n-hexene 2,5-di(methylthio)-3-methylthiophene 4-methyl-2,6-bis(1,1-dimethylethyl)phenol cyclohexanepropanal 2-methylmaleamide tert-butoxybenzene sulfanilic acid 1-(2-pyrazinyl)-4-methyl-2-pentanol 2-methylnonane 4-morpholinetetrolic acid anisyl propionate dipropyl acetal acetaldehyde phenyl trimethylacetate phenyl valerate 4-methylthio-3,5-dimethylphenyl-N-methylcarbamate phenyl ether carbonate dispiro[5.1.5.3]hexadecan-7-one undecanoic acid phenol endo-2-hydroxybicyclo[3.3.1]nonane neopentyl phenyl ether octadecanoic acid 1-(2-pyrazinyl)-4-methyl-2-pentanol 1-methylallyl phenyl ether 2,4-di-tert-butylanisole

0.46 0.91 1.95 3.62 4.41 0.61 0.76 1.49 2.15 0.76 2.86 1.21 2.18 11.51 12.25 11.62 12.09 13.50 6.93 8.70

0.19 0.24 0.57 1.64 2.00 0.73 4.75 7.78 0.63 10.23 0.59 6.45 5.73 5.89 4.52 5.02 5.73 5.28 3.51 4.76 1.18 7.31 3.32 3.36 3.83 4.78

Peak numbers correspond to those in the TICs.

Figure 7. Total ion chromatograms of (a) the THF extract of the raw Beypazari lignite and (b) the THF extract of the DMSO-swollen and lyophilized Beypazari lignite treated with supercritical carbon dioxide.

Conclusions

Figure 6. Pore size distribution of (A) raw and lyophilized Beypazari and (B) raw and lyophilized Elbistan lignite samples.

cane, and 3,5-dimethylcyclohexanol; these compounds were absent in the extracts of the raw Elbistan lignite.

(1) Elbistan lignite, which is a much younger lignite, reached its equilibrium swelling ratio of 1.46 in EDA, and Beypazari lignite reached its equilibrium constant value of 1.27 after about 42 h in EDA. (2) The lyophilization technique caused the coals to dry to the extent that atypical structural changes were observed by SEM. For instance, all of the particles contained extensive cracks. Cracking of the particles

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Table 5. Chemicals Identified and Their Relative Percentages in the TICs of THF Extracts of Raw or Lyophilized and Then Supercritical Carbon Dioxide Treated Elbistan Lignite Samples peak number in the TIC of the THF extract of Elbistan lignitea raw lyophilized lignite supercritical (A) CO2-treated lignite (B) 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 27 28 29 30 31 32 33

a

1 2 3 4 5 6 8

12 13 14 17 18 20 21 24 27 28 29 31 32 33 34 35 36 37 38 39 40

empirical formula

chemical name

C6H12O C2H4O2S C5H13NO C7H11NO2S C11H21N C8H15NO2 C14H22O C12H18O C9H17NO3 C9H16O C14H22O C10H16O2 C7H10N2O3 S C16H10 C17H26O3 C9H14O C10H18O C13H28 C27H56 C6H10O2S C10H17NO3 C12H16O4 C7H14O4 C5H12O2SN C10H18O C19H32O3 C11H14N4S C13H20O C9H10N2O3 C10H17N C7H16O6S2 C17H22OS2 C6H10N2O3 C7H12O C15H15N C11H17NO C7H16S2 C10H18O C18H38 C8H16O

oxetane thiovanic acid 2-isopropylaminoethanol N-acetylmethionine skytanthine 1,4-bis(4-cyclohexylbutyl) 2,4-di-tert-butylphenol (hexyloxy)benzene O,N-permethylated N-acetylvaline (E)-4-nonenal n-octyl phenyl ether 7-nonynoic acid benzenesulfonic acid benzo(def)phenanthrene undecanoic acid 5H-inden-5-one 4-(1-hydroxy-1-methylethyl)-1-methylcyclohexene n-tridecane heptacosane diallyl sulfone 3-acetoxy-6-hydroxytropane propanoic acid methyl-2-deoxyl-L-fucoside trimethyltin O-acetate 3-decyn-2-ol 3β-12β-17β-trihydroxy-5R-androstane 1,2,4-thiadiazol-3-amine, 5-(ethylimino)-4,5-dihydro-4-(4-methylphenyl) [(4,4-dimethylpentyl)oxy]benzene 2-hydroxyimino-N-(p-methoxyphenyl)acetamide 1-aminoadamandate 1,5-dimesyloxypentane 3-(2-phenyl-1,3-dithian-2-yl)cycloheptanone cycloalanylserine 7-norbornanol 1,4,6-trimethyl-2-azafluorene R-dimethylamino-4-ethyl-o-cresol trimethylene bisethyl sulfide dihydrocarveole hexadecane 3,5-dimethylcyclohexanol

relative percentage of compound A B 0.70 2.37 2.99 2.70 0.82 0.97 9.43 8.75 9.36 9.56 7.84 0.86 4.01 2.48 2.18 5.56 4.35 5.23 0.75 1.37 7.40 3.16 5.73 1.43 0.72 9.38 8.21 8.02 9.30 7.87 9.11 8.07

1.69 0.28 2.67 0.40 1.76

3.70 1.13 1.56 0.48 0.76 0.90

1.22 7.27 8.10 8.63 8.43 12.22 5.64 0.40 10.86 0.61 2.49 13.76 3.85 0.44

Peak numbers correspond to those in the TICs.

Figure 8. Total ion chromatograms of (a) the THF extract of the raw Elbistan lignite and (b) the THF extract of the EDAswollen and lyophilized Elbistan lignite treated with supercritical carbon dioxide.

was very severe, and it seemed that the particle size of the samples decreased by a factor of about 10 after lyophilization. (3) Lyophilization did not change the pore size distribution of the coal samples, but it mechanically reduced the particle size of the coal particles, and the effect was pronounced in the case of Elbistan lignite, which was relatively much younger in age compared to the Beypazari lignite. (4) While lyophilization seemed to increase the BET surface area of the Elbistan lignite from 2.9 to 4.7 m2/ g, it did not cause any effect on the BET surface area of the Beypazari lignite, which indicated almost a constant value of about 5.0 m2/g. The pore size distribution seemed to be unaltered after lyophilization in all of the samples. (5) The amount of material extracted with THF from raw and lyophilized coals after supercritical carbon dioxide treatment was consistently greater in the case of the lyophilized coal samples. EF049706V

Lyophilization-Induced Structural Changes in Solvent-Swollen and

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