Efficient oxidation of ethyl benzene using in situ generated molybdenum acetylide oxo-peroxo complex as recyclable catalyst

Applied Catalysis A: General 531 (2017) 45–51

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Efficient oxidation of ethyl benzene using in situ generated molybdenum acetylide oxo-peroxo complex as recyclable catalyst Sonali B. Khomane a , Dhananjay S. Doke a,b , M.K. Dongare a,c , S.B. Halligudi d , S.B. Umbarkar a,b,∗ a

Catalysis Division, CSIR-National Chemical Laboratory, Pune, 411008, India Academy of Scientific and Innovative Research, CSIR, Anusandhan Bhawan, New Delhi, 110 001, India c Malati Fine Chemicals Pvt. Ltd, 4/A Durvankurdarshan Society, Pune, 411008, India d Mont Vert Seville, Wakad, Pune, 411057, India b

a r t i c l e

i n f o

Article history: Received 24 September 2016 Received in revised form 28 November 2016 Accepted 3 December 2016 Available online 5 December 2016 Keywords: Homogeneous catalysis Molybdenum acetylide complex Alkyl aromatics Carbonyl compounds Oxo-peroxo species

a b s t r a c t Selective oxidation of various alkanes/alkyl aromatics to corresponding carbonyl compounds has been carried out with very high conversion (∼98%) and selectivity (up to 100%) for carbonyl compounds using cyclopentadienyl molybdenum acetylide complex, CpMo(CO)3 (C CPh) (1) as catalyst and tert-butyl hydrogen peroxide (TBHP) as an oxidant and turnover number (TON) of 88 was obtained with turnover frequency (TOF) of 2.45 h−1 . Mo acetylide oxo-peroxo species is formed in situ by reaction of 1 with TBHP during the course of reaction as catalytically active species. Interestingly even though the catalytically active species is homogeneous in nature it could be recycled very easily by recovering the catalytically active species as solid after addition of diethyl ether, and separating the products into organic phase. In the case of ethyl benzene oxidation, even after three recycles no appreciable loss in ethyl benzene conversion and acetophenone selectivity was observed. This complex showed high catalytic activity for the oxo functionalization of other alkyl aromatics and alkanes such as substituted ethyl benzenes, toluene as well as cyclohexane. TBHP was found to be more efficient oxidant than hydrogen peroxide for this oxidation. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The selective catalytic oxidation of hydrocarbons/alkyl aromatics is one of the important methods for the preparation of valuable organic chemical which is widely used in agrochemicals, pharmaceuticals, and high-tonnage commodities [1–4]. Hydrocarbon is of great importance in petrochemical industry. Petroleum feedstocks such as alkanes, olefins, and aromatics can transform into a plenty of valuable organic chemicals [5]. Catalytic oxidation reaction using metal complexes are important for the developments of ‘no waste’ chemical technology based on the ‘atom economy’ principle. Activation of the carbon–hydrogen bonds is significantly difficult in alkane and alkyl aromatics due to its kinetic stability, leading to the high inertness of carbon-hydrogen bond making its

∗ Corresponding author at: Catalysis Division, CSIR-National Chemical Laboratory, Pune, 411008, India. E-mail addresses: [email protected], [email protected] (S.B. Umbarkar). http://dx.doi.org/10.1016/j.apcata.2016.12.003 0926-860X/© 2016 Elsevier B.V. All rights reserved.

oxidation difficult [6].Oxidation of alkane/alkyl aromatics in the presence of metal catalysts requires elevated temperature or the presence of strongly acidic reaction media. Hence the selective functionalization of aliphatic hydrocarbons is a major challenge. But very few methods are available for converting alkanes into valuable products. Due to the high kinetic stability, high energy is required which leads to complete oxidation of alkanes instead of selective oxidation, so processes have to be developed for the selective and efficient alkane oxidation. Generally the oxidants such as dichromate, permanganate, nitric acid, halides or ozone, etc are used for the catalytic oxidation of hydrocarbon [7]. Owing to obvious environmental hazards associated with classical stoichiometric oxidants, such as acids or dichromates, new environment-friendly catalytic processes using recyclable catalysts, clean oxidants like molecular oxygen or H2 O2 under mild reaction conditions needs to be implemented. Hydrogen peroxide has obvious advantages due to low cost and benign nature. However, decomposition at higher temperature poses some limitations and reduction in the catalytic activity. As compared to hydrogen peroxide tert-butyl hydrogen peroxide is quite stable towards thermal decomposition. Also, the

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2. Experimental section 2.1. Materials and instrumentation Scheme 1. In situ generation of oxo-peroxo Mo (VI) acetylide complex with TBHP.

oxidation ability of tert-butyl hydrogen peroxide is better than H2 O2 . The selective oxidation of ethyl benzene to acetophenone is one of the important reaction. Acetophenone is used as raw material for the synthesis of pharmaceuticals, fragrance, chewing gum, resins, alcohols, esters, aldehydes, and is a test substrate for asymmetric transfer of hydrogen, as flavouring agents in many sweets and drinks, and as a solvent for cellulose ether [7–9]. Acetophenone and 1-phenylethanol are the main products of ethyl benzene oxidation along with benzaldehyde and benzoic acid as undesired by-products in many cases in the presence of tert-butyl hydrogen peroxide or air (O2 ) under high-pressure conditions [10]. Of these two products, acetophenone has higher commercial value; hence, it is desirable to increase selectivity for acetophenone in the ethyl benzene reaction. Molybdenum(VI) complexes are functionally and structurally similarity to molybdo-enzymes and hence are capable of catalyzing a variety of oxidation reactions [11–14]. Molybdenum oxides based heterogeneous catalysts as well as homogeneous molybdenum complexes have shown very high efficiency for the oxidation/epoxidation reactions [15–17]. Molybdenum carbonyl complexes with a wide range of ligands (halides N- containing and cyclopentadienyl) have been used for epoxidation of a variety of olefins [18–21]. Kuhn and coworkers [22] have used CpMo(CO)3 R (R = CH3 , CF3 or Cl) complexes extensively for epoxidation of olefins with very high conversion and selectivity. During the epoxidation in situ formed Mo-oxo peroxo moiety has been shown to catalytically active species [23]. Previously, we have used molybdenum acetylide complex, CpMo(CO)3 (C CPh); Cp = ␩5 -C5 H5 (1) as an efficient oxidation catalyst for cis-dihydroxylation of various olefins as well as selective oxidation of amines to nitroso compounds, alcohols to aldehydes and sulphides to sulphoxides and sulphones [24–27]. In continuation of our efforts to investigate the catalytic oxidation of a variety of organic substrates, oxidation of ethyl benzene and other alkanes have been studied using complex 1 and tert-butyl hydrogen peroxide as an oxidant.To our knowledge, there are no reports in the literature on the use of the above complex for oxidation of ethyl benzene. Here we report the use of complex 1 for selective oxidation of ethyl benzene to acetophenone.

Fig. 1. FTIR spectrum of complex 1.

All chemicals used for oxidation of ethyl benzene were procured from known companies and used without further purification. Ethyl benzene purchased from Merck, tert- butyl hydrogen peroxide (5.0–6.0 M) in decane was used as received from Aldrich. The exact molarity of the TBHP was determined using iodometric titration and was found to be 5.5 M. Acetonitrile (ACN) was procured from Finar India. CpMo(CO)3 ( C CPh) complex (1) has been prepared using reported method [25]. 2.2. Ethylbenzene oxidation Round bottom flask (25 ml) was charged with 0.5 g (0.0047 mol) ethyl benzene, TBHP (5.5 M in decane)1.7 g (0.019 mol), catalyst CpMo(CO)3 (C CPh) (1 mol%) and 10 g CH3 CN as a solvent. The reaction mixture was stirred at 80 ◦ C (temperature of the oil bath) for 24 h. The reaction was monitored using GC (Agilent 6890 Gas Chromatograph equipped with an HP-5 dimethyl polysiloxane (60 m length, 0.25 mm internal diameter, 0.25 um film thickness)) with flame ionization detector. Products were confirmed using GC–MS (Model GC Agilent 6890N with HP5 MS 30 m capillary column, MS Agilent 5973 Network MSD. 3. Result and discussion Molybdenum acetylide carbonyl complex CpMo(CO)3 ( C CPh) 1 was prepared as per the reported literature [25]. Previously we have established the nature of catalytically active species formed when 1 was used for oxidation of various organics like olefins, anilines, aldehydes and sulphides using hydrogen peroxide as oxidant [26]. As the oxidant used for ethyl benzene oxidation was tert-butyl hydrogen peroxide, the formation of Mo-oxo peroxo intermediate (Scheme 1) was reconfirmed by FTIR spectroscopy by addition of TBHP to the CpMo(CO)3 ( C CPh) complex (Fig. 1). When TBHP was added to complex 1, the disappearance of peaks at 1941, 2034 cm−1 corresponding to carbonyl group was observed confirming the loss of carbonyl ligands from low valent Mo centre. The appearance of a peak at 949 cm−1 confirmed the formation of high valent molybdenum oxo (Mo O) bond. The appearance of peaks at 841 can be assigned to O O stretching vibrations of peroxo species whereas formation of weak bands at 655 and 564 cm−1 can be attributed to asymmetric and symmetric stretching vibrations of peroxo group of Mo(O2 ). The IR peaks for the Mo oxo and peroxo species are in good agreement with the values reported in the literature [24–28] for different molybdenum (oxo) peroxo complexes. Even after the

(a) After addition of oxidant (TBHP) (b) before addition of (TBHP)

S.B. Khomane et al. / Applied Catalysis A: General 531 (2017) 45–51

47

100 90 80 70

Scheme 2. Ethyl benzene oxidation.

addition of TBHP to the complex 1 the bands due to C H stretching vibrations of the phenyl ring in the range 2854–2955 cm−1 and the C C stretching vibrations of the ring at 1464 and 1377 cm−1 were present. The presence of a peak at 2107 cm−1 confirmed the retention of acetylide group attached to Mo centre even after addition of an oxidant. The formation of molybdenum oxo-peroxo complex after addition of TBHP to the corresponding carbonyl complex has been shown previously by Kuhn et al. in case of cyclopentadienyl molybdenum tricarbonyl alkyl complex [22,23]. The detailed kinetic studies have shown an initial formation of dioxo complex which after subsequent reaction with excess TBHP forms oxoperoxo molybdenum alkyl complex. The structure of oxo-peroxo has been confirmed by single crystal X-ray diffraction analysis [23]. Hence the formation of cyclopentadienyl Mo-oxo peroxo acetylide formation in presence of TBHP is confirmed from characterisation data as well as comparison with literature reports. Initially, complex 1 was used for oxidation of ethyl benzene in acetonitrile solvent at 80 ◦ C (Scheme 2) using TBHP as oxidant with 1 mol% catalyst loading with respect to the substrate.Very high ethyl benzene conversion (88%) was obtained after 20 h with 100% selectivity for acetophenone with TON of 88 (Fig. 2). However, when the same reaction was carried out using hydrogen peroxide as oxidant, there was no ethyl benzene conversion observed, hence TBHP was used as an oxidant in all further reactions. 3.1. Effect of solvent, reaction temperature, catalyst loading and oxidant ratio To optimize the various reaction parameters for maximum yield of acetophenone, the effect of solvent, reaction temperature, catalyst loading and oxidant ratio on the ethyl benzene oxidation using complex 1 as catalyst precursor and TBHP as oxidant was studied. Initially, the effect of various solvents such as methanol, acetonitrile, tert-butyl alcohol, carbon tetra chloride on the oxidation efficiency of ethyl benzene was examined (Table 1). For comparison the reaction was also carried out without solvent, however, very poor ethyl benzene conversion of only 5% was obtained even after 20 h with 97% selectivity for acetophenone (Table 1 entry 1).When methanol was used as a solvent (Table 1, entry 2) there was an increase in conversion to 44% but a decrease in selectivity to 85%.In higher alcohol, t-butanol (Table 1, entry 3) the conversion decreased to 26% with 100% selectivity for ace-

%

60 50 40 30 20 10 0 2

4

6

8

10

12

14

16

18

20

22

-1

Time(h ) Fig. 2. Ethyl benzene oxidation using complex 1; 䊏 ethyl benzene conversion, selectivity for 䊉 acetophenone,  phenyl ethanol. Reaction conditions: ethyl benzene −0.5 g(0.0047 mol), TBHP- 1.7 g (0.019 mol), complex 1 1 mol%,CH3 CN −10 g, temperature −80 ◦ C.

tophenone. When chlorinated solvent like CCl4 (Table 1,entry 4) was used, results were similar to t-butanol. The trend in conversion a different solvent can be correlated to its dipole moment. The order for ethyl benzene conversion was observed to be in the order CH3 CN > MeOH > CCl4 > no solvent. This trend is in good agreement with the dipole moments of the solvents. Acetonitrile has maximum dipole moment (3.44) which has shown maximum conversion of 88%.This observation is in agreement with the literature reports where ethyl benzene has shown maximum conversion in CH3 CN using nano Fe based catalyst and TBHP as oxidant [29]. Among the screened solvents, acetonitrile was found to be highly efficient for ethyl benzene oxidation in terms of conversion as well as acetophenone selectivity. Hence, acetonitrile was selected as a solvent for further study. 3.2. Effect of reaction temperature Temperature is known to affect the conversion as well as selectivity for desired product. Higher temperature accelerates the rate of the reaction. When ethyl benzene oxidation using complex 1 was carried out at room temperature (∼28 ◦ C) and 40 ◦ C (oil bath temperature) there was no reaction observed even after 20 h. Hence the temperature of the oil bath was further increased gradually from 60 to 80 ◦ C (Fig. 3) with a marginal increase in the ethyl benzene conversion after 4 h from 17 to 22% as well as in acetophenone selectivity from 82 to 84%.When the temperature of the oil bath was further increased to 100 ◦ C, though the temperature of the reaction

Table 1 Effect of solvent on ethyl benzene oxidation using complex 1a. Sr. No.

1.

Solvent

Without #

2.

MeOH

3.

t-BuOH

4.

CCl4 #

5.

CH3 CN

Time, h

4 20 4 20 4 20 4 20 4 20

Conversion, %

4 5 10 44 17 26 20 28 22 88

Selectivity, %

TON

Acetophenone

Phenylethanol

92 97 90 85 88 100 92 100 84 100

8 3 10 15 12 – 8 – 16 –

Reaction conditions: ethyl benzene − 0.5 g (0.0047 mol),TBHP- 1.7 g (0.019 mol), complex 1 1 mol%,solvent–10 g, temperature- 80 ◦ C, # reflux temperature.

5 44 26 28 88

48

S.B. Khomane et al. / Applied Catalysis A: General 531 (2017) 45–51 102

102

90

98

70

96 94

60

92 50

90

40

88

100

80

98

70

96 94

60

92

50

90

40

88

84

30

86

82

20

86

30 20 10 4

6

8

10

12

14

16

18

selectivity (%)

80

conversion (%)

100

Selectivity(%)

Conversion(%)

90

84 82

20

4

-1

6

8

10

12

-1

14

16

18

20

Time(h )

Time (h )

Fig. 3. Effect of reaction temperature (oil bath temperature)on ethyl benzene oxidation; ethyl benzene conversion at  100 ◦ C, 䊉 80 ◦ C, 䊏 60 ◦ C and acetophenone selectivity at 100 ◦ C,  80 ◦ C, 䊐 60 ◦ C. Reaction conditions: ethyl benzene – 0.5 g(0.0047 mol), TBHP-1.7 g (0.019 mol), complex 1 1 mol%, CH3 CN–10 g.

Fig. 4. Effect of amount of oxidant on ethylbenzene oxidation; ethyl benzene conversion at  2 and, 䊏 1 equivalent TBHP; acetophenone selectivity at  2 and 䊐 1 equivalent TBHP. Reaction conditions: ethyl benzene- 0.5 g(0.007 mol),complex 1 1 mol%,CH3 CN −10 g, temperature- 80 ◦ C.

mixture did not exceed 80 ◦ C there was an increase in ethyl benzene conversion to 59% after 4 h with a corresponding increase in acetophenone selectivity of 93%. However, when high boiling solvent like benzonitrile was used to carry out the reaction at a higher temperature (100 ◦ C) the ethyl benzene conversion was 25% with 100% selectivity for acetophenone after 4 h. 3.3. Effect of catalyst loading To further optimize the reaction conditions for optimizing the ethyl benzene conversion as well as acetophenone selectivity, the catalyst loading with respect to ethyl benzene was varied from 0.5 mol% to 2 mol% and the results are given in Table 2. With the increase in catalyst loading the conversion after 4 h increased from 15% (0.5 mol%) to 44% for 2 mol% loading. The acetophenone selectivity after 4 h also increased from 80% for 0.5 mol% loading to 93% for 2 mol% loading. With an increase in catalyst loading the availability of catalytically active centres increased leading the acceleration of the reaction and in turn increasing conversion after 4 h. However, to have a balance between catalyst loading and reaction performance, 1 mol% catalyst loading was used throughout the experiments to further optimize other parameters. 3.4. Effect of amount of oxidant The effect of the amount of oxidant on ethyl benzene conversion and acetophenone selectivity was studied by using 1 and 2 equivalent TBHP with respect to ethyl benzene and the results are shown in Fig. 4. The ethyl benzene conversion after 4 h was not

significantly higher when 2 equivalent (22%) conversion TBHP was used instead of 1 equivalent TBHP (19%) conversion. However, after 20 h the conversion increased to only 31% in case of 1 equivalent TBHP, whereas 88% conversion was obtained with 2 equivalent TBHP after 20 h with 100% selectivity for acetophenone. Overall it was observed that at lower ethyl benzene conversion acetophenone selectivity was also lower with the corresponding formation of 1-phenyl ethanol which eventually converted to acetophenone at higher conversions leading to 100% selectivity for acetophenone. This clearly indicated the stepwise oxidation of ethyl benzene to 1-phenylethanol and subsequently to acetophenone. 4. Catalyst recycles study Interestingly even though the catalyst is homogeneous in nature, it could be recycled very easily and even after three recycles there was no significant decrease in the ethyl benzene conversion and acetophenone selectivity (Table 3). After completion of the reaction, the catalytically active species settled at the bottom as solid. Further addition of diethyl ether precipitated the catalyst completely. After simple decantation (without separating and washing the catalyst), the reaction mixture was separated from the catalyst and fresh charge was added to continue the reaction. The catalyst was recycled successfully for three cycles without significant loss in the catalytic activity with almost similar TOF values (2.4 h−1 ). To confirm the integrity of the catalytically active species during recycle, the solid obtained after the 1 st cycle was characterized by FTIR (Fig. 5). The spectrum clearly shows the presence of Mo = O at 988 O O of peroxo species at 922–797 cm−1 Mo O

Table 2 Effect of catalyst loading on ethyl benzene oxidation using complex 1a . Sr. No.

Catalyst loading, mol%

Time, h

Conversion, %

1.

0.5

2.

1

3.

1.5

4.

2

4 20 4 20 4 20 4 20

15 28 22 88 37 95 44 98

a

Selectivity, %

TON

Acetophenone

1-Phenylethanol

80 90 84 100 84 100 93 100

20 10 16 – 16 – 7 –

Reaction conditions: ethyl benzene- 0.5g(0.0047 mol), TBHP −1.7g(0.019 mol), CH3 CN −10 g, temperature −80 ◦ C.

56 84 84 49

S.B. Khomane et al. / Applied Catalysis A: General 531 (2017) 45–51

49

Table 3 Recycle study for ethyl benzene oxidation using complex 1a . Sr. No.

Cycle

Conversion, %

1 2 3 4

0 1 2 3

98 97 97 96

a

TOF, h−1

Selectivity, % Acetophenone

Phenyl ethanol

100 100 100 100

0 0 0 0

2.45 2.43 2.43 2.4

Reaction conditions: ethyl benzene −0.5 g(0.0047 mol),CH3 CN- 8 g, complex 1 2 mol%, TBHP- 1.7 g(0.019 mol), time −20 h, temperature −80 ◦ C. 0.40

6. Test of the scope of oxidation reaction

0.35

1938

0.30

Asorbance

0.25 922 797

0.20 0.15

2039

988

3065

578 (b) 3107

0.10

(a)

0.05 0.00 500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumber(cm ) Fig. 5. Comparison of the FTIR spectra for (a) fresh complex 1 and (b) catalytically active species separated after 1st run.

at 578 cm−1 cm−1 and 3065 cm−1 of Cp and absence of 1938 and 2039 cm−1 of (CO). The FTIR results confirmed the retention of all organic ligands on Mo centre during oxidation reaction.

5. Oxidation mechanism Based on the characterisation data for pre-catalyst and catalytically active species the mechanism for ethyl benzene oxidation is proposed (Scheme 3). Typically oxidation of alkyl aromatics proceeds via the radical mechanism. To confirm this mechanism in present case the ethyl benzene oxidation was carried out under optimised reaction conditions and after 8 h of reaction (41% ethyl benzene conversion) 0.05 g (TEMPO) was added as radical quencher and reaction was continued further till 15 hHowever after 10 h of addition of radical quencher, the ethyl benzene conversion increased only marginally from 41% to 50%, which confirmed the radical mechanism. Based on the catalytic activity data, characterisation as well as comparison with literature reports, following mechanism is proposed for ethyl benzene oxidation using cyclopentadienyl Mo acetylide carbonyl complex as pre-catalyst. In the first step the cyclopentadienyl molybdenum acetylide carbonyl complex (species I) reacts with TBHP to form corresponding oxo-peroxo complex (species II) after losing carbonyl ligand as confirmed by FTIR analysis (Fig. 1).In next step, the excess TBHP reacts with Mo oxo peroxo complex (species II) to form species III which subsequently generates Me3 COO. and Me3 CO. radical. Previously we have shown by DFT calculation the attack of TBHP on cyclopentadienyl Mo oxo-peroxo acetylide complex during oxidation using TBHP [30]. The DFT calculations support the formation of species III. After generation of peroxy radical, it attacks ethyl benzene to form ethyl benzene peroxy intermediate which subsequently forms acetophenone and 1-phenyl ethanol [31].

Wider applicability of complex 1 for oxidation of a range of alkyl aromatics was evaluated under optimised reaction conditions and the results are given in Table 4. Activated alkanes like ethyl benzene and diphenylmethane (Table 4, entry 1 & 2) showed very high activity with 88 and 89% conversion respectively with 100% selectivity for ketone. However in the case of toluene conversion decreased to only 29% with 100% selectivity for benzoic acid (Table 4, entry 3). In the case of cyclohexene, oxidation at the allylic position was observed with 60% conversion and 55% selectivity for cyclohexanol and 45% selectivity for cyclohexenone (Table 4, entry 4). When cyclic alkanes like cyclohexane was oxidized very low conversion (13%) was obtained with 77% selectivity for cyclohexanol and 33% selectivity for cyclohexanone (Table 4, entry 5). When cyclohexanol was oxidized under identical conditions 45% conversion with 100% selectivity for cyclohexanone was obtained (Table 4, entry 6). The above results showed a very high efficiency of complex 1 for oxidation of alkanes using TBHP as an oxidant under mild reaction conditions. Very recently cobalt based mixed oxides with flower like a core-shell structure of Co-Zn-Al oxides supported on alumina has been reported for ethyl benzene oxidation using TBHP as oxidant [32]. However even at higher temperature (120 ◦ C) maximum 70% conversion has been reported with 80% selectivity for acetophenone, benzaldehyde and benzoic acid as other byproducts with the loss of one carbon. Homogeneous and heterogenised copper complex [bis L-tyrosinato Cu(II)] have also been reported for oxidation of ethylbenzene using H2 O2 as oxidant at a lower temperature (∼65 ◦ C) [33]. However, use of NaHCO3 as co-catalyst was essential for this reaction. Ethyl benzene conversion was lower (36–52%) using the heterogenised and homogeneous Cu complex. The range of homogeneous and heterogeneous catalysts has been utilized for oxidation of ethyl benzenes to acetophenone using different oxidants like H2 O2 , TBHP, and air/O2 . Recent publication reports use of Ti-Zr-Co alloy catalyst for aerobic oxidation of ethyl benzene under high temperature (150–200 deg C) and high pressure (∼2 MPa) conditions with maximum 60% conversion and ∼60% selectivity for acetophenone. Maximum 73% selectivity for acetophenone has been reported at lower conversion (∼14%) [34]. Cobalt-N C supported on carbon nanotube has also been reported for ethyl benzene oxidation at 120◦ C and 0.8 MPa with maximum 20% conversion and 73% acetophenone selectivity [35]. The same group has also reported the effect of different supports including ceria on ethylbenzene oxidation of Co N C catalyst which showed maximum ∼30% conversion and 75% acetophenone selectivity under similar reaction conditions of 120◦ C and 0.8 MPa [10]. High temperature (140–160 ◦ C), high pressure (∼0.5–2 MPa) oxidation of ethyl benzene has been reported using carbon nano tubes (CNTs) a metal free catalyst with maximum 35% conversion and upto 60% selectivity for acetophenone [31]. Several Mn based heterogeneous catalysts have been reported for ethyl benzene oxidation. Manganese porphyrin based porous coordination polymers have been used recently for ethyl benzene oxidation using TBHP as oxidant at 65 ◦ C with upto 80% conversion and >99% acetophenone selectivity [36]. The activity of the heterogeneous catalysts was bet-

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Scheme 3. The probable free radical mechanism for oxidation of ethyl benzene with TBHP over Mo catalyst.

ter than its homogeneous counterpart however during recycle the catalytic activity decreased marginally. Manganese incorporated into silica matrix of TUD-1 with varying Si/Mn ratio (115, 44, 18) have shown maximum ∼40% conversion with ∼55% acetophenone selectivity using TBHP as oxidant at 80 ◦ C [37]. Mn supported on SBA-1 cubic mesoporous silica has been reported for ethyl benzene oxidation using TBHP as oxidant with maximum 20% conversion and 57% acetophenone selectivity at 80 ◦ C [38] .Ethyl benzene oxidation in super critical CO2 at 150 bar pressure at 120 ◦ C using

Mn supported on nano sized SiO2 /Al2 O3 mixed oxides has been reported to give 91% conversion with 98% acetophenone selectivity [39]. Very recently Silva et al. have reported the use of mononuclear Fe(III) complexes as homogeneous catalysts for oxidation of aromatic compounds including ethyl benzene using H2 O2 as oxidant under Ar atmosphere [40]. Total 30% conversion with only 23% selectivity for acetophenone has been reported. No catalyst recycle has been reported for this system. Homogeneous liquid phase

Table 4 Test of the scope of oxidation reaction using complex 1a. Sr. No.

substrate

Products (selectivity, %)

1

Conversion%

TON

88

88

89

89

29

29

60

60

13

13

45

45

(100)

2

(100)

3

(100)

4

(55)

(45)

5

(77)

(23)

6

(100) Reaction conditions: Substrate- 0.5 g (0.0047 mol), TBHP- 1.7 g (0.019 mol), Catalyst −1 mol%, CH3 CN–10 g, Time- 20 h; Temp- 80 ◦ C.

S.B. Khomane et al. / Applied Catalysis A: General 531 (2017) 45–51

oxidation of ethyl benzene using CoBr2 or Mn(OAc)2 a catalyst in acetic acid solvent has been reported [41]. Reactions conditions have been varied the batch as well as continuous mode oxidation using H2 O2 or oxygen as oxidant at 80–120 ◦ C temperature. Oxygen as oxidant under high pressure (12 bar at 120 ◦ C) conditions gave almost complete conversion with 80–84% acetophenone selectivity under continuous mode. Metal free ethyl benzene oxidation using molecular oxygen has been reported using combination of N-hydroxyquinolinimide (NHQI) and 4-carboxylN-hydroxyphthalimide (Car-NHPI) as catalysts. Maximum 70% conversion and 66% acetophenone selectivity have been reported 120 ◦ C [42]. In view of the reported catalysts for ethyl benzene oxidation, some typical advantages of the present catalytic systems are very high conversion and very high selectivity for acetophenone and very easy catalyst recycle. The selectivity for 1-phenylethanol is almost zero at higher conversions. 7. Conclusions

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21] [22] [23] [24]

Organometallic complex, CpMo(CO)3 ( C CPh) was found to be very efficient catalyst precursor for oxidation of ethyl benzene to acetophenone with very high conversion and 100% selectivity for acetophenone using TBHP as an oxidant under mild reaction conditions. Even though the complex is homogenous in nature, the catalytically active species could be recycled efficiently even up to three cycles without appreciable loss in the activity. Acknowledgement SBU acknowledges the financial support by Department of Science and Technology for the project SR/S1/IC-08/2004. References [1] [2] [3] [4] [5]

A. Gunay, K.H. Theopold, Chem. Rev. 110 (2010) 1060–1081. L.I. Matienko, L.A. Mosolova, G.E. Zaikov, Chem. Rev. 78 (2009) 211–230. S.J. Li, Y.G. Wang, Tet. Lett. 46 (2005) 8013–8015. F.B. Feng, W.X. Feng, Appl. Organomet. Chem. 29 (2015) 63–86. A.K. Suresh, M.M. Sharma, T. Sridhar, Ind. Eng. Chem. Res. 39 (2000) 3958–3997. [6] K.T.V. Rao, P.S.N. Rao, P. Nagaraju, P.S.S. Prasad, N. Lingaiah, J. Mol. Catal. A 303 (2009) 84–89. [7] C. Ricca, F. Labat, N. Russo, C. Adamo, E. Sicilia, J. Phys. Chem. C 118 (2014) 12275–12284.

[25] [26] [27] [28]

[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

51

S. Devika, M. Palanichamy, V. Murugesan, Appl. Catal. A 407 (2011) 76–84. G. Yang, Y. Ma, J. Xu, J. Am. Chem. Soc. 126 (2004) 10542–10543. Y. Chen, S. Zhao, Z. Liu, Phys. Chem. Chem. Phys. 17 (2015) 14012–14020. K. Krohn, K. Khanbabaee, Liebigs Ann. Chem. 1993 (1993) 905–909. K. Krohn, K. Bruggmann, D. Doring, P.G. Jones, Chem. Ber. 125 (1992) 2439–2442. K. Krohn, H. Rieger, K. Bruggmann, Synthesis 90 (1990) 1141–1143. P. Patnaik, Handbook of Inorganic Chemicals, McGraw Hill, 2003, pp. 385. M. Abrantes, M.A. Santos, J. Mink, F.E. Kuhn, C.C. Romao, Organometallics 22 (2003) 2112–2118. G. Grivani, S. Tangestaninejad, M.H. Habibi, V. Mirkhani, Catal.Commun. 6 (2005) 375–378. A.M. Martins, C.C. Romao, M. Abrantes, M.C. Azevedo, J. Cui, A.R. Dias, M.T. Duarte, M.A. Lemos, T. Lourenco, R. Poli, Organometallics 24 (2005) 2582–2589. F. Porta, L. Prati, J. Mol. Catal. A 157 (2000) 123–129. S. Tollari, M. Cuscela, F. Potta, J. Chem. Soc. Chem. Commun. (1993) 1510–1511. S. Tollari, S. Bruni, C.L. Bianchi, M. Rainoni, F. Porta, J. Mol. Catal. 83 (1993) 311–322. A. Bordoloi, S.B. Halligudi, Adv. Synth. Catal. 349 (2007) 2085–2088. S.A. Hauser, R.M. Reich, J. Mink, A. Pöthig, M. Cokoja, F.E. Kühn, Catal. Sci. Technol. 5 (2015) 2282–2289. A.M. Al-Ajlouni, D. Veljanovski, A. Capapé, J. Zhao, E. Herdtweck, M.J. Calhorda, F.E. Kühn, Organometallics 28 (2009) 639–645. A.V. Biradar, B.R. Sathe, M.K. Dongare, S.B. Umbarkar, J. Mol. Catal. A 285 (2008) 111–119. A.V. Biradar, T.V. Kotbagi, M.K. Dongare, S.B. Umbarkar, Tetrahedron Lett. 49 (2008) 3616–3619. A.V. Biradar, M.K. Dongare, S.B. Umbarkar, Tetrahedron Lett. 50 (2009) 2885–2888. M.G. Chandgude, A.V. Biradar, T.V. Kotbagi, V.G. Puranik, M.K. Dongare, S.B. Umbarkar, Catal. Lett. 142 (2012) 1352–1360. (a) T. Fujihara, K. Hoshiba, Y. Sasaki, T. Imamura, Bull. Chem. Soc. Jpn. 73 (2000) 383–390; (b) C. Bianchi, F. Porta, Vacuum 47 (1996) 179–182. D. Habibi, A.R. Faraji, M. Arshadi, J.L.G. FierroJ, Mol. Catal. A 372 (2013) 90–99. P. Chandra, S.L. Pandhare, S.B. Umbarkar, M.K. Dongare, K. Vanka, Chem. Eur. J. 19 (2013) 2030–2040. J. Luo, F. Peng, H. Yu, H. Wang, W. Zheng, ChemCatChem 5 (2013) 1578–1586. R. Xie, G. Fan, L. Yang, F. Li, Catal Sci. Technol. 5 (2015) 540–548. M. Ghorbanloo, A. Mohamadi, H. Yahiro Nano, Chem. Res. 1 (2016) 118–126. T. Liu, H. Cheng, L. Sun, F. Liang, C. Zang, Z. Ying, W. Lin, F. Zhao, Appl. Catal. A 512 (2016) 9–14. Y. Qiu, C. Yang, J. Huo, Z. Liu, J. Mol. Catal. A 424 (2016) 276–282. S. Sun, M. Pan, X. Hu, W. Shao, J. Li, F. Zhang, Catal. Lett. 146 (2016) 1087–1098. G. Imran, M.P. Pachamuthu, R. Maheswari, A. Ramanathan, S.J.S. Basha, Porous Mater. 19 (2012) 677–682. G. Imran, R. Maheswari, Mater. Chem. Phys. 161 (2015) 237–242. M. Arshadi, M. Ghiaci, A. Rahmanian, H. Ghaziaskar, A. Gil, Appl. Catal. B 119–120 (2012) 81–90. G.C. Silva, et al., J. Mol. Catal. A (2016), http://dx.doi.org/10.1016/j.molcata. 2016.08.037. B. Gutmann, P. Elsner, D. Roberge, C.O. Kappe, ACS Catal. 3 (2013) 2669–2676. Q. Zhao, K. Chen, W. Zhang, J. Yao, H. Li, J. Mol. Catal. A 402 (2015) 79–82.