Method for preparing light aromatic hydrocarbon through catalytic pyrolysis of waste polypropylene plastic

Abstract

The invention discloses a method for preparing light aromatic hydrocarbon by catalytic pyrolysis of waste polypropylene plastics, which comprises the following steps: by taking a hollow ZSM-5 molecular sieve as a carrier, loading metal Ga by an ion exchange method, introducing Zr by an impregnation method to obtain a zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst, and carrying out two-stage ex-situ reaction process, firstly, deeply depolymerizing the polypropylene plastics to obtain the light aromatic hydrocarbon. Macromolecular long-chain hydrocarbon is converted into micromolecular short-chain hydrocarbon, so that a more suitable reactant is provided for subsequent aromatization; then short-chain hydrocarbon is fully diffused in pore channels of the zirconium-gallium modified hollow ZSM-5 molecular sieve, and is subjected to hydrogen transfer and aromatization reaction under the synergistic effect of acidic sites of the molecular sieve. Meanwhile, the introduction of nitrogen or carbon dioxide atmosphere inhibits the generation of alkane by-products caused by hydrogen transfer to a certain extent, and further improves the selectivity of aromatic hydrocarbon.

Description

Technical Field

[0001] This invention relates to the field of waste plastic treatment technology, specifically to a method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polypropylene plastic. Background Technology

[0002] Polypropylene (PP) possesses high molecular weight and hydrophobic properties, making it highly resistant to microbial enzymes, light, and water. This has led to its widespread application in jewelry packaging, sterile medical devices, and construction. However, the rapid increase in global annual demand for plastics, coupled with low natural degradation rates and limited lifespan, has resulted in a dramatic increase in waste plastic production, exacerbating environmental pollution. Furthermore, high recycling costs, outdated recycling technologies, and consumer habits have hindered effective waste plastic treatment. Waste plastics are chemically stable under natural conditions and difficult to degrade naturally, causing serious environmental damage. The pollution of the human living environment by waste plastics is becoming increasingly severe. Therefore, researchers are exploring efficient and effective waste plastic treatment technologies to achieve resource utilization and contribute to the green development of society.

[0003] Catalytic pyrolysis, particularly using microporous molecular sieve zeolites such as ZSM-5 as catalysts, has proven to be an effective technology for upgrading plastic waste into high-value-added chemicals (such as aromatics). Monocyclic aromatics such as benzene, toluene, ethylbenzene, and xylene are important raw materials for the synthesis of various chemical products in chemical processes. Due to its unique pore structure and tunable acidity, ZSM-5 exhibits excellent performance in improving aromatic selectivity during plastic pyrolysis. By doping with metals (such as Ni, Pt, Ru, and Zn), the distribution of acidic sites and the synergistic effect of metal-zeolite interactions can be optimized, thereby increasing the yield of aromatics. However, catalytic cracking processes require high temperatures, and the random cleavage of C-C bonds often results in a broad carbon number distribution in the products, accompanied by high energy consumption and reduced catalyst lifetime, thus reducing economic efficiency. Furthermore, hydrogen transfer easily occurs during the dehydrogenation aromatization of polyolefins, leading to the formation of alkanes, which is even more detrimental to aromatization. Existing methods for catalytic pyrolysis of waste polyolefin plastics still suffer from technical bottlenecks such as poor aromatic selectivity, excessively high proportion of heavy chain hydrocarbons, and severe catalyst deactivation due to carbon buildup. Therefore, there is an urgent need to develop a method for the efficient production of light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics, so as to improve the yield of light aromatic hydrocarbons. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polypropylene plastics.

[0005] This invention is achieved through the following technical solutions:

[0006] A method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polypropylene plastics involves synthesizing a Zr-Ga bimetallic modified porous hollow ZSM-5 molecular sieve catalyst via a hydrothermal method, and then using a non-in-situ catalytic method to carry out a cyclization and aromatization reaction on the waste polypropylene raw material under a nitrogen or carbon dioxide atmosphere to directly convert polypropylene into light aromatic hydrocarbons.

[0007] The specific steps of this method are as follows:

[0008] (1) Preparation of the original ZSM-5 molecular sieve:

[0009] Sodium hydroxide, tetrapropylammonium hydroxide, aluminum nitrate and tetraethyl orthosilicate were placed in deionized water, stirred evenly, and then subjected to a hydrothermal reaction at 180-200℃ to obtain a precursor. After centrifuging, washing and drying the precursor, it was calcined in a tube furnace at 500-600℃ to obtain the original ZSM-5.

[0010] (2) Preparation of hierarchical porous hollow ZSM-5 molecular sieve:

[0011] After mixing tetrapropylammonium hydroxide with the original ZSM-5 prepared in step (1), the mixture was subjected to hydrothermal reaction at 80-100℃, centrifugation, washing and drying to obtain hierarchical hollow ZSM-5.

[0012] (3) Synthesis of zirconium-gallium modified hollow ZSM-5 molecular sieve:

[0013] The hierarchical hollow ZSM-5 molecular sieve obtained in step (2) and gallium nitrate were added to deionized water, and gallium metal was loaded by ion exchange. After centrifugation and washing, the process was repeated three times to obtain gallium-modified ZSM-5. The gallium-modified hollow ZSM-5 molecular sieve and zirconium nitrate were added to deionized water, and zirconium metal was loaded by impregnation. The solution was then placed in a forced-air drying oven. The dried solid residue was ground and placed in a tube furnace. After calcination at 500-600℃, the final zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst was obtained.

[0014] (4) Two-stage pyrolysis-aromatization reaction:

[0015] The entire reaction takes place in a pyrolysis fixed bed, and the reaction proceeds in two stages;

[0016] First reaction zone: The zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst prepared in step (3) is placed in the first reaction zone of a fixed bed tube furnace. Under a nitrogen or carbon dioxide atmosphere, the zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst is heated from room temperature to 500-600℃ at a rate of 20-40℃ / min and maintained at this temperature for 60-80 min.

[0017] Second reaction zone: Waste polypropylene is placed in the second reaction zone of a fixed-bed tube furnace. After the temperature of the first reaction zone reaches the set value, the waste polypropylene in the second reaction zone is heated from room temperature to 500-600℃ at a rate of 20-40℃ / min and maintained at this temperature for 40-60 min to achieve preliminary pyrolysis. A large amount of volatiles generated from the preliminary pyrolysis of the waste polypropylene in the second reaction zone are then carried by the carrier gas to the first reaction zone, where they are further catalyzed by the molecular sieve for aromatization.

[0018] (5) Product collection:

[0019] The two-stage reaction products of initiating depolymerization and molecular sieve-catalyzed aromatization are collected by a condenser to obtain light aromatic hydrocarbons.

[0020] Preferably, in step (1), the ratio of sodium hydroxide, tetrapropylammonium hydroxide, aluminum nitrate and tetraethyl orthosilicate is 0.1-0.5 g : 15-20 mL : 0.5-1 g : 10-15 mL; the reaction time in the hydrothermal reactor is 46-50 h; and the calcination time of the catalyst in the tube furnace is 6-8 h.

[0021] Preferably, in step (2), the concentration of the tetrapropylammonium hydroxide solution is 0.2-0.3 M; and the reaction time in the hydrothermal reactor is 22-26 h.

[0022] Preferably, in step (3), the temperature of the ion exchange method is 70-80℃ and the time is 2-3 h; the temperature of the impregnation method is 70-80℃ and the time is 2-3 h; and the calcination time in the tube furnace is 5-8 h.

[0023] Preferably, in step (4), the ratio of zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst to waste polypropylene plastic raw material is (1-2):1. During the reaction, nitrogen or carbon dioxide is introduced into the fixed bed as the carrier gas, the carrier gas flow rate is 30-40 mL / min, the pyrolysis temperature of the first stage of the fixed bed is 500-600℃, the pyrolysis temperature of the second stage of the fixed bed is 500-600℃, and the total reaction time is 40-60 min.

[0024] Preferably, in step (5), the condensation device consists of a cold trap and a collection bottle connected by a rubber tube. The collection bottle is placed in an insulated box, which uses an ice-water mixture as a cooling medium. The collection bottle contains 30-40 mL of isopropanol to collect the product.

[0025] Compared with the prior art, the present invention has the following characteristics:

[0026] (1) The present invention uses hollow ZSM-5 molecular sieve as support, loads metal Ga by ion exchange method, and introduces Zr by impregnation method. The two metals optimize the distribution of LB acid in molecular sieve, thereby significantly improving the aromatization activity of zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst.

[0027] (2) This invention employs a two-stage, non-in-situ reaction process. First, it deeply depolymerizes polypropylene plastic, converting large long-chain hydrocarbons into small short-chain hydrocarbons, providing more suitable reactants for subsequent aromatization. Subsequently, the short-chain hydrocarbons diffuse fully within the pores of the zirconium-gallium modified hollow ZSM-5 molecular sieve, undergoing hydrogen transfer and aromatization reactions under the synergistic effect of the acidic sites on the molecular sieve. Simultaneously, the introduction of nitrogen or carbon dioxide atmosphere, especially carbon dioxide, suppresses the formation of alkane byproducts caused by hydrogen transfer, further improving the selectivity of aromatic hydrocarbons.

[0028] In summary, this invention proposes a novel method for the resource-based treatment of waste polypropylene, which can achieve the harmless treatment of plastics while efficiently converting them into light aromatic hydrocarbon products. This invention not only provides a new technical approach for the recycling and reuse of typical waste polypropylene, but also effectively reduces the resource waste and environmental pollution caused by traditional recycling methods, demonstrating promising application prospects and environmental benefits. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the two-stage catalytic pyrolysis system for polypropylene plastics according to the present invention. It includes a gas cylinder (gas supply system), a vertical tubular furnace, a liquid collection and cooling device, and a tail gas treatment device connected in sequence. The vertical tubular furnace includes a quartz basket and a quartz mesh, which are used to store polypropylene raw materials and catalysts, respectively.

[0030] Figure 2 The chromatogram is of the product obtained in Example 1. Detailed Implementation

[0031] The following is a further description of the invention, but not a limitation thereof.

[0032] Example 1:

[0033] Includes the following steps:

[0034] (1) 0.4 g sodium hydroxide, 62.28 mL tetrapropylammonium hydroxide, 2 g aluminum nitrate and 47.6 mL tetraethyl orthosilicate were dissolved in 62.8 mL deionized water and stirred thoroughly until completely dissolved. The solution was then transferred to a hydrothermal reactor and kept at 180 °C for 48 h. After centrifugation, the product was washed six times each with alcohol and deionized water, dried in an oven, and then kept at 550 °C for 6 h at a heating rate of 1 °C / min to obtain pristine ZSM-5. Then, 5 g of pristine ZSM-5 and 150 mL of 0.2 M tetrapropylammonium hydroxide were dissolved and mixed thoroughly, and the solution was transferred to a hydrothermal reactor and kept at 90 °C for 24 h. After centrifugation, the product was washed six times each with alcohol and deionized water, and dried to obtain hierarchical porous hollow ZSM-5 molecular sieve.

[0035] (2) 2.82 g of hierarchical hollow molecular sieve and 0.55 g of gallium nitrate were thoroughly mixed in the solution and stirred at 75 °C for 3 h. After that, the mixture was centrifuged and washed 3 times, and the process was repeated 3 times. Then, 0.076 g of zirconium nitrate was added, and after the solution was thoroughly mixed, the mixture was stirred at 75 °C for 3 h. After drying in an oven at 80 °C, the mixture was calcined in a tube furnace at 550 °C for 6 h to obtain the catalyst (gallium loading of 5%).

[0036] (3) Place 0.5 g of the catalyst prepared in step (2) and 0.5 g of polypropylene plastic into separate containers. Figure 1 The two-stage tube furnace, as shown, contains a quartz mesh and a quartz basket, and carbon dioxide is initially introduced at a flow rate of 30 mL / min. The first reaction zone is heated from room temperature to 500°C over 30 minutes, and then maintained at 500°C for 60 minutes. Once the first reaction zone reaches 500°C, the second reaction zone is heated from room temperature to 550°C over 15 minutes, and then maintained at 550°C for 45 minutes.

[0037] (4) The pyrolysis vapor generated after catalysis in step (3) is passed through a cooling device, and the pyrolysis oil is absorbed by an absorption bottle containing 50 mL of isopropanol in a cold bath. Then, an absorption bottle containing at least 200 mL of alcohol is connected for tail gas treatment. The tail gas is then recovered using a 5 L aluminum foil gas bag and considered as pyrolysis gas. Finally, the collected liquid phase product is analyzed; see the chromatogram below. Figure 2 The yield of aromatic hydrocarbons was 84.04%, as detailed in Table 1.

[0038] Table 1

[0039] Example 2

[0040] Referring to Example 1, the difference is that carbon dioxide is replaced with nitrogen in step (3).

[0041] Finally, the collected liquid phase products were analyzed, and the yield of aromatic hydrocarbons was 73.78%, as detailed in Table 1.

[0042] Example 3

[0043] Referring to Example 1, the difference is that in step (2), 2.955 g of hierarchical porous hollow molecular sieve and 0.055 g of gallium nitrate were thoroughly mixed in the solution and stirred thoroughly at 75°C for 3 h. After that, the mixture was centrifuged and washed 3 times, and this process was repeated 3 times. Then, 0.076 g of zirconium nitrate was added, and after the solution was thoroughly mixed, the mixture was stirred thoroughly at 75°C for 3 h. After being thoroughly dried in an oven at 80°C, the mixture was calcined in a tube furnace at 550°C for 6 h to obtain the catalyst (gallium loading of 0.5%).

[0044] Finally, the collected liquid phase products were analyzed, and the yield of aromatic hydrocarbons was 79.78%, as detailed in Table 1.

[0045] Example 4

[0046] Referring to Example 3, the difference is that carbon dioxide was replaced with nitrogen in step (3), and the collected liquid phase product was finally detected. The yield of aromatic hydrocarbons was 68.32%, as detailed in Table 1.

[0047] Example 5

[0048] Referring to Example 1, the difference is that step (2) is: 2.97 g of hierarchical hollow molecular sieve and 0.076 g of zirconium nitrate are thoroughly mixed in the solution and stirred thoroughly at 75°C for 3 h. After being thoroughly dried in an oven at 80°C, it is then placed in a tube furnace at 550°C for 6 h to obtain the catalyst Zr / ZSM-5-H.

[0049] Finally, the collected liquid phase products were analyzed, and the yield of aromatic hydrocarbons was 31.30%, as detailed in Table 1.

[0050] Example 6

[0051] Referring to Example 5, the difference is that carbon dioxide was replaced with nitrogen in step (3). Finally, the collected liquid phase product was analyzed, and the yield of aromatic hydrocarbons was 32.57%, as detailed in Table 1.

[0052] Example 7

[0053] Includes the following steps:

[0054] (1) 0.4 g sodium hydroxide, 62.28 mL tetrapropylammonium hydroxide, 2 g aluminum nitrate and 47.6 mL tetraethyl orthosilicate were dissolved in 62.8 mL deionized water and stirred thoroughly until completely dissolved. The solution was then transferred to a hydrothermal reactor and kept at 180 °C for 48 h. After centrifugation, the product was washed six times each with alcohol and deionized water, dried in an oven, and then kept at 550 °C for 6 h at a heating rate of 1 °C / min to obtain pristine ZSM-5. Then, 5 g of pristine ZSM-5 and 150 mL of 0.2 M tetrapropylammonium hydroxide were dissolved and mixed thoroughly, and the solution was transferred to a hydrothermal reactor and kept at 90 °C for 24 h. After centrifugation, the product was washed six times each with alcohol and deionized water, and dried to obtain hierarchical porous hollow ZSM-5 molecular sieve.

[0055] (2) Place 0.5 g of catalyst and 0.5 g of polypropylene plastic in the quartz mesh and quartz basket of the two-stage tube furnace, respectively, and start purging with carbon dioxide at a flow rate of 30 mL / min. The first stage reaction temperature is increased from room temperature to 500 °C over 30 min, and then maintained at 500 °C for 60 min. After the first stage reaction temperature reaches 500 °C, the second stage reaction temperature is increased from room temperature to 550 °C over 15 min, and then maintained for 45 min.

[0056] (3) The pyrolysis vapor generated after catalysis in step (2) is passed through a cooling device, and the pyrolysis oil is absorbed by an absorption bottle containing 50 mL of isopropanol in a cold tank. Then, an absorption bottle containing ethanol (not less than 200 mL) is connected for tail gas treatment. Finally, it is recovered using a 5 L aluminum foil gas bag and regarded as pyrolysis gas. Finally, the collected liquid phase product is tested, and the yield of aromatic hydrocarbons is 18.03%. See Table 1 for details.

[0057] Example 8

[0058] Referring to Example 7, the difference is that in step (2), carbon dioxide is replaced with nitrogen.

[0059] Finally, the collected liquid products were analyzed, and the yield of aromatic hydrocarbons was 11.23%, as detailed in Table 1.

[0060] Example 9

[0061] Includes the following steps:

[0062] (1) 0.5 g of polypropylene plastic was placed in a quartz basket of a vertical tube furnace in a two-stage tube furnace, and carbon dioxide was introduced at a flow rate of 30 mL / min. The first stage reaction was heated from room temperature to 500°C over 30 min, and then maintained at 500°C for 60 min. After the first stage reaction temperature reached 500°C, the second stage reaction temperature was heated from room temperature to 550°C over 15 min, and then maintained for 45 min.

[0063] (2) The pyrolysis vapor generated in step (1) is passed through a cooling device, and the pyrolysis oil is absorbed by an absorption bottle containing 50 mL of isopropanol in a cold tank. Then, an absorption bottle containing ethanol (not less than 200 mL) is connected for tail gas treatment. Finally, it is recovered using a 5 L aluminum foil gas bag and regarded as pyrolysis gas. The collected liquid phase product is finally tested, and the yield of aromatic hydrocarbons is 4.91%. See Table 1 for details.

[0064] Example 10

[0065] Referring to Example 9, the difference is that carbon dioxide is replaced with nitrogen in step (1).

[0066] Finally, the collected liquid products were analyzed, and the yield of aromatic hydrocarbons was 5.80%, as detailed in Table 1.

Claims

1. A method for producing light aromatic hydrocarbons from waste polypropylene plastic by catalytic pyrolysis, characterized in that, The Zr-Ga bimetallic modified multi-pore hollow ZSM-5 molecular sieve catalyst is synthesized by a hydrothermal method, and then a non-in-situ catalysis method is adopted to perform a cyclization aromatization reaction on waste polypropylene raw materials in a nitrogen or carbon dioxide atmosphere, so as to directly convert the polypropylene into light aromatic hydrocarbons.

2. The method of claim 1, wherein, The Zr-Ga bimetallic modified multi-pore hollow ZSM-5 molecular sieve catalyst has a zirconium loading of 1-2% and a gallium loading of 0.5-5%.

3. The method of claim 1, wherein, The specific steps are as follows: (1) Preparation of original ZSM-5 molecular sieve: sodium hydroxide, tetrapropylammonium hydroxide, aluminum nitrate and tetraethyl orthosilicate are placed in deionized water, and a precursor is obtained by hydrothermal reaction at 180-200 DEG C. After centrifugation, washing and drying, the precursor is calcined at 500-600 DEG C to obtain the original ZSM-5; (2) Preparation of multi-level pore hollow ZSM-5 molecular sieve: tetrapropylammonium hydroxide is uniformly mixed with the original ZSM-5 prepared in step (1), and then subjected to hydrothermal reaction at 80-100 DEG C, centrifugation, washing and drying to obtain the multi-level pore hollow ZSM-5; (3) Synthesis of zirconium-gallium modified hollow ZSM-5 molecular sieve: the multi-level pore hollow ZSM-5 molecular sieve obtained in step (2) is added to deionized water with gallium nitrate, and an ion exchange method is adopted to load the metal gallium, and then centrifugation and washing are performed to obtain the gallium modified ZSM-5; the gallium modified hollow ZSM-5 molecular sieve is added to deionized water with zirconium nitrate, and then an impregnation method is adopted to load the zirconium metal, and then the solution is placed in a blast drying oven, the solid residue after drying is ground and placed in a tube furnace, and the final zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst is obtained after calcination at 500-600 DEG C; (4) Two-stage pyrolysis-aromatization reaction: the whole reaction is carried out in a pyrolysis fixed bed, and the reaction is carried out in two stages; the first reaction zone: the zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst prepared in step (3) is placed in the first reaction zone of the fixed bed tube furnace, and the zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst is heated from room temperature to 500-600 DEG C at a rate of 20-40 DEG C / min under a nitrogen or carbon dioxide atmosphere, and maintained at this temperature for 60-80 min; the second reaction zone: the waste polypropylene is placed in the second reaction zone of the fixed bed tube furnace, and after the temperature of the first reaction zone reaches the set value, the waste polypropylene in the second reaction zone is heated from room temperature to 500-600 DEG C at a rate of 20-40 DEG C / min, and maintained at this temperature for 40-60 min; a large amount of volatile matter is generated by the preliminary pyrolysis of the waste polypropylene in the second reaction zone, and then the volatile matter generated by pyrolysis is carried away by the carrier gas to the first reaction zone, and further catalytic aromatization is carried out under the catalysis of the molecular sieve; (5) Product collection.

4. The method of claim 3, wherein, In step (1), the amount ratio of sodium hydroxide, tetrapropylammonium hydroxide, aluminum nitrate and tetraethyl orthosilicate is 0.1-0.5 g: 15-20 mL: 0.5-1 g: 10-15 mL; the reaction time in the hydrothermal reactor is 46-50 h; and the calcination time of the catalyst in the tube furnace is 6-8 h.

5. The method of claim 3, wherein, In step (2), the concentration of the tetrapropyl ammonium hydroxide solution is 0.2-0.3 M; the reaction time in the hydrothermal reactor is 22-26 h.

6. The method of claim 3, wherein, In step (3), the temperature of the ion exchange method is 70-80℃, and the time is 2-3 h; the temperature of the impregnation method is 70-80℃, and the time is 2-3 h; the calcination time in the tube furnace is 5-8 h.

7. The method of claim 3, wherein, In step (4), the ratio of the zirconium-gallium modified hollow ZSM-5 molecular sieve catalyst to the waste polypropylene plastic raw material is (1-2):1, the nitrogen or carbon dioxide is used as the carrier gas in the fixed bed during the reaction, the carrier gas flow rate is 30-40 mL / min, the pyrolysis temperature of the first section of the fixed bed is 500-600℃, the pyrolysis temperature of the second section of the fixed bed is 500-600℃, and the total reaction time is 40-60 min.

8. The method of claim 3, wherein, In step (5), the light aromatic hydrocarbons are collected by a condensation device, the condensation device is composed of a cold trap and a liquid collection bottle connected by a rubber tube, the liquid collection bottle is placed in a heat preservation box, the heat preservation box uses an ice-water mixture as the cooling medium, and the liquid collection bottle is filled with isopropyl alcohol to collect the product.

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