Qingjie
Tang
,
Ziliang
Yuan
,
Shiwei
Jin
*,
Kaiyue
Yao
,
Hanmin
Yang
*,
Quan
Chi
and
Bing
Liu
*
Key Laboratory of Catalysis and Materials Sciences of the Ministry of Education, South-Central University for Nationalities, Wuhan, 430074, People's Republic of China. E-mail: jinsw@mail.scuec.edu.cn; yhm20181011@163.com; liubing@mail.scuec.edu.cn; Fax: +86 27 67842572; Tel: +86 27 67842572
First published on 7th November 2019
There has been a great deal of attention to the development of heterogeneous non-noble metal catalysts for the selective and mild hydrogenation of nitro compounds into primary amines. Herein, a biomass-derived carbon material supported Ni catalyst (Ni/C) was facilely prepared by a one-pot pyrolysis process, and the as-prepared Ni/C catalyst demonstrated a high catalytic activity for the hydrogenation of nitro compounds into primary amines at room temperature. The Ni/C catalyst not only demonstrated a high catalytic activity but also showed a good tolerance to other functional groups. Structurally diverse primary amines were achieved in yields from 92% to 99% within a few hours at room temperature under 5 bar H2. Furthermore, the Ni/C catalyst showed good reusability without the loss of its activity.
In recent years, both H2 and other hydrogen donors such as isopropanol and formic acid have been extensively studied for the hydrogenation of nitro compounds into primary amines over different kinds of catalysts.9,10 H2 has been considered as an economic and sustainable reducing agent with only the release of water. Catalytic hydrogenation of nitro compounds into primary amines with H2 has been performed by either homogeneous or heterogeneous catalytic systems. The use of homogenous catalytic systems generally demonstrated high catalytic activity for the hydrogenation of nitro compounds because of the flexible contact of the active sites with the substrates.11 However, it is difficult to recycle the homogeneous catalysts. Heterogeneous catalytic systems can overcome the drawbacks of homogeneous catalytic systems and thus have attracted much more interest for the development of effective heterogeneous catalytic systems for the hydrogenation of nitro compounds into primary amines.
Heterogeneous noble metal catalysts such as supported Pd, Au, and Ru have been early used in the hydrogenation of nitroarenes to synthesize functionalized anilines due to their extraordinary activities.12,13 However, the high cost of noble metal catalysts greatly limits their practical applications. In addition, it is a great challenge in the chemoselective reduction of nitroarenes bearing reducible groups (C–Br, CO, CC, and so on).14
In recent years, many non-noble metal catalysts, mainly including Co, Ni and Fe, have been studied for the hydrogenation of nitro compounds into primary amines.15–18 Of various inexpensive catalysts, supported Ni catalysts have attracted substantial interest for catalytic hydrogenation reactions due to the excellent catalytic performance of Ni nanoparticles.19,20 Among various kinds of supported Ni catalysts, carbon material supported Ni catalysts have been widely studied because carbon materials demonstrated some excellent merits such as high surface area, unique electronic properties, and high chemical, thermal, and mechanical stability.21 However, the synthesis of most carbon-based catalysts typically forgoes environmentally benign approaches in favor of complex and expensive procedures that include acid oxidation, washing, impregnation, and reduction. The high cost of many novel carbon nanomaterials (such as graphene and carbon nanotubes) makes their large-scale use less practical. From the viewpoints of green and sustainable chemistry, it is highly desirable to prepare Ni/C catalysts with high catalytic activity for the hydrogenation of nitro compounds into primary amines via facile and green strategies from renewable resources.
Biomass is an abundant renewable resource which has attracted great interest for the preparation of various kinds of carbon-based materials.22,23 Among various kinds of biomass, cotton is mainly composed of cellulose fibers with abundance in nature and a low cost, which can serve as a good candidate for the preparation of carbon materials.24 Herein, one kind of Ni/C catalyst was successfully prepared via the one-pot pyrolysis of a cotton–Ni(NO3)2 composite and used for the hydrogenation of nitro compounds towards the chemoselective synthesis of structurally diverse amines.
The XRD patterns of the as-obtained Ni/C catalyst are shown in Fig. 1. Three peaks at 44.4°, 51.9° and 76.5° were clearly observed in the XRD patterns of the Ni/C catalyst, which corresponded to crystalline facets of Ni (111), Ni (200), and Ni (220), respectively.20 From Fig. 1, we can see that the XRD pattern of the catalyst after 5 reaction cycles is almost the same, indicating that our catalyst has good stability. Therefore, we can conclude that the Ni2+ absorbed on cotton has been successfully reduced to metallic Ni by H2 at 400 °C. Generally, the XRD peak at 26.0° was assigned to graphitic carbon. The disappearance of this peak in the XRD pattern of our prepared Ni/C catalyst suggested that the carbon in the Ni/C catalyst might be amorphous.
The TEM image of the as-prepared Ni/C catalyst is shown in Fig. 2. It clearly shows that Ni nanoparticles were homogeneously distributed on the surface of the carbon materials, and no significant aggregation of Ni nanoparticles was observed before and after the reaction. These results revealed that Ni nanoparticles would have a strong interaction with the carbon materials, which was unlike the traditional impregnation method. The particle size distribution of Ni nanoparticles in Fig. 2 clearly indicated that nickel nanoparticles have a narrow size distribution, and the average size of nickel nanoparticles was calculated to be 9.5 nm. Moreover, the high-resolution TEM (HRTEM, Fig. 2c) image revealed that the lattice fringe of the Ni nanoparticles had an interplanar spacing of 0.205 nm, which is ascribed to the (111) plane of metallic Ni nanoparticles.
Fig. 2 TEM image (a), particle size distribution image (b), (c) HRTEM image of the Ni/C catalyst and (d) TEM image after reaction. |
XPS was used to examine the oxidation states of elements on the surface of the Ni/C catalyst. The XPS survey spectrum confirms the presence of mainly Ni, C, N, and O in the catalyst (Fig. 3). As shown in Fig. 3, the C 1s XPS spectra of the Ni/C catalyst show one main peak at 284.6 eV, corresponding to sp2-hybridized graphitic carbon (CC). In addition, two other weak peaks at the binding energies of 286.1 eV and 288.9 eV should be attributed to C–O and O–CO species, respectively.25 As shown in Fig. 3, the two fitted peaks at ∼852.7 and ∼869.6 eV were assigned to the binding energies of metallic Ni 2p3/2 and Ni 2p1/2, respectively. The fitted Ni 2p3/2 peak at 855.6 and the Ni 2p1/2 peak at 873.3 eV can be assigned to the divalent valence state Ni2+,20 which was due to the surface oxidation of metallic Ni nanoparticles during storage in the air. Obviously, most of the nickel nanoparticles were present in their oxidized state as detected by XPS, which revealed that the metallic nickel on the surface of nickel nanoparticles was majorly oxidized into its oxidized state either by the treatment with 1% O2 in the nitrogen atmosphere or by the air during the storage.
Fig. 3 XPS spectra of the Ni/C catalyst. (a) The survey spectrum; (b) the C 1s XPS spectrum; (c) the Ni 2p XPS spectrum. |
N2 adsorption–desorption of the Ni/C catalyst shows an H1 type hysteresis loop and a type IV isotherm (Fig. 4a), which is a characteristic isotherm for mesoporous materials. However, the sorption isotherm does not level out in a plateau at relative pressures p/p0 > 0.9, indicating the existence of macropores. The pore size distribution in Fig. 4b also shows that three types of macropores, mesopores and micropores were present in the Ni/C catalyst. In addition, all of the macropores, mesopores and micropores had a large size distribution. The total surface area of the Ni/C catalyst was determined to be 41.8 m2 g−1, and the total pore volume was calculated to be 0.09 cm3 g−1.
Entry | Solvent | Con. (%) | Selectivity (%) | |
---|---|---|---|---|
1 | 2 | |||
a Reaction conditions: nitrobenzene (1 mmol), Ni/C catalyst (20 mg), solvent (10 mL), H2 (10 bar), 40 °C and 4 h. | ||||
1 | Hexane | 29.2 | 71.2 | 28.8 |
2 | Dichloromethane | 26.5 | 100 | 0 |
3 | THF | 50.0 | 70.7 | 29.3 |
4 | iso-PrOH | 53.9 | 74.2 | 25.8 |
5 | Acetonitrile | 64.5 | 100 | 0 |
6 | EtOH | 70.0 | 88.7 | 11.3 |
7 | MeOH | 90.2 | 82.4 | 17.6 |
8 | H2O | 100 | 100 | 0 |
The best results were achieved in water, which produced the quantitative conversion of nitrobenzene into aniline at 40 °C after 4 h (Table 1, entry 8). The use of water as the reaction solvent is much more preferable because water is a green solvent at a low cost without any toxicity. To the best of our knowledge, there has been no report on the use of supported Ni catalysts for the hydrogenation of nitro compounds under such mild conditions. In recent years, single metal atom catalysts have aroused great interest for chemical reactions because of their high activity, but the recently reported single Ni atom catalyst for the hydrogenation of nitro compounds was carried out at 120 °C and 3 MPa H2 pressure.6
The effect of the reaction temperature on the hydrogenation of nitrobenzene was studied over the Ni/C catalyst, and the results are shown in Fig. 5. It was found that the Ni/C catalyst demonstrated excellent catalytic activity even at room temperature, and a moderate nitrobenzene conversion of 54.4% was achieved at room temperature after 2 h. The hydrogenation of nitrobenzene was sensitive to the reaction temperature. Nitrobenzene conversion greatly increased to 78.6% by the increase of the reaction temperature to 40 °C, and a full nitrobenzene conversion was achieved at 60 °C after 2 h. The selectivity of the intermediate phenylhydroxylamine decreased from 55.6% at 25 °C to 12.2% at 40 °C, and it was not observed at 60 °C. Generally, the intermediates of the hydrogenation of nitro compounds were not stable and could not be detected by gas chromatography at high reaction temperatures. Therefore, few literature studies have reported on the intermediates of the hydrogenation of nitro compounds over supported metal catalysts at high reaction temperatures.
Fig. 5 The effect of the reaction temperature on the hydrogenation of nitrobenzene. Reaction conditions: nitrobenzene (1 mmol), Ni/C catalyst (20 mg), water (10 mL), H2 (10 bar), 25–60 °C and 2 h. |
Then the effect of the hydrogen pressure on the hydrogenation of nitrobenzene over the Ni/C catalyst was studied at room temperature, and the results are shown in Fig. 6. The conversion of nitrobenzene was observed to increase with an increase of hydrogen pressure. For example, the conversion of nitrobenzene was 41.5% under 2.5 bar H2 pressure, and then it increased to 76.3% under 5 bar H2 pressure. On further increasing the hydrogen pressure to 10 bar, the nitrobenzene conversion continuously increased to 90.0%. The increase of nitrobenzene conversion with an increase of hydrogen pressure should be caused by the increase of hydrogen concentration in the reaction solution with an increase of hydrogen pressure. Aniline and the intermediate phenylhydroxylamine were produced at 25 °C under different hydrogen pressure, and these results again suggested that the stability of the phenylhydroxylamine intermediate was sensitive to the reaction temperature, not hydrogen pressure.
Fig. 6 The effect of hydrogen pressure on the hydrogenation of nitrobenzene. Reaction conditions: nitrobenzene (1 mmol), Ni/C catalyst (20 mg), water (10 mL), H2 (2.5–10 bar), 25 °C and 4 h. |
The time course of the product distribution was recorded for the hydrogenation of nitrobenzene at 25 °C and 5 bar H2 over the Ni/C catalyst (Fig. 7). The conversion of nitrobenzene gradually increased during the reaction process, and its content greatly decreased at an early reaction stage, which was due to the high concentration of nitrobenzene at the early reaction stage. The yield of aniline also gradually increased during the reaction process, and it reached 100% after 7 h at 5 bar H2 pressure and 25 °C. The content of phenylhydroxylamine first increased to 40.8% from the beginning to 5 h and then it decreased from 40.8% at 5 h to zero after 7 h. These results suggested that the hydrogenation of nitrobenzene into phenylhydroxylamine was a slow step. Generally, two reaction pathways are accepted for the hydrogenation of nitrobenzene into aniline.26 One way involves phenylhydroxylamine as the intermediate, which is called the direct pathway (Scheme 2). The other way proceeds via the azoxybenzene intermediate. Clearly, the hydrogenation of nitrobenzene into aniline proceeds via the direct way over the Ni/C catalyst (Fig. 8).
Fig. 7 Time course of the product distribution on the hydrogenation of nitrobenzene. Reaction conditions: nitrobenzene (1 mmol), Ni/C catalyst (20 mg), water (10 mL), H2 (5 bar), 25 °C. |
Fig. 8 Catalyst recycling experiments of the Ni/C catalyst. Reaction conditions: nitrobenzene (1 mmol), Ni/C catalyst (20 mg), water (10 mL), H2 (5 bar), 25 °C, 6 h. |
Entry | Structure | Time (h) | Con. (%) | Yield (%) |
---|---|---|---|---|
a Reaction conditions: nitrobenzene (1 mmol), Ni/C catalyst (20 mg), water (10 mL), H2 (5 bar), 25 °C. b Reaction conditions: solvent (water 7 ml, methanol 3 ml). | ||||
1a | 7 | 100 | 100 | |
2a | 8.5 | 100 | 99 | |
3a | 7 | 100 | 97 | |
4a | 6.5 | 100 | 94 | |
5b | 5.5 | 100 | 96 | |
6b | 6.5 | 100 | 99 | |
7a | 7.5 | 100 | 99 | |
8b | 7.5 | 100 | 98 | |
9b | 8 | 100 | 92 | |
10b | 7.5 | 98 | 96 | |
11b | 7.5 | 98.6 | 97 | |
12a | 6 | 100 | 99 |
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