Catalyst, method and device for solvent-free photo-thermal catalysis of waste polystyrene plastic into benzoic acid crystal

Abstract

The invention belongs to the technical field, and particularly relates to a catalyst, a method and a device for solvent-free photo-thermal catalysis of waste polystyrene plastic into benzoic acid crystals, a photo-thermal catalysis material is graphite-loaded molybdenum carbide Mo2C / C, and the photo-thermal catalysis material comprises the following components in parts by weight: 1-5 parts of ammonium molybdate tetrahydrate, 0.5-10 parts of citric acid, 30-80 parts of sodium chloride and water. The novel precious metal-free catalytic material has high photo-thermal conversion efficiency, does not use an organic solvent, is low in cost and is green and environment-friendly; the product purity is high, the production steps are simple, and operation and subsequent actual production are easy.

Description

Technical Field

[0001] The present invention belongs to the technical field of waste plastic treatment, and particularly relates to a catalyst, a method and a device for photocatalytically converting waste polystyrene plastic into benzoic acid crystals without a solvent by using light and heat. Background Art

[0002] Plastics, as an inexpensive, easy-to-process and stable synthetic polymer material, have brought great convenience to the production and life of human society. However, due to their extremely difficult-to-decompose characteristics and the random disposal after use, serious environmental problems have been brought about, and the existing high-energy-consuming treatment methods for waste plastics do not meet the requirements.

[0003] Traditional plastic recycling methods mainly include incineration, landfilling, mechanical recycling, etc. However, a large amount of carbon dioxide and toxic gases will be released during the incineration power generation process, and it takes hundreds of years for landfilled garbage to degrade. Although mechanical recycling can reuse plastics, it will damage the mechanical properties of plastics, which belongs to downgraded recycling and can only obtain devalued products. In contrast, chemical recycling can convert plastic waste into value-added products such as fuel, organic chemicals, and green hydrogen, which is one of the better ways for waste plastic treatment. However, in the existing chemical recycling process, additional conditions (such as high temperature, high pressure, etc.), high energy input, and the use of organic solvents are often required, which seriously reduce the economic benefits and environmental protection significance of the value-added recycling of waste plastics and are difficult to be applied on a large scale in practice.

[0004] Photocatalysis reduces the dependence on fossil fuels by using light energy and reduces environmental pollution. It is one of the preferred methods for plastic degradation at present. It drives chemical reactions by converting solar energy into heat energy and improves the energy utilization efficiency. However, there are also technical problems such as insufficient light absorption and heat conversion efficiency of the catalytic material, resulting in low overall efficiency. On the other hand, catalysts with high efficiency usually choose to use precious metals, with high costs, which limits large-scale application; and there is no good catalytic environment and the design of the device reactor is complex, with low conversion rate, etc.

[0005] The description of Chinese Patent CN 112961047A records a method for photocatalytic selective oxidation of polystyrene to synthesize benzoic acid, and the steps are as follows: Dissolve bismuth nitrate, sodium molybdate, and cetyltrimethylammonium bromide in a certain amount of water and mix evenly, and then prepare bismuth molybdate; Mix a nickel nitrate solution, an iron nitrate solution, and a formaldehyde solution evenly, and adjust the pH value with sodium hydroxide, and then prepare Ni-Fe hydrotalcite; Place bismuth molybdate and Ni-Fe hydrotalcite in a certain amount of water, and finally obtain Bi 2 MoO 6 -Ni-Fe hydrotalcite composite photocatalyst. Take Bi 2 MoO 6The Bi-Ni-Fe hydrotalcite composite photocatalyst is dispersed in a solvent and polystyrene, and oxygen is introduced, heated, and irradiated with light to obtain benzoic acid. The conversion rate of polystyrene can be increased by controlling the molar ratio of bismuth to nickel and iron in the catalyst, and benzoic acid with a high yield can be obtained. However, this patent requires the use of pure oxygen as an oxidant, and the air pressure needs to reach 1 MPa, which is 10 times the atmospheric pressure. Such high-pressure pure oxygen gas is not only expensive, has explosion safety problems, but also has complex operations. Therefore, such relatively harsh conditions are not conducive to wide application. In addition, this patent also requires the use of flammable, toxic, and corrosive benzotrifluoride. Although it can solve the problem of waste plastic pollution to a certain extent, it brings higher risks during implementation and may even cause more serious environmental pollution, severely limiting its practical value.

[0006] The specification of Chinese Patent CN 108940332A records a preparation method of a highly active MoS 2 / g-C 3 N 4 / Bi 24 O 31 Cl 10 composite photocatalyst. Using bismuth nitrate, ammonium chloride, and citric acid as raw materials, Bi 24 O 31 Cl 10 is prepared by an improved solution combustion method; using melamine and acetic acid as raw materials, g-C 3 N 4 is prepared by a one-step thermal polymerization method; using ammonium molybdate and thiourea as raw materials and dimethylformamide as a solvent, MoS 2 is prepared by a hydrothermal method; after ultrasonic dispersion treatment of Bi 24 O 31 Cl 10 is ultrasonically mixed and reacted with g-C 3 N 4 and MoS 2 in a methanol solution, and then washed, centrifuged, and dried to obtain the composite photocatalyst. It is simple and easy to operate, has low cost, and good repeatability, and has broad application prospects in the fields of photocatalytic water splitting, photocatalytic oxidation of environmental pollutants, etc. However, the composite photocatalyst in this patent has a complex composition and a relatively cumbersome preparation process. It requires toxic solvent dimethylformamide and environmentally harmful thiourea during its synthesis process, which violates the concept of green development and limits its practical value.

[0007] In addition, the traditional polystyrene recycling methods, costs, and output values are summarized as follows:

[0008] 1. Mechanical recycling (physical recycling): Process: Waste polystyrene is directly processed into recycled pellets through steps such as crushing, melting, and extrusion granulation. Applicable scenarios: Suitable for waste materials with relatively high cleanliness (such as EPS foam boxes and household appliance packaging materials). The technology is mature and the equipment is simple. Advantages: Low investment, quick results, suitable for decentralized recycling scenarios. Disadvantages: High requirement for raw material cleanliness, and the performance decreases after multiple recycling times.

[0009] 2. Pyrolysis and catalytic pyrolysis (traditional chemical recycling): Process: Under high temperature (above 400 °C) and the action of a catalyst, polystyrene is decomposed into styrene monomers, oils, or mixed hydrocarbon products. Applicable scenarios: Treating mixed or contaminated waste plastics, and the products can be used as chemical raw materials or fuels. Advantages: High degree of resource utilization, and the added value of the products is relatively high. Disadvantages: High energy consumption, complex equipment, requiring noble metal catalysts (such as platinum), and high costs.

[0010] 3. Energy recovery: Process: Utilizing its high calorific value (about 40 MJ / kg), waste polystyrene is incinerated to generate electricity or heat. Applicable scenarios: Low-quality waste materials that cannot be physically / chemically recycled. Advantages: Reducing landfill volume and directly generating energy. Disadvantages: Generating greenhouse gases (such as CO 2 ) and potential pollutants (such as dioxins), with great environmental protection disputes.

[0011] 4. Downcycling: Process: After modifying waste polystyrene, it is used to produce coatings, adhesives, asphalt enhancers, etc. Applicable scenarios: Small-scale or specific industrial uses. Advantages: Expanding application fields and reducing raw material consumption. Disadvantages: High technical threshold and limited market acceptance. Summary of the Invention

[0012] To solve the above technical problems, the present invention provides a catalyst, method, and device for photocatalytic conversion of waste polystyrene plastics into benzoic acid crystals without solvents. The novel non-noble metal catalytic material has high photothermal conversion efficiency, no use of organic solvents, low cost, and is green and environmentally friendly; the product has high purity, simple production steps, and is easy to operate and suitable for subsequent actual production.

[0013] A catalyst for photocatalytic conversion of waste polystyrene plastics into benzoic acid crystals without solvents in the present invention for solving the above technical problems, the photocatalytic material is molybdenum carbide supported on graphite (Mo 2 C / C), and it includes the following components in parts by weight: ammonium molybdate tetrahydrate 1 - 5, citric acid 0.5 - 10, sodium chloride 30 - 80, and water.

[0014] In an optimized scheme, the photocatalytic material includes the following components in parts by weight: ammonium molybdate 3, citric acid 10, sodium chloride 50, and water 20.

[0015] In the present invention, the photocatalytic material is β-phase molybdenum carbide supported on two-dimensional mesoporous graphite, which has a large specific surface area (432.9 m 2 ·g -1 ), excellent photothermal performance (able to convert visible light energy into heat energy of 270 °C within 5 seconds), good physical and chemical stability, and excellent hydrophilic and lipophilic properties.

[0016] The catalytic material in the present invention is synthesized as follows:

[0017] (1) Weigh ammonium molybdate tetrahydrate, citric acid, and sodium chloride in an agate mortar, then add ultrapure water and grind thoroughly to mix the materials evenly;

[0018] (2) Place the mixture in an oven and dry it at 55 - 65 °C; the optimized drying temperature is 60 °C.

[0019] (3) Place the fully dried material in a tubular furnace and calcine it at 600 - 1000 °C under argon protection for 1 - 6 h;

[0020] (4) Wash the calcined product with ultrapure water to separate the sodium chloride template, and obtain black Mo 2 C / C after suction filtration and drying.

[0021] In step (2), the drying time is 8 - 18 h, and the optimized time is 12 h.

[0022] In step (3), the heating rate is 2 °C / min.

[0023] In the optimized scheme, in step (3), the calcination temperature is 750 °C, the time is 2 h, the argon dosage has no requirement, and the pressure value is normal pressure.

[0024] In step (4), the drying temperature is 60 - 120 °C for 6 - 12 h; in the optimized scheme, the temperature is 60 °C and the time is 12 h.

[0025] In the present invention, the photocatalyst is molybdenum carbide supported on graphite, which has high light absorption efficiency and photothermal conversion efficiency, and can quickly realize the upgrading conversion of polystyrene plastics.

[0026] The method for photocatalytically converting waste polystyrene plastics into benzoic acid crystals in the present invention includes the following steps:

[0027] Step 1: Weigh the above catalyst and polystyrene powder (obtained by crushing a real polystyrene foam box), mix them thoroughly, and then evenly spread the mixed powder at the bottom of the reaction pool; the mass ratio of the catalyst to the polystyrene powder is 0.8 - 1.2:1; in the optimized scheme, the optimal mass ratio of the catalyst to the polystyrene powder is 1:1.

[0028] Step 2: Connect the reaction cell and the product collection device using a spherical stainless steel interface clamp. Then fix the reaction device and place the xenon lamp directly above the reactor, and irradiate the mixed powder with light at an intensity of 0.5 - 1.3 W / cm 2 Optimize the light intensity to 1 W / cm 2 , as different light intensities will affect the reaction rate.

[0029] Step 3: After 10 - 14 h, turn off the light source. When the reaction ends, the condensed benzoic acid crystals can be collected on the product collection device.

[0030] In the present invention, a device for the solvent - free photocatalytic conversion of waste polystyrene plastics into benzoic acid crystals:

[0031] The device structure is provided with a light source, a reactor, and a photocatalyst:

[0032] The light source is an artificial light source (xenon lamp, LED lamp) or a natural light source (sunlight).

[0033] The reactor is provided with a reaction cell and a product collection device. The reaction cell is connected to the product collection device through a flange. The reaction cell is a cylindrical quartz cup for placing the mixed polymer of the photocatalyst and polystyrene plastic powder. The product collection device is a cylindrical quartz tube with a flange at the bottom, a branch pipe welded on the side, and sealed with a high - permeability quartz sheet at the top.

[0034] The proportion of the length of the product collection device in the total height of the device is 75%.

[0035] The length of the product collection device is 90 mm.

[0036] The principle of photocatalytic decomposition of waste polystyrene plastics into benzoic acid crystals in the present invention:

[0037] After the light irradiation starts, due to the excellent photothermal effect of the photocatalyst, a significant gas convection phenomenon occurs in the device. At the same time, the generated products also rise with the air flow to the product collection device. Also, because the inner surface temperature of the product collection device is approximately at room temperature, which is much lower than the boiling point and melting point of benzoic acid, the generated benzoic acid gradually condenses on the inner surface of the product collection device, and thus pure benzoic acid crystals are obtained.

[0038] In the photocatalytic process, the catalyst is the most crucial, directly determining whether the reaction can occur, the reaction rate, and the product type; the light source is the second - most crucial, affecting the reaction rate and the purity of benzoic acid; the device structure only affects the product collection and slightly affects the reaction rate. The best effect can be achieved through the coordination of these three conditions.

[0039] The beneficial effects in the present invention:

[0040] A: Low cost, the catalysts used are all inexpensive materials, and no precious metal materials are required to be added;

[0041] B: Using light (solar energy) as the only energy input, it can achieve value-added recycling of waste polystyrene at low cost.

[0042] C: The reaction conditions are mild, and the conversion of real waste polystyrene can be achieved at room temperature and atmospheric pressure, which is conducive to large-scale practical applications.

[0043] D: The product separation is simple. The product of this method is high-purity benzoic acid crystals, and no complex product separation procedure is required.

[0044] In this invention, the low cost, mild conditions, and simple procedure undoubtedly lay a solid foundation for large-scale practical applications. Compared with other treatment methods, comparative experiments with low cost and similar or better effects are carried out; for example, the treatment with reagents or the treatment with the addition of precious metals.

[0045] This invention does not require the use of organic solvents. Moreover, the external energy used in the catalytic process is abundant and clean solar energy, realizing the conversion of solar energy to chemical energy. The method in this invention not only saves raw materials but also is low-carbon and environmentally friendly, and the product has high purity, without the need for subsequent complicated product purification and separation, which is conducive to actual production. Description of the Drawings

[0046] Figure 1 It is the front view of the reaction device in this invention.

[0047] Figure 2 It is Mo 2 The X-ray diffraction pattern of C / C.

[0048] Figure 3 It is Mo 2 The ultraviolet-visible-infrared diffuse reflectance absorption spectrum of C / C.

[0049] Figure 4 It is the photo-thermal conversion curve of the material in this invention under the light condition of 1 W / cm 2

[0050] Figure 5 It is the photo-thermal imaging diagram of the material in this invention within 40 seconds under the light condition of 1 W / cm 2

[0051] Figure 6 It is the reactor device diagram (left) in this invention, and the benzoic acid crystals generated on the reactor wall during the photo-thermal catalysis of polystyrene (right).

[0052] Figure 7 It is the NMR diagram of the benzoic acid crystals generated by the photo-thermal catalysis of polystyrene in this invention.

[0053] Figure 8 It is the image of the reaction rate of photocatalytic decomposition of polystyrene into benzoic acid crystals for a series of examples in the present invention.

[0054] Figure 9 It is the principle of the method for photocatalytic decomposition of polystyrene into benzoic acid crystals in the present invention. Detailed implementation manners

[0055] The following further elaborates on the present invention in conjunction with the detailed implementation manners:

[0056] In the following Examples 1-5, when the amount of ammonium molybdate tetrahydrate is 100-500 mg, the fixed amount of citric acid is 100 mg, the fixed amount of sodium chloride is 5 g, the calcination temperature is 750 °C, and the time is 2 h.

[0057] The equipment related to the equipment structure composition in the present invention mainly consists of three elements: a light source, a reactor, and a photocatalyst. The light source is an artificial light source (xenon lamp, LED lamp) or a natural light source (sunlight).

[0058] The reactor is as Figure 1 As shown, the reactor designed in the present invention mainly consists of two parts: a reaction cell and a product collection device. The reaction cell is connected to the product collection device through a flange. The reaction cell is a cylindrical quartz cup for placing the polymer mixture of the photocatalyst and polystyrene plastic powder. The product collection device is a cylindrical quartz tube with a flange at the bottom, a branch pipe welded on the side, and sealed with a high-transparency quartz sheet at the top.

[0059] Example 1

[0060] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C). The synthesis method is as follows: First, weigh 100 mg of ammonium molybdate tetrahydrate, 100 mg of citric acid, and 5 g of sodium chloride in an agate mortar. Then add ultrapure water and grind thoroughly to mix the materials evenly. After that, place the mixture in an oven and dry it at 60 °C for 12 h. Then place the fully dried material in a tubular furnace and calcine it at 750 °C under argon protection for 2 h, with a heating rate of 2 °C / min. Finally, wash the calcined product with ultrapure water to separate the sodium chloride template, and obtain black Mo 2 C / C-1 after suction filtration and drying. The drying temperature is 60 °C and the time is 12 h.

[0061] Example 2

[0062] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2C / C) was synthesized as follows: First, 200 mg of ammonium molybdate tetrahydrate, 100 mg of citric acid, and 5 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C for 12 h. Next, the thoroughly dried material was placed in a tube furnace and calcined at 750 °C under argon protection for 2 h with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-2, and the drying temperature was 60 °C for 12 h.

[0063] Example 3

[0064] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C) was synthesized as follows: First, 300 mg of ammonium molybdate tetrahydrate, 100 mg of citric acid, and 5 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C for 12 h with a heating rate of 2 °C / min. Next, the thoroughly dried material was placed in a tube furnace and calcined at 750 °C under argon protection for 2 h with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-3, and the drying temperature was 60 °C for 12 h.

[0065] Example 4

[0066] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C) was synthesized as follows: First, 400 mg of ammonium molybdate tetrahydrate, 100 mg of citric acid, and 5 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C for 12 h with a heating rate of 2 °C / min. Next, the thoroughly dried material was placed in a tube furnace and calcined at 750 °C under argon protection for 2 h with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-4, and the drying temperature was 60 °C for 12 h.

[0067] Example 5

[0068] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2C / C) was synthesized as follows: First, 500 mg of ammonium molybdate tetrahydrate, 100 mg of citric acid, and 5 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C for 12 h with a heating rate of 2 °C / min. Next, the thoroughly dried material was placed in a tube furnace and calcined at 750 °C under argon protection for 1 - 6 h with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-5 was obtained, with a drying temperature of 60 °C and a time of 12 h.

[0069] Example 6

[0070] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C) was synthesized as follows: First, 450 mg of ammonium molybdate tetrahydrate, 1000 mg of citric acid, and 8 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 65 °C for 8 h. Next, the thoroughly dried material was placed in a tube furnace and calcined at 1000 °C under argon protection for 1 h with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-6 was obtained, with a drying temperature of 80 °C and a time of 10 h.

[0071] Example 7

[0072] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C) was synthesized as follows: First, 150 mg of ammonium molybdate tetrahydrate, 50 mg of citric acid, and 3 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 55 °C for 18 h. Next, the thoroughly dried material was placed in a tube furnace and calcined at 600 °C under argon protection for 6 h with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-7 was obtained, with a drying temperature of 100 °C and a time of 8 h.

[0073] Example 8

[0074] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2C / C) was synthesized as follows: First, 350 mg of ammonium molybdate tetrahydrate, 200 mg of citric acid, and 4 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C for 10 h. Next, the thoroughly dried material was placed in a tube furnace and calcined at 900 °C for 2 h under argon protection, with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-8, and the drying temperature was 120 °C for 6 h.

[0075] Example 9

[0076] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C) was synthesized as follows: First, 450 mg of ammonium molybdate tetrahydrate, 800 mg of citric acid, and 7 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C for 14 h. Next, the thoroughly dried material was placed in a tube furnace and calcined at 800 °C for 4 h under argon protection, with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-9, and the drying temperature was 70 °C for 9 h.

[0077] Example 10

[0078] Catalytic material: The photothermal catalytic material is molybdenum carbide supported on graphite (Mo 2 C / C) was synthesized as follows: First, 500 mg of ammonium molybdate tetrahydrate, 500 mg of citric acid, and 6 g of sodium chloride were weighed into an agate mortar. Subsequently, ultrapure water was added and the materials were thoroughly ground to mix them evenly. Then, the mixture was placed in an oven and dried at 60 °C. Next, the thoroughly dried material was placed in a tube furnace and calcined at 700 °C for 3 h under argon protection, with a heating rate of 2 °C / min. Finally, the calcined product was washed with ultrapure water to separate the sodium chloride template, and after suction filtration and drying, black Mo 2 C / C-10, and the drying temperature was 60 °C for 12 h.

[0079] The photothermal catalytic material obtained in Example 3 above was measured using an X-ray diffractometer (XRD), and the obtained X-ray diffraction image is as Figure 2 shown. The diffraction peaks corresponding to graphite and molybdenum carbide are shown in the figure, confirming that the synthesized photocatalyst is molybdenum carbide supported on graphite.

[0080] The photothermal catalytic materials in Examples 1-5 were measured using a UV-Vis-NIR spectrometer, and the obtained UV-Vis-IR absorption spectra are as Figure 3As shown, all materials exhibit extremely strong absorption throughout the entire wavelength range, demonstrating their excellent light absorption efficiency.

[0081] Furthermore, the photothermal catalytic materials obtained in Examples 1-5 can rapidly achieve photothermal generation under the illumination condition of 1 W / cm 2 . As Figure 4 shown, all the prepared materials can reach above 200 °C within 30 seconds, which fully demonstrates the excellent photothermal conversion efficiency of the materials.

[0082] Example 11

[0083] Operating steps of the method for photothermal catalytic conversion of actual polystyrene waste plastics into benzoic acid crystals:

[0084] Weigh 100 mg of the catalysts in Examples 1-5 and mix them thoroughly with 100 mg of polystyrene powder (obtained by crushing real polystyrene foam boxes). Then evenly spread the mixed powder at the bottom of the reaction cell. Next, use a spherical stainless steel interface clamp to connect the reaction cell and the product collection device. After that, fix the reaction device and place the xenon lamp directly above the reactor, and irradiate the mixed powder with light at an intensity of 1 W / cm 2 . After 12 hours, turn off the light source. After the reaction ends, collect the condensed benzoic acid crystals on the product collection device, as Figure 6 shown.

[0085] Perform relevant measurements on the obtained benzoic acid crystals, as Figure 5 , Figures 7 - 8 shown.

[0086] Figure 5 As can be seen from the visual imaging (measured by a FLIR A300 infrared thermal imager) shown, after the photocatalytic materials prepared in Examples 1-5 of the present invention are mixed with real polystyrene waste plastics at a ratio of 1:1, they can also efficiently heat up under the illumination condition of 1 W / cm 2 , and then rapidly achieve the upgrading conversion of polystyrene plastics.

[0087] The condensed degradation products in the present invention are confirmed by liquid nuclear magnetic resonance spectroscopy (measured by a JNM-ECZ600R / M1 nuclear magnetic resonance spectrometer), as Figure 7 shown. As can be seen from Figure 7 , polystyrene degrades into benzoic acid, and the purity of this crystal is high. It is directly confirmed by nuclear magnetic resonance spectroscopy as high-purity benzoic acid.

[0088] Figure 8 is the image of the reaction rate of photothermal catalytic decomposition of polystyrene waste plastics into benzoic acid crystals for Examples 1-5 (directly weighed by a BSA323S electronic balance), where Example 3 (Mo 2The reaction rate of C / C-3) was the highest, and 74.1 mg of benzoic acid crystals were obtained within 12 hours, with the yield of benzoic acid reaching 63.1%.

[0089] Example 12

[0090] Weigh 100 mg of the catalysts in Examples 6-10 respectively and mix them thoroughly with 100 mg of polystyrene powder (obtained by crushing a real polystyrene foam box). Then evenly spread the mixed powder at the bottom of the reaction cell. The mass ratio of the catalyst to the polystyrene powder is 1:1. Then use a spherical stainless steel interface clamp to connect the reaction cell and the product collection device. After that, fix the reaction device and place the xenon lamp directly above the reactor, and irradiate the mixed powder with light at an intensity of 1 W / cm 2 . Turn off the light source after 6 hours and 12 hours respectively. After the reaction ends, collect the condensed benzoic acid crystals on the product collection device. Measure the yield and productivity of the obtained benzoic acid crystals, as shown in Table 1 below.

[0091] Table 1

[0092]

[0093] . It can be seen from Table 1 above that with different light irradiation times, the yield and productivity are different. The yield and productivity are relatively high at 12 h, and the obtained benzoic acid crystals have high purity.

[0094] Example 13

[0095] Weigh 100 mg of the above-mentioned catalyst respectively and mix it thoroughly with 100 mg of polystyrene powder (obtained by crushing a real polystyrene foam box). Then evenly spread the mixed powder at the bottom of the reaction cell. The mass ratio of the catalyst to the polystyrene powder is 0.8:1; use a spherical stainless steel interface clamp to connect the reaction cell and the product collection device. After that, fix the reaction device and place the xenon lamp directly above the reactor, and irradiate the mixed powder with light at an intensity of 0.5 W / cm 2 . Turn off the light source after 10 h. After the reaction ends, collect the condensed benzoic acid crystals on the product collection device.

[0096] Example 14

[0097] Weigh 100 mg of the above-mentioned catalyst and 100 mg of polystyrene powder (obtained by crushing a real polystyrene foam box), and mix them thoroughly. Then evenly spread the mixed powder at the bottom of the reaction cell. The mass ratio of the catalyst to the polystyrene powder is 1.2:1; use a spherical stainless steel interface clamp to connect the reaction cell and the product collection device. After that, fix the reaction device and place the xenon lamp directly above the reactor, and irradiate the mixed powder with light at an intensity of 1.3 W / cm 2When irradiating the mixed powder with light, different light intensities will affect the reaction rate. After 14 hours, turn off the light source, and after the reaction ends, the condensed benzoic acid crystals can be collected on the product collection device.

[0098] The device structure is equipped with a light source, a reactor, and a photocatalyst: the light source is an artificial light source (xenon lamp, LED lamp) or a natural light source (sunlight). The reactor is equipped with a reaction cell and a product collection device, and the reaction cell is connected to the product collection device through a flange; the reaction cell is a cylindrical quartz cup for placing the polymer mixture of the photocatalyst and polystyrene plastic powder; the product collection device is a cylindrical quartz tube with a flange at the bottom, a branch pipe welded on the side, and sealed with a high-transparency quartz sheet at the top. The length of the product collection device accounts for 75% of the total height of the device. The length of the product collection device is 90 mm.

[0099] The principle of the method for photocatalytic decomposition of polystyrene waste plastic into benzoic acid crystals is as Figure 9 shown. After the light irradiation starts, due to the excellent photothermal effect of the photocatalyst, a significant gas convection phenomenon appears in the device. At the same time, the generated products also rise with the airflow to the product collection device. Also, because the inner surface temperature of the product collection device is approximately room temperature, which is much lower than the boiling point and melting point of benzoic acid, the generated benzoic acid gradually condenses on the inner surface of the product collection device, and pure benzoic acid crystals are obtained.

[0100] Experiment 1: Different types or intensities of light sources

[0101] The light source intensity mainly affects the reaction rate and the purity of benzoic acid. This photocatalytic reaction uses light as the only energy input, and the reaction is a photo-thermal synergistic catalysis. Part of the incident light is converted into heat energy, and the other part is converted into photo-generated charges, which jointly promote the progress of the reaction. Therefore, when the light intensity increases, the reaction will proceed faster, but it will cause a slight decrease in the purity of benzoic acid crystals. See Table 2 below.

[0102] Table 2 Experimental comparison (100 mg of benzoic acid used, 100 mg of PS used)

[0103]

[0104] Experiment 2: Different types and dosages of photocatalysts

[0105] The type of photocatalyst will determine: ① whether the reaction can proceed; ② the reaction rate; ③ the type and purity of the reaction products; ④ the yield of benzoic acid. An excellent photocatalyst can achieve a higher reaction rate and excellent product selectivity, resulting in a higher purity of the obtained benzoic acid. React the photocatalytic materials in Examples 1-5 with PS, as shown in Table 3 below.

[0106] Table 3 Experimental comparison table (light intensity at the surface of the catalyst is 1 W·cm -1 )

[0107]

[0108]

[0109] As can be seen from the above table, when the amounts of the catalyst and polystyrene are fixed, for the same reaction time, Example 3 has the highest benzoic acid yield; when the catalyst is fixed as in Example 3, the highest benzoic acid yield can be obtained when the amounts of the catalyst and polystyrene are 100 mg and 100 mg respectively.

[0110] Test 3: Different structures in the reaction device

[0111] React the photocatalytic material in Example 3 with PS in reaction devices of different lengths, as shown in Table 4 below.

[0112] Table 4 Test comparison (amount of benzoic acid: 100 mg; amount of PS: 100 mg; light intensity at the catalyst surface: 1 W·cm -1 )

[0113]

[0114] In the device, it is mainly the product collection part that affects the effect. When the length of the collection device is too low, the collection of products will be affected because insufficient condensation area and air flow circulation space cannot be provided. Some products will directly escape from the device, and another part will be lost due to over-oxidation by the catalyst with the air flow circulation, thus affecting the yield. When the length of the collection device is appropriate, not only can the condensation of benzoic acid be fully ensured, but also the air flow circulation can be used to continuously supplement oxygen from the air into the device (as the reaction proceeds, the air pressure in the device decreases due to the consumption of oxygen, so outside air will continuously enter the device through the upper branch pipe of the collection device under the action of atmospheric pressure) to ensure the continuous progress of the reaction. When the branch pipe connected to the atmosphere on the product collection device is blocked, the whole system becomes a closed system. As the reaction proceeds, the oxygen concentration in the system gradually decreases, and the reaction rate drops significantly because oxygen cannot be supplemented through the branch pipe. When the collection device is too long, since the light intensity will decay with the increase of the irradiation distance, when the light intensity at the catalyst surface is the same as that of the collection device with an appropriate length, more electrical energy input is required, resulting in a reduction in energy utilization efficiency.

[0115] Test 4: Cost accounting

[0116] Conduct cost accounting for 5 traditional polystyrene recycling methods and the method in the present invention, as shown in Table 5 below:

[0117] Table 5 Comparison table of cost accounting for different recycling methods

[0118]

[0119] In the traditional recycling methods of the above-mentioned polystyrene, only chemical recycling can achieve the value-added recycling of waste polystyrene. Compared with the traditional chemical methods, the present invention can not only achieve the value-added recycling of waste polystyrene, but also greatly reduce the production cost (about 5,000 - 7,000 yuan / ton, estimated) due to many advantages such as simple and inexpensive equipment, mild reaction conditions, low cost and reusable catalyst, and less energy input (solar energy), which is much lower than the traditional chemical recycling method. Moreover, the value of the obtained product is equivalent to that of the product of the traditional chemical recycling method (the price of benzoic acid is 7,000 - 16,000 yuan / ton). Therefore, the economic effect and practical value of the present invention are much higher than those of the traditional recycling methods.

[0120] The above-mentioned embodiments / tests are merely examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. A solvent-free photothermal catalysis of waste polystyrene plastics into benzoic acid crystals, characterized in that: The photothermal catalytic material is graphite-supported molybdenum carbide Mo2C / C, which comprises the following components in parts by weight: 1-5 parts of ammonium molybdate tetrahydrate, 0.5-10 parts of citric acid, 30-80 parts of sodium chloride and water.

2. The solvent-free photothermal catalysis of waste polystyrene plastic into benzoic acid crystals according to claim 1, characterized in that: The photothermal catalytic material comprises the following components in parts by weight: 3 parts ammonium molybdate, 10 parts citric acid, 50 parts sodium chloride and 20 parts water.

3. A solvent-free photothermal catalytic catalyst for converting waste polystyrene plastics into benzoic acid crystals according to claim 1 or 2, characterized in that: The catalytic material: The synthesis method is as follows: (1) Weigh ammonium molybdate tetrahydrate, citric acid and sodium chloride in an agate mortar, add water and grind evenly; (2) drying the mixture in an oven at 55-65°C; (3) placing the fully dried material in a tube furnace and calcining it at 600-1000° C. under argon protection for 1-6 hours; (4) The calcined product is washed with water to separate the sodium chloride template, and black Mo2C / C is obtained after filtration and drying.

4. The solvent-free photothermal catalysis of waste polystyrene plastic into benzoic acid crystals according to claim 3 is characterized by: The heating rate in step (3) is 2-5°C / min.

5. The solvent-free photothermal catalysis of waste polystyrene plastic into benzoic acid crystals according to claim 3, characterized in that: The drying time in step (2) is 8-18 hours, and the optimized time is 12 hours.

6. A method for solvent-free photothermal catalysis of waste polystyrene plastics into benzoic acid crystals, characterized in that: The following steps are involved: Step 1, weigh the catalyst and polystyrene powder described in weight 1, mix them thoroughly, and then spread the mixed powder evenly on the bottom of the reaction tank; the mass ratio of the catalyst to the polystyrene powder is 0.8-1.2:1; Step 2: Use a spherical stainless steel interface clamp to connect the reaction pool and the product collection device, then fix the reaction device and place a xenon lamp directly above the reactor with an intensity of 0.5-1.3W / cm 2 irradiating the mixed powder with light; Step 3: After 10-14 hours, turn off the light source. When the reaction is completed, the condensed benzoic acid crystals can be collected on the product collection device.

7. The method for converting waste polystyrene plastic into benzoic acid crystals by photothermal catalysis without solvent according to claim 6, characterized in that: In the step 1, the mass ratio of the catalyst to the polystyrene powder is 1:1; in the step 2, the strength is 1W / cm 2 The mixed powder is irradiated with light.

8. A device for solvent-free photothermal catalysis of waste polystyrene plastics into benzoic acid crystals, characterized in that: The equipment structure is provided with a light source, a reactor and a photocatalyst; the reactor includes a reaction pool and a product collecting device, and the reaction pool and the product collecting device are connected as a whole through a flange; the reaction pool is a cylindrical quartz cup for placing a mixed polymer of a photocatalyst and polystyrene plastic powder; the product collecting device is a cylindrical quartz tube with a flange at the bottom, a welded branch pipe on the side, and a top sealed with a high-transparency quartz sheet.

9. The device for converting waste polystyrene plastic into benzoic acid crystals without solvent photothermal catalysis according to claim 8, characterized in that: The length of the product collecting device accounts for 75% of the total height of the device; further, the length of the product collecting device is 90 mm.

10. The device for converting waste polystyrene plastic into benzoic acid crystals without solvent photothermal catalysis according to claim 8, characterized in that: The light source is an artificial light source or a natural light source; the artificial light source is a xenon lamp or an LED lamp, and the natural light source is sunlight.

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