Themolecularrepresentationsofcoal–Areview
JonathanP.Mathewsa,b,⇑,AlanL.ChaffeecaTheJohnandWillieLeoneFamilyDepartmentofEnergy&MineralEngineering,ThePennsylvaniaStateUniversity,126HoslerBuilding,UniversityPark,PA16802,USATheEMSEnergyInstitute,ThePennsylvaniaStateUniversity,126HoslerBuilding,UniversityPark,PA16802,USAcSchoolofChemistry,MonashUniversity,Vic3800,Australiabarticleinfoabstract
Between1942and2010therewere>134proposedmolecularlevelrepresentations(models)ofcoal.Whiletheyspannedtherankrange,bituminousrepresentationsarethebulk,withfarfewerlignite,andveryfewsubbituminousoranthraciterepresentations.Theyhavetransitionedfrompredominantly2Dpenandpaperdrawingsinto3Dcomputationalstructures,andhaverecentlyincreasedincomplexity,andtoalimiteddegree,inscale.Advancesinanalyticaltechniquesaswellasmodelingsoftware,andcomputationpowerhaveresultedinimprovedpartialrepresentationsofcoalstructure.Computeraideddesignhashelpedtoovercomesomeofthechallengesinmodelconstructionforafewmodels.Yetgen-erallyitisthecapturingoftheconstitutionofcoalthatremainselusive.Evaluationofphysicalparame-tersandbehaviorobservationshasaidedourconfidenceintherepresentationsbutmodelsaretypicallygeneratedforaspecificuse.Nomodelhasfacedthegambitof‘‘tests’’.Ó2011ElsevierLtd.Allrightsreserved.Articlehistory:Received14June2011Receivedinrevisedform6November2011Accepted8November2011Availableonline30December2011Keywords:CoalstructureCoalmolecularmodelingCoalmodelsContentsIntroduction...........................................................................................................1Coalmodelshistoricoverview............................................................................................1Modelconstructionstrategies.............................................................................................7Molecularrepresentationsofcoal..........................................................................................84.1.Themodelsoflignite/browncoals....................................................................................84.2.Themodelsofsubbituminouscoals...................................................................................94.3.Themodelsofbituminouscoals.....................................................................................104.4.Themodelsofanthracitecoals......................................................................................125.Concludingcomments..................................................................................................12AppendixA.Supplementarydata...........................................................................................12References...........................................................................................................121.2.3.4.1.IntroductionThecoalliterature,overthelast66years,hasgeneratedasur-prisinglylargenumber(>133)ofmolecularlevelrepresentationsofcoal(orcoalextracts):1940s[1–3],1950s,1960s[4–9],1970s[10–13],1980s[14–33],1990s[34–61],and>2000s[62–82].AfewareverywellknownsuchasGiven[5],Wiser[22],Wender[11],Solomon[19],andShinn[23]models,butfarmoreareob-scure.Whileexcellentreviewsofcoalstructureexist[83–88]andarecentkerogenreview[89],thatincludescoalmodels,theirfocus⇑Correspondingauthor.E-mailaddresses:jmathews@psu.edu(J.P.Mathews),alan.chaffee@sci.monash.edu.au(A.L.Chaffee).0016-2361/$-seefrontmatterÓ2011ElsevierLtd.Allrightsreserved.doi:10.1016/j.fuel.2011.11.025hasmainlybeenoncoalstructure.Thispaperprovidesadedicatedreviewofthehistoryandadvancesinthestructuralrepresenta-tionsofcoal.Theusefulnessandapplicationofcoalmolecularrep-resentationswasrecentlyevaluated[90].2.CoalmodelshistoricoverviewFigs.1–4showtheselectedmodelsofcoalgroupedaccordingtorank.ThefirstcoalmodelwasgeneratedattheStateCollegeofPennsylvania(nowthePennsylvaniaStateUniversity)in1942[1].Whilethismodelsetthestageforfurtheradvancementsthegenerationofa2Dstructurewasaccompaniedwitha‘‘real’’vanderWaalsrepresentation.The3rddimension(spacefillingmodels)incoalrepresentationswouldnotappearagainuntiltheSpiro[20],2J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–14SpiroandKosky[21]andMeyers[18]modelsofthe1980sandthefirstcomputationalviewsgeneratedbyCarlsonintheearly1990s[35].Throughthe1940smodelswereproposedbyGillet[2,3].Ma-jorimprovementsweremadeintheworkofGiven(alsoatPennState)inthe1960safteradecadelullinwhichnomodelsweregenerated.Thewell-knownGivenmodelfirstappearedinthejour-nalNature[91]beforebeingreproducedinthejournalFuel[5](Fig.3b).Itwasslightlyrefinedwithatweakingofthehydroaro-maticstructurein1961[92].Whilethestructurewasnota‘‘mod-el’’perse,itsintendedfunctionwastoshowthetypesofhydrogenstructuresinthevitrain-richbituminouscoal,ithasbeenwidelyadoptedasastructuralrepresentation.Asecondary,lesserknown,variantofa‘‘typicalstructure’’ofabituminouscoalfollowed[92].Thechangewasamoveawayfroma9,10-dihydroanthracenetoa9,10-dihydrophenanthreneconfigurationoffusedhydroaromaticrings(Fig.3c).Anadditionaladaptedmodelfromtheearlierworkwasalsopublished[8]aswasasimplified3Dmodelconsistingofgluedflatandbenthexagonaldisks[9].TheHillhigh-volatilebituminousmodelof1962wasasignifi-cantimprovementinscale(C597H1428O81N14S6),andappearedonthejournalcoverinawhiterepresentationoverablackback-ground,with‘‘COAL’’inlargeredlettering(Fig.3h)[7].Yetitisoneofthemanyrelativelyobscurerepresentations.The1970sbroughtthewellknownWiserbituminousmodel(Fig.3i)[22]alongwiththenowrejectedpolyadamantanepostulationofCha-krabarttyandBerkowitz[10].Their‘‘structuralalternative’’carbonskeletalrepresentationshadanincreasingscalerepresentingtheranktransitionfrom76%Cto90%C(wt.basis).Thisinjectedpas-sionintothecoalstructuredebate,butwasalmostimmediatelyre-jectedasastructuralentity[93–96].ThenotedcoalscientistsWender[11]andPitt[12]alsoproducedtheirstructuresattheendofthedecade.TheWendermodelsrepresentedanearlyat-tempttoillustratethestructuraldifferencesasafunctionofrankwithfourverydifferentpartial-structureentities(Figs.1aand5aandb).ThePittmodelsalsocapturedcoalificationfrom80%to90%Cforavitraincoalbyalterationofthe80%Cmodel(functionalgroupmodification,oxygenremoval,andaromatization)[12].HeredyandWender[14]generatedtheirbituminousmodelin1980withthenowfamiliarmodelformat:smalleraromaticringstructureswithcross-links.Previousmodelstendedtobebasedonundulating(non-condensed)ringcatenation,asinthecaseofPitt,orhydroaromaticandaromaticringcatenation,asseenintheFuchsmodel(Fig.3a).Oberlinetal.[15]generatedaKuckersitemodel(araretypeIIkerogenthatis‘‘bituminous’’innature)withartisticflair,forthefirsttimerepresentingacoalasanassemblageofsmallerfragmentswithstrongrelationshiptotheirorganicpre-cursors.Italsocapturedthephenomenonofaromatic(p–p)stack-ingwhichMeyersalsocaptured[18].Whitehurstetal.[16]developedsmallskeletalmodelsfortherankrangeasanaidfordiscussion.Iwataetal.[17]generatedsimplemolecularrepresen-tationsofthreeJapanesebituminouscoals.Thelargestofthese‘‘average’’structurescontained45atoms.Meyers[18]generateda3D‘‘stick’’modelvariantoftheWiser[22]model.ThewellknownSolomon(Fig.3g)[19]bituminousmodelwasalsopro-ducedaroundthistime.Histhree-fragmentmodelincludedhydro-genbondingexplicitly.Spiro[20]generated3Dspace-fillingmodelsoffourofthewellknownmodels(Wiser,Given,Solomon,andHerdyandWender(modelconstructedbutnotshowninthepaper)).Toenable3Dspace-fillingstructures,threeofthestructuresrequiredalterationduetostericinterferences.Heconcludedthatthethirddimensionshouldbeincludedwithotherparametersformodelgenerationformoreappropriatemodelstructures.LaterSpiroandKosky[21]ex-tendedtherankrangewithlow-intermediateandhigh-rankmole-culesin3Drepresentationsandalsoattempted‘‘eureka’’typevolumedisplacementmeasurements,thefirstevaluationofacoalmodelphysicalparameter.TheShinn[23]bituminousmodel(Fig.3j)isperhapsthemostcomprehensive1980srepresentationandcapturedtheragingdebateconcerninganextractable/mobilephase[97,98]withhis2Dbituminouscoalmodel.Thiswascreatedatalargerscale(10,000amu)andcontainedthreerelativelysmallunconnectedmolecularentities,basedonliquefactionproducts,heldwithinalarger‘‘encasement’’molecule.Fig.1.Themolecularrepresentationsofligniteandbrowncoals:(a)adaptedfromWender(*indicatingconnectivitypoints)[11],(b)Philipetal.[24],(c)Wolfrum[25],(d)MillyaandZingaro[26],(e)TrompandMoulijn[30],(f)HuttingerandMichenfelder[28],(g)Kumagaietal.[58],and(h)Patrakovetal.[68].Structuresarereprintedwithpermissionofthecopyrightholders.J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–143Fig.2.Themolecularrepresentationsofsubbituminouscoal:(a)Shinn[51],(b)Nomuraetal.[59],and(c)Hatcher[34].Structuresarereprintedwithpermissionofthecopyrightholders.Severallignite/browncoalmodelsappearedin1984book‘‘Thechemistryoflow-rankcoals’’[24–26].Anotherearlybrowncoalmodelwasgeneratedin1987byHuttingerandMichenfelder[28]andincludedcations(calcium,potassium,iron,aluminum,andsodium)asastructuralentity,butnotwater(Fig.1f).Thehigh-vitrinitecokingcoalmodelofLazarovandMarinov[29]wasalargemoleculewithtwoadditionalhydrogen-boundmoietiesandintramolecularhydrogen-bonding.Alongwithtraditionalhydrogen-bonding,hydrogen-bondingtonitrogenstructureswasalsoindicatedinthis2Dstructure.TheonlysemianthracitemodelwasgeneratedalongwithasmalllignitemodelbyTrompandMoulijn[30]inthesameyear(Fig.5d).Thesemianthracitemodelhadarangeofaromaticringsizesuptosixcondensedrings.Hatch-eretal.[31,34,37,43]generatedarangeofmodelsfromlignitetosubbituminouscoal.Thesemodelsemphasizedcoalificationandtherelationshipbetweencoalandligninandwerestronglyinflu-encedbythepriorAdlerligninmodel[99].Thesemodelsspecifi-callyaccountfordemethoxylation,catecholformation,aryl-etherbondcleavage,andcarboncross-links.Thefirstapplicationofcomputational3DrepresentationstocoalmodelswerebyCarlson[35](1992),butthemolecularmod-elingofaromaticstackinginpitch,byVorpagelandLavin[100]inthesameyear,isalsoapplicabletocoal[101].Thispermittedcomparisonandrelativecontributionofbondingandnon-bondinginteractionenergiesofpreviouslypublishedmodelsofWiser(Fig.3i),Solomon(Fig.3g),andShinn(Fig.3j).Thecomputationalapproachallowedcalculationofaphysicalparameter,simulatedheliumdensity,forthefirsttimesincetheSpiroandKosky[21]waterdisplacementevaluation.FollowingtheCarlsonworkgener-ating3Dcomputationalrepresentationsbecamecommon.Nomuraetal.[38]introducedtheAkabirabituminousmodelalsowitharangeofringsizesandlongchainaliphaticsbasedonextractdata,Curiepointpyrolysisandwithcrosspolarizationmagicanglespin-ning(CPMAS)NMRdata.The3DmodelwasmanuallyassembledviaareverseengineeringapproachsimilartothatofShinn.Murataetal.[39]alsousedpreviouslygenerated[17]smallstructuralenti-ties(simplifiedcoalmodels)representingarankrangeofJapanesecoals,thatweretreatedasoligomersforsuccessfulagreementwithexperimentalversusdensitycalculations.Infollowingpapersthey(Dongetal.[40])utilizedcomputationalaideddesigntoexaminethedensityoffivevariantsoftheearliermodeltodiscerncross-linking(modelflexibility)applicability.Modelswithoutextensivecovalentcross-linkingweremoreconsistentwithdensitycalcula-tionsbetteragreeingwithphysicalmeasurements.Theyutilizedperiodicboundaryconditionsandmoleculardynamics/mechanicstogenerate‘‘semistableconformers’’.Faulonetal.[41]contributedtocomputeraideddesignincoalmodelconstructionwiththeSIG-NATUREprogramdesignedtogenerateandassemblemolecularrepresentationsbasedonassembling(flash-pyrolysis)fragmentsviadesignatedcross-linking/hydroaromaticclusterformationtobeconsistentwithelemental,NMR,andotherparameterswithinallowedlimits.Multiplesolutionscouldbegeneratedandcon-structed.Theyalsofollowedupwithphysicalevaluationofdensityandporesizedistribution[45].ThePORprogram[45]immersedmodelsinavolumeof1A3cellsanddeterminedifthecellswerewithinvanderWallsradiiorininaccessibleoraccessiblepore4J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–14Fig.3.Themolecularrepresentationsofbituminouscoal:(a)FuchsandSandoffstructure[1],(b)Given[5],(c)adaptedfromGiven[8],(d)Meyers[18],(e)CartzandHirsch[4],(f)Ladner(asprintedinGibson)[133],(g)Solomon[19],(h)HillandLyon[7],(i)adaptedfromWiser[22],(j)Shinn[23].Structuresarereprintedwithpermissionofthecopyrightholders.space.Othershaveusedcomputer-basedstochiasticgenerationap-proachestoaidovercomingtheissuesofscale-limiteddiversity.Whiletheapproachisnotcommonforgeneratingrepresentations,itismorecommonlyemployedtoobtainalarge-scaledistributionofstructuralfeaturesusuallycoupledtoreactivestudies.Theap-proachhasbeenusedforasphaltenes,kerogen[102],andcoal[50].J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–145Fig.4.Continuationofthemolecularrepresentationsofbituminouscoal:thespace-fillingrepresentationofSpiro(a–f)[20](a)Wisermodel,(b)Givenmodel,(c)Solomonmodel,(d)Wisermodel,(e)Solomonmodel,(f)Solomonmodelglobularconfiguration,(g)LazarovandMarinov[29],(h)ZaoZhuangcoalmodel(Nomura,Artok,Murata,Yamamoto,Hama,Gao,andKidena),(i)Takanahashietal.UpperFreeportmodel,(j)NarkiewiczandMathews[73],k-l)inertinite-richandvitrinite-richmodelsofVanNiekerkandMathews[82].Structuresarereprintedwithpermissionofthecopyrightholders.6J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–14Fig.5.Themolecularrepresentationsofanthracitecoal,aandb)adaptedfromWender(*indicatingconnectivitypoints)[11](c)SpiroandKosky[21],(d)semianthracitemodelofTrompandMoulijn[30],(e)Fragmentsofan85%and75%aromaticcarbon‘‘anthracite’’byVishnyakovetal.[55],(f)representationofJeddoAnthracite,adaptedfromPappanoetal.[60].Structuresarereprintedwithpermissionofthecopyrightholders.J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–147Coal-specificmodelingadvancesfollowedwiththeCS2/NMPex-tractworkofIinogroup,specificallywithextractfractionsbeingproducedfromtheacetoneinsolublesub-fractionswithbothpyr-idinesolubleandinsolublemodelsbeingproduced(Takanohashietal.[46]).AmorecomplexmodelofZaoZhuangChinesebitumi-nouscoalfollowedutilizingasimilarextractionprocessbutwithamoreextensivefractionationtogenerateamodelwithfivecuts,somewithmultiplefragments,thatwererecombinedintoanasso-ciatedstructure(‘‘anisotropicmodel’’)[49].Shinnproducedasub-bituminousmodelin1996(Fig.2a)[51].Nomuraetal.[52]generatedlow-rankAustralianandIndonesiancoalsmodels.Ohk-awaetal.[53]utilizedcomputeraidedconstructiontotheAkabirabituminouscoalwithconnectionsbetweennodesbeingselectedbasedonlowersterichindranceasdeterminedbyenergyevalua-tionsforenhancedconstructionefficiency.Ibayashietal.[103]andTanakaetal.[104]alsoutilizedanefficientconstructionalap-proachbasedona‘‘geneticalgorithm’’todeterminelowestenergyconfigurations.BasedontheirearlierworkTakanohashietal.[54]generatedanassociatedversionoftheUpperFreeportArgonnePremiumcoalextracts.ThemodelconsistedofthreeCS2/NMPsub-fractions:acetonesoluble,pyridinesoluble,andpyridineinsoluble.Theyexaminedthestabilityofassociatedmolecularstructuresandtheinteractionwithvarioussolvents,includingswelling[61,105].Vishnyakovetal.[55]generatedanthracitemodelsforuseinexaminingmethaneadsorptionandphasestransitionsinslitsofvariouswidths.Jonesetal.[57]createdamodelofPittsburghNo.8bituminouscoalforutilizationinacharformationstudy.Kumagaietal.[58]studiedaYallournlow-rankcoalandspecifi-callymodeledtheroleofwater,afirstinthecomputationalandnon-computationalcoalmodels.Theyweresuccessfulinsimulat-ingirreversiblevolumetricchangeswithmoisturerelease[58].Pappano[63]generatedfourmodelsofanthracitestructures,someofwhichcontained80orsoringsinmultiplestacks,onlyoneofthesemodelsisavailableoutsideofthethesis[60].Attheturnofthe21stcentury,thestate-of-the-artincoalrepresentationwascomputational3Dmodelsoflimitedscale,andonecouldargue,limitedapplicabilitybeyondtheirintendeduse.Theadaptationofcomputationalapproachesintheearly1990slimitedthescaletothatavailableviasoftwareandorcomputationallimitations.Thesewere,inmanycases,smallerthanthemodelsgeneratedin2Dwithouttheaidofcomputers.AdvancementstotheUpperFreeportassociatedmodelcontin-uedwithresidueinclusionandNMRsimulationresultinginex-tractsandresiduemodelmodifications[65,67].Thiswasthefirstwholecoalmodelthathadcontributionsfromallofthefractions.Laterthismodelwasutilizedtodeterminerelaxationbehavioroftheassociatedstructureandconcurrentvolumechangebysimu-latedheattreatmentcapturedwithmoleculardynamics[106].Mathewsetal.[64]alsogeneratedmolecularstructuresatthistimeforUpperFreeportandLewiston–Strocktonvitrinites.Bothmodelsweresinglemolecularentitiesthatignoredmolecularweightdistribution.Theywereutilizedincharmodelgenerationsimulatingthedevolatilizationprocess[107,108].AHyperCoalex-tractmodelwasgeneratedbyTakanohashietal.[66]usedcompu-tationalapproaches(moleculardynamics)toidentifyphysicalcross-links;thesebeinghydrogenbondedandcationbridgingbe-tweencarboxylicgroups.Thefirstliptinite-rich(Barzas)coalmolecularrepresentationwasgeneratedbyPatrakovetal.[68],thisstructurewaslarge(10,260amu)consistingofcopiousquanti-tiesofcondensed-linked(butnotcross-linked)hydroaromaticrings(Fig.1h).Vuetal.[69]developeda3Dmodelofalow-rankcoalifiedwood(Australian).Thismodelalsoincludedwaterasastructuralentity,andexaminedcoal–water,coal–cationinteractions.Molec-ulardynamicsimulationsdemonstratedrestrictedmobilityofwaterintheporesduebothtotheconfiningnatureandcoal–waterhydrogenbonding.Thiswasenhancedwiththepresenceofcat-ions.Domazetisetal.[70,71,76,78,109]alsogeneratedlow-rankcoalmodelstoexamineinorganicinteractionswiththecoalwithhigherlevelmodelingapproaches.AleapinscaleoccurredwiththeNarkiewiczandMathews[73]modelofPocahontasNo.3coal.ThisArgonnePremiumcoalhasbeenwellstudiedandastructuralreviewwasavailable[110].Thismodelcontained>20,000atomsandcontainedanextensivemolecularweightdistributionandwas‘‘squashed’’preferentiallytoinducestructuralalignment.ItwaslaterusedtoaidinvisualizingissuesofinterestinCO2seques-trationwithCO2,CH4,andH2Obeing‘‘loaded’’intothemodel[111].Thelarge-scaleeffortofvanVanNiekerkandMathews[82]alsocontainedamolecularweightdistribution,solvent-swell-ing,[74]andsolubilitycalculations[112]forthemolecularentitiesforinertinite-richandvitrinite-richSouthAfricancoals.Advance-mentsinscalewerealsomadewithanautomatedaromaticstruc-turegenerationapproachdirectlyfromHRTEMlatticefringeimages(Fringe3D)[113,114].Withthisapproach,>20,000atomnon-cross-linkedslicemodelrepresentationscanbegeneratedeasilythatincludestheorientation,stackingandthosedistribu-tionsforthearomaticmoietiesofcoals.Scriptingpopulatedthestructurewithheteroatomsandaliphaticstructurestomeetde-siredlevels.Thelatestcoalmodel(Morwellbrowncoal)hasbeengeneratedinaccordancewith2DNMRandFTIRdata[80]andusedtofollowthermalmaturationviaareactive(bond-breakingandbond-forming)moleculardynamicsapproach(ReaxFF)[115].3.ModelconstructionstrategiesThereareavarietyofconstructionapproachesusedinthegen-erationofcoalmodels.Whileitisnotimplicitlystated,theearlymodelswerelikelyconstructedwithlittlemorethanintellect,pa-per,andpencilwiththemodeldraftbeingconvertedintoa2Ddrawingwiththeaidofchemistrystencils.The3DvanderWaalsorDriedingphysicalmodels[18,20,21]wereinitially,simplyextensionsofthisapproach.Latermodelswerethenconstructedinasimilarmanner,oranearliermodelwasmanipulatedtorepre-sentthenewcoalofinterest.Thislatterapproachiscommoninthelow-rankmodelswhereligninmodels[99]arecommonlyadaptedthustakingadvantageofitsknownchemistry.Thescaleofcoalmodelsareheavilyinfluencedbytheintendeduse,withthefewlargerscalemodelsbeinggeneratedforuseinfollowingprocessessuchaspyrolysisorliquefaction.Small-scalemodelswereforcoal-ificationtransitionsortosimplyaidintheillustrationofstructuraldifferences.Theearlymodelswerebasedsolelyonchemistryandtoalim-itedextentonexpectedbehaviors.Spiro[20]introducedthefirstofthephysicalparameters,sterichindranceandalteredthestruc-turesofseveraloftheearlymodelstobetteraccountforthethirddimension.Thisapproachwasexpandedoncethecomputationalrepresentationappearedanddensitywasalsoconsidered[35,39,40,44,45,47,116].Itistheconstructionofmodelsthatisthetimeconsumingandchallengingundertaking.Thefirstcom-puteraideddesign(elucidation)andconstructionforcoalsap-pearedintheearly1990s[37,41]andseveraldifferentgroups[53,103,104]proposedelucidationandconstructionalapproaches.Initiallytheconstraintsofthesoftwareandhardwarerestrictedthescaleofthemodeltosizessmallerthanthelargestpenandpapermodels.Considerationofstructuralisomersandstatisticalsam-plingfurtherexpandedtherealizationthatmanyrepresentationscouldbeconstructed[44]withtherelativelysimpleconstraintsofaveragevaluessuchaselementalcompositionandlimitedNMRparameters(sometimesonlyaromaticity).Latermodelscap-italizedontheNMRadvanceswithprotonNMRofanexpanded8J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–14datasetfromdipolardephasingandspectralpeakdeconvolution.Structuralfragmentsweregleanedbydestructivetechniquessuchasflash-pyrolysis[37,41,64]orwetchemistryapproachesthatim-posednecessarylimitationsonthecomponentmolecules.Struc-turaldiversityhowever,waslimitedbymodelscale.Therewerebehaviorevaluations,suchassolvent-swelling,wereperformedwithsimple[54,61,105]andcomplexmodels[74]demonstratingthattheextentofswellingwithwater[52]andcommonsolventscouldbecaptured.Inthecaseofalignitecoal,thestructuralcol-lapsewithwaterremovalbeyondthegel-point[52]aidedintheconfidencethatstructuralmodelscouldcapturesalientstructuralfeaturesandbasicbehaviors.Othermorenovelvalidationap-proacheshaveincludedNMRparameter/spectrapredictions[65,67,117],HRTEMsimulation[73],theoreticalsolubilitydistribu-tion[74,112],andwideangleX-rayscattering[118].Significantad-vancesinscalewererequiredformeaningfulincorporationofmolecularweight.Thelargestofthecoalmodelscurrentlyare>20,000atoms.Concurrently,advancesarebeingmadeintherep-resentationsofkerogens,humicacids,andcarbonmaterialswithReverseMonteCarloconstructionapproaches[119–123]beingsomeofthemanysignificantadvances.Nomodelhasyetcapturedthestructuralfeaturesofthemacer-alsthatcomprisethecoalsstructurealthoughrepresentationsofvitrinite[41,64],inertinite-rich[82],and‘‘liptinite-rich’’[68]havebeengenerated.HumicandaromaticportionswereincludedinboththeWolfrum[25]andtheDomazetis–Jamesbrowncoalmod-els[71].Furtheradvanceswitha‘‘supermodel’’,anassociationofsmallmolecularcomponentscomprisingsolubilityfractionsoftheUpperFreeportcoal,improvedthequalityofrepresentationsasdidhigher-levelcomputersimulationcapturingcations[71,75]andsimulationsincludingwater[69,81,111]asstructuralfactures.Recently,modelconstructiondirectlyfromHRTEMlatticefringeshasbeenattemptedtodirectlycapturethearomaticstackingandalignmentofthearomaticmoietyofcoal[113].Thisapproachhasgreatpotentialtoenablebetterrepresentationsofthosestruc-turalfeaturesatscaleandwithsignificantlyreducedconstructionandelucidationeffort.Afterall,itistheeffectiveuseofcoalmod-els,ratherthantheirconstruction,wheremoresignificantscien-tificcontributionscanbemade.4.Molecularrepresentationsofcoal4.1.Themodelsoflignite/browncoalsThedevelopmentofstructuralunderstandingoflignitelaggedbehindthatofbituminouscoalsandthefirstexplicitmodelofalig-niticcoalwaspublished,onlyin1976byWender(Fig.1a)[11].DrawingonUSexperience,Wenderputforwardaseriesof‘repre-sentativepartialstructures’forcoalsofdifferentrank,butqualifiedthemdeliberatelybycommenting:‘Thestructuresshownarenotcoalmodels.Buttheyrepresentaconvenientwayofcataloguingrep-resentativechemicalstructuressothatthereactionsofthevariousranksofcoalscanbeunderstood,atleastinapreliminaryway.Theyareusefulasanaidtomemoryandabasisforprediction;theyare,inshort,framesofreference.’Wender’smodel(Fig.1a)containedonly92atomswithamolecularformulaC42H40O10andhepresentsverylittlediscussionastoitsdevelopment.Nevertheless,itcap-turesanumberofessentialfeaturesoflignitestructure.Forexam-ple,itconsistsofsinglearomaticringslinkedandcross-linkedbyaliphaticsidechains.Theconceptofcross-linkingalsoisexplicitlycaptured.Theoxygenisinavarietyofforms(carboxylicacid,ke-tone,phenol,alcohol,ether,furan).Themodelalsoincorporatesanaryl-methoxygroupandsomeC3-aliphaticsidechains.Thelat-terareknowntobecommonbuildingunitsoflignin,oneofthemajorinputsoforganicmattertocoal.Inasimilarvein,thegroupofIwataetal.[17]developedrepresentativemodelsforJapanesecoalsofvariousrank.ThemodelforTempokuligniteproposedbyIwata[124]wasbasedonarepeatingstructuralunit(C21H20O5).Byvirtueofitssize,itdoesnotincorporatethestruc-turalheterogeneityinherentintheWendermodel,butitdoescon-ceiveligniteasapolymericentity.Bythemid-1980s,asligniteliquefactionworkwaswellunder-wayinresponsetotheoilcrisis[125],chemistswereactivelythinkingaboutlignitestructureandanumberofmodelsemerged.AnumberofthesewerefirstpresentedatanAmericanChemicalSocietysymposium,laterpublishedasanACSbook‘TheChemistryofLow-RankCoals’[126].AmodelputforwardbyPhilipetal.[24]proposedanaveragestructuralunit(C115H125O17NS)basedonachainofbenzofuranunits,togetherwithhydroaromaticandali-phaticsidechains(Fig.1b).Themodelwascreatedafterconsider-ationofliquefactionproductsofTexasligniteandisunderstoodasbeingderivedfromcellulose,ligninandotherplantcomponentsbydehydrationanddeoxygenationoftheoriginalorganicmatter.Thismodelintroducesanumberoffurtherimportantstructuralfea-turesofligniteforthefirsttime;specificallytheheteroatomsN(asindole)andS(asthiol),thepresenceofanesterifiedaliphaticsidechain,andtheinvolvementofH-bondingasamechanismofholdingstructuralsub-unitstogether.ThemodelbyWolfrum[25](Fig.1c)wasdevelopedfollowingdetailedanalyticalstudiesofRhenishbrowncoal.Beinglarger(C227H183O35N4S3CaFeAl),itincorporatesmorestructuralheteroge-neity.Aromaticringsarepresentaswellascondensedaromatic(andsomehydroaromatic)ofuptosevenringsinsizearelinkedbysmallaliphaticgroups.Heteroatomsarepresentinanumberoffunctionalforms–amide,amine,hydroxylamineandindoleforN;thiol,thiopheneandthioetherforS.Thecoordinationofme-talatoms(presumablyascations,thoughthisisnotwhatisshown)topolarfunctionalgroups,arenowexplicitlyrecognized.Interest-inglytheWolfrummodeldoesnotincorporateanylongaliphaticchainsorincludeanyesterlinkages.Themodel(C122H146O23N2SM4)byMillyaandZingaro[26],per-tinenttoWilcoxlignite,doesincorporatelongaliphaticchainslinkingsmallerhydroaromaticunits(Fig.1d).ConsequentlythisprovidesthehighestH/Cratio(1.25)ofanymodelintheliterature.Thisisthefirstmodelthatdepictsligniteasacompositeofdiscretefragmentsandthatthenfurthersuggestshowthefragmentsmaybeboundtogether–thatisbymetalcations,whichactas‘coordi-natingbridges’,principallybybridgingbetweenphenolandcar-boxylicacidsites.In1987,TrompandMoulijn[30]publishedamodeloflignite(C161H185O48N2S1M4,Fig.1e)asanaidetounderstandingitspyro-lysisbehavior.Inthismodelthereisaclearprevalenceofstruc-turalunitscontaininganisolatedaromaticring(C6),usuallywithmethoxyand/orhydroxylsubstitution,andconnectedtoaC3ali-phaticsidechain(C6AC3unit).Thisisthecharacteristicbuildingblockoflignin.Themodelalsocontainsesterlinkedaliphaticsidechains,mono-andmulti-valentcations(M)exchangedatcarbox-ylicacidsites,togetherwithasmallnumberofheteroaromaticentities.Thismodelis,arguably,asignificantconceptualadvanceinasmuchasitbringstogetherknowledgeofthelignite’soriginswiththatfromreactionbehaviortoformulateanimprovedmoreappropriatemodel.ThemodelbyHuttingerandMichenfelder[28](C270H240O90N3S3M10,Fig.1f),fromthesameyear,wasthelargest(forlignite/browncoal)putforwardtothattime.Therelationtoaligninpre-cursorstructureisnotasclear,butitdoescontainamixtureofali-phaticchainsandhydroaromaticringsystemscondensedtovariousdegrees.Thesignificanceofexchangedcationstothestruc-turalintegrityofligniteisagainrecognizedinthismodel.Priortothismodel,therehadbeenlittleconcernabouttheessentialthreedimensionalnatureofthestructuresbeingproposed,buttheseJ.P.Mathews,A.L.Chaffee/Fuel96(2012)1–149authorscomment:‘Toexaminethestructureinthreedimensionsaspacefillingmodelwasmade(whichwaspossiblewithoutanystericproblems).Thismodelshowedasatisfyingspacefilling,especiallywithrespecttotheextensionofthestructureintothethirddimension.’Theauthorspresentthismodelasasingleentity(nopotentialcross-linkingsitesareinferred),butnotethatthisleavesafundamentalquestionastohowthelinkagesbetweenseveralstructuralunitsmayberealized.Hatcherandcolleagues,inaseriesofpapersstartingfromthelate1980s[31,34,43],proposedastrongrelationshipbetweenthestructureofthewoodycomponentlignite(whichcanoftenbeisolatedbyhandsampling)andligninasitsprincipalevolution-aryprecursor.UsingAdler’sligninmodel[99]asastartingpoint,theyprovidedaseriesofmodels,thatdemonstratedconsistencywithexperimental(solidstate1H-NMR)datathroughthesequencelignin?browncoal(ligniteB)?ligniteA?subbituminouscoal.Theyalsoproposedaseriesofrelativelystraightforwardchemicalreactionsthatcouldaccountfortheseobservedchemicalchanges.ThepreservationoftheC6AC3motifthroughouttheevolutionofthese‘fossilwood’modelsisclearlyevident(Fig.2c).AslightlymodifiedHatcherlignitemodelalsoappearsinapaperdebatingthestructureofcoal[98].Nomuraetal.(1997)alsogeneratedtwo3DmodelsforanAustralian(Yallourn)andanIndonesianbrowncoal[52].Bythemid1990s,ascomputationalchemistrybegantoemergeasadistinctspecialistdiscipline,interestinthemolecularrepre-sentationofcoalwasrekindledascoalscientistsbegantosenseitspotentialutilityinunderstandingthebehaviorofcoalcontain-ingsystemsinatruly3Dcontext.Kumagaietal.[58],whofirstap-pliedthistoYallournbrowncoal,createda3DperiodiccellconsistingofonetetramerplusonepentamerbasedontheunitstructureC21H20O7(totalmolecularweightof3464amu).A2Drepresentationoftheunitstructure,constructedonthebasisofthedatafromelementalanalysisand13C-NMRspectroscopy,isshowninFig.1g.Clearlythemodelissimplisticinthesensethatitfailstoincorporateacross-sectionoffunctionalgroups,polycon-densedhydroaromaticringsystems,heteroatoms,cations,etc.–allofwhichhadbeenembodiedinpreviousmodels.Howeveritrep-resentsasignificantadvanceontwofronts.First,themodelincor-porateswater.SinceYallournbrowncoalcontainsca.60%waterbyweight[127]–or,inotherwords,morewaterthan‘coal’–thisinclusionisveryappropriateandsignificantforstructuralinvesti-gations.Second,theabilitytocalculatetherelativeenergyofthestructureanditerativelyreduceittoaminimumvalue(i.e.,geo-metricallyoptimizethestructure)facilitatedthecomparisonof3Dstructuresonaquantitativebasisforthefirsttime.Forthispur-pose,Kumagaietal.[58]usedamolecularmechanics(forcefield)approach,inwhichtheenergyofatotalsystemwasevaluatedfromthesumoftheenergiesduetobondedinteractions(bond:Eb;angle:Ea;torsion:El,inversion:Ei)plustheenergiesduetonon-bondedinteractions(vanderWaals:EvdW;electrostatic:Eel;hydrogenbond:Ebb),makinguseoftheDrieding[128]forcefield:E¼ðEbþEaþElþEiÞþðEvdWþEelþEbbÞSincetheyuseda3Dperiodiccell,thecalculatedenergiesapplyoveranextended3D(infinite)context,asopposedtovacuum.Theywerethuswereabletoinvestigateconformationalandvolumetricchangesinthemacromolecularstructureofthemodelaswaterwasprogressivelyremovedfromthestructure.Vuetal.[69]proposedastructuralunit(C100H80O2)forcoalde-rivedfossilwood.Thiswasbasedon13C-solid-stateNMR,ultimateandfunctionalgroupanalyticaldatatakenforasampleofPodocar-pussp.collectedfromtheLoyYangmine,LatrobeValley,Victoria.Thismodelis,itcanbeargued,pertinenttojustonecomponentofthecomplexheterogeneousarrayofmorphologicallydistinctcomponentsthatmakeuplignite.Themodelisbasedonan11-merof(different)degradedligninsubunits,threeofwhichwerethenassembledwithwaterintoa3Dperiodiccell(27.3Â27.3Â27.3Å3),soastoachieveadensityconsistentwiththecoalsample.Thisgroupinvestigatedthetimedevelopment(moleculardynamics)ofabrowncoalmodelsystem,soastocharacterizeforthefirsttimemolecularmotionandaveragestructuralproperties(e.g.,moleculardiffusionandaveragebonddistances)atambientconditions(298K,1atm).Thisisanimprovementoverstandardmolecularmechanicsmethodssinceitincorporatestheinherentstructuralmotionofmatterattheconditionsofinterest.Thisworkdemonstratedthatthediffusionofwatermoleculesinthevicinityofthelignite(degradedlignin)issubstantiallyreducedcomparedtopurewater,largelyasaresultofhydrogenbondinginteractions.Cationexchanged(Na-andCa-)formsofthemodelwerealsoinvestigatedand,inthiscase,themobilityofwatermoleculeswasfurtherreduced.TheliptobiolithmodelofPatrakovetal.[68]isabouttheonlyrepresentationofacuticularcoal(85%C).ThisSiberianbrowncoalwasconstructedbasedonnon-isothermalliquefactionproducts,waslarge(C727H790N2S4O36)andalsocon-structedin3D.ThestructureisshowninFig.1h.Thehydroaromat-icfragmentscomprisingthestructurevarybetween1and8rings,withmoreringsbeingaliphaticthanaromatic.Domazetisandcoworkers[70,71,75,129]havepublishedaser-iesofpapersthatfocusontheinteractionbetweeninorganiccat-ionsandbrowncoal.Thisgrouppreparedamodel(C258H256N2O78S)basedonthestructuraldatafromtheliterature[71].Ratherthanusingamolecularmechanicsapproach,thisgroupappliedsemi-empiricalandabinitiomethodstocomparethestructuresoftheplainmodelversusthoseofderivativeNa,Ca,Mgsaltspreparedbymodificationoftheoriginalmodel.Theyinvestigatedsimpleadditionofsalttothemodel[Coal(Na+ClÀ)],ion-exchangeofcationsonthemodel½CoalðCOÀNaþ2ÞðÞandtheef-fectsoffurtherhydrationofthesemodels½CoalðCOÀþ2ÞðNaÞÁ10H2O.Formostcases,theauthorsreportedin-creasedstabilitywhensaltsand/orwater[76]wereaddedtothebasiccoalmodel.ForFe(andNi)thisgroupconsideredarangeofmorecomplexionconfigurations,withthemetalassumingmono-dentate,multi-dentateandbridgingconfigurations,forexample:½CoalðCOÀþ3OHÀÞÀþ32Þ2ðFeÞ3ð7ðH2OÞ,½CoalðCO2Þ3Feand½CoalðCOÀþ3ÀÀ2Þ2ðFeÞ2lÀðOHÞ2ðOHÞ2ðHOPhÞ2ðH2OÞ2,respectively.Thelattercasewassimulatedbytheco-additionofphenolgroups(HOPh)tothemodel.Itwasfoundthatpolymericmulti-nuclearcomplexesbasedonFeandNiweregenerallyenergeticallyfavoredtoformincoal[128],aswaswateradsorptionuptoatleast20%[76].Furtherabinitiostudiesbythisgroupconcludedthattheinteractionbetweenmetalcomplexesandbrowncoalbecomesimportantincoalpyrolysis[130]andcharformation[107]aswellasforcatalysisofsteamgasificationreactions[77].Recently,Salmonetal.[80]createdamacro-modelforanAngiospermwoodisolatedsamplefromAustralianMorwellbrowncoal.UsinganapproachsimilartoHatcheretal.[31],themodelwasconstructedtomatchexperimentaldataviamanipulationofanangiospermligninstructureintheliterature[131].Ageopoly-merconsistingoffiveC232H212O94unitswasusedinreactiveforcefield(ReaxFF)simulationstoinvestigatethermaltransformationsasameansofsimulatingcoalmaturation[115].4.2.ThemodelsofsubbituminouscoalsThesubbituminouscoalmodelsarefewinnumberand,alongwithanthracite,arethepoorcousinsofthefarmoreabundantbituminousmodels.Shinnwhoisbetterknownforhisbituminousmodel[23]generated,withasimilarapproach,asubbituminousmodelinaconferencepaperin1996[51].Themodel(showninFig.2a)consistsofmostlysmallringstructures(<4),withlimitedhydroaromaticstructures,lotsofcross-linking,andmultiple10J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–14examplesofoxygenfunctionalitywithintwomacromolecules.Comparisonbetweenthesubbituminousandbituminousmodelswashelpfulinvisualizingstructuraldifferences.Healsopresentedtheemergingvisionofhowthecomputationaladvancescouldim-pactcoalmolecularmodeling:computerconstructionofmolecularconnectivity,3Denergyminimizedstructures,reactionpathwaypredictionswithquantummechanicalapproaches,incorporationofgreatermoleculardetail,andMonte-Carloconstructionap-proachestodisplaymultiplepotentialstructures.Itwouldtakeseveraldecadesbeforethesewererealized,andoftheseap-proaches,theuseofcoalmodelsinapredictivecapacityadvancedtheleast.Nomuraetal.[59]generatedasizable‘‘structuralunit’’subbitu-minouscoalmodel(522carbonatoms)forTaiheiyocoal(Fig.2b),butitwasnotwidelycitedbeingpublishedinareport,ratherthanapeer-reviewedjournal.TheworkofHatcher[34]alsocapturedsubbituminousstructurethroughcarefulcoalificationevaluationsbasedonanimprovedcoalsampling,selectionofobviouscoalifiedwood,andtheapplicationofsolidstate13CNMR,andflash-pyroly-sisGCMS.Thismodelcapturedligninstructurewithappropriatecoalificationtransitions(Fig.2c).Takanohashietal.generatedamuchsmallerscaleHyperCoalmodelcapturingmodelinformationfromrawandextractedWyodakcoal[66].Themodelwasanasso-ciatedmodelofthreefragments(containing3–4hydroaromaticrings)bondtogetherviaaphenolichydrogenbondandacal-cium–oxygenionicbond.Moleculardynamicswasusedtoprobeanydissociationwithtemperature,butnonewasobservedat300°Cinagreementwithexpectations.Giventhecurrentimpor-tanceofsubbituminouscoaltoelectricitygenerationintheUSwithlowsulfuremissions,thelargereservebaseworldwide,andare-emergingdesireforcoal-to-liquidsitseemslikelythatthesubbituminouscoalomissionswillbere-evaluatedbythecoalstructuralmodelingcommunity.Forcompleteness,therearealsotwosmallmolecularrepresentationsforsubbituminouscoalusedinvisualizingcoalifcationstructuraldifferences[16,17].4.3.ThemodelsofbituminouscoalsThebituminouscoalsarethebestrepresentedofthecoalrankinthemodelrepresentationarena.Thefirstbituminousmodelwasalsothefirstcoalmodel.TheFuchsmodel[1]atfirstglanceappearstobeclosertoastructurewewouldexpectforananthra-citecoalorchar.The43conjoinedringsactuallycontainonly24conjugatedaromaticsystemsinthreeseparateislandsintercon-nectedwithhydroaromatic,cyclopentaneethers,anddiphenyleneketoneoxide.Themodelwasutilizedtoexplainpyrolysisbehaviorandwascompatiblewith’’theproximateandultimateanalysesofbituminouscoals,withtheresultsofgroupdeterminations,andwiththeexperiencesofoxidation,reduction,andthermaldecomposition.’’Interestingly,themodel(84.3%C)waspresentedin3Dspace,to-getherwitha2Ddrawing(Figs.3a),andwasutilizedtoexemplifyaprocess,animpressivefeatin1942!Afewofthestructuralfea-tures,suchasdiphenyleneketoneoxideappearinlaterbutoftenlower-rankstructures.TheGilletmodelsconsistedofsmallmolec-ularentities(forexamplehexaceneandperylene-likestructuresalongwithmorecomplexmulticyclicdibenzofuran-peroxidemol-eculesforacokingcoal[2,3].Ofthese,peryleneanditsvariousderivativesofwerefavoredas‘‘hypotheticalstructuralformulae’’.Whilenotamolecularlevelrepresentation,theschematicofHirsch[132]appearedin1954andproposedthe3Dnatureof‘‘open’’,‘‘liquid’’,and‘‘anthracitic’’structurescrossingtherankrangeandpresentedconjugatedaromaticstructuresofabenzopy-rene(5-rings),coronene(7-rings)anda30-ringstructureforaro-maticlamellaediametersfor80%,89%,and94%carboncoals.ThisinfluentialworkwasbasedonlandmarkX-rayscattingdata.ThehighlycitedGivenmodelfirstappearedinthejournalNaturein1959[91]andlaterinFuelin1960(Fig.3b)[5].Themodelwastweakedin1961withachangeintheorientationofthehydroaromaticstructuresfromdihydro-9,10-anthracenetodihy-dro-9,10-phenanthracenetypestructuresasthehydroaromaticlinkages[92].Anadditionalsimilar,yetuniquecoal,modelap-pearedin1962andwasslightlymodifiedin1964(Fig.3c)[8].However,thesestructuresarenotwellcited.ThewellknownGi-ven‘‘model’’washighlyinfluentialandproposedstructureswerecognizetodayinmanyofthemodels‘‘Thusamoleculeincoalapparentlyconsistsofanumberofrathersmallaromaticsystems,say1–3fusedrings,highlysubstitutedbyaliphaticgroupingsthatservemainlytolinktogethertheorderedregionsandmostlydonotterminateinmethylgroups.’’[91]Thestructurepresentedwasa20-ringaromatic,hydrocylic,andhighly-linearstructurewitha4-ringappendageaddingthethirddimensiontoanotherwisenearlyplanarmolecule.The1960modelofCartzandHirsch[4]fleshedouta2Dhydroaromaticrepresentationforan84.5%carbonbasedonextensiveX-raydiffractionanalysesdeterminedinearlierwork(Fig.3e)[132].Thepresenceofsterichindranceinthemodelwaspresentedbya2Ddrawingwithout-of-planebonds.Thestructurewasacombinationof13ringsincluding5-memberedandhydroaromaticstructures.Theoxygenfunctionalitywassim-pler,asexpectedwithahigherrankcoal,andincludedphenolicstructures.LadnercreatedhisdistillatemodeltorecognizetheNMR-determinedcontributionoftertiaryCHcomponentsthatin-creasedthehydroaromaticcycliccomponentsovermethylenelinkages[6].AlargerLadnerstructure,modifiedfromtheGivenmodel,appearsinalaterGibsonpaper[133]asapersonalcommu-nication(Fig.3f).AleapinmodelcreativitywasgeneratedbyHillandLyon[7].Theirmodelforahigh-volatilebituminouscoalwasasignificantincreaseinsizeandstructuraldiversitywithanoverallmolecularweightapproaching10,000amu;fivetimeslargerthanpreviousmodels(Fig.3h).Ringsizesvariedfrombenzenetoadibenzo-ova-lenestructure.Theoxygenfunctionalityincludedcarboxylicacid,phenolic,aryl-Ocross-links,aliphaticethers,furananalogs,andquinoneanalogs.Withmodernanalyticalapproachesmanyofthesefunctionalitiesarenolongersupportedincoalsofthisrank,buttheirinclusionwasindicativeofthegrowingrealizationofthecomplexityofcoalandthemovementawayfrompolymericstruc-tures.FunctionaldiversitywasalsopresentwithSandNforms:pyridine,pyrrole,thiophene,thiol,amines,analine,andtheirana-logs.Therepresentationalsohasamolecularweightdistributionwithtwosmallnon-connectedmolecules,thefirstoccurrenceofthisfeatureincoalmodelliterature.Givencreatedamodelfor‘‘bright’’(vitrain)bituminouscoalthatwas‘‘lashedtogetherbychemicalbonds’’[134],asanapparentprecursortohiswellrecognizedmodel[5]althoughotherslightvariantsofthemodelexist[91,92].A‘‘classicmodel’’,itisverydifferentthantheChakrabarttyandBerkowitzpolyadamantanespostulation[10]discussedearlier.TheotherclassicmodelsarethewellcitedpapersofSolomon(Fig.3g)[19],Wiser(Fig.3i)[22],andShinn(Fig.3j)[23].Theseclassicmodelsalsohadthedistinctionofbeingthefirstcomputationallymodeledcoalstruc-tures[35]although‘‘real’’3Dmodelshadpreviouslybeencon-structed(Given,Wiser,Solomon,andHeredyandWender[14])withvanderWallspheres(space-fillingmodels)[20]and,also,fortheoriginalmolecularrepresentationofcoalbyFuchsandSandoff[1].Therearehowever,many,many,morebituminousmodelsthatareworthyofrecognition.Wender[11]generatedhisframesofref-erencecoalificationmodelsasanaidforviewingthe‘‘CatalyticSyn-thesisofChemicalsfromCoal’’(papertitle),afamiliartrendformanyofthebituminouscoalmodels.Okaetal.[135]alsogeneratedsim-ilarsized(small)modelswithacomputeraideddesignapproachforcomponentswithanoverallformulaofC24.7H23.05S0.85N0.44.J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–1411TheBartleetal.[13]smallmodelstructuresofsupercritical-gasex-tractfractionsarealsonoteworthyduetothecarefulchemicalspeciation.ThePitt[12]modelwas‘‘snakelike’’undulatingperi-condensed25ringstructure(aromatic,hydroaromatic,and1,4-benzoquinone)foran80%Ccoal.The90%Cvarianthavinggreateraromaticity,nobenzoquinone,andsomecatacondensedrings.Itwasthelastofthecoalmodelstohaveanextendedmostlyperi-condensedsinglemoleculerepresentations.Modelsfollowingthisperiodtendedtohavethefamiliarcross-linked,mostlysingularmolecular,representations.TheHeredyandWender[14]modelisonesuchexamplewith4molecules(naphthalene,phenan-threne,andfluorenehyroaromaticderivates)cross-linkedto-gether.TheMerrick[27]modelisalsoasimilarexampleofthisstyleofmodel.Othersmallrepresentationssuchasthetwobitu-minousmodelsofIwataetal.[17]aretypicalofsmallmolecularrepresentationshowingcompositionaldifferences.Whitehurstetal.[16]generatedarankrangeofsmall-scaleskeletalstructuresforanaidinliquefactiondiscussions.TheWiser[22]model(Fig.3i),largerandmorecomplex,wasdrawnforsimilarreasonsand,alongwiththeShinn(Fig.3j)[23]andSolomon(Fig.3g)[19]models,areperhapsthosethataremostrespectedandrele-vantbituminousmodelsfromthiseraofpenandpaperconstruction.Meyers[18]generated3DstructureswiththeDriedingmolec-ularmodelingkit.Thesemetalrodsinterconnectedandgeneratedstructuresthatwhensuspendedin3Dspacecouldeasilybecon-fusedwithcomputationallygeneratedstructuresthatstillutilizethisnametodescribethatmolecularview.ThephotographofthearrangementisshowninFig.3d.Inthismannerstackingwascapturedwithtwofreearomaticstructuresformingathreearo-maticstackdescribingthe‘‘crystallinestructure’’coupledwithacross-linkedcomponentalso.Apolymericchainsegmentwasalsogenerated.Alsoofnoteforinnovativestructuralgeneration,isthegluedtogetherbentandflathexagondisksofFrancis[9](1961),forasimplified3DversionoftheGivenmodel.Threedimensionalspace-fillingstructuresappearedfortheWiser,Given,Solomon,andHeredymodels(Fig.4a–f)[20].Theinclusionofthethirddimensiondemonstratedstraininthemodels,andallbutonewerealtered.TheSolomonmodelwasalsopresentedinastacked‘‘glob-ularconfiguration’’(Fig.5f).Thisworkcontinuedwithalaterpaperextendingtherankrange(low-rankandahigh-rank(anthracite)coalmodels[21].Thebituminouscoalmodelhassubsequentlybeengeneratedin3Dspacewithcomputationalmodelingap-proachandutilizedinCO2sequestrationstudy[136].Solomon[19](Fig.3g)createdhishypotheticalbituminousmodeltoaidintheunderstandingofthermaldecompositionforaPittsburghseamcoal.Itcontainedmultiplecomponentscross-linkedorassociatedwithhydrogenbonds.The‘‘cracking’’ofthemoleculesgeneratinglightgases,tarsandchar.Incomparison,theWisermodel(Fig.3i)didnotshowhydrogenbondingandtherewasasinglemoleculecomprisingcross-linkedentitieswithetherandaliphaticlinkages.Theintentofthemodelwastoindicate‘‘whatmustbedonetobituminouscoaltoconvertittotheliquidstate’’.Ithasbeenpointedoutthattherearesimilaritiesbetweencoal-pyrolysisandcoal-liquefactioninprimaryconversions[137,138].Thus,similaritiesareexpectedintheuseofcoalmodelstoshowtheinitialstagesoftheseprocesses.TheShinn[23]modelcontinuesinasimilarvein,showingproductstructuresforbothsingleandtwo-stageliquefactionfromfragmentingthecross-linkedinitialmodel(initialmodelisshowninFig.3j).LazarovandMarinov[29]generatedan‘‘assembled’’cokingcoalmodelwithsimilarapproachbasedonproductanalysisofsolublefrac-tionsofabutylated(toincreaseextractionyield)coaltocreateathree-moleculemodelwithcross-linkedrelativelylarge,andmostlypericondensed,aromaticandhydroaromaticcomponentsincludinghydrogenbonding(Fig.4g).Itiseasytoimaginethatafewadditionalcross-linksandsomearomatizationwouldgeneratelargearomaticraftsresponsibleforalignedcokestructures.Theyalsoproducedbutlyatedextractmodelsanddelvedinstatisticalconsiderationsforthenumberofmodelsthatcouldbegeneratedfromthatdata[29,139].The1990’swerethebeginningofthecomputationalapproachestoconstructionandstructuralrepresentation.Initiallytheseneces-sarilysmallstructures,duetosoftwareandcomputationalcon-straints,addedlittletotheexistingcapabilitytorepresentandutilizecoalrepresentations[37].However,itwassoonevidentthattherewasgreatpotentialforcomputationstudiesaccompaniedbyincreasedcomputationaccessandcapability[45,46,50,54].Carlson[35]demonstratedandquantifiedtheimportanceofvanderWaalinteractionsandhydrogenbonding,alongwithdensitymeasure-mentsonthe3Dmodels.Healsoperformedaromaticstackingexperimentsandobserved,withtheminimizationapproachused,thattheringpairsstackedinaparallel,overlappingfashionbutusuallywithsomeringrotationrelativetotheotheraromaticstructure.StrainedmoleculesandvanderWaalstabilizationwasalsoinvestigated.Anexpandedmodelingstudyofpolyaromatics[100]isalsoworthyofnote;althoughitwaspitch-relatedandnotcoalspecific,itisrelevanttothehigh-rankcoals[101].Compu-tationalmodelsofUpperFreeportandLewiston–Stocktonbitumi-nousvitrainsweregenerated[64]alongwithamodelforPittsburghNo.8coalmodel[57].AseriesofmodelsforAkabiracoal[38–40,53,116]exploreddensity,cross-linkingapproaches,andconstruction.Theverylongchainaliphatics,reminiscentofterpenechemistry,(onechaincon-tained26carbonatoms)foundintheoriginallarger-scalemodelsarerelativelyrareinthemodelingliterature.Thesecomponentslikelycomefromtheliptinitegroupandthecarefulselectionofvitrinite-richcoalsandcoalifiedlogsinthebituminousworkmaybepartiallyresponsiblefortheiromission.However,theirinclusionthroughoutthestructuremaybemisleading,forbehav-ioralstudies,iftheyexistindiscreetassociatedcomponents.Thiscomplexnatureofcoalsisoneofthereasonsthattheirstructuralelucidationissochallenging.Recentlarge-scalemodelingeffortshaveadaptedthisassociatedapproach[82]forthelongaliphaticchainsthatareevidentfromrapid-pyrolysismassspectra.Theinclusionofphysicalevaluationsviacomputationalapproach[45]wasasignificantaidtomodelingeffortsasitfurthercon-strainedtherepresentationsandyieldedmorereasonableporosityvalues.Longchainswereomittedfrombituminousvitrinite(coalscrapingsobtainedfromobviouscoalifiedtreesfromundergroundmines)models[64].Therearefewrepresentationsofnon-vitraincoals.Alsothereareremarkablyfewofthebituminouscoalmodelsthataddressthestructuresofcokingcoals,besidesPittsburghcoal[19,72].Thereareafewexceptionseitherconsideringthecoal[29,56]orthethermoplastictransformations[140,141].Notableinitsconstructionstrategyutilizingmethylenechloridesoluble,pyridine-solubleandpyridine-insolublefractions(Fig.4h)foracokingZaoZhungcoal,Nomuraetal.generatedastructureshow-ingthesethreeseparatecomponentswithconsiderableattentiontodetailinthestructuralevaluation[56].Furthergainscamefromsimilarcarefulconsiderationofsolu-bilityfractionsandtheexplorationofswelling,andtheassociatedmodelsforUpperFreeportcoalviaTakanohashiandco-workers[54,61,62,65,67,105,106,142].Thesuper-molecularstructureofUpperFreeportcoalisshowninFig.4i[67].Modifications,basedonsimulatedNMRspectra,furtherrefinedtheCS2/NMPinsolubleportion’sstructure[67].Thesemodelshoweverarestillrelativelysmall.Whileusefulforillustrativepurposes,behavioralstudieswouldarelimitedinmodelsofthisscale.Theeraofthelarger-scalepenandpaperbituminousmodelswasstartedbyHill&Lyon(Fig.3h)[7],Wiser(Fig.3i)[22],andShinn(Fig.3j)[23]between1962and1984.Itwasmuchlaterinthestartofthenewcentury12J.P.Mathews,A.L.Chaffee/Fuel96(2012)1–14beforelarge-scalecomputationalmodelswereconstructedforPocahontascoalwiththeimplicitgoalofincorporatingamolecularweightdistribution[73].ThisNarkiewiczandMathewsmodel(Fig.5j)wasamuchlargerstructure(C13,781H8,022O140N185S23),20timesthescaleoftheShinnmodel.ItwasalsousedtovisualizeCO2sequestration-relevantissues[111].Thismodelwasalsopref-erentiallyaligned,viaasquashingprotocoltoforcealignment,an-otherfirstforbituminouscoalmolecularrepresentations.Thereasonforthelargerscalewasduetothedesiretoincorporateasignificantmolecularweightdistribution(215separatemolecularentities,rangingbetween78and3286amu)forimprovedrepre-sentationandutility.TheslightlysmallervanNiekerkandMath-ewsmodelsforinertinite-rich(Fig.4k)andvitrinite-rich(Fig.4l)SouthAfricancoalshavealsobeenconstructedatscale>14,000carbonatoms[82]forsimilarreasonsandforconsiderationofsol-ventinteractions[143]andsolventextraction[112].Thescalehoweverneedstobeexpandedfurtherifwewishtocaptureevenaminisculeportionofthecontinuumthatiscoalstructure.RecentworkutilizingFringe3D,toobtainadistributionofPAHringsizesfromHRTEMlatticefringeimages,coupledwithscriptingforali-phaticandheteroatominclusionandasemi-automatedautomatedcross-linkingapproachhasenabledamodelforasubbituminouscoal[81]andtheIllinoisNo.6coaltobegeneratedatevengreaterscale(50,789atomswithin728cross-linkedaromaticclusters)[144].Interestingly,theexpansionofscalehascausedvisualizationissuesthatarebeingsolvedwithanovelreactive-coarse-grainingapproachthatgenerates2Dlatticestructures(reminiscentoftheBathelatticecoalnetworkrepresentations)fromcomplex3Datomisticrepresentations[145,146].4.4.ThemodelsofanthracitecoalsTheanthracitemodelsare,likethesubbituminousmodels,farlessfrequentthanthebituminouscoalexamples.Limitedrepre-sentationsthatessentiallycapturetheincreaseinringnumberswithcoalificationarenumeroushowever,suchastheseminalHirsch[132]X-rayscatteringworkwhichincludedthenowfa-mouscoalrankschematicrepresentation(open,liquid,andanthra-citestructures)andtheWender[11]‘‘frameofreference’’representation(relevantstructuresshowninFig.5aandb).TheanthraciterepresentationofSpiroandKosky[21]wasa2D(Fig.5c)andaspace-fillingmodelsimilartothelow-andinterme-diate-rank(bituminous)models.AsemianthracitemodelhasbeenproposedbyTrompandMoulijn(Fig.5d)[30]but,likemanymod-elspresentedhere,itsconstructionwasnotexplainedbeyondextendingtheShinn[23]model.Anthracitemodelswerealsogen-eratedforamethanesimulationbyVishnyakovetal.[55]buttheconstructiondetailsarevague.TwofragmentsoftheirmodelareshowninFig.5e.ThePappano[63]modelsaretheonlyanthraciteatomisticrep-resentationswhichcapturethestackingandlargearomaticsheets.HisfourmodelsrepresentthenstillminedcoalsampledfromthePennsylvaniaanthracitefields.AsallofthecommerciallyminedUSanthracitecomesfromthesefields[147]itisnotsurprisingthatbeyondthisPennStateworkandthesomewhatobscureRussianwork[55],littleanthracitemodelinghasbeenperformed.Thisisdespiteitsimportanceinothercountriesandtheexpectationthatitis,perhaps,theeasiestofthecoalrankstomodelasthereare:lessmaceralstructuraldivergence,lessoxygen,andlesshydrogenenablingmoresimplisticgenerationofhighlyaromaticcarbonrichcoals.Oneofthemodels(Jeddo)isshownasa3DrepresentationinFig.5f[60,63].Themodelwasgeneratedwithlaserdesorptionion-izationmassspectroscopy,X-raydiffractiondatatheaidoftheSIG-NATURE[44]andPORprograms[45]forconstructionandphysicalevaluation.Provocatively,thisstructurestrayedfromtheexpectedlinearformofthegraphicsheetswithcurvatureinonesheetimpartingsympatheticcurvature(duetonon-bondinginterac-tions)inadjacentsheets.5.ConcludingcommentsThereareanabundanceofstructuresthatcapture,toacertaindegree,thestructuralfeaturesofcoal.With>134structuresbeinggeneratedoverthelast70yearsthefieldhasbeenactive,yetonlyafewstructuresarewellknown.Thefieldhasbeendominatedbytherepresentationsofbituminouscoal.Farfewerligniteandbrowncoalmodelsexist.Surprisingly,giventheirimportancetoUScoalproduction,veryfewsubbituminousmodelsareavailable.Thelimitedanthracitemodelshoweverareexpectedgivenitsmostlyveryregionalnatureandahistoricimportance.Modelshavemovedfrompen-and-paperconstructstomodelsgeneratedwithcomputationalaids.Manyofthestructureshavebeensmall(100’sofatoms)withprogress,onlyrecently,ingeneratingstruc-turesatgreaterscaleforimprovedrepresentationqualityandapplicabilityinfollowingbehaviors.Progressisbeingmadeinbothanalyticaltechniquesandthecomputationalpowercoupledwithsoftwareadvancesthatareenablingimprovedstructuralrepresen-tation.Yetnocoalmodelexiststhatrepresentsmaceralcontribu-tions(althoughseveralmaceral-richstructuresexist).Thisislikelytochangeasconstructionstrategiesenablemorerapidmod-elbuildingapproachestolargerscalemodelsthatcanaccommo-datemultiplesimulationdirections.AppendixA.SupplementarydataSupplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,atdoi:10.1016/j.fuel.2011.11.025.References[1]FuchsW,SandoffAG.Theoryofcoalpyrolysis.IndustEngChem1942;34:567.[2]GilletA.Lamoleculedehouille.BulletinDesSocietesChimiquesBelges1948;57(7–9):298–306.[3]GilletA.Constitutionofcoal.Research1949;2:407–14.[4]CartzL,HirschPB.AcontributiontothestructureofcoalsfromX-raydiffractionstudies.PhilTransRoySocSerAMathPhysSci1960;A252:557.[5]GivenPH.Thedistributionofhydrogenincoals.Fuel1960;39:147–53.[6]LadnerWR,StaceyAE.Somediscussiononpossiblecoalstructures.Fuel1961;40(5):452–4.[7]HillGR,LyonLB.Anewchemicalstructureforcoal.IndustEngChem1962;54(6):36–9.[8]GivenPH.Thechemicalstudyofcoalmacerals.In:HobsonGD,ColomboU,editors.Advancesinorganicgeochemistry:proceedingsoftheinternationalmeetinginMilan,1962,Macmillan:NewYork;1964.p.39–48.[9]FrancisW.Towardsanunderstandingofthechemicalstructureofcoal.Incoal,itsformationandcomposition.2nded.London:EdwardArnoldPublishersLtd.;1961.p.717–53.[10]ChakrabarttySK,BerkowitzN.Studiesonthestructureofcoals.3.Someinferencesaboutskeletalstructures.Fuel1974;53(4):240–5.[11]WenderI.Catalyticsynthesisofchemicalsfromcoal.CatalRev–SciEng1976;14(1):97–129.[12]PittGL.Structuralanalysisofcoal.In:PittGJ,MillwardGR,editors.Coalandmoderncoalprocessing:anintroduction.NewYork:AcademicPress;1979.p.27–50.[13]BartleKD,MartinTG,WilliamsDF.Chemicalnatureofasupercritical-gasextractofcoalat350°C.Fuel1975;54(4):226–35.[14]HeredyLA,WenderI.Molecularstructureforbituminouscoal.PreprPap–AmChemSocDivFuelChem1980;25:38–45.[15]OberlinA,BoulmierJL,VilleyM.Electronmicroscopicstudyofkerogenmicrotexture.Selectedcriteriafordeterminingtheevolutionpathandevolutionstageofkerogen.InKerogeninsolubleorganicmatterfromsedimentaryRocks,Durand,B.,Ed.Paris:EditionsTechnip;1980.[16]WhitehurstDD,MitchellTO,FarcasiuM.Coalliquefaction:thechemistryandtechnologyofthermalprocesses.NewYork:AcademicPress;1980.p.xv,378p.[17]IwataK,ItohH,OuchiK,YoshidaT.Averagechemical-structureofmildhydrogenolysisproductsofcoals.FuelProcessTechnol1980;3(3–4):221–9.[18]MeyersRA.Coalstructure.In:MeyersRA,editor.Coalhandbook.NewYork:MarcelDekker;1981.[19]SolomonPR.Coalstructureandthermaldecomposition.In:BlausteinBD,BockrathBC,FriedmanS,editors.Newapproachesincoalchemistry,vol.ACSJ.P.Math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