Method for catalytically synthesizing methyl benzoate based on choline chloride eutectic solvent

Abstract

The invention relates to an application of a deep-eutectic solvent in catalyzing a reaction of benzoic acid and methanol to synthesize methyl benzoate, and discloses a method for catalytically synthesizing methyl benzoate by using a choline chloride deep-eutectic solvent. The corresponding eutectic solvent can be prepared by mixing choline chloride and p-toluenesulfonic acid according to a certain proportion, heating and stirring. Mixing the prepared eutectic solvent with benzoic acid and methanol, and heating and stirring at a certain temperature to perform esterification reaction; after the reaction is finished, methyl benzoate can be obtained through the processes of liquid separation, evaporation and the like, and meanwhile, the recycled eutectic solvent can also be obtained. Compared with corresponding traditional inorganic acid catalytic esterification, the method has the great advantages of being small in equipment corrosion degree, simple in technological process, capable of saving cost, recyclable in catalyst, high in environment friendliness and the like, can be used for large-scale industrial production of methyl benzoate and has certain commercial value.

Description

Technical Field

[0001] This invention belongs to the field of organic chemical synthesis, specifically relating to a method for synthesizing methyl benzoate using a green, environmentally friendly, and highly reusable choline chloride-based eutectic solvent as a catalyst. Background Technology

[0002] Methyl benzoate is an important chemical raw material. It is used in various fields such as food additives, pesticides, fragrances, enzyme activity inhibitors, and organic solvents. Methyl benzoate is mainly synthesized from benzoic acid and methanol using sulfuric acid as a homogeneous catalyst. However, strong acid catalysts can lead to severe equipment corrosion, byproduct generation, and difficulties in catalyst recovery. To explore green catalysts, researchers conducted a series of experiments, such as using potassium phosphomolybdate catalysts, ternary solid catalysts, and alkaline earth layer benzoates. However, these catalysts failed to achieve better yields and excellent recyclability. Furthermore, this esterification reaction is a reversible, endothermic forward reaction, limited by the reaction temperature and the boiling point of methanol. The generated water cannot be effectively removed, thus limiting the conversion rate. In conclusion, designing catalysts that can effectively remove water from the reaction system to promote the forward shift of chemical equilibrium is crucial. To meet these requirements, reactive extraction technology has broad application prospects. According to reactive extraction, selecting a suitable solvent that is both water-affinity and ester-repellent is essential. In traditional synthesis processes, excess alcohol is usually used as both a reactant and a solvent in the system. However, esters are generally miscible with alcohols.

[0003] In recent years, eutectic solvents have gradually become a research hotspot and have been widely used in many fields. Deuterated esters (DESs) are a new type of environmentally friendly solvent, serving as an alternative to toxic and non-degradable ionic liquids. DESs are composed of hydrogen bond donors and acceptors. These solvents possess a widely dispersed hydrogen bond network, resulting in melting points significantly lower than their components. Furthermore, DESs exhibit significant stability at room temperature and offer substantial advantages in preparation, including non-toxicity, biodegradability, and cost-effectiveness. DESs composed of choline chloride (ChCl) and p-toluenesulfonic acid (PTSA) have been extensively studied. Choline chloride offers advantages such as low cost, low toxicity, and biodegradability. Meanwhile, p-toluenesulfonic acid is a strong acid, suitable for ester systems. These characteristics make DES [ChCl:PTSA] particularly promising and have sparked extensive research. Especially in the field of catalysis, researchers have designed [ChCl:PTSA] solutions with different ratios and achieved excellent catalytic performance. Overall, DESs generated from PTSA and ChCl exhibit strong acidity and high polarity. Their strong acidity enables them to be used as highly efficient catalysts. Their high polarity means that DESs can repel low-polarity esters, thus allowing them to function as extractants.

[0004] This invention aims to disclose a method for synthesizing methyl benzoate based on choline chloride-based eutectic solvent catalysis. This method features mild process conditions, high esterification yield, high environmental friendliness, and high catalyst reusability, and has strong application prospects for large-scale industrial production. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing processes by disclosing a green, environmentally friendly, and highly efficient method for synthesizing methyl benzoate using choline chloride-based eutectic solvents. This method can significantly reduce the problems associated with traditional acid catalysis.

[0006] The method of the present invention includes the following steps: (1) Weigh choline chloride and p-toluenesulfonic acid in a certain proportion and dry them under vacuum at 80°C for 2 hours. Then put them into a flask in a certain proportion and stir, and heat to 80°C for 1 hour. Finally, a transparent and clear liquid is obtained. The obtained liquid product is the eutectic solvent catalyst based on choline chloride. The catalyst is sealed and stored in a drying oven.

[0007] (2) Weigh out a certain amount of benzoic acid and methanol, as well as the eutectic solvent catalyst prepared in step (1), and add them to a three-necked flask equipped with a thermometer, a reflux condenser and a magnetic stirrer. Stir and heat to the specified temperature to carry out the reaction for a certain time.

[0008] (3) After the reaction is complete and cooled, the reaction system is transferred to a separatory funnel for separation to obtain the ester phase and the eutectic solvent phase. The ester phase is dried in a drying oven and subjected to atmospheric distillation to remove methanol, finally yielding methyl benzoate; the eutectic solvent phase is placed in a rotary evaporator to remove water, and extracted to remove small amounts of benzoic acid and methanol, and then dried in a vacuum drying oven to obtain the recycled eutectic solvent catalyst.

[0009] In step (1), the molar ratio of choline chloride and p-toluenesulfonic acid is converted to p-toluenesulfonic acid at a ratio of 0.1 to 0.9.

[0010] In step (2), the molar ratio of benzoic acid to methanol is 1:1, 1:2, and 1:3.

[0011] The reaction temperatures in step (2) are 70°C, 80°C, and 90°C.

[0012] In step (2), the ratio of the eutectic solvent catalyst to the total mass of benzoic acid and methanol is 0.5, 1, and 1.5.

[0013] The repeatability experiment specifically involves reusing the obtained eutectic solvent catalyst under optimal conditions for a total of 5 cycles, and calculating the corresponding methyl benzoate yield.

[0014] In summary, compared with existing methyl benzoate technologies, the technical solution of the present invention achieves the following beneficial effects: (1) The eutectic solvent of choline chloride used in this invention is used to catalyze the synthesis of methyl benzoate from benzoic acid and methanol. It has high catalytic activity and can obtain a high yield of methyl benzoate.

[0015] (2) The catalyst of the present invention is simple to prepare. It only requires mixing choline chloride and p-toluenesulfonic acid in a certain proportion and heating to form a clear and transparent solution. No other operations are required.

[0016] (3) The eutectic solvent catalyst of the present invention has high environmental friendliness, low corrosion to equipment, and high reusability.

[0017] (4) The catalyst raw materials of the present invention are inexpensive and easy to obtain, the preparation process is simple and the cost is low, which can further reduce the production cost. Attached Figure Description

[0018] Figure 1 Differential scanning calorimetry (DSC) plot of the prepared catalyst; Figure 2 The FT-IR spectra of the prepared catalysts are shown in ((a) ChCl, (b) PTSA and (c) [ChCl: PTSA]). Figure 3 The graph shows the effect of different molar ratios of benzoic acid and methanol on the yield. Figure 4 A yield graph showing the effect of different temperatures; Figure 5 The yield graph shows the effect of different catalyst qualities. Figure 6 The yield graphs for catalysts after 5 cycles are shown ((a) methyl benzoate yield and (b) catalyst mass). Figure 7 Before and after catalyst cycling 1 1H NMR spectra ((a) before reaction (b) after reaction). Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

Example 1

[0020] Preparation of eutectic solvent catalysts Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.1 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES1. The catalyst was sealed and stored in a desiccant oven.

[0021] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.2 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES2. The catalyst was sealed and stored in a drying oven.

[0022] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.3 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES3. The catalyst was sealed and stored in a drying oven.

[0023] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.4 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES4. The catalyst was sealed and stored in a drying oven.

[0024] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.5 (molar fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES5. The catalyst was sealed and stored in a desiccant oven.

[0025] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.6 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES6. The catalyst was sealed and stored in a drying oven.

[0026] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.7 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES7. The catalyst was sealed and stored in a drying oven.

[0027] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.8 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated as DES8. The catalyst was sealed and stored in a drying oven.

[0028] Choline chloride and p-toluenesulfonic acid were weighed at a ratio of 0.9 (mole fraction of p-toluenesulfonic acid) and vacuum dried at 80°C for 2 hours. The two were then placed in a flask at a specific ratio and stirred, then heated to 80°C and stirred for 1 hour, yielding a clear, transparent liquid. This liquid product is the eutectic solvent catalyst based on choline chloride, designated DES99. The catalyst was sealed and stored in a drying oven.

Example 2

[0029] The eutectic solvent DES5 ([ChCl:PTSA]), in which the proportion of p-toluenesulfonic acid in the catalyst synthesized in Example 1 was 0.5, was used to react benzoic acid and methanol to synthesize methyl benzoate. The reaction conditions were: reaction temperature 80°C, reaction time 6 h, molar ratio of benzoic acid to methanol 1:2, reaction pressure atmospheric pressure, and the mass ratio of catalyst to the sum of the masses of benzoic acid and methanol 1:1. After the reaction was completed, the reaction product was taken out, and the product composition was analyzed by gas chromatography, revealing a methyl benzoate yield of 94.18%.

[0030] The effect of reaction temperature on the reaction activity of the eutectic solvent catalyst prepared in Example 2 when used in the reaction of benzoic acid and methanol is shown in Table 1; the effect of the molar ratio of benzoic acid to methanol on the reaction activity of the eutectic solvent catalyst prepared in Example 2 when used in the reaction of benzoic acid and methanol is shown in Table 1; the effect of the amount of eutectic solvent catalyst prepared in Example 2 on the reaction activity of benzoic acid and methanol is shown in Table 1.

[0031] Table 1 Effect of different reaction parameters on the yield of methyl benzoate

Example 3

[0032] Repeatability test of eutectic solvent After the reaction was completed and cooled, the reaction system was transferred to a separatory funnel for separation, yielding an ester phase and a eutectic solvent phase. The ester phase was dried in a drying oven and then distilled at atmospheric pressure to remove methanol, ultimately yielding methyl benzoate. The eutectic solvent phase was concentrated in a rotary evaporator to remove water, and then extracted to remove trace amounts of benzoic acid and methanol. It was then dried in a vacuum drying oven to obtain the recycled eutectic solvent catalyst. The obtained catalyst was used to synthesize benzoic acid and methanol under the same optimal conditions, repeated 5 times. The optimal reaction conditions were: reaction temperature 80℃, reaction time 6h, benzoic acid to methanol molar ratio 1:2, reaction pressure atmospheric pressure, and catalyst to benzoic acid and methanol mass ratio 1:1. In the 5 repeated cycles, the yield of methyl benzoate was not less than 92%, demonstrating good recycling ability. The recycling data for Example 3 are shown in Table 2.

[0033] Table 2. Methyl benzoate yield after 5 cycles

[0034] Appendix Figure 1 The display shows differential scanning calorimetry (DSC) of eutectic solvent catalysts prepared by mixing choline chloride and p-toluenesulfonic acid in different proportions, determining their eutectic point temperatures and ensuring that the DES meets the requirement of homogeneity of the system before the reaction. The melting point of the DES is lower than that of each component. This is due to the presence of a broad internal hydrogen bond network. Furthermore, the melting point of the DES depends on the molar ratio of HBD and HBA. Clearly, the DES with a 1:1 molar ratio of ChCl and PTSA has the lowest melting point, -28 °C. This melting point is significantly lower than that of individual components of ChCl and PTSA, indicating the successful synthesis of the eutectic solvent and its suitability for use as a room-temperature DES.

[0035] Appendix Figure 2The displayed spectra are for choline chloride, p-toluenesulfonic acid, and a eutectic solvent in a 1:1 synthesis ratio. Fourier transform infrared spectroscopy confirms the hydrogen bonding interaction between ChCl and PTSA in DES. For ChCl, the OH bond extension mode appears at 3245 cm⁻¹. -1 At this location, the CH expansion joint appears at 2908 cm. -1 and 2957 cm -1 Location. N + (CH3)3 Symmetrical and asymmetric extension bands appeared at 3019 cm. -1 and 920-930 cm -1 Meanwhile, an asymmetric bending mode appears at 951 cm. -1 For PTSA, SO + The asymmetrical extension movement of 3 appeared at 1170 cm. -1 and 1124 cm -1 The symmetrical extensional motion of SO3 appears at 1007 cm. -1 The CS bending motion occurred at 1031 cm. -1 The DES composed of ChCl and PTSA [ChCl:PTSA] exhibits a broadened peak and shift compared to these two spectra. A peak can be observed at 3245 cm⁻¹ in ChCl. -1 The characteristic region of the OH group at the wavelength is broadened and shifts to a higher wavelength region. This indicates that the OH group participates in a broader hydrogen bond network and also confirms the successful preparation of the eutectic solvent.

[0036] Appendix Figure 3 The yield plots show the effect of different molar ratios of benzoic acid and methanol, and the influence of reaction parameters was evaluated by univariate analysis. The yield of methyl benzoate increases with increasing molar ratio of benzoic acid to methanol. This is because increasing the molar ratio of benzoic acid to methanol promotes a positive shift in chemical equilibrium. However, excess methanol reduces the concentration of benzoic acid, leading to a decrease in yield. Simultaneously, excess methanol may increase the solubility of both phases, potentially hindering esterification. A high molar ratio of benzoic acid to methanol is also detrimental to catalyst recovery, as it may increase separation costs and reduce the DES content in the DES phase. Therefore, the optimal molar ratio of benzoic acid to methanol is 1:2.

[0037] Appendix Figure 4The yield graph shows the effect of different temperatures, highlighting temperature as a crucial parameter in the esterification process. Increasing the reaction temperature leads to a gradual increase in the yield of methyl benzoate. According to reports, higher temperatures accelerate molecular motion and increase contact frequency, thereby improving conversion. Therefore, relatively high temperatures are preferred during esterification. Higher temperatures promote the reaction and increase the yield of methyl benzoate. However, considering the boiling point of methanol, the reaction temperature should not be too high. Furthermore, temperature affects the miscibility of the reactants, thus influencing the purity of methyl benzoate in the ester phase. Therefore, the optimal reaction temperature for this experiment was 80℃.

[0038] Appendix Figure 5 The yield plots show the effect of different catalyst masses, investigating the influence of different catalyst dosages (relative to the total mass of reactants) on the reaction. DES [ChCl:PTSA] is a typical Brunter acidic eutectic solvent. The Brunter acidic sites of DES can protonate compounds, validating the catalytic activity of DES. Increasing the DES dosage from 50 wt% to 100 wt% significantly improved the yield of methyl benzoate. However, further increasing the DES dosage resulted in a slight increase in yield. Similar phenomena have appeared in many reports, with the presence of catalytically active sites being the most frequently mentioned. They believe that acidic sites play a crucial role in the esterification process. Meanwhile, using PTSA as a catalyst, experiments were conducted under the same conditions, achieving a methyl benzoate yield of 70.68%. This demonstrates the solvent effect of DES [ChCl:PTSA] on the esterification reaction. DES can separate the ester phase and absorb reactants and water into the DES phase, thereby driving the shift in chemical equilibrium. The optimal DES dosage is 100 wt%.

[0039] Appendix Figure 6 The yield graph for five catalyst cycles is shown, demonstrating that the DES catalyst exhibits better reusability and environmental safety compared to other liquid catalysts. To evaluate the reusability of DES, experiments were conducted under identical conditions. Throughout the five cycles, the yield of methyl benzoate consistently remained above 92%, indicating consistent performance of the DES catalyst. Furthermore, the catalyst properties remained relatively unchanged. Minor mass fluctuations can be attributed to the dissolution of small amounts of DES in the ester phase.

[0040] Appendix Figure 7 Showing the catalyst before and after cycling 1 The H NMR spectrum showed no significant peak shift, indicating that the structure was not severely damaged, which further demonstrates the reusability of the synthesized eutectic solvent.

Claims

1. A method for synthesizing methyl benzoate using a choline chloride-based eutectic solvent as a catalyst, characterized in that... Includes the following steps: (1) Weigh choline chloride and p-toluenesulfonic acid in a certain proportion and dry them under vacuum at 80°C for 2 hours. Then put them into a flask in a certain proportion and stir, and heat to 80°C for 1 hour. Finally, a transparent and clear liquid is obtained. The obtained liquid product is the eutectic solvent catalyst based on choline chloride. The catalyst is sealed and stored in a drying oven. (2) Weigh out a certain amount of benzoic acid and methanol, as well as the eutectic solvent catalyst prepared in step (1), and add them to a three-necked flask equipped with a thermometer, a reflux condenser, and a magnetic stirrer. Stir and heat to the specified temperature to carry out the reaction for a certain time; (3) After the reaction is complete and cooled, the reaction system is transferred to a separatory funnel for separation to obtain the ester phase and the eutectic solvent phase. The ester phase is dried in a drying oven and subjected to atmospheric distillation to remove methanol, ultimately yielding methyl benzoate. The eutectic solvent phase is placed in a rotary evaporator to remove water and is extracted to remove small amounts of benzoic acid and methanol. It is then dried in a vacuum drying oven to obtain the recovered eutectic solvent catalyst.

2. The method according to claim 1, characterized in that... In step (1), the molar ratio of choline chloride to p-toluenesulfonic acid is 0.1 to 0.9, and the proportion used for catalysis is 0.

5.

3. The method according to claim 1, characterized in that... In step (2), the molar ratio of benzoic acid to methanol is 1:1, 1:2, and 1:

3.

4. The method according to claim 1, characterized in that... The reaction temperatures in step (2) are 70°C, 80°C, and 90°C.

5. The method according to claim 1, characterized in that... In step (2), the ratio of the eutectic solvent catalyst to the total mass of benzoic acid and methanol is 0.5, 1, and 1.5.

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