Polymer Anchored Catalysts for Oxidation of Styrene Using TBHP and

Article pubs.acs.org/IECR

Polymer Anchored Catalysts for Oxidation of Styrene Using TBHP and Molecular Oxygen Sweta Sharma, Shishir Sinha, and Shri Chand* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India ABSTRACT: Catalytic oxidation of styrene was investigated for polymer anchored Cu(II) and Mn(II) complexes prepared by Schiff base tridentate ligand. The effect of different oxidants, tert-butyl hydroperoxide (TBHP) under atmospheric pressure and molecular oxygen (O2) at high pressure in a batch reactor, were studied for maximum conversion of styrene and selectivity of styrene oxide product. The effect of various reaction parameters such as temperature, styrene to TBHP mole ratio, and catalyst amount were studied using TBHP as oxidant. The maximum conversion of styrene (87.3%) was obtained using Cu(II) complex, with maximum selectivity to styrene oxide (76.2%) at styrene to TBHP mole ratio of 1:3, 70 °C, and 25 mg catalyst. The O2 oxidant showed maximum conversion of 45.5% at 80 °C and 0.5 MPa pressure, with a styrene oxide selectivity of 22.4%. The catalytic activity was improved to 70.0% by addition of TBHP as initiator during the use of O2 oxidant. The Cu(II) catalyst showed better catalytic activity in comparison to Mn(II) catalyst using both TBHP and O2 oxidant. Gupta et al.24 reviewed polymer supported Schiff base complexes as catalysts for various alkenes and reported that Mn(III), Ru(III), Cu(II), Fe(III), Co(II), and Ni(II) were active metals for oxidation reactions. Chang et al.25 reported the cyclohexene oxidation using O2 as oxidant catalyzed by chloromethylated polystyrene supported tridendate Schiff base Cu(II), Ni(II), Co(II), and Mn(II) complexes; Cu(II) complex was found to be most active (51.9% conversion). Maurya et al.26 studied the styrene oxidation using Schiff base oxovanadium complexes using TBHP and H2O2 as oxidant and found the maximum selectivity to benzaldehyde. In the present study, the effect of different oxidants on the catalytic activity of styrene oxidation is determined for polymer anchored Cu(II) and Mn(II) complex catalysts prepared by Schiff base tridentate ligand. The catalytic activity was evaluated using (i) TBHP as oxidant at atmospheric pressure and (ii) O2 as oxidant under high pressure reaction conditions in a batch reactor (Parr reactor). In the literature, few data are available on the effect of pressure on styrene oxidation.27,28 The effect of TBHP and O2 oxidants on styrene conversion and styrene oxide selectivity were discussed by studying the effect of various reaction parameters.

1. INTRODUCTION Catalytic oxidation of styrene is a useful reaction from both academic and commercial point of view for the synthesis of styrene oxide, which is an important intermediate for large variety of fine chemicals such as perfumes and drugs etc. Traditionally, oxidation of styrene is carried out by using stoichiometric amounts of organic peracids as oxidant.1 However, peracids are very expensive and hazardous, provide poor selectivity for styrene oxide, and generate undesirable side products. Therefore, it is desirable to replace the conventional process with a more effective (in terms of styrene oxide selectivity) and environment friendly process which employs clean oxidants and generate little waste.2−7 Different homogeneous catalysts have been used for styrene oxidation using various oxidants.8−11 However, to overcome the issue of the removal of catalyst from the reaction products, low catalytic activity, and stability associated with homogeneous catalysts, more efficient and environment friendly heterogeneous catalysts have been proposed for styrene oxidation.12−14 In heterogeneous catalysis, a support is generally used to provide the mechanical strength to the catalyst, as well as increase the effective surface area. A large variety of supports have been used, which include inorganic supports such as molecular sieves, silica, alumina, and zeolites and organic supports such as polymers.15−18 During the last decade, polymeric supports have drawn the attention of researchers because of their various advantages. Polymer anchored metal complexes are gaining importance as efficient heterogeneous catalysts in a variety of organic transformations including oxidation and hydrogenation reactions because they are insoluble, recyclable, and generate less solid waste than nonsupported catalysts, which make the overall process more environmental friendly.19,20 Sherrington et al.21 synthesized various polymeric supports by the immobilization of Mo(VI) metal and tested for epoxidation of alkene in presence of tert-butylhydroperoxide. Heterogenized transition metal complexes, especially those with Schiff base on polymer support, were found to be efficient catalysts for the oxidation of various olefins and alcohols using different oxidants.22,23 © 2012 American Chemical Society

2. EXPERIMENTAL SECTION To find an optimal process condition for styrene oxidation to produce styrene oxide, the performances of TBHP and O2 oxidants using polymer anchored Cu(II) and Mn(II) catalysts were compared. H2O2 has been mainly used as an oxidant for different oxidation reactions.29,30 However, for styrene oxidation, H2O2 as an oxidant leads to poor selectivity to styrene oxide and decomposes at higher temperature.31 Therefore, in the present work, both TBHP and O2 were used as oxidants. Special Issue: CAMURE 8 and ISMR 7 Received: Revised: Accepted: Published: 8806

August 30, 2011 December 20, 2011 January 4, 2012 January 4, 2012 dx.doi.org/10.1021/ie201953y | Ind. Eng.Chem. Res. 2012, 51, 8806−8814

Industrial & Engineering Chemistry Research

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PS-[Mn(Hfsal-aepy)Cl](C-2). The PS-[Mn(Hfsal-aepy)Cl] complex was synthesized similarly, using MnCl2·4H2O as the catalyst precursor. 2.2. Catalyst Characterization. Elemental analyses of the ligand and complexes were carried out with an Elementar model Vario-EL-III, U.S.A. The metal loading in the polymer anchored complexes were estimated by AAS (GBC Avanta atomic absorption spectrophotometer, Australia). The BET surface area was determined by N2 adsorption/desorption isotherms at 77 K, using ASAP 2020 (Micromeritics, U.S.A.). Prior to the analysis, the samples were degassed at 90 °C for 8 h. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of 0.99. The Fourier transform infrared (FT-IR) spectra of the polymer ligand and metal complexes were recorded using Nicolet equipment (NEXUS Aligent 1600, U.S.A.) from KBr pellets in the range 4000−400 cm−1. The thermal stability of the catalysts was determined by Perkin-Elmer (Pyris Diamond, U.S.A.) equipment in air at a heating rate of 10 °C/min. 2.3. Reaction with TBHP as an Oxidant. The reaction with TBHP was carried out at atmospheric pressure in a 50 mL flask equipped with a reflux water condenser and placed in a thermostatted oil bath. In a typical experiment, 25 mg of catalyst and 10 mmol of styrene were added to the reaction mixture, followed by the addition of TBHP (20 mmol) in 10 mL acetonitrile. The reaction mixture was heated at 60 °C for 6 h under continuous stirring. Thereafter, experiments were performed for different catalyst amounts, temperatures, and styrene to TBHP ratios to obtain the optimal process conditions for styrene oxide production. 2.4. Reaction with O2 as an Oxidant. The catalytic oxidation of styrene using O2 as oxidant was carried out in a 300 mL batch stainless steel Parr reactor (4843, U.S.A.). The reactor is equipped with temperature and pressure controller, agitator, thermocouple, gas inlet valve, and sampling valve. In a typical reaction condition, the reactor was charged with 50 mmol of styrene, 100 mg of catalyst in 40 mL of acetonitrile, and the reaction was carried out in the temperature range 60−85 °C at 0.1−0.5 MPa. The reaction mixture was stirred at 600 rpm for 7 h. The reaction products formed by TBHP and O2 oxidants were analyzed by using a gas chromatography (GC) instrument (Netel, India), using a SGE capillary column of length 30 m, i.d. 0.25 mm, film thickness 0.25 μm, and a flame ionization detector (FID). Iso-octane was used as internal standard. The products were confirmed through gas chromatography−mass spectrometry (GC-MS) (Perkin-Elmer Clarus 500, U.S.A.).

Experiments have been divided into four sections. In the first section, polymer anchored metal complexes Cu(II) and Mn(II) were prepared by Schiff base tridentate ligand, and in the second section, Cu(II) and Mn(II) catalysts were characterized by various techniques. In the third section, these catalysts were used to optimize the process conditions for styrene oxidation by using TBHP as an oxidant, and in the fourth section, O2 was used as an oxidant. 2.1. Preparation of Catalyst. Chloromethylated polystyrene (18.9% Cl (i.e., 5.3 mmol Cl/g resin)) cross-linked with 5% divinyl benzene, gifted from Thermax Limited, Pune (India) was used as the polymer support. 3-Formyl salicylic acid (Hfsal), covalently bonded to chloromethylated polystyrene (PS) crosslinked with divinylbenzene, reacted with 2-(2-aminoethyl)pyridine (aepy) to give the Schiff base tridentate ligand PS-[Hfsal-aepy], as reported in the literature.32 The anchored ligand then reacted with CuCl2·2H2O and MnCl2·4H2O to form the metal complexes PS[Cu(Hfsal-aepy)Cl and PS-[Mn(Hfsal-aepy)Cl]. The whole scheme is shown in Figure 1.

Figure 1. Preparation of polymer anchored Schiff base complexes.

PS-[Cu(Hfsal-aepy)Cl] (C-1). The polymer anchored ligand PS-[Hfsal-aepy] (2.0 g) was swelled in methanol (30 mL) for 30 min. A solution containing 3.75 g (22 mmol) of CuCl2·2H2O in 30 mL methanol was prepared separately. The copper salt solution was then added to the solution containing PS-[Hfsal-aepy] dropwise, and the resulting mixture was heated at 60 °C in a thermostatted oil bath and refluxed for 18 h with continuous stirring. The liquid mixture was then cooled to room temperature, and the resultant resin was filtered, washed thoroughly with hot methanol (60 °C), and dried at 80 °C in an oven.

3. RESULTS AND DISCUSSION Polymer anchored metal complexes C-1 and C-2, prepared by schiff base for styrene oxidation, were characterized by various techniques. The physiochemical data, BET surface area, pore volume, and pore size of the ligand and polymer anchored complex catalysts are shown in Table 1. The AAS results confirmed the presence of copper and manganese metals in the complexes formed. The low metal content in the complexes may be due to the limited availability of reactive functionalities.33

Table 1. Physical and Analytical Data of Polymer Anchored Ligand and Polymer Anchored Metal Complexes elements (%) cmpd

color

C

H

N

chloromethylated PS-DVB resin PS-[Hfsal-aepy] (L) PS-[Cu(Hfsal-aepy)Cl] (C-1) PS-[Mn(Hfsal-aepy)Cl] (C-2)

golden yellow yellow black orange

75.6 71.7 56.2 59.8

6.4 6.3 7.4 8.4

0.6 9.2 8.6 7.7 8807

metal loading (wt %)

BET (m2/g)

pore vol. (cm3/g)

pore size (Å)

1.97 1.24

25.09 20.45 16.47 15.68

0.0279 0.0114 0.0045 0.0038

44.5 43.2 40.8 38.4

dx.doi.org/10.1021/ie201953y | Ind. Eng.Chem. Res. 2012, 51, 8806−8814


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BET analysis showed that when metal is anchored onto the support, a decrease in surface area is observed in the metal complexes, which may be due to pore blocking. The pore sizes were found in the range 38.4−44.5 Å. Figure 2 shows the N2

Figure 4. TGA of C-1 and C-2 complexes.

complexes.38 The third step (400−600 °C) weight loss, starting immediately after completion of the second step, was mainly due to the decomposition of the metal complexes. After 600 °C, no further weight loss in the catalysts was observed. At temperatures higher than 600 °C, some residue was obtained possibly as a result of the formation of respective metal oxides. 3.1. Oxidation of Styrene. With the objective to understand the effect of TBHP and O2 oxidants for C-1 and C-2 catalysts, the styrene oxidation was conducted in two steps. In the first step, the reaction was conducted at atmospheric pressure in a flask using TBHP as oxidant, and the effects of different styrene to TBHP mole ratios, temperatures, and catalyst amounts were studied. In the second step, the reaction was carried out at high pressure in a batch reactor (Parr reactor) using O2 as an oxidant, and the effect of various reaction parameters such as temperature and pressure were analyzed. It was found that, styrene oxide, benzaldehyde, and benzoic acid were the main reaction products, along with minor amounts of phenyl acetaldehyde and 1-phenylethane-1, 2-diol for both TBHP and O2 oxidants. The possible scheme of the product formation is shown in Figure 5. A similar kind of product formation is shown by Hulea and Maurya et al.12,39 The effect of different oxidants (TBHP and O2) on styrene conversion and products selectivity, particularly in terms of styrene oxide, at different reaction conditions is discussed in detail in the following section. 3.2. Oxidation of Styrene using TBHP as an Oxidant. The catalytic activity of styrene was evaluated using C-1 catalyst and TBHP oxidant. No reaction occurred in the absence of catalyst, indicating the absence of any homogeneous reaction. The effect of various reaction parameters such as styrene to TBHP mole ratio, reaction temperature, and amount of catalyst was quantified in terms of conversion of styrene and selectivity of different products particularly styrene oxide. Effect of Styrene to TBHP Mole Ratio. The effect of the styrene to TBHP mole ratio on the conversion and styrene oxide selectivity was studied at different mole ratios at 60 °C for the C-1 catalyst. In all the experiments, the amount of styrene is kept constant (10 mmol) and the amount of TBHP was changed to alter the styrene to TBHP mole ratio. Results indicated that, for all the mole ratios of styrene to TBHP, styrene conversion increases with time. The overall styrene conversion was increased from 37.1% to 72.3% and the selectivity to styrene oxide increased from 63% to 75% as the ratio of styrene to TBHP was increased from 1:1 to 1:3 after 6 h of reaction time, as shown in Figure 6. However, at ratios more than 1:3, improvement in styrene conversion and selectivity to styrene oxide were not appreciable. It was further

Figure 2. N2 adsorption/desorption isotherms: (a) chloromethylated PS-DVB bead, (b) ligand, (c) C-1, and (d) C-2.

adsorption/desorption isotherms of support, ligand, and metal complexes and exhibits the type IV adsorption/desorption isotherms, which are typical characteristics of macroporous materials.34,35 The FT-IR spectra of chloromethylated polystyrene bead polymer ligands and metal complexes are shown in Figure 3.

Figure 3. FT-IR spectrum of (a) chloromethylated PS-DVB bead, (b) ligand, (c) C-1, and (d) C-2.

The FT-IR spectra of chloromethylated polystyrene shows two strong peaks at wavenumbers of 690 and 1250 cm−1 (Figure 3a), which correspond to the CCl bond. These peaks were found to be absent in the polymer anchored ligand (Figure 3b), which indicate that there is no CCl bonding.36 The IR spectra of polymer ligand exhibited band at 1670 cm−1 due to azomethine group (CN), which shifted to lower wavenumber (1625, 1620 cm−1) positions in polymer anchored complexes (Figure 3c, d), suggesting the coordination of azomethine nitrogen to the metal ion.37 The bands of medium intensity covering the 2700−2900 cm−1 regions are consistent with the presence of −CH2 groups. The thermal stability of the polymer anchored complexes was investigated by thermogravimetric analysis (TGA). The C-1 and C-2 catalysts decomposed mainly in three major steps in the temperature range 100−600 °C, as shown in Figure 4. The first step of weight loss was in the temperature range 80−170 °C, and it was due to the loss of physisorbed water. The second weight loss was in the range 170−400 °C, on account of dissociation of the covalently bonded ligand with the metal 8808



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Figure 5. Product formation from oxidation of styrene.

data showed that the conversion of styrene was very low (∼12%) below 50 °C. As the temperature was increased from 50 to 70 °C, it was noted that the conversion of styrene increased for all the time with increase in temperature, which shows that the rate constant of step I (Figure 5) increases with an increase in temperature. Figure 7 indicates that both styrene

Figure 6. (A) Effect on styrene conversion with time at different styrene to TBHP mole ratios. (B) Selectivity of products at different styrene to TBHP mole ratios. Reaction conditions: styrene (10 mmol), C-1 (25 mg), temperature (60 °C), acetonitrile (10 mL).

observed that, with the increase in the styrene to TBHP ratio, the selectivity of benzaldehyde decreased and selectivity to other products increased. This suggests that, with the increase in the styrene to TBHP ratio, the formation of styrene oxide initially increases, which shows that the rate of reaction for step I (shown in Figure 5) increases with an increase in TBHP concentration. However, higher concentration of TBHP results in higher byproduct formation. This result suggests that a large amount of oxidant is necessarily not required to improve the conversion and selectivity of the desired product and 1:3 is the optimum styrene to TBHP ratio. Therefore, all other parameters for C-1 catalyst were evaluated at styrene to TBHP mole ratio of 1:3. Effect of Temperature. The influence of reaction temperature on styrene oxidation was evaluated by changing the reaction temperature from 30 °C (ambient temperature) to 80 °C at a styrene to TBHP mole ratio of 1:3. The experimental

Figure 7. (A) Effect on styrene conversion with time at different temperatures. (B) Selectivity of products at different temperatures. Reaction conditions: styrene (10 mmol), TBHP (30 mmol), C-1 (25 mg), acetonitrile (10 mL).

conversion and selectivity to styrene oxide increase with temperature until 70 °C. The data showed that, at all temperatures, selectivity to styrene oxide was highest among the products. The maximum styrene conversion (87.3%) was obtained at 70 °C after 6 h. At the same temperature, selectivity of styrene oxide and benzaldehyde were 78.3% and 15.7%, respectively. Further increase in temperature caused a reduction in styrene conversion, and at 80 °C, the conversion of styrene was 84.5%, which was slightly lower than conversion at 70 °C 8809



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catalyst particle reduces with increases in catalyst amount after an optimal value. The reduction in TBHP concentration causes decreases in the overall styrene conversion and prohibits further oxidation of styrene oxide to other products. Therefore, an optimal amount of catalyst should be chosen to get maximum conversion and selectivity of the desired product to give highest yield. The styrene oxide yield was maximum with 25 mg catalyst; therefore, 25 mg catalyst has been considered an optimal catalyst amount. The optimized reaction conditions for styrene oxidation were found to be at a styrene to TBHP mole ratio of 1:3, temperature 70 °C, and using 25 mg of C-1 catalyst. Mn(II) complex catalyst (C-2) was compared at these reaction conditions, and the results are shown in Table 2. It was

(87.3%). However, the selectivity to styrene oxide was almost constant (79.0%) at 80 °C, whereas the selectivity to benzaldehyde decreased to 10.1% as benzaldehyde was further oxidized to benzoic acid. Such results are consistent with the reported literature which showed that, at high temperature, the vinyl C−H bonds of styrene molecules are very active and therefore readily oxidized to other products.15 Therefore, 70 °C was chosen as the reaction temperature for further studies. Effect of Catalyst Amount. The effect of catalyst amount on styrene oxidation was investigated by changing the catalyst amount in the range 10−50 mg, keeping the styrene to TBHP mole ratio and temperature at 1:3 and 70 °C, respectively. The results are shown in Figure 8. It was observed that conversion

Table 2. Comparison of Cu(II) and Mn(II) Catalysts catalysts

conv. (%)

Ssoa

Sbzab

Sothersc

TOF (h−1)d

Cu(II) Mn(II)

87.3 75.4

78.3 69.0

15.7 21.3

6.0 9.8

187.0 223.0

a

Sso, selectivity to styrene oxide. bSbza, selectivity to benzaldehyde. Sothers, selectivity to other products. dTurn over frequency (TOF) = [moles of substrate converted/{time (h) × moles of metal}]. c

found that overall styrene conversion was higher for Cu(II) catalyst compared to Mn(II) catalyst. It was further noted that, for both catalysts, styrene oxide was found to be the major reaction product (selectivity >75.0%) with benzaldehyde also forming with TBHP oxidizing agent. A minor amount of benzoic acid and other products were also detected. However, the selectivity of styrene oxide is higher for the Cu(II) catalyst than for the Mn(II) catalyst; at the same time, selectivity of benzaldehyde was higher for Mn(II) catalyst. A high turn over frequency (TOF) value was obtained for Mn(II) catalyst (223.0 h−1). 3.3. Oxidation of Styrene using O2 Oxidant. In the next step, the oxidation of styrene was carried out at in a batch reactor (Parr reactor) using O2 as oxidant and C-1 catalyst. As no reaction was observed at atmospheric pressure at all the temperatures, experiments were performed at higher pressure. It was observed that, at higher pressure with O2 oxidant, selectivity of styrene oxide was lower compared to selectivity of benzaldehyde. At the same time, traces of benzoic acid along with other minor products were also detected. The reaction showed an induction period for first 2 h of reaction, which was not observed when TBHP was used as an oxidant. Therefore, a small amount of TBHP as initiator was added and finally oxidation of styrene with O2 as an oxidant was conducted without and with TBHP. 3.4. Oxidation of Styrene using O2 Oxidant without TBHP. First, preliminarily experiments were performed at various stirring speeds to study the effect of external mass transfer resistance on styrene oxidation in presence of O2 oxidant for C-1 catalyst at 80 °C and 0.3 MPa pressure. Figure 9 shows that there is no significant effect of stirring speed on styrene conversion, which indicates that there is no external mass transfer effect. Therefore, further experiments were conducted at a medium stirring speed of 600 rpm. Effect of Temperature and Pressure. To find the effect of temperature and pressure on the performance of styrene oxidation with O2 oxidant, both the temperature and pressure were varied simultaneously. The reaction was carried out by varying the reaction temperature from 30 to 90 °C and the O2 pressure from 0.1−0.6 MPa using 100 mg catalyst, 50 mmol styrene, and 40 mL acetonitrile. It was observed that no reaction occurred below 60 °C irrespective of pressure (the

Figure 8. (A) Effect on styrene conversion with time at different catalyst amounts. (B) Selectivity of products at different catalyst amounts. Reaction conditions: styrene (10 mmol), TBHP (30 mmol), temperature (70 °C), acetonitrile (10 mL).

of styrene at all the time was higher for 25 mg catalyst,as compared to 10 mg catalyst. The overall styrene conversion increased from 54.4% to 87.3% as catalyst amount was varied from 10 mg to 25 mg. However, with further a increase in the catalyst amount to 35 mg, the overall conversion decreased to 70.6%, which further decreased to 67.7% at 50 mg. The possible reason for decrease in styrene conversion at catalyst amount >25 mg can be explained by the decrease in effective reactant (styrene and TBHP) concentration on a single catalyst particle for the higher catalyst amount. This decrease in reactant concentration on catalyst particle, where the reaction is actually taking place, causes diminution in rate of reaction, which finally results in lower conversion. The current hypothesis can be further verified by the increase in styrene oxide selectivity with the increase in catalyst amount. At 35 mg and 50 mg catalyst, the selectivities were 82.4% and 84.6%, respectively, with simultaneous decreases in the selectivity of other products. These trends further indicate that the concentration of TBHP (oxidant) on single 8810



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also with increases in pressure at all temperatures. Yet, at all temperatures and pressures, selectivity to benzaldehyde has been higher than the selectivity to styrene oxide. In case of TBHP oxidant, the opposite trend was observed. This is because O2 is a stronger oxidant than TBHP is, which further oxidizes most of the styrene oxide to benzaldehyde by step II, as shown in Figure 5. With further increases in temperature to 85 °C, styrene conversion decreased to 40.4% at 0.5 MPa pressure. At this temperature, vaporization of the reactant mixture was observed, which reduced the overall conversion. Less vaporization is observed with increase in pressure at 85 °C, which results in increase in conversion of styrene with pressure. Figure 11A shows that, at 0.5 MPa pressure, for different temperatures, styrene conversion increased with time. However, Figure 9. Effect of stirring speed on styrene conversion. Reaction conditions: styrene (50 mmol), temperature (80 °C), pressure (0.3 MPa), C-1 (100 mg), acetonitrile (40 mL).

result is not shown here). The reaction started at 60 °C and 0.5 MPa pressure with 7.2% conversion. Figure 10A shows the

Figure 11. (A) Effect on styrene conversion at different temperatures with time. (B) Selectivity of styrene oxide at different temperatures. Reaction conditions: styrene (50 mmol), pressure (0.5 MPa), C-1 (100 mg), acetonitrile (40 mL).

at 85 °C, styrene conversion was lower than conversion at 80 °C at all times after 180 min, which further indicates that vapor phase styrene oxidation is lower than the liquid phase styrene oxidation. Therefore, the maximum styrene conversion was obtained at 80 °C and 0.5 MPa. At the same time, selectivity to styrene oxide at all temperatures remained almost constant (∼23%), as shown in Figure 11B. With these results, it can be concluded that, with O2 as an oxidant, the maximum production of styrene oxide can be achieved at 80 °C and 0.5 MPa. 3.5. Oxidation of Styrene using O2 Oxidant with TBHP as Initiator. The results presented in Figures 10 and 11 for styrene oxidation using O2 as an oxidant at different process conditions have shown low conversion for the initial two hours. Therefore, a small amount of TBHP was added as an initiator

Figure 10. (A) Effect on styrene conversion at different temperatures and pressures. (B) Selectivity of styrene oxide at different temperatures and pressures. Reaction conditions: styrene (50 mmol), Cu(II) catalyst (100 mg), acetonitrile (40 mL).

effect of temperature and pressure on styrene conversion. It was observed that, at all temperatures, the conversion of styrene increased with pressure. Similarly, at all pressures, conversion increased with the temperature until 80 °C. Figure 10A shows that, as temperature increased from 60 to 80 °C and pressure increased from 0.1 to 0.5 MPa, styrene conversion increased to 45.5%. At all temperatures and pressure, selectivity to styrene oxide was low and selectivity to benzaldehyde was higher, as shown in Figure 10B. However, it was observed that selectivity to styrene oxide increased with increases in temperature at all pressures and 8811



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because, with TBHP as an oxidant, no induction period was observed (Figures 6−8). The experiments were performed with O2 oxidant and TBHP as an initiator at an optimal pressure of 0.5 MPa to find the optimal operating temperature for styrene oxide production. Figure 12A shows that, with increases in temperature, the conversion of styrene has increased at all times. This was due to the increase in the rate constant at higher temperatures. It should be noted that no initial induction period was observed at any temperature when a small amount of TBHP was used as an initiator. It was further observed that, at all temperatures, the conversion of styrene was higher when TBHP was used as an initiator, compared to when no TBHP was used as initiator. Figure 12B shows that the selectivity of styrene oxide was lower than the selectivity of benzaldehyde, with benzaldehyde observed as major reaction product. Table 3 shows the reaction performance at different temperatures at 0.5 MPa pressure with and without TBHP as an initiator. It was thus observed that TBHP as an initiator increases the overall conversion but with reduced selectivity of styrene oxide at all temperatures. This is because the addition of

TBHP, which is also an oxidizing agent, helped in furthering the oxidation of styrene oxide to benzaldehyde. However, the maximum production (10.43% yield) of styrene oxide was obtained at 85 °C when TBHP was used as an initiator. Therefore, using O2 as an oxidant and TBHP as initiator will help in reducing the induction period and will maximize the styrene oxide yield. The activity of the C-2 complex catalyst was evaluated at 80 °C and 0.5 MPa oxygen pressure, and 36.5% conversion was found with 74.5% selectivity to benzaldehyde. The low conversion in (C-2) catalyst for both TBHP and O2 oxidants may be due to poor catalyst loading compared to that of C-1, as shown in the AAS analysis. 3.6. Recyclability of the Catalysts. The reuse of heterogeneous catalysts is an important aspect that makes them economical and preferable over homogeneous catalysts. The C-1 and C-2 catalysts were used for recycling studies. Figure 13 shows the recycling of C-1 and C-2 catalysts using

Figure 12. (A) Effect of TBHP initiator on styrene conversion at different temperatures with time. (B) Selectivity of products at different temperatures with time, using TBHP as initiator. Reaction conditions: styrene (50 mmol), temperature (85 °C), pressure (0.5 MPa), C-1 (100 mg), acetonitrile (40 mL), TBHP (1 mL).

Figure 13. Recyclability of polymer anchored C-1 and C-2 complexes (A) using TBHP as oxidant and (B) using O2 as oxidant with TBHP as initiator.

Table 3. Comparison of Effect of Temperature on Styrene Oxidation for O2 Oxidant without TBHP and with TBHP Initiatora with TBHPb

without TBHP selectivity (%)

a

temp. (°C)

conv. (%)

styrene oxide

75 80 85

41.3 45.5 40.4

21.2 22.4 23.1

selectivity (%)

benzaldehyde

yield (%) styrene oxide

TOF (h−1)

conv. (%)

styrene oxide

benzaldehyde

yield (%) styrene oxide

TOF (h−1)

69.5 71.4 69.3

8.7 10.1 9.3

95.1 104.8 93.0

52.5 62.4 70.0

15.6 12.8 14.9

66.7 71.3 68.9

8.2 7.9 10.4

121.0 143.7 161.2

Reaction conditions: styrene, 50 mmol; pressure, 0.5 MPa; C-1, 100 mg; acetonitrile, 40 mL. bTBHP: 1 mL. 8812



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TBHP oxidant and O2 oxidant using TBHP as initiator. In a typical experiment, the catalyst was filtered after a reaction time of 6 h, thoroughly washed with acetonitrile, and dried at 90 °C. These catalysts were further tested under similar reaction conditions. The results showed that no appreciable difference in catalytic activity, as well as in product selectivity, was observed for four cycles.

4. CONCLUSIONS Styrene oxidation was evaluated on polymer anchored C-1 and C-2 catalysts. The catalysts were prepared and characterized by various techniques. The effects of two different oxidants, TBHP and O2, on the catalytic activity of styrene was studied. The maximum conversion (87.3%) of styrene was found when TBHP was used as an oxidant at a styrene to TBHP mole ratio of 1:3 at 70 °C and 25 mg catalyst with maximum selectivity to styrene oxide (78.3%) at atmospheric pressure using C-1 catalyst. The effect of O2 oxidant at higher pressure in a batch reactor showed maximum conversion at 80 °C and 0.5 MPa pressure, with a styrene oxide selectivity of 22.4%. However, in this case, the selectivity of styrene oxide was very low compared to the selectivity of styrene oxide when TBHP was used as an oxidant. As the O2 oxidant initially showed an induction period, a small amount of TBHP (1 mL) was added as an initiator to the reaction mixture, which increased the styrene conversion to 70.0% at 85 °C and 0.5 MPa pressure. However, the selectivity of styrene oxide was further reduced, but the overall yield of styrene oxide was higher in this case compared to the case when TBHP was not used as an initiator. C-2 catalyst was found to be less active than C-1 catalyst for both the oxidants. The results of styrene conversion for TBHP and O2 oxidants showed that type of oxidant effects the conversion and product selectivity. TBHP showed maximum selectivity to styrene oxide, whereas O2 showed maximum selectivity to benzaldehyde.

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REFERENCES

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Polymer Anchored Catalysts for Oxidation of Styrene Using TBHP and

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