A spinel catalyst with modified acid sites, its preparation method and application

Abstract

This invention relates to the field of photocatalysis, and in particular to an acid-site modified spinel catalyst, its preparation method, and its application. The purpose of this invention is to provide a phosphoric acid-functionalized metal site transition metal catalyst material and its preparation method. By introducing phosphoric acid to promote a reversible valence state transition process at the metal sites, it drives an "active / controllable" electron transfer process without the need for additional consumable metal complexes as activators, thus solving the bottleneck problems of low light-induced efficiency and high energy consumption in plastic degradation in existing technologies.

Description

Technical Field

[0001] This invention relates to the field of photocatalysis technology, and in particular to an acid-site modified spinel catalyst, its preparation method, and its application. Background Technology

[0002] The widespread use of plastic materials has led to a continuous increase in plastic pollution, with microplastics (MPs) ranging in size from 1 to 5 mm posing a significant risk to human health. Landfilling and incineration, currently the most widely used methods for plastic disposal, cause serious environmental problems. Many existing studies employ acidic or alkaline media to pretreat plastic polymers (such as polyethylene terephthalate, PET) to degrade them into small-molecule intermediates, which are then converted into CO2 or high-value-added chemicals through catalytic conversion processes (J. Energy Chem., 2023, 78: 487-96 and J. Energy Chem., 2024, 91: 522-41). However, such chemical pretreatment processes not only inevitably increase the economic cost of plastic recycling but also cause secondary environmental pollution. It is worth noting that, compared to polymers with ester or amide bonds in their main chain (such as PET, polyurethane, polycarbonate, polylactic acid, and polyamide), and polymers with chlorine atoms or phenyl substituents in their main chain -CC- bonds (such as polyvinyl chloride and polystyrene), polyolefins such as polyethylene exhibit extremely strong chemical inertness due to the lack of chemically active functional groups and low polarity. This makes them difficult to hydrolyze and extremely challenging to activate. More importantly, these polyolefins account for a large proportion of global plastic waste, and their treatment has become an urgent industry problem to be solved.

[0003] Solar-driven photocatalysis is clean and sustainable, mediating chemical conversion processes under mild reaction conditions. Studies have confirmed its ability to achieve photocatalytic conversion of polyethylene plastic waste via reactive oxygen species (ROS) (ACS Catalysis, 2022, 12(8): 4659-79). However, current photocatalytic conversion processes for polyethylene generally suffer from low product yields and poor selectivity for single oxidation products (J. Energy Chem., 2022, 69: 369-88). Furthermore, reports indicate that catalytic hydrocracking technology utilizing the synergistic effect of metal and acid sites at high temperatures is an effective approach to degrading polyethylene waste. Acid sites can break C / C bonds, while metal sites can stabilize reaction intermediates and inhibit catalyst coking (Chem, 2023, 9(8): 2318-36). This mechanism, which activates polyethylene using Brønsted acid sites, provides a new approach for photocatalytic plastic conversion under environmental conditions.

[0004] CoMn₂O₄-based catalysts have been actively studied in catalytic oxidation and other fields due to their unique properties, particularly the abundant valence state variations of manganese and their strong redox activity. Meanwhile, CoMn₂O₄ can also serve as a support for certain catalysts. It typically exhibits moderate Lewis acidity and weak Brønsted acidity, thus stronger Brønsted acidity can be introduced into CoMn₂O₄ through surface modification with phosphates or sulfates. Compared to other Brønsted acid groups, phosphate groups have relatively small pKa values, implying stronger proton-donating capabilities.

[0005] In the field of photocatalytic degradation of polyolefin plastics, the publicly available research results (Nat. Com., 2025, 16, 2876, Nat. Com., 2025, 16, 3129, CCS Chem., 2025, 7, 3519–3529, and Energy Environ. Prot., 2025, 39(3): 40−52) show that the development of photocatalysts is still limited to some semiconductor materials (such as TiO2 and g-C3N4). These materials require high doses of catalyst (4.5-9 wt%) in practical applications to compensate for their insufficient electron transfer efficiency; simultaneously, to activate the free radical initiation process, additional external high-concentration transition metal complexes (such as iron / copper salts and bipyridine, amine ligands, etc.) must be added as activators. The aforementioned technical solutions have significant drawbacks: high pretreatment costs and significant pollution, slow reaction rates, short catalyst lifetimes and high costs, poor product selectivity and expensive separation, narrow applicability, and high energy consumption of the light source. Therefore, there is an urgent need to develop novel material systems that integrate highly efficient charge carrier activity and catalytic-activation functions to overcome these technical bottlenecks. Summary of the Invention

[0006] The purpose of this invention is to provide a transition metal material with phosphoric acid functionalized metal sites and its preparation method. By introducing phosphoric acid to promote a reversible valence state transition process at the metal sites, an "active / controllable" electron transfer process is driven without the need to add additional consumable metal complexes as activators, thus solving the bottleneck problems of low light efficiency and high energy consumption in the prior art.

[0007] To address the aforementioned technical problems, this invention provides a method for preparing an acid-site modified spinel catalyst, comprising the following steps: S11: Add manganese salt, cobalt salt, phthalic acid and lauric acid to an organic solvent and react at 120-130℃ for 5-7 h to obtain the precursor product; S12: The precursor product is calcined in air at 450-550°C for 1-3 h and cooled to 20-30°C to obtain CoMn2O4; S13: Add CoMn2O4 to water containing phosphorus compounds, react for 10-12 h, and then separate the solid and liquid phases to obtain the solid reactant. S14: Calcine the solid reactants at 450-500℃ for 3-5 h to obtain the acid-site modified spinel catalyst.

[0008] This invention utilizes phthalic acid and lauric acid, which are organic acid complexation-precipitation agents. Lauric acid is often used as a surfactant / morphology modifier to assist hydrothermal reactions, reduce aggregation, and regulate morphology.

[0009] Preferably, the manganese salt is selected from manganese nitrate, the cobalt salt is selected from cobalt nitrate, and the organic solvent is selected from methanol. Manganese nitrate and cobalt nitrate are chosen because they have good water solubility, high degree of dissociation, and uniform ion distribution, which facilitates precise proportioning of components in the co-precipitation reaction. Nitrates decompose easily after calcination, leaving no impurities and resulting in high product purity. Methanol has strong solubility, is compatible with phthalic acid and lauric acid, improves the dispersibility of organic acids, regulates precipitation growth, reduces particle agglomeration, and makes the CoMn2O4 crystal morphology more uniform.

[0010] Preferably, the mass ratio of the manganese salt, cobalt salt, phthalic acid and lauric acid is 1-4:1-3:6-10:40.

[0011] Preferably, in step S13, after the reaction, the mixture is cooled to 20-30°C, centrifuged, washed with ethanol, vacuum dried, and pulverized.

[0012] Preferably, the phosphorus compound is trimethyl phosphate or ammonium phosphate. Trimethyl phosphate and ammonium phosphate are chosen as phosphorus sources because they have good solubility, uniform dispersion, and are easily and fully compounded with CoMn2O4.

[0013] Preferably, the molar ratio of the phosphorus compound to CoMn2O4 is 0.3-1.2. Precise control of the phosphorus doping amount optimizes the crystal structure and improves adsorption and catalytic performance, while avoiding structural collapse and reduced activity caused by excessive phosphorus.

[0014] Preferably, in step S14, the heating rate during calcination is 2-10℃ / min. Controlling the heating rate allows phosphorus to diffuse uniformly into the crystal lattice, avoiding rapid heating that could cause powder breakage and agglomeration. Simultaneously, it facilitates gradual and orderly crystal growth, reduces internal defect imbalance, ensures uniform phosphorus doping, and improves the material's crystallinity and catalytic stability.

[0015] The present invention also provides an acid-site modified spinel catalyst prepared by the above preparation method.

[0016] The present invention also provides a method for degrading plastics, wherein the plastics and the acid-site modified spinel catalyst are added to water for degradation for 12-24 h under light conditions.

[0017] Preferably, the plastic is polyethylene.

[0018] Furthermore, the polyethylene includes general polyethylene (PE) with a weight average molecular weight (Mw) of 10,000-50,000 g / mol and low-density polyethylene (LDPE) with a weight average molecular weight (Mw) of 65-75 kDa.

[0019] Preferably, the illumination conditions are achieved using a 250-350 W xenon lamp.

[0020] Preferably, the acid-site modified spinel catalyst is used in water at an amount of 0.25-2.5 mg / mL.

[0021] Compared with the prior art, the above-described technical solution of the present invention has the following advantages: This invention addresses the difficulties in activation and cleavage of polyolefins such as LDPE due to the nonpolarity of their molecular chains and the high dissociation energy of their CH bonds. It employs an impregnation method to phosphorylate CoMn2O4, introducing Brønsted acid sites to promote the protonation of LDPE carbon chains to form carbocations, significantly reducing the C / C bond cleavage energy barrier. Simultaneously, it optimizes the catalyst's band structure, enhances visible light absorption, and promotes the in-situ generation of reactive oxygen species such as OH.

[0022] Meanwhile, this invention introduces the Brønsted acid functionalization strategy into the polyolefin photocatalytic degradation system for the first time, providing a new principle for the green removal of microplastics in water and setting a new paradigm for the design of inert plastic-directed activation catalysts. Attached Figure Description

[0023] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0024] Figure 1 This is CoMn2O4-nPO from Example 1 of the present invention. x Comparative Example 1 CoMn2O4-SO4 2- The XRD patterns of CoMn2O4@(NH4)3PO4 in Example 2; wherein, PDF#01-1126 is card record numbered 01-1126 in the International Center for Diffraction Data (ICDD) Powder Diffraction File database; Figure 2 This is CoMn2O4-nPO from Example 1 of the present invention. x Comparative Example 1 CoMn2O4-SO42- Infrared spectra of CoMn2O4@(NH4)3PO4 from Example 2; Figure 3 This is CoMn2O4-nPO from Example 1 of the present invention. x High-magnification transmission electron microscopy (TEM) elemental scanning map and energy dispersive spectroscopy (EDS) image; Figure 4 This is CoMn2O4-SO4 from Comparative Example 1 of the present invention. 2- High-magnification TEM elemental scanning map and EDS map; Figure 5 These are the high-magnification TEM elemental scan and EDS image of CoMn2O4@(NH4)3PO4 in Example 2 of this invention; Figure 6 This is CoMn2O4-nPO from Example 1 of the present invention. x Yield plot for (n=0,0.06,0.20,0.30,0.55); Figure 7 This is CoMn2O4-nPO from Example 1 of the present invention. x Comparative Example 1 CoMn2O4-SO4 2- Performance comparison with CoMn2O4@(NH4)3PO4 in Example 2. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0026] Example 1: Preparation method of transition metal materials with phosphorylated metal sites (1) Preparation of original CoMn2O4 0.24 g of manganese nitrate tetrahydrate and 4 g of lauric acid were dissolved in 60 mL of methanol. 0.18 g of cobalt nitrate hexahydrate was added to the solution, and the mixture was sonicated for 5 min and stirred for 10 min. 0.8 g of phthalic acid was dissolved in methanol and added dropwise to the solution, and the mixture was stirred for 10 min. The mixture was then transferred to a 200 mL reactor and reacted at 125 °C for 6 hours. After cooling to 25 °C, the mixture was centrifuged at 4000–7650 rpm, washed with ethanol, and dried under vacuum. The dried precursor product was collected, pulverized, and calcined at 500 °C for 2 hours in air. After cooling to room temperature (25 °C), CoMn₂O₄ powder was obtained.

[0027] (2) Preparation of phosphorylated CoMn2O4 (CoMn2O4-nPOx) 150, 300, 500, and 600 μL of trimethyl phosphate solution were dissolved in 1.4 mL of deionized water, respectively. The solution volumes were calculated based on the theoretical P:Co molar ratios of 0.3, 0.6, 1, and 1.2. Next, 1.0 g of previously synthesized CoMn₂O₄ powder was added. The mixture was aged at room temperature for 12 hours, then dried overnight at 80 °C to obtain a solid sample. Finally, the sample was calcined in air at 450 °C for 4 hours at a heating rate of 10 °C / min, and then cooled to room temperature to obtain the sample CoMn₂O₄-nPO₄. x (n=0.06,0.20,0.31,0.55).

[0028] Example 2: Preparation of CoMn2O4@(NH4)3PO4 2.0 g of (NH4)3PO4·3H2O was dissolved in 70 mL of deionized water and stirred. This amount was calculated based on the theoretical P to Co molar ratio of 1. Then, 2.3 g of CoMn2O4 powder obtained in step (1) of Example 1 was added to the (NH4)3PO4 aqueous solution and stirred vigorously for 10 hours. The mixture was filtered and washed several times with deionized water. The obtained precipitate was then dried overnight under vacuum at 80 °C and then calcined in static air at a heating rate of 2 °C / min for 4 hours at a temperature of 500 °C. After cooling to room temperature, the final CoMn2O4@(NH4)3PO4 sample was obtained.

[0029] Comparative Example 1: Preparation of transition metal materials with sulfated functionalized metal sites 465.4 mg of CoMn2O4 powder prepared in Example 1 (1) was dispersed in 10 mL of deionized water, and 10 mL of 200 mM (NH4)2SO4 aqueous solution (corresponding to the theoretical molar ratio of SO4) was added to the mixture. 2- / Co is 1). After vacuum drying for 18 hours, the resulting solid was calcined in air at 500°C for 4 hours to obtain CoMn2O4-SO4. 2- .

[0030] Comparative Example 2: The CoMn2O4 powder prepared in step (1) of Example 1 was used.

[0031] Application Example 1: Cut a 30×50 cm piece 2 Polyethylene plastic wrap (PE, Miaojie PE Plastic Wrap M160E) was used and rinsed three times with deionized water. 200 mL of deionized water was placed in a beaker and heated to 75°C using a constant temperature water bath. The plastic wrap was then immersed in the water bath for 30 minutes and removed. The water sample containing microplastics was allowed to cool to room temperature; this water sample was referred to as the leachate.

[0032] Sodium chloride (NaCl) was selected as the flotation agent. 500 mL of saturated NaCl solution was added to the 40 mL leachate sample, then transferred to a separatory funnel and allowed to stand for 24 hours. After removing the bottom precipitate, the upper layer was filtered through a 10 μm stainless steel membrane.

[0033] Photocatalytic degradation of microplastics was carried out in a 50 mL batch reactor. 10 mg of photocatalyst (i.e., the catalyst products prepared in Examples 1 and 2, and Comparative Examples 1 and 2) was dispersed in 40 mL of the above-mentioned leachate and continuously stirred. Before irradiation, the suspension was continuously stirred in the dark at 10 bar O2 pressure for 20 minutes to reach adsorption-desorption equilibrium. Then, the suspension was continuously irradiated with magnetic stirring for 12 hours using a xenon lamp (300 W) as the light source.

[0034] Application Example 2: Take a 30×6 cm 2 A 150 mg low-density polyethylene (LDPE, 70 kDa) plastic film was cut into three pieces, and 50 mg of photocatalyst was evenly spread on each of the three LDPE plastic film pieces. The plastic film was folded, small holes were punched in it, and then it was fixed to the bottom of the reactor. Subsequently, 20 mL of deionized water was slowly added. Finally, a 300 W xenon lamp was used as the light source, and the reactor was irradiated at 10 bar O2 pressure for 24 hours.

[0035] Effect evaluation: X-ray powder diffraction (XRD) analysis was performed on an X'Pert-Pro MPD using Cu Ka radiation (λ = 1.5406 Å) with a 2θ range of 10° to 80° and a scan step of 0.02°. High-resolution transmission electron microscopy (HRTEM) and high-intensity electron microscopy (HAADF) were performed using a Tecnai G2 F20.

[0036] Fourier transform infrared (FT-IR) measurements were performed on a BRUKER VERTEX 70. B acid / L acid Fourier transform infrared (FT-IR) measurements were performed using a Bruker Tensor 27 FTIR spectrometer based on pyridine adsorption. Samples were pretreated under vacuum at 300°C for 1 hour to remove surface-adsorbed contaminants. Background spectra were collected after the samples were cooled to room temperature. Pyridine vapor was introduced into the reaction cell for 60 min, followed by extraction of physically adsorbed pyridine by vacuum for 30 min. Finally, the samples were heated to 110°C and held for 30 min at a heating rate of 10°C / min. Measurements were performed in the wavenumber range of 1400–1700 cm⁻¹. -1Infrared signals are collected internally. Similarly, the temperature is raised to other specified temperatures (40°C and 70°C) for desorption, and then cooled to collect the corresponding spectra.

[0037] The method for detecting gas concentrations using gas chromatography (GC) is as follows: H2 and N2 are detected using a thermal conductivity detector (TCD) equipped with GDX-104 and 5A packed columns; CO, CO2, and CH4 are detected using a flame ionization detector (FID) equipped with a TDX-01 packed column. The vaporization chamber temperature is 150°C, the column temperature is maintained at 60°C, the TCD detector temperature is 100°C, and the FID detector temperature is 150°C.

[0038] Original CoMn2O4 and all CoMn2O4-nPO x The X-ray diffraction (XRD) pattern of the sample was indexed as a spinel structure of CoMn2O4 (PDF card number 01-1126), indicating that surface phosphorylation did not change the original spinel structure of the catalyst.

[0039] Fourier transform infrared (FTIR) spectroscopy measurements were performed to confirm the presence of phosphate groups on the catalyst surface, such as... Figure 2 1590 and 3420 cm -1 The peaks at these locations belong to surface hydroxyl groups and adsorbed water molecules, respectively. For CoMn2O4-nPO4... x 990 and 1180cm -1 The peaks at T are respectively attributed to T d The t1 vibrational frequency of free phosphate ions in a symmetrical state and the phosphate ion in a connected bidentate state. 1110 cm⁻¹ -1 The peak at 920 cm⁻¹ belongs to the P=O vibration. -1 The peak at 2100 cm⁻¹ belongs to the asymmetric stretching vibration of the POP group; the peak at 2100 cm⁻¹ is observed in phosphate-containing samples. -1 The peak at that point corresponds to the stretching vibration of the p-OH bond. CoMn2O4-nPO x High-resolution TEM (HRTEM) image of the sample ( Figure 3 The crystal shows lattice fringes of 0.49 and 0.19 nm, corresponding to the (211) and (101) planes of CoMn2O4, respectively, consistent with XRD results. Figure 1 Consistent with the EDS plot, the phosphorus element is well dispersed on the CoMn2O4 substrate.

[0040] The photocatalytic cracking of LDPE was carried out in a 50 mL batch reactor under light irradiation (through a 300 W xenon lamp) in the liquid phase at room temperature (25 °C). Unless otherwise specified, other reaction conditions were 50 mg catalyst, 150 mg LDPE, 20 mL water, 10 bar O2, and irradiation for 4 hours. Figure 6 and Figure 7 In the middle, compared with the original CoMn2O4, all CoMn2O4-nPO x The samples all showed significantly higher product yields after 4 hours of irradiation. The main gaseous products were CH4, CO2, CO, and H2, while the main liquid products were CH3OH, CH3COOH, and CH3CH2OH. Compared with other samples, CoMn2O4-0.31PO x It has the highest yield of gaseous products, including CH4 (11 μmol / g). plastic ), CO2 (7997 μmol / g) plastic CO (79 μmol / g) plastic ) and H2 (1382 μmol / g plastic In addition, different CoMn2O4-nPO4 x The photocatalytic activity of the catalyst showed a linear relationship with the strength of its corresponding Brønsted acid, indicating that the Brønsted acid centers contributed significantly to the photocatalytic activity. Increasing the initial feed ratio of the organophosphate precursor further enhanced the phosphorylation of the catalyst, and the actual P / Co atomic ratio was determined to be 0.23 by EDS. (CoMn₂O₄-0.31PO) x In comparison, CoMn2O4-0.55PO x The ratio of Brønsted acid to Leachate acid decreased from 17.6% to 7.1%, and the degradation performance of LDPE also decreased accordingly, indicating that an excessively high synthesis feed ratio is not conducive to improving the catalytic performance of the catalyst.

[0041] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing an acid-site modified spinel catalyst, characterized in that, Includes the following steps: S11: Add manganese salt, cobalt salt, phthalic acid and lauric acid to an organic solvent and react at 120-130℃ for 5-7 h to obtain the precursor product; S12: The precursor product is calcined in air at 450-550°C for 1-3 h and cooled to 20-30°C to obtain CoMn2O4; S13: Add CoMn2O4 to water containing phosphorus compounds, react for 10-12 h, and then separate the solid and liquid phases to obtain the solid reactant. S14: Calcine the solid reactants at 450-500℃ for 3-5 h to obtain the acid-site modified spinel catalyst.

2. The preparation method according to claim 1, characterized in that: The manganese salt is selected from manganese nitrate, the cobalt salt is selected from cobalt nitrate, and the organic solvent is selected from methanol.

3. The preparation method according to claim 1, characterized in that: The mass ratio of the manganese salt, cobalt salt, phthalic acid, and lauric acid is 1-4:1-3:6-10:

40.

4. The preparation method according to claim 1, characterized in that: The phosphorus compound is trimethyl phosphate or ammonium phosphate.

5. The preparation method according to claim 1, characterized in that: The molar ratio of the phosphorus compound to CoMn2O4 is 0.3-1.

2.

6. The preparation method according to claim 1, characterized in that: In step S14, the heating rate during calcination is 2-10℃ / min.

7. An acid-site modified spinel catalyst prepared by the preparation method according to any one of claims 1-6.

8. A method for degrading plastics, characterized in that: Under light irradiation, the plastic and the acid-site modified spinel catalyst of claim 7 were added to water for degradation for 12-24 h.

9. The plastic degradation method according to claim 8, characterized in that: The plastic is polyethylene.

10. The plastic degradation method according to claim 8, characterized in that: The acid-site modified spinel catalyst is used in water at a concentration of 0.25-2.5 mg / mL.

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