Microporous and Mesoporous Materials 187 (2014) 7–13

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Microporous and Mesoporous Materials

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Direct synthesis, characterization and catalytic application of SBA-15mesoporous silica with heteropolyacid incorporated into theirframework

1387-1811/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.micromeso.2013.12.007

⇑ Corresponding author. Tel./fax: +86 25 52090617.E-mail address: ymzhou@seu.edu.cn (Y. Zhou).

Xiaoli Sheng, Jie Kong, Yuming Zhou ⇑, Yiwei Zhang, Zewu Zhang, Shijian ZhouSchool of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China

a r t i c l e i n f o

Article history:Received 26 May 2013Received in revised form 9 September 2013Accepted 9 December 2013Available online 13 December 2013

Keywords:HeteropolyacidMesoporous SBA-15Acidic catalysisCatalysis stability

a b s t r a c t

The Keggin phosphotungstic acid, H3PW12O40 (HPW) incorporated into SBA-15 ordered mesoporous silicawere synthesized via a method involving the introduction of HPW in an acidified solution of P123 triblockcopolymer (EO20PO70EO20), the SBA-15 mesostructuring agent (direct synthesis). Samples with similarHPW loadings were also prepared by impregnation of SBA-15. A comparison between direct incorpora-tion of HPW into mesoporous silica and impregnation of HPW on mesoporous silica was done. Character-ization by elemental analysis, XRD, N2 adsorption, TEM, DRS-UV and FTIR spectroscopy showed that aftercalcination HPW in the direct synthesized samples was better dispersed or may even be partially embed-ded in the pore walls. Moreover, their catalytic behaviors were investigated in the alkylation of o-xylenewith styrene. Results show that direct synthesized sample has the better catalytic performances in termsof yield and stability. This behavior may be owing to high dispersion of the HPW species on the SBA-15and the strong interaction between the HPW and the support, thus prevent HPW leaching from thesupport.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Hetropoly acids (HPA) have witnessed rapid growth in the lastdecade as solid acid catalyst [1–3]. Polyoxometalates with Kegginstructure have been chosen as catalyst because of their easyavailability and extreme stability in solution as well as in solidstate. 12-Tungstophosphoric acid (HPW), in particular, has beenthe target catalyst among the Keggin series in many earlier reportsbecause of the strongest acidity [4–5]. However, the maindisadvantage is their very low surface area (<10 m2 g�1) and henceit becomes necessary to disperse HPW on supports that possesslarge surface area. The use of SBA-15 mesoporous molecular sievesis attractive, because it possesses well-ordered pore structures,high thermal stability and high surface area [6]. One possibleroute to obtain these supported HPW catalysts is the directimpregnation of the support with a heteropoly acid solutionfollowed by evaporation of the solvent [7–9]. Although theconventional wet impregnation method is easy to increase itssurface area by supporting HPW onto various carriers. However,weak interaction between the HPW and the support resulted inits leaching in polar media [10]. The reaction stabilities of the

catalysts are still not satisfactory. Thus, it is necessary to furtherimprove the catalytic performances of the catalysts.

Leaching of HPW from ordered mesoporous silica in polar reac-tion media can be prevented by surface modification of the sup-port. In our previous work, the deposition of basic aluminaclusters; doping of the silicate with La atoms; and functionaliza-tion of the silicate walls with aminosilane groups for anchoringthe HPW molecules were reported to be successful [11–13]. Incor-poration of HPA into the pores of a mesoporous material can alsobe achieved by encapsulating HPA during the synthesis of the silicamaterial itself [14–20]. Indeed, Yang et al. incorporated HPW intoSBA-15 support by adding HPW to the SBA-15 synthesis mixture[15,17]. Toufaily et al. reported a similar approach to incorporateHPW into MSU type ordered mesoporous silica [18]. Shi et al. ob-tained SBA-15-supported HPW catalysts by adding P and Wsources into the initial sol–gel system during hydrolysis of tetra-ethyl orthosilicate to form the Keggin-type HPA in situ [14,16].

In this work, The Keggin phosphotungstic acid (HPW) incorpo-rated into SBA-15 ordered mesoporous silica were synthesized viaa method involving the introduction of HPW in an acidified solutionof P123 triblock copolymer (EO20PO70EO20), the SBA-15 mesostruc-turing agent (direct synthesis). There are some important differ-ences between other synthesis procedures and ours. The maindifference is related to the timing of the introduction of HPW.

We introduced HPW before the hydrolysis of TEOS, whereas inReferences [15,17,18] TEOS was already present and had reacted

Surfactant (S0) –P123: (EO) 70(PO)20(EO)70

S0 + HCl(aq.) (S0/H3O+)(Cl-) (S0/H3O+)(Cl-/H5SiO4+) SBA-15

TEOS T

(S0/H3O+)(Cl-;PW123-) (S0/H3O+)(Cl-;PW12

3-/H5SiO4+)

TEOS

HPW/SBA-15-DS

T

Surfactant (S0) –P123: (EO) 70(PO)20(EO)70

S0 + HCl(aq.) (S0/H3O+)(Cl-) (S0/H3O+)(Cl-/H5SiO4+) SBA-15

TEOS T

(S0/H3O+)(Cl-;PW123-) (S0/H3O+)(Cl-;PW12

3-/H5SiO4+)

TEOS

HPW/SBA-15-DS

T

Fig. 1. Proposed synthesis mechanism for HPW/SBA-15-DS samples.

8 X. Sheng et al. / Microporous and Mesoporous Materials 187 (2014) 7–13

prior to HPW addition. Shi et al. introduced P and W sources(Na2WO4 and Na2HPO4) instead of HPW in the synthesis mixture.

The aim of this paper is to compare the differences betweendirect incorporation of HPW into mesoporous silica and impregna-tion of HPW on mesoporous silica. The effect of the incorporation/impregnation of HPW on the structure of mesoporous solids wasinvestigated by different techniques. The catalytic properties ofthe catalysts were assessed in the alkylation of o-xylene withstyrene. Special attention was paid to catalyst stability andreusability.

2. Experimental

2.1. Catalyst preparation

SBA-15 samples were synthesized following a previously pub-lished method using the P123 tri-block copolymer (EO20PO70EO20)as a structure directing agent [6]. After the hydrothermal step,samples were thoroughly washed with distilled water and weredried at 120 �C for 24 h. Calcination was performed in air at540 �C for 6 h (heating rate: 2 �C min�1). The Keggin phosphotung-stic acid (H3PW12O40-HPW) was dispersed on the silica supporteither using an aqueous incipient wetness impregnation techniqueor via a direct synthesis method.

The aqueous incipient wetness impregnation technique waspreviously employed by many authors for the immobilization anddispersion of HPA on high surface area supports [7–9]. We inserted12-Tungstophosphoric Acid (HPW) by stirring 2 g of freshly cal-cined SBA-15 in a 10 mL aqueous solution containing 0.27 mmolof H3PW12O40 6H2O for 6 h at 353 K. The impregnated powdersare dried at 120 �C overnight and calcined in air at 300 �C for 4 h.The resulting HPW/SBA-15 samples prepared via impregnationare denoted as HPW/SBA-15-PS (PS refers to the impregnation syn-thesis method).

Incorporation of HPW via the direct synthesis route was achievedvia the following adaptation of the SBA-15 synthesis procedure(shown in Fig. 1). P123 polymer, 4.00 g; 30 g of water; and 120 g ofHCl (2 M) were mixed following the standard method for SBA-15synthesis. This solution was stirred for over 3 h at 40 �C and thenadded to a second solution containing 0.27 mmol of H3PW12O40

6H2O in a 10 mL aqueous solution. The mixture was stirred for24 h before addition of 9.4 g tetraethyl orthosilicate (TEOS). Duringhydrolysis of TEOS a white precipitate was formed. After stirring foranother 30 min, the mixture then transferred into a Teflon-linedautoclave and aged for 48 h at 80 �C. The resulting solid was filtered,washed with deionized water and dried at 100 �C for 24 h. The calci-nation step was performed in air at 540 �C for 6 h (heating rate:2 �C min�1). The resulting HPW/SBA-15 samples were labeled asHPW/SBA-15-DS (DS refers to the direct synthesis method).

2.2. Characterization

Elemental analysis of samples was performed by means ofX-ray fluorescenece (XRF) analysis on a SWITZERLAND ARL9800XRF. The corresponding weight ratio of anhydrous HPW on drySBA-15 of the different catalysts is exhibited in Table 1.

Powder X-ray diffraction (XRD) patterns were obtained with aRigaku D/max-rC Siemens diffractometer using nickel-filtered CuKa as monochromatic X-ray radiation. The scattering intensitieswere measured over an angle range of 0.58 < 2h < 40 with a stepsize D(2h) = 0.028 and a step time of 8 s.

The nitrogen adsorption and desorption isotherms were mea-sured at �196 �C on an ASAP-2020 (Micromertics USA). The spe-cific surface area, ABET, was determined from the linear part ofthe BET equation (P/P0 = 0.05–0.25). The pore size distributionwas derived from the desorption branch of the N2 isotherm usingthe Barrett–Joyner–Halenda (BJH) method. The total pore volumewas estimated from the amount of nitrogen adsorbed at a relativepressure (P/P0) of ca. 0.995. Pore structures of the samples wereexamined by TEM (Jeol, JEM-2000EXII).

Infrared spectra were recorded on a Bruker Tensor 27 (German)using DRIFT techniques, scanned from 4000 to 400 cm�1. The sam-ple was ground with KBr and pressed into a thin wafer. The sam-ples were evacuated at 300 �C for 4 h before the measurement.

The diffuse reflectance UV–vis spectra were collected using aSHIMADZU UV3600 (Japan) scanning spectrophotometer. Thepowder sample was loaded into a quartz cell, and the spectra werecollected over the range of 200–800 nm reference to BaSO4.

2.3. Catalytic tests

The alkylation reactions were carried out in a continuouslystirred batch reactor under reflux conditions using a three-neck100-mL round-bottom flask equipped with a condenser. Preliminaryruns were conducted with 7.50 g (0.0721 mol) of styrene, 57.35 g(0.5402 mol) of o-xylene (mole ratio of o-xylene to styrene, 7.5:1)and 1.50 g of catalyst (20% w/w of styrene) at 120 �C for 180 min.The required amount of o-xylene was initially added to the reactorat the reaction temperature, followed by the desired amount ofcatalyst, a known amount of styrene was then added to the reactionmixture at the same temperature. After the reaction, unreactedo-xylene was distilled out under atmospheric pressure and then acollected part was called as crude product. The crude product wasanalyzed with GC-9890A gas chromatograph equipped with OV-1capillary column and a flame ionization detector (FID). The yield ofPXE was defined as follows:

yield of PXEð%Þ ¼ actual product weightall theoretical product weight� 100

actual product weight ¼ crude product weight�PXEðchromatographyÞ%

ð1Þ

Table 1Texture properties of various catalysts (inside parentheses, the data after reaction).

Entry Sample SBETa (m2 g�1) Vtotal (cm3g�1) Pore sizeb (nm) HPW (wt.%) (calculated) HPWc (wt.%) (found)

1 SBA-15 810 1.27 6.45 – –2 HPW/SBA-15-PS 622 (720) 0.92 (1.07) 6.02 (5.96) 40 39.53 HPW/SBA-15-DS 430 (456) 0.82 (0.87) 6.45 (6.45) 40 41.9

a BET method.b BJH model applied to the desorption branch of the isotherm.c From XRF analysis.

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

HPW/SBA-15-DS

HPW/SBA-15-PS

(110)

(200)(110)

(100)

inte

nsity

(a.u

.)

2 Theta(degree)

(a)

SBA-15

5 10 15 20 25 30 35 40

inte

nsity

(a.u

.)

2 Theta(degree)

(b)

HPW

HPW/SBA-15-DS

HPW/SBA-15-PS

Fig. 2. Low-angle (a) and high-angle (b) XRD patterns of pure SBA-15 and SBA-15materials modified with HPW via the PS and DS methods.

X. Sheng et al. / Microporous and Mesoporous Materials 187 (2014) 7–13 9

3. Results and discussion

3.1. Characterization of the HPW/SBA-15 catalysts

The differences in HPW aggregation on the SBA-15 supports fol-lowing the two synthesis methods were evidenced by XRD (Fig. 2).All materials showed a sharp peak indexed as (100) and a smallerpeaks indexed as (110), which are typical of the 2-D hexagonal(p6 mm) SBA-15 material [6]. As for the HPW/SBA-15-DS sample,the peak height of the main (100) reflection is nearly one secondas the initial SBA-15 and the reflection of (200) is little intense.The diffraction peak of impregnated HPW appears to have almostthe same intensity. Generally, the appearance of the main (100)reflection means the existence of mesopore in all HPW/SBA-15samples. Therefore, this finding suggests that the mesoporousstructure of SBA-15 is not destroyed after the introduction ofHPW irrespective of the synthesis methods. The actual concentra-tion of HPW in the samples was estimated by means of XRF anal-ysis. XRF results are listed in Table 1. The amount of HPW found inHPW/SBA-15-PS sample is less than that in HPW/SBA-15-DS sam-ple possibly due to the reaction between surfactant and HPW dur-ing the synthesis of the HPW/SBA-15-DS sample. In addtion, asshown in Fig. 1(b), the high-angle region did not show the charac-teristic diffraction pattern of crystalline HPW phase for the HPW/SBA-15-DS sample, which is an evidence of the dispersed natureof HPW in HPW/SBA-15-DS samples. On the contrary, samples pre-pared by the impregnation of the same HPW loading onto SBA-15presented the characteristic XRD pattern of crystalline HPW.

Fig. 3a shows the N2 adsorption–desorption isotherms of SBA-15, HPW/SBA-15-PS, and HPW/SBA-15-DS. As can be seen, all sam-ples exhibit typical IV type isotherms and H1 type hysteresis loopsat high relative pressures. This indicates that SBA-15 with a fairlyuniform pore size distribution was successfully prepared by thetwo synthesis methods. However, the volumes adsorbed inflectedsharply at relative pressure (P/P0) 0.63 for SBA-15, 0.50 for theHPW/SBA-15-PS and HPW/SBA-15-DS samples. The desorptionbranch extended to a lower relative pressure suggesting a partialloss of structural organization and the formation of some narrowerpores. The BJH pore size distribution (calculated from the analysisof the desorption branch of the isotherms) is presented in Fig. 3b. Itcan be seen that SBA-15 samples exhibit a fairly uniform pore sizedistribution (5–7 nm). It is well worth mentioning that that theHPW/SBA-15-DS sample has pore sizes around 5–7 nm, similar tothe SBA-15. The result is consistent with the reported literature[19].

In the DS synthesis method, the size of the majority of mesop-ores is not affected by the presence of HPW, which is in agreementwith the proposed synthesis model (Fig. 1) in which the pore size isimposed by the P123 micelles, which have the same diameter inthe presence and absence of HPW. On the contrary, impregnationof the calcined SBA-15 sample with HPW resulted in a reductionof the mesopore diameter, which shows that HPW inside the mes-opores occupies space and decreases the pore width. On the otherhand, the results obtained from the textural analysis of the samplesbased on nitrogen adsorption are presented in Table 1. The HPW/

SBA-15-DS samples have a specific surface area of 430 m2/g, whichis significantly lower than that of the unmodified SBA-15 (810 m2/g). The HPW/SBA-15-DS samples have a higher specific surfacearea of 622 m2/g. It is clear that the surface area and pore volumedecrease with the introduction of the HPW. The loss of surface areaupon HPW loading using impregnation can be related to theagglomeration of HPW molecules on the external surface of thematerial resulting in pore blockage. Interestingly, the HPW/SBA-15-DS sample shows very similar pore size to that of SBA-15, in

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

800

1000

HPW/SBA-15-DS

HPW/SBA-15-PS

(a)

Vol

ume

adso

rbed

(cm

3 STP

/g)

Relative Pressure(P/P0)

SBA-15

0 5 10 15 20

0

5

10

15

20

25

HPW/SBA-15-DS

HPW/SBA-15-PS

SBA-15

Pore

Vol

ume(

cm3 /g

)

Pore Diameter(nm)

(b)

Fig. 3. (a) Low-temperature nitrogen adsorption–desorption isotherms and (b) Poresize distribution patter of purely siliceous SBA-15 HPW/SBA-15-PS and HPW/SBA-15-DS samples.

10 X. Sheng et al. / Microporous and Mesoporous Materials 187 (2014) 7–13

good agreement with the pore size distribution calculated from theBJH isotherm model (Fig. 3b). This result clearly indicates thatHPW in the direct synthesis samples was better dispersed ormay even be partially embedded in the pore walls, compared tothat of HPW/SBA-15-PS sample.

The TEM images for HPW/SBA-15-DS and HPW/SBA-15-PS sam-ples are presented in Fig. 4. Obviously, the pores of the impreg-nated sample are hexagonal analogous to those of pure silicaSBA-15 [6]. The DS sample has a different morphology with irreg-ular particle size confirming that the HPW had an impact on theformation process of the mesoporous material. This result suggeststhat the ordering of the HPW/SBA-15-DS sample is less than that ofthe HPW/SBA-15-PS sample, which is in agreement with the re-sults of XRD.

The IR spectra of the pure HPW, HPW/SBA-15-PS and HPW/SBA-15-DS are shown in Fig. 5. Pure HPW shows IR bands approxi-mately at 1080 (P-O in the central tetrahedron), 980 (terminalW = O) and 890 and 800 (W-O-W) cm�1 corresponding to asym-metric vibration associated with Keggin ion [21]. For the HPW/SBA-15-PS and HPW/SBA-15-DS samples, although the absence ofvibration band at 1080 cm�1 could be probably owing to theconcealment by the strong and broad background of SBA-15, the

vibration bands at approximately 980, 890 and 800 cm�1 can beclearly observed. This unambiguously demonstrates that theprimary structure of HPW Keggin anions is preserved after themodified via the two synthesis methods. On the other hand, it isnoteworthy that the W-O and W-O-W bands of [PW12O40]3� inthe HPW/SBA-15-PS and HPW/SBA-15-DS samples appear atslightly red-shift positions compared to those of the pureHPW, indicating the presence of a strong interaction between[PW12O40]3� and SBA-15 supports [22]. As can be seen, the bandof 980 cm�1 in the HPW/SBA-15-PS and HPW/SBA-15-DS samplesappear at the 976 cm�1 and the 950 cm�1, respectively. It couldbe considered that interaction between [PW12O40]3� and SBA-15supports of the HPW/SBA-15-DS sample is more than that of theHPW/SBA-15-PS sample.

The interaction between HPW and SBA-15 can also be con-firmed by DRS-UV analyses as shown in Fig. 6. It is known thatthe diffuse reflectance UV–vis spectroscopy is a sensitive probefor the identification and characterization of metal ion coordina-tion, as well as its existence in the framework or in the extraframe-work position of metal-containing zeolites [23]. As can be seen, astrong signal in the UV–visible spectra at k = 265 nm is observedfor the HPW/SBA-15-PS(PS-1) catalyst, which is assigned tothe oxygen-metal charge transfer of tungstophosphate anionPW12O40

3� [24]. However, for the HPW/SBA-15-DS (DS-2) sample,the signal at the k = 265 nm is shifted slightly at the k = 240 nm,implying that in this case PW12O40

3� Keggin type structure of theligand environment has been affected. This suggests that phospho-tungstic acid in the direct synthesis method may part into the holewall of mesoporous material, which changed the interactionbetween the carrier and phosphotungstic acid, thus affecting itscharacteristic absorption peak of the peak position.

3.2. Catalytic activity

The catalytic activity of different catalysts is showed in Table 2.The detailed reaction scheme is shown in Scheme 1, reaction (1) isthe PXE formation reaction whereas reaction (2) and (3) representthe formation of styrene oligomers and more substitutes,respectively.

As listed in Table 2, the homogeneous HPW show very high cat-alytic performances for the reaction, however, it is difficult to sep-arate the HPW from the product mixture. Furthermore, the SBA-15support itself shows no catalytic performance. It is noteworthythat the HPW/SBA-15-DS catalyst exhibits the much higher con-version (100%) and higher product yield (91.5%) than those ofHPW/SBA-15-PS catalyst. In this case a significant difference isfound between the two catalysts as a function of the differentpreparation methods. Combined with the data of Table 1, it canbe seen that although the catalyst in the impregnation synthesismethod has a larger specific surface area, but its pore size is small,is not conducive to the macromolecular reaction. In contrast, theHPW/SBA-15-DS catalyst has a larger pore size, is beneficial tothe reaction. Possibly, this behavior may be explained by the high-er dispersion as revealed in Fig. 5. From these reasons, it can be de-duced that high dispersion of HPW onto SBA-15 and the largesurface area with suitable pore size might account for the high cat-alytic activity.

The important questions that must be addressed while studyingalkylation processes over a solid catalyst relate to the stability ofthe catalyst to leaching of the active component and the possibilityof catalyst recycling. The catalytic reusability of the HPW/SBA-15-PS and HPW/SBA-15-DS catalysts was evaluated by carrying outthe reaction with used catalyst under the optimized conditions.After each run, the catalyst was recovered by filtration, thenwashed with ethanol, dried and used again. The data obtainedare shown in Fig. 7. It can be seen that only 7% reduction in the

b d

a

b

cc

b dd

aa

b

ccccc

Fig. 4. TEM images of various samples: (a) HPW/SBA-15-DS in [100] direction, (b) HPW/SBA-15-DS in [110] direction, (c) HPW/SBA-15-PS in [100] direction, and (d) HPW/SBA-15-PS in [110] direction.

1200 1100 1000 900 800 700 600

(c)

(b)

Tra

nsm

itta

nce

(a.u

)

Wavenumber cm-1

1080 980 890 800

976

950

(a)

Fig. 5. FT-IR spectra of pure HPW (a), HPW/SBA-15-PS (b) and HPW/SBA-15-DS (c).

200 300 400 500 6000

1

2

DS-2

PS-1

Abs

orba

nce

Wavelength / nm

Fig. 6. UV–vis diffuse reflectance spectra of HPW/SBA-15-PS(PS-1) and HPW/SBA-15-DS(DS-2) catalysts.

X. Sheng et al. / Microporous and Mesoporous Materials 187 (2014) 7–13 11

activity is observed after 4 runs on the HPW/SBA-15-DS catalyst. Incontrast, the deactivation of the HPW/SBA-15-PS catalyst is muchfaster and the PXE yield drops to a very low level of 40% afterthe fourth reaction cycle. The poor catalytic stability of HPW/SBA-15-PS may be due to the possibility that HPW leaching fromthe catalyst support into the liquid solvent, which may result inthe low conversion. On the other hand, the decrease of yield arisingfrom catalysts lost during separation and transfer of catalysts intothe next reaction cycle cannot be excluded. This observation re-veals satisfied reusability for HPW/SBA-15-DS, which means thatHPW have only a slight tendency to leach from the SBA-15 carrierin reaction. This result is also implying that a strong interaction

between the HPW and the SBA-15 in the direct synthesis method,thus prevent HPW leaching from the catalyst support.

In order to further confirm the interaction between the HPWand the SBA-15, the catalysts after reaction have been studied bythe N2 adsorption–desorption isotherms. The results obtained fromthe textural analysis of the samples based on nitrogen adsorptionare presented in Table 1 (inside parentheses). It is clear that thesurface area and pore volume increase after the reaction, thismay be due to the blockage of the micropores with grafted HPWspecies is exposed, led to the pore volume and specific surface areaincrease. On the other hand, it is also possible that at the end of thereaction, HPW leaching from the catalyst support into the liquid

Table 2Activity of various catalysts a in alkylation of o-xylene with styrene.a

Catalyst Styrene conversion (%) PXE Yieldc (%) PXE Selectivityd (%)

SBA-15 – – –HPWb 100 97.9 9:1HPW /SBA-15-PS 91 68.3 9:1HPW /SBA-15-DS 100 91.5 9:1

a Reaction conditions: o-xylene: styrene = 7.5:1, reaction temperature = 120 �C,reaction time = 3.0 h, catalyst loading = 20% (w/w of styrene).

b Homogeneous catalyst, 0.30 g.c Isolated yield based on the amount of styrene.d Ratio of para-to-ortho product.

+

Cat

O-Xylene Styrene Phenylxylyl ethane(PXE) (1)

nCat

Styrene Styrene Oligomer (2)

O-Xylene Styrene

Cat

more substitutes (3)

Scheme 1. Reaction scheme of alkylation of o-xylene with styrene over aheterogeneous catalyst.

1 2 3 40

20

40

60

80

100 HPW/SBA-15-DSHPW/SBA-15-PS

PX

E Y

ield

(%)

Reaction cycle

Fig. 7. Catalytic stability of the HPW/SBA-15-DS and HPW/SBA-15-PS catalysts inthe alkylation of o-xylene with styrene (Reaction conditions: o-xylene:sty-rene = 7.5:1, reaction temperature = 120 �C, reaction time = 3.0 h, catalyst load-ing = 20% (w/w of styrene).

12 X. Sheng et al. / Microporous and Mesoporous Materials 187 (2014) 7–13

solvent may lead to a number of pores on the surface of the carrierincreases, which could contribute to the surface area of the cata-lysts. In addition, from the data (see Table 1), it can be seen thatonly 6% increase in the specific surface area is observed after 4 runson the HPW/SBA-15-DS catalyst. In contrast, in the case of theHPW/SBA-15-PS catalyst the specific surface area was increasedby 16% after the fourth reaction cycle. These data can also confirmmuch HPW leaching from the HPW/SBA-15-PS catalyst, but littleHPW leaching from the HPW/SBA-15-DS catalyst after four cata-lytic cycles. Therefore, it is reasonable that a strong interaction be-tween the HPW and the SBA-15 in the direct synthesis methodresults in efficient immobilization of HPW, which maintains itshigh activity in acid-catalyzed reactions.

4. Conclusions

In this work, a procedure is described for direct incorporation ofKeggin-type heteropolyacids into ordered mesoporous silica byusing a mixture of the introduction HPW in an acidified solution ofP123 triblock copolymer. After incorporation or impregnation, theheteropolyacid anions preserved their Keggin structure on the sur-face of mesoporous SBA-15.

Characterization results from XRF, XRD, N2 adsorption, TEM,DRS-UV and FTIR spectroscopy indicate that HPW was better dis-persed or may even be partially embedded in the pore walls inthe direct synthetic method. The HPW/ SBA-15-DS catalyst ishighly efficient in the alkylation of o-xylene with styrene. It hadthe best catalytic performances with styrene conversion up to100% and PXE yield up to 91.5%. The catalyst could be used formore than four times without any significant loss of activity andleaching of tungsten species in the reaction mixture. The goodstability can be attributed to the strong interaction between theSBA-15 and HPW molecules.

Acknowledgments

The authors are grateful to the financial supports of NationalNatural Science Foundation of China (Grant No. 21306023,21376051, 21106017 and 51077013), Fund Project for Transforma-tion of Scientific and Technological Achievements of Jiangsu Prov-ince of China (Grant No. BA2011086), Specialized Research Fundfor the Doctoral Program of Higher Education of China (GrantNo.20100092120047), Key Program for the Scientific Research Guid-ing Found of Basic Scientific Research Operation Expenditure ofSoutheast University (Grant No. 3207043101) and InstrumentalAnalysis Fund of Southeast University.

References

[1] F.E. Celik, H. Lawrence, A.T. Bell, J. Mol. Catal. A Chem. 288 (2008) 87–96.[2] A. Heydari, S. Khaksar, M. Sheykhan, M. Tajbaksh, J. Mol. Catal. A Chem. 287

(2008) 5–8.[3] A. Kumar, P. Singh, S. Kumar, R. Chandra, S. Mozumdar, J. Mol. Catal. A Chem.

276 (2007) 95–101.[4] J. Kaur, I.V. Kozhevnikov, Chem. Commun. 19 (2002) 2508–2509.[5] T. Rajkumar, G. Ranga Rao, J. Mol. Catal. A Chem. 295 (2008) 1–9.[6] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky,

Science 279 (1998) 548–552.[7] J. Wang, H.O. Zhu, Catal. Lett. 93 (2004) 209–212.[8] Q.Y. Liu, W.L. Wu, J. Wang, X.Q. Ren, Y.R. Wang, Micropor. Mesopor. Mater. 76

(2004) 51–60.[9] H.X. Jin, Q.Y. Wu, W.Q. Pang, Mater. Lett. 58 (2004) 3657–3660.

[10] A. Popa, V. Sasca, J. Halasz, Appl. Surf. Sci. 255 (2008) 1830–1835.[11] X.L. Sheng, Y.M. Zhou, Y.W. Zhang, Y.Z. Duan, M.W. Xue, Chem. Eng. J. 179

(2012) 295–301.[12] X.L. Sheng, Y.M. Zhou, Y.W. Zhang, Y.Z. Duan, Z.W. Zhang, Micropor. Mesopor.

Mater. 161 (2012) 25–32.[13] X.L. Sheng, Y.M. Zhou, Y.W. Zhang, Y.Z. Duan, M.W. Xue, Catal. Lett. 142 (2012)

360–367.[14] C. Shi, R. Wang, G. Zhu, Eur. J. Inorg. Chem. 23 (2005) 4801–4807.[15] L. Yang, Y. Qi, X. Yuan, J. Shen, J. Kim, J. Mol. Catal. A Gen. 229 (2005) 199–205.

X. Sheng et al. / Microporous and Mesoporous Materials 187 (2014) 7–13 13

[16] C. Shi, R. Wang, G. Zhu, Eur. J. Inorg. Chem. 15 (2006) 3054–3061.[17] L. Yang, J. Li, X. Yuan, J. Mol. Catal. A Gen. 262 (2007) 114–118.[18] J. Toufaily, M. Soulard, J.L. Guth, Colloid Surf. A Physicochem. Eng. Aspects 312

(2008) 285–291.[19] B.C. Gagea, Y. Lorgouilloux, Y. Altintas, J. Catal. 265 (2009) 99–108.

[20] A. Popa, V. Sasca, E.E. Kiss, Mater. Res. Bull. 46 (2011) 19–25.[21] P. Staiti, S. Freni, S. Hocevar, J. Power Sources 79 (1999) 250–255.[22] S.S. Wu, J. Wang, W.H. Zhang, X.Q. Ren, Catal. Lett. 125 (2008) 308–314.[23] S.C. Laha, P. Mukherjee, S.R. Sainkar, J. Catal. 207 (2002) 213–223.[24] J.C. Hu, Y.D. Wang, L.F. Chen, Micropor. Mesopor. Mater. 93 (2006) 158–163.