In Situ Synthesis of Hierarchical Carbon-Encapsulated Pd Nanoparticles as an Efficient Semi-Hydrogenation Catalyst

Abstract

The process of directly using atmospheric H₂ for the catalytic semi-hydrogenation of alkynes to alkenes has significant applications in the polyolefin industry. Herein, we report a facile approach to synthesize a hierarchical carbon-encapsulated Pd catalyst for the highly selective semi-hydrogenation of nitrophenylacetylene. The catalyst featured a structure of (Pd@NG)/(Pd@C), which demonstrated that an oligo-layer of nitrogen-doped graphene (NG)-encapsulated Pd particles are supported on the carbon matrix, semi-embedded by another type of Pd particle. The catalyst, named Pd@NC, achieved 99% selectivity for nitrostyrene at 97% nitrophenylacetylene conversion and demonstrated an excellent stability. A good selectivity arose from the bridging effect of hierarchical porous carbon, where hydrogen activation and alkyne hemihydrogenation took place on palladium particles and NG, respectively. The NG layer provided excellent protection against the over-hydrogenation of the reaction.

1. Introduction

The semi-hydrogenation of alkynes holds significant industrial importance, particularly in petrochemical, pharmaceutical, and fine chemical production. This process involves the hydrogenation of C≡C bonds and the activation of hydrogen. Hydrogen activation generally takes place on the surface of metals or metal oxides, where hydrogen molecules dissociate into active hydrogen species [1]. Through the mechanism of hydrogen spillover [2], these active hydrogen atoms migrate to the hydrogenation sites to complete the hydrogenation process. However, the addition of active hydrogen (H*) to C-C unsaturated bonds is so fast that it can occur at temperatures as low as 85 K [3]. Therefore, a major challenge in current alkyne hydrogenation technology is the over-hydrogenation of alkyne. Specifically, alkenes produced during the reaction cannot desorb promptly, leading to their further conversion into alkanes, which reduces the selectivity of the semi-hydrogenation process.

Over the past few decades, industrial processes have predominantly relied on PdAg/Al₂O₃ catalysts [4,5,6] or Lindlar catalysts [7] for the semi-hydrogenation of alkynes. In PdAg/Al₂O₃ catalysts, the introduction of Ag limits the exposure of highly active Pd sites, significantly suppressing over-hydrogenation. Similarly, in Lindlar catalysts, Pd forms compounds with Pb, partially passivating Pd’s catalytic activity. This ensures that the catalytic hydrogenation of alkynes proceeds only to alkenes without further hydrogenation to alkanes. Alternative alloy systems, such as PdCu [8,9,10], PdIn [11], PdGa [12], and PdZn [13,14,15], have been explored. However, the inclusion of a second metal with properties similar to Pd complicates the recycling of Pd.

To address challenges related to reducing noble metal loading and improving catalyst stability, single-atom catalysts have recently emerged as a promising strategy for selective alkyne hydrogenation. For instance, Huang et al. [16,17] enhanced the selectivity and stability of ethylene during Pd-catalyzed hydrogenation by constructing Pd single-atom and Pd₁-Cu₁ diatomic structures confined within defect-rich ND@G frameworks. These unique structures promote reactant molecule adsorption and hydrogen dissociation. The encapsulation of single metal atoms, clusters, or nanoparticles in zeolite molecular sieves [18,19,20,21,22] or MOFs [23,24] has also been shown to enhance the selective hydrogenation efficiency. However, while single-atom catalysis offers remarkable catalytic activity for the semi-hydrogenation of alkynes, the controllable preparation of high-loading single-atom catalysts remains a challenge. Shen et al. [25] addressed this by using nanoscale interfacial confinement, wherein a 2D Pb monolayer was confined on the surface of Pd nanoparticles, forming interfaces with a high selectivity for semi-hydrogenation.

The development of carbon-coated metal catalysts provides another promising route for selective hydrogenation. These catalysts typically feature few-layer graphene shells deposited on metal cores, which can include transition metals or their oxides, such as Fe [26], Co [27], Ni [28,29], and Cu [30]. A limited number of studies have reported noble metal cores, such as Pt [31] and Pd [32], for the semi-hydrogenation of alkynes. The carbon shell structure prevents direct contact between alkyne molecules and the catalyst surface, suppressing side reactions such as over-hydrogenation, coking, and oligomerization. Through hydrogen spillover, active hydrogen migrates from the metal surface through carbon defects to hydrogenation sites, where it reacts with alkyne molecules to achieve semi-hydrogenation. This is an effective strategy for achieving selective hydrogenation. However, despite numerous attempts to develop noble-metal-based carbon-coated catalysts, the complexity of their preparation remains a significant obstacle. Therefore, developing simple and accessible synthetic methodologies for single-metal-based carbon-coated catalysts is crucial.

Inspired by existing reports, we proposed a catalyst design according to the conjecture that the metal nanoparticle size plays a crucial role in alkyne semi-hydrogenation, where larger nanoparticles exhibit a higher activity, but a lower stability, whereas smaller nanoparticles offer an improved selectivity and stability [33]. The catalyst with hierarchical metal particles is expected to be obtained through the pyrolysis of MOFs. In recent years, MOF pyrolysis has garnered significant attention [34,35,36,37,38]. For example, Zhou [39] prepared carbon materials through the vacuum pyrolysis of MOF precursors, while Zhang et al. [40] used MOF pyrolysis to encapsulate PdPh heterojunctions partially within a carbon shell, achieving the efficient semi-hydrogenation of nitrophenylacetylene to nitrostyrene.

This study focused on a novel and precise semi-hydrogenation process using carbon-coated noble metal nanoparticles. Nitrophenylacetylene was selected as a model substrate to investigate the role of Pd@NC catalysts in the selective semi-hydrogenation of alkynes. The nitro group further validated the high selectivity of the catalyst, as nitro groups are more readily reduced by hydrogen compared to C≡C bonds, as exposed Pd metal will rapidly catalyze the conversion of nitro groups into amines. In this work, a one-pot synthesis yielded two distinct carbon-encapsulated Pd species with different sizes. Larger Pd nanoparticles were coated by amorphous carbon structures, formed as matrix of the catalyst, where the large metal particles facilitated hydrogen activation. In contrast, smaller Pd nanoparticles were semi-embedded in the support surface and coated with a few layers of nitrogen-doped graphene, and were identified as the primary active site for the semi-hydrogenation of C≡C triple bonds. The synergistic effects of the dual-site bridge formed in situ enabled the selective hydrogenation of nitro-phenylacetylene to nitro-styrene under mild conditions.

2. Results and Discussion

Characterization

Based on the characterization results from FTIR spectroscopy (Figure S1), we preliminarily confirmed the successful synthesis of PCN-222-Pd (1.00 Pd). To further verify its structure, an X-ray diffraction analysis (XRD) was conducted on PCN-222-Pd (1.00 Pd). As shown in Figure 1a, the PCN-222-Pd (1.00 Pd) exhibited several characteristic peaks at 4.8°, 7.1°, and 8.0°, which are consistent with the simulated results for PCN-222. This consistency indicates that the prepared PCN-222-Pd (1.00 Pd) possesses a well-ordered lattice structure, further confirming the successful synthesis of the catalyst precursor PCN-222-Pd (1.00 Pd). The PCN-222-Pd (1.00 Pd) was further pyrolyzed under an argon atmosphere to obtain Pd@NC (1.00 Pd). The surface area and pore size of Pd@NC (1.00 Pd) were determined using the BET method with nitrogen adsorption–desorption (Figure 1b). The N₂ adsorption–desorption isotherms of the catalyst were classified as type IV, and the specific surface area was 151.892 m² g⁻¹. The inset of Figure 1b reveals the presence of both micropores and mesopores, which were inherited from the PCN-222 structure. In Figure 1c, the SEM of Pd@NC (1.00 Pd) clearly shows that Pd@NC (1.00 Pd) has a rod-like structure similar to PCN222, with a rough surface and the distribution of smaller fine granular structures. Figure 1d displays two prominent Raman peaks of Pd@NC (1.00 Pd) at 1351 cm⁻¹ and 1575 cm⁻¹, which were attributed to the characteristic D-band (disordered, sp³ hybridized carbon) and G-band (ordered, sp² hybridized carbon) of carbon-based nanostructures. The G-band represents the degree of graphitization of carbon materials, while the D-band is associated with the structural defects of materials. The Raman spectrum of Pd@NC (1.00 Pd) suggests that graphitization occurred after pyrolysis at 600 °C (Figure 1d). This study investigated the synthesis of PCN-222-Pd catalyst precursors with varying levels of Pd content. When the Pd loading was 50% or 100%, elemental Pd was present due to the reduction of Pd(II) to Pd(0) in the high Pd concentration.

High-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) (Figure S2) clearly illustrated the structure of the Pd@NC (1.00 Pd) catalyst, as shown in Figure 2a. The lattice fringes of the Pd particles exhibited an interplanar spacing of 0.235 nm (Figure S3) [23], which is close to the (111) plane of Pd. The graphene layers near the Pd nanoparticles were ultra-thin, with approximately 1~4 layers, forming a structure where a few layers of graphene completely encapsulate the Pd nanoparticles. The interplanar spacing was 0.335 nm [24], which is close to the lattice spacing of graphene. Additionally, the few-layer graphene structure encapsulating the Pd nanoparticles formed a secondary structure that was semi-embedded in the amorphous carbon matrix, resulting in a distinct supported hierarchical carbon-encapsulated metal materials. Besides the semi-embedded Pd@NC with a small Pd size (about 6~8 nm), another type of Pd nanoparticle was uniformly distributed within the carbon matrix derived from PCN-222. The inset of Figure 2b show that the size distribution of this type of Pd nanoparticle approximately ranged from 9 to 13 nm. Figure 2c–e present the HRTEM mapping images, showing that the small-sized palladium metal particles in Pd@NC (1.00 Pd) were very likely to be completely encapsulated by nitrogen-doped graphene. To verify this hypothesis, Pd@NC (0.50 Pd) was placed in a CO atmosphere at room temperature for 48 h. Then, as shown in Figure S4, the CO-FTIR spectroscopy revealed that the adsorbed CO on Pd@NC (0.50 Pd) had a small vibration peak signal in the range of 1950 cm⁻¹ to 2100 cm⁻¹. This shows that, for Pd@NC (0.50 Pd) containing only small-sized Pd nano-particles, a large amount of CO cannot be adsorbed with the metal to form a strong adsorbed carbon monoxide signal, thereby verifying that the small-sized palladium nano-particles are completely encapsulated in nitrogen-doped graphene. Moreover, the two types of Pd nanoparticles can be distinguished in Figure 2e.

Due to the significantly higher electronegativity of nitrogen compared to carbon, doping a carbon matrix with nitrogen atoms can greatly enhance the interaction between the carbon materials and Pd nanoparticles. This enhanced interaction mainly stems from the charge transfer between heteroatoms and palladium atoms. This strategy can effectively alter the electronic structure of active Pd sites. As shown in Figure 3a, the C=O and C-O functional groups in the carbon matrix of the Pd@NC (1.00 Pd) catalyst were assigned to the binding energy positions of 289 eV and 287 eV, indicating the presence of a certain amount of oxygen-containing functional groups on the carbon matrix. These oxygen-containing functional groups on the carbon support can maintain part of the Pd nanoparticles in an oxidized state. Meanwhile, as shown in Figure 3b, the integral peak area of the pyrrolic nitrogen of the Pd@NC (1.00 Pd) catalyst was greater than that of pyridinic nitrogen, revealing that the carbon support of the Pd@NC (1.00 Pd) catalyst was in an electron-rich state. As shown in Figure 3c, the binding energy of the palladium particles in Pd@NC (1.00 Pd) slightly shifted to a lower binding energy compared to Pd@NC (0.50 Pd), indicating electron transfer between the palladium clusters and the carbon matrix. The nitrogen-doped carbon matrix transferred electrons to the Pd nanoparticles in the Pd@NC (1.00 Pd) catalyst, while the oxygen-containing functional groups on the carbon matrix made this electron transfer easier. For the Pd@NC (0.50 Pd) catalyst, the integral peak area of pyrrolic nitrogen was approximately equal to that of pyridinic nitrogen, indicating that the electron-rich state of the carbon matrix in Pd@NC (0.50 Pd) was not as pronounced as in Pd@NC (1.00 Pd).

3-Nitrophenylacetylene (1 mmol) was used as the substrate; 20 mg of catalyst was added, respectively; isopropanol (5 mL) was used as the solvent; and the reaction was carried out at 70 °C under 1 bar of hydrogen pressure. The product was detected by GC-MS after 10 h. As shown in Figure 4a, both Pd@NC (0.50 Pd) and Pd@NC (1.00 Pd) have a high selectivity of 99% for 3-nitrostyrene, while Pd@NC (0.50 Pd) only has a conversion of 2%, but Pd@NC (1.00 Pd) has a conversion as high as 97%. As shown in Figure S5, the XRD of Pd@NC (1.00 Pd) showed that it had obvious diffraction peak signals of single-element palladium particles, proving that there were large-sized palladium particles in it. After pyrolysis, the diffraction peak signals of palladium particles were also observed. There were a number of large Pd particles that effectively activated hydrogen into active hydrogen, which achieved highly efficient semi-hydrogenation with the alkyne molecules adsorbed on the carbon shell. In the Pd@NC (0.50 Pd), as shown in Figures S6–S8, the absence of large particles of Pd prevented the activation of hydrogen into active hydrogen, thus preventing the semi-hydrogenation of alkynes. In the kinetic test of Pd@NC (1.00 Pd), samples of the reaction mixture were taken every two hours and analyzed using GC-MS. As shown in Figure 4b, as the reaction progressed, the substrate 3-nitrophenylacetylene gradually decreased, while the product 3-nitrostyrene steadily increased. After 9.5 h of reaction time, the conversion rate reached 97%, and no other by-products were detected by GC-MS. This result indicates that the catalyst exhibited a superior selectivity. In the stability test of the Pd@NC (1.00 Pd) catalyst, a recycling experiment was conducted under the following conditions: 1 mmol of 3-nitrophenylacetylene, 20 mg of catalyst, and 5 mL of isopropanol at a temperature of 70 °C and under 1 bar of hydrogen pressure. After the reaction, the product was analyzed by GC-MS, and the catalyst was separated by centrifugation. The separated catalyst was then reused under the same conditions for a new cycle of the experiment. As shown in Figure 4c, the results indicate that the Pd@NC (1.00 Pd) catalyst demonstrated a 99% nitrostyrene selectivity at a 97% nitrophenylacetylene conversion and exhibited a superior stability at 70 °C after eight cycles.

3. Experimental Section

3.1. Materials and Chemicals

PdCl₂ was provided by Eybridge Reagent Co., Ltd., Shanghai, China. ZrCl₄, 4-carboxybenzaldehyde, benzoic acid (BA), and propionic acid were provided by Aladdin Reagent Co., Ltd., Shanghai, China. Pyrrole and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Methanol, anhydrous ethanol, and N,N-dimethylformamide (DMF) were purchased from the Tianjin Yongda Chemical Reagent Co., Ltd., Tianjin, China. All the chemical reagents were of analytic grade and used without any further purification.

3.2. Solvothermal “One-Pot” Synthesis of PCN-222-Pd

An amount of 0.0586 g of PdCl₂ (0.32 mmol) was weighed using a balance and added to a 500 mL three-neck round-bottom flask. A total of 50 mL of N,N-dimethylformamide (DMF) was measured using a graduated cylinder and sonicated for 1 h to partially dissolve the PdCl₂ in the DMF. Next, 0.2560 g of boxyphenyl porphyrins (TCPPs) (0.32 mmol) was weighed and added to the above mixture. Subsequently, the mixture was stirred and refluxed in an oil bath at 120 °C for 2 h. Then, 0.60 g of ZrCl₄ and 14 g of benzoic acid were weighed using a balance, and 160 mL of DMF was measured using a graduated cylinder; these were then directly added to the reaction mixture. The temperature was maintained at 120 °C and the reaction was continued for 48 h. After the reaction was complete, the mixture was cooled to room temperature, centrifuged (7500 r/min, 4 min), and washed with water and ethanol three times each; the obtained product was then dried in a vacuum oven at 60 °C. The final product was a dark purple solid, PCN-222-Pd (1.00 Pd), where 1.00 Pd indicates a molar ratio of PdCl₂ to TCPP of 1, and PCN-222-Pd (0.50 Pd), where 0.50 Pd indicates a molar ratio of PdCl₂ to TCPP of 0.5.

3.3. In Situ Synthesis of Pd@NC (xPd) Catalysts

The synthesis of Pd@NC (xPd) was conducted using a one-pot method (Scheme 1). A total of 140 mg of the synthesized PCN-222-Pd (0.50 Pd) and PCN-222-Pd (1.00 Pd) were weighed and spread evenly in a porcelain boat. They were then calcined in a tubular furnace at 600 °C for 2 h in an argon atmosphere (heating rate of 5 °C/min to 600 °C), followed by natural cooling to obtain Pd@NC (0.50 Pd) and Pd@NC (1.00 Pd).

3.4. Characterization Method

The characterization method used in this study included powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption isotherms, gas chromatography–mass spectrometry (GC-MS), and Fourier-transform infrared (FTIR) spectroscopy. The XRD spectra were obtained using a PANalytical Empyrean diffractometer with Cu Kα radiation. The SEM and TEM images were taken at 5.0 kV and 200 kV, respectively. The HRTEM was performed using an American FEI Talos F200X G2, Hillsboro, OR, USA; The XPS spectra were obtained using a PHI Quantera SXM spectrometer, Chigasaki, Japan; ULVAC-PHI, with the carbon contamination of C1s at 284.8 eV used to determine the binding energy. The specific surface area of the samples was determined by nitrogen adsorption–desorption isotherms at 77 K using the Brunauer–Emmett–Teller (BET) analysis method, and the pore-size distribution curves were obtained using the Barret–Joyner–Halenda (BJH) equation. GC-MS was performed using a Bruker 450GC-320MS, Ettlingen, Germany; The FTIR spectra were recorded using a Bruker Vertex spectrometer, Ettlingen, Germany, with a spectral resolution of 16 cm⁻¹ in the range of 30,000–10 cm⁻¹, using a KBr pellet.

3.5. Catalytic Evaluation Test

As depicted in Scheme 2, In this experiment, 3-nitrophenylacetylene (3-NPA) (1 mmol), the catalyst (20 mg), and isopropanol (5 mL) were placed into a 25 mL flask. The reaction was performed at 65 °C under a hydrogen atmosphere, maintained with a hydrogen balloon. Upon the completion of the reaction, the catalyst was recovered by filtration, and the resultant liquid was subjected to a gas chromatography (GC) analysis. The catalyst was subsequently washed several times with isopropanol and reused in further experiments without additional treatment. The obtained product was analyzed using gas chromatography–mass spectrometry (GC-MS). The conversion and selectivity were calculated using the following equations:

$$\mathrm{Conversion}(\%)={\displaystyle \frac{\mathrm{nitrophenylacetylene} \mathrm{feed}\left(\mathrm{mol}\right)-\mathrm{nitrophenylacetylene} \mathrm{residue}\left(\mathrm{mol}\right)}{\mathrm{nitrophenylacetylene} \mathrm{feed}\left(\mathrm{mol}\right)}}\times 100\%$$

$$\mathrm{Selectivity}(\%)={\displaystyle \frac{\mathrm{nitrostyrene} \mathrm{product}\left(\mathrm{mol}\right)}{\mathrm{nitrophenylacetylene} \mathrm{feed}\left(\mathrm{mol}\right)-\mathrm{nitrophenylacetylene} \mathrm{residue}\left(\mathrm{mol}\right)}}\times 100\%$$

4. Mechanism Explanation

In the DMF solution, the TCPP ligand formed a complex with palladium chloride, resulting in TCPP-Pd. However, a small portion of palladium did not enter the porphyrin ring of TCPP and remained as dispersed palladium ions in the DMF solution. These palladium ions underwent partial reduction to Pd(0) when exposed to the DMF solvent. After synthesizing TCPP-Pd, zirconium chloride and benzoic acid were added to the solution and the mixture was heated for 48 h. During this process, porous PCN-222 was formed. Dispersed Pd(0) nanoparticles were observed to distribute on the outer surface and within the inner core of the rod-shaped PCN-222. The confined spaces in the porous structure of PCN-222 led to the formation of smaller palladium nanoparticles from the TCPP-coordinated Pd.

Under an argon atmosphere, the pyrolysis of PCN-222 material in the absence of oxygen resulted in varying degrees of carbonization of the nanocarbon materials. A thick amorphous carbon layer formed on large Pd nanoparticles, exhibiting a high thermal stability and mechanical strength, which served to anchor the semi-embedded chainmail-like structure [41,42,43]. This is referred to as a carbon matrix. Meanwhile, semi-embedded small Pd nanoparticles in situ grew on the surface of the carbon matrix, surrounded by highly graphitized, ultrathin, few-layer graphene. This chainmail-like structure is considered to be the hydrogenation site for selective semi-hydrogenation. The large-sized Pd nanoparticles encapsulated by the amorphous carbon activated hydrogen molecules into active hydrogen. The active hydrogen then reached the hydrogenation sites via hydrogen spillover, thereby establishing a dual-site bridge mechanism between the hydrogen activation sites and the hydrogenation sites. There was no direct contact between the yielded alkene with metal because of the carbon encapsulation; therefore, the semi-hydrogenation remained highly selective.

5. Conclusions

We prepared Pd@NC (1.00 Pd) from pyrolyzing an MOF precursor with varying levels of palladium content. The active sites were nitrogen-doped, few-layer, carbon-encapsulated palladium nanoparticles semi-embedded on an amorphous carbon matrix and large-sized palladium nanoparticles. The superior selectivity and conversion of this catalyst can be attributed to the synergistic mechanism of their two active sites, which simultaneously facilitated the activation of hydrogen and the adsorption of alkynes. They enabled the precise semi-hydrogenation of active hydrogen through hydrogen spillover. This development offers vital potential for the strategic design of dual-site bridge semi-hydrogenation catalysts.