Polymerized microcrystalline catalyst loaded with phosphotungstic acid encapsulated Cu and Fe and preparation method and application thereof

Abstract

The application discloses a polymeric microcrystal catalyst loaded with phosphotungstic acid and encapsulating Cu and Fe, and a preparation method and application thereof. The catalyst comprises a plurality of spherical bodies formed by agglomeration of silica nanospheres, and phosphotungstic acid is loaded on the surface and in the channels of the silica nanospheres in the form of a macromolecular heteropoly acid; active components Cu and Fe are in the form of isolated ions to replace Si in a skeleton and are encapsulated in hollow silica nanospheres; the content of metal elements Cu and Fe in the catalyst is 2-3 wt% relative to the total mass of the catalyst, the molar ratio of Cu to Fe is 0.8-1.2:1, and Cu and Fe in the catalyst are highly dispersed; the lignin conversion rate reaches 75.1%, and the mass yield and selectivity of a main product, diethyl maleate, reach 78.4 wt% and 73.8%, respectively. The preparation process of the catalyst in the method is simple and environment-friendly.

Description

Technical Field

[0001] This invention relates to the technical field of biomass catalytic conversion and high-value utilization, and in particular to a polymeric microcrystalline catalyst for Cu and Fe encapsulated with phosphotungstic acid, its preparation method, and its application in lignin oxidative depolymerization. Background Technology

[0002] To reduce global dependence on fossil fuels and environmental pollution, the conversion of lignocellulose into biofuels and chemicals is of great significance. Lignin, the main component of lignocellulose, is an aromatic polymer containing the most abundant structural unit β-O-4, along with smaller amounts of α-O-4, β-5, and β-β structural units, which are interconnected by C-C and CO bonds. Due to the complex and amorphous chemical structure of lignin, its efficient conversion into high-value-added platform chemicals presents both opportunities and significant challenges.

[0003] In the catalytic degradation of lignin, the oxidative depolymerization process can generate ketones, aldehydes, and organic acids, which are highly functional and complex and not easily obtained from petroleum fossil fuels, and can be directly utilized, attracting great interest from researchers. In the presence of oxidants, lignin macromolecules generally depolymerize through the following three methods: (1) interunit bond breaking, (2) propane side chain oxidative modification and cleavage to prepare phenolic monomers such as vanillin, and (3) oxidative cleavage of aromatic rings to generate dicarboxylic acids (DCAs) such as adipic acid and maleic acid / fumaric acid. Currently, most researchers focus on the generation of phenolic chemicals, while research on DCAs, which have great application prospects, is relatively limited. Dicarboxylic acids, especially maleic acid, can be used in unsaturated polyester resins, pharmaceuticals, and the food industry. Commercial diethyl maleate mainly comes from the hydrolysis of maleic anhydride, which in turn comes from the oxidation process of fossil fuel chemicals. Therefore, it is of great significance to find an effective and environmentally friendly method to replace fossil fuels in the generation of maleic acid.

[0004] In recent years, Li et al. (ZPCai, XHLi, Selective production of diethylmaleate via oxidative cleavage of lignin aromatic unit, Chem, 2019, 5(9), 2365-2377) have utilized homogeneous catalysts and polyoxometalate ionic liquids ([BSmim]CuPW) 12 O 40 This catalyst catalyzes the oxidative depolymerization of lignin to produce diethyl maleate in high yield and with high selectivity. Since homogeneous catalysts are difficult to separate and recycle, heterogeneous catalysts have a better prospect for industrial applications.

[0005] Li et al. (Li L, Kong J, Hierarchical hollow silicalite encapsulated Cu-Fe oxides for selectively oxidative depolymerization of lignin to diethylmeleate. J Catal, 2025, 448: 116157.) prepared a series of polycrystalline materials (Cu-Fe oxides) encapsulated in hierarchical hollow silicalite nanofibers. x -Fe y @HhNS), used for the stepwise oxidation of lignin to diethyl maleate (DEM), DEM yield is attributed to the framework Cu in Cux-Fey@HhNS. 2+ Deconstruction of lignin Cα-Cβ, followed by isolation of the Fe framework 3 + While this technology achieves the cleavage of aromatic rings and a well-balanced micropore / mesopore ratio, the depolymerization process of lignin macromolecules is inefficient. The complex structure of lignin molecules and the large steric hindrance make it difficult for the large benzene ring structure to diffuse to the surface of the solid catalyst, resulting in a low overall product yield.

[0006] Choi et al. (Jae-Young Kim, Joon Weon Choi, Effect of molecular size of lignin on the formation of aromatic hydrocarbon during zeolite catalyzed pyrolysis, Fuel, 2019, 240, 92100.) prepared three lignin polymers with different molecular sizes (F3>F2>F1) and applied a microporous Y-type molecular sieve catalyst to the pyrolysis reaction of these three lignins. The results showed that the larger the molecular size of the lignin polymer, the lower the yield of its degradation products. Furthermore, the lignin macromolecules first depolymerized under harsh reaction conditions to form small aromatic compounds before further transformation could occur within the molecular sieve channels. This catalyst is difficult to apply under milder reaction conditions.

[0007] Chinese invention patent CN112547134B discloses a polymeric microcrystalline catalyst encapsulated with Cu and Fe oxides, its preparation method, and its application. The catalyst is reacted with organosoluble bagasse lignin at 120–160°C and 0.5–2.0 MPa O2 pressure for 4–32 hours. Subsequently, the polymeric microcrystalline catalyst encapsulated with Cu and Fe oxides is separated from the product, yielding high-value-added monophenolic products and lipid dicarboxylic acid esters. However, the yield of diethyl maleate obtained under the optimal reaction temperature is only 35.3 wt%, and the yield of the main products needs to be improved. Summary of the Invention

[0008] Given the significant diffusion resistance and lack of effective active sites in existing solid catalysts during heterogeneous catalysis of lignin, this invention provides a polymeric microcrystalline catalyst with excellent catalytic performance, environmental friendliness, and low cost, which encapsulates Cu and Fe with phosphotungstic acid, and its preparation method.

[0009] Another objective of this invention is to provide the application of the polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid in the selective oxidative depolymerization of lignin, wherein the yield and selectivity of the main product diethyl maleate obtained by depolymerization reach 38.1 wt% to 78.4 wt% and 58.1% to 73.8%, respectively.

[0010] The objective of this invention is achieved through the following technical solution.

[0011] A polymeric microcrystalline catalyst encapsulating Cu and Fe with phosphotungstic acid (PTA) comprises multiple aggregates of silica nanospheres forming spheres. PTA is supported on the surface and within the pores of the silica nanospheres in the form of a macromolecular heteropolyacid. Active components Cu and Fe replace Si in the framework in isolated ionic states and are encapsulated within hollow silica nanospheres. In the catalyst, the content of Cu and Fe elements, based on the total mass of the catalyst, is 2–3 wt%, with a Cu to Fe molar ratio of 0.8–1.2:1, and Cu and Fe exhibit high dispersion within the catalyst. The polymeric microcrystalline catalyst has a hierarchical pore structure, which includes micropores, mesopores, and macropores. The micropores are mainly distributed on the walls of the hollow nanospheres, with a pore size of 0.6–0.9 nm. The mesopores and macropores are mainly distributed between or within the hollow nanospheres, with a pore size of 2.1–220 nm.

[0012] The preparation method of the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid includes the following steps: 1) Tetraethyl orthosilicate and alkaline solution were mixed and stirred at room temperature for hydrolysis. Then, deionized water, copper salt, and iron salt were added and stirred overnight to form a gel. The resulting mixture was transferred to a hydrothermal reactor and heat-treated at 150–170 °C for 72–120 h. After cooling to room temperature, the mixture was centrifuged, the solid fraction was dried, and calcined at 500–550 °C to obtain the support hier-Cu. 50 -Fe 50 -S-1; 2) After dissolving phosphotungstic acid hydrate in deionized water, add the hier-Cu carrier obtained in step 1). 50 -Fe 50 -S-1, stir and mix evenly, and let the resulting mixture dry at room temperature for 24-48 hours. Then, dry it in an oven and calcine it at a high temperature of 300-350℃ to obtain a polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid.

[0013] Preferably, the alkaline solution is any one of tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium hydroxide.

[0014] Preferably, in step 1), the copper salt is copper citrate; and the iron salt is an organometallic iron salt.

[0015] Preferably, in step 1), the organometallic iron salt is either ferric citrate or ferric acetylacetonate.

[0016] Preferably, in step 1), the molar ratio of tetraethyl orthosilicate to alkaline solution is 1:0.27-0.81; the molar ratio of tetraethyl orthosilicate to deionized water is 1:35-40; the molar ratio of tetraethyl orthosilicate to copper salt is 1:0.016-0.024; and the molar ratio of tetraethyl orthosilicate to iron salt is 1:0.016-0.024. In step 2), the carrier hier-Cu 50 -Fe 50 The mass ratio of -S-1 to phosphotungstic acid hydrate is 1:0.05 to 0.5.

[0017] Preferably, in step 1), the time for mixing and hydrolyzing the tetraethyl orthosilicate and the alkaline solution at room temperature is 2-3 hours; In step 2), the hydrothermal reactor is a stainless steel hydrothermal reactor with a polytetrafluoroethylene liner. In step 1), the high-temperature calcination is carried out in an air atmosphere in a muffle furnace, with a heating rate of 5-10 °C / min and a calcination time of 4-6 h. In step 2), the high-temperature calcination is carried out in an air atmosphere in a muffle furnace, with a heating rate of 2-4 °C / min and a calcination time of 3-5 h; In steps 1) and 2), the drying temperature is 60–100°C; the drying time in step 1) is 12–18 hours. The application of the polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid in the selective oxidative depolymerization of lignin: Biomass or organic lignin, reaction solvent and polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid are mixed and reacted at 155-175℃ and 0.9-1.1MPa O2 pressure for 12-28h. The polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid is separated from the product, and the degradation product obtained is a high-value-added lipid dicarboxylic acid ester chemical.

[0018] The application of the polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid in the selective oxidative depolymerization of lignin: the biomass is derived from sugarcane bagasse, and the organic soluble lignin is derived from any one of poplar wood, sugarcane bagasse, wheat straw, corn cob, rice straw, and cotton straw; the reaction solvent is ethanol; The separation of the polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid from the product involves filtration followed by thorough washing with anhydrous ethanol to obtain ethanol-soluble degradation products. The lipid dicarboxylic esters include diethyl maleate, diethyl succinate, diethyl fumarate, and diethyl malate. The total yield of the obtained degradation products is 28.3–106.2 wt%, of which the yield and selectivity of diethyl maleate are 43.1 wt%–78.4 wt% and 60.4%–73.8%, respectively.

[0019] Compared with existing technologies, the present invention has the following advantages: 1) The multi-level porous structure (micro-meso-macropores) of the catalyst containing the polymerized hollow nanosphere structure of the present invention can promote the diffusion of lignin macromolecules, provide for the further degradation of intermediate products, and regulate the existence and dispersion of metal active sites, thereby improving the depolymerization efficiency of lignin.

[0020] 2) In this invention, the active components Cu and Fe are distributed in a highly dispersed form within the pores of the catalyst, resulting in a larger exposed surface area of ​​the active sites, making it easier for reactant molecules to contact and react; the well-dispersed active sites have a more consistent structure, which is beneficial for controlling the reaction path and reducing the occurrence of side reactions; the dispersed active sites are less likely to migrate and aggregate, especially under high-temperature reaction conditions, making them less prone to sintering and deactivation.

[0021] 3) The hierarchical porous molecular sieve catalyst synthesized in this invention improves the acidity of the catalyst by loading different contents of phosphotungstic acid. The phosphotungstic acid and Cu and Fe in the support produce a synergistic effect, promoting O2 transport and enhancing the catalyst's oxidative depolymerization ability. This makes it easier for large lignin molecules to depolymerize into smaller aromatic products, which then enter the pores and bind to active sites, improving the yield and selectivity of the final product, diethyl maleate. The lignin conversion rate of this invention reaches 75.1%, and the yield and selectivity of the main product, diethyl maleate, reach 78.4 wt% and 73.8%, respectively.

[0022] 4) Compared with other oxidative degradation processes, this invention has significant advantages at high temperatures of 155-175℃, and has potential application value for realizing the comprehensive development and utilization of biomass and solving environmental pollution and energy shortage problems.

[0023] 5) The catalyst of this invention has a simple preparation process, is environmentally friendly, and can achieve high lignin depolymerization efficiency and product selectivity. Attached Figure Description

[0024] Figure 1 The image shows a magnified TEM image of a single aggregated sphere of catalyst 30PW / CuFe@S-1 from Example 1.

[0025] Figure 2 The image shows the TEM images of two aggregated spheres of catalyst 30PW / CuFe@S-1 in Example 1.

[0026] Figure 3 This is an elemental distribution diagram of the catalyst 30PW / CuFe@S-1 in Example 1 of the present invention.

[0027] Figure 4 The TEM image shows the catalyst 50PW / CuFe@S-1 of Example 5 of this invention.

[0028] Figure 5 This is an elemental distribution diagram of the catalyst 50PW / CuFe@S-1 in Example 5 of the present invention.

[0029] Figure 6 The N2 adsorption-desorption isotherms are for different catalysts 30PW / CuFe@S-1, 20PW / CuFe@S-1, and 50PW / CuFe@S-1 in Examples 1, 3, and 5 of this invention.

[0030] Figure 7 The images show the pore size distribution of different catalysts 30PW / CuFe@S-1, 20PW / CuFe@S-1, and 50PW / CuFe@S-1 in Examples 1, 3, and 5 of this invention.

[0031] Figure 8The activity tests for the selective oxidation of lignin to diethyl maleate by catalytic organosol in Examples 6-10 of this invention are shown in the following graphs: line graphs of diethyl maleate selectivity and lignin conversion rate with phosphotungstic acid loading for catalysts 10PW / CuFe@S-1, 20PW / CuFe@S-1, 30PW / CuFe@S-1, 40PW / CuFe@S-1, and 50PW / CuFe@S-1; and bar graphs of diethyl maleate yield with phosphotungstic acid loading.

[0032] Figure 9 The following are activity tests for the selective oxidation of lignin in organosols to prepare diethyl maleate in Examples 6, 11-14 of this invention: line graphs showing the selectivity of diethyl maleate and the conversion rate of lignin with reaction time for catalyst 30PW / CuFe@S-1, and bar graphs showing the yield of diethyl maleate with reaction time.

[0033] Figure 10 The following are activity tests for the selective oxidation of lignin in organosols to prepare diethyl maleate in Examples 6, 15, and 16 of this invention: line graphs showing the selectivity of diethyl maleate and lignin conversion of catalyst 30PW / CuFe@S-1 as a function of reaction temperature, and bar graphs showing the yield of diethyl maleate as a function of reaction temperature.

[0034] Figure 11 The XRD patterns are of the catalysts 10PW / CuFe@S-1, 20PW / CuFe@S-1, 30PW / CuFe@S-1, 40PW / CuFe@S-1, and 50PW / CuFe@S-1 in Examples 1 to 5 of this invention.

[0035] Figure 12 This is the GC-FID spectrum of the 30PW / CuFe@S-1 material in Example 1 of this invention catalyzing the formation of volatile degradation products from organically dissolved bagasse lignin.

[0036] Figure 13 This is the MS spectrum of the 30PW / CuFe@S-1 material used in Example 1 of this invention to catalyze the production of diethyl maleate from organically dissolved bagasse lignin.

[0037] Figure 14 XPS characterization of Cu 2 using catalysts 10PW / CuFe@S-1, 20PW / CuFe@S-1, and pure CuFe@S-1 from Examples 1 and 2 of this invention. p Atlas.

[0038] Figure 15 XPS characterization of Cu 2 for catalysts 30PW / CuFe@S-1, 40PW / CuFe@S-1, and 50PW / CuFe@S-1 in Examples 3-5 of this invention.p Atlas, Figure 16 XPS characterization of Fe2+ in the 30PW / CuFe@S-1 catalyst of Example 1 of this invention p Atlas.

[0039] Figure 17 For the series of Examples 1 to 5 x NH3-TPD acid characterization curves of PW / CuFe@S-1 and pure CuFe@S-1 catalysts. Detailed Implementation

[0040] To better understand the present invention, it will be further described below with reference to the accompanying drawings and specific embodiments. However, the implementation of the present invention is not limited thereto. The described embodiments are some, but not all, of the embodiments of the present invention. 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.

[0041] Note: According to the International Union of Pure and Applied Chemistry (IUPAC), pores with a diameter less than 2 nm are called micropores; pores with a diameter greater than 50 nm are called macropores; and pores with a diameter between 2 and 50 nm are called mesopores. S-1 is short for Silicate-1, indicating MFI type silicon dioxide.

[0042] To address the problems of homogeneous catalysts in lignin depolymerization, and also to the issues in heterogeneous catalytic oxidation where the complex structure and steric hindrance of lignin molecules hinder the diffusion of large benzene rings to the solid catalyst surface, and the low conversion rate, product yield, and selectivity caused by metal active site agglomeration and high-temperature sintering, this invention aims to improve the catalytic depolymerization efficiency of lignin. This invention uses tetraethyl orthosilicate (TEOS) as the silicon source and tetrapropylammonium hydroxide (TPAOH) as a microporous structure directing agent. Copper salts, represented by copper citrate, and iron salts, represented by iron citrate, are used. Through the hydrolysis of tetraethyl orthosilicate, solid copper and iron nanospheres are formed. The low pH of iron citrate inhibits the growth of the nanospheres. During the agglomeration of the nanospheres into larger spheres, the agglomerates are hydrothermally treated in an aqueous TPAOH solution, transforming the solid nanospheres into hollow ones, forming micropores on the walls of the hollow nanospheres, as well as mesoporous and macroporous structures between or within the microspheres. Phosphotungstic acid is then loaded onto a polymeric microcrystalline catalyst encapsulating Cu and Fe species to enhance the catalyst's acidity. The strong Brønsted acid sites of phosphotungstic acid preferentially catalyze the breaking of C–O bonds in lignin macromolecules, generating small-molecule aromatic intermediates. These small molecules more easily diffuse into the micropores, contacting Cu / Fe sites in the framework and W centers of PW, undergoing aromatic epoxidation ring-opening in the presence of O2. The prepared hierarchical porous polymeric nanosphere catalyst with encapsulated Cu and Fe supported by phosphotungstic acid exhibits excellent catalytic performance, especially in the selective oxidative depolymerization of lignin, demonstrating high conversion rate, product yield, and selectivity. Furthermore, the catalyst preparation process of this invention is simple and easy to control.

[0043] Specifically, the preparation method of the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to the present invention includes the following steps: 1) Mix tetraethyl orthosilicate and alkaline solution at room temperature and stir to hydrolyze. Add deionized water, copper salt and iron salt and stir overnight to form a gel. Transfer the resulting mixture to a hydrothermal reactor and heat treat at 150-170℃ for 72-120h. After cooling to room temperature, centrifuge, dry the solid part, and calcine at 500-550℃ to obtain the carrier. 2) After dissolving phosphotungstic acid hydrate in deionized water, add the carrier obtained in step 1), stir and mix evenly. After drying the mixture at room temperature for 24-48 hours, place it in an oven to dry and calcine at 300-350℃ to obtain a polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid.

[0044] In the above preparation method, the alkaline solution can be any one of tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium hydroxide. The copper salt is preferably copper citrate; the iron salt is preferably an organometallic iron salt, and more preferably any one of ferric citrate and ferric acetylacetonate.

[0045] In the above preparation method, the preferred molar ratio of tetraethyl orthosilicate to alkaline solution is 1:0.27-0.81; the preferred molar ratio of tetraethyl orthosilicate to deionized water is 1:35-40; the preferred molar ratio of tetraethyl orthosilicate to copper salt is 1:0.016-0.024; the preferred molar ratio of tetraethyl orthosilicate to iron salt is 1:0.016-0.024; and the preferred mass ratio of carrier to phosphotungstic acid hydrate is 1:0.05-0.5.

[0046] The polymeric microcrystalline catalyst for Cu and Fe encapsulated with phosphotungstic acid prepared by the above method comprises spheres formed by the aggregation of multiple silica nanospheres. Phosphotungstic acid is supported on the surface and within the pores of the silica nanospheres in the form of a macromolecular heteropolyacid. The active components Cu and Fe replace Si in the framework in an isolated ionic state and are encapsulated in hollow silica nanospheres. Based on the total mass of the catalyst, the content of metal elements Cu and Fe relative to the spheres in the catalyst is 2-3 wt%, and the molar ratio of Cu to Fe is 0.8-1.2:1. Cu and Fe in the catalyst are highly dispersed. The polymeric microcrystalline catalyst exhibits a multi-level pore structure and pore size distribution, including micropores, mesopores, and macropores. Among them, micropores are mainly distributed on the sphere walls of hollow nanospheres, with a pore size of 0.6-0.9 nm; mesopores and macropores are mainly distributed between or within the hollow nanospheres, with a pore size of 2.1-220 nm.

[0047] Example 1: Preparation of catalyst 30PW / CuFe@S-1 (1) TEOS (15.4 mL) and tetrapropylammonium hydroxide (TPAOH) (16.5 mL) were mixed and stirred at room temperature for 2 h. Then, deionized water (36 mL), copper citrate (0.22 g, 0.7 mmol), and ferric citrate (0.34 g, 1.4 mmol) were added, and the mixture was stirred overnight at 25 °C. The molar composition of the resulting gel was 1 TEOS: 0.27 TPAOH: 0.02 CuO: 0.01 Fe2O3: 32 H2O. The gel mixture was then transferred to a lined polytetrafluoroethylene hydrothermal reactor and heated in an oven at 170 °C for 72 h. After cooling to room temperature, the mixture was centrifuged, and the solid fraction was dried at 60 °C for 12 h and calcined in air at 550 °C for 6 h at a heating rate of 5 °C / min. The resulting hierarchical porous nanosphere sample was named hier-Cu. 50 -Fe 50 -S-1, where hier represents hierarchical pores, and the molar ratios of Si / Cu and Si / Fe in this catalyst are both 50.

[0048] (2) Dissolve 0.3 g of phosphotungstic acid hydrate in 0.6 mL of deionized water, and add 1.0 g of the obtained hier-Cu to the resulting solution. 50 -Fe50 Sample S-1 was mixed thoroughly, and the resulting mixture was then dried at room temperature for 24 hours, dried at 100°C for 12 hours, and calcined in air at 350°C for 4 hours at a heating rate of 2°C / min to obtain 30PW / CuFe@S-1. The actual loadings of Cu and Fe in the Cu and Fe-encapsulated polymeric microcrystalline catalyst prepared in this embodiment were tested using inductively coupled plasma atomic emission spectrometry (ICP-AES). The results were 2.6 wt% and 2.3 wt%, respectively, which are basically consistent with the theoretical loadings of the two metals.

[0049] The morphology and structure of the Cu and Fe polymeric microcrystalline catalyst supported on phosphotungstic acid and encapsulated in this embodiment were characterized by transmission electron microscopy (TEM). The TEM characterization results are shown in [Figure number missing]. Figure 1 and Figure 2 As shown, from a locally magnified view Figure 1 It can be clearly observed that the material contains multiple nanospheres that have aggregated to form relatively regular spheres, which are formed by the stacking and aggregation of nanospheres. Figure 2 TEM characterization results showed that each nanosphere exhibited a hollow structure, with an average particle size of 60.5 nm. The larger spheres formed by aggregation had an average particle size of approximately 210 nm. This is because the low pH value of copper citrate and ferric citrate during the hydrolysis of tetraethyl orthosilicate in step (1) limited the growth of the nanospheres, causing the solid nanospheres to aggregate and form larger spheres. After being treated with phosphotungstic acid in step (2), the solid nanospheres formed a hollow structure.

[0050] Elemental analysis was performed on the Cu and Fe polymeric microcrystalline catalyst supported on phosphotungstic acid and encapsulated in this embodiment, and the elemental distribution diagram is shown below. Figure 3 As shown, by Figure 3 It can be concluded that phosphotungstic acid is loaded on the surface and within the pores of silica nanospheres in the form of a macromolecular heteropolyacid. The active components Cu and Fe replace Si in the framework in an isolated ionic state and are dispersed in the hollow silica nanospheres.

[0051] The pore structure of the prepared encapsulated Cu and Fe oxide polymeric microcrystalline catalyst was characterized by N2 physical adsorption-desorption, and the results are as follows: Figure 6 and 7 As shown. Figure 6 The N2 adsorption-desorption isotherm for catalyst 30PW / CuFe@S-1 is represented by... Figure 6It can be seen that the adsorption capacity increases significantly in the lower relative pressure range (P / P0 < 0.45), indicating the presence of a microporous structure in the catalyst. In the medium relative pressure range (P / P0 = 0.45-1.0), a significant hysteresis loop appears, indicating the presence of a mesoporous structure in the catalyst. Figure 7 The figure shows the pore size distribution of the catalyst. As can be seen from the figure, the catalyst has a relatively wide pore size distribution. The micropore size ranges are approximately 0.68–0.81 nm and 1.5–2.0 nm, while the mesopore and macropore size ranges are approximately 2.0–126.6 nm (the pore size distribution curve shows that both mesopores and macropores are distributed, and appear to be continuous in the figure; the pore volume and specific surface area measured by nitrogen adsorption-desorption method can only distinguish between micropores, not between mesopores and macropores). Micropores are mainly distributed on the walls of hollow nanospheres. These nanospheres aggregate to form relatively regular, larger spheres, exhibiting obvious mesoporous and macroporous structures between or within the nanospheres. The total specific surface area, micropore specific surface area, and external specific surface area of ​​the catalyst are all 270 m². 2 / g, 185m 2 / g and 85m 2 / g, the total pore volume, micropore volume, and mesopore / macropore volume of the catalyst are all 0.16 cm³. 3 / g, 0.10cm 3 / g and 0.06cm 3 / g.

[0052] The catalyst of this invention contains abundant mesopores and macropores, which helps reduce the diffusion resistance of lignin macromolecules to the catalyst active sites and the desorption of degradation products. Combined with the abundant surface active sites and confinement effect of the microporous structure, it promotes the further conversion of lignin degradation intermediates and improves the yield and selectivity of the final product. The main problems faced by existing catalysts are their small specific surface area and relatively simple pore structure (containing only micropores or mesopores). Catalysts containing only microporous structures (such as ZSM-5), although possessing abundant surface active sites, suffer from low lignin macromolecule diffusion due to the confinement effect of the pores, resulting in low lignin depolymerization efficiency and low product yield and selectivity. Catalysts containing only mesopores and macropores (such as MCM-41), while improving the mass transfer of macromolecular reactants, exhibit lower hydrothermal stability and product selectivity. In this invention, lignin macromolecules first diffuse into the mesopores and macropores of the catalyst, where they undergo oxidative depolymerization at the phosphotungstic acid active sites to generate phenolic compounds. These smaller phenolic compounds further diffuse into the micropores, where the Fe and Cu active sites further open the rings to form lipid dicarboxylic acid esters such as diethyl maleate. Therefore, this invention combines the advantages of pore structure and metal oxide active sites to achieve the stepwise depolymerization of lignin, demonstrating significant advantages over existing catalysts.

[0053] Example 2: Preparation of catalyst 10PW / CuFe@S-1 (1) TEOS (15.4 mL) and TPAOH (16.5 mL) were mixed and stirred at room temperature for 2 h. Then, deionized water (36 mL), copper citrate (0.22 g, 0.7 mmol), and ferric citrate (0.34 g, 1.4 mmol) were added, and the mixture was stirred overnight at 25 °C. The TPAOH used was 25% in water. The molar composition of the resulting gel was 1 TEOS: 0.27 TPAOH: 0.02 CuO: 0.01 Fe2O3: 32 H2O. The mixture was then transferred to a lined polytetrafluoroethylene hydrothermal reactor and heated in an oven at 170 °C for 72 h. After cooling to room temperature, the mixture was centrifuged, and the solid fraction was dried at 60 °C for 12 h and calcined in air at 550 °C for 6 h at a heating rate of 5 °C / min. The resulting hierarchical porous nanosphere sample was named hier-Cu. 50 -Fe 50 -S-1, where hier represents hierarchical pores, and the molar ratios of Si / Cu and Si / Fe in this catalyst are both 50.

[0054] (2) Dissolve 0.1 g of phosphotungstic acid hydrate in 0.6 mL of deionized water, and add 1.0 g of the obtained hier-Cu to the resulting solution. 50 -Fe 50 The -S-1 sample was mixed evenly, and then the resulting mixture was left to air dry at room temperature for 24 hours, dried at 100℃ for 12 hours, and calcined in air at 350℃ for 4 hours with a heating rate of 2℃ / min to obtain 10PW / CuFe@S-1.

[0055] Example 3: Preparation of catalyst 20PW / CuFe@S-1 (1) TEOS (15.4 mL) and TPAOH (16.5 mL) were mixed and stirred at room temperature for 2 h. Then, deionized water (36 mL), copper citrate (0.22 g, 0.7 mmol), and ferric citrate (0.34 g, 1.4 mmol) were added, and the mixture was stirred overnight at 25 °C. The molar composition of the resulting gel was 1 TEOS: 0.27 TPAOH: 0.02 CuO: 0.01 Fe2O3: 32 H2O. The mixture was then transferred to a lined polytetrafluoroethylene hydrothermal reactor and heated in an oven at 170 °C for 72 h. After cooling to room temperature, the mixture was centrifuged, and the solid fraction was dried at 60 °C for 12 h and calcined in air at 550 °C for 6 h at a heating rate of 5 °C / min. The resulting hierarchical porous nanosphere sample was named hier-Cu. 50 -Fe 50-S-1, where hier represents hierarchical pores, and the molar ratios of Si / Cu and Si / Fe in this catalyst are both 50.

[0056] (2) Dissolve 0.2 g of phosphotungstic acid hydrate in 0.6 mL of deionized water, and add 1.0 g of the prepared hier-Cu to the resulting solution. 50 -Fe 50 The -S-1 sample was mixed evenly, and then the resulting mixture was left to air dry at room temperature for 24 hours, dried at 100℃ for 12 hours, and calcined in air at 350℃ for 4 hours with a heating rate of 2℃ / min to obtain 20PW / CuFe@S-1.

[0057] The adsorption-desorption characterization results of catalyst 20PW / CuFe@S-1 are as follows: Figure 6 and 7 As shown, Figure 6 The N2 adsorption-desorption isotherms show that the adsorption capacity increases significantly at lower relative pressures, and there is a significant hysteresis loop at moderate relative pressures, indicating that both microporous and mesoporous structures exist in the catalyst. Figure 7 The figure shows the pore size distribution of the catalyst. As can be seen, the catalyst has a wide pore size distribution, with micropores ranging from approximately 0.68 to 0.86 nm and 1.3 to 2.0 nm, and mesopores and macropores ranging from approximately 2.0 to 185.8 nm. The total specific surface area, micropore specific surface area, and external specific surface area of ​​the catalyst are 296 m². 2 / g, 191m 2 / g and 105m 2 / g, the total pore volume, micropore volume, and mesopore / macropore volume of the catalyst are all 0.17 cm³. 3 / g, 0.10cm 3 / g and 0.07cm 3 / g.

[0058] Example 4: Preparation of catalyst 40PW / CuFe@S-1 (1) TEOS (15.4 mL) and TPAOH (16.5 mL) were mixed and stirred at room temperature for 2 h. Then, deionized water (36 mL), copper citrate (0.22 g, 0.7 mmol), and ferric citrate (0.34 g, 1.4 mmol) were added, and the mixture was stirred overnight at 25 °C. The molar composition of the resulting gel was 1 TEOS: 0.27 TPAOH: 0.02 CuO: 0.01 Fe2O3: 32 H2O. The mixture was then transferred to a lined polytetrafluoroethylene hydrothermal reactor and heated in an oven at 170 °C for 72 h. After cooling to room temperature, the mixture was centrifuged, and the solid fraction was dried at 60 °C for 12 h and calcined in air at 550 °C for 6 h at a heating rate of 5 °C / min. The resulting hierarchical porous nanosphere sample was named hier-Cu. 50 -Fe 50 -S-1, where hier represents hierarchical pores, and the molar ratios of Si / Cu and Si / Fe in this catalyst are both 50.

[0059] (2) Dissolve 0.4 g of phosphotungstic acid hydrate in 0.6 mL of deionized water, and add 1.0 g of the prepared hier-Cu to the resulting solution. 50 -Fe 50 The -S-1 sample was mixed evenly, and then the resulting mixture was left to air dry at room temperature for 24 hours, dried at 100℃ for 12 hours, and calcined in air at 350℃ for 4 hours with a heating rate of 2℃ / min to obtain 40PW / CuFe@S-1.

[0060] Example 5: Preparation of catalyst 50PW / CuFe@S-1 (1) TEOS (15.4 mL) and TPAOH (16.5 mL) were mixed and stirred at room temperature for 2 h. Then, deionized water (36 mL), copper citrate (0.22 g, 0.7 mmol), and ferric citrate (0.34 g, 1.4 mmol) were added, and the mixture was stirred overnight at 25 °C. The molar composition of the resulting gel was 1 TEOS: 0.27 TPAOH: 0.02 CuO: 0.01 Fe2O3: 32 H2O. The mixture was then transferred to a lined polytetrafluoroethylene hydrothermal reactor and heated in an oven at 170 °C for 72 h. After cooling to room temperature, the mixture was centrifuged, and the solid fraction was dried at 60 °C for 12 h and calcined in air at 550 °C for 6 h at a heating rate of 5 °C / min. The resulting hierarchical porous nanosphere sample was named hier-Cu. 50 -Fe 50 -S-1, where hier represents hierarchical pores, and the molar ratios of Si / Cu and Si / Fe in this catalyst are both 50.

[0061] (2) Dissolve 0.5 g of phosphotungstic acid hydrate in 0.6 mL of deionized water, and add 1.0 g of the prepared hier-Cu to the resulting solution. 50 -Fe 50 The -S-1 sample was mixed evenly, and then the resulting mixture was left to air dry at room temperature for 24 hours, dried at 100℃ for 12 hours, and calcined in air at 350℃ for 4 hours with a heating rate of 2℃ / min to obtain 50PW / CuFe@S-1.

[0062] The prepared catalyst 50PW / CuFe@S-1 was characterized by TEM, and the results are shown in the figure. Figure 4 As shown. By Figure 4 It can be seen that the catalyst exhibits a polymerized nanosphere structure with an average particle size of 75 nm, and the larger spheres formed by agglomeration have an average particle size of about 200 nm.

[0063] Elemental analysis of the catalyst 50PW / CuFe@S-1 prepared in this embodiment yielded the elemental distribution diagram shown below. Figure 5 As shown, by Figure 5 Similarly, it can be concluded that phosphotungstic acid is loaded on the surface and within the pores of silica nanospheres in the form of a macromolecular heteropolyacid. The active components Cu and Fe replace Si in the framework in an isolated ionic state and are dispersed in the hollow silica nanospheres.

[0064] The adsorption-desorption characterization results of catalyst 50PW / CuFe@S-1 are as follows: Figure 6 and 7 As shown, Figure 6 The N2 adsorption-desorption isotherms show that the adsorption capacity increases significantly at lower relative pressures, and there is a significant hysteresis loop at moderate relative pressures, indicating that both microporous and mesoporous structures exist in the catalyst. Figure 7 The figure shows the pore size distribution of the catalyst. As can be seen, the catalyst has a wide pore size distribution, with micropores ranging from approximately 0.68 to 0.80 nm and 1.2 to 2.0 nm, and mesopores and macropores ranging from approximately 2.0 to 147.6 nm. The total specific surface area, micropore specific surface area, and external specific surface area of ​​the catalyst are 244 m². 2 / g, 166m 2 / g and 78m 2 / g, the total pore volume, micropore volume, and mesopore / macropore volume of the catalyst are all 0.14 cm³. 3 / g, 0.09cm 3 / g and 0.05cm 3 / g.

[0065] Depend on Figure 8The XRD spectra of the catalysts in Examples 1-5 show that the increase in phosphotungstic acid loading did not affect the crystal structure of the catalyst. No obvious characteristic diffraction peaks of iron oxide, copper oxide and metal composite oxide were observed in the spectra, indicating that Fe and Cu were highly dispersed and did not cause metal agglomeration due to the introduction of phosphotungstic acid, which is beneficial to the improvement of catalytic activity.

[0066] Example 6: A method for the selective oxidation of organosol lignin to prepare diethyl maleate: Organosoluble bagasse lignin (0.1 g), the 30PW / CuFe@S-1 catalyst prepared in Example 1 (0.2 g), and ethanol (20 mL) were added to a 50 mL high-pressure reactor and mixed. The reactor was purged three times with high-purity oxygen, and then purged with 1.0 MPa O2. The reactor was placed in a heating jacket and heated to 165 °C for 24 h. After the reaction, the reactor was cooled to room temperature, the reaction mixture was filtered, and thoroughly washed with anhydrous ethanol. Dimethyl phthalate was added as an internal standard to the obtained ethanol-soluble reaction solution, and then diluted to 25 mL. Gas chromatography-mass spectrometry (GC-MS, capillary column: Agilent HP-5MS, 5% phenyl methyl siloxane, 30 m × 0.320 mm × 0.25 μm) was used for qualitative and quantitative analysis. The temperature program was: 50 °C for 1 min, then increased to 270 °C at a rate of 10 °C / min and held for 5 min. Add 150–175 mL of deionized water to 25 mL of the adjusted reaction solution to precipitate unreacted lignin (regenerated lignin). The lignin conversion rate is calculated by the ratio of the mass difference between the added lignin raw material and the regenerated lignin to the mass of the raw material lignin. The GC-FID chromatogram of the reaction solution is attached. Figure 12 As shown, analysis revealed two types of products at different retention times: phenolic compounds and lipid dicarboxylic acids. Diethyl maleate, with the largest peak area, was identified as the major product, and its mass spectrum is attached. Figure 13 As shown in the chromatogram, this further confirms the presence of diethyl maleate. The mass of the obtained degradation product was determined using the internal standard method, and the mass ratio of the degradation product to the added lignin feedstock was the product yield. Furthermore, the selectivity of diethyl maleate was determined by the ratio of the mass obtained from gas chromatography to the mass of all degradation products.

[0067] Tests showed that under the action of a 30PW / CuFe@S-1 catalyst, with an added lignin raw material of 0.1g, the mass of regenerated lignin obtained was 0.0249g. Using the above formula for calculating the conversion rate of lignin, the conversion rate of lignin under the action of this catalyst was found to be 75.1%. By integrating the peak areas of diethyl maleate and the internal standard dimethyl phthalate, the mass of diethyl maleate was calculated using the internal standard method to be 0.0784g. The total mass of other lipid dicarboxylic acid esters and phenolic compounds was 0.0278g, of which the masses of diethyl succinate, diethyl fumarate, and diethyl malate were 0.0020g, 0.0041g, and 0.0049g, respectively. The calculated yield and selectivity of diethyl maleate were 78.4 wt% and 73.8%, respectively. Compared with the low product yield obtained in the existing lignin oxidative depolymerization process, the petrochemical diethyl maleate in this embodiment has a higher yield and selectivity, which has obvious advantages.

[0068] Example 7: The oxidation process of organosoluble bagasse lignin using 10PW / CuFe@S-1 catalyst in this example is the same as in Example 6, except that the catalyst is replaced with the 10PW / CuFe@S-1 prepared in Example 3. Organosoluble bagasse lignin (0.1g), 10PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) were heated to 165℃ in a reactor for depolymerization for 24h.

[0069] The test results showed that the lignin conversion rate was 68.4% under the reaction conditions, and the yield and selectivity of diethyl maleate were 58.4 wt% and 68.8%, respectively.

[0070] Example 8: The oxidation process of organosoluble bagasse lignin using 20PW / CuFe@S-1 catalyst in this example is the same as in Example 6, except that the catalyst is replaced with the 20PW / CuFe@S-1 prepared in Example 4. Organosoluble bagasse lignin (0.1g), 20PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) were heated to 165℃ in a reactor for depolymerization for 24h.

[0071] The test results showed that the lignin conversion rate was 72.9% under the reaction conditions, and the yield and selectivity of diethyl maleate were 77.3 wt% and 71.9%, respectively.

[0072] Example 9: The oxidation process of organosoluble bagasse lignin using 40PW / CuFe@S-1 catalyst in this example is the same as in Example 6, except that the catalyst is replaced with the 40PW / CuFe@S-1 prepared in Example 5. Organosoluble bagasse lignin (0.1g), 40PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) were heated to 165℃ in a reactor for depolymerization for 24h.

[0073] The test results showed that the lignin conversion rate was 65.3% under the reaction conditions, and the yield and selectivity of diethyl maleate were 77.7 wt% and 76.4%, respectively.

[0074] Example 10: The oxidation process of organosoluble bagasse lignin using 50PW / CuFe@S-1 catalyst in this example is the same as in Example 6, except that the catalyst is replaced with the 50PW / CuFe@S-1 prepared in Example 6. Organosoluble bagasse lignin (0.1g), 50PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) were heated to 165℃ in a reactor for depolymerization for 24h.

[0075] The test results showed that the lignin conversion rate was 64.5% under the reaction conditions, and the yield and selectivity of diethyl maleate were 51.4 wt% and 68.8%, respectively.

[0076] Figure 8 The activity tests of the selective oxidation of organosoluble lignin to prepare diethyl maleate in Examples 6-10 of the present invention are shown. Line graphs of diethyl maleate selectivity and lignin conversion rate of catalysts 10PW / CuFe@S-1, 20PW / CuFe@S-1, 30PW / CuFe@S-1, 40PW / CuFe@S-1, and 50PW / CuFe@S-1 as a function of phosphotungstic acid loading are shown, as well as bar graphs of diethyl maleate yield as a function of phosphotungstic acid loading. Figure 8 The effect of different phosphotungstic acid loadings on catalyst performance was shown. As the phosphotungstic acid loading increased from 10% to 30%, the diethyl maleate yield significantly increased from 58.4 wt% to 78.4 wt%, and the lignin conversion rate increased from 68.4% to 75.1%. When the phosphotungstic acid loading continued to increase to 40% and 50%, the diethyl maleate yield decreased to 77.7 wt% and 51.4 wt%, respectively, and the lignin conversion rate also decreased to 65.3% and 64.5%. These results indicate that the catalytic performance first increases and then decreases with increasing phosphotungstic acid loading, with 30% being the optimal loading. The catalyst with a phosphotungstic acid loading of 50% achieved the lowest diethyl maleate yield of only 51.4 wt%, but this is still a significant improvement compared to the highest diethyl maleate yield of 35.6% achieved by the catalyst in Chinese invention patent CN112547134B.

[0077] Example 11: The oxidation process of organosoluble bagasse lignin in this example is the same as in Example 6, except that organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in the reactor for 12h for depolymerization.

[0078] The test results showed that the lignin conversion rate was 62.6% under the reaction conditions, and the yield and selectivity of diethyl maleate were 49.9 wt% and 76.6%, respectively.

[0079] Example 12: The oxidation process of organosoluble bagasse lignin in this example is the same as in Example 6, except that organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in the reactor for 16h for depolymerization.

[0080] Tests showed that under these reaction conditions, the lignin conversion rate was 71.7%, and the yield and selectivity of diethyl maleate were 64.8 wt% and 76.8%, respectively.

[0081] Example 13: The steps of the 30PW / CuFe@S-1 catalytic organosoluble bagasse lignin oxidation process in this example are the same as in Example 6, except that organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in the reactor for 20h for depolymerization.

[0082] The test results showed that the lignin conversion rate was 74.2% under the reaction conditions, and the yield and selectivity of diethyl maleate were 71.6 wt% and 75.8%, respectively.

[0083] Example 14: The oxidation process of organosoluble bagasse lignin in this example is the same as in Example 6, except that organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in the reactor for 28h for depolymerization.

[0084] The test results showed that the lignin conversion rate was 78.7% under the reaction conditions, and the yield and selectivity of diethyl maleate were 65.1 wt% and 65.3%, respectively.

[0085] Figure 9The activity tests of the selective oxidation of lignin to diethyl maleate by catalytic organosol lignin in Examples 6, 11-14 of the present invention are shown. Line graphs of diethyl maleate selectivity and lignin conversion rate of catalyst 30PW / CuFe@S-1 as a function of reaction time and bar graphs of diethyl maleate yield as a function of reaction time are shown. Figure 9 The effect of reaction time on catalyst performance was shown. As the reaction time increased from 12 h to 24 h, the diethyl maleate yield significantly increased from 49.9 wt% to 78.4 wt%, and the lignin conversion rate increased from 62.6% to 75.1%. When the reaction time was further increased to 28 h, the diethyl maleate yield decreased to 65.1 wt%. These results indicate that the catalytic performance first increases and then decreases with increasing reaction time, with 24 h being the optimal reaction time. The catalyst achieved the lowest diethyl maleate yield of only 49.9 wt% at a reaction time of 12 h, but this is still a significant improvement compared to the highest diethyl maleate yield of 35.6% achieved by the catalyst in Chinese invention patent CN112547134B.

[0086] Example 15: The oxidation process of organosoluble bagasse lignin in this example is the same as in Example 6, except that organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 155℃ in the reactor for 24h for depolymerization.

[0087] The test results showed that the lignin conversion rate was 69.5% under the reaction conditions, and the yield and selectivity of diethyl maleate were 58.7 wt% and 71.6%, respectively.

[0088] Example 16: The oxidation process of organosoluble bagasse lignin in this example is the same as in Example 6, except that organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 175°C in the reactor for 24h for depolymerization.

[0089] Tests showed that under these reaction conditions, the lignin conversion rate was 79.5%, and the yield and selectivity of diethyl maleate were 43.1 wt% and 60.4%, respectively.

[0090] Figure 10 The activity tests of the selective oxidation of lignin to diethyl maleate by catalytic organosol in Examples 6, 15, and 16 of the present invention are shown. Line graphs of diethyl maleate selectivity and lignin conversion rate of catalyst 30PW / CuFe@S-1 as a function of reaction temperature and bar graphs of diethyl maleate yield as a function of reaction temperature are also shown. Figure 10The effect of reaction temperature on catalyst performance was shown. As the reaction temperature increased from 155℃ to 165℃, the diethyl maleate yield significantly increased from 58.7 wt% to 78.4 wt%, and the lignin conversion rate increased from 69.5% to 75.1%. When the reaction temperature continued to increase to 175℃, the diethyl maleate yield decreased to 43.1 wt%. These results indicate that the catalytic performance first increases and then decreases with increasing reaction temperature, with 165℃ being the optimal reaction temperature. The catalyst at a reaction time of 175℃ achieved the lowest diethyl maleate yield of only 43.1 wt%, but this is still a significant improvement compared to the highest diethyl maleate yield of 35.6% achieved by the catalyst in Chinese invention patent CN112547134B.

[0091] Example 17: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin are the same as in Example 6, except that the conditions of step (1) in the catalyst preparation process of Example 1 are changed to heating the polytetrafluoroethylene hydrothermal reactor in an oven at 170°C for 96 hours. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165°C in the reactor for depolymerization for 24 hours.

[0092] The test results showed that the lignin conversion rate was 72.4% under the reaction conditions, and the yield and selectivity of diethyl maleate were 71.5 wt% and 73.6%, respectively.

[0093] Example 18: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin are the same as in Example 6, except that the conditions of step (1) in the catalyst preparation process of Example 1 are changed to heating the polytetrafluoroethylene hydrothermal reactor in an oven at 170°C for 120 hours. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165°C in the reactor for depolymerization for 24 hours.

[0094] The test results showed that the lignin conversion rate was 74.4% under the reaction conditions, and the yield and selectivity of diethyl maleate were 72.1 wt% and 74.1%, respectively.

[0095] Example 19: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin are the same as in Example 6, except that the conditions of step (2) in the catalyst preparation process of Example 1 are changed to calcination in air at 300°C for 4 hours. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165°C in a reactor for depolymerization for 24 hours.

[0096] The test results showed that the lignin conversion rate was 73.7% under the reaction conditions, and the yield and selectivity of diethyl maleate were 73.9 wt% and 72.7%, respectively.

[0097] Example 20: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin are the same as in Example 6, except that the organometallic salt used in step (1) of the catalyst preparation process in Example 1 is replaced with iron acetylacetone. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0098] The test results showed that the lignin conversion rate was 72.6% under the reaction conditions, and the yield and selectivity of diethyl maleate were 72.4 wt% and 71.5%, respectively.

[0099] Example 21: The steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin in this example are the same as in Example 6, except that the molar ratio of tetraethyl orthosilicate to deionized water used in step (1) of the catalyst preparation process in Example 1 is 1:40. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) were heated to 165℃ in a reactor for 24h for depolymerization.

[0100] The test results showed that the lignin conversion rate was 74.6% under the reaction conditions, and the yield and selectivity of diethyl maleate were 76.9 wt% and 72.8%, respectively.

[0101] Example 22: The steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin in this example are the same as in Example 6, except that the heating rate in step (1) of the catalyst preparation process in Example 1 is changed to 10℃ / min. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0102] The test results showed that the lignin conversion rate was 74.4% under the reaction conditions, and the yield and selectivity of diethyl maleate were 76.3 wt% and 72.4%, respectively.

[0103] Example 23: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin are the same as in Example 6, except that the heating rate in step (2) of the catalyst preparation process in Example 1 is changed to 4℃ / min. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0104] The test results showed that the lignin conversion rate was 73.1% under the reaction conditions, and the yield and selectivity of diethyl maleate were 73.5 wt% and 71.8%, respectively.

[0105] Example 23: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organic soluble bagasse lignin are the same as in Example 6, except that the calcination time in step (1) of the catalyst preparation process in Example 1 is changed to 4h. Organic soluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0106] Tests showed that under these reaction conditions, the lignin conversion rate was 74.1%, and the yield and selectivity of diethyl maleate were 68.3 wt% and 67.8%, respectively.

[0107] Example 24: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organic soluble bagasse lignin are the same as in Example 6, except that the calcination time in step (2) of the catalyst preparation process in Example 1 is changed to 5h. Organic soluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0108] The test results showed that the lignin conversion rate was 69.1% under the reaction conditions, and the yield and selectivity of diethyl maleate were 62.3 wt% and 56.8%, respectively.

[0109] Example 25: The steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin in this example are the same as in Example 6, except that the drying temperature in step (1) of the catalyst preparation process in Example 1 is changed to 100℃. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0110] The test results showed that the lignin conversion rate was 74.9% under the reaction conditions, and the yield and selectivity of diethyl maleate were 77.5 wt% and 72.8%, respectively.

[0111] Example 26: In this example, the steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin are the same as in Example 6, except that the drying temperature in step (2) of the catalyst preparation process in Example 1 is changed to 60℃. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0112] The test results showed that the lignin conversion rate was 74.7% under the reaction conditions, and the yield and selectivity of diethyl maleate were 77.9 wt% and 72.9%, respectively.

[0113] Example 27: The steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin in this example are the same as in Example 6, except that the drying time in step (1) of the catalyst preparation process in Example 1 is changed to 18h. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0114] The test results showed that the lignin conversion rate was 74.8% under the reaction conditions, and the yield and selectivity of diethyl maleate were 77.2 wt% and 73.2%, respectively.

[0115] Example 28: The steps of the 30PW / CuFe@S-1 catalytic oxidation of organosoluble bagasse lignin in this example are the same as in Example 6, except that the drying time in step (2) of the catalyst preparation process in Example 1 is changed to 18h. Organosoluble bagasse lignin (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0116] The test results showed that the lignin conversion rate was 74.6% under the reaction conditions, and the yield and selectivity of diethyl maleate were 77.4 wt% and 73.4%, respectively.

[0117] Example 29: The oxidation process of organosoluble bagasse lignin using the 30PW / CuFe@S-1 catalyst in this example is the same as in Example 6, except that 0.9 MPa O2 is introduced. Organosoluble bagasse lignin (0.1 g), 30PW / CuFe@S-1 catalyst (0.2 g), and ethanol (20 mL) are heated to 165 °C in a reactor for depolymerization for 24 h.

[0118] Tests showed that under these reaction conditions, the lignin conversion rate was 73.1%, and the yield and selectivity of diethyl maleate were 73.6 wt% and 74.4%, respectively.

[0119] Example 30: The experimental steps in this example are the same as in Example 6, except that the raw material is replaced with bagasse lignin instead of organic soluble bagasse. Bagasse (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) are heated to 165℃ in a reactor for depolymerization for 24h.

[0120] The yield and selectivity of diethyl maleate under these reaction conditions were tested to be 39.3 wt% and 69.0%, respectively.

[0121] Example 31: The experimental steps in this example are the same as in Example 6, except that the raw material is replaced with corn cob instead of organic soluble bagasse lignin. Corn cob (0.1g), 30PW / CuFe@S-1 catalyst (0.2g), and ethanol (20mL) were heated to 165℃ in a reactor for depolymerization for 24h.

[0122] The yield and selectivity of diethyl maleate under these reaction conditions were tested to be 38.1 wt% and 58.1%, respectively.

[0123] Since the bagasse and corn cobs used in Examples 30 and 31 were not treated with organic solvents to remove cellulose and hemicellulose, the yield and selectivity obtained in Comparative Example 6 were reduced.

[0124] Figure 14 XPS characterization of Cu 2 using catalysts 10PW / CuFe@S-1, 20PW / CuFe@S-1, and pure CuFe@S-1 from Examples 1 and 2 of this invention. p Atlas, Figure 15 XPS characterization of Cu 2 for catalysts 30PW / CuFe@S-1, 40PW / CuFe@S-1, and 50PW / CuFe@S-1 in Examples 3-5 of this invention. p Atlas, Figure 16 XPS characterization of Fe2+ in the 30PW / CuFe@S-1 catalyst of Example 1 of this invention p The graph shows that as the PW loading increases from 0 to 50 wt%, Cu... 2+ The proportion gradually decreased, Cu + The proportion continued to increase, and 30PW / CuFe@S-1 formed the optimal Cu 2+ / Cu + Redox pairs, by Figure 16 It can be seen that Fe 2 p The six peaks in the spectrum can be divided into three categories, with binding energies of 708.3 and 722.3 eV belonging to Fe. 2+ The binding energies of 712.6 and 725.9 eV belong to Fe. 3+The binding energies of 717.6 and 731.0 eV belong to Fe. 3+ The satellite peaks indicate that Fe 2+ and Fe 3+ The species coexist in the catalyst, primarily in the form of Fe. 3+ The existence of species demonstrates that the active components Cu and Fe replace Si in the framework in an isolated ionic state, dispersed in hollow silica nanospheres.

[0125] Figure 17 For the series of Examples 1 to 5 x The NH3-TPD acidity characterization curves of PW / CuFe@S-1 and pure CuFe@S-1 catalysts are shown in the figure. As the phosphotungstic acid (PW) loading increases, the desorption peak area and position of the catalyst undergo significant changes: the total acid content shows a continuous upward trend, reaching its maximum at a PW loading of 30 wt%, and the total acid content remains high even in samples with subsequent PW loadings of 40 wt% and 50 wt%. Regarding the peak area changes, the areas of the medium-strong acid peaks B and C, and the strong acid peaks D and E, significantly increase after PW loading, while the area of ​​the weak acid peak A only increases slightly. This indicates that the introduction of PW not only significantly increases the total acid content but also effectively optimizes the acid distribution. Simultaneously, the medium-strong acid peak C and the strong acid peak D shift significantly towards the high-temperature region, indicating that PW loading significantly enhances the acid strength of the catalyst.

[0126] As can be seen from the above examples, phosphotungstic acid significantly improves the yield and selectivity of diethyl maleate, mainly due to its multiple roles in the catalyst: First, as a strong Brønsted acid, phosphotungstic acid effectively promotes the mild breaking of C–O and C–C bonds in lignin, accelerating the initial depolymerization of macromolecules. These small-molecule aromatic compounds generated from the oxidative depolymerization of lignin more easily enter the micropores and contact the Cu / Fe sites in the framework and the W centers of PW, undergoing aromatic epoxidation ring-opening in the presence of O2, thereby achieving shape-selective catalysis; Figure 11 The XRD patterns revealed no obvious characteristic diffraction peaks of iron oxides, copper oxides, or metal composite oxides. The introduction of phosphotungstic acid did not cause metal agglomeration, and the Cu and Fe active components remained highly dispersed, ensuring sufficient exposure of active sites and stable catalytic performance. The synergistic effect of these factors achieved highly efficient and selective conversion of lignin to diethyl maleate. The yield and selectivity of diethyl maleate obtained by the phosphotungstic acid-encapsulated Cu and Fe polymeric microcrystalline catalyst for lignin depolymerization reached 78.4 wt% and 73.8%, respectively, far exceeding the 35.6 wt% yield and 70.7% selectivity of diethyl maleate obtained under optimal conditions in Chinese invention patent CN112547134B.

[0127] Existing solid catalyst-catalyzed oxidation methods for lignin depolymerization mostly produce phenolic compounds, including vanillin and vanillic acid. It has been reported that the total yield of phenolic compounds is 10-11 wt%. This may be due to the significant diffusion resistance between the lignin macromolecule and the active site of the catalyst, resulting in low product yield and selectivity (R. Behling, S. Valange, G. Chatel, Heterogeneous catalytic oxidation for lignin valorization into valuable chemicals: what results? What limitations? What trends? GreenChem., 2016, 18(7), 1839-1854). Existing technologies (ZP Cai, XHLi, Selective production of diethyl maleate via oxidative cleavage of lignin aromatic unit, Chem, 2019, 5(9), 2365-2377) use ionic liquids as homogeneous catalysts to promote the ring-opening of lignin to produce lipid dicarboxylic acid esters such as diethyl maleate. However, the heterogeneous solid catalyst of this invention is easier to recover and separate than the homogeneous ionic liquid catalyst, and it is also lower in cost and more environmentally friendly.

[0128] In addition, existing technology (Li L, Kong J, Hierarchical hollow silicaliteencapsulated Cu-Fe oxides for selectively oxidative depolymerization of lignin to diethyl meleate. J Catal, 2025, 448: 116157.) has prepared a series of polycrystalline materials (Cu) with copper and iron oxides coated in hierarchical hollow nano-silica zeolite. x -Fe y However, the depolymerization process of lignin macromolecules in this technology is inefficient. The complex structure of lignin molecules and the large steric hindrance make it difficult for the large benzene ring structure to diffuse to the surface of the solid catalyst, resulting in a low yield of the total product.

[0129] In particular, the active components of the polymer microcrystalline catalyst in Chinese invention patent CN112547134B are oxides of Cu and Fe, with Cu and Fe active components respectively in the form of CuO. x and FeO yThe active components Cu and Fe exist in the form of oxides. In this invention, the active components Cu and Fe are highly dispersed within the pores of the catalyst, resulting in a larger exposed surface area of ​​the active sites, making it easier for reactant molecules to contact and react. The well-dispersed active sites have a more consistent structure, which is beneficial for controlling the reaction pathway and reducing side reactions. Furthermore, the introduction of phosphotungstic acid makes the catalyst more acidic, possessing stronger oxygen adsorption and activation capabilities, thereby enhancing the depolymerization ability of lignin. Chinese invention patent CN112547134B yields a maximum of 35.6 wt% for the main product, diethyl maleate, while this invention, at the designed reaction temperature range of 155–175°C, achieves a yield and selectivity of 38.1 wt%–78.4 wt% and 58.1%–73.8% for diethyl maleate, respectively, demonstrating a significant advantage. Simultaneously, this invention, based on the excellent lignin oxidation activity of the polymeric microcrystalline catalyst encapsulated with Cu and Fe supported by phosphotungstic acid, has significant advantages such as green and renewable raw materials, a simple process flow, and high oxidation efficiency.

[0130] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A polymeric microcrystalline catalyst for Cu and Fe encapsulated with phosphotungstic acid, characterized in that, The catalyst comprises spheres formed by the aggregation of multiple silica nanospheres. Phosphotungstic acid is supported on the surface and within the pores of the silica nanospheres in the form of a macromolecular heteropolyacid. The active components Cu and Fe replace Si in the framework in an isolated ionic state and are encapsulated in hollow silica nanospheres. In the catalyst, the content of Cu and Fe elements, based on the total mass of the catalyst, is 2-3 wt%, the molar ratio of Cu to Fe is 0.8-1.2:1, and Cu and Fe in the catalyst are highly dispersed. The polymeric microcrystalline catalyst has a hierarchical pore structure, which includes micropores, mesopores, and macropores. The micropores are mainly distributed on the walls of the hollow nanospheres, with a pore size of 0.6–0.9 nm. The mesopores and macropores are mainly distributed between or within the hollow nanospheres, with a pore size of 2.1–220 nm.

2. The method for preparing the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid as described in claim 1, characterized in that... Includes the following steps: 1) Mix tetraethyl orthosilicate and alkaline solution at room temperature and stir to hydrolyze. Add deionized water, copper salt and iron salt and stir overnight to form a gel. Transfer the resulting mixture to a hydrothermal reactor and heat treat at 150-170℃ for 72-120h. After cooling to room temperature, centrifuge, dry the solid part, and calcine at 500-550℃ to obtain the carrier. 2) After dissolving phosphotungstic acid hydrate in deionized water, add the carrier obtained in step 1), stir and mix evenly. After drying the mixture at room temperature for 24-48 hours, place it in an oven to dry and calcine at 300-350℃ to obtain a polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid.

3. The method for preparing the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to claim 2, characterized in that: The alkaline solution is any one of tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium hydroxide.

4. The method for preparing the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to claim 2, characterized in that: In step 1), the copper salt is copper citrate; the iron salt is an organometallic iron salt.

5. The method for preparing the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to claim 4, characterized in that: In step 1), the organometallic iron salt is either iron citrate or iron acetylacetonate.

6. The method for preparing the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to claim 2, characterized in that: In step 1), the molar ratio of tetraethyl orthosilicate to alkaline solution is 1:0.27-0.81; the molar ratio of tetraethyl orthosilicate to deionized water is 1:32-40; the molar ratio of tetraethyl orthosilicate to copper salt is 1:0.016-0.024; and the molar ratio of tetraethyl orthosilicate to iron salt is 1:0.016-0.

024. In step 2), the mass ratio of the carrier to phosphotungstic acid hydrate is 1:0.05 to 0.

5.

7. The method for preparing the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to claim 2, characterized in that: In step 1), the tetraethyl orthosilicate and the alkaline solution are mixed and stirred for 2-3 hours at room temperature for hydrolysis. In step 2), the hydrothermal reactor is a stainless steel hydrothermal reactor with a polytetrafluoroethylene liner. In step 1), the calcination is carried out in an air atmosphere in a muffle furnace, with a heating rate of 5-10°C / min and a calcination time of 4-6 hours. In step 2), the calcination is carried out in an air atmosphere in a muffle furnace, with a heating rate of 2-4°C / min and a calcination time of 3-5 hours. In steps 1) and 2), the drying temperature is 60–100°C; in step 1), the drying time is 12–18 hours.

8. The application of the Cu and Fe polymeric microcrystalline catalyst supported on phosphotungstic acid encapsulated as described in claim 1 in the selective oxidative depolymerization of lignin, characterized in that: Biomass or organic lignin, reaction solvent, and a polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid are mixed and reacted at 155–175 °C and 0.9–1.1 MPa O2 pressure for 12–28 h. The polymeric microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid is then separated from the product, and the resulting degradation product is a lipid dicarboxylic acid ester mainly composed of diethyl maleate. The reaction solvent is ethanol.

9. The application of the Cu and Fe polymeric microcrystalline catalyst supported on phosphotungstic acid encapsulated according to claim 8 in the selective oxidative depolymerization of lignin, characterized in that: The biomass is derived from sugarcane bagasse, and the organic lignin is derived from any one of poplar wood, sugarcane bagasse, wheat straw, corn cob, rice straw, and cotton straw.

10. The application of the polymeric microcrystalline catalyst of Cu and Fe encapsulated with phosphotungstic acid according to claim 8 in the selective oxidative depolymerization of lignin, characterized in that: The separation of the polymer microcrystalline catalyst containing Cu and Fe encapsulated with phosphotungstic acid from the product is achieved by filtration followed by thorough washing with anhydrous ethanol to obtain an ethanol-soluble degradation product. The lipid dicarboxylic acid esters include diethyl maleate, diethyl succinate, diethyl fumarate, and diethyl malate; wherein the yield and selectivity of the product diethyl maleate are 38.1 wt%–78.4 wt% and 58.1%–73.8%, respectively.

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