Việc sử dụng trấu silica như một chất xúc tác. Đây là nghiên cứu mới của ông Đinh Tấn Thành, Công ty TNHH Cao su Kỹ thuật Tiến Bộ, cùng nhóm cộng sự sản xuất silica gel khí từ vỏ trấu, góp phần làm giảm ô nhiễm môi trường, tiết kiệm chi phí. Quy trình chế tạo silica aerogel được thực hiện theo các bước: Vỏ trấu sau khi được rửa sạch và sấy khô để loại bỏ tạp chất, sẽ được tiến hành xử lý làm giảm thành phần kim loại sau đó nung hai cấp ở 500oC và 700oC. Quy trình xử lý tạo ra silica vô định hình, đạt mức độ tinh khiết cao, thành phần silica trong tro trấu trên 90%. Ngoài ra, thành phần oxit kim loại khác với hàm lượng không đáng kể. Sau đó điều chế dung dịch silicat natri. Từ silica thu được ở bước 1, hòa tan với dung dịch sút để điều chế dung dịch silicat natri 6 và 8% Si02. Tiếp đến tạo Silica sol, chất này được tạo thành từ dung dịch silicat natri và axit citric có pH 3,5. Sau đó tạo gel. Silica sol dần dần thành gel nước và được ủ ở nhiệt độ 60oC trong 24 giờ giúp ổn định cấu trúc gel. Gel nước sau khi ủ, được rửa bằng nước nhiều lần loại bỏ muối citrate có trong gel trước khi biến tính bằng hỗn hợp tetramethyl chloro silane (TMCS). Cuối cùng là sấy khô tự nhiên và xử lý nhiệt. Gel sau khi biến tính được để bay hơi tự nhiên ở nhiệt độ thường, sau đó xử lý nhiệt ở 200oC để tạo ra silica aerogel khí. Theo nhóm nghiên cứu, do có diện tích bề mặt cao và cấu trúc xốp nhẹ, vật liệu này có thể dùng làm chất cảm biến, chất xúc tác cho một số phản ứng hóa học, ứng dụng trong việc chế tạo tế bào quang điện, vật liệu cách nhiệt, cách âm và các vật liệu cao cấp khác...
Trang 1jo u r n al h om epa ge :w w w e l s e v i e r c o m / l oc a t e / c a t t o d
Review
Farook Adama,∗, Jimmy Nelson Appaturia, Anwar Iqbalb
a School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
b Kulliyah of Science, International Islamic University Malaysia, 25200 Kuantan, Pahang, Malaysia
a r t i c l e i n f o
Article history:
Received 15 October 2011
Received in revised form 18 April 2012
Accepted 21 April 2012
Available online 15 June 2012
Keywords:
Rice husk
Biomass
Silica
Catalyst
Transition metal
a b s t r a c t
Inthisreviewarticle,wereporttherecentdevelopmentandutilizationofsilicafromricehusk(RH) fortheimmobilizationoftransitionmetalsandorganicmoieties.Siliconprecursorwasobtainedinthe formofsodiumsilicateandasricehuskash(RHA).Sodiumsilicatewasobtainedbydirectsilica extrac-tionfromricehuskviaasolventextractionmethodwhilericehuskashwasobtainedbypyrolyzing theRHintherangeof500–800◦Cfor5–6h.Transitionmetalswereimmobilizedintothesilicamatrix viathesol–geltechniquewhiletheorganicmoietieswereincorporatedusingagraftingmethod 3-(Chloropropyl)triethoxy-silane(CPTES)wasusedasabridgetolinktheorganicmoietiestothesilica matrix.Allthecatalystsexhibitedgoodphysicalandcatalyticpotentialinvariousreactions
© 2012 Elsevier B.V All rights reserved
Contents
1 Introduction 3
2 Synthesismethodologies 4
2.1 Silicafromricehusk:bycalcinationandsolventextraction 4
2.2 Modificationofsilica:incorporationofmetalandimmobilizationoforganic 4
3 Transitionmetal-basedcatalystsfromricehusk 4
3.1 Chromium 4
3.2 Molybdenum 5
3.3 Tungsten 5
3.4 Iron 6
3.5 Cobalt 7
4 Metalbasedcatalystfromricehuskash 7
4.1 Friedel–Craftsreactionusingironcatalysts 7
4.2 Ricehuskashsupportedrutheniumcatalyst 7
4.3 RHAsupportedgallium,indium,ironandaluminumforthebenzylationofxylenesandbenzene 8
4.4 RHAsupportedaluminum,galliumandindiumforthetert-butylationofaromatics 8
4.5 Photocatalysisreactionusingsilica–tinnanotubes 8
4.6 OxidationofbenzeneoverbimetallicCu–Cesilicacatalysts 9
4.7 Benzoylationofp-xyleneonironsilicacatalyst 9
4.8 SynthesisofnanocrystallinezeoliteLfromRHA 9
5 Organic–inorganichybridcatalysts 10
5.1 One-potsynthesisviasol–gelmethod 10
5.2 Graftingmethod 11
5.3 Esterificationusingorganic–inorganichybridcatalysts 11
5.4 Silicafromricehuskashimmobilizedwith7-amino-1-naphthalenesulfonicacid 11
5.5 Silicafromricehuskashimmobilizedwithsulfanilicacid 12
∗ Corresponding author Tel.: +60 46533567; fax: +60 46574854.
E-mail addresses: farook@usm.my , farook dr@yahoo.com (F Adam).
0920-5861/$ – see front matter © 2012 Elsevier B.V All rights reserved.
Trang 2References 13
1 Introduction
Silicaisthemostabundantoxideintheearth’scrust,yetdespite
thisabundance,silicaispredominantlymadebysyntheticmeans
foritsuseintechnologicalapplicationsanditisoneofthevaluable
inorganicmultipurposechemicalcompounds[1]
Althoughsilicahasasimplechemicalformula(SiO2),itcanexist
inavarietyofforms,eachwithitsownstructuralcharacteristics,as
wellaschemicalandphysicalproperties.Silicacanexistintheform
ofgel,crystallineandamorphousmaterial.Generally,thestructure
ofSiO2isbaseduponaSiO4tetrahedron,whereeachsiliconatomis
bondedtofouroxygenatomsandeachoxygenatomisboundtotwo
siliconatoms.Thesurfaceofsilicaconsistsoftwotypesoffunctional
group:silanolgroups(Si O H)andsiloxanegroups(Si O Si).The
silanolgroupsarethelocusofactivityforanyprocess-takingplace
onthesurface,whilethesiloxanesitesareconsiderednon-reactive
[1].Porousamorphoussilicacontainsthreetypesofsilanolonits
surface:isolated,geminalandvicinal[2]
Theunequaldistributionofthesilanolsinthematrix,
result-ingfrom irregular packingof the SiO4 tetrahedralunit aswell
astheincompletecondensation,resultsinaheterogeneous
sur-face(i.e.,non-uniformityinthedispersionofsilanolgroups)for
synthesizedsilica.Thevarioussilanolscanhavedifferent
adsorp-tionactivitiesandcurrentknowledgeindicatesthattheisolated
silanolsare themorereactivespecies Withincreasing
temper-atureofheattreatment, thesilicasurfacebecomeshydrophobic
duetothecondensationofsurfacehydroxylgroupsresultingin
theformationofsiloxanebridges.Commercialsilicamanufacture
is a multi-stepprocess involvinghighheat and pressure,
mak-ingit less costeffective and not very environmentally friendly
[3]
Thediscoveryofmesoporousmaterialsbyresearchersfromthe
MobilOilCompanyinitiatedanintenseresearcheffortresulting
inmorethan3000publications,especially intheareaof
meso-porousmaterialsmadefromsilica.Theinertnessofsilicaaidedwith
theeaseofstructuraltailoringhasmadeitagoodinorganic
mate-rialonwhichtosupportotherorganicandinorganicmoieties[4]
Inmostpublishedreports,themajorsilicaprecursorsusedwere
commerciallymadealkoxysilane compounds suchas
tetraethy-lorthosilicate(TEOS),sodiumsilicateandtetramethylorthosilicate
[5].Nakashimaetal reportedthatacute exposuretoTEOS can
leadtodeath.Thus,thereisaneedtofindasafer,lessexpensive
andmoreenvironmentallyfriendlysilicaprecursor[6].Naturally
occurringsilicas,especiallythosefoundinagrowaste,canprovide
analternativesourcetoreplacecommercialsilicaprecursors.Rice
husksawdust[7],andrapeseedstalk[8]areamongthewidely
stud-iedagrowasteswhichhavebeenconvertedintomorevaluableend
products
Rice(OryzasativaL.)isaprimarysourceoffoodforbillionsof
peopleanditcovers1%oftheearth’ssurface.Globally,
approxi-mately600milliontonnesofriceareproducedeachyear.Forevery
1000kgofpaddymilled,about220kg(22%)ofhuskisproduced
[9].Ricehusk(RH)isthereforeanagriculturalresidueabundantly
availableinriceproducingcountries.Muchofthehuskproduced
fromtheprocessingofriceiseitherburntordumpedaswaste.RHis
composedof20%ash,38%cellulose,22%lignin,18%pentoseand2%
otherorganiccomponents[10,11].Eventhoughsomeofthishusk
isconvertedintoendproductssuchasfeedstock[12]and
adsor-bent[13]mostisburntopenly,causingenvironmentalandhealth
Scheme 1.Various utilizations of the rice husk silica.
problemsespeciallyinpooranddevelopingcountries.Therefore,it
isveryimportanttofindpathwaystofullyutilizethericehusk Silicacanbepyrolyzedatelevatedtemperaturetoformricehusk ash(RHA)oritcanbeextractedfromricehuskintheformofsodium silicatebyusingasolventextractionmethod.Inmostapplications, ricehuskashismorefavorablecomparedtoricehusk.Ricehusk ashisageneraltermdescribingallformsoftheashproducedfrom burningricehusk.Inpractice,theformofashobtainedvaries con-siderablyaccordingtotheburningtemperature.Thesilicaintheash undergoesstructuraltransformationsdependingontheconditions (time,temperature,etc.)ofcombustion.At550–800◦Camorphous ashisformedandattemperaturesgreaterthanthis,crystallineash
isformed[14].Thesetypesofsilicahavedifferentpropertiesand
itisimportanttoproduceashofthecorrectspecificationforthe particularenduse(seeScheme1).Eventhoughtheuseofsodium silicateextractedfromricehuskusingsolventisstilllimited,our studieshaveshownthatitcanbeutilizedformanypurposes Tran-sitionmetalscaneasilybesupportedonsilicaviasodiumsilicate extractedfromricehusk.Thesetransformedmetalsilicateshave goodpotentialasheterogeneouscatalysts
Severalresearchershavereporteddifferenttypesofsynthesis procedurestopreparemesoporoussilicafromricehuskfor incor-porationofmetals.Tsayetal.[15]haveusedaluminumsulfate, nickel nitrate and aqueous ammonia to prepareNi/RHA–Al2O3
via simple impregnation and ion exchange methods Chen
et al [16] have reported the preparation Cu/RHA using the deposition–precipitationmethodandcalcinationat673K,andthe materialwastested for partialoxidationof methanol(POM)to obtainH2.Changetal.[17]describedthesynthesisofCu/RHAfor thedehydrogenationofethanolusingcoppernitratetrihydrateas thecoppersourceviaanincipientwetnessimpregnationroute Duetothehighinterestinusingricehusksilicainadsorptionand catalysis,severalstudieshavebeencarriedoutonthesynthesis
ofmesoporousmolecularsieveM41Smaterials.Grisdanuraketal
[18] reported the synthesis of MCM-41 mesoporous materials usingCTABasstructure-directingagent(SDA),fortheadsorption
Trang 3of chlorinated volatile organic compounds and photocatalytic
degradationof herbicide (alachlor)[19] and
tetramethylammo-nium[20].Someresearchershadalsouseddirecthydrothermal
synthesis[21]andgasificationprocesses[22]toobtainMCM-41
fromrice husk ash.In 2009,Janget al.[23] synthesizedhighly
siliceous MCM-48fromRHA usinga cationicneutral surfactant
mixtureasthestructure-directingtemplate.Thematerialswere
usedforCO2adsorption.Todate,variousparameterssuchasthe
sourceofsilica,effectofsurfactantandconcentration,temperature
andpHhavebeenconsideredasmajorpivotalfactorsthat
influ-encetheformationofstructuralmaterialwiththedesiredporesize
distributionforcatalyticstudies.Inthepresentarticle,wereview
work performed on the use of silica, obtained from rice husk
eitherviacombustionorbysolventextraction,tosupportvarious
transitionmetalsandorganicmoietiesforheterogeneouscatalysis
2 Synthesis methodologies
2.1 Silicafromricehusk:bycalcinationandsolventextraction
Initially,adhereddirtandsoilonRHcanberemovedbywashing
withplentyoftapwaterandrinsingwithdistilledwater.The
metal-licimpuritiesinRHcanbereducedtonegligiblelevelsbystirring
withnitricacid[24]orrefluxingwithhydrochloricacid[17].Direct
extractionofsilicacanbeperformedbystirringtheacidtreated
RH(afterdrying)withsodiumhydroxidesolution.Duringthis
pro-cess,silicaisextractedintheformofsodiumsilicatetogetherwith
otherorganicmoieties,accordingtothemethodpatentedbyAdam
andFua[25].Thesodiumsilicateobtainedisconvertedtosilicaby
addingsuitableamountsofmineralacid
Ricehuskash(RHA)canbeobtainedbypyrolyzingtheRHat
temperaturesrangingfrom500◦Cto800◦Cfor5–6hinamuffle
furnace.TheRHAwasthendissolvedusingsodiumhydroxideto
obtainsodiumsilicate.Modificationswereundertakentothis
pro-cedureaccordingtocatalystpreparationparameters.Thus,Chang
etal.[17]pyrolyzedRHat900◦Cfor1hinafurnaceandN2flow
toobtainablackcrudeproduct,whichwasthenpyrolyzedagainin
airunderthesameconditionstoobtainawhiteash.In2006,
Chan-drasekharetal.[26]studiedcriticallytheeffectofacidtreatment,
calcinationtemperatureandtherateofheatingofRHandshowed
thattheseparametersinfluencedthesurfacearea,reactivitytoward
limeandbrightnessoftheash
2.2 Modificationofsilica:incorporationofmetaland
immobilizationoforganic
SilicaprecipitationfromRH andframework transitionmetal
incorporation was undertaken using the sol–gel technique A
greaterdegreeofcontrolonthefinalpropertiesofacatalystcan
beobtainedbyusingthesol–geltechnique,whichisduetothe
abilityofthemetalprecursortobemixedhomogeneouslywiththe
molecularprecursorofthesupport[27].Metaloxidecanbetrapped
withinthepolymerizinggel,permittingprecipitationfromsolution
wherethemetalioncanoccupyneighboringpositionsinthegel
matrix.Furtherprocessingandcalcinationdecomposesthe
resul-tantamorphousmixtureofmetaloxide,hydroxidesandmetalsalts
leadingtotheformationofanM O Mbond[28].Toobtaina
struc-turalmaterial,cetyltrimethylammoniumbromide(CTAB)asaSDA
wasaddedintothesodiumsilicatesolution.Severalresearchers
havereporteddifferentsyntheticroutesforpreparationofsilica
incorporatedmetalcatalysts.Changetal.[29]incorporatednickel
nitrateintothesilica matrixviaanionexchange method.They
alsousedaqueouscopperandchromiumnitratesolutionsto
syn-thesizeCu/Cr/RHAviaincipientwetnessimpregnation.Themetal
salt solution was added slowly to thesupport and thoroughly
stirred at room temperature Recently, Chen et al [16] used deposition–precipitationtoincorporatethecoppernitrateinRHA
Inthistechnique,themetalsaltwasdissolvedinureasolutionand addedtoRHAtoyieldasuspension.Thesuspensionwasheatedat
90◦CandthepHwasadjustedto2–3byaddingnitricacid Theimmobilizationoforganicmoietieswascarriedoutintwo steps.FirsttheCPTESwasreactedwiththesodiumsilicatefrom RHAinasinglestep.ThisledtotheformationofRHACCl,which containedthe Clfunctionalgroupattheendoftheorganicchain Thischlorinefunctionalgroupwasthenreactedwiththerequired organicligandinasubstitutionreactiongivingrisetothe immobi-lizedRHAC-Rcatalysts,whereRistheligand
3 Transition metal-based catalysts from rice husk
3.1 Chromium Our interest inchromium-incorporated silicafromrice husk began with theaim of incorporating chromium into the silica matrixfromricehuskusingthesol–geltechnique[30].The cat-alytic potentialofthechromium-loadedcatalystswastestedin theoxidationofcyclohexane,cyclohexeneandcyclohexanol.The as-synthesizedchromium–silicacatalyst’ssurfaceareawasonly 0.542m2g−1.Subsequentpreparationresultedinasurfaceareaof 1.20m2g−1whenitwascalcinedat500◦Cfor5h.Surface direct-ingagentwasnotaddedduringthepreparation.Thesecatalysts containedonlyCr(III)species.Calcinedchromium–silicacatalyst wasobservedtobehighlyhygroscopic.Calcination ofthe cata-lysthadimprovedtheselectivityofcyclohexanonebutlowered theselectivityofcyclohexanolintheoxidationofcyclohexane.The conversionofcyclohexanewas27.13% whentheas-synthesized chromium–silica catalyst was used while the conversion was 12.69%whencalcinedchromium–silicacatalystwasusedinstead Onlyaslightchangewasobservedintermsofcyclohexene conver-sionandproductselectivitywhenthesecatalystswereused.Both catalystsyielded100%cyclohexanoneselectivity
Byprolongingtheagingperiod andbyincorporatingsurface directingagent,thesurfaceareacouldbeincreased.Thesurface areawasincreasedto3.95m2g−1,and theconversionof cyclo-hexane was100% in6h Cyclohexanolandcyclohexanonewere formedinapproximately80:20ratio.Theselectivityoftheproducts wereimprovedwhen4-(methylamino)benzoicacidwasaddedto thecatalystpreparationmediumtoincreasethesurface hydropho-bicityandtheselectivityofcyclohexanolandcyclohexanonewas foundtobearatioof50:50.Thegreaterhydrophobiccharacterof chromium–silicacatalystmodifiedwith4-(methylamino)benzoic acidenhancestheinteractionofthecyclohexanemoleculewiththe polarcatalystsurfaceforadsorptionandsubsequent transforma-tion.Thenitrogenatomlonepairin4-(methylamino)benzoicacid mayformhydrogenbondswiththehydroxylgroupsthusretarding theconversionofcyclohexanoltocyclohexanone.AgainonlyCr3+
specieswereidentifiedtobetheactivesite[31] Theeffectof pHontheoxidation stateofchromiumandits influenceintheoxidationofstyrenewasalsostudiedtoidentify whichchromiumspecieswasmoreactiveintheoxidation reac-tion[32].ThecatalystswerepreparedatpH10,pH7andpH3
AtpH10,onlyCr(VI)specieswerefoundwhileatpH7andpH
3,Cr(VI)andCr(III)speciesco-existed.ChromiumloadingatpH
10(7.3w/w%)washighest,anditwaslowestatpH3(2.3w/w%)
AtpH10,theinteractionbetweenthenegativelychargedsilicate particlesandpositivelychargedchromiumionishigh,thus increas-ingthepossibilityofSi O Crbondformationandtheadsorption
ofchromiumhydroxide,Cr(OH)3 onthesilica support.Asnitric acidwasfurtheraddedtoreducethepH,adsorbedCr(OH)3can
bere-dissolvedintothesolutionasCr(III)ionsthusresultingin
Trang 4Fig 1. The SEM image of tungsten–silica catalysts from rice husk prepared at (a) pH 10, (b) pH 7 and (c) pH 3 [37]
lowerchromiumcontent.ItishypothesizedthattheSi O Crbond
especiallyinthecatalystpreparedatpH3wasstrongenoughto
preventtheoxidationofCr(III)speciestoCr(VI)speciesduring
cal-cination.TheCr(VI)speciesfoundinthechromium–silicacatalyst
preparedatpH10 andpH7wasdue totheoxidationof Cr(III)
speciesinCr(OH)3[33].Withtheaidofcetyltrimethylammonium
bromide(CTAB)asasurface-directingagent,thesurfaceareaof
thecatalystswasimprovedto143–564m2g−1.Higherchromium
containingcatalystsyieldedlowersurfaceareaandviceversa.The
surfaceofthecatalystswascomposedofrockyparticles.Catalysts
preparedinacidicmediawerefoundtobemoreactivein
catalyz-ingtheoxidationofstyreneusinghydrogenperoxideasoxidant
Benzaldehydewasobtainedasthemajorproduct.Themaximum
conversionofstyrenewas99.9%with63.1%selectivityto
benzalde-hyde.Thehighercatalyticactivityofthechromium–silicacatalysts
preparedinacidicmediaisrelatedtothehighersurfaceareaand
theco-existenceofCr(III)andCr(VI)species.Therateofhydrogen
peroxidedecompositionisincreasedinacidicreactionmedia
Re-characterizationofthecatalystafterreactionindicatedareduction
inchromiumcontentand thechromiumdetectedby AAS
anal-ysiswas1.0w/w% aftercatalytic testing However,the leached
chromiumspeciesdidnotcontributesignificantlytocatalytic
activ-ity.Thiswasconfirmedbyleachingtests.Thecatalystwasfoundto
bereusableseveraltimeswithoutlossofcatalyticactivity
3.2 Molybdenum
Thesamereactionconditionswereusedtostudytheeffectof
pHontheincorporationofmolybdenumintotheframework of
silicafromricehusk[34].AASanalysisdemonstratedthat
high-estconcentrationofmolybdenumwasinthecatalystspreparedin
acidicmedia.SpectroscopicanalysesshowedthepresenceofMo(V)
andMo(VI)speciesonthesurfaceofthemolybdenum–silica
cat-alystpreparedatpH3whileonlyMo(VI)specieswasdetectedon
thesurfaceofthecatalystpreparedatpH10andpH7.Thepore
systeminthecatalystsnarrowedasthepHwasreduced.Thisis
duetothedepositionofmolybdenumspeciesintothelargerpores
thusresultinginaunimodalporesystem.Anotherreasoncouldbe
duetothepresenceofnitrateions.AtpH10andpH7,the
pres-enceofNO −ionscanshifttheequilibriumofthesurfactantand
silicateassembly.TheNO3− ionblockstheadsorptionofsilicate ionsonmicellesanddelaystheformationofthesilica/surfactant mesophases.Thiscancauseincompleteinteractionbetween sili-catespeciesandsurfactant,resultinginsmallerporesbeingformed
bythetemplate.Thelargerporeswereformedbythe agglomer-ationofsilica nanoparticlesduringthehydrolysis–condensation process[35,36].Shortorderedporearrangementsexistedinthe molybdenum–silica catalystsprepared at pH 10 and started to deteriorate as the pH was reduced The SEM images indicate thatthecatalystshadrockyparticleswithsphericalsurfaces[33] Molybdenum–silica catalystprepared at pH3 showeda higher styreneconversionandbenzaldehydeselectivitycomparedtothe othertwocatalysts.Benzaldehyde(Bza)wasobtainedasthemajor product.Theconversionwas82.2%andtheBzaselectivitywasca 82.8%.Asignificantamountofmolybdenumleachedoutfromthe supportwhenitwasusedforthefirsttime.Duetothelossofthe activesites,styreneconversiondroppedabout50%whenthe cat-alystwasreused.However,thecatalystsremainedheterogeneous duringconsecutivereuse.Re-characterizationoftheusedcatalyst indicatedthatonlyMo(VI)specieswerefoundonthesurfaceof thecatalyst.Theporesystemofthecatalystchangedfrombeing unimodaltobimodalaftercatalyticreactionduetoleaching.The re-characterizationofusedmolybdenum–silicacatalystindicatesthat themajorityofthemolybdenumspecieswasphysicallyadsorbed
onthesurfaceofthecatalystandmostprobablythiswastheMo(V) species
3.3 Tungsten Tungstenspecieswereinsertedintothesilicamatrixusingthe samemethodandconditionsasmentionedabove[37].Thehighest tungstenconcentrationwasfoundinthecatalystspreparedatpH
3whilethelowestwasfoundinthecatalystspreparedatpH10 Theincreasingtrendintheimmobilizationoftungstencontentas thepHwasdecreasedcanberelatedtotheinteractionbetween tungstatespecies(WO4)2 −andthesilicatespecies.AtpH10,lack
ofinteractionbetweenthesetwospeciesduetonegativecharge repulsion,yieldedcatalystswithlowerincorporationoftungsten Theinteractionbecamestrongerasthenegativecharacterofthe silicaoligomersreducedasthepHapproachedtheisoelectricpoint
Trang 5O Si O
W
6+
(b) Si
O
Si
O Si
O W
O Si
(a)
Fig 2. The structure of (a) isolated tungsten species and (b) isolated (WO 4 ) 2−
species.
ofsilica(∼pH2).Thus,thecatalystswithhigheramountsof
tung-stenformedunderconditionsofacidicpH.TheSEMimageofthe
catalystshowedthatbrightspotsstartedtoappearastheacidityof
thecatalystspreparationwasincreased.Theimagesareshownin
Fig.1.EDXanalysisdetectedaslightlyhighertungsten
concentra-tiononthebrightspotscomparedtothedarkareasasshowninthe
SEMimagesofthecatalyst(Fig.1).Isolatedtetrahedral(WO4)2−
speciesweretheonlytungstenspeciesfoundonthesurfaceofthe
tungsten–silicacatalystpreparedatpH10
UV–visdiffusereflectancespectroscopicanalysissuggestedthe
presenceofdifferentkindsoftungstenspecies.Isolated
tetrahe-dral(WO4)2 −species,isolatedtungstenspeciesorlowoligomeric
tungstenoxidespecieswasfoundinthecatalystpreparedatpH7
Ontheotherhand,tungstenoxidewasdetectedtogetherwith
iso-latedtetrahedral(WO4)2 −species,isolatedtungstenspeciesorlow
oligomerictungstenoxidespeciesonthesurfaceoftungsten–silica
catalystpreparedatpH3
Isolatedtungstenspeciesreferstotungstenionsincorporated
insidethesilicaframeworkasshowninFig.2(a),whereasthe
iso-latedtetrahedral(WO4)2−speciesisthespeciesthatwasformed
onthesurfaceofthecatalystasshowninFig.2(b).XRDanalysis
indicatesthatthepeakrelatedtotheamorphoussilicaat2=23◦
startedtosplitinto3relativelynarrowbandswithsharppeaks
Newpeaksstartedtoappearaswellat2=27◦,29◦,33◦,34◦,42◦,
47◦and48◦whenthepHofthesynthesismediumwasdecreased,
indicatingphasesegregationleadingtowardtheformationoflarger
WO3crystalsonthecatalystsurface[38].Thesplitbecamemore
obviousintungsten–silicacatalystpreparedatpH7andpH3.All
thecatalystshave abimodalporesystem.Theformationofthe
bimodalporesystemcouldbeduetothepresenceofnitrateions
[39]andduetothetungstenprecursor.Normallymetalspeciesare
abletospeedupthecondensationandhydrolysisprocess
How-everthisdidnothappeninthiscase.Thiscanberelatedtothe
bulkysizeof(WO4)2 −species.Thebulkysizeof(WO4)2 −species
mayhavepreventedthehydrolysisandcondensationprocessfrom
takingplaceleadingtotheformationofdifferentporesizes.Some
researchersconcludedthattheFT-IRbandaround963cm−1inthe
tungsten–silicamaterialindicatedtheincorporationof tungsten
speciesinsidethesilicamatrix.Thisbandstartedtodiminishwhen
theaciditywasdecreased.ThisindicatestheagglomerationofWO3
crystalsleadingtotheformationofextra-frameworkWO3onthe
supportsurface,especiallyintungsten–silicacatalystpreparedin
acidicmedium[39,40].Asimilarphenomenonwasalsoobservedby
uswhenweincorporatedindiumintothematrixofsilicafromrice
huskash[41].AstheIn3+ionconcentrationwasincreased,thisband
startedtodisappearindicatingtheformationofextra-framework
metaloxideonthesurface.Thestructureofsurfaceactivesitesof
tungsten–silicacatalystspreparedinanacidicmediumisshownin
Fig.3
Thehigheststyreneconversionof61.9%and 100%selectivity
towardbenzaldehydewasachievedwhenatungsten–silica
cata-lystpreparedinacidicmediumwasused.Higherconcentrationsof
tungstenandthepresenceofdifferentkindsoftungstenspecies
havebeen identifiedtobethemain factorscontributingtothe
higheractivityofthecatalystpreparedatpH3.Thereactionwas
Fig 3.Proposed surface active sites of tungsten–silica catalyst prepared at pH 3 [37]
proposedtobecatalyzedbypertungsticacidlikeintermediates, withstyrene oxideas theintermediate activereagent Asmall amountof tungsten species wasfoundto beleached from the supportand catalyzethereactionhomogeneously.Thephysical propertiesofthecatalystwerenotaffectedbythelossoftungsten activesites[38]duetoleaching
3.4 Iron Ironisacheaptransitionmetalwhichisnon-toxictohuman healthandwhichhasbeenknowntocatalyzemanyorganic reac-tions.When4-(methylamino)benzoicacidwasusedasasurface directingagentitincreasedthecatalystsurfacearea[42].The 4-(methylamino)benzoic acidwasproposedtobeattachedtothe silicamatrixviathenitrogenatom.TheformationoftheSi Nbond
isshowninFig.4 Thesurfaceareaofthecatalystincreasedfrom267to331m2g−1 aftermodification.Theincreaseinsurfaceareawasaccompanied
byaporesizereduction asexpected.Theporesizeofthe cata-lystdecreased from9.2to6.0nm.A seriesofcross-linkedlines arrangedinanorderlymannerwasobservedintheTEMimageof 4-(methylamino)benzoicacidmodifiediron–silicacatalyst,which werenotpresentintheTEMimageofunmodifiedcatalyst.This couldbeduetotheamineactingasatemplateduringthesyntheses Both catalystswere tested in theFriedel–Crafts benzylation
oftoluenegiving100%tolueneconversion.Themono-substituted (ortho-andpara-)productswerefoundtobethemajorcomponents
intheproduct.Theunmodifiediron–silicacatalystwasfoundtobe lessselectivetothemono-substituted(ortho-andpara-)product comparedtothe4-(methylamino)benzoicacidmodifiediron–silica catalyst
Inanotherstudy,apurelyiron–silicacatalystwasfoundtobe veryactiveintheoxidationofphenolusinghydrogenperoxideas oxidantundermildconditions[43].Oxidationofphenolusingthis catalystyieldedcatecholandhydroquinoneastheonlyproducts TwosignalsrelatedtotheQ3andQ4siliconcentersat−100.4and
−108.7ppmwereobservedwhenthecatalystwassubjectedto29Si MASNMRanalysis.Signalsrelatedtothespinningsidebandswere observed,suggestingtheparamagneticnatureofFe(III)species[44]
whichwasdetectedatca.10and−210ppm
Oxidation of phenolhas beenassociated witha free radical mechanismbymanyauthors[45–47].Inthisresearch,wehad pro-posedanon-freeradicalmechanism.Freeradicalmechanismsare knowntoproducebenzoquinonewhichcanlaterbetransformed
topolymericmaterialsandtar.Howevertheseproductswerenot detectedinourstudy.Theintermediatewasformedonthesurface
ofthecatalystassistedbytheformationofcoordinatebondsby thereactantstotheFe3+activesites.Thepolarnatureofthe cata-lyststronglysuggeststhereactantswereadsorbedonthecatalyst surfaceviahydrogenbond
Trang 6HO O
H3C
Si O
O
O- Na+
Strongly basic
HO O
N
CH3 Si
O
O O
Si
Si
Si
+ NaOH
Fig 4.The formation of Si N bond [42]
3.5 Cobalt
Cobaltcatalysts,includingnanoparticles,havebeenprepared
usingricehusk silicaasthesupport.Mostof theprocedures in
theliteraturewereexpensive,tediousandtimeconsuming
How-ever,weintroducedasimplewaytopreparecobalt–silicacatalyst
andnanoparticles.Thesol–gelmethodwasusedtopreparethe
cobaltricehusksilicananoparticlesundermild conditions[48]
Thecobaltnanoparticlespreparedwereintherangeof2–15nm
TheFT-IR spectraof thenanoparticlesindicated some
similari-tieswithtungsten–silicacatalystsmentionedinSection3.3.The
bandat967cm−1disappeareduponcobaltadditionintothe
sil-icaframework Thisis due tothepresence of cobaltsilicate or
hydrosilicate[49].FT-IRandXRDanalysesindicatethatthecobalt
nanoparticlescomprisedCo3O4 and CoO Cobaltnitrate
decom-posedintoanintermediatecobaltsilicatephasefirstandlaterinto
Co3O4duringthedryingprocess.Cobaltricehusksilica
nanopar-ticlesprepared viathis methodexhibitboth ferromagnetic and
antiferromagneticproperties.Thecorrespondingsaturation
mag-netization(Ms),coercivity(Hc)andremanentmagnetization(Mr)
werenotedtobe0.245emu/g,340.09Oeand0.0115emu/g
respec-tively.Msforcobalt–silicananoparticleswasmuchlowercompared
toMbulkCo=166emu/g[50].DecreaseinMsismostlydue tothe
smallercobaltrice husk silicananoparticlessynthesized in this
study.Hardmagnet behaviorisshownfromthehysteresisloop
byshowinglargeHc(>100Oe)[51].Thehysteresisofthismaterial
showthepresence ofa ‘curvature’shape whichindicates
ferro-magnetic(FM)natureanda‘straight’shapewhichcorrespondto
antiferromagnetic(AFM)properties.Similarmagnetichysteresis
plotwasreportedforCoOnanoparticleswithsizerangingfrom10
to80nmpreparedbysol–gelmethod[52].Theantiferromagnetic
propertyofthecatalystisduetothepresenceofCoO
nanoparti-cles.TheConanoparticlespreparedinthisworkareproposedto
followthecore–shellmodel,inwhichthecoreisattributedto
fer-romagneticmetallicandtheshellconsistsofantiferromagneticCoO
species[53]
4 Metal based catalyst from rice husk ash
4.1 Friedel–Craftsreactionusingironcatalysts
TheFriedel–Crafts(benzylation)reactionbetweentolueneand
benzylchloridehasbeencarriedoutusingsolid,environmentally
friendlyandreusablecatalysts(RHA-FeandRHA-Fe700)[11].The
mono-substitutedbenzyltoluenewasthemajorproductandboth catalystsyieldedmorethan92%oftheproductat100◦C,in1h, withoutsolvent
Thecatalystsshowpromisingactivitywithalmostequal dis-tribution of ortho- and para-isomers Sixteen minor products consistingofvariousdi-substituted isomerswerealsodetected Theortho-substitutedproduct waspresent in largerproportion (49.53%)comparedtothepara-substituted(46.01%)productwhen using RHA-Fe700 as the catalyst The higher yield of ortho-substituted product was due basically, to the presence of 2 ortho-positionsforsubstitutiononthetoluenemolecule.For
RHA-Fe,about48.1%and44.8%ofortho-andpara-substitutedproducts wereobservedrespectively.However,RHA-Fe700gavea signif-icantly loweryield of the di-substituted productscompared to RHA-Fe
It was foundthat theRHA-Fe700 gave slightly higher yield (∼97.1%)forthemono-substitutedproductandsignificantlylower yield(∼2.8%)for thedi-substitutedproductsduringthesecond reusabilitystudies.However,therewasnotmuchdifferenceinthe distributionoftheortho-andpara-derivatives
4.2 Ricehuskashsupportedrutheniumcatalyst RHA-Ru(as-synthesized)andRHA-Ru700(calcinedat700◦C) heterogeneouscatalystswerepreparedsimilarlyusingricehusk ashsilica as the support.The effect of calcinationon the sur-face and bulk structure of the catalyst was investigated and compared with as-synthesized RHA-Ru catalyst using several physico-chemicaltechniques[54].XRD studiesshowedRHA-Ru was largely amorphous (2=22◦) with some crystalline peaks presentin RHA-Ru700.Ruthenium wasshowntobepresent in the form of its dioxide (RuO2) in RHA-Ru700 These materials werefurtherinvestigatedusingN2sorptionstudies.Theisotherm and hysteresis loop were shown to be of type IV with type H3hysteresisrespectively forbothcatalystsaccordingtoIUPAC classification The BET surface areaof RHA-Ru was65.1m2g−1 comparedtoRHA-Ru700(10.4m2g−1).Thesignificantreduction
in the surface area was attributed to a collapse in the pore structure at 700◦C due tothe condensationof adjacentsilanol groups
FineneedlelikestructurewasseenintheSEMmicrographsfor RHA-Ru700.Theneedleslookedlikethinflatelongatedpiecesof fiberwithsharpedgesandofnanodimension.Thewidthofthe needleswasestimatedtobeabout200nm.However,thiswasnot
Trang 7Table 1
The effect of different xylene isomers on the percentage conversion and product distribution at 80 ◦ C and Xyl/BC molar ratios of 15:1 [55]
Time (min) Selectivity (%) Time (min) Selectivity (%)
RHA-Ga
RHA-In
RHA-Fe
a Turnover rate for 50% conversion in mol g −1 s −1
b Two mono-substituents 2,4-DMDPM and 2,6-DMDPM in a percentage ratio of about 79:21.
observedinRHA-Ru.RHA-Ruingeneralhadaporousmatrixdue
totheamorphousstructureofthecatalyst
4.3 RHAsupportedgallium,indium,ironandaluminumforthe
benzylationofxylenesandbenzene
LiquidphaseFriedel–Craftsreactionof xylenes(o-Xyl,m-Xyl
andp-Xyl) withbenzylchloride(BC)overthepreparedcatalyst
(RHA-Fe,RHA-GaandRHA-In)wascarriedoutat80◦C[55].The
differencesinactivityandselectivitybetweenthexyleneisomers
andcatalystsareshowninTable1
FromTable1,theRHA-Feshowedthehighestcatalyticactivity
whereas RHA-In and RHA-Ga gave higher selectivity to
2,5-dimethyldiphenylmethane (2,5-DMDPM) withina shorter time
The rate of reaction decreased in the following order:
RHA-Fe>RHA-In>RHA-Ga.Ironhasaredoxpotentialof+0.77Vwhile
galliumandindiumhavearedoxpotentialof−0.44V.Thehigher
redoxpropertyofironwasexpectedtoplayacrucialrolefor
initiat-ingtheBCcarbocationandshowedsuperiorcatalyticactivityover
therest.However,thehigheractivityofRHA-InoverRHA-Gacould
beduetotheloweramountofnon-frameworkGaspeciespresent
onthesurfaceofRHA-Ga.Thecatalystcouldbereusedseveraltimes
withoutsignificantchangeintheiractivityandselectivity[55]
In2009,AhmedandAdam[56]usedaluminum,galliumandiron
incorporatedRHAforthebenzylationofbenzene(Bz)withBC.Iron
basedcatalyst,showedexcellentactivity,whereasRHA-Ga gave
goodselectivitytowarddiphenylmethane(DPM).However,
RHA-Alwasalmostinactiveinthisreactionduetothelowredoxproperty
oftheAl3+ion.Amongthemainadvantagesofthesecatalystwas
thattherewas,noneedforcalcinationaftercatalystpreparation
andmoreimportantwasthefactthatRHA-GaandRHA-Fewere
notmoisturesensitiveandcanbehandledandstoredundernormal
conditions
4.4 RHAsupportedaluminum,galliumandindiumforthe
tert-butylationofaromatics
Thetert-butylationofsomesubstitutedbenzenes(tolueneand
chlorobenzene)withtert-butylchloride(TBC)wascarriedoutusing
RHA-Al,RHA-GaandRHA-Inat80◦C[57].Attheinitialstageofthe
reaction,thetert-butylcationwasformedsubsequentlyviathe
rad-icalmechanismprocess,whichinturnattacksthebenzeneringfor
theformationoftert-butylbenzene(TBB)anddi-tert-butyl
ben-zene(DTBB)viatheSN1mechanism(mainreaction).However,a
protoneliminationreaction(sidereaction)alsooccurred,resulting
intheformationofisobutenedimmers(IBD)andisobutenetrimers
(IBT) Theextent ofthese sideproducts wasfoundtodecrease
significantlywithtime,indicatingthereversibilityofthe
oligomer-izationreactions.Thecatalystswerestableagainstleachingand
werereusableseveraltimesbutwithanobservabledropin cat-alyticactivity.RHA-Galostalmost20%ofitsactivityaftereachrun, whereas,RHA-Inwasstableuntilthe3rdrunandthenlost∼13%
ofitsactivityatthe5thrun.Thedeactivationwassuggestedtobe inducedbythepoisoningeffectofthebulkysideproductsthatwere stronglyadsorbedonthecatalystsurface
Basedontheproductanalysis,amechanismwasproposedfor the tert-butylation of aromatics It was suggested the reaction proceedsinitiallythroughtheradicalmechanismforthe conver-sionofTBCtotert-butylcarbocations.However,thecarbocations remainedadsorbedonthecatalyst,possiblyattheframework posi-tionreplacingtheextra-frameworkNa+ionsformingtert-butoxide Thesetert-butoxidespeciescaneitherattackthearomatictoform thetert-butylproducts(SN1)orcanundergoeliminationreaction (E1)fortheformationofIBmonomers.Thelatterspecies(i.e.,IB) hasextraordinaryreactivitytowardpolymerizationunderalltypes
ofacidicconditions(i.e.,LewisorBrønsted).Itisnoteworthythat thepolymerizationreactioncanbeinitiatedbyunconverted tert-butylcarbocationorlibratedHCl.Thecapabilityofthecatalystfor convertingtheTBCtoTBcarbocationdependsmerelyonitsredox potentialandthenumberofactivesitesonitssurface.However, theproductionoftert-butylatedproductsdependsonitsabilityto activatethearomaticfortheSN1reactionaswellasthehigh nucle-ophilicityofthearomatic,i.e.,thepresenceofelectrondonatingand notelectronwithdrawingsubstituentsinthebenzenering[57] 4.5 Photocatalysisreactionusingsilica–tinnanotubes
Silica–tin nanotubes (RHA-10Sn) with external diameter of 2–4nmandinternaldiameterof1–2nmweremadebyasimple sol–gelmethodatroomtemperature[24].Thesenanotubespossess
ahollowinnercorewithopentubeends(Fig.5(a))
The specific surface area of RHA-10Sn was found to be
607m2g−1 comparedtoRHA-silica(315m2g−1).Theincreasein surfaceareasuggeststhattinparticlewerewelldispersedwithin thesilicamatrix.Nocrystallinephasewasdetectedinthehighangle powderXRDanalysis.Theroot-mean-squareroughnessandheight distributionofRHA-10Snwerefoundtobe111.5and322.6(nm) fromAFManalysis(Fig.5(b)).Thesehighvaluescorrelatewellto thehighlyporoustubularmaterialwithahighBETsurfacearea Thephotocatalytic activityofRHA-10Snwasstudiedtoward degradationof methyleneblue (MB)under UV-irradiation.Asa controlexperiment,darkreaction(withoutUVandcatalyst)and photolysiswasconductedtocomparewiththeadsorptionand pho-tocatalyticstudies.About96%of MBremainedunchanged after
60mininthedarkreaction.ThedegradationofMBwasconfirmed withthereduction in concentrationafter 960min Thecatalyst RHA-10Sngave maximum degradationcompared toRHA-silica Thisbehaviorisduetothewidebandgap(E =3.6eV)ofSnandhigh
Trang 8Fig 5. (a) The TEM micrographs at 110 K, and (b) the 3-D AFM topography image of RHA-10Sn [24]
surfacearea.Thedegradationproductswereidentifiedasinorganic
anionssuchasnitrate,chlorideandsulfateusingion
chromatogra-phyanalysis[24]
4.6 OxidationofbenzeneoverbimetallicCu–Cesilicacatalysts
Aseries ofmesoporous RHA silicasupported Cu–Cebimetal
catalystwaspreparedwithcetyltrimethylammoniumbromide(as
atemplate).ThesecatalystswerelabeledasRHA-10Cu5Ce,
RHA-10Cu20Ce,andRHA-10Cu50Ce.TG/DTGanalysisof thecatalysts
confirmedthecompleteremovalofthetemplateat773K.TheXRD
patternshowedthatRHAandmetalincorporatedsilicacatalysts
haveamorphouscharacteristicsduetothepresenceofabroadpeak
intheregionof20–30◦2.However,anobservedshiftofthe
diffrac-tionbandforRHA–10Cu50Ce,tothe25–35◦2regioncanbedue
tothepoorcrystallizationofCeO2withincreaseinCeloading[58]
Thesecatalystswereusedforasinglestepoxidationofbenzene
withH2O2 asoxidantandacetonitrileassolventat343Kunder
atmosphericpressure.Theincorporationoftwodifferentmetals
withsilicaplaysacrucialroleinthecatalyticactivityduetoa
syn-ergyeffectbetweenthemetalions.Theequationforthecatalytic
oxidationispresentedinScheme2
In a typical run, 84.3% benzene conversion and 96.4%
phe-nol selectivity was achieved using 70mg of RHA-10Cu20Ce at
343Kwithotherparameterskeptconstant(H2O2=22mmol;
ben-zene=11mmol;acetonitrile=116mmolandreactiontimeof5h)
The high activity and phenol selectivity observed under mild
reactionconditionscouldbecorrelatedtotheenhancedtextural
propertiessuchasthespecificsurfacearea(329m2g−1),largepore
volume(0.95m3g−1)andgooddispersionofloadedCuandCeions
whichgavemoreactivecentersontheamorphoussilica.However,
themonometalceria(RHA-20Ce)orcopper(RHA-10Cu)showed
lowactivity(23.5%or47.7%)andphenolselectivity(34.6%or79.4%)
incomparisontothebimetalliccatalysts.Thisisanindicationthat
theexistenceofcopperandceriatogetherinthecatalyticsystem
wasnecessaryforimprovingtheoxidationofbenzene
The oxidation of benzene over different metal loaded
cata-lystsresultedinthesameproducts.However,theselectivityfor
phenolwassignificantlylowerandasaconsequence,ahigher
per-centage ofhydroquinone and 1,4-benzoquinonewere obtained
Thecatalyticoxidation followed theorderRHA-10Cu5Ce<
RHA-10Cu20Ce<RHA-10Cu50Cewhiletheorderofphenolselectivity
was RHA-10Cu50Ce<RHA-10Cu5Ce<RHA-10Cu20Ce The
cata-lyst,RHA-10Cu20Cewasfoundtobethemostsuitable for this
reactionbasedonitsreusability(uptothreerecycleswithsome
lossincatalyticactivity)[58]
4.7 Benzoylationofp-xyleneonironsilicacatalyst RHAwasusedtosynthesizeRHA-5Fe,RHA-10Fe,RHA-15Feand RHA-20Feviathesol–geltechnique(pH5.0)atroomtemperature
[59].Theacidityofthecatalystswasconfirmedbypyridine adsorp-tion, and FT-IR spectra show typical bands around 1551cm−1 and1565cm−1(attributedtoBrønstedacidsites)and1450cm−1 (attributedtoLewisacidsites).Thesurfaceofthecatalysts exhib-ited irregular shaped particles, compared to RHA-silica which showedagglomeratesofsphericalparticles
TheliquidphaseFriedel–Craftsacylationreactionofp-xylene (p-xyl)withbenzoylchloride(BzCl)wascarriedoutoverthe as-synthesizedcatalyst.TheRHA-10Fecatalystexhibitedthehighest activityforbenzoylationofp-xyl.TheconversionofBzClandthe selectivitytoward 2,5-dimethylbenzophenone (2,5-DMBP)were foundtobe98.4and88.9%respectivelyat413K[59]
Asthemolarratioincreasedfrom1:5to1:20,(BzCl:p-xyl)the BzClconversionalsoincreased.Atamolarratioof1:20,high con-versionofBzCl(86.0%)wasobserved.Atthelowerconcentration
ofBzCl,moreactivesitesofcatalystareavailableforadsorption, whichresultsintheformationofactiveelectrophilicbenzoylinium cationsthatcanreactwithp-xyl.Inaddition,theselective forma-tionof2,5-DMBPwasnotaffectedasthemolarratiowaschanged from1:5to1:20.Thebenzoylationoverdifferentmetalloaded cat-alystsresultedinthesameproducts.However,theselectivityof 2,5-DMBPwasreducedslightlyaftertheFeloadingincreasedmore than10wt.%.Whentheamountofironincreasedfrom5to10wt.%, theBzClconversionincreasedfrom77.7to98.4%.However,further increaseofmetalloadingto15and20wt.%didnothavemucheffect
onthecatalyticactivity.TheRHA-SiO2,didnotshowanyactivity forthebenzoylationreactionunderthesamereactionconditions Hence,thepresenceofironwascrucialforboostingthecatalytic activity.TheRHA-10Fewassuccessfullyreusedseveraltimes How-evertheamountofFeonthecatalystwasfoundtobereduced from7.22to4.96w/w%.Adecreaseinconversion(42.4%)wasalso observedforthesecondcyclewithinsignificantdecreasein selec-tivityof2,5-DMBP(86.8%).Thereductioninconversionisdueto thereducednumberofmetalactivesitesonthecatalystandmay alsobeduetotheblockageoftheporesystembyproducts[59] Themechanismforthecatalysisinvolvestheformationofan adsorbedBzCltransitionspecies(faststep).Thisreactswithp-xyl
toform2,5-DMBP(abimolecularslowstep)withthesimultaneous eliminationofHCl[59]
4.8 SynthesisofnanocrystallinezeoliteLfromRHA Wongetal.[60]havereportedthemicroscopicinvestigation
ofaluminosilicatezeoliteL(structurecodeLTL)nanocrystalsusing
Trang 9Scheme 2. The oxidation of benzene to phenol with the Cu–Ce silica catalyst [58]
RHAasthereactivesilicasourceinatemplate-freehydrothermal
system.Unliketheconventionalcylindrical-shapedzeoliteL,the
nanocrystalline zeolite L synthesizedfrom RHAexhibits a
one-dimensionalchannelstructure withtablet-like features(shorter
c-dimensionforbetterdiffusionofproductsand reactants).The
frameworkstructureofzeoliteLconsistsofcancrinite(CAN)cages
andhexagonalprisms(D6R),alternatingtoformcolumnsthatrun
paralleltothec-axis.Theresearchinterestinthesynthesisofzeolite
Lisbasedonitsexcellentcatalyticpropertiesandwideapplications
inhost–guestchemistry.Microscopicandspectroscopicanalyses
showedthatthenucleationofzeoliteLtookplaceintheveryearly
partofthereaction.ThisrapidformationofLTLnanocrystalsisdue
totheuseofRHAasthereactivesilicasourceintheprecursor
solu-tion.FullycrystallizedzeoliteLwasachievedafter24hresulting
inaproductwithameancrystallitesizeof210nm.TEMimages
(Fig.6)confirmedthearrangementofhexagonalpattern,whichis
thedistinctivefeatureofzeoliteL
5 Organic–inorganic hybrid catalysts
5.1 One-potsynthesisviasol–gelmethod Therearevarioussynthesismethodsthathavebeenutilizedto attachorganicgroupstosilicasurfaceviatheformationof cova-lentbonds.Thesearepost-syntheticfunctionalization(grafting), co-condensation(directsynthesis),productionofperiodic meso-porousorganosilanes(PMO)and“ship-in-bottle”techniques.More recently,Adametal.hadsuccessfullyimmobilized chloropropy-ltriethoxysilane (CPTES) onto the silica network via a one-pot synthesisusingthesol–gelmethod[61]
The 29Si MAS NMR spectrum of the resulting organo-silica product,RHACCl(Fig.7(a))showschemicalshiftsattributedtoQ4
andQ3[Qn=Si(Osi)n(OH)4−n],i.e.atı=−109.92and−100.65ppm
Achemicalshiftat−65.2ppmindicatestheformationofSi O Si linkageofCPTEStothesiliconatomofthesilicaviathreesiloxane
Trang 10Fig 7.The MAS NMR spectra of RHACCl: (a) the 29 Si MAS NMR spectrum for RHACCl and (b) the 13 C MAS NMR spectrum for RHACCl [61]
bonds, SiO2( O )3Si CH2CH2CH2Cl (T3).The chemical shift at
−57.4ppm wasdue totwo siloxane bondstothesilica matrix,
i.e.SiO2( O )2Si(OH)CH2CH2CH2Cl.The13CMASNMRofRHACCl
(Fig.7(b))showedthreepeakswithchemicalshiftat10.37,26.70
and47.69ppmwhich correspondstotheC1,C2andC3carbons
fromCPTESrespectively[61]
5.2 Graftingmethod
Graftingisamethodtofunctionalizeormodifythesurfaceof
mesostructuredsilicawithorganicgroups.Thisprocesswascarried
outusingRHACClwithsaccharine(Sac)(anartificialsweetening
agent) [62] and melamine (Mela)[63] The synthesis of
silica-saccharine(RHAC-Sac)andsilica-melamine(RHAPrMela)catalysts
werecarriedoutusingdrytolueneandtriethylamine
(deprotonat-ingagent)underrefluxconditionsat110◦C
EDXconfirmedthepresenceofchlorine(RHACCl;3.07%),
nitro-gen(RHAPrMela;3.65%)andsulfur(RHAC-Sac;2.29%)respectively
RHAPrMelaexhibitedahollownanotubelikestructureand
RHAC-Sacshowedagglomeratedparticles
The results of 29Si MAS NMR studies for both RHA-Sac
and RHAPrMela indicated the successful immobilization
of these organic molecules on the solid support
Chemi-cal shifts were observed which were attributed to Q4 and
Q3 silicon atoms A chemical shift at −64.78 and −57.41
(ppm) indicates the formation of Si O Si linkages via
three siloxane bonds, (SiO2)( O )3Si CH2CH2CH2 Sac and
(SiO2)( O )3Si CH2CH2CH2 Mela (T3) respectively A
chemi-calshiftat −57.4and −49.16(ppm)indicatestheformation of
two siloxane linkages, i.e (SiO2)( O )2Si(OH)CH2CH2CH2 Sac
and (SiO2)( O )2Si(OH)CH2CH2CH2 Mela (T2), to the silica
respectively
The13CMASNMRofRHA-SacisshowninFig.8(a).Severalbroad
chemicalshiftsat124and130ppmwhichwereeasilyassignableto
thearomaticcarbonatC8,C4,C6andC9areapparent.Thechemical
shiftofthecarbonofthelactamring(C10)canbeseenat160ppm
The13CMASNMRforRHAPrMelashowstwostrongchemical
shiftsat161.52and169.67ppmwiththeirrespectivespinningside
bands(marked*),indicatingthatthecarbonatomsinmelamineare
notequivalent.Toprovetheexistenceofthespinningsidebands,
the13CMASNMR wasrecordedatdifferentspin frequenciesof
7MHz(Fig.8(b)),and5MHz(Fig.8(c)).Theresultclearlyshowed
theshiftinginthespinningsidebandswhilethemainchemical
shiftsofthemelamineringwerenotaffected.Thechemicalshiftat
161.52ppmwasassignedtothetwocarbonatomswithfreeamine
groupsC5(Scheme3)whicharechemicallyequivalent.Thesecond
chemicalshiftat169.67ppmwasassignedtothecarbonatomofthe
melamineringwhichisbondedtothepropylgroupC4(Scheme3) throughtheC3carbonatom
5.3 Esterificationusingorganic–inorganichybridcatalysts
Asimple,environmentallyfriendly,cheap,time-savingand non-toxiccatalyst(RHA-Sac[62]andRHAPrMela[63])wasusedforthe esterificationreactionusingethanolandaceticacid.Aconversionof 66%wasachievedat85◦C,and6hreactiontimewith(acid:alcohol) 1:1molarratio.Thecatalystcontainsweakbasicsites(strong con-jugateacid)andtheaminegroupwhichwasbelievedtoplayan importantroleinthiscatalyticactivity.However,similarcatalytic activitywasalsoobtainedwhenthehomogeneouscatalyst(Sac) wasused.Minimallossofcatalyticefficiencywasobservedwhen thesolidcatalystwasreusedafterregenerationat150◦C
Theesterification of aceticacid withethanol wasalso stud-iedat 85◦C usingRHAPrMela.About73%conversionwith100% selectivity (ethyl acetate) was achieved in the esterification The higher conversion was obtained due to the strong basic character of the secondary amine in RHAPrMela compared to RHA-Sac The esterification of severalalcohols were also stud-ied over RHAPrMela The alcohols studied were 1-propanol (conversion=47%),1-butanol(conversion=42%),2-propanol (con-version=25%),tert-butanol(conversion=14%)andbenzylalcohol (conversion=20%).Theconversiongenerallydecreasedasthe rel-ativemolecularmassofthealcohol increased.Primaryalcohols alsoshowedahigherconversionratecomparedtothe2◦ orthe
3◦derivativesasshownforpropanolandbutanol.Thesevariations couldbedue tosteariceffects asdemonstrated for1-propanol, 2-propanol,tert-butanolandbenzylalcohol.However,itmustbe notedthatthesestudieswerenotcarriedoutattheoptimal con-ditionsfor therespectivealcohols,butrathertheconditionsfor ethanolwasused
5.4 Silicafromricehuskashimmobilizedwith 7-amino-1-naphthalenesulfonicacid
RHAwasfunctionalizedwith3-(chloropropyl)triethoxysilane and7-amino-1-naphthalenesulfonicacidtopreparea heteroge-neouscatalystfortheesterificationofn-butylalcoholwithdifferent mono-anddi-acidswithstrongBrønstedacidsites.Eventhough thesurface area of thecatalyst was only111m2g−1,it gave a conversion of 88% and 100% selectivity toward the ester The esterificationreactionwasproposedtotakeplaceattheterminal
SO3Hgroup.Thesulfonicgroupcanadsorbthecarboxylicacid andformaneightmemberedtransitionstateforsubsequentattack