Method for preparing single-benzene-ring liquid chemicals through solvent-free hydrogenation degradation of polystyrene waste plastics based on bifunctional catalyst

Abstract

The invention discloses a method for preparing a single-benzene-ring liquid chemical through solvent-free hydrogenation degradation of polystyrene waste plastics based on a bifunctional catalyst. According to the method, Nb2O5 is taken as a carrier, 1-3wt% of noble metal ruthenium is loaded to prepare a bifunctional catalyst, and high-molecular-weight polystyrene is efficiently degraded into single-benzene-ring liquid chemicals such as benzene, methylbenzene and ethylbenzene through hydrogenation reaction under a solvent-free condition. Precious metal ruthenium hydrogenates a polystyrene long chain to generate a short-chain hydrocarbon intermediate, then a carrier # imgabs0 # acid site drives beta-cracking and hydrogen transfer reaction, and a single-benzene-ring product is directionally generated through the synergistic effect of the short-chain hydrocarbon intermediate and the carrier # imgabs0 # acid site. The method has the advantages of no solvent, reduced equipment cost, high reaction efficiency, simple reaction operation, mild reaction conditions, simple catalyst preparation process, high activity, selectivity and good stability, and is suitable for industrial production, and no other solvents preempt acidic sites. The method not only can realize cyclic utilization of resources, but also can alleviate the current environmental pollution problem.

Description

Technical Field

[0001] The present invention belongs to the technical fields of catalytic chemistry and waste plastic resource utilization, and relates to a solvent-free hydrogenation degradation method based on a bifunctional catalyst for efficiently converting high-molecular-weight polystyrene waste plastics into high-value single-benzene-ring liquid chemicals such as benzene, toluene, and ethylbenzene. Background Art

[0002] Polystyrene (PS) has advantages such as low density, low cost, excellent processability, and dimensional stability, and has become a general-purpose plastic with an annual global output of over ten million tons, being widely used in fields such as packaging, construction, and medical equipment. However, its chemical properties are very stable and natural degradation is extremely difficult, causing it to continuously accumulate in the environment, accounting for one-fourth of the total waste. According to a 2023 report by the World Wide Fund for Nature, approximately 30% of the annual global waste polystyrene enters the marine ecosystem, and microplastics mainly composed of polystyrene waste plastics have even been found in the bodies of marine organisms, seriously threatening ecological safety and human health. Therefore, finding a method for recycling waste polystyrene is crucial for the ecological environment.

[0003] So far, the treatment methods for polystyrene waste plastics mainly include two categories: mechanical methods and chemical methods. Although the mechanical method, which mainly involves in-factory direct recycling and mechanical pulverization, is essentially an environmentally friendly and economically sustainable polystyrene recycling solution, it has a high cost and a recycling limit. Its mechanical properties will degenerate periodically with the increase in the number of recycling times, resulting in a gradual decline in product quality. For example, as reported in CN114524844A, the tensile strength of polystyrene decreases by 40% after three recycles. Although it has short-term economic viability, it cannot solve the problem of quality deterioration caused by molecular chain breakage. Therefore, the potential of using chemical methods to treat polystyrene waste plastics is higher. However, although chemical treatment can achieve the reuse of polystyrene at the molecular level, most of them have problems such as low product added value or harsh reaction conditions. For example, CN112961047A provides a method for synthesizing benzoic acid by photocatalytic oxidation of polystyrene. This method requires a reaction time of 24 hours, which is too long, and the reaction product is limited to benzoic acid. CN115947977A provides a method for catalytic degradation of polystyrene under a supercritical CO2-assisted environment by mixing an auxiliary solution with the plastic. Although the degradation efficiency of this method can be as high as 90%, it requires maintaining a high pressure of above 7 Mpa and a high temperature of 300 °C to achieve, and the equipment safety risks and energy consumption issues cannot be underestimated. In addition, CN202110546377.7 uses a Pt-based catalyst to carry out the catalytic degradation reaction of PS in a hydrogen atmosphere, which also requires a high pressure of 5 MPa and a reaction time of 12 hours, and the selectivity of monobenzene ring aromatic hydrocarbons is less than 60%. Similarly, US20220380012A1 uses a solvent-based hydrogenation system to catalytically degrade polystyrene, but it needs to use tetrahydrofuran solvent and maintain a high pressure of 10 MPa, while the catalyst cycle stability is only 3 times, and the solvent residue leads to a 15% increase in the subsequent separation cost.

[0004] In view of the above research problems, there is an urgent need to explore a degradation method with high efficiency and high conversion rate under mild conditions. Summary of the Invention

[0005] In view of the above problems in polystyrene degradation of the present invention: harsh reaction conditions (requiring a hydrogen pressure of above 5 MPa and a high temperature of above 300 °C), strong solvent dependence (tetrahydrofuran solvent leads to an increase in separation cost), low product selectivity (the selectivity of monobenzene ring aromatic hydrocarbons is less than 60%), and poor catalyst cycle stability (the maximum use is 3 times), etc., a solvent-free hydrogenation degradation method based on a Ru / Nb2O5 bifunctional catalyst is provided. It is a method that can efficiently and highly selectively catalytically degrade high-molecular-weight polystyrene waste plastics into high-value-added monobenzene ring liquid chemicals under mild conditions of 225 - 275 °C and 2 - 3 MPa. The selectivity of monobenzene ring compounds such as benzene, toluene, and ethylbenzene in the liquid product is higher than 90%, and the catalyst can be recycled more than 5 times.

[0006] Technical solution of the present invention:

[0007] A method for preparing monobenzene ring liquid chemicals by solvent-free hydrogenation degradation of polystyrene waste plastics based on a bifunctional catalyst, comprising the following steps:

[0008] (1) Using an inorganic solid acid as a carrier, a bifunctional metal catalyst loaded with 1-3 wt% of noble metal ruthenium is prepared;

[0009] (2) The bifunctional metal catalyst is first calcined in an oxygen-argon mixed gas atmosphere at 400 °C for 4 h, and then pre-reduced in a hydrogen-argon mixed gas atmosphere at 300 °C for 3 h to obtain a catalyst;

[0010] (3) Under solvent-free reaction conditions, the polystyrene waste plastics and the catalyst obtained in step (2) are mixed at a mass ratio of 10:1 - 20:1, and the reaction is carried out at a temperature of 225 - 275 °C and a hydrogen pressure of 2 - 3 Mpa for 4 - 9 h;

[0011] (4) After the reaction, the liquid product is separated, and the catalyst can be recycled ≥ 5 times after being calcined and regenerated.

[0012] In step (1), the inorganic solid acid is Nb2O5.

[0013] In step (2), the volume ratio of oxygen to argon in the oxygen-argon mixed gas is 1:1.

[0014] In step (2), the volume ratio of hydrogen to argon in the hydrogen-argon mixed gas is 1:1.

[0015] The liquid conversion rate of polystyrene in this method is greater than 80%, and the selectivity of liquid products such as benzene, toluene, and ethylbenzene, which are monobenzene ring high-value-added chemicals, is greater than 90%. The monobenzene ring chemicals can be directly used as pharmaceutical intermediates or high-octane fuel additives.

[0016] Advantages of the present invention: The present invention breakthroughly designs a method for catalytic degradation of PS by using a bifunctional metal catalyst under solvent-free conditions, which can efficiently degrade PS under mild conditions of 225-275 °C and a hydrogen pressure of 2-3 MPa, and convert it into high-value single-benzene-ring liquid chemicals, which can be used as precursors for synthesizing various products or directly utilized. The catalyst utilizes the synergistic effect of noble metal Ru and inorganic solid acid Nb2O5. Ru preferentially hydrogenolyzes the long chain of polystyrene to generate short-chain hydrocarbon intermediates; while the acidic sites of Nb2O5 promote β-scission and hydrogen transfer reactions to direct the formation of single-benzene-ring products, thereby realizing the catalytic degradation of high-molecular-weight polystyrene waste plastics. This synergistic effect is particularly significant under solvent-free conditions, avoiding the competitive adsorption of solvent molecules on the acidic sites. The catalyst used in this method is simple to prepare, has stable performance, and is easy to regenerate. The degradation process is simple to operate, has low energy consumption, and mild degradation conditions. The solvent-free condition makes the reaction more efficient, and it can be operated in a conventional autoclave, greatly reducing the equipment cost. Therefore, this method has very important research significance for the degradation of polystyrene waste plastics, can greatly relieve the economic pressure brought by "white pollution", and has broad industrial application prospects. Brief Description of the Drawings

[0017] Figure 1 It is a thermogravimetric diagram of polystyrene waste plastics. Detailed Embodiments

[0018] The following further illustrates the detailed embodiments of the present invention in combination with the drawings and technical solutions.

[0019] Example 1 Preparation of Ru / Nb2O5 Catalyst

[0020] Add a certain amount of RuCl3 (0.57 g / 100 ml) solution of deionized water and Nb2O5 support to a 250 ml round-bottom flask, so that the ruthenium loading is 1-3 wt%, and stir at room temperature for more than 12 h. After the impregnation, remove the moisture by rotary evaporation and dry it in an oven at 80 °C for 12 h to ensure complete removal of the moisture. Subsequently, the prepared catalyst is calcined at 400 °C for 3 h in a mixed atmosphere of oxygen and argon (volume ratio 1:1). Before each reaction, the catalyst is pre-reduced at 300 °C for 3 h in a mixed atmosphere of hydrogen and argon (volume ratio 1:1) in a tubular furnace.

[0021] The preparation methods of catalysts with other supports (Al2O3, SiO2, TiO2) are the same as above.

[0022] Example 2 Explore the differences between catalytic degradation and pyrolysis in an autoclave reactor under the same reaction conditions

[0023] Weigh 4.0 g of polystyrene plastic and 0.4 g of Ru / Nb2O5 catalyst respectively. After continuous reaction for 6 h at 250 °C under 2 MPa of H2, cool it to room temperature and analyze the products. The liquid yield reaches 78.3%, and the selectivity of monobenzene ring compounds such as toluene and ethylbenzene reaches 90%. When the reaction is carried out under the same reaction conditions without adding a catalyst, the polystyrene only melts and no degradation reaction occurs, and the solid residue reaches 98%. Figure 1 This can be used as evidence to illustrate the necessity of this catalyst.

[0024] Example 3 explores the influence of different reaction temperatures on the catalytic reaction of polystyrene waste plastics in an autoclave reactor.

[0025] Weigh 4.0 g of polystyrene waste plastic and 0.4 g of Ru / Nb2O5 catalyst respectively. After continuous reaction for 6 h under the condition of 2 MPa of H2, explore the influence of different reaction temperatures on the catalysis. The main temperatures explored are 200 °C, 225 °C, 250 °C, 275 °C and 300 °C.

[0026]

[0027] It can be seen from the above data that the reaction temperature has an important influence on the formation of monobenzene ring compounds. At the appropriate temperature range of 225 °C - 275 °C, the Ru / Nb2O5 bifunctional catalyst can act synergistically efficiently. At this time, the metal Ru can not only break the polystyrene long chain, but also will not cause excessive hydrogenation of the reaction products due to too high temperature. And the strong Bronsted acid sites of Nb2O5 have the best activity at this temperature, and preferentially attack the C-C bond at the β-position of the benzene ring through protonation, promoting the directional formation of monobenzene ring products. The reason why the reaction temperature is the most suitable at this time is that when the temperature is too low, the hydrogenolysis of Ru metal cannot be fully activated, resulting in low efficiency of breaking the polystyrene long chain, and the B acid sites need a certain amount of thermal energy to trigger the cleavage of the C-C bond at the β-position of the benzene ring through protonation, and its catalytic ability is insufficient at low temperature, and the intermediate is difficult to be directionally converted into monobenzene ring products. When the temperature is too high, the hydrogenation ability of Ru is significantly enhanced, resulting in further hydrogenation of monobenzene ring products to form saturated hydrocarbons such as cyclohexane, and at the same time triggering the cracking of short-chain hydrocarbons to form gaseous alkanes. High temperature will also cause surface reconstruction of the Nb2O5 support or inactivation of acidic sites, weakening its β-cracking guiding ability, resulting in random cleavage of C-C bonds and formation of by-products such as polycyclic aromatic hydrocarbons. Therefore, too high or too low temperature will break the bifunctional balance of Ru / Nb2O5, and the temperature range of 225 - 250 °C is the optimal solution for the two to act synergistically to obtain monobenzene ring aromatic compounds.

[0028] Example 4 explores the influence of different reaction pressures on the catalytic reaction of polystyrene waste plastics in an autoclave reactor.

[0029] Weigh 4.0 g of polystyrene waste plastic and 0.4 g of catalyst. Under the condition of 250 °C, after continuous reaction for 6 h, the influence of different reaction pressures on catalysis was explored. The pressures of 0 MPa, 1 MPa, 2 MPa, 3 MPa and 4 MPa were mainly explored.

[0030]

[0031] It can be seen from this example that the reaction pressure has an important influence on the formation of monobenzene ring compounds. Under the medium pressure of 2 - 3 MPa, the hydrogen concentration is sufficient to activate the hydrogenolysis of Ru, effectively breaking the polystyrene long chain into short-chain intermediates. At the same time, the β-scission reaction guided by the B acid sites can proceed fully. At this time, the supply amount of hydrogen can not only promote the target reaction, but also prevent the monobenzene ring products from being over-hydrogenated and damaged due to excessive hydrogen. When the H2 pressure is too low, the low H2 concentration limits the hydrogenolysis of Ru metal, and the polystyrene long chain cannot be effectively broken. At the same time, the strong Bronsted acid sites of Nb2O5 require appropriate H2 to assist the hydrogen transfer reaction to stabilize the intermediates. Due to insufficient pressure, it is difficult for the intermediates to be directionally formed into monobenzene ring products through β-scission, and the reaction stays in the initial degradation stage. When the pressure is too high, excessive H2 significantly enhances the hydrogenation ability of Ru, resulting in the further hydrogenation of monobenzene ring products into saturated hydrocarbons such as cyclohexane. At the same time, short-chain hydrocarbons are cracked into gaseous alkanes, and the selectivity of monobenzene ring decreases. Also, under high pressure, hydrogen molecules will occupy the B acid sites of Nb2O5, weakening its guiding effect on β-scission, resulting in random cleavage of C-C bonds and generating by-products such as polycyclic aromatic hydrocarbons. Excessive H2 will also hinder the contact between the catalyst and the reactants, leading to a decrease in reaction efficiency. Therefore, the pressure also affects the catalytic ability of Ru / Nb2O5. The H2 pressure of 2 - 3 MPa can well assist the bifunctional catalyst to degrade polystyrene directionally.

[0032] Example 5 explores the influence of the acidity of the catalyst support on the reaction of catalytic polystyrene waste plastic in a high-pressure autoclave reactor

[0033] Weigh 4.0 g of polystyrene waste plastic and 0.4 g of catalyst. Under the conditions of 250 °C and 2 MPa H2, after continuous reaction for 6 h, the influence of different support acidities on catalysis was explored. Three types of catalysts, namely Ru / Nb2O5 ( acid), Ru / Al2O3 (Lewis acid), Ru / SiO2 (Lewis acid), Ru / TiO2 (Lewis acid), and Ru / C (no acid), were mainly explored.

[0034]

[0035] It can be seen from this example that the strong Bronsted acid of the Nb2O5 support is the core factor for high selectivity of single benzene rings. Its B acid sites, through protonation, preferentially attack the C-C bonds at the β-position of the benzene ring in the polystyrene chain, triggering β-cleavage reactions and directly generating single benzene ring aromatic compounds. At the same time, B acid also accelerates the active H transfer reaction, stabilizes the single benzene ring structure and inhibits the formation of polycyclic aromatic hydrocarbons. In contrast, Lewis acid supports (such as Al2O3, SiO2) tend to initiate α-cleavage, generating long-chain hydrocarbons or polycyclic by-products, while acid-free supports (such as Ru / C) lack directional catalytic ability and have a high product randomness. In addition, to fully exert the acidic catalytic effect of Nb2O5, reaction conditions need to be combined to ensure that its acidic sites are not destroyed by high temperatures and can cooperate with the hydrogenolysis of Ru. At the same time, the solvent-free condition avoids the competitive adsorption of solvent molecules on the acidic sites, further strengthening the synergistic effect. Temperature provides activation energy, pressure regulates the H2 concentration, and B acid directs the reaction path. The three work together to finally achieve the efficient conversion of polystyrene into single benzene ring aromatic compounds under mild conditions.

[0036] Example 6 explored the influence of catalysts with different metal loadings on the catalytic degradation reaction of polystyrene waste plastics in an autoclave reactor.

[0037] Weigh 4.0 g of polystyrene waste plastics and 0.4 g of catalyst. Under the conditions of 250 °C and a hydrogen pressure of 2 MPa, after continuous reaction for 6 h, the influence of different reaction loadings on catalysis was explored, mainly exploring 1 wt%, 2 wt% and 3 wt%.

[0038]

[0039] Gas chromatography-mass spectrometry analysis was performed on the liquid phase. The final results showed that as the loading increased, the selectivity of polystyrene conversion to single benzene ring aromatic compounds increased, reaching as high as 91% at 3 wt%, indicating that the loading of the catalyst has an important influence on the reaction. However, too high a loading will lead to increased economic costs, and 2 wt% is preferred.

[0040] Example 7 explored the influence of different reaction times on the catalytic degradation reaction of polystyrene waste plastics in an autoclave reactor.

[0041] Weigh 4.0 g of polystyrene waste plastics and 0.4 g of catalyst. Under the conditions of 250 °C and 2 MPa, the influence of different reaction times on catalysis was explored, mainly exploring 2 - 9 h.

[0042]

[0043] Perform gas chromatography-mass spectrometry analysis on the liquid phase. The final results show that as the reaction time increases, the selectivity of polystyrene conversion to monocyclic aromatic hydrocarbon compounds increases. The conversion rate of monocyclic aromatic hydrocarbon compounds is the highest at a reaction time of 6 hours, and the reaction effect is the best. Excessive reaction time leads to an increase in side reactions, indicating that the reaction time has an important impact on the reaction.

[0044] Example 8 explores the stability of the catalyst in an autoclave reactor

[0045] Weigh 4.0 g of waste polystyrene plastic and 0.4 g of the catalyst. Under the conditions of a hydrogen pressure of 2 MPa, a reaction temperature of 250 °C, and a reaction time of 6 hours, separate the catalyst from the product and calculate the conversion rate of polystyrene to monocyclic aromatic hydrocarbon compounds. Re-roast and reduce the recovered catalyst. Recycle the regenerated catalyst five times. During the five cycles, the liquid yield remains at 78.0 - 79.5%, and the monocyclic selectivity is 89 - 91%, indicating that the catalyst has good stability.

[0046] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for preparing single benzene ring liquid chemicals by solvent-free hydrogenation degradation of polystyrene waste plastics based on a bifunctional catalyst, characterized in that: Here are the steps: (1) A bifunctional metal catalyst prepared by using an inorganic solid acid as a carrier and loading 1-3 wt% of precious metal ruthenium; (2) calcining the bifunctional metal catalyst at 400° C. in an oxygen-argon mixed gas atmosphere for 4 h, and then pre-reducing it at 300° C. in a hydrogen-argon mixed gas atmosphere for 3 h to obtain a catalyst; (3) under solvent-free reaction conditions, polystyrene waste plastic and the catalyst obtained in step (2) are mixed in a mass ratio of 10:1-20:1, and the reaction time is 4-9 hours at a temperature of 225-275° C. and a hydrogen pressure of 2-3 MPa; (4) After the reaction, the liquid product is separated and the catalyst can be recycled for ≥5 times after calcination and regeneration.

2. The method for preparing single benzene ring liquid chemicals by solvent-free hydrogenation degradation of polystyrene waste plastics based on a bifunctional catalyst according to claim 1, characterized in that: In step (1), the inorganic solid acid is Nb2O5.

3. The method for preparing single benzene ring liquid chemicals by solvent-free hydrogenation degradation of polystyrene waste plastics based on a bifunctional catalyst according to claim 1, characterized in that: In step (2), the volume ratio of oxygen to argon in the oxygen-argon mixed gas is 1:

1.

4. The method for preparing single benzene ring liquid chemicals by solvent-free hydrogenation degradation of polystyrene waste plastics based on a bifunctional catalyst according to claim 1, characterized in that: In step (2), the volume ratio of hydrogen to argon in the hydrogen-argon mixed gas is 1:1.

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