H5CoW12O40 supported on nano silica from rice husk ash: A green bifunctional catalyst for the reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl compounds

Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128

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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

H5 CoW12 O40 supported on nano silica from rice husk ash: A green bifunctional catalyst for the reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl compounds Ezzat Rafiee a,∗ , Maryam Khodayari a , Masoud Kahrizi a , Reza Tayebee b a b

Faculty of Chemistry, Razi University, 67149 Kermanshah, Iran Department of Chemistry, School of Sciences, Sabzevar Tarbiat Moallem University, Sabzevar, Iran

a r t i c l e

i n f o

Article history: Received 3 January 2012 Received in revised form 28 February 2012 Accepted 4 March 2012 Available online 13 March 2012 Keywords: Nano catalyst 12-Tungestocobaltic acid Bifunctional catalyst Supported catalyst ␤-Keto enol ethers

a b s t r a c t Rice husk ash (RHA) is an abundant agricultural by-product. The present research work deals with the production of nano silica powders, with high surface area and in amorphous form, from RHA using optimized technique. 12-Tungestocobaltic acid, H5 CoW12 O40 (CoW), was supported on silica from RHA to produce silica supported CoW (CoW/NSiO2 ) as a nano catalyst. The characterization data derived from FT-IR reveal that the Keggin structure of CoW remains intact in CoW/NSiO2 . TEM image showed that the catalyst had spherical shape with an average particle size of 10 nm. The acidity of the catalyst was measured by potentiometric titration with n-butylamine. To our surprise, this very strong solid acid catalyst showed an excellent distribution of acid sites, suggesting that the catalyst possesses a higher number of surface active sites compared to CoW supported on commercial silica (CoW/SiO2 ), CoW and K5 CoW12 O40 . A high catalytic activity was found over CoW/NSiO2 . Finally, CoW/NSiO2 has been used as a highly effective catalyst for benzylation of linear 1,3-dicarbonyl compounds with benzylic alcohols and synthesis of ␤-keto enol ethers from cyclic 1,3-dicarbonyl compounds. The present methodology offers a practical, simple, mild, environmentally friendly, and timesaving method under solvent-free conditions. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nanotechnology is a growing and important field in the modern science. Nano-sized catalysts continue to attract interest for different researcher areas due to their different physical and chemical properties when compared to bulk material. The extremely small sized particles maximized the surface area exposed to the reactant, allowing more reactions to occur at the same time, thus speeding up the process. Catalysis by heteropoly acids (HPAs) and related compounds is a field of increasing importance as nano catalysts [1–4]. They are promising candidates for catalyst design due to their controllable acid and redox properties and have been actually utilized in some industrial processes [5,6]. K5 CoW12 O40 in our previous works showed excellent reactivity as an electron transfer catalyst [7,8]. By changing this salt to acidic form (H5 CoW12 O40 hereafter CoW) Brönsted acidity is added to electron transfer ability of this catalyst. But there is a problem here, CoW in acidic form is a homogeneous catalyst in most of organic processes. The use of heterogeneous catalysts in different areas of the organic

∗ Corresponding author. Tel.: +98 831 427 4559; fax: +98 831 427 4559. E-mail addresses: e.rafi[email protected], ezzat rafi[email protected] (E. Rafiee). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2012.03.005

synthesis has now reached significant levels, not only for the possibility to perform environmentally benign synthesis, but also for the good yields frequently, accompanied by excellent selectivity that can be achieved. The use of a support allowing the CoW to be dispersed over a large surface may result in an increase of its catalytic activity. The property as well as the catalytic behavior of supported CoW depends on the carrier, catalyst loading and conditions of pre-treatment among other variables. Diverse supports have been tested, acidic or neutral solids such as active carbon, SiO2 and ZrO2 being suitable as carriers [1] but SiO2 , which is relatively inert toward HPAs, is the one most often used. At present, nano scale silica materials are prepared using several methods, including vapor-phase reaction, sol–gel, and thermal decomposition technique [9–12]. However, their high cost of preparation has limited their wide applications. The production of reactive nano scale silica from rice husk (RH) is a simple process compared to other conventional production techniques [13]. RH is an abundantly available waste material in rice producing countries where there is a need for its disposal or utilization. It would be of benefit to the environment to recycle the waste to produce eco-materials having a high-end value. The white ash obtained from its controlled combustion or calcination at relatively moderate temperatures produces a main residue containing silica in

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Table 1 Comparison of catalytic activity of HPC catalysts in benzylation of ethyl acetoacetate. Entry

Catalyst

1 2 3 4 5 6

– H5 CoW12 O40 /NSiO2 H5 CoW12 O40 /NSiO2 K5 CoW12 O40 H5 CoW12 O40 H5 CoW12 O40 /SiO2

Time (min) 60 60 3 10 10 5

Yield (%)a

Conversion (%)

5 10b 95 95 94 89

6 11 95 96 94 91

a Isolated yield; reaction conditions: benzyl alcohol (1 mmol), ethylacetoacetate (1 mmol), catalyst (0.4 g), 80 ◦ C under solvent-free conditions. b Reaction proceed at room temperature.

the form of an amorphous phase [14,15,9,16,17]. The small particle diameter and high surface area of the ultra-fine silica powders give rise to many technological applications [9,18] especially as support to heterogenization of catalyst [19]. Supporting the CoW on silica nano particles with high surface area improves the catalytic performance in various organic reactions and also they can exhibit quite different characteristics from the bulk CoW and K5 CoW12 O40 . The construction of carbon–carbon bonds is a fundamental task in organic synthesis. Alkylation of 1,3-dicarbonyl compounds is a useful transformation involving C C bond formation. However, because of the poor leaving ability of the hydroxyl group, excess amount of Brönsted or Lewis acids is required for this kind of transformation [20–23]. Recently, many chemists have focused their attention on the acid-catalyzed alkylation processes that are considered to be the most promising approaches in this field. Various Lewis acids such as InCl3 [24,25], InBr3 [26], FeCl3 [27–31], M(OTf)3 [32,33], Lewis acidic ruthenium complex [34], as well as montmorillonite [35], p-toluenesulfonic acid [36] have been demonstrated to facilitate the dehydrative substitution of allylic and benzylic alcohols with 1,3-dicarbonyl compounds. Unfortunately many of these methods for the benzylation reaction prepared so far has major or minor disadvantage such as long reaction times, low yield of the products, tedious work up procedures and use of the toxic solvents. In this study we report efficient CoW/NSiO2 for benzylation of a wide variety of 1,3-dicarbonyl compounds under solvent-free conditions. Activity of mentioned catalyst has been compared with reactivity of CoW supported on commercial silica and K5 CoW12 O40 . Previously K5 CoW12 O40 has been used as a reusable catalyst in some organic transformation [7,8]. But to the best of our knowledge this is the first time that acidic form of CoW has been supported, characterized and used as catalyst.

Fig. 1. Comparison of catalytic activity of different catalysts in benzylation of ethylacetoacetate.

Fig. 2. Potentiometric titration curves of catalysts.

2. Experimental 2.1. General remarks All organic materials were purchased commercially from Fluka Chemical Corp. and Merck & Co., Inc. and were used without further purification. Aerosil 300 silica from Degussa was used. FT-IR spectra were recorded with KBr pellets using a Shimadzu 470 spectrophotometer. NMR spectra were recorded on a Bruker Avance 200 MHz NMR spectrometer with CDCl3 as solvent and TMS as internal standard. CHN compositions were measured by Hekatech elemental analysis model Euro EA 3000. Transmission electron microscopy (TEM) examination was performed with a TEM microscope Philips CM 120 kV Netherland. 2.2. Preparation of nano silica from RH RH was washed thoroughly with water to remove the soluble particles and dust or other contaminants present in them whereby the heavy impurities like sand are also removed. It was then dried in an air oven at about 110 ◦ C for 24 h. The dried RH was refluxed with an acidic solution (0.1 M HCl) for nearly 90 min with frequent stirring. It was cooled and kept intact for about 20 h, then decanted, thoroughly washed with warm distilled water until the rinse became free from acid. The wet solid was subsequently dried in an oven at 110 ◦ C for 24 h. The obtained white powder was burned inside a programmable furnace (Model Nobertherm controller B 170) at 700 ◦ C with rate 10 ◦ C/min and 2 h as soaking time. We designated this as RHA. 20 g of RHA sample was stirred in 160 mL of 2.5 M sodium hydroxide solution. The solution was heated in a covered beaker for 3 h with constant stirring. It was filtered and the residue was washed with 40 mL boiling distilled water. The obtained viscous, transparent and colorless solution was allowed to cool down to room temperature. Then H2 SO4 (5.0 M) was added under constant stirring at controlled conditions until pH = 2 and then added NH4 OH until pH = 8.5, and allowed to room temperature for 3 h. Nano silica was prepared by refluxing of extracted silica with HCl (6.0 M) for 4 h and then washed repeatedly using deionized water to make it acid free. It was then dissolved in 2.5 M of sodium hydroxide stirring. H2 SO4 was added until pH = 8. The precipitate silica was washed

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128

O O

OH

H3C

1b

O

Catalyst ( 0.4 g)

OCH2CH3

Solvent-free, 80 °C

123

O

H 3C

OCH2CH3 Ph

4bc

2c Scheme 1. Model reaction.

the cobalt (II) complex was oxidized to the cobalt (III) complex by potassium persulfate. The crystals of K5 CoW12 O40 ·20 H2 O were dried. After this synthesis, CoW was prepared from its potassium salt in acidic conditions and extracted from ether [7].

repeatedly with warm deionized water to make it alkali free, dried at 50 ◦ C for 48 h in oven.

2.3. Preparation of CoW CoW was synthesized from its potassium salt via acidic extraction method. The synthesis of K5 CoW12 O40 ·3H2 O started with the preparation of sodium tungstodicobalt(II)ate from cobaltous acetate (2.5 g, 0.01 mol) and sodium tungstate (19.8 g, 0.06 mol) in acetic acid and water. The sodium salt was then converted to the potassium salt by treatment with potassium chloride (13 g). Finally,

2.4. Preparation of CoW/NSiO2 For the preparation of 40 wt.% CoW/NSiO2 catalyst, 3.0 g of nano silica was dispersed in 30 mL water by sonication. Solution of CoW (2 g in 10 mL water) was added and stirred overnight at room

° OH

OH

R1

R2

R1

R2

°

CoW/NSiO2 CoW/NSiO2

O

H2O

O

R3

R4 1

R

R

R1

2

R2

OH O R4

R3

4

O R

+ OH °

O R4

3

(a) Electron transfer pathway O

OH2 R

R

2

R

Pathway 2

R

R

2

H

O

R1

R

1

R4

R2

R2 R1

R

2

R3

R

4

OH O

O R3

R2

OH 1

.. OH O

O +

R2

R1

O O R3

-H2O

OH

H2O

R2 R

OH2 1

R

1

CoW/NSiO2

R

2

OH2

CoW/NSiO2 2

Pathway 1

Pathway 3

R

1

CoW/NSiO2

O

R

1

OH

CoW/NSiO2

-H2O

CoW/NSiO2

.. HO

1

CoW/NSiO2

O

R1

R

4

H+

R2

5 (b) Brönsted acid pathway Scheme 2. Proposed mechanisms for catalytic addition of diketones to alcohols.

O

R2 R1

R4 R3

O O

O O R3

R4

R1

R2

4 Main product

6

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Table 2 Benzylation of different 1,3-dicarbonyl compounds catalyzed by 40% CoW/NSiO2 .

O O

H3C

ROH 1

2

CoW/NSiO2 (0.4 g) O Solvent-free, 80° C

O O

R'

H3C

O

R

Entry

Alcohol

Diketone

O

R

6

R

O

2a: R'= CH3 2b: R'= OCH3 2c: R'= OCH2CH3

R

4

3

O 1a: R= (Ph)2CH 1b: R= PhCH2 1c: R= 4-Cl-PhCH2

R'

R

O

R

6

5

Product

Time (min)

Yield (%)a

Conversion (%)

3

92

95

5

84

94

5

73

96

2

91

95

3

86

96

3

95

95

5

91

95

3

70

93

3

75

94

O O H3C 1

1a

CH3

Ph Ph O O

2a

H3C 2

1a

2b

3

1a

2c

OCH3

Ph Ph O O H3C OCH2CH3 Ph Ph O O H3C

4

1b

CH3 Ph O O

2a

H3C 5

1b

Ph

2b

OCH3

O O H3C 6

1b

Ph O O

2c

H 3C

1c

7

OCH2CH3

OCH2CH3

Cl

2c

Ph 8

1a

3

9

1b

3

a

Isolated yield.

O

O

Ph

O

O

Ph

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128 Table 3

O

O +

125

O ROH

OR

CoW/NSiO2

Synthesis of ␤-keto enol ethers in the presence of CoW/NSiO2 .a . Entry 1 2 3 4 5 6 7

Alcohol

Amount of the catalyst (g)

Time (min)

Yield (%)b

MeOH EtOH EtOH EtOH Propanol Butanol Hexanol

0.1 0.02 0.05 0.1 0.1 0.1 0.1

10 45 20 10 15 15 15

94 86 (95)c 88 92 93 94 85

0.1

20

73

0.1

20

76

0.1

2 (h)

15

0.1

15

84

0.1

15

92

0.1

25

81

0.1

15

90

OH 8

OH 9

OH 10

OH 12

OH

13

MeO OH

14

O2N OH

15 a b c

Reaction conditions: dimedone (1 mmol), alcohol (3 mL), at 80 ◦ C. Isolated yield. Reaction proceeded at 60 ◦ C.

temperature. Finally, the solvent was removed by using rotary evaporator, dried and calcinated at 150 ◦ C for 2 h.

2.5. General procedure for benzylation of the 1,3-dicarbonyl compound The solid acid catalyst (0.4 g), was added to a mixture of alcohol (1 mmol) and 1,3-dicarbonyl compound (1 mmol) at 80 ◦ C. The reaction mixture was crushed for the short period of time. The progress of the reaction was monitored by thin-layer chromatography (TLC). After completion of the reaction, the mixture was washed with acetonitrile, and then the catalyst was removed by a repeated centrifugation (4000–6000 rpm, 30 min) and decantated. Catalyst was collected and treated with 1,2-dichloroethane (DCE) for removing coke followed by vacuum drying and calcinations at 150 ◦ C, 1 h for reusing. The filtrate was concentrated and the product was purified by column chromatography on silica-gel using EtOAc/hexane (1:3) as eluent. All the products were identified by comparing their spectral data with those of the authentic samples

[37–40]. Analytical data for 5a as a new compound are presented below: 5-Benzhydryloxy-3,3-dimethyl-cyclohex-2-enone (5a): 1 H NMR (200 MHz, CDCl3 ) ı: 6.8–7.21 (m, 10H), 5.95 (s, 1H), 4.82 (s, 1H), 2.81 (s, 2H), 1.84 (s, 2H), 1.05 (s, 6H); 13 C NMR (100 MHz, CDCl3 ) ı: 192.1, 170.5, 142.1, 127.8, 127.5, 126.4, 125.5, 86.1, 77.9, 54.2, 43.7, 26.2, 15.4; Anal. calcd for C21 H22 O2 : C 82.32, H 7.24; found: C 82.34, H 7.22; HRMs calcd for C21 H22 O2 : M, 306.4036; found: 306.40.

2.6. General procedure for synthesis of the ˇ-keto enol ethers A mixture of dimedone (0.14 g, 1.0 mmol) and alcohol (4 mL) was stirred well in the presence of the catalyst (0.4 g) at 80 ◦ C. After complete consumption of the starting materials as indicated by TLC, the reaction was filtered. The excess alcohol in the filtrate was removed by rotary evaporation and the crude purified by column chromatography over silica-gel (ethyl acetate/hexane, 1:4).

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Fig. 3. Effect of various amount of CoW/NSiO2 in model reaction.

3. Results and discussion 3.1. Catalytic reaction As a starting point for optimization of the reaction conditions, the reaction of benzyl alcohol (1 mmol) and ethylacetoacetate (1 mmol) under solvent-free conditions was chosen as a model reaction (Scheme 1). Control experiment showed that the substrates did not react in the absence of the catalyst (Table 1, entry 1). Only 10% yield of the corresponding product was obtained in the presence of CoW/NSiO2 at room temperature (Table 1, entry 2). Whereas by increasing the temperature to 80 ◦ C, a significant improvement was observed and yield of the product was increased to 95% after 5 min (Table 1, entry 3). Thus 80 ◦ C was chosen as reaction temperature in further investigations. CoW/NSiO2 is a bifunctional catalyst with Brönsted acidity and electron transfer properties. It can catalyze the reaction in two different ways: Brönsted acid and electron transfer mechanisms (Scheme 2(a) and pathway 1 of (b)). In order to investigate which way is the main mechanism, the model reaction was carried out in the presence of K5 CoW12 O40 as heterogeneous catalyst with only electron transfer property [7] and the obtained result was compared to CoW/NSiO2 (Table 1, entries 3 and 4). The reaction proceeds well in the presence of K5 CoW12 O40 via electron-transfer mechanism and excellent yield of the product was obtained. But, the reaction time was increased in comparison with CoW/NSiO2 (Fig. 1). Thus, it seems that high catalytic activity of CoW/NSiO2 catalyst was mainly related to its Brönsted acidity. To test this hypothesis, a radical scavenger (acrylonitrile) was added to the model reaction in the presence of K5 CoW12 O40 as catalyst after 3 min (when 50% of the corresponding product was obtained). After continuation of the reaction until 10 min, yield of the product was 55% and it seems that the reaction was stopped by addition of radical scavenger. This test was checked in the model reaction with CoW/NSiO2 as catalyst to know which way was the main mechanism of the catalyst, Brönsted acidity, electron transfer ability or both of them. By addition of the radical scavenger the reaction completed after 6 min. It means that the benzylation reaction probably proceeded via both pathways. Possibly, Brönsted acidity was more active than electron transfer pathway. More investigation about the mechanism of the catalyst is under investigation in our laboratory. In addition to bifunctionality, the catalytic behavior of CoW/NSiO2 presumably depends on the carrier. In order to investigate the effect of nano silica (SBET = 623 m2 /g [13]) on catalytic activity, the model reaction was carried out in the presence of bulk

Fig. 4. FT-IR of the (a) pure CoW, (b) nano silica and (c) CoW/NSiO2 .

CoW and CoW/SiO2 (CoW support on Aerosil silica (SBET = 300 m2 /g) as a commercial support) catalysts (Table 1, entries 5 and 6). The results show that the catalytic activity of CoW/NSiO2 was higher than the activities of bulk CoW and CoW/SiO2 . Considering that the above-mentioned catalytic reaction is mainly acid-catalyzed organic reaction, the acidity of the catalysts plays an important role in this transformation. The nature of the supports apparently affected the acidity of the supported CoW, which was probed by a potentiometric titration with an organic base [41]. The acidic environment around the electrode membrane mainly determines this method, based on the measured potential difference. The measured electrode potential is an indicator of the acidic properties of the dispersed solid particles. An aliphatic amine, as n-butylamine, with a basic dissociation constant of approximately 10−6 , allows a potentiometric titration of strong acids. The titration curves obtained for CoW, CoW/SiO2 and CoW/NSiO2 are presented in Fig. 2. It is considered that the initial electrode potential (Ei ) indicates the maximum strength of the acid sites and that the value from which the plateau is reached (mmol amine/g catalyst) indicates the total number of acid sites that are present in the titrated solid (n value). The acidic strength of the acid sites can be classified according to the following ranges: Ei > 100 mV (very strong acid sites), 0 < Ei < 100 mV (strong acid sites), −100 < Ei < 0 mV (weak acid sites) and Ei < −100 mV (very weak acid sites) [41]. All kind of catalysts presented very strong acid sites, Ei > 100 mV. Immobilization of CoW to silica or nano silica was accompanied by

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128

Fig. 5. TEM image of the CoW/NSiO2 (a) and the diagram of particle size distribution of the CoW/NSiO2 particles (b).

a gradual increase in total number of surface acidic sites. But “n” value in the case of CoW/NSiO2 was about 2 and more than other ones due to its higher surface area and higher dispersion of acidic protons. As a result, it seems that higher activity of CoW/NSiO2 catalyst is related to surface area enhancement and higher dispersion of the acidic protons of nano catalyst and possibly also bifunctionality of this catalyst. For establishing the best reaction conditions, the model reaction was carried out in the presence of different amounts of CoW/NSiO2 catalyst. Improvement in time of the reaction was observed as the catalyst quantity increased from 0.1 to 0.4 g. Further increasing in the catalyst quantity showed no improvement in the yield or reaction time (Fig. 3). Thus, 0.4 g of CoW/NSiO2 was the suitable choice for catalyst loading. Encouraged by these results, we turned our attention to various cyclic and linear 1,3-dicarbonyls and benzylic alcohols (Table 2). Reaction of the linear 1,3-dicarbonyls such as acetylacetone with different types of benzyl alcohols proceeded smoothly providing 73–95% yields (entries 1–7). However, symmetrical ether 6 from self-condensation of alcohol was observed in some cases. To investigate that this ether (6) is as an intermediate to produce product 4 in different pathway, dibenzyl ether was used as starting

127

material with ethyl acetoacetate in optimum reaction conditions. After 15 min corresponding product 6 was obtained in 94% yield. Thus, it seems there is another pathway for benzylation of the 1,3-dicarbonyl compounds (Scheme 2(b)). Employing cyclic 1,3-dicarbonyls, such as 5,5dimethylcyclohexane-1,3-dione (dimedone), using the same reaction conditions, produced corresponding 2-benzylic-1,3dicarbonyl compounds in very low yield (<10%), and an unknown compounds were generated as main products. This compound was isolated by column chromatography and characterized by FTIR, NMR, and CHN techniques. The results indicated that the unknown compound is the corresponding ␤-keto enol ethers (5a and 5b). Due to the wide range applications of ␤-keto enol ethers as synthons in several key intermediate compounds [42,43], various alcohols were treated with dimedone as a cyclic 1,3-dicarbonyl compound (Table 3). Toward these studies, effect of amount of the catalyst and reaction temperature was investigated (Table 3, entries 2–4). The present conversion with primary alcohols proceeded rapidly to form the products in high to excellent yields. This protocol was then applied to secondary, vinyl or benzyl alcohols, which are structurally different. Reactions with these type alcohols took longer reaction times. Even, 4-nitrobenzyl alcohol responded well with longer duration and good yield. To our surprise, t butyl alcohol as substrate afforded 16% of corresponding product after 1 h (Table 3, entry 10), as previous approaches gave negative results for similar reactions [44,45]. More amount of the catalyst (0.12 mg) was not improved reaction time or yield of this reaction. It should be mentioned that no acetal adducts were detected, even in the case of linear 1,3-diketones for which it has been reported [46]. A possible mechanism of the reaction in the presence of CoW/NSiO2 as a Brönsted acid catalyst is proposed (Scheme 2(b)). The benzylation of linear 1,3-dicarbonyl compounds possibly proceeded by pathway 1. By action of CoW/NSiO2 the alcohol was protonated to generate a stable benzyl cation after dehydration. This carbocation could be quickly combined with the employed 1,3-dicarbonyl compound to produce, after the release of H+ , the final alkylated product. Production of the symmetrical ether as the side reaction explains by pathway 2. In the presence of CoW/NSiO2 , the alcohol is protonated to give oxonium ion. Subsequent dehydration of oxonium ion results carbocation intermediate. Binding of cation with the same alcohol, followed by the release of H+ , generated the symmetrical ether. The suggested mechanism to explain the reaction between cyclic 1,3-dicarbonyl compounds with alcohols is shown in pathway 3. Similar to other routes, by action of the catalyst a stable benzyl cation was produced from initial alcohol after dehydration. Under the present conditions, due to the resonance of oxygen with adjacent ␲-bonding electrons, cyclic 1,3-dicarbonyl compound as a nucleophile combined with the stable carbocation to produce, after the release H+ , ␤-keto enol ethers.

3.2. Characterization of the catalyst 3.2.1. FT-IR of CoW/NSiO2 Fig. 4 shows the FT-IR spectra of the pure CoW, nano silica and CoW/NSiO2 . The FT-IR spectrum of the pure CoW indicates that typical bands for CoW, belong to Co O, 1105 cm−1 ; W = Ot (terminal oxygen), 943 cm−1 ; W Oc W (corner sharing oxygen), 887 cm−1 and W Oe W (edge sharing oxygen), 779 cm−1 . These characteristic peaks of Keggin structure were observed in FT-IR spectrum of CoW/NSiO2 , although some bands were overlapped with those of the support. FT-IR of the supported CoW indicates that the primary Keggin structure is preserved after supporting CoW on nano silica.

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Acknowledgments The authors thank the Razi University Research Council and Kermanshah Oil Refining Company for support of this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molcata.2012.03.005 References

Fig. 6. Reusability of the CoW/NSiO2 in the reaction of benzyl alcohol (1 mmol), ethylacetoacetate (1 mmol), catalyst (0.4 g), 80 ◦ C under solvent-free conditions after 30 min.

3.2.2. TEM of CoW/NSiO2 The particle size and shape of the CoW/NSiO2 was observed by transmission electron microscopy (Fig. 5(a)). Shape of the CoW/NSiO2 is spherical with an average particle size of 10 nm. Fig. 5(b) shows the diagram of particle size distribution of these particles.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

3.3. Recyclability One of the advantages of using heterogeneous catalyst is the possibility of their reusability and/or regeneration to reduce wastage and production of wasteful material. The stability of the supported catalyst has been studied by running the reaction successively with the same catalyst in the same reaction. Fig. 6 shows the results of this study in the model reaction in the presence of CoW/NSiO2 . Calcinations of the fresh and reused catalyst can help to increase reactivity and reusability. Also, the treatment with 1,2dichloroethane (DCE) was attempted for removing coke from the catalyst. The catalyst could, after a specified workup, be reused several times, albeit with slightly decreasing activity. Fig. 6 shows a slightly loss of activity after four successive runs in longer reaction times. It could thus be drawn that CoW/NSiO2 could be a satisfied catalyst for this reaction with high activity and good reusability. 4. Conclusion The studies show that the silica support with the fine grain size, large surface area, high degree of purity and the amorphous state is successfully obtained through a consecutive preparation method from RHA as an agricultural waste. CoW/NSiO2 was synthesized by impregnation method as an efficient catalyst. According to TEM results, the average particle size of CoW/NSiO2 was 10 nm. FT-IR of the supported CoW indicated that the primary Keggin structure was preserved after supporting CoW on nano silica. The catalytic activity of CoW/NSiO2 was studied for direct benzylation reactions of linear 1,3-dicarbonyl compound under solvent-free conditions. Results showed that reaction of the cyclic 1,3-dicarbonyl compounds with alcohols produced ␤-keto enol ethers as main products. The catalytic reactions proceeded with excellent yields in very short reaction times. It seems CoW/NSiO2 is a bifunctional catalyst with Brönsted acidity and electron transfer properties. It can catalyze the reaction in two different ways. Also, this catalyst can be a satisfied catalyst for this reaction with good reusability. The present methodology offers a practical, simple, mild, environmentally friendly, and timesaving method under solvent-free conditions.

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