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. 2024 Mar 13;9(12):14375–14380. doi: 10.1021/acsomega.3c10475

Prepared Sulfonic-Acid-Based Ionic Liquid for Catalytic Conversion of Furfuryl Alcohol to Ethyl Levulinate

Aiyun Hu †,‡,*, Shan Li †, Sheng Zhang †, Haijun Wang ‡
PMCID: PMC10975587  PMID: 38559984

Abstract

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Efficient utilization of Brønsted acids has been advanced through the synthesis of a novel pyridinium propyl sulfonic acid ionic liquid catalyst, [PSna][HSO4]. Employing niacin and 1,3-propanesulfonic lactone, the synthesis aimed to achieve a catalyst that combines atom-efficiency with stability. Optimal catalytic activity was demonstrated at a temperature of 110 °C over a 2 h reaction time, resulting in a furfuryl alcohol conversion and ethyl levulinate yield of 97.79% and 96.10%, respectively. Notably, the extraction and recovery of [PSna][HSO4] exhibited commendable repeatability with up to five cycles, maintaining furfuryl alcohol conversion and ethyl levulinate yield at 93.74% and 88.17%, which highlights the catalyst’s durability. Density flooding theory (DFT) calculations were employed to determine the most probable reaction pathways and identify all possible transition states and the reaction energy barriers overcome at each step of the reaction.

1. Introduction

Biofuels derived from biomass offer sustainable alternatives to fossil fuels for transportation and chemical production.1−3 They are carbon-neutral, as the emitted carbon dioxide is absorbed by plants.4 Biomass is widely available, renewable, and recyclable, making it an attractive energy source.5−7 Ethyl levulinate (EL), derived from biomass, has desirable properties and finds applications in biofuels, fuel additives, pharmaceutical intermediates, and green solvents.8,9 However, traditional catalysts for EL production can cause pollution, equipment corrosion, and separation issues.10,11 Therefore, the selection of catalysts for EL synthesis should consider environmental and economic factors, aiming for ease of separation, eco-friendliness, stability, high activity, and cost-effectiveness.12,13 In industry, furfuryl alcohol (FA) obtained directly from pentose hydrolysis is commonly used as a raw material for producing EL.14−16 This approach not only saves resources but also simplifies the purification process for the final products.17,18

The conversion of FA alcohols to EL requires Brønsted acid catalysts (e.g., H2SO4 and HF), but high yields cause pollution, making them unsuitable for industry.19 Guo et al. synthesized solid acid ArSO3H-HMCSs, with higher catalytic activity than other carbon-based materials.20 Nis and Kaya Ozsel studied the conversion of corn stover, sorghum, and cellulose into high-value chemicals using an ionic liquid-water medium, optimizing parameters for a maximum conversion rate and product selectivity.21 Gonzalez Maldonado et al. found that protonated FA reacts more readily with ethanol than neutral FA, indicating an easier addition reaction.22

Ionic liquids offer advantages like high boiling point, low volatility, and thermal stability.23 They also exhibit good catalytic activity and stability in biomass reactions.13,24 Metal chlorides are used as cocatalysts, and acidic ionic liquids combined with metal chlorides show excellent catalytic ability in biomass degradation.25 Researchers have studied acidic ionic liquids and developed new ones to replace traditional catalysts.26,27 Introducing sulfonic acid groups enhances the acidity of ionic liquids.28

In this study, a [PSna][HSO4] ionic liquid catalyst was synthesized for converting FA to EL. [PSna] is based on nicotinic acid, with a pyridine ring serving as the cation. The cation is connected to a propanesulfonic acid group through a reaction with 1,3-propane sultone, enhancing the catalyst’s acidity. The resulting nicotinic acid propanesulfonic acid is combined with H2SO4 to obtain [PSna][HSO4]. Acidic ionic liquids play a crucial role in the conversion of various substances, both biological and chemical.

2. Experimental Section

2.1. Materials

Nicotinic acid (Na), 1,3-propane sultone, ethanol, and sulfuric acid (98%) were purchased from China Pharmaceutical Reagents Co, Ltd. and used without further purification.

2.2. Preparation of Catalyst [PSna][HSO4]

To synthesize niacin propyl sulfonic acid ionic liquid ([PSna][HSO4]), niacin (NA) was dissolved in ethanol and heated at 60 °C until fully dissolved. 1,3-Propanesulfonic acid lactone was then added, and the mixture was reacted at 60 °C for 12 h to obtain a white precipitate called PSna. The solid precipitate was collected, washed with solvents, and dried in a vacuum drying oven to yield solid PSna with a 95.8% yield.

For the synthesis of [PSna][HSO4], dried PSna was dissolved in water, and concentrated sulfuric acid was added. The reaction was carried out at 50 °C for 6 h. After completion, the water in the reaction system was removed by rotary evaporation, resulting in a brownish-yellow viscous ionic liquid. The viscous ionic liquid [PSna][HSO4] was further dried in a vacuum drying oven to ensure minimal moisture content in the final product. The synthesis route is shown in Figure 1.

Figure 1.

Figure 1

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Synthesis route of [PSna][HSO4].

2.3. Alcoholysis of FA to EL

In a stainless steel reaction kettle, [PSna][HSO4], FA, and ethanol were mixed together. The kettle was then placed in a preheated oil bath with stirring. After a certain reaction time, the kettle was removed and rapidly cooled to room temperature. The catalyst was extracted using ethyl acetate, and the concentrations of the reactant and product were analyzed using gas chromatography (GC). The FA conversion and EL yield were calculated using specific equations.

2.3. 1
2.3. 2

After the reaction was complete, the reaction mixture was collected, and the final reaction solution was rotary evaporated to remove ethanol and other low-boiling substances. Then, it was extracted three times using ethyl acetate. The remaining residue was placed in a vacuum drying oven at 60 °C for 12 h to undergo vacuum drying.

2.4. Characterization of [PSna][HSO4] Catalyst

FT-IR spectra of the synthesized material were obtained using a Bruker FT-IR spectrometer.29 Samples were prepared using the KBr pellet method and scanned on a Nicolet 360 FTIR spectrometer, covering a wavenumber range of 500–4000 cm–1.

Thermogravimetric analysis (TGA) was conducted using a Mettler-Toledo TGA/DSC3+ analyzer under a nitrogen gas flow of 50 mL/min. The testing was performed within a temperature range of 50 to 800 °C, with a heating rate of 10 °C/min.30

Nuclear magnetic resonance hydrogen spectrum characterization was conducted on the four synthesized sulfonic acid ionic liquids using a Bruker 600, AVANCE III instrument.31 D2O was used as the reagent. The results of the nuclear magnetic characterization included chemical shifts and peak counts for different hydrogen positions.

3. Results and Discussions

The synthesized ionic liquid [PSna][HSO4] underwent FT-IR analysis (Figure 2a). Peaks in the range of 500–4000 cm–1 indicated carboxyl groups in nicotinic acid. Peaks at 1450 cm–1, 1640 cm–1, 3070 cm–1, and 3140 cm–1 corresponded to C=N, C=C, and C–H vibrations in the pyridine ring of nicotinic acid. Peaks at 1057 and 1014 cm–1 represented symmetric and asymmetric stretching vibrations of O=S=O and −SO32–, respectively, confirming a nicotinic acid and 1,3-propane sultone presence. An absorption band at 805 cm–1 was denoted as a S–O stretching vibration, while peaks at 1020 and 1218 cm–1 indicated the stretching vibrations of the sulfonic group and SO42–. These results confirm the successful synthesis of the desired ionic liquid by reacting the amphoteric compound with H2SO4.

Figure 2.

Figure 2

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(a) FT-IR spectra of PSna and [PSna][HSO4]. (b) TG spectra of [PSna][HSO4].

The thermal stability of [PSna][HSO4] ionic liquid was analyzed using thermogravimetric analysis (TGA). Results in Figure 2b show the decomposition temperature. Within the 40–200 °C range, [PSna][HSO4] remains relatively stable. However, temperatures exceeding 250 °C cause rapid decomposition and collapse of its structure. Therefore, the ionic liquid is stable at temperatures below 200 °C and can be used as a catalyst for synthesizing EL from FA.

The synthesized ionic liquid was characterized using 1H NMR spectroscopy to confirm its structure and verify its desired composition. The analysis of the NMR spectra revealed the number of hydrogen peaks and their corresponding chemical shifts. Based on spectral analysis, the synthesized product was identified as the intended ionic liquid. The 1H NMR analysis results are as follows: 9.29 (q, J = 2.2 Hz, 1H), 8.92–8.82 (m, 2H), 8.05–8.01 (m, 1H), 4.65 (t, J = 7.6 Hz, 2H), 2.80 (t, J = 7.3 Hz, 2H), and 2.40–2.11 (m, 2H).

Hydrolysis of FA requires a Brønsted acid catalyst. The acidity of ionic liquid was assessed using the Hammett method. A solution of nitroaniline indicator in water was prepared, and its absorption at λmax = 380 nm was measured. Solutions with different concentrations of para-nitroaniline and [PSna][HSO4] were prepared. Protonation ability was determined by measuring absorbance changes. The acidity constant (H0) of the ionic liquid was calculated using H0 = pK(I)aq + log([I]/[IH+]), where pK(I)aq is the pKa value of nitroaniline. H0 values are presented in Table 1, with the pKa of para-nitroaniline as 0.99.

Table 1. H0 Values of [PSna][HSO4] in Water.

ionic liquid absorbance [I] (%) [IH+] (%) H0
none 1.41 100 0  
[PSna][HSO4] 0.60 42.51 57.49 0.86
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3.2. Catalytic Performance Research

3.2.1. The Effect of Catalyst Type

The acidic IL [PSna][HSO4] catalyzes the conversion of FA to EL (Table 2). Using ethanol as solvent and hydrogen bond donor, FA conversion was negligible at 110 °C without additional catalysts. However, when the zwitterionic compound PSna was added as a catalyst, FA to EL conversion was observed. After 2 h at 110 °C, the FA conversion rate was 54.36% with a yield of 42.13% EL. The presence of sulfonic acid groups in PSna imparts acidity, catalyzing the FA to EL conversion, leading to enhanced catalytic activity.

Table 2. Catalytic Performance of Different Catalystsa.
entry catalysts CFA (%) YEL (%) SEL (%)
1 none 0 0 0
2 PSna 54.36% 42.13% 77.50%
3 [PSna][HSO4] 97.79% 96.10% 98.27%
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a

Reaction condition: FA (0.098 g, 1 mmol), catalyst (0.05 g, 51%), ethanol (10 mL).

The addition of [PSna][HSO4] as a catalyst greatly increased FA conversion. After a 2 h reaction at 110 °C, FA conversion reached 97.79% with a yield of 96.10% EL. These results demonstrate the high efficiency of the acidic ionic liquid, achieving remarkable conversion rates and yields even under mild conditions. It highlights the importance of incorporating Brønsted acid sites, which enhance the overall reaction efficiency.

3.2.2. The Effect of Catalyst Dosage

The impact of varying the [PSna][HSO4] catalyst amount (0.025 to 0.100 g) on reaction outcomes was investigated. Results in Figure 3 show that increasing the catalyst amount from 0.025 to 0.050 g improved FA conversion rates (88.73% to 97.79%) and EL yields (85.94% to 96.10%) at 110 °C for 2 h. This enhancement is attributed to increased availability of acid catalytic sites, facilitating better contact between FA and those sites. Further increasing the catalyst amount led to stable FA conversion rates but a slight decline in EL yield. This is due to increased viscosity and excess acid causing polymerization of FA and formation of byproducts, reducing the desired EL. Optimal catalyst amount is determined as 0.050 g.

Figure 3.

Figure 3

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Influence of the catalyst [PSna][HSO4] dosage on the alcoholysis reaction of FA to EL.

3.2.3. The Influence of Reaction Temperature and Time on the Catalytic Reaction

The effect of reaction temperature and time on the catalytic results of [PSna][HSO4] was studied. Figure 4 shows that temperature had a significant influence on the reaction outcomes. Increasing the temperature from 90 to 110 °C, while maintaining the same reaction time, resulted in higher FA conversion rates and EL yields. At 110 °C for 2 h, FA conversion reached 97.79% with a yield of 96.10%. Higher temperatures within a certain range promoted the hydrolysis reaction by reducing the viscosity of the ionic liquid and enhancing molecular collisions. Raising the temperature to 120 °C with a reaction time of 1.5 h yielded similar conversion rates and yields as 110 °C for 2 h. Prolonging the reaction time at 120 °C slightly increased FA conversion and EL yield. Considering all factors, the optimal conditions for preparing EL from FA using 0.050 g [PSna][HSO4] were a reaction temperature of 110 °C and a reaction time of 2 h (Figure 4).

Figure 4.

Figure 4

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Influence of different reaction temperatures and times on (a) FA conversion and (b) EL yield.

3.2.4. Catalyst Repeatability

Catalyst recycling is important, but centrifugation is not suitable for ionic liquids. Instead, extraction methods using ethyl acetate are used. After each cycle, the reaction liquid was collected and evaporated to remove low-boiling solvents, and then ethyl acetate was added to separate the phases. The catalytic activity of [PSna][HSO4] slightly decreased after five cycles. The yield of EL reduced from 96.10% to 88.17% (Figure 5) due to the loss of active sites and byproduct accumulation. Therefore, the catalytic performance remained relatively stable.

Figure 5.

Figure 5

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Recycle experiment of [PSna][HSO4].

3.3. Study of Reaction Mechanism

Preliminary findings suggest that FA can follow two possible reaction pathways in the presence of acidic catalysts (Figure 6). The reaction barriers and transition states for these pathways were identified through theoretical calculations. The hydroxyl group in FA reacts with H+ to form intermediate product A and water. In the first pathway, intermediate A reacts with ethanol to produce furfuryl ether. Then, under the action of H+, a ring-opening reaction occurs to form the intermediate product B (2-hydroxy-6-methoxy-3-octene), followed by a hydrogen transfer, resulting in the formation of the acidic EL product. In the second pathway, ethanol reacts directly with intermediate A, causing the opening of the furan ring to form the intermediate product B. Subsequently, under the catalysis of H+, it undergoes rearrangement and finally forms the acidic EL product. Comparing the barriers, the first pathway requires a higher barrier for the conversion of ethyl furfuryl ether. Both pathways eventually result in the opening of the furan ring. The low reaction barriers indicate the spontaneous occurrence of these pathways. The acidity of the reaction system and conditions influence the preferred pathway, with the lower energy barrier being favored. These pathways have been extrapolated based on experimental observations, but it is possible that other pathways with lower energy barriers exist. Future theoretical calculations are needed to further understand the FA transformation process under acidic catalysts.

Figure 6.

Figure 6

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(a) Calculated Gibbs free-energy profile using M062X/def2tzvp with schematic geometries of intermediate A to intermediate products B. (b) Calculated Gibbs free-energy profile using M062X/def2tzvp with schematic geometries of intermediate A to protonated EL.

3.4. Kinetics Analysis

The effect of acidity on the catalytic performance of [PSna][HSO4] in FA hydrolysis for EL synthesis was investigated (Figure 7). The reaction kinetics were studied at different temperatures (353, 363, and 373 K) using 0.050 g of [PSna][HSO4]. Ethanol did not significantly influence the reaction kinetics. The FA hydrolysis was found to be a pseudo-first-order reaction, with rate constants of 0.45, 0.61, and 0.90 h–1 at the respective temperatures. The relationship between the rate constant and temperature was determined using the Arrhenius equation, resulting in an activation energy of 12.09 kJ/mol. The abundance of Brønsted acid sites in the catalyst can lower the activation energy of the reaction. The experimental conditions included FA (0.098 g, 1 mmol), [PSna][HSO4] (0.05 g), ethanol (10 mL), and temperatures ranging from 353 to 373 K.

Figure 7.

Figure 7

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(a) First-order kinetic curves of FA to EL. (b) The Arrhenius equation at different temperatures.

The ionic liquid [PSna][HSO4], with 100% Brønsted acid utilization, shows enhanced catalytic performance due to its lower activation energy compared to nonhomogeneous solid-supported acids. This is largely because [PSna][HSO4] has more Brønsted acid sites and a lower molecular weight in its amphoteric compound PSna, providing more active sites. These factors lead to its superior catalytic activity.

4. Conclusions

A bifunctional compound, PSna, containing a sulfonic acid group, was synthesized and combined with H2SO4 to create a strong acidic ionic liquid catalyst [PSna][HSO4], with an acidity value (H0) of 0.86. Optimal conditions for the FA hydrolysis process to synthesize EL were identified, achieving a FA conversion rate of 97.79% and an EL yield of 96.10% at 110 °C in 2 h. Energy barrier calculations and reaction kinetics studies confirmed that [PSna][HSO4] substantially lowers the activation energy. The research underscores the importance of selecting suitable cationic and anionic components for creating efficient ionic liquids for use in biocatalysis and organic synthesis.

Acknowledgments

This study is supported by the National Key R&D Program of China (Grant No. 2021YFC2101100) and Major Projects in Inner Mongolia Autonomous Region (2020). We would like to thank MogoEdit (https://www.mogoedit.com) for its English editing during the preparation of this manuscript.

The authors declare no competing financial interest.

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