Preparation method and application of metal-free hindered Lewis acid-base pair modified graphite phase nitrogen carbide photocatalyst

Abstract

A preparation method of the metal-free hindered Lewis acid-base pair modified graphite phase nitrogen carbide photocatalyst comprises the following steps: putting melamine into a muffle furnace, heating to 550 DEG C at a constant heating rate, keeping for 4 hours, and marking a prepared sample as CN; and preparing FLPs modified g-C3N4, and carrying out alkalization treatment. According to the metal-free hindered Lewis acid-base pair modified graphite phase nitrogen carbide photocatalyst, FLPs are constructed on CN so as to enhance the photocatalytic degradation effect of TC, and the biotoxicity of an intermediate product is effectively reduced; the reduction of the toxicity lays a favorable foundation for subsequent integration of a photochemical process and biodegradation in a wastewater treatment system, not only provides a strategy for developing a high-performance photocatalyst, but also opens up a new way for reasonably regulating and controlling an intermediate product by simulating an FLPs structure.

Description

Technical Field

[0001] The present invention relates to the technical field of photocatalytic degradation, and in particular to a preparation method and application of a metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst. Background Art

[0002] The rapid development of global industry and agriculture has been accompanied by environmental pollution, especially pollution caused by organic pollutants. Among them, tetracycline (TC) has attracted significant environmental attention due to its widespread use in biomedicine and animal husbandry. Studies have shown that more than 70% of TC is not fully absorbed by humans or animals and is subsequently released into the environment, posing a significant risk to the ecosystem and human health. The high physicochemical stability and biotoxicity of TC make it difficult to remove by conventional degradation methods (such as biodegradation and physical treatment processes), thus exacerbating its persistence in the environment.

[0003] Photocatalysis has become one of the most promising advanced oxidation processes for pollutant degradation. The key challenge in photocatalytic degradation of TC lies in how to disrupt the stability of TC molecules and reduce the thermodynamic energy barrier of its ring-opening reaction. To enhance the pollutant activation efficiency, researchers have been working on optimizing the catalyst performance through two main strategies: precisely regulating its micro / nanostructure and rationally constructing efficient electron-localized active sites. For example, Yang et al. developed ultrathin Bi2WO6 atomic layers to increase the surface area and pollutant adsorption amount, and simultaneously introduced chloride ions to create electron-localized active sites through a multifunctional regulation mechanism, significantly enhancing the activation of pollutant molecules. However, the polarization effect of pollutants by only a single electron donor site is limited. Therefore, developing a dual-site strategy that can simultaneously exert "push-pull" electron effects on pollutants has become a promising direction for improving catalytic performance. This method can not only improve the electron transfer efficiency but also optimize the adsorption and activation of pollutant molecules through synergistic effects.

[0004] In recent years, frustrated Lewis pairs (FLPs) have received extensive attention. Different from single electron donor sites, FLPs have both Lewis acid and Lewis base dual active sites, which can effectively activate various molecules through synergistic effects. Defect engineering has become a highly effective method for constructing FLPs and has been successfully implemented in various semiconductor photocatalysts (such as CeO2, TiO2, and Bi2WO6). In these systems, the spatial separation between defect sites (as Lewis acid centers) and adjacent surface hydroxyl groups (as Lewis base centers) usually remains at about This optimal spatial configuration not only prevents the formation of stable Lewis acid-base adducts but also creates a highly reactive microenvironment. It endows FLPs with excellent activation ability for small molecules (such as H2, CO2, and H2O), promoting their efficient conversion in photocatalytic reactions.

[0005] Among various photocatalytic materials, graphitic carbon nitride (g-CN), as a novel metal-free semiconductor catalyst, has attracted much attention due to its excellent chemical stability, tunable band structure, and visible-light responsiveness. However, the practical application of g-CN synthesized by conventional methods in photocatalytic wastewater treatment is severely limited by its inherent drawbacks such as insufficient surface active sites and high recombination rate of photo-generated electron-hole pairs, which jointly damage its photocatalytic efficiency. FLPs can effectively trigger the activation of various molecules through a unique "push-pull" effect, demonstrating unprecedented catalytic ability. For example, Yu et al. constructed FLPs on g-C3N4, where P and N serve as Lewis acid and base sites, respectively. The formed FLPs not only promoted charge carrier separation but also narrowed the band gap and improved substrate activation. These synergistic effects together enhanced the photocatalytic activity, and the degradation efficiency of TC in the PMS / visible light system by the modified sample was as high as 83.4%.

[0006] Therefore, based on the above background, it is necessary to propose a preparation method of a metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst to solve the problem of enhancing the activation efficiency of pollutants during photocatalytic degradation of TC. Summary of the Invention

[0007] The present invention aims at the problem of how to destroy the stability of TC molecules and reduce the thermodynamic energy barrier of its ring-opening reaction during photocatalytic degradation of TC, and proposes a preparation method and application of a metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst.

[0008] The preparation method of the present metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst includes the following steps:

[0009] S1. Place melamine in a muffle furnace, heat it to 550 °C at a constant heating rate, and keep it for 4 hours. The prepared sample is labeled as CN.

[0010] S2. Prepare FLPs-modified g-C3N4 and perform alkalization treatment.

[0011] Further, in S1, specifically: Place 20 g of melamine in a muffle furnace, heat it to 550 °C at a heating rate of 15 °C / min, and keep it for 4 hours. The prepared sample is labeled as CN.

[0012] Further, in S2, specifically: Add 0.5 g of CN to a 60 mL alkaline solution containing 15 mL of 1 mol / L NaOH solution, and stir for 1 hour; then, transfer the suspension to a polytetrafluoroethylene-lined autoclave and heat it at 160 °C for 12 hours; after the reaction is completed, centrifuge the solid product, wash it with deionized water, and dry it under vacuum at 60 °C to obtain FLPs-modified CN, labeled as CN-OH-15.

[0013] CN-OH-15 prepared according to the above method can be applied to the photocatalytic degradation catalyst of tetracycline TC.

[0014] Implementing the present invention has the following beneficial effects:

[0015] The metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst of the present invention constructs frustrated Lewis pairs (FLPs) on carbon nitride (CN) to enhance the photocatalytic degradation effect of tetracycline (TC); FLPs generate carbon vacancies as Lewis acid sites through alkalization post-treatment, and adjacent surface hydroxyl groups serve as Lewis base sites; FLP engineering endows CN with excellent photoelectrochemical properties, significantly improves the charge separation efficiency, and optimizes the energy band structure to enhance the redox potential.

[0016] FLPs exhibit excellent TC molecular activation ability through a unique "push-pull" electron effect; the activated TC molecules are easily oxidized by interfacial photo-generated holes, making the photocatalytic degradation rate constant of the CN-OH-15 sample as high as 0.041 min -1 ; the degradation mechanism mainly involves demethylation, decarbonylation, and ring-opening reactions, effectively reducing the biotoxicity of intermediate products; the reduction of toxicity lays a favorable foundation for the subsequent integration of the photochemical process and biodegradation in the wastewater treatment system.

[0017] The present invention not only provides a strategy for developing high-performance photocatalysts but also opens up a new way for reasonably regulating intermediate products by mimicking the FLPs structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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 following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0019] Figure 1 Schematic diagram for the construction of frustrated Lewis acid-base pairs (FLPs) and activation of substrates;

[0020] Figure 2 XRD characterization diagram;

[0021] Figure 3 EPR characterization diagram;

[0022] Figure 4 FT-IR spectrogram;

[0023] Figure 5 Fine FT-IR spectrogram;

[0024] Figure 6 It is an XPS characterization analysis diagram;

[0025] Figure 7 It is a solid-state NMR characterization diagram;

[0026] Figure 8 It is a TEM diagram of CN(a) and CN-OH-15(b);

[0027] Figure 9 It is a specific surface area and pore size analysis diagram;

[0028] Figure 10 It is a DRS analysis diagram;

[0029] Figure 11 It is a band gap analysis diagram;

[0030] Figure 12 It is a photocurrent analysis diagram;

[0031] Figure 13 It is an electrochemical impedance analysis diagram;

[0032] Figure 14 It is a steady-state fluorescence spectrum analysis diagram;

[0033] Figure 15 It is a transient fluorescence spectrum analysis diagram;

[0034] Figure 16 It is a photocatalytic degradation performance diagram of tetracycline;

[0035] Figure 17 It is a photocatalytic degradation kinetic analysis diagram of tetracycline;

[0036] Figure 18 It is an ion interference test diagram;

[0037] Figure 19 It is a degradation performance analysis diagram under different water quality conditions;

[0038] Figure 20 It is a degradation effect analysis diagram of different pollutants;

[0039] Figure 21 It is a cycling performance diagram;

[0040] Figure 22 It is an XRD analysis diagram before and after cycling;

[0041] Figure 23 It is an infrared spectrum analysis diagram before and after cycling;

[0042] Figure 24 It is a diagram of heat treatment to remove surface hydroxyl groups of the material;

[0043] Figure 25Performance control chart after surface hydroxyl removal;

[0044] Figure 26 Experimental chart of scavenger;

[0045] Figure 27 Analysis chart of reactive oxygen species;

[0046] Figure 28 Experimental chart of scavenger for CN sample;

[0047] Figure 29 Flat band potential analysis chart;

[0048] Figure 30 VB-XPS analysis chart;

[0049] Figure 31 Energy band structure analysis chart;

[0050] Figure 32 Hole-mediated interfacial oxidation mechanism chart;

[0051] Figure 33 Free radical-mediated free oxidation mechanism chart;

[0052] Figure 34 a and b are in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experimental spectra, c is in-situ Raman experimental spectrum, d is a schematic diagram of chemisorption tendency, e is a quantitative toxicity assessment chart, and f is a toxicological assessment analysis chart;

[0053] Figure 35 Analysis chart of tetracycline degradation pathway. Detailed implementation manners

[0054] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings 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 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 shall fall within the protection scope of the present invention.

[0055] The preparation method of the metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst includes the following steps:

[0056] S1. Place 20 g of melamine in a muffle furnace, heat it to 550 °C at a heating rate of 15 °C / min, and hold for 4 hours. The prepared sample is labeled as CN;

[0057] S2. Preparation of FLPs-modified g-C3N4 and alkalization treatment: Add 0.5 g of CN to 60 mL of alkaline solution containing 10, 15, and 20 mL of 1 mol / L NaOH solution, and stir for 1 hour; subsequently, transfer the suspension to a polytetrafluoroethylene-lined autoclave and heat at 160 °C for 12 hours; after the reaction is completed, centrifuge the solid product, wash it with deionized water, and dry it under vacuum at 60 °C to obtain FLPs-modified CN, labeled as CN-OH-10, CN-OH-15, and CN-OH-20 respectively. For comparison, a control sample labeled as CN-H2O was prepared using the same hydrothermal post-treatment method with NaOH replaced by water.

[0058] Perform performance analysis on the obtained CN, CN-OH-10, CN-OH-15, CN-OH-20, and CN-H2O:

[0059] As Figure 1 shown, Figure 1 It is a schematic diagram for the construction and activation of substrates by frustrated Lewis pairs (FLPs); FLPs are constructed on the surface of bulk carbon nitride (CN) by an alkalization post-treatment using a disorder engineering strategy. The alkalization process creates a highly corrosive and hydroxyl-rich environment, promoting the simultaneous formation of defect structures and surface hydroxyl groups on CN. These modified hybrid structures have a spatially adjacent arrangement, providing an ideal configuration for the formation of FLPs. FLPs induce a highly localized and electron-deficient distribution on the surface of CN, significantly promoting the adsorption of reactant molecules and their subsequent activation in the photocatalytic reaction.

[0060] Perform XRD detection on the samples, please refer to Figure 2 , Figure 2 It is an XRD characterization diagram; the samples after hydrothermal and alkalization post-treatment show little change compared with CN, indicating that these post-synthesis modifications do not significantly change the crystal structure of CN.

[0061] To understand the electronic structure modification caused by alkalization post-treatment, electron paramagnetic resonance (EPR) analysis was used, please refer to Figure 3 , Figure 3 It is an EPR characterization diagram. The first derivative spectra of all samples show characteristic peaks at g = 2.001, corresponding to the unpaired electrons in the heptazine unit of CN. And with the increase of the alkalization treatment concentration, the characteristic signal gradually weakens, which can be attributed to the gradual formation of carbon vacancies in the CN skeleton. This indicates that the alkaline solution induces selective hydrolysis of unstable regions in the CN structure during the treatment process, resulting in the controllable generation of carbon vacancies.

[0062] To characterize the surface functional groups and chemical modifications of the samples, a multi-technique spectroscopic method was used, please refer to Figure 4 , Figure 4This is the FT-IR spectrogram. The FTIR spectrum shows characteristic vibration modes, including triazine ring bending (810 cm -1 ), aromatic heterocyclic stretching (1200 - 1600 cm -1 ), and hydroxyl / amine stretching (2900 - 3500 cm -1 ).

[0063] Please refer to Figure 5 , Figure 5 for the detailed FT-IR spectrogram; the detailed spectral analysis identified two new peaks at 3289 cm -1 and 1086 cm -1 in the alkali-treated sample, corresponding to surface hydroxyl groups and C-OH bonds, respectively.

[0064] Please refer to Figure 6 , Figure 6 for the XPS characterization analysis diagram; the high-resolution XPS analysis of the O1s region further confirmed these findings. An additional peak appeared at 533.2 eV for CN-OH-15, which was clearly assigned to the C-O-H group, in contrast to CN and CN-H2O.

[0065] Further insights into the chemical structure were obtained through solid-state proton nuclear magnetic resonance (1H ssNMR) spectroscopy. As Figure 7 shown, Figure 7 this is the solid-state NMR characterization diagram; the spectrum showed characteristic peaks at 5.07 ppm and 9.9 ppm, corresponding to primary amines and secondary amines, respectively. Notably, the primary amine peak of CN-OH-15 shifted down to 4.47 ppm, indicating weakened interlayer van der Waals forces and the formation of thinner nanosheets. Additionally, a new peak appeared at 1.28 ppm for CN-OH-15, directly demonstrating the successful introduction of surface hydroxyl groups through post-alkali treatment. These hydroxyl groups are crucial for establishing FLPs as they undergo cooperative electron interactions with adjacent carbon vacancies (Lewis acid sites) as Lewis base sites.

[0066] The morphological changes induced by post-alkali treatment were analyzed by transmission electron microscopy (TEM), as Figure 8 shown. All samples maintained the characteristic layered stacking structure of carbon nitride. The original CN showed thick layered aggregates, while CN-OH-15 transformed into ultrathin nanosheets, which was attributed to the combined effect of alkaline etching and exfoliation during the treatment. This morphological reconstruction led to a significant increase in the specific surface area, with CN-OH-15 reaching 115.6 m 2 / g, an order of magnitude higher than that of the original CN (11.0 m 2 / g). Please refer to Figure 9 and Table 1. The synergistic effect of morphological evolution and surface area expansion significantly enhanced the accessibility of reaction sites and promoted the intimate contact between the photocatalyst and pollutant molecules.

[0067] Table 1 Specific surface area, pore width and porosity of CN and CN-OH-x series samples

[0068]

[0069] The degree of alkalization during the post-treatment of CN plays a decisive role in the morphological size, carbon vacancy concentration and surface hydroxyl density, and thus significantly affects its optical properties and energy band structure. The original CN shows a characteristic ultraviolet absorption edge at about 470 nm, while the CN-OH-x series (x = 10, 15, 20) shows a narrower broadband light absorption. Please see Figure 10 for details. Comparative analysis shows that the absorption edge of the alkalized samples exhibits an obvious blue shift compared with the original CN, which is attributed to the quantum confinement effect, that is, the bulk CN is transformed into thinner nanostructures by controlling alkalization. It is worth noting that the band gap of CN-OH-15 is expanded to 2.73 eV, while that of the original CN is 2.66 eV. Please see Figure 11 for details. The expansion of the band gap indicates the enhanced redox potential of CN-OH-15, which is particularly beneficial for photocatalytic applications that require strong oxidation ability.

[0070] To study the influence of FLPs on the charge carrier dynamics during the photocatalytic process, a systematic study was carried out by combining photoelectrochemical characterization and advanced spectroscopic techniques. As Figure 12 shown, although the photocurrent generation of CN-H2O after hydrothermal post-treatment is reduced compared with the original CN, which indicates that the formation of a more perfect interface may instead hinder the charge separation efficiency. In contrast, the FLPs-modified CN samples show a significantly enhanced photocurrent response, indicating that FLPs act as effective "relay stations" for interfacial charge separation. Electrochemical impedance spectroscopy (EIS) measurements further support this observation. Please see Figure 13 for details, especially CN-OH-15 shows a smaller semicircle radius in the Nyquist plot, indicating a reduced interfacial charge transfer resistance and an improved charge separation efficiency. Please see Figure 14 for details. The photoluminescence (PL) spectrum shows that the emission intensity of the FLPs-modified CN samples is significantly reduced, indicating that FLPs act as effective charge carrier traps and suppress the recombination of photo-generated electron-hole pairs. Please see Figure 15 for details. Time-resolved fluorescence spectroscopy further verifies this. The charge carrier lifetime of CN-OH-15 is extended to 3.08 ns, which is significantly longer than that of the original CN (2.21 ns). These photoelectrochemical results together demonstrate the key role of FLPs in promoting charge carrier separation and migration.

[0071] Please see Figure 16, Photocatalytic performance evaluation showed that CN-OH-15 achieved 87% TC degradation within 60 minutes, significantly superior to the original CN (52.0%). Kinetic analysis indicated that the degradation rate constant of CN-OH-15 was 0.045 min-1, which was 4.1 times that of the original CN (0.011 min-1), see Figure 17 . This significant improvement emphasizes the crucial role of FLPs interfacial modification in optimizing photocatalytic performance. To evaluate the practical applicability of this photocatalytic system, its performance under different wastewater conditions was systematically evaluated. Common inorganic ions (Mg 2+ , Ca 2+ , NO3 - , Cl - , HCO3 - ) were introduced into the reaction system, and the influence on the TC degradation efficiency of CN-OH-15 was relatively small, see Figure 18 . The reaction kinetics decreased slightly in the presence of HCO3 - , which could be attributed to the pH change during the reaction. In addition, CN-OH-15 performed excellently in different real water matrices, with the TC removal rate exceeding 85% within 60 minutes, see Figure 19 . CN-OH-15 also demonstrated the multifunctional degradation ability towards pollutants with various functional groups, highlighting its great potential in practical wastewater treatment applications, see Figure 20 . More importantly, CN-OH-15 maintained consistent photocatalytic performance in four consecutive reaction cycles, and there was no significant decrease in the TC removal efficiency, see Figure 21 . The changes in the XRD patterns and FTIR spectra after repeated use were negligible, see Figure 22 and 23 , further confirming the structural stability of CN-OH-15.

[0072] To clarify the precise role of frustrated Lewis pairs (FLPs) in enhancing the photocatalytic degradation of tetracycline (TC), we conducted a systematic control experiment by calcining CN-OH-15 at 350 °C to destroy the FLPs structure. Fourier transform infrared spectroscopy (FT-IR) confirmed that the surface hydroxyl groups were completely eliminated after heat treatment, see Figure 24 . The degradation performance of CN-OH-15 with destroyed FLPs decreased significantly, and the reaction rate constant dropped to 0.027 min-1, lower than that of the intact CN-OH-15 (0.045 min-1), see Figure 25。This significant performance decline provides strong evidence for the crucial role of FLPs in enhancing photocatalytic activity, highlighting the necessity of maintaining Lewis acid (carbon vacancy) and base (surface hydroxyl) sites for optimal catalytic efficiency. Subsequently, quenching experiments were conducted to verify the contribution of reactive species to the TC degradation process in CN-OH-15, see Figure 26 。The significant inhibition effect of triethanolamine (TEOA) confirmed the dominant role of photo-generated holes (h + ) in the degradation process. The moderate inhibition effects of isopropanol (IPA) and β-carotene indicated a minor contribution of hydroxyl radicals (·OH) and singlet oxygen ( 1 O2), while the inhibitory effect of superoxide dismutase (SOD) was negligible, indicating an insignificant role of superoxide anion radicals (·O2 - ). These experimental observations were confirmed by quantitative reactive oxygen species (ROS) detection, which showed weak signals of ·OH, 1 O2 and ·O2 - under illumination, see Figure 27 。In contrast to the hole-mediated degradation mechanism observed in CN-OH-15, pristine CN mainly proceeds through a radical-mediated pathway, where ROS (including ·OH, 1 O2 and ·O2 - ) play a dominant role, while the contribution of photo-generated holes is small, see Figure 28 。This difference was further supported by electron paramagnetic resonance (EPR) spectroscopy, which showed a higher concentration of ROS generated in pristine CN compared to CN-OH-15.

[0073] See Figure 29 and 30 , comprehensive electronic structure characterization by Mott–Schottky measurements and valence band X-ray photoelectron spectroscopy (XPS) analysis provided us with an in-depth understanding of the mechanism differences. The valence band position of CN-OH-15 (1.89 eV) is more positive than that of CN (1.58 eV) and CN-H2O (1.71 eV), see Figure 31 and Table 2, indicating an enhanced oxidation potential of its photo-generated holes. CN-OH-15 has a wider bandgap and a weaker reduction potential, which simultaneously limits the generation of ·O2 - and the exciton dissociation process for the formation of singlet oxygen ( 1 O2). Based on these comparative analyses, it is proposed that FLPs improve photocatalytic performance through a dual mechanism: (1) regulating the valence band position to enhance the oxidation potential of photo-generated holes; (2) providing effective adsorption sites for TC molecules, resulting in a hole-mediated interfacial oxidation mechanism as the main one, see Figure 32. In contrast, the pristine CN lacking effective adsorption sites mainly follows the less efficient free radical-mediated free oxidation mechanism, see Figure 33 , which is significantly limited by the rapid quenching of ROS in the aqueous environment.

[0074] Table 2 Data analysis table of the energy band positions of the samples

[0075]

[0076]

[0077] In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were carried out to dynamically explore the reaction mechanism mediated by FLPs. In the in-situ DRIFTS spectra, negative bands indicate the consumption of reactants, while positive bands indicate the accumulation of products. Figure 34 a shows the simulated adsorption process of TC on CN-OH-15. The sharp positive bands at 3450 cm -1 and 3702 - 3726 cm -1 , corresponding to N-H stretching vibration and free O-H groups respectively, indicate the enhanced adsorption interaction between the surface hydroxyl groups of CN-OH-15 and TC molecules. In addition, the enhancement of the bands in the 1200 - 1500 cm -1 region, attributed to C-N heterocyclic vibration, indicates the strong adsorption of TC on carbon vacancies. During the photocatalytic reaction process, as Figure 34 b, the band at 3425 cm -1 corresponding to surface hydroxyl groups continuously decreases, indicating its active participation in TC degradation. At the same time, the negative bands corresponding to C=O stretching vibration (1775 cm -1 ), C-H bending vibration (1330 cm -1 ), and C-N stretching vibration (1126, 1020 cm -1 ) provide clear evidence for the decarbonylation, demethylation, and deamination processes. The positive band appearing in the 1418 - 1496 cm -1 region, assigned to the C=C stretching vibration of aromatic compounds, indicates extensive oxidative ring-opening degradation of the TC carbon skeleton.

[0078] These results are supported by in-situ Raman experiments, as Figure 34 c. The Raman peaks corresponding to the edge carbon vacancies and surface hydroxyl groups of CN-OH-15 are at 750 cm -1 and 980 cm -1At this point, it decreased significantly after dark adsorption equilibrium, indicating that FLPs participated in the adsorption of TC. As the reaction proceeded, the gradual recovery of these peaks indicated the dynamic desorption of fragmented functional groups on the FLPs sites, effectively regenerating the active sites and preventing catalyst deactivation. The characteristic spectra of TC, including C=O stretching vibration (1630 cm -1 ), C-O / C-N stretching vibration (1320 cm -1 ), C-C skeletal vibration (930 - 1140 cm -1 ), and methyl bending vibration (1436 cm -1 ), showed significant attenuation during the photoreaction process, providing direct spectral evidence for the demethylation and decarbonylation processes.

[0079] Considering that the hydroxyl and amide groups on the aromatic ring endow TC with strong electron-donating properties, TC molecules showed a higher chemisorption tendency on CN-OH-15, which was confirmed by its more negative surface potential, as Figure 34 d. The abundant FLPs in CN-OH-15 had a significant electron "push-pull" effect and played a key role in TC activation. The Lewis acid (LA) sites with empty orbitals effectively accepted the lone pair electrons from the activated hydroxyl and amino groups. At the same time, the Lewis base (LB) sites provided lone pair electrons to activate the adjacent methyl groups and carbocations. This dual-site activation mechanism promoted the effective polarization of TC molecules, followed by continuous degradation under the action of various ROS.

[0080] Subsequently, the intermediate products of TC degradation were systematically identified and the degradation pathway was elucidated. Please see Figure 35 . Under the action of holes and ROS, the N-methyl groups of FLP-activated TC underwent stepwise removal to form TC-418 and TC-419 intermediates. Another parallel degradation pathway involved OH-mediated demethylation of TC, followed by rapid dehydration and condensation to form the TC-428 intermediate. Then, these intermediate products were gradually degraded to TC-341 through deamidation and dehydroxylation reactions, and to form TC-274, TC-246, and TC-290 intermediates through ring cleavage, decarbonization, and deamination reactions. Finally, these intermediates were further transformed into small molecules including TC-262 and TC-248.

[0081] Using the Ecological Structure-Activity Relationship (ECOSAR) program based on quantitative structure-activity relationship (QSAR) analysis, the biotoxicity of TC functional groups and degradation intermediate products was quantitatively evaluated. The chronic toxicity of TC molecules to Daphnia magna mainly came from functional groups such as benzyl alcohol and amines, as Figure 34e. These functional groups are effectively polarized by FLPs, initiating a continuous degradation process including deamination, demethylation, and dehydroxylation during the photocatalytic process. To evaluate the environmental impact, we conducted a toxicological assessment of the degradation intermediates, such as Figure 34 f. The results showed that compared with the parent TC molecule, the biotoxicity (to Daphnia magna), developmental toxicity, and mutagenicity of the intermediates were significantly reduced. These findings indicate that the activation induced by FLPs not only promotes the complete degradation of TC but also regulates the degradation pathway to minimize the formation of toxic intermediates, thus improving the environmental safety of the photocatalytic treatment process.

[0082] The present invention successfully designed and synthesized a CN catalyst with abundant FLPs for the photocatalytic degradation of TC. The FLPs are composed of carbon vacancies as Lewis acid sites and adjacent surface hydroxyl groups as Lewis base sites. This unique configuration of FLPs significantly promotes the separation of photogenerated carriers and enhances the oxidation potential of photogenerated holes. More importantly, FLPs can effectively activate TC molecules through the "push-pull" electron effect, thereby promoting molecular polarization and subsequent degradation. The optimized CN-OH-15 catalyst exhibits a significant photocatalytic degradation rate constant (0.046 min-1), which is four times that of the original CN. The degradation process mainly follows a hole-mediated interfacial oxidation pathway, involving extensive demethylation, decarbonylation, and ring-opening reactions. The intermediates are non-toxic or low-toxic, greatly improving the biodegradability of the wastewater and facilitating the combination of this green photochemical detoxification process with traditional biological wastewater treatment. The unique electronic and structural properties of FLPs, as well as their ability to regulate the degradation pathway and minimize toxic by-products, make them highly attractive for dealing with complex environmental challenges.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a metal-free Lewis acid-base pair modified graphitic carbon nitride photocatalyst, characterized in that, It includes the following steps: S1. Place melamine in a muffle furnace, heat it to 550 °C at a constant heating rate, and keep it for 4 hours. The prepared sample is labeled as CN; S2. Prepare FLPs-modified g-C3N4 and perform alkalization treatment.

2. The preparation method of the metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst according to claim 1, characterized in that, Specifically in S1: Place 20 g of melamine in a muffle furnace, heat it to 550 °C at a heating rate of 15 °C / min, and keep it for 4 hours. The prepared sample is labeled as CN.

3. The preparation method of the metal-free Lewis pair modified graphitic carbon nitride photocatalyst according to claim 2, characterized in that, Specifically in S2: Add 0.5 g of CN to 60 mL of an alkaline solution containing 15 mL of 1 mol / L NaOH solution, and stir for 1 hour; subsequently, transfer the suspension to a polytetrafluoroethylene-lined autoclave and heat it at 160 °C for 12 hours; after the reaction is completed, centrifuge the solid product, wash it with deionized water, and dry it under vacuum at 60 °C to obtain FLPs-modified CN, labeled as CN-OH-15.

4. The preparation method of the metal-free frustrated Lewis pair modified graphitic carbon nitride photocatalyst according to any one of claims 1-3, characterized in that, The prepared CN-OH-15 is applied to the photocatalytic degradation catalyst for tetracycline TC.

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