Immobilization of metalloporphyrin on a silica shell with bimetallic oxide core for ethylbenzene oxidation

Abstract

In this study, metalloporphyrin has been immobilized on a core–shell structured SiO₂@CeO₂ doped with transition metals such as Fe, Cu, Co and Mn. The as-prepared catalysts have been characterized via N₂ adsorption–desorption, XRD, TEM, FT-IR spectroscopy, and UV-vis spectroscopy. It is found that metalloporphyrin is anchored onto a SiO₂ shell with a thickness of about 20 nm and on a MO_x/CeO₂ core (M = Fe, Cu, Co and Mn) with a diameter about 120 nm, which may benefit the diffusion of substrates through the pores in the thin shell into the metal oxide cores and the formation of a synergistic effect between metalloporphyrin and metal oxides. Moreover, CoTPP-(MO_x/CeO₂)@SiO₂ catalysts (M = Fe, Cu, Co and Mn) exhibit different physical and chemical properties, such as surface areas, particle sizes and catalytic performances owing to the addition of transition metals into CeO₂. Moreover, the catalyst doped with Co exhibits a higher catalytic performance than the other catalysts for ethylbenzene oxidation. Thus, the addition of transition metals, such as Co, Cu, Fe and Mn, plays an important role in the catalytic performance of the catalysts for ethylbenzene oxidation via adjusting the physical and chemical properties of the core–shell catalysts.

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1. Introduction

There has been substantial growing interest in applying metalloporphyrin as a model of cytochrome P450 monooxygenases to the selective oxidation reactions of chemical compounds under mild conditions.^1–3 However, owing to easy degradation and difficult reuse of unsupported metalloporphyrin, heterogenization through immobilization of metalloporphyrin onto supports is employed to not only solve the problem of deactivation of the catalysts, but also provide a special microenvironment for oxidation reaction by supports.^4–6 These supports may include organic materials (e.g. ion exchange resin,^7,8 natural organic macromolecule polymer,⁹ peptides,¹⁰ metal–organic frameworks (MOFs)¹¹) and inorganic materials (e.g. silica gel,^12,13 zeolite,^14,15 montmorillonite,¹⁶ alumina¹⁷).

Because of the outstanding properties of core–shell structured materials, SiO₂@CeO₂ is used to support metalloporphyrin. Guo et al.⁴ found that these catalysts may exhibit a comparable higher catalytic performance for hydrocarbon oxidation than their homogeneous metalloporphyrin counterpart. Here, because of the ability of storing and releasing oxygen, CeO₂ has been used as the core of the core–shell structured supports.^18–24 Moreover, transition metals are usually added to CeO₂ to enhance the catalytic properties of the catalysts because transition metals improve the thermal stability and redox properties of CeO₂.^25–27 Blanco et al.²⁸ prepared low content Mn-doped CeO₂ composite using a co-precipitation method and found that cerium oxide can make manganese species stable at high oxidation state with high catalytic activity. Pérez-Alonso et al.²⁹ obtained a series of Fe–Ce catalysts using a co-precipitation method and found that the samples may form a solid solution of cerium and iron, that is to say, Fe cations were dissolved in CeO₂ cubic lattice. The interaction between the cerium–iron displays a higher rate of CO conversion in the synthesis of hydrocarbons in Fischer–Tropsch reactions.

In this study, we synthesized MO_x/CeO₂@SiO₂ with a core–shell structure to immobilize metalloporphyrin. The catalytic performance of the as-prepared catalysts for ethylbenzene oxidation with molecular oxygen as oxidant is used to explore the role of MO_x/CeO₂ and the synergistic effect between metalloporphyrin and the MO_x/CeO₂ core. Techniques such as N₂ adsorption–desorption, XRD, FT-IR spectroscopy, UV-vis spectroscopy, TEM and TPR were employed. The synthesis and reaction schematic is shown in Scheme 1.

Scheme 1 The synthesis and reaction schematic.

2. Experimental

2.1 Synthesis of cobalt(II) 5-(4-carboxyphenyl)-10,15,20-triphenyl porphyrin (CoTPP)

The compound was synthesized according to the literature.^5,30,31 220 mL of propanoic acid, 5.56 g of benzaldehyde and 2.62 g of 4-carboxybenzaldehyde were added into a three-neck flask and refluxed under stirring, and then 30 mL propanoic acid with 4.69 g pyrrole was added dropwise through a funnel. The mixture was refluxed for 1 h with stirring. The product was cooled overnight, filtered, washed with water, and then purified. 5-(4-Carboxyphenyl)-10,15,20-triphenyl porphyrin was obtained. 1.0 g of the obtained porphyrin was dissolved in 100 mL of N,N-dimethylformamide (DMF), and after the addition of 2.50 g of CoCl₂·6H₂O, the mixture was heated to reflux under stirring until the porphyrin was exhausted. After cooling overnight, the solution was filtered and washed repeatedly with deionized water, and the product, denoted as CoTPP, was obtained.

2.2 Synthesis of bimetallic oxides core with silica shell and silica solid

Surface modified FeO_x/CeO₂ was synthesized, according to the literature.^32,33 0.054 g of FeCl₂·3H₂O and 1.0786 g of Ce(NO₃)₃·6H₂O was added into a flask and dissolved in 50 mL of deionized water. With stirring, a NaOH (0.5 g/50 mL) solution was slowly added dropwise into the abovementioned solution. After stirring for 30 min, the original solution was aged for 18 hours at 80 °C. Then, the precipitate was filtered and washed twice with deionized water and anhydrous ethanol, and then dispersed in 40 mL aqueous solution containing 0.74 g of sodium citrate. After stirring at 90 °C for 3 h, the product was centrifuged and then redispersed in 40 mL of H₂O, 120 mL anhydrous ethanol and 5 mL ammonium hydroxide before adding 40 mL of ethanol solution containing 0.17 mL tetraethyl orthosilicate (TEOS). The product was centrifuged, washed with ethanol and water and dried after agitating for 6 h at 38 °C. The coated (FeO_x/CeO₂)@SiO₂ core–shell particles were obtained. Adopting the similar method, other (MO_x/CeO₂)@SiO₂ (M = Mn, Cu, and Co) core–shell particles and SiO₂ solid were synthesized.

2.3 Synthesis of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Mn and Co) and CoTPP–SiO₂ catalysts

The CoTPP modified with (3-aminopropyl)triethoxysilane (APTES) was prepared according to the literature,^4,5,30 9 mL of thionyl chloride, 100 mg of CoTPP and 30 mL of chloroform (CHCl₃) were added into a round bottom flask and refluxed for 3 h with stirring. After the reaction, excess thionyl chloride and chloroform were removed under reduced pressure. The obtained solid mixture was redissolved in 30 mL of chloroform, and a mixture of APTES (0.03 g), triethylamine (0.54 g) and chloroform (30 mL) was slowly added into the flask. The reaction was refluxed for 1 h.

About 0.24 g of (MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe or Mn) or SiO₂ was dispersed in 60 mL of toluene under ultrasonication for 20 min at room temperature. The CoTPP modified with APTES was then added dropwise into the flask with vigorous stirring at 75 °C. The reaction was completed within 24 h and then the precipitate was filtered, washed with toluene and dried under vacuum at 80 °C. The obtained samples were denoted as CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn).

2.4 Characterization of catalysts

Surface area was measured by nitrogen adsorption–desorption at −196 °C on an Autosorb-6b apparatus from Quanta Chrome Instruments. The samples were degassed at 100 °C for 12 h prior to the adsorption experiments. FT-IR spectra were obtained on a Vertex 70 (Bruker) Fourier transform infrared spectrometer. UV-vis diffuse reflectance spectra of solid samples were collected on a Shimadzu 2450 spectrophotometer. The morphology of samples was measured by transmission electron microscopy (TEM, F20) with an electron microscope operating at an 80 kV voltage. The phase analysis of samples was carried out by X-ray powder diffraction (XRD-6100).

2.5 Measurement of catalytic performance

Typically, 10 mL ethylbenzene and 30 mg catalyst were loaded in a 50 mL Teflon-lined stainless steel reactor and then sealed and heated to 120 °C for 5 h under 0.8 MPa O₂ pressure. The samples after reaction were analyzed by gas chromatography Shimadzu GC-2014 equipped with a capillary column (RTX-5, 30 m, ϕ 0.25 mm) with internal standard method using bromobenzene and 1,4-dichlorobenzene as references. The reused catalyst was obtained through recovering by centrifugation, washing with ethanol and drying at 80 °C in air.

3. Results and discussion

3.1 BET

Fig. 1 displays the N₂ adsorption–desorption isotherms of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe and Mn). All the catalysts, as shown in Fig. 1, exhibit type III isotherm.³⁴ The isothermal lines of the catalysts have a hysteresis loop with the P/P₀ position of the inflection point corresponding to a diameter in the micropores and mesopores, which suggests the easy diffusion of substrates onto the surface of the metal oxides in the core through the SiO₂ shell and improved catalytic performance of the samples.³⁴ Moreover, according to the N₂ adsorption–desorption isotherms, the surface areas and pore volumes of the catalysts are calculated and presented in Table 1. As illustrated in Table 1, CoTPP-(FeO_x/CeO₂)@SiO₂, compared with CoTPP-(MnO_x/CeO₂)@SiO₂, has a larger surface area (24 m² g⁻¹ vs. 16 m² g⁻¹). However, CoTPP-(MnO_x/CeO₂)@SiO₂ has a pore volume of 0.0636 cm³ g⁻¹, larger than that of CoTPP-(FeO_x/CeO₂)@SiO₂. This means more mesopores may exist in CoTPP-(MnO_x/CeO₂)@SiO₂ than in CoTPP-(FeO_x/CeO₂)@SiO₂. As a result, CoTPP-(CuO_x/CeO₂)@SiO₂ and CoTPP-(CoO_x/CeO₂)@SiO₂, compared with CoTPP-(FeO_x/CeO₂)@SiO₂, have a similar pore volume but lower surface area. It is easy to deduce that CoTPP-(CuO_x/CeO₂)@SiO₂ and CoTPP-(CoO_x/CeO₂)@SiO₂ possess more mesopores than CoTPP-(FeO_x/CeO₂)@SiO₂. Normally, the existence of mesopores on the shell enables the redox and oxygen storage capability of ceria core doped with transition metal to express and enhance the catalytic performance of the particles for ethylbenzene oxidation.

Fig. 1 N₂ adsorption–desorption isotherms of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe and Mn) catalysts.

Table 1 The surface area and pore volume of the samples

Samples	Surface area/m^2 g^−1	Pore volume/cm^3 g^−1
CoTPP-(FeOx/CeO2)@SiO2	24	0.040
CoTPP-(MnOx/CeO2)@SiO2	16	0.064
CoTPP-(CoOx/CeO2)@SiO2	14	0.035
CoTPP-(CuO/CeO2)@SiO2	9	0.034

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3.2 XRD

In Fig. 2, the characteristic peaks of ceria in the XRD patterns of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn) are observed at 28.5°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7° and 79.01°, which efficiently match with the standard spectrum of cerianite (space groups: Fm3̄m, JCPDS no. 34-0394).²⁹ Besides these, no more peaks can be found, including the peak of MO_x (M = Fe, Cu, Co and Mn), which is usually easily observed. This indicates that MO_x (M = Fe, Cu, Co and Mn) are likely present in an amorphous state and/or a relatively high dispersion.³⁸

Fig. 2 The XRD patterns of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn) catalysts.

The particle sizes of the metal oxides are obtained via the following formula:

D_(hkl) = Kλ/β cos θ

where (hkl) is the plane of ceria (herein we adopt (111) plane), β is the integral half high width, which should be converted into radians, and θ is the diffraction angle. Scherrer constant K is assumed to be 0.89, and the X-ray wavelength λ is set to 0.154056 nm. The theoretical calculation results show that the core of (CoO_x/CeO₂)@SiO₂ is assembled by numerous 10.7 nm nanoparticles, and for (CuO/CeO₂)@SiO₂, (FeO_x/CeO₂)@SiO₂ and (MnO_x/CeO₂)@SiO₂, the particle sizes are 9.4 nm, 11.6 nm and 8.9 nm, respectively. It is well established in bimetallic systems that the less reducible metal inhibits the aggregation of the easily reduced metal.^28,29,39 Apparently, these transition metals doped in ceria play a crucial role in the particle sizes of the catalysts.

3.3 TEM

The TEM images are presented in Fig. 3 and the core–shell structure is clearly exhibited. Obviously, it can be found that the bimetallic oxide core is well coated by a thin SiO₂ shell with a thickness of about 20 nm. However, because of the electrostatic repulsion between the surface of the bimetallic oxide and silica, transition metals doped in CeO₂ particles may affect the formation of core–shell structure, which may not allow the coating of silica shell on CeO₂ particles.^4,35,36 As shown in Scheme 2, we make use of sodium citrate as a surfactant to modify and adjust the electrostatic properties of the bimetallic oxide cores, which may benefit the growth of silica to form a core–shell structure.⁴ Furthermore, it can be found that the nano metal oxide particles with a diameter of about 10 nm, as calculated according to the XRD data, may aggregate to form larger particles 120 nm in diameter, as measured according to the TEM data. Obviously, a multi-particle core with a diameter of 120 nm might be formed via the accumulation of single particles having a size of 10 nm. In this study, the structure of the catalysts with a shell of about 20 nm and a core of 120 nm can enhance the catalytic performance of the core–shell catalysts because the thin shell may benefit the diffusion of substrates into the metal oxide core. This will lead to a synergistic effect between the metalloporphyrin grafted in the shell and the metal oxides in the core.

Fig. 3 TEM images of (a) CoTPP-(CoO_x/CeO₂)@SiO₂, (b) CoTPP-(CuO_x/CeO₂)@SiO₂, (c) CoTPP-(FeO_x/CeO₂)@SiO₂, and (d) CoTPP-(MnO_x/CeO₂)@SiO₂.

Scheme 2 Formation of citrate-modified MO_x/CeO₂ (M = Fe, Cu, Mn and Co) microspheres.

3.4 FT-IR spectroscopy

Fig. 4 displays the FT-IR spectra of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe and Mn) catalysts. As shown in Fig. 4a, there is a very large overlapped band at 3418 cm⁻¹, which is ascribed to the bands of adsorbed water or N–H group in the sample. In a comparison of Fig. 4a and b, the adsorption bands observed at 1090 cm⁻¹ or lower wavenumbers are ascribed to the stretching vibrations of Si–O–Si and Si–O–H in the supports.^4,5 The bands at 1404 cm⁻¹ and 3410 cm⁻¹ are attributed to the bending and stretching vibrations of N–H group, respectively.^4,5 In addition, the bands of N–H groups exhibit a blue-shift of about 29 cm⁻¹, which may be caused by the surrounding environment. The adsorption bands appearing at 1624 cm⁻¹ due to the C=O group in amide (N–C=O) demonstrate the formation of amide bonds between metalloporphyrin and the support via dehydrolysis reaction among the functional groups of COOH and NH₂.^4,5 This indicates that metalloporphyrin is covalently bonded to the silica shell. Moreover, FT-IR spectroscopy can only measure the surface properties of the shell and not the core in the catalysts, and the chemical properties on the shell of the catalysts are evidently similar. Hereafter, there is no difference among the FT-IR spectra of the CoTPP-(MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe and Mn) catalysts.

Fig. 4 FT-IR spectra patterns of CoTPP-(MO_x/CeO₂)@SiO₂ (a) and (MO_x/CeO₂)@SiO₂ (b) (M = Co, Cu, Fe and Mn) catalysts.

3.5 UV-vis spectroscopy

UV-vis spectroscopy can be used to verify the existence of porphyrin rings due to their characteristic S and Q bands. For example, cobalt porphyrin, as shown in Fig. 5, exhibits bands at 414 nm and 532 nm, which are attributed to Soret band and Q band of metalloporphyrin.^4,5,30 As for CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn), two additional bands exist at 466 nm and 675 nm when compared with (MO_x/CeO₂)@SiO₂ carriers in Fig. 5b, which can be ascribed to S and Q bands of immobilized cobalt porphyrin. However, due to the influence of supports, the S and Q bands of immobilized cobalt porphyrin get red-shifted.³⁷ It is evident that metalloporphyrin has been anchored onto the supports. Moreover, the S and Q bands of CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn) are very similar. This indicates that the cores in the catalysts have little influence on the chemical properties of the metalloporphyrin on the shell owing to the coating of silica shells.

Fig. 5 UV-vis spectra patterns of CoTPP-(MO_x/CeO₂)@SiO₂ (a) and (MO_x/CeO₂)@SiO₂ (b) samples (M = Co, Cu, Fe and Mn).

3.6 Measurement of catalytic performance

The selective oxidation of ethylbenzene using molecular oxygen as an oxidant is employed to measure the performance of the catalysts. As shown in Table 2, the blank, i.e. autocatalysis system, has the lowest ethylbenzene conversion of 6.9% compared with CoTPP and (MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe and Mn) catalysts, whose ethylbenzene conversions reach 9.8%, 13.1%, 13.6%, 17.2% and 13.3%, respectively. Evidently, both CoTPP and (MO_x/CeO₂)@SiO₂ can accelerate the ethylbenzene oxidation reaction rate to some extent; thus, they can enhance the conversion of ethylbenzene. However, when CoTPP was immobilized on (MO_x/CeO₂)@SiO₂ particle to form a CoTPP-(MO_x/CeO₂)@SiO₂ catalyst, the activity of the catalyst was remarkably enhanced. Namely, the ethylbenzene conversion reaches at least up to 28% or even higher in the first run. Moreover, on comparing CoTPP-(MO_x/CeO₂)@SiO₂ with CoTPP–SiO₂, it was found that the participation of bimetallic oxide (MO_x/CeO₂) considerably increased the conversion of ethylbenzene. This should be ascribed to the synergistic effect between metalloporphyrin on the shell and the metal oxides in the core. In addition, metalloporphyrin may suffer from leaching and oligomerization during the oxidation reaction, which may result in the deactivation of the catalysts. Herein, it can be clearly observed that the CoTPP-(MO_x/CeO₂)@SiO₂ (M = Co, Cu, Fe and Mn) catalysts still can retain the selectivity to acetophenone to about 77% even when used up to five times. Moreover, with respect to the catalytic activity of CoTPP-(CoO_x/CeO₂)@SiO₂ sample, the extent it reduced in ethylbenzene conversion is slower than other samples; thus, it possesses relatively higher stability and activity for ethylbenzene oxidation. This may be due to the influence of cobalt doping on the physical and chemical properties of the catalyst, such as particle size³⁹ and redox and oxygen storage capability (OSC) of ceria.⁴⁰ As for CoTPP-(FeO_x/CeO₂)@SiO₂, the ethylbenzene conversion is almost similar to that of (FeO_x/CeO₂)@SiO₂, which suggests the deactivation of CoTPP on the surface of the catalyst. Nevertheless, the ethylbenzene conversions over CoTPP-(MO_x/CeO₂)@SiO₂ (M = Co, Cu and Mn) catalysts are higher than those over their corresponding supports. It can be deduced that CoTPP over (MO_x/CeO₂)@SiO₂ (M = Co, Cu and Mn) has not been completely deactivated. All in all, the addition of transition metal, such as Co, Cu and Mn, has a crucial effect on the activities of the catalysts for ethylbenzene oxidation owing to their influence on the physical and chemical properties of the catalyst.

Table 2 Comparison of catalytic performance of the catalysts for ethylbenzene oxidation^f

Samples	R1		R2		R3		R4		R5
	C^b (%)	S^c (%)	C (%)	S (%)	C (%)	S (%)	C (%)	S (%)	C (%)	S (%)
a 30 mg catalyst.b Conversion of ethylbenzene (%).c Selectivity to acetophenone (%).d 26 mg (MOx/CeO2)@SiO2.e 4 mg CoTPP.f Reaction conditions: ethylbenzene 10 mL, O2 0.8 MPa pressure, temperature 120 °C.
^aCoTPP-(FeOx/CeO2)@SiO2	28.2	76.6	19.9	77.2	17.3	77.5	16.6	77.1	16.8	78.0
^aCoTPP-(CuOx/CeO2)@SiO2	30.6	71.4	22.0	77.5	15.2	79.6	14.7	79.7	14.2	78.6
^aCoTPP-(CoOx/CeO2)@SiO2	28.7	76.8	22.9	77.5	17.6	77.1	17.9	75.2	16.1	77.9
^aCoTPP-(MnOx/CeO)@SiO2	28.8	75.8	17.3	76.0	16.6	76.6	15.3	76.6	15.0	76.3
CoTPP–SiO2	30.1	75.0	13.2	79.4	11.7	77.6	9.7	77.5	10.2	77.5
^d(FeOx/CeO2)@SiO2	17.2	74.8	—		—		—		—
^d(CuOx/CeO2)@SiO2	13.6	76.4	—		—		—		—
^d(CoOx/CeO2)@SiO2	13.1	75.8	—		—		—		—
^d(MnOx/CeO2)@SiO2	13.3	75.3	—		—		—		—
^eCoTPP	9.8	76.7	—		—		—		—
Blank	6.9	76.8	—		—		—		—

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4. Conclusions

In summary, we have synthesized CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn) with a core–shell structure. In the catalysts, CoTPP is anchored onto a SiO₂ shell with a thickness of about 20 nm and on a MO_x/CeO₂ core with a diameter of about 120 nm, which may benefit the diffusion of substrates through the pore in the thin shell into the metal oxide cores and a synergistic effect between CoTPP and metal oxides. Moreover, because of the addition of transition metals into CeO₂, CoTPP-(MO_x/CeO₂)@SiO₂ (M = Fe, Cu, Co and Mn) present different physical and chemical properties such as surface areas, particle sizes and catalytic performance. In particular, CoTPP-(CoO_x/CeO₂)@SiO₂ exhibits a higher catalytic performance than the other catalysts for ethylbenzene oxidation. Herein, the addition of transition metals, such as Co, Cu, Fe and Mn, has a crucial effect on the properties of the catalysts for ethylbenzene oxidation owing to their influence on the physical and chemical properties of the catalyst.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (no. 21103045, 1210040, 1103312) and the Fundamental Research Funds for the Central Universities.

Notes and references

1. L. J. Que and W. B. Tolman, Nature, 2008, 455, 333.
2. R. A. Sheldon, Green Chem., 2007, 9, 1273.
3. D. Mansuy, Catal. Today, 2008, 138, 2.
4. X. Guo, Y. Y. Li, D. H. Shen, J. Gan, M. Tian and Z. G. Liu, Appl. Catal., A, 2012, 413, 30.
5. D. H. Shen, L. T. Ji, Z. G. Liu, W. B. Sheng and C. C. Guo, J. Mol. Catal. A: Chem., 2013, 379, 15.
6. S. Yuvaraj, M. Gabriele, P. Giuseppe, M. Eugenio, P. Roberto, D. A. Arnaldo and D. N. Corrado, Adv. Mater. Lett., 2012, 3, 442.
7. T. Ishida, S. Okamoto, R. Makiyama and M. Haruta, Appl. Catal., A, 2009, 353, 243.
8. O. Junichi, M. Masaki, I. Akimasa, K. Tatsuya, H. Hirotaka, M. Noriko, T. Yoshimasa and S. Yutaka, Anal. Sci., 1991, 7, 555.
9. X. Y. Yang, S. H. Lin and M. R. Wiesner, J. Hazard. Mater., 2014, 264, 161.
10. C. J. Taggart, E. Z. Welch, M. F. Mulqueen, V. B. Dioguardi, A. G. Cauer, B. Kokona and R. Fairman, Biomacromolecules, 2014, 15, 4544.
11. M. Zhao, S. Ou and C.-D. Wu, Acc. Chem. Res., 2014, 47, 1199.
12. F. L. Benedito, S. Nakagaki, A. A. Saczk, P. G. P. Zamora and C. M. M. Costa, Appl. Catal., A, 2003, 250, 1.
13. S. Nakagaki, G. K. B. Ferreira, A. L. Marçal and K. J. Ciuffi, Curr. Org. Synth., 2014, 11, 67.
14. V. R. Rani, M. R. Kishan, S. J. Kulkarni and K. V. Raghavan, Catal. Commun., 2005, 6, 531.
15. M. J. F. Calvete, M. Silva, M. M. Pereira and H. D. Burrows, RSC Adv., 2013, 3, 22774.
16. A. M. Machado, F. Wypych, S. M. Drechsel and S. Nakagaki, J. Colloid Interface Sci., 2002, 254, 158.
17. M. R. D. Nunzio, B. Cohen, S. Pandey, S. Hayse, G. Piani and A. Douhal, J. Phys. Chem. C, 2014, 118, 11365.
18. X. W. Liu, K. B. Zhou, L. Wang, B. Y. Wang and Y. D. Li, J. Am. Chem. Soc., 2009, 131, 3140.
19. L. J. Yang, S. Zhou, T. Ding and M. Meng, Fuel Process. Technol., 2014, 124, 155.
20. P. Gawade, B. Bayram, A. M. C. Alexander and U. S. Ozkan, Appl. Catal., B, 2012, 128, 21.
21. X. Guo, Y. Y. Li, D. H. Shen, Y. Y. Song, X. Wang and Z. G. Liu, J. Mol. Catal. A: Chem., 2013, 367, 7.
22. G. R. Rao, S. K. Meher, B. G. Mishra and P. H. K. Charan, Catal. Today, 2012, 198, 140.
23. Z. L. Wu, M. J. Li, J. Howe, H. M. Meyer and S. H. Overbury, Langmuir, 2010, 26, 16595.
24. Z. L. Wu, M. J. Li and S. H. Overbury, J. Catal., 2012, 285, 61.
25. H. Li, G. F. Wang, F. Zhang, Y. Cai, Y. D. Wang and I. Djerdj, RSC Adv., 2012, 2, 12413.
26. P. Zhao, C. N. Wang, F. He and S. T. Liu, RSC Adv., 2014, 4, 45665.
27. B. K. Cho, J. Catal., 1991, 131, 74.
28. G. Blanco, M. A. Cauqui and J. J. Delgado, Surf. Interface Anal., 2004, 36, 752.
29. F. J. P. Alonso, M. L. Granados and M. Ojeda, Chem. Mater., 2005, 17, 2329.
30. X. Guo, D. H. Shen, Y. Y. Li, M. Tian, Q. Liu, C. C. Guo and Z. G. Liu, J. Mol. Catal. A: Chem., 2011, 351, 174.
31. Z. G. Liu, L. T. Ji, X. L. Dong, Z. Li, L. L. Fu and Q. A. Wang, RSC Adv., 2015, 5, 6259.
32. Z. G. Liu, R. X. Zhou and X. M. Zheng, J. Mol. Catal. A: Chem., 2007, 267, 137.
33. T. Aubert, F. Grasset, S. Mornet, E. Duguet, O. Cador, S. Cordier, Y. Molard, V. Demange, M. Mortier and H. Haned, J. Colloid Interface Sci., 2010, 341, 201.
34. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquérol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603.
35. T. Tago, S. Tashiro, Y. Hashimoto, K. Wakabayashi and M. Kishida, J. Nanopart. Res., 2003, 5, 55.
36. F. Grasset, R. Marchand, A. M. Maric, D. Fauchadour and F. Fajardie, J. Colloid Interface Sci., 2006, 299, 726.
37. G. Huang, T. M. Li, S. Y. Liu and M. G. Fan, Appl. Catal., A, 2009, 371, 161.
38. Z. G. Liu, S. H. Chai, A. Binder, Y. Y. Li, L. T. Ji and S. Dai, Appl. Catal., A, 2013, 451, 282.
39. Z. Lendzion-Bielun, M. M. Bettahar and S. Monteverdi, Catal. Commun., 2010, 11, 1137.
40. A. B. Kehoe, D. O. Scanlon and G. W. Watson, Chem. Mater., 2011, 23, 4464.

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