A method for synthesizing long-chain esters of hydroxybenzoic acids

Abstract

The application discloses a synthesis method of long-chain ester of hydroxybenzoic acid, which comprises the following steps: (1) mixing hydroxybenzoic acid and long-chain fatty alcohol according to a molar ratio of 1:1.1-1.5, adding a magnetic solid acid catalyst and a solvent to form a reaction system; (2) under nitrogen protection, heating to 150-180 DEG C, and then stirring for 1-3 hours; (3) after the reaction is completed, the magnetic solid acid catalyst is recovered by an external magnetic field, and the solvent is removed by reduced pressure distillation to obtain a long-chain ester of hydroxybenzoic acid crude product; (4) the crude product is purified by recrystallization or column chromatography to obtain a target product. The method realizes efficient and green synthesis of the long-chain ester of hydroxybenzoic acid, and the product selectivity and reaction yield are significantly improved compared with traditional methods.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of organic synthesis, and particularly relates to a synthesis method of long-chain ester of hydroxybenzoic acid. BACKGROUND

[0002] In the field of organic synthesis, long-chain ester of hydroxybenzoic acid has a wide range of applications. For example, C12-C18 ester of p-hydroxybenzoic acid plays an important role in multiple industries due to its excellent antibacterial properties, low toxicity, and chemical stability. In the cosmetics industry, such as octyl p-hydroxybenzoate, it can be used as a sunscreen and also as an emulsion stabilizer; in the pharmaceutical field, it can be used as a drug release carrier and ointment base; in the food industry, it can be used as a preservative to replace sodium benzoate; and in the field of material science, it can be used as an antioxidant for high polymer materials.

[0003] Currently, according to the public literature and patents (such as CN118955282A, CN117902977A, CN114989996B, etc.), the mainstream preparation method of long-chain ester of hydroxybenzoic acid has many problems. Although the concentrated sulfuric acid catalysis method has low cost and simple reaction conditions, it is highly corrosive, which can cause serious equipment wear and tear, and increase the annual maintenance cost by 30%. There are many side reactions, such as carbonization and sulfonation, and the selectivity of the product is less than 85%. A large amount of alkali is needed for post-treatment, which can produce high-salinity wastewater with a chemical oxygen demand (COD) of more than 5000 mg / L. The solid acid catalysis method, such as molecular sieve and heteropoly acid (e.g., CN118955282A), has a large mass transfer resistance for long-chain alcohols, such as C16 esterification reaction time of more than 8 hours, and the catalyst is easily deactivated by carbon deposition, with a cycle number of less than 5 times. The enzyme catalysis method, such as lipase immobilization (CN114989996B), has high enzyme cost, reaching 2000 yuan per gram, and the reaction system needs to be strictly controlled in terms of temperature and pH value, which makes industrialization difficult. Moreover, the current technology also faces challenges such as kinetic barriers, large molecular volume and high viscosity of long-chain alcohols, which result in low mass transfer efficiency and a diffusion coefficient of less than 1×10 -10 m² / s; unsustainable catalysts, liquid acid pollution of the environment, and easy blocking of active sites of solid acid by long-chain molecules; high separation energy consumption, and more than 40% of the total cost for energy consumption in traditional processes requiring multiple steps of distillation / extraction, etc.

[0004] Therefore, there is an urgent need to develop a new synthesis method of long-chain ester of hydroxybenzoic acid, which has better mass transfer efficiency and thus reaction progress, reduces reaction time, simplifies process steps, reduces comprehensive energy consumption, and reduces environmental pollution. SUMMARY

[0005] The present application aims to solve a series of problems existing in the preparation method of the existing long-chain ester of hydroxybenzoic acid. In view of the kinetic barrier brought by the increase of the carbon chain of the long-chain ester of hydroxybenzoic acid, the mass transfer efficiency of the long-chain alcohol at the reaction interface is improved, the reaction process is accelerated, and the reaction time is reduced. For the problem of unsustainable catalyst, a kind of catalyst which is both environmentally friendly and can maintain high activity is developed, which avoids the pollution of liquid acid to the environment and the blockage of active sites of solid acid. Reduce the energy consumption of separation, reduce the complex operation of multi-step distillation / extraction, and reduce the production cost.

[0006] The present application finally finds that by selecting a sulfonated mesoporous carbon@magnetic Fe3O4 core-shell catalyst (S-MC / Fe3O4), the efficient and green synthesis of long-chain ester of hydroxybenzoic acid can be realized, the product selectivity is improved, the catalyst consumption is reduced, the wastewater discharge is reduced, the comprehensive energy consumption is reduced, and the greenization and sustainable development of the synthesis process of long-chain ester of hydroxybenzoic acid are promoted.

[0007] The specific scheme is as follows:

[0008] A synthesis method of long-chain ester of hydroxybenzoic acid, comprising the following steps:

[0009] (1) mixing hydroxybenzoic acid and long-chain fatty alcohol according to a molar ratio of 1:1.1-1.5, adding a magnetic solid acid catalyst and a solvent to form a reaction system;

[0010] (2) under the protection of nitrogen, heating to 150-180℃, and then stirring for 1-3 hours;

[0011] (3) after the reaction is completed, the magnetic solid acid catalyst is recovered by an external magnetic field, the solvent is removed by reduced pressure distillation, and a crude product of long-chain ester of hydroxybenzoic acid is obtained;

[0012] (4) the crude product is purified by recrystallization or column chromatography to obtain the target product;

[0013] The magnetic solid acid catalyst is a Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst.

[0014] In one of the schemes, the hydroxybenzoic acid is one or more of p-hydroxybenzoic acid, o-hydroxybenzoic acid or m-hydroxybenzoic acid; the long-chain fatty alcohol is a straight-chain or branched primary alcohol with a carbon chain length of C8-C24, preferably a straight-chain or branched primary alcohol with a carbon chain length of C12-C18.

[0015] In one of the schemes, the solvent in step (1) is selected from one or more of xylene, toluene, chlorobenzene, cyclohexane or petroleum ether, and xylene and / or toluene are preferably used as the solvent.

[0016] In one of the schemes, the magnetic solid acid catalyst is prepared by a method comprising the following steps:

[0017] (a) mixing FeCl3.6H2O with sodium citrate at a molar ratio of 1:1.5-2.5, and reacting at 180-220°C for 10-15 hours; after the reaction is completed, the product is separated, purified, and dried to obtain nanoscale Fe3O4 magnetic particles;

[0018] (b) mixing the nanoscale Fe3O4 magnetic particles obtained in step (a) with sucrose as a carbon source and cetyltrimethylammonium bromide (CTAB) as a template agent, at a weight ratio of nanoscale Fe3O4 magnetic particles:sucrose:CTAB of 1:5-10:1-2, carbonizing at 800-1000°C under nitrogen for 1-3 hours, after the carbonization is completed, removing the CTAB template by soaking in dilute hydrochloric acid, then washing with deionized water until neutral, and drying to obtain Fe3O4 coated with mesoporous carbon;

[0019] (c) introducing fuming sulfuric acid vapor and reacting at 100-150°C for 1-3 hours to introduce sulfonic acid groups.

[0020] In one of the schemes, step (c) is specifically gas-phase sulfonation using fuming sulfuric acid vapor, and the sulfonic acid group density in the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst is 1-1.8 mmol / g, with 1.2-1.5 mmol / g being preferred.

[0021] In one of the schemes, the amount of the magnetic solid acid catalyst is 3wt%-8wt% of the total mass of the reactants, preferably 4wt%-6wt%.

[0022] In one of the schemes, step (2) uses a gradient temperature program, which specifically includes:

[0023] Initial stage: from room temperature to 120-140°C at a rate of 2-5°C / min, and holding for 10-30 min.

[0024] Reaction stage: heating to the target temperature of 150-180°C at a rate of 1-3°C / min, holding at the target temperature, and stirring for 1-3 hours.

[0025] In one of the schemes, the stirring rate in step (2) is 300-800 rpm, preferably 400-600 rpm.

[0026] In one of the schemes, the strength of the external magnetic field in step (3) is 0.3-1.0T, and the catalyst recovery time is 30-120 seconds.

[0027] In one of the schemes, the catalyst is regenerated by ethanol washing and vacuum drying at 60-80 DEG C after recovery, and the reaction yield remains above 90% after 20 cycles compared with the first time.

[0028] In one of the schemes, the recrystallization solvent in step (4) is ethanol or methanol, and the eluent for column chromatography is a mixed solvent of petroleum ether and ethyl acetate with a volume ratio of (5:1) to (15:1).

[0029] Beneficial effects

[0030] The present application realizes efficient and green synthesis of long-chain ester of hydroxybenzoic acid by selecting a specific catalyst and optimizing the process, and significantly improves the industrial application potential. Specifically:

[0031] Firstly, the sulfonated mesoporous carbon@magnetic Fe3O4 core-shell catalyst (S-MC / Fe3O4) solves the inherent defects of traditional catalytic system: the mesoporous carbon layer with a nanoscale pore size (pore size 3-5 nm) provides a mass transfer channel with high specific surface area for long-chain alcohols, which improves the diffusion coefficient of long-chain alcohols in the reaction process, thereby shortening the esterification reaction time; a certain concentration of sulfonic acid groups loaded on the surface of the catalyst shell and the mesopores of the mesoporous carbon layer endow the catalyst with high acid active sites, so that the esterification reaction proceeds fully, and the product selectivity is above 95%, avoiding the carbonization and sulfonation side reactions of traditional concentrated sulfuric acid method.

[0032] Meanwhile, the inner core of the catalyst is magnetic Fe3O4, which enables the catalyst to be quickly recovered by a magnetic field, and after 20 cycles, the reaction yield remains above 90% compared with the first time, and the single catalyst loss is reduced to below 0.5%, significantly reducing solid waste emissions. In addition, the reaction system does not require acid-base neutralization, and the COD value of wastewater is reduced from 5000 mg / L to below 200 mg / L, significantly improving the green degree of the process.

[0033] Further, in the reaction process, nitrogen protection and gradient heating are used, which on the one hand activates the acid sites of the catalyst to promote the initial esterification reaction, and on the other hand inhibits the oxidation of long-chain alcohols, and under the action of stirring, the reaction yield is improved. In the product purification stage, after removing the solvent by vacuum evaporation, the product purity can be improved to above 98wt% by recrystallization or column chromatography, which significantly reduces the energy consumption compared with the traditional multi-step distillation. The overall process greatly reduces the comprehensive production cost, and is suitable for flexible adaptation of C8-C24 linear / branched alcohols. The high-purity (≥98%) and low-residue (heavy metals <1 ppm) long-chain ester of hydroxybenzoic acid product can be widely used in cosmetics, pharmaceuticals and other fields, and the product production can be moved from laboratory level to ten-thousand-ton industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1TEM image of Fe3O4@mesoporous carbon-SO3H catalyst particles of Example 1. DETAILED DESCRIPTION

[0035] While the application has been described with a certain degree of particularity, it is to be understood that the present application is being

[0036] [Magnetic solid acid catalyst]

[0037] One of the focuses of the present application is the structural design of the catalyst. The magnetic solid acid catalyst is selected as the catalyst for the esterification reaction of long-chain esters of hydroxybenzoic acid, and specifically the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst. The preparation of such materials can refer to the methods for preparing magnetic mesoporous carbon materials in journal articles (Carbon 2005. 43: 2536-2543; Journal of Hazardous Materials 2011. 192: 1140-1147; Journal of Colloid and Interface Science 2012. 369: 366-372; Fuel Processing Technology 2013. 106: 376-384; Advanced Materials 2012. 24: 485-491). In order to better apply to the preparation of long-chain esters of hydroxybenzoic acid, the present application needs to further increase the high-acid active sites of the catalyst in the method for preparing magnetic mesoporous carbon materials, forming a composite catalyst system with a magnetic material inside, a mesoporous carbon layer with a large specific surface area outside, and a certain concentration of sulfonic acid groups on the surface of the mesoporous carbon layer and in the mesopores for promoting esterification. The material is prepared by the following steps:

[0038] (a) Fe3O4magnetic particles are synthesized by a hydrothermal method, FeCl3·6H2O is mixed with sodium citrate at a molar ratio of 1:1.5~2.5, and the reaction is carried out at 180~220℃ for 10~15 hours; after the reaction is completed, the nanoscale Fe3O4magnetic particles are separated, purified and dried;

[0039] In step (a), sodium citrate acts as a reducing agent and a stabilizer in this process, promoting the reduction of Fe 3+ to Fe 2 + , thereby forming Fe3O4with two valence states in inverse spinel type and controlling the morphology of Fe3O4nanoparticles;

[0040] After the reaction is completed, the Fe3O4 particles generated in the reaction are further separated from the reaction solution by centrifugation or magnetic separation, and then washed with deionized water and ethanol for 3-5 times to remove unreacted FeCl3, sodium citrate and byproducts (such as NaCl), so as to prevent residual impurities from affecting the subsequent carbon coating; then vacuum drying at 60-80°C for 8-20 hours to obtain pure Fe3O4 magnetic particle powder. If not washed, the residual sodium citrate may carbonize at high temperature to form a disordered carbon layer, blocking the mesoporous channels and reducing the specific surface area.

[0041] (b) mixing the nanoscale Fe3O4 magnetic particles obtained in step (a) with sucrose as a carbon source and cetyltrimethylammonium bromide (CTAB) as a template agent, according to the weight ratio of nanoscale Fe3O4 magnetic particles: sucrose: CTAB: 1: 5-10: 1-2, carbonizing at 800-1000°C under nitrogen for 1-3h, after the carbonization is completed, removing the CTAB template by soaking in dilute hydrochloric acid, and then washing with deionized water until neutral, and drying to obtain mesoporous carbon-coated Fe3O4;

[0042] In step (b), the CTAB forms a micellar template by self-assembly, guiding the sucrose to carbonize to form a mesoporous structure. By controlling the weight ratio of CTAB to carbon source and the carbonization reaction temperature, the pore size of the mesoporous structure can be adjusted. In the present application, the average pore size of the mesoporous structure is controlled at 2-8nm, preferably 3-5nm.

[0043] After the carbonization reaction is completed, the CTAB template needs to be removed by soaking in dilute hydrochloric acid, and then washed with deionized water until neutral to ensure the opening of the mesoporous structure. The concentration of dilute hydrochloric acid can be selected as 0.5M-1M HCl aqueous solution.

[0044] (c) introducing sulfonic acid groups by gas phase sulfonation method at 100-150°C for 1-3 hours.

[0045] Specifically, in step (c), the mesoporous carbon-coated Fe3O4 obtained in step (b) is placed in a sealed reactor, and fuming sulfuric acid vapor is introduced, wherein the partial pressure is controlled at 50-80 kPa, and the reaction is carried out at 100-150°C for 1-3 hours, so as to introduce sulfonic acid groups (-SO3H).

[0046] By controlling the reaction temperature, reaction time and partial pressure of fuming sulfuric acid vapor, the final introduced sulfonic acid group density in the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst can be 1-1.8mmol / g, and preferably 1.2-1.5mmol / g, which is determined by titration method.

[0047] The specific surface area of the finally obtained Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst is 800-1200m2 / g, preferably 850-1000 m 2 / g.

[0048] The average particle size of the Fe3O4@ mesoporous carbon-SO3H core-shell structure catalyst particle is 200-500 nm, preferably 250-400 nm, measured by transmission electron microscopy.

[0049] As can be seen, the magnetic solid acid catalyst designed in the application, specifically the Fe3O4@ mesoporous carbon-SO3H core-shell structure catalyst, significantly improves the synthesis efficiency and process sustainability of long-chain esters of hydroxybenzoic acid through the synergistic effect of multi-level structure:

[0050] The Fe3O4 magnetic particles located inside endow the catalyst with rapid magnetic separation and recovery capability, which can greatly reduce the energy consumption of solid-liquid separation and avoid the loss of active sites caused by traditional filtration, making it possible to recycle and maintain high esterification activity;

[0051] The high specific surface area of the mesoporous carbon layer in the catalyst and the uniform pore size inside provide a diffusion channel with low mass transfer resistance for the long-chain fatty alcohol as one of the raw materials, which improves the reaction interface contact efficiency and greatly shortens the reaction time compared with traditional solid acid catalysis;

[0052] The high-density sulfonic acid groups on the surface and in the pores serve as strong acidic active sites, which suppresses the dehydration and carbonization of long-chain alcohols and other side reactions, thereby improving product selectivity and eliminating the need for corrosive reagents such as concentrated sulfuric acid, thereby reducing high-salinity wastewater discharge from the source.

[0053] [Esterification reaction of long-chain esters of hydroxybenzoic acid]

[0054] After selecting the magnetic solid acid catalyst, specifically the Fe3O4@ mesoporous carbon-SO3H core-shell structure catalyst, as the esterification catalyst, the application performs synthesis of long-chain esters of hydroxybenzoic acid, including the following steps:

[0055] (1) Mixing step: mixing hydroxybenzoic acid and long-chain fatty alcohol at a molar ratio of 1:1.1-1.5, adding a magnetic solid acid catalyst and a solvent to form a reaction system;

[0056] In the mixing step (1), the hydroxybenzoic acid is one or more of p-hydroxybenzoic acid, o-hydroxybenzoic acid, or m-hydroxybenzoic acid; the long-chain fatty alcohol is a straight-chain or branched primary alcohol with a carbon chain length of C8-C24, preferably a straight-chain or branched primary alcohol with a carbon chain length of C12-C18.

[0057] The solvent is selected from one or more of xylene, toluene, chlorobenzene, cyclohexane or petroleum ether, wherein preferably xylene and / or toluene is used as the solvent, the above-mentioned solvent is compatible with the magnetic solid acid catalyst, does not destroy the structure of the catalyst or reduce its activity, has a relatively low system viscosity at 150-180℃, and can maintain high mass transfer efficiency, it should be noted that in order to avoid evaporation of the solvent at the reaction temperature, the reaction needs to be carried out in a refluxing reaction device or under pressurized conditions.

[0058] Based on the considerations of catalytic activity, reaction efficiency and cost, the amount of the magnetic solid acid catalyst is 3wt%-8wt% of the total mass of the reactants, preferably 4wt%-6wt%.

[0059] (2) Reaction step: under nitrogen protection, the temperature is raised to 150-180℃, and then stirred for 1-3 hours;

[0060] Preferably, a gradient temperature rising program is used in the reaction step (2), which specifically includes:

[0061] Initial stage: from room temperature to 120-140℃ at a rate of 2-5℃ / min, and keep for 10-30min;

[0062] Reaction stage: raise the temperature to the target temperature of 150-180℃ at a rate of 1-3℃ / min, keep the temperature after reaching the target temperature, and stir for 1-3 hours.

[0063] In the reaction process, nitrogen protection and gradient temperature rising are used, on the one hand to activate the acid sites of the catalyst to promote the initial esterification reaction, and on the other hand to inhibit the oxidation of long-chain alcohols, and under the action of stirring to promote the improvement of reaction yield. The stirring rate in step (2) is 300-800 rpm, preferably 400-600 rpm.

[0064] (3) Separation step: after the reaction is completed, the magnetic solid acid catalyst is recovered by an external magnetic field, and the solvent is removed by vacuum distillation to obtain the crude product of long-chain ester of hydroxybenzoic acid;

[0065] In the separation step, the magnetic solid acid catalyst containing Fe3O4 magnetic particles is recovered by setting an external magnetic field, wherein the magnetic field strength is 0.3-1.0T, considering that the average particle size of the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst particles is 200-500nm, the duration of the external magnetic field is 30-120 seconds, and the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst particles are separated from the reaction liquid.

[0066] The recovered catalyst is washed with ethanol, and then regenerated by vacuum drying at a temperature of 60-80℃ to obtain the regenerated Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst, which can be recycled for 20 times, and the reaction yield can still be maintained at more than 90% compared with the first reaction.

[0067] The residual reaction liquid is subjected to vacuum distillation to remove the solvent, and white solid is obtained, which is the crude product of long-chain ester of hydroxybenzoic acid.

[0068] (4) purification step: the crude product is purified by recrystallization or column chromatography to obtain the target product;

[0069] In the recrystallization solvent in the purification step (4), ethanol or methanol is used, and the column chromatography eluent is a mixed solvent of petroleum ether and ethyl acetate, with a volume ratio of (5:1) to (15:1).

[0070] Since the catalyst can be recovered by controlling magnetic separation in the present application, there is no solid catalyst in the reaction liquid, and the low efficiency of traditional solid-liquid separation such as sedimentation, centrifugation or filtration is avoided for high-viscosity reaction liquid. At the same time, the traditional concentrated sulfuric acid catalyst is easy to cause sulfonation, carbonization and other side reactions, generating sulfonate, tar-like substances, and the product is complex, which needs multi-step distillation to separate different impurities. Compared with the above, the product purity of the crude product of the present application is already very high, and only one recrystallization or one column chromatography is needed after removing the solvent by one vacuum evaporation to purify the target product long-chain ester of hydroxybenzoic acid to more than 98wt%, which greatly reduces the complexity of the process and reduces the energy consumption and comprehensive production cost.

[0071] In order to further understand the present application, the preferred embodiments of the present application are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present application, and are not limitations of the claims of the present application.

[0072] Example 1

[0073] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst

[0074] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle, and hydrothermally reacted at 200℃ for 12 hours.

[0075] The Fe3O4 particles were collected by magnetic separation, and then washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60℃ for 12 hours to obtain the Fe3O4 magnetic particles.

[0076] Fe3O4 magnetic particles (1.0 g), sucrose (8.0 g), and CTAB (1.5 g) were mixed and ultrasonically dispersed in 50 mL of ethanol for 1 hour. Carbonization was performed at 900°C for 2 hours at a temperature increase rate of 5°C / min under nitrogen protection. After cooling, the CTAB template was removed by immersing in 1M HCl for 24 hours, washing with deionized water until neutral, and drying at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0077] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. The Fe3O4@mesoporous carbon-SO3H catalyst was obtained by sulfonization at 120°C for 2 hours. Figure 1 FIG. 1 is a TEM image of the Fe3O4@mesoporous carbon-SO3H catalyst particles of Example 1, which was collected under a JEOL JEM-2100 transmission electron microscope (accelerating voltage 200 kV).

[0078] (2) Esterification reaction

[0079] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, 1.58 g of catalyst, and 100 mL of xylene were added to a 500 mL three-necked flask, and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0080] Under nitrogen protection, the temperature was increased to 160°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 2 hours.

[0081] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds, and the catalyst was recovered. The recovered catalyst was washed with ethanol three times and vacuum dried at 80°C for 4 hours, and was used for subsequent recycling after regeneration.

[0082] The remaining reaction liquid was subjected to vacuum distillation at a pressure of 10 kPa and a temperature of 80°C to remove xylene, and a white solid crude product was obtained.

[0083] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was decreased to -10°C at a rate of 0.5°C / min. The crystals were filtered after precipitation, and dried to obtain n-hexadecyl p-hydroxybenzoate. The purity of the product was 99.6% as determined by HPLC.

[0084] Example 2

[0085] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst

[0086] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water and ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle for hydrothermal reaction at 200°C for 12 hours.

[0087] The Fe3O4 magnetic particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times, respectively, and dried at 60 °C for 12 hours to obtain Fe3O4 magnetic particles.

[0088] 1.0 g of Fe3O4 magnetic particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Carbonization was performed at 5 °C / min to 900 °C for 2 hours under nitrogen protection, and after cooling, the CTAB template was removed by soaking in 1M HCl for 24 hours. After washing with deionized water until neutral, drying was performed at 80 °C to obtain mesoporous carbon-coated Fe3O4.

[0089] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 80 kPa (60 kPa in Example 1), and sulfonated at 120 °C for 3 hours (2 hours in Example 1) to obtain Fe3O4@mesoporous carbon-SO3H catalyst.

[0090] (2) Esterification reaction

[0091] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to 100 mL of xylene in a 500 mL three-necked flask, and a condensation reflux device was formed, and a uniform suspension was formed by stirring.

[0092] Under nitrogen protection, the temperature was raised to 160 °C at a rate of 3 °C / min, and the temperature was maintained, and the reaction was stirred at a rate of 500 rpm for 2 hours.

[0093] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds, and the catalyst was recovered. The recovered catalyst was washed with ethanol for 3 times, and dried at 80 °C for 4 hours under vacuum, and used for subsequent recycling after regeneration.

[0094] The remaining reaction liquid was adjusted to a pressure of 10 kPa and a temperature of 80 °C for vacuum distillation to remove xylene, and a white solid crude product was obtained.

[0095] The crude product was dissolved in 150 mL of hot ethanol at 70 °C, and the temperature was lowered to -10 °C at a rate of 0.5 °C / min. After the crystals were precipitated, they were filtered and dried to obtain n-hexadecyl p-hydroxybenzoate. The purity of the product was determined by HPLC to be 99.2%.

[0096] Example 3

[0097] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst

[0098] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water and ultrasonically dispersed for 30 min. Then, the mixture was transferred to a polytetrafluoroethylene-lined reactor and hydrothermally reacted at 200°C for 12 h.

[0099] Fe3O4 particles were collected by magnetic separation, washed four times with deionized water and four times with ethanol, and dried under vacuum at 60°C for 12 hours to obtain Fe3O4 magnetic particles.

[0100] 1.0g of Fe3O4 magnetic particles, 8.0g of sucrose, and 2.0g of CTAB were mixed and added to 50mL of ethanol for ultrasonic dispersion for 1 hour. Under nitrogen protection, the mixture was heated to 850°C at a rate of 5°C / min and carbonized for 2 hours. After cooling, the mixture was soaked in 1M HCl for 24 hours to remove the CTAB template, washed with deionized water until neutral, and dried at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0101] The above materials were placed in a closed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. Sulfonation was carried out at 120°C for 2 hours to obtain Fe3O4@mesoporous carbon-SO3H catalyst.

[0102] (2) Esterification reaction

[0103] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, 1.58 g of catalyst and 100 mL of xylene were added to a 500 mL three-necked flask, a condenser reflux apparatus was formed, and the mixture was stirred to form a uniform suspension.

[0104] Under nitrogen protection, the temperature was raised to 160° C. at a rate of 3° C. / min, and the temperature was maintained with stirring at a rate of 500 rpm for 2 hours.

[0105] After the reaction, a 0.5 T magnetic field was applied for 60 seconds to recover the catalyst, which was washed three times with ethanol and dried under vacuum at 80° C. for 4 hours for subsequent regeneration and recycling.

[0106] The remaining reaction solution was adjusted to a pressure of 10 kPa and a temperature of 80° C. and subjected to reduced pressure distillation to remove xylene to obtain a crude product as a white solid.

[0107] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was lowered to -10°C at 0.5°C / min. The crystals were precipitated and filtered. After drying, hexadecyl parahydroxybenzoate was obtained. The purity of the product was determined by HPLC to be 99.0%.

[0108] Example 4

[0109] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst

[0110] FeCl3-6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined autoclave for hydrothermal reaction at 200 °C for 12 hours.

[0111] The Fe3O4particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60 °C for 12 hours to obtain Fe3O4magnetic particles.

[0112] 1.0 g of Fe3O4magnetic particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Carbonization was carried out at a temperature increasing rate of 5 °C / min to 900 °C for 2 hours under nitrogen protection. After cooling, the CTAB template was removed by soaking in 1M HCl for 24 hours. After washing with deionized water until neutral, drying was carried out at 80 °C to obtain mesoporous carbon-coated Fe3O4.

[0113] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. Sulfonation was carried out at 120 °C for 2 hours to obtain Fe3O4@ mesoporous carbon-SO3H catalyst.

[0114] (2) Esterification reaction

[0115] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to a 500 mL three-necked flask together with 100 mL of toluene (xylene in Example 1), and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0116] Under nitrogen protection, the temperature was increased to 140 °C at a rate of 3 °C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 3 hours.

[0117] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80 °C for 4 hours for subsequent recycling after regeneration.

[0118] The remaining reaction liquid was subjected to vacuum distillation at a pressure of 5 kPa and a temperature of 60 °C to remove toluene, and a white solid crude product was obtained.

[0119] The crude product was dissolved in 150 mL of hot ethanol at 70 °C, and the temperature was decreased to -10 °C at a rate of 0.5 °C / min. After the crystals were precipitated, they were filtered and dried to obtain n-hexadecyl p-hydroxybenzoate. The purity of the product was determined by HPLC to be 99.1%.

[0120] Example 5

[0121] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst

[0122] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle for hydrothermal reaction at 200°C for 12 hours.

[0123] The Fe3O4 particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60°C for 12 hours to obtain Fe3O4 magnetic particles.

[0124] 1.0 g of Fe3O4 magnetic particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Under nitrogen protection, the temperature was raised to 900°C at a rate of 5°C / min for carbonization for 2 hours. After cooling, the CTAB template was removed by soaking in 1M HCl for 24 hours, washed with deionized water until neutral, and dried at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0125] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa, and sulfonated at 120°C for 2 hours to obtain Fe3O4@mesoporous carbon-SO3H catalyst.

[0126] (2) Esterification reaction

[0127] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to a 500 mL three-necked flask with 100 mL of dimethylbenzene, and a condensation reflux device was formed, and a uniform suspension was formed by stirring.

[0128] Under nitrogen protection, the temperature was raised from room temperature to 130°C at a rate of 4°C / min, and held for 20 min; then the temperature was raised to the target temperature of 160°C at a rate of 2°C / min, and held after reaching the target temperature, and stirred for 2 hours.

[0129] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0130] The remaining reaction liquid was adjusted to a pressure of 10 kPa and a temperature of 80°C for vacuum distillation to remove dimethylbenzene, and a white solid crude product was obtained.

[0131] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was lowered to -10°C at a rate of 0.5°C / min. After the crystals were precipitated, they were filtered and dried to obtain p-hydroxybenzoic acid hexadecyl ester. The purity of the product was determined by HPLC to be 99.7%.

[0132] Example 6

[0133] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined autoclave for hydrothermal reaction at 200°C for 12 hours.

[0134] The Fe3O4particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60°C for 12 hours to obtain Fe3O4magnetic particles.

[0135] 1.0 g of Fe3O4magnetic particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Carbonization was carried out at a temperature increasing rate of 5°C / min to 900°C for 2 hours under nitrogen protection. After cooling, the CTAB template was removed by immersing in 1M HCl for 24 hours. After washing with deionized water until neutral, drying was carried out at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0136] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. Sulfonation was carried out at 120°C for 2 hours to obtain Fe3O4@ mesoporous carbon-SO3H catalyst.

[0137] (2) Esterification reaction

[0138] 10.0 g of p-hydroxybenzoic acid, 35.2 g of n-tetracosanol (C 24 H 50 O, 0.086 mol), and 1.58 g of catalyst were added to a 500 mL three-necked flask, and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0139] Under nitrogen protection, the temperature was increased to 170°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 3 hours.

[0140] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0141] The remaining reaction liquid was subjected to vacuum distillation at a pressure of 10 kPa and a temperature of 80°C to remove dimethylbenzene, and a white solid crude product was obtained.

[0142] The crude product was dissolved in 200 mL of n-heptane, and after dissolving at 70°C, the temperature was decreased to -10°C at a rate of 0.5°C / min. The crystals were filtered after precipitation, and dried to obtain n-tetracosyl p-hydroxybenzoate. The purity of the product was 98.1% as determined by HPLC.

[0143] Example 7

[0144] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst

[0145] Directly use the catalyst after 20 cycles of Example 1.

[0146] (2) Esterification reaction

[0147] Put 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, 1.58 g of catalyst, and 100 mL of xylene into a 500 mL three-necked flask, and form a condensation reflux device, stir to form a uniform suspension.

[0148] Under nitrogen protection, heat to 160°C at a rate of 3°C / min, maintain the temperature, and stir the reaction at a rate of 500 rpm for 3 hours.

[0149] After the reaction is completed, apply a magnetic field of 0.5T for 60 seconds to recover the catalyst. The recovered catalyst is washed with ethanol for 3 times, vacuum dried at 80°C for 4 hours, and used for subsequent recycling after regeneration.

[0150] Adjust the pressure of the remaining reaction liquid to 10 kPa and the temperature to 80°C to remove xylene by vacuum distillation to obtain a white solid crude product.

[0151] Dissolve the crude product in 150 mL of hot ethanol at 70°C, cool to -10°C at a rate of 0.5°C / min, filter after the crystals are precipitated, and dry to obtain p-hydroxybenzoic acid hexadecyl ester. The product purity is 98.9% as determined by HPLC.

[0152] Comparative Example 1

[0153] (1) Catalyst replacement

[0154] Use 98% concentrated sulfuric acid instead of Fe3O4@mesoporous carbon-SO3H catalyst.

[0155] (2) Esterification reaction

[0156] Put 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, 1.5 mL of concentrated sulfuric acid, and 100 mL of xylene into a 500 mL three-necked flask, and form a condensation reflux device, stir to form a uniform suspension.

[0157] Under nitrogen protection, heat to 160°C at a rate of 3°C / min, maintain the temperature, and stir the reaction at a rate of 500 rpm for 2 hours.

[0158] After the reaction is completed, neutralize to pH=7 with 10% NaHCO3 aqueous solution, separate to remove the aqueous phase, and wash the organic phase with water for 3 times.

[0159] The remaining organic phase was adjusted to 10 kPa and distilled at 80°C to remove xylene to obtain a crude product.

[0160] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and cooled to -10°C at a rate of 0.5°C / min. The crystals were filtered and dried to obtain p-hydroxybenzoic acid cetyl ester. The purity of the product was 95.5% as determined by HPLC.

[0161] Comparative Example 2

[0162] (1) Synthesis of Fe3O4@carbon-SO3H catalyst

[0163] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water and ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle for hydrothermal reaction at 200°C for 12 hours.

[0164] The Fe3O4 particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60°C for 12 hours to obtain Fe3O4 magnetic particles.

[0165] 1.0 g of Fe3O4 magnetic particles and 8.0 g of sucrose were mixed and ultrasonically dispersed in 50 mL of ethanol for 1 hour. Under nitrogen protection, the temperature was raised to 900°C at a rate of 5°C / min for carbonization for 2 hours. After washing with deionized water until neutral, drying was performed at 80°C to obtain Fe3O4 without mesoporous carbon coating.

[0166] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. The material was sulfonated at 120°C for 2 hours to obtain Fe3O4@carbon-SO3H catalyst.

[0167] (2) Esterification reaction

[0168] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, 1.58 g of catalyst, and 100 mL of xylene were added to a 500 mL three-necked flask, and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0169] Under nitrogen protection, the temperature was raised to 160°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 5 hours.

[0170] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0171] The remaining reaction solution was adjusted to a pressure of 10 kPa and a temperature of 80°C, and subjected to vacuum distillation to remove xylene, to obtain a white solid crude product.

[0172] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and cooled to -10°C at a rate of 0.5°C / min. After the crystals were precipitated, they were filtered and dried to obtain p-hydroxybenzoic acid hexadecyl ester. The purity of the product was determined by HPLC to be 89.2%.

[0173] Comparative Example 3

[0174] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H catalyst

[0175] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle for hydrothermal reaction at 200°C for 12 hours.

[0176] The Fe3O4particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times each, and vacuum dried at 60°C for 12 hours to obtain Fe3O4magnetic particles.

[0177] 1.0 g of Fe3O4magnetic particles, 8.0 g of sucrose, and 3.0 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Under nitrogen protection, the temperature was raised to 950°C at a rate of 5°C / min for carbonization for 2 hours. After cooling, the CTAB template was removed by immersion in 1M HCl for 24 hours. The product was washed with deionized water until neutral and dried at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0178] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. The reactor was heated to 120°C for 2 hours to obtain the Fe3O4@mesoporous carbon-SO3H catalyst.

[0179] (2) Esterification reaction

[0180] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to 100 mL of xylene in a 500 mL three-necked flask, and a condensation reflux device was formed. The mixture was stirred to form a uniform suspension.

[0181] Under nitrogen protection, the temperature was raised to 160°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 2 hours.

[0182] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0183] The remaining reaction solution was subjected to vacuum distillation at 10 kPa and 80°C to remove xylene, to obtain a white solid crude product.

[0184] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was lowered to -10°C at a rate of 0.5°C / min. The crystals were filtered and dried to obtain p-hydroxybenzoic acid hexadecyl ester. The purity of the product was 91.5% as determined by HPLC.

[0185] Comparative Example 4

[0186] (1) Synthesis of SiO2@mesoporous carbon-SO3H catalyst

[0187] 1.0 g of SiO2 particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and ultrasonically dispersed in 50 mL of ethanol for 1 hour. Under nitrogen protection, the temperature was raised to 900°C at a rate of 5°C / min and carbonized for 2 hours. After cooling, the CTAB template was removed by immersion in 1M HCl for 24 hours, washed with deionized water until neutral, and dried at 80°C to obtain mesoporous carbon-coated SiO2.

[0188] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. The temperature was raised to 120°C, and the material was sulfonated for 2 hours to obtain the SiO2@mesoporous carbon-SO3H catalyst.

[0189] (2) Esterification reaction

[0190] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to 100 mL of xylene in a 500 mL three-necked flask, and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0191] Under nitrogen protection, the temperature was raised to 160°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 2 hours.

[0192] After the reaction was completed, the catalyst was separated by centrifugation (8000 rpm for 30 minutes), washed with ethanol 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0193] The remaining reaction solution was subjected to vacuum distillation at 10 kPa and 80°C to remove xylene, to obtain a white solid crude product.

[0194] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was lowered to -10°C at a rate of 0.5°C / min. The crystals were filtered and dried to obtain p-hydroxybenzoic acid hexadecyl ester. The purity of the product was 97.1% as determined by HPLC.

[0195] Comparative Example 5 (low temperature reaction)

[0196] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H catalyst

[0197] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle for hydrothermal reaction at 200°C for 12 hours.

[0198] The Fe3O4particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60°C for 12 hours to obtain Fe3O4magnetic particles.

[0199] 1.0 g of Fe3O4magnetic particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Under nitrogen protection, the temperature was raised to 900°C at a rate of 5°C / min for carbonization for 2 hours. After cooling, the CTAB template was removed by soaking in 1M HCl for 24 hours, washed with deionized water until neutral, and dried at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0200] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. The temperature was raised to 120°C for 2 hours to obtain the Fe3O4@mesoporous carbon-SO3H catalyst.

[0201] (2) Esterification reaction

[0202] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to a 500 mL three-necked flask with 100 mL of dimethylbenzene, and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0203] Under nitrogen protection, the temperature was raised to 140°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 6 hours.

[0204] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0205] The remaining reaction liquid was adjusted to a pressure of 10 kPa and a temperature of 80°C for vacuum distillation to remove dimethylbenzene, and a white solid crude product was obtained.

[0206] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was lowered to -10°C at a rate of 0.5°C / min. The crystals were filtered after precipitation, and dried to obtain n-hexadecyl p-hydroxybenzoate. The product purity was 93.4% as determined by HPLC.

[0207] Comparative Example 6

[0208] (1) Synthesis of Fe3O4@mesoporous carbon-SO3H catalyst

[0209] FeCl3·6H2O (5.4 g, 0.02 mol) and sodium citrate (8.8 g, 0.03 mol) were dissolved in 80 mL of deionized water, ultrasonically dispersed for 30 minutes, and then transferred to a polytetrafluoroethylene-lined reaction kettle for hydrothermal reaction at 200°C for 12 hours.

[0210] The Fe3O4particles were collected by magnetic separation, washed with deionized water and ethanol for 4 times respectively, and vacuum dried at 60°C for 12 hours to obtain Fe3O4magnetic particles.

[0211] 1.0 g of Fe3O4magnetic particles, 8.0 g of sucrose, and 1.5 g of CTAB were mixed and added to 50 mL of ethanol for ultrasonic dispersion for 1 hour. Under nitrogen protection, the temperature was raised to 900°C at a rate of 5°C / min for carbonization for 2 hours. After cooling, the CTAB template was removed by soaking in 1M HCl for 24 hours, washed with deionized water only once, and dried at 80°C to obtain mesoporous carbon-coated Fe3O4.

[0212] The above material was placed in a sealed reactor, and fuming sulfuric acid vapor was introduced at a partial pressure of 60 kPa. The material was sulfonated at 120°C for 2 hours to obtain the Fe3O4@mesoporous carbon-SO3H catalyst.

[0213] (2) Esterification reaction

[0214] 10.0 g of p-hydroxybenzoic acid, 21.6 g of n-hexadecanol, and 1.58 g of catalyst were added to a 500 mL three-necked flask together with 100 mL of xylene, and a condensation reflux device was formed. A uniform suspension was formed by stirring.

[0215] Under nitrogen protection, the temperature was raised to 160°C at a rate of 3°C / min, and the temperature was maintained. The reaction was stirred at a rate of 500 rpm for 2 hours.

[0216] After the reaction was completed, a magnetic field of 0.5T was applied for 60 seconds to recover the catalyst. The recovered catalyst was washed with ethanol for 3 times, and vacuum dried at 80°C for 4 hours for subsequent recycling after regeneration.

[0217] The remaining reaction liquid was subjected to vacuum distillation at a pressure of 10 kPa and a temperature of 80°C to remove xylene, and a white solid crude product was obtained.

[0218] The crude product was dissolved in 150 mL of hot ethanol at 70°C, and the temperature was lowered to -10°C at a rate of 0.5°C / min. The crystals were filtered after precipitation, and dried to obtain n-hexadecyl p-hydroxybenzoate. The purity of the product was determined by HPLC to be 89.7%.

[0219] The catalysts obtained in Examples 1-7 and Comparative Examples 1-6 were characterized as shown in Table 1 :

[0220] Table 1 : Catalyst microstructure of Examples 1-7 and Comparative Examples 1-6

[0221]

[0222] wherein,

[0223] - The sulfonic group density was tested using the ASTM D664 standard, using acid-base titration, with an error range of ± 0.05 mmol / g.

[0224] - The specific surface area test method was ET nitrogen adsorption-desorption: based on the Brunauer-Emmett-Teller (BET) theory, the specific surface area was calculated from the nitrogen adsorption isotherm at low temperature (77 K). A Micromeritics ASAP 2460 full-automatic specific surface analyzer was used, with an accuracy of ± 3%.

[0225] - The average particle size of the catalyst was first measured directly from the TEM images by measuring the particle size distribution, then at least 300 particles were manually or automatically labeled using ImageJ software, and the average particle size and standard deviation were calculated; the lens used was a JEOL JEM-2100 transmission electron microscope (accelerating voltage 200 kV).

[0226] - The mesopore average pore diameter (nm) was calculated from the nitrogen desorption branch based on the Barrett-Joyner-Halenda (BJH) model:

[0227] 1. Isotherm acquisition: same as the BET test conditions, record the complete adsorption-desorption curve (P / P0=0.01-0.99);

[0228] 2. Hysteresis loop analysis: identify the hysteresis loop of the desorption branch (usually of type H1, indicating uniform cylindrical mesopores);

[0229] 3. Pore diameter calculation: use the BJH formula rp=RTln(P0 / P)2γVm+t (γ=8.88x10-3N / m, t is the adsorption layer thickness), combined with the Kelvin equation to derive the pore size distribution;

[0230] 4. Statistical average: take the pore size distribution peak value or volume-weighted average value as the mesopore average pore diameter.

[0231] A Quantachrome Autosorb-iQ analyzer was used, with automatic software fitting, with an accuracy of ± 0.2 nm, dedicated to mesopore / macropore material pore size analysis.

[0232] The indexes of the esterification reaction process of examples 1-7 and comparative examples 1-6 are shown in table 2:

[0233] Table 2: indexes of the esterification reaction process of examples 1-7 and comparative examples 1-6

[0234]

[0235] The esterification rate reflects the reaction selectivity of the application, i.e. the molar percentage of the target product long-chain ester of hydroxybenzoic acid in all products before purification after the reaction, reflecting the inhibition ability of side reactions. The main product and by-product are separated and quantified by HPLC / GC-MS, and the calculation formula is as follows:

[0236] Esterification rate = molar amount of long-chain ester of hydroxybenzoic acid / total molar amount of products x 100%.

[0237] The yield reflects the percentage of the actual product quality and the theoretical product quality, reflecting the reaction efficiency and catalyst activity. The weighing method is used, i.e. the ratio of the product quality after purification to the theoretical yield (based on the molar ratio):

[0238] Yield = actual mass of long-chain ester of hydroxybenzoic acid / theoretical yield x 100%.

[0239] The purity is the mass percentage of the target product in the final product, reflecting the effectiveness of the purification process. The HPLC area normalization method is used: the purity is calculated by the chromatographic peak area.

[0240] Here, the actual reaction yield of example 7 is 80.5%, compared with the yield of example 1 of 88.2%, and the reaction yield is 80.5% / 88.2%=91.3% compared with the first time, i.e. after the catalyst system is used for 20 times, the reaction yield is still maintained at more than 90% compared with the first time.

[0241] According to the data in table 1 and table 2, it can be seen that examples 1-7 satisfying the application have significant advantages in the final reaction indexes (esterification rate / yield / purity) compared with comparative examples 1-6 which are not within the synthesis process range of the application. Meanwhile, those skilled in the art can also know that the synthesis process according to the application also has obvious advantages in energy consumption, wastewater discharge, etc.

[0242] It should be noted that the specific embodiments are only an explanation of the application, and are not a limitation of the application. Those skilled in the art can make non-creative modifications to the embodiments according to the needs after reading the specification, but as long as they are within the scope of the claims of the application, they are protected by the patent law.

Claims

1. A method for synthesizing a long-chain hydroxybenzoic acid ester, characterized in that: The steps include: (1) Mixing hydroxybenzoic acid and long-chain fatty alcohol in a molar ratio of 1:1.1-1.5, adding a magnetic solid acid catalyst and a solvent to form a reaction system; (2) Under nitrogen protection, heat to 150-180°C, and then stir and react for 1-3 hours; (3) After the reaction is completed, the magnetic solid acid catalyst is separated and recovered by an external magnetic field, and the solvent is removed by vacuum distillation to obtain a crude product of a long-chain hydroxybenzoic acid ester; (4) Purifying the crude product by recrystallization or column chromatography to obtain the target product; The magnetic solid acid catalyst is a Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst, and the magnetic solid acid catalyst is prepared by a method comprising the following steps: (a) FeCl3·6H2O and sodium citrate are mixed in a molar ratio of 1:1.5-2.5 and reacted at 180-220°C for 10-15 hours; after the reaction is completed, the nano-sized Fe3O4 magnetic particles are separated, purified and dried; (b) mixing the nano-sized Fe3O4 magnetic particles obtained in step (a) with sucrose as a carbon source and cetyltrimethylammonium bromide as a template, and carbonizing the mixture at a weight ratio of nano-sized Fe3O4 magnetic particles: sucrose: CTAB of 1:5-10:1-2 under nitrogen at 800-1000°C for 1-3 hours. After the carbonization, the mixture is soaked in dilute hydrochloric acid to remove the CTAB template, and then washed with deionized water until neutral, and dried to obtain mesoporous carbon-coated Fe3O4; (c) introducing fuming sulfuric acid vapor and reacting at 100-150°C for 1-3 hours to introduce sulfonic acid groups; In which, step (c) is specifically to use fuming sulfuric acid vapor for gas phase sulfonation, the density of sulfonic acid groups in the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst is 1-1.8 mmol / g, and the specific surface area of ​​the Fe3O4@mesoporous carbon-SO3H core-shell structure catalyst finally obtained is 800-1200m 2 / g, average particle size of 200-500nm; The hydroxybenzoic acid is one or more of p-hydroxybenzoic acid, o-hydroxybenzoic acid or m-hydroxybenzoic acid; the long-chain fatty alcohol is a straight-chain or branched primary alcohol with a carbon chain length of C8 to C24.

2. The synthesis method according to claim 1, wherein The solvent in step (1) is selected from one or more of xylene, toluene, chlorobenzene, cyclohexane or petroleum ether.

3. The synthesis method according to claim 1, wherein The amount of the magnetic solid acid catalyst is 3 wt% to 8 wt% of the total mass of the reactants.

4. The synthesis method according to claim 1, characterized in that The step (2) adopts a gradient temperature increase program, which specifically includes: Initial stage: heat from room temperature to 120-140℃ at a rate of 2-5℃ / min, and keep warm for 10-30min; Reaction stage: Raise the temperature to the target temperature of 150-180°C at a rate of 1-3°C / min. After reaching the target temperature, keep warm and stir the reaction for 1-3 hours.

5. The synthesis method according to claim 1, characterized in that The stirring rate in step (2) is 300-800 rpm.

6. The synthesis method according to claim 1, characterized in that In step (3), the strength of the external magnetic field is 0.3-1.0 T, and the catalyst recovery time is 30-120 seconds.

7. The synthesis method according to claim 1, characterized in that The catalyst was recovered and washed with ethanol and vacuum dried at 60-80° C. for regeneration. After being recycled 20 times, the reaction yield remained above 90% compared with the first time.

8. The synthesis method according to claim 1, characterized in that In step (4), the recrystallization solvent is ethanol or methanol, and the column chromatography eluent is a mixed solvent of petroleum ether and ethyl acetate in a volume ratio of (5:1) to (15:1).

Images