heteropolyoxometalates which are included in microporous silica, csxh3−xpmo12o40/sio2 and...

Applied Catalysis A: General 218 (2001) 91–99

Heteropolyoxometalates which are included in microporous silica,CsxH3−xPMo12O40/SiO2 and CsyH5−yPMo10V2O40/SiO2, asinsoluble solid bifunctional catalysts: synthesis and selective

oxidation of benzyl alcohol in liquid–solid systems

Ge Peng a, Yonghui Wang a, Changwen Hu a,∗, Enbo Wang a, Shouhua Feng b,Yongchun Zhou b, Hong Ding b, Yanyong Liu c

a Institute of Polyoxometalate Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR Chinab Key Laboratory of Inorgonic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, PR China

c Chemical Technology Division, Institute of Research and Innovation, Takada 1201, Kashiwa, Chiba 277, Japan

Received 18 December 2000; received in revised form 8 March 2001; accepted 2 April 2001

Abstract

Heteropolyoxometalates (HPOMs) which are included in microporous silica, CsxH3−xPMo12O40/SiO2 and CsyH5−yPMo10

V2O40/SiO2, have been synthesized by a sol–gel technique and characterized by IR, UV–VIS, X-ray and N2 adsorptionisotherms. IR and UV–VIS data indicate that the identities of CsxH3−xPMo12O40 and CsyH5−yPMo10V2O40 are preservedwithin these synthesized compounds. TEM image and BET adsorption isotherms confirm the presence of nanometer particlesand microporous structures. X-ray powder patterns prove that the cesium salts of the HPOMs are uniformly dispersed in thesilica network. As insoluble solid bifunctional catalysts, they show high catalytic activity and selectivity for selective oxidationof benzyl alcohol into benzyl aldehyde by H2O2 in liquid–solid systems. The highest activity is observed at x = 1.5 or y = 2.5in CsxH3−xPMo12O40/SiO2 or CsyH5−yPMo10V2O40/SiO2, respectively. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Microporous heteropolyoxometalates; Nanometer particles; Synthesis; Structural characterization; Selective oxidation; Benzylalcohol; Benzyl aldehyde

1. Introduction

The catalytic function of heteropolyoxometalates(HPOMs) has attracted much attention particularlyin the last two decades, for these compounds providea good basis for the molecular design of mixed ox-ide catalysts and have high capabilities in practicaluses [1–5]. They have been widely used as acid and

∗ Corresponding author. Tel.: +86-431-5640694;fax: +86-431-5640694.E-mail address: [email protected] (C. Hu).

oxidation catalysts in solution as well as in the solidstate [6]. One of the recent interests in catalysis bymicroporous HPOMs is shape selectivity [7,8]. Whileshape-selective catalysis by zeolites is of great interestin both science and technology [9–11], there are somerestrictions of zeolites for general applications. Theacid strength of zeolites is not so strong, the pore sizeis not variable, and the constituent elements are lim-ited [11]. Recently, Okuhara et al. have reported theacid catalysis and adsorption features of water-tolerantHPOM Cs2.5H0.5PW12O40 [12–14]. However, it isstill difficult to separate them from reaction solution

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92 G. Peng et al. / Applied Catalysis A: General 218 (2001) 91–99

because of their milk-like state. As a result, many ef-forts have been made to support HPOMs with variousmaterials. For example, we have thoroughly studiedthe preparation of polyoxometalate-intercalated lay-ered double hydroxides [15–17]. Although they areinsoluble solid catalysts, their relatively low surfaceareas restrict their applications. Izumi et al. have pre-pared silica-supported microporous HPOM catalystssuch as H3PW12O40/SiO2 and H4SiW12O40/SiO2 byadding H3PW12O40 and H4SiW12O40 during the hy-drolysis of tetraethyl orthosilicate (TEOS) [18]. In thepresent study, we have prepared some silica-supportedKeggin-type MoV-series HPOMs through encapsu-lating them into a silica matrix by a sol–gel tech-nique involving the hydrolysis of TEOS. Our studiesshow that these compounds are good bifunctional(acid and oxidation) catalysts with high surface areasand exhibit high catalytic activity and selectivity forselective oxidation of benzyl alcohol (BAL) intobenzyl aldehyde (BAD) in liquid–solid systems.

2. Experimental

2.1. Materials

1-Butanol, TEOS, cesium carbonate and BALare of analytical grade. H5PMo10V2O40·32H2O and

Table 1Loads, ICP analysis and IR data for various catalystsa

Catalyst Load (wt.%) Molar ratio IR data (cm−1)

Cs P Mo V vP or Si–Oa vMo=OdvMo–Ob–Mo vMo–Oc–Mo

Cs0.0H3.0PMo12O40/SiO2 11.2 0.0 1.1 12.2 – 1093.5 976.0 882.2 808.3Cs0.5H2.5PMo12O40/SiO2 19.6 0.5 0.9 11.8 – 1088.2 967.7 869.4 796.6Cs1.5H1.5PMo12O40/SiO2 20.9 1.5 1.2 12.1 – 1087.8 964.4 868.3 795.1Cs2.5H0.5PMo12O40/SiO2 25.2 2.5 1.1 12.0 – 1090.0 964.6 869.1 796.0Cs3.0PMo12O40/SiO2 26.1 3.1 1.0 12.1 – 1090.2 963.6 868.8 792.7

Cs0.0H5PMo10V2O40/SiO2 3.6 0.0 1.0 10.2 2.1 1086.9 960.0 870.9 799.4Cs1.0H4.0PMo10V2O40/SiO2 8.0 1.0 1.2 10.1 2.1 1086.4 962.3 868.2 795.2Cs2.5H2.5PMo10V2O40/SiO2 9.7 2.5 1.1 10.1 2.2 1089.0 960.5 866.3 796.9Cs4.0H1.0PMo10V2O40/SiO2 13.7 4.0 0.9 9.9 2.0 1088.8 958.4 865.1 789.5Cs5.0PMo10V2O40/SiO2 17.2 5.2 1.0 10.2 1.9 1087.3 957.9 863.5 788.0

H3PMo12O40 a – – 1.1 12.2 – 1062.7 963.4 879.5 795.6H5PMo10V2O40 b – – 1.0 10.2 2.1 1057.9 960.4 880.6 794.0SiO2 – – 1093.3 – –

a IR spectra data of a and b are cited from [19,20].

H3PMo12O40·30H2O were prepared according to theliterature methods [19,20].

2.2. Preparation of the catalysts

According to the literature method of synthe-sizing Cs2.5H0.5PW12O40 [21], CsxH3−xPMo12O40or CsyH5−yPMo10V2O40 was prepared by addingslowly dropwise the required amount of aqueous ce-sium carbonate (0.47 M) to aqueous H3PMo12O40or H5PMo10V2O40 (0.75 M) with vigorous stirringat room temperature. The precipitate obtained wasaged in parent solution for 20 h at room tempera-ture, followed by evaporation in vacuum at 45◦C,grinding into 60-mesh-pass particles, and finally cal-cination in vacuum at 300◦C for 3 h. According tothe literature method of preparing silica-includedheteropolyoxo-tungstates reported by Izumi et al.[18], CsxH3−xPMo12O40/SiO2 were prepared as fol-lows. To a suspension of a certain amount of the Cssalt (according to the loads of Cs salts in Table 1) in18 ml of distilled water was added dropwise a mixtureof 1-butanol (9.0 ml) and TEOS (23.0 ml) under stir-ring at room temperature. The resulting mixture wasstirred at 40◦C for 1 h and then at 80◦C for 3 h. Duringthis course TEOS gradually hydrolyzed into a sol andthen into a gel. The acidity required by the hydrolysisof TEOS was provided by the HPOMs. At the same

G. Peng et al. / Applied Catalysis A: General 218 (2001) 91–99 93

time the Cs salt was dispersed in the network of sil-ica gel and thus the gel of the silica-included Cs saltwas obtained. The hydrogel obtained was dehydratedslowly at 60◦C for 3 h and dried in vacuum at 80◦Cfor 12 h, followed by grinding into 60-mesh-pass par-ticles. The dried gel was washed with water at 90◦Cuntil the filtrate was neutral. To fasten the silica net-work, the dried gel was calcinated in vacuum at 150◦Cfor 3 h. CsyH5−yPMo10V2O40/SiO2 was also preparedby the above-mentioned method.

2.3. Characterization of the catalysts

ICP analysis was performed on a Perkins-Elmer/Plasma 40 instrument to determine the Cs/P/Mo/Vmole ratio. TEM images were obtained on a HitachiH-600 transmission electron microscope. UV andIR spectra were recorded on a Schmado UV-2201UV–VIS spectrophotometer and a Nicolet Magna 560IR spectrophotometer, respectively. X-ray powderdiffraction (XRD) patterns were obtained at room tem-perature with an automated Rigaku D/max-3C diffrac-tometer using Cu K� radiation. Wavelength-dispersivespectrometry (WDS) was performed by means of flu-orescence analysis. The BET specific surface areasand pore size distributions were calculated from nitro-gen adsorption isotherms determined at 77.5 K usingan ASAP 2010 M surface analyzer; the pretreatmenttemperature was 250◦C.

2.4. Catalytic reaction andproduct analysis

The selective oxidation of BAL was chosen asa test reaction to estimate the catalytic activity ofsilica-included HPOMs CsxH3−xPMo12O40/SiO2 andCsyH5−yPMo10V2O40/SiO2. Each reaction was per-formed at 90◦C in a round-bottomed flask (25 ml)containing the reactant BAL 2 ml and suspended cat-alyst powder 0.1–0.2 g with magnetic stirring for 5 hand H2O2 (30%) was added dropwise every half anhour (total 1 ml). The reaction solution was analyzedby an SP502 gas chromatograph (GC) equipped witha flame ionization detector in the temperature range260–280◦C with a quartz capillary column (30 min length). Catalytic activities for the reaction wereevaluated by conversion of BAL.

3. Results and discussion

3.1. ICP and IR

CsxH3−xPMo12O40/SiO2 and CsyH5−yPMo10V2O40/SiO2 with various Cs contents were pre-pared corresponding to the final x and y valuesx = 0, 0.5, 1.5, 2.5, 3 and y = 0, 1, 2.5, 4, 5.The results of elemental analysis and loads forCsxH3−xPMo12O40/SiO2 and CsyH5−yPMo10V2O40/SiO2 are listed in Table 1. The observed mole ra-tios of Cs/P/Mo/V are close to those of the chemicalcompositions. The loads observed for these HPOMsare smaller than the theoretical ones, which sug-gests that the non-trapped HPOMs were dissolvedwhen washed with hot water. It is also found thatthe loads increase with the value of x or y inCsxH3−xPMo12O40/SiO2 or CsyH5−yPMo10V2O40/SiO2 growing. Therefore, the presence of Cs in thesecompounds is important to the load. Fig. 1 showsthe WDS mappings of Cs2.5H2.5PMo10V2O40/SiO2corresponding to Si, Cs, Mo and V elemental dis-tributions. According to a high silica content anduniform elemental distributions, it can be deducedthat CsxH3−xPMo12O40 and CsyH5−yPMo10V2O40particles distribute in SiO2 uniformly withoutclustering.

Infrared spectra of Cs1.5H1.5PMo12O40, Cs1.5H1.5PMo12O40/SiO2 and pure SiO2 are shown in Fig. 2and the IR data of all the synthesized compoundsare summarized in Table 1. The vibration peaks at1060–1090, 960–970, 860–880 and 790–800 cm−1

are characteristic of a Keggin structure, suggestingthat the Keggin structures of these HPOMs are pre-served. However, the characteristic peak at 1088 cm−1

has shifted by about 30 cm−1 and broadened com-pared with that of the parent CsxH3−xPMo12O40or CsyH5−yPMo10V2O40 owing to the overlappingof P–Oa and Si–O–Si vibrations. Therefore, it isbelieved that a strong chemical interaction, not asimple physical adsorption, exists between HPOMsand silica surface [2]. Lefebvre and Thouvenot et al.also showed an interaction between HPOMs and sil-ica through 31P MAS NMR [22,23]. The model forthis strong chemical interaction between HPOMs andsilica network generated during the hydrolysis ofTEOS in the present work will be depicted in detail inSection 3.5.


Fig. 1. WDS mappings of the elemental distributions for Cs2.5H2.5PMo10V2O40/SiO2 (sharing the same scale).

3.2. UV–VIS and TEM

The UV–VIS spectra of Cs1.5H1.5PMo12O40/SiO2and Cs2.5H2.5PMo10V2O40/SiO2 are shown in Fig. 3.A characteristic UV absorption peak occurs at263–266 nm in both spectra, similar to that of thePMo12O40

3− species. A maximum absorption at310 nm for Cs1.5H1.5PMo12O40/SiO2 or at 380 nmfor Cs2.5H2.5PMo10V2O40/SiO2, belonging to theOd–Mo or Od–V charge–transfer band of Keggin an-ions [24], indicates that the primary Keggin structureis retained after immobilizing Cs1.5H1.5PMo12O40 orCs2.5H2.5PMo10V2O40 into silica. The TEM image ofsilica-included Cs1.5H1.5PMo12O40 is shown in Fig. 4.It is observed that the relatively uniform particles arewithin 10–20 nm in diameter. A similar particle sizeis also observed for the other silica-included HPOMs.Such a small particle size can lead to a large surfacearea and thus is beneficial to enhance the oxidativecatalytic activity of the synthesized catalysts.

3.3. XRD

X-ray powder diffraction patterns of Cs1.5H1.5PMo12O40 are shown in Fig. 5(a) before and (b)

after being included in silica, in which marked differ-ences are observed. In Fig. 5(a), crystallite structureof Cs1.5H1.5PMo12O40 is observed at 2θ = 3–60◦.However, the XRD pattern in Fig. 5(b) presents nosharp peaks, but two obviously broadened peaks at2θ = 10◦ and 2θ = 15–35◦, respectively. This broad-ened XRD pattern results from the uniform dispersalof crystallite Cs1.5H1.5PMo12O40 (without clustering)in the amorphous silica network (without calcina-tion at high temperatures) formed by the hydrolysisof TEOS. When the load of Cs1.5H1.5PMo12O40 insilica is higher than 30 wt.%, the existence of itscrystalline phase is distinctively observed. Similarly,when we supported H4SiW12O40 in silica by meansof immersion and the load was lower than 20 wt.%,the XRD pattern of H4SiW12O40/SiO2 did not showthe crystalline phase of H4SiW12O40; however, thiscrystalline phase did exist when the load was higher(40 wt.%) [25].

3.4. BET isotherms

The nitrogen adsorption at 77.5 K on Cs1.5H1.5PMo12O40/SiO2 obtained by hydrolysis of TEOS wasmeasured and the adsorption isotherms and pore size


Fig. 2. IR spectra of (a) Cs1.5H1.5PMo12O40; (b) Cs1.5H1.5-PMo12O40/SiO2; and (c) pure SiO2.

distribution are shown in Fig. 6. Cs1.5H1.5PMo12O40/SiO2 exhibits a high BET specific surface area, thatis, 595 m2 g−1. The presence of micropores is con-firmed for Cs1.5H1.5PMo12O40/SiO2 by the sharpincrease of the adsorbed amount of nitrogen at avery low relative pressure. The sharp peak in thepore size distribution curve indicates that the poresin Cs1.5H1.5PMo12O40/SiO2 mainly have a diameterof ca. 0.60 nm, suggesting that these microporesare uniform. The formation of the micropores inCs1.5H1.5PMo12O40/SiO2 originates from the porousstructure of silica particle gels obtained from hydrol-ysis of TEOS under the acidic conditions. The meso-porosities of the silica gels are confirmed by the poresize distribution curve for the pure SiO2. The medianpore diameter of silica is ca 3.8 nm. During the hydrol-ysis of TEOS in the presence of Cs1.5H1.5PMo12O40,

Fig. 3. UV–VIS spectra of (a) Cs1.5H1.5PMo12O40/SiO2; and (b)Cs2.5H2.5PMo10/V2O40/SiO2.

the Keggin anions are entrapped by the silica network,resulting in the decrease of the pore size and formationof the microporous Cs1.5H1.5PMo12O40/SiO2 com-posite. The entrapped Keggin anions appear to be ahighly concentrated aqueous solution in the silica net-work and the micropores seem to be narrow enough toprevent the removal of the Cs1.5H1.5PMo12O40/SiO2molecules from them. On the other hand, some

Fig. 4. TEM image of Cs1.5H1.5PMo12O40/SiO2.


Fig. 5. XRD patterns of (a) Cs1.5H1.5PMo12O40; and (b)C1.5H1.5PMo12O40/SiO2 (load: 20 wt.%).

organic reactant molecules such as BAL (ca. 0.57 nm,calculated from the C=C, C–C and C–OH bonddistances, 0.140, 0.142 and 0.147 nm, respectively)are permitted to enter the pores (ca. 0.60 nm) of thesilica and perform the selective oxidation reactionthere.

Fig. 6. Plots of nitrogen adsorption isotherm and pore size distri-bution for Cs1.5H1.5PMo12O40/SiO2.

3.5. Structural model for CsxH3−xPMo12O40/SiO2and CsxH5−xPW10V2O40/SiO2

According to the above analysis, we put forward anetwork structural model for supported microporousHPOMs: CsxH3−xPMo12O40/SiO2 and CsyH5−y

PMo10V2O40/SiO2. In the silanol gel obtained fromhydrolysis of TEOS in the presence of Keggin-typePOMs CsxH3−xPMo12O40 and CsyH5−yPMo10V2O40,the Si lies in the center of a Si(OH or O)4 tetrahe-dron, and these tetrahedra are linked with each otherby oxygen bridges into a pore-containing networkstructure. At the same time, Keggin-type HPOMs,CsxH3−xPMo12O40 and CsyH5−yPMo10V2O40, enterinto the interior of pores in the network so that the pro-tons on the spherical surface of the polyanions can in-teract with the OH groups of silanol or the bridge oxy-gens of Si–O–Si in the silica network, leading to theformation of hydrogen bonds. Additionally, as the OHgroups of silanol are protonated in the acidic mediumof CsxH3−xPMo12O40 or CsyH5−yPMo10V2O40, anacid–base reaction between the HPOMs (acting as aBrönsted acid) and the silanol group (acting as a Brön-sted base) occurs inside the pores. That is, one protonof CsxH3−xPMo12O40 or CsyH5−yPMo10V2O40 willreact with the OH of silanol and form a SiOH2

+group, which should act as a counter ion for thepolyanion. Thus the (≡SiOH2

+)(CsxH2−xPMo12O40)and (≡SiOH2

+)(CsxH4−xPMo10V2O40) species areformed. Therefore, the particles of HPOMs are firmlyimmobilized into the interior of pores, which is dif-ferent from the surface adsorption produced by theusual immersion method. However, the above two in-teractions (including hydrogen bonding and acid–basereaction) take place when the protons of the HPOMsare not completely substituted by Cs atoms. Once theprotons are totally substituted, namely, in the caseof Cs3.0PMo12O40/SiO2 or Cs5.0PMo10V2O40/SiO2,the interaction between HPOMS and the silica net-work is hydrogen bonding of the OH groups fromsilanol and oxygen atoms on the surface of theHPOMs. As a result, CsxH3−xPMo12O40/SiO2 andCsxH5−xPMo10V2O40/SiO2 will not easily leak intopolar solvents such as water. Thus the CsxH3−x

PMo12O40/SiO2 and CsxH5−xPMo10V2O40/SiO2 hy-brids are of practical importance as water-tolerantcatalysts, similar to Cs2.5H0.5PW12O40/SiO2 reportedby Okuhara et al. [13].


3.6. Selective oxidation reaction of BAL into BAD

The catalytic behaviors of CsxH3−xPMo12O40/SiO2and CsyH5−yPMo10V2O40/SiO2 were firstly exam-ined through the selective oxidation of BAL (Eq. (1))in the presence of a small amount of H2O2 (30%) inorder to avoid colloidal dispersion of the salts andacidification of the reactant. In the case of differentloads, various catalysts with the same equivalent ofHPOMs were added in the reaction.

(1)

Table 2 summarizes variations of activity and se-lectivity of CsxH3−xPMo12O40/SiO2 and CsyH5−y

PMo10V2O40/SiO2 towards the selective oxidation ofBAL by H2O2 in a liquid–solid mixture. As can beseen from Table 2, although the catalysts used aredifferent, the product selectivities of them towardsBAD are all almost 100% and little formation ofbenzyl acid (BZA) is observed. However, striking dif-ferences occur among the catalytic activities of thesecatalysts. For the support SiO2, little catalytic activityis observed. With respect to CsxH3−xPMo12O40/SiO2or CsyH5−yPMo10V2O40/SiO2, apparent dispari-ties in activity are observed with the content ofCs varying. Fig. 7 illustrates the changes in cat-alytic activity of CsxH3−xPMo12O40/SiO2 and

Table 2Activity and selectivity of various catalysts for oxidation reactionin liquid–solid systems

Catalyst BAL conver-sion (%)

Selectivity (%)

BAD BZA

H3.0PMo12O40/SiO2 5.88 100 0Cs0.5H2.5PMo12O40/SiO2 60.0 98.0 2.0Cs1.5H1.5PMo12O40/SiO2 69.1 98.6 1.4Cs2.5H0.5PMo12O40/SiO2 55.3 99.2 0.8Cs3.0PMo12O40/SiO2 8.5 97.8 2.2

Cs0.0H5.0PMo10V2O40/SiO2 16.0 98.5 1.4Cs1.0H4.0PMo10V2O40/SiO2 25.6 97.9 2.1Cs2.5H2.5PMo10V2O40/SiO2 52.5 96.9 3.0Cs4.0H1.0PMo10V2O40/SiO2 18.2 96.8 3.1Cs5.0PMo10V2O40/SiO2 6.6 97.6 2.4

SiO2 3.3 96.0 3.9

Fig. 7. Changes in the catalytic activity of CsxH3−xPMo12-O40/SiO2 (�); and CsyH5−yPMo10V2O40/SiO2 (�) with differ-ent Cs contents for the selective oxidation of BAL.

CsyH5−yPMo10V2O40/SiO2 with different Cs con-tents. The catalytic activity is expressed by the initialrate (the unit of r0 is mol-BAL × (mmol-HPOM ×h)−1). The highest activity is observed at x = 1.5 ory = 2.5, suggesting that 1:1 (Cs/H) mole ratio valueis favorable for both acid and oxidation (bifunctional)catalysis. The reason for this may be that the oxida-tion of BAL into BAD not only requires the catalyst’soxidative property, but also correlates with its acidamount, that is to say, high reactivity occurs when thecatalyst possesses an appropriate acid amount. Thefact that very low catalytic activities are observed forthe HPOMs with H completely substituted by Cs (i.e.x = 3 or y = 5) and without Cs substitution (i.e.x = 0 or y = 0) provides potential evidence for thisassumption. Studies on the acid amount and catalyticmechanism are underway. On the other hand, it isfound that Cs1.5H1.5PMo12O40/SiO2 is much more ac-tive than Cs2.5H2.5PMo10V2O40/SiO2; the reason forthis needs further investigation. In addition, a majoradvantage of CsxH3−xPMo12O40/SiO2 or CsyH5−y

PMo10V2O40/SiO2 is the presence of microporousstructure to which selective oxidation is accessible,and thus the reaction will perform in the micropores.The time courses of Cs1.5H1.5PMo12O40/SiO2 andCs2.5H2.5PMo10V2O40/SiO2 are shown in Fig. 8.


Fig. 8. Time courses of the selective oxidation of BAL:Cs1.5H1.5PMo12O40/SiO2 (�); Cs2.5H2.5PMo10V2O40/SiO2 (�);and pure SiO2 (�).

A similar, nearly steady conversion is observedfor the two catalysts. Following a long inductionperiod is a steady rate of acceleration and an al-most constant yield is finally reached after 5 h. TheCs1.5H1.5PMo12O40/SiO2 once used can be separatedand recovered for the next reaction cycle by washingwith distilled water and then with alcohol, and thenby calcination at 150◦C under O2 atmosphere for 2 h.An obvious decrease in catalytic activity and selec-tivity for BAD was not observed after five reaction

Fig. 9. Plot of the catalyst life and the product selectivity: (�)selectivity for BAD; (�) selectivity for BZA; and (�) conversionof BAL. Reaction conditions: time, 2 h; temperature, 90◦C; catalystCs1.5H1.5PMo12O40/SiO2, 0.15 g; H2O2, 1 ml (30%); BAL, 2 ml.

cycles (Fig. 9). The fact that the reaction solution con-tained no dissolved HPOMs, which was determinedby UV–VIS, confirms that these HPOMs did not leakfrom the silica support during the catalytic reaction.

4. Conclusions

Microporous HPOMs, CsxH3−xPMo12O40/SiO2and CsyH5−yPMo10V2O40/SiO2, have been syn-thesized by encapsulating CsxH3−xPMo12O40 andCsyH5−yPMo10V2O40 uniformly into a silica matrixvia hydrolysis of TEOS. The intrinsic properties ofthe polyanion are still retained after immobilizingCsxH3−xPMo12O40 and CsyH5−yPMo10V2O40 intoSiO2. The microporous structure and nanometer sizeof CsxH3−xPMo12O40/SiO2 and CsyH5−yPMo10V2O40/SiO2 are confirmed and these micropores areuniform. As a novel class of solid bifunctional cat-alysts for liquid–solid reactions, they are insolublein polar solvents, readily separable and possess largesurface areas owing to their microporous structure.Cs1.5H1.5PMo12O40/SiO2 is an excellent catalyst forthe selective oxidation of BAL into BAD, exhibitinghigh catalytic activity and selectivity. These stud-ies will be extended to the synthesis of new hybridnetworks based on hydrosoluble polymer chains co-valently crosslinked or electrostatically interactingwith Keggin ions. It is hoped that the silica-includedHPOMs will find wide applications to various typesof acid, oxidation or bifunctional catalysis organicreactions in liquid–solid systems.

Acknowledgements

The Natural Science Fund Council of China isacknowledged for financial support (No. 20071007).The present work is also supported by the Founda-tion for University Key Teacher by the Ministry ofEducation of China.

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heteropolyoxometalates which are included in microporous silica, csxh3−xpmo12o40/sio2 and...

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