Catalysis Communications 41 (2013) 6–11

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Catalysis Communications

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Short Communication

Catalytic application of silver nanoparticles immobilized to ricehusk-SiO2-aminopropylsilane composite as recyclable catalyst inthe aqueous reduction of nitroarenes

Jamal Davarpanah, Ali Reza Kiasat ⁎Chemistry Department, College of Science, Shahid Chamran University, Ahvaz 61357-4-3169, Iran

⁎ Corresponding author. Tel./fax: +98 611 3331746.E-mail address: akiasat@scu.ac.ir (A.R. Kiasat).

1566-7367/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.catcom.2013.06.020

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 April 2013Received in revised form 11 June 2013Accepted 13 June 2013Available online 27 June 2013

Keywords:Rice huskSilver nanoparticlesRice husk aminosilaneNitro group reduction

In the present work, amorphous silica nanoparticles were taken from low-cost rice husk (RH-SiO2) by carry-ing out an acid chemical treatment followed by the process of burning. The nanoparticles were modified withaminopropyltrimethoxysilane (APTMS) using sol–gel method. The target nanocomposite (RHPrNH2@Ag)was formed by embedding the silver nanoparticles into the rice husk propylamine composite (RHPrNH2).The catalytic activity of RHPrNH2@Ag nanocomposite was successfully tested in the aqueous reduction ofnitroarenes to the corresponding amines by using NaBH4 as reducing agent.

© 2013 Elsevier B.V. All rights reserved.

NO2 NH2

H2O, Reflux

RHPrNH2@AgNaBH4 ,

1. Introduction

Increased environmental consciousness has motivated researchinto the effective utilization of agricultural byproducts. Rice husk isan important agricultural waste that abundantly produced globallyevery year. Due to its high silicon content, rice husk has become asource for preparation of elementary silicon and a number of siliconcompounds, especially silica (sodium silicate) [1], silicon carbide [2]and silicon nitride [3].

Silica is the major inorganic constituent of the rice husk, which bycarrying out an acid chemical treatment followed by the process ofburning, it is possible to extract high-surface area amorphous silica.Recently, Adam et al. managed to incorporate various types of transi-tion metals into rice husk silica and reported their catalytic potentialsin the oxidation, acylation and benzylation reactions [4–6].

Metal nanoparticles continue to attract interest for different re-searcher areas due to their different physical and chemical propertieswhen compared to bulk metals. Because of the surface reactivity ofsilver nanoparticle, which makes it desirable for use in catalysis, it isthermodynamically unstable and easily aggregate. Particles growthor aggregation leads to loss of nanosize and important properties,which achieved at nanolevel [7]. In response to this challenge, someessential methods have been developed, for example, application ofpolymeric stabilizing agents, electron-beam lithiography, electro-chemical deposition, template growth technique and functionalizedsupports methods [8].

rights reserved.

On the other hand, aromatic amines are important starting ma-terials and intermediates for the manufacture of a great variety ofchemicals. They are generally synthesized by the chemical reductionof nitroarenes [9]. Although a variety of methods has been well docu-mented [10–15] for this purpose, however, some of these protocolsbear drawbacks such as long reaction time, use of toxic and expensivecatalyst, carcinogenic solvent, unavailability and reusability of thecatalyst. Therefore, there is still need for a green catalyst which candominate one or more drawbacks and also an environmentally benignprocedure to synthesize aromatic amines.

By considering all the above-mentioned points and in continuationof our research [9,16,17] to develop green chemistry by using water asreaction medium and molecular host–guest systems [18,19], herein,amorphous nanosilica was successfully extracted from rice husk(RH-SiO2), functionalized with propylamine groups (RHPrNH2) andused as a host to support silver NPs (RHPrNH2@Ag). The catalytic ac-tivity of the synthesized RHPrNH2@Ag was investigated for the reduc-tion of nitroarenes to the corresponding aromatic amines by usingNaBH4 as reducing agent in aqueous medium (Scheme 1).

R R

Scheme 1. Reduction of nitroarenes in the presence of RHPrNH2@Ag.

Table 1Specific surface area (SBET), diameter pore and total pore volume of RHPrNH2@Ag.

Sample BET surface area(m2 g−1)

Diameter(nm)

Pore volume(cm3 g−1)

RHPrNH2@Ag 139 15.96 0.979

Fig. 1. The SEM micrograph of extracted nanosilica.

7J. Davarpanah, A.R. Kiasat / Catalysis Communications 41 (2013) 6–11

2. Experimental

2.1. General

Chemical materials were purchased from Fluka and Merck compa-nies and usedwithout further purification. Rice husk (RH)was collect-ed from a rice mill in Khozestan–Baghmalek (Iran). Products werecharacterized by comparison of their physical data, IR and 1H NMRand 13C NMR spectra with known samples. The purity determinationof the products and reaction monitoring were accomplished by TLCon silica gel PolyGramSILG/UV 254 plates. NMR spectrawere recordedin CDCl3 on a Bruker Advance DPX 400 MHz instrument spectrometerusing TMS as internal standard. FT-IR spectra of the powders were

Scheme 2. The simple reaction sequence for the p

recorded using BOMEMMB-Series 1998 FT-IR spectrometer. Nitrogenadsorption measurements were conducted at 77.4 K on a Belsorp18(Bel Japan Inc.). The specific surface area and the pore size distributionwere calculated by Brunauer–Emmett–Teller (BET) method andBarrett–Joyner–Halenda (BJH) model, respectively.

2.2. Extraction of silica from rice husk

The rice husk was stirred with 1.0 M nitric acid at room tempera-ture for about 24 h. It was thoroughly washed with distilled wateruntil the pH of the rinse became constant, dried in an oven at 100 °Cfor 24 h andwas burned in amuffle furnace at 800 °C for 6 h. Silica ex-traction from RH was carried out by stirring the rice husk in 500 mL,1.0 M sodium hydroxide for 24 h at room temperature. Then the mix-ture was filtered and titrated with 3.0 M HNO3. The titration was con-tinued until the solution pH reached 5.0 and aged for 24 h. The silicagel/precipitate was filtered, washed thoroughly with distilled waterand dried at 100 °C for 18 h [20].

2.3. Synthesis of rice husk propylamine silane (RHPrNH2)

About 3.0 g of the nanosilica (obtained from RH) was stirred in350 mL of 1.0 MNaOH at room temperature. The solution was filteredto remove undissolved particles and 6.0 mL solution of APTMS wasadded to the resulting sodium silicate. The solution was then titratedslowly (1.0 mL min−1) with 3.0 M nitric acid with constant stirring.The change in pH was monitored by using a pH meter. A white gelstarted to form when the pH decreased to less than 11.0. The titrationwas done slowly until pH 5 was reached. The gel obtained was agedfor 20 h. Then the gel was separated by centrifuge. The separation pro-cess was repeated 6 times with distilled water. The final washing wasdone with acetone. The sample was then dried at room temperature[21].

2.4. Immobilization of silver nanoparticles on RHPrNH2

1.5 g of RHPrNH2 was dispersed in 100 mL freshly prepared aque-ous of 0.003 M NaBH4, the mixture was stirred for 1 h in ice bath. AnAgNO3 solution (100 mL of 0.001 M) was added drop wise with con-stant stirring to solution. After 2 h, the ice bath was removed and the

reparation of RHPrNH2@Ag nanocomposite.

400900140019002400290034003900

Tra

nsm

itta

nce

(%T

)

Wavenumber (cm-1)

RHPrNH2AgRHPrNH2RH-SiO2

Fig. 2. The FT-IR spectra of RH-SiO2, RHPrNH2 and RHNH2@Ag.

8 J. Davarpanah, A.R. Kiasat / Catalysis Communications 41 (2013) 6–11

catalyst remained on the stir plate until reached to room temperatureand stirred for 3 h. Finally, the nanocomposite was filtered, washedwith water for several times and dried at 45 °C. The immobilized sil-ver nanoparticles were stored in a dark colored bottle.

2.5. Procedure for reduction of nitro compounds, catalyzed byRHPrNH2@Ag

In a 50 mL round bottom flask, nitroaromatic compound (1 mmol),NaBH4 (4–6 mmol) and catalyst (0.5 g) were added and completelymixed at room temperature. Then water (5 mL) was added to the mix-ture and the resulting suspension was stirred under reflux conditionsfor the time shown in Table 1. After completion of the reaction as indi-cated by TLC [using Et2O/n-hexane as eluent: 1/5], the insolublesupported nanocatalyst was filtered off and the filtrate was extractedwith diethyl ether (2 × 10 mL). The organic phase was dried over calci-um chloride, and evaporated in vacuo to give the product. All thecompounds were characterized on the basis of spectroscopic data(IR, 1H & 13C NMR) and by comparison with those reported in the liter-ature [9–15].

Fig. 3. The TEM image

3. Results and discussion

Amorphous nanosilicawas easily extracted from rice husk (RH). Themorphological characterization and the particle size distribution ofextracted nanosilica were performed by measuring SEM. According toFig. 1, nanoparticles were obtained with spherical shape. The RH-SiO2

nanospheres size distribution has an average of 62 nm.RH-SiO2 nanospheres were modified with aminopropyltrime-

thoxysilane (APTMS) by sol–gel method. Embedding of the silvernanoparticles into the rice husk propyl amine composite (RHPrNH2)was easily carried out by chemical reduction of AgNO3 by NaBH4 inwater and in the presence of RHPrNH2. The schematic diagram forthe synthesis of the target nanocomposite, RHPrNH2@Ag, is shownin Scheme 2.

The FT-IR spectra of the RH-SiO2, RHPrNH2 and RHPrNH2@Agsamples are shown in Fig. 2. The broad band around 3450 cm−1 isdue to the stretching vibration of SiO\H bond and the HO\H vibra-tion of water molecules adsorbed on the silica surface. The bandaround 1635 cm−1 is also due to the bending vibration of water mol-ecules bound to the silica matrix. The strong peak at 1099 cm−1 isdue to asymmetric stretching vibration of the structural siloxanebond, Si\O\Si. The bands at 800 cm−1 and 466 cm−1 in all spectraare due to the deformation of Si\O bond. The band at 964 cm−1,which in RHPrNH2 and RHPrNH2@Ag was disappeared, is attributedto Si\OH stretching vibration [22]. RHPrNH2 and RHPrNH2@Agshowed a band at 1382 cm−1 which is characteristic for the NH3

group.TEMmicrographs provide more accurate information on the parti-

cle size and morphology of immobilization silver NPs on the supportsurface. As shown in Fig. 3. The TEM image represents the stabiliza-tion of silver nanoparticles on the amino rice husk support and indi-cates that the average particle size of silver is about 20 nm and alsoit demonstrated that the prepared nanoparticles were separatedwidely with narrow size distribution and did not agglomerate.

TGA and differential thermo-gravimetric (DTG) curves of RHPrNH2@Ag are shown in Fig. 4. The thermal analysis further demonstrates thatthe PrNH2 groups are successfully grafted on theRHnanosilica structure.Weight loss in the temperature range of 330–430 °C (ca. 44.0 wt.%) canbe assigned to the decomposition of the organic part of catalyst, which is

of RHPrNH2@Ag.

Fig. 4. TGA and DTG profile of RHPrNH2@Ag.

9J. Davarpanah, A.R. Kiasat / Catalysis Communications 41 (2013) 6–11

attributed to propylamine groups. One step of weight loss in the com-bined TGA–DTG curves show that the catalyst is stable at the tempera-ture employed in the catalytic studies and Ag nanoparticles are stableup to 330 °C.

Fig. 5 shows the XRD spectra of RH-SiO2 and RHPrNH2@Ag. Thelack of sharp peaks in the X-ray diffraction pattern indicates theamorphous nature of the samples which only had a broad diffractionband at 2θ angle of ca. 21.8. The XRD result of RHPrNH2@Ag was sim-ilar to other metal incorporated rice husk silica complexes reportedearlier [4–6,21,22]. The broad peaks at about 2θ = 38.4, 44.6, 64.6and 77.9 are ascribed to the Ag particles according to the data of theJCPDS file [JCPDS 4-783].

The BET adsorption isotherm obtained for RHPrNH2@Ag is shownin Fig. 6. The hysteresis loop was observed in the range of 0.4 b P /P0 b 1.0, which is associated with capillary condensation takingplacewithin themesopores. The isotherms present a sharp adsorptionstep in the P/P0, which implied that the materials possess large poresize with narrow distributions. The BET surface area of RHPrNH2@Agwas found to be 139 m2 g−1, while that of RH-SiO2 was reported as

Fig. 5. The X-ray diffraction pattern for

347 m2 g−1 [21]. This was further confirmed by the pore size distribu-tions calculated by Barrett–Joyner–Halenda (BJH) method from theadsorption branches. The average pore diameter of 15.96 nm (BJHmodel) was obtained for RHPrNH2@Ag. The characteristic data onthe sample are summarized in Table 1.

To evaluate the catalytic activity of RHPrNH2@Ag as heterogeneouscatalyst in the reduction of NO2 group, reduction of nitrobenzene wasexamined to determinewhether the use of RHPrNH2@Agwas efficientand to investigate the optimize conditions. In a typical experiment, ni-trobenzene (1 mmol) and NaBH4 (6 mmol) were stirred under refluxconditions in water. TLC analysis of the reaction mixture did not showcompletion of the reaction after 8 h and the reaction media was con-taminated by azobenzene. When the reaction was performed in thepresence of the catalyst, it proceeded rapidly to give the aniline. Asshown in Table 2, 0.5 g of the catalyst and 6 mmol of NaBH4 are thebest operative experimental conditions (Table 2, Entry 4).

Subsequently, with optimal conditions in hand, the generality andsynthetic scope of this protocol were demonstrated by reduction ofother nitro aromatic compounds. Reduction of different nitroarenes

(a) RH-SiO2 and (b) RHPrNH2@Ag.

Fig. 6. Pore size distribution (a) and nitrogen adsorption–desorption isotherm (b) ofRHPrNH2@Ag.

Table 3Reduction of nitroarenes by NaBH4 in the presence of RHPrNH2@Ag in water (NaBH4/nitro compound: 6:1, temperature: 100 °C).

Entry R Product Time (min) Catalyst (g) Yield (%)

1 50 0.5 100

2 40 0.4 98

3 65 0.5 98

4 75 0.6 95

5 70 0.5 95

6 50 0.5 97

7 45 0.5 85

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carrying activated and deactivated groups were carried out by use ofNaBH4 in the presence of RHPrNH2@Ag in water under reflux condi-tions (Table 3). The reduction of nitroarenes took place smoothlyand easily to afford the corresponding amines as exclusive productsaccording to TLC. It is worthy to note that the azoxy, azo and hydrazocompounds as the usual side products in reduction of nitroareneswere not observed in this method.

Table 2Reduction of nitrobenzene in the presence of RHPrNH2@Ag under different reactionconditions in water.

Entry NaBH4

(mmol)Time(min)

Catalyst(g)

Yield(%)

1 6 180 0 Trace2 6 90 0.2 453 6 50 0.3 724 6 50 0.5 1005 5 50 0.5 836 4 50 0.5 687 4 90 0.5 70

Reusability of the RHPrNH2@Ag catalyst was tested by consecu-tively recovering and then reusing the catalyst nanoparticles up tofour times. The reduction reaction of nitro benzenewas carried out re-peatedly under one constant set of operating conditions (NaBH4/nitrobenzene: 6:1, temperature: 100 °C) (Table 4).

4. Conclusions

In the present study nanosilica was easily extracted from rice huskash and functionalized with APTMS. RHPrNH2 was used as a supportfor the stabilization of Ag NPs. The catalytic activity of the synthesizedRHPrNH2@Ag was successfully investigated for the reduction ofnitroarenes with NaBH4 in water. This environmentally friendlyroute for the reduction of aromatic nitro compounds can be used forlaboratory synthesis and we feel that it may be a suitable additionto methodologies already present in the literature.

Acknowledgment

We are grateful to the Research Council of Shahid Chamran Uni-versity for the financial support.

Table 4Recyclability of RHPrNH2@Ag.

No of runs Time (min) Yield (%)

1 50 1002 60 1003 70 934 70 93

11J. Davarpanah, A.R. Kiasat / Catalysis Communications 41 (2013) 6–11

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