ContentslistsavailableatScienceDirect
Journal
of
Hazardous
Materials
j o ur na l ho me p a g e : w w w . e l s e v i e r . c o m / l o ca t e / j h a z m a t
Silica
materials
recovered
from
photonic
industrial
waste
powder:
Its
extraction,
modification,
characterization
and
application
Liang-Yi
Lin,
Jien-Ting
Kuo,
Hsunling
Bai
∗InstituteofEnvironmentalEngineering,NationalChiaoTungUniversity,1001UniversityRd.,Hsinchu300,Taiwan
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received2February2011
Receivedinrevisedform3May2011 Accepted5May2011
Available online 11 May 2011 Keywords:
Resourcerecovery
Photonicindustrialwastepowder Mesoporoussilicamaterials CO2adsorbent
a
b
s
t
r
a
c
t
Thisstudyexploredthepossibilityofrecoveringwastepowderfromphotonicindustryintotwouseful resources,sodiumfluoride(NaF)andthesilicaprecursorsolution.Analkalifusionprocesswasutilized toeffectivelyseparatesilicatesupernatantandthesediment.Theobtainedsedimentcontainspurified NaF(>90%),whichprovidesfurtherreusepossibilitysinceNaFiswidelyappliedinchemicalindustry. Thesupernatantisavaluablesilicatesourceforsynthesizingmesoporoussilicamaterialsuchas MCM-41.TheMCM-41producedfromthephotonicwastepowder(PWP),namelyMCM-41(PWP),possessed highspecificsurfaceareas(1082m2/g),narrowporesizedistributions(2.95nm)andlargeporevolumes
(0.99cm3/g).Theamine-modifiedMCM-41(PWP)wasfurtherappliedasanadsorbentforthecaptureof
CO2greenhousegas.Breakthroughexperimentsdemonstratedthatthetetraethylenepentamine(TEPA)
functionalizedMCM-41(PWP)exhibitedanadsorptioncapacity(82mgCO2/gadsorbent)ofonlyslightly
lessthanthatoftheTEPA/MCM-41manufacturedfrompurechemical(97mgCO2/gadsorbent),andits
capacityishigherthanthatofTEPA/ZSM-5zeolite(43mgCO2/gadsorbent).Theresultsrevealedboth
thehighpotentialofresourcerecoveryfromthephotonicsolidwasteandthecost-effectiveapplication ofwaste-derivedmesoporousadsorbentforenvironmentalprotection.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Sincetheevolutionofcomputerscienceandinformation tech-nology,demandsonlargescalesiliconintegratedcircuitprocesses increaserapidlyduringrecentyears.However,hugeamountsof wasteproductshavebeencreatedintheformsofwastesolvents, sludgeandsolidwaste,etc.Silicon(Si)-containingwastepowder isacommonwasteproductofchemical vapordeposition(CVD) processinthinfilmtransistor-liquidcrystaldisplay(TFT-LCD)and semiconductorplants.InatypicalCVDprocess,gaseousreagentsof silane(SiH4)andammonia(NH3)areintroducedandsiliconderived compoundssuchassilicaand/orsiliconnitride(SiNX)thinfilmsare formedonthesubstrates.Thecleaningagentofnitrogen trifluo-ride(NF3)issubsequentlyintroducedtocleanuptheaccumulative particlesformedonthewallofthereactionchamber.Thewaste powdersarethencollectedbybaghouseslocatedintheexhaust oftheCVDprocess. Suchwastepowdersare light-densitywith bulkyvolumeandarethusdifficulttobetransportedanddisposed of.Therefore,additionalexpensesonwastetreatmentandlandfill disposalareneeded.
Wasterecoveryisaneffectivestrategytoavoidthefastdrainof naturalresources.Thishasbeenespeciallyimportantonusingthe
∗ Correspondingauthor.Tel.:+88635731868;fax:+88635725958.
E-mailaddresses:[email protected],[email protected](H.Bai).
naturalresourcesforthecaptureofabundantCO2greenhousegas. Forexample,silicamaterialshavebeentested[1]fortheirpotential asadsorbentsoncapturingCO2.However,suchsilicaadsorbents weremainlymadefrompurechemicalsources,whichhavebeen inshortageduetolargedemandsofsiliconinsemiconductor, pho-tonicandsolarcellindustries,etc.
Theperiodicmesoporoussilicamaterialsreceivedtremendous attentionsincethefirstdiscoverybyMobiloilresearchers[2].These mesoporoussilicamaterialswithhighsurfacearea(>1000m2/g), narrow pore distribution and well-defined pore structures are usefulintheapplicationfieldsofcatalysisandadsorption[3,4]. Generally,suchmaterialscanbesynthesizedusingvarioussources of silica,includingexpensiveorganicprecursors suchassilicon alkoxides[5]andcost-effective reagentslikecolloidalsilicaand sodium silicate [6]. Theuse of cheapersilica source insteadof expensivesilicaprecursorinthesynthesisprocesswouldbeagreat contributiontoindustrialapplications,especiallyforthe environ-mentalprotectionapplicationswhichrequiremassivequantityof adsorbentsorcatalysts.
Recently, several research studies have attempted to reuse solidwastesformanufacturingmesoporoussilicamaterials.This includes coal fly ash [7] and rice husk ash [8] which are by-productsofthecoal-firedpowerplantandagriculturalactivities, respectively.Changetal.[9]firstlysynthesizedmesoporous alu-minosiliicateusingflyashastheprecursor.Theresultsconfirmed thepresenceofhexagonalporearrangementwithahighsurface 0304-3894/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.
areaof735m2/g.Besides,HuiandChao[7]reportedtheeffects ofsynthesisparametersonthemesostructureofSi-MCM-41with thesupernatantofthecoalflyashastheprecursorsolution.They demonstratedtheeffectofinitialpHvalueontheincorporation amountsofTi,AlandFespeciesduringthesynthesis.
Unlikethe coalflyashor rice husk ashwhich contains lots ofcomplicatedmetaloxides[10,11],theprimarycomponentsof photonicwastepowdermightonlycontainSi-,N-andF-species. Thusit would bemore faciletobe utilizedas thesilicasource sincehighpurityofsilicacouldbeobtained.Totheauthors’ knowl-edge,theidentificationofthewastepowderfromCVDprocessesof eithersemiconductororphotonicindustryaswellasthepossibility ofwastepowderrecoveryforfurtherenvironmentalapplications havenotbeenreportedyet.
Inthepresentinvestigation,attemptshavebeenmadeto eval-uatethechemicalcompositionofthephotonicwastepowderas wellas torecoverthesupernatantand thesedimentinto valu-ableresources.Thesupernatantisusedasthesilicasourceforthe synthesisofMCM-41.Thechemicalcomposition,porestructure, morphologyaswellasCO2adsorptionperformanceoftheobtained MCM-41fromthephotonicwastepowderarepresentedand com-paredwiththoseoftheMCM-41obtainedfrompurechemical.In ordertounderstandtheindustrialapplicability,theperformanceof thesetwomesoporousmaterialsarealsocomparedtothatofthe commercialZSM-5zeoliteadsorbentforadsorbingCO2greenhouse gas.
2. Experimental
2.1. Wastepowdercharacterizationandextraction
ThephotonicwastepowderwasobtainedfromaTFT-LCDplant andcharacterizedwithoutanypretreatment.Theelemental anal-ysisof thewastepowderwasdeterminedbyenergy-dispersive X-ray spectroscopy in a scanning electron microscope (SEM-EDS,HITACHI-S4700).Thefunctionalgroupsofthewastepowder werecharacterizedbytheFTIRinstrument(BrukerVector22IR). Powderlow angle X-raydiffraction pattern of the waste pow-derwasrecordedbyRigakuX-raydiffractometerequippedwith nickel-filtered CuK␣ (=1.5405 ˚A) radiation. The diffractogram wasrecordedinthe2rangeof5–80◦withascanningspeedof 2◦perminute.Thespecificsurfacearea,porevolumeandaverage porediameter(BJHmethod)ofthesampleweremeasuredbyN2 adsorption–desorptionisothermsat−196◦CusingaBETsurface areaanalyzer(Micromeritics,ASAP2000).Thethermal behavior ofthewastepowderwasdeterminedinthetemperaturerange of25–900◦Cusingathermo-gravimetricanalyzer(TGA,Netzsch TG209F1,Germany).
Theextractionofsilicawascarriedoutthroughanalkalifusion treatment[9].Inatypicalprocess,thewastepowderandNaOH powderwerethoroughlymixedataweightratioof1:1.2andfused at550◦Cfor1h.ThereceivedfusedproductwasthenmixedwithDI waterataweightratioof1:5withcontinuousstirringfor24h.The resultingmixturewasthencentrifugedtoseparatethesediment forfurthercharacterizationofitscomponents.Andthesupernatant wasalsoanalyzedbyICP-MS(SCIEXELAN5000–Inductively Cou-pledPlasma-MassSpectrometer)andutilizedforthesynthesisof MCM-41.
2.2. Synthesisandcharacterizationofthemesoporoussilica MCM-41
MesoporoussilicamaterialsofMCM-41weresynthesizedby hydrothermaltreatmentmethodusingeitherwastederivedsilica orpuresilicasourceastheprecursor solutions.
Cetyltrimethyl-ammonium bromide (CTAB) was employed as the structure-directingtemplateinthesynthesis.Forthephotonicwastederived MCM-41,themolarcompositionofthegelmixturewas1SiO2:0.2 CTAB:0.89 H2SO4:120 H2O, where the SiO2 precursor source was obtainedfrom the supernatant of photonic waste powder extractionprocessdescribedinSection2.1.Inatypicalsynthesis procedure,50mlofsupernatantwasfirstlystirredvigorouslyfor 30min.Then,thepHofthesolutionwasadjustedto10.5using2M H2SO4followedbyfurtherstirringtoformagel.Afterthat4.6gof CTAB(dissolvedin16mlofDIwater)wasaddeddropbydropinto theabovemixtureandthecombinedmixturewasstirredforthree additionalhours.Theresultinggelmixturewastransferredintoa Tefloncoatedautoclaveandkeptinanovenat145◦Cfor36h.After coolingtoroomtemperature,theresultantsolidwasrecoveredby filtration,washed withDIwaterand driedinanovenat110◦C for6h.Finally,theorganictemplatewasremovedviaamuffle fur-naceinairat550◦Cfor6h.TheMCM-41materialsynthesizedfrom photonicwastepowder(PWP)wasnamedasMCM-41(PWP).
Forcomparisonpurpose,thesynthesisofMCM-41usingpure sodiummetasilicate nanohydrate(Na2SiO3·9H2O) wasalso per-formedfollowingsimilarproceduredescribedpreviouslyforthe synthesisofMCM-41(PWP).Thedetailedprocedurewasdescribed inKarthiket al.[12].Themolarcomposition ofthegelmixture was1SiO2:0.2CTAB:120H2O:0.89H2SO4.TheMCM-41material derivedfrompurechemicalofNa2SiO3·9H2Owasdenotedas MCM-41(NaSi).
The calcined MCM-41 materials were characterized by BET surface areaanalyzer,XRD analysisand theEDS instrument as described previouslyfor thecharacterizationof photonicwaste powders.Inaddition,TEMimagesofthecalcinedMCM-41 materi-alswereobtainedwithaTEMinstrument(JEOLJEM1210)operated at 120keV and the samples (5–10mg) were ultrasonicated in ethanolanddispersedoncarbonfilmsupportedoncoppergrids (200mesh).
2.3. TEPA-impregnatedMCM-41forCO2capture
Inordertoenhancetheadsorptionperformanceforthecapture ofCO2,alladsorbents(MCM-41(PWP),MCM-41(NaSi)andZSM5 zeolite)weremodifiedwiththeamineagentof tetraethylenepen-tamine(TEPA)ataweightratioof1:1bythewetimpregnation method described in Lu et al. [13]. To obtain the adsorption capacityandbreakthroughcurveoftheadsorbents,CO2 adsorp-tion experiment was carried out in a packed column with an internaldiameterof0.75cm.Theadsorptioncolumnwaspacked with1.0gof adsorbents(packing height=∼5cm)and placed in a temperature-controlledoven.In a typical process,adsorbents werepretreatedunderaN2flowof0.2L/minat110◦Cfor1h,and thencooledto25◦C.Subsequently,thegasflowwasswitchedto 20%(v/v)CO2gasstream(balancedwithN2)underaflowrateof 0.1L/min.TheconcentrationofCO2wascontinuouslymeasuredby aCO2analyzer(MolecularAnalyticsAGM4000GasAnalyzer).The CO2 adsorptioncapacity(q,mg/g)atacertaintime(t,time)was estimatedas q= 1 m
t 0 Q×(Cin−Ceff)dt (1)wheremistheweightofadsorbent(g),Qisthegasflowrate(L/min), andCinandCeffaretheinfluentandeffluentCO2 concentrations (mg/L),respectively,whichareexpressedintermsofpercentin vol-ume(%).Theadsorptioncapacityofzerogas(N2only)wasdeducted fromtheadsorptioncapacitiesofadsorbents.
Fig.1. (a)EDSspectrumand(b)FTIRspectrumofthephotonicwastepowder.
3. Resultsanddiscussion
3.1. Characterizationofthephotonicwastepowder(PWP)
TheresultsofEDS andFTIRanalysesforthephotonicwaste powderareshowninFig.1,andtheweightpercentagesofeach elementinthephotonicwastepowderarelistedinTable1.As can beseen from Fig.1a, thefour observed elementsof Si, O, NandFinthephotonicwastepowderwereexpectedsincethe solidwastewasderivedfromtheCVDprocessinwhichthethree gaseousreactantswereSiH4,NF3andNH3.Furthermore,theFTIR spectrumofphotonicwastepowdershowninFig.1bexhibited sig-nificantabsorptionbandsat3336,3139,1400,1080,960,742and 482cm−1.Thebandsat3336and3139cm−1arerelatedtoN–H stretching,while742and482cm−1canbeassignedtoSiF62−[14]. Besides,thebandsat1400,1080and960cm−1 canbeassigned totheNH4+[14],Si–O–SiandSi–OHstretchingvibrations, respec-tively[15].Consideringtheaboveresults,theprimarycomponents inphotonicwastepowdercouldbetheadmixtureof(NH4)2SiF6 andSiO2.
TheXRDanalysiswasthencarriedouttofurtheridentifythe crystallinecomponentsin thephotonicwastepowder withthe resultdepictedinFig.2.Strong(NH4)2SiF6diffractionpeakswere identified[16].Incontrast,thediffractionpeakofsilicawasnot observed fromtheXRD pattern, suggesting that SiO2 wasonly presentinlittleamountanditmightbetrappedinthe(NH4)2SiF6 lattice.Howeverthepreciseweightpercentagesof(NH4)2SiF6and
2 theta (degree) 80 70 60 50 40 30 20 10 Intensity (counts) 0 200 400 600 800 1000 1200 1400 1600 1800 (NH4)2SiF6
Fig.2. XRDpatternofthephotonicwastepowder.
Temperature (ºC) 900 800 700 600 500 400 300 200 100 0 Weight remained (%) 0 10 20 30 40 50 60 70 80 90 100 TGA Temperature (oC) 800 700 600 500 400 300 200 100 0
Mass Change Rate (%/min)
-25 -20 -15 -10 -5 0 5 DTG (a) (b)
Fig.3. (a)TGAanalysisand(b)DTGprofileofthephotonicwastepowder.
SiO2 couldnotbeobtainedfromTable1sincetheEDSdataonly provided a rough estimateof theelemental content, while the ICP-MScouldonly detectthe Sicontent in thephotonic waste powder.
Theweightpercentagesof(NH4)2SiF6 andSiO2 presentedin thephotonicwastepowderweredeterminedusingTGAweight loss and differentialthermo-gravimetric (DTG) analyses. It can be seen from Fig. 3a that the photonic waste powder sample showedaninitialweightlossat<150◦C,whichcouldbeascribed totheevaporationofthephysicallyadsorbedwateronthe sur-faceofthematerials.Asthetemperatureexceeded150◦C,there wasa significantweightlossfor thephotonicwastepowderat around 237◦C as clearly observed from Fig. 3b. Mel’nichenko et al.[17] investigated themechanism of thesolid-phase reac-tionbetween(NH4)2SiF6andSiO2,andproposedthat(NH4)2SiF6 couldbethermallydecomposed between220and300◦Cin the presenceofSiO2.Generally,thefollowingchemicalreactionsare expectedtotakeplacewhen(NH4)2SiF6washeatedupto900◦C [16]: (NH4)2SiF6 220◦C −→ SiF4(g)+2NH4F(s) (2) NH4F(s) 850◦C −→ NH4F(g) (3)
Considering the abovereactions, (NH4)2SiF6 would be com-pletelydecomposed duringthethermaltreatmentupto900◦C. However, it is noted that there was still 15wt.% of residues remainedafterheatingupthephotonicwastepowderto900◦C. ThisisprobablyduetothepresenceofSiO2 sinceitisthermally stableupto900◦C.ThetotalSiweightfraction(from(NH4)2SiF6 andSiO2)of20.4%calculatedbytheTGA/DTGresultwasveryclose totheSimassfractionof22.4%detectedbytheICP-MSresultshown inTable1.Therefore,theprimarycomponentsinphotonicwaste powderweredeterminedtobearound85%of(NH4)2SiF6and15% ofSiO2fromtheresultsofTGA,FTIRandSEM-EDSanalysis. 3.2. Characterizationofsedimentandsupernatantliquidafter extraction
Fig.4showstheEDSspectrumofthesedimentandits corre-spondingXRDpattern.ItcanbefoundfromFig.4athatNaandF weretheprimaryelementsinthesediment,withnegligibleSiand Ospecies,whichwasalsoconfirmedbytheresultofEDSanalysis showninTable1.TheresidualSiO2 couldbeduetoinsufficient NaOHamountsappliedintheextractionprocess.Themajor com-ponentofNaFpresentedinthesedimentwasalsodemonstratedby
Table1
ElementalanalysisofphotonicwastepowderandsedimentanalyzedbytheSEM-EDSandICP-MSanalyses.
Element Si F O N Na
Photonicwastepowder(wt.%)a 28.82 47.82 17.54 5.82 –
Sediment(wt.%)a 5.22 33.34 2.99 –c 58.45
Photonicwastepowder(wt.%)b 22.35 – – – –
Supernatant(ppm)b 13,080 – – – 33,180
aSampleanalyzedbytheSEM-EDSanalysis. b SampleanalyzedbytheICP-MSanalysis. c Notdetected.
Fig.4.(a)EDSspectrumand(b)XRDpatternofthesedimentaftersilicaextraction.
Fig.4b.Thefivesharpdiffractionpeakslocatedat34,39,56,67and 70.5◦wereinagreementwiththeNaFstandardXRDpeaks[18,19]. Besides,therewasabroadpeakcenteringat22◦asobservedfrom Fig.4b,whichcanbeascribedtotheamorphousSiO2 [20].This resultisconsistentwiththeEDSspectrumwhichshowedthatSi andOspecieswerealsopresentedinminoramounts.
Thethermalbehaviorofthesedimentwassubsequently inves-tigatedusingTGAandDTGanalyses.ItcanbeseenfromFig.5athat therewasonlyaround5%weightlossforthesedimentduringthe thermaltreatmentupto900◦C.TheDTGcurveshowedthree dis-tinguishedpeaksfromroomtemperatureto900◦C(Fig.5b).The firststep(<150◦C)isduetothelossofphysicallyadsorbedwater onthesurfaceofthesedimentandthesecondstep(150–600◦C) isattributedtothelossofchemicallyadsorbedwaterbondedto Si–OHthroughhydrogenbond[21]sincethesedimentcontained
Temperature (ºC) 900 800 700 600 500 400 300 200 100 0 Weight remained (%) 95 96 97 98 99 100 TGA Temperature (oC) 900 800 700 600 500 400 300 200 100 0
Mass Change Rate (%/min) -0.15 -0.12 -0.09 -0.06 -0.03 DTG (a) (b)
Fig.5. (a)TGAanalysisand(b)DTGprofileofthesediment.
onlyalittleamountofSiO2.From600to900◦C,theweightloss isexpectedtobeassociatedwiththefurthercondensationofthe Si–OHgroupsfromtheamorphousSiO2[20].
Asaresult,onecanconcludethatFwaseffectivelycapturedby NaOHandthesodiumfluoride(NaF)sedimentswereformedafter theextractionprocess.TheNaFpuritywasquitehigh(>90%)in thesedimentasobservedfromtheTGA/DTGdata,whichprovides furtherpossibilityforreuse.Sodiumfluorideisoneofthe well-knownchemicalcompoundswhicharewidelyutilizedinindustries asthesourceoffluorideionindiverseapplications.Inotherwords, thephotonicwastepowdercanbeconverted intotwovaluable resources,thesupernatantasthesilicaprecursorandthesediment ofsodiumfluoride.
Thecompositionofthesupernatantwasalsoanalyzedby ICP-MSinordertoinvestigatetheconcentrationsofSiandNaions. Table1showsthattheconcentrationsofSiandNaionswere13,080 and33,180ppm,respectively.TheconcentrationratiobetweenNa andSiis2.54inthisstudy.ItislowerthantheNa/Siratioof sil-icasupernatant recovered fromflyash[9]in which theSi and Naconcentrationswere572and12,000ppm,respectively.Chang etal.[9]statedthatthehighconcentrationratioofNa/Siinthe precursortendedtohinderthemesostructureandthusthe MCM-41obtainedfromflyashsupernatantcouldnotexceed1000m2/g. Incomparison,thesilicasupernatantobtainedfromthephotonic wastepowdercouldbeabettersourceforproducinghighquality waste-derivedMCM-41.
3.3. CharacterizationofMCM-41(PWP)
Fig.6ashowstheEDS spectrumoftheMCM-41sample syn-thesizedfromthephotonicwastepowder.Itcanbeseenthatonly SiandOelementswerepresentinMCM-41(PWP)indicatingthat alkalifusiontreatmentisaviableprocesstoeffectivelyseparate silicatespeciesfromthePWPderivedsilica.Fig.6billustratesthe
Fig.6. (a)EDSspectrumofMCM-41(PWP)sampleand(b)XRDpatternsofthe MCM-41(PWP)andMCM-41(NaSi)samples.
Relative pressure (P/P0) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Amount of N 2 adsorbed (cm 3 -g -1 ) 0 100 200 300 400 500 600 700 800 Adsorption Desorption
Photonic waste powder MCM-41(PWP) MCM-41(NaSi)
Fig.7.N2adsorption–desorptionisothermsofthephotonicwastepowder,calcined
MCM-41(NaSi)andMCM-41(PWP)samples.
powderXRD patternsoftheMCM-41(PWP) and MCM-41(NaSi) samples.OnecanseethatMCM-41(NaSi)sampleexhibiteda well-defined(100)diffractionpeaklocatedat2of2.3–2.6◦andthree reflectionsof(110),(200)and(210)at4.2,4.7and6.2◦, respec-tively,whichcouldbeindexedonahexagonallatticeofmesoporous silica materials[6,12,22].Similarobservation wasalsoseen for theMCM-41(PWP) sample, thus indicating that highly ordered mesostructurewasobtainedusingthesupernatantextractedfrom photonicwastepowderasthesilicasource.
The N2 adsorption–desorption isotherms of photonic waste powder,MCM-41(NaSi)andMCM-41(PWP) areshowninFig.7. It is clearthat photonicwastepowder exhibited a typicaltype II isotherm of non-porous materials according to the IUPAC classification.Ontheotherhand,bothMCM-41(NaSi)and MCM-41(PWP)possessedtypeIVisothermswhicharethecharacteristic ofmesoporous materials,featuring a narrowstep due to capil-larycondensationofN2withintheprimarymesopores[5,23].The isothermsoftheMCM-41(NaSi)andMCM-41(PWP)alsoshowed thetypeH1hysteresisloopassociatedwithwell-arranged cylin-dricalchannelswithuniformshapeandporesize[24].However, thelesssteepcondensationstepforMCM-41(PWP)sample indi-catesalesserdegreeofuniformmesostructureascomparedtothat ofMCM-41(NaSi)sample.Thiswasexpectedsincethesilica pre-cursorofMCM-41(PWP)couldnotbeinsuchahighpurityasthe purechemicalofNa2SiO3forproducingMCM-41(NaSi).
Fig.8displaystheBJHporesizedistributionsofMCM-41(NaSi) andMCM-41(PWP)samples.ItisclearthatMCM-41(NaSi)sample showedanarrowporesizedistributionwiththepeakporesizeat 2.7nm,suggestingtheuniformporosityoftheobtainedmaterials. TheMCM-41(PWP)alsoshowedanarrowporesizedistribution withthepeaksizeataround3.0nm.Butasmallpeaklocatedat around4nmwasalsoobserved.Thebimodalmesoporositywould resultinlessuniformityofmesostructureofMCM-41(PWP).
ThephysicochemicalpropertiesofBETspecificsurfacearea, spe-cificporevolumeandaverageporediameterderivedfromtheN2 adsorption–desorptionmeasurementsaresummarizedinTable2. To check thestability of using thephotonic waste powder for preparing theMCM-41(PWP), duplicate experiments were per-formedand theBETcharacterization ofthetwo MCM-41(PWP) samplesprepared fromtwo differentbatches ofwaste powder showedthattherewasnegligibleeffectonthesamplepore struc-ture.Thiscouldbeattributedtothefactthatthesolidwastepowder wasobtainedfromthesameTFT-LCDplant,whichcontrolstheir processpreciselyandstably.ItcanbeobservedthatMCM-41(PWP) possessedhighspecificsurfacearea(1082m2/g),narrowporesize
Pore diameter (nm) 7 6 5 4 3 2
dV/d logD, Pore volume (cm
3/ g) 0 2 4 6 8 10 12 MCM-41(NaSi) MCM-41(PWP)
Fig.8. BJHporediameterdistributionsofcalcined MCM-41(NaSi)and MCM-41(PWP)samples.
distribution(2.95nm)andlargeporevolume(0.99cm3/g)which are fairlyclosed to theproperties of MCM-41(NaSi). Theslight shrinkageintheporevolumeofMCM-41(PWP)couldbedueto thehigheramountsofNaionsinthesupernatant[9].However, theeffectofthehigherNaconcentrationwasveryminorwhich couldbeattributedtoprecisepH-controlsduringthesynthesisof theMCM-41(PWP).
Thetextural mesostructure oftheMCM-41(NaSi) and MCM-41(PWP)sampleswasfurtherrevealedbyTEMimagesshownin Fig.9aandb,respectively.TheTEMimageofMCM-41(PWP) mate-rial(Fig.9b)clearlyshowsawellorderedlongrangehexagonal arrayof mesopores similarto thatof theMCM-41(NaSi) mate-rial(Fig.9a),and theselongrange crystallographicfeaturesare consistent withtheresults ofXRD and BET analyses. Thepore diameter observed in Fig. 9b was ca. 3.2nm which was fairly closeto thenarrowpore sizedistribution (BJH)determinedby N2adsorption–desorptionmeasurements,indicatingthatthepore structureofMCM-41(PWP)ishighlyordered.
3.4. ApplicationasadsorbentsforCO2capture
Resultsof theCO2 adsorptioncapacities forbothraw adsor-bentsandthosemodifiedwithTEPAarelistedinTable2.Forraw adsorbents,theporesizeofZSM-5zeolite,0.44nm,isvery simi-lartothemoleculardiameterofCO2,0.35nm.Thusitcanserveas amolecularsieveandhasamuchhigherCO2adsorptioncapacity of40mg/gthanthemesoporousadsorbentsofMCM-41.However, whencoatedwithTEPA,thesmallporeopeningofZSM-5zeolite limitedtheaccessofTEPAmolecules.Thustheexcessiveamount ofTEPAreagentscoatedontheexternalsurfacewouldresultin theblockageoftheporeopeningandtheCO2capturecouldnot beenhanced.Besides,TEPA-modifiedZSM-5zeolitehadagel-like morphology[25]anditssurfaceareacouldnotbemeasured.Onthe contrary,theTEPA-modifiedmesoporoussilicamaterials, MCM-41(NaSi)andMCM-41(PWP),havelargerporesizeswhichwould allowtheloading ofmore TEPA moleculesinto thepore chan-nelsresultinginhigherCO2capturecapacitiesof97and82mg/g, respectively.
Fig.10 displays thebreakthrough curves of CO2 over TEPA-modified adsorbents at 25◦C via the packed column reactor. Initially all adsorbents could have adsorption efficiencies of near 100%, but the breakthrough time of the TEPA/ZSM-5 zeoliteadsorbent wastheshortest, followedbythe TEPA/MCM-41(PWP)andthentheTEPA/MCM-41(NaSi).Theslightdifference in the CO2 adsorption capacities between the
TEPA/MCM-Table2
PhysicalpropertiesandCO2adsorptioncapacitiesofphotonicwastepowderandmesoporousadsorbents.
Samplename SBETa(m2/g) Dpb(nm) Vpc(cm3/g) Adsorptioncapacity
(mgCO2/gadsorbent)
Photonicwastepowder 30 – 0.07 –
MCM-41(NaSi) 1102 3.0 1.13 4 MCM-41(PWP) 1082±13 2.95±0.05 0.99±0.02 2 ZSM-5 361 0.44d 0.12e 40 TEPA/MCM-41(NaSi) 153 4.1 0.17 97 TEPA/MCM-41(PWP) 18 –f 0.12 82 TEPA/ZSM-5 – – – 43
aBETsurfacearea.
b PorediametercalculatedbyBJHtheory. c PorevolumecalculatedbyBJHtheory.
d MeanporewidthcalculatedbyHorvath–Kawazoemodel. ePorevolumecalculatedbyt-plotmethod.
f Notdetected.
41(NaSi)and TEPA/MCM-41(PWP)canbedescribed bytheirN2 adsorption–desorptionisotherms as shownin Fig. 11and their pore structure data listed in Table 2. Apparently, TEPA/MCM-41(NaSi)exhibitedabetterresolvedcapillarycondensationstep thanthatofTEPA/MCM-41(PWP).Thus theTEPA/MCM-41(NaSi)
Fig.9.TEMimagesof(a)MCM-41(NaSi)and(b)MCM-41(PWP).Thescalebarin theTEMimagesis50nm.
Time (min) 10 9 8 7 6 5 4 3 2 1 0 Ceff /C in 0.0 0.2 0.4 0.6 0.8 1.0 TEPA/MCM-41(NaSi) TEPA/MCM-41(PWP) TEPA/ZSM-5
Fig.10.CO2breakthroughcurvesforTEPAloadedadsorbentsofMCM-41(NaSi),
MCM-41(PWP)andZSM-5zeolite.
stillmaintainedbetterameso-structureascompared tothatof TEPA/MCM-41(PWP). The density of TEPA is about 0.99cm3/g andtheporevolumesofMCM-41(NaSi)and MCM-41(PWP)are 1.13cm3/gand0.99cm3/g,respectively.ThusthemaximumTEPA loadinginsidetheporechannelswascalculatedtobe53%and50%,
Relative pressure (p/p0) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Amount of N 2 adsorbed (cm 3 / g, STP) 0 20 40 60 80 100 120 Adsorption Desorption TEPA/MCM-41(NaSi) TEPA/MCM-41(PWP)
Fig. 11.N2 adsorption–desorption isotherms of the TEPA/MCM-41(NaSi) and
Wavenumber (cm-1) 1000 1500 2000 2500 3000 Transmission (%)
a
b
N-H2 -NCOO Si-O-Si -NH3+ O-Si/-NH2+ O-Sic
C-H2 1630 1320 15631470 2933 2814 O-H 1637 1415 1080Fig.12.FTIRspectraof(a)calcinedMCM-41(PWP),(b)freshTEPA/MCM-41(PWP), and(c)TEPA/MCM-41(PWP)afterCO2adsorption.
respectively,fortheMCM-41(NaSi)andMCM-41(PWP).Asaresult, theMCM-41(NaSi)had ahigherCO2 adsorptioncapacitydueto thefactthatithadahigherporevolumetoaccommodateTEPA moleculesinsidethepores.AndasseeninTable2,afterloadedwith TEPA,theBETsurfaceareaoftheTEPA/MCM-41(NaSi)ishigherthan thatoftheTEPA/MCM-41(PWP).
TheCO2adsorptionmechanismcouldbedescribedbythe func-tionalgroupsontheadsorbentsurfacesasobservedintheFTIR spectrashowninFig.12fortheMCM-41(PWP)(Fig.12a)andfresh TEPA/MCM-41(PWP)(Fig.12b).OnecanseethatMCM-41(PWP) showed significant bands at 1637 and 1080cm−1 correspond-ingtoH–O–HbandandSi–O–Siasymmetricstretchingvibration, respectively. On the other hand, the FTIR spectrum of fresh TEPA/MCM-41(PWP) exhibited significantbands at 2933,2814, 1630,1563,1470and1080cm−1.Thebandsat2933and2814cm−1 are related to C–H2 stretching from CH2CH2CH2–NH2 (RNH2), while1563and1470cm−1canbeassignedtoN–H2vibrationinthe primaryaminegroup(RNH2)[6].Andthebandat1630cm−1canbe assignedtotheNH3+deformationoftheprotonatedprimaryamine group or secondary amine group (–NH3+O–Si/–NH2+O–Si). The presenceoftheC–H,N–H2 and –NH3+O–Si/–NH2+O–Siconfirms that TEPA was successfully grafted on the surface of MCM-41(PWP).Additionally,thespectrumofTEPA/MCM-41(PWP)after CO2adsorptiontestwasalsomeasured(Fig.12c).Itwasobserved thatonedistinguishedbandappearedat1415cm−1whichcanbe correspondedtotheNCOOskeletalvibration,indicatingthe for-mationofalkylammoniumcarbamatethroughtheCO2adsorption process.Generally,thefollowingchemicalreactionsareexpected totakeplacewhenCO2moleculesreactwithTEPA[26]:
CO2+2RNH2→ RNHCOO−+RNH3+ (4)
CO2+2R1R2NH→R1NHCOO−+R2NH2+(orR2NCOO−+R1NH2+) (5)
CO2+R2NH+RNH2 → R2NCOO−+RNH3+(orRNHCOO−
+R2NH2+) (6)
4. Conclusions
Thisstudysuccessfullydemonstratedthefeasibilityofrecycling thephotonicwastepowdertoavoidthefastdrainofthenatural
resources.ItcanbeemployedfortheSi-relatedindustriessuchas semiconductor,opto-electronicandsolarcellindustriesasawaste reductionstrategy.Itwasconfirmedthatalkalifusionatthe tem-peratureof550◦Cisaviableprocesswhichcaneffectivelyseparate thesilicatespeciesfromthewastepowder.Theextractedsodium silicatewascapableofreplacingcommercialsilicaprecursorfor theproductionofmesoporousMCM-41materialwithhighsurface area(1082cm2/g)andlargeporevolume(0.99cm3/g).Meanwhile, nearpurifiedsodiumfluoride(>90%)wasalsoobtainedasthe sed-imentduringtheextractionprocess,whichcanalsobeavaluable resourceoffluoride.Asaresult,twovaluableproductswere pro-ducedduringthewasterecoveryprocess.Adsorptionbreakthrough experimentsconfirmedthatamine-modifiedMCM-41(PWP),with ahighadsorptioncapacityof82mgCO2/gadsorbent,canbeused asaneffectiveadsorbenttoreduceCO2greenhousegasemission. Acknowledgments
Theauthorsgratefullyacknowledgethefinancialsupportfrom theNationalScienceCounciloftheRepublicofChinathroughGrant No.:NSC98-2221-E-009-023-MY3.
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![Fig. 4 b. The five sharp diffraction peaks located at 34, 39, 56, 67 and 70.5 ◦ were in agreement with the NaF standard XRD peaks [18,19]](https://thumb-ap.123doks.com/thumbv2/9libinfo/7892225.155670/4.918.470.851.839.1112/five-sharp-diffraction-peaks-located-agreement-standard-peaks.webp)
