8
Applied Catalysis A: General 392 (2011) 28–35 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Silica supported transition metal substituted polyoxotungstates: Novel heterogeneous catalysts in oxidative transformations with hydrogen peroxide Ana C. Estrada a , Isabel C.M.S. Santos b , Mário M.Q. Simões b , M. Grac ¸ a P.M.S. Neves b,, José A.S. Cavaleiro b , Ana M.V. Cavaleiro a,∗∗ a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal article info Article history: Received 14 July 2010 Received in revised form 20 October 2010 Accepted 20 October 2010 Available online 30 October 2010 Keywords: Oxidation Hydrogen peroxide Fe Mn Supported metal-substituted polyoxometalates abstract The preparation and characterization (FT-IR, FT-Raman, diffuse reflectance, elemental analysis) of novel catalysts with iron or manganese substituted polyoxotungstates [XM III (H 2 O)W 11 O 39 ] n(X = P, M = Fe or Mn; X = Si or B, M = Fe) immobilized on a functionalized silica matrix are reported. The new materials were tested as heterogeneous catalysts in the oxidation of cis-cyclooctene and cyclooctane at 80 C, using environmentally safe hydrogen peroxide as oxidant and acetonitrile as solvent. Some of these novel heterogeneous catalysts could be reused several times without appreciable loss of catalytic activity. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Polyoxometalates (POMs) are a large class of metal–oxygen anions comprising group 5 and 6 transition metals [1]. They present a large variety of composition, structure and electronic properties which makes them remarkable materials for applications in diverse areas including catalysis, chemical analysis, medicine or materials science [1,2]. However, the majority of the applications of POMs seem to be in the area of catalysis; Katsoulis has reported that about 85% of patent and applied research literature concerns the catalytic activity of POMs [3]. In fact, research considering the development of homogeneous and heterogeneous catalytic systems with polyox- ometalates is a matter of high interest [4–8]. A common drawback of homogeneous systems is the difficult catalyst/products separa- tion and the poor catalyst reusability. To surpass these problems, many strategies have been adopted in order to immobilize POMs on appropriate supports without loss of their intrinsic activity and selectivity. So, polyoxometalates, namely the Keggin anions, have been immobilized on solid supports such as silicas, activated car- bons, zeolites, aluminas, zirconia and titania [9–16]. Corresponding author. Tel.: +351 234370710; fax: +351 234 370084. ∗∗ Corresponding author. Tel.: +351 234370734; fax: +351 234 370084. E-mail addresses: [email protected] (M.G.P.M.S. Neves), [email protected] (A.M.V. Cavaleiro). Transition metal mono-substituted Keggin-type heteropoly- tungstates, [XW 11 O 39 M(H 2 O)] n, X=P, Si, B, (XW 11 M), in which a transition metal cation is co-ordinated to the binding sites of a lacunary heteropolyanion [XW 11 O 39 ] n, have generated substan- tial interest as oxidative catalysts in the last 25 years. These species, with a metal centre surrounded by an all inorganic environment, have been used in several types of oxidative reactions with a variety of oxidants and substrates, due to the range of possibilities for their chemical modification without affecting the Keggin-type primary structure, through choice of the heteroatom X or of the substituting metal, and to properties like thermal robustness, stability against oxidants and reversible redox reactions [7,8]. Also, they may be combined with different cationic species, allowing their solubilisation in different media or their deposition in, for example, films and supports. Considering the [XW 11 O 39 M(H 2 O)] nanions supported on sil- ica, not much information is available. A few studies report [XW 11 O 39 M(H 2 O)] n, X = P, Si or B, immobilized on different types of silica [11,17–25], namely functionalized ones [11,17–23]. Most of these studies refer the use of Co, Zn or Ni substituted polyox- ometalates supported on silicas functionalized with NH 2 groups. Excluding photocatalysis, there are only a few reports describing the use of silica supported XW 11 M in oxidative heterogeneous catalytic systems [18,19,21], but none with Fe or Mn substi- tuted Keggin anions. Johnson and Stein studied the oxidation of cyclohexene with O 2 in the presence of both PW 11 Co and 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.10.026

Silica supported transition metal substituted polyoxotungstates: Novel heterogeneous catalysts in oxidative transformations with hydrogen peroxide

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Applied Catalysis A: General 392 (2011) 28–35

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

ilica supported transition metal substituted polyoxotungstates: Noveleterogeneous catalysts in oxidative transformations with hydrogen peroxide

na C. Estradaa, Isabel C.M.S. Santosb, Mário M.Q. Simõesb, M. Graca P.M.S. Nevesb,∗,osé A.S. Cavaleirob, Ana M.V. Cavaleiroa,∗∗

Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, PortugalDepartment of Chemistry, QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal

r t i c l e i n f o

rticle history:eceived 14 July 2010eceived in revised form 20 October 2010ccepted 20 October 2010vailable online 30 October 2010

a b s t r a c t

The preparation and characterization (FT-IR, FT-Raman, diffuse reflectance, elemental analysis) of novelcatalysts with iron or manganese substituted polyoxotungstates [XMIII(H2O)W11O39]n− (X = P, M = Fe orMn; X = Si or B, M = Fe) immobilized on a functionalized silica matrix are reported. The new materialswere tested as heterogeneous catalysts in the oxidation of cis-cyclooctene and cyclooctane at 80 ◦C,using environmentally safe hydrogen peroxide as oxidant and acetonitrile as solvent. Some of these novel

eywords:xidationydrogen peroxideen

heterogeneous catalysts could be reused several times without appreciable loss of catalytic activity.© 2010 Elsevier B.V. All rights reserved.

upported metal-substitutedolyoxometalates

. Introduction

Polyoxometalates (POMs) are a large class of metal–oxygennions comprising group 5 and 6 transition metals [1]. They presentlarge variety of composition, structure and electronic propertieshich makes them remarkable materials for applications in diverse

reas including catalysis, chemical analysis, medicine or materialscience [1,2]. However, the majority of the applications of POMseem to be in the area of catalysis; Katsoulis has reported that about5% of patent and applied research literature concerns the catalyticctivity of POMs [3]. In fact, research considering the developmentf homogeneous and heterogeneous catalytic systems with polyox-metalates is a matter of high interest [4–8]. A common drawbackf homogeneous systems is the difficult catalyst/products separa-ion and the poor catalyst reusability. To surpass these problems,

any strategies have been adopted in order to immobilize POMs

n appropriate supports without loss of their intrinsic activity andelectivity. So, polyoxometalates, namely the Keggin anions, haveeen immobilized on solid supports such as silicas, activated car-ons, zeolites, aluminas, zirconia and titania [9–16].

∗ Corresponding author. Tel.: +351 234370710; fax: +351 234 370084.∗∗ Corresponding author. Tel.: +351 234370734; fax: +351 234 370084.

E-mail addresses: [email protected] (M.G.P.M.S. Neves), [email protected]. Cavaleiro).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.10.026

Transition metal mono-substituted Keggin-type heteropoly-tungstates, [XW11O39M(H2O)]n−, X = P, Si, B, (XW11M), in whicha transition metal cation is co-ordinated to the binding sites of alacunary heteropolyanion [XW11O39]n−, have generated substan-tial interest as oxidative catalysts in the last 25 years. These species,with a metal centre surrounded by an all inorganic environment,have been used in several types of oxidative reactions with avariety of oxidants and substrates, due to the range of possibilitiesfor their chemical modification without affecting the Keggin-typeprimary structure, through choice of the heteroatom X or of thesubstituting metal, and to properties like thermal robustness,stability against oxidants and reversible redox reactions [7,8]. Also,they may be combined with different cationic species, allowingtheir solubilisation in different media or their deposition in, forexample, films and supports.

Considering the [XW11O39M(H2O)]n−anions supported on sil-ica, not much information is available. A few studies report[XW11O39M(H2O)]n−, X = P, Si or B, immobilized on different typesof silica [11,17–25], namely functionalized ones [11,17–23]. Mostof these studies refer the use of Co, Zn or Ni substituted polyox-ometalates supported on silicas functionalized with NH2 groups.

Excluding photocatalysis, there are only a few reports describingthe use of silica supported XW11M in oxidative heterogeneouscatalytic systems [18,19,21], but none with Fe or Mn substi-tuted Keggin anions. Johnson and Stein studied the oxidationof cyclohexene with O2 in the presence of both PW11Co and

talysis

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SiW9O37{Co(H2O)}3]10− [19]. Others have used PW11Co in the oxi-ation of aldehydes with O2 [21] or in the aerobic oxidation of-pinene [18]. However some papers may be found with silica sup-orted Fe-sandwich type anions [26,27]. None of these works refershe catalytic oxidation of cis-cyclooctene and cyclooctane or to these of H2O2 as oxidant.

Following our previous work, concerning the use of transitionetal-substituted Keggin-type anions in the oxidative transfor-ation of organic compounds, using hydrogen peroxide as an

nvironmentally friendly oxidant, under homogeneous condi-ions [28–35], we are now interested in the development ofelated heterogeneous systems using silica-supported XW11M.n the study presented here, a series of novel materials basedn triethylpropylammonium-functionalized silica and the transi-ion metal mono substituted polyoxotungstates [PW11Fe(H2O)-39]4−(PW11Fe), [SiW11Fe(H2O)O39]5−(SiW11Fe), [BW11Fe(H2O)-39]6−(BW11Fe), and [PW11Mn(H2O)O39]4−(PW11Mn), were pre-ared and characterized by several analytical and spectroscopicechniques. Their use in catalytic oxidation was studied usingis-cyclooctene and cyclooctane as model substrates, hydrogen per-xide as oxidant and acetonitrile as solvent.

. Experimental

.1. Reagents and methods

Acetonitrile (Panreac), 30% (w/w) aqueous hydrogen per-xide (Riedel-de-Häen), cis-cyclooctene (Aldrich), cyclooctaneAldrich) and 3-bromopropylsilica (∼9.42% functionalized;00–400 mesh, 60 A pore size, 500 m2/g surface area, Aldrich)ere used as received. All other solvents used herein were

btained from commercial sources and used as received oristilled and dried using standard procedures. Potassium saltsf the transition metal substituted polyoxotungatates used inhis work (K4PW11Fe(H2O)O39·6H2O, K5SiW11Fe(H2O)O39·9H2O,6BW11Fe(H2O)O39·9H2O, K4PW11Mn(H2O)O39·6H2O) wererepared according to described procedures [36,37].

Infrared absorption spectra were obtained on a Mattson 7000T-IR spectrometer, using KBr pellets. Diffuse reflectance spectraere registered on a Jasco V-560 spectrometer, using BaSO4 as

eference. FT-Raman spectra were recorded on a RFS-100 BrukerT-Spectrometer, equipped with a Nd:YAG laser with excitationavelength of 1064 nm, with laser power set to 200 mW. Elemental

nalysis for P, Fe, Mn, W and Si were performed by ICP spectrom-try (University of Aveiro, Central Laboratory of Analysis) and theolutions were prepared by digesting the materials in HF undericrowave heating. C, N and H elemental analyses were carried

ut on a Leco CHNS-932 apparatus. Thermogravimetric measure-ents were carried out between 30 and 800 ◦C at 10 ◦C min−1 onTGA-50 Shimadzu thermobalance. The values of the total weight

oss (%TG) were calculated assuming decomposition to a mixturef oxides at the end of the experiment. GC-FID analyses were per-ormed on a Varian 3900 chromatograph using helium as the carrieras (30 cm/s). GC/MS analyses were performed on a Finnigan TraceC/MS (Thermo Quest CE instruments) using helium as the car-

ier gas (35 cm/s). In both cases fused silica capillary DB-5 typeolumns (30 m × 0.25 mm i.d., 0.25 �m film thickness) were used.he chromatographic conditions were as follows: initial temper-ture: 80 ◦C (cis-cyclooctene and cyclooctane); temperature rate:0 ◦C/min (cis-cyclooctene and cyclooctane); final temperature:20 ◦C (cis-cyclooctene), 230 ◦C (cyclooctane); injector temper-

ture: 220 ◦C (cis-cyclooctene), 250 ◦C (cyclooctane); detectoremperature: 220 ◦C (cis-cyclooctene), 250 ◦C (cyclooctane).

Aliquots were taken from the reaction mixtures, at regular inter-als, for peroxide determination by titration with ceric sulphate38].

A: General 392 (2011) 28–35 29

2.2. Immobilization of XW11M on the silica support

For the functionalization of the silica matrix, 5.0 mL of triethy-lamine, N(Et)3, were added to 500 mg of 3-bromopropylsilica. Themixture was refluxed in 50 mL of toluene under nitrogen atmo-sphere and vigorous stirring for 36 h [39]. The modified silica[silicaN(Et)3Br] was filtered, washed with toluene and dried in adesiccator. Anal. (wt.%): Found: C, 12.28; H, 2.91; N, 1.87. Thesevalues correspond to an average triethylpropylammonium groupsloading of 1.2 mmol g−1.

The preparation of the catalyst materials was performed byadding an aqueous solution containing the potassium salt of thepolyoxoanion (250 mg of XW11M dissolved in 10.0 mL of H2O) to500 mg of silicaN(Et)3Br. The mixture was stirred at room tem-perature during 8 h. The solid obtained, silicaN(Et)3/XW11M, wasfiltered, carefully washed with different solvents (H2O and diethylether) and dried under vacuum at room temperature for two days.

The calculated analytical values presented below correspond tothe molar proportion SiO2:triethylpropylammonium:POM:Br:H2Oequal to 13:1:0.06:0.76:2. These proportions are only approximate,as the experimental values could as well be fitted to other relations(e.g.: 12 SiO2 instead of 13).

SilicaN(Et)3/BW11Fe: Anal. (wt.%): Found: C, 8.68; H, 2.15; N,1.32; Fe, 0.29; Si, 28.60; W, 7.80; total decomposition loss, 17.9;Calcd.: C, 9.12; H, 2.15; N, 1.18; Fe, 0.28; Si, 30.81; W, 10.24;total decomposition loss, 20.6; Average loading, 45 �mol g−1. FT-IR (cm−1): 1093 (vs), 943 (m), 894 (m), 813 (s), 465 (vs). FT-Raman(cm−1): 966 (vs), 889 (m), 241 (s), 212 (s). FT-Raman BW11Fe, potas-sium salt (cm−1): 968 (vs), 910 (m), 523 (m), 242 (s), 213 (s).

SilicaN(Et)3/SiW11Fe: Anal. (wt.%): Found: C, 8.70; H, 2.24; N,1.30; Fe, 0.28; Si, 30.05; W, 8.80; total decomposition loss, 15.5;Calcd.: C, 9.11; H, 2.14; N, 1.18; Fe, 0.28; Si, 30.93; W, 10.23;total decomposition loss, 20.5; Average loading, 47 �mol g−1. FT-IR (cm−1): 1096 (vs), 954 (m), 910 (s), 797 (s), 465 (vs). FT-Raman(cm−1): 974 (vs), 889 (m), 237 (s), 220 (s). FT-Raman SiW11Fe,potassium salt (cm−1): 982 (vs), 897 (m), 525 (m), 243 (s), 220 (s).

SilicaN(Et)3/PW11Fe: Anal. (wt.%): Found: C, 8.71; H, 2.21; N,1.30; Fe, 0.27; P, 0.16; Si, 29.40; W, 7.68; total decompositionloss, 17.7; Calcd.: C, 9.11; H, 2.13; N, 1.18; Fe, 0.28; P, 0.16; Si,30.78; W, 10.23; total decomposition loss, 20.5; Average loading,46 �mol g−1. FT-IR (cm−1): 1098 (vs), 954 (m), 945 (m), 882 (s), 799(s), 747 (sh), 465 (vs). FT-Raman (cm−1): 1041(w), 988 (vs), 978 (vs),891 (m), 228 (s), 215 (s). FT-Raman PW11Fe, potassium salt (cm−1):1049(w), 995 (vs), 981(vs), 904 (m), 518 (m), 449(m), 231 (s), 215(s).

SilicaN(Et)3/PW11Mn: Anal. (wt.%): Found: C, 8.73; H, 2.26; N,1.28; Mn, 0.30; P, 0.17; Si, 28.90; W, 7.80; total decompositionloss, 19.7; Calcd.: C, 9.11; H, 2.13; N, 1.18; Mn, 0.28; P, 0.16; Si,30.78; W, 10.23; total decomposition loss, 20.5; Average loading,49 �mol g−1. FT-IR (cm−1): 1099 (vs), 962 (w), 887 (w), 807 (s), 464(vs). FT-Raman (cm−1): 1070 (w), 1048 (w), 996 (sh), 985 (vs), 907(m), 229 (s), 215 (s). FT-Raman PW11Mn, potassium salt (cm−1):1079 (w), 1054(w), 992 (vs), 975(vs), 903 (m), 516 (m), 229 (s), 215(s).

2.3. Catalytic studies

In a first run typical experiment, the substrate (1.0 mmol), ace-tonitrile (1.5 mL), the catalyst (25 mg, 1.1–1.2 �mol of POM) andthe required amount of 30% (w/w) aqueous H2O2 (1.0 mmol in thecase of cis-cyclooctene and 9.8 mmol in the case of cyclooctane)

were stirred in a closed vessel (5.0 mL micro-reaction vessel fromSupelco) at 80 ◦C, in the absence of light. Aliquots were withdrawnfrom the reaction mixture and injected into the GC after centrifu-gation. The recyclability of the heterogenized POM catalysts wastested as follows: after 6 h of reaction, the heterogeneous cata-

30 A.C. Estrada et al. / Applied Catalysis A: General 392 (2011) 28–35

Scheme 1.

eme 2

lwrTpcts

3

3

s(cd

F

Sch

yst was separated from the reaction mixture by centrifugation andashed with different solvents (CH3CN and C2H5OH) to remove the

emaining substrate and oxidant, as well as the reaction products.he recovered POM catalyst was dried under vacuum at room tem-erature and used in a new reaction under identical experimentalonditions, with readjustment of all quantities, without changinghe molar ratios and reaction concentrations. Product yields andubstrate conversions were determined by gas chromatography.

. Results and discussion

.1. Catalyst synthesis and characterization

The procedures used to prepare the functionalizedilica (silicaN(Et)3Br) and the POM-supported materialssilicaN(Et)3/XW11M) are illustrated in Schemes 1 and 2 [39]. Theharacteristic colours presented by the silicaN(Et)3/XW11M pow-ers suggest the successful immobilization of iron or manganese

ig. 1. FT-IR spectra of (a) K-BW11Fe, (b) silicaN(Et)3/BW11Fe and (c) silicaN(Et)3Br. On th

.

substituted polyoxotungstates. From the elemental analyses it ispossible to determine average loadings of 45–49 �mol per gramof material. The analytical results also suggest that no alteration ofthe structure of the transition metal substituted polyoxotungstatesoccurs during the immobilization process.

Keggin-type anions have characteristic FT-IR spectra, presentingin the range of 700–1000 cm−1 bands attributable to asymmetricstretching vibrations of the polyoxometalate bridges (W–O–W),terminal bonds (W–O) and X–O [40,41]. When the spectra ofthe silicaN(Et)3/BW11Fe material (Fig. 1b) and the correspondingpotassium salt of anion (Fig. 1a) are compared, it can be seen thatthree of the bands of the POM are present at similar frequencies,although only one of these bands (at 894 cm−1) may be clearly iden-tified due to the overlapping of the others with the silica bands in

the same region (Fig. 1c); their presence may only be deduced fromthe relative intensities [39]. The same behaviour is observed forthe SiW11Fe, PW11Fe and PW11Mn spectra, where only the bandsat 910, 882 and 887 cm−1, respectively are clearly attributed to

e right, enlargement of the FT-IR spectra in the range of 1000–700 cm−1.

A.C. Estrada et al. / Applied Catalysis A: General 392 (2011) 28–35 31

11Fe:

tift

cfsbsbsa

Fig. 2. FT-Raman spectra of (I) SiW11Fe; (II) BW

he POMs. The bands corresponding to P–O asymmetric stretch-ng vibration in the case of PW11Mn and PW11Fe are hidden uponormation of the new material due to the dominating intensity ofhe bands near 1100 cm−1 associated with the support.

The FT-Raman spectra of the supported anions are more elu-idative than the FT-IR data. Similar Raman patterns are foundor silicaN(Et)3/XW11M when compared with those of the potas-ium salts of the polyoxometalates and the characteristic POM

ands are seen without any interference due to silica. Fig. 2hows representative examples. All spectra present one strongand, near 970–980 cm−1 for the potassium, K-XW11M, salts,lightly shifted to lower wavenumbers in the silica materials,ttributed to vs(W–Oterminal) [40,41]. FT-Raman spectra of PW11Fe

Fig. 3. Diffuse reflectance electronic spectra of solids. (I): (a) K-PW11Mn and (

(a) potassium salt and (b) silicaN(Et)3/XW11Fe.

and PW11Mn have another strong band near 985–990 cm−1,due to �as(P–O) vibrations [41]. These results, together withthe FT-IR data for the same composites, suggest that polyox-otungstates are effectively immobilized on the functionalizedsilica.

The diffuse reflectance spectrum of the silicaN(Et)3/PW11Mn issimilar to the spectrum of K-PW11Mn (Fig. 3I), showing a band inthe visible region, with the maximum at 510 nm, which is attributed

III

to a d–d electronic transition of Mn , and the tail of a charge trans-fer band (due to O → W or O → Mn transitions [36,42]) between 300and 400 nm. For iron substituted Keggin anions, only the chargetransfer bands are observed on the spectra of both POM and silicabased POM material (Fig. 3II).

b) silicaN(Et)3/PW11Mn. (II): (a) K-PW11Fe and (b) silicaN(Et)3/PW11Fe.

32 A.C. Estrada et al. / Applied Catalysis A: General 392 (2011) 28–35

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3.2.2. Oxidation of cyclooctaneThe results of the oxidation of cyclooctane (2) in the presence

of silicaN(Et)3/XW11Fe materials are shown in Table 1 and are

Sch

It is worth to refer that the functionalization of silica with qua-ernary ammonium groups seems to lead, in the conditions usedn this work, to more stable materials, with considerable catalyticdvantage in comparison with some supports based on silica func-ionalized with – NH2 groups. In fact, for comparison, we preparedW11Fe (X = Si, B) supported on a mesoporous silica functionalizedith NH2 and used it in the catalytic oxidation of cyclooctane in

onditions similar to those here described. The silica support wasrepared by a known procedure [43] and left in contact with acidi-ed POM solutions for impregnation. However, extensive leachingas observed in these catalytic studies. This fact may be due, pos-

ibly, to the facile deprotonation of the NH3+ groups in the reaction

edia. In this type of materials, POMS are supported by electro-tatic interaction with the quaternary ammonium or NH3

+ groups.

.2. Catalytic results

In the present work all the oxidative reactions were per-ormed with H2O2 as oxidant and acetonitrile as solvent, at 80 ◦C,ollowing our previous studies with the same XW11M in homoge-eous conditions [28,31,33]. The oxidation of cis-cyclooctene (1)as performed in the presence of silicaN(Et)3/PW11Mn and the

upported iron-substituted POMs were used for reactions withyclooctane (2). For both substrates the results obtained were com-ared with those obtained under homogeneous conditions and theeactions were carried out in identical conditions. The conversionsnd product selectivity were evaluated after 6 h of reaction. Thetructures of the substrates and obtained products are shown incheme 3..

.2.1. Oxidation of cis-cycloocteneThe oxidation of cis-cyclooctene (1) in the presence of

ilicaN(Et)3/PW11Mn affords in the first run, with 65% of con-ersion, 1,2-epoxycyclooctane (3) as the only product (100%electivity). The reactions do not proceed in the absence of the cat-lyst and only 2% of conversion was registered in the presence ofhe modified support, silicaN(Et)3Br (Fig. 4).

The values of conversion obtained with heterogeneous sys-em in the first two runs (65%) are three times higher than thene obtained with the corresponding TBA salt homogeneous cat-lyst (22%) (Fig. 4). This remarkable behaviour can be justified byhe fact that the concurrent dismutation of hydrogen peroxide is

ore rapid in the homogeneous conditions than in the presence

f silicaN(Et)3/PW11Mn. In fact, in the homogeneous conditions,0% of the H2O2 is consumed after 6 h whereas only 60% is used

n the heterogeneous system. The catalytic action of PW11Mn inelation to the dismutation of H2O2 has been referred before [44]nd seems to be more important when PW11Mn is in solution. This

.

better performance under heterogeneous conditions was main-tained for five runs where a substrate conversion of 26% was stillattained.

With the purpose of clarifying the heterogeneous characteris-tics of the novel supported catalysts, in opposition to a putativehomogeneous reaction catalysed by the leached POM, the cata-lyst was removed after 2 h of reaction by centrifugation and theremaining solution was left to react for 4 additional hours. Theconversion of cis-cyclooctene increased only 1.1% between 2 and6 h of reaction. Moreover, solution UV–vis spectra were obtainedafter catalysis, for each of the five catalytic cycles. So, following cen-trifugation and separation of the catalyst, silicaN(Et)3/PW11Mn, theUV–vis spectra were run which clearly show that no leaching hasoccurred for the several cycles (Fig. 5). Also the FT-IR spectrum forsilicaN(Et)3/PW11Mn after the fifth catalytic cycle shows no decom-position or degradation of the POM since the bands correspondingto PW11Mn are still there. These results suggest that XW11M doesnot leach to the solution, confirming the heterogeneous nature ofthe catalytic system.

Fig. 4. Conversion of cis-cyclooctene in the presence of homogeneous (PW11Mn, TBAsalt) or heterogeneous [silicaN(Et)3/PW11Mn] catalysts and with only the modifiedsupport [silicaN(Et)3Br], after 6 h of reaction. Reaction conditions: 1.0 mmol of cis-cyclooctene, 1.5 �mol of PW11Mn (homogenous) or 25 mg of silicaN(Et)3/PW11Mn(heterogeneous), 1.0 mmol of H2O2 (aqueous 30% w/w), 1.5 mL of CH3CN, at 80 ◦C.

A.C. Estrada et al. / Applied Catalysis A: General 392 (2011) 28–35 33

F −4

ccc

cnptrhwFFtsHH

pc(ci

as

Fig. 6. Time course of cyclooctane oxidation reaction. Reaction conditions: 25 mg of

TO

ig. 5. UV–vis spectra of (a) K-PW11Mn in water solution (8 × 10 M, the sameoncentration if all the POM was leached) and (b) the five reaction solutions afterentrifugation and separation of the catalyst, silicaN(Et)3/PW11Mn, after the fiveatalytic cycles.

ompared with the ones obtained with the corresponding homoge-eous catalyst. These results are in accordance with those alreadyublished by us [31,33]. No substrate conversion is observed inhe absence of catalyst and silicaN(Et)3Br support show only aesidual activity (2%) in cyclooctane oxidation (entry 13). Theighest conversions, 74% and 71%, are attained in the first runith silicaN(Et)3/PW11Fe and silicaN(Et)3/BW11Fe, respectively.

or silicaN(Et)3/SiW11Fe a lower conversion was obtained (37%).ig. 6 shows the time course of the cyclooctane oxidation inhe presence of the several catalysts; silicaN(Et)3/PW11Fe andilicaN(Et)3/BW11Fe show a similar profile. The consumption of2O2 was always higher than 86% (Table 1), corresponding to low2O2 efficiency values (4–14%).

Considering the recovery and reuse of the catalyst, the besterformance was observed with silicaN(Et)3/PW11Fe. In fact, goodonversion values are still observed in the second and third cycles65–66%). The advantages (recovery and reuse) of the new materialan compensate the lower conversion values obtained in compar-

son with those under homogeneous conditions (entry 4).

Table 1 shows that the selectivity of the products is notffected by the reuse of catalyst. The main products in case ofilicaN(Et)3/PW11Fe and silicaN(Et)3/SiW11Fe are cyclooctanone

able 1xidation of cyclooctane with aqueous H2O2 using homogeneous and heterogeneous iron

Entry Catalyst Run Conv.(%)a

1 SilicaN(Et)3/PW11Feb 1 742 2 663 3 654 TBA4PW11Fe(H2O)O39

d – 895 SilicaN(Et)3/BW11Feb 1 716 2 477 3 168 TBA4H2BW11Fe(H2O)O39·H2Oe – 959 SilicaN(Et)3/SiW11Feb 1 3710 2 2011 3 1912 TBA4HSiW11Fe(H2O)O39

d – 6213 SilicaN(Et)3Brf – 2

a Determined by gas chromatography.b Reaction conditions: 25 mg of catalyst (1.1–1.2 �mol of POM); CH3CN (1.5 mL); cycloc Not determined.d From Ref. [31]. Reaction conditions: 1.5 �mol of catalyst; CH3CN (1.5 mL); cyclooctane From Ref. [33]. Reaction conditions: 1.5 �mol of catalyst; CH3CN (1.5 mL); cyclooctanf Similar reaction conditions, although using 25 mg of silicaN(Et)3Br.

catalyst (1.1–1.2 �mol of POM); CH3CN (1.5 mL); cyclooctane (1 mmol); 30% (w/w)H2O2 (9.8 mmol); 80 ◦C.

(4) and cyclooctane hydroperoxide (6). With silicaN(Et)3/BW11Fecyclooctanone (4) is always the main product.

It is worth to note that the selectivity obtained under heteroge-neous conditions is different from the one attained in homogeneousconditions. In fact, when comparing both systems the formationof higher amounts of cyclooctanol (5) (15–29% against 0–8%) con-comitant with minor amounts of 1,2-epoxycyclooctane (3) (2–10%against 0%) are observed. It has been proposed that the exten-sive hydroperoxidation of cyclooctane, observed when an excessof hydrogen peroxide is used in the presence of iron substitutedPOMs in homogeneous conditions, is related to the extensiveformation of O2 through the dismutation of H2O2 [31,45]. The dif-ferent selectivity observed is possibly a consequence of cyclooctanehydroperoxide (6) decomposition due to the silica acidity.

The solid materials recovered after the reactions were charac-terized, in order to check the catalysts stability. Fig. 7 illustratesFT-Raman spectra of K-BW11Fe and silicaN(Et)3/BW11Fe (after twoand three catalytic cycles). The bands indicate that the BW11Fe

structure is intact after three heterogeneous catalytic cycles, eventhough the loss of activity, in this case, is significant. Similar exper-iments after the fourth cycle did not show the presence of the POMon the silica.

-substituted polyoxotungstates after 6 h of reaction.

H2O2 consumption (%) Products selectivity (% mol)a

3 4 5 6

97 6 40 16 3897 7 38 16 39n.dc 5 40 16 3997 – 26 – 7498 3 62 16 1999 2 63 15 20n.dc 3 62 29 688 – 21 2 7786 3 41 20 3690 11 39 19 31n.dc 10 40 18 3162 – 30 8 6267 – 100 – –

octane (1 mmol); 30% (w/w) H2O2 (9.8 mmol); 80 ◦C; after 6 h of reaction.

e (1 mmol); 30% (w/w) H2O2 (9.8 mmol); 80 ◦C; after 6 h of reaction.e (1 mmol); 30% (w/w) H2O2 (9.8 mmol); 80 ◦C; after 6 h of reaction.

34 A.C. Estrada et al. / Applied Catalysis A: General 392 (2011) 28–35

F 3/BW1

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ig. 7. Comparison of FT-IR and FT-Raman spectra of (a) K-BW11Fe; (b) silicaN(Et)ycle.

. Conclusions

Iron and manganese substituted polyoxotungstates were immo-ilized on triethylpropylammonium-functionalized silica andharacterized by different techniques. FT-Raman, FT-IR and ele-ental analysis provide enough evidence of the presence of the

ron and manganese substituted polyoxotungstates supported onhe modified silica. The catalytic activity of the immobilizedOMs on the novel materials was studied for cis-cyclooctenend cyclooctane oxidation with aqueous 30% hydrogen perox-de at 80 ◦C and compared with corresponding homogenousystems. In heterogeneous conditions, cis–cyclooctene was oxi-ized with a remarkable higher conversion than the obtained

n homogeneous solution and with 100% selectivity for the 1,2-poxycyclooctane. The oxidation of cyclooctane gave conversionss high as 71–74%, with moderate selectivity for cyclooctanone,yclooctane hydroperoxide and cyclooctanol, together with minormounts of 1,2-epoxycyclooctane. These products distribution isomewhat different from that obtained in homogeneous system.

The immobilized iron and manganese substituted polyoxo-ungstates materials have demonstrated to be active and reusableatalysts in clean oxidation reactions using hydrogen peroxide asxidant.

cknowledgments

Thanks are due to the University of Aveiro, FCT (Fundacão para aiência e a Tecnologia) and FEDER for funding QOPNA and CICECO..C.E. thanks FCT for her Ph.D. grant.

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