Nickel and cobalt nanoparticles modified hollow mesoporous carbon microsphere catalysts for efficient catalytic reduction of widely used dyes

Abstract

Recently, with increasing consciousness of environmental protection, Chemists are focusing on green chemistry, which allows the transformation of highly toxic organic wastes into reusable or low-toxicity compounds under mild reaction conditions. In this work, hollow mesoporous carbon microspheres (h-MCM) with an ultrahigh surface area were synthesized by a co-sol–emulsion–gel method and were used as supports for Ni and Co nanoparticles (NPs). The prepared nanocatalysts (Ni/h-MCM and Co/h-MCM) have high Ni and Co NPs dispersion on the surface of the h-MCM as well as in its hollow core. The prepared catalysts exhibit a high surface area, mesoporous shell and an accessible interior space. These properties are beneficial for enhancing the catalytic activity of the prepared catalysts. In the catalytic reduction of methylene blue, methyl orange, and rhodamine B, the obtained Ni/h-MCM and Co/h-MCM exhibited excellent catalytic activity as compared with other reported catalysts. This work may promote the design of non-precious metal based nanocatalysts for environmental catalysis.

---

Introduction

In recent years, catalysts based on noble metal nanoparticles (NMNPs), especially Au, Pd, Pt and Ru NPs have attracted considerable attention due to their excellent catalytic properties.^1–4 However, the scarce resources and the high cost of noble metals greatly limit the catalytic application of the NMNPs based catalysts. Researchers nowadays are trying their best to develop new types of non-noble metal catalysts that exhibit excellent catalytic performance to replace NMNPs based catalysts. The widespread and low cost transition metal (TM), especially Co and Ni have attracted considerable interest in catalysis applications because they can catalyze the hydrogenation, reduction, oxidation and hydrogen evolution reactions.^5–9 However, because of the high surface energy, the naked TM NPs are apt to agglomerate during the catalytic process, and subsequently greatly reducing their intrinsic activity.¹⁰ Thus it is a critical challenge to stabilize the TM NPs on catalyst supports while retaining their nanoscale properties. Although the commercially available Al₂O₃, SiO₂ and active carbon can be used as catalyst supports for fabrication Co and Ni based catalysts, but their low surface area, micropore structure still can lead the aggregation of the TM NPs and decrease the catalytic activity of the prepared catalysts. It is well known that the mesoporous materials with high surface area and large pore volume can provide a large surface area as well as efficiently immobilize the TM NPs in the mesopores with desirable dispersion. Therefore, researchers have focused their research on mesoporous carbon materials, which meet the requirements for NP supports attribute to their large surface area, tunable pore structure, excellent chemical and mechanical stability.^11,12 The recently reported hollow mesoporous carbon microspheres (h-MCM) has mesoporous hollow structure, low density, high surface area, accessible interior space and graphitic carbon framework.^13–15 These properties make h-MCM ideal support for fabricating TM NPs based catalysts with high dispersion.

On the other hand, the widely used organic dyes including methylene blue (MB), methyl orange (MO), and rhodamine B (RhB) are high toxicity, because they are carcinogenic and mutagenic.^16–18 The discharge of organic dyes polluted water to the environment can cause serious water pollution. Therefore, it is urgent to develop efficient methods to treat the dye-polluted wastewater. And till now, several methods such as adsorption, oxidation, the Fenton reaction, as well as an electrochemical process, have been reported for the treatment of dye-polluted wastewater. However, there are some drawbacks of these methods, for example they are not quite effective and cost-effective.^19–24 Compare with the above mentioned methods, the catalytic reduction treatment method exhibits high efficiency for reducing the toxicity of the organic dyes, attribute the C=N or N=N can be throughly hydrogenated.^25,26 Other than the NMNPs based catalysts, TM NPs based catalysts such as Cu NPs, Co NPs and Ni NPs based catalysts have also been reported for the catalytic reduction of organic dyes.^27–30 However, developing simple and efficient approach to prepare TM NPs modified mesoporous carbon materials catalysts with high surface area and good TM NPs dispersion is highly desired but remains a challenge.

Based on the above considerations, here we prepare the h-MCM materials with ultrahigh surface area using the co-sol–emulsion–gel method.³¹ Ni and Co NPs were successfully supported on the h-MCM with high dispersion. The novel Ni/h-MCM and Co/h-MCM nanocatalysts exhibit excellent catalytic activity for the reduction of the organic dyes MB, MO, and RhB. In addition, the prepared nanocatalysts can be magnetically recycled and reused for at least five times without obvious decrease in the catalytic activity. This study provides a useful platform based on low-cost and magnetically recyclable Ni and Co based nanocatalyst for efficient reduction of organic dyes.

Experimental

Materials

The reagents that were used for the preparation of h-MCM including tetraethyl orthosilicate (TEOS), cetyltrimethyl ammonium bromide (CTAB), resorcinol, and N₂H₄·H₂O were purchased from Tianjin Heowns Biochemical Technology Co., Ltd. Ni(NO₃)₂, Co(NO₃)₂ and the organic dyes MB, MO and RhB, were supplied by the Sinopharm Chemical Reagent Co. Ltd (China). Other reagents and solvents were of analytical grade and used without further purification.

Preparation of the h-MCM, Ni/h-MCM and Co/h-MCM

According to the similar co-sol–emulsion–gel procedure reported by Cao et al.,³¹ the h-MCM was successfully obtained. For the preparation of the Ni/h-MCM nanocatalyst, the procedure was typically: 100 mg of h-MCM and 75 mg of Ni(NO₃)₂·6H₂O were dissolved in 30 mL H₂O. After stirring for 6 h, the mixture was rotary dried, and then the obtained composite was annealed from 30 °C to 400 °C with a ramp rate of 5 °C min⁻¹ and held for 2 h at 400 °C under H₂/N₂ (v/v = 1 : 2, 90 mL min⁻¹) atmosphere. Then obtain the Ni/h-MCM nanocatalyst. The fabrication procedure for the Co/h-MCM was the same as the Ni/h-MCM nanocatalyst.

Catalytic reduction of organic dyes MB, MO and RhB

Typically, 20 μL aqueous solution of the organic dye (0.01 M) was mixed with 2.7 mL of deionized H₂O, followed by adding 250 μL of a freshly prepared aqueous solution of NaBH₄ (0.1 M). And then, the prepared nanocatalyst (15 mg mL⁻¹) were added (20 μL for catalytic reduction of RhB and MO, 5 μL for catalytic reduction of MB), and the catalytic reduction process was started. The progress of the reaction was monitored by UV-vis spectroscopy.

Characterizations

Transmission electron microscopy (TEM, Tecnai G2 F30) and scanning electron microscopy (SEM, JSM-6701F) were used to characterize the morphology of the h-MCM, Ni/h-MCM and Co/h-MCM catalysts. The powder X-ray diffraction (PXRD, Rigaku D/max-2400), Raman spectrometer (Jobin Yvon Lab Ram HR evolution) and a micromeritics ASAP 2010 (USA) were used to investigate the physical properties of the samples. The magnetic measurement of Ni/h-MCM and Co/h-MCM catalyst was performed using a quantum design vibrating sample magnetometer (VSM). An Agilent Cary 5000 UV-vis spectrophotometer was utilized for monitoring the extent of the catalytic reduction of MB, MO and RhB.

Results and discussion

Materials characterizations

The h-MCM was prepared through the co-sol–emulsion–gel process,³¹ carbonization and HF etching process. And then the Ni/h-MCM and Co/h-MCM were obtained by the reduction of the Ni(NO₃)₂ and Co(NO₃)₂ adsorbed h-MCM respectively. Fig. 1a shows the Raman spectra of the h-MCM, Ni/h-MCM and Co/h-MCM, two characteristic peaks named D band and G band can be clearly observed. The D band located at 1317 cm⁻¹ can be assigned to the defect sites in the hexagonal framework of graphite materials, while the G band can be assigned to sp² hybridized carbon atoms of the graphite structure.³² The relative I_D/I_G values for h-MCM, Ni/h-MCM and Co/h-MCM are 1.08, 1.08, and 1.06, these values reveal that the h-MCM, Ni/h-MCM and Co/h-MCM graphite framework have comparatively defect sites, should mainly exist as C=O and COOH. These functional groups in the h-MCM, Ni/h-MCM and Co/h-MCM may act as the condensation centers, and beneficial for the adsorption of the Ni²⁺ and Co²⁺ ions. From the I_D/I_G values, it can be seen that after being supported with Ni and Co NPs, the framework of h-MCM has almost not been changed. The XRD patterns of the prepared samples were displayed in Fig. 1b. The broad peak located between 20–30° is assigned to the graphitic interlayer (002) peak of the h-MCM support. Compared to the h-MCM, the Ni/h-MCM has three sharp peaks at 2θ = 44.5°, 52° and 76.4°, respectively assigned to the Ni(111), Ni(200) and Ni(220) characteristic peaks.³³ In the Co/h-MCM, two characteristic peaks of metallic Co (Co(111) and Co(220)) can also be clearly observed.³⁴ Thus, the XRD results demonstrate that the Ni and Co exist as metallic NPs and successfully been supported on the h-MCM support.

Fig. 1 Raman spectra of the h-MCM, Ni/h-MCM and Co/h-MCM (a), XRD spectra of the h-MCM, Ni/h-MCM and Co/h-MCM (b).

To investigate the morphology of the prepared samples, SEM and TEM characterizations have been performed (Fig. 2). As can be seen in Fig. 2a that the spherical h-MCM NPs have an average diameter of about 150 nm. The TEM image displayed in Fig. 2b confirms that, the h-MCM has a hollow core and mesoporous shell. The shell thickness is about 20–30 nm. When adsorbed with Ni(NO₃)₂ and Co(NO₃)₂, and respectively reduced under H₂ atmosphere, the Ni/h-MCM and Co/h-MCM nanocatalysts were obtained. Fig. 2c and f show the TEM image of the Ni/h-MCM and Co/h-MCM nanocatalysts, respectively. The Ni and Co NPs were successfully modified on the h-MCM as well as in the hollow core. There is almost no NPs aggregation, indicating the Ni and Co NPs were highly dispersed. Furthermore, the h-MCM support almost maintained its original structure after the modification process, indicating the adsorption and reduction process did not damage the h-MCM structure. In the HRTEM image of the Ni/h-MCM, the Ni(111) lattice d = 0.204 nm can be clearly observed (Fig. 2d). Fig. 2g shows the Co(111) lattice of d = 0.205 nm. In addition, the average particle size of the Ni NPs is about 5 nm (Fig. 2e), and the Co NPs have an average particle size of 7 nm (Fig. 2h), as measured from the TEM images. The EDS spectra displayed in Fig. S1† indicate the composition of the Ni/h-MCM and Co/h-MCM nanocatalysts.

Fig. 2 SEM (a) and TEM (b) images of the h-MCM, TEM image of Ni/h-MCM (c), HRTEM image of the Ni NPs (d) and Ni NPs size distribution of Ni/h-MCM (e), TEM image of Co/h-MCM (f), HRTEM image of the Co NPs (g) and Co NPs size distribution of Co/h-MCM (h).

The N₂ adsorption–desorption isotherms of the h-MCM, Ni/h-MCM and Co/h-MCM nanocatalysts were shown in Fig. 3a. The three samples all exhibit type-IV isotherms. From the loops, it can be seen that at relatively high pressure (P/P₀ > 0.6), steep increase of N₂ absorption was observed, indicating that all the samples exhibit mesoporous structure.³⁵ The Brunauer–Emmett–Teller (BET) surface areas are 1059.8, 921.5 and 666.1 m² g⁻¹, respectively for h-MCM, Ni/h-MCM and Co/h-MCM; and the pore volumes for the three samples were found to be 1.8, 1.3, 1.15 m³ g⁻¹, respectively. The results indicated that the obtained Ni/h-MCM and Co/h-MCM nanocatalysts exhibit ultrahigh surface areas and large pore volumes. The decrease of the BET surface areas and the pore volumes for the Ni/h-MCM and Co/h-MCM nanocatalysts compared with h-MCM was probably attributed to the modification of the Ni and Co NPs on the support. Moreover, the pore size distribution of the h-MCM, Ni/h-MCM and Co/h-MCM nanocatalysts were described in Fig. 3b. Slightly decrease of the pore size in Ni/h-MCM and Co/h-MCM nanocatalysts compared with h-MCM can be observed, mainly due to the Ni and Co NPs that grafted in the mesopore of the h-MCM support. In addition, the textual parameters of the prepared samples were presented in Table 1, and the changing trends in BET surface areas, pore volumes and pore size of the prepared samples before and after modifying Ni and Co NPs can be clearly seen.

Fig. 3 The N₂ sorption isotherms of h-MCM, Ni/h-MCM and Co/h-MCM (a), and the pore size distribution of h-MCM, Ni/h-MCM and Co/h-MCM (b).

Table 1 Textual parameters of the h-MCM, Ni/h-MCM and Co/h-MCM

Entry	Sample	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
1	h-MCM	1058.9	1.8	6.7
2	Ni/h-MCM	921.5	1.3	5.9
3	Co/h-MCM	666.1	1.15	5.6

---

As is well known that, metallic Ni and Co NPs exhibit magnetism. Thus, the prepared Ni/h-MCM and Co/h-MCM nanocatalysts also have the magnetism property as confirmed by the magnetization curves (Fig. 4). The magnetism property is very useful for a prepared nanocatalyst because it can be simply separated from the reaction mixture by using an external magnet. Fig. 4 inset shows the separation procedure of the Ni/h-MCM and Co/h-MCM nanocatalysts.

Fig. 4 Room temperature magnetization curves of the Ni/h-MCM and Co/h-MCM. Inset is the magnetic manipulation of the nanocatalysts.

Reduction of MB, MO and RhB

Organic dyes including MB, MO and RhB that are widely used in manufacturing industries are well-known environmental pollutants. Various noble metals such as Pd, Au, Ag and Pt based catalysts have been developed for the purpose of disposing these environmental pollutants.^36,37 While the current work utilizes the cost-effective and widespread metallic Ni and Co to prepare Ni/h-MCM and Co/h-MCM nanocatalysts to catalytic reduction the MB, MO and RhB organic dyes pollutants.

Initially, the selected organic dyes, blue colored MB, yellow colored MO and pink colored RhB have the absorption peak at λ_MB = 663 nm, λ_MO = 465 nm, λ_RhB = 554 nm. When only the reductant NaBH₄ was introduced into the dyes solution, the color of the dyes solution can maintain a long time, indicating that without an effective catalyst the reduction reaction cannot be occurred. When the Ni/h-MCM nanocatalyst was added into the dyes solutions containing excess NaBH₄, the reaction started as observed through the UV-vis spectra (Fig. 5a–c). Fig. 5a shows that the maximum absorption peak of MB at 663 nm gradually decreased and finally disappeared in 537 s. And also the blue color mixture changed to a colourless solution, indicating the completion of the reduction reaction. Fig. 5b and c show the absorption peaks change trend of the catalytic reduction of the MO and RhB respectively. From which it can be observed that, the reduction of the MO and RhB can be completed in 550 s and 335 s, respectively. These results reveal that the Ni/h-MCM nanocatalyst can effectively catalyze reduction the organic dyes. Like the Ni/h-MCM, the Co/h-MCM nanocatalyst can also effectively catalyze reduction the above mentioned organic dyes. The UV-vis spectra displayed in Fig. S2a–c† clearly demonstrated that the Co/h-MCM nanocatalyst can completely catalytic reduction of the MB, MO and RhB organic dyes in 353 s, 338 s and 185 s, respectively.

Fig. 5 Successive UV-vis spectra for the reduction of MB (a), MO (b) and RhB (c) by NaBH₄ with Ni/h-MCM nanocatalyst. (d) Kinetic curve for the reduction of MB, MO and RhB catalyzed by Ni/h-MCM nanocatalyst.

The reaction mechanism invoked for catalytic reduction of MB, MO and RhB organic dyes over the Ni/h-MCM and Co/h-MCM nanocatalysts was detailed illustrated in Scheme 1, which invoked a two-electron transfer process.^16,26,27 Firstly, BH₄⁻ was absorbed on the metal NPs (Ni or Co NPs), and transferred a hydride to the surface of the metal NPs to form the Ni–H or Co–H bonds; secondly, the organic dye molecules (MB, MO and RhB) were absorbed onto the surfaces of the Ni/h-MCM or Co/h-MCM nanocatalyst, and two active H atoms on the surface of the metal NPs tend to be captured by the organic dye molecule. Finally, the C=N or N=N bonds were hydrogenated and the reduction products were desorbed from the metal NPs and reactivated the Ni/h-MCM and Co/h-MCM nanocatalysts.

Scheme 1 The mechanism of the catalytic reduction of organic dyes (MB, MO, RhB).

In the organic dyes catalytic reduction reaction, the NaBH₄ used was excess as compared with the dyes, thus it should maintain constant during the reduction reaction. Therefore, the organic dyes catalytic reduction reactions could be considered as the pseudo-first order kinetics. The linear relationship between the ln(C_t/C₀) vs. the reaction time also confirms the pseudo-first order reaction kinetics (Fig. 5d and S2d†). Using the eqn (1), all the rate constants K estimated from the slopes of the linear relationship are given in Table 2. Furthermore, the catalytic activity comparison of the Ni/h-MCM and Co/h-MCM nanocatalysts with the other reported noble metal catalysts and non-precious metal catalysts are also listed in Table 2. The Ni/h-MCM and Co/h-MCM nanocatalysts have higher catalytic activity than other non-precious metal based catalysts in the catalytic reduction of organic dyes, and also comparatively catalytic activity with other reported noble metal based catalysts.^25,38–42 The excellent catalytic activity of the Ni/h-MCM and Co/h-MCM nanocatalysts was mainly attributed its unique structures: ultrahigh surface area, mesoporous shell, hollow core and the highly dispersed active sites, these factors may enhance the ease accessibility of the active sites and enhance the mass transfer effect, thus enhance the catalytic activity of the prepared nanocatalysts.

Table 2 Compare catalytic activity for the reduction of MB, MO, and RhB between the current work and some other catalysts reported in the literatures

Samples	Dyes	K (s^−1)	Ref.
Ni/h-MCM	MB	0.011	This work
	MO	0.003
	RhB	0.005
Co/h-MCM	MB	0.023	This work
	MO	0.003
	RhB	0.006
Crystalline palladium nanoparticle	MB	0.017	38
	MO	0.002
Fe3O4@C@Au	MB	0.006	39
MnFe2O4@SiO2@AgMRC	MO	0.04	25
	RhB	0.02
	MB	0.002
Au–CNx	RhB	0.0003	40
Cu microsphere	MB	0.006	41
Ni nanotube arrays	MB	0.037	42

---

Furthermore, the reusability and stability of the Ni/h-MCM and Co/h-MCM nanocatalysts were also investigated. As the prepared nanocatalysts exhibit magnetism property, they were easily recovered from the reaction mixture by using an external magnet. Using the catalytic reduction of MB as the probe reaction, both of the Ni/h-MCM and Co/h-MCM nanocatalysts can be reused for at least 5 runs without an obvious decrease in the catalytic activity (Fig. 6). And from the TEM images of the reused Ni/h-MCM and Co/h-MCM nanocatalysts (Fig. S3†), it can be seen that the morphology of the reused nanocatalysts was well maintained.

Fig. 6 The reusability of the Ni/h-MCM and Co/h-MCM nanocatalysts for the catalytic reduction of MB.

Conclusions

In summary, we have successfully prepared the novel Ni and Co modified mesoporous carbon microspheres catalysts, named Ni/h-MCM and Co/h-MCM. The obtained nanocatalysts have been carefully characterized and their catalytic activity in the catalytic reduction of organic dyes MB, MO and RhB have been detailed investigated. The prepared Ni/h-MCM and Co/h-MCM nanocatalysts can also be recovered easily using an external magnet from the reaction mixture and reused without obvious decrease in catalytic activity, indicative of its reusability and stability.

Acknowledgements

We thank the financial support from the Fundamental Research Funds for the Central Universities (lzujbky-2015-14 and lzujbky-2015-21).

References

1. Y. Hartadi, D. Widmann and R. J. Behm, ChemSusChem, 2015, 8, 456–465.
2. M. Martin-Martinez, L. M. Gómez-Sainero, J. Bedia, A. Arevalo-Bastante and J. J. Rodriguez, Appl. Catal., B, 2016, 184, 55–63.
3. K. Ghosh, R. A. Molla, M. A. Iqubal and S. M. Islam, Green Chem., 2015, 17, 3540–3551.
4. R. Fang, H. Liu, R. Luque and Y. Li, Green Chem., 2015, 17, 4183–4188.
5. S. Rosler, J. Obenauf and R. Kempe, J. Am. Chem. Soc., 2015, 137, 7998–8001.
6. X. Jin, M. Zhao, C. Zeng, W. Yan, Z. Song, P. S. Thapa, B. Subramaniam and R. V. Chaudhari, ACS Catal., 2016, 6, 4576–4583.
7. V. Polshettiwar, B. Baruwati and R. S. Varma, Green Chem., 2009, 11, 127–131.
8. C. van der Wijst, X. Duan, I. Skeie Liland, J. C. Walmsley, J. Zhu, A. Wang, T. Zhang and D. Chen, ChemCatChem, 2015, 7, 2991–2999.
9. M. R. Nabid, Y. Bide and Z. Habibi, RSC Adv., 2015, 5, 2258–2265.
10. X. Li, C. Zeng, J. Jiang and L. Ai, J. Mater. Chem. A, 2016, 4, 7476–7482.
11. H. Zhang, O. Noonan, X. Huang, Y. Yang, C. Xu, L. Zhou and C. Yu, ACS Nano, 2016, 10, 4579–4586.
12. S. Mohapatra, S. R. Rout, R. K. Das, S. Nayak and S. K. Ghosh, Langmuir, 2016, 32, 1611–1620.
13. D.-S. Yang, C. Kim, M. Y. Song, H.-Y. Park, J. C. Kim, J.-J. Lee, M. J. Ju and J.-S. Yu, J. Phys. Chem. C, 2014, 118, 16694–16702.
14. Y. Dai, H. Jiang, Y. Hu, Y. Fu and C. Li, Ind. Eng. Chem. Res., 2014, 53, 3125–3130.
15. L. Chen, X. Cui, M. Wang, Y. Du, X. Zhang, G. Wan, L. Zhang, F. Cui, C. Wei and J. Shi, Langmuir, 2015, 31, 7644–7651.
16. M. Atarod, M. Nasrollahzadeh and S. M. Sajadi, J. Colloid Interface Sci., 2016, 462, 272–279.
17. U. Kurtan, M. Amir and A. Baykal, Appl. Surf. Sci., 2016, 363, 66–73.
18. X. S. Wang, J. Liang, L. Li, Z. J. Lin, P. P. Bag, S. Y. Gao, Y. B. Huang and R. Cao, Inorg. Chem., 2016, 55, 2641–2649.
19. E. M. Yusni and S. Tanaka, Electrochim. Acta, 2015, 181, 130–138.
20. M. S. Tehrani and R. Zare-Dorabei, RSC Adv., 2016, 6, 27416–27425.
21. D. Tao, Y. Higaki, W. Ma and A. Takahara, Chem. Lett., 2015, 1572–1574.
22. S. Ou, J.-P. Zheng, G.-Q. Kong and C.-D. Wu, Dalton Trans., 2015, 7862–7869.
23. H. S. Oliveira, L. D. Almeida, V. A. A. de Freitas, F. C. C. Moura, P. P. Souza and L. C. A. Oliveira, Catal. Today, 2015, 240, 176–181.
24. A. Mizrahi and I. M. Litaor, Desalin. Water Treat., 2014, 52, 5264–5275.
25. U. Kurtan, M. Amir, A. Yildiz and A. Baykal, Appl. Surf. Sci., 2016, 376, 16–25.
26. P. Veerakumar, S. M. Chen, R. Madhu, V. Veeramani, C. T. Hung and S. B. Liu, ACS Appl. Mater. Interfaces, 2015, 7, 24810–24821.
27. Z.-Z. Wang, S.-R. Zhai, B. Zhai and Q.-D. An, Eur. J. Inorg. Chem., 2015, 1692–1699.
28. T. Kamal, S. B. Khan and A. M. Asiri, Cellulose, 2016, 23, 1911–1923.
29. A. Chinnappan, A. H. Tamboli, W.-J. Chung and H. Kim, Chem. Eng. J., 2016, 285, 554–561.
30. Y. Yang, Y. Ren, C. Sun and S. Hao, Green Chem., 2014, 16, 2273–2280.
31. J. Hou, T. Cao, F. Idrees and C. Cao, Nanoscale, 2016, 8, 451–457.
32. Z. Dong, C. Dong, Y. Liu, X. Le, Z. Jin and J. Ma, Chem. Eng. J., 2015, 270, 215–222.
33. W. L. Zhen, J. T. Ma and G. X. Lu, Appl. Catal., B, 2016, 190, 12–25.
34. X. Z. Li, Y. Y. Fang, L. X. Wen, F. Li, G. L. Yin, W. M. Chen, X. C. An, J. Jin and J. T. Ma, Dalton Trans., 2016, 5575–5582.
35. Z. Shi, W. Jiao, L. Chen, P. Wu, Y. Wang and M. He, Microporous Mesoporous Mater., 2016, 224, 253–261.
36. W. Zuo, G. Chen, F. Chen, S. Li and B. Wang, RSC Adv., 2016, 6, 28774–28780.
37. V. Vaiano, G. Iervolino, D. Sannino, J. J. Murcia, M. C. Hidalgo, P. Ciambelli and J. A. Navio, Appl. Catal., B, 2016, 188, 134–146.
38. V. Vilas, D. Philip and J. Mathew, Mater. Chem. Phys., 2016, 170, 1–11.
39. Z. Gan, A. Zhao, M. Zhang, W. Tao, H. Guo, Q. Gao, R. Mao and E. Liu, Dalton Trans., 2013, 8597–8605.
40. T. Bhowmik, M. K. Kundu and S. Barman, RSC Adv., 2015, 5, 38760–38773.
41. Y. Zhang, P. Zhu, L. Chen, G. Li, F. Zhou, D. Lu, R. Sun, F. Zhou and C.-p. Wong, J. Mater. Chem. A, 2014, 2, 11966–11973.
42. X.-Z. Li, K.-L. Wu, Y. Ye and X.-W. Wei, CrystEngComm, 2014, 16, 4406–4413.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22498a

---