Method for catalytic depolymerization of lignin to monophenolic chemicals using derivatives of zeolitic imidazolate framework materials

Abstract

The application discloses a method for preparing monophenol chemicals by catalyzing lignin depolymerization through zeolite imidazolate framework material derivatives. The method uses lignin as raw material, uses zeolite imidazolate framework material derivatives as catalyst, and uses small-molecule organic alcohol as hydrogen-donor solvent. After inert gas replacement and pressure charging to 0.5-3 MPa, the reaction temperature is controlled to 200-240 DEG C, and stirring is carried out for 2-10 h, so that selective depolymerization of lignin can be realized without external hydrogen, and monophenol chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol are obtained. The method is environment-friendly, stable in performance, highly efficient in catalysis and high in selectivity, and realizes selective conversion of lignin into monophenol chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol without external hydrogen.

Description

Technical Field

[0001] This invention relates to the preparation of monophenolic chemicals by catalytic depolymerization of lignin, specifically to a method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives, belonging to the field of efficient utilization technology of agricultural and forestry waste and high-value utilization technology of renewable resources. Background Technology

[0002] Currently, the world's energy demand is growing, and the non-renewable nature of traditional fossil fuels and the environmental pollution caused by their use are prompting researchers to seek renewable and clean energy sources. Biomass is the most abundant renewable organic carbon resource on Earth, and converting it into renewable fuels and chemicals is one of the important ways to achieve "dual carbon" goals and sustainable development.

[0003] Lignin is the second largest biomass resource in terms of reserves and the only renewable aromatic carbon resource in nature. Lignin is composed of phenylpropane structural units linked by carbon-carbon and carbon-oxygen bonds. This complex three-dimensional amorphous structure makes it difficult to efficiently convert and utilize. Over 98% of lignin is used as low-value fuel, resulting in resource waste and environmental pollution. Therefore, to fully utilize the aromatic structural units in lignin, it is urgent to develop efficient and highly selective catalytic systems for converting lignin to achieve its high-value utilization.

[0004] The structural formula of 4-propyl-2,6-dimethoxyphenol is: In the food industry, 4-propyl-2,6-dimethoxyphenol is used as a flavoring and seasoning; it is also an important organic synthetic intermediate for the preparation of various fine chemicals. For example, oxidation yields 2,6-dimethoxy-1,4-benzoquinone, which has anticancer effects; demethoxylation yields 4-propylphenol, a liquid crystal raw material, or 4-propylguaiacol, a raw material for daily chemical fragrance formulations. Currently, the preparation of 4-propyl-2,6-dimethoxyphenol mainly adopts a petrochemical route, which is subject to harsh conditions, cumbersome steps, and numerous side reactions. Therefore, it is crucial to seek efficient and mild alternatives to petroleum-based synthetic routes, and the syringyl structural unit in lignin provides a natural raw material for the synthesis of 4-propyl-2,6-dimethoxyphenol.

[0005] US Patent US005807952A discloses a method for preparing phenolic chemicals by pyrolysis of wood in the presence of a strong alkali. When the amount of potassium hydroxide is approximately 0.1–5 wt.%, the pyrolysis temperature is 400–600 °C, and the reaction time is 1–3 minutes, 15–60% of phenolic chemical products can be obtained. However, this method uses a homogeneous catalyst, making separation and recycling difficult, and generates a large amount of waste alkaline liquid, which is environmentally unfriendly. The harsh reaction conditions add challenges to its application.

[0006] Chinese invention patent application 202211391788 discloses a method for preparing monophenolic products from lignin depolymerization catalyzed by Ni4Ru2 / HZSM-5. The introduction of ruthenium not only promotes the dispersion of metallic nickel but also enhances the hydrogenation activity of the catalytic system, thereby promoting lignin depolymerization. The conversion rate of lignin depolymerization catalyzed by this system is 65.7–89.9%, and the yield of monophenolic products is 6.7–18.7 wt.%. However, this catalytic system uses the precious metal ruthenium as a catalyst, and hydrogen is added externally during the depolymerization process, increasing the cost of catalytic conversion of lignin.

[0007] Chinese invention patent 2021113705176 discloses a method for hydrogenolysis of lignin catalyzed by nickel supported on sodium lignin sulfonate-based porous carbon. This method uses nickel supported on sodium lignin sulfonate-based porous carbon as a catalyst, small organic molecule alcohols as the reaction medium, and lignin as the base material to selectively convert lignin into monophenolic chemicals. The catalyst support is obtained by calcining, washing, and drying a precursor. The precursor uses sodium lignin sulfonate as the carbon source, ZnCl2 and KOH as pore-forming agents, and is obtained by adding deionized water for complete dissolution, reaction, and drying. However, the catalyst preparation process of this technology is complex, involving two steps: pore-forming and reduction. The pore-forming process requires the strong base KOH, which is environmentally unfriendly; the reduction process requires high-purity hydrogen, posing energy consumption and safety issues. Furthermore, the nickel active center in this technology is loaded onto the catalyst via impregnation, resulting in insufficient interaction between the metal and the support, leading to poor dispersion of the metal active center in the catalyst and underutilization of the metal active center sites. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, this invention aims to provide an environmentally friendly, stable, highly efficient, and selective method for catalytically depolymerizing lignin using zeolite-like imidazole ester framework material derivatives; under conditions without external hydrogen, lignin is selectively converted into monophenolic chemicals, primarily 4-propyl-2,6-dimethoxyphenol.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazolium ester framework material derivatives involves using lignin extracted from biomass as raw material, zeolite-like imidazolium ester framework material derivatives as catalysts (Co@NC catalyst), and small organic molecule alcohols as hydrogen donor solvents. After purging with an inert gas, the reaction is pressurized to 0.5–3 MPa, controlled at a temperature of 200–240 °C, and stirred for 2–10 h. This allows for the selective depolymerization of lignin into monophenolic chemicals, primarily 4-propyl-2,6-dimethoxyphenol. The zeolite-like imidazolium ester framework material derivative is obtained by pyrolysis of a cobalt-based zeolite-like imidazolium ester framework material (ZIF-67) under an inert gas atmosphere.

[0011] To further achieve the purpose of this invention, preferably, the precursor is prepared by the following method: cobalt nitrate hexahydrate and 2-methylimidazole are dissolved in deionized water, mixed and stirred at room temperature for 12-24 hours, centrifuged and dried to obtain ZIF-67 precursor.

[0012] Preferably, the molar ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 0.015 to 0.030:1.

[0013] Preferably, the inert gas is any one of nitrogen, argon, and helium.

[0014] Preferably, the pyrolysis refers to the decomposition of a dry solid under an inert atmosphere by heating, and the pyrolysis process is carried out in a tube furnace; the pyrolysis temperature is 600-900℃, and the pyrolysis time is 1-4h.

[0015] Preferably, the lignin extraction method is as follows: the dried biomass raw material and the extract are placed together in a hydrothermal reactor and heated at 100-120°C for 2-6 hours. After cooling, the mixture is filtered, and the solid phase is washed with anhydrous ethanol. The washing liquid and filtrate are combined, and deionized water is added to precipitate the solid. The mixture is allowed to stand for 12-24 hours, and after filtration, the solid phase is collected and dried to obtain the lignin solid raw material. The biomass raw material is any one of bagasse, bamboo, corn cob, poplar, pine, and birch, which is crushed and then sieved through an 80-120 mesh sieve.

[0016] Preferably, the extract is a mixture of anhydrous ethanol and dilute sulfuric acid in a volume ratio of 2 to 5:1, and the concentration of the dilute sulfuric acid is 0.2 to 0.5 M; the amount of extract used per gram of biomass raw material is 10 to 20 mL.

[0017] Preferably, the hydrogen-donating solvent, an organic small molecule alcohol, is any one of methanol, ethanol, ethylene glycol, isopropanol, and butanol.

[0018] Preferably, the mass ratio of the zeolite-like imidazole ester framework material derivative catalyst to lignin is 0.5 to 1.5:1.

[0019] Preferably, the monophenolic chemicals include 4-ethylphenol, 2-methoxy-4-ethylphenol, 2-methoxy-4-propylphenol, 4-ethyl-2,6-dimethoxyphenol, 4-propyl-2,6-dimethoxyphenol, and isopropyl p-hydroxyphenylpropionate.

[0020] Compared with existing technologies, the present invention has the following advantages:

[0021] 1) The catalyst preparation process in this invention is green and environmentally friendly, requiring no external pore-forming agent. It utilizes the intrinsic changes of the precursor during pyrolysis to obtain a hierarchical porous catalyst. Compared to reducing catalysts with high-purity hydrogen, this invention utilizes the reducing properties of carbon itself to pyrolyze the catalyst in an inert atmosphere, making it more economical and safer. Simultaneously, the one-step pyrolysis reduction of the active metal component using carbon in the precursor increases the interaction between the metal and the support. Cobalt is highly dispersed on the carbon support, allowing for full utilization of the cobalt active sites during catalysis. This enables the catalyst to possess both solvent-based hydrogen production and lignin hydrogenolysis capabilities. Not only improve The catalytic performance of lignin hydrogenolysis, At the same time, it also achieved Selective depolymerization of lignin Purpose .

[0022] 2) This invention utilizes the unique properties of the precursor pyrolysis process. In-situ generation Carbon nanotubes possess excellent hydrogen storage capacity and unique surface and pore structures, making them suitable for catalyzing hydrogenolysis and shape-selective catalysis. Main structure Therefore, carbon nanotubes structure The presence of this catalyst improves the adsorption of lignin and the selectivity of the main product 4-propyl-2,6-dimethoxyphenol.

[0023] 3) The zeolite-like imidazole ester framework material derivative catalyst used in this invention has a wide pore size distribution and a large specific surface area, which can effectively reduce the mass transfer resistance between lignin macromolecules and catalyst active sites, thereby enhancing the mass transfer between lignin and catalyst active sites, improving the performance of catalytic hydrogenolysis of lignin, and providing nanoscale space for the main product 4-propyl-2,6-dimethoxyphenol by selective hydrogenolysis of lignin.

[0024] 4) The nitrogen-doped carbon structure of the catalyst in this invention can effectively regulate the physicochemical properties and electronic structure of the catalyst, promoting the electronic interaction between cobalt and nitrogen. (Carbon and nitrogen) Structure of cobalt The anchoring effect promotes the dispersion of cobalt nanoparticles, improves catalyst activity, and achieves higher main product selectivity under milder conditions. It also prevents cobalt loss during catalysis, enhancing catalyst stability.

[0025] 5) This invention uses lignin as raw material, zeolite-like imidazole ester framework material derivatives as catalysts, and 10 mL of isopropanol as the hydrogen donor solvent. After inert gas replacement and pressurization to 0.5–3 MPa, the reaction temperature is controlled at 200–240 °C, and the mixture is stirred for 2–10 h. The lignin conversion rate is 79.8–89.4%, the yield of monophenol products is 11.0–16.1 wt.%, the yield of the main product 4-propyl-2,6-dimethoxyphenol is 4.2–7.5 wt.%, and the selectivity is 36.0–52.0%. Calculated using isopropanol as the hydrogen donor solvent, after 0.1 g of lignin is hydrogenated, the lignin consumes hydrogen in the reaction and is hydrogenated into monophenol products under the action of the catalyst. The amount of hydrogen produced per 10 mL of isopropanol is 2.9–9.7 mmol.

[0026] 6) The zeolite-like imidazole ester framework material derivative-catalyzed lignin depolymerization technology used in this invention to prepare monophenolic chemicals has the advantages of renewable raw materials, simple reaction process, mild reaction conditions, and environmentally friendly catalyst. This zeolite-like imidazole ester framework material derivative-catalyzed lignin depolymerization technology partially solves the problems of low lignin conversion rate and low yield and selectivity of the target product in traditional technologies. Attached Figure Description

[0027] Figure 1 The image shows the XRD pattern of ZIF-67, the precursor of the zeolite-like imidazole ester framework material in Example 1 of this invention.

[0028] Figure 2 This is the nitrogen adsorption-desorption isotherm of ZIF-67, the precursor material of the zeolite-like imidazole ester framework in Example 1 of the present invention.

[0029] Figure 3 This is a pore size distribution diagram of ZIF-67, the precursor material of the zeolite-like imidazole ester framework in Example 1 of the present invention.

[0030] Figure 4 The image shows the XRD pattern of the zeolite-like imidazole ester framework material derivative catalyst Co@NC-800 in Example 1 of this invention.

[0031] Figure 5 The nitrogen adsorption-desorption isotherm of the catalyst Co@NC-800, a zeolite-like imidazole ester framework material derivative, in Example 1 of this invention.

[0032] Figure 6 This is a pore size distribution diagram of the Co@NC-800 catalyst, a zeolite-like imidazole ester framework material derivative catalyst, in Example 1 of the present invention.

[0033] Figure 7 The images show scanning electron microscope (SEM) images and EDS mapping diagrams of the zeolite-like imidazole ester framework material derivative catalyst Co@NC-800 in Example 1 of this invention.

[0034] Figure 8 The image shows the X-ray energy dispersive spectroscopy (EDS) spectrum of Co@NC-800, a catalyst derived from a zeolite-like imidazole ester framework material in Example 1 of this invention.

[0035] Figure 9 This is the gas chromatogram of the product obtained from the depolymerization of lignin from organic-soluble bagasse catalyzed by Co@NC-800 in Example 6 of the present invention.

[0036] Figure 10 This is the mass spectrum of 4-propyl-2,6-dimethoxyphenol, the main product obtained by the depolymerization of lignin from organic-soluble bagasse catalyzed by Co@NC-800 in Example 6 of the present invention. Specific implementation methods

[0037] To better understand the present invention, the invention will be further described below with reference to the accompanying drawings and embodiments, but the implementation of the present invention is not limited thereto.

[0038] This invention uses ZIF-67, a zeolite-like imidazolium ester framework material, as a precursor. Leveraging its ordered periodic network structure and thermal instability, a Co@NC catalyst is obtained through pyrolysis under an inert gas atmosphere. Cobalt serves as the active center for catalytic hydrogen production from isopropanol and hydrogenolysis of lignin. The nitrogen-doped carbon structure effectively modulates the catalyst's electronic structure, and the hierarchical porous structure promotes diffusion between lignin macromolecules and the catalyst, enhancing the catalytic hydrogenolysis performance of lignin. Simultaneously, the carbon nanotube structure in the catalyst exhibits shape-selective catalysis, enabling the selective conversion of lignin into the main product 4-propyl-2,6-dimethoxyphenol, overcoming the limitations of traditional catalysts. Catalytic lignin depolymerization productsThe catalyst exhibits strong selectivity for breaking single chemical bonds in lignin, effectively reducing side reactions and lowering energy consumption for subsequent product separation, thus laying the foundation for the industrialization of high-value utilization of lignin. Compared to Chinese invention patent 2021113705176, this invention reduces the two-step process (pore formation and reduction) to a single pyrolysis step, yielding hierarchical porous carbon-supported non-precious metal materials. The catalyst preparation process does not use the strong base KOH for pore formation, making it more environmentally friendly. Furthermore, the catalytic system does not require external hydrogen gas; instead, it uses a solvent to supply hydrogen, making production safer. Under the same conditions, the selectivity for lignin conversion, monophenol biomass chemical yield, and the yield of the main product 4-propyl-2,6-dimethoxyphenol is slightly improved. Specifically, the present invention relates to a method for preparing monophenolic chemicals by catalyzing the depolymerization of lignin using a zeolite-like imidazolium ester framework material derivative: using lignin extracted from biomass as raw material, a zeolite-like imidazolium ester framework material derivative as catalyst, and an organic small molecule alcohol as hydrogen donor solvent, the reaction is pressured to 0.5–3 MPa after being replaced with an inert gas, the reaction temperature is controlled at 200–240°C, and the reaction is stirred for 2–10 h, so that lignin is selectively depolymerized into monophenolic chemicals mainly composed of 4-propyl-2,6-dimethoxyphenol; the zeolite-like imidazolium ester framework material derivative is obtained by pyrolysis of cobalt-based zeolite-like imidazolium ester framework material (ZIF-67) under an inert gas atmosphere.

[0039] The characteristics and effects of the catalyst of this invention are mainly reflected in the following aspects:

[0040] This invention uses ZIF-67 as a precursor, utilizing its ordered periodic network structure and thermal instability. After pyrolysis in an inert gas atmosphere, ZIF-67 can be reduced to a Co@NC catalyst in one step by utilizing the reducing properties of carbon, while retaining some of the properties of the ZIF-67 precursor. The catalyst prepared by utilizing the pyrolysis characteristics of ZIF-67 has a hierarchical porous structure, which is beneficial for mass transfer between lignin and the catalyst.

[0041] Compared to methods that use external carbon and nitrogen sources to load active metals, this invention uses ZIF-67 as a precursor. The catalyst obtained through pyrolysis exhibits stronger metal-support interactions, and the Co-N bonds in the precursor are retained during pyrolysis. The anchoring effect of Co and N ensures that the cobalt at the catalytic active center remains highly dispersed, improving catalyst activity. Furthermore, it prevents Co loss during catalysis, enhancing catalyst stability. Example 13 shows that after four catalyst cycles, there was no significant loss of metallic cobalt, indicating good catalyst stability.

[0042] Example 1: Preparation of Co@NC-800 catalyst, a zeolite-like imidazolium ester framework material derivative.

[0043] The preparation of the catalyst Co@NC-800, a zeolite-like imidazole ester framework material derivative, includes the following two steps:

[0044] (1) Preparation of ZIF-67 precursor: Weigh 1g of 99% pure cobalt nitrate hexahydrate solid and 27.5g of 99% pure 2-methylimidazole solid, and dissolve them in 15mL and 100mL of deionized water, respectively, and stir for 5min. Mix the two solutions and stir at room temperature for 24h. Centrifuge to obtain purple solid, wash several times with deionized water and methanol, dry overnight in a 60℃ oven, and grind to obtain purple powder ZIF-67.

[0045] (2) Preparation of Co@NC-800 catalyst: The ZIF-67 prepared in (1) was placed in a tube furnace and kept at 800℃ for 2 hours under an inert gas atmosphere to obtain black powder Co@NC-800.

[0046] The structures of the ZIF-67 precursor and the Co@NC-800 catalyst were characterized by X-ray diffraction and nitrogen physical adsorption-desorption. The results are as follows: Figure 1-6 As shown. Figure 1 The diffraction peaks of ZIF-67 are consistent with those of the simulated ZIF-67 model and the XRD patterns from related studies, indicating that the ZIF-67 structure was successfully prepared. Figure 2 As can be seen, the adsorption isotherm of the catalyst precursor ZIF-67 is a classic Type I isotherm. The adsorption capacity increases sharply in the range of relatively low pressure (P / P0 < 0.45), reflecting that the precursor pore size is less than 1 nm. The pore size distribution diagram also confirms this. Figure 3 The pore size distribution of ZIF-67 exhibits a bimodal shape in the micropore range.

[0047] The Co@NC-800 catalyst was obtained through pyrolysis, such as... Figure 4 The characteristic diffraction peaks at 2θ of 44.2°, 51.5°, and 75.8° correspond to the (111), (200), and (220) crystal planes of metallic cobalt. This indicates that the active center of the catalyst is elemental cobalt. Figure 5 It is evident that the adsorption isotherm of Co@NC-800 is a type IV isotherm, and a significant hysteresis loop appears in the range of moderate relative pressures (P / P0 = 0.4–1.0), indicating the presence of a mesoporous structure in the catalyst. Figure 6 As can be seen, the catalyst contains micropores, mesopores, and macropores simultaneously, with an average pore size of 4.1 nm. Meanwhile, the catalyst has a specific surface area of ​​252 m². 2 / g. The catalyst's large specific surface area and wide pore size distribution greatly improve the accessibility of lignin macromolecules to the catalyst's active sites, thus enhancing mass transfer.

[0048] Figure 7 It is evident that the Co@NC-800 catalyst retains, to some extent, the dodecahedral morphology of the ZIF-67 precursor. As pyrolysis proceeds, carbon nanotube structures appear on the catalyst surface. This is due to the remodeling of ZIF-67 at high temperatures, and the presence of Co... 2+ Gradually reduced to Co, Co catalyzes the graphitization of the ligands. With increasing graphitization, carbon nanotube structures are formed, resulting in a morphology with a hexahedral core and multiple carbon nanotubes extending from the surface. EDSMapping analysis of the catalyst revealed that Co is uniformly dispersed within it.

[0049] Figure 8 As can be seen, the N1s spectrum of the Co@NC-800 catalyst shows a Co-N peak with high intensity, indicating a strong metal-support interaction. Simultaneously, a graphite N peak was also observed. This nitrogen-doped carbon structure can effectively regulate the physicochemical properties and electronic structure of the catalyst, thereby enhancing its catalytic performance.

[0050] Example 2: Preparation of Co@NC-900 catalyst, a zeolite-like imidazolium ester framework material derivative.

[0051] The preparation of the catalyst Co@NC-900, a zeolite-like imidazole ester framework material derivative, includes the following two steps:

[0052] (1) Preparation of ZIF-67, a precursor of cobalt-based zeolite imidazole ester framework material: 1 g of 99% pure cobalt nitrate hexahydrate solid and 27.5 g of 99% pure 2-methylimidazolium solid were weighed and dissolved in 15 mL and 100 mL of deionized water, respectively, and stirred for 5 min. The two solutions were mixed and stirred at room temperature for 24 h. After centrifugation, a purple solid was obtained. The solid was washed several times with deionized water and methanol and then dried overnight in a 60℃ oven. After grinding, purple powder ZIF-67 was obtained.

[0053] (2) Preparation of Co@NC-900, a catalyst derived from cobalt-based zeolite imidazole ester framework material: ZIF-67 prepared in (1) was placed in a tube furnace and pyrolyzed at 900℃ for 2h under an inert gas atmosphere to obtain black powder Co@NC-900.

[0054] Example 3: Co@NC-800 catalyzed depolymerization of bagasse lignin

[0055] (1) Extraction of lignin: 10.0 g of bagasse, 120 mL of anhydrous ethanol and 30 mL of 0.3 M dilute sulfuric acid were added to a hydrothermal reactor and reacted at 110 °C for 4 h. After cooling to room temperature, the mixture was filtered, and four times the volume of deionized water was added to the filtrate to precipitate the lignin. After standing for 12 h, the mixture was filtered, dried and ground to obtain bagasse lignin.

[0056] (2) Catalytic depolymerization of lignin: 0.1 g bagasse lignin, 0.125 g Co@NC-800 catalyst, and 10 mL isopropanol were added to the reactor. After purging the reactor with argon five times, 1.0 MPa argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected, and the gas phase products were qualitatively and quantitatively analyzed using a gas chromatograph (GC-TCD, capillary column model: CP-Molsieve, specifications: 30 m × 0.53 mm × 0.25 μm. Instrument temperature program: from room temperature to 40 °C, stabilize for 0.5 min, and hold at 40 °C for 13 min).

[0057] When determining the molar amount of the gaseous products, the system is under normal temperature and pressure, and the mixture of hydrogen and internal standard argon can be considered an ideal gas mixture. According to the ideal gas law and the law of partial volumes of an ideal gas, the argon content n in the gaseous products is... Ar (mmol) and hydrogen content n H2 (mmol) can be calculated according to formulas (1-1) to (1-2).

[0058]

[0059]

[0060] Among them, P Ar (Pa) represents the argon gas pressure used in the reaction; V Ar (mL) represents the volume of argon gas introduced into the reaction; R is the gas constant, with a value of 8.314 J / (mol·K); T(K) is the room temperature. f is the correction factor, S H2 S represents the peak area in the hydrogen spectrum. Ar This represents the peak area in the argon gas spectrum.

[0061] The reaction mixture was filtered, and the filter cake was washed with anhydrous ethanol. It was then soaked in 20 mL of tetrahydrofuran, filtered, washed, and dried. The filter cake was the recovered catalyst and could be recycled. The liquid product and the anhydrous ethanol washing solution were transferred to a 25 mL volumetric flask, and dimethyl phthalate (DMT) was added as an internal standard. The solution was then diluted to volume with anhydrous ethanol. 1.5 mL of the solution was pipetted from the volumetric flask, filtered through a 0.22 μm filter membrane, and transferred to a sample vial. Qualitative and quantitative analysis was performed using gas chromatography-mass spectrometry (GC-MS, capillary column: HP-5MS 5% phenyl Methyl silox, 30 m × 0.25 mm × 0.25 μm). The temperature program was: 50 °C for 1 min, then increased to 250 °C at a rate of 10 °C / min and held for 31 min. Qualitative and quantitative analysis of the liquid product was performed to determine the type and yield of each product. Add 4 times the volume of deionized water to the remaining reaction solution, let stand for 24 hours, and precipitate unreacted lignin. Filter the solution and dry it to constant weight in a vacuum drying oven to obtain brown solid regenerated lignin.

[0062] Table 1 Distribution and yield of volatile products obtained from lignin depolymerization

[0063]

[0064] Corresponding lignin conversion rate (C L ), total yield of monophenolic products (Y) MP The yield of the main product 4-propyl-2,6-dimethoxyphenol (Y) S2 ) and its selectivity (S S2 W can be calculated using formulas (1-3) to (1-6). F (g) and W R (g) represents the mass of the original lignin and the solid precipitated after adding water following the reaction; W MP (g) and W S2 (g) represents the mass of the monophenolic product and 4-propyl-2,6-dimethoxyphenol, respectively.

[0065]

[0066]

[0067]

[0068]

[0069] The GC-FID spectrum of the product is as follows Figure 9 The product with the strongest peak signal is the main product, which is determined by its mass spectrum. Figure 10The main product was confirmed to be 4-propyl-2,6-dimethoxyphenol. Table 1 further shows the retention time and yield of different products. It can be seen that under the action of Co@NC-800 catalyst, the reactivity of the basic structural units of lignin is as follows: syringyl (S unit) > guaiacol (G unit) > p-hydroxyphenyl (H unit).

[0070] Calculations showed that in this example, the lignin conversion rate was 88.7%, the total yield of monophenolic products was 16.1 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 7.4 wt.%, and the selectivity was 46.0%. The amount of hydrogen produced was 5.7 mmol.

[0071] Example 4: Co@NC-900 catalyst depolymerizes bagasse lignin

[0072] The difference between this embodiment and embodiment 3 is that:

[0073] 0.1 g of bagasse lignin, 0.125 g of Co@NC-900 catalyst, and 10 mL of isopropanol were added to a reactor. After purging the reactor with argon five times, 1.0 MPa of argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gases were collected. The solid-liquid mixture after the reaction was filtered, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed using GC-TCD and GC-MS, respectively.

[0074] Calculations showed that in this embodiment, the lignin conversion rate of bagasse was 86.6%, the yield of monophenol products was 14.5 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 6.3 wt.%, and the selectivity was 43.6%. The amount of hydrogen produced was 5.2 mmol.

[0075] Example 5: Co@NC-800 catalyst depolymerizes bagasse lignin

[0076] The difference between this embodiment and embodiment 3 is that:

[0077] 0.1 g of bagasse lignin, 0.1 g of Co@NC-800 catalyst, and 10 mL of isopropanol were added to a reactor. After purging the reactor with argon five times, 1.0 MPa of argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The liquid product was obtained after filtration, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0078] Calculations showed that in this embodiment, the lignin conversion rate of bagasse was 85.4%, the yield of monophenol products was 12.1 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 4.6 wt.%, and the selectivity was 38.6%. The amount of hydrogen produced was 2.9 mmol.

[0079] Example 6: Co@NC-800 catalyst depolymerizes bagasse lignin

[0080] The difference between this embodiment and embodiment 3 is that:

[0081] 0.1 g of bagasse lignin, 0.125 g of Co@NC-800 catalyst, and 10 mL of isopropanol were added to a reactor. After purging the reactor with argon five times, 1.0 MPa of argon gas was introduced, and the temperature was increased to 220 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The liquid product was obtained after filtration, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0082] Calculations show that in this example, the lignin conversion rate of bagasse was 89.4%, the yield of monophenolic products was 15.3 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 7.0 wt.%, and the selectivity was 45.8%. The amount of hydrogen produced was 4.5 mmol.

[0083] Example 7: Co@NC-800 catalyst depolymerizes bagasse lignin

[0084] The difference between this embodiment and embodiment 3 is that:

[0085] 0.1 g of bagasse lignin, 0.125 g of Co@NC-800 catalyst, and 10 mL of isopropanol were added to a reactor. After purging the reactor with argon five times, argon gas was introduced at 0.5 MPa, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The liquid product was obtained after filtration, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0086] Calculations showed that in this embodiment, the lignin conversion rate of bagasse was 87.4%, the yield of monophenolic products was 13.8 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 6.4 wt.%, and the selectivity was 46.0%. The amount of hydrogen produced was 9.7 mmol.

[0087] Example 8: Co@NC-800 catalyst depolymerizes bagasse lignin

[0088] The difference between this embodiment and embodiment 3 is that:

[0089] 0.1 g of bagasse lignin, 0.125 g of Co@NC-800 catalyst, and 10 mL of isopropanol were added to a reactor. After purging the reactor with argon five times, 1.0 MPa of argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 2 h. After cooling to room temperature, the reaction gas was collected. The liquid product was obtained after filtration, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0090] Calculations showed that in this embodiment, the lignin conversion rate of bagasse was 82.0%, the yield of monophenolic products was 11.0 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 4.2 wt.%, and the selectivity was 38.5%. The amount of hydrogen produced was 5.4 mmol.

[0091] Example 9: Co@NC-800 catalyst depolymerizes bagasse lignin

[0092] The difference between this embodiment and embodiment 3 is that:

[0093] 0.1 g of bagasse lignin, 0.125 g of Co@NC-800 catalyst, and 10 mL of anhydrous ethanol were added to a reactor. After purging the reactor with argon five times, argon gas was introduced at 1.0 MPa, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The liquid product was obtained after filtration, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0094] Calculations showed that in this embodiment, the lignin conversion rate of bagasse was 87.0%, the yield of monophenol products was 15.0 wt.%, the yield of 4-propyl-2,6-dimethoxyphenol was 5.4 wt.%, and the selectivity was 36.0%. The amount of hydrogen produced was 6.8 mmol.

[0095] Example 10: Depolymerization of bamboo lignin using Co@NC-800 catalyst

[0096] The difference between this embodiment and embodiment 3 is that:

[0097] (1) Extraction of lignin: 10.0g of bamboo, 120mL of anhydrous ethanol and 30mL of 0.3M dilute sulfuric acid were added to a hydrothermal reactor and reacted at 110℃ for 4h. After cooling to room temperature, the mixture was filtered, and four times the volume of deionized water was added to the filtrate to precipitate the lignin. After standing for 12h, the mixture was filtered, dried and ground to obtain bamboo lignin.

[0098] (2) Catalytic depolymerization of lignin: 0.1 g bamboo lignin, 0.125 g Co@NC-800 catalyst, and 10 mL isopropanol were added to the reactor. After purging the reactor with argon five times, 1.0 MPa argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The solid-liquid mixture after the reaction was filtered, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0099] Calculations showed that in this embodiment, the bamboo lignin conversion rate was 80.1%, the monophenol product yield was 14.2 wt.%, the 4-propyl-2,6-dimethoxyphenol yield was 6.1 wt.%, and the selectivity was 42.7%. The amount of hydrogen produced was 3.3 mmol.

[0100] Example 11: Co@NC-800 catalyst depolymerizes birch lignin

[0101] The difference between this embodiment and embodiment 3 is that:

[0102] (1) Extraction of lignin: 10.0 g of birch wood, 120 mL of anhydrous ethanol and 30 mL of 0.3 M dilute sulfuric acid were added to a hydrothermal reactor and reacted at 110 °C for 4 h. After cooling to room temperature, the mixture was filtered, and four times the volume of deionized water was added to the filtrate to precipitate the lignin. After standing for 12 h, the mixture was filtered, dried and ground to obtain birch lignin.

[0103] (2) Catalytic depolymerization of lignin: 0.1 g of birch lignin, 0.125 g of Co@NC-800 catalyst, and 10 mL of isopropanol were added to the reactor. After purging the reactor with argon five times, 1.0 MPa of argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The solid-liquid mixture after the reaction was filtered, and dimethyl phthalate (DMT) was added as an internal standard to the liquid phase product. The gas phase product and the liquid phase product were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0104] Calculations showed that in this embodiment, the birch lignin conversion rate was 79.8%, the monophenol product yield was 14.5 wt.%, the 4-propyl-2,6-dimethoxyphenol yield was 7.5 wt.%, and the selectivity was 52.0%. The amount of hydrogen produced was 3.9 mmol.

[0105] Example 12: Co@NC-800 catalyst depolymerizes birch lignin

[0106] The difference between this embodiment and embodiment 3 is that:

[0107] (1) Extraction of lignin: 10.0 g of poplar wood, 120 mL of anhydrous ethanol and 30 mL of 0.3 M dilute sulfuric acid were added to a hydrothermal reactor and reacted at 110 °C for 4 h. After cooling to room temperature, the mixture was filtered, and four times the volume of deionized water was added to the filtrate to precipitate the lignin. After standing for 12 h, the mixture was filtered, dried and ground to obtain poplar lignin.

[0108] (2) Catalytic depolymerization of lignin: 0.1 g poplar lignin, 0.125 g Co@NC-800 catalyst, and 10 mL isopropanol were added to the reactor. After purging the reactor with argon five times, 1.0 MPa argon gas was introduced, and the temperature was increased to 230 °C at a rate of 5 °C per minute and maintained for 4 h. After cooling to room temperature, the reaction gas was collected. The solid-liquid mixture after the reaction was filtered, and dimethyl phthalate (DMT) was added as an internal standard to the liquid product. The gaseous and liquid products were qualitatively and quantitatively analyzed by GC-TCD and GC-MS, respectively.

[0109] Calculations showed that in this embodiment, the birch lignin conversion rate was 85.8%, the monophenol product yield was 16.0 wt.%, the 4-propyl-2,6-dimethoxyphenol yield was 7.3 wt.%, and the selectivity was 45.5%. The amount of hydrogen produced was 4.2 mmol.

[0110] Example 13: Catalyst recycling performance

[0111] The catalyst from Example 3 was soaked in tetrahydrofuran for 12 hours, filtered, and dried to constant weight. The resulting solid catalyst was then subjected to a recycling experiment. After four cycles, the lignin conversion rate was 79.0%, the monophenol product yield was 12.8 wt.%, the 4-propyl-2,6-dimethoxyphenol yield was 3.6 wt.%, the selectivity was 28%, and the amount of hydrogen produced was 5.4 mmol. When the catalyst was recycled directly without further treatment, the overall catalyst activity decreased slightly. This was because unreacted substrate and oligomers deposited on the catalyst surface, covering some active sites and leading to a decrease in product yield.

[0112] Atomic absorption spectrometry (AAS) was performed on the catalyst to determine the change in cobalt content before and after recycling. The instrument used for AAS was a Hitachi Z-2300 flame atomic absorption spectrophotometer (AAS), employing an air-acetylene flame atomizer with an atomization temperature range of 2100-2400℃. Before recycling, the cobalt content in the Co@NC-800 catalyst was 29 wt.%, and after four cycles, the cobalt content was 28.7 wt.%. This shows that the cobalt content remained almost unchanged before and after recycling, indicating that there was essentially no cobalt loss from the catalyst. Combined with X-ray energy dispersive spectroscopy (EDS) results, this demonstrates that the anchoring effect of nitrogen (N) in the catalyst on Co effectively prevents Co loss during catalysis, thereby improving the catalyst's stability.

[0113] As can be seen from the above embodiments, the method for preparing monophenolic chemicals by catalytic depolymerization of lignin using a zeolite-like imidazole ester framework material derivative of the present invention achieves selective depolymerization of lignin, with a lignin conversion rate of 79.8-89.4%, a monophenolic product yield of 11.0-16.1 wt.%, and a main product, 4-propyl-2,6-dimethoxyphenol, yield of 4.2-7.5 wt.%, with a selectivity of 36.0-52.0%. Furthermore, using isopropanol as the hydrogen donor solvent, after 0.1 g of lignin is supplied for hydrogenolysis, the lignin consumes hydrogen gas in the reaction and is hydrogenolyzed into monophenolic products under the action of the catalyst. The amount of hydrogen gas generated per 10 mL of isopropanol is 2.9-9.7 mmol. In particular, the technology used in this invention has the characteristics of renewable raw materials, simple reaction process, mild reaction conditions, and environmentally friendly catalyst, and can achieve intermittent or continuous reactions. The present invention has the above-mentioned technical effects, mainly due to the following characteristics:

[0114] 1) Compared with the method of preparing catalysts by adding external carbon sources and heteroatom doping sources, loading active metals, and finally reducing them, the catalyst of the present invention uses ZIF-67 with a highly ordered periodic network structure of coordination bonds as a precursor to prepare Co@NC catalyst by one-step pyrolysis. The interaction between the metal and the support is stronger, the dispersion of active metal cobalt is higher, and the catalytic active site of cobalt is fully utilized. The catalyst can have both catalytic solvent hydrogen production and lignin hydrogenolysis capabilities, which improves the performance of catalytic lignin hydrogenolysis and realizes the selective depolymerization of lignin.

[0115] 2) The catalyst preparation does not require high-purity hydrogen reduction; instead, it utilizes the reducing properties of carbon through pyrolysis in an inert atmosphere, making it more economical and safer. Simultaneously, the reduction of the active metal component by carbon in the precursor enhances the interaction between the metal and the support, improving the catalyst's activity and stability. After four catalyst cycles, there was no significant loss of cobalt metal, indicating good catalytic stability.

[0116] 3) Catalyst preparation does not require external strong alkali to create pores. Multi-level porous catalysts can be obtained by utilizing the characteristics of ZIF-67 pyrolysis process, which enriches the pore structure of the catalyst in a more environmentally friendly way and enhances the mass transfer between lignin and catalyst.

[0117] In summary, this invention, through the nitrogen-doped carbon structure of the catalyst during lignin hydrogenolysis, effectively modulates the physicochemical properties and electronic structure of the catalyst, thereby enhancing its lignin hydrogenolysis performance. Under environmentally friendly and milder conditions, higher selectivity for the main product is achieved, with a selectivity for the main product 4-propyl-2,6-dimethoxyphenol ranging from 36.0% to 52.0%. Furthermore, the catalyst of this invention exhibits good substrate applicability to lignin from various biomass sources and can efficiently catalyze the hydrogenolysis of bagasse, bamboo, poplar, and birch lignin.

[0118] The implementation of the present invention is not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives, characterized in that, Using lignin extracted from biomass as raw material, a zeolite-like imidazolium ester framework material derivative as catalyst, and an organic small molecule alcohol as hydrogen donor solvent, the reaction is pressurized to 0.5–3 MPa after being replaced with an inert gas, and the reaction temperature is controlled at 200–240 °C. The reaction is stirred for 2–10 h to selectively depolymerize lignin into monophenolic chemicals, mainly 4-propyl-2,6-dimethoxyphenol. The zeolite-like imidazolium ester framework material derivative is obtained by pyrolysis of a cobalt-based zeolite-like imidazolium ester framework material under an inert gas atmosphere. The precursor is prepared by dissolving cobalt nitrate hexahydrate and 2-methylimidazole in deionized water, mixing them, stirring at room temperature for 12–24 h, centrifuging, and drying to obtain the ZIF-67 precursor. The organic small molecule alcohol is any one of methanol, ethanol, ethylene glycol, isopropanol, and butanol. The lignin extraction method is as follows: The dried biomass raw material and the extract are placed together in a hydrothermal reactor and heated at 100–120°C for 2–6 hours. After cooling, the mixture is filtered, and the solid phase is washed with anhydrous ethanol. The washing liquid and filtrate are combined, and deionized water is added to precipitate the solid. The mixture is allowed to stand for 12–24 hours, filtered, and the solid phase is collected and dried to obtain solid lignin raw material. The biomass raw material is any one of bagasse, bamboo, corn cob, poplar, pine, and birch, and is pulverized and sieved through an 80–120 mesh sieve. The monophenolic chemicals mentioned are 4-ethylphenol, 2-methoxy-4-ethylphenol, 2-methoxy-4-propylphenol, 4-ethyl-2,6-dimethoxyphenol, 4-propyl-2,6-dimethoxyphenol, and isopropyl p-hydroxyphenylpropionate.

2. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives according to claim 1, characterized in that, The extract is a mixture of anhydrous ethanol and dilute sulfuric acid in a volume ratio of 2 to 5:1, and the concentration of the dilute sulfuric acid is 0.2 to 0.5 M; the amount of extract used per gram of biomass raw material is 10 to 20 mL.

3. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives according to claim 1, characterized in that, The molar ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 0.015 to 0.030:

1.

4. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives according to claim 1, characterized in that, The inert gas mentioned is any one of nitrogen, argon, and helium.

5. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives according to claim 1, characterized in that, The pyrolysis refers to the decomposition of a dry solid under an inert atmosphere by heating, and the pyrolysis process is carried out in a tube furnace; the pyrolysis temperature is 600-900℃, and the pyrolysis time is 1-4h.

6. The method for preparing monophenolic chemicals by catalytic depolymerization of lignin using zeolite-like imidazole ester framework material derivatives according to claim 1, characterized in that, The mass ratio of the zeolite-like imidazole ester framework material derivative catalyst to lignin is 0.5–1.5:1.

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