Cu₁Ni₂/Al₂O₃ Catalyst from Its Hydrotalcite Precusor with Highly Active Sites for Efficient Hydrogenation of Levulinic Acid Toward 2-Methyltetrahydrofuran

Abstract

2-Methyltetrahydrofuran (2-MTHF), a hydrogenated derivative of levulinic acid (LA), is a biomass-derived platform compound with diverse and significant applications as a biofuel, gasoline additive, green solvent, and pharmaceutical synthesis intermediate. This study investigates the preparation of a Cu₁Ni₂/Al₂O₃ catalyst through the calcination–reduction of CuNiAl hydrotalcite as a precursor, which was subsequently utilized in the hydrogenation of LA to produce 2-MTHF. The calcination–reduction process applied to CuNiAl hydrotalcite results in a lattice confinement effect. This method not only disperses the active metal sites but also alters the bonding patterns of the active metals, thereby enhancing the activity and stability of the Cu₁Ni₂/Al₂O₃ catalyst. The results indicate that complete conversion of LA and a 2-MTHF yield of 87.6% can be achieved under optimal conditions of 190 °C, 5 MPa hydrogen, and a reaction time of 5 h, demonstrating an efficient one-step conversion process. Additionally, the catalyst’s recyclability was assessed through multiple reuse tests, with a loss of activity of only 9.2% after six cycle experiments, suggesting its feasibility and reliability for industrial applications.

1. Introduction

2-Methyltetrahydrofuran (2-MTHF), a biomass-derived platform compound from the hydrogenation of levulinic acid (LA), is widely acknowledged as a high-value bio-based fuel and gasoline additive owing to its hydrophobicity and full miscibility with gasoline across all proportions [1,2]. Catalysts play a pivotal role in governing the efficiency and selectivity of 2-MTHF production [3,4,5,6]. Initially, noble metal catalysts [7,8,9,10,11,12], such as iridium (Ir) [13], palladium (Pd) [14], platinum (Pt) [15], and ruthenium (Ru) [16], were predominantly employed. Although these catalysts exhibit remarkable catalytic activity and selectivity, their high costs limit their applicability in large-scale production. As a result, researchers have increasingly focused on more cost-effective non-noble metal catalysts, including those based on nickel (Ni), copper (Cu), and cobalt (Co) [17,18]. Cu-based and Ni-based catalysts are widely used for the selective hydrogenation of C=C, C≡C, and C–O bonds [19,20,21]. These catalysts have demonstrated excellent performance under various conditions, including high conversion rates, yields, and recyclability. Notably, Ni- and Cu-based catalysts have been shown to efficiently catalyze the conversion of LA to 2-MTHF with formic acid or isopropanol applied as the hydrogen sources [22], rivaling the activity and selectivity of noble metal counterparts.

Ni catalysts show exceptional C=O hydrogenation activity but suffer from intermediate over-hydrogenation, leading to pentanol formation. In contrast, Cu catalysts achieve selective C–O bond activation, yet their kinetically constrained H₂ dissociation substantially impedes the reaction progression [23]. In the investigation of LA hydrogenation for 2-MTHF production, CuNi bimetallic catalysts have demonstrated exceptional catalytic activity and stability [24,25]. For instance, Obregon et al. conducted systematic investigations into the factors influencing the hydrogenolysis of LA. They evaluated a series of Ni-based, Cu-based, and bimetallic Ni-Cu catalysts in green solvents for the hydrogenative conversion of LA to 2-MTHF. Among these, the Ni/Al₂O₃ catalyst exhibited the highest catalytic activity, whereas the Cu/Al₂O₃ counterpart demonstrated superior selectivity toward the target product. Notably, the bimetallic Ni-Cu/Al₂O₃ catalyst demonstrated a synergistic effect, outperforming its monometallic analogs in both activity and selectivity [26]. Xie et al. synthesized a mesoporous Al₂O₃-ZrO₂ support via the sol–gel method and subsequently fabricated Cu-Ni/Al₂O₃-ZrO₂ catalysts through impregnation. Remarkably, these catalysts achieved a 2-MTHF selectivity of up to 99.8% under complete LA conversion while maintaining negligible degradation in both activity and selectivity after five consecutive reaction cycles [27]. Huang et al. reported that using n-hexane as the reaction medium and 10Cu-5Ni/Al₂O₃ as the catalyst, complete conversion of LA was achieved under conditions of 180 °C and 4 MPa H₂, with a selectivity of 98% toward 2-MTHF [23].

Among research efforts, hydrotalcite-supported non-noble catalysts have increasingly attracted attention due to their tunable active sites, high dispersibility, and exceptional stability [28,29,30]. Specifically, hydrotalcite-supported catalysts can feature both acidic and basic active centers [31,32], enabling them to facilitate both the dehydrogenation of alcohols and the hydrogenation of C=O bonds in reactions such as the transfer hydrogenation of LA. Moreover, utilizing hydrotalcite as a precursor for the preparation of supported metal catalysts offers significant advantages over traditional methods such as coprecipitation and impregnation [33,34]. The low lattice energy effect exhibited by hydrotalcite laminates allows for uniform distribution of metal ions within the interlayer spaces [35,36]. This homogeneous distribution is subsequently converted into highly dispersed metal catalysts through a reduction step, resulting in high dispersibility and thermal stability, as well as uniform particle size. Additionally, the strong interaction between the metal and the support effectively mitigates the sintering and aggregation of metal particles at elevated temperatures, thereby maintaining both the high activity and stability of the catalyst [37,38].

Reports on the use of Cu and Ni in bimetallic form in hydrotalcite catalysts are still relatively limited. Bimetallic catalysts are known to leverage the complementary effects of two metals, enhancing both catalytic efficiency and selectivity [39]. Therefore, CuNi bimetallic hydrotalcite catalysts, with their potential synergistic effects and promising catalytic performance, hold significant research value and potential [32].

In this study, the Cu₁Ni₂/Al₂O₃ catalyst was synthesized via a calcination–reduction method using CuNiAl hydrotalcite as the precursor. The catalyst was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis, and temperature-programmed desorption (TPD) techniques to investigate the correlation between its structural characteristics and catalytic performance. Additionally, the impacts of reaction time, temperature, and hydrogen pressure on the catalytic activity were examined. The stability of the Cu₁Ni₂/Al₂O₃ catalyst was evaluated through multiple reuse cycles in the one-step hydrogenation of LA to produce 2-MTHF.

2. Results

2.1. Characterization of CuNiAl Hydrotalcites

2.1.1. CuNiAl Hydrotalcites XRD Analysis

The XRD patterns of the hydrotalcite-like compounds synthesized according to

Table 1. Types and amounts of precursor salts in the preparation of CuNiAl hydrotalcite.

Hydrotalcite	Cu:Ni:Al	MCu (mmol)	MNi (mmol)	MAl (mmol)
CuNiAl	5:3:2	50	30	20
CuNiAl	3:3:2	30	30	20
CuNiAl	1:3:2	10	30	20
CuNiAl	1:2:1	10	20	10

are shown in Figure 1, revealing that the ion ratio conditions significantly influenced the crystal structure of the resulting CuNiAl compounds. In the figure, distinct and intense diffraction peaks are observed at 11.8°, 23.5°, 60.8°, and 61.4°, corresponding to the (003), (006), (110), and (113) crystal planes, respectively, indicating the successful formation of layered double hydroxides (LDHs) with a well-defined layered structure [40]. The sharp and narrow nature of these peaks suggests that the synthesized LDHs possess a highly ordered crystal structure, single-phase crystalline form, and good structural integrity. Due to the substitution of Mg²⁺ in the hydrotalcite structure by Cu²⁺ and Ni²⁺, characteristic peaks corresponding to the (012), (015), and (018) crystal planes are observed at 34.9°, 39.1°, and 47.2°, respectively, which are attributed to Cu and Ni.

When the Cu:Ni:Al ratio is 5:3:2, the relatively high concentration of Cu²⁺ compared to Al³⁺ and Ni²⁺ induces a pronounced Jahn–Teller effect, leading to distortion of the layers and disruption of the crystal structure, which results in the formation of other Cu-containing phases. At a Cu:Ni:Al ratio of 3:3:2, the Jahn–Teller distortion is alleviated, resulting in hydrotalcite-like compounds with fewer impurities. When the Cu:Ni:Al ratio is 1:3:2, the Jahn–Teller effect is significantly suppressed, yielding a single-phase hydrotalcite. However, the excessive Al³⁺ content leads to reduced crystallinity. Therefore, based on the experimental results, a Cu:Ni:Al ratio of 1:2:1 is identified as the optimal composition for preparing hydrotalcite precursors, as it promotes the formation of LDHs with a highly ordered structure while minimizing impurity formation.

2.1.2. CuNiAl Hydrotalcites FT-IR Analysis

Figure 2 presents the FT-IR spectra of CuNiAl hydrotalcites synthesized under conditions of pH 9–10, a crystallization time of 24 h, and Cu:Ni:Al molar ratios of 1:2:1, 1:3:2, 3:3:2, and 5:3:2. As shown, the absorption peak near 3470 cm⁻¹ corresponds to the stretching vibration of O–H bonds within and between layers, while the distinct peak at 1377 cm⁻¹ arises from the V₃ vibrational mode of interlayer CO₃²⁻ [41]. A weak absorption peak at 1642 cm⁻¹ is attributed to the bending vibration of interlayer water molecules. In the lower wavenumber region, the peak around 780 cm⁻¹ corresponds to the V₂ mode of CO₃²⁻, with other peaks attributed to M–O, M–O–M, and O–M–O vibrations of interlayer cations and oxygen [42]. These peaks collectively confirm the characteristic FT-IR features of a typical LDH structure.

Comparative analysis of the four hydrotalcites reveals consistent peak positions but varying intensities, attributed to compositional changes within the layers caused by differing Cu:Ni:Al ratios. Notably, an increase in copper content enhances the Jahn–Teller effect of Cu²⁺, expanding interlayer spacing and increasing concentrations of CO₃²⁻, O–H, and H₂O concentrations. This phenomenon results in the intensification of the corresponding FT-IR peaks. These results confirm that CuNiAl hydrotalcites were successfully synthesized with different CuNi ratios and retained the expected functional groups.

2.1.3. CuNiAl Hydrotalcites SEM Analysis

The SEM images of CuNiAl hydrotalcite are presented in Figure 3, revealing the differences in the morphology of hydrotalcite at different CuNiAl ratios. Under the condition of a Cu:Ni:Al ratio of 1:2:1, the synthesized hydrotalcite exhibits a distinct hexagonal crystal structure, indicating well-defined crystallinity and high structural order. This hexagonal morphology plays a crucial role in enhancing the specific surface area, thereby improving catalytic performance. When the ratio is adjusted to 1:3:2, the crystallinity decreases, and hexagonal integrity is compromised. This structural degradation may arise from altered metal ion interactions, leading to poor control over crystal morphology during growth.

Furthermore, adjusting the ratio to 3:3:2 further reduces crystallinity while increasing layer thickness. This phenomenon is attributed to the Jahn–Teller effect of Cu²⁺, which induces layer distortion and thickening, ultimately altering crystal morphology and properties. At a 5:3:2 ratio, crystallinity sharply declines with heterogeneous morphology, directly linked to excessive Cu²⁺ concentration that promotes CuO impurity formation. These impurities disrupt the layered structure and introduce angular irregular particles, potentially impairing catalytic activity and selectivity.

In summary, Cu:Ni:Al ratio variations critically govern hydrotalcite crystal structure, morphology, and functionality. Among all ratios tested, the 1:2:1 formulation demonstrates optimal crystallinity and application potential.

2.2. Characterization of Cu₁Ni₂/Al₂O₃ Catalyst

2.2.1. Cu₁Ni₂/Al₂O₃ Catalyst XRD Analysis

The calcination process induces structural transformation and reorganization within the catalyst precursor, generating ordered crystalline phases and porous architectures that strongly influence the catalyst’s specific surface area and pore size distribution. The reduction process, conversely, converts metal oxides in the precursor into metallic species or lower-valence oxides, thereby enriching catalytically active sites. A systematic comparison of XRD patterns at different calcination and reduction temperatures provides deeper insights into temperature-dependent structural modifications and their mechanistic links to catalytic activity. This comparative analysis lays the foundation for optimizing preparation protocols to improve both catalytic performance and operational stability.

Figure 4 illustrates the XRD patterns obtained at various calcination temperatures. As the calcination temperature increases, the mixed diffraction peaks of NiO and CuO become progressively sharper, without distinct characteristic peaks for individual NiO and CuO, indicating the formation of a homogeneous composite oxide through metal–metal interactions. At a calcination temperature of 300 °C, the characteristic peaks corresponding to the hydrotalcite structure—specifically the (003), (006), (110), and (113) planes—disappear, signaling the initial transformation of CuNiAl hydrotalcite into trimetallic oxides. Although mixed diffraction peaks of NiO and CuO are observed at 300 °C, they remain relatively broad, indicating only partial transformation of the hydrotalcite structure. Upon reaching 400 °C, the diffraction peaks become sharper, indicating that higher temperatures promote the conversion of hydrotalcite into mixed metal oxides. At 500 °C, sharp mixed diffraction peaks of NiO and CuO indicate the formation of a substantial amount of mixed metal oxides. However, at 600 °C, the mixed diffraction peaks of NiO and CuO remain largely unchanged, suggesting that most hydrotalcite has been converted into mixed metal oxides by this temperature. Nevertheless, excessively high temperatures result in the formation of spinel-like compounds, such as CuO, NiO, and Al₂O₃, which reduce the specific surface area of the catalyst and consequently diminish its performance. Therefore, the optimal calcination temperature is determined to be 500 °C.

Figure 5 shows the XRD patterns of catalysts prepared at varying reduction temperatures. With increasing reduction temperature, the characteristic diffraction peaks of NiO and CuO gradually weaken and eventually vanish, confirming their reduction to metallic species. Concurrently, the characteristic diffraction peak of the CuNi alloy emerges at 2θ = 43.36°, demonstrating that CuO and NiO are reduced to form the CuNi alloy at elevated temperatures [43,44,45]. With increasing reduction temperatures, the diffraction peaks become sharper and more pronounced, providing direct evidence of grain growth and improved crystal integrity. The observed grain growth is typically correlated with improved catalytic activity and structural stability. Additionally, the full width at half maximum (FWHM) of the diffraction peaks can be used to estimate grain size, as it is inversely proportional to grain size according to Scherrer’s formula; thus, sharper peaks imply larger grains and fewer crystal defects.

Above 400 °C, the variation in diffraction peaks in the XRD pattern diminishes, indicating that most metal oxides have been converted to their metallic states. This transformation is crucial for catalyst activity, as the metallic forms of Cu and Ni typically exhibit higher catalytic activity than their oxide counterparts. Additionally, the formation of alloys often enhances the catalyst’s resistance to poisoning and improves stability in harsh environments.

2.2.2. Cu₁Ni₂/Al₂O₃ Catalyst SEM Analysis

The surface morphology of the catalysts was examined through SEM images to infer their structural properties and potential catalytic performance. The calcination temperature and duration significantly influenced the physical properties of the catalysts, including particle size, pore structure, and the uniform distribution of the active phase. Figure 6a shows the catalyst reduced at 300 °C after 2 h of calcination at 300 °C calcination temperature.

As shown in Figure 6a, the catalyst subjected to a calcination and reduction temperature of 300 °C for 2 h exhibits a compact and coarse surface morphology. This texture may imply a lower specific surface area and limited exposure of active sites, potentially resulting in reduced catalytic activity. In contrast, Figure 6b reveals that the catalyst treated at a calcination and reduction temperature of 400 °C for 4 h displays a more open surface architecture with evident porosity with uniform particle size distribution. Such a configuration is generally associated with enhanced catalytic performance due to the increased availability of active sites and improved transport of reactants and products. Such structural evolution is closely correlated with calcination and reduction conditions [46,47]. Figure 6c demonstrates that extending the treatment duration to 6 h at a calcination and reduction temperature of 500 °C further develops the porous structure of the catalyst surface, leading to a more pronounced porosity, which is conducive to an increased surface area and, consequently, higher catalytic efficiency. Lastly, Figure 6d shows the catalyst treated at 600 °C for 8 h, where the surface porosity begins to deteriorate, and sintering between particles becomes apparent. This sintering phenomenon typically results in a reduction in the effective surface area, thereby compromising the catalyst’s activity.

A comparison of the four samples reveals a structural evolution from dense to moderately porous and eventually to severely sintered as the calcination temperature and time increase. The calcination and reduction temperatures need to be carefully controlled to optimize the pore structure of the catalysts and the degree of exposure of active sites. Calcination and reduction temperatures of 400 °C and 500 °C, combined with moderate calcination durations (4 h and 6 h), are optimal for forming an efficient catalyst structure. In contrast, high-temperature treatment at 600 °C may negatively affect the performance of the catalyst due to the sintering effect.

A comparison of the four samples reveals a structural transition from dense to moderately porous and eventually to severely sintered as calcination temperature and duration increase. The calcination and reduction temperatures need to be carefully controlled to optimize the pore structure of the catalysts and the degree of exposure of the active sites. After a series of studies, calcination and reduction temperatures of 400 °C and 500 °C, combined with moderate calcination times (4 h and 6 h), are optimal for the formation of an efficient catalyst structure. In contrast, high-temperature treatment at 600 °C may deteriorate the performance of the catalyst due to excessive sintering.

2.2.3. Cu₁Ni₂/Al₂O₃ Catalyst BET Analysis

The pore structure and specific surface area of the Cu₁Ni₂/Al₂O₃ catalyst were analyzed using the nitrogen adsorption–desorption isotherm technique. As shown in Figure 7, the BET adsorption isotherm corresponds to a type IV isotherm, indicating that the catalyst mainly consists of mesopores. Initially, mesopores undergo monolayer adsorption at lower relative pressures. As the relative pressure (P/P₀) increases, multilayer adsorption occurs. At higher relative pressures, capillary condensation begins, and hysteresis loops appear on the isotherm. As the relative pressure continues to rise, the hysteresis loop plateaus, suggesting that the system becomes saturated with coalescing liquid and adsorption levels off. Approaching a relative pressure of 1, adsorption in the macropores increases, causing the curve to rise. The analysis of specific surface area and pore volume provides critical insights into the material’s pore structure and its potential for adsorption and catalytic applications.

The BET specific surface area method was employed in the experiments to evaluate the pore structure of the catalysts. Given that calcination temperature significantly impacts catalyst activity, catalysts at various calcination temperatures were selected for analysis. The data presented in

Table 2. BET specific surface area of Cu₁Ni₂/Al₂O₃ catalysts at four different calcination temperatures.

Catalysts	SBET/m2·g−1
Cu1Ni2/Al2O3(300)	127.43
Cu1Ni2/Al2O3(400)	121.53
Cu1Ni2/Al2O3(500)	113.34
Cu1Ni2/Al2O3(600)	98.36

indicate that the specific surface area of the catalyst decreases from 127.43 m²·g⁻¹ to 98.36 m²·g⁻¹ as the calcination temperature increases from 300 °C to 600 °C. This change can be attributed to two primary factors: first, the removal of hydroxyl groups, interlayer water, and carbonates during high-temperature treatment of CuNiAl hydrotalcite leads to a partial collapse of the hydrous laminar structure, resulting in a reduced specific surface area. Second, higher calcination temperatures promote sintering, leading to the formation of dense oxide phases, which further reduce porosity and surface area.

The experimental results indicate that the catalysts prepared at a calcination and reduction temperature of 500 °C exhibit a superior specific surface area, opening up new possibilities for their application in areas such as catalysis, adsorption, and separation.

2.2.4. Cu₁Ni₂/Al₂O₃ Catalyst TPD Analysis

Investigating the hydrogen adsorption capacity of a catalyst is essential for understanding the dissociation and activation of hydrogen molecules during catalytic reactions [48]. The metal active sites on the catalyst can temporarily adsorb hydrogen molecules, which can subsequently decompose into hydrogen ions for exchange within the reaction system. To obtain information on the type and quantity of desorbed hydrogen, H₂ temperature-programmed desorption (H₂-TPD) experiments are an effective tool. In this study, the Cu₁Ni₂/Al₂O₃ catalyst synthesized under optimal conditions was subjected to H₂-TPD analysis.

According to the data in Figure 8, the catalyst exhibits a prominent desorption peak near 100 °C and a shoulder peak around 127 °C within the temperature range of 50 °C to 500 °C. The peaks below 100 °C are attributed to the desorption of physically adsorbed hydrogen, while those above 100 °C indicate the promotion of hydrogen molecules by the metal active sites, leading to chemisorption and resulting in hydrogen being chemisorbed on the catalyst surface in a dissociated state. The catalyst possessed excellent hydrogen adsorption capability [43]. This suggests that the CuNi alloy in the catalyst exhibits strong interactions and provides a significant number of metal active sites, thereby enhancing the transfer of hydrogen in the reaction.

NH₃ temperature-programmed desorption (NH₃-TPD) is primarily used to characterize the adsorption capacity of ammonia on solid surfaces and to release ammonia at different temperatures. This method allows for the evaluation of the catalyst’s surface acid–base properties, providing insights into its affinity for adsorbed substances and the activity of catalytic reactions [49]. There is a direct positive correlation between the adsorption capacity of NH₃ and the acidity of the adsorption sites on the sample surface; generally, stronger acidity leads to greater NH₃ adsorption, necessitating higher temperatures for desorption [50]. By measuring the amount of NH₃ in the desorbed gas at different temperatures, the acid site distribution of the sample can be qualitatively analyzed. In catalysis studies, desorption peaks below 200 °C are typically classified as weak acid sites, those between 200 °C and 400 °C indicate moderate to strong acid sites, and peaks above 400 °C suggest the presence of strong acid sites on the sample surface. From Figure 9, it is evident that the Cu₁Ni₂/Al₂O₃ catalyst is predominantly characterized by weak and moderate acid sites, with a scarce strong acid sites. The introduction of Cu facilitates the formation of a substantial number of CuNi alloys compared to a single Ni-based catalyst. This metal-to-metal interaction within the CuNi alloys enhances the number of acidic sites on the surface of the Cu₁Ni₂/Al₂O₃ catalyst.

CO₂ temperature-programmed desorption (CO₂-TPD) is widely employed to characterize the surface basicity of solid catalysts. In this experiment, the catalyst was exposed to CO₂ gas at a specific temperature, allowing CO₂ molecules to adsorb onto the catalyst surface. As the temperature increases, the previously adsorbed CO₂ gradually desorbs. This process provides valuable information about the number, strength, and distribution of basic sites on the catalyst surface. The presence and strength of basic sites play a vital role in facilitating hydrogen ion transfer and promoting interactions with reactant molecules, thereby affecting overall catalytic performance. The CO₂-TPD curves presented in Figure 10 indicate that a significant number of aluminates were formed in the Cu₁Ni₂/Al₂O₃ catalyst prepared using hydrotalcite as a precursor and subjected to a calcination and reduction process. This suggests that the Cu₁Ni₂/Al₂O₃ catalyst is primarily characterized by weak and moderate to strong basicity. Additionally, the presence of both acidic and basic centers in the Cu₁Ni₂/Al₂O₃ catalyst enables it to effectively activate the C=O bond of acetylpropionic acid during catalytic transfer hydrogenation [43]. In addition, these sites promote O–H bond cleavage in isopropanol, thereby facilitating the production of active hydrogen.

2.3. Analysis of Influencing Factors

2.3.1. Effect of Reaction Temperature

Studying the effect of reaction temperature on the conversion of LA to 2-MTHF plays a critical role, as it directly influences the conversion rate, selectivity, and product yield. While higher temperatures can accelerate the reaction and enhance LA conversion, they may also lead to increased formation of by-products and reduced selectivity for 2-MTHF. Conversely, lower temperatures may lead to insufficient catalytic activity and significantly inhibit the progression of the reaction. Therefore, experimentally exploring the impact of reaction temperature is essential for achieving an optimal balance between high efficiency and selectivity, thereby improving the economic viability and environmental sustainability of industrial production.

The experimental data, as presented in Figure 11, indicated that the conversion of LA reached 55.6% at 170 °C, and the selectivities of 1,4-pentanediol(1,4-PDO) and 2-MTHF were 40.1% and 49.9%, respectively. These results imply that the lower temperature may have constrained the efficiency of hydroconversion, resulting in the intermediate 1,4-PDO not being fully converted during the reaction. At 190 °C, the LA conversion reached 100%, while the selectivity of 2-MTHF increased significantly to 87.2%, and that of 1,4-PDO decreased sharply to 1.4%. The yields of 2-MTHF and 1,4-PDO decreased as the temperature continued to increase, presumably because the high temperature promoted the formation of the by-product pentanol.

2.3.2. Effect of Reaction Time

To clarify the kinetic properties of the catalyst in promoting the conversion of LA to 2-MTHF through hydrogenation and to explore the optimal reaction time, a series of experiments with varying reaction times were conducted. These experiments aim to elucidate the reasons behind the variation in catalyst activity with reaction time, allowing for the identification of the ideal reaction duration that maximizes yield and selectivity. This optimization is crucial for improving the efficiency and economic viability of the conversion process. Such findings hold significant implications for industrial production and contribute to the development of more effective production strategies, ensuring optimal resource utilization. The experimental results are presented in Figure 12.

After a reaction time of 1 h, the conversion of LA reached 54.6%, with 2-MTHF selectivity of 57.8% and 1,4-PDO at a selectivity of 40.7%, while other by-products were minimal. These results indicate a relatively clear reaction pathway during the shorter reaction cycle. When the reaction time was extended to 4 h, LA conversion increased significantly to 91.5%, while the selectivity for 1,4-PDO decreased and that for 2-MTHF increased. This suggests that 1,4-PDO, an intermediate, was gradually converted into the target product, 2-MTHF, as the reaction progressed [51]. When the reaction was extended to 5 h, the conversion of LA reached 100%, with the selectivity for 2-MTHF at 85.8%. Continuing the reaction for an additional hour to a total of 6 h resulted in a further increase in the selectivity of 2-MTHF to 90.2%. These findings indicate that the conversion of the intermediate 1,4-PDO to 2-MTHF became more efficient as the reaction time was extended.

2.3.3. Effect of Reaction Hydrogen Pressure

In hydrogenation reactions, hydrogen pressure significantly influences the reaction rate, product yield, and selectivity. Elevated hydrogen pressure increases the concentration of hydrogen molecules in the reaction system, thereby enhancing the conversion efficiency of reactants and accelerating the formation of target products. The core of the hydrogenation process lies in the interaction between hydrogen molecules and reactant molecules; thus, an increase in hydrogen pressure facilitates greater participation of hydrogen molecules in the reaction, expediting the overall chemical process. Moreover, according to Le Chatelier’s principle, increasing the concentration of a reactant in a chemical reaction shifts the equilibrium toward the formation of more products. Consequently, elevating the hydrogen pressure in a hydrogenation reaction effectively drives the reaction toward the generation of hydrogenated products, resulting in a higher yield of the target product and a reduction in by-product formation. It is essential to carefully optimize and control hydrogen pressure during the reaction to achieve a highly efficient and selective hydrogenation process while ensuring safety and economic viability. This study explores the optimization of efficiency and product selectivity in the hydrogenation reaction by controlling hydrogen pressure at a reaction temperature of 190 °C and a reaction time of 5 h while maintaining safety and cost-effectiveness.

The experimental results are presented in Figure 13. At a hydrogen pressure of 2 MPa, the conversion rate of LA was 68.1%, with the conversion rate continuing to increase as hydrogen pressure rose. When the pressure was elevated to 5 MPa, the conversion of LA reached 100%, and the selectivity for 2-MTHF increased from 82.3% to 87.6%. This demonstrates that appropriately increasing the hydrogen pressure can enhance the yield of 2-MTHF while inhibiting the formation of by-products [18]. However, at 6 MPa, the selectivity for by-products increased, leading to a decrease in the selectivity for 2-MTHF. Therefore, the optimal hydrogen pressure was determined to be 5 MPa.

2.3.4. Research on Recycling Effect

To thoroughly investigate the stability and recyclability of the bimetallic Cu₁Ni₂/Al₂O₃ catalyst in the one-step conversion of LA to 2-MTHF, a series of recycling performance tests were conducted. These tests aimed to assess changes in catalyst performance during repeated use and to validate its reliability and economic advantages for long-term applications. The catalyst was recovered from the reaction mixture via centrifugal separation. Subsequently, the catalyst was washed three times alternately with ethanol and deionized water to remove any residual reactants, products, or impurities from its surface. After washing, the catalyst was dried overnight at 60 °C. Prior to each experimental cycle, the catalyst was activated at 550 °C for 3 h in a 10% H₂/Ar atmosphere to prevent surface oxides from impacting the experimental results. These procedures enabled a comprehensive assessment of the catalyst’s durability and recyclability over multiple cycles. The results lay a solid foundation for the practical deployment of this catalyst in industrial chemical processes.

The experimental results, presented in Figure 14, demonstrate the catalyst’s performance during reuse. Overall, the catalyst maintained notable stability and good reusability. In six cycling experiments conducted under identical reaction conditions, the yield of 2-MTHF decreased slightly from the initial 87.6% to 78.4%, resulting in an activity loss of only 9.2%. Analysis indicated that the catalyst’s activity declined at a higher rate during the first three cycles, likely due to alterations in the surface microstructure and active sites during initial use. However, from the fourth to sixth cycles, the reduction in activity significantly slowed, suggesting that the catalyst reached a more stable state. These findings confirm the catalyst’s durability during prolonged operation. These findings underscore the importance of proper pretreatment and initial adjustments of catalysts in practical industrial applications to ensure optimal performance over extended periods.

The results strongly affirm that the bimetallic Cu₁Ni₂/Al₂O₃ catalyst not only effectively catalyzes the one-step conversion of LA to 2-MTHF but also retains a high degree of stability and reliability after multiple cycles. This sustained performance supports its potential for commercial deployment in chemical manufacturing processes.

2.3.5. Investigation of Reaction Mechanism

The conversion of LA to 2-MTHF was achieved through consecutive hydrogenation reactions over a Cu₁Ni₂/Al₂O₃ catalyst. The synergistic interaction between Cu and Ni facilitated a series of transformations, including LA hydrogenation to GVL, GVL ring-opening hydrogenation to 1,4-PDO, and 1,4-PDO cyclization to 2-MTHF [52]. Reaction conditions (time, temperature, and hydrogen pressure) significantly influenced the conversion rate and yield, as revealed by experimental analysis. The reaction pathway is shown in Figure 15.

During the initial hydrogenation, LA was rapidly converted to 2-MTHF and 1,4-PDO, indicating a high hydrogenation rate due to the Cu-Ni synergy. Ni demonstrated superior activity in LA hydrogenation to GVL, attributed to its high d-electron density, which enhanced C=C bond adsorption and activation, promoting efficient reduction [53]. Cu showed higher activity in the ring-opening hydrogenation of GVL to 1,4-PDO, forming coordination interactions with the cyclic C–O bond to lower dissociation energy and enable ring cleavage [23]. Additionally, acidic sites on the Al₂O₃ support facilitated GVL ring-opening and downstream hydrogenation of GVL [50].

The cyclization of 1,4-PDO to 2-MTHF was highly sensitive to reaction conditions. Elevated temperatures (>180 °C) enhanced molecular motion and reduced activation energy, significantly accelerating conversion [27]. Short reaction times (<1 h) led to insufficient 1,4-PDO conversion, while prolonged times (>5 h) caused excessive accumulation, catalyst deactivation, and decreased selectivity due to by-product adsorption [40]. Hydrogen pressure also played a key role: low pressure (<3 MPa) limited hydrogenation, while optimal pressure (5 MPa) improved H₂ activation at Ni sites, promoting Cu–H formation and accelerating cyclization [52]. Excessive pressure (>5 MPa), however, induced over-hydrogenation and side reactions, reducing 2-MTHF yield.

In summary, the coordinated regulation of temperature, time, and hydrogen pressure is crucial for optimizing 2-MTHF production and balancing reaction rate, selectivity, and catalyst stability.

3. Materials and Methods

3.1. Raw Materials

The reagents Cu(NO₃)₂·3H₂O, Ni(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaOH, and Na₂CO₃ (AR ≥ 99%) were obtained from Shanghai Maclean’s Biochemical Science and Technology Co., Ltd. (Shanghai, China). Isopropanol (AR ≥ 99%) was obtained from Shanghai Aladdin Biochemical Science and Technology Co., Ltd. (Shanghai, China). Furthermore, N₂ (99.99%), H₂/Ar (10% (vol)) and H₂ (99.99%) were obtained from Fujian Dehe Chemical Co. (Fuzhou, China).

3.2. Methods of Catalyst Preparation

3.2.1. Synthesis of CuNiAl Hydrotalcite

The CuNiAl hydrotalcite precursor was prepared via the coprecipitation method as follows. Initially, to prepare solution A, 10 mmol/L Cu(NO₃)₂·3H₂O, 10 mmol/L Al(NO₃)₃·9H₂O, and 20 mmol/L Ni(NO₃)₂·6H₂O were dissolved in 150 mL of deionized water at room temperature. Simultaneously, NaOH and Na₂CO₃ were dissolved in deionized water in a predetermined ratio to create solution B. The types and quantities of precursor salts utilized in the synthesis process of CuNiAl hydrotalcite were summarized in Table 1. Next, solution A was placed in a temperature-controlled water bath at 60 °C and stirred continuously and uniformly using a magnetic stirrer while the pH was monitored in real time using a pH meter. Subsequently, solution B was slowly added to solution A while maintaining the pH in the range of 9–10. The mixture was then heated to 65 °C and stirred vigorously for 24 h. After the reaction, the slurry was centrifuged and washed until the supernatant reached neutrality. Finally, the solid was dried in an oven at 80 °C for 12 h and then ground into powder to obtain the CuNiAl hydrotalcite.

3.2.2. Preparation of Cu₁Ni₂/Al₂O₃ Catalyst

A specified amount of CuNiAl hydrotalcite was weighed and placed into a crucible, which was then transferred to a muffle furnace. The sample was calcined at a predetermined temperature for a specified duration to obtain CuNiAl mixed metal oxides. Subsequently, the resulting mixed oxides underwent hydrogen reduction treatment to form the active catalyst.

The detailed steps for preparing the Cu₁Ni₂/Al₂O₃ catalyst are as follows: First, about 0.1 g of the as-prepared CuNiAl hydrotalcite precursor was placed into a U-shaped quartz tube packed with quartz wool. Argon gas was then introduced at a constant flow rate of 40 mL/min, and the system was heated to 100 °C at a uniform ramp rate of 10 °C/min. This temperature was maintained for 1 h to perform the initial pretreatment. Afterward, the system was allowed to cool naturally to room temperature. Following the cooling step, the argon atmosphere was switched to a 10% H₂/Ar gas mixture at the same flow rate of 40 mL/min. The temperature was then increased at a rate of 10 °C/min to a predetermined reduction temperature, at which point the sample was held for 3 h to ensure complete reduction. After the reduction process, the Cu₁Ni₂/Al₂O₃ catalyst was cooled to room temperature under a continuous flow of argon gas.

3.3. Catalyst Testing Methods

3.3.1. Characterization

XRD analysis was performed using a Smartlab SE X-ray diffractometer (Rigaku Company, Tokyo, Japan). Cu Kα radiation (λ = 0.15418 nm) was used as the X-ray source, with an accelerating voltage of 40 kV and a tube current of 40 mA. The scan rate was set at 2°/min, and the 2θ range was scanned from 5° to 90°.

FT-IR spectroscopy was performed using an iN10 Fourier transform infrared spectrometer (ThermoScientific Company, Waltham, MA, USA), with a scanning range of 4000–400 cm⁻¹. Samples were prepared using the KBr pellet method, in which the sample and KBr were mixed at a mass ratio of 1:20 and ground in an agate mortar. The homogeneous mixture was then pressed into a pellet for subsequent analysis.

The morphology of the samples was examined using a GeminiSEM 300 field-emission scanning electron microscope (ZESS Company, Oberkochen, Germany). Prior to analysis, the powder samples were ultrasonically dispersed in ethanol for 10 min, dropped onto aluminum foil, dried, and then coated with a thin layer of gold by sputtering.

The specific surface area, pore volume, and pore size distribution of the samples were measured using an ASAP 2460 automated physisorption analyzer (ASAP 2460, Micromeritics Company, Norcross, GA, USA). The surface area was calculated based on the BET method.

TPD experiments were conducted using an AutoChem II 2920 chemisorption analyzer (Micromeritics Company, Norcross, GA, USA). Approximately 0.1 g of the sample was placed in a U-shaped quartz tube and reduced at 350 °C for 1 h under a 10% H₂/Ar flow (40 mL/min) to ensure complete reduction of surface oxides. The gas was then switched to pure He (40 mL/min) for 1 h to remove residual hydrogen. Subsequently, the temperature was lowered to 50 °C, and the target gas (e.g., H₂, NH₃, or CO₂) was introduced for 1 h to allow sufficient adsorption. Finally, pure He was reintroduced to purge physically adsorbed gases from the sample surface. During the temperature-programmed desorption process, the effluent gas was monitored using a thermal conductivity detector (TCD) while heating at a rate of 5 °C/min.

3.3.2. Catalyst Activity Test

The 60-mesh catalyst was mixed with levulinic acid and isopropanol in a specified ratio and loaded into a 100 mL stainless steel high-pressure reactor, which was lined with quartz to prevent metal contamination. The reactor was purged with hydrogen gas 4–5 times and then pressurized to the desired level to ensure a stable gas environment. The reactor was heated to the predetermined temperature, with real-time temperature monitoring, and the mechanical stirring device was initiated at a stirring rate of 500 rpm to maintain uniform mixing. Upon completion of the reaction, heating and stirring were stopped, and the reactor was cooled to room temperature before opening to collect the reaction mixture. The resulting mixture was subsequently separated and purified to obtain the target product. The product was analyzed to evaluate the reaction efficiency and selectivity, providing data support for subsequent research.

3.3.3. Analysis of Reaction Products

Product samples were individually analyzed using an Agilent 7890A (Agilent Technologies Company, Santa Clara, CA, USA) gas chromatograph. A more detailed analysis was conducted on the same system, with 1,4-dioxane as the internal standard. During analysis, 1,4-dioxane was employed as an internal standard to enhance the accuracy and reliability of the quantitative measurements. The chromatograph was equipped with a high-performance flame ionization detector (FID) operated at an optimized temperature of 260 °C, which effectively enhanced signal sensitivity and ensured the accuracy of detection. To detect the target components in the reaction solution derived from acetylpropionic acid, an HP-INNOWAX capillary column installed on the Agilent 7890A was used for chromatographic separation.

Levulinic acid conversion rate:

$$C_{LA}={\displaystyle \frac{n_{LA}-n_{\mathrm{1,4}}\times A_{LA}\times f_{LA}}{A_{\mathrm{1,4}}\times f_{\mathrm{1,4}}}}\times n_{LA}\times 100\%$$

Product selectivity:

$$S_{\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}={\displaystyle \frac{A_{\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}\times n_{\mathrm{1,4}}}{A_{\mathrm{1,4}}{f}_{\mathrm{1,4}}\times n_{\mathrm{1,4}}-A_{LA}{f}_{LA}\times n_{\mathrm{1,4}}}}\times 100\%$$

Calculation of production rates:

$$Y_{\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}=C_{LA}\times S_{\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}\times 100\%$$

4. Conclusions

In this study, the Cu₁Ni₂/Al₂O₃ catalyst was synthesized using a calcination–reduction method with CuNiAl hydrotalcite as the precursor. The microscopic properties of the Cu₁Ni₂/Al₂O₃ catalyst were characterized using SEM, XRD, BET, and TPD techniques. The study also investigated the specific effects of reaction conditions, including temperature, time, and hydrogen pressure, on the conversion of LA, as well as the selectivity and yield of 2-MTHF. Based on the experimental findings, the following conclusions were drawn:

(1)

Through the comprehensive use of characterization techniques such as XRD, FT-IR, and SEM, high-quality hydrotalcites with high crystallinity, structural stability, and expected functional groups can be obtained under well-controlled synthesis conditions and an optimal Cu:Ni:Al = 1:2:1 molar ratio of 1:2:1;

(2)

XRD analysis indicated that CuNiAl hydrotalcite gradually transformed into a homogeneous composite tri-metal oxide with increasing calcination temperature; however, excessively high temperatures led to catalyst structural degradation. SEM analysis highlighted morphological changes of the catalysts under different calcination conditions, demonstrating that appropriate calcination temperature and time fostered an optimal pore structure. The ideal preparation conditions were established as a calcination temperature of 500 °C, a calcination time of 6 h, and a reduction temperature of 400 °C. BET results confirmed that catalysts prepared under these optimal conditions exhibited a high specific surface area, facilitating catalytic reactions. Additionally, TPD experiments validated that the catalysts had strong hydrogen adsorption capacity and suitable acid–base properties, supporting their high catalytic performance;

(3)

The systematic investigation of reaction temperature, time, and hydrogen pressure revealed that conducting the reaction at 190 °C and 5 MPa H₂ for 5 h resulted in the complete conversion of LA, achieving a 2-MTHF yield of 87.6%. This demonstrated a highly efficient one-step conversion of LA to 2-MTHF;

(4)

The recycling performance tests of the bimetallic Cu₁Ni₂/Al₂O₃ catalyst were conducted to assess its stability and economic viability during reuse. The results indicated that the catalyst exhibited excellent reusability and stability, with only a slight decrease in activity after six cycles. This demonstrates its feasibility and reliability for industrial applications.