Liu Yefeng; Lü Ying; Li Zeyu; Shen Yue; Wang Ruixin

(School of Chemical Engineering and Technology, North University of China, Taiyuan 030051)

Abstract: In this study, the precursor 4-(4-carboxy-phenoxy) phthalonitrile (CPPN) was first bonded onto the silica gel surface modified with poly(glycidyl methacrylate) (PGMA) (PGMA/SiO2) to prepare CPPN-PGMA/SiO2, and metal phthalocyanine (MPc; M=Co, Fe, Cu, Mn) was supported on the PGMA/SiO2 surface to prepare MPc-PGMA/SiO2 by synchronous synthesis and immobilization with phthalonitrile and metal salt in the solution. The chemical composition and surface morphology were characterized by the Fourier transform infrared (FTIR) spectroscopy, UV-Vis spectroscopy,scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetry analysis (TGA).The catalytic performance of MPc-PGMA/SiO2 in epoxidation of styrene was also investigated with molecular oxygen acting as the oxidant. The results show that MPc-PGMA/SiO2 can efficiently and selectively catalyze molecular oxygen for oxidation of styrene to styrene oxide under mild conditions. However, the catalytic activity differs substantially depending on the central metal, and a highest catalytic activity is achieved by CoPc-PGMA/SiO2. The CoPc-PGMA/SiO2 amount and temperature can also affect the catalytic oxidation of styrene, and at normal atmospheric pressure, a maximum conversion rate of styrene (99%) and selectivity of styrene oxide (53%) are obtained using 0.1 g of CoPc-PGMA/SiO2 (22.61 μmol of CoPc) at 100 °C for 6 h. CoPc-PGMA/SiO2 also has excellent reusability, and the conversion rate of styrene is still over 90% after 5 cycles.

Key words: metal phthalocyanine; poly glycidyl methacrylate; synchronous synthesis and immobilization; catalytic oxidation; styrene

1 Introduction

Epoxidation of olefins is important in organic synthesis due to the wide application of epoxides in organic coatings, surfactants, and organic synthetic intermediates[1-2]. Molecular oxygen is considered as a cost effective and environmentally friendly oxidant for oxidation of organic compounds. However, it is chemically inert due to the unique triplet groundstate configuration, and thus catalysts are needed for molecular oxygen to obtain better oxidation efficiency under mild conditions. Currently, the most commonly used catalysts for this purpose include metal oxides and transition metal complexes such as metalloporphyrin,metal phthalocyanine (MPc), and metal Schiff-base complexes[3-7]. Despite the increasing use of these complexes as biomimetic catalysts, their application may be limited due to easy aggregation, poor antioxidant capacity, difficult separation from the reaction system,and secondary pollution[8-12]. One approach to solve these problems is to convert the homogeneous catalyst into a heterogeneous one by immobilizing the catalyst on an appropriate carrier, such as chitosan, graphene oxide, carbon nanotube, MCM-41, SiO2, metal-organic frameworks (MOFs), porous carbon materials, zeolite,polypropylene fibers, and mesoporous polymers[13-22].In particular, SiO2has the advantages of high specific surface area, structural stability, easy modification,low cost, and high availability. In our previous studies,metalloporphyrin and other catalysts have been successfully immobilized on polymer modified SiO2particles to improve their catalytic activity[23-24].

MPc has similar biomimetic catalytic activity as metalloporphyrin along with higher stability than metalloporphyrin. In this study, the precursor 4-(4-carboxyphenoxy)phthalic dinitrile (CPPN) was first bonded onto the silica gel surface modified with poly(glycidyl methacrylate) (PGMA) (PGMA/SiO2)to prepare CPPN-PGMA/SiO2particles, and MPc(M=Co, Fe, Cu, Mn) was deposited on PGMA/SiO2to prepare MPc-PGMA/SiO2by synchronous synthesis and immobilization with phthalonitrile and metal salt in the solution. This method is simple and can avoid the loss of phthalocyanine, while the homogeneous catalyst can be converted into a heterogeneous one without affecting the catalytic activity and selectivity. The particular microenvironment of the macromolecular chain can protect MPc against autoxidation and improve the chemical and thermal stability, and it can also facilitate the separation of the catalyst from the reaction medium and thus improve the catalytic degradation efficiency and reusability.

2 Experimental

2.1 Reagents and instruments

Silica gel (100—160 mesh; about 125 μm in diameter with a pore size of 6 nm, a pore volume of 1.0 mL/g,and a surface area of 350 m2/g) was purchased from the Ocean Chemical Co., Ltd. (Qingdao, China);glycidyl methacrylate (GMA) was obtained from the Maya Reagent Co., Ltd.; γ-methacryloxypropyl trimethoxysilane (KH-570) was purchased from the Nanjing Silicone Materials Co., Ltd. (Jiangsu, China);p-hydroxybenzoic acid was obtained from the Tianjin Guangfu Fine Chemical Research Institute (Tianjin,China); 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was obtained from the Sinopharm Chemical Reagent Co., Ltd.;4-nitrophthalonitrile was purchased from the Shijiazhuang Aifa Chemical Technology Co., Ltd. (Shijiazhuang,China); CPPN and polyglycidyl methacrylate-grafted silica (PGMA/SiO2) were synthesized in our laboratory according to the literature report[25]; other chemicals and reagents were commercially available and analytically pure.

The instruments used in this study included a L1600300 Spectrum II Fourier transform infrared spectrometer (FTIR, LIantrisant, UK); a 2802 UV/Vis ultraviolet/visible spectrophotometer (Youniko Company, Shanghai); a scanning electron microscope (SEM, JSM-7001F, Japan Electron Optics Laboratory Co., Ltd.); a synchronous thermal analyzer (TG, ZCT-2000, Metler Toledo Instruments Co., Ltd.), operating in air with a temperature rising rate of 10 °C/min; an Axis Ultra DLD X-ray photoelectron spectrometer (XPS, Kratos Analytical,UK); and a GC9800 gas chromatograph (GC, Shanghai Kechuang Chromatographic Instrument Co., Ltd.).

2.2 Synchronous synthesis and immobilization of MPc on CPPN-PGMA/SiO2

Briefly, 1 g of PGMA/SiO2was added to 40 mL of DMF, and then 0.3 g of CPPN and 3.6 mmol of triethylamine were added and heated to 100 °C in N2atmosphere. After reaction for 8 h, the products were filtered, washed thoroughly with distilled water and anhydrous alcohol, and dried in vacuum to obtain the CPPN-PGMA/SiO2particles. After that, 1.0 g of CPPNPGMA/SiO2was added to 30 mL of n-pentanol, and then 0.6 g of 4-nitrophthalonitrile, a certain amount of metal salts (cobalt acetate, copper acetate, manganese acetate, or ferrous chloride) and 2.5 mL of DBU were added, and then the reaction occurred at 140 °C for 12 h in N2atmosphere, with the reaction products being cooled down, filtered, washed repeatedly with DMF until no detection of phthalocyanine in the filtrate,and dried in vacuum to obtain the blueish green MPc-PGMA/SiO2particles (M=Co, Fe, Cu, Mn), in which the immobilization amount of the MPc was 22.61 μmol/g.

2.3 Catalytic oxidation performance of MPc-PGMA/SiO2

MPc-PGMA/SiO2was used to catalyze the molecular oxygen for oxidation of styrene. Brie fly, 2 mL of styrene,30 mL of DMF and a certain amount of MPc-PGMA/SiO2(M=Co, Fe, Cu, Mn) were added into a 100 mL four-necked round-bottomed flask, while oxygen was fed continuously, with the reaction taking place at 100 °C under stirring. At the end of the reaction, the mixture was cooled down and filtered, and samples were collected from the filtrate and diluted for analysis by a gas chromatograph equipped with a SE-30 capillary column(30 m × 0.25 mm × 0.25 μm) under conditions covering a carrier gas pressure of 0.2 MPa, a gasi fication chamber temperature of 260 °C, a detector temperature of 260 °C,and an initial oven temperature of 90 °C. Samples were kept at the initial temperature for 18 min, heated to 200 °C at a temperature increase rate of 20 °C/min and kept at that temperature for another 10 min. High purity nitrogen was used as the carrier gas. Toluene was used as the internal standard, and the retention time of styrene,benzaldehyde and styrene oxide was 15.1 min, 19.3 min and 23.9 min, respectively.

3 Results and Discussion

3.1 Preparation of MPc-PGMA/SiO2

CPPN was prepared by the reaction between the nitro group of 4-nitrophthalonitrile and the hydroxyl group of p-hydroxybenzoic acid, and then phthalonitrile was grafted onto the PGMA/SiO2surface via the ringopening reaction of the carboxyl group in CPPN with the epoxy group located on the PGMA/SiO2surface for the preparation of CPPN-PGMA/SiO2particles. In the presence of DBU and a metal salt, the cyclization reaction occurs between the phthalonitrile group on the CPPN-PGMA/SiO2surface and 4-nitrophthalonitrile in the solution, resulting in the synchronous synthesis and immobilization of MPc on the PGMA/SiO2surface and the consequent formation of MPc-PGMA/SiO2particles,as shown in Figure 1.

3.2 Structure and morphology of immobilized CoPc

Figure 2 shows the SEM images of activated silica gel and CoPc-PGMA/SiO2particles. Many tiny particles, which are identi fied to be CoPc functionalized polymers, are observed on the surface of CoPc-PGMA/SiO2(Figure 2 B).

Figure 3 (A) shows the FTIR spectra of PGMA/SiO2,CPPN-PGMA/SiO2, and CoPc-PGMA/SiO2. The spectrum of PGMA/SiO2shows the distinct absorption peaks at 1 100 cm-1and 805 cm-1(Si-O-Si stretching vibration), 1 736 cm-1(C=O stretching vibration),3 001 cm-1and 2 955 cm-1(stretching vibration of -CH2and -CH3on the main chain of PGMA), 1 485 cm-1and 1 450 cm-1(bending vibration of -CH2and -CH3on the main chain of PGMA), and 908 cm-1(characteristic absorption of the epoxy group on the side chain of PGMA), respectively. In the spectrum of CPPN-PGMA/SiO2, a new absorption peak appears at 1601 cm-1due to the stretching vibration of the benzene ring, but the characteristic peak of the epoxy group at 908 cm-1disappears, indicating the occurrence of the ring-opening reaction. The new absorption peak at 2 200 cm-1is ascribed to cyano C≡N, indicating the successful grafting of CPPN onto PGMA/SiO2. In the spectrum of CoPc-PGMA/SiO2,the new absorption peak at 1 597 cm-1is assigned to the stretching vibration of the phthalocyanine ring skeleton;while those at 1 533 cm-1and 1 342 cm-1are assigned to the asymmetric and symmetric stretching of the nitro group, indicating the successful synchronous synthesis and immobilization of CoPc on the PGMA/SiO2surface.

Figure 3 (B) shows the UV-Vis spectra of CPPN-PGMA/SiO2and CoPc-PGMA/SiO2. Compared to CPPN-PGMA/SiO2, a strong absorption peak appears at 710 nm in the UV-Vis spectrum of CoPc-PGMA/SiO2, which is assigned to the Q-band absorption resulted from the π - π*transition of π electrons in CoPc[26], and thus can further con firm the synchronous synthesis and immobilization of CoPc onto the PGMA/SiO2surface.

Figure 3 (C) shows the thermal weight loss of PGMA/SiO2,CPPN-PGMA/SiO2, and CoPc-PGMA/SiO2. It can be seen that a slight weight loss occurs at a temperature of lower than 250 °C due to the desorption of water molecules.PGMA/SiO2begins to decompose at approximate 250 °C and is completely decomposed at 740 °C, with a total weight loss of 37.6%. The weight loss of CPPN-PGMA/SiO2is 41.4%. Thus, the bonding degree of CPPN on the PGMA/SiO2surface is calculated to be 3.8%. CoPc-PGMA/SiO2is completely decomposed at 850 °C with a total weight loss of 52.2%. The residue is considered to be cobalt oxide and SiO2, and the amount of CoPc on the surface of CoPc-PGMA/SiO2is about 16.3 g/(100 g).

Figure 1 Synchronous synthesis and immobilization of MPc on the surface of PGMA/SiO2 particles

XPS is an effective tool for the analysis of chemical composition, valence state, electron cloud distribution, and energy level structure on the material surface. In this study,XPS was performed to analyze changes in the valence state of Co in CoPc-PGMA/SiO2before and after epoxidation of styrene, as shown in Figure 4. Figure 4 (A) shows the fullscan XPS spectra of the fresh and recycled CoPc-PGMA/SiO2. The C, O, N, Si, and Co elements are detected,indicating the presence of CoPc in the as-prepared solid CoPc-PGMA/SiO2catalysts. Figure 4 (B) shows the highresolution XPS spectra of Co 2p in the fresh and recycled CoPc-PGMA/SiO2. Figure 4 B (I) shows two independent peaks of Co 2p for the fresh CoPc-PGMA/SiO2, with one at 781.1 eV corresponding to the Co 2p3/2of the divalent Co,and the other one at 797.1 eV corresponding to the Co 2p1/2of the divalent Co. The peaks at 786.0 eV and 802.8 eV correspond to the satellite peaks of Co 2p3/2and Co 2p1/2,thus indicating that Co mainly exists in the form of Co2+in CoPc-PGMA/

Figure 2 SEM images of SiO2 (A) and CoPc-PGMA/SiO2 (B)

3.3 Factors affecting the bonding of CPPN on PGMA/SiO2

Figure 5 (A) shows the bonding amount of CPPN on the surface of PGMA/SiO2for different alkaline solutions(NaOH, Na2CO3, NaHCO3, and TEA) that can promote the ring-opening reaction of the carboxyl group with the epoxy group. It is interesting to note that the lowest bonding amount of CPPN is observed for NaOH, which is the most alkaline solution tested and thus is highly corrosive to SiO2. Although TEA is second to NaOH in alkalinity, it can be well dissolved in the reaction system and thus shows the best catalytic activity. Thus, TEA is considered as the most appropriate catalyst, and CPPNPGMA/SiO2particles with 0.16 mmol/g of CPPN can be prepared with the use of TEA. Figure 5 (B) shows the variation of the bonding amount of CPPN with temperature. It shows that the bonding amount of CPPN is low at low temperatures, but it increases with the increase of temperature and reaches a maximum of 0.16 mmol/g at 100 °C. Thus, the optimal temperature for the bonding of CPPN to PGMA/SiO2is 100 °C.

3.4 Factors affecting the synchronous synthesis and immobilization of MPc on CPPN-PGMA/SiO2

Figure 3 (A) FTIR spectra of PGMA/SiO2, CPPN-PGMA/SiO2 and CoPc-PGMA/SiO2; (B) UV-Vis diffuse re flectance spectra of CPPN-PGMA/SiO2 and CoPc-PGMA/SiO2; and(C) TG curves of PGMA/SiO2, CPPN-PGMA/SiO2 and CoPc-PGMA/SiO2

Figure 6 (A) shows the variation of the immobilization amount of CoPc with reaction time. It shows that the immobilization amount of CoPc increases linearly at first with an increasing reaction time. However, it increases more slowly after 10 h and reaches a maximum of 16.3 g/(100 g) at the 12thh, after which it remains constant with further increase of reaction time. Thus, the optimal reaction time is determined to be 12 h.

Figure 4 (A) XPS spectra of fresh (I) and recycled (II)CoPc-PGMA/SiO2; (B) high-resolution spectra of Co 2p in fresh (I) and recycled (II) CoPc-PGMA/SiO2

Figure 6 (B) shows the variation of the immobilization amount of CoPc with temperature. It can be seen that the amount of immobilized CoPc increases dramatically with an increasing temperature until the boiling point reaches 140 °C, at which the amount of immobilized CoPc is 16.3 g/(100 g). Thus, the optimal temperature is determined to be 130—140 °C. Figure 6 (C) shows that the amount of immobilized CoPc increases with an increasing DBU amount and reaches a maximum of 16.3 g/(100 g) at 2.5 mL of DBU, and further increasing the DBU amount results in only a slight decrease in the amount of immobilized CoPc.Thus, the optimal DBU amount is determined to be 2.5 mL.

3.5 Performance of MPc-PGMA/SiO2 in catalytic oxidation of styrene

The variation of the conversion rate of styrene with reaction time in the presence or absence of CoPc-PGMA/SiO2is shown in Figure 7 (A). It shows that the conversion rate of styrene is only 18% in the 7thh in the absence of CoPc-PGMA/SiO2. The addition of CoPc-PGMA/SiO2results in a rapid increase of the conversion rate of styrene with the reaction time, and styrene can be completely oxidized after 5 h. The CoPc immobilized on the polymer/silicon dioxide carrier becomes insoluble,and consequently the homogeneous catalyst is converted to a heterogeneous catalyst that can still effectively activate molecular oxygen for oxidation of styrene. As a result, a high conversion rate can be obtained at 100 °C and under atmospheric pressure.

Figure 5 (A) Effect of basic compound on the bonding amount of CPPN, T=100 °C, (B) Effect of temperature on the bonding amount of CPPN, t=8 h■—TEA; ●—NaHCO3; ▲—Na2CO3; ★—NaOH

The main products resulted from the oxidation of styrene include styrene oxide and benzaldehyde[30-31], the former of which can be further oxidized into phenylacetaldehyde and acetophenone, while the latter of which can be further oxidized into benzoic acid, as shown in Figure 8. Figure 7 (B) shows the gas chromatogram of the products resulted from the catalytic oxidation of styrene by CoPc-PGMA/SiO2for 7 h. Several peaks are observed at 6.2 min, 9.5 min, 15.1 min, 18.3 min, and 23.9 min,which are attributed to DMF, methylbenzene, styrene,benzaldehyde, and styrene oxide, respectively. It shows that the oxidation products are only styrene oxide and benzaldehyde, indicating that styrene is not over-oxidized.It is also noted that the dominant oxidation product is styrene oxide, indicating the high selectivity.

Figure 7 (A) Change of styrene conversion rate over time at T=100 °C with and without catalyst, (B) Gas chromatogram of styrene oxidation products■—in precence of cat.; ●—in absence of cat.

Figure 6 The function of amount of immobilized CoPc with(A) reaction time, (B) temperature, and (C) DBU dosage

The epoxidation of styrene occurs via the free radical mechanism[32-34]. Firstly, O2is adsorbed on the surface of CoPc-PGMA/SiO2and is bound with central metal ions Co2+, and the unpaired electron of Co2+is delocalized onto the oxygen ligand, resulting in the formation of high valence Co(III)OO-that can interact with styrene to form the transient free radicals a. Some free radicals may undergo O-O bond rupture to form styrene oxide and Co3+is converted back to Co2+; while some free radicals may undergo coordination reactions between the benzene ring and the center cobalt ions to form the intermediates b, which in turn can be decomposed to form cyclic peroxide intermediates c, and Co3+is converted back to Co2+. Finally, the intermediate c is decomposed to form benzaldehyde, as shown in Figure 9.

Figure 10 (A) and (B) show the variation of the conversion rate of styrene and the selectivity of styrene oxide with reaction time for different MPc-PGMA/SiO2(M=Co, Cu, Mn, or Fe) particles. It shows that the conversion rate of styrene and the selectivity of styrene oxide decrease in the following order: CoPc-PGMA/SiO2＞ CuPc-PGMA/SiO2＞ MnPc-PGMA/SiO2＞ FePc-PGMA/SiO2, indicating that CoPc-PGMA/SiO2has the best catalytic activity. Thus, the conversion rate of styrene and the selectivity of styrene oxide differ significantly depending on the central metal, which can form high valence active intermediates with oxygen molecules at high temperatures. At normal atmospheric pressure, the maximum conversion rate of styrene (99%) and selectivity of styrene oxide (53%) are obtained using 0.1 g of CoPc-PGMA/SiO2(22.61 μmol of CoPc) at 100 °C for 6 h. It also shows that the conversion rate of styrene and the selectivity of styrene oxide first increase with an increasing reaction time. However, the conversion rate of styrene shows no signi ficant change after 6 h, but the selectivity of styrene oxide shows a decreasing trend, which can be attributed to the over-oxidation or ring opening of styrene oxide that can be converted into phenylacetaldehyde or acetophenone.Thus, the optimal reaction time is 6 h.

Figure 8 Oxidation pathways and products of styrene

Figure 9 Schematic diagram of styrene oxidation mechanism

3.6 Effects on the catalytic performance of CoPc-PGMA/SiO2

Figure 11 (A) shows the effect of the CoPc-PGMA/SiO2amount on the catalytic oxidation of styrene. It shows that the amount of the active catalytic component CoPc increases with an increasing CoPc-PGMA/SiO2amount,and as a consequence the conversion rate of styrene and the selectivity of styrene oxide also increase and reach a maximum at 0.1 g of CoPc-PGMA/SiO2. However, further increasing the CoPc-PGMA/SiO2amount results in no signi ficant change in the selectivity of styrene oxide but a decrease of the conversion rate of styrene. This is probably because the interactions between the active intermediates in the presence of excess catalyst can reduce the conversion rate of the target product acetophenone, which is consistent with the results for metalloporphyrin.[23]Thus, the optimal CoPc-PGMA/SiO2amount is determined to be 0.1 g(approximately 22.61 μmol of CoPc). Figure 11 (B)shows the effect of reaction temperature on the catalytic oxidation of styrene. It shows that the conversion rate of styrene and the selectivity of styrene oxide increase with an increasing temperature and reach a maximum of 99%and 53% at 100 °C, respedtively. However, the selectivity of styrene oxide shows a slightly decreasing trend at temperatures higher than 100 °C, which can be attributed to the further oxidation of styrene oxide due to oxidation and polymerization of styrene. Thus, the optimal temperature is determined to be 100 °C.

Figure 10 The function of conversion of styrene (A) and selectivity of styrene oxide (B) with time■—CoPc-PGMA/SiO2; ●—CuPc-PGMA/SiO2;▲—MnPc-PGMA/SiO2; ★—FePc-PGMA/SiO2

Figure 11 The function of conversion of styrene and selectivity of styrene oxide with CoPc-PGMA/SiO2 dosage (A) and temperature (B)

3.7 Reusability of CoPc-PGMA/SiO2

The used CoPc-PGMA/SiO2was washed thoroughly with DMF and ethanol to remove oxidation products adsorbed on the surface and dried in vacuum for recycled use. A given amount of CoPc-PGMA/SiO2and styrene was added to take part in the reaction at 100 °C for 6 h,with the conversion of styrene at different cycles shown in Figure 12. It shows that the conversion rate of styrene is decreased in the second cycle, which can be attributed to the desorption of a small amount of CoPc adsorbed on the surface of the catalyst in the first cycle. However, the conversion rate is maintained at about 90% in subsequent cycles, indicating that the immobilized catalyst has excellent stability. It is also noted that no significant changes occur in the color (bluish green) of CoPc-PGMA/SiO2, indicating that the structure remains intact without detachment of phthalocyanine. The immobilization of CoPc contributes to the increased stability of the catalyst,which can avoid the autoxidation due to excess catalyst,and thus CoPc-PGMA/SiO2can be used repeatedly.

Figure 12 Reusability of CoPc-PGMA/SiO2 for the oxidation of styrene

Fresh CoPc-PGMA/SiO2and CoPc-PGMA/SiO2, which had been used for 5 cycles, were characterized by FTIR spectroscopy in order to determine its stability, as shown in Figure 13 (A). It is evident that the FTIR spectrum of the recovered CoPc-PGMA/SiO2is quite similar to that of the fresh CoPc-PGMA/SiO2. The absorption peaks at 1 597 cm-1, 1 533 cm-1, and 1 342 cm-1are attributed to the stretching vibration of the phthalocyanine skeleton and the asymmetric and symmetric stretching vibration of the nitro group, respectively, thus indicating that the structure of phthalocyanine remains intact after repeated use.

Figure 13 (B) shows the UV-Vis diffuse reflectance spectra of CoPc-PGMA/SiO2before and after catalytic oxidation of styrene. The strong Q-band absorption peak is still observed at 710 nm in the UV-Vis spectra of CoPc-PGMA/SiO2after several cycles, indicating no signi ficant changes in CoPc immobilized on the PGMA/SiO2carrier after catalytic oxidation of styrene. There is also no change in the positions of diffraction peaks in the full-scan XPS and high-resolution Co 2p spectra (Figure 4 ), indicating no change in CoPc of CoPc-PGMA/SiO2after catalytic oxidation of styrene. Thus, it is concluded that CoPc-PGMA/SiO2can maintain an intact structure after repeated use, and thus it has excellent resuablity and stability.

3.8 Comparison with other catalysts

The catalytic activity of CoPc-PGMA/SiO2is compared with that reported in previous studies, as shown in Table 1. It shows that the conversion rate of styrene and the selectivity of styrene oxide are generally higher than those reported in most of previous studies.

4 Conclusions

Figure 13 (A) FTIR spectra of fresh and recovered CoPc-PGMA/SiO2; (B) UV-Visible diffuse re flectance spectra of fresh and recovered CoPc-PGMA/SiO2

In this study, CoPc-PGMA/SiO2has been successfully synthesized by the synchronous synthesis and immobilization method. The results show that alkalinity and temperature have significant effects on the amount of CPPN bound on PGMA/SiO2, and the DBU amount and temperature can affect the synchronous synthesis and immobilization of MPc on PGMA/SiO2. The maximum immobilization amount of MPc is 16.3 g/(100 g). The asprepared MPc-PGMA/SiO2can effectively activate the molecular oxygen under mild conditions for oxidation of styrene into styrene oxide. The catalytic activity reduces in the following order: CoPc-PGMA/SiO2＞ CuPc-PGMA/SiO2＞ MnPc-PGMA/SiO2＞ FePc-PGMA/SiO2.However, excess CoPc-PGMA/SiO2or high temperature can reduce the selectivity of styrene oxide. Under normal atmospheric pressure, the maximum conversion rate of styrene (99%) and selectivity of styrene oxide (53%) areobtained using 0.1 g of CoPc-PGMA/SiO2(22.61 μmol of CoPc) at 100 °C for 6 h. CoPc-PGMA/SiO2has good reusability and the conversion rate of styrene is still over 90% after 5 cycles.

Table 1 Comparison of catalytic activity of catalysts deposited on different supports towards styrene oxidation

Acknowledgements: This work was supported by the Natural Science Foundation of the Shanxi Province of China (No.201801D121069).