ChineseJournalofCatalysis36(2015)446–453
催化学报2015年第36卷第3期|www.chxb.cn a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m 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
Article HighlyhydrothermallystableFePO4–SBA‐15synthesizedusinga
novelone‐pothydrothermalmethod
RunqinWanga,c,RongheLina,YunjieDinga,b,*,JiaLiua,c,WentingLuoa,c,HongDua,c,YuanLüa
DalianNationalLaboratoryforCleanEnergy,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China StateKeyLaboratoryofCatalysis,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China cUniversityofChineseAcademyofSciences,Beijing100049,China a
b
A R T I C L E I N F O
A B S T R A C T
Articlehistory:
Received9October2014
Accepted18November2014
Published20March2015
Keywords:
Ironphosphate
SBA‐15
Mesoporousmaterial
Hydrothermalstability
Steam
Metalloading
The hydrothermal stability of FePO4–SBA‐15 synthesized using a novel one‐pot hydrothermal
method(OP)wassystematicallyinvestigatedusingtwomethods:treatmentwithpuresteamat800
°C or with boiling water at 100 °C. The structural changes in the samples were monitored using
small angle X‐ray diffraction and N2‐physisorption methods. It was found that the hydrothermal
stabilitiesofOPsamplesremainedhighandshowedlittledifferenceovertheFePO4‐dopingrange
5–40wt%.Theseresultsdifferfrompreviousreportsthattheloadingofheterogeneousmetalat‐
oms significantly influences the hydrothermal stability of the host ordered mesoporous material.
Forcomparison,thehydrothermalstabilitiesofFePO4–SBA‐15synthesizedusinganimpregnation
method(IMP)andcommerciallyobtainedSBA‐15werealsostudied.Theorderofthesamplehy‐
drothermalstabilitieswasOP>IMP>>SBA‐15.TheformedFePO4protectivelayershelpedtopre‐
vent mesostructure degradation during hydrothermal treatment, therefore modified samples
showedsuperiorhydrothermalstabilitiescomparedwithpureSBA‐15.Thesuperiorperformance
ofOPsamplesoverIMPsamplesismainlyattributedtotheformationofstableSi–O–Febondsand
moremicroporesinOPsamples.
©2015,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
SincethediscoveryoftheM41Ssilicatefamilyin1992[1,2],
highly ordered mesoporous silicates have attracted great in‐
terest because of their properties such as high surface areas,
largeporevolumes,andtunableporesizes.Muchresearchhas
focusedontheuseoforderedmesoporoussilicatematerialsas
catalystsandcatalystsupports[3–5].
Catalystreactivityandstabilityarethemostimportantfac‐
torsincatalysis,thereforethehydrothermalstabilitiesofmate‐
rials,especiallyin100%steamat600–800°C,arecrucialfac‐
tors in industrial applications such as steam reforming and
catalytic cracking [4,6,7]. Much research has been performed
on improving the hydrothermal stabilities of mesoporous sili‐
cates. It has been reported that mesoporous silicates with
thicker walls, more micropores, and silica walls with higher
degreesofpolymerizationaremorestableunderhydrothermal
conditions [8–14]. Some effective approaches have been de‐
velopedtoimprovethehydrothermalstabilitiesofmesoporous
materials, such as high‐temperature treatments [8,15], car‐
bon‐proppingthermaltreatments[8],andadditionofinorganic
salts [16–19]. These approaches increase the polymerization
degreeofthesilicaframeworkorprotectmesoporouschannels
against collapse, thereby improving the hydrothermal stabili‐
*Correspondingauthor.Tel/Fax:+86‐411‐84379143;E‐mail:[email protected] ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(21103170).
DOI:10.1016/S1872‐2067(14)60202‐3|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.36,No.3,March2015 RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
tiesofmesoporoussilicates.Ithasalsobeendemonstratedthat
the introduction of a metal into mesoporous silica greatly im‐
provesitshydrothermalstability,andthemetalloadingsignif‐
icantly affects the hydrothermal stability, especially in treat‐
mentsinpuresteamat800°C[20–24].Lietal.[20]reported
that Al–SBA‐15 samples, prepared using a post‐synthesis
method,withlowerAlcontentsweremorestableundersteam
at800°CthansampleswithhigherAlcontents.Withincreasing
Al content, more micropores on the pore walls are buried by
the Al layer, and micropores are important in improving the
hydrothermal stability of mesoporous silicate. At high Al con‐
tents, the Al species easily form agglomerates during steam
treatment, therefore the protective Al layer is destroyed and
many ≡Si–O–Si≡ bonds are exposed to the steam. However,
Selvaraj et al. [24] found that Cr–SBA‐15 samples with higher
amounts of Cr showed better hydrothermal stabilities than
those with low Cr amounts. They suggested that Cr–SBA‐15
withhigheramountsofCrhadmoreSi–O–Crbonds,whichare
relativelystabletofurtherattackbywatermolecules,andmore
tetrahedralCr6+/Cr5+ionscancreatemorenegativechargeson
the pore wall surfaces, which could repel attacks by water
molecules and OH− groups on the ≡Si–O–Si≡ bonds of the
framework. They also found that Ga–SBA‐15 behaves like
Cr–SBA‐15inhydrothermaltreatment[21].Thereisstillade‐
bateontheeffectofmetalatomsonthehydrothermalstability
ofamesoporousmaterial.
Inourpreviouswork[25],FePO4–SBA‐15(OP)synthesized
usinganovelone‐pothydrothermalmethodshowedgoodcat‐
alyticactivityandexcellentstabilityduringoxybrominationof
methane for 1000 h; this reaction requires severe reaction
conditions,i.e.,ahightemperatureofabout600°Candcorro‐
sive HBr/H2O as the feedstock. These results suggest that the
OPsampleisverystableandresistanttosevereconditionsfora
longtime.Adeeperunderstandingandexplorationoftherea‐
sonsforthehydrothermalstabilitiesofOPsampleswillhelpin
the development of hydrothermally stable catalysts and other
materials.Inthisstudy,weinvestigatedthehydrothermalsta‐
bilitiesofOPsampleswithlowandhighFePO4loadings,name‐
ly5and40wt%,bytreatingthemwithboilingwaterat100°C
orpuresteamat800°C.Inaddition,wecomparedthehydro‐
thermal stabilities of OP samples with FePO4–SBA‐15 (IMP)
samplespreparedusinganimpregnationmethodandcommer‐
cially available SBA‐15. X‐ray diffraction (XRD) and N2 phy‐
sisorptionwereusedtodeterminethechangesinthestructural
propertiescausedbythehydrothermaltreatments. 2. Experimental
447
35°Cfor2htoobtainsolutionB.SolutionAwasaddeddrop‐
wisetosolutionBwithstirring,andthensubsequentlystirred
vigorouslyfor20hat35°C.Themixturewasthenagedinan
autoclave for 24 h at 90 °C. The resultant solid was filtered,
washedwithdeionizedwater,anddriedat60°Cfor12hinair.
Calcination involved two steps: heating at 250 °C for 3 h, and
thenat600°Cfor4h.Themolarcompositionoftheinitialsolu‐
tionwas1.0TEOS:0.017P123:nFe:1.5H3PO4:208H2O(n=
0.02091 and 0.26490). The obtained FePO4–SBA‐15 samples
with FePO4 loadings of 5 wt% and 40 wt% were denoted by
5OPand40OP,respectively.
For comparison, FePO4–SBA‐15 was also prepared using a
previously reported incipient wetness impregnation method
withFe(NO3)3·9H2OandH3PO4asprecursors[26].SBA‐15was
purchasedfromtheChangchunJilinUniversityHighTech.Co.,
Ltd. The obtained samples with FePO4 loadings of 5 wt% and
40wt%weredenotedby5IMPand40IMP,respectively.
2.2. Hydrothermalstabilityevaluation
Thehydrothermalstabilitywasinvestigatedbytreatingthe
OPsamplesinaclosedbottleat100°Cfor7dunderstaticcon‐
ditions. The obtained solid products were denoted by
5OP‐b100and40OP‐b100. The high‐temperature hydrothermal stability was investi‐
gated by exposing the OP, IMP, and SBA‐15 samples to pure
steam(100%watervapor)at600,700,and800°Catautoge‐
nouspressurefor24h.Theobtainedsamplesweredenotedby
xOP‐sT, xIMP‐sT, and SBA‐15‐sT, respectively, where x (%) is
the FePO4 loading (x = 5 or 40), and T is the hydrothermal
treatmenttemperature(T=600,700,or800°C).
2.3. Characterization
The structural properties of the samples were determined
by N2 physisorption using a physical adsorption instrument
(Quantachrome, USA). Before the measurements, the samples
wereoutgassedat300°Cinavacuumfor3h.Thespecificsur‐
face areas were calculated using the BET method. The total
porevolumeswereestimatedfromtheamountsadsorbedata
relativepressureof 0.99. The microporevolumes weredeter‐
minedusingV–tplots.Theporesizedistributionswerederived
from the desorption branches of the isotherms using the BJH
method,exceptinthecasesof5OPand5IMP,whichwerede‐
rivedfromtheadsorptionbranchesoftheisothermsusingthe
BJHmethod.PowderXRDpatternswererecordedwithaPAN‐
alyticalX'Pert‐PropowderX‐raydiffractometerusingCuKα(40
kV,40mA)radiation.
2.1. Synthesis
3. Resultsanddiscussion
FePO4–SBA‐15 was prepared using a previously reported
one‐pot hydrothermal method [25]; the synthetic procedure
wasasfollows.AcertainamountofFe(NO3)3·9H2Oandtetrae‐
thylorthosilicate(TEOS,8.2mL)werehydrolyzedindeionized
water (10 mL) for 30 min to obtain solution A. A nonionic
triblockcopolymersurfactant(EO20PO70EO20(P123),4g)was
dissolvedin85wt%H3PO4anddeionizedwaterandstirredat
3.1. EffectofFePO4loadingonhydrothermalstabilitiesofOP
samples
The metal loading has an important effect on the hydro‐
thermalstabilitiesoforderedmesoporousmaterialssupported
metalsamples[20–23,27].WeinvestigatedtheeffectofFePO4
RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
600
1.1
0.9
(a)
Intensity
loading on the hydrothermal stability of the OP samples. Two
methods were used to evaluate the OP sample hydrothermal
stability:treatmentwithboilingwaterat100°Cfor7dorwith
pure steam at 800 °C for 24 h. Small‐angle XRD and N2 phy‐
sisorption were used to examine the structural properties of
theOPsamplesbeforeandafterhydrothermaltreatments.
Fig. 1(a) shows the small‐angle XRD patterns of 5OP,
5OP‐b100,and5OP‐s800.Itcanbeseenthatallthesesamples
hadsimilarXRDpatterns,withonlyonecleardiffractionpeak
at 0.9°–1.1°, which can be attributed to the (100) facets of
SBA‐15;thepeakscorrespondingtothe(110)and(200)facets
were less prominent. Furthermore, a comparison of the fresh
and treated 5OP samples shows that the relative intensity of
the (100) peak changed slightly after treatment with boiling
water at 100 °C or pure steam at 800 °C. It is therefore con‐
cludedthat5OPretainedanorderedhexagonalmesostructure
after the hydrothermal treatments. However, after treatment
with pure steam at 800 °C, the (100) diffraction peak of 5OP
shiftedtoalarger 2θvalue, suggestingthatthesamplemeso‐
poresshrankduringtreatmentwithpuresteamat800°C.Sim‐
ilarly, as shown in Fig. 1(b), the XRD patterns of fresh and
treated 40OPindicatedthatthese samples still hadhexagonal
mesostructures after the different hydrothermal treatments;
however,thesamplemesoporesshrankduringtreatmentwith
puresteamat800°Cfor24h.Theresultsforthe40OPsamples
are similar to those for 5OP, which implies that the SBA‐15
silicates modified with FePO4 have excellent hydrothermal
stabilitiesoverawideFePO4‐dopingrange.
TheXRDresultswereconfirmedusingN2physisorption.Fig.
2(a) and (b) shows the N2 adsorption‐desorption isotherms
and corresponding pore size distributions of 5OP, 5OP‐b100,
5OP-s800
5OP-b100
5OP
(b)
1.1
0.9
Intensity
448
40OP-s800
40OP-b100
40OP
0.5
1.0
1.5
2.0
2.5 3.0
2/( o )
3.5
4.0
4.5
5.0
Fig.1.Small‐angleXRDpatternsof5OPsamples(a)and40OPsamples
(b)beforeandafterhydrothermaltreatment.
and5OP‐s800,respectively.Toeliminateinterferencefromthe
tensilestrengtheffectoftheadsorbedphase[28],theporesize
distributionof5OPwasderivedfromtheadsorptionbranchof
(b)
(a)
9.5
dV/d(D) (a.u.)
Volume (ml/g)
500
400
300
5OP
200
5OP-b100
100
5OP-s800
0
0.0
600
0.2
0.4
0.6
Relative pressure (p/p0)
0.8
7.8
6.5
5OP
5OP-s800
0
1.0
5OP-b100
(c)
2
4
6
8 10 12 14
Pore diameter (nm)
16
18
20
7.9
(d)
400
dV/d(D) (a.u.)
Volume (ml/g)
500
300
40OP
200
40OP-b100
100
0
0.0
0.2
0.4
0.6
Relative pressure (p/p0)
6.6
40OP
40OP-s800
0.8
5.0
1.0
40OP-b100
0
2
4
6
40OP-s800
8 10 12 14
Pore diameter (nm)
16
18
20
Fig.2.N2adsorption‐desorptionisothermsandcorrespondingporesizedistributionsfor5OPsamples(a,b)and40OPsamples(c,d)beforeandafter
hydrothermaltreatment.
RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
the isotherm using the BJH method. The isotherm curves of
5OP,5OP‐b100,and5OP‐s800werealltypeIVcurveswithH1
hysteresisloops,whicharetypicalfeaturesoforderedhexago‐
nal mesostructures. The pore size distributions of these sam‐
ples were all narrow and centered at 7.8, 9.5, and 6.5 nm for
5OP,5OP‐b100,and5OP‐s800,respectively.Itshouldbenoted
that the pore diameter of the 5OP sample decreased after
treatment with pure steam at 800 °C, indicating mesopore
shrinkage during the high‐temperature hydrothermal treat‐
ment;thisisconsistentwiththeXRDresults.Similarly,theN2
adsorption‐desorptionisothermsandcorresponding pore size
distributionsoffreshandtreated40OP(Fig.2(c)and(d))show
thattheorderedmesostructuresof40OPwerewellpreserved
after the different hydrothermal treatments, and mesopore
shrinkageoccurredduringsteamtreatmentat800°C.
Table1liststhestructuralparametersofthesamplesbefore
and after hydrothermal treatments. It was found that the
changes in the structural parameters of the two OP samples
with different FePO4 loadings (5 and 40 wt%) after hydro‐
thermaltreatmentwereverysimilar.Aftertreatmentinboiling
waterat100°C,thetotalporevolumesoftheOPsamplesde‐
creased slightly, and their micropore volumes decreased
greatly. However, after treatment with steam at 800 °C, the
totalporevolumesoftheOPsamplesdecreasedconsiderably,
andtheirmicroporesnearlydisappeared.TheBETsurfacear‐
eas of the OP samples decreased significantly, and their re‐
duced surface areas were very similar after the two hydro‐
thermaltreatments.TheBETsurfaceareasofthetwofreshOP
samples were reduced by 40.2%–44.9% after treatment with
boiling water at 100 °C, and by 73.2%–79.4% after treatment
withpuresteamat800°C.Theseriousdecreasesinthesurface
areas are caused by extensive destruction of micropores and
collapseofmesopores,asshowninTable1.
The above results show that OP samples with FePO4 load‐
ingsof5and40wt%hadverysimilarbehaviorduringthedif‐
Table1 Physical propertiesofthe FePO4‐SBA‐15 samples beforeandafter hy‐
drothermaltreatments.
Da
Vtb
Vmc
SBETd
Reduced
(nm) (cm3/g) (cm3/g) (m2/g) areae(%)
5OP
7.8
0.93 0.169
898 —
5OP‐b100
9.5
0.96 0.022
537 40.2 5OP‐s800
6.5
0.35 0
185 79.4 40OP
6.6
0.85 0.087
615 —
40OP‐b100
7.9
0.84 0.013
339 44.9 40OP‐s800
5.0
0.33 0
165 73.2 SBA‐15
6.6
1.06
0
532
—
SBA‐15‐s700
—
0.41
0
181
66.0
SBA‐15‐s800
—
0.18
0
17
96.8
5IMP
3.9
0.92
0.007
506
—
5IMP‐s800
—
0.39
0
56
88.9
40IMP
6.6
0.38 0
184 —
40IMP‐s700
3.3
0.22
0
87
52.7
40IMP‐s800
2.8
0.20
0
65
64.7
aThe porediameters werecalculatedby thedesorptionoradsorption
branchesoftheisotherms.
bTotalporevolume.cMicroporevolume.dBETspecificsurfacearea.
e The reduced surface area was calculated according to the formula:
(SBET(pre‐steamed)–SBET(post‐steamed))/SBET(pre‐steamed).
449
ferent hydrothermal treatments, i.e., the FePO4 loading does
nothaveagreatimpactonthehydrothermalstabilitiesofthe
samples synthesizedusingthe one‐pothydrothermal method;
these results are totally different from those previously re‐
ported for mesoporous‐silica‐supported metal oxides
[20,21,24].Lietal.[20]reportedthatAl–SBA‐15sampleswith
lowAlcontentsweremorestablethanthosewithhighAlcon‐
tents under steam at 800 °C. However, Selvaraj et al. [21,24]
foundthatCr–SBA‐15orGa–SBA‐15withhighamountsofCror
Ga had better hydrothermal stabilities under steam at 800 °C
thansampleswithlowerCrorGacontents.Theysuggestedthat
formationofaprotectivemetallayerorstableSi–O–metalspe‐
cies might result in good hydrothermal stabilities of these
mesoporous‐silica‐supported metal oxides. In our previous
report[25],foranOPsamplewithaFePO4loadingof10wt%,
diffuse reflectance ultraviolet‐visible (UV‐vis) spectroscopy
showed that large amounts of FePO4 species were present in
bulkFePO4,andasmallamountofFePO4specieswerepresent
intheSBA‐15framework;FePO4granuleswithmicrondiame‐
ters were also observed in scanning electron microscopy im‐
ages, and the elemental components were confirmed using
energy‐dispersive X‐ray spectroscopy. In this study, to under‐
stand why the hydrothermal stabilities of OP samples with
differentFePO4loadingswerethesame,thepresenceofFePO4
intheOPsampleswasexaminedusingwide‐angleXRD.Fig.3
shows that the OP samples with different FePO4 loadings had
similar XRD patterns, with distinct peaks ascribable to FePO4
crystals, even for a low FePO4 loading of 5 wt%. We deduced
thattheprotectiveFePO4layeronthesurfacesofbothOPsam‐
ples might protect silica against attack by water molecules
during hydrothermal treatments, leading to their very similar
stabilitiesinboilingwaterat100°Corpuresteamat800°C.
Inadditiontothemesostructures,thepresenceofFeinthe
OPsamplesafterhydrothermaltreatmentwasalsosignificant.
The wide‐angle XRD patterns of the samples after hydrother‐
maltreatmentareshowninFig.3.TheFein40OP‐b100wasin
theformsFePO4andFePO4·2H2O.For40OP‐s800,mostdiffrac‐
tion peaks were ascribed to FePO4 crystals, but the residual
peakswereunidentified. After treatment at600°C,theseuni‐
Sample
FePO4
FePO42H2O
Intensity (a.u.)
40OP
40OP-s800
40OP-b100
5OP
40IMP
10
20
30
40
50
2/( o )
60
70
Fig.3.Wide‐angleXRDpatternsofsamples.
80
90
450
RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
dentified diffraction peaks remained, excluding the possibility
of FePO4·nH2O. The FePO4 phase of 40OP was stable during
hydrothermaltreatment.
3.2. DifferencesamonghydrothermalstabilitiesofOP,IMP,and
SBA‐15samples
To further understand the hydrothermal stabilities of OP
samples,wecomparedthehydrothermalstabilitiesofOPsam‐
ples,IMPsamples,andcommerciallyavailableSBA‐15.Fig.4(a)
showsthesmall‐angleXRDpatternsofSBA‐15beforeandafter
hydrothermaltreatments.ItcanbeseenthatSBA‐15hadthree
strongdiffractionpeaksat1.0°,1.6°,and1.9°indexedto(100),
(110), and (200) facets with P6mm symmetry, respectively,
(a)
1.0
suggesting a highly ordered mesostructure. After treatment
withsteamat600°C,theintensityofthe(100)facetdiffraction
peak decreased, whereas those of the (110) and (200) facet
peaks became almost invisible; this suggests that the
mesostructure became disordered but was still hexagonal.
However, when the steam treatment temperature was in‐
creased to 700 or 800 °C, no diffraction peak was detected,
indicating that the SBA‐15 mesostructure was completely de‐
stroyed by steam treatment at 700 or 800 °C for 24 h. These
results were confirmed using N2 physisorption. Fig. 5(a) and
5(b) show the N2 adsorption‐desorption isotherms and pore
size distributions of SBA‐15 before and after hydrothermal
treatment,respectively.TheisothermoffreshSBA‐15wastype
IVwithanH1hysteresisloop;thesearetypicalfeaturesofor‐
(b)
0.9
(c)
Intensity
1.0
1.1
1.6 1.9
1.0
SBA-15
40IMP
SBA-15-s600
5IMP
5IMP-s700
5IMP-s800
40IMP-s600
40IMP-s700
40IMP-s800
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
2/( o )
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
2/( o )
SBA-15-s700
SBA-15-s800
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
2/( o )
700 (a)
600 (c)
600
500
400
300
200
300
200
100
0
0
(b)
0.2
0.4
0.6
0.8
Relative pressure (p/p0)
1.0
5IMP
5IMP-s800
400
100
0.0
(e)
0.0
0.2
0.4
0.6
0.8
Relative pressure (p/p0)
1.0
0.0
(d) 3.9
6.6
40IMP
40IMP-s600
40IMP-s700
40IMP-s800
Volume (ml/g)
SBA-15
SBA-15-s700
SBA-15-s800
500
Volume (ml/g)
Volume (ml/g)
Fig.4.Small‐angleXRDpatternsofSBA‐15(a),5IMP(b),and40IMP(c)samplesbeforeandafterhydrothermaltreatment.
0.2
0.4
0.6
0.8
Relative pressure (p/p0)
6.6
(f)
5IMP
5IMP-s800
dV/d(D) (a.u.)
SBA-15
SBA-15-s700
SBA-15-s800
dV/d(D) (a.u.)
dV/d(D) (a.u.)
3.3
1.0
40IMP
40IMP-s600
40IMP-s700
40IMP-s800
2.8
0 2 4 6 8 10 12 14 16 18 20
Pore diameter (nm)
0 2 4 6 8 10 12 14 16 18 20
Pore diameter (nm)
0 2 4 6 8 10 12 14 16 18 20
Pore diameter (nm)
Fig.5.N2adsorption‐desorptionisothermsandcorrespondingporesizedistributionsofSBA‐15(a,b),5IMP(c,d),and40IMP(e,f)samplesbefore
andafterhydrothermaltreatment.
RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
dered hexagonal mesostructures. These structures were com‐
pletelydestroyedaftertreatmentinpuresteamat700or800
°C, leading to large decreases in the total pore volumes and
specific surface areas, as shown in Table 1. However, as dis‐
cussed in Section 3.1, the OP samples can withstand steam
treatmentevenat800°Cfor24h.TheOPsamplesarethere‐
foremorehydrothermallystablethanSBA‐15,suggestingthat
loading FePO4 on SBA‐15 using the one‐pot hydrothermal
method significantly enhanced the hydrothermal stability of
SBA‐15.Theseresultsareinagreementwithreportsthatmetal
addition, using aone‐pothydrothermal method,improvesthe
hydrothermal stabilities of mesoporous silicate materials
[20,29,30].
IMP samples with high and low FePO4 loadings (5 and 40
wt%) were used to investigate the hydrothermal stability of
commercially available SBA‐15‐supported FePO4. Fig. 4(b)
shows the small‐angle XRD patterns of 5IMP samples before
and after hydrothermal treatment. The pattern of 5IMP dis‐
playedastrongdiffractionpeakat0.9°,indexedto(100)facet
with P6mm symmetry, suggesting an ordered mesostructure.
After treatment with steam at 700 °C, a weakened diffraction
peak was detected, suggesting that the mesostructure was
preserved to some extent. However, no diffraction peak was
detectedfor5IMP‐s800,suggestingthatthemesostructurewas
completely destroyed after hydrothermal treatment in pure
steam at 800 °C. These results were confirmed using N2 phy‐
sisorption. Fig. 5(c) and (d) shows the N2 adsorption‐ desorp‐
tion isotherms and pore size distributions of 5IMP samples
before and after hydrothermal treatments, respectively. To
eliminate interference by the tensile strength effect of the ad‐
sorbed phase [28], the pore size distribution of 5IMP was de‐
rivedfromtheadsorptionbranchoftheisothermusingtheBJH
method. It was found that the isotherm of 5IMP was type IV
withanH1hysteresisloopandnarrowporesizedistribution,
which are typical features of ordered hexagonal mesostruc‐
tures.AsinthecaseofpureSBA‐15,themesostructuresofthe
impregnated samples were completely destroyed after treat‐
mentwithpuresteamat800°C,resultinginlargedecreasesin
thetotalporevolumesandspecificsurfaceareas,asshownin
Table1.TheseobservationswereconsistentwiththeXRDre‐
sults.
The small‐angle XRD patterns of 40IMP before and after
hydrothermaltreatments,showninFig.4(c),indicatethatfresh
40IMP displayed only a weak diffraction peak at 1.0°. This
mightbebecauseofthelargeFePO4loadingpartlyblockingthe
SBA‐15 pores. Fig. 5(e) and (f) shows the N2 adsorp‐
tion‐desorptionisothermsandporesizedistributionsof40IMP
samplesbeforeandafterhydrothermaltreatments,respective‐
ly. The isotherm of 40IMP was type IV with an H1 hysteresis
loop, which are typical features of ordered hexagonal
mesostructures.Thesamplesobtainedbytreatmentwithpure
steamat600and700°C,i.e.,40IMP‐s600and40IMP‐s700,also
hadtypeIVisothermswithH1hysteresisloops,indicatingthat
the hexagonal mesostructures were well preserved. The pore
sizedistributionsof40IMP,40IMP‐s600,and40IMP‐s700were
allnarrow.However,the40IMP‐s800isothermshowedahys‐
teresisloopwithaflatslope,differentfromthoseoftheother
451
three 40IMP samples, indicating that the ordered mesostruc‐
tureof40IMPwasdestroyedaftersteamtreatmentat800°C.
Basedonthe aboveanalysis, wecan concludethatthehydro‐
thermalstabilitiesofthesamplesfollowtheorderOP>IMP>>
SBA‐15.
3.3. InterpretationofprotectiveroleofFePO4
Si–O–Sibondscanbeattractedbywaterandhydrolyzedto
Si–OHinboilingwater,asshowninEq.(1): ≡Si–O–Si≡ +HO–H↔ ≡Si–OH+HO–Si≡
(1)
Si–OH can be dehydroxylated again to Si–O–Si by thermal
treatment, and this process is dominant under high‐tempera‐
ture steam [8,29]. Here, it is reasonable to suppose that the
good hydrothermal stabilities of the OP and IMP samples can
be ascribed to the protective layer formed by deposition of
FePO4 species on the SBA‐15 surface. The protective layer of
FePO4canrepelattackbywatermoleculesonSi–O–Sibonds;it
can also cover Si–OH bonds and prevent their condensation
with each other, thereby protecting the surface framework of
SBA‐15 from further disintegration. As reported previously
[20,29],asimilarprotectivelayerhasbeenusedtoexplainthe
improvements in the hydrothermal stabilities of Al–MCM‐41
andAl–SBA‐15.AlthoughtheamountofAlspeciesinMCM‐41
isnothighenoughtoformAl‐richsurfacespeciestocoverall
theMCM‐41surfaces,ithasbeensuggestedthatthesurfaceAl
species protect not only the adjacent Si atoms but also those
distant from the Al species [29,31]. The same mechanism can
beusedtoexplainthesuperiorhydrothermalstabilitiesofOP
andIMPmaterialscomparedwiththatofSBA‐15.
Although the OP and IMP samples both have protective
FePO4 layers, there are differences between these samples.
First,theOPsampleshavemuchlargernumbersofmicropores
than the IMP samples, as shown in Table 1. According to the
report by Zhang et al. [8], the large number of micropores
should contribute to the higher structural stability on treat‐
mentwithsteam.Secondly,thelocationsofFePO4intheOPand
IMPmaterialsaredifferentasshownusingdiffusereflectance
UV‐vis spectroscopy in our previous work [25]. It was found
thatbulkFePO4andstructuralironwerebothformedintheOP
material,whereasonlyisolatedFePO4specieswereformedon
the outer surface of SBA‐15 for IMP material. The Si–O–metal
bondsformedintheOPmaterialsaremorestablethanthe ≡
Si–O–Si≡ bonds[24,32];thismightbeanotherreasonforthe
superior hydrothermal stabilities of OP materials compared
withIMPmaterials.Lastly,inadditiontothesedifferencesbe‐
tween the properties of these SBA‐15‐supported FePO4 sam‐
ples,thephasesoftheFePO4speciesaredifferent.Incontrast
totheFePO4crystalsformedinOPwithFePO4loadingsof5and
40 wt%, highly dispersed FePO4 was formed in 40IMP, with
onlyaverybroaddiffractionpeakrelatedtoamorphoussilica
asconfirmedinFig.3;thisisconsistentwithapreviousreport
[33]. We therefore speculate that these differences contribute
to the superior hydrothermal stabilities of OP over IMP sam‐
ples.However,whetherthereareanydifferencesbetweenthe
protective layers formed by FePO4 crystals and highly dis‐
persedFePO4remainstoberesolvedinfurtherstudies.
452
RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
4. Conclusions
ChemB,2005,109:8723
OP samples withlow andhigh FePO4 contents synthesized
usinganovelone‐pothydrothermalmethodshowedthesame
hydrothermal stabilities in boiling water at 100 °C or pure
steamat800°C.Thisisdifferentfromreportsintheliterature
thattheloadingonSBA‐15‐supportedmetaloxideshasanim‐
portant effect on the hydrothermal stability in pure steam at
800 °C. A comparison of the hydrothermal stabilities of OP,
IMP,andpureSBA‐15samplesshowedthattheirhydrothermal
stabilityorderwasOP>IMP>>SBA‐15.TheprotectiveFePO4
layer on the surfaces of mesoporous silicates might protect
silica against attack by water molecules, therefore the protec‐
tiveFePO4layeronOPandIMPmightcontributetothebetter
hydrothermalstabilities.Thesuperiorhydrothermalstabilities
ofOPsamplesoverIMPsamplesmightberelatedtothemuch
larger proportion of micropores, the crystal phase of FePO4,
andthepresenceofFeintheSBA‐15framework,whichisevi‐
dentfromtheformationofSi–O–Febonds.
[9] MokayaR.ChemCommun,2001:633
[10] LiuH,WangMY,HuHJ,LiangYG,WangY,CaoWR,WangXH.J
SolidStateChem,2011,184:509
[11] Li Q, Wu Z X, Feng D, Tu B, Zhao D Y. J Phys Chem C, 2010, 114:
5012
[12] ShindoT,NakazawaY,OzawaS.JPorousMater,2009,16:481
[13] ModyHM,KannanS,BajajHC,ManuV,JasraRV.JPorousMater,
2008,15:571
[14] LiuZY,ZhuZB,WangRY,ZhuXD.ChinJCatal(刘子玉, 朱子彬,
王仁远, 朱学栋. 催化学报),2008,29:928
[15] SangchoomW,MokayaR.JMaterChem,2012,22:18872
[16] RyooR,JunS.JPhysChemB,1997,101:317
[17] JunS,KimJM,RyooR,AhnYS,HanMH.MicroporousMesoporous
Mater,2000,41:119
[18] KimJM,JunS,RyooR.JPhysChemB,1999,103:6200
[19] SongMJ,ZouCL,NiuGX,ZhaoDY.ChinJCatal(宋明娟, 邹成龙,
牛国兴, 赵东元. 催化学报),2012,33:140
[20] Li Q, Wu Z X, Tu B, Park S S, Ha C S, Zhao D Y. Microporous
MesoporousMater,2010,135:95
[21] Selvaraj M, Kawi S, Park D W, Ha C S. Microporous Mesoporous
Mater,2009,117:586
References
[22] Selvaraj M, Kawi S, Park D W, Ha C S. J Phys Chem C, 2009, 113:
7743
[1] KresgeCT,LeonowiczME,RothWJ,VartuliJC,BeckJS.Nature,
1992,359:710
[2] BeckJS,VartuliJC,RothWJ,LeonowiczME,KresgeCT,Schmitt
[3]
[4]
[5]
[6]
[7]
[8]
KD,ChuCTW,OlsonDH,SheppardEW,MccullenSB,HigginsJ
B,SchlenkerJL.JAmChemSoc,1992,114:10834
TaguchiA,SchüthF.MicroporousMesoporousMater,2005,77:1
CormaA.ChemRev,1997,97:2373
PeregoC,MilliniR.ChemSocRev,2013,42:3956
DaJW,SongCM,QianL,SuJM,XuXZ.JPorousMater,2008,15:
189
EswaramoorthiI,DalaiAK.IntJHydrogenEnergy,2009,34:2580
ZhangFQ,YanY,YangHF,MengY,YuCZ,TuB,ZhaoDY.JPhys
[23] MokayaR.JPhysChemB,2000,104:8279
[24] SelvarajM,KawiS.ChemMater,2007,19:509
[25] WangRQ,LinRH,DingYJ,LiuJ,WangJH,ZhangT.ApplCatalA,
2013,453:235
[26] Lin R H, Ding Y J, Gong L F, Dong W D, Chen W M, Lu Y. Catal
Today,2011,164:34
[27] Selvaraj M, Park D W, Ha C S. Microporous Mesoporous Mater,
2011,138:94
[28] GroenJC,Perez‐RamirezJ.ApplCatalA,2004,268:121
[29] ShenSC,KawiS.JPhysChemB,1999,103:8870
[30] ShaoYF,WangLZ,ZhangJL,AnpoM.MicroporousMesoporous
Mater,2008,109:271
GraphicalAbstract
Chin.J.Catal.,2015,36:446–453 doi:10.1016/S1872‐2067(14)60202‐3
HighlyhydrothermallystableFePO4–SBA‐15synthesizedusinganovelone‐pothydrothermalmethod
RunqinWang,RongheLin,YunjieDing*,JiaLiu,WentingLuo,HongDu,YuanLü
DalianInstituteofChemicalPhysics,ChineseAcademyofSciences;UniversityofChineseAcademyofSciences
600
600
5 wt% OP
400
300
200
Hydrothermal treatment
400
300
200
100
100
0
0
0.0
40 wt% OP
500
Volume (ml/g)
Volume (ml/g)
500
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
0.0
0.2
0.4
0.6
0.8
Relative pressure (p/p0)
Sample after being boiled at 100 oC
Sample after being steamed at 800 oC
Fresh sample
1.0
FePO4–SBA‐15samplespreparedusinganewone‐potmethodshowedexcellenthydrothermalstabilitiesineitherpuresteamat800°C
orboilingwaterat100°C,regardlessoftheFePO4loading(5or40wt%).
RunqinWangetal./ChineseJournalofCatalysis36(2015)446–453
[31] Sano T, Nakajima Y, Wang Z B, Kawakami Y, Soga K, Iwasaki A.
MicroporousMater,1997,12:71
[32] WangKX,LinYJ,MorrisMA,HolmesJD.JMaterChem,2006,16:
453
4051
[33] WangY,Wang X X,SuZ, Guo Q,Tang Q H,ZhangQ H,WanH L.
CatalToday,2004,93‐95:155
水热合成一锅法制备FePO4–SBA-15及其水热稳定性能
王润琴a,c, 林荣和a, 丁云杰a,b,*, 刘
佳a,c, 罗文婷a,c, 杜
虹a,c, 吕
元a
a
中国科学院大连化学物理研究所洁净能源国家实验室(筹), 辽宁大连116023
中国科学院大连化学物理研究所催化基础国家重点实验室, 辽宁大连116023
c
中国科学院大学, 北京100049
b
摘要: 通过两种水热处理方式, 即800 oC水汽条件和100 oC沸水处理, 考察了一锅法制备的FePO4–SBA-15 (OP)的水热稳定性. 水热
处理前后样品的结构变化通过小角X射线衍射和N2物理吸附表征. 研究发现, 经水热条件下原位生成FePO4修饰后的OP样品具有
良好的水热稳定性, 并且FePO4的担载量(5%和40%)对OP样品的水热稳定性几乎没有影响. 这与文献报道的金属担载量会影响介
孔材料水热稳定性的结果不同. 此外, 还对比研究了浸渍法制备的FePO4/SBA-15 (IMP)和商品SBA-15的水热稳定性. 结果表明, 各
样品水热稳定性由强到弱的顺序是OP > IMP >> SBA-15. OP和IMP样品水热稳定性优于纯硅分子筛SBA-15的原因可能是FePO4保
护层能抑制介孔材料在水热环境下的结构塌陷. OP样品水热稳定性较IMP样品好的原因可能主要是由于OP样品中存在稳定的
Si–O–Fe键和较多的微孔.
关键词: 磷酸铁; SBA-15; 介孔材料; 水热稳定性; 水蒸气; 金属担载量
收稿日期: 2014-10-09. 接受日期: 2014-11-18. 出版日期: 2015-03-20.
*通讯联系人. 电话/传真: (0411)84379143; 电子信箱: [email protected]
基金来源: 国家自然科学基金(21103170).
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).