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Synthesis and characterization of carbon@HPW core/shell nanorod using potato as a novel precursor: Efficient catalyst for C—N coupling reaction
⁎Corresponding author at: Department of Inorganic Chemistry, Faculty of Chemistry, Razi University, Kermanshah 67149, Iran. Tel./fax: +98 833 4274559. ezzat_rafiee@yahoo.com (Ezzat Rafiee) e.rafiei@razi.ac.ir (Ezzat Rafiee)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Carbon support with nanorod (CSN) structure was synthesized by natural potato as green and very cheap source via hydrothermal method. A novel type of core/shell nanorod catalyst was synthesized by the immobilization of 12-tungstophosphoric acid (HPW) on the surface of the CSN (C@HPW). Characterization of the core/shell nanorod catalyst was carried out using transmission electron microscopy (TEM), X-ray diffraction (XRD) pattern, energy dispersive X-ray (EDX) spectroscopy, Fourier transform infrared (FTIR) spectrophotometry, and Raman spectrophotometer. The TEM results show that morphology of carbon catalyst support could be changed from nanorod to core/sell nanorod by the immobilization of HPW. The characterization data derived from FTIR spectroscopy reveal that HPW on the support exists in the Keggin structure. The C@HPW core/shell nanorod was found to be a unique, effective, and eco-friendly catalyst for the C—N coupling reactions for a broad range of aryl and alkyl amines and alcohols under aerobic conditions at room temperature. The excellent conversion at low reaction time shows that the catalyst has strong and sufficient sites, which are responsible for its catalytic performance. The acidity measurements of CSN and C@HPW by means of potentiometric titration with n-butylamine have been used to estimate their relative acid strength. Leaching of the HPW from the CSN was minimized by optimization of calcination temperature. The reused catalyst for at least five repeating cycles shows excellent activity.
Keywords
Core/shell nanorod
12-Tungstophosphoric acid
Natural potato
Carbon
C—N coupling reaction
1 Introduction
Introduction of green, efficient, and selective catalysts for production of amine derivatives without additive is of great challenge in the synthesis of pharmaceuticals, agrochemicals, dyes, surfactants, functionalized materials, and fine chemicals. The most frequently used method for the preparation of N-alkyl and aryl amines is the coupling of amines with aryl halides and arylboronic acids in the presence of different reducing agents (Teo et al., 2011; Nasrollahzadeha et al., 2014; Sawant et al., 2013). An alternative environmentally benign approach to these methods is the N-alkylation of amines with alcohols which have no toxic by-products (Wang et al., 2013; Valot et al., 1999; Fujita et al., 2003; Qu et al., 2014; Gonzalez-Arellano et al., 2010; Martínez et al., 2009; Liu et al., 2012).
Very recently, more and more groups have turned their attention to Brønsted and Lewis acid-catalyzed condition such as heteropoly acids (HPAs) for direct N-alkylation with alcohols (Wang et al., 2008a,b; Sanz et al., 2007; Reddy et al., 2007). The HPAs in the bulk form possess very low surface area and high solubility in polar medium, which limits the exertion of potentially catalytic performance and makes some difficulties in catalyst recovery (Damyanova et al., 1999). Many efficient supports were developed for preparing nanoscale HPA particles at heterogeneous systems (Long et al., 2010; Okuhara, 2003; Rafiee and Eavani, 2011). Using environmentally friendly heterogeneous catalysts based on natural ingredients is of interest and remarkable to many researchers.
Materials of natural origin have been studied and proposed for a wide range of biomedical applications (Lakshmi and Laurencin, 2007). Using agricultural by-products and natural plant resources as attractive precursors for preparing carbon compounds is promising (Balathanigaimani et al., 2008; Hu et al., 2007; Wu et al., 2005; Kim et al., 2006). However, carbon compounds prepared from natural precursors have not been studied as much as other carbon sources, but are expected to possess various advantages over other carbon compounds, and have many applications. Besides, these materials are cheap, natural abundant, and eco-friendly compounds (Wang et al., 2008a, 2008b; Zhao et al., 2009). Hydrothermal method is promising route, offering low-cost, low temperature, and environmentally friendly production for novel carbon compounds from natural precursors without using toxic chemicals (Hu et al., 2010; Titirici and Antonietti, 2010).
Carbon catalyst support prepared from natural precursors can reduce the preparation cost; therefore, 12-tungstophosphoric acid (HPW) coated carbon produced from natural abundant potato was synthesized as a green and very cheap carbon support nanorod (CSN) via hydrothermal treatment at aqueous solvent as a green synthetic method. We are interested in creating various nanostructure patterns as a definitive step toward the fabrication of catalysts with different activities for organic reactions (Okuhara, 2003; Ghaderi-Shekhiabadi et al., 2014; Rafiee et al., 2009a,b). Therefore, the green core/shell nanorod catalyst was synthesized and employed in the C—N coupling reactions for a broad range of aryl and alkyl amines and alcohols under aerobic condition at room temperature.
2 Experimental
2.1 Material and methods
Potato was obtained from west of IRAN. Analytical grades of 12-tungstophosphoric acid (H3PW12O40 = HPW), solvents, amines, and alcohols were purchased from Merck and were used without further purification. The transmission electron microscopy (TEM) images were recorded on a Jeol JEM-2100 transmission electron microscope with an accelerating voltage of 200 kV. TEM samples were prepared by dispersing some solid products into ethanol and then sonicating for approximately 30 s. A few drops of the suspension were deposited on copper grids, which were then put into the desiccators for drying. X-ray diffraction (XRD) pattern was recorded by an Inel French, EQUINOX 3000 model X-ray diffractometer using Cu Kα radiation. Energy dispersive X-ray (EDX) measurements were made with an IXRF model 550i attached to SEM. EDX samples were prepared by the coating of solid particles into a conductive layer. The Fourier transform infrared (FTIR) spectrophotometry of the samples was recorded at room temperature using a Bruker, ALPHA spectrophotometer at a spectral resolution of 4 cm−1 using KBr pellets in the range of 400–4000 cm−1 with a delicate beam condenser and a liquid nitrogen cooled MCT-Detector. Raman scattering study was performed with a Laser Raman Spectrometer (Thermo Nicolet Almega XR Raman, USA) with Nd:YLF laser and 4 cm−1 resolution, using a laser excitation wavelength at 785 nm. Laser power is 100 mW but this usually uses 30 mW in order to keep the sample safe with 32 scans around 3 min. UV–vis spectra were recorded with an Agilent (8453) UV–vis spectrophotometer diode-array spectrometer using quartz cells of 1 cm optical path. Thin layer chromatography (TLC) on precoated silica gel Fluorescent 254 nm (0.2 mm) on aluminum plates was used for monitoring the progress of reactions. The potential variation was measured with a Hanna 302 pH meter using a double junction electrode.
2.2 Preparation of CSN
The CSN was prepared as follows: in a typical method, the natural potato was manually peeled and stabilized in vacuum oven at 30 °C. Then 5 g of the stabilized sample was dissolved in distilled water (50 ml) under magnetic stirring. The homogeneous solution was transferred to a 100 ml Teflon-line stainless steel autoclave. The autoclave was sealed and maintained at 180 °C for 12 h in a digital temperature-controlled oven. After thermal treatment, the autoclave was cooled to room temperature and the resulting black precipitate was collected and centrifuged with distilled water for several times and finally dried in vacuum oven at 50 °C.
2.3 Preparation of C@HPW core/shell nanorod catalyst
0.3 g of HPW was dissolved in 1 mL of distilled water. This solution was added dropwise to a suspension of 0.5 g CSN in distilled water (50 mL) under sonication. The mixture was stirred at room temperature for overnight. Finally, the solvent was removed by using rotary evaporator and dried. After preparation, the catalyst was calcined for 2 h at different temperatures (100, 150, 200, 250, and 300 °C). It finds that, the 150 °C as calcination temperature was observed very low of the leaching of the HPW in the heterogeneous catalytic system.
2.4 Acidity measurement
For the potentiometric titration, 0.05 g of solid was suspended in acetonitrile (90 mL) and stirred for 3 h. The suspension was titrated with a 0.05 mol/L solution of n-butylamine in acetonitrile. The potential variation was measured with a Hanna 302 pH meter using a double junction electrode.
2.5 General procedure for the C—N coupling reactions
In a typical reaction, a suspension of amine (1.0 mmol) and alcohol (1.0 mmol) was added to the mixture of the catalyst (13.8 mmol%, 0.001 g) in acetonitrile (1.5 mL). The resulting mixture was mixed at room temperature for an appropriate time under aerobic conditions. Progress of the reaction was monitored by TLC. After completion of the reaction, the catalyst was removed by a repeated centrifugation (2000 rpm, 3 min) and decantated. Catalyst was collected and treated with 1,2-dichloroethane (DCE) for removing coke followed by vacuum drying and calcination at 150 °C for 2 h for reusing. Solvent of the reaction mixture was evaporated to generate the crude product. The product was concentrated and purified by column chromatography on silica-gel using EtOAc/heptane (1:3) as eluent.
3 Results and discussion
3.1 Characterization results
The CSN was synthesized according to the literature using natural potato under hydrothermal treatment (Salavati-Niasari and Ghaderi-Sheikhiabadi, 2014). The C@HPW core/shell nanorods were prepared by coating HPW on the surface of CSN at room temperature for overnight. Finally, the mixture was dried by rotary evaporator and calcined (Scheme 1). The prepared CSN and C@HPW core/shell nanorod catalyst was characterized by TEM, XRD, EDX, Raman, and FTIR.
Fig. 1a and b shows typical TEM images of the CSN and C@HPW catalyst, respectively. Fig. 1a indicates that the support consists of relatively uniform rods with diagonals of about 200 nm and more than 1 μm length. TEM image observation of the C@HPW catalyst shows that HPW is well coated on the carbon support surface and created core/shell nanorod structure with diagonal of about 250 nm (Fig. 1b). The size distribution by means of microstructure measurement software was obtained with the relative precision by measuring more than ten nanorods from TEM images (Fig. 1c and d).
Stability of the HPW anion and CCS in the C@HPW catalyst was investigated by XRD and FTIR spectroscopic analysis. The XRD pattern for the C@HPW catalyst is shown in Fig. 2. When HPW (Guo et al., 2008) is impregnated on CCS (Salavati-Niasari and Ghaderi-Sheikhiabadi, 2014), characteristic peaks assigned to HPW and CCS are comparable to those for the C@HPW, which implies retention of their crystalline character (Fig. 2). The presence of the intense peaks indicates that there is no significant change in the structure of the HPW and CCS during the preparation of nanocatalyst.
Fig. 3a, b, d shows FTIR of CSN, C@HPW, and HPW, respectively. In Fig. 3a the peaks at 1469, 1615, and 1702 cm–1 due to various C⚌C stretching vibration can be observed. The absorption bonds at 2923, 3415, and 3729 cm–1 are related to O—H and ⚌C—H stretching vibration. The HPW Keggin structure is well known and consists of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedral. These groups are connected to each other by corner-sharing oxygens (Pope, 1983). This structure gives rise to four types of oxygen, being responsible for the fingerprint bands of the Keggin ion between 700 and 1200 cm–1. HPW shows typical bands for absorption at 1080 cm–1 P—O stretching vibration, 984 cm–1 W⚌O stretching vibration, and 896 and 814 cm–1 W—O—W stretching vibration (Fig. 3b). The shift of the heteropolyanion vibration W—O—W at 800 cm−1 of HPW to the value at 814 cm−1 of C@HPW (Fig. 3b) may indicate that it is the oxygen atoms of the HPA anion which are bonded by the intermolecular interactions with the support surface. The interaction probably occurs through Coulombic forces of the protons surface of CSN and the negatively charged HPW units (Micek-Ilnicka et al., 2012; Pawelec et al., 2001).
Fig. 4 shows Raman spectrum of the C@HPW catalyst. The peak at 999 cm–1 corresponds to the stretching frequency of ⚌C⚌C⚌ (Heimann et al., 1999), but there is no obvious peak around 2100 cm–1 corresponding to the stretching frequency of —C≡C— (Heimann et al., 1999). On the other hand, the peak at 1580 cm–1 (G band) is attributed to an E2g mode of graphite and the peak at 1320 cm–1 (D band) corresponds to vibration of carbon atoms with dangling bonds in plane terminations of disordered (Fig. 4, inset-Cuesta et al., 1994).
The acidity measurements of CSN and C@HPW by means of potentiometric titration with n-butylamine were used to estimate their relative acid strength according to the two values (a) Ei as initial potential and (b) number of acid sites (Fig. 5). The initial electrode potential (Ei) indicates the maximum strength of the acid sites and the value from which the plateau is reached (mmol amine/g solid) indicates the total number of acid sites that are present in the titrated solids (Rafiee et al., 2008). The acidic strength of the solids can be classified according to the following range: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), −100 < Ei < 0 mV (weak sites) and Ei < −100 mV (very weak sites). According to potentiometric titration curves (Fig. 5), C@HPW presented very strong acidic sites (Ei = 580 mV).
EDX spectrum can provide qualitative information about the types of different chemical elements in the catalyst support and matrix. Fig. 6a shows an EDX spectrum of CSN that contained the purified carbon (Casciardi et al., 2010; Khanna et al., 2008). EDX spectra of C@HPW core/shell nanorod show uniform distribution of carbon, tungsten, phosphor, and oxygen elements in catalyst matrix (Fig. 6b, c, d, e, and f).
3.2 Catalytic behavior
At the onset of our catalytic studies, the C—N coupling was examined by treating aniline (1.0 mmol) with an excess of benzyl alcohol (2.0 mmol) as alkylating agent in the presence of C@HPW core/shell nanorod as acid catalyst. It was found that C@HPW is capable catalyst with a yield of 98%, when acetonitrile was used as the solvent at 60 °C. With this result in hand, experiments were conducted to optimize the reaction conditions for the C—N coupling and the results are summarized in Table 1.
|
||||||
|---|---|---|---|---|---|---|
| Entry | PhNH2:PhCH2OH molar ratio | Catalyst (g) | Temperature (°C) | Solvent | Time (min) | Yielda (%) |
| 1 | 1:2 | C@HPW (0.002) | 60 | Acetonitrile | 10 | 98 |
| 2 | 1:2 | C@HPW (0.002) | r.t. | Acetonitrile | 12 | 98 |
| 3 | 1:2 | C@HPW (0.001) | r.t. | Acetonitrile | 10 | 90 |
| 4 | 1:2 | C@HPW (0.003) | r.t. | Acetonitrile | 10 | 96 |
| 5 | 1:2 | C@HPW (0.001) | r.t. | n-Heptane | 30 | 72 |
| 6 | 2:1 | C@HPW (0.001) | r.t. | Acetonitrile | 60 | 81 |
| 7 | 1:1 | C@HPW (0.001) | r.t. | Acetonitrile | 10 | 98 |
| 8 | 1:1 | HPW (0.001) | r.t. | Acetonitrile | 10 | 68 |
| 9 | 1:1 | CSN (0.001) | r.t. | Acetonitrile | 5 h | – |
| 10 | 1:1 | – | r.t. | Acetonitrile | 5 h | – |
In the first set of experiments model reaction was evaluated with 0.002 g of C@HPW in acetonitrile as solvent at 60 °C and room temperature. Results show that the corresponding product was afforded in similar time and yield (Table 1, entries 1 and 2). Therefore, room temperature was chosen as reaction temperature. To investigate the effect of catalyst loading, the reaction was carried out in the presence of different amounts of the catalyst. The best result was obtained in the presence of just 0.001 g of C@HPW core/shell nanorod catalyst, and the use of higher amounts of the catalyst slightly decreased the reaction time (Table 1, entries 2–4). The C—N coupling product is obtained in two studied solvents but more efficiently in acetonitrile (Table 1, entries 3 and 5). Equimolar amount of aniline and benzyl alcohol resulted in excellent yield of C—N coupling product compared to other ratios (Table 1, entries 3, 6, and 7). The HPW as catalyst produced the C—N coupling product with 68% yield, and HPW-free CSN did not exhibit activity within coupling reaction (Table 1, entries 8, 9). In the absence of the catalyst, no product was formed even after 5 h in optimized reaction conditions (Table 1, entry 10).
To evaluate the scope and generality of this new protocol, various amines and alcohols were tested (Tables 2 and 3). We started by changing the electronic nature of the substituent on amine (Table 2). Electron donating groups at the orto, meta, and para-position of the aryl amines afforded high yields in comparison with electron withdrawing groups (Table 2, entries 1–9). Formation of hydrogen bond between the amine and nitro present on the 2- nitro-phenylamine slightly affects on the yield of corresponding C—N coupling product compared to 4-nitro-phenylamine (Table 2, entries 5 and 7) (Lu et al., 2014). In the case of sterically demanding group of the aryl amine yield of the corresponding product was acceptable (Table 2, entry 10). Alkyl amines were also tolerated under similar reaction conditions, giving the final products in moderate yields with benzyl alcohol (Table 2, entries 11–13). The reaction using different 4-substituted electron donating groups compared to electron withdrawing groups’ benzylic alcohols with aniline gave practically the better results (Table 2, entries 14–17). It seems that formation of hydrogen bond between the hydroxide and nitro group on the 2- nitro-benzyl alcohol inhibits the progress of the reaction (Table 2, entry 18) (Reddy et al., 2013). Low yields were observed in the reaction of aniline with alkyl alcohols (Table 2, entries 19 and 20). Beside when coupling reaction was carried out in methanol or ethanol as solvent, yields of coupling products are very low in comparison with entries 19 and 20 (32% and 10%, respectively).
|
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|---|---|---|---|---|---|
| Entry | Amine | Alcohol | Product | Time (min) | Yield b,c (%) |
| 1 |
|
|
|
10 | 98 |
| 2 |
|
|
|
5 | 94 |
| 3 |
|
|
|
40 | 76 |
| 4 |
|
|
|
60 | 79 |
| 5 |
|
|
|
50 | 91 |
| 6 |
|
|
|
50 | 98 |
| 7 |
|
|
|
30 | 86 |
| 8 |
|
|
|
30 | 79 |
| 9 |
|
|
|
60 | 71 |
| 10 |
|
|
|
60 | 72 |
| 11 |
|
|
|
60 | 85 |
| 12 |
|
|
|
120 | 49 |
| 13 |
|
|
|
120 | 85 |
| 14 |
|
|
|
10 | 90 |
| 15 |
|
|
|
15 | 92 |
| 16 |
|
|
|
25 | 81 |
| 17 |
|
|
|
40 | 58 |
| 18 |
|
|
|
360 | 15 |
| 19 |
|
|
|
120 | 53 |
| 20 |
|
|
|
120 | 61 |
| Entry | Amine | Alcohol | Time (min) | Yield b,c (%) |
|---|---|---|---|---|
| 1 |
|
|
45 | 91 |
| 2 |
|
|
45 | 82 |
| 3 |
|
|
35 | 90 |
| 4 |
|
|
60 | 98 |
| 5 |
|
|
50 | 81 |
| 6 |
|
|
50 (5 h) | 9 (62) |
| 7 |
|
|
50 | 88 |
| 8d |
|
|
60 | 85 |
| 9d |
|
|
45 | 91 |
We next turned our attention to the C—N coupling of heterocyclic aromatic and secondary amines with different alcohols (Table 3). At first, 2-aminopyridine, 3-aminopyridine, and 4-aminopyridine with benzyl alcohol reacted efficiently to give excellent and same yields of corresponding products (Table 3, entries 1–3). The position of this extra nitrogen atom seems to have no influence on results, as well as the existence of two nitrogen atoms on the six membered ring of the amine (Table 3, entry 4). We considered morpholine as a suitable candidate for further evaluation of the influence of the C@HPW on the C—N coupling of a cyclic secondary amine. Our catalytic system showed good activity for the arylation of morpholine. However, in the cases of 4-chlorobenzyl alcohol a low yield at long reaction time was also observed (Table 3, entries 5–7). Alkylation of secondary amines shows that, basicity and steric factor of amine substrates were important. The reaction of diethyl and diphenyl amine with benzyl alcohol required higher reaction temperature (60 °C) to obtain excellent yields (Table 3, entries 8 and 9).
Ethylene diamine and ethylene glycol are interesting substrates, because these contain double functional groups from amine and alcohol, both susceptible to C—N coupling reaction. In both cases, products of first C—N coupling (a1 and b1) were obtained after 2 and 3 h, respectively. Other coupling products (a2 and b2) were not observed after certain reaction times (Scheme 2).
3.3 Reusability of the C@HPW core/shell nanorod catalyst
Finally the reusability of the catalyst was investigated. It should be noted that the reusability of the catalyst without calcination was found unfavorable. In this case, when the supernatant portion of the C@HPW catalyzed reaction mixture was subjected to UV–vis spectrum, it exhibits the presence of absorption band at 265 nm assigned to Keggin type of PW12O403−. It was due to the leaching of HPW from the support into the liquid phase. To check the leaching stability of the catalyst, 0.001 g of the catalyst with calcination after each run was stirred in 1.5 mL of acetonitrile at room temperature for 10 min. UV–vis spectra of the diluted solution were recorded after the removal of the solid. The content of HPW in solution was determined with the aid of calibration curves (Fig. 7).
Therefore, this catalyst could be recovered and subsequently reused several times in acetonitrile just after calcination at 150 °C for 2 h. C@HPW was recyclable very easy with slight loss of catalytic activity. The model reaction was carried out by using 0.01 g of the catalyst and the experiments were properly scaled up. After each run, the catalyst was washed and dried as described in Section 2.5. As shown in Fig. 8, only 7% reduction in product yield was observed after five times of the reuse. We consider the loss of catalyst during the recovery process was 5 wt.% after five successive runs. According to the FTIR spectra in Fig. 3c, it seems that catalyst is stable in the reaction and after recovery process and Keggin structure of the HPW preserved.
3.4 Mechanism of the reaction
The plausible mechanism (Scheme 3) for the synthesis of corresponding C—N coupling product in model reaction in the presence of C@HPW involves the initial interaction of C@HPW as acid catalyst with benzyl alcohol for protonation and C—O activation. Subsequent dehydration of oxonium ion was proceeded which was attacked by the amine source, followed by the release of H+, and generates the final C—N coupling product.
4 Conclusion
Natural potato provides a good support to design core/shell nanorod structure. To the best of our knowledge, there is no report on the fabrication of carbon nanostructures as catalyst support with natural potato. The C@HPW core/shell nanorod was synthesized by natural and green precursors via hydrothermal method. The catalyst was also characterized by TEM, XRD, EDX, FTIR, and Raman spectrophotometry. This green catalyst showed excellent activity in C—N coupling reactions at room temperature. The acidity measurements of C@HPW presented very strong acidic sites. Leaching of the HPW from the CSN was minimized by optimization of calcination temperature. The catalyst could be recovered and reused at least five times without significant decrease in catalytic activity.
This study is an initial attempt toward synthesis of the C@HPW core/shell nanorod catalyst. Modification of synthetic approach and preparation, properties, and application in a wide range of other chemical reactions are in progress and will be reported in due course.
Acknowledgment
The authors thank the Razi University Research Council for the support of this work.
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