Lyophilization-Induced Structural Changes inSolvent-Swollen and Supercritical Carbon Dioxide

Treated Low-Rank Turkish Coals and Characterizationof Their Extracts

Serkan Bas,† Billur Sakintuna,† Burak Birkan,† Yusuf Menceloglu,†Alpay Taralp,† Zeki Aktas,‡ and Yuda Yurum*,†

Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, Istanbul 34956, andDepartment of Chemical Engineering, Ankara University, Besevler, 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 fractionin previously impregnated, solvent-swollen low-rank Turkish coals as well as to potentially removeinherently volatile components of the coal matrix. Experiments were performed subsequently toassess if there existed a correlation between the conformational stability of swollen coal in certainsolvents and the magnitude of lyophilization-induced structural changes. Lyophilization-inducedalterations in the macroscopic and macromolecular structure of the coal and the effect oflyophilization on the structure of supercritical carbon dioxide extracts of the coal are reported inthe present work. Freeze-dried samples were treated with supercritical carbon dioxide in asupercritical system at 50 bar and 80 °C. Then the soluble components accessible within rawsamples and supercritically treated samples were digestively extracted in tetrahydrofuran, for24 h at 20 °C. Extracts obtained were analyzed using a GC-MS system. Structural changes ofthe coal particles upon lyophilization were observed by SEM. Lyophilization seemed to increasethe BET surface area of the coal samples. Lyophilization did not change the pore size distributionof the coal samples, but it mechanically reduced the particle size of the coal particles. In bothtypes of coal, the amount of material that had extracted into THF after supercritical carbon dioxidetreatment was greater in the case of samples that had been previously lyophilized.

Introduction

Extensive knowledge based on coal structure is fun-damental to comprehend the physical properties of coaland the chemistry of conversion processes. Inter- andintramolecular interactions have long been recognizedas principle determinants of the overall physical andchemical properties of coal. One key experimentalstrategy to investigate inter- and intramolecular as-sociation forces has been based on solvent swelling andextraction. By way of this method, insight has beengained into the macromolecular network structure ofcoal. Effort1-6 has been invested to elucidate the cor-relation between the extent of swelling and parametersrelated to the macromolecular network. In particular,some noteworthy investigations have focused on betterunderstanding the influence of parameters such asaverage molecular weight between cross-link points,

using the theories of polymer physical chemistry.7 Thisreadily observable fact, namely, solvent-induced swell-ing of coal, has long been related to the macromolecularstructure of coal. In particular, the Flory-Rehner theoryand variants thereof, which have been used to ap-proximate the molecular weight between cross-linkpoints,5,8-12 have been frequently utilized to associatethe macromolecular network parameters with the de-gree of swelling in good solvents. The Flory-Rehner7

theory assumes that the deformation of the elementarychains of the network is closely related, even in proceed-ing down to the molecular level. Thus, it would followto reason that any observed macroscopic deformationof a given sample should correlate linearly with achange in the statistical distribution of chain lengthsof the coal macromolecule. In other words, the coalmacromolecule would be anticipated to expand consis-tently on the segmental scale whenever the macroscopicswelling is related to molecular characteristics such asthe cross-link density.

* To whom correspondence should be addressed. Phone: 90-216-483 9512. Fax: 90-216-483 9550. E-mail: yyurum@sabanciuniv.edu.

† 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.

(7) Flory, P. J. Principles of Polymer Chemistry; Cornell UniversityPress: 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.

1056 Energy & Fuels 2005, 19, 1056-1064

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

With due consideration of the types of potentiallysignificant inter- and intramolecular interactions andtheir contributions to swelling behavior, some key pointsshould be emphasized. In particular, (1) covalent cross-link bonds (>50 kcal/mol in strength) act as pointinteractions, (2) reversible cross-links can be dislocatedat elevated temperatures or by suitable exchange reac-tions and solvent swelling, and (3) hydrogen bonds (∼5kcal/mol) act to brace together molecules in a continu-ously associated form. Multiple bonds with such coop-erative interactions are established in the macromolec-ular structure and are estimated to have bond strengthson the order of 20 kcal/mol or more. Indeed, manystudies indicate that noncovalent bonds in coal influencestrongly the physical properties of coal.13,14 A mixedsolvent, such as N-methyl-2-pyrrolidinone/carbon disul-fide (CS2/NMP), has been noted to give high extractionyields of more than 50 wt % (daf) for several bituminouscoals at room temperature, even though no noteworthychemical reaction was shown to happen during theextraction.15,16 This finding supplies a provision insupport of the noncovalent network model of coal as wasproposed originally by Nishioka,17 namely, the associ-ated or physical network model. A cross-linked (covalentbonds), three-dimensional macromolecular model hasbeen extensively recognized to elucidate the polymericnature of coal.18 It was, however, accepted that intra-and intermolecular interactions, known as noncovalentbonds, play an important role in coal structure.17,19-25

Noncovalent bonds in coal contain ionic forces, charge-transfer interactions, and interactions due to δ-electronsin polycyclic aromatic compounds.17 The large quantityof these interactions is highly rank-dependent. Theseinteractions are considered to be more stable thanhydrogen bonds and dispersion forces, and to merelypartially be solvated even with one of the best recog-nized solvents, pyridine.20 It has been proposed thatmajor sites are cross-linked by these noncovalent bondsand act as if they are covalently cross-linked.17,21-24 Thereal structure of coal may be a mixture of assembliescovalently cross-linked and structures connected bynoncovalent interactions. The level to which coal mol-ecules may have these two types of structures isunidentified. However, several appearances of evidencefor noncovalent networks were attained from the sur-veys of solvent swelling of coal between 1992 and1993.17,21-24 These studies emphasized the (1) irrevers-ibility of swelling,21 (2) reliance of swelling on coalconcentration,22 and (3) larger swelling of coal residuethan coal extract.23

The perfusion of solvent into coal has been a subjectof much debate. Solvent molecules incrementally inter-calate into the compliant, noncovalent network andinfiltrate into the coal structure. As the solvent mol-ecules make their way into coal, the coal swells.26 Theuse of an appropriate solvent as swelling agent aids bothin the penetration of solvent within pores and in theexit of solubilized coal compounds migrating outward.27

In a number of reports, it was noted that the solubilityparameters of coal can supply systematic knowledgerelated to the physical interactions of any solid fuel.28

There are two factors which considerably complicate themeasurement of coal swelling by solvents: the porousnature of coals and the macromolecular network.29 Thisnetwork contains a noteworthy amount of extractableorganic material,30 which is related to cross-linked andnon-cross-linked structures of coal.

To separate the solvent in the solvent-swollen lignite,and to investigate the effect of swelling on the macro-scopic structure of coal and the effect of supercriticalcarbon dioxide extraction, solvent-swollen lignite wasdried by lyophilization. Lyophilization, commonly re-ferred to as freeze-drying, is the process of removingwater from a product by desorptive sublimation underreduced pressure. In the present work, lyophilizationwas employed to separate the organic solvent which wasused to swell the coal. The solvents were separated byvacuum sublimation during lyophilization, leaving be-hind a solvent-free coal. To extend our understandingof the fundamentals governing the nature of the swell-ing, we designed experiments to investigate further theinfluence of the conformational stability of swollen coalin suitable solvents on the magnitude of lyophilization-induced structural changes. The magnitude of lyo-philization-induced structural changes was investigatedtypically by employing scanning electron microscopy and13C NMR and FTIR spectroscopic techniques. Lyo-philization-induced alterations in the macroscopic andmacromolecular structure of the coal and the effect oflyophilization on the structure of the supercriticalcarbon dioxide extracts of the coal are reported in thepresent work.

Experimental Section

Turkish Elbistan and Beypazari lignite samples were usedin the present work. Elemental and proximate analyses ofthese are given in Table 1. The lignite samples were groundto -100 mesh size before use.

The swelling behavior of the lignite samples was measuredby Liotta’s method.31 In this method, a volumetric swellingmethod using a glass tube is commonly used. Volumetricswelling ratios, Q, are calculated by the ratio of heights orvolumes before and after swelling. Approximately 100 mg ofa sample was placed in a 6 mm o.d. tube and centrifuged for10 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 weremixed, the tube was centrifuged after 24 h, and the height ofthe sample in the tube (h2) was measured. Coal swellingkinetics was followed until equilibrium was established.

(13) Iino, M.; Takanohashi, T. Structures and Dynamics of Asphalt-enes; 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.

(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.

Structural Changes in Low-Rank Turkish Coals Energy & Fuels, Vol. 19, No. 3, 2005 1057

The swollen Elbistan and Beypazari lignite samples weredried in a vacuum oven until a constant weight was obtained.The dried lignite samples were frozen directly in liquid N2 andthen were freeze-dried at room temperature and a pressureof 0.120 mbar for 60 h in a Christ brand ALPHA 1-2 LDlyophilizer. The freeze-dried lignite samples were weighedafter the experiments. Each experiment was repeated at leasttwo times.

The freeze-dried and raw samples were examined with aGemini brand scanning electron microscope. The change in thestructure after freeze-drying was determined by comparisonwith the structures of the raw samples. All samples werecoated with gold.

Freeze-dried samples were packed in a 50 mL TharTechDesign high-pressure reaction view cell equipped with astirring bar. An Isco model 260 D automatic syringe pump wasused to pressurize the view cell with carbon dioxide (99.99%pure) to approximately 3 MPa, and the view cell was heatedto a reaction temperature of 80 ( 1 °C. Then the remainingcarbon dioxide was slowly added to the system until thedesired temperature and pressure (5 MPa) were reached. Aftersettling to the final extraction conditions, the reaction wasallowed to proceed with stirring for 2 h. At the end of thereaction CO2 was slowly vented from the view cell into asolution of dichloromethane. The raw samples and supercriti-cally treated samples were then extracted in tetrahydrofuran(THF) digestively, for 24 h at 20 °C.

THF extracts obtained were analyzed using a Shimadzubrand QP5050A gas chromatography-mass spectrometry(GC-MS) system. Pure helium was used as the carrier gas inthe GC-MS system. The flow rate of the carrier gas was 3mL/min. A capillary DB-5 ms column (length 30 m, diameter0.25 mm, and thickness 0.25 µm) was used in the analyses.Both the temperatures of the injection port and column ovenwere constant at 135 °C.

To measure the changes in the surface area and the poresize distribution of the coal samples, adsorption isotherms weremeasured with a Quantachrome NOVA 2200 series surfacearea and porosimetry system. The determination is based onthe measurements of the adsorption isotherms of nitrogen at77 K. The surface areas of the samples were determined byusing the BET equation in the relative pressure range between0.05 and 0.3, at five adsorption points. Before the measure-ments were started, moisture and gases such as nitrogen andoxygen which were adsorbed on the solid surface or held inthe open pores were removed under reduced pressure at 100°C for 5 h. The pore volume and the pore size distribution ofthe 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 hasbeen investigated by the whole coal swelling strategy.Changes of the volumetric swelling ratios of Beypazari

and Elbistan lignites in EDA and DMSO are presentedin parts A and B, respectively, of Figure 1. It was foundthat the rates of solvent uptake and kinetics of swellingare strongly influenced by factors such as the nature ofthe coal, the size of the coal particles,32,33 the nature ofthe solvent used,22,28,34-36 the size and shape of thesolvent molecules,37,38 the accessibility of solvents to coalmacromolecules, solvent sorption and diffusion pro-cesses in coals,39,40 the temperature of heat treat-ment,35,41,42 the moisture content of the coal,42 and otherfeatures related to its pretreatment.42,43 Coals do notdissolve; rather they swell when they come into contactwith a good solvent. The best solvents contain atomswith an available unshared electron pair, such asnitrogen or oxygen.44 Coals swell more in hydrogen-bonding solvents45 such as EDA and DMSO. Swellingratios measured in the present study for Beypazarilignite reached equilibrium constant values of 1.27 afterabout 42 h in EDA and 1.42 in DMSO after about 45 h.Elbistan lignite, a much younger lignite (Pleistocene-Pliocene, 2-5 million years B.P.),46 reached equilibriumswelling ratios of 1.46 in EDA after 41 h and 1.23 inDMSO after 45 h. These values were relatively lower

(32) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379.(33) Krzesinska, M. Erdol, 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;

Yurum, Y., Ed.; NATO ASI Series C, Vol. 244; Kluwer AcademicPublishers: Dordrecht, The Netherlands, 1988; p 85.

(46) Karayigit, A. I.; Akdag, Y. Turk. J. Earth Sci. 1996, 7, 1.

Table 1. Proximate and Elemental Analyses of Beypazariand Elbistan Lignites

Beypazarilignite

Elbistanlignite

Proximate Analysismoisture, as received (%) 22.3 34.6fixed carbon (% db) 25.4 27.7volatile matter (% db) 31.8 46.7mineral matter (% db) 42.8 25.6

Elemental Analysiscarbon (%, dmmf) 62.7 65.2hydrogen (%, dmmf) 4.7 5.4nitrogen (%, dmmf) 0.8 2.1sulfur, total (%, dmmf) 4.0 5.4oxygen, by difference (%, dmmf) 27.8 21.9

Figure 1. Change of volumetric swelling ratios of (A) Bey-pazari and (B) Elbistan lignites in ethylenediamine anddimethyl sulfoxide with time.

1058 Energy & Fuels, Vol. 19, No. 3, 2005 Bas et al.

than those measured for higher rank coals in the samesolvents.47 In light of the outcome of some preliminarysolvent-swelling experiments, the use of DMSO wasadopted prior to lyophilization of the Beypazari lignitewhile the use of EDA was adopted prior to lyophilizationof the Elbistan lignite, as the degree of swelling of thecoal samples utilized in the present work had beencomparatively superior in the above-mentioned solvents.

Nishioka et al.19 and Larsen et al.48 stated that coalswelling provides the macromolecule with the op-portunity to undergo conformational rearrangementsand to adopt a lower free energy, more highly associatedstructure. The swelling of coal refers to an increase involume due to absorption of solvent. When such a coalsample is dried, a high distortion is observed.49 Thesechanges can be interpreted as a consequence of thereorientation of the macromolecular chains, the drivingforce coming from the free energy of mixing of the

solvent and the coal structure. The coals seem to expandgreatly in size and crack.

The effect of the lyophilization technique underatmospheric conditions on the physical and chemicalstructure of the lignite samples was investigated in thepresent work. The mass of the dry coal samples stayedalmost unaltered after lyophilization. The structuralchanges concomitant with this technique were observedby scanning electron microscopy. Images of the raw andlyophilized coal samples are presented in Figures 2-5.In Figures 2 and 3, the micrographs of raw Beypazariand Elbistan lignite samples are shown, respectively.The particles of the raw Beypazari lignite samplesseemed to be irregular in shape and contained somelayered structures, and the particles were free of cracks.The much younger Elbistan lignite did not show par-ticles with layered structures, but it once again ap-peared sponge-looking and crackless. Micrographs of thelyophilized Beypazari and Elbistan lignite particles arepresented in Figures 4 and 5, respectively. All of theparticles in these figures contained extensive cracks.Cracking of the particles was very severe, and it seemed

(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.

Figure 2. Micrographs of raw Beypazari lignite samples.

Structural Changes in Low-Rank Turkish Coals Energy & Fuels, Vol. 19, No. 3, 2005 1059

that the particle size of the samples had decreased byan order of magnitude after lyophilization. Changes inthe BET surface area of the raw and lyophilized coalsamples are presented in Table 2. While lyophilizationseemed to increase the BET surface area of the Elbistanlignite from 2.9 to 4.7 m2/g, it did not cause any changein the BET surface area of the Beypazari lignite, whichwas indicated by the almost constant value of about 5.0m2/g. Pore diameters of the two lignite samples, bothraw and treated, stayed at a constant value of 2.2 nm.The pore size distributions of both raw and lyophilizedlignite samples are presented in Figure 6. The pore sizedistribution seemed to be unaltered after lyophilizationin all of the samples. In the case of the lyophilizedElbistan lignite sample, the differential pore volume

appeared to be expanded slightly relative to that of theraw lignite sample for the low-diameter pores of ap-proximately 25 Å pore diameter. The immediate conclu-sion that could be reached at this point was thatlyophilization did not change the pore size distributionof the coal samples but it mechanically reduced theparticle size of the coals, and the effect was particularlypronounced in the case of Elbistan lignite, which wasrelatively much younger in age compared to the Bey-pazari lignite. The lyophilization experiments conductedin this work were instructive in this instance andshowed clearly that there was a strong relation betweenmechanical stability of the coals in swelling solvents andthe magnitude of lyophilization-induced structural al-terations.

A significantly higher extract yield than that pro-duced by Soxhlet extraction can be obtained by extract-ing coal with organic solvents in supercritical states.50

Carbon dioxide is preferred as the supercritical solventfor workable applications since it is inexpensive, non-toxic, inflammable, and environmentally tolerable andhas a low critical temperature and a moderate critical

(50) Vahrman, M. Fuel 1970, 49, 5.

Figure 3. Micrographs of raw Elbistan lignite samples.

Table 2. Surface Areas and Pore Diameters of the CoalSamples

sampleBET surfacearea (m2/g)

pore diameter(nm)

raw Beypazari lignite 5.8 2.20lyophilized Beypazari lignite 5.3 2.20raw Elbistan lignite 2.9 2.19lyophilized Elbistan lignite 4.7 2.20

1060 Energy & Fuels, Vol. 19, No. 3, 2005 Bas et al.

pressure. The attention surrounding supercritical car-bon dioxide treatment may be attributed to the excep-tional advantages offered by the process, such as highmass-transfer rates, favorable selectivities, and lowoperating temperatures. Some volatile material wascarried with supercritical carbon dioxide treatment. Thepercentage of the material loss after this treatment isgiven in Table 3. It seemed that material carried withthe supercritical carbon dioxide increased for the ly-ophilized samples. In the case of raw Beypazari lignitewhile the material lost with supercritical carbon dioxidewas abou 0.4%, the material loss increased in the caseof the lyophilized sample of the same lignite. The valuesfor the material loss after supercritical carbon dioxidetreatment for raw and lyophilized Elbistan lignitesamples were 2.9% and 3.5%, respectively. In the

present work, the amount of material extracted withTHF from coals after supercritical carbon dioxide treat-ment was greater in the case of lyophilized samples inboth of the coals. THF extraction yields increased fromalmost 1% to 8% in Beypazari lignite and stayed nearlyconstant at about 4% in Elbistan lignite samples (Table3).

Supercritical extraction often forms a basis to carryout coal structure investigations. In the current inves-tigation, this method was used to gain insight into thestructure of matrix-resident incipients. The mild condi-tions of the supercritical extraction process give rise toonly slight changes in the structure of the extractablematerial.51,52 GC-MS was used to study the distribution

(51) Bartle, K. D.; Martin, T. G.; Williams, D. F. Fuel 1975, 54, 226.

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

samplematerial loss after

supercritical CO2 treatment (%)yield of THF

extraction (%)total

yield (%)

raw Beypazari lignite 0.4 0.9 1.3lyophilized Beypazari lignite 7.1 7.5 14.6raw Elbistan lignite 2.9 4.7 7.6lyophilized Elbistan lignite 3.5 4.3 7.8

Structural Changes in Low-Rank Turkish Coals Energy & Fuels, Vol. 19, No. 3, 2005 1061

of organic compounds in the extract obtained from thesupercritical process. The compounds identified in thetotal ion chromatograms (TICs) of the extracts (Figures7 and 8) and their relative percentages are presentedin Tables 4 and 5. The first result that could be observedfrom the data presented in these tables was that mostof the compounds were oxygenated compounds in theextracts of the raw and treated Beypazari and Elbistanlignite samples. Among the oxygenated compounds,alcohols, phenols, aldehydes, carboxylic acids, esters,and ether were present. Amines, amides, sulfanilicacids, and pyrazinyl compounds were among the nitrog-enous compounds detected. Thiophenes and sulfidesdescribed were the majority of the sulfur compounds.n-Hexene, 2-methylnonane, n-tridecane, heptacosane,and hexadecane were the aliphatic hydrocarbons de-tected in the extracts. The fluorescence spectra of thesolubilized coal in the paper by Kashimura et al.53 givean indication of the presence of aromatic ring systemswith three to six condensed rings. In contrast, the

aromatic compounds determined in the present studywere usually monoaromatic compounds, though phenan-threne and fluorene compounds were also observed inthe extracts of the Elbistan lignite. It seemed that therewas little evidence for highly condensed aromatic ringsin the extracts from raw and treated coal samples. Theextracts also contained cyclic aliphatic ring compounds.The presence of 1-aminoadamantane in the THF extractof the lyophilized supercritical carbon dioxide treatedElbistan lignite sample is quite interesting.

Swelling and lyophilization of the coal samples facili-tated the extraction of a greater number of oxygenated,nitrogenous, and sulfur-containing compounds from thecoal structure. Phenol, neopentyl phenyl ether, octade-canoic acid, 1-(2-pyrazinyl)-4-methyl-2-pentanol, 1-methylallyl phenyl ether, and 2,4-di-tert-butylanisolewere some of the compounds present in the extracts ofthe treated Beypazari lignite but absent in the extractsof the raw Beypazari lignite. The extracts of the treatedElbistan lignite contained 7-norbornanol, 1,4,6-trimeth-yl-2-azafluorene, R-(dimethylamino)-4-ethyl-o-cresol, tri-methylene bisethyl sulfide, dihydrocarveole, hexade-

(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.

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

1062 Energy & Fuels, Vol. 19, No. 3, 2005 Bas et al.

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

Conclusions

(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 constantvalue of 1.27 after about 42 h in EDA.

(2) The lyophilization technique caused the coals todry to the extent that atypical structural changes wereobserved by SEM. For instance, all of the particlescontained extensive cracks. Cracking of the particles

Table 4. Chemicals Identified and Their Relative Percentages in the TICs of THF Extracts of Raw or Lyophilized andThen Supercritical Carbon Dioxide Treated Beypazari Lignite Samples

peak number in the TICof the THF extracta

relative percentageof compound

rawBeypazarilignite (A)

lyophilizedsupercritical CO2-treated

Beypazari lignite (B)empiricalformula chemical name A B

1 1 C7H14O2 DL-trans-1,3-dimethoxycyclopentane 0.46 0.192 2 C5H13N methyldiethylamine 0.91 0.243 3 C6H12 1-n-hexene 1.95 0.574 4 C7H10S3 2,5-di(methylthio)-3-methylthiophene 3.62 1.645 5 C15H24O 4-methyl-2,6-bis(1,1-dimethylethyl)phenol 4.41 2.006 6 C9H18O2 cyclohexanepropanal 0.61 0.737 C5H8N2O2 2-methylmaleamide 0.76 4.758 8 C10H14O tert-butoxybenzene 1.49 7.789 9 C6H7NO3S sulfanilic acid 2.15 0.6310 10 C10H16N2O 1-(2-pyrazinyl)-4-methyl-2-pentanol 0.76 10.2311 11 C10H22 2-methylnonane 2.86 0.5912 12 C8H11NO3 4-morpholinetetrolic acid 1.21 6.4513 13 C11H14O3 anisyl propionate 2.18 5.7314 14 C8H18O2 dipropyl acetal acetaldehyde 11.51 5.8915 15 C11H14O2 phenyl trimethylacetate 12.25 4.5216 16 C11H14O2 phenyl valerate 11.62 5.0217 17 C11H15NO2S 4-methylthio-3,5-dimethylphenyl-N-methylcarbamate 12.09 5.7318 18 C9H10O3 phenyl ether carbonate 13.50 5.2819 19 C16H26O dispiro[5.1.5.3]hexadecan-7-one 6.93 3.5120 20 C17H26O3 undecanoic acid 8.70 4.76

21 C6H6O phenol 1.1822 C9H16O endo-2-hydroxybicyclo[3.3.1]nonane 7.3123 C11H16O neopentyl phenyl ether 3.3224 C19H38O2 octadecanoic acid 3.3625 C10H16N2O 1-(2-pyrazinyl)-4-methyl-2-pentanol 3.8326 C10H12O 1-methylallyl phenyl ether 4.7827 C15H24O 2,4-di-tert-butylanisole

a Peak numbers correspond to those in the TICs.

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

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

Structural Changes in Low-Rank Turkish Coals Energy & Fuels, Vol. 19, No. 3, 2005 1063

was very severe, and it seemed that the particle size ofthe samples decreased by a factor of about 10 afterlyophilization.

(3) Lyophilization did not change the pore size dis-tribution of the coal samples, but it mechanicallyreduced the particle size of the coal particles, and theeffect was pronounced in the case of Elbistan lignite,which was relatively much younger in age compared tothe Beypazari lignite.

(4) While lyophilization seemed to increase the BETsurface area of the Elbistan lignite from 2.9 to 4.7 m2/g, it did not cause any effect on the BET surface area ofthe Beypazari lignite, which indicated almost a constantvalue of about 5.0 m2/g. The pore size distributionseemed to be unaltered after lyophilization in all of thesamples.

(5) The amount of material extracted with THF fromraw and lyophilized coals after supercritical carbondioxide treatment was consistently greater in the caseof the lyophilized coal samples.

EF049706V

Table 5. Chemicals Identified and Their Relative Percentages in the TICs of THF Extracts of Raw or Lyophilized andThen Supercritical Carbon Dioxide Treated Elbistan Lignite Samples

peak number in the TICof the THF extract of

Elbistan lignitea relativepercentage

of compoundraw

lignite(A)

lyophilizedsupercritical

CO2-treated lignite (B)empiricalformula chemical name A B

1 1 C6H12O oxetane 0.70 1.692 2 C2H4O2S thiovanic acid 2.37 0.283 3 C5H13NO 2-isopropylaminoethanol 2.99 2.674 4 C7H11NO2S N-acetylmethionine 2.705 5 C11H21N skytanthine 0.82 0.406 6 C8H15NO2 1,4-bis(4-cyclohexylbutyl) 0.97 1.767 C14H22O 2,4-di-tert-butylphenol 9.438 8 C12H18O (hexyloxy)benzene9 C9H17NO3 O,N-permethylated N-acetylvaline 8.7510 C9H16O (E)-4-nonenal 9.3611 C14H22O n-octyl phenyl ether 9.5612 12 C10H16O2 7-nonynoic acid 7.84 3.7013 13 C7H10N2O3 S benzenesulfonic acid 0.86 1.1314 14 C16H10 benzo(def)phenanthrene 4.01 1.5615 C17H26O3 undecanoic acid 2.4816 C9H14O 5H-inden-5-one 2.1817 17 C10H18O 4-(1-hydroxy-1-methylethyl)-1-methylcyclohexene 5.56 0.4818 18 C13H28 n-tridecane 4.35 0.7619 C27H56 heptacosane 5.2320 20 C6H10O2S diallyl sulfone 0.75 0.9021 21 C10H17NO3 3-acetoxy-6-hydroxytropane 1.3722 C12H16O4 propanoic acid 7.4023 C7H14O4 methyl-2-deoxyl-L-fucoside 3.1624 24 C5H12O2SN trimethyltin O-acetate 5.73 1.2225 C10H18O 3-decyn-2-ol 1.4326 C19H32O3 3â-12â-17â-trihydroxy-5R-androstane 0.7227 27 C11H14N4S 1,2,4-thiadiazol-3-amine, 5-(ethylimino)-4,5-dihydro-4-(4-methylphenyl) 9.38 7.2728 28 C13H20O [(4,4-dimethylpentyl)oxy]benzene 8.21 8.1029 29 C9H10N2O3 2-hydroxyimino-N-(p-methoxyphenyl)acetamide 8.02 8.6330 C10H17N 1-aminoadamandate 9.3031 31 C7H16O6S2 1,5-dimesyloxypentane 7.87 8.4332 32 C17H22OS2 3-(2-phenyl-1,3-dithian-2-yl)cycloheptanone 9.11 12.2233 33 C6H10N2O3 cycloalanylserine 8.07 5.64

34 C7H12O 7-norbornanol 0.4035 C15H15N 1,4,6-trimethyl-2-azafluorene 10.8636 C11H17NO R-dimethylamino-4-ethyl-o-cresol 0.6137 C7H16S2 trimethylene bisethyl sulfide 2.4938 C10H18O dihydrocarveole 13.7639 C18H38 hexadecane 3.8540 C8H16O 3,5-dimethylcyclohexanol 0.44

a Peak numbers correspond to those in the TICs.

Figure 8. Total ion chromatograms of (a) the THF extract ofthe raw Elbistan lignite and (b) the THF extract of the EDA-swollen and lyophilized Elbistan lignite treated with super-critical carbon dioxide.

1064 Energy & Fuels, Vol. 19, No. 3, 2005 Bas et al.