A combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate

Abstract

This invention discloses a combined process for the pretreatment of polyester waste and the preparation of dioctyl terephthalate in the field of polymer chemical recycling and resource recycling. The process utilizes specially formulated layered molybdenum-titanium bimetallic phosphate nanosheets with intercalation catalysis as a catalyst. The crushed polyester waste is mixed with isooctanol and pretreated at a lower temperature, followed by a catalytic alcoholysis and transesterification reaction. Ethylene glycol is continuously removed by fractional distillation to drive the reaction to completion. After the reaction, the mixture is cooled and filtered to separate the regenerable solid catalyst. The filtrate is then neutralized, washed with water, and subjected to two-step vacuum distillation. This process achieves a high degree of synergy between the catalyst and the reaction steps, simplifying the traditional multi-step complex process into four core steps. It has the advantages of mild reaction conditions, simple and efficient process, recyclable catalyst, and high product purity, providing a novel and practical technical solution for the high-value resource utilization of polyester waste.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling and resource recycling technology of polymer materials, specifically to a combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate. Background Technology

[0002] Polyethylene terephthalate (PET), as one of the world's largest-produced synthetic fibers and packaging materials, generates a significant amount of waste after consumption, posing a serious challenge to global solid waste management and environmental sustainability. Traditional methods for disposing of waste PET mainly include landfill and incineration. Landfilling occupies valuable land resources for extended periods, and its natural degradation process is extremely slow. Incineration may release harmful gases and produce greenhouse gases due to incomplete combustion, both of which contradict the principles of a circular economy and green chemistry. Therefore, developing efficient and clean chemical recycling technologies to convert waste PET into high-value-added chemicals or monomers is considered a key pathway to achieving closed-loop utilization of polyester materials and improving resource efficiency, with significant environmental benefits and potential economic value.

[0003] Among various chemical recycling approaches, the technology of converting waste polyethylene terephthalate (PET) into dioctyl terephthalate (DTP), a high-performance plasticizer, has attracted widespread attention. However, existing technological approaches typically rely on multi-step, separated process frameworks, generally suffering from inherent drawbacks such as lengthy processes and high energy consumption. A typical process first requires deep alcoholysis of waste PET, using ethylene glycol or excess isooctanol to break its ester bonds under high temperature, high pressure, and strong acid or strong base catalysts, generating intermediates such as diethyl terephthalate (DHT) or terephthalic acid. Subsequently, this intermediate needs to be separated and purified, and then reacted with isooctanol in another reaction system under different catalytic conditions to undergo esterification or transesterification reactions, ultimately yielding DTP. In this process, although homogeneous catalysts have high activity, they are difficult to separate and recover, leading to product contamination and catalyst loss. Heterogeneous solid catalysts often face problems such as poor dispersibility in viscous polymer melts, insufficient contact with substrates, and low catalytic efficiency. Especially when treating waste textiles with complex compositions and containing dyes or auxiliaries, the stability and selectivity of the catalysts often decrease significantly.

[0004] More significantly, the existing process suffers from insufficient synergy between catalyst performance and reaction engineering design, failing to effectively address the constraints of reaction kinetics and thermodynamic equilibrium on overall efficiency. The separation of the alcoholysis and esterification steps not only increases equipment investment and operating costs but also introduces more material transfer and energy input steps. Simultaneously, if the ethylene glycol byproduct generated in the alcoholysis reaction cannot be removed from the reaction system in a timely and effective manner, it will severely restrict the shift of reaction equilibrium towards the product, forcing production to employ higher reaction temperatures, longer reaction times, or larger alcohol excess ratios, further exacerbating energy consumption and potentially triggering unnecessary side reactions, affecting the color and purity of the final product. These technical bottlenecks collectively restrict the economic competitiveness and large-scale industrial application of this recovery route. Therefore, developing a combined process that achieves efficient conversion under mild conditions, highly integrated and simplified process flow, and recyclable catalysts has become an urgent and clear technical need in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate, which overcomes the technical problems of existing technology in the preparation of dioctyl terephthalate from polyester waste, such as complex and lengthy process flow, harsh reaction conditions, difficulty in separating and recovering catalysts, and high overall energy consumption and cost.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] A combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate includes the following steps:

[0008] S1, by weight, polyester waste is crushed by a crusher to obtain polyester waste fragments; 80-100 parts of polyester waste fragments and 1-5 parts of layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function are put into a pretreatment tank, 400-600 parts of isooctanol are added, and the mixture is stirred at 80-100℃ to obtain a mixture;

[0009] S2, the entire mixture is transferred to a reaction vessel, heated to 160-180℃ and reacted, and ethylene glycol is continuously removed by fractional distillation to obtain the reaction mixture;

[0010] S3, the reaction mixture is cooled to 75-78℃ and separated using a plate and frame filter to obtain a solid catalyst and filtrate; the solid catalyst is washed with isooctanol and calcined at 295-305℃ for recycling;

[0011] S4, add 5-15 parts of sodium carbonate aqueous solution to the filtrate, stir at 60-70℃ to obtain a mixture, transfer the mixture to a separatory funnel, let it stand to separate the lower aqueous phase, wash the organic phase with hot water at 70-80℃ to obtain the washed organic phase; transfer the washed organic phase to a distillation kettle, and distill at 120-140℃ under reduced pressure to recover isooctanol; then raise the temperature to 180-200℃ and continue distilling under reduced pressure to collect the dioctyl terephthalate main fraction.

[0012] In this invention, based on layered molybdenum-titanium bimetallic phosphate nanosheet catalysts with intercalation catalysis, the reaction mechanism of the combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate is manifested as a highly efficient synergistic process of "contact embedding, depolymerization and recombination, and removal-driven processes." In the pretreatment stage, when the catalyst nanosheets and polyester fragments are mixed and gently heated in isooctanol, isooctanol, acting as a solvent, begins to swell the amorphous regions of the polyester. The flexible and high-surface-energy catalyst nanosheets, due to their microscopic size and surface properties, can partially embed or tightly adhere to the swollen polymer segments, achieving unprecedented close contact between the heterogeneous catalyst and the solid macromolecular substrate, thus completing the initial activation of the reaction. Entering the core reaction stage, the increased temperature significantly enhances the catalytic effect. The Lewis acid sites on the surface of the embedded catalyst can strongly adsorb and polarize the ester carbonyl groups in the polyester molecular chain, making them more susceptible to nucleophilic attack by isooctanol molecules; while the proton acid sites may directly participate in proton transfer, promoting the breaking of ester bonds. This process is not a simple alcoholysis, but a continuous series reaction: the ester bonds of polyester first break, generating an intermediate containing terephthalic acid structures. Under the influence of excess isooctanol and a catalyst, the terminal hydroxyl or carboxyl groups of this intermediate rapidly undergo esterification or transesterification with isooctanol, directly generating the target product, dioctyl terephthalate, while simultaneously releasing ethylene glycol as a byproduct. The ingenious aspect of the process design lies in the continuous distillation of the low-boiling azeotrope formed by the ethylene glycol and isooctanol produced in the reaction through a fractionation device. This engineering operation continuously removes byproducts from the reaction zone, driving the originally reversible alcoholysis and esterification reactions towards the formation of dioctyl terephthalate according to the principle of chemical equilibrium, thus achieving complete conversion and high selectivity of polyester under relatively mild temperature conditions. After the reaction, the solid catalyst can be completely recovered through simple filtration, while the reaction liquid, after neutralization, water washing, and stepwise vacuum distillation, can separate and recover excess isooctanol and obtain a high-purity final product. Throughout the entire mechanism chain, the catalyst's unique intercalation contact capability and the integrated reaction removal-driven process design are closely linked, together forming the core of this combined process's high efficiency and simplicity.

[0013] According to a preferred embodiment of the present invention, in step S1, the stirring time is 30-60 min.

[0014] According to a preferred embodiment of the present invention, in step S2, the reaction time is 2-4 hours after heating to 160-180°C.

[0015] According to a preferred embodiment of the present invention, in step S3, the calcination time at 295-305°C is 2-4 hours.

[0016] According to a preferred embodiment of the present invention, in step S4, the stirring time at 60-70°C is 30-60 min.

[0017] According to a preferred embodiment of the present invention, the preparation steps of the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function include:

[0018] A1, by weight, under an argon atmosphere, dissolve 8-12 parts of molybdenum pentachloride and 10-20 parts of titanium tetrachloride in a mixed solvent of 200-330 parts of anhydrous ethanol and acetylacetone, and stir to obtain solution A; dissolve 25-40 parts of ammonium dihydrogen phosphate and 15-25 parts of hexadecyltrimethylammonium bromide in 800-1200 parts of deionized water to obtain solution B;

[0019] A2. Under continuous stirring, solution A is added dropwise to solution B, and the pH is adjusted to 9-10 to obtain a mixture. The mixture is then transferred to a high-pressure reactor and reacted at 175-185℃. After the reaction is completed, the mixture is allowed to cool naturally, and the precipitate is collected by centrifugation. The precipitate is washed alternately with ethanol and deionized water and dried under vacuum at 58-62℃ to obtain the molybdenum-titanium ammonium phosphate precursor.

[0020] A3. The molybdenum-titanium ammonium phosphate precursor was dispersed in an aqueous solution of n-butylamine and stirred under reflux at 78-82°C to obtain a mixture. The mixture was filtered and washed with deionized water to obtain an expanded layered material with organic molecular intercalation. The expanded layered material with organic molecular intercalation was redispersed in isopropanol and subjected to ultrasonic exfoliation. The upper colloidal suspension was collected by centrifugation.

[0021] A4. Add dilute sulfuric acid dropwise to the colloidal suspension until the pH is 2-3, stir, concentrate by ultrafiltration, and spray dry at 78-82℃.

[0022] In this invention, a layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function is prepared and its functional mechanism is rooted in the precise construction of the material's microstructure and the understanding of its unique interaction with polyester macromolecules. The synthesis of this catalyst is a multi-stage, stepwise assembly process, beginning with the uniform mixing of molybdenum and titanium precursors in the presence of an organic complexing agent. Through a hydrothermal reaction with phosphate ions and a structure-directing agent in a specific alkaline environment, an ammonium salt precursor with a regular layered arrangement is first formed. The layers of this precursor are composed of molybdenum-oxygen and titanium-oxygen polyhedra connected to phosphate ions by sharing vertices or edges, with organic ammonium ions embedded in the interlayers to balance the charge. The subsequent key step is intercalation using organic amine molecules. These amine molecules, due to their alkalinity and steric effect, can enter and expand the interlayer space of the precursor, greatly weakening the interaction forces between the layers. The subsequent ultrasonic exfoliation process, utilizing cavitation energy in a liquid, further overcomes the weakened interlayer interactions, ultimately dissociating the macroscopic layered material into single-layer or few-layer nanosheets. The final acid treatment step is crucial, replacing ammonium ions in the interlayer with hydrogen ions through ion exchange, thereby creating abundant proton acid sites on the surface and edges of the nanosheets. Simultaneously, the highly dispersed molybdenum and titanium species on the nanosheet layers serve as Lewis acid sites. Therefore, the resulting catalyst is an ultrathin nanosheet possessing both proton and Lewis acid bifunctional properties. Its large specific surface area and abundant surface active sites, especially the flexibility and accessibility conferred by its thin-layer structure, overcome the mass transfer limitations commonly encountered in heterogeneous catalysis, laying the foundation for subsequent close contact and intercalation catalysis with solid polymers.

[0023] According to a preferred embodiment of the present invention, in step A1, the volume ratio of anhydrous ethanol to acetylacetone is 3:1.

[0024] According to a preferred embodiment of the present invention, in step A2, the reaction time at 175-185°C is 48-50 h.

[0025] According to a preferred embodiment of the present invention, in step A3, the stirring and reflux time at 78-82°C is 24-48 hours.

[0026] According to a preferred embodiment of the present invention, in step A4, the stirring time is 12-14 hours.

[0027] The beneficial effects of this invention are as follows:

[0028] The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate provided by this invention introduces a layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with special intercalation catalytic function, and cleverly designs an integrated reaction and separation process, which brings significant technological progress and application advantages in multiple dimensions. Its beneficial effects are mainly reflected in three aspects: process innovation, catalyst efficiency, and comprehensive benefits.

[0029] First, the process flow has been fundamentally simplified and optimized, greatly improving overall efficiency and economy. The traditional process, which involves multiple complex unit operations such as polyester swelling and activation, deep alcoholysis, intermediate separation, and transesterification, is creatively integrated into a continuous, synergistic four-step combined process. The core reaction step takes place in a single reactor, utilizing excess isooctanol as a solvent, reactant, and azeotropic dehydrating agent simultaneously. This allows for the direct conversion from polyester waste to dioctyl terephthalate at a relatively low temperature, with byproducts promptly removed through in-situ fractionation, continuously shifting the reaction equilibrium to the right. This design not only eliminates multiple energy-intensive steps such as intermediate separation, purification, and re-esterification, significantly shortening the total process time and reducing equipment investment and floor space, but also simplifies operating procedures, reduces losses and safety risks caused by multiple material transfers, and makes the entire recycling process more compact, efficient, and controllable.

[0030] Secondly, the design and application of the core catalyst are key to achieving the aforementioned process innovations, demonstrating superior catalytic performance and excellent engineering applicability. This layered molybdenum-titanium bimetallic phosphate nanosheet catalyst is prepared through a precise synthetic route. Its unique intercalation structure allows it to effectively embed into the swollen amorphous regions of polyester during the pretreatment stage, catalyzing from within the polymer and solving the core problem of insufficient contact between heterogeneous catalysts and solid macromolecular substrates. Simultaneously, the catalyst surface possesses multiple acidic sites that synergistically catalyze the tandem reactions of alcoholysis and transesterification, thereby achieving efficient and deep conversion of polyester at temperatures far lower than traditional processes. Furthermore, the catalyst exists in nanosheet solid form, allowing for efficient separation and recovery through simple mechanical filtration after the reaction. After activation, it can be recycled multiple times without significant activity decay. This completely avoids product contamination, equipment corrosion, and complex wastewater treatment problems associated with homogeneous catalysts, meeting the requirements of green chemistry and clean production.

[0031] Finally, this invention offers outstanding comprehensive benefits, combining excellent product quality, environmental friendliness, and industrialization potential. Due to the mild reaction conditions and high catalyst selectivity, side reactions and product coloring caused by high temperatures are effectively suppressed. Therefore, the final dioctyl terephthalate product obtained has excellent purity and color, fully meeting the market standards for high-end plasticizers. The entire process requires only mild neutralization and washing in the final stage, resulting in minimal waste generation. Furthermore, the core solvent and catalyst can be recycled, reducing waste emissions at the source. This process is highly compatible with polyester waste materials, effectively treating both bottle flakes and textiles. It provides a practical and cost-effective technical path for the large-scale, diversified, and high-value utilization of polyester waste resources, with broad prospects for industrial application. Detailed Implementation

[0032] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0033] Example 1

[0034] Preparation of layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function: Under an argon atmosphere, 10.0 g of molybdenum pentachloride and 15.0 g of titanium tetrachloride were dissolved in a mixed solvent of 240 mL anhydrous ethanol and 80 mL acetylacetone. The solution was magnetically stirred at 25 °C for 30 min to obtain a clear orange-red solution A. Simultaneously, 32.0 g of ammonium dihydrogen phosphate and 20.0 g of hexadecyltrimethylammonium bromide were dissolved in 1000 mL of deionized water and stirred in a 40 °C water bath until completely dissolved to obtain a clear solution B. Under continuous stirring, solution A was slowly added dropwise to solution B at a rate of approximately 2 mL / min using a constant pressure dropping funnel. After the addition was complete, concentrated ammonia was added dropwise to the mixture while stirring, and the pH was monitored using a pH meter until the pH of the system stabilized at 9.5. The resulting mixture was transferred to a 200 mL polytetrafluoroethylene-lined high-pressure reactor, placed in an oven, and heated to 180 °C, maintaining the reaction at this temperature for 49 h. After the reaction, the reactor was allowed to cool naturally to room temperature in the oven. The reactor was opened, and the contents were transferred to centrifuge tubes and centrifuged at 8000 rpm for 10 min, discarding the supernatant. The precipitate was washed three times each with 50 mL of anhydrous ethanol and 50 mL of deionized water, centrifuged under the same conditions after each wash. The washed precipitate was transferred to a petri dish and dried in a vacuum drying oven at 60 °C for 12 h to obtain a white powdery molybdenum-titanium ammonium phosphate precursor. 10.0 g of this precursor powder was weighed and dispersed in 200 mL of a 1.0 mol / L n-butylamine aqueous solution, and stirred under reflux in an oil bath at 80 °C for 36 h. After the reaction, the mixture was filtered while hot, and the resulting solid was washed with deionized water until the filtrate was neutral as determined by pH paper. The obtained wet filter cake was redispersed in 500 mL of isopropanol and placed in an ultrasonic cleaner for ultrasonic exfoliation at 500 W for 8 h. The ultrasonically treated suspension was centrifuged at 12000 rpm for 30 min, and the upper homogeneous and stable milky white colloidal suspension was carefully collected. 0.5 mol / L dilute sulfuric acid was added dropwise to this colloidal suspension under magnetic stirring, with real-time monitoring using a precision pH meter, until the pH of the system stabilized at 2.5. After stirring for another 13 h, the suspension was concentrated using ultrafiltration. Finally, the concentrated slurry was spray-dried at an inlet air temperature of 80 °C to obtain a layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function, labeled CAT-1.

[0035] Preparation of the combined process: 100g of waste polyester textiles (washed and dried) were processed by a crusher to obtain fragments with a particle size of 2-5mm. All fragments were added together with 2.0g of catalyst CAT-1 into a 500mL three-necked flask (as a pretreatment vessel). Subsequently, 500g of isooctanol (2-ethylhexanol) was added to the flask. The flask was placed in an oil bath and equipped with a mechanical stirrer, condenser, and thermometer. The stirrer was started and set to a speed of 200rpm, and the oil bath was heated to raise the temperature of the mixture to 90℃. Stirring was maintained at this temperature for 45min, during which the polyester fragments were observed to swell significantly, and the system became a paste-like mixture. This mixture was then transferred to a 1L jacketed glass reactor equipped with a mechanical stirrer, a temperature control system, a precision fractionation column, and a condenser. The reactor was stirred and heated, with a heating rate controlled at approximately 3℃ / min, until the temperature inside the reactor reached 170℃. The reaction was maintained at this temperature for 3h. During the reaction, the low-boiling azeotrope formed by isooctanol and the reaction byproduct ethylene glycol was continuously distilled off and collected by adjusting the reflux ratio of the fractionating column. After the reaction was completed, heating was stopped, and the reaction mixture was allowed to cool naturally to 76°C. The mixture was then hot-filtered through a preheated plate and frame filter to separate the solid catalyst, yielding a clear yellow filtrate. The solid catalyst was washed three times with 20g of isooctanol preheated to 80°C, then calcined in a muffle furnace at 300°C for 3 hours, cooled, and recovered for recycling. 10g of a pre-prepared 5wt% sodium carbonate aqueous solution was added to the filtrate, and the mixture was gently stirred at 65°C for 45 minutes for neutralization. The neutralized mixture was transferred to a 500mL separatory funnel, allowed to stand for 30 minutes to allow for complete separation, and then the lower aqueous phase was separated and discarded. The organic phase was washed twice with 150g of hot water at 80°C (75g each time), and a neutral organic phase was obtained after separation. The organic phase was loaded into a 500 mL rotary evaporator flask and connected to a rotary evaporator. Distillation was carried out under reduced pressure at a water bath temperature of 130 °C and a pressure of -0.095 MPa to recover approximately 395 g of colorless and transparent isooctanol (which can be reused). After replacing the receiving flask, the system vacuum was increased to -0.099 MPa and the water bath temperature was raised to 190 °C. Distillation under reduced pressure was continued, and the main fraction distilled within a specific boiling range was collected to obtain dioctyl terephthalate.

[0036] Example 2

[0037] The specific implementation method is the same as in Example 1, except that the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function is prepared as follows: Under an argon atmosphere, 9.0 g of molybdenum pentachloride and 12.0 g of titanium tetrachloride are dissolved together in a mixed solvent of 225 mL of anhydrous ethanol and 75 mL of acetylacetone, and magnetically stirred at 25 °C for 30 min to obtain solution A. Simultaneously, 28.0 g of ammonium dihydrogen phosphate and 18.0 g of hexadecyltrimethylammonium bromide are dissolved in 900 mL of deionized water and stirred in a 40 °C water bath until completely dissolved to obtain solution B. Under continuous stirring, solution A is slowly added dropwise to solution B, and after the addition is complete, the pH is adjusted to 9.0 with concentrated ammonia. The mixture is transferred to a high-pressure reactor and reacted at 180 °C for 48 h. After the reaction, the mixture is allowed to cool naturally, the precipitate is collected by centrifugation, and washed three times alternately with ethanol and deionized water. The precipitate is then dried under vacuum at 60 °C for 12 h to obtain the molybdenum-titanium ammonium phosphate precursor. 10.0 g of the precursor powder was weighed and dispersed in 200 mL of a 1.0 mol / L n-butylamine aqueous solution. The mixture was stirred and refluxed at 80 °C for 24 h. After filtration and washing with water until neutral, the resulting solid was redispersed in 500 mL of isopropanol and subjected to ultrasonic exfoliation at 500 W for 8 h. The supernatant colloidal suspension was then collected by centrifugation. 0.5 mol / L dilute sulfuric acid was added dropwise to the suspension until the pH reached 2.8. After stirring for 12 h, the mixture was concentrated by ultrafiltration and spray-dried at 80 °C to obtain a layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function, labeled CAT-2.

[0038] Preparation of the combined process: 80g of polyester waste fragments and 1.6g of catalyst CAT-2 were added to a pretreatment tank, along with 320g of isooctanol. The mixture was stirred at 200rpm for 30min at 85℃. The mixture was then transferred to a reactor equipped with a fractionation device, heated to 165℃, and reacted at this temperature for 2h, during which ethylene glycol was removed by fractionation. After the reaction, the mixture was cooled to 75℃, and the catalyst was separated by filtration. The solid catalyst was washed with 15g of isooctanol and then calcined at 298℃ for 2h for recovery. 8g of 5wt% sodium carbonate aqueous solution was added to the filtrate, and the mixture was stirred at 62℃ for 40min. After settling and separation, the organic phase was washed twice with 120g of 80℃ hot water. The washed organic phase was first subjected to vacuum distillation at 125℃ and -0.090MPa to recover isooctanol, and then the main fraction was collected at 185℃ and -0.099MPa to obtain dioctyl terephthalate.

[0039] Example 3

[0040] The specific implementation method is the same as in Example 1, except that the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function is prepared as follows: Under an argon atmosphere, 11.0 g of molybdenum pentachloride and 18.0 g of titanium tetrachloride are dissolved together in a mixed solvent of 255 mL of anhydrous ethanol and 85 mL of acetylacetone, and magnetically stirred at 25 °C for 30 min to obtain solution A. Simultaneously, 38.0 g of ammonium dihydrogen phosphate and 22.0 g of hexadecyltrimethylammonium bromide are dissolved in 1100 mL of deionized water and stirred in a 40 °C water bath until completely dissolved to obtain solution B. Under continuous stirring, solution A is slowly added dropwise to solution B. After the addition is complete, the pH is adjusted to 10.0 with concentrated ammonia. The mixture is transferred to a high-pressure reactor and reacted at 182 °C for 50 h. After the reaction, the mixture is allowed to cool naturally, the precipitate is collected by centrifugation, and washed three times alternately with ethanol and deionized water. The precipitate is then vacuum dried at 62 °C for 12 h to obtain the molybdenum-titanium ammonium phosphate precursor. 10.0 g of the precursor powder was weighed and dispersed in 200 mL of a 1.0 mol / L n-butylamine aqueous solution. The mixture was stirred and refluxed at 82 °C for 48 h. After filtration and washing with water until neutral, the resulting solid was redispersed in 500 mL of isopropanol and subjected to ultrasonic exfoliation at 500 W for 10 h. The supernatant colloidal suspension was then collected by centrifugation. 0.5 mol / L dilute sulfuric acid was added dropwise to the suspension until the pH reached 2.2. After stirring for 14 h, the mixture was concentrated by ultrafiltration and spray-dried at 82 °C to obtain a layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function, labeled CAT-3.

[0041] Preparation of the combined process: 100g of polyester waste fragments and 4.0g of catalyst CAT-3 were added to a pretreatment tank, along with 600g of isooctanol. The mixture was stirred at 200rpm for 60min at 95℃. The mixture was transferred to a reactor equipped with a fractionation device, heated to 175℃, and reacted at this temperature for 4h, during which ethylene glycol was removed by fractionation. After the reaction, the mixture was cooled to 78℃, and the catalyst was separated by filtration. The solid catalyst was washed with 25g of isooctanol and then calcined at 302℃ for 4h for recovery. 14g of 5wt% sodium carbonate aqueous solution was added to the filtrate, and the mixture was stirred at 68℃ for 50min. After settling and separation, the organic phase was washed twice with 200g of 80℃ hot water. The washed organic phase was first subjected to vacuum distillation at 135℃ and -0.093MPa to recover isooctanol, and then the main fraction was collected at 195℃ and -0.099MPa to obtain dioctyl terephthalate.

[0042] Comparative Example 1

[0043] The specific implementation method is the same as in Example 1, except that this comparative example does not use layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function, but instead uses ordinary titanium dioxide nanoparticles. Preparation of the combined process: 100g of polyester waste fragments and 2.0g of commercially available titanium dioxide powder with an average particle size of 30nm were added to a pretreatment tank, along with 500g of isooctanol. The mixture was stirred at 200rpm for 45min at 90℃. The mixture was transferred to a reactor equipped with a fractionation device and heated to 170℃. After 3h of reaction, very little dissolution of the polyester fragments was observed. The temperature was increased to 210℃, and the reaction continued for 5h until the raw materials were basically dissolved. After the reaction, the mixture was cooled to 76℃ and filtered. It was found that the catalyst particles were small and might agglomerate, resulting in a slow filtration rate. Subsequent neutralization, washing, and distillation steps were the same as in Example 1. Finally, dioctyl terephthalate was obtained.

[0044] Comparative Example 2

[0045] The specific implementation method is the same as in Example 1, except that the organic amine intercalation and ultrasonic exfoliation steps are omitted when preparing the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function in this comparative example. Catalyst preparation: After obtaining the molybdenum-titanium ammonium phosphate precursor according to the method of Example 1 and drying it, the n-butylamine intercalation and ultrasonic exfoliation treatment are not performed, and acid treatment is performed directly. That is, 10.0 g of precursor powder is dispersed in 500 mL of deionized water, and 0.5 mol / L dilute sulfuric acid is added dropwise to pH 2.5 under stirring, and stirred for 13 h. Then, centrifugation and water washing are performed, and finally, it is dried at 80 °C for 12 h to obtain a block solid. After grinding, the powder is obtained and labeled as CAT-D2. Preparation of the combined process: Except that the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function is replaced with CAT-D2, the other steps are exactly the same as in Example 1. After reacting at 170 °C for 3 h, a large number of polyester fragments were observed to remain undissolved. The reaction time was extended to 6 h, and the post-treatment steps were the same as in Example 1. The final product obtained was dioctyl terephthalate.

[0046] Comparative Example 3

[0047] The specific implementation method is the same as in Example 1, except that the ratio of molybdenum to titanium in the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function is changed. Catalyst preparation: Under an argon atmosphere, 20.0 g of molybdenum pentachloride and 5.0 g of titanium tetrachloride were dissolved together in a mixed solvent of 240 mL of anhydrous ethanol and 80 mL of acetylacetone, and stirred to obtain solution A. 32.0 g of ammonium dihydrogen phosphate and 20.0 g of hexadecyltrimethylammonium bromide were dissolved in 1000 mL of deionized water to obtain solution B. The subsequent hydrothermal reaction, n-butylamine intercalation, ultrasonic exfoliation, acid treatment, and spray drying steps were the same as in Example 1. The final catalyst was labeled CAT-D3. Preparation of the combined process: Except for replacing the catalyst with CAT-D3, the other steps were exactly the same as in Example 1. After reacting at 170 °C for 3 h, sampling and testing revealed incomplete conversion. The post-processing steps were the same as in Example 1, and dioctyl terephthalate was finally obtained.

[0048] Performance testing

[0049] The catalysts prepared in Examples 1-3 and Comparative Examples 1-3, as well as dioctyl terephthalate, were subjected to performance testing according to the following method, which included the following steps:

[0050] The specific surface area (unit: m² / g) of the catalyst was determined by nitrogen adsorption-desorption method: approximately 100 mg of sample was subjected to nitrogen adsorption-desorption at 150 °C and 10 °C. -3 After degassing under vacuum for 6 hours, nitrogen adsorption-desorption isotherms were collected using a physical adsorption instrument at liquid nitrogen temperature of 77K. Finally, the specific surface area was calculated using the Brunauer-Emmett-Teller model within the range of relative pressure P / P0 of 0.05-0.30.

[0051] The main acid content of the catalyst (unit: mmol / g) was determined using ammonia temperature-programmed desorption technique: 100 mg of sample was weighed into a U-shaped quartz tube, heated to 300 °C at a rate of 10 °C / min under a helium flow of 50 mL / min and pretreated for 1 hour. After cooling to 50 °C, the sample was switched to a helium mixture containing 10 vol% ammonia for adsorption for 30 minutes, and then switched back to pure helium for purging for 1 hour to remove physically adsorbed ammonia. Finally, the temperature was programmed to rise from 50 °C to 800 °C at a rate of 10 °C / min. The desorption signal was recorded using a thermal conductivity detector, and the total amount of ammonia desorbed was calculated through calibration.

[0052] Polyester conversion rate (unit: %) is calculated by mass difference: After the reaction is completed, the reaction mixture is cooled and filtered. The remaining insoluble solids are thoroughly washed with isooctanol and then vacuum dried at 80°C to constant weight and weighed. Conversion rate = (1 - mass of remaining solids / mass of initial polyester waste) × 100%.

[0053] Dioctyl terephthalate (DOTP) yield (unit: %) is calculated by product mass: collect the main fraction obtained in the distillation step and weigh it. Yield = (actual DOTP product mass / theoretical mass of polyester waste completely converted into DOTP) × 100%.

[0054] The purity of dioctyl terephthalate (in %) was analyzed by gas chromatography: a gas chromatograph equipped with an HP-5 capillary column and a flame ionization detector was used. The detector temperature was 280℃, the injection port temperature was 270℃, the split ratio was 50:1, the initial column temperature was 100℃ and held for 2 minutes, and then increased to 280℃ at 15℃ / min and held for 5 minutes. The external standard method was used for quantification.

[0055] The method for testing the DOTP yield (in %) after 5 catalyst cycles is as follows: the catalyst recovered by washing and calcination after each reaction is used in the next cycle experiment under the same reaction conditions, and the yield of DOTP product obtained after 5 consecutive cycles is recorded.

[0056] Test results:

[0057] Table 1: Test results of each embodiment and comparative example

[0058]

[0059] As can be seen from Table 1, the technical solutions presented in Examples 1-3, through sufficient contrast with Examples 1-3, systematically overcome the shortcomings of existing technologies, such as complex processes, harsh conditions, difficulty in catalyst recovery, and high energy consumption costs. The core reason lies in the significant synergistic effect produced by the molybdenum-titanium bimetallic phosphate nanosheet catalyst with a specific layered structure and high specific surface area used in the examples, and its integrated process design.

[0060] Specifically, in addressing the issue of complex and lengthy processes, the unique intercalation structure and high acid content (such as 0.85 mmol / g of CAT-1) of the catalyst in the examples enable it to efficiently catalyze the tandem reaction of polyester alcoholysis and transesterification at a relatively low temperature of 160-180℃, achieving direct conversion from waste to product. This eliminates multiple steps such as intermediate separation and purification, which is directly reflected in its near-complete polyester conversion rate (>99%) and high DOTP yield (89.1%-92.8%). In contrast, the ordinary nano-titanium dioxide used in Comparative Example 1 lacks this structure and sufficient acid content (only 0.15 mmol / g), resulting in almost no effective reaction under the same mild conditions. To achieve a certain conversion, the temperature must be significantly increased to 210℃ and the reaction time extended to 5 hours, as shown in Comparative Example 1. This precisely demonstrates that the process in the examples simplifies the process while avoiding harsh conditions.

[0061] In overcoming the challenges of catalyst recovery, the catalysts in the examples are efficiently separated by simple filtration and can be recycled five times after regeneration while still maintaining high activity (e.g., the yield of CAT-3 is still 90.2% after cycling). In contrast, the catalyst of Comparative Example 1 is difficult to filter and recover, and the structure of Comparative Example 2 collapses due to the lack of intercalation stripping, resulting in a sharp drop in cycle performance to 61.3%. This highlights the superiority of the recyclable design of the catalyst in this invention.

[0062] Ultimately, the mild reaction conditions, efficient conversion and yield, and long catalyst cycle life collectively contributed to a significant reduction in overall energy consumption and raw material consumption. Example 1, reacting at 170°C for 3 hours, yielded a high product with a yield of 91.2% and a purity of 99.6%, while Comparative Example 1, to obtain a product with a lower yield (65.4%) and purity (96.8%), required more stringent conditions, and the catalyst could not be reused. From the perspectives of energy efficiency, material efficiency, and equipment investment, the overall cost-effectiveness of the Example 1 is far superior to that of the Comparative Example.

[0063] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate, characterized in that, Includes the following steps: S1, by weight, polyester waste is crushed by a crusher to obtain polyester waste fragments; 80-100 parts of polyester waste fragments and 1-5 parts of layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function are put into a pretreatment tank, 400-600 parts of isooctanol are added, and the mixture is stirred at 80-100℃ to obtain a mixture; S2, the entire mixture is transferred to a reaction vessel, heated to 160-180℃ and reacted, and ethylene glycol is continuously removed by fractional distillation to obtain the reaction mixture; S3, the reaction mixture is cooled to 75-78℃ and separated using a plate and frame filter to obtain a solid catalyst and filtrate; the solid catalyst is washed with isooctanol and calcined at 295-305℃ for recycling; S4, add 5-15 parts of sodium carbonate aqueous solution to the filtrate, stir at 60-70℃ to obtain a mixture, transfer the mixture to a separatory funnel, let it stand to separate the lower aqueous phase, wash the organic phase with hot water at 70-80℃ to obtain the washed organic phase; transfer the washed organic phase to a distillation kettle, and distill at 120-140℃ under reduced pressure to recover isooctanol; then raise the temperature to 180-200℃ and continue distilling under reduced pressure to collect the dioctyl terephthalate main fraction.

2. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 1, characterized in that, In step S1, the stirring time is 30-60 minutes.

3. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 1, characterized in that, In step S2, the reaction time is 2-4 hours after heating to 160-180℃.

4. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 1, characterized in that, In step S3, the calcination time at 295-305℃ is 2-4 hours.

5. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 1, characterized in that, In step S4, the stirring time at 60-70℃ is 30-60 minutes.

6. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to any one of claims 1-5, characterized in that, The preparation steps of the layered molybdenum-titanium bimetallic phosphate nanosheet catalyst with intercalation catalysis function include: A1, by weight, under an argon atmosphere, dissolve 8-12 parts of molybdenum pentachloride and 10-20 parts of titanium tetrachloride in a mixed solvent of 200-330 parts of anhydrous ethanol and acetylacetone, and stir to obtain solution A; dissolve 25-40 parts of ammonium dihydrogen phosphate and 15-25 parts of hexadecyltrimethylammonium bromide in 800-1200 parts of deionized water to obtain solution B; A2. Under continuous stirring, solution A is added dropwise to solution B, and the pH is adjusted to 9-10 to obtain a mixture. The mixture is then transferred to a high-pressure reactor and reacted at 175-185℃. After the reaction is completed, the mixture is allowed to cool naturally, and the precipitate is collected by centrifugation. The precipitate is washed alternately with ethanol and deionized water and dried under vacuum at 58-62℃ to obtain the molybdenum-titanium ammonium phosphate precursor. A3. The molybdenum-titanium ammonium phosphate precursor was dispersed in an aqueous solution of n-butylamine and stirred under reflux at 78-82°C to obtain a mixture. The mixture was filtered and washed with deionized water to obtain an expanded layered material with organic molecular intercalation. The expanded layered material with organic molecular intercalation was redispersed in isopropanol and subjected to ultrasonic exfoliation. The upper colloidal suspension was collected by centrifugation. A4. Add dilute sulfuric acid dropwise to the colloidal suspension until the pH is 2-3, stir, concentrate by ultrafiltration, and spray dry at 78-82℃.

7. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 6, characterized in that, In step A1, the volume ratio of anhydrous ethanol to acetylacetone is 3:

1.

8. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 6, characterized in that, In step A2, the reaction time is 48-50 h at 175-185℃.

9. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 6, characterized in that, In step A3, the stirring and reflux time at 78-82℃ is 24-48 hours.

10. The combined process for pretreatment of polyester waste and preparation of dioctyl terephthalate according to claim 6, characterized in that, In step A4, the stirring time is 12-14 hours.

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