Production of 5-Ethoxymethylfurfural from glucose using bifunctional catalysts

Zhang, R., Chen, J., Song, X., Li, J., Xiong, J., Yu, Z., and Lu, X. (2025). "Production of 5-Ethoxymethylfurfural from glucose using bifunctional catalysts," BioResources 20(2), 2962–2978.

Abstract

5-Ethoxymethylfurfural (5-EMF) is a promising liquid fuel or fuel additive due to its high energy density and stability. The conversion of glucose to 5-EMF involves a three-step tandem reaction: isomerization, dehydration, and etherification. However, low catalytic efficiency in these steps has limited 5-EMF yields. To address this, a Fe/ZSM-5 bifunctional catalyst with both Brønsted and Lewis acid sites was developed and characterized using XRD, SEM, XPS, BET, Py-FTIR, and NH₃-TPD techniques. The catalyst’s performance in glucose conversion was systematically evaluated. Optimal conditions—20 wt% Fe loading, 180 °C reaction temperature, 10 h reaction time, and a catalyst-to-glucose mass ratio of 1:1—resulted in 97.1% glucose conversion and a 38.4% 5-EMF yield. Reaction kinetics followed a first-order model with an activation energy of 32.6 kJ/mol. The catalyst maintained over 94% glucose conversion after five cycles, demonstrating its stability. These findings underscore the potential of the Fe/ZSM-5 bifunctional catalyst for efficient glucose valorization to 5-EMF and provide key insights for process optimization.

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Production of 5-Ethoxymethylfurfural from Glucose Using Bifunctional Catalysts

Rui Zhang ,^a,* Jiale Chen,^a Xishang Song,^a Jiantao Li,^a Jian Xiong,^b Zhihao Yu,^c and Xuebin Lu ^b

DOI: 10.15376/biores.20.2.2962-2978

Keywords: Fe/ZSM-5 bifunctional catalyst; Isomerisation; Dehydration; Etherification; Glucose;
5-Ethoxymethylfurfural

Contact information: a: School of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China; b: School of Ecology and Environment, Tibet University, Lhasa 850000, China; c: School of Environmental Science and Engineering, Tianjin University, Tianjin 300350, China; *Corresponding author: rzhang@tcu.edu.cn

Graphical Abstract

INTRODUCTION

With the global depletion of fossil fuel reserves, the expansion of renewable energy sources has become an urgent priority. The efficient utilization of lignocellulosic biomass presents a promising alternative to fossil fuels. As a major agricultural nation, China generates substantial amounts of agricultural waste annually, which serves as a significant source of lignocellulosic biomass (Zhao et al. 2024). Harnessing this resource is vital for promoting the transition to a sustainable energy structure. Lignocellulosic biomass primarily consists of cellulose, hemicellulose, and lignin. Among these components, cellulose is a linear, high-molecular-weight polysaccharide composed of repeating (1-4)-β-linked D-glucose units connected via β-1,4-glycosidic bonds. Hydrolysis of cellulose specifically cleaves these β-1,4-glycosidic bonds, liberating individual D-glucose molecules. These glucose units can subsequently be converted into biomass-derived platform chemicals, including 5-hydroxymethylfurfural (5-HMF), levulinic acid (LA), and γ-valerolactone (GVL), through further catalytic transformations (Mo et al. 2024). Notably, 5-HMF can be etherified to produce 5-ethoxymethylfurfural (5-EMF), a biomass-derived compound with an energy density of 8.7 kWh/L, which is comparable to that of conventional gasoline (8.8 kWh/L) (Gawade et al. 2018). This makes 5-EMF a promising candidate for the development of next-generation biofuels, contributing to the diversification and sustainability of energy sources.

The development of efficient catalysts for the conversion of 5-HMF to 5-EMF is a critical challenge in this process. Sulfuric acid can be used as a catalyst to achieve a 5-EMF yield of 79.8% after 18 h of reaction at 70 °C (Yang et al. 2019). Che et al. (2012) employed H₄SiW₁₂O₄₀/MCM-41-loaded nanosphere catalysts with ethanol as the solvent, obtaining a 5-EMF yield of 77.2% after 4 h of reaction at 90 °C. These high yields were attributed to the effective design of the catalysts, where active components, such as metal oxides, were loaded onto zeolite carriers to enhance the yield and selectivity of the target products. Compared with zeolite-based solid acid catalysts, novel solid acid catalysts derived from carbonized biomass through sulfonation exhibit advantages such as improved biocompatibility and lower production costs (Rong et al. 2014). For instance, Yao et al. (2016) developed a sulfonated carbon-based solid acid catalyst, PCM-SO₃H-1, which facilitated the synthesis of 5-EMF from 5-HMF in a biphasic ethanol-dimethylsulfoxide system. Under optimized conditions (100 °C, 8 h), a maximum 5-EMF yield of 85.6% was achieved. While the direct synthesis of 5-EMF from 5-HMF achieves higher yields, the high cost of 5-HMF remains a significant barrier to the sustainable and cost-effective industrial production of 5-EMF. Therefore, developing more economical and efficient catalytic processes for the conversion of biomass-derived feedstocks to 5-EMF remains an area of active research.

Table 1. The Preparation of 5-EMF Catalyzed by Solid Acid Catalysts

The one-step conversion of glucose obtained from the hydrolysis of cellulose to 5-ethoxymethylfurfural (5-EMF) has become an important research priority. As summarized in Table 1, high yields of 5-EMF can be achieved when fructose is used as the reaction substrate. For instance, using a self-developed Glu-Fe₃O₄-SO₃H catalyst, researchers achieved an 81% yield of 5-EMF through a 24-hour catalytic reaction at 80 °C (Thombal et al. 2016). Similarly, Teeling et al. (2005) synthesized 5-EMF from fructose in an anhydrous phase using AlCl₃ as the catalyst, achieving a yield of 71.2%. Acidic ion-exchange resins, such as Amberlyst-15, are widely employed in esterification and etherification reactions due to the presence of functional groups such as sulfonic acid (-SO₃H) and carboxylic acid (-COOH). Zuo et al. (2018) reported the use of commercial Amberlyst-15 ion exchange resin as a catalyst for the alcoholysis of fructose and inulin, achieving 5-EMF yields of 77.3% and 65.2%, respectively. When Amberlyst-15 was modified with chromium (Cr), the yields of 5-EMF were determined to be 46.7% and 50.2% when catalyzing glucose and sucrose, respectively (Son et al. 2012; EI-Nassan 2021). Although the yield of 5-EMF from glucose was lower compared to fructose, the significantly lower cost of glucose makes it a more viable feedstock for large-scale industrial applications. Consequently, the synthesis of 5-EMF from glucose has become a major area of research interest. In the one-pot synthesis process, glucose is initially isomerized to fructose via Lewis acid catalysis. The fructose is subsequently dehydrated to produce 5-HMF, catalyzed by Brønsted acid, followed by the etherification of 5-HMF with ethanol to form 5-EMF, also catalyzed by Brønsted acid (Wang et al. 2021; Zhang et al. 2022). However, the catalytic activity of existing catalysts is often insufficient for each step of this multistep reaction. This limitation is primarily attributed to the inappropriate ratio of Brønsted acid and Lewis acid sites in the catalysts. Therefore, the development of bifunctional catalysts with an optimal balance of Brønsted and Lewis acid sites is essential to enhance the efficiency and selectivity of the one-pot synthesis of 5-EMF from glucose.

In this study, an Fe/ZSM-5 bifunctional catalyst was developed to facilitate glucose isomerization (catalyzed by Lewis acid), dehydration (catalyzed by Brønsted acid), and etherification (catalyzed by Brønsted acid). The catalyst was designed to achieve efficient conversion of glucose to 5-EMF through the modulation of Brønsted and Lewis acid sites. The catalytic performance of the prepared Fe/ZSM-5 bifunctional catalysts was systematically evaluated, and their microphysical and chemical structures were thoroughly characterized. Furthermore, the reaction kinetics of glucose conversion to 5-EMF over the Fe/ZSM-5 bifunctional catalysts was investigated. This study will provide valuable insights into the catalytic behavior of bifunctional catalysts and offer a basis for optimizing multiphase catalytic systems for the conversion of glucose to 5-EMF.

EXPERIMENTAL

Materials

Concentrated sulphuric acid (analytically pure) was purchased from Tianjin Ruimingwei Chemical Co. Ltd, China. Analytically pure activated carbon, H-β zeolite, sodium hydroxide (NaOH), ferric chloride (FeCl₃), zirconium trichloride (ZrCl₃), copper chloride (CuCl₂), chromium trichloride (CrCl₃), ZSM-5 zeolite, guaranteed reagent of ethanol, dextrose, 5-EMF (99.9%), and phenolphthalein (99%) were purchased from Tianjin Biotest Technology Development Co. Ltd, China.

Catalyst Preparation

Four metal-loaded bifunctional catalysts (Zr, Cu, Fe, and Cr) supported on ZSM-5 molecular sieves were prepared separately using the wet impregnation method (Hoang et al. 2022). In this process, metal ions in an aqueous metal salt solution were mixed with a specific mass ratio of ZSM-5 zeolite. The mixture was then sonicated for 1 h and stirred at room temperature for 3 h. After impregnation, the mixture was dried at 105 °C for 12 h and ground into a fine powder. The resulting catalyst was then calcined in air at 550 °C for 6 h with a linear temperature ramp of 2 °C/min. The final product was metal/ZSM-5 bifunctional catalyst containing both Brønsted and Lewis acid sites.

Characterisation of Catalysts

The crystalline phase of the samples was characterised by X-ray diffraction (XRD, Rigaku Smartlab 9KW, Japan) using a Rigaku Smartlab 9 KW, and analysed using MDI Jade 6.0.

Sample morphology was photographed using a high-resolution field emission scanning electron microscope (SEM, TESCAN MIRA LMS, Czech Republic) equipped with a Schottky field emission electron gun.

For quantitative and valence analysis of the elemental composition of the materials, X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, United States of America) was used to measure the inner electron binding energy of the atoms and their chemical shifts.

Fourier Transform Infrared Spectroscopy (FTIR, Niocolet is10, United States of America) test was used to characterize the molecular mechanism and chemical composition of materials. N₂ adsorption desorption experiments were performed on the catalysts using a TriStar II 30200 adsorption meter (BET, TriStar II 3020, United States of America).

The surface acid content of the catalysts were determined using a MicrotracBELCatII chemisorbentimeter (NH₃-TPD, Microtrac BELCat II, Japan).

Pyridine infrared spectroscopy (Bruker Tensor 27, German) was utilised to differentiate the acidity (Brønsted acid, Lewis acid) of the molecular sieve samples.

Catalytic Reaction

A reaction mixture consisting of 10 g/L glucose and a specific amount of catalyst was added to a 50 mL anhydrous ethanol solution in a flange mechanical stirring reactor. The procedure was to connect 99.99% N₂ in the inlet channel of the reactor, open the outlet channel of the reactor, pass high-purity nitrogen into the reactor uniformly, and maintain it for 15 min in order to exhaust the air in the reactor, and then close the outlet channel of the reactor until the pressure in the kettle reaches 0.1MPa to close the inlet channel of the reactor to begin the warming reaction. The catalytic conversion of glucose was then initiated at a reaction temperature of 140 °C and a reaction time of 5 min. Upon completion of the reaction, the reactor was immersed in cold water to cool. The products were filtered, diluted, and analyzed using high-performance liquid chromatography (HPLC, LC-20A, Japan) equipped with a refractive index detector. The analysis was performed using a BioRad Aminex HPX-87H column (300 × 7.8 mm) at 65 °C, with 5 mM H₂SO₄ as the mobile phase at a flow rate of 0.6 mL/min.

RESULTS AND DISCUSSION

Screening of Loaded Metal Catalysts

In the conversion of glucose to synthesize 5-EMF, glucose is first isomerized to fructose; this fructose is then dehydrated to produce 5-HMF, which is subsequently etherified to yield 5-EMF. The isomerization of glucose to fructose, however, is highly challenging under non-catalytic conditions (Dharmapriya and Huang 2024). Overcoming this critical step is essential to ensure efficient progression of the subsequent dehydration and etherification reactions. Without a catalyst, side reactions are more likely to dominate, competing with the desired pathway and thereby reducing the yield of 5-EMF. Therefore, in the present study, metal-loaded bifunctional catalysts containing both Brønsted and Lewis acid sites were prepared by loading four metals (Zr, Cu, Fe, and Cr) onto ZSM-5 molecular sieves using the wet impregnation method. The catalytic performance of these catalysts was evaluated through the conversion of glucose too. As shown in Fig. 1, all four metal-loaded bifunctional catalysts successfully catalyzed the glucose conversion reaction, with glucose conversion yields exceeding 90%. Among them, the Fe/ZSM-5 bifunctional catalyst exhibited the best catalytic performance, with a 5-EMF yield of 30.8%. The Cu/ZSM-5, Zr/ZSM-5, and Cr/ZSM-5 bifunctional catalysts also promoted the synthesis of 5-EMF from glucose, but their catalytic activities were lower than that of Fe/ZSM-5 bifunctional catalyst. The lower activity of these three catalysts may be attributed to their inability to provide an adequate number of suitable Brønsted and Lewis acid sites, as well as an unfavorable distribution of these acid sites on the ZSM-5 molecular sieves. Based on these findings, the Fe/ZSM-5 bifunctional catalyst was selected for the catalytic conversion of glucose to 5-EMF in the subsequent study.

Fig. 1. Effect of different metal-loaded bifunctional catalysts on 5-EMF catalytic properties (glucose conversion, 5-EMF yield, 5-EMF selectivity) for glucose synthesis

Fig. 2. Effect of Fe loading on the yield of 5-EMF (glucose 10 g/L, catalyst 10 g/L, reaction time 6 h, reaction temperature 180 °C)

Modulation of Catalytic Performance of Fe/ZSM-5 Bifunctional Catalysts

The ratio of Brønsted acid and Lewis acid sites on Fe/ZSM-5 bifunctional catalysts plays a crucial role in the isomerization of glucose to fructose, the dehydration of fructose to biomass-derived macromolecules, and the subsequent etherification reaction. This ratio can be effectively tuned by adjusting the metal loading on the ZSM-5 molecular sieves (Dutta et al. 2012). As shown in Fig. 2, the yield of 5-EMF initially increased and then decreased with increasing Fe loading, indicating that an optimal Fe loading is beneficial for glucose conversion. The best catalytic performance was observed when the Fe loading reached 20 wt.%, resulting in a 5-EMF yield of 34.4%. At this point, the ratio and number of Brønsted and Lewis acid sites on the Fe/ZSM-5 bifunctional catalyst were optimized (Liu et al. 2012; Jia et al. 2013).

The reaction temperature and reaction time are critical factors influencing the reaction rate and product distribution during glucose conversion. As shown in Fig. 3, within the temperature range of 140 to 220 °C, the glucose conversion rate steadily increased with temperature. When the temperature reached 180 °C, the yield of 5-EMF reached its maximum of 34.4%. However, although the glucose conversion rate continued to increase, the yield of 5-EMF began to decrease once the temperature exceeded 180 °C. This suggests that while moderate high temperatures promote glucose conversion to 5-EMF, excessively high temperatures lead to the formation of by-products that reduce the yield of 5-EMF. Therefore, 180 °C was selected as the optimal reaction temperature for further experiments.

Fig. 3. Effect of reaction temperature on the yield of 5-EMF (glucose 10 g/L, catalyst 10 g/L, reaction time 6 h)

As shown in Fig. 4, the yield of 5-EMF exhibited an initial increase followed by a decrease within the reaction time range of 6 to 14 h. The yield of 5-EMF peaked at 38.2% when the reaction time reached 10 h. Beyond this point, the formation of by-products negatively impacted the yield of the target product, as reflected in the trend of 5-EMF selectivity. Based on these results, 10 h was determined to be the optimal reaction time.

The catalyst dosage is a crucial factor in evaluating the catalytic performance. As shown in Fig. 5, the glucose conversion steadily increased with the increase of Fe/ZSM-5 bifunctional catalyst dosage. However, the yield of 5-EMF peaked at a catalyst dosage of 0.5 g. Further increases in catalyst dosage resulted in a decrease in the 5-EMF yield, rather than an increase. This decrease may be due to the excess catalyst, which may have converted the 5-EMF into by-products, thus reducing the yield of the target product. Thus, 0.5 g of Fe/ZSM-5 bifunctional catalyst was selected as the optimal catalyst dosage.

Fig. 4. Effect of reaction time on the yield of 5-EMF (glucose 10 g/L, catalyst 10 g/L, reaction temperature 180 °C)

Fig. 5. Effect of catalyst dosage on the yield of 5-EMF (glucose 10 g/L, reaction temperature 180 °C, reaction time 10 h

Fig. 6. X-ray diffraction (XRD) characterisation image of Fe/ZSM-5 bifunctional catalyst

Characterization of Fe/ZSM-5 Bifunctional Catalysts

The XRD characterization results (Fig. 6) indicate that the Fe/ZSM-5 bifunctional catalysts exhibited a series of peaks between 14° and 30° and around 45°, confirming that they retained the complete ZSM-5 (MFI) structure. Additionally, distinct peaks at approximately 24.2°, 33.2°, 35.7°, and 54.1° in the XRD pattern of the Fe/ZSM-5 bifunctional catalysts suggest that Fe₃O₄ was successfully loaded onto the ZSM-5 zeolite without disrupting its structure (Chen et al. 2020).

Further characterization of the Fe/ZSM-5 bifunctional catalysts was conducted using XPS to analyze elemental composition and chemical morphology. As shown in Fig. 7(a), the total energy spectrum displayed five characteristic peaks corresponding to Fe 2p, O 1s, C 1s, Si 2p, and Al 2p. The O 1s spectrum exhibited three sub-peaks between binding energies of 526.70 and 534.90 eV, corresponding to C-O bonds, C=O bonds, and a satellite peak, which is consistent with the fine mapping of the C1s spectrum. The fine mapping of Fe2p (1/2, 3/2) revealed two characteristic peaks near binding energies of 712 and 725.03 eV, indicating that the iron in the catalysts predominantly was present in the forms of Fe²⁺ and Fe³⁺.

Fig. 7. X-ray photoelectron spectroscopy (XPS) characterisation image of Fe/ZSM-5 bifunctional catalyst

In this study, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses were performed to examine the dispersion morphology and surface structure of the elements on the Fe/ZSM-5 bifunctional catalysts with varying metal loadings. As shown in Fig. 8, the loading of FeO did not significantly alter the surface properties of the carrier, and the crystal structure of ZSM-5 remained intact. However, agglomeration of metal ions was observed, which may lead to a decrease in surface area (Liu et al. 2018). The energy spectra (Fig. 8) revealed that the Fe elements were uniformly distributed on the catalyst surface. The best dispersion was observed at a 20 wt% Fe loading, which aligns with the experimental results. Fourier Transform Infrared Spectroscopy (FTIR) was employed to identify and analyze the chemical bonds and functional groups present on the Fe/ZSM-5 bifunctional catalysts. The results (Fig. 9) showed vibrational peaks for surface hydroxyl groups and water molecules that were near 1630 cm^-1. Symmetric stretching absorption peaks for Al-O and Si-O chemical bonds appeared in the range of 450 to 800 cm^-1 (Alam et al. 2018). A peak at 700 cm^-1 was attributed to the Fe-O bond vibration (Wen et al. 2018; Kumar et al. 2019). Additionally, the peaks around 1100 cm^-1 corresponded to the asymmetric stretching of Si-O-Al, Si-O-Si, and Al-O-Al bonds. A double-frequency peak at 1220 cm^-1 was identified as the characteristic absorption of the five-membered ring of the ZSM-5 system (Yao et al. 2016). Overall, these findings further confirm that Fe was successfully loaded onto ZSM-5 without compromising its structural integrity.

Fig. 8. Scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) characterisation images (a-e, SEM. f-j, EDS) of Fe/ZSM-5 bifunctional catalyst. (a) 5 wt%, (b) 10 wt%, (c) 15 wt%, (d) 20 wt%, (e) 25 wt%

Fig. 9. FT-IR characterisation of Fe/ZSM-5 bifunctional catalyst

The N₂ adsorption-desorption isotherms are presented in Fig. 10. The results indicate that the isotherms of the Fe/ZSM-5 bifunctional catalysts with different Fe loadings all exhibited typical type IV adsorption curves. This behavior is indicative of nitrogen condensation during the medium pressure stage, which appeared as a H3-type hysteresis loop due to capillary condensation within the pores (Kumar et al. 2019). This suggests that all catalyst carriers possessed the characteristic structural features of slit-shaped mesoporous materials. The pore size distributions of the samples were obtained through BJH analysis of the adsorption data (Fig. 10(b)). As shown in Table 2, an increase in Fe loading led to a decrease in both the specific surface area and pore size of the Fe/ZSM-5 bifunctional catalysts. In light of the catalytic performance data from the previous section, the optimal catalyst performance for 5-EMF yield was observed at a 20 wt.% Fe loading.

Fig. 10. Nitrogen adsorption desorption characterisation image of Fe/ZSM-5 bifunctional catalyst

Table 2. Specific Surface Area of Fe/ZSM-5 Bifunctional Catalysts with Different Loadings

The results from ammonia temperature-programmed desorption (NH₃-TPD) analysis (Fig. 11) show that the 20 wt% Fe/ZSM-5 bifunctional catalyst contained two types of acidic sites with varying acid strengths.

Fig. 11. Characterisation of ammonia programmed temperature rise desorption (NH₃-TPD)

The adsorption and desorption of NH₃ from the weakly acidic sites occurred within the temperature range of 110 to 250 °C, while the strongly acidic sites were observed in the range of 370 to 450 °C (Yu et al. 2024). Based on the ratio of the peak areas, the concentration ratio of weak to strong acidic sites on the Fe/ZSM-5 bifunctional catalyst surface was found to be 17:1. From the IR characterization, it can be inferred that the weak acidic sites on the catalyst surface are likely derived from -OH and -COOH groups on the catalyst support, while the strong acidic sites are attributed to the Lewis acid sites provided by the Fe species loaded onto the ZSM-5 zeolite. The total acidity of the Fe/ZSM-5 bifunctional catalysts was determined to be 0.87 mmol/g.

Fig. 12. Py-IR characterisation of Fe/ZSM-5 bifunctional catalyst

Figure 12 shows the Py-IR spectra of the Fe/ZSM-5 bifunctional catalysts at temperatures of 180, 260, and 340 °C. The pyridine adsorption peaks corresponding to the Lewis acid and Brønsted acid sites appeared around 1445 and 1545 cm^-1, respectively, while the peak near 1490 cm^-1 resulted from the combination of both sites (Kumari et al. 2019). The acidic sites of the Fe/ZSM-5 bifunctional catalysts were primarily attributed to the Fe component within the framework, whereas Brønsted acidity was exclusively provided by the bridging hydroxyl group of Si-O-Fe (Trachta et al. 2022). The acid amount and distribution of the zeolite catalysts were calculated by combining the NH₃-TPD and Py-IR characterization results (Table 3). The total acid amount of the Fe/ZSM-5 bifunctional catalyst decreased with increasing pyridine desorption temperature, indicating that the catalyst lost a portion of its acidic sites at higher temperatures.

Table 3. Acid Density of Fe/ZSM-5 Bifunctional Catalyst

Kinetic Analysis of Glucose Conversion to 5-EMF

Glucose dehydration reaction is a pseudo-first-order reaction, dependent on the concentrations of the reactants (Bounoukta et al. 2021). If the glucose dehydration reaction follows pseudo-first-order kinetics, a plot of ln(C₀/C) versus reaction time would be linear, with the slope equal to the negative value of the reaction rate constant (K) (Wang et al. 2015). To elucidate the reaction process, a simplified kinetic model for the macroscopic glucose alcoholysis reaction was proposed, as shown in Fig. 13.

Fig. 13. Reaction kinetics model

The reaction process consisted of two main steps: (1) the conversion and synthesis of glucose to 5-EMF, and (2) the conversion of glucose into by-products. The glucose conversion and synthesis to 5-EMF was simplified into a single-stage reaction, and the corresponding differential equation was derived, as follows,

where C_G is the concentration of glucose feedstock (g/L) and the initial concentration is denoted as C_G0; C_5-EMF is the concentration of 5-EMF (g/L); Y_5-EMF is the yield of 5-EMF (mol%); k₁ and k₂ are the reaction rate constants for the formation of by-products and 5-EMF, respectively (h^-1); k is the rate constant for the degradation of glucose; and t is the reaction time.

Table 4. Reaction Rate Constants at Different Temperatures

The yield of 5-EMF (Y_5-EMF) obtained at different temperatures was substituted into Eq. 5, and the data were analyzed using MATLAB. The resulting kinetic parameters are presented in Table 4. The value of the rate constant (k) increased with the rise of temperature, indicating that higher temperatures accelerated the formation of 5-EMF. This suggests that the reaction rate is enhanced at elevated temperatures, which is consistent with the results obtained from the temperature-dependent experiments. Additionally, the R² values of the fitted curves for the reaction kinetics at various temperatures, as shown in Fig. 14, further confirm that glucose dehydration follows first-order kinetics with a good fit.

Fig. 14. Reaction kinetic curves and activation energy for glucose dehydration

The Arrhenius formula reveals the relationship between the rate of reaction and temperature in an exponential form (Eq. 6) and a logarithmic form (Eq. 7).

Glucose dehydration is more consistent with a first-order kinetic reaction, and the activation energy curves were plotted and fitted with 1/RT as the horizontal coordinate and -lnk as the vertical coordinate. The activation energy E_a of the reaction with the participation of the Fe/ZSM-5 bifunctional catalyst was calculated to be 32.6 kJ/mol, with a pre-exponential factor A = 5620 min^-1. The E_a values obtained in this study for the synthesis of 5-EMF from glucose catalyzed by the Fe/ZSM-5 bifunctional catalyst were lower than those values reported in the literature (64.2 to 73.2 kJ/mol) (Hsiao et al. 2021). This indicated that the Fe/ZSM-5 bifunctional catalyst is favorable for the conversion of glucose to 5-EMF.

Reusability of Fe/ZSM-5 Bifunctional Catalyst

To investigate the stability and reusability of Fe/ZSM-5 bifunctional catalysts, catalyst reuse experiments were conducted. In the ethanol reaction system, the catalytic glucose conversion reaction was carried out at 180 °C for 10 h. As shown in Fig. 15, after five cycles, the catalytic performance for glucose conversion remained stable, with conversion rates consistently above 94%. The yield of 5-EMF remained stable during the first four cycles, reaching 22.8% in the fifth cycle.

Fig. 15. Reusability of Fe/ZSM-5 bifunctional catalyst

Others

This study focused on the preparation of 5-EMF using pure glucose as the starting point for the reaction. Further challenges would need to be faced when cellulose is taken as the starting point, including a saccharification process to prepare crude glucose. The complexity of the substrate composition, the regulation of the reaction pathway, and other challenges should be considered when preparing 5-EMF from the glucose-rich crude fraction as the starting point. The glucose-rich crude fraction may contain lignin derivatives, organic acids, phenolic compounds, etc., and the presence of these substances may affect the selectivity and yield of the reaction target. In addition, the presence of impurities is likely to cause side reactions, leading to a decrease in the yield of the target product. Future research, involving glucose-rich crude products, will be needed.

CONCLUSIONS

The optimum Fe/ZSM-5 bifunctional zeolite catalyst, with Fe loading 20 wt%, was determined through experimental optimization. Microstructural and chemical characterization revealed that the wet impregnation method did not alter the original structure of the ZSM-5 molecular sieves, and the Fe in the catalyst primarily was present in the forms of Fe²⁺ and Fe³⁺. The yield of 5-EMF reached 38.2% with the optimal Fe/ZSM-5 bifunctional catalyst. The ratio of weak to strong acid sites on the catalyst surface was 17:1, indicating that the weak acid sites played a dominant role in the reaction.
The reaction kinetics of glucose dehydration followed a first-order kinetic model, and the addition of the catalyst significantly lowered the activation energy of the reaction. The activation energy for the Fe/ZSM-5 bifunctional catalyst in the reaction (32.6 kJ/mol) was lower than those values reported in the literature, indicating that the Fe/ZSM-5 bifunctional catalyst was favorable for catalyzing the conversion of glucose to 5-EMF.
The Fe/ZSM-5 bifunctional catalyst was easy to recycle and had catalytic activity with some effect. After five cycles, the conversion of glucose was always maintained above 94%, and the 5-EMF yield was maintained at 22.84%, which indicates the direction to optimize the Fe/ZSM-5 bifunctional catalyst in the future.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant No. 51908400, 52066017, 51876180, 22308253), Tianjin Natural Science Foundation key project (23JCZDJC00430).

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Article submitted: December 28, 2024; Peer review completed: February 8, 2025; Revised version received: February 15, 2025; Accepted: February 21, 2025; Published: March 3, 2025,

DOI: 10.15376/biores.20.2.2962-2978