Magnetically separable chicken feathers: a biopolymer based heterogeneous catalyst for the oxidation of organic substrates

Abstract

Magnetically separable poultry chicken feathers were found to be an efficient, green and heterogeneous catalyst for the oxidation of alcohols and sulfides to the corresponding carbonyl compounds and sulfoxides, respectively using t-butyl hydroperoxide (TBHP) as oxidant with complete selectivity and high conversions. The developed catalyst exhibited higher stability, activity and better recycling ability than the bare magnetic nanoparticles. The designed catalyst could readily be recovered using an external magnet without showing any significant leaching during the reaction.

---

Recently, the use of magnetic nanoparticles (MNPs) as a heterogeneous catalyst in organic transformations has received considerable interest. The main advantage of these catalysts lies in the fact that the magnetic nanoparticles can be efficiently isolated from the reaction mixture through a simple magnetic separation using external magnet. However, the main problem^1–3 associated with MNPs is their agglomeration and oxidation by air, which can adversely affect their catalytic activity. These problems can be overcome by the coating of magnetic nanoparticles with an organic or inorganic core. These coatings not only prevent agglomeration but also provide a barrier to prevent oxidation of MNPs. So far several natural and synthetic polymers have been used in the preparation of coated MNP.⁴

Chicken feathers (CF), a waste material are a cheap, biodegradable and natural biopolymer produced in large amounts from poultry industries. They are mainly (>90%) composed of the protein keratin, bio polymer with a high degree of cysteine cross-linkings.^5,6 In the recent decades, increasing environmental concerns have led interest towards the potential uses of renewable resources.^7–9 Their limited applicability and tedious disposal procedures are becoming a major concern in the recent decades.¹⁰ Chicken feather has been found to be a high performance material in various fields; for instance, in making plastics, as a sorbents, feather meal and other products.^11–13 Recently researchers have found potential applications of waste feathers by converting them into biodegradable polymers for expending the industrial applications.^14–16 In addition to these applications, feather keratin has widely been used for developing water-resistant thermoplastic materials for various applications.^17,18

Chicken feather is made up of polypeptide chains which are environmentally green, abundantly available, inexpensive and renewable. It holds the potential to act as a support and also as source of various functional groups like –COOH, –NH₂, –SH and –OH for various important chemical reactions. However the use of chicken feathers in catalytic applications is rarely known in the literature.¹⁹

Selective oxidation of alcohols and sulphides to corresponding aldehydes/ketones and sulfoxides is one of the most significant transformations in organic chemistry.^3,20,21 Traditionally, a number of stoichiometric oxidants such as chromium(VI) reagents, permanganates, metallic salts and N-chlorosuccinimide (NCS) are known; however, these reagents generate a huge amount of undesirable wastes which causes environmental problems.^22,23 Subsequently a number of noble²⁴ and non-noble metal²⁵ based catalysts in combination with greener oxidants such as molecular oxygen, hydrogen peroxide and t-butyl hydroperoxide (TBHP) have been reported. However, limited efficiency, requirement of basic additives, difficult separation of catalysts and poor product yields are certain drawbacks.²⁶ Thus, considering the economy and environmental friendliness, it is highly desirable to develop less expensive, easily separable, reusable non-noble heterogeneous catalysts, particularly, based on bio-derived materials under base-free conditions using green oxidants and green media (e.g., water) as the solvent. In this context, several biodegradable supports such as chitosan,²⁷ wool,²⁸ cellulose,²⁹ and starch³⁰ have been reported. In addition, the presence of magnetic element in heterogeneous catalysts is additionally desirable owing to their facile and efficient magnetic separation after reactions.³¹

In the present paper we report the successful synthesis of magnetically separable chicken feather (Fe₃O₄@CF) and its application as heterogeneous catalyst for oxidation of various alcohols and sulfides using TBHP as oxidant (Scheme 1).

Scheme 1 Fe₃O₄@CF catalyzed oxidation reactions.

Result and discussion

Synthesis and characterization of the catalyst

The required chicken feather coated magnetic (Fe₃O₄) nanoparticles were prepared by a hydrothermal method as proposed by Zhang et al.³² with the little modifications (Scheme 2). The obtained heterogeneous chicken feather coated MNPs denoted as Fe₃O₄@CF were found to be quite stable without their agglomeration and could easily be recovered by external magnet without any detectable leaching.

Scheme 2 Synthesis of Fe₃O₄@CF nanoparticles.

The structural changes after the modification of magnetic nanoparticles with CF (Fe₃O₄@CF) were determined by FTIR as shown in Fig. 1. Fig. 1a, shows the FTIR spectrum of bare MNPs. The observed band at 579 cm⁻¹ is associated with the stretching vibration of Fe–O. Further two bands around 3426 and 1635 cm⁻¹ ascribed to the stretching and bending vibrations of surface hydroxyl groups, respectively. Fig. 1b represents the FTIR spectrum of bare chicken feather. The absorption peaks at 1647, 1536 and 3423 cm⁻¹ are attributed to the characteristic absorption bands of the amide I and amide II and (–NH, –OH) stretching, respectively. However, in the FTIR spectrum of Fe₃O₄@CF (Fig. 1c), an additional peak at around 579 cm⁻¹, corresponding to the Fe–O vibrations confirmed the presence of iron NPs (Fig. 1c). The peak shifting of bending vibration of amide I and amide II from 1647 and 1536 cm⁻¹ to 1641 and 1515 cm⁻¹, respectively was most likely due to the interaction of amide groups of feather with metal nanoparticles (Fig. 1c). Furthermore, the peak at 1072 cm⁻¹ corresponding to C–O stretching vibration of –OH of chicken feather is disappeared in modified feather due to the interaction with MNPs (Fig. 1c). Based on these results, it was concluded that the Fe₃O₄@CF nanoparticles were successfully obtained along with a strong interaction between amine groups of the supported CF with Fe₃O₄ nanoparticles.

Fig. 1 FTIR spectra of: (a) bare MNP; (b) bare CF; (c) Fe₃O₄@CF.

Afterwards, the crystalline structure of Fe₃O₄@CF was determined by XRD. As shown in Fig. 2a, CF has amorphous structure, however, in Fe₃O₄@CF (Fig. 2b) the characteristic diffraction peaks (2θ = 30.21°, 35.61°, 43.31°, 53.8°, 57.31° and 63.01°) corresponding to the (220), (311), (400), (422), (511) and (440) reflections of cubic spinel structure with Fe₃O₄ phase confirmed the successful formation of CF coated MNPs.³³

Fig. 2 XRD patterns of: (a) bare CF; (b) Fe₃O₄@CF.

The morphology of the synthesized Fe₃O₄@CF catalyst was initially determined by transmission electron microscopy (TEM). However, the TEM image (Fig. 3a) did not provide appropriate information; hence we analyzed the sample by HR-TEM as shown in Fig. 4. Fig. 4a–c, clearly indicates the homogeneous distribution of black coloured CF coated MNPs with a narrow size distribution in the range of 3–15 nm. The SAED pattern in Fig. 3b exhibited a number of rings which clearly indicates a highly polycrystalline nature of the magnetite without the presence of any other crystalline phases. Furthermore, the elemental composition as determined by EDX analysis (Fig. 3c and 5d) indicates the presence of C, O and Fe elements in the synthesized Fe₃O₄@CF. Images of elemental mapping of C, Fe and the combined composition (Fig. 5a–c) clearly reveal the presence of iron oxide nanoparticles bounded with CF.

Fig. 3 (a) TEM image; (b) SAED pattern; (c) EDX of Fe₃O₄@CF.

Fig. 4 HR-TEM images of Fe₃O₄@CF; (a, c) of fresh catalyst; (b) particle size distribution in fresh catalyst; (d) recovered catalyst after 10 run.

Fig. 5 Elemental mapping of Fe₃O₄@CF for (a) C; (b) Fe; (c) mixture of C, Fe; (d) EDX image of Fe₃O₄@CF.

The surface textural properties of the synthesized material Fe₃O₄@CF was determined by N₂ adsorption/desorption as shown in Fig. 6. The N₂ adsorption/desorption isotherms of the core–shell magnetic Fe₃O₄@CF nanoparticles showed a curve, and the BET surface area, BJH pore volume and pore size was found to be 59.71 m² g⁻¹, 0.16 cm³ g⁻¹ and 3–15 nm, respectively. The loading of iron in the synthesized catalyst was found to be 27 wt% (1.6 mmol g⁻¹) as determined by ICP-AES analysis.

Fig. 6 N₂ adsorption/desorption isotherms of Fe₃O₄@CF and the inset is the corresponding pore size distribution.

Catalytic activity

After the successful synthesis of Fe₃O₄@CF, we intended to explore the catalytic activity of the catalyst for oxidation of alcohols using aqueous TBHP as oxidant. 4-Methoxy benzyl alcohol was chosen as a model substrate to perform the optimization experiments by varying the reaction parameters. The results of these experiments are summarized in Table 1. Initially the reaction was carried out in different solvents such as acetonitrile, dichloromethane n-hexane, chloroform and water as shown in Fig. 7. Among the various solvents studied, water was found to be a promising solvent for this transformation which further makes the developed protocol more attractive from environmental and economical viewpoints. The enhanced activity of the CF coated MNPs in water is most likely due to the possible strong hydrogen bonding between water molecules and amino acid chains. Owing to this bonding the amino acid chain may get stretched and provide more accessibility of active iron sites to the substrate molecules. However the organic solvents such as chloroform, dichloromethane, owing to their inability to form hydrogen bonding with functionalities of CF showed poor efficiency of this transformation.

Table 1 Results of optimization experiments^a

Entry	Cat (mol%)	Temp. °C	Time (h)	Conv.^b (%)
a Reaction conditions: 4-methoxy benzyl alcohol (1 mmol), TBHP (1.5 eq.), at 60 °C for 3 h.b Determined by GC-MS.c Blank reaction without catalyst.
1	0.2	60	2	58
2	0.5	60	2	67
3	1.0	60	2	97
4	2.0	60	2	98
5	1.0	60	12	5^c
6	1.0	RT	10	—
7	1.0	80	2	72

---

Fig. 7 Effect of solvent.

Further, we checked the effect of catalyst loading by varying the catalyst amount from 0.2 to 2 mol% (Table 1, entry 1–4) under otherwise identical experimental conditions. The conversion was found to be increased with increasing the iron loading from 0.2 to 1.0 mol% (Table 1, entry 1–3); however further increase in loading did not influence the yield to any significant extent (Table 1, entry 4). A very poor conversion was obtained in the absence of catalyst under otherwise identical reaction conditions (Table 1, entry 5). Further, the effect of reaction temperature was investigated. The reaction was found to be very slow at room temperature (Table 1, entry 6); however 60 °C was found to be optimum and gave maximum conversion of 4-methoxy benzyl alcohol to aldehyde with 97% isolated yield. Further increase in temperature (80 °C) afforded the poor yield (Table 1, entry 7) of 4-methoxy benzaldehyde along with the formation of benzoic acid as over-oxidation product.

Next, the scope of the reaction was explored by performing the oxidation of a variety of aliphatic and aromatic primary alcohols under the optimized reaction conditions. The results are presented in Table 2 (entries 1–10). All the substrates were efficiently converted to the corresponding carbonyl compounds in good to excellent yields. Benzyl alcohols substituted with electron-donating groups (Table 2, entries 4–5) were found to be more reactive and required lesser reaction time in completion of the reaction as compared to those having electron withdrawing substituents (Table 2, entries 7–9). Furthermore, chicken feather coating enhances the stability as well as catalytic efficiency of the MNPs, resulted to nearly two times higher activity than the bare MNPs for oxidation of 4-methoxy benzyl alcohol under identical reaction conditions (Table 2, entry 5). This enhancement is probably due to the interaction of functional groups of CF with substrate molecule through hydrogen bonding, which probably helps in abstracting the proton from alcohol molecule. Bare chicken feather (CF) without magnetic core did not show any activity for the oxidation of alcohol under otherwise similar reaction conditions (entry 5). This finding suggests that iron is the main active catalytic site for this transformation. Furthermore, we investigated the oxidation of secondary alcohols such as benzhydrol, cyclohexanol and cyclopentanol under the optimized reaction conditions (Table 2, entries 10–12). Secondary alcohols were selectively converted to the corresponding carbonyl compounds without any evidence for the formation of any by-product.

Table 2 Fe₃O₄@CF-catalyzed oxidation of alcohols^a

Entry	Alcohol	Product	Yield^b (%)
a Reaction conditions: substrate (1 mmol), catalyst (1.0 mol%), ^tBuOOH (1.5 eq.), at 60 °C for 3 h.b Isolated yields.c Using bare MNPs as catalyst.d Recycling experiment using recovered bare MNPs.e Using bare chicken feather powder as catalyst.f Yield of corresponding acid while using 3 mmol TBHP, 100 °C temp. and 6 h reaction time.
1			98
2			97
3			93
4			91
5			97, 51^c, 20^d^,^e, 98^f
6			92
7			90
8			92
9			85
10			94
11			90
12			93

---

Furthermore, we compared the catalytic activity of developed protocol with the literature known methods for oxidation of alcohols using molecular oxygen, hydrogen peroxide and TBHP as oxidant (Table 3). As shown in Table 3, the reported methods either use expensive, toxic metallic catalysts or require tedious synthetic procedures and afforded poor product yield in comparatively longer reaction time. However, the present methodology uses a waste material chicken feather based magnetic nanoparticles which are readily synthesized, low cost, easily accessible, easily recoverable with the effect of external magnet, exhibited excellent catalytic activity and afforded excellent conversions/yields under comparatively mild conditions. These benefits make the developed methodology a facile and promising tool from environmental and economical viewpoints.

Table 3 Comparison of Fe₃O₄@CF with known methods

Substrate	Catalyst/temp. (°C)	Oxidant	Solvent	Yield (%)	Ref.
Benzyl alcohol	RuO2·xH2O/80 °C	O2	PhCH3	16	34
1-Octanol	5% Pt–1%Bi–Al2O3/60 °C	O2	PhCH3	76	34
Benzyl alcohol	0.5% Pd/Al2O3/80 °C	O2	scCO2	87	34
2-Heptanol	Na2WO4/90 °C	H2O2	Solvent free	95	35
2-Decanol	Na2WO4/90 °C	H2O2	Solvent free	92	35
Benzyl alcohol	Fe2O3/SBA-15/95 °C	H2O2	H2O	95	36
Benzyl alcohol	[Cn*RuIII(CF3CO2)3·H2O](Cn*)N,N′,N′′-trimethyl-1,4,7-triazacyclononane)/RT, 12 h	TBHP	CH2Cl2	88	37

---

Although the exact mechanism of the reaction is not clear at this stage, in analogy to the existing reports³⁸ a plausible mechanism of the reaction is shown in Scheme 3. Based on the experimental results, it is clear that iron is the active catalyst for the reaction, which converts to oxo-iron intermediate through reaction with TBHP. In the next step, oxo-iron intermediate abstracts hydrogen from alcohol molecule and transforms it into corresponding carbonyl compound with the regeneration of the original catalyst as shown in Scheme 3. Water and t-butanol are formed as by-products during the reaction.

Scheme 3 Possible mechanism of the reaction.

Oxidation of sulfides to sulfoxides

Furthermore, we investigated the oxidation of a variety of substituted aromatic and aliphatic sulfides under optimized experimental conditions. The results are summarized in Table 4. We used several sulfides as substrates for oxidation and obtained corresponding sulfoxides without by-products such as sulfones.

Table 4 Fe₃O₄@CF catalyzed oxidation of sulfides to sulfoxides using aq. TBHP as oxidant^a

Entry	R	R^1	Yield^b (%)
a Reaction conditions: substrate (1 mmol), catalyst (1.0 mol%), aq. TBHP (1.5 mmol) at 60 °C using water as solvent; reaction time 3 h.b Isolated yield.
1	n-C3H7	n-C3H7	98
2	n-C4H9	n-C4H9	97
3	i-C3H7	i-C3H7	98
4	C6H5	C6H5	96
5	C6H5CH2	C6H5	97
6	C6H5	CH3	98
7	C6H5	n-C4H9	96
8	C6H5CH2	C6H5CH2	94
9	C6H4(OCH3-p)	C6H5	92

---

Recycling of the catalyst

Furthermore, we have tested the recycling of the developed heterogeneous catalyst by choosing the 4-methoxy benzyl alcohol as the model substrate. After the completion of the reaction, the catalyst could be easily recovered by external magnet and reused for subsequent ten runs. The recovered catalyst showed an efficient recycling ability without giving any significant change in the yield of the desired product under identical reaction conditions (Fig. 8).

Fig. 8 Results of recycling experiments.

In order to check the stability of the catalyst we also characterized the recovered catalyst after tenth run by FTIR, HR-TEM and ICP-AES analysis. Fig. 4d, shows the HR-TEM image of recovered catalyst which seems to be identical with the fresh one. Furthermore, nearly identical FTIR spectra of both fresh and recovered catalyst (Fig. 9) indicates that the synthesized catalyst is quite stable. Furthermore, we determined the iron content in the recovered catalyst after 10 runs. The loading of iron in the recovered catalyst was found to be 27 wt% (1.6 mmol g⁻¹) as determined by ICP-AES analysis, which is similar to the fresh catalyst.

Fig. 9 FTIR spectra of Fe₃O₄@CF: (a) fresh catalyst; (b) recovered catalyst after 10 run.

Experimental section

Materials

Chicken feather was supplied by local poultry farm, the feathers were cleaned (washed and treated with ethanol), dried in oven. Feather barbs separated from quill and ground in a Retsch ball mill to get feather powder. Ferric chloride and ferrous chloride was purchased from Sigma-Aldrich. All other chemicals were of the highest available grade and were used without further purification.

Characterization techniques

The synthesized catalyst, bare Fe₃O₄ and bare CF was characterized using various analytical techniques. The Fourier transform infrared (FT-IR) spectra were recorded by the KBr method with a Perkin Elmer spectrometer between 500 and 4000 cm⁻¹. Samples for XRD analysis was prepared on glass slide and the diffraction plot of CF, Fe₃O₄@CF was analyzed with Bruker D8 Advance diffractometer at 40 kv and 40 mA with Cu Kα radiation (λ = 0.15418 nm). Scanning electron microscopy (SEM) was performed by Jeol Model JSM-6340F. High resolution transmission electron microscopy (HR-TEM) of the samples was executed using a JEOL JEM 2100 Microscope. N₂ adsorption–desorption using a Micromeritics ASAP2010 instrument of samples evacuated at 200 °C for 4 h was used to determine specific BET surface area (S_BET) and pore volume. Pore size was calculated from the desorption branch of the adsorption–desorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The iron content in catalyst was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES DRE, PS-3000UV, Leeman Labs Inc, USA). Sample for ICP-AES was prepared by digesting 0.01 g catalyst with conc. HNO₃ at 70 °C for 30 min to oxidize all organic materials and leach out iron metal in the oxidized form. The obtained solution was filtered and final volume was made up to 10 mL by adding de-ionized water.

Preparation of Fe₃O₄@CF

Iron oxide nanoparticles were synthesized by co-precipitation method, in which NaOH solution was slowly added to the 40 mL solution of ferric chloride (4.86 g, 0.03 mol) and ferrous chloride (2.976 g, 0.015 mol) and the resulting mixture was stirred at 60 °C under nitrogen atmosphere. During this process, magnetite Fe₃O₄ NPs were precipitated out which were collected by an external magnet and subsequently washed with methanol, acetone and deionized water. The collected MNPs were dried in a vacuum and then dispersed in deionized water. To this suspension, 3 g of feather powder was added and the mixture was sonicated for approximately 2 h and then stirred for subsequent 5 h at room temperature (30 °C). The obtained Fe₃O₄@CF material was separated by external magnet and washed with methanol, acetone and deionized water to remove impurities. The final material so obtained was dried at 60 °C under vacuum.

General experimental procedure for oxidation reaction

A 50 mL round bottomed flask was charged with substrate (1.0 mmol), catalyst (1 mol%), water (5 mL), TBHP (70 wt% aq.; 1.5 eq.) and the resulting mixture was stirred at 60 °C for 3 h. The reaction progress was monitored by TLC. After completion of the reaction, the mixture was cooled to room temperature and the catalyst was separated by external magnet. The recovered catalyst was washed with methanol, dried under vacuum and used for recycling experiments. The filtrate so obtained was extracted with dichloromethane (20 mL). The organic layer was dried over anhydrous MgSO₄ and then, concentrated under reduced pressure to give crude product. The crude product was purified by column chromatography over silica gel using ethyl acetate : hexane (1 : 9) as eluent. The conversion of substrate was determined by GC-MS.

Acknowledgements

We kindly acknowledge Director, IIP for his kind permission to publish these results. MB is thankful to CSIR, New Delhi for providing technical HR under the XII five year projects.

References

1. W. Lim and I. S. Lee, Nano Today, 2010, 5, 412–434.
2. R. Narayanan and M. A. E. Sayed, J. Phys. Chem. B, 2005, 109, 12663–12676.
3. H. Atashin and R. Malakooti, J. Saudi Chem. Soc., 2013 DOI:10.1016/j.jscs.2013.09.007.
4. J. Dobson, Drug Dev. Res., 2006, 67, 55–60.
5. R. Lederer, Integument, Feathers, and Molt Ornithology: The Science of Birds, http://www.ornithology.com.
6. K. Sarvanan and B. Dhurai, Journal of Textile and Apparel Technology and Management, 2012, 7, 1–6.
7. B. C. Ranu, A. Saha and R. Dey, Drug Discovery Dev., 2010, 13, 658–668.
8. C. J. Jun Li and B. M. Trost, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 13197–13202.
9. R. J. White, V. Budarin, R. Luque, J. H. Clark and D. J. Macquarriea, Chem. Soc. Rev., 2009, 38, 3401–3418.
10. W. I. A. Saber, M. M. El-Metwally and M. S. El-Hersh, Res. J. Microbiol., 2010, 5, 21–35.
11. M. Bernhart and O. O. Fasina, Waste Manag., 2009, 29, 1392–1398.
12. S. Huda and Y. Yang, Compos. Sci. Technol., 2008, 68, 790–798.
13. V. McGovern, Environ. Health Perspect., 2000, 108, 366–369.
14. J. R. Barone, J. Polym. Environ., 2009, 17, 143–151.
15. J. R. Barone, Composites, Part A, 2005, 36, 1518–1524.
16. J. R. Barone and W. F. Schmidt, Compos. Sci. Technol., 2005, 65, 173–181.
17. E. Jin, N. Reddy, Z. Zhu and Y. Yang, J. Agric. Food Chem., 2011, 59, 1729–1738.
18. N. Reddy, J. Jiang and Y. Yang, J. Polym. Environ., 2014, 22, 310–317.
19. P. P. Latha, M. Bhatt and S. L. Jain, Tetrahedron Lett., 2015, 56, 5718–5722.
20. L. P. Mahesh, R. M. Suleman, B. B. Hanumant and S. U. Balu, Oxidation of benzylic alcohols and sulfides over LaCoO₃ and TBHP, ARKAT USA, Inc., 2007, pp. 70–75.
21. R. Abdolreza, J. Maasoumeh, N. Atena and S. Mehri, Inorg. Chem. Commun., 2012, 15, 230–234.
22. G. J. Brink, I. W. C. Arend and R. A. Sheldon, Science, 2000, 287, 1636–1639.
23. G. M. Haresh, S. C. Ganesh and K. Ashok, Green Chem., 2006, 8, 344–348.
24. (a) H. Miyamura, R. Matsubara, Y. Miyazaki and S. Kobayashi, Angew. Chem., Int. Ed., 2007, 46, 4151–4154; (b) M. Mahyari, A. Shaabani, M. Behbahani and A. Bagheri, Appl. Organomet. Chem., 2014, 28, 576–583; (c) C. Shang and Z. P. Liu, J. Am. Chem. Soc., 2011, 133, 9938–9947; (d) S. Rautiainen, O. Simakova, H. Guo, A. R. Leino, K. Kordas, D. Murzin, M. Leskela and T. Repo, Appl. Catal., A, 2014, 485, 202–206.
25. (a) C. Bai, A. Li, X. Yao, H. Liu and Y. Li, Green Chem., 2016, 18, 1061–1069; (b) J. Zhu, J. L. Faria, J. L. Figueiredo and A. Thomas, Chem.–Eur. J., 2011, 17, 7112–7117; (c) J. Zhu, K. Kailasam, A. Fischer and A. Thomas, ACS Catal., 2011, 1, 342–347; (d) K. I. Shimizu, K. Kon, M. Seto, K. Shimura, H. Yamazaki and J. N. Kondo, Green Chem., 2013, 15, 418–424.
26. M. Jafarpour, A. Rezaeifard, V. Yasinzadeh and H. Kargar, RSC Adv., 2015, 5, 38460–38469.
27. (a) S. Ahmad, B. Boroujeni, M. Laeini and M. Sadegh, Appl. Organomet. Chem., 2016, 30, 154–159; (b) A. Murugadossa and H. Sakuraia, J. Mol. Catal. A: Chem., 2011, 341, 1–6.
28. S. Wu, H. Zhang, Y. Wang, Q. Su, Z. Lei and L. Wu, Lett. Org. Chem., 2014, 11, 774–779.
29. D. Baruah, U. P. Saikia, P. Pahari and D. Konwar, Tetrahedron Lett., 2015, 56, 2543–2547.
30. S. Verma, D. Tripathi, P. Gupta, R. Singh, G. M. Bahuguna, L. N. Shivakumar, R. K. Chauhan, S. Saran and S. L. Jain, Dalton Trans., 2013, 42, 11522–11527.
31. (a) J. Zhu, P. C. Wang and M. Lu, New J. Chem., 2012, 36, 2587–2592; (b) S. Ahmad, B. M. Borjian and S. M. Hamidzad, J. Chem. Sci., 2015, 127, 1927–1935.
32. S. Zhang, H. Niu, Z. Hu, Y. Cai and Y. Shi, J. Chromatogr. A, 2010, 1217, 4757–4764.
33. W. Li, Y. Tian, B. Zhang, L. Tian, L. Xiangjie, H. Zhang, N. Ali and Q. Zhang, New J. Chem., 2015, 39, 2767–2777.
34. T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–3058.
35. R. Noyori, M. Aokib and K. Satoc, Chem. Commun., 2003, 1977–1986.
36. F. Sadri, A. Ramazani, A. Massoudi, M. Khoobi, R. Tarasi, A. Shafiee, V. Azizkhani, L. Dolatyari and S. Woo Joo, Green Chem. Lett. Rev., 2014, 7, 257–264.
37. W. Fung, Y. Wing and C. Che, J. Org. Chem., 1998, 63, 2873–2877.
38. N. Sehlotho and T. Nyokong, J. Mol. Catal. A: Chem., 2004, 209, 51–57.

---