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Article

Transfer Hydrogenation of Vanillin with Formic Acid over Graphene-Encapsulated Nitrogen-Doped Bimetallic Magnetic Pd/Fe@N/C Catalyst

by
Hualiang Zuo
1,2,3,
Yulong Lei
2 and
Jianguo Liu
2,*
1
Zhongshan Advanced New Functional Materials Engineering Technology Research Center, Zhongshan Polytechnic, Zhongshan 528400, China
2
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China
3
Institute of Chemical Technology, Faculty of Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(9), 906; https://doi.org/10.3390/catal15090906
Submission received: 30 July 2025 / Revised: 12 September 2025 / Accepted: 14 September 2025 / Published: 18 September 2025
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Abstract

The improved biomass-derived aldehyde compounds represent a valuable route to the production of high-value-added fuels and chemicals. However, the majority of mature catalytic systems exhibit low hydrodeoxygenation (HDO) activity, even under harsh reaction conditions. In this study, it was observed that a Pd/Fe magnetic bimetallic catalyst, in conjunction with formic acid (FA) as a hydrogen source and nitrogen-containing carbon material as a support, exhibited remarkable catalytic performance for the conversion of phenyl aldehydes in oxygenates derived from crude lignin. In the hydrogenation of vanillin, the Pd/Fe@N/C catalyst demonstrated superior catalytic activity under mild reaction conditions of 80 °C. When ethyl acetate was used as the solvent, the product was vanillyl alcohol (VA), and when cyclohexane was employed as the solvent, the product was p-methyl guaiacol (MMP). The yields achieved were 84.5% and 92.3%, respectively. It is recommended that further exploration of the FLOW reactor system be considered at a later stage due to the magnetic and easily separable characteristics of the catalyst. The excellent mass transfer and heat transfer performance of the FLOW reactor system will further ensure that the reaction conditions are moderate and will strive to achieve normal-temperature conversion.

1. Introduction

Biomass represents the most abundant carbon-based renewable resource on Earth due to its composition of diverse oxygen-containing functional groups and its capacity for conversion into a wide range of high-value chemicals [1]. Selective hydrogenation is a common strategy to achieve biomass conversion, and since most biomass contains multiple reducible functional groups, controlling the activity and selectivity of hydrogenation reactions is a significant challenge [2].
The selective hydrogenation of biomass-derived compounds plays a critical role in producing biofuels, value-added chemicals, and functional materials from renewable feedstocks, contributing to the development of sustainable chemical processes. However, the structural complexity of biomass-derived molecules, which often contain carbonyl, hydroxyl, and carboxyl groups within the same framework, leads to competitive reactions that can result in undesired byproducts and low selectivity under conventional hydrogenation conditions. Vanillin is derived from the thermochemical or biological depolymerization of lignin biomass and is a promising biomass platform compound with great potential for catalytic hydrogenation to high-value-added compounds and biofuels [3,4]. Selective hydrogenation of vanillin mainly includes hydrogenation reduction and hydrodeoxygenation of aldehyde groups. Vanilla alcohol can be obtained by hydrogenation reduction of aldehyde groups. Vanilla alcohol possesses a sweet and nutty flavor similar to vanillin and is characterized by enhanced durability and stability. It is widely used in food, medicine and perfume industries [5]. Due to its high oxygen content, vanillin has the disadvantages of low energy density, unstable combustion and excessive corrosion as a biofuel. The hydrodeoxygenation of aldehyde groups to generate p-methyl guaiacol (MMP) has been demonstrated to be an effective method of reducing oxygen content and enhancing the quality of biofuel oil. However, the presence of multiple functional groups on vanillin results in uncontrolled side reactions during hydrogenation, thereby complicating the selective conversion of vanillin [6]. The crux of the challenge in vanillin hydrogenation pertains to the precise modulation of hydrogenation products, rendering them amenable to control. The same catalyst usually achieves only complete conversion of one product or partial conversion of two products. For example, Duan et al. used a Ni-Co-P/HAP catalyst to catalyze the vanillin hydrodeoxygenation reaction. The reaction was conducted at a temperature of 200 °C for 3 h, resulting in the production of MMP [7]. In a related study, Alijani et al. employed a Pd-activated carbon catalyst for the hydrogenation of vanillin, yielding a selectivity to vanillyl alcohol of less than 80% [8]. Therefore, it is of significant importance to achieve the complete conversion of vanillin and MMP by adjusting reaction conditions with the same catalyst.
Plenty of heterogeneous catalysts, including Ni [9], Cu [10], Co [11], Pt [12], Au [13], Ru [14], Ir [15], Ru [16] and Pd [17,18,19,20], have been reported as effective catalysts for the reduction of aldehydes. Among these transition metal catalysts, it has been found that Pd nanoparticle-supported catalysts exhibit numerous advantages in aldehyde hydrogenation reduction. Pd-based catalysts demonstrate elevated hydrogenation activity under mild process conditions and maintain their performance upon repeated use in solvent media. However, in the context of vanillin hydrogenation, Pd-based catalysts usually prove incapable of achieving controllable conversion of both products. This is due to the fact that excessive hydrogenation of Pd leads to partial conversion of vanillin and MMP, thereby compromising the selectivity of these two products. Therefore, the development of Pd-based metal catalysts that can facilitate vanillin controlled hydrogenation under mild conditions to yield two products remains a significant challenge. Furthermore, Pd-promoted bimetallic Fe-based catalysts have been reported to exhibit superior performance compared to their monometallic counterparts in the hydrogenation of biomass-derived model compounds [21]. Motivated by these findings, this work aims to design a Pd-Fe bimetallic catalyst with high activity for the selective hydrogenation of vanillin under mild conditions, facilitating its controlled conversion into valuable products (Figure 1).
It is widely acknowledged that the properties of the support have a significant impact on the state of the metal, the adsorption properties of the reagent and the mass transfer process. Various materials, including carbon [21], polymer [22], Al2O3 [23], mesoporous silica [24], zeolite [25], carbon nanotubes [26], MOFs [27], graphene [28,29,30] and other carriers, have been employed as supports for Pd nanoparticles to enhance their catalytic performance. Among these, graphene has demonstrated advantageous properties, such as a high specific surface area and excellent electrical conductivity, positioning it as an ideal support for palladium-based catalysts. The high specific surface area of graphene promotes the uniform dispersion of Pd and Fe nanoparticles [31], facilitates mass and heat transfer, and increases the frequency of collisions between reactants and the active sites on the catalyst surface. Furthermore, a larger specific surface area correlates with a higher density of surface active sites, as the proportion of exposed atoms on the surface increases with surface area, thereby providing more accessible active sites for catalytic reactions. These surface active sites and edge-unsaturated coordination atoms have been shown to stabilize reaction intermediates and lower activation barriers [32]. Excellent conductivity is conducive to electron transfer during the reaction. It has been demonstrated that graphene exhibits excellent catalytic properties in hydrogenation reactions [33]. A significant approach to enhancing the catalytic performance of metals in heterogeneous organic compounds is through the strengthening of their physical and chemical interactions with metal-based catalysts. Carbon materials have the capacity to customize pore structures and modify catalytic surface sites through the introduction of heteroatoms [30,31]. To date, a variety of methodologies have been developed for the purpose of modifying the properties of carbon materials. These methods involve the activation of carbon materials using various reagents or the incorporation of elements such as nitrogen, sulfur, and phosphorus into the carbon materials [32,33,34]. It has been hypothesized that N modification could promote the activity and selectivity of catalysts by introducing extra anchor points, adjusting the electronic structure of the central metal, and interacting with the proton-active center [35,36]. The presence of N species in carbon materials alters the electronic state of the carbon atoms, expands graphite structure and produces defect sites. These defects endow the carbon materials with excellent catalytic activity and stability, enabling vanillin to be selectively hydrogenated to vanillin and MMP.
Most of the methods reported in the literature for the hydrogenation reduction of vanillin require the use of hazardous high-pressure hydrogen gas. Conventionally, hydrogen is produced via water-gas shift reactions (WGS) that rely on fossil resources, consuming large amounts of energy and releasing significant quantities of carbon dioxide in the process. In this context, catalytic transfer hydrogenation (CTH) has emerged as a promising alternative, as it utilizes readily available and inexpensive hydrogen donors such as alcohols and organic acids, and does not require complex experimental equipment such as high-pressure reactors. Among these hydrogen donors, formic acid (FA) is particularly attractive, as it is a by-product of the acid hydrolysis of biomass and can also be sustainably produced from the hydrolysis of cellulosic biomass. Compared to conventional hydrogenation, the CTH of formic acid enables hydrogenation under milder reaction conditions, eliminating the need for high-pressure hydrogen and high temperatures while maintaining high catalytic efficiency.
In this work, Pd/Fe bimetallic catalysts supported on nitrogen-containing carbon materials were prepared for the selective reduction of vanillin to vanillyl alcohol or MMP under mild reaction conditions. The synergistic catalytic action of Pd and Fe, combined with N-doped carbon support, provides additional active sites, enhancing catalytic efficiency while enabling facile catalyst separation and recovery. Among the catalysts evaluated, the Pd/Fe@N/C catalyst showed the highest catalytic activity at 80 °C under mild reaction conditions. By controlling the solvent, selective synthesis of either VA or MMP from Vanillin was achieved. Under optimized conditions, the yields of VA and MMP reached 84.5% and 92.3%, respectively. Furthermore, the catalyst exhibited magnetic properties, allowing easy separation from the reaction mixture, which positions it as a promising candidate for application in continuous-flow reactor technology. The continuous FLOW process has excellent mass and heat transfer performance, which will further moderate the reaction conditions and strive to achieve normal-temperature conversion.

2. Results and Discussion

2.1. Synthesis and Characterization of the Catalysts

The contents of Pd and Fe in the catalysts determined by ICP-OES are listed in Table 1. The content of Pd in Pd@N/C was 6.9 wt.% and the value in Pd/Fe@N/C was 0.7% wt.%, which was approximately one-tenth of the former one. The contents of Fe in Fe@N/C and Pd/Fe@N/C were 13.0% and 15.1%, respectively, indicating that the contents of Fe were at the same level.
Figure 2 shows SEM images and TEM images of the nitrogen-doped graphene-encapsulated Pd/Fe@N/C catalyst. The surface of the Pd/Fe@N/C catalyst exhibited a well-defined porous structure, with Pd and Fe particles uniformly dispersed throughout the material. Smaller Pd particles were noticed to be evenly interspersed among the Fe particles, with both types of particles encapsulated by a thin C shell, effectively preventing metal particle loss during subsequent reactions. The HR-TEM analysis revealed that the N-doped Pd/Fe@N/C catalysts comprised metal nanoparticles encapsulated within fewer than five graphene layers. The particle size distribution of the metal particles was found to be relatively broad, with the predominant particle size measuring 31.7 nm.
The detailed composition and elemental valence states of the Pd/Fe@N/C catalysts were evaluated using XPS. Analysis verifies that the Pd/Fe@N/C catalyst comprises carbon, nitrogen, oxygen, palladium and iron. As shown in Figure 3a, the high-resolution C 1s spectrum exhibits a peak at 284.8 eV corresponding to C–C/C=C bonds. The incorporation of N results in the appearance of a peak at 286.3 eV, indicative of the C–N bonding. Furthermore, two peaks at approximately 288.6 and 290.8 eV are attributed to C–O and O–C=O bonds, respectively [37,38]. The N 1s spectra (Figure 3b) could be deconvolved into four peaks with binding energies of 394.6, 398.6, 400.8, and 404.2 eV, indicating that N atoms doped into C had four distinct bonding characteristics in the form of pyridine-N, amine/M-Nx (demonstrating chemical coordination of N species and metals), pyrrole-N, and graphite-N [39]. The 3d orbital peaks of Pd are located at 336.1 and 341.4 eV due to carbothermal reduction resulting from high-temperature treatment, which corresponds to 3d5/2 and 3d3/2 of metal Pd (0) (Figure 3c). Partial oxidation of Pd during the preparation and storage processes is also observed, with two additional low-intensity peaks at 338.1 eV and 344.5 eV corresponding to the 3d5/2 and 3d3/2 orbitals of Pd (II). Furthermore, the Fe 2p spectrum (Figure 3d) exhibits two distinct peaks at 711.7 and 722.8 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 transitions, respectively, confirming the presence of iron in the catalyst.
The XRD pattern (Figure 4) confirms the structural characteristics of the Pd/Fe@N/C catalysts. A broad peak between 20° and 30°, corresponding to the (002) plane of graphitic carbon (ICDD: 00-041-1487), is observed. Moreover, distinct diffraction peaks corresponding to Pd (011) and Pd (020) (ICDD: 96-152-3107), along with peaks attributable to Fe3O4 species (ICDD: 03-065-3107), are clearly observed. The formation of the Pd/Fe@N/C catalyst is confirmed in combination with the XPS results. Of the various modification strategies, incorporating different metals—especially secondary active metals—into magnetic metal oxides has been shown to effectively improve catalytic performance [26].
The magnetic properties of Pd/Fe@N/C was studied using a vibrating sample magnetometer (VSM). As shown in Figure 5, the saturation of the magnetization was observable when applied magnetic fields exceeded 15 kOe. The saturation magnetization, as determined by the hysteresis loop, was found to be 6.1 emu g−1. The magnetic property of the catalyst facilitates its separation, which renders it highly promising for application in continuous-flow (FLOW) reaction systems.

2.2. Pd/Fe@N/C Catalyzed Selective Hydrogenation of Vanillin

The Pd/Fe@N/C bimetallic catalyst coated with N-doped graphene was successfully synthesized, and its catalytic activity for vanillin catalytic transfer hydrogenation was systematically investigated. As shown in Figure 6a, a series of catalysts were prepared for vanillin hydrogenation. The results demonstrated that despite reducing the quantity of active metal Pd to just 10% of that used in the Pd@N/C catalyst, the catalytic hydrogenation activity for vanillin was significantly enhanced due to the bimetallic catalytic effect provided by the transition metal Fe. However, when pure Fe was used as the active center, catalytic hydrogenation of vanillin did not proceed, regardless of the valence state of the Fe center atom. In addition, a comparative analysis of Fe@C catalysts with and without N doping revealed that the presence of Pd atoms was essential for N doping to enhance catalytic activity. In contrast, in the bimetallic system, N doping significantly increased the catalytic activity of Pd/Fe@C catalysts. Notably, a physical mixture of Pd@N/C and Fe@N/C did not show this significant promotion of catalytic activity. This result indicates the significance of the manner in which Fe is added. It is imperative to incorporate Fe during the preparation of the Pd catalyst to ensure its efficacy. It is worth noting that after N doping on the C carrier, the proportion of the Pd/Fe bimetallic active component was relatively reduced. Nevertheless, its catalytic activity was still the best. Clearly, both Pd and Fe metals play an essential role in significantly enhancing the catalytic activity of catalysts while substantially reducing the precious metal Pd content. It has been proven that the high catalytic efficiency of Pd-Fe catalysts for phenyl aldehydes is due to the synergistic effect of Pd and Fe [40]. The effect of solvent on hydrogenation performance was further studied (Figure 6b). The experimental conditions encompass a formic acid/vanillin ratio of 10, a reaction temperature of 60 °C, and a reaction time of 60 min. A general trend was observed wherein decreasing solvent polarity led to a decrease in vanillyl alcohol selectivity while increasing the selectivity for 2-methoxy-4-methylphenol (MMP). When water was used as the solvent, the selectivity for vanillyl alcohol (VA) reached 90%. In contrast, using cyclohexane as the solvent resulted in a complete loss of VA selectivity, while the selectivity for MMP increased to 100%. This observation can be attributed to the inefficient hydrogenolysis of vanillin in weakly polar solvents, where the reaction rate is significantly slower than in polar solvents. In polar solvents, the hydrogenolysis of the intermediate (vanillyl alcohol) is effectively promoted, thereby accelerating the rate of hydrodeoxygenation.
Consequently, water and cyclohexane were selected as representative solvents for further screening studies. The reaction conditions were first optimized using water as the solvent, focusing on the FA/vanillin molar ratio, reaction temperature, and reaction time (Figure 7a). When the FA/vanillin molar ratio increased from 1 to 10, the yield of VA correspondingly increased from 42.1% to 81.5%. However, further increasing the FA/vanillin molar ratio resulted in a notable decrease in VA selectivity and a significant increase in the selectivity for MMP. This indicates that excess FA promotes continued hydrodeoxygenation of vanillin to MMP. Furthermore, the yield of VA increased with rising temperature, reaching a maximum at 80 °C. The optimal reaction conditions in water were determined to be: 0.5 mmol vanillin, 5 mg catalyst, a FA/vanillin ratio of 10:1, 4 mL of H2O, and a reaction temperature of 80 °C for 1 h. Under these conditions, a vanillin conversion of 93.2% and a VA selectivity of 89.6% were obtained. Similarly, Figure 7b shows the results when cyclohexane was used as the solvent. The selectivity for MMP reached a maximum value of 99% at FA/vanillin molar ratios of 10 and 20. However, further increasing the FA/vanillin ratio led to a slight decrease in MMP selectivity, likely due to the formation of excessive hydrodeoxygenation by-products. As the temperature increased, the yield of MMP gradually increased, peaking at 80 °C. The optimal conditions for cyclohexane as the solvent were as follows: 0.5 mmol vanillin, 5 mg catalyst, an FA/vanillin ratio of 10:1, 4 mL of cyclohexane, and a reaction temperature of 80 °C for 5 h. Under these conditions, a vanillin conversion of 92.3% and a MMP selectivity of 99% were obtained. Compared with the most active and selective catalysts of similar systems reported in the literature (Table 2), these results indicate that the developed catalyst has excellent catalytic performance. In particular, it could achieve comparable catalytic results without the need for high-pressure hydrogen and at a much lower temperature.
Furthermore, the scope of the hydrodeoxygenation reactions of biomass-derived aromatic aldehydes was investigated, and the results are summarized in Table 3. As shown in the table, aldehyde substrates bearing methyl, isopropyl, and strong electron-donating groups such as hydroxyl and methoxy exhibited excellent conversion and high selectivity for the formation of the corresponding alcohols. When the methyl group was positioned ortho, meta, or para to the aldehyde group, all three methyl-benzaldehyde substrates exhibited high conversion rates and selectivity, with product yields exceeding 93% in each case. The experiments outlined above have demonstrated that the nitrogen-doped graphene-encapsulated Pd, Fe bimetallic catalysts (Pd/Fe@N/C) exhibit high levels of reactivity.
As illustrated in Figure 8, the Pd/Fe@N/C catalyst demonstrated excellent stability and recyclability. After five consecutive catalytic cycles, no substantial decline in catalyst activity or product selectivity was observed. Additionally, no detectable metal leaching was found in the reaction solution, confirming the catalyst’s excellent chemical stability and recoverability. TEM analysis conducted after five recycling cycles further confirmed that the catalyst’s structure remains largely unchanged, with the integrity of the coating structure clearly maintained. These findings indicates that the coating structure exhibits high structural stability to the catalyst. The exceptional stability and recoverability of the catalyst is attributed to the distinctive nitrogen-doped graphene coating on the Pd-Fe bimetallic core, which not only plays an active role in the hydrogenation process but also effectively protects the active sites from deactivation [38]. Due to the low metal loss and the easy magnetic separation of the catalyst, the reduction of nitroaromatic compounds has been successfully applied in fixed-bed systems [41]. For practical industrial applications, future work will focus on integrating this catalyst into continuous-flow (FLOW) systems to enhance the efficiency and scalability of biomass-derived compound hydrogenation processes.

2.3. Proposed Mechanism of Pd/Fe@N/C Catalyzed Hydrogenation of Vanillin

In light of the preceding analyses and literature reports, it can be concluded that FA played two significant roles in the HDO of benzylic ketones. First, it provides heterolytic hydrogen H+/H− species for the reduction of C=O bonds. Second, it facilitates the formation of formate intermediates, which accelerate the cleavage of C–O bonds, leading to the formation of deoxygenated products. Based on these insights, a feasible mechanism was proposed for the HDO of benzylic ketones in the FA and Pd/Fe@N/C system. As shown in Scheme 1, the HDO of benzylic ketones involves the following three parts. Firstly, the generation of active H+/Pd-H− species is evident: basic amino groups and Pd dual sites promote FA decomposition to form Pd-HCOO− and H+ species. Subsequently, Pd–HCOO− is subject to decomposition, yielding Pd-H− species and CO2 through b-hydride elimination. This process gives rise to the active H+/Pd-H− species. Secondly, the HDO of benzylic ketones is a process in which the carbonyl group is hydrogenated to alcohol intermediates by the attack of active H+/Pd-H− species. This is followed by partially esterification to form formate species. Then, the alkylaromatics are produced via the decarboxylative deoxygenation of formate species or the hydrogenolysis of alcohol intermediates, in which the former is thermodynamically favorable. The third part of the process is the decomposition of excess FA by the release of H2 and CO2 [31,32].

3. Experimental Section

3.1. Materials and Catalyst Preparation

Pd(NO3)2·6H2O (AR, 0.0003 mol, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Fe2(SO4)3 (AR, 0.003 mol, Sinopharm Chemical Reagent Co., Ltd.), melamine (99%, 0.03 mol, Sigma-Aldrich, Saint Louis, MO, USA), and citric acid (C6H8O7, ≥99.0%, 0.03 mol, Sigma-Aldrich) were dissolved in anhydrous ethanol (50 mL, AR, Sinopharm Chemical Reagent Co., Ltd.). For the catalyst containing only Pd, the amount of Pd(NO3)2·6H2O was 0.003 mol. The mixture was then aged at 70 °C under stirring (300 rpm) for 4 h until a green bubble gel was obtained, then dried at 100 °C for 24 h in a drying oven to remove the excess solvent. The resulting green solid was then calcined in a fixed-bed reactor at 700 °C for 3 h under a high-purity N2 (99.999%) flow of 40 mL·min−1. The heating rate was controlled at 2 °C·min−1. The obtained black solids were treated in 1 M H2SO4 aqueous solution (Sigma-Aldrich) at 70 °C until the solution was colorless in order to remove the insecure and uncovered Pd particles. The black solids were then thoroughly washed with deionized water until the pH of the waste solution reached 7. Finally, the black solids were freeze-dried at −48 °C for 12 h in vacuum by using a freeze dryer. The dried black solids were marked as M1/M2 @ X/C, where M1 = Pd; M2 = Fe; X is the N doping, e.g., Pd/Fe@N/C, Pd@N/C (without the addition of Fe2(SO4)3) and Fe@N/C (without the addition of Pd(NO3)2·6H2O). For comparison, in the preparation process of other two catalysts, melamine was not added, and they were marked as Fe@C and Pd/Fe@C.

3.2. Catalysts Characterization

The transmission electron microscopy (TEM) measurements were conducted on a JEM-2100F field-emission high-resolution transmission electron microscope (JEM-2100F, JEOL Ltd., Kyoto, Japan) to determine the morphology of the catalysts. The working voltage was 200 kV. Scanning electron microscopy (SEM) measurements were performed on an SU-70 analytical scanning electron microscope (Hitachi, Tokyo, Japan) operating at 10 kV. X-ray photoelectron spectroscopies (XPS) were collected over a photoelectron spectrometer (Thermo Scientific, Waltham, MA, USA) based on monochromatic Al Kα source (hν = 1486.6 eV). For calibrating the electron binding energy (BE) of the elements, the BE of the C1s was set to 284.8 eV. Phase analysis was carried out on an X-ray powder diffractometer (XRD, X-Ray Diffraction) of the X’Pert Pro MPD type from PANanalytical, Almelo, Netherlands. The parameters were as follows: Cu target Kα radiation, λ = 0.150 nm, voltage was set at 40 kV with a current of 40 mA, scanning ranged from 5° to 80° with a scanning step of 0.02°, and the scanning rate was 2°·min−1. The patterns were processed on JADE 6.0 software using the powder diffraction file (PDF) database of the International Center for Diffraction Data (ICDD). The contents of Pd and Fe in the catalysts were detected using an ICP-OES spectrometer (Avio 200) from PerkinElmer, Shelton, CT, USA. Approximate 40 mg of the sample was fully dissolved in 6 mL of aqua regia and then diluted to a final volume of 500 mL using distilled water. The diluted sample was analyzed. The magnetic properties of the catalyst was studied by a Vibrating Sample Magnetometer (VSM) from Lake Shore, Westerville, OH, USA. It has a room-temperature measurement sensitivity of 5×10−7 emu.

3.3. Catalytic Tests

The selective hydrogenation of aldehydes was carried out in a 50 mL stainless steel autoclave (Shanghai Yanzheng Experimental Instrument Co., Ltd., Shanghai, China). The procedure was as follows: The autoclave was filled with 0.5 mmol of substrate, 0.05 g of catalyst, the desired amount of formic acid (0.5 mmol, 2.5 mmol, 5.0 mmol, 10.0 mmol, 20.0 mmol or 30.0 mmol) and 4 mL of solvent. Then the heating process was initiated, with the temperature increasing to the setting temperature at a rate of 10 °C·min−1. The mixture should then be stirred at a speed of 300 r·min−1. After completion of the reaction, the reaction kettle was naturally cooled to room temperature. The liquid product and solid residue were then separated by filtration and centrifugation. The liquid product was collected in a transparent glass bottle for further analysis. In order to test the catalyst recycling performance, the solid component was washed with ethanol and deionized water after each reaction and then dried. This ensured the component was thoroughly rinsed and dried for subsequent reuse in the recycling process. The liquid product was analyzed using GC-2014C (Shimadzu, Kyoto, Japan) equipped with a capillary column (HP-5, 30 m × 250 mm × 0.25 μm) and a flame ionization detector (FID). The temperature program was set at a ramp rate of 10 °C·min−1 from 60 °C to 300 °C and held for 3 min. The main products were analyzed quantitatively using n-octanol (0.05 g was added to the sample) as the internal standard. The product was determined by TRACRE 1300ISQ GC-MS (Thermo Fisher Science, USA) using the same column and conditions as GC and retention times that determined using standards.

4. Conclusions

Pd/Fe magnetic bimetallic catalysts supported on nitrogen-doped carbon materials were successfully prepared. These catalysts enabled the selective reduction of vanillin to vanillyl alcohol (VA) and 2-methoxy-4-methylphenol (MMP) under mild conditions, without the formation of by-products. The N-doped graphene support was found to provide a greater number of active sites via the synergistic catalysis of Pd and Fe, while the magnetic properties of the catalyst facilitated easy separation and recyclability. For the hydrogenation of vanillin, the Pd/Fe@N/C catalyst exhibited excellent catalytic activity under mild conditions using formic acid at 80 °C. When ethyl acetate was used as the solvent, vanillyl alcohol was obtained with a yield of 84.5%, while the use of cyclohexane as the solvent led to the exclusive formation of MMP with a yield of 92.3%. Given the magnetic separability and high stability of the catalyst, further exploration of its application in a continuous-flow (FLOW) reactor system is recommended. FLOW reactors offer superior mass and heat transfer capabilities, enabling precise control of reaction conditions and facilitating efficient temperature management, which will be beneficial for the scalable, green hydrogenation of biomass-derived compounds.

Author Contributions

Conceptualization, J.L.; methodology, J.L.; software, H.Z.; validation, J.L., H.Z. and Y.L.; formal analysis, H.Z.; investigation, H.Z. and Y.L.; resources, J.L.; data curation, Y.L.; writing—original draft preparation, H.Z.; writing—review and editing, J.L.; visualization, Y.L.; supervision, J.L.; project administration, J.L.; funding acquisition, H.Z. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Featured Innovation Project for Ordinary Universities in Guangdong Province (2023KTSCX364), 2023 High-level Talents Research Initiative Project (KYG2301), National Natural Science Foundation of China (51976225), and Fundamental Research Funds for the Central Universities (2242022R10058).

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Catalysts 15 00906 g001
Figure 1. Preparation of the Pd/Fe@N/C catalyst and its application in the selective hydrogenation of vanillin.
Figure 1. Preparation of the Pd/Fe@N/C catalyst and its application in the selective hydrogenation of vanillin.
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Catalysts 15 00906 g002
Figure 2. Representative SEM images (a), TEM images (b,c) and the metal particle size distribution (d) of Pd/Fe@N/C catalyst.
Figure 2. Representative SEM images (a), TEM images (b,c) and the metal particle size distribution (d) of Pd/Fe@N/C catalyst.
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Catalysts 15 00906 g003
Figure 3. XPS of Pd/Fe@N/C. (a) C 1s spectrum; (b) N 1s spectrum; (c) Pd 3d spectrum; and (d) Fe 2p spectrum.
Figure 3. XPS of Pd/Fe@N/C. (a) C 1s spectrum; (b) N 1s spectrum; (c) Pd 3d spectrum; and (d) Fe 2p spectrum.
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Catalysts 15 00906 g004
Figure 4. XRD patterns of the Pd/Fe@N/C catalyst.
Figure 4. XRD patterns of the Pd/Fe@N/C catalyst.
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Catalysts 15 00906 g005
Figure 5. Magnetic hysteresis loop of the Pd/Fe@N/C catalyst at room temperature.
Figure 5. Magnetic hysteresis loop of the Pd/Fe@N/C catalyst at room temperature.
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Catalysts 15 00906 g006
Figure 6. Comparative activity of vanillin hydrogenation over different catalysts and the study of solvent effect. (a) Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, H2O (4 mL), 5 mmol FA, 80 °C, 2 h. (b) Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, 4 mL solvent, 5 mmol FA, 60 °C, 1 h.
Figure 6. Comparative activity of vanillin hydrogenation over different catalysts and the study of solvent effect. (a) Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, H2O (4 mL), 5 mmol FA, 80 °C, 2 h. (b) Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, 4 mL solvent, 5 mmol FA, 60 °C, 1 h.
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Catalysts 15 00906 g007
Figure 7. Research on the reaction factors of Pd/Fe@N/C catalyzed hydrogenation of Vanillin. (a) Reaction system with water as solvent. (b) Reaction system with cyclohexane as solvent. Reaction conditions: for the investigation of FA/vanillin ratio, the reaction temperature was 80 °C and the reaction time was 1 h; for the investigation of reaction temperature, the FA/vanillin ratio was 10 and the reaction time was 1 h; for the investigation of reaction time, the FA/vanillin ratio was 10 and the temperature was 80 °C.
Figure 7. Research on the reaction factors of Pd/Fe@N/C catalyzed hydrogenation of Vanillin. (a) Reaction system with water as solvent. (b) Reaction system with cyclohexane as solvent. Reaction conditions: for the investigation of FA/vanillin ratio, the reaction temperature was 80 °C and the reaction time was 1 h; for the investigation of reaction temperature, the FA/vanillin ratio was 10 and the reaction time was 1 h; for the investigation of reaction time, the FA/vanillin ratio was 10 and the temperature was 80 °C.
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Catalysts 15 00906 g008
Figure 8. Pd/Fe@N/C catalyst catalyzed hydrogenation cycle experiment of vanillin. Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, H2O (4 mL), 5 mmol FA, 80 °C, 1 h.
Figure 8. Pd/Fe@N/C catalyst catalyzed hydrogenation cycle experiment of vanillin. Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, H2O (4 mL), 5 mmol FA, 80 °C, 1 h.
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Catalysts 15 00906 sch001
Scheme 1. A feasible mechanism for the HDO of benzylic ketones in the FA and Pd/Fe@N/C system.
Scheme 1. A feasible mechanism for the HDO of benzylic ketones in the FA and Pd/Fe@N/C system.
Catalysts 15 00906 sch001
Table 1. The ICP-OES results for the contents of Pd and Fe in different catalysts.
Table 1. The ICP-OES results for the contents of Pd and Fe in different catalysts.
CatalystPd Content (wt.%)Fe Content (wt.%)
Pd@N/C6.9undetectable
Fe@N/Cundetectable13.0
Pd/Fe@N/C0.715.1
Table 2. A comparison of the catalytic results of similar systems in the literature with those of this work.
Table 2. A comparison of the catalytic results of similar systems in the literature with those of this work.
EntryCatalystMain ProductReaction ConditionsConv. (%)Sel. (%)Reference
1Pd/CVA30 °C, 7 bar H2, water, 90 min>99>99Ref. [6]
2Pd/NoritMMP50 °C, 5 bar H2, isopropanol, 300 min100>99.5Ref. [8]
3Pd/HHTMMP50 °C, 5 bar H2, isopropanol, 240 min100>99.5Ref. [8]
4Ni/ZrPMMP220 °C, 5 bar H2, isopropanol, 30min97.2590.89Ref. [9]
5Cu/AC-600MMP120 °C, 20 bar H2, water, 300 min99.993.2Ref. [10]
6Ni–Co–P/HAP (hydroxyapatite)MMP200 °C, formic acid, isopropanol, 300 min97.8693.97Ref. [7]
7Cu/AC-600MMP180 °C, 2-propanol, 300 min99.899.1Ref. [10]
8Pd/Fe@N/CVA80 °C, formic acid, water, 60 min93.289.6This work
9Pd/Fe@N/CMMP80 °C, formic acid, cyclohexane, 300 min92.399.0This work
Table 3. Hydrogenation reduction of various aldehyde by the catalyst of Pd/Fe@N/C.
Table 3. Hydrogenation reduction of various aldehyde by the catalyst of Pd/Fe@N/C.
Entry aSubstrateProductConv. b (%)Sel. b (%)
1Catalysts 15 00906 i001Catalysts 15 00906 i00296.197.4
2Catalysts 15 00906 i003Catalysts 15 00906 i004>9998.0
3Catalysts 15 00906 i005Catalysts 15 00906 i00697.596.2
4Catalysts 15 00906 i007Catalysts 15 00906 i00885.995.3
5Catalysts 15 00906 i009Catalysts 15 00906 i010>9993.3
6Catalysts 15 00906 i011Catalysts 15 00906 i01284.486.4
a Reaction conditions: substrate (0.5 mmol), 5 mg catalyst, H2O (4 mL), 5 mmol FA, 80 °C,1 h. b Determined by gas chromatography with flame ionization detector (GC-FID); conversion of aldehyde, Conv. (%); selectivity of the corresponding product, Sel. (%).
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Zuo, H.; Lei, Y.; Liu, J. Transfer Hydrogenation of Vanillin with Formic Acid over Graphene-Encapsulated Nitrogen-Doped Bimetallic Magnetic Pd/Fe@N/C Catalyst. Catalysts 2025, 15, 906. https://doi.org/10.3390/catal15090906

AMA Style

Zuo H, Lei Y, Liu J. Transfer Hydrogenation of Vanillin with Formic Acid over Graphene-Encapsulated Nitrogen-Doped Bimetallic Magnetic Pd/Fe@N/C Catalyst. Catalysts. 2025; 15(9):906. https://doi.org/10.3390/catal15090906

Chicago/Turabian Style

Zuo, Hualiang, Yulong Lei, and Jianguo Liu. 2025. "Transfer Hydrogenation of Vanillin with Formic Acid over Graphene-Encapsulated Nitrogen-Doped Bimetallic Magnetic Pd/Fe@N/C Catalyst" Catalysts 15, no. 9: 906. https://doi.org/10.3390/catal15090906

APA Style

Zuo, H., Lei, Y., & Liu, J. (2025). Transfer Hydrogenation of Vanillin with Formic Acid over Graphene-Encapsulated Nitrogen-Doped Bimetallic Magnetic Pd/Fe@N/C Catalyst. Catalysts, 15(9), 906. https://doi.org/10.3390/catal15090906

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