Microwave-assisted extraction of Shenfu coal and its macromolecule structure

Availableonlineatwww.sciencedinectcomMININGScience directSCIENCE ANDTECHNOLOGYELSEVIERwww.elsevicr.com/locate/jcumtMicrowave-assisted extraction of Shenfu coal andits macromolecule structureCHEN Hong LI Jian-wei, LEI Zhao, GE Ling-mei'Department of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, ChinaPetrochemical Engineering, Shenyang University of Technolog, Shenyang, Liaoning 111003, ChinaAbstract: Coals consist of some molecules trapped within an organic matrix from which some organic compounds can be extractedby solvents. The Soxhlet technas been widely used for extracting organic compounds. Microwave heating methods may beuccessfully applied in the field of coal science. Acetone extraction yields and the chemical composition of the extract were inves-tigated using a typical Chinese coal, Shenfu coal, with microwave-assisted extraction. The acetone extract and residue were analyzed by GC/MS and carbon-13 nuclear magnetic resonance spectroscopy respectively. The carbon spectra were converted intoseveral numerical parameters, fa, Ha, Xb, which indicate the difference in macromolecular structure between Shenfu coal and itsresidue. Furthermore, a hybrid genetic algorithm was employed using these parameters to approximate a coal macromolecule byassembling the structural fragments or functional groups into a large and complicated structKeywords: microwave-assisted extraction; GC/MS analysis; carbon-13 nuclear magnetic resonance; coal macromolecule structure1 Introductionof slovak brown coal[]At present, solid-state"C cross-polarization magicCoal is an important energy source because of angle spinning(CP/MAS)nuclear magnetic reso-arge reserves, and a wide distribution. In addition nance NMR)is a standard high-resolution solid-statecoal provides energy security. Utilizing coal effi- NMR method used for solid hydrocarbons. CP/MASciently in gasification, liquefaction or pyrolysis re- NMR has been extensively applied to the investiga-uires a better understanding of the coal structure. tion of coal structure. It is able to measure fa(carbonCoal processing technologies, especially value-added aromaticity)and the fraction of each type of carbon inones operating under mild conditions, mainly depend coal and can thus estimate the average chemicalon the composition and structure of the coal. There. structure of a coalfore, acquiring information on coal structure is veryAcetone is easily separated from an extract and canimportant for the proper utilization of coaL.be reused after treatment. The worked described inExtraction by organic solvents is one traditional this paper used acetone as an extraction solvent. Ex-and useful method used to investigate the chemical traction yield is investigated, the chemical composi-structure of coals. Many extraction tests have been tion of the acetone-extractable fraction is analyzed byperformed to select satisfactory solvents and extrac- GC/MS and the residue after acetone extraction istion methods. Different solvents, such as pyridine, examined by NMR. We employ a hybrid genetic al-tetrahydrofuran(THF) and cyclohexanone have been gorithm, which is composed of a genetic algorithmstudied as have methods like Soxhlet extraction, frac- and an Elitist Model, to simulate a fragment oftional extraction and ultrasonic-assisted extraction -7. Shenfu coal macromolecule structureRecently, some papers have suggested that the mi-crowave heating of raw materials is a progressive 2 Experimentaltechnology that has been successfully applied to coaltreatmen10 A favorable effect from microwave 2.1 Coal sampletreatment was observed in the desulphurisation anddetail the desulphurization and disintegration process a-e Shenfu coal was ground to a particle size ofdrying of coal. As an example, Marland described in中国煤化工 zed in hydrochloeceived II July 2008: accepted O5ber 2008CNMHGProject 40472082 supported by the National Natural Scicnce Foundation of ChinaCorrespondingauthorTel:+86-29-85608232;E-mailaddresshongchen1967@126.comVol 19 No. 1ric acid and hydrofluoric acid solution. It was then were identified by comparing the mass spectradried for two hours in a vacuum at 373 K prior to the NIST98 library data.experiments. The ultimate and proximate analyses 2.4 Solid-state CNMR measurementswere obtained with a Vario EL Ill Elemental Deter-minator. The results for Shenfu coal were(wt%): Ad4.27; Aaf, 35.42 M,d, 7 29; Cdaf, 75.70: Hdaf, 5.208; VARIAN INOVA 300M type NMR spectromeNaaf, 1.348: Odaf, 16.60 and Sdaf, 0.305. The elemental ter. Cross-polarization magic angle spinninganalysis of the residue after acetone extraction was: CP/AS) was employed under the following condi-Caf, 75.84: Hdaf, 4.949; Ndaf, 1.754; Odaf, 13.90 and tions: the weight of the sample was 80-100 mg, theSdt,0.264.spinning speed was 6 kHz, theC resonance2.2 Microwave-assisted extraction MaEquency was 75.425 MHz, the H resonance frequencywas 300.553 MHz the number of scans was 2500An MDS-6 auto-frequency modulation microwave the pulse width was 4.0 microseconds, the contactdigestion/extraction oven with temperature and pres- time was 5 ms and the circulation delay time was 4 swave-assisted extraction experiments. The maximum NUTS data processing softwar ther processed withsure control, made in China, was used for the micro- The resulting spectra were furmicrowave power was 1000 W and the upper controllimit of the temperature was 523 K (inside the vessel). 3 Results and discussionThe upper pressure limit was 5.0 MPa. At first thetemperature and time in the oven were adjusted to a 3.1 Effect of temperature and time on mAE yieldproper range suitable for the sample. Then an accu-rately-weighed and demineralized sample (ca. 4 Fig. I shows the extraction yield of Shenfu coalgrams), together with ca. 40 mL of solvent, was put using acetone as solvent over a temperature range ofnto an extraction vessel made of polytetrafluoro-to 393 K. The extraction yield increased gradu-ethylene. The extraction vessel was placed into a ro-ally to a maximum value and then decreased as thetating pan fixed at the bottom of the MDS-6 diges-extraction temperature rose.tionextraction oven. After extracting for 10-60 minutes the extracts and residual coal were centrifuged at2000 rev/min for 10 minutes. The filteredwere separated from the solvent with a R205evaporator and dried under vacuum to aweight. The extraction yield was calculated byExtraction yield(wt%, daf)extract(g) x100%(coal(g, daf)Temperature(K)where coal (g, daf) was the weight of the dry ash-freeFig. 1 Effect of temperature on the extraction yieldsample. The acetone-extracts from mae were thentransferred to a chromatography column and elutedwith n-hexane toluene and tetrahydrofuran in turnThis can be explained in the following way. WithThe fractions obtained from elution were called oil low temperature MAE the solvent drives some small(n-Hexane soluble fraction), asphaltene (n-hexane molecules, or the so-called mobile phase, from theinsoluble but toluene soluble fraction)and pre-as- coal matrix into the solvent without destroying thephaltene( toluene insoluble but THF soluble fraction). coal backbone structure. At higher temperatures(393The solvent was removed from these fractions and K)some branch chains will break off the coal macGC/MS analysis was performed on themromolecule and leave active sites behind. Some of theextract will enter into the resulting vacancy and form2.3 GC/MS analysisstable bonds with the coal matrix leading to the dropGC/MS analysis was performed with an HP6890/ in extraction yieldHP5973 GC/MS system. A DB-5ms column(300 When the irradiation time was increased from 10 to0. 25 mm) was used with helium as the carrier gas 60 minutes the extraction yield increased slightly andat a flow rate of 0.7 mL/min. The temperature in the reached a maximum at 45 minutes. We thought thatoven was held at 120C for 2 min, then ramped to factors like coal rank, solvent volume to coal mass220C at 7C/min, then ramped to 280C/min at 5 ratio or the coal particle size might have an effect onoC/min and finally maintained for 8 minutes at 280 the extraction yield. This will be studied laterC The mass spectrometer was operated in electronimpact(70 eV)mode and the temperature of the ion3.2rV凵中国煤化工source was 230C. The mass range scanned wasCNMHGand the Soxhletfrom 30 to 500 amu. Data were acquired and proc. techniessed using Chemstation software. The compounds extraction yields for MAE (373 K) and the SoxhletCHENMicrowave-assisted extraction of shenfu coal and its macromolecule structurtechniques were 7.85% and 5.42%, respectively. The strongly absorb microwave energy because they haveprinciple of microwave heating is based on the direct a permanent dipole moment that is affected by theeffect of microwaves on molecules by ionic conduc- microwaves. Acetone, with high dielectric constanttion or polarization of the molecules. Hence micro- and relatively strong polarity, favors replacing themicrowave ng is highly efficient. The absorption of coal electron-donor participants under MAE.Thisthree mechanisms. First, the sample can be immersed Soxhlet extraction, of coal.in a solvent, or a mixture of solvents, that stronglyabsorbs microwave energy. Second, the sample can3.3 GC/MS analysis of the acetone-extractablebe extracted in a combined solvent containing sol-fractionvents with both high and low dielectric losses, mixeTable 1 lists the compounds identified in the oils,in various proportions. Third, a sample with a high asphaltenes and preasphaltenes of the acetone-exdielectric loss can be extracted in a microwave trans- tractable fraction of Shenfu coal. Fig. 2 shows theparent solvent 2). Extraction and partitioning of sol- total ion chromatograms(TICs)of the oil, asphalteneutes can occur by any combination of these three and preasphaltene fractionsMAE modes. Polar molecules and ionic solutionsTable 1 Compounds identified in oil, asphaltene and preasphaltene from the acetone-extractable fractionCompounds1,3-Dimethyl-3, 4, 5, 6-tetrahydro-(IH)pyrimidineButylatedhydroxytoluene2 3-(1, 1-DimethylethylH-methoxy-phen3 Heptadecane3 1, 2, 3, 4-Tetrahydro-5, 6, 7, 8-tetramethylnaphthalene 3 9-Methylacridineethyl-4(I-m4 3-Methyl-2-proplonyl-benzoic acid7 Octadecane5 1. 6-Dimethyl-4I-methylethyl)naphthalene5 1, 4-Dimethyl-7-(I-methylethyl-azulene8 Phytane66-Ethyl-1 2,3 4-tetrahydro-l, I 4, 4-tetramethylnapbNonadecane7 14. 5-TrimethyInaphthalene7 Hexadecanoic acid methyl ester8 ,2,3, 4, 4a.9, 10, 10a-0ctahydro-I, 1, 4a-trimethylphe 8 n-Hexadecanoic acid11 2, 7, 10-Trimethyl-Octadecane9 1, 2, 3, 4-Tetramethylnaphthalene12 2, 6, 10, 15-Tetra-methylheptadecane 10 7-Buryl-l-bexylnaphthalene11 1-Methyl-7-(1-methylethyl)-phenanthrene13 Heneicosane11 4, 4-Diacetyldiphenyimethane124 4-Methylenebis-(2, 6-bis(I, l-dimethylethyI)F-phenol4. 5. 8, 10, 13-Hexame-thylhe13 2-Methyl-3-phenyl-lH-indole20 Tricosane14 1-Methyl-7-I-methylethyl)phenanthrene15 1-Methyl((4-propylpheny)ethynyl -benzeneirene20 3, 6-Dimethylphenanthrene21 2, 5-DimethylpheaanthreneThe number of organic compounds(OCs)identi- 3.3.1 Oilsfied in the acetone-extractable fraction from the oilsAs shown in Table 1, the oils comprise normal alasphaltenes and preasphaltenes were eighteen, kanes, branched alkanes and cycloalkanes. Normaltwenty-one and thirteen, respectively. Mass spectra of alkanes range from C12 to C28 and Cl2 has the mostmany of the OCs correspond well when compared to abundance, being 29.791%. The total content, and thelibrary spectra. Others are fairly or largely consistent number, of branched alkanes is lower than that ofwith corresponding library compoundsnormal alkanes. Five branched alkanes have beenlude normal alkanes, branched alkanes, cycloalkanes, thy hexadecane is the most abundant. As listed in Ta-non-substituted arenes, substituted arenes and or- blegano-oxygen and organo-nitrogen compounds Some was中国煤某某化macromolecular compounds in the extracts cannot be in theCN GEree and adsorbeddetected with GC/MSeasuy exuacueu Dy MAEVol 19 No. 12535Time(min)(b)Asphaltene(e) PreasphalteneFig 2 Total ion chromatogram of Shenfu coal fractions from MAE3.3.2 Asphaltenestenes too. as also shown in Table 1. the oxo-com-Also listed in Table I are the majority of the spe- pounds detected include octadecanoic acid, mecies detected in the asphaltenes. These are aromatic ester, n-hexadecanoic acid and 4, 4-methylene-rings, including benzene, naphthalene, phenanthrene bis[2, 6-bis(1, 1-dimethylethyl)l-Phenol. It is supposedand anthracene rings, which are probably located in that the polar substances in coal are entangled in thethe polycyclic nuclei. Among these species 1- three-dimensional network of the coal and cannot beMethyl-7-(1-methylethyl)-phenanthrene and alkyl easily extracted. When MaE is applied the hot ace-naphthalenes have higher abundance. The alkyl sub- tone can disrupt the cross-linked structure, which re-stituents are mainly small radicals, such as methyl, sults in small molecules. This will be beneficial forbutyl and hexyl, for which microwaves have a strong extraction of the polar fractionpenetration ability. The heating time is thus shortened,the extraction efficiency is improved and more com-3.4 NMR parameters of coalpounds are extractedIn the previous section the composition of the ace-3.3.3 Preasphalkenestone-extractable fraction as determined by a GC/MSThe compounds separated from preasphaltene are method was described. However, the structure of thealso listed in Table 1. The structural features of these residue is still unclear. It is well known that spectralsubstances are not similar to asphaltenes. In methods, such as NMR, IR, or XPS, can be used topreasphaltenes there are fewer polycyclic compounds obtain structural information from the residue.and more heterocyclic compounds. Hetero-nitrogen Among these methods nMr is the most suitable forcompounds that were detected include 9-Methy- our aim, since we need to acquire structural informa-lacridine and 2-Methyl-3-phenyl-lH-indole. More- tion of both the surface layer and also deeper insidever, the nitrogen-heteroatom compounds were theYDSce analytical tech-greater in number and total amount than were the nique中国煤化 IMR technique tooxo-heteroatom compounds. The preasphaltene sub- carrystances containing oxygen and nitrogen were greaterCN MH Graf Shenfu coalin number and total amount than those of the asphal- (daf) and its residue. The peaks correspond to differCHEN Hong et aldecile struent carbon-functional groups, including carbonyl priate tool for structure determination of coal. SomeC=O, CHO), carboxyl( COOH)and aromatic carbon. useful information, for example the aromaticity factorThirteen different carbon functional groups are found a), the proportion of aromatic carbon atoms in thein the results. The corresponding data are presented in carbon-functional groups, can be computed from thTable 2, which indicates thatC NMR is an appro- NMR data from Eq ( 2)g20025050050100150200250Chemical shift (10Chemical shift (10(a) Shenfu coal(b)ResidueFig 3 Curve-fitted"C-NMR spectra of SF coalTable 2 parameters of the macromolecular structure derived from C NMR of Shenfu coal and its residueSampleSFcoal066203440.1340.1060.0710.1090.187Residue0678032019010001010702950659017000f(2) 3.5 Flow chart of the program. ppmWe employed a hybrid genetic algorithm to simuns late fragments of Shenfu coal macromolecular strucand A2o-14opm is the total area of the carbon-functional ture based on our analyses. The technique for settinggroups. The value f, is subdivided into f, which isthe hybrid genetic algorithm parameters and a flowchart is shown in Fig 4.the fraction of carbonyl and carboxyl carbons, and f rwhich is the fraction of sp-hybridized carbons pre-aliphatic and aromatic carbonin aromatic rings. The value of f is subdi-ble aoin and updateinto protonated( )and non-protonated aro-matic carbons. The non-protonated aromatic carbonsUpda te pc arraydate bondingare further subdivided into the fractions of phenolic(r), alkylated ( and bridgehead )carbons4 Output resultsThe fraction of aliphatic carbons is labeled f,. Thisvalue is divided into the fraction of CH and CH2Fig 4 Program flow chart of the hybrid genetic algorithmgroups(H)and the fraction of CH3 groups (f)Following the flow chart, a fragment of Shenfucoal macromolecule with a formula weight of 3000The aliphatic carbons that are bonded to oxygen are was predicted in Fig. 5labeled as fr. The values of these parameters havebeen obtained by curve-fitting analysis of theI3C-NMR spectrum and are listed in Table 2- CHeHCe, For a comparison to the Shenfu coal ( daf) f a, mostclearly as the H/C atomic ratio decreases there is anfa of the residue. In terms of thetwo-component concept, some small moleculesdropped off the coal macromolecule, which leads to aH/C decrease and an fa increase. According to theseparameters, structural differences between Shenfucoal (daf) and its residue after acetone extractionsuggest that extracted substances should be consid-中国煤化工ered simple components of the building blocks fromwhich the coal matrix has been formed. The coal daf)CNMHGOframework was not destroyed after extractionFig. 5 A fragment of Shenfu coal macromolecule structureMining Science and TechnologyVoL 19 No. 14 Conclusionsvent at room temperature: effect of coal rank and syner-gism of the mixed solvent. Fuel, 1988(67): 1639-16471)Results from the present study indicate that us-ra K, Shimada M, Mae K, Yoo s H. Extraction ofg microwave-assisted extraction for not more thanoal below 350C in a flowing non- polar solvent. Fuel,2001(80):1573-1582ne hour can yield a higher extraction than that of se. [4] Qin Z H, Wei X Y, Jiang C, Sun H, Xin J N, Zong ZMThe acetone extractable fraction consists of aliphaticFTIR analysis of extracts from two bituminoushydrocarbons, aromatic hydrocarbons, oxygen andwith CS using extraction in batches. Journal ofnitrogen containing compounds and heterocyclicUniversity of mining Technolog, 2005, 34(5)compounds. Aliphatic hydrocarbons are mainly in the [5] Ding ML, Zong ZM, ZongY, ouyang X D, Huang YGoil, aromatic hydrocarbons containingoxygen andZhou L, Zheng Y X, Zhou x, Wei Y B, Wei x Y Groupnitrogen atoms and heterocyclic compounds are in theseparation and analysis of a carbon disulfide-solubleasphaltenes and preasphaltenes. Moreover, oxygenfraction from Shenfu coal by column chromatographcontaining compounds are in greater number and totalJournal of China University of Mining Technologyamount compared to nitrogen containing compounds2008,18(1):27-32Yet nitrogen-heteroatomic compounds are in greater[6 Shui H F, Wang Z C, Wang G Q Effect of hydrothermaltreatment on the extraction of coal in the CS/NMPnumber and amount compared to oxo-heteroatomicmixed solvent. Fuel, 2006(85): 1798-1802compounds[7 LiC Q, Takanohashi T, Yoshida T Effect of acid treat-2)A Shenfu coal macromolecular model is assemment on thermal extraction yield in ashless coal producFuel,2004(83):127-732a self-adaptive genetic algorithm. The formula weight [8 Meikap B C, Purohit n K, Mahadevan v Eftect of miis 2974.5. with an error of 0.88% to 3000. Themodels composition of C(75.10%), H (6.27%), Nrheological characteristics of coal-water slurriesnal of Colloid and interface Science, 2005(281)(41%),0(16 14%),s(1.08%)is very close to theanalytical results of C(75.70%), H(5.208%), N[9] Wang TX, Zong Z M, Zhang J W, Wei Y B, Zhao w, Li(1348%,0(16.60%),andS(0.305%).BM, Wei X Y Microwave-assisted hydroconversion ofdemineralized coal liquefaction residues from ShenfuAcknowledgementsand Shengli coals. Fuel, 2008(87): 498-507.[10] Seehra M S, KaIra A, Manivannan A Dewatering of finecoal slurries by selective heating with microwaves. Fuel,The authors gratefully acknowledge the financial2007(86):829834support from the National Natural Science Founda- [11] Marland S, Han B, Merchant A, Rowson N. The effection of China(No. 40472082)of microwave radiation on coal grindability. Fuel, 200079):1283-1288.[12]Sun Y F, Takaoka M, Takeda N, Matsumoto T, Oshita K.ReferenceApplication of microwave-assistedh fromon to theanalysis of PCBs and CBZs in fly ask[1] Renganathan K, Zondlo J W, Mintz E A, Kneisl P, StillerA H. Preparation of an ultra-low ash coal extract under2006(137):106-112mild conditions, Fuel Process Technol, 1988(18): 273178[2] Iino M, Takanohashi T, Ohsuga H, Toda K. Extraction中国煤化工CNMHG