Method for preparing monocyclic aromatic hydrocarbon through polystyrene hydrocracking

Abstract

The invention relates to the field of plastic degradation, in particular to a polystyrene hydrocracking method. Comprising the following steps: respectively adding a metal sulfide catalyst and polystyrene into a suspended bed reactor, sealing the suspended bed reactor, introducing hydrogen with a certain pressure, and starting a reaction; the polystyrene hydrocracking method provided by the invention adopts the non-noble metal catalyst, and is more economical. The problem that polystyrene is easy to generate hydrogen by self-cyclization and polycyclic aromatic hydrocarbon is formed is solved; meanwhile, the problems of benzene ring hydrogenation and aromatic hydrocarbon selectivity reduction are solved, and the yield of monocyclic aromatic hydrocarbon is up to 90%. The method can realize complete cracking of waste polystyrene, and has good economic benefits.

Description

Technical Field

[0001] The present invention relates to the field of plastic degradation, and is a method for hydrocracking of polystyrene. Background Art

[0002] Plastics mainly come from petroleum. From 1950 to 2017, the cumulative production of plastics reached as high as 9.2 billion tons. However, only 0.7 billion tons of plastics were recycled, 1 billion tons were incinerated, and 5.3 billion tons were discarded into the natural environment. The natural cracking time of plastic waste is 200 to 1000 years, and it will accumulate in the environment for a long time. It not only wastes petroleum and land resources, but also pollutes the environment and endangers human health. Polystyrene is one of the seven major types of plastics, and its consumption in 2019 reached as high as 21 million tons. The downstream applications of its products are mainly concentrated in the fields of electronic appliances, building materials, packaging materials, and daily necessities. However, at present, less than 1% of polystyrene is recycled.

[0003] At present, the treatment methods for recycled waste polystyrene mainly include mechanical recycling, energy recycling, and landfill treatment. The above treatment methods for waste polystyrene will have huge economic and environmental negative effects. Among the developed routes, the traditional pyrolysis and catalytic cracking processes of polystyrene are simple, but the catalyst is severely carbon-deposited, and light gases and solid residues will be produced. In addition, there are also plasma-assisted rapid hydrocracking of polystyrene, photocatalytic oxidation of polystyrene, and azoalkane-assisted polystyrene cracking routes. Although these routes can be carried out at lower temperatures, the yield of high-value oil products is low and the feasibility is poor. The hydrocracking of polystyrene can almost completely convert into aromatic hydrocarbons and alkanes. At present, there are few reports on the hydrocracking of polystyrene. Patent 202310316107.6 discloses a route for cracking polystyrene into monocyclic aromatic hydrocarbons and monocyclic alkanes under the action of a supported ruthenium catalyst and methanol. The hydrocracking of polystyrene mainly uses noble metals such as Pt and Ru as catalysts, and the catalytic performance is excellent, but noble metal catalysts are extremely prone to poisoning. Waste plastics contain a large amount of impurities, which are extremely likely to poison noble metals. Therefore, there is an urgent need to develop an implementable method for recycling waste polystyrene, with the key point being to find a suitable hydrocracking catalyst that can completely upgrade waste polystyrene into valuable aromatic hydrocarbons under harsh conditions. Metal sulfides are two-dimensional layered materials and have been used in various process such as catalytic hydrogenation, hydrodesulfurization, hydrodenitrogenation, hydrodeoxygenation, hydrocracking, hydrodemetallization, hydrodealkylation, and hydroring-opening in coal chemical industry and oil refining industry. At present, there is no report on the one-pot catalytic hydrocracking of polystyrene to produce light aromatic hydrocarbons. Summary of the Invention

[0004] The present invention provides a method for catalytic hydrocracking of polystyrene to produce light aromatics in one pot, which can effectively solve the problems of low polystyrene conversion rate, coking, reactor bed plugging and catalyst deactivation during the polystyrene cracking process. Moreover, the catalyst cost is low, while the selectivity of mono-aromatics is improved, having good economic benefits.

[0005] The present invention provides a method for preparing mono-cyclic aromatics by hydrogenating polystyrene, comprising the following steps: adding a metal sulfide catalyst and polystyrene into a suspension bed reactor respectively, sealing the suspension bed reactor, and introducing hydrogen under a certain pressure.

[0006] The metal element in the above metal sulfide catalyst is at least one of Fe, Co, Ni, Mo and W. The mass ratio of the catalyst to polystyrene is 1:19 to 1:10000.

[0007] The molar ratio of the H 2 to polystyrene monomer is 1:0.8 to 1:2.

[0008] The hydrocracking of polystyrene is carried out at a temperature of 300 °C to 450 °C.

[0009] The time for the hydrocracking of polystyrene is 0.5 h to 12 h.

[0010] The present invention is the application of the above catalyst in the cracking of polystyrene. Under the above reaction conditions, the hydrogen activated by the metal sulfide cracks the polystyrene chain to generate mono-cyclic aromatics (at least one or more of benzene, toluene, ethylbenzene, isopropylbenzene). Compared with the cracking of polystyrene catalyzed by noble metals, this route not only avoids the further hydrogenation of aromatic products, but is also more economical. In addition, it avoids the side reaction of aromatization of the polystyrene chain to generate by-products such as poly-cyclic aromatics. The present invention proposes a new reaction route to upgrade polystyrene, and adjusts the reaction temperature and reaction time to further optimize the reaction activity and product selectivity. Experiments show that by using the metal sulfide catalyst in the present invention, 10 MPa H 2, under the condition of reacting at 400 °C for 2 hours, comparing the pyrolysis of polystyrene with the cracking of polystyrene catalyzed by the metal sulfide described in the present invention, it is found that in the pyrolysis of polystyrene, the yield of monocyclic aromatic hydrocarbons follows a trend: metal sulfide catalyzed polystyrene cracking > polystyrene pyrolysis. The yield of monocyclic aromatic hydrocarbons in the product of metal sulfide catalyzed polystyrene cracking is as high as 90.71 wt%. Under the same other conditions of the present invention, tests were carried out at 0.5 h, 1 h, 2 h, 4 h, 8 h, and 12 h respectively, and it was found that with the extension of the reaction time, the conversion of polystyrene was more complete, and the yields of toluene, ethylbenzene, and isopropylbenzene increased significantly. The present invention was tested at reaction temperatures of 360 °C, 380 °C, 400 °C, 420 °C, and 425 °C respectively under the same other conditions. By regulating the reaction temperature, it was found that when the reaction temperature was reduced to 360 °C, the cracking degree of polystyrene decreased, and when the reaction temperature was increased to 425 °C, polystyrene was almost completely cracked, indicating that the increase in temperature accelerated the reaction kinetics.

[0011] The present invention solves the problems of benzene ring hydrogenation and reduction of aromatic hydrocarbon selectivity; solves the problem that polystyrene is prone to self-cyclization to produce hydrogen and form polycyclic aromatic hydrocarbons; and also solves the problem of easy deactivation of the catalyst. Thus, it can be seen that the method provided by the present invention is environmentally friendly, the catalyst has high activity and stability, and the product has high value. It can achieve the cracking of polystyrene, successfully avoid the problems of coking and lightening during the cracking process, and has very good economic benefits. Brief Description of the Drawings

[0012] Figure 1 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 1;

[0013] Figure 2 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 2;

[0014] Figure 3 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 3;

[0015] Figure 4 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 4;

[0016] Figure 5 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 5;

[0017] Figure 6 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 6;

[0018] Figure 7 It is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 7. Detailed Description of the Invention

[0019] The present invention is not limited by the following embodiments, and specific implementation manners can be determined according to the technical solutions of the present invention and actual situations. Various chemical reagents and chemical supplies mentioned in the present invention are well-known and commonly used chemical reagents and chemical supplies in the prior art unless otherwise specified; the percentages in the present invention are mass percentages unless otherwise specified.

[0020] The method for hydrocracking of the polystyrene material is carried out according to the following method: A metal sulfide catalyst and polystyrene are respectively added into a suspension bed reactor, the suspension bed reactor is sealed, and hydrogen is introduced at a certain pressure. The metal element in the above metal sulfide catalyst is at least one of Fe, Co, Ni, Mo, and W. The mass ratio of the catalyst to polystyrene is 1:19 to 1:10000. The molar ratio of H 2 to benzene in polystyrene is 1:0.8 to 1:2. The hydrocracking of the polystyrene is carried out at a temperature of 300°C to 450°C. The hydrocracking time of the polystyrene is 0.5 h to 12 h. By adopting the suspension bed hydrogenation process, the present invention can achieve complete conversion of polystyrene, with a high yield of monocyclic aromatic hydrocarbons, and can realize the cracking of the polystyrene material, having extremely high economy.

[0021] Example 1:

[0022] The method for hydrocracking of the polystyrene is carried out according to the following method: Molybdenum disulfide is added into a suspension bed reactor (the molybdenum disulfide is prepared by a hydrothermal method. 1.94 g of L-cysteine is dissolved in 50 mL of deionized water, transferred to a hydrothermal autoclave, and then 0.58 g of molybdenum oxide is added to the above hydrothermal autoclave. After sealing the above hydrothermal autoclave, it is heated at 200°C for 12 hours, cooled, filtered, washed, evaporated to dryness and ground, and finally molybdenum disulfide is obtained. The length of the molybdenum disulfide lamella is 5 nm to 20 nm, and the stacking layer number is 1 to 5 layers, as Figure 1 shown, Figure 1 which is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 1), and polystyrene is added into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:1000; the suspension bed reactor is sealed, and hydrogen at 10 MPa is introduced. The molar ratio of H 2 to benzene in polystyrene is 1:1. The reaction is carried out under the reaction conditions of a temperature of 400°C; the residence time of the mixture in the suspension bed reactor is 2 hours; a polystyrene cracking product is obtained.

[0023] Example 2:

[0024] The method for hydrocracking of polystyrene is carried out as follows: Add molybdenum disulfide catalyst into a suspension bed reactor (the molybdenum disulfide is prepared by a hydrothermal method. Dissolve 2.30 g of L-cysteine in 50 mL of deionized water, transfer it to a hydrothermal autoclave, then add 1.00 g of ammonium heptamolybdate into the above hydrothermal autoclave. Seal the above hydrothermal autoclave and heat it at 180 °C for 24 hours. After cooling, filter, wash, evaporate to dryness and grind to finally obtain molybdenum disulfide. The obtained molybdenum disulfide has a sheet length of 5 nm to 10 nm and a stacking layer number of 1 to 10 layers, as Figure 2 shown, Figure 2 which is the transmission electron microscopy image of the molybdenum disulfide catalyst prepared in Example 2). Add polystyrene into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:500; Seal the suspension bed reactor, introduce 12 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1.2:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 1 hour; Obtain the polystyrene cracking product.

[0025] Example 3:

[0026] The method for hydrocracking of polystyrene is carried out as follows: Add molybdenum disulfide catalyst into a suspension bed reactor (the molybdenum disulfide is prepared by a hydrothermal method. Dissolve 1.94 g of L-cysteine in 30 mL of deionized water, transfer it to a hydrothermal autoclave, then add 1.30 g of molybdenum acetylacetonate into the above hydrothermal autoclave. Seal the above hydrothermal autoclave and heat it at 160 °C for 24 hours. After cooling, filter, wash, evaporate to dryness and grind to finally obtain molybdenum disulfide. The obtained molybdenum disulfide has a sheet length of 2 nm to 10 nm and a stacking layer number of 1 to 3 layers, as Figure 3 shown, Figure 3 which is the transmission electron microscopy image of the molybdenum disulfide catalyst prepared in Example 3). Add polystyrene into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:500; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 8 hours; Obtain the polystyrene cracking product.

[0027] Example 4:

[0028] The method for hydrocracking polystyrene is carried out as follows: Add molybdenum disulfide catalyst into a suspension bed reactor (the molybdenum disulfide is prepared by a hydrothermal method. Dissolve 1.94 g of L-cysteine in 50 mL of deionized water, transfer it to a hydrothermal kettle, then add 0.58 g of molybdenum oxide into the above hydrothermal kettle. Seal the above hydrothermal kettle and heat it at 220 °C for 12 hours. After cooling, filter, wash, evaporate and dry, and then grind to finally obtain molybdenum disulfide. The obtained molybdenum disulfide has a lamellar length of 10 nm to 30 nm and a stacking layer number of 1 to 10 layers, as Figure 4 shown, Figure 4 is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 4). Add polystyrene into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:200; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 380 °C; The residence time of the mixture in the suspension bed reactor is 2 hours; Obtain polystyrene cracking products.

[0029] Example 5:

[0030] The method for hydrocracking polystyrene is carried out as follows: Add cobalt molybdenum sulfide catalyst into a suspension bed reactor (the cobalt molybdenum sulfide is prepared by a hydrothermal method. Dissolve 2.05 g of L-cysteine in 50 mL of deionized water, transfer it to a hydrothermal kettle, then add 0.30 g of molybdenum oxide and 0.50 g of cobalt acetate into the above hydrothermal kettle. Seal the above hydrothermal kettle and heat it at 200 °C for 22 hours. After cooling, filter, wash, evaporate and dry, and then grind to finally obtain cobalt molybdenum sulfide. The obtained cobalt molybdenum sulfide has a lamellar length of 3 nm to 10 nm and a stacking layer number of 1 to 5 layers, as Figure 5 shown, Figure 5 is the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 5). Add polystyrene into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:200; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 8 hours; Obtain polystyrene cracking products.

[0031] Example 6:

[0032] The method for hydrocracking polystyrene is carried out as follows: Add a nickel molybdenum sulfide catalyst to a suspension bed reactor (the molybdenum disulfide is prepared by a hydrothermal method. Dissolve 2.04 g of L-cysteine in 50 mL of deionized water, transfer it to a hydrothermal autoclave, then add 0.30 g of molybdenum oxide and 0.50 g of nickel acetate to the above hydrothermal autoclave. Seal the above hydrothermal autoclave and heat it at 200 °C for 22 hours. After cooling, filter, wash, evaporate to dryness and then grind to finally obtain nickel molybdenum sulfide. The obtained nickel molybdenum sulfide has a lamellar length of 5 nm to 10 nm and a stacking layer number of 1 to 5 layers, as Figure 6 shown, Figure 6 the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 6), add polystyrene to the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:200; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 425 °C; The residence time of the mixture in the suspension bed reactor is 2 hours; Obtain a polystyrene cracking product.

[0033] Example 7:

[0034] The method for hydrocracking polystyrene is carried out as follows: Add a molybdenum disulfide catalyst to a suspension bed reactor (the molybdenum disulfide is prepared by a hydrothermal method. Dissolve 1.22 g of thiourea in 50 mL of deionized water, transfer it to a hydrothermal autoclave, then add 0.71 g of ammonium heptamolybdate to the above hydrothermal autoclave. Seal the above hydrothermal autoclave and heat it at 180 °C for 24 hours. After cooling, filter, wash, evaporate to dryness and then grind to finally obtain molybdenum disulfide. The obtained molybdenum disulfide has a lamellar length of 5 nm to 25 nm and a stacking layer number of 1 to 10 layers, as Figure 7 shown, Figure 7 the transmission electron microscope image of the molybdenum disulfide catalyst prepared in Example 7), add polystyrene to the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:500; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 1 hour; Obtain a polystyrene cracking product.

[0035] Comparative Example 1

[0036] The method for hydrocracking polystyrene is carried out as follows: Add 30 g of polystyrene to a suspension bed reactor and seal the suspension bed reactor. Introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 2 hours; Obtain a polystyrene pyrolysis product.

[0037] Comparative Example 2

[0038] The method for hydrogenation cracking of polystyrene is carried out as follows: Add molybdenum disulfide catalyst into a suspension bed reactor, and the molybdenum disulfide is purchased from Aladdin. Add polystyrene into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:200; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 2 hours; Obtain polystyrene cracking products.

[0039] Comparative Example 3

[0040] The method for hydrogenation cracking of polystyrene is carried out as follows: Add a commercial Pt / C catalyst (Pt loading is 5 wt.%) into a suspension bed reactor, and add polystyrene into the suspension bed reactor according to the mass ratio of the catalyst to polystyrene of 1:200; Seal the suspension bed reactor, introduce 10 MPa of hydrogen, and the molar ratio of H 2 to benzene in polystyrene is 1:1. React under the reaction conditions of a temperature of 400 °C; The residence time of the mixture in the suspension bed reactor is 1 hour; Obtain polystyrene cracking products.

[0041] In the present invention, in Examples 1 to 7 and Comparative Examples 1 to 3, the results of hydrogenation cracking of polystyrene are shown in Table 1. It can be seen from Table 1 that the present invention has very good implementation effects. After the hydrogenation reaction of the polystyrene material, the conversion rate is as high as 99.9%, and the yield of monocyclic aromatic hydrocarbons can be as high as 90.71%. The composition of the monocyclic aromatic hydrocarbons is at least one or more of benzene, toluene, ethylbenzene, and isopropylbenzene. The method for hydrogenation cracking of polystyrene provided by the present invention uses a non-noble metal catalyst, which is more economical; The present invention solves the problem of benzene ring hydrogenation and reduces the aromatic hydrocarbon selectivity; Solves the problem that polystyrene is prone to self-cyclization to produce hydrogen and form polycyclic aromatic hydrocarbons; At the same time, it also solves the problem that the catalyst is prone to deactivation. It can achieve complete cracking of waste polystyrene and has good economic benefits.

[0042] The above technical features constitute the embodiments of the present invention, which have strong adaptability and implementation effects. Non-essential technical features can be increased or decreased according to actual needs to meet the requirements of different situations.

[0043] Table 1

[0044]

[0045] The method for hydrocracking polystyrene provided by the present invention uses a non-noble metal catalyst, which is more economical. It solves the problem that polystyrene is prone to form hydrogen by self-cyclization and form polycyclic aromatic hydrocarbons; at the same time, it solves the problem of benzene ring hydrogenation and reduces the aromatic selectivity, and the yield of monocyclic aromatic hydrocarbons is as high as 90%. It can achieve complete cracking of waste polystyrene and has good economic benefits.

Claims

1. A method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene, characterized in that, it comprises the following steps: Under the action of a catalyst and H 2 , hydrogenate and crack polystyrene; The catalyst is a metal sulfide.

2. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to claim 1, characterized in that, the metal element in the metal sulfide catalyst is at least one or more than two of Fe, Co, Ni, Mo and W.

3. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to claim 1, characterized in that, in the metal sulfide catalyst, the metal element Mo.

4. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to claim 1, characterized in that, the metal sulfide is a two-dimensional layered material, the length of the metal sulfide catalyst sheet (the length of the straight line connecting the two farthest points on the sheet) is less than 1 μm, and the stacking layer number is less than 50 layers; Preferably, the length of the metal sulfide sheet is less than 100 nm, and the stacking layer number is less than 20 layers. More preferably, the length of the metal sulfide sheet is less than 10 nm, and the stacking layer number is less than 10 layers.

5. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to claim 1, characterized in that, the mass ratio of the catalyst to polystyrene is 1:19 to 1:10000. Preferably, the mass ratio of the catalyst to polystyrene is 1:100 to 1:1000. More preferably, the mass ratio of the catalyst to polystyrene is 1:250 to 1:

500.

6. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to claim 1, characterized in that, The molar ratio of H 2 to benzene in polystyrene is from 1:0.8 to 1:2, preferably 2 the molar ratio of H to benzene in polystyrene is from 1:1 to 1:2, more preferably 2 the molar ratio of H to benzene in polystyrene is from 1:1 to 1:1.

5.

7. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to claim 1, characterized in that, The specific process is as follows: Add the metal sulfide catalyst and polystyrene into a suspension bed reactor respectively, seal the suspension bed reactor, and start the reaction after introducing hydrogen at a certain pressure; the reaction pressure is 8 MPa to 20 MPa, preferably 10 MPa to 15 MPa, and more preferably 10 MPa to 12 MPa.

8. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to any one of claims 1 to 6, characterized in that, the hydrogenation cracking of polystyrene is carried out at a temperature of 300 °C to 450 °C. Preferably, the hydrogen cracking is carried out at a temperature of 380 °C to 420 °C, and more preferably the hydrogen cracking is carried out at a temperature of 400 °C; the time for the hydrogenation cracking of polystyrene is 0.5 h to 12 h. Preferably, the time for the hydrogenation cracking is 2 h to 12 h, and more preferably the time for the hydrogenation cracking is 4 h to 8 h.

9. The method for preparing monocyclic aromatic hydrocarbons by hydrogenating polystyrene according to any one of claims 1 to 7, characterized in that, the polystyrene is waste polystyrene plastic.

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