Periodic mesoporous organosilicas functionalized with iron(III) complexes: preparation, characterization and catalysis on direct hydroxylation of benzene to phenol

Abstract

Three reusable, recoverable, iron(III) complexes incorporating periodic mesoporous organosilica PMO-Fe₁–PMO-Fe₃ were prepared. The composition, morphology, and textural properties with ordered mesoporous channel structure of the prepared materials were characterized by FT-IR, ICP, TGA, BET, SEM and TEM. Their catalytic performances were tested in the direct hydroxylation of benzene to phenol in liquid phase using H₂O₂ as oxidant. As expected, the incorporation of active-iron-centers within hydrophobic periodic mesoporous organosilica prevents over-oxidation of the designed phenol and hence significantly improves the selectivity of phenol with nearly two-folds compared with their corresponding homogeneous iron precursors Fe₁–Fe₃. In addition, these heterogeneous catalysts can be recovered and reused for at least four times without any significant loss of catalytic activity and selectivity.

---

Introduction

The one-step hydroxylation of benzene to phenol using green oxidants such as molecular O₂,^1–8 H₂O₂ ^9–21 and N₂O^22–25 is of great importance for phenol production and has consistently attracted much attention from both economic and eco-friendly standpoints. It is not only because phenol is a valuable chemical intermediate in petrochemical, agrochemical, polymer and plastic industries,²⁶ but also due to the disadvantages of current industrial process, three-step cumene process, with low yield and high energy consumption.^27,28 Among these oxidants, H₂O₂ possesses an overwhelming advantage in the viewpoint of both environmentally benign process and economic efficiency because of relatively mild reaction conditions for catalytic hydroxylation of benzene.^9–21

In the past several decades, a variety of heterogeneous and homogeneous metal-based catalysts have been developed for the direct hydroxylation of benzene with hydrogen peroxide as the oxidant.^9–21,29 In particular, the introduction of iron, copper and vanadium oxides and their complexes into mesoporous supports, such as titanium silicalite (TS-1),^30,31 silica materials,^10,19,32 graphitic carbon nitride,^33–37 activated carbon,^38–40 and carbon nanotube⁴¹ has been widely studied. However, it remains challenging to achieve selective hydroxylation of benzene to phenol with H₂O₂ because of over-reaction to produce unwanted by-products such as p-benzoquinone and catechol. Increasing the benzene conversion always results in the poor selectivity of phenol by varying reaction conditions, for example, prolonging reaction time. On the contrary, highly selective hydroxylation could be achieved at the expense of the activity of the catalysts.⁴² Furthermore, the cost of the catalysts is not favoured for scaling up in industry. Therefore, exploring novel heterogeneous catalytic systems for the direct hydroxylation to phenol with appropriate recyclability, activity and selectivity poses still a great challenge for chemists.

Periodic mesoporous organosilicas (PMOs) are a new class of silica based mesoporous materials containing organic groups as integral part of their structures, which can be synthesized by simultaneous hydrolysis and condensation of alkoxysilane precursors bearing organic and functional groups in the presence of structure-directing agents. This category of materials possess features of high surface area, tunable pore size/volume, strong lipophilicity and robustness.^43,44 These features arouse increasingly interest in catalytic community.^43–45 By metallating PMO materials bearing functional groups, a catalytic system functionalized with transition metal complex can be assembled. Such a catalytic system is recyclable and reusable while partially possessing the catalytic feature of a transition metal complex.^11,46–49 The pore-structure and high surface areas of PMO materials increases the robustness of the system by minimizing the bleaching of the transition metal complex which is chemically assembled into the materials. Therefore, PMO-based catalytic systems are promising heterogeneous catalysts for many chemical processes involving transition metal complexes.^11,46–49 However, despite the advantages of PMO as mentioned above, the studies in constructing catalytic systems based on PMO for direct hydroxylation of benzene to phenol has rarely been reported in the literature.¹¹

Previously, we reported iron and copper complexes with multidentate ligands for the catalytic hydroxylation of benzene.^13,17,18 These catalytic systems showed decent catalytic performances with benzene conversion of nearly 50%. Due to over oxidation, the selectivity was less than 30%, which is far from ideal. Due to the pore-structure of PMOs and the lipophilicity of their pores, benzene ought to be favoured to retain in the pores while the product phenol which is less lipophilic than benzene tends to leave the pores and thus diffuses away from the catalytic centers. Therefore, using PMOs as supports loaded chemically with transition metal complexes may be an approach to improve the selectivity.¹¹ Herein, we report the preparation and characterization of three PMOs functionalized with iron(III) complexes, PMO-Fe₁, PMO-Fe₂, and PMO-Fe₃. The catalytic activities of the resulting materials were evaluated by the hydroxylation of benzene to phenol under mild condition with H₂O₂ as the oxidant. Compared to their iron(III) precursors, these iron-functionalized PMOs show comparable activity but with higher yield and selectivity of phenol. The results indicate that the successful incorporation of the iron(III) complexes into PMOs could result in significant improvement in both the selectivity and the durability of the catalysts.

Experimental

Materials and instrumentation

The organic solvents used in this work were appropriately dried when necessary. Fe(III) complexes Fe₁, Fe₂ and 3-iodopropyltrimethoxysilane (IPTES) were synthesized following literature procedures.^13,50 Tetraethylorthosilicate (TEOS) (98%), n-cetyltri-methylammonium bromide (CTAB) (98%), aminopropyltriethoxysilane (APTES) and ferric acetylacetonate (Fe₃) were provided by Aladdin. All other chemicals and solvents were of AR grade and were used as received from commercial suppliers. FT-IR spectra were recorded on an Agilent 640 using a CaF₂-cell with a spacer of 0.1 mm. Thermo-gravimetric analyses were carried out on a STA-409PC using an oxidant atmosphere (air, 80 mL min⁻¹) with a heating program consisting of a heating ramp of 10 K per minute from 303 to 1073 K. N₂ adsorption–desorption isotherms were recorded with a Micromeritics ASAP2020 automated sorption analyzer. Specific surface areas were calculated from the adsorption data in the low pressures range using the Brunauer–Emmett–Teller (BET) model. Pore size was determined following the Barret–Joyner–Halenda (BJH) method. SEM images were conducted on a S-4800 microscope. TEM images were obtained using Jeol JEM-2001F microscope. Varian 710-ES series ICP-OES was used for performing trace metal analysis.

Synthesis of PMO-Fe₁ and PMO-Fe₂

In a typical synthesis of PMO-Fe, A solution of Fe₁ (1.57 g, 5.8 mmol) in MeCN (15 mL) was added in a mixture of K₂CO₃ (1.24 g, 9.0 mmol) and ICH₂CH₂CH₂Si(OEt)₃ (IPTES, 3.32 g, 10 mmol), and the reaction mixture was stirred at 70 °C under N₂ overnight. After the reaction, removal of K₂CO₃ and MeCN gave a crude product Fe₁-Si(OEt)₃. Fe₁-Si(OEt)₃ and tetraethylorthosilicate (TEOS) were used as silica sources, and cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent. A gel was formed with a particular molar ratio of Si : CTAB : NH₃ (25%): H₂O : EtOH = 1.0 : 0.14 : 4.5 : 134 : 11.8. CTAB (1.5 g, 4.1 mmol) was dissolved in H₂O (70 mL), and then NH₃ solution (20 mL, 25%) was added. The mixture solution was stirred at 40 °C for 30 min to give a clear solution. A premixed solution of Fe₁-Si(OEt)₃ (0.57 g, 1 mmol) and TEOS (5.80 g, 28 mmol) in EtOH (40 mL) was added to the above solution under vigorous stirring. After 3 h at 60 °C, the resulted gel was transferred to a polyethylene container, which was then heated to 90 °C for 4 days. The solid obtained was washed with H₂O and dried in air at 60 °C. The structure-directing agent was finally removed by extraction of the solid with dilute ethanolic NH₄NO₃ solution (100 mL, 6 g L⁻¹) at 70 °C for 1 day. PMO-Fe₁ (4.63 g, 72.9%) was obtained by filtration and then dried in vacuo. The synthesis of PMO-Fe₂ (4.47 g, 68.9%) was same as the synthesis of PMO-Fe₁, except that complex Fe₂ instead of complex Fe₁.

Synthesis of PMO-Fe₃

Ferric acetylacetonate (Fe₃, 1.41 g, 4.0 mmol) was added in a MeCN solution (30 mL) of NH₂CH₂CH₂CH₂Si(OEt)₃ (APTES, 2.21 g, 10 mmol), and the reaction mixture was stirred at 70 °C under N₂ overnight. After the reaction, removal of solvent gave a crude product Fe₃-Si(OEt)₃. Fe₃-Si(OEt)₃ and tetraethylorthosilicate (TEOS) were used as silica sources, and cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent. A gel was formed with a particular molar ratio of Si : CTAB : NH₃ (25%): H₂O : EtOH = 1.0 : 0.14 : 4.5 : 134 : 11.8. CTAB (1.5 g, 4.1 mmol) was dissolved in H₂O (70 mL), and then NH₃ solution (20 mL, 25%) was added. The mixture solution was stirred at 40 °C for 30 min to give a clear solution. A premixed solution of Fe₃-Si(OEt)₃ (0.96 g, 1 mmol) and TEOS (5.80 g, 28 mmol) in EtOH (40 mL) was added to the above solution under vigorous stirring. After 3 h at 60 °C, the resulted gel was transferred to a polyethylene container, which was then heated to 90 °C for 4 days. The solid obtained was washed with H₂O and dried in air at 60 °C. The structure-directing agent was finally removed by extraction of the solid with dilute ethanolic NH₄NO₃ solution (100 mL, 6 g L⁻¹) at 70 °C for 1 day. PMO-Fe₃ (5.04 g, 74.6%) was obtained by filtration and then dried in vacuo.

Catalytic assessment

A typical experiment is as follows: benzene (0.9 mL, 10 mmol), acetonitrile (3.8 mL) and PMO-Fe (60 mg) were placed into the reaction vessel (10 mL), which is equipped with cooling condenser and placed in an oil-bath. The reaction was heated at an appropriate temperature for period of a time. When the reaction temperature reached 70 °C, aqueous H₂O₂ (30 wt%, 1.5 mL, 15 mmol) was slowly and carefully added in one-go. When the reaction was stopped, the volume was calibrated to 10 mL with CH₃CN. To the calibrated reaction solution was added MgSO₄ (4.0 g) to remove the water in the reaction before being analyzed by gas chromatography (GC 7820A) with a packed column of Restek capillary SE-54 and using toluene as an internal standard. The temperature of the GC column was set at 60 °C for 1 min and then was programmed to rise to 160 °C at the rate of 10 °C min⁻¹. Under the employed conditions, the retention time for benzene, toluene, benzoquinone and phenol were 3.3 ± 0.2 min, 4.4 ± 0.2 min, 6.6 ± 0.2 min, and 7.4 ± 0.2 min, respectively.

In establishing the calibration curves for the quantitative analysis of both benzene and phenol, 5 standard samples containing quantitative benzene, phenol and toluene, respectively, were prepared in appropriate ratios, heated and worked up in the same manner as that for the above reaction. The samples were analyzed by GC with the same GC parameters. Plotting the ratios of area-readings of benzene and phenol over that of toluene against their quantities in the standard samples produced two linear equations, y_b = 0.0053x_b + 0.1272 (R² = 0.9998) and y_p = 0.0040x_p − 0.0105 (R² = 0.9988) for benzene and phenol, respectively, where y is designated as the area-reading, x as the quantity of the substance, and b stands for benzene and p for phenol. The two calibrating equations were used for the quantitative analysis of the two substances in all samples. To calibrate any uncertainty resulting from batches, a synthesized sample was always analyzed along with the reaction samples in each batch. Reaction yield/selectivity and conversion rate are calculated as follows: phenol (mmol)/benzene-reacted (mmol) × 100% and benzene-reacted (mmol)/benzene initially used (mmol) × 100%, respectively.

Results and discussion

Synthesis and characterization of catalysts

Iron complexes Fe₁ and Fe₂ were synthesized following the procedures we reported previously.¹³ To prepare a PMO, it is crucial to synthesize a precursor with terminal alkoxysilane. Thus, 3-iodopropyltrimethoxysilane (IPTES) and aminopropyltriethoxysilane (APTES) were employed to react with Fe₁, Fe₂ and Fe₃ via nucleophilic substitution or Schiff base condensation, which afforded Fe₁-Si(OEt)₃, Fe₂-Si(OEt)₃ and Fe₃-Si(OEt)₃, respectively (Scheme 1). In order to avoid unwanted polymerization of silane, these terminal alkoxysilane-functional precursors were synthesized under inert conditions and were used without any purification for next hydrothermal process by using cetyltrimethylammoniumbromide (CTAB) as the structure-directing template and tetraethylorthosilicate (TEOS) as silica sources under an appropriate concentration of ammonia, as shown in Fig. 1. The molar ratio of reactants, ammonia and solvent is referred the optimal one reported by Zhao and coworkers,¹¹ which is Si : CTAB : NH₃ (25%): H₂O : EtOH = 1.0 : 0.14 : 4.5 : 134 : 11.8. Under this particular molar ratio, iron contents for the prepared PMOs, PMO-Fe₁–PMO-Fe₃, could achieved to more than 7.84 mg g⁻¹ as determined by inductively coupled plasma (ICP), Table 1. The ICP results support the successful incorporation of iron complexes into PMOs.

Scheme 1 Synthesis of organosilica precursors Fe₁-Si(OEt)₃, Fe₂-Si(OEt)₃ and Fe₃-Si(OEt)₃.

Fig. 1 The synthesis procedure of iron-functionalized PMOs: (a) MeCN, K₂CO₃, 70 °C, N₂; (b) CTAB, TEOS, 90 °C, 4 d; (c) removal of CTAB. IPTES = 3-iodopropyltrimethoxysilane and APTES = aminopropyltriethoxysilane.

Table 1 ICP results of Fe content in iron-functionalized PMOs^a

PMO-Fe	Mass (mg)	Fe content (mmol)	Fe content (mg g^−1)
a Iron content after four cycles is placed in brackets.
PMO-Fe1	60	0.0097 (0.0093)	9.05 (8.68)
PMO-Fe2	60	0.0094 (0.0090)	8.77 (8.40)
PMO-Fe3	60	0.0084 (0.0081)	7.84 (7.56)

---

The FTIR spectra (Fig. 2) of PMO-Fe₁–PMO-Fe₃ showed the characteristic bands of the PMO-type materials around 3420, 1630 and 1040 cm⁻¹ for ν(O–H), δ(O–H) and ν(Si–O), respectively.⁵¹ The relatively weak bands between 3100–2800 cm⁻¹ were assigned to asymmetric and symmetric stretching vibrations of C–H bonds. The peaks in the range from 1620 to 1400 cm⁻¹ are assigned to the organic unit in complexes Fe₁–Fe₃. In particular, a new broad band centered at 1620 cm⁻¹ in PMO-Fe₃ are due to the combinational vibration of newly formed C=N bonds, and a complete disappearance of the band at 1572 cm⁻¹, which is assigned to the initial C=O bonds in Fe₃. All these observations demonstrated the successful incorporation of iron complexes within the periodic mesoporous organosilica.

Fig. 2 FTIR spectra of iron-functionalized PMOs: (a) PMO-Fe₁, (b) PMO-Fe₂, (c) PMO-Fe₃ and iron complexes: (d) Fe₁, (e) Fe₂, (f) Fe₃.

The TGA profiles of PMO-Fe₁–PMO-Fe₃ showed three regions of weight loss (Fig. 3). At temperature below 150 °C, a slight weight loss of these PMOs was presumably related to the removal of water and organic solvents remained from extraction processes. A second region of weight loss followed at temperatures between 200 and 390 °C, which may be attributed to the organic species in the embedded complexes. In the range of 390–700 °C, the weight loss is mainly because of the decomposition of the trimethylsilyl groups and the condensation of silanols of the silica framework.⁵² And the total weight loss up to 800 °C of the PMOs are in the range of 27–30%.

Fig. 3 TGA analysis of iron-functionalized PMOs: (a) PMO-Fe₁, (b) PMO-Fe₂, (c) PMO-Fe₃.

The isothermal N₂ adsorption/desorption measurements were carried out for the three iron-functionalized PMOs to determine their surface areas and pore size distributions (Fig. 4). By using the BJH model, the pore size distribution of PMO-Fe derived from the adsorption branch is presented in the Fig. 4(A). In the mesopore range, a strong peak centered at 2.3, 2.0 nm for PMO-Fe₂, PMO-Fe₃ can be observed respectively. From the N₂ adsorption isotherm under the lowest pressure, the BET surface areas of PMO-Fe₁, PMO-Fe₂, and PMO-Fe₃ were measured to be 684, 988, and 783 m² g⁻¹, respectively (Table 2). The increase in the surface area with the order of PMO-Fe₂ > PMO-Fe₃ > PMO-Fe₁ is due to smaller pore diameter of both PMO-Fe₂ and PMO-Fe₃ than PMO-Fe₁ with analogous total pore volumes (Table 2). The NLDFT equilibrium model for the pore size distributions indicates the presence of mesopores in all the PMOs, and the average pore diameters for PMO-Fe₁, PMO-Fe₂, and PMO-Fe₃ were found to be 5.0, 2.9, and 4.1 nm, respectively (Table 2).

Fig. 4 Pore distribution (A) and N₂ adsorption-desorption (B) of iron-functionalized PMOs: (a) PMO-Fe₁, (b) PMO-Fe₂, (c) PMO-Fe₃.

Table 2 BET surface area, pore volume and NLDFT pore diameter of PMO-Fe₁–PMO-Fe₃

PMO-Fe	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	NLDFT pore diameter (nm)
PMO-Fe1	684	0.75	5.0
PMO-Fe2	988	0.64	2.9
PMO-Fe3	783	0.75	4.1

---

The morphology of these iron-functionalized PMOs was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From the SEM images (Fig. 5a–f), all the PMOs are granular in morphology with particle sizes ranging between 500 nm and 600 nm. TEM analysis (Fig. 5g–i) shows mesostructural morphologies for these iron-functionalized PMOs, which are in accordance with the NLDFT calculation from N₂ adsorption/desorption measurements.

Fig. 5 SEM of iron-functionalized PMOs: (a and b) PMO-Fe₁, (c and d) PMO-Fe₂, (e and f) PMO-Fe₃ and TEM of iron-functionalized PMOs: (g) PMO-Fe₁, (h) PMO-Fe₂, (i) PMO-Fe₃.

Catalytic test

Catalytic activity of iron-functionalized PMOs as catalysts for the hydroxylation of benzene to phenol was evaluated at 70 °C using acetonitrile as solvent and H₂O₂ as oxidant. Full details of the optimization of the reaction conditions are reported in our recent work.¹³ To achieve the best catalytic results, it should be noted that longer reaction times are required for Fe₃ (Table 3, entries 3 and 4) and heterogeneous catalysts PMO-Fe₁–PMO-Fe₃ compared to their corresponding homogeneous iron precursors Fe₁–Fe₃ (Table 3, entries 1, 2 and 4–7).

Table 3 The catalytic results of PMO-Fe₁–PMO-Fe₃ and their corresponding Fe(III) complexes

Entry	Catalyst	Conversion (%)	Yield (%)	Selectivity (%)
a Reaction conditions: Fe1 or Fe2 or Fe3 (0.02 mmol), benzene (0.9 mL, 10 mmol), H2O2 (1.2 mL, 12 mmol), acetonitrile as solvent (3.8 mL), temperature = 70 °C, time = 2 h.b Reaction time was prolonged from 2 h to 8 h.c Reaction conditions: PMO-Fe (60 mg), benzene (0.9 mL, 10 mmol), H2O2 (1.5 mL, 15 mmol), acetonitrile as solvent (3.8 mL), temperature = 70 °C, time = 12 h.
1	Fe1^a	36.4	10.6	29.1
2	Fe2^a	38.7	10.0	25.8
3	Fe3^a	5.6	1.7	30.4
4	Fe3^b	38.2	9.6	25.1
5	PMO-Fe1^c	28.5	15.2	53.3
6	PMO-Fe2^c	33.1	14.6	44.1
7	PMO-Fe3^c	34.8	14.3	41.1

---

The observed efficiencies of PMO-Fe₁–PMO-Fe₃ catalysts indicate that the heterogenization process preserves the catalytic properties of the active-iron-centers for direct hydroxylation of benzene (Table 3). Importantly, we notice that the heterogenized PMO-Fe₁–PMO-Fe₃ catalysts show only slightly lower catalytic activity (benzene conversion) with nearly double selectivity toward the production of phenol compared with the corresponding homogeneous iron precursors Fe₁–Fe₃. This is not uncommon since usually heterogenisation often results in lower catalytic activity due to the slow diffusion rate in the pore.⁵³ The selectivity to produce phenol was significantly improved as the prevention of phenol over-oxidation could be achieved due to most of the incorporated active-iron species are placed inside the mesopore.²¹ Meanwhile, the better selectivity for production of phenol could be also attributed to the presence of the silanol groups on PMOs, which act as Brönsted acidic sites⁵⁴ and can easily adsorb the weakly basic benzene onto the acidic framework of the PMO catalysts through electrostatic interactions.¹¹

Recyclability is an important feature for a heterogeneous catalytic system. To check the recyclability, PMO-Fe₁–PMO-Fe₃ were repetitively examined in direct hydroxylation of benzene without any regeneration. As shown in Fig. 6, for all the catalysts, both conversion and selectivity showed no substantial decrease after four cycles. After each cycle, catalyst was removed by simple centrifugation, washing for several times with appropriate solvent before being dried under vacuum. We observed that the amount of Fe present in the PMO-Fe₁–PMO-Fe₃ after 4 reuses is almost the same as the fresh catalyst (Table 1), indicative of the true heterogeneity and good stability of the catalyst. This conclusion is further confirmed by FTIR spectral comparison (Fig. 7), which shows the spectra are almost the same between before the use and after 4 reuses. Additionally, only negligible amount of leached Fe in solution could be detected by ICP-AES (the concentration of Fe in all cases were <4 ppm) after 4 recycles.

Fig. 6 Recyclability tests of PMO-Fe₁–PMO-Fe₃ for the direct hydroxylation of benzene to phenol.

Fig. 7 FTIR spectral variation of the catalysts after 4 repetitive reactions.

Conclusions

In conclusion, we have successfully synthesized a type of heterogeneous catalysts with iron complex incorporated into PMOs. Highly mesoporous architecture together with the iron complex centers enables the PMOs to serve as an ideal scaffold for heterogeneous catalysis. For the purpose of catalytic application, it has been demonstrated that the incorporation of the homogeneous precursors, iron complexes, into PMOs not only improves the selective conversion of benzene toward the phenol production, but also affords an excellent recyclability of the heterogeneous PMOs catalysts, making the catalytic process more benign from environmental and commercial point of view. Thus, the current work presents a heterogeneous green catalytic system for direct production of phenol from benzene with good recyclability and improved catalytic selectivity.

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (Grant no. 21301071, 21571083) and the Government of Zhejiang Province (Qianjiang Professorship for XL) for supporting this work.

Notes and references

1. Y. Liu, K. Murata and M. Inaba, Catal. Commun., 2005, 6, 679.
2. R. Bal, M. Tada, T. Sasaki and Y. Iwasawa, Angew. Chem., Int. Ed., 2006, 45, 448.
3. Y.-Y. Gu, X.-H. Zhao, G.-R. Zhang, H.-M. Ding and Y.-K. Shan, Appl. Catal., A, 2007, 328, 150.
4. M. Tada, Y. Uemura, R. Bal, Y. Inada, M. Nomura and Y. Iwasawa, Phys. Chem. Chem. Phys., 2010, 12, 5701.
5. S. S. Acharyya, S. Ghosh, R. Tiwari, C. Pendem, T. Sasaki and R. Bal, ACS Catal., 2015, 5, 2850.
6. G. Luo, X. Lv, X. Wang, S. Yan, X. Gao, J. Xu, H. Ma, Y. Jiao, F. Li and J. Chen, RSC Adv., 2015, 5, 94164.
7. A. Okemoto, Y.-h. Tsukano, A. Utsunomiya, K. Taniya, Y. Ichihashi and S. Nishiyama, J. Mol. Catal. A: Chem., 2016, 411, 372.
8. S. Shang, B. Chen, L. Wang, W. Dai, Y. Zhang and S. Gao, RSC Adv., 2015, 5, 31965.
9. L. Balducci, D. Bianchi, R. Bortolo, R. D'Aloisio, M. Ricci, R. Tassinari and R. Ungarelli, Angew. Chem., Int. Ed., 2003, 42, 4937.
10. P. K. Khatri, B. Singh, S. L. Jain, B. Sain and A. K. Sinha, Chem. Commun., 2011, 47, 1610.
11. P. Borah, X. Ma, K. T. Nguyen and Y. Zhao, Angew. Chem., Int. Ed., 2012, 51, 7756.
12. Y. Leng, J. Liu, P. Jiang and J. Wang, Chem. Eng. J., 2014, 239, 1.
13. B. B. Xu, W. Zhong, Z. H. Wei, H. L. Wang, J. Liu, L. Wu, Y. G. Feng and X. M. Liu, Dalton Trans., 2014, 43, 15337.
14. B. Puértolas, A. K. Hill, T. García, B. Solsona and L. Torrente-Murciano, Catal. Today, 2015, 248, 115.
15. Y. Morimoto, S. Bunno, N. Fujieda, H. Sugimoto and S. Itoh, J. Am. Chem. Soc., 2015, 137, 5867.
16. C. Wang, L. Hu, Y. Hu, Y. Ren, X. Chen, B. Yue and H. He, Catal. Commun., 2015, 68, 1.
17. L. Wu, W. Zhong, B. Xu, Z. Wei and X. Liu, Dalton Trans., 2015, 44, 8013.
18. E. Gu, W. Zhong, H. Ma, B. Xu, H. Wang and X. Liu, Inorg. Chim. Acta, 2016, 444, 159.
19. M. Jourshabani, A. Badiei, Z. Shariatinia, N. Lashgari and G. Mohammadi Ziarani, Ind. Eng. Chem. Res., 2016, 55, 3900.
20. J. Yang, G. Sun, Y. Gao, H. Zhao, P. Tang, J. Tan, A. Lu and D. Ma, Energy Environ. Sci., 2013, 6, 793.
21. M. Yamada, K. D. Karlin and S. Fukuzumi, Chem. Sci., 2016, 7, 2856.
22. H. Xin, A. Koekkoek, Q. Yang, R. van Santen, C. Li and E. J. M. Hensen, Chem. Commun., 2009, 7590.
23. R. Navarro, S. Lopez-Pedrajas, D. Luna, J. M. Marinas and F. M. Bautista, Appl. Catal., A, 2014, 474, 272.
24. L. Li, Q. Meng, J. Wen, J. Wang, G. Tu, C. Xu, F. Zhang, Y. Zhong, W. Zhu and Q. Xiao, Microporous Mesoporous Mater., 2016, 227, 252.
25. F. Zhang, X. Chen, J. Zhuang, Q. Xiao, Y. Zhong and W. Zhu, Catal. Sci. Technol., 2011, 1, 1250.
26. M. Weber and M. Kleine-Boymann, in Ullmann's Encyclopedia of Industrial Chemistry, 2004.
27. R. J. Schmidt, Appl. Catal., A, 2005, 280, 89.
28. R. Molinari and T. Poerio, Asia-Pac. J. Chem. Eng., 2010, 5, 191.
29. S. Fukuzumi and K. Ohkubo, Asian J. Org. Chem., 2015, 4, 836.
30. D. Bianchi, L. Balducci, R. Bortolo, R. D'Aloisio, M. Ricci, G. Spanò, R. Tassinari, C. Tonini and R. Ungarelli, Adv. Synth. Catal., 2007, 349, 979.
31. D. Barbera, F. Cavani, T. D'Alessandro, G. Fornasari, S. Guidetti, A. Aloise, G. Giordano, M. Piumetti, B. Bonelli and C. Zanzottera, J. Catal., 2010, 275, 158.
32. K. Lemke, H. Ehrich, U. Lohse, H. Berndt and K. Jähnisch, Appl. Catal., A, 2003, 243, 41.
33. G. Shiravand, A. Badiei, G. M. Ziarani, M. Jafarabadi and M. Hamzehloo, Chin. J. Catal., 2012, 33, 1347.
34. G. Ding, W. Wang, T. Jiang, B. Han, H. Fan and G. Yang, ChemCatChem, 2013, 5, 192.
35. X. Chen, J. Zhang, X. Fu, M. Antonietti and X. Wang, J. Am. Chem. Soc., 2009, 131, 11658.
36. J. Xu, Q. Jiang, T. Chen, F. Wu and Y.-X. Li, Catal. Sci. Technol., 2015, 5, 1504.
37. J. Xu, Q. Jiang, J.-K. Shang, Y. Wang and Y.-X. Li, RSC Adv., 2015, 5, 92526.
38. J.-S. Choi, T.-H. Kim, K.-Y. Choo, J.-S. Sung, M. B. Saidutta, S.-O. Ryu, S.-D. Song, B. Ramachandra and Y.-W. Rhee, Appl. Catal., A, 2005, 290, 1.
39. J. Xu, H. Liu, R. Yang, G. Li and C. Hu, Chin. J. Catal., 2012, 33, 1622.
40. Y. Zhong, G. Li, L. Zhu, Y. Yan, G. Wu and C. Hu, J. Mol. Catal. A: Chem., 2007, 272, 169.
41. S. Song, H. Yang, R. Rao, H. Liu and A. Zhang, Appl. Catal., A, 2010, 375, 265.
42. Y. Kong, X. Xu, Y. Wu, R. Zhang and J. Wang, Chin. J. Catal., 2008, 29, 385.
43. N. Mizoshita, T. Tani and S. Inagaki, Chem. Soc. Rev., 2011, 40, 789.
44. P. Van Der Voort, D. Esquivel, E. De Canck, F. Goethals, I. Van Driessche and F. J. Romero-Salguero, Chem. Soc. Rev., 2013, 42, 3913.
45. S. S. Park, M. Santha Moorthy and C.-S. Ha, NPG Asia Mater., 2014, 6, e96.
46. X. Liu, Y. Maegawa, Y. Goto, K. Hara and S. Inagaki, Angew. Chem., Int. Ed., 2016, 55, 7943.
47. Y. Maegawa and S. Inagaki, Dalton Trans., 2015, 44, 13007.
48. N. Ishito, H. Kobayashi, K. Nakajima, Y. Maegawa, S. Inagaki, K. Hara and A. Fukuoka, Chem.–Eur. J., 2015, 21, 15564.
49. M. Waki, Y. Maegawa, K. Hara, Y. Goto, S. Shirai, Y. Yamada, N. Mizoshita, T. Tani, W.-J. Chun, S. Muratsugu, M. Tada, A. Fukuoka and S. Inagaki, J. Am. Chem. Soc., 2014, 136, 4003.
50. X. Huang and X. Du, ACS Appl. Mater. Interfaces, 2014, 6, 20430.
51. A. S. M. Chong and X. S. Zhao, J. Phys. Chem. B, 2003, 107, 12650.
52. V. Dufaud, F. Beauchesne and L. Bonneviot, Angew. Chem., Int. Ed., 2005, 44, 3475.
53. N. End and K. U. Schoning, Top. Curr. Chem., 2004, 242, 241.
54. A. Jentys, K. Kleestorfer and H. Vinek, Microporous Mesoporous Mater., 1999, 27, 321.

---