Introduction

Oxygenation of organic substrates has been extensively studied, and olefin oxidation is especially interesting because of the industrial importance of this type of reaction. Among such oxidation reactions, olefin epoxidation is extremely important for the preparation of highly functionalized organic compounds, epoxy resins, paints, perfumes, and surfactants [1–6]. Various epoxides are used as intermediates in organic synthesis, pharmaceuticals, and polymer production, and act as precursors for the synthesis of complex molecules because of the strained oxirane ring [7]. The traditional procedure for epoxidation of alkenes is oxidation by peracids [8]. However, peracids are very expensive, hazardous to handle, and nonselective for the epoxide formation, and also lead to formation of undesirable products, creating a lot of waste material [9, 10]. Alkyl hydroperoxides are used on a large scale in industrial epoxidation, for example, in Halcon-Arco and Sumitomo processes [11]. The recycling of the byproduct, tert-BuOH, has been realized in the Sumitomo process. In order to make the epoxidation process cleaner, safer, and more efficient, the use of catalysts is mandatory. Over the last few decades, the application of soluble compounds of early transition metals such as rhenium [12], titanium [13], vanadium [14], manganese [15], and molybdenum [16] has been used for alkene epoxidation. Good activities and selectivities have been pointed out as the main advantages of homogeneous catalysts. However, some industrial problems such as corrosion, deposition on the reactor wall, difficulty in recovery, and separation of the catalyst from reaction mixtures are associated with these homogeneous catalysts.

In recent years, the design and synthesis of catalytically active supported metal complexes have received considerable attention because of easy separation of the product from the reaction medium along with the recovery and reuse of these expensive catalysts. These heterogeneous catalysts offer several practical advantages over their homogeneous soluble counterparts [17, 18]. Various approaches have been focused on the incorporation of metal-based catalysts onto or into inert supports by different methods, such as organic supports including polybenzimidazole [19], functionalized polystyrene [20], ion exchange resins [21], and epoxy resins [22]. Inorganic oxides with hydroxyl-covered surfaces [23], amorphous silica [24], modified MCM-41 [25], alumina [26], or intercalation in clays [27] have been used for immobilization of homogeneous compounds and have been applied as catalysts for alkene epoxidation. Recently, anchoring of catalytically active transition metal complexes onto a polymer matrix has received considerable attention because of their potential advantages in practical synthesis, i.e., (1) the catalyst is easily separated from the reaction mixture by filtration, and (2) the recovered catalyst can be reused. Tangestaninejad et al. [28] and Sherrington’s group [29] have synthesized polymer resin immobilized catalysts and have investigated their catalytic activity in alkene epoxidation with tert-butyl hydroperoxide (TBHP). Transition metal complexes have attracted a lot of attention as possible oxidation catalysts for the selective oxidation of olefins in mild conditions [30, 31]. Many catalytic epoxidation methods, including asymmetric epoxidation, have been developed [32], but selective epoxidation of alkenes using heterogeneous catalysts and clean oxidants under mild conditions is still a challenge [33].

Copper is one of the most important elements in biological systems. Recently, copper complexes have been widely used in oxidation reactions under heterogeneous phase [34–36]. Heterogeneous Schiff base complexes containing donor atoms such as oxygen and nitrogen have been investigated for alkene epoxidation [37]. The activity of polymer-supported Schiff base complexes of transition metal ions varies with the type of Schiff base ligands, coordination sites, and metal ions used in their formation. Therefore, we have decided to prepare a heterogeneous Cu(II) catalyst bearing an N, O-donor ligand. The synthesis procedures for the polymer-supported complex combine the advantages of both homogeneous (activity) and heterogeneous (separation, recovery, recycling) catalysts. Therefore, an immobilized catalyst allows overcoming the problems with separation and reuse, and greatly amends the properties of the catalyst and its commercial value.

In this article, we have developed a polymer-supported Cu(II) Schiff base complex by reacting aldehyde functionalized polymer resins with 2-aminophenol and CuCl2. The heterogeneous catalyst was characterized by various spectroscopic methods (IR, UV–Vis diffuse reflectance spectroscopy), elemental analysis, thermogravimetric analysis, and scanning electron microscopy. Catalytic performance was investigated in liquid phase epoxidation of styrene as probe reaction. The influences of various reaction parameters on the conversion of styrene, yield, and selectivity of styrene oxide have been studied in order to optimize the reaction conditions. Under optimized reaction conditions, epoxidation of various olefins was also investigated. Catalytic activities of polymer-supported and homogeneous complex were compared. The results illustrated that the heterogeneous copper catalyst was more effective for the epoxidation reaction with a variety of olefin compounds using TBHP as oxidant and was found to be easily recoverable and recyclable up to five times.

Results and discussion

Characterization of the polymer-supported Cu(II) Schiff base complex

The outline for the preparation of polymer-supported Cu(II) Schiff base complex is given in Scheme 1. Due to insolubility of the polymer-supported Cu(II) complex in all common organic solvents, characterization was limited to the physicochemical properties, chemical analysis, SEM, TGA, IR, and UV–Vis spectral data. Elemental analysis of the free ligand and complex supported the formulation of the complex. Chemical analysis suggested 1.76% Cu in the immobilized Cu(II) Schiff base complex.

Scheme 1
scheme 1

 

Full size image

Scanning electron micrographs of the polymer-supported Schiff base ligand and the prepared catalyst were recorded to understand the morphological changes occurring on the surface of the polymer matrix. In Fig. 1 the SEM images of the polymer-anchored Schiff base ligand (a) and the immobilized copper complex on modified polystyrene (b) are shown. SEM pictures showed a morphological difference between the free Schiff base ligand and the catalyst. The presence of copper was also indicated by energy dispersive spectroscopy analysis of X-rays (EDX).

Fig. 1
figure 1

FE SEM image of polymer-supported Schiff base (a) and polymer-supported Cu(II) Schiff base catalyst (b)

Full size image

The mode of attachment of 2-aminophenol and metal onto the support was confirmed by IR spectral bands. In the FT-IR spectrum of polymer-bound aldehyde, a new peak appeared at 1,702 cm−1 that was assigned to carbonyl group stretching vibration of the aldehyde. In the polymer bound aldehyde, the sharp C–Cl peak (due to –CH2Cl groups) at 1,264 cm−1 in the starting polymer was seen as a weak band or was absent [38]. The formation of the Schiff base ligand on the polymeric support is indicated by a peak at 1,624 cm−1 assigned to C=N stretching frequency. This band showed a decrease in intensity and shifted to lower wave numbers after complexation. It suggested the coordination of the Schiff base to the central Cu(II) ion through the azomethine nitrogen. The phenolic (C–O) stretching frequency was observed in the region of 1,286 cm−1 (of ligand), which shifted to lower frequency at 1,270 cm−1 in the complex, indicating coordination of metal through the phenolic oxygen. The complex showed a characteristic frequency ν Cu–Cl at 354 cm−1 [39]. In the complex, bands at 628 cm−1 and 530 cm−1 were assigned to ν Cu–O and ν Cu–N stretching frequencies, respectively [40, 41].

Thermal stability of the polymer-supported Schiff base ligand and Cu(II) complex was investigated at a heating rate of 10 °C/min in air over a temperature range of 30–600 °C. It is shown that the thermal stability of the metal complex was slightly greater than that of its precursor Schiff base ligand. The complex was stable up to 350 °C, and above this temperature it decomposed. Thermogravimetric study suggested that the polymer-supported Cu(II) complex degraded at considerably high temperature.

The electronic spectrum of the polymer-supported Cu(II) complex was recorded in diffuse reflectance spectrum mode as a MgCO3/BaSO4 disc because of the solubility limitations in common organic solvents. In the complex five bands at ca. 310, 380, 446, 485, and 674 nm were observed. The absorption around 310 nm may be attributed to π→π* transitions of the phenyl moiety. The bands at 380 and 446 nm were due to LMCT. The band at 446 nm indicated the phenolic oxygen to copper charge transfer [42, 43]. In the complex, the bands around at 485 and 674 nm were due to d–d transition, which was assigned to the square-planar geometry [36, 44, 45]. From the different microanalytical and spectral data, the probable structure of the polymer-supported Cu(II) complex may be described according to Scheme 1.

Catalytic activity in the epoxidation of styrene

Catalytic activities of polymer-supported Cu(II) Schiff base catalyst were investigated by epoxidation of styrene to produce styrene oxide. In search of optimal reaction conditions to achieve the maximum yield of styrene oxide, the effects of solvent, temperature of the reaction, oxidant, reaction time, reactant ratio (styrene to TBHP), and amount of catalyst were studied. During the catalytic oxidation reaction of styrene, two products (styrene oxide and benzaldehyde) were identified (Scheme 2).

Scheme 2
scheme 2

 

Full size image

The styrene epoxidation has been examined using different solvents. The selected solvent should possess certain criteria, such as that it should be stable and should dissolve the substrate and oxidant. Table 1 shows the influence of solvent on the conversion of styrene and selectivity of styrene oxide by using polymer-supported Cu(II) Schiff base complex as a catalyst. The selectivity of styrene oxide was in the order ACN > DMF > MeOH > (Me)2CO > MePh. Results indicate that the epoxidation reaction did not proceed in toluene solvent. Additionally, poor conversion and selectivity were found when the reaction was carried out in acetone medium. The reaction, however, showed low activity and epoxidation selectivity in methanol; the main product was benzaldehyde. For the present reaction, the copper catalyst exhibits the highest catalytic activity and selectivity for styrene oxide in acetonitrile medium, because it is a polar solvent, has a high dielectric constant, and dissolves a wide range of chemical compounds. On the other hand, DMF is the next best solvent. It is a dipolar aprotic solvent with high dielectric constant. The results suggest the possibility that the combination of two solvents might result in good activity, yield, and selectivity because both solvents have a high dielectric constant and polarity. Now the reaction was examined in various solvent volume ratios (ACN/DMF). The highest selectivity (90%) of styrene oxide and conversion of styrene (88%) were achieved for a volume ratio of 9:1 of ACN to DMF. Both DMF and ACN molecules strongly interact with the active sites of the catalyst. The affinity of ACN and DMF to the active site might promote separation of styrene oxide from the site. So, the deep oxidation of styrene oxide was prevented and the selectivity increases [46]. However, the detailed mechanism is unknown at present.

Table 1 Styrene epoxidation using different solvents
Full size table

In this study, the effects of different oxidants such as TBHP, H2O2, PhIO, and molecular oxygen over this catalyst were examined in the epoxidation reaction of styrene. The results of this study are given in Table 2. The reaction showed almost no conversion when molecular oxygen acted as oxidant. TBHP was the best oxidant among the three oxidants that were used in catalytic epoxidation reaction in the present study. Oxidants like H2O2 and PhIO were clearly less efficient than TBHP, as evident from the percentage of conversion of styrene. The significant difference in the styrene oxide yields in case of using tert-butyl hydroperoxide in comparison to hydrogen peroxide can be explained by an inhibition effect due to water formation during epoxidation in case of hydrogen peroxide. Hydrogen peroxide was not applicable because this water strongly blocked the active sites of the metal catalyst [47].

Table 2 Styrene epoxidation using different oxidants
Full size table

Liquid phase epoxidation of styrene was carried out with three different molar ratios of styrene to TBHP. In all cases, styrene oxide was obtained as the major product along with small amounts of benzaldehyde as by-product. It was observed that the highest conversion of styrene (88%) and selectivity (90%) of styrene oxide were obtained at a styrene to TBHP molar ratio of 1:2 over the heterogeneous copper catalyst at 60 °C in 8 h as shown in Fig. 2. When the molar ratio of styrene to TBHP was increased from 1:1 to 1:2, the conversion as well as selectivity of styrene oxide increased with decreasing the selectivity of the byproducts. The catalytic activity was found to be decreased when the styrene to oxidant molar ratio was 1:3; probably a high oxygen concentration produced by more oxidants inhibited the reaction [48]. Thus, the optimal molar ratio of styrene to TBHP is 1:2.

Fig. 2
figure 2

Variation of styrene to TBHP molar ratio with conversion of styrene, yield, and selectivity of styrene oxide with polymer-supported Cu(II) Schiff base complex. Reaction conditions: styrene (5 mmol), catalyst (0.05 g), ACN/DMF (9:1), 60 °C

Full size image

To determine the best temperature, the epoxidation of styrene has been thoroughly investigated with different temperatures over polymer-supported Cu(II) Schiff base complex with time. The epoxidation reaction was carried out with various reaction temperatures from 40 to 80 °C using the reactant as a 1:2 molar ratio of styrene to TBHP with ACN/DMF (9:1) solvent mixture for a duration of 10 h. Results are shown in Fig. 3. The optimum temperature was found to be 60 °C. At lower temperatures the conversion of styrene was low. At a higher temperature, the yield and selectivity of styrene oxide decrease. Under these reaction conditions, the conversion increased with time and then reached a maximum value of 88% at 8 h. When the reaction time goes on, the conversion is kept almost constant. The yield and selectivity of epoxide were 79 and 90% at 8 h.

Fig. 3
figure 3

Variation of reaction temperature with conversion of styrene, yield, and selectivity of styrene oxide with polymer-supported Cu(II) Schiff base complex with variation of time. Reaction conditions: styrene (5 mmol), TBHP (10 mmol), catalyst (0.05 g), ACN/DMF (9:1)

Full size image

The amount of catalyst was varied from 0.025 to 0.1 g keeping the molar ratio of styrene:TBHP at 1:2 and the reaction temperature at 60 °C. The reaction was carried out for 8 h, and the products were analyzed. The results are presented in Fig. 4. With the increase in catalyst amount from 0.025 to 0.050 g the conversion of styrene increased from 76 to 88%. This was due to the availability of more active sites that favored the accessibility of a large number of molecules of substrates and oxidant to the catalyst. Styrene conversion was almost the same with a further increase of catalyst amount to 0.10 g.

Fig. 4
figure 4

Effect of catalyst amount with conversion of styrene, yield, and selectivity of styrene oxide with polymer-supported Cu(II) Schiff base complex. Reaction conditions: styrene (5 mmol), TBHP (10 mmol), ACN/DMF (9:1), 60 °C, 8 h

Full size image

Thus, the required parameters for the epoxidation of styrene, to get selectively styrene oxide, were as follows: styrene (5 mmol), TBHP (10 mmol), ACN/DMF (9:1), catalyst (0.05 g), reaction time (8 h), and temperature (60 °C). The major product, styrene oxide, was formed with 90% selectivity in the above reaction conditions.

Many efficient heterogeneous copper catalysts have been reported for the styrene epoxidation with TBHP [35, 49–53]. However, this catalytic system was superior to reaction time [50–53], conversion [49, 50], selectivity [35, 49–51, 53], and reaction temperature [50] in comparison to others. The results are summarized in Table 3. Comparison of this catalyst with the above-reported systems reveals that the present system gives better yields than the above-reported catalysts.

Table 3 Epoxidation of styrene with TBHP catalyzed by a variety of copper catalysts
Full size table

Catalytic epoxidation with different olefins

Finally, in order to explore further the catalytic activity and selectivity of the catalyst, epoxidation of other olefins was investigated under optimized reaction conditions, which proved to be best for styrene (Table 4, entry 1). This catalyst efficiently converted both cyclic and linear alkenes to their corresponding epoxides with TBHP. 1-Hexene and 1-octene as linear alkenes were efficiently converted to their corresponding epoxides. Cyclic olefins such as cyclohexene and cyclooctene were also epoxidized in high yields. First, the ability of the prepared catalyst was investigated in the epoxidation of cyclooctene. The results showed that the conversion is 82% with 100% epoxide selectivity. Epoxidation of cyclohexene occurred with 81% conversion and 95% epoxide selectivity, and allylic oxidation products (cyclohexene-1-one and cyclohexene-1-ol) were also detected in the reaction mixture. In the case of isopropenylbenzene, the corresponding epoxide was obtained in 85% epoxide selectivity. In the oxidation of α-pinene, the major product was α-pineneoxide, while verbenone and verbenol were produced as minor products. Limonene selectively epoxidized to the 1,2-epoxy product with other coproducts. Oxidation of linear alkenes such as 1-hexene and 1-octene were accompanied by allylic oxidation.

Table 4 Epoxidation of olefins catalyzed by polymer-supported Cu(II) Schiff base catalyst
Full size table

Comparison of homogeneous Cu(II)-amp and heterogeneous PS-Cu(II)-amp in the alkene epoxidation

In order to examine the effect of supporting on the catalytic activity of copper complex in the alkene epoxidation with TBHP, we repeated all the reactions with both types of catalyst and with same reaction conditions. The obtained results showed that in the alkene epoxidation the conversion, selectivity, and stability of the heterogeneous catalyst were better than those of the homogeneous catalyst (Table 4). On the other hand, the heterogeneous catalyst can be recovered several times without loss of its activity. The most important disadvantage of homogeneous Cu(II)-amp was the catalyst degradation in the presence of oxidant, whereas the heterogenized catalyst can be reused several times without significant loss of its activity.

Recyclability and stability

The recyclability of the catalyst is important for the catalysis reaction. To investigate the reusability of polymer-supported Cu(II) Schiff base complex, the catalyst was separated by filtration after the first catalytic reaction finished. For the next reaction cycle, we recovered the catalyst by washing with solvent and drying under a vacuum, then subjected it to the second run under the same reaction conditions. The catalytic run was repeated with further addition of substrates in appropriate amount under optimum reaction conditions. The nature and yield of the final products were comparable to that of the original one. Table 4 (entry 1) illustrates the reusability of the catalyst for the epoxidation of styrene during five cycles. It was found that the catalytic activity did not change significantly after five repeated runs.

To check the leaching of copper metal into the solution during the reaction, the following experiment was carried out under the optimum reaction conditions. The reaction was stopped after it had proceeded for 3 h with styrene conversion of 54%. The separated filtrate was allowed to react for another 5 h under the same reaction condition, but no further styrene conversion was observed. The UV–Vis spectroscopy was also used to determine the stability of the heterogeneous catalyst. The UV–Vis spectra of the reaction solution, at the first run, did not show any absorption peaks characteristic of copper metal, indicating that leaching of copper metal did not take place during the course of the oxidation reaction. These results suggest that the present catalyst was heterogeneous in nature.

Conclusions

In summary, we have successfully anchored the Cu(II) Schiff base complex into the polymer matrix. This catalyst, which can be prepared easily, is a heterogeneous and efficient catalyst for epoxidation of linear and cyclic alkenes with TBHP in acetonitriline/N,N-dimethylformamide mixed solvent (9:1). The developed heterogeneous catalyst shows high catalytic activity in styrene epoxidation with conversion (88%) as well as selectivity of styrene oxide (90%). Demanding olefins such as limonene can also be epoxidized with high selectivity. The complex was also applied for the heterogeneous catalysis of other olefins to their corresponding epoxides. The results show that the immobilized catalyst is highly active and selective to the epoxide formation. Hence, a better catalytic efficiency has been observed in polymer-supported Cu(II) Schiff base complex. Another important factor is the stability and recyclability of the catalyst under the optimized reaction conditions. Leaching tests indicated that the catalytic reaction is mainly heterogeneous in nature. The reusability of this catalyst is high; it can be reused five times without a significant decrease in its initial activity.

Experimental

Analytical grade reagents and freshly distilled solvents were used throughout the experiment. Liquid substrates were predistilled and dried by appropriate molecular sieve. Distillation and purification of the solvents and substrates were done by standard procedures [54]. Chloromethylated poly(styrene-divinyl benzene) and olefins were supplied by Sigma-Aldrich Chemical Co. (USA). Copper(II) chloride and 2-aminophenol were procured from Merck and were used without further purification.

A Perkin-Elmer 2400 C elemental analyzer was used to collect microanalytical data (C, H, N). FT-IR spectra of the samples were recorded on a Perkin-Elmer FTIR 783 spectrophotometer using KBr pellets. Diffuse reflectance UV–Vis spectra were taken using a Shimadzu UV-2401PC double-beam spectrophotometer having an integrating sphere attachment for solid samples. A Mettler Toledo TGA/SDTA 851 instrument was used for the thermogravimetric (TGA) analysis. Morphologies of the functionalized polystyrene and complex were analyzed using a scanning electron microscope ZEISS EVO40 (England) equipped with EDX facility. Copper content in the catalyst was determined using a Varian AA240 atomic absorption spectrometer.

Polymer-supported N-(2-hydroxyphenyl)benzaldimine (PS-amp, 3)

Chloromethylated polystyrene beads (1) were functionalized with aldehyde group according to a literature procedure [55]. The aldehyde-bearing beads (2) were allowed to swell in 50 cm3 methanol, and a solution of 4.0 g 2-aminophenol in 25 cm3 methanol was added dropwise over a period of 1 h with constant stirring, and then it was refluxed for 8 h. After cooling to room temperature, the reddish brown-colored polymer beads were filtered, washed thoroughly with methanol and petroleum ether, and dried in a vacuum. Yield: 35%; IR (KBr): \( \bar{v} = \) 3,414, 2,924, 1,700, 1,624, 1,509, 1,453, 1,286 cm−1; UV–Vis: λ max = 320–330 nm.

Polymer-supported N-(2-hydroxyphenyl)benzaldimine copper(II) chloride complex (PS-Cu(II)-amp, 4)

The polymer-supported Schiff base (2.0 g) was kept in contact with 10 cm3 methanol in a round-bottom flask. A methanolic solution of CuCl2 (10 cm3, 1% w/v) was added over a period of 45 min and was refluxed for 6 h. The black-colored copper-loaded beads were filtered carefully, washed with ethanol, and dried in a vacuum. Yield: 20%; IR (KBr): \( \bar{v} = \) 3,376 (sh), 2,920, 1,699, 1,605, 1,511, 1,453, 1,270, 628, 530, 354 cm−1; UV–Vis: λ max = 310, 380, 446, 485, 674 nm.

General procedure for catalytic epoxidation reactions catalyzed by PS-Cu(II)-amp

The liquid phase epoxidation reactions were carried out in a two-necked round-bottom flask fitted with a water condenser and placed in an oil bath at different temperatures under vigorous stirring. Substrates (5 mmol) were dissolved in ACN/DMF (9:1) mixed solvent for different sets of reactions together with 0.05 g catalyst, to which 10 mmol of TBHP (70% in aq. solution) was added. When the reaction was carried out under O2 atmosphere, oxygen gas was purged into the flask continuously. The withdrawal of the reaction mixture was done at regular time intervals and analyzed with a Varian 3400 gas chromatograph equipped with a 30 m CP-SIL8CB capillary column and a flame ionization detector. Chlorobenzene was used as an internal standard. All reaction products were identified by using an Agilent GC-MS.