Polyolefin low-temperature aromatization method based on zeolite encapsulated metal catalyst

Abstract

The invention relates to the field of polyolefin catalytic aromatization, and discloses a low-temperature polyolefin aromatization method based on a zeolite-encapsulated metal catalyst, which comprises the following steps: heating and melting a zeolite-encapsulated metal catalyst as a lower bed layer and a mixture of polyolefin plastic and a lamellar ZSM-5 molecular sieve as an upper bed layer to carry out aromatization reaction; the zeolite encapsulated metal catalyst comprises a matrix and metal encapsulated in the matrix, and the matrix is an HZSM-5 molecular sieve. By designing a double-bed series catalytic route, polyolefin is firstly converted into low-carbon olefin, and then olefin aromatization reaction is performed, so that the reaction temperature and the content of heavy aromatic hydrocarbon are reduced; according to the catalyst for catalyzing the olefin aromatization reaction in the lower bed layer, the metal nanoparticles are encapsulated in zeolite crystals, the pore channel confinement effect is exerted to avoid generation of a large-size carbon deposition precursor, the diffusion speed of reactants and product molecules is adjusted to inhibit a deep reaction, a stable catalytic system is formed, and the catalytic activity of the catalyst is improved. The aromatic hydrocarbon yield and the service life of the catalyst can be improved.

Description

Technical Field

[0001] The invention relates to the field of polyolefin catalytic aromatization, in particular to a polyolefin low-temperature aromatization method based on zeolite-encapsulated metal catalyst. Background Art

[0002] As a common plastic material, polyolefins are widely used in packaging, agriculture, construction, automobiles and other fields, which greatly facilitates our lives and promotes industrial development. However, with the continuous increase in global plastic production, the disposal of plastic waste has become an increasingly serious environmental problem.

[0003] The aromatization reaction of polyolefins is an effective way to recycle waste plastics. Traditional polyolefin aromatization reactions usually use supported metal-molecular sieve catalysts, which are prone to metal sintering and carbon deposition deactivation during the reaction, resulting in a decrease in catalytic efficiency. In addition, the direct aromatization reaction of polyolefins requires a relatively high reaction temperature (>500°C), accompanied by the generation of a large amount of heavy aromatic products, resulting in a low proportion of high-value methylated aromatic products (toluene, xylene, and trimethylbenzene). The use of precious metal Pt to control the breakage of CC bonds can selectively obtain aromatic components (Science 2020, 370, 437-441), successfully lowering the reaction temperature, but the products are mainly heavy aromatics, and precious metals are expensive and easily poisoned, making them difficult to apply industrially.

[0004] Therefore, it is particularly important to develop a method that can effectively convert polyolefins into methylated aromatics at lower temperatures, among which the design of reaction routes and the development of efficient and stable catalysts are the key to solving this problem. Summary of the invention

[0005] The present invention aims to overcome the above-mentioned problems existing in the aromatization reaction of polyolefins in the prior art, and provides a method for efficiently catalyzing the aromatization of polyolefins using a dual-catalyst series system. By designing a dual-bed series catalytic route, polyolefins are first converted into low-carbon olefins, and then an olefin aromatization reaction is carried out, thereby successfully reducing the reaction temperature and the content of heavy aromatic hydrocarbons. The catalyst for catalyzing the olefin aromatization reaction in the lower bed is different from the traditional supported metal-molecular sieve catalyst. By encapsulating metal nanoparticles in zeolite crystals to form a stable catalytic system, the aromatic hydrocarbon yield and the service life of the catalyst can be improved.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A low-temperature aromatization method for polyolefins based on zeolite-encapsulated metal catalysts comprises the following steps: using the zeolite-encapsulated metal catalyst as a lower bed layer, and a mixture of polyolefin plastic and a sheet ZSM-5 molecular sieve as an upper bed layer, heating and melting them to carry out aromatization reaction; The zeolite encapsulated metal catalyst comprises a substrate and a metal encapsulated in the substrate, wherein the substrate is a HZSM-5 molecular sieve with a silicon-aluminum ratio of 20 to 30; the metal is one or more of Ga, Cu, Fe, Zn, and Cr; and the molar ratio of the metal atom to the silicon in the HZSM-5 molecular sieve is 0.01 to 0.1; The preparation method of the zeolite encapsulated metal catalyst is as follows: dissolving a silicon source, an aluminum source, a template agent and a metal salt in water to form a mixed solution, then performing a hydrothermal crystallization reaction, and calcining the product to obtain the zeolite encapsulated metal catalyst.

[0007] The present invention designs a double-bed series catalytic route, firstly reacts polyolefin with the upper bed layer of sheet HZSM-5 molecular sieve, and the sheet ZSM-5 molecular sieve can efficiently crack polyolefin into light olefin; then the olefin aromatization reaction is carried out through the zeolite encapsulated metal catalyst of the lower bed, which can successfully reduce the reaction temperature and the content of heavy aromatics. Compared with the existing process, the method of the present invention has a lower reaction temperature and a higher yield of methylated aromatics, and no reducing reaction gas CO, H2, etc. needs to be added during the reaction. The method of the present invention can successfully achieve high selectivity of preparing methylated aromatics from polyolefin at a relatively low reaction temperature (below 400°C); and the series catalytic system of the present invention has excellent stability, and the activity will not be significantly reduced after long-term use.

[0008] The zeolite encapsulated metal catalyst used in the lower bed of the present invention is different from the traditional supported metal-molecular sieve catalyst. By encapsulating metal nanoparticles in zeolite crystals, a stable catalytic system is formed, which can improve the yield of aromatics, and has obvious progress in anti-carbon deposition, which is conducive to the long-term use of the catalyst. In the zeolite encapsulated metal catalyst, the metal is encapsulated in the zeolite pores, and the distribution of its active sites is more regular and uniform, and the pore confinement effect can avoid the generation of large-size carbon deposit precursors. In contrast, the active sites of traditional supported catalysts are unevenly distributed on the surface of the carrier, and local overactivity is prone to occur, resulting in the aggregation and deposition of carbon deposit precursors. In addition, due to the limitation of the pores, the diffusion rate of the reactants and product molecules entering the zeolite pores is relatively slow, which makes the reaction process more gentle, which is conducive to suppressing the generation of overreaction and carbon deposition. The reaction on the surface of the traditional supported catalyst is relatively more violent, and it is easier to trigger a deep reaction to generate carbon deposition.

[0009] Preferably, the molar ratio of SiO2, Al2O3, template, metal atom and water in the mixed solution is 1:0.02~0.03:0.45~0.47:0.01~0.1:30~35.

[0010] Preferably, when preparing the zeolite-encapsulated metal catalyst, the metal salt is first coordinated with EDTA to form a complex, which is then added to the mixed solution.

[0011] Preferably, the silicon source is TEOS, the aluminum source is aluminum nitrate, and the template is TPAOH.

[0012] Preferably, the temperature of the hydrothermal crystallization reaction is 150-200° C., and the time is 36-60 hours.

[0013] Preferably, the product is calcined at a temperature of 500-600° C. and for a time of 3-5 h.

[0014] Preferably, the mass ratio of the polyolefin plastic, the layered ZSM-5 molecular sieve and the zeolite-encapsulated metal catalyst is 1:0.05-0.2:0.1-0.3.

[0015] Preferably, quartz sand is added between the upper bed layer and the lower bed layer as a partition.

[0016] Preferably, the aromatization reaction temperature is 350-450° C., and the reaction time is 2-4 h.

[0017] Preferably, an inert atmosphere is provided by a carrier gas during the aromatization reaction, and the carrier gas flow rate is 4-6 mL / min.

[0018] Preferably, the polyolefin plastic is one or more of polyethylene, polypropylene, and polystyrene, and has a weight average molecular weight (Mw) of 150,000 to 260,000 Da.

[0019] Therefore, the present invention has the following beneficial effects: (1) The present invention provides a more efficient polyolefin aromatization method, which has a lower reaction temperature and a higher yield of methylated aromatics than the existing process, and does not require the addition of reducing reaction gases such as CO and H2 during the reaction; (2) The catalyst of the present invention is cheap and readily available, avoiding the use of precious metals, thereby greatly reducing the preparation cost of the catalyst and facilitating large-scale production of the catalyst; (3) Compared with the conventional supported metal-zeolite catalyst, the zeolite-encapsulated metal catalyst used in the present invention has significant improvement in anti-carbon deposition, which is beneficial to the long-term use of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is the XRD pattern of Ga@HZSM-5 (Ga / Si=0.1) prepared in Example 1 of the present invention.

[0021] Figure 2 This is a thermogravimetric analysis diagram of Ga@HZSM-5 (Ga / Si=0.1) after reaction in Example 1 of the present invention.

[0022] Figure 3It is a scanning electron microscope image of Fe@HZSM-5 (Fe / Si=0.03) prepared in Example 2 of the present invention.

[0023] Figure 4 It is the gas chromatogram of the liquid product in Example 2 of the present invention.

[0024] Figure 5 It is a transmission electron microscope image of Cr@HZSM-5 (Cr / Si=0.03) prepared in Example 4 of the present invention.

[0025] Figure 6 It is a transmission electron microscope image of Zn / HZSM-5 prepared in Comparative Example 2 of the present invention.

[0026] Figure 7 It is a physical picture of the reaction residue in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0028] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are conventional methods in the art.

[0029] Overall embodiment: A low-temperature aromatization method for polyolefins based on zeolite-encapsulated metal catalysts comprises the following steps: using the zeolite-encapsulated metal catalyst as a lower bed layer, and a mixture of polyolefin plastic and a sheet ZSM-5 molecular sieve as an upper bed layer, heating and melting them to carry out aromatization reaction; The zeolite encapsulated metal catalyst comprises a substrate and a metal encapsulated in the substrate, wherein the substrate is a HZSM-5 molecular sieve with a silicon-aluminum ratio of 20 to 30; the metal is one or more of Ga, Cu, Fe, Zn, and Cr; and the molar ratio of the metal atom to the silicon in the HZSM-5 molecular sieve is 0.01 to 0.1; The preparation method of the zeolite encapsulated metal catalyst is as follows: dissolving a silicon source, an aluminum source, a template agent and a metal salt in water to form a mixed solution, then performing a hydrothermal crystallization reaction, and calcining the product to obtain the zeolite encapsulated metal catalyst.

[0030] As a specific implementation manner, the molar ratio of SiO2, Al2O3, template, metal atoms and water in the mixed solution is 1:0.02~0.03:0.45~0.47:0.01~0.1:30~35; preferably, the molar ratio of SiO2, Al2O3, template, metal atoms and water in the mixed solution is 1:0.02:0.46:0.01~0.1:32.

[0031] As a specific implementation, when preparing the zeolite-encapsulated metal catalyst, the metal salt is first coordinated with EDTA to form a complex, and then added to the mixed solution.

[0032] As a specific implementation, the silicon source is TEOS, the aluminum source is aluminum nitrate, and the template agent is TPAOH.

[0033] As a specific implementation manner, the temperature of the hydrothermal crystallization reaction is 150-200° C., and the time is 36-60 hours.

[0034] As a specific implementation method, the calcination temperature of the product is 500-600° C., and the calcination time is 3-5 hours.

[0035] As a specific implementation, the mass ratio of polyolefin plastic, layered ZSM-5 molecular sieve and zeolite-encapsulated metal catalyst is 1:0.05~0.2:0.1~0.3; preferably, the mass ratio of polyolefin plastic, layered ZSM-5 molecular sieve and zeolite-encapsulated metal catalyst is 1:0.1:0.2.

[0036] As a specific implementation method, quartz sand is added between the upper bed layer and the lower bed layer as a partition.

[0037] As a specific implementation method, the aromatization reaction temperature is 350-450° C., and the reaction time is 2-4 hours.

[0038] As a specific implementation method, an inert atmosphere is provided by a carrier gas during the aromatization reaction, and the carrier gas flow rate is 4-6 mL / min.

[0039] As a specific implementation, the polyolefin plastic is one or more of polyethylene, polypropylene, and polystyrene, and the weight average molecular weight (Mw) is 150,000-260,000 Da.

[0040] By designing a double-bed series catalytic route, polyolefins are first converted into low-carbon olefins, and then olefin aromatization reaction is carried out, successfully reducing the reaction temperature and the content of heavy aromatics; the catalyst for catalyzing the olefin aromatization reaction in the lower bed is different from the traditional supported metal-molecular sieve catalyst. By encapsulating metal nanoparticles in zeolite crystals to form a stable catalytic system, the yield of aromatics and the service life of the catalyst can be improved.

[0041] Embodiment 1: A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: TEOS, Al(NO3)3·9H2O, TPAOH, Ga(NO3)3·9H2O and water were mixed and stirred to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:Ga2O3:H2O of 1:0.02:0.46:0.05:32. After hydrothermal crystallization at 180°C for 48 h, the product was air-calcined at 550°C for 4 h to prepare a zeolite-encapsulated metal catalyst, denoted as Ga@HZSM-5 (Ga / Si=0.1). Its XRD is shown in Figure 1 As shown; (2) Catalytic aromatization of polyethylene: Weigh 0.2 g of the prepared Ga@HZSM-5 (Ga / Si=0.1) catalyst and mix it with 0.2 g of quartz sand and put it into a quartz tube, fix it with quartz wool, and then add 0.5 g of quartz sand to separate the upper and lower layers; mix 1 g of polyethylene powder (M w =150000Da) was mixed with 0.1g of ZSM-5 molecular sieve and ground into granules, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 5mL / min, and the fixed bed was reacted at 400℃ for 2h. The product was condensed and tested by gas chromatography. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatic yield was 54.8%, of which methylated aromatics accounted for 94.5%, and the catalyst carbon deposition content after the reaction was 2.1%. The thermogravimetric data are shown in Table 1. Figure 2 as shown in .

[0042] Embodiment 2: A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: Fe(NO3)3·9H2O and sodium ethylenediaminetetraacetate (EDTA) were first coordinated to obtain a Fe-EDTA complex to prevent Fe from agglomerating in an alkaline medium; then the Fe-EDTA complex was added to a mixed gel containing TEOS, Al(NO3)3·9H2O, TPAOH and H2O to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:Fe-EDTA:H2O of 1:0.02:0.46:0.03:32. After hydrothermal crystallization at 150°C for 60 h, the product was air-calcined at 500°C for 5 h to prepare a zeolite-encapsulated metal catalyst, denoted as Fe@HZSM-5 (Fe / Si=0.03); its scanning electron microscope image is shown in FIG. Figure 3 As shown in; (2) Catalytic aromatization of polypropylene: Weigh 0.2 g of the prepared Fe@HZSM-5 (Fe / Si=0.03) catalyst and mix it with 0.2 g of quartz sand and put it into a quartz tube, fix it with quartz wool, and then add 0.5 g of quartz sand to separate the upper and lower layers; mix 1 g of polypropylene powder (M w =200000Da) was mixed with 0.1g of ZSM-5 molecular sieve and ground into granules, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 5mL / min, and the fixed bed was reacted at 400℃ for 2h. The product was condensed and tested by gas chromatography. The results are shown in Table 1; the polypropylene conversion rate was 100%, the aromatic yield was 47.8%, of which methylated aromatics accounted for 92.4%, and the liquid product distribution was as follows Figure 4 As shown in ; the carbon deposit content of the catalyst after the reaction is 2.5%.

[0043] Embodiment 3: A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: TEOS, Al(NO3)3·9H2O, TPAOH, Zn(NO3)2·6H2O, Ga(NO3)3·9H2O and water were mixed and stirred to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:ZnO:Ga2O3:H2O of 1:0.02:0.46:0.02:0.015:32. After hydrothermal crystallization at 200°C for 36 h, the product was air-calcined at 600°C for 3 h to prepare a zeolite-encapsulated metal catalyst, denoted as ZnGa@HZSM-5 (Zn / Si=0.02, Ga / Si=0.03). (2) Catalytic aromatization of polystyrene: Weigh 0.2 g of the prepared ZnGa@HZSM-5 (Zn / Si=0.02, Ga / Si=0.03) catalyst and mix it with 0.2 g of quartz sand and put it into a quartz tube, fix it with quartz wool, and then add 0.5 g of quartz sand to separate the upper and lower layers; mix 1 g of polystyrene powder (M w =260000Da) was mixed with 0.1g of flaky ZSM-5 molecular sieve, ground and granulated, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 4mL / min, and the fixed bed was reacted at 450℃ for 2h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polystyrene conversion rate was 100%, the aromatics yield was 62.5%, of which methylated aromatics accounted for 96.4%, and the catalyst carbon deposition content after the reaction was 1.8%.

[0044] Embodiment 4: A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: First, Cr(NO3)3·9H2O and sodium ethylenediaminetetraacetate (EDTA) were coordinated to obtain a Cr-EDTA complex to prevent Cr from agglomerating in an alkaline medium; then, the Cr-EDTA complex was added to a mixed gel containing TEOS, Al(NO3)3·9H2O, TPAOH and H2O to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH: Cr-EDTA:H2O of 1:0.02:0.46:0.03:32. After hydrothermal crystallization at 180°C for 60 h, the product was air-calcined at 550°C for 4 h to prepare a zeolite-encapsulated metal catalyst, denoted as Cr@HZSM-5 (Cr / Si=0.03); its transmission electron microscopy image is shown in FIG. Figure 5 As shown; (2) Catalytic aromatization of polyethylene: Weigh 0.2 g of the prepared Cr@HZSM-5 (Cr / Si=0.03) catalyst and mix it with 0.2 g of quartz sand and put it into a quartz tube, fix it with quartz wool, and then add 0.5 g of quartz sand to separate the upper and lower layers; w =150000Da) was mixed with 0.1g of flaky ZSM-5 molecular sieve, ground and granulated, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 4mL / min, and the fixed bed was reacted at 400℃ for 3h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatics yield was 52.8%, of which methylated aromatics accounted for 94.5%, and the catalyst carbon deposition content after the reaction was 2.0%.

[0045] Embodiment 5: A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: First, CuSO4 and sodium ethylenediaminetetraacetate (EDTA) were coordinated to obtain a Cu-EDTA complex to prevent Cu from agglomerating in an alkaline medium; then, the Cu-EDTA complex was added to a mixed gel containing TEOS, Al(NO3)3·9H2O, TPAOH and H2O to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:Cu-EDTA:H2O of 1:0.02:0.46:0.05:32. After hydrothermal crystallization at 180°C for 48 h, the product was air-calcined at 550°C for 4 h to prepare a zeolite-encapsulated metal catalyst, denoted as Cu@HZSM-5 (Cu / Si=0.05); (2) Catalytic aromatization of polyethylene: Weigh 0.2 g of the prepared Cu@HZSM-5 (Cu / Si=0.05) catalyst and mix it with 0.2 g of quartz sand and put it into a quartz tube, fix it with quartz wool, and then add 0.5 g of quartz sand to separate the upper and lower layers; mix 1 g of polyethylene powder (M w =150000Da) was mixed with 0.1g of flaky ZSM-5 molecular sieve, ground and granulated, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 6mL / min, and the fixed bed was reacted at 350℃ for 4h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatics yield was 43.5%, of which methylated aromatics accounted for 91.4%, and the catalyst carbon deposition content after the reaction was 2.7%.

[0046] Embodiment 6: A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: First, CuSO4 and Cr(NO3)3·9H2O were coordinated with sodium ethylenediaminetetraacetate (EDTA) to obtain Cu-EDTA complex and Cr-EDTA complex respectively to prevent metal agglomeration in alkaline medium; then, Cu-EDTA complex and Cr-EDTA complex were added to a mixed gel containing TEOS, Al(NO3)3·9H2O, TPAOH and H2O to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:Cu-EDTA:Cr-EDTA complex:H2O of 1:0.02:0.46:0.01:0.03:32. After hydrothermal crystallization at 180℃ for 48 h, the product was air-calcined at 550℃ for 4 h to prepare a zeolite-encapsulated metal catalyst, denoted as CuCr@HZSM-5 (Cu / Si=0.01, Cr / Si=0.03); (2) Catalytic aromatization of polyolefins: 0.2 g of the prepared CuCr@HZSM-5 (Cu / Si=0.01, Cr / Si=0.03) catalyst was weighed and mixed with 0.2 g of quartz sand and placed in a quartz tube, fixed with quartz wool, and then 0.5 g of quartz sand was added to separate the upper and lower layers; 1 g of polyolefin powder (0.5 g of polyethylene, M w =150000Da; Polypropylene 0.5g, M w=200000Da) was mixed with 0.1g of flaky ZSM-5 molecular sieve, ground and granulated, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 5mL / min, and the fixed bed was reacted at 400℃ for 2h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatics yield was 57.9%, of which methylated aromatics accounted for 94.8%, and the catalyst carbon deposition content after the reaction was 1.9%.

[0047] Comparative Example 1 (no metal encapsulated in the zeolite molecular sieve): A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite molecular sieve: TEOS, Al(NO3)3·9H2O, TPAOH and water were mixed and stirred to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:H2O of 1:0.02:0.46:32. After hydrothermal crystallization at 180°C for 48 h, the product was air-calcined at 550°C for 4 h to prepare a zeolite molecular sieve, which was recorded as HZSM-5. (2) Catalytic aromatization of polyethylene: Weigh 0.2 g of prepared HZSM-5 and 0.2 g of quartz sand, mix them into a quartz tube, fix them with quartz wool, and then add 0.5 g of quartz sand to separate the upper and lower layers; w =150000Da) was mixed with 0.1g of flaky ZSM-5 molecular sieve, ground and granulated, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 5mL / min, and the fixed bed was reacted at 400℃ for 2h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatics yield was 36.8%, of which methylated aromatics accounted for 89.7%, and the catalyst carbon deposition content after the reaction was 8.1%.

[0048] Comparative Example 2 (using supported metal-molecular sieve catalyst): A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of supported metal-molecular sieve catalyst: TEOS, Al(NO3)3·9H2O, TPAOH and water were mixed and stirred to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:H2O of 1:0.02:0.46:32. After hydrothermal crystallization at 180°C for 48 h, the product was air-calcined at 550°C for 4 h to prepare a zeolite molecular sieve, denoted as HZSM-5. HZSM-5 was mixed with a 1.7 mol / L aqueous Zn(NO3)2·6H2O solution, with a volume ratio of HZSM-5 to Zn(NO3)2·6H2O solution of 1:1. After ultrasonic drying, the mixture was air-calcined at 400°C for 2 h to prepare a supported metal-molecular sieve catalyst, denoted as Zn / HZSM-5. Its transmission electron microscopy image is shown in the figure. Figure 6 As shown in; (2) Catalytic aromatization of polyethylene: 0.2 g of prepared Zn / HZSM-5 was weighed and mixed with 0.2 g of quartz sand and placed in a quartz tube, fixed with quartz wool, and then 0.5 g of quartz sand was added to separate the upper and lower layers; 1 g of polyethylene powder (M w =150000Da) was mixed with 0.1g of flaky ZSM-5 molecular sieve, ground and granulated, mixed evenly with 1g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 5mL / min, and the fixed bed was reacted at 400℃ for 2h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatics yield was 42.5%, of which methylated aromatics accounted for 91.3%, and the catalyst carbon deposition content after the reaction was 5.1%.

[0049] Comparative Example 3 (mixed loading of lamellar ZSM-5 molecular sieve and zeolite-encapsulated metal catalyst): A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, comprising the following steps: (1) Preparation of zeolite-encapsulated metal catalyst: TEOS, Al(NO3)3·9H2O, TPAOH, Ga(NO3)3·9H2O and water were mixed and stirred to obtain a mixed solution with a molar ratio of SiO2:Al2O3:TPAOH:Ga2O3:H2O of 1:0.02:0.46:0.2:32. After hydrothermal crystallization at 180°C for 48 h, the product was air-calcined at 550°C for 4 h to prepare a zeolite-encapsulated metal catalyst, denoted as Ga@HZSM-5 (Ga / Si=0.1). (2) Catalytic polyethylene aromatization reaction: 1g polyethylene powder (M w=150000Da) was mixed with 0.1g of ZSM-5 zeolite and 0.2g of the prepared Ga@HZSM-5 (Ga / Si=0.05) catalyst, ground and granulated, mixed evenly with 1.5g of quartz sand, and loaded into the upper layer of the quartz tube; N2 carrier gas was introduced with a carrier gas flow rate of 5mL / min, and the fixed bed was reacted at 400℃ for 2h. The product was condensed and tested by gas chromatograph. The results are shown in Table 1; the polyethylene conversion rate was 100%, the aromatics yield was 38.8%, of which methylated aromatics accounted for 79.5%, and the carbon deposition content of the catalyst after the reaction was 3.9%. The actual picture is as follows Figure 7 as shown in .

[0050] Table 1: Test results of polyolefin aromatization reaction

[0051] It can be seen from the data in Table 1 that the method of the present invention in Examples 1 to 6 can significantly improve the aromatics yield and methylated aromatics selectivity by connecting the ZSM-5 molecular sieve in series with the zeolite encapsulated metal catalyst. This fully demonstrates that the double-bed series catalytic system changes the traditional one-step aromatization route of polyolefins in the process of catalyzing the aromatization of polyolefin plastics, splits the polyolefin aromatization reaction into two-step reactions in series, and successfully reduces the reaction temperature.

[0052] It can be seen from the experimental results of Comparative Examples 1 and 2 that, compared with pure ZSM-5 molecular sieve, the presence of metal greatly improves the catalytic performance of olefin aromatization reaction in the lower bed and reduces the carbon deposit content.

[0053] It can be seen from the experimental results of Example 1 and Comparative Example 2 that when the traditional supported metal-zeolite catalyst is used in Comparative Example 2, the carbon deposit content is significantly increased, resulting in a decrease in the yield of aromatics. The aromatization activity and stability of the zeolite-encapsulated metal catalyst of the present invention are significantly higher than those of the supported metal-molecular sieve catalyst.

[0054] It can be seen from the experimental results of Comparative Example 3 that the yield of aromatics is significantly reduced and the content of heavy aromatics increases when the dual-bed loading of the two catalysts is replaced by mixed loading. This indicates that the dual-bed loading of the present invention is crucial to the tandem aromatization reaction, and the mixed-loading layered ZSM-5 molecular sieve and zeolite encapsulated metal catalyst cannot achieve the ideal two-step aromatization route, resulting in a decrease in the selectivity of methylated aromatics.

[0055] It can be seen from the experimental results of Examples 3 to 5 that as the reaction temperature increases, the yield of aromatics gradually increases.

[0056] The present invention provides a more efficient polyolefin aromatization method, which has a lower reaction temperature and a higher yield of methylated aromatics than the existing process, and does not require the addition of reducing reaction gases such as CO and H2 during the reaction. At the same time, the catalyst involved in the present invention is cheap and easy to obtain, avoiding the use of precious metals, which greatly reduces the preparation cost of the catalyst and is conducive to the large-scale production of the catalyst. Compared with traditional supported metal catalysts, zeolite-encapsulated metal catalysts can give full play to the pore confinement effect to avoid the generation of large-sized carbon deposit precursors, and adjust the diffusion rate of reactant and product molecules to inhibit deep reactions, thereby improving aromatization performance.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should be included in the scope of the claims of the present invention.

Claims

1. A method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalysts, characterized in that: The method comprises the following steps: using a zeolite-encapsulated metal catalyst as a lower bed layer, and a mixture of polyolefin plastic and a layer of ZSM-5 molecular sieve as an upper bed layer, heating and melting them to carry out an aromatization reaction; The zeolite encapsulated metal catalyst comprises a substrate and a metal encapsulated in the substrate, wherein the substrate is a HZSM-5 molecular sieve with a silicon-aluminum ratio of 20 to 30; the metal is one or more of Ga, Cu, Fe, Zn, and Cr; and the molar ratio of the metal atom to the silicon in the HZSM-5 molecular sieve is 0.01 to 0.1; The preparation method of the zeolite encapsulated metal catalyst is as follows: dissolving a silicon source, an aluminum source, a template agent and a metal salt in water to form a mixed solution, then performing a hydrothermal crystallization reaction, and calcining the product to obtain the zeolite encapsulated metal catalyst.

2. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1, characterized in that: The molar ratio of SiO2, Al2O3, template, metal ions and water in the mixed solution is 1:0.02~0.03:0.45~0.47:0.01~0.1:30~35.

3. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1 or 2, characterized in that: When preparing the zeolite-encapsulated metal catalyst, the metal salt is first coordinated with EDTA to form a complex, and then added into the mixed solution.

4. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1 or 2, characterized in that: The silicon source is TEOS, the aluminum source is aluminum nitrate, and the template agent is TPAOH.

5. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1, characterized in that: The temperature of the hydrothermal crystallization reaction is 150~200℃, and the time is 36~60h.

6. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1 or 5, characterized in that: The calcination temperature of the product is 500~600℃, and the calcination time is 3~5h.

7. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1, characterized in that: The mass ratio of polyolefin plastic, lamellar ZSM-5 molecular sieve and zeolite encapsulated metal catalyst is 1:0.05~0.2:0.1~0.

3.

8. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1 or 7, characterized in that: Quartz sand is added between the upper bed layer and the lower bed layer as a partition.

9. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1, characterized in that: The temperature of the aromatization reaction is 350~450°C, and the reaction time is 2~4h. During the aromatization reaction, an inert atmosphere is provided by a carrier gas, and the carrier gas flow rate is 4~6 mL / min.

10. The method for low-temperature aromatization of polyolefins based on zeolite-encapsulated metal catalyst according to claim 1, characterized in that: The polyolefin plastic is one or more of polyethylene, polypropylene and polystyrene.

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