Eggshell/Fe₃O₄ nanocomposite: novel magnetic nanoparticles coated on porous ceramic eggshell waste as an efficient catalyst in the synthesis of 1,8-dioxo-octahydroxanthene

Abstract

An eggshell/Fe₃O₄ nanocomposite was synthesized simply via an economic and novel method, using recycled eggshell biowaste as a starting material and an aqueous solution of FeSO₄ as a coating agent without any additional alkali or a protective atmosphere. Furthermore, the catalytic activity of the magnetic nanocomposite was investigated in the synthesis of 1,8-dioxo-octahydroxanthenes under solvent-free conditions. The reaction proceeds to completion in a short reaction time with an excellent yield. The suggested strategy for synthesizing 1,8-dioxo-octahydroxanthenes is of great interest because a novel, green, and low-cost magnetic nanocomposite is used as a heterogeneous catalyst, on account of its convenient preparation and high reusability.

---

Introduction

Natural products with a xanthene heterocyclic motif such as 9-aryl-1,8-dioxo-octahydroxanthenes show a broad spectrum of important biological and pharmaceutical uses in the field of medicinal chemistry, which include as antibacterial, antiviral, anti-inflammatory agents and anticancer agents^1–3 and novel CCR1 receptor antagonists,⁴ as well as their efficiency in the field of material science as dyes,⁵ laser technologies for visualization of biomolecules, fluorescent markers⁶ and luminescent sensors.⁷ In this regard, there are several reports on their synthesis in the presence of different catalysts. However, the reported methods suffer from drawbacks such as long reaction times, moderate yields, the use of toxic and expensive catalysts and reagents with high catalyst loading, which are not in agreement with the aspect of green chemistry. Thus, the value of a green and highly effective protocol for this important class of heterocycles depends mainly on identifying a simple, cheap, highly effective and green catalyst.¹³

Iron oxide nanoparticles such as magnetite (Fe₃O₄) have attracted considerable attention in recent years due to their potential applications in magnetic media, catalysis, color imaging, ferrofluids, and biomedicine.^20–22 The most popular methods for the preparation of magnetic nanoparticles (MNPs) are thermal decomposition^23–25 and coprecipitation.^26–28 In the coprecipitation method, Fe₃O₄ nanoparticles are prepared from a mixture of Fe²⁺ and Fe³⁺ salt solutions by adding an alkali under a protective atmosphere.^26–28 Since nanoparticles synthesized by the coprecipitation method have a tendency to be polydispersed in size, the addition of an inorganic coating agent with high thermal stability²⁹ can enhance the stability of the MNPs.^30,31

Eggshell (ES) wastes are abundant and inexpensive biomaterials which are composed of more than 90% calcium carbonate.^32,33 ES shows high thermal stability, relatively low density and phase continuity in a composite, compared to mineral CaCO₃.³⁴ In addition, the porous structure of ES means it uses less material to form a higher surface area compared to an artificial material.³⁵ In this regard, the porous structure of the ES with wide nucleation sites to minimize the aggregation of particles is an excellent host for supporting MNPs. So, in this work, a new and highly effective strategy is used for the synthesis of a novel ES/Fe₃O₄ nanocomposite, coating ES bioceramic with MNPs via a co-precipitation method. To our knowledge, this is the first report of green synthesis of MNPs supported on ES waste as a natural and biocompatible composite. In addition, the catalytic activity of the nanocomposite was investigated in the synthesis of 1,8-dioxo-octahydroxanthenes via one pot multi-component condensation of aryl aldehydes and dimedone in solvent-free and thermal conditions (Scheme 1).

Scheme 1 Synthesis of 1,8-dioxo-octahydroxanthenes; biowaste derivatives catalyzed by an ES/Fe₃O₄ nanocomposite.

Results and discussions

Characterization of ES/Fe₃O₄ nanocomposite

In order to investigate the highest MNP deposition on the ES surface, ES powder was treated with different concentrations of Fe²⁺ ion solution under the same conditions as mentioned in the Experimental section. As shown in Fig. 1, the highest recovery of the metal ion was observed at a concentration of 1000 mg L⁻¹. It was proven that the recovery decreases at higher concentrations, but the amount of adsorbed metal ions increases due to the higher initial metal ion concentration. Therefore, a concentration of 1000 mg L⁻¹ was used as the maximum concentration of the initial ion solution in the synthesis of the nanocomposite. Furthermore, the structure and chemical composition of the synthesized composite with an optimized amount of Fe ions were fully characterized.

Fig. 1 Adsorption of Fe ions on the eggshell support.

XRD analysis

The eggshell has a basic nature which can provide an alkaline media with a pH of 9.5 by forming Ca(OH)₂ in its surface structure. Therefore, ES acts as an alkali as well as a coating agent to precipitate Fe²⁺ without the use of any more external base and a protective atmosphere. According to eqn (1), Fe(OH)₂ was oxidized by O₂ in air to form FeOOH. A combination of Fe(OH)₂ and FeOOH at 60 °C produced Fe₃O₄ particles^37–39 which grew on the porous surface of the ES. It indicated that the use of an ES support can produce Fe₃O₄ with no need to involve Fe³⁺ as a precursor. The phase composition of the nanocomposite can be deduced from the comparison of the XRD patterns (Fig. 2) with those known from the literature.⁴⁰

Fig. 2 XRD patterns of (a) magnetic ES/Fe₃O₄ nanocomposite and (b) raw eggshell.

It can be seen from the XRD patterns that the main peak at 2θ = 29.4 is related to the major phase of CaCO₃ in the eggshell structure⁴¹ and the peaks at (220), (311), (400), (422), (511), (440) and (731) are relevant to the phase of Fe₃O₄.⁴² All of the diffraction peaks arising from the structure of the nanocomposite are similar to those reported in the literature.^40,41 Furthermore, the presence of iron was proved with an energy dispersion X-ray (EDX) analysis of the nanocomposite that showed high levels of Fe (38.1%), Ca (27.0%), O (24.9%) and carbon (9.4%) with small amounts of P (0.2%), Zn (0.2) and Na (0.2%). The XRD investigation revealed that the as-synthesized magnetic Fe₃O₄ and ES NPs have average diameters of 13 and 18 nm respectively, as calculated with the Scherrer equation (D = Kλ/(β cos θ)).

FTIR spectra analysis

Comparing the FTIR spectra of ES/Fe₃O₄ MNPs and eggshell (Fig. 3) shows the typical carbonate absorptions at about 2517 (HCO₃), 1799 (CO), 1424, 710 (C–O) and 876 cm⁻¹ (OCO) which are according to the literature.⁴² The broad band at about 3427 cm⁻¹ is due to OH stretching vibrations in Ca(OH)₂, formed during the adsorption of water on the surface of the composite structure. Furthermore, the strong FTIR band observed at around 599 cm⁻¹ can be attributed to the Fe–O–Fe stretching vibration mode of Fe₃O₄.⁴³

Fig. 3 FTIR spectra of (a) ES/Fe₃O₄ nanocomposite and (b) raw ES.

Thermo gravimetric analysis (TGA)

The thermal stability of the catalyst was investigated using thermal gravimetric analysis (TGA) and compared with raw ES (Fig. 4). The differential scanning calorimetry (DSC) and TGA analysis which was performed from room temperature to 900 °C indicated that the main weight loss in both the raw ES (Fig. 4b) and the ES/Fe₃O₄ composite (Fig. 4a) is related to the decomposition of ES to CO₂ and CaO, proving the existence of carbonate in the eggshell structure.⁴⁰ The measured weight loss below 600 °C is due to the release of physically adsorbed water.³⁶ So, the catalyst remained stable at the reaction temperature (80 °C).

Fig. 4 TGA curve of (a) ES/Fe₃O₄ MNPs and (b) raw ES.

Electron microscopic investigation

Transmission microscopy (TEM) and field emission scanning electron microscopy (FESEM) images of eggshell supported magnetic nanoparticles and raw eggshell have been provided in Fig. 5a and b respectively. From the comparison of the TEM image of the raw eggshell with the one supporting MNPs, it can be verified that the Fe₃O₄ MNPs were coated on the surface of the ES as a nanoribbon structure. The TEM investigation also showed an average diameter of 13 nm for the magnetic Fe₃O₄ nanoribbon which is in good agreement with the data arising from the Scherrer equation. Furthermore, the FESEM image showed the good dispersion capability of the ES/Fe₃O₄ nanocomposite, which should be due to the decreasing of the electrostatic force and high surface energy of the magnetic particles after being coated onto ES. In general, the great dispersion of the Fe₃O₄ nanoribbons on the porous structure of the eggshell with spherical morphology provides a large contact area for catalyzing the reaction.

Fig. 5 TEM and FESEM images of (a) ES/Fe₃O₄ nanocomposite and (b) raw ES.

VSM analysis

A VSM plot of the ES/Fe₃O₄ magnetic nanocomposite was shown in Fig. 6. The symmetric hysteresis displays the magnetic behaviors of the composite. This is because that the diameter of the as-synthesized magnetic nanocomposite (18 nm) is smaller than that of the critical threshold of Fe₃O₄ (25 nm).⁴³ As shown in Fig. 6a and b, the saturation magnetization of the magnetic nanocomposite is smaller than that of pure Fe₃O₄. This small saturation magnetization (7.68 emu g⁻¹) may be attributed to the small particle size effect of the magnetic nanocomposite since a noncollinear spin arrangement occurs primarily at or near the surface.⁴³ This phenomena results in the reduction of the magnetic moment in magnetic nano particles.⁴⁴

Fig. 6 VSM plot of (a) magnetic nanocomposite and (b) pure Fe₃O₄.

Synthesis of 1,8-dioxo-1,2,3,4,5,6,7,8-octahydroxanthene derivatives

The activity of the nanocatalyst was tested using a one-pot three-component condensation of dimedone (2 eq.) with different aromatic aldehydes (1 eq.) under solvent-free and thermal conditions to obtain the corresponding 1,8-dioxo-octahydroxanthenes. Our attempts to synthesise compound 4 in the absence of the catalyst produced only compound 3 and no cyclization product was obtained even after 5 h (Scheme 1). Next, the optimum amount of nanocatalyst was evaluated in the range of 0.02–0.2 g, as summarized in Fig. 7. The optimum amount of catalyst was 0.1 g. Further increasing the amount of catalyst did not show a significant improvement of the yield or reaction time.

Fig. 7 Optimization of catalyst amount.

In order to investigate the role of Fe₃O₄ MNPs in enhancing the catalytic activity of the ES, we decided to carry out comparative reactions in the presence of the ES/Fe₃O₄, raw ES, pure Fe₃O₄ and physically mixed ES and Fe₃O₄ (Table 1). As described in Table 2 (entries 1–3), the raw ES is the best catalyst after the ES/Fe₃O₄ nanocomposite and the physically mixed ES–Fe₃O₄ could not improve its catalytic activity (entry 3), whereas the chemical mixture of ES and Fe₃O₄ could improve the catalytic activity of the ES effectively.

Table 1 Synthesis of 3,3,6,6-tetramethyl-9-(4-nitro-phenyl)-1,8-dioxo-1,2,3,4,5,6,7,8-octahydroxanthene in the presence of different catalysts

Entry	Catalyst	Time	Yield^a (%)
a Yield refers to isolated pure product.
1	ES/Fe3O4	15 min	95
2	ES	2 h	93
3	ES + Fe3O4	2 h	86
4	Fe3O4	2 h	82
5	CaCO3/Fe3O4	2 h	74

---

Table 2 Nano ES/Fe₃O₄-catalyzed synthesis of 3,3,6,6-tetramethyl-9-phenyl-1,8-dioxo-1,2,3,4,5,6,7,8-octahydroxanthene derivatives

Entry	Ar	Yield^a (%)	Mp (°C) [ref.]
a Yield refers to isolated pure product.
1	C6H5	92	199–201 ref. 8
2	4-Cl–C6H4	93	233–235 ref. 8
3	3-NO2–C6H4	95	168–170 ref. 8
4	2,4-Cl2–C6H4	97	247–249 ref. 8
5	4-Br–C6H4	98	244–246 ref. 58
6	2-OH–C6H4	90	182–184 ref. 58
7	4-OH–C6H4	98	246–248 ref. 8
8	4-CH3O–C6H4	96	241–243 ref. 8
9	4-NO2–C6H4	95	222–224 ref. 8
10	4-(CH3)2N–C6H4	94	219–221 ref. 8

---

Since almost 90% of the eggshell is composed of calcium carbonate and this is the main active site of the eggshell catalyst, we carried out a comparative reaction in the presence of CaCO₃/Fe₃O₄ (0.1 g) in the same conditions. Compared with the nanocomposite, the reaction time was longer and the product yield was low. This proves that eggshell has a high porosity with a naturally much higher surface area. In summary, the ES/Fe₃O₄ nanocomposite showed a higher yield in a very short reaction time in comparison with the other catalysts.

Therefore, the role of the ES/Fe₃O₄ system as a multifunctional bio-derived catalyst with improved catalytic activity was proven. Furthermore, the generality of this reaction was examined using different aldehydes (Table 2). In all cases, the reactions gave the corresponding products in excellent yields (90–98%) and in very short reaction times (15 min). This method offers significant improvements with regards to the scope of the transformation, simplicity, and green aspects by avoiding expensive, hazardous or corrosive catalysts.

A possible mechanism for the formation of the products is shown in Scheme 2. In this reaction, intermediate 3 was formed through the Knoevenagel reaction between dimedone and aldehyde, and subsequently, elimination of water occurred from intermediate 3 to give compound 4.

Scheme 2 Plausible mechanistic pathway for the synthesis of 1,8-dioxo-1,2,3,4,5,6,7,8-octahydroxanthene derivatives.

Eco-friendly conditions, ease of separation, stability and the reusability of heterogeneous catalysts are the most important benefits and make them useful for green commercial and industrial applications. The reusability of the nanocatalyst was examined for the synthesis of 1,8-dioxo-octahydroxanthenes (Table 2, entry 2). The catalyst was recovered magnetically after each run, washed three times with hot EtOH, dried in an oven at 120 °C, and tested for its activity in subsequent runs (Fig. 8). It was found that the catalyst could be reused five times without any loss of activity. Thus, the new procedure is green, cost effective, clean and more efficient than reported methods, with ease of separation of the catalyst from the reaction mixture. This claim is justified through representative examples from more recently published literature using conventional catalysts, illustrated in Table 3.

Fig. 8 Recycling experiment using the nanocatalyst.

Table 3 A comparison of various methods in the synthesis of Ca₂Fe₂O₅ phase

Entry	Catalyst	Reaction conditions	Time/yield (%)	Reference
1	Amberlyst-15	Catalyst amount: 200 mg CH3CN (10 mL), reflux	5 h/94	45
2	p-Dodecylbenzenesulfonic acid	Catalyst amount: 10 mol% H2O (20 mL), ultrasonic irradiation	1 h/94	10
3	NaHSO4·SiO2	Catalyst amount: 100 mg CH3CN (10 mL), reflux	6.5 h/93	46
4	Silica chloride	Catalyst amount: 100 mg CH3CN (10 mL), reflux	6 h/90	49
5	1-Methylimidazolium trifluoroacetate:[Hmim]TFA	Catalyst amount: 100 mg 80 °C	3.5 h/82	47
6	Alum	Catalyst amount: 10 mol% H2O (10 mL), 80 °C	25 min/94	48
7	SmCl3	Catalyst amount: 20 mol% solvent-free, 120 °C	8 h/97	50
8	Cellulose sulfonic acid	Solvent-free, 110 °C	5 h/95	51
9	[Bmim]HSO4	Solvent-free, 80 °C	1.5 h/85	52
10	[Bmim]ClO4	Catalyst amount: 4 mmol 100 °C	1 h/94	53
11	Sulfamic acid	Catalyst amount: 10 mol% solvent-free, heat	11 h/94	54
12	LiBr	Catalyst amount: 15 mol% solvent-free, heat	1 h/84	55
13	Tetrabutylammonium hydrogen sulfate	Catalyst amount: 10 mol% aqueous 1,4-dioxane (20 mL), reflux	3 h/94	56
14	Tetrabutylammonium bromide	Catalyst amount: 40 mol% solvent-free, 120 °C	5 h/97	56
15	InCl3·4H2O	Catalyst amount: 22 mg 80 °C	40 min/96	57
16	Nano Fe3O4	Catalyst amount: 10 mol% solvent-free, 100 °C	15 min/90	58
17	ZrOCl2·8H2O	Catalyst amount: 12 mg solvent-free, 80 °C	25 min/95	59
18	ES/Fe3O4	Catalyst amount: 100 mg solvent-free, 80 °C	15 min/95	Present work

---

Experimental section

Materials and instrument

All chemicals used in this work, were of analytical reagent grade, purchased from Merck. IR spectra were obtained with a MATSON 1000 FT-IR spectrophotometer. X-ray diffraction (XRD) with an X-Pert Philips PW 340/60 diffractometer (40 kV and 30 mA) and Cu K_α radiation (λ = 0.154 nm) was used to analyze the structure of the composite. TGA experiments were carried out using an STA 409 PC Luxx thermal analysis machine (NETZSCH, Germany) under a flow of nitrogen. The morphology of the cross section of the film was examined with a scanning electron microscope (SEM) (Seron Tech. AIS 2100) and transition electron microscopy (TEM) (Philips, CM 120). The magnetic property of the MNPs was measured using an Alternating Gradient Force Magnetometer (AGFM, Iran).

General procedure for the preparation of magnetic ES/Fe₃O₄ nanocomposite

The eggshell powder was prepared according to our previous method.³⁶ Following this, 1.0 g of the ES powder was mixed simply with 50.0 mL of FeSO₄ solution at a concentration of 25–1000 mg L⁻¹. The suspension was stirred vigorously in a beaker at 60 °C. The black ES/Fe₃O₄ nano-composite was produced after 2 h. The resulting particles were magnetically separated and washed with deionized water three times to remove any excess salts from the suspension. The products were then dried at 60 °C for further characterization.

General procedure for the preparation of 1,8-dioxo-octahydroxanthenes

In a typical general procedure, a mixture of 4-chlorobenzaldehyde (0.14 g, 1 mmol) and dimedone (0.28 g, 2 mmol) in solvent-free conditions at 80 °C, were stirred thoroughly in the presence of a catalytic amount of catalyst (0.1 g) to afford the corresponding 1,8-dioxo-octahydroxanthenes in excellent yields. After the completion of the reaction (TLC), hot EtOH was added to the reaction mixture and stirred for 5 min. Then the solid catalyst was magnetically separated from the soluble products and washed with hot EtOH. After cooling, the crude products were precipitated. Pure 1,8-dioxo-octahydroxanthenes were obtained in high yields without the use of any more purification. All compounds were known in the literature^8–19 and the NMR and IR spectra of the products were in agreement with earlier data.^8–19 The selected spectral data of four representative 1,8-dioxo-1,2,3,4,5,6,7,8-octahydroxanthenes are given in the ESI.†

Conclusions

In summary, an efficient, green, and alkali-free synthesis of a magnetic ES/Fe₃O₄ nanocomposite has been established without any protective atmosphere by simply stirring FeSO₄ aqueous solution with eggshell powder as a coating agent via a thermal co-precipitation method. The Fe₃O₄ NPs showed magnetic properties even after coating. Moreover, a green, rapid and highly efficient protocol for the one-pot synthesis of 1,8-dioxo-octahydroxanthenes has been described under thermal and solvent-free conditions using the ES/Fe₃O₄ nanocomposite as a heterogeneous and green catalyst with high catalytic activity and reusability. Finally, the synthesis of the nanocomposite based on porous ceramic ES waste is interesting because of the potential to lower the cost of designing new materials in various fields, especially in organic transformation, and reducing environmental problems.

Acknowledgements

The financial support of the Graduate University of Advanced Technology is gratefully acknowledged (project 7/4736).

Notes and references

1. R. D. Jonathan, K. R. Srinivas and E. B. Glen, Eur. J. Med. Chem., 1988, 23, 111.
2. J. Robak and R. J. Gryglewski, Pol. J. Pharmacol., 1996, 48, 555; H. K. Wang, S. L. Lee and K. H. Morris-Natschke, Med. Res. Rev., 1997, 17, 367; V. Rukavishnikov, M. P. Smith, G. B. Birrell, J. F. W. Keana and O. H. Griffith, Tetrahedron Lett., 1998, 39, 6637.
3. N. Mulakayala, P. V. N. S. Murthy, D. Rambabu, M. Aeluri, R. Adepu, G. R. Krishna, C. M. Reddy, K. R. S. Prasad, M. Chaitanya, C. S. Kumar, M. V. B. Rao and M. Pal, Bioorg. Med. Chem. Lett., 2012, 22, 2186.
4. A. Naya, M. Ishikawa, K. Matsuda, K. Ohwaki, T. Saeki, K. Noguchi and N. Ohtake, Bioorg. Med. Chem. Lett., 2003, 11, 875; H. Wang, L. Lu, S. Zhu, Y. Li and W. Cai, Curr. Microbiol., 2006, 52, 1.
5. P. Srihari, S. S. Mandal, J. S. S. Reddy, R. Srinivasa Rao and J. S. Yadav, Chin. Chem. Lett., 2008, 19, 771.
6. G. Pohlers, J. C. Scaiano and R. F. Sinta, Chem. Mater., 1997, 9, 3222; C. G. Knight and T. Stephens, Biochem. J., 1989, 258, 683.
7. J. F. Callan, A. P. de Silva and D. C. Magri, Tetrahedron, 2005, 61, 8551; O. Sirkecioglu, N. Talinli and A. Akar, J. Chem. Res., Synop., 1995, 502; A. Banerjee and A. K. Mukherjee, Stain Technol., 1981, 56, 83; J. P. Poupelin, G. Saint-Rut and O. J. Foussard-Blanpin, Med. Chem., 1978, 13, 67.
8. T. S. Jin, J. S. Zhang, J. C. Xiao, A. Q. Wang and T. S. Li, Synlett, 2004, 866.
9. F. Darvish, S. Balalaei, F. Chadegani and P. Salehi, Synth. Commun., 2007, 37, 1059.
10. T. S. Jin, J. S. Zhang, A. Q. Wang and T. S. Li, Ultrason. Sonochem., 2006, 13, 220.
11. S. Rostamizadeh, A. M. Amani, G. H. Mahdavinia, G. Amiri and H. Sepehrian, Ultrason. Sonochem., 2010, 17, 306.
12. N. Mulakayala, G. P. Kumar, D. Rambabu, M. Aeluri, M. V. B. Rao and M. Pal, Tetrahedron Lett., 2012, 53, 6923.
13. A. Ilangovan, S. Muralidharan, P. Sakthivel, S. Malayappasamy, S. Karuppusamy and M. P. Kaushik, Tetrahedron Lett., 2013, 54, 491.
14. M. Seyyedhamzeh, P. Mirzaei and A. Bazgir, Dyes Pigm., 2008, 76, 836.
15. E. Mosaddegh, M. R. Islami and A. Hassankhani, Arabian J. Chem., 2012, 5, 77.
16. A. Hasaninejad, M. Dadar and A. Zare, Chem. Sci. Trans., 2012, 1, 233.
17. F. Shirini, M. Abedini and R. Pourhasan, Dyes Pigm., 2013, 99, 250.
18. F. Shirini and N. Ghaffari Khaligh, Dyes Pigm., 2012, 95, 789.
19. B. Hamers, E. Kosciusko-Morizet, C. Müller and D. Vogt, ChemCatChem, 2009, 1, 103.
20. L. Shen, P. E. Laibinis and T. A. Hatton, Langmuir, 1999, 15, 447.
21. J. Q. Cao, Y. X. Wang, J. F. Yu, J. Y. Xia, C. F. Zhang, D. Z. Yin and U. O. Hafeli, J. Magn. Magn. Mater., 2004, 277, 165.
22. Q. Li, Y. M. Xuan and J. Wang, Exp. Therm. Fluid Sci., 2005, 30, 109.
23. J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeong, Nat. Mater., 2004, 3, 891.
24. F. X. Redl, C. T. Black, G. C. Papaefthymiou, R. L. Sandstorm, M. Yin, H. Zeng, C. B. Murray and S. P. O'Brien, J. Am. Chem. Soc., 2004, 126, 14583.
25. S. Sun, H. Zeng, D. B. Robinson, S. Raoux, P. M. Rice, S. X. Wang and G. Li, J. Am. Chem. Soc., 2004, 126, 273.
26. A. Bee, R. Massart and S. Neveu, J. Magn. Magn. Mater., 1995, 149, 6.
27. J. P. Jolivet, C. Chaneac and E. Tronc, Chem. Commun., 2004, 481.
28. X. Wang, J. Zhuang, Q. Peng and Y. Li, Nature, 2005, 437, 121.
29. Y. Zhu, L. P. Stubbs, F. Ho, R. Liu, C. P. Ship, J. A. Maguire and N. S. Hosmane, ChemCatChem, 2010, 2, 365.
30. S. I. Stoeva, F. Huo, J. S. Lee and C. A. Mirkin, J. Am. Chem. Soc., 2005, 127, 15362.
31. M. A. Correa-Duarte, M. Grzelczak, V. Salgueirino-Maceira, M. Giersig, L. M. Liz-Marzan, M. Farle, K. Sierazdki and R. Diaz, J. Phys. Chem. B, 2005, 109, 19060.
32. E. Mosaddegh and A. Hassankhani, Chin. J. Catal., 2014, 35, 351.
33. G. Suresh Kumar, A. Thamizhave and E. K. Girija, Mater. Lett., 2012, 76, 198.
34. P. S. Guru and S. Dash, Adv. Colloid Interface Sci., 2014, 209, 49.
35. J. Z. Zhang, J. G. Wang and J. J. Ma, J. Zhejiang Univ., Sci., A, 2005, 6, 1095.
36. (a) E. Mosaddegh and A. Hassankhani, Catal. Commun., 2013, 33, 70; (b) E. Mosaddegh, Ultrason. Sonochem., 2013, 20, 1436.
37. P. Refait and J. M. R. Genin, J. Mater. Res., 1993, 34, 797.
38. A. A. Olowe and J. M. R. Genin, J. Mater. Res., 1991, 32, 965.
39. C. Hui, C. Shen, T. Yang, L. Bao, J. Tian, H. Ding, C. Li and H. J. J. Gao, J. Phys. Chem. C, 2008, 112, 11336.
40. S. Sheng-Nan, W. Chao, Z. Zan-Zan, H. Yang-Long, S. S. Venkatraman and X. Zhi-Chuan, Chin. Phys. B, 2014, 23, 037503.
41. Y. Wei, B. Han, X. Hu, Y. Lin, X. Wang and X. Deng, Procedia Eng., 2012, 27, 632.
42. E. Mosaddegh, A. Hassankhani and H. Karimi-Maleh, Mater. Sci. Eng., C, 2015, 46, 264.
43. L. Levy, Y. Sahoo, K. Kim, E. J. Bergey and P. N. Prasad, Chem. Mater., 2002, 14, 3715.
44. S. F. Si, C. H. Li, X. Wang, D. P. Yu, Q. Peng and Y. D. Li, Cryst. Growth Des., 2005, 5, 391.
45. B. Das, P. Thirupathi, I. Mahender, V. S. Reddy and Y. K. Rao, J. Mol. Catal. A: Chem., 2006, 247, 233.
46. B. Das, P. Thirupathi, K. R. Reddy, B. Ravikanth and L. Nagarapu, Catal. Commun., 2007, 8, 535.
47. M. Dabiri, M. Baghbanzadeh and E. Arzroomchilar, Catal. Commun., 2008, 9, 939.
48. B. R. Madje, M. B. Ubale, J. V. Bharad and M. S. Shingare, S. Afr. J. Chem., 2010, 63, 36.
49. H. A. Soliman and T. A. Salama, Chin. Chem. Lett., 2013, 24, 404.
50. A. Llangovan, S. Malayappasamy, S. Muralidharan and S. Maruthamuthu, Chem. Cent. J., 2011, 5, 81.
51. H. A. Oskooie, L. Tahershamsi, M. M. Heravi and B. Bag-hernejad, E-J. Chem., 2010, 7, 717.
52. K. Niknam and M. Damya, J. Chin. Chem. Soc., 2009, 56, 659.
53. S. Makone and S. Mahurkar, Green Sustainable Chem., 2013, 3, 27.
54. A. Saini, S. Kumar and J. S. Sandhu, Synlett, 2006, 1928.
55. B. Rajitha, B. S. Kumar and Y. T. Reddy, Tetrahedron Lett., 2005, 46, 8691.
56. H. N. Karade, M. Sathe and M. P. Kaushik, ARKIVOC, 2007, xiii, 252.
57. B. Karamia, S. Nejatib and K. Eskandari, Curr. Chem. Lett., 2015, 4, 169.
58. M. A. Ghasemzadeh, A. Safaei-Ghomi and S. Zahedi, J. Serb. Chem. Soc., 2013, 78, 769.
59. E. Mosaddegh, M. R. Islami and A. Hassankhani, Arabian J. Chem., 2012, 5, 77.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17639e

---