Optimal Surface Amino-Functionalization Following Thermo-Alkaline Treatment of Nanostructured

24 Pages·2016·4.56 MB·English

materials
Article
Optimal Surface Amino-Functionalization Following
Thermo-Alkaline Treatment of Nanostructured Silica
Adsorbents for Enhanced CO Adsorption
2
ObduliaMedina-Juárez,MiguelÁngelGarcía-Sánchez,UlisesArellano-Sánchez,
IsaacKornhauser-StrausandFernandoRojas-González*
DepartmentofChemistry,UniversidadAutónomaMetropolitana-Iztapalapa,Av.SanRafaelAtlixco186,Col.
Vicentina,MéxicoCity09340,Mexico;[email protected](O.M.-J.);[email protected](M.Á.G.-S.);
[email protected](U.A.-S.);[email protected](I.K.-S.)
* Correspondence:[email protected];Tel.:[phone]
AcademicEditor:ThomasFiedler
Received:28September2016;Accepted:31October2016;Published:4November2016
Abstract: Special preparation of Santa Barbara Amorphous (SBA)-15, mesoporous silica with
highlyhexagonalordered,thesematerialshavebeencarriedoutforcreatingadsorbentsexhibiting
anenhancedandpartiallyselectiveadsorptiontowardCO . Thiscreationstartsfromanadequate
2
conditioning of the silica surface, via a thermo-alkaline treatment to increase the population of
silanolspeciesonit. CO adsorptionisonlyreasonablyachievedwhentheSiO surfacebecomes
2 2
aminatedafterputincontactwithasolutionofanaminoalkoxidecompoundintherightsolvent.
Unfunctionalizedandamine-functionalizedsubstrateswerecharacterizedthroughX-raydiffraction,
N sorption,Ramanspectroscopy,electronmicroscopy,29Sisolid-stateNuclearMagneticResonance
2
(NMR),andNH thermalprogrammeddesorption. Theseanalysesprovedthatthethermo-alkaline
3
procedure desilicates the substrate and eliminates the micropores (without affecting the SBA-15
capillaries),presentintheoriginalsolid. NMRanalysisconfirmsthatthehydroxylatedsolidanchors
more amino functionalizing molecules than the unhydroxylated material. The SBA-15 sample
subjected to hydroxylation and amino-functionalization displays a high enthalpy of interaction,
areasonwhythissolidissuitableforastrongdepositionofCO butwiththepossibilityofobserving
2
alow-pressurehysteresisphenomenon. Contrastingly, CH adsorptiononamino-functionalized,
4
hydroxylatedSBA-15substratesbecomesalmostfivetimeslowerthantheCO one,thusgivingproof
2
oftheirselectivitytowardCO . AlthoughtheamountofretainedCO isnotyetsimilartoorhigher
2 2
thanthosedeterminedinotherinvestigations,themethodologyhereindescribedisstillsusceptible
tooptimization.
Keywords: CO enhanced capture; SBA-15 habilitation for CO sorption; desilication; silanol
2 2
functionalization;covalentcoordinatedCO deposition
2
1. Introduction
1.1. TheProblem
The effective and abundant capture of CO on the surface of nanoparticulate substrates is
2
afundamentalexperimentalaim,sincethisgasplaysaleadingroleintheupsurgeofthegreenhouse
effect; this last phenomenon being nowadays one of the most important ecological problems to
deal with. Several strategies for achieving satisfactory CO levels of capture already exist; one
2
implementation of particular and novel interest consists in the trapping and eventual in situ
transformation of this gas into useful and environmentally friendly compounds, via chemical
procedures. Hence, withtheobjectiveinmindofreachinganefficientcarboncaptureandstorage
Materials2016,9,898;doi:10.3390/ma9110898 www.mdpi.com/journal/materials
Materials2016,9,898 2of24
(CCS) [1–3], three alternatives are available at present: (a) CCS pre-combustion of carbon sources
beforeactualfuelcombustion;(b)oxycombustion(i.e.,carboncompoundcombustionundernearly
pure oxygen); and (c) post-combustion capture. Carbon capture after combustion is unavoidable
duetotheamountofindustrialprocessesthatdependontheoverwhelmingburningoffossilfuels;
anexampleofthisbeingcementproductionamongmanyotherprocessesinvolvingtheburningof
hugeamountsofcarbon. Nowadays,thevastmajorityofCCSresearchisrelatedtotheoccurrence
ofthefollowingphenomena: (a)absorption[4,5];(b)cryogenicseparation[6];(c)separationthrough
membranes [7]; and (d) adsorption [8–10]. The materials employed to perform after combustion
CCSoperationsrequire,besidesbeingselective,someothercrucialcharacteristicssuchas: (i)tobe
endowedwithahighcompoundcapturecapacity;(ii)toberenewable;(iii)lowcost;(iv)durable;and
(v)practical.
1.2. CarbonCaptureandStorage(CCS)Alternatives
Inparticular,severalsorptionexperimentsareveryusefulasCCSalternatives. Accordingtothe
natureofthesubstrateemployed,CO sorptionorcaptureoccursthroughtwopossibleprocesses:
2
- Absorption processes, which involve the bubbling of the adsorptive in solutions containing
speciessuchasamines,carbonates,andions. Inthiscase,thesolventbecomesveryimportant,
sinceionordertoachieveagoodcaptureofCO ,solventsofalowpolarityandrelativelylow
2
vapor pressure are very much preferred. Nevertheless, absorption processes involve a great
disadvantagethatisconcernedwiththeshortlifetimesthatthetrappedspeciescanattainas
thesemoleculesusuallyinteract(react)withthesolventspecies.
- Adsorptionprocessesimplyingtheemploymentofstructuressuchaszeolites[11],hydrotalcites[12],
polymerfibers[13],activatedcarbons[14],metaloxidenetworks[15],functionalizednanoporous
materials[16–18],and,recently,metaloxideframeworks(MOFs)[19–22].
1.3. CO ChemisorptionandPhysisorption
2
ThesubstratesthataremoreusuallychosenforCO chemisorptioncapturearemetaloxides(e.g.,
2
CaOandMgO);thesesolidscanformcarbonatesinthepresenceofCO .However,ashortcomingarises
2
withtheemploymentofthesesubstrates,inviewofthefactthathightemperaturesarerequiredfor
achievingtheirtotalregeneration,i.e.,287and800◦CforMgOandCaO,respectively[15].Lithiumsalts,
suchasLi ZrO andLi SiO [23,24],havebeenusedtocaptureCO (between450and700◦C),thus
2 3 4 4 2
apparentlybeinggoodalternativesforreachinganefficientCO adsorption,irrespectivelyofthehigh
2
productioncostinvolvedwiththiskindofsubstrates.
AnotherfinealternativeforeffectiveCO chemisorptionistheuseofhydrotalcites,eversince
2
Leónetal.[25]demonstratedthattheirreversiblereactionbetweenthesemineralsandCO isdue
2
tothecreationofunidentateadsorptionsites. Likewise,theformationofbidentatesitesgenerates
surfacebicarbonates,whichbehaveasweakadsorptionsites,usefulforhighlyreversiblereactions.
Theconcentrationoftheadsorbatephaseiscloselyrelatedtothenumberofirreversibleadsorptionsites.
Activecarbonpossessesattractivecharacteristicsforthephysisorptionofdiversespeciessuchas
highuptakecapacity,hydrophobicityandlowcost;however,thissubstrateshowsapoorselectivity
betweenCO andN . Luetal.[26]observedthatcarbonnanotubesdepictedahigherCO adsorption
2 2 2
than zeolites when surface functionalized with amine groups (NH ) through the attachment of
2
3-aminopropyltriethoxysilane (APTES). Due to the previous arguments, a convenient adsorbent
forCO capturedependsonthetemperatureandpressureatwhichtheadsorptionprocessistotake
2
place. According to these requirements, surface-functionalized mesoporous materials stand apart
fromtherestofadsorbentsbecauseoftheirabilitytophysisorborchemisorbdiversecompounds,
dependingontheirphysicochemicalinteractions(weakorstrong)withtheadsorbent.
Recently,theimplementationofMOFshasbroughtaboutgreatadvantages: (i)agoodstructural
stability;and(ii)ahighporositythischaracteristicbeingreflectedinsurfaceareasofabout2900m2·g−1
Materials2016,9,898 3of24
and large total pore volumes between 1 and 2 cm3·g−1 [19] if compared to solids such as zeolites
andmolecularsieves. However,agreatdisadvantagesurgeswhentryingtoemployMOFSforCO
2
adsorption,inviewofthelowselectivityofthesesolidstowardsCO ,especiallywhenthisadsorptive
2
ismixedwithothergases. Additionally,thesematerialsareoverwhelmedbyhighsynthesiscosts;
furthermore,thecapturecapacityofMOF’sdecreasesunderhumidconditions,eventhoughthislast
eventcouldbedealtwiththroughsurfacefunctionalizationviadiverseorganicgroups[20–22].
2. StagesFollowedConcerningtheChoiceandImplementationofProficientSilicaAdsorbents
forEnhancedCO Capture
2
2.1. AdsorbentChoice
TheCO moleculeisaLewisacidsincethismoleculecanacceptapairofelectronsfromabase
2
inordertoestablishacovalentcoordinatedbond. Therefore,thecrucialstepsfordesigningaproper
CO adsorbentincludetheselectionofasubstratewhosesurfacecanbehabilitatedtopossessagood
2
numberofbasicsites;i.e.,thesurfacehastobeendowedwithchemicalspeciesthatcanbecombined
withothermatchingmoleculesforattainingthedesiredcharacteristics.
Nanoporous silica supports [16–18] are provided, in general, with a high surface area and
aconsiderabletotalporevolume.Thesecharacteristicsmakethesesubstratesappropriateforadsorbing
an assortment of chemical species. The employment of amphiphilic triblock copolymers as pore
templatingagentshasallowedthecreationofwell-orderedmesoporousnetworks,whichhavebeen
labeledasSBA-n(SantaBarbara)materials[27]. Amongthem,theSBA-15[28]substrateresultsto
haveanidealsurfaceforperforminganefficientandenhancedCO adsorption. Ontheonehand,this
2
materialisformedbyacollectionofcylindricalcapillaries,allpracticallyhavingthesamediameters,
inahexagonalarrangementandlyingparalleltoeachotheralongmicrometriclengths. Ontheother
hand,thecapillariesopenatbothendsthatareformingthesemesoporousstructuresareendowedof
suchsizesastoallowthefreetransitofdifferentmoleculesalongthenanocylinderaxis. Thisisavery
importantpropertysincetheadsorptivemoleculescanenter,whiletheadsorbatemoleculescanexit
thestructureeasily.
Besides,thefunctionalizationofthesurfacevia,forinstance,amino-alkoxides(i.e.,thetypeof
compoundsthatwewillbeemployingfortheaminationoftheSiO surface)canalsobenefitfrom
2
thissizecharacteristic,giventhatthedepositionofmoleculesusuallyoccursafteranadsorptionfrom
solutionprocess. Anotherimportantpropertyofamorphoussilicaisthepresenceofsilanol(Si–OH)
speciesthroughoutthesurface. Thesespeciesaredeterminantwhenthesurfaceistobefunctionalized
withagivenmolecule. InthecaseofCO ,thismoleculeisinitselfaweakLewisacid,whiletheSi–OH
2
groupsarelikewise. Then,inorderforthesilicasurfacetocaptureCO ,ithastobefunctionalized
2
withpertinentspecies,e.g.,amino-substitutedalkoxides. Thealkoxidegroupscanbeattachedtothe
surfaceafterreactingwiththesilanolspecies;inturn,theCO moleculesbecomecoordinatedtothe
2
NH groupsoftheamino-functionalizingcompound.
2
2.2. HabilitationoftheSiO SurfacetoAttainanEfficientandSufficientAmine-FunctionalizationProcess
2
Silica by itself manifests no significant affinity toward CO adsorption, for this reason, it is
2
necessary to perform the chemical modification of this surface. This transformation can rely on
the silanol groups, since the character and concentration of these species are crucial for attaining
a widespread amino-functionalization of the SiO surface. Importantly, because of the thermal
2
treatment required to eliminate remnant pore templating species, a relatively small population of
silanolgroups(–SiOH)subsistsinthefinalnetwork. Therefore,ahydroxylationprocessviatreatment
oftheSiO substratewithalowlyconcentratedNaOHsolution,setattherighttemperature,hasbeen
2
foundtobeasuitableoperationforachievinganefficienthydroxylationofthesurface. Thereasonof
thelowNaOHconcentrationtobeusedinthisprocessresidesinpreventinganexcessivedesilication
(i.e.,dissolutionandleaching)oftheSiO matrixwhenputincontactwithanalkalinesolution[29].
2
Materials2016,9,898 4of24
Nonetheless,thepreviouschemicalprocedurecanstillresultuseful,notonlyforattainingmoreSiOH
speciesonthesurface,buttoeliminatethemicroporespresentintheparentsilicamaterial.
2.3. SupplementaryAspectsinRelationtotheChoiceofSantaBarbaraAmorphousSilicaNo. 15(SBA-15)for
EnhancedCO Trapping
2
OtheraspectsrelatedtotheconvenienceofchoosingSiO assubstrateforCO adsorptionare
2 2
thepossibilityofreadilyregeneratingtheadsorbentsurfacebymeansofamildthermaltreatmentin
anoven,thusreducingthepowerrequirementsnecessarytoperformthesurfaceregenerationprocess.
Anotherstrikingcharacteristicistheselectivitythatthesilicasurfaceshowstowardtheadsorptionof
CO insteadofadsorptivessuchasCH .
2 4
Furthermore,thedepositionofafunctionalizingmoleculeonthesilicasurfacecanbemadefrom
eitheranorganicorinorganicsolutionofthespeciesinquestion. Inthecaseofamine-functionalization,
itisbettertoperformthisprocessemployinganappropriateorganicliquidfordissolvingtheamino
alkoxidecompoundchosenforfunctionalizingthesurface,e.g.,tolueneinsteadofethanol. Thereason
ofthisisthechemicalcharacterofthesilanolgroups,whichcaninteractsynergisticallyornotwiththe
solventmoleculeswithrespecttoaneffectivedepositionofthefunctionalizingcompound[30].
TheobjectiveofthisworkconsistsinshowingevidenceofamoreabundantandefficientCO
2
captureonthebasisofsomeappropriatetexturalchangesthatcanbeinflictedonmodelSBA-15silica
adsorbentsviathermalandchemicalsurfacefunctionalizationprocedures. Aconcomitantaimisto
increasethenumberofsilanolgroups(i.e.,–Si–OH)onthesilicasurface,sincethisisakeyaspectfor
achievinganefficientfunctionalizationofthesurfaceviachemicalanchoragemethods. Theultimate
goalistoaccomplishanabundantcaptureofmiscellaneousadsorptives(otherthanCO )fortheir
2
storageandeventualtransformationintoothermoleculesofdiversepracticalinterest.
Thisworkisorganizedasfollows. First,theexperimentalsectiondetailingthesynthesisofthe
silicasubstratesispresented. Next,thethermo-alkalinehabilitationofthesurfaceofthesesolidsis
described. Thefollowingstepconsistsincarryingoutthefunctionalizationofthepreviousmaterials
withanespeciallyselectedamino-alkoxidecompound.Subsequently,N sorptionisothermsat76Kare
2
showntogetherwithalistofthemostimportantstructuralparametersoftheporoussolidsinvolved
inthisinvestigation. CO adsorptionisotherms,thecenterofthisresearch,intherangeof263–313K
2
onthediverseSBA-15materialsarepresented. Additionally,FourierTransformInfrared(FTIR),X-ray
Diffraction (XRD), Transmission Electron Microscopy (TEM), Thermogravimetric Analysis (TGA),
solid-state Nuclear Magnetic Resonance (NMR), and Temperature Programed Desorption (TPD)
characterizationexperimentsaredepicted. Finally,CH adsorptionisothermsat298Konsomeofthe
4
SBA-15adsorbentsarepresented,inordertoascertaintheselectivityofthesesubstratesbetweenthis
lastadsorptiveandCO .
2
3. MaterialsandMethods
3.1. Materials
Tetraethylorthosilicate,TEOS(Aldrich,SaintLouis,MO,USA,98%),hydrochloricacid(J.T.Baker,
Chemicals,CenterValley,PA,USA,36.5%–38.0%w/w),poly(ethyleneglycol)Aldrich(formolecular
biology),extrapuresodiumhydroxidepellets(Riedel-deHaën,LowerSaxony,Germany,99%–100%),
3-Aminopropyltriethoxysilane(APTES,ofSigma-Aldrich,99%),toluene(Merck,Kenilworth,NJ,USA)
anddenaturedethylalcohol95%v/v,Pluronic123(BASF,FlorhamPark,NJ,USA)wereused.
3.2. SynthesisofPrecursorSBA-15Solids
The following molar ratio sequence was employed to prepare SBA-15
silica:TEOS:197H O:6.2HCl:0.017P123, the synthesis was performed according to the method
2
devised by Beck et al. [24] and Zhao et al. [31]. The ensuing material was washed, filtered, put in
Materials2016,9,898 5of24
contactwithethylalcoholunderstirring,anddriedfor14hat373K.Finally,thesolidwascalcinedat
723Kfor6h,whileemployingaheatingrampof1K·min−1.
3.3. SoftThermo-AlkalineTreatmentPerformedoverSBA-15PrecursorMaterials
ThemesoporoussilicaadsorbentsoftheSBA-15(S15denoted)kindweresubjectedtoathermo-
(313K)alkalinetreatmentthroughtheemploymentofdiluted(<1M)NaOHsolutions. Specifically,
a 250 mg sample of each S15 substrate was subjected to a series of treatments under the previous
conditions,eachoftheseoperationstakingplaceeverytwohours. Subsequently,adesilication(i.e.,
hydroxylation)processwasperformedbyaddinga0.025MNaOHsolutionat313K(2h).Theresultant
solidwaswashedwithdeionizedwater,filtered,anddriedat373Kfor2h. Theensuingsubstratewas
labeledasS15H.
3.4. ChemicalAnchoringof3-Aminopropyltriethoxysilane(APTES)ontheSurfaceofSiO MesoporousSolids
2
ThechemicalbondingofAPTESonthesurfacesoftheS15andS15Hsilicasubstrateswasachieved
oncethesesolidswerecalcinedinairat723Kfor2h. Theadvantageofthisrouteistoguaranteethe
totalremovalofthesurfactantmoleculesafterthesubstratehasbeenannealedat723K.
BeforeAPTESsurfacedeposition,theprecursorSiO solidsweretreatedat473Kundervacuum
2
for2hinordertoremoveorganicimpuritiesandalsotopromotetheactivationofthesilanol(SiOH)
groupsalreadyresidingonthesurface. Themolarratioinvolvedinthisfunctionalizationreaction
correspondedto3SiO :2APTESandthisprocedurewascarriedoutat353KunderaN atmosphere
2 2
and continuous magnetic stirring of a reflux system during 20 h, while employing toluene as the
APTESdispersionmedium. Thefinalsolidswereseparatedbyfiltrationanddriedat373Kfor2h.
ThesematerialswerelabeledasS15NH andS15HNH ,giventhattheseaminatedmaterialsproceed
2 2
fromtherespectiveS15andS15Hsamples.
3.5. CO SorptionExperimentsatDifferentTemperatures
2
PriortoCO sorptionexperiments,eachsamplewasoutgassedat373Kunderhighvacuumfor6h.
2
Theadsorptiontemperaturerangeutilizedfortheseexperimentswaschosenbetween273and298K
instepintervalsof10K.Finally,theadsorptioncapacityoftheamine-functionalizedSiO substrates
2
wasdeterminedexperimentally,inanautomaticadsorptioninstrument(QuantachromeAutosorb1LC,
QuantachromeInstruments,BoyntonBeach,FL,USA).Additionaltexturalandthermal(isostericheat
ofadsorption)propertieswere,calculatedfromthecorrespondingCO adsorptionisothermsevaluated
2
atthedifferenttemperatures. CH adsorptionat273K,onsomeofthesilicasamples,wasalsocarried
4
outwiththeaimofvisualizingtheselectivityofthemodifiedSBA-15adsorbentstowardsCO .
2
4. CharacterizationTechniques
- FTIR:Thistechniquewillbehelpfulforrealizingthepresenceofsilanolandsiloxanespecieson
thesurfaceoftheadsorbents. FTIRsignalswereobtainedfromaPerkinElmerParagon1000
(FT-IR)instrumentequippedwithanAttenuatedTotalReflectance(ATR)tool.
- TGA: This analysis was performed on a Perkin Elmer Diamond instrument on a N flow of
2
50mL·min−1,subjectedtoattemperaturerampof5◦C·min−1alongatemperatureintervalof
323–823K.
- Raman: Thesespectraareusefulforprovidinganoverallviewofthestructuralconsolidation
oftheadsorbentspreparedinthiswork; inadditiontothestructureofthesubstrateitself, in
particular,ofsilicabondsaswellasfreeorvicinalsilanolgroupsthatlieonthesilicasurface.
Throughthistechniqueitcouldbepossible,inprinciple,torealizethearrangementofadsorbate
moleculesthroughouttheadsorbentsurface. TheinstrumentemployedwasaRamanMicroscope
(ThermoFisherScientific,Waltham,MA,USA)witha636nmlaserdeliveringamaximumpower
of10mW.
Materials2016,9,898 6of24
- TEM:ImageswereobtainedfromaHRTEMJeol2100Fmicroscope(JEOLLtd.,Tokyo,Japan)
operatingatanaccelerationvoltageof200keV.Thisinstrumentwasemployedtoobservethe
structuralarrangementofthesamplesandtorealizeifthepreparationprocedurehadaffected(or
not)theshapeandstructureoftheSiO substrates.
2
- XRD: It is not only useful for checking the ordered hexagonal arrangement of the nanotubes
making the SBA-15 hexagonal arrangement but also for determining the pore width and
the thickness of their pore walls. The XRD parameters of the network were measured from
aBrukerD8Advanceinstrument(BrukerAXS,Madison,WI,USA)employingaCu-Kαradiation
wavelengthof1.54Åinthelowregion(0.6◦ to5.0◦ inthe2θscale).
- N Sorption: ThetexturalpropertiesofthesilicaadsorbentsweredeterminedfromanASAP
2
2020automaticinstrument(MicromeriticsInstrumentCorp.,Norcross,GA,USA),employingN
2
atitsboilingpoint(76KatMexicoCity’saltitude). Thematerialswerepreviouslyoutgassedat
373Kfor12h. underaturbomolecularvacuumof10−6mbar. Thesurfaceareawasdetermined
from either the BET or t-methods, while the pore size distribution was obtained through the
Non-LocalizedFunctionalTheory(NLDFT)approach. Thekernelemployedforthiscalculation
wasthatcorrespondingtothefillingofcylindricalporesalongtheboundaryN adsorptioncurve
2
at77K.
- NH TPD:Thetotalacidity(Brönsted+Lewis)wasqualitativelyrealizedfromaMicromeritics
3
TPD/TPR2900NH programmeddesorption(TPD)device(MicromeriticsInstrumentCorp.)
3
providedwithaTCDdetector. Theexperimentalprocedureconsistedinoutgassingthesample
underamixtureofN andairflowingatindividualratesof60cm3·min−1beforebeingmixed
2
fromroomtemperatureupto773K;thesystemwasthenkeptatthistemperaturefor15min.
ThiswasfollowedbytheconcurrenceofN andNH flowsof60cm3·min−1each,uptoafinal
2 3
temperatureof1073K.Subsequently,thesystemwaslefttocooldownto303Kforaboutone
hourunderthesameNH andN flows. Subsequently, anadditionalcleaningofthesurface
3 2
at303KwasperformedwithaHeflowof60cm3·min−1 duringonehour. ThedefinitiveTPD
experimentwascarriedoutunderaN flowof60cm3·min−1usedasacarriergasandfollowing
2
toaheatingrampof10◦C·min−1upto1073K;whileregisteringtheamountofNH desorbedat
3
eachtemperature.
- SolidNMR:SolidStateNuclearMagneticResonanceexperimentswererunonaBrukerAdvance
II300spectrometer(BrukerBioSpin,TheWoodlands,TX,USA),operatingat59.62MHzfor29Si.
ThemagicanglespinningtechniquewasaltogetheremployedwiththeNMRresultsasareference
oftetramethylsilane(TMS).Thistechniqueisveryimportantinordertoascertainthenumber
ofbondsestablishedbetweenthesurfacesilanolgroupsandtheethoxygroupsoftheAPTES
functionalizingspecies.
- CO andCH adsorptionandselectivity: Finally,CO andCH adsorptionstudiesatdifferent
2 4 2 4
temperatureswereperformedonaQuantachromeAutosorb-1LCinstrument;forthistask,the
sampleswerepreviouslyoutgassedat373Kfor6h. CO adsorptionisothermswereobtained
2
from263to303KinordertoobtaintheCO enthalpyofadsorptionatpressuresfrom0.001to1bar.
2
5. ResultsandDiscussion
5.1. Middle-InfraredSpectroscopyStudies
TheFTIRspectrainFigure1evidencevibrationalchangesofthemolecules;therein,itispossibleto
noteaprominentsignalwideningcorrespondingtophysisorbedwaterintheS15Hsamplessubjectedto
thehydroxylationtreatment,ifcomparedtothespectrumoftheS15precursorspecimen.Thisdifference
surgesevenifallsampleswerepreviouslyannealedundervacuumat473Kfor2h. Ontheother
hand,theintramolecularSi–OH(963cm−1)andSi–O–Siflexion(802cm−1)signalsappearoverlapped
formingabroadbandintheS15Hspectrum. However,forhydroxylatedsamples,thebandassigned
to free silanol groups (Si–OH) appearing at 3769 cm−1 [32,33] results masked by the widening of
Materials2016,9,898 7of24
the vibrational band of physisorbed water. Nevertheless, a larger amount of free SiOH groups
sitesinteractwithwaterthroughhydrogenbondsandpromotesarapidlyhydration,whichcanbe
reflectedinalargeramountofphysisorbedwater,againwithrespecttothesamequantityintheS15
precursorsample.
Materials 2016, 9, 898 7 of 24
Materials 2016, 9, 898 7 of 24
(a) S15
HO abs
(b) S15H 2
(a) S15 Si-O-Si Fexion
HO abs
(b) S15H 2
Si-O-Si Fexion
T
%
% T (a)
Free Si-OH
(a)
Free Si-OH (b)
H2O ads (b) Si-OH
HO ads
2 Si-OH
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 cm-21000Si-O1-S5i0 s0tretch1i0n0g0 500
Figure 1. Fourier Transform Infrared (FTIR) spectrcam o-1f: (aS)i -OS-aSni tsatr eBtcahribngara Amorp hous (SBA)-15
Figure 1. Fourier Transform Infrared (FTIR) spectra of: (a) Santa Barbara Amorphous (SBA)-15
preFpcirugerucsuroerr so1s.ir l isFciolaiuc(arSi 1(eSr51 )T;5)ar;an andnsdf(ob (r)bmS) BSIBAnAf-r1-a15r5es dsaam m(FppTlleIeR aa)ff ttseeprre bbceetirinna ggo ttfrr:ee aa(atte)e ddS wawniittthah 0B0.0.a02r25b5 aMrMa N ANaOmaOHorH aptha aot utaesmt e(pmSeBprAaet)ru-a1rt5eu re
ofp3o1rf e33c1Ku3r (sKSo 1r(S 5s1Hil5i)cH.a) (. S15); and (b) SBA-15 sample after being treated with 0.025 M NaOH at a temperature
of 313 K (S15H).
The samples under scrutiny were now functionalized through a chemical bonding method, i.e.,
a pTrhoTechesseas sm eammplppellsoeyus inundngde ArePrs cTsrcEurSut itaninnydyw twheearrtee w nnaoosww a lffruuennadccttyiio odnneaasllciizrzieebdde dtth hirnro outhugegh he axa pchecrheimemmiecniactla abll obsneodcntiidnoginn .mg Inem tthheotishd oc, aid.se,e.i,, . e.,
apaar opccoreovscaselseesnm te mpbloponlyodiyn iisgn geAs AtPaPTbTEliEsShSae andnd dbte htthwaatetw ewnaa stsh aaell rrseeuaarddfayyc dde eeossfcc rtrhiibbee edsdu ibinns tthtrhaete eee xaxpnpedre irmtihmeen ebtnratilda slgesicnetgicot naiom. nIin.n Itonh sitsph ceiascsiceeas, se,
acao(i v.ceao.,lv eAanlPteTnbEto Snb)od (nFidsig eiuss rteaes b2tla)i.bs Ihlnies hdthebidse FtbwTetIewRe neseptnhe cetthrseuu msrfua, ricftea icsoe fp otohfs esthisbeul esb utsotbr snatotreatetae sn iadlinctdah evthibberr ibadrtgiiodinngaginl agbm aanimndoisn asops sewpceeielclsi ea(ssi . e.,
AP(tTiy.Eep.Si,c )Aa(lPF TipgEruiSmr)e a(F2r)yig. uIanrmet hi2ni)se. IFnbT atIhnRidsss pF Te(–cINRtr uHspm2)e,,c itatrsuis mipn,o istt hsiiseb plceoastsoseib noloeft teoAs nPiloTictEaeS vs iil[bi3cr4aa, 3tvi5oib]n.r aaNltibooatnincaedl sbtaahnsadtw st eahlsle awbseatlynl pdasisc al
pritcmyoprarirceyaslpa mopnridinmienabgra ytn oda Sms1(5i–nHNe, Hsbua2bn),sdatsrs ai(tn–esNt hpHer2e)cv,a isoaesu osilfnyA stPuhTbe jEecScatse[e3d 4 ot,3of5 a]A. tPNreToaEttSmic ee[3nt4ht ,aw3t5it]th.h e N0b.o0at2ni5cde Ms tc hNoaratr OetshHpe o anbrade nitndhgse to
S15cmoHor,rsest suipbnostnetrndasitnee gso tfpo ra eSllv1, 5ioeHsup,s esluycibasslultyrba jeitfec stt ehpdersetevo isoaiugtsnrelayal sts muabreejne tcctowemdit phtoa0 rae.0 dt2r e5toaMt mthNeonsateO woHibtthaa ri0en.0et2dh5 e iMnm toNhseta OipnHatre enanrstee Stoh1fe5a ll,
espmsepoceiscati lmliynetinef ntthsheae tso eifs s saiugllbn, jaeelscspteaedrce itaocl lonymo i pfa latkhraeeldsinet eos eitgxhnpoaoslsesu oarebr.et a cinoemdpianretdh etop atrheonsteS o1b5tsapineecdim inen ththea ptaisresnutb Sje1c5t ed
tosnpoeacilmkaelnin tehaetx ipso ssuubrjee.cted to no alkaline exposure.
Si-O-Si Flexion
(820 cm-1)
(b) Si-O-Si Flexion
(820 cm-1)
(b) (a)
(a)
Free SiOH
% T F3r7e4e0 ScimO-H1 -NH+
% T 3740 cmN -- 1H 3 s1t0re8t cchmin-1g N-H Flexion ( 1 5-N10H 3c3+m-1)
N - H 3 s1t0re8t cchmin-1g N((11-55H88 5F5 lcecmmxio--11)n) C ( 1- N 511 3s05tr 0ce mtccm-h1)i-n1g
C-N stretching
1350 cm-1
Si-CH-R
(a) S15NH2 (12002 cm-1) Si-O-Si stretching
((ab)) SS1155NHHN2H2 S(1i-2C0H02 -cRm-1) S (i-1O0-6S8i csmtre-1t)ching
4000 3(5b0)0 S15H30N0H02 2500 2000 1500 10 0 (01068 cm50-10)
4000 3500 3000 2500cm-12000 1500 1000 500
Figure 2. FTIR spectra of SBA-15 materials: (a) Sc1m5 -1precursor; and (b) S15H SBA-15 + Na OH 0.025 M
FigFsuaigrmeupr2ele. 2s,. FFTTbIIoRRt hssp peecccthrtaara roafco tSfeBrSAizB-eA1d5- 1m5aaftmteerari atelsrf:i ua(anls)c :tSi1o(5na )aplriSez1ca5utiropsnor re;c auwnridstoh (r b;) taShn1e5d H( sbSaB)mASe-1 155H a+m NSoBauOAnH-t1 50.o+0f2 N5 aM3O- H
saamminpolepsr, opyblottrhie thochxayrsailcatnerei z(AedP TEaSf)t.e r functionalization with the same amount of 3-
0.025 M samples, both characterized after functionalization with the same amount of
aminopropyltriethoxysilane (APTES).
3-aminopropyltriethoxysilane(APTES).
5.2. Raman Spectra of Precursor and Thermo Alkaline Treated Silica Samples
5.2. Raman Spectra of Precursor and Thermo Alkaline Treated Silica Samples
Raman spectra are useful in order to provide additional structural characterization, besides of
that Rcoarmreasnp ospnedcintrga taor et hues esfuubl sitnr aotred ietrs etlof , pirno vpiadreti caudldairt itoon eavl isdtreunccteu rthale cphraersaecntecrei zoaft ifornee, boers ivdiecsin oafl
tshilaatn cool rgrreosuppons dthinagt atroe tlhyien sgu obns ttrhaete s iiltisceal fs,u irnf apcae.r ticular to evidence the presence of free or vicinal
silanol groups that are lying on the silica surface.
Materials2016,9,898 8of24
5.2. RamanSpectraofPrecursorandThermoAlkalineTreatedSilicaSamples
Ramanspectraareusefulinordertoprovideadditionalstructuralcharacterization,besidesof
thatcorrespondingtothesubstrateitself,inparticulartoevidencethepresenceoffreeorvicinalsilanol
Materials 2016, 9, 898 8 of 24
groupsthatarelyingonthesilicasurface.
TThhee ttrraaddiittiioonnaall RRaammaann ssppeeccttrraa ooff vviittrreeoouuss SSiiOO22 aarree cchhaarraacctteerriizzeedd bbyy bbaannddss aappppeeaarriinngg aatt aarroouunndd
441100,, 880000,, 11006655, ,aanndd 11220000 ccmm−1− a1nadn darae raesasosscoiactieadte tdo tvoibvriabtriaotnios nosf othfet haemaomrpohropuhso usislicsail inceatnweotrwko [r3k6][.3 I6n] .
oInuro cuarsec,a hsoe,wheovwere, vtheer, btahnedbsa tnhdats etxhiastt aetx aisrtouatndar 4o1u0n, d50401, 0a,n5d0 601,0a cnmd−16 c1a0nc bme− li1nkcaend tboe thlien kveibdratotiotnhse
ovfi bsrialotixoannseo gfrsoiluopxsa n(Sei–gOro–uSpi)s, (wSih–iOch– Sair),ew ashsiicghnaerde taos sriignngesd otfo driivnegrsseo fsdizievse r[s3e7]s.i zTehse[ 3si7g].nTalh aets aigronualnadt
4a1ro0 ucnmd−14 i1s 0ascsmo−ci1atiesda stsoo tchiea tbeedntdointhge mboendde ivnigbrmatoiodne ovfi borxaytigoenn oaftoomxysg iennsiadteo am rsinings midaedae roinf mgmoraed theaonf
fmouorr esitlhicaonn faotuormssil i(cno >n 4a)t;o tmhiss (pne>ak4 )i;s tuhsisuaplelya kkinsouwsnu aalsly thken oRw bnanads.t hTeheR sbigannadl. aTt haerosuignnda l49a0t acrmou−1n ids
a4s9s0occimat−e1d itso atshseo cbiraetaetdhitnogt hmeobdree avtihbirnagtiomn oodf erivnigbsr adteiopnictoifngri nfogusrd seiplicicotnin agtofomusr; tshiliisc osnignataol mussu;athlliys
bsieginnga lkunsouwalnly abs etihneg Dkn1 obwannda; sththe epDea1kb aapndpe;athrienpge aatk aarpopuenadr i6n0g5 actmar−1o uisn adss6o0c5iactmed− 1toi sthases borceiaattehdintgo
mthoedber evaibthraintigonm oofd reinvgibs rfaotriomnedof bryin tghsrefoe rsmiliecdonb yattohmrese, sainlidc obnraantodmeds ,aasn Dd2 bbraannddse [d38a]s. DA2dbdaitniodnsa[l3l8y], .
tAhde dpietiaokn aatl layr,othuendp e9a8k0 actma−r1o ius nadss9o8c0iactemd− w1iitsha tshseo cviiabtreadtiwonitahl mthoedvei borfa (tOioHna)-lgmrooudpes owfi(tOh Hre)s-gpreocut ptos
Swi.i threspecttoSi.
IInn tthhee RRaammaann ssppeeccttrraa ooff tthhee pprriissttiinnee SS1155 ssaammppllee,, iitt ccaann bbee oobbsseerrvveedd sshhaallllooww RR,, DD11,, aanndd DD22 bbaannddss
wwhhiicchh aarree aassssoocciiaatteedd ttoor rininggsso offd diviveresreses isziezsese xeixsitsintigngin inth tehsei lsicilaicnae ntwetowrkor(kF i(gFuirgeu3re). 3A).d Adidtidointiaolnlya,ltlhye,
tthweo tbwaon dbsanapdps eaaprpinegaraitnagr oaut nadro8u5n0da n8d501 0a6n0dc 1m0−601 ccamn−1b ecalnin kbeed li[n3k9e]dto [t3h9e] etom tihssei oenmsilsoscioatnesd laotcaarteodu nadt
a8r5o0u,n90d0 8,5905,0 9–0100,0 905,0a–n1d00100,5 a0n–d1 1100050c–m1−1010a cnmd−t1h aantdc athnabt ecadnu beet oduteet rtoah teetdrraahlesdilriaclo snilaictoonm asthomavsi nhgavfoinugr,
ftohurer,e ,thtwreoe,o trwoon loyr oonnelyb roindeg ibnrgidogxiyngge onxaytgoemns a,troemspse, crteivspeleyct[i4v0e]l.yN [4o0n]-.b Nriodng-ibnrgidogxiyngge onxaytgoemns actooumlds
ccoourrleds cpoornrdestpoohnydd troo hxyyldgroroxuypl sgroorutposd oifrf etore dnitffmereetnatls m,seutachls,a ssupcohl yassi lpicoalytesialilckaatlein aelkcaaltiinoen sca[t3i9o]n.sI n[3t9h]e.
IRna tmhea nRsapmecatnr usmpeocftrtuhmea olkf atlhine eatlrkeaaltiende straematpelde (sSa1m5Hpl)e, a(Sn1i5nHte)n, saenD in1tbeannsde aDp1p beaanrsda atpaproeuarnsd a4t9 a0rocmun−d1 .
4B9e0s icdmes−,1.a Bneisnitdeenss,e aann dinbternoased abnadn dbaropapde abrasnfrdo map6p0e0artso f1r1o0m0 c6m00− 1t.oT 1h1i0s0s icgmna−1l. pTahthisw saigynsaulg pgaetshtswtahye
seuxgisgteesntcse tohfea esxliisgthetnlyceh oigfh ap solpiguhltaltyio hnigohf spiloicpaurliantgiosnf oorfm seildicbay rfinougsr sfiolircmoneda tboym fso.uFru rstihliecromn oarteo,mthse.
Finutretnhseermbaonred, tlhocea itnetdenasteh biagnhder lofrceaqteude natc iheisghsuegr gfreesqtusethnecipesr essuegngceestosf taheh pigrheesernacme oouf an thoigfhteetrr aamheodurnatl
osifl itceotrnahaetodmrasl shialivcoinng antoombsr ihdagviinngg onxoy bgreindsgiansgc ooxnysgeeqnuse nasc ecoonfstehqeueanlkcael ionfe thtree aaltkmaelinnte ttorewathmicehntt htoe
wsahmicphl etshew searmespulebsje wcteerde. subjected.
(a) S15 VSi-O Elongation
(b) S15H
(b)
a)
u.
sity ( Si-O-Si Bending
n
nte Si-O-Si Streching
I
(a)
2000 1800 1600 1400 1200 1000 800 600 400
cm-1
Figure 3. Raman spectra of SBA-15 materials: (a) precursor SBA-15 substrate (S15); and (b) silica after
Figure3.RamanspectraofSBA-15materials:(a)precursorSBA-15substrate(S15);and(b)silicaafter
being treated with NaOH 0.025 M (S15H).
beingtreatedwithNaOH0.025M(S15H).
5.3. Thermogravimetric Analysis (TGA) Analysis of Precursor and Functionalized Silica Samples
5.3. ThermogravimetricAnalysis(TGA)AnalysisofPrecursorandFunctionalizedSilicaSamples
The S15 precursor simple was subjected to TGA analysis (Figure 4). The weight loss of 6.0 wt %
TheS15precursorsimplewassubjectedtoTGAanalysis(Figure4). Theweightlossof6.0wt%
observed at around 373 K could be due to water desorption. Afterwards, a slightly decreasing slope
observedataround373Kcouldbeduetowaterdesorption. Afterwards,aslightlydecreasingslope
is developed as the temperature is raised and in such a way that when the analysis is fulfilled at 823
isdevelopedasthetemperatureisraisedandinsuchawaythatwhentheanalysisisfulfilledat823
K an additional mass loss of 2.0 wt % is detected. A slight mass loss of about 2.0 wt % is possibly due
to the partial dehydroxylation of the SiO2 matrix, or in other words, the Si–OH groups condense to
generate water molecules and Si–O–Si siloxane species, consequently the total mass loss is about 8.0
wt %. Nonetheless, for the S15H sample subjected to hydroxylation, a mass loss of 11.0 wt % arises
thus confirming that this sample was more hydrated than the precursor solid. This, in principle, may
be due to the fact that the hydroxylation treatment causes the rupture of siloxane groups to generate
vicinal Si–OH groups, which can be easily rehydrated and, as the temperature is raised during the
Materials2016,9,898 9of24
Kanadditionalmasslossof2.0wt%isdetected. Aslightmasslossofabout2.0wt%ispossibly
duetothepartialdehydroxylationoftheSiO matrix,orinotherwords,theSi–OHgroupscondense
2
togeneratewatermoleculesandSi–O–Sisiloxanespecies,consequentlythetotalmasslossisabout
8.0wt%. Nonetheless,fortheS15Hsamplesubjectedtohydroxylation,amasslossof11.0wt%arises
thusconfirmingthatthissamplewasmorehydratedthantheprecursorsolid. This,inprinciple,may
beduetothefactthatthehydroxylationtreatmentcausestheruptureofsiloxanegroupstogenerate
vicinalSMia–teOriaHls 20g1r6o, 9u, 8p98s , whichcanbeeasilyrehydratedand,asthetemperatureisraised9 odf u24r ingthe
TGAanalysis,becomedehydratedpromoting,atthesametime,thecondensationofthem,since,asis
TGA analysis, become dehydrated promoting, at the same time, the condensation of them, since, as
wellknoMisw awtenreia,lllss k2iln0i1oc6wa, 9n,h 8,y 9s8di l ircaat hioynd-rcaotinond-econnsdaetinosnatiporno pcreoscseessseasr aerer erevveerrssiibbllee pphheennoommenean [a41[]4. 1]. 9 of 24
TGA analysis, become dehyd10r0ated promoting, at the same time, the condensation of them, since, as
is well known, silica hydration-condensation processes are reversible S p1h5enomena [41].
S15H
96
100
S15
ht S15H
eig 9962
w
al
u
eightResid 9828
al w%
du 84
si 88
e
R
%
80
84300 400 500 600 700 800
Temperature (K)
Figure4F.igTuhree 4rm. Tohgerrmavogimraevtimriectra8i0nc3 0aa0lnyaslyissisu 4un00nddeerr aa N5N0022 aatmtmoso6ps0h0pehree roef t7oh0f0e tSh1e5 Sp18a05r0enpta sraemnptlsea amndp loef athned ofthe
S15H solid proceeding from the corresponding alkaline hydroxylation of the previous substrate.
S15HsolidproceedingfromthecorrespondingaTlekmapleirnateureh (yKd) roxylationofth eprevioussubstrate.
FTihgeu rTeG 4A. T shteurdmyo cgorarvreimspeotrnicd iannga ltyos itsh uen sdaemr pa lNes2 fautmncotsiopnhearliez oefd twhei tSh1 5A pPaTrEenSt asraem sphloe wannd i no fF tihgeu re 5;
ThneaTmGeSlA1y5,H sft osuro dltidhy ep crSoo1cr5erNeedsHipn2go afnnroddmi nS t1gh5eHt cooNrtrHhese2p sosunabdmsintprgal ateelsksa.f luTinhnee ch tySid1or5noHxayNlilazHtei2od nh ywodf itrthohex pyArleaPvtTeiodEu sSs asamubrpestlresa htseou. fwfenrs intheF igure5;
highest mass loss of all. The first TGA region is linked to desorption of the physisorbed water at
namely,fortheS15NH andS15HNH substrates. TheS15HNH hydroxylatedsamplesuffersthe
highestamrouansTsdh el3 oT7sG3s AKo s;f tui.aedl2.y,l . c3o.T0r rhewestp ofi%nr dsfitonrTg Gttho2Ae t hSer1 es5agNmiHopn2l essip sfeuclniimnctkieonend,a laitnzoedd d 5we.0is to2hw rAtp P%tiTo EfnSo rao rtfeh esthh Soe1w5pnHh iNny HsFii2sg ousorrebli de5;d. water
nNaemveerlyth, efloers st,h teh eS 1s5igNnHif2i caanndt mS1a5sHs NloHss2 dseupbiscttreadt efsr.o Tmh e5 4S31 5KH oNnHw2a rhdysd, rios xcyaluasteedd bsaym ApPleT EsuS ftfheresr mthael
at arouhndiedgghr3ea7sdt3a mtiKoans;s. i S.lepo.se,sc i3ofi.fc0 aallwlly. ,tT thh%ee Sffi1or5srNt tTHhG2e AmS ar1tee5grNiioanHl pi2sre lssipennketecsdi am tmoe ands,ess aloonrspdst oio5fn .10 4o.f1w twhtet% %ph wfyohsriisleoth rtbheeed SS 11w55aHHteNNr HaHt2 2 solid.
Neverthasoreollieudsn csdo, rt3rh7es3ep sKoin;g dni.sei tfi.,o c 3aa. 0nm twamsts a%los sssfo lorof s t1sh7e.d5 e Sw1p5ti Nc%tHe. Td2 hsfiprs oeicmmimp5lein4es,3 aaK nddiof fn5e.rw0e nawcretd bs%e, tiwfsoerce anthu tesh eeSds1e5 bsHpyNeAcHimP2 eTsnoEsli Sdof.t hermal
degradaN1t9ie.o4v newr.tthS %epl,ee swcsi,hfi ticchhae l eslyvig,idnthiefnieccaSens1 t5t hmNaatH sas mlomassjao dtre ewrpieiacilgtehpdtr felorsosemsn o t5cs4cu3a rKms foaonsr wsthlaoer dshssy, odisrf oc1xa4yu.l1saetwded tb ay%n Adw PsTuhEbilSsee tqthuheeernmStla1yl5 HNH
2 2
solidcodArerPgeTrsEapdSo afntuiodnncst. itSoopneaacliimzfiecadas lslsyim,l otphslese .S o1f5N17H.52 mwatte%ri.alT phreisseinmtsp al imesasas ldoisfsf eorf e1n4.c1e wbt e%tw weheilne tthhee Ss1e5sHpNeHci2m ensof
solid corresponds to a mass loss of 17.5 wt %. This implies a difference between these specimens of
19.4wt%,whichevidencesthatamajorweightlossoccursforthehydroxylatedandsubsequently
19.4 wt %, which evidences1 0t0hat a major weight loss occurs for the hydroxylated and subsequently
APTESfunctionalizedsimple. S15NH2
APTES functionalized simple.
S15HNH2
96
100
S15NH2
ht S15HNH2
g
ei 92
w 96
al
u
ghtesid 88
eiR 92
al w%
u
esid 8884
R
%
80
84300 400 500 600 700 800
Temperature (K)
Figure 5. Thermogravimetr8ic0 analysis, under N2 flow, of the S15NH2 and S15HNH2 APTES-functionalized
300 400 500 600 700 800
silica specimens.
Temperature (K)
Fig5u.4re. X5F-.irgTauyhr eeD r5mi.f Tfrohagecrrtmaiovonigm (rXaevRtirmDice)ta rAnicna alanylyaslsiysiss,i usa,tn u Ldnoedwrer NA Nn2 gfflllooeww ,, ooff ththe eS1S51N5HN2H anda Sn1d5HS1N5HH2 NAPHTEAS-fPuTnEctSio-nfualnizcetdio nalized
2 2 2
silica specimens.
silicaspTehciem XeRnDs. diffractograms obtained for both precursor and functionalized silica materials are
shown in Figure 6. Therein, it is observed for all cases, the structural parameters of a pore network
5.4. X-ray Diffraction (XRD) Analysis at Low Angle
The XRD diffractograms obtained for both precursor and functionalized silica materials are

shown in Figure 6. Therein, it is observed for all cases, the structural parameters of a pore network
Materials2016,9,898 10of24
5.4. X-rayDiffraction(XRD)AnalysisatLowAngle
TheXRDdiffractogramsobtainedforbothprecursorandfunctionalizedsilicamaterialsareshown
Materials 2016, 9, 898 10 of 24
inFigure6. Therein,itisobservedforallcases,thestructuralparametersofaporenetworkconsisting
coofncsyilsitnindgri coafl cpyolirnedirnicaalh pexoareg oinn aal hpeaxcakginognawl ipthacrkeilnagti vweiltyh srhealartpivpeelyak sshaaprpp epaerainksg aaptp(1e0a0r)in,(g1 1a0t )(,1a0n0d),
((210100)),p alnadn e(s2,0w0)h piclhanaerse, pwrhoiocfho afrteh ipsraososfe ortfi othni.s assertion.
(100) S15
S15H
S15NH2
S15HNH2
Sample 2theta d100 (nm) Vp (cm3/g) Wd (nm)
a) S15 0.97 9.11 0.899 8.64
u. S15H 0.88 10.04 0.818 9.73
y ( S15NH2 0.97 9.11 0.534 7.83
sit S15HNH2 0.97 9.11 0.441 7.77
n
e
nt
I
(110) (200)
0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0
2 Theta
FFiigguurree 66.. LLoowwa annggleleX -Xra-ryaDy iDffrifafcrtaioctnio(nX R(XDR)dDi)f frdaicfftroagcrtaomgrsaomfSs 1o5f, SS1155H, ,SS1155HN, HS1,5aNnHd2S, 1a5nHdN SH15HsoNlidHs2.
2 2
solids.
AscanbeseeninFigure6,thereisnochangeintheinterplanar100distancesdepictedbyboth
As can be seen in Figure 6, there is no change in the interplanar 100 distances depicted by both
S15 precursor and S15NH functionalized samples. However, when comparing the W pore sizes
S15 precursor and S15NH22 functionalized samples. However, when comparing the Wdd pore sizes
obtainedfromEquation(1)[42]fortheS15andS15NH specimens,itisreadilyrealizedanevident
obtained from Equation (1) [42] for the S15 and S15NH22 specimens, it is readily realized an evident
poresizediminutionprovokedbytheAPTESpresenceinsidetheporesoftheSBA-15substrate.
pore size diminution provoked by the APTES presence inside the pores of the SBA-15 substrate.
(cid:32) (cid:33)11//22
 vp 
wd =1.213·d100· 1 +V v+V  (1)
w 1.213d δ p p mic  (1)
d 100 1
 
where V andV arethemesoporeandmicroporevoVlumeVs, respectively, δ isthesilicasubstrate
p mic  p mic 
density,andd isthe100interplanardistanceobtainedfromtheXRDpattern.
100
wherIen Vthp eancads eVomifc tahree stahme pmleessotrpeoartee danwdi tmhiaclrkoaplionree svoolulutimones,,i trecaspnebcetivneoltye,d δa iss litghhe tsiinliccrae assuebisntrtahtee
dinetnerspitlya,n aanrdd ids1t0a0 nisc et;hteh 1is0e0f finectetrcpolnafinramr sdtihstaatntchee ohbytdarionxeydl afrtioomn ttrheea tXmReDn tpiantctreerans.e stheporessizeofthe
precuInrs tohres ocalisde wofi tthhoeu stasmigpnliefisc tarnetaltyedal tweritihn galtkhaelianrer asyolauntdiosnh, aitp ceaonf bcey lninodterdic aal spliogrhets itnhcartecaosne sitnit tuhtee
ivnotiedrpvloalnuamr edoisftathneces;u tbhsitsr aetfefe.cCt ocnotnrafisrtminsg tlyh,atth tohsee hsaymdrpolxeyslfautniocnti otrneaaltimzeednwt iinthcrAeaPsTeEs Sthseu fpfeorreeds spiazret ioafl
tshtreu cptruercaulrdsiosar rrsaoyli,dth wusitchoonuctlu sdiignngiftihcaatntthlye Sa1lt5eHriNngH 2thsea maprrlaeya cacnepdt ashhaipghe eostf acmyoliunndtriocfacl hpemoriesso rtbheadt
AcoPnTsEtiStuitne cvoomidp avroislounmteo othf ethS1e5 sNuHbs2trsaatme.p Cleo,netvreanstiifntghley,t wthoossaem spamlesplaerse fpuuntcitniocnoanltiazcetdw witihtht hAePsaTmESe
s3uSfiOfe2r:e2dA PpTarEtSiaml sotlraurcrtautrioa.l Tdhisisarsruagyg, etshtsusth caotnthcleuhdyindgro txhyalat titohne pSr1o5cHesNsHfa2v osaremdpalela argcceegpetn ae rahtiigohneostf
afrmeeoSuin–tO oHf cshiteems.isItoirsbtehdu AsPadTeEqSu iant ecotomsptraersissotnh atto tthhee aSb1u5nNdHan2 cseamanpdled, iesvtreinb uift itohne otwfOo Hsamgrpoluepss aoren pthuet
isnu rcfoancetaocft swiliictha athdes osrabmenet s3SdieOte2:r2mAiPnTeEthSe mococluarr rreantcioe.o Tfhsuisb ssueqgugeensttss uthrafat ctehree ahcytdiornosx.ylation process
favored a large generation of free Si–OH sites. It is thus adequate to stress that the abundance and
5.5. TransmissionElectronMicroscopy(TEM)
distribution of OH groups on the surface of silica adsorbents determine the occurrence of subsequent
surfaTceh ereTaEctMionmsi.c rographsshowninFigure7corroboratethattheS15precursorspecimenconsistsof
anorderedarrayofcylindricalcapillariesdisposedinahexagonalpacking,somethingthatisevident
5in.5t.h Tersaenismmaisgseiosn. Electron Microscopy (TEM)
The TEM micrographs shown in Figure 7 corroborate that the S15 precursor specimen consists
of an ordered array of cylindrical capillaries disposed in a hexagonal packing, something that is
evident in these images.