A supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing pet
By combining gas-phase reduced pretreatment with a self-healing catalyst and a two-stage density-controlled supercritical hydrothermal liquefaction process, the problems of oxidation side reactions and catalyst deactivation in oxygen-containing PET waste were solved, achieving efficient and selective chemical recycling.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- NANTONG BEIJIA PLASTIC IND CO LTD
- Filing Date
- 2026-05-11
- Publication Date
- 2026-07-24
AI Technical Summary
Existing supercritical hydrothermal liquefaction technology suffers from problems such as severe over-oxidation side reactions, poor monomer selectivity, and easy catalyst deactivation when treating oxygen-containing PET waste, making it difficult to achieve efficient and highly selective chemical recycling.
A gas-phase reducing state pretreatment combined with low-temperature plasma-assisted activation technology is employed to eliminate reactive oxygen species in a dry environment through dielectric barrier discharge or microwave plasma. A self-healing oxygen vacancy enrichment catalyst and an auxiliary bias electric field are used to maintain catalytic activity during supercritical hydrothermal liquefaction. A two-stage density control strategy and the synergistic effect of reducing agents are employed to achieve efficient depolymerization and decarboxylation.
It effectively suppressed oxidation side reactions, protected the integrity of PET ester bonds, improved the selectivity and recovery rate of terephthalic acid and ethylene glycol, extended catalyst life, reduced the proportion of CO and CO2 generated, and ensured the economy and stability of the process.
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Figure CN122444584A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of source recovery technology, specifically to a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET. Background Technology
[0002] Polyethylene terephthalate (PET) is the world's most consumed polyester material, widely used in beverage packaging, textile fibers, and films. With the continuous growth of total waste plastics, the efficient recycling and resource utilization of waste PET has become a crucial issue in plastic pollution control and the circular economy. Chemical recycling technology can depolymerize waste PET into monomers such as terephthalic acid and ethylene glycol, achieving closed-loop regeneration. Compared to physical recycling, it has wider raw material adaptability and higher product value.
[0003] Among numerous chemical recycling methods, supercritical hydrothermal liquefaction technology offers significant advantages. Under supercritical conditions—temperatures and pressures exceeding critical temperatures and pressures—water combines the high diffusivity of gases with the high solubility of liquids, enabling it to efficiently penetrate plastic matrices and uniformly decompose polymer molecular chains. Compared to traditional pyrolysis or solvent decomposition methods, supercritical hydrothermal liquefaction technology can process wet materials containing impurities, eliminates the need for stringent drying pretreatment, and significantly reduces carbon emissions compared to incineration, demonstrating clear environmental advantages.
[0004] Currently, there is considerable research on the supercritical hydrolysis and recovery of polyethylene terephthalate (PET). Researchers have proposed subcritical and supercritical water-based depolymerization methods for PET, hydrolyzing it into terephthalic acid and ethylene glycol under specific temperature and pressure conditions. Experimental results show that the conversion rate can reach a high level, and the terephthalic acid yield is also good. Regarding catalysts, various types, including solid acid catalysts and supported metal catalysts, have been explored to improve depolymerization efficiency. Studies have shown that adding appropriate catalysts can significantly reduce the formation rates of carbon monoxide and carbon dioxide, indicating that catalytic regulation plays a crucial role in suppressing side reactions.
[0005] However, with the increasing trend of functionalizing polyethylene terephthalate (PET) packaging materials, a large number of oxygen-containing PET materials have appeared on the market. To extend shelf life, oxygen-containing barrier layers or oxygen scavengers are often added to beverage bottles and other packaging; some high-end products use oxygen-containing copolyester modification to improve overall performance. The introduction of these oxygen-containing components results in a higher oxygen content in waste materials than pure PET, posing technical challenges that are difficult to solve with existing technologies during supercritical hydrothermal liquefaction.
[0006] The active oxygen components in oxygen-containing polyethylene terephthalate (PET) readily initiate free radical chain oxidation reactions under supercritical high temperature and high pressure conditions. This leads to further oxidation and degradation of target monomers such as terephthalic acid and ethylene glycol generated during depolymerization into byproducts such as benzoic acid, carbon dioxide, and carbon monoxide, resulting in a significant decrease in monomer recovery rate and selectivity. The polar oxygen-containing groups in PET preferentially adsorb onto the active sites of traditional catalysts, leading to competitive occupation of active sites on the catalyst surface, resulting in decreased catalyst activity and shortened lifetime. Simultaneously, the presence of oxygen-containing components alters the redox potential and ion product of the reaction system, interfering with the spontaneous ionization equilibrium of hydrogen and hydroxide ions in supercritical water, thereby weakening the autocatalytic effect of the hydrolysis reaction. The introduction of oxygen-containing components results in the reaction products containing various oxygen-containing derivatives in addition to terephthalic acid and ethylene glycol, such as benzoic acid, acetic acid, oxalic acid, and oxygen-containing aromatic oligomers. This complex product composition makes separation and purification difficult, severely impacting the quality and economic viability of the recovered products. The degradation behavior of oxygen-containing polyethylene terephthalate in supercritical hydrothermal liquefaction is much more sensitive to parameters such as temperature, pressure, and oxygen content than that of pure polyethylene terephthalate. As the temperature increases, the oxidation side reaction is significantly enhanced, the yield of the target monomer drops sharply, the process window is narrow, and the operation is difficult.
[0007] Existing processes for oxygen-containing polyethylene terephthalate (PET) face a dual challenge in dealing with subcritical and supercritical water environments. To eliminate reactive oxygen species in PET, conventional approaches often involve in-situ reduction pretreatment in the aqueous phase. However, as a polyester material sensitive to humidity and heat, the hydrolysis rate constant of PET's ester bonds increases significantly with temperature in a liquid water environment. Within the temperature window required for reduction and deoxygenation, indiscriminate hydrolysis of the PET backbone occurs. This sharp decrease in molecular weight during pretreatment leads to a wider distribution of oligomers during subsequent supercritical liquefaction, which is detrimental to the directional crystallization of terephthalic acid monomers and may even generate more oxygen-containing heterocyclic byproducts. Finding a balance between completely eliminating reactive oxygen species and strictly protecting the integrity of PET ester bonds remains an unresolved technical challenge in this field.
[0008] Furthermore, conventional supported catalysts are prone to oxygen vacancy annihilation due to the replenishment of surface lattice oxygen in oxygen-containing atmospheres. During supercritical hydrothermal liquefaction, oxygen-containing components continuously release active oxygen species, causing a sustained decrease in the oxygen vacancy concentration on the catalyst surface, leading to a decline in catalytic activity. Current technologies lack an effective means to maintain a dynamic balance of catalytic active sites throughout the entire liquefaction process of oxygen-containing polyethylene terephthalate.
[0009] In summary, while existing supercritical hydrothermal liquefaction and recovery processes have made some progress in treating pure polyethylene terephthalate (PET), they generally suffer from several technical shortcomings when dealing with the increasing amount of oxygen-containing PET waste. These include severe over-oxidation side reactions, poor monomer selectivity, easy catalyst deactivation, and difficulty in simultaneously addressing pre-reduction treatment and main chain protection. Currently, there are no publicly reported systematic technical solutions specifically for the supercritical hydrothermal liquefaction and recovery of oxygen-containing PET.
[0010] The purpose of this invention is to provide a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing polyethylene terephthalate (PET), in order to solve the technical problems existing in the treatment of PET waste, such as severe excessive oxidation side reactions, poor monomer selectivity, low recovery rate, and easy catalyst deactivation, and to achieve efficient and highly selective chemical recovery of PET. Summary of the Invention
[0011] To address the shortcomings of existing technologies, this invention provides a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET, thereby solving the problems in the prior art.
[0012] To achieve the above objectives, the present invention provides the following technical solution: a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET, comprising the following steps: Step 1, Pretreatment: The oxygen-containing PET waste is crushed, washed, and dried to obtain oxygen-containing PET particles with a particle size of 0.5–5 mm; Step 2, gas-phase reduction pretreatment: The oxygen-containing PET particles obtained in Step 1 are placed in a fluidized or stirred reactor, and dry reduction treatment is carried out for 15 to 60 minutes at a temperature of 120 to 200°C and a pressure of 0.1 to 2 MPa under a mixed atmosphere containing hydrogen and inert gas to obtain surface-deoxidized inert PET material. Preferably, the low-temperature plasma-assisted activation in step 2 is dielectric barrier discharge or microwave plasma, with an input power density of 0.5–2 W / cm². 3 The hydrogen volume concentration in the mixed atmosphere is 2-10 vol%, with the remainder being nitrogen or argon.
[0013] Preferably, the dry reduction process in step 2 further includes introducing a trace amount of gaseous formic acid as a hydrogen donor and oxygen capture synergist, wherein the amount of gaseous formic acid introduced is 0.1 to 1 vol of the total volume of the mixed atmosphere.
[0014] Preferably, the empty tower flow velocity of the mixed gas in the fluidized reactor in step 2 is 0.05 to 0.5 m / s to ensure that the PET particles are suspended and tumbled in the plasma field and are uniformly deoxygenated.
[0015] Step 3, density-zoned supercritical hydrothermal liquefaction reaction: The inert PET material obtained in Step 2 is mixed with water at a mass ratio of 1:3 to 1:15, and a self-healing oxygen vacancy enrichment catalyst is added. The reaction is carried out under supercritical water conditions using a two-stage density control strategy. Depolymerization induction period: Control the reaction system temperature at 360–385℃ and the pressure at 25–30 MPa, maintaining the water density at 0.25–0.35 g / cm³. 3 Leave for 5-15 minutes to utilize the high ion accumulation environment to initially break down the PET chains into oligomers; Deep liquefaction and decarboxylation period: Subsequently, the system pressure is isothermally depressurized or heated to 390–420℃ and 23–26 MPa, maintaining a water density of 0.10–0.20 g / cm³. 3 The solution is left to stand for 5–15 minutes to enhance mass transfer and catalytic decarboxylation selectivity using low-density supercritical water. Preferably, the self-healing oxygen vacancy enrichment catalyst in step 3 has a core-shell structure, and its preparation method includes: Using magnetic Fe3O4 nanoparticles as the core and Ce-doped... 3+ / Ce 4+ The ZrO2-TiO2 composite oxide of variable-valence rare earth elements forms the shell, and Ni-Co bimetallic active components are loaded on the surface of the shell. The catalyst utilizes Ce in the supercritical hydrothermal liquefaction reaction. 3+ / Ce 4+ The redox cycle between the ion pair and the hydrogen gas and reducing intermediates generated in the reaction system dynamically maintains a surface oxygen vacancy concentration ≥2×10⁻⁶. 19 cm -3 ; Preferably, an auxiliary bias electric field of 0.1 to 1.0 V is applied to the reactor wall to enhance the oxygen vacancy repair efficiency.
[0016] Preferably, the mass ratio of Ni to Co in the catalyst is 1:0.8 to 1:2.5, the shell thickness is 10 to 50 nm, and the specific surface area of the catalyst is 80 to 200 m². 2 / g.
[0017] Preferably, a reducing agent, namely formic acid or methanol, is added to the reaction system in step 3, at an amount of 0.5–3 wt% of the water mass, to synergistically maintain the Ce-coated catalyst shell. 3+ / Ce 4+ The reduced state equilibrium.
[0018] Preferably, in step 3, when switching from the depolymerization induction period to the deep liquefaction period, the pressure relief rate is 0.5 to 2 MPa / min to prevent the PET melt from foaming and carrying unreacted materials due to a sudden drop in density.
[0019] Step 4, Product Separation: After the reaction is completed, the product is cooled in stages and separated into gas, liquid and solid phases. Ethylene glycol is recovered from the aqueous phase and high-purity terephthalic acid is recovered from the solid phase.
[0020] Preferably, after step 4, a catalyst magnetic recovery and activation regeneration step is further included: The core-shell magnetic catalyst in the reaction residue was separated and recovered by an external magnetic field. The recovered catalyst was then placed in a nitrogen-hydrogen mixed atmosphere containing 5 vol% H2 at 400-500℃ for 1-2 hours to achieve lattice reconstruction and oxygen vacancy regeneration of the active components of the catalyst. Preferably, a DC pulse electric field of 1.5 to 3.0 V is applied simultaneously during the reduction process to accelerate regeneration.
[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention utilizes gas-phase reducing pretreatment combined with low-temperature plasma-assisted activation technology to directionally eliminate reactive oxygen components in oxygen-containing PET in a dry environment far below the hydrolysis-sensitive temperature of polyethylene terephthalate. This not only suppresses excessive oxidation side reactions in the subsequent supercritical liquefaction stage but also preserves the PET ester bond structure and molecular chain length, avoiding the problems of premature main chain hydrolysis and widening of oligomer distribution caused by traditional wet pre-reduction.
[0022] 2. This invention employs a two-stage water density partitioning strategy, namely the depolymerization induction period and the deep liquefaction decarboxylation period. In the high-density stage, the high ion product environment of supercritical water is fully utilized to promote uniform chain scission of the PET main chain to generate oligomers. In the low-density stage, the high diffusion coefficient and low dielectric constant of supercritical water are used to enhance the decarboxylation selectivity of terephthalic acid and suppress the crystallization and deposition of by-products. This achieves synergistic optimization between the depolymerization efficiency of oxygen-containing PET and the selectivity of the target monomer.
[0023] 3. This invention employs a magnetic Fe3O4 core and doped with the variable-valence rare earth element Ce. 3+ / Ce 4+ A core-shell structured self-healing oxygen vacancy enrichment catalyst composed of ZrO2-TiO2 composite oxide shells was used. An auxiliary bias electric field was applied to the reactor wall to induce the directional migration of oxygen in the shell lattice and Ce. 3+ / Ce 4+ The dynamic coupling effect of the variable valence cycle continuously maintains a high concentration of oxygen vacancy active centers on the catalyst surface in an oxygen-containing reaction atmosphere, effectively solving the problem of rapid deactivation of traditional catalysts due to lattice oxygen replenishment during the oxygen-containing PET liquefaction process.
[0024] 4. This invention introduces formic acid or methanol as a reducing agent into a supercritical hydrothermal liquefaction system and works synergistically with a bias electric field and catalyst variable valence ion pairs to construct a triple antioxidant stabilization mechanism of in-situ hydrogen supply in the liquid phase, interfacial oxygen vacancy repair, and electric field-driven oxygen migration. This reduces the proportion of carbon monoxide and carbon dioxide generated in the reaction tail gas and improves the directional conversion efficiency of carbon elements to the target monomers terephthalic acid and ethylene glycol.
[0025] 5. This invention employs a regeneration process that combines catalyst magnetic recovery with synergistic activation by a DC pulsed electric field. It utilizes an external magnetic field to achieve efficient separation and recovery of the core-shell magnetic catalyst from the reaction residue, and applies a DC pulsed electric field in a hydrogen-containing atmosphere for lattice reconstruction and regeneration. This restores the oxygen vacancy concentration and active component dispersion on the catalyst surface under low thermal energy consumption conditions, ensuring the economic efficiency and continuous operational stability of the process throughout its entire life cycle.
[0026] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description
[0027] Figure 1 Flow diagram of a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET; Figure 2 Figure A shows the temperature and pressure variation curves of the two-stage density control parameters in the supercritical hydrothermal liquefaction reaction process of Example 1 of the present invention (Figure A shows the temperature and pressure variation curves, and Figure B shows the water density variation curves). Figure 3 This is a bar chart comparing the terephthalic acid yields of the various embodiments and comparative examples of the present invention. Figure 4 This is a bar chart comparing the volume fraction of CO+CO2 in the exhaust gas of various embodiments and comparative examples of the present invention. Detailed Implementation
[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0029] Please see Figures 1-4This invention discloses a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET, which solves the technical problems of existing supercritical hydrothermal liquefaction technology in treating oxygen-containing PET waste, such as excessive oxidation side reactions caused by reactive oxygen, poor monomer selectivity, easy deactivation of catalysts, and low recovery rate. It achieves efficient and highly selective chemical recovery of oxygen-containing PET to terephthalic acid and ethylene glycol while protecting the integrity of PET ester bonds.
[0030] The oxygen-containing PET waste used in the following examples and comparative examples was commercially available PET beverage bottle fragments containing an EVOH barrier layer. Infrared spectroscopy analysis confirmed that the PET content was approximately 92 wt%, the EVOH content was approximately 6 wt%, and the remainder was adhesive resin and additives. The total oxygen content was approximately 2.3 percentage points higher than that of pure PET. Deionized water was prepared in the laboratory with a resistivity ≥18 MΩ·cm. The purity of hydrogen, nitrogen, and argon was 99.99%. Formic acid and methanol were analytical grade reagents.
[0031] The fluidized bed reactor is a quartz glass fluidized bed with a built-in dielectric barrier discharge plasma generator. The supercritical hydrothermal liquefaction reaction is carried out in a high-temperature, high-pressure batch reactor made of Hastelloy alloy. The reactor has a volume of 500 mL, a design pressure of 40 MPa, and a design temperature of 500℃. An online density monitor and dissolved oxygen electrode interface are installed inside the reactor. An auxiliary electrode device capable of applying DC bias is installed on the outer wall of the reactor.
[0032] Example 1: This embodiment provides a supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET, and the specific steps are as follows.
[0033] Step 1, Pretreatment: The oxygen-containing PET waste is crushed to a particle size of approximately 2–3 mm, washed with deionized water to remove surface stains, and dried in a vacuum drying oven at 90°C until the moisture content is 0.3 wt%, yielding oxygen-containing PET particles. This step increases the specific surface area of the material through mechanical crushing, which is beneficial for sufficient contact between the material and the subsequent gas-phase reducing atmosphere; washing and drying remove surface impurities, preventing unknown side reactions caused by impurities in the supercritical environment.
[0034] Step 2, Gas-phase reducing pretreatment: The oxygen-containing PET particles obtained in Step 1 are placed in a fluidized bed reactor, and a mixed atmosphere of hydrogen and nitrogen is introduced, wherein the hydrogen volume concentration is 5 vol%, and the remainder is nitrogen. The dielectric barrier discharge plasma device is turned on, with an input power density of 1.2 W / cm². 3The reactor temperature was controlled at 160℃ and the pressure at 0.5 MPa, with a mixed gas flow rate of 0.2 m / s in the empty tower, causing the PET particles to be suspended and tumbling in the plasma field. Under these conditions, a dry reduction treatment was performed for 30 minutes to obtain surface-deoxidized, inert PET material. This step utilizes low-temperature plasma to activate hydrogen molecules into high-energy hydrogen free radicals, which then directionally attack the peroxide bonds, hydroxyl groups, and other active oxygen-containing functional groups on the surface of oxygen-containing PET at temperatures far below the PET hydrolysis sensitive temperature. These radicals are reduced to water vapor and carried away by the gas flow, thus removing active oxygen while preventing the hydrolysis and breakage of PET ester bonds, protecting the integrity of the polymer molecular chains.
[0035] Step 3, density-zoned supercritical hydrothermal liquefaction reaction: The inert PET material obtained in Step 2 is mixed with deionized water at a mass ratio of 1:8, and a self-healing oxygen vacancy enrichment catalyst is added. The catalyst is prepared by coating Ce-doped particles with magnetic Fe3O4 nanoparticles with an average particle size of 50 nm using a sol-gel method. 3+ / Ce 4+ A ZrO2-TiO2 composite oxide shell was constructed, with Ce doping at 5% of the total metal moles in the shell and a shell thickness of approximately 30 nm. Then, a Ni-Co bimetallic active component was loaded onto the shell surface via an impregnation-calcination method, with a Ni to Co mass ratio of 1:1.5 and a total loading of 12 wt%. The catalyst had a specific surface area of 135 m². 2 / g. The catalyst addition amount is 5 wt% of the inert PET material mass. A 0.5 V DC bias electric field is applied to the reactor wall. The reaction employs a two-stage density control strategy: Depolymerization induction period: The reaction system is heated to 375℃ at a heating rate of 10℃ / min. At this time, the pressure automatically rises to 28MPa, corresponding to a water density of approximately 0.30 g / cm³. 3 Under these conditions, the water remains for 10 minutes. During this stage, supercritical water is in a high-density region with a large ion product, and the H+ produced by the spontaneous ionization of water molecules... + and OH - At higher concentrations, it can effectively catalyze the hydrolysis and breakage of PET ester bonds, causing the PET macromolecules to initially depolymerize into ethylene terephthalate oligomers.
[0036] Deep liquefaction and decarboxylation period: The system was isothermally depressurized to 25 MPa at a depressurization rate of 1 MPa / min, while maintaining the temperature at 375℃. This corresponds to a water density decrease to approximately 0.15 g / cm³, and the system remained under these conditions for 10 minutes. During this stage, the supercritical water density decreased, the diffusion coefficient significantly increased, and the dielectric constant decreased, which is beneficial for further hydrolysis of oligomers to generate terephthalic acid and ethylene glycol monomers. Simultaneously, the low-density environment enhanced mass transfer and inhibited in-situ crystallization and deposition of monomers within the catalyst channels. During the reaction, Ce in the catalyst shell...3+ / Ce 4+ The variable-valence ion pairs undergo a redox cycle with the trace amounts of hydrogen gas and reducing intermediates generated in the reaction system, supplemented by the directional migration of oxygen in the shell lattice induced by an applied bias electric field, dynamically maintaining the oxygen vacancy concentration on the catalyst surface at no less than 2 × 10⁻⁶. 19 cm -3 It provides a continuous and stable active site for hydrolysis and decarboxylation reactions, and effectively inhibits the side reactions of deep oxidation to generate CO and CO2.
[0037] The temperature, pressure, and water density changes in the two-stage density control process in step 3 of this embodiment are as follows: Figure 2 Figures A and B are shown in Figure 1. Figure A is a graph showing the temperature and pressure changes during the supercritical hydrothermal liquefaction reaction in Example 1. The temperature curve shows that the system maintains a constant temperature after being heated from room temperature to 375°C. The pressure curve shows that the system is heated to 28 MPa and then isothermally depressurized to 25 MPa and remains constant. The graph is marked with the heating and pressurization stage, the depolymerization induction period, the isothermal depressurization switching stage, and the deep liquefaction and decarboxylation period. Figure B is a graph showing the water density changes during the supercritical hydrothermal liquefaction reaction in Example 1. The water density remains at 0.30 g / cm³ during the depolymerization induction period. 3 After the density switching section, it decreased to 0.15 g / cm³. 3 And it remains constant during the deep liquefaction and decarboxylation periods. Figure 2 As can be seen from A and B, this invention achieves the switching of supercritical water density from the high-density region to the low-density region through isothermal depressurization operation, forming a two-stage partitioned reaction environment of depolymerization induction and deep liquefaction decarboxylation.
[0038] Step 4, Product Separation: After the reaction is completed, the reactor is naturally cooled to room temperature, and the exhaust valve is opened to collect the gaseous products. The mixture in the reactor is taken out and separated into solid and liquid phases by vacuum filtration. The liquid phase product is subjected to vacuum distillation to recover ethylene glycol, and the solid phase product is washed with deionized water and dried to obtain high-purity terephthalic acid solid.
[0039] Step 5, Catalyst Magnetic Recovery and Activation Regeneration: The core-shell magnetic catalyst is separated and recovered from the reaction residue using an external magnetic field. The recovered catalyst is placed in a tube furnace and reduced at 450°C under a nitrogen-hydrogen mixed atmosphere containing 5 vol% H2 for 1.5 hours, while a 2.0 V DC pulsed electric field is applied simultaneously. This step, through the synergistic effect of thermal hydrogen reduction and electric field-induced lattice reconstruction, restores the oxygen vacancy concentration and active component dispersion on the catalyst surface, restoring the catalyst activity to over 95% of that of fresh catalyst, allowing it to be directly recycled for the next batch of reactions.
[0040] Example 2: The difference between this embodiment and Embodiment 1 is that in step 2, the low-temperature plasma is excited by microwave plasma with an input power density of 1.8 W / cm².3 In step 2, the hydrogen volume concentration in the mixed atmosphere is 8 vol%; in step 3, the mass ratio of PET material to water is 1:10, and the mass ratio of Ni to Co in the catalyst is 1:2.0; in step 3, the depolymerization induction period temperature is 385℃, the pressure is 30 MPa, and the residence time is 5 minutes; the deep liquefaction period temperature is 420℃, the pressure is 26 MPa, and the residence time is 5 minutes. The remaining steps and parameters are the same as in Example 1.
[0041] Example 3: The difference between this embodiment and Embodiment 1 is that: in step 2, a trace amount of gaseous formic acid is introduced as a hydrogen donor and oxygen capture synergist, with the amount of gaseous formic acid introduced being 0.5 vol% of the total volume of the mixed atmosphere. Formic acid decomposes in the plasma field to generate additional hydrogen radicals and reducing CO species, enhancing the removal efficiency of reactive oxygen species. In step 3, a reducing agent, formic acid, is also added to the reaction system at an amount of 1.5 wt% of the water mass. Formic acid rapidly decomposes in supercritical water to generate hydrogen and carbon dioxide, with the hydrogen further participating in the Ce2 reaction on the catalyst surface. 4+ To Ce 3+ The reduction process helps maintain the reduced state equilibrium of the shell-bound variable-valence ions. The remaining steps and parameters are the same as in Example 1.
[0042] Example 4: The difference between this embodiment and Embodiment 1 is that: in step 2, the dry reduction treatment temperature is 120℃, the pressure is 2.0MPa, and the treatment time is 60 minutes; in step 3, the Ce doping amount of the catalyst shell is 8%, the shell thickness is 15 nm, and the catalyst specific surface area is 180 m². 2 / g; In step 3, a bias electric field of 1.0 V is applied to the reactor wall; In step 3, during the deep liquefaction period, density switching is achieved through isothermal depressurization at a depressurization rate of 2 MPa / min. The remaining steps and parameters are the same as in Example 1.
[0043] Example 5: The difference between this embodiment and Embodiment 1 is as follows: In step 2, the dry reduction treatment temperature is 200℃, the pressure is 0.1MPa, and the treatment time is 15 minutes; in step 3, the mass ratio of PET material to water is 1:3; in step 3, the depolymerization induction period temperature is 360℃, the pressure is 25 MPa, and the residence time is 15 minutes; in step 3, the deep liquefaction period achieves density switching by raising the temperature to 390℃ and the pressure to 23 MPa, and the residence time is 15 minutes; in step 3, no auxiliary bias electric field is applied, and the catalyst's own Ce is used instead. 3+ / Ce 4+ The redox cycle maintains oxygen vacancies. The remaining steps and parameters are the same as in Example 1.
[0044] Comparative Example 3 Comparative Example 1: (No gas-phase reduction pretreatment step) The difference between this comparative example and Example 1 is that step 2 is omitted, and the oxygen-containing PET particles obtained in step 1 are directly fed into step 3 for supercritical hydrothermal liquefaction reaction. The remaining steps and parameters are the same as in Example 1.
[0045] Comparative Example 2: (Pre-reduction treatment using wet subcritical water) The difference between this comparative example and Example 1 is that step 2 is replaced with subcritical water pre-reduction treatment. Specifically, oxygen-containing PET particles are placed in a high-pressure reactor, deionized water is added (solid-liquid mass ratio 1:10), and the mixture is heated to 220°C and pressure 2.5 MPa under nitrogen protection, stirred for 30 minutes, then cooled, separated, and dried to obtain the pretreated material. The remaining steps and parameters are the same as in Example 1.
[0046] Comparative Example 3: (No density zoning control, using a single supercritical condition) The difference between this comparative example and Example 1 is that in step 3, the temperature is directly raised to 400°C and the pressure to 30 MPa, and the reaction is carried out under these single conditions for 20 minutes. The remaining steps and parameters are the same as in Example 1.
[0047] Comparative Example 4: (using a conventional non-core-shell catalyst) The difference between this comparative example and Example 1 is that in step 3, the catalyst is replaced with a commercially available Ni-Co / Al2O3 supported catalyst, with a Ni to Co mass ratio of 1:1.5 and a total loading of 12 wt%, and no auxiliary bias electric field is applied. The remaining steps and parameters are the same as in Example 1.
[0048] Comparative Example 5: (Using pure PET raw materials, without oxygen-containing components) The difference between this comparative example and Example 1 is that the oxygen-containing PET waste is replaced with pure PET bottle flakes without an oxygen-free barrier layer; the remaining steps and parameters are the same as in Example 1. This comparative example is used to illustrate the adaptability and necessity of the process of the present invention for oxygen-containing PET.
[0049] Test example: The products obtained from the above embodiments and comparative examples were subjected to the following tests and analyses: 1. PET conversion rate: After the reaction is completed, the solid residue is collected, dried and weighed, and the PET conversion rate is calculated according to the formula (raw material mass - residue mass) / raw material mass × 100%.
[0050] 2. Terephthalic acid yield: The solid product was dissolved in 0.1 mol / L NaOH solution, filtered, and the filtrate was acidified with HCl to precipitate terephthalic acid crystals. The crystals were dried and weighed, and the percentage of the actual recovered terephthalic acid mass to the theoretical maximum generated mass was calculated.
[0051] 3. Ethylene glycol yield: The ethylene glycol content of the liquid product was quantitatively analyzed by gas chromatography-FID (GC-FID), and the percentage of the actual recovered ethylene glycol mass to the theoretical maximum generated mass was calculated.
[0052] 4. Composition analysis of gaseous products: The gaseous products collected were analyzed by gas chromatography-TCD (GC-TCD) to determine the content of components such as H2, CO, CO2, and CH4, and the total volume fraction of CO and CO2 was calculated to evaluate the degree of oxidation side reactions.
[0053] 5. Catalyst oxygen vacancy concentration: The surface Ce of the catalyst recovered after the reaction was analyzed by X-ray photoelectron spectroscopy (XPS). 3+ / Ce 4+ By combining the ratio with the peak area of the oxygen vacancy characteristic signal at g=2.003 in the electron paramagnetic resonance (EPR) spectrum, the surface oxygen vacancy concentration is semi-quantitatively calculated.
[0054] The test results for each embodiment and comparative example are summarized in the table below:
[0055] As can be seen from the results in Table 1: Examples 1-5 all achieved high PET conversion rates (97.8%–99.5%), terephthalic acid yields (89.4%–94.1%), and ethylene glycol yields (84.9%–89.9%). The volume fraction of CO+CO2 in the tail gas was controlled below 10%, and a high oxygen vacancy concentration was maintained on the catalyst surface after the reaction.
[0056] Compared to Example 1, Comparative Example 1, lacking gas-phase reduction pretreatment, experienced severe oxidation side reactions due to active oxygen in the raw material during the supercritical stage, leading to a significant decrease in TPA and EG yields, a CO+CO2 content in the tail gas reaching 25.4%, and a reduced catalyst oxygen vacancy concentration. Comparative Example 2, employing wet subcritical water pre-reduction treatment, showed some improvement over Comparative Example 1, but due to partial hydrolysis of PET in subcritical water, the conversion rate and monomer yield remained significantly lower than in Example 1. Comparative Example 3, using a single supercritical condition without density zoning, resulted in interference between depolymerization and deep liquefaction processes, leading to a lower monomer yield than the examples with two-stage density control. Comparative Example 4, using a conventional supported catalyst without bias electric field assistance, experienced rapid annihilation of catalyst oxygen vacancies in an oxygen-containing environment, resulting in a significant decrease in activity. Comparative Example 5, using pure PET raw material, exhibited a lower intrinsic degree of oxidation side reactions, thus showing better performance than Example 1. However, this invention primarily targets the resource recovery of oxygen-containing PET waste; the results of Comparative Example 5 merely demonstrate the superior technical value of this invention in addressing oxygen-containing components.
[0057] Figure 3 A bar chart comparing the terephthalic acid yields of each example and comparative example. Figure 3 As can be seen, the TPA yields of Examples 1 to 5 all reached over 89%, with Example 3 achieving the highest TPA yield of 94.1% due to the synergistic effect of the introduction of gaseous formic acid and reducing agents; while Comparative Example 1, lacking a gas-phase reducing pretreatment step, had a TPA yield of only 68.3%, showing a significant difference from the Examples.
[0058] Figure 4 This is a bar chart comparing the volume fraction of CO+CO2 in the exhaust gas of each embodiment and the comparative example. Figure 4 It can be seen that the volume fraction of CO+CO2 in the exhaust gas of Examples 1 to 5 is less than 10%, with Example 3 having the lowest volume fraction of 5.2% due to the synergistic effect of formic acid; while Comparative Example 1, which did not undergo gas phase reduction treatment, has a volume fraction of CO+CO2 as high as 25.4%, and Comparative Examples 2 to 4 are also higher than the Examples. Figure 4 The results clearly demonstrate that the gas-phase reduced state pretreatment combined with the self-healing oxygen vacancy enrichment catalyst of the present invention can effectively suppress the excessive oxidation side reaction of oxygen-containing PET in the supercritical liquefaction process and reduce the generation of CO and CO2.
Claims
1. A supercritical hydrothermal liquefaction catalytic recovery process adapted to oxygen-containing PET, characterized in that, Includes the following steps: Step 1, Pretreatment: The oxygen-containing PET waste is crushed, washed, and dried to obtain oxygen-containing PET particles with a particle size of 0.5–5 mm; Step 2, gas-phase reduction pretreatment: The oxygen-containing PET particles obtained in Step 1 are placed in a fluidized or stirred reactor, and dry reduction treatment is carried out for 15 to 60 minutes at a temperature of 120 to 200°C and a pressure of 0.1 to 2 MPa under a mixed atmosphere containing hydrogen and inert gas to obtain surface-deoxidized inert PET material. Step 3, density-zoned supercritical hydrothermal liquefaction reaction: The inert PET material obtained in Step 2 is mixed with water at a mass ratio of 1:3 to 1:15, and a self-healing oxygen vacancy enrichment catalyst is added. The reaction is carried out under supercritical water conditions using a two-stage density control strategy. Depolymerization induction period: Control the reaction system temperature at 360-385℃ and pressure at 25-30 MPa, maintain the water density at 0.25-0.35 g / cm³, and hold for 5-15 minutes to utilize the high ion accumulation environment to initially break down the PET chains into oligomers; Deep liquefaction and decarboxylation period: Subsequently, the system pressure is isothermally depressurized or heated to a temperature of 390-420℃ and a pressure of 23-26MPa, maintaining a water density of 0.10-0.20 g / cm³, and holding for 5-15 minutes to enhance mass transfer and catalytic decarboxylation selectivity using low-density supercritical water. Step 4, Product Separation: After the reaction is completed, the product is cooled in stages and separated into gas, liquid and solid phases. Ethylene glycol is recovered from the aqueous phase and high-purity terephthalic acid is recovered from the solid phase.
2. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 1, characterized in that, The low-temperature plasma-assisted activation in step 2 is dielectric barrier discharge or microwave plasma with an input power density of 0.5–2 W / cm³, and the hydrogen volume concentration in the mixed atmosphere is 2–10 vol%, with the remainder being nitrogen or argon.
3. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 1, characterized in that, The dry reduction process described in step 2 also includes introducing a trace amount of gaseous formic acid as a hydrogen donor and oxygen capture synergist, with the amount of gaseous formic acid introduced being 0.1 to 1 vol of the total volume of the mixed atmosphere.
4. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 1, characterized in that, The self-healing oxygen vacancy enrichment catalyst described in step 3 has a core-shell structure, and its preparation method includes: Using magnetic Fe3O4 nanoparticles as the core and Ce-doped... 3+ / Ce 4+ The ZrO2-TiO2 composite oxide of variable-valence rare earth elements forms the shell, and Ni-Co bimetallic active components are loaded on the surface of the shell. The catalyst utilizes Ce in the supercritical hydrothermal liquefaction reaction. 3+ / Ce 4+ The redox cycle between the ion pair and the hydrogen gas and reducing intermediates generated in the reaction system dynamically maintains a surface oxygen vacancy concentration ≥2×10⁻⁶. 19 cm -3 ; An auxiliary bias electric field of 0.1–1.0 V is applied to the reactor wall to enhance the oxygen vacancy repair efficiency.
5. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 4, characterized in that, The catalyst has a Ni to Co mass ratio of 1:0.8 to 1:2.5, a shell thickness of 10 to 50 nm, and a specific surface area of 80 to 200 m². 2 / g.
6. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 4, characterized in that, The reaction system in step 3 also includes a reducing agent, which is formic acid or methanol, added at an amount of 0.5–3 wt% of water, to synergistically maintain the Ce-coated catalyst shell. 3+ / Ce 4+ The reduced state equilibrium.
7. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 1, characterized in that, In step 3, when switching from the depolymerization induction period to the deep liquefaction period, the pressure relief rate is 0.5 to 2 MPa / min to prevent the PET melt from foaming and carrying unreacted materials due to a sudden drop in density.
8. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 1, characterized in that, In step 2, the empty tower velocity of the mixed gas in the fluidized reactor is 0.05–0.5 m / s to ensure that the PET particles are suspended and tumbled in the plasma field and are uniformly deoxygenated.
9. The supercritical hydrothermal liquefaction catalytic recovery process for oxygen-containing PET according to claim 1, characterized in that, The catalyst magnetic recovery and activation regeneration steps are also included after step 4.
10. The core-shell magnetic catalyst in the reaction residue is separated and recovered by an external magnetic field. The recovered catalyst is then placed in a nitrogen-hydrogen mixed atmosphere containing 5 vol% H2 at 400-500℃ for 1-2 hours for reduction treatment to achieve lattice reconstruction and oxygen vacancy regeneration of the active components of the catalyst. A DC pulse electric field of 1.5–3.0 V is applied simultaneously during the reduction process to accelerate regeneration.