Preparation of Nanosized Gold Particles by Microwave Irradiation and Kinetics Study for Reduction of 4-Nitroaniline under Various Conditions

Preparation of Nanosized Gold Particles by Microwave Irradiation and Kinetics Study for Reduction of 4-Nitroaniline under Various Conditions

Jae Jin Kim and Weon Bae Ko

Department of Chemistry, Sahmyook University, Seoul 139-742, Korea (Received September 15, 2015, Revised October 8, 2015, Accepted October 14, 2015)

Abstract: Nanosized gold particles were synthesized by microwave irradiation in a mixture composed of potassium tet- rachloroaurate(III) n-hydrate, sodium citrate dihydrate and Tween 20. The synthesized gold particles were characterized by UV-vis spectrophotometer, scanning electron microscopy, and X-ray diffraction techniques. Using UV-vis spectroscopy, it was confirmed that gold nanoparticles act as a catalyst in the reduction of 4-nitroaniline with sodium borohydride to form 1,4-diaminobenzene. Additionally, we studied the kinetics of this reductive reaction in the presence of these gold nanopar- ticles under various conditions.

Keywords: nanosized gold particles, 4-nitroaniline, reductive reaction, catalytic activity

Introduction

Recently, metal nanopaticles have attracted significant attention among researchers due to their outstanding plas- monic activity, chemical stability, and catalytic properties.

In particular, nanometer sized gold particles are frequently employed in the preparation of hybrid nanocomposites that have uses in electrochemical and molecular electronics appli- cation.

It is known that gold nanoparticles are able to catalyze the reduction of nitro compounds to their amino analogs with sodium borohydride.

In this reaction, the hydrogen is trans- ferred from the borohydride anion to the aromatic nitro com- pound by the gold nanoparticles.

The catalytic activity of many metal and semiconductor nanoparticles in the reductive reaction of 4-nitroaniline is also known.

4-Nitroaniline is a known toxin and is found in wastewater discharged from chemical manufacturing industries. The chemical is dangerous to aquatic organisms and may cause long-term damage to the environment.

1,4-diaminobenzene is known as an important component in the industrial chem- icals, such as rubber antioxidants, precursor to aramid textile fibers, thermoplastics and elastomers.

Preparation of 1,4- diaminobenzene is obtained by the catalytic hydrogenation of 4-nitroaniline.

The mechanism of reductive reaction from 4-nitroaniline to the 1,4-diaminobenzene has been reported to follow either a direct route or a condensation route.

The condensation route is more advantages than the direct route, as the nitro com- pound is adsorbed on the surface of nanosized gold parti- cles.

We have studied the catalytic activity of nanosized gold particles for the reductive reaction of 4-nitroaniline to 1,4- diaminobenzene in an aqueous solution of NaBH

.

We used UV-vis spectroscopy at various temperatures to investigate the progress of the reductive reaction of 4-nitroaniline to 1,4- diaminobenzene.

Experimental

1. Chemical reagents

Potassium tetrachloroaurate(III) n-hydrate (KAuCl

·nH

O) was purchased from Wako Pure Chemical Industries, Ltd.

Sodium citrate dihydrate (C

H

Na

O

·2H

O) was obtained from Samchun Chemicals. Tween 20 was supplied by Sigma- Aldrich.

2. Preparation of nanosized gold particles

0.01 M Potassium tetrachloroaurate(III) n-hydrate (KAu- Cl

·nH

O) solution was added to a beaker, containing 50 ml

†

Corresponding author E-mail: [email protected]

of distilled water. 10 ml of 1% sodium citrate dihydrate (C

H

Na

O

·2H

O) solution was added to the above solution containing 1 wt% aqueous solution of Tween 20. The mixed solution was irradiated by microwave for 1 min.

3. Evaluation of the catalytic activity of the nanosized gold particles in the reductive reaction of 4-nitroaniline

The prepared nanosized gold particles were used as a catalyst to reduce 4-nitroaniline to 1,4-diaminobenzene with sodium borohydride in an aqueous solution. This reductive reaction was examined by UV-vis spectroscopy (Shimazu UV-1691PC).

4. Characterization of synthesized nanosized gold particles

The crystallite structure and size of gold nanoparticles were confirmed by X-ray diffraction (XRD, D8 Advance, Bruker) with CuK

radiation source λ = 0.15406 nm at 40 kV and 40 mA. The surface shape of the nanosized gold particles were observed by scanning electron microscopy (SEM, JSM-6510, JEOL Ltd) at an accelerating voltage of 0.5 to 30 kV.

Results and Discussion

1. Analysis of SEM

Figure 1 presents a typical SEM image of the nanosized

gold particles synthesized in deionized water solution under microwave irradiation for 1 min. The surface of the synthe- sized nanosize gold particles showed the shape of snow flake in Figure 1.

2. Analysis of XRD

The nanosized gold particles were analyzed by X-ray dif- fraction (Figure 2). The 2θ diffraction peaks were showed at 38.24°, 44.35°, 65.31° and 78.06°, which were assigned to the (110), (200), (220) and (311) planes, respectively. Table 1 shows the mean crystallite size of gold nanoparticles, which were calculated using Scherrer’s equation :

Figure 1. SEM image of the nanosized gold particles.

Figure 2. XRD pattern of the nanosized gold particles.

where, S is the crystallite size, θ is the Bragg diffraction angle of the XRD peak in radians, β is the full width at half maximum (FWHM) of the diffraction peak, and λ is the wavelength of X-rays (λ = 0.15406 nm). The average crys- tallite size of gold nanoparticles was calculated to be 13.68 nm in Table 1.

S = 0.9λ β cosθ --- Table 1. Mean Crystallite Size of the Nanosized Gold Particles,

Calculated by Scherrer’s Equation

Peaks 2θ value FWHM D (nm)

a 38.24° 0.473 19.406

b 44.35° 0.709 10.371

c 65.31° 0.551 11.981

d 78.06° 0.670 12.983

Average

crystallite size 13.685

Figure 3. UV-vis spectra of the reductive reaction of 4-nitroaniline at (a) 25

C and (b) 80

C. The reduction was carried out using nanosized

gold particles.

3. UV-vis spectroscopy analysis of the nanosized gold particles as a catalyst

The reduction of 4-nitroaniline to 1,4-diaminobenzene with NaBH

using nanosized gold particles as a catalyst is shown in Figure 3. NaBH

was used as the reducing agent. UV-vis spectroscopy was used to examine the reduction of 4-nitroan- iline to 1,4-diaminobenzene.

As soon as NaBH

was added into an aqueous solution 4-nitroaniline, the catalytic gold nanoparticles began relaying electrons from BH

to the aryl nitro compound substrate.

Under the experimental conditions used in this study, the BH

anion on its own is incapable of reducing the nitro com- pound to the corresponding amino compound.

It is an industrially important step meaning reductive reaction from nitro compounds to amino compounds because aromatic amino compounds are primarily used as intermediates in the manufacture of dyes and pigments. The catalytic activity of the nanosized gold particles is efficient and erases the kinetic barrier of the reaction to assist with electron relay for the reductive reaction.

Figure 3 shows the time-reliant UV-vis absorption spectra obtained for the sodium borohydride reductive reaction of 4-nitroaniline at 25

C and 80

C in the presence of nanosized gold particles. 4-Nitroaniline shows a strong absorption band at 380 nm, and as 4-nitroaniline was converted to 1,4-diaminobenzene, the color of the solution changed from yellow to colorless. The amount of the nano- sized gold particles added in the solution was very small;

hence, they did not affect the absorption spectra of 4-nitroan- iline.

As shown in Figure 4, the absorbance peak at 380 nm

decreased gradually and disappeared with the reduction of 4- nitroaniline. At the same time, a new peak at 240 nm appeared and increased in intensity because of the formation of 1,4-diaminobenzene.

4. Kinetic study on the reductive reaction of 4-nitroaniline catalyzed by nanosized gold particles

Figure 4 shows the kinetic profile of the reduction of 4- nitroaniline.

Kinetic analysis indicated that the reduction of 4-nitroaniline can be approximated as pseudo-first-order kinetics, according to the Langmuir-Hinshelwood (L-H) equation, which also takes into account the adsorption prop- erties of the catalyst surface for the substrate:

ln( C/C

) = −kt

where C

is the initial concentration of dyestuff solution and C is the concentration at the time of measurement, t. When the concentration of reductive reaction was fixed, k was determined by the variation of concentration with time. Here, the value of k can be a measure of catalyst efficiency.

Figure 4 shows the variation of ln(C/C

) with time taken for the reductive reaction of 4-nitroaniline using the nano- sized gold particles. The linear profile of these data points indicated that the reduction of 4-nitroaniline follows the first- order kinetics.

Conclusion

Nanosized gold particles were synthesized by using potas- sium tetrachloroaurate(III) n-hydrate, sodium citrate dihy- drate, and Tween 20 under microwave irradiation. The nanosized gold particles were dehydrated in a drying oven at 60

C. XRD analysis confirmed that the prepared products were gold nanoparticles. The 2θ diffraction peaks at 38.24°, 44.35°, 65.31°, and 78.06° were assigned to the (110), (200), (220), and (311) planes, respectively. The average crystallite size of the gold nanoparticles was 13.68 nm and SEM showed that the gold nanoparticles had the shape of snow- flake. In addition, the gold nanoparticles, as a catalyst, could enhance the reduction of 4-nitroaniline to 1,4-diaminoben- zene in the presence of sodium borohydride. Kinetic analysis indicated that this reaction follows the first-order reaction kinetics. The kinetics of the reductive reaction were also dependent on temperature, i.e., at higher temperatures (80

C), the reduction of 4-nitroaniline to 1,4-diaminobenzene was Figure 4. The kinetics of the reductive reaction of 4-nitroaniline

at 25

C ( ▲) and 80

C ( ◆) by the nanosized gold particles.

faster than that at 25

C.

Acknowledgements

This study was supported by Sahmyook University research funding in Korea.

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