Fan Yang‡
a,
Sen Dong‡a,
Chunxia Wang‡b and
Yongfeng Li*a
aState Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, Changping 102249, China. E-mail: yfli@cup.edu.cn; Fax: +86-010-89739028; Tel: +86-010-89739028
bBeijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
First published on 25th May 2016
A heterogeneous catalytic assembly of the different Pd/PdO ratio nanoparticles supported on oxide carbon nanotubes (OCNTs) can be controllably synthesized by a one-pot gas–liquid interfacial plasma (GLIP) method via adjusting the glow produced parameter. The Pd/PdO nanoparticles have uniform particle size distribution. It is found that the catalysts with different Pd/PdO ratios could affect catalytic activity during the reduction of 4-nitrophenol (4-NP) in water by NaBH4. The best turn over frequency (TOF) value is up to 3000 h−1, which is much higher than the case only using Pd nanoparticles as catalysts. Moreover, the catalysts allowed hydrogenation of nitroarenes in water by atmospheric pressure H2, and it displayed remarkable activity toward the hydrogenation of nitroarenes using low Pd loading in good yields. The catalyst can be simply and efficiently used for ten consecutive runs without significant decrease in activity.
In the past several years, our group has focused on the development of green catalysts and environmentally friendly procedures to various organic molecular transformations reactions by a series of carbon materials supported Pd and Au nanoparticles.33–38 These catalysts can be successfully applied in catalyst reactions in water due to the functionalized carbon materials, showing good hydrophobic/hydrophilic property. Moreover, the catalysts are fabricated by a gas–liquid interfacial plasma method (GLIP), which shows many merits for synthesis of metal nanoparticles, such as high process rate, preparation of nanomaterials in large scale, avoiding the use of toxic stabilizers and reducing agents, ambient reaction temperature, no need to stir during the nanoparticle formation process, and produced the glow along with the emission of argon.39–41 The schematic illustration of the GLIP method for synthesis of metal nanoparticles is shown in Fig. 1. Most recently, we have revealed that Pd(NO3)2·2H2O can be decomposed to PdO as well as reduced to Pd under glow condition.36,37 Therefore, we successful fabricate the Pd/PdO nanoparticles which show better catalytic activity than Pd nanoparticles due to the boundary effect of Pd and PdO component. In continuation of our previous work,36 it is the first time for us to report a controllable synthesis of different ratios Pd/PdO nanoparticles supported on the oxidized multi-walled carbon nanotube by the GLIP method via adjusting the glow produced parameter, and the catalytic performance of different ratios Pd/PdO nanoparticle are examined by the hydrogenation of nitroaromatics to aromatic amines.
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Fig. 1 Schematic illustration for synthesis of metal nanoparticles supported on OCNTs by the GLIP method. |
Sample | Pd-1 | Pd-2 | Pd-3 | Pd-4 |
---|---|---|---|---|
a Average size obtained from the size distribution histogram.b The sum of Pd and PdO loading calculated by XPS.c The sum of Pd and PdO loading calculated by ICP-OES. | ||||
Pressure | 140 | 140 | 190 | 500 |
Electricity (mA) | 0.3 | 0.5 | 1.4 | 4.4 |
Voltage (V) | 268 | 265 | 270 | 260 |
Pd-n sizea (nm) | 4.2 | 3.5 | 3.9 | 3.8 |
Pd loadingb (wt%) | 12.0 | 10.0 | 11.6 | 11.0 |
Pd loadingc (wt%) | 11.2 | 10.6 | 10.8 | 11.0 |
Pd/PdOb | 61/39 | 45/55 | 33/67 | 23/77 |
In addition, the SAED of Pd-n is indicated in Fig. 3a. The lattice spacing measured from the diffraction rings are 0.34, 0.23, 0.21, 0.20, 0.17, 0.14, 0.13 and 0.12 nm, corresponding to reflections C(002), Pd(111), C(100), Pd(200), C(004), Pd(220), C(110) and Pd(311), respectively. Fig. 3b shows the XRD patterns of Pd-n, which shows the diffraction peaks at 25.9°, 34.0°, 40.4°, 43.4°, 46.5°, 68.3° and 81.8°, corresponding to the C(002), PdO(101), Pd(111), C(100), Pd(200), Pd(220) and Pd(311). These evidences have confirmed that the Pd/PdO possess a face-centered crystalline structure.
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Fig. 3 (a) A SAED image of Pd-3; (b) XRD spectra of Pd-1 (black curve), Pd-2 (red curve), Pd-3 (blue curve) and Pd-4 (purple curve). |
In order to certify the Pd element state and the Pd/PdO ratio of the Pd-n, XPS, a powerful technique was used to confirm the component of the Pd/PdO hybrid materials. Wide XPS scan indicate that the Pd-n catalysts are composed of Pd, C, and O, as shown in Fig. 4a. The peaks of the chemical components C 1s and O 1s are at 284.17 and 533.17 eV, respectively. A small peak corresponding to Pd 3p is observed at 562.08 eV. The Pd catalysts show two peaks for Pd 3d5/2 and Pd 3d3/2 which are split into two types of Pd electronic states (Pd0 and PdO) centred around 335.97, 337.87, 341.17 and 343.37 eV, as shown in Fig. 4b–e. Moreover, the Pd/PdO ratio can be calculated by peak-differentiation-imitating, and the results are summarized in Table 1. These results suggest that the Pd catalyst contains Pd and PdO active species, and the ratio of the Pd/PdO can be controlled through adjusting the plasma parameters. These results demonstrate that the ratio of the PdO is increased with the higher power output, and the PdO ratio can be controlled between 40% and 80% approximately. The synergistic action of the Pd/PdO ratio may play an important role in highly active catalytic species. However, there are few reports about how the Pd/PdO ratio affects the catalytic activity.42,43 Therefore, in our case the hydrogenation of nitroarenes reaction was used to evaluate the catalyst efficiency in order to reveal the affection of Pd/PdO ratio for the reduction reaction.
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Fig. 4 (a) XPS spectra of Pd-1 (black curve), Pd-2 (green curve), Pd-3 (red curve) and Pd-4 (blue curve); high-resolution XPS Pd 3d image of Pd-1 (b), Pd-2 (c), Pd-3 (d) and Pd-4 (e). |
Initially, we employed the reduction of 4-nitrophenol as the model reaction to evaluate the Pd-n catalytic performance in micro-reaction vial at room temperature in aqueous solution with NaBH4 as reduction agent. The evolution of UV-vis spectra with reaction time for the reduction of 4-NP to 4-AP by using different catalysts was monitored, as shown in Fig. 4. With the time increasing, the absorption band at 400 nm was decreased, and the intensity of the new absorption band at 300 nm was increased, being ascribed to the formation of 4-AP. Two isosbestic points are observed at 277 and 317 nm. The turnover frequency (TOF) is a significant parameter for evaluating the catalytic activity in heterogeneous catalytic reaction. The TOF values of various catalysts are 857, 1000, 3000 and 1500 h−1 for Pd-1, Pd-2, Pd-3 and Pd-4 catalysts, respectively. In comparison with other Pd heterogeneous, the Pd-n catalyst demonstrates obviously superior catalytic activity,44–49 and the Pd-3 catalyst shows the best catalytic activity. However, the reusability of the Pd-n was not performed well, probably due to the PdO active species was reduced by the strong reductant (NaBH4). Above speculation was proved by which PdO peak was disappeared in the XPS spectra of the recycle Pd-n (Fig. 5).
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Fig. 5 (a) Successive absorption spectra of the conversion from 4-NP to 4-AP with Pd-n catalysts: Pd-1 (a), Pd-2 (b), Pd-3 (c) and Pd-4 (d). |
Therefore, we try to use H2 as reductant for further estimating their high catalytic activity, we first use Pd-n as catalyst for selective hydrogenation by using p-nitrotoluene as the model substrate (Table 2). We employed the reactions with a very low catalyst loading (0.75 mol%) under mild reaction conditions (atmospheric H2 pressure and 60 °C in water). The Pd-1 and Pd-2 catalysts show good activity for 4-nitrotoluene hydrogenation reaction under current conditions (entries 1 and 2). To our great delight, Pd-3 gave the best results (entry 3). Interestingly, a slightly decrease in activity was observed by using Pd-4 as catalyst (entry 4). We have also checked the case only using Pd/OCNTs, PdO/OCNTs and a mixture of Pd/OCNTs and PdO/OCNTs catalysts, and the results show much lower activity compared with Pd-n catalysts (entry 5, 13 and 14). These results demonstrate that the PdO component in the catalyst can enhance the activity, and the PdO ratio is an important factor affecting the reaction. The reaction temperature also shows some influence on the yield of the product, and the reaction carried out at 60 °C shows the best results (entries 3, 6 and 7). We have further identified several solvents, including THF, EtOAc, EtOH, and the reactions are carried out under the benchmark conditions. To our surprise, the EtOAc and EtOH are compatible with this catalytic system (entries 8 and 9), but they still show little lower yield compared with H2O. The THF did not work well with this catalyst (entry 10). The blank test without Pd/PdO nanoparticles was also carried out, and we found that the reaction did not proceed at all, which means that the Pd/PdO active species is necessary for this reduction reaction. In sharp contrast to the case of Pd/PdO, the reaction catalyzed by Pd/C proceeds rather slowly and stops at a quite low conversion. This result is rationalized by the lack of a suitable hydrophobic environment provided by the OCNTs.50 Therefore, the PdO enhance the Pd nanoparticles activity and OCNTs provide the hydrophilic–hydrophobic environmental, resulting in the high activity of Pd-3 for catalytic hydrogenation of nitroarenes in water.
Entry | Cat. | Solvent | Temperature (°C) | Yieldb (%) |
---|---|---|---|---|
a Reaction conditions: 4-nitrotolune (0.5 mmol), cat. Pd (0.75 mol%), and in 1 mL solvent.b Isolated yield.c Cat. Pd-5 is PdO/OCNTs (0.75 mol%).d Cat. Pd-6 is Pd/OCNTs (0.25 mol%) and PdO/OCNTs (0.5 mol%). | ||||
1 | Pd-1 | H2O | 60 | 81 |
2 | Pd-2 | H2O | 60 | 81 |
3 | Pd-3 | H2O | 60 | 94 |
4 | Pd-4 | H2O | 60 | 88 |
5 | Pd/OCNTs | H2O | 60 | 67 |
6 | Pd-3 | H2O | 40 | 85 |
7 | Pd-3 | H2O | 80 | 68 |
8 | Pd-3 | EtOAc | 60 | 91 |
9 | Pd-3 | EtOH | 60 | 92 |
10 | Pd-3 | THF | 60 | 52 |
11 | OCNTs | H2O | 60 | 0 |
12 | Pd/C | H2O | 60 | 37 |
13 | Pd-5c | H2O | 60 | 68 |
14 | Pd-6d | H2O | 60 | 60 |
With the optimized reaction conditions in hand, we have further extended the Pd-3 catalyst to various nitroarenes to examine the generality of the hydrogenation reactions (Table 3). Satisfyingly, in all cases, the hydrogenation of nitrobenzene proceeded smoothly and completely to give aniline in excellent isolated yields with high selectivity. The electronic features of the nitroarenes did not affect the reaction yield or selectivity. Various 4-substituted nitroarenes, bearing either electron-donating or electron-withdrawing groups, such as –COOMe, –COOH, –COCH3, –CN, –OMe and –OH, provided the corresponding products in good to excellent yields (entries 1–4 and 7, 8). As for the 4-chloronitrobenzene (1e), the hydrogenation reaction was accompanied by dehalogenation reaction, giving the 4-chloroaniline 69% yield (entry 5). Nitrobenzene (1f) also proceeded smoothly to give aniline in excellent yield (entry 6). Importantly, other position substituted nitroarenes group, including 2-COOH (1i), 3-OH (1j), were compatible with the catalysis system (entries 9 and 10). For compound with two nitro groups, 1,3-dinitrobenzene (1k), both nitro groups were reduced to amines (entry 11). Moreover, heterocycle nitroarene 8-nitroquinoline (1l) was converted to 8-aminoquinoline in quantitative yields (entry 12). These results indicate that the Pd-3 shows high activity in nitroarene hydrogenation.
Entry | Ar-NO2 (1) | Ar-NH2 (2) | Time (h) | Yieldb (%) |
---|---|---|---|---|
a Reaction conditions: nitroarenes (0.5 mmol), Pd-3 (0.75 mol%), and atmospheric H2 in 1 mL H2O at 60 °C.b Isolated yield. | ||||
1 | 4-MeOOCC6H4NO2 (1a) | 4-MeCOOC6H4NH2 (2a) | 4 | 87 |
2 | 4-COOHC6H4NO2 (1b) | 4-COOHC6H4NH2 (2b) | 18 | 79 |
3 | 4-CH3OCC6H4NO2 (1c) | 4-CH3OCC6H4NH2 (2c) | 12 | 91 |
4 | 4-CNC6H4NO2 (1d) | 4-CNC6H4NH2 (2d) | 18 | 89 |
5 | 4-ClC6H4NO2 (1e) | 4-ClC6H4NH2 (2e) | 3 | 69/87 |
6 | C6H5NO2 (1f) | C6H5NH2 (2f) | 4 | 98 |
7 | 4-CH3OC6H4NO2 (1g) | 4-CH3OC6H4NO2 (2g) | 4 | 97 |
8 | 4-OHC6H4NO2 (1h) | 4-OHC6H4NH2 (2h) | 24 | 95 |
9 | 2-COOHC6H4NO2 (1i) | 2-COOHC6H4NH2 (2i) | 18 | 90 |
10 | 3-OHC6H4NO2 (1j) | 3-OHC6H4NH2 (2j) | 24 | 99 |
11 | 3-NO2C6H4NO2 (1k) | 3-NH2C6H4NH2 (2k) | 15 | 98 |
12 | 8-Nitroquinoline (1l) | 8-Aminoquinoline (2l) | 4 | 99 |
Reusability is one of the most important features for a heterogeneous catalyst, which is superior to a homogenous one. First, to confirm that the reaction is indeed catalyzed by solid Pd-3 rather than by homogenous palladium species, we have carried out the leaching experiments. After the catalytic hydrogenation of p-nitrotoluene was carried out for 1 h under standard conditions, the Pd-3 was removed from the vessel by centrifugation with p-toluidine produced in 56% yield. No further reaction took place after removing the catalyst in 3 h, then the Pd-3 was put back into the mixture. As a result, the hydrogenation reaction is restarted, and the product aniline was obtained in 95% in 3 h. In addition, the ICP-OES was not detected the Pd nanoparticles in the reaction mixture. The potential recyclability of the catalyst Pd-3 was explored in the model hydrogenation of 4-nitrotoluene. The reaction was carried out under normal atmospheric pressure, and in water at 60 °C for 3 h. The completion of the reaction was monitored by TLC analysis. After completion of the reaction, the catalyst was filtered from the reaction and washed by EtOH for next run. This process was repeated for 10 cycles, giving all cycles in high yield without prolong the reaction time (Fig. 6a). After the reactions for 10 cycles, the Pd-3 was again examined by TEM and XPS. The particle size is a little bit increased to 4.1 nm, as shown in TEM image (Fig. 6b). The XPS spectra of recycled Pd-3 show no difference compare with the fresh Pd-3. These results illustrate that the Pd/PdO nanoparticle are stabilized in the current reduction systems.
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Fig. 6 (a) The yields for 10 cycles of the Pd-3 catalyzed 4-nitrotoluene hydrogenation reaction; (b) a TEM image for recovery of Pd-3 catalyst after 10 cycles hydrogenation of 4-nitrotoluene. |
Based on the above results, the reason for the high activity of Pd-3 catalyzed hydrogenation of nitroarenes is schematically shown in Scheme 1. Initially, the Pd-3 was dispersed in H2O, and the H2 and nitro group was absorbed on the surface of the Pd-3. Next, an H–H bond cleavage occurs in a rate-determining step to give the [Pd]–H species.51 Such species are responsible for the rapid reduction of nitroarenes into the corresponding anilines. It is proposed that the high catalytic activity of Pd-3 for hydrogenation of nitroarenes is due to the synergistic effect between Pd and PdO crystallographic plane.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06900b |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2016 |