Epoxidation of alkenes efficiently catalyzed by Mo salen supported on surface-modified halloysite nanotubes

Article abstract of DOI:10.1016/S1872-2067(14)60244-8

Halloysite-nanotube-supported Mo salen (HNTs-Mo-SL) catalysts were successfully prepared using a facile chemical surface modification and self-assembly method. The morphologies, sizes, structure, and dispersion of the as-prepared catalysts were investigated by transmission electron microscopy, X-ray diffraction, and Fourier-transform infrared, inductively coupled plasma, and X-ray photoelectron spectroscopy, which confirmed the existence of the Mo salen structure and successful synthesis of the HNTs-Mo-SL catalyst. The immobilized catalyst was found to be highly reactive in the epoxidation of a wide range of alkenes, including linear, cyclic, and aromatic alkenes. The immobilized catalyst exhibited a higher catalytic activity for alkene epoxidation than homogeneous Mo. In contrast experiments, it was determined that the salen structure played an important role in immobilizing MoO(O₂)₂(DMF)₂ and improving the conversion and efficiency of alkene epoxidation, which could not be obtained using other ligands, such as the N atom as a single ligand. Furthermore, the bonding between Mo and the salen ligands and the possible mechanism of alkene epoxidation catalyzed by the catalyst were determined. The catalyst could be reused several times without significant loss of catalytic activity. Given that halloysite nanotubes are cheap and easy to obtain, this catalyst offers a novel alternative for the rational design of catalysts with desired features.

Full text of DOI:10.1016/S1872-2067(14)60244-8

ChineseꢀJournalꢀofꢀCatalysisꢀ36ꢀ(2015)ꢀ348–354
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ꢀ ꢀ ꢀ
ꢀ ꢀ ꢀ ꢀ
ava i l a b l e a t w w w. s c i e n c ed i r e c t . c o m
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j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e /c h n j c
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ꢀ
Article
EpoxidationꢀofꢀalkenesꢀefficientlyꢀcatalyzedꢀbyꢀMoꢀsalenꢀsupportedꢀ
onꢀsurface‐modifiedꢀhalloysiteꢀnanotubesꢀ
YuꢀLong,ꢀBingꢀYuan,ꢀJiantaiꢀMaꢀ*
ꢀ
StateꢀKeyꢀLaboratoryꢀofꢀAppliedꢀOrganicꢀChemistryꢀandꢀCollegeꢀofꢀChemistryꢀandꢀChemicalꢀEngineering,ꢀLanzhouꢀUniversity,ꢀLanzhouꢀ730000,ꢀGansu,ꢀ
Chinaꢀ ꢀ
A R T I C L E ꢀ I N F O ꢀ
A B S T R A C T ꢀ
Articleꢀhistory:ꢀ
Halloysite‐nanotube‐supportedꢀMoꢀsalenꢀ(HNTs‐Mo‐SL)ꢀcatalystsꢀwereꢀsuccessfullyꢀpreparedꢀusingꢀ
aꢀfacileꢀchemicalꢀsurfaceꢀmodificationꢀandꢀself‐assemblyꢀmethod.ꢀTheꢀmorphologies,ꢀsizes,ꢀstructure,ꢀ
andꢀdispersionꢀofꢀtheꢀas‐preparedꢀcatalystsꢀwereꢀinvestigatedꢀbyꢀtransmissionꢀelectronꢀmicroscopy,ꢀ
X‐rayꢀdiffraction,ꢀandꢀFourier‐transformꢀinfrared,ꢀinductivelyꢀcoupledꢀplasma,ꢀandꢀX‐rayꢀphotoelec‐
tronꢀspectroscopy,ꢀwhichꢀconfirmedꢀtheꢀexistenceꢀofꢀtheꢀMoꢀsalenꢀstructureꢀandꢀsuccessfulꢀsynthesisꢀ
ofꢀtheꢀHNTs‐Mo‐SLꢀcatalyst.ꢀTheꢀimmobilizedꢀcatalystꢀwasꢀfoundꢀtoꢀbeꢀhighlyꢀreactiveꢀinꢀtheꢀepoxi‐
dationꢀofꢀaꢀwideꢀrangeꢀofꢀalkenes,ꢀincludingꢀlinear,ꢀcyclic,ꢀandꢀaromaticꢀalkenes.ꢀTheꢀimmobilizedꢀ
catalystꢀexhibitedꢀaꢀhigherꢀcatalyticꢀactivityꢀforꢀalkeneꢀepoxidationꢀthanꢀhomogeneousꢀMo.ꢀInꢀcon‐
trastꢀexperiments,ꢀitꢀwasꢀdeterminedꢀthatꢀtheꢀsalenꢀstructureꢀplayedꢀanꢀimportantꢀroleꢀinꢀimmobi‐
Receivedꢀ13ꢀSeptemberꢀ2014ꢀ
Acceptedꢀ3ꢀNovemberꢀ2014ꢀ
Publishedꢀ20ꢀMarchꢀ2015ꢀ
Keywords:ꢀ
Salenꢀmolybdenumꢀ
Alkeneꢀepoxidationꢀ
Halloysiteꢀnanotubeꢀ
Surfaceꢀmodificationꢀ
High‐efficiencyꢀcatalyst
lizingꢀMoO(O
2
)
2
(DMF) ꢀandꢀimprovingꢀtheꢀconversionꢀandꢀefficiencyꢀofꢀalkeneꢀepoxidation,ꢀwhichꢀ
2
couldꢀnotꢀbeꢀobtainedꢀusingꢀotherꢀligands,ꢀsuchꢀasꢀtheꢀNꢀatomꢀasꢀaꢀsingleꢀligand.ꢀFurthermore,ꢀtheꢀ
bondingꢀbetweenꢀMoꢀandꢀtheꢀsalenꢀligandsꢀandꢀtheꢀpossibleꢀmechanismꢀofꢀalkeneꢀepoxidationꢀcata‐
lyzedꢀbyꢀtheꢀcatalystꢀwereꢀdetermined.ꢀTheꢀcatalystꢀcouldꢀbeꢀreusedꢀseveralꢀtimesꢀwithoutꢀsignifi‐
cantꢀlossꢀofꢀcatalyticꢀactivity.ꢀGivenꢀthatꢀhalloysiteꢀnanotubesꢀareꢀcheapꢀandꢀeasyꢀtoꢀobtain,ꢀthisꢀcat‐
alystꢀoffersꢀaꢀnovelꢀalternativeꢀforꢀtheꢀrationalꢀdesignꢀofꢀcatalystsꢀwithꢀdesiredꢀfeatures.ꢀ
©ꢀ2015,ꢀDalianꢀInstituteꢀofꢀChemicalꢀPhysics,ꢀChineseꢀAcademyꢀofꢀSciences.
PublishedꢀbyꢀElsevierꢀB.V.ꢀAllꢀrightsꢀreserved.
1
.ꢀ ꢀ Introductionꢀ
difficultyꢀinꢀseparationꢀandꢀrecoveryꢀofꢀtheꢀcatalystsꢀfromꢀtheꢀ
reactionꢀproducts.ꢀOneꢀwayꢀtoꢀovercomeꢀtheseꢀproblemsꢀisꢀtoꢀ
immobilizeꢀhomogeneousꢀcatalystsꢀonꢀsolidꢀsupports.ꢀ
Theꢀepoxidationꢀofꢀalkenesꢀisꢀoneꢀofꢀtheꢀmostꢀwidelyꢀstudiedꢀ
reactionsꢀinꢀorganicꢀchemistry.ꢀTheꢀresultantꢀepoxidesꢀareꢀim‐
portantꢀ intermediatesꢀ inꢀ organicꢀ synthesisꢀ andꢀ essentialꢀ pre‐
cursorsꢀinꢀtheꢀsynthesisꢀofꢀvariousꢀimportantꢀsubstances,ꢀsuchꢀ
asꢀ plasticizers,ꢀ perfumes,ꢀ andꢀ epoxyꢀ resins.ꢀ Catalyzingꢀ theseꢀ
epoxidationꢀ reactionsꢀ byꢀ transitionꢀ metalꢀ complexesꢀ hasꢀ at‐
tractedꢀ muchꢀ attentionꢀ [1–5].ꢀ Aꢀ numberꢀ ofꢀ solubleꢀ Moꢀ com‐
plexesꢀwithꢀdifferentꢀligandsꢀhaveꢀbeenꢀsynthesizedꢀandꢀusedꢀ
asꢀ homogeneousꢀ catalystsꢀ forꢀ theꢀ epoxidationꢀ ofꢀ variousꢀ al‐
kenesꢀ [6,7].ꢀ However,ꢀ homogeneousꢀ catalystsꢀ haveꢀ someꢀ in‐
dustrialꢀproblems,ꢀsuchꢀasꢀdepositionꢀonꢀtheꢀreactorꢀwallꢀandꢀ
Inꢀ recentꢀ years,ꢀ variousꢀ typesꢀ ofꢀ supportedꢀ catalystsꢀ haveꢀ
beenꢀ reportedꢀ inꢀ organicꢀ transformations.ꢀ Manyꢀ approachesꢀ
haveꢀ beenꢀ usedꢀ toꢀ loadꢀ Moꢀ complexesꢀ onꢀ differentꢀ supports,ꢀ
suchꢀ asꢀ silicaꢀ [8–12],ꢀ modifiedꢀ MCM‐41ꢀ [13–16],ꢀ zeolitesꢀ
[17,18],ꢀandꢀmulti‐wallꢀcarbonꢀnanotubesꢀ(MWNTs)ꢀ[19,20],ꢀtoꢀ
obtainꢀ heterogeneousꢀ catalysts.ꢀ However,ꢀ muchꢀ cheaperꢀ clayꢀ
mineralsꢀhaveꢀnotꢀbeenꢀreportedꢀasꢀsupportsꢀforꢀMoꢀcomplexes.ꢀ
Clayꢀ mineralsꢀ areꢀ attractiveꢀ inorganicꢀ hostsꢀ forꢀ functionalꢀ in‐
organic–organicꢀ hybridsꢀ becauseꢀ ofꢀ theirꢀ swellingꢀ propertiesꢀ
andꢀchargedꢀsurfaces,ꢀwhichꢀcanꢀbeꢀeasilyꢀmodified.ꢀClaysꢀhaveꢀ
*ꢀCorrespondingꢀauthor.ꢀTel:ꢀ+86‐931‐8912577;ꢀFax:ꢀ+86‐931‐8912882;ꢀE‐mail:ꢀmajiantai@lzu.edu.cnꢀ
DOI:ꢀ10.1016/S1872‐2067(14)60244‐8ꢀ|ꢀhttp://www.sciencedirect.com/science/journal/18722067ꢀ|ꢀChin.ꢀJ.ꢀCatal.,ꢀVol.ꢀ36,ꢀNo.ꢀ3,ꢀMarchꢀ2015ꢀ ꢀ ꢀ

ꢀ
YuꢀLongꢀetꢀal.ꢀ/ꢀChineseꢀJournalꢀofꢀCatalysisꢀ36ꢀ(2015)ꢀ348–354ꢀ
349ꢀ
beenꢀ successfullyꢀ usedꢀ inꢀ differentꢀ fields,ꢀ evenꢀ asꢀ vectorsꢀ forꢀ
deliveryꢀofꢀDNAꢀtoꢀcellsꢀ[21–23].ꢀAsꢀaꢀnaturallyꢀabundantꢀclay,ꢀ
HO
O
O
O
O
O
O
O
O
O
O
O
H
H
H
C
Si
Si
N
N
Si
Si
NH2
NH2
halloysiteꢀnanotubesꢀ(HNTs,ꢀAl
2
Si
2
O
5
(OH)
4
·2H O)ꢀhaveꢀaꢀtubu‐
2
larꢀ structureꢀ similarꢀ toꢀ thatꢀ ofꢀ MWNTs,ꢀ andꢀ theyꢀ areꢀ cheap,ꢀ
non‐toxic,ꢀandꢀbiocompatible.ꢀTheꢀtubularꢀstructureꢀofꢀHNTsꢀisꢀ
composedꢀofꢀoneꢀsilicaꢀsheetꢀwithꢀoneꢀaluminaꢀsheetꢀonꢀeachꢀ
silicateꢀlayer,ꢀandꢀtheyꢀhaveꢀaꢀdiameterꢀofꢀ50ꢀnmꢀandꢀaꢀlumenꢀofꢀ
C2H5OH, reflux,10 h
O
O
O
H
C
HO
O
O
O
O
15ꢀ nmꢀ [23].ꢀ HNTsꢀ haveꢀ beenꢀ usedꢀ inꢀ variousꢀ fields,ꢀ suchꢀ asꢀ
O
Mo
O
O
nanocontainersꢀ forꢀ activeꢀ anticorrosionꢀ coatingsꢀ [28–31]ꢀ andꢀ
supportsꢀ forꢀ catalystsꢀ inꢀ differentꢀ typesꢀ ofꢀ reactionsꢀ [32–36],ꢀ
andꢀhaveꢀusefulꢀstructuralꢀpropertiesꢀ[24–27].ꢀ
H
O
O
Si
Si
N
C
MoO(O2)2(DMF)2
CH3CN, r.t., 48 h
O
O
O
Herein,ꢀweꢀreportꢀtheꢀsynthesisꢀofꢀMoꢀcatalystsꢀonꢀmodifiedꢀ
HNTꢀsupports.ꢀToꢀtheꢀbestꢀofꢀourꢀknowledge,ꢀtheꢀsynthesisꢀofꢀ
Moꢀsalenꢀ(Mo‐SL)ꢀsupportedꢀonꢀsurface‐modifiedꢀHNTsꢀhasꢀnotꢀ
beenꢀ previouslyꢀ reported.ꢀ Weꢀ investigatedꢀ theꢀ stabilityꢀ andꢀ
catalyticꢀ activityꢀ forꢀ epoxidationꢀ reactionsꢀ ofꢀ aꢀ varietyꢀ ofꢀ al‐
kenesꢀ withꢀ theꢀ tert‐butylꢀ hydroperoxideꢀ oxidant.ꢀ Inꢀ addition,ꢀ
weꢀinvestigatedꢀtheꢀcoordinationꢀbetweenꢀMoꢀandꢀsalenꢀligandsꢀ
andꢀtheꢀ possibleꢀmechanismꢀofꢀalkeneꢀepoxidationꢀ usingꢀthisꢀ
catalyst.ꢀ
H
C
N
O
O
O
Mo
O
O
O
Fig.ꢀ 1.ꢀ Synthesisꢀ processꢀ andꢀ structureꢀ ofꢀ theꢀ HNTs-APTMS-Mo-SLꢀ
catalyst.ꢀ
Fig.ꢀ 1.ꢀ Weꢀ usedꢀ AAPTMSꢀ insteadꢀ ofꢀ APTMSꢀ usingꢀ theꢀ sameꢀ
methodꢀtoꢀobtainꢀanotherꢀcatalyst,ꢀHNTs‐AAPTMS‐Mo‐SLꢀ(Fig.ꢀ
2
(a)).ꢀTheꢀcatalystꢀwithoutꢀtheꢀsalenꢀstructureꢀ(HNTs‐APTMS‐
ꢀ
2.ꢀ ꢀ Experimentalꢀ
Mo,ꢀFig.ꢀ2(b))ꢀwasꢀalsoꢀprepared.ꢀ ꢀ
2.1.ꢀ ꢀ Materialsꢀ
2.3.ꢀ ꢀ Characterizationꢀofꢀcatalystsꢀ
HalloysiteꢀwasꢀobtainedꢀfromꢀHebei,ꢀChina.ꢀ3‐
trimethoxysilaneꢀ (APTMS)ꢀ andꢀ [3‐(2‐aminoethyl)
ꢀ
Aminopropyl‐
ꢀ
Theꢀ sizeꢀ andꢀ morphologyꢀ ofꢀ theꢀ HNTsꢀ wasꢀ observedꢀ byꢀ aꢀ
Tecnaiꢀ G2ꢀ F30ꢀ transmissionꢀ electronꢀ microscopeꢀ (TEM).ꢀ Theꢀ
samplesꢀwereꢀobtainedꢀbyꢀplacingꢀaꢀdropꢀofꢀaꢀcolloidalꢀsolutionꢀ
ontoꢀaꢀcopperꢀgridꢀandꢀevaporatingꢀtheꢀsolventꢀinꢀairꢀatꢀroomꢀ
temperature.ꢀ X‐rayꢀ powderꢀ diffractionꢀ (XRD)ꢀ patternsꢀ wereꢀ
obtainedꢀwithꢀaꢀShimadzuꢀXRD‐6000ꢀdiffractometerꢀusingꢀCuꢀ
ꢀ
aminopro‐
pyl]trimethoxysilaneꢀ (AAPTMS)ꢀ wereꢀ purchasedꢀ fromꢀ Alfaꢀ
Aesarꢀandꢀusedꢀwithoutꢀfurtherꢀpurification.ꢀMolybdenumꢀtri‐
oxide,ꢀ N,N‐dimethylformamideꢀ (DMF),ꢀ tert‐butylꢀ hydroperox‐
ideꢀ (TBHP),ꢀ andꢀ solventsꢀ wereꢀ allꢀ analyticalꢀ gradeꢀ reagentsꢀ
purchasedꢀfromꢀShanghaiꢀChemicalꢀReagentꢀCo.ꢀLtd.ꢀ(Shanghai,ꢀ
China)ꢀandꢀusedꢀwithoutꢀfurtherꢀpurification.ꢀDeionizedꢀwaterꢀ
wasꢀusedꢀforꢀallꢀofꢀtheꢀexperiments.ꢀ ꢀ
K ꢀradiationꢀatꢀ40ꢀkV.ꢀElementalꢀanalysisꢀ(EA)ꢀwasꢀcarriedꢀoutꢀ
α
toꢀanalyzeꢀtheꢀelementalꢀcompositionꢀofꢀtheꢀcatalystsꢀusingꢀanꢀ
elementalꢀ analyzerꢀ (Varioꢀ EL,ꢀ Elementarꢀ Analysensystemeꢀ
GmbH,ꢀ Hanau,ꢀ Germany).ꢀ Fourier‐transformꢀ infraredꢀ (FT‐IR)ꢀ
spectraꢀwereꢀrecordedꢀonꢀanꢀImpactꢀ400ꢀFT‐IRꢀspectrometerꢀ
2
.2.ꢀ ꢀ Preparationꢀofꢀcatalystsꢀ
(Nicolet,ꢀWaltham,ꢀMA,ꢀUSA),ꢀandꢀtheꢀsamplesꢀwereꢀmeasuredꢀ
TheꢀHNTs‐SL‐Moꢀcatalystsꢀwereꢀpreparedꢀbyꢀaꢀfacileꢀchem‐
icalꢀ surfaceꢀ modificationꢀ andꢀ self‐assemblyꢀ method.ꢀ APTMSꢀ
0.5ꢀmL)ꢀandꢀHNTsꢀ(1ꢀg)ꢀwereꢀdispersedꢀinꢀtolueneꢀ(25ꢀmL).ꢀ
withꢀKBrꢀpellets.ꢀX‐rayꢀphotoelectronꢀspectraꢀ(XPS)ꢀwereꢀrec‐
ordedꢀonꢀaꢀPHI‐5702ꢀphotoelectronꢀspectrometer,ꢀandꢀtheꢀCꢀ1sꢀ
lineꢀatꢀ284.8ꢀeVꢀwasꢀusedꢀasꢀtheꢀbindingꢀenergyꢀreference.ꢀTheꢀ
Moꢀ contentꢀ wasꢀ measuredꢀ withꢀ anꢀ inductivelyꢀ coupledꢀ plas‐
ma‐atomicꢀemissionꢀspectrometerꢀ(ICP‐AES).ꢀ
(
Theꢀ systemꢀ wasꢀ vacuumizedꢀ inꢀ theꢀ processꢀ ofꢀ ultrasonicꢀ dis‐
persionꢀforꢀ1ꢀh.ꢀThen,ꢀrefluxingꢀwasꢀperformedꢀatꢀ110ꢀ°Cꢀforꢀ24ꢀ
hꢀinꢀaꢀnitrogenꢀenvironment.ꢀWeꢀobtainedꢀHNTs‐APTMSꢀafterꢀ
theꢀsolidꢀwasꢀfilteredꢀoutꢀandꢀdriedꢀunderꢀvacuumꢀafterꢀwash‐
ingꢀwithꢀtolueneꢀseveralꢀtimes.ꢀHNTs‐APTMSꢀ(0.5ꢀg)ꢀwasꢀdis‐
solvedꢀinꢀtepidꢀethanolꢀ(40ꢀmL).ꢀThen,ꢀwhileꢀstirringꢀatꢀ40ꢀ°C,ꢀ
ethanolꢀ (70ꢀ mL)ꢀ containingꢀ salicylaldehydeꢀ (0.55ꢀ mL)ꢀ wasꢀ
addedꢀinꢀ2ꢀh.ꢀThisꢀsystemꢀwasꢀrefluxedꢀatꢀ80ꢀ°Cꢀforꢀ10ꢀh.ꢀAfterꢀ
filtering,ꢀ washing,ꢀ andꢀ drying,ꢀ weꢀ obtainedꢀ aꢀ yellowꢀ solid,ꢀ
whichꢀwasꢀHNTsꢀsupportingꢀsalenꢀligandsꢀ(HNTs‐APTMS‐SL).ꢀAꢀ
2.4.ꢀ ꢀ CatalyticꢀepoxidationꢀofꢀalkenesꢀbyꢀTBHPꢀ
TheꢀepoxidationꢀofꢀalkenesꢀwasꢀcarriedꢀoutꢀusingꢀTBHPꢀasꢀ
mixtureꢀ ofꢀ HNTs‐APTMS‐SLꢀ (300ꢀ mg)ꢀ andꢀ MoO(O
2 2
) (DMF)
2
ꢀ
(30ꢀmg)ꢀsynthesizedꢀaccordingꢀtoꢀaꢀprocedureꢀdescribedꢀinꢀtheꢀ
literatureꢀ[37,38]ꢀwasꢀstirredꢀinꢀacetonitrileꢀ(15ꢀmL)ꢀatꢀroomꢀ
temperatureꢀforꢀ48ꢀh.ꢀAfterwards,ꢀtheꢀsolidꢀwasꢀfilteredꢀoutꢀandꢀ
driedꢀunderꢀvacuumꢀafterꢀwashingꢀwithꢀacetonitrile.ꢀThisꢀgaveꢀ
theꢀHNTs‐APTMS‐Mo‐SLꢀcatalyst.ꢀ ꢀ
Theꢀcompleteꢀsynthesisꢀprocessꢀofꢀtheꢀcatalystꢀisꢀshownꢀinꢀ
Fig.ꢀ2.ꢀStructureꢀofꢀHNTs‐AAPTMS‐Mo‐SLꢀ(a)ꢀandꢀHNTs‐APTMS‐Moꢀ(b).

350ꢀ
YuꢀLongꢀetꢀal.ꢀ/ꢀChineseꢀJournalꢀofꢀCatalysisꢀ36ꢀ(2015)ꢀ348–354ꢀ
anꢀ oxidizingꢀ agent.ꢀ Inꢀ aꢀ 25‐mLꢀ roundꢀ bottomꢀ flaskꢀ equippedꢀ
withꢀ aꢀ magneticꢀ stirrerꢀ bar,ꢀ aꢀ mixtureꢀ ofꢀ alkeneꢀ (2.5ꢀ mmol),ꢀ
TBHPꢀ(5ꢀmmol),ꢀcatalystꢀ(1ꢀmol%ꢀMo),ꢀandꢀsolventꢀ(8ꢀmL)ꢀwasꢀ
preparedꢀandꢀrefluxedꢀunderꢀstirringꢀatꢀdifferentꢀtemperatures.ꢀ
Afterꢀfilteringꢀoffꢀtheꢀcatalyst,ꢀtheꢀfiltrateꢀwasꢀdehydratedꢀandꢀ
analyzedꢀbyꢀaꢀgasꢀchromatograph–massꢀspectrometerꢀ(GC‐MS,ꢀ
Agilentꢀ7820A/5977E).ꢀ
*
*
*
*
*
*
*
(4)
(3)
*
*
*
(2)
3.ꢀ ꢀ Resultsꢀandꢀdiscussionꢀ
3.1.ꢀ ꢀ SynthesisꢀofꢀheterogeneousꢀHNTs‐supportedꢀMoꢀcatalystꢀ
(1)
TheꢀstructureꢀandꢀmorphologiesꢀofꢀtheꢀHNTs,ꢀHNTs‐APTMS‐
ꢀ
10
20
30
40
/( )
50
60
70
80
o
Mo‐SL,ꢀHNTs‐AAPTMS‐Mo‐SL,ꢀandꢀHNTs‐APTMS‐Moꢀwereꢀob‐
ꢀ
2
servedꢀ byꢀ TEM.ꢀ Fig.ꢀ 3(a)ꢀ showsꢀ theꢀ TEMꢀ imageꢀ ofꢀ naturallyꢀ
occurringꢀHNTs.ꢀTheꢀpredominantꢀformꢀwasꢀaꢀhollowꢀtubularꢀ
structureꢀ inꢀ theꢀ submicrometerꢀ range.ꢀ Theꢀ sizesꢀ ofꢀ theꢀ innerꢀ
andꢀouterꢀdiametersꢀofꢀtheꢀhalloysiteꢀtubulesꢀwereꢀaboutꢀ15–20ꢀ
andꢀ40–50ꢀnm,ꢀrespectively,ꢀdependingꢀonꢀtheꢀdepositꢀcondi‐
Fig.ꢀ4.ꢀXRDꢀpatternsꢀofꢀHNTsꢀ(1),ꢀHNTs‐APTMS‐SLꢀ(2),ꢀHNTs‐
SL‐Moꢀ(3),ꢀandꢀHNTs‐APTMS‐Moꢀ(4).ꢀ
ꢀ
APTMS‐
tiveꢀ intensitiesꢀ ofꢀ allꢀ theꢀ diffractionꢀ peaksꢀ ofꢀ HNTsꢀ matchedꢀ
wellꢀwithꢀJCPDSꢀ29‐1487ꢀ[36,37].ꢀTheꢀtypicalꢀdiffractionꢀpeaksꢀ
ofꢀ HNTs‐APTMS‐SLꢀ andꢀ HNTs‐APTMS‐Mo‐SLꢀ wereꢀ similarꢀ toꢀ
thoseꢀofꢀtheꢀunfunctionalizedꢀHNTs.ꢀHowever,ꢀtheꢀintensitiesꢀofꢀ
theꢀ mainꢀ peaksꢀ slightlyꢀ decreased,ꢀ whichꢀ revealedꢀ thatꢀ theꢀ
crystalꢀ structureꢀ ofꢀ theꢀ HNTsꢀ wasꢀ wellꢀ maintainedꢀ afterꢀ theꢀ
introductionꢀofꢀorganicꢀfunctionalꢀgroups.ꢀSomeꢀofꢀtheꢀcharac‐
teristicꢀ peaksꢀ decreasedꢀ afterꢀ immobilizationꢀ ofꢀ
tions.ꢀ Characterizationꢀ ofꢀ HNTs‐APTMS‐Mo‐SLꢀ byꢀ TEMꢀ isꢀ
ꢀ
shownꢀ inꢀ Fig.ꢀ 3(b).ꢀ Itꢀ showsꢀ thatꢀ theꢀ Moꢀ complexꢀ wasꢀ com‐
pletelyꢀattachedꢀtoꢀtheꢀmodifiedꢀHNTs,ꢀandꢀthereꢀwasꢀnoꢀiso‐
latedꢀ Moꢀ complexꢀ inꢀ theꢀ TEMꢀ image.ꢀ Therefore,ꢀ withoutꢀ anyꢀ
monodispersedꢀMoꢀ complex,ꢀitꢀcanꢀbeꢀ concludedꢀthatꢀ theꢀ Moꢀ
complexꢀwasꢀdistributedꢀonꢀtheꢀsurfaceꢀofꢀtheꢀsurface‐modifiedꢀ
HNTs.ꢀForꢀcomparison,ꢀtheꢀTEMꢀimagesꢀofꢀHNTs‐AAPTMS‐
ꢀ
Mo‐
ꢀ
MoO(O
2 2 2
) (DMF) .ꢀFurthermore,ꢀanotherꢀfiveꢀpeaksꢀoverꢀHNTs‐
ꢀ
SLꢀ andꢀ HNTs‐APTMS‐Moꢀ areꢀ shownꢀ inꢀ Figs.ꢀ 3(c)ꢀ andꢀ 3(d).ꢀ
HNTs‐AAPTMS‐Mo‐SLꢀshowedꢀslightꢀaggregationꢀbecauseꢀofꢀaꢀ
massꢀofꢀMoꢀcomplexꢀandꢀHNTs‐APTMS‐Moꢀshowedꢀlessꢀaggre‐
gationꢀbecauseꢀofꢀweakꢀcoordinationꢀbetweenꢀMoꢀandꢀsingleꢀNꢀ
atoms.ꢀ
APTMS‐SLꢀ andꢀ HNTs‐APTMS‐Mo‐SLꢀ (markedꢀ withꢀ asterisks)ꢀ
thatꢀ canꢀ beꢀ attributedꢀ toꢀ theꢀ Mo(VI)ꢀ speciesꢀ ratherꢀ thanꢀ
MoO(O
2
)
2
(DMF) .ꢀThisꢀisꢀevidenceꢀofꢀtheꢀsuccessfulꢀsynthesisꢀofꢀ
2
theꢀMoꢀsalenꢀstructure.ꢀFromꢀtheꢀaboveꢀresults,ꢀHNTs‐APTMS‐
ꢀ
Mo‐SLꢀwasꢀsuccessfullyꢀsynthesized.ꢀInꢀaddition,ꢀtheꢀpeaksꢀat‐
tributedꢀtoꢀtheꢀMo(VI)ꢀspeciesꢀofꢀHNTs‐APTMS‐Moꢀwereꢀclearlyꢀ
smallerꢀ thanꢀ thoseꢀ ofꢀ HNTs‐APTMS‐Mo‐SL,ꢀ whichꢀ mightꢀ beꢀ
becauseꢀofꢀtheꢀsmallerꢀamountꢀofꢀMoꢀ(Tableꢀ2).ꢀ
TheꢀcrystalꢀstructureꢀandꢀphaseꢀpurityꢀofꢀtheꢀHNTs,ꢀHNTs‐
APTMS‐SL,ꢀ HNTs‐APTMS‐Mo‐SL,ꢀ andꢀ HNTs‐APTMS‐Moꢀ wereꢀ
determinedꢀbyꢀXRD,ꢀasꢀshownꢀinꢀFig.ꢀ4.ꢀTheꢀpositionsꢀandꢀrela‐
ꢀ
TheꢀFT‐IRꢀspectraꢀofꢀtheꢀsamplesꢀareꢀshownꢀinꢀFig.ꢀ5.ꢀTwoꢀ
−
1
characteristicꢀbandsꢀofꢀHNTsꢀatꢀ3696ꢀandꢀ3621ꢀcm ꢀwereꢀpre‐
sent,ꢀ whichꢀ areꢀ dueꢀ toꢀ theꢀ stretchingꢀ vibrationꢀ ofꢀ O–Hꢀ inꢀ theꢀ
inner‐surfaceꢀhydroxylꢀgroupsꢀofꢀAl–O–H.ꢀTheꢀabsorptionꢀpeakꢀ
(
)
( )
−1
atꢀ910ꢀcm ꢀwasꢀascribedꢀtoꢀtheꢀdeformationꢀvibrationꢀofꢀtheꢀ
aboveꢀ hydroxylꢀ groups.ꢀ Interlayerꢀ orꢀ adsorbedꢀ waterꢀ isꢀ indi‐
−1
catedꢀbyꢀtheꢀstretchingꢀvibrationꢀatꢀ3450ꢀcm ꢀandꢀtheꢀcorre‐
−1
spondingꢀdeformationꢀvibrationꢀatꢀ1637ꢀcm ꢀ[36,37].ꢀMostꢀofꢀ
theꢀabsorptionꢀpeaksꢀofꢀtheꢀcatalystsꢀdidꢀnotꢀchangeꢀafterꢀmodi‐
fication,ꢀ exceptꢀ forꢀ aꢀ decreaseꢀ inꢀ theꢀ intensityꢀ ofꢀ theꢀ OHꢀ
stretchingꢀband.ꢀItꢀwasꢀfoundꢀthatꢀtheꢀbasicꢀstructuresꢀofꢀtheꢀ
HNTsꢀandꢀtheꢀfunctionalizedꢀHNTsꢀwereꢀtheꢀsameꢀafterꢀmodifi‐
cationꢀ andꢀ graftingꢀ betweenꢀ theseꢀ groupsꢀ andꢀ APTMSꢀ (orꢀ
AAPTMS).ꢀ Theꢀ relativelyꢀ highꢀ intensityꢀ ofꢀ theꢀ bandꢀ atꢀ 2933ꢀ
(
c)
(d)
−1
2
cm ꢀisꢀcharacteristicꢀofꢀ–CH ꢀstretchingꢀvibrations,ꢀwhichꢀalsoꢀ
indicatedꢀthatꢀAPTMSꢀ(orꢀAAPTMS)ꢀsuccessfullyꢀbondedꢀtoꢀtheꢀ
−1
HNTs.ꢀOverꢀHNTs‐APTMS‐SL,ꢀtheꢀbandsꢀatꢀ1660ꢀandꢀ1251ꢀcm ꢀ
ꢀ
wereꢀ inducedꢀ byꢀ C=Nꢀ andꢀ C–Oꢀ stretchingꢀ vibrations,ꢀ whichꢀ
verifiedꢀtheꢀsuccessfulꢀsynthesisꢀofꢀtheꢀsalenꢀligand.ꢀItꢀisꢀworthꢀ
notingꢀthatꢀtheꢀC=NꢀandꢀC–Oꢀstretchingꢀvibrationsꢀinꢀtheꢀspec‐
traꢀ ofꢀ HNTs‐APTMS‐Mo‐SLꢀ andꢀ HNTs‐AAPTMS‐Mo‐SLꢀ wereꢀ
Fig.ꢀ 3.ꢀ TEMꢀ imagesꢀ ofꢀ HNTsꢀ (a),ꢀ HNTs‐APTMS‐Mo‐SLꢀ (b),ꢀ HNTs‐
AAPTMS‐Mo‐SLꢀ(c),ꢀandꢀHNTs‐APTMS‐Moꢀ(d).ꢀ
similarꢀtoꢀthoseꢀofꢀHNTs‐APTMS‐SL,ꢀindicatingꢀaꢀstrongꢀinter‐
ꢀ

ꢀ
YuꢀLongꢀetꢀal.ꢀ/ꢀChineseꢀJournalꢀofꢀCatalysisꢀ36ꢀ(2015)ꢀ348–354ꢀ
Tableꢀ1ꢀ ꢀ ꢀ
351ꢀ
Elementalꢀanalysisꢀdataꢀofꢀtheꢀcatalysts.ꢀ
(5)
(4)
(3)
Catalystꢀ
Cꢀ(%)ꢀ
ꢀ ꢀ 3.62ꢀ
ꢀ ꢀ 4.79ꢀ
ꢀ ꢀ 9.27ꢀ
11.3ꢀ
Nꢀ(%)ꢀ
1.31ꢀ
2.76ꢀ
1.23ꢀ
2.69ꢀ
Hꢀ(%)ꢀ
HNTs‐APTMSꢀ
0.84ꢀ
1.10ꢀ
1.02ꢀ
1.48ꢀ
HNTs‐AAPTMSꢀ
HNTs‐APTMS‐SLꢀ
HNTs‐AAPTMS‐Mo‐SLꢀ
1251 C-O
3.2.ꢀ ꢀ CatalyticꢀepoxidationꢀofꢀalkenesꢀusingꢀTBHPꢀasꢀanꢀoxidantꢀ
(2)
2
933-CH
2
1660 C=N
TheꢀcatalyticꢀactivitiesꢀofꢀtheꢀthreeꢀHNT‐supportedꢀMoꢀcata‐
lystsꢀandꢀhomogeneousꢀMoꢀcatalystꢀwereꢀinitiallyꢀinvestigatedꢀ
inꢀtheꢀepoxidationꢀofꢀcyclohexeneꢀinꢀtheꢀpresenceꢀofꢀTBHP.ꢀTheꢀ
catalyticꢀ activitiesꢀ ofꢀ theseꢀ fourꢀ catalystsꢀ asꢀ wellꢀ asꢀ PANIꢀ mi‐
(
1)
1637 O-H
O-H
500 3000 2500 2000 1500 1000 500
9
10 O-H
crosphere‐Moꢀ[38]ꢀandꢀMoO
x
/HAP‐γ‐Fe
2
O ꢀ[11]ꢀareꢀshownꢀinꢀ
3
3
Tableꢀ2.ꢀTheꢀsameꢀamountꢀofꢀMoꢀwasꢀusedꢀinꢀtheꢀcontrastꢀtests.ꢀ
Theꢀ turnoverꢀ frequencyꢀ (TOF)ꢀ ofꢀ HNTs‐APTMS‐Mo‐SLꢀ wasꢀ
1
Wavenumber (cm )
Fig.ꢀ5.ꢀFT‐IRꢀspectraꢀofꢀHNTsꢀ(1),ꢀHNTs‐APTMS‐Moꢀ(2),ꢀHNTs‐
SLꢀ(3),ꢀHNTs‐APTMS‐Mo‐SLꢀ(4),ꢀandꢀHNTs‐AAPTMS‐Mo‐SLꢀ(5).ꢀ
−
1
ꢀ
APTMS‐
quiteꢀ goodꢀ (23.75ꢀ h ),ꢀ andꢀ twiceꢀ thatꢀ ofꢀ MoO
/HAP‐γ‐Fe O ꢀ
x 2 3
−1
(
10.94ꢀh ).ꢀHowever,ꢀitꢀwasꢀslightlyꢀlessꢀthanꢀthatꢀofꢀPANIꢀmi‐
−1
crosphere‐Moꢀ(27.67ꢀh ),ꢀwhichꢀcanꢀbeꢀattributedꢀtoꢀtheꢀhigherꢀ
reactionꢀtemperatureꢀ(60ꢀ°C).ꢀNevertheless,ꢀtheꢀconversionꢀofꢀ
cyclohexeneꢀ catalyzedꢀ byꢀ theꢀ HNTs‐APTMS‐Mo‐SLꢀ affordedꢀ
95%,ꢀwhichꢀisꢀmuchꢀhigherꢀthanꢀthatꢀobtainedꢀusingꢀtheꢀPANIꢀ
actionꢀbetweenꢀMoꢀandꢀtheꢀONꢀbidentateꢀsalenꢀligand.ꢀThere‐
fore,ꢀ FT‐IRꢀ reasultsꢀ confirmedꢀ theꢀ existenceꢀ ofꢀ theꢀ Moꢀ salenꢀ
structureꢀbyꢀtheꢀpost‐treatmentꢀprocess.ꢀ
Inꢀ additionꢀ toꢀ XRDꢀ andꢀ FT‐IRꢀ analysis,ꢀ theꢀ XPSꢀ techniqueꢀ
provedꢀ toꢀ beꢀ extremelyꢀ usefulꢀ toꢀ confirmꢀ theꢀ presenceꢀ ofꢀ
APTMSꢀandꢀMo,ꢀbecauseꢀtheyꢀareꢀsensitiveꢀtoꢀtheꢀsurfaceꢀandꢀ
provideꢀinformationꢀnotꢀonlyꢀaboutꢀtheꢀelectronicꢀstateꢀofꢀMo,ꢀ
butꢀalsoꢀaboutꢀtheꢀotherꢀatoms.ꢀAsꢀshownꢀinꢀFig.ꢀ6,ꢀtheꢀpeaksꢀofꢀ
Siꢀ 2p,ꢀ Cꢀ 1s,ꢀ Nꢀ 1s,ꢀ andꢀ Oꢀ 1sꢀ appearedꢀ atꢀ standardꢀ positions.ꢀ
Moreover,ꢀtheꢀXPSꢀanalysisꢀofꢀMo(VI)ꢀisꢀshownꢀinꢀFig.ꢀ6(a)ꢀandꢀ
microsphere‐Moꢀ (83%)ꢀ andꢀ MoO
lystsꢀ despiteꢀ theirꢀ higherꢀ reactionꢀ temperatures.ꢀ PANIꢀ micro‐
sphere‐MoꢀdoesꢀnotꢀhaveꢀtheꢀMoꢀsalenꢀstructureꢀandꢀonlyꢀusesꢀ
x
/HAP‐γ‐Fe O3ꢀ(90.8%)ꢀ cata‐
2
aꢀNꢀatomꢀasꢀaꢀsingleꢀligandꢀcoordinatedꢀtoꢀMoO(O
2
)(DMF) .ꢀTheꢀ
2
activeꢀsiteꢀofꢀMoO /HAP‐γ‐Fe ꢀisꢀtheꢀMoꢀoxide.ꢀTherefore,ꢀitꢀ
x
O
2 3
canꢀ beꢀ assumedꢀ thatꢀ theꢀ Moꢀ salenꢀ structureꢀ isꢀ conduciveꢀ toꢀ
improvingꢀtheꢀconversionꢀandꢀefficiencyꢀofꢀalkeneꢀepoxidation.ꢀ
Onꢀtheꢀotherꢀhand,ꢀbyꢀcomparingꢀtheꢀconversionꢀofꢀtheꢀepoxi‐
dationꢀ reactionꢀ catalyzedꢀ byꢀ HNTs‐APTMS‐Mo‐SLꢀ (95%),ꢀ
HNTs‐APTMS‐Moꢀ (87%)ꢀ withoutꢀ theꢀ salenꢀ structure,ꢀ andꢀ
HNTs‐APPTMS‐Mo‐SLꢀ(90%),ꢀwhichꢀhasꢀbothꢀMoꢀsalenꢀandꢀMoꢀ
coordinatedꢀ toꢀ oneꢀ Nꢀ atom,ꢀ itꢀ isꢀ suggestedꢀ thatꢀ theꢀ Moꢀ salenꢀ
structureꢀofꢀtheꢀcatalystsꢀplayedꢀanꢀimportantꢀroleꢀinꢀthisꢀreac‐
(
b).ꢀAsꢀexpected,ꢀtheꢀpeaksꢀatꢀ237ꢀandꢀ403ꢀeVꢀcanꢀbeꢀattributedꢀ
toꢀMoꢀ3dꢀandꢀMoꢀ3p,ꢀwhichꢀconfirmedꢀtheꢀexistenceꢀofꢀMo(VI)ꢀ
withꢀtheꢀsameꢀvalenceꢀstateꢀasꢀMoꢀinꢀMoO(O
(DMF) ꢀ[38].ꢀTheꢀ
)
2 2
2
XPSꢀ reasultsꢀ providedꢀ evidenceꢀ ofꢀ theꢀ successfulꢀ synthesisꢀ ofꢀ
theꢀHNT‐supportedꢀMoꢀcatalyst.ꢀ
Inꢀaddition,ꢀEAꢀwasꢀusedꢀtoꢀverifyꢀtheꢀpresenceꢀofꢀC,ꢀN,ꢀandꢀ
H.ꢀAsꢀshownꢀinꢀTableꢀ1,ꢀafterꢀtheꢀfirstꢀstep,ꢀAPTMSꢀandꢀAPPTMSꢀ
wereꢀsuccessfullyꢀattachedꢀtoꢀtheꢀHNTsꢀbecauseꢀofꢀtheꢀpresenceꢀ
ofꢀ theꢀ correspondingꢀ percentagesꢀ ofꢀ C,ꢀ N,ꢀ andꢀ H.ꢀ Salicylalde‐
tion.ꢀMoO(O
)ꢀconnectedꢀtoꢀtheꢀsalenꢀstructureꢀprobablyꢀhadꢀaꢀ
2
uniqueꢀmechanismꢀthatꢀwasꢀtheꢀmostꢀefficientꢀcatalyticꢀmecha‐
nismꢀ forꢀ epoxidationꢀ ofꢀ alkenesꢀ withꢀ theꢀ TBHPꢀ oxidant.ꢀ Ac‐
cordingꢀtoꢀtheꢀreportedꢀmechanismꢀ[39],ꢀtheꢀpossibleꢀepoxida‐
tionꢀprocessꢀcatalyzedꢀbyꢀtheꢀMoꢀsalenꢀcatalystꢀisꢀillustratedꢀinꢀ
hydeꢀ thenꢀ reactedꢀ withꢀ theꢀ terminalꢀ aminoꢀ groupꢀ ofꢀ HNTs‐
ꢀ
APTMSꢀorꢀHNTs‐APPTMSꢀtoꢀformꢀtheꢀsalenꢀstructure.ꢀTheꢀcor‐
respondingꢀ percentagesꢀ ofꢀ C,ꢀ N,ꢀ andꢀ Hꢀ wereꢀ detectedꢀ inꢀ ac‐
cordanceꢀwithꢀHNTs‐APTMS‐SLꢀorꢀHNTs‐APPTMS‐SL.ꢀ
Schemeꢀ1.ꢀHeterogeneousꢀMoO(O
2
)(DMF) ꢀhadꢀaꢀlowerꢀcatalyt‐
2
icꢀactivityꢀ(73%)ꢀthanꢀsupportedꢀMo.ꢀWhenꢀtheꢀreactionꢀtimeꢀ
50000
40000
30000
20000
10000
0
28000
30000
O 1s
Mo 3d
Mo 3p
26000
24000
22000
OKLL
2
8000
26000
4000
22000
0000
18000
C 1s
Mo 3p
N 1s
Mo 3d
20000
2
18000
16000
14000
12000
Si 2p
2
(c)
(a)
(b)
10000
0
200 400 600 800 1000
Binding energy (eV)
220 225 230 235 240 245 250 255
Binding energy (eV)
385 390 395 400 405 410 415 420
Binding energy (eV)
ꢀ
Fig.ꢀ6.ꢀXPSꢀspectrumꢀofꢀHNTs‐APTMS‐Mo‐SLꢀ(a)ꢀandꢀtheꢀspecificꢀMoꢀXPSꢀspectraꢀofꢀHNTs‐APTMS‐Mo‐SLꢀ(b,ꢀc).ꢀ

352ꢀ
YuꢀLongꢀetꢀal.ꢀ/ꢀChineseꢀJournalꢀofꢀCatalysisꢀ36ꢀ(2015)ꢀ348–354ꢀ
Tableꢀ2ꢀ ꢀ ꢀ
EpoxidationꢀofꢀcyclohexeneꢀtoꢀtheꢀcorrespondingꢀepoxideꢀwithꢀTBHPꢀusingꢀdifferentꢀcatalysts.ꢀ
ꢀ
aꢀ −1
Catalystꢀ
Amountꢀ(mg)ꢀ
ꢀ 61ꢀ
Moꢀ(%)ꢀ
ꢀ 4.11ꢀ
ꢀ 4.98ꢀ
ꢀ 2.63ꢀ
28.90ꢀ
28.90ꢀ
28.90ꢀ
ꢀ 7.35ꢀ
20ꢀ
Timeꢀ(h)ꢀ
ꢀ 4ꢀ
Conversionꢀ(%)ꢀ
Selectivityꢀ(%)ꢀ
TOFꢀ (h )ꢀ
HNTs‐APTMS‐Mo‐SLꢀ
HNTs‐AAPTMS‐Mo‐SLꢀ
HNTs‐APTMS‐Moꢀ
95ꢀ
90ꢀ
87ꢀ
73ꢀ
74ꢀ
76ꢀ
83ꢀ
ꢀ ꢀ 90.8ꢀ
96ꢀ
95ꢀ
96ꢀ
94ꢀ
97ꢀ
95ꢀ
97ꢀ
ꢀ ꢀ 98.7ꢀ
23.75ꢀ
22.50ꢀ
21.75ꢀ
18.25ꢀ
ꢀ 9.25ꢀ
ꢀ 6.33ꢀ
27.67ꢀ
10.94ꢀ
ꢀ 50ꢀ
ꢀ 94ꢀ
ꢀ ꢀ 9ꢀ
ꢀ ꢀ 9ꢀ
ꢀ ꢀ 9ꢀ
120ꢀ
ꢀ 25ꢀ
ꢀ 4ꢀ
ꢀ 4ꢀ
ꢀ 4ꢀ
ꢀ 8ꢀ
12ꢀ
ꢀ 6ꢀ
ꢀ 8ꢀ
MoO(O
MoO(O
MoO(O
2
)
2
)
2
)
2
2
2
(DMF)2ꢀ
(DMF)2ꢀ
(DMF)2ꢀ
ꢀbꢀ
PANIꢀmicrosphere‐Mo
ꢀc
MoO
x
/HAP‐γ‐Fe
2
O
3
ꢀ
ꢀ
aꢀ
Turnoverꢀfrequency,ꢀi.e.,ꢀmolesꢀofꢀcyclohexeneꢀconvertedꢀperꢀmoleꢀofꢀMoꢀperꢀhour.ꢀ
bꢀ
cꢀ
Ref.ꢀ[38].ꢀReactionꢀconditions:ꢀcyclohexeneꢀ(1.84ꢀmL),ꢀTBHPꢀ(4.8ꢀmL),ꢀCHCl
Ref.ꢀ[11].ꢀReactionꢀconditions:ꢀcyclohexeneꢀ(5ꢀmmol),ꢀTBHPꢀ(5ꢀmmol),ꢀCHCl
3
ꢀ(8ꢀmL),ꢀ60ꢀ°C.ꢀ
ꢀ(8ꢀmL),ꢀ70ꢀ°C.ꢀ
3
O
O
O
Tableꢀ3ꢀshowsꢀtheꢀcatalyticꢀactivityꢀofꢀHNTs‐APTMS‐Mo‐SLꢀ
usedꢀ forꢀ theꢀ epoxidationꢀ ofꢀ aꢀ wideꢀ rangeꢀ ofꢀ alkenes.ꢀ Itꢀ wasꢀ
foundꢀthatꢀthisꢀcatalystꢀefficientlyꢀconvertedꢀcyclic,ꢀlinear,ꢀandꢀ
aromaticꢀ alkenesꢀ toꢀ theirꢀ correspondingꢀ epoxides.ꢀ Itꢀ showedꢀ
thatꢀcarbocyclicꢀalkenesꢀwereꢀefficientlyꢀconvertedꢀtoꢀtheꢀcor‐
respondingꢀ epoxidesꢀ withꢀ highꢀ conversionꢀ andꢀ TOFꢀ (Tableꢀ 3,ꢀ
entriesꢀ 1ꢀ andꢀ 2).ꢀ However,ꢀ oxidationꢀ ofꢀ aromaticꢀ andꢀ linearꢀ
alkenesꢀ requiredꢀ longerꢀ reactionꢀ timesꢀ andꢀ higherꢀ reactionꢀ
temperatureꢀthanꢀcarbocyclicꢀalkenesꢀ(entriesꢀ3–7).ꢀ
O
Mo
O
O
O
O
H
Si
N
C
O
3 3
(CH ) COH
3 3
(CH ) COOH
O
H
N
H
(
H
3
C)
3
O
3 3
(H C) O
O
O
O
O
O
O
O
O
O
O
Mo
O
O
Mo
Experimentsꢀwereꢀconductedꢀtoꢀtestꢀtheꢀcycleꢀperformanceꢀ
andꢀ theꢀ stabilityꢀ ofꢀ theꢀ catalyst.ꢀ Asꢀ shownꢀ inꢀ Fig.ꢀ 7,ꢀ theꢀ
HNTs‐APTMS‐Mo‐SLꢀ catalystꢀ couldꢀ beꢀ filteredꢀ andꢀ reusedꢀ atꢀ
leastꢀeightꢀtimesꢀwithoutꢀsignificantꢀlossꢀofꢀactivityꢀ(Fig.ꢀ7(a)).ꢀ
ToꢀinvestigateꢀtheꢀstabilityꢀofꢀtheꢀMoꢀsalenꢀstructure,ꢀtheꢀcata‐
lyticꢀperformanceꢀofꢀtheꢀseparatedꢀliquidsꢀwasꢀtestedꢀbyꢀaddingꢀ
freshꢀ cyclohexeneꢀ andꢀ TBHPꢀ intoꢀ theꢀ filtratesꢀ afterꢀ eachꢀ run.ꢀ
Epoxidationꢀreactionsꢀunderꢀtheꢀsameꢀreactionꢀconditionsꢀgaveꢀ
theꢀsameꢀresultsꢀasꢀtheꢀblankꢀexperiment.ꢀTheꢀperformanceꢀofꢀ
theꢀHNTs‐APTMS‐Moꢀcatalystsꢀdecreasedꢀduringꢀtheꢀreuseꢀex‐
perimentsꢀ(Fig.ꢀ7(b)).ꢀAꢀtotalꢀofꢀ0.67ꢀmgꢀMoꢀ(27%ꢀofꢀtheꢀorigi‐
nalꢀ Moꢀ amount)ꢀ wasꢀ detectedꢀ byꢀ ICPꢀ inꢀ theꢀ separatedꢀ liquidꢀ
afterꢀ theꢀ eighthꢀ cycle,ꢀ whichꢀ explainedꢀ theꢀ sharpꢀ decreaseꢀ inꢀ
catalyticꢀactivity.ꢀHowever,ꢀtheꢀseparatedꢀliquidꢀafterꢀtheꢀeighthꢀ
cycleꢀcatalyzedꢀbyꢀHNTs‐APTMS‐Mo‐SLꢀcontainedꢀ0.09ꢀmgꢀMoꢀ
O
O
O
O
O
O
H
H
Si
C
O
O
Si
N
C
Schemeꢀ1.ꢀEpoxidationꢀprocessꢀcatalyzedꢀbyꢀHNTs‐APTMS‐Mo‐SL.ꢀ
wasꢀ extendedꢀ toꢀ 12ꢀ h,ꢀ conversionꢀ slightlyꢀ increasedꢀ toꢀ 76%.ꢀ
TheꢀreactionꢀcatalyzedꢀbyꢀHNTs‐APTMS‐Moꢀhadꢀhigherꢀconver‐
sionꢀ withꢀ lowerꢀ temperatureꢀ andꢀ shorterꢀ reactionꢀ timeꢀ thanꢀ
thatꢀ reportedꢀ forꢀ theꢀ PANIꢀ microsphere‐Moꢀ catalyst.ꢀ Hence,ꢀ
supportedꢀ MoO(O
2
)(DMF) ꢀ isꢀ anꢀ excellentꢀ catalystꢀ forꢀ alkeneꢀ
2
epoxidationꢀ reaction.ꢀ Amongꢀ theꢀ catalystsꢀ investigated,ꢀ theꢀ
HNT‐supportedꢀMoꢀcatalystsꢀcoordinatedꢀwithꢀligandsꢀshowedꢀ
theꢀbestꢀcatalyticꢀactivity,ꢀandꢀHNT‐supportedꢀMoꢀsalenꢀwasꢀtheꢀ
bestꢀcatalyst.ꢀ
(
3.6%ꢀ ofꢀ theꢀ originalꢀ Moꢀ amount).ꢀ Theꢀ experimentsꢀ demon‐
Tableꢀ3ꢀ ꢀ
stratedꢀtheꢀexcellentꢀstabilityꢀofꢀtheꢀMoꢀsalenꢀstructure.ꢀ
EpoxidationꢀofꢀvariousꢀalkenesꢀwithꢀTBHPꢀcatalyzedꢀbyꢀHNTs‐APTMS‐
ꢀ
Mo‐SL.ꢀ
4
.ꢀ ꢀ Conclusionsꢀ
Con‐ Selec‐
TOF
Entryꢀ
Alkeneꢀ
t/hꢀ Solventꢀ T/°Cꢀ versionꢀ tivityꢀ
−1
(h )
Aꢀrobust,ꢀhighlyꢀeffective,ꢀandꢀreusableꢀHNTs‐Mo‐SLꢀcatalystꢀ
(%)ꢀ
(%)ꢀ
wasꢀ successfullyꢀ synthesized,ꢀ characterized,ꢀ andꢀ usedꢀ forꢀ theꢀ
epoxidationꢀofꢀcycloocteneꢀusingꢀtheꢀTBHPꢀoxidant.ꢀThisꢀsup‐
portedꢀheterogeneousꢀcatalystꢀwasꢀfoundꢀtoꢀbeꢀhighlyꢀeffectiveꢀ
forꢀtheꢀepoxidationꢀofꢀaꢀwideꢀrangeꢀofꢀalkenes,ꢀincludingꢀlinear,ꢀ
cyclic,ꢀandꢀaromaticꢀalkenes.ꢀTheꢀHNTs‐Mo‐SLꢀcatalystꢀexhibit‐
edꢀaꢀhigherꢀactivityꢀthanꢀhomogeneousꢀMoꢀandꢀsupportedꢀMoꢀ
withoutꢀ theꢀ salenꢀ structureꢀ (HNTs‐APTMS‐Mo).ꢀ Itꢀ wasꢀ foundꢀ
thatꢀtheꢀsalenꢀstructureꢀplayedꢀanꢀimportantꢀroleꢀinꢀimmobiliz‐
1
ꢀ
ꢀ 4ꢀ CH
2
Cl
2
2
ꢀ 40ꢀ ꢀ 95ꢀ
ꢀ 40ꢀ >99ꢀ
96ꢀ 23.75
98ꢀ 25.0
ꢀ
ꢀ
2ꢀ
ꢀ 4ꢀ CH
2
Cl
3ꢀ
ꢀ 6ꢀ CH
ꢀ 6ꢀ CH
ꢀ 6ꢀ CH
ꢀ 8ꢀ CH
3
3
3
3
CN ꢀ 80ꢀ ꢀ 87ꢀ
CN ꢀ 80ꢀ ꢀ 89ꢀ
CN ꢀ 80ꢀ ꢀ 83ꢀ
CN ꢀ 80ꢀ ꢀ 80ꢀ
90ꢀ 14.50
91ꢀ 14.83
90ꢀ 13.83
93ꢀ 10.0ꢀ
ꢀ
4ꢀ
ꢀ
5ꢀ
ꢀ
6ꢀ
ꢀ
7ꢀ
12ꢀ tolueneꢀ 110ꢀ ꢀ 91ꢀ
99ꢀ 7.58
ꢀ
ingꢀ MoO(O
2 2 2
) (DMF) ꢀ andꢀ improvingꢀ theꢀ conversionꢀ andꢀ effi‐
Reactionꢀconditions:ꢀalkeneꢀ(2.5ꢀmmol),ꢀTBHPꢀ(5ꢀmmol),ꢀHNTs‐APTMS‐
ꢀ
Mo‐SLꢀ(60ꢀmg),ꢀsolventꢀ(8ꢀmL).ꢀ
ciencyꢀofꢀalkeneꢀepoxidation,ꢀwhichꢀcannotꢀbeꢀobtainedꢀusingꢀ

ꢀ
YuꢀLongꢀetꢀal.ꢀ/ꢀChineseꢀJournalꢀofꢀCatalysisꢀ36ꢀ(2015)ꢀ348–354ꢀ
353ꢀ
1
00
100
95
90
85
80
75
70
100
100
90
80
70
60
50
9
9
8
8
7
7
5
0
5
0
5
0
90
80
70
60
(
a)
(
b)
50
0
2
4
6
8
0
2
4
6
8
Run number
Run number
Fig.ꢀ7.ꢀCatalystꢀ reusabilityꢀforꢀ HNTs‐APTMS‐Mo‐SLꢀ (a)ꢀandꢀ HNTs‐APTMS‐Moꢀ(b).ꢀReactionꢀconditions:ꢀcyclohexeneꢀ(2.5ꢀmmol),ꢀTBHPꢀ(5ꢀmmol),ꢀ
HNTs‐APTMS‐Mo‐SLꢀ(60ꢀmg),ꢀHNTs‐APTMS‐Moꢀ(95ꢀmg),ꢀsolventꢀ(8ꢀmL),ꢀ40ꢀ°C.ꢀ
otherꢀligands,ꢀsuchꢀasꢀtheꢀNꢀatomꢀasꢀaꢀsingleꢀligand.ꢀFurther‐
more,ꢀtheꢀcoordinationꢀbetweenꢀMoꢀandꢀtheꢀsalenꢀligandꢀandꢀ
theꢀpossibleꢀmechanismꢀofꢀalkeneꢀepoxidationꢀcatalyzedꢀbyꢀthisꢀ
catalystꢀwereꢀinvestigated.ꢀMoreover,ꢀtheꢀcatalystꢀhadꢀexcellentꢀ
stabilityꢀ andꢀ couldꢀ beꢀ easilyꢀ recycledꢀ inꢀ theꢀ reactionꢀ system.ꢀ
Givenꢀthatꢀhalloysiteꢀnanotubesꢀareꢀcheapꢀandꢀeasyꢀtoꢀobtain,ꢀ
thisꢀcatalystꢀoffersꢀaꢀnovelꢀalternativeꢀforꢀtheꢀrationalꢀdesignꢀofꢀ
catalystsꢀwithꢀdesiredꢀfeatures.ꢀ
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EpoxidationꢀofꢀalkenesꢀefficientlyꢀcatalyzedꢀbyꢀMoꢀsalenꢀsupportedꢀonꢀ ꢀ
surface‐modifiedꢀhalloysiteꢀnanotubesꢀ ꢀ ꢀ
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O
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ꢀ
O
Mo
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YuꢀLong,ꢀBingꢀYuan,ꢀJiantaiꢀMa*ꢀ
LanzhouꢀUniversityꢀ
O
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Si
N
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ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
（
Halloysite Nanotubes-APTMS-SL-Mo）
(
3 3
CH ) COH
(CH
3
)
3
COOH
O
H
(
H
3
C)
3
O
high conversion,
selectivity, TOF
carbocyclic
linear
aromatic
O
O
O
O
O
Mo
O
Halloysite‐nanotube‐supportedꢀmolybdenumꢀsalenꢀcatalystsꢀwereꢀsuccessfullyꢀpre‐
paredꢀbyꢀfacileꢀchemicalꢀsurfaceꢀmodification,ꢀandꢀtheyꢀwereꢀstableꢀandꢀshowedꢀex‐
cellentꢀcatalyticꢀactivityꢀforꢀtheꢀepoxidationꢀofꢀalkenes.ꢀ
O
O
O
H
Si
N
C
O

354ꢀ
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