Liquid alloy catalyst for waste plastic depolymerization recovery, preparation method and application

Abstract

The invention discloses a liquid alloy catalyst for waste plastic depolymerization recovery, a preparation method and application. The liquid alloy catalyst is composed of an active metal, an adjusting metal and a solvent metal. The active metal and the adjusting metal form a heterogeneous electronic structure in the solvent metal, and the electron density of the active metal in the heterogeneous electronic structure is higher than that of the adjusting metal; namely, the active metal provides a catalytic site for the liquid alloy catalyst, the adjusting metal is used for adjusting and controlling the electronic structure of the active metal, and the solvent metal provides a liquid substrate for the liquid alloy catalyst; the active metal is rhodium, platinum, palladium, nickel, cobalt or iron; the adjusting metal is iridium, tin, zinc, copper, vanadium or manganese; the solvent metal is one or a mixture of two or more of gallium, indium, bismuth and lead. The liquid alloy catalyst for waste plastic depolymerization recovery is used for depolymerization recovery of waste polyolefin plastic, and harmless treatment and closed-loop recovery of polyolefin waste can be achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of harmless and resourceful treatment of waste plastics. Specifically, it is a liquid alloy catalyst for depolymerization and recycling of waste plastics, a preparation method and application thereof. BACKGROUND

[0002] In the field of synthetic polymer materials, plastics, as the largest industrial product in global consumption, have accumulated more than 10 billion tons of production. Due to the inherent characteristics of polymer materials, traditional plastic products have a long degradation period in the natural environment, which lasts for hundreds of years. This results in about 84% of waste plastics that cannot be effectively recycled, thereby forming a persistent ecological burden.

[0003] Existing polyolefin waste recycling technologies mainly include thermal cracking, hydrogenolysis, solvent sorting, biodegradation, and catalytic cracking processes. Among them, solvent sorting is a commonly used physical separation technology in polyolefin waste recycling. Its core is to use the "solubility or insolubility" characteristics of different plastics in specific solvents to achieve precise sorting of mixed plastics. Its advantage is that it can efficiently separate polyolefins with similar properties (such as HDPE and LDPE). However, it also has problems such as high solvent cost and high energy consumption. Catalytic cracking technology introduces solid catalysts such as zeolite molecular sieves, alumina carriers, and transition metal oxides to achieve controlled polymer chain scission at moderate temperatures of 200-500 ℃. Compared with conventional thermal cracking technology, catalytic cracking technology can significantly improve the selectivity of light olefins and aromatic hydrocarbons. It also has the advantages of wide reaction temperature range, low energy consumption, and direct use of products as petroleum chemical raw materials.

[0004] However, existing catalytic cracking technologies have the following technical defects: ① When treating plastic waste, the catalyst is prone to coking and carbon deposition, and the catalytic performance is not high, especially when treating a mixture of multiple plastics, the selectivity of the product monomer is poor; ② The preparation of the catalyst requires the use of noble metals (such as platinum Pt, palladium Pd, etc.) or rare earth elements, which have a high material cost proportion; ③ The catalyst recovery and regeneration after deactivation due to carbon deposition are difficult; ④ Traditional catalytic cracking processes require heating polyolefin raw materials to a molten state before contacting with the catalyst for reaction, resulting in a complicated process and high energy consumption.

[0005] Liquid alloy catalysts are a type of catalytic material with liquid metal alloy as the matrix. This type of catalyst is in a liquid state under reaction conditions, combining the fluidity of a liquid with the catalytic activity of a metal, and is one of the research hotspots in the field of catalysis in recent years. The core limitation of existing liquid alloy catalysts (such as pure Ga-In alloy, Ga-Sn alloy, Bi-In alloy, etc.) is the lack of interface electronic state regulation. The main performance is that the active sites of existing liquid alloy catalysts are mainly non-polar metal elemental sites (such as Ga 0 , Sn 0), the electronic state is single and uniform, and the polarization ability of the C−H bond in the polyolefin molecule is weak, usually relying on the synergistic effect of high temperature (> 300 ℃), high pressure (> 5 bar) or hydrogen co-reactant to activate the C−H bond; and the C−C bond β-scission process in the polyolefin molecule is random and has poor directional control ability. This leads to the fact that the monomer propylene (C3H6) space-time yield is generally less than 30 mmolC3H6·g cat ⁻ 1 ·h⁻ 1 , and the C3H6 monomer selectivity is less than 40%. In addition, the existing liquid alloy catalyst is easy to be poisoned by impurities (such as additives in plastics and chlorine elements) due to the unstable electronic state during the catalytic reaction, so its cycle stability is usually less than 15 hours.

[0006] Therefore, developing a new catalytic cracking system with high catalytic activity, anti-coking performance and recyclability has become a technical problem to be solved in the field of efficient depolymerization of polyolefins. SUMMARY

[0007] Therefore, the technical problem to be solved by the present application is to provide a liquid alloy catalyst for waste plastic depolymerization and recycling, a preparation method and application, to solve the technical problems of poor catalytic performance, anti-coking performance and cycle stability of the existing catalytic cracking technology in plastic depolymerization and recycling.

[0008] To solve the above technical problems, the present application provides the following technical solutions:

[0009] A liquid alloy catalyst for waste plastic depolymerization and recycling, which is composed of an active metal, a regulating metal and a solvent metal; the active metal and the regulating metal form a heterogeneous electronic structure in the solvent metal, and the electronic density of the active metal in the heterogeneous electronic structure is higher than that of the regulating metal; that is, the active metal provides catalytic sites for the liquid alloy catalyst, the regulating metal is used to regulate the electronic structure of the active metal, and the solvent metal provides a liquid matrix for the liquid alloy catalyst;

[0010] The active metal is rhodium (Rh), platinum (Pt), palladium (Pd), nickel (Ni), cobalt (Co) or iron (Fe); the regulating metal is iridium (Ir), tin (Sn), zinc (Zn), copper (Cu), vanadium (V) or manganese (Mn); and the solvent metal is one or two or more of gallium (Ga), indium (In), bismuth (Bi) and lead (Pb).

[0011] The sum of the mass of the active metal and the regulating metal accounts for 1 wt%~75 wt% of the total mass of the liquid alloy catalyst, and the mass ratio of the active metal to the regulating metal is 6: (1~12).

[0012] A preparation method of a liquid alloy catalyst for depolymerization recycling of waste plastics, comprising the following steps:

[0013] Step S1: respectively dissolve the precursor salts of the active metal and the adjusting metal into deionized water to obtain a metal precursor solution;

[0014] Step S2: add an acidic solution to the metal precursor solution and stir to mix uniformly to obtain an acidified precursor solution; add an alkaline solution dropwise to the acidified precursor solution to adjust the pH to alkaline, and then fully stir to perform a precipitation reaction; after the precipitation reaction is completed, centrifugal separation is performed on the reaction system; the solid product obtained by centrifugation is washed to neutral with an ethanol solution and then dispersed in an ethanol solution again to obtain a de-alkali cation dispersion; the washed solid product is a hydroxide precipitate containing an ethanol solution; if vacuum drying is directly performed, the hydroxide precipitate particles will be tightly combined through hydrogen bonds and Van der Waals forces to form large agglomerates, which will make it difficult to decompose into fine oxide particles during subsequent calcination; in the present application, the solid product is washed and then dispersed in an ethanol solution to form a uniform and stable de-alkali cation dispersion before vacuum drying, so that the hydroxide precipitate particles exist in a "monodispersed" or "weakly agglomerated" state during the drying process, thereby avoiding agglomeration during the drying process and obtaining loose solid powder, which is beneficial to decomposition into fine oxide particles after calcination, facilitates grinding, and is also beneficial to uniform mixing of the oxide precursor mixture and the solvent metal in the subsequent step;

[0015] Step S3: vacuum dry the de-alkali cation dispersion, and then calcine in an air atmosphere; after calcination is completed, a metal oxide precursor mixture is obtained;

[0016] Step S4: grind the metal oxide precursor mixture and mix it uniformly with a solvent metal powder to obtain a mixture powder; place the mixture powder in a hydrogen atmosphere and perform a heating reduction treatment; after the heating reduction treatment is completed, the above-mentioned liquid alloy catalyst for depolymerization recycling of waste plastics is obtained.

[0017] In the preparation method of the liquid alloy catalyst for depolymerization recycling of waste plastics, in step S1, the precursor salt of the active metal is nickel nitrate hexahydrate, palladium nitrate dihydrate, or rhodium trichloride trihydrate; the precursor salt of the adjusting metal is stannous chloride dihydrate, zinc nitrate hexahydrate, or vanadium trichloride; in the metal precursor solution, the mass concentration of the precursor salt of the active metal is (0.01~0.08) g mL⁻ 1 .

[0018] In the step S2, the acid solution is a concentrated nitric acid solution with a mass fraction of 65 wt%-68 wt% or a perchloric acid solution with a mass fraction of 60 wt%-65 wt%; the amount of the acid solution is controlled so that the molar ratio of hydrogen ions to active metal ions in the acidified precursor solution is 1:(0.3-1.0); the reason for adding a specific amount of concentrated nitric acid or perchloric acid solution before the co-precipitation with alkali solution is that the metal ions in the metal precursor solution will hydrolyze to form hydroxide precipitates or basic salt precipitates under neutral or weak alkaline conditions, and these precipitates may also wrap unsolved precursor salts or impurities during the formation process, thereby causing the purity of the hydroxide precipitates generated by the alkali precipitation to decrease; the addition of the acid solution can keep the metal ions in the metal precursor solution in a fully dissolved state before co-precipitation, and the addition of concentrated nitric acid or perchloric acid can also oxidize the impurities, so that the subsequent precipitate product obtained by adding alkali has higher purity. The acid radical ions left by the concentrated nitric acid or perchloric acid can be decomposed into gas and removed in the subsequent calcination process without being left in the metal oxide. In addition, the metal precursor solution is kept under acidic conditions for co-precipitation with alkali, and the added OH- is neutralized with H+ in the solution first, and the pH of the entire reaction system slowly increases, which can effectively avoid the precipitation of particles that are too large or agglomerated due to the excessively high local OH- concentration, thereby facilitating the formation of uniform and fine precipitates.

[0019] The basic solution is a sodium hydroxide solution with a mass fraction of 30 wt%-35 wt%; the amount of the basic solution is controlled so that the pH is adjusted to 10-11; the stirring time of the precipitation reaction is 2-4 h; the centrifugal separation conditions are: a rotation speed of 12000-16000 rpm and a time of 15-25 min; the ethanol solution is a mixture of anhydrous ethanol and water in a volume ratio of 1:1; and the mass fraction of the solid product in the alkali-cation dispersion solution is 5 wt%-15 wt%.

[0020] In the step S3, the vacuum drying conditions are: a drying temperature of 100-150 ℃ and a drying time of 8-15 h; and the calcination conditions are: a calcination temperature of 400-600 ℃ and a calcination time of 2-4 h. Under the calcination conditions, the active metal hydroxide and the adjusting metal hydroxide generated by co-precipitation can be completely dehydrated and converted into corresponding oxides.

[0021] In step S4 of the preparation method of the liquid alloy catalyst for depolymerization recycling of waste plastics, the particle size of the ground metal oxide precursor mixture is 5-30 μm. Grinding the metal oxide precursor mixture to this particle size range has a large specific surface area and is not prone to agglomeration during mixing, which is beneficial to the full mixing of the metal oxide precursor mixture and the solvent metal powder, reduces local composition unevenness caused by agglomeration, and has a large contact area with hydrogen during the subsequent heating reduction treatment reaction, so that the reaction is more complete, and the reduction reaction efficiency is improved. The solvent metal powder is one or a mixture of two or more of gallium powder, indium powder, bismuth powder, and lead powder, and the particle size of the solvent metal powder is 40-150 μm, which can ensure the full diffusion of hydrogen between the solvent metal particles and the gradual melting of the solvent metal to wrap the reduced active metal / adjusting metal during the heating process, forming a uniform liquid alloy. The powder with this particle size range has good filling and air permeability, and is suitable for programmed temperature reduction in a fixed bed or fluidized bed reactor to ensure the stability of the reaction system. The heating reduction treatment method is: first, the temperature is raised from room temperature to 450-550 ℃ at a rate of 5-10 ℃·min⁻ 1 , and then kept for 1-2 h, then raised to 700-800 ℃ at a rate of 10-15 ℃·min⁻ 1 , and then kept for 1-2 h, then reduced to 450-550 ℃ at a rate of 10-15 ℃·min⁻ 1 , and then kept for 1-4 h, and finally naturally cooled to room temperature and transferred to an inert gas atmosphere for storage. Under the preparation process conditions of the present application, the temperature control program of the mixture powder heating reduction treatment is scientifically designed to accurately control the surface structure and electronic distribution characteristics of the liquid alloy catalyst, form active site distribution and electron density conducive to the directional rupture of C−C bonds, and enable the liquid alloy catalyst to efficiently break C−C bonds during the polyolefin depolymerization reaction, thereby improving the reaction efficiency and monomer selectivity. Specifically:

[0022] The first stage (450-550 ℃ for 1-2 h): the reaction temperature of this stage is higher than the melting point of all solvent metals to ensure that the solvent metal is completely melted to provide a liquid diffusion medium for the active metal and the adjusting metal; however, the reaction temperature of this stage needs to be lower than the reduction temperature of the refractory metal oxide to avoid the aggregation problem caused by the premature reduction of the refractory metal. In this reaction stage, the easily reducible metal oxides (such as NiO and CoO) are first slowly reduced, and the generated metal elements can be preliminarily diffused into the molten solvent metal matrix, avoiding local aggregation caused by the simultaneous reduction of a large amount of metal at high temperature in the subsequent stage; if the temperature of this stage is raised too quickly or the reaction temperature is too high, the easily reducible metal will be reduced too quickly, forming large particles and affecting the uniformity of its dispersion;

[0023] The second stage (heating to 700-800 DEG C and keeping for 1-2 hours): this stage mainly reduces the difficultly-reducible metal oxides (such as Rh2O3, PtO2, IrO2 and the like, and the reduction temperature is relatively high), and ensures that all the metal oxides are completely converted into metal elements; at this temperature, the viscosity of the solvent metal (such as Ga and In) is significantly reduced, the atomic diffusion resistance is reduced, the atomic diffusion coefficient is significantly increased at high temperature (the diffusion rate is exponentially related to the temperature), the active metal (such as Rh and Pt) and the adjusting metal (such as Ir and Sn) can be effectively promoted to uniformly distribute in the molten solvent metal, the local concentration gradient possibly existing in the first stage is broken, and a more uniform alloy phase is formed;

[0024] The third stage (cooling to 450-550 DEG C and keeping for 1-4 hours): at this stage, the solvent metal is still in a molten state (the melting point is far lower than 450 DEG C), but the temperature is reduced, so that the atomic diffusion rate is slowed down, and the grain coarsening caused by excessive diffusion at high temperature can be avoided; the slow diffusion process can further eliminate the local component unevenness, so that the active metal and the adjusting metal form a stable dispersed state in the solvent metal matrix.

[0025] Under the preparation process conditions of the present application, by reasonably controlling the temperature variation rate, reaction temperature and holding time and the like of each stage of the heating reduction treatment, the uniform diffusion of the active metal and the adjusting metal in the solvent metal matrix can be effectively promoted, the introduced adjusting metal can effectively reduce the electron transfer energy barrier, the dispersion stability of the active sites of the active metal can be maintained, and the liquid alloy catalyst prepared has uniform composition and stable performance.

[0026] An application of a liquid alloy catalyst for depolymerization and recycling of waste plastics, the liquid alloy catalyst for depolymerization and recycling of waste plastics is used for depolymerization and recycling of waste polyolefin plastics.

[0027] The application of the liquid alloy catalyst for depolymerization and recycling of waste plastics, the liquid alloy catalyst is used for depolymerization and recycling of waste polyolefin plastics through a polyolefin depolymerization and recycling system; the polyolefin depolymerization and recycling system comprises a depolymerization reactor, an electrified heating device, a depolymerization product recovery device, a depolymerization product separation device, a temperature monitor and a raw material adding device; the depolymerization reactor has a cavity structure, a reaction zone is arranged in the cavity structure, and the waste polyolefin plastics and the liquid alloy catalyst are subjected to a depolymerization reaction in the reaction zone.

[0028] The discharge port of the raw material adding device is communicated with the feed port of the depolymerization reactor, the discharge port of the depolymerization reactor is communicated with the feed port of the depolymerization product separation device, and the discharge port of the depolymerization product separation device is communicated with the feed port of the depolymerization product recovery device; the electrified heating device provides heat for the depolymerization reaction of the waste polyolefin plastic and the liquid alloy catalyst in the reaction zone; the temperature detection point of the temperature monitor is arranged in the reaction zone; and the temperature signal output end of the temperature monitor is electrically connected with the temperature signal input end of the electrified heating device.

[0029] The electrified heating device is an electromagnetic induction heater, the induction coil of the electromagnetic induction heater surrounds the periphery of the reaction zone; the electrified heating device is further provided with a water cooling device, and the water cooling device exchanges heat with the electrified heating device; and the feed port of the depolymerization reactor is located on the cavity side wall or the top of the reaction zone top section of the depolymerization reactor.

[0030] The application of the liquid alloy catalyst for waste plastic depolymerization recovery, the waste polyolefin plastic is pure polyolefin plastic, polyolefin plastic mixed with polyvinyl chloride or polyolefin plastic added with a modified additive; the polyolefin plastic is one or a mixture of two or more of high-density polyethylene plastic, low-density polyethylene plastic, polypropylene plastic or polystyrene plastic; and the modified additive includes ethylene-butene copolymer, styrene-butadiene-styrene block copolymer and glass fiber.

[0031] The technical scheme of the application achieves the following beneficial technical effects:

[0032] 1、The liquid alloy catalyst prepared by the application is liquid at the depolymerization reaction temperature of the waste plastic, has high atomic fluidity, can provide a large number of uniformly distributed catalytic active sites, realizes efficient adsorption and activation of the depolymerization reaction substrate, has ideal catalytic activity and monomer selectivity; in addition, the surface atoms of the liquid metal alloy catalyst have high fluidity during the catalytic reaction process, can spontaneously migrate and fill surface defects, thereby avoiding poisoning or sintering of the catalytic active sites, significantly improving the stability and service life of the catalyst; at the same time, the liquid alloy catalyst has excellent corrosion resistance and can stably operate under harsh reaction conditions; in addition, the low surface energy property of the liquid metal alloy makes it difficult to adsorb amorphous carbon, coke-like substances and the like generated during the carbon reaction process, thereby effectively reducing the generation of carbon deposition, reducing the deactivation probability of the catalyst and improving the stability of the catalytic performance of the catalyst; furthermore, the liquid alloy catalyst and the waste plastic to be treated can form a clear solid-liquid two-phase system during the depolymerization reaction process, and the liquid alloy catalyst can be efficiently separated and recovered after the depolymerization reaction is completed.

[0033] 2, The liquid alloy catalyst for waste plastic depolymerization recovery has excellent electric heat conduction performance, and can realize rapid heat response and efficient heat management of the reaction system. The liquid alloy catalyst prepared by the application is used for depolymerization and recovery of polyolefin waste by using a matching polyolefin depolymerization and recovery system, without melting pretreatment. Local hot spots are formed in the contact area of solid polyolefin waste and liquid alloy catalyst by high-frequency electromagnetic field, an electrified reaction interface based on eddy current effect is constructed, the liquid alloy catalyst is in an excited state, and the C-C bond activation energy barrier is precisely controlled, the polymer chain is broken in a specific direction, the chain termination reaction and free radical intermediate coupling are inhibited, so that the selectivity of depolymerization monomer is significantly improved, and the energy consumption is greatly reduced.

[0034] 3, The liquid alloy catalyst NiSn-GaIn prepared by the method of the application solves the electronic state defects of the existing liquid alloy catalyst through precise interface electronic control strategy, which is specifically shown in the following two aspects: on the one hand: in NiSn-GaIn, Ni and Sn form a hetero-electronic structure (Ni δ ⁻-Sn δ+ ) in the solvent metal GaIn, the electronic density of Ni is higher than that of Sn, the rich electron characteristics of Ni δ ⁻ can efficiently activate the C-H bond of polyolefin and accelerate the C-H bond breaking; at the same time, the electron-deficient characteristics of Sn δ+ can adsorb the C-C bond breaking intermediate in a specific direction, promote the beta-breaking path (inhibit the generation of by-products caused by alpha-breaking), so as to significantly improve the catalytic reaction efficiency and directional catalytic ability of NiSn-GaIn; on the other hand: NiSn-GaIn can utilize the high surface entropy characteristics of GaIn matrix to form a "dynamic stabilization effect" on the electronic state of Ni δ ⁻-Sn δ+ active site, which can not only effectively inhibit the strong electronic combination of impurities (such as phthalate additives and chlorinated hydrocarbons in plastics) and active sites, reduce the poisoning of catalytic active sites, but also repair the electronic state distortion caused by the adsorption of trace impurities by dynamically adjusting the interface electron cloud, so as to maintain the stability of the electronic state of the catalytic active site.

[0035] 4. The preparation method of the liquid alloy catalyst for waste plastic depolymerization recovery, first, the corresponding precursor salt of the active metal and the adjusting metal is dissolved in deionized water in a certain proportion and an acidic solution is added, then co-precipitation is carried out by adding alkali, which can avoid composition segregation caused by simple physical mixing; the metal hydroxide obtained by co-precipitation is dispersed into an ethanol solution again and then vacuum dried, then the mixture of the active metal hydroxide and the adjusting metal hydroxide obtained is calcined to obtain a metal oxide precursor mixture, finally, the metal oxide precursor mixture is mixed with a solvent metal, and then two-stage heating and one-stage cooling reduction treatment are carried out to obtain the liquid alloy catalyst, in which the individual metals in the liquid alloy catalyst are not simply melted to form a physical mixture, realizing selective and strong electronic interaction between the active metal and the adjusting metal in the liquid or molten state matrix of the solvent metal through interface electronic regulation and control, so that the electron density of the active metal is higher than that of the adjusting metal, that is, the active metal presents an electron-rich state, thereby the active metal and the adjusting metal form a heterogeneous electronic structure (such as Ni δ ⁻-Sn δ+ ) in the solvent metal, the strong interaction between the metals and the electronic transfer caused thereby bring multiple key effects:

[0036] (1) Precise electronic structure regulation: Unlike simple mixed alloys, the liquid alloy catalyst prepared by the present application has a specific atomic-level coordination environment, which significantly changes the electron density of the active metal surface and effectively reduces the d-band center. This change optimizes the adsorption strength and activation energy barrier of the active metal catalytic active site to the C−C bond in the polyolefin molecule, and the electron-deficient characteristics of the adjusting metal can directionally adsorb the C−C bond breaking intermediates, promoting the β-breaking path (inhibiting the generation of by-products caused by α-breaking), laying a foundation for efficient C−C bond breaking and directional generation of target monomers (such as C3H6).

[0037] (2) Structure dispersion and anchoring: The strong interaction between the adjusting metal and the active metal enables the active sites of the active metal to be firmly "anchored" on the interface of the liquid matrix formed by the solvent metal. This "electronic-structure synergistic effect" effectively inhibits the bulk diffusion, surface encapsulation, and particle agglomeration and sintering of the active metal under high-temperature reaction conditions, thereby maintaining the high dispersion and long-term stability of the active sites of the active metal.

[0038] (3) Dynamic stable liquid carrier environment: The solvent metal provides a dynamic stable "liquid carrier" for the highly dispersed active metal-adjusting metal composed active component due to its unique electronic properties, excellent fluidity, and high thermal and electrical conductivity. This not only facilitates rapid heat transfer and efficient electron transfer during the reaction process, ensuring efficient operation of the catalytic cycle, but also provides convenience for subsequent separation of the catalyst and the product.

[0039] (4) The liquid alloy catalyst prepared by the method of the present invention has its active metal and regulating metal anchored in the liquid solvent metal matrix in a highly dispersed form, and the surface structure and electronic properties of the liquid alloy catalyst are precisely controlled to form a distribution of catalytic active sites and electron density that are conducive to the directional breaking of C−C bonds. This enables the liquid alloy catalyst to achieve efficient and directional breaking of C−C bonds in the polyolefin depolymerization process, significantly improving the catalytic activity, anti-coking properties, cycle stability, catalytic reaction efficiency and monomer selectivity of the liquid alloy catalyst. Attached Figure Description

[0040] Figure 1 A schematic diagram of the polyolefin waste depolymerization and recycling system used in Embodiment 4 of the present invention;

[0041] Figure 2 A schematic diagram of the electromagnetic heating of the reaction zone in the polyolefin waste depolymerization and recycling system used in Example 4 of this invention;

[0042] Figure 3 HAADF-STEM and corresponding EDS mapping images of the liquid alloy catalyst NiSn-GaIn prepared in Example 1 of this invention;

[0043] Figure 4a and Figure 4b The images show the In-situ XPS spectra of the liquid alloy catalyst NiSn-GaIn prepared in Example 1 of this invention at 25 °C and 500 °C, respectively.

[0044] Figure 5a and Figure 5b The images show the K-edge X-ray absorption near-edge structure spectra (K-edge XANES spectra) of Ni and Sn, respectively, of the liquid alloy catalyst NiSn-GaIn prepared in Example 1 of this invention and the reference sample.

[0045] Figure 6 A comparison chart of the conversion rate and selectivity of gaseous hydrocarbons in Example 4 of this invention and the recovery of waste polypropylene plastic using other reference samples;

[0046] Figure 7 A comparison chart of the depolymerization rate, C3H6 monomer selectivity, and C3H6 space-time yield of waste polypropylene plastics recovered by other depolymerization / recycling processes in Example 4 of this invention;

[0047] Figure 8 A comparison chart of the conversion rate of NiSn-GaIn liquid alloy catalyst prepared in Example 1 for the recovery of mixed polyolefin plastics and the selectivity of gaseous hydrocarbons in this invention embodiment;

[0048] Figure 9A comparison chart of conversion rate of different types of commercial polyolefin plastic products after consumption and selectivity of gaseous hydrocarbons recovered by the liquid alloy catalyst NiSn-GaIn prepared in Example 1 is shown in the embodiment of the present application.

[0049] Figure 10 A distribution chart of solid phase and liquid phase residues after demixing of commercial plastic by the liquid alloy catalyst NiSn-GaIn prepared in Example 1 is shown in the embodiment of the present application.

[0050] The reference signs in the figure are as follows: 1-depolymerization reactor; 2-electrified heating device; 3-depolymerization product recovery device; 4-top section of reaction zone; 5-reaction zone; 6-depolymerization product separation device; 7-water cooling device; 8-temperature monitor; 9-raw material adding device; 10-induction coil. DETAILED DESCRIPTION

[0051] Example 1

[0052] In this embodiment, the preparation method of the liquid alloy catalyst for depolymerization and recovery of waste plastics comprises the following steps:

[0053] Step S1: 2.973 g of Ni(NO3)2·6H2O and 0.760 g of SnCl2·2H2O were placed in 80 mL of deionized water, and ultrasonic treatment was performed for 30 min to completely dissolve them, obtaining a metal precursor solution;

[0054] Step S2: A concentrated nitric acid solution with a mass fraction of 68 wt% was added to the above metal precursor solution and stirred and mixed uniformly, obtaining an acidified precursor solution, and the molar ratio of hydrogen ions to active metal ions in the acidified precursor solution was 1:0.6; A 32 wt% sodium hydroxide solution was added dropwise to the acidified precursor solution to adjust the pH to 10.5, and then magnetic stirring was performed for 2 h to fully perform the precipitation reaction; After the precipitation reaction was completed, the reaction system was centrifuged at a speed of 14000 rpm for 18 min; The solid product obtained by centrifugation was washed with an ethanol solution (a mixture of anhydrous ethanol and water in a volume ratio of 1:1) for 3 times and then redispersed in an ethanol solution, obtaining an alkali cation-free dispersion liquid; The mass fraction of the solid product in the alkali cation-free dispersion liquid was 15 wt%;

[0055] Step S3: The alkali cation-free dispersion liquid was vacuum dried at 120 ℃ for 12 h, and then calcined in an air atmosphere, and the calcination conditions were: calcination temperature 500 ℃, calcination time 3 h; After calcination was completed, a metal oxide precursor mixture was obtained;

[0056] Step S4: The metal oxide precursor mixture was ground to a particle size of 10-30 μm, and then mixed uniformly with 3.0 g of indium powder (particle size of 75-150 μm) and 6.0 g of gallium powder (particle size of 50-100 μm) to obtain a mixture powder; the mixture powder was placed in a reactor in a hydrogen atmosphere and subjected to a heating reduction treatment according to the following procedure: first, the temperature was raised from room temperature to 500 ℃ at a temperature increase rate of 10 ℃·min⁻ 1 , and held for 1.5 h, then the temperature was raised to 800 ℃ at a temperature increase rate of 12.5 ℃·min⁻ 1 , and held for 1.5 h, then the temperature was lowered to 500 ℃ at a temperature decrease rate of 12.5 ℃·min⁻ 1 , and held for 2.5 h to perform a reduction reaction, and then naturally cooled to room temperature to obtain the liquid alloy catalyst; finally, the prepared liquid alloy catalyst was transferred to an inert gas atmosphere for storage.

[0057] The liquid alloy catalyst prepared in this example for depolymerization and recycling of waste plastics is denoted as NiSn-GaIn; wherein the active metal is Ni, the adjusting metal is Sn, the solvent metal is Ga and In, and the mass ratio of Ni, Sn, Ga and In is 6:4:60:30.

[0058] Example 2

[0059] In this example, the preparation method of the liquid alloy catalyst for depolymerization and recycling of waste plastics comprises the following steps:

[0060] Step S1: 6.260 g of Pd(NO3)2·2H2O and 22.753 g of Zn(NO3)2·6H2O were placed in 380 mL of deionized water, and ultrasonic treatment was performed for 30 min to completely dissolve them, to obtain a metal precursor solution;

[0061] Step S2: a concentrated nitric acid solution with a mass fraction of 68 wt% was added to the above metal precursor solution and stirred to mix uniformly, to obtain an acidified precursor solution, and the molar ratio of hydrogen ions to active metal ions in the acidified precursor solution was 1:0.3; a sodium hydroxide solution with a mass fraction of 32 wt% was added dropwise to the acidified precursor solution to adjust the pH to 10, and then magnetic stirring was performed for 3 h to fully perform a precipitation reaction; after the precipitation reaction was completed, the reaction system was centrifuged at a speed of 12000 rpm for 20 min; the solid product obtained by centrifugation was washed with an ethanol solution (a mixture of anhydrous ethanol and water in a volume ratio of 1:1) for 3 times and then redispersed in an ethanol solution to obtain an alkali cation-free dispersion liquid; the mass fraction of the solid product in the alkali cation-free dispersion liquid was 10 wt%;

[0062] Step S3: The alkali-free cation dispersion liquid is vacuum dried at 100 ℃ for 15 h, and then calcined in an air atmosphere, with a calcination temperature of 400 ℃ and a calcination time of 4 h; after calcination, a metal oxide precursor mixture is obtained;

[0063] Step S4: The metal oxide precursor mixture is ground to a particle size of 10-30 μm, and then uniformly mixed with 2.5 g of lead powder (particle size of 45-100 μm) to obtain a mixture powder; the mixture powder is placed in a reactor in a hydrogen atmosphere and subjected to a programmed heating reduction treatment as follows: first, the temperature is raised from room temperature to 450 ℃ at a rate of 8 ℃·min⁻ 1 , and held for 2 h, then the temperature is raised to 700 ℃ at a rate of 10 ℃·min⁻ 1 , and held for 2 h, then the temperature is lowered to 450 ℃ at a rate of 10 ℃·min⁻ 1 , and held for 4 h for reduction reaction, and then naturally cooled to room temperature to obtain a liquid alloy catalyst; finally, the prepared liquid alloy catalyst is transferred to an inert gas atmosphere for storage.

[0064] The liquid alloy catalyst prepared in this example for depolymerization and recycling of waste plastics is denoted as PdZn-Pb; wherein the active metal is Pd, the adjusting metal is Zn, and the solvent metal is Pb, and the mass ratio of Pd, Zn and Pb is 25:50:25.

[0065] Example 3

[0066] In this example, the preparation method of the liquid alloy catalyst for depolymerization and recycling of waste plastics comprises the following steps:

[0067] Step S1: 0.440 g of RhCl3·3H2O and 0.086 g of VCl3 are placed in 15 mL of deionized water, and ultrasonic treatment is performed for 30 min to completely dissolve them to obtain a metal precursor solution;

[0068] Step S2: adding a concentrated nitric acid solution with a mass fraction of 68 wt% into the above metal precursor solution and stirring to mix uniformly, to obtain an acidified precursor solution, the molar ratio of hydrogen ions to active metal ions in the acidified precursor solution being 1:1; adding a sodium hydroxide solution with a mass fraction of 32 wt% dropwise into the acidified precursor solution to adjust the pH to 11, and then performing a magnetic stirring for 4 h to fully perform a precipitation reaction; after the precipitation reaction is completed, centrifugally separating the reaction system at a rotating speed of 16000 rpm for 15 min; washing the solid product obtained by centrifugation with an ethanol solution (a mixture of anhydrous ethanol and water in a volume ratio of 1:1) for 3 times and then re-dispersing the solid product in the ethanol solution, to obtain an alkali-cation-free dispersion liquid; the mass fraction of the solid product in the alkali-cation-free dispersion liquid being 5 wt%;

[0069] Step S3: vacuum drying the alkali-cation-free dispersion liquid at 150 ℃ for 8 h, and then calcining in an air atmosphere, the calcination conditions being: a calcination temperature of 600 ℃ and a calcination time of 2 h; after the calcination is completed, obtaining a metal oxide precursor mixture;

[0070] Step S4: grinding the metal oxide precursor mixture to a particle size of 10-30 μm, and then uniformly mixing the metal oxide precursor mixture with 19.8 g of bismuth powder (a particle size of 50-100 μm), to obtain a mixture powder; placing the mixture powder in a reactor in a hydrogen atmosphere and performing a heating reduction treatment according to the following procedure: first, increasing the temperature from room temperature to 550 ℃ at a temperature increasing rate of 5 ℃·min⁻ 1 , and then maintaining the temperature for 1 h; subsequently, increasing the temperature to 750 ℃ at a temperature increasing rate of 15 ℃·min⁻ 1 , and then maintaining the temperature for 1 h; then, decreasing the temperature to 450 ℃ at a temperature decreasing rate of 15 ℃·min⁻ 1 , and then maintaining the temperature for 2 h to perform a reduction reaction, and then naturally cooling to room temperature, to prepare a liquid alloy catalyst; finally, transferring the prepared liquid alloy catalyst to an inert gas atmosphere for storage for standby use.

[0071] The liquid alloy catalyst prepared in this example for depolymerization recovery of waste plastics is denoted as RhV-Bi; wherein, the active metal is Rh, the adjustment metal is V, and the solvent metal is Bi, and the mass ratio of Rh, V and Bi is 0.86:0.14:99.

[0072] The selection of the types of the active metal, the adjustment metal and the solvent metal and the ratio thereof for preparing the liquid alloy catalyst in Example 1 to Example 3 are shown in Table 1.

[0073] Table 1: Selection of metal types and ratio for preparing the liquid alloy catalyst in different examples

[0074]

[0075] Example 4

[0076] This example uses the liquid alloy catalyst (NiSn-GaIn) prepared in Example 1 in a polyolefin depolymerization recycling system for depolymerization recycling of waste plastics. The waste plastics are polyolefin waste, i.e. waste polypropylene plastics (PP plastics).

[0077] As shown in Figure 1 and Figure 2 , the polyolefin depolymerization recycling system used in this example includes a depolymerization reactor 1, an electrified heating device 2, a depolymerization product recovery device 3, a depolymerization product separation device 6, a temperature monitor 8, and a raw material adding device 9; the depolymerization reactor 1 has a cavity structure, and a reaction zone 5 is arranged in the cavity structure, in which the depolymerization reaction of waste plastics and liquid alloy catalyst is carried out;

[0078] The discharge port of the raw material adding device 9 is communicated with the feed port of the depolymerization reactor 1, the discharge port of the depolymerization reactor 1 is communicated with the feed port of the depolymerization product separation device 6, and the discharge port of the depolymerization product separation device 6 is communicated with the feed port of the depolymerization product recovery device 3; the electrified heating device 2 provides heat for the depolymerization reaction of waste plastics and liquid alloy catalyst in the reaction zone 5; the temperature detection point of the temperature monitor 8 is arranged in the reaction zone 5; the temperature signal output end of the temperature monitor 8 is electrically connected with the temperature signal input end of the electrified heating device 2, so as to ensure that the electrified heating device can maintain the set reaction temperature; the raw material adding device is used for adding polyolefin waste, liquid alloy catalyst or mixture of the two, and the raw material adding device can dynamically match the thermodynamic state in the depolymerization reactor by real-time regulation of the feed rate, so as to realize continuous operation of the system at a low manual intervention rate and reduce the equipment loss rate. In the process of collecting depolymerization products, according to the priority of the reaction depolymerization monomers, the depolymerization product separation device and the depolymerization product recovery device are used to separate and purify the depolymerization products, so as to realize efficient collection of the depolymerization products, and further realize harmless treatment and high-value utilization of polyolefin waste.

[0079] The electrified heating device 2 is an electromagnetic induction heater, and the induction coil 10 of the electromagnetic induction heater surrounds the periphery of the reaction zone 5; the electrified heating device drives the charge carriers to form local hot spots at the contact interface of polyolefin waste and liquid alloy catalyst through a pulsed electric field, so that the liquid alloy catalyst is in an excited state, which can effectively activate the C−H bond of the polyolefin waste molecular chain, directionally cut the C−C bond, inhibit the occurrence of side reactions, and significantly improve the selectivity of the target product. The electrified heating device 2 is also provided with a water cooling device 7, and heat exchange is carried out between the water cooling device 7 and the electrified heating device 2, so as to prevent the electrified heating device from being overloaded at high temperature and ensure its safe and stable operation; the feed port of the depolymerization reactor 1 is located on the cavity side wall above the reaction zone top section 4 of the depolymerization reactor 1.

[0080] In this embodiment, the depolymerization reactor 1 is a tubular reactor (vacuum heat pipe with an outer diameter of 34 mm, an inner diameter of 30 mm, a wall thickness of 2 mm, and a pipe length of 600 mm, has heat preservation effect, hard texture is not easy to damage, and the whole body is transparent to facilitate observation of the state of the reaction), and the protective gas generating device matched with the depolymerization reactor 1 is a nitrogen cylinder. The electromagnetic induction heater is an Ambrell EASYHEAT 0224 type induction heater (maximum power is 2.4 kW, frequency is 150-400 kHz). The water cooling device 7 is an Ambrell COOLING-SYSTEMS type water cooling system (flow rate is 2.8 L·min⁻ 1 , maximum input pressure is 5.6 bar, and the highest water temperature is 35 ℃), and the temperature monitor 8 selects a SCIT-2M2 type separated infrared temperature detector, and the temperature detection point is arranged at the contact surface of the polyolefin waste and the liquid alloy catalyst in the reaction zone, so as to accurately monitor the dynamic temperature of the reaction interface.

[0081] In other embodiments, a supporting device can also be installed in the reaction zone 5 for supporting the polyolefin waste and the liquid alloy catalyst to prevent corrosion or damage of the equipment by the liquid alloy catalyst. The supporting device can be made of aluminum silicate cotton material. The aluminum silicate cotton is a light-weight refractory fiber material made of SiO2 (48 wt%-52 wt%), Al2O3 (43 wt%-49 wt%), Fe2O3 (0.9 wt%-0.13 wt%), CaO (<1 wt%), and MgO (<1 wt%), which has the characteristics of high temperature resistance, fire resistance, corrosion resistance, gas permeability, and solid-liquid barrier, so as to meet the functional requirements of the supporting device for carrying the polyolefin waste and the liquid alloy catalyst.

[0082] The working process of the polyolefin depolymerization and recycling system in this embodiment is as follows: the liquid alloy catalyst prepared in Example 1 is added to the reaction zone 5, and inert gas is used to remove air in the polyolefin depolymerization and recycling system; the electrified heating device 2 is turned on, the heating temperature is set in the range of 450-550 ℃ (in other embodiments, the setting of the heating temperature should ensure that the liquid alloy catalyst is completely melted), and the water cooling device 7 is turned on to cool the electrified heating device 2; the raw material adding device 9 is opened to add the polyolefin waste to be treated, which forms a contact surface with the pre-added liquid alloy catalyst and performs a depolymerization reaction; the temperature detection point of the temperature monitor 8 is arranged on the contact surface; the gaseous product after depolymerization is separated and purified by the depolymerization product separation device 6; and finally, the target product is collected by the depolymerization product recovery device 3.

[0083] The embodiment also provides a detection system including a gas chromatograph and a mass spectrometer for analyzing the gas composition and proportion at the outlet of the depolymerization reactor 1, collecting data into a computer for unified collection and processing, so as to calculate the conversion rate of the polyolefin waste, the space-time yield of the polymer monomer (such as ethylene and propylene), and other experimental data.

[0084] Example 5

[0085] The embodiment differs from example 4 only in that the liquid alloy catalyst used is different, that is, the liquid alloy catalyst PdZn-Pb used in the embodiment is prepared in example 2; other raw materials, equipment and process conditions are all the same as in example 4.

[0086] Example 6

[0087] The embodiment differs from example 4 only in that the liquid alloy catalyst used is different, that is, the liquid alloy catalyst RhV-Bi used in the embodiment is prepared in example 3; other raw materials, equipment and process conditions are all the same as in example 4.

[0088] The depolymerization effects of examples 4 to 6 on PP plastic are compared and evaluated as follows.

[0089] Table 2 is the determination results of the depolymerization conversion rate, propylene (C3H6) monomer selectivity and propylene space-time yield of examples 4 to 6 on PP plastic.

[0090] Table 2 depolymerization effects of different examples on waste polypropylene plastic

[0091]

[0092] As shown in table 2, the depolymerization conversion rates of examples 4 to 6 on PP plastic can all reach a high level of more than 95 %, the C3H6 space-time yield reaches a high level of 138.71~141.48 mmol C3H6·g cat ⁻ 1 ·h⁻ 1 , and the C3H6 monomer selectivity and C3H6 space-time yield of example 4 are both higher than those of examples 5 and 6. This shows that the overall depolymerization and recycling performance of the liquid alloy catalyst NiSn-GaIn on polypropylene plastic is better than that of the liquid alloy catalyst PdZn-Pb and RhV-Bi. Therefore, the micro morphology and chemical structure of the liquid alloy catalyst NiSn-GaIn and its depolymerization and catalytic performance on polyolefin waste are further studied and analyzed.

[0093] (1) Transmission electron microscopy (TEM) analysis of NiSn-GaIn

[0094] As Figure 3As shown by fine characterization analysis by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the results show that the liquid alloy catalyst NiSn-GaIn still maintains a liquid phase state at room temperature, and no identifiable metal or metal oxide nanoparticle phase is observed in its microstructure. In addition, Figure 3 The elemental distribution analysis results of energy dispersive X-ray spectroscopy (EDS) in FIG. 8 show that the active metal Ni and the adjustment metal Sn are highly uniformly dispersed in the matrix composed of solvent metals Ga and In, which reflects the reasonable embedding of Ni and Sn as active metal and adjustment metal in structure, and also fully proves the successful preparation of the liquid alloy catalyst NiSn-GaIn.

[0095] (2) X-ray photoelectron spectroscopy (XPS spectrum) analysis of NiSn-GaIn and reference samples

[0096] Figure 4a and Figure 4b The reference sample Ni-GaIn in FIG. 9 is prepared by the same method as in Example 1, and the mass ratio of Ga, In and Ni in Ni-GaIn is 6:3:1. As shown in Figure 4a and Figure 4b When the temperature is increased to 500 ℃, the Ni 2p peak in NiSn-GaIn moves to a lower binding energy, and the Sn 3d peak moves to a higher binding energy. This indicates that the electron donor ability of Sn is significantly enhanced with the increase of temperature, forming a Sn δ+ to Ni δ - heterojunction electron transfer channel. In contrast, the Ni 2p peak in Ni-GaIn cannot be observed at a high temperature of 500 ℃, indicating that its electron transfer is limited without Sn regulation. This shows that the doping of Sn significantly reduces the electron transfer energy barrier and promotes the directional transport of interface charges.

[0097] In addition, although the high temperature increases the mobility of Ga and In, the strong interaction between Sn and Ni anchors the Ni δ - in the Ga-In liquid matrix interface, thereby effectively inhibiting the bulk diffusion or surface encapsulation of Ni. This “electronic-structural synergistic effect” not only maintains the dispersion stability of the Ni active site, but also optimizes the C−C bond dissociation energy barrier by regulating the d-band center position, providing theoretical confirmation at the electronic structure level for the improvement of the catalytic performance of NiSn-GaIn.

[0098] (3) X-ray absorption near-edge structure spectrum (XANES spectrum) analysis of NiSn-GaIn and reference samples

[0099] According to Figure 5a and Figure 5bThe XANES spectrum of NiSn-GaIn can be seen that, compared with Ni foil (i.e. Ni foil in the figure), the Ni K-edg position of NiSn-GaIn obviously moves to the negative photon energy direction, while the Ni K-edd position of Ni-GaIn is close to that of Ni foil, indicating that Ni is in a negative valence state in NiSn-GaIn, and close to zero valence state in Ni-GaIn Figure 5a The reference sample Ni-GaIn is prepared by the same method as Example 1, and the mass ratio of Ga, In and Ni in Ni-GaIn is 6:3:1). The Sn K-edg of SnNi-GaIn is between Sn foil and SnO2, indicating that the electron density is reduced, and Sn in SnNi-GaIn exists in a positive valence state (0<δ<+4), as shown in Figure 5b It can be seen that the electronic state of Ni and Sn obtained by XANES analysis is consistent with the in-situ XPS test result, indicating that there is a Ni-Sn metal bond in NiSn-GaIn, and the electrons are transferred from Sn to Ni δ ⁻.

[0100] The hetero-electronic structure of Ni δ ⁻-Sn δ+ in NiSn-GaIn and the metastable characteristics of Sn valence state are verified by XANES analysis, which shows that the liquid alloy catalyst SnNi-GaIn prepared in Example 1 realizes precise regulation of the electronic state, and is directly related to the C-C bond activation ability of the catalyst. The charge transfer effect introduced by Sn doping can effectively reduce the d-band center position of Ni in SnNi-GaIn, optimize the adsorption strength of the reaction intermediate, and thus improve its selectivity for polyolefin depolymerization.

[0101] (4) Comparison of conversion rate of recycled waste polypropylene plastic and selectivity of gaseous carbon hydrocarbons of NiSn-GaIn and reference samples

[0102] Figure 6 is a comparison chart of the conversion rate of recycled waste polypropylene plastic and the selectivity of gaseous carbon hydrocarbons of Example 4 and other reference samples; wherein, GaIn, Sn-GaIn and Ni-GaIn are prepared by the same method as the preparation of NiSn-GaIn in Example 1; and the mass ratio of Ga and In in GaIn is 2:1, the mass ratio of Ga, In and Sn in Sn-GaIn is 6:3:1, and the mass ratio of Ga, In and Ni in Ni-GaIn is 6:3:1. The polyolefin waste depolymerization recycling system and recycling process conditions for recycling waste polypropylene plastic by using GaIn, Sn-GaIn and Ni-GaIn are the same as those in Example 4. From Figure 6As can be seen from Table 2, the C3H6monomer selectivity of the NiSn-GaIn liquid alloy catalyst for recovering waste polypropylene plastic is 64.98 mol%, which is increased by 24.41% and 55.57% respectively than the C3H6monomer selectivity of the Ni-GaIn and GaIn liquid alloy catalysts for recovering waste polypropylene plastic. In addition, the conversion rate of waste polypropylene plastic and the C3H6monomer selectivity of the liquid alloy catalyst NiSn-GaIn are far higher than those of the GaIn, Ni-GaIn and Sn-GaIn liquid alloy catalysts.

[0103] From Figure 6 As can be seen from Table 2, the C3H6monomer selectivity of the NiSn-GaIn liquid alloy catalyst for recovering waste polypropylene plastic is 64.98 mol%, which is increased by 24.41% and 55.57% respectively than the C3H6monomer selectivity of the Ni-GaIn and GaIn liquid alloy catalysts for recovering waste polypropylene plastic. In addition, the conversion rate of waste polypropylene plastic and the C3H6monomer selectivity of the liquid alloy catalyst NiSn-GaIn are far higher than those of the GaIn, Ni-GaIn and Sn-GaIn liquid alloy catalysts.

[0104] (5) Comparative analysis of NiSn-GaIn and other depolymerization / recovery processes for recovering waste polypropylene plastic

[0105] Figure 7 The comparative diagram of the depolymerization rate, C3H6monomer selectivity and C3H6space-time yield of the waste polypropylene plastic recovered by Example 4 and other depolymerization / recovery processes. Figure 7 In Table 3, “conductive medium” represents the process of recovering waste polypropylene plastic by using carbon felt; “molecular sieve” represents the process of recovering waste polypropylene plastic by using molecular sieve H-ZSM-5; “catalytic cracking catalyst” represents the process of recovering waste polypropylene plastic by using catalytic cracking catalyst “E-CAT”; “composite catalyst” represents the process of recovering waste polypropylene plastic by using composite catalyst “Fe-SBA-15”; “isomerization ethenolysis catalyst” represents the process of recovering waste polypropylene plastic by using Na / γ-Al2O3; “example” represents the process of recovering waste polypropylene plastic by using Example 4. Figure 7 As can be seen from Table 3, the process of recovering waste polypropylene plastic by using Example 4 is far superior to the no catalyst group, the molecular sieve group, the catalytic cracking catalyst group, the composite catalyst group and the isomerization ethenolysis catalyst group in terms of the depolymerization rate, C3H6monomer selectivity and C3H6space-time yield.

[0106] (6) Evaluation of the catalytic effect of NiSn-GaIn on the depolymerization reaction of mixed polyolefin plastic

[0107] Figure 8 In this context, LDPE, HDPE, PS, POE, SBS, PP+LDPE, PP+HDPE, PP+PS, PP+POE, PP+SBS, and Mix represent the processes used in Example 4 to depolymerize and recycle the following polyolefin plastics using the NiSn-GaIn and polyolefin waste depolymerization and recycling systems: low-density polyethylene, high-density polyethylene, polystyrene, ethylene-butene copolymer, styrene-butadiene-styrene block copolymer, a mixture of polypropylene and low-density polyethylene (mixed at a mass ratio of 1:1), a mixture of polypropylene and high-density polyethylene (mixed at a mass ratio of 1:1), a mixture of polypropylene and polystyrene (mixed at a mass ratio of 1:1), a mixture of polypropylene and ethylene-butene copolymer (mixed at a mass ratio of 1:1), a mixture of polypropylene and styrene-butadiene-styrene block copolymer (mixed at a mass ratio of 1:1), and a mixture of the above six polyolefin plastics (mixed at equal mass ratios); and the process conditions for each recycling process are the same as in Example 4.

[0108] like Figure 8 As shown, the selectivity of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) to C2H4 monomers was 33.18 mol% and 42.87 mol%, respectively, while the selectivity of polystyrene (PS) to C3H6 was 45.95 mol%. These results indicate that the liquid alloy catalyst NiSn-GaIn, through the polyolefin depolymerization and recovery system in Example 4, can not only depolymerize PE plastics of LDPE and HDPE into monomers, but also induce the formation of short-chain olefins from the aromatic rings of polystyrene (PS) side groups through C−C bond cleavage. For structurally complex thermoplastic elastomer polyolefin plastics, such as ethylene-octene block copolymer (POE) and styrene-butadiene-styrene triblock copolymer (SBS), the liquid alloy catalyst NiSn-GaIn, through the polyolefin depolymerization and recovery system in Example 4, can still decompose them into C2H4, with selectivities reaching 40.61 mol% and 35.79 mol%, respectively.

[0109] Due to their similar densities, it is difficult to obtain a pure plastic stream composed of individual PE or PP units through simple crushing, washing, and sorting steps. Therefore, we also conducted depolymerization experiments on blended polyolefin plastics. Figure 8The results in Table 6 show that the olefin selectivity of PP+LDPE, PP+HDPE and PP+PS mixed at a mass ratio of 1:1 is 64.08 mol%, 58.58 mol% and 83.16 mol%, respectively, and C3H6 and C4H8 are dominant. The cracking products of PP+POE and PP+SBS are mainly composed of C2H4 and C3H6, and the olefin selectivity is 59.65 mol% and 56.17 mol%, respectively. In addition, the cracking products of the mixture of the above six polyolefin plastics mixed at an equal mass ratio are mainly composed of C2H4, C3H6 and C4H8, and the olefin selectivity is 67.80 mol%. It can be seen that the blended polyolefin plastics and typical additives thereof can be depolymerized into light olefins with high selectivity by using the liquid alloy catalyst NiSn-GaIn through the polyolefin depolymerization recycling system in Example 4.

[0110] (7) Evaluation of the catalytic effect of NiSn-GaIn on the depolymerization reaction of commercial polyolefin plastic products

[0111] The actual plastic waste is usually a mixed waste stream composed of various polymers and their modified additives as well as food residues, inks and labels, which poses additional challenges to plastic recycling technologies. Therefore, we also selected commercial plastic products commonly used in daily life, including take-out meal boxes, foam boxes, reagent barrels, food packaging bags, drinking water bottle caps, milk bottles, toothpaste tubes, beverage straws, medical sealing bags and mixed commodity plastics (an equal proportion mixture of the above 9 commercial plastic products), to verify the effectiveness of NiSn-GaIn and the polyolefin waste depolymerization recycling system in solving the problem of plastic pollution. The depolymerization recycling process conditions of each group of experiments are the same as those in Example 4.

[0112] Figure 9 The results in Table 6 show that the olefin selectivity of PP+LDPE, PP+HDPE and PP+PS mixed at a mass ratio of 1:1 is 64.08 mol%, 58.58 mol% and 83.16 mol%, respectively, and C3H6 and C4H8 are dominant. The cracking products of PP+POE and PP+SBS are mainly composed of C2H4 and C3H6, and the olefin selectivity is 59.65 mol% and 56.17 mol%, respectively. In addition, the cracking products of the mixture of the above six polyolefin plastics mixed at an equal mass ratio are mainly composed of C2H4, C3H6 and C4H8, and the olefin selectivity is 67.80 mol%. It can be seen that the blended polyolefin plastics and typical additives thereof can be depolymerized into light olefins with high selectivity by using the liquid alloy catalyst NiSn-GaIn through the polyolefin depolymerization recycling system in Example 4. Figure 9 The results in Table 6 show that the olefin selectivity of PP+LDPE, PP+HDPE and PP+PS mixed at a mass ratio of 1:1 is 64.08 mol%, 58.58 mol% and 83.16 mol%, respectively, and C3H6 and C4H8 are dominant. The cracking products of PP+POE and PP+SBS are mainly composed of C2H4 and C3H6, and the olefin selectivity is 59.65 mol% and 56.17 mol%, respectively. In addition, the cracking products of the mixture of the above six polyolefin plastics mixed at an equal mass ratio are mainly composed of C2H4, C3H6 and C4H8, and the olefin selectivity is 67.80 mol%. It can be seen that the blended polyolefin plastics and typical additives thereof can be depolymerized into light olefins with high selectivity by using the liquid alloy catalyst NiSn-GaIn through the polyolefin depolymerization recycling system in Example 4.

[0113] The test results prove that the NiSn-GaIn and polyolefin waste depolymerization and recycling system has feasibility in solving the actual pollution of mixed polyolefin waste streams, and the liquid alloy catalyst NiSn-GaIn can maintain continuous operation for 120 hours in the depolymerization and recycling treatment of mixed commercial plastic waste, and the stability is more than 8 times of the existing liquid alloy catalyst (<15 hours).

[0114] As shown in Figure 10 , after the mixed commercial plastics are depolymerized by the NiSn-GaIn and polyolefin waste depolymerization and recycling system, the carbon number distribution range of the solid phase and liquid phase residues is C9~C 26 ; wherein, the component content of C 16 is the highest (18.40 wt%), followed by C 15 (14.03 wt%), which are the main components of aviation kerosene. According to the current market price, the economic added value is 30~35% higher than that of ordinary diesel.

[0115] In summary, the liquid alloy catalyst and application system disclosed by the application have the advantages of high catalytic activity, low cost and high stability. The application adopts a specific preparation method to form a heterogeneous electronic structure (such as Ni δ ⁻-Sn δ+ ) with uneven electronic distribution of active metals and adjusting metals in the liquid alloy catalyst. The catalytic performance of the catalyst (NiSn-GaIn) prepared by using non-noble metal components in the PP plastic depolymerization is significantly better than that of the noble metal system, and the PP depolymerization conversion rate is more than 95%, the C3H6 monomer selectivity breaks through 60 mol%, and the C3H6 space-time yield reaches 140 mmol C3H6·g cat ⁻ 1 ·h⁻ 1 . The liquid alloy catalyst prepared by the application is especially suitable for polyolefin blend cracking, wherein the light olefin selectivity of the NiSn-GaIn catalyzed polyolefin blend depolymerization is stably maintained at 59.93~74.63 mol%.

[0116] The liquid alloy catalyst prepared by the application is in liquid state at room temperature and is easy to separate and recover after depolymerization; meanwhile, the liquid alloy catalyst has excellent corrosion resistance, anti-coking and anti-carbon deposition performance, can effectively inhibit catalyst agglomeration and deactivation and reduce maintenance cost. The application constructs an electrified reaction interface based on Joule heat effect, uses high-frequency electromagnetic field to form local hot spots in the solid-liquid contact area, excites the activation energy barrier of the liquid alloy catalyst for directional regulation and control of C-C bond, inhibits chain termination reaction and free radical coupling, thereby realizes precise breaking of polymer chain, significantly improves monomer selectivity and reduces energy consumption. Therefore, the liquid alloy catalyst prepared by the application combined with the matched polyolefin waste depolymerization and recovery system can realize harmless treatment and high-value resource conversion of polyolefin waste.

[0117] Obviously, the above examples are only examples for clearly illustrating but not limitation to the embodiments. Other different forms of changes or variations can be made on the basis of the above description for ordinary skilled in the art. Here, all the embodiments are not required to be exhausted and the changes or variations derived therefrom are still within the protection scope of the patent application claims.

Claims

1. A liquid alloy catalyst for the depolymerization and recycling of waste plastics, characterized in that, It consists of an active metal, a regulating metal, and a solvent metal; the active metal and the regulating metal form a heterogeneous electronic structure in the solvent metal, and the electron density of the active metal in the heterogeneous electronic structure is higher than that of the regulating metal; that is, the active metal provides catalytic sites for the liquid alloy catalyst, the regulating metal is used to regulate the electronic structure of the active metal, and the solvent metal provides a liquid matrix for the liquid alloy catalyst. The active metal is rhodium, platinum, palladium, nickel, cobalt, or iron; the regulating metal is iridium, tin, zinc, copper, vanadium, or manganese; and the solvent metal is one or a mixture of two or more of gallium, indium, bismuth, and lead.

2. The liquid alloy catalyst for the depolymerization and recycling of waste plastics according to claim 1, characterized in that, The total mass of the active metal and the regulating metal accounts for 1 wt% to 75 wt% of the total mass of the liquid alloy catalyst, and the mass ratio of the active metal to the regulating metal is 6:(1~12).

3. A method for preparing a liquid alloy catalyst for the depolymerization and recycling of waste plastics, characterized in that, Includes the following steps: Step S1: Dissolve the precursor salts corresponding to the active metal and the regulating metal in deionized water to obtain metal precursor solutions; Step S2: Add an acidic solution to the metal precursor solution and stir to mix evenly to obtain an acidified precursor solution; Add an alkaline solution dropwise to the acidified precursor solution to adjust the pH to alkaline, then stir thoroughly to carry out the precipitation reaction; after the precipitation reaction is completed, centrifuge the reaction system. The solid product obtained by centrifugation was washed with ethanol solution until neutral and then redispersed in ethanol solution to obtain a de-alkaline cationic dispersion. Step S3: After vacuum drying the alkaline cationic dispersion, calcine it in an air atmosphere. After calcineation, a mixture of metal oxide precursors is obtained. Step S4: Grind the metal oxide precursor mixture and mix it evenly with the solvent metal powder to obtain a mixture powder; place the mixture powder under a hydrogen atmosphere for heating and reduction treatment; After the heating reduction treatment is completed, the liquid alloy catalyst for the depolymerization and recycling of waste plastics as described in claim 1 or 2 is obtained.

4. The method for preparing the liquid alloy catalyst for waste plastic depolymerization and recycling according to claim 3, characterized in that, In step S1, the precursor salt corresponding to the active metal is nickel nitrate hexahydrate, palladium nitrate dihydrate, or rhodium trichloride trihydrate; the precursor salt corresponding to the adjusting metal is stannous chloride dihydrate, zinc nitrate hexahydrate, or vanadium trichloride; the mass concentration of the precursor salt corresponding to the active metal in the metal precursor solution is (0.01~0.08) g·mL⁻ 1 .

5. The method for preparing the liquid alloy catalyst for waste plastic depolymerization and recycling according to claim 3, characterized in that, In step S2, the acidic solution is a concentrated nitric acid solution with a mass fraction of 65 wt% to 68 wt% or a perchloric acid solution with a mass fraction of 60 wt% to 65 wt%; the amount of acidic solution is controlled so that the molar ratio of hydrogen ions to active metal ions in the acidification precursor solution is 1:(0.3~1.0). The alkaline solution is a sodium hydroxide solution with a mass fraction of 30 wt% to 35 wt%; the amount of alkaline solution added is controlled to adjust the pH to 10 to 11; the stirring time for the precipitation reaction is 2 to 4 h; the centrifugation conditions are: rotation speed 12000 to 16000 rpm, time 15 to 25 min; the ethanol solution is a mixture of anhydrous ethanol and water in a 1:1 volume ratio; the mass fraction of the solid product in the alkaline cation dispersion is 5 wt% to 15 wt%.

6. The method for preparing the liquid alloy catalyst for waste plastic depolymerization and recycling according to claim 3, characterized in that, In step S3, the vacuum drying conditions are: drying temperature 100~150 ℃, drying time 8~15 h; the calcination conditions are: calcination temperature 400~600 ℃, calcination time 2~4 h.

7. The method for preparing the liquid alloy catalyst for waste plastic depolymerization and recycling according to claim 3, characterized in that, In step S4, the particle size of the metal oxide precursor mixture after grinding is 5~30 μm; the solvent metal powder is one or a mixture of two or more of gallium powder, indium powder, bismuth powder, and lead powder, and the particle size of the solvent metal powder is 40~150 μm; the heating reduction treatment method is as follows: first at 5~10 ℃·min⁻ 1 The heating rate was increased from room temperature to 450-550 ℃ and held for 1-2 h, followed by a rate of 10-15 ℃·min⁻ 1 The temperature was increased to 700-800 ℃ at a heating rate and held for 1-2 h, followed by a heating rate of 10-15 ℃·min⁻ 1 The temperature was lowered to 450~550 ℃ and held for 1~4 h, and finally cooled naturally to room temperature and transferred to an inert gas atmosphere for storage.

8. The application of a liquid alloy catalyst for the depolymerization and recycling of waste plastics, characterized in that, The liquid alloy catalyst for the depolymerization and recycling of waste plastics as described in claim 1 or 2 is used for the depolymerization and recycling of waste polyolefin plastics.

9. The application of the liquid alloy catalyst for the depolymerization and recycling of waste plastics according to claim 8, characterized in that, The liquid alloy catalyst is depolymerized and recycled through a polyolefin depolymerization and recycling system. The polyolefin depolymerization and recycling system includes a depolymerization reactor (1), an electrified heating device (2), a depolymerization product recovery device (3), a depolymerization product separation device (6), a temperature monitor (8), and a raw material addition device (9). The depolymerization reactor (1) has a cavity structure, and a reaction zone (5) is set inside the cavity structure. The waste polyolefin plastic and the liquid alloy catalyst undergo depolymerization reaction in the reaction zone (5). The outlet of the raw material adding device (9) is connected to the inlet of the depolymerization reactor (1), the outlet of the depolymerization reactor (1) is connected to the inlet of the depolymerization product separation device (6), and the outlet of the depolymerization product separation device (6) is connected to the inlet of the depolymerization product recovery device (3); the electrified heating device (2) provides heat for the depolymerization reaction of waste polyolefin plastics and liquid alloy catalyst in the reaction zone (5); the temperature detection point of the temperature monitor (8) is set in the reaction zone (5); the temperature signal output terminal of the temperature monitor (8) is electrically connected to the temperature signal input terminal of the electrified heating device (2); The electrified heating device (2) is an electromagnetic induction heater, and the induction coil (10) of the electromagnetic induction heater is surrounded around the reaction zone (5). The electrified heating device (2) is also equipped with a water cooling device (7), and the water cooling device (7) and the electrified heating device (2) exchange heat. The feed inlet of the depolymerization reactor (1) is located on the cavity sidewall or top above the top section (4) of the reaction zone of the depolymerization reactor (1).

10. The application of the liquid alloy catalyst for the depolymerization and recycling of waste plastics according to claim 8, characterized in that, Waste polyolefin plastics are pure polyolefin-based plastics, polyolefin-based plastics mixed with polyvinyl chloride, or polyolefin-based plastics with added modifiers; polyolefin-based plastics are one or a mixture of two or more of high-density polyethylene plastics, low-density polyethylene plastics, polypropylene plastics, or polystyrene plastics; modifiers include ethylene-butene copolymers, styrene-butadiene-styrene block copolymers, and glass fibers.

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