Gum acacia–CuNp–silica hybrid: an effective, stable and recyclable catalyst for reduction of nitroarenes

Abstract

A gum acacia–CuNp–silica hybrid behaved as an efficient catalyst for the sodium borohydride reduction of nitroarenes. The hybrid (3 × 10⁻³ g) in combination with 0.1 M NaBH₄ was capable of reducing 4-nitrophenol into 4-amino phenol within 2.3 minutes at room temperature. The reduction did not require any exclusion of oxygen or moisture. The catalyst retained its full activity even after one month storage under the laboratory conditions. It could be easily recycled for six repeated cycles with only marginal loss of catalytic activity. The conditions for borohydride reduction of 4-nitrophenol have been optimized, and under these conditions the reduction of 4-nitroaniline and 4-nitrobenzoic acid was possible within 3.15 and 8.3 minutes, respectively.

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1. Introduction

Aromatic amines are being used in many industries such as rubbers, paints, and plastics.¹ They are also used for the preparation of a number of biologically and pharmaceutically active molecules.² Several procedures^3–6 are known for the reduction of nitroarenes to aryl amines, but sodium borohydride reduction is the easiest and cleanest method as it can be performed in aqueous medium under mild reaction conditions.⁷ The transition metal based catalysts are invariably required for this conversion due to sluggish hydrolysis⁸ of sodium borohydride. The metal nanoparticles (Nps)^9–12 are especially attractive catalysts for the reduction as they have high surface-to-volume ratios and unique electronic and surface properties. The chemoselective metal nanoparticle catalyzed reduction of aromatic nitro compounds is reported under the combined microwave and ultrasound irradiation.¹³ The use of CuNps in combination with ammonium formate also led to the chemoselective reduction of aromatic nitro compounds though the synthesis required high stoichiometric ratio of copper nanoparticles (3 equiv.), excess of the reductant (5 equiv.) to nitro-compound ratio, long reaction time (8–12 hours), and argon protection.¹⁴ CuNps in THF/H₂O have shown excellent catalytic property for the borohydride reduction of aromatic nitro compounds.⁴

The handling, recovery, and recycling of the Nps are the three major issues of homogeneous catalytic reactions,¹⁵ while the agglomeration of Nps can decrease the catalytic performance of the Nps under the heterogeneous conditions. The use of copper nanoparticle (CuNp) can find an edge over silver and gold nanoparticle because of its lower cost¹⁶ and comparable thermal, electrical, and antimicrobial properties.¹⁷ The major limitation of CuNp is its spontaneous oxidation to CuO or Cu₂O at ambient conditions (during and after its synthesis).¹⁸ This problem can be alleviated by protecting the Nps with a nonoxidizable shell of polymers, preferably a biopolymer.¹⁹ The immobilization of Nps on solid supports²⁰ with robust surface chemistry, good stability, and porosity can further minimize the over-stoichiometric use, oxidation and the agglomeration of NPs. Solid supported silver nanoparticles (AgNps) are known to have remarkable activity in catalyzing the reduction of aromatic nitro compounds.²¹ In our recent studies we noticed that polysaccharide inspired silica–AgNps hybrids have all the desired properties of heterogeneous catalysts such as high mechanical strength, water insolubility, high surface area, and porosity.^22,23

In the present article we report on the synthesis and catalytic activity of gum acacia (Ac)–CuNp–silica nanohybrid (C_A). Ac is highly branched polysaccharide having a protein–polysaccharide complex as a minor component. It is isolated from exudates of widely distributed and abundantly available trees of Acacia senegal and Acacia seyal.²² The gum acacia polysaccharide is previously known for having capping and stabilizing effect on CuNps.²⁴ The Ac stabilized CuNps have been hybridized with silica using sol gel technique to derive the catalyst (C_A). The catalytic activity of C_A has been demonstrated for sodium borohydride reduction of nitroarenes taking 4-nitrophenol as a representative nitroarene. To understand the generality of the catalyst (C_A), the reduction of 4-nitroaniline and 4-nitrobenzoic acid was also attempted under the optimized reduction conditions for 4-nitrophenol.

2. Experimental

2.1. Materials and instrumentation

Tetramethoxysilane (98% TMOS; Merck, Germany) was used as silica precursor. Gum acacia, sourced from Acacia senegal (Merck, India) was used. 4-Nitrophenol (4-NP) (Merck, India), 4-nitroaniline (4-NA) (Loba Chemie), phenylenediamine (4-PDA), 4-nitrobenzoic acid (4-NBA), 4-aminobenzoic acid (4-ABA) (Alfa SR) were used. NaBH₄ (CDH), CuSO₄, hydrazine (80%) (Merck, India) were used. Silica gel G (Merck) was used. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuKα1 radiation with wavelength of 1.54 Å in the wide angle region (from 10 to 70 on 2-theta scale). IR was done by forming KBr pellets using Perkin Elmer FT-IR Spectrum 2 spectrophotometer (resolution 4 cm⁻¹) in the range of 400–4000 cm⁻¹. SEM analyses were performed on Nova Nano FE-SEM 450 (FEI). UV-visible spectrum was recorded on a Cyber Lab double beam UV-visible spectrophotometer. Electrospray Ionisation-MS was done on Waters Q-TOF Premier-HAB-213 instrument. ¹H NMR was carried out on 500 MHz (JEOL-500) in DMSO-d6.

2.2. Synthesis of CuNps

To Ac aqueous solution (12 mL of 2% (w/v), CuSO₄ solution 8 mL of 1.5% (w/v)) was added and the mixture was stirred on a magnetic stirrer for 10 minutes. This mixture was stirred again for 60–70 minutes after addition of hydrazine (0.8 mL). Subsequently, the reactor was kept at room temperature for 1 h under vigorous stirring without any inert protective gas until the complete reduction of the metallic salt. The polysaccharide capped metal nanoparticle²⁴ solution was obtained after the reaction was complete and the color of the solution changed to brown. The formation of CuNp solution was also confirmed by a SPR peak at 618 nm and TEM.

2.3. Synthesis of the gum acacia–CuNp–silica composite (Ac–CuNp–Si)

To the known volume of CuNp solution obtained above, known volumes of tetramethoxysilane (TMOS) (silica precursor) and methanol (MeOH) (co-solvent) were added. The resulting solution was polymerized till a hybrid hydrogel was obtained. The gel was dried (overnight at 40 °C) and finely powdered before use as catalyst.

2.4. Synthesis of the controls

The control silica (CS) and control Ac hybrid (Ac–Si) were synthesized by polymerizing 0.5 mL TMOS with 0.5 mL MeOH and 4 mL of H₂O; and 0.5 mL of TMOS, 0.5 mL of MeOH, 1 mL of 2% w/v Ac solution, and 3 mL of H₂O respectively.

2.5. General procedure for spectrophotometric determination of the reduction of 4-nitrophenol using C_A as catalyst

The conversion of 4-NP to 4-AP was monitored spectrophotometrically. The reduction of the 4-NP to 4-AP was optimized by carrying out the reaction at different volumes 4-NP, (0.1 to 0.3 mL of 2 mM), catalyst amount (1 to 7 × 10⁻³ g), and NaBH₄ concentration (1 mL of 0.05 M to 0.2 M), while adding the required volume of water in each set so that the total reaction volume was 3 mL. 4-NP in aqueous medium has a maximum absorption (λ_max) at 317 nm, but a red shift of the peak of 4-NP was observed (from 317 to 403 nm) immediately after the addition of NaBH₄. This was due to the formation of 4-nitrophenolate ions in the alkaline medium of NaBH₄. The catalytic reduction of 4-NP to 4-AP showed decolorization of the 4-NP which was monitored spectrophotometrically by plotting absorbance versus time. C_A catalyzed sodium borohydride reduction of 4-NP was studied through the decrease in the peak height at 403 nm with respect to the reaction time. At the same time, a new peak appeared at 300 nm whose intensity gradually increased due to the formation of 4-aminophenol.²⁵ Under the optimized conditions for 4-NP reduction, reduction of 4-nitro aniline (4-NA) and 4-nitrobenzoic acid (4-NBA) were also carried out where appearance of p-phenylene diamine (p-PDA) peak at 238 nm and 4-aminobenzoic acid (4-ABA) peak at 270 nm were monitored.

The shelf life of C_A was explored by storing it at room temperature and monitoring its catalytic activity (after definite intervals) towards the borohydride reduction of 4-NP as described above.

2.6. Experimental procedure for the reduction of 4-nitrophenol catalyzed by C_A (for the recycling study and product identification)

Ten times scaled up reaction (at the optimized conditions) was carried out to identify the products by TLC and for the recycling study for ease in handling the product. In short the reduction of the 4-NP to 4-AP was performed using 3 mL of 2 mM 4-NP, 3 × 10⁻² g C_A, and 10 mL 0.1 M NaBH₄, while adding the requisite amount of distilled water so that the total reaction volume was 30 mL. After the completion of the reaction (followed by TLC), the reaction mixture was centrifuged to separate the catalyst. The recovered catalyst was recycled after a thorough wash with distilled water and acetone (to separate all the organics). The filtrate containing the reaction products was extracted with ethyl acetate (30 mL). The ethyl acetate extract was concentrated under reduced pressure. The crude product thus obtained was then purified by column chromatography using hexane/ethyl acetate as an eluent to afford the pure product. The products were characterized by TLC with authentic sample of 4-aminophenol in the solvent ethylacetate/cyclohexane (2 : 1). Electrospray ionization-mass spectrometry (ESI-MS), ¹H NMR and % yield calculation was done from hundred times scaled up reaction to get the product in practically good yield.

3. Results and discussion

3.1. Synthesis of copper nanoparticles (CuNps)

The CuNp solution was synthesized by reducing copper sulphate with hydrazine²⁴ in presence of Ac aqueous solution as capping and stabilizing agent. A change in color of the solution from blue to dark brown and appearance of characteristic absorption band with a peak at ∼618 nm confirmed the formation of the CuNps (Fig. 1).

Fig. 1 (A) UV visible spectrum (B) TEM picture of GA capped of CuNps.

3.2. Synthesis of Ac–CuNp–Si nanohybrid

To a known volume of tetramethoxysilane (TMOS), a known volume of methanol (MeOH) (Table 1), and a known volume of CuNp solution were added. The polymerization of TMOS was allowed until the hybrid hydrogel was formed. MeOH acted as co-solvent to ensure the miscibility of TMOS with Ac capped CuNp solution, while Ac templated and catalyzed the polymerization of TMOS. A series of hybrid hydrogels (Table 1) were obtained which were dried and powdered well before their use as catalyst. The best hybrid hydrogel (C_A) in terms of homogeneity and gelling time was obtained when the ratio of CuNp : TMOS : MeOH : H₂O was 2 : 1 : 1 : 6 while other combination of reactants resulted into either turbid hydrogels or precipitation (Table 1). The transparent monolith was termed as “Gel” while formation of an opaque heterogeneous mixture was termed as precipitation. To optimize the material synthesis, the ratio between TMOS and MeOH was kept fixed at 1 : 1 while the other reactants were varied. The choice of 1 : 1 ratio of TMOS and MeOH was based upon our previous experience during the synthesis of similar materials.²⁶

Table 1 Optimization of the hybrid synthesis using varying volumes of Ac capped CuNp solution (H₂O was added to make up the total reaction volume to 5 mL in each set)

Hybrid	CuNps (mL)	TMOS (mL)	MeOH (mL)	Ratio (CuNp : TMOS : MeOH : H2O)	Gel/ppt time (min)	Nature of gel	Yield (g)
CA	1	0.5	0.5	2 : 1 : 1 : 6	100	Transparent	0.210
CB	2	0.5	0.5	4 : 1 : 1 : 4	36	Turbid gel	0.230
CC	3	0.5	0.5	6 : 1 : 1 : 2	16	ppt	0.285
CD	4	0.5	0.5	8 : 1 : 1 : 0	8	ppt	0.295

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3.3. Characterization of C_A

3.3.1. FTIR. The FTIR spectrum of the C_A is compared with the spectra of CS and Ac–Si in Fig. 2. In CS, peaks from the Si–O–Si modes are seen below 1250 cm⁻¹. Peaks due to SiO–H, Si–OH, and O–Si–O stretchings are observed at 3398 cm⁻¹, 1068 cm⁻¹, and 759 cm⁻¹ respectively.

Fig. 2 FTIR (A) CS; (B) Ac; (C) Ac–Si; (D) C_A.

In pure Ac, the stretching peaks of O–H and C–H are visible at 3374 cm⁻¹ and 2923 cm⁻¹ respectively. Strong peak due to COO– asymmetric stretching is visible at 1600 cm⁻¹ and COO⁻ symmetric stretching is seen as weak absorption band at 1404 cm⁻¹.

The presence of SiO–H, Si–OH, and O–Si–O stretching peaks at 3404 cm⁻¹, 939 cm⁻¹, 1084 cm⁻¹, and 771 cm⁻¹ respectively indicate the incorporation of silica in the Ac–Si hybrid. The C–H stretching vibration in the hybrid is seen at 2920 cm⁻¹. The SiO–H, Si–OH, O–Si–O, and C–H stretching peaks in the hybrid are slightly shifted from that of the CS which indicated that the presence of Ac gum has affected the association state of the surface silanols of silica matrix. The presence of the gum acacia content in the hybrid is indicated by the COO– asymmetric stretching as strong peak at 1632 cm⁻¹, and COO– symmetric stretching at 1384 cm⁻¹. The strong peaks at 1200–900 cm⁻¹ are the finger print of the Ac.²⁷ The COO– asymmetric and symmetric stretching peaks of Ac are also seen affected by the silica hybridization due to their mutual interaction which has changed their association states.

While in C_A, peaks due to SiO–H, Si–OH and O–Si–O bonds are seen at 3389 cm⁻¹, 939 cm⁻¹, 1068 cm⁻¹, and 771 cm⁻¹ respectively. The peak due to Si–O absorption (1068 cm⁻¹) did not shift from that of CS. C–H stretching vibration is seen at 2918 cm⁻¹. The strong peak at 1619 cm⁻¹ can be attributed to the COO– asymmetric stretching of Ac gum while the weak absorption band at 1384 cm⁻¹ can be assigned to COO– symmetric stretching. The strong peaks at 1200–900 cm⁻¹ may be assigned for the finger print of carbohydrates.²⁷ The Si–OH and O–Si–O asymmetric stretching peaks showed significant shift towards the lower wave numbers in CuNp incorporated hybrid. This blue shift may be assigned to the involvement of COO– and –OH groups in the capping of the copper nanoparticles.²⁸ A new absorption peak was observed at 461 cm⁻¹, which is related to the interaction between the Cu-NPs and the Ac.²⁹

3.3.2. XRD. XRD patterns indicated that both C_A and Ac–Si have amorphous nature as that of CS (Fig. 3). All of them showed similar amorphous humps but at different diffraction angle centers. Incorporation of Ac gum and CuNps during the hybrid formation lowered the 2θ value. The diffraction angle centers of the CS, Ac–Si and C_A are 2θ 23.72°, 23.36°, and 2θ 23° respectively. Absence of diffraction peaks of CuNps (which are in small amount) in C_A may be due to their overlapping with the very broad amorphous hump of silica matrix.²² Missing copper peak is due to intrinsic limitation of XRD in detecting very small size CuNps (∼18 nm as revealed by TEM).

Fig. 3 XRD of the C_A in comparison to Ac–Si and CS.

3.3.3. FESEM. SEM pictures of C_A and EDX are compared with Ac–Si in the Fig. 4(a) and (b) respectively. Incorporation of Cu is evidenced by EDX of C_A where the presence of Cu, K and L signals are visible.³⁰ Both hybrids exhibited different surface morphologies. Ac–Si show scattered irregular bulk particles of different sizes whose surface is seen deposited with irregular small particles, while in C_A, the deposition of slightly agglomerated particles of regular morphology (spherical to nearly spherical) is seen. These particles are silica coated Ac stabilized CuNps which are seen deposited at the surface of bulk particles of the hybrid.

Fig. 4 (a) FESEM image of Ac–Si at 5k (A) & 2k (A′) magnifications, C_A at 5k (B) & 2k (B′) magnifications; (b) EDAX of Ac–Si (A); C_A (B)

3.3.4. TEM. TEM micrograph of CS and Ac–Si hybrid are shown in Fig. 5(A) and (B) where CS is seen as a cluster of very fine globular silica nanoparticles. Ac–Si hybrid showed relatively larger particles as silica nanoparticles are now coated with Ac gum. The TEM picture of Ac gum capped CuNp solution Fig. 1(B) revealed that the average particle size of CuNps is ∼18 nm. In TEM image of C_A (Fig. 6(A) and (B)) CuNps are clearly seen as intense dark spots well dispersed within silica matrix. TEM histogram (inset of Fig. 6(A)) revealed that the hybrid has particle size ranging from 8 to 34 nm (average size particle size is ∼18 nm). Besides, these Nps have nearly spherical core shell morphology which evidenced Ac coating of the CuNPs. Such supported structures are known to alter the surface properties of metal Nps and therefore their catalytic activity.³¹ Similar observation of low crystallite size has been reported in the synthesis of polymer capped metal Nps and metal-oxide nanocomposites.³²

Fig. 5 (A) TEM micrograph of control silica (B) Ac–Si hybrid.

Fig. 6 Transmission electron micrographs of C_A at different magnification (A) and (B); TEM histogram seen as inset of (A).

3.4. Catalytic activity of C_A

The catalytic activity of C_A was studied for the sodium borohydride reduction of nitroarenes in aqueous medium. 4-NP was used as the representative nitroarene whose reduction was monitored spectrophotometrically.²¹ Aqueous solution of 4-NP had a maximum absorption (λ_max) at 317 nm. A red shift of the peak (from 317 to 403 nm) was observed immediately after the addition of NaBH₄. The shift can be attributed to the formation of 4-nitrophenolate ions which are formed due to the alkaline borohydride. However the peak at 403 nm did not change in absence of the catalyst even after 24 h. The addition of C_A immediately initiated the reduction and the color of the reaction mixture gradually faded from yellow to colorless in 2.3 min due to the reduction of 4-NP to 4-AP (Fig. 8). The catalyst proved many times more efficient than the CS and Ac–Si (Table 2).

Table 2 Comparison of the catalytic activity of the C_A and controls in reducing 4-NP

Material	Vol. of GA soln (2% w/v) (mL)	Cu Nps (mL)	H2O (mL)	TMOS (mL)	MeOH (mL)	Gelling time (min)	Yield of catalyst (g)	Reduction time (min)
Ac–Si	1	—	3	0.5	0.5	165	0.210	18
CA	—	1	3	0.5	0.5	18	0.200	2.3
CS	—	—	3	0.5	0.5	240	0.150	No reduction

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The presence of 4-amino phenol in ethylacetate extract of reaction product was confirmed by thin layer chromatography (TLC) using ethylacetate/cyclohexanol (2 : 1) as solvent³³ and 4-aminophenol as the reference compound, and through ESI-MS analysis (Fig. 7). Even the ten and hundred times scaled up reactions completed in 2.3 and 5 minutes respectively which indicated that the catalyst retains its efficiency even for the scaled up reactions. ESI-MS spectrum showed as base peak³⁴ [M + H]⁺ at m/z 110.06 which clearly demonstrated the formation of 4-aminophenol (molecular weight = 109 amu) as the reduction product. [M + 2H]²⁺ peak is also visible at m/z 111. ¹H NMR (DMSO-d6, 500 MHz) of the product also confirmed the formation of the 4-nitrophenol. The peaks are seen at δ 8.3 (s, 1H), δ 6.47 (s, 4H), δ 4.36 (s, 2H).

Fig. 7 ESI-MS spectrum of the catalytic reduction product of 4-nitrophenol.

Fig. 8 (A) Reduction of 4-NP (B) 4-NA (C) 4-NBA using C_A.

The plot of absorbance versus time was recorded spectrophotometrically (Fig. 9(A)) for monitoring the catalytic reduction of 4-NP to 4-AP. The kinetics of C_A catalyzed sodium borohydride reduction of 4-NP was studied by monitoring the decrease in the peak height at λ 403 nm with time (Fig. 8(A)). The plot of [A] vs. time exhibited a straight line with (R² value 0.964), indicating the reaction to be zero order (Fig. 9(B)). The rate constant was computed to be 1.049 × 10⁻³ mol L⁻¹ min⁻¹. Other researchers have also reported zero order kinetics for nanoparticle catalyzed reduction of nitrophenol.^29,35,36 The mechanism of the heterogeneous catalysis is based on hydride transfer from borohydride to NP at the catalyst surface.³⁷

Fig. 9 (A) Plot of [A] versus time (B) plot of zero order rate constant vs. concentration of 4 NP (for the catalytic reduction of 4-NP to 4-AP conditions: [4-NP] = 2.0 mm; [NaBH₄] = 0.1 m; C_A = 3.0 × 10⁻³ g).

Both Np and borohydride are first adsorbed at the C_A surface prior to the hydride transfer. In principle, the change in the absorbance at both the wavelengths (λ 295 nm and λ 403 nm) can be studied to follow the kinetics of the reduction, but in the present study, λ 403 nm was used for better accuracy of results.²¹ The reduction of 4-NP was studied at different concentration of NaBH₄, 4-NP, and C_A (catalyst) (Table 3).

Table 3 Conversion of p-NP to p-AP using C_A as catalyst for different volumes of 2 mM 4-NP in presence of 1 mL of NaBH₄ of various concentrations

S. no.	p-NP mL	NaBH4 [M]	Catalyst (g)	H2O (mL)	Time (min)
1	0.1	0.1	3 × 10^−3	1.9	Immediate
2	0.3	0.1	3 × 10^−3	1.7	2.3
3	0.5	0.1	3 × 10^−3	1.5	8.3
4	0.7	0.1	3 × 10^−3	1.3	12.0
5	0.3	0.05	3 × 10^−3	1.7	5
6	0.3	0.15	3 × 10^−3	1.7	Immediate
7	0.3	0.20	3 × 10^−3	1.7	Immediate
8	0.3	0.1	1.0 × 10^−3	1.7	6
9	0.3	0.1	5 × 10^−3	1.7	2.10
10	0.3	0.1	7 × 10^−3	1.7	1.30

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3.4.1. Effect of NaBH₄ concentration. The effect of NaBH₄ concentration on the catalytic reduction reaction was studied for the fixed concentrations of 4-NP and C_A. It was observed that increase in BH₄⁻ concentration increased the reaction rate in the studied concentration range (0.05 to 0.2 M). The sodium borohydride content in the reaction system was kept very high as compared to 4-NP so that the rate of the reduction can be assumed to be independent of the concentration of sodium borohydride.

3.4.2. Effect of catalyst amount. The effect of catalyst concentration on the catalytic reduction was studied at various catalyst amount (1 × 10⁻³ to 7 × 10⁻³ g) while keeping the concentrations of 4-NP and NaBH₄ fixed. As the catalyst amount was increased, the reduction became faster.

The reduction was monitored with time using 3 × 10⁻³ g C_A as catalyst, 0.3 mL of 2 mM 4-NP; 1 mL of 0.1 M NaBH₄. It was observed that the reduction completed within 2.3 minutes. However on reducing the volume of 4 NP to 0.1 mL or increasing the NaBH₄ concentration to >0.1 M, the reaction took place immediately so its kinetics could not be monitored. The reaction time could be reduced to 1.7 min by increasing the C_A amount to 7 × 10⁻³ g. While the progress of the reaction could be monitored best when 3 × 10⁻³ C_A was used as catalyst with 0.3 mL of 2 × 10⁻³ M 4-NP; 1 mL of 0.1 M NaBH₄. Under identical condition, no reduction was observed using CS as catalyst while Ac–Si could catalyze the reduction which completed in 18 min (Table 2), indicating gum acacia has some role in the catalytic activity of C_A. The shelf life of catalyst was fairly good; it retained its full catalytic activity up to one month storage at room temperature under dry conditions.

3.5. Recycling

In order to simplify the catalyst recovery for recycling, the reaction was scaled up ten times. In brief 3.0 mL NP, 10 mL 0.1 NaBH₄, 30 mg C_A were used for the reduction. After the first cycle, the catalyst was easily recovered, dried, and reused for the next cycle. It was observed that even for the scaled up reaction, 2.5 min were required for the reduction (Table 4). The recycling was done for six repetitive cycles. In the second and third cycles the reduction time was 6.8 and 8.15 min respectively while in the sixth cycle it took 28 min. Thus though there is a loss in the efficiency of the catalyst in each cycle, the reduction time in the sixth cycle remained still attractive which can be minimized by marginally increasing the catalyst amount.

Table 4 Recycling study

Cycle	Time (min)
1	2.5
2	8.25
3	9.30
4	15.20
5	23.30
6	25.50
7	28.30

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To evaluate the utility of the catalyst for other nitroarenes, 4-NA and 4-NBA were also reduced under the conditions of the reduction of 4-NP. The details are given below.

3.6. Reduction of 4-nitro aniline to 4-phenylenediamine

The reduction of 4-NA to p-PDA was monitored³⁸ by successive UV-vis absorbance measurements of the reaction mixture containing 4-NA and NaBH₄ as 4-NA and p-PDA have distinct peaks at 380 and 238 nm, respectively. With the progress of reaction, the light yellowish-colored 4-NA solution gradually fades and turns colorless with the formation of p-PDA. In the presence of C_A, the average reaction time (RT) was ∼3.15 min (Fig. 8(B)). The catalyst expedited the reaction ∼28-fold than what was observed with gold NPs as catalysts (RT = 86 min)^14,39 and ∼240-fold faster than the reduction using copper NP catalysts (RT = 8–12 h) under identical conditions.⁴⁰ The product from the reaction mixture was extracted with acetone and concentrated under reduced pressure. The crude product thus obtained was then purified by column chromatography using n-butanol/acetic acid/water as eluent. The identity of the product was established by TLC with authentic sample of 4-phenylenediamine as reference in solvent system n-butanol : acetic acid : water 40 : 10 : 50 (upper phase).^{41 1}H NMR (CDCl₃, 500 MHz) of the product also confirmed the formation of the p-phenylenediamine. The peaks are present at δ 6.54 (s, 4H), δ 3.34 (s, 4H).

3.7. Reduction of 4-nitrobenzoic acid to 4-aminobenzoic acid

C_A (3 × 10⁻³ g) was used as heterogeneous catalyst for reducing 0.3 mL of mM solution of 4-NBA in presence of 1 mL of 0.1 M NaBH₄ and the products were diluted after separation from the catalyst and detected by UV-vis spectroscopy (Fig. 8(C)). Reaction did not take place without catalyst under similar conditions. However, it happened quickly and the absorption band of 4-ABA appeared in 8.3 min (ref. 26 and 27) (Table 5). The product was extracted with ether and concentrated under reduced pressure and purified by column chromatography. 4-Amino benzoic acid was detected by TLC (solvent system; n-propanol : water : chloroform (5 : 2 : 1)) with the authentic sample.^{42 1}H NMR (DMSO-d6, 500 MHz) of the product also confirmed the formation of the 4-aminobenzoic acid. The peaks are at δ 11.9 (s, 1H), δ 7.60 (s, 2H), δ 6.63 (s, 2H), δ 5.83 (s, 2H).

Table 5 NaBH₄ reduction of different nitroarenes using the C_A catalyst

S. no.	Nitroarene	Reference λ (nm)	Observed λ (nm)	Reduction time (min)	Yield (%)	Ref.
1	4-NP	403, 295	405, 295	2.3	73%	14, 19
2	4-NBA	270	270	8.3	77%	26
3	4-NA	305, 240	304, 238	3.15	74%	38

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The reduction time for 4 NP, 4-NA, and 4 NBA were 2.3, 3.1 and 8.3 min respectively (Fig. 8, Table 5). The difference in the reduction time for the nitroarenes under study can be assigned to different electronic effects of phenolate ion, amino and carboxylate ions (that exist in the alkaline medium). Higher electron density at p-substituent to nitro group facilitated the reductions which in the present study were phenolate ion, amino group and carboxylate anion respectively. It appears that some interaction between the substrate and catalyst takes place in the rate determining step and the reduction rate is independent of nitroarene concentration, which is in conformity with zero order kinetics for p-NP. However the proper understanding of the mechanism of the reduction by C_A requires deeper investigations.

The products (4-amino phenol, p-phenylenediamine, and 4-amino benzoic acid obtained from the reduction of 4-nitrophenol, 4-nitroaniline, and 4-nitrobenzoic acid respectively) could be successfully identified with TLC with the respective authentic samples in support of the spectrophotometric results. The % yield of 4-amino phenol, p-phenylenediamine, and 4-amino benzoic acid are summarized in Table 5.

4. Conclusions

We have successfully synthesized CuNps anchored onto gum acacia hybrid silica host lattice. The XRD, SEM, EDX, XRD, and TEM were used for the characterization of the catalyst. The chemical composition by EDX revealed the formation of elemental CuNps instead of their oxides. From the TEM analysis, the average sizes of the Nps were found to be ∼18 nm. These supported copper NPs were found to be an excellent catalyst towards reduction of 4-NP to 4-AP, 4-NA to 4-PDA, and 4-NBA to 4-ABA at room temperature. The catalyst is easily recyclable and has good shelf life at room temperature. This suggests that this heterogeneous catalyst can be commercially exploited for the reduction of aromatic nitro compounds.

Acknowledgements

Council of Scientific and Industrial Research (C.S.I.R.) is acknowledged for the financial support to carry out this work. Authors acknowledge NIT, Jaipur for SEM studies, Nanophosphor centre, Allahabad University for XRD, and CDRI, Lucknow for FTIR facility. Authors thank I.I.T. Kanpur for the ESI-MS, and ¹H NMR facility.

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