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Original article
13 (
4
); 4916-4925
doi:
10.1016/j.arabjc.2020.01.011

High performance of nitrogen-doped carbon-supported cobalt catalyst for the mild and selective synthesis of primary amines

, , , ,
 Key Laboratory of Catalysis and Materials Sciences of the Ministry of Education, South-Central University for Nationalities, Wuhan 430074, PR China

⁎Corresponding authors. Jinsw@mail.scuec.edu.cn (Shiwei Jin), zehuizh@mail.ustc.edu.cn (Zehui Zhang)

Received: , Accepted: ,
© The Author(s) 2020
Disclaimer:
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

A nitrogen-doped carbon-supported Co catalyst (Co/N-C-800) was discovered to be highly active for the reductive amination of carbonyl compounds with NH3 and the hydrogenation of nitriles into primary amines using H2 as the hydrogen source. Structurally diverse carbonyl compounds were selectively transformed into primary amines with good to excellent yields (82.8–99.6%) under mild conditions. The Co/N-C-800 catalyst showed comparable or better catalytic performance than the reported noble metal catalysts. The Co/N-C-800 catalyst also showed high activity for the hydrogenation of nitriles, affording the corresponding primary amines with high yields (81.7–99.0%). An overall reaction mechanism is proposed for the reductive amination of benzaldehyde and the hydrogenation of benzonitrile, which involves the same intermediates of phenylmethanimine and N-benzylidenebenzylamine.

Keywords

Nitrogen-doped carbon
Co catalysts
Primary amines
Reductive amination
Hydrogenation
Nitriles
1

1 Introduction

Primary amines are of significant importance for the synthesis of pharmaceuticals and agrichemicals, and also building blocks for polymers and dyes (Devi et al., 2016; Kathuria and Dhamiwal, 2016; Morofuji et al., 2013). Several approaches have been used for the synthesis of primary amines, mainly including reduction of amides or nitriles (Bornschein et al., 2014; Mukherjee et al., 2015; Das et al., 2012), amination of aryl halides (Vo and Hartwig, 2009; Green and Hartwig, 2015), reductive amination of carbonyl compounds (Talwar et al., 2014; Dangerfield et al., 2010), and direct amination of alcohols (Mutti et al., 2015; Pronin et al., 2013). The amination of aryl halides exhibited low atomic efficiency by the release of some halides wastes. Although the direct amination of alcohols or the reduction of amides or nitriles could produce amines without the release of hard-disposal wastes, these methods generally performed under harsh reaction conditions with relatively low selectivity of primary amines. Compared with other methods, the reductive amination of carbonyl compounds with ammonia has been considered to be a preferred method for the synthesis of primary amines because of the low cost of starting materials and the mild and green reaction conditions (Enthaler, 2010; van der Vlugt, 2010).

To date, many works have been reported for the reductive amination of carbonyl compounds with NH3 and H2 both in homogeneous and heterogeneous catalytic systems. For example, Chusov and List (2014), Huang et al. (2016), Jagadeesh et al. (2015), Hahn et al. (2019) and so on. However, The homogeneous catalytic systems (Gallardo-Donaire et al., 2016; Li et al., 2019; Cui et al., 2019) demonstrated several drawbacks such as the difficulty in catalyst recycling, the use of additives and ligands, and harsh reaction conditions (40–65 bar H2 and 120–135 °C) (Scheme 1). Compared with the homogeneous catalysts, the heterogeneous catalysts can be recycled and reused (Scheme 1). But the reported heterogeneous noble-metal catalytic systems also demonstrated some drawbacks including high price of precious catalysts, high H2 pressure (Formenti et al., 2019; Senthamarai et al., 2018; Chandra et al., 2018; Robertson and Personick, 2019), limited substrate scope, low selectivity for primary amines (Leung et al., 2018), and instability of the catalysts (Kwon et al., 2019; Gomez et al., 2004) (Scheme 1). Thus, it is highly desirable to develop heterogeneous non-noble-metal catalysts with high activity and selectivity for the cost-effective synthesis of primary amines via reductive amination of carbonyl compounds with NH3 and H2.

Synthesis of primary amines via reductive amination with NH3 and H2.
Scheme 1 Synthesis of primary amines via reductive amination with NH3 and H2.

Currently, nitrogen-doped carbon-supported metal catalysts have gained extensive attention for advanced catalysis (Bjelic et al., 2018; Bose et al., 2018; Wu et al., 2014; Kang et al., 2017; Bjelić et al., 2019; Bjelić et al., 2018; Bjelić et al., 2018;). The incorporation of nitrogen in the carbon architecture leads to a better particle dispersity, and a higher catalyst activity and stability (Westerhaus et al., 2013). In our previous work (Zhou et al., 2017), the nitrogen doped carbon material supported cobalt catalysts (Co/N-C-T, in which T represents the pyrolysis temperature), which was prepared from the one-pot pyrolysis of ZIF-67 (ZIF: zeolitic imidazolate framework), has been discovered to be active for the reduction of nitro compounds into primary amines and the subsequent one-pot reductive amination for the synthesis of secondary amines with CO/H2O as the hydrogen donor via water-gas-shift reaction. In the continuation of the interest in the synthesis of useful amines, the Co/N-C-T catalysts were studied for the synthesis of primary amines via the reductive amination of carbonyl compounds with NH3 and H2 for the first time (Scheme 1). Interestingly, benzonitrile was observed to be one of the intermediates for the reductive amination of benzyaldehyde. Thus, the hydrogenation of nitriles was also performed over the nitrogen-doped carbon supported-Co catalyst. To the best of our knowledge, there have been no other heterogeneous non-noble-metal catalysts reported for the simultaneous reductive amination of carbonyl compounds and the hydrogenation of nitriles.

2

2 Experimental section

2.1

2.1 Synthesis of the Co/N-C-800 catalyst

The synthesis of the catalyst Co/N-C-T was following our previously reported processor (Zhou et al., 2017). Typically, Co(NO3)2·6H2O (99%, 0.45 g) in 3 mL of water was added to a solution of 2-methylimidazole (99%, 5.5 g) in 20 mL of water. Then the mixture was stirred at 25 °C for 6 h. The resulting purple precipitates were obtained via centrifugation and washed with water and methanol for twice, respectively, and finally dried in a vacuum oven to obtain ZIF-67. ZIF-67 (2.0 g) was pyrolyzed at 800 °C for 8 h in a nitrogen atmosphere at a heating rate of 3 °C/min from room temperature to obtain the Co/N-C-T.

2.2

2.2 Reductive amination of carbonyl compounds

Typically, 50 mL autoclave equipped with a magnetic stirrer and a temperature controller was charged with tetrahydrofuran (THF, 10 mL), 28 wt% aq·NH3 solution (2 mL), benzaldehyde (1 mmol), and the Co/N-C-800 catalyst (20 mg). Then the autoclave was purged with H2 for five times to completely remove the air. After being sealed, the autoclave was charged with 5 bar H2 at room temperature. Then the autoclave was heated from room temperature to 110 °C within 5 min and the reaction was performed at 110 °C for 4 h with a magnetic stirring at 1000 rpm. Afterwards, the autoclave was cooled to room temperature and depressurized. The reaction mixture was diluted with THF and analyzed by gas chromatography (GC) using nitrobenzene as the internal standard. We have ensure each reaction was performed under identical conditions including the use of the same autoclave, the same speeding rate and the same magnetic bar to exclude the mass transport constrains. We have stated it in the experimental section in the revised manuscript.

2.3

2.3 Hydrogenation of nitriles

In a typical run, 50 mL autoclave was charged with THF (10 mL), 28 wt% aq·NH3 solution (0.2 mL), benzonitrile (1 mmol), and the Co/N-C-800 catalyst (20 mg). Then the autoclave was purged with H2 for five times to completely remove the air. After being sealed, the autoclave was charged with 10 bar H2 at room temperature. Then the autoclave was heated from room temperature to 110 °C within 5 min and the reaction was performed at 110 °C for 5 h with a magnetic stirring at 1000 rpm. Afterwards, the autoclave was cooled to room temperature, and depressurized. The reaction mixture was diluted with THF and analyzed by GC using nitrobenzene as the internal standard.

2.4

2.4 Analytic methods

Products analysis was performed on Agilent 7890A GC with autosampler and a flame ionization detector. The products were separated by a HP-5 capillary column (30 m × 530 μm × 1.5 μm). The temperature of the column was initially kept at 80 °C for 3 min, and then increased at a rate of 20 °C min−1 to 220 °C. Products were identified by the comparison of the retention time with the authentic chemicals, and further confirmed by GC–MS (Agilent 7890A GC/5973 MS, HP-5 column). The amounts of products were determined based on GC data using the internal standard method.

3

3 Results and discussion

3.1

3.1 Reductive animation of benzaldehyde with different catalysts

In our previous work, three kinds of the Co/N-C-T catalysts with the pyrolysis temperatures at 600, 800 and 900 have been studied for the reduction of nitro compounds with CO/H2O as the hydrogen donor (Zhou et al., 2017). To extend the scope of the as-prepared catalysts, the reductive animation of benzaldehyde with NH3/H2 was studied over the Co/N-C-T catalysts. Similarly, the catalytic activity of the Co/N-C-T catalysts towards the synthesis of benzylamine from the reductive animation of benzaldehyde also decreased in an order of Co/N-C-600, Co/N-C-800 and Co/N-C-900 (Table 1, Entries 1–3). The Co/N-C-T catalysts with a larger size of Co nanoparticles provided less active sites to contact with the substrates at the same amount of cobalt loading, thus demonstrating lower activity. Similar phenomenon was also observed by other researchers when using other types of metallic nanoparticles with different sizes for chemical reactions (Zhang et al., 2020). The Co/N-C-600 catalyst produced the highest benzaldehyde conversion of 96.1% and the highest benzylamine selectivity of 58.4% after 1 h at 90 °C and 5 bar H2. As discussed later, N-benzylidenebenzylamine (product B) and benzonitrile (product C) were the intermediates, while benzyl alcohol was the byproduct. Although the Co/N-C-600 produced the highest conversion, it also generated the highest amount of the byproduct of benzyl alcohol (Table 1). Taking the activity and the total selectivity of benzylamine and the intermediates into consideration, the Co/N-C-800 catalyst should have the potential to attain the highest yield of benzylamine.

Table 1 Catalyst screening for the reductive animation of benzaldehyde.a
Entry Catalyst Con. (%) Sel. A (%) Sel. B (%) Sel. C (%) Sel. D (%)
1 Co/N-C-600 96.1 58.4 26.8 0 12.9
2 Co/N-C-800 90.8 48.9 44.1 1.3 4.5
3 Co/N-C-900 88.1 18.0 76.0 1.4 4.3
a Reaction conditions: Benzaldehyde (1 mmol), THF (8 mL), 26.5 wt% NH3·H2O (2 mL, 28 mmol), H2 5 bar, Co/N-C-T (20 mg, 0.13 mmol Co), 90 °C, 1 h. the selectivity and conversion was calculated according to the GC results; Con. = reacted benzaldehyde/initial benzaldehyde; Sel. B = (2 × mmol of product B/1 mmol)/reacted benzaldehyde*100%; Sel. (A, C, D) = mmol of product (A, C, D)/reacted benzaldehyde*100%.

3.2

3.2 Optimization of the reaction conditions

The reaction conditions were then optimized for the synthesis of benzylamine from the reductive animation of benzaldehyde over the Co/N-C-800 catalyst. Firstly, the reductive amination was performed at different temperatures (Fig. 1). The Co/N-C-800 catalyst was even active at room temperature, affording 36.8% conversion and 93.8% selectivity of the intermediate N-benzylidenebenzylamine after 4 h. The conversion greatly increased to 95.9% at 70 °C, and full conversion was achieved at 110–150 °C after 4 h. As shown in Fig. 1, the increase of the reaction temperature facilitated the transformation of the intermediate to benzylamine, but a high reaction temperature also benefitted the hydrogenation of benzaldehyde into benzyl alcohol (Song et al., 2018). The optimal reaction temperature was 110 °C, which produced full conversion and 91.8% yield of benzylamine after 4 h at 5 bar H2 with 28 mmol of NH3. It should be noted that the Co/N-C-800 catalyst demonstrated higher catalytic activity than the MOF-derived cobalt nanoparticles for the same type reaction, which was performed at 120 °C under 40 bar H2 pressure (Jagadeesh et al., 2017).

The results of the reductive amination of benzylaldehyde at different temperatures. Reaction conditions: THF (8 mL), 26.5. wt.% NH3·H2O (2 mL, 28 mmol NH3), H2 5 bar, Co/N-C-800 (20 mg, 0.13 mmol Co), 4 h. A = benzylamine; B = N-benzylidenebenzylamine; C = Benzyl alcohol.
Fig. 1 The results of the reductive amination of benzylaldehyde at different temperatures. Reaction conditions: THF (8 mL), 26.5. wt.% NH3·H2O (2 mL, 28 mmol NH3), H2 5 bar, Co/N-C-800 (20 mg, 0.13 mmol Co), 4 h. A = benzylamine; B = N-benzylidenebenzylamine; C = Benzyl alcohol.

In addition, both H2 pressure and the amount of NH3 showed a great effect on the selectivity of the products, and a little effect on the benzaldehyde conversion except Entry 1 (Table 2). The lowest selectivity of benzylamine and the highest selectivity of N-benzylidenebenzylamine were produced at the lowest H2 pressure of 1 bar (Table 2, Entries 1 vs 2–3) or the lowest amount of NH3 (7 mmol, Table 2, Entries 6 vs 3–5). These results indicated that benzylamine was formed via the reaction of the intermediate N-benzylidenebenzylamine with H2 and NH3 (Chandra et al., 2018), which was discussed later. Interestingly, benzonitrile as one of the intermediates (discussed later) was only detected at 1 bar H2 with 28 mmol of NH3 (Table 2, Entry 1), indicating that the low H2 pressure lowered the reaction rate. While dibenzylamine was detected with 7 or 14 mmol of NH3 at 5 bar H2 (Table 2, Entries 5 & 6), suggesting that a high amount of NH3 inhibited the hydrogenation of N-benzylidenebenzylamine to produce dibenzylamine as the byproduct (discussed later).

Table 2 The reductive animation of benzaldehyde under different conditionsa.
Entry H2 (bar) NH3 (mmol) Con. (%) Selectivity (%)
A B C D E
1 1 28 83.4 16.3 66.2 0 3.4 9.2
2 2.5 28 97.1 88.3 5.6 0 5.4 0
3 5 28 100 92.0 0 0 5.7 0
4 5 21 100 83.6 4.5 0 6.2 0
5 5 14 96.4 64.8 23.3 3.0 5.9 0
6 5 7 95.1 39.4 43.3 7.1 6.2 0
a Reaction conditions: 110 °C, 4 h, THF (8 mL), Co/N-C-800 (20 mg, 0.13 mmol Co) and a certain amount of 26. 5 wt% NH3·H2O. the selectivity and conversion was calculated according to the GC results; con. = reacted benzaldehyde/initial benzaldehyde; Sel. B = (2 × mmol of product B/1 mmol)/reacted benzaldehyde*100%; Sel. (A, C, D, E) = mmol of product (A, C, D, E)/reacted benzaldehyde*100%.

3.3

3.3 Substrate scope of the reductive amination

Next, the substrate scope of the reduction amination was studied. The reactions were performed under the optimal reaction conditions as described above for the reductive amination of benzaldehyde. Substituted aromatic aldehydes with electron-withdrawing or electron-donating groups were smoothly transformed into the corresponding amines with excellent yields after 4 h at 110 °C and 5 bar H2 (89.1–95.1%, Table 3, Entries 1–7). 2-Naphthaldehyde as a representative fused-ring aromatic aldehyde was also successfully converted into 2-aminonaphthalene with a high yield of 88.6% (Table 2, Entry 8). The reductive amination of the heterocyclic substrate of furfural produced furfurylamine with a high yield of 90.2% (Table 3, Entry 9). The Co/N-C-800 catalyst showed comparable activity to the noble Rh/Al2O3 catalyst (Table 3, Entries 9 vs 10), and better catalytic performance than the noble Ru/Al2O3 (Robertson and Personick, 2019), Pd/C (Robertson and Personick, 2019) and Ru/PVP/HAP (Zhong et al., 2015) catalysts (Table 3, Entries 9 vs 11–13). The Co/N-C-800 catalyst was also effective for the reductive amination of aliphatic and cyclic aldehydes into amines (Table 3, Entries 14–17). Of particular note is that the reductive amination of citronellal gave 91.0% yield of the corresponding amine without the reduction of C⚌C bond (Table 3, Entry 16). Under the same reaction conditions for the reductive amination of aldehydes, the reductive amination of cyclohexanone gave low conversion of 18.7% (Table 3, Entry 18), suggesting ketones were much less active. To our delight, the representative ketones including aromatic, aliphatic, cyclic and even steric hindrance ones could also be successfully transformed to the corresponding amines under more demanding reaction conditions (130 °C and 20 bar H2) for a certain reaction time (Table 3, Entries 19–22).

Table 3 The results of the reductive amination of various carbonyl compoundsa.
Entry Substrate T. (°C) H2 (bar) Con. (%) Yield (%)
1 110 5 100 92.0
2 110 5 100 92.7
3 110 5 100 93.4
4 110 5 98.9 89.1
5 110 5 100 94.9
6 110 5 100 95.1
7 110 5 100 92.1
8 110 5 100 88.6
9 110 5 98.9 90.2
10b1 80 20 100 91.5
11c 80 30 100 75
12b2 80 20 100 10.6
13d 100 4 100 60
14 110 5 100 90.0
15 110 5 100 88.2
16 110 5 100 91.0
17 110 5 100 99.6
18 110 5 18.7 18.0
19e 130 20 98.1 95.5
20e 130 20 100 91.1
21f 130 20 100 98.8
22f 130 20 100 82.8
a Reaction conditions: Substrate (1 mmol), THF (8 mL), Co/N-C-800 (20 mg, 0.13 mmol Co), 26.5 wt% NH3·H2O (2 mL, 28 mmol), 4 h.
b The b1Rh/Al2O3 and b2Pd/C catalyst used (Robertson and Personick, 2019).
c The Ru/γ-Al2O3 catalyst was used, (Robertson and Personick, 2019).
d The Ru/PVP/HAP catalyst was used, (Zhong et al., 2015).
e The reaction time was 12 h.
f The reaction time was 18 h.

3.4

3.4 Catalytic hydrogenation of nitriles

Besides the reductive amination of carbonyl compounds, the hydrogenation of nitriles is also an attractive method for the synthesis of primary amines. As listed in Table 4 (Entry 1) and Fig. 2(a), benzonitrile was one of the intermediates for the reductive amination of benzylaldehyde, suggesting that the Co/N-C-800 catalyst also had the ability to catalyse the hydrogenation of nitriles into primary amines.

Table 4 The results of the hydrogenation of benzonitrile under different conditions.a
Entry Solvent H2 (bar) T (°C) NH3 (mmol) C. (%) Selectivity (%)
A B C
1 Water 5 150 – 100 46.7 – 11.0
2 EtOH 5 150 – 100 33.3 8.8 29.2
3 THF 5 150 – 100 58.2 22.8 18.8
4 THF 5 110 – 15.8 63.9 35.4 0.5
5 THF 10 110 – 38.4 66.5 30.4 0.9
6 THF 10 110 1.4 70.8 70.7 28.1 0.8
7 THF 10 110 2.8 100 84.4 14.0 0.7
8b THF 10 110 2.8 100 98.0 – 0.9
a Reaction conditions: benzonitrile (1 mmol), solvent (10 mL), Co/N-C-800 (20 mg), 4 h.
b The reaction time was 5 h. the selectivity and conversion was calculated according to the GC results; con. = reacted benzonitrile/initial benzonitrile; Sel. B = (2 × mmol of product B/1 mmol)/reacted benzonitrile *100%; Sel. (A, C) = mmol of product (A, C)/reacted benzonitrile *100%.
Time course of products distribution. (a) The reductive amination of benzaldehyde. Reaction conditions: Benzaldehyde (1 mmol), THF (8 mL), Co/N-C-800 (20 mg), 110 °C, 5 bar H2, 28. wt.% NH3·H2O (2 mL); (b) The hydrogenation of benzonitrile. Reaction conditions: benzonitrile (1 mmol), THF (10 mL), Co/N-C-800 (20 mg, 0.13 mmol Co), 110 °C, 10 bar, 26.5 wt% NH3·H2O (0.2 mL, 2.8 mmol NH3).
Fig. 2 Time course of products distribution. (a) The reductive amination of benzaldehyde. Reaction conditions: Benzaldehyde (1 mmol), THF (8 mL), Co/N-C-800 (20 mg), 110 °C, 5 bar H2, 28. wt.% NH3·H2O (2 mL); (b) The hydrogenation of benzonitrile. Reaction conditions: benzonitrile (1 mmol), THF (10 mL), Co/N-C-800 (20 mg, 0.13 mmol Co), 110 °C, 10 bar, 26.5 wt% NH3·H2O (0.2 mL, 2.8 mmol NH3).

Firstly, the hydrogenation of benzonitrile was carried out in green solvents of water and ethanol at 150 °C and 5 bar H2 (Table 4, Entries 1 & 2). Although full conversion was obtained in water, the selectivity of benzylamine was only 46.7%, dibenzylamine (Product C) and benzyl alcohol were the byproducts with the selectivity of 11.0 and 42.0%, respectively. The formation of benzyl alcohol revealed that water participated the reaction (Molnar et al., 2017), as the oxygen in benzyl alcohol should come from water, which was the only oxygen source in the reaction system. A low benzylamine selectivity of 33.3% was also observed in ethanol at full conversion (Table 4, Entry 2). Besides the intermediate N-benzylidenebenzylamine (Product B) and dibenzylamine, N-ethylbenzylamine was also produced with a selectivity of 24.3% by the reaction of benzylamine with ethanol (Yang et al., 2016). Then, the hydrogenation of benzonitrile was performed in THF, as it was a good solvent for the reductive amination. The highest selectivity of benzylamine was obtained in 58.2% at a full conversion at 150 °C (Table 4, Entries 3 vs 1, 2), but dibenzylamine as the byproduct was still present with a selectivity of 18.8%. The formation of dibenzylamine was greatly inhibited by decreasing the reaction temperature to 110 °C at the expense of lowering the catalyst activity with a conversion of 15.8% at 5 bar H2 after 4 h (Table 4, Entry 4). Benzonitrile conversion greatly increased to 38.4% at 10 bar H2 with similar products distribution (Table 4, Entries 5 vs 4). As listed in Table 2, the increase of NH3 amount promoted the transformation of the intermediate N-benzylidenebenzylamine into benzylamine. As expected, the conversion further increased to 70.8% with 0.1 mL of 26.5 wt% NH3·H2O (1.4 mmol of NH3) (Table 4, Entries 6 vs 5). Further increasing the amount of NH3 to 2.8 mmol, full conversion and 84.4% selectivity of benzylamine were produced after 4 h (Table 4, Entry 7), and a high benzylamine yield of 98.0% was obtained after 5 h (Table 4, Entry 8).

Then the scope of the developed catalytic system was explored with structurally diverse nitriles. Full conversions were achieved with different aromatic nitriles at 110 °C and 10 bar (Table 5, Entries 1–5). Aromatic nitriles with electron-donating groups showed higher activity than those with electron-withdrawing groups, but the corresponding amines yields were a little lower (Table 5, Entries 2, 3 vs 4, 5). According to the previous work (Wang et al., 2020), H2 molecules underwent heterolysis to give the H+ and H- species. H+ should be combined with nitrogen atoms, while H- should combine on the surface of metallic cobalt nanoparticles. The H+ would move freely and the first step of the hydrogenation of nitrile was the addition of H+ to —C≡N according to our previous work (Zhou et al., 2019), and the substituted groups with electron-donating groups would enhance the electron density of the —C≡N bond, which benefited the hydrogenation of nitriles. Hetero-aromatic and fused-ring aromatic nitriles were also successfully hydrogenated into the corresponding amines with excellent yields (Table 5, Entries 6–7). Compared with aromatic nitriles, the non-aromatic nitriles were less active. For example, only 19% conversion of 2-phenylacetonitrile was observed under the reaction conditions for the hydrogenation of aromatic nitriles (Table 5, Entry 8). To our delight, the aliphatic nitriles could be smoothly hydrogenated to the amines under relative harsh conditions (150 °C, 40 bar H2; Table 5, Entries 9 & 10). In addition, the non-active cyclic nitrile and 1-adamantanecarbonitrile with large steric hindrance were also successfully converted to the amines with high yields (Table 5, Entries 11 & 12).

Table 5 Catalytic hydrogenation of nitriles over Co/N-C-800 catalyst.a
Entry Substrate NH3 (mmol) Time (h) Con. (%) Yield A (%) Yield C (%)
1 2.8 5 100 98.0 0.9
2 2.8 8 100 94.0 5.7
3 2.8 8 100 94.0 5.1
4 2.8 4 100 81.7 16.4
5 2.8 4 100 85.0 13.6
6 2.8 5 100 97.0 0.9
7 2.8 5 100 99.0 0
8 2.8 5 19.0 19.0 0
9b 5.6 14 100 86.2 13.4
10b 5.6 14 100 98.0 1.2
11b 5.6 14 100 94.0 5.2
12b 5.6 14 100 85.0 14.0
a Reaction conditions: Substrate (1 mmol), THF (10 mL), Co/N-C-800 (20 mg, 0.13 mmol Co), H2 10 bar, 110 °C.
b The reactions were performed at 40 bar H2 and 150 °C.

3.5

3.5 Mechanism study for the synthesis of primary amines

Fig. 2(a) shows the products distribution of the reductive amination of benzaldehyde. The intermediate N-benzylidenebenzylamine (Scheme 2, Compound A) should be formed from the condensation of benzylamine with benzaldehyde. Thus, it wonders how the initial benzylamine (Compound E) was produced. Benzylamine can be generated by the “direct way” (Xu et al., 2012), which involves the condensation of benzaldehyde with NH3 to generate phenylmethanimine (Compound C) and the subsequent hydrogenation (Scheme 2, Path A). However, phenylmethanimine was not detected due to its instability (Han et al., 2015). Benzonitrile was also observed as the other intermediate (Fig. 2a), and it should be produced via the dehydrogenation of phenylmethanimine, indirectly confirming phenylmethanimine was the intermediate of Path A. The initially formed benzylamine easily reacted with benzaldehyde to give N-benzylidenebenzylamine (Yang et al., 2013), which had the highest content at 40 min (Fig. 2a). As listed in Table 2, increasing the amount of NH3 promoted the transformation of N-benzylidenebenzylamine into benzylamine, while inhibited the side reaction of the hydrogenation of N-benzylidenebenzylamine to dibenzylamine (Compound D). The addition of one NH3 molecule to N-benzylidenebenzylamine gave rise to the instable geminal diamine (Compound C), which was then hydrogenated to benzylamine (Scheme 2, Path B) (Chandra et al., 2018). Besides the formation of dibenzylamine, the other competitive side reaction was the hydrogenation of benzaldehyde into benzyl alcohol.

Proposed mechanism for the synthesis of primary amines over the Co/N-C-800 catalyst.
Scheme 2 Proposed mechanism for the synthesis of primary amines over the Co/N-C-800 catalyst.

N-benzylidenebenzylamine was also identified to be the intermediate for the hydrogenation of benzonitrile (Fig. 2b). Path A was also suitable for the hydrogenation of benzonitrile, in which phenylmethanimine was generated by the hydrogenation of benzonitrile. Unlike the reductive amination, N-benzylidenebenzylamine was formed via the condensation of phenylmethanimine with benzylamine, and one molecule of NH3 was released (Scheme 2, Path C) (Blandez et al., 2014). Then, N-benzylidenebenzylamine underwent Path B to generate benzylamine. The selectivity of benzylamine from the hydrogenation of benzonitrile could be improved by the addition of ammonia (Table 2, Entries 4–6), as the increase of ammonia amount accelerated the transformation of N-benzylidenebenzylamine into benzylamine (Scheme 2, Path B). In addition, benzyl alcohol was produced in a yield of 4.5% (Table 1, Entry 2). Benzyl alcohol should be generated from the hydrogenation of benzaldehyde, which was formed by the hydrolysis of phenylmethanimine. To the end, an overall reaction pathway for the reductive amination of benzaldehyde and the hydrogenation of benzonitrile is depicted in Scheme 2. All the results suggested that the Co/N-C-800 catalyst was robust for several types of chemical reactions.

3.6

3.6 Recycle of the Co/N-C-800 catalyst

Finally, the stability of the Co/N-C-800 catalyst was investigated by the hydrogenation of benzonitrile at 110 °C and 10 bar H2. The Co/N-C-800 catalyst could be easily collected from the reaction mixture by an external magnet (Fig. 3). Then it was washed with water and ethanol, respectively. After drying, the spent catalyst was reused for the next run under the identical conditions. The catalytic activity of the Co/N-C-800 catalyst could be fully restored after seven times (Fig. 3). In each run, the conversion of benzonitrile was 100% and the selectivity of the product benzylamine was round 98%. The reaction solution was analyzed by ICP/AES, and the Co content in the reaction solution was below the detection limit, suggesting that there was no leaching of Co from the catalyst.

Recycling experiments of the Co/N-C-800 catalyst. Reaction conditions: benzonitrile (1 mmol), THF (10 mL), Co/N-C-800 (20 mg, 0.13 mmol Co), 110 °C, 10 bar H2, 26.5 wt% NH3·H2O (0. 2 mL, 2.8 mmol NH3).
Fig. 3 Recycling experiments of the Co/N-C-800 catalyst. Reaction conditions: benzonitrile (1 mmol), THF (10 mL), Co/N-C-800 (20 mg, 0.13 mmol Co), 110 °C, 10 bar H2, 26.5 wt% NH3·H2O (0. 2 mL, 2.8 mmol NH3).

4

4 Conclusions

To summarize: an unprecedented method was developed for the synthesis of primary amines via the reductive amination and the hydrogenation of nitriles over the non-noble Co/N-C-800 catalyst. The Co/N-C-800 catalyst was easily prepared by the one-step pyrolysis of ZIF-67. Controlled experiments indicated that metallic Co nanoparticles were the active sites for this reaction, and nitrogen atoms provided the sites for the nucleation and growth of cobalt nanoparticle. In addition, nitrogen atoms also had strong electronic interaction with Co nanoparticles to stabilize them, and also improving the catalytic activity. The reduction amination of various carbonyl compounds over Co/N-C-800 catalyst can produce the corresponding amines with good to excellent amines yields (82.8–99.6%). The catalytic activity of Co/N-C-800 catalyst was comparable and superior to the current reported noble metal catalysts. To the best of our knowledge, this is the first report that a none-noble heterogeneous catalyst for the reductive amination of various carbonyl compounds into primary amines with a high conversion and selectivity. meanwhile, the Co/N-C-800 catalyst also showed a high catalytic activity and selectivty to different nitriles with excellent yields (81.7–99.0%) in THF. More importantly, an overall reaction pathway was proposed for the two reactions according to a series of experiments. For the reductive amination of benzaldehyde and the hydrogenation of benzonitrile, phenylmethanimine and dibenzylimine were the two main intermediates for the two reactions. These results indicated that the Co/N-C-800 catalyst was active for several types of reactions. The Co/N-C-800 catalyst exhibited excellent stability, possibly due to the strong interaction between Co nanoparticles and nitrogen atoms. Compared with the already disclosed synthesis methodologies, this novel catalytic system holds multiple advantages, including cost effectiveness (using nonprecious metals as catalyst), environmental friendly (under mild reaction condition in water), simplicity (facile magnetic separation of the catalyst) and a broad substrate scope. Thus, this method provides great potential for industrial application in the production of primary amines from carbonyl compounds and nitriles. Furthermore, there is a large room to enlarge the scope of the Co/N-C-800 catalyst for a broad spectrum of the green synthesis of fine chemicals.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21203252).

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