A method for preparing fumaric acid by catalytic oxidation of dechlorinated product of polyvinyl chloride using ionic liquid

Abstract

The application belongs to the technical field of high-value utilization of waste chloroplast and chemical synthesis, and more particularly relates to a method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination product using an ionic liquid. The polyvinyl chloride dechlorination product is mixed with an ionic liquid, and a selective oxidation reaction is carried out in the presence of nitrogen monoxide and oxygen to obtain an oxidation product with fumaric acid as the target. Compared with traditional fumaric acid preparation processes such as the maleic anhydride isomerization method or the furfural oxidation method, the application opens up a new path for efficient and green fumaric acid synthesis by using waste polyvinyl chloride dechlorination product as raw material in an ionic liquid catalytic system. The method not only effectively reduces the cost of raw materials, but also has mild reaction conditions and is environmentally friendly, and the yield of fumaric acid can reach 92% at most, which has significant industrial application potential.

Description

Technical Field

[0001] This invention belongs to the field of high-value utilization and chemical synthesis technology of waste chlorine-containing plastics, and more specifically, it relates to a method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids. Background Technology

[0002] Plastics are long-chain polymer compounds derived from petroleum, used in many aspects of life due to their ductility, flexibility, durability, and economic benefits. Global plastic production was reportedly 359 million tons in 2019 and is projected to reach 500 million tons by 2025. Polyvinyl chloride (PVC) is one of the most important plastics derived from petroleum, and its superior properties have led to its widespread use in daily life over the past few decades. However, the difficulty of mechanically recycling PVC and its non-biodegradable nature make its recycling a significant challenge. Developing practical chemical processes to convert waste PVC into high-value-added chemicals can not only alleviate environmental pressures but also transform plastic waste into new carbon resources, which is crucial for promoting a circular economy and sustainable development.

[0003] Fumaric acid, also known as fumaric acid, with the chemical formula C4H4O4, is a carboxylic acid derived from butene. It is a colorless, flammable crystal. Fumaric acid is an important chemical, widely used in the food industry, biopharmaceuticals, and the synthesis of polymers due to its strong acidity, low hygroscopicity, antibacterial and preservative properties, and reactivity. Notably, its esterification product, dimethyl fumarate, is often used to treat relapsing-remitting multiple sclerosis, and it also possesses strong antifungal properties. Therefore, fumaric acid has wide applications and significant value, with a promising future market prospect.

[0004] Currently, the main industrial method for producing fumaric acid is the maleic anhydride isomerization process. Other processes include furfural oxidation, maleic acid enzymatic isomerization, paraffin fermentation, and biomass fermentation. However, the maleic anhydride process, enzymatic isomerization, and paraffin fermentation all rely on petroleum-based feedstocks. With the scarcity and rising prices of petroleum resources, the sustainability and market competitiveness of these methods are increasingly weakening. Although furfural oxidation can be derived from biomass, it suffers from low yields and high catalyst toxicity, failing to meet green and environmental protection requirements. Biological production of fumaric acid offers advantages such as low raw material costs and environmental friendliness. Yields can be effectively improved through metabolic regulation, genetic modification, and enzyme engineering, making it a key development direction for many countries. As early as the beginning of the 20th century, researchers began using mold fermentation to produce fumaric acid, later expanding to yeasts and bacteria. Among these, Rhizopus species (such as Rhizopus oryzae and Rhizopus oligospermum) are a focus of research. For example, Chinese scholars once used *Rhizopus oryzae* to ferment in 12% glucose medium for 11 days, achieving a fumaric acid yield of 53.15 g / L and a yield of 45%. However, this method suffered from a long fermentation cycle and numerous byproducts. American scholars used *Rhizopus oryzae* to ferment for 48 hours, achieving a fumaric acid yield of 33 g / L and a yield of 33.7%. These studies demonstrate the potential of microbial fermentation methods, but also reflect the need for further optimization in terms of efficiency and product purity.

[0005] Driven by the principles of green chemistry, this invention proposes a synergistic strategy to turn waste into treasure: coupling the degradation of waste polyvinyl chloride (PVC) with the production of fumaric acid in the same process. This not only provides a high-value outlet for the green transformation of PVC, but also aligns with the trend of developing novel synthesis processes using environmentally friendly media such as ionic liquids, opening up a completely new direction for the sustainable preparation of fumaric acid. Summary of the Invention

[0006] This invention addresses the urgent need for processing of polyvinyl chloride (PVC) and the problems of low yield and long production cycle in fumaric acid production. It provides a method for preparing fumaric acid from PVC dechlorination products using ionic liquid catalytic oxidation. This method uses PVC or waste PVC dechlorination products as raw materials and utilizes an ionic liquid-mediated nitric oxide / oxygen shuttle oxidation system to selectively oxidize and break conjugated carbon chains, yielding an oxidation product with fumaric acid as the main diacid product.

[0007] To achieve the above and other related objectives, the present invention provides the following technical solutions: A method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids includes the steps of: mixing polyvinyl chloride dechlorination products with ionic liquids, and carrying out selective oxidation reactions in the presence of nitric oxide and oxygen to obtain fumaric acid as the target oxidation product.

[0008] It should be noted that in this invention, nitric oxide and oxygen can form a recyclable nitrogen oxide oxidation system in an ionic liquid medium. The ionic liquid acts as both a catalyst and enhances the transfer and contact efficiency of nitrogen oxides in the reaction system, as well as improving the contact between dechlorination products and oxidizing active species. Compared to direct oxidation of PVC, this invention first removes or significantly reduces the chlorine content, and then performs directional oxidation of the conjugated carbon chain, thus reducing the risk of hydrogen chloride release and chlorination byproduct formation.

[0009] Furthermore, the polyvinyl chloride dechlorination product is obtained by crushing, washing, drying and dechlorinating waste polyvinyl chloride raw materials; the waste polyvinyl chloride raw materials include one or more of polyvinyl chloride pipes, polyvinyl chloride films, polyvinyl chloride cable sheaths, polyvinyl chloride scraps and recycled polyvinyl chloride materials.

[0010] Furthermore, the ionic liquid includes one or more of quaternary phosphonium salt ionic liquids, quaternary ammonium salt ionic liquids, or imidazolium salt ionic liquids.

[0011] Furthermore, the anions of the ionic liquid include one or more of benzenesulfonate, alkylsulfonate, acetate, trifluoroacetate, and trichloroacetate.

[0012] Furthermore, the cation of the ionic liquid includes one or more of trihexyltetradecylphosphine, trihexyloctylphosphine, tributyloctylphosphine, triethylbutylamine, tetraethylammonium, and 1-alkyl-3-methylimidazolium.

[0013] Furthermore, the ionic liquid includes one or more of [trihexyltetradecylphosphine][acetic acid], [tributyloctylphosphine][acetic acid], [triethylbutylammonium][acetic acid], [trihexyltetradecylphosphine][benzenesulfonic acid], [tetraethylammonium][benzenesulfonic acid], and [trihexyloctylphosphine][benzenesulfonic acid].

[0014] Furthermore, the mass ratio of the ionic liquid to the polyvinyl chloride dechlorination product is 1-5:1; preferably 2-5:1.

[0015] Furthermore, the temperature of the selective oxidation reaction is 60-150℃, preferably 80-100℃; the reaction time is 6-48h, preferably 18-36h.

[0016] Furthermore, after the selective oxidation reaction is completed, the mixture is cooled and the tail gas is discharged. Fumaric acid and other small molecule oxidation products are transferred from the solvent phase or crystallized out. Fumaric acid is then separated by one or more of the following methods: solid-liquid separation, recrystallization, or extraction.

[0017] Furthermore, the partial pressure of the nitric oxide is 0.1-1 MPa, preferably 0.1-0.5 MPa; and the partial pressure of the oxygen is 1-5 MPa, preferably 2-4 MPa.

[0018] Furthermore, after the selective oxidation reaction is completed, the ionic liquid is separated from the fumaric acid, and the separated ionic liquid is recycled for the next batch of selective oxidation reaction.

[0019] The beneficial effects of this invention are: First, by dechlorinating before oxidation, the problems of hydrogen chloride release, equipment corrosion, and chlorination byproducts in the direct oxidation process of PVC are avoided.

[0020] Second, the conjugated polyene structure in the dechlorination product is used as a reaction site to achieve the directional conversion to fumaric acid.

[0021] Third, ionic liquids can be recycled, and the reaction does not require the addition of external metal catalysts, making the system easier to separate and recycle.

[0022] In summary, compared to traditional fumaric acid preparation processes such as maleic anhydride isomerization or furfural oxidation, this invention opens up a new, efficient, and environmentally friendly route for fumaric acid synthesis using waste polyvinyl chloride dechlorination products as raw materials under an ionic liquid catalytic system. This method not only effectively reduces raw material costs but also features mild reaction conditions, is environmentally friendly, and achieves a fumaric acid yield of up to 92%, demonstrating significant potential for industrial applications. Attached Figure Description

[0023] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a liquid chromatogram of the reaction solution after the reaction in Example 1 of the present invention. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0025] This invention provides a method for preparing fumaric acid from polyvinyl chloride dechlorination products using ionic liquid catalytic oxidation, comprising the following steps: 1. Prepare dechlorinated PVC. The PVC raw materials are selected from waste PVC pipes, PVC films, PVC cable sheaths, PVC scraps, and recycled PVC. To reduce the influence of plasticizers, fillers, and stabilizers on the subsequent oxidation reaction, the raw materials can be shredded, washed, dried, and sieved before dechlorination. The dechlorination reaction can be carried out according to the method described in the literature, Kamil Oster et al., “Dehydrochlorination of PVC in multi-layered blister packs using ionic liquids”, Green Chemistry, 2020, Vol. 22, pp. 5132-5142, DOI: 10.1039 / D0GC01312A, to obtain dechlorinated PVC.

[0026] 2. The dechlorinated polyvinyl chloride (PVC) is mixed with an ionic liquid and subjected to a selective oxidation reaction in the presence of nitric oxide and oxygen to obtain fumaric acid as the target oxidation product. Specifically: (1) Ionic liquid-mediated selective oxidation of NO / O2: The dechlorinated polyvinyl chloride and ionic liquid are added to a pressure-resistant reactor, which is then filled with nitric oxide and oxygen. The reactor is heated under stirring to carry out a selective oxidation reaction.

[0027] The mass ratio of ionic liquid to dechlorinated polyvinyl chloride is 1-5:1, preferably 2-5:1. The partial pressure of nitric oxide is 0.1-1 MPa, and the partial pressure of oxygen is 1-5 MPa. The oxidation reaction temperature is 60-150℃.

[0028] Under the above conditions, the polyvinyl chloride dechlorinated product undergoes oxidative cleavage under the shuttle oxidation of nitrogen oxides and oxygen, generating a dicarboxylic acid product with fumaric acid as the target product.

[0029] The selective oxidation reaction of this invention does not require an external metal catalyst. Compared with metal salt or metal oxide catalytic systems, this system reduces metal residue and post-processing difficulties, and facilitates the recycling of ionic liquids.

[0030] (2) Product separation and ionic liquid circulation: After the reaction is complete, the mixture is cooled and the exhaust gas is discharged. Water, methanol, ethanol, or a water-alcohol mixture is added to the reaction system to allow fumaric acid and other small-molecule oxidation products to transfer from the solvent phase or crystallize out. The fumaric acid product can be obtained by filtration, recrystallization, or extraction.

[0031] The ionic liquid after fumaric acid separation can be reused in the next batch of reactions after depressurization to remove solvent and volatiles. Preferably, the ionic liquid retains high catalytic activity even after being recycled more than 5 times.

[0032] The following describes the specific implementation of the method for preparing fumaric acid using ionic liquid catalytic oxidation of polyvinyl chloride dechlorination products.

[0033] The raw materials used in the following specific embodiments were all purchased from the market. The raw materials, such as ionic liquid, polyvinyl chloride, and nitric oxide, were purchased from companies such as Anhui Zesheng Technology Co., Ltd., Shanghai Titan Technology Co., Ltd., and Jingong Special Products Co., Ltd. The purity of the drugs was chemically pure.

[0034] Specifically, the polyvinyl chloride resin was purchased from the exploration platform of Shanghai Titan Technology Co., Ltd., brand name Adamas, original number 100677FC, CAS number 9002-86-2, grade reagent grade; the sodium hydroxide was purchased from the exploration platform of Shanghai Titan Technology Co., Ltd., brand name Greatit, original number G19852H-ZX, CAS number 1310-73-2, purity not less than 96%, analytical grade; the fumaric acid reference standard was purchased from the exploration platform of Shanghai Titan Technology Co., Ltd., brand name CATO, original number CCPD101302-5g.

[0035] Tetraethylammonium benzenesulfonate, triethylbutylammonium acetate, tributyloctylphosphonium acetate, trihexyloctylphosphonium benzenesulfonate, and trihexyltetradecylphosphonium acetate were purchased from Anhui Zesheng Technology Co., Ltd. These were custom-prepared ionic liquids according to their respective cation and anion compositions, with a bulk mass fraction of not less than 98% and a moisture content not exceeding 0.5%. Each ionic liquid was dried under reduced pressure at 80°C for 12 hours before use, cooled to room temperature, and then sealed for storage.

[0036] Nitric oxide, oxygen, and nitrogen were purchased from Hangzhou Jingong Special Gases Co., Ltd. High-purity nitric oxide was used, oxygen volume fraction was not less than 99.99%, and nitrogen volume fraction was not less than 99.999%. All other conventional chemical reagents, unless otherwise specified, were of analytical grade. Solvents used in liquid chromatography were of chromatographic grade, and deionized water or ultrapure water was used in the experiments.

[0037] Fumaric acid reference standard was used for qualitative and quantitative analysis by liquid chromatography. The target product was confirmed by comparing the retention times of the target peak in the reaction sample with those of the fumaric acid reference standard, and by combining the standard spike test. A standard curve between fumaric acid concentration and chromatographic peak area was established using the external standard method, and the content and yield of fumaric acid in the reaction product were calculated accordingly.

[0038] The gas pressures mentioned in this invention refer to the initial pressure after the corresponding gas is introduced into the closed reactor at room temperature and before the reaction is heated.

[0039] Definitions of conversion rate and fumaric acid yield: The conversion rate of the PVC dechlorination product is calculated based on the change in the mass of carbon in the methanol-insoluble solids before and after the reaction. After the reaction, methanol is added to the reaction system, allowing the soluble organic products generated by the reaction to enter the liquid phase, while the unreacted PVC dechlorination product, filler, and other methanol-insoluble components remain in the solid phase. After solid-liquid separation, the methanol-insoluble solids are collected, washed, dried, and weighed. Simultaneously, elemental analysis is used to determine the mass fraction of carbon in the PVC dechlorination product before the reaction and the methanol-insoluble solids after the reaction.

[0040] The conversion rate of polyvinyl chloride dechlorination products is calculated using the following formula:

[0041] In the formula: m0 is the mass of the dechlorination product of polyvinyl chloride before the reaction; w C,0 The mass fraction of carbon in the dechlorination products of polyvinyl chloride before the reaction; m r w represents the mass of the methanol-insoluble solid after the reaction. C,r This represents the mass fraction of carbon in the methanol-insoluble solid after the reaction.

[0042] In this application, the fumaric acid yield is the carbon molar yield calculated based on the carbon element in the initial polyvinyl chloride repeating unit. After the reaction, the reaction solution is subjected to solid-liquid separation, and the resulting liquid phase is analyzed by high-performance liquid chromatography after being brought to a final volume or diluted. The concentration of fumaric acid in the reaction solution is determined according to the fumaric acid standard curve, and the molar amount of fumaric acid is calculated based on the total volume of the reaction solution. When polyvinyl chloride products are used as raw materials, the molar amount of the initial polyvinyl chloride repeating unit is calculated based on the actual polyvinyl chloride content in the raw material, deducting the influence of fillers, plasticizers, and other non-polyvinyl chloride components. The molar mass of the polyvinyl chloride repeating unit is 62.5 g / mol.

[0043] During the dechlorination process of polyvinyl chloride (PVC), the main product removed is hydrogen chloride, and the carbon element in the PVC main chain does not suffer substantial loss. Therefore, the subsequent fumaric acid yield is calculated based on the theoretical molar amount of carbon in the PVC repeating unit before dechlorination. The fumaric acid yield of 92% mentioned in this application refers to 92% of the carbon element contained in the initial PVC repeating unit being converted into fumaric acid in the reaction solution, i.e., the carbon molar yield of fumaric acid is 92%, not that the mass of fumaric acid accounts for 92% of the total mass of the added PVC dechlorination products.

[0044] In this application, the high-performance liquid chromatography (HPLC) method determines the fumaric acid content in the reaction solution after the reaction is complete, and the resulting fumaric acid yield is a liquid chromatography analytical yield. This yield is not equivalent to the separation yield obtained after fumaric acid is concentrated, acidified, recrystallized, filtered, and dried. The actual yield of the product obtained after fumaric acid separation and purification is defined separately as the fumaric acid separation yield.

[0045] Example 1 Preparation of dechlorinated polyvinyl chloride (PVC): 10 g of PVC and 20 g of 1-butyl-3-methylimidazolium chloride were added to a 100 mL three-necked flask. The reaction system was heated to 160 °C under a nitrogen atmosphere and stirred for 8 h to obtain a reaction mixture containing PVC dechlorinated PVC. After the reaction was completed, the reaction system was cooled to room temperature, and water was added to precipitate the PVC dechlorinated PVC. The solid product was obtained by solid-liquid separation. The obtained solid was washed with water to remove residual 1-butyl-3-methylimidazolium chloride, and then dried under vacuum to obtain the PVC dechlorinated PVC.

[0046] In a 100 mL autoclave equipped with mechanical stirring, 6 g of tetraethylammonium [benzenesulfonic acid] and 3 g of polyvinyl chloride dechlorination product were added and stirred. The mixture was then charged with 0.3 MPa of NO and 2 MPa of O2 at room temperature. The reaction temperature was controlled at 80 °C, the stirring rate at 150 r / min, and the reaction was carried out for 24 h. The conversion rate was measured to be ≥99%, and the yield was 92%. After the reaction was completed, 10 g of water was added to the reaction system and stirred thoroughly to dissolve the ionic liquid in the aqueous phase, while fumaric acid precipitated as a solid, thus forming a solid-liquid two-phase system. The solid fumaric acid was then separated from the ionic liquid by filtration.

[0047] Examples 2-4 Following the process conditions of Example 1, the mechanical stirring speed was controlled at 150 r / min, and the ionic liquid, NO and O2 pressures, reaction time, and other parameters were the same as in Example 1. The conversion rate and yield of the products were varied by changing the reaction temperature, as shown in Table 1. It can be seen that the oxidative cleavage of the conjugated polyene segments is insufficient at too low a temperature (70°C); the conversion rate and fumaric acid yield significantly improve when the temperature is increased to 80°C; while further increasing the temperature can maintain a high conversion rate, further oxidation of fumaric acid or increased side reactions lead to a decrease in the fumaric acid yield.

[0048] Table 1. Effect of different reaction temperatures on the dechlorination oxidation reaction of polyvinyl chloride.

[0049] Examples 5-8 Following the process conditions of Example 1, the mechanical stirring speed was controlled at 150 r / min, and the ionic liquid, NO and O2 pressures, and reaction temperature were the same as in Example 1. The conversion rate and yield of the products were varied, and are shown in Table 2. It can be seen that insufficient reaction time results in incomplete conversion of the raw materials and insufficient formation of fumaric acid. Increasing the reaction time improves both conversion and yield, achieving a higher yield after 24 hours. However, further extending the reaction time beyond 24 hours increases the likelihood of continued oxidation of fumaric acid, thus leading to a decrease in yield.

[0050] Table 2. Effect of different reaction times on the dechlorination oxidation reaction of polyvinyl chloride.

[0051] Examples 9-13 Following the process conditions of Example 1, the mechanical stirring speed was controlled at 150 r / min, and the ionic liquid, O2 pressure, reaction temperature, and time were the same as in Example 1. The conversion rate and yield of the product were varied by changing the NO charging pressure. Table 3 shows that without the addition of nitric oxide, it is difficult to achieve efficient and selective fumaric acid production using only oxygen. An appropriate amount of nitric oxide can significantly improve the conversion rate and fumaric acid yield. However, when the nitric oxide partial pressure is too high, excessive oxidation and nitrogen-containing side reactions increase, thus increasing the NO charging pressure can actually lead to a decrease in yield.

[0052] Table 3. Effect of different NO charging pressures on the dechlorination oxidation reaction of polyvinyl chloride.

[0053] Examples 14-17 Following the process conditions of Example 1, the mechanical stirring speed was controlled at 150 r / min, and the ionic liquid, NO pressure, reaction temperature, and time were the same as in Example 1. The conversion rate and yield of the product were varied by changing the O2 pressure. Table 4 shows that oxygen is essential for maintaining the nitric oxide / oxygen oxidation cycle and achieving carbon chain cleavage. The conversion rate tends to stabilize after the oxygen partial pressure reaches 2.0 MPa, and further increasing the oxygen partial pressure has limited effect on improving the fumaric acid yield. In other words, before the oxygen partial pressure reaches 2.0 MPa, decreasing the O2 pressure leads to a decrease in conversion rate and yield. After the oxygen partial pressure reaches 2.0 MPa, increasing the O2 pressure does not significantly change the yield; in fact, the yield may even decrease.

[0054] Table 4. Effect of different O2 charging pressures on the dechlorination oxidation reaction of polyvinyl chloride.

[0055] Examples 18-21 Following the process conditions of Example 1, the mechanical stirring speed was controlled at 150 r / min, and the amount of ionic liquid, NO and O2 pressures, reaction temperature, and time were the same as in Example 1. The conversion rate and yield of the products were changed by altering the type of ionic liquid. Table 5 shows that different combinations of cations and anions collectively affect the polarity, viscosity, wetting and dispersing ability of the dechlorination products, and absorption and transfer ability of nitrogen oxides of the ionic liquid. In the tested systems, tetraethylammonium benzenesulfonate yielded the highest fumaric acid yield, indicating that the effectiveness of ionic liquids cannot be judged solely by cation chain length or a single polarity parameter.

[0056] Table 5. Effects of different types of ionic liquids on the dechlorination oxidation reaction of polyvinyl chloride.

[0057] More specifically, it is worth noting that different ionic liquids have a significant impact on the conversion rate of PVC dechlorination products and the yield of fumaric acid. The type of cation and the alkyl chain length of the ionic liquid affect its polarity, viscosity, and wetting and dispersing ability towards PVC dechlorination products, while the type of anion affects the dissolution, transport, and activation processes of nitric oxide and oxygen in the reaction medium. Therefore, the influence of ionic liquids on this reaction is not determined by a single structural factor, but rather by the combined effect of the cation structure and the type of anion.

[0058] Under the experimental conditions described in this group, when [tetraethylammonium][benzenesulfonic acid] was used as the ionic liquid, the conversion rate of PVC dechlorination products was no less than 99%, and the fumaric acid yield reached 92%, which was higher than other ionic liquid systems. This indicates that a suitable structural match was formed between the quaternary ammonium cation with a shorter alkyl chain and the benzenesulfonate anion. This ionic liquid can maintain good system polarity and gas mass transfer ability, and can also promote sufficient contact between PVC dechlorination products and active nitrogen oxide species, thus facilitating the selective oxidative cleavage of the conjugated carbon chain. In contrast, quaternary phosphine salt ionic liquids with longer alkyl chains have relatively higher hydrophobicity and viscosity, which may limit the mass transfer of oxygen and nitrogen oxides to some extent; different anions also have different stabilization and transfer abilities for active nitrogen and oxygen species, resulting in different fumaric acid yields.

[0059] The above results indicate that both the alkyl chain length of the cation and the type of anion in the ionic liquid affect the selective oxidation reaction, but there is a synergistic matching relationship between the two. The fumaric acid yield does not exhibit a simple monotonic change with variations in cation chain length or ionic liquid polarity. By selecting appropriate combinations of cations and anions, the mass transfer efficiency and selectivity of the nitric oxide / oxygen oxidation system can be improved, with [tetraethylammonium][benzenesulfonic acid] showing superior reaction performance.

[0060] Examples 22-25 Following the process conditions of Example 1, the mechanical stirring speed was controlled at 150 r / min, and the type of ionic liquid, NO and O2 pressures, reaction temperature, and time were the same as above. The conversion rate and yield of the products were varied by changing the amount of ionic liquid used. Table 6 shows that without the addition of ionic liquid, the dechlorination product had insufficient contact with the gaseous oxide, resulting in a fumaric acid yield of only 5%. Increasing the amount of ionic liquid significantly improved the reaction efficiency, but beyond a certain amount, changes in the system viscosity and mass transfer state prevented further increases in the fumaric acid yield.

[0061] Table 6. Effect of different amounts of ionic liquids on the dechlorination oxidation reaction of polyvinyl chloride.

[0062] Example 26 After the reaction is complete, the fumaric acid product is separated, the ionic liquid is recovered, and the solvent and volatile components are removed under reduced pressure. The recovered ionic liquid is directly used in the next batch of polyvinyl chloride dechlorination oxidation reaction, and the cycle is repeated 5 times.

[0063] Specifically, taking Example 1 as an example: Following the reaction conditions of Example 1, [tetraethylammonium][benzenesulfonic acid] was used as an ionic liquid to selectively oxidize the dechlorination products of polyvinyl chloride, and the yield of fumaric acid obtained in the first reaction was 92%.

[0064] After the initial reaction, the reaction system was cooled and the remaining gas was released. The fumaric acid product was then separated using the aforementioned product separation method. The ionic liquid phase after fumaric acid separation was subjected to vacuum solvent removal to remove residual water, alcohol solvents, and other volatile components, yielding the recovered ionic liquid.

[0065] The recovered ionic liquid was used for the selective oxidation reaction of the next batch of PVC dechlorination products. Except for the use of the recovered ionic liquid, the PVC dechlorination products, nitric oxide partial pressure, oxygen partial pressure, reaction temperature, reaction time, and stirring conditions were all set as in Example 1. After each batch of reaction, the fumaric acid product was separated using the same method, and the ionic liquid was recovered and subjected to vacuum desolvation. This process was repeated five times consecutively; that is, after the first reaction, the recovered ionic liquid was used to continue five batches of selective oxidation reactions.

[0066] In the fifth reuse, the yield of fumaric acid remained at 93% relative to the yield of the first reaction. This yield retention was calculated using the following formula: Yield retention rate = Fumaric acid yield at the 5th reuse ÷ Fumaric acid yield in the first reaction × 100%.

[0067] The initial fumaric acid yield was 92%. Therefore, based on a 93% yield retention rate, the fumaric acid yield in the fifth reuse was approximately 85%. These results indicate that [tetraethylammonium][benzenesulfonic acid] can maintain good reaction medium and catalytic promoting effects after multiple recovery and recycling, demonstrating good recyclability.

[0068] In summary, ionic liquids, nitric oxide, and oxygen all play crucial roles in the reaction. The reaction cannot proceed effectively without oxygen; while some oxidation occurs without nitric oxide, the yield of fumaric acid is significantly reduced; and without ionic liquids, reactant dispersion and gas transport conditions deteriorate, resulting in low conversion and fumaric acid yields. The complete system increases the conversion rate of PVC dechlorination products to at least 99% and achieves a fumaric acid molar yield of 92%, demonstrating the synergistic effect among the three components.

[0069] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids, characterized in that, The process includes the following steps: mixing polyvinyl chloride dechlorinated material with an ionic liquid and carrying out a selective oxidation reaction in the presence of nitric oxide and oxygen to obtain an oxidation product targeting fumaric acid.

2. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The dechlorinated polyvinyl chloride product is obtained by crushing, washing, drying and dechlorinating waste polyvinyl chloride raw materials; the waste polyvinyl chloride raw materials include one or more of polyvinyl chloride pipes, polyvinyl chloride films, polyvinyl chloride cable sheaths, polyvinyl chloride scraps and recycled polyvinyl chloride materials.

3. The method for preparing fumaric acid using ionic liquid-catalyzed oxidation of polyvinyl chloride dechlorination products according to claim 1, characterized in that, The ionic liquid includes one or more of quaternary phosphonium salt ionic liquids, quaternary ammonium salt ionic liquids, or imidazolium salt ionic liquids.

4. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The anions of the ionic liquid include one or more of benzenesulfonate, alkylsulfonate, acetate, trifluoroacetate, and trichloroacetate.

5. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The cation of the ionic liquid includes one or more of trihexyltetradecylphosphine, trihexyloctylphosphine, tributyloctylphosphine, triethylbutylamine, tetraethylammonium, and 1-alkyl-3-methylimidazolium.

6. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The ionic liquid includes one or more of the following: [trihexyltetradecylphosphine][acetic acid], [tributyloctylphosphine][acetic acid], [triethylbutylammonium][acetic acid], [trihexyltetradecylphosphine][benzenesulfonic acid], [tetraethylammonium][benzenesulfonic acid], and [trihexyloctylphosphine][benzenesulfonic acid].

7. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The mass ratio of the ionic liquid to the polyvinyl chloride dechlorination product is 1-5:

1.

8. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The selective oxidation reaction is carried out at a temperature of 60-150℃ and for a reaction time of 6-48h.

9. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, After the selective oxidation reaction is completed, the mixture is cooled and the tail gas is discharged. Fumaric acid and other small molecule oxidation products are transferred from the solvent phase or crystallized out. Fumaric acid is then separated by one or more of the following methods: solid-liquid separation, recrystallization, or extraction.

10. The method for preparing fumaric acid by catalytic oxidation of polyvinyl chloride dechlorination products using ionic liquids according to claim 1, characterized in that, The partial pressure of nitric oxide is 0.1-1 MPa; the partial pressure of oxygen is 1-5 MPa.

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