Forming a Cu-Based Catalyst for Efficient Hydrogenation Conversion of Starch into Glucose

Abstract

A pellet-forming as-catalyst, CuO/Al₂O₃, was prepared by the precipitation–tablet molding method and characterized by the Brunner–Emmet–Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) techniques and TEM. The characterization results showed that the formed CuO/Al₂O₃ was in situ reduced to Cu/Al₂O₃ and Cu₂O/ Al₂O₃ catalysts in the reaction system. The catalytic performance of catalyzing hydrogenation starch into glucose was investigated in an autoclave over CuO/Al₂O₃. The yield of glucose reached 83.16% at a temperature of 160 °C, a pressure of 1.8 MPa, a 100 g starch solution of 15 wt%, a catalyst dosage of 2.25%, a reaction time of 4 h, and a rotational speed of 630 r/min. The reusability of the catalyst was evaluated, and the glucose yield did not decrease obviously even after being reused for five consecutive cycles. Starch was converted into glucose through the synergistic action of Cu⁺ and Cu⁰ catalysis. This work is expected to provide valuable insights into the design of catalysts and the hydrogenation process for efficient starch hydrogenation.

1. Introduction

The rapid development of industry has led to an exponential increase in energy demand. As fossil resources become increasingly scarce, crude oil prices continue to rise [1]. In order to alleviate the consumption of fossil raw materials and the deterioration of the environment, a large number of renewable biomass resources are used to produce various materials, fuels, and chemicals [2]. Biomass is a promising renewable resource, with carbohydrates as its main component. Converting carbohydrates into fuels and chemicals is one effective way to alleviate the energy crisis [3,4]. Therefore, the development of sustainable renewable resources has garnered much attention. Starch is a major component of various biomasses and is one of the most abundant renewable resources on Earth, and it can be used as a sustainable energy source [5]. Starch is widely present in cassava, potatoes, yams, taros, and rice, and it can be used to produce beer and in the sugar industry (glucose and maltose) [6]. Among the numerous platform compounds of biomass conversion, glucose is an important platform compound that can be further converted into downstream products such as fructose, 5-hydroxymethylfurfural, pyruvic acid, sorbitol, ethylene glycol, etc., and is widely used in industries such as chemicals, food, medicine, and energy [7,8,9]. Moreover, glucose is also widely used in the human body, fermentation, and food processing and is the main raw material for wine-making, pharmaceuticals, and chemical production [10]. Starch can be divided into amylose and amylopectin, with amylose being composed of about 400 1, 4-D-glucose polymers and amylopectin having about 4% branching through 1, 6-glycosidic bonds on the basis of 1, 4-D-glucose polymerization [11]. Hydrolyzing starch to produce glucose mainly involves breaking the 1, 4-glycosidic bond or 1, 6-glycosidic bond within polymer molecules [12].

Currently, the main methods for preparing glucose from starch include enzymatic hydrolysis [13], acid hydrolysis [14], and supercritical water hydrolysis [15,16,17]. The enzymatic hydrolysis of starch catalyzed by glucose amylase is the most common and effective method in industrial applications. However, this method still has certain limitations because of its low efficiency in hydrolyzing α-1, 6 bonds, resulting in a long reaction cycle [18]. Acid hydrolysis is an efficient method for glucose production, but it requires a large amount of acid and alkali in the refining process, which burdens subsequent wastewater treatment. On the other hand, the catalytic hydrogenation of starch to produce glucose can reduce the need for acid and alkali treatments [19]. In the experiments conducted by Wang et al. [20], the influence of ultrasound on starch hydrolysis during different stages of enzymatic hydrolysis was studied. It was found that using ultrasound treatments minimized enzyme activity loss and significantly improved the reaction rate. However, the use of ultrasound is expensive and increases costs. Vijay et al. [21] used activated carbon-supported tungstosilicic acid as a catalyst for starch hydrolysis and demonstrated the selectivity of glucose production through hydrothermal methods. No byproducts, such as pyruvic acid and formic acid, were produced during the experiment. Even after five consecutive reactions, the catalyst maintained its activity and selectivity without significant loss. However, the subsequent neutralization caused by acid catalyst immersion is environmentally undesirable. Therefore, considering a more efficient and economical method to promote the glycosylation reaction is a worthwhile research direction. The direct catalytic hydrogenation of biomass derivatives provides a simple operation method where, after the reaction, the catalyst can be filtered for reuse, reducing environmental pollution caused by acid and alkali neutralization. This process offers new insights into the production of various platform compounds from biomass [22].

Transition metal oxides are widely used as inexpensive and environmentally friendly catalysts in industrial catalysis [23]. The solvent for catalytic hydrogenation using metal oxide catalysts is water, which does not require disposal. Currently, precious metal catalysts such as Pt, Ru, and Pb have demonstrated excellent hydrogenation activity and selectivity. However, researchers have been striving to develop affordable and readily available non-precious metal hydrogenation catalysts. It has been discovered that inexpensive copper (Cu)-based catalysts [24,25] exhibit good performance in breaking C-O bonds and demonstrate excellent catalytic selectivity [26,27]. Because of their low cost and excellent catalytic performance, Cu-based catalysts have become a focus of research in the methanol oxidation carbonylation reaction [28,29]. However, there is relatively little research on the use of CuO for biomass degradation. In this context, Yang et al. [30], from our research group, studied the use of CuO as a catalyst for cellulose degradation to produce glucose. In this study, a CuO/Al₂O₃ as-catalyst was prepared using a precipitation-tableting method, and its catalytic performance in starch hydrogenation meant to produce glucose was investigated.

2. Results and Discussion

2.1. Crystallinity and Textural Properties

The crystallinity of the resulting samples can be determined with powder XRD measurements. As shown in Figure 1a, the diffraction peaks at 2θ = 32.4°, 34.6°, 39.1°, and 57.7° are ascribed to the (110), (002), (200), and (202) planes of CuO (JCPDS 05-0661), respectively. It is worth noting that Cu/Al₂O₃ was produced via reduction from CuO/Al₂O₃ in situ, while the XRD diffraction peaks around 43.3°, 50.4°, and 74.1° correspond to the (111), (200), and (220) planes of Cu metal [31]. The diffraction peaks of Cu are in good agreement with JCPDS 04-0836, indicating the formation of Cu. It is worth noting that the diffraction peaks at 2θ = 29.55°, 36.4°, and 73.52° were detected, which are the same as the (110), (111), and (311) planes of Cu₂O (JCPDS 05-0667), suggesting Cu₂O is contained in Cu/Al₂O₃. Furthermore, for two samples, the typical characteristic peaks of Al₂O₃ can be also observed; the diffraction peaks at 31.93° can be scripted to the (220) plane of γ-Al₂O₃, which is consistent with JCPDS 10-0425.

The N₂ adsorption–desorption isotherms for the CuO/Al₂O₃ and Cu/Al₂O₃ solids are presented in Figure 1b. Two samples present type Ⅲ isotherms according to the IUPAQ classification with a hysteresis loop of the H₃ type, indicating that the samples were in the presence of well-developed mesoporous structures with monolayer–multilayer adsorption and aggregates of plate-like particles, giving rise to slit-shaped pores [32]. The presence of mesoporous structures is further confirmed by pore size distribution curves in Figure 1c. The pore size distribution is relatively concentrated and mainly presents mesoporous structures. It can be seen from

Table 1. Specific surface area, pore size, and pore volume of Cu-based catalysts.

Samples	SBET/(m2/g)	VT/(cm3/g)	Dpore/nm
Cu/Al2O3	24.7	0.26	7.9
CuO/Al2O3	51.2	0.32	12.5

S_BET—BET surface area; V_T—total pore volume, dpore—average pore diameter, P/P₀ = 0.992.

that the specific surface area and pore volume of the Cu/A decreases significantly after reduction, from the initial 51.2 m²/g and 0.32 m³/g to 24.7 m²/g and 0.26 m³/g, respectively, which can be attributed to the electrostatic action of Cu to make a part of the surface pores blocked [33].

2.2. Morphology and Microstructure

In order to confirm the influence of the reduction process on the two samples, the morphologies of CuO/Al₂O₃ before and after reduction in situ via H₂ were investigated with TEM; the result is shown in Figure 2a,b. Before the reaction, the structure of the catalyst precursor, CuO/Al₂O₃, was loosely distributed, while the catalyst, Cu/Al₂O₃, is obviously aggregated after the reaction. This is because the catalyst precursor, CuO/Al₂O₃, was reduced by H₂ to form the catalyst, Cu/Al₂O₃, in situ, and the Cu dispersed on the surface of the Al₂O₃ carrier is gathered up because of electrostatic attraction between the metals. It can also be seen from the figure that the particle size of the catalyst decreases after reduction, which is in accordance with the conclusion of Figure 1c. The typical TEM micrograph and the particle size distribution histograms of CuO and Cu nanoparticles on the Al₂O₃ support are shown in Figure 2c,d, presenting a mean CuO particle size of 2.36 nm and a mean Cu particle size of 2.09 nm.

2.3. Surface Characterization of the Two Materials

The XPS technique was used to investigate the surface chemical structure and Cu, Al, and O components of CuO/Al₂O₃, and the standard charge was calibrated using a C 1s binding energy of 284.8 eV. The results are shown in Figure 3. As depicted in Figure 3a, the presence of Cu (2p), Al (2p), and O (1s) was discovered by an XPS survey scan of the CuO/Al₂O₃ particles. Figure 3b shows the O 1s spectra. The O 1s spectra of the CuO/Al₂O₃ as-catalyst were divided into three peaks using the Gauss decomposition method. According to a previous study [34], the lowest binding energy corresponds to lattice oxygen (O_α) in mixed metal oxide, the medium binding energy is assigned to surface-adsorbed oxygen (O_β) species, and the highest binding energy is attributed to surface hydroxyl species (O_γ). Here, three peaks centering on 529.5 eV (O_α), 531.0 eV (O_β), and 532 eV (O_γ) are assignable to lattice oxygen (Al-O, Cu-O), chemically adsorbed oxygen, and adsorbed water [35]. As depicted in Figure 3c, in the high-resolution XPS spectra of the Cu 2p core level of the reacted CuO/Al₂O₃ particles, the signal can be divided into five peaks at around 933.8 eV, 941.3 eV, 943.5 eV, 953.8 eV, and 962.2 eV [36]. According to the literature [37], the peaks of Cu²⁺ detected at 933.7–934.1 eV and 953.4–954.8 eV on each sample can be assigned to Cu 2p3/2 and Cu 2p1/2, respectively. Here, representative peaks of Cu 2p_3/2 (933.8 eV) and Cu 2p_1/2 (953.8 eV) represent the CuO of Cu 2p. The characteristic Cu²⁺ satellite peaks can be observed at 941.3 eV, 943.5 eV, and 962.2 eV, which are a result of hybridization between the 3d Cu orbital and the oxygen 2p orbital [38]. According to our previous study, the as-catalyst, CuO/Al₂O₃, was not detected in the presence of Cu⁺, and only Cu²⁺ formed on CuO/Al₂O₃ in the XPS analysis. The results were consistent with the results of XRD of CuO/Al₂O₃. Furthermore, as seen in Figure 3d, the Al 2p binding energy of CuO/Al₂O₃ was 74.5 eV, indicating Al (III) species [39].

After the hydrogenation reaction, the catalyst was characterized by XPS; the results are shown in Figure 4. As exhibited in Figure 4a, the compositions of the catalytical materials are still composed of three elements: Al, O, and Cu. Figure 4b shows the O 1 s spectra. The O 1 s spectra of the Cu/Al₂O₃ catalyst can be divided into three peaks by the Gauss decomposition method. Three peaks centering on 529.5 eV (O_α), 531.0 eV (O_β), and 532 eV (O_γ) are assignable to lattice oxygen (Al-O, Cu-O), chemically adsorbed oxygen, and adsorbed water. However, it can be seen from the peak area that the lattice oxygen content was relatively reduced at this time, which may be due to the reduction of CuO into Cu and Cu₂O caused by H₂ gas during the reaction. Furthermore, as shown in Figure 4c, the Al 2p binding energy of Cu/Al₂O₃ is about 74.5 eV, indicating that the valency of the Al³⁺ species did not change. However, compared with Figure 3c, the bond energy of Cu in Figure 4d has changed. As shown in Figure 4d, two peaks centering on 932.0 eV and 951.8 eV are scripted to Cu⁰ [40], which is consistent with XRD. The two peaks at 933.0 eV and 953.2 eV can be assignable to Cu⁺ 2p3/2 and Cu⁺ 2p1/2, respectively, indicating the existence of Cu⁺ in Cu/Al₂O₃. It is noteworthy that the active component of the catalyst that plays a catalytic role in the reaction is Cu⁰ and Cu⁺. The abundant Cu⁺ species on the surface of the Al₂O₃ can effectively dissociate H from hydrogen [41]. Moreover, the Cu⁺ species plays a remarkable role in polarizing the C–O bond, which can be effective in breaking the 1, 6-glucoside bond in starch.

2.4. Catalytic Activity

The catalytic activities of the resulting Cu/Al₂O₃ catalyst were evaluated for the hydrogenation of starch under a batch reactor. First, the influence of the reaction temperature in yielding glucose was investigated by proceeding with the reaction in a range of 140–170 °C under 1.8 MPa H₂, a rotation speed of 630 r/min, a 100 g starch solution of 15% (wt%), and a catalyst dosage of 2.25% for 4 h; the results were shown in Figure 5a.

As can be seen from Figure 5a, within a range of 140 °C to 160 °C, the glucose yield increases with the temperature. The highest glucose yield, 83.18%, was achieved at 160 °C. However, the glucose yield decreases with increasing reaction temperatures thereafter. This is because the starch solution and glucose solution will undergo coking at high temperatures, which can encapsulate the catalyst and cause it to lose activity because of coking, leading to a decrease in glucose yield. Therefore, the optimal temperature can be found at 160 °C for the subsequent optimization of starch hydrogenation.

Figure 5b illustrates the relationship between hydrogen pressure and glucose yield from 1 to 4 MPa at 160 °C, a rotation speed of 630 r/min, a 100 g starch solution of 15% (wt%), and a catalyst dosage of 2.25% for 4 h. As seen in Figure 5b, the glucose yield gradually increases with increasing pressure under a pressure range between 1 and 1.8 MPa. Under the conditions of 1.8 MPa and 2.0 MPa, the highest glucose yield, 83.18%, is achieved because of the optimal solubility of hydrogen in water and the resulting optimal catalytic effect. However, when the pressure reaches above 2.0 MPa, the hydrogen may continue to react with glucose to produce products such as glycerol, ethylene glycol, etc., resulting in a slight decrease in the yield of glucose with further a increase in reaction pressure. Therefore, an optimum pressure of 1.8 MPa was selected to investigate the other conditions.

To investigate the impact of starch concentration on catalytic reactions, we investigated the effect of starch concentration on glucose yield by using starch solutions with concentrations of 10, 15, 20, 25, and 30 wt% under reaction conditions of 160 °C, 1.8 MPa pressure, 630 r/min rotation speed, 4 h reaction time, and 2.25% catalyst dosage; the results are shown in Figure 5c.

As shown in Figure 5c, it can be observed that the glucose yield increases with the increase in starch concentration from 10% to 15%. An optimum glucose yield of 83.18% is achieved using a 15% starch solution under hydrogenation conditions. However, when the starch concentration exceeds 15%, the glucose yield significantly decreases. This can be attributed to the fact that, within a certain range, an increase in starch concentration facilitates sufficient contact between starch and the catalyst, thereby benefiting the reaction process. However, excessively high concentrations result in the formation of multilayer starch adsorption on the catalyst surface during the reaction, leading to a decrease in the effective contact area and, consequently, a reduction in catalytic activity. Therefore, the optimal starch concentration under the experimental conditions is 15%.

The dosage of the catalyst also plays an important role in the starch hydrogenation reaction. The catalyst dosage varied between 1% and 2.5% (the mass of the catalyst divided by the total mass of starch and water is equal to the mass percentage of the catalyst), and its effect on the starch yield was studied. Reactions were performed under the same experimental conditions as above, that is a 160 °C reaction temperature, a 4 h reaction time, 630 r/min agitation speed, and a 100 g starch solution of 15 wt%. The further results are shown in Figure 5d.

As indicated in Figure 5d, the starch yield increases first and then decreases. In detail, a gradual increase in the starch yield is up to 83.18% when the catalyst dosage increases from 1% to 2.25%, which is probably due to more active sites resulting from the increase in the catalyst dosage. When the catalyst dosage increases from 2.25% to 2.5%, the decrease in the starch yield is small. On the one hand, too many catalysts are pushed together, and the effective active sites are relatively reduced, resulting in a decrease in catalytic efficiency. On the other hand, excess catalysts may hinder the adsorption and transformation of the product on the catalyst surface because of competitive adsorption. Consequently, the yield of the product is lowered. Therefore, taking all factors into consideration, especially the glucose yield, the optimal dosage of the catalyst is 2.25%.

In order to test the influence of reaction time, especially on the reaction pathway of starch hydrogenation, an experiment was carried out at 630 r/min with 1.8 MPa H₂ under the same conditions as above, that is, a 100 g starch solution (15 wt%), a 2.25% catalyst dosage, and a160 °C reaction temperature for reaction times ranging from 2 h to 6 h. The results are shown in Figure 5e. Based on the data presented in Figure 5e, it is evident that the glucose yield steadily increases as the reaction time extends from 2 to 4 h. The glucose yield reaches a maximum value of 83.18% when the reaction time is 4 h. However, beyond the 4 h mark, the glucose yield exhibits a noticeable decline, which indicates that a longer reaction duration does not necessarily result in a higher yield for starch hydrogenation. Within the first 4 h, starch can fully interact with the catalyst, leading to optimal reaction efficiency. However, as the reaction time exceeds 4 h, side products are generated during the reaction process, causing a decline in glucose yield. Therefore, the optimal reaction time is preferred to be 4 h.

The agitation rate also plays a critical role; especially in the solid–liquid–gas tri-phase reaction system, the mass transfer should not be ignored given the limited diffusion. Figure 5f illustrates the relationship between stirring rates and the glucose yield from 520 r/min to 730 r/min under the same conditions as above, that is, at a 160 °C reaction temperature, a 4 h reaction time, 1.8 MPa H₂, a catalyst dosage of 2.25%, and a 100 g starch solution of 15 wt%. As depicted in Figure 5f, the glucose yield rises to reach the required value after the agitation rate is more than 630 r/min. However, the potential risk and energy consumption should also be considered in this work. Therefore, the optimal reaction conditions can be set at 630 r/min.

2.5. Reusability and Supporter Comparison

The reusability of CuO/Al₂O₃ was investigated with repeated use (five times) of the starch hydrogenation under the same reaction conditions as above, that is, at a 160 °C reaction temperature, a 4 h reaction time, 1.8 MPa H₂, a catalyst dosage of 2.25%, a 100 g starch solution of 15 wt%, and a 630 r/min agitation speed; the results are shown in Figure 6a. The 2.25 g catalyst amount used in the reusability studies remains unchanged, which minimizes the influence of catalyst losses that may occur in the ultrasonic washing of the catalyst. After each cycle, the catalyst was filtered, washed with deionized water, and dried at 105 °C for 24 h. The recovery rate of the catalyst was over 90% because of a careful recycling operation. As seen in Figure 6a, no significant loss in glucose yield can be observed up to five times, which suggests that the catalytic activity remains stable and that this type of catalyst possesses a certain level of stability and potential industrial value.

Moreover, some other supports, including active carbon, SiO2, and ZSM-5, were also selected to prepare the Cu catalysts in this work. The effect of catalysts prepared by these carriers on starch hydrogenation was investigated under optimized conditions (a 160 °C reaction temperature, a 4 h reaction time, 1.8 MPa H₂, a catalyst dosage of 2.25%, a 100 g starch solution of 15 wt%, and a 630 r/min agitation speed); the results are shown in Figure 6b. As exhibited in Figure 6b, under the same reaction conditions, Cu/Al₂O₃ was verified to be the best catalyst among all these different supporter catalysts.

3. Catalytic Mechanism Discussion

A simple Cu-based catalyst is a promising choice because of its high activity in the hydrogenolysis of C-O bonds, and it has already achieved high activity in the hydrogenolysis of glycerol [42]. Here, Cu⁰ is essential as an active species for high catalytic activity. Cu⁰ seems to be essential for high catalytic activity; however, the role of Cu⁺¹ cannot be excluded [32]. The active Cu⁺ species of the catalyst polarize the C-O bonds and increase the activity of the catalyst for the hydrogenation of the C-O bond [43].

Based on the discussions above, we propose a possible reaction pathway, as shown in Scheme 1. The hydrolysis of starch into glucose can be considered two parallel reactions. In the presence of H₂, CuO/Al₂O₃ is reduced and converted into the active form of metallic Cu⁰/Al₂O₃ and Cu₂O at high temperatures. The active Cu⁰ and Cu₂O synergistically catalyze the hydrolysis of the C-O bond in the α-1, 4-glycosidic linkage of long-chain polysaccharides in starch. Meanwhile, in the hydrogenation of starch using water as a solvent, hot water molecules at the reaction temperature can produce acid in situ to act as the H⁺ donor, which combines with the broken C-O bond to generate glucose [30]. In summary, the active Cu⁰ and Cu⁺ catalyze the hydrogenolysis of long-chain starch into low-molecular-weight glucose. The pH value of the reaction solution is acidic, and the theory of in situ acid production is valid because no additional acidic substances are added during the reaction. Therefore, under the synergistic effects of hydrothermal catalysis and Cu⁰ and Cu⁺ catalysis, starch is converted into glucose. Firstly, CuO is reduced to Cu⁰ and Cu₂O by H₂; then, it catalyzes the breaking cleavage of the C-O bond to generate glucose. At the same time, at the reaction temperature, hot water acts as the H⁺ donor in the process of in situ acid production, which combines with the broken C-O bond to hydrolyze into low-molecular-weight glucose.

4. Materials and Methods

4.1. Materials

Soluble starch (AR) was acquired from Beijing Aoboxing Biotechnology Co., Ltd. (Beijing, China). Cu(NO₃)₂.6H₂O (AR ≥ 99.5%) was purchased from Sinopharm Chemical Reagent (Shanghai, China). Boehmite (IR) was obtained from Dezhou Jinghuo Technology Co., Ltd. (Dezhou, China). Polyethylene glycol (PEG-4000) was purchased from Guangdong Guanghua Chemical Co., Ltd. (Guangdong, China). Deionized water was self-made, while aqueous ammonia (25 wt%) was obtained from National Pharmaceutical Group Chemical Reagent Co., Ltd. (Shanghai, China). Moreover, ultrahigh-purity N₂ (99.999%) and H₂ (99.999%) were both provided by Nanjing Special Gases (Nanjing, China). In addition, deionized water without carbon dioxide was used in all the experimental processes. The materials in this work were used without further purification.

4.2. Catalyst Preparation

The active component of this catalyst is non-precious metal copper. The preparation process of the catalyst involved first taking a certain amount of copper nitrate and preparing a 3 mol/L aqueous solution in a three-mouth flask. Then, 25% aqueous ammonia was added to form Cu (OH)₂ precipitate, and the pH was maintained around 10 at 50 °C, followed by aging for 6 h. After vacuum filtration and washing, the precipitate was dried and put aside. Next, 30% copper hydroxide, 55% Boehmite, 10% polyethylene glycol, and 5% deionized water were mixed evenly and then pressed into shape by a press machine. The molded solid was placed in an oven and dried at 110 °C for 10 h. Finally, the molded solid was calcined in a muffle furnace at 500 °C for 4 h to obtain CuO/Al₂O₃.

4.3. Catalyst Characterization

Textural properties were characterized through nitrogen sorption—desorption isotherms at −196 °C using a BK122W analyzer (JWGB Sci & Tech Ltd., Beijing, China). Before analysis, samples were degassed under vacuum for 6 h at 150 °C. The specific surface areas (SSAs) and porosity were achieved by calculations using the BET and BJH methods, respectively.

The morphology of the catalyst and the Ru NPs were measured with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Before testing, all of the samples were crushed into mesh-like powders via grinding in a crystal mortar. The SEM micrographs were acquired using a Hitachi S-4800 (Hitachi, Tokyo, Japan) electron microscope working at 200 kV.

The crystallinity of the samples after being crushed into mesh-like powders was determined by X-ray diffraction (XRD) patterns and recorded on a Rigaku Smartlab^TM 9 Kw diffractometer (Rigaku, Osaka, Japan) equipped with Cu Kα radiation (λ = 1.542 Å) operating at 40 kV and 100 mA in an angle range of 2θ = 5–60° with a step size of 0.03. To confirm the thermo-stability and CuO/γ-Al₂O₃ or Cu-based/γ- Al₂O₃ ratio of the resulting catalysts, an STA 449F3 thermogravimetric analyzer (TGA) (NETZSCH, Berlin, Germany) at a heating rate of 10 °C/min from 30 °C to 800 °C under N₂ flow was used in this work. Here especially, the samples were sealed with high-purity N₂ before being delivered for testing.

X-ray photoelectron spectroscopy (XPS) measurements of the crushed catalyst powders were performed with a K-Alpha spectrometer (Thermo Scientific, New York, NY, USA) with a monochromatic Al-Kα ray source under ultra-high vacuum conditions. In order to exclude external contamination, the samples were sealed with high-purity N₂ before being delivered for testing. Binding energy (BE) values were calibrated to the referenced C 1s peak (284.8 eV) during data processing of the XPS spectra. The peak decompositions assigned to the Cu 2p, Al 2p, and O 1s binding energy values were analyzed by applying mixed Gaussian–Lorentzian profiles and a Shirley nonlinear sigmoid-type baseline fitting of varying proportions (30–80%).

4.4. Catalytic Hydrogenation of Starch to Glucose

The hydrogenation of starch was performed in a 150 mL stainless autoclave under vigorous stirring. In a typical reaction with 2% as-catalyst, 0.6 mL of formic acid, and 100 g of 15 wt% starch solution, the reactor was purged with hydrogen four times to remove air. The reactor was heated in a temperature range of 90–120 °C and then kept at the preset hydrogen pressure from 1 to 4 MPa. Here, the as-prepared Cu/Al₂O₃ precursor in the batch reactor was reduced in situ. During hydrogenation, the initial stirring rate was kept at the desired value (530–750 r/min). After the reaction, the solid catalyst was separated, washed, and dried under vacuum-drying at 60 °C for the next run. The residue solution was immediately filtered with a needle–drum membrane filter of 0.45 μm and analyzed via high-performance liquid chromatography (HPLC) (Shimadzu, Tokyo Japan) with refractive index (RI) detection. The HPLC column applied in this work was a SUGAR SC-1011 column (4.6 nm × 150 mm, Shodex, Tokyo, Japan) at 80 °C with a flow of 1 mL/min using deionized water as the mobile phase. Glucose yield was calculated using Equation (1), as follows:

$$y_{Glucose}=\frac{{\left[\mathrm{G}\mathrm{l}\mathrm{u}\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{e}\right]}_{\mathrm{f}}}{\mathrm{m}}\times 100\%$$

(1)

where, [Glucose]_f represents the final quality of glucose as detected, and m represents the initial quality of starch.

5. Conclusions

In summary, a CuO/Al₂O₃ catalyst was prepared via the precipitation-tableting method. XRD and XPS characterization confirmed the in situ reduction of CuO/Al₂O₃ into Cu/Al₂O₃ and Cu₂O/Al₂O₃. The hydrogenation process of starch to glucose over a Cu-based catalyst was investigated. Optimal reaction conditions were determined as follows: a temperature of 160 °C, a pressure of 1.8 MPa, a 100 g starch solution of 15 wt%, a catalyst dosage of 2.25%, a reaction time of 4 h, and a rotational speed of 630 r/min. Under these optimized conditions, the glucose yield reached 83.16%. Notably, the catalyst exhibited stable performance, showing no significant decrease in glucose yield even after being reused for five consecutive cycles. Starch was converted into glucose through synergistic actions, including H⁺, Cu⁰, and Cu₂O catalysis. This work is expected to provide valuable insights into the design of catalyst and hydrogenation processes for efficient starch hydrogenation.