An Ecofriendly and Efficient Solid Catalyst for Nonsolvent Liquid

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Supported Undecaphosphotungstate: An Ecofriendly and Efficient Solid Catalyst for Nonsolvent Liquid-Phase Aerobic Epoxidation of Alkenes Pragati 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 an efficient catalytic system for solvent-free liquid-phase aerobic epoxidation of alkenes under mild reaction conditions. The superiority of the present catalyst lies in achieving 85% conversion for cyclohexene with 100% selectivity toward cyclohexene oxide. Further, the catalyst 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 industrial as well as laboratory scale as obtained epoxides are widely used as raw materials for epoxy resins, paints, and surfactants and are intermediates in various organic synthetic reactions.1 Recently, owing to the known disadvantages of traditional oxidants, epoxidation based on ecofriendly processes have gained much attention. Among all, epoxidation with H2O2 is most important as it generates only water as a byproduct. However, at the same time, the majority of catalytic systems using H2O2 as an oxidant also face the problems of (i) lower H2O2 efficiency, (ii) low epoxide selectivity, and (iii) lower reactivity toward cyclic olefins. These problems could be overcome by the use of molecular oxygen, as it has higher reactivity toward cyclic olefins. Thus the development of epoxidations with molecular oxygen or air alone has technologically and environmentally attracted much attention.24 However, little success has been achieved for the selective epoxidation of alkenes with O2 in the liquid phase5 because of catalyst deactivation and difficulty of CH bond/O2 activation. Hence, the search for a stable catalyst is in demand. Catalysts based on polyoxometalates (POMs) are excellent candidates due to their inherent stability toward oxygen donors,68 and their better capacity to utilize green oxidants as well as higher 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 or more of the MO octahedra from the fully occupied POMs (XM12O40n) have gained much attention in the area of oxidation catalysis. The catalytic evaluation of the lacunary POMs has been reported by different groups.1014 Especially, detailed studies have been carried out on oxidation reactions using H2O2 over lacunary silicotungstates by Mizuno et al.1520 A literature survey shows that studies on the lacunary phosphotungstates10,2124 are very scarce. It was also found that, no literature is available on the catalytic aspects of supported lacunary phosphotungstates. To the best of our knowledge there are two articles on the catalytic activity of supported lacunary phosphotungstate and that was by our research group only.25,26 r 2011 American Chemical Society

In our early reports we found that undecatunstophosphate supported onto zirconia is an efficient bifunctional catalyst. It is known that support also plays an important role in modifying the catalytic properties of the catalyst. As an extension of our work, we here report the use of undecaphosphotungstate (PW11) supported onto neutral alumina (Al2O3) for nonsolvent aerobic epoxidation of alkenes. To optimize the parameters, detailed study was carried on oxidation of styrene (Sty) by varying different parameters such as reaction temperature, catalyst amount, and reaction time. Further, the heterogeneity test was also performed for the best catalyst, to confirm the absence of leaching of the active species from the support surface. The catalyst was also regenerated and reused. Under the optimized conditions oxidation of cyclic alkenes was also evaluated over both the catalysts. The superiority of the work lies in achieving excellent results, especially for epoxidation of cyclohexene (Cy6).

’ EXPERIMENTAL SECTION Materials. All chemicals used were of A. R. grade. Zirconium oxychloride (ZrOCl2 3 8H2O) (Loba Chemie, Mumbai), anhydrous disodium hydrogen phosphate (Na2HPO4) (Merck, Mumbai), sodium tungstate dihydrate (Na2WO4 3 2H2O) (SD fine chemicals, Mumbai), and neutral alumina (Merck, Mumbai) were used as received. Acetone and sodium hydroxide were obtained from Merck and were used as received. Synthesis of Undecatungstophosphate (PW11). The undecatungstophosphate 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 stoichiometric ratio. The solution was heated to 8090 °C, and the pH was adjusted to 4.8 with concentrated nitric acid. The volume was Received: April 8, 2011 Accepted: June 21, 2011 Revised: June 3, 2011 Published: June 21, 2011 9069

dx.doi.org/10.1021/ie200747a | Ind. Eng. Chem. Res. 2011, 50, 9069–9076

Industrial & Engineering Chemistry Research

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diffractometer. The conditions were as follows: Cu KR radiation (1.54 Å), scanning angle from 0 to 60°. Adsorptiondesorption isotherms of samples were recorded on a Micromeritics ASAP 2010 surface area analyzer at 196 °C. The BET specific surface area was calculated by using the standard Bruanuer, Emmett, and Teller method on the basis of the adsorption data. Further the pore size distributions were calculated applying the Barrett JoynerHalenda (BJH) method to the desorption branches of the isotherm. Catalytic Reaction. The oxidation reaction was carried out in a batch-type reactor operated under atmospheric pressure. In a typical reaction, a measured amount of catalyst was added to a three-necked flask containing alkene (100 mmol) at 80 °C (for Sty) 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 of the reaction, the liquid product was extracted with dichloromethane, dried with magnesium sulfate, and analyzed on a gas chromatograph (Nucon 5700 model). Product identification was done by comparison with standard samples and finally by a combined gas chromatographymass spectrometer (HewlettPackard column) using HP-1 capillary column (30 m, 0.5 mm id) with EI and 70 eV ion source. The conversion as well as selectivity was calculated on the basis of mole percent of alkenes. conversion ð%Þ ¼

selectivity ð%Þ ¼ Figure 1. FT-IR spectra for PW11 and (a) Al2O3 and (b) (PW11)2/ Al2O3.

then reduced to half by evaporation, and the heteropolyanion was separated by liquidliquid extraction with 80100 mL of acetone. The extraction was repeated until the acetone extract showed an absence of NO3- ions (ferrous sulfate test). The extracted solid was dried in air. The obtained sodium salt of undecatungstophosphate was designated as PW11. Synthesis of PW11 Supported onto Al2O3 (PW11/Al2O3). A series of catalysts containing 1040% PW11 were synthesized by impregnating Al2O3 (1 g) with an aqueous solution of PW11 (0.10.4 g in 1040 mL of conductivity water) with stirring for 35 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 using JSM 5910 LV combined with an INCA instrument for EDXSEM. FT-IR spectrum of the sample was obtained by using the KBr wafer on a PerkinElmer instrument. TGA of the samples was carried out on a Mettler Toledo Star SW 7.01 in the temperature range of 50600 °C under nitrogen atmosphere with a flow rate of 2 mL/min and a heating rate of 10 °C/min. The magic-angle spinning (MAS) solid state NMR study was carried out on a BRUKER NMR spectrometer under ambient conditions. 31P MAS NMR spectra were recorded on a Bruker Advance DSX300 NMR spectrometer at 121.48 MHz using a 7 mm rotor probe with 85% phosphoric acid as an external standard. The XRD pattern was obtained by using a Philips PW-1830

ðinitial mol%Þ ðfinal mol%Þ ðinitial mol%Þ

moles of product formed 100 moles of substrate consumed

The turn over number (TON) was calculated using the following equation TON ¼

moles of product moles of catalyst

’ RESULTS AND DISCUSSION A detailed characterization of PW11 can be found in our earlier communication.25 In the present article we report the main characterization of the support as well as PW11/Al2O3 for the reader’s convenience. Leaching is a negative property for any catalyst. Any leaching of the catalyst from the support would make the catalyst unattractive. When polyoxometalates react with a mild reducing agent such as ascorbic acid it develops blue coloration, which can be used for the quantitative characterization for the leaching of polyoxometalates from the support.27 No development of blue color indicates no leaching. The same procedure was repeated with Sty, Cy6, cis-cyclooctene (Cy8) and with the filtrate of the reaction 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. The obtained 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 9070

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Figure 2.

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31

P MAS NMR of (PW11)2/Al2O3.

show bands at 752 and 853 cm1, 950 cm1, and at 1091 and 1045 cm1 corresponding to the symmetric stretching of WOW, WdO, and PdO bonds, respectively. The positions are in good agreement with those of PW11, which has been reported earlier25 confirming the presence of these groups in the synthesized materials. TGA of PW11 shows 8% weight loss in the temperature range 100150 °C due to the loss of crystalline water. It also shows 2% weight loss at 400 °C. This may be due to the decomposition of the Keggin structure. TGA of (PW11)2/Al2O3 shows 5% weight loss 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. The TGA studies show an increase in the stability of PW11 after supporting onto Al2O3. 31 P MAS NMR spectrum for PW11 shows an intense peak at 11.30 ppm while the solution NMR shows a peak at 10.6 ppm indicating the formation of the monolacunary species. The observed shift in MAS NMR as compared to that of solution NMR is as expected. Edwards et al. have reported that for supported phosphotungstate catalysts, the majority of the active species are present in different forms such as dispersed, intact, or fragmented Keggin units on the surface of the support.28 They report four different types of resonances at 4, 8, 11, and 13 ppm. The resonance at 4 ppm is due to a adsorbed phosphorus species derived from a highly fragmented Keggin unit. The resonance at 8 ppm may indicate the presence of phosphate character due to the partial fragmentation of the Keggin unit to produce, “11defect” Keggin species and species of the type WnOyO3POH, in which a phosphate is coordinated to a tungstenoxygen cluster fragment. The resonance at 11 and 13 ppm could be due to intact Keggin units interacting with surface hydroxyl groups. The 31P MAS NMR spectra for (PW11)2/Al2O3 is represented in Figure 2. It shows one strong resonance at 7.2 ppm. The strong resonance at 7.2 ppm corresponds to the presence of “11-defect Keggin species” and is in good agreement with reported one. This observation clearly indicates fine dispersion of 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

Table 1. Surface Area of PW11/ Al2O3 Series catalysts

surface area (m2/g)

Al2O3

80.1

(PW11)1/Al2O3

89.1

(PW11)2/Al2O3 (PW11)3/Al2O3

91.9 61.5

(PW11)4/Al2O3

46.5

Al2O3, electrostatic interaction is expected which results in high dispersion of PW11 on Al2O3. Thus the obtained results are in good agreement with the proposed explanation. The powder X-ray diffraction pattern of PW11 shows (Supporting Information, Figure S1) that the synthesized complex is crystalline. The major peaks are seen in the range 710°, 1622°, and 2530° which is believed to be the typical 2θ range for the Keggin structure.29 The XRD pattern of (PW11)2/Al2O3 shows the amorphous nature of the materials indicating that the crystallinity of the PW11 is lost on supporting it onto Al2O3 (Supporting Information, Figure S1). Further, it does not show any diffraction lines of lacunary PW11 indicating a very high dispersion of solute as a noncrystalline form on the support surface. The values of surface area for the whole series of catalysts are shown in Table 1. The larger surface area of all catalysts as compared to that of the support was because of the supporting of PW11, as expected. Initially the value for surface area increases with an increase in loading from 10% to 20%. On further increase in the amount of PW11 from 20% to 40%, the surface area decreases. This may be due to the formation of multilayers of active species, PW11, onto support surface due to higher loading. This results in penetration of the active species in the pores resulting in blocking/stabilization of active sites on the monolayer and so the total surface area decreases. A detailed pore size distribution as well as the adsorption desorption isotherm was also evaluated for (PW11)2/Al2O3 and is represented in Figure 3. The nitrogen adsorption isotherm presents a type-II isotherm with a hysteresis loop in the desorption isotherm in the high range of relative pressure (Figure 3). A type-II isotherm is 9071



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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.

Figure 3. (a) BET isotherm adsorptiondesorption isotherm; (b) pore size distribution plot for (PW11)2/Al2O3.

obtained when adsorption occurs on nonporous powders. The inflection point or knee of the isotherm usually occurs near the completion of the first adsorbed monolayer and, with increasing relative pressure, second and higher layers are completed until at saturation the number of adsorbed layers becomes infinite. The pore size distribution curve shows two maxima in the range of 315 nm corresponding to the pores belonging to the range of microporosity. Uniformity in the pore size over a broad distribution curve was obtained. As Al2O3 is a neutral support, an electrostatic type of interaction is expected which results in uniform dispersion of PW11 on Al2O3.

Thus from the pore size distribution as well as 31P MAS NMR data, it can be confirmed that there is a strong interaction between PW11 and Al2O3. Oxidation of Sty Using O2. The catalytic activity was evaluated for epoxidation of alkenes using molecular oxygen as an oxidant. To ensure the catalytic activity, all reactions were carried out without catalyst. It was found that no oxidation takes place. The support, Al2O3 was also used as catalyst for epoxidation of alkenes and no conversion was found. A detail study was carried out on epoxidation of Sty by varying different parameters, such as reaction temperature, amount of catalyst and reaction time to optimize the conditions. Effect of Temperature. To determine the optimum temperature the reaction was investigated at four different temperatures 50, 60, 80, and 100 °C, keeping other parameters fixed (catalyst amount, 25 mg; reaction time, 4 h). The results for the same are presented in Figure 4. The results show that conversion increased with increasing temperature. Only a negligible improvement in conversion was observed on increasing temperature from 80 to 100 °C. So the temperature of 80 °C was found optimal for the maximum conversion of Sty. As we are optimizing conditions for maximum conversion, % selectivity was not taken into consideration. Effect of (PW11)2/Al2O3 Amount. The effect of the catalyst amount on the conversion of Sty is represented in Figure 5. It is seen from the figure that the conversion increases up to 75 mg, 9072



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Table 2. Aerobic Epoxidation of Alkenes Using (PW11)2/ Al2O3 oxidanta O2

alkene

conversion (%)b

products

selectivity (%)

TON

Sty

58

StyO

56

4265

Cy6c Cy8c

85 3.5

Cy6O Cy8O

>99 >99

6029 2584

Amount of catalyst, 25 mg; temperature, 80 °C (Sty), 50 °C (cyclic olefins). Alkene: 100 mmol; oxidant O2, 1 atm; time, 4 h. b Conversion based on substrate; Cy8O = cyclooctene oxide; Cy6O = cyclohexene oxide c 24 h; amount of active PW11 on the support, 4.16 mg. a

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

after that it stays almost constant. This may be because in the case of catalysis using heteropolyacids, in the presence of nonpolar molecules such as hydrocarbons, the reaction follows the adsorption phenomenon rather than a typical pseudoliquid behavior.30 Similar behavior is expected in the case of lacunary POMs. The nonpolar molecules such as hydrocarbons just adsorb on the surface without entering the bulk. Thus, on further increase in the amount, there may be blocking of the active sites, and so increase in conversion is not significant. This shows that it follows the adsorption phenomenon rather than the typical pseudoliquid behavior. At the same time, the difference in the selectivity of products is also observed up to 50 mg of the catalyst. It has been reported by Yoshida et al.20 that in the lacunary species the surface W(=O)2 atoms are active enough to catalyze the reaction. Also the terminal oxo ligands at the lacunary site are active enough to react with the metal cations as well as the protons which efficiently catalyze the oxidation of alkenes. As the amount of the catalyst increases the number of W atoms available for the reaction to progress increases which results in a difference in the selectivity of products. On further increase in the catalyst amount the distribution of selectivity of products becomes constant. This may be due to the combined effect of the acidity of the tungsten atoms as well as the effect of the surface adsorption phenomenon which stabilizes the distribution of products. From the viewpoint of epoxide importance, 25 mg of the catalyst amount was found optimum. Effect of Reaction Time. It was observed that the 20% loaded catalyst gives the best results with 25 mg of the catalyst. The effect of reaction time on the conversion of Sty is represented in Figure 6. It is observed from Figure 6 that with increase in reaction time the % conversion also increases. Initially, the increase in the conversion is fast, but after 9 h a slow increase in the conversion is observed. This may be due to the fact that as Sty is consumed during 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 the catalyst surface is faster. As a result the overall rate of the reaction slows 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 was styrene 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 unstable intermediate, epoxide, is converted to the more stable product BA via a bond cleavage mechanism. Owing to the known industrial importance of StyO, the reaction time was optimized at 4 h. The optimum conditions for maximum selectivity toward StyO (i.e., 56%) over (PW11)2/Al2O3 are catalyst amount of 25 mg, temperature of 80 °C, and time of 4 h. Epoxidation of Cyclic Olefins. Epoxidation of cyclic olefins was carried out under the optimized conditions of Sty, except the temperature was 50 °C. The conversion as well as selectivity for epoxidation of alkenes under optimized conditions is presented in Table 2. The mechanistic pathway for the oxidation of alkenes using a dioxygen species has been reported by Neumann and Dahan.31 It has been proposed that oxidation of the substrate in the presence of a transition metal compound proceeds by a metal-catalyzed auto-oxidation reaction by forming an MO2 intermediate. This type of auto-oxidation reaction therefore, gives possibility of achieving epoxidation of alkenes by an addition reaction. As a result epoxide formation is favored. In the presence case the formation 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 elimination of the catalyst resulting in further oxygenated products. The activation time required for the catalyst in the case of cyclic olefins is more as compared to terminal alkenes,31 as a result in the present case 24 h reaction time was required to optimize the parameters. The observed order for the reactivity of cyclic olefins was Cy6 > Cy8. The higher conversion for the lower number of cyclic carbon indicates that the Cy8 is more strained. The lower conversion for Cy8 is mainly due the bulkiness of the cyclic ring. The large ring size, as well as ring strain, partially prevents the oxidation 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 under solvent-free conditions. Reaction Mechanism. It is known that for oxidation reactions with transition metal atoms, especially polyoxometalates, O2 first binds to the metal center and then transfers an oxygen atom to the olefin. Thus, the activation of the metal center results via the generation of the active species which may be superoxo or oxo species. In the present case, the supported undecatungstophosphate is also expected to follow the same mechanism via the formation of an active tungsten-oxo or superoxo intermediate. To confirm the formation of the active intermediate, the catalyst 9073



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Table 4. Oxidation of Sty with and without Catalyst Using O2 selectivity (%) catalyst

conversion (%)

BA

StyO

(PW11)2/Al2O3 (4 h)

58.0

44.0

56.0

filtrate (6 h)

58.1

44.0

56.0

a

Catalyst amount, 25 mg; temperature, 80 °C ; Sty, 100 mmol; O2 1 atm; time, 4 h. a

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

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

Table 3. Oxidation of Alkenes with Recycled Catalysts Using 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 R3-(PW11)2/Al2O3

56.0 55.8

44.0 44.2

56.0 55.8

Oxidation of Cy6b (PW11)2/Al2O3

85.0

100

R1-(PW11)2/Al2O3

84.2

100

R2-(PW11)2/Al2O3

84.0

100

R3-(PW11)2/Al2O3

83.1

100

Catalyst amount, 25 mg; temperature, 80 °C (for Sty), 50 °C (for Cy6); alkene, 100 mmol O2 1 atm; time, 4 h. b Time = 24 h. a

was isolated under reaction conditions and characterized by FT-IR spectra. The FT-IR spectra of the catalysts before and after passage of O2 are represented in Figure 7. It is seen from the figure that the typical vibrations at 950 and 853 cm1, which are assigned to the terminal WdO and bridged WOW stretching, indicate that the basic Keggin unit remains intact during the catalytic oxidation. Apart from that, an additional peak at 834 cm1 is observed.

This corresponds to the formation of tungstensuperoxo species and is in good agreement with the reported value.31 Thus, experimental evidence of FT-IR confirms that the formed tungsten superoxo species is the actual active species responsible for epoxidation. On the basis of the FT-IR observation, we propose the probable mechanism (Scheme 1) for epoxidation of cyclohexene. Catalytic Activity of Regenerated Catalysts. The catalyst remains insoluble in the present reaction conditions and can be separated easily by simple filtration followed by washing. The regenerated catalysts were washed with dichloromethane and dried at 100 °C. Oxidation of alkenes was carried out with the recycled catalysts, under the optimized conditions. In both cases, the catalysts could be used for more cycles. The data for the catalytic activity is represented in Table 3. As seen from the data there was no appreciable change observed in selectivity; however, a little decrease in conversion was observed which shows that the catalysts are stable and can be regenerated for repeated use up to three cycles. Heterogeneity Test. For the rigorous proof of heterogeneity, a test32 was carried out by filtering the catalyst from the reaction mixture at 80 °C after 4 h. The heterogeneity test was evaluated for oxidation of Sty as an example, and similar observations are expected with the other alkenes. The reaction vessel was filled with Sty, to which 25 mg (PW11)2/Al2O3 was added under experimental conditions. After this period, the reaction mixture was filtered to a second flask and stirred without catalyst for 2 more 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 an SE-30 column. No appreciable change in the conversion as well as selectivity was found indicating that both the catalysts fall into category C (Table 4).32 On the basis of these results, it can be concluded that there is no leaching of the PW11 from the support and the present catalyst is truly heterogeneous in nature. Comparison with Reported Catalysts. Table 5 represents comparative data with other reported catalysts used for aerobic epoxidation of alkenes. It is seen from Table 5 that in the case of Sty, 100% conversion was obtained with NaCoX9633 but the selectivity for StyO is 33%. In the case of Co2+X8 only 44% conversion was obtained with 60% selectivity for StyO. The present catalyst gives 56% conversion with 56% selectivity for StyO. In the case of oxidation of cyclic olefins, especially for Cy6, results are very unique and outstanding. The present catalyst gives 85% conversion and >99% selectivity for Cy6O. While in the case of NaCoX9634 only 26% conversion is obtained with 48% selectivity for Cy6O. In the case of Cy8 also the % conversion was low (3.5%), but single selective product was obtained. Further, all reported reactions were carried out with DMF as solvent under 60 psi pressure (4.1 atm) conditions while the present reactions are nonsolvent reactions under ambient 9074



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

alkene

(PW11)2/Al2O3 NaCoX96

conversion (%) 56

StyO/56

DMF 20

100

StyO/33

16.0

44

StyO/60

13.0

85

Cy6O/>99

6029

26

Cy6O/48

100:4:1:25 10:4:4:200 Cy6

NaCoX96 a

solvent (mL)

Sty

Co2+X (PW11)2/Al2O3

reaction conditionsa

8:24:1:25 2:8:4:200

DMF 40

products/selectivity

TON 4265

Substrate (mmol):reaction time (h):pressure (atm):amount of catalyst (mg). Conversion: 1 atm = 14.5 psi.

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

’ CONCLUSIONS We have introduced an efficient solid catalytic system comprising undecaphosphotungstate and neutral alumina for aerobic epoxidation of alkenes. The superiority of the present catalyst lies, 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). The novelty of the work is that the designed heterogeneous catalyst gives higher selectivity toward epoxides, especially in the case of Cy6. Further, in all the cases, the regenerated catalysts can be used successfully without any significant loss in catalytic activity up to three cycles. ’ ASSOCIATED CONTENT

bS

Supporting Information. Powder XRD pattern of (a) PW11 and (b) Al2O3 and (PW11)2/Al2O3. This material is available 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 and Industrial Research, CSIR New Delhi, for providing financial assistance. We are thankful to Indian Institute of Science, Bangalore for recording 31P MAS NMR spectra. ’ REFERENCES (1) Mizuno, N.; Yamaguchi, K.; Kamata, K. Epoxidation of olefins with hydrogen peroxide catalyzed by polyoxometalates. Coord. Chem. Rev. 2005, 249, 1944–1956. (2) Yamaguchi, K.; Mizuno, N. Heterogeneously catalyzed liquidphase oxidation of alkanes and alcohols with molecular oxygen. New J. Chem. 2002, 26, 972–974. (3) Sheldon, R.; Kochi, J. Metal Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (4) Hill, C. Advances in Oxygenated Processes; Baumstark, A., Ed.; JAI Press, Inc.: London, 1988, Vol. 1 (5) Q Tang, Q.; Wang, Y.; Liang, J.; Wang, P.; Zhang, O.; Wan, H. Co2+-exchanged faujasite zeolites as efficient heterogeneous catalysts for epoxidation of styrene with molecular oxygen. Chem. Commun. 2004, 440–441. (6) Neumann, R. Polyoxometalate complexes in organic oxidation chemistry. Prog. Inorg. Chem. 1998, 47, 317.

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An Ecofriendly and Efficient Solid Catalyst for Nonsolvent Liquid

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