Startseite In situ supported rhodium catalyst on mesoporous silica for chemoselective hydrogenation of nitriles to primary amines
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In situ supported rhodium catalyst on mesoporous silica for chemoselective hydrogenation of nitriles to primary amines

  • Abdelaziz Nait Ajjou EMAIL logo , André Robichaud und Ateeq Rahman
Veröffentlicht/Copyright: 13. Februar 2025
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Green Processing and Synthesis
Aus der Zeitschrift Green Processing and Synthesis Band 14 Heft 1

Abstract

Heterogeneous rhodium supported on mesoporous silica (Rhinsitu/mesSiO2) was easily prepared, in one step, by incorporating rhodium units onto the silica framework during the sol–gel process. Rhinsitu/mesSiO2 was revealed to be an excellent active and selective catalyst for the hydrogenation of aromatic, heteroaromatic, aliphatic nitriles, and aliphatic nitriles bearing aromatic or heteroaromatic rings to primary amines, regardless of their steric hindrance. The catalytic system that is efficiently recyclable operates under mild conditions in the presence of ammonia.

Graphical Abstract

Rhinsitu/mesSiO2 was prepared in one step by incorporating rhodium units onto the silica during the sol–gel process. It was revealed as an excellent active and selective catalyst for the hydrogenation of aromatic, heteroaromatic, aliphatic nitriles, and aliphatic nitriles bearing aromatic or heteroaromatic rings to primary amines.

Keywords: rhodium-supported catalyst; nitriles; hydrogenation; primary amines; mesoporous silica

1 Introduction

Amines are crucial chemical compounds utilized in the synthesis of a diverse range of organic chemicals that hold significant industrial value [1,2,3,4]. Their usage is extensive; for example, they are employed as agrochemicals, pharmaceuticals, prodrugs, anti-corrosives, cosmetics, and dyes [1,2,3,4,5]. The synthesis of amines varies based on the initial organic compound or feedstock, such as biomass [1,2,3,4,6]. On an industrial scale, amines are produced through several methods, primarily including amination of alkenes, reductive amination of alcohols and aldehydes, and reduction of nitriles [1]. Although these methods benefit from readily available raw materials, they face several challenges, including high temperature and pressure requirements, catalyst deactivation, and low selectivity due to the formation of numerous side products [1].

Recently, the great interest in molecular hydrogen as a renewable green energy source, has focused attention on the search for suitable systems for its storage. Among the solutions recently proposed are liquid organic hydrogen carriers (LOHCs) such as RCH2NH2/RCN couples [7]. Therefore, the development of an environmentally friendly and cheap process to synthesize primary amines from nitriles is important not only from a synthetic viewpoint but also for the fight against global warming [7]. Nowadays, amines are still prepared by using at least stoichiometric amounts of hydride-reducing agents for nitrile reduction [8,9,10,11] and for direct reductive amination (DRA) of aldehydes and ketones [3,12,13]. However, these reducing agents, such as NaBH4 and mainly NaBH3CN, and their by-products have the disadvantages of being toxic and potential contaminants for the final products, which hinders their large-scale use and are not acceptable in the concept of green chemistry. Unlike a process based on hydride-reducing agents, catalytic hydrogenation of nitriles to amines is interesting from economical and ecological aspects, and various interesting processes have been developed, especially during the last decade [14,15,16,17,18,19,20]. Generally, nitrile reduction generates a mixture of amines (1°, 2°, and 3°) and imine intermediates. It has been demonstrated that chemoselectivity can be influenced by the catalyst, the support, the structure of the nitrile to reduce, and the reaction conditions [14,15,17]. Also, to increase chemoselective formation of primary amines, ammonia is most widely used [14,15]. Additionally, mineral bases [14,15] or in situ trapping agents such as acids [14,15,21], CO2 [22], di-tert-butyl decarbonate [22,23], and acetic anhydride [14,15,22] were also successfully utilized.

In catalytic hydrogenation of nitriles, significant interest is in the formation of primary amines. Most recently, homogeneous hydrogenation processes, notably based on Pincer metal complexes, have gained significant development for the highly selective hydrogenation of nitriles to primary amines. However, most of these homogeneous processes need dry solvents that are, in many cases, harmful and are performed under harsh conditions of pressure and temperature, with the obvious problem of recovering the catalysts in their active form and recycling them [14,16,18,19,20]. In contrast to homogeneous processes, the heterogeneous hydrogenation of nitriles has been extensively studied. However, many processes are still limited by their inadequate selectivity to primary amines, the difficult fabrication of the catalysts, and the drastic conditions used [14,16,17,18,19]. However, excellent heterogeneous processes for the selective hydrogenation of nitriles to the desired amines (1°, 2°, or 3°) have been lately reported as a result of the judicious control of various parameters such as the catalyst nature and the support used, as well as the hydrogenation conditions. Thus, various supported catalysts for the chemoselective formation of primary amines in good to excellent yields, under mild conditions have been recently published, specifically [Co(OAc)2/Phen@/α-Al2O3]-800 [24], Ru nanoparticles/K-doped Al2O3 [25], B12@CeO2-8 [26], Fe3C@G-CNT-700 [27], nano-N3C/Al2O3 [28], Ni/ZnAlOx-600 [29], and MC/Ni [30].

Rhodium-based catalysts have been widely used for the hydrogenation of different unsaturated groups [31], except for nitrile hydrogenation to amines. To date, the reported homogeneous and heterogeneous catalysts for the formation of tertiary, secondary, and particularly primary amines are very scarce [14,15,16,18]. For instance, Freifelder studied low-pressure hydrogenation of aliphatic nitriles to primary amines catalyzed by Rh/Al2O3 at room temperature and in the presence of ammonia [32]. Excellent results were obtained, especially in the case of nitriles containing tertiary amine or ether moieties. Under their conditions, the catalyst did not show any sign of N-benzyl group hydrogenolysis. In the absence of ammonia, secondary amines were formed predominantly. Rh/Al2O3 was also investigated by Chatterjee and co-workers for the selective hydrogenation of dinitriles in supercritical carbon dioxide, and they reported that adiponitrile was converted to 6-aminocapronitrile, without the formation of 1,6-hexamethylenediamine [33]. Yoshida et al. reported that rhodium hydride complexes RhH[P(iPr)3]3 and Rh2H2(μ-N2){P(cyclohexyl)3}4 are excellent homogeneous catalysts for the hydrogenation of aromatic and aliphatic nitriles to the primary amines under ambient conditions, with RhH[P(iPr)3]3 being more efficient [34]. RhH[P(iPr)3]3 was further used by Xie et al. for the hydrogenation of phenyl acetonitrile and benzonitrile under CO2 pressure [22]. While phenylacetonitrile gave excellent yield of primary amine, benzylamine was obtained in moderate yield in the case of benzonitrile. Various aliphatic nitriles have been hydrogenated successfully to primary amines using Rh/C in acetic acid and under flow conditions at 90°C [35]. An excellent transfer hydrogenation of aliphatic and aromatic nitriles has been revealed by using the catalyst Fe3O4@nSiO2–NH2RhCu@mSiO2, and formic acid as hydrogen donors under mild conditions [36]. Finally, we reported that [Rh(COD)Cl]2 in aqueous ammonia generated a water‐soluble complex, which is a highly selective catalyst for the hydrogenation of aromatic and sterically hindered aliphatic nitriles to primary amines with excellent and moderate yields [37].

Generally, the support exerts a large influence on the catalytic activity and selectivity of heterogeneous catalytic reactions, partly due to its interaction with the incorporated metallic active species. For the heterogeneous hydrogenation of nitriles, diverse supports such as alumina, silica, and carbon have been used to support catalysts like Pd, Rh, Ni, and Co. Highly ordered mesoporous materials such as MCM-41 and HMS are interesting supports, owing to the large surface area and the uniform and large pore size that promote high loading and dispersion of the metal catalysts [38,39,40,41]. The processes used to synthesize the supported catalysts also have a great influence on the catalyst performance. Post-grafting and impregnation methods are the usual approaches to prepare supported catalysts, and for the hydrogenation of nitriles, the impregnation approach is predominant. In contrast to the previous approaches, in situ synthesis of the supported catalysts is an elegant process that allows its formation in one step, and the leaching of the active metals is minimized or circumvented. It has been demonstrated in many different reactions that the in situ catalysts lead to better results than the catalysts prepared by the impregnation method [42,43,44]. As far as nitrile hydrogenation is concerned, to the best of our knowledge, few mesoporous catalysts have been studied [45,46,47], and the catalysts prepared by the in situ approach are very scarce [44]. Also, we are not aware of any process based on a supported rhodium catalyst, which is described in the same report, for the hydrogenation of aromatic, heteroaromatic, and aliphatic nitriles to primary amines. Herein, we report a convenient method based on an in situ supported rhodium catalyst on mesoporous silica for the hydrogenation of aromatic, heteroaromatic, aliphatic nitriles, and aliphatic nitriles substituted with aromatic or heteroaromatic rings.

2 Materials and methods

2.1 Materials and instruments

Nitriles and catalyst precursors were acquired from Sigma-Aldrich Company. NMR spectra were performed on a Bruker AC-200 spectrometer at 200 (1H) and 50 MHz (13C), using tetramethylsilane (TMS) as the internal standard and CDCl3 in most cases as solvent. Surface areas and pore sizes were determined by Brunauer–Emmett–Teller (BET) analysis and by Professor Serge Kaliaguine (Université Laval, Quebec). X-ray diffraction (XRD) analysis was performed by Professor Ralf Brüning (Mount Allison University, New Brunswick).

2.2 In situ catalyst and mesoporous silica synthesis

A typical procedure for the preparation of Rhinsitu/mesSiO2 and mesoporous silica mesSiO2 is as follows: To a warmed HCl solution (33 mL, 0.03 mol·L−1) containing the desired amount of the catalyst precursor (RhCl3·xH2O), dodecylamine (13.5 mmol) was added. Then, the mixture was stirred vigorously for a few minutes. At room temperature, a solution of tetraethyl orthosilicate (50 mmol) in 21 mL of EtOH/iPrOH (17 mL/4 mL) was added, and a gel started to form. After 20 min, the resulting gel was allowed to age for 18 h at ambient temperature. The product thus obtained was filtered and then washed thoroughly with water, followed by methanol. The final catalyst was obtained after calcination in air at 600°C for 4 h. The rhodium content was determined by atomic absorption spectroscopy.

The same procedure was used to prepare the mesoporous silica mesSiO2 but without the presence of any catalytic precursor.

Both samples were characterized by small-angle XRD diffraction and BET analysis.

2.3 Supported catalysts prepared by impregnation

A typical procedure for the preparation of Mimpr/mesSiO2 is as follows: Supported catalysts Mimpr/mesSiO2 were prepared using the incipient wetness technique by dissolving 0.08 mmol of the organometallic precursor Pd(OAc)2, RuCl3, Pt(COD)Cl2 or [Rh(COD)Cl]2 (0.04 mmol) in MeOH (20 mL) for 10 min. Mesoporous silica (mesSiO2, 1 g) was added, and the mixture was stirred at room temperature for 1 h. The solid was then filtered, washed thoroughly with methanol, and dried at room temperature for 30 min before use.

2.4 Typical procedure for the hydrogenation of nitriles

In a glass liner, MeOH/NH3 (10 mL) and nitrile (2.5 mmol) were introduced successively. Then, a calculated amount of Rhinsitu/mesSiO2 in order to adjust rhodium loading (molar ratio of Rh to the nitrile, typically 0.8 mol%) was added. The glass liner was then placed in a 45 mL autoclave and the mixture was stirred at room temperature for 5 min. The autoclave was purged three times with N2 and then three times with H2 (50 psi). After the pressure was adjusted to 80 psi of hydrogen, the autoclave was heated at 90°C in an oil bath for the desired reaction time. After the reaction was completed, the autoclave was cooled to room temperature, and the remaining hydrogen was cleared out. Then, the mixture was filtered and washed with diethyl ether or ethyl acetate. The organic filtrate was dried and concentrated; then, CDCl3/TMS was added to the resulting products, followed by 1H NMR and 13C NMR analysis. For the purified samples, SiO2 column chromatography was used with ethyl acetate and methanol as the eluent.

For the recycling experiments, it is critical that the reactions are executed under the same conditions as those of the initial hydrogenation reaction. This includes utilizing the same amount of benzonitrile and maintaining an identical weight ratio (%w/w) of the catalyst to benzonitrile. Generally, the quantity of catalyst recovered is less than the amount originally employed. Therefore, to ensure an adequate supply of catalysts for the first recycling experiments, recovered catalysts with the same rhodium loading from several benzonitrile hydrogenations were consolidated and calcined at 600°C for 4 h. The solid obtained from this process was then reused in the recycling experiments. This approach was similarly applied to the second round of recycling experiments. It is vital for the recovered catalysts to be calcined to remove any organic residues that may have built up on their surfaces. Without this procedure, the outcomes may be unsatisfactory in terms of both activity and selectivity toward primary amines.

3 Results and discussion

3.1 Characterization of mesSiO2 and Rh insitu /mesSiO2

The results of small-angle XRD analysis of mesoporous silica (mesSiO2) and the catalyst Rhinsitu/mesSiO2 are illustrated in Figure 1. Both samples exhibit a diffraction peak at approximately 2θ = 1.8°, indicating the presence of a mesoporous structure [38,39,40,41]. The observed broadening of the diffraction peaks, along with their low intensities, implies that the mesopores lack a well-ordered arrangement.

Figure 1 Small-angle XRD patterns of mesSiO2 and Rhinsitu/mesSiO2.
Figure 1

Small-angle XRD patterns of mesSiO2 and Rhinsitu/mesSiO2.

The nitrogen adsorption–desorption isotherms and the associated NLDFT pore size distributions were carried out to investigate the mesoporous structure of the two samples in detail (Figures 2 and 3). The mesoporous architectures of both samples are confirmed by the patterning of type IV isotherms with H1 hysteresis loops [39,40,41]. As illustrated in Figure 2, mesSiO2 displays a larger hysteresis loop compared to Rhinsitu/mesSiO2, likely due to the presence of larger pores in mesSiO2. Figure 3 reveals an average pore size of 50.9 Å for mesSiO2, whereas the predominant signal for Rhinsitu/mesSiO2 is centered around 25.8 Å. The calculated surface areas and pore volumes for mesSiO2 and Rhinsitu/mesSiO2 are 526 and 505 m²·g−1, and 1.62 and 0.8 mL·g−1, respectively.

Figure 2 N2 adsorption–desorption isotherms of mesSiO2 and Rhinsitu/mesSiO2.
Figure 2

N2 adsorption–desorption isotherms of mesSiO2 and Rhinsitu/mesSiO2.

Figure 3 NLDFT pore size distributions of mesSiO2 and Rhinsitu/mesSiO2.
Figure 3

NLDFT pore size distributions of mesSiO2 and Rhinsitu/mesSiO2.

3.2 Catalytic hydrogenation of nitriles to primary amines

At the inception of our project focused on the reduction of nitriles, we conducted an in-depth investigation into the hydrogenation of benzonitrile, which was selected as the model substrate. This process was examined in organic solvents such as methanol and tetrahydrofuran, as well as in water, all without the inclusion of ammonia. The experiments were carried out at temperatures ranging from 50°C to 100°C and under hydrogen pressures between 50 and 200 psi. We evaluated a variety of homogeneous catalysts, including Pd(OAc)2, RuCl3, [Rh(COD)Cl]2, [Ir(COD)Cl]2, and Pt(COD)Cl2, both individually and in conjunction with ligands such as TPPTS (P(m‐C6H4SO3Na)3) or BQC (2,2′‐biquinoline‐4,4′‐dicarboxylic acid dipotassium salt). Additionally, we tested heterogeneous catalysts like Pd/C, Pd/SiO2, and Pt/SiO2. Unfortunately, the selectivity for the primary amine was found to be unsatisfactory. Furthermore, the introduction of ammonium salts such as NH4Cl or AcONH4 did not yield any significant enhancement in selectivity. These salts are known to improve the selectivity for primary amines in the reductive amination of carbonyl compounds, as ammonium ions can protonate the primary amine, resulting in alkylammonium ions which are less nucleophilic. Consequently, this inhibits the formation of highly alkylated imines and amines [15].

We then performed some reactions in aqueous ammonia, and we were delighted to discover that [Rh(COD)Cl]2 is an excellent recyclable catalyst for the hydrogenation of nitriles to primary amines [37]. The hydrogenation, however, has to be performed under 400 psi of H2 to minimize the formation of the corresponding amides and alcohols. Consequently, using nonaqueous ammonia, our goal is to discover a good heterogeneous catalyst that operates under mild temperature and H2 pressure conditions and that leads principally to primary amines.

Primarily, we prepared four different heterogeneous catalysts by the impregnation method, using Pd(OAc)2, RuCl3, Pt(COD)Cl2 and [Rh(COD)Cl]2 as the catalytic precursors, and mesoporous silica (mesSiO2) as the support. The supported catalysts (Mimpr/mesSiO2) were then tested for the hydrogenation of benzonitrile in MeOH/NH3 at 90°C and H2 pressure of 80 psi (Table 1). While ruthenium and platinum catalysts gave average conversions, palladium and rhodium catalysts led to the full transformation of benzonitrile with the highest selectivity to benzylamine reached in the case of Rhimpr/mesSiO2 (94%, Table 1, entry 4). When benzonitrile was subjected to hydrogenation with extended reaction time, no over-hydrogenation products were observed (Table 1, entry 5). Unfortunately, the analysis of the product mixture with atomic absorption indicated the leaching of the catalyst from the support. Additionally, the recycling of the catalyst Rhimpr/mesSiO2 resulted in the production of benzylamine (83%), dibenzylamine (4%), and benzylidenebenzylamine (13%), indicating a notable reduction in selectivity for benzylamine. This decline in selectivity could be attributed to various factors, including a reduction in the rhodium content or the buildup of organic compounds on the catalyst’s surface, as it was reused following a basic wash with methanol.

Table 1

Hydrogenation of benzonitrile catalyzed by Mimpr/mesSiO2 a

Run MLn Time (h) Conversion (%) 1 (%) 2 (%) 3 (%)
1 Pd(OAc)2 24 100 78 20 2
2 RuCl3 24 52 32 4 16
3 Pt(COD)Cl2 24 45 39 4 2
4 [Rh(COD)Cl]2 24 100 94 2 4
5 [Rh(COD)Cl]2 36 100 95 3 2
6b [Rh(COD)Cl]2 36 100 83 4 13

aReaction conditions: benzonitrile (2.5 mmol), H2 (80 psi), MeOH/NH3 (7 N, 10 mL), 90°C, Mimpr/mesSiO2 (1 g, 3.2 mol%). bFirst recycling of the catalyst in run 5.

As a consequence, we prepared a new supported catalyst in one step by the in situ technique using the same method for the preparation of mesoporous silica, with the exception of the addition of the rhodium precursor during the sol–gel process. The newly prepared catalyst Rhinsitu/mesSiO2 was tested for the hydrogenation of benzonitrile under the same conditions. By increasing the rhodium loading from 0.16 to 0.8 mol% (Rh to benzonitrile ratio) and by adjusting the reaction time, the selectivity to benzylamine increased to the detriment of dibenzylamine, and particularly benzylidenebenzylamine (Table 2). With 0.8 mol% loading, excellent results were obtained after 3 h with the selective formation of benzylamine with 92% yield, and the supported catalyst was recycled twice with the same activity and selectivity (Table 2, entries 5–7). For the recycling experiments, the catalysts must be reactivated by heat treatment at 600°C for 4 h before its use (see Section 2.4). By extending the reaction time to 6 h or increasing the rhodium loading to 1.6 mol%, excellent results were obtained with no extensive formation of over-hydrogenation products, mainly cyclohexanemethylamine (Table 2, entries 8–10). Knowing that the reaction in water will lead to other by-products, the hydrogenation of benzonitrile was still tested by replacing CH3OH/NH3 with aqueous ammonia. Not surprisingly, a complex mixture was obtained with benzylamine as the majority product (69%), followed by benzamide (18%), benzylidenebenzylamine (6%), benzyl alcohol (6%), and dibenzylamine (1%) (Table 2, entry 11).

Table 2

Hydrogenation of benzonitrile catalyzed by Rhinsitu/mesSiO2 a

Run Rh Mol% Time (h) Conversion (%) 1 (%) 2 (%) 3 (%) 4 (%)
1 0.16 5 77 37 12 28 0
2 0.16 21 100 88 12 0 0
3 0.32 18 100 91 9 0 0
4 0.8 1 98 77 4 17 0
5 0.8 3 100 92 5 0 3
6b 0.8 3 100 93 6 1 0
7c 0.8 3 100 92 4 4 0
8 0.8 6 100 92 5 0 3
9 0.8 12 100 95 4 0 1
10 1.6 3 100 96 4 0 0
11d 0.8 12 100 69 1 6 0

aReaction conditions: benzonitrile (2.5 mmol), Rhinsitu/mesSiO2, H2 (80 psi), MeOH/NH3 (7 N, 10 mL), and 90°C. bFirst recycling of the catalyst in run 5. cSecond recycling of the catalyst in run 5. dReaction performed with aqueous ammonia instead of MeOH/NH3. The following products were also obtained: benzamide (18%) and benzyl alcohol (6%).

In Table 3, we present most of the reported homogeneous and heterogeneous rhodium catalysts for the reduction of benzonitrile to benzylamine. As can be seen, Rh2H2(μ-N2){P(cyclohexyl)3}4 [34] and Rh-PVP [48,49] were inactive for the formation of benzylamine, while very low yields were obtained in many cases, for instance, Rh/MCM-41, Rh/C, and Rh/Al2O3 in scCO2 [45] and Rh/C in CH2Cl2/H2O in the presence of NaH2PO4 [50]. Moderate yields were reached by RhH[P(iPr)3]3 with a slight improvement under CO2 pressure [22,34] and by 5% Rh/γ-Al2O3 in CH2Cl2/H2O in the presence of NaH2PO4 [50]. With the addition of ammonia, Rh@S-1 led to a good yield of benzylamine [44], and excellent yield was gained with [Rh(COD)Cl]2, as we reported in aqueous ammonia [37]. Finally, a 98% yield was obtained with Fe3O4@nSiO2–NH2–RhCu@mSiO2, but formic acid was used as the hydrogen donor [36]. Thus, our present results showed the superiority of our catalytic system Rhinsitu/mesSiO2 to the reported catalysts for the hydrogenation of benzonitrile in terms of activity and selectivity to benzylamine.

Table 3

Reduction of benzonitrile to benzylamine catalyzed by reported rhodium catalysts

Catalyst Solvent Temperature (°C) H2 (atm) Yield (%) Reference
Rhinsitu/mesSiO2 MeOH/NH3 90 5.44 96 This work
Rh2H2(μ-N2){P(cyclohexyl)3}4 THF 20 1 0 [34]
Rh-PVP MeOH 25 0.99 0 [48]
Rh-PVP MeOH 30 2.96 0 [49]
Rh/MCM-41 scCO2 50 19.74 1.3 [45]
5% Rh/C scCO2 50 19.74 7.2 [45]
5% Rh/Al2O3 scCO2 50 19.74 4.7 [45]
5%Rh/C CH2Cl2/H2O/NaH2PO4 30 5.92 23 [50]
RhH[P(iPr)3]3 THF 20 1 45 [34]
RhH[P(iPr)3]3 THF/scCO2 23-25 4.93 61 [22]
5%Rh/γ-Al2O3 CH2Cl2/H2O/NaH2PO4 30 5.92 52 [50]
Rh@S-1 MeOH/NH3 80 9.87 68 [44]
[Rh(COD)Cl]2 NH4OH 100 27.22 90 [37]
Fe3O4@nSiO2–NH2-RhCu@mSiO2 HCOOH–NEt3 40 — 98 [36]

With the important results obtained in the case of benzonitrile, the limits and the hydrogenating power of Rhinsitu/mesSiO2 were examined with other nitriles (Table 4). Although 2-cyanopyridine is a very difficult compound to selectively hydrogenate into primary amine, an interesting yield of 65% is achieved in our case (Table 4, entry 2). For comparison, we present in Table 5 many reported catalysts for the hydrogenation of 2-cyanopyridine. For example, B12@CeO2-8 in iPrOH/NH4OH [26], 10% Pd/C [51], and 10% Pd(OH) x /C in iPrOH/Et3N [51] were inactive even under drastic temperature and pressure conditions (Table 5, entries 2–4). Under the same conditions, other catalysts, such as Pt/C and Ru/C, are poorly selective despite the total conversion of 2-cyanopyridine (Table 5, entries 5 and 6). Good results were reached, however, with 10% Pd/C but in the presence of sulfuric acid (57%) [52] and with Co-N-C@MgO-700 (70%) [51] by using high catalyst loading and high temperature and pressure of H2. Also, only a 51% yield was obtained by a hydrogen transfer process based on 5–10% Pd/C catalyst and HCO2H/Et3N at room temperature (Table 5, entry 10) [53]. Finally, the only best result so far was reported for the Ni-NPs@SiO2-500 catalyst (83%). However, high hydrogen pressure is needed (Table 5, entry 11) [54]. Rhinsitu/mesSiO2 catalyst was also proven to be highly effective for the selective hydrogenation of 3-cyanoyridine to 3-(aminomethyl)pyridine with full conversion and 93% selectivity (Table 4, entry 3). In the cases of hexanenitrile, phenylacetonitrile, and 3-phenylpropionitrile, equally excellent results were obtained with full conversions of the starting nitriles and selectivities of 91–94% (Table 4, entries 4–6). The efficiency of our hydrogenation catalytic system was further assessed in the case of 2-pyridylacetonitrile and 3-pyridyacetonitrile that were fully converted to the corresponding primary amines with the respective selectivities of 94 and 93% using 1.6 mol% Rh loading (Table 4, entries 7 and 8). These two compounds have been rarely studied, and the reported yields were obtained under harsh conditions:

Table 4

Hydrogenation of different nitriles catalyzed by Rhinsitu/mesSiO2 a

Run Nitrile Rh Mol% Time (h) Conversion (%) Selectivity to 1° amine (%)
1
0.8 3 100 92
2b
0.8 6 100 65
3
0.8 6 100 93
4
0.8 6 100 91
5
0.8 3 100 92
6
0.8 6 100 94
7
1.6 24 100 94
8
1.6 24 100 93
9
1.6 24 89 100
10c
1.6 24 96 100
11
1.6 24 100 >99

aReaction conditions: nitrile (2.5 mmol), Rhinsitu/mesSiO2, H2 (80 psi), MeOH/NH3 (7 N, 10 mL), 90°C. bIsolated yield. Other unknown products were formed, including ring reduction and very small amount of secondary amine. cH2 pressure is 200 psi.

Table 5

Reduction of 2-cyanopyridine catalyzed by reported catalysts

Run Catalyst Solvent Temperature (°C) H2 (atm) Conversion (%) Selectivity (%) Reference
1 Rhinsitu/mesSiO2 MeOH/NH3 90 5.44 100 65 This work
2 B12@CeO2-8 iPrOH/NH4OH 140 49.35 0 0 [26]
3 10% Pd/C iPrOH/Et3N 130 39.48 >99 <1 [51]
4 10% Pd(OH) x /C iPrOH/Et3N 130 39.48 >99 <1 [51]
5 10% Pt/C iPrOH/Et3N 130 39.48 >99 31 [51]
6 5% Ru/C iPrOH/Et3N 130 39.48 >99 49 [51]
7 10% Pd/C CH2Cl2/H2O/H2SO4 30 5.92 100 57 [52]
8 Co-N-C@MgO-700 iPrOH/NH4OH 80 19.74 26 22 [51]
9a Co-N-C@MgO-700 iPrOH/NH4OH 130 39.48 >99 70 [51]
10 5–10% Pd/C HCO2H/Et3N rt — — 51b [53]
11 Ni-NPs@SiO2-500 MeOH/NH3 80 34.54 — 83b [54]

aThe amount of the catalyst is tripled compared to run 8. bPercentage (%) represents the reported yield.

2-Pyridylacetonitrile:

  1. [Rh(COD)Cl]2, NH4OH, H2 (400 psi), 100°C: 54% [37].

  2. Ni-NPs@SiO2-500, MeOH/NH3, H2 (508 psi), 80°C: 85% [54].

3-Pyridylacetonitrile:

  1. [Rh(COD)Cl]2, NH4OH, H2 (400 psi), 100°C: 67% [37].

  2. B12@CeO2-8, iPrOH/NH4OH, H2 (435 psi), 120°C: 98% [26].

Sterically hindered nitriles, 2-phenylpropionitrile and 2-phenylbutyronitrile, have also been rarely reported, and the processes used were with high temperatures and high hydrogen pressure [37,54,55,56]. Fe/Fe-O@SiO2 was shown to catalyze the hydrogenation of 2-phenylpropionitrile to primary amine with 93% yield. However, the reaction has to be performed at 120°C under 725 psi of H2 in the presence of ammonia [55]. Concerning 2-phenylbutyronitrile, moderate to excellent yields were obtained, and once again, high hydrogen pressure with a temperature ranging from 80°C to 120°C was used:

  1. [Rh(COD)Cl]2, NH4OH, H2 (400 psi), 100°C: 50% [37].

  2. Ni-NPs@SiO2-500, MeOH/NH3, H2 (508 psi), 80°C: 82% [54].

  3. Fe/Fe-O@SiO2, iPrOH/NH3, H2 (725 psi), 120°C: 90% [55].

  4. Cobalt-terephthalic acid MOF@C-800, Toluene/NH3, H2 (363 psi), 120°C, 88% [56].

Rhinsitu/mesSiO2 catalyst for both cases led to excellent results as 2-phenylpropylamine and 2-phenylbutylamine were selectively obtained with 96 and >99% yields, respectively, from 2-phenylpropionitrile and 2-phenylbutyronitrile (Table 4, entries 10 and 11). These results indicated that our catalytic system is compatible with sterically hindered nitriles.

Scheme 1 outlines the proposed hydrogenation mechanism for generating primary amines [15,46,57,58,59] and clarifies the effect of the NH3 additive. Initially, the starting nitrile leads to the formation of the primary amine RCH2–NH2 ( B ) through two consecutive hydrogenation reactions, with aldimine ( A ) acting as an intermediate. The reaction between the primary amine and aldimine yields 1-amino-dialkylamine RCH(NH2)NHCH2R ( C ), which can release ammonia to yield dialkylimine RCH═NCH2R ( D ). This dialkylimine can subsequently undergo hydrogenation to produce the secondary amine (RCH2)2NH ( E ). Alternatively, it can also arise directly from C following hydrogenolysis and the liberation of ammonia.

Scheme 1 Plausible mechanism for the formation of primary amines.
Scheme 1

Plausible mechanism for the formation of primary amines.

The presence of excess ammonia serves to inhibit the formation reactions of D and E , thereby favoring the production of primary amine B . It was suggested that ammonia interacts with A to yield gem-diamine F , which subsequently transforms into B through hydrogenolysis [15]. The protection of primary imine A as gem-diamine F decreases its availability for the addition reaction with primary amine B . This results in a deceleration of secondary amine E formation due to the inhibition of the C and D formation reactions [15]. Additionally, it has been observed that dialkylimine D reacts with ammonia to generate primary amine B and aldimine A . This transamination equilibrium shift consequently diminishes the formation of D and E [15].

4 Conclusions

The hydrogenation of nitriles to amines by homogeneous or heterogeneous rhodium-based catalysts is very rare. Rhinsitu/mesSiO2, prepared in a single step by the sol–gel process, has been shown to be an excellent catalyst for the synthesis of primary amines from nitriles in the presence of ammonia, and the catalyst is recyclable without loss of activity or catalytic selectivity. Although rare rhodium-based reports have been published, the diversity of nitriles examined in the same report is limited, particularly with the absence of heteroaromatic nitriles and nitriles that contain heteroaromatic groups. Under moderate conditions of temperature (90°C) and hydrogen pressure (80 psi), Rhinsitu/mesSiO2 allowed the selective hydrogenation of aromatic, heteroaromatic, aliphatic, and aliphatic nitriles containing phenyl or heteroaryl groups. It was also shown to be compatible with bulky nitriles such as 2-phenylbutyronitrile.

Acknowledgement

We are grateful to Janssen Pharmaceutica (Beerse, Belgium) and to la Faculté des Études Supérieures et de la Recherche (FESR) of Université de Moncton for financial support for this research. We are grateful to Professor Ralf Brüning of Mount Allison University (New Brunswick) for XRD analysis and to Professor Serge Kaliaguine of Université Laval (Quebec) for BET analysis.

  1. Funding information: This study was mainly funded by Janssen Pharmaceutica, Beerse, Belgium, and by Faculté des Études Supérieures et de la Recherche of Université de Moncton.

  2. Author contributions: Abdelaziz Nait Ajjou: resources, supervision, conceptualization, methodology, investigation, formal analysis, writing, review, and editing; André Robichaud: methodology, carried out experiments, investigation, data analysis, and formal analysis, Ateeq Rahman: methodology, carried out experiments, investigation, and data analysis. All authors read and approved the final manuscript.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2024-09-04
Accepted: 2024-12-15
Published Online: 2025-02-13

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/gps-2024-0193
Schlagwörter für diesen Artikel
rhodium-supported catalyst; nitriles; hydrogenation; primary amines; mesoporous silica
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  22. Synthesis and optimization of gemcitabine-loaded nanoparticles by using Box–Behnken design for treating prostate cancer: In vitro characterization and in vivo pharmacokinetic study
  23. A comparative analysis of single-step and multi-step methods for producing magnetic activated carbon from palm kernel shells: Adsorption of methyl orange dye
  24. Sustainable green synthesis of silver nanoparticles using walnut septum waste: Characterization and antibacterial properties
  25. Efficient electrocatalytic reduction of CO2 to CO over Ni/Y diatomic catalysts
  26. Greener and magnetic Fe3O4 nanoparticles as a recyclable catalyst for Knoevenagel condensation and degradation of industrial Congo red dye
  27. Recycling of HDPE-giant reed composites: Processability and performance
  28. Fabrication of antibacterial chitosan/PVA nanofibers co-loaded with curcumin and cefadroxil for wound healing
  29. Cost-effective one-pot fabrication of iron(iii) oxychloride–iron(iii) oxide nanomaterials for supercapacitor charge storage
  30. Novel trimetallic (TiO2–MgO–Au) nanoparticles: Biosynthesis, characterization, antimicrobial, and anticancer activities
  31. Green-synthesized chromium oxide nanoparticles using pomegranate husk extract: Multifunctional bioactivity in antioxidant potential, lipase and amylase inhibition, and cytotoxicity
  32. Therapeutic potential of sustainable zinc oxide nanoparticles biosynthesized using Tradescantia spathacea aqueous leaf extract
  33. Chitosan-coated superparamagnetic iron oxide nanoparticles synthesized using Carica papaya bark extract: Evaluation of antioxidant, antibacterial, and anticancer activity of HeLa cervical cancer cells
  34. Antioxidant potential of peptide fractions from tuna dark muscle protein isolate: A green enzymatic approach
  35. Clerodendron phlomoides leaf extract-mediated synthesis of selenium nanoparticles for multi-applications
  36. Optimization of cellulose yield from oil palm trunks with deep eutectic solvents using response surface methodology
  37. Nitrogen-doped carbon dots from Brahmi (Bacopa monnieri): Metal-free probe for efficient detection of metal pollutants and methylene blue dye degradation
  38. High energy density pseudocapacitor based on a nanoporous tungsten(VI) oxide iodide/poly(2-amino-1-mercaptobenzene) composite
  39. Green synthesized Ag–Cu nanocomposites as an improved strategy to fight multidrug-resistant bacteria by inhibition of biofilm formation: In vitro and in silico assessment study
  40. In vitro evaluation of antibacterial activity and associated cytotoxicity of biogenic silver nanoparticles using various extracts of Tabernaemontana ventricosa
  41. Fabrication of novel composite materials by impregnating ZnO particles into bacterial cellulose nanofibers for antimicrobial applications
  42. Solidification floating organic drop for dispersive liquid–liquid microextraction estimation of copper in different water samples
  43. Kinetics and synthesis of formation of phosphate composites from low-grade phosphorites in the presence of phosphate–siliceous shales and oil sludge
  44. Removal of minocycline and terramycin by graphene oxide and Cr/Mn base metal–organic framework composites
  45. Microfluidic preparation of ceramide E liposomes and properties
  46. Therapeutic potential of Anamirta cocculus (L.) Wight & Arn. leaf aqueous extract-mediated biogenic gold nanoparticles
  47. Antioxidant-rich Micromeria imbricata leaf extract as a medium for the eco-friendly preparation of silver-doped zinc oxide nanoparticles with antibacterial properties
  48. Influence of different colors with light regime on Chlorella sp., biomass, pigments, and lipids quantity and quality
  49. Experimental vibrational analysis of natural fiber composite reinforced with waste materials for energy absorbing applications
  50. Green synthesis of sea buckthorn-mediated ZnO nanoparticles: Biological applications and acute nanotoxicity studies
  51. Production of liquid smoke by consecutive electroporation and microwave-assisted pyrolysis of empty fruit bunches
  52. Synthesis of MPAA based on polyacrylamide and gossypol resin and applications in the encapsulation of ammophos
  53. Application of iron-based catalysts in the microwave treatment of environmental pollutants
  54. Enhanced adsorption of Cu(ii) from wastewater using potassium humate-modified coconut husk biochar
  55. Adsorption of heavy metal ions from water by Fe3O4 nano-particles
  56. Green synthesis of parsley-derived silver nanoparticles and their enhanced antimicrobial and antioxidant effects against foodborne resistant bacteria
  57. Unwrapping the phytofabrication of bimetallic silver–selenium nanoparticles: Antibacterial, Anti-virulence (Targeting magA and toxA genes), anti-diabetic, antioxidant, anti-ovarian, and anti-prostate cancer activities
  58. Optimizing ultrasound-assisted extraction process of anti-inflammatory ingredients from Launaea sarmentosa: A novel approach
  59. Eggshell membranes as green carriers for Burkholderia cepacia lipase: A biocatalytic strategy for sustainable wastewater bioremediation
  60. Research progress of deep eutectic solvents in fuel desulfurization
  61. Enhanced electrochemical synthesis of Ni–Fe/brass foil alloy with subsequent combustion for high-performance photoelectrode and hydrogen production applications
  62. Review Article
  63. Sustainable innovations in garlic extraction: A comprehensive review and bibliometric analysis of green extraction methods
  64. Rapid Communication
  65. In situ supported rhodium catalyst on mesoporous silica for chemoselective hydrogenation of nitriles to primary amines
  66. Special Issue: Valorisation of Biowaste to Nanomaterials for Environmental Applications
  67. Valorization of coconut husk into biochar for lead (Pb2+) adsorption
  68. Corrigendum
  69. Corrigendum to “An updated review on carbon nanomaterials: Types, synthesis, functionalization and applications, degradation and toxicity”
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Heruntergeladen am 27.10.2025 von https://www.degruyterbrill.com/document/doi/10.1515/gps-2024-0193/html
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