Synthesis, characterization and application of Silica supported Copper

Research Article pubs.acs.org/journal/ascecg

Synthesis, Characterization, and Application of Silica-Supported Copper-Doped Phosphotungstic Acid in Claisen−Schmidt Condensation Shaheen Siddiqui, Mohd Umar Khan, and Zeba N. Siddiqui* Department of Chemistry, Aligarh Muslim University, Aligarh, 202002 Uttar Pradesh, India S Supporting Information *

ABSTRACT: An efficient, novel, and recyclable silicasupported copper-doped phosphotungstic acid (CuPTA/ SiO2) has been synthesized by an impregnation method. The catalyst has been characterized by FTIR, XRD, SEM/ EDX, ICP−AES, EPR, XPS, and NH3-TPD analyses. The catalytic application of the catalyst has been explored for Claisen−Schmidt condensation. The catalyst could be recycled for six runs without any appreciable loss in catalytic activity. The catalytic activity of the recovered catalyst after six runs was confirmed by FTIR, XRD, SEM/EDX, ICP−AES, and EPR analyses. The Claisen−Schmidt products were obtained in excellent yield in a shorter time period. KEYWORDS: Heteropoly acid, CuPTA/SiO2, Phenoxy pyrazolyl chalcones, Solvent-free reaction, Claisen−Schmidt condensation

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formations including hydrolysis,8 ring-opening reactions of epoxides,9 Friedel−Crafts alkylation,10 acylation of esters,11 dimerization reaction,12 Dakin−West reaction,13 pentaerythritol−melamine phosphate (PER−MP) reaction,14 intramolecular rearrangement,15 hydroalkylation of olefins,16 etc. However, for the effective accessibility of the catalyst to the substrate in terms of surface area, these compounds need to be incorporated by impregnation on suitable support materials. In this context, among different solid supports, silica is most preferred because of its many advantageous properties such as excellent chemical and thermal stability, good accessibility, neutral or mildly acidic nature, tunable surface area, and porosity.17 For the past decade, focusing on green chemistry by using environmentally benign media and reaction conditions has been the most attractive areas in synthetic organic chemistry. Green chemistry efficiently makes use of raw materials, eradicates waste, and dodges the use of toxic and/or hazardous reagents and solvents in the manufacture and application of chemical products.18 Because of the growing concern about the environment and human health, solvent-free organic synthesis has become a major area of focus in organic chemistry. As solvents constitute a major part of waste generation in organic synthesis, a solventfree approach contributes to sustainability by eliminating or minimizing the waste generation.19 Overall, the advantages of

INTRODUCTION In recent years, heterogeneous solid acid catalysts have received more interest in organic syntheses. Heterogeneous solid acid catalysts are more preferable over conventional acids as these are easily recoverable from reaction mixtures by simple filtration, and are reusable.1 Among different solid acid catalysts, heteropoly acids (HPAs) have gained much attention because of their exclusive properties such as well-defined structure, Brønsted acidity, and possibility to modify their acid−base properties.2 HPAs can be regarded as green catalysts because of their noncorrosive, environmentally benign, and economically feasible nature.3,4 These catalysts have the ability to accept and release electrons and possess high proton mobility. They are mainly used as acid and oxidation catalysts. The major disadvantages of HPAs as catalysts lie in their low thermal stability, small surface area, and homogeneous nature. HPAs can be made eco-friendly insoluble acid catalysts with high thermal stability and high surface area by exchanging their proton with a metal ion and supporting them on suitable supports. The support provides an opportunity to spread HPAs over a large surface area, which effectively increases catalytic activity. Modification of HPAs, particularly from the Brønsted acidic sites to Lewis acidic sites in Keggin-type HPAs, is highly beneficial in organic reactions.5 The Brønsted acidic protons, exchanged by cations, lead to the formation of microporous solid Lewis acidic catalysts which are effective in various catalytic organic transformations. Among the common known Keggin-type HPAs, phosphotungstic acid (H3PW12O40; PTA), in anhydrous acid form, is the strongest acid.6,7 Its effectiveness as a catalyst has been explored in various organic trans© 2017 American Chemical Society

Received: May 10, 2017 Revised: July 18, 2017 Published: July 27, 2017 7932

DOI: 10.1021/acssuschemeng.7b01467 ACS Sustainable Chem. Eng. 2017, 5, 7932−7941

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of CuPTA/SiO2

Figure 1. FTIR spectra of (A) PTA, (B) CuPTA, (C) SiO2, (D) CuPTA/SiO2, and (E) recycled catalyst after six runs.

solvent-free organic synthesis are shorter reaction time period, cleaner reaction pathways, easy workup procedures, and being environmentally more benign as compared with the classical reactions.20,21 In view of these issues, development of new strategies, which make an organic transformation green with efficient, selective, and high yield, has always been a prominent task in synthetic organic chemistry. Pyrazole-containing organic compounds have been given great interest by virtue of their enormous biological activities and vast applications such as analytical reagents, ligands for the transition-metal-catalyzed cross-coupling reactions, and dyestuffs.22−28 Numerous varieties of chalcones, containing a pyrazole ring, have been synthesized and evaluated for biological activities such as antimicrobial, anti-inflammatory, antioxidant, cytotoxic activities, etc.29−34 Claissen−Schmidt condensation by far is the most widely used reaction for the synthesis of chalcones. There are many synthetic methods for the synthesis of chalcones using catalysts including Lewis acid,35 Bronsted acid,36 solid acid,37 and solid base38 via Claisen−Schmidt condensation. However, there are only a few reports of synthesizing pyrazolyl chalcones using HPAs.39

Prompted by all these observations, we herein report an easy synthesis of copper-doped phosphotungstic acid which was supported on silica gel by an impregnation method (CuPTA/ SiO2) and employed as a catalyst for Claisen−Schmidt condensation.

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EXPERIMENTAL SECTION

Preparation of Silica-Supported Copper-Doped Phosphotungstic Acid (CuPTA/SiO2). Copper-doped phosphotungstic acid (CuPTA) was prepared by exchanging the protons of phosphotungstic acid (PTA) with Cu2+ ions. For this, 3.45 g of PTA was dissolved in 20 mL of distilled water in a 50 mL beaker, and another solution containing 0.604g of Cu(NO3)2·6H2O dissolved in 10 mL of water was added to it dropwise with stirring for 1 h. The solution was left to equilibrate overnight, filtered, and oven-dried at 120 °C overnight to obtain CuPTA. With the impregnation method, 100 mg of CuPTA was dissolved in distilled water (20 mL) and added to 500 mg of silica gel with continuous stirring for 1 h. The resultant mixture was allowed to stand for 3 h, and the excess water was evaporated in a water bath. The dried catalyst mass was kept overnight for further drying in an air oven at 120 °C (Scheme 1). The amount of copper incorporated into the PTA was found to be 8.1% by ICP−AES analysis. 7933


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Figure 2. Powder XRD pattern of (a) PTA (b), SiO2 (c), CuPTA/SiO2 (d), and recovered catalyst after six runs.

Figure 3. SEM images of (a) silica gel (b) freshly prepared catalyst CuPTA/SiO2 and (c) recovered catalyst after six runs. obtained filtrate was concentrated to obtain the products which were

General Procedure for the Synthesis of Phenoxy Pyrazolyl Chalcones (3a−m) via Claisen−Schmidt Condensation. The phenoxy pyrazolyl chalcones were synthesized via Claisen−Schmidt condensation by heating a mixture of different active methyl compounds 2a−e (3 mmol) and 5-aryloxy-3-methyl-1-phenylpyrazole-4-carbaldehydes 1a−c (3 mmol) in the presence of CuPTA/SiO2 (0.1 g, 0.128 mmol) at 80 °C under solvent-free conditions for a specified period (Table 6). After completion of the reaction (as monitored by TLC), the reaction mixture was kept at room temperature for 30 min, and ethanol (5 mL) was added to it. The reaction mixture was then filtered to recover the catalyst, and the

recrystallized with methanol to obtain pure products 3a−m (Table 6).

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RESULTS AND DISCUSSION

The catalyst was prepared as outlined in Scheme 1. The catalyst was characterized by spectral techniques including FTIR, XRD, SEM/EDX, elemental mapping, EPR, TPD-NH3, XPS, and ICP−AES analyses. 7934


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Figure 4. EDX analysis of the catalyst (CuPTA/SiO2).

Figure 5. Elemental Mapping images (a) Cu and (b) W of the freshly prepared catalyst CuPTA/SiO2.

EDX analysis (Figure 4) of the catalyst showed the presence of Cu, O, Si, and W elements confirming the formation of the catalytic system as visualized. Elemental mapping images (Figure 5) of the catalyst showed uniform distribution of the elements Cu and W in the desired catalytic system. Total acidity and strength of the acidic sites present on the catalyst were determined by NH3-absorption−desorption analysis. The TPD-NH3 profile of CuPTA/SiO2 is shown in Figure 6. A strongly desorbed peak in the range 200−300 °C confirmed the higher acidic strength of the catalyst.42 XPS analysis was performed to investigate the elemental composition of the catalyst. The peaks corresponding to

In the IR spectra of pure PTA (Figure 1A), the presence of sharp and strong absorption bands at 1080 and 984 cm−1 was attributed to PO and WO stretching modes of vibrations, respectively, whereas asymmetric stretching modes of vibrations for WOW were present at 893 (corner-sharing WO W) and 801 (edge-sharing WOW) cm−1, respectively. These features indicated the Keggin type structure of PTA.40 As per literature record, doping of HPA with metal causes a reduction in intensity of bands.41 In the present case, when doping of PTA was performed with Cu, the only observed change was a reduction in intensity of bands at 1080, 984, 892, and 799 cm−1 (Figure 1B). This fact establishes that the Keggin structure is retained after doping with Cu. When Cu-doped PTA was supported on silica gel, the peak at 1091 cm−1 was obtained because of asymmetric SiOSi stretching and a very weak band at 802 cm−1 because of the symmetric SiO Si stretching mode of vibrations, whereas a broad band at 3452 cm−1 was due to the SiOH band. The bands due to the Keggin structure had merged with the peak of silica gel (Figure 1C,D). The XRD patterns of PTA (Figure 2a), silica gel (SiO2; Figure 2b), freshly prepared catalyst (CuPTA/SiO2; Figure 2c), and recovered catalyst after six runs (Figure 2d) were recorded at the 2θ = 10−90° range. The diffractogram of the catalyst was similar to that of the support SiO2 lacking diffraction lines because of heteropolyanions. This indicated that the CuPTA was highly dispersed on the support surface and present as noncrystalline species. For a study of the surface morphology of the catalyst, scanning electron microscopy (SEM) images of the catalyst were employed. The SEM micrograph of the catalyst (Figure 3b) showed an even distribution of copper-doped PTA on the surface of the silica gel.

Figure 6. TPD-NH3 spectra of CuPTA/SiO2. 7935


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Figure 7. XPS analysis of the catalyst.

Scheme 2. Synthesis of 3a

tungsten, silicon, oxygen, and copper are shown in Figure 7. XPS spectra showed the presence of the following: O2− 1s (64.82%, BE (binding energy) = 526.38 eV) which corresponds to O−H bonding, Si 2p (26.07%, BE = 102.64 eV) which corresponds to Si−O bonding, Cu 2p3 (8.08%, BE = 938.51 eV) which corresponds to Cu−O bonding,43 and W6+ 4f (1.82%, BE = 34.72 eV) which corresponds to W−O bonding (Figure 7).44 The X-EPR spectra of H3PW12O40 and its Cu2+ form were carried out and compared. The two spectra were different in nature and in number of hyperfine splitting peaks. The spectrum of metal-free H3PW12O40 indicated only a single band at g = 2.238. However, the copper-doped catalyst (Cu− H3PW12O40) exhibited two EPR bands at g = 2.232 and 2.041 which confirmed the anisotropic nature of Cu2+ species present in the catalyst.45,46 ICP−AES analysis showed the wt % of Cu to be 8.1% which corresponded to the loading amount of 1.26 mmol/g of catalyst. Optimization of Reaction Conditions. The optimum reaction conditions were obtained by using a model reaction (Scheme 2) to show the effect of various parameters such as the effect of different catalysts and solvents, CuPTA/SiO2 loading, amount of CuPTA/SiO2, and different temperatures. First, the model reaction was employed on 1a and 2a to give 3a without any catalyst, and a small quantity (28%) of the product was formed after prolonged heating (Table 1, entry 1). The model reaction was then examined with different metal salts such as FeCl3, Co(NO3)2, NiCl2, CuCl2, ZnCl2, Cu(NO3)2, CuSO4, and Cu(CH3COO)2. It was observed that a trace amount of the product was obtained with Co(NO3)2 (Table 1, entry 2), whereas FeCl3, NiCl2, CuCl2, ZnCl2, CuSO4, and Cu(CH3COO)2 afforded a very poor yield of the product after a longer time period (Table 1, entries 3, 4, 5, 6, 7,

Table 1. Effect of Different Catalysts on the Model Reactiona entry

catalyst

timeb

yieldc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

no catalyst Co(NO3)2 FeCl3 NiCl2 CuCl2 ZnCl2 CuSO4 Cu(CH3COO)2 Cu(NO3)2 ammonium tungstate ammonium molybdate PTA AgPTA CuPTA CuPTA/ZrO2 CuPTA/Al2O3 PTA/SiO2 CuPTA/SiO2 SiO2

9h 4.5 h 3h 1.5 h 2h 1.5 h 6h 3.5 h 1h 2h 2.5 h 1h 45 min 30 min 35 min 28 min 50 min 10 min 12 h

28 trace 42 49 32 52 38 46 68 trace trace 48 54 62 72 68 74 94 no product formation

a

Reaction conditions: 3-methyl-(2-nitrophenoxy)-1-phenylpyrazole-4carbaldehyde 1a (3 mmol), 1,3-dimethyl-2,4,6-pyrimidinetrione 2a (3 mmol), catalyst (100 mg), solvent-free conditions, T = 80 °C. b Reaction progress monitored by TLC. cIsolated yield.

and 8). However, the reaction with Cu(NO3)2 completed in a relatively smaller time period affording a moderate yield of the product (Table 1, entry 9). With heteropoly acid salts, such as ammonium tungstate and ammonium molybdate, only trace amounts of the product were obtained (Table 1, entries 10 and 11). However, the reaction with PTA afforded an improved yield of the product (Table 1, entry 12). When the reaction was 7936


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ACS Sustainable Chemistry & Engineering conducted with metal-doped PTA such as AgPTA and CuPTA the rate of reaction was enhanced and time period reduced giving good yield of the product (Table 1, entries 13 and 14). With encouragement from this result and consideration of the surface-enhancing property of the supporting materials, the model reaction was also examined with different supports such as ZrO2, Al2O3, and SiO2. It was observed that, using ZrO2 and Al2O3 as supports, the reaction afforded a moderate yield of the product (Table 1, entries 15 and 16). When SiO2 was used as a supporting material for PTA (PTA/SiO2), a good yield of the product was obtained (Table 1, entry 17). However, using CuPTA supported on silica (CuPTA/SiO2), the reaction completed in a relatively shorter reaction time (10 min) affording an excellent yield of the product (94%; Table 1, entry 18) showing the effect of doped Cu metal on the catalytic activity of the catalyst, whereas, for the model reaction carried out with SiO2 alone as catalyst, no product formation was observed (Table 1, entry 19). For a demonstration of the superiority of solvent-free reaction conditions over different organic solvents, the model reaction was carried out in CH3OH, C2H5OH, (CH3)2CHOH, and polyethylene glycol (PEG-200), and the results are shown in Table 2. When CH3OH, C2H5OH, (CH3)2CHOH, and

Table 3. Effect of CuPTA Loading on Support SiO2 for the Model Reactiona

solvent

temperature

timeb

yieldc (%)

1 2 3 4 5 6 7

solvent-free CH3OH C2H5OH (CH3)2CHOH PEG CH3COOC2H5 H2O

80 °C reflux reflux reflux 100 °C reflux reflux

10 min 40 min 45 min 53 min 52 min 58 min 2h

94 68 65 59 52 48 36

CuPTA/SiO2 (% w/w)

timeb

yieldc (%)

1 2 3 4 5

5 10 15 20 25

1.5 h 50 min 35 min 10 min 10 min

58 62 76 94 91

a

Reaction conditions: 3-methyl-(2-nitrophenoxy)-1-phenylpyrazole-4carbaldehyde 1a (3 mmol), 1,3-dimethyl-2,4,6-pyrimidinetrione 2a (3 mmol), different CuPTA loadings on SiO2, solvent-free conditions, T = 80 °C. bReaction progress monitored by TLC. cIsolated yield.

period (10 min) was obtained (Table 4, entry 5). Further increasing the amount of the catalyst to 120 mg showed no Table 4. Effect of Amount of Catalyst on the Model Reactiona

Table 2. Effect of Different Solvents on the Model Reactiona entry

entry

entry

CuPTA/SiO2 (mg)

timeb

yieldc (%)

1 2 3 4 5 6

no catalyst 20 50 80 100 120

9h 1h 50 min 45 min 10 min 10 min

28 56 62 70 94 94

a

Reaction conditions: 3-methyl-(2-nitrophenoxy)-1-phenylpyrazole-4carbaldehyde 1a (3 mmol), 1,3-dimethyl-2,4,6-pyrimidinetrione 2a (3 mmol), different amounts of CuPTA/SiO2, solvent-free conditions, T = 80 °C. bReaction progress monitored by TLC. cIsolated yield.

effect on the yield of the product, whereas decreasing the amount (80, 50, and 20 mg) led to formation of the product in moderate yield. The reaction temperature plays an important role in influencing the rate of reaction. An increase in temperature may enhance the rate of the reaction by stimulating the reacting molecules leading to an improved yield of the product and reaction time. As indicated in Table 5, the effect of increasing

a

Reaction conditions: 3-methyl-(2-nitrophenoxy)-1-phenylpyrazole-4carbaldehyde 1a (3 mmol), 1,3-dimethyl-2,4,6-pyrimidinetrione 2a (3 mmol), CuPTA/SiO2 (100 mg), solvent or solvent-free conditions, T = 80 °C. bReaction progress monitored by TLC. cIsolated yield.

PEG-200 were used as solvents, moderate yields of the products were obtained (Table 2, entries 2, 3, 4, and 5), whereas CH3COOC2H5 and H2O afforded a poor yield of the product (Table 2, entries 6 and 7). However, when the reaction was carried out under thermal solvent-free conditions, the maximum yield of the product was obtained in the minimum time period (Table 2, entry 1). For a demonstration of the effect of catalyst loading on SiO2 (support) for the synthesis of 3a, different catalyst loadings were employed such as 5%, 10%, 15%, 20%, and 25% w/w. It was observed that both yield of the product and reaction time were improved upon increasing the catalyst loading up to 20% w/w (Table 3, entries 1, 2, 3, and 4). Further, an increase in catalyst loading resulted in a slightly reduced yield of the product with no effect on reaction time (Table 3, entry 5). For a determination of the appropriate amount of the catalyst for catalyzing the reaction, the model reaction was carried out in the presence of different amounts (20, 50, 80, 100, and 120 mg) of CuPTA/SiO2. It was observed that 100 mg of the catalyst was adequate for the successful completion of the reaction. When the reaction was carried out in the absence of CuPTA/SiO2, it took a longer time for completion with a very poor yield of product. When 100 mg of the catalyst was used, the maximum yield of the product (94%) in the shortest time

Table 5. Effect of Temperature on the Model Reactiona entry

temperature

timeb

yieldc (%)

1 2 3 4 5

room temperature 40 °C 60 °C 80 °C 100 °C

6h 3.5 h 2h 10 min 9 min

trace 52 73 94 60

a

Reaction conditions: 3-methyl-(2-nitrophenoxy)-1-phenylpyrazole-4carbaldehyde 1a (3 mmol), 1,3-dimethyl-2,4,6-pyrimidinetrione 2a (3 mmol), CuPTA/SiO2 (100 mg), solvent-free conditions at different temperatures. bReaction progress monitored by TLC. cIsolated yield.

temperature (from 25 to 80 °C) is directly related to the improved yield of the product. Upon further increasing the temperature, the reaction completed in a shorter time but afforded charred product. Thus, 80 °C was found to be the optimum temperature for the desired reaction. Catalytic Reaction. The phenoxy pyrazolyl chalcones were synthesized via Claisen−Schmidt condensation by heating a mixture of different active methyl compounds 2a−e (3 mmol) and 5-aryloxy-3-methyl-1-phenylpyrazole-4-carbaldehydes 1a−c (3 mmol) in the presence of 100 mg of CuPTA/SiO2 at 80 °C 7937


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Table 6. Synthesis of Phenoxy Pyrazolyl Chalcones (3a−m) Using CuPTA/SiO2 as a Catalyst under Solvent-Free Conditions

a

Newly synthesized compound. bReported compound.38 cReaction progress monitored by TLC. dIsolated yield.

Recycling Study of the Catalyst. For examination of the reusability of the catalyst (CuPTA/SiO2), recycling studies were carried out. Thus, in a model reaction, phenoxy aldehyde 1a (3 mmol), active methyl compound 2a (3 mmol), and CuPTA/SiO2 (100 mg) were taken in a round-bottom flask and heated at 80 °C under solvent-free conditions for specified time period (10 min). After completion of the reaction, 5 mL of ethanol was added, and the reaction mixture was filtered. The recovered catalyst was thoroughly washed with ethanol (4 × 10 mL), dried in an oven at 120 °C for 2 h, and used for the subsequent cycles. The same procedure was applied for all recycling studies. The results (Table 7) revealed that the catalyst exhibited good catalytic activity up to six consecutive cycles. The recovered catalyst after the sixth cycle was characterized by FTIR, powder XRD, SEM, EPR, and ICP−AES analysis. In

under solvent-free conditions for a period of 10−12 min. After completion of the reaction (as monitored by TLC), the reaction mixture was kept at room temperature for 30 min, and 5 mL of ethanol was added to it. The reaction mixture was then filtered to recover the catalyst, and the obtained filtrate was concentrated to obtain the products which were recrystallized with methanol to obtain pure products. The products were obtained in excellent yield (93−94%; Table 6; Scheme 3). Reaction Mechanism. A plausible mechanism for the synthesis of phenoxy pyrazolyl chalcones (3a−m) is depicted in Scheme 4.13 The carbonyl group of aldehydes 1a−c is activated by coordinating a lone pair of electrons of the oxygen atom with the catalyst (CuPTA/SiO2), facilitating the attack of the double bond of ketones 2a−e to form β-hydroxyl compound X which undergoes dehydration to form the product 3a−m. 7938


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Scheme 3. Synthesis of Phenoxy Pyrazolyl Chalcones Using CuPTA/SiO2 as a Catalyst under Thermal Solvent-Free Conditions

Scheme 4. Proposed Mechanism for the Synthesis of Phenoxy Pyrazolyl Chalcones (3a−m)

the FTIR spectra of the recycled catalyst, the characteristic peaks (1080, 984, 893, and 801 cm−1) remained the same (Figure 1E). In the XRD pattern (Figure 2d), peaks remained the same, and also no change in the morphology of the catalyst in the SEM micrograph (Figure 3c) was observed as compared to the fresh catalyst. The EPR study of the recycled catalyst revealed that the oxidation state of Cu (+2) remained the same with g values (g∥ 2.232 and g⊥ 2.041). ICP−AES analysis showed the percentage of Cu in the recovered catalyst (7th cycle) as 7.8% .This shows that no leaching of Cu occurs during the course of reaction, and Cu remains bonded to PTA up to the sixth cycle. However, in the seventh cycle, the yield of the product decreased (85%) as a result of the leaching of Cu, causing some deactivation of the catalyst (Figure 8).

Table 7. Recycling Study of the Catalyst for the Model Reactiona catalytic cycle

timeb (min)

yieldc (%)

I II III IV V VI VII

10 10 10 10 10 10 10

94 94 94 93 93 92 85

a

Reaction conditions: 3-methyl-(2-nitrophenoxy)-1-phenylpyrazole-4carbaldehyde 1a (3 mmol), 1,3-dimethyl-2,4,6-pyrimidinetrione 2a (3 mmol), CuPTA/SiO2 (100 mg), solvent-free conditions, T = 80 °C. b Reaction progress monitored by TLC. cIsolated yield.

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(2) Zhang, D. Y.; Duan, M. H.; Yao, X. H.; Fu, Y. J.; Zu, Y. G. Preparation of a novel cellulose-based immobilized heteropoly acid system and its application on the biodiesel production. Fuel 2016, 172, 293−300. (3) Kozhevnikov, I. V. Sustainable heterogeneous acid catalysis by heteropoly acids. J. Mol. Catal. A: Chem. 2007, 262, 86−92. (4) Timofeeva, M. N. Acid catalysis by heteropoly acids. Appl. Catal., A 2003, 256, 19−35. (5) Kozhevnikov, I. V. Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions. Chem. Rev. 1998, 98, 171−198. (6) Kozhevnikov, I. V. E. Advances in catalysis by heteropolyacids. Russ. Chem. Rev. 1987, 56, 811−825. (7) Bardin, B. B.; Bordawekar, S. V.; Neurock, M.; Davis, R. J. Acidity of Keggin-type heteropolycompounds evaluated by catalytic probe reactions, sorption microcalorimetry, and density functional quantum chemical calculations. J. Phys. Chem. B 1998, 102, 10817−10825. (8) Klein, M.; Pulidindi, I. N.; Perkas, N.; Gedanken, A. Heteropoly acid catalyzed hydrolysis of glycogen to glucose. Biomass Bioenergy 2015, 76, 61−68. (9) Azizi, N.; Saidi, M. R. Highly efficient ring opening reactions of epoxides with deactivated aromatic amines catalyzed by heteropoly acids in water. Tetrahedron 2007, 63, 888−891. (10) Rafiee, E.; Khodayari, M. Synthesis and characterization of a green composite of H3PW12O40 and starch-coated magnetite nano particles as a magnetically-recoverable nano catalyst in Friedel-Crafts alkylation. J. Mol. Catal. A: Chem. 2015, 398, 336−343. (11) Ninomiya, W.; Sadakane, M.; Matsuoka, S.; Nakamura, H.; Naitou, H.; Ueda, W. An efficient synthesis of α-acyloxyacrylate esters as candidate monomers for bio-based polymers by heteropolyacidcatalyzed acylation of pyruvate esters. Green Chem. 2009, 11, 1666− 1674. (12) Davis, M. C.; Groshens, T. J. Dimerization of resveratrol trimethyl ether by phosphotungstic acid, structure confirmation of resformicol A and B. Tetrahedron Lett. 2012, 53, 3521−3523. (13) Rafiee, E.; Tork, F.; Joshaghani, M. Heteropoly acids as solid green Brønsted acids for a one-pot synthesis of β-acetamido ketones by Dakin−West reaction. Bioorg. Med. Chem. Lett. 2006, 16, 1221− 1226. (14) Ding, T.; Liu, Q.; Shi, R.; Tian, M.; Yang, J.; Zhang, L. Synthesis, characterization and in vitro degradation study of a novel and rapidly degradable elastomer. Polym. Degrad. Stab. 2006, 91, 733−739. (15) Bhure, M. H.; Rode, C. V.; Chikate, R. C.; Patwardhan, N.; Patil, S. Phosphotungstic acid as an efficient solid catalyst for intramolecular rearrangement of benzyl phenyl ether to 2-benzyl phenol. Catal. Commun. 2007, 8, 139−144. (16) Wang, G. W.; Shen, Y. B.; Wu, X. L.; Wang, L. Solvent-free hydroalkylation of olefins with 1, 3-diketones catalyzed by phosphotungstic acid. Tetrahedron Lett. 2008, 49, 5090−5093. (17) Newman, A. D.; Brown, D. R.; Siril, P.; Lee, A. F.; Wilson, K. Structural studies of high dispersion H3PW12O40/SiO2 solid acid catalysts. Phys. Chem. Chem. Phys. 2006, 8, 2893−2902. (18) Clark, J. H. Green chemistry: challenges and opportunities. Green Chem. 1999, 1, 1−8. (19) Tiwari, D.; Devi, A.; Chandra, R. Synthesis of cardinal based phenol resin with aid of microwaves. Int. J. Drug Dev. Res. 2011, 3 (2), 171−175. (20) Tundo, P.; Anastas, P.; Black, D. S.; Breen, J.; Collins, T. J.; Memoli, S.; Miyamoto, J.; Polyakoff, M.; Tumas, W. Synthetic pathways and processes in green chemistry. Introductory overview. Pure Appl. Chem. 2000, 72, 1207−1228. (21) Siddiqui, Z. N.; Farooq, F.; Musthafa, T. N. M. A highly efficient, simple, and eco friendly microwave-induced synthesis of indolylchalcones and pyrazolines. Green Chem. Lett. Rev. 2011, 4, 63− 68. (22) Rizk, H. F.; El-Badawi, M. A.; Ibrahim, S. A.; El-Borai, M. A. Synthesis of some novel heterocyclic dyes derived from pyrazole derivatives. Arabian J. Chem. 2011, 4, 37−44.

Figure 8. Recycling study of CuPTA/SiO2 on the model reaction.

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CONCLUSION In conclusion, we have designed a simple method for the synthesis of novel CuPTA/SiO2. The catalyst efficiently catalyzed the Claisen−Schmidt condensation reaction toward the synthesis of phenoxy pyrazolyl chalcones under thermal solvent-free conditions. The catalyst could be recycled for six runs during the course of the reaction. The widespread scope, clean reaction profile, reusability of the catalyst, improved rate of reaction, and product yield are the advantages of the present protocol.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01467. General experimental details and spectroscopic data of newly synthesized compounds including their spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +919412653054. ORCID

Zeba N. Siddiqui: 0000-0002-0952-8608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistance in the form of a major research project (CST/SERPD/D-283) from CST-UP is gratefully acknowledged. The authors would also like to thank USIF, A.M.U., for SEM/EDX and elemental mapping, IIT Bombay for ICP−AES analysis, IIT Madras for TPD-NH3 analysis, IIT Kanpur for XPS analysis, and SAIF Chandigarh, Punjab University, for providing NMR and mass spectral data.

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REFERENCES

(1) Van Santen, R. A.; Neurock, M. Molecular Heterogeneous Catalysis: A Conceptual and Computational Approach; John Wiley & Sons, 2009. 7940


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Synthesis, characterization and application of Silica supported Copper

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