Effectsofstepchangeofsynthesistemperatureonsynthes

Effectsofstep-changeofsynthesistemperatureonsynthesisofzeolite4AfromcoalflyashK.S.Hui,C.Y.H.Chao*DepartmentofMechanicalEngineering,TheHongKongUniversityofScienceandTechnology,ClearWaterBay,Kowloon190,HongKongReceived8December2004;receivedinrevisedform2September2005;accepted16September2005Availableonline2November2005AbstractPureform,singlephaseandhighcrystallinezeolite4Asamplesweresynthesizedbyapplyingstep-changeofsynthesistemperatureduringhydrothermaltreatmentofcoalflyashpreparedsolution.Theresultsindicatethatstep-changeofsynthesistemperatureduringhydrothermaltreatmentplaysanimportantroleinreducingtheoverallsynthesistimewhilemaintainingahighdegreeofcrystallinityforthesamples.Inordertoobtainsimilarqualityofzeolite4Asample,areductionbyhalfofthetotalsynthesistimecouldbeachievedbyapplyingstep-changeofsynthesistemperaturecomparedtousingconstanttemperaturethroughoutthesynthesisprocess.Generally,thecrystallinityofthesynthesizedzeolite4Asampleswasintherangeof75–87%.Theyielding(quantity)ofpurezeolite4Asamplesobtainedinalltheexperimentswasabout15–18%.Inaddition,crystalsofsmallerparticlesizeexistinginanarrowerparticlesizedis-tributionweresynthesized.
2005ElsevierInc.Allrightsreserved.Keywords:Coalflyash;Zeolite4A;Synthesis;Temperature1.IntroductionLargequantitiesofcoalflyashareproducedinelectricpowerplantsthroughouttheworldeveryyear.Theamountofcoalflyashformedisapproximately500milliontonsperyearandispredictedtoincrease.Efficientdisposalofcoalflyashhasbeenaworldwideissuebecauseofthemassiveamountofashproducedandharmfuleffectsonenviron-ment[1–3].Asatechniqueforrecyclingcoalflyash,synthe-sisofzeolitesfromcoalflyashhasreceivedextensiveattentionoverthelastdecade.Duetotheiruniformporesizesandlargesurfaceareas,zeolitesareveryusefulmateri-alsforawiderangeofapplicationssuchasionexchanger,molecularsieves,gasesadsorbentsandcatalysts.Manyresearchershaveinvestigatedthesynthesisofzeolitesfromcoalflyash[3–40].However,inmanyofthesestudies,thetotalconversiontimewasgenerallylong(24–72hormore)andthesynthesistemperaturewas90–225C.Thesyntheticzeoliteproductsstillcontainedasignificantamountofresid-ualflyash.Moreover,theobtainedzeolites,suchasNaP1,sodalite,analcime,usuallyhadamorecondensedstructure(lowporevolume).Thepresenceofnon-zeoliticphasesintheconvertedproductslimitsthecationexchangecapacityoftheproductsandgreatlyhindersthepotentialapplica-tionsofzeolitesfromcoalflyash.Thus,theapplicabilityofsyntheticzeolitesfromcoalflyashisgreatlyreduced.Inspiteofthelargeamountofstudiesfocusingontheconversionofcoalflyashintozeolites,thereareonlyafewstudies[25,34,41]focusingontheproductionofpureformandsinglephasezeolite4Afromcoalflyash.Holl-manetal.[25]adoptedatwo-stagesynthesismethodtosynthesizepureformzeolitessuchaszeoliteX,zeolite4A(withsometracesofsodalite)andNaP1.TheyadoptedanextractionprocesstoextractSisourcefromcoalflyash,andwiththeadditionofAlsource,tosynthesizezeo-litesfromtheextractedsolution.Atasynthesistempera-tureof90C,thereactiontimewas48,48and67–72hforthesynthesisofzeoliteX,NaP1andzeolite4A(withsometracesofsodalite)respectively.Tanakaetal.[34]have1387-1811/$-seefrontmatter
2005ElsevierInc.Allrightsreserved.doi:10.1016/j.micromeso.2005.09.005*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](C.Y.H.Chao).(2006)145–151synthesizedpureformandsinglephasezeolite4Afromwastesolutions.Thereactiontimewas24hat85C.Changetal.[41]usedafusionmethodtosynthesizezeoliteAandthereactiontimewas3daysat60C.Fromthesestudies,synthesisofzeolitesusuallytookalongtime(24hto3days).Forpotentialindustrialapplications,pro-cessestosynthesizepureformzeoliteswithhighcrystallin-itywithinashortertotalsynthesistimeshouldbeexplored.Zeolite4Ahasfoundwidespreadapplicationinbothlaboratoriesandindustries.Forthesepurposes,ithasbeenmanufacturedindustriallyonagreaterscalethananyotherzeolite[42].Inbatchzeolitecrystallization,theformationofunwantedcompetingphasesisespeciallyeasy.Thefor-mationofzeolite4Afromanamorphousaluminosilicateprecursoranditssubsequentcollapseintohydroxy-sodaliteisanelegantexampleofOstwald
srule;thefirstpolymorphofacompoundformedfromsolutionistheleastthermody-namicallystableandisthenreplacedinsuccessionbymorethermodynamicallystablepolymorphs[43].Suchsucces-sivetransformationsareverycommoninzeolitechemistry,makingthecontrolledformationofsinglephaseandhighcrystallinezeolite4Ainzeolite4A/hydroxyl-sodalitesys-temparticularlyworthyofdetailedstudy.Theaimsofthepresentworkweretoexploreatechniquetoproducepure,singlephaseandhighcrystallinezeolite4Afromcoalflyash,inordertoprovideanothermethodforrecyclingofcoalflyash.Emphasiswasputonrestrictingtheformationofhydroxyl-sodalitephasezeoliteduringthecrystallizationofzeolite4A.Thefirststudyofapplyingstep-changeofsynthesistemperature
duringcrystalliza-tionofzeolite4Afromcoalflyashwaspresented.Theinflu-enceofstep-changeofsynthesistemperature
duringthecourseofsynthesison(1)crystallinityand(2)particlesizedistributionofzeolite4Asampleswerestudied.2.Experime