Mesoporous polyacrylic acid supported silver nanoparticles as an efficient catalyst for reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source

Accepted Manuscript Mesoporous polyacrylic acid supported silver nanoparticles as an efficient catalyst for reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source Usha Mandi, Anupam Singha Roy, Sudipta K. Kundu, Susmita Roy, Asim Bhaumik, Sk. Manirul Islam PII: DOI: Reference:

S0021-9797(16)30179-5 http://dx.doi.org/10.1016/j.jcis.2016.03.037 YJCIS 21161

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

3 November 2015 15 March 2016 17 March 2016

Please cite this article as: U. Mandi, A.S. Roy, S.K. Kundu, S. Roy, A. Bhaumik, Sk. Manirul Islam, Mesoporous polyacrylic acid supported silver nanoparticles as an efficient catalyst for reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/ 10.1016/j.jcis.2016.03.037

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Mesoporous polyacrylic acid supported silver nanoparticles as an efficient catalyst for reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source Usha Mandi,a Anupam Singha Roy,a,b Sudipta K. Kundu,c Susmita Roy,a Asim Bhaumik,*,c Sk. Manirul Islam* ,a a

Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India

b

Department of Chemical Sciences, Indian Institute of Science Education and Research

(IISER) Kolkata, Mohanpur - 741246, India c

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata -

700032, India

Abstract Silver nanoparticle immobilized mesoporous cross-linked polyacrylic acid (Ag-MCP-1) has been synthesized via aqueous-phase polymerization of acrylic acid followed by the surface immobilization with silver nanoparticles. The nanocomposite material has been characterized by different spectroscopic techniques. Powder X-ray diffraction patterns revealed the formation of silver nanoparticles, while transmission electron microscope image showed that Ag nanoparticles are formed and uniformly dispersed in the mesoporous polyacrylic acid. The Ag-MCP-1 nanocomposite can be used as an efficient heterogeneous catalyst in the reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source. This nanocomposite can be reused more than five times without any significant decrease in its catalytic activity. Keywords: Mesoporous material, Polyacrylic acid, Silver nanoparticles, Amination, Nitrobenzenes, Glycerol.

1

*

Authors to whom correspondence should be addressed.

Dr. Sk. Manirul Islam, Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India. Phone: +91-33-2582-8750, Fax: +91-33-2582-8282, E-mail: [email protected] Prof. Asim Bhaumik, Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata-700032, India, Phone: +91-33-2473-4971; Fax: +91-332473-2805, E-mail: [email protected]

Current trends for designing high performance nanomaterials include the synergy between tiny inorganic nanocrystals and organic polymers, particularly fine dispersion of nanosized metal particles at the surface of a suitable polymer matrix [1]. Hydrogel nanocomposites (HNs), which include the incorporation of inorganic nanoparticles inside three-dimensional polymeric networks, have been attracted great interests in recent years because of their advantages over pure hydrogels or inorganic nanoparticles [2-4]. Recently, silver nanoparticles have been widely utilized in this field due to remarkably enhancement some properties of hydrogel such as mechanical toughness, large deformability, excellent electrical conductivity, antimicrobical effects, optical properties and high transparency [5,6]. Silver should be well dispersed on the surface of the hydrogels without the formation of large aggregates otherwise the properties of silver dramatically reduce [7]. To improve the dispersion of nanoparticles inside the hydrogel matrix, and also partially prevent the formation of aggregates, Poly (acrylic acid) (PAA) can be used as a stabilizer for nanoparticles synthesis [8]. The past studies of our own have demonstrated that the supported silver nanoparticles showed superior catalytic activity in various reactions. Silver has highest electrical conductivity as well as enhanced catalytic activity, antibacterial properties and good biocompatibility [9-12]. Hence, we have developed a novel material 2

which exhibits both organic and inorganic characteristics with the selection of the proper organic mesoporous PAA and inorganic Ag particles phases. Aromatic amines are important building blocks for the synthesis of pharmaceuticals, natural products, functional materials, agrochemicals, dyes and ligands synthesized through transition-metal-catalyzed reactions [13-15]. N-substituted amines are usually prepared by the alkylation of amines with halides [16]. But this method often suffers from environmental problems, high cost of the starting materials and low selectivity to the desired products [17, 18]. Several approaches have been developed to overcome these drawbacks, in many cases, N-substituted

amines

could

also

be

synthesized

by

hydroamination

[19],

hydroaminomethylation [20], self-coupling of primary amine [21], and reductive amination [22]. Recently, the catalytic alkylation of amines with alcohols [23] using the hydrogen autotransfer process [24], also known as borrowing hydrogen [25] or self-supplying system for active hydrogen [26] has been proposed as an environmental benign procedure to produce N-substituted amines. The alkylation of amines by alcohols under harsh conditions has been reported independently by Grigg and Watanable in the early 1980s [27]. In recent years, milder conditions have been achieved by Yamaguchi and co-workers [28,29] and by Beller [30]. Several catalytic systems for the one-pot reductive amination of aldehydes with nitroarenes have been developed [31-36]. Both nitroarenes and alcohols are cheap, readily available so the direct amination of nitroarenes with alcohols would be a more sustainable. Alcohol can be employed as an alkylating agent as well as the hydrogen source to reduce nitrobenzene. Therefore, a large excess of alcohol is needed in the reaction medium. Based on the efforts for clean and economic synthesis of N-substituted amines using alcohol as the reducing and alkylation agent [37-40], we have tried to develop a method for a one-pot synthesis of N3

substituted amines and imines from nitrobenzene using glycerol as the hydrogen source. Herein, we report the mesoporous cross-linked polymer supported silver nanocatalyst (AgMCP-1) and its efficient catalysis for the clean and atom-efficient mono-N-alkylation of a range of amines with alcohols using glycerol as hydrogen source.

2. Experimental 2.1. Materials Acrylic acid (monomer), N,N,N′,N′-tetramethylethylenediamine (TEMED, promoter), N,N-methylene-bis-acrylamide (BA, cross-linker) and ammonium persulfate (APS, initiator) were obtained from Loba Chemie. Tris(hydroxymethyl) aminomethane (TRIS) was obtained from Sigma-Aldrich and used as received. Silver nitrate and sodium borohydride were obtained from E-Merck. All other chemicals used in this investigation were of analytical grade. 2.2 Characterization techniques A Bruker D8 Advance X-ray diffractometer using Ni–filtered Cu Kα (λ=0.15406 nm) radiation was used to obtain the powder X-ray diffraction (XRD) pattern of MCP-1 and AgMCP-1 samples. A JEOL JEM 2010 transmission electron microscope and JEOL JEM 6700F field-emission scanning electron microscope were used for the analysis of size, shape and morphology of the nanocomosite. UV-Vis spectra were taken on a Shimadzu UV-2401PC spectrophotometer. FT-IR spectra of the samples were taken on KBr pellets using a PerkinElmer FT-IR 783 spectrophotometer. 2.3. Synthesis of MCP-1 MCP-1 material was prepared following our previously reported procedure [41]. 0.01 mol (3.64 g) of cetyltrimethylammonium bromide (CTAB) and 0.04 mol (2.88 g) of acrylic acid were dissolved in 30 mL of water. Then 0.01 mol (1.54 g) BA was added to the above 4

mixture. Finally, 0.02 mol (4.56 g) APS and 0.01 mol (1.16 g) TEMED dissolved in 20 mL of water and added into the above solution under stirring condition. NaOH (25% aqueous) solution was slowly added to the above mixture and stirring for 1 h. The resultant mixture was autoclaved at 348 K for 3 days. The precipitate was filtered out and washed with deionized water. The surfactant was removed from the as-synthesized material followed by the method as mentioned earlier [41]. 2.4. Synthesis of Colloidal Ag Nanoparticles 0.1 mL aqueous solution of 1% AgNO3 was added to TRIS (0.5 mg) solution (10 mL) and stirred for 2 min. Then, 0.25 mL of NaBH4 (0.08%) aqueous solution was added dropwise. The stirring was continued for another 10 min, and the final colloidal solution was stored at 4°C. 2.5. Synthesis of Ag-MCP-1 nanocomposite 0.1 g of MCP-1 was dispersed in 10 mL of Ag-NPs colloidal solution and stirred for 1 h at room temperature. The colour of MCP-1 was changed to black while stirring which indicates the loading of Ag-NPs onto the surface of MCP-1. After centrifugation, black coloured Ag-MCP-1 (Scheme 1) material was obtained. This mesoporous material was washed further with copious amounts of water and dried at room temperature. The loading of Ag-NPs onto MCP-1 was further confirmed by spectral measurements (Ag loading 7.25 wt%).

5

Scheme 1. Schematic diagram showing the formation of Ag-MCP-1 nanocomposite. 2.6. General procedure for the coupling reaction of nitrobenzenes with alcohols to synthesize substituted imines In a round bottomed flask, nitrobenzene (1 mmol), alcohol (1 mmol), K2CO3 (13.9 mg, 0.1 mmol), 1.0 g glycerol, 2 mL toluene and Ag-MCP-1 (25 mg) was added and the reaction mixture was stirred under refluxing for 12 h under N 2 atmosphere. After cooling, 20 mL ethanol was added to the reaction mixture and the conversion and selectivity of the desired product were determined by a Varian 3400 gas chromatograph equipped with a 30 m CP-SIL 8CB capillary column and Trace DSQ II GC-MS equipped with a 60 m TR-50MS capillary column. 2.7. General procedure for the coupling reaction of nitrobenzenes with alcohols to synthesize substituted amines In a round bottomed flask, nitrobenzene (1 mmol), alcohol (1 mmol), K2CO3 (13.9 mg, 0.1 mmol), 1.0 g glycerol, 2 mL xylene and Ag-MCP-1 (25 mg) was added and the reaction mixture was stirred under refluxing for 24 h under N2 atmosphere. The conversion and selectivity of the desired product were determined by the above method.

6

3. Results and Discussion 3.1. Characterization of Ag-MCP-1 nanocomposite TEM Analysis: HR TEM images of Ag-MCP-1 samples at different magnifications are shown in Figure 1. The presence of silver nanoparticles in Ag-MCP-1 is confirmed from these TEM images. From these TEM images it is clear that the silver nanoparticles of size ca.~ 5-25 nm are dispersed in the polymeric matrix. Fine dispersion of Ag nanoparticles (Figure 1A-B) will have high impact on the catalytic activity of the materials, whereas the presences of white spots corresponding to small mesopores are seen in Figure 1C.

Figure 1. HR TEM images (A, B and C) of Ag-MCP-1 nanocomposite and FFT pattern is shown in Figure 1D). XRD studies: In Figure 2A), the small angle PXRD of extracted and Ag loaded MCP-1 samples are shown. Both MCP-1 and Ag-MCP-1 samples exhibit a broad peak centred at 2θ value of 2.40 and 2.47, respectively indicating the presence of mesophases in these samples. 7

The shifting of the peak of Ag-MCP-1 to the higher 2θ value could be attributed to high loading of Ag nanoparticles in the interior surface of MCP-1. Figure 2B) shows the wide angle powder XRD pattern of extracted MCP-1 and Ag-MCP-1. The sharp intense peaks of the XRD pattern confirm the formation of Ag nanoparticles in polyacrylic acid matrix. Bragg's reflections at 2θ = 38.20, 44.36, 64.50, and 77.65o corresponds to (111), (200), (220) and (311) lattice planes of silver nanoparticles embedded in MCP-1 [42, 43]. Whereas, MCP1 exhibits a broad peak centred at 2θ = ~22o owing to its amorphous nature [44].

Figure 2. A) Small-angle PXRD of MCP-1 and Ag-MCP-1, B) wide angle PXRD patterns of MCP-1 and Ag-MCP-1 materials. SEM analysis: In order to examine the surface morphology of the Ag-MCP-1 nanocomposite, FE-SEM analysis has been done. Figure 3(a) shows the FE-SEM image obtained for the AgMCP-1 material. This FE-SEM image indicates the porous structure of the Ag-MCP-1 material and the spherical particles become aggregated among themselves to form a large assembly of particles. Figure 3(b) shows an EDX spectrum of the Ag-MCP-1 nanocomposite which shows the presence of Ag particles. UV-Vis spectra: UV-Vis absorption spectra of the pure MCP-1 and Ag-MCP-1 nanocomposite are shown in Figure 3(c). The UV–Vis spectrum of MCP-1 does not show 8

any characteristic band, whereas the Ag-MCP-1 hybrid material shows a strong absorption band corresponding to the surface plasmon resonance at 403 nm [42].

Figure 3. (a) FE-SEM image of Ag-MCP-1, (b) EDX spectra of Ag-MCP-1 nanocomposite, (c) DRS UV-vis of MCP-1 and Ag-MCP-1 materials and (d) FT-IR spectra of (I) MCP-1 and (II) Ag-MCP-1 nanocomposite. FTIR spectra: Figure 3(d) shows the IR spectra of the MCP-1 and Ag-MCP-1. The carbonyl asymmetric stretching band [v(C=O)] of -CO2H is found at 1730 cm-1 in the spectrum of MCP-1. On the otherhand, for the Ag-MCP-1 the v(C=O) of -CO2H shifts to lower wavenumber and appeared around at 1726 cm-1 due to the strong interection between silver nanoparticles and MCP-1 [45].

9

N2 adsorption/desorption and BET surface area analysis: The N2 adsorption–desorption isotherm for Ag-MCP-1 material has been shown in Figure 4. The BET surface area of the material is 31 m2g-1. This decrease in surface area from parent MCP-1 material (surface area 56 m2g-1) can be attributed to be the incorporation of Ag nanoparticles in the porous framework of polymer. The pore size of the material has been determined by NLDFT method and this is shown in the inset of Figure 4. The average pore size of the material is centred at 2.46 and 3.57 nm which proved that the Ag-MCP-1 material contains a wide range of pores [46, 47]. Thus, BET analysis data revealed that after the successful immobilization of the Ag nanoparticles over MCP-1 the parent cross-linked porous polymer has retained its mesoporosity. Large desorption hysteresis could be attributed to the cross-linked polymeric network [41].

Figure 4. N2 adsorption/desorption isotherm of Ag-MCP-1 material and pore size distribution calculated by NLDFT method, is shown in the inset.

3.2 Catalytic activity Heterogeneous nanocatalysts are also in great demand for clean technology and sustainable development [48-52], so we have decided to investigate the catalytic activity of 10

Ag-MCP-1 in the reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source. Reductive imination of nitrobenzenes with alcohols catalysed by Ag-MCP-1 nanocatalyst: The catalytic activity of Ag-MCP-1 was explored in the imination of p-methyl nitrobenzene and benzyl alcohol with 1 to 1 molar ratio as the model substrates (Scheme 2).

NO2

OH

NH 2

Ag-MCP-1

N

K 2CO 3, N 2, 120 oC 1a

2a

Toluene, 12 h, Glycerol

3a

4a

Scheme 2. Reductive imination of p-methyl nitrobenzene with benzyl alcohol catalysed by Ag-MCP-1 nanocatalyst.

The performance of a silver-catalyzed reductive coupling is known to be governed by the number of factors such as base, solvent and catalyst. Firstly, we have carried out the imination of alcohols with nitrobenzenes in the presence of various bases such as K2CO3, Na2CO3, NaOH and KOH and solvents such as Toluene, THF, DMF and 1,4-dioxane and the results are summarized in Table 1. Without using any base (Table 1, entry 1) 15% conversion of p-methyl nitrobenzene and 80% selectivity to N-benzylidene-4-methylaniline were obtained. As seen in Table 1, the use of K2CO3 as base and toluene as solvent resulted in good conversion of the substrate (Table 1, entry 2). Similarly the use of other bases and solvents were investigated (Table 1, entries 3-9) but in every cases lower conversion of the nitrobenzene and lower selectivity to imine were observed. It is seen that, when the reaction time was increased upto 24 h complete conversion of p-methyl nitrobenzene to N-benzyl-4methylaniline was observed (Table 1, entry 10). In the absence of any catalyst, low conversion (~5 %) of the substrate was observed (Table 1, entry 11). The substrate was 11

allowed to react with MCP-1 as catalyst using K2CO3 and toluene, it was found that 35% conversions of p-methyl nitrobenzene was obtained (Table 1, entry 12). From the above discussions, it can be seen that the best yield was obtained in the imination of nitrobenzenes with alcohols using K2CO3 as base and toluene as solvent. When AgO, Ag nanoparticles and MCP-1 are used as catalysts, 26-45% conversion of p-methyl nitrobenzene is observed (Table 1, entries 13-15). But when the combination of Ag and MCP-1, Ag-MCP-1, is used as catalyst the conversion and the selectivity of the desired product is high (Table 1, entry 2). The synergistic effect of Ag and MCP-1 may be responsible for this high catalytic activity. Table 1. Optimization of the condition of the coupling reaction of p-methyl-nitrobenzene and benzyl alcohol.a Entry

Catalyst

Base

Solvent

Conversion

Selectivity

(%)

(%)b 3a

4a

1

Ag-MCP-1

_

Toluene

15

20

80

2

Ag-MCP-1

K2CO3

Toluene

70

3

97

3

Ag-MCP-1

Na2CO3

Toluene

32

10

90

4

Ag-MCP-1

NaOH

Toluene

5

0

>99

5

Ag-MCP-1

KOH

Toluene

5

0

>99

6

Ag-MCP-1

K2CO3

THF

25

0

>99

7

Ag-MCP-1

K2CO3

DMF

42

8

92

8

Ag-MCP-1

K2CO3

1,4-dioxane

27

12

88

12

9

Ag-MCP-1

K2CO3

TFMB

42

11

89

10c

Ag-MCP-1

K2CO3

Toluene

100

0

>98d

11

_

K2CO3

Toluene

~5

0

>99

12c

MCP-1

K2CO3

Toluene

35

30

70

13

AgO

K2CO3

Toluene

26

100

0

14

Ag

K2CO3

Toluene

45

87

12

K2CO3

Toluene

32

45

53

nanoparticles 15

MCP-1

Reaction conditions: a) 1 mmol benzyl alcohol, 1 mmol nitrobenzene, 25 mg catalyst, 10 mol% base, 1.0 g glycerol, 2 mL solvent, Time 12 h; b) the products were confirmed by GCMS, the yields was determined by GC. C) The reaction was prolonged to 24 h, d) N-benzyl-4methylaniline (5a) formed.

After having optimized the reaction conditions for the coupling reaction of p-methyl nitrobenzene and benzyl alcohol, imination of various nitrobenzenes and benzyl alcohols using K2CO3 as base and toluene as solvent have been carried out in the presence of 25 mg of Ag-MCP-1 as a catalyst at 120 oC under N2 atmosphere and glycerol as hydrogen source (Table 2). The reaction proceeds very smoothly at 120 oC in 12 h. Using Ag-MCP-1 nanocatalyst the yields are good to excellent. We explored the protocol with a variety of nitrobenzenes with benzyl alcohols (Table 2, entries 1-5). The nitrobenzenes reacted well, providing the imine products in 83–99% yields (Table 2, entries 1-5). The electron-donating as well as electron-withdrawing groups on aromatic rings were tolerated, although the latter gave slightly reduced yields. 1-nitronaphthalene with benzyl alcohol also gave the 13

corresponding imine product in good yield 79% (Table 2, entry 6). Treatment of p-methyl nitrobenzenes with various benzyl alcohols furnished the corresponding imines in good to excellent yields (Table 2, entries 7–10). Electron-donating groups on benzyl alcohol increases the reactivity to give the corresponding imines in 90-95% yields (Table 2, entries 7-9), while electron-withdrawing group, on the benzyl alcohol decreased the reactivity (Table 2, entry 10). Table 2. Reductive imination of nitrobenzenes with alcohols catalysed by Ag-MCP-1 nanocatalyst.a Entry Nitrobenzene

Alcohol

Product

Yieldb (%)

NO2

1

88

OH

N

2

NO2

99

OH

N

NO2

3

90

OH N

O

O NO2

4

83

OH N

Cl

Cl

NO2

5

86

OH

N

Br

Br

6

NO2

OH

N

14

79

NO2

7

90

OH N

NO2

8

92

OH

N

O

O

NO2

9

95

OH

N

NO2

10

70

OH

N

Cl

Cl

a

Reaction conditions: 1 mmol benzyl alcohol, 1 mmol nitrobenzene, 25 mg catalyst, 10 mol% K2CO3, 1.0 g glycerol, 2 mL toluene, 120 oC, 12 h. bYield determined by GC analysis.

Reductive amination of nitrobenzenes with alcohols catalysed by Ag-MCP-1 nanocatalyst: We have started our investigation for amination of p-methyl nitrobenzene with benzyl alcohol using glycerol as hydrogen source and xylene as solvent as a model reaction in the presence of Ag nanocatalyst Ag-MCP-1 as shown in Scheme 3.

NO2

Ag-MCP-1

OH

H N

K 2CO 3, N 2, 150 oC 1a

2a

Xylene, 24 h, Glycerol

5a

Scheme 3. Reductive amination of p-methyl nitrobenzene with benzyl alcohol catalysed by Ag-MCP-1 nanocatalyst.

15

Table 3. Reductive amination of nitrobenzenes with alcohols catalysed by Ag-MCP-1 nanocatalysta Entry

Nitrobenzene

Alcohol

Yieldb

Product

(%) NO2

1

93

OH H N

2

NO2

89

OH H N

NO2

3

84

OH H N

O

O NO2

4

81

OH H N

Cl

Cl

5

6

NO2

NO2

80

HN

OH

86

OH H N

7

NO2

89

OH H N

a

Reaction conditions: 1 mmol benzyl alcohol, 1 mmol nitrobenzene, 25 mg catalyst, 10 mol% K2CO3, 1.0 g glycerol, 2 mL xylene, 150 oC, 24 h. bYield determined by GC analysis.

In the reductive coupling of benzyl alcohol and p-methyl nitrobenzene, Nbenzylidene-4-methylaniline was formed as the main product and also very small amount of 16

N-benzyl-4-methylaniline was observed as a side product. From this result we conclude that it may be possible to synthesize secondary amines by this method, too. To explore the generality and scope of this reaction structurally varied alcohols were subjected to react with substituted nitrobenzene using 25 mg of catalyst at 150 oC under refluxing in xylene and N2 atmosphere to afford corresponding N-substituted aniline in good to excellent yields. The results are reported in Table 3. Various nitrobenzenes underwent reaction with benzyl alcohols produced the corresponding amines in 81-93% yields (Table 3, entries 1-4). 1nitronaphthalene with benzyl alcohol also gave the corresponding amine product in good yield 80% (Table 3, entry 5). Electron-donating groups on benzyl alcohol gave the corresponding amines in 86-89% yields (Table 3, entries 6-7).

4. Recyclability of Ag-MCP-1 nanocatalyst The recyclability of the Ag-MCP-1 nanocatalyst is investigated in reductive coupling of nitrobenzenes with alcohols. After the completion of the reaction, the catalyst is separated from the reaction mixture and washed thoroughly with water, acetone. Almost consistent activity is observed for next five consecutive cycles (Figure 5). As seen from the following figure that the catalyst can be efficiently recycled and reused for repeating cycles.

Figure 5. Recycling efficiency of Ag-MCP-1 in reductive coupling of nitrobenzenes with alcohols. 17

5. Heterogeneity Test Hot Filtration Test A hot filtration test has been carried out in the reductive amination of nitrobenzene with benzyl alcohol using Ag-MCP-1 nanocatalyst to investigate whether the reaction occurred in a heterogeneous manner or silver being leached out from the solid support. The catalyst was filtered out from the reaction mixture after 9 h at 150 0C and the remaining filtrate was allowed to react up to the 24 h (Figure 6). It is noticed that after the removal of the catalyst from the reaction mixture after 9 h reaction time, the amination reaction do not proceed further. This result suggested the heterogeneous nature of the Ag-MCP-1 nanocatalyst.

Figure 6. Heterogeneity Test in the reductive amination of nitrobenzene with benzyl alcohol

18

Comparison with other reported system Reductive coupling of various substituted nitrobenzenes with benzyl alcohols using various catalysts has been studied [53-56]. Table 4 provides a comparative result for the present system with those reported systems and, it is seen that our catalytic system exhibited higher yields of the desired product compared to the others. Table 4. Comparison of catalytic activity of the present catalyst in reductive coupling of nitrobenzene and benzyl alcohol with other reported systems. Catalyst

Amination Reaction Condition

Imination Yield

Reaction Condition

(%) RuCl3/PPh3

Au/Fe2O3

Au/Ag-Mo

phosphine-

Nitrobenzene (1.0 mmol), alcohol (1.0 mmol), glycerol (1.5 g),TFMB (0.5 mL), RuCl3 (2.5 mol%), PPh3 (10 mol%), K2CO3 (10 mol %), 130 °C, Ar, 24 h. 5 mmol (0.615 g) nitrobenzene, 50 mmol (5.400 g) benzyl alcohol, 0.1 g catalyst, 160 oC, 8 h, 20 mL pressure tube, Ar 1 mmol benzyl alcohol, 1 mmol nitrobenzene, 40 mg catalyst, 10 mol% K2CO3, 1.0 g glycerol, 0.5 mL xylene, 150 oC, 24 h

Nitrobenzene (0.3 mmol), 2 (3x10 -3 amine mmol), tBuOK in ruthenium(II) alcohol (1.83x10-3 mmol), H2 (1 atm) at 110-120 oC for 24 h

Ref. Yield (%)

86

_

_

53

87

_

_

54

86

55

_

56

91

89

19

1 mmol benzyl alcohol, 1 mmol nitrobenzene, 40 mg catalyst, 10 mol% K2CO3, 1.0 g glycerol, 0.5 mL toluene, 120 0C, 24 h

complex Ag-MCP-1

1 mmol benzyl alcohol, 1 mmol nitrobenzene, 25 mg catalyst, 10 mol% K2CO3, 1.0 g glycerol, 2 mL xylene, 150 oC, 24 h

93

1 mmol benzyl alcohol, 1 mmol nitrobenzene, 25 mg catalyst, 10 mol% K2CO3, 1.0 g glycerol, 2 mL toluene, 120 oC, 12 h

88

This Work

Conclusions From the above experimental data we can conclude that a new Ag-MCP-1 nanocatalyst can be synthesized by the immobilization of silver nanoparticles on the mesoporous cross-linked polymer support. Ag-MCP-1 has been successfully used as an efficient recyclable heterogeneous catalyst for reductive coupling of nitrobenzenes with alcohols. This catalytic system tolerates a wide range of substituted and non-substituted organic substrates. The above catalytic process is green and offers various advantages, like easy work-up process, reusability of the catalyst for several reaction cycles. These advantages make the system cheap and environmental friendly, and thus have huge potential to be explored for the synthesis of value added organic fine chemicals.

Acknowledgements SMI acknowledges Department of Science and Technology (DST), Council of Scientific and Industrial Research (CSIR). UM thankful to the UGC, New Delhi for her senior research fellowship. SR thankful to KU for her URS fellowship. AB thanks DST for financial support through DST-UKIERI project grant. References 1. Y. Li, K. Wu, I. Zhitomirsky, Colloids. Surfaces A 356 (2010) 63–70. 2.

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Graphical Abstract Mesoporous polyacrylic acid supported silver nanoparticles as an efficient catalyst for reductive coupling of nitrobenzenes and alcohols using glycerol as hydrogen source Usha Mandi,a Anupam Singha Roy,a,b Sudipta K. Kundu,c Susmita Roy,a Asim Bhaumik,*,c Sk. Manirul Islam* ,a a

Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India

b

Department of Chemical Sciences, Indian Institute of Science Education and Research

(IISER) Kolkata, Mohanpur - 741246, India c

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata -

700032, India

25