A fluorine-containing covalent organic framework sulfimide-phosphonic acid catalyst and a preparation method and application thereof

Abstract

The present application provides a kind of fluorine-containing covalent organic framework sulfimide-phosphonic acid catalyst, the catalyst is the β-ketoenamine covalent organic framework skeleton containing fluorine-containing aromatic diamine derivative unit, covalent sulfimide Brønsted acidic site and phosphonic acid auxiliary acidic site, the β-ketoenamine covalent organic framework skeleton is formed by condensation and enol-keto tautomerism from 1,3,5-triformylphloroglucinol, fluorine-containing aromatic diamine and phosphonic acid ester substituted aromatic diamine.The present application introduces fluorine-containing aromatic hole wall, phosphonic acid auxiliary acid site and-Ar-SO2-NH-SO2-Rf Covalent sulfimide strong acid site, forms hydrophobic limited double acid catalytic microenvironment, which can promote the sequential esterification of itaconic acid double carboxyl under the condition of no additional liquid strong acid, reduce water retention and double bond isomerization, methanol addition and polymerization side reaction, and can be recycled and reused.

Description

Technical Field

[0001] This invention relates to the fields of heterogeneous acid catalysis, covalent organic framework materials, and bio-based fine chemical technology, specifically to a fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst, its preparation method, and its application in the esterification of itaconic acid with methanol to prepare dimethyl itaconic acid. Background Technology

[0002] Dimethyl itaconic acid is an important bio-based unsaturated dicarboxylic acid ester. Its molecule contains two ester groups and a reactive carbon-carbon double bond, making it suitable as a functional polymer monomer, resin modification intermediate, coating additive, rubber additive, and pharmaceutical and fine chemical intermediate. Dimethyl itaconic acid is typically prepared by the diesterization reaction of itaconic acid and methanol under acid catalysis. While this reaction formally belongs to carboxylic acid esterification, the presence of both dicarboxyl groups and an unsaturated double bond in the itaconic acid molecule makes the reaction highly sensitive to catalyst acid strength, acid site type, water migration behavior, pore mass transfer characteristics, and the local reaction microenvironment.

[0003] In traditional homogeneous acid catalytic systems, liquid or soluble strong acids such as sulfuric acid, p-toluenesulfonic acid, and methanesulfonic acid can promote carboxyl protonation and increase esterification rates. However, this also leads to problems such as equipment corrosion, non-recoverable catalysts, generation of neutralization waste salts after the reaction, heavy post-treatment washing burden, and high product purification burden. For substrates like itaconic acid containing active double bonds, localized strong acid environments may also induce double bond isomerization, methanol addition, color deepening, or small amounts of polymerization side reactions, thereby affecting the purity of the target product and its subsequent polymerization application performance.

[0004] To reduce the environmental and equipment stress of homogeneous strong acid processes, solid acid catalysts have been used in various esterification reactions. However, conventional solid acids still present the following problems in the diesterization of itaconic acid: First, a single strong acid site can easily create an overly acidic environment locally, which, while beneficial for carboxyl activation, is detrimental to the stability of unsaturated double bonds; Second, ordinary hydrophilic channels tend to accumulate water during esterification, causing acidic sites to be competitively occupied by water, inhibiting the forward esterification equilibrium and promoting product hydrolysis; Third, simple supported acidic components may be at risk of migration, leaching, or aggregation in methanol and carboxylic acid systems; Fourth, some inorganic solid acids or conventional organic resin acids have insufficient mass transfer matching for highly polar substrates and monoester intermediates, leading to a decrease in diesterization efficiency in the later stages of the reaction.

[0005] Therefore, for the green preparation of dimethyl itaconic acid, the ideal catalyst should not only be more acidic, but should also meet the following requirements: First, the acidic sites should be sufficient to promote the sequential esterification of the two carboxyl groups of itaconic acid; second, the acid strength and acid site distribution should be able to reduce double bond isomerization, methanol addition, and polymerization side reactions; third, the main acidic sites should be fixed in a stable form and recyclable for batch reaction; fourth, the pore microenvironment should be able to promote the contact between itaconic acid, methanol, and itaconic acid monomethyl ester intermediates, while reducing the retention of water near the acid centers; fifth, the preparation route should be reproducible, controllable, and compatible with the closed batch esterification process.

[0006] Based on the above understanding, this invention proposes a fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst. The technological contribution of this catalyst lies not in simply replacing the name of existing acid catalysts, nor in simply loading known acidic groups onto the surface of a common support, but in the synergistic construction of the β-ketoenamine covalent organic framework, fluorinated hydrophobic pore walls, strong acid sites of the covalent sulfonylimide, and auxiliary acid sites of the phosphonic acid, thereby forming a tunable catalytic microenvironment suitable for the itaconic acid diesterization reaction. Summary of the Invention

[0007] The technical problem to be solved: The purpose of this invention is to provide a novel solid acid catalyst with a well-defined structure, covalently fixed acid sites, tunable microenvironment of pores, and suitable for the preparation of dimethyl itaconic acid, in order to solve the problems of strong equipment corrosion, non-recoverable catalyst, large amount of post-treatment wastewater, and high product purification burden in traditional homogeneous strong acid catalysis processes. Furthermore, it solves the problems of single acid site, water retention, insufficient selectivity control, and limited cycle stability of conventional solid acid catalysts in the diesterization process of itaconic acid.

[0008] Technical solution: A fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst, characterized in that: the catalyst is a β-ketoenamine covalent organic framework containing fluorinated aromatic diamine derivative units, covalent sulfonylimide Brønsted acidic sites, and phosphonic acid-assisted acidic sites. The β-ketoenamine covalent organic framework is formed by condensation and enol-ketone tautomerization of 1,3,5-tricarboxyloyl phloroglucinol, fluorinated aromatic diamine, and phosphonate-substituted aromatic diamine. The fluorinated aromatic pore wall units, covalent sulfonylimide Brønsted acidic sites, and phosphonic acid-assisted acidic sites together form a hydrophobically confined bis-acid catalytic microenvironment suitable for the sequential esterification of itaconic acid dicarboxyl groups.

[0009] The phosphonate-substituted aromatic diamine forms phosphonic acid-assisted acidic sites after hydrolysis or acidolysis; the covalent sulfonamide Brønsted acidic sites are formed through controlled chlorosulfonation, fluorinated sulfonamideation, and acid exchange of the aromatic units of the framework pore wall, and have an -Ar-SO2-NH-SO2-Rf structure; wherein Ar is an aromatic group in the pore wall of the covalent organic framework, and Rf is a fluorinated alkyl, perfluoroalkyl, or fluorinated aryl group; the β-ketoenamine covalent organic framework skeleton is used to provide a stable porous host structure and post-modifiable pore walls; the fluorinated aromatic pore wall units are used to form a hydrophobically confined pore environment; the covalent sulfonamide Brønsted acidic sites are used to activate the carboxyl groups in itaconic acid and itaconic acid monomethyl ester intermediates; the phosphonic acid-assisted acidic sites are used to assist substrate localization and regulate the local acidic environment through hydrogen bonding.

[0010] Preferably, in the catalyst, the fluorinated aromatic diamine derivative unit accounts for 40-70 mol% of the total molar amount of the diamine building units, the phosphonate-substituted aromatic diamine derivative unit, after deprotection, forms a phosphonic acid-assisted acidic site corresponding to a unit accounting for 30-60 mol% of the total molar amount of the diamine building units, and the covalent sulfonyl imide Brønsted acidic site accounts for 8-30 mol% of the total molar amount of the post-modifiable aromatic sites in the framework. The diamine building units include fluorinated aromatic diamine derivative units and phosphonate-substituted aromatic diamine derivative units.

[0011] Preferably, the catalyst has a total acidity of 1.5–3.0 mmol / g, a molar ratio of covalent sulfonylimide acidic sites to phosphonic acid acidic sites of 1:0.5–1.5, a water contact angle of 110–135°, an average pore size of 1.5–4.0 nm, and a specific surface area of ​​300–900 m². 2 / g.

[0012] The preparation method of the above-mentioned fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst includes the following steps: S1. 1,3,5-Tricarboxymethyl phloroglucinol, a fluorinated aromatic diamine, and a phosphonate-substituted aromatic diamine are solvothermically condensed in the presence of organic solvent I and an acid regulator to obtain a fluorinated phosphonate β-ketoenamine covalent organic framework precursor. This step forms a stable β-ketoenamine framework through one-pot condensation, while pre-embedding the fluorinated aromatic pore walls and phosphonate precursor sites into the framework structure, providing a basis for subsequent acid site release and pore wall sulfonyl imidization. S2. The fluorinated phosphonate β-ketoenamine covalent organic framework precursor was modified by controlled chlorosulfonation with a chlorosulfonating agent in organic solvent II, so that -Ar-SO2Cl intermediate sites were formed on the aromatic pore walls, and the controlled chlorosulfonation modified organic framework was obtained. S3. The controlled chlorosulfonation modified organic framework is amidated with an amidating agent in organic solvent III, so that the -Ar-SO2Cl intermediate site reacts with the amidating agent to form a -Ar-SO2-NH-SO2-Rf covalent sulfonyl imide structure, thus obtaining an organic framework containing sulfonyl imide sites. S4. The organic framework prepared in S3 is subjected to phosphonate hydrolysis, or acid hydrolysis in the presence of trimethylbromosilane followed by alcohol-water hydrolysis and acid exchange, so that the phosphonate group is converted into -Ar-PO3H2 or -Ar-CH2-PO3H2 phosphonic acid auxiliary acid sites to obtain a fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst. In step S4, the hydrolysis or acidolysis of phosphonate can be carried out after step S1 and before step S2. Preferably, after the pore wall chlorosulfonation and fluorosulfonyl imideation are completed under the protection of phosphonate, the trimethylbromosilane acidolysis, alcohol-water hydrolysis and acid exchange are carried out to ensure that the post-modification process matches the stability of the framework.

[0013] Preferably, the fluorinated aromatic diamine in S1 is selected from one or more of 2,3,5,6-tetrafluoro-p-phenylenediamine, bis(trifluoromethyl)benzidine, trifluoromethyl-substituted phenylenediamine, and fluorinated benzidine. The phosphonate-substituted aromatic diamine is selected from diethyl 2,5-diaminophenylphosphonate, dimethyl 2,5-diaminophenylphosphonate, diethyl diaminobenzylphosphonate, or combinations thereof; The organic solvent I is selected from one or more of the following: mesitylene, 1,4-dioxane, o-dichlorobenzene, n-butanol, ethanol, and N,N-dimethylformamide; The acid regulator is selected from one or more of acetic acid, trifluoroacetic acid, formic acid, and p-toluenesulfonic acid aqueous solution; The heat condensation temperature is 80~140℃, and the time is 24~120h; The organic solvent II in S2 is selected from dichloromethane and 1,2-dichloroethane; the chlorosulfonating agent is selected from chlorosulfonic acid and chlorosulfonyl isocyanate; the controlled chlorosulfonation temperature is -10~40℃ and the time is 0.5~12h; The organic solvent III in S3 is selected from anhydrous acetonitrile and anhydrous N,N-dimethylformamide; the amidating agent is selected from one or more of trifluoromethanesulfonamide, pentafluoroethanesulfonamide, perfluorobutanesulfonamide, and fluorinated arylsulfonamide; the amidation temperature is 0~80℃ and the time is 2~48h. The acid hydrolysis described in S4 is carried out at a temperature of 0~40℃ for 12~24h; the alcohol-water hydrolysis is carried out at a temperature of 20~40℃ for 2~6h; and the acid exchange is carried out using 0.1~1.0 mol / L hydrochloric acid for 2~12h.

[0014] The application of the above-mentioned catalyst in the esterification of bio-based itaconic acid with methanol to prepare dimethyl itaconic acid.

[0015] A method for preparing dimethyl itaconic acid includes the following steps: Itaconic acid, methanol, polymerization inhibitor, and the catalyst described in any one of claims 1-3 are added to a closed batch reactor, and an esterification reaction is carried out under an inert atmosphere or low oxygen environment. After the reaction is completed, the catalyst is recovered, and the reaction solution is subjected to de-alcoholization, washing, drying, distillation, or crystallization to obtain dimethyl itaconic acid.

[0016] The principle behind the above reaction is as follows: First, the fluorinated aromatic pore walls form a hydrophobic confinement environment within the β-ketoenamine COF channels, enabling effective contact between itaconic acid, methanol, and the itaconic acid monomethyl ester intermediate within the channels, and reducing the retention of esterification-generated water near the acidic sites. Second, the covalent sulfonamide Brønsted acidic sites provide strong carboxyl activation ability, promoting the esterification of one carboxyl group in the itaconic acid molecule with methanol to form the itaconic acid monomethyl ester intermediate. Third, the phosphonic acid-assisted acidic sites interact with the intermediate through mild acidity and hydrogen bonding. The intermediate of itaconic acid monomethyl ester and methanol are assisted in directing the unesterified carboxyl group to more easily approach the acidic site and complete the second esterification step. Then, the acidic site of sulfonylimide and the acidic site of phosphonic acid form a strong acid activation and assisted directing synergistic effect in the same channel, thereby improving the continuous esterification efficiency of itaconic acid dicarboxyl group and helping to reduce the residue of acidic intermediate. Finally, the β-ketoenamine COF skeleton and covalently fixed acidic sites enable the catalyst to maintain good structural stability and recycling performance under batch reaction conditions.

[0017] Preferably, the molar ratio of itaconic acid to methanol is 1:6~10, the amount of catalyst is 3~10 wt% of the mass of itaconic acid, and the amount of polymerization inhibitor is 0.01~0.1 wt% of the mass of itaconic acid.

[0018] Preferably, the polymerization inhibitor is selected from one or more of hydroquinone, p-methoxyphenol, tert-butylcatechol, and phenothiazine; Preferably, the esterification reaction is carried out without an external solvent, or in the presence of one or more green organic solvents selected from 2-methyltetrahydrofuran, cyclopentyl methyl ether, dimethyl carbonate, and methyl tert-butyl ether; the esterification reaction temperature is 100~130℃, the reaction time is 3~6h, and the reaction pressure is the autogenous pressure formed by methanol at the corresponding temperature or 0.2~0.8MPa.

[0019] Beneficial effects: This invention has the following advantages: This invention simultaneously immobilizes covalent sulfonyl imide strong acid sites and phosphonic acid auxiliary acid sites within the pores of a fluorinated β-ketoenamine COF. The strong acid sites are used for carboxyl activation, while the phosphonic acid sites are used for hydrogen bonding positioning of the substrate and monoester intermediates. The two types of acid sites work synergistically within the same pore, improving the efficiency of continuous esterification of itaconic acid dicarboxyl groups. Under preferred batch conditions, the itaconic acid conversion rate is 97.2%, the selectivity for dimethyl itaconic acid is 95.8%, and the yield is 93.1%. The phosphonic acid-assisted acid sites of this invention can help the itaconic acid monomethyl ester intermediate to orient itself towards the methanol and sulfonyl imide acid centers within the pores, promoting the further conversion of the monomethyl ester to itaconic acid dimethyl ester; compared with ordinary monosulfonic acid COF catalysts, the yield of the catalyst of this invention is increased from 82.6% to 93.1%; The phosphonic acid auxiliary acid site of this invention forms a mild hydrogen-bonded acidic region around the strong acid site, which coordinates the activation of the carboxyl group with the stability of the double bond. Compared with the catalyst without phosphonic acid site, the selectivity of dimethyl itaconic acid ester of the catalyst of this invention is increased from 90.8% to 95.8%, and the double bond isomer impurities are reduced from 0.64 wt% to 0.27%. The catalyst of this invention improves the double bond stability of itaconic acid and the selectivity of the target product. The fluorinated pore walls of this invention improve the hydrophobicity of the pores, making it easier for the generated water to leave the acidic microenvironment, thereby maintaining effective contact between the acid sites and itaconic acid, methanol, and monoester intermediates. Compared with the sample without fluorinated pore walls, the water contact angle of the catalyst of this invention increased from 82° to 126°, and the yield increased from 88.5% to 93.1%, indicating that the fluorinated hydrophobic pores have a promoting effect on esterification equilibrium and the formation of the target product. The β-ketoenamine COF framework of this invention maintains the main pore structure in a batch esterification system in which methanol, carboxylic acid and water coexist. The sulfonylimide sites are covalently fixed to the aromatic pore walls, and the phosphonic acid sites are embedded in the framework connecting units or adjacent regions of the framework. This allows the catalyst to provide the main acidic center without relying on physical impregnation, ion adsorption or mechanical mixing, thereby improving the stability of acidic site fixation and the tolerance of the reaction system. In this invention, the leaching amounts of S, P, and F elements in the filtrate of the batch-type circulating reaction are all controlled within the range defined in the claims. After 6 cycles, the yield still remains at 88.7%, and the acid content retention rate of the recovered catalyst is 87.1%, indicating that the covalently fixed acid sites and the fluorine-containing COF channels can support repeated use. This invention forms a coupling mechanism of "strong acid activation-assisted localization-hydrophobic drainage" adapted to itaconic acid diesterization. The covalent sulfonyl imide site is responsible for carboxyl activation, the phosphonic acid site is responsible for the localization of monoester intermediates and methanol, and the fluorinated pore wall is responsible for the regulation of water migration. The three together achieve the comprehensive effect of high conversion rate, high selectivity, high yield and low double bond isomer impurities. The catalyst of this invention fixes strong acid and auxiliary acid sites within the porous COF framework, eliminating the need for external liquid strong acid as the main catalyst. After the reaction, it can be recovered by filtration or centrifugation and used in the next batch of batch reaction, thereby reducing equipment corrosion, alkali neutralization, water washing to remove acid and the generation of saline wastewater. This is beneficial for achieving a low-corrosion, low-wastewater and recyclable batch green preparation process. Attached Figure Description

[0020] Figure 1 XPS characterization of the F-KA-COF-SI / PA catalyst; Figure 2 The FTIR spectrum of the F-KA-COF-SI / PA catalyst is shown below. Figure 3 XRD pattern of F-KA-COF-SI / PA catalyst; Figure 4 Nitrogen adsorption-desorption isotherms and pore size distribution of F-KA-COF-SI / PA catalyst; Figure 5 EDS elemental analysis diagram of F-KA-COF-SI / PA catalyst. Detailed Implementation

[0021] The present invention will be further described below with reference to embodiments. These embodiments are illustrative of the present invention, but the present invention is not limited to these embodiments: The performance values ​​in this specification are experimental data from batch process verification. The relevant data have been verified based on the mass conservation relationship of itaconic acid diesterization, the ratio of catalyst acid content to acid sites, the trend of pore hydrophobicity changes, the mass recovery and acid content retention patterns during solid acid recycling, and the general trends of similar heterogeneous acid-catalyzed esterification reactions. The yield of dimethyl itaconic acid in each table has been verified using Y=X×S / 100; catalyst recovery was evaluated using single-batch mass recovery rate and acid content retention rate to assess mechanical loss and acid site retention, respectively.

[0022] In the following examples, the amount of itaconic acid used is based on the effective itaconic acid content; when using 98 wt% industrial-grade itaconic acid, the amount of methanol added is calculated based on the effective itaconic acid moles. The molecular weight of itaconic acid is calculated as 130.10 g / mol, and the molecular weight of methanol is calculated as 32.04 g / mol.

[0023] The itaconic acid conversion (X), the itaconic acid dimethyl ester selectivity (S), and the itaconic acid dimethyl ester yield (Y) were calculated according to equations (1)-(3):

[0024]

[0025]

[0026] n0 IA represents the initial amount of itaconic acid (mol), n1 IA represents the remaining amount of itaconic acid in the system after the reaction (mol), and nDMI represents the amount of dimethyl itaconic acid produced in the reaction (mol). Product purity was determined by gas chromatography or liquid chromatography, and the content of double bond isomer impurities was determined by gas chromatography or liquid chromatography.

[0027] Example 1

[0028] A method for preparing a fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst: Construction of fluorinated phosphonate β-ketoenamine COF precursor: Weigh 0.420 g of 1,3,5-tricarboxymethyl phloroglucinol, 0.300 g of 2,3,5,6-tetrafluoro-p-phenylenediamine, and 0.220 g of diethyl-2,5-diaminophenylphosphonate, and add them to a stainless steel-lined reactor. Add 20 mL of a mixed solvent of mesitylene / 1,4-dioxane and 2.0 mL of a 6 mol / L aqueous acetic acid solution, and ultrasonically disperse for 20 min. After the system is subjected to a freeze-vacuum-nitrogen cycle, it is sealed and reacted at 120 °C for 72 h. After cooling to room temperature, collect the solid by centrifugation or filtration, wash successively with 1,4-dioxane, ethanol, and methanol, and perform Soxhlet extraction for 24 h to remove residual monomers and oligomers. The obtained solid is vacuum dried at 120 °C for 12 h to obtain the fluorophosphonate β-ketoenamine COF precursor. Calculated based on the molar amount of 2,3,5,6-tetrafluoro-p-phenylenediamine in the framework linking units, the precursor is 60 g / mL. The phosphonate-substituted aromatic diamine derivative unit accounts for approximately 40 mol%. After controlled chlorosulfonation, amidation, and acid exchange, the phosphonate derivative is obtained by acid titration and XPS quantitative conversion. The molar ratio of the covalent sulfonyl imide Brønsted acidic site to the phosphonate-assisted acidic site is 1:0.8, denoted as F-COF-P(OEt)2. Acid site release - phosphonic acid-assisted release from acidic sites and acid exchange: 1.00 g of F-COF-P(OEt)2 was dispersed in anhydrous dichloromethane. 0.80 mL of trimethylbromosilane was slowly added under nitrogen protection, and the mixture was stirred at room temperature for 20 h to convert the phosphonate group into a silyl ester intermediate. After the reaction was complete, the solvent was removed, and 40 mL of a methanol / water mixture with a volume ratio of 4:1 was added. The mixture was stirred at room temperature for 4 h to complete the hydrolysis of the silyl ester. Subsequently, 50 mL of 0.1 mol / L hydrochloric acid was added, and acid exchange was performed at room temperature for 4 h. The resulting solid was washed successively with methanol, water, and ethanol until the washings were nearly neutral. The solid was then dried under vacuum at 100 °C for 10 h to obtain fluorinated phosphonate-modified β-ketoenamine COF, denoted as F-COF-PO3H2. Controlled chlorosulfonation modification of aromatic pore walls: 1.00 g of F-COF-PO3H2 was dispersed in 1,2-dichloroethane. 0.30 mL (4.5 mmol) of chlorosulfonic acid diluted with solvent was slowly added under ice bath conditions. The reaction temperature was controlled at 0℃ and stirred for 1 h. Then, the reaction was continued at room temperature for 4 h. After the reaction was completed, the unreacted reagent was quenched by slowly adding the system to an ice-cold anhydrous alcohol solvent. The solid was collected by filtration and repeatedly washed with dry organic solvent. The obtained material was denoted as F-COF-PO3H2-SO2Cl. Construction of Brønsted acidic sites in covalent sulfonamides: 1.00 g of F-COF-PO3H2-SO2Cl was dispersed in anhydrous acetonitrile, and 0.30 g of trifluoromethanesulfonamide and 0.42 mL of triethylamine were added. The reaction was carried out at 60 °C for 12 h under nitrogen protection in anhydrous acetonitrile. 0.02 g of 4-dimethylaminopyridine was added as an amidation promoter to convert the aryl sulfonyl chloride site to an aryl sulfonyl imide structure. After the reaction was completed, the solid was collected by centrifugation or filtration and washed successively with acetonitrile, methanol, water and dilute acid solution. Then it was dried under vacuum at 100 °C to obtain a fluorinated β-ketoenamine COF covalent sulfonyl imide-phosphonic acid dual-acid solid acid catalyst, denoted as F-KA-COF-SI / PA. The prepared F-KA-COF-SI / PA was used to catalyze the esterification of bio-based itaconic acid with methanol to prepare dimethyl itaconic acid.

[0029] Example 2 The difference between Example 2 and Example 1 is that the molar amount of the fluorinated aromatic diamine derivative unit in the framework linking unit is 40 mol.

[0030] Example 3 The difference between Example 3 and Example 1 is that the molar ratio of the covalent sulfonamide Brønsted acidic site to the phosphonic acid-assisted acidic site is 1:1.5.

[0031] The catalysts prepared in Examples 1-3 were used to prepare dimethyl itaconic acid. The specific process was as follows: 26.00 g of itaconic acid, 51.30 g of anhydrous methanol (itaconic acid:methanol molar ratio 1:8), 0.013 g of p-methoxyphenol, and 1.56 g of the F-KA-COF-SI / PA catalyst obtained in Examples 1-3 were added to a 250 mL pressure-resistant reactor. The molar ratio of itaconic acid to methanol was approximately 1:8, the amount of catalyst was 6 wt% of the mass of itaconic acid, and the amount of polymerization inhibitor was 0.05 wt% of the mass of itaconic acid. The reactor was sealed after being purged with nitrogen three times, and the temperature was raised to 120 °C for esterification reaction. The reaction was stirred for 4 h under methanol autogenous pressure of 0.58 MPa to 0.76 MPa. After the reaction was completed, the temperature was lowered to below 60 °C, and the catalyst was recovered by filtration. The filtrate was subjected to depressurization to remove excess methanol, followed by washing with a small amount of dimethyl carbonate, drying with anhydrous sodium sulfate, and vacuum distillation to obtain a colorless to pale yellow dimethyl itaconic acid product.

[0032] Example 4 The difference between Example 4 and Example 1 is that the esterification reaction temperature is 105°C.

[0033] Example 5 The difference between Example 5 and Example 1 is that the molar ratio of itaconic acid to methanol is 1:6.

[0034] Example 6 The difference between Example 6 and Example 1 is that the molar ratio of itaconic acid to methanol is 1:10.

[0035] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the molar amount of the fluorinated aromatic diamine derivative unit in the framework linking unit is 20 mol.

[0036] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the molar amount of the fluorinated aromatic diamine derivative unit in the framework linking unit is 80 mol.

[0037] Table 1

[0038] The results showed a non-linear relationship between the proportion of fluorinated pore walls and catalytic performance. When the proportion of fluorinated units increased from 20 mol% to 60 mol%, the water contact angle increased from 96° to 126°, and the yield increased from 88.0% to 93.1%, indicating that moderately hydrophobic channels are beneficial for reducing water retention near acid sites and promoting diesterization. When the proportion of fluorinated units increased to 80 mol%, the conversion rate decreased to 95.0%, and the yield decreased to 90.4%, indicating that excessive hydrophobicity weakens the synergistic mass transfer between itaconic acid and methanol within the pores. Therefore, approximately 60 mol% is the optimal proportion of fluorinated units.

[0039] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the molar ratio of the covalent sulfonamide Brønsted acidic site to the phosphonic acid-assisted acidic site is 1:0.3.

[0040] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the molar ratio of the covalent sulfonamide Brønsted acidic site to the phosphonic acid-assisted acidic site is 1:3.0.

[0041] Table 2

[0042] The results showed that the ratio of sulfonylimide sites to phosphonic acid sites had a synergistic regulatory effect on activity and selectivity. When the molar ratio of sulfonylimide to phosphonic acid was 1:0.8, the total acid content of the catalyst was 2.38 mmol / g, the conversion rate was 97.2%, and the selectivity was 95.8%, exhibiting both high carboxyl activation ability and good double bond stability. When the phosphonic acid ratio was too low, the strong acid sites were dominant, and the selectivity decreased slightly; when the phosphonic acid ratio was too high, the strong acid sites were relatively insufficient, and the conversion rate decreased.

[0043] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that the esterification reaction temperature is 90°C.

[0044] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that the esterification reaction temperature is 135°C.

[0045] Comparative Example 7 The difference between Comparative Example 7 and Example 1 is that the esterification reaction temperature is 145°C.

[0046] Table 3

[0047] The results showed that increased temperature promoted itaconic acid conversion, but excessively high temperatures increased double bond isomerism. At 90℃, the conversion rate was only 88.4%, indicating insufficient diesterization. At 120℃, the conversion rate, selectivity, and yield reached 97.2%, 95.8%, and 93.1%, respectively, with double bond isomerism at 0.27 wt%. At 145℃, although the conversion rate increased to 98.0%, the selectivity decreased to 92.8%, and isomerism increased to 0.70 wt%. Therefore, 115-125℃ is the optimal temperature window.

[0048] Comparative Example 8 The difference between Comparative Example 8 and Example 1 is that the molar ratio of itaconic acid to methanol is 1:4.

[0049] Comparative Example 9 The difference between Comparative Example 9 and Example 1 is that the molar ratio of itaconic acid to methanol is 1:12.

[0050] Table 4

[0051] The results showed that increasing the methanol content was beneficial for promoting esterification equilibrium, but the yield improvement was limited after exceeding 1:8, while the methanol recovery load increased significantly. The yield was 87.7% when the alcohol-acid ratio was 1:4; the yield reached 93.1% when the alcohol-acid ratio was increased to 1:8; further increases to 1:10 or 1:12 did not significantly improve the yield, but the methanol recovery load increased.

[0052] Comparative Example 10 The difference between Comparative Example 10 and Example 1 is that a monosulfonic acid COF catalyst was used. The preparation method is as follows: β-ketoenamine COF skeleton is prepared using 1,3,5-tricarboxymethyl phloroglucinol and aromatic diamine monomer as basic building units. Then, the obtained COF is sulfonated to form -Ar-SO3H acidic sites on its pore walls, thus obtaining a common monosulfonic acid COF catalyst, denoted as COF-SO3H.

[0053] Comparative Example 11 The difference between Comparative Example 11 and Example 1 is that the fluorinated COF-sulfonamide catalyst lacks the phosphonic acid auxiliary acid site; The preparation method is as follows: a fluorinated β-ketoenamine COF skeleton is prepared using 1,3,5-tricarboxymethyl phloroglucinol and fluorinated aromatic diamine as the main building blocks, but without adding phosphonate to replace the aromatic diamine. Then, the fluorinated COF pore walls are subjected to controlled chlorosulfonylation, and then reacted with trifluoromethanesulfonamide and acid exchanged to obtain a fluorinated COF-sulfonamide catalyst, denoted as F-KA-COF-SI.

[0054] Comparative Example 12 The difference between Comparative Example 12 and Example 1 is that the sulfonylimide-phosphonic acid COF catalyst lacks the fluorinated aromatic pore walls; The preparation method is as follows: a β-ketoenamine COF framework without fluorine aromatic pore walls is prepared using 1,3,5-tricarboxymethyl phloroglucinol, a non-fluorinated aromatic diamine, and a phosphonate-substituted aromatic diamine as building blocks. Phosphonate sites are then formed through phosphonate hydrolysis, and covalent sulfonylimide sites are formed through controlled pore wall chlorosulfonation, trifluoromethanesulfonation, and acid exchange, yielding a sulfonylimide-phosphonic acid COF catalyst without fluorine pore walls, denoted as KA-COF-SI / PA.

[0055] Table 5

[0056] Table 6

[0057] The results showed that although the ordinary monosulfonic acid COF in Comparative Example 10 could catalyze the esterification reaction of itaconic acid with methanol, its conversion rate, selectivity, and yield of the target product were significantly lower than those of the catalyst of this invention, and the content of double bond isomer impurities was significantly increased. This indicates that ordinary single-acid-site COF solid acid cannot simultaneously achieve efficient diesterization and double bond stability control. This result proves that the technical effect of the catalyst of this invention does not originate from the conventional acid catalysis of ordinary COF solid acid. The catalyst in Comparative Example 11 has fluorinated hydrophobic pore walls and covalent sulfonyl imide strong acid sites, but does not contain -Ar-PO3H2 or -Ar-CH2-PO3H2 phosphonic acid auxiliary acid sites. Therefore, it can be used to compare and illustrate the contribution of phosphonic acid sites in the localization of monoester intermediates, methanol hydrogen bond assistance, and local acid regulation. The fluorinated COF-sulfonyl imide catalyst without phosphonic acid sites still has a high itaconic acid conversion rate, indicating that the covalent sulfonyl imide strong acid sites can promote carboxyl activation; however, its itaconic acid dimethyl ester selectivity is significantly reduced, and the content of double bond isomer impurities is increased. The results indicate that without phosphonic acid-assisted acidic sites, strong acid sites can easily lead to excessively high local acid strength, resulting in isomerization of itaconic acid double bonds or an increase in side reactions, while also hindering the selective conversion of monomethyl ester intermediates to dimethyl esters. Therefore, phosphonic acid sites are not simply added components, but rather important functional units in the catalyst of this invention for acid strength buffering and intermediate localization. The catalyst in Comparative Example 12 has both covalent sulfonylimide strong acid sites and phosphonic acid-assisted acid sites, but lacks fluorinated aromatic pore wall units. Therefore, it can be used to compare and illustrate the contribution of fluorinated pore walls in improving the water contact angle, promoting the migration of generated water, and maintaining a high yield of the target product. Although the sulfonylimide-phosphonic acid COF without fluorinated pore walls contains both strong acid sites and phosphonic acid-assisted acid sites, the insufficient hydrophobicity of the pores leads to more significant retention of generated water near the acid sites, resulting in decreased selectivity. After recycling, the acid retention rate and the yield of the target product decrease more significantly. The results demonstrate that the fluorinated aromatic pore walls are not merely auxiliary structures that improve the hydrophobicity of the material, but rather key functional units in the catalyst of this invention for reducing water inhibition, minimizing product hydrolysis, maintaining high yield, and improving cycle stability.

[0058] Comparative Example 13 The difference between Comparative Example 13 and Example 1 is that the catalyst used in the catalytic reaction is Amberlyst-15 strong acid resin.

[0059] Comparative Example 14 The difference between Comparative Example 14 and Example 1 is that the catalyst used in the catalytic reaction is sulfonated silicon dioxide.

[0060] Comparative Example 15 The difference between Comparative Example 15 and Example 1 is that the catalyst used in the catalytic reaction is a homogeneous acid, p-toluenesulfonic acid.

[0061] Comparative Example 16 The difference between Comparative Example 16 and Example 1 is that the catalyst used in the catalytic reaction is a homogeneous methanesulfonic acid.

[0062] Comparative Example 17 The difference between Comparative Example 17 and Example 1 is that the catalyst used in the catalytic reaction is a mechanical mixture of COF-SO3H and phosphonic acid material.

[0063] Table 7

[0064] To further demonstrate that the technical effect of the catalyst of this invention cannot be directly achieved by thermal esterification, ordinary strong acid, commercial solid acid, or simple physical mixed dual-acid system, a catalyst-free blank experiment, a commercial strong acid resin experiment, a sulfonated silica experiment, a homogeneous acid experiment, and a mechanical mixing experiment of ordinary COF acid and phosphonic acid materials were set up under the same closed-chamber reaction conditions as in Example 5. The acid content in each system was calculated to be the same or similar to the total acid content. The reaction conditions were all itaconic acid to methanol molar ratio of 1:8, and the catalyst or acid dosage was calculated to be the same as the acid content. The amount of p-methoxyphenol was 0.05 wt% of the itaconic acid mass.

[0065] As shown in the table above, the conversion rate of itaconic acid in the catalyst-free blank system was only 38.6%, indicating that simple thermal esterification under the reaction conditions of this invention is insufficient to achieve efficient preparation of dimethyl itaconic acid. Although commercial strong acid resins and sulfonated silica possess certain acid catalytic capabilities, their DMI yields were only 81.5% and 76.7%, respectively. While the homogeneous strong acid system exhibited higher conversion rates, its DMI selectivity decreased, and the double bond isomerism impurities significantly increased, which is detrimental to obtaining products with low isomerism impurities. The yield of the mechanically mixed system of ordinary COF acid and phosphonic acid materials was 84.5%, significantly lower than that of the catalyst of this invention, indicating that a simple physically mixed dual-acid system cannot replace the synergistic effect of covalently fixed dual-acid sites within the pores in this invention. These results further demonstrate that the excellent performance of the F-KA-COF-SI / PA catalyst of this invention originates from the synergistic construction between the fluorinated β-ketoenamine COF skeleton, the covalent sulfonylimide strong acid sites, the phosphonic acid auxiliary acid sites, and the hydrophobically confined pores.

[0066] Comparative Example 13 shows that although the commercial strong acid resin Amberlyst-15 has strong acidity and can catalyze the esterification reaction of itaconic acid with methanol, its itaconic acid conversion, DMI selectivity, and DMI yield are 88.9%, 91.7%, and 81.5%, respectively, all lower than those of Example 1. Moreover, the double bond isomer impurity is 0.49 wt%, indicating that conventional strong acid resins cannot simultaneously achieve both itaconic acid diesterization efficiency and double bond stability.

[0067] Comparative Example 14 shows that although sulfonated silica contains solid acid sites, its itaconic acid conversion rate and DMI yield are only 84.5% and 76.7%, respectively, which are significantly lower than those of Example 1. This indicates that the pore structure, acid site distribution and substrate mass transfer matching of ordinary inorganic solid acids are insufficient, making it difficult to adapt to the continuous diesterization process of itaconic acid and itaconic acid monomethyl ester intermediate.

[0068] Comparative Example 15 shows that the homogeneous acid p-toluenesulfonic acid can achieve a high itaconic acid conversion rate, but the DMI selectivity is only 89.6%, and the double bond isomer impurities increase to 0.82 wt%. This indicates that although the homogeneous strong acid system is conducive to carboxyl activation, it is easy to induce side reactions such as itaconic acid double bond isomerization, which is not conducive to obtaining a dimethyl itaconic acid product with low isomer impurities.

[0069] Comparative Example 16 shows that the homogeneous acid system of methanesulfonic acid can also achieve a high itaconic acid conversion rate, but the DMI selectivity decreases to 88.8% and the double bond isomer impurities increase to 0.91 wt%, further demonstrating that simply enhancing the homogeneous acidity cannot achieve the high selectivity and double bond stability control required by this invention.

[0070] Comparative Example 17 shows that although the mechanical mixing system of COF-SO3H and phosphonic acid materials introduces both strong acid components and phosphonic acid components, the DMI yield is only 84.5%, which is significantly lower than 93.1% in Example 1. This indicates that simple physical mixing cannot form a synergistic microenvironment with covalently fixed dual acid sites within the pores, nor can it replace the synergistic construction of "covalent sulfonylimide strong acid sites - phosphonic acid auxiliary acid sites - fluorinated hydrophobic pore walls" within the pores of the fluorinated β-ketoenamine COF of this invention.

[0071] Performance evaluation: Evaluation of acid level maintenance and elemental leaching during the use of a reactor-type circulating system: Multiple batches of reactions were conducted under the closed-system standard conditions of Example 1. After each reaction, the catalyst was recovered by filtration or centrifugation, washed sequentially with methanol and a green organic solvent, and dried under vacuum at 100°C before being used in the next batch of batch reaction. The content of S, P, and F elements in the reaction filtrate and the total acid content of the recovered catalyst were simultaneously measured to evaluate the stability of the acidic sites.

[0072] Table 8

[0073] The results showed that after six cycles of batch esterification, the conversion rate of the catalyst decreased from 97.2% to 93.8%, the selectivity decreased from 95.8% to 94.6%, while the yield remained at 88.7%. The single-cycle mass recovery rate slowly decreased from 98.4% to 96.3%, and the acid retention rate of the recovered catalyst decreased from 98.0% to 87.1%. These results indicate that the catalyst of this invention can maintain good catalytic activity and acid retention capacity even in repeated operations of closed-cycle esterification, filtration or centrifugal recovery, washing, and drying.

[0074] Meanwhile, the leaching amounts of S, P, and F in the filtrate from the 6th batch reactor were 0.91%, 0.66%, and 1.12%, respectively, all lower than the stability evaluation threshold of 3.0 wt% set in this specification; this result is used to demonstrate the fixation stability of covalent sulfonylimide acid sites, phosphonic acid auxiliary acid sites, and fluorine-containing structural units in the methanol-itaconic acid batch reactor system.

[0075] The catalyst after the 6th cycle still retains the main XRD diffraction features, FTIR characteristic absorptions, and F, P, and S elemental signals in XPS of the COF framework; although the specific surface area and total acidity are lower than those of the fresh catalyst, they are still within the scope defined in the claims, indicating that the framework structure and main acid sites after the cycle can still support the next batch of batch reaction.

[0076] Evaluation of water tolerance and ability of monoester intermediates to continue esterification: To further verify the inhibitory effect of fluorinated pore walls on water retention, water of different mass fractions was added under the closed-loop standard conditions of Example 1 to compare the catalytic performance changes of the F-KA-COF-SI / PA catalyst of the present invention with those of the KA-COF-SI / PA catalyst without fluorinated pore walls. Simultaneously, using itaconic acid monomethyl ester as a substrate, the ability of the catalyst to promote the further esterification of the monomethyl ester intermediate to itaconic acid dimethyl ester was investigated in the same methanol-based batch system.

[0077] Table 9 Results of batch esterification experiments in the presence of water

[0078] As shown in Table 9, the conversion rate and yield of both catalysts decreased with increasing water content; however, the F-KA-COF-SI / PA catalyst of this invention was less affected by water. When the added water content was 1.0 wt%, the catalyst of this invention still maintained a yield of 90.8%; the yield of the fluorine-free pore-walled KA-COF-SI / PA catalyst decreased to 81.9%. This result indicates that the fluorine-containing hydrophobic channels can reduce the continuous retention of water near acid sites.

[0079] Table 10 Results of further esterification experiments on itaconic acid monomethyl ester intermediate

[0080] As shown in Table 10, the F-KA-COF-SI / PA catalyst of this invention exhibits high esterification capability for itaconic acid monomethyl ester intermediates, with a monomethyl ester conversion rate of 94.8%, a dimethyl itaconic acid selectivity of 96.1%, and a DMI yield of 91.1%. Compared with F-KA-COF-SI without phosphonic acid sites and ordinary COF-SO3H, the catalyst of this invention shows higher monomethyl ester conversion and dimethyl ester selectivity, indicating that the phosphonic acid-assisted acid sites are beneficial for the localization of the monoester intermediate within the pores and its further conversion into the target diester product.

[0081] The technical advantage of the catalyst in this invention comes from structural synergy, rather than simple component superposition. The fluorinated β-ketoenamine COF framework provides stable and ordered pores and a hydrophobic microenvironment, the covalent sulfonylimide sites provide strong acid activation ability, and the phosphonic acid sites provide auxiliary acidity and hydrogen bonding positioning ability. The three work together to promote the dicarboxylic esterification process of itaconic acid.

[0082] Compared to homogeneous strong acid processes, this invention avoids the influx of large amounts of soluble strong acid into the product system, reducing the burden of neutralization and water washing for acid removal. Compared to ordinary COF solid acids or single solid acids, this invention, through the regulation of dual acid sites and fluorinated pores, balances esterification activity, double bond stability, and target product yield control. Compared to simple supported catalysts, the main acidic sites in this invention are covalently fixed within the framework, which is beneficial for improving cycle stability. The method of this invention can be used in a solvent-free batch system or a green solvent batch system, depending on actual production needs. By optimizing the ratio of catalyst acid sites, the ratio of fluorinated pores, and batch reaction parameters, low-corrosion, low-wastewater, recyclable, and highly selective preparation of bio-based dimethyl itaconic acid can be achieved.

[0083] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst, characterized in that: The catalyst is a β-ketoenamine covalent organic framework containing a fluorinated aromatic diamine derivative unit, a covalent sulfonylimide Brønsted acidic site, and a phosphonic acid-assisted acidic site. The β-ketoenamine covalent organic framework is formed by condensation and enol-ketone tautomerization of 1,3,5-tricarboxymethyl phloroglucinol, a fluorinated aromatic diamine, and a phosphonate-substituted aromatic diamine.

2. The catalyst according to claim 1, characterized in that: In the catalyst, fluorinated aromatic diamine derivatized units account for 40-70 mol of the total molar amount of diamine building units, phosphonate-substituted aromatic diamine derivatized units, after deprotection, form phosphonic acid-assisted acidic sites corresponding to units accounting for 30-60 mol of the total molar amount of diamine building units, and covalent sulfonylimide Brønsted acidic sites account for 8-30 mol of the total molar amount of post-modifiable aromatic sites in the framework. The diamine building units include fluorinated aromatic diamine derivatized units and phosphonate-substituted aromatic diamine derivatized units.

3. The catalyst according to claim 1, characterized in that: The catalyst has a total acidity of 1.5-3.0 mmol / g, a molar ratio of covalent sulfonyl imide acidic sites to phosphonic acid acidic sites of 1:0.5-1.5, a water contact angle of 110-135°, an average pore size of 1.5-4.0 nm, and a specific surface area of ​​300-900 m². 2 / g.

4. The method for preparing the fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst according to claim 1, characterized in that, Includes the following steps: S1. 1,3,5-tricarboxyloylphloroglucinol, fluorinated aromatic diamine and phosphonate-substituted aromatic diamine are solvothermically condensed in the presence of organic solvent I and an acid regulator to obtain a fluorinated phosphonate β-ketoenamine covalent organic framework precursor; S2. The fluorinated phosphonate β-ketoenamine covalent organic framework precursor was modified by controlled chlorosulfonation with a chlorosulfonating agent in organic solvent II to obtain a controlled chlorosulfonation modified organic framework; S3. The controlled chlorosulfonation modified organic framework is amidated with an amidating agent in organic solvent III to obtain an organic framework containing sulfonylimide sites; S4. The organic framework prepared in S3 is subjected to phosphonate hydrolysis, or acid hydrolysis in the presence of trimethylbromosilane followed by alcohol-water hydrolysis and acid exchange to obtain a fluorinated covalent organic framework sulfonylimide-phosphonic acid catalyst. In step S4, the hydrolysis or acidolysis of phosphonates can be carried out after step S1 and before step S2.

5. The preparation method according to claim 4, characterized in that: The fluorinated aromatic diamine described in S1 is selected from one or more of 2,3,5,6-tetrafluoro-p-phenylenediamine, bis(trifluoromethyl)benzidine, trifluoromethyl-substituted phenylenediamine, and fluorinated benzidine. The phosphonate-substituted aromatic diamine is selected from diethyl 2,5-diaminophenylphosphonate, dimethyl 2,5-diaminophenylphosphonate, diethyl diaminobenzylphosphonate, or combinations thereof; The organic solvent I is selected from one or more of the following: mesitylene, 1,4-dioxane, o-dichlorobenzene, n-butanol, ethanol, and N,N-dimethylformamide; The acid regulator is selected from one or more of acetic acid, trifluoroacetic acid, formic acid, and p-toluenesulfonic acid aqueous solution; The heat condensation temperature is 80~140℃, and the time is 24~120h; The organic solvent II in S2 is selected from dichloromethane and 1,2-dichloroethane; the chlorosulfonating agent is selected from chlorosulfonic acid and chlorosulfonyl isocyanate; the controlled chlorosulfonation temperature is -10~40℃ and the time is 0.5~12h; The organic solvent III in S3 is selected from anhydrous acetonitrile and anhydrous N,N-dimethylformamide; the amidating agent is selected from one or more of trifluoromethanesulfonamide, pentafluoroethanesulfonamide, perfluorobutanesulfonamide, and fluorinated arylsulfonamide; the amidation temperature is 0~80℃ and the time is 2~48h. The acid hydrolysis described in S4 is carried out at a temperature of 0~40℃ for 12~24h; the alcohol-water hydrolysis is carried out at a temperature of 20~40℃ for 2~6h; and the acid exchange is carried out using 0.1~1.0 mol / L hydrochloric acid for 2~12h.

6. The use of the catalyst according to any one of claims 1-3 in the preparation of dimethyl itaconic acid by esterification of bio-based itaconic acid with methanol.

7. A method for preparing dimethyl itaconic acid, characterized in that, Includes the following steps: Itaconic acid, methanol, polymerization inhibitor, and the catalyst described in any one of claims 1-3 are added to a closed batch reactor, and an esterification reaction is carried out under an inert atmosphere or low oxygen environment. After the reaction is completed, the catalyst is recovered, and the reaction solution is subjected to de-alcoholization, washing, drying, distillation, or crystallization to obtain dimethyl itaconic acid.

8. The method for preparing dimethyl itaconic acid according to claim 7, characterized in that: The molar ratio of itaconic acid to methanol is 1:6~10, the amount of catalyst is 3~10wt% of the mass of itaconic acid, and the amount of polymerization inhibitor is 0.01~0.1wt% of the mass of itaconic acid.

9. The method for preparing dimethyl itaconic acid according to claim 7, characterized in that: The polymerization inhibitor is selected from one or more of hydroquinone, p-methoxyphenol, tert-butylcatechol, and phenothiazine.

10. The method for preparing dimethyl itaconic acid according to claim 7, characterized in that: The esterification reaction is carried out without any external solvent, or in the presence of one or more green organic solvents selected from 2-methyltetrahydrofuran, cyclopentyl methyl ether, dimethyl carbonate, and methyl tert-butyl ether; the esterification reaction temperature is 100~130℃, the reaction time is 3~6h, and the reaction pressure is the autogenous pressure formed by methanol at the corresponding temperature or 0.2~0.8MPa.

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