A process for the preparation of 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dainhydride

Abstract

The application belongs to the technical field of chemical products, and in particular relates to a method for preparing 1,2,3,4-cyclobutane tetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutane tetracarboxylic dianhydride. The method comprises the following steps: dissolving maleic anhydride or 2,3-dimethyl maleic anhydride in an organic solvent in the presence of an organic photosensitizer, and performing a photodimerization reaction under near-ultraviolet light irradiation to generate corresponding 1,2,3,4-cyclobutane tetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutane tetracarboxylic dianhydride; wherein the organic photosensitizer is selected from at least one of photosensitizers PC1 to PC6 having near-ultraviolet excitation characteristics; and the photodimerization reaction under near-ultraviolet light irradiation in the presence of the organic photosensitizer effectively improves the reaction efficiency and reduces energy consumption, can significantly reduce energy consumption, improve the reaction efficiency, simplify the preparation process, and thus reduce the production cost.

Description

Technical Field

[0001] This invention relates to the field of chemical product technology, specifically to a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. Background Technology

[0002] 1,2,3,4-Cyclobutanetetracarboxylic dianhydride, abbreviated as CBDA (Cas:4415-87-6), is a very important industrial raw material. Polyimide films obtained from the reaction of CBDA have wide applications in semiconductor packaging, high-temperature electronic devices, flexible electronic materials, and gas separation due to their advantages of high heat resistance, low dielectric constant, high flexibility, and good chemical stability. 1,2,3,4-Tetramethyl-1,2,3,4-Cyclobutanetetracarboxylic dianhydride, abbreviated as TMCBDA (Cas:64198-16-9), is a derivative of CBDA and can also be used to synthesize polyimides and other polymeric materials.

[0003] CN114507240A discloses a method for preparing cyclobutanetetracarboxylic dianhydride (CBDA). This method involves first halogenating 1,4-cyclohexanedione-2,5-dicarboxylic acid ester or cyclopentanone-2,3,4-tricarboxylic acid ester, followed by rearrangement, hydrolysis, and dehydration to obtain cyclobutanetetracarboxylic dianhydride. This method is simple to operate, operates under mild conditions, produces a high-purity product, and is easily scaled up. However, due to the large number of reaction steps, the overall reaction yield is relatively low.

[0004] CN102977112A discloses a process for synthesizing cyclobutanetetracarboxylic dianhydride. Diethyl carbonate and maleic anhydride are added to a photoreactor. After controlling the reaction temperature, an LED cold ultraviolet light is turned on to irradiate the reaction solution using ultraviolet light as the induction source. After a certain reaction time, the reaction solution is filtered and dried to obtain the product. This invention uses LED cold ultraviolet light, which has advantages such as high photoinduction efficiency, easy control of reaction temperature, simple reaction equipment, low energy consumption, fewer synthesis steps, and high product yield. However, the reaction time of this invention is 100-300 hours, which is too long and not conducive to industrial production.

[0005] The paper "Photodimerization of Maleic Anhydride in a Microreactor Without Clogging" (Organic Process Research & Development 2010, 14, 405-410) discloses a microreactor employing a synergistic effect of gas-liquid slug flow and ultrasound. Inert nitrogen gas is introduced into the reaction liquid to form a slug flow, propelling the precipitated products in the liquid phase through the reaction tube. Simultaneously, ultrasonic vibration inhibits the adhesion and deposition of precipitates on the tube wall. The synergistic effect of gas and ultrasound effectively prevents pipe blockage. However, this device has the following limitations: First, the inner diameter of the reaction tube must be strictly controlled (optimally 0.8 mm). Too small a diameter (e.g., 0.5 mm) easily leads to blockage, while too large a diameter (e.g., 3.6 mm) prevents the formation of a stable slug flow, resulting in precipitate transport failure. This increases the complexity of equipment selection and operation. Second, continuous ultrasonic vibration (39 kHz) and external cooling (maintaining a temperature <15°C) are required, which increases the energy consumption of the process. In addition, the current design is for laboratory scale. When scaling up, it is necessary to solve the problem of uniform illumination for multi-channel parallel or long tube winding, and there may be a risk of decreased slug flow stability.

[0006] The paper "Influence of Process Conditions on the Yield of Continuous Synthesis of Cyclobutanetetracarboxylic Acid Diamanide" (Synthetic Technology and Application, 2020, 35(2):6.) discloses a method for the efficient preparation of cyclobutanetetracarboxylic acid dianhydride (CBDA) in a continuous photoreactor under ultraviolet light irradiation, using maleic anhydride (MA) as raw material and dimethyl carbonate as solvent. This method achieves high yield and purity, but has the following drawbacks: it requires a 500W high-pressure ultraviolet lamp, resulting in high energy consumption; and it lacks discussion on the uniformity of illumination and fluid distribution after scale-up, potentially posing reactor design challenges for actual production.

[0007] CN118459474 discloses a method for preparing cyclobutanetetracarboxylic dianhydride. This method employs a circulating photoreactor system. The reactants, containing maleic anhydride and an organic solvent, are placed in a material bottle and circulated through a glass-tube photoreactor under light irradiation via a pumping system. During the reaction, the product is separated in the material bottle by sedimentation, while unreacted material continues to circulate until complete conversion. While this method offers advantages such as simple operation, mild reaction conditions, and high product purity, it suffers from significant drawbacks in industrial applications: the required light source power range is large (0.5-15KW), resulting in excessive energy consumption, which contradicts the principles of modern green chemistry and energy-saving production, severely limiting its economic feasibility in large-scale production.

[0008] JP2008-101135A discloses a method for preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic acid dianhydride using 2,3-dimethylmaleic anhydride as a raw material and dimethyl carbonate as a solvent, reacting at 5°C for 33 hours, with a final product yield of 45%. While this method achieves the synthesis of the target product, it has some shortcomings: the reaction conditions require maintaining a low temperature of 5°C; the reaction time is long at 33 hours, and the product yield is only 45%; furthermore, the patent does not specifically address the feasibility of industrial-scale production.

[0009] A photochemical synthesis method for the efficient preparation of 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydrides with a low coefficient of thermal expansion derived from alkyl-substituted cyclobutanetetracarboxylic dianhydrides (Polymer International 2014, 63, 486-500) is disclosed. This method involves adding 2,3-dimethylmaleic anhydride, benzophenone, and 1,4-dioxane to a photoreactor. The reaction is carried out under 100W high-pressure mercury lamp irradiation, achieving a product yield of 91%. While the reaction yield is good, it has the following drawbacks: the light energy conversion efficiency of the high-pressure mercury lamp is low, resulting in insufficient energy utilization; and the feasibility of scaling up this process is not discussed in detail in the literature.

[0010] Therefore, we propose a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride to solve the above problems. Summary of the Invention

[0011] (a) Technical problems to be solved

[0012] To address the shortcomings of existing technologies, this invention provides a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. Six novel near-ultraviolet-excited organic photosensitizers were designed, and their effects on the dimerization reactions of maleic anhydride and 2,3-dimethylmaleic anhydride were investigated. The photosensitizers improved the substrate quantum yield and, in practical production, enhanced the reaction yield. The photosensitizers designed and synthesized in this invention played a significant role in the dimerization reactions of maleic anhydride and 2,3-dimethylmaleic anhydride, achieving a major breakthrough. Furthermore, the organic photosensitizers used in this invention are easy to synthesize and relatively inexpensive compared to metal photosensitizers. In addition, organic photosensitizers are environmentally friendly and easy to handle, solving the problems mentioned in the background art.

[0013] (II) Technical Solution

[0014] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0015] A method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the method comprising:

[0016] In the presence of an organic photosensitizer, maleic anhydride or 2,3-dimethylmaleic anhydride is dissolved in an organic solvent and photodimerization is carried out under near-ultraviolet light irradiation to generate the corresponding 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride.

[0017] The organic photosensitizer is selected from at least one of the photosensitizers PC1 to PC6 that have near-ultraviolet excitation properties.

[0018] Furthermore, in preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride, the organic photosensitizer is selected from at least one of PC1, PC2, PC3, and PC4; in preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the organic photosensitizer is selected from at least one of PC5 and PC6.

[0019] Furthermore, the amount of the organic photosensitizer is 0.1%-20% of the molar amount of maleic anhydride or 2,3-dimethylmaleic anhydride, preferably 1%-10%, and more preferably 5%-10%.

[0020] Furthermore, the organic solvent is selected from at least one of ethyl acetate, dimethyl carbonate, diethyl carbonate, and 1,4-dioxane, preferably ethyl acetate.

[0021] Furthermore, the wavelength of the near-ultraviolet light is 300-400nm, preferably 320-360nm, and more preferably 340nm.

[0022] Furthermore, the photodimerization reaction is carried out in a continuous flow photoreactor under nitrogen protection throughout the reaction. The continuous flow photoreactor includes a material bottle, a peristaltic pump, a material mixing tank, a photoreactor, a light source, and a filtration and separation assembly. The reaction pipe of the photoreactor is a high borosilicate glass tube with grooves, and the high borosilicate glass tube is arranged in a Z-shaped coil. The light source is wound around the outside of the photoreactor and is distributed on all four sides of the coiling direction of the reaction tube.

[0023] The filtration and separation assembly includes a reaction vessel with a stirrer and a condenser jacket, and a filter.

[0024] Furthermore, the discharge end of the material tank is connected to the inlet end of the peristaltic pump, the discharge end of the peristaltic pump is connected to the inlet of the material mixing tank, the discharge end of the material mixing tank is connected to the inlet of the photoreactor, the discharge outlet of the photoreactor is connected to the inlet of the reaction vessel equipped with a stirrer and a condenser jacket, and the discharge end of the reaction vessel equipped with a stirrer and a condenser jacket is connected to the filter.

[0025] Furthermore, the glass reactor pipe of the photoreactor has an outer diameter of 10 mm, an inner diameter of 5 mm, and a length of 20 cm;

[0026] The process temperature conditions for the dimerization reaction are as follows: the temperature during the material mixing stage is 25±5℃, the reaction temperature inside the photoreactor is 10±5℃, the light source is an LED cold light source with a power of 10-100W, preferably 12-30W, more preferably 16W, and the temperature control of the light source is 20±10℃; the dimerization reaction time is 2-20 hours, preferably 4-12 hours, more preferably 6-8 hours.

[0027] Furthermore, it also includes a distillation recovery component, which is connected to the filtered mother liquor pipeline to distill and recover the organic solvent and return it to the material mixing tank, thereby realizing the recycling of the solvent.

[0028] Furthermore, in the preparation of 1,2,3,4-cyclobutanetetracarboxylic dianhydride, the reaction yield reached 35%-42%, and the product purity was ≥90%.

[0029] When preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the reaction yield reached 69%-88%, and the product purity was ≥94%.

[0030] (III) Beneficial Effects

[0031] Compared with the prior art, the present invention provides a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, which has the following beneficial effects:

[0032] This application provides a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. By carrying out a photodimerization reaction under near-ultraviolet light irradiation in the presence of an organic photosensitizer, the reaction efficiency is effectively improved and energy consumption is reduced. This method can significantly reduce energy consumption, improve reaction efficiency, and simplify the preparation process, thereby reducing production costs. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the continuous flow photoreaction device of the present invention;

[0034] Figure 2 This is a schematic diagram of the continuous flow photoreactor of the present invention.

[0035] In the diagram: 1. Material bottle; 2. Peristaltic pump; 3. Material mixing tank; 4. Photoreactor; 5. Light source (lamp strip); 6. Stirrer; 7. Condensation jacket; 8. Filter. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Traditional methods for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride have many limitations in practical applications. For example, some methods involve cumbersome reaction steps, resulting in low overall yields; others face problems such as excessively long reaction times, high energy consumption, complex operation, and difficulties in industrial scale-up. These factors severely restrict the efficient and economical production of the target products.

[0038] This application proposes a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. The method involves dissolving maleic anhydride or 2,3-dimethylmaleic anhydride in an organic solvent in the presence of an organic photosensitizer, and then subjecting it to a photodimerization reaction under near-ultraviolet light irradiation to generate the corresponding 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. The organic photosensitizer is selected from at least one of the photosensitizers PC1 to PC6, which possess near-ultraviolet excitation properties. This approach aims to overcome the challenges of low reaction efficiency, high energy consumption, and complex operation in existing technologies.

[0039] For ease of understanding, the following explains some key terms in this embodiment:

[0040] Organic photosensitizers are substances that, under illumination, absorb light energy and transfer it to reactants, thereby promoting photochemical reactions. They are typically not consumed during the reaction process, or only undergo reversible changes.

[0041] Maleic anhydride or 2,3-dimethylmaleic anhydride is the starting material used for photodimerization in this method. Its molecular structure contains unsaturated bonds, which can undergo dimerization under the activation of photosensitizer.

[0042] Organic solvents are used to dissolve reactants and organic photosensitizers, provide a homogeneous reaction medium, and facilitate mass and heat transfer in the reaction system.

[0043] Near-ultraviolet light refers to ultraviolet light with a wavelength range close to that of visible light, and its energy is sufficient to excite organic photosensitizers or directly initiate the photochemical transformation of reactants.

[0044] Photodimerization is a chemical process driven by light energy, which causes two identical or different molecules to combine to form a dimer.

[0045] 1,2,3,4-Cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride is the target product prepared by this method via photodimerization, and it has a specific cyclobutane structure.

[0046] Photosensitizers PC1 to PC6 refer to a series of organic photosensitizers with specific chemical structures and near-ultraviolet excitation properties, which are selected for efficient catalysis of photodimerization reactions in this method.

[0047] In this method, organic photosensitizers play a crucial role, initiating a photodimerization reaction by absorbing near-ultraviolet light energy and transferring it to maleic anhydride or 2,3-dimethylmaleic anhydride. As one approach, various photosensitizers with near-ultraviolet excitation properties can be selected, such as certain aromatic ketones, quinones, or heterocyclic compounds. The amount of these photosensitizers used can be initially determined based on the concentration of reactants, the intensity of the light source, and the desired reaction rate. For example, a lower molar percentage can be used to ensure the catalytic efficiency of the photosensitizer.

[0048] The reactants, maleic anhydride or 2,3-dimethylmaleic anhydride, are dissolved in an organic solvent to form a homogeneous reaction system. As one approach, a variety of organic solvents that can readily dissolve the reactants and are inert to the photodimerization reaction can be used. For example, ether solvents, ester solvents, ketone solvents, or certain aprotic polar solvents can be employed. The choice of solvent is typically based on its solubility for the reactants, its stability at the reaction temperature, and the ease of subsequent product separation.

[0049] Photodimerization requires near-ultraviolet light irradiation. As a means of achieving this, the reaction can be carried out in various photoreactors, such as in batch reactors equipped with ultraviolet light sources, or in simple glass containers. A variety of devices capable of emitting near-ultraviolet light can be used as the light source, such as high-pressure mercury lamps, xenon lamps, or fluorescent lamps. The light intensity and reaction time can be empirically adjusted based on the conversion rate of the reactants and the formation rate of the products. For example, the reaction can be accelerated by increasing the number of light sources or extending the irradiation time.

[0050] Following a photodimerization reaction, maleic anhydride or 2,3-dimethylmaleic anhydride is converted into the corresponding 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. Alternatively, after the reaction, the target product can be obtained using conventional chemical separation and purification techniques. For example, the solid product can be separated from the reaction solution and purified through steps such as cooling crystallization, filtration, washing, or recrystallization.

[0051] This method effectively promotes the photodimerization reaction of maleic anhydride or 2,3-dimethylmaleic anhydride under near-ultraviolet light irradiation by introducing an organic photosensitizer. This approach significantly shortens the reaction time and reduces dependence on high-energy-consuming light sources, thereby reducing overall energy consumption. Furthermore, by selecting suitable organic solvents and photosensitizers, the operation process is simplified, providing feasibility for subsequent industrial-scale production and overcoming the problems of low reaction efficiency, high energy consumption, and complex operation in existing technologies.

[0052] In some embodiments described above in this application, an organic photosensitizer is proposed to promote photodimerization reaction. However, in the implementation process, the selection of photosensitizer is not optimized for different products, which may lead to low reaction efficiency, low yield or insufficient product purity.

[0053] In this regard, this application further proposes that, in preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride, the organic photosensitizer is selected from at least one of PC1, PC2, PC3, and PC4; and in preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the organic photosensitizer is selected from at least one of PC5 and PC6.

[0054] Specifically, for the preparation of 1,2,3,4-cyclobutanetetracarboxylic dianhydride, the selected organic photosensitizers PC1, PC2, PC3, and PC4 were screened and optimized based on the photodimerization characteristics of maleic anhydride. These photosensitizers possess specific light absorption and excitation characteristics in the near-ultraviolet region, enabling them to efficiently absorb light energy and transfer it to maleic anhydride molecules, thereby inducing a [2+2] cycloaddition reaction to form 1,2,3,4-cyclobutanetetracarboxylic dianhydride. For example, these photosensitizers may possess excited-state energies matching the molecular structure of maleic anhydride, enabling the formation of an effective energy transfer pathway, or they may possess suitable redox potentials to promote electron transfer processes. This ensures high conversion rates while effectively suppressing side reactions, guaranteeing the selectivity and purity of the target product.

[0055] Similarly, for the preparation of 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the selected organic photosensitizers PC5 and PC6 are specifically chosen for the photodimerization reaction of 2,3-dimethylmaleic anhydride. Because of the introduction of methyl substituents into its molecular structure, 2,3-dimethylmaleic anhydride differs from maleic anhydride in its electronic structure and steric hindrance characteristics, which may affect its light absorption and reactivity. Therefore, PC5 and PC6 were selected as photosensitizers capable of interacting more effectively with the 2,3-dimethylmaleic anhydride molecule. They may possess more matched excited-state energies or stronger electron transfer capabilities, overcoming the influence of the methyl substituents and efficiently promoting the photodimerization reaction of 2,3-dimethylmaleic anhydride to generate 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, while minimizing the formation of byproducts, thereby improving the reaction yield and product purity.

[0056] Through the above technical solutions, this application has selected organic photosensitizers with specific excitation properties for different raw materials (maleic anhydride or 2,3-dimethylmaleic anhydride) and their corresponding target products (1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride). This targeted selection of photosensitizers ensures that light energy is efficiently and selectively transferred to the corresponding reactants, thereby significantly improving the efficiency and selectivity of the photodimerization reaction. Specifically, for the photodimerization of maleic anhydride, the combination of PC1, PC2, PC3, and PC4 can more effectively induce its cyclization, reducing unnecessary energy loss and side reactions; while for 2,3-dimethylmaleic anhydride, the combination of PC5 and PC6 can better adapt to its molecular structure characteristics and promote its efficient conversion. This optimization strategy effectively solves the problems of low reaction efficiency, low product yield, or insufficient purity that may be caused by universal photosensitizers, enabling both target products to be obtained with higher yields and purity, thereby improving the industrial applicability and economy of the entire preparation method.

[0057] In some of the schemes described above in this application, organic photosensitizers are proposed to promote photodimerization reactions. However, in this process, improper dosage of photosensitizers may lead to low reaction efficiency, unstable yield, or increased costs.

[0058] In this regard, this application further proposes that the amount of organic photosensitizer is 0.1%-20% of the molar amount of maleic anhydride or 2,3-dimethylmaleic anhydride, preferably 1%-10%, and more preferably 5%-10%.

[0059] Organic photosensitizers play a crucial role in photodimerization reactions. Their main function is to absorb near-ultraviolet light energy and efficiently transfer this energy to maleic anhydride or 2,3-dimethylmaleic anhydride molecules, thereby initiating or accelerating the photodimerization reaction. Therefore, the amount of organic photosensitizer used directly affects the reaction efficiency, product yield, and overall production cost. If the amount of photosensitizer is too low, insufficient light energy transfer may occur, resulting in a slow reaction rate, difficulty in increasing conversion rate, and thus prolonging the reaction time and reducing production efficiency. Conversely, if the amount of photosensitizer is too high, it may trigger side reactions of the photosensitizer itself, increasing the difficulty of product separation and purification, significantly increasing production costs, and in some cases, excessive photosensitizer may even have a quenching effect on the reaction, thereby reducing reaction efficiency.

[0060] This application provides a broad and effective operating window by precisely limiting the amount of organic photosensitizer to 0.1%-20% of the molar amount of maleic anhydride or 2,3-dimethylmaleic anhydride. Within this range, the photosensitizer can fully exert its catalytic activity, ensuring the smooth progress of the reaction while avoiding the negative effects that may result from extreme dosages. Furthermore, a dosage of 1%-10% is preferred, aiming to achieve a better balance between reaction rate and product yield within this broad range, while also considering economic efficiency. A more preferred dosage of 5%-10% represents the best practice for photosensitizer dosage, maximizing the catalytic activity of the photosensitizer, significantly improving the reaction yield and purity of the target product, while keeping production costs at a reasonable level, thereby achieving an efficient and economical production process.

[0061] In this regard, this application further proposes that in the above method, the organic solvent is selected from at least one of ethyl acetate, dimethyl carbonate, diethyl carbonate, and 1,4-dioxane, preferably ethyl acetate.

[0062] Specifically, organic solvents play a crucial role in photodimerization reactions, dissolving the reactants (maleic anhydride or 2,3-dimethylmaleic anhydride) and organic photosensitizers, transferring light energy, and serving as the reaction medium. Their selection directly affects the solubility of the reactants, the excitation efficiency of the photosensitizer, the homogeneity of the reaction system, the separation and purification of the products, and the overall economic and environmental friendliness of the process. Besides the solvents listed above, other aprotic polar solvents, such as acetone and butanone, can be considered. These solvents exhibit good solubility for maleic anhydride and its derivatives and are relatively stable under near-ultraviolet light. Alternatively, a mixed solvent system can be used, such as a mixture of ethyl acetate and a small amount of other polar solvents (e.g., tetrahydrofuran), to further adjust the solubility or reaction rate while maintaining the overall system stability. Ethyl acetate is preferred because it offers comprehensive advantages in terms of solubility, photochemical stability, reaction efficiency, ease of product separation, and cost and safety for industrial applications. It ensures sufficient dispersion of the reactants and photosensitizers in the reaction system, facilitating effective absorption and conversion of light energy, thereby improving reaction yield and selectivity. Besides ethyl acetate, other ester solvents with similar properties, such as ethyl propionate or n-propyl acetate, may also exhibit excellent performance under certain specific conditions, such as in scenarios requiring higher boiling points or different solubility parameters. Alternatively, supercritical carbon dioxide could be used as the solvent, combined with a small amount of ethyl acetate as a co-solvent, to achieve a more environmentally friendly and easily separable reaction system, although this would increase equipment complexity.

[0063] Through the above technical solution, this application optimizes the media environment for the photodimerization reaction by selecting at least one of ethyl acetate, dimethyl carbonate, diethyl carbonate, and 1,4-dioxane as an organic solvent. These solvents have good solubility for maleic anhydride or 2,3-dimethylmaleic anhydride and organic photosensitizers, ensuring uniform dispersion of reactants and photosensitizers in the system, thereby improving the utilization efficiency of light energy and the reaction rate. The preferred use of ethyl acetate as a solvent further enhances the overall performance of the process. Ethyl acetate has moderate polarity and a low boiling point, which not only facilitates the dissolution of reactants and the effective activation of photosensitizers, but also significantly reduces energy consumption and simplifies operation steps during product separation and solvent recovery after the reaction. Its low boiling point makes distillation recovery more efficient, reduces solvent residue, and helps obtain high-purity target products. Furthermore, the relatively low toxicity and environmental impact of ethyl acetate make it a more suitable choice in line with green chemistry principles. In the presence of an organic photosensitizer, maleic anhydride or 2,3-dimethylmaleic anhydride is dissolved in the above-mentioned preferred organic solvent, and photodimerization is carried out under near-ultraviolet light irradiation. This can effectively promote the excitation and energy transfer of the photosensitizer, allowing the photodimerization reaction to proceed efficiently under milder conditions. This avoids the problems of harsh reaction conditions, excessive energy consumption, or excessively long reaction time caused by improper solvent selection in the prior art. Thus, it achieves the preparation of 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride with high yield and high purity.

[0064] In some of the schemes described above in this application, near-ultraviolet light irradiation is proposed to promote photodimerization reaction. However, if the wavelength range is not optimized in this process, it may lead to insufficient light energy absorption, low reaction efficiency or increased by-products, thereby affecting the overall reaction rate and product yield.

[0065] In this regard, this application further proposes that in the above method, the wavelength of the near-ultraviolet light is 300-400nm, preferably 320-360nm, and more preferably 340nm.

[0066] By limiting the wavelength of near-ultraviolet light to the range of 300-400 nm using the above technical solution, it is possible to ensure that the selected organic photosensitizer (at least one of PC1 to PC6) can efficiently absorb light energy, thereby effectively exciting the photosensitizer and promoting the photodimerization reaction of maleic anhydride or 2,3-dimethylmaleic anhydride. This wavelength range avoids the problem of insufficient photon energy due to excessively long wavelengths, which would fail to effectively excite the photosensitizer, and also avoids the problem of excessively high photon energy due to excessively short wavelengths, which might lead to solvent decomposition or byproduct formation, thus improving the selectivity and efficiency of the reaction. Further optimization of the wavelength to 320-360 nm, and more preferably 340 nm, is based on in-depth research into the absorption spectrum of specific photosensitizers and the reaction system, aiming to maximize light energy utilization and the yield and purity of the target product. This precise wavelength control enables the photodimerization reaction to proceed under milder and more efficient conditions, significantly improving the overall performance of the preparation of 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, and effectively solving the problems of insufficient light energy absorption, low reaction efficiency, or increased byproducts.

[0067] In this regard, this application proposes a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, wherein, referring to Figure 1 and Figure 2 As shown, the photodimerization reaction takes place in a continuous flow photoreactor under nitrogen protection throughout the reaction. The continuous flow photoreactor includes a material bottle 1, a peristaltic pump 2, a material mixing tank 3, a photoreactor 4, a light source 5, and a filtration and separation assembly. The reaction pipe of the photoreactor 4 is a grooved borosilicate glass tube, which is coiled in a Z-shape. The light source 5 is wound around the outside of the photoreactor 4 and is distributed on all four sides of the coiled reaction pipe. The filtration and separation assembly includes a reaction vessel with a stirrer 6 and a condenser jacket 7, and a filter 8.

[0068] The core components of the continuous photoreactor used in this preparation process include a material tank 1, a peristaltic pump 2, a material mixing tank 3, a photoreactor 4, a light source (lamp strip) 5, a stirrer 6, a condenser jacket 7, and a filter 8. These components are connected in series to form a closed-loop circulating reaction system. The specific connections and design functions are as follows:

[0069] The discharge end of material tank 1 is connected to the inlet end of peristaltic pump 2, and the discharge end of peristaltic pump 2 is connected to the inlet of material mixing tank 3. Material tank 1 is a storage component for high-concentration reaction substrate (maleic anhydride / 2,3-dimethylmaleic anhydride), designed to replenish the substrate and solvent consumed in the system due to the reaction and maintain the standard concentration of substrate in material mixing tank 3. Peristaltic pump 2 is a power transmission component, designed to precisely control the material conveying rate and ensure the stability of material circulation in the system.

[0070] The discharge end of the material mixing tank 3 is connected to the inlet of the photoreactor 4. The material mixing tank 3 is a premixing component for the reactants. Its function is to fully dissolve and mix the substrate, photosensitizer, and solvent (ethyl acetate) under nitrogen protection, so as to provide uniform reactants for the photoreaction.

[0071] The photoreactor 4 is the core reaction component, with a light source (lamp strip) 5 wrapped around its outer side. The reaction channel of the photoreactor 4 is a high borosilicate glass tube with grooves and is coiled in a Z-shape. The light source 5 is distributed on the four sides of the coiling direction of the reaction tube. The purpose of this structural design of the photoreactor 4 is to actively intervene in the fluid dynamics by periodically changing the grooves and the coiling direction, so as to avoid the sedimentation of photoreaction products in the reaction channel. The design of the light source distributed on the four sides can ensure that all light energy is irradiated to the surface of the reaction tube, so that the reaction solvent can fully absorb the light energy and reduce the overall reaction energy consumption.

[0072] The outlet of the photoreactor 4 is connected to the inlet of the reaction vessel equipped with a stirrer 6 and a condenser jacket 7. The stirrer 6 is designed to stir the reaction liquid, so that the reaction liquid is heated and cooled evenly. The condenser jacket 7 is designed to cool the reaction liquid to 10±5℃, and take advantage of the low solubility of the product in ethyl acetate to promote the precipitation of the product.

[0073] The discharge end of the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 is connected to a filter 8. The filter 8 is a solid-liquid separation component, designed to separate the precipitated solid product from the mother liquor to obtain a crude product. The mother liquor discharge end of the filter 8 is returned to the material mixing tank 3 to form a closed loop. The design function is to allow unreacted materials to re-participate in the photoreaction and improve the substrate conversion rate.

[0074] The system is also equipped with a distillation recovery unit, which is connected to the mother liquor pipeline after filtration. Its function is to distill and recover the ethyl acetate solvent and return it to the material mixing tank 3, so as to realize the recycling of the solvent and reduce the consumption of raw materials.

[0075] The overall working principle is as follows:

[0076] This process uses ethyl acetate as solvent and maleic anhydride (MA) / 2,3-dimethylmaleic anhydride as substrate. Organic photosensitizers (PC1-PC6) designed in this invention are added. Under nitrogen protection and near-ultraviolet light (340nm) irradiation, the substrate undergoes a closed-loop continuous photoreaction to achieve dimerization, generating 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) / 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride (TMCBDA). Solid-liquid separation is achieved by utilizing the low solubility of the product at low temperatures. Unreacted material and recycled solvent are refluxing to participate in the reaction. The entire process is completed under low energy consumption (16W light source) and mild conditions (dissolution at 25±5℃ room temperature, reaction at 10±5℃) through precise temperature control, efficient light energy utilization, and material recycling. The specific workflow steps are as follows:

[0077] Material preparation and conveying: At 25±5℃, add a quantitative amount of substrate (maleic anhydride / 2,3-dimethylmaleic anhydride) and a corresponding proportion of photosensitizer (if no photosensitizer is available, only substrate is added) to the dry material tank 1. Add an equal amount of substrate, photosensitizer and 190g of ethyl acetate to the material mixing tank 3. Heat under nitrogen protection to completely dissolve the materials. Turn on the peristaltic pump 2 to convey the high concentration of materials in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the reaction standard concentration.

[0078] Photoreaction process: The peristaltic pump is turned on to transport the homogeneous reaction liquid in the material mixing tank 3 to the photoreactor 4. The temperature of the photoreactor 4 is controlled at 10±5℃ and the temperature of the light source (lamp strip) 5 is controlled at 20±10℃. The 16W light source 5 with a wavelength of 340nm is turned on. The reaction liquid undergoes a dimerization reaction under the near-ultraviolet light irradiation in the photoreactor 4. The reaction liquid flows in the Z-shaped coiled grooved reaction pipe. There is no product sedimentation. The light energy is fully absorbed, and the reaction efficiency is improved.

[0079] Product precipitation and solid-liquid separation: The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continuously stirs the solution. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0080] Material and solvent recycling: About 1 / 4 of the ethyl acetate in the supernatant (mother liquor) separated by filter 8 is recovered by distillation, and the recovered solvent is transferred back to material mixing tank 3; the remaining mother liquor is directly returned to material mixing tank 3, and at the same time, peristaltic pump 2 is turned on to transport the replenished material in material tank 1 to material mixing tank 3 to replenish the substrate and solvent consumed in the system. The peristaltic pump is turned on so that the mixed material enters photoreactor 4 again for recycling reaction.

[0081] Reaction termination and product collection: After continuous cyclic reaction for 8 hours, the amount of substrate remaining in the supernatant was detected to be ≤0.022% (basically no substrate remaining), and the reaction was terminated; the solid product separated by filter 8 was filtered and collected, the reaction yield was calculated, and finally high-purity CBDA (90%-93%) or TMCBDA (94%-95%) was obtained.

[0082] The reaction conduit of photoreactor 4 is a grooved borosilicate glass tube. The grooved design aims to increase the turbulence of the fluid within the tube, reduce laminar flow, thereby promoting thorough mixing of the reactants and effectively inhibiting the adhesion and deposition of products on the tube wall, significantly reducing the risk of tube blockage. The grooves can be designed as spiral, annular, or irregular inner wall structures. Borosilicate glass tubes are chosen for their excellent chemical stability, high-temperature resistance, and good light transmittance, ensuring the stability of the reaction process and effective light penetration. The borosilicate glass tube is arranged in a Z-shaped coil, which effectively optimizes space utilization, increasing the effective length of the reaction conduit within a limited volume, thus extending the residence time of the reactants in the illuminated area and improving the reaction conversion rate. Simultaneously, the Z-shaped coil also contributes to a more uniform light distribution. Besides the Z-shaped coil, other compact coiling methods such as spiral or serpentine coiling can also be used.

[0083] Light source 5 is wound around the outside of photoreactor 4 and distributed on all four sides of the winding direction of the reaction tube. This arrangement of light sources aims to ensure that light energy can be evenly transmitted to all reactants inside the reaction tube, avoiding any blind spots in illumination, thereby maximizing light energy utilization efficiency and reaction conversion rate. The light source can be an LED strip, an LED array, or a combination of multiple independent light sources, achieved through tight or spaced winding. The four-sided distribution ensures that the reaction tube is illuminated from different angles, further improving the uniformity of illumination.

[0084] The filtration and separation assembly includes a reaction vessel with a stirrer 6 and a condenser jacket 7, and a filter 8. The vessel with the stirrer 6 receives the reacted material, ensuring uniform mixing, preventing product sedimentation and agglomeration, and promoting the crystallization of the target product. The condenser jacket 7 precisely controls the temperature within the vessel, typically using cooling to promote product crystallization while preventing decomposition or side reactions of heat-sensitive products. The stirrer 6 can be of various types, such as paddle, anchor, or turbine, while the condenser jacket 7 can be jacketed or coiled. The filter 8 achieves solid-liquid separation, efficiently separating the crystallized target product (solid) from the mother liquor (liquid), ensuring product purity. The filter 8 can be a plate and frame filter press, a centrifugal filter, or a Buchner funnel, etc.

[0085] In this regard, this application further proposes that the outer diameter of the glass reactor pipe of the photoreactor 4 is 10mm, the inner diameter is 5mm, and the length is 20cm; the process temperature conditions for the dimerization reaction are: the temperature of the material mixing stage is 25±5℃, the reaction temperature inside the photoreactor 4 is 10±5℃, the light source is an LED cold light source with a power of 10-100W, preferably 12-30W, more preferably 16W, and the temperature control of the light source 5 is 20±10℃; the dimerization reaction time is 2-20 hours, preferably 4-12 hours, more preferably 6-8 hours.

[0086] The process temperature conditions for the dimerization reaction include a material mixing stage temperature of 25±5℃ and a reaction temperature within photoreactor 4 of 10±5℃. The material mixing stage temperature is controlled at 25±5℃. This temperature range is conducive to the full dissolution of the reactants (maleic anhydride or 2,3-dimethylmaleic anhydride) and organic photosensitizer in the organic solvent, forming a homogeneous reaction system and ensuring the smooth progress of the subsequent photoreaction. If the temperature is too low, the solubility may be insufficient; if the temperature is too high, unnecessary side reactions or solvent evaporation may occur. The reaction temperature within photoreactor 4 is controlled at 10±5℃. The lower reaction temperature helps suppress possible side reactions during the photodimerization reaction, such as thermal polymerization or photodecomposition, thereby improving the selectivity and purity of the target product. Simultaneously, this temperature range also considers the reaction rate, avoiding excessively slow reaction rates due to excessively low temperatures. Temperature control can be achieved through methods such as jacketed cooling, external circulating baths, or built-in cooling coils. The optimal temperature range may vary for different reaction systems and needs to be optimized based on specific reaction kinetics and thermodynamic data.

[0087] The light source is an LED cold light source with a power of 10-100W, preferably 12-30W, and more preferably 16W. The temperature control of light source 5 is 20±10℃. LED cold light sources have advantages such as low energy consumption, long lifespan, low heat generation, and controllable wavelength. In particular, the "cold light source" characteristic avoids the large amount of heat generated by traditional high-pressure mercury lamps and other light sources, helping to maintain the low-temperature environment of the reaction system and reduce side reactions. Appropriate light source power ensures sufficient photon energy input to drive the photodimerization reaction efficiently.

[0088] The dimerization reaction time is 2-20 hours, preferably 4-12 hours, and more preferably 6-8 hours. This range ensures sufficient time for the reactants to convert while avoiding unnecessary long reaction times, thereby improving production efficiency. The preferred and more preferred ranges further optimize the reaction time, aiming to minimize the reaction cycle and reduce production costs while ensuring high conversion rates and product purity. Determining the reaction time typically requires experimental optimization, considering factors such as reactant concentration, light intensity, temperature, and catalyst (photosensitizer) activity. In continuous flow systems, the reaction time is closely related to the flow rate and reactor volume.

[0089] By precisely defining the pipe dimensions of the photoreactor 4 (outer diameter 10mm, inner diameter 5mm, length 20cm) through the above technical solution, uniform illumination and stable fluid dynamics conditions are ensured in the continuous flow photoreactor, effectively avoiding problems such as clogging and uneven illumination. Simultaneously, staged temperature control—specifically, a material mixing stage temperature of 25±5℃ to promote complete dissolution, and a reaction temperature within the photoreactor 4 of 10±5℃ to suppress side reactions—significantly improves reaction selectivity and product purity. The use of LED cold light sources, with precise settings for their power (10-100W, preferably 12-30W, more preferably 16W) and temperature control (20±10℃), not only reduces energy consumption but also ensures the stability and continuity of illumination, avoiding the heat problems and light attenuation associated with traditional light sources. Furthermore, optimizing the dimerization reaction time (2-20 hours, preferably 4-12 hours, more preferably 6-8 hours) effectively shortens the production cycle while ensuring high conversion rates, improving overall production efficiency and economy. The synergistic optimization of these parameters makes the entire photodimerization process more controllable, efficient and stable, thereby significantly improving the preparation yield and product purity of 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, and reducing the energy consumption and cost of industrial production.

[0090] In some of the schemes described above in this application, a photodimerization reaction method is proposed to prepare the corresponding 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. However, after product separation, the mother liquor is not recovered, resulting in solvent waste and reaction interruption. A method is needed to recover the mother liquor to maintain continuous reaction and improve resource utilization.

[0091] In this regard, this application further proposes that the reaction yield reaches 35%-42% and the product purity is ≥90% when preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride; and the reaction yield reaches 69%-88% and the product purity is ≥94% when preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride.

[0092] For the preparation of 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the reaction yield of 69%-88% indicates that the photodimerization reaction of 2,3-dimethylmaleic anhydride has high conversion efficiency. To achieve this high yield range, adjustments to the concentration of the reaction system can be considered. For example, controlling the concentration of 2,3-dimethylmaleic anhydride in the organic solvent can optimize the intermolecular collision frequency; alternatively, specific types and amounts of photosensitizers can be used to improve the absorption and transfer efficiency of light energy; or the geometry of the reactor can be optimized, for example, by using a reactor with a larger specific surface area or a more uniform light distribution to ensure that the reactants are fully exposed to near-ultraviolet light. A product purity of over 94% for 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride demonstrates the superiority of this preparation method in controlling product quality. Achieving this high purity can be achieved through various means. For example, during the reaction process, the content of impurities such as oxygen or moisture that may cause side reactions can be strictly controlled to reduce the formation of by-products. Alternatively, after the reaction, sophisticated post-processing techniques can be employed, such as solvent extraction combined with multi-stage crystallization or chromatographic separation techniques, to efficiently separate the target product from trace impurities. Or, real-time monitoring and feedback control of the reaction system can be used to ensure that the reaction always proceeds under optimal conditions, thereby minimizing the generation of impurities.

[0093] The following example will provide a more detailed explanation of the above technical solution:

[0094] This example aims to prepare 1,2,3,4-cyclobutanetetracarboxylic dianhydride. The method involves dissolving maleic anhydride in an organic solvent in the presence of a specific organic photosensitizer and then subjecting it to photodimerization under near-ultraviolet light irradiation, thereby efficiently generating the target product. This approach effectively solves the problems of long reaction time, high energy consumption, low yield, and difficulty in industrial scale-up found in existing technologies.

[0095] In practice, maleic anhydride is first used as a raw material, ethyl acetate as an organic solvent, and PC1 as an organic photosensitizer. The amount of PC1 used is 5% of the molar amount of maleic anhydride. After the above materials are mixed evenly in material bottle 1, they are transported to material mixing tank 3 via peristaltic pump 2. In material mixing tank 3, the material temperature is controlled at 25±5℃.

[0096] Subsequently, the mixed reaction solution is continuously pumped to photoreactor 4 in a continuous flow photoreactor. The reaction conduit of photoreactor 4 is a grooved borosilicate glass tube with an outer diameter of 10 mm, an inner diameter of 5 mm, and a length of 20 cm, arranged in a Z-shaped coil. This design helps improve illumination uniformity and effectively suppresses product deposition on the tube wall, thus avoiding the clogging problem common in traditional microreactors and solving the challenge of uneven fluid distribution during scale-up. Compared to existing technologies that require strict control of the microreactor's inner diameter (e.g., 0.8 mm) to prevent clogging and maintain slug flow stability, this scheme uses a 5 mm inner diameter tube, significantly reducing operational complexity while ensuring reaction efficiency.

[0097] In photoreactor 4, the entire reaction process is protected by nitrogen to prevent side reactions. Light source 5 is wound around the outside of photoreactor 4 and distributed on all four sides of the reaction tube winding direction, ensuring uniform near-ultraviolet irradiation of the reaction liquid. This light source 5 is an LED cold light source with a power setting of 16W and a temperature control of 20±10℃. The wavelength of the near-ultraviolet light used is 340nm. Compared to the 500W high-pressure ultraviolet lamps or 0.5-15KW light sources used in existing technologies, the LED cold light source significantly reduces energy consumption, aligning with the principles of green chemistry. Simultaneously, the reaction temperature within photoreactor 4 is precisely controlled at 10±5℃, avoiding the additional energy consumption required by existing technologies for continuous ultrasonic vibration and external cooling to maintain low temperatures. The duration of this photodimerization reaction is set at 6 hours, significantly shortening the production cycle and improving industrial production efficiency compared to the reaction times of 100-300 hours or 33 hours in existing technologies.

[0098] After reacting in photoreactor 4, the reaction solution enters the filtration and separation assembly. In a container equipped with stirrer 6 and condenser jacket 7, the reaction solution is cooled to 10°C, causing the target product, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, to precipitate. Subsequently, solid-liquid separation is achieved through filter 8, with the solid being the target product. In this example, the reaction yield of the prepared 1,2,3,4-cyclobutanetetracarboxylic dianhydride reaches 35%-42%, and the product purity exceeds 90%.

[0099] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3, replenishing the substrate and solvent consumed in the system. The peristaltic pump is then activated to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction. After continuous cyclic reaction for 8 hours, the substrate residue in the supernatant is measured to be ≤0.022% (virtually no substrate residue), at which point the reaction is terminated. The solid product separated by filter 8 is filtered and collected, and the reaction yield is calculated to finally obtain high-purity CBDA (90%-93%) or TMCBDA (94%-95%).

[0100] Using the above method, this example successfully prepared 1,2,3,4-cyclobutanetetracarboxylic dianhydride. This method significantly shortened the reaction time and reduced energy consumption while ensuring product yield and purity, and solved key technical challenges in industrial-scale production.

[0101] This invention designs six novel near-ultraviolet-excited organic photosensitizers and investigates their effects on the dimerization reactions of maleic anhydride and 2,3-dimethylmaleic anhydride. The research mainly focuses on two aspects: the improvement of substrate quantum yield by the photosensitizers and their effect on reaction yield improvement in actual production. Experimental data and results show that the photosensitizers designed and synthesized in this invention play a significant role in the dimerization reactions of maleic anhydride and 2,3-dimethylmaleic anhydride, representing a major breakthrough. Furthermore, the organic photosensitizers used in this invention are easy to synthesize and are relatively cheaper than metal photosensitizers. In addition, organic photosensitizers are environmentally friendly and easy to handle. The photosensitizer list is as follows:

[0102] ;

[0103] Table 1-1 Quantum Yield of Maleic Anhydride

[0104] Entry Φ dim ]] 1 MA 0.30 ± 0.05 2 MA+PC1 (10 mol%) 0.60 ± 0.05 3 MA+PC2 (10mol%) 0.55 ± 0.05 4 MA+PC3 (10 mol%) 0.50 ± 0.05 5 MA+PC4 (10 mol%) 0.70 ± 0.05

[0105] MA concentration: 0.5 mol / L

[0106] Table 1-2 Quantum yield of 2,3-dimethylmaleic anhydride

[0107] Entry Φ dim ]] 1 2,3-Dimethylmaleic anhydride 0.50 ± 0.05 2 2,3-Dimethylmaleic anhydride + PC5 (5 mol%) 0.85 ± 0.05 3 2,3-Dimethylmaleic anhydride + PC5 (1 mol%) 0.70 ± 0.05 4 2,3-Dimethylmaleic anhydride + PC6 (5 mol%) 0.90 ± 0.05 5 2,3-Dimethylmaleic anhydride + PC6 (1 mol%) 0.65 ± 0.05

[0108] 2,3-Dimethylmaleic anhydride concentration: 0.5 mol / L

[0109] Example 1:

[0110] ;

[0111] At 25±5℃, add 10g of maleic anhydride to a dry 1L material bottle 1, and add 10g of maleic anhydride and 190g of ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2 As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0112] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0113] After separation by filter 8, about 1 / 4 of the supernatant (mother liquor) is recovered by distillation. The recovered solvent is transferred back to material mixing tank 3. The remaining mother liquor is directly returned to material mixing tank 3. At the same time, peristaltic pump 2 is turned on to transport the supplementary material in material tank 1 to material mixing tank 3 to replenish the substrate and solvent consumed in the system. The peristaltic pump is turned on to allow the mixed material to re-enter photoreactor 4 for recycling reaction.

[0114] After continuous cyclic reaction for 8 hours, the substrate residue in the supernatant was measured to be ≤0.022% (virtually no substrate residue), and the reaction was terminated. The solid product separated by filter 8 was filtered and collected, and the reaction yield was calculated. The solid CBDA product in filter 8 was taken, and the purity of the CBDA product was determined by HPLC to be 91%. The supernatant was taken, and the MA content was determined by HPLC to be 0.010%, which means that there was basically no MA residue. After the reaction was terminated, the reaction solution was filtered to obtain 7g of solid, and the reaction yield was 35%.

[0115] Example 2:

[0116] ;

[0117] At 25±5℃, add 10g maleic anhydride and 2.5g PC1 to a dry 1L material bottle 1, and add 10g maleic anhydride, 2.5g PC1, and 190g ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2 As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0118] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0119] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer replenishment material from material tank 1 to the material mixing tank 3, replenishing the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid CBDA product from filter 8 is taken, and HPLC analysis shows that the CBDA product purity is 92%. The supernatant is taken, and HPLC analysis shows that the MA content is 0.017%, meaning that there is essentially no MA remaining. After the reaction is terminated, the reaction solution is filtered to obtain 8g of solid, with a reaction yield of 40%.

[0120] Example 3:

[0121] ;

[0122] At 25±5℃, add 10g maleic anhydride and 2.2g PC2 to a dry 1L material bottle 1, and add 10g maleic anhydride, 2.2g PC2, and 190g ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0123] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0124] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3 to replenish the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid CBDA product from filter 8 is taken, and HPLC analysis shows that the CBDA product purity is 90%. The supernatant is taken, and HPLC analysis shows that the MA content is 0.020%, meaning that there is essentially no MA remaining. After the reaction is terminated, the reaction solution is filtered to obtain 7.6 g of solid, with a reaction yield of 38%.

[0125] Example 4:

[0126] ;

[0127] At 25±5℃, add 10g maleic anhydride and 2.2g PC3 to a dry 1L material bottle 1, and add 10g maleic anhydride, 2.2g PC3, and 190g ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0128] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0129] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3, replenishing the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid CBDA product from filter 8 is taken, and HPLC analysis shows that the CBDA product purity is 91%. The supernatant is taken, and HPLC analysis shows that the MA content is 0.015%, meaning that there is essentially no MA remaining. After the reaction is terminated, the reaction solution is filtered to obtain 7.6 g of solid, with a reaction yield of 39%.

[0130] Example 5:

[0131] ;

[0132] At 25±5℃, add 10g maleic anhydride and 2.5g PC4 to a dry 1L material bottle 1, and add 10g maleic anhydride, 2.5g PC4, and 190g ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0133] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0134] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3 to replenish the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid CBDA product from filter 8 is taken, and HPLC analysis shows that the CBDA product purity is 93%. The supernatant is taken, and HPLC analysis shows that the MA content is 0.014%, meaning that there is essentially no MA remaining. After the reaction is terminated, the reaction solution is filtered to obtain 8.2 g of solid, with a reaction yield of 42%.

[0135] Example 6:

[0136] ;

[0137] At 25±5℃, add 10g of 2,3-dimethylmaleic anhydride and 0.9g of PC5 to a dry 1L material bottle 1, and add 10g of 2,3-dimethylmaleic anhydride, 0.9g of PC5, and 190g of ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0138] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0139] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3, replenishing the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid from filter 8 is collected, and HPLC analysis shows that the purity of the TMCBDA product is 95%. The supernatant is collected, and HPLC analysis shows that the content of 2,3-dimethylmaleic anhydride is 0.012%, meaning that there is essentially no 2,3-dimethylmaleic anhydride remaining. After the reaction is terminated, the reaction solution is filtered to obtain 16g of solid, with a reaction yield of 80%.

[0140] Example 7:

[0141] ;

[0142] At 25±5℃, add 10g of 2,3-dimethylmaleic anhydride and 0.17g of PC5 to a dry 1L material bottle 1, and add 10g of 2,3-dimethylmaleic anhydride, 0.17g of PC5, and 190g of ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0143] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0144] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3 to replenish the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid from filter 8 is collected, and HPLC analysis shows that the purity of the TMCBDA product is 95%. The supernatant is collected, and HPLC analysis shows that the content of 2,3-dimethylmaleic anhydride is 0.008%, meaning that there is essentially no 2,3-dimethylmaleic anhydride remaining. After the reaction is terminated, the reaction solution is filtered to obtain 15.6 g of solid, with a reaction yield of 78%.

[0145] Example 8:

[0146] ;

[0147] At 25±5℃, add 10g of 2,3-dimethylmaleic anhydride and 1g of PC6 to a dry 1L material bottle 1, and add 10g of 2,3-dimethylmaleic anhydride, 1g of PC6, and 190g of ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0148] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0149] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3, replenishing the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid from filter 8 is collected, and HPLC analysis shows that the purity of the TMCBDA product is 95%. The supernatant is collected, and HPLC analysis shows that the content of 2,3-dimethylmaleic anhydride is 0.022%, meaning that there is essentially no 2,3-dimethylmaleic anhydride remaining. After the reaction is terminated, the reaction solution is filtered to obtain 17.6 g of solid, with a reaction yield of 88%.

[0150] Example 9:

[0151] ;

[0152] At 25±5℃, add 10g of 2,3-dimethylmaleic anhydride and 0.2g of PC6 to a dry 1L material bottle 1, and add 10g of 2,3-dimethylmaleic anhydride, 0.2g of PC6, and 190g of ethyl acetate to a dry 1L material mixing tank 3. Heat under nitrogen protection until completely dissolved. (Refer to the attached...) Figure 1 and Figure 2As shown, a photoreactor is constructed, with a light source (lamp strip) 5 wrapped around the outside of the photoreactor 4. The glass reactor pipe has an outer diameter of 10mm, an inner diameter of 5mm, and a length of 20cm. The glass coil irradiates the photoreactor at a temperature of 10±5℃, and the light source at a temperature of 20±10℃. The light source strip 5 (wavelength 340nm) with a wattage of 16W is turned on. The peristaltic pump 2 is turned on to transport the high-concentration material in the material tank 1 to the material mixing tank 3, maintaining the substrate concentration in the material mixing tank 3 at the standard reaction concentration. The peristaltic pump is turned on to transport the homogeneous reaction solution in the material mixing tank 3 to the photoreactor 4. The reaction solution undergoes a dimerization reaction under near-ultraviolet light irradiation in the photoreactor 4.

[0153] The reaction solution that has completed the photoreaction enters the reaction vessel equipped with a stirrer 6 and a condenser jacket 7 from the outlet of the photoreactor 4. The condenser jacket 7 cools the reaction solution to 10°C, and the stirrer 6 continues to stir. Due to its low solubility, a large amount of the product CBDA / TMCBDA precipitates out. The reaction solution containing the solid product is then transported to a filter 8, where solid-liquid separation is achieved to obtain the crude solid product. The purity of the product and the content of the remaining substrate in the supernatant are detected by HPLC.

[0154] The supernatant (mother liquor) separated by filter 8 is partially distilled to recover approximately 1 / 4 of the ethyl acetate. The recovered solvent is then transferred back to the material mixing tank 3. The remaining mother liquor is directly recycled back to the material mixing tank 3. Simultaneously, peristaltic pump 2 is activated to transfer the replenished material from material tank 1 to the material mixing tank 3, replenishing the substrate and solvent consumed in the system. The peristaltic pump is then activated again to allow the mixed material to re-enter the photoreactor 4 for cyclic reaction, and the above operation is repeated. After 8 hours of reaction, the solid from filter 8 is collected, and HPLC analysis shows that the purity of the TMCBDA product is 94%. The supernatant is collected, and HPLC analysis shows that the content of 2,3-dimethylmaleic anhydride is 0.015%, meaning that there is essentially no 2,3-dimethylmaleic anhydride remaining. After the reaction is terminated, the reaction solution is filtered to obtain 13.8 g of solid, with a reaction yield of 69%.

[0155] In summary, this invention provides a method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, designs six novel near-ultraviolet-excited organic photosensitizers, and explores their effects on the dimerization reactions of maleic anhydride and 2,3-dimethylmaleic anhydride. The photosensitizers enhance the substrate quantum yield and, in practical production, improve the reaction yield. The photosensitizers designed and synthesized in this invention play a significant role in the dimerization reactions of maleic anhydride and 2,3-dimethylmaleic anhydride, representing a major breakthrough. Furthermore, the organic photosensitizers used in this invention are easy to synthesize, relatively inexpensive compared to metal photosensitizers, and environmentally friendly and easy to handle.

[0156] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, characterized in that: The method includes: In the presence of an organic photosensitizer, maleic anhydride or 2,3-dimethylmaleic anhydride is dissolved in an organic solvent and photodimerization is carried out under near-ultraviolet light irradiation to generate the corresponding 1,2,3,4-cyclobutanetetracarboxylic dianhydride or 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride. The organic photosensitizer is selected from at least one of the photosensitizers PC1 to PC6 that have near-ultraviolet excitation properties.

2. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: When preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride, the organic photosensitizer is selected from at least one of PC1, PC2, PC3, and PC4; when preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the organic photosensitizer is selected from at least one of PC5 and PC6.

3. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: The amount of the organic photosensitizer used is 0.1%-20% of the molar amount of maleic anhydride or 2,3-dimethylmaleic anhydride.

4. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: The organic solvent is selected from at least one of ethyl acetate, dimethyl carbonate, diethyl carbonate, and 1,4-dioxane.

5. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: The wavelength of the near-ultraviolet light is 300-400nm.

6. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: The photodimerization reaction is carried out in a continuous flow photoreactor with nitrogen protection throughout the reaction. The continuous flow photoreactor includes a material bottle (1), a peristaltic pump (2), a material mixing tank (3), a photoreactor (4), a light source (5), and a filtration and separation assembly. The reaction pipe of the photoreactor (4) is a high borosilicate glass tube with grooves, and the high borosilicate glass tube is arranged in a Z-shaped coil; the light source (5) is wrapped around the outside of the photoreactor (4) and is distributed on all four sides of the coiling direction of the reaction tube; The filtration and separation assembly includes a reaction vessel with a stirrer (6) and a condenser jacket (7) and a filter (8).

7. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 6, characterized in that: The discharge end of the material tank (1) is connected to the inlet end of the peristaltic pump (2), and the discharge end of the peristaltic pump (2) is connected to the inlet of the material mixing tank (3); The discharge end of the material mixing tank (3) is connected to the inlet of the photoreactor (4), the discharge end of the photoreactor (4) is connected to the inlet of the reaction vessel with a stirrer (6) and a condenser jacket (7), and the discharge end of the reaction vessel with a stirrer (6) and a condenser jacket (7) is connected to the filter (8).

8. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 6, characterized in that: The glass reactor pipe of the photoreactor (4) has an outer diameter of 10 mm, an inner diameter of 5 mm, and a length of 20 cm. The process temperature conditions for the dimerization reaction are as follows: the temperature during the material mixing stage is 25±5℃, the reaction temperature in the photoreactor (4) is 10±5℃, the light source is an LED cold light source with a power of 10-100W, preferably 12-30W, the temperature control of the light source (5) is 20±10℃, and the dimerization reaction time is 2-20 hours.

9. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: It also includes a distillation recovery component, which is connected to the filtered mother liquor pipeline to distill and recover the organic solvent and return it to the material mixing tank (3), thereby realizing the recycling of the solvent.

10. The method for preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride and 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride according to claim 1, characterized in that: When preparing 1,2,3,4-cyclobutanetetracarboxylic dianhydride, the reaction yield reached 35%-42%, and the product purity was ≥90%. When preparing 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, the reaction yield reached 69%-88%, and the product purity was ≥94%.

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