A method for recycling polystyrene

Abstract

This application provides a method for recycling polystyrene. Under light irradiation, polystyrene and an oxidant undergo an oxidative depolymerization reaction in the presence of graphite-phase carbon nitride to generate oxidation products. The oxidant is selected from oxygen or air. The method provided in this application utilizes graphite-phase carbon nitride as a catalyst to recover polystyrene, achieving high-value-added conversion of polystyrene with high conversion rate and high selectivity. The oxidation products include high-value-added chemical products benzoic acid, acetophenone, and benzaldehyde, as well as the byproduct CO. x The yield can reach over 95%, and the total selectivity of the oxidation products for high-value-added chemical products benzoic acid, acetophenone, and benzaldehyde can reach over 75%. The catalyst used in this application is highly stable, inexpensive, easy to synthesize, and easy to recover and separate. The method provided in this application is simple, safe, has a high reaction rate, and low reaction cost, making it an environmentally friendly method for recovering polystyrene.

Description

Technical Field

[0001] This application relates to the field of polymer recycling technology, and in particular to a method for recycling polystyrene. Background Technology

[0002] Currently, the production of plastic products is rapidly increasing, and the disposal of waste plastics has become a global problem. Large quantities of plastics are directly landfilled or discarded into the natural environment after use, causing serious environmental pollution. With the development of emerging recycling and reuse methods, waste plastics have been recognized as an important carbon resource. Chemical recycling is considered the most valuable recycling method among all recycling approaches.

[0003] Chemical recycling methods for polystyrene include pyrolysis and hydrodepolymerization. Pyrolysis requires temperatures above 400°C and the presence of catalysts such as metal oxides, making its reaction conditions extremely demanding. Hydrodepolymerization typically requires supported noble metal catalysts (such as platinum-tungsten dioxide-zirconia), high-pressure pure hydrogen exceeding 2 MPa, and high temperatures above 250°C, and its products are low-value mixed hydrocarbons. Given the stringent reaction conditions and low-value products of these methods, there is an urgent need to develop a simple, easy-to-implement, and high-conversion method for recycling polystyrene to achieve high-value-added conversion of waste polystyrene. Summary of the Invention

[0004] The purpose of this application is to provide a method for recycling polystyrene to achieve high-value-added conversion of polystyrene. The specific technical solution is as follows:

[0005] This application provides a method for recycling polystyrene, comprising the following steps: under light irradiation, polystyrene and an oxidant undergo an oxidative depolymerization reaction in the presence of graphite-like carbon nitride to generate oxidation products; wherein the mass ratio of the graphite-like carbon nitride to polystyrene is (1-5):1; the oxidant is selected from oxygen or air, and the gas pressure of the oxidant is 0.1 MPa-1.5 MPa; the reaction temperature of the oxidative depolymerization reaction is 120℃-160℃, and the reaction time is 8h-24h.

[0006] In one embodiment of this application, a solvent is further added to the oxidative depolymerization reaction. The solvent is selected from at least one of acetonitrile and trifluorotoluene, and the ratio of the volume of the solvent to the mass of the polystyrene is (1-10):1 mL / mg.

[0007] In one embodiment of this application, the wavelength of the light in the illumination conditions is 280nm-980nm.

[0008] In one embodiment of this application, the oxidation products include benzoic acid, acetophenone, benzaldehyde, and CO. x, where x is 1 or 2.

[0009] In one embodiment of this application, the yield of the oxidation product is 50%-98%.

[0010] In one embodiment of this application, the total selectivity of the oxidation product for the benzoic acid, the acetophenone, and the benzaldehyde is 50%-80%.

[0011] In one embodiment of this application, the method further includes pretreating the polystyrene and then performing the oxidative depolymerization reaction. The pretreatment process includes treating the polystyrene in air or oxygen at a temperature of 200°C-320°C for 1-10 hours.

[0012] The beneficial effects of this application are:

[0013] This application provides a method for recycling polystyrene. Under light irradiation, polystyrene and an oxidant undergo an oxidative depolymerization reaction in the presence of graphite-phase carbon nitride to generate oxidation products. The oxidant is selected from oxygen or air. The method provided in this application utilizes graphite-phase carbon nitride as a catalyst to recover polystyrene, achieving high-value-added conversion of polystyrene with high conversion rate and high selectivity. The oxidation products include high-value-added chemical products benzoic acid, acetophenone, and benzaldehyde, as well as the byproduct CO. x The yield can reach over 95%, and the total selectivity of the oxidation products for high-value-added chemical products benzoic acid, acetophenone, and benzaldehyde can reach over 75%. The catalyst used in this application is highly stable, inexpensive, easy to synthesize, and easy to separate and recover. The method provided in this application is simple, safe, has a high reaction rate, and low reaction cost, making it an environmentally friendly method for recovering polystyrene.

[0014] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0016] Figure 1 This is a schematic diagram of the photo-oxidative degradation reaction of polystyrene according to one embodiment of this application;

[0017] Figure 2 This is a diagram showing the selectivity of the products in Examples 1 and 5-6 of this application.

[0018] Figure 3 This is a diagram showing the effect of selective yield on the products in Examples 1, 7 and Comparative Examples 2-3 of this application;

[0019] Figure 4 This is a graph showing the selectivity of the products in Examples 1, 8-9 and Comparative Examples 4-5 of this application;

[0020] Figure 5 The X-ray diffraction (XRD) spectra of the catalyst before and after 5 cycles in the cyclic experiment;

[0021] Figure 6 The 1H NMR spectrum of benzoic acid in the products of the amplified cyclic experiment;

[0022] Figure 7 This is a graph showing the trend of product amount in a single cycle and the cumulative product amount in a scaled-up cyclic experiment. Detailed Implementation

[0023] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0024] This application provides a method for recycling polystyrene, comprising the following steps: under light irradiation, polystyrene and an oxidant undergo an oxidative depolymerization reaction in the presence of graphite-like carbon nitride (g-C3N4) to generate oxidation products; wherein the mass ratio of graphite-like carbon nitride to polystyrene is (1-5):1; the oxidant is selected from oxygen or air, and the gas pressure of the oxidant is 0.1MPa-1.5MPa; the reaction temperature of the oxidative depolymerization reaction is 120℃-160℃, and the reaction time is 8h-24h. The method provided in this application uses graphite-like carbon nitride as a catalyst to recycle polystyrene, achieving high-value-added conversion of polystyrene with high conversion rate and high selectivity. Specifically, the oxidation products include high-value-added chemical products benzoic acid, acetophenone, and benzaldehyde, as well as the byproduct CO. x The yield of oxidation products is 50%-98%, and the selectivity of the oxidation products for high-value-added chemical products is 50%-80%. Where x is 1 or 2, CO x This includes CO and / or CO2.

[0025] For example, the mass ratio of graphitic carbon nitride to polystyrene can be 1:1, 2:1, 2.5:1, 3:1, 4:1, 5:1, or any value between them. When the mass ratio of graphitic carbon nitride to polystyrene is too low, the reaction rate is too low or even impossible to catalyze the oxidative depolymerization reaction; when the mass ratio is too high, it will result in catalyst waste. By controlling the mass ratio of graphitic carbon nitride to polystyrene within the above ranges, catalyst waste can be avoided while ensuring a high reaction rate.

[0026] For example, the gas pressure of the oxidant can be 0.1 MPa, 0.2 MPa, 0.5 MPa, 1 MPa, 1.2 MPa, 1.5 MPa, or any range thereof. When the gas pressure of the oxidant is below 0.1 MPa, the oxidant concentration is too low, and the yield of the oxidation product is too low; when the gas pressure of the oxidant is above 1.5 MPa, it is prone to over-oxidation, and the oxidation product contains excessive CO. x The selectivity for benzoic acid, acetophenone, and benzaldehyde increases, while the overall selectivity decreases. Adjusting the gas pressure of the oxygen agent within the aforementioned range is beneficial for simultaneously increasing the yield of oxidation products and the overall selectivity for benzoic acid, acetophenone, and benzaldehyde.

[0027] For example, the reaction temperature for oxidative depolymerization can be 120℃, 130℃, 140℃, 150℃, 160℃, or any range therein. If the reaction temperature is below 120℃, the yield of the oxidation products will be too low. While a reaction temperature above 160℃ can increase the yield and reaction rate, it also increases the concentration of CO in the oxidation products. x The limited selectivity of the reaction is detrimental to improving the overall selectivity for benzoic acid, acetophenone, and benzaldehyde. By controlling the reaction temperature within the aforementioned range, it is beneficial to simultaneously increase the yield of the oxidation products and the overall selectivity for benzoic acid, acetophenone, and benzaldehyde.

[0028] For example, the reaction time for oxidative depolymerization can be 8h, 10h, 12h, 16h, 20h, 24h, or any value in between. When the reaction time is less than 8h, the reaction time is too short, resulting in a low yield of oxidation products. When the reaction time is greater than 24h, although the yield of oxidative depolymerization can be improved, it also increases the concentration of CO in the oxidation products. x The limited selectivity of the reaction time is detrimental to improving the overall selectivity for benzoic acid, acetophenone, and benzaldehyde. By controlling the reaction time within the aforementioned range, it is beneficial to simultaneously increase the yield of the oxidation products and the overall selectivity for benzoic acid, acetophenone, and benzaldehyde.

[0029] In summary, by adjusting the mass ratio of graphite-like carbon nitride to polystyrene, the gas pressure of the oxidant, the reaction time of the oxidative depolymerization reaction, and the reaction temperature within the range specified in this application, high conversion rate and high selectivity of polystyrene recovery can be achieved. Unless otherwise specified in this application, high selectivity refers to high overall selectivity for benzoic acid, acetophenone, and benzaldehyde.

[0030] The oxidative degradation of polystyrene is initiated by the partial oxidation of polystyrene by reactive oxygen species, unlike the traditional pyrolysis route where the reaction initially involves random breaking of C-C bonds, leading to the depolymerization of oligomers as intermediates. For example... Figure 1 As shown, under the action of a catalyst (g-C3N4), oxygen (O2), and light (hv), the C content in polystyrene (PS) increases. α Sites and / or C phenyl site (C) phenyl The CH group at the site (referring to at least one of the ortho, meta, or para positions on the benzene ring) is oxidized to C-OH, C-OH, C-OH. β The CH at the site is oxidized to a C-OH or C=O group, forming an activated intermediate PS-O containing oxygen-containing functional groups. Then, the C-C bonds on the intermediate PS-O are further oxidized, resulting in C-C cleavage to form intermediate products, including intermediates with functionalized chains. intermediates with oxidized aromatic rings The intermediates with oxidized aromatic rings are further oxidized and degraded into CO. x The intermediates with functionalized chains are oxidized to benzaldehyde, acetophenone, benzoic acid, and CO. x . Figure 1 In this context, R represents a polystyrene molecular chain (not shown), such as R including but not limited to... etc. R1 and R2 represent oxidized molecular chains (not shown) in the intermediate PS-O, and R3 to R9 represent oxidized molecular chains (not shown) in the intermediate with functionalized chains. R1 to R9 include, but are not limited to, oxidized molecular chains (not shown) in the intermediate. etc. R10 represents a molecular chain (not shown) on an intermediate where the aromatic ring has been oxidized; R10 includes, but is not limited to, [other molecular chains]. The degree of polymerization of polystyrene is determined by the following formulas: R1 and R2 have a degree of polymerization greater than R3-R10, and the asterisk (*) in R and R1-R10 indicates a group connection. The degree of polymerization of polystyrene refers to the total number of monomer units in the polystyrene molecular chain.

[0031] This application uses graphite-like carbon nitride as a catalyst. There are no particular limitations on the preparation method of the catalyst, as long as it achieves the purpose of this application. Exemplarily, it is obtained by sintering a nitrogen-rich precursor (e.g., urea, melamine, dicyandiamide, and thiourea) in air or a nitrogen atmosphere at 500°C-600°C for 3-5 hours.

[0032] In one embodiment of this application, a solvent is further added to the oxidative depolymerization reaction. The solvent is selected from at least one of acetonitrile and trifluorotoluene. The mass ratio of the solvent to polystyrene is (1-10):1 mL / mg, preferably (1-7):1 mL / mg, more preferably (1-3):1 mL / mg. Exemplarily, the mass ratio of the solvent to polystyrene is 1.5:1 mL / mg. By selecting a solvent within the above range and adjusting the volume ratio of the solvent to the mass of polystyrene within the above range, a liquid-phase reaction environment can be provided for the oxidative depolymerization reaction, achieving high conversion rate and high selective recovery of polystyrene.

[0033] In one embodiment of this application, the wavelength of the light under illumination conditions is 280nm-980nm, preferably 380nm-780nm, and more preferably 390nm-500nm. For example, the wavelength of the light is 400nm. By adjusting the wavelength of the light within the above range, it is beneficial to oxidize polystyrene to form activated intermediates, thereby oxidizing and degrading polystyrene into benzoic acid, acetophenone, benzaldehyde, and CO. x This enables the recycling of polystyrene.

[0034] In one embodiment of this application, the method for recycling polystyrene further includes pretreating the polystyrene and then subjecting it to an oxidative depolymerization reaction. The pretreatment process includes treating the polystyrene in air or oxygen at a temperature of 200°C-320°C for 1-10 hours, preferably at 200°C-320°C for 1-6 hours. This pretreatment process improves the solubility and flowability of the polystyrene, thereby increasing the reaction rate of polystyrene degradation and the selectivity of the oxidation products for benzoic acid.

[0035] In this application, the catalyst g-C3N4 can be recovered after the reaction by washing with a solvent. For example, acetonitrile can be used to wash the catalyst after the reaction. The catalyst used in this application has good reusability and can be recycled in the oxidative depolymerization reaction for the recovery of polystyrene while maintaining good reactivity and selectivity.

[0036] In this application, the reaction vessel used includes, but is not limited to, a stainless steel high-pressure reactor with a sapphire window and a magnetic stirring system, as long as it can achieve the purpose of this application.

[0037] In this application, illumination is provided by a light source. There are no particular limitations on the light source used, as long as it serves the purpose of this application. The power range of the light source used in this application is 200W-400W. For example, a 300W xenon lamp is used as the light source.

[0038] In this application, there is no particular limitation on the weight-average molecular weight of polystyrene, as long as it can achieve the purpose of this application. For example, the weight-average molecular weight of polystyrene can be 800-110,000, specifically, the weight-average molecular weight of polystyrene can be 800, 2500, 12,000, 50,000, 110,000, etc.

[0039] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0040] Test methods and equipment:

[0041] Yield and selectivity testing and calculation

[0042] The products of the oxidative depolymerization reaction were analyzed using a gas chromatography system (Agilent 7820A) and a liquid chromatography system (Agilent 1200). Gas components such as CO2, O2, N2, and CO were separated using a Porapark Q column and a molecular sieve column (5A type) and then detected by a thermal conductivity detector (TCD). The content of each gas component was calculated using 0.079 MPa N2 as a quantitative internal standard (IS1), and then calculated according to the following formula:

[0043]

[0044] After the reaction, 2.5 mmol of nitrobenzene was added to the solution as a quantitative internal standard (IS2). The liquid phase products were separated using a C18 column with acetonitrile / water gradient elution. The specific calculation method is as follows:

[0045]

[0046] The peak areas in the chromatogram exhibit a linear response relationship within a certain concentration range: S(A), S(B), S(IS1), and S(IS2) are the peak areas of components A, B, the gaseous internal standard (IS1), and the liquid internal standard (IS2), respectively; k(A) and k(B) are the relative correction factors for the internal standards of components A and B, respectively; and n(A), n(B), n(IS1), and n(IS2) are the molar numbers of components A, B, the gaseous internal standard (IS1), and the liquid internal standard (IS2), respectively. Therefore, the method for calculating the content of a substance (taking benzoic acid as an example) is as follows:

[0047]

[0048] The yield and overall selectivity are calculated based on the number of moles of carbon in the corresponding substances.

[0049]

[0050]

[0051] In this application, for a certain product T (benzoic acid, acetophenone, benzaldehyde, or CO) x The selectivity of ) is calculated as follows:

[0052]

[0053] N(T) is the number of moles of carbon in T.

[0054] Catalyst structure characterization

[0055] The XRD pattern of the catalyst was measured using an X-ray energy dispersive spectroscopy (EDS) instrument.

[0056] Characterization of oxidation products

[0057] The 1H NMR spectra of the separated liquid products were acquired using a 400MHz NMR spectrometer (Bruker AVANCE III). Deuterated acetone was used as the solvent (containing 1 wt% tetramethylsilane as the zero-point standard for the 1H NMR chemical shift).

[0058] Characterization of polymers

[0059] The weight-average molecular weight (Mw) of polystyrene samples was analyzed using a gel permeation chromatography system (PL-GPC220).

[0060] Reaction rate testing and calculation

[0061] CO obtained by gas chromatography xThe reaction rate is calculated by taking the amount of carbon in all detected substances (including COx, benzoic acid, acetophenone, and benzaldehyde) and dividing it by the catalyst mass (m) and reaction time (t).

[0062]

[0063] The unit for amount of substance is mmol, the unit for reaction time is hours (h), the unit for catalyst mass is grams (g), and the unit for reaction rate is mmol. 碳 ·g 催化剂 -1 h -1 .

[0064] Catalyst preparation:

[0065] Preparation of g-C3N4: 5g of urea was placed in a crucible and heated to 550℃ in a muffle furnace at a rate of 2℃ / min. The temperature was maintained in air for 4h. After cooling, the urea was washed with ethanol and deionized water, centrifuged, and dried to obtain about 1g of yellow powder g-C3N4.

[0066] Example 1

[0067] In a 100 mL stainless steel autoclave (equipped with a sapphire window and magnetic stirring system), add 20 mg of polystyrene (Mw = 35,000), 50 mg of g-C3N4, and 30 mL of acetonitrile (CH3CN). Introduce oxygen as an oxidant at a pressure of 1 MPa. Heat the autoclave to 150 °C, maintain this temperature, and irradiate with a 300 W xenon lamp with a 400 nm wavelength filter for 24 hours.

[0068] Examples 2-9

[0069] Except for adjusting the parameters according to Table 1, everything else is the same as in Example 1.

[0070] Example 10

[0071] Except for treating the polystyrene in air at 220°C for 5 hours, the rest is the same as in Example 1.

[0072] Example 11

[0073] Except for treating the polystyrene in air at 300°C for 1 hour, the rest is the same as in Example 1.

[0074] Comparative Examples 1-5

[0075] Except for adjusting the parameters according to Table 1, everything else is the same as in Example 1.

[0076] The reaction parameters and test results of each embodiment and comparative example are shown in Table 1 and Table 2.

[0077] Table 1

[0078]

[0079] Table 2

[0080]

[0081] As can be seen from Examples 1-3, when the mass ratio of catalyst to polystyrene is within the range of this application, not only can high value-added conversion of polystyrene be achieved, but also high yield and high selectivity can be achieved.

[0082] As can be seen from Example 1 and Comparative Example 1, selecting the catalyst g-C3N4 of this application can not only achieve high-value-added conversion of polystyrene, but also has high yield and high selectivity.

[0083] As can be seen from Examples 1 and 4, selecting an oxidant within the scope of this application can not only achieve high-value-added conversion of polystyrene, but also achieve high yield and high selectivity.

[0084] From Example 1, Examples 5-6 and Figure 2 It can be seen that when the gas pressure of the oxidant is within the range of this application, the yield of the oxidation product is high, and the overall selectivity for benzoic acid, acetophenone and benzaldehyde is also high.

[0085] From Example 1, Example 7, Comparative Examples 2-3 and Figure 3 It can be seen that when the reaction temperature of the oxidative depolymerization reaction is below 120℃, the yield of the oxidation products is too low; when the temperature of the oxidative depolymerization reaction is above 160℃, although the yield and reaction rate of the oxidative depolymerization reaction increase, the oxidation products contain less CO. x Increased selectivity is detrimental to improving the overall selectivity for benzoic acid, acetophenone, and benzaldehyde. Therefore, by controlling the reaction temperature of the oxidative depolymerization reaction within the range of this application, it is possible not only to achieve high-value-added conversion of polystyrene but also to achieve high yield and high selectivity.

[0086] From Examples 1, 8-9, Comparative Examples 4-5 and Figure 4 It can be seen that if the reaction time of the oxidative depolymerization reaction is too short, the yield of the oxidation products will be too low; if the reaction time of the oxidative depolymerization reaction is too long, although the yield of the oxidative depolymerization reaction increases, the oxidation products will contain less CO. xIncreased selectivity is detrimental to improving the overall selectivity for benzoic acid, acetophenone, and benzaldehyde. By controlling the reaction time of the oxidative depolymerization reaction within the range of this application, not only can high-value-added conversion of polystyrene be achieved, but also with high yield and high selectivity.

[0087] Referring to Table 2, it can be seen from Examples 1 and 10-11 that pretreatment can not only improve the reaction rate and yield of polystyrene degradation, but also improve the selectivity of benzoic acid in organic matter.

[0088] <Cyclic Experiment>

[0089] To evaluate the stability of the catalyst, a cyclic experiment was conducted based on Example 1. First, 20 mg of polystyrene (Mw = 35,000), 50 mg of g-C3N4, and 30 mL of acetonitrile were added to a 100 mL stainless steel high-pressure reactor. Oxygen was introduced as the oxidant, with a gas pressure of 1 MPa. The high-pressure reactor was heated to 150°C and then maintained at 150°C. After irradiation with a 300 W xenon lamp with a 400 nm wavelength filter for 8 hours, the polystyrene and reaction solution were separated from the catalyst. This process was repeated 5 times. After each cycle, the catalyst was separated and used as the catalyst for the next cycle.

[0090] Yields, overall selectivity, and yields for each cycle of benzoic acid, acetophenone, benzaldehyde, and CO2. x The selectivity and reaction rate of each catalyst are shown in Table 3. The catalyst g-C3N4 used in this application has good reusability and can maintain good reactivity and selectivity. Figure 5 XRD patterns of the catalyst of this application before and after 5 cycles are shown, where a represents the unused catalyst and b represents the catalyst after 5 cycles. Figure 5 It can be seen that the characteristic peaks at 13° and 27° did not change before and after the catalyst was used, indicating that its catalytic performance remained basically unchanged before and after use.

[0091] Table 3

[0092]

[0093] <Amplification Loop Experiment>

[0094] Based on Example 1, the following cyclic reaction system was designed for scale-up cyclic experiments to study the feasibility of the method provided in this application for the large-scale recovery of polystyrene. 500 mg of polystyrene (Mw = 35,000), 200 mg of g-C3N4, and 40 mL of acetonitrile were added to a 100 mL stainless steel high-pressure reactor. Oxygen was introduced as the oxidant, with a gas pressure of 1 MPa. The high-pressure reactor was heated to 150°C and then maintained at 150°C. It was irradiated with a 300 W xenon lamp with a 400 nm wavelength filter. After 8 hours, constituting one reaction cycle, the solution containing products such as benzoic acid was filtered out, and 40 mL of acetonitrile was injected using a high-pressure injection pump. After 20 cycles, 90% of the polystyrene was oxidized and degraded, producing a total of 360 mg of aromatic oxygen-containing compounds: benzoic acid, acetophenone, and benzaldehyde. The selectivity of the oxidation products for these three compounds was 74%, 15%, and 11%, respectively. The solution discharged from the reaction system was first concentrated, then separated by silica gel column chromatography using petroleum ether as the eluent. The solution containing benzoic acid was collected, and the solvent was evaporated to obtain the pure chemical, of which 240 mg of benzoic acid was recovered. The 1H NMR spectrum of benzoic acid is shown below. Figure 6 As shown, the peaks with chemical shifts of 8.04 ppm, 7.64 ppm, and 7.52 ppm are signals of benzoic acid, while the peaks with chemical shifts of 2.05 ppm and 1.29 ppm correspond to deuterated acetone solvent and petroleum ether, respectively.

[0095] Figure 7 The trend graphs of single-cycle product amount and cumulative product amount in 20 cycles of a scaled-up cyclic experiment are presented. The single-cycle product amount and cumulative product amount are expressed in moles of carbon atoms, in mmol. Using the polystyrene recovery method of this application for cyclic reaction can avoid deep oxidation of the target product, enabling the continuous conversion of large quantities of polystyrene into high-value-added chemical products. Therefore, the method provided in this application has great potential for polystyrene recovery in industrial production.

[0096] In summary, the method provided in this application for polystyrene recovery can achieve high-value-added conversion of polystyrene with high conversion rate and high selectivity. The catalyst in this application has high stability, is inexpensive, easy to synthesize, and easy to recover and separate. The method in this application is simple, safe, has a high reaction rate, and low reaction cost, making it an environmentally friendly method for polystyrene recovery.

[0097] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, or article that includes said element.

[0098] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0099] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for recycling polystyrene, comprising the following steps: Under light irradiation, polystyrene and an oxidant undergo an oxidative depolymerization reaction in the presence of graphitic carbon nitride to generate oxidation products; among which... The mass ratio of the graphite-like carbon nitride to polystyrene is (1-5):1; The oxidant is selected from oxygen or air, and the gas pressure of the oxidant is 0.1 MPa-1.5 MPa; The reaction temperature for the oxidative depolymerization reaction is 120℃-160℃, and the reaction time is 8h-24h.

2. The method according to claim 1, wherein, A solvent is also added to the oxidative depolymerization reaction. The solvent is selected from at least one of acetonitrile and trifluorotoluene, and the ratio of the volume of the solvent to the mass of the polystyrene is (1-10):1 mL / mg.

3. The method according to claim 1, wherein, The wavelength of the light under the specified illumination conditions is 280nm-980nm.

4. The method according to claim 1, wherein, The oxidation products include benzoic acid, acetophenone, benzaldehyde, and CO. x , where x is 1 or 2.

5. The method according to claim 4, wherein, The yield of the oxidation product is 50%-98%.

6. The method according to claim 4, wherein, The total selectivity of the oxidation products for benzoic acid, acetophenone and benzaldehyde is 50%-80%.

7. The method according to claim 1, further comprising pretreating the polystyrene and then performing the oxidative depolymerization reaction, wherein the pretreating process includes: Treat in air or oxygen at a temperature of 200℃-320℃ for 1-10 hours.

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