Phosphotungstate-Based Ionic Silica Nanoparticles Network for Alkenes Epoxidation

13 Pages·2015·2.21 MB·English

Article
Phosphotungstate-Based Ionic Silica Nanoparticles
Network for Alkenes Epoxidation
XiaotingLi,PingpingJiang*,ZhuangqingWangandYuandanHuang
Received:28October2015;Accepted:14December2015;Published:24December2015
AcademicEditor:MichalisKonsolakis
TheKeyLaboratoryofFoodColloidsandBiotechnology,MinistryofEducation,
SchoolofChemicalandMaterialEngineering,JiangnanUniversity,Wuxi214122,China;
[email protected](X.L.);[email protected](Z.W.);[email protected](Y.H.)
* Correspondence:[email protected];Tel.:[phone]
Abstract: An inorganic-organic porous silica network catalyst was prepared by linking silica
nanoparticles using ionic liquid and followed by anion-exchange with phosphotungstate.
Characterization methods of FT-IR, TG, SEM, TEM, BET, etc., were carried out to have
a comprehensive insight into the catalyst. The catalyst was used for catalyzing cyclooctene
epoxidationwithhighsurfacearea,highcatalyticactivity,andconvenientrecovery. Theconversion
and selectivity of epoxy-cyclooctene could both reach over 99% at 70 ˝C for 8 h using hydrogen
peroxide (H O ) as an oxidant, and acetonitrile as a solvent when the catalyst was 10 wt. %
2 2
ofcyclooctene.
Keywords: silicananoparticles;ionicliquid;network;phosphotungstate;catalyst;epoxidation
1. Introduction
Inorganic-organichybrid[1–8]materialshaveattractedconsiderableinterestasthecombination
of the features of different parts can generate high performance. As the development of nanoscale
materials, SiO has indeed stimulated remarkable scientific interest because of its excellent
2
performance and promising applications in scientific and technological fields. In contrast to other
inorganicmaterials,thepreparationofSiO nanoparticleshasbeenverymaturewithawidesource
2
of raw materials, and they possess the merits of high specific surfaces, as well as excellent thermal
stabilities. Furthermore, the presence of a large number of silanol (Si–OH) groups on the surface
makesiteasilyforsurfaceorganicfunctionalization[9,10]. Ionicliquids,aclassofnewtypeofgreen
environmental protection organic compounds with outstanding properties, were introduced as the
organic part, recently. The resulting materials can be applied into numerous fields, like catalysts,
anionexchange,selectivegastrapping,drugdelivery,orelectrochemistry[11].
The modification of silica nanoparticles with ionic liquids has been already reported by some
researchers [12–14]. Ionic liquids are usually just grafted or absorbed onto the surface for next use.
However, this is no longer a hot spot and novel method. Meanwhile, it remains separation
problemsifthenanoparticlesaresmallerthanacertainsize. Forafewyearsnow,thenewly-arising
challenge in the field of nanoparticle research is focusing on the development of specific materials
based on assemblies of nanoparticles, which approaches to make use of the nanoparticle collective
properties [15–20]. Thus, a silica network is being prepared by connecting silica nanoparticles with
ionicliquidtotaketheadvantageofcovalentlinkofionicliquidbytwodifferentparts.
Epoxidesareimportantrawmaterialsinchemicalindustrialproduction[21]. Polyoxometalates
(POMs),asisknowntoall,havebeenwidelyusedasepoxidationcatalystswithH O fortheirhigh
2 2
efficient active centers [22–24]. However, POMs, themselves, are low efficient and soluble in some
Catalysts2016,6,2;doi:10.3390/catal6010002 www.mdpi.com/journal/catalysts
Catalysts2016,6,2 2of13
epoxidation systems which results in the difficulty in separation of the catalysts. Therefore, ionic
liquid-basedPOMhybridcatalystscomeintobeing.
In this study, we first prepared a silica network by connecting silica nanoparticles with
covalent-linked ionic liquid. This material was proved to be porous with high surface area.
Phosphotungstic acid anions were then introduced into the material by ion exchange between
Keggin-type H PW O and ionic liquid as well as protonation of amino group. Ionic liquid
3 12 40
had played an important role in two aspects: linking silica nanoparticles and introduction of
an active center. The catalysts were used in catalytic epoxidation of olefin for the first time.
Structural characteristics and catalytic performance of as-prepared catalysts were carried out to
have a comprehensive insight into the catalyst. The study potentially propels the development of
nanoparticle networks as promising materials for various fields to take advantage of the collective
propertiesofnanoparticles.
2. ResultsandDiscussion
2.1. Characterization
2.1.1. FT-IRAnalysis
ThebasicmodificationmoietiesontheSiO particleswerecharacterizedbyFT-IR,whichwere
2
showninFigure1. Thebandsat3450and1630cm´1 werecorrespondingtostretchingandbending
vibrationsofsurfaceSi–OHgroupsonthesurfaceofSiO . Aftergraftingwithsilanecouplingagent,
2
thepeakintensitydecreasedandnewbandsthatassignedtotheC–Hstretchingandbendingrocking
moderespectivelyappearedintheregionof2970cm´1and1460cm´1(Figure1b–e),whichindicated
Catalytshtes s2u0c1c6es, s6fu l functionalization on the surface of SiO particles. The other bands at 1100 cm´1 and 3
2
814cm´1wereattributedtosymmetricandanti-symmetricstretchingvibrationofSi–O–Si,andband
a bandat 9a6t0 8cm90´ 1cwma−s1a tatrpipbeuaterdedt oitnh eFSiig–Ourset re1tech, inwghvicibhr atwioans ofaSssi–iOgnHe.dF otroP Wasy(0m.0m58e)t/rSiciO 2stIrmet+cChli´ng of
SiO , a band at 890 cm´1 appeared in Figure 1e, which was assigned to asymmetric stretching of
W–O –W2 in the corner-shared octahedral of Keggin-typeHPW. Other characteristic bands at 1080, 983,
b
W–O –W in the corner-shared octahedral of Keggin-type HPW. Other characteristic bands at 1080,
b
and 80958 3c,man−1d w80e5rec mal´l 1owveerrleaaplpleodv ewrliatphp tehde wbaitnhdtsh eofb aSnidOs2o. fSiO .
2
(a)
a.u.) (b) 2970 1630 960
e ( (c)
c
n
mitta (d) 3440 1460
ans (e) 814
r
T
890
1100
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber/cm-1
Figure1.FT-IRspectraof(a)SiO ;(b)SiO -Im;(c)SiO -Cl;(d)SiO Im+Cl´SiO ;(e)PW(0.058)/SiO
2 2 2 2 2 2
FiguIrme +C1l.´ FSTiO-IR. spectra of (a) SiO2; (b) SiO2-Im; (c) SiO2-Cl; (d) SiO2 Im+Cl− SiO2;
2
(e) PW(0.058)/SiO Im+Cl− SiO .
2 2
2.1.2. ThermalStabilityandStructure
2.1.2. TherTmhael TSGtabanilaitlyys aesndo fSStriOuc,tuSrieO -Cl, SiO -Im, SiO Im+Cl´ SiO , and PW (0.058)/SiO Im+Cl´
2 2 2 2 2 2
SiO wereshowninFigure2. ForthepureSiO nanoprticles(Figure2a),theweightlossof5.65wt.%
2 2
Thec oTuGld abneaolnylsyeos bosfe rSvieOd2b, eSfoirOe21-0C0l˝, CS,iwOh2-iIcmhw, SasiOas2s iIgmn+eCdlt−o SthieOd2e, saonrdp tiPoWno f(0w.0at5e8r.)/NSoiOw2e iIgmh+tClols−s SiO2
were schoouwldn bine sFeiegnureev e2n. Fwoitrh ththee piunrcere SaisOing noafntoemprptiecrlaetsu r(eFitgou8r0e0 2˝aC)., Itnher ewaleitiyg,htth leosfosr omfe 5d.6io5n wictu. n%it scould
2
be only observed before 100 °C, which was assigned to the desorption of water. No weight loss could
be seen even with the increasing of temperature to 800 °C. In reality, the formed ionic units of
imidazolium presents a higher thermal stability than the aromatic precursors. The precursors started to
decompose before 200 °C, while the imidazolium mainly decomposes around 300 °C. From
Figure 2b,c, SiO -Cl started to decompose at 180 °C, SiO -Im started to decompose at 120 °C, and the
2 2
main weight loss was around 400 °C. From Figure 2d, the first stage ranging from 25 to 120 °C was
ascribed to the elimination of adsorbed water. After 120 °C the unreacted organic groups started to
decompose and the weight loss around 300 °C was mainly attributed to the decomposition of
imidazolium ionic units, which could verify the occurrence of nucleophilic reactions, on one hand. For
the PW(0.058)/SiO Im+Cl− SiO (Figure 2e), the mass loss after 300 °C also contained the collapse of
2 2
PW anions in the remainder form of P O and WO .
2 5 3
2.1.3. SEM, EDS, TEM, and DLS Characterization
The morphology changes between SiO nanoparticles and the modified product of SiO -Cl, SiO -Im,
2 2 2
and SiO Im+Cl− SiO were provided by the SEM and TEM images presented in Figure 3. Figure 3a,b
2 2
were the SEM images for SiO nanoparticles before reaction, which presented scattered and small
2
particles size of SiO . The particle size could be observed in TEM in Figure 3f,g with an average of
2
15 nm diameter which was in slight agglomeration. After functionalization, the particle morphology of
SiO -Cl and SiO -Im had almost no change (showed in (c) and (d)) compared to SiO nanoparticles.
2 2 2
Figure 3e,h showed the SEM and TEM micrographs after linking nanoparticles through the ionic
Catalysts2016,6,2 3of13
of imidazolium presents a higher thermal stability than the aromatic precursors. The precursors
started to decompose before 200 ˝C, while the imidazolium mainly decomposes around 300 ˝C.
FromFigure2b,c, SiO -Clstartedtodecomposeat180˝C,SiO -Imstartedtodecomposeat120˝C,
2 2
and the main weight loss was around 400 ˝C. From Figure 2d, the first stage ranging from 25 to
120˝Cwasascribedtotheeliminationofadsorbedwater. After120˝Ctheunreactedorganicgroups
Cstaatratleydsttso 2d0e1c6o,m 6 poseandtheweightlossaround300˝Cwasmainlyattributedtothedecomposition4

ofimidazoliumionicunits,whichcouldverifytheoccurrenceofnucleophilicreactions,ononehand.
lFiqouridth-leikPe Wb(o0n.0d5. 8T)/hSei OproIdmu+cCt,l ´byS iOobse(Frvigautiroen 2oef) , “thiselamndass”s, lcoosnsnaefcteterd3 0in0to˝ Ca allasorgecro ngtaroinuepd, ltihkee
2 2
scpoolnlagpes-ecaokfeP wWitahn diiosnpserinsetdh heorleems.a inderformofP O andWO .
2 5 3
100
(b)
95 (a)
%)
s ( 90 (e)
as (d)
M
85
(c)
80
75
100 200 300 400 500 600 700 800
Temperature (oC)
FFigiguurere2 .2T. hTehrmeromgoragvriamveimtriectrciucr vceusrvoefs( ao)fS (iOa)2 ;S(ibO)S;i O(b2)-C Sli;O(c)-SCiOl; 2(-cIm) S;i(Od)S-IiOm2; I(md)+ CSli´OS iIOm2+;Canl−d
2 2 2 2
(e)PW(0.058)/SiO Im+Cl´SiO .
SiO ; and (e) PW2(0.058)/SiO2 Im+Cl− SiO .
2 2 2
2.1.3. SEM,EDS,TEM,andDLSCharacterization
(a) (b) (c) (d) (e)
The morphology changes between SiO nanoparticles and the modified product of SiO -Cl,
2 2
SiO -Im, andSiO Im+Cl´ SiO wereprovidedbytheSEMandTEMimagespresentedinFigure3.
2 2 2
Figure 3a,b were the SEM images for SiO nanoparticles before reaction, which presented scattered
2
and small particles size of SiO . The particle size could be observed in TEM in Figure 3f,g with
2

anaverageof15nmdiameterwhichwasinslightagglomeration.Afterfunctionalization,theparticle
morphologyofSiO -ClandSiO -Imhadalmostnochange(showedin(c)and(d))comparedtoSiO
2 (f) 2 (g) (h) 2
nanoparticles.Figure3e,hshowedtheSEMandTEMmicrographsafterlinkingnanoparticlesthrough
the ionic liquid-like bond. The product, by observation of “islands”, connected into a larger group,
likesponge-cakewithdispersedholes.
In order to get a profound insight into the microscopic structure, scanning SEM-energy

dispersive spectroscopy (EDS) elemental mapping images of catalyst PW(0.058)/SiO -Im-SiO was
2 2
produced,and(ti)h eresultsweresShio wninFigure3i.OSi ,O,N,Cl,P,andWN wereuniformlydistributed
inthiscatalyst.
DLS (Dynamic Light Scattering) was carried out after dispersing the samples in ethanol with
previoussonicationandtheresultswereshowninFigure4. Italsoindicatedthattheparticlesizeof
the product SiO Im+Cl´ SiO was much larger than the one before reaction, which corresponded
2 2
to the SEM and TEMCmlicrographs. ForPF igure 4A, a diffraWction peak could be observed around
30nm.TheresultwaslargerthanthemeasurementbyTEMmicrographs,whichwasduetotheslight
agglomeration. WhenreactingwithN-(3-triethoxysilylpropyl)-4,5-dihydroimidazol,theparticlesize
wasslightlyincreasedinFigure4B.Nevertheless,fortheresultedSiO Im+Cl´SiO ,themostaverage
2 2
sizeshowedinFigure4Cwasat700nmafternucleophilicreaction. Thesmallersizedistributionwas
duetotheincompleteorpartialreactionofthefunctionalizedsilicaparticles. Overall, thissizewas
Figure 3. SEM, TEM and EDS images at different magnifications. (a,b) for SEM of SiO ;
2
muchlargerthanSiO ,alsoindicatingthechangeafterthereaction.
(c) for SEM of 2SiO -Cl; (d) for SEM of SiO -Im; (e) for SEM of SiO Im+Cl− SiO ;
2 2 2 2
(f,g) for TEM of SiO ; and (h) for TEM of SiO Im+Cl− SiO respectively; (i) for EDS
2 2 2
elemental mapping images of the Si, O, N, Cl, P, and W, respectively.
Catalysts 2016, 6 4

liquid-like bond. The product, by observation of “islands”, connected into a larger group, like
sponge-cake with dispersed holes.
100
(b)
95 (a)
%)
s ( 90 (e)
as (d)
M
85
(c)
80
75
100 200 300 400 500 600 700 800
Temperature (oC)
Figure 2. Thermogravimetric curves of (a) SiO ; (b) SiO -Cl; (c) SiO -Im; (d) SiO Im+Cl−
2 2 2 2
Catalysts2016,6,2 4of13
SiO ; and (e) PW(0.058)/SiO Im+Cl− SiO .
2 2 2
(a) (b) (c) (d) (e)

(f) (g) (h)
Catalysts 2016, 6 5

In order to get a profound insight into the microscopic structure, scanning SEM-energy dispersive

spectroscopy (EDS) elemental mapping images of catalyst PW(0.058)/SiO -Im-SiO was produced, and
2 2
the results were( is) hown in Figure S3ii . Si, O, N, Cl, P, aOn d W were uniformNly distributed in this catalyst.
DLS (Dynamic Light Scattering) was carried out after dispersing the samples in ethanol with previous
sonication and the results were shown in Figure 4. It also indicated that the particle size of the product
SiO Im+Cl− SiO was much larger than the one before reaction, which corresponded to the SEM and
2 2
TEM micrographs. ForC Fligure 4A, a diffrPa ction peak could bWe observed around 30 nm. The result was
larger than the measurement by TEM micrographs, which was due to the slight agglomeration. When
reacting with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol, the particle size was slightly increased in
Figure 4B. Nevertheless, for the resulted SiO Im+Cl− SiO , the most average si ze showed in Figure 4(C)
2 2
was at 700 nm after nucleophilic reaction. The smaller size distribution was due to the incomplete or
FigFuirgeu r3e.3 .SSEEMM,, TTEEMMan adnEdD ESiDmSag iemsaatgdeisff earte ndtimffaegrennifitc amtiaognsn.i(fai,cba)tfioornSsE.M (ao,fbS)i Ofo2;r( cS)EfoMrS EoMf SoifO2;
partia(lc r)e SafiocOrt2i -oSCnlE; oM(df) tfohofre SSfEuiMOncot-fiCoSilnO; a2(l-idIzm)e ;df(o es)ri floSircEaSM EpMa rootffi cSSlieOisO2. IOm-I+vmCe;lr´ a(lSeli,)O t2fho;i(srf ,gsSi)EzfoeMr wT EoaMsf mSofiuOSciOh 2Il;maar+ngCdel(r−h t)hSfoaiOrn S;i O 2, also
indicatinTgE tMheo cfhSiaOn2gIem a+Cftle´r2 StihOe2 rreeaspcetciotivne. ly;(i)forEDSel2ementalmappingimagesofthe2S i,O,N,Cl,P, 2
(f,ga) nfdoWr ,TreEspMec toivfe lSy.iO2; and (h) for TEM of SiO2 Im+Cl− SiO2 respectively; (i) for EDS
elemental mapp ing images of the Si, O, N, Cl, P , and W, respectively.
1.0 1.0 1.0
0.8 0.8 0.8
Distribution/a.u.00..46 Distribution/a.u.00..46 Distribution/a.u.00..46
0.2 0.2 0.2
0.0 0.0 0.0
0.1 1 10 100 1000 10000 0.1 1 10 100 1000 10000 0.1 1 10 100 1000 10000
Radius/nm Radius/nm Radius/nm
1
(A) (B) (C)
Figure4.DLSmeasurementof(A)thestartingSiO nanoparticles;(B)SiO -Im;and(C)resultantSiO
Figure 4. DLS measurement of (A) the s2tarting SiO nanop2articles; (B) SiO -Im2; and
2 2
Im+Cl´SiO .
(C) resultant 2SiO Im+Cl− SiO .
2 2
2.1.4. NitrogenSorption
2.1.4. Nitrogen Sorption
The porous characteristic of the materials were investigated by Brunauer-Emmet-Teller (BET)
method. The N adsorption-desorption isotherms and corresponding pore size distribution curves
The porous cha2racteristic of the materials were investigated by Brunauer-Emmet-Teller (BET)
were shown in Figure 5 (the curves of PW/SiO Im+Cl´ SiO were particularly similar, so
2 2
methPoWd.( 0T.0h5e8 )N/S2i OadsIomrp+Ctiol´n-SdiOesowrpatsiocnh oisseotahserrempsr easnendt actoivrree).spAolnldsianmgp pleosred isspizleay deidsttryibpuictailonty pcue-rIvVes were
2 2
showisno tihne rFmigsuwriet h5a (ctlheaer caudrsvoerps toiofn P-dWes/oSripOtio nImhy+sCtelr−e sSiisOloowpsearet thpearrteilcautilvaerlpyr essismurielaorf, 0s.o8 <PPW/(P0.0<518)/SiO
2 2 0 2
Im+C(Fl−ig SuirOe52 ,wleafts) cahsowseel laass rberporaedsepnotraetsivizee).d Aistlrli bsaumtiopnle(Fs idgiusrpel5a,yreigdh tty).pFicoarlt htyepSeiO-I2Vn aisnootphaerrtmiclse sw,iththe a clear
poresizewasmainlydistributedinlessthan2nm,whichwasduetothemicroporesofnanoparticles
adsorption-desorption hysteresis loops at the relative pressure of 0.8 < P/P < 1 (Figure 5, left) as well
0
as broad pore size distribution (Figure 5, right). For the SiO nanoparticles, the pore size was mainly
2
distributed in less than 2 nm, which was due to the micropores of nanoparticles themselves. The size
formed above 2 nm was mainly due to the accumulation between the particles. However, when linked
by ionic liquid, the SiO network and its catalyst showed more regular in pore size distribution between
2
20–40 nm. Data of surface area, pore diameter, and pore volume were presented in Table 1. The BET
specific surface area of the prepared SiO particles (entry 1) was as high as 381 m2g−1 and the average
2
pore size was 11.2 nm. When modified by organic reagents and then linked by nucleophilic reaction,
specific surface of SiO network (entry 2) was obviously lower than the pure SiO due to the introduction
2 2
of the organic moieties. After introducing phosphotungstic acid anions into the material by ion exchange,
the specific surface of PW(x)/SiO Im+Cl− SiO (entries 3–6) also decreased and were lower than the
2 2
Catalysts2016,6,2 5of13
themselves. Thesizeformedabove2nmwasmainlyduetotheaccumulationbetweentheparticles.
However, when linked by ionic liquid, the SiO network and its catalyst showed more regular in
2
poresizedistributionbetween20–40nm. Dataofsurfacearea,porediameter,andporevolumewere
presentedinTable1.TheBETspecificsurfaceareaofthepreparedSiO particles(entry1)wasashigh
2
as381m2¨g´1 andtheaverageporesizewas11.2nm. Whenmodifiedbyorganicreagentsandthen
linkedbynucleophilicreaction,specificsurfaceofSiO network(entry2)wasobviouslylowerthan
2
thepureSiO duetotheintroductionoftheorganicmoieties.Afterintroducingphosphotungsticacid
2
anionsintothematerialbyionexchange,thespecificsurfaceofPW(x)/SiO Im+Cl´SiO (entries3–6)
2 2
also decreased and were lower than the SiO network. Meanwhile, compared to the pure SiO , the
2 2
SiO networkanditscatalystsexhibitedanincreaseofporesizeandporevolume. Inparticular,the
2
poresizeseemedmorecentralized. Thisincreasedthecontactareaofthesubstrateandcatalystsand
led to better catalytic effect. Meanwhile, increased PW anions loading of PW(x)/SiO Im+Cl´ SiO
2 2
ledtoagradualdecreaseinporevolumeandaverageporesize(entries3–6).

(A)  (B)

Figure5.Nitrogenadsorption-desorptionisotherms(A)andporesizedistribution(B)ofthesamples
SiO ,SiO Im+Cl´SiO ,andPW/SiO Im+Cl´SiO .
2 2 2 2 2
Table1.BETparametersoftheSiO ,SiO Im+Cl´SiO ,andPW(x)/SiO Im+Cl´SiO .
2 2 2 2 2
Entry Compound SBET(m2¨g´1) Vp(cm3¨g´1) AveragePoreSize(nm)
1 SiO 381 0.93 11.2
2
2 SiO Im+Cl´SiO 242 1.50 24.1
2 2
3 PW(0.035)/SiO Im+Cl´SiO 182 1.24 24.0
2 2
4 PW(0.058)/SiO Im+Cl´SiO 176 1.07 24.0
2 2
5 PW(0.074)/SiO Im+Cl´SiO 168 1.01 22.4
2 2
6 PW(0.17)/SiO Im+Cl´SiO 159 0.93 20.3
2 2
2.1.5. MoreEvidenceoftheSynthesisoftheNetwork
Inordertofurtherconfirmtheoccurrenceofnucleophilicsubstitutionbetweentheimidazoline
functional group and chloroalkyl group, anion exchange experiment was carried out as an indirect
proof. Once the nucleophilic substitution happened, the chloride ion was easily exchanged by the
fluorinatedaniontoobtaintheSiO Im+BF ´ SiO andNaCl. First,wedetectedchlorideionsinthe
2 4 2
solvent and washing phase by the signature of the halide salts in the X-ray diffraction pattern after
1
Catalysts2016,6,2 6of13
Catalysts 2016, 6 7
drying (Figure 6). In the XRD pattern, the obtained salt reflection was marked with a star and was
Catalysts 2016, 6 7
consistentwithNaCl. TheotherBraggpeaksbelongedtothefluorinatedsaltwhichwasinanexcess
the neiwnloyr deexrcthoacnognefidr mantihoense xccohualndg eb.e Noebxvt,ioEuDslXy aonbaslyesrivseodf wthheilhey bthried bmoarotenr iawlaSsiO h2idImde+nB Fb4y´ tShieO 2c,arbon
the neowbltyai neexdchafatnergtehde aexncihoannsg ceoruealdct iboen aolbsovisohuoswlye dotbhseeerxviestde nwcehiolfen tehwel yb-oinrtorno dwucaesd haindidoenns (bFyig uthree 7c).arbon
peak. After the exchange reaction, it was noted that chlorine could still be observed in a small amount.
peak. ATyfpteicra tlhpee aekxschoafnflgueo rrienaecftrioomn, tiht ewnaesw nlyoteexdc hthanatg ecdhlaonriionnes ccoouulldd sbteilol bbvei ooubssleyrovbesde rivne ad swmhaillel tahmeount.
The presence of chlorine was due to the residue alkyl chloride groups which did not react with APTES
boronwashiddenbythecarbonpeak. Aftertheexchangereaction,itwasnotedthatchlorinecould
The presence of chlorine was due to the residue alkyl chloride groups which did not react with APTES
or the srteilslibdeueo bcshelrovreidnein ioanssm walhliacmh owuenrte. Tnhoet epxrecsheanncgeeodf. chlorine was due to the residue alkyl chloride
or the residue chlorine ions which were not exchanged.
groupswhichdidnotreactwithAPTESortheresiduechlorineionswhichwerenotexchanged.
u.)
ensity(a.u.)Intensity(a. ((aa)) ** ** ** ** ** **
nt
I
(b)
(b)
10 20 30 40 50 60 70
10 20 2 T3h0eta(degre4e0s) 50 60 70
2 Theta(degrees)
Figure 6. XRD analyses of (a) the exchange solvent, after separation and washing;
Figure 6. XRD analyses of (a) the exchange solvent, after separation and washing; (b) NaBF .
Figure 6. XRD analyses of (a) the exchange solvent, after separation and was4hing;
(b) NTahBeoFb4t.a Tinhede soablttareiflneecdti osnalwt arsefmleacrktieodnw withasa mstaarrakneddw waistcho an ssistatern atnwdit hwNasa Cclo.nsistent with NaCl.
(b) NaBF . The obtained salt reflection was marked with a star and was consistent with NaCl.
4

Figure7.EDXspectraoftheresultingmaterials.
Figure 7. EDX spectra of the resulting materials.
Figure 7. EDX spectra of the resulting materials.
2.2. CatalyticPerformances
2.2. Catalytic Performances
2.2. CatalyItticis Pkenrofwornmtaonaclelst hat the Keggin-type [PW O ]3´ (PW) can be degraded to peroxo-active
12 40
It issp kencioews,n[P tWo 4aOll8 (tOha2t) 8t]h3´e sKpeegcigeisn,-btyypexec [ePssWhydOrog]e3n− p(PerWox)i dcea,nw bheic dheigsrthadeeadct itvoe psepreocxieos-iancatilvkeen sepsecies,
12 40
[PWIt iOse pk(onOxoidw)ant]i3 o−ton s sa[pl2le5 ct]h.ieTasth ,e thsbeoy lKv eeengxtgcheianss-sta yghpryeed a[trPoinWgfle1un2e Onpc40ee]ro3o−nx (tihPdeWea, c)t wicvahintiyc bhoe f cidaset agtlryhatedi ceradec attcoitv ipoe nersaopnxedoca-icaeecst toivinneit rsialpelekceineess,
4 8 2 8
[PW4Ois8c(oOm2)m8]o3n− lysupseecdieisn, epboyx ideaxtcioenssr eahcytidornoagseann epfefircoiexnidteso, lvwenhti.cMh oirse ovthere, uasicntgivHe 2Osp2eacsiaens oixny geanlkenes
epoxidations [25]. The solvent has a great influence on the activity of catalytic reaction and acetonitrile
sourcecandogreatbenefitsontheenvironmentandindustry[26]. Thus,Table2listedthecatalytic
epoxidations [25]. The solvent has a great influence on the activity of catalytic reaction and acetonitrile
is compmerofonrlmy aunsceedo ifnd eifpfeorxeindtactaiotanl yrsetascftoironth aese apno xeidffaitciioennto fsocylvcelonotc. tMenoerienoavceert,o unsitirnilge HusiOng aasq uaeno uosxygen
2 2
is com3m0%onHly Ouseads iann eopxoidxaindta.tioTnh erereascutilotsns haso waned efthfiactiethnet scoatlavleynstt. PMWo/rSeioOveIrm, u+sCiln´g SHiO2O2h aads manu cohxygen
source can d2o 2great benefits on the environment and industry [26]. Thus2, Table 2 lis2ted the catalytic
sourceb ectatenr dcaot aglyretiact ebffeencteftihtasn otnh ethKee gegnivni-rtoynpemHenPt WandO ind.uOstnrey r[e2a6so].n Twhausst,h Teasbulpee r2-h lyidstreodp hthobei ccaotfalytic
3 12 40
performance of different catalysts for the epoxidation of cyclooctene in acetonitrile using aqueous 30%
H PW O , which made it difficult to contact with the oily substrates. Meanwhile, the cations of
perform3ance1 2of4 d0ifferent catalysts for the epoxidation of cyclooctene in acetonitrile using aqueous 30%
HH2OO2 aiamss iaadnna zoooxxliiidudamanntat..n TTdhhNee H rree2ss+uuillnttssi msshhidooawwzeeoddli n tthehaamtt atthhdeee cictaaettaaasllyiyessrtt fPPoWrWth//SSeiirOOed2 oIIxmmo++fCCPllW−− SS.iPiOOhy2 shhiacaaddl mmlouaudcchihn bgbeeottftteeHrr PccWaattaallyyttiicc
2 2 2 2
effect than the Keggin-type H PW O . One reason was the super-hydrophobic of H PW O , which
3 12 40 3 12 40
effect than the Keggin-type H PW O . One reason was the super-hydrophobic of H PW O , which
made it difficult to contact wit3h the1 2oil4y0 substrates. Meanwhile, the cations of imidazol3ium1 a2nd40 NH + in
2
made it difficult to contact with the oily substrates. Meanwhile, the cations of imidazolium and NH + in
2
imidazoline made it easier for the redox of PW. Physical loading of HPW on the pure SiO nanoparticles
2
imidazoline made it easier for the redox of PW. Physical loading of HPW on the pure SiO nanoparticles
2
Catalysts2016,6,2 7of13
on the pure SiO nanoparticles was carried out and the catalytic activity was very low, which also
2
illustrated the serviceability of SiO Im+Cl´ SiO for loading the PW anions. In order to further
2 2
compare the catalytic activity of the network catalysts with traditional silica grafted ionic liquid
catalyst, we also performed a catalytic reaction in the presence of the imidazolium and the silica
particleslabeledasHPW/SiO -ILwhichwaspreparedbynucleophilicreactionbetweenSiO -Cland
2 2
methylimidazole, and then reacted with phosphotungstic acid. The catalytic activity showed that
theconversionwaslowerthanthenetworkcatalystswithsimilarselectivity(Table2,entry4),which
demonstratedthattheformednetworkcatalystscouldleadtohighconversionandselectivity.
Table2.Catalyticperformancesofcyclooctenewithvariouscatalysts.
Conversion(%) Selectivity(%)
Catalysts
2h 4h 6h 8h 2h 4h 6h 8h
SiO - - - - - - - -
2
HPW 4.9 8.6 11.2 14.2 70.3 67.9 61.8 43.2
HPW/SiO 0.8 1.6 4.4 6.6 36.8 34.5 35.8 32.4
2
HPW/SiO -IL 30.5 52.7 72.4 79.8 98.7 98.1 97.4 97.0
2
PW(0.035)/SiO Im+Cl´SiO 54.3 68.4 76.5 84.1 98.5 98.2 97.9 96.9
2 2
PW(0.058)/SiO Im+Cl´SiO 77.2 88.8 94.4 99.4 99.4 99.4 99.3 99.2
2 2
PW(0.074)/SiO Im+Cl´SiO 68.5 78.2 91.1 93.5 99.3 98.4 98.9 98.9
2 2
PW(0.17)/SiO Im+Cl´SiO 45.3 63.6 70.2 77.6 98.7 98.1 98.7 98.2
2 2
Reaction conditions: cyclooctene: 2 mmol, acetonitrile: 5 mL, H2O2: 6 mmol, catalysts: 0.02 g,
temperature70˝C.
Furthermore, Table 2 also showed that the loading of PW anions of PW/SiO Im+Cl´SiO
2 2
affected the catalytic activity of the catalysts. This was correlation with the amount of active sites
inthecatalystandtheinsidestructureofmaterial. Obviously, PW(0.058)/SiO Im+Cl´SiO showed
2 2
the best activity. When the loading of PW anions was less than 0.058, the conversion decreased
accordingly. However,excessivePWloadingresultedintheagglomerationofexcessiveHPWinthe
channel,whichmightchangethespecificsurfaceareaandporevolume,therefore,ledtothedecrease
ofcatalyticactivity.
ToinvestigatetheimportanceoftheILmoiety,theimidazolinebasedPW/SiO -Imwasprepared
2
bystirring0.6gHPWwith1gSiO -Im,andtheloadingofPWwereca. 0.082mmol/g. Thecatalytic
2
activity and reusability were compared in Table 3. Imidazoline-modified SiO can be protonated
2
withHPW.TheresultsshowedthatthecatalystofPW/SiO -Imofferedevenhigherconversionthan
2
PW(0.058)/SiO Im+Cl´SiO inthefirstrun. ThisisbecauseoftheincreasedprotonatingofSiO -Im
2 2 2
per unit mass leading to more PW loading, and the PW(0.058)/SiO Im+Cl´SiO also contains the
2 2
part of SiO -Cl. However, the reusability of PW/SiO -Im was far less active than the catalyst of
2 2
PW(0.058)/SiO Im+Cl´SiO . Thereweretwopossiblereasons: onewasthatthestabilityofNH +in
2 2 2
imidazolinewaslowandtheotherwasthattherecoveryefficiencyofindividualSiO nanoparticles
2
wasmuchlowerthanthecollectiveSiO networkonaccountofthecatalystssize.
2
Table 3. Catalytic reusability of PW(0.058)/SiO Im+Cl´SiO and PW(0.082)/SiO -Im for the
2 2 2
epoxidationofcyclooctene.
PW(0.058)/SiO Im+Cl´SiO PW(0.082)/SiO -Im
Runa 2 2 2
Conversion(%) Selectivity(%) Conversion(%) Selectivity(%)
1 94.4 99.3 96.0 98.2
2 88.7 97.5 27.1 87.3
3 82.1 95.1 25.8 81.6
4 75.2 94.6 18.6 78.3
The reusability experiment was carried out by adding the residual catalyst in the next reaction after
centrifugation and drying without any change of other components. a Reaction conditions: cyclooctene:
2mmol,acetonitrile:5mL,H2O2:6mmol,catalysts:0.02g,temperature:70˝C,reactiontime:6h.
Catalysts 2016, 6 9

Table 3. Catalytic reusability of PW(0.058)/SiO Im+Cl−SiO and PW(0.082)/SiO -Im for
2 2 2
the epoxidation of cyclooctene.
PW(0.058)/SiO Im+Cl−SiO PW(0.082)/SiO -Im
Run a 2 2 2
Conversion (%) Selectivity (%) Conversion (%) Selectivity (%)
1 94.4 99.3 96.0 98.2
2 88.7 97.5 27.1 87.3
3 82.1 95.1 25.8 81.6
4 75.2 94.6 18.6 78.3
The reusability experiment was carried out by adding the residual catalyst in the next reaction after
centrifugation and drying without any change of other components. a Reaction conditions: cyclooctene:
2 mmol, acetonitrile: 5 mL, H O : 6 mmol, catalysts: 0.02 g, temperature: 70 °C, reaction time: 6 h.
2 2
2.2.1. Effects of Temperature and Time
Catalysts2016,6,2 8of13
Each chemical reaction was accompanied by thermal effects. The same reaction carried out at
differ2e.n2.t1 .teEmffepcetrsaotufrTeesm wpeorualtdu rreeasnudlt Tiinm qeuite different results. The reaction time was an important basis
to judge it effective or not. The effects of temperature and time on the epoxidation of cyclooctene using
Each chemical reaction was accompanied by thermal effects. The same reaction carried out at
catalydsitf fPerWen(t0t.e0m5p8e)/rSatiuOr2eIsmw+oCull−dSirOes2u wlteinreq suhitoewdnif fienr eFnitgruerseu l8ts. .ItT wheasre oabctsieornvteidm ethwata swaitnh imthpeo irntacnretase of
tempebraastiusrteo, jtuhdeg emiotleefcfueclatirv eenoerrgnyot,. anTdh ereelfafetcivtse ocfotnemtepnet raotfu raecatinvdatetidm emoonletchueleesp oinxicdraetaiosend,o fwhich
cycloocteneusingcatalystPW(0.058)/SiO Im+Cl´SiO wereshowninFigure8. Itwasobservedthat
increased the effective collision between t2he reactant2 molecules, thereby improving the conversion.
with the increase of temperature, the molecular energy, and relative content of activated molecules
However, the increase of temperature also increased the possibility of ring rupture, resulting in the
increased,whichincreasedtheeffectivecollisionbetweenthereactantmolecules,therebyimproving
decretahsee coofn vseerlesicotniv.iHtyo. wWevheern, ttheeminpcerreaatsuereo freteamchpeedra 7tu0r e°Cal,s othien crreeaacsteidonth heapdo tsesnibdielidty toof brainlagnrcuep. tHuroew, ever,
the corensvueltrisnigonin dtehcerdeaescreeda saet o8f0s °eClec atinvdit yt.hWe sheelnectetimvipteyr awtuarse irne aac hsehdar7p0 d˝eCc,ltihnee rweahcitciohn mhaigdhtte nbdee dduteo to the
balance. However,theconversiondecreasedat80˝Candtheselectivitywasinasharpdeclinewhich
excessive activation of the catalyst. And with the increase of reaction time, the conversion increased
might be due to the excessive activation of the catalyst. And with the increase of reaction time, the
while selectivity decreasing. As a whole, the conversion and selectivity were both higher than 90% at
conversion increased while selectivity decreasing. As a whole, the conversion and selectivity were
70 °Cb ointh 6h hig;h sepretchiafnic9a0ll%y,a 9t470.4˝%C iann6d h9;9s.p3e%cifi, crealslpy,e9c4ti.4v%elyan. d 99.3%,respectively.
100
100
(b)
90 99 (a)
%
C atalysts 2016, Conversion/6 7800 ((bcc)) Selectivity/%9978 (c) 10
60
amount was over5 0200 (ma)g. Taking economics and green c9h6emistry into consideration, a low catalyst
amount (200 mg) 4w0 as used in further experiments. 95
2 4 Tim6e/h 8 10 2 4 Tim6e/h 8 10
(A) (B)
2.2.3. Effect of Solvent
Figure8. Effectoftemperatureandtimeon(A)conversion;and(B)selectivity. (a): 60˝C;(b): 70˝C;
Figure 8. Effect of temperature and time on (A) conversion; and (B) selectivity. (a): 60 °C;
In ordera ntdo( ci)n:v80es˝Cti.gate the general application of the synthetic catalyst in different solvents, the
(b): 70 °C; and (c): 80 °C.
influences of solvents on the oxidation of cyclooctene were studied and the results were summarized in
2.2.2. EffectofCatalystDosage
Table 4. Acetonitrile, methanol, ethanol, chloroform, 1,2-dichloroethane, and ethyl acetate were used as
2.2.2. Effect of Catalyst Dosage
solvents. TThhee irneflsuuletnsc edseomfodnifsfterraetnetdc athtaalty stthaem sooulnvtesnwtse rweeinrev egsteignaetreadl arnedlevthaentr etsou ltaslkweenreess heopwonxiidnation,
Figure 9. It was indicated that the conversion increased with the increase of catalyst amount, but
espeTchiael lyin ffolure ancceetso noift ridleif,f emreentht acnaotla, lyasntd acmholournotfso rwme.r eIn inpvaertsitciuglaatre,d thaen dc atthaely triecs urletasc twioenr ew sahso wmno rein
furtherimprovedslightlywhentheamountwasover300mg. Theselectivityhadalmostnochange
suFiitgaubrlee 9i.n I tt hwea sp ionldairc astoeldv ethnat.t tDheu ec otnov ethrsei ohni ginhc rbeoaisleindg w pitohi ntht ea inndc rsetaasbel eo fs cealetacltyivsti taym oofu ntht,e b uptr ofduurtchte, r
whentheamountwasover200mg. Takingeconomicsandgreenchemistryintoconsideration,alow
aicmetpornocivatrteiadlley sswltiaagmsh,t oltyhu unwts,(h 2ce0hn0o mtshegen) awamsa sothuuens ert dewaicnatsif ouonrvt hemerr e3de0xiu0pm emr iimgn. e oTnuthsre. e sxepleercitmiveintyt. had almost no change when the
100
100 (d)
(b)
90
(c) 99 (d)
%80 % (c)
Conversion/567000 ((ba)) Selectivity/9978 (a)
40
96
30
20 95
2 4 Tim6e/h 8 10 2 4 Tim6e/h 8 10
(A) (B)
Figure9. Effectoftemperatureandtimeon(A)conversionand(B)selectivity. (a): 0.01g;(b):0.02g;
Figure 9. Effect of temperature and time on (A) conversion and (B) selectivity. (a): 0.01g;
(c):0.03g;(d):0.04g.
(b):0.02g; (c): 0.03g; (d): 0.04g.
Table 4. Effect of solvents on epoxidation of cyclooctene.
Boiling Point Conversion Selectivity
Entry Solvent
(°C) 4 h 8 h 4 h 8 h
1 Acetonitrile 81.6 88.8 99.4 99.4 99.2
2 Methanol 64.5 92.9 96.8 99.1 96.6
3 Ethanol 78.2 52.0 77.1 98.8 98.8
4 Chloroform 61.2 98.9 100.0 98.6 96.6
5 1,2-Dichloroethane 83.5 8.8 15.0 94.0 93.6
6 Ethyl acetate 78.3 4.5 5.5 85.8 88.2
Reaction conditions: cyclooctene: 2 mmol, solvent: 5 mL, H O : 6 mmol, catalysts: 0.02 g, temperature: 70 °C.
2 2
3. Experimental Section
3.1. Materials and Methods
N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and 3-aminopropyl-trimethoxysilane (APTES) were
purchased from Meryer (Meryer, Shanghai, China). Other commercially-available chemicals were
bought from local suppliers (Sinopharm Chemical Reagent, Beijing, China). All reagents were purified
by standard procedures before use. FT-IR spectra were obtained as potassium bromide pellets in a
Catalysts2016,6,2 9of13
2.2.3. EffectofSolvent
Inordertoinvestigatethegeneralapplicationofthesyntheticcatalystindifferentsolvents,the
influencesofsolventsontheoxidationofcyclooctenewerestudiedandtheresultsweresummarized
in Table 4. Acetonitrile, methanol, ethanol, chloroform, 1,2-dichloroethane, and ethyl acetate were
used as solvents. The results demonstrated that the solvents were general relevant to alkenes
epoxidation,especiallyforacetonitrile,methanol,andchloroform. Inparticular,thecatalyticreaction
was more suitable in the polar solvent. Due to the high boiling point and stable selectivity of the
product,acetonitrilewas,thus,chosenasthereactionmediuminourexperiment.
Table4.Effectofsolventsonepoxidationofcyclooctene.
Conversion Selectivity
Entry Solvent BoilingPoint(˝C)
4h 8h 4h 8h
1 Acetonitrile 81.6 88.8 99.4 99.4 99.2
2 Methanol 64.5 92.9 96.8 99.1 96.6
3 Ethanol 78.2 52.0 77.1 98.8 98.8
4 Chloroform 61.2 98.9 100.0 98.6 96.6
5 1,2-Dichloroethane 83.5 8.8 15.0 94.0 93.6
6 Ethylacetate 78.3 4.5 5.5 85.8 88.2
Reactionconditions:cyclooctene:2mmol,solvent:5mL,H2O2:6mmol,catalysts:0.02g,temperature:70˝C.
3. ExperimentalSection
3.1. MaterialsandMethods
N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and 3-aminopropyl-trimethoxysilane (APTES)
were purchased from Meryer (Meryer, Shanghai, China). Other commercially-available chemicals
were bought from local suppliers (Sinopharm Chemical Reagent, Beijing, China). All reagents
were purified by standard procedures before use. FT-IR spectra were obtained as potassium
bromide pellets in a Nicolet 360 FT-IR thermoscientific spectrometer in the 4000–400 cm´1 region
(Thermo Fisher Scientific, Waltham, MA, USA). The elemental analyses were performed on a CHN
elemental analyzer (Elementar, Hanau, HE, Germany). TG analysis was carried out with a STA409
instrument in dry air at a heating rate of 10 ˝C¨min´1 (Mettler Toledo, Zurich, Switzerland).
SEM image was performed on a HITACHI S-4800 field-emission scanning electron microscope
(Hitachi, Tokyo, Japan). Transmission electron microscopic (TEM) photographs of the prepared
samples were taken in JEOL JEM 2100 electron microscope under an accelerating voltage of 200 kV
(JEOL, Tokyo, Japan). The metal loading of the host materials of Tungsten were determined by
inductively-coupled plasma atomic emission spectroscopy (ICP-AES) on a Perkin-Elmer AA-300
spectrophotometer (Shimadzu, Kyoto, Japan). Nitrogen adsorption/desorption isotherms were
measured at ´196 ˝C using a Quantachrome Quadrasorb SI automated gas sorption system
(Micromeritics instrument corp, Atlanta, GA, USA). Samples were degassed under vacuum
for 5 h at 120 ˝C. The micropore volume was obtained with the t-plot method, while the
Brunauer-Emmet-Teller (BET) method was applied to calculate the specific surface area. Pore size
distributions were evaluated from desorption branches of nitrogen isotherms using the BJH model.
The total pore volume was determined at P/P 0.95. The X-ray diffraction (XRD) pattern of
0
the material was recorded on a Bruker D8 advanced powder X-ray diffractometer using Cu Ka
(k=1.5406Å)astheradiationsourcein2θrangeof4˝–70˝ withastepsizeof4˝ andasteptimeof
1s(BrukerAxsGmbh,Karlsruhe,BW,Germany). DLSexperimentswerecarriedoutwithprevious
sonicationofthesamples. Theruntimeofthemeasurementswas10s. Everysizedistributioncurve
was obtained by averaging six measurements. The apparatus was an ALV/DLS/SLS-5022F light
scatteringelectronicspectrometer(ALV-GmbH,Langen,Germany).
Catalysts 2016, 6 11

Nicolet 360 FT-IR thermoscientific spectrometer in the 4000–400 cm−1 region (Thermo Fisher
Scientific, Waltham, MA, USA). The elemental analyses were performed on a CHN elemental analyzer
(Elementar, Hanau, HE, Germany). TG analysis was carried out with a STA409 instrument in dry air at
a heating rate of 10 °C·min−1 (Mettler Toledo, Zurich, Switzerland). SEM image was performed on a
HITACHI S-4800 field-emission scanning electron microscope (Hitachi, Tokyo, Japan). Transmission
electron microscopic (TEM) photographs of the prepared samples were taken in JEOL JEM 2100
electron microscope under an accelerating voltage of 200 kV (JEOL, Tokyo, Japan). The metal loading
of the host materials of Tungsten were determined by inductively-coupled plasma atomic emission
spectroscopy (ICP-AES) on a Perkin-Elmer AA-300 spectrophotometer (Shimadzu, Kyoto, Japan).
Nitrogen adsorption/desorption isotherms were measured at −196 °C using a Quantachrome Quadrasorb
SI automated gas sorption system (Micromeritics instrument corp, Atlanta, GA, USA). Samples were
degassed under vacuum for 5 h at 120 °C. The micropore volume was obtained with the t-plot method,
while the Brunauer-Emmet-Teller (BET) method was applied to calculate the specific surface area. Pore
size distributions were evaluated from desorption branches of nitrogen isotherms using the BJH model.
The total pore volume was determined at P/P 0.95. The X-ray diffraction (XRD) pattern of the material
0
was recorded on a Bruker D8 advanced powder X-ray diffractometer using Cu Ka (k = 1.5406 Å) as the
radiation source in 2θ range of 4°–70° with a step size of 4° and a step time of 1s (Bruker Axs Gmbh,
Karlsruhe, BW, Germany). DLS experiments were carried out with previous sonication of the samples.
The run time of the measurements was 10 s. Every size distribution curve was obtained by averaging six
measurements. The apparatus was an ALV/DLS/SLS-5022F light scattering electronic spectrometer
(ALV-GmbH, Langen, Germany).
Catalysts2016,6,2 10of13
3.2. Catalyst Preparation
3.2. CatalystPreparation
Silica nanoparticles, as well as the surface functionalization of the silica nanoparticles with
Silica nanoparticles, as well as the surface functionalization of the silica nanoparticles with
N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and APTES, were prepared according to literature with
N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and APTES, were prepared according to literature
minowr imthomdiinfiocramtioond i[fi2c7a]t.i oTnh[e2 7ty].pTichaelt pypreicpaalrpatrieopna rpartoiocnedpurorece odfu trheeo cfathtaelycastta wlyasst wina sSicnhSecmhee m1.e 1.
N Cl
N
OH OSi N N OSOi OHOH OSi Cl OSOi
OH O O
OH Anhydrous toluene, reflux OOSi N N OH OOSi Cl
Anhydrous toluene, reflux
OH
Catalysts 20O1HO6H, 6 OOSi N N OH OOSi Cl 12
(A) (B)
N
N
OSOi NN OSOi Cl OOOSiOOSi N NCl SOiO OSOiCl OOOSi Cl
OOSi N N OOSi Cl 1) Anhydrous toluene, 80℃ OH NN
2) H3PW12O40
OOSi N N OOSi Cl OSiO SOiO
NCNl HO HO
OSOi
OOSi CNlN SOiO

(C)
ScheSmcheem 1e.1 .TTyyppiciaclapl reppraeraptaiornatpiroonce dpurroecoefdthuereca taolfy sttshPeW (cxa)/taSliyOs2tIsm +PCWl´(Sxi)O/S2.iO(A)SIymnt+hCesli−s oSfiO .
2 2
SiO -Im;(B)SynthesisofSiO -Cl;(C)SynthesisofPW(x)/SiO Im+Cl´SiO .
2 2 2 2
(A) Synthesis of SiO -Im; (B) Synthesis of SiO -Cl; (C) Synthesis of PW(x)/SiO Im+Cl− SiO .
2 2 2 2
3.2.1. SynthesisofSilicaNanoparticles
3.2.1. Synthesis of Silica Nanoparticles
74µLammoniasolution(25%–28%)and1.98g(110mmol)waterwereaddedto100mLabsolute
methanolina250mLroundbottomflask. Thesolutionwasstirredfor5minbeforeaddingdropwise
74 μL ammonia solution (25%–28%) and 1.98 g (110 mmol) water were added to 100 mL absolute
10.41 g (500 mmol) TEOS. The final solution was stirred for three days at ambient temperature.
methanol in a 250 mL round bottom flask. The solution was stirred for 5 min before adding dropwise
The resulting solid was centrifuged and washed with methanol and water several times, and dried
10.41u ngd (e5r0v0a cumummo.l) TEOS. The final solution was stirred for three days at ambient temperature.
The resulting solid was centrifuged and washed with methanol and water several times, and dried
3.2.2. SynthesisofModifiedSilicaNanoparticles
under vacuum.
0.6 g (0.01 mol) previously prepared silica nanoparticle was dispersed in 50 mL anhydrous
toluenebysonicationfor60min. Then0.005molN-(3-triethoxysilylpropyl)-4,5-dihydroimidazol(or
3.2.2. Synthesis of Modified Silica Nanoparticles
APTES) was added dropwise. The solution was stirred under 110 ˝C for 24 h. The product was
filtered, washed in a Soxhlet apparatus with diethyl ether and dichloromethane for 24 h, and then
0.6d rgi e(d0.a0t15 0m˝Colu) npdreervvioaucusluym p.rTehpearoebdta sinileidcap onwandoeprawratiscnlea mweads adsiSsipOer-sIemd ainnd 5S0i Om-LC la,nrehsypdercotiuvse ltyo.luene
2 2
by sonication for 60 min. Then 0.005 mol N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol (or APTES)
3.2.3. SynthesisofSilicaNanoparticleNetwork
was added dropwise. The solution was stirred under 110 °C for 24 h. The product was filtered, washed
The synthesis was driven by a nucleophilic substitution occurring between an imidazoline
in a Soxhlet apparatus with diethyl ether and dichloromethane for 24 h, and then dried at 50 °C under
functional group and a chloroalkyl group present on the surface of the silica nanoparticles.
vacuum. The obtained powder was named as SiO -Im and SiO -Cl, respectively.
0.5 g silica nanoparticles modified with N-(3-t2riethoxysilylpr2opyl)-4,5-dihydroimidazol and 0.5 g
silica nanoparticles modified with APTES were introduced into a 100 mL round bottom flask with
3.2.3.5 S0ymntLheasnihs yodfr oSuilsictao lNueanneo.paTrhtieclseo lNuteiotwnowrka s stirred over 2 days at 70 ˝C and filtered, washed,
The synthesis was driven by a nucleophilic substitution occurring between an imidazoline functional
group and a chloroalkyl group present on the surface of the silica nanoparticles. 0.5 g silica nanoparticles
modified with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazol and 0.5 g silica nanoparticles modified with
APTES were introduced into a 100 mL round bottom flask with 50 mL anhydrous toluene. The solution
was stirred over 2 days at 70 °C and filtered, washed, finally dried under vacuum. A pale yellow powder
was obtained and labelled as SiO Im+Cl− SiO . Elemental analysis: found C: 7.25 wt. %, H: 1.45 wt. %,
2 2
N: 1.96 wt. %.
3.2.4. Synthesis of Phosphotungstate-Loaded Silica Nanoparticle Network
Catalysts of PW(x)/SiO Im+Cl− SiO with different H PW O loadings on SiO Im+Cl− SiO were
2 2 3 12 40 2 2
prepared by following strategy. 1.0 g silica nanoparticle network was dispersed in 20 mL deionized
water and then added dropwise into an aqueous solution (20 mL) with various amount of H PW O
3 12 40