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Article

Reductive Amination of Cyclohexanone via Bimetallic Rh-Ni Catalysts: A Pathway to Improved Catalytic Efficiency

by
Karen Morales
1,
Camila Sandoval
1,
Andreia Peixoto
2,
Ricardo Chimentão
3,
Jordi Llorca
4 and
Doris Ruiz
1,*
1
Laboratory of Heterogeneous Catalysis for Valorization and Selective Processes (CATSVAL), Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción 4070371, Chile
2
REQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal
3
Laboratorio de Investigación de Procesos Catalíticos y Adsorción (LIPROCAD), Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción 4070409, Chile
4
Department of Chemical Engineering, Institute of Energy Technologies and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(9), 803; https://doi.org/10.3390/catal15090803 (registering DOI)
Submission received: 27 May 2025 / Revised: 11 August 2025 / Accepted: 20 August 2025 / Published: 23 August 2025
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Abstract

Reductive amination of cyclohexanone with NH3 and H2 over Rh and Rh-Ni catalysts on SiO2 has been studied. Research has focused on the catalytic efficiency of monometallic and bimetallic catalysts in the production of cyclohexylamine, a key intermediate in the synthesis of numerous fine chemicals. Through the wet impregnation method, Rh and Rh-Ni catalysts with varying nickel loadings (1, 2, 5, and 10 wt.%) were synthesized and characterized using techniques such as N2 physisorption, TEM, HAADF-STEM, XRD, XPS, H2-TPR, and NH3-TPD. The catalytic reactions were conducted under controlled conditions using a glass-coated reactor, using ammonia as nitrogen source. Rh-Ni bimetallic catalysts exhibited the highest conversion rates on reductive amination, attributed to enhanced dispersion and advantageous surface properties. High metal dispersion and small particle sizes were confirmed by TEM, HAADF-STEM, and XRD. XPS analysis confirmed the reduced state of Rh and mainly oxidized state of Ni, while H2-TPR and NH3-TPD results indicated improved reducibility and acidity, respectively, which are critical for catalytic activity. Monometallic Rh/SiO2 catalyst showed 83.4% of conversion after 300 min and selectivity of 99.1% toward the desired product cyclohexylamine. The addition of nickel, a cheap and easily available metal, increases the activity without compromising selectivity. At 300 min of the reaction, the 2 wt.% NiRh/SiO2 catalyst exhibited the highest conversion, yield, and selectivity for the desired product cyclohexylamine, 99.8%, 96.4%, and 96.6% respectively. Additionally, this catalyst is recyclable after the fourth cycle, showing 99.5% selectivity and 74.0% yield for cyclohexylamine at 75.7% conversion. Recycling tests confirmed the stability of bimetallic catalysts, maintaining performance over multiple cycles without significant deactivation.

Graphical Abstract

1. Introduction

Amines, especially primary amines, are crucial in synthetic organic chemistry due to their role in the synthesis of pharmaceuticals, agrochemicals, fine chemicals, polymers, color pigments, and corrosion inhibitors [1,2,3,4,5]. In pharmaceuticals, amines are key intermediates in forming active compounds. In agrochemicals, they are used in synthesizing herbicides and insecticides. For fine chemicals, amines facilitate carbon–nitrogen bond formation. In polymers, they are involved in creating polyamides and polyurethanes. In pigments, amines are used in producing azo dyes, and as corrosion inhibitors, they protect metal surfaces. Due to their versatile valorization, the development of efficient synthetic routes to primary amines is an interesting research focus. Advances in catalytic methodologies that enhance selectivity and sustainability, in line with green chemistry principles, are expected in this area [6,7,8,9].
Among the proposed methods for amine synthesis are direct amination of alcohols [10,11,12,13,14,15], elimination of carbonyl group from amides [16,17], reduction of nitro compounds [18,19], and reductive hydrogenation of nitriles [20,21]. However, one of the most convenient methods for synthesizing amines is reductive amination of carbonyl compounds, which, under optimal conditions, generates water as the only byproduct [22,23,24,25].
In this regard, reductive amination with ammonia (NH3) and molecular hydrogen (H2) usually requires severe conditions, such as high NH3 or H2 pressures or high temperatures [26,27,28,29,30]. Therefore, its applications vary depending on the substrate nature, formation of unwanted secondary and tertiary amines, or further hydrogenation of carbonyl groups and other substituents [31,32,33,34,35]. Thus, selectivity toward primary amines is still a challenge.
Heterogeneous catalysts play a crucial role in facilitating chemical transformations, and their optimization is vital for enhancing reaction efficiency and selectivity. In this context, the amination of cyclohexanone is a reaction of significant industrial relevance, as it leads to the production of cyclohexylamine. Consequently, the development of efficient and recyclable heterogeneous catalysts, with high selectivity toward primary amines under mild conditions using NH3 and H2, is both a desirable and challenging goal [36,37].
Overalkylation remains a common issue, particularly with linear aliphatic amines [38,39,40,41]. In this context, an excess of NH3 is often used to enhance selectivity toward primary amines [35,42]. Various noble metal catalysts (Ir, Au, Ru, Pd, Pt, and Rh) [11,43,44,45] as well as non-noble metal catalysts (Cu, Co, and Ni) [46,47,48,49,50] have been reported for the efficient synthesis of a wide range of amines. Rhodium (Rh) exhibits high activity in hydrogenation and dehydrogenation steps and offers excellent selectivity toward primary amines. Nickel (Ni), an earth-abundant and economically attractive metal, also exhibits good performance in hydrogenation and N-alkylation, positioning it as a promising alternative to noble metals. Although the reaction does not inherently require additional H2, small amounts are frequently introduced to facilitate imine hydrogenation [38,51] and to prevent catalyst deactivation due to coke formation [52]. These reactions are typically carried out under reflux or mild H2 pressure, at temperatures ranging from 120 °C to 200 °C.
The present study seeks to bridge existing knowledge gaps by systematically evaluating the catalytic performance of Rh and Rh-Ni catalysts in the amination of cyclohexanone, with a particular emphasis on elucidating the differences in catalytic efficiency between monometallic and bimetallic supported catalysts. Specifically, this work focuses on the reductive amination of cyclohexanone (CH), as illustrated in Scheme 1, aiming to selectively obtain the primary amine cyclohexylamine. We report a mild procedure, proceeding at 100 °C in cyclohexane, using 4 bar of NH3 and 2 bar of H2, and heterogeneous catalysts based on Rh and Rh-Ni supported on silica, with dual functions enabling both reductive amination of ketones and hydrogenation of imines. Notably, no formation of unwanted secondary or tertiary amines was detected.

2. Results and Discussion

2.1. Catalyst Characterization

The textural properties of the samples, including B.E.T. surface area, pore volume, and pore size, are detailed in Table 1. The SiO2 support displays a surface area of 319 m2 g−1. Upon incorporation of rhodium species by wet impregnation, the surface area and pore volume decrease. Specifically, Rh/SiO2 catalysts exhibit a surface area of 291 m2 g−1 and pore volume of 0.82 cm3 g−1. The observed reductions in surface area and pore volume upon impregnation with rhodium and nickel precursors suggest a partial filling of the pores by rhodium and nickel species.
The slight increase in B.E.T. surface area observed for the 2 wt.%NiRh/SiO2 catalyst may be attributed to partial redispersion of Rh species or to the removal of residual surface species during Ni incorporation, which may improve pore accessibility. All samples exhibit a type IV isotherm with a H1 hysteresis loop (Figure S1), characteristic of mesoporous solids [52].
The metal particle size and metal dispersion of the catalysts were evaluated using transmission electron microscopy (TEM). The synthesis procedure resulted in metal nanoparticles with a Gaussian size distribution centered around 2 nm for most samples, except for the 10 wt.% NiRh/SiO2 catalyst, which exhibited a broader particle size distribution (Figure 1). In this case, a bimodal distribution was observed, attributed to the presence of large Ni particles coexisting with small Rh nanoparticles (Figure 2).
High-angle annular dark-field imaging (HAADF)–scanning transmission electron microscopy (STEM) combined with EDX analysis was also performed for the 10 wt.% NiRh/SiO2 catalyst to correlate nanoparticle size with chemical composition. As expected, the average particle size obtained by HAADF-STEM was slightly different from that determined by TEM, given the higher spatial resolution and the smaller sampling area of HAADF-STEM. The HAADF-STEM analysis confirmed the presence of a bimodal size distribution, consistent with the broad size range observed in TEM.
Figure 2a shows a representative HAADF-STEM image of the 10 wt.% NiRh/SiO2 catalyst. A clear bimodal distribution is observed, centered at about 3.2 and 6.4 nm. This behavior can be attributed to the high Ni content, which favors the formation of larger Ni particles due to increased agglomeration tendencies. Figure 2b presents a higher-magnification HAADF-STEM image along with two EDX spectra recorded on a 9.0 nm particle (bottom) and on a 2.0 nm particle (top). The EDX spectrum of the smaller particle shows a Rh signal only, whereas the largest average particle exhibits both Rh and Ni signals. These findings suggest the coexistence of monometallic Rh nanoparticles and larger Ni-containing nanoparticles.
Figures S2 and S3 show representative HAADF-STEM images and particle size distributions for the Rh/SiO2 (2.6 nm) and 1 wt.% NiRh/SiO2 (3.2 nm). The mean particle size for 1 wt.% NiRh/SiO2 is larger than the mean particle size observed for the catalyst containing only Rh. In contrast, 5 wt.% NiRh/SiO2 (Table 2) showed the smallest average particle size, centered at 1.58 nm, along with the highest dispersion determined by TEM. The decrease in average particle size observed for catalysts with low (1–5 wt.%) Ni loadings may result from redispersion during the thermal treatment following Ni impregnation. This behavior can be explained by two well-known effects in bimetallic systems: electronic interactions between the metals, where the introduction of Ni modifies the surface energy of the pre-existing Rh nanoparticles, promoting redispersion and limiting sintering; and geometric effects, whereby the incorporation of Ni leads to surface dilution of Rh (Table S1) and enhanced anchoring at the metal–support interface, which suppresses particle growth [53].
The metallic phase of the catalysts was characterized by X-ray diffraction (XRD). The XRD patterns (Figure 3) displayed characteristic Ni (111) reflections at 2θ ≈ 44°, whose intensity increased with higher Ni content in the samples. Additional reflections corresponding to the (200) and (222) planes were detected at approximately 55° and 76°, respectively, albeit with lower intensity. A broad and low-intensity signal around 22° was attributed to the amorphous SiO2 support. The presence of Rh (111) was confirmed by a weak diffraction peak at 2θ ≈ 41°. The low intensity of the Rh signal is consistent with small metallic particle sizes and high dispersion, as expected for samples with low Rh content and further corroborated by TEM and HAADF-STEM analysis.
Binding energies (BEs) of Rh (3d), Ni (2p), Si (2p), and O (1s) were determined by XPS for all catalysts studied. The Rh 3d5/2 core-level spectra show the presence of two different rhodium species on the catalyst surface (see Table 3 and Figure 4 below).
The peak-fitted XPS spectrum in the Rh 3d region showed the presence of two orbit peaks of Rh 3d3/2 and Rh 3d5/2. The orbit peak 3d5/2 exhibited two deconvoluted peaks, one centered at 307.1 eV and another at 309.1 eV, which are attributed to Rh° and Rh3+, respectively [54].
The addition of Ni slightly modifies the chemical environment and the binding energy (BE) of Rh, which can be associated with evident Rh–Ni contact; however, it does not suggest the formation of an alloy. The peak-fitted XPS spectrum in the Ni 2p region showed the presence of two orbit peaks of Ni 2p3/2 and 2p1/2. The peak 2p3/2 exhibited two deconvoluted peaks, one centered at 855.7 eV and another at 856.5 eV, which are attributed to Ni2+ species. The deconvoluted peaks centered at 860.7 and 862.9 eV are ascribed to Ni2+ satellite species [55]. Although the signal was weak in some cases, satellite peaks located approximately 6 eV above the main peak (characteristic of Ni2+) were consistently observed. Only the 10 wt.% NiRh/SiO2 catalyst exhibited a distinct signal centered at 852.5 eV, corresponding to nickel in its reduced state. This observation supports the presence of reduced rhodium, as identified by XRD, and is consistent with the particle sizes determined by TEM and HAADF-STEM.
No significant variation in rhodium surface exposure was observed with increasing nickel content; however, a slightly lower Rh/Si ratio of 0.0029 was measured for the 10 wt.% NiRh/SiO2 catalyst, which may be attributed to partial coverage by nickel. The surface exposure of nickel remained nearly constant regardless of its nominal content. Nevertheless, a slight increase in the Ni/Rh surface ratio was detected, suggesting an enrichment of nickel relative to rhodium on the surface. Among all samples, the 2 wt.% NiRh/SiO2 catalyst exhibited the highest surface concentrations of both metals, with a Rh/Ni ratio of 0.3254, in agreement with the high surface area, pore volume, and dispersion observed by N2 physisorption, TEM, and XRD analyses.
TPR profiles showed distinct H2 consumption behaviors among the catalysts (Figure 5, Table 4). The 10 wt.% NiRh/SiO2 catalyst exhibited the highest H2 consumption (2.732 mmol g−1), which can be attributed to its higher total metal content and the significant presence of reducible Ni species, as clearly reflected in its intense and low-temperature TPR peak. In contrast, the catalysts with intermediate Ni loadings (2 wt.% and 5 wt.%) displayed lower hydrogen consumption values (0.766 and 0.396 mmol g−1, respectively).
Interestingly, the 1 wt.% NiRh/SiO2 catalyst exhibited a high H2 consumption value (1.421 mmol g−1, See Table 4 below), despite its lower Ni content. This unusual behavior correlates with its high surface Rh/Ni atomic ratio (1.5286) observed by XPS, indicating a surface strongly enriched in Rh species, which are more easily reducible under the applied TPR conditions. Additionally, this catalyst exhibited a secondary hydrogen consumption peak at higher temperatures, absent in the other samples. This second reduction signal may be associated with the presence of more strongly interacting metal species or metal–support interactions unique to this composition. Given the low Ni content and relatively high H2 consumption of this catalyst, it is plausible that a fraction of the metal species exists in a more stabilized, less reducible form, possibly as highly dispersed or strongly anchored Ni or Rh species. These species may require higher temperatures to undergo reduction, thus generating the additional peak observed in the TPR profile. Overall, these results suggest that the reduction behavior of this catalyst depends not only on its bulk metal content, but also on its surface composition and the distribution of reducible species. In addition, the analysis allowed the determination of the activation temperatures of the metals prior to each reaction (<250 °C, reported in the Section 3).
NH3-TPD analysis (Figure 6) was used to evaluate the surface acidity of the catalysts. The monometallic Rh/SiO2 sample shows a prominent desorption peak centered around 325 °C, associated with moderately acidic sites attributed to Rh species. Upon incorporation of Ni, changes are observed both in the intensity and position of the desorption features. In particular, the peak at ~325 °C becomes less intense and shifts slightly toward higher temperatures in bimetallic samples, indicating a modification of the acid strength and environment of the active sites. Additionally, in the 5 wt.% and 10 wt.%NiRh/SiO2 catalysts, a broad shoulder appears at higher temperatures (~680–700 °C), suggesting the formation of stronger acid sites, likely associated with the presence of Ni species. Although no direct correlation is observed between total acid density and Ni content, the evolution of the desorption profiles reflects qualitative changes in the acid site distribution and strength induced by Ni. These results support that Ni incorporation alters the surface acid properties, although not in a strictly linear or purely quantitative way.
The 2 wt.% NiRh/SiO2 catalyst has the highest surface area (SBET), pore volume, diameter, and fraction of metal exposed on the surface, as corroborated by XPS. This high metallic dispersion, together with a low total amount of desorbed NH3 determined by NH3-TPD, suggests a moderate to low surface acidity, a condition that is favorable for reductive amination reactions with NH3 and H2. In this context, the balance between high metallic accessibility and low density of acid sites is associated with greater selectivity toward the formation of primary amines, by minimizing overalkylation routes or formation of byproducts derived from unwanted consecutive reactions.

2.2. Reductive Amination

The reductive amination of cyclohexanone was performed to obtain the target product cyclohexylamine (Scheme 1). Reactions were conducted in a batch reactor using activated Rh/SiO2 (reduced state), using inactivated Rh/SiO2 (oxidized state), and in the absence of catalyst. The use of reduced Rh/SiO2 enabled a maximum conversion of 83.4% to be achieved, with an apparent first-order rate constant of 0.0072 min−1 at 300 min (Figure 7a). Throughout the reaction, a high selectivity of 99.1% toward cyclohexylamine was maintained (Figure 7b).
When an oxidized catalyst is used, the presence of metal species in an oxidized state (such as Rh3+) hinders the activation of key reactants such as H2, NH3, or carbonyl compounds, thereby limiting product formation. In the absence of an active catalyst capable of lowering the activation energy, the reaction proceeds with low and constant conversion, due to either energetic barriers or the establishment of an unfavorable equilibrium. The results indicate that the H2 present in the system is not sufficient to reduce the oxidized catalyst in situ. In the absence of a catalyst, conversion is also limited due to thermodynamic constraints. In contrast, when a previously reduced catalyst is employed, the adsorption and activation of reactive species are promoted, enabling a significantly higher conversion toward the primary amine.
In the study of the reaction conditions, an increase in conversion of cyclohexanone with partial pressure of H2 was found. The highest conversion (93.6%) was achieved under 2 bar NH3 and 4 bar H2 (Figure 8a). The presence of NH3 was shown to be critical for directing the reaction toward the formation of cyclohexylamine. As depicted in Figure 8b,c, the highest selectivity (>99%) was observed under 2 bar NH3/4 bar H2 and 4 bar NH3/2 bar H2, indicating that ammonia promotes imine formation over complete hydrogenation of the carbonyl group. An influence of H2/NH3 ratio on selectivity was also found. In the presence of an excess of H2 (4 bar), a slight increase in selectivity toward cyclohexylamine was observed, attributed to the hydrogenation of the imine intermediate.
In absence of H2, the addition of only NH3 resulted in 3% conversion, suggesting that hydrogen is essential for the imine hydrogenation via condensation. In particular, the stabilization of the imine intermediate and its subsequent hydrogenation require hydrogenation. Furthermore, hydrogen likely contributes to maintaining the catalyst in its reduced state and may inhibit carbonaceous deposit formation, thereby minimizing catalyst deactivation. Under conditions lacking NH3 (4 bar He and 2 bar H2), a moderate conversion was observed, with cyclohexanol being the predominant product (Figure 8d). This confirms that carbonyl hydrogenation is the dominant pathway in nucleophile absence.
Comparable trends have been reported for the amination of dodecanol, where Ru/C exhibited the highest conversion and selectivity toward dodecylamine under 200 °C, 4 bar NH3, and 2 bar H2 [56]. In contrast, the addition of NH3 without H2 in an inert atmosphere did not lead to the formation of the desired products, unlike alcohol-based systems, in which H2-mediated dehydrogenation initiates the entire transformation process [42,45]. These findings indicate that the decisive step, regardless of the initial substrate or conditions, is the hydro/dehydrogenation sequence rather than the amination pathway leading to imine formation. This underscores the importance of employing catalysts with metals active in H2 chemisorption and capable of promoting hydro/dehydrogenation reactions.
As shown in Figure 9a, a higher conversion is observed at low reaction times due to the small amount of substrate molecules to be transformed by Rh. On the other hand, at higher concentrations, cyclohexanone molecules can compete for the same active site, which can generate a saturation of the catalyst and reduce its effectiveness. In Figure 9b, selectivity toward cyclohexylamine as a function of cyclohexanone conversion is presented. The distribution of products at 300 min is observed in Figure 9c; in all cases, the selectivity toward cyclohexylamine was ≥99% (Figure 9b). It is highlighted that CH/Rh = 50 reaches complete conversion.
Adding a second, non-noble metal to the Rh/SiO2 catalyst was chiefly intended to improve its catalytic performance and selectivity for producing primary amines. This was largely achieved by electronic changes brought about by the inclusion of the second metal. The observed enhancement upon Ni addition is attributed to interactions between Rh and Ni, which induce electron transfer from Ni to Rh. This electron enrichment of Rh stabilizes its reduced state under reaction conditions, suppressing deactivation via oxidation. Techniques such as XPS, H2-TPR, and HAADF-STEM confirm the modified electronic environment and the enhanced reducibility of Rh upon Ni incorporation.
This adjustment was meant to enable the ongoing regeneration of Rh in its reduced form, thereby enhancing both the durability and the ability to reuse the material.
The first reaction using the monometallic Rh/SiO2 catalyst achieved an 83.4% conversion and demonstrated a 99.1% selectivity for cyclohexylamine after 300 min. In comparison, the bimetallic 2 wt.% NiRh/SiO2 catalyst achieved the highest conversion of 99.8% and a selectivity of 96.6% at the same reaction time (Figure 10a,c). Product distribution at 240 min is shown in Figure 10b. However, when Ni loading exceeded 5 wt.%, the conversion slightly decreased to 95%, and the selectivity toward cyclohexanol increased modestly, as seen in Figure 10c,d. These results suggest that while moderate Ni addition enhances Rh activity, excessive Ni content may alter the catalyst surface in a way that promotes side reactions, particularly the hydrogenation of the carbonyl group to cyclohexanol. This behavior cannot be solely explained by the increased total metal content (Ni + Rh), as the catalytic activity does not follow a linear correlation with metal loading. For instance, the catalyst with the highest metal loading (10 wt.% NiRh/SiO2) did not display a proportional increase in conversion compared to 2 wt.% NiRh/SiO2. Rather, this behavior is related to changes in the catalyst’s surface and acidic properties caused by higher Ni content, as confirmed by TEM and XPS analyses.
HAADF-STEM results suggest that Rh and Ni do not form bimetallic alloys. Instead, the evidence points toward a partial decoration of Rh nanoparticles by Ni species. This structural arrangement implies spatial proximity between the two metals, which can promote synergistic effects without requiring alloy formation. Based on this configuration, Rh remains the primary active site for the reductive amination of cyclohexanone. Ni, although not directly involved in the main reaction pathway, likely contributes by modulating the electronic properties of Rh, facilitating H2 activation, or stabilizing reaction intermediates. Therefore, Ni acts as an electronic and structural promoter that enhances the overall catalytic performance of the material. Specifically, the average metal particle size increased with Ni loading, which in turn favored the formation of the byproduct cyclohexanol at elevated nickel concentrations.
One of the main advantages of using solid catalysts or applying heterogeneous catalysis processes is that the material can be easily recovered and reused in downstream processes. Cyclohexanone amination was performed using 100 mg of the 2 wt.% NiRh/SiO2 catalyst at 100 °C, under pressure of 4 bar NH3 and 2 bar H2, in accordance with standard procedures. Once the reaction concluded, the catalyst was isolated by filtration, rinsed with a non-polar solvent, dried, and reused without requiring an additional activation step, while maintaining identical reaction conditions.
As shown in Figure 11a, a decrease in conversion was observed throughout the number of cycles, which may be associated with surface oxidation of metal phases or blocking of active sites by adsorbed species. However, a 99.5% selectivity for cyclohexylamine was again achieved at 300 min of reaction (Figure 11b). The initial decrease in conversion or the longer induction period observed at the beginning of the experiment is often ascribed to the partial oxidation of the Rh surface or to the presence of adsorbed species left over from the previous catalytic cycle. Regarding selectivity, it even increased slightly from the first cycle to the fourth reuse, reaching 99.5% selectivity toward cyclohexylamine. This suggests that nickel in its oxidized state may play a significant role in enhancing selectivity and that the use of solid catalysts facilitates their recovery and reuse, reducing process costs and allowing high yields of the target product (Figure 11c).
There are few studies available that allow for a direct comparison, under similar conditions, of the amination of cyclohexanone with NH3 via heterogeneous catalysis. Some reported systems include Cu–Cr–La/γ-Al2O3 catalysts in the liquid phase or Cu/ZrO2 catalysts in the liquid–gas phase. Other studies, under conditions further removed from those investigated here, report the amination of cyclohexanone in the gas phase using group VIII metals. In all these works, the reported activity/selectivity varies depending on the reaction conditions; however, through different routes, the formation of primary cyclohexylamine is consistently observed.
Our system achieves maximum conversion and outstanding results compared with those reported for cyclohexanone amination, doubling the yield or selectivity of conventional heterogeneous systems (Cu–Cr–La/γ-Al2O3, Cu/ZrO2, Ni/SiO2) [28,29,45,47,57]. This represents a significant contribution to the field of cyclohexanone amination with NH3. The NiRh/SiO2 catalyst clearly outperforms conventional Cu- and Ni-based systems in terms of conversion, selectivity, and yield. The high selectivity suggests effective control over both the formation and subsequent hydrogenation of the intermediate imine, thereby avoiding the generation of byproducts such as secondary amines or condensed imines, which are commonly reported for Cu or Ni systems.
Some published studies conducted under similar conditions report recyclability with high conversions as well; however, our system stands out by achieving 99.5% selectivity after the fourth cycle, with a controlled loss in conversion, possibly attributable to partial deactivation of active sites or accumulation of adsorbed species. In this regard, the result is highly promising.
The performance of our system meets industrial-level efficiency and selectivity criteria. The catalysts exhibit remarkable stability and do not require reactivation to maintain high activity and selectivity. Furthermore, the use of NH3 as the primary nucleophile and the selective production of cyclohexylamine with minimal byproduct formation suggest a cleaner process, aligned with the principles of green chemistry.

3. Materials and Methods

Chemicals: RhCl3∙3H2O (≥99.9%, Aldrich, Darmstadt, Germany), Ni (NO3)2·6H2O (99.999%, Sigma-Aldrich, St. Louis, MO, USA), SiO2 (Sigma-Aldrich HP), cyclohexanone (≥99.0%, Merck, Darmstadt, Germany), and cyclohexane (≥99.9%, Merck, Darmstadt, Germany). Ammonia and hydrogen (99.995%) were purchased from Linde Gas (Santiago, Chile).
Catalysis Synthesis: The monometallic catalysts were synthesized from metallic precursor rhodium chloride by wet impregnation method with nominal loading of 1.7 wt.% Rh in 80 mL of water and SiO2 (120–150 µm). Four bimetallic catalysts of 1, 2, 5, and 10 wt.% Ni (Table S1) were synthesized by successive impregnation on Rh/SiO2. All catalysts were calcined at 400 °C at 10 °C min−1 for 2 h. Then, catalysts were activated under hydrogen flow at 30 mL min−1 at 10 °C min−1 until temperature reduction. From H2-TPR analysis, Rh/SiO2, 1 wt.% NiRh/SiO2, 2 wt.% NiRh/SiO2, 5 wt.% NiRh/SiO2, and 10 wt.% NiRh/SiO2 catalysts were reduced at 130, 240, 240, 250, and 250 °C, respectively.

Characterization of Catalysts and Reactions

Specific Surface Area, Pore Diameter, and Pore Volume: The surface area, pore diameter, and pore volume were determined by N2 physisorption at 77 K in a Micromeritics TriStar 3020 instrument (Norcross, GA, USA). The catalysts were first degassed at 393 K for 3 h. Brunauer–Emmett–Teller in the range of P/P° = 0.05–0.3 and Barrett, Joyner, and Halenda (BJH) methods were used to calculate surface area and pore volume, respectively.
Transmission Electron Microscopy (TEM): Transmission electron microscopy analysis was performed using a Jeol Model JEM-1200 EXII instrument (Tokyo, Japan), analyzing around 200 metallic particles in each sample.
High-Angle Annular Dark-Field Imaging–Scanning Transmission Electron Microscopy: Images were acquired on an FEI Tecnai F20 microscope (Hillsboro, OR, USA) equipped with a field emission source, a point-to-point resolution of 0.19 nm, and 200 kV. The samples were suspended in methanol, deposited onto a coated copper grid, and dried prior to analysis.
X-ray Diffraction (XRD): The XRD analysis was carried out using a Bruker diffractometer model D4Endeavor (Ludwigsburg, Germany), equipped with a Cu-Kα X-ray source. The analysis conditions were 40 kV and 20 mA. The diffractograms were recorded in a range of Bragg angles (2θ) between 2° and 90° at 0.02 counts per second.
X-ray Photoelectron Spectroscopy (XPS): The oxidation state of metals was determined in a Kratos Axis Ultra HAS instrument (Manchester, UK) with a Mg-Kα X-ray (1253.6 eV) source. XPS binding energies were referenced to the C 1s orbital (BE = 285.0 eV).
Temperature-Programmed Reduction (H2-TPR): The reduction temperature was performed in a TPR/TPD 2900 Micromeritics instrument (Norcross, GA, USA) with a thermal conductivity detector. For each analysis, 100 mg of catalyst was used. The carrier gas was 5% H2/Ar in flow of 40 mL min−1 at 10 °C min−1 from room temperature to 1000 °C.
Temperature-Programmed Desorption of Ammonia (NH3-TPD): The measurements were carried out using a TPR/TPD 2900 Micromeritics instrument with a thermal conductivity detector. Acidity of each catalyst was calculated from the amount of NH3 desorbed from room temperature to 1000 °C at 10 °C min−1.
Reactions: Catalytic tests were performed in a 100 mL glass-coated reactor equipped with a heating jacket, a gas entrainment impeller, a sampling line, and temperature, pressure, and stirring rate controllers. Ammonia and hydrogen were supplied directly to the reactor through a 3-way valve. Reductive amination of cyclohexanone was carried out under pressure of NH3 (4 bar) and H2 (2 bar) in 50 mL of cyclohexane at 100 °C, using a cyclohexanone/Rh molar ratio of 100:1 and the stirring rate of 800 rpm.
Recycling Tests: The recycling tests were carried out in the same conditions as described before. After each reaction, the catalyst was separated from the reaction mixture and washed with 20 mL of hexane to remove traces of non-polar organic matter from the surface of the catalyst, without affecting its metal structure. Then, it was dried in an oven at 60 °C prior to use, without any additional reactivation.
Chromatography: Samples from the reactor were withdrawn at different time intervals and analyzed with a GC-FID Shimadzu 2014 (Kyoto, Japan) equipped with a β-Dex 225 column (length 30 m × inner diameter 0.25 mm × film thickness 0.25 μm). The flame ionization detector and injector temperatures were 220 and 200 °C, respectively. The temperature program was 110 °C for 11 min. Helium was used as a carrier gas with a split ratio of 15. Cyclohexanone GC response factor was determined by analyzing solutions with known concentrations. Mass spectrometry allowed a comparison of the mass spectra (m/z) of the reaction compounds cyclohexanone, cyclohexylamine, and cyclohexanol.

4. Conclusions

The reductive amination of cyclohexanone with ammonia and hydrogen over NiRh/SiO2 catalysts has been successfully demonstrated, underscoring the potential for tuning catalytic activity and selectivity through bimetallic modification. The study highlights the superior performance of bimetallic catalysts compared to their monometallic counterparts. Specifically, the monometallic Rh/SiO2 catalyst exhibited high selectivity of 99.1% and a moderate conversion of 83.4%, while the incorporation of nickel significantly enhanced the catalytic performance.
In the comparative analysis of bimetallic systems, 2 wt.% NiRh/SiO2 catalyst emerged as the most effective, achieving a conversion of 99.8%, a yield of 96.4%, and a selectivity toward cyclohexylamine of up to 96.6% at 300 min of reaction. This catalyst not only demonstrated superior initial performance but also exhibited remarkable recyclability, maintaining a selectivity of 99.5% and a yield of 74.0% after four consecutive reaction cycles. These results are indicative of the robustness and sustainability of the bimetallic system in prolonged operational scenarios. The findings demonstrated that the integration of a non-noble metal such as nickel into Rh/SiO2 significantly increases catalytic efficiency. This modification not only optimizes the synthesis of primary amines via reductive amination but also presents a cost-effective and stable alternative to traditional noble-metal-based catalysts. The enhanced performance, coupled with the economic viability of nickel incorporation, positions the NiRh/SiO2 system as a promising candidate for other applications where selective synthesis of primary amines is required. In conclusion, bimetallic modification of Rh-based catalysts with nickel leads to substantial improvements in catalytic performance through conversion, yield, selectivity, and recyclability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15090803/s1, Figure S1. N2 physisorption isotherms of Rh and RhNi supported catalysts. Figure S2. HAADF-STEM of Rh/SiO2 catalyst: (a) Rh as bright dots, (b) HAADF-STEM-EDX, (c) bright-field TEM image and (d) higher magnification HAADF-STEM image. Figure S3. Micrographs of 1 wt.% NiRh/SiO2 catalyst: (a) HAADF-STEM-EDX, (b) HAADF-STEM image and particle size distribution and (c) higher magnification HAADF-STEM image. Table S1. Nominal Rh and Ni loadings in the synthesized catalysts.

Author Contributions

Conceptualization, D.R. and K.M.; methodology, K.M. and C.S.; XPS analysis, A.P. and R.C.; HAADF-STEM analysis, J.L. and R.C.; writing, K.M. and D.R.; supervision, D.R.; project administration, D.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by ANID, FONDECYT grant numbers 1220355 and 1201936, and ANID PhD scholarship grant number 21211283.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank the Instituto de Geología Económica Aplicada (GEA) of the University of Concepción for the facility for performing X-ray analyses.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
CHCyclohexanone
XPSX-ray Photoelectron Spectroscopy
NH3-TPDTemperature-Programmed Desorption of Ammonia
TEMTransmission Electron Microscopy
HAADFHigh-Angle Annular Dark-Field Imaging
STEMScanning Transmission Electron Microscopy
XRDX-ray Diffraction
H2-TPRTemperature-Programmed Reduction Using H2
B.E.T.Brunauer–Emmett–Teller
BJHBarrett, Joyner, and Halenda

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Catalysts 15 00803 sch001
Scheme 1. Reaction scheme for reductive amination of cyclohexanone.
Scheme 1. Reaction scheme for reductive amination of cyclohexanone.
Catalysts 15 00803 sch001
Catalysts 15 00803 g001
Figure 1. TEM micrographs and histograms of (a) Rh/SiO2, (b) 1 wt.% NiRh/SiO2, (c) 2 wt.% NiRh/SiO2, (d) 5 wt.% NiRh/SiO2, and (e) 10 wt.% NiRh/SiO2.
Figure 1. TEM micrographs and histograms of (a) Rh/SiO2, (b) 1 wt.% NiRh/SiO2, (c) 2 wt.% NiRh/SiO2, (d) 5 wt.% NiRh/SiO2, and (e) 10 wt.% NiRh/SiO2.
Catalysts 15 00803 g001
Catalysts 15 00803 g002
Figure 2. HAADF-STEM of 10 wt.% NiRh/SiO2 catalyst: (a) micrograph and size distribution, (b) HAADF-STEM-EDX.
Figure 2. HAADF-STEM of 10 wt.% NiRh/SiO2 catalyst: (a) micrograph and size distribution, (b) HAADF-STEM-EDX.
Catalysts 15 00803 g002
Catalysts 15 00803 g003
Figure 3. XRD profiles of Rh and RhNi supported catalysts.
Figure 3. XRD profiles of Rh and RhNi supported catalysts.
Catalysts 15 00803 g003
Catalysts 15 00803 g004aCatalysts 15 00803 g004b
Figure 4. XPS spectra of (a) Rh/SiO2, (b) 1 wt.% NiRh/SiO2, (c) 2 wt.% NiRh/SiO2, (d) 5 wt.% NiRh/SiO2, and (e) 10 wt.% NiRh/SiO2.
Figure 4. XPS spectra of (a) Rh/SiO2, (b) 1 wt.% NiRh/SiO2, (c) 2 wt.% NiRh/SiO2, (d) 5 wt.% NiRh/SiO2, and (e) 10 wt.% NiRh/SiO2.
Catalysts 15 00803 g004aCatalysts 15 00803 g004b
Catalysts 15 00803 g005
Figure 5. TPR profiles of Rh and RhNi supported catalysts.
Figure 5. TPR profiles of Rh and RhNi supported catalysts.
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Catalysts 15 00803 g006
Figure 6. NH3-TPD analysis of Rh and RhNi supported catalysts.
Figure 6. NH3-TPD analysis of Rh and RhNi supported catalysts.
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Catalysts 15 00803 g007
Figure 7. Reductive amination of cyclohexanone: (a) Conversion using reduced Rh/SiO2, using oxidized Rh/SiO2, and without catalyst. (b) Selectivity toward cyclohexylamine and cyclohexanol as a function of cyclohexanone conversion over Rh/SiO2 catalyst. Conditions: 1 mol% Rh; 0.04 mol L−1; 50 mL cyclohexane; 4 bar NH3; 2 bar H2; 100 °C; 800 rpm.
Figure 7. Reductive amination of cyclohexanone: (a) Conversion using reduced Rh/SiO2, using oxidized Rh/SiO2, and without catalyst. (b) Selectivity toward cyclohexylamine and cyclohexanol as a function of cyclohexanone conversion over Rh/SiO2 catalyst. Conditions: 1 mol% Rh; 0.04 mol L−1; 50 mL cyclohexane; 4 bar NH3; 2 bar H2; 100 °C; 800 rpm.
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Catalysts 15 00803 g008
Figure 8. Ammonia and hydrogen pressure effect on cyclohexanone amination over Rh/SiO2. (a) Conversion of cyclohexanone as a function of time. (b) Selectivity toward cyclohexylamine as a function of time. (c) Selectivity toward cyclohexylamine as a function of conversion. (d) Selectivity toward cyclohexanol as a function of conversion. Conditions: 1 mol % Rh; 0.04 mol L−1; 4 bar NH3; 2 bar H2; 50 mL cyclohexane; 100 °C; 800 rpm; 300 min.
Figure 8. Ammonia and hydrogen pressure effect on cyclohexanone amination over Rh/SiO2. (a) Conversion of cyclohexanone as a function of time. (b) Selectivity toward cyclohexylamine as a function of time. (c) Selectivity toward cyclohexylamine as a function of conversion. (d) Selectivity toward cyclohexanol as a function of conversion. Conditions: 1 mol % Rh; 0.04 mol L−1; 4 bar NH3; 2 bar H2; 50 mL cyclohexane; 100 °C; 800 rpm; 300 min.
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Catalysts 15 00803 g009
Figure 9. Effect of metal molar ratio on CH amination. (a) Conversion of cyclohexanone over Rh/SiO2. (b) Selectivity toward cyclohexylamine as a function of cyclohexanone conversion. (c) Product distribution at 300 min. Conditions: 0.04 mol L−1; 4 bar NH3; 2 bar H2; 50 mL cyclohexane; 100 °C; 800 rpm.
Figure 9. Effect of metal molar ratio on CH amination. (a) Conversion of cyclohexanone over Rh/SiO2. (b) Selectivity toward cyclohexylamine as a function of cyclohexanone conversion. (c) Product distribution at 300 min. Conditions: 0.04 mol L−1; 4 bar NH3; 2 bar H2; 50 mL cyclohexane; 100 °C; 800 rpm.
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Catalysts 15 00803 g010
Figure 10. CH reductive amination over NiRh/SiO2 catalysts. (a) Conversion over time. (b) Product distribution at 240 min. (c) Selectivity toward cyclohexylamine as a function of cyclohexanone conversion. (d) Selectivity toward cyclohexanol as a function of cyclohexanone conversion. Conditions: 100 mg of catalyst; 0.04 mol L−1; 50 mL cyclohexane; 4 bar NH3; 2 bar H2; 100 °C; 800 rpm.
Figure 10. CH reductive amination over NiRh/SiO2 catalysts. (a) Conversion over time. (b) Product distribution at 240 min. (c) Selectivity toward cyclohexylamine as a function of cyclohexanone conversion. (d) Selectivity toward cyclohexanol as a function of cyclohexanone conversion. Conditions: 100 mg of catalyst; 0.04 mol L−1; 50 mL cyclohexane; 4 bar NH3; 2 bar H2; 100 °C; 800 rpm.
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Catalysts 15 00803 g011
Figure 11. Recycling of 2 wt.% NiRh/SiO2 catalyst on cyclohexanone amination. (a) Conversion. (b) Product distribution at 300 min reaction. (c) Selectivity toward cyclohexylamine as a function of cyclohexanone conversion. Conditions: 1 mol% Rh; 0.04 mol L−1; 50 mL cyclohexane; 4 bar NH3; 2 bar H2; 800 rpm; 300 min.
Figure 11. Recycling of 2 wt.% NiRh/SiO2 catalyst on cyclohexanone amination. (a) Conversion. (b) Product distribution at 300 min reaction. (c) Selectivity toward cyclohexylamine as a function of cyclohexanone conversion. Conditions: 1 mol% Rh; 0.04 mol L−1; 50 mL cyclohexane; 4 bar NH3; 2 bar H2; 800 rpm; 300 min.
Catalysts 15 00803 g011
Table 1. Textural properties of Rh and RhNi over SiO2 catalysts from N2 physisorption.
Table 1. Textural properties of Rh and RhNi over SiO2 catalysts from N2 physisorption.
Catalyst *SB.E.T
(m2 g−1) a		dpore
(nm) b		Vpore
(cm3 g−1) c
SiO2319 10.6 0.85
Rh/SiO229111.40.82
1 wt.% NiRh/SiO228010.90.77
2 wt.% NiRh/SiO230410.90.84
5 wt.% NiRh/SiO227210.50.72
10 wt.% NiRh/SiO226310.10.67
a Surface area by physisorption–B.E.T. model, b pore diameter, and c pore volume. * From now on, wt. is omitted from figures.
Table 2. Metal particle size (d) and dispersion (D) by TEM.
Table 2. Metal particle size (d) and dispersion (D) by TEM.
CatalystdTEM
(nm)		DTEM
(%)
Rh/SiO21.9546.8
1 wt.% NiRh/SiO21.6555.3
2 wt.% NiRh/SiO21.9546.8
5 wt.% NiRh/SiO21.5858.1
10 wt.% NiRh/SiO21.88–7.1248.6
Table 3. Binding energies (eV) of internal levels and atomic ratios of reduced Rh and RhNi catalysts.
Table 3. Binding energies (eV) of internal levels and atomic ratios of reduced Rh and RhNi catalysts.
CatalystRhSiNiRatio
BE(eV) 3d5/2Rh°
Rh3+		BE(eV) 2pBE(eV) 2p3/2(Ni/Si)at(Rh/Si)at(Rh/Ni)at
Rh/SiO2307.181.0103.5--0.0040-
308.719.0
1 wt.% NiRh/SiO2307.270.8103.6855.5 ± 5.8 †0.00240.00361.5286
309.029.2
2 wt.% NiRh/SiO2307.178.4103.4855.7 ± 5.1 †0.01710.00560.3254
309.121.6
5 wt.% NiRh/SiO2307.178.8103.4855.7 ± 5.5 †0.00740.00320.4378
309.121.2
10 wt.% NiRh/SiO2307.084.1103.4855.5 ± 5.5 †
852.5 *		0.00680.00290.4236
309.215.9
† Shift (eV) of the satellite peak relative to the main peak, * Ni° at 852.5 eV.
Table 4. H2-TPR and NH3-TPD data of Rh and RhNi catalysts.
Table 4. H2-TPR and NH3-TPD data of Rh and RhNi catalysts.
CatalystH2 Consumption
(mmol gcat−1)		NH3 Desorption
(mmol gcat−1)
Rh/SiO20.4690.514
1 wt.% NiRh/SiO21.4210.214
2 wt.% NiRh/SiO20.7660.132
5 wt.%NiRh/SiO20.3960.261
10 wt.% NiRh/SiO22.7320.328
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Morales, K.; Sandoval, C.; Peixoto, A.; Chimentão, R.; Llorca, J.; Ruiz, D. Reductive Amination of Cyclohexanone via Bimetallic Rh-Ni Catalysts: A Pathway to Improved Catalytic Efficiency. Catalysts 2025, 15, 803. https://doi.org/10.3390/catal15090803

AMA Style

Morales K, Sandoval C, Peixoto A, Chimentão R, Llorca J, Ruiz D. Reductive Amination of Cyclohexanone via Bimetallic Rh-Ni Catalysts: A Pathway to Improved Catalytic Efficiency. Catalysts. 2025; 15(9):803. https://doi.org/10.3390/catal15090803

Chicago/Turabian Style

Morales, Karen, Camila Sandoval, Andreia Peixoto, Ricardo Chimentão, Jordi Llorca, and Doris Ruiz. 2025. "Reductive Amination of Cyclohexanone via Bimetallic Rh-Ni Catalysts: A Pathway to Improved Catalytic Efficiency" Catalysts 15, no. 9: 803. https://doi.org/10.3390/catal15090803

APA Style

Morales, K., Sandoval, C., Peixoto, A., Chimentão, R., Llorca, J., & Ruiz, D. (2025). Reductive Amination of Cyclohexanone via Bimetallic Rh-Ni Catalysts: A Pathway to Improved Catalytic Efficiency. Catalysts, 15(9), 803. https://doi.org/10.3390/catal15090803

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