Reduction of aromatic nitro compounds catalyzed by biogenic CuO nanoparticles

Abstract

Synthesis of CuO nanoparticles using the peel of Musa balbisiana and its application in reduction of nitro aryl compounds are reported here. CuO nanoparticles were characterized by using XRD, XPS, PL, SEM and TEM techniques. In XRD analysis, significant peaks appeared at 18.3, 24.4, 33.6, 34.6, 35.2, 38.2 and 42.7 respectively. The BET surface area and total pore volume of CuO were found to be 7.479 m² g⁻¹ and 0.1259 m³ g⁻¹, respectively. The generated CuO nanoparticles exhibited interplanar lattice fringe spacings of 0.16 nm. SEM images indicate the formation of flower-like CuO architecture. The hierarchical CuO architecture is found to be made up of nanosheets which were self assembled to form flower-like nanostructures. The synthesized CuO nanoparticles show efficient catalytic activity in reduction of nitro aryl compounds to corresponding amino compounds with a high yield of conversion (74–96%). The reaction was carried out in a green solvent i.e. water. The catalyst was found to be active for several runs. It was further confirmed by several pieces of experimental evidence.

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1. Introduction

Cupric oxide (CuO) has received much attention for applications in fields such as catalysis, magnetic storage media, solar energy conversion, gas sensing, field emission transistors, energy conversion and storage, electronics, sensors and environmental science.^1–3 Nanoparticles in comparison to bulk solids have a significantly high catalytic activity and exhibit novel characteristics of quantum effects.^4,5 The prospect of potential applications of CuO nanostructures has led to substantial research and development efforts to form various types of nanostructures. It is still a challenge to develop a simple, rapid, easy to control and energy-efficient method for preparation of CuO nanostructures with suitable catalytic activity.

Synthesis of aniline derivatives is of great importance in the chemical industry due to their versatility in biologically active natural products, pharmaceuticals, dyes and ligands for transition metal-catalyzed reactions.⁶ The most promising and practical method for the synthesis of aniline is hydrogenation of nitroarene. Although the hydrogenation of simple nitro compounds is readily carried out with various commercial catalysts, the selective reduction of the nitro group with H₂ in presence of other reducible groups is challenging. Thus, the development of an efficient catalytic system for the selective reduction of nitro compounds is highly desired.

There are several approaches to realize the reduction process: metal/acid reduction,^7,8 catalytic hydrogenation,⁹ electrolytic reduction,¹⁰ homogeneous catalytic transfer hydrogenation,¹¹ heterogeneous catalytic transfer hydrogenation,^12,13 etc. However, these methods have certain shortcomings:¹⁴ metal/acid system has poor selectivity and acid brings severe corrosion to the equipment; catalytic hydrogenation at high pressure; homogeneous or heterogeneous catalytic transfer hydrogenation could perform well only in presence of expensive metals like palladium, platinum, ruthenium etc., and separation of the target product is difficult.¹⁵ In the past decades, substances containing copper, as an important organic reaction catalyst, have been given much attention by material scientists and chemists.^16,17 Furthermore, the excellent catalytic efficiency of copper nanostructure made it a research focus both in preparation and application in organic synthesis. Copper has a low price, low toxicity, and copper nanostructure can be recycled to reduce the cost.

Here we reported a simple and efficient method for synthesis and characterization of CuO nanoparticles by using the peel of Musa balbisiana. The simple, efficient method of reduction of nitro aryl compounds to corresponding aniline in presence of NaBH₄ and CuO nanoparticles is reported here. Moreover the morphology, size and catalytic activity of CuO nanoparticles synthesized by commercial K₂CO₃ or Na₂CO₃ reagent is compared with the CuO nanoparticles prepared by peel extract of Musa balbisiana. We propose a very simple, efficient procedure which is suitable for medium and large scale reduction of nitro aryl to corresponding amino compounds.

2. Experimental

2.1. Synthesis of CuO nanoparticles

In this method, the peel of Musa balbisiana was dried and burnt. To the 1 g of ash of the peel, 25 ml of distilled water was added and filtered. 5 ml 1 M CuSO₄·5H₂O solution was added to the filtrate and stirred for 15 min. Light blue precipitate was obtained. After filtration, the precipitate was heated for 2 h at 300 °C temperature for the formation of powder CuO nanoparticles.

2.2. Characterization

Scanning electron microscopy (SEM) characterization was performed on JEOL JSM – 6360 at 15 kV. X-ray diffraction (XRD) measurement were carried out by Rigaku X-ray diffractometer (Model: ULTIMA IV, Rigaku, Japan) with Cu-Kα X-ray source (λ = 1.54056 Å) at voltage 40 kV. The X-ray photoelectron spectroscopy (XPS) analysis was done on instrument ESCA-3000 (VG Scientific, UK). The source used is AlKalpha having energy 1486.6 eV The high resolution transmission electron microscopy (HRTEM) images were recorded by a JEOL Model 2100 EX, Japan operated at voltage of 200 kV.

Shimadzu IR Affinity-1 spectrophotometer was used for FT-IR analysis. Specific surface area, pore volume, average pore diameter were measured with the Autosorb-1 (Quantachrome, USA). Specific surface area of the sample was measured by adsorption of nitrogen gas at 77 K and applying the Brunauer–Emmett–Teller (BET) calculation. Pore size distributions were derived from desorption isotherms using the Barrett–Joyner–Halenda (BJH) method.^18,19 The ¹H NMR spectra were recorded at room temperature in CDCl₃ solution on a Bruker DPX-300 spectrometer and chemical shifts were reported relative to SiMe₄. Mass spectra were recorded on ESQUIRE 3000 mass spectrometer.

2.3. Kinetic study

For kinetic studying the catalytic activity of as-prepared CuO nanoparticles, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄ is performed as a probe reaction in UV/Visible spectrophotometer. The effect of concentration of nanoparticles solution on the speed of catalytic reduction was studied by using different quantities (0.02, 0.04, 0.06, 0.08 and 0.1 mol%) of nanoparticles.

2.4. Optimization of reaction

The reduction reaction was first optimized by using different solvent like water, acetonitrile, ethanol, DMSO etc. under the same reaction condition of temperature (30 °C) and pressure (1 atm) and amount of CuO (1 mol%) catalyst.

2.5. Catalytic reduction of aromatic nitro compounds

To 5 ml of water added 10 mM aromatic nitro compounds and 5 equiv. NaBH₄ and then stirred for about 1 h at temperature 30 °C in presence of 1 mol% CuO nanoparticles in water. The progress of the reaction was monitored by thin layer chromatography (TLC). The reaction mixture was extracted with ethyl acetate. The combined organic layer was dried with NaSO₄ and concentrated via rotary evaporation. The CuO catalyst could be used consecutively for five times for the reduction of 4-NP (1^st recycle 96%, 2^nd recycle 95% and 3^rd recycle 94%, 4^th recycle 92% and 5^th recycle 91%) to 4-AP was obtained. The product was purified by column chromatography by using hexane–ethyl acetate as solvent system in different concentration to obtain the pure compound. The structure of the compounds was further confirmed by ¹H NMR, ¹³C NMR, GC-MS analysis.

3. Results and discussion

3.1. Characterisation of CuO nanoparticles

In the present investigation, a simple approach in synthesis of CuO nanoparticles using peel of Musa balbisiana is demonstrated. The peel of the plant was burnt and filtered. 1 kg of the ash prepared from peel of Musa balbisiana contains 233.60 g of K⁺, 2.00 g of Na⁺, 161.40 g of CO₃²⁻ and 6.62 g of Cl⁻.²⁰ These ions may be responsible for formation of Cu(OH)₂, which further undergo in formation of CuO nanoparticles by thermal process [Scheme 1S, ESI†].

3.2. X-ray diffraction analysis

The formation of CuO nanoparticles was confirmed by XRD analysis. In XRD analysis, the planes (020), (021), (110), (002), (111), (042), (130), (131), (151), (200), (152) and (202) indicate the formation of monoclinic crystallite without having any peak due to the possible Cu₂O and Cu(OH)₂ impurity.²¹ The significant 2θ values appeared at 18.3, 24.4, 33.6, 34.6, 35.2, 38.2, 42.7 corresponds to (020), (021), (002), (111), (042), (138) and (131) planes respectively. The corresponding d values are 4.85, 3.62, 2.62, 2.56, 2.51, 2.36 and 2.15 Å respectively. These are very close to those in the JCPDS File no. 5-0661 (Fig. 1). The result is similar as reported data.^21,22

Fig. 1 XRD spectrum of CuO nanoparticles.

3.3. FT-IR analysis

The FT-IR spectrum of CuO nano pellets is shown in Fig. 1S [ESI†]. Small band at 2361 cm⁻¹ assigned to carboxylic group (COO–) vibration. Peak at 1632 cm⁻¹ assigned to amide (CONH–) group while the peak at 1115 cm⁻¹ revealed the presence of (O–H) stretching for alkyl group. These characteristic peaks indicated the presence of very small amount of protein/amino acid molecules along with CuO nanoparticles. The characteristic peaks of CuO positioned from 987 cm⁻¹ to 450 cm⁻¹. The peaks positioned at around 641, 604 and 532 cm⁻¹ corresponds to the characteristic stretching vibrations of Cu–O bond in the monoclinic CuO nanoparticles. The peaks observed at 532 and 604 cm⁻¹ are due to Cu–O stretching along the [−202] and 450 cm⁻¹ from Cu–O stretching along the [202] direction in CuO crystal.²³

3.4. X-ray photoelectron spectroscopy (XPS) analysis

The Fig. 2A and B shows the XPS spectrum of CuO nanoparticles. The spectrum obtained was calibrated binding energy (BE) at 284.5 eV for a C 1s electron. The XPS BE at 941.5 and 951.5 eV corresponds to Cu 2p_3/2 and Cu 2p_1/2 respectively. The BE at 538 eV corresponds to O 1s of CuO nanoparticles. The result is strongly supported by reported data.²⁴ The XPS analysis revealed that there are no Cu₂O and Cu(OH)₂ impurities in the sample.

Fig. 2 XPS spectra of CuO nanoparticles, (A) Cu 2P and (B) O 1s.

3.5. Scanning electron microscopic (SEM) analysis

The SEM images indicate the formation of flower like morphology of CuO nanoparticles. Fig. 3A and B showed the formation of flower-like CuO architecture with an average diameter of about 50–90 nm. Higher magnification showed that the flower-like hierarchicals are composed of nanosheet subunits as building blocks, which were self-organized to form flower nanostructure.

Fig. 3 SEM images (A and B) of CuO nanoparticles.

3.6. High resolution transmission electron microscope (HR-TEM) analysis

The HR-TEM analysis results showed the formation of flower like cluster of CuO nanostructure. The nanoparticles overlap each other which strongly support the formation of flower like nanostructure along with spherical and oval shape (Fig. 4A and B). The size of CuO nanoparticles was found in the range of 3.0 ± 0.2–25.0 ± 1.3 nm. The average size of the particles is 15.5 ± 0.8 nm. The difference between the two atomic layers is 0.16 nm.

Fig. 4 TEM images (A and B) of CuO nanoparticles.

3.7. EDX analysis

The EDX spectra indicated the formation of CuO nanoparticles [Fig. 2S] [ESI†]. The element C, O, K, Cl etc. may be due to presence of mineral with the extract of Musa balbisiana.

3.8. Photoluminescence (PL) analysis

In photoluminescence analysis, two emission peaks were observed at 394 nm (violet) and 475 nm (blue) (Fig. 5). The first one corresponds to the band-edge emission and second one is due to artefact.²⁵

Fig. 5 Photoluminescence spectrum of CuO nanoparticles.

3.9. Characterization of CuO nanoparticles synthesised by K₂CO₃ and Na₂CO₃

In order to understand the role of ions like K⁺, Na⁺, CO₃²⁻ etc. we had performed an experiment using commercially available K₂CO₃ and Na₂CO₃ reagent. In this experiment CuO nanoparticles was synthesized using 1 mM K₂CO₃ with 1 mM CuSO₄·5H₂O then filtered and heated at about 300 °C for 2 h. Further CuO nanoparticles were also synthesized by using 1 mM Na₂CO₃ with same procedure. The synthesized CuO nanoparticles were characterized using XRD, SEM and TEM analysis.

In case of CuO nanoparticles synthesized by K₂CO₃, the planes (020), (021), (110), (002), (111), (042), (130), (131), (151), (200) and (152) indicated the formation of monoclinic crystallite. The significant 2θ values appeared at 18.2, 24.5, 33.5, 34.6, 35.2, 38.2, 42.7 corresponds to (020), (021), (002), (111), (042), (138) and (131) planes respectively [Fig. 3S] [ESI†]. The SEM images indicated the formation of flower like morphology of CuO nanoparticles with an average diameter of about 50–90 nm [Fig. 4S] [ESI†]. It showed that the hierarchical flowers are composed of nano subunits which were self-organized to form flower nanostructure. However, the morphology is quite different from the CuO nanoparticles when synthesised by peel extract of Musa balbisiana. The TEM image indicated the formation of pentagonal, hexagonal CuO nanoparticles. The size of the nanoparticles ranged 3.3 ± 0.5 to 30.5 ± 1.4 nm [Fig. 5S] [ESI†]. The average size of the CuO nanoparticles 20.0 ± 1.4 nm.

In case of CuO nanoparticles synthesized by Na₂CO₃, the significant 2θ values appeared at 18.3, 24.4, 33.5, 34.6, 35.2, 38.2, 42.6 corresponds to (020), (021), (002), (111), (042), (138) and (131) planes respectively [Fig. 6S] [ESI†]. The SEM images indicated the formation of flower like CuO nanoparticles [Fig. 7S] [ESI†] with an average diameter of about 45–80 nm. However, the morphology is quite different from the CuO nanoparticles when synthesised by using Musa balbisiana. The TEM image indicated the formation of pentagonal, hexagonal, spherical CuO nanoparticles. The size of the nanoparticles ranged 3.6 ± 0.8 to 35.5 ± 1.1 nm [Fig. 8S] [ESI†]. The average size of the nanoparticles is 23.5 ± 1.5 nm. The average size in both cases is greater than as synthesized CuO nanoparticles by peel of Musa balbisiana. Further it was observed that with variation of concentration of Na₂CO₃ or K₂CO₃ (0.1, 0.5 and 1.0 mM), the size and shape of the CuO nanoparticles does not change significantly. Therefore from the study, it reveals that the alkaline precursors like K⁺, CO₃²⁻, Na⁺, Cl⁻ etc. may be responsible for synthesis of CuO nanoparticles. It is the novelty of our study.

3.10. Kinetic study of catalytic activity

The relationship between the rate of the catalytic reduction of 4-NP in the presence of CuO nanoparticles with respect to time can be used to describe by follow equation.^26,27 The rate of the reactionwhere, K is the adsorption coefficient of the reactant on CuO, k is the reaction rate constant and C is the concentration of the reactant at any time t.

When the initial concentration is very small (C₀ small), the equation can be simplified to an pseudo first order kinetic

ln(C₀/C) = kKt = kt (2)

where, k = kK is the pseudo first order reaction rate constant.

The reduction of 4-NP in presence of NaBH₄ containing CuO obeys pseudo-first-order kinetics.

Here, in order to evaluate the catalytic activity of the CuO nanoparticles, the reduction of 4-NP (5 × 10⁻³ M) to 4-AP by NaBH₄ (0.5 mM) was employed as a probe reaction.²⁶ The presence of both the reactant 4-nitrophenolate anion (λ_max = 400 nm) and the product 4-AP (λ_max = 310 nm) can be convincingly demonstrated by the UV/Visible absorption spectroscopy. After the addition of CuO nanoparticles, the electron donor (BH₄⁻) and electron acceptor (4-nitrophenolate ion) are adsorbed on the surface of the CuO and catalytic reduction starts by the transfer of electron from BH₄⁻ to 4-nitrophenolate ion. Thus CuO facilitates the reduction of 4-NP to 4-AP by lowering the activation energy of the reaction and play the role of catalyst. The UV/Vis spectrum of the mixture of 25 ml of 4-NP (0.25 × 10⁻³ M) and 1 ml NaBH₄ (0.5 mM) was studied during the catalytic reduction. In the absence of CuO catalyst, the peak at 400 nm remains unchanged suggesting that the reduction did not proceed at all. However, when 0.02 mol% of CuO nanoparticles was introduced, the intensity of absorption peak at 400 nm was found to decrease and a new peak at 310 nm appears with a fading of the dark yellow color of the 4-nitrophenolate ion in solution (Fig. 6A). This is due to the reduction of 4-NP to 4-AP. The reduction was found to be completed approximately within 25–30 min as indicated by the decrease the intensity of the yellow colour. In the present study, the concentration of BH₄⁻ was so high compared with that of 4-NP and it could be considered to remain constant during the reaction. However the rate of reaction depends on the quantity of the CuO nanoparticles.

Fig. 6 (A) UV/Visible spectrum of the absorption spectra of mixture of 4-nitrophenol and NaBH₄ in presence of CuO nanoparticles (0.10 mol%) at different time intervals. (B) The logarithm of the ratio between the original concentration of 4-nitrophenol and its final concentration versus corresponding time (min).

By plotting ln(C₀/C) versus the corresponding time (min) yields linear relationship as shown in the Fig. 6B. The rate constant in reduction reaction of 4-NP was investigated by using different concentration (0.02, 0.04, 0.06, 0.08 and 0.1 mol% respectively) of CuO nanoparticles solution. From the study it was found that the rate constant was increased with increasing the concentration of nanoparticles. With increasing the concentration of nanoparticles, the surface area of active site of CuO nanocatalyst increased. So the rate of the reaction was increased gradually. From the Table 1S [ESI†], it was found that the rate constant was found highest (0.0885 min⁻¹), when 0.1 mol% of CuO nanoparticles was used in the reduction reaction.

3.11. Optimization of reaction conditions

Initial attempts to optimize the reaction conditions for the reduction of nitro aryl compounds to the corresponding amino compounds were done using 4-NP as the substrate in presence of different solvents and 1 mol% of CuO catalyst (Table 1). The reaction was observed by using different solvent like water, acetonitrile, ethyl acetate, chloroform, DMF, ethanol, methanol and DMSO. From the Table 1, it was observed that water was best solvent in reduction of 4-NP to 4-AP when 1 mol% CuO catalyst and 5 equiv. NaBH₄ as reducing agent were used at 30 °C temperature. It is interesting to note that 96% yield was found in water within 0.5 h of reaction time. In case of other solvent the product was formed in longer period of time with comparatively low yield.

Table 1 Optimization of the reaction conditions for the conversion of 4-nitrophenol to 4-aminophenol with different solvents, 5 equiv. NaBH₄ and 1 mol% CuO catalyst

Entry	Solvent	Time^a (h)	Yield^b (%)
a Reactions performed at 30 °C and monitored using TLC until all the 4-nitrophenol compounds was found to have been consumed.b Isolated yield after column chromatography of the crude product with 2% standard deviation.
1	Water	0.5	96
2	Ethyl acetate	2	75
3	Acetonitrile	1	95
4	Chloroform	5	72
5	Methanol	6	83
6	Ethanol	9	85
7	DMSO	7	77
8	DMF	8	79

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3.12. Reduction of nitro aryl compounds to corresponding amino compounds

The catalytic application of the synthesized CuO nanoparticles towards the reduction of nitro aryl compounds has been investigated. A variety of nitro compounds are smoothly reduced to their corresponding amino compounds with high yield. During the reaction, other functional groups such as bromo, fluoro, methoxy and chloro remain intact (Table 2). The yield of the compounds is significant in all cases. The highest yield is found in formation of 4-AP. The TOF value is also significant in case of 4-AP. Therefore, these observations indicated the synthesized CuO nanoparticles are a suitable catalyst for reduction of nitro aryl compounds with high yield in water.

Table 2 CuO catalyzed reduction of nitro aryl to amino compounds

Entry	Substrate	Product	Time^a (h)	Yield^b (%)	TOF^c (h^−1)
a Reactions performed at 30 °C and monitored using TLC until all the aromatic nitro compounds was found to have been consumed.b Isolated yield after column chromatography of the crude product with 2% standard deviation.c TOF: turn over frequency.
1			0.50	96	1920
2			2.00	85	425
3			1.80	83	461
4			1.60	88	550
5			1.70	74	435
6			2.0	80	400

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The progress of the reduction reaction was monitored by silica-gel TLC. Upon completion of the reaction, the product was separated by silica-gel column chromatography using an appropriate eluent system. The isolated compounds were identified by ¹H NMR, ¹³C NMR, GC-MS analysis [Scheme 2S] [ESI†]. These data of all the products were comparable with the commercialized compounds. As shown in Table 2, all of the nitro aryl compounds afforded their corresponding amino compounds in excellent yield. Our protocol provides a milder and facile methodology for reduction reaction that is useful for organic synthesis.

The catalytic activity of CuO nanoparticles when synthesised from K₂CO₃ was also studied. From the Table 2S [ESI†], it was observed that the yield of the products are quite less compared to the CuO nanocatalyst when synthesized from peel of Musa balbisiana under the same reaction condition. On the other hand the required time is high to complete the reaction. This is because the rate of reaction depends upon the size, morphology, active surface of the nanocatalyst. Moreover, the average size of the CuO nanoparticles was quite greater than the CuO nanoparticles synthesized from peel extract of the plant. The study revealed that CuO nanoparticles are highly efficient catalyst when synthesized from peel of Musa balbisiana.

Since, we have tested the reusability of CuO nanocatalyst in the reduction of 4-NP. It is believed that CuO nanocatalyst can be reused for this reduction reaction as well. In reduction of 4-NP to 4-AP, the activity of catalyst was observed five times. The CuO nanocatalyst was recovered by simple filtration and was washed with hot water–ethanol to remove any absorbed products. The catalyst was reused without obvious loss of their catalytic activity, up to five cycles. Recyclability of the catalyst is shown in Fig. 7. It was confirmed that efficiency remain almost same after five times recycle of catalyst (1^st recycle 96%, 2^nd recycle 95% and 3^rd recycle 94%, 4^th recycle 93% and 5^th recycle 91% 4-AP was obtained). It was further confirmed by using XRD and TEM analysis after 5^th recycle [Fig. 9S and 10S] [ESI†]. TEM and XRD investigation also showed that the activity, morphology and size distribution of the CuO nanocatalyst remain unchanged after use of 5 times in catalytic reaction.

Fig. 7 Recycling of the catalyst after reduction of 4-nitrophenol to 4-aminophenol.

The reactions were carried out by maintaining the stoichiometry of reactant and recovered catalyst. Further investigation was done for precise evidence. The fresh and recovered catalyst was investigated through N₂ adsorption–desorption study. The BET surface area and total pore volume of CuO was found to be 7.479 m² g⁻¹ and 0.1259 m³ g⁻¹ respectively for the fresh catalyst. The specific surface area of the recovered catalysts decrease marginally to 170 (5^th run) compared to 247 m² g⁻¹ of freshly prepared catalyst (Fig. 11S) [ESI†]. The decrease of the surface area of the catalyst after reaction may be due to the partial destruction of the support by the small amount of base used in the reaction. It was observed that the adsorption–desorption hysteresis loop of the catalyst used in the 5^th run ranging between P/P₀ = 0.3 and 0.9 shifted to P/P₀ = 0.6 and 0.9, respectively. This may be due to the change in the structure of pores. The BJH pore size distribution curve of the recovered catalyst shows a slight broadening of the distribution pattern compared to fresh catalyst (Fig. 12S) [ESI†] indicating breakdown of the pore walls forming larger pores. However, it is strongly supported that there is no loss of efficiency of the catalyst after 5^th recycle.

We speculated the proposed mechanism as showed in Scheme 1. It is in consistent with previous literatures.^7,28 In heterogeneous systems, it is demonstrated that there are four steps in the reduction of aromatic nitro compounds: (i) absorption of hydrogen, (ii) absorption of aromatic nitro compounds to the metal surfaces, (iii) electron transfer mediated by metal surfaces from BH₄⁻ to aromatic nitro compounds. (iv) Desorption of aromatic amino compounds. To understand the mechanism of the reduction process in this method, some of the known intermediates like nitrosobenzene was subjected to this reduction protocol and was found to reduce completely to aniline as the only product within 0.5 h respectively. The mechanism for the reduction of nitroarene probably follows both reduction pathways via directly from hydroxyl amine and via azobenzene intermediates. The reduction probably takes place on the active surface of CuO nanoparticles by the liberated hydrogen formed by decomposition of sodium borohydride. The proposed mechanism is further supported by reduction of nitrobenzene using hydrogen gas (60 psi at room temperature) and employing excess active copper (>1 equiv.).²⁹

Scheme 1 Proposed mechanism on reduction of nitro aryl to amino compounds in presence CuO nanoparticles.

The catalytic activity of CuO nanoparticles was compared with the earlier report^30–34 in reduction of 4-NP to 4-AP. From the study [Table 3] it was observed that the conversion of 4-NP to 4-AP was significantly high (96%) with in 0.5 h compared with reported results. Only in case of catalyst (Py)Zn(BH₄)₂, the yield was higher (97%) than our present work. However, the required time was quite high (1.5 h). So, CuO nanoparticles can be considered as one of the suitable catalyst in reduction of nitro aryl compounds.

Table 3 Comparison of nano-CuO catalyst for reduction of 4-nitrophenol to 4-aminophenol with earlier report

Sl. no.	Catalyst	Temp.	Time (h)	Yield (%)	Ref.
1	Ru(0)-supported on AT-Mont	150 °C	12	65	30
2	Cu NPs	50 °C	5	66	31
3	(Py)Zn(BH4)2	RT	1.5	97	32
4	NiCo2O4	80 °C	1.5	90	33
5	CuBr2	RT	3.5	88	34
6	CuO	30 °C	0.5	96	Present work

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4. Conclusion

It is very simple process of synthesis of CuO nanoparticles using peel of Musa balbisiana with flower like hierarchical architectures. Due to its great chemical flexibility and synthetic tenability, the present route provides a very simple efficient pathway of synthesizing 3D CuO nanostructures with surface area 7.479 m² g⁻¹. The rate of catalytic reaction increased with raising the concentration of catalyst. Moreover, we have developed a simple, efficient and inexpensive catalytic method for the reduction of nitro aryl compounds to amino compounds using CuO nanocatalyst and NaBH₄ with high yield product and high TOF value. The CuO nanocatalyst was found to be highly active and could be recycled for five consecutive runs without significant loss of catalytic activity.

Acknowledgements

The authors thank Director, CSIR-North East Institute of Science & Technology, Jorhat, Assam for valuable advice. The authors also thank to SAIF, Shillong for TEM & SEM analysis. I.S. thankful to CSIR New Delhi for her fellowship. The authors thankful to SEED Division, DST, New Delhi for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10397a

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