Magnetically separable cobalt ferrite nanocatalyst for ...

Magnetically separable… by K K Senapati and P Phukan,Bulletin of the Catalysis Society of India, 9 (2011) 1-8

Magnetically separable cobalt ferrite nanocatalyst for aldol condensations of aldehydes and ketones Kula Kamal Senapati and Prodeep Phukan* Department of Chemistry, Gauhati University, Guwahati – 781014, Assam, India E-mail: [email protected]

ABSTRACT Synthesis of spinel cobalt ferrite magnetic nanoparticles with an average sizes in the range 40-50 nm has been achieved by a combined sonochemical and co-precipitation technique in aqueous medium without any surfactant or organic capping agent. The uncapped nanoparticles were utilized directly for aldol reaction in ethanol. After the reaction is over, the nanoparticules were compartmented by using an external magnet.

KEYWORDS Cobalt ferrite, co-precipitation, sonochemical, magnetic nanocatalyst, aldol, carbon-carbon bond formation.

1. Introduction In recent years, the use of nanoparticles as catalysts in organic transformations has attracted considerable interest, as nanoparticles provide a larger number of active sites per unit area[1-2]. Although efficiency of a catalyst can be enhanced enormously by synthesizing them in nanometer scale, recovery of nanocatalyst from the reaction mixture is the major drawback as they cannot be efficiently filtered out of the reaction medium. In this respect, magnetic nanoparticles hold out significant potential as a reusable catalyst as they can be recovered simply by magnetic separation which intern prevents loss of catalyst and increases reusability. Recently, there are several reports in the literature for utilization of magnetic nanoparticles as support for immobilization of homogeneous catalyst [3-10]. Cobalt ferrites are the most widely studied bi-metallic magnetic oxides, because of their large magnetic anisotropy compared to other oxide ferrite [11, 12]. Several methods have been developed for the synthesis of cobalt ferrite nanoparticles which include important examples such as mechanochemical method [13], sonochemical reactions [14], co-precipitation [15-21], micro-emulsion procedure [22-23], and others [24-29]. However, in many cases, the nanoparticles formed get agglomerated with nonuniform shape and cannot be stored for a long period. Sometimes, the particles formed are poorly crystalline and milling or high temperature annealing is required to obtain highly crystalline structure. Also, the magnetic hardening occurs only after high temperature annealing. Microemulsion methods are very effective for synthesizing nanoparticles with well-defined size and narrow size distribution, but these methods are not suitable to produce in large quantities. The most advantageous method for production of nanoparticles is the coprecipitation method where the particles were prepared by coprecipitating a mixture of cobalt(II) and iron(II) salts with hydroxide ions using potassium nitrate [15] or air [30] as oxidizing agent. Very recently, Kulkarni and his coworkers reported a co-precipitation method for oleic acid capped CoFe2O4 nanoparticles [22]. They found that, coericivity of the CoFe2O4 nanoparticles is dependent on the amount of the capping agent as well as the annealing temperature. Although several reports are available in the literature for the synthesis of cobalt ferrite, most of them did not focus on the stability of the colloidal dispersions during storage. An organic capping agent such as oleic acid is necessary to make a stable colloidal dispersion of the nanoparticles [21, 22]. 1


In this report, we wish to present a combine sonochemical and coprecipitation technique for the synthesis of highly stable and nearly monodispersed cobalt ferrite nanocrystals in aqueous medium. Catalytic property of the magnetic nanoparticles was evaluated by carrying out aldol condensation of aldehydes and ketones in ethanolic solution. After the reaction, the catalyst could be recovered efficiently by using and external magnet. 2. Experimental Section 2.1. Synthesis of cobalt ferrite nanoparticles Cobalt-ferrite nanocrystals have been synthesized by a combined sonochemical and coprecipitation technique [31]. Two aqueous solutions of FeCl3 (1.5 g, 9.3 mmol, 50 mL) and CoCl2.6H2O (1g, 4.2 mmol, 50 mL) were mixed in a 200 mL flat bottom flask and placed in an ultrasonic bath. An aqueous KOH solution (3M, 25 mL) was added dropwise under argon atmosphere with continuous ultrasonic irradiation (frequency 40 KHz and power of 40 KW). Prior to mixing, all these three solutions were sonicated for 30 min to remove dissolved oxygen. The temperature of the sonicator bath was raised up to 60 ºC and the mixture was further sonicated for 30 minitues in air atmosphere. The black precipitate formation was observed during that time. Energy dispersive X-ray spectroscopy (EDX) analysis at this point confirms the formation of cobalt ferrite. The reaction mixture was centrifuged (14000 rpm) at ambient temperature for 15 minutes. The mixture was further subjected to successive sonication (30 min) and centrifugation (15 min) for five times. The black precipitate was then separated, washed with copious amount of distilled water, ethanol and kept overnight in an incubator at 60 ºC for ageing. The precipitate was further dried in oven at 100 ºC for one hour and subsequently kept under high vacuum (10-2 bar) for one hour. Finally, the black particles were taken in 50 mL of dry ethanol and subjected to successive sonicaiton (30min) and centrifugation (15 min) repeatedly till a brown colored solution appears. The precipitate was separated, dried and used for further applications. 2.2. Characterization The formation cobalt ferrite particles were first characterized by Electron Dispersive X-ray analysis combined with the Scanning Electron Microscope (SEM). Scanning Electron Microscopy was done on “LEO 1430 VP” Scanning Electron Microscope combined with Oxford EDX system (INCA X-ray microanalysis). For SEM analysis, the sample in ethanol was dispersed on aluminium foil rapped on the aluminum stub used for sample mounting. The sample was air dried and the stub was mounted into the SEM chamber. The sample was scanned by electron beam at an accelerating voltage of 15 kV and a working distance of 14 mm using the secondary electron detector. The particle sizes were measured in the image of 300 nm resolution at the magnification of 18 KX. The crystalline nature of the synthesized cobalt ferrite sample was further observed by X-ray diffraction pattern. X-ray diffraction measurement of the nanoparticles were measured on a Bruker AXS D8 using Cu Kα radiation (λ= 1.54178 Aº). The samples were scanned from 10o to 70o (2Ө) at the speed of 5o/min. TEM studies were made on a 200 KV Transmission Electron microscope (JEOL JEM2100). The sample for TEM analysis was prepared by taking a micro drop of dilute ethanolic solution of the particle deposited on a carbon-coated copper grid (400 mesh size). The nanoparticle formed was observed at bright field TEM images at the

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magnifications of 25 KX and 300 KX. The average size of the nanoparticles from the TEM analysis was found to be 40-50 nm which is consistent with the XRD pattern the particle size obtained from the XRD analysis. The same particle sizes were also evident from SEM morphology. The ESR spectra were recorded using a JEOL JES FA200 ESR spectrometer. The magnetic properties of the cobalt ferrite nano crystals were analyzed in a Lakshore Vibrating Sample Magnetometer system. The magnetic properties of the as synthesized nano particle was measured in vibrating sample magnetometer taking 20 mg of solid sample fixed to the tips of the vibrating rod. 2.3. Typical procedure for aldol condensation In a typical reaction, aldehyde (1 mmol), acetophenone (1 mmol), cobaltferrite catalyst (0.4 mmol) were taken in ethanol (3 ml) and stirred at 50 °C for appropriate time. The reaction progress was monitored by TLC. After completion of the reaction, dichloromethane was added and the catalyst was separated by magnetic decantation. After recovery of the catalyst, organic solvent was evaporated under reduced pressure and the reaction mixture was purified by column chromatography using ethyl acetate / petroleum ether as the eluent. 3. Results and discussions 3.1 Synthesis of CoFe2O4 nanoparticles Cobalt ferrite nanoparticles were synthesized by a combined sonochemical and coprecipitation technique in aqueous medium without any surfactant or organic capping agents [31]. The synthesis was carried out in an alkaline pH with repeated ultrasonic irradiation. At the initial stage of the formation of the black particles, the composition was determined by using EDX technique. Final analysis of the particle by EDX confirms the desired composition of the CoFe2O4 nanoparticles. Formation of stable nanoparticle could be due to repetitive ultrasonic treatment during synthesis [32,33]. Cavitation effect during ultrasounic treatment might be responsible for generation of tremendous pressure and temperature. This effect creates some short lived localized hot-spots which may induce in situ calcinations to obtain directly cobalt ferrite particles. Hence, extra high temperature is not necessary in this method to get crystalline nanoparticles. We observed that it is not possible to produce stable nanoparticles if the synthesis is carried out without repetitive ultrasonic treatment. At the initial stage of the synthesis, formation of a dark brown precipitate was observed which on repetitive ultrasonication, transforms into black particles. Interestingly, the resulting black nanomaterial as well as its dispersion exhibit excellent stability and can be stored without any stabilizer for 2 months in a refrigerator. Agglomeration of the nanoparticles was not observed during storage. The morphology and properties remain unaltered during storage which is evident from electron microscopic analysis. Further, the solid nanopaticles can be easily dispersed in aqueous or alcoholic medium to form colloidal solution. 3.2. Structural characteristics The XRD pattern (Figure 1) of the nanopaticles shows the presence of all the characteristic signals which are present in spinel cobalt ferrite (JCPDS–International center diffraction data, PDF cards 3-864 and 22-1086) [21, 28]. The particle size of the cobalt ferrite nanoparticles obtained from the XRD pattern using scherer's formula [34] was found to be 41 nm.

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Figure 1: XRD pattern of as-synthesized cobalt ferrite nano particle The structural composition and crystallinity of the cobalt ferrite nanoparticles was further examined by using SEM and TEM. Figure 2 shows the SEM image of the nanoparticles. The iron/cobalt ratio in the nanocrystals as determined by EDX analysis was found to be 2.02 which is very much close to the atomic ratio in the formula CoFe2O4.

Figure 2. SEM image of cobalt ferrite samples Figure 3 shows the TEM image of the cobalt-ferrite nanocrystals deposited on a carbon coated copper grid. The average size of the nanoparticles from the TEM analysis was found to be 40-50 nm (Figure 3a & b) which is consistent with the particle size obtained from XRD analysis. The diffraction pattern (SAED) obtained from the TEM (Figure 3c) showed spinel phase CoFe2O4, with the rings corresponding to reflections from the (220), (311), (422), (440) and (642) planes. The interplanar distance of the (111) reflections observed in HRTEM image (Figure 3d) is 0.45 nm corresponds to the spinel phase crystalline nanoparticle [16].

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1 00 10 0 nm nm

b

a

20 2 0 nm nm

Figure 3.

TEM micrographs of the synthesized nanosized cobalt ferrite samples: a. Bright Field TEM (300KX), b. Bright Field TEM (25KX) c. SAED pattern d. HRTEM 3.3. Magnetic properties The magnetic properties of the as prepared cobalt nanoparticles were investigated using a vibrating sample magnetometer (VSM), variable temperature electron spin resonance (ESR) spectrometer. The ESR spectra was recorded in the temperatrue range from 300K to 100K (Figure 4). The value of g-factor (=2.01) determined from EPR measurement is in good agreement with that reported in the literature for nanostructured CoFe2O4 particles [14]. When the spectra was recorded by varying the temperarute, line broadening of the EPR signal was observed with decreasing temperature with no significant shift in resonance magnetic field, which can be attributed to strong magnetocrystalline anisotropy. 1000 800 600

Intensity

400 200 0

300K

-2 0 0 -4 0 0

200K

-6 0 0 -8 0 0

80K -1 0 0 0 200

400

600

F ie ld ( x 1 0 G )

Figure 4. ESR spectra of the cobalt ferrite sample In the VSM measurement, the magnetization-hysteresis (M-H) loop (Figure 5) was taken at room temperature with a maximum applied field of ± 2T. From the hysteresis loop, the saturation magnetization (Ms), coercivity (Hc) and rentivity (Mr) were evaluated. The Ms value was found to be 64.49 emug-1 for the CoFe2O4 nanoparticles. Similarly, the Hc values of 895.4 Oe and the Mr values of 32.36 emug-1 were obtained for the nanocatalyst. 3.4. Catalytic activity for aldol condensation reaction: The aldol condensation reaction is an important and widely used synthetic transformation in organic synthesis for the formation of carbon-carbon bond [35]. Chalcones are commonly synthesized by a base catalysed aldol condensation of aldehyde

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with a ketone. We have examined the catalytic activity of the synthesized cobalt ferrite particles for aldol condensation between different aromatic aldehydes and acetophenone derivatives (Scheme 1).

Figure 5. M-H loops of CoFe2O4 nanoparticles O

O

O

CoFe2O4 MNPs

+ 1

R

EtOH, 50-60 oC R1

R2

R2

Scheme 1: Aldol condensation reaction in presence of cobalt ferrite MNPs Treatment of aldehydes (1 mmol) with ketones (1 mmol) in the presence of CoFe2O4-MNPs catalyst (40 mol%) in 5 mL ethanol at 50-60oC gave the corresponding α,β-unsaturated ketones in moderate to good yields. The typical results of the CoFe2O4 MNPs-catalyzed aldol reactions of aromatic aldehydes and aryl ketones, under optimized reaction conditions, are shown in Table 1. Table 1: Aldol condensation of aromatic aldehydes and ketones by CoFe2O4 MNPs Entry Substrate Reactant Product Time Yield (h) (%) O

O

1

O

H

O

O

2

3

O

70

1.4

65

2

75

O

H

Me

Me

O

O

O

H

MeO

1.5 Cl

O

4

60

O

H

Cl

2

MeO

6


O

5

O

O

1.1

H

Br

68

Br

O

O

6

O

0.5

H

O 2N

76

O 2N

Cl

O

O

7

O

Cl

H

O

O

8

65

1

66

3

61

2.5

63

O

H

F

F

O

9

O

O

H

OH

HO

O

10

1.2

O

O

H

MeO

OMe

It is seen from Table 1 that chalcones were formed in moderate to good yields and stable under the reaction conditions and no reversibility was observed. Moreover, the self-condensation of aldehydes was not observed and hence, no side products of catalytic aldol reactions were found. More beneficially, the catalyst can be separated from the reaction mixture by an external magnetic field after the completion of reaction.

Conclusions In conclusion, a new type of CoFe2O4 magnetic nanocatalyst has been developed with average sizes in the range 40-50 nm using a combined sonochemical and co-precipitation process. These MNPs were used as efficient quasi-homogenous catalyst for aldol condensation of aromatic aldehydes with acetophenone derivatives under mild conditions. Catalyst could be recovered from the reaction mixture by magnetic compartmentation with the aid of an external magnet after completion of reaction. Acknowledgement: Financial support from DST (India) for TEM facility at CIF, IIT Guwahati (Grant No. SR/S5/NM-01/2005) and Ramanna Fellowship to P. Phukan (Grant No. SR/S1/RFPC-07/2006) is gratefully acknowledged. References:

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