Microwaveassistedzeolitesynthesisfromcoalflyashinh

Microwave-assistedzeolitesynthesisfromcoalflyashinhydrothermalprocessMikiInadaa,*,HidenobuTsujimotoa,YukariEguchib,NaoyaEnomotob,JunichiHojobaDepartmentofChemistryandBiochemistry,GraduateSchoolofEngineering,KyushuUniversity,6-10-1Hakozaki,Higashi-ku,Fukuoka812-8581,JapanbDepartmentofAppliedChemistry,FacultyofEngineering,KyushuUniversity,6-10-1Hakozaki,Higashi-ku,Fukuoka812-8581,JapanReceived22November2004;receivedinrevisedform29January2005;accepted5February2005Availableonline19March2005AbstractCoalflyashes,whichincludemuchamountofsilicaandalumina,canbeconvertedintozeolitebyhydrothermalalkalinetreatment.Inthepresentwork,theeffectofmicrowaveirradiationonthezeoliteformationwasinvestigatedwithemphasisonthechangeinyieldofzeoliteduringthereaction.Theflyashwasmixedwith2MNaOHsolutionandheatedbyoilbathormicrowavefor2h.ZeoliteNa-P1formedaftertheconventionaltreatmentusingoilbath,butnozeoliticproductwasobtainedbymicrowaveheating.Whenmicrowavewasappliedinthecourseofhydrothermaltreatment,zeolitizationwaspromotedbytheearly-stageirradiation.ThisisduetothestimulateddissolutionofSiO2andAl2O3fromcoalflyash.Ontheotherhand,themicrowaveirradiationinthemiddletolaterstageretardedthecrystallizationofzeolite.Themicrowaveiseffectivetoproducethezeolitefromcoalflyashinashortperiodbycontrolofirradiationscheduleintheearlystage.q2005ElsevierLtd.Allrightsreserved.Keywords:Coalflyash;Zeolite;Microwave1.IntroductionZeolitesynthesisfromcoalflyashisawellknownprocessfortheapplicationofcoalflyash.Sincecoalflyashincludesalargeamountofsilicaandalumina,theyareeasilyconvertedintozeolitebyhydrothermaltreatmentinanalkalinesolution[1,2].Thisprocessproceedsatalowtemperaturewithoutmuchenergyconsumption,andtheproductsareexpectedtobehigh-valueindustrialmaterialsforionexchangerandgasadsorbent,etc.Theflyashisderivedfrommineralsincludedincoal.Duringcoalcombustion,themineralspartiallymelttomakeflyashparticlesinwhichcrystallinephasessuchasquartzandmulliteremaininthecore,whereasaglassphaseofaluminosilicatecoversthesurface.Theglassphaseplaysanimportantroleinthezeoliteformationbecauseofthehighsolubilityintoalkalinesolution.Theauthorshavestudiedonthesynthesisconditionandformationmechanismofzeolitefromcoalflyash[2],andproposedtheimportanceofSiO2–Al2O3compositiontocontroltheyieldandtypeofresultingzeolite[3].Nowadays,amicrowavetechniquehasbeenextendedasoneofhydrothermalprocessesforsynthesisoffineoxidepowders[4].Themicrowave-assistedhydrothermalprocesswaseffectiveforzeolitesynthesisfromsilica–aluminagelprecursor[5].Thezeolitizationofcoalflyashbymicrowaveheatingwasalsoreportedtobeusefulforshorteningthereactiontime[6].Microwaveisabsorbeddirectlyintowaterassolvent,andenablestherapidheatingcomparedtoaconventionalheatingprocess.Theelectromagneticwavecauseswaterdipolerotation,probablyresultingintheactivationofwatermoleculesbybreakofhydrogenbonds.Theshort-timezeolitizationispreferableforefficientindustrialproductionbecausethelong-timereactioncon-sumesmoreenergy.Wehaveinvestigatedtheeffectofmicrowaveirradiationontheformationofzeolitefromcoalflyash[7],andrecentlyfoundtheunexpectedphenomenathattheinitialradiationiseffectiveratherthantheoverallirradiationduringthezeolitization.Thispaperreportstheexperimentalresultsanddiscussionbasedontheformationmechanismofzeolite.Fuel84(2005)1482–1486:10.1016/j.fuel.2005.02.002*Correspondingauthor.Tel./fax:C81926423545.E-mailaddress:[e-mail](M.Inada).2.Experimentalprocedure2.1.MaterialsAcoalflyashwassuppliedfromanelectricpowerstationinJapan.Table1summarizesthechemicalandmineralogicalcompositionsoftheflyash.ThecontentsofmetalcomponentsweredeterminedbyX-rayfluorescenceanalysisandthechemicalcompositionwaspresentedintheformofstableoxide.Thecontentsofmajorcrystallinephases,quartzandmulliteassumingtheformof3Al2O3$2SiO2,weredeterminedbyX-raydiffractionanalysisusingMgOpowderasstandard.Commercialmulliteandquartzpowderswereusedtomaketheircalibrationcurves.ThecontentsofSiO2andAl2O3inamorphousglassphase,whichshouldbeincludedasaluminosilicateglass,werecalculatedbysubtractingtheSiO2andAl2O3compositionsofcrystalphasesfromthebulkcompositions.About50wt%ofglassphasemainlytakespartinthezeoliteformation.2.2.ZeolitesynthesisandevaluationCoalflyash(10–15g)wastreatedwith2MNaOHaqueoussolution(80–120ml)for2hintheambientatmosphere.Theconventionalheatingwasperformedbyusing500mlTeflonflaskequippedwitharefluxcondenserandanoilbathunderstirringwithamagneticstirrer.Theoilbathtemperaturewas1208C,resultinginthesolutiontemperatureofabout1008C.Themicrowaveheatingprocesswasperformedbyusingahouseholdoven(2.45GHz,500W)restructuredtoequipthevesselwiththecondenserundertheambientatmosphere.Thesolutionwasstirredwithapropellerfor2hmicrowaveheating.Themicrowavewasirradiatedcontinuouslytothesolution,theboilingofwhichindicatedthetemperatureof1008Corabove.Conventionalandmicrowaveheatingprocesseswerecombinedinthisstudy.Inthecombinedexperiment,themicrowavewasappliedfor15mininthecourseoftheconventionalheating,inwhichtheves