PMMA (polymethyl methacrylate) depolymer and preparation method thereof

Abstract

The invention provides a PMMA (polymethyl methacrylate) depolymer and a preparation method thereof, and belongs to the technical field of high polymer materials. The method comprises the following steps: adding PMMA into a chlorine-containing solvent, adding a catalyst, carrying out a heating depolymerization reaction under visible light irradiation, filtering, recovering the catalyst, and separating and recovering the chlorine-containing solvent to obtain a PMMA depolymer; the catalyst is prepared by the following steps: mixing hydrogenated N-doped chiral mesoporous core-shell TiO2 powder and graphene coated C3N4, adding silver nitrate, bismuth nitrate, ammonia water, ammonium metavanadate and sodium dihydrogen phosphate, adjusting the pH value of the solution to be neutral, and stirring for reaction. According to the method, the PMMA degradation reaction rate is increased, complete degradation of PMMA is promoted, conditions are mild, complete degradation reaction under visible light irradiation is achieved, and industrial application is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and specifically relates to a PMMA depolymer and a preparation method thereof. Background Art

[0002] As a commonly used polymer material, plexiglass is widely used. However, it will cause certain pollution to the environment after being discarded. Therefore, how to recycle waste organic glass reasonably, effectively and at low cost is a problem worthy of research.

[0003] At present, most plastic recycling methods rely on macroscopic mechanical crushing, cleaning and reprocessing. Therefore, compared with the virgin polymer, the performance of the recycled material will decline. By chemically decomposing to the original monomers, more thorough purification can be achieved, and then the ideal performance can be restored by repolymerization. Due to the chemical stability and carbon-carbon main chain of polymethyl methacrylate (PMMA), its depolymerization has long been a difficult problem.

[0004] Chinese Patent Application CN118871520A discloses a thermal catalytic plastic depolymerization process, a method for depolymerizing plastic waste using cement with a specific composition as a catalyst. This method provides high-quality liquid depolymerization products that can be used as cracker raw materials with high efficiency. However, the effect of this catalyst on the thermal catalytic depolymerization of highly stable PMMA is not good. Summary of the Invention

[0005] The purpose of the present invention is to provide a PMMA depolymer and a preparation method thereof, which accelerate the PMMA degradation reaction rate, promote the complete degradation of PMMA, have mild conditions, and achieve complete degradation reaction under visible light irradiation, and realize industrial application.

[0006] The technical solution of the present invention is realized as follows: The present invention provides a preparation method of a PMMA depolymer. Add PMMA into a chlorine-containing solvent, add a catalyst, heat and depolymerize under visible light irradiation, filter, recover the catalyst, separate and recover the chlorine-containing solvent to obtain the PMMA depolymer; the catalyst is prepared by mixing hydrogenated N-doped chiral mesoporous core-shell TiO2 powder and graphene-coated C3N4, adding silver nitrate, bismuth nitrate, ammonia water, ammonium metavanadate and sodium dihydrogen phosphate, adjusting the pH value of the solution to neutral, and stirring and reacting.

[0007] As a further improvement of the present invention, the chlorine-containing solvent is selected from at least one of orthodichlorobenzene, metadichlorobenzene, and paradichlorobenzene; the addition amount of the catalyst is 1-2 wt% of the total mass of PMMA, the temperature of the heat depolymerization reaction is 140-160 °C, the time is 4-7 h, and the light intensity of the visible light is 500-1000 Lx.

[0008] As a further improvement of the present invention, the preparation method of the catalyst is as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: Add NH2-MIL-125 into ethanol, add tetrabutyl titanate and chiral amino acid pore-forming agent, stir and mix evenly, add concentrated hydrochloric acid, stir for hydrothermal reaction, wash the product, dry, and calcine to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: Carry out hydrogenation treatment on the N-doped chiral mesoporous core-shell TiO2 powder under high-temperature and high-hydrogen-pressure conditions to obtain modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of graphene oxide-coated C3N4: Heat and calcine melamine, add the product into the graphene oxide aqueous dispersion, stir and mix evenly, and dry to obtain graphene oxide-coated C3N4; S4. Preparation of graphene-coated C3N4: Reduce the graphene oxide-coated C3N4 by hydrazine hydrate vapor to obtain graphene-coated C3N4; S5. Preparation of basic reagent: Add silver nitrate, bismuth nitrate and complexing agent into water, and dropwise add ammonia water until a transparent solution is formed; S6. Preparation of ammonium metavanadate solution: Dissolve ammonium metavanadate with nitric acid to obtain ammonium metavanadate solution; S7. Preparation of catalyst: Add the modified N-doped chiral mesoporous core-shell TiO2 powder and graphene-coated C3N4 into water, add the basic reagent, dropwise add ammonium metavanadate solution and sodium dihydrogen phosphate, stir and mix evenly, adjust the pH value of the solution, continue to stir and react under dark conditions, filter, wash, dry, and grind to obtain the catalyst.

[0009] As a further improvement of the present invention, in step S1, the mass ratio of NH2-MIL-125, tetrabutyl titanate, chiral amino acid pore-forming agent, ethanol and concentrated hydrochloric acid is 8-10:5-9:1-2:40-50:10-20, the temperature of the stirring hydrothermal reaction is 110-130 °C, the time is 40-52 h, the temperature of the calcination is 700-800 °C, the time is 1-3 h, and the chiral amino acid pore-forming agent is selected from at least one of N-acetyl-L-lysine, N-acetyl-L-glutamic acid, and N-lauroyl-L-lysine.

[0010] As a further improvement of the present invention, in step S2, the temperature of the high-temperature and high-hydrogen-pressure condition is 180-220 °C, the hydrogen pressure is 15-25 Bar, and the time of the hydrogenation treatment is 4-6 d.

[0011] As a further improvement of the present invention, the temperature of the calcination in step S3 is 500 - 600 °C, the time is 1 - 3 h, the solid-liquid ratio of the melamine and graphene oxide aqueous dispersion is 1:3 - 5 g / mL, and the concentration of the graphene oxide aqueous dispersion is 0.1 - 0.2 mg / mL.

[0012] As a further improvement of the present invention, the time of the hydrazine hydrate vapor reduction in step S4 is 10 - 12 h.

[0013] As a further improvement of the present invention, the molar ratio of silver nitrate, bismuth nitrate and complexing agent in step S5 is 3 - 5:2 - 3:2 - 3, and the complexing agent is EDTA or disodium EDTA; the concentration of nitric acid in step S6 is 3 - 5 mol / L.

[0014] As a further improvement of the present invention, the mass ratio of the modified N-doped chiral mesoporous core-shell TiO2 powder, graphene-coated C3N4, basic reagent, ammonium metavanadate solution and sodium dihydrogen phosphate in step S7 is 5 - 7:2 - 4:7 - 11:0.5 - 1:0.2 - 0.5, the pH value of the adjusting solution is neutral, and the time of the continuous stirring reaction under dark conditions is 3 - 5 h.

[0015] The present invention further protects a PMMA depolymer obtained by the above preparation method.

[0016] The catalyst prepared by the present invention has excellent visible light responsiveness. First of all, TiO2 is a common photocatalyst, but due to the wide bandgap of TiO2 (the bandgaps of anatase and rutile TiO2 are 3.2 and 3.0 eV respectively), only ultraviolet light can excite the photocatalytic reaction of TiO2. Therefore, its industrial application is greatly limited.

[0017] The present invention uses a Ti-based MOF (N-containing Ti-based metal-organic framework MOF, namely NH2-MIL-125(Ti)) as the carrier and core material, deposits TiO2 on its surface through a sol-gel reaction to form a core-shell structure, and through calcination, NH2-MIL-125 decomposes into TiO2 and realizes N doping, effectively reducing the bandgap of TiO2 and expanding its spectral response from the ultraviolet light region to the visible light region. In addition, under the action of a chiral mesoporous porogen, the shell layer obtains a mesoporous TiO2 shell layer with an asymmetric helical stacking structure, and this asymmetric structure will introduce defects into its shell layer and form a Ti 3+ structure, thereby enabling the catalyst to have visible light response ability.

[0018] Further, under the long-term hydrogenation at high temperature and high hydrogen pressure, the surface lattice of TiO2 can be further disordered, and the TiO2 in the shell layer can be further formed into an amorphous structure, which can lead to Ti 3+and the formation of oxygen vacancies, thus greatly expanding the visible-light response of TiO2 and forming black modified N-doped chiral mesoporous core-shell TiO2 powder.

[0019] Ag3PO4 has a high valence band potential and extremely strong oxidation ability, but there are problems such as photocorrosion and high recombination rate of photogenerated carriers, which limit its application. In the present invention, melamine is calcined and coated with graphene oxide, and after reduction, graphene-coated C3N4 is obtained. It is mixed with the modified N-doped chiral mesoporous core-shell TiO2 powder to in-situ generate Ag3PO4 and bismuth vanadate. The formation of a carrier migration channel between different semiconductors improves the separation of photogenerated electron-hole pairs, and a Z-scheme heterojunction is constructed through synergistic action, effectively retaining the strong reducibility of C3N4 and the high oxidability of Ag. The separation efficiency of photogenerated electron-hole pairs is improved, thereby inhibiting the photocorrosion phenomenon and enhancing the photostability. In addition, the energy band structures of TiO2 and BiVO4 can achieve rapid separation of photogenerated charges in the visible-light region through the construction of a heterojunction, thereby realizing the rapid degradation of organic substances under visible light and improving the photocatalytic performance of the prepared catalyst. In addition, the in-situ generation of Ag3PO4 in the solvent reaction not only realizes the uniform deposition of Ag3PO4 nanoparticles on its surface, but also effectively controls the size of Ag3PO4 nanoparticles, greatly improving the photocatalytic activity of the composite material.

[0020] However, ordinary physical coating, such as directly coating C3N4 on Ag3PO4, will lead to a decrease in the charge transfer efficiency of the material and a weakening of the photocatalytic performance. In the present invention, by introducing a π-conjugated material - graphene-coated C3N4, the migration efficiency of carriers at the interface is greatly improved, thereby promoting the separation of photogenerated carriers. The loading of TiO2 and BiVO4 particles further inhibits the photocorrosion of Ag3PO4, significantly enhancing the photostability and visible-light photocatalytic activity of the catalyst.

[0021] The present invention has the following beneficial effects: Traditional chain-end initiated depolymerization relies on unstable end groups that promote depolymerization at moderate temperatures. However, the new method of the present invention introduces a visible-light initiated backbone-initiated depolymerization strategy, overcoming the limitations of traditional methods. This process allows depolymerization to occur independently of the polymer synthesis route, molecular weight, and end-group functionality. There is a fundamental difference between the traditional chain-end initiated method and the newly introduced backbone-initiated method. The chain-end initiated process requires specific weak links at the polymer end. The backbone-initiated method does not require unstable groups, thus enabling the depolymerization of commercial PMMA.

[0022] On the one hand, the present invention generates chlorine radicals in situ directly from the solvent, and these chlorine radicals dehydrogenate from the polymer backbone. Regardless of the synthesis route of polymethacrylate (such as free radical or ionic polymerization), end groups, and molecular weight (up to 1.6 million Daltons), nearly quantitative (>98%) depolymerization can be achieved. Conducting multi-gram scale depolymerization and endowing the possibility of time control make this method a general recycling route. On the other hand, under the action of the prepared catalyst, the degradation reaction rate is accelerated, the complete degradation of PMMA is promoted, and the complete degradation reaction under visible light irradiation is realized.

[0023] The inventors surprisingly found that in the presence of a chlorine-containing solvent under visible light irradiation, significant depolymerization (>95%) occurs, even at a relatively low temperature (150 °C). To further verify the end group independence of this process, PMMA samples with different end groups were tested: PMMA derived from free radical polymerization (C–H and C=C end groups); PMMA derived from anionic polymerization (C–H end groups); PMMA derived from atom transfer radical polymerization (ATRP) (C–Cl end groups); PMMA capped with dithiobenzoate converted to PMMA capped with cyano (C–C end groups); all of these changes showed >95% depolymerization, indicating that this process is independent of end group chemistry. However, this depolymerization reaction is light-dependent, and when the light irradiation stops, the depolymerization stops. Brief Description of the Drawings

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0025] Figure 1 SEM image of the catalyst prepared in Preparation Example 1. Detailed Description of the Embodiments

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0027] NH2-MIL-125, porous material MIL-125(Ti)-NH2 or NH2-MIL-125(Ti), CAS: 1309760-94-8; graphene oxide, with an average thickness of 5 nm, an average sheet diameter of 20 μm, and an average oxygen content of 35%; EDTA, ethylenediaminetetraacetic acid; disodium EDTA, disodium ethylenediaminetetraacetate. PMMA resin, Wanhua Chemical, HD01, shrinkage rate 0.2 - 0.6%, density 1.19 - 1.20 g / cm 3 .

[0028] Preparation Example 1 Preparation of the catalyst The method is as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: Add 8 g of NH2-MIL-125 to 40 g of ethanol, add 5 g of tetrabutyl titanate and 1 g of N-acetyl-L-lysine, stir and mix for 15 min, add 10 g of concentrated hydrochloric acid, stir, perform hydrothermal reaction at 110 °C for 40 h, wash the product, dry it, and calcine it at 700 °C for 1 h to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: Perform hydrogenation treatment on the N-doped chiral mesoporous core-shell TiO2 powder at 180 °C and a hydrogen pressure of 15 Bar for 4 d to obtain modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of graphene oxide-coated C3N4: Heat 10 g of melamine to 500 °C and calcine it for 1 h. Add the product to 30 mL of a 0.1 mg / mL graphene oxide aqueous dispersion, stir and mix for 15 min, and dry it to obtain graphene oxide-coated C3N4; S4. Preparation of graphene-coated C3N4: Reduce the graphene oxide-coated C3N4 by hydrazine hydrate vapor for 10 h to obtain graphene-coated C3N4; S5. Preparation of the basic reagent: Add 0.3 mol of silver nitrate, 0.2 mol of bismuth nitrate, and 0.2 mol of EDTA to 100 mL of water, and dropwise add 1 mol / L ammonia water until a transparent solution is formed; S6. Preparation of ammonium metavanadate solution: Dissolve 0.2 mol of ammonium metavanadate in 10 mL of 3 mol / L nitric acid to obtain an ammonium metavanadate solution; S7. Preparation of the catalyst: Add 5 g of modified N-doped chiral mesoporous core-shell TiO2 powder and 2 g of graphene-coated C3N4 to 100 mL of water, add 7 g of the basic reagent, dropwise add 0.5 g of the ammonium metavanadate solution and add 0.2 g of sodium dihydrogen phosphate, stir and mix for 15 min, adjust the pH value of the solution to neutral, continue to stir and react for 3 h under dark conditions, filter, wash, dry, and grind to obtain the catalyst, Figure 1SEM image of the prepared catalyst. As can be seen from the figure, the catalyst forms a composite structure composed of a variety of composite substances.

[0029] Preparation of the catalyst in Preparation Example 2 The method is as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: Add 10 g of NH2-MIL-125 to 50 g of ethanol, add 9 g of tetrabutyl titanate and 2 g of N-acetyl-L-glutamic acid, stir and mix for 15 min, add 20 g of concentrated hydrochloric acid, stir, carry out hydrothermal reaction at 130 °C for 52 h, wash the product, dry it, and calcine it at 800 °C for 3 h to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: Carry out hydrogenation treatment on the N-doped chiral mesoporous core-shell TiO2 powder at 220 °C under a hydrogen pressure of 25 Bar for 6 d to obtain modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of graphene oxide-coated C3N4: Heat 10 g of melamine to 600 °C and calcine it for 3 h. Add the product to 50 mL of a 0.2 mg / mL graphene oxide aqueous dispersion, stir and mix for 15 min, and dry it to obtain graphene oxide-coated C3N4; S4. Preparation of graphene-coated C3N4: Reduce the graphene oxide-coated C3N4 with hydrazine hydrate vapor for 12 h to obtain graphene-coated C3N4; S5. Preparation of the basic reagent: Add 0.5 mol of silver nitrate, 0.3 mol of bismuth nitrate, and 0.3 mol of disodium EDTA to 100 mL of water, and dropwise add 1 mol / L ammonia water until a transparent solution is formed; S6. Preparation of ammonium metavanadate solution: Dissolve 0.3 mol of ammonium metavanadate in 10 mL of 5 mol / L nitric acid to obtain an ammonium metavanadate solution; S7. Preparation of the catalyst: Add 7 g of modified N-doped chiral mesoporous core-shell TiO2 powder and 4 g of graphene-coated C3N4 to 100 mL of water, add 11 g of the basic reagent, dropwise add 1 g of the ammonium metavanadate solution and add 0.5 g of sodium dihydrogen phosphate, stir and mix for 15 min, adjust the pH value of the solution to neutral, continue to stir and react for 5 h under dark conditions, filter, wash, dry, and grind to obtain the catalyst.

[0030] Preparation of the catalyst in Preparation Example 3 The method is as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: Add 9 g of NH2-MIL-125 to 45 g of ethanol, add 7 g of tetrabutyl titanate and 1.5 g of N-acetyl-L-glutamic acid, stir and mix for 15 min, add 15 g of concentrated hydrochloric acid, stir, carry out hydrothermal reaction at 120 °C for 48 h, wash the product, dry it, and calcine it at 750 °C for 2 h to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: Carry out hydrogenation treatment on the N-doped chiral mesoporous core-shell TiO2 powder at 200 °C and a hydrogen pressure of 20 Bar for 5 d to obtain modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of graphene oxide-coated C3N4: Heat 10 g of melamine to 550 °C and calcine it for 2 h. Add the product to 40 mL of a 0.15 mg / mL graphene oxide aqueous dispersion, stir and mix for 15 min, and dry it to obtain graphene oxide-coated C3N4; S4. Preparation of graphene-coated C3N4: Reduce the graphene oxide-coated C3N4 by hydrazine hydrate vapor for 11 h to obtain graphene-coated C3N4; S5. Preparation of alkaline reagent: Add 0.4 mol of silver nitrate, 0.25 mol of bismuth nitrate and 0.25 mol of EDTA to 100 mL of water, and dropwise add 1 mol / L ammonia water until a transparent solution is formed; S6. Preparation of ammonium metavanadate solution: Dissolve 0.25 mol of ammonium metavanadate in 10 mL of 4 mol / L nitric acid to obtain an ammonium metavanadate solution; S7. Preparation of catalyst: Add 6 g of modified N-doped chiral mesoporous core-shell TiO2 powder and 3 g of graphene-coated C3N4 to 100 mL of water, add 8 g of alkaline reagent, dropwise add 0.7 g of ammonium metavanadate solution and add 0.4 g of sodium dihydrogen phosphate, stir and mix for 15 min, adjust the pH value of the solution to neutral, continue to stir and react for 4 h under dark conditions, filter, wash, dry, and grind to obtain the catalyst.

[0031] Comparative Preparation Example 1 Compared with Preparation Example 3, the difference is that N-acetyl-L-glutamic acid is not added in step S1.

[0032] Specifically as follows: Preparation of N-doped core-shell TiO2 powder: Add 9 g of NH2-MIL-125 to 45 g of ethanol, add 7 g of tetrabutyl titanate, stir and mix for 15 min, add 15 g of concentrated hydrochloric acid, stir, carry out hydrothermal reaction at 120 °C for 48 h, wash the product, dry it, and calcine it at 750 °C for 2 h to obtain N-doped core-shell TiO2 powder.

[0033] Comparative Preparation Example 2 Compared with Preparation Example 3, the difference lies in that Step S1 is different.

[0034] Specifically as follows: Preparation of N-doped TiO2 powder: 9 g of NH2-MIL-125 was calcined at 750 °C for 2 h to obtain N-doped TiO2 powder.

[0035] Comparative Preparation Example 3 Compared with Preparation Example 3, the difference lies in that Step S2 was not carried out.

[0036] Comparative Preparation Example 4 Compared with Preparation Example 3, the difference lies in that Step S3 is different and Step S4 was not carried out.

[0037] Specifically as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: 9 g of NH2-MIL-125 was added to 45 g of ethanol, 7 g of tetrabutyl titanate and 1.5 g of N-acetyl-L-glutamic acid were added, stirred and mixed for 15 min, 15 g of concentrated hydrochloric acid was added, stirred, hydrothermally reacted at 120 °C for 48 h, the product was washed, dried, and calcined at 750 °C for 2 h to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: The N-doped chiral mesoporous core-shell TiO2 powder was hydrogenated at 200 °C under a hydrogen pressure of 20 Bar for 5 d to obtain modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of C3N4: 10 g of melamine was heated to 550 °C and calcined for 2 h to obtain C3N4; S4. Preparation of basic reagent: 0.4 mol of silver nitrate, 0.25 mol of bismuth nitrate and 0.25 mol of EDTA were added to 100 mL of water, and 1 mol / L ammonia water was added dropwise until a transparent solution was formed; S5. Preparation of ammonium metavanadate solution: 0.25 mol of ammonium metavanadate was dissolved in 10 mL of 4 mol / L nitric acid to obtain an ammonium metavanadate solution; S6. Preparation of catalyst: 6 g of modified N-doped chiral mesoporous core-shell TiO2 powder and 3 g of C3N4 were added to 100 mL of water, 8 g of basic reagent was added, 0.7 g of ammonium metavanadate solution was added dropwise and 0.4 g of sodium dihydrogen phosphate was added, stirred and mixed for 15 min, the pH value of the solution was adjusted to neutral, and the reaction was continued under dark conditions with stirring for 4 h, filtered, washed, dried, ground to obtain the catalyst.

[0038] Comparative Preparation Example 5 Compared with Preparation Example 3, the difference lies in that bismuth nitrate was not added in Step S5 and ammonium metavanadate was not added in Step S7.

[0039] The details are as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: Add 9 g of NH2-MIL-125 to 45 g of ethanol, add 7 g of tetrabutyl titanate and 1.5 g of N-acetyl-L-glutamic acid, stir and mix for 15 min, add 15 g of concentrated hydrochloric acid, stir, perform hydrothermal reaction at 120 °C for 48 h, wash the product, dry it, and calcine it at 750 °C for 2 h to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: Perform hydrogenation treatment on the N-doped chiral mesoporous core-shell TiO2 powder at 200 °C under a hydrogen pressure of 20 Bar for 5 d to obtain modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of graphene oxide-coated C3N4: Heat 10 g of melamine to 550 °C and calcine it for 2 h. Add the product to 40 mL of a 0.15 mg / mL graphene oxide aqueous dispersion, stir and mix for 15 min, and dry it to obtain graphene oxide-coated C3N4; S4. Preparation of graphene-coated C3N4: Subject the graphene oxide-coated C3N4 to hydrazine hydrate vapor reduction for 11 h to obtain graphene-coated C3N4; S5. Preparation of basic reagent: Add 0.65 mol of silver nitrate and 0.25 mol of EDTA to 100 mL of water, and dropwise add 1 mol / L ammonia water until a transparent solution is formed; S6. Preparation of catalyst: Add 6 g of modified N-doped chiral mesoporous core-shell TiO2 powder and 3 g of graphene-coated C3N4 to 100 mL of water, add 8 g of basic reagent, add 0.4 g of sodium dihydrogen phosphate, stir and mix for 15 min, adjust the pH value of the solution to neutral, continue stirring and reacting for 4 h under dark conditions, filter, wash, dry, and grind to obtain the catalyst.

[0040] Comparative Preparation Example 6 Compared with Preparation Example 3, the difference lies in that the modified N-doped chiral mesoporous core-shell TiO2 powder is not added in step S7.

[0041] The details are as follows: S7. Preparation of catalyst: Add 9 g of graphene-coated C3N4 to 100 mL of water, add 8 g of basic reagent, dropwise add 0.7 g of ammonium metavanadate solution and add 0.4 g of sodium dihydrogen phosphate, stir and mix for 15 min, adjust the pH value of the solution to neutral, continue stirring and reacting for 4 h under dark conditions, filter, wash, dry, and grind to obtain the catalyst.

[0042] Comparative Preparation Example 7 Compared with Preparation Example 3, the difference lies in that the graphene-coated C3N4 is not added in step S7.

[0043] The details are as follows: S7. Preparation of the catalyst: Add 9 g of modified N-doped chiral mesoporous core-shell TiO2 powder into 100 mL of water, add 8 g of basic reagent, dropwise add 0.7 g of ammonium metavanadate solution and add 0.4 g of sodium dihydrogen phosphate, stir and mix for 15 min, adjust the pH value of the solution to neutral, continue to stir and react for 4 h under dark conditions, filter, wash, dry, and grind to obtain the catalyst.

[0044] Test Example 1 Photocatalytic performance test Take 0.1 g of the catalyst prepared in Preparation Examples 1-3 or Comparative Preparation Examples 1-7 and 100 mL of 4 mg / L rhodamine B solution and put them into a reaction tube. Magnetically stir for 20 min in the dark to make the catalyst and rhodamine B dye reach the adsorption-desorption equilibrium. Select a 300 W xenon lamp as the visible light source, stir and irradiate, and conduct a photocatalytic degradation experiment. After 5 min, take 2 mL of the solution, centrifuge and take the supernatant, and measure the absorbance of the rhodamine B solution at 554 nm on a UV-visible spectrophotometer, and calculate the degradation rate of the rhodamine B solution using the following formula.

[0045] Degradation rate (%) = (C0 - C) / C0 × 100% In the formula: C0 is the initial concentration of the rhodamine B solution when the adsorption-desorption equilibrium is reached; C is the concentration of the rhodamine B solution after irradiation for a certain period of time.

[0046] The results are shown in Table 1.

[0047] Table 1

[0048] As can be seen from the above table, the catalysts prepared in Preparation Examples 1-3 of the present invention have good visible light photocatalytic degradation effects.

[0049] Example 1 This example provides a method for preparing a PMMA depolymer. Add PMMA resin into p-dichlorobenzene, add the catalyst prepared in Preparation Example 1, the addition amount of the catalyst is 1 wt% of the total mass of PMMA, under visible light irradiation, the light intensity is 500 Lx, heat to 140 °C, carry out a depolymerization reaction for 5 h, filter, recover the catalyst, separate and recover the chlorinated solvent, and obtain the PMMA depolymer.

[0050] Example 2 This example provides a method for preparing a PMMA depolymer. Add PMMA resin into o-dichlorobenzene, add the catalyst prepared in Preparation Example 2, the addition amount of the catalyst is 2 wt% of the total mass of PMMA, under visible light irradiation, the light intensity is 1000 Lx, heat to 160 °C, carry out a depolymerization reaction for 5 h, filter, recover the catalyst, separate and recover the chlorinated solvent, and obtain the PMMA depolymer.

[0051] Example 3 This example provides a method for preparing a PMMA depolymer. Add PMMA resin to m-dichlorobenzene, and add the catalyst prepared in Preparation Example 3. The addition amount of the catalyst is 1.5 wt% of the total mass of PMMA. Under visible light irradiation with a light intensity of 700 Lx, heat to 150 °C and carry out the depolymerization reaction for 5 h. Then filter, recover the catalyst, and separate and recover the chlorine-containing solvent to obtain the PMMA depolymer.

[0052] Comparative Example 1 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 1.

[0053] Comparative Example 2 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 2.

[0054] Comparative Example 3 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 3.

[0055] Comparative Example 4 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 4.

[0056] Comparative Example 5 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 5.

[0057] Comparative Example 6 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 6.

[0058] Comparative Example 7 Compared with Example 3, the difference lies in that the catalyst is prepared from Comparative Preparation Example 7.

[0059] Comparative Example 8 Compared with Example 3, the difference lies in that the solvent m-dichlorobenzene is replaced with xylene.

[0060] Comparative Example 9 Compared with Example 3, the difference lies in that no catalyst is added.

[0061] Test Example 1 Evaluate the reactions in Examples 1 - 6 and Comparative Examples 1 - 9, and test the depolymerization rate of each group. The results are shown in Table 2.

[0062] Table 2

[0063] As can be seen from the above table, the methods in Examples 1 - 6 of the present invention can well depolymerize PMMA materials to obtain depolymers.

[0064] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a PMMA depolymerization product, characterized in that, PMMA is added to a chlorine-containing solvent, a catalyst is added, and a depolymerization reaction is carried out under heating under visible light irradiation. After filtration, the catalyst is recovered, the chlorine-containing solvent is separated and recovered, and a PMMA depolymer is obtained; the catalyst is prepared by mixing hydrogenated N-doped chiral mesoporous core-shell TiO2 powder and graphene-coated C3N4, adding silver nitrate, bismuth nitrate, ammonia water, ammonium metavanadate and sodium dihydrogen phosphate, adjusting the pH value of the solution to neutral, and carrying out a stirring reaction.

2. The preparation method according to claim 1, characterized in that, The chlorine-containing solvent is selected from at least one of orthodichlorobenzene, metadichlorobenzene, and paradichlorobenzene; the addition amount of the catalyst is 1-2 wt% of the total mass of PMMA, the temperature of the heating depolymerization reaction is 140-160 °C, the time is 4-7 h, and the light intensity of the visible light is 500-1000 Lx.

3. The preparation method according to claim 1, wherein The preparation method of the catalyst is as follows: S1. Preparation of N-doped chiral mesoporous core-shell TiO2 powder: NH2-MIL-125 is added to ethanol, tetrabutyl titanate and a chiral amino acid pore-forming agent are added, stirred and mixed evenly, concentrated hydrochloric acid is added, and a hydrothermal reaction is carried out under stirring. The product is washed, dried, and calcined to obtain N-doped chiral mesoporous core-shell TiO2 powder; S2. High-temperature and high-hydrogen-pressure hydrogenation treatment: The N-doped chiral mesoporous core-shell TiO2 powder is hydrogenated under high-temperature and high-hydrogen-pressure conditions to obtain a modified N-doped chiral mesoporous core-shell TiO2 powder; S3. Preparation of graphene-coated C3N4: Melamine is heated and calcined, and the product is added to an aqueous dispersion of graphene oxide, stirred and mixed evenly, and dried to obtain graphene oxide-coated C3N4; S4. Preparation of graphene-coated C3N4: The graphene oxide-coated C3N4 is reduced by hydrazine hydrate vapor to obtain graphene-coated C3N4; S5. Preparation of an alkaline reagent: Silver nitrate, bismuth nitrate and a complexing agent are added to water, and ammonia water is added dropwise until a transparent solution is formed; S6. Preparation of ammonium metavanadate solution: Ammonium metavanadate is dissolved in nitric acid to obtain an ammonium metavanadate solution; S7. Preparation of the catalyst: The modified N-doped chiral mesoporous core-shell TiO2 powder and graphene-coated C3N4 are added to water, an alkaline reagent is added, an ammonium metavanadate solution and sodium dihydrogen phosphate are added dropwise, stirred and mixed evenly, the pH value of the solution is adjusted, and the stirring reaction is continued under dark conditions. After filtration, washing, drying, and grinding, the catalyst is obtained.

4. The preparation method according to claim 3, characterized in that, In step S1, the mass ratio of NH2-MIL-125, tetrabutyl titanate, chiral amino acid pore-forming agent, ethanol and concentrated hydrochloric acid is 8-10:5-9:1-2:40-50:10-20, the temperature of the stirring hydrothermal reaction is 110-130 °C, the time is 40-52 h, the temperature of the calcination is 700-800 °C, the time is 1-3 h, and the chiral amino acid pore-forming agent is selected from at least one of N-acetyl-L-lysine, N-acetyl-L-glutamic acid, and N-lauroyl-L-lysine.

5. The preparation method according to claim 3, characterized in that, In step S2, the temperature of the high-temperature and high-hydrogen-pressure condition is 180-220 °C, the hydrogen pressure is 15-25 Bar, and the hydrogenation treatment time is 4-6 d.

6. The preparation method according to claim 3, characterized in that, The temperature of the calcination described in step S3 is 500 - 600 °C, the time is 1 - 3 h, the solid-liquid ratio of the melamine and graphene oxide aqueous dispersion is 1:3 - 5 g / mL, and the concentration of the graphene oxide aqueous dispersion is 0.1 - 0.2 mg / mL.

7. The preparation method according to claim 3, characterized in that, The time of the hydrazine hydrate vapor reduction described in step S4 is 10 - 12 h.

8. The preparation method according to claim 3, wherein The molar ratio of silver nitrate, bismuth nitrate and complexing agent described in step S5 is 3 - 5:2 - 3:2 - 3, and the complexing agent is EDTA or disodium EDTA; the concentration of nitric acid described in step S6 is 3 - 5 mol / L.

9. The preparation method according to claim 3, characterized in that, The mass ratio of the modified N-doped chiral mesoporous core-shell TiO2 powder, graphene-coated C3N4, alkaline reagent, ammonium metavanadate solution and sodium dihydrogen phosphate described in step S7 is 5 - 7:2 - 4:7 - 11:0.5 - 1:0.2 - 0.5, the pH value of the adjusted solution is neutral, and the time of the continuous stirring reaction under dark conditions is 3 - 5 h.

10. A PMMA depolymer obtained by the preparation method according to any one of claims 1 - 9.

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