Rational Design of Ce–Ni Bimetallic MOF-Derived Nanocatalysts for Enhanced Hydrogenation of Dicyclopentadiene

Abstract

The development of highly efficient catalysts for the hydrogenation of dicyclopentadiene (DCPD) remains a critical challenge. In this study, we designed a series of bimetallic Ce–Ni metal–organic framework (MOF)-derived nanocatalysts by precisely tuning the Ce/Ni ratio and calcination temperatures. The optimized catalyst, Ni–CeO_2(7:3) @C–400 °C, featuring highly dispersed carbon-coated Ni nanoparticles, achieved complete hydrogenation of DCPD to tetrahydrodicyclopentadiene (THDCPD) with 100% conversion and nearly 100% selectivity within 2 h under 100 °C and 2 MPa. The porous carbon framework significantly facilitated the diffusion and accessibility of DCPD molecules, combined with Ce species reconstructing the electronic structure of Ni active centers through electronic interactions, synergistically enhancing the hydrogenation efficiency. Furthermore, the catalyst demonstrated good structural stability. This work not only provides a robust strategy for the rational design of bimetallic MOF-derived catalysts but also highlights their potential for practical applications in industrial hydrogenation processes.

1. Introduction

The use of high-energy-density (HED) liquid fuels offers significant potential for enhancing the range, payload capacity, and speed of various aerospace vehicles, including guided missiles and supersonic aircraft. Among the HED fuel options, Jet Propellant 10 (JP-10) stands out as a widely utilized jet fuel [1], characterized by its high energy density, low freezing point, favorable flash point, remarkable thermal stability, and minimal pollutant emissions and toxicity. The hydrogenation of Dicyclopentadiene (DCPD) to produce tetrahydro–dicyclopentadiene (THDCPD) is a critical step in the synthesis of JP-10 fuel. However, this reaction typically involves a high energy barrier, necessitating the incorporation of catalytic materials to facilitate the process.

Among various catalytic materials, supported noble metal catalysts such as Pd/C, Pd/Al₂O₃, and Ru/SBA-15 have demonstrated the ability to efficiently cleave H₂ and moderately adsorb DCPD. Under relatively mild conditions, these catalysts can achieve over 90% conversion of DCPD and selectivity for endo-tetrahydro-dicyclopentadiene (endo-THDCPD). However, the practical application of noble metals is limited due to their susceptibility to leaching, deactivation, and high costs. Porous Raney–Ni is the only commercially utilized catalyst for the hydrogenation of DCPD, achieving yields of over 96% for endo-THDCPD at 120 °C [2]. Nevertheless, Raney–Ni presents safety concerns and is prone to rapid deactivation, hindering efficient continuous production [3,4]. Therefore, there is an urgent need to develop cost-effective hydrogenation catalysts for DCPD that offer both high efficiency and stability.

Recently, metal–organic frameworks (MOFs) have garnered significant attention as catalysts owing to their unique properties [5,6], such as high surface area, tunable pore architectures, and facile functionalization. Upon calcination under an inert atmosphere at elevated temperatures, MOFs can be converted into porous carbon-supported metal derivatives, which not only preserve the inherent advantages of MOFs but also demonstrate improved stability [7,8]. These MOF-derived materials have demonstrated exceptional catalytic performance in various hydrogenation reactions, such as propylene hydrogenation [9,10], CO₂ hydrogenation, and the hydrogenation of unsaturated aldehydes. For instance, Jia et al. [11] demonstrated that pyrolysis of nitrogen-rich ZnNi MOFs yielded Ni nanoparticles supported on N-doped porous carbon, where Zn not only inhibited Ni aggregation but also generated additional pores via in situ volatilization during the pyrolysis process, resulting in a catalyst achieving 100% DCPD conversion and >99% THDCPD selectivity at 120 °C and 3 MPa H₂ pressure. Despite the promising potential of MOF-derived materials in DCPD hydrogenation, the development of high-performance catalysts capable of achieving efficient and highly selective [12,13] hydrogenation under relatively mild conditions remains both highly desirable and challenging [14,15].

In this study, the Ni–CeO_2(x:y) @C–T catalyst system was prepared by regulating the thermal decomposition of the Ce/Ni–MOF precursor, where x:y is the ratio of Ce:Ni in the synthesis of the precursor Ce_xNi_y–MOF–808 and T is the thermal decomposition temperature of the precursor. The effect of the Ce/Ni ratio and thermal decomposition temperature on the catalytic performance of DCPD hydrogenation was systematically investigated. The porous carbon framework significantly facilitated the diffusion and at 400 °C exhibited the optimal catalytic performance. The results showed that when the Ce/Ni atomic ratio was 3:7, the Ni–CeO_2(3:7) @C–400 °C catalyst has better catalytic effect. At this temperature, the confined structure formed by carbonization of the MOFs skeleton effectively inhibited the sintering of Ni nanoparticles (~4.2 nm), and XPS and H₂–TPR confirmed the formation of strong electronic interactions between Ce–O–Ni at the interface. Through the electron transfer from CeO₂ to Ni, electron-rich Ni⁰ active centers were generated. This electron regulation significantly promoted the dissociation and activation of H₂, enabling the catalyst to achieve a conversion rate of >99% and 100% tetrahydro product selectivity within 2 h at 100 °C and 2 MPa H₂. The cyclic experiments indicated that the activity only decreased by 7.2% after 5 reactions, which was attributed to the stabilizing effect of the carbon support on the active components. Comparative studies showed that at low temperatures (≤300 °C), due to incomplete Ni reduction, the active sites were insufficient, while at high temperatures (≥500 °C), Ni agglomeration (particle size increased to 12.8 nm) and the weakening of metal-support interaction occurred. This work provides a new paradigm for the controllable construction of dual-metal MOFs derivative catalysts and the study of the correlation between electronic structure and performance.

2. Results and Discussion

2.1. Synthesis and Characterization of Catalysts

To investigate the effects of varying metal ratios and temperatures on bimetallic MOF derivatives, we first present the synthetic pathway for bimetallic Ni–CeO_2(x:y) @C–300 °C derivatives, as illustrated in Figure 1. In the initial step, nickel nitrate hexahydrate and cerium ammonium nitrate hexahydrate were fully dissolved in a DMF solution, while trimesic acid (1,3,5–BTC) was dissolved in an ethanol solution. The two solutions were then thoroughly mixed to facilitate the successful coordination of the ligands in BTC with the Ce and Ni metal centers. Notably, the yield of Ce₃Ni₇–MOF–808 reached a remarkable 80%. In the second step, the two metal elements were converted into nanoparticles, with nickel deposited on the surface of CeO₂, through calcination in a hydrogen-rich atmosphere within a high-temperature tube furnace. The bimetallic derivatives, designated as Ni–CeO_2(x:y) @C–T, correspond to the Ce₃Ni₇–MOF–808 ratio, where @C indicates the carbonized MOF [16], and T denotes the specific temperatures employed during synthesis.

The structure and crystallinity of the MOF derivatives were analyzed using X-ray diffraction (XRD) powder patterns. The characteristic peaks of the pristine bimetallic MOF spectra aligned well with those documented in the literature, confirming the successful synthesis of Ce_xNi_y–MOF–808. As shown in Figure 2a, the diffraction patterns of Ni–CeO_2(3:7) @C–T at different temperatures (300 °C, 400 °C, and 500 °C) exhibited distinct peaks corresponding to Ni (JCPDS 04-0850) [17] and CeO₂ (JCPDS 34-0394) [18], consistent with reported data. At 300 °C, a prominent Ce–MOF characteristic peak [19,20] was still detectable, indicating that the MOF derivative had not yet fully formed. This observation aligns with the subsequent thermogravimetric analysis, which revealed no distinct Ni characteristic peaks, resulting in a conversion rate of less than 100% and a selectivity below 100% for the hydrogenation of dicyclopentadiene (DCPD). By 500 °C, the Ce–MOF characteristic peaks had vanished, signifying that the molecular framework had completely collapsed, allowing for the complete formation of the derivatives. The spectrum at this temperature clearly displayed the characteristic peaks of Ni and CeO₂, indicating their predominance. At 400 °C, a small number of Ce–MOF characteristic peaks remained, suggesting that the skeleton had not entirely collapsed, while the peaks for Ni and CeO₂ were more pronounced.

Further structural information about Ni−CeO_2(3:7) @C–T was obtained using Fourier-transform infrared spectroscopy (FT–IR). Figure 2b displays the FTIR spectra of Ni–CeO_2(3:7) @C–T in the range of 500 to 4000 cm⁻¹ [21,22]. The absorption peaks at 560 and 513 cm⁻¹ are attributed to Ce–O stretching vibrations, indicating the formation of coordination bonds between cerium and oxygen atoms. In the low-frequency region, absorption bands between 1300 and 990 cm⁻¹, as well as 890 and 746 cm⁻¹, correspond to the in-plane bending vibrations of C–H in the benzene ring. Additionally, the new absorption peaks at 1560 and 1390 cm⁻¹ are associated with the antisymmetric and symmetric stretching vibrations of the COO⁻ groups in the benzene-1,3,5-tricarboxylic acid (BTC) ligand. A distinct vibrational absorption in the range of 1600 to 1400 cm⁻¹ indicates the presence of the benzene ring, while the pronounced peak at 1650 cm⁻¹ corresponds to the C–O stretching vibration of the DMF solvent. Notably, a significant absorption band occurs around 1700 cm⁻¹ [23,24]. The compound exhibits a broad absorption band in the 3300 to 3500 cm⁻¹ range, which is indicative of –OH stretching vibrations from molecular intergranular water or physically adsorbed water within the framework [20,25]. The band associated with the coordinated water molecules shifts to a lower wavenumber for the –OH band, suggesting strong coordination to the metal. The intensity of the characteristic peaks of the functional groups gradually diminishes with increasing temperature, indicating that corresponding valence bonds are broken or unbonded at elevated temperatures, consistent with subsequent thermogravimetric (TG) results.

As shown in Figure 2c, at 300 °C, the characteristic peaks of the functional groups are pronounced, indicating that the structure is intact at this stage with no bond breakage. However, at 400 °C, a significant reduction in the intensity of these peaks suggests partial collapse of the organic skeleton. By 500 °C, the characteristic functional groups are less distinct, with some peaks disappearing altogether, indicating a complete structural collapse. In addition, we investigated the subject–object interactions by examining the thermal stability of Ni–CeO_2(3:7) @C through thermogravimetric analysis (TGA). The Ni–CeO_2(3:7) @C samples exhibited three distinct stages of mass loss: the loss of water and DMF molecules, the loss of certain hydroxyl groups, and the decomposition of the backbone structure. The decomposition temperature of the Ni–CeO_2(3:7) @C backbone was found to be 323 K. Notably, this decomposition temperature was significantly lower than expected, indicating strong interactions within the Ni–CeO_2(3:7) @C structure. Further analysis of the surface area, pore volume, and pore structure of Ce₃Ni₇–MOF–808 was conducted using N₂ adsorption measurements at 77 K, as illustrated in Figure 2d. These measurements confirmed the successful synthesis of mesoporous Ce₃Ni₇–MOF–808, which exhibited type IV adsorption behavior characteristic of mesoporous materials. The calculated structural parameters from the N₂ isotherm revealed a surface area of 34.39 m²·g⁻¹ and a predominant pore size of 6.079 nm. This slight reduction in specific surface area and pore volume compared to Ce₃Ni₇–MOF–808 can be attributed to the deposition of Ce and Ni particles on the surface and within the pore channels.

The morphologies of Ni–CeO_2(3:7) @C at 300 °C, 400 °C, and 500 °C were characterized to assess structural changes. As shown in Figure 3, the morphology of Ni–CeO_2(3:7) @C–T after hydrogen reduction differs significantly from that of Ce–MOF–808. The surface becomes slightly rougher with the incorporation of the metal element Ni, yet there is no noticeable aggregation of large particles. These findings indicate that the Ce and Ni nanoparticles (NPs) are highly dispersed. In Figure 3a,d, at 300 °C, the MOF–808 structural skeleton remains largely intact, exhibiting an overall irregular rod-like morphology with a few regular prismatic features. Conversely, Figure 3b,e illustrate that at 400 °C, the Ni–CeO_2(3:7) @C–400 °C structure has completely transformed into derivatives, with no regular prismatic structures present, and a more uniform particle size is observed. Figure 3c,f depict the morphology at 500 °C, where numerous derivatives and evidence of carbonization are apparent. The low magnification transmission electron microscopy (TEM) image in Figure 3g further confirms that the surface of the Ni–CeO_2(3:7) @C–400 °C derivatives does not exhibit significant agglomeration. Figure 3g reveals that the derivatives comprise particles with a crystal face spacing of 0.32 nm, which is intermediate between the CeO₂ grain size of 0.31 nm (111) (JCPDS 34-0394) [18,26] and the Ni grain size of 0.203 nm (111) (JCPDS 04-0850) [27,28].

This is attributed to the smaller lattice of the Ni nanoparticles compared to that of CeO₂, leading to a modification in the crystal plane spacing due to the formation of the Ce/Ni bimetallic crystalline phase. Additionally, the high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) image with elemental mapping of Ni–CeO_2(3:7) @C–400 °C is presented in Figure 3g. The C, O, and Ce elements are distributed uniformly throughout the backbone, whereas the Ce element is less concentrated on the surface of the derivatives. In contrast, the Ni element is primarily located on the surface, appearing more prominent due to its complete reduction to Ni monomers during the hydrogen reduction process, which enhances the contact area. The reduction at a temperature of 400 °C, when the structure has fully collapsed, aligns with the thermogravimetric (TG) results. This indicates that hydrogen reduction at different temperatures affects the distribution and content of Ni elements, predominantly concentrating them on the surface of the derivatives. The exposure of Ni’s active sites is beneficial for the hydrogenation reaction of dicyclopentadiene (DCPD), significantly influencing the DCPD conversion and 4H-selectivity. Moreover, the successful synthesis of the samples was corroborated by X-ray diffraction (XRD), Fourier-transform infrared (FT–IR) spectroscopy, and scanning electron microscopy (SEM) analyses, all of which confirmed the desired structural and compositional characteristics.

2.2. Catalytic Hydrogenation Properties of Ni–CeO_2(x:y) @C–T

Dicyclopentadiene (DCPD) is a diolefin characterized by a double bond, featuring both a norbornene ring (NB-bond) and a cyclopentene ring (CP-bond). Due to the differing hydrogenation activation energies for these two bonds, the hydrogenation reaction proceeds preferentially. The NB-bond is hydrogenated first to yield dihydro product (DHDCPD), followed by the hydrogenation of the CP-bond to produce the final tetrahydro product (THDCPD), as illustrated in Figure 4a [11,21]. THDCPD serves as a crucial intermediate in the synthesis of various fine chemicals, particularly in the development of high-energy-density fuels [11,29,30]. The activation energies required for the hydrogenation of norbornene and cyclopentene are 22.8 kJ·mol⁻¹ and 40.9 kJ·mol⁻¹, respectively, indicating that cyclopentene poses a greater challenge for activation [24]. However, DCPD tends to depolymerize into monoolefins at elevated temperatures, potentially impacting the selectivity of THDCPD (Sel.THDCPD). Therefore, the hydrogenation of DCPD to THDCPD was employed as a probe reaction to evaluate the catalytic performance of the synthesized samples.

In the context of the Ni–CeO_2(x:y) @C–T, optimizing the metal ratio is essential for maximizing the activation capabilities of Ni. We conducted DCPD hydrogenation experiments to assess the hydrogenation properties of the Ni–CeO_2(x:y) @C–T [31] derivatives at various temperatures, aiming to achieve optimal DCPD selectivity. Based on thermogravimetric analysis (TGA) results indicating structural collapse at 400 °C, we selected different Ce/Ni ratios at this temperature. Given that Ni⁰ is the active species, our focus was on the synergistic effect between Ni and Ce on DCPD hydrogenation. As shown in Figure 4b, varying the ratios of x and y in Ce_xNi_y revealed that at ratios of 1:9 and 2:8, the DCPD conversion rate increased but did not reach 100%. The selectivity for DHDCPD decreased while the selectivity for THDCPD increased, yet neither reached 100%. Notably, at the ratio of 3:7, both DCPD conversion and THDCPD selectivity achieved 100%, marking a successful target outcome. In contrast, ratios of 4:6, 1:1, 6:4, and 7:3 failed to meet the target requirements. This may be attributed to insufficient mesoporous coordination to fully support the reduction of Ni in an H₂ atmosphere, leading to potential agglomeration and a subsequent decrease in active sites for DCPD.

Figure 4b indicates that the pristine bimetallic MOF, without H₂ reduction, exhibited low DCPD conversion under stringent conditions (100 °C, 2 MPa, and 2 h), yielding only the unsaturated hydrogenation product DHDCPD. Conversely, for Ni–CeO_2(x:y) @C–T, the hydrogenation performance at varying temperatures (300 °C, 400 °C, and 500 °C) with a Ce:Ni ratio of 3:7 is depicted in Figure 4c. At 300 °C, DCPD conversion was 11.15%, yielding 85.49% DHDCPD and 12.54% THDCPD. In stark contrast, at 400 °C and 500 °C, DCPD conversion reached 100%, with THDCPD produced exclusively at 100% selectivity.

As shown in Figure 5a, all three catalysts synthesized at 400 °C demonstrated a Dicyclopentadiene (DCPD) conversion rate of 100%. However, only Ni–CeO_2(3:7) @C–400 °C catalyst achieved 100% selectivity for tetrahydro DCPD (THDCPD), while the other two catalysts exhibited selectivities of 65.42% and 89.92%, respectively.

Table 1. Comparison of hydrogenation performance of different nickel-based catalytic materials.

Catalytic Materials	Reaction Conditions (Temperature/H2 Pressure/Time)	DCPD Conversion Rate (%)	DHDCPD Selectivity (%)	THDCPD Selectivity (%)	References
Ni–CeO2(3:7) @C–400 °C	100 °C/2 MPa/2 h	100	0	>99.9	This work
Raney Ni	120 °C/−/−	96	0	96	[2]
Ni–53–500	60 °C/3 MPa/2 h	100	0	>99.9	[7]
MET–1000	120 °C/3 MPa/−	100	<1	>99	[11]
Ni@C/g–C3N4	20 °C/1 MPa/3 h	>99	1.1	98.9	[29]

presents a summary of the catalytic performance of various nickel-based catalysts for the hydrogenation of Dicyclopentadiene (DCPD). Nickel-based hydrogenation catalysts share certain common deactivation mechanisms, primarily arising from the impairment of active sites or interference from the reaction environment. During the reaction, particularly under high-temperature conditions, nickel nanoparticles tend to migrate and sinter, leading to increased particle size, reduced specific surface area, and a decrease in the number of active sites. Carbon deposition can cover active sites, block the pores of the support, and impede the contact between reactants and active centers. When active sites on the catalyst surface are strongly adsorbed by impurities in the reaction system, these impurities can irreversibly occupy the active sites, significantly reducing H₂ dissociation efficiency and reactant adsorption capacity. Supports (e.g., carbon, CeO₂, MOF-derived materials, etc.) may undergo structural damage (e.g., pore collapse, crystal phase transformation) under high-temperature, high-pressure, or strongly polar environments. This can lead to the loss of support functionality, agglomeration of nickel nanoparticles, or hindered reactant diffusion. These mechanisms may induce deactivation either individually or synergistically, with agglomeration and carbon deposition being the most prevalent causes for nickel-based hydrogenation catalysts. The carbon-coated structure of Ni–CeO_2(7:3) @C–400 °C catalyst effectively inhibited the sintering and loss of Ni nanoparticles, which solved the problem of easy deactivation of traditional nickel-based catalysts. Moreover, the electron interaction between Ce and Ni forms an electron-rich Ni⁰ active center, which cooperates with the diffusion-promoting effect of porous carbon, resulting in the formation of an electron-rich Ni⁰ active center, compared with single metal catalyst or noble metal catalyst, it has more efficient H₂ dissociation-activation ability. Compared with Raney–Ni (120 °C) and ZnNi MOFs-derived catalysts (3 MPa), 100% conversion and selectivity can be achieved at 100 °C and 2 MPa with low energy consumption.

To further investigate the catalytic properties, H₂ temperature-programmed desorption (H₂–TPD) measurements were performed to evaluate the binding energy and adsorption state of hydrogen in the Ni–CeO_2(x:y) @C–400 °C catalyst. As shown in Figure 5b, the H₂–TPD spectrum of Ni–CeO_2(1:9) @C–400 °C shows a peak of physisorption at 81 °C, which is lower than that of Ni–CeO_2(3:7) @C–400 °C and Ni–CeO_2(7:3) @C–400 °C (87 °C and 86 °C, respectively), suggesting that H₂ forms molecular states on the metal surface by physisorption. The larger peak area of Ni–CeO_2(3:7) @C–400 °C indicates that the higher the total number of active sites on the surface where H₂ can be adsorbed, the higher the hydrogenation capacity. While the chemisorption peaks of sample Ni–CeO_2(3:7) @C–400 °C and sample CeO_2(7:3) @C–400 °C appeared at 293 °C and 288 °C, respectively, at which time stronger bonding energy is required for the dissociation and drug use of H₂ in the Ni active sites to form Ni–H bonds. Comparison of the two peaks reveals that Ni–CeO_2(3:7) @C–400 °C is shifted to the high temperature region compared to Ni–CeO_2(7:3) @C–400 °C, which may be due to the introduction of Ce optimizing the electronic structure of Ni through the electronic effect to form a more stable adsorption site. Figure S1a,b present scanning electron microscopy (SEM) images of the DCPD derivatives after hydrogenation.

The morphology of the Ce:Ni ratio of 1:9 showed minimal changes before and after hydrogenation, maintaining overall structural integrity. In contrast, the 3:7 ratio exhibited more pronounced morphological changes, transitioning from a needle and rod cluster structure to a predominantly rectangular columnar structure, with a more regular arrangement and no visible cracks. The 7:3 ratio displayed a morphology similar to that of the 3:7 ratio but featured additional cracks and signs of structural degradation. Comparing the three sample groups, the 3:7 Ce:Ni ratio demonstrated superior structural integrity, indicating a stronger Ce–Ni bond. Under high-pressure hydrogenation conditions (100 °C, 2 MPa), this ratio achieved a complete structure, with both the hydrogenation conversion rate and 4H selectivity reaching 100%, consistent with the H₂–TPD results. This improvement can be attributed to the formation of Ce–Ni bonds, which enhance the hydrogen dissociation and activation of Ce and Ni in Ni–CeO_2(x:y) @C–T, preventing excessive H-coverage of the Ni⁰ active site. The resolution peaks in the MOFs around 288 °C, in conjunction with thermogravimetric analyses, suggest that the observed adsorption is linked to the loss of DMF molecules and hydroxyl groups within the MOF structure. In summary, the results indicate that the mesoporous structure of Ce–MOF–808, combined with the synergy of Ni nanoparticles, enhances substrate accessibility and the reduction performance of the carrier.

Further analysis of the X-ray photoelectron spectroscopy (XPS) spectra at the three ratios reveals the presence of both cerium (Ce) and nickel (Ni) in the bimetallic catalysts, as illustrated in Figure S2a. The high-resolution Ce 3d spectra obtained in Figure 5c show peaks at 884.09 eV (v′), 889.37 eV (v‴), 899.46 eV (u′), and 908.35 eV (u‴), which are attributed to the Ce⁴⁺ oxidation state. In contrast, the peaks at 885.29 eV (v″), and 903.71 eV (u‴) correspond to Ce³⁺, indicating that Ce³⁺ plays a predominant role in the hydrogenation of dicyclopentadiene (DCPD) under a reducing hydrogen atmosphere.

The differences in the ratios of Ce³⁺ and Ce⁴⁺ are minimal, which can be attributed to the catalyst reduction occurring through high-temperature calcination in a standard gas environment. Theoretically, a higher content of Ce⁴⁺ is obtained due to the electron loss of Ce³⁺. As shown in the Ni 2p spectrum in Figure 5d, Ni serves as the main active site in the Ni–CeO_2(x:y) @C–400 °C catalyst, which demonstrates excellent DCPD hydrogenation performance. The XPS spectra of the Ni 2p orbitals in Ni–CeO_2(x:y) @C–400 °C exhibit three primary peaks at 853.16 eV, 855.47 eV, and 859.93 eV, corresponding to Ni⁰, Ni²⁺, and Ni satellite states, respectively. The reduction of the Ni–CeO_2(1:9) @C–400 °C catalyst resulted in a greater proportion of Ni⁰ compared to the other two catalysts. Conversely, the Ni–CeO_2(7:3) @C–400 °C catalyst displays a lower concentration of active Ni⁰, accompanied by a higher proportion of Ni²⁺. The synergistic effects of these two valence states correlate with the DCPD hydrogenation performance. Notably, the Ni⁰ active species in the Ni–CeO_2(3:7) @C–400 °C catalyst are most evident and stable, as indicated by the prominent peaks at 853.16 eV and 859.93 eV, confirming their critical role in enhancing catalytic activity.

The effects of various reduction temperatures and the adjustment of the coordination numbers of Ce and Ni on the catalytic performance for DCPD hydrogenation were systematically investigated. The de-coordination behavior of Ce₃Ni₇@MOF-808 was initially characterized through thermogravimetric (TG) analysis. As shown in Figure 2c, the ordered mesoporous structure (OMS) achieved with a Ce to Ni ratio of 3:7 is predominantly influenced by temperature regulation. However, the MOF-808 framework exhibited increased defects at varying temperatures. Although the differences in active elemental content between Ni–CeO_2(3:7) @C–400 °C and the analogous Ni–CeO_2(2:8) @C–400 °C and Ni–CeO_2(4:6) @C–400 °C are minimal, the extent of defects in the OMS at different temperatures significantly impacts both the conversion and selectivity of DCPD [31]. This is attributed to the interplay between the degree of defects in the OMS and the availability of the active Ni monomers, which are generated through reduction at the corresponding temperatures. Additionally, the acidic sites within the synthetic ligand 1,3,5-benzenetricarboxylic acid (BTC) contribute to the catalytic environment, encompassing weak, moderate, and strong acidic sites, as previously reported by Ezugwu et al. [32]. The ammonia (NH₃) adsorption peaks corresponding to weak acidic sites are associated with the physical adsorption of NH₃ on the MOF surface or within its pores. Notably, the peaks for acidic sites in the Ni–CeO_2(3:7) @C–400 °C backbone are not conducive to NH₃ adsorption, leading to a significant decrease in adsorption capacity [33]. Conversely, the medium to strong acidity of Ni–CeO_2(3:7) @C–400 °C arises from the hydroxyl-exposed Ce⁴⁺/Ce³⁺ in Ce₆O₄(OH)₄ clusters [34,35].

The valence states and interfacial charge transfer of chemical elements on the catalyst surface were investigated using X-ray photoelectron spectroscopy (XPS) analysis. The XPS spectra, as presented in Figure S3a, reveal the presence of Ce, Ni, O, and C signals. Notably, in Figure S3b, it is observed that as the temperature increases, the backbone bond of MOF–808 at 288 eV in the C 1s spectrum gradually diminishes. This aligns with previous analyses indicating that the MOF structure nearly collapses completely at 500 °C. At 400 °C, the backbone remains partially intact, and the synergy with the generated CeO₂ significantly enhances the reduction of Ni to Ni⁰, generating a substantial number of active centers, consistent with the observed DCPD hydrogenation results.

The signal corresponding to elemental Ce in Ni–CeO_2(3:7) @C–T (at 300 °C, 400 °C, and 500 °C) is attributed to CeO₂ formed during the synthesis of the derivatives [36]. In Figure 6a the high-resolution spectra of Ce 3d for Ni–CeO_2(3:7) @C–400 °C, peaks at 883.14 eV (v′), 889.52 eV (v‴), 899.21 eV (u′), and 908.15 eV (w′) are associated with Ce⁴⁺, while peaks at 885.11 eV (v″) and 903.83 eV (u‴) correspond to Ce³⁺. Here, Ce³⁺ plays a significant role, and both elemental Ce and Ni positively influence the hydrogenation of DCPD in a reducing H₂ atmosphere [37,38]. For Ni–CeO_2(3:7) @C–500 °C, the high-resolution spectrum shows peaks at 883.14 eV (v′), 889.52 eV (v‴), 901.69 eV (u‴), 908.15 eV (w′), and 917.58 eV (w″), indicating a higher proportion of Ce⁴⁺, which is mainly attributed to the supply of electrons by Ce³⁺ and the transformation of Ni²⁺ into Ni⁰, resulting in a more reactive metal center and enhanced hydrogenation activity.

In the Ni 2p spectrum in Figure 6b, three main peaks at 853.05 eV, 855.19 eV, and 858.88 eV correspond to Ni⁰, Ni²⁺, and Ni satellite peaks, respectively. The lower content of Ni observed may result from the oxidation of Ni⁰ on the surface of Ni-CeO_2(3:7) @C–T particles to Ni²⁺ in air. The binding energies of Ce⁴⁺, Ce³⁺, Ni⁰, and Ni²⁺ are all shifted towards higher binding energy [39,40], primarily due to the collapse of the MOF structure and the partial breakage of chemical bonds at elevated temperatures.

Quantification of metal elements in the XPS spectra was performed by analyzing the area ratios of the Ce⁴⁺ and Ce³⁺ peaks, as well as the Ni⁰ and Ni²⁺ peaks. As shown in Figure 5d, with increasing temperatures, a significant reduction of Ni²⁺ to Ni⁰ is noted at 400 °C, indicating that Ni⁰ plays a dominant role in DCPD hydrogenation, corroborating previous findings. The area ratios of Ce⁴⁺ and Ce³⁺ suggest a trend opposite to the degradation of the BTC ligand linkage, where defects arising from BTC linkage removal and the microenvironmental regulation of Ni by Ce synergistically enhance the electron density at the Ni active site on the Ce surface.

Furthermore, the introduction of an optimal ratio of Ce and Ni influences the exposure of Ni and its electron density. From the thermogravimetric (TG) test results, it was determined that the skeleton completely collapsed at 500 °C, with substantial loss of the BTC ligand. The peak area ratio of Ce⁴⁺/Ce³⁺ at 400 °C showed minimal variation compared to that at 300 °C, and the characteristic peak of Ce⁴⁺ at 917.58 eV was not observed (w″). This indicates that the synergistic interaction between bimetallic Ce and Ni leads to Ce³⁺ being the dominant valence at 300 °C and 400 °C, while Ce⁴⁺ prevails at 500 °C. Overall, the characteristic peaks for both Ce⁴⁺ and Ce³⁺ shift towards higher binding energies as the temperature increases [41,42], suggesting that the derivatives formed by Ni–CeO_2(3:7) @C–400 °C exhibit optimal DCPD hydrogenation performance, characterized by a rich presence of Ni⁰.

Based on the relevant literature and the above experimental results, we propose a plausible reaction mechanism for DCPD hydrogenation on Ni–CeO_2(3:7) @C–400 °C. As illustrated in Figure 7, the mesoporous structure of porous carbon facilitates the diffusion of DCPD molecules to the catalyst surface. This enhances contact with highly dispersed Ni nanoparticles (NPs), thereby reducing the distance between them and increasing the overall surface area of the catalyst. Consequently, this addresses the issue of limited substrate diffusion commonly encountered in traditional catalysts. Additionally, CeO₂ transfers electrons to Ni through the Ce–O–Ni interface, which elevates the electron density of Ni⁰ and improves its adsorption activation capacity for H₂ and DCPD. Acting as an active center in the catalytic reaction, Ni⁰ dissociates H₂ into reactive hydrogen species (H*) due to electron transfer from Ce in an electron-rich state. The NB bond of DCPD preferentially reacts with H* to form DHDCPD, followed by further hydrogenation of the CP bond leading to THDCPD formation. This mechanism aligns with findings from XPS analysis (Figure 5c,d), confirming the pivotal role played by bimetallic synergy alongside mass transfer facilitated by the carbon carrier.

Based on the results obtained, the cycling performance of the Ni–CeO_2(3:7) @C–400 °C catalyst was evaluated. After each hydrogenation cycle, the catalyst substrate was centrifuged, dried, and then reintroduced to the same amount of DCPD and methanol solvent. The reaction was conducted under conditions of 100 °C, 2 MPa, and for 2 h. The performance results are presented in Figure 8a, which shows that the conversion and selectivity for THDCPD remained at 100% for the first three cycles. In the fourth and fifth cycles, while the conversion of THDCPD slightly decreased to approximately 97%, the overall conversion continued to be 100%. XRD characterization (Figure 8b) indicated that there were no significant changes in the characteristic peaks of CeO₂ and Ni in the catalyst before and after the reaction; there was neither broadening nor weakening of the Ni diffraction peak, with only a minor presence of miscellaneous peaks introduced by DCPD adsorption. These results demonstrate that both the dispersion state of the entire framework and that of Ni nanoparticles remain stable, with no evident agglomeration observed. Furthermore, the X-ray photoelectron spectroscopy (XPS) spectrum of the post-hydrogenation sample is depicted in Figure S4a, confirming the presence of Ce, Ni, C, and O, which aligns with the previously measured spectrum. The Ce 3d spectrum in Figure 8c demonstrates an increase in the content of Ce³⁺ and a decrease in Ce⁴⁺ after hydrogenation. In Figure 8d, the Ni 2p spectra show a higher Ni⁰ ratio post-hydrogenation compared to the pre-hydrogenation state. The enhanced Ce³⁺/Ni⁰ ratio following hydrogenation can be attributed to three key factors: direct reduction by H₂, the Ce-Ni electron synergistic effect, and the optimal matching between catalyst structure and reaction conditions. Specifically, molecular hydrogen (H₂) serves as a reducing agent to provide electrons, facilitating electron transfer between Ce and Ni that promotes their respective reduction processes. Meanwhile, the carbon carrier material, in combination with appropriate temperature and pressure conditions, ensures the efficiency of the reduction reaction. This structural and compositional change enhances the catalyst’s electron transfer capability, increases the concentration of main active sites, and ultimately improves both the hydrogenation efficiency and cyclic stability of the catalyst for DCPD hydrogenation.

3. Materials and Methods

3.1. Materials and Reagents

Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, ≥98.0%(Sinopharm Chemical Reagent, Shanghai, China), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), N,N-dimethylformamide (DMF, 99.5%, Macklin, Shanghai, China), 1,3,5-trimesic acid (BTC, 99%, Bailiwick, Chaska, MN, USA), ethanol, methanol (99.9%, Tongguang, Nantong, China), DCPD (≥96%, endo- and exo-DCPD, Aladdin, Shanghai, China), hydrogen (H₂ 5%, Ar 95%, Huan Yu Jing Hui Co., Beijing, China), and deionised water (18.2 MΩ·cm⁻¹, made by Hitech Laboratory Purification and Hydration System, Shanghai, China). All of the above chemicals were purified by secondary purification.

3.2. Synthesis of Ce_xNi_y–MOF–808

In the synthesis process, 0.3 mmol of cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) and 0.7 mmol of nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) were firstly taken and dissolved in 18 mL of N,N-dimethylformamide (DMF). Meanwhile, in another vessel, 2 mmol of 1,3,5-benzenetricarboxylic acid (BTC) was added to 18 mL of ethanol. A DMF solution containing cerium and nickel sources was mixed with an ethanol solution containing BTC to obtain a green transparent solution. This mixed solution needs to be sonicated again for 10 min. Before transferring to the 50 mL autoclave again, it was ensured that all the reactants were homogeneously mixed. The mixed solution was heated to 120 °C at a rate of 5 °C·min⁻¹ and held for 12 h to promote the reaction between the metal ions and organic ligands.

Upon completion of the reaction, the autoclave is allowed to cool down naturally to room temperature, where unreacted solvent and by-products may be present at the liquid-phase interface. The solid reaction product was removed and washed twice with DMF to remove the residual unreacted material, and then washed once with ethanol to further purify the product. Finally, the washed reaction product was dried in a vacuum oven at 60 °C for 10 h to ensure removal of all residual solvents to obtain the Ce_xNi_y–MOF–808 (x = 3, y = 7) product. After synthesis, the samples were placed in a vacuum desiccator to prevent oxidation of the samples.

To investigate the influence of the Ce/Ni ratio on catalytic performance, different ratios of metals were adjusted to synthesize bimetallic MOF–808 materials with varying metal proportions. Ce_xNi_y–MOF–808 (x = 1, y = 9; x = 2, y = 8; x = 4, y = 6; x = 1, y = 1; x = 6, y = 4; x = 7, y = 3) products prepared using the same synthesis method as that for Ce_xNi_y–MOF–808 (x = 3, y = 7) except for adjusting the feeding ratios of Ce(NO₃)₃·6H₂Oand Ni(NO₃)₂·6H₂O.

3.3. Synthesis of the Derivative Ni–CeO_2(x:y) @C–T

The synthesised Ce_xNi_y–MOF–808 samples were placed in a tube furnace and firstly hydrogen (H₂) was introduced to allow a steady flow at a constant airflow rate for 30 min to ensure a homogeneous reaction environment. Subsequently, the samples were subjected to calcination reduction at high temperatures of 300 °C, 400 °C, and 500 °C, respectively, with the reaction time set to 3 h. Through this process, the samples underwent reduction transformation by removing the organic components and metal oxides, and the derivative materials containing Ni–CeO_2(x:y) @C–T with different colour characteristics were finally obtained, which were earthy yellow, dark grey and black. After successfully making the derivatives, the samples were placed in a vacuum drying oven to prevent oxidation of the samples.

Ni–CeO_2(x:y) @C–T (x = 1, y = 9; x = 2, y = 8; x = 4, y = 6; x = 1, y = 1; x = 6, y = 4; x = 7, y = 3) products prepared using the same synthesis method as that for Ni–CeO_2(x:y) @C–T except for using the corresponding Ce_xNi_y–MOF–808.

3.4. Catalyst Characterization

X-ray diffraction measurements were carried out using a Bruker D8-Advance XRD diffractometer (Bruker, Ettlingen, Germany) with Cu Kα radiation in the 2θ range of 5–60° at a rate of 10°·min⁻¹; the sample morphology was observed using a transmission electron microscope (TEM) with a JEM-22200FS (JEOL Ltd., Tokyo, Japan) and an SEM with a SU8010 (Hitachi, Sapporo-shi, Japan); the surface condition of the samples was analysed using a Thermo Scientific K-Alpha XPS spectrometer (Waltham, MA, USA) to analyse the surface state of the samples; FT-IR Fourier transform infrared spectroscopy on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA); thermogravimetric (TG) analysis on an STA449F3 spectrometer (NETZSCH, Selb, Germany) in a nitrogen atmosphere in the temperature range of 40–800 °C at a temperature increase rate of 10 °C·min⁻¹ test; nitrogen (N₂) adsorption and desorption isotherms were measured on a Quantachrome Autosorb-iQ instrument (Boynton Beach, FL, USA) at a temperature of −196 °C, and the data were further processed to analyse the pore characteristics and the specific surface area of the samples by the density-functional theory (DFT) and Brunauer–Emmet–Teller (BET) methods. The hydrogen temperature-controlled analysis experiments were carried out on a Micromeritics Auto Chem II chemisorption analyser (Norcross, GA, USA). The hydrogen–temperature controlled digestion (H₂–TPD experiments were carried out on a Micromeritics Auto Chem II chemisorption analyser. The composition of the DCPD hydrogenation reaction products was analysed by GC–MS, Agilent 7890/5975C–GC/MSD meteorological chromatography-mass spectrometry (Santa Clara, CA, USA) and flame ion detector.

3.5. DCPD Hydrogenation Catalytic Process

DCPD catalytic hydrogenation reaction experiments were carried out in a sealed high pressure reactor. The reaction solvents were optimised from ethanol, methanol and cyclohexane and screened with methanol. In a typical reaction, DCPD (200 μL), methanol (5 mL) and catalyst (20 mg) were added to the reactor. Subsequently, the entire reactor was purged 2 times at room temperature to remove the internal air and kept sealed for 10 min to ensure its hermeticity. Subsequently, separate N₂ and H₂ purges were performed in sequence to ensure a pure H₂ atmosphere inside the reactor [3]. After purging the reactor vessel with H₂ to 2 MPa, the stirring speed was set to 600 rpm to exclude the effect of internal mass transfer. The temperature of the reactor was raised to a reaction temperature of 100 °C, and the reactor was set to react for a certain time before being lowered to room temperature. The products were separated by centrifugation and the upper clear liquid was diluted with methanol and analysed by GC–MS. DCPD conversion and product selectivity could be calculated from the equation.

4. Conclusions

In summary, the Ce_xNi_y–MOF–808 catalyst material was synthesized by adjusting the ratio of the two metals, Ce and Ni, using a traditional impregnation method. The synthesized catalyst underwent H₂ reduction treatment at various temperatures to yield the Ni–CeO_2(x:y) @C–T catalyst derivatives. Comparisons across different ratios and temperatures revealed that the Ni–CeO_2(3:7) @C–400 °C exhibited the best performance, maintaining structural stability before and after hydrogenation. The Ni–CeO_2(3:7) @C–400 °C catalyst demonstrated excellent catalytic activity for the hydrogenation of dicyclopentadiene (DCPD), achieving a DCPD conversion of 100% and THDCPD selectivity of approximately 100% under specific conditions (100 °C, 2 MPa) over a short reaction time of 2 h. The construction of the bimetallic Ce–Ni framework within the MOF structure results in high stability and performance of the cerium-nickel nanoparticles during DCPD hydrogenation. Experimental and analytical results indicate that the presence of Ce enhances the availability of active Ni sites, which significantly improves the adsorption and activation of H–H and C=C bonds, crucial steps for accelerating the hydrogenation process of DCPD.