Supported Undecaphosphotungstate

8/4/2019 Supported Undecaphosphotungstate

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Published: June 21, 2011

r 2011 American Chemical Society 9069 dx.doi.org/10.1021/ie200747a| Ind. Eng. Chem. Res. 2011, 50, 90699076

ARTICLE

pubs.acs.org/IECR

Supported Undecaphosphotungstate: An Ecofriendly and EfficientSolid Catalyst for Nonsolvent Liquid-Phase Aerobic Epoxidation of

AlkenesPragati A Shringarpure and Anjali Patel*Chemistry Department, Faculty of Science, M. S. University of Baroda, Vadodara 390 002, India

bS Supporting Information

ABSTRACT: An ecofriendly solid catalytic system comprising undecaphosphotungstate and neutral alumina was found to be anefficient catalytic system for solvent-free liquid-phase aerobic epoxidation of alkenes under mild reaction conditions. The superiorityof the present catalyst lies in achieving 85% conversion for cyclohexene with 100% selectivity toward cyclohexene oxide. Further, thecatalyst can be regenerated and can be reused up to three cycles without any significant loss in the catalytic activity.

INTRODUCTION

Epoxidation of olefins is an important reaction on the in-dustrial as well as laboratory scale as obtained epoxides are widelyused as raw materials for epoxy resins, paints, and surfactants andare intermediates in various organic synthetic reactions.1 Re-cently, owing to the known disadvantages of traditional oxidants,epoxidation based on ecofriendly processes have gained muchattention. Among all, epoxidation with H2O2 is most importantas it generates only water as a byproduct. However, at the sametime, the majority of catalytic systems using H2O2 as an oxidantalso face the problems of (i) lower H2O2 efficiency, (ii) lowepoxide selectivity, and (iii) lower reactivity toward cyclic olefins.These problems could be overcome by the use of molecularoxygen, as it has higher reactivity toward cyclic olefins.

Thus the development of epoxidations with molecular oxygenor air alone has technologically and environmentally attractedmuch attention.24 However, little success has been achieved forthe selective epoxidation of alkenes with O2 in the liquid phase

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because of catalyst deactivation and difficulty of CH bond/O2activation. Hence, the search for a stable catalyst is in demand.

Catalysts based on polyoxometalates (POMs) are excellentcandidatesdue to their inherent stability toward oxygen donors,68

and their better capacity to utilize green oxidants as well ashigher selectivity toward epoxides.9,1 Recently, lacunary POMs{XM11O39

(n+4), XM10O32(n+5), where X = Si, P; n = 4, 3; M =

Mo(VI),W(VI)}, which are formed by the removal of one ormore of the MO octahedra from the fully occupied POMs(XM12O40

n) have gained much attention in the area of oxida-tion catalysis. The catalytic evaluation of the lacunary POMshas been reported by different groups.1014 Especially, detailedstudies have been carried out on oxidation reactions using H2O2over lacunary silicotungstates by Mizuno et al.1520

A literature survey shows that studies on the lacunaryphosphotungstates10,2124 are very scarce. It was also found that,no literature is available on the catalytic aspects of supportedlacunary phosphotungstates. To the best of our knowledge thereare two articles on the catalytic activity of supported lacunaryphosphotungstate and that was by our research group only.25,26

In our early reports we found that undecatunstophosphatesupported onto zirconia is an efficient bifunctional catalyst. It isknown that support also plays an important role in modifying thecatalyticpropertiesofthecatalyst.Asanextensionofourwork,wehere report the use of undecaphosphotungstate (PW11) sup-ported onto neutral alumina (Al2O3) for nonsolvent aerobicepoxidation of alkenes. To optimize the parameters, detailedstudy was carried on oxidation of styrene (Sty) by varyingdifferent parameters such as reaction temperature, catalystamount, and reaction time. Further, the heterogeneity test wasalso performed for the best catalyst, to confirm the absence ofleaching of the active species from the support surface. The

catalyst was also regenerated and reused. Under the optimizedconditions oxidation of cyclic alkenes was also evaluated over

both the catalysts. The superiority of the work lies in achievingexcellent results, especially for epoxidation of cyclohexene (Cy6).

EXPERIMENTAL SECTION

Materials. All chemicals used were of A. R. grade. Zirconiumoxychloride (ZrOCl2 3 8H2O) (Loba Chemie, Mumbai), anhy-drous disodium hydrogenphosphate(Na2HPO4) (Merck, Mumbai),sodium tungstate dihydrate (Na2WO4 3 2H2O) (SD fine chemi-cals, Mumbai), and neutral alumina (Merck, Mumbai) were usedas received. Acetone and sodium hydroxide were obtained from

Merck and were used as received.Synthesis of Undecatungstophosphate (PW11). The un-decatungstophosphate was synthesized by the method reported

by Brevard et al.23 Sodium tungstate dihydrate (0.22 mol, 72.5 g)and anhydrous disodium hydrogen phosphate (0.02 mol, 2.84 g)

were dissolved in 150200 mL of conductivity water in stoichio-metric ratio. The solution was heated to 8090 C, and the pH

was adjusted to 4.8with concentrated nitricacid. The volumewas

Received: April 8, 2011Accepted: June 21, 2011Revised: June 3, 2011


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9070 dx.doi.org/10.1021/ie200747a |Ind. Eng. Chem. Res. 2011, 50, 90699076

Industrial & Engineering Chemistry Research ARTICLE

then reduced to half by evaporation, and the heteropolyanionwas separated by liquidliquid extraction with 80100 mL ofacetone. The extraction was repeated until the acetone extractshowed an absence of NO3- ions (ferrous sulfate test). Theextracted solid was dried in air. The obtained sodium salt ofundecatungstophosphate was designated as PW11.

Synthesis of PW11 Supported onto Al2O3 (PW11/Al2O3). Aseries of catalysts containing 1040% PW11 were synthesized byimpregnating Al2O3 (1 g) with an aqueous solution of PW11(0.10.4 g in 1040 mL of conductivity water) with stirring for35 h and then drying at 100 C for 10 h. The obtained materials

were designated as (PW11)1/Al2O3, (PW11)2/Al2O3, (PW11)3/Al2O3, and (PW11)4/Al2O3.

Characterization. Elemental analysis was carried out usingJSM 5910 LV combined with an INCA instrument for EDX-SEM. FT-IR spectrum of the sample was obtained by using theKBr wafer on a PerkinElmer instrument. TGA of the samples wascarried out on a Mettler Toledo Star SW 7.01 in the temperaturerange of 50600 C under nitrogen atmosphere with a flow rateof 2 mL/min and a heating rate of 10 C/min. The magic-anglespinning (MAS) solid state NMR study was carried out on aBRUKER NMR spectrometer under ambient conditions. 31PMAS NMR spectra were recorded on a Bruker Advance DSX-300 NMR spectrometer at 121.48 MHz using a 7 mm rotorprobe with 85% phosphoric acid as an external standard. The

XRD pattern was obtained by using a Philips PW-1830

diffractometer. The conditions were as follows: Cu KR radiation(1.54 ), scanning angle from 0 to 60. Adsorptiondesorptionisotherms of samples were recorded on a Micromeritics ASAP2010 surface area analyzer at 196 C. The BET specific surfacearea was calculated by using the standard Bruanuer, Emmett, andTeller method on the basis of the adsorption data. Further thepore size distributions were calculated applying the Barrett

JoynerHalenda (BJH) method to the desorption branches ofthe isotherm.Catalytic Reaction. The oxidation reaction was carried out in

a batch-type reactor operated under atmospheric pressure. In atypical reaction, a measured amount of catalyst was added to athree-necked flask containing alkene (100 mmol) at 80 C (forSty) and 50 C (for cyclic alkenes). The reaction was started by

bubbling O2 into the liquid. The reaction was carried out by varying different parameters such as reaction temperature,amount of the catalyst, and reaction time. After completion ofthe reaction, the liquid product was extracted with dichloro-methane, dried with magnesium sulfate, and analyzed on a gaschromatograph (Nucon 5700 model). Product identification wasdone by comparison with standard samples and finally by a

combined gas chromatographymass spectrometer (Hewlett-Packard column) using HP-1 capillary column (30 m, 0.5 mm id) with EI and 70 eV ion source. The conversion as well asselectivity was calculated on the basis of mole percent of alkenes.

conversion % initial mol% final mol%

initial mol%

selectivity% moles of product formed

moles of substrate consumed 100

The turn over number (TON) was calculated using the followingequation

TON moles of productmoles of catalyst

RESULTS AND DISCUSSION

A detailed characterization of PW11 canbe found in our earliercommunication.25 In the present article we report the maincharacterization of the support as well as PW11/Al2O3 for thereaders convenience.

Leaching is a negative property for any catalyst. Any leachingof the catalyst from the support would make the catalystunattractive. When polyoxometalates react with a mild reducingagent such as ascorbic acid it develops blue coloration, which can

be used for the quantitative characterization for the leaching ofpolyoxometalates from the support.27 No development of bluecolor indicates no leaching. The same procedure was repeated

with Sty, Cy6, cis-cyclooctene (Cy8) and with the filtrate of thereaction mixture after the reaction, and no leaching was found.Thus the study indicates the presence of chemical interaction

between PW11 and the support.The EDS analysis was performed for (PW11)2/Al2O3 in order

to determine the elemental composition of the catalysts. Theobtained results are close to the theoretical values: theoretical

values are W, 64.4; P, 0.99; practical values are W, 63.7; P, 0.97.The FT-IR spectra for the Al2O3, PW11, and (PW11)2/Al2O3

are shown in Figure 1. The FT-IR spectra of (PW11)2/Al2O3

Figure 1. FT-IR spectra for PW11 and (a) Al2O3 and (b) (PW11)2/Al2O3.


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show bands at 752 and 853 cm1

, 950 cm1

, and at 1091 and1045 cm1 corresponding to the symmetric stretching ofWOW, WdO, and PdO bonds, respectively. The positionsare in good agreement with those of PW11 , which has beenreported earlier25 confirming the presence of these groups in thesynthesized materials.

TGA of PW11 shows 8% weight loss in the temperature range100150 C due to the loss of crystalline water. It also shows 2%

weight loss at 400 C. This may be due to the decomposition ofthe Keggin structure. TGA of (PW11)2/Al2O3 shows 5% weightloss in the temperature range 70130 C due to loss of adsorbed

water. Further, it does not show any weight loss up to 550 C,indicating the synthesized catalyst is stable up to 550 C. TheTGA studies show an increase in the stability of PW11 after

supporting onto Al2O3.31P MAS NMR spectrum for PW11 shows an intense peakat 11.30 ppm while the solution NMR shows a peak at 10.6ppm indicating the formation of the monolacunary species. Theobserved shift in MAS NMR as compared to that of solutionNMR is as expected.

Edwards et al. have reported that for supported phosphotung-state catalysts, the majority of the active species are present indifferent forms such as dispersed, intact, or fragmented Kegginunits on the surface of the support.28 They report four differenttypes of resonances at 4, 8, 11, and 13 ppm. Theresonance at 4 ppm is due to a adsorbed phosphorus speciesderived from a highly fragmented Keggin unit. The resonance at8 ppmmay indicate the presenceof phosphate character due to

the partial fragmentation of the Keggin unit to produce, 11-defectKeggin speciesand species of the type WnOyO3POH,in which a phosphate is coordinated to a tungstenoxygencluster fragment. The resonance at 11 and 13 ppm could

be due to intact Keggin units interacting with surface hydroxylgroups.

The 31P MAS NMR spectra for (PW11)2/Al2O3 is representedin Figure 2. It shows one strong resonance at 7.2 ppm. Thestrong resonance at 7.2 ppm corresponds to the presence of11-defect Keggin species and is in good agreement withreported one. This observation clearly indicates fine dispersionof PW11 on to Al2O3. A slight shift from the reported value may

be due to the difference in the type of support used. In the case of

Al2O3, electrostatic interaction is expected which results in highdispersion of PW11 on Al2O3. Thus the obtained results are ingood agreement with the proposed explanation.

The powder X-ray diffraction pattern of PW11 shows

(Supporting Information, Figure S1) that the synthesized com-plex is crystalline. The major peaks are seen in the range 7 10,1622,and2530which is believed to be the typical2 rangefor the Keggin structure.29 The XRD pattern of (PW11)2/Al2O3shows the amorphous nature of the materials indicating that thecrystallinity of the PW11 is lost on supporting it onto Al2O3(Supporting Information, Figure S1). Further, it does not showany diffraction lines of lacunary PW11 indicating a very highdispersion of solute as a noncrystalline form on the supportsurface.

The values of surface area for the whole series of catalysts areshown in Table 1. The larger surface area of all catalysts ascompared to that of the support was because of the supporting ofPW11, as expected. Initially the value for surface area increases

with an increase in loading from 10% to 20%. On further increasein the amount of PW11 from 20% to 40%, the surface areadecreases. This may be due to the formation of multilayers ofactive species, PW11, onto support surface due to higher loading.This results in penetration of the active species in the poresresulting in blocking/stabilization of active sites on the mono-layer and so the total surface area decreases.

A detailed pore size distribution as well as the adsorptiondesorption isotherm was also evaluated for (PW11)2/Al2O3 andis represented in Figure 3.

The nitrogen adsorption isotherm presents a type-II isothermwith a hysteresis loop in the desorption isotherm in the highrange of relative pressure (Figure 3). A type-II isotherm is

Figure 2. 31P MAS NMR of (PW11)2/Al2O3.

Table 1. Surface Area of PW11/ Al2O3 Seriescatalysts surface area (m2/g)

Al2O3 80.1

(PW11)1/Al2O3 89.1

(PW11)2/Al2O3 91.9

(PW11)3/Al2O3 61.5

(PW11)4/Al2O3 46.5


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obtained when adsorption occurs on nonporous powders. Theinflection point or knee of the isotherm usually occurs near thecompletion of the first adsorbed monolayer and, with increas-ing relative pressure, second and higher layers are completeduntil at saturation the number of adsorbed layers becomesinfinite.

The pore size distribution curve shows two maxima in therange of 315 nm corresponding to the pores belonging to therange of microporosity. Uniformity in the pore size over a broaddistribution curve was obtained. As Al2O3 is a neutral support, anelectrostatic type of interaction is expected which results inuniform dispersion of PW11 on Al2O3.

Thus from the pore size distribution as well as 31P MAS NMRdata, it can be confirmed that there is a strong interaction

between PW11 and Al2O3.Oxidation of Sty Using O2. The catalytic activity was eval-

uated for epoxidation of alkenes using molecular oxygen as anoxidant. To ensure the catalytic activity, all reactions were carriedout without catalyst. It was found that no oxidation takes place.The support, Al2O3 was also used as catalyst for epoxidation ofalkenes and no conversion was found. A detail study was carriedout on epoxidation of Sty by varying different parameters, such asreaction temperature, amount of catalyst and reaction time tooptimize the conditions.

Effect of Temperature. To determine the optimum tempera-

ture the reaction was investigated at four different temperatures50, 60, 80, and 100 C, keeping other parameters fixed (catalystamount, 25 mg; reaction time, 4 h). The results for the same arepresented in Figure 4.

The results show that conversion increased with increasingtemperature. Only a negligible improvement in conversion wasobserved on increasing temperature from 80 to 100 C. So thetemperature of 80 C was found optimal for the maximumconversion of Sty. As we are optimizing conditions for maximumconversion, % selectivity was not taken into consideration.

Effect of (PW11)2/Al2O3 Amount. The effect of the catalystamount on the conversion of Sty is represented in Figure 5. It isseen from the figure that the conversion increases up to 75 mg,

Figure3. (a) BET isotherm adsorptiondesorptionisotherm; (b) poresize distribution plot for (PW11)2/Al2O3.

Figure 4. Effect of reaction temperature: catalyst amount, 25 mg;reaction time, 4 h.

Figure 5. Effect of catalyst amount: temperature, 80 C; reaction time,4 h.


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after that it stays almost constant. This may be because in the caseof catalysis using heteropolyacids, in the presence of nonpolarmolecules such as hydrocarbons, the reaction follows the adsorp-

tion phenomenon rather than a typical pseudoliquid behavior.30

Similar behavior is expected in the case of lacunary POMs. Thenonpolar molecules such as hydrocarbons just adsorb on thesurface without entering the bulk. Thus, on further increase inthe amount, there may be blocking of the active sites, and soincrease in conversion is not significant. This shows that itfollows the adsorption phenomenon rather than the typicalpseudoliquid behavior. At the same time, the difference in theselectivity of productsis also observed up to 50 mg of the catalyst.It has been reported by Yoshida et al.20 that in the lacunaryspecies the surface W(=O)2 atoms are active enough to catalyzethe reaction. Also the terminal oxo ligands at the lacunary site areactive enough to react with the metal cations as well as theprotons which efficiently catalyze the oxidation of alkenes.

As the amount of the catalyst increasesthe number of W atomsavailable for the reaction to progress increases which results in adifference in the selectivity of products. On further increase in thecatalyst amount the distribution of selectivity of products be-comes constant. This may be due to the combined effect of theacidity of the tungsten atoms as well as the effect of the surfaceadsorption phenomenon which stabilizes the distribution ofproducts. From the viewpoint of epoxide importance, 25 mg ofthe catalyst amount was found optimum.

Effect of Reaction Time. It was observed that the 20% loadedcatalyst gives the best results with 25 mg of the catalyst. Theeffect of reaction time on the conversion of Sty is represented inFigure 6.

It is observed from Figure 6 that with increase in reaction time

the % conversion also increases. Initially, the increase in theconversion is fast, but after 9 h a slow increase in the conversion isobserved. This may be due to the fact that as Sty is consumedduring the reaction, the amount of the reactant (i.e., Sty)decreases, which then requires time to bind with the oxidant.Further, the rate of desorption of the products formed from thecatalyst surface is faster. As a result the overall rate of the reactionslows down resulting in a slow increase in the % conversion

with time.The distribution of the product changes with increase in the

reaction time. After the completion of 2 h the major product wasstyrene oxide (StyO) and the minor product was benzaldehyde(BA). As the reaction time increases the product selectivity shifts

toward BA. With increase in the reaction time the unstableintermediate, epoxide, is converted to the more stable productBA via a bond cleavage mechanism. Owing to the knownindustrial importance of StyO, the reaction time was optimizedat 4 h.

The optimum conditions for maximum selectivity towardStyO (i.e., 56%) over (PW11)2/Al2O3 are catalyst amount of25 mg, temperature of 80 C, and time of 4 h.

Epoxidation of Cyclic Olefins. Epoxidation of cyclic olefinswas carried out under the optimized conditions of Sty, except thetemperature was 50 C. The conversion as well as selectivity forepoxidation of alkenes under optimized conditions is presentedin Table 2.

The mechanistic pathway for the oxidation of alkenes using adioxygen species has been reported by Neumann and Dahan.31 Ithas been proposed that oxidation of the substrate in the presenceof a transition metal compound proceeds by a metal-catalyzedauto-oxidation reaction by forming an MO2 intermediate. Thistype of auto-oxidation reaction therefore, gives possibility ofachieving epoxidation of alkenes by an addition reaction. As aresult epoxide formation is favored. In the presence case theformation of WdO2 species, at the vacant cavity/lacunary

position, favors the epoxidation of substrate.In the case of cyclic olefins an allylic attack is preferred giving

rise to epoxide which in turn rearranges by reductive eliminationof the catalyst resulting in further oxygenated products. Theactivation time required for the catalyst in the case of cyclicolefins is more as compared to terminal alkenes,31 as a result inthe present case 24 h reaction time was required to optimize theparameters.

The observed order for the reactivity of cyclic olefins wasCy6 > Cy8. The higher conversion for the lower number of cycliccarbon indicates that the Cy8 is more strained. The lowerconversion for Cy8 is mainly due the bulkiness of the cyclic ring.The large ring size, as well as ring strain, partially prevents theoxidation process which results in lower conversion of the

substrate.The superiority of the present contribution lies in obtaining

85% conversion for Cy6 with >99% selectivity for Cy6O undersolvent-free conditions.

Reaction Mechanism. It is known that for oxidation reactionswith transition metal atoms, especially polyoxometalates, O2 firstbinds to the metal center and then transfers an oxygen atom tothe olefin. Thus, the activation of the metal center results via thegeneration of the active species which may be superoxo or oxospecies. In the present case, the supported undecatungstopho-sphate is also expected to follow the same mechanism via theformation of an active tungsten-oxo or superoxo intermediate.To confirm the formation of the active intermediate, the catalyst

Figure 6. Effect of reaction time: catalyst amount, 25 mg; temperature,80 C.

Table 2. Aerobic Epoxidation of Alkenes Using (PW11)2/Al2O3

oxidanta alkene conversion (%)b products selectivity (%) TON

O2 Sty 58 StyO 56 4265

Cy6c 85 Cy6O >99 6029

Cy8c 3.5 Cy8O >99 2584

aAmount of catalyst, 25 mg; temperature, 80 C (Sty), 50 C (cyclicolefins). Alkene: 100 mmol; oxidant O2, 1 atm; time, 4 h.

b Conversionbased on substrate; Cy8O = cyclooctene oxide; Cy6O = cyclohexene oxidec 24 h; amount of active PW11 on the support, 4.16 mg.


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was isolated under reaction conditions and characterized byFT-IR spectra.

The FT-IR spectra of the catalysts before and after passage ofO2 are represented in Figure 7. It is seen from the figure that thetypical vibrations at 950 and 853 cm1, which are assigned to theterminal WdO and bridged WOW stretching, indicate thatthe basic Keggin unit remains intact during the catalytic oxida-tion. Apartfrom that, an additional peak at 834 cm1 is observed.

This corresponds to the formation of tungstensuperoxo spe-cies and is in good agreement with the reported value.31

Thus, experimental evidence of FT-IR confirms that theformed tungsten superoxo species is the actual active speciesresponsible for epoxidation. On the basis of the FT-IR observa-tion, we propose the probable mechanism (Scheme 1) forepoxidation of cyclohexene.

Catalytic Activity of Regenerated Catalysts. The catalystremains insoluble in the present reaction conditions and can beseparated easily by simple filtration followed by washing. The

regenerated catalysts were washed with dichloromethane anddried at 100 C. Oxidation of alkenes was carried out with therecycled catalysts, under the optimized conditions. In both cases,the catalysts could be used for more cycles. The data for thecatalytic activity is represented in Table 3. As seen from the datathere was no appreciable change observed in selectivity;however,a little decrease in conversion was observed which shows that thecatalysts are stable and can be regenerated for repeated use up tothree cycles.

Heterogeneity Test. For the rigorous proof of heterogeneity,a test32was carried out by filtering the catalyst from the reactionmixture at 80 C after 4 h. The heterogeneity test was evaluatedfor oxidation of Sty as an example, and similar observations areexpected with the other alkenes. The reaction vessel was filled

with Sty, to which 25 mg (PW11)2/Al2O3 was added underexperimental conditions. After this period, the reaction mixture

was filtered to a second flask and stirred without catalyst for 2more hours (i.e., total of 6 h). Both the reaction mixtures (i.e.,after 4 h and 6 h) were analyzed by gas chromatography using anSE-30 column. No appreciable change in the conversion as wellas selectivity was found indicating that both the catalysts fall intocategory C (Table 4).32 On the basis of these results, it can beconcluded that there is no leaching of the PW11 from the supportand the present catalyst is truly heterogeneous in nature.

Comparison with Reported Catalysts. Table 5 representscomparative data with other reported catalysts used for aerobicepoxidation of alkenes.

It is seen from Table 5 that in the case of Sty, 100% conversion

was obtained with NaCoX9633but the selectivity for StyO is 33%.In the case of Co2+X8 only 44% conversion was obtained with60% selectivity for StyO. The present catalyst gives 56% conver-sion with 56% selectivity for StyO. In the case of oxidation ofcyclic olefins, especially for Cy6, results are very unique andoutstanding. The present catalyst gives 85% conversion and>99% selectivity for Cy6O. While in the case of NaCoX96

34 only26% conversion is obtained with 48% selectivity for Cy6O. In thecase of Cy8 also the % conversion was low (3.5%), but singleselective product was obtained.

Further, all reported reactions were carried out with DMF assolvent under 60 psi pressure (4.1 atm) conditions while thepresent reactions are nonsolvent reactions under ambient

Figure 7. FT-IR spectra of (PW11)2/Al2O3 (a) before addition of O2and (b) (PW11)2/Al2O3-a after addition of O2 (under experimentalconditions).

Scheme 1. Epoxidation of Cyclohexene via the Formation ofTungsten-Oxo Species

Table 3. Oxidation of Alkenes with Recycled CatalystsUsing O2

selectivity

catalystsa conversion (%) BzA StyO Cy6O

Oxidation of Sty

(PW11)2/Al2O3 58.0 44.0 56.0

R1-(PW11)2/Al2O3 56.7 44.0 56.0

R2-(PW11)2/Al2O3 56.0 44.0 56.0

R3-(PW11)2/Al2O3 55.8 44.2 55.8

Oxidation of Cy6b

(PW11)2/Al2O3 85.0 100

R1-(PW11)2/Al2O3 84.2 100

R2-(PW11)2/Al2O3 84.0 100R3-(PW11)2/Al2O3 83.1 100

a Catalyst amount, 25 mg; temperature, 80 C (for Sty), 50 C (forCy6); alkene, 100 mmol O2 1 atm; time, 4 h.

b Time = 24 h.

Table 4. Oxidation of Sty with and without Catalyst Using O2

selectivity (%)

catalyst conversion (%) BA StyO

(PW11)2/Al2O3 (4 h)a 58.0 44.0 56.0

filtrate (6 h) 58.1 44.0 56.0a

Catalyst amount, 25 mg; temperature, 80C ; Sty, 100 mmol; O2 1

atm; time, 4 h.


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pressure. It is also interesting to note that the present catalystgives very high TON as compared to the reported catalysts.

CONCLUSIONS

We have introduced an efficient solid catalytic system com-prising undecaphosphotungstate and neutral alumina for aerobicepoxidation of alkenes. The superiority of the present catalystlies, especially, in obtaining 85% conversion of Cy6 and 100%selectivity for Cy6O with very high TON. Apart from that, the

amount of active species required is very small (4.16 mg). Thenovelty of the work is that the designed heterogeneous catalystgives higher selectivity toward epoxides, especially in the case ofCy6. Further, in all the cases, the regenerated catalysts can beused successfully without any significant loss in catalytic activityup to three cycles.

ASSOCIATED CONTENT

bS Supporting Information. Powder XRD pattern of (a)PW11 and (b) Al2O3 and (PW11)2/Al2O3. This material isavailable free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

ACKNOWLEDGMENT

P. Shringarpure is thankful to Council of Scientific andIndustrial Research, CSIR New Delhi, for providing financialassistance. We are thankful to Indian Institute of Science,Bangalore for recording 31P MAS NMR spectra.

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Table 5. Comparison of Conversion and Selectivity Values for Epoxidation of Alkenes

catalyst alkene reaction conditionsa solvent (mL) conversion (%) products/selectivity TON

(PW11)2/Al2O3 100:4:1:25 56 StyO/56 4265

NaCoX96 Sty DMF 20 100 StyO/33 16.0

Co2+ X 10:4:4:200 44 StyO/60 13

(PW11)2/Al2O3 Cy6 8:24:1:25 85 Cy6O/>99 6029

NaCoX96 2:8:4:200 DMF 40 26 Cy6O/48a Substrate (mmol):reaction time (h):pressure (atm):amount of catalyst (mg). Conversion: 1 atm = 14.5 psi.


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