Acid-base dual site metal-free catalyst, preparation method and application thereof

Abstract

The present application relates to the technical field of polyester waste resource utilization, and particularly relates to an acid-alkali dual-site non-metallic catalyst, a preparation method and application thereof.The acid-alkali dual-site non-metallic catalyst provided by the present application is suitable for oxidative depolymerization of polyester, especially polyethylene terephthalate, has low catalyst dosage, and has excellent reaction kinetic performance, environmental protection and recyclable characteristics.When the mass ratio of PET waste to the catalyst is 100:1 and the reaction time is 30 min at 180 DEG C, the PET degradation rate is 99.97%, the ethylene glycol yield is 91.53%, and the dimethyl terephthalate yield is 90.62%.Further increasing the mass ratio of PET waste to the catalyst to 150:1, the PET degradation rate still remains 97.68%, which indicates that the acid-alkali dual-site non-metallic catalyst provided by the present application has excellent depolymerization performance under high PET load conditions.

Description

Technical Field

[0001] This invention relates to the field of polyester waste resource utilization technology, and in particular to an acid-base dual-site metal-free catalyst, its preparation method, and its application. Background Technology

[0002] Polyethylene terephthalate (PET), as a petroleum-based polyester material, has a dense aromatic ring structure and highly crystalline ester bonds in its molecular chain, which gives it excellent chemical stability. However, this also makes it extremely difficult to degrade in the natural environment, resulting in the accumulation of polyethylene terephthalate.

[0003] Chemical recycling, as a sustainable technology solution, is gradually attracting the attention of technical personnel. Chemical recycling methods depolymerize polyethylene terephthalate (PET) into its original monomers, such as terephthalic acid (TPA), dimethyl terephthalate (DMT), and ethylene glycol (EG), achieving closed-loop recycling and improving the overall utilization rate of PET. Currently, the main chemical recycling methods include hydrolysis, alcoholysis, hydrogenolysis, and glycolysis.

[0004] Chinese patent CN114890898A discloses an alcoholysis recovery method based on a two-component catalyst, which uses PET as raw material, ethylene glycol as solvent, nitrogen-containing polycyclic organic compounds and metal salts as catalysts to alcoholyze PET into repolymerizable diethyl terephthalate at 120-200℃. Chinese patent CN113874346A proposes using sodium methoxide as a catalyst for PET depolymerization, generating DMT and monoethylene glycol (MEG). Chinese patent CN114656313A discloses a method for the synergistic conversion of PET plastics into monocyclic aromatic hydrocarbons using natural gas hydrates, employing a titanium dioxide-supported Pt / Ni bimetallic catalyst to achieve synergistic coupling of PET depolymerization, hydrodeoxygenation, and natural gas hydrate reforming.

[0005] The methods described above have achieved some success in the depolymerization of polyethylene terephthalate (PET), but they typically require harsh reaction conditions, such as high temperature and pressure or large doses of metal-based catalysts. Therefore, obtaining efficient yet mild catalysts has become a core issue in the depolymerization and recovery of PET. Summary of the Invention

[0006] In view of this, the present invention provides an acid-base dual-site metal-free catalyst, its preparation method and application. The acid-base dual-site metal-free catalyst provided by the present invention is highly efficient and mild, and can significantly reduce the depolymerization cost of polyethylene terephthalate.

[0007] This invention provides a metal-free acid-base dual-site catalyst, comprising lignin carbon and nitrogen and a second non-metallic element doped in the lignin carbon; the oxygen-containing functional groups of the lignin carbon include -COOH, -OH and C=O; the nitrogen element exists in the forms of pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen; the second non-metallic element includes one or more of sulfur, phosphorus and boron; the metal-free acid-base dual-site catalyst contains oxygen vacancies.

[0008] This invention also provides a method for preparing the metal-free acid-base dual-site catalyst described above, comprising the following steps:

[0009] The lignin and non-metallic compound are ground, mixed, and then calcined to obtain the acid-base dual-site metal-free catalyst; the non-metallic compound includes one or more of sulfur-containing compounds, boron-containing compounds, and phosphorus-containing compounds, as well as nitrogen-containing compounds.

[0010] Preferably, the lignin is derived from lignocellulose biomass components; the lignin includes one or more of enzymatically hydrolyzed lignin, alkali lignin, acid-base densified lignin, sodium lignin sulfonate, and organic solvent lignin.

[0011] Preferably, the nitrogen-containing compound includes one or more of dicyandiamide, melamine, urea, and L-histidine; the sulfur-containing compound is thiourea; the boron-containing compound includes sodium borate; and the phosphorus-containing compound is phytic acid.

[0012] Preferably, the mass ratio of lignin to non-metallic compound is 1:(1~25); when the non-metallic compound is a nitrogen-containing compound and a boron-containing compound, the mass ratio of the nitrogen-containing compound to the boron-containing compound is (1~5):1; when the non-metallic compound is a nitrogen-containing compound and a sulfur-containing compound, the mass ratio of the nitrogen-containing compound to the sulfur-containing compound is (1~15):(1~10); when the non-metallic compound is a nitrogen-containing compound and a phosphorus-containing compound, the mass ratio of the nitrogen-containing compound to the phosphorus-containing compound is (1~5):1.

[0013] Preferably, the calcination process further includes heating; the heating includes a first heating, a holding period, and a second heating; the rate of the first heating is 5 °C / min; the holding period is 500~700 °C for 1~2 hours; and the rate of the second heating is 5 °C / min.

[0014] Preferably, the calcination temperature is 200~1200 ℃, and the holding time is 1~4 h; the calcination is carried out in an inert atmosphere.

[0015] The present invention also provides the application of the acid-base dual-site metal-free catalyst described in the above-described scheme or the acid-base dual-site metal-free catalyst obtained by the preparation method described in the above-described scheme in polyester depolymerization.

[0016] Preferably, the method of application includes the following steps: depolymerizing polyester and an acid-base dual-site metal-free catalyst in an oxygen atmosphere; wherein the acid-base dual-site metal-free catalyst is the acid-base dual-site metal-free catalyst described in the above scheme or the acid-base dual-site metal-free catalyst obtained by the preparation method described in the above scheme.

[0017] Preferably, the depolymerization reaction is carried out at a temperature of 140~200 °C for 5~60 min; the depolymerization reaction is carried out in the presence of an organic solvent.

[0018] This invention provides a metal-free catalyst with two acid-base sites. The metal-free catalyst with two acid-base sites provided by this invention is a Lewis-Brønst heteroatom-doped N@C catalyst with two acid-base sites. It possesses abundant acid-base active sites and vacancy defects, enabling efficient activation of O2 to achieve the oxidative depolymerization and utilization of polyethylene terephthalate. Specifically, as... Figure 6 As shown, the present invention achieves the above-mentioned beneficial effects through the following two aspects:

[0019] (1) Material structure optimization: This invention modifies the electronic structure of carbon materials by using lignin as a carbon source and co-doping it with nitrogen-containing and non-metallic substances containing a second heteroatom, thereby promoting the formation of oxygen vacancies (VO) and defects, significantly increasing the specific surface area and pore volume, and thus enhancing the adsorption capacity of the acid-base dual-site metal-free catalyst with reactants. Furthermore, it contains no precious metal components, avoiding the problems of high cost and difficult recycling of precious metal catalysts, making it more competitive in industrial applications.

[0020] (2) Synergistic effect of acid-base dual active sites: The synergistic effect of acidic and basic sites improves the activation efficiency of oxygen and enhances the nucleophilic attack on the ester carbon. On the one hand, the addition of nitrogen brings abundant pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen, which increases the basicity of the acid-base dual-site metal-free catalyst, promotes the adsorption and activation of O2, improves the activation efficiency of O2, and generates O2-· intermediates, which react with methanol to generate OOH- and CH3O-, and jointly carry out nucleophilic attack. On the other hand, oxygen-containing functional groups in lignin (such as -COOH, -OH, C=O) provide acidic sites for the acid-base dual-site metal-free catalyst. These acidic sites activate the carbonyl group, form carbocation intermediates, enhance the electrophilicity of the carbonyl carbon, and reduce the dissociation energy of the ester bond, thereby further promoting the depolymerization and upgrading of polyester.

[0021] This invention also provides a method for preparing the acid-base dual-site metal-free catalyst described in the above-mentioned scheme. The preparation method provided by this invention, through specific carbon and nitrogen sources and C / N ratios, controls the calcination temperature and dops with a second heteroatom to obtain a heteroatom-doped N@C catalyst with both acid-base active sites and high catalytic activity. This invention not only converts inexpensive lignin into a high-performance catalyst, but also promotes the depolymerization and utilization of various polyester wastes by effectively activating O2. The preparation method provided by this invention has simple steps and inexpensive raw materials, achieving the dual advantages of efficient resource utilization and cost-effectiveness.

[0022] This invention also provides the application of the acid-base dual-site metal-free catalyst described in the above-described scheme or the acid-base dual-site metal-free catalyst obtained by the preparation method described in the above-described scheme in polyester depolymerization. The acid-base dual-site metal-free catalyst provided by this invention is suitable for the oxidative depolymerization of polyester, especially polyethylene terephthalate (PET). It has low catalyst dosage and possesses excellent reaction kinetics, environmental friendliness, and recyclability. At 180 °C, the mass ratio of PET waste to catalyst (M...)... PET :M Cat When the ratio of ethylene glycol to ethylene glycol was 100:1, the reaction time was 30 min, resulting in a PET degradation rate of 99.97%, an ethylene glycol yield of 91.53%, and a dimethyl terephthalate yield of 90.62%. Further improvements in M... PET :M Cat Even at a ratio of 150:1, the PET degradation rate remained at 97.68%, indicating that the acid-base dual-site metal-free catalyst provided by this invention exhibits excellent depolymerization performance under high PET loading conditions. This invention provides a green, economical, and efficient solution for the upgrading and utilization of PET, revolutionizing the treatment of plastic polyester waste and demonstrating broad industrial application prospects. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in this invention 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 some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0024] Figure 1 This is an X-ray energy dispersive spectroscopy (XPS) analysis of the metal-free catalyst with acid-base dual sites of the present invention;

[0025] Figure 2 This is an electron paramagnetic resonance (EPR) analysis diagram of the metal-free catalyst with acid-base dual sites of the present invention;

[0026] Figure 3This is the Raman spectrum analysis of the metal-free catalyst with acid-base dual sites of the present invention;

[0027] Figure 4 This is a temperature-programmed desorption (TPD) diagram of the acid-base dual-site metal-free catalyst of the present invention;

[0028] Figure 5 This is a temperature-programmed desorption (TPD) diagram of CO2 using the acid-base dual-site metal-free catalyst of the present invention.

[0029] Figure 6 This is a diagram illustrating the mechanism of action of the acid-base dual-site metal-free catalyst of the present invention. Detailed Implementation

[0030] This invention provides a metal-free acid-base dual-site catalyst, comprising lignin carbon and nitrogen and a second non-metallic element doped in the lignin carbon; the oxygen-containing functional groups of the lignin carbon include -COOH, -OH and C=O; the nitrogen element exists in the forms of pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen; the second non-metallic element includes one or more of sulfur, phosphorus and boron; the metal-free acid-base dual-site catalyst contains oxygen vacancies.

[0031] This invention also provides a method for preparing the metal-free acid-base dual-site catalyst described above, comprising the following steps:

[0032] The lignin and non-metallic compound are ground, mixed, and then calcined to obtain the acid-base dual-site metal-free catalyst; the non-metallic compound includes one or more of sulfur-containing compounds, boron-containing compounds, and phosphorus-containing compounds, as well as nitrogen-containing compounds.

[0033] In this invention, the lignin is preferably derived from lignocellulose biomass components; the lignin preferably includes one or more of enzymatically hydrolyzed lignin, alkali lignin, acid-base densified lignin, sodium lignin sulfonate, and organic solvent lignin; the alkali lignin is preferably commercially available alkali lignin.

[0034] In this invention, the nitrogen-containing compound preferably includes one or more of dicyandiamide, melamine, urea, and L-histidine; the sulfur-containing compound is preferably thiourea; the boron-containing compound preferably includes sodium borate; and the phosphorus-containing compound is preferably phytic acid.

[0035] In this invention, the mass ratio of lignin to nonmetallic compound is preferably 1:(1~25), more preferably 1:15 or 1:25.

[0036] In this invention, when the non-metallic compound is a nitrogen-containing compound and a boron-containing compound, the mass ratio of the nitrogen-containing compound to the boron-containing compound is preferably (1~5):1, more preferably (3~4):1.

[0037] In this invention, when the non-metallic compound is a nitrogen-containing compound and a sulfur-containing compound, the mass ratio of the nitrogen-containing compound to the sulfur-containing compound is preferably (1~15):(1~10), and more preferably (4~5):1.

[0038] In this invention, when the non-metallic compound is a nitrogen-containing compound and a phosphorus-containing compound, the preferred mass ratio of the nitrogen-containing compound to the phosphorus-containing compound is (1~5):1, more preferably (3~4):1. Using the above-mentioned ratio of non-metallic compounds, this invention can achieve better catalytic efficiency and catalytic effect.

[0039] In this invention, the grinding and mixing time is preferably 1 to 60 min, more preferably 20 to 40 min.

[0040] In this invention, the calcination process preferably includes heating; the heating preferably includes a first heating, a holding period, and a second heating; the rate of the first heating is preferably 5 °C / min; the holding period is preferably 500~700 °C, more preferably 550~650 °C, and the holding period is preferably 1~2 hours; the rate of the second heating is preferably 5 °C / min.

[0041] In this invention, the calcination temperature is preferably 200~1200 °C, more preferably 600~1000 °C, and even more preferably 800 °C; the holding time is preferably 1~4 h, more preferably 1~2 h; the calcination is carried out in an inert atmosphere; the inert atmosphere is preferably argon. The acid-base dual-site metal-free catalyst prepared by this invention at 800 °C exhibits the best effect in the oxidative depolymerization of PET.

[0042] During calcination, nitrogen is incorporated into the carbon support, increasing the electron density of the acid-base dual-site metal-free catalyst, altering its electronic structure, and promoting the formation of oxygen vacancies and defects. This creates basic sites, effectively activating O2 to generate O2-·, a highly oxidizing agent, thus enhancing the depolymerization efficiency of PET by the acid-base dual-site metal-free catalyst. Simultaneously, sulfur contributes to the formation of acidic sites, strengthening the adsorption of substrates by the catalyst. The synergistic effect of these two elements further increases the density of active sites on the surface of the acid-base dual-site metal-free catalyst, enhancing O2 activation and substrate adsorption, thereby promoting the depolymerization and upgrading of PET waste.

[0043] Furthermore, during calcination, the organic matter in the raw materials carbonizes, forming a porous structure that significantly increases the specific surface area and surface energy. This promotes the bonding of non-metallic elements with lignin, forming a stable composite structure and achieving uniform distribution and stable anchoring of non-metallic elements in the acid-base dual-site metal-free catalyst. In particular, calcination under an inert atmosphere effectively prevents the acid-base dual-site metal-free catalyst from reacting with oxygen or other components at high temperatures, avoiding oxidative damage and ensuring stable doping of non-metallic elements. Simultaneously, it maintains the structure and porosity of the acid-base dual-site metal-free catalyst, thus guaranteeing its high performance.

[0044] The present invention also provides the application of the acid-base dual-site metal-free catalyst described in the above-described scheme or the acid-base dual-site metal-free catalyst obtained by the preparation method described in the above-described scheme in polyester depolymerization.

[0045] In this invention, the polyester preferably includes one or more of polyethylene terephthalate, polybutylene terephthalate, polylactic acid, and polycarbonate; the polyester can specifically be polyester waste, and there are no special requirements for the source, use, morphology, and pigment of the polyester waste.

[0046] In this invention, the method of application preferably includes the following steps: depolymerizing polyester and an acid-base dual-site metal-free catalyst in an oxygen atmosphere; wherein the acid-base dual-site metal-free catalyst is the acid-base dual-site metal-free catalyst described in the above scheme or the acid-base dual-site metal-free catalyst obtained by the preparation method described in the above scheme.

[0047] In this invention, the mass ratio of the polyester and the acid-base dual-site metal-free catalyst is preferably (50~500):(1~50), more preferably 100:10, 100:1 or 300:1.

[0048] In this invention, the pressure of the oxygen is preferably 0.1~1 MPa, more preferably 0.5 MPa.

[0049] In this invention, the reaction vessel is preferably purged with oxygen before the depolymerization reaction.

[0050] In this invention, the temperature of the depolymerization reaction is preferably 140~200 °C, more preferably 180 °C, and the holding time is preferably 5~60 min, more preferably 30 min; the depolymerization reaction is preferably carried out in the presence of an organic solvent; the organic solvent is preferably an alcohol; the alcohol is preferably methanol; the mass ratio of the polyester to the volume of the organic solvent is preferably (0.1~3) g:(5~40) mL, more preferably 0.5 g:20 mL. By controlling the above parameters, this invention can significantly improve the degradation effect.

[0051] In this invention, the reaction system is preferably cooled after the depolymerization reaction; the final temperature of the cooling is preferably room temperature.

[0052] In this invention, the cooling process preferably further includes mixing, dissolving, and crystallizing the product system with chloroform, followed by separating the catalyst and the product.

[0053] The acid-base dual-site metal-free catalyst provided by this invention exhibits excellent performance in the oxidation, depolymerization, and upgrading of various plastic polyester wastes, especially in the treatment of PET waste, whether it contains PET of different pigments, shapes, or origins, or is a mixed PET waste, it can achieve good results.

[0054] In this invention, the formula for calculating the PET conversion rate is:

[0055] PET conversion rate (%) = {(mass of initial PET - mass of PET after reaction) / mass of initial PET} × 100%.

[0056] In this invention, the formulas for calculating EG yield and DMT yield are as follows:

[0057] EG yield (%) = {Amount of EG quantified by GC after reaction (mol) / Amount of EG theoretically produced (mol)} × 100%;

[0058] DMT yield (%) = {Amount of DMT quantified by GC (mol) / Amount of DMT theoretically produced (mol)} × 100%.

[0059] To further illustrate the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] Example 1

[0061] This embodiment prepared a metal-free catalyst with two acid-base sites, and the specific steps are as follows:

[0062] 0.3 g of commercial lignin, 1.5 g of thiourea, and 3 g of dicyandiamide were thoroughly ground in a mortar to obtain a precursor. The obtained precursor was placed in a ceramic boat and calcined in an argon atmosphere in a quartz tube furnace. The temperature was increased to 550 °C at a rate of 5 °C / min and held for 1 hour, followed by further heating to 800 °C and holding for 1 hour to obtain a metal-free acid-base dual-site catalyst (SN@C-800 °C catalyst).

[0063] The SN@C-800℃ catalyst prepared in this example was characterized by XPS, and the results are as follows: Figure 1 As shown. According to Figure 1 As can be seen, the peaks of N and S are clearly observed, indicating the successful doping of the two atoms.

[0064] The EPR analysis of the SN@C-800℃ catalyst prepared in this example was performed, and the results are as follows: Figure 2 As shown. According to Figure 2 As can be seen, g=2.003, indicating the existence of VO.

[0065] Raman analysis was performed on the SN@C-800℃ catalyst prepared in this example, and the results are as follows: Figure 3 As shown. According to Figure 3 It can be seen that the spectrum shows I D / I G (Defect carbon / graphite carbon) = 1.17, indicating that the SN@C-800℃ catalyst of this invention has a good degree of defect.

[0066] The SN@C-800℃ catalyst prepared in this example was subjected to NH3 temperature-programmed desorption, and the results are as follows: Figure 4 As shown. According to Figure 4 It can be seen that the SN@C-800℃ catalyst of the present invention has abundant acidic sites.

[0067] The SN@C-800℃ catalyst prepared in this example was subjected to CO2 temperature-programmed desorption, and the results are as follows: Figure 5 As shown. According to Figure 5 It can be seen that the SN@C-800℃ catalyst of this invention has abundant basic sites.

[0068] This embodiment also uses the SN@C-800℃ catalyst prepared in this embodiment to depolymerize PET waste. The specific steps are as follows:

[0069] 0.5 g of PET fragments, 0.02 g of SN@C-800℃ catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. After reacting at 180℃ for 30 min, the mixture was cooled to room temperature. Chloroform was added to dissolve the crystallized DMT. The catalyst and product were separated by filtration, and the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative detection of DMT and EG content. The test results are shown in Table 1.

[0070] Table 1. Test results of PET depolymerization using SN@C-800℃ catalyst

[0071]

[0072] As can be seen from Table 1, the SN@C-800℃ catalyst provided by this invention can effectively degrade PET waste.

[0073] Example 2

[0074] This embodiment prepared a metal-free catalyst with two acid-base sites using different calcination temperatures. The specific steps are as follows:

[0075] 0.3 g of commercial lignin and 3 g of dicyandiamide were thoroughly ground in a mortar to obtain a precursor. The precursor was placed in a ceramic boat and calcined in an argon atmosphere in a quartz tube furnace. The temperature was raised to 550 °C at a rate of 5 °C / min and held for 1 hour. Then, it was heated to 800 °C and held for 1 hour to obtain a metal-free catalyst with two acid-base sites (N@C-800 °C-LS).

[0076] In this embodiment, the calcination temperatures were changed to 600 ℃ and 1000 ℃, respectively, to prepare N@C-600℃-LS and N@C-1000℃-LS.

[0077] This embodiment also uses the N@C catalyst prepared in this embodiment for depolymerizing PET waste. The specific steps are as follows:

[0078] 0.5 g of PET fragments, 0.02 g of an N@C catalyst prepared in this embodiment, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. After reacting at 180°C for 30 min and cooling to room temperature, chloroform was added to dissolve the crystallized DMT. The catalyst and product were separated by filtration, and the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative detection of DMT and EG content. The test results are shown in Table 2.

[0079] Table 2. Test results of N@C catalyst depolymerization of PET at different calcination temperatures.

[0080]

[0081] according to Figure 2 It can be seen that the acid-base dual-site metal-free catalyst prepared at 800 °C has the best overall performance, with a PET conversion rate as high as 98.33%.

[0082] Example 3

[0083] This embodiment prepared a metal-free catalyst with two acid-base sites using lignin from different sources. The specific steps are as follows:

[0084] 0.3 g of enzymatically hydrolyzed lignin (EL) and 3 g of dicyandiamide were thoroughly ground in a mortar to obtain a precursor. The obtained precursor was placed in a ceramic boat and calcined in an argon atmosphere in a quartz tube furnace. The temperature was increased to 550 °C at a rate of 5 °C / min and held for 1 hour, followed by further heating to 800 °C and holding for 1 hour to obtain a metal-free acid-base dual-site catalyst (N@C-800℃-EL).

[0085] In this embodiment, the carbon source was changed to sodium lignin sulfonate (SL), alkali lignin (AL), or acid-base densified lignin (ADL), and N@C-800℃-SL, N@C-800℃-AL, and N@C-800℃-ADL were prepared respectively.

[0086] This embodiment also uses the N@C-800℃ catalyst prepared in this embodiment for depolymerizing PET waste. The specific steps are as follows:

[0087] 0.5 g of PET fragments, 0.02 g of catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. After reacting at 180 °C for 30 min and cooling to room temperature, chloroform was added to dissolve the crystallized DMT. The catalyst and product were separated by filtration, and the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative detection of DMT and EG content. The test results are shown in Table 3.

[0088] Table 3. Test results of PET depolymerization using N@C-800℃ catalysts with different lignin sources (carbon sources).

[0089]

[0090] As can be seen from Table 3, the acid-base dual-site metal-free catalyst prepared from alkali lignin has the best overall performance, with a PET conversion rate of over 85%.

[0091] Example 4

[0092] This embodiment prepared a metal-free catalyst with two acid-base sites, using different nitrogen-containing compounds as nitrogen sources. The specific steps are as follows:

[0093] 0.3 g of commercial lignin and 3 g of melamine (M) were thoroughly ground in a mortar to obtain a precursor. The obtained precursor was placed in a ceramic boat and calcined in an argon atmosphere in a quartz tube furnace. The temperature was increased to 550 °C at a rate of 5 °C / min and held for 1 hour, followed by further heating to 800 °C and holding for 1 hour to obtain a metal-free acid-base dual-site catalyst (N@C-800℃-M).

[0094] In this embodiment, the nitrogen source was changed to urea (U) or L-histidine (H) to prepare N@C-800℃-U and N@C-800℃-H, respectively.

[0095] This embodiment also uses the N@C-800℃ catalyst prepared in this embodiment for depolymerizing PET waste. The specific steps are as follows:

[0096] 0.5 g of PET fragments, 0.02 g of catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. After reacting at 180 °C for 30 min and cooling to room temperature, chloroform was added to dissolve the crystallized DMT. The catalyst and product were separated by filtration, and the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative detection of DMT and EG content. The test results are shown in Table 4.

[0097] Table 4. Test results of PET depolymerization using N@C at 800℃ catalyst with different nitrogen sources.

[0098]

[0099] As can be seen from Table 4, the acid-base dual-site metal-free catalyst prepared with urea as the nitrogen source has the best catalytic effect.

[0100] Example 5

[0101] This embodiment prepared a metal-free catalyst with two acid-base sites using different C / N ratios. The specific steps are as follows:

[0102] 0.3 g of commercial lignin and 1.5 g of dicyandiamide (C / N=1:5) were thoroughly ground in a mortar to obtain a precursor. The obtained precursor was placed in a ceramic boat and calcined in an argon atmosphere in a quartz tube furnace. The temperature was increased to 550°C at a rate of 5°C / min and held for 1 hour, followed by further heating to 800°C and holding for 1 hour to obtain a metal-free catalyst with two acid-base sites (N@C-800°C-1:5).

[0103] In this embodiment, the C / N ratio was changed to 1:15 or 1:20 to prepare N@C-800℃-1:15 and N@C-800℃-1:20, respectively.

[0104] This embodiment also uses the N@C-800℃ catalyst prepared in this embodiment for depolymerizing PET waste. The specific steps are as follows:

[0105] 0.5 g of PET fragments, 0.02 g of catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. After reacting at 180 °C for 30 min and cooling to room temperature, chloroform was added to dissolve the crystallized DMT. The catalyst and product were separated by filtration, and the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative determination of DMT and EG content. The test results are shown in Table 5.

[0106] Table 5. Test results of PET depolymerization using N@C catalyst at -800℃ with different C / N ratios.

[0107]

[0108] As can be seen from Table 5, the prepared acid-base dual-site metal-free catalyst exhibits the best catalytic effect when the C / N ratio is 1:15.

[0109] Example 6

[0110] This embodiment prepared a metal-free catalyst with two acid-base sites by using different heteroatom doping methods. The specific steps are as follows:

[0111] 0.3 g of commercial lignin, 1.5 g of boric acid, and 3 g of dicyandiamide were thoroughly ground in a mortar to obtain a precursor. The obtained precursor was placed in a ceramic boat and calcined in an argon atmosphere in a quartz tube furnace. The temperature was increased to 550 °C at a rate of 5 °C / min and held for 1 hour, followed by further heating to 1000 °C and holding for 1 hour to obtain BN@C-1000 °C.

[0112] In this embodiment, 1.5 g of boric acid was replaced with 1.5 g of phytic acid to prepare PN@C-1000℃.

[0113] This embodiment also uses the N@C-1000℃ catalyst prepared in this embodiment to depolymerize PET waste. The specific steps are as follows:

[0114] 0.5 g of PET fragments, 0.02 g of catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. After reacting at 180 °C for 30 min and cooling to room temperature, chloroform was added to dissolve the crystallized DMT. The catalyst and product were separated by filtration, and the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative determination of DMT and EG content. The test results are shown in Table 6.

[0115] Table 6. Test results of PET depolymerization using N@C catalysts with different heteroatom doping at -1000℃.

[0116]

[0117] As shown in Table 6, the products of boron- and phosphorus-doped acid-base dual-site metal-free catalysts do not contain EG, and the PET conversion rate is higher among the boron-doped acid-base dual-site metal-free catalysts.

[0118] Example 7

[0119] This embodiment utilizes the SN@C-800℃ catalyst prepared in Example 1 to depolymerize PET waste at different reaction temperatures. The specific steps are as follows:

[0120] 0.5 g of PET fragments, 20 mg of SN@C-800℃ catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. The reaction was carried out at 160–200℃ for 30 min, followed by cooling to room temperature. Chloroform was added to dissolve the crystallized DMT. After separating the catalyst and product by filtration, the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative determination of DMT and EG content. The test results are shown in Table 7.

[0121] Table 7. Test results of SN@C-800℃ catalyst depolymerization of PET at different reaction temperatures.

[0122]

[0123] As can be seen from Table 7, the SN@C-800℃ catalyst exhibits the best overall catalytic effect at the reaction temperature of Example 1. However, as the reaction temperature increases, the catalytic effect remains relatively stable at a high level without significant decrease, indicating that the catalyst of this invention possesses certain temperature resistance.

[0124] Example 8

[0125] This embodiment utilizes the SN@C-800℃ catalyst prepared in Example 1 to depolymerize PET waste using different reaction times. The specific steps are as follows:

[0126] 0.5 g of PET fragments, 20 mg of SN@C-800℃ catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. The reaction was carried out at 180 °C for 0–45 min, followed by cooling to room temperature. Chloroform was added to dissolve the crystallized DMT. After separating the catalyst and product by filtration, the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative determination of DMT and EG content. The test results are shown in Table 8.

[0127] Table 8. Test results of SN@C-800℃ catalyst depolymerization of PET at different reaction times.

[0128]

[0129] As shown in Table 8, with the extension of reaction time, the PET conversion rate increased to 100%, achieving complete degradation of polyester.

[0130] Example 9

[0131] This embodiment utilizes the SN@C-800℃ catalyst prepared in Example 1 to depolymerize PET waste under different O2 pressures. The specific steps are as follows:

[0132] 0.5 g of PET fragments, 20 mg of SN@C-800℃ catalyst, and 20 mL of methanol were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0–0.75 MPa. The reaction was carried out at 180 °C for 30 min, followed by cooling to room temperature. Chloroform was added to dissolve the crystallized DMT. After separating the catalyst and product by filtration, the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative determination of DMT and EG content. The test results are shown in Table 9.

[0133] Table 9. Test results of SN@C-800℃ catalyst depolymerization of PET under different O2 pressures.

[0134]

[0135] As shown in Table 9, the conversion rate of PET gradually increases with the increase of oxygen partial pressure, and can reach a conversion rate of 99.67%, demonstrating a significant catalytic effect.

[0136] Example 10

[0137] This embodiment utilizes the SN@C-800℃ catalyst prepared in Example 1 to depolymerize PET waste with different PET loadings. The specific steps are as follows:

[0138] PET fragments (0.1–3 g), SN@C-800℃ catalyst (20 mg), and methanol (20 mL) were added to a stainless steel reactor. The reactor was first purged three times with oxygen, then pressurized to 0.5 MPa. The reaction was carried out at 180 °C for 30 min, followed by cooling to room temperature. Chloroform was added to dissolve the crystallized DMT. After separating the catalyst and product by filtration, the filtrate was analyzed using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative determination of DMT and EG content. The test results are shown in Table 10.

[0139] Table 10. Test results of SN@C-800℃ catalyst depolymerization of PET under different PET loadings.

[0140]

[0141] As can be seen from Table 10, the PET loading has no significant effect on the PET conversion rate. However, when the PET loading is too high, the products EG and DMT decrease, and other products may be generated.

[0142] Example 11

[0143] This embodiment utilizes the SN@C-800℃ catalyst prepared in Example 1, and conducts depolymerization tests using different PET wastes. The specific steps are as follows:

[0144] Add 0.5 g of PBT, PLC, or PC, 20 mg of SN@C-800℃ catalyst, and 20 mL of methanol to a stainless steel reactor. First, purge the stainless steel reactor three times with oxygen, then pressurize to 0.5 MPa. React at 180 ℃ for 30 min, then cool to room temperature. Add chloroform to dissolve the crystallized DMT. After separating the catalyst and product by filtration, analyze the filtrate using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). GC analysis was performed using an Agilent 7890 instrument equipped with a flame ionization detector (FID) and an HP-5 capillary column (30 m × 0.25 mm × 0.25 μm) for quantitative detection of DMT and EG content. The test results are shown in Table 11.

[0145] Table 11 Test results of depolymerization of different polyesters using SN@C-800℃ catalyst

[0146]

[0147] As can be seen from Table 11, the SN@C-800℃ catalyst of this invention has a high depolymerization effect on different polyesters, and the conversion rate of polyester substrates is above 99%.

[0148] The embodiments of the present invention have been described above; however, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of the invention, and all such substitutions and modifications should fall within the scope of the invention.

Claims

1. The application of an acid-base dual-site metal-free catalyst in polyester depolymerization, wherein the acid-base dual-site metal-free catalyst is characterized in that, Includes lignin carbon and nitrogen and a second nonmetallic element doped in the lignin carbon; The oxygen-containing functional groups of the lignin carbon include -COOH, -OH and C=O; The nitrogen element exists in pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. The second nonmetallic element includes one or more of sulfur, phosphorus, and boron; The acid-base dual-site metal-free catalyst contains oxygen vacancies.

2. The application according to claim 1, characterized in that, The preparation method of the aforementioned acid-base dual-site metal-free catalyst includes the following steps: The lignin was ground and mixed with a non-metallic compound and then calcined to obtain the metal-free catalyst with two acid-base sites. The non-metallic compounds include one or more of sulfur-containing compounds, boron-containing compounds, and phosphorus-containing compounds, as well as nitrogen-containing compounds.

3. The application according to claim 2, characterized in that, The lignin is derived from lignocellulose biomass components; The lignin includes one or more of the following: enzymatically hydrolyzed lignin, alkali lignin, acid-base densified lignin, sodium lignin sulfonate, and organic solvent lignin.

4. The application according to claim 2 or 3, characterized in that, The nitrogen-containing compound includes one or more of dicyandiamide, melamine, urea, and L-histidine; The sulfur-containing compound is thiourea; The boron-containing compound includes sodium borate; The phosphorus-containing compound is phytic acid.

5. The application according to claim 2, characterized in that, When the nonmetallic compound is a nitrogen-containing compound and a boron-containing compound, the mass ratio of the nitrogen-containing compound to the boron-containing compound is (1~5):1; When the nonmetallic compound is a nitrogen-containing compound and a sulfur-containing compound, the mass ratio of the nitrogen-containing compound to the sulfur-containing compound is (1~15):(1~10). When the non-metallic compound is a nitrogen-containing compound and a phosphorus-containing compound, the mass ratio of the nitrogen-containing compound to the phosphorus-containing compound is (1~5):

1.

6. The application according to claim 2 or 5, characterized in that, The process before calcination also includes heating; The heating process includes a first heating, a heat preservation, and a second heating. The first heating rate is 5 °C / min; The insulation temperature is 500~700℃, and the insulation time is 1~2 hours; The second heating rate is 5 °C / min.

7. The application according to claim 2, characterized in that, The calcination temperature is 200~1200 ℃, and the holding time is 1~4 h; The calcination is carried out in an inert atmosphere.

8. The application according to claim 1, characterized in that, The method of application includes the following steps: Polyester and a metal-free catalyst with acid-base dual sites were used to carry out depolymerization reaction in an oxygen atmosphere.

9. The application according to claim 8, characterized in that, The depolymerization reaction is carried out at a temperature of 140~200 ℃ and for a holding time of 5~60 min; The depolymerization reaction is carried out in the presence of an organic solvent.

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