Amine-functionalized boehmite nanoparticle-supported

ChineseJournalofCatalysis37(2016)1263–1274
催化学报2016年第37卷第8期|www.cjcatal.org available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc Article Amine‐functionalizedboehmitenanoparticle‐supported molybdenumandvanadiumcomplexes:Efficientcatalystsfor epoxidationofalkenes
MahdiMirzaee*,BahramBahramian,MariehMirebrahimi
DepartmentofChemistry,ShahroodUniversityofTechnology,Shahrood,Iran
A R T I C L E I N F O
A B S T R A C T
Articlehistory:
Received26March2016
Accepted30April2016
Published5August2016
Keywords:
Boehmitenanoparticles
Heterogeneouscatalysts
Hexa‐carbonylmolybdenum
Oxo‐sulfatevanadium
Epoxidation
Boehmitenanoparticleswithahighsurfaceareaandahighdegreeofsurfacehydroxylgroupswere
covalentlyfunctionalizedby3‐(trimethoxysilyl)‐propylaminetosupportvanadium‐oxo‐sulfateand
molybdenumhexacarbonylcomplexes.ThesesupportedcatalystswerethencharacterizedbyFou‐
rier‐transforminfraredspectroscopy,powderX‐raydiffraction,thermogravimetryanddifferential
thermalanalysis,X‐ray‐photoelectronspectroscopy,elementalanalysis,inductivelycoupledplasma,
and transmission electron microscopy techniques. The catalysts were subsequently used for the
epoxidationofcis‐cyclooctene, and the experimental procedureswere optimized.Theprogressof
thereactionswasinvestigatedbygas‐liquidchromatography.Recyclingexperimentsrevealedthat
these nanocatalysts could be repeatedly used several times for a nearly complete epoxidation of
cis‐cyclooctene.Theoptimizedexperimentalconditionswerealsousedsuccessfullyfortheepoxida‐
tionofsomeothersubstitutedalkenes.
©2016,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
Epoxidesarewellknownasoneofthemostvaluableinter‐
mediatesfortheproductionofcommerciallyimportantchemi‐
calssuchaspolyglycols,polyamides,polyurethanesandmany
otherpolymers,pharmaceuticals,foodadditives,epoxypaints,
dye‐stuffs, flavour and fragrance compounds [1–8], and
non‐toxicPVC‐plasticizersandstabilizers[8–10].Theyarealso
valuableadditivestolubricantsandadhesives[8,11,12].
In addition to conventional Mo complexes, several other
solublecomplexesofV,W,Re,Mn,Ru,Co,Fe,andTihavebeen
employed as active homogenous catalysts for the epoxidation
of alkenes [2,13–15]. However, these catalysts are toxic and
expensive, and they must be completely separated from the
productsandwaste.Inthisrespect,thedevelopmentofheter‐
ogeneoussystemsforimprovingtherecoveryofvaluablecata‐
lystsisaveryimportantaspectfromaneconomicandecologi‐
calviewpoint.Furthermore,immobilizationofthecatalystson
supports with a high surface area in heterogeneous systems
canefficientlyimprovethecatalystreactivityandenhancethe
turnover number (TON) of the catalytic procedure [2]. An‐
choring catalysts on a solid surface disperses the active sites
uniformlyandprotectsthemfromdimerizationbyoxo‐bridge
formation [16]. Accordingly, various organic and inorganic
materialshavebeenusedasinsolublesupportsinheterogene‐
ous catalytic procedures. The organic insoluble supports in‐
clude different types of polymers such as polybenzimidazole
[13,17],polystyrene[13,18,19],polymethacrylate[13],polyan‐
iline[20],andpoly‐divinylbenzene‐allylamine[21].Inaddition,
ion exchange and epoxy resins [13,22], graphene oxide [23],
multi‐walled carbon nanotubes [16,24], ordered mesoporous
carbon [25], silica [7,13,26], titanium silicate [13], MCM‐41
*Correspondingauthor.Tel/Fax:+98‐23‐32395441;E‐mail:[email protected] DOI:10.1016/S1872‐2067(16)62451‐8|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.37,No.8,August2016 1264
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
[26–29], MCM‐48 [29], SBA‐15 [1,16,26,30,31], alumina [32],
montmorillonite K‐10 [13,33], coated magnetic particles
[13,34],zeolites[16,35–37]andlayeredbimetalhydroxides[5]
have been used as insoluble supports. Generally, inorganic
supportshaveahigherchemicalstabilityandsurfaceareathan
organic ones. Additionally, the surface of some metal oxides
and metal hydroxides is covered with hydroxyl groups. These
groups create hydrophilic surfaces that provide valuable sites
forgraftingawiderangeoffunctionalities.Subsequently,sup‐
porting various transition metals on these functionalities can
produce active and stable catalysts for liquid‐phase organic
reactions. Boehmite (γ‐AlOOH)isaninorganicinsoluble supportthat
containsextrahydroxylgroupsonitssurface.Ithasbeenused
as an absorbent, flame retardant [38], and an important pre‐
cursor for preparing advanced catalysts, alumina, and alumi‐
na‐derivedceramics[38,39].Wehaveusedpureboehmitefor
thesuccessfulmulti‐componentsynthesisofhighlysubstituted
imidazoles [40]. Choudhary et al. [41] have also used pure
boehmiteforthecatalyticepoxidationofstyrene,althoughthey
reported that it was deactivated by the production and accu‐
mulation of water, and so it required continuous removal of
water. Consequently, in this work, we used grafted boehmite
nanoparticles (BNPs) with 3‐(trimethoxysilyl)‐propylamine
(MSPA) to support two complexes of molybdenum and vana‐
dium. This strategy has been used for anchoring various metal
complexes on different solid supports. However, almost all of
them have changed the terminal amine group in MSPA to a
Schiffbaseandthenuseditforthesupportingmetalcomplex‐
es.Wehavealsousedthisapproachinourpreviouswork[42].
Inthiswork,graftedMSPAwasusedforthestraightanchoring
ofmetalcomplexesandthenthesupportedcatalystswereused
intheepoxidationofvariousalkenesubstrates.
2. Experimental
2.1. Materialsandmethods
The required solvents and reagents were purchased from
Merck or Fluka, and used without further purification. Fouri‐
er‐transform infrared (FT‐IR) spectra were recorded on a
BomemMBseriesFT‐IRinstrumentat4cm−1resolutionusing
KBrpellets.1HNMRspectrawereobtainedind6‐DMSOusinga
BrukerAVANCE300MHzspectrometer.PowderX‐raydiffrac‐
tion(PXRD)patternswerecollectedonaPhillipsPW‐1800or
STOEdiffractometerwithCuKαirradiation.Thermogravimetric
(TG) analyses were performed in a Rheometric Scientific
STA‐1500orBAHRThermoanalyseGmbH withaheatingrate
of10°Cmin−1inair.A400‐WHglampwasusedforactivation
ofthehexacarbonylmolybdenumcomplex.Elementalanalyses
were performed in a Hereans CHN‐O‐Rapid Analyzer. Trans‐
missionelectronmicroscopy(TEM)wasperformedonaJEOL
JSM‐6360LV transmission electron microscope. Gas‐liquid
chromatography(GLC)wasperformedonaShimadzuGC‐16A
instrument using a 2‐m column packed with silicon DC‐200.
X‐rayphotoelectronspectroscopy(XPS)wasperformedonan
ESCA 2000 system from VG Microtech using a monochroma‐
tizedaluminumKαanode.
2.2. PreparationofBNPs
Aluminium 2‐butoxide was prepared by the reaction of al‐
uminium with 2‐butanol according to the general preparation
procedure for aluminium alkoxides [43]. The prepared alu‐
miniumalkoxidewasvacuum‐distilledtoobtainahigh‐quality
aluminium alkoxide precursor. For a hydrothermal‐assisted
hydrolysis, according to our previous report [44], aluminium
2‐butoxidewasdilutedin2‐butanolto50%,andthenloadedin
aglasscontainer.Theglasscontainerwasplacedina300‐mL
stainless steel autoclave. The gap between the autoclave
chamber and the glass container was covered with 50 mL of
distilled water, and then the autoclave was heated at 100 °C.
Aluminium 2‐butoxide was hydrolysed by diffusion of water
vapourintothesolutioninthehydrothermalchamber.Allma‐
nipulations of the aluminium alkoxides were performed on a
Schlenkline.Theirtransferintotheautoclavewasperformedin
a moisture‐free atmosphere to prevent the hydrolysis of alu‐
miniumalkoxidespriortointroducingitintothehydrothermal
chamber. After 5 h, the autoclave was cooled down and the
powder produced (BNPs) was filtered off and dried at 100 °C
overnight.
2.3. Preparationofsupportedmolybdenumandvanadium
complexesonBNPsandamine‐functionalizedBNPs
For the preparation of amine‐functionalized BNPs (AF‐
BNPs), 1.00 g of BNPs was refluxed with MSPA (61 µL, 0.35
mmol)indrytoluene(50.0mL)for24h(Scheme1).Thesolid
substance obtained was filtered off, washed three times with
drytoluene,anddriedat100°Covernight.
TheMo(CO)6andVOSO4·H2Ocomplexeswereusedtosup‐
port on BNPs or AFBNPs (Scheme 1). For this purpose,
Mo(CO)6(0.276g,1.05mmol)wasactivatedunderultraviolet
light in tetrahydrofuran (THF, 20.0 mL) for 30 min. This acti‐
vated molybdenum complex was then added to dry BNPs or
AFBNPs, and the mixture was subsequently refluxed for 6 h.
The mixture was then cooled down and filtered off, and the
resulting precipitate was washed several times with THF and
dried at 100 °C overnight. In a similar procedure, VOSO4·H2O
(0.200 g, 1.05 mmol) was refluxed with BNPs or AFBNPs in
ethanol (20 mL) for 7 h. The resulting powder was washed
severaltimeswithethanolanddriedat100°Covernight.
2.4. Catalyticepoxidationofalkenes In a typical process, an appropriate round bottom flask
equipped with a magnetic stirrer bar and a condenser was
chargedwithanalkene,anoxidant,thecatalyst(Mo‐AFBNPsor
V‐AFBNPs),andsolvent,andthemixturewasthenrefluxed.All
thereactionswereperformedatleasttwotimes,andthereac‐
tionprogresswasmonitoredbyGLC.Becausedifferentalkenes
have different reactivities toward oxidation, these reactions
were continued until no further progress was observed. The
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
characterizations of the main products and by‐products were
performed by comparison of their retention times with those
forthestandardsamples.Thealkeneconversionsandproduct
selectivities were calculated using their peak areas by the
standardadditionmethod.Inthecaseofcis‐andtrans‐stilbene,
1HNMRwasalsousedtoanalyzetheproducts. 2.5. Catalystrecycling
(020)
1265
(051, 200)
(120)
(140, 031)
Intensity
(3)
(2)
In a typical experiment, after recovering the catalyst from
the reaction mixture, it was washed several times with CCl4,
and then used in the same procedure under the same condi‐
tions. (1)
10
15
20
25
30
35
40
45
50
55
60
2/( )
o
3. Resultsanddiscussion
Fig.2.PXRDpatternsforBNPs(1),Mo‐AFBNPs(2),andV‐AFBNPs(3).
(4)
Weight loss (%)
(2)
(1)
Weight loss (%)
Transmittance
(3)
4000
3500
3000
2500
2000
1500
1000
500
1
Wavenumber (cm )
Fig. 1. FT‐IR spectra for BNPs (1), AFBNPs (2), Mo‐AFBNPs (3), and
V‐AFBNPs(4).
-5
0
(c)
-20
-40
-60
-80
-10
-20
-30
-40
-50
-60
-70
10
8
6
4
2
0
-2
(b)
(a)
-10
-15
-20
-25
100
200
300
400
500
o
Temperature ( C)
DTA (%)
5
0
-5
-10
-15
-20
-25
0
-5
-10
-15
-20
-25
0
DTA (%)
BNPs were prepared by the hydrothermal‐assisted sol‐gel
processing of an aluminum 2‐butoxide solution in 2‐butanol.
Heat flow (mV)
3.1. Preparationandcharacterizationofsupported molybdenumandvanadiumcomplexesonBNPsandAFBNPs
The most promising property of these hydrothermally pro‐
cessed BNPs is the formation of a crystalline single‐phase
product with no organic residue [44]. This was confirmed by
theFT‐IRspectrum,PXRDpattern,andTG/DSCthermogramof
thissample. IntheFT‐IRspectrumofBNPs(Fig.1),therearetwodifferent
stretching vibrations at 3280 and 3075 cm−1, which were as‐
signed to the bridged and terminal hydroxyl groups of BNPs,
respectively. These stretching vibrations were accompanied by
twobendingvibrationsforthebridgedandterminalhydroxyl
groups at 1150 and 1075 cm−1, respectively. BNPs also had
some distinguishable Al–O related vibrations at 740, 610, and
480cm−1intheIRspectrum.ThePXRDpatternforBNPs(Fig.
2) also confirmed the crystallization of single‐phase boehmite
by comparison with JCPDS card No. 21‐1307. In the TG/DSC
curveofBNPs (Fig.3(a)),thereweretwoendothermic peaks:
Weight loss (%)
There are different methods that have been used for the
preparation of BNPs. Among them, a hydrothermal‐assisted
sol‐gelprocessonaluminumalkoxidewasusedhere[44].This
methodhassomeadvantagessuchaspreparationinaone‐pot
process and processing at a low temperature. The BNPs ob‐
tained were then grafted with MSPA because they showed no
acceptable interaction with the molybdenum or vanadium
complexes. The supported molybdenum and vanadium AF‐
BNPs catalysts were then used in the epoxidation of
cis‐cyclooctene, and the experimental conditions were opti‐
mizedfortheparametersinvolved,suchasthecatalyst,solvent,
andoxidant.Hotfiltrationwasalsousedtoconfirmthehetero‐
geniccharacterofbothcatalyticprocedures.Inaddition,recy‐
cling experiments were performed to determine if these cata‐
lysts could be reused. Both catalysts were then used for the
epoxidation of different alkenes under the optimum experi‐
mentalconditions. 600
Fig. 3. Thermal analysis curves for BNPs (a), Mo‐AFBNPs (b), and
V‐AFBNPs(c).
1266
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
(a)
(b)
(c)
Fig.4.TEMimagesforBNPs(a),Mo‐AFBNPs(b),andV‐AFBNPs(c).
onebelow100°Candtheotheroneatapproximately450°C.
Thefirstonewasattributedtotheeliminationofthephysically
adsorbedwater,andtheotheronewasattributedtothedehy‐
droxylation of boehmite and crystallization of γ‐alumina. The
absenceoftheC–HstretchingvibrationsintheIRspectrumand
an exothermic peak at approximately 250 °C in the TG/DSC
curve of BNPs, respectively, confirmed the loss of all organic
residuesinthehydrothermallyprocessedBNPs.Calculationof
the particle size using the PXRD pattern according to the
Scherer equation shows an average particle size of 10 nm for
BNPs.ThiswasconfirmedbyTEMimageofBNPs(Fig.4(a)).In
this image, needle‐shaped BNPs were seen with a length of
over50nmandawidthofupto10nm.AccordingtotheBET
analysis,theefficientsurfaceareaforBNPswas326m2g−1.
BNPs themselves have promising catalytic properties for
multi‐componentsynthesisofhighlysubstitutedimidazoles,as
performedbyourresearchteamandreportedelsewhere[40].
Inaddition,ithasbeenshownthatforanefficientepoxidation
of alkenes under mild conditions, the presence of a catalyst
containinganactivemetalsiteisvital.Theoxidantcouldcoor‐
dinatetothissite,andbeactivatedfortheepoxidationprocess.
However, BNPs showed no reactivity in the epoxidation of al‐
kenes (Table 1, entry 30), similar to the report by Choudhary
and co‐workers [41]. Furthermore, supporting the molyb‐
denum and vanadium complexes on BNPs failed (Scheme 1).
TheIR spectra andinductivelycoupled plasma (ICP)analyses
showed no evidence to confirm an interaction between them
evenafter6–7hreflux.Consequently,wetriedtofunctionalize
BNPs using MSPA. The functionalization was performed by
refluxingBNPs andMSPAindrytoluenefor 24h. Duringthis
process,the–O3Si(CH2)3NH2groupwasgraftedonthehydrox‐
yl‐covered surface of BNPs by the elimination of methanol
(Scheme 1), which produced AFBNPs. The IR spectrum for
AFBNPs(Fig.1)showedanewbandat1553cm−1,whichwas
assigned to the N–H stretching vibration of the grafted amine
groupsinadditiontotheBNPscharacteristicbands.Elemental
analysis showed that the nitrogen content of AFBNPs was
0.46%, which means that 0.33 mmol of the pending amine
BNPs
OH
OH
OH
OH
OH
Mo(CO)6 + THF
hv (30 min)
Mo(CO)5THF
Reflux
6h
Reflux
6h
BNPs
OH
OH
OH
OH
OH
Reflux
7h
MeO Si
MeO
NH2
Reflux, 24 h
-MeOH
OH
O
Si
O
OH
OH
OH
O
O Si
OH OH
OH
Mo-AFBNPs
NH2
O
AFBNPs
OH
OH
OH
OH
OH
OH
BNPs
MeO
OC CO
H2N Mo CO
OC CO
OH
VOSO4 . H2O
Reflux
7h
H2N
O
O Si
OH OH
OH
V O
O S
O
O
V-AFBNPs
Scheme1.AttempttosupportmolybdenumandvanadiumcomplexesonBNPsandAFBNPs.
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
groupswerecovalentlybondedtothesurfaceof1.00gofBNPs.
AFBNPsalsoshowednoreactivityinthecatalyticepoxidation
ofalkenes(Table1,entry31),asitwaspredictableaccordingto
theabove‐mentioneddiscussion.TheefficientBETsurfacearea
ofAFBNPswas321m2g−1,whichdoesnotshowasignificant
changeincomparisonwithBNPs.
AFBNPswerethenusedtosupportmolybdenumandvana‐
dium complexes. For this purpose, Mo(CO)6 and VOSO4·H2O
wereused.However,Mo(CO)6isnotlabileintheligandsubsti‐
tutionreaction,soitssolutioninTHFwasactivatedbyirradia‐
tion with ultraviolet light. This process resulted in the for‐
mationofaMo(CO)5(THF)complex,whichsubstituteditsTHF
ligandwiththegraftedaminegrouponBNPsafterrefluxfor6h
(Mo‐AFBNPs).TheVOSO4·H2Ocomplexwasalsorefluxedwith
AFBNPs in ethanol for 7 h to support vanadium on AFBNPs
(V‐AFBNPs).
IntheFT‐IRspectrumofMo‐AFBNPs(Fig.1),thecarbonyl
stretching vibration band for the coordinated molybdenum
complex was covered by the characteristic broad band of the
BNPsintheregionof1900–2500cm−1.Thiswasattributedto
the small amountofcatalystthat wasloadedonAFBNPs. The
ICP analysis showed that approximately 90% of the nitrogen
sitesonAFBNPswereusedtoanchor0.3mmolofthemolyb‐
denumcomplexon1.00gofMo‐AFBNPs.ThePXRDpatternof
Mo‐AFBNPs(Fig.2)confirmstheretentionoftheBNPscrystal‐
line structure after functionalization and metal loading. There
are two endothermic peaks in the TG/DTA curves for
Mo‐AFBNPs (Fig. 3(b)). One of them is below 100 °C, and the
otherisabove200°C.Thefirstwasattributedtotheelimina‐
tionofphysicallyadsorbedwater,andthesecondcouldbeat‐
tributed to the elimination of carbonyl ligands from the coor‐
dinated molybdenum complex. There is also an exothermic
peak at approximately 300 °C owing to the ignition of the or‐
ganic parts of the pending groups on the BNPs. This thermo‐
gramalsoshowsanotherweightlossintherangeof400–500
°C, which was accompanied with an endothermic peak in the
DTAcurveowingtothedehydroxylationofboehmiteandcrys‐
tallizationofγ‐alumina, similartoBNPs. The totalweightloss
ofthissampleupto600°Cwasabout24%.Calculationofthe
particlesizeusingthePXRDpatternforMo‐AFBNPsaccording
totheSchererequationshowsnochangeintheparticlesizein
80
4000
Intensity (a.u.)
(3)
3000
comparisontoBNPs,andthiswasconfirmedbytheTEMimage
(Fig.4(b)).TheefficientsurfaceareaforMo‐AFBNPswas323
m2g−1,whichdoesnotshowasignificantvariationincompari‐
sonwithBNPsandAFBNPs.
TheIRspectrumofV‐AFBNPs(Fig.1)showsanewbandat
956 cm−1 owing to the V=O stretching vibration of the VOSO4
complex, which confirms the coordination of the vanadium
complex to the functionalized BNPs. The ICP analysis also
showed that over 80% of the nitrogen sites on AFBNPs were
usedtoanchor0.28mmolofthevanadiumcomplexon1.00g
of V‐AFBNPs, which is lower than the metal loading in
Mo‐AFBNPs.ThePXRDpatternofV‐AFBNPs(Fig.2)alsocon‐
firms retention of the crystalline structure of the BNPs after
metal loading. In the TG/DTA curve of V‐AFBNPs (Fig. 3(c)),
there is an endothermic peak below 100 °C with 6% weight
lossintheTGcurveowingtotheeliminationofthephysically
adsorbedwater.Similartothemolybdenumcounterpart,there
is an exothermic peak at approximately 300 °C owing to the
ignitionoftheorganicpartsofAFBNPs.Additionally,andsimi‐
lartoBNPsandMo‐AFBNPs,thisthermogramshowsaweight
lossintherangeof400–500°C,accompaniedbyanendother‐
mic peak in the DTA curve, which was similarly assigned as
resultingfromdehydroxylationofboehmiteandcrystallization
of γ‐alumina. The total weight loss of V‐AFBNPs up to 600 °C
wasabout24%,similartoMo‐AFBNPs.Calculationoftheparti‐
clesizeusingthePXRDpatternforV‐AFBNPsaccordingtothe
Schererequation,andsimilartothemolybdenumcounterpart,
shows no change in the particle size in comparison to BNPs.
ThiswasalsoconfirmedbytheTEMimage(Fig.4(c)). The efficient surface area for V‐AFBNPs was 318 m2 g−1,
whichdoesnotshowasignificantvariationincomparisonwith
BNPs, AFBNPs, and Mo‐AFBNPs. The XPS analysis was also
performedforfurtherevidenceofthesurfacecompositionsof
both the Mo‐AFBNPs and V‐AFBNPs catalysts inferred by the
FT‐IRandICPanalyses.WidescanXPSspectraofbothcatalysts
over a wide binding energy range from 0 to 600 eV are dis‐
playedinFig.5.InthecaseofMo‐AFBNPscatalyst(Fig.5),the
XPS Mo 3d5/2 and Mo 3d3/2 peaks were observed at 232 and
236eV,respectively,indicatingthepresenceofamolybdenum
complexonthesurfaceofthiscatalyst.However,inthecaseof
the V‐AFBNPs catalyst (Fig. 5), the XPS V 2p3/2 and V 2p1/2
O 1s
900
Mo 3d5/2
Mo 3d3/2
1267
O 1s
600
C 1s
40
300
V 2p3/2
V 2p1/2
(4)
2000
0
215
1000
235
255
0
510
(2)
520
530
540
(1)
Al 2s
Al 2p
Si 2p
0
0
50
100
150
200
250
300
350
Binding energy (eV)
400
450
500
550
Fig.5.XPSanalysisofMo‐AFBNPs(1),V‐AFBNPs(2),Mo3dregion(3)ofMo‐AFBNPs,andV2pandO1sregion(4)ofV‐AFBNPs.
600
1268
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
peaksat517and522eV,respectively,indicatesthepresenceof
avanadiumcomplexonthesurfaceofthiscatalyst.TheXPSSi
2pandC1speaksat99and284eV,respectively,confirmedthe
attachment of the silylpropyl amine group on the surface of
boehmite nanoparticles in both catalysts. In addition, the XPS
Al2p,Al2s,andO1speaksat72,117,and530eV,respectively,
owing to the BNPs, were present in the spectra of both cata‐
lysts. In brief, the surface compositions characterized by the
FT‐IR,ICP,andXPSanalysessupporteachother.
3.2. Alkeneepoxidationwithtert‐butylhydroperoxidecatalyzed
byMo‐AFBNPsandV‐AFBNPs
Epoxidationofcyclooctenehasbeenwidelyusedasamodel
reactiontodemonstratethecatalyticactivitiesofnewsynthe‐
sized catalysts. This reaction was chosen because cyclooctene
epoxideistheonlyproductanditdoesnothaveanyotherby‐
product.Thus,weusedthisreactiontoshowandoptimizethe
catalyticactivitiesofourcatalysts.Theoptimizationofthetitle
reaction was investigated by changing different parameters
including the solvent, oxidant, catalyst, and time. Our experi‐
ments showed that in the presence of these catalysts,
tert‐butylhydroperoxide(TBHP)wastheonlyoxidizingsystem
that can promote the epoxidation of cis‐cyclooctene in CCl4.
Other oxygen sources such as hydrogen peroxide and sodium
periodate(inthepresenceof0.01goftetrabutylphosphonium
bromide as the phase‐transfer reagent), and urea‐hydrogen
peroxideinCCl4orCH3CNdonotshowanacceptableactivity.It
seems that the protic solvents, which were inevitably present
withthoseoxidants,competedtobindtothemetalcomplexes,
andblockedthecatalyticsites.Furthermore,undermildcondi‐
tions,theepoxidationofcis‐cyclooctenebyTBHPdidnotpro‐
ceedintheabsenceofthecatalyst.
AsshowninTable1(entries1–6and17–22),theuseofdif‐
ferent solvents showed the same trend as other literature re‐
ports on homogeneous alkene epoxidation by the Mo(CO)6
catalyst[45].CoordinatingsolventssuchasCH3CNandCH3OH
compete with TBHP to occupy the coordination sites on the
transition metal catalyst. Therefore, in the presence of these
solvents, the observed yields are very low. Among other sol‐
vents,CCl4hasthehighestyieldofepoxideproductforboththe
Mo‐AFBNPs and V‐AFBNPs catalysts. This could be attributed
to the higher boiling point of CCl4 in comparison to the other
solvents.
We then determined how the amount of the amine group
anchoredonBNPsinfluencedtheprogressofthereaction(Ta‐
ble1,entries6–8).Forthispurpose,differentamountsofMSPA
(6:0.35,7:0.7,and8:0.23mmol)wereusedtoanchor1.00gof
Table1
Epoxidationofcis‐cylooctenewithTBHPunderrefluxconditionsafter2h.
Pendingaminegroup
Catalystloadinga
(mmol)/BNPs(g)
(mol%)
1
Mo‐AFBNPs(40)
CH3CN(3)
0.35
1.7
2
Mo‐AFBNPs(40)
CH3OH(3)
0.35
1.7
3
Mo‐AFBNPs(40)
(CH3)2CO(3)
0.35
1.7
4
Mo‐AFBNPs(40)
CH2Cl2(3)
0.35
1.7
5
Mo‐AFBNPs(40)
CHCl3(3)
0.35
1.7
6
Mo‐AFBNPs(40)
CCl4(3)
0.35
1.7
7
Mo‐AFBNPs(40)
CCl4(3)
0.70
2.0
8
Mo‐AFBNPs(40)
CCl4(3)
0.23
0.8
9
Mo‐AFBNPs(30)
CCl4(3)
0.35
1.29
10
Mo‐AFBNPs(20)
CCl4(3)
0.35
0.86
11
Mo‐AFBNPs(10)
CCl4(3)
0.35
0.43
12
Mo‐AFBNPs(20)
CCl4(3)
0.35
0.86
13
Mo‐AFBNPs(20)
CCl4(3)
0.35
0.86
14
Mo‐AFBNPs(20)
CCl4(3)
0.35
0.86
15
Mo‐AFBNPs(20)
CCl4(2)
0.35
0.86
16
Mo‐AFBNPs(20)
CCl4(1)
0.35
0.86
17
V‐AFBNPs(20)
CH3CN(3)
0.35
0.8
18
V‐AFBNPs(20)
CH3OH(3)
0.35
0.8
19
V‐AFBNPs(20)
(CH3)2CO(3)
0.35
0.8
20
V‐AFBNPs(20)
CH2Cl2(3)
0.35
0.8
21
V‐AFBNPs(20)
CHCl3(3)
0.35
0.8
22
V‐AFBNPs(20)
CCl4(3)
0.35
0.8
23
V‐AFBNPs(10)
CCl4(3)
0.35
0.4
24
V‐AFBNPs(30)
CCl4(3)
0.35
1.2
25
V‐AFBNPs(40)
CCl4(3)
0.35
1.6
26
V‐AFBNPs(20)
CCl4(3)
0.35
0.8
27
V‐AFBNPs(20)
CCl4(3)
0.35
0.8
28
V‐AFBNPs(20)
CCl4(2)
0.35
0.8
29
V‐AFBNPs(20)
CCl4(1)
0.35
0.8
30
BNPs(20)
CCl4(3)
0.35
—
31
AFBNPs(20)
CCl4(3)
0.35
—
aCalculatedfor0.7mmolcyclooctene.bGLCyieldsarebasedonthestartingcyclooctene.
Entry
Catalyst(mg)
Solvent(mL)
Oxidant/Substrate molarratio
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
2
2.5
2
2
2
2
2
2
2
2
2
2
2
1
3
2
2
2
2
Epoxideyieldb
(%)
Noreaction
Noreaction
18
23
40
53
55
40
53
63
46
42
79
83
86
96
8
10
35
61
68
73
57
67
63
53
76
80
83
Noreaction
Noreaction
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
BNPs. These functionalized BNPs were then used to support
Mo(CO)6.Thesecatalystswerethenusedfortheepoxidationof
0.7 mmol of cis‐cyclooctene with 1.05 mmol of TBHP. The re‐
sultsobtainedshowedthatafter2htheconversionsofentries
6 and 7 were the same and higher than entry 8, respectively.
Therefore,0.35mmolofthependingaminegrouppergramof
BNPswasselectedforfurtherinvestigations.Accordingtothe
ICPanalysis,theamountofsupportedmolybdenumcomplexes
on samples 6, 7, and 8 were 0.3, 0.35, and 0.14 mmol g−1, re‐
spectively. The same activities for entries 6 and 7 may be at‐
tributedtoalmostthesamemetalloadingonAFBNPs.
The amount of catalyst used for the epoxidation of
cis‐cyclooctene with TBHP was also investigated. Fig. 6 and
Table 1 (entries 6, 9–11, and 22–25) confirm that increasing
theamountofboththeMo‐AFBNPsandV‐AFBNPscatalystsup
to20mgincreasedthecycloocteneepoxideyield.
Theoxidanttosubstrateratioisoneofthemostcrucialpa‐
rameters in the catalytic epoxidation processes. The results
tabulatedinTable1(entries10,12–14,22,26, and27) show
that theoptimumoxidantto substrate ratio forbothcatalysts
was 2. Increasing this ratio to 2.5 and 3 for Mo‐AFBNPs and
V‐AFBNPs,respectively,didnotchangethe epoxidationyields
significantly, but decreasing the ratio to 1 lowered the yields
significantly. The amount of solvent was also investigated. The results
(Table1,entries13,15,16,22,28,and29)showthatdecreas‐
ingtheamountofsolventfrom3to1mLincreasestheepoxide
yield.
Fig.7showstheeffectofreactiontimeontheepoxidationof
cis‐cyclooctenewithTBHP.Acomparisonbetweenthereactivi‐
tiesoftheMo‐AFBNPsandV‐AFBNPscatalystsrevealsthatthe
supportedmolybdenumcatalystismorereactive,andafter90
min, its conversion reaches a maximum, and then becomes
constant.However,thesupportedvanadiumcomplexrequires
180mintoreachthisgoal.
Moreover,theheterogenic characterofthesecatalysts was
checkedbyahotfiltrationtest.Forthispurpose,eachcatalyst
was removedbycentrifugingthe catalyticreactionmixtureat
the halfway point of the optimum reaction time, which had
100
Yield (%)
80
73
63
60
40
Mo-AFBNPs
V-AFBNPs
20
0
0
10
20
30
Amount of catalyst (mg)
40
50
Fig. 6. Epoxidation of 0.7 mmol of cis‐cyclooctene with 1.4 mmol of
TBHP,catalyzedbyMo‐AFBNPsorV‐AFBNPsin3mLofCCl4after2h
underrefluxconditions.
1269
96
100
93
80
Yield (%)
60
Mo-AFBNPs
V-AFBNPs
40
20
30
60
90
120
Time (min)
150
180
210
Fig. 7. Epoxidation of 0.7 mmol of cis‐cyclooctene with 1.4 mmol of
TBHPcatalyzedby20mgofMo‐AFBNPsorV‐AFBNPsin3mLofCCl4
underrefluxconditions.
been determined earlier. The reaction was then allowed to
continue and reach the optimum reaction time without the
presence of the catalyst. The results obtained showed that
whenthecatalystwasremovedfromthereactionmedium,the
catalytic procedure stopped and did not continue anymore.
This test proves that the catalytic reactions proceeded in het‐
erogeneousconditionsandthecatalystswerenotleachedinto
thereactionmedium.
Wealsostudiedthereactionprogressinthepresenceofthe
relatedhomogenouscatalysts. Theresults showedthatbyus‐
ingMo(CO)6orVOSO4·H2Oasthecatalystunderthesameex‐
perimentalconditionsfortheheterogeneouscounterparts,the
reaction yields reduced to 85% and 55%, respectively. The
greater reactivities of the heterogeneous Mo‐AFBNPs and
V‐AFBNPscatalystsmayberelatedtothehomogenousdisper‐
sionofthecatalyticactivesitesontothesurfaceofBNPs,which
efficientlyincreasedtheeffectivecollisionofthereagentstothe
metalsites,andconsequently,increasedthecycloocteneepox‐
ide yield in comparison to the homogeneous counterparts. In
addition,anchoringthemolybdenumandvanadiumcomplexes
onto the BNPs surface protect them from dimerization by
oxo‐bridge formation, which leads to the deactivation of the
catalytic sites and lowers the yields in homogenous reaction
conditions.
The new catalysts Mo‐AFBNPs and V‐AFBNPs can also be
usedfortheepoxidationofawiderangeofsubstitutedalkenes
(Table2).Theblankexperiments,performedwithoutcatalysts,
with BNPs or AFBNPs, showed a very low conversion even
after5hreflux.InthepresenceofMo‐AFBNPsandV‐AFBNPs,a
widerangeofboththecyclicandlinearalkenescouldefficient‐
lyandselectivelybeconvertedtoepoxides.Basedontheepox‐
idation mechanisms suggested earlier [44], the higher elec‐
tron‐donating ability of an alkene double bond is expected to
show more epoxidation reactivity. Therefore, cyclooctene and
cyclohexene withinnerdouble bonds shouldexhibitmoreac‐
tivityincomparisonwith1‐hexeneand1‐octene,whichcontain
terminal double bonds. In addition, cyclooctene is more reac‐
tive than cyclohexene owing to some cyclooctene confor‐
1270
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
Table2
EpoxidationofsomealkeneswithTBHPcatalyzedbyMo‐AFBNPsorV‐AFBNPsunderrefluxconditions.
Entry
1
2
Mo‐AFBNPsa
Epoxideb
Conversionb(%)
(%)(largescalec)
Alkene
3
4
5
6
7
Time
(h)
Conversionb(%)
V‐AFBNP
Epoxideb
(%)(largescalec)
Time(h)
96
96(92)
1.5
93
93(88)
3
98d
70(67)
1.5
92d
85(81)
3
60
60(55)
6
58
58(54)
10
68
68(63)
6
60
60(55)
10
88e
81(77)
6
87e
66(61)
10
98f
86(81)
1.5
89f
80(76)
3
99g
99transh
3
98g
98transh
6
92g
79cisand13transi
3
90g
79cisand11transi
6
8
GLCyieldbasedonthestartingalkenes.
bReactionconditions:alkene0.7mmol,TBHP1.4mmol,catalyst20mg,CCl41mL.
cLargescalereactionconditions:alkene14mmol,TBHP28mmol,catalyst0.4g,CCl420mL.
dThebyproduct(cyclohexanone)is18%forMo‐AFBNPsand7%forV‐AFBNPs.
eThebyproduct(benzaldehyde)is7%forMo‐AFBNPsand21%forV‐AFBNPs.
fThebyproduct(acetophenone)is12%forMo‐AFBNPsand9%forV‐AFBNPs.
gBoth1HNMRandGLCdataapprovedthereportedyields.
hTransisomeristheonlyproduct,accordingto1HNMRdata.
iCisandtransisomersarebothproduced,accordingto1HNMRdata.
a
mationsthatfavortheformationofreactionintermediatesthat
decrease the activation energy [46]. In contrast, 1‐octene is
oxidized slower than 1‐hexene, and this observation could be
attributedtothemoresterichindranceofthehexylgroupcon‐
nected to the double bond present in 1‐octene in comparison
withthebutylgroupin1‐hexene.Alargerhexylgroupcanef‐
fectivelyhindertheapproachof1‐octenetothecatalystmetal
center, and slow down the reaction. Cis‐ and trans‐stilbene
werealsocompletelyconvertedtoepoxidesbytheMo‐AFBNPs
andV‐AFBNPscatalysts.Surprisingly,trans‐stilbenegivesonly
a trans epoxide, and cis‐stilbene gives a mixture with a high
cis/trans epoxide molar ratio (Table 2). This shows the high
selectivitiesofthesecatalystsintheepoxidationofalkenes.In
addition, we scaled‐up the reaction and reported the results
obtainedinTable2.Theseresultsshowonlya4%–6%reduc‐
tionintheepoxideyieldsofdifferentalkenesincomparisonto
the small‐scale experiment, and they confirmed the potential
useofthesecatalyticprotocolsinindustrialprocedures. BasedontheSobczak’sreport[47]andotherexperimental
andtheoreticalreports[2,7,15,45],probablemechanismswere
proposedfortheepoxidationofcyclooctenewithTBHPbythe
Mo‐AFBNPs and V‐AFBNPs catalysts (Schemes 2 and 3). Ac‐
cordingtoourexperimentalresults,thereactionratesdisplaya
first‐order dependence on the catalyst and substrate concen‐
trations.Therearealsosomecrucialstagesintheseprocesses.
The V‐AFBNPs catalyst initially contains the oxo ligand on its
metal center but, for Mo‐AFNBPs, the pending molybdenum
complex is oxidized in the first step, and an oxo‐molybdenum
complexisformed.Thenextstepforbothcatalystsinvolvesthe
transfer of the TBHP hydroxyl proton to a terminal oxygen
atomoftheseoxocomplexes,whichresultsinthecoordination
of the tert‐butylperoxide anion to the Lewis acidic metal cen‐
ters. Then the alkene substrate is coordinated to the metal
center,and,asanucleophile,isinsertedintothemetaloxygen
bond of the coordinated peroxide electrophile anion. This
mechanism easily explains the faster reaction of the elec‐
tron‐rich alkenes in comparison to the electron‐poor ones. In
the next step, the epoxide product is formed, and the
tert‐butylperoxideanionisconvertedtothetert‐butoxideani‐
on.Thentheperoxideproductisreleasedandthecatalyticcy‐
cle is continued by substitution of a new tert‐butylperoxide
insteadofthetert‐butoxideanion.
3.3. Catalystrecoveryandreuse
Asdiscussedearlier,thereusabilityofsolid‐supportedcata‐
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
O O
OH H2N Mo O
O
HO O
O Si
OH OH
OH
TBOH
TBHP
II
1271
III
O O
OH H2N Mo O
O
HO O
O Si
OH OH
OH
O O
H2N Mo O
OC CO
OH H2N Mo CO
OC CO
O
O Si
OH OH
OH
I
OH
HO O
O
O Si
H
OH OH
OH
O O
OH H2N Mo O
O
HO O
O Si
OH OH
OH
TBOH
V
O
IV
Scheme2.ProposedmechanismforepoxidationofalkeneswithTBHPbyMo‐AFBNPs.
TBOH
TBHP
O
OH
O
Si
O
OH HO
OH
I
O
H2
N
O
S
OH
O
(IV) V
O
H2
N
O
Si
O
OH HO
OH
O
O
O
(V)
V
O
O
O
TBHP
II
TBOH
S
IV
O
O
O
Si
O
OH HO
OH
N
H2
S
O
O
OH
V
(V)
O
O
S
O
O
O
OH
H
O O
C(Me)3
III
O
Si
O
OH HO
OH
N
H2
V
(V)
OH
O O
C(Me)3
Scheme3.ProposedmechanismforepoxidationofalkeneswithTBHPbyV‐AFBNPs.
lysts is one of their most important benefits. Therefore, the
reusability of the Mo‐AFBNPs and V‐AFBNPs catalysts was
monitored by means of multiple sequential epoxidations of
cis‐cyclooctenewithTBHP(Table3).Aftereachcycle,thereac‐
tion mixture was centrifuged, and the supernatant solutions
and recovered catalysts were analyzed by ICP. This analysis
showed that there was no metal contaminant in the superna‐
tant solution, and no decrease in the metal loading of the re‐
coveredcatalyst.Italsoconfirmed thatthemetalionsdidnot
leachfromtheheterogeneouscatalysts,andthatthereactions
were performed entirely under heterogeneous conditions. In
addition,retentionofthecatalyticactivity,uptofivecycles,for
recovered Mo‐AFBNPs, makes it a better candidate for the
epoxidation of alkenes. The reason for the small gradual de‐
creasein theyields withV‐AFBNPs wasattributedtothe poi‐
soningeffectofwatervapor,whichmaybeadsorbedduringthe
recovering process and blocks the catalytic sites on the metal
center.
Turnover frequency (TOF) is a virtuous parameter for the
comparisonofthecatalyticactivities,soweusedittoshowthe
1272
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
Table3
Epoxidationofcis‐cylooctenewithTBHPunderrefluxconditionsusingrecycledcatalysts.
Numberof
cycles
Mo‐AFBNPscatalysta
Epoxideyieldc(%) TOFd(h–1) Leachede(%)
V‐AFBNPscatalystb
Content (mmol/g)
f
Epoxideyield (%)
c
1
96
75
ND
0.3
93
2
92
72
ND
0.29
91
3
90
70
ND
0.29
90
4
90
70
ND
0.3
87
5
90
70
ND
0.29
85
aReactionconditions:cyclooctene0.7mmol,TBHP1.4mmol,catalyst20mg,CCl41mL,90min.
bReactionconditions:cyclooctene0.7mmol,TBHP1.4mmol,catalyst20mg,CCl41mL,180min.
cGLCyieldsarebasedonthestartingcyclooctene.
dTOF=moleofreactantyield/(100moleofcatalysttime)
eMetalleachedfromcatalystintothecatalyticsolutionmediumdeterminedbyICP. fMetalcontentofrecycledcatalystdeterminedbyICP.
ND—Notdetected
benefits of our catalytic systems over other reported catalytic
protocols. Very recently, Feng et al. [21] have supported a di‐
oxo‐molybdenum complex onto an organic copolymer and
reached a TOF of 5 in the epoxidation of cis‐cyclooctene with
TBHP.Sakthiveletal.[31]havesupportedMo(CO)6onSBA‐15
and reported a TOF of 32 in the same reaction.
Moahmmadikish et al. [48] have supported a molybdenum
complexontonanomagnetiteandreportedaTOFof19withthe
same catalytic reaction conditions. Yang et al. [49] have also
supportedamolybdenumSchiffbasecomplexontoSBA‐15and
reachedaTOFof25inthesamereaction.Moghadametal.[50]
have reported a TOF of 55.4 for the catalytic epoxidation of
cis‐cyclooctene with TBHP by a molybdenum complex sup‐
ported onto multi‐walled carbon nanotubes. All of these re‐
ports have lower TOF values than the TOF value obtained for
Mo‐AFBNPsinthepresentresearchwork.Inaddition,theTOF
values for Mo‐AFBNPs and V‐AFBNPs (75 and 39 h–1, respec‐
tively)inthepresentreportaresignificantlygreaterthantheir
Schiff‐basecounterparts,Mo‐IFBNPsandV‐IFBNPs(44and29
h–1,respectively),inourpreviousreport[42].
4. Conclusions
TOFd
(h–1)
Leachede(%)
Contentf
(mmol/g)
39
38
37
36
35
ND
ND
ND
ND
ND
0.28
0.28
0.27
0.27
0.28
Acknowledgments The authors would like to thank the vice‐president's office
for research affairs of Shahrood University of Technology for
the financial support of this work. We also would like to
acknowledge the kind helps of Ms. Adonis Amoli and Mr.
MahmoodNorouzi.
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GraphicalAbstract
Chin.J.Catal.,2016,37:1263–1274 doi:10.1016/S1872‐2067(16)62451‐8
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胺功能化的拟薄水铝石纳米颗粒负载的Mn-V复合物: 高效的烯烃环氧化催化剂
Mahdi Mirzaee *, Bahram Bahramian, MariehMirebrahimi
沙赫鲁德科技大学化学系, 沙赫鲁德, 伊朗
1274
MahdiMirzaeeetal./ChineseJournalofCatalysis37(2016)1263–1274
摘要: 将具有高比表面积和表面高度羟基化的拟薄水铝石纳米颗粒与3-(3甲氧基硅烷)-正丙胺进行共价结合而官能团化,
再用于负载硫酸氧钒和六羰基钼络合物。所得样品采用红外光谱、粉末X射线衍射、热重-差热分析、X射线光电子能谱、
元素分析、电感耦合等离子体和透射电镜等技术进行了表征, 并用于顺-环辛烯的环氧化反应中, 优化了诸如溶剂和氧化剂
等反应条件. 反应过程采用气-液色谱进行监测. 重复使用实验表明, 该纳米催化剂可重复使用多次, 并保持顺-环辛烯接近
完全环氧化. 所得到的优化反应条件也成功用于其它的取代烯烃的环氧化反应中.
关键词: 拟薄水铝石纳米颗粒; 多相催化剂; 六羰基钼; 硫酸氧钒; 环氧化
收稿日期: 2016-03-26. 接受日期: 2016-04-30. 出版日期: 2016-08-05.
*通讯联系人. 电话/传真：+98-23-32395441; 电子信箱: [email protected]
本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).