Significant improvement of styrene oxidation over zinc phthalocyanine supported on multi-walled carbon nanotubes

Journal of Molecular Catalysis A: Chemical 402 (2015) 29–36

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Significant improvement of styrene oxidation over zinc phthalocyanine supported on multi-walled carbon nanotubes Yi Wan a , Qian Liang a , Zhongyu Li a,b,c,∗ , Song Xu a , Xiaojun Hu b,∗ , Qiaoli Liu c , Dayong Lu c a Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China b Key Laboratory of Regional Environment and Ecoremediation (Ministry of Education), Shenyang University, Shenyang 110044, PR China c Department of Materials Science and Engineering, Jilin Institute of Chemical Technology, Jilin 132022, PR China

a r t i c l e

i n f o

Article history: Received 22 December 2014 Received in revised form 9 March 2015 Accepted 14 March 2015 Available online 17 March 2015 Keywords: Hybrid catalyst Multi-walled carbon nanotube Zinc phthalocyanine Styrene oxidation

a b s t r a c t A novel hybrid catalyst (ZnPc–MWCNTs) was facilely prepared by a ultrasonic impregnation method. The as-prepared hybrid materials were characterized by FT-IR spectra, X-ray diffraction (XRD), diffuse reflectance spectra (DRS), scanning electron microscope (SEM), Raman spectroscopy and transmission electron microscopy (TEM). This hybrid material was employed as catalyst for styrene oxidation in presence of hydrogen peroxide as oxidant. The results showed that styrene was oxidized efficiently in the ZnPc–MWCNTs/H2 O2 system. The enhancement of the catalytic activity of ZnPc–MWCNTs hybrid materials was investigated in terms of different oxidation conditions. It was found that the optimum oxidation condition was 40 mg catalyst, 8 h reaction time and reaction temperature of 60 ◦ C. Under this condition, the conversion and the selectivity of styrene oxide was up to 94% and 90.6%, respectively. Moreover, the catalytic mechanism of styrene oxidation to benzaldehyde by the ZnPc–MWCNTs hybrid was also discussed. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Styrene oxidation at the side chain is of considerable interest in catalytic studies. Consequently, there is a growing concern in the synthesis of fine chemicals via this versatile reaction, such as the oxidative conversion of styrene to styrene oxide [1]. Styrene oxide is a very valuable chemical which has widespread applications as an important intermediate for the production of pharmaceuticals and fine chemicals. Since styrene has terminal olefinic group, its epoxidation is difficult, requiring long period (several hours) for obtaining appreciable styrene oxide yield [2]. Conventionally, styrene oxide is produced by the epoxidation of styrene using stoichiometric amounts of peracids [3]. However, peracids are expensive, corrosive, unsafe to handle and their use in the styrene epoxidation creates lot of waste due to formation of undesirable products. Therefore, it is a great practical interest to find such a catalyst which is much better for the epoxidation of styrene.

∗ Corresponding authors. Tel.: +86 519 86334771; fax: +86 519 86334771. E-mail addresses: [email protected], [email protected] (Z. Li), [email protected] (X. Hu). http://dx.doi.org/10.1016/j.molcata.2015.03.010 1381-1169/© 2015 Elsevier B.V. All rights reserved.

Metal-catalyzed oxidation of styrene can give rise to a whole variety of organic products. Metallophthalocyanine (MPc) has analogous structure to metalloporphyrin, and it has been researched in applications of solar cell materials, optical data materials and catalytic oxidation materials due to its optical and electronic properties [4–8]. MPcs are known to catalyze the oxidation of alkenes and alkanes. Metallophthalocyanines like Zn(II) phthalocyanines are readily available oxidation catalysts and found to transfer oxygen from various oxygen donors to alcohols, alkenes, phenols and thiols, numerous studies are carried out [9,10]. However, metallophthalocyanines are easy to aggregate, leading to markedly decrease the catalytic activity. Thus, design and synthesis of new hybrid materials to prevent aggregation of phthalocyanines become very urgent and necessary. Nowadays, the heterogenization of transition metal complexes onto robust supports is especially attractive due to combining the advantages of homogeneous (activity and selectivity) and heterogeneous (facile recovering and recycling of catalyst) catalyses. Mangematin and Sorokin [11] described the catalytic behaviors of iron tetraaminophthalocyanine grafted onto silica for oxidation reaction of cyclooctene, cyclohexene and styrene. Carbon nanotubes (CNTs) received much attention because of their outstanding

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Fig. 1. Schematic representation of synthesis of ZnPc–MWCNTs.

thermal, mechanical, and electrical properties. It is well known that the CNTs are extremely promising as supports for metal catalyst for organic synthesis and fuel cell applications [12–17]. Up to now, some studies have indicated that CNTs as the catalyst support can prevent the aggregation of nanoparticles, increase the catalytic active sites and improve the catalytic activity due to the high surface areas, special hollow interiors and excellent electronic properties. To our knowledge, catalytic oxidation of styrene metal complex embedded in multi-walled carbon nanotubes (MWCNTs) has not been reported by now. Therefore, we reported firstly the fabrication of ZnPc embedded in MWCNTs by an ultrasonic impregnation method, and studied the catalytic activity of styrene oxidation of the as-prepared ZnPc–MWCNTs hybrid materials. 2. Experimental 2.1. Preparation of ZnPc–MWCNTs hybrid materials All the reagents are analytical grade and used without further purifications. Zinc phthalocyanine (ZnPc) was synthesized from phthalonitrile, zinc acetate and 1-pentanol as described procedure [18]. The facile ultrasonic impregnation method of ZnPc–MWCNTs hybrid materials is shown in Fig. 1. Before preparing the ZnPc–MWCNTs hybrid materials, multi-walled carbon nanotubes must be purified in order to remove amorphous carbon fibers, amorphous carbon particles and other impurities such as graphite particles. In brief, 2 g of MWCNT was grinded in the agate mortar for 20 min, and then added to 100 ml of concentrated nitric acid, sonicated for 30 min to obtain a well dispersed MWCNT solution. The suspension was magnetic stirred and regurgitated at 120 ◦ C for 10 h, then cooled down to room temperature naturally. Washed with absolute ethanol and distilled water repeatedly, then dried under vacuum at 60 ◦ C for 12 h to obtain the carboxylic acid-functionalized MWCNTs. The synthesis procedure of ZnPc–MWCNTs is as follows: 0.8 g ZnPc were added into 100 ml ethanol under magnetic stirring. Then 0.1 g oxidized-MWCNTs were added and sonicated for 2 h. The mixture solution was then centrifuged and the supernatant was collected. Finally, the ZnPc–MWCNTs were obtained after dried in a vacuum at 80 ◦ C for 4 h.

were measured by a UV–vis scanning spectrophotometer (Shimadzu UV-2550) using an integrating sphere and BaSO4 as white standard. Fourier transform infrared spectra (FT-IR) of samples were collected with a Nicolet (PROTéGé 460) spectrometer in the range from 400 to 4000 cm−1 . The thermal stability of the materials was carried out using the TG-209-F3 thermogravimetric analysis meter (Nestal Company, Germany). Raman spectra were measured at room temperature using a LabRAM XploRA Raman spectrometer (Horiba Jobin Yvon, French) with a 532 nm laser focused on a spot of about 3 nm in diameter. 2.3. Evaluation of catalytic activity The oxidation reaction of styrene was carried out in a threenecked 100 ml round bottom flask equipped with a reflux condenser. Typically, 40 mg of catalyst, 2.8 ml (27.3 mmol) of 30 wt.% aqueous H2 O2 and 0.46 ml (3.9 mmol) of styrene along with 5 ml acetonitrile were mixed together. The pH value of the solution was adjusted to 8 and the solution was stirred at 60 ◦ C for 8 h. After the reaction, ZnPc–MWCNTs were separated by filtration. Then the chloroform was used to extract the organic phase. The content of liquid products was analyzed by HP5890 gas chromatography (GC) equipped with a flame detector and a HP-5 capillary column (0.32 mm × 0.25 ␮m, Agilent, USA). The products were confirmed by the retention time of the standard samples. The sample (0.5 ml) was taken during the reaction at certain intervals each sample was injected into the GC to measure the content of resultant of reaction.

2.2. Characterization X-ray diffraction (XRD) measurements were carried out at room temperature using a Rigaku D/MAX-2500PC diffractometer (Rigaku Co., Japan) with Cu K␣ radiation (␭ = 0.15406 nm) operated at 40 kV and 100 mA. Morphologies of the prepared samples were observed on a JSM-6360LA scanning electron microscope (SEM, JEOL, Japan) and a JEM-2100 transmission electron microscope (TEM, JEOL, Japan). UV–vis diffuse reflection spectra (DRS) of the photocatalysts

Fig. 2. FT-IR spectra of oxidized ZnPc–MWCNTs (a), ZnPc (b) and MWCNTs (c).

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Fig. 3. SEM images of ZnPc–MWCNTs (a), oxidized-MWCNTs (b).

3. Result and discussion

3.3. TEM images

3.1. FT-IR Spectroscopy

The TEM morphological features of the oxidized-MWCNTs, ZnPc–MWCNTs and ZnPc are shown in Fig. 4a. It is found that each oxidized-MWCNTs is mainly long and folded pipe and its diameter is in the range of 20–30 nm (Fig. 4a). The resulting hybrid materials exhibited a stretched and shortened feature with phthalocyanines uniformly assembling on the convex surface of MWCNTs (Fig. 4b). However, ZnPc exhibited poor dispersibility with clusters aggregated while the density of ZnPc on the MWCNTs was improved (Fig. 4c).

The FT-IR spectra of ZnPc, oxidized-MWCNTs and ZnPc–MWCNTs are depicted in Fig. 2. For ZnPc, two strong bands at 1486 and 1330 cm−1 are assignable to the stretching of C C and C N, respectively. Another feature that should be given attention is that the peaks at 1277, 1087 and 1057 cm−1 are obviously strong, which are also characteristics of phthalocyanine macrocycles. For carboxylic acid-functionalized MWCNTs, the absorption band at 1717 cm−1 is attributed to the carbonyl stretching. The broad peak is detected at 3443 cm−1 due to the absorption of carboxyl stretching from the COOH group. The peaks appeared in ZnPc can be found in ZnPc–MWCNTs, but are not present in oxidized MWCNTs. All these observations indicated evidently that zinc phthalocyanines were attached on the surface of MWCNT successfully.

3.2. SEM images Fig. 3 shows the scanning electron microscopy (SEM) images of oxidized-MWCNTs and ZnPc–MWCNTs. It can be seen clearly that ZnPc particles are well dispersed in each threadlike MWCNT matrix (Fig. 3a), showing good miscibility with MWCNTs. The diameter of ZnPc–MWCNTs was thicker compared with oxidized-MWCNTs, and the MWCNTs were well coated by ZnPc particles which densely and evenly deposited around the pipe wall of these MWCNTs to form a core–shell composite structure.

3.4. XRD patterns Fig. 5 shows the XRD patterns of the oxidized-MWCNTs, ZnPc and ZnPc–MWCNTs. It is found that the ZnPc has main peaks at 2.21◦ , 9.23◦ , 18.68◦ , 26.12◦ and 30.46◦ , corresponding to an interplanar space of 19.98 Å, 4.80 Å, 2.40 Å, 1.75 Å and 1.52 Å, respectively. Compared to ZnPc, the characteristic peaks of the ZnPc–MWCNTs became weak because the ZnPc was well dispersed on the surface of MWCNTs. Therefore, XRD patterns of zinc phthalocyanine–MWCNTs indicated that the hybrid catalysts of ZnPc–MWCNTs were prepared successfully. 3.5. UV–vis diffuse reflectance spectra Fig. 6 shows the UV–vis absorption spectra of the ZnPc (in DMF) solutions, and the UV–vis diffuse reflectance absorption spectra (DRS) of ZnPc, MWCNTs and ZnPc–MWCNTs. As shown in Fig. 6a, the Q-band absorption at 600–700 nm was assigned to a ␲–␲* transition from the highest occupied molecular orbital (HOMO), of

Fig. 4. TEM images of oxidized-MWCNTs (a), ZnPc–MWCNTs (b) and ZnPc (c).

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Weight (%)

100

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a 40

b 20 300

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500

600

700

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Temperature (°C) Fig. 7. TGA curves of ZnPc–MWCNTs (a), ZnPc (b) recorded at a rate of 20 ◦ C/min under a N2 atmosphere.

Fig. 5. XRD patterns of oxidized MWCNTs (a), ZnPc–MWCNTs (b) and ZnPc (c).

˛1␮ symmetry to the lowest unoccupied molecular orbital (LUMO) of eg symmetry. As can be seen from the DRS spectra in Fig. 6b, the DRS spectrum of the ZnPc–MWCNTs was similar to that of ZnPc, confirming the successful loading of phthalocyanine onto the MWCNTs. Moreover, the ZnPc–MWCNTs show a broader spectral absorption in the visible/near-IR region, indicating that the ZnPc–MWCNTs can be activated by the corresponding visible/nearIR light. Two absorption peaks were observed at 553–685 nm, originating from Davydov splitting due to molecular exciton coupling, which typically occurs due to an excited state resonance interaction in loosely bound molecular systems. It is known that the Q band absorption is attributed to the ␲–␲* transition of the respective ␲-conjugating molecules [19] and the absorption transitions around 685 and 553 nm are attributed to monomeric and dimeric ZnPc species, respectively [20]. For ZnPc (Fig. 6b), the strong absorbance peak around 553 nm indicates that the ZnPc species exits mainly in the dimeric form, while for the ZnPc–MWCNTs, it is clear that the Q band absorption declines after the ZnPc immobilized on MWCNTs. This may be attributed to the desorption and deaggregation of ZnPc immobilized on the surface of MWCNTs. Noticeably, the monomeric absorption band (around 685 nm) increases relative to the dimeric after ZnPc immobilized on MWCNTs, which indicates that the desorption rate of dimeric ZnPc is faster than the monomeric.

Intensity

c b

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500 Fig. 6. UV–vis spectra of ZnPc (a); UV–vis diffuse reflectance spectra of MWCNTs, ZnPc and ZnPc–MWCNTs (b).

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Raman shift (cm ) Fig. 8. Raman spectra of oxidized-MWCNTs (a), ZnPc–MWCNTs(b) and ZnPc (c).

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% Conversion % Selectivity

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Fig. 9. Effect of conversion and selectivity on (a) type of catalyst; (b) reaction time; (c) amount of catalyst and (d) reaction temperature.

3.6. Thermogravimetric analysis The thermostability of ZnPc–MWCNTs was further estimated by thermogravimetric analysis (TGA; see Fig. 7). The ZnPc shows a characteristic thermogram with the first weight loss occurring between 450 ◦ C and 640 ◦ C, and the second weight loss occurs from 640 ◦ C to 800 ◦ C. Based on the weight loss and residue quantity at 800 ◦ C, the weight percentage of ZnPc–MWCNTs hybrid materials and the ZnPc is about 46.7–26.9%, respectively. Thus, the results indicated that ZnPc–MWCNTs possessed good thermal stability under 600 ◦ C. 3.7. Raman spectroscopy In order to further study the interaction between ZnPc particles and MWCNTs, Raman spectra of MWCNTs, ZnPc and ZnPc–MWCNTs hybrid materials were characterized (Fig. 8). It can be seen that the Raman spectrum of oxidized-MWCNTs show two prominent bands at 1340 cm−1 (D-band) and 1570 cm−1 (G-band) with almost same intensity. After conjugation with MWCNTs, the Raman spectrum of the ZnPc–MWCNTs hybrid materials is similar to that of ZnPc in the range of 500–3000 cm−1 . However, the intensity gets weak, while the peaks between 1340 cm−1 and 1570 cm−1 turn to broader. Moreover, the variation of the relative intensity of the D band to the G band can provide direct evidence for the

covalent modification of MWCNTs. Compared MWCNTs, ZnPc and ZnPc–MWCNTs, little variation of the ratio of the D band to the G band (ID /IG ) can be observed, which suggested that ZnPc associated with the surface of MWCNTs through non-covalent modification [21]. 3.8. Catalytic activity of ZnPc–MWCNTs hybrid materials Styrene oxidation reaction gives styrene oxide and benzaldehyde in a perceptible yield. However, styrene oxide is the major product under the present conditions. In order to obtain maximum conversion of styrene, catalytic activity for the oxidation of styrene was evaluated by varying different parameters (Fig. 9), and turnover number (TON) was also calculated (Fig. 10). 3.8.1. Effect of catalytic activity on different catalysts Fig. 9a shows the effect of catalytic activity of styrene oxidation on different catalysts. It was found that the catalytic activity and TON were relatively low without catalyst (TON was 203). Compared to ZnPc, the catalytic activity was significantly enhanced after using ZnPc–MWCNTs hybrid materials. The conversion of styrene increases dramatically from 65.6% to 94.0% and the selectivity of styrene oxide remains about 90%. Because pure ZnPc is easy to aggregate, leading to markedly decrease their catalytic activity (TON was 274). The MWCNTs as catalyst support for ZnPc could

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Temperature (°C) Fig. 10. Effect of turnover number (TON) on (a) type of catalyst; (b) reaction time; and (c) reaction temperature.

improve the catalytic performance (TON was 375) due to preventing the aggregation of ZnPc and increasing the catalytic active sites. 3.8.2. Effect of catalytic activity on reaction time The time dependence of catalytic oxidation of styrene was studied by performing the reaction of styrene (3.9 mmol) with 30% H2 O2 (27.3 mmol) in presence of 40 mg of the catalyst at 60 ◦ C with constant stirring (Fig. 9b). The conversion of styrene and the selectivity of styrene oxide were monitored at different reaction time. It was found that the conversion of styrene increased with the increase of reaction time because of the formation of reactive intermediate (O ZnPc), which is finally converted into the target product. The conversion of styrene has reached 94.0% after 8 h (TON from 258 up to 375), and the percentage of conversion has increased slowly after 10 h (TON was 387). The selectivity of styrene oxide remains high during the reaction process. Consequently, the best reaction time of styrene converted to styrene oxide was 8 h.

3.8.3. Effect of catalytic activity on amounts of catalyst Fig. 9c shows the effect of catalytic activity on the amounts of catalyst. The amount of catalyst has a significant effect on the oxidation of styrene. Lower conversion of styrene observed at lower quantity of catalyst may result from the fewer formation of intermediate (O ZnPc), while the increase of amount of catalyst could make more reactants contacted with active sites of catalyst. It was observed that 40 mg of catalyst is optimal for 3.9 mmol styrene at 60 ◦ C. The conversion as well as rate of reaction increased as the amount of the catalyst increased up to 40 mg. With further increase in the catalyst amount, increase in the reaction rate was limited and the selectivity towards styrene oxide was remained about 90%. 3.8.4. Effect of catalytic activity on reaction temperature The effect of temperature on the oxidation of styrene was also investigated, keeping other parameters fixed: amount of catalyst (40 mg), styrene (3.9 mmol), 30% H2 O2 (27.3 mmol) and reaction

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Fig. 11. Mechanistic approach of oxidation of styrene to benzaldehyde catalyzed by ZnPc–MWCNTs.

time (8 h). As shown in Fig. 9d, at lower temperature, the conversion was less (TON was 180 at 30 ◦ C), whereas as higher temperature (60 ◦ C) the maximum conversion was observed (TON was 375). However, the catalytic activity began to decrease at 70 ◦ C (TON was 374) because the high temperature accelerated the decomposition of H2 O2 . On the other hand, the selectivity of styrene oxide had little effect on temperature and the selectivity of styrene oxide remained almost constant. Hence, the optimization of condition was achieved at 60 ◦ C. 3.9. Catalytic mechanism of ZnPc–MWCNTs hybrid materials In styrene oxidation system, as shown in above, using 40 mg of ZnPc–MWCNTs at 60 ◦ C for 8 h is the optimum reaction condition. Under this condition, the conversion of styrene reached 94.0%, and the selectivity of styrene oxide was 90.6%, which are higher than previous literature reported [22]. The introduction of MWCNTs prevented the aggregation of ZnPc efficiently and increased the catalytic active sites. The possible reaction mechanism for styrene oxidation is shown in Fig. 11. The hydrogen peroxide reacted with acetonitrile under alkaline environment leads to the peroxyimidic acid CH3 C( NH) O O H which is known to be an active oxidant agent [23]. Moreover, the catalytic activity of metallophthalocyanine is generally attributed to its special electronic structure. In the case of styrene oxidation, the electrons are transferred from the filled orbitals of Zn2+ to the empty ␲* orbitals of oxygen to form the active zinc oxide intermediates (Zn O), which could enhance the interaction between the center of zinc and the oxygen. Compared with ZnPc, ZnPc–MWCNTs have higher catalytic activity because the active metal sites of ZnPc are fixed in the rigid phthalocyanine cores and MWCNTs can make catalytic active sites on the walls easy to contact with styrene. 4. Conclusion A novel hybrid catalyst, ZnPc–MWCNTs was facilely prepared by a ultrasonic impregnation method. The as-prepared ZnPc–MWCNTs materials were used as catalyst for the catalytic oxidation of styrene in the ZnPc–MWCNTs/H2 O2 system. It was found that the introduction of MWCNTs into the ZnPc catalyst

system could lead to a significant enhancement of catalytic activity. The catalytic oxidation results indicated that the maximum conversion of styrene and selectivity of styrene oxide reached 94% and 90%, respectively. Therefore, the MWCNTs as catalyst support for ZnPc could improve the catalytic performance due to preventing the aggregation of ZnPc and increasing the catalytic active sites.

Acknowledgements This work was financially supported by Open Fund of Key Laboratory of Regional Environment and Eco-remediation of Ministry of Education, China (SYU-KF-E-12), Open Fund of Key Laboratory of Contaminated Environment Control and Regional Ecology Safety, Shenyang University, China (SYU-KF-L-12), Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, China (BM2012110), Project for Six Major Talent Peaks of Jiangsu Province, China (2011-XCL-004) and Natural Science Foundation of Changzhou City, China (CJ20140053).

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