Construction method of high-crystallinity polyheptazine imide / polytriazine imide homojunction and application of homojunction in hydrogen production by photocatalytic reforming of PET (Polyethylene Terephthalate)

Abstract

The invention belongs to the technical field of industrial catalysis, and discloses a construction method of a high-crystallinity polyheptazinyl imide / polytriazine imide homojunction and application of the homojunction in hydrogen production by photocatalytic reforming of PET (Polyethylene Terephthalate). According to the method, the mixture of melamine, KSCN and NH4Cl, KCl and LiCl are taken as raw materials, and the construction of the high-crystallinity PHI / PTI homojunction is successfully realized by a one-step molten salt assisted calcination strategy. The homojunction photocatalyst can be used for catalyzing and reforming polyethylene glycol terephthalate (PET) for the first time in a low-alkali solution (COH-= 1M) medium at normal temperature and normal pressure, so that synthesis of high-added-value chemicals such as formate, glyoxal and the like and green hydrogen fuel is realized; the catalyst has huge potential in green energy production and efficient utilization of plastic wastes.

Description

Technical Field

[0001] The present invention belongs to the technical field of industrial catalysis, and particularly relates to a method for constructing a highly crystalline polyheptazine imide / polyazine imide homojunction and its application in photocatalytic reforming of PET to produce hydrogen in a highly efficient and low-alkali medium. Background Art

[0002] Due to their excellent durability, light weight, and cost-effectiveness, plastics have become indispensable basic materials in various industries, especially playing an important role in fields such as packaging, transportation, electronics, and construction. The widespread use of plastics has promoted the development of modern industry but also brought environmental problems that cannot be ignored. Among them, polyethylene terephthalate (PET), as a common plastic material, accounts for about 10% of the global plastic production annually. Although PET has a wide range of applications in daily life, its degradation process in the environment generates microplastics, which are usually less than 5 nanometers and can quickly penetrate into water and atmospheric systems and are almost everywhere. The accumulation of microplastics not only pollutes the ecological environment but may also pose potential health hazards to organisms, especially the increasing impact on aquatic organisms, birds, and human health has attracted extensive attention from the scientific community and the public (Nat. Rev. Earth Environ. 2021, 2: 659-660). Current plastic waste management strategies, mainly ammonolysis and chemical pyrolysis, can achieve the conversion of plastics to a certain extent, but are restricted by huge energy consumption and secondary pollution problems, which limit their feasibility and efficiency in large-scale applications (Small 2024, 20(46): 2403347). In contrast, photoreforming (PR) technology, as an emerging waste plastic recycling method, uses solar energy to convert waste plastics into value-added chemicals and can simultaneously generate hydrogen (H2), providing a potential sustainable solution to the energy crisis (Chem. Eng. J. 2023, 467: 143534). This technology promotes the efficient recovery of resources by using short-chain molecules generated during the hydrolysis of plastics as carbon sources and through photocatalytic oxidation reactions, showing great application potential. However, the inherent hydrophobicity of plastics requires the use of high-concentration alkaline media (such as C OH -(Nature, 2024, 631: 884 - 890). This harsh condition not only exacerbates the corrosion problem of the equipment but also significantly increases the operating cost. In addition, the highly alkaline environment may trigger environmental problems that violate the principles of green chemistry, increasing the complexity and sustainability risks of the system. Therefore, there is an urgent need to develop more efficient and environmentally friendly recycling methods and advanced photocatalytic systems to ensure that these technologies can operate under milder conditions, thereby reducing energy consumption, minimizing secondary pollution, and achieving a dual optimization of economic and environmental benefits. Promoting the innovation and progress of this technology is of crucial strategic significance for establishing a circular economy model for plastic waste management, promoting the sustainable use of resources, and addressing the increasingly severe environmental challenges.

[0003] During photoreforming, the photo - oxidized plastics act as sacrificial electron donors, and the photo - generated holes (h + ) produced by the photocatalyst are oxidized to high - value - added products, while the electrons in the conduction band (CB) are used to photoreduce protons in water to generate the green fuel H2. This process can not only effectively convert waste plastics but also support the generation of clean energy. In recent years, how to construct an efficient bifunctional photocatalytic system that can both improve the efficiency of H2 production and promote the efficient oxidation of waste plastics under mild conditions (such as low temperature and low alkali concentration) has become a hot topic in the field of photocatalysis research (Chem. Rev. 2023, 123: 4443 - 4509). Researchers have been continuously exploring ways to optimize the structure of the catalyst, improve the reaction conditions, and enhance the stability of the system to achieve this goal, thereby promoting the coordinated development of waste plastic recycling and green energy production. In the early research on photoreforming PET plastics, Reisner and his team at the University of Cambridge, UK, successfully developed a Cd / CdO x quantum dot photocatalyst for hydrolyzing PET in a 10M NaOH solution to produce ethylene glycol (EG) and terephthalic acid (TPA) (Energy Environ. Sci. 2018, 11, 2853 - 2857). Driven by simulated sunlight, the degraded PET molecules are photocatalytically oxidized to various organic chemicals, and at the same time, green H2 is also produced. This research has opened up a new direction for using solar energy to convert plastic waste. Subsequently, the team coupled cyanide - functionalized carbon nitride with nickel phosphide (CN x |Ni2P) and successfully achieved a photoreforming H2 production rate of 141 μmol·g -1 ·h -1(J.Am.Chem.Soc.2019,141,15201-15210). This breakthrough provides important experimental evidence for further exploring the reuse of solar reforming of plastic waste. However, the use of toxic Cd-based materials and extremely strong alkaline conditions poses environmental challenges. Based on the progress of these studies, the use of g-C3N4 / CuFeO2 heterojunction photocatalysts has achieved efficient photocatalytic reforming of waste plastics degradation under mild conditions. Among them, in a 5M KOH solution, this system generated 196±24 μmol·g -1 ·h -1 of H2 during the photocatalytic reforming of PET plastics and produced a large amount of valuable organic chemicals, demonstrating the great potential of photocatalytic technology in the treatment of waste plastics (Chem.Eng.J.2023,475,146413). In addition, Kang et al. from San Diego State University in the United States also conducted relevant research, explored the relationship between the concentration of ethylene glycol (EG) monomers and product formation, and developed a MoS2 / g-C3N4 photocatalyst. In an experiment with 2 hours of simulated sunlight irradiation, this system mainly produced formate and achieved 3.93 μmol·g -1 of H2 under 2M KOH conditions (ACS Mater.Letter.2023,5:3032-3041). These studies enhanced light absorption and promoted the catalytic process by constructing heterojunction structures, enabling the catalytic reaction to occur under milder alkaline conditions. However, the limitations of the energy band structure and surface / interface properties still restrict the further improvement of H2 production efficiency. These studies significantly enhanced the light absorption ability and effectively promoted the separation efficiency of photogenerated carriers by carefully designing and constructing heterojunction structures. Such a design enables the catalytic reaction to proceed smoothly under milder alkaline conditions, thereby improving the catalytic performance and reaction efficiency. However, although the heterojunction structure has improved the reaction efficiency to a certain extent, the unsatisfactory energy band structure and the limitations of surface / interface properties are still the key factors restricting the further improvement of H2 production efficiency. To address this issue, future research may need to explore more in-depth optimization strategies for energy band regulation and surface engineering. Recently, Professor Yang Bai et al. from Jilin University successfully created a CDs-g-C3N4 / Pt photocatalyst by introducing carbon quantum dots (CDs) into the carbon nitride material. This system achieved a remarkable 515±168 μmol·g -1 ·h -1The H2 production rate demonstrates the great potential of carbon nitride-based materials in realizing visible-light PET plastic reforming and H2 production under low-concentration alkaline conditions (Appl. Catal. B Environ. 2022, 316, 121662). However, despite this, g-C3N4-based materials still face problems such as limited specific surface area and high photo-generated carrier recombination rate in practical applications, which severely restrict the further improvement of their photocatalytic performance. Since the carriers in g-C3N4 materials are prone to recombination after generation, the effective separation efficiency of photo-generated electrons and holes is reduced, thereby affecting the efficiency and stability of the catalytic reaction. Therefore, how to effectively solve these limitations, optimize the structure of the photocatalyst, and improve its rate in key reactions such as H2 generation remains the core problem that needs to be overcome urgently in the current field of photocatalysis. Promoting the further optimization of the photocatalyst structure is not only the key way to improve photocatalytic performance but also the necessary step to realize the transformation of this technology to practical applications and promote its wide application in fields such as energy production and environmental protection. Currently, constructing homojunctions to regulate the lattice structure and improve the specific surface area has become an effective strategy to enhance the optical absorption ability of the catalyst and promote the separation and migration of photo-generated carriers, thereby improving photocatalytic performance. This method can optimize the electronic structure of the catalyst, enabling it to capture light energy more efficiently and achieve efficient electron transfer in photocatalytic reactions. However, to our knowledge, the homojunction system based on redox bifunctionality and applied to plastic degradation has not been fully developed and effectively utilized so far. Although in other fields, such as water splitting and air pollution control, the redox bifunctional homojunction shows good catalytic performance, its application in the degradation of plastic waste is still in its infancy, which provides broad space and potential for future research and application. Summary of the Invention

[0004] The present invention provides a method for constructing a highly crystalline homojunction g-C3N4-based photocatalyst for efficient visible-light PET reforming. Through an innovative molecular engineering strategy and a unique design concept, combined with the advantages of a highly crystalline structure, this method can significantly improve the separation efficiency of electrons and holes, thereby effectively reducing the recombination of photo-generated carriers. In addition, the optimized highly crystalline structure also enhances the light absorption ability of the photocatalyst. By precisely regulating the distribution of active sites on the catalyst surface, this method can effectively optimize the adsorption and activation process of reactants, improve the utilization rate of reactants on the catalyst surface, and thus promote the H2 production rate of the PET photocatalytic reforming reaction. Overall, the photocatalyst of the present invention exhibits remarkable catalytic performance in the PET photoreforming to H2 reaction and provides new ideas and solutions for improving the efficiency of photocatalytic H2 production and promoting the practical application of this technology.

[0005] A method for constructing a highly crystalline heptazine imide / polyazine imide homojunction, comprising the following steps:

[0006] The mixture of melamine, KSCN and NH4Cl was thoroughly ground and then placed in a crucible. Subsequently, KCl and LiCl were added to the crucible, and then it was placed in a muffle furnace for calcination. After the calcination was completed, it was washed repeatedly with ethanol and water for multiple times, and then placed in an oven for drying.

[0007] The mass ratio of the described melamine, the mixture of KSCN and NH4Cl, KCl and LiCl is 2:6:1.65:1.35; among them, in the mixture of KSCN and NH4Cl, the mass ratio of KSCN and NH4Cl is 5:1, 4:2, 3:3, 2:4 or 1:5.

[0008] The grinding time is 5 min;

[0009] The calcination temperature is 500 - 580 °C, the calcination time is 2 - 8 h, and the heating rate is 2 - 5 °C / min;

[0010] The drying temperature is 60 °C.

[0011] The highly crystalline polyheptazine imide / polytriazine imide homojunction prepared by the present invention is used for the application of photocatalytic reforming of PET to produce hydrogen.

[0012] The polyheptazine imide / polytriazine imide homojunction photocatalyst, under normal temperature and pressure, in a low-alkali solution C OH - = 1 M medium, for the first time uses visible light to catalytically reform polyethylene terephthalate (PET) to realize the synthesis of high-value-added chemicals (such as formate, glyoxal, etc.) and green hydrogen (H2) fuel.

[0013] Experimental results show that under the irradiation condition of visible light (λ > 420 nm) and without adding any sacrificial agent, the H2 production rate of the best catalyst is as high as 1413 μmol·g -1 ·h -1 , and the hydrogen production performance is improved by 19 times compared with the unmodified sample.

[0014] The beneficial effects of the present invention are:

[0015] The present invention successfully constructs a highly crystalline polyheptazine imide (PHI) / polytriazine imide (PTI) homojunction by using a one-step molten salt-assisted calcination strategy. This homojunction photocatalyst can, under normal temperature and pressure, in a low-alkali solution (C OH -In a 1 M medium, for the first time, visible light was used to catalytically reform polyethylene terephthalate (PET) to achieve the synthesis of high-value chemicals (such as formate, glyoxal, etc.) and green hydrogen (H2) fuel. The experimental results show that under visible light (λ>420 nm) irradiation conditions and without adding any sacrificial agent, the H2 production rate of the best catalyst is as high as 1413 μmol·g -1 ·h -1 , which is 19 times higher than the hydrogen production performance of the unmodified sample. This performance not only refreshes the records of all currently reported g-C3N4-based materials in the field of PET photoreforming for hydrogen production, but also demonstrates the great potential of this catalyst in green energy production and efficient utilization of plastic waste. Description of the Drawings

[0016] Figure 1 It is a schematic flow chart for preparing a highly crystalline PHI / PTI homojunction by a molten salt-assisted calcination strategy.

[0017] Figure 2 It is the XRD pattern of different samples obtained in Example 1.

[0018] Figure 3 It is the HRTEM image of CN-TH3-LiK.

[0019] Figure 4 It is the 13 C-NMR pattern of CN-TH3-LiK and pure-phase g-C3N4.

[0020] Figure 5 It is the transient photocurrent response pattern of CN-TH3-LiK and pure-phase g-C3N4.

[0021] Figure 6 It is the liquid-phase NMR pattern of the PET photoreforming reaction before and after using the CN-TH3-LiK photocatalyst 1 H-NMR.

[0022] Figure 7 It is the PET photoreforming hydrogen production performance pattern of different samples.

[0023] Figure 8 It is the comparison pattern of PET photoreforming hydrogen production performance under different alkaline concentration media. Detailed Embodiments

[0024] The present invention will be described in detail below in conjunction with embodiments, so that those skilled in the art can better understand the present invention, but the present invention is not limited to the following embodiments.

[0025] Comparative Example 1

[0026] Place 10.0 g of melamine in an alumina porcelain boat, then put it into a muffle furnace and calcine it at 550 °C with a heating rate of 5 °C / min for 4 hours to obtain g-C3N4. Wash it repeatedly with ethanol and water. After washing, place it in an oven at 60 °C and dry it until completely dry.

[0027] Example 1

[0028] Fully grind 2.0 g of melamine with a mixture of 6 g of KSCN and NH4Cl and put it into a 50.0 mL crucible. Then add 1.65 g of KCl and 1.35 g of LiCl to the crucible, and then place it in a muffle furnace at 550 °C, 5 °C / min, and calcine for 4 h to obtain sample B. Wash sample B repeatedly with ethanol and water. After washing, place it in an oven at 60 °C and dry it until completely dry. The reaction process is as Figure 1 shown.

[0029] In the mixture of KSCN and NH4Cl, the mass ratio of KSCN to NH4Cl is 5:1, 4:2, 3:3, 2:4, and 1:5. According to the amount of NH4Cl added, the obtained samples are denoted as CN-TH1-LiK, CN-TH2-LiK, CN-TH3-LiK, CN-TH4-LiK, and CN-TH5-LiK, respectively.

[0030] Figure 2 The X-ray diffraction (XRD) patterns of the samples in the examples of the present invention are shown. The structural characteristics of PHI and PTI can be clearly seen from the XRD patterns. The main diffraction peaks at 8.2° and 28.1° are attributed to the typical in-plane repeated stacking and interlayer stacking characteristics of PHI, respectively. The diffraction peaks at 12.0°, 21.3°, 26.6°, and 30.4° correspond to the (100), (110), (002), and (102) crystal planes of PTI, respectively (Chem. Eng. J. 2025, 509, 161255; J. Alloy. Compd. 2023, 938, 168484). In addition, XRD also reflects that the PHI / PTI heterojunction material has high crystallinity characteristics, which is consistent with Figure 3 the HRTEM test results of CN-TH3-LiK provided.

[0031] To further verify the successful preparation of the PHI / PTI heterojunction catalyst, the 13 C-NMR characterization was also tested. From Figure 4In the NMR spectrum, it can be seen that there are three peaks at 157.5, 164.2, and 168.8 ppm in the CN-TH3-LiK sample. The first peak corresponds to the signal of the C atom (C-N3) in the heptazine structure. At the same time, the second peak represents the signal of the C atom (N2-CN or terminal CN2(NH x )) with a slight shift (δ = 1.4 ppm) relative to the g-C3N4 reference peak at 165.6 ppm. This shift may be due to the presence of K / Li ions, which change the electron cloud density of the material. The weak vibration peak between 168 and 169 ppm is attributed to the unprotonated cyclic nitrogen atom CN2(N-) adjacent to C (ACS Appl. Energy Mater. 2024, 7, 6090-6095). The above results indicate that the PHI / PTI homojunction catalyst was successfully prepared, which is consistent with the XRD test results.

[0032] Figure 5 Figure 6 shows the transient photocurrent response diagrams of CN-TH3-LiK and g-C3N4. It can be clearly seen from the figure that the best sample exhibits significantly better photocurrent response characteristics than the pure-phase g-C3N4. This indicates that the best sample, CN-TH3-LiK, can more effectively separate photo-generated electrons and holes under light illumination, significantly improving the charge separation efficiency. The higher charge separation efficiency not only helps to reduce the recombination rate of photo-generated carriers but also effectively promotes the photocatalytic reaction, thus increasing the H2 yield and reaction efficiency in the photoreforming of PET for hydrogen production.

[0033] Application tests:

[0034] The photocatalytic hydrogen production activity test was carried out in a 250 mL Pyrex top-irradiation photoreactor. To facilitate the collection of gas products, the reactor was connected to a closed gas circulation system. The light source was a 300 W xenon lamp equipped with a cut-off filter (λ > 420 nm), and the experimental temperature was controlled at 20 °C.

[0035] (1) PET pretreatment:

[0036] A one-step hydrolysis process was used to prepare a PET-derived solution for photocatalytic applications. 12.5 g of PET powder was immersed in 250 mL of 2 M NaOH aqueous solution, the temperature was set at 40 °C, and it was stirred at a speed of 300 rpm. This pretreatment process lasted for 24 hours with continuous stirring to initiate the depolymerization process. After the reaction, the system was naturally cooled to room temperature without external intervention, and 25 mL of the supernatant was carefully collected for subsequent photocatalytic evaluation.

[0037] (2) Photocatalytic reaction:

[0038] Disperse 20 mg of polyheptazine imide / polyazine imide homojunction photocatalyst in 25 mL of pure water, and add 25 mL of plastic supernatant (solution C OH - = 1 M). In the comparative experiment of adding co-catalyst, the Pt co-catalyst was synthesized by in-situ photodeposition method: an aqueous solution of a specific amount of H2PtCl6·6H2O (2 wt%) was used as the precursor. Finally, the reaction cell was irradiated using a solar simulator, with argon as the carrier gas. During the irradiation process, the temperature was kept constant at 20 °C and continuous stirring was carried out at a speed of 400 rpm. Subsequently, a GC7920-TA gas chromatograph equipped with a molecular sieve chromatographic column (Beijing Zhongmei Jinyuan Technology Co., Ltd.) was used, with high-purity argon (99.999%) as the carrier gas to collect and analyze the hydrogen production data.

[0039] Figure 6 shows the liquid nuclear magnetic resonance ( 1 1H-NMR) spectra before and after the photoreforming reaction using the CN-TH3-LiK photocatalyst. It can be clearly observed from the figure that after the photoreforming reaction, various organic value-added products were generated in the solution, including formate, glyoxal, etc. The appearance of these products indicates that the photoreforming reaction not only successfully converts PET into H2, but also generates organic compounds with certain added value. This result demonstrates the great potential of the photoreforming reaction in energy conversion and resource recycling.

[0040] Figure 7 shows the test results of the PET photoreforming hydrogen production performance of different samples in the comparative examples and examples of the present invention. From the experimental data, it can be seen that the highly crystalline PHI / PTI homojunction exhibits excellent catalytic performance in the PET photoreforming hydrogen production process, significantly improving the H2 production rate. The hydrogen production rate of PET photoreforming using the CN-TH3-LiK photocatalyst can reach 1412.62 μmol·g -1 ·h -1 , which fully demonstrates the great application potential of the PHI / PTI homojunction as a g-C3N4-based homojunction photocatalyst in the PET photoreforming hydrogen production reaction, providing an important basis for further developing efficient photocatalysts and promoting the application and development of photocatalytic hydrogen production technology.

[0041] Figure 8 shows the comparison results of the PET photoreforming hydrogen production performance in the examples of the present invention and other current literatures. Through comparative analysis, it can be found that the best photocatalyst in this study under normal temperature and pressure conditions and in a low-concentration alkaline medium (C OH -When the reaction environment is 1 M), it exhibits the best H2 production performance in photocatalytic reforming so far. This research result not only verifies the excellent performance of the PHI / PTI homojunction as a photocatalyst but also provides valuable reference for further optimizing and developing efficient homojunction photocatalysts.

[0042] The highly crystalline PHI / PTI homojunction catalyst obtained by the molecular engineering strategy of the present invention can significantly improve the electron migration rate of the catalyst, reduce the recombination of electron-hole pairs, and thus effectively improve the efficiency of photocatalytic electron transfer (PET) for hydrogen production by reforming. This highly crystalline homojunction structure not only optimizes the separation and transport of electrons during the photocatalytic reaction but also enhances the stability and durability of the catalyst. At normal temperature and pressure, in a low-alkaline solution (C OH - = 1 M) medium, and under visible light irradiation (λ > 420 nm) conditions, without adding any sacrificial agent, the H2 production rate of the best catalyst is as high as 1413 μmol·g -1 ·h -1 , which is nearly 19 times higher than the hydrogen production performance of the unmodified sample. This excellent performance not only breaks the record of all reported g-C3N4-based materials in the field of PET photocatalytic reforming for hydrogen production but also provides a new direction for the development of efficient and sustainable photocatalytic H2 production. By finely tuning the molecular structure, this catalyst demonstrates great application potential in the field of photocatalysis, especially in key technology fields such as environmental protection and clean energy conversion.

Claims

1. A method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction, characterized in that: The steps include: A mixture of melamine, KSCN and NH4Cl is fully ground and put into a crucible, and then KCl and LiCl are added into the crucible, and then it is placed in a muffle furnace for calcination. After the calcination is completed, it is repeatedly washed with ethanol and water for many times and placed in an oven for drying.

2. The method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction according to claim 1, characterized in that: The mass ratio of the melamine, the mixture of KSCN and NH4Cl, KCl and LiCl is 2:6:1.65:1.

35.

3. The method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction according to claim 1, characterized in that: In the mixture of KSCN and NH4Cl, the mass ratio of KSCN to NH4Cl is 5:1, 4:2, 3:3, 2:4 or 1:

5.

4. The method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction according to claim 1, characterized in that: The grinding time is 5 min.

5. The method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction according to claim 1, characterized in that: The calcination temperature is 500-580° C., and the calcination time is 2-8 hours.

6. The method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction according to claim 1, characterized in that: During calcination, the heating rate is 2 to 5°C / min.

7. The method for constructing a highly crystalline polyheptazine imide / polytriazine imide homojunction according to claim 1, characterized in that: The drying temperature is 60°C.

8. Use of the polyheptazinonimide / polytriazinonimide homojunction prepared by the construction method according to any one of claims 1 to 7 in photocatalytic reforming of PET to produce hydrogen.

9. The use according to claim 8, characterized in that At room temperature and pressure, low alkali solution C OH - =1M medium, in visible light environment, adding poly(heptazinonimide) / poly(triazinonimide) homojunction photocatalytic reforming of polyethylene terephthalate (PET) to produce hydrogen.

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