African Journal of Pure and Applied Chemistry Vol. 7(2), pp. 50-55, February 2013 Available online at http://www.academicjournals.org/AJPAC DOI: 10.5897/AJPAC12.060 ISSN 1996 - 0840 ©2013 Academic Journals

Full Length Research Paper

Highly efficient epoxidation of olefins with hydrogen peroxide oxidant using modified silver polyoxometalate catalysts

Emmanuel Tebandeke 1*, Henry Ssekaalo 1 and Ola F. Wendt 2

1Department of Chemistry, Makerere University, P. O. Box 7062 Kampala, Uganda. 2Organic Chemistry, Department of Chemistry, Lund University, P. O. Box 124, 221 00 Lund, Sweden.

Accepted 31 January, 2013

The catalytic epoxidation of olefins is an important reaction in chemical industry since epoxides are versatile and important intermediates in the synthesis of fine chemicals and pharmaceuticals. The current epoxidation procedures often use stoichiometric organic and sometimes toxic inorganic oxidants that are less favourable from an environmental and economical point of view. In contrast to such processes, catalytic epoxidation processes employing H2O2 oxidant are preferred for green processing. Herein is reported a highly efficient green process for the epoxidation of olefins using H2O2 oxidant and modified silver polyoxometalate catalysts at 65°C. The preparation and activity of the catalysts are described. The method enjoys >90% conversion and ≥99% selectivity to the epoxide for a variety of cyclic and linear olefins including terminal ones. The catalysts are easily recovered by filtration and are reusable several times.

Key words: Epoxidation, olefins, hydrogen peroxide, polyoxometalate, silver catalysts.

INTRODUCTION

The catalytic epoxidation of olefins is very important in compared to organic peroxides and peracids (Jones, the chemical industry since epoxides are versatile and 1999; Lane and Burgess, 2003). Indeed, in certain important intermediates in the synthesis of many fine circumstances, H 2O2 oxidant could be considered to be chemicals and pharmaceuticals (Hudlucky, 1990; better than even molecular oxygen insofar as O 2/organic Sheldon, 1981) . A large number of catalytic epoxidation mixtures can sometimes spontaneously ignite to give processes are known, but still the chlorohydrin process hazardous explosions (Lane and Burgess, 2003). and catalytic processes based on organic peroxides and Although, several transition metal compounds have organic peracids are used extensively (Jørgensen, 1989). been employed as catalysts in the H 2O2-based These processes are unfavourable both from an eco- epoxidation, these systems have several disadvantages nomic and environmental point of view, being costly and (Battioni et al., 1988; Romao et al., 1997). Various producing large amounts of waste. In contrast to such catalytic systems for H 2O2-based epoxidation by poly- processes, catalytic epoxidation with hydrogen peroxide oxometalates or transition metal-substituted poly- (H 2O2) oxidant is very rewarding because: (i) it generates oxometalates have been reported, but most of these only water as a by-product, (ii) it has a high content of require either a great excess of the olefin with respect to active oxygen species, and (iii) it is rather cheap H2O2 or a great of excess H 2O2 with respect to the olefin to ensure high yields of and selectivity to epoxides (Duncan et al., 1995; Kamata et al., 2003; Mizuno et al., 2005). *Corresponding author. E-mail: Sharpless and co-workers (Rudolph et al., 1997) [email protected], [email protected], reported a pyridine ligand-accelerated methyltrioxo- [email protected]. Tel: +256 752 592 655. rhenium (MTO) catalysed epoxidation of various olefinic

Tebandeke et al. 51

compounds using H 2O2 with excellent yields to epoxides. with constant stirring and the mixture refluxed at 90°C for 1 h. However, the catalyst reusability is not reported and MTO Ag(NO) 3 (2.5 g) in a minimum amount of water was added to is very expensive not to mention the by-products that are precipitate out the catalyst and the mixture refluxed for another one hour. The temperature of the mixture was adjusted and maintained formed. This is a common problem in homogeneous at room temperature overnight. The mixture was filtered and the catalysis since the catalyst is dissolved in the reaction resultant solid dried in a fume hood. The catalyst was then calcined mixture, the separation of the catalyst is difficult and in in air for 2 h to yield the modified silver polyoxometalate catalyst most cases the catalyst cannot be reused. Because of (Ag/Ag-POM). The potassium salt of the polyoxometalate (K-POM) this, a lot of undesirable waste is produced and also the was prepared using a similar procedure but without addition of AgNO 3 and it was precipitated out using KCl. cost of epoxide production is high. Some processes utilising heterogeneous solid catalysts pH 7, 90°C for the epoxidation of olefins have also been described. Olefins are converted to epoxides in the presence of Reflux 2 h (1) heterogeneous catalysts comprised of carbon molecular sieves containing transition metals (Gaffney et al., 1994) . However, there is always a high possibility of leaching of Catalyst preparation in the presence of a reductant metals from the solid catalyst during the epoxidation process, causing loss of catalytic activity and selectivity The pH of a 50 ml solution containing Na 3PW 12 O40 (5.0 g) in and also making difficult the separation of the leached out CH 3CO 2H/CH 3CO 2Na buffer at 80°C was adjusted to approximately 7 with Na 2CO 3. AgNO 3 (0.25 g) in a minimum amount of water was components from the reaction mixture (Arends and added dropwise with constant stirring and the mixture refluxed at Sheldon, 2001) . Other heterogeneous catalysts have 90°C for 1 h. Ascorbic acid (0.1 g) dissolved in a minimum amount been reported employing H 2O2 with a titanium silicate of water was added and the mixture stirred. Ag(NO) 3 (2.5 g) in a zeolite catalyst or the same modified with Cu, Pd or Pt minimum amount of water was added to precipitate out the catalyst compounds (Clerici and Romano, 1999) . However, since and the mixture stirred further. The temperature of the mixture was adjusted and left at room temperature overnight. The mixture was the titanium zeolite catalysts are acidic in nature, they then filtered and the resultant solid left to dry in a fume hood. The also catalyse isomerisation and epoxide ring opening, catalyst was then calcined in air for 2 h to yield the modified silver thereby reducing the selectivity for epoxide formation (Fu polyoxometalate catalyst* (Ag/Ag-POM*). et al., 1999) . In these respects, effective catalysts for the epoxidation of olefins using H 2O2 at ambient conditions are still being sought. Catalytic runs

Herein, we report that modified silver polyoxometalate The catalysed epoxidation reaction under atmospheric pressure catalysts are highly efficient for the epoxidation of olefins (Equation 2) involved refluxing a mixture of the olefin substrate (2 using H 2O2 oxidant. In the present work, a catalytic mmol), aqueous hydrogen peroxide (50%, 4 mmol), catalyst (25 system comprised of silver and polyoxometalates mg) and acetonitrile (5 ml) at 65°C. Portions of the reaction mixture (POMs), after undergoing heat treatment is employed in were withdrawn at regular intervals and analysed on a Varian 3300 the epoxidation process. gas chromatograph and GC-MS Model Agilent 6890N, in order to identify the oxidation products. Quantification of the reactants and products in the aforementioned techniques was achieved by use of internal standards. The used catalyst was recovered by filtration MATERIALS AND METHODS and washing with n-hexane.

General considerations Ag/Ag-POMAg/Ag-POM + H2O2 O + H2O Solvents and commercially available reagents were used as Reflux 65°C 65OC received from Aldrich. The elemental composition of the samples was determined using ICP-AES on a PerkinElmer model OPTIMA (2) 3000 DV Instrument. The IR spectra of the catalysts were recorded on a Shimadzu FTIR 8201 PC instrument using KBr discs. Gas chromatography analyses were performed with a Varian 3300 gas RESULTS AND DISCUSSION chromatograph with an FID and fitted with a 30 m CP-Sil 5 CB capillary column. All GC-MS studies were performed on an Agilent Catalyst characterization 6890N Instrument.

The optimum pH for the preparation of modified silver Catalyst preparation in the absence of a reductant polyoxometalate catalysts was found to be 7.0. Higher pH values beyond pH 8.0 resulted into partial loss of The preparation of the modified silver polyoxometalate catalyst structure of the polyoxometalate (POM) and thus

(Equation 1) involved adjustment of the pH of a 50 ml aqueous reduction in the catalyst yield. Elemental analysis data solution of Na PW O (5.0 g) in CH CO H/CH CO Na buffer at 3 12 40 3 2 3 2 are shown in Table 1. The high weight percent of Ag in 80°C to approximately 7 with Na 2CO 3. To the resultant solution, AgNO 3 (0.25 g) in a minimum amount of water was added drop wise the Ag/Ag-POM and Ag/Ag-POM * catalysts is due to fact

52 Afr. J. Pure Appl. Chem.

Table 1. Elemental composition of the different catalysts as determined by ICP.

Catalyst (pH) Ag (%) K (%) P (%) W (%) K-POM (pH 7) -- 9.1 0.88 57.9 Ag/Ag-POM (pH 7) 22.3 0.1 0.62 50.2 Ag/Ag-POM* (pH 7) 21.8 0.2 0.71 51.5 \ *Catalyst prepared under reducing conditions.

Figure 1. An overlay of the IR spectra of K-POM (a), Ag/Ag-POM (b), Ag/Ag-POM*(c) and Ag/Ag-POM400 (calcined at 400°C).

that some Ag + ions are consumed during the precipitation conversion and 99% selectivity to the corresponding of Ag-POM. epoxides. However, olefins like cyclohexene showed An overlay of the IR-spectra of the K-POM support and lower selectivity (Entry 8, Table 2). The excess water supported silver catalysts is presented in Figure 1. The from the 30% aqueous hydrogen peroxide seemed to vibrational stretches in the IR spectra of the support and facilitate the epoxide ring opening. When 50% aqueous the catalysts are highly coincidental indicating that the hydrogen peroxide was used there was an improvement anion of the K-POM largely retains its structure after in both conversions and selectivity to the corresponding inclusion of silver. However, as the calcination tem- epoxides. perature increased beyond 400°C, there is a slight Control experiments using the support K-POM as change in the spectra due to dehydration and probable catalyst, showed very low selectivity for the epoxide partial degradation of the structure of the POM. (Entry 9, Table 2). Calcination temperatures around 400°C produced the most efficient catalyst in terms of conversion and selectivity. The uncalcined catalysts Epoxidation reaction showed lower selectivity in the epoxidation reaction. The catalysts prepared in the presence of a reductant showed Initially the epoxidation of different olefins with 30% higher conversions; however, their selectivities to the aqueous hydrogen peroxide catalysed by calcined Ag/Ag- epoxide were lower (Entries 4 and 5, Table 2). POM was investigated. Substrates like cyclooctene and The above results prompted the application of the norbonene could be epoxidized with about 95% modified silver polyoxometalate catalyst calcined at 400°C

Tebandeke et al. 53

Table 2. Conversion and selectivity in the epoxidation of cyclohexene to cyclohexene oxide (CHO) by various catalytic systems using 50% H 2O2 oxidant at 65 °C.

Entry Catalyst Conversion (%) CHO Selectivity (%) 1 Ag/Ag-POM calcined at120°C 83.8 90.5 2 Ag/Ag-POM calcined at 400°C 92.7 99.0 3 Ag/Ag-POM calcined at 450°C 91.7 99.1 4 Ag/Ag-POM* calcined at 120°C 87.9 89.3 5 Ag/Ag-POM* calcined at 400°C 96.3 92.8 6 Recycled Ag/Ag-POM120 89.6 90.5 7 Recycled Ag/Ag-POM400 91.9 98.1 8a Ag/Ag-POM calcined at 400°C 86.4 89.5 9 K-POM calcined at 120°C 89.6 21.8

Reaction conditions: olefin (2 mmol), catalyst (25 mg), H2O2 (4 mmol), acetonitrile (5 ml), and reaction a time 24 h. oxidant (30% H 2O2, 4 mmol).

Table 3. Conversion and selectivity in the epoxidation of various olefins using 50% hydrogen peroxide oxidant and Ag/Ag-POM400 at 65°C.

Entry Olefin Time (h) Conversion (%) Product Selectivity (%) 1 24 99.2 99.4 O

2 24 99.1 99.6

O

3 24 92.7 99.0 O

4 24 96.5 99.0 O

5 24 99.0 99.1

O

6 24 96.7 O 99.5

7 48 94.8 99.3

O

8 48 97.4 O 99.4

9 48 95.5 O 99.7

10 a 36 97.6 99.3 O

a Reaction conditions: olefin (2 mmol), catalyst (Ag/AgPOM400, 25 mg), 50% H 2O2 (4 mmol), acetonitrile (5 ml), olefin (20 mmol), catalyst (Ag/Ag-POM400, 100 mg), 50% H 2O2 (40 mmol).

(Ag/Ag-POM400) to other substrates and the results are could be achieved for a number of cyclic olefins and shown in Table 3. Greater than 90% conversions and linear olefins. It is interesting to note that even terminal close to 99% selectivity to the corresponding epoxides olefins like 1-hexene, could be epoxidized with 94% yield

54 Afr. J. Pure Appl. Chem.

and 99% selectivity (Entry 7, Table 3). Interestingly, the Chemical Sciences (IPICS), under the International present catalyst could be easily recovered by filtration Science Programme (ISP), Uppsala University, Sweden and reused without loss of catalytic activity. for funding this study. We thank Tommy Olsson for The conversions and yields observed with the present running the ICP analyses. OFW also thanks the Swedish modified silver polyoxometalate catalysts are higher than Research Council for financial support. those reported for the epoxidation of olefins using H 2O2 catalysed by polyoxometalates (Bailey et al., 1995; Duncan et al., 1995; Ishii et al., 1988; Venturello et al., REFERENCES 1985) or transition metal substituted polyoxometalates Arends IWCE, Sheldon RA (2001). Activities and stabilities of (Kuznetsova et al., 1996; Neumann and Gara, 1994; heterogeneous catalysts in selective liquid phase oxidations: Recent Neumann and Khenkin, 1996; Tangestaninejad and developments. Appl. Catal. A-Gen. 212(1-2):175-187. Yadollahi, 1998). The observed yields are also superior Bailey AJ, Griffith WP, Parkin BC (1995). Heteropolyperoxo-and to those reported when the olefin was used in a great isopolyperoxo-tungstates and-molybdates as catalysts for the oxidation of tertiary amines, alkenes and alcohols. J. Chem. Soc. excess over H 2O2 oxidant (5:1) in order to ensure high Dalton Trans. 11:1833-1837. yields and selectivity (Kamata et al., 2003; Mizuno et al., Battioni P, Renaud JP, Bartoli JF, Reina-Artiles M, Fort M, Mansuy D 2005). The exclusion of solvents in catalytic processes is (1988). Monooxygenase-like oxidation of hydrocarbons by hydrogen a major challenge in green chemistry and, gratifyingly, peroxide catalyzed by manganese porphyrins and imidazole: selection of the best catalytic system and nature of the active oxygen the epoxidation with the present catalytic system, could species. J. Am. Chem. Soc. 110(25):8462-8470. be performed in neat olefin without a solvent. For Clerici MG, Romano U (1999). US Patent No. 4937216. p.1-8 example, cyclooctene was epoxidized without addition of Duncan DC, Chambers RC, Hecht E, Hill CL (1995). Mechanism and a solvent with approximately the same selectivity, albeit dynamics in the H3 [PW12O40]-catalyzed selective epoxidation of terminal olefins by H O . Formation, reactivity, and stability of {PO slightly longer reaction time (Entry 10, Table 3). 2 2 4 [WO(O 2)2] 4}3. J. Am. Chem. Soc. 117(2):681-691. Several POM based catalysts have been reported for Fu Z, Yin D, Yin D, Li Q, Zhang L, Zhang Y (1999). Synthesis, the epoxidation of olefins using H 2O2 oxidant . However, characterization and catalytic properties of titanium and boron co- many of them also seem to catalyse the epoxide ring substituted silicalite zeolites. Micropor. Mesopor. Mater. 29(3):351- 359. opening, thereby reducing the selectivity for epoxide Gaffney AM, Nandi MK, Pitchai R, Zhang HY (1994) .US Patent No. formation. This explains why for example, high selectivity 5319114. US. p.1-12. for epoxidation reactions using polyoxometalates was Hudlucky M (1990). Oxidations in Organic Chemistry. Washington DC: observed when the olefin was present in a greater excess Am. Chem. Soc. Washington DC. p.1 . Ishii Y, Yamawaki K, Ura T, Yamada H, Yoshida T, Ogawa M (1988). compared to the H 2O2 (Kamata et al., 2003; Mizuno et al., Hydrogen peroxide oxidation catalyzed by heteropoly acids combined 2005), so that the H 2O2 is used up in the process fast with cetylpyridinium chloride. Epoxidation of olefins and allylic enough to prevent over oxidation. alcohols, ketonization of alcohols and diols, and oxidative cleavage of In this present work, the inclusion of silver onto a 1, 2-diols and olefins. J. Org. Chem. 53(15):3587-3593. Jones CW (1999). Applications of hydrogen peroxide and derivatives: polyoxometalate and eventual calcination improved the Royal Society of Chemistry p.80. catalytic activity of the system in terms of conversion and Jørgensen KA (1989). Transition-metal-catalyzed epoxidations. Chem. selectivity. However, the nature of active silver species in Rev. 89(3):431-458. the catalyst is not yet clear. The observed catalytic Kamata K, Yonehara K, Sumida Y, Yamaguchi K, Hikichi S, Mizuno N (2003). Efficient Epoxidation of Olefins with >99% Selectivity and Use activity could be attributed to silver or a combination of of Hydrogen Peroxide. Science 300(5621):964-966. silver and the polyoxometalate. Further studies are Kuznetsova NI, Kuznetsova LI, Likholobov VA (1996). Catalytic ongoing to shed some light on the mechanism and the properties of Cr-containing heteropolytungstates in H 2O2 participated role of silver in the epoxidation process. reactions: H 2O2 decomposition and oxidation of unsaturated hydrocarbons with H 2O2. J. Mol. Cat. A-Chem. 108(3):135-143. Lane BS, Burgess K (2003). Metal-catalyzed epoxidations of alkenes with hydrogen peroxide. Chem. Rev. 103(7):2457-2474. Conclusion Mizuno N, Yamaguchi K, Kamata K (2005). Epoxidation of olefins with hydrogen peroxide catalyzed by polyoxometalates. Coord. Chem. Rev. 249(17-18):1944-1956. We have successfully prepared modified silver poly- Neumann R, Gara M (1994). Highly active manganese-containing oxometalate catalysts. These silver containing poly- polyoxometalate as catalyst for epoxidation of alkenes with hydrogen oxometalates are efficient in the epoxidation of olefins peroxide. J. Am. Chem. Soc. 116(12):5509-5510. Neumann R, Khenkin AM (1996). A new dinuclear rhodium (III) with H 2O2 oxidant producing the corresponding epoxides 10 sandwich'polyoxometalate. ,[(WZnRhIII 2)(ZnW 9O34 )2] -.Synthesis, in excellent yields and selectivity. The catalysts are easily characterization and catalytic activity. J. Mol. Cat. A-Chem. 114(1- recovered by filtration and are reusable several times. 3):169-180. Romao CC, Kuhn FE, Herrmann WA (1997). Rhenium (VII) oxo and imido complexes: Synthesis, structures, and applications. Chem. Rev. 97(8):3197-3246. ACKNOWLEDGEMENTS Rudolph J, Reddy KL, Chiang JP, Sharpless KB (1997). Highly Efficient

Epoxidation of Olefins Using Aqueous H 2O2 and Catalytic We are grateful to the International Programmme in Methyltrioxorhenium/Pyridine: Pyridine-Mediated Ligand Acceleration.

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J. Am. Chem. Soc. 119(26):6189-6190. Venturello C, D'Aloisio R, Bart JCJ, Ricci M (1985). A New Sheldon RA (1981). Aspects of homogeneous catalysis: Reidel, peroxotungsten heteropoly anion with special oxidizing properties: Dordrecht. 4:3. synthesis and structure of tetrahexylammonium tetra Tangestaninejad NI, Yadollahi B (1998). Zinc containing (diperoxotungsto) phosphate 3. J. Mol. Cat. 32(1):107-110. 5 polyoxometalate, PZnMo 2W 9O39 -, can act as efficient catalyst for oxidation of organic compound by 30% H 2O2. Chem. Lett. p.511.