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Article

# Bimetallic Pd- and Co-Containing Mesoporous Carbons as Efficient Reusable Nanocatalysts for Hydrogenations of Nitroarenes and Enones Under Mild and Green Conditions

Mohamed Enneiymy <fnref n="1" />,<fnref n="2" />, Cyril Vaulot <fnref n="1" />,<fnref n="2" /> ID, Loïc Vidal <fnref n="1" />,<fnref n="2" />, Camelia Matei Ghimbeu <fnref n="1" />,<fnref n="2" />,* ID, Claude Le Drian <fnref n="1" />,<fnref n="2" /> and Jean-Michel Becht <fnref n="1" />,<fnref n="2" />,*

## Abstract

Easy and rapid preparations of magnetic Co- and Pd-containing mesoporous carbons (IM1, IM2 and DM) from green phenolic resins, amphiphilic templates and metallic salts via two synthetic routes are reported. Catalysts IM1 and IM2 are prepared via an indirect method involving two steps, i.e., the preparation of Co-containing mesoporous carbons with different Co contents (2.5 and 12.5%) and the further introduction of Pd (2.3%) via impregnation using a solution of a Pd salt and a process of thermal reduction. The mesoporous carbon obtained contains two distinct crystalline metallic phases, i.e., Co particles of 5.0 nm (IM1) and Pd nanoparticles of ~1.3 nm (IM1), while the increase in Co content triggers higher Co particle sizes of 23 nm and Pd particle sizes of 1.3 and 6.8 nm (IM2). Differently, the catalyst DM is prepared via direct synthesis, in one step, including all precursors and both metal salts. This results in Pd₅₀-Co₅₀ nanoalloys of 6.5 nm uniformly dispersed in the carbon matrix. The reactivity and reusability of catalysts IM1, IM2 and DM were then ascertained in organic synthesis for hydrogenations of nitroarenes and enones. It turned out that no reactions were observed in the presence of the catalyst DM due to the presence of Co in Pd₅₀-Co₅₀, which deactivates the catalytic activity of Pd. Gratifyingly, catalysts IM1 and IM2 were very efficient for mild hydrogenations of both nitroarenes and enones using only 5 mequiv. of supported Pd in EtOH at room temperature. The smaller Pd particle sizes (1.3 nm) and the high surface-to-volume area are probably responsible for the high reactivity observed. Catalysts IM1 and IM2 can be recovered by application of an external magnetic field. However, a more efficient magnetic recovery of catalyst IM2 compared to IM1 was observed due to its higher Co content. Catalyst IM2 can be successfully reused at least seven times without a loss of efficiency. Finally, almost-Pd-free products can be obtained directly after reaction without any purification step, since the Pd leaching is very low (<0.1% of the initial amount), thus decreasing waste and increasing the reaction's efficiency.

**Keywords:** heterogeneous catalysis; magnetic catalysts; palladium; hydrogenations

## 1. Introduction

Aromatic primary amines (APA) constitute key intermediates for the preparation of numerous compounds, finding widespread applications in the synthesis of pharmaceu-

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Academic Editor: Oriana Piermatti

Received: 10 October 2025

Revised: 7 November 2025

Accepted: 14 November 2025

Published: 2 December 2025

**Citation:** Enneiymy, M.; Vaulot, C.; Vidal, L.; Matei Ghimbeu, C.; Le Drian, C.; Becht, J.-M. Bimetallic Pd- and Co-Containing Mesoporous Carbons as Efficient Reusable Nanocatalysts for Hydrogenations of Nitroarenes and Enones Under Mild and Green Conditions. *Catalysts* **2025**, *15*, 1126. https://doi.org/10.3390/catal15121126

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Institut de Science des Matériaux de Mulhouse (IS2M)—UMR CNRS 7361, Université de Haute-Alsace, F-68100 Mulhouse, France</footnote>

<footnote n="2">² Institut de Science des Matériaux de Mulhouse (IS2M)—UMR 7361, Université de Strasbourg, F-67000 Strasbourg, France</footnote>

<footnote>* Correspondence: camelia.ghimbeu@uha.fr (C.M.G.); jean-michel.becht@uha.fr (J.-M.B.)</footnote>
ticals, dyes, chemical fibres, pesticides and polymers [1,2]. The development of efficient preparation methods of APA has, therefore, elicited huge interest [3]. The Béchamp reduction of nitroarenes using a stoichiometric amount or an excess of Fe/HCl is certainly one of the oldest and most popular methods, but it suffers from severe drawbacks, including slow reaction rates, harsh conditions and the formation of significant amounts of waste [4]. Other routes to obtain APA include nucleophilic aromatic substitution reactions. However, drastic reaction conditions, a poor substrate scope or the use of toxic organic solvents limit their applications in fine chemistry [5]. More recently, Cu-catalyzed couplings between expensive aryl iodides [6] or areneboronic acids [7] and an excess of ammonia (NH₃), or the Buchwald–Hartwig Pd-catalyzed amination have been disclosed [6].

Another important route to APA consists of the reduction of nitroarenes, easily obtained by nitration of unfunctionalized aromatic compounds via an electrophilic aromatic substitution [8]. In this respect, various reductions of nitroarenes in the presence of a soluble Pd-catalyst combined with a sensitive ancillary ligand in the presence of NaBH₄ or H₂ as a reducing agent are traditionally used [9–11]. However, palladium is a rare natural resource that must be preserved, and its price is subject to erratic fluctuations (from €98,000/kg in 2022 to €29,000/kg currently). Since soluble Pd catalysts are almost impossible to recover for reuse, various efficient heterogeneous reusable Pd-[12,13] but also Pt-[14], Rh-[15] and Au-[16] catalysts have been discussed for applications in the reduction of nitroarenes using NaBH₄ [12] or H₂ [13] as a reducing agent. It is noteworthy that the reduction of nitroarenes using NaBH₄, often in excess, is not chemospecific and generates significant amounts of salt by-products, which become problematic when syntheses are performed on large scales [17]. The use of H₂ as a reducing agent is much more convenient from an industrial point of view, since no by-products are generated. However, in some cases, harsh conditions (high pressure of H₂, high temperature) and hazardous solvents are required [18,19].

In recent years, several groups have shown that heterogeneous reusable catalysts containing Pd NPs allow mild hydrogenations of nitroarenes at 1 atm H₂ and rt. For example, Pd-containing porous organic polymers [20], Schiff-base polymers [21], halloysite bearing melamine-based polymers modified by cyclodextrins [22] or even bimetallic Pd-Pt [23] or Cu-Ni [24] catalysts have been disclosed. In most cases, the Pd-catalyst can be recovered and reused without a noticeable loss of efficiency [13]. But the generalization of their use in organic synthesis is severely limited by their long and tedious multi-step preparations, requiring, in most cases, hazardous starting materials or substrates [21]. In addition, the efficient and quantitative recovery of the catalyst by filtration or centrifugation can be another limitation. To overcome this latter drawback, various magnetic heterogeneous Pd-catalysts have been described [25]. Magnetic catalysts present the major advantage of being able to be recovered selectively in a few seconds from any reaction mixture by the simple application of an external magnet. Despite the fact that numerous reusable magnetic catalysts have been reported for C-C bond forming reactions, many fewer have been reported for the efficient and versatile reduction of nitroarenes. In most cases, magnetic catalysts are based on Fe₃O₄ nanoparticles. In this way, reductions of nitroarenes have been reported using Pd NPs grafted on Fe₃O₄ yolk-shell structured periodic mesoporous organosilica [26], on porous silica shell carbon-coated cobalt NPs [27], on porous magnetic core-shell POP nanospheres [28], on graphene oxide/carbon nanotubes-Fe₃O₄ [29] on carbon-encapsulated Fe NPs [30], or on core-shell and cyclodextrin-functionalized Fe₃O₄ [31] in the presence of 1 atm H₂ [27], or NaBH₄ [26,28,31]. Similar to their non-magnetic heterogeneous reusable counterparts, the major drawbacks of the magnetic catalysts developed to date lies in their multi-step preparations, which can be long and tedious [27]; their significant amounts of supported Pd used for reductions (>10 mequiv. in most cases) and often long reaction
times [27]. Additionally, NaBH₄ is often used as the reducing agent, therefore generating inorganic wastes at the end of the reaction [26,28,31].

In addition, saturated ketones are other particularly important compounds found in numerous flagrances and pharmaceutically active compounds [32,33]. Enones, obtained via well-known condensation reactions [34], constitute traditionally valuable precursors of saturated ketones that are easily obtained via chemospecific reductions in the carbon-carbon double bond of enones. However, despite the huge applications of saturated ketones in various fields, efficient and easily accessible heterogeneous reusable catalysts for chemospecific reductions of enones to their corresponding ketones are scarce [35]. For example, the reduction of enones has been performed under continuous flow synthesis using Ni Raney or Pd/C and 1 atm of H₂ [36]. Triocylamine-stabilized Rh NPs supported on γ-Al₂O₃ have also been used for the hydrogenation of enones [37]. However, the preparation of this catalyst involves phase vapour deposition of the precious metal from mesitylene-solvated Rh atoms. Ionic liquid-stabilized Pd NPs in glycerol have also been researched for uses in the preparation of several chemoselective hydrogenations and, in particular, of enones. Almost-Pd-free saturated ketones have been obtained after reaction, but a rapid decrease in the efficiency of the catalyst during reuse has been observed due to the agglomeration of Pd NPs [38]. In 2025, Pd NPs supported on a terpyridine-Zn-based MOF were developed for the hydrogenation of C=C bonds but a decrease in the yield was observed during the reuse of the catalyst. Finally, a four-step preparation of a Pd-containing magnetic catalyst composed of a Fe₃O₄ core and recovered by a carbon shell has been disclosed [39]. This catalyst was successfully used for reductions of enones in EtOH using 1 atm pressure of H₂ but used a significant amount (18.8 mequiv.) of supported Pd. In addition, the possibility of reusing this catalyst has not been determined.

Today, it is therefore of great importance to develop easily and rapidly accessible heterogeneous reusable Pd catalysts for versatile uses in organic chemistry. For this purpose, some years ago, some of us decided to focus our attention on the development of efficient reusable Pd-catalysts obtained in only one or two steps from non-toxic or even biosourced precursors in "green" conditions. For this purpose, mesoporous carbons are a promising choice owing to their short, efficient and environmental-friendly synthesis routes [40]. Additionally, they present a high surface area with tuneable pores, active sites and surface functionalities, which provide good catalyst dispersion and control over particle size. Inspired by these results, Pd-containing mesoporous carbons have been prepared and used successfully for Suzuki–Miyaura reactions [41–43] or hydrogenations of nitroarenes under mild conditions [44,45]. However, up to now, no easily accessible and highly efficient magnetic Pd-containing mesoporous carbon catalysts for general use in the hydrogenation of both nitroarenes and enones have been reported. In addition, it is noteworthy that some magnetic Co-containing carbon catalysts have been reported in the literature; however, their utilization in organic synthesis remains much less investigated than that of Fe₃O₄-based counterparts [42]. Notably, Co-based catalysts are particularly promising since they exhibit superior magnetic saturation compared to Fe-based catalysts (for example MS(Co) = 158 emu·g⁻¹ vs. MS (magnetite) < 92 emu·g⁻¹), along with outstanding chemical and thermal stability [46]. Accordingly, we aimed to design Co-containing Pd catalysts, wherein the cobalt particles further improve the magnetic recovery of the catalyst at the end of the reaction We describe herein the easy and rapid preparation methods and characterizations of reusable magnetic Co- and Pd-containing mesoporous carbon nanocatalysts and their applications for mild, efficient and versatile reductions of nitroarenes and enones.
## 2. Results and Discussion

### 2.1. Preparations of Catalysts IM1, IM2 and DM

Inspired by previous reports from our group [47], the Pd- and Co-containing mesoporous carbons IM1 and IM2 were prepared using a two-step bottom-up approach via an indirect method (Scheme 1, top). In the first step, two types of Co-containing mesoporous carbons were prepared with different amounts of Co salt, followed by the introduction of the Pd NPs. For this purpose, self-assembly of phloroglucinol-glyoxal resin in the presence of the surfactant Pluronic F-127 and Co(NO₃)₂·6H₂O in ethanol, followed by thermal treatment at 600 °C under an Ar atmosphere, afforded us the magnetic mesoporous carbons IMCo1 (2.5% Co) and IMCo2 (12.5% Co). During the thermal annealing, the resin decomposes during the formation of the carbon framework and the surfactant is degraded, leading to micro- and meso-pores, while the Co salt undergoes thermal decomposition to form cobalt oxides, which are reduced to metallic Co by carboreduction reactions. Usually, the obtained composite exhibits metal particles distributed within the internal walls or throughout the carbon matrix rather than on the external surface, as demonstrated in the literature [48,49].

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of Pd-Co mesoporous carbon catalysts by a two-step indirect method (IM1, IM2) and a one-pot direct method (DM).</figcaption></figure>

Both Co-carbon materials were then impregnated with a solution of Pd(OAc)₂ in chloroform followed by reduction at 300 °C under an Ar/H₂ atmosphere. This step allows the impregnation of the Pd salt in the carbon pores and its further decomposition and reduction to Pd metal. Therefore, the desired heterogeneous bimetallic mesoporous carbon catalysts IM1 (2.5% Co and 2.3% Pd) and IM2 (12.5% Co and 2.3% Pd) were obtained. In order to ascertain the influence of the method of preparation of the catalysts on their properties and catalytic efficiency, a mesoporous carbon DM containing Pd₅₀-Co₅₀ nanoalloys was also prepared via a direct method (Scheme 1, bottom) as reported earlier by our group [47]. This synthesis was similar to that of IMCo1 and IMCo2; therefore, the Pd₅₀-Co₅₀ particles were expected to be more confined in the carbon framework than in the pores [48,49]. The amount of Pd present in the fresh catalysts was ascertained after the complete mineralization of the catalysts followed by complexation (for further details see Supplementary Materials). It is noteworthy that materials IM1, IM2 and DM can be stored for several months without particular precautions.
## 2.2. Characterizations of Catalysts IM1, IM2 and DM

### 2.2.1. X-Ray Diffraction (XRD)

The XRD diffractograms (Figure 1) show the patterns of Co-based and Pd-Co-based mesoporous carbon materials. When a two-step synthesis is used, it can be seen that in the first step of the synthesis, resulting in the formation of **IMCo1** and **IMCo2**, it is mainly the metallic Co phase that is formed as a mixture of the hexagonal (hcp) and cubic (fcc) phases [50]. However, the hexagonal phase is more predominant in both materials. It is worth noting that increasing the amount of Co in the material from 2.5 to 12.5 wt.% (**IMCo1** to **IMCo2**), leads to a significant increase in the intensity of the peaks, indicating an increase in the order of the coherent domains and crystallite size (8.4 nm to 23.7 nm). In the second step of the synthesis, when the Pd is incorporated (**IM1** and **IM2**), the peaks related to the Co phase are preserved, and, in addition, very small peaks associated with a Pd phase are observed, especially for **IM2** materials. The crystallite size of the Pd phase was found to be 3.2 nm and 8.4 nm, respectively, in **IM1** and **IM2**. Therefore, the Co content in the materials increased as well as the final size of the Pd nanoparticles. The results suggest the existence of two different crystalline phases, Co and Pd, with different crystallite sizes, which is opposite to the Pd-Co materials obtained by the one-pot method (**DM**). In this material, only peaks corresponding to Pd (fcc) crystalline structure are seen at ~40.0°, 46.0°, 68.0° and 82°. The position of the (111) peak at higher 2θ angles compared to single phase Pd, as well as the absence of Co peaks, indicate the formation of Pd–Co alloys [47]. The crystallite size is ~5.2 nm, which is between that of **IM1** and **IM2**. Moreover, for all materials, in addition to Pd and Co peaks, a very broad peak is noticed at low 2θ angles (~20°) due to the carbon framework, which present a high degree of disorder.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** X-ray diffractions of Co mesoporous carbons **IMCo1**, **IMCo2** and Pd-Co catalysts **IM1**, **IM2** and **DM**.</figcaption></figure>

### 2.2.2. TEM Analyses

TEM and STEM analyses were conducted to gain insights into the particle size distribution (Figure 2). For **IMCo1**, the Co average particle size was ~5.0 nm. They are observed as highly dispersed small, dark entities in the porous carbon network. By increasing the
amount of Co in the carbon to 12.5% (IMCo2), the average particle size significantly increased to 23.0 nm, a tendency that was somewhat expected (Figure 2a,b). The particles are easily visible with spherical to oval shapes. When the Pd phase is added to IMCo1 and IMCo2 to obtain IM1 and IM2, the Pd particles exhibit ultra-small sizes (1.3 nm on average), as illustrated in Figure 2c,d. This observation is in line with our previous results [43]. However, for IM2, the small Pd particles (1.3 nm) co-exist with larger particles (6.8 nm on average), as indicated by the double particle size distribution histogram. EDX analysis of this sample (see Figure S1 in the Supplementary Materials) shows that the large particles contain both Pd and Co, suggesting that a Pd-Co mixture or alloy was formed due to a reaction between the Co particles and the Pd acetate precursor. However, considering the XRD results and the occurrence of Co crystalline phases, it is more likely that the predominant phase is a Pd-Co mixture; otherwise, if a Pd-Co alloy had been formed preferentially, no Co peaks would have been observed. Nevertheless, the presence of a small quantity of this phase cannot be excluded either. Differently, for the DM material (Pd<sub>50</sub>Co<sub>50</sub>), one single particle population is seen, with average particle sizes of 6.5 nm, homogenously distributed in the carbon framework (Figure 2e). For all materials, the NPs are well dispersed in the carbon matrix (pores or walls), which is an important issue to ensure efficient catalysts. Regarding the Pd surface chemistry, our previous studies demonstrated that the particles are covered by a small layer of palladium oxide [43,44].

<figure><img src="image_5.png" /><figcaption>**Figure 2.** TEM images and Co NP size distribution histograms of (a) IMCo1 and (b) IMCo2 and (c) DM. Inset: particle size distribution histograms; STEM images and Pd NPs of (d) IM1 and (e) IM2.</figcaption></figure>

### 2.2.3. Nitrogen Adsorption/Desorption Isotherms

The N<sub>2</sub> adsorption/desorption isotherms and their pore size distributions are illustrated below (Figure 3). All materials exhibit a mixture of type I/type IV isotherm according to IUPAC classification, which is specific to micro- and meso-porous materials (Figure 3a). The steep increase in the N<sub>2</sub> adsorbed volume in the low-pressure region is associated with the monolayer filing of micropores, whereas the hysteresis loop at higher relative pressures indicates the N<sub>2</sub> condensation in mesopores. The micropore isotherm region is rather similar for all materials, translating into similar BET surface area comprised between
705–843 m² g⁻¹ (Table 1). Worth to note that for the indirect method, the S<sub>BET</sub> is the highest when low amount of metal is present in the carbon structure (IMCo1), and a decrease in the surface occurs when the amount of Co is increased (IMCo2) and further when the second metal (Pd) is introduced in the carbon structure. These particles can block some pores, explaining therefore this behaviour. Nevertheless, the direct method (DM) leads to a high specific surface area, rather approaching that of IMCo1. Worth to mention that the Pd-Co NPs obtained by the direct method are more likely to be located in the carbon walls or surface rather than the pores, due to their confinement during the concomitant synthesis as the carbon network [48,49]. This allows to preserve a high BET surface area. In respect to the high-pressure hysteresis, it can be seen that IMCo1 and IM1 materials exhibit a type H4 shape, with high adsorbed N₂ volumes (0.71 and 0.81 cm³ g⁻¹), while the other materials present a H2 type shape, with lower adsorbed N₂ volumes (0.50 to 0.56 cm³ g⁻¹ for IM2 and DM, respectively) and a smaller hysteresis loop area. Indeed, the IMCo1 and IM1 shows larger mesopore size (average ~7 nm vs. ~4 nm for the other materials, Figure 3b) and higher mesopore volume (Table 1). Micropores below 2 nm are seen as well for all material (Figure 3b). The significant differences between the pore sizes and volumes can be attributed to the location of some particles in the pores, but also to the impact of the metal salt type/quantity on the micelle's formation, which directly influence the mesopore formation.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** (a) Nitrogen adsorption isotherms for Co-containing mesoporous carbons IMCo1, IMCo2 and catalysts IM1, IM2 and DM. (b) Pore size distributions using the NLDFT method.</figcaption></figure>

<table><caption>**Table 1.** Textural properties of materials determined from N₂ sorption measurements.</caption><thead><tr><th>Material</th><th>S<sub>BET</sub><br>m<sup>2</sup>g<sup>-1</sup></th><th>V<sub>T</sub><br>cm<sup>3</sup>g<sup>-1</sup></th><th>V<sub>micro</sub><br>cm<sup>3</sup>g<sup>-1</sup></th><th>V<sub>meso</sub><br>cm<sup>3</sup>g<sup>-1</sup></th></tr></thead><tbody><tr><td>IMCo1</td><td>843</td><td>0.71</td><td>0.38</td><td>0.33</td></tr><tr><td>IM1</td><td>749</td><td>0.81</td><td>0.31</td><td>0.50</td></tr><tr><td>IMCo2</td><td>752</td><td>0.55</td><td>0.30</td><td>0.23</td></tr><tr><td>IM2</td><td>705</td><td>0.50</td><td>0.29</td><td>0.21</td></tr><tr><td>DM</td><td>817</td><td>0.56</td><td>0.37</td><td>0.19</td></tr></tbody></table>

## 2.3. Reductions of Nitroarenes

The reaction conditions were optimized using 4-nitroacetophenone as a starting material (Table 2). It turned out that the hydrogenation was chemospecific and afforded, almost quantitatively, the desired primary aromatic amine **1a** in only 1.5 h using either the catalyst **IM1** or **IM2** under 1 atm of H₂ at rt in EtOH (entries 1–2). The possibility of recovering the
catalysts **IM1** or **IM2** using a magnet was then determined. It turned out that the catalyst **IM2** (containing 12.5% Co) was rapidly and quantitatively recovered, whereas the catalyst **IM1** (containing only 2.5% Co) remained partially suspended in the reaction medium even in the presence of an external magnet, and only <70% of its initial amount was recovered. The study was therefore pursued with the catalyst **IM2**. Lowering the amount of supported Pd gave **1a** with only a 65% yield (entry 3) but increasing the reaction time to 2.5 h afforded **1a** with a 99% yield (entry 4). The amount of supported Pd could be reduced to 1 mequiv. but required a longer reaction time (entry 5).

<table><caption>**Table 2.** Optimization of the reaction conditions for the reduction of nitroarenes.</caption><thead><tr><th>Entry <sup>(a)</sup></th><th>Catalyst</th><th>Pd (Mequiv.)</th><th>Reaction Time (h)</th><th>Yield % <sup>(b)</sup></th></tr></thead><tbody><tr><td>1</td><td><b>IM1</b></td><td>5</td><td>1.5</td><td>99</td></tr><tr><td>2</td><td><b>IM2</b></td><td>5</td><td>1.5</td><td>99</td></tr><tr><td>3</td><td><b>IM2</b></td><td>2.5</td><td>1.5</td><td>65</td></tr><tr><td>4</td><td><b>IM2</b></td><td>2.5</td><td>2.5</td><td>99</td></tr><tr><td>5</td><td><b>IM2</b></td><td>1</td><td>18</td><td>99</td></tr><tr><td>6</td><td><b>DM</b></td><td>5</td><td>2</td><td>No reaction</td></tr><tr><td>7</td><td><b>Pd/C</b></td><td>5</td><td>2</td><td>60</td></tr><tr><td>8</td><td><b>Pd Encat NP30</b></td><td>5</td><td>2</td><td>26</td></tr><tr><td>9</td><td><b>Pd(PPh<sub>3</sub>)<sub>4</sub></b></td><td>5</td><td>2</td><td>No reaction</td></tr></tbody></table>

It is worth noting that no reaction was observed in the presence of the catalyst **DM** (entry 6), probably due to the inactivity of the Pd-Co alloy particles. This can be probably related to the larger particles of Pd-Co alloy in **DM** compared to the Pd particle size in **IM1** and **IM2** [38,51]. Another hypothesis can be linked to the crystalline structure of Pd, which was significantly changed by substitution with Co. In our previous work, we demonstrated that incorporating cobalt into the palladium structure disrupts the hydrogenation process of Pd particles. Specifically, when the Pd-Co alloy contains more than 25% Co, the formation of Pd hydride was entirely suppressed [50]. Therefore, the Pd:Co ratio and particle size need to be further optimized to better understand the catalytic behavior of this alloy. Additionally, blank experiments were performed using Co-containing **IMCo1** or **IMCo2** material for the hydrogenation of 4-nitroacetophenone. No reaction occurred (the starting material was recovered unchanged) proving that the only role of Co nanoparticles is to allow the rapid and efficient magnetic recovery of the catalysts **IM1** and **IM2** after a reaction. The activity of some commercially available Pd catalysts was then evaluated under the optimized conditions. It turned out that Pd/C (entry 7) or Pd Encat NP30 (entry 8) gave the primary amine **1a** in low yields, while no reaction was observed using Pd(PPh<sub>3</sub>)<sub>4</sub>, and the starting material was recovered unchanged (entry 9).

The heterogeneous and/or homogeneous nature of the catalysis was then determined. For this purpose, the hydrogenation was performed under the conditions described in Table 1, entry 2. After 30 min of reaction (yield at that point: 36%) the catalyst **IM2** was magnetically recovered. The reaction medium was then reacted for another 1.5 h at rt under 1 atm of H<sub>2</sub> (yield at that point: 38%). Therefore, soluble Pd entities play only a minor role

<footnote><sup>(a)</sup> Reactions performed using 4-nitroacetophenone (2 mmol), the indicated amount of supported Pd (mequiv.) and reaction time in EtOH (10 mL). <sup>(b)</sup> Isolated yields after magnetic recovery of the catalyst, concentration under vacuum and drying of the reaction product under vacuum.</footnote>
in the reduction of nitroarenes. Next, the amount of Pd present in the reaction mixture was determined after the magnetic recovery of the catalyst **IM2**, the complete mineralization of the reaction mixture and the determination of the amount of Pd present by complexation (for further details see the Supplementary Materials). It is important to note that < 0.1% of the initial amount of supported Pd was present. Therefore, almost-Pd-free products were obtained at the end of the reaction, avoiding long and tedious purification steps to eliminate the residual amounts of Pd. Next, recycling experiments were performed using the catalyst **IM2** (conditions described in Table 1, entry 2). It turned out that **IM2** was quantitatively recovered and used at least seven times with > 90% yields (Figure 4b). Since the loss of Pd after each use of the catalyst **IM2** was <0.1% of the initial amount and no increase in the reaction time was necessary to obtain **1a** in almost >90% yields, it can be concluded, in accordance with previous reports [41,42,44,45], that no significant deactivation of the catalyst **IM2** occurred during its reuse.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) Evolution of the reduction reaction after the magnetic recovery and removal of the catalyst IM2 at 30 min; (b) Reuse of the catalyst IM2 under the conditions in Table 2, entry 2.</figcaption></figure>

Finally, the scope of the hydrogenation reaction using the catalyst **IM2** was determined (Figure 5). Versatile primary aromatic amines bearing electro-attracting groups (compounds **1a–d**, **1f**), electro-donating groups (compounds **1g–i**) or even 3-aminopyridine (**1j**) were prepared in excellent yields under 1 atm of H₂ at rt in EtOH in 1.5 h or 4 h, depending on the nature of the starting material. Noteworthily, the hydrogenation of 4-nitrophenol (4-NP) is of particular interest from an industrial point of view, since 4-NP is a highly toxic compound present in numerous industrial wastes [52]. A solution commonly used to remove 4-NP is to reduce it to the corresponding 4-aminophenol (4-AP) that is much less toxic and finds widespread applications as building block for the preparation of active pharmaceutical ingredients (API) or polymers [52]. As shown in Figure 5, 4-NP was almost quantitatively hydrogenated using catalyst **IM2**.

The activity of the catalyst **IM2** was then compared to that of other magnetic representative Pd-catalysts described in the literature (Table 3). It turned out that **IM2** presents a good and comparable TOF compared to most of the other catalysts described in Table 3 for the reduction of nitroarenes. In addition, **IM2** offers the best compromise between rapid and easy preparation from biosourced non-toxic precursors and activity for the hydrogenation
tion of nitroarenes under environmental-friendly conditions. Noteworthily, compared to the most active catalyst from Table 3 (entry 7), **IM2** is prepared in only two steps, and no inorganic salts (due to the use of NaBH₄ as the reducing agent in entry 7 instead of H₂) are generated during the reduction of nitroarene.

<figure><img src="image_8.png" /><figcaption>Reaction time: a) 1.5 h and b) 4 h</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Reductions of various nitroarenes.</figcaption></figure>

<table><caption>**Table 3.** TOFs for the hydrogenation of nitroarenes in the presence of reported magnetic reusable catalysts.</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>Synthesis—<br>Number of Steps</th><th>Reducing Agent,<br>Solvent</th><th>Pd (Mequiv.)</th><th>TOF (h<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1</td><td>IM2 (this work)</td><td>2</td><td>H<sub>2</sub>, EtOH</td><td>5</td><td>50–133</td></tr><tr><td>2</td><td>Fe<sub>3</sub>O<sub>4</sub>@PDA@POP@Pd [28]</td><td>4</td><td>H<sub>2</sub>, EtOH</td><td>4.7</td><td>35–106</td></tr><tr><td>3</td><td>Pd@Co/C-SiO<sub>2</sub>-NH<sub>2</sub> [27]</td><td>3</td><td>H<sub>2</sub>, isopropanol</td><td>1.6</td><td>250–417</td></tr><tr><td>4</td><td>GO/CNT- Fe<sub>3</sub>O<sub>4</sub>@Pd [29]</td><td>5</td><td>H<sub>2</sub>, EtOH</td><td>10</td><td>2–34</td></tr><tr><td>5</td><td>Pd/Fe<sub>3</sub>O<sub>4</sub> [53]</td><td>2</td><td>H<sub>2</sub>, EtOH</td><td>10</td><td>20–133</td></tr><tr><td>6</td><td>Pd NPs@Pct-CMC/Fe<sub>3</sub>O<sub>4</sub> [54]</td><td>5</td><td>NaBH<sub>4</sub>, EtOH</td><td>50</td><td>0.09</td></tr><tr><td>7</td><td>Fe<sub>3</sub>O<sub>4</sub>@N-C@Pd Y-S [55]</td><td>4</td><td>NaBH<sub>4</sub>, EtOH-H<sub>2</sub>O</td><td>10</td><td>42–1960</td></tr><tr><td>8</td><td>Pd/ox-CEINs [30]</td><td>4</td><td>CO<sub>2</sub>NH<sub>4</sub>, EtOH</td><td>10</td><td>70–208</td></tr></tbody></table>

PDA: Polydopamine; POP: Porous organic polymer; GO: Graphine Oxide; CNT: Carbon nano-Tube; Pct-CMC: Pectin-CarboxyMethyl Cellulose; N-C: Nitrogen-doped Carbon; Y-S: Yolk-Shell; CEINs: Carbon-encapsulated iron nanoparticles.

## 2.4. Reductions of Enones

The reaction conditions were optimized using chalcone as starting material and the catalysts **IM1**, **IM2** and **DM** (Table 4). It turned out that the hydrogenation of chalcone was chemospecific and almost quantitatively afforded 1,3-diphenylpropanone **2a** in only 1.5 h under 1 atm of H₂ at rt in EtOH (entries 1–2). Lowering the amount of supported Pd gave **2a** in a lower 60% yield (entry 3), but increasing the reaction time to 2.5 h afforded **2a** in 99% yield (entry 4). Reducing the amount of supported Pd to 1 mequiv. was possible, but a longer reaction time was required (entries 5–6). Finally, no reaction was observed using the catalyst **DM** and the starting material was recovered unchanged (entry 7).
<table><caption>**Table 4.** Optimization of the reaction conditions for the reduction of enones.</caption><thead><tr><th>Entry <sup>(a)</sup></th><th>Catalyst</th><th>Pd (Mequiv.)</th><th>Reaction Time (h)</th><th>Yield % <sup>(b)</sup></th></tr></thead><tbody><tr><td>1</td><td>IM1</td><td>5</td><td>1.5</td><td>99</td></tr><tr><td>2</td><td>IM2</td><td>5</td><td>1.5</td><td>99</td></tr><tr><td>3</td><td>IM2</td><td>2.5</td><td>1.5</td><td>60</td></tr><tr><td>4</td><td>IM2</td><td>2.5</td><td>2.5</td><td>99</td></tr><tr><td>5</td><td>IM2</td><td>1</td><td>1.5</td><td>15</td></tr><tr><td>6</td><td>IM2</td><td>1</td><td>18</td><td>99</td></tr><tr><td>7</td><td>DM</td><td>5</td><td>24</td><td>No reaction.</td></tr></tbody></table>

Since the magnetic recovery of the catalyst **IM2** was more efficient than that of **IM1**, we determined the possibility of reusing **IM2** for the reduction of chalcone (Figure 6) under optimized conditions (Table 4, entry 2). After reaction, **IM2** was magnetically recovered, washed, dried and reused. Gratifyingly, **IM2** could be used at least seven times with no significant decrease in the yield. It is noteworthy that the hydrogenation of enones in the presence of a reusable magnetic Pd catalyst has been much more sparsely studied compared to the reduction of nitroarenes. In some cases, magnetic catalysts have been developed and successfully used for reductions of enones but the possibility of reuse was not determined [39]. In addition, Manorama's group has developed Pd NPs immobilized on the surface of amine-terminated $Fe_3O_4$ and $NiFe_2O_4$ for hydrogenations of enones using 10 mequiv. of supported Pd. The efficiency of the catalyst remains unaltered even after 10 repeated cycles for each of the reactions [56]. Therefore, the catalyst **IM2** also presents an interest for such reactions, and moreover it is easily prepared, efficient and reusable without a significant loss of efficacy upon reuse.

<figure><img src="image_10.png" /><figcaption>**Figure 6.** Reuse of catalyst **IM2** under conditions of Table 3, entry 1.</figcaption></figure>

<footnote>(a) Reactions performed using chalcone (2 mmol), the indicated amount of supported Pd (mequiv.) and reaction time in EtOH (10 mL). (b) Isolated yields after magnetic recovery of the catalyst, concentration under vacuum and drying of the reaction product under vacuum.</footnote>
**3. Materials and Methods**

**3.1. Chemicals and Reagents**

All reagents and solvents were obtained from commercial sources and were used without further purification. Pluronic F127 (Sigma Aldrich, St. Louis, MO, USA) is a triblock copolymer with an average formula of (PEO)₁₀₆(PPO)七十(PEO)₁₀₆. The silica gel was purchased from Merck and had a 0.063–0.2 mm granulometry (70–230 mesh ASTM). The external magnet used for magnetic separation was a S-25-07-N (Supermagnete, Uster, Switzerland).

**3.2. Preparations of Catalysts**

Two-step indirect synthesis

STEP 1: The carbon source, phloroglucinol (1.65 g) and the surfactant Pluronic F-127 (3.27 g) were dissolved in ethanol (80 mL) with stirring, followed by the addition of the metallic salt Co(NO₃)₂·6H₂O (0.106 g for **IMCo1** or 0.530 g for **IMCo2**). A glyoxal solution (40% in water, 1.62 mL) cross-linker and citric acid (0.2 g) chelating agent were then added. The obtained solution was deposited in several Petri dishes and left to evaporate the solvent under the fume hood at room temperature for 15 h in order to form a polymer film. The latter was removed by mechanical scratching, dried at 80 °C for 24 h and then pyrolysed at 600 °C for 2 h under an atmosphere of argon to obtain the Co-containing mesoporous carbons **IMCo1** (2.5% Co) and **IMCo2** (12.5% Co) [57].

STEP 2: The Co-containing mesoporous carbons **IMCo1** and **IMCo2** (1.74 g), in anhydrous ethanol (10 mL), were impregnated with a solution of Pd(OAc)₂ (0.086 g) in CHCl₃ (2 mL) at room temperature. The mixtures were stirred under a fume hood until complete evaporation of the solvent had been achieved, followed by thermal reduction under a mixture of Ar/H₂ (10% of H₂) at 300 °C to obtain **IM1** (2.3% Pd) and **IM2** (2.3% Pd) [46,58].

One-pot synthesis

**DM** catalyst containing Pd₅₀Co₅₀ alloy confined in mesoporous carbon, was prepared according to our previous work [47], and similarly to the step 1 described above. The main difference was that both Co and Pd salts (Co(NO₃)₂·6H₂O and PdCl₂) were introduced in the reaction mixture to obtain simultaneously PdCo catalyst.

**3.3. Characterizations of Catalysts**

The crystalline structure of the catalysts was assessed by X-Ray Diffraction (XRD) using a Bruker D8 Advanced diffractometer (Bruker, Karlsruhe, Germany) possessing a flat-plate Bragg–Brentano θ-θ geometry and a high-resolution energy dispersive 1D detector (LynxEye XE-T, Cu Kα₁,₂, λ = 1.54 Å). The crystallite sizes were calculated based on the peak positions and broadness by the Scherrer equation. The particle size distribution was obtained by high-resolution transmission electron microscopy (HR-TEM) with a JEOL ARM-200F instrument (Jeol, Tokyo, Japan) working at 200 kV. Several TEM and STEM images were used to count a high number of particles to be representative for each sample. N₂ sorption isotherms were measured at 77 K with a Micromeritics ASAP 2420 instrument (Micromeritics, Atlanta, GA, USA). The catalysts were vacuum outgassed at 150 °C for 12 h to remove any moisture. The BET specific surface area (S<sub>BET</sub>) was calculated in the relative pressure range of 0.01–0.1 P/P₀. The pore size distribution (PSD) was determined by using the nonlocal density functional theory (NLDFT) model for carbon slit pores. The total pore volume (V<sub>T</sub>) was calculated at P/P₀ = 0.99, the micropore volume (V<sub>micro</sub>) according to the Dubinin–Radushkevich method, and the mesopore volume (V<sub>meso</sub>) was determined by a subtraction approach (V<sub>T</sub> − V<sub>micro</sub>).
### 3.4. Reduction of Nitroarenes

A solution of nitroarene (2 mmol) in EtOH (10 mL) was vigorously stirred in the presence of the catalyst **IM2** (5 méquiv. Pd) under H₂ at atmospheric pressure at rt for 2 h (for the preparation of aromatic amines **1a**, **1e-j**) or 4 h (for the preparation of aromatic amines **1b-d**). The catalyst **IM2** was then magnetically recovered, washed twice with EtOH (2 × 5 mL), magnetically recovered and dried under vacuum before reuse. The solvent was evaporated, and the aromatic primary amines were dried under vacuum. If necessary, the aromatic amines can be purified by flash-chromatography on silica gel using mixtures of AcOEt and cyclohexane as an eluant. All synthesized compounds were characterized by ¹H, ¹³C and ¹⁹F NMR (if relevant) spectroscopy [59]. The corresponding spectra are available in the Supplementary Materials.

### 3.5. Reduction of Enones

A solution of enone (2 mmol) in EtOH (10 mL) was vigorously stirred in the presence of the catalyst **IM2** (5 méequiv. Pd) under H₂ at atmospheric pressure at rt for 2 h. The catalyst **IM2** was then magnetically recovered, washed twice with EtOH (2 × 5 mL), magnetically recovered and dried under vacuum before reuse. The solvent was evaporated, and ketones were dried under vacuum. If necessary, the ketones can be purified by flash-chromatography on silica gel using mixtures of AcOEt and cyclohexane as an eluant. The synthesized compound was characterized by ¹H and ¹³C spectroscopy [59] The corresponding spectra are available in the Supplementary Materials.

## 4. Conclusions

Herein, short and efficient preparations of reusable magnetic Co- and Pd-containing mesoporous carbon catalysts from non-toxic precursors and solvents have been described. The catalysts **IM1** and **IM2** were obtained in two steps via an indirect route and contained separated Co and Pd NPs with different particle sizes, while the catalyst **DM** was prepared in one step via a direct route, producing Co₅₀-Pd₅₀ nanoalloys embedded in the carbon network. The activity of the catalysts **IM1**, **IM2** and **DM** for chemospecific versatile hydrogenations of nitroarenes and enones was determined. Similar results were obtained for the reductions by using both **IM** catalysts, but **IM2** afforded the best compromise between activity, quantitative magnetic recovery, and reusability using only 5 mequiv. of supported Pd under 1 atm of H₂ in EtOH at rt for 1.5 h. The catalyst **IM2** was successfully used at least seven times for both reductions of nitroarenes and enones with only residual amounts of Pd in the final products. This could be related to the small and dispersed Pd particle size (1.3 nm) which favourably enhanced the catalytic reaction. No reduction was observed for the catalyst **DM**, probably due to the presence of inactive Co₅₀-Pd₅₀ nanoalloys for these reductions. Therefore, complementary studies are required to find the optimal Pd-Co alloy composition and particle size. To the best of our knowledge, the catalyst **IM2** is one of the most rapidly, easily accessible and efficient reusable magnetic catalysts for very mild hydrogenations of nitroarenes or enones under environment friendly conditions. Current work is in progress by our group in order to extend the use of the catalyst **IM2** for other versatile reactions in fine chemistry.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15121126/s1; Description of the NMR Spectra of Compounds **1a-j** and **2a**, Determinations of the Pd content of catalysts and of the Pd leached in the reaction medium, Figure S1: STEM images of catalyst IM2 and its corresponding EDX mapping, Copy of the NMR Spectra of Compounds **1a-j** and **2a**.
**Author Contributions:** Conceptualization, C.M.G. and J.-M.B.; methodology, C.M.G. and J.-M.B.; software, L.V. and C.V.; validation, C.M.G., J.-M.B., C.V., L.V. and C.L.D.; formal analysis, L.V. and C.V.; investigation, M.E.; resources, C.M.G. and J.-M.B.; data curation, L.V. and C.V.; writing—original draft preparation, C.M.G. and J.-M.B.; writing—review and editing, C.M.G. and J.-M.B.; visualization, all authors; supervision, C.M.G. and J.-M.B.; project administration, J.-M.B., C.L.D. and C.M.G.; funding acquisition, C.L.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author(s).

**Acknowledgments:** We are grateful to the Fondation pour l'Ecole Nationale Supérieure de Chimie de Mulhouse for a doctoral grant and generous financial support to M. Enneiymy, to B. Rety for helpful technical assistance, to J.-M. Le Meins for the XRD via IS2M technical platform.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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