Sonochemical syntheses of nano-sized dioxomolybdenum complexes: An efficient, selective and reusable heterogeneous nanocatalyst for oxidation of alkenes

Applied Catalysis A: General 456 (2013) 240–248

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Sonochemical syntheses of nano-sized dioxomolybdenum complexes: An efficient, selective and reusable heterogeneous nanocatalyst for oxidation of alkenes Saeed Rayati ∗ , Payam Abdolalian Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran

a r t i c l e

i n f o

Article history: Received 11 October 2012 Received in revised form 22 February 2013 Accepted 3 March 2013 Available online xxx Keywords: Epoxidation Ultrasonic irradiation Olefins Heterogeneous nanocatalyst Molybdenum Schiff base complexes

a b s t r a c t Two Schiff base ligands derived from 2-hydroxy-4-methoxyacetophenone and 2,2 dimethylpropylenediamine (H2 L1 ) or 1,2-cyclohexanediamine (H2 L2 ) have been synthesized and characterized by physico-chemical and spectroscopic methods. Then dioxomolybdenum complexes have been prepared by reaction of Schiff base ligands and MoO2 (acac)2 under ultrasonic irradiation to give nanoparticles of MoL1 and MoL2 . FT-IR, UV–vis, 1 H NMR and 13 C NMR spectroscopy, elemental analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were used to characterize and investigate the nanocatalysts. A practical catalytic method for efficient and highly selective oxidation of wide range of olefins with anhydrous tert-butyl hydroperoxide over the prepared molybdenum nanocatalysts was investigated. Under mild reaction conditions, oxidation of cyclooctene led to the formation of corresponding epoxide with a yield of more than 90%. The catalysts were reused five consecutive times without detectable catalyst leaching or significant loss of activity. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Since epoxides are very important chemicals in organic synthetic chemistry [1–5] and can serve as versatile precursors for the synthesis of many natural products and drug molecules [6,7], therefore, the development of new methods for selective oxidation of olefins to epoxides is of great interest. Various oxygen donors such as sodium hypochlorite [8], sodium peroxide [9], m-chloroperbenzoic acid [10], sodium chlorite [11], sodium perborate tetrahydrate [12], oxone [13], idosylbenzene [14], tert-butyl hydroperoxide [15–22], hydrogen peroxide [23–25], tetra-n-butylammonium hydrogen monopersulfate [26–29] and urea hydrogen peroxide [30–33] have been used in the catalytic epoxidation of olefins. However, there were always some problems due to long reaction time, the cost of expensive catalyst, involving toxic solvents or difficult experiment operation. Therefore, many improvements such as the use of ultrasonic irradiation [34–39] or use of heterogeneous catalysts [40–45] instead of homogeneous ones were investigated. Compared to the homogeneous catalysts, the heterogenized catalysts offer the opportunity of easy catalyst handling and recycling and result in higher turnover numbers. However, the latter are generally less active and therefore

∗ Corresponding author. Tel.: +98 21 22850266; fax: +98 21 22853650. E-mail addresses: [email protected], [email protected] (S. Rayati). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.03.003

many attempts have been made to synthesis nanocatalysts as efficient alternatives for the traditional heterogeneous catalysts. The use of ultrasonic irradiation for the preparation of nanomaterials [46] is one of the most promising techniques to achieve such a goal. The advantages of ultrasound procedures such as good yields, short reaction times and mild reaction conditions are due to the reduced particle size, increased surface area and the improvement of mass transfer between the liquid and the catalyst surface due to large solid–liquid interfacial area [47]. In the present investigation, two nanostructured dioxomolybdenum Schiff base complexes have been synthesized under ultrasonic irradiation and used as catalyst for oxidation of different olefins with tert-butyl hydroperoxide (TBHP).

2. Experimental 2.1. Instruments and reagents Infrared spectra were recorded as KBr pellets using Unicam Matson 1000 FT-IR. A Bruker FT-NMR 500 (500 MHz) spectrometer was utilized to obtain NMR spectra. Melting points were measured on an Electrothermal 9100 apparatus. The electronic absorption spectra were recorded on a single beam spectrophotometer (Cam Spec-M330). The high-power ultrasonic cleaning unit Bandelin Super Sonorex RK-100H with ultrasonic peak output 320 W and HF power 80 Weff has been used. Scanning electron microscopy (SEM)

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OMe

MeO

O

O Mo

N

O

O

O O

Mo

O N

N

N

Me

Me

Me

OMe

MeO

O

Me

241

Me

Me

MoL1

MoL2 Fig. 1. The Mo(VI) complexes used in this study.

images were obtained on a Hitachi S-1460 field emission scanning electron microscope using Acc voltage of 15 kV. The oxidation products were analyzed with a gas chromatograph (Shimadzu, GC-14B) equipped with a SAB-5 capillary column (phenyl methyl siloxane 30 m × 320 mm × 0.25 mm) and a flame ionization detector. 2,2 -Dimethylpropylenediamine,1,2-cyclohexanediamine, 2-hydroxy-4-methoxyacetophenone, molybdenyl acetylacetonate and tert-butyl hydroperoxide (solution 80% in di-tert-butylperoxide) were used as received from commercial suppliers. Solvents were dried and distilled by standard methods before use. Other chemicals were purchased from Merck or Fluka chemical companies. 2.2. Preparation of the Schiff base ligands (H2 L1 and H2 L2 ) The Schiff base ligands were prepared by standard methods [20]. The solution of 2-hydroxy-4-methoxyacetophenone (0.332 g, 2 mmol) were mixed with 2,2 -dimethylpropylenediamine (0.102 g, 1 mmol) or 1,2-cyclohexanediamine (0.114 g, 1 mmol) for preparation of H2 L1 and H2 L2 , respectively in ethanol (20 mL). The bright yellow solutions were stirred and heated to reflux for 1 h. The desired dark yellow solutions were precipitated by mixing with diethyl ether and filtered off and finally dried in air. H2 L1 : Yield: 84%, 0.335 g, M.p. = 155, Selected FT-IR data,  (cm−1 ): 3440 (O H), 2962 (C H), 1611 (C N), 1550 (C C). 1 H NMR (ı): 1.10 (s, 6H, NCH C(CH ) CH N), 2.34 (s, 6H, 2 3 2 2 N CCH3 ), 3.33 and 3.62 (4H, NCH2 C(CH3 )2 CH2 N), 3.74 (s, 6H, OMe), 6.24–7.54 (m, 6H, ArH), 17.17 (s, 2H, OH). 13 C{1 H} NMR (ı): 14.37 (NCH2 C(CH3 )2 CH2 N), 24.45 ((CH3 )C N), 35.80 (NCH2 C(CH3 )2 CH2 N), 55.20 (OCH3 ), 55.44 (NCH2 C(CH3 )2 CH2 N), 102.22–170.63 (aromatic C), 172.00 (C N). H2 L2 : Yield: 88%, 0.362 g, M.p.: >151, Selected FT-IR data,  (cm−1 ): 3460 (O H), 2931 (C H), 1617 (C N), 1540 (C C). 1 H NMR (ı): 1.47–1.51, 1.68–1.76, 1.91–1.93, 2.02–2.09 (complex m, 8H, CH2 of cyclohexyl), 2.25 (s, 6H, N CCH3 ), 3.81, 3.84 (d, 2H, CH of cyclohexyl), 3.79 (s, 6H, OMe), 6.21–7.26 (m, 6H, ArH), 12.78 (s, 2H, OH). 13 C{1 H} NMR (ı): 24.19 ((CH3 )C N), 55.21 (OCH3 ), 14.20, 32.48, 61.36 (cyclohexyl carbons), 102.12–169.77 (aromatic C), 170.73 (C N).

1H

NMR (ı): 1.10 (s, 6H, NCH2 C(CH3 )2 CH2 N), 2.37 (s, 6H, N CCH3 ), 3.42–3.52 (4H, NCH2 C(CH3 )2 CH2 N), 3.75 (s, 6H, OMe), 6.26–7.56 (m, 6H, ArH). 13 C{1 H} NMR (ı): 15.03 (NCH2 C(CH3 )2 CH2 N), 23.92 (N CCH3 ), 36.18 (NCH2 C(CH3 )2 CH2 N), 55.58 (OMe), 101.18–173.52 (aromatic C), 177.36 (C N). MoL2 -bulk: 80%, 0.261 g, D.p.: >238, Selected FT-IR data,  (cm−1 ): 2932 (C H), 1602 (C N), 1540 (C C), 842 and 903 (Mo O). 1 H NMR (ı): 1.47–1.52, 1.71–1.80, 1.91–1.97, 2.02–2.10 (complex m, 8H, CH2 of cyclohexyl), 2.36 (s, 6H, N CCH3 ), 3.82, 3.86 (d, 2H, CH of cyclohexyl), 3.80 (s, 6H, OMe), 6.28–7.42 (m, 6H, ArH). 13 C{1 H} NMR (ı): 24.96 ((CH )C N), 54.24 (OCH ), 22.82, 46.96, 3 3 54.72 (cyclohexyl carbons), 100.33–168.38 (aromatic C), 203.16 (C N). The dioxo-molybdenum(VI) complexes have been shown in Fig. 1. 2.4. Preparation of dioxo-molybdenum(VI) complexes at nano-size under ultrasonic irradiation A chloroform solution (50 mL) of the Schiff base ligands, H2 L1 (0.200 g, 0.50 mmol) and H2 L2 (0.205 g, 0.50 mmol), was poured dropwise under ultrasonic irradiation into to a 50 mL molybdenyl acetylacetonate (0.163 g, 0.50 mmol) in tetrahydrofuran (THF) during 40 min. After the end of the titration the solution was kept in the ultrasonic bath for a period of 40 min. The obtained precipitates were filtered and dried. Yield (MoL1 -nano: 88%, 0.21 g; MoL2 -nano: 92%, 0.23 g). 2.5. General heterogeneous oxidation procedure Catalytic experiments were carried out in a 5 mL test tube. In a typical procedure, to a chloroform solution of cyclooctene (3.3 mmol), 0.033 mmol dioxo-molybdenum(VI) complex and 9.9 mmol TBHP was added. The molybdenum complex is completely insoluble in chloroform and therefore the catalytic system is heterogeneous in nature. The reaction mixture was stirred for 6 h at room temperature (RT). The reaction products were monitored at periodic time intervals using gas chromatography. The oxidation products were identified by comparison with authentic samples (retention times in GC).

2.3. Preparation of molybdenum(VI) complexes 3. Results and discussion The molybdenum complexes were prepared as follows: the Schiff base ligand, H2 L1 (0.399 g, 1 mmol) or H2 L2 (0.410 g, 1 mmol) was dissolved in 20 ml of ethanol. An ethanolic solution of molybdenyl acetylacetonate (0.326 g, 1 mmol) was added and the reaction mixture was refluxed for 1 h. The colored solution was concentrated to yield colored powders. The products washed with warm ethanol. MoL1 -bulk: Yield: 77%, 0.252 g, D.p.: >219, Selected FT-IR data,  (cm−1 ): 2957 (C H), 1600 (C N), 1527 (C C), 847 and 908 (Mo O).

3.1. Characterization of the ligands and dioxo-molybdenum(VI) complexes 3.1.1. IR spectral studies A practical list of IR spectral data is presented in Table 1. Comparison of the spectra of the complexes with the ligands provides evidence for the coordination mode of ligand in

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Table 1 IR spectral data of ligands and molybdenum complexes. Compound

H2 L1 H2 L2 MoL1 MoL2

Selective IR bands (cm−1 ) Mo O

C N

– – 847, 908 842, 903

1612 1617 1600 1602

catalysts. A sharp band appearing at 1612–1617 cm−1 due to (C N) (azomethine), shifts to lower wave number by 12–15 cm−1 and appears at 1600–1602 cm−1 . This indicates the involvement of azomethine nitrogen in coordination to the metal center [48]. IR spectra of the Mo complexes show two characteristic strong bands in the regions 847–842 and 908–903 cm−1 , which could be assigned to the symmetric and asymmetric stretching vibrations of cis-dioxo (MoO2 ) respectively [49–52]. The infrared spectra of the molybdenum complexes as bulk or nano size are identical (Fig. 2). 3.1.2. Electronic spectral studies Table 2 provides electronic spectral data of the molybdenum(VI) complexes along with their assignments. The electronic absorption spectrum (in methanol) show three absorption bands due to the ␲→␲*, n→␲* transitions and LMCT respectively. The absorption band observed at 231 nm or 222 nm is attributed to the ␲→␲* transition of the imino group [53]. The spectra of the complexes show a band at 299 or 280 nm, which is due to the n to ␲* transition and another band at 373 nm assignable to a ligand to metal charge transfer (LMCT) due to the promotion of an electron from the highest occupied molecular orbital (HOMO) of the ligand to the lowest unoccupied molecular orbital (LUMO) of molybdenum atom [54].

Fig. 2. IR spectra of MoL1 -nano (A) and MoL1 -bulk (B). Table 2 UV–vis data for molybdenum complexes in methanol. Compound MoL

1

MoL2

max (nm) (ε, M−1 cm−1 )

Assignment

231 (27,780) 299 (18,732) 373 (8090)

␲→␲* n→␲* LMCT

222 (110,870) 280 (30,890) 373 (10,955)

␲→␲* n→␲* LMCT

3.1.3. Scanning electron microscopy (SEM) images of the complexes Scanning electron micrographs (SEMs) were recorded to obtain the shape and diameter of the MoL1 -nano and MoL2 -nano

Fig. 3. SEM images of (A) MoL1 -nano, (B) MoL2 -nano nanoparticles and (C) MoL1 -bulk.

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243

Table 3 Experiments on catalytic oxidation of cyclooctene at room temperature.a No.

Catalyst

Oxidant

Conversionb (%)

Time (h)

1 2 3 4 5 6

None MoL1 -nano MoL1 -nano MoL1 -bulk MoL2 -nano MoL2 -bulk

TBHP None TBHP TBHP TBHP TBHP

0 0 67 20 54 19

6 6 6 6 6 6

a Reaction conditions: the molar ratio of catalyst:alkene:TBHP was 1:100:300. Solvent: chloroform. b GC yields based on the starting olefin.

diameter of 40–60 nm (MoL1 -nano) or 40–60 nm (MoL1 -nano) that is in agreement with other results. 3.2. Heterogeneous catalytic epoxidation of olefins The catalytic performance of MoL1 -nano and MoL2 -nano was investigated in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP) as oxygen donor. A series of blank experiments (Table 3) revealed that the presence of catalyst and oxidant are essential for an effective catalytic reaction. In the absence of catalyst (entry 1 in Table 3) or oxidant (entry 2 in Table 3), the reactions did not proceed even under reflux. Nano-sized catalysts (MoL1 or MoL2 ) show better catalytic activity in the epoxidation of cyclooctene with respect to the bulk ones. In order to find the suitable reaction conditions, the effect of various reaction parameters that may affect the conversion and selectivity of the reaction were studied. Solvent, catalyst concentration, temperature, the nature and concentration of oxidant are the factors that have been evaluated. The influence of different solvents on the oxidation of cyclooctene was studied using bulk and nanoparticle of MoL1 and MoL2 as catalyst and results are presented in Fig. 6. Chloroform, 1,2-dichloroethane (DCE), dichloromethane (DCM) and acetonitrile were used as solvent and the highest conversion (67% in MoL1 -nano and 54% in MoL2 -nano) was obtained in chloroform. The higher conversions in chloroform with respect to the other solvents possibly are due to the non-coordinating behavior of the chloroform. Coordinative solvent such as acetonitrile compete with TBHP to bind to the molybdenum center and inhibited the reaction [55,56]. Fig. 7 shows that the highest cyclooctene conversion was obtained after 6 h and further oxidation of cyclooctene did not occur after 6 h. In order to investigate the effect of oxidizing agent in the oxidation reaction, two other peroxides (hydrogen peroxide and urea hydrogen peroxide (UHP)) were used, and results show that no significant product formation was observed in the case of UHP and hydrogen peroxide. Therefore, TBHP is a more efficient oxidant with respect to the others (Table 4). Different catalyst concentrations have been used in the oxidation of cyclooctene (Fig. 8). It was observed that oxidation of cyclooctene required 0.033 mmol of catalyst for completion. Fig. 4. The particle size distribution for MoL1 (A) and MoL2 (B) after sonication and MoL1 -bulk (C).

complexes (Fig. 3). The results showed that spherical particles were well distributed and the average particle size was from 50 to 80 nm for MoL1 -nano and 40 to 70 nm for MoL2 -nano (Fig. 4A and B) while average particle size for MoL1 -bulk is 250–300 nm (Fig. 4C). This confirms that sonication causes a significant decrease in the particle size. Fig. 5 shows TEM images of MoL1 -nano and MoL2 -nano. As can be seen, the products are formed from particles with mean

Table 4 Epoxidation of cyclooctene with different oxidants catalyzed by MoL1 and MoL2 .a Entry

Oxidant

MoL1 -bulk

MoL1 -nano

MoL2 -bulk

MoL2 -nano

1 2 3

H2 O2 UHP TBHP

0 0 20.0

1.5 1.5 67.0

0 0 19.0

1.0 1.5 54

a Reaction conditions: the molar ratios for using cat.:cyclooctene:oxidant are 1:100:300. The reactions were run for 6 h at 25 ◦ C.

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80

70

70

60

60

MoL1-bulk

50

MoL1-nano MoL2-bulk

40

MoL2-nano

30

MoL1 (Bulk) MoL1 (Nano) MoL2 (Bulk)

Conversion %

Conversion %

Fig. 5. TEM images of (A) MoL1 -nano and (B) MoL2 -nano.

50

MoL2 (Nano)

40 30 20

20

10

10

0

0 Dichloromethane

Chloroform

Dichloroethane

0.0111 mmol

Acetonitrile

Fig. 6. The effect of various solvents on the epoxidation of cyclooctene with TBHP using molybdenum complexes. Reaction conditions: cyclooctene (100 mmol), catalyst (1 mmol), solvent (3 mL), TBHP (300 mmol). The reactions were run for 6 h at room temperature.

Different oxidant (TBHP) concentrations have been studied in the oxidation of cyclooctene (Fig. 9). It was observed that oxidation of cyclooctene required 300 mmol of TBHP for completion. In order to investigate the effect of temperature on the activity of the catalyst, four different temperatures (15, 25, 35 and 45 ◦ C) were used while keeping other parameters fixed (i.e. 3.3 mmol cyclooctene, 9.9 mmol TBHP and 0.033 mmol catalyst). Effect of change in temperature on the rate of reaction in the presence of nano-catalysts is more than bulk ones. The results presented in Fig. 10 show that in 45 ◦ C reaction temperature gives the maximum percentage conversion of cyclooctene (ca. 100%) after 1 h in the presence of MoL1 /MoL2 -nano whereas increase in temperature has lesser effect on the conversion of cyclooctene in the presence of bulk catalysts.

0.0222 mmol

0.0333 mmol

0.0444mmol

Fig. 8. The influence of catalyst concentration on the oxidation of cyclooctene using TBHP catalyzed by MoL1 -nano and MoL2 -nano. The reactions were run under air in chloroform and the molar ratio of cyclooctene:TBHP:catalyst was 3.3:9.9:X.

The effect of ultrasound irradiation on the epoxidation of cyclooctene with TBHP in the presence of nano or bulk molybdenum catalysts was also investigated. As shown in Fig. 11 application of ultrasonic waves has different effects in the presence of nano or bulk catalysts. For example, if the reaction runs with MoL1 -nano, the reaction was completed after 50 min of ultrasonic irradiation, while during this time 38.5% conversion was obtained with bulk catalyst (Fig. 11). In order to establish the general applicability of this catalytic system, under the optimized conditions, oxidation of different olefins were subjected in the presence of the catalytic amount of MoL1 nano and MoL2 -nano and the results are presented in Table 5. Since temperature of the reaction also has influence on the performance of the catalyst for olefin conversion, therefore, catalytic activity of 70

80

60

Conversion %

70

50

60

MoL1 (Bulk) MoL1 (Nano) MoL2 (Bulk)

50

40

40

30

30

MoL2 (Nano)

20

20 10

10

0 0

1

2

3

4

5

6

Time (h) Fig. 7. Effect of reaction time in the oxidation of cyclooctene with TBHP in the presence of MoL1 -nano in chloroform.

0

3.3 mmol

6.6 mmol

9.9 mmol

13.2 mmol

Fig. 9. The influence of oxidant concentration on the oxidation of cyclooctene catalyzed by MoL1 -nano and MoL2 -nano. The reactions were run under air in chloroform and the molar ratio of cyclooctene:TBHP:catalyst was 3.3:X:0.033.

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245

Table 5 Epoxidation of olefins using TBHP catalyzed by MoL1 -nano or MoL2 -nano in chloroform.a Entry

Alkene

MoL1

Product

MoL2 ◦

Time in 25 C (min)

Conversion (selectivity) %b

Time in 45 ◦ C (min) 1

O

2

O 3

O 4

O

5

O

Cl

Time in 25 ◦ C

Conversion (selectivity) %

Time in 45 ◦ C

360

67 (100)

360

54 (100)

60

100 (100)

60

100 (100)

360

54 (100)

360

52 (100)

60

100 (100)

60

100 (100)

360

57 (100)

360

50 (100)

60

100 (100)

60

97 (100)

360

62 (66)c

360

54 (77)

60

80 (35)

60

75 (38)

360

52 (42)d

360

46 (65)

60

76 (30)

60

70 (29)

360

45 (40)e

360

41 (41)

60

73 (21)

60

69 (28)

360

32 (35)f

360

34 (50)

60

50 (32)

60

52 (46)

360

51 (69)g

360

49 (60)

60

57 (48)

60

52 (49)

360

28 (100)

360

24 (100)

60

44 (100)

60

40 (100)

360 60

43 (100) 84 (100)

360 60

41 (100) 76 (100)

Cl

6

O

Me

Me

7

O MeO

MeO Me

8

Me O

9

O 10

5 a b c d e f g

5

O

The molar ratio of catalyst:alkene:TBHP was 1:100:300. Conversions and selectivities were determined by GC based on the starting alkene. Benzaldehyde is the by-product. 4-Chlorobenzaldehyde is the by-product. 4-Methylbenzaldehyde is the by-product. 4-Methoxybenzaldehyde is the by-product. Acetophenone is the by-product.

Table 6 Comparison of the activity of Mo(VI) complexes in the oxidation of cyclooctene. Alkene

Mo complex

Conversion (%)

Reaction time (h)

Oxidant

Ref.

Cyclohexene Cyclooctene Cyclohexene Cyclooctene Cyclooctene Cyclooctene Cyclooctene

MO2 Ar/ZAPS-PVPA MoO2 alaacacAmpMCM-41 MoO2 leuacacAmpMCM-41 Immobilized Schiff base MoO2 (VI) catalysts MoO2 pycaAmpMCM-41 [Mo(O)2-(salen)–POM] MoL1

98 87 86 98 92 100 100

8 9 9 8 4 6 1

TBHP TBHP TBHP TBHP TBHP TBHP TBHP

[59] [60] [60] [61] [62] [63] Present work

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S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248

A 100

B 100

80 60

2

0

4

80 60 40 20

6

80

45°c

60

35°c

35°c

40

25°c

20

25°c

0

15°c

15°c

0

4

100

45°c

4

2

0

D

2

15°c

0

6

100

0

25°c

20

15°c

0

35°c

40

25°c

20

45°c

60

35°c

40

C

80

45°c

0

2

4

6

6

Fig. 10. Effect of temperature on the oxidation of cyclooctene in the presence of MoL1 -nano (A), MoL2 -nano (B), MoL1 -bulk (C) and MoL2 -bulk (D).

the catalysts has been tested for two temperatures. The activity of the catalyst was found to increase as the temperature was raised from 25 to 45 ◦ C. It is observed that the time required to achieve the maximum conversion is around 1 h at 45 ◦ C, whereas, the same was achieved at 6 h at 25 ◦ C. Oxidation of cyclooctene, cyclohexene, 1methylcyclohexene, 1-octene and indene lead to the corresponding epoxide as the sole product, while in the oxidation of styrene some benzaldehyde (Table 5, entry 4), in the oxidation of substituted styrene, some substituted benzaldehyde (Table 5, entries 5–7) and also in the oxidation of ␣-methyl styrene, acetophenone (Table 5, entry 8) were detected as by product. Production of benzaldehyde in the oxidation of styrene is due to the over-oxidation of styrene oxide with TBHP. The lower catalytic activity of ␣-methyl styrene with respect to the styrene is due to steric hindrance of the methyl substituent in the ␣-methyl styrene. Epoxidation of terminal alkene (oct-1-ene) proceeds in moderate yield (43% in the case of MoL1 nano and 41% for MoL2 -nano). Much reactivity observed in the case of MoL1 -nano in comparison with MoL2 -nano seems to be due to its lower steric hindrance around molybdenum center. In the proposed catalytic cycle TBHP will be activated by coordination to the molybdenum center and formed a hepta-coordinated molybdenum intermediate (I, Scheme 1) [57,58]. Then olefin as a nucleophile will attack to the oxygen atom of the coordinated TBHP. Many molybdenum Schiff base complexes have been reported for alkene epoxidation. The results of the oxidation of cyclooctene,

80

The reusability of a heterogeneous catalyst is of prime importance not only from the economic point of view but also due to the easy work-up procedure. The homogeneous molybdenum complexes cannot be recovered even once, in contrast the heterogeneous catalysts can be filtered and reused multiple times without significant loss of catalytic activity. The stability of the molybdenum complexes was monitored using multiple sequential epoxidation reaction. For each of the repeated reactions, the catalyst was filtered, washed with chloroform and methanol and dried before being used. The catalyst was consecutively reused five times (Table 7) without a detectable catalyst leaching or a significant loss of its activity. No detectable leaching (tested by atomic absorption spectroscopy) was observed after each catalytic reaction. Also the IR spectra of the catalysts remained intact during the reaction. These observations show that the catalysts are stable during the oxidation reaction.

MoL1, Nano MoL1, Bulk

100

60 40 20 0

3.3. Catalyst reuse and stability

Conversion %

Conversion %

100

in the presence of various heterogeneous Mo(VI) complexes H2 O2 or TBHP are summarized in Table 6 [59–63]. Comparison of this catalytic system with previously reported systems shows that higher conversion of cyclooctene in the shorter reaction time was achieved.

10

20

30

40

50

Time or ultrasonic irradiation (min)

80

MoL2, Nano MoL2, Bulk

60 40 20 0

10 20 30 40 50 Time of ultrasonic irradiation (min)

Fig. 11. Effect of ultrasonic waves on the oxidation of cyclooctene with TBHP in the presence of nano and bulk catalysts.

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OMe

MeO

Me

MeO

+TBHP O

H

O

O

N

O

O

Mo

TBOH

Me C

O O O

N

N Me

O O

N R

Me

OMe

Me

Mo

R

Me

247

Me

I Me

MeO H O

C

O O

Mo N Me

Me

Me

MeO

OMe

Me

H O

O O

O O O

Mo N

N R

Me

O

Me

Me C

OMe

Me

O O

N R

Me

Scheme 1. Proposed mechanism.

Table 7 The results obtained from catalyst reuse in the oxidation of cyclooctene with TBHP by molybdenum complexes. Run

1 2 3 4 5

Conversion % MoL1 -nano

MoL2 -nano

100 100 97 94 93

100 100 99 98 97

Reaction conditions: the molar ratios for using cat.:cyclooctene:oxidant are 1:100:300. The reactions were run for 1 h at 45 ◦ C.

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