Method for preparing alpha, omega-binary acid through decommissioned polyolefin cracking based on stage oxidation

Abstract

The invention provides a method for preparing alpha, omega-binary acid by decommissioning polyolefin cracking based on stage oxidation, which comprises the following steps: in the absence of a catalyst or in the presence of a catalyst, mixing decommissioning polyolefin with water to form a dispersion system, and reacting at the temperature of 100-200 DEG C and the partial pressure of oxidant gas of 0.1-5.0 MPa to prepare alpha, omega-binary acid. The method comprises the following steps: introducing oxidant gas into a high-pressure closed reactor, carrying out a stirring reaction to obtain small-molecular carboxylic acid, carrying out oxidation treatment on the small-molecular carboxylic acid by using nitric acid to obtain alpha, omega-dibasic acid with a carbon atom number of 4-10, and efficiently preparing high-purity alpha, omega-dibasic acid with a carbon atom number of 4-10. And [omega]-binary acid.

Description

Technical Field

[0001] This invention relates to the field of high-value recycling of decommissioned polyolefins, and specifically to a method for producing α,ω-diacids from decommissioned polyolefins through staged oxidation. Background Technology

[0002] Polyolefins (such as polyethylene and polypropylene) have long dominated the market for general-purpose plastics due to the chemical and thermal stability imparted by their saturated C-C backbone. Their production has continued to climb and they have been widely used in packaging, daily chemicals, automobiles, and home appliances. However, with the accumulation of their waste, a large amount of decommissioned polyolefins are being discharged in an unregulated manner. The natural degradation of decommissioned polyolefins takes about 200 years, which will cause serious environmental pollution problems such as "white pollution" and microplastics.

[0003] Currently, the main methods for treating decommissioned polyolefins are physical recycling and chemical recycling. Physical recycling includes incineration and landfill. However, incineration leads to greenhouse gas emissions, while landfill poses risks of secondary pollution, such as soil and groundwater contamination. In short, while physical recycling is simple and low-cost, it is only suitable for materials from a single source. Furthermore, due to pollution, impurities, and thermo-oxidative aging caused by repeated processing, the material's performance continues to deteriorate, making high-quality closed-loop regeneration impossible. Chemical recycling, as an important supplementary pathway, aims to break down polymers into oligomers or small molecule products for recycling. Pyrolysis and catalytic pyrolysis can generate olefins and aromatics, while hydrocracking can produce saturated alkanes with high selectivity. Oxidative cracking, as an alternative pathway, typically activates C–C bonds at lower temperatures and introduces polar oxygen-containing functional groups such as hydroxyl, carbonyl, and carboxyl groups during chain scission, potentially directly yielding small molecules or oligomers that can be functionalized, thus increasing the added value of the products. However, current chemical recovery methods generally rely on high temperatures and metal catalysts, resulting in high equipment and operating costs, and their adaptability to raw materials containing additives or mixed components remains unclear.

[0004] Previous studies have shown that introducing oxygen into catalyst-free systems can significantly lower the chain-breaking temperature threshold of polyolefins and potentially yield carboxylic acid products. However, due to the difficulty in finely controlling the free radical process, the system is prone to side reactions such as crosslinking, condensation, and deep oxidation, resulting in low carboxylic acid yields and high coking rates, severely limiting the practical application of this technology. While high-temperature gasification and aqueous liquefaction technologies offer advantages in adaptability to complex feedstocks and operational stability, and can suppress some coking through aqueous media, they still require stringent high-temperature and high-pressure reaction conditions, leading to high energy consumption, high equipment costs, and operational difficulties. However, the presence of an aqueous phase in these two methods can improve heat and mass transfer, alleviate local overheating, and suppress unfavorable side reactions, suggesting that the synergy of oxidants and aqueous media may be key to improving selectivity and stability. On the other hand, the introduction of heterogeneous catalysts provides a new direction for optimizing oxidative cracking reactions, potentially further reducing reaction temperatures and improving the selectivity of carboxylic acid products. In recent years, two-dimensional (plate-like / layered) porous metal oxides and their spinel / solid solution complexes (such as TiO2, ZrO2, CeO2, Co3O4, ZnO, ZnCo2O4, CoFe2O4, NiFe2O4, Ce–Zr–O, etc.) using metal-organic frameworks (MOFs) as precursors have shown advantages in oxidation reactions with high specific surface area and tunable pores. Further loading noble metals, such as Pt, Pd, and Au, onto these supports can also enhance low-temperature activity and the ability to regulate product distribution. However, existing catalytic oxidative cracking technologies still face many bottlenecks: they are sensitive to fluctuations in feedstock composition and lack a clear and stable operating window under aqueous conditions; catalysts are prone to deactivation due to carbon deposition and loss of active components, and regeneration processes and recycling strategies are still imperfect, making it difficult to meet the needs of continuous industrial operation.

[0005] More importantly, even if products mainly composed of carboxylic acids are obtained through existing oxidative cracking technologies, their composition is still mainly mixed carboxylic acids, containing a large number of impurity components such as monocarboxylic acids, keto acids, and lactones. This will hinder their subsequent high-value utilization. For example, in the stepwise polymerization of synthetic polyesters, the presence of monocarboxylic acids will prematurely end the oligomers, resulting in low molecular weight products. α,ω-diacarboxylic acids are important monomers for materials such as polyesters, polyamides, lubricants, and plasticizers. Their highly selective preparation is of great significance for the high-value utilization of decommissioned polyolefins. Current technologies have not yet been able to achieve highly selective preparation of high-purity α,ω-diacarboxylic acids, and there is still a significant gap from industrial application.

[0006] Therefore, there is an urgent need for an efficient method to recover decommissioned polyolefins to obtain high-purity α,ω-dicarboxylic acids. Summary of the Invention

[0007] The purpose of this invention is to provide a method for producing α,ω-diacarboxylic acids from decommissioned polyolefins through staged oxidation. This method provides a staged oxidation treatment technology that uses water as the continuous phase and can use heterogeneous catalysts. It effectively suppresses coking and over-oxidation side reactions while ensuring a high carboxylic acid yield. Furthermore, it efficiently converts decommissioned polyolefins into highly selective α,ω-diacarboxylic acids through subsequent directional oxidation steps, taking into account environmental friendliness, process controllability, and high added value of the product.

[0008] To achieve the above objectives, this technical solution provides a method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation, comprising the following steps: Oxidative preparation of small molecule carboxylic acids: In the absence of a catalyst or in the presence of a catalyst, decommissioned polyolefins are mixed with water to form a dispersion system. Under the conditions of 100~200°C and oxidant gas partial pressure of 0.1~5.0 MPa, oxidant gas is introduced into a high-pressure closed reactor for stirring reaction to obtain small molecule carboxylic acids, wherein the small molecule carboxylic acids are mainly small molecule carboxylic acids with 4~10 carbon atoms. Preparation of α,ω-diacarboxylic acids by oxidation: α,ω-diacarboxylic acids are obtained by oxidizing small molecule carboxylic acids with nitric acid, wherein the number of carbon atoms in the α,ω-diacarboxylic acids is 4~10.

[0009] It should be noted that, compared with existing polyolefin oxidative cracking technologies, this invention uses water as the continuous phase and adopts a staged oxidation route of "aqueous phase oxidation - nitric acid oxidation". This allows for chain scission and selective oxidation of decommissioned polyolefins without the use of organic reagents and under conditions of no catalysis or selective heterogeneous catalysis. The overall process is characterized by mild conditions, low energy consumption, low carbon buildup, and simple post-processing. Furthermore, the final product obtained from the oxidation is an α,ω-diacid, which can be directly used as a platform chemical in the fields of polyester, polyamide, lubricants, and plasticizers, thus meeting the needs of the high-value recycling industry of decommissioned polyolefin plastics.

[0010] In the oxidation preparation stage of small molecule carboxylic acids, the mass yield of carboxylic acid products in this stage is >95%, and the selectivity of α,ω-diacids is >70%.

[0011] Furthermore, this scheme uses water as the continuous phase. Preferably, after adding water and decommissioned polyolefin to form a dispersion system under normal pressure and completing the initial mechanical dispersion, the pressure and temperature are slowly increased under closed conditions to a temperature of 100~200°C and an oxidant gas partial pressure of 0.1~5.0 MPa. This has the advantage of weakening the violent initiation of free radicals caused by instantaneous exothermic reaction and sudden increase in dissolved oxygen, thus stabilizing the controllability of early chain segment activation.

[0012] Furthermore, the volume fraction of the aqueous phase in the dispersion system is greater than 50%. Preferably, the volume fraction of the aqueous phase in the dispersion system is greater than 60%. The presence of the aqueous phase allows decommissioned polyolefins to form special macroscopic and microscopic structures. Macroscopically, a large number of bubbles are generated, and microscopically, a porous structure is formed. This greatly increases the interfacial area of ​​the three phases of water, oxidant gas, and polyolefin, which is beneficial to mass and heat transfer. Oxidation reaction is essentially an exothermic reaction. If the heat is not transferred away in time, local high-temperature hot spots will form, leading to the mineralization and carbonization of the cracking products. Therefore, this scheme specifically controls the volume fraction of the aqueous phase to be greater than 50%. The advantage of this is that it ensures macroscopic continuity and surface wetting, forming a heat transfer / mass transfer diffusion zone. That is, the heat transfer / mass transfer buffer of the aqueous phase inhibits cross-linking, coking, and deep oxidation. If the aqueous phase is insufficient, dry areas and bubble retention are likely to occur, thereby inducing a chain deviation of hot spot-cross-linking-coking.

[0013] Furthermore, the reaction temperature is 140~180℃, because excessively high temperatures will lead to deep oxidation and cross-linking of decommissioned polyolefins and cracking products. The specific reaction time can be optimized within 0.5~24 h according to the physicochemical properties of decommissioned polyolefins.

[0014] Furthermore, the partial pressure of the oxidant gas is 0.2–2.0 MPa. This provides sufficient active species for the chain scission of decommissioned polyolefins, and the synergistic effect of the oxidant gas and aqueous phase ensures that small molecule carboxylic acid products account for >50 wt.% of the total mass of decommissioned polyolefins. Additionally, the partial pressure can be moderately increased within this range while the reaction time and temperature can be reduced to align the chain scission and functionalization rhythms and suppress side reactions. Moreover, higher oxidant pressures lead to the mineralization of the pyrolysis products.

[0015] Furthermore, the oxidant gas can be one or more of air, oxygen, ozone, nitrogen dioxide, and nitric oxide. It should be noted that the oxidant gas selected in this scheme is not limited to oxygen; any oxidizing gas can be used. Under scale-up conditions, oxygen / oxygen-enriched air is preferred to balance safety, supply stability, and cost.

[0016] Furthermore, the oxidant gas is introduced into the high-pressure closed reactor via a one-time filling, constant-pressure continuous aeration, or segmented oxygen supplementation. Preferably, a partial pressure closed-loop control is established to ensure that oxygen pressure fluctuations are less than ±0.05 MPa, thereby reducing the impact of hypoxia-hyperxia fluctuations on carboxylic acid selectivity.

[0017] Furthermore, after introducing oxidant gas into a high-pressure closed reactor, the decommissioned polyolefin undergoes chain scission and selective oxidation reactions to obtain gaseous, liquid, and solid products. The liquid product is then separated to obtain a small molecule carboxylic acid. Further, the gaseous product can be condensed / washed and safely fed into a gas collection device for analysis, while the solid product is dried, weighed, and characterized for conversion rate and side reaction analysis. The liquid product then enters an extraction / crystallization / neutralization process to obtain the target carboxylic acid or its salt. Preferably, extraction or rotary evaporation is used to separate the liquid product to obtain the small molecule carboxylic acid. For polybasic acid scenarios, neutralization to form a salt can be performed first, followed by selective crystallization and then acidification to recover the target small molecule carboxylic acid.

[0018] The extractant for extracting liquid products can be one or more of the following: low-carbon alcohols (such as isopropanol and n-butanol), low-boiling esters (such as ethyl acetate and ethyl propionate), or green carbonates (such as diethyl carbonate), in different proportions. The extractant is determined by considering the partition coefficient, water compatibility, energy consumption for recovery, and environmental friendliness. In addition, water-miscible alcohols can be extracted after salting out or pre-concentration to improve the separation efficiency. For partially miscible or immiscible ester / carbonate systems, atmospheric or vacuum distillation is preferred.

[0019] Furthermore, in this scheme, water is used as the solvent for the overall preparation of α,ω-diacids, while ethanol is preferred as the extractant. Both can be recycled. The ethanol / water system is first subjected to phase separation, and then the ethanol is recovered by vacuum distillation.

[0020] Furthermore, the solid content of the decommissioned polyolefin is 1~100 wt.%, based on the total mass of the decommissioned polyolefin in the entire reaction system. Preferably, the solid content of the decommissioned polyolefin is 1~50 wt.%, more preferably 5~30 wt.%; excessively high solid content will deteriorate rheology and heat transfer, while excessively low solid content will dilute the treatment efficiency, resulting in insufficient production capacity.

[0021] Furthermore, the decommissioned polyolefin is one or more of the following: low-density polyethylene, high-density polyethylene, linear low-density polyethylene, isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene, polystyrene, ethylene, and α-olefin copolymers, mixed in any proportion. When the mixing ratio of the decommissioned polyolefin changes, the predetermined selectivity and yield of the product are maintained primarily through the coordinated adjustment of multiple parameters such as water volume fraction, solid content, oxidant gas pressure, temperature, reaction time, and stirring rate.

[0022] Furthermore, the decommissioned polyolefins undergo crushing-washing-drying pretreatment before feeding. The recommended particle size of the decommissioned polyolefins is 0.1~2.0 mm to balance dispersion stability and transport operability. Any additives and inorganic fillers that may be present do not need to be treated and will not significantly affect the method.

[0023] Regarding the selection of catalysts: This method can prepare small molecule carboxylic acids by oxidation without a catalyst, or by oxidation with a catalyst. Of course, the presence of a catalyst can accelerate and improve the efficiency.

[0024] In some embodiments, the catalyst is selected from porous metal oxide catalysts and their composites prepared by heat treatment of metal-organic framework materials, or the catalyst is selected from noble metal supported catalysts and their doped composite catalysts.

[0025] Furthermore, the catalyst, decommissioned polyolefin, and water are mixed to form a suspension for oxidative cracking. The mass ratio of catalyst to decommissioned polyolefin is 0.1:100 to 20:100. Preferably, in an immobilized environment, the catalyst can be coated / loaded onto the inner surface of an inert support or a high-pressure closed reactor, with the catalyst coating preferably being 50-300 μm thick to ensure mass transfer efficiency.

[0026] When the catalyst is selected from porous metal oxide catalysts and their composites prepared by heat treatment of metal-organic framework materials, the catalyst is a two-dimensional porous metal oxide or its composite, wherein the two-dimensional porous metal oxide is selected from: TiO2, ZrO2, HfO2, CeO2, La2O3, Y2O3, V2O5, Nb2O5, Ta2O5, Cr2O3, MnO x The complex refers to one or more two-dimensional porous metal oxides such as Fe2O3 / Fe3O4, Co3O4, NiO, CuO / Cu2O, ZnO, MoO3, and WO3. The complex refers to spinel, solid solution, or composite oxides of two-dimensional porous metal oxides, such as ZnCo2O4, CoFe2O4, NiFe2O4, MnCo2O4, ZnMn2O4, CuFe2O4, Ce–Zr–O, etc. It should be noted that the two-dimensional pores and larger specific surface area improve the efficiency of activation and intermediate reactions at low temperatures.

[0027] Furthermore, the two-dimensional porous metal oxide is obtained by thermal decomposition / calcination at 250~650 °C from a MOF precursor containing the corresponding metal, wherein the MOF precursor is selected from one or more of ZIF, MIL, UiO, and PBA types.

[0028] Furthermore, the BET specific surface area of ​​the two-dimensional porous metal oxide is preferably 50~200 m². 2 / g, with a preferred pore size of 5~20 nm, and retaining the sheet-like / layered morphology and pores derived from MOF.

[0029] Furthermore, when preparing two-dimensional porous metal oxides by thermal decomposition, the heating rate of thermal decomposition is 1~5 °C / min, the holding time is 0.5~6 h, and the atmosphere is air or diluted O2.

[0030] Furthermore, when preparing two-dimensional porous metal oxides by calcination, calcination is carried out under an inert gas (N2 / Ar), followed by short-term air activation to form an oxide phase.

[0031] When the catalyst is selected from noble metal supported catalysts or doped composite catalysts, the noble metal in the noble metal supported catalyst or doped composite catalyst is selected from one or more of Ru, Rh, Pd, Ag, Ir, Pt, and Au, and the noble metal loading is 0.01~5.0 wt.%.

[0032] Furthermore, the methods for introducing precious metals include impregnation-calcination equal-volume impregnation, ion exchange, colloidal deposition, or in-situ introduction via MOF, followed by activation in air at 200–450 °C.

[0033] In addition, after the reaction for the oxidation preparation of small molecule carboxylic acids is completed, the catalytic activity and metal / support valence state can be recovered through solid-liquid separation. If necessary, the carbon deposits can be removed in an air atmosphere at 300~500°C, and / or activated at 200~350°C in an inert or reducing atmosphere, such as ≤5 vol% H2 / N2 or H2 / Ar, to restore the catalytic activity and metal / support valence state. The regeneration cycle is determined based on the decreasing trend of activity and selectivity, and the relationship between the number of cycles and performance is recorded.

[0034] Furthermore, the high-pressure closed reactor can be a pressure-resistant stirred tank, a circulating jet reactor, or a high-pressure vessel with internal components. Preferably, a paddle-type agitator or a circulating jet-reflux scheme can be used in the high-pressure closed reactor to enhance macroscopic circulation and microscopic shear, ensuring solid phase suspension / semi-suspension and interface renewal.

[0035] Preferably, the stirring in the high-pressure closed reactor uses a 45° inclined blade propeller / three-blade propeller, which can optimize the rotation speed with power per unit volume and oxidizing gas concentration as indicators, avoiding excessive shearing that causes foam accumulation or emulsification difficulties.

[0036] Preferably, the material selection for the high-pressure closed reactor takes into account the synergistic corrosion of oxidizing gas solubility, temperature, acidity and potential oxygen-providing components: corrosion-resistant materials such as 316L and C276 are preferred, fluorinated elastomers and polytetrafluoroethylene are used in key sealing parts, and cathodic protection or coatings are provided to extend service life.

[0037] In addition, safety strategies are set up for this high-pressure sealed reactor. These strategies include: using two-stage overpressure protection (PSV + rupture disc) and dual-channel temperature interlock, setting up inert gas sweep lines and corresponding gas flame arresters, and matching tail gas absorption and explosion relief units when ozone / NOx is involved. The materials are also used in limited quantities after assessment of their compatibility and corrosivity.

[0038] Furthermore, the start-up and shutdown strategy of this high-pressure sealed reactor is as follows: before heating, nitrogen is purged, material is injected, dispersed, sealed, pressure is increased slowly, temperature is increased slowly, and steady-state operation is maintained; when shutting down, the temperature is reduced, pressure is reduced, inerting is performed, and discharge is carried out.

[0039] Regarding the small molecule carboxylic acids obtained during preparation: The oxidation process for preparing small molecule carboxylic acids yields one or more of the following: dicarboxylic acids, monocarboxylic acids, ketone-containing carboxylic acids, and lactone-containing carboxylic acids, in varying proportions. Furthermore, the yield of small molecule carboxylic acids can be >50 wt.%, and the carbon number distribution within the C4–C15 region can be controlled by adjusting multiple parameters, including water volume fraction, solid content, oxidant gas pressure, temperature, reaction time, and stirring rate.

[0040] In the oxidation preparation of α,ω-diacarboxylic acids, this scheme uses nitric acid to oxidize small molecule carboxylic acids to obtain α,ω-diacarboxylic acids, wherein the mass yield of carboxylic acid products of α,ω-diacarboxylic acids is >70%, and the selectivity of α,ω-diacarboxylic acids is >98%.

[0041] Furthermore, 30-70 wt.% nitric acid is added to the small molecule carboxylic acid, and the reaction is carried out at 50-120 °C for 0.5-6 h to selectively oxidize and sever the remaining monocarboxylic acid, keto acid, and lactone, thereby increasing the selectivity of α,ω-diacarboxylic acid in the product to >98 ​​wt.%.

[0042] Preferably, 65 wt.% nitric acid is added to the small molecule carboxylic acid, and the reaction is carried out at 100 °C for 1 h, so that the residual monocarboxylic acid, keto acid, and lactone components are further oxidized and the terminal structure is adjusted, thereby increasing the proportion of α,ω-diacarboxylic acid and reducing the proportion of non-target oxygen-containing byproducts.

[0043] After the reaction in the oxidative preparation stage of α,ω-diacarboxylic acid is completed, the α,ω-diacarboxylic acid product is recovered by any of the following methods: reduced pressure denitration / dehydration, crystallization separation, or acid-base conversion-re-acidification. The 4-10% α,ω-diacarboxylic acid obtained in this scheme can be used as raw material for polyester / polyamide monomers, as well as surfactants, lubricants, and plasticizers, and has good application value. In addition, according to the target product application, such as polyester / polyamide monomers, surfactants, lubricants, etc., quality indicators such as acid value, iodine value, and color can be adjusted. For example, for polyester monomers, priority is given to improving the selectivity of α,ω-diacarboxylic acids and reducing lactone / ketone residues; for surfactants, some monocarboxylic acids and hydroxy acids can be retained to simplify subsequent esterification.

[0044] In addition, regarding the overall reaction system of the method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation, an online / offline synergistic in-situ monitoring system can be configured to monitor temperature, total pressure, and stirring power online, while the carbon conservation relationship of the products can be established offline using methods such as infrared spectroscopy, gas chromatography-mass spectrometry, acid-base titration, proton nuclear magnetic resonance spectroscopy, and carbon nuclear magnetic resonance spectroscopy.

[0045] Furthermore, if the catalyst is to be used for long-term continuous operation, the online regeneration bypass or parallel switching capability can be reserved in the design stage. The regeneration system corresponds to the deactivation mechanism, and a combination of air regeneration / mild oxidation and low-temperature activation is adopted to improve the long-term stability of the catalyst.

[0046] Compared with existing technologies, this technical solution has the following characteristics and beneficial effects: Compared with existing polyolefin oxidative cracking technologies, this invention does not require strong oxidants, organic reagents, or catalysts. The process is green, the process conditions are mild, the energy consumption is low, the carbon deposition is low, the post-processing is simple, the cost is lower without catalytic conditions, and problems such as catalyst deactivation are avoided, resulting in more stable operation. The product is an α,ω-diacid that can be directly used as a platform chemical in the fields of polyester, polyamide, lubricant, and plasticizer, which can meet the needs of the high-value recycling industry of retired polyolefin plastics. Attached Figure Description

[0047] Figure 1 The graph shows the conversion rate of oxidation and the yield of carboxylic acid at each stage of the embodiments.

[0048] Figure 2 This is a distribution diagram of the carboxylic acid products obtained by oxidation in stage 3 of Example 3.

[0049] Figure 3 This is a distribution diagram of the carboxylic acid products obtained from the oxidation stage in Example 3.

[0050] Figure 4 This is a comparison chart of carboxylic acid yield and α,ω-diacid selectivity in stage one and stage two oxidation in Example 3.

[0051] Figure 5 This is a scanning electron microscope image of the catalyst prepared in Example 8.

[0052] Figure 6 The thermogravimetric analysis curves are of the residual solids from the oxidation stage of Example 3 and Comparative Example 2. Detailed Implementation

[0053] 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. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0054] This invention provides a method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation. To make the objectives, technical solutions, and effects of this invention clearer, the following provides a more detailed description. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0055] Yield of small molecule carboxylic acid preparation by oxidation in stage one Y The calculation formula is as follows:

[0056] In the above formula, m 1 represents the mass of the carboxylic acid product extracted after the first oxidation reaction is completed. m PO To improve the quality of decommissioned polyolefins.

[0057] Yield of α,ω-diacids prepared by oxidation in stage two Y 2. The calculation formula is as follows:

[0058] In the above formula, m 2 represents the mass of the carboxylic acid product extracted after the oxidation reaction in stage two is completed.

[0059] The structures of the carboxylic acid products prepared in stage one and by oxidation were determined by a combination of Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. FTIR spectroscopy conditions: Thermo Fisher Nicolet iS50, ATR mode, 650–4000 cm⁻¹ -1 4 cm resolution -1 32 scans / spectrums; NMR spectroscopy test conditions: Bruker AVANCE NEO 500 MHz, acetone-d6 ( delta ~2.05 ppm) is used as an internal standard, and records are made. 1 H NMR, 13 C NMR and HSQC spectra.

[0060] The carbonyl index (defined as the ratio of carbonyl groups to methylene groups) and number-average sequence length of the carboxylic acid products prepared by stage one and stage two oxidation were determined using 1H NMR spectroscopy. 1 Determined by ¹H NMR. Carbonyl index of carboxylic acid products. CI Sum of average sequence length n The calculation formula is as follows:

[0061]

[0062] In the above formula, I 1 represents the integral area of ​​the saturated methylene signal farthest from the functional group in the polyhydrocarbon backbone; I 2 (2.06–2.13 ppm) is adjacent to the carbonyl group. α -Methylene R–CH2–C(=O) –)signal integration area; I 3 (2.20–2.39 ppm) is a carboxylic acid. α -methylene (R–CH2–COOH); I 4 (2.39–2.48 ppm) represents the integrated area of ​​the signal at the ortho-methylene position of the carbonyl group (R–C(=O)–CH2–R′). I 5 (2.48–2.55 ppm) is gamma The methylene group near the carboxyl group in ketocarboxylic acids is –C(=O)–CH2–C. H 2–COOH) signal integration area; I 6 (2.55–2.58 ppm) represents the integral area of ​​the methylene signal in succinic acid; I 7 (2.61–2.78 ppm) is gamma -The methylene group near the carbonyl group in ketocarboxylic acids –C(=O)–C H The signal integration area of ​​2–CH2–COOH.

[0063] The composition and distribution of carboxylic acid products prepared in stage one and stage two oxidation were determined by gas chromatography-mass spectrometry (GC-MS) under the following conditions: Agilent 7890B-5977B, HP-5ms UI column (30 m × 0.25 mm × 0.25 μm). The carboxylic acid composition was determined by mass spectrometry. The carboxylic acid products were methylated with BF3 / methanol solution (50 wt.%), with methylheptadecanate as an internal standard. An internal standard calibration curve was established using a mixed solution of dimethyl esters corresponding to α,ω-diacarboxylic acids with 4 to 10 carbon atoms to obtain the mass selectivity of the carboxylic acid products, thereby calculating the yield of the corresponding products.

[0064] The coke content was determined by thermogravimetric-differential scanning calorimetry (TGA / DSC). Test conditions: Mettler TGA / DSC 3+; aluminum oxide crucible; sample size approximately 5–10 mg; programmed temperature rise in three stages: ① 50–200 °C, 2 °C / min, 50 mL nitrogen / min (removal of moisture and volatile components); ② 200–600 °C, 2 °C / min, 50 mL nitrogen / min (oligomery thermal decomposition); ③ 600–1000 °C, 5 °C / min, 50 mL air / min (hard coke oxidation combustion). The coke content (the mass fraction of coke relative to the mass of decommissioned polyolefins) was calculated based on the proportion of coke mass lost during the air oxidation stage. Combined with the residual solid mass, the coke content produced under each experimental condition was obtained and used for coking trend assessment. The coke content is tested using the residual solids after the first-stage oxidation as the test object, and is used to evaluate the inhibitory effect of water on the "hot spot-crosslinking-coking" process. The second-stage nitrate oxidation is mainly aimed at the further oxidation, transformation and purification of the carboxylic acid products obtained in the first stage, and coke is not usually used as the main evaluation index.

[0065] In the optional catalyst preparation embodiments, the metal salts used (such as ZrCl4, Ce(NO3)3·6H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, etc.), organic ligands (2-methylimidazole, terephthalic acid H2BDC, trimesic acid H3BTC, etc.), and solvents (DMF, methanol, ethanol, deionized water, etc.) are all of analytical grade or higher and used directly. Precursor crystallization is carried out in a polytetrafluoroethylene-lined reactor, and thermal conversion is performed using a tube / box furnace or muffle furnace with programmed temperature rise and atmosphere switching functions. The specific steps are as follows: the target metal salt and organic ligand are dissolved in alcohol / DMF / water or a mixture thereof; the metal:ligand molar ratio and pH are adjusted; crystallization is carried out by stirring at room temperature or by solvothermal treatment at 80~150 °C for 2~24 h; the precursor is obtained by centrifugation and washing 2~3 times, and vacuum drying at 60~80 °C for 8~12 h. To obtain two-dimensional (lamellar / layered) morphologies, dilute solutions, higher ligand / metal ratios, or small amounts of morphology modifiers such as acetic acid / formic acid are preferred to inhibit isotropic growth. The dried MOF precursor is spread evenly in an alumina / quartz boat, and the temperature is increased at 5 °C / min to 250–650 °C and held for 0.5–6 h to complete the thermal conversion; the atmosphere can be air or diluted O2. The resulting two-dimensional porous metal oxide / spinel material has a BET of 50–200 μm. 2 / g, pore size 5~20 nm, and retains the interconnected pore system and lamellar / layered morphology derived from MOF.

[0066] Based on this, two routes can be used to construct noble metal / oxide composite catalysts: (i) In-situ introduction of MOF—the selected noble metal precursor (such as the corresponding chloride / nitrate) is added to the metal salt / organic ligand gel system of the MOF to be prepared according to the target loading amount, and the nucleation and crystallization are controlled by ultrasonic homogenization and a small amount of acid / base or ligand excess. The noble metal-infused MOF precursor is obtained under solvothermal conditions; then the temperature is raised to 250~650 °C at 5 °C / min and held for 0.5~6h in air or diluted O2 to thermally convert it into a two-dimensional porous noble metal / metal oxide composite material, maintaining the plate / layer interconnected pores and high dispersion. (ii) Post-loading—A support is first prepared, and then one or more of Ru, Rh, Pd, Ag, Ir, Pt, and Au are introduced by equal-volume impregnation, ion exchange, or colloidal deposition, with a loading of 0.01–5.0 wt.%. After drying, it is activated in air at 200–450 °C. If necessary, it is gently treated at 200–300 °C with <5% H₂ / N₂ to adjust the surface valence state and dispersion. The resulting catalyst can be used directly as a powder. After use, carbon deposits are removed in air at 300–500 °C, and / or activated at 200–350 °C in an inert or reducing atmosphere, such as ≤5 vol% H₂ / N₂ or H₂ / Ar, to restore catalytic activity and metal / support valence state. It can be recycled. The catalyst is preferably used in a stage-one aqueous oxidation process to further improve the conversion rate and the yield of the target carboxylic acid; stage-two oxidation generally does not involve a catalyst.

[0067] Example 1: In this embodiment, the decommissioned polyolefin is selected as high-density polyethylene (HDPE). M w = 18.6 kDa, = 4.35), with a solid content of 5 wt.% (based on the total mass of degraded polyolefin in the reaction system). Deionized water was injected to make water a continuous phase (volume fraction > 50%). Oxygen was used as the oxidant gas, injected in a single pass at a pressure of 1 MPa. The reaction temperature was set at 160 °C. Before the experiment, high-density polyethylene resin was dispersed in water and placed in the reactor. The reactor was purged with nitrogen for 30 minutes, followed by oxygen for 30 minutes, and then 1 MPa of oxygen was injected to seal the entire reactor. The reactor temperature was raised to 160 °C, and stirring was started at 400 rpm. The reaction was stopped after 1 hour. After the reaction, the mixture was cooled to room temperature, and the gaseous product was collected using a gas bag. An equal volume of anhydrous ethanol was added to extract the liquid product. After collection and centrifugation, the supernatant was evaporated under reduced pressure to obtain a liquid product mainly composed of carboxylic acid. The residual solid was dried under vacuum at 70 °C for 12 hours to obtain a solid product. All materials used in the experiment were used directly without any treatment. This embodiment only performs stage 1 oxidation, and the resulting liquid product is a mixture of small molecule carboxylic acids. The reaction conversion rate in this embodiment is 30.9%, the carboxylic acid yield is 36.9 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield is 24.0 wt.%, the carbonyl index is 0.42, and the number-average sequence length is 6.77.

[0068] Example 2: The experimental conditions were as follows: reaction time was 4 hours, and other experimental conditions were the same as in Example 1. This example only performed stage 1 oxidation, and the resulting liquid product was a mixture of small molecule carboxylic acids. The reaction conversion rate was 78.7%, the carboxylic acid yield was 87.8 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield was 59.3 wt.%, the carbonyl index was 0.46, and the number-average sequence length was 6.36.

[0069] Example 3: The experimental conditions were as follows: reaction time was 6 hours, and other experimental conditions were the same as in Example 1. The reaction conversion rate was 84.7%, the carboxylic acid yield was 97.8 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield was 70.6 wt.%, the carbonyl index was 0.52, and the number-average sequence length was 5.86.

[0070] Based on this, the obtained stage one small molecule carboxylic acid mixture was further subjected to stage two carboxylic acid oxidation treatment: Using the mass of the stage one carboxylic acid mixture as a baseline, nitric acid solution was added to the stage one carboxylic acid mixture at a mass ratio of approximately 8–15:1. The mixture was refluxed at 100°C for 1 hour. After the reaction was completed, free nitric acid and volatile nitrogen-containing byproducts were removed under reduced pressure, and the product was subjected to rotary evaporation to obtain a light yellow solid product, mainly composed of α,ω-diacarboxylic acids with 4–10 carbon atoms. After stage two treatment, the carboxylic acid yield was 72.0 wt.% (mass ratio of solid product to raw material), and the selectivity of α,ω-diacarboxylic acids was >99%.

[0071] Example 4: The experimental conditions were as follows: reaction time was 8 hours, and other experimental conditions were the same as in Example 1. This example only performed stage 1 oxidation, and the resulting liquid product was a mixture of small molecule carboxylic acids. The reaction conversion rate was 88.0%, the carboxylic acid yield was 99.2 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield was 68.5 wt.%, the carbonyl index was 0.56, and the number-average sequence length was 5.60.

[0072] Example 5: The experimental conditions were as follows: reaction time was 12 hours, and other experimental conditions were the same as in Example 1. This example only performed stage 1 oxidation, and the resulting liquid product was a mixture of small molecule carboxylic acids. The reaction conversion rate was 89.0%, the carboxylic acid yield was 76.2 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield was 53.3 wt.%, the carbonyl index was 0.57, and the number-average sequence length was 5.48.

[0073] Example 6: The experimental conditions were: oxygen pressure 0.5 MPa, reaction time 6 hours, and other experimental conditions the same as in Example 1. This example only involved stage 1 oxidation, and the resulting liquid product was a mixture of small-molecule carboxylic acids. The reaction conversion rate was 43.3%, the carboxylic acid yield was 43.4 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield was 30.3 wt.%, the carbonyl index was 0.44, and the number-average sequence length was 6.56.

[0074] Example 7: The experimental conditions were: oxygen pressure of 2 MPa, reaction time of 6 hours, and other experimental conditions were the same as in Example 1. This example only involved stage 1 oxidation, and the resulting liquid product was a mixture of small-molecule carboxylic acids. The reaction conversion rate was 99.9%, the carboxylic acid yield was 88.2 wt.% (the mass ratio of liquid product to raw material), the α,ω-diacid yield was 59.3 wt.%, the carbonyl index was 0.76, and the number-average sequence length was 4.63.

[0075] Example 8: Precursor solutions were prepared according to a Ce:H₂BDC ratio of 1:1.5. Ce metal salt solution: Ce salt was dissolved in N,N-dimethylformamide / water at a volume ratio of 8 / 2. Ligand solution: H₂BDC was dissolved in N,N-dimethylformamide. The noble metal precursor chloroplatinic acid was added to the metal salt solution and adjusted to the desired Pt loading of 1.0 wt.%, then homogenized by sonication. A small amount of acetic acid, an acid regulator, was then added to control nucleation / crystallization at a ratio of acetic acid:Ce = 10~20:1 (molar ratio), and homogenized again. The mixture was transferred to a polytetrafluoroethylene-lined reactor, and deionized water was added to maintain the N,N-dimethylformamide / water ratio of 8 / 2 (volume ratio). Crystallization was carried out at 120 °C for 24 h using a solvothermal method. After filtration, the sample was washed sequentially with N,N-dimethylformamide and ethanol, and then vacuum dried at 60 °C to obtain the Pt@CeBDC precursor. This precursor was then spread evenly in a dish and thermally converted at 400 °C for 4 h at a rate of 5 °C / min under air atmosphere to obtain a two-dimensional porous Pt@CeO2-BDC catalyst. Figure 3 This is a scanning electron microscope (SEM) image of the Pt@CeO2-BDC catalyst. The prepared Pt@CeO2-BDC catalyst was used in the aqueous oxidative cracking reaction of decommissioned polyolefins. The feed ratio was set as decommissioned polyolefin: Pt@CeO2-BDC = 100:20 (mass ratio). The reaction time was 4 hours, and other experimental conditions were the same as in Example 1. This example only performed stage 1 oxidation, and the resulting liquid product was a mixture of small-molecule carboxylic acids. The reaction conversion rate was 89.9%, the carboxylic acid yield was 95.8 wt.% (mass ratio of liquid product to feedstock), the α,ω-diacid yield was 62.0 wt.%, the carbonyl index was 0.35, and the number-average sequence length was 7.75.

[0076] Comparative Example 1: The experimental conditions were: nitrogen was used instead of oxygen, the reaction time was 6 hours, and other experimental conditions were the same as in Example 1. The reaction conversion rate was 0%.

[0077] Comparative Example 2: The experimental conditions were as follows: no water solvent was introduced, the reaction time was 6 hours, and other experimental conditions were the same as in Example 1. The reaction conversion rate was 65.6%, the carboxylic acid yield was 10.5 wt.% (the mass ratio of liquid product to raw material), the carbonyl index was 0.28, and the number-average sequence length was 9.26. Furthermore, the coke content in this comparative example was 3.1 wt.%, which is three times higher than the coke content obtained in Example 3 (0.9 wt.%). This demonstrates that water, as a continuous phase, plays a role in preventing hotspot formation—crosslinking—coking.

[0078] The results of the stage 1 oxidation in Examples 1-8 and Comparative Examples 1-2 are summarized in Table 1. Using the technical solution of this invention, in the stage 1 oxidation, water is used as the solvent, and an equivalent amount of oxidant gas is introduced to convert decommissioned polyolefins into carboxylic acid products. The yields are summarized as follows: Figure 1 Furthermore, the results of Examples 2 and 8 show that, under the condition of an appropriate amount of catalyst, decommissioned polyolefins can achieve higher conversion rates and carboxylic acid yields. This is mechanistically attributed to the synergistic effect of two-dimensional porous oxides / noble metals-supports, which allows surface oxygen vacancies and metal active sites to promote O2 activation and selective C–C cleavage. Example 8 verifies the acceleration and selection effects of the optional catalyst in this invention. Combined with the results of Comparative Examples 1 and 2, it can be jointly demonstrated that the synergy between the aqueous phase and the oxidizing gas is key to achieving stable, controllable, and highly selective stage 1 oxidation process in this invention, while the optional catalytic system can further accelerate conversion and optimize the distribution of the target product. Under the premise of ensuring a high carboxylic acid yield in stage 1 oxidation, stage 2 oxidation can purify the product to α,ω-diacids. The specific compositions of the carboxylic acid products obtained in stages 1 and 2 of Example 3 are as follows: Figure 2 , 3 The carboxylic acid yields and α,ω-dicarboxylic acid selectivity in stages one and two, for example... Figure 4 This demonstrates that the technical solution of this invention can effectively convert decommissioned polyolefins into α,ω-diacids through staged oxidation, while achieving high yield and high selectivity of the product.

[0079] Table 1: Summary of Oxidation Results in Examples and Comparative Examples

[0080] In summary, this method proposes for the first time a staged oxidation-based approach to produce α,ω-diacarboxylic acids from decommissioned polyolefins. In the first stage of oxidation, the water solvent reduces local hot spots and prevents coking. Combined with the action of the oxidant, this enables low-temperature, controllable pyrolysis of decommissioned polyolefins into small-molecule carboxylic acids. The product is a small-molecule carboxylic acid that can be stably produced without a catalyst. The method can also selectively introduce MOF-derived two-dimensional porous metal oxides and their low-loaded noble metal catalysts to further improve the conversion rate and the yield of the target carboxylic acid. In the second stage of oxidation, the small-molecule carboxylic acid obtained in the first stage can be further oxidized, converted, and purified into α,ω-diacarboxylic acids, avoiding the complex separation and purification steps required in practical applications. The resulting α,ω-diacarboxylic acids can be used as raw materials for polyester and polyamide monomers, as well as surfactants and lubricating additives, demonstrating significant application value. This patent proposes a high-value recycling route for decommissioned polyolefins under mild conditions and without organic reagents, providing guidance for subsequent research and contributing to the green and sustainable development of polymer engineering.

[0081] It should be understood that the application of this invention is not limited to the examples described above. Those skilled in the art can make improvements or variations based on the above description, and all such improvements and variations should fall within the protection scope of the appended claims. The above embodiments are only used to illustrate the technical solutions of this invention and are not intended to limit it. Although this invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to depart from the spirit and scope of the technical solutions of the embodiments of this invention.

Claims

1. A method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation, characterized in that, Includes the following steps: Oxidative preparation of small molecule carboxylic acids: In the absence of a catalyst or in the presence of a catalyst, decommissioned polyolefins are mixed with water to form a dispersion system. Under the conditions of 100~200°C and oxidant gas partial pressure of 0.1~5.0 MPa, oxidant gas is introduced into a high-pressure closed reactor for stirring reaction to obtain small molecule carboxylic acids, wherein the small molecule carboxylic acids are mainly small molecule carboxylic acids with 4~10 carbon atoms. Preparation of α,ω-diacarboxylic acids by oxidation: α,ω-diacarboxylic acids are obtained by oxidizing small molecule carboxylic acids with nitric acid, wherein the number of carbon atoms in the α,ω-diacarboxylic acids is 4~10.

2. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, The mass yield of carboxylic acid products of small molecule carboxylic acids is >95%, and the selectivity of α,ω-diacarboxylic acids is >70%; the mass yield of carboxylic acid products of α,ω-diacarboxylic acids is >70%, and the selectivity of α,ω-diacarboxylic acids is >98%.

3. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, After adding water and degraded polyolefins to form a dispersion system under normal pressure and completing the initial mechanical dispersion, the system is slowly pressurized and heated under closed conditions, and the volume fraction of the aqueous phase in the dispersion system is greater than 50%.

4. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, The oxidant gas is one or more of air, oxygen, ozone, nitrogen dioxide, and nitric oxide. The oxidant gas is introduced into the high-pressure closed reactor in a one-time filling, constant pressure continuous ventilation, or segmented oxygen replenishment manner.

5. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, After an oxidant gas is introduced into a high-pressure closed reactor, decommissioned polyolefins undergo chain scission and selective oxidation reactions to obtain gaseous, liquid, and solid products. The liquid products are then separated to obtain small molecule carboxylic acids.

6. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, The retired polyolefin is one or more of low-density polyethylene, high-density polyethylene, linear low-density polyethylene, isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene, polystyrene, ethylene and α-olefin copolymers mixed in any proportion.

7. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, Based on the total mass of decommissioned polyolefins in the entire reaction system, the solid content of decommissioned polyolefins is 1~100 wt.%, and the mass ratio of catalyst to decommissioned polyolefins is 0.1:100~20:

100.

8. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, The catalyst is selected from porous metal oxide catalysts and their composites prepared by heat treatment of metal-organic framework materials, or the catalyst is selected from noble metal supported catalysts and their doped composite catalysts.

9. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, Add 30-70 wt.% nitric acid to a small molecule carboxylic acid and react at 50-120 °C for 0.5-6 h.

10. The method for producing α,ω-diacids from decommissioned polyolefins based on staged oxidation according to claim 1, characterized in that, α,ω-dicarboxylic acids can be recovered by any of the following methods: denitrification / dehydration, crystallization separation, or acid-base conversion-re-acidification.

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